U.S. patent application number 09/925663 was filed with the patent office on 2002-03-14 for optical amplifier and light source.
Invention is credited to Nilsson, Lars Johan Albinsson, Paschotta, Ruediger Eberhard.
Application Number | 20020030881 09/925663 |
Document ID | / |
Family ID | 10803953 |
Filed Date | 2002-03-14 |
United States Patent
Application |
20020030881 |
Kind Code |
A1 |
Nilsson, Lars Johan Albinsson ;
et al. |
March 14, 2002 |
Optical amplifier and light source
Abstract
Single- or few-moded waveguiding cladding-pumped lasers,
superfluorescent sources, and amplifiers, as well as lasers,
including those for high-energy pulses, are discloswed, in which
the interaction between the waveguided light and a gain medium is
substantially reduced. This leads to decreased losses of guided
desired light as well as to decreased losses through emission of
undesired light, compared to devices of the prior art. Furthermore,
cross-talk and inter-symbol interference in semiconductor
amplifiers can be reduced. We also disclose devices with a
predetermined saturation power. As a preferred embodiment of the
invention, we disclose a single (transverse) mode optical fiber
laser or amplifier in which the active medium (providing gain or
saturable absorption) is shaped as a ring, situated in a region of
the fiber's cross-section where the intensity of the signal light
is substantially reduced compared to its peak value. The fiber can
be cladding-pumped.
Inventors: |
Nilsson, Lars Johan Albinsson;
(Southampton, GB) ; Paschotta, Ruediger Eberhard;
(Zuerich, CH) |
Correspondence
Address: |
REIDLAW, L.L.C.
1926 SOUTH VALLEYVIEW LANE
SPOKANE
WA
99212-0157
US
|
Family ID: |
10803953 |
Appl. No.: |
09/925663 |
Filed: |
August 7, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09925663 |
Aug 7, 2001 |
|
|
|
09326752 |
Jun 4, 1999 |
|
|
|
6288835 |
|
|
|
|
09326752 |
Jun 4, 1999 |
|
|
|
PCT/GB97/03353 |
Dec 4, 1997 |
|
|
|
Current U.S.
Class: |
359/341.1 ;
359/341.5 |
Current CPC
Class: |
H01S 3/06716 20130101;
H01S 3/08045 20130101; H01S 3/0672 20130101; H01S 3/06733 20130101;
H01S 2301/02 20130101; H01S 3/094007 20130101; H01S 3/09403
20130101; H01S 3/2308 20130101; H01S 3/06729 20130101; H01S 3/063
20130101 |
Class at
Publication: |
359/341.1 ;
359/341.5 |
International
Class: |
H01S 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 4, 1996 |
GB |
GB962531.7 |
Claims
We claim:
1. An amplifying optical device comprising: a first waveguiding
structure comprising a first core and cladding and configured to
guide optical radiation, the first waveguiding structure defined by
a cross section defining a central area of the first core; an
amplifying region surrounding the central area of the first core; a
second waveguiding structure comprising a second core and
configured to guide optical pump power; and at least one pump
source configured to supply optical pump power; and wherein: the
second core is at least partly formed by at least part of the
cladding; the second waveguiding structure comprises the amplifying
region; the pump source is optically coupled to the second
waveguiding structure; and in use at least a portion of the optical
radiation guided in the first waveguiding structure overlaps the
amplifying region.
2. The amplifying optical device of claim 1, and further wherein
the amplifying region is located substantially in the cladding.
3. The amplifying optical device of claim 1, and further wherein
the first waveguiding structure and the second waveguiding
structure are fabricated in a single optical fiber, and further
wherein the first core supports a single transverse optical mode at
an operating signal wavelength.
4. The amplifying optical device of claim 3, and further wherein:
the second core is defined by a refractive index; the second core
is located adjacent to at least one region comprising at least one
of a vacuum, a gas, a liquid, a polymer and a glass amplifying
region, and wherein the region is defined by a refractive index
which is lower than the refractive index of the second core; the
amplifying region is made from an oxide glass system selected from
the group comprising silica, doped silica, silicate, and phosphate;
and the amplifying region comprises at least one rare earth dopant
selected from the group comprising Ytterbium, Erbium, Neodymium,
Praseodymium, Thulium, Samarium, Holmium, Europium, Terbium, and
Dysprosium.
5. The amplifying optical device of claim 3, and further wherein:
the first waveguiding structure is defined by a normalized modal
intensity, and the normalized modal intensity of the optical
radiation guided in the first waveguiding structure is between 0.01
and 0.001 micrometers squared; and the ratio of the normalized
modal intensity of the optical radiation guided in the first
waveguiding structure averaged over the amplifying region to the
normalized modal intensity of the optical pump power when averaged
over the amplifying region is between about 1.5 and about 10.
6. The amplifying optical device of claim 5, and further wherein:
the first waveguiding structure is fabricated from at least one
glass system; the amplifying region comprises a rare-earth dopant
comprising Erbium; and the amplifying region is characterized by a
dopant concentration, a disposition and a length, and wherein the
dopant concentration, the disposition and the length of the
amplifying region are arranged such that the amplifying optical
device amplifies in the wavelength range of about 1480 nm to about
1570 nm.
7. The amplifying optical device of claim 6, and wherein in
operation at least some of the optical pump power propagates along
the amplifying region at least two times.
8. The amplifying optical device of claim 6, and further wherein
the first core is characterized by a diameter and wherein the
amplifying region surrounding the first core is disposed in a ring
defined by an inner diameter between one and one point five times
the diameter of the first core.
9. The amplifying optical device of claim 6, and further wherein:
the amplifying region is ring-shaped and is centered on the first
waveguiding structure; and the amplifying region is made from an
erbium-doped oxide glass system selected from the group comprising
silica, doped silica, silicate, aluminosilicate and phosphate.
10. The amplifying optical device of claim 9, and further wherein
the second core is defined by a refractive index, and second core
is located adjacent to at least one region comprising at least one
of a vacuum, a gas, a liquid, a polymer and a glass, and wherein
the region is defined by a refractive index which is lower than the
refractive index of the second core.
11. The amplifying optical device of claim 6, and further wherein
the optical radiation comprises a signal mode and unwanted modes
selected from the group comprising radiation modes and guided
modes, the amplifying optical device further comprising an absorber
configured to differentially attenuate the unwanted modes with
respect to the signal mode.
12. The amplifying optical device of claim 6, and further wherein
the first core is situated off-center to the cross section of the
second waveguiding structure.
13. The amplifying optical device of claim 6, and further wherein
the second core is defined by a shape other than a circular
shape.
14. The amplifying optical device of claim 6, and further wherein
the optical radiation is in the form of a pulse and the amplifying
optical device is configured to reduce the distortion of the
pulse.
15. The amplifying optical device of claim 5, and further wherein:
the first waveguiding structure is fabricated from at least one
glass system; the amplifying region is doped with Ytterbium; the
pump source has a wavelength in the band from about 870 nm to about
950 nm; the amplifying region absorbs at least about 30% of the
optical pump power launched into the second waveguiding structure;
and the amplifying region is characterized by a dopant
concentration of Ytterbium, a disposition and a length, and wherein
the dopant concentration, the disposition and the length of the
amplifying region are arranged such that the amplifying optical
device amplifies in a wavelength range selected from the group of
about 970 nm to about 990 nm, and about 1010 nm to 1030 nm.
16. The amplifying optical device of claim 15, and further wherein:
the amplifying region is ring-shaped and is centered on the cross
section of the first waveguiding structure; and the amplifying
region is made from an ytterbium-doped oxide glass system selected
from the group consisting of silica, doped silica, silicate,
aluminosilicate and phosphate.
17. The amplifying optical device of claim 16, and further wherein
the second core is defined by a refractive index, and second core
is located adjacent to at least one region comprising at least one
of a vacuum, a gas, a liquid, a polymer and a glass, and wherein
the region is defined by a refractive index which is lower than the
refractive index of the second core.
18. The amplifying optical device of claim 15, and further wherein
the optical radiation comprises a signal mode and unwanted modes
selected from the group comprising radiation modes and guided
modes, the amplifying optical device further comprising an absorber
configured to differentially attenuate the unwanted modes with
respect to the signal mode.
19. The amplifying optical device of claim 15, and further wherein
the device is seeded by optical radiation having a wavelength
selected from the group wavelength ranges of about 970 nm to about
990 nm, and about 1010 nm to 1030 nm.
20. The amplifying optical device of claim 15, and further wherein
the device is configured such that at least some of the optical
pump power propagates along the amplifying region at least two
times.
21. The amplifying optical device of claim 15, and further
comprising an optical feedback device comprising a first and a
second reflector, and where the first reflector is selected from
the group comprising a reflector, a mirror, a fiber Bragg grating
and a cleaved facet and the second reflector is selected from the
group comprising a reflector, a mirror, a fiber Bragg grating and a
cleaved facet.
22. The amplifying optical device of claim 21, and further wherein
at least one of the first and second reflectors is configured to
suppress optical feedback outside the wavelength range.
23. The amplifying optical device of claim 5, and further wherein:
the first waveguiding structure is fabricated from at least one
glass system; the amplifying region contains Neodymium; and the
amplifying region is characterized by a dopant concentration, a
disposition and a length, and wherein the dopant concentration, the
disposition and the length of the amplifying region are arranged
such that the amplifying optical device amplifies in the wavelength
range of about 850 nm to about 950 nm.
24. The amplifying optical device of claim 23, and further wherein
the device is configured such that at least some of the optical
pump power propagates along the amplifying region at least two
times.
25. The amplifying optical device of claim 23, and further
comprising an optical feedback device comprising a first and a
second reflector, and wherein the first reflector is selected from
the group comprising a mirror, a reflector, a fiber Bragg grating
and a cleaved facet and where the second reflector is selected from
the group comprising a mirror, a reflector, a fiber Bragg grating
and a cleaved facet.
26. The amplifying optical device of claim 23, and further wherein
the device is seeded by optical radiation having a wavelength
between 850 nm and 950 nm.
27. An amplifying optical device comprising: a first waveguiding
structure comprising a first core and cladding and configured to
guide optical radiation, the first waveguiding structure defined by
a cross section defining a central area of the first core; at least
one pump source configured to supply optical pump power; and an
amplifying region that surrounds the central area of the first
core; and wherein the pump source is optically coupled to the
amplifying region; the amplifying region substantially surrounds
the first core in the cross-section of the first waveguiding
structure; and in use the optical radiation guided in the first
waveguiding structure overlaps the amplifying region.
28. The amplifying optical device of claim 27, and further
comprising an optical feedback device configured to ensure that a
portion of the optical radiation guided by the first waveguiding
structure is amplified more than once by any one section of the
amplifying region.
29. The amplifying optical device of claim 28, and further
comprising a second waveguiding structure comprising a second core
configured to guide the optical pump power; and wherein the second
waveguiding structure comprises the amplifying region; the second
core is at least partly formed by at least part of the cladding;
and the pump source is optically coupled to the second waveguiding
structure.
30. The amplifying optical device of claim 28, and further wherein
the first waveguiding structure is fabricated from at least one
glass system, and wherein the amplifying region contains at least
one rare-earth dopant selected from the group comprising Ytterbium,
Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium,
Europium, Terbium, and Dysprosium.
31. The amplifying optical device of claim 28, and further wherein
the optical radiation comprises a signal mode and unwanted modes
selected from the group comprising radiation modes and guided
modes, the amplifying optical device further comprising an absorber
configured to differentially attenuate the unwanted modes with
respect to the signal mode.
32. The amplifying optical device of claim 27, and further wherein
the amplifying region comprises a silica glass doped with a
rare-earth dopant and a dopant selected from the group comprising
germanium, aluminum and phosphorus.
33. The amplifying optical device of claim 27, and further wherein
the first core is defined by an outer diameter and wherein the
amplifying region surrounding the first core is disposed in a ring
with an inner diameter between one and one point five times the
outer diameter of the first core.
34. The amplifying optical device of claim 28, and further wherein
the feedback device comprises a first and a second reflector, and
where the first reflector is selected from the group comprising a
fiber Bragg grating and a cleaved facet and where the second
reflector is selected from the group comprising a fiber Bragg
grating and a cleaved facet.
35. The amplifying optical device of claim 27, and further
comprising an optical switch configured to be switched between a
blocking state and a non-blocking state, and wherein the amplifying
optical device is configured to be operable such that energy is
stored in the amplifying region when the optical switch is in the
blocking state, and the energy is released in the form of at least
one optical pulse when the optical switch is in the non-blocking
state.
36. The amplifying optical device of claim 27, and further
comprising at least one saturable absorber configured to absorb the
optical radiation.
37. The amplifying optical device of claim 36, and wherein the
first waveguiding structure defines a plurality of longitudinal
sections, and further wherein the saturable absorber and the
amplifying region are located in the same longitudinal section of
the first waveguiding structure.
38. The amplifying optical device of claim 36, and wherein the
wherein the first waveguiding structure defines a plurality of
longitudinal sections in the direction of propagation of the signal
beam, and further wherein the saturable absorber and the amplifying
region are at least partially located in different longitudinal
sections of the first waveguiding structure.
39. The amplifying optical device of claim 36, and wherein in use
the saturable absorber is bleached in a time varying manner.
40. The amplifying optical device of claim 39, and further wherein
the time varying manner is periodic.
41. The amplifying optical device of claim 36, and wherein the
amplifying region comprises a Yb.sup.3+-sensitized Er.sup.3+-doped
glass, and the saturable absorber comprises an Er.sup.3+-doped
glass.
42. The amplifying optical device of claim 41, and further and
wherein: the waveguiding structure is defined in a glass optical
fiber; the cross section of the waveguiding structure is defined by
a center; the amplifying region is made from an oxide glass system
selected from the group comprising silica, doped silica, silicate,
and phosphate; and the saturable absorber is located proximate the
center of the first waveguiding structure.
43. The amplifying optical device of claim 42, and further wherein
the optical pump power has a wavelength of 1020-1080 nm.
44. The amplifying optical device of claim 42, and further
comprising a second waveguiding structure comprising a second core
and configured to guide the optical pump power; and wherein: the
second core is at least partly formed by at least part of the
cladding; the pump source is optically coupled to the second
waveguiding structure; and the pump source emits at a wavelength in
the range 900-950 nm.
45. The amplifying optical device of claim 1, and further wherein
the first waveguiding structure is fabricated from at least one
glass system and wherein the amplifying region contains at least
one rare-earth dopant selected from the group comprising Ytterbium,
Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium,
Europium, Terbium, and Dysprosium.
46. The amplifying optical device of claim 1, and further
comprising a master oscillator configured to generate an optical
seed; and wherein: the amplifying optical device is defined by an
intrinsic lasing threshold and an intrinsic saturation energy; the
master oscillator is optically coupled to the first waveguiding
structure; the optical seed is an optical pulse seed; the
amplifying region is arranged around the first core such that the
amplifying optical device has high energy storage at its intrinsic
lasing threshold; the amplifying region amplifies the optical pulse
seed; and the amplifying optical device is configured to be
operated such that the amplified optical pulse seed has an energy
exceeding the intrinsic saturation energy of the amplifying optical
device.
47. A method of pumping at least one optical fiber amplifier with a
fiber laser, the fiber laser comprising: a first waveguiding
structure comprising a first core and cladding and configured to
guide optical radiation, the first waveguiding structure defined by
a cross section defining a central area of the first core; an
amplifying region; a second waveguiding structure comprising a
second core and configured to guide the optical pump power; and
wherein the amplifying region surrounds the central area of the
first core; the second waveguiding structure contains the
amplifying region; and the second core is at least partly formed by
at least part of the cladding; the method comprising: configuring
the fibre laser to provide optical feedback; pumping the fibre
laser with optical pump power to provide a lasing output; and
optically pumping the optical fibre amplifier with the lasing
output.
48. A method of amplifying optical pulses to energies exceeding the
intrinsic saturation energy of an amplifying optical device,
comprising: providing a first waveguiding structure comprising a
first core and cladding; providing a source of optical pump power;
providing a second waveguiding structure comprising a second core
at least partly formed by at least part of the cladding, and an
amplifying region around the first core; guiding optical radiation
using the first waveguiding structure; and guiding the optical pump
power using the second waveguiding structure such that the
amplifying region interacts with the optical radiation guided in
the first waveguiding structure and the optical pump power guided
in the second waveguiding structure.
49. A method of using a waveguiding saturating absorber comprising:
providing a waveguiding structure having a core and a cladding;
guiding optical radiation in the waveguiding structure; and
providing an absorbing region situated within the cladding and
disposed such that the absorbing region provides an absorption of
the optical radiation guided in the core such that in use at least
10% of the absorption is bleached by the optical radiation guided
by the core in at least a part of the waveguiding saturating
absorber at least part of the time.
50. The method of claim 49 wherein the amplifying region is
configured in a ring around the first core.
51. A method of amplifying optical signals with an optical fiber
amplifier wherein the fiber amplifier comprises: a first
waveguiding structure comprising a first core and cladding and
configured to guide optical radiation, the first waveguiding
structure defined by a cross section defining a central area of the
first core; an amplifying region; a second waveguiding structure
comprising a second core and configured to guide the optical pump
power; and wherein: the amplifying region surrounds the central
area of the first core; the second waveguiding structure contains
the amplifying region; and the second core is at least partly
formed by at least part of the cladding; the method comprising:
pumping the fiber amplifier with optical pump power to provide
gain; and coupling an optical signal to the first waveguiding
structure of the fiber amplifier.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/326,752 filed Jun. 4, 1999, now U.S. Pat.
No.________, which is in turn a Continuation-in-part of PCT Patent
Application No. PCT/GB97/03353, filed Dec. 4, 1997, which in turn
claims priority to Great Britain patent application Serial No.
GB962531.7, filed Dec. 4, 1996, now abandoned.
FIELD OF THE INVENTION
[0002] The invention relates to optical amplifiers and light
sources. By way of example, though not exclusively, the invention
relates to single- or few-moded waveguiding lasers,
superfluorescent sources, optical amplifiers, high pulse-energy
devices, energy-storage devices, cladding-pumped devices,
semiconductor signal amplifiers, and waveguiding saturable
absorbers.
BACKGROUND OF THE INVENTION
[0003] The tightly confined modal fields of single- or few-moded
waveguiding lasers, superfluorescent sources, and amplifiers lead
to a very strong interaction between any waveguided light and the
active medium in the waveguiding core. Therefore, a comparatively
small amount of gain medium is sufficient for providing the gain in
these devices. Specifically, the gain for a given stored energy, as
well as for a given absorbed pump power, is high. This is often
beneficial, since it means that the pump power requirements for a
given desired laser output power or amplifier gain can be low.
[0004] However, for several devices, this efficient interaction
between mode and gain medium can be detrimental. The following
example refers to certain types of amplifiers and lasers, but of
course the skilled man will realise that the same or similar
problems can occur in, for example, superfluorescent sources.
[0005] In a laser or amplifier, the achievable single-pass gain is
limited to, say, 50 dB. The reason is that at this gain, a
significant fraction of the pump power is converted to amplified
spontaneous emission (ASE). A 10 dB higher gain results in
approximately 10 dB more ASE, so at these gains, the extra pump
power required to increase the gain further will be prohibitively
high. Since the ASE limits the gain of the device, it also limits
the energy stored in the gain media. This in turn obviously limits
the amount of energy that a pulse can extract from the device.
Consequently, the pulse energy that can be obtained from
waveguiding lasers and amplifiers is limited. Instead, bulk (i.e.,
not waveguiding) lasers and amplifiers for which the extractable
energy for a given gain can be several orders of magnitude lower
are often employed to provide much higher pulse energies. However,
the robustness and stability of bulk lasers is often inferior to
waveguiding ones.
[0006] Moreover, the gain limit can also be problematic for lasers
and amplifiers irrespective of whether the stored energy is a major
concern, if the high gain appears at another wavelength than the
desired one. The reason is that ASE (or lasing) at the gain peak
will suppress the gain achievable at the desired wavelength,
possibly to a value below what is required for a good amplifier or
laser. This applies to all types of amplifiers and lasers.
[0007] Furthermore, in optically pumped lasers and amplifiers, a
suitable interaction between the gain medium and the amplified or
generated signal beam is not enough; also the interaction between
the pump beam and the gain medium must be appropriate. However, in
some types of lasers and amplifiers (typically cladding-pumped
ones), the interaction with the pump beam is significantly smaller
than the interaction with the signal beam. Then, for a device that
efficiently absorbs the pump, the interaction with the signal beam
will be much stronger than what is required. Unfortunately, this
excess interaction is often accompanied by excess losses for the
signal beam, since:
[0008] 1. The scattering loss of an active medium is normally
higher than it can be for a passive medium. For instance,
rare-earth-doped fibers have scattering losses of, e.g., several
orders of magnitude higher than standard, passive, single-mode
fibers.
[0009] 2. A fraction of the active medium often has inferior
properties. For instance, in Er-doped fibers, pairs of
Er.sup.3+-ions can form. These result in an unbleachable loss. The
strong interaction then leads to a high loss.
[0010] 3. The active medium in its amplifying state can also absorb
light (so-called excited-state absorption, ESA). Again, a stronger
interaction leads to more power lost through ESA.
[0011] Moreover, a bleachable medium (e.g., an unpumped gain medium
with a ground-state absorption) can be used as a saturable
absorber. An efficient interaction leads to a low saturation power.
A reduced interaction leads to a higher saturation power, which can
be more suitable for some applications, especially if the
interaction, and hence the saturation power, can be controlled.
[0012] Clearly, although often beneficial, the tight confinement of
the guided light is a problem for some devices.
SUMMARY OF THE INVENTION
[0013] An aim of the present invention is to improve the
interaction between light guided along a waveguide and rare-earth
dopants within an active medium.
[0014] Accordingly in one non-limiting embodiment of the present
invention, there is provided apparatus comprising a waveguide and
an amplifying region wherein the waveguide comprises a core and a
cladding and the amplifying region comprises rare-earth dopants and
wherein the amplifying region comprises a ring around the core of
the waveguide.
[0015] Various aspects of the invention are defined in the appended
claims, and in passages throughout the present application.
[0016] According to a first embodiment of the present invention,
there is provided an amplifying optical device comprising a first
waveguiding structure comprising a first core and cladding and
configured to guide optical radiation, at least one pump source
configured to supply optical pump power, an amplifying region
situated in the cladding; and wherein the pump source is optically
coupled to the amplifying region; and wherein in use the optical
radiation guided in the first waveguiding structure overlaps the
amplifying region.
[0017] The invention also provides a method of pumping at least one
optical fiber amplifier with a fiber laser, the method comprising
providing a first waveguiding structure fabricated from at least
one glass system and comprising a first core and cladding;
providing a second waveguiding structure comprising a second core
at least partly formed by the cladding and an amplifying region
comprising Ytterbium; providing a source of optical pump power in
optical communication with the second waveguiding structure and
having a wavelength in the band from about 870 nm to about 950 nm;
providing an optical feedback device; guiding optical radiation
using the first waveguiding structure; guiding the optical pump
power using the second waveguiding structure such that the
amplifying region interacts with the optical radiation guided in
the first waveguiding structure and the optical pump power guided
in the second waveguiding structure to amplify the optical
radiation guided by the first waveguiding structure; using the
optical feedback device to ensure that a plurality of times a
portion of the optical radiation guided by the first waveguiding
structure is amplified more than once by the amplifying region;
providing an amplifying region characterized by a dopant
concentration, a disposition and a length, and wherein the dopant
concentration, the disposition and the length of the amplifying
region are arranged such that the fiber laser emits optical
radiation at an emission wavelength in the region of about 970 nm
to about 990 nm; and coupling the optical radiation at the emission
wavelength in the region of about 970 nm to 990 nm into the at
least one optical amplifier.
[0018] A second method provided by the invention is a method of
amplifying optical pulses to energies exceeding the intrinsic
saturation energy of an amplifying optical device, comprising:
providing a first waveguiding structure comprising a first core and
cladding; providing a source of optical pump power; providing a
second waveguiding structure comprising a second core at least
partly formed by at least part of the cladding, and an amplifying
region; guiding optical radiation using the first waveguiding
structure; and guiding the optical pump power using the second
waveguiding structure such that the amplifying region interacts
with the optical radiation guided in the first waveguiding
structure and the optical pump power guided in the second
waveguiding structure.
[0019] A third method provided by the invention is the method of
using a waveguiding saturating absorber comprising: providing a
waveguiding structure having a core and a cladding; guiding optical
radiation in the waveguiding structure; providing an absorbing
region situated within the cladding and disposed such that it
provides an absorption of the optical radiation guided in the core
such that in use at least 10% of the absorption is bleached by the
optical radiation guided by the core in at least a part of the
waveguiding saturating absorber at least part of the time.
[0020] According to a second embodiment of the present invention,
there is provided an amplifying optical device comprising: a first
waveguiding structure configured to guide optical radiation which
can propagate in a fundamental mode; a pump source configured to
supply optical pump power; and a second waveguiding structure
configured to guide the optical pump power, wherein the pump source
is optically coupled to the second waveguiding structure; and
wherein in use the optical radiation is characterized by an optical
power distribution of the fundamental mode having a contour of
equal intensity perpendicular to the local longitudinal axis of the
first waveguiding structure the contour enclosing about 75% of the
optical power of the fundamental mode; and wherein the second
waveguiding structure contains an amplifying region situated to
interact with the optical pump power guided in the second
waveguiding structure when the amplifying optical device is in use;
and wherein the amplifying region is situated to lie outside the
contour of equal intensity; and wherein during use at least 0.1% of
the optical radiation guided by the first waveguiding structure
overlaps the amplifying region.
[0021] Embodiments of the invention provide devices that are
considerably improved by a predetermined reduction of the
interaction between a signal light beam and an active medium (per
unit volume) compared to prior-art designs, without necessarily
changing the properties of the gain medium or reducing the
confinement of the signal light (although a reduced confinement can
also be beneficial for the disclosed devices). The active medium
serves to amplify or generate the signal light beam, or, if
unpumped, can act as a saturable absorber.
[0022] The reduction in interaction is achieved by placing the bulk
of the active medium in regions where the intensity of the signal
beam is substantially smaller than its peak intensity, in a
cross-section of the waveguiding device perpendicular to the
direction of propagation of the signal beam. This can provide
advantages for the following devices:
[0023] 1. Lasers (e.g., Q-switched and gain-switched ones) and
amplifiers in which it is desirable to store large energies. In
these devices (as well as for so-called energy-storage devices in
general), the reduced interaction leads to a larger stored energy
before practical upper limits on the gain is reached.
[0024] 2. Optical amplifiers (typically semiconductor ones) for
which even the energy of a single signal bit can be comparable to
the stored energy. In those, already the amplification of a bit
extracts enough energy to reduce the gain. This leads to four-wave
mixing, cross-talk, and inter-symbol interference. This can be
reduced with the higher stored energy that, for a given gain,
accompanies the reduced interaction.
[0025] 3. Amplifiers and lasers in which an efficient pump
absorption necessitates large amounts of gain media, which in
prior-art devices leads to excessive small-signal absorption,
background absorption, or excited state absorption at the operating
wavelength, or excessive gain at another wavelength. A reduced
interaction then leads to reduced losses. Moreover, a reduced
interaction can reduce the gain at the undesired wavelength
relative to that at the desired one, and thereby the problems
associated with a too high gain at the wrong wavelength. This
applies to lasers in which there is a significant unpumped loss
(typically, reabsorption loss or out-coupling loss). These points
are especially relevant for cladding-pumped devices. For example,
to ensure sufficient pump absorption, the fiber may need to be so
long that one or both of those problems arise.
[0026] 4. Saturable absorbers, in which the saturation power is
otherwise too small.
[0027] Embodiments of the invention can overcome or alleviate some
of the problems described above and can at least partially achieve
one or more of the following:
[0028] 1. To reduce the susceptibility to so-called quenching and
background losses, in particular for cladding-pumped devices.
[0029] 2. To obtain efficient emission at wavelengths otherwise
inaccessible for devices where there is a significant unpumped
loss, in particular for cladding-pumped devices.
[0030] 3. To improve the energy storage capabilities, for
energy-storage devices.
[0031] 4. To reduce signal cross-talk and inter-symbol interference
for signal amplifiers.
[0032] 5. To allow for a larger, predetermined, saturation
power.
[0033] Embodiments of the invention can provide the following
devices and embodiments, and the use of the following amplifying
and/or absorbing waveguiding structures in such devices:
[0034] 1. An amplifying optical fiber in which the active medium is
placed partly or wholly outside the waveguiding core, e.g., in a
ring around the core. The gain medium can also reside inside the
core in regions where the normalized modal intensity of the signal
beam is small. The fiber can be made of a glass, partly doped with
Pr.sup.3+, Tm.sup.3+, Sm.sup.3+, Ho.sup.3+, Nd.sup.3+, Er.sup.3+,
or Yb.sup.3+, or a combination thereof, and it can be
cladding-pumped.
[0035] 2. A cladding-pumped amplifier or laser in which the
difference between the overlaps of the pump and signal beams with
gain medium is substantially reduced compared to prior-art
designs.
[0036] 3. A ring-doped optical fiber for high-energy pulse
amplification or generation or other energy storage applications.
The fiber can for instance be made of a glass, partly doped with
Pr.sup.3+, Tm.sup.3+, Sm.sup.3+, Ho.sup.3+, Nd.sup.3+, Er.sup.3+,
or Yb.sup.3+, or a combination thereof, and it can be
cladding-pumped. Moreover, the device can incorporate a
longitudinally distributed saturable absorber to suppress the
build-up of ASE. In one embodiment, the gain medium is a
Yb.sup.3+-sensitized Er.sup.3+-doped glass, and the saturable
absorber is an Er.sup.3+-doped glass, and they are located so that
the signal intensity is higher in the saturable absorber than in
the gain medium.
[0037] 4. A Q-switched or gain-switched fiber laser based on an
amplifying fiber with a relatively higher saturation energy
combined with a saturable absorber fiber having a relatively lower
saturation energy. The difference in saturation energy stems, at
least to a significant part, from differences in the geometry of
the fibers. The active media in the different fibers can be the
same or different, and can for instance be a glass doped with a
rare earth, e.g., Pr.sup.3+, Tm.sup.3+, Sm.sup.3+, Ho.sup.3+,
Nd.sup.3+, Er.sup.3+, or Yb.sup.3+, or a combination thereof.
[0038] 5. A ring-doped, cladding-pumped ytterbium-doped fiber for
amplification or generation of light in the range 950 nm to 1050
nm.
[0039] 6. A ring-doped, cladding-pumped neodymium-doped fiber for
amplification or generation of light in the range 850 nm to 950
nm.
[0040] 7. A ring-doped, cladding-pumped erbium-doped fiber for
amplification or generation of light in the range 1450 nm to 1600
nm.
[0041] 8. An amplifying planar waveguide structure in which the
active medium is placed partly or wholly outside the waveguiding
core, thus interacting with the signal beam only where the
normalized intensity of the modal field is small. The waveguide can
be cladding-pumped. Moreover, the design can be specifically
adapted to correspond to any of the fiber devices listed above.
[0042] 9. A semiconductor amplifier for signal amplification, in
which the gain region is placed partly or wholly outside the
waveguiding core, thus interacting with the signal beams only where
their normalized modal intensities are small. Thereby, the
saturation energy of the device will be increased, which
subsequently reduces the inter-symbol interference and
inter-wavelength cross-talk.
[0043] 10. A waveguiding structure with a saturable absorption, in
which the absorbing medium is placed partly or wholly outside the
waveguiding core, thus interacting with the signal beam only where
its normalized modal intensity is small.
[0044] Evanescent-field devices, including ring-doped fiber devices
have not been considered for devices of the type proposed here, nor
has any device been proposed or demonstrated based on ring-doping
(or evanescent field interaction) that provide significant benefits
of the type considered here, compared to traditional devices in
which the gain-medium resides in the core in places where the
interaction with the signal beam is large. Specific differences
between embodiments of the invention and a prior art device are as
follows:
[0045] 1. It has not been one of the specific devices considered
here.
[0046] 2. It has not used a single-moded or few-moded waveguiding
core.
[0047] 3. It has not been a device in which the energy extraction
results in cross-talk or inter-symbol interference.
[0048] 4. The control of the emission wavelength that we propose
has not been obtained.
[0049] 5. The device has not substantially reduced the effect of
losses at the signal wavelength.
[0050] 6. It has not been a cladding-pumped device.
[0051] 7. It has not been a device for high-energy pulses.
[0052] 8. It has not been an optical fiber doped with erbium or
another rare-earth for high-energy pulses.
[0053] 9. The output of the device could not be launched into a
standard single-mode fiber through splicing or butt-coupling, nor
has the device allowed for an easy launch of signal light.
[0054] 10. The output beam has not been tightly confined.
[0055] 11. It has been a device doped in regions of the core where
the modal intensity is large.
[0056] 12. It has been a device doped in a large area around the
core (e.g., homogeneously in the cladding), hence rendering it
inefficient for cladding-pumping.
[0057] 13. It has not been a fiber structure, or at least not an
all-fiber structure.
[0058] 14. It has not been a solid-state device.
[0059] 15. The interaction length has been limited to a few
centimeters.
[0060] 16. It has not been a high-gain device.
[0061] 17. It has not been a device pumped by an optical beam
guided along the amplifying medium.
[0062] 18. It has not been possible to manufacture the device with
standard manufacturing techniques for rare-earth doped fibers like
MCVD and solution doping.
[0063] 19. The purpose of the design has not been to obtain a
smaller interaction between the gain medium and the signal light
than would otherwise be possible, nor have any substantial benefits
of a substantially smaller interaction been proposed, discussed, or
demonstrated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0064] The invention will now be described by way of example with
reference to the accompanying drawings, throughout which like parts
are referenced to by like references, and in which:
[0065] FIG. 1 illustrates a Ring-doped optical fiber;
[0066] FIG. 2 illustrates the dependencies of the refractive index,
the gain medium, and the modal field across a transverse
cross-section through the center of the fiber in FIG. 1;
[0067] FIG. 3 illustrates a planar waveguide structure amplifying
the evanescent field of a signal beam;
[0068] FIG. 4 illustrates a double-clad ring-doped optical
fiber;
[0069] FIGS. 5a and 5b illustrate examples of the proposed
devices;
[0070] FIG. 6 illustrates the extractable energy and small-signal
gain at 1550 nm for a ring-doped erbium-doped fiber (EDF) pumped by
0.1 W, 0.2 W, and 0.5 W at 1480 nm in the core;
[0071] FIG. 7 illustrates the extractable energy and small-signal
gain at 1550 nm for a ring-doped erbium-doped fiber (EDF) pumped by
0.1 W, 0.2 W, and 0.5 W at 980 nm in the core;
[0072] FIG. 8 illustrates the normalized modal intensity .PSI. vs.
ring position for the ring-doped EDFs of FIGS. 6, 7, and 10;
[0073] FIG. 9 illustrates the extractable energy ("pulse energy
above cw") vs. launched pump power for a core-pumped fiber
amplifier with an Yb.sup.3+-doped ring;
[0074] FIG. 10 illustrates the extractable energy and small-signal
gain at 1550 nm for a ring-doped EDF cladding-pumped by 1 W and 5 W
at 980 nm;
[0075] FIG. 11 illustrates a view of a fiber having a saturable
absorber in the central part of the core and a ring-shaped gain
medium around the absorber;
[0076] FIG. 12 illustrates a semiconductor amplifier for signal
amplification;
[0077] FIG. 13a to 13c illustrates devices in which unwanted,
higher-order modes are suppressed by the inclusion of an
absorber;
[0078] FIG. 14 shows an amplifying optical device;
[0079] FIG. 15 shows an amplifying optical device containing a
second waveguiding structure;
[0080] FIG. 16 shows a preferred embodiment of an amplifying
optical device in which there is a significant saturable small
signal absorption;
[0081] FIG. 17 shows a schematic of a high-power optical
amplifier;
[0082] FIG. 18 shows a master oscillator power amplifier MOPA;
[0083] FIG. 19 shows a fiber laser;
[0084] FIG. 20 shows a fiber laser which includes a reflector;
[0085] FIG. 21 shows a preferred embodiment of a fiber laser
containing Ytterbium as a dopant;
[0086] FIG. 22 shows a Q-switched laser;
[0087] FIG. 23 shows a fiber laser being used to pump three optical
fiber amplifiers;
[0088] FIG. 24 shows a waveguiding saturating absorber;
[0089] FIG. 25 shows an amplifying optical device in which an
amplifying region is situated outside the first core;
[0090] FIG. 26 shows a passive Q-switched laser;
[0091] FIG. 27 shows a passive Q-switched laser which includes a
pump reflector;
[0092] FIG. 28 shows an optical fiber having a ring-doped
amplifying region located in the cladding;
[0093] FIG. 29 shows a wavelength-tracking filter; and
[0094] FIG. 30 shows a single-frequency laser.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0095] FIG. 1 depicts a ring-doped optical fiber. A transparent
cladding (10) (typical radius 50 .mu.m-250 .mu.m) surrounds a
transparent waveguiding core (30) of a higher refractive index,
with a diameter of typically a few to ten .mu.m (micrometers). The
core is surrounded by a gain medium (20), which can amplify a
signal beam, guided by the core. The gain medium (20) can be pumped
by an optical pump beam, which can amplify a signal beam, guided by
the core.
[0096] FIG. 2 illustrates the normalized modal intensity
distribution .PSI. the refractive index profile with the core (30),
and the dopant profile (20), in a transverse cross-section through
the center of the fiber. For an optical fiber in glass, the
cladding refractive index is typically around 1.5, and the
numerical aperture is typically around 0.1-0.3.
[0097] FIG. 3 shows a waveguiding amplifier or laser. As for the
fiber, a transparent cladding (110) surrounds a transparent
waveguiding core (130) of a higher refractive index. A gain medium
(120) is situated near the core.
[0098] FIG. 4 is similar to FIG. 1, except that the inner cladding
(10) is now surrounded by an outer cladding (210), of a lower
refractive index. Thus, the inner cladding can guide light, and
serves to guide a pump beam launched into the inner cladding. The
signal beam is guided by the core (30).
[0099] FIGS. 5a and 5b illustrate examples of an erbium-doped fiber
amplifier and a fiber laser respectively. For the amplifier of FIG.
5a, a signal beam in an optical fiber is launched into a
wavelength-selective coupler (310). Also an optical pump beam from
the pig-tailed pump source (320) is launched into the coupler,
which combines the pump and signal beams and launches them both
into an erbium-doped fiber (330). In the fiber, the erbium-ions
serve to transfer energy from the pump beam to the signal beam,
which is thereby amplified. The amplified signal is then, for
example, launched into another fiber for further transmission.
[0100] For the laser of FIG. 5b, a beam from an optical pump source
(370) is coupled via a lens (350) into a fiber (340) doped with a
gain medium. The ends of the fiber provide some means (360) for
reflecting a signal beam, possibly with wavelength discrimination,
thus providing feedback for the laser. The reflector at the pump
input end transmits the pump and reflects the signal, while the
out-coupling reflector in the other end transmits a significant
fraction of the signal beam. Other components are also often used
in the devices of FIGS. 5a and 5b, e.g., an isolator for the
amplifier; however those have been omitted for clarity.
[0101] Although it is clear that the ideas and concepts disclosed
below apply to many different geometries, the discussion below will
for conciseness be focused on ring-doped fibers. Moreover, it will
be assumed that the structures are longitudinally uniform, although
this is not necessarily so.
[0102] Other waveguiding geometries can also be used. For example,
the core can be of a more complicated shape than the traditional
ones illustrated in the drawings. The invention also extends to
cores that fulfill the same or a similar function as traditional
ones do, and allow for an active medium to be incorporated in a
region where the normalized modal intensity is small.
[0103] Moreover, while the embodiments primarily deal with devices
pumped by an optical beam propagating along the core, other pumping
schemes are also possible, like flash-lamp pumping and side-pumping
with diode bars, electrical pumping, chemical pumping, and
more.
[0104] While advantages are described mainly in terms of localizing
the active medium in regions where the normalized modal field is
small, the active medium can also extend to regions where it is
large.
[0105] Principle
[0106] The disclosed devices provide advantages compared to
prior-art, core-doped, devices by suppressing gain and thus
radiation losses at undesired wavelengths and/or by reducing the
propagation losses in the device. Below follows a description of
how these advantages can be obtained. We restrict the discussion to
homogeneously broadened gain media; substantial benefits can be
realized also in inhomogeneously broadened devices. The description
focuses on cladding-pumped devices.
[0107] It is known that with a gain medium for which the shape of
the gain spectrum depends on the population inversion, the emission
wavelength of a fiber can be modified by changing the strength of
the interaction between a signal beam and the gain medium. For
instance, the fiber length can be changed. This also changes the
absorption of the pump. However, we will demonstrate below that in
cladding-pumped devices, the same control can be obtained through
ring-doping, while separately controlling the absorption of the
pump. In particular, the pump absorption can be kept sufficiently
large, as will be further described in the following.
[0108] The following relation can be used for evaluating the gain G
in a waveguiding device with a homogeneously broadened gain medium
[see C. R. Giles and E. Desurvire, "Modeling erbium-doped fiber
amplifiers", J. Lightwave Technol. 9, 271-83, (1991)]: 1 G = 10 ln
10 [ e N 0 ( x , y ) n 2 ( x , y ) ( x , y ) x y - a N 0 ( x , y )
( 1 - n 2 ( x , y ) ) ( x , y ) x y ] L [ dB ] ( 1 )
[0109] where N.sub.0 is the concentration of amplifying centra,
n.sub.2 is the degree of excitation, .PSI. is the normalized mode
intensity, .sigma..sup.a and .sigma..sup.e are the absorption and
emission cross-section of the active centra, respectively, and L is
the length of the gain region. Equation 1 can be written in a
simplified form:
G=(10/In10) N.sub.0 A.sub.doped.PSI..sub.doped
[n.sub.2.sigma..sup.3-(1-n.- sub.2).sigma..sup.a]L [dB] (2)
[0110] where N.sub.0, n.sub.2, and .PSI..sub.doped have been
appropriately averaged over the doped area A.sub.doped. (In the
literature, the product A.sub.doped .PSI..sub.doped is often
replaced by the so-called overlap .GAMMA..)
[0111] There are two assumptions in Eqs. 1 and 2, namely, that the
gain is homogeneously broadened and that only two levels in the
gain medium are significantly populated. However, even for devices
that do not meet these assumptions, the problems that we address
exist and can generally be countered by designing devices according
to our present invention. In the notation, there is also the
implicit, unimportant, assumption that the gain stems from a number
of active centra, each of which has been ascribed cross-sections
for stimulated emission and absorption. Other types of gain media
also exist, and the results will be valid also for them. To
proceed, we will also assume that the degree of inversion is
wavelength-independent. This is normally true to a good
approximation. If not, this results in a slight inhomogeneity in
the gain spectrum. For simplicity, we have also assumed that other
losses are small compared to either the gain G or the bleachable
absorption (10/In10)N.sub.0A.sub.dope- d
.PSI..sub.doped.sigma..sup.aL. Again, this is a non-restrictive
assumption, and the equations can be easily modified to include any
other loss. For instance, a filter can be used for controlling the
gain spectrum and laser output wavelength, both in prior-art
devices and the devices disclosed here.
[0112] It follows from Eq. 2 that the gains G.sub.1, G.sub.2, and
G.sub.3 at three different wavelengths .lambda..sub.1,
.lambda..sub.2, and .lambda..sub.3 are related to each other in the
following way:
G.sub.3=G.sub.2(.PSI..sub.3, doped/.PSI..sub.2, doped)
(.sigma..sub.3.sup.3/.sigma..sub.1.sup.3-.sigma..sub.3.sup.a/.sigma..sub.-
1.sup.a)/(.sigma..sub.2.sup.3/.sigma..sub.1.sup.3-.sigma..sub.2.sup.a/.sig-
ma..sub.1.sup.a)
+G.sub.1(.PSI..sub.3, doped/.PSI..sub.1, doped)
(.sigma..sub.3.sup.3/.sigm-
a..sub.2.sup.e-.sigma..sub.3.sup.a/.sigma..sub.2.sup.a)/(.sigma..sub.1.sup-
.e/.sigma..sub.2.sup.3-.sigma..sub.1.sup.a/.sigma..sub.2.sup.a)
[dB] (3)
[0113] Equation 3 makes the important point that for given
cross-sections, the only parameters that affect this relation are
the normalized mode intensities, averaged over the doped region.
Let now .lambda..sub.1 be the pump wavelength. The pump is then
absorbed by an amount .alpha..sub.p.sup.operating.ident.-G.sub.1 in
the operating state of the device. In order to operate efficiently,
.alpha..sub.p.sup.operating needs to be sufficiently large, say, at
least 5 dB. Also, we assume that we require a certain gain G.sub.2
at a wavelength .lambda..sub.2. .alpha..sub.p.sup.operating and
G.sub.2 are then parameters already specified. This also implies a
certain gain G.sub.3 at other wavelengths .lambda..sub.3, but if
this gain is too large, prohibitive amounts of power will be lost
to ASE. Insofar as the cross-sections cannot be significantly
modified, this can only be remedied by designing the device for
appropriate values of the normalized modal intensities. The
description of such designs is a central part of the present
invention.
[0114] To simplify the further description, we now assume that the
pump does not stimulate any emission; hence, .sigma..sub.1.sup.3=0.
Equation 3 then becomes
G.sub.3=G.sub.2 (.sigma..sub.3.sup.3.PSI..sub.3,
doped/.sigma..sub.2.sup.3- .PSI..sub.2, doped)
+.alpha..sub.p.sup.operating (.sigma..sub.3.sup.e .PSI..sub.3,
doped/.sigma..sub.p .sup.a.PSI..sub.p, doped)
[(.sigma..sub.2.sup.a/.sigma..sub.2.sup.e)-(.sigma..sub.3.sup.a/.sigma..s-
ub.3.sup.e)][dB] (4)
[0115] The value of the first term depends on the relative sizes of
.PSI..sub.2, doped and .PSI..sub.3, doped at .lambda..sub.2 and
.lambda..sub.3. In a fiber, the spot-sizes at .lambda..sub.2 and
.lambda..sub.3 can differ. Then, ring-doping implies that the gain
at the wavelength with the larger spot-size gets relatively larger
than at the other wavelength, compared to a homogeneously doped
core. Depending on how close the wavelengths are to each other,
this is often not a significant effect.
[0116] In contrast, in cladding-pumped devices, the second term in
Eq. 4 can to a significant extent be controlled by designing the
device for an appropriate value of (.PSI..sub.3, doped/.PSI..sub.p,
doped). Normally, it is very different in a cladding-pumped device
and in a core-pumped device. In the core, the normalized pump
intensity .PSI..sub.p is approximately equal to the inverse of the
pumped area for both core-pumped and cladding-pumped devices, so
the same is true for .PSI..sub.p, doped in a core-doped device. It
follows that in a core-doped device, .PSI..sub.p, doped will be
much larger in a core-pumped device than in a cladding-pumped
device. Thus, the effective area ratio r.sub.effective
.ident.(.PSI..sub.3, doped/.PSI..sub.p, doped) will be much larger.
(We will also use "effective area ratio" for the ratio .PSI..sub.2,
doped/.PSI..sub.p, doped.) Consequently, a core design which is
suitable for the core-pumped device may be inappropriate for a
cladding-pumped device because the effective area ratio becomes too
large. In prior-art cladding-pumped devices, r.sub.effective is
large, typically around 100. Then, the second term in Eq. 4 is
potentially large for some undesired wavelength .lambda..sub.3,
which makes it difficult to absorb the pump without getting a high
gain at the undesired wavelength. Therefore, laser systems with
significant reabsorption that work well in a core-doped,
core-pumped, geometry will not be efficient core-doped,
cladding-pumped lasers. (In a device doped in the core,
r.sub.effective is approximately equal to the area ratio
r.ident.A.sub.pumped/A.sub.doped- , where A.sub.pumped is the
pumped area and A.sub.doped is the doped area. Hence, for a
cladding-pumped device homogeneously doped throughout the core,
r=A.sub.cladding/A.sub.core.)
[0117] Consider instead a ring-doped, cladding-pumped device. Since
.PSI..sub.p is approximately constant over the inner cladding,
.PSI..sub.p, doped will not change much with the transverse
disposition of the gain medium. However, since the light at
.lambda..sub.3 is confined to the core, .PSI..sub.3, doped
decreases rapidly if the amplifying region is moved away from the
core. This obviously reduces the interaction between the gain
medium and the signal beam. Hence, the devices disclosed here
allows r.sub.effectve to be substantially reduced, e.g., to values
in the range 1-10, whereby the gain at unwanted wavelengths can be
suppressed compared to the gain at a desired wavelength.
[0118] First, we treat the case where the scattering (or
absorption) loss of the gain region is larger than that of a
transparent, passive region. For simplicity, we assume that there
is no scattering loss outside the gain region. Starting from Eq. 2,
we can then derive the following expression between the scattering
loss and the gain G.sub.1 and G.sub.2 at two different
wavelengths:
.alpha..sub.2.sup.scatter =.sigma..sub.2.sup.scatter
[G.sub.2(.sigma..sub.1.sup.a+.sigma..sub.1.sup.e)-(.PSI..sub.2,
doped/.PSI..sub.1, doped)
G.sub.1(.sigma..sub.2.sup.a+.sigma..sub.2.sup.e-
)]/(.sigma..sub.1.sup.a.sigma..sub.2.sup.3-.sigma..sub.1.sup.e.sigma..sub.-
2.sup.d) [dB] (5)
[0119] In Eq. 5, we have arbitrarily made the non-restrictive
assumption that each active center scatters with a cross-section
.sigma..sub.2.sup.scatter. Also, we have for simplicity assumed
that scattering is small compared to the gain. It follows that the
scattering losses can become high already at a small value of the
ratio between stimulated emission and scattering
(.sigma..sub.2.sup.scatter/.sigma..sub- .2.sup.e) if
r.sub.effective.apprxeq.100, i.e., in a core-doped, cladding-pumped
device. Then, already a value (.sigma..sub.2.sup.scatter/-
.sigma..sub.2.sup.3) as low as {fraction (1/1000)} can result in
significant losses. In contrast, in ring-doped cladding-pumped
devices, acceptable values of
(.sigma..sub.2.sup.scatter/.sigma..sub.2.sup.e) will be one or two
orders of magnitude larger.
[0120] Next, we will show how ring-doping also can reduce the
sensitivity to quenching.
[0121] Very often, some active centra in a gain medium are defect.
These quenched centra retain their ground-state absorption (GSA),
but, if they absorb a photon, they are not efficiently excited.
This leads to a so-called unsaturable absorption, the spectrum of
which is approximately proportional to the small-signal
ground-state absorption spectrum of the medium. For instance, this
type of unsaturable absorption has been observed in the important
Yb.sup.3+:glass and Er.sup.3+:glass gain media. The small-signal
absorption is given by:
.alpha..sub.2.sup.ss=.sigma..sub.2.sup.a[G.sub.2(.sigma..sub.1.sup.a+.sigm-
a..sub.1.sup.e)-(.PSI..sub.2, doped/.PSI..sub.1, doped)
G.sub.1(.sigma..sub.2.sup.a+.sigma..sub.2.sup.e)]/(.sigma..sub.1.sup.a.si-
gma..sub.2.sup.e-.sigma..sub.1.sup.e.sigma..sub.2.sup.a) [dB]
(6)
[0122] If, for instance, 3% of the active centra are quenched, we
get an unsaturable absorption of 0.03.times..alpha.2ss. Equation 6
is very similar to Eq. 5, and the same result holds: A
cladding-pumped device with the currently disclosed design will be
typically 10-100 times less sensitive to quenching than are
core-doped designs of the prior-art. (This does not apply to
four-level systems, for which .alpha.2ss=0 dB.)
[0123] Next, we consider the case of excited-state absorption at
the signal wavelength .lambda..sub.2. Again, a stronger interaction
leads to more power lost through ESA, at least for a device with
significant small-signal absorption, as the following equations
will show. The excited-state absorption can be written as:
.alpha..sub.2.sup.ESA=.sigma..sub.2.sup.ESA
[G.sub.2/.sigma..sub.2.sup.a-(- .PSI..sub.2, doped/.PSI..sub.1,
doped)G.sub.1/.sigma..sub.1.sup.a]/[(.sigm-
a..sub.2.sup.e-.sigma..sub.2.sup.ESA)/.sigma..sub.2.sup.a-.sigma..sub.1.su-
p.e/.sigma..sub.1.sup.a][dB] (7)
[0124] For a transition to the ground-state, the total
excited-state absorption can be significant already for values of
.sigma..sub.2.sup.ESA/(.sigma..sub.2.sup.3-.sigma..sup.ESA) of
{fraction (1/1000)}. Again, in cladding-pumped devices, the
sensitivity can be reduced one or two order of magnitudes by
ring-doping. (For four-level transitions, .sigma..sub.2.sup.a=0, so
the sensitivity to ESA is independent of any ring-doping, and equal
to that of traditional core-doped, core-pumped devices.)
[0125] Equations 1-7 thus demonstrate how ring-doping makes the
disclosed devices less susceptible to absorption loss and
scattering losses and to emission losses to ASE at an undesired,
high-gain wavelength. The improvements are a direct consequence of
the reduction of the effective area ratio r.sub.effective.noteq.r
to values around 1-10. In contrast, in prior-art devices, the
signal light in the core is confined to an area approximately 100
times smaller than that of the pump, so the area ratios
r.apprxeq.r.sub.effective.apprxeq.100. While the area ratio can
well be made larger, a smaller area ratio is troublesome since a
smaller area of the inner cladding can make it difficult to launch
the pump into the device, and since a larger signal spot-size leads
either to a large bend sensitivity or to a multi-mode core.
[0126] In addition to the general designs described up to this
point, we next describe some particular cladding-pumped fiber
lasers and amplifiers with sizable advantages compared to the prior
art.
[0127] Ytterbium-doped fiber operating in wavelengths between 975
and 985 nm
[0128] For Yb.sup.3+-doped devices at these wavelengths, the
suppression of quasi-four-level emission around 1030 nm can be
especially troublesome for cladding-pumped devices designed
according to the prior art. For a wavelength of 975 nm
(corresponding to the peak of the cross-sections) with
representative cross-section values (cf. Table 1), Eq. 3 gives the
following relation between the gain at 975 nm, the gain at 1028 nm,
and the pump absorption of the pumped (i.e., partly bleached)
fiber:
G.sub.1028=0.25 G.sub.975+0.74 (.PSI..sub.doped/.PSI..sub.p,
doped).alpha..sub.p.sup.operating [dB] (8)
[0129] Here, we have assumed that .PSI..sub.975,
doped=.PSI..sub.1028, doped, which is a reasonable approximation
for guided modes at nearby wavelengths. Now, assume that we want
the laser to work at 975 nm, with 3.5% reflectivity at one end and
100% at the other one. Then, if the background losses are
negligible, G.sub.975=7.28 dB.
[0130] Consider a representative core-doped prior-art design with
r.apprxeq.r.sub.effective=100. Then, for every dB of pump
absorption we get 74 dB of gain at 1028 nm. Since the gain at
unwanted wavelengths must be below approximately 50 dB, we would
have to restrict the single-pass pump absorption to below 1 dB or
20%. This would be a highly inefficient laser.
[0131] Instead, we propose to use ring-doping. Then, the pump
absorption can be 5 dB or more, which allows for a good laser
efficiency. Note that increasing the end-face reflectivity at 975
nm will not help us much, as the high gain at 1028 nm largely
follows from the requirements on pump absorption, while it is
comparatively insensitive of the gain at 975 nm. For the same
reason, the gain at 1028 nm will not be much higher for a high-gain
amplifier at 975 nm than it is for a low-gain laser, so the
disclosed design provides benefits for both applications.
[0132] A high-power laser at 975 nm can be used for pumping
Er.sup.3+. Also other wavelengths can be used for this, e.g., 980
nm and 985 nm. However, also those wavelengths are severely
affected by unwanted emission around 1030 nm.
1TABLE 1 Cross-sections for absorption and stimulated emission used
in some numerical examples. Absorption Emission cross- cross-
Active Wavelength section/ section/ medium /nm 10.sup.-25 m.sup.2
10.sup.-25 m.sup.2 Remark Nd.sup.3+:glass 800 20 0 Pump to
.sup.4F.sub.5/2. Nd.sup.3+:glass 870 10 10 Nd.sup.3+:glass 1050 0
30 Unwanted wavelength Yb.sup.3+:glass 912 8.25 0.275 Pump
Yb.sup.3+:glass 975 25.85 25.85 Yb.sup.3+:glass 980 6.76 8.57
Yb.sup.3+:glass 985 1.77 2.97 Yb.sup.3+:glass 1030 0.45 6.3
Unwanted wavelength Er.sup.3+:glass 980 2 0 Pump to
.sup.4I.sub.11/2 Er.sup.3+:glass 1531 5 5 Er.sup.3+:glass 1550 2.4
3.8 Er.sup.3+:glass 1564 1.6 3 Unwanted wavelength
[0133] In the following, we will show that lasing at 975 nm will be
particularly sensitive to any unbleachable Yb.sup.3+, the existence
of which has been reported in [R. Paschotta, J. Nilsson, P. R.
Barber, A. C. Tropper, and D. C. Hanna, "Lifetime quenching in Yb
doped fibers", submitted to Optics Communications]. This
sensitivity can be order of magnitudes higher in core-doped designs
according to prior art, compared to the devices of the present
invention.
[0134] From Eq. 6, we get
.alpha..sub.975.sup.ss=1.07 G.sub.975+6.48
(.PSI..sub.doped/.PSI..sub.p, doped).alpha..sub.p.sup.operating
[dB] (9)
[0135] Hence, with a prior-art design for cladding-pumping, we get
several thousand decibels of small-signal absorption at 975 nm for
a desired pump-absorption of around 5 dB. For r.sub.effective=100,
already an unsaturable fraction of 1% of this (the lowest value
reported in R. Paschotta, J. Nilsson, P. R. Barber, A. C. Tropper,
and D. C. Hanna, "Lifetime quenching in Yb doped fibers", submitted
to Optics Communications) leads to an unsaturable absorption of
around 30 dB, which is unacceptable. With the new devices, the
sensitivity is drastically reduced. Even further reductions are
possible by lasing at other wavelengths, e.g.,
.alpha..sub.980.sup.ss=0.83 G.sub.980+1.34
(.PSI..sub.doped/.PSI..sub.p, doped).alpha..sub.p.sup.operating
[dB] (10)
[0136] at 980 nm, and
.alpha..sub.985.sup.ss=0.64 G.sub.985+0.37
(.PSI..sub.doped/.PSI..sub.p, doped).alpha..sub.p.sup.operating
[dB] (11)
[0137] at 985 nm. The sensitivity to quenching is much reduced, and
can be quite small in a ring-doped device.
[0138] While the analytic considerations above clearly demonstrate
the advantages of the disclosed devices, they do not quantify the
advantages in terms of the most important laser characteristics,
namely, pump threshold P.sub.th and slope efficiency
.eta..sub.slope. In order to provide a more complete description of
the improvements compared to prior art, we next present
calculations of P.sub.th and .eta..sub.slope from simulations with
a spectrally and spatially resolved numerical model [B. Pedersen,
A. Bjarklev, J. H. Povlsen, K. Dybdal, and C. C. Larsen, "The
design of erbium-doped fiber amplifiers", J. Lightwave Technol. 9,
1105-1112 (1991)]. The only significant simplification in the model
is that the pump is always assumed to be uniformly distributed
across the inner cladding. Besides that, the gain medium is assumed
to be homogeneously broadened, which is reasonable for
Yb.sup.3+:glass systems. With the model, we analyzed fibers of
different core-doped and ring-doped designs. In all cases, we kept
the doped area constant, equal to the core size, while the outer
radius r.sub.d.sup.outer of the doped area and hence
.PSI..sub.doped was varied. The area ratio was 80, and .PSI..sub.p,
doped =(3080 .mu.m.sup.2).sup.-1. Other parameters are given in
Table 2.
[0139] A first studied cavity had one laser mirror formed by a
bare, cleaved, fiber end, providing a broadband reflectivity of
3.5%, while a narrow-band reflector (typically, a fiber
bragg-grating) provided a 99.9% reflectivity in a desired laser
wavelength range 975 nm to 977 nm. Outside this range, the
reflectivity was zero, as can be achieved with an AR-coated or an
angle-cleaved fiber end.
[0140] Lasing in the desired wavelength range was prevented by
strong ASE at long wavelengths (1028 nm -1035 nm) until
r.sub.d.sup.outer became 5 .mu.m. Then, the diameter of the inner
ring is 4.2 .mu.m and r.sub.effective=5.8 in good agreement with
earlier estimates. The results are presented in Table 3.
2TABLE 2 Values used in detailed Yb-calculations. Other parameters
as in TABLE 1. Quantity Symbol Value Numerical aperture NA 0.1 Core
diameter 7 .mu.m Cut-off wavelength .lambda..sub.c 915 nm Doped
area A.sub.doped 38.5 .mu.m.sup.2 Yb concentration [Yb.sup.3+] 2.7
.times. 10.sup.25 m.sup.-3 Signal overlap with core
.GAMMA..sub.core 0.796 Area of inner cladding A.sub.pump 3080
.mu.m.sup.2 Pump overlap with core .GAMMA..sub.p,core 1/80
Effective area ratio for core-doped .gamma..sub.reffective 63.7
device Small-signal pump-absorption .alpha..sub.p.sup.ss 1.21 dB/m
Metastable lifetime .tau. 0.76 ms Background loss 0 dB/m
Reflectivity, pump launch end 99.9% at desired wavelength, 0
elsewhere Reflectivity at other end Either 3.5% broadband, or 50%
at desired wave- length and 0% elsewhere
[0141]
3TABLE 3 Laser characteristics of 10 m long unquenched fiber
operating at 976 nm, with a HR fiber grating and a bare, cleaved
end providing the laser cavity reflections. The small-signal
absorption .alpha..sup.ss applies to a wavelength of 977 nm. The
transmitted pump power P.sub.p.sup.transmitted is expressed as a
fraction of the launched pump power. r.sub.d.sup.inner is the
radius of the gain medium. r.sub.d.sup.outer/ r.sub.d.sup.inner/
.alpha..sup.ss/ .mu.m .mu.m r.sub.effective dB/m P.sub.th/W
.eta..sub.slope .times. 100 P.sub.p.sup.transmitted 3.5-5 0-3.6
64-12 170-31 No lasing for a pump 22% - power of 5 W.ASE 46% around
1030 nm dominates theoutput 5.5 4.2 5.8 15.3 2.01 .+-. 0.1 69 .+-.
2 26% 6.0 4.9 2.9 7.47 2.14 .+-. 0.1 66 .+-. 2 29% 6.5 5.5 1.6 4.03
2.37 .+-. 0.1 62 .+-. 2 33% 7.0 6.1 1.0 2.58 2.78 .+-. 0.1 61 .+-.
2 34% 7.5 6.6 0.57 1.43 3.62 .+-. 0.1 51 .+-. 2 44%
[0142] Clearly, in contrast to prior-art devices, the device
disclosed here can lase at 976 nm with a good efficiency. The range
of acceptable effective area ratios is 1-6. The slope efficiency
with respect to absorbed pump was approximately 93%--a quite high
number which in reality will be lowered by background losses. These
were assumed negligible in the calculations.
[0143] A shorter fiber length favors lasing at shorter wavelengths
in a two-level system like this. However, shortening the fiber to 5
m is not sufficient for lasing at 976 nm in a core-doped design.
Moreover, at this length, a significant fraction of the pump is not
absorbed. Hence, making the fiber sufficiently short to ensure 976
nm lasing in a core-doped design is not an attractive option, even
if the pump is double-passed through the cavity. The conclusion is
that prior-art designs are inadequate for lasing at 976 nm for the
considered area ratio.
[0144] Above, the smaller ring diameters appear to be better than
the larger ones (provided that lasing is obtained). However, if a
fraction of the Yb-ions are quenched, this will change, as is
evident from Table 4.
4TABLE 4 Laser characteristics of 5 m long fiber operating at 976
nm, with 2% of the Yb.sup.3- - ions quenched. A HR fiber grating
and a bare, cleaved end provided the laser cavity reflections.
r.sub.d.sup.outer/ r.sub.d.sup.inner/ .eta..sub.slope .times. .mu.m
.mu.m r.sub.effective .alpha..sup.ss/dB/m P.sub.th/W 100
P.sub.p.sup.transmitted 3.5-4.5 0-3.5 64-22 170-57 No lasing for
pump 33%- power of 10 W.ASE 47% around 1030 nm dominates the output
5.0 3.5 12 30.6 1.69 .+-. 0.1 29 .+-. 2 50% 5.5 4.2 5.8 15.3 1.77
.+-. 0.1 34 .+-. 2 53% 6.0 4.9 2.9 7.47 2.04 .+-. 0.1 33 .+-. 2 58%
6.5 5.5 1.6 4.03 2.57 .+-. 0.1 29 .+-. 2 64% 7.0 6.1 1.0 2.58 3.58
.+-. 0.1 26 .+-. 2 68%
[0145] At 980 nm and 985 nm, the fiber behaved similarly as at 976
nm, except that 985 nm would not lase for an output reflectivity of
3.5%. A grating with 50% reflectivity at the output end allowed for
lasing at 985 nm. In contrast, lasing at 976 nm in an unquenched
fiber was only marginally improved by a grating also at the output
end, and for a partly quenched fiber, results were worse with a
grating than with a bare end. Also, as predicted in Eqs. 9-11, the
longer wavelengths are less sensitive to quenching than are the 976
nm lasers.
[0146] These and other detailed numerical model calculations have
shown:
[0147] The earlier analytic considerations are largely accurate in
determining whether or not a laser can work efficiently.
[0148] The disclosed devices perform much better as lasers at 975
nm-985 nm than do prior-art designs.
[0149] The best value of the effective area ratio is around 3-10
for this laser.
[0150] The sensitivity to quenching is reduced with a smaller
effective area ratio.
[0151] The susceptibility to quenching is smaller at 980 nm and
especially at 985 nm than it is at 976 nm.
[0152] Neodymium-doped fiber operating on the
.sup.4F.sub.3/2.fwdarw..sup.- 4I.sub.9/2 transition (850 nm-950
nm)
[0153] A device designed in a similar way as the Yb.sup.3+-doped
cladding-pumped fiber will also improve on prior-art designs for
this Nd.sup.3+-transition. For Nd.sup.3+-doped devices at these
wavelengths, the suppression of the dominant
.sup.4F.sub.3/2.fwdarw..sup.rI.sub.11/12 at 1050 nm transition is a
problem, especially for cladding-pumped devices. For a wavelength
of 870 nm, typical cross-sections (cf. Table 1) gives the following
relation between the gains at 870 nm and around 1050 nm and the
pump absorption of the pumped fiber:
G.sub.1050=3 G.sub.870+1.5 (.PSI..sub.doped/.PSI..sub.p,
doped).alpha..sub.p.sup.operating [dB] (12)
[0154] The relations will be similar for other wavelengths in this
transition. Equation 12 reveals that the gain at 1050 nm will be at
least three times larger than that at 870 nm. This limits the 870
nm gain to 15 dB--a comparatively low but still useful number.
However, with a prior-art, core-doped device, it will not be
possible to absorb the pump properly, since the gain at 1050 nm
becomes prohibitively high already for a single-pass pump
absorption of less than 0.5 dB (.apprxeq.10%). On the other hand,
in a ring-doped device, r.sub.effective can be reduced by a factor
10 or more, .alpha..sub.p.sup.operating of at least 5 dB (=68%) is
possible, while still allowing for a single-pass gain at 870 nm of
10 dB.
[0155] In Eq. 12, we for simplicity assumed that .PSI..sub.doped is
equal at 1050 nm and 870 nm. However, for ring-doping,
.PSI..sub.doped will be larger at 1050 nm than at 870 nm. This
means that the factor "3" in Eq. 12 actually will be larger. For
instance, with a numerical aperture of 0.1 and a core diameter of 6
.mu.m, a doped ring with r.sub.d.sup.inner=4 .mu.m and
r.sub.d.sup.outer=5 .mu.m gives (.PSI..sub.1050, doped
/.PSI..sub.870, doped)=1.6. Then, G.sub.1050=4.8 G.sub.870+1.5
(.PSI..sub.doped/.PSI..sub.p, doped).alpha..sub.p.sup.operating.
Nevertheless, appropriate designs allow enough gain for efficient
lasing at 870 nm before the gain at 1050 nm becomes unrealistically
large. The 870 nm gain can be even higher in modified designs: If
the core has a higher cut-off wavelength of, e.g., 950 nm, the core
will be multi-moded at 870 nm. Since the higher-order
LP.sub.11-mode penetrates further into the cladding than the
fundamental LP.sub.01-mode does, the LP.sub.11-mode gain at 870 nm
is higher than the gain of the LP.sub.01-mode. Hence, higher-order
mode lasing at 870 nm becomes relatively easier to achieve compared
to the 1050 nm lasing in the fundamental mode.
[0156] Erbium-doped fiber operating on the
.sup.4I.sub.13/2.fwdarw..sup.4I- .sub.15/2 transition (1450 nm-1600
nm)
[0157] The concerns of this device are similar to those of the
cladding-pumped Yb-doped fiber described above. For instance, if we
want the device to operate at 1531 nm, emission at 1564 nm or
longer wavelengths is a potential problem in an aluminosilicate
host. From Eq. 3 and Table 1, we get
G.sub.1564=0.6 G.sub.1531+0.7 (.PSI..sub.doped/.PSI..sub.p, doped)
.alpha..sub.p.sup.operating [dB] (13)
[0158] Clustering is a well-known problem in erbium-doped fibers,
and results in a saturable absorption. Equation 6 gives
.alpha..sub.1531.sup.ss=G.sub.1531+5 (.PSI..sub.doped /.PSI..sub.p,
doped) .alpha..sub.p.sup.operating [dB] (14)
[0159] These numbers are similar to the ones for Yb.sup.3+operating
at 976 nm, so ring-doping allows for similar improvements as for
Yb.sup.3+.
[0160] The wavelength range 1550 nm-1565 nm is technologically
important for optical communication systems. In this range, lasing
at 1550 nm may be particularly bard to achieve, because the gain
at, e.g., 1564 nm may become prohibitively large. From Eq. 3, we
get
G.sub.1564=0.79 G.sub.1550+0.15 (.PSI..sub.doped/.PSI..sub.p,
doped).alpha..sub.p.sup.operating [dB] (15)
[0161] Also in this relatively benign case, adequate pump
absorption can be troublesome in a prior-art design for unfavorable
values of r.sub.effective.ident.(.PSI..sub.doped/.PSI..sub.p,
doped), so a ring-doped fiber will be advantageous. As it comes to
the unsaturable absorption, we have that
.alpha..sub.1550.sup.ss=0.6 G.sub.1550+2.0
(.PSI..sub.doped/.PSI..sub.p, doped) .alpha..sub.p.sup.operating
[dB] (16)
[0162] Core doped devices can then have a small signal absorption
of 1000 dB. Even an unsaturable fraction as low as 1% of this small
signal absorption will create an unacceptable unsaturable loss of
10 B. Consequently we conclude that ring-doped designs are
better.
[0163] Principle
[0164] The type of high-energy pulse amplifiers and lasers we
consider are so-called energy-storage devices in which a pulse
extracts significant amounts of energy stored in the gain medium.
The energy supplied by the pump during the generation/amplification
of a single pulse can be negligible. The amount of energy stored in
the device then sets an upper limit on how much energy can be
extracted by a pulse. This is a significant difference compared to
other laser and amplifiers, for which power extraction is typically
limited by the supplied pump power, and in any case not by the
stored energy.
[0165] In order to obtain high-energy pulses from such an energy
storage laser or an amplifier, we need both a large stored (and
extractable) energy and a sufficiently high gain. While the gain
efficiency of waveguiding amplifiers means that it is often easy to
meet the second objective, the same gain efficiency can make it
difficult to store large amounts of energy in the device: The gain
efficiency implies that a comparatively small amount of extractable
energy in the gain medium leads to a high gain. However, as already
pointed out, since ASE limits the achievable gain of the device, it
also limits the energy that can be stored [J. Nilsson and B.
Jaskorzynska, "Modeling and optimization of low repetition-rate
high-energy pulse amplification in cw-pumped erbium-doped fiber
amplifiers", Opt. Lett. 18, 2099-2101 (1993).].
[0166] The gain G in a transverse mode is related to the energy E
stored in the gain medium through the following relation: 2 G = (
10 / ln 10 ) [ doped E ( a + e ) / h - L ] = ( 10 / ln 10 ) [ doped
E / U sat - L ] = ( 10 / ln 10 ) doped E extractable / U sat = ( 10
/ ln 10 ) E extractable / E sat [ dB ] ( 17 )
[0167] Here, hv is a photon energy, .alpha.L is the unpumped loss
of the medium, U.sub.sat.ident.hv/(.sigma..sup.a+.sigma..sup.a) is
the saturation energy fluence, E.sup.extractable is the energy over
the bleaching level, i.e., the maximum energy that can be extracted
from the device, and E.sub.sat.ident.U.sub.sat/.PSI..sub.doped is
the saturation energy. The important point is that G is
proportional to .PSI..sub.doped. Hence, a smaller value of
.PSI..sub.doped leads to a smaller gain per unit extractable
energy. Therefore, for a gain medium located in a region where the
normalized modal intensity of the signal beam is small, the
extractable energy for a given gain will be high. Then, if the gain
is sufficiently large for the device in question, a device with low
values of .PSI..sub.doped will be capable of generating or
amplifying pulses to high energies.
[0168] Here, we disclose the use of devices that, although the
light is tightly confined in a single- or few-moded waveguide, have
a small value of .PSI..sub.doped for high-energy pulse amplifiers
and lasers, e.g. Q-switched and gain-switched ones. Note that any
effect this may have on the relative gain at different wavelengths
can be counteracted by simply making the device longer or
increasing the concentration of active centra.
[0169] In addition to the general geometries described earlier, we
will now describe some specific geometries and devices.
[0170] Core-pumped ring-doped pulse fiber amplifier or fiber
laser
[0171] In the important class of core-pumped devices, the pump and
the signal are guided by the same core. For instance, most
erbium-doped fiber amplifiers (EDFAs) are of this type. Typically,
the gain medium can be a Tm.sup.3+, Sm.sup.3+, Ho.sup.3+,
Nd.sup.3+, Er.sup.3+, or Yb.sup.3+-doped glass. The desired
weakness of the interaction between the signal beam and the gain
medium normally then implies that also the interaction with the
pump beam is weak, whereby the pumping of the medium becomes weaker
and the pump absorption smaller. Nevertheless, the disclosed
devices can show significant improvements. We can distinguish two
cases:
[0172] 1. The pump and signal wavelengths are close, so the signal
and pump mode profiles are close to each other. In this case, it is
just a matter of finding suitable values of .PSI. for placing the
ring. These will depend on the lifetime and cross-sections of the
dopant, the pump power and pulse energy, and other parameters. FIG.
6 shows how the extractable energy and small-signal gain at 1550 nm
depends on the position of the ring for a ring-doped EDF
core-pumped by 0.1 W, 0.2 W, and 0.5 W at 1480 nm. FIG. 6
illustrates the extractable energy and small signal gain at 1550 nm
for a ring-doped erbium-doped fiber (EDF) pumped by 0.1 W, 0.2 W,
and 0.5 W at 1480 nm in the core. The ring thickness was
sufficiently thin to make variations of the normalized intensity of
the modal field negligible over its thickness. Other parameters are
listed in Tables 5 and 6 under "normal core" amplifier and a "large
core" amplifier. In all cases, a higher pump power gives a higher
small-signal gain and a larger extractable energy. Moreover, the
fiber length was optimized for maximum small-signal gain in all
cases. The advantages compared to the prior-art EDFs (also shown)
are substantial. FIG. 8 shows model calculation results on how
doped depends on the ring position for the ring-doped EDF. The
method used for these and other similar calculations in this
specification follows [J. Nilsson and B. Jaskorzynska, "Modeling
and optimization of low repetition-rate high-energy pulse
amplification in cw-pumped erbium-doped fiber amplifiers", Opt.
Lett. 18, 2099-2101 (1993).].
[0173] 2. The pump and signal mode profiles are different. In this
case, the pump is unlikely to penetrate far into the cladding, so
the doped region must be inside the core or immediately outside the
core. Unfortunately, for positions for which the signal intensity
is suitable, the pump intensity tends to be much too weak. A good
design should then aim at reducing this problem as far as possible.
FIG. 7 is similar to FIG. 6, except that the pump wavelength is now
980 nm. In particular, FIG. 7 illustrates the extractable energy
and small-signal gain at 1550 nm for a ring-doped erbium-doped
fiber (EDF) pumped by 0.1 W, 0.2 W and 0.5 W at 980 nm in the core.
The ring thickness was sufficiently thin to make variations of the
normalized modal intensity negligible over its thickness. Other
parameters are listed in Tables 5 and 6 under "normal core
amplifier". For comparison, also results for EDFAs homogeneously
doped throughout the core are shown, both for the "normal core"
amplifier and a "large core" amplifier. In all cases, a higher pump
power gives a higher small-signal gain and a larger extractable
energy. Moreover, the fiber length was optimized for maximum
small-signal gain in all cases. We see that the results are now
worse, and that the benefits of ring-doping are smaller. However,
performance is still superior compared to that of prior-art
designs.
5TABLE 5 Geometrical and dopant parameters for energy-storage
EDFAs. Normal-core Large-core Quantity Symbol amplifier amplifier
Core diameter 5 .mu.m 11 .mu.m Numerical aperture NA 0.171 0.100
Cut-off wavelength .lambda..sub.c 1118 nm 1437 nm Signal overlap
with core .GAMMA..sub.core 0.651 0.795 Area of inner cladding for
cladding- A.sub.pump 1571 .mu.m.sup.2 1571 .mu.m.sup.2 pumping Pump
overlap with core for cladding- .GAMMA..sub.p,core 1/80 1/16.5
pumping Effective area ratio for core-doped -- 52.1 13.1 device
Background loss -- 0 dB/m 0 dB/m
[0174]
6TABLE 6 Spectroscopic parameters for energy-storage EDFAs.
Quantity Symbol Value Metastable lifetime .tau. 10.9 ms Absorption
cross-section at 1480 nm .sigma..sup.a1480 1.87 .times. 10.sup.-25
m.sup.2 Emission cross-section at 1480 nm .sigma..sup.a1480 0.75
.times. 10.sup.-25 m.sup.2 Absorption cross-section at 980 nm
.sigma..sup.a980 2 .times. 10.sup.-25 m.sup.2 Absorption
cross-section at 1550 nm .sigma..sup.a1550 2.45 .times. 10.sup.-25
m.sup.2 Emission cross-section at 1550 nm .sigma..sup.a1550 3.83
.times. 10.sup.-25 m.sup.2 Pump intensity required at 1480 nm
I.sub.sat 0.0470 to invert 35.7% of the population mW/.mu.m.sup.2
Pump intensity required at 980 nm I.sub.sat 0.0930 to invert half
the population mW/.mu.m.sup.2
[0175] As an alternative, the core can be single-mode at the signal
wavelength, and multi-moded for the pump. It is well-known that
pump-light in higher-order modes will penetrate further into the
cladding, thereby improving the pumping of the gain medium.
Moreover, for so-called upconversion devices, the pump wavelength
is shorter than the signal wavelength, with the favorable
side-effect that the pump extends further into the ring, even if it
is in the same mode as the signal.
[0176] FIG. 9 shows measured results on high-energy pulse
amplification for a ring-doped, core-pumped Yb.sup.3+-doped fiber
amplifier according to the present embodiments. FIG. 9 illustrates
the extractable energy ("pulse energy above cw") vs launched pump
power for a core-pumped fiber amplifier with an Yb.sup.3+-doped
ring. The fiber was pumped at 1000 nm, and amplified signal pulses
at 1047 nm. The highest recorded extracted pulse energy (above the
cw-level) of more than 60 .mu.J can be compared to published 10
.mu.J total pulse energy from large-area core amplifier (albeit at
a lower pump power of 160 mW) [D. T. Walton, J. Nees, and G.
Mourou, "Broad-bandwidth pulse amplification to the 10-.mu.J level
in an ytterbium-doped germanosilicate fiber", Opt. Lett. 21,
1061-1063 (1996)], as used in the prior art for high
pulse-energies. The ring-doped fiber had
.PSI..sub.doped.apprxeq.0.02 .mu.m.sup.2. A smaller value can allow
for even larger extracted energies, as long as the pump power is
large enough to create a significant gain.
[0177] The emission cross-section of erbium in glass is smaller
than for many other gain media, like Nd.sup.3+:glass at 1050 nm and
many transition metals. It follows from Eq. 17 that the stored
energy will be smaller in these media. Therefore, the improvements
with ring-doping can be relatively larger than for
Er.sup.3+:glass.
[0178] Cladding-pumped devices
[0179] We now describe cladding-pumped ring-doped fibers for
high-energy pulse amplification and generation. Because of the
typically higher pump powers used with these devices and because of
the separately controllable normalized pump and signal mode
intensities in the doped region, the disclosed cladding-pumped
devices will by far outperform any prior-art core-doped single- or
few-moded waveguiding device. A typical device will be a
rare-earth-activated glass fiber optically pumped by a pump beam
launched into the inner cladding (cf. FIG. 3).
[0180] FIG. 10 shows how the extractable energy and small-signal
gain at 1550 nm depends on the position of the ring for a
ring-doped EDF cladding-pumped by 1 W and 5 W at 980 nm. In
particular, FIG. 10 illustrates the extractable energy and
small-signal gain at 1550nm for a ring-doped EDF cladding-pumped by
1 W and 5 W at 980 nm. The ring thickness was sufficiently thin to
make variations of the normalized modal intensity negligible over
its thickness. Other parameters are listed in Tables 5 and 6 under
"normal-core amplifier". For comparison, also results for EDFAs
homogeneously doped throughout the core are shown, both for the
"normal core" amplifier and a "large core" amplifier. In all cases,
a higher pump power gives a higher small-signal gain and a larger
extractable energy. Moreover, the fiber length was optimized for
maximum small-signal gain in all cases. In all cases, the fiber
length was optimized for maximum small-signal gain. Other
parameters were the same as in FIG. 10, and are listed in Tables 5
and 6. The advantages compared to the prior-art EDFs (also shown)
are substantial. The increase of the extractable energy can
approach two orders of magnitude in the devices studied here.
[0181] In view of these results, we propose a ring-doped
cladding-pumped optical fiber where the ring is located at a
position where the mode intensity is, e.g., one or two orders of
magnitude smaller than it is in its center. In order to get
sufficient absorption, sensitization can be used, e.g., as in
ytterbium-sensitized erbium-doped fibers.
[0182] A ring-shaped gain medium outside the core can be better
pumped by a beam in the cladding. Thus, while cladding-pumping has
normally been considered to facilitate launching of
non-diffraction-limited sources like diode bars, we also propose to
use cladding-pumped, ring-doped fibers even when high-brightness,
near-diffraction-limited pumps that could be efficiently launched
into the core are available. The high brightness will still be
favorable because the area of the inner cladding can be small. In
these devices, cladding-modes can see a higher gain than the
desired core-mode does, whereby some measure for suppressing
cladding-modes would be required.
[0183] Even if only a small part of the stored energy is extracted
from a ring-doped amplifier, the high stored energy can still be
advantageous, since it for instance reduces the distortions of the
chirped-pulse amplification with small distortions of the pulse
shape.
[0184] Passively Q-switched and gain-switched lasers
[0185] In passively Q-switched lasers, energy and thereby ASE
builds up in a region of gain. The ASE then transfers energy to a
saturable absorber. The saturable absorber must be so that the
absorption change per unit stored energy is smaller than it is in
the gain region. In prior-art devices, this is achieved by using a
saturable absorber with large absorption and/or stimulated emission
cross-sections, compared to those of the gain medium. Ring-doped
fibers open up for Q-switched lasers where the gain section and the
saturable absorber are made from the same material, e.g. an
erbium-doped glass. This is possible since
E.sub.sat.ident.hv/[.PSI..sub.doped (.sigma..sup.a+.sigma..sup.e)]
can be two orders of magnitude higher in the ring-doped fiber than
in the core-doped one, even though the material-dependent
quantities (.sigma..sup.a+.sigma..sup.e) are equal in the two
different fibers.
[0186] The gain section can also be a core-doped fiber with a large
area core, however, this does not work as well as a properly
designed ring-doped fiber.
[0187] In a first embodiment, a ring-doped fiber is cascaded with a
core-doped fiber, each of which are doped with a similar dopant
with a non-negligible ground-state absorption, to form a laser
cavity. A pump beam is launched into a gain section, consisting of
the ring-doped fiber, thereby building up a gain and stored energy.
A cw pump beam can be used, and the fiber can be cladding-pumped.
The gain section generates ASE, through which energy is transferred
from the gain section to a core-doped fiber constituting a
saturable absorber. The pump also acts to bleach the
pump-absorption in the ring-doped fiber, whereby the pump
penetrates deeper into the cavity, and possibly helps in bleaching
the saturable absorber. The transfer of energy from the gain
section to the absorber section increases the net gain in the
cavity to a point where it exceeds threshold. Then, energy is
radiated from the cavity in the form of a Q-switched pulse. This
substantially reduces the stored energy, and hence the gain, in the
cavity, so that the ASE becomes negligible, and the pump power that
penetrates to the saturable absorber becomes small. The saturable
absorber then relaxes to a state that is at least partly absorbing.
Thereby, the absorption in the saturable absorber has increased
substantially before the gain section starts to generate ASE again,
whereupon the cycle is repeated.
[0188] A second embodiment is similar to the first embodiment,
except that there is provided a pump-absorber or a pump reflector
between the gain-section and the saturable absorber. This
substantially reduces the pumping of the saturable absorber.
[0189] A third embodiment is similar to the first or second
embodiment, except that the active centra in the gain medium and
the saturable absorber are different. The pump wavelength can be
chosen so that it cannot bleach the saturable absorber.
[0190] FIG. 11 is a view of a fiber having a saturable absorber
(640) in the central part of the core (30), and a ring-shaped gain
medium (620) around the absorber. In the illustrated example, the
gain medium resides in the core, but it can be placed partly or
wholly in the cladding (10).
[0191] FIG. 12 illustrates a semiconductor amplifier for signal
amplification. The semiconductor amplifier provides gain for a
guided optical signal beam in a region where the normalized modal
intensity is small and comprises a cladding (410), a gain region or
an active layer (420), a core or index guiding layer (430), a
substrate (480), and a contact layer (490). Also the approximate
location of a signal beam is indicated (470). The refractive index
of the active layer (420) can be depressed with respect to the
remainder of the cladding (410) in order to suppress gain
guiding.
[0192] FIGS. 13a to c illustrate devices in which unwanted,
higher-order modes are suppressed by the inclusion of an absorber.
FIG. 13a shows a fiber with an amplifying ring 10 and an absorbing
ring 510 configured to suppress high-order modes. The absorption of
the desired fundamental mode is small or even negligible. FIG. 13b
shows a planar waveguide with amplification of the evanescent field
by a gain region 120, within an absorbing superstructure 520.
Again, the absorption of the desired fundamental mode is small or
even negligible. Undesired higher-order modes penetrate further
into the absorber, whereby they are suppressed. FIG. 13c shows a
double clad ring-doped fiber in which a signal-absorbing region 530
has been incorporated into the cladding, thereby preventing any
build-up of signal light in the cladding.
[0193] Device with a distributed saturable absorber
[0194] Above, two media with different saturation characteristics
were combined in a cascade. However, the two gain media can also
reside side by side in the same fiber. An example of this is
illustrated in FIG. 11. A fiber having a core (30) and a cladding
(10) is doped with a saturable absorber (640) and a gain medium
(620). Here, the saturable absorber is located in a region where
the normalized modal intensity is larger than it is in the region
of the gain medium. Hence, if the absorber and gain media are
similar (except that the gain medium is pumped), and the
cross-section for stimulated emission of the gain medium is similar
to the absorption cross-section of the absorber, the small-signal
gain of the fiber can be negative or small, even though the
extractable energy of the gain medium is larger than the energy
required to bleach the saturable absorber. Hence, the ASE in the
fiber can be suppressed, while the energy that can be extracted
from the device, if for instance a signal pulse is launched into
it, can be large.
[0195] In contrast to the prior art, a ring-shaped gain medium
allows the active centra in the absorber and the gain media to be
of the same or similar types, as long as it is possible to pump the
centra in the gain medium while leaving those in the absorber
medium unpumped. A particular studied embodiment consisted of an
Er.sup.3+-doped saturable absorber and an Yb.sup.3+-sensitized
Er.sup.3+-doped gain medium. The Er.sup.3+in the gain medium was
excited indirectly (i.e., via the Yb.sup.3+) by an optical pump
beam launched into the fiber core. The launched pump power was 1 W
at a wavelength of 1064 nm, which is a wavelength that will not
excite the Er.sup.3+in the saturable absorber. The fiber had a
numerical aperture of 0.16, and a core diameter of 7 .mu.m. The
diameter of the saturable absorber (640) was 1 .mu.m, while the
inner and outer radii of the ring-shaped gain medium (620) were 3.4
.mu.m and 3.5 .mu.m, respectively. The Er.sup.3+-concentration was
2.38.times.10.sup.25 m.sup.-3 in both the absorber and gain media,
and the Yb.sup.3+-concentration was 2.97.times.10.sup.26 m.sup.-3
in the gain medium. The absorption and emission cross-sections at
the peak (wavelength 1536 nm) were both 6.8.times.10.sup.-25
m.sup.2. Hence, the small-signal absorption at that wavelength was
2.1 dB/m in the saturable absorber, and 1.3 dB in the (unpumped)
gain medium. Moreover, at 1064 nm, the cross-sections for
stimulated emission and absorption of the Yb.sup.3+-ions were at
2.times.10.sup.26 m.sup.2 and 5.times.10.sup.-28 m.sup.2,
respectively. The metastable lifetimes of the Er.sup.3+and the
Yb.sup.3+were 10.2 ms and 1.3 ms, respectively. The energy was
transferred from the Yb.sup.3+to the Er.sup.3+with a rate
coefficient k.sub.tr of 1.05.times.10.sup.-21 m.sup.3/s [J.
Nilsson, P. Scheer, and B. Jaskorzynska, "Modeling and optimization
of short Yb.sup.3+-sensitized Er.sup.3+-doped fiber amplifiers",
IEEE Photon. Technol. Lett. 6, 383-385 (1994).]. The spectral
characteristics of the gain and absorber region followed those for
Er.sup.3+and Yb.sup.3+in a phosphosilicate glass. Numerical
calculations, following those in [J. Nilsson and B. Jaskorzynska,
"Modeling and optimization of low repetition-rate high-energy pulse
amplification in cw-pumped erbium-doped fiber amplifiers", Opt.
Lett. 18, 2099-2101 (1993).] and [J. Nilsson, P. Scheer, and B.
Jaskorzynska, "Modeling and optimization of short
Yb.sup.3+-sensitized Er.sup.3+-doped fiber amplifiers", IEEE
Photon. Technol. Lett. 6, 383-385 (1994).] and using the parameters
above, showed that the extractable energy in this device was
approximately 0.6 mJ at 1536 nm and 1.1 mJ at 1560 nm.
[0196] In the example above, the ring-shaped gain region was thin
and hence the extractable energy per unit length small. This
implied that the length of the fiber became so long (several
hundred meters) that background losses could become important, and
the calculated energy, neglecting background losses, difficult to
achieve. By placing a ring-shaped gain medium outside the core
(where the normalized modal intensity is smaller), its gain can be
kept constant while the stored energy in the gain medium is
increased (cf. Eq. 17). Hence, the fiber can be shorter. For a
cladding-pumped fiber having an inner cladding with a radius of 10
.mu.m, a saturable absorber (640) with a radius of 0.5 .mu.m
(small-signal absorption 2.2 dB/m at 1536 nm), and a gain medium
(620) with an inner radius of 4.5 .mu.m and an outer radius of 5.5
.mu.m (small-signal absorption 3.3 dB/m at 1536 nm), calculations
gave an extractable energy of 0.8 mJ at 1536 nm and 1.4 mJ at 1560
nm, for a fiber length of 50 m. The fiber length can be further
reduced by using a larger-area gain region (e.g., a thicker doped
ring). The other parameters of the fiber were the same as above. A
problem with this approached is that the preferred host material
for a Yb.sup.3+-sensitized Er.sup.3+-doped gain medium
(phosphosilicate glass) has a higher refractive index than the
preferred cladding (fused silica). Hence, some extra measure may be
needed to level the refractive index of the gain medium with that
of cladding.
[0197] The calculations have also shown that ASE in the
long-wavelength end of the .sup.4I.sub.13/2.fwdarw..sup.4I.sub.15/2
emission spectrum, where the emission cross-sections become
relatively larger compared to the absorption cross-section, can
build up and partly bleach the absorption and compress the gain.
This can be avoided by introducing an unsaturable loss at these
long wavelengths. Bending the fiber provides a method for making
the fiber lossy at 1600 nm, while keeping the unsaturable loss
small at 1536 nm. For example, with the fiber parameters above and
with a bend radius of 9 mm, the bend-loss is approximately 0.033
dB/m at 1600 nm, 0.012 dB/m at 1560 nm, and 0.0061 dB/m at 1536 nm,
i.e., it is five times smaller at 1536 nm than at 1600 nm. Another
alternative for an unsaturable loss at longer wavelengths is to use
an unsaturable absorber in addition to the saturable absorber. For
this particular transition, Tm.sup.3+:glass and Tb.sup.3+:glass are
suitable systems for an optical fiber, as the absorption of
suitable pump wavelengths (e.g., 1064 nm or at least 1047 nm in the
case of Tm.sup.3+) is small, as is the absorption for a signal at
1536 nm. Yet another alternative is to use different host media for
the gain and the absorber media. A suitable host medium for the
absorber makes its spectrum wider, and can thus prevent the
build-up of ASE at long wavelengths.
[0198] In the example above, a pump wavelength of 1064 nm was
assumed. Other wavelengths are also possible. However, the pump
should not pump the Er.sup.3+directly, since then also the
saturable absorber will be excited. Moreover, for pumps on the
short-wavelength side of the Yb.sup.3+absorption peak, emission
around 980 nm from the Yb.sup.3+can build up in the fiber and
bleach the Er.sup.3+-ions in the absorber.
[0199] Even if the centra providing the gain and the saturable
absorption are different, a design according to FIG. 11 can improve
to prior-art devices in that the gain efficiency of the gain medium
is relatively lower than it otherwise would be.
[0200] Saturable absorber
[0201] The saturation power P .sub.sat of a saturable absorber is
given by P
.sub.sat.ident.hv/[.PSI..sub.doped(.sigma..sup.a+.sigma..sup.e).tau.],
where .tau. is the lifetime of a metastable state. For some
devices, a medium that would otherwise be a suitable saturable
absorber (e.g., because of a suitable spectral response) is
inappropriate because its saturation power is too small. This can
be the case for an EDF saturable absorber, with P .sub.sat
typically smaller than 1 mW. We here disclose that ring-doping
allows .PSI..sub.doped to be chosen so that a larger, predetermined
value of P .sub.sat can be obtained. For this application, a
few-moded fiber can be acceptable for single-mode applications, as
higher-order modes will experience a higher loss which can render
the power in them negligible.
[0202] Signal amplifiers for reduced cross-talk
[0203] In some optical amplifiers, especially semiconductor ones,
even the energy of a single signal bit (e.g., 0.1-100 fJ) can be
non-negligible comparable to the stored energy. Then, already the
amplification of a single bit extracts enough energy to reduce the
gain. This leads to four-wave mixing and cross-talk in
multi-wavelength amplifiers and inter-symbol interference in
single-wavelength amplifiers. This can be avoided with the higher
stored energy that, for a given gain, accompanies the reduced
interaction in the devices disclosed in this invention.
[0204] FIG. 12 illustrates an embodiment. A semiconductor amplifier
provides gain for one or several guided optical signal beams in a
region where the normalized modal intensity is small. The device
can be electrically pumped. The refractive index of the gain region
can be depressed in order to suppress gain-guiding, since this can
otherwise occur in semiconductor optical amplifiers in which the
gain per unit length is large. This would lead to a large
normalized modal intensity in the gain-region, thereby preventing
substantial reductions of the interaction.
[0205] Suppression of unwanted modes
[0206] Often, lasing on a specific transverse mode is desired, and
then normally on the fundamental mode of the core. If so, it may be
necessary to suppress other, undesired, modes. Higher-order guided
modes of the core extend further into the cladding and thus see a
significantly higher gain than does the fundamental mode in a
ring-doped device. Although we normally envisage single-moded cores
as preferred designs, higher-order modes can also be present due to
fabrication errors, etc. However, these modes are less strongly
guided and will be more sensitive to bending. Hence, with a fiber,
simply bending it can reinstate a net gain advantage for the
fundamental mode.
[0207] Another alternative is to incorporate a region outside the
gain region that absorbs the signal (at desired and possibly also
at undesired wavelengths) but has a low loss for the pump. This
absorbing region is located so that it preferentially absorbs light
in undesired modes. These can be higher-order modes of the core,
and also cladding-modes. See FIG. 13.
[0208] Several possibilities exist for creating the absorbing
region. In the case of a Yb-doped device, Pr.sup.3+and Er.sup.3+can
be suitable such absorbers. For Nd.sup.3+at 850nm-950 nm,
Yb.sup.3+can be used. For Er.sup.3+, Tm.sup.3+and Sm.sup.3+are
potential candidates, just to mention some possibilities with
rare-earth doping. Sm.sup.3+can also suppress unwanted 1050 nm
radiation in Nd.sup.3+-doped samples. Optionally, some additional
measures can be taken to quench the dopant, to prevent it from
bleaching.
[0209] Amplifying optical devices
[0210] FIG. 14 shows an amplifying optical device 1400 comprising a
first waveguiding structure 1401 comprising a first core 1402 and a
cladding 1403, and configured to guide optical radiation 1404; a
pump source 1405 configured to supply optical pump power 1406, an
amplifying region 1407 situated in the cladding 1403; wherein the
pump source 1405 is optically coupled to the amplifying region
1407, and wherein in use the optical radiation 1404 guided in the
first waveguiding structure 1401 overlaps the amplifying region
1407.
[0211] Also identified in FIG. 14 is an amplifying optical
waveguide structure 1408 which comprises the first waveguiding
structure 1401 and the amplifying region 1407. The purpose of the
amplifying optical device 1400 is to generate or amplify optical
radiation 1404.
[0212] The amplifying optical waveguide structure 1400 can also
operate with an amplification less than unity for radiation at
certain wavelengths (especially in the absence of optical pump
power 1406) and can then be used to absorb optical radiation
1404.
[0213] The amplifying optical waveguide structure 1408 has first
and second ends 1409 and 1410. The first waveguiding structure
extends to first and second ends 1409 and 1410, so that optical
beams can be coupled into and out of the first waveguiding
structure 1401. It is also possible to couple light into the first
waveguiding structure 1401 through the side of the amplifying
optical waveguide structure 1408. Points at which optical beams can
enter or exit an optical waveguiding structure can be referred to
as input and output ports, as the case may be. An optical beam
launched through a port can exit through the same port, if, for
instance, the amplifying optical device is a reflecting
traveling-wave amplifier.
[0214] FIG. 14 illustrates the optical radiation 1404 and the
optical pump power 1406 with the intensity profile in a
cross-section of the beams.
[0215] By amplifying optical device 1400 we mean a device for
generating, amplifying, or absorbing the optical radiation 1404. By
way of example only and without limitation, an amplifying optical
device 1400 can be an optical amplifier, a master oscillator power
amplifier (MOPA), an amplified spontaneous emission (ASE) source, a
superfluorescent source, an energy storage device, a high-pulse
energy device, a cladding-pumped device, a semiconductor signal
amplifier, or a laser which by way of example and without
limitation can include a laser, a fiber laser, a Q-switched laser,
a mode-locked laser, or a semiconductor laser.
[0216] By a pump source 1405, we mean a device for supplying
optical pump power 1406. By way of example only and without
limitation, a pump source 1405 can include a gas laser, a solid
state laser, a semiconductor laser, a chemical laser or a
semiconductor light emitting diode. The semiconductor laser can be
implemented with a diode bar or a broad-stripe laser diode, and can
be used either for end-pumping or for side-pumping. A pump source
1405 can also be provided by natural illumination, for example by
daylight. The preferred pump source 1405 is a high-power
semiconductor laser diode.
[0217] Though FIG. 14 shows a single pump source 1405, multiple
pump sources can be employed in order to obtain higher powers
and/or for pump redundancy. The pump power 1406 from the multiple
pump sources can be coupled into the amplifying region 1407 via at
least one of the first end 1409, the second end 1410 and the
side.
[0218] Though FIG. 14 shows optical pump power 1406 being launched
through a facet at a first end 1409 of the amplifying optical
waveguide structure, other schemes for launching the power are also
possible, as will be described in this document.
[0219] By first core 1402, we mean the region of the first
waveguiding structure 1401 where the intensity of the optical
radiation 1404 is relatively high compared to the intensity of the
optical radiation 1404 propagating in the cladding 1403 in the same
transverse section. By way of example only and without limitation,
the first core 1402 can be that region with a refractive index
greater than the refractive index of the cladding 1403. However,
for the purposes of this invention, the first core 1402 may
sometimes be defined as the region of the first waveguiding
structure 1401 bounded by a contour of equal optical intensity of
the fundamental mode and which contains 75% of the optical
radiation 1404 at a wavelength corresponding to the second mode
cut-off of the first waveguiding structure 1401. Note however that
the device cannot be operated at the wavelength corresponding to
the second mode cut-off of the first waveguiding structure
1401.
[0220] The first waveguiding structure 1401 can be a single mode
waveguiding structure, or can support several higher-order modes.
The first waveguiding structure 1401 can be a planar waveguiding
structure or can be an optical fiber.
[0221] Though not shown in FIG. 14, it is possible that a
waveguiding structure has branches so that a light beam propagating
in one waveguiding structure is divided into two beams propagating
in different waveguiding structures. Correspondingly, two beams can
be combined to one.
[0222] In some instances, the propagation of high-order modes,
which can be leaky, is undesirable. These can be suppressed by
bending the first waveguiding structure 1401, or by introducing a
absorber into the cladding 1403 configured such that there is a
high-differential loss between the undesired high-order modes and
the desired propagating mode or modes.
[0223] The amplifying region 1407 can contain at least one rare
earth dopant selected from the group consisting of Ytterbium,
Erbium, Neodymium, Praseodymium, Thulium, Samarium, and Holmium. It
can also contain Europium, Terbium, and/or Dysprosium. The
amplifying region 1407 can contain at least one transition
metal.
[0224] For embodiments where the amplifying region 1407 is located
in both the first core 1402 and the cladding 1403, the rare-earth
dopant within the portion of the amplifying region 1407 residing in
the cladding 1403 is not be the same as the rare-earth dopant in
the first core 1407.
[0225] It can also be that the amplifying region 1407 amplifies
optical radiation 1404 at different wavelengths using, e.g.,
different rare-earth dopants, with at least two beams of optical
radiation 1404 at different wavelengths propagate simultaneously or
alternately through the waveguide. Therefore, we take the location
of the amplifying region 1407 to be determined by the properties of
the amplifying region at the wavelength of optical radiation 1404
of interest.
[0226] The amplifying region 1407 can be excluded from the first
core 1407 leading to a so-called ring-doped design. A particular
advantage of this embodiment is that the amplifying region 1407 in
operation will contain a significantly larger stored energy than
corresponding core-doped designs. This can be important for devices
that emit optical pulses with pulse energies being a significant
fraction of the energy stored in the amplifying region 1407. This
can also be important for amplifiers with high requirements on
linearity, for example in analog Community Antenna Television
(CATV) applications - especially if the signal contains
low-frequency components and the output power is high whereupon
undesirable non-linear distortion can occur.
[0227] The first waveguiding structure 1401 can be configured such
that in use the first waveguiding structure 1401 is a single-mode
waveguide, and the optical radiation 1404 guided by the first
waveguiding structure 1401 has a Gaussian equivalent spot size (b
1/e.sup.2 intensity diameter) greater than about eight times the
wavelength as measured in vacuum of the optical radiation 1404
guided by the first waveguiding structure 1401.
[0228] The first waveguiding structure 1401 can be of a more
complicated shape than the traditional ones illustrated in the
drawings. For example, it can be non-circular or utilize
complicated core designs such as found in W-fibers, multiple
cladding fibers (including those with areas in the cladding with a
raised refractive-index), segmented core designs, and so-called
alpha profiles.
[0229] The amplifying optical device 1400 can be advantageous in
several ways as explained previously. For example, in order to
incorporate a large enough amplifying region 1407 to absorb enough
pump power some amplifying optical waveguide structures 1408 need
to be long, e.g. 0.1 kilometers up to several kilometers. Around a
wavelength of 1000 nm, low-loss first waveguiding structures made
from doped silica fibers exhibit a background loss of at least
approximately 1 dB/km. A 1 dB background loss may be acceptable in
an amplifying optical device 1400. However, an amplifying region
1407 incorporated into the first core 1402 tends to significantly
increase the background loss, often by more than one order of
magnitude. This reduces the maximum acceptable lengths of the
amplifying optical waveguide structure 1408, which can have an
impact on the pump absorption. By placing the amplifying region
1407 in the cladding 1403 rather than in the first core 1402 in
which most of the power of the optical radiation 1404 propagates
enables us to use a passive first core 1402 (e.g. made from a pure
silica or a silica doped with at least one of fluorine, germanium,
phosphorus, tantalum, aluminium and titanium). The background
losses of the first waveguiding structure 1401 can then be reduced
so that a longer amplifying optical waveguide structures 1408 that
can absorb more pump power 1406 can be used in order to generate
higher power optical radiation 1404.
[0230] FIG. 15 shows an amplifying optical device 1500 based on the
amplifying optical device 1400, which further comprises a second
waveguiding structure 1501 comprising a second core 1502 and
configured to guide the optical pump power 1406, and wherein the
second waveguiding structure 1501 contains the amplifying region
1407 and wherein the second core 1502 is at least partly formed by
at least part of the cladding 1403, and wherein the pump source
1405 is optically coupled to the second waveguiding structure
1501.
[0231] The first core 1402 can form a part of the second core
1502.
[0232] Also identified in FIG. 15 is a cladding-pumped amplifying
optical waveguide structure 1508 which comprises the first and
second waveguiding structures 1401, 1501 and the amplifying region
1407.
[0233] In some instances, unwanted modes in either the first or
second waveguiding structures 1401, 1501 can be excited. The
higher-order modes in the first waveguiding structure 1401 can be
suppressed by bending the first waveguiding structure 1401. The
higher-order modes in the first and second waveguiding structures
1401 and 1501 can be suppressed by introducing an absorber into the
cladding 1403 configured such that there is a high-differential
loss between the undesired modes and the desired mode or modes.
Unwanted modes in the second waveguiding structure 1501 can also be
removed by mode stripping--for example by removing outer coatings
and placing the fiber into high-index fluid.
[0234] Examples of the first waveguiding structure 1401 and the
amplifying region 1407 were described previously with reference to
FIGS. 1, 2, 3, 4, 11, 12, 13a, 13b and 13c. The amplifying region
1407 is shown as the ring-doped dopant profile 20 in FIG. 1. As
FIG. 14 illustrates, the amplifying region 1407 can comprise at
least one doped region of arbitrary shape.
[0235] Waveguiding can also be achieved with a micro-structured
design, for example, the first or second waveguiding structures
1401, 1501 can contain longitudinally extensive holes.
[0236] A preferred design of the amplifying optical device 1500 is
one in which in use the optical radiation 1404 can be guided by the
first waveguiding structure 1401 without coupling to the second
waveguiding structure 1501.
[0237] The cross-sectional area of the second core 1502 can be
greater than 1000 square microns and smaller than 100,000 square
microns.
[0238] The second core 1502 is preferably adjacent to a region 1503
having a lower refractive index than the second core 1502, the
amplifying optical device 1502 being such that the region 1503
provides total internal reflection of the optical pump power 1406.
The region 1503 can comprise a vacuum, a gas, a liquid, a polymer
or a glass. If the amplifying optical waveguiding structure 1508 is
an optical fiber, the polymer can be applied as a coating during
the fiber drawing process. The amplifying optical waveguiding
structure 1508 then forms an example of a double-clad optical
fiber, as illustrated in FIG. 4. A double-clad optical fiber is a
preferred amplifying optical waveguiding structure 1508.
Alternatively the second core 1502 can be surrounded by a metal or
a periodic layer for reflecting light.
[0239] It is preferred that the first waveguiding structure 1401
and the second waveguiding structure 1501 are fabricated in a
single optical fiber.
[0240] It is preferred that the first waveguiding structure 1401 is
fabricated from at least one glass system, preferably an oxide
glass system selected from the group consisting of silica, doped
silica, silicate, and phosphate. The second waveguiding structure
1501 can also be fabricated from the at least one glass system. By
doped silica we mean (WITHOUT LIMITATION ETC) silica doped with
fluorine and/or at least one of the oxides of the following -
germanium, phosphorus, boron, tantalum, titanium, aluminum, tin,
where the oxide dopant concentration is typically up to around 10%.
By silicate, we mean doped silica where the dopant concentration is
greater than about 10%. By phosphate we mean a phosphate compound
glass which includes phosphoria with the addition of other glass
forming or modifying agents. In addition, the dopants included in
any of the above glass systems can include rare earth and
transition elements.
[0241] The amplifying optical devices 1400 and 1500 can also
contain limited amounts of gain medium in the first core 1402 while
still retaining the basic characteristics of a device doped in the
cladding 1403. We also note that the amplifying optical waveguide
structures 1408 and 1508 can be longitudinally varying, e.g., with
a section doped in the first core 1402 rather than the cladding
1403. Nevertheless, the advantages of the novel amplifying optical
devices disclosed here remain to the extent that most of the power
transferred to the optical radiation 1404 can be transferred from
parts of amplifying region 1407 located in the cladding 1403.
[0242] Nevertheless, the advantages of the novel amplifying optical
devices disclosed here remain to the extent that most of the power
transferred to (or from) the optical radiation 1404 can be
transferred from (or to) parts of the amplifying region 1407
located in the cladding 1403.
[0243] In order to obtain an efficient device, it is preferred to
locate the amplifying region 1407 close enough to the first core
1402 so that the all of the optical pump power 1406 absorbed by the
amplifying region 1407 can be transferred to the optical radiation
1404. Otherwise, pump-to-signal power conversion efficiency is
reduced.
[0244] It is well-known that the shape of the second core 1502 as
well as the location of the amplifying region 1407 relative to the
second core 1502 affects the rate at which the amplifying region
1407 absorbs the optical pump power 1406. In particular, an
amplifying region 1407 located near the center of a circularly
symmetric second core 1502 may fail to absorb the optical pump
power 1406 efficiently. Well-known methods for improving the pump
absorption are to locate the amplifying region 1407 off-center, to
use a non-circular second core 1502, or to bend the amplifying
optical waveguide structure 1508.
[0245] For efficient absorption of the optical pump power 1406, it
is preferred that the amplifying region 1407 is transversely
disposed to regions within the second waveguiding structure 1501
where the intensity of the optical pump power 1406 is high.
[0246] FIG. 15 shows a second waveguiding structure 1501 that
confines light in both directions transverse to the first
waveguiding structure 1401 so that the first and second waveguiding
structure 1401 and 1501 are parallel to each other. However, for
instance, in a planar structure like the one illustrated in FIG. 3,
the second core 1502 can be quite wide in one direction and
effectively only confine light in one transverse direction. In such
a structure, the optical pump power 1406 can also propagate at an
angle to the first optical waveguiding structure 1401. In order to
ensure sufficient pump absorption, the amplifying optical waveguide
structure 1408 can contain several waveguiding structures 1401 with
amplifying regions 1407. The different waveguiding structures 1401
can be coupled to each other in series or in parallel, or can be
independent.
[0247] FIG. 16 shows a preferred embodiment of an amplifying
optical device 1600 comprising an optical fiber 1610 containing an
amplifying region 1407 that is characterized by a dopant
concentration 1601, a disposition 1602 and a length 1603, and
wherein the dopant concentration 1601, the disposition 1602 and the
length 1603 of the amplifying region 1407 are arranged such that
the amplifying optical device 1600 amplifies at an operating
wavelength at which there is a significant saturable small signal
absorption, for example as found in a two- or three-level
system.
[0248] In a preferred embodiment, the optical fiber 1610 is
fabricated from silica-based glasses. Several suitable dopants that
increase the refractive index but are otherwise optically passive
are well-known and include germania, phosphorus, alumina and
tantalum. These can be used for defining the first core 1402. The
cladding 1403 also functions as a second core 1502, and can be
formed of pure silica except in the amplifying region 1407 which is
formed by doping the cladding 1403 with a rare earth such that a
ring is formed around the first core 1402.
[0249] It is well-known that co-dopants can be used to increase the
solubility of rare earth dopants in silica. This is otherwise poor.
Preferred co-dopants include alumina and phosphorous. The
incorporation of alumina, phosphorous, and/or a rare earth in
silica is known to increase the refractive index compared to pure
silica. This increase can be significant, and depending on the
design of the optical fiber 1610, it can be undesirable. If so, it
can be negated by further co-doping the amplifying region 1407 with
a index-lowering element like fluorine. Alternatively, we can use a
cladding 1403 made from silica doped with an index-raising agent
(e.g., tantalum or one of the other index-raising elements listed
above). This way, the refractive index of the cladding 1403 can
equal or even exceed that of the amplifying region 1407, even if
this is co-doped with, say, alumina or phosphorous. All of these
methods for modifying the properties of silica are well-known.
[0250] Preferred embodiments of the amplifying optical device 1600
enables efficient cladding pumping of the amplifying optical device
1600 at this operating wavelength compared to corresponding
core-doped designs utilizing the same rare earth dopant.
[0251] A preferred embodiment is designed to operate in the
wavelength range of about 1480 nm to about 1570 nm. The dopant is
either Erbium or Erbium co-doped with Ytterbium. For Erbium,
typical values for the length 1603 are in the range 5 to 100
meters, dopant concentration 1601 is approximately 0.1% to 0.5% by
weight, and the disposition 1602 is a ring around the first core
1401 with an inner diameter of about one to two times the core
diameter and a thickness of 2 to 5 microns. For Erbium co-doped
with Ytterbium, typical values for the length 1603 are in the range
0.5 to 50 meters, dopant concentration 1601 is approximately Erbium
0.1% by weight and Ytterbium concentration is 10 to 20 times the
Erbium concentration, and the disposition 1602 is a ring around the
first core 1401 with an inner diameter of about one to two times
the core diameter and a thickness of 2 to 5 microns. More examples
are given with respect to FIGS. 6, 7 and 10. These design values
are given for illustrative purposes only and are meant to be
non-limiting. For example, it can be preferable in some instances
to design a first waveguide structure 1401 with a very large first
core 1402, in which case the length 1603 and the disposition 1602
would be different.
[0252] Another preferred embodiment is designed to operate in
either the wavelength range of about 970 nm to 990 nm or the
wavelength range of about 1010 nm to 1030 nm. The dopant is
Ytterbium. Typical values for the length 1603 are in the range 0.5
to 50 meters, dopant concentration 1601 is approximately 0.1% to 2%
by weight, and the disposition 1602 is a ring around the first core
1401 with an inner diameter of about one to two times the core
diameter and a thickness of 1 to 3 microns.
[0253] While the intrinsic fluorescence from Ytterbium in glass
peaks around 980 nm, it is well-known that cladding-pumped
single-moded Ytterbium-doped fiber lasers normally emit around
1060-1120 nm. Emission at 980 nm is possible, but normally only at
a significantly reduced efficiency. In contrast, a surprising and
important result is that the design here allows efficient operation
of cladding-pumped single-moded Ytterbium-doped fiber lasers at 980
nm. The pump source 1450 is preferably in the wavelength band from
about 870 nm to about 950 nm, and preferably between 900 and 940
nm. It is preferred that the amplifying region 1407 absorbs at
least about 30% of the optical pump power 1406 launched into the
second waveguiding structure 1501.
[0254] Yet another preferred embodiment is designed to operate in
the wavelength range of about 850 nm to 950 nm. The dopant is
Neodymium. Typical values for the length 1603 are in the range 0.5
to 50 meters, dopant concentration 1601 is approximately 0.1% to 2%
by weight, and the disposition 1602 is a ring around the first core
1401 with an inner diameter of about one to two times the core
diameter and a thickness of 1 to 3 microns.
[0255] FIG. 28 shows a cross-sectional view of a preferred
embodiment of an optical fiber 1610, having a ring-doped amplifying
region 1407 located in the cladding 1403 and centered on the first
core 1402. Also provided is a second core 1502 comprising the first
core 1402 and cladding 1403. The second core 1502 is rectangularly
shaped and is located within an outer cladding 2803. The refractive
index of the first core 1402 is higher than that of the cladding
1403, which is in turn higher than that of the outer cladding 2803.
The refractive index of the amplifying region can be equal to that
of the cladding 1403 by way of co-dopants. The amplifying region
1407 can be doped with erbium, ytterbium, and/or another rare earth
element. The optical fiber 1610 can be made from glass, preferably
doped and undoped silica. The fiber can be surrounded by a coating
made from a polymer or another material. Alternatively, the outer
cladding 2803 can be made from a polymer.
[0256] It is preferred that the first core 1402 is single-moded at
a desired wavelength of optical radiation 1404.
[0257] FIG. 17 shows a schematic of a high-power optical amplifier
1700 comprising at least one optical fiber 1610, an optical pump
source 1405, a coupler 1701, an input port 1702, an output port
1703, a first isolator 1704 and a second isolator 1705.
[0258] FIG. 17 also shows a filter 1706 that can be added in order
to suppress amplified spontaneous emission at undesired
wavelengths. The filter 1706 can be a fiber optic Bragg grating, a
long-period grating, an absorbing medium, an acousto-optic filter,
or an interferometric filter such as implemented with a Mach
Zehnder or a Fabry Perot. The filter 1706 can be tunable and it can
alternatively be placed following the output port 1703.
[0259] The coupler 1701 can be a dichroic mirror, a wavelength
division multiplexing coupler, or a pump-injecting fused fiber
coupler formed by side-coupling the core of a multimode optical
fiber to the second core 1502 by fusing and twisting the fibers
together.
[0260] The optical pump source 1405 is preferably a high-power
semiconductor laser-diode coupled into the optical fiber 1610.
[0261] For high-power applications (greater than about 1 Watt), the
second isolator 1705 is preferably not utilized and the coupler
1701 is preferably a dichroic mirror or a pump-injecting fused
fiber coupler. Optical pump power 1406 can also be launched into
the second waveguiding structure 1501 through the side of the
fiber, the pump power 1406 being reflected into the second core
1502 by a V-groove formed in the fiber. Optical isolators are often
used to suppress reflections in the signal beam, and can also be
used for protecting the optical pump source 1405. However, optical
isolators are in general lossy and can also distort the beam, and
will therefore normally only be used if deemed necessary. This
depends on, for example, if high-gain strictly single-pass
traveling wave amplification is required and also on if a pump
source can be reached and damaged by cw or pulsed signal light,
reflected pump light, or (in case of multiple pump sources) light
from another pump source.
[0262] The different optical components in FIG. 17 and other
embodiments can be optical fiber devices or they can provide fiber
pigtails for use as input and output ports. If so, it is preferred
that the different components are connected by splicing them
together to form a continuous optical fiber waveguide.
Alternatively, it is possible to fabricate several optical
components with a single optical fiber.
[0263] FIG. 18 shows a master oscillator power amplifier MOPA 1800
which comprises a master oscillator 1801 optically coupled to the
first waveguiding structure 1401 of an optical amplifier 1802. The
master oscillator 1801 generates an optical seed 1803 which is
amplified by the optical amplifier 1802 to provide an output 1804
of higher power than the optical seed 1803.
[0264] The optical amplifier 1802 can be the optical amplifier 1700
shown in FIG. 17.
[0265] The master oscillator 1801 can be a semiconductor laser, a
semiconductor distributed feedback laser, a fiber laser, a fiber
distributed feedback laser, a fiber ring laser, a gas laser, a bulk
laser, a source of amplified spontaneous emission or a light
emitting diode filtered by an optical filter which can be an
optical fiber Bragg grating.
[0266] The optical seed 1803 can be continuous wave, or can be an
optical pulsed seed.
[0267] In a preferred embodiment, the disposition of the amplifying
region 1407 is arranged such that the optical amplifier 1802 can
store a large amount of energy before it reaches its intrinsic
lasing threshold. By large, we mean large compared to a
corresponding design in which the amplifying region 1407 is
situated in the first core 1402. Optical amplifiers designed in
accordance with FIGS. 14 and 15 will perform surprisingly well for
this application, far better than prior art optical amplifiers
having rare-earth dopants in the first core 1402.
[0268] The optical amplifier 1802 is preferably be the optical
amplifier 1700 which can be designed to have high energy storage at
its intrinsic lasing threshold when used with an optical pulsed
seed, the optical amplifier 1700 being designed such that it is
able to be operated such that the amplified optical pulse seed has
an energy exceeding the intrinsic saturation energy of the optical
amplifier 1700.
[0269] The optical amplifier 1802 preferably utilizes the optical
fiber described with FIG. 11, either in a core-pumped or a
cladding-pumped version.
[0270] The optical seed 1803 can also be provided by an external
source not associated with the MOPA 1800. Thus the present
invention provides for both the MOPA apparatus as well as the use
of optical amplifiers to amplify optical pulses to energies
exceeding the intrinsic saturation energy of the optical amplifier
1802.
[0271] The small-signal gain (without any optical radiation
incident at the input port) of the optical amplifier 1802 is
normally no more than 40 dB and efficient energy extraction is
difficult at a gain above 20 dB for pulsed operation. In order to
reach the intrinsic saturation energy of the optical amplifier
1802, it is therefore preferred that the master oscillator 1801
seeds the optical amplifier 1802 with pulses of at least 0.01% and
preferably at least 1% of the desired output pulse energy. Seeds
1803 with still higher pulse energy are preferable. An intermediate
amplifier can be required between the master oscillator 1801 and
the optical amplifier 1802 to ensure that the optical seed 1803 is
sufficiently large. The master oscillator 1801 can emit optical
pulses. Alternatively, there can be an optical time gate between
the master oscillator 1801 and the optical amplifier 1802 that
opens and closes to create an optical pulse pattern. It is
preferred that the pump source 1405 is a semiconductor laser
diode.
[0272] It is preferable to reduce the reflections outside the first
waveguiding structure 1401. This can be achieved by using
antireflection coatings or an angled end to the first waveguiding
structure 1401.
[0273] Similarly, for cw operation, the power of the optical seed
1803 that seeds the optical amplifier 1802 should be at least 0.01%
and preferably at least 1% of the desired power of the output 1804.
Still higher-power seeds 1803 are preferable. An intermediate
amplifier can be provided to reach adequate power levels for the
optical seed 1803.
[0274] In a preferred embodiment, the optical amplifier 1802
comprises an optical fiber 1610 containing an amplifying region
1407 that is doped with Ytterbium. The optical fiber 1610 is
characterized by a dopant concentration 1601, a disposition 1602
and a length 1603, and wherein the dopant concentration 1601, the
disposition 1602 and the length 1603 of the amplifying region 1407
are configured such that the optical amplifier 1802 amplifies in
either the wavelength range of about 970 nm to 990 nm or the
wavelength range of about 1010 nm to 1030 nm.
[0275] It is preferred that the amplifying region 1407 absorbs at
least about 30% of the optical pump power 1406 launched into the
second waveguiding structure 1501. Typical values for the length
1603 are in the range 0.5 to 50 meters, dopant concentration 1601
is approximately 0.1% to 2% by weight, and the disposition 1602 is
a ring around the first core 1401 with an inner diameter of about
one to two times the core diameter and a thickness of 1 to 3
microns.
[0276] It is preferred that the bandwidth of the optical seed 1803
is greater than about 50 MHz in order to avoid complications
arising from Brillouin scattering.
[0277] FIG. 19 shows a fiber laser 1900 comprising a pump source
1405, an amplifying optical waveguide structure 1408 and an optical
feedback device 1901, wherein the optical feedback device 1901 is
configured to ensure that a portion of the optical radiation 1404
guided by the first waveguiding structure 1401 is amplified more
than once by any one section of the amplifying region 1407.
[0278] The amplifying optical waveguide structure 1408 may be the
cladding pumped amplifying optical waveguide structure 1508.
[0279] The optical feedback device 1901 may comprise a wavelength
division multiplexing coupler 1902, an isolator 1903 and an output
coupler 1904. The optical pump power 1406 is launched into the
first waveguiding structure 1401 via the wavelength division
multiplexing coupler 1902. This enables the optical radiation 1404
to circulate in a closed loop structure 1910. The closed loop
structure 1910 in this example is a laser cavity 1906 that is
configured in a ring.
[0280] The wavelength division multiplexing coupler 1902 may be a
dichroic mirror, or a fused wavelength division multiplexing fiber
coupler
[0281] The fiber laser 1900 may be a core-pumped fiber laser with
the optical pump power 1406 being coupled into the first
waveguiding structure 1401.
[0282] Higher output powers are obtainable with cladding pumping
whereby the optical pump power 1406 is coupled into the second
waveguiding structure 1501. Closed loop designs such as shown in
FIG. 19 is preferably use a dichroic mirror or a pump-injecting
coupler formed by coupling the core of a multimode optical fiber to
the second core 1502 by fusing and twisting the fibers
together.
[0283] FIG. 20 shows a fiber laser 2000 where the optical feedback
device 1901 comprises at least one reflector 2005. The reflector
2005 is shown as a fiber Bragg grating 2001 and a cleaved facet
2002. Alternatively, optical loop mirrors, dielectric mirrors,
metallic mirrors or any other form of reflector can be utilized.
The two reflectors 2005 in FIG. 20 and the amplifying optical
waveguide structure 1408 are configured to form a laser cavity
1906, which in the example shown in FIG. 20 is a linear laser
cavity. However, other types of laser cavity are also possible,
such as ring cavities, A reflector 2005 can be partly transmitting
to form an output coupler 1904 through which a part of the optical
radiation 1404 that is incident on the reflector 2005 is
transmitted, and emitted at an output port 1905 of the fiber laser
2000. In FIG. 20, the cleaved facet 2002 also forms the output
coupler 1904.
[0284] The amplifying optical waveguide structure 1408 is
preferably the cladding pumped amplifying optical waveguide
structure 1508.
[0285] The reflector 2005 can be a diffraction grating placed
externally to the first waveguiding structure 1401 and optically
coupled to it.
[0286] The fiber Bragg grating 2001 is formed in the first
waveguiding structure 1401 so that it interacts with the optical
radiation 1404 guided by the first waveguiding structure 1401. The
fiber Bragg grating 2001 can be formed in the first core 1402.
[0287] It is preferable to reduce the reflections outside the
desired wavelength range. This can be achieved by using
antireflection coatings or an angled end to the first waveguiding
structure 1401.
[0288] FIG. 21 shows a preferred embodiment of a fiber laser 2100.
The fiber laser 2100 comprises a laser cavity 1906. The optical
fiber 1610 comprises the amplifying optical waveguide structure
1408. The amplifying region 1407 comprises Ytterbium and the fiber
laser design is tailored such that it emits in a wavelength region
of about 970 nm to about 990 nm. The design details of similar
amplifier designs optimised to amplify between about 970 nm to 990
nm were described in the description pertaining to FIG. 16.
[0289] Although the coupler 1701 is shown implemented as an optical
fiber coupler, any of the implementations described previously can
be used. The coupler 1701 is shown as being optically connected
with a splice 2101 to the optical fiber 1610, but this is not
essential. It is preferred to fabricate the coupler 1701 using the
optical fiber 1610 for the throughput connection.
[0290] It is preferred that the amplifying region 1407 is disposed
in a ring surrounding the first core 1402.
[0291] The optical radiation 1404 guided by the first waveguide
structure 1401 is characterized by an operating wavelength. It is
preferred that the first waveguide structure 1401 is configured to
support only one transverse guided optical mode, albeit in two
orthogonal polarizations--note that in practice a single-mode
optical fiber normally supports two orthogonally polarized
modes.
[0292] A lens 2102 can be used to collimate optical radiation
emitted by the optical fiber 1610 and coupled out from the laser
cavity 1906. The out-coupled optical radiation can be passed
through an optical isolator 2103.
[0293] The laser cavity 1906 can be configured in a linear
configuration with the optical feedback device 1901 comprising the
fiber Bragg grating reflector 2001 and the cleaved fiber facet 2002
shown in FIG. 21.
[0294] The optical feedback device 1901 can alternatively be
provided by any of the implementations described previously,
including a closed-loop structure.
[0295] The cleaved fiber facet 2002 also constitutes an output
coupler 1904 through which optical radiation1404 is emitted from
the laser cavity 1906. Other options for an output coupler 1904
include any other partly transmitting and partly reflecting
arrangement like fiber Bragg gratings and dielectric and metallic
thin-film mirrors and also optical fiber and waveguide couplers,
which can split a beam traveling in a waveguide into two beams
traveling in different waveguides. One beam can then remain in the
laser cavity 1906 while the other can be coupled out of the laser
cavity 1906.
[0296] For improved wavelength selection and suppression of
emission at unwanted wavelengths, it is preferred that the fiber
Bragg grating 2001 reflects predominantly at a desired wavelength
of emission, while the reflectivity at undesired but amplified
wavelengths is low, e.g., below 0.1%. A lower value can be even
better.
[0297] FIG. 22 shows a Q-switched laser 2200 comprising a pump
source 1405, an amplifying optical waveguide structure 1408, at
least one optical feedback device 1901, and an optical switch 2201,
wherein the optical feedback device 1901 is configured to ensure
that a portion of the optical radiation 1404 guided by the first
waveguiding structure 1401 is amplified more than once by any one
section of the amplifying region 1407, the amplifying optical
device being able to be operated such that energy is stored in the
amplifying region 1407 with the optical switch 2201 in a blocking
state, the energy being released in the form of an optical pulse
when the optical switch 2201 is in a non-blocking state.
[0298] The optical switch 2201 is located inside the laser cavity
1906 in such a way that it can block optical radiation 1404
propagating in the laser cavity 1906.
[0299] In FIG. 22, the optical feedback device 1901 are exemplified
by a mirror 2205 and a waveguide facet 2202 which also serves as a
partly reflecting output coupler 1904. The waveguide facet 2202 is
preferably perpendicular.
[0300] Intracavity reflections can arise at any interface between
two media. This is often undesired, in which case the reflections
should be suppressed. For example, reflections from the amplifying
optical waveguide structure can be suppressed by using the angled
facet 2206 illustrated in FIG. 22. Alternatively, a facet can be
coated by an anti-reflection coating, e.g., in the form of a
dielectric stack.
[0301] Reflections can also need to be suppressed in other types of
amplifying optical devices 1400, and can be similarly
suppressed.
[0302] The optical switch 2201 may not be physically connected to
the amplifying optical waveguide structure 1408. In that case, an
intracavity lens 2203 can be used to optically connect the optical
switch 2201 to the amplifying optical waveguide structure 1408.
[0303] Optical pump power 1406 can be coupled from a pump source
1405 to the amplifying optical waveguide structure via a dichroic
mirror 2204 and a lens 2102. The dichroic mirror 2204 can also
separate the optical pump power 1406 and the optical radiation 1404
emitted from the amplifying optical waveguide structure 1408. The
lens 2102 can also serve to collimate the optical radiation 1404.
Other means for coupling the optical pump power 1406 into the
amplifying optical waveguide 1408 can alternatively be used,
including those previously discussed.
[0304] It is well known that Q-switched lasers can be operated in
various modes, such as multiple pulses being emitted when the
switch is opened, a single pulse being emitted, or a pulse being
emitted on alternate switch signals.
[0305] A preferred embodiment utilizes an optical fiber as the
amplifying optical waveguide structure 1408.
[0306] Another preferred embodiment utilizes the optical fiber
described with FIG. 11 as the amplifying optical waveguide
structure 1408, either in a core-pumped or a cladding-pumped
version.
[0307] The optical switch 2201 can be implemented using a waveguide
switch fabricated from Lithium Niobate, Gallium Arsenide, or a
fiber-optic acousto-optic switch, or a bulk optical switch such as
an acousto optic modulator, an acousto optic tunable filter, a Kerr
cell, a Pockels cell, an elasto-optic modulator, or a liquid
crystal switch.
[0308] The pump source 1405 can be a multi-moded semiconductor
laser or a high-brightness, near-diffraction limited diode
laser.
[0309] FIG. 23 shows a laser 2301 being used to pump at least one
amplifying optical device 2300 via optical fibers 2302 and a power
splitter 2303. The laser 2301 can be any laser designed in
accordance with this invention.
[0310] The amplifying optical device 2300 is preferably be an
Erbium Doped Fiber Amplifier EDFA.
[0311] A preferred embodiment is the pumping of Erbium Doped Fiber
Amplifiers EDFAs with the fibre laser 2100. It is preferred that
the outputs from more than one fiber laser 2100 are coupled
together prior to the pumping of the Erbium Doped Fiber Amplifiers
in order to provide pump redundancy.
[0312] Advantages of using the fibre laser 2100 for this
application include highly-efficient pumping of the Ytterbium
ring-doped amplifying region 1407 by relatively low-cost, high
power and very-reliable multimode semiconductor lasers for
efficient generation of single-moded radiation in the wavelength
band of about 970 nm to about 990 nm.
[0313] The amplifying optical devices 2300 can be conventional
Erbium-doped fiber amplifiers such as amplifiers installed in
numerous numbers in the telecommunication networks worldwide. Use
of the fiber laser 2100 will allow the upgrading of such amplifiers
to enable the amplification of numerous wavelength channels
simultaneously at high output power (such as required in dense
wavelength division multiplexing systems).
[0314] The amplifying optical devices 2300 can also be Erbium
Ytterbium doped fiber amplifiers, or an amplifier designed in
accordance with this invention, or any other optically pumped
optical amplifiers including planar waveguiding ones.
[0315] The amplifying optical devices 2300 can also be optically
pumped lasers, including fiber and planar distributed feedback
lasers doped with erbium and/or ytterbium configured for pumping by
the fiber laser 2100.
[0316] FIG. 24 shows a waveguiding saturating absorber 2400
comprising a waveguiding structure 2401 having a core 2402 and a
cladding 2403 configured to guide optical radiation 2404 and an
absorbing region 2405 situated within the cladding 2403 and
disposed such that it provides an absorption of the optical
radiation 2404 guided in the core 2402.
[0317] A preferred method to use the waveguiding saturating
absorber 2400 is where in use at least 10% of the absorption is
bleached by the optical radiation 2404 guided by the waveguiding
structure 2401 in at least a part of the waveguiding saturating
absorber 2400 at least part of the time The waveguiding saturating
absorber 2400 can absorb optical radiation 2404 guided by the
waveguiding structure 2401. However, at the same time, power
absorbed in the absorbing region 2405 bleaches the absorption,
which thus becomes smaller for optical radiation 2404 of higher
power. The intrinsic saturation power P .sub.sat is an important
parameter for a waveguiding saturating absorber. If the power of
the optical radiation 2404 is much smaller than P .sub.sat, the
waveguiding saturating absorber will be essentially un-bleached and
absorb a certain fraction of the optical radiation 2404. If the
power of the optical radiation 2404 is much larger than P .sub.sat
the waveguiding saturating absorber can be substantially bleached
and unable to absorb any more power, in which case the fraction of
the power that is absorbed decreases as the power increases.
[0318] By placing the absorbing region 2405 in the cladding 2403,
the intrinsic saturation power Psa, can be significantly higher,
e.g., by one or two orders of magnitude, than if the absorbing
region 2405 is placed in the core 2402. This is often advantageous.
Because of the tight confinement of guided optical radiation 2404,
(especially if the waveguiding structure 2401 only supports a
single transverse mode at the wavelength of the radiation 2404),
the optical intensity inside the core 2402 becomes high enough to
bleach the absorbing region 2405 already at relatively low optical
powers. For instance, an erbium-doped fiber in which erbium doped
in the core 2402 provides absorption normally has an intrinsic
saturation power of less than 1 mW for optical radiation 2404
around 1530 nm. It is often desirable to have a higher value of P
.sub.sat than that. A larger value of P .sub.sat can be
accomplished by placing the absorbing region 2405 (e.g., doped with
erbium) in the cladding 2403. Thereby, we can increase P .sub.sat
and furthermore by a careful choice of the location of the
absorbing region 2405 achieve a specific value of Psa, in order to
optimize the characteristics of the waveguiding saturating absorber
2400 in a particular application. In contrast, if the absorber
(e.g., the erbium-doped medium) is spread evenly out in the core of
a standardized design, P .sub.sat will primarily be determined by
the intrinsic properties of the absorber (e.g., the erbium doped
medium), which are normally difficult to control.
[0319] Complex designs of the core 2402 can be used to increase the
saturation power P .sub.sat , (e.g., the effective size of the core
2402 can be increased) the scope for change is much smaller than if
the absorbing region 2405 can reside in the cladding 2405. Complex
designs of the core include segmented cores, W-fibers, multiple
cladding fibres, and cores with so-called alpha profiles.
[0320] Any of the amplifying optical waveguide structures 1408 can,
if un-pumped, accordingly operate as a waveguiding saturating
absorber 2400 with a large and controllable intrinsic saturation
power P .sub.sat at wavelengths where the un-pumped gain region
1407 exhibits a saturable absorption. The dopants used must
therefore operate as a two or three level system at the operating
wavelength of the waveguiding saturating absorber 2400. Specific
examples of amplifying optical waveguide structures are shown in
FIGS. 14 and 15.
[0321] It is preferred that the saturable absorber is in a solid
state.
[0322] It is preferred that the saturable absorber is a glass doped
with a rare earth element.
[0323] A waveguiding saturating absorber 2400 can for instance be
used for rejecting low-power radiation and as an optical switch in
Q-switched and mode-locked lasers.
[0324] If the optical radiation 2404 is coherent and double-passed
through the waveguiding saturating absorber 2400 following
reflection in one end of the waveguiding saturating absorber 2400,
the optical radiation 2404 can form a standing-wave pattern which
bleaches the waveguiding saturating absorber 2400 according to the
standing-wave pattern. In this case, bleaching will predominantly
occur at the wavelength of the optical radiation 2404, while the
absorption at neighboring wavelengths can be higher. This effect
can be used in wavelength-tracking filters and for stabilization of
single-frequency lasers.
[0325] FIG. 29 shows a wavelength-tracking filter 2900 having a
waveguiding saturating absorber 2400, a circulator 2901, and an
optical feedback device 1901, which can be a mirror 2205 butted
directly to the waveguiding saturating absorber 2400.
[0326] FIG. 30 shows a single-frequency laser 3000 comprising an
amplifying optical structure 3008 and a waveguiding saturating
absorber 2400. The amplifying optical structure 3008 can be an
amplifying optical waveguiding structure 1408, or a similar
waveguiding structure but with the amplifying region placed in the
first core 1402. The amplifying optical structure 3008 can be
pumped by a pump source 1405 which can be launched via a wavelength
division multiplexing coupler 1902. A laser cavity 1906 is formed
by two optical feedback devices 1901. The optical feedback devices
1901 can be a fiber Bragg grating 2001 and a cleaved facet 2002, or
any other optical feedback device.
[0327] The amplifying optical structure 3008 and the waveguiding
saturating absorber 2400 can be formed from separate structures
joined together. Alternatively, if the amplifying optical structure
3008 is an amplifying optical waveguide structure 1408, they can be
formed from a single amplifying optical waveguide structure 1408, a
part of which is pumped and a part of which is unpumped. A part of
the amplifying optical waveguide structure 1408 can remain unpumped
because of a limited pump penetration, or the configuration of the
laser can be such that pump light cannot reach the waveguiding
saturating absorber 2400.
[0328] For a narrow-band wavelength-tracking filter, it is
preferred that the waveguiding saturating absorber 2400 is long,
e.g., several centimeters up to several meters.
[0329] The waveguiding structure 2401 is preferably be single-moded
and is preferably a polarization maintaining or single-polarization
waveguiding structure.
[0330] FIG. 25a shows an amplifying optical device 1500 comprising
a first waveguiding structure 1401 configured to guide optical
radiation 1404 which can propagate in a fundamental mode, a pump
source 1405 configured to supply optical pump power 1406, and a
second waveguiding structure 1501 configured to guide the optical
pump power 1406. The pump source 1405 is optically coupled to the
second waveguiding structure 1501.
[0331] FIGS. 25b, 25c and 25d show optical power distributions 2510
of the fundamental mode of the optical radiation 1404 propagating
along different first waveguide structures 1401.
[0332] In use, the optical radiation 1404 can be characterized by
an optical power distribution 2510 of the fundamental mode having a
contour 2505 of equal intensity perpendicular to the local
longitudinal axis of the first waveguiding structure 1401, the
contour 2505 enclosing about 75% of the optical power of the
fundamental mode. The area enclosed by the contour 2505 defines a
high intensity region 2501.
[0333] The second waveguiding structure 1501 contains an amplifying
region 1407 situated to interact with the optical pump power 1406
guided in the second waveguiding structure 1501 when the amplifying
optical device 1400 is in use; and wherein the amplifying region
1407 is situated to lie outside the contour 2505 of equal
intensity.
[0334] It is preferred that during use at least 0.1% of the optical
radiation 1404 guided by the first waveguiding structure 1401
overlaps the amplifying region 1407.
[0335] In FIG. 25b, 25c and 25d, the contour 2505 and the
high-intensity region 2501 is exemplified with optical power
distributions 2510 from three different kinds of first waveguiding
structures 1401. FIG. 25b shows the first waveguiding structure
1401 of a normal circular single-moded step-index fiber. The
high-intensity region 2501 is circular and will large coincide with
the core. The first core 1402 and possible location of the
amplifying region 1407 are also sketched in this case. FIG. 25c
shows the optical power distribution 2510 which can be propagated
along a more complicated first waveguiding structure 1401. The
high-intensity region 2501 is split up into two main regions. FIG.
25d shows the optical power distribution 2510 which can be
propagated along a ring core fibre resulting in a ring-shaped
high-intensity region 2501.
[0336] The first waveguiding structure 1401 can be a single mode
waveguide, or can support several (up to 10.sup.th order)
higher-order modes.
[0337] It is preferred that the disposition and design of the
amplifying region 1407 is arranged such that when in use, the
intensity of the optical radiation 1404 guided by the first
waveguiding structure 1401 and the optical pump power 1406 guided
by the second waveguiding structure 1501 are within the amplifying
region 1407 on average approximately equal to each other within
about 10 dB. This averaging would be performed over the
longitudinal and transverse extent of the gain region 1407, and be
weighted by the concentration of amplifying centers (e.g.,
Ytterbium or other rare-earth ions) in different parts of the
amplifying region 1407.
[0338] Alternatively it is preferred that the effective area ratio
r.sub.effective is in the range 1 to 10.
[0339] It is preferred that at least 90% of the amplifying region
1407 is located outside the high intensity region 2501. Insofar as
the concentration of the amplifying centers (e.g., Ytterbium or
other rare-earth ions) can vary within the amplifying region 1407,
the meaning is understood to be that at least 90% of the amplifying
centers should lie outside the high-intensity region 2501.
[0340] FIG. 26 shows a passive Q-switched laser 2600 comprising a
pump source 1405, a coupler 1701, a cladding-pumped amplifying
optical waveguide structure 1508 which is joined to an optical
waveguide structure 2601 doped with a saturable absorber 2603.
[0341] The design of the first waveguiding structure 1401 of the
cladding-pumped amplifying optical waveguide structure 1508 can be
the same or similar to the design of the optical waveguide
structure 2601.
[0342] The amplifying region 1407 in the cladding-pumped amplifying
optical waveguide structure 1508 can be doped with the same active
dopant as the saturable absorber 2603 in the optical waveguide
structure 2601. The operation of the passive Q-switched laser 2601
has been described previously in the text.
[0343] The saturable absorber 2603 is preferably placed in the core
of the optical waveguide structure 2601.
[0344] The cladding-pumped amplifying optical waveguide structure
1508 is joined (physically or optically) to the optical waveguide
structure 2601, so that the optical waveguide structure 2601 is
optically coupled to the first waveguiding structure 1401 of the a
cladding-pumped amplifying optical waveguide structure 1508.
[0345] The cladding-pumped amplifying optical waveguide structure
1508 and the optical waveguide structure 2601 can be optical
fibers, and they can be joined together by a splice 2101.
[0346] A laser cavity 1906 is formed with the cladding-pumped
amplifying optical waveguide structure 1508 and the optical
waveguide structure 2601 and optical feedback device 1901.
[0347] The laser cavity 1906 can be of the linear type, as
illustrated in FIG. 26, in which case an output port 1703 and an
output coupler 1904 can for example be formed by the facet of the
cladding-pumped amplifying optical waveguide structure 1508.
[0348] Alternatively, the optical fiber described with FIG. 11 can
be used as a passively Q-switched fiber laser, in which case a
saturable absorber in a first core 1402 and a amplifying medium
1407 doped in a cladding 1403 or in the edges of the first core
1402 co-exist along the fiber, rather than being disposed in
separate sections of fiber. Either core-pumping or cladding-pumping
can be use.
[0349] FIG. 27 shows a passive Q-switched laser 2700 comprising a
pump source 1405 end-coupled into an amplifying optical waveguide
structure 1408 which is joined via a pump-reflector 2702 to an
optical waveguide structure 2601 doped with a saturable absorber
2603.
[0350] The amplifying region 1407 in the amplifying optical
waveguide structure 1408 can be doped with the same active dopant
as the saturable absorber 2603 in the optical waveguide structure
2601. The design of the first waveguiding structure 1401 of the
amplifying optical waveguide structure 1508 can be the same or
similar to the design of the optical waveguide structure 2601. The
saturable absorber 2603 is preferably placed in the core of the
optical waveguide structure 2601. The operation of the passive
Q-switched laser 2601 has been described previously in the
text.
[0351] The pump reflector 2702 can be an optical fiber Bragg
grating or any other reflector which is transparent to the signal.
Alternatively a pump absorber or a wavelength selective coupler can
be used such as a fused-fiber wavelength division multiplexing
coupler in order to selectively transmit the signal in preference
to the pump between the first waveguiding structure 1401 of the
amplifying optical waveguide structure 1408 and the optical
waveguide structure 2601.
[0352] For pulsed laser emission, an optical feedback device 1901
and an output coupler 1904 must also be provided.
[0353] All of the features disclosed in this specification
(including any accompanying claims, abstract, and drawings), and/or
all of the steps of any method or process so disclosed, can be
combined in any combination, except combinations where at least
some of such features are mutually exclusive.
[0354] Each feature disclosed in this specification (including any
accompanying claims, abstract, and drawings), can be replaced by
alternative features serving the same, equivalent, or similar
purpose, unless expressly stated otherwise. Thus, unless expressly
stated otherwise, each feature disclosed is one example only of a
generic series of equivalent or similar features.
[0355] The invention is not restricted to the details of the
foregoing embodiments. The invention extends to any novel one, or
any novel combination, of the steps of any method or process so
disclosed.
[0356] In the embodiments described above a ring-shaped (generally
cylindrical) doped region has been used. However, the doped region
does not of course have to be rotationally symmetric, nor evenly
distributed along the length of the fiber or waveguide.
[0357] While the above invention has been described with
particularity to specific embodiments and examples thereof, it is
understood that the invention comprises the general novel concepts
disclosed by the disclosure provided herein, as well as those
specific embodiments and examples, and should not be considered as
limited by the specific embodiments and examples disclosed and
described herein.
* * * * *