U.S. patent application number 12/104082 was filed with the patent office on 2008-08-14 for multimode long period fiber bragg grating machined by ultrafast direct writing.
This patent application is currently assigned to Matsushita Electric Industrial Co., Ltd. Invention is credited to Ming Li, Xinbing Liu, Tetsuo Ohara, Rajminder Singh, Jimmy Yi-Jie-Jia.
Application Number | 20080193085 12/104082 |
Document ID | / |
Family ID | 36261822 |
Filed Date | 2008-08-14 |
United States Patent
Application |
20080193085 |
Kind Code |
A1 |
Singh; Rajminder ; et
al. |
August 14, 2008 |
MULTIMODE LONG PERIOD FIBER BRAGG GRATING MACHINED BY ULTRAFAST
DIRECT WRITING
Abstract
An optical fiber with an integral photonic crystal structure.
The optical fiber core is formed of a non-photosensitive material
having an initial index of refraction. The optical fiber core
includes a substantially cylindrical surface, a longitudinal core
axis, a core radius, and a number of index-altered portions having
an altered index of refraction different from the initial cladding
index of refraction. The index-altered portions are arranged within
the non-photosensitive material of the optical fiber core to form a
photonic crystal structure. The photonic crystal structure may be a
one dimensional, a two dimensional, or a three dimensional photonic
crystal structure.
Inventors: |
Singh; Rajminder;
(Cambridge, MA) ; Li; Ming; (Cambridge, MA)
; Yi-Jie-Jia; Jimmy; (Cambridge, MA) ; Liu;
Xinbing; (Cambridge, MA) ; Ohara; Tetsuo;
(Cambridge, MA) |
Correspondence
Address: |
RATNERPRESTIA
P.O. BOX 980
VALLEY FORGE
PA
19482
US
|
Assignee: |
Matsushita Electric Industrial Co.,
Ltd
Osaka
JP
|
Family ID: |
36261822 |
Appl. No.: |
12/104082 |
Filed: |
April 16, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10976524 |
Oct 29, 2004 |
7376307 |
|
|
12104082 |
|
|
|
|
Current U.S.
Class: |
385/37 |
Current CPC
Class: |
H01S 5/141 20130101;
G02B 6/02085 20130101; G02B 2006/0209 20130101; G02B 6/02147
20130101; G02B 6/02095 20130101; H01S 5/4068 20130101; H01S 5/12
20130101 |
Class at
Publication: |
385/37 |
International
Class: |
G02B 6/26 20060101
G02B006/26 |
Claims
1. An optical fiber with an integral photonic crystal section,
comprising: an optical fiber core formed of a non-photosensitive
material having an initial index of refraction, the optical fiber
core includes; a substantially cylindrical surface; a longitudinal
core axis; a core radius; and a plurality of index-altered portions
having an altered index of refraction different from the initial
index of refraction; wherein the plurality of index-altered
portions are arranged within the non-photosensitive material of the
optical fiber core to form a photonic crystal structure.
2. An optical fiber according to claim 1, wherein the
non-photosensitive material of the optical fiber core includes at
least one of: fused silica; borosilicate; quartz; zirconium
fluoride; silver halide; chalcogenide glass; optical plastic; clear
fused quartz; aluminosilicate; polymethylmeth-acrylate;
polystyrene; acrylic; or arsenic trioxide.
3. An optical fiber according to claim 1, wherein the optical fiber
core is a multimode optical fiber core.
4. A optical fiber according to claim 3, wherein the core radius of
the multimode optical fiber core is in the range of about 10 .mu.m
to about 200 .mu.m.
5. A multimode optical fiber according to claim 4, wherein the core
radius of the multimode optical fiber core is in the range of about
25 .mu.m to about 31.25 .mu.m.
6. An optical fiber according to claim 1, wherein the altered index
of refraction of the plurality of index-altered portions is altered
by selective irradiation of the non-photosensitive material by
pulses of ultrafast laser light.
7. An optical fiber according to claim 1, wherein: the
non-photosensitive material of the optical fiber core has an
initial crystallinity; and the plurality of index-altered portions
have an altered crystallinity which is less than the initial
crystallinity of the non-photosensitive material.
8. An optical fiber according to claim 7, wherein the altered
crystallinity of the plurality of index-altered portions is altered
by selective irradiation of the non-photosensitive material by
pulses of ultrafast laser light.
9. An optical fiber according to claim 1, wherein the photonic
crystal structure formed in the optical fiber core is a one
dimensional photonic crystal structure.
10. An optical fiber according to claim 1, wherein the photonic
crystal structure formed in the optical fiber core is a two
dimensional photonic crystal structure.
11. An optical fiber according to claim 1, wherein the photonic
crystal structure formed in the optical fiber core is a three
dimensional photonic crystal structure.
12. An optical fiber according to claim 1, wherein the photonic
crystal structure formed in the optical fiber core includes a
defect.
13. An optical fiber with integral diffractive coupling optics,
comprising: an optical fiber core formed of a non-photosensitive
material having an initial index of refraction, the optical fiber
core includes; a substantially planar end surface; a substantially
cylindrical surface; a longitudinal core axis; a core radius; and a
coupling section adjacent to the substantially planar end surface
with a plurality of index-altered portions having an altered index
of refraction different from the initial index of refraction;
wherein the plurality of index-altered portions are arranged within
the coupling section of the optical fiber core to form the integral
diffractive coupling optics.
14. An optical fiber according to claim 13, wherein the
non-photosensitive material of the optical fiber core includes at
least one of: fused silica; borosilicate; quartz; zirconium
fluoride; silver halide; chalcogenide glass; optical plastic; clear
fused quartz; aluminosilicate; polymethylmeth-acrylate;
polystyrene; acrylic; or arsenic trioxide.
15. An optical fiber according to claim 13, wherein the optical
fiber core is a multimode optical fiber core.
16. An optical fiber according to claim 15, wherein the core radius
of the multimode optical fiber core is in the range of about 10
.mu.m to about 200 .mu.m.
17. An optical fiber according to claim 13, wherein the altered
index of refraction of the plurality of index-altered portions is
altered by selective irradiation of the non-photosensitive material
by pulses of ultrafast laser light.
18. An optical fiber according to claim 13, wherein: the plurality
of index-altered portions are a plurality of concentric circular
annular portions centered on the longitudinal core axis of the
optical fiber core; and the plurality of concentric circular
annular portions are sized and arranged such that the integral
diffractive coupling optics formed in the coupling section of the
optical fiber core is a circular two dimensional diffractive
optical lens.
19. An optical fiber according to claim 13, wherein: the plurality
of index-altered portions are a plurality of concentric elliptical
annular portions centered on the longitudinal core axis of the
optical fiber core; the plurality of concentric elliptical annular
portions are sized and arranged such that the integral diffractive
coupling optics formed in the coupling section of the optical fiber
core is an elliptical two dimensional diffractive optical lens.
20. An optical fiber according to claim 13, wherein: the plurality
of index-altered portions are a plurality of parallel lines
perpendicular to the longitudinal core axis of the optical fiber
core; the plurality of parallel lines are sized and arranged such
that the integral diffractive coupling optics formed in the
coupling section of the optical fiber core is a one dimensional
diffractive optical lens.
Description
[0001] This application claims the benefit of U.S. patent
application Ser. No. 10/976,524, filed Oct. 29, 2004 the contents
of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to structures formed
in optical fibers by ultrafast laser direct writing. More
particularly these structures may be long period Bragg gratings,
photonic crystal structures, and/or diffractive optical elements
formed within the cores of multimode optical fibers.
BACKGROUND OF THE INVENTION
[0003] A Bragg grating is a periodic or aperiodic perturbation of
the effective absorption coefficient and/or the effective
refractive index of an optical waveguide. More simply put, a Bragg
grating can reflect a predetermined narrow or broad range of
wavelengths of light incident on the grating, while passing all
other wavelengths of the light. Such structures provide a desirable
means to manipulate light traveling in the optical waveguide.
[0004] A fiber Bragg grating (FBG) is a Bragg grating formed in an
optical fiber. FBG's may be formed from photo-imprinted gratings in
optical fibers. Photo-imprinting involves the irradiation of an
optical waveguide with a laser beam of ultraviolet light to change
the refractive index of the core of the waveguide. By irradiating
the fiber with an intensive pattern that has a periodic (or
aperiodic) distribution, a corresponding index perturbation is
permanently induced in the core of the waveguide. The result is an
index grating that is photo-imprinted in the optical waveguide.
This method requires that the glass be photosensitive, an effect
discovered in 1978 by Dr. Kenneth Hill of the Communications
Research Centre Canada.
[0005] The FBG may become a very selective spatial reflector in the
core of the fiber. Any change to the spatial period of the grating,
or index of refraction, causes a proportional shift in the
reflected and transmitted spectrum. FBG's have proven attractive in
a wide variety of optical fiber applications, such as: narrowband
and broadband tunable filters; optical fiber mode converters;
wavelength selective filters, multiplexers, and add/drop
Mach-Zehnder interferometers; dispersion compensation in
long-distance telecommunication networks; gain equalization and
improved pump efficiency in erbium-doped fiber amplifiers; spectrum
analyzers; specialized narrowband lasers; and optical strain gauges
in bridges, building structures, elevators, reactors, composites,
mines and smart structures.
[0006] Since their market introduction in 1995, the use of optical
FBG's in commercial products has grown exponentially, largely in
the fields of telecommunications and stress sensors. The demand for
more bandwidth in telecommunication networks has rapidly expanded
the development of new optical components and devices (especially
Wavelength Division Multiplexers). FBG's have contributed to the
phenomenal growth of some of these products, and are recognized as
a significant enabling technology for improving fiber optic
communications.
[0007] Photo-imprinted FBG's may have low insertion losses and are
compatible with existing optical fibers used in telecommunication
networks, but as the optical power being transmitted in a
photo-imprinted FBG increases, some undesirable effects may arise.
One drawback of photo-imprinted FBG's is the requirement that the
optical fiber have a photosensitive core. Photosensitive materials
typically have absorption coefficients higher than are desirable
for high power applications, as well as potentially undesirable
non-linearities that may become large at high optical powers.
Photo-imprinted FBG's are also susceptible to degradation over
time, particularly is the photosensitive material of the fiber core
is heated or exposed to UV radiation.
[0008] In their article, FIBER BRAGG GRATINGS MADE WITH A PHASE
MASK AND 800-NM FEMTOSECOND RADIATION (Optics Letters, Vol. 28, No.
12, pgs. 995-97 (2003)), Stephen J. Mihailov, et al. disclose a
first order FBG formed in a single mode fiber using a femtosecond
laser. The single mode fiber used was a standard SMG-28
telecommunications fiber with a non-photosensitive Ge doped core.
The authors were able to form a first order Bragg grating structure
in this core. This direct laser written single mode FBG was found
to have superior thermal stability as compared to a photo-imprinted
FBG.
[0009] Although the direct laser written single mode FBG of Stephen
J. Mihailov, et al. may overcome many of the disadvantages of the
photo-imprinted FBG's, the present invention includes a number of
additional improvements that may provide superior performance,
particularly at higher power levels, and increased versatility of
the Bragg grating structures that may be formed. Additionally, the
present invention includes additional diffractive structures that
may be formed in optical fibers to control and monitor light
propagating in the fiber.
SUMMARY OF THE INVENTION
[0010] An exemplary embodiment of the present invention is a
multimode long period fiber Bragg grating (LPFBG) for a
predetermined wavelength band. The LPFBG formed of a
non-photosensitive material having an initial index of refraction.
The multimode optical fiber core includes a substantially
cylindrical surface, a longitudinal core axis, a core radius, and a
number of index-altered portions having an altered index of
refraction different from the initial cladding index of refraction.
Each of the index-altered multimode optical fiber core has a first
transmission surface and second transmission surface that is
substantially parallel to the first transmission surface. Also,
these index-altered portions are arranged within the
non-photosensitive material of the multimode optical fiber core
such that the first transmission surface of one portion of the
plurality of index-altered portions is substantially parallel to
the second transmission surface of a neighboring portion to form a
long period Bragg grating structure.
[0011] Another exemplary embodiment of the present invention is a
fiber Bragg grating (FBG) for a predetermined wavelength band. The
FBG includes: an optical fiber core having a substantially
cylindrical surface, a longitudinal core axis, and a core radius;
and a cladding layer formed of a non-photosensitive material on the
substantially cylindrical surface of the optical fiber core. The
optical fiber core has a core index of refraction and the
non-photosensitive material of the cladding layer has an initial
cladding index of refraction that is lower than the core index of
refraction. The cladding layer includes an outer cladding radius
and a number of index-altered portions having an altered index of
refraction different from the initial cladding index of refraction.
Each of the index-altered portions of the cladding layer extends
into the cladding layer from the substantially cylindrical surface
of the optical fiber core. Also, these index-altered portions are
arranged within the non-photosensitive material of the cladding
layer to form a Bragg grating structure.
[0012] A further exemplary embodiment of the present invention is
an optical fiber with integral photonic crystal section. The
optical fiber includes an optical fiber core formed of a
non-photosensitive material having an initial index of refraction.
The optical fiber core includes a substantially planar end surface,
a substantially cylindrical surface, a longitudinal core axis, a
core radius, and a coupling section adjacent to the substantially
planar end surface with a number of index-altered portions. The
index-altered portions have an altered index of refraction that is
different from the initial index of refraction and are arranged
within the coupling section of the optical fiber core to form a
photonic crystal structure.
[0013] An additional exemplary embodiment of the present invention
is an optical fiber with integral diffractive coupling optics. The
optical fiber includes an optical fiber core formed of a
non-photosensitive material having an initial index of refraction.
The optical fiber core includes a substantially planar end surface,
a substantially cylindrical surface, a longitudinal core axis, a
core radius, and a coupling section adjacent to the substantially
planar end surface with a number of index-altered portions. The
index-altered portions have an altered index of refraction that is
different from the initial index of refraction and are arranged
within the coupling section of the optical fiber core to form the
integral diffractive coupling optics.
[0014] Yet another exemplary embodiment of the present invention is
a wavelength stabilized, high power, uncooled laser source. The
wavelength stabilized, high power, uncooled laser source includes
one or more high power laser(s) and a multimode optical fiber with
a LPFBG that is optically coupled to the high power laser(s). The
multimode optical fiber includes a multimode core formed of a
non-photosensitive material having an initial index of refraction.
This multimode core includes a substantially cylindrical surface, a
longitudinal core axis, a core radius, and a number of
index-altered portions. The index-altered portions have an altered
index of refraction that is different from the initial index of
refraction and are arranged within the non-photosensitive material
of the multimode core to form a long period Bragg grating
structure. This long period Bragg grating structure reflects a
predetermined fraction of light in a predetermined wavelength band
that is propagating in the multimode core back into the high power
laser(s). This desirably locks the output wavelength band of the
wavelength stabilized, high power, uncooled laser source to the
predetermined wavelength band.
[0015] Yet a further exemplary embodiment of the present invention
is a multimode optical fiber with a helical fiber Bragg grating.
The optical fiber includes a multimode optical fiber core formed of
a non-photosensitive material having an initial index of
refraction. The optical fiber core has a substantially cylindrical
surface, a longitudinal core axis, a core radius, and a helical
index-altered portion having an altered index of refraction
different from the initial index of refraction. This helical
index-altered portion includes a longitudinal index-altered portion
axis that is coaxial to the core axis of the core, an index-altered
portion outer radius, an index-altered portion inner radius which
is less than the index-altered portion outer radius, and a
longitudinal pitch. Also, the helical index-altered portion is
arranged within the non-photosensitive material of the multimode
optical fiber core to form a long period Bragg grating
structure.
[0016] Yet an additional exemplary embodiment of the present
invention is an optical fiber with an alternative helical fiber
Bragg grating. The optical fiber includes: an optical fiber core
having a substantially cylindrical surface, a longitudinal core
axis, and a core radius; and a cladding layer formed of a
non-photosensitive material on the substantially cylindrical
surface of the optical fiber core. The optical fiber core has a
core index of refraction and the non-photosensitive material of the
cladding layer has an initial cladding index of refraction that is
lower than the core index of refraction. The cladding layer
includes an outer cladding radius and a helical index-altered
portion having an altered index of refraction different from the
initial cladding index of refraction. This helical index-altered
portion includes a longitudinal index-altered portion axis that is
coaxial to the core axis of the core, an index-altered portion
outer radius, an index-altered portion inner radius which equal to
the core radius of the optical fiber core, and a longitudinal
pitch. Also, the helical index-altered portion is arranged within
the non-photosensitive material of the cladding layer to form a
Bragg grating structure.
[0017] It is to be understood that both the foregoing general
description and the following detailed description are exemplary,
but are not restrictive, of the invention.
BRIEF DESCRIPTION OF THE DRAWING
[0018] The invention is best understood from the following detailed
description when read in connection with the accompanying drawing.
It is emphasized that, according to common practice, the various
features of the drawing are not to scale. On the contrary, the
dimensions of the various features are arbitrarily expanded or
reduced for clarity. Included in the drawing are the following
figures.
[0019] FIG. 1A is a cut-away side plan drawing illustrating an
exemplary multimode long period fiber Bragg grating (LPFBG)
according to the present invention cut along line 1A of FIG.
1B.
[0020] FIG. 1B is an end plan drawing illustrating the exemplary
multimode LPFBG of FIG. 1A.
[0021] FIG. 1C is a cut-away side plan drawing illustrating an
alternative exemplary multimode LPFBG according to the present
invention cut along line 1C of FIG. 1D.
[0022] FIG. 1D is an end plan drawing illustrating the alternative
exemplary multimode LPFBG of FIG. 1C.
[0023] FIG. 2A is a cut-away side plan drawing illustrating another
exemplary multimode LPFBG according to the present invention cut
along line 2A of FIG. 2B.
[0024] FIG. 2B is an end plan drawing illustrating the exemplary
multimode LPFBG of FIG. 2A.
[0025] FIG. 2C is a cut-away side plan drawing illustrating a
further exemplary multimode LPFBG according to the present
invention cut along line 2C of FIG. 2D.
[0026] FIG. 2D is an end plan drawing illustrating the exemplary
multimode LPFBG of FIG. 2C.
[0027] FIG. 3A is a cut-away side plan drawing illustrating an
additional exemplary multimode LPFBG according to the present
invention cut along line 3A of FIG. 3B.
[0028] FIG. 3B is an end plan drawing illustrating the exemplary
multimode LPFBG of FIG. 3A.
[0029] FIG. 3C is a cut-away side plan drawing illustrating an
exemplary multi-wavelength multimode LPFBG according to the present
invention cut along line 3C of FIG. 3D.
[0030] FIG. 3D is an end plan drawing illustrating the exemplary
multi-wavelength multimode LPFBG of FIG. 3C.
[0031] FIG. 3E is a side plan drawing illustrating yet another
exemplary multimode LPFBG according to the present invention.
[0032] FIG. 3F is an end plan drawing illustrating the exemplary
multimode LPFBG of FIG. 3E.
[0033] FIGS. 4A, 4B, and 4C are cut-away side plan drawings
illustrating yet further exemplary multimode LPFBG's according to
the present invention.
[0034] FIGS. 5A and 5B are cut-away side plan drawings illustrating
exemplary apodized multimode LPFBG's according to the present
invention.
[0035] FIGS. 6A and 6B are cut-away side plan drawings illustrating
other exemplary multimode LPFBG's according to the present
invention.
[0036] FIG. 7 is a cut-away side plan drawing illustrating an
exemplary multi-wavelength multimode LPFBG according to the present
invention.
[0037] FIG. 8 is a cut-away side plan drawing illustrating an
exemplary multi-wavelength multimode LPFBG optical tap according to
the present invention.
[0038] FIG. 9 is a cut-away side plan drawing of an exemplary
multi-wavelength multimode fiber Bragg grating (FBG) according to
the present invention illustrating two alternative Bragg grating
structures.
[0039] FIG. 10 is a cut-away side plan drawing illustrating an
exemplary multi-wavelength multimode FBG optical tap according to
the present invention
[0040] FIG. 11A is a side plan drawing illustrating an exemplary
multimode helical FBG according to the present invention.
[0041] FIG. 11B is an end plan drawing illustrating the exemplary
multimode helical FBG of FIG. 11A.
[0042] FIG. 12A is a cut-away side plan drawing illustrating an
exemplary multimode fiber with an integral one-dimensional photonic
crystal according to the present invention.
[0043] FIG. 12B is a cut-away side plan drawing illustrating an
exemplary multimode fiber with an integral three-dimensional
photonic crystal according to the present invention.
[0044] FIG. 13A is a cut-away side plan drawing illustrating an
exemplary multimode fiber with integral diffractive coupling optics
according to the present invention cut along line 13A of FIG.
13B.
[0045] FIG. 13B is an end plan drawing illustrating the exemplary
multimode fiber with integral diffractive coupling optics of FIG.
13A.
[0046] FIG. 13C is a cut-away side plan drawing illustrating an
alternative exemplary multimode fiber with integral diffractive
coupling optics according to the present invention cut along line
13C of FIG. 13D.
[0047] FIG. 13D is an end plan drawing illustrating the exemplary
multimode fiber with integral diffractive coupling optics of FIG.
13C.
[0048] FIG. 13E is a cut-away side plan drawing illustrating
another exemplary multimode fiber with integral diffractive
coupling optics according to the present invention cut along line
13E of FIG. 13F.
[0049] FIG. 13F is an end plan drawing illustrating the exemplary
multimode fiber with integral diffractive coupling optics of FIG.
13E.
[0050] FIG. 14 is a block schematic diagram illustrating an
exemplary wavelength stabilized, high power, uncooled laser source
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0051] The extremely high intensities achievable in ultrafast laser
machining of materials allow the material to be changed in a number
of ways. The most common way that a material may be changed during
ultrafast laser machining is for the material to be removed from
the surface via ablation. Alternatively, various properties of the
material may be changed such as the crystallinity and/or the
refractive index. These material changes may occur on the surface
of the material or, for substantially transparent materials, the
ultrafast pulses may be focused within the material to cause these
changes to take place inside of the bulk of the material. These
internal changes may occur only above a specific fluence, so that
the intervening material may be unaffected by the ultrafast laser
pulses. Careful control of the pulse energy, pulse duration, and
focus of the pulses may allow for the creation of precise regions
with changed properties that have sharp boundaries.
[0052] Thus, the use of ultrafast lasers for direct writing of
Bragg grating structures in optical fibers may have the advantage
of providing sharp contrasts between index-altered portions of the
fiber and surrounding unaltered portions of the fiber.
Additionally, the use of an ultrafast laser machining system
designed for direct writing of structures in optical fibers, such
as the exemplary systems disclosed in US patent application
ULTRAFAST LASER MACHINING SYSTEM FOR FORMING MULTIMODE LONG PERIOD
FIBER BRAGG GRATING, filed concurrently with the present
application, allows for the creation complex structures within
optical fibers, particularly in multimode optical fibers.
[0053] Such an ultrafast laser machining system may be focused to a
small region within an optical fiber. The fluence of each pulse of
laser light of this exemplary ultrafast laser machining system may
be controlled such that only this small region near the beam spot
is machined by the pulse. The optical fiber may be moved in three
dimensions so that the beam spot of the laser pulses is scanned
within in the optical fiber, tracking through the portions of the
non-photosensitive material of the optical fiber to be machined.
The location of the machined region may be viewed through a stereo
imaging system throughout the machining process to allow for more
accurate machining of the diffractive structure. In this manner, a
complex diffractive structure may be written three dimensionally
within an optical fiber with a high level of precision.
[0054] Single mode optical fibers have relatively small fiber
cores, typically less that 9 .mu.m for telecommunication
wavelengths. The creation of diffractive structures within the
cores of single mode fiber may require highly accurate and precise
control of the beam spot of an exemplary ultrafast laser machining
system. Multimode fibers, however, may have significantly more
space for forming structures within the core. This additional space
may be desirable to lower the machining accuracy requirements of
the laser machining system used to form these exemplary structures.
Typical multimode fiber core radii range from about 10 .mu.m to
about 200 .mu.m, with 25 .mu.m and 31.25 .mu.m being the most
common multimode fiber core radii for telecommunication
wavelengths. Also, the multiple transverse modes utilized by light
propagating in multimode fibers may lead to a large number of
potential structural forms for controlling and monitoring light in
these fibers.
[0055] Thus, applying ultrafast laser machining techniques to
multimode optical fibers may create a significant expansion of the
potential uses of direct laser written structures in optical fibers
over the first order, single mode FBG's disclosed in Stephen J.
Mihailov, et al.'s article. Also, the use of highly accurate and
precise ultrafast laser machining systems may allow for addition
diffractive structures to be formed within single mode fibers.
[0056] Exemplary embodiments of the present invention include a
number of diffractive structures formed within optical fibers such
as: multimode long period FBG's (LPFBG's); multimode optical fibers
with helical FBG structures; optical fibers with integral photonic
crystal sections and/or diffractive coupling optics; and optical
fibers with FBG's formed in the cladding layer. These modified
optical fibers may be useful in a variety of situations, including:
wavelength stabilized, high power, uncooled laser sources;
dispersion compensation applications; optical filters; and in many
optical telecommunications applications to name a few.
[0057] FIGS. 1A and 1B illustrate exemplary multimode LPFBG 106
designed for a predetermined wavelength band. This exemplary
multimode LPFBG is formed by a number of cylindrical index-altered
portions 104 with substantially planar transmission surfaces
located in multimode optical fiber core 100. The transmission
surfaces of each index-altered portion are substantially parallel,
as are the facing transmission surfaces of neighboring
index-altered portions. These index-altered portions have an index
of refraction which has been altered, from the initial index of
refraction of the non-photosensitive material of multimode optical
fiber core 100, desirably by selective irradiation of portions of
the non-photosensitive material by pulses of ultrafast laser
light.
[0058] Multimode optical fiber core 100 includes a substantially
cylindrical surface, a longitudinal core axis, and a core radius.
Cladding layer 102 may be desirably formed on the substantially
cylindrical surface of multimode optical fiber core 100. Multimode
optical fiber core 100 is desirably formed of a non-photosensitive
material that has an index of refraction, which may be altered by
high intensity, ultrafast laser irradiation. The fractional index
change between multimode optical fiber core 100 and index-altered
portions 104 is dependent on the selection of the
non-photosensitive material. Many materials exhibit a fractional
index change between 10.sup.-5 and 10.sup.-3, with approximately
10.sup.-4 being typical, although it is noted that arsenic trioxide
may exhibit a fractional index change as high as 10.sup.-2.
Crystalline or semi-crystalline materials may also exhibit higher
fractional index changes. In these materials the crystallinity of
the index-altered portions 104 may be altered by the ultrafast
laser machining, leading to a relatively higher fractional index
change compared to non-crystalline materials. The
non-photosensitive material of multimode optical fiber core 100 may
desirably include one or more of: fused silica; borosilicate;
quartz; zirconium fluoride; silver halide; chalcogenide glass;
optical plastic; clear fused quartz; aluminosilicate;
polymethylmeth-acrylate; polystyrene; acrylic; and/or arsenic
trioxide.
[0059] Cladding layer 102 has a cladding index of refraction that
is desirably lower than the initial index of refraction of the
non-photosensitive material of the multimode optical fiber core.
The cladding layer may be formed of any material typically used for
optical fiber cladding, although it may be desirable to use a
non-photosensitive material similar to the non-photosensitive
material of multimode optical fiber core 100, particularly if it is
desired to form diffractive structures in cladding layer 102, as
shown in FIG. 9.
[0060] Index-altered portions 104 of the exemplary long period
Bragg grating structure shown in FIGS. 1A and 1B have constant
longitudinal thickness 108 and constant longitudinal pitch 110.
Longitudinal thickness 108 and longitudinal pitch 110 are selected
such that the resulting long period Bragg grating structure is
preferentially coupled to the predetermined wavelength band. The
longitudinal pitch of a long period Bragg grating structure
determines the peak wavelength reflected by the structure, and the
longitudinal pitch of the index-altered portions affects the Q of
the grating structure and, thus, the full width half maximum of the
reflected wavelength band. Desirably, longitudinal pitch 110 is
greater than the longest wavelength of the predetermined wavelength
band in the material. Longitudinal thickness 108 is desirably less
than half of longitudinal pitch 110. The minimum longitudinal
thickness is determined by the minimum feature size that may be
formed by the ultrafast laser machining system used to form the
LPFBG, typically greater than 10 nm.
[0061] In telecommunication wavelength bands, the index-altered
portions 104 of an exemplary long period Bragg grating structure
may desirably have a longitudinal thickness in the range of 1 .mu.m
to 20 .mu.m, preferably in the range of 5 .mu.m to 10 .mu.m. Their
longitudinal pitch may desirably be in the range of 1 .mu.m to 500
.mu.m, preferably in the range of 15 .mu.m to 20 .mu.m.
[0062] It is noted that the number of periods of the long period
Bragg grating structure, the filling factor, and the fractional
index change between multimode optical fiber core 100 and
index-altered portions 104 determine the fraction of light
reflected (or transmitted) in the predetermined wavelength band.
The filling factor is a measure of the cross-sectional area of
multimode fiber core 100 filled by index-altered portions 104. For
example, the exemplary LPFBG of FIGS. 1A and 1B has a higher
filling factor than the exemplary LPFBG of FIGS. 1C and 1D.
Therefore, if the number of periods and the fractional index change
of these two exemplary LPFBG's are the same, the fraction of light
reflected (or transmitted) in the predetermined wavelength band by
the exemplary LPFBG of FIGS. 1A and 1B is greater (less) than that
of the exemplary LPFBG of FIGS. 1C and 1D.
[0063] The desired fraction of light reflected back along multimode
optical fiber core 100 by long period Bragg grating structure 106
may be up to 99.9%, though for a number of applications, such as
laser wavelength locking, the reflected light fraction may be
preferably in the range of 3% to 20%. Although the exemplary
multimode LPFBG of FIG. 1A has only seven index-altered portions
104, forming six periods of the long period Bragg grating
structure, it is noted that long period Bragg grating structures of
100 or more periods may be more typical. The number of periods in
an LPFBG is only limited by the length of the optical fiber. For
some applications, long period Bragg grating structures with
thousands, or even tens of thousands, of periods may be
desirable.
[0064] FIGS. 1C and 1D illustrate another exemplary multimode LPFBG
106 formed by a number of cylindrical index-altered portions 112
with substantially planar transmission surfaces located in
multimode optical fiber core 100. The difference between the
exemplary multimode LPFBG of FIGS. 1C and 1D and that of FIGS. 1A
and 1B is index-altered portion radius 114 of cylindrical
index-altered portions 112 which is less than core radius of
multimode optical fiber core 100. In the exemplary embodiment of
FIGS. 1C and 1D, index-altered portion radius 114 provides a
parameter that may selected to preferentially couple the exemplary
long period Bragg grating structure to a desired subset of
transverse modes of multimode optical fiber core 100.
[0065] As shown in FIG. 4A, the index-altered portion radius of
cylindrical index-altered portions 400 may be varied between
different index-altered portions in the longitudinal direction of
multimode optical fiber core 100 to preferentially couple exemplary
long period Bragg grating structure 402 to a more specific subset
of transverse modes of multimode optical fiber core 100.
[0066] FIGS. 2A and 2B illustrate alternative exemplary multimode
LPFBG 106 designed for a predetermined wavelength band. This
exemplary multimode LPFBG is formed by a number of annular
index-altered portions 200 with substantially planar transmission
surfaces located in multimode optical fiber core 100. Each of these
annular index-altered portions includes: a longitudinal
index-altered portion axis coaxial to the longitudinal core axis of
multimode optical fiber core 100; an index-altered portion outer
radius, which, in the exemplary embodiment of FIGS. 2A and 2B, is
equal to the core radius; and index-altered portion inner radius
202. Index-altered portion inner radius 202 may be selected to
preferentially couple long period Bragg grating structure 106 to a
desired subset of transverse modes of multimode optical fiber core
100.
[0067] FIGS. 2C and 2D illustrate another exemplary multimode LPFBG
106 formed by a number of annular index-altered portions 204
located in multimode optical fiber core 100. The exemplary
multimode LPFBG of FIGS. 2C and 2D differs from the exemplary
multimode LPFBG of FIGS. 2A and 2B in that index-altered portion
outer radius 206 of annular index-altered portions 204, as well as
index-altered portion inner radius 202, may be varied to
preferentially couple long period Bragg grating structure 106 to a
desired subset of transverse modes of multimode optical fiber core
100. Also, as shown in FIG. 4B, one or both of the index-altered
portion outer radius and the index-altered portion inner radius 202
of annular index-altered portions 404 may be varied between
different index-altered portions in the longitudinal direction of
multimode optical fiber core 100 to preferentially couple exemplary
long period Bragg grating structure 406 to a more specific subset
of transverse modes of multimode optical fiber core 100.
[0068] FIGS. 3A and 3B illustrate further exemplary multimode LPFBG
302 designed for a predetermined wavelength band. This exemplary
multimode LPFBG is formed by a number of index-altered portions 300
located in multimode optical fiber core 100. Each of these
index-altered portions is in the shape of an annular arcuate
segment that includes: an index-altered portion outer radius, which
is less than the core radius of multimode optical fiber core 100 in
the exemplary embodiment of FIGS. 3A and 3B; an index-altered
portion inner radius; and an angular extent around the longitudinal
core axis of multimode optical fiber core 100.
[0069] Although not necessary, it may be desirable for the angular
extent of annular arcuate segments to be equal to approximately
360.degree./n, where n is an integer greater than 1. Such annular
arcuate segments may be desirable to preferentially couple to
subsets of transverse modes of multimode optical fiber core 100
described by Laguerre polynomials.
[0070] In the exemplary embodiment of FIGS. 3A and 3B, annular
arcuate segments 300 are longitudinally arranged in a single line
parallel to the longitudinal core axis of multimode optical fiber
core 100.
[0071] It is noted that annular arcuate segments 300 may
alternatively be aligned in a helix about the longitudinal core
axis, as shown in FIGS. 3E and 3F. In this alternative exemplary
embodiment, the annular arcuate segments are desirably
longitudinally arranged in the helix with neighboring annular
arcuate segments having an angular separation of approximately
360.degree./n, where n is an integer greater than 1 representing
the number of annular arcuate segments in one period of the helix.
FIGS. 3E and 3F have four annular arcuate segments per period of
the helix and, thus, annular arcuate segments 300 of this exemplary
multimode LPFBG may be aligned into four subsets of annular arcuate
segments 308, 310, 312, and 314.
[0072] FIGS. 3C and 3D illustrate an exemplary multi-wavelength
multimode LPFBG formed by two subsets of annular arcuate segments
300 located in multimode optical fiber core 100. The annular
arcuate segments of subset 304 have a first longitudinal thickness
and a first longitudinal pitch within multimode optical fiber core
100. The first longitudinal thickness and pitch of these annular
arcuate segments are selected such that the portion of the long
period Bragg grating structure formed by subset 304 is
preferentially coupled to a first subband of wavelengths of the
predetermined wavelength band. The annular arcuate segments of
subset 306 have a second longitudinal thickness and a second
longitudinal pitch within multimode optical fiber core 100, which
are selected such that the portion of the long period Bragg grating
structure formed by subset 306 is preferentially coupled to a
second subband of wavelengths of the predetermined wavelength band.
This second subband of the predetermined wavelength band is
desirably different than the first subband, allowing the exemplary
multimode LPFBG of FIGS. 3C and 3D to couple two subbands of the
predetermined wavelength band. The selection of two subsets of
annular arcuate segments in FIGS. 3C and 3D is merely illustrative
and is not meant to be limiting.
[0073] It is noted that any or all of the index-altered portion
outer radius, the index-altered portion inner radius, or the
angular extent of the annular arcuate segments may be selected to
preferentially couple long period Bragg grating structure 302 to a
desired subset of transverse modes of multimode optical fiber core
100. Also, as shown in FIG. 4C, the index-altered portion outer
radius, index-altered portion inner radius, and/or angular extent
of annular arcuate segments 408 may be varied between different
index-altered portions in the longitudinal direction of multimode
optical fiber core 100 to preferentially couple exemplary long
period Bragg grating structure 410 to a more specific subset of
transverse modes of multimode optical fiber core 100.
[0074] FIGS. 5A and 5B illustrate exemplary apodized multimode
LPFBG's. FIG. 5A illustrates exemplary apodized multimode LPFBG
which includes a plurality of index-altered portions having
cylindrical shape. The index-altered portion radii of these
cylindrical index-altered portions are varied between different
index-altered portions in the longitudinal direction of multimode
optical fiber core 100 such that long period Bragg grating
structure 500 is an apodized long period Bragg grating structure.
FIG. 5B illustrates similar apodized multimode LPFBG structure 502
formed of either annular or annular arcuate index-altered portions.
In this exemplary structure at least one of the index-altered
portion outer radii or the index-altered portion inner radii (or
the angular extent for angular arcuate segments) of the plurality
of index-altered portions are varied between different
index-altered portions in the longitudinal direction of multimode
optical fiber core 100, desirably forming exemplary apodized long
period Bragg grating structure 502.
[0075] FIGS. 6A and 6B illustrate two additional exemplary
multimode LPFBG's. In FIG. 6A, exemplary long period Bragg grating
structure 602 is formed by index-altered portions 600 which have
transmission surfaces that are conic surfaces, and in FIG. 6B,
exemplary long period Bragg grating structure 606 is formed by
index-altered portions 604 which have curved transmission surfaces.
The curved transmission surfaces of index-altered portions 604 may
be aspherical curved, as shown in FIG. 6B, or they may be spherical
curved surfaces. Exemplary long period Bragg grating structures,
such as those of FIGS. 6A and 6B, in which the index-altered
portions have non-planar transmission surfaces may be desirable for
converting transverse modes of light propagating in multimode fiber
core 100. Such control of the relative power in various transverse
modes of the propagating field may desirably improve coupling
efficiencies in spliced fiber couplers or other fiber coupling
means. Although the exemplary index-altered portions with
non-planar transmission surfaces are shown in FIGS. 6A and 6B
extending across the width of multimode fiber core 100, it is
contemplated that non-planar transmission surface index-altered
portions may also be formed with index-altered portion radii less
than the fiber core radius and/or may be formed as annuli or
annular arcuate segments.
[0076] FIGS. 7, 8, 9, and 10 illustrate several exemplary
multi-wavelength multimode LPFBG's. FIG. 7 illustrates an exemplary
multi-wavelength multimode LPFBG in which the index-altered
portions are separated longitudinally into two subsets,
index-altered portions 702, which form first portion 700 of the
long period Bragg grating structure, and index-altered portions
710, which form second portion 708. Index-altered portions 702 in
first portion 700 have a first longitudinal thickness 704 and a
first longitudinal pitch 706 within multimode optical fiber core
100 which are selected such that first portion 700 of the long
period Bragg grating structure is preferentially coupled to a first
subband of wavelengths of the predetermined wavelength band.
Index-altered portions 710 in second portion 708 have a second
longitudinal thickness 712 and a second longitudinal pitch 714
within multimode optical fiber core 100 which are selected such
that second portion 708 of the long period Bragg grating structure
is preferentially coupled to a second subband of wavelengths of the
predetermined wavelength band, which is different than the first
subband of wavelengths. Thus, the resulting long period Bragg
grating structure may desirably act as two separate multimode
LPFBG's.
[0077] It is noted that although FIG. 7 includes only two portions
the long period Bragg grating structure coupled to different
subband of wavelengths of the predetermined wavelength band, this
choice is merely for simplified illustration and is not limiting.
Also, although exemplary subsets of index-altered portions 702 and
710 are shown in FIGS. 6A and 6B as cylindrical portions extending
across the width of multimode fiber core 100, it is contemplated
that cylindrical index-altered portions with index-altered portion
radii less than the fiber core radius and/or annular or annular
arcuate index-altered portions may be used to form exemplary
multi-wavelength multimode LPFBG's. The use of these alternative
index-altered portions may allow for the various portions of the
resulting long period Bragg grating structure to be preferentially
coupled to different subsets of transverse modes of the multimode
fiber core as well as different subbands of wavelengths. Further
the use of annular arcuate index-altered portions in
multi-wavelength multimode LPFBG's may allow for a reduction of the
longitudinal length of the long period Bragg grating structure, as
shown in FIG. 3C.
[0078] FIG. 8 illustrates another exemplary multi-wavelength
multimode LPFBG 800 which may function as a wavelength dispersive
optical tap. In this exemplary embodiment, oblique cylindrical of
index-altered portions 802 have tilted planar transmission
surfaces. These are planar transmission surfaces are tilted within
multimode optical fiber core 100 such that the longitudinal core
axis of the optical fiber core has a predetermined angle of
incidence with the surfaces. These tilted planar transmission
surfaces allow multi-wavelength multimode LPFBG 800 to reflect a
predetermined fraction of propagating light 804 through cladding
102 so that the intensity of propagating light 804 may be
monitored. It is noted that other long period Bragg grating
structures, particularly those with asymmetric index-altered
portions and/or index-altered cladding portions (such as those
shown in FIGS. 9 and 10), may predictably scatter light through
cladding layer 102 and, thus, may also be used to form optical taps
in multimode optical fibers.
[0079] Additionally, the longitudinal thickness and the
longitudinal pitch of oblique cylindrical of index-altered portions
802 is continuously varied along the longitudinal direction of the
multimode optical fiber core to form a chirped long period Bragg
grating structure. This allows the various wavelengths of
propagating light 804 (.lamda..sub.1, .lamda..sub.2, .lamda..sub.3,
.lamda..sub.4, and .lamda..sub.5) to be reflected through cladding
102 at different points by multi-wavelength multimode LPFBG 800. In
this way the spectral composition of propagating light 804 may be
monitored.
[0080] FIG. 9 illustrates an additional exemplary multi-wavelength
multimode LPFBG with the two portions of the long period Bragg
grating structure formed by different types of index-altered
portions. Cylindrical index-altered portions 900 extend from
multimode fiber core 100 into cladding layer 102, while annular
index-altered cladding portions 902 are formed entirely within
cladding layer 102. Index-altered portions 900 and index-altered
cladding portions 902 may extend part way through cladding layer
102, as shown in FIG. 9, or all of the way to the outer cladding
surface. The extension of index-altered portions into the cladding
layer may increase the coupling of some higher order transverse
modes to the long period Bragg grating structure, while the
formation of index-altered cladding portions entirely within
cladding layer 102 may reduce perturbations to lower order
transverse modes caused by the long period Bragg grating structure.
In single mode fibers, the formation of index-altered cladding
portions within the cladding layer may allow coupling of evanescent
portions of the propagating light either to reflect a fraction of
the light in the predetermined wavelength band back along the
optical fiber core or to scatter a fraction of the light in the
predetermined wavelength band out of the fiber to form an optical
tap. As with index-altered portions formed entirely within a
multimode fiber core, various parameters of index-altered portions
formed partially or entirely within the cladding layer of an
optical fiber may be varied between different index-altered
portions in the longitudinal direction of the optical fiber such
that the resulting Bragg grating structure is an apodized Bragg
grating structure.
[0081] It is noted that, because index-altered cladding portions
preferentially couple to higher order transverse modes and
evanescent portions of the propagating light, the predetermined
fraction of light reflected back along the optical fiber core by
FBG's formed entirely in the cladding layer may be less than by
FBG's formed in the fiber core, but fractions in the range of 0.01%
to 10% may be reflected by such Bragg grating structures.
[0082] It is contemplated that both annular and annular arcuate
index-altered portions may be extended into the cladding layer, as
well. Also, index-altered portions formed entirely in multimode
fiber core 100 may be combined with index-altered portions extended
into cladding layer 102 and/or index-altered cladding portions are
formed entirely within cladding layer 102.
[0083] FIG. 10 illustrates another exemplary chirped multimode
LPFBG 1000, formed by index-altered annular arcuate segments 1002
formed entirely within cladding layer 102. As illustrated by the
exemplary scattering of propagating light 804 through cladding
layer 102, exemplary chirped multimode LPFBG 1000 may be used as a
wavelength dispersive optical tap to monitor the spectral
composition of propagating light 804.
[0084] FIGS. 11A and 11B illustrate a multimode optical fiber with
a helical FBG formed in multimode optical fiber core 100. Helical
index-altered portion 1100 includes: a longitudinal index-altered
portion axis which is coaxial to the longitudinal core axis of
multimode core 100; index-altered portion outer radius 1104;
index-altered portion inner radius 1102; and longitudinal pitch
1106. The index-altered portion outer radius 1104 and index-altered
portion inner radius 1102 may varied to preferentially couple the
helical FBG to a subset of the transverse modes of multimode
optical fiber core 100. Longitudinal pitch 1106 may be altered to
selectively couple the helical FBG to a predetermined wavelength
band. The longitudinal thickness of helical index-altered portion
1100 may also be varied to further define the predetermined
wavelength band coupled to the helical FBG. It is noted that
helical index-altered portion 1100 may be formed entirely within
multimode optical fiber core 100, as shown in FIGS. 11A and 11B, or
may be extended into cladding layer 102. Additionally, a helical
FBG may include a helical index-altered cladding portion formed
entirely within the cladding layer of either a single mode or
multimode optical fiber.
[0085] It is contemplated that an exemplary multimode long period
fiber Bragg grating may also be formed in which the index-altered
portions are arranged in a non-periodic pattern. The resulting long
period Bragg grating structure may desirably be formed to have a
predetermined transmission spectrum in the predetermined wavelength
band for light propagating in the multimode optical fiber core,
thus allowing the spectrum of light transmitted through the fiber
to be altered to a desired spectral shape.
[0086] Another exemplary embodiment of the present invention is an
optical fiber with an integral photonic crystal section. These
integral photonic crystal structures may be formed using an
ultrafast laser machining system alter portions of an optical fiber
core in a manner similar to the methods used to form the Bragg
grating structures described above. The inclusion of photonic
crystal sections within the core of single mode and multimode
optical fibers may allow even greater control of the light
propagated along these fibers. Additionally, these integral
photonic crystal structures may be useful for improving coupling
efficiencies between optical fibers and other optical components,
including other optical fibers. Further, highly selective
wavelength specific couplers may be created using these integral
photonic crystal structures. Such couplers may be particularly
desirable for use in dense wavelength division multiplexing optical
communication systems.
[0087] FIGS. 12A and 12B illustrate such exemplary structures
formed in the cores of multimode optical fibers. Multimode optical
fibers have been selected for the examples for illustrative
purposes. In FIG. 12A, multimode optical fiber core 100 includes
cylindrical index-altered portions 104, which have an altered index
of refraction different from the initial index of refraction of the
non-photosensitive material of multimode optical fiber core 100.
Cylindrical index-altered portions 104 are arranged within
multimode optical fiber core 100 to form one dimensional photonic
crystal structure 1200. One dimensional photonic crystal structure
1200 appears similar to long period Bragg grating structure 106 of
FIG. 1A, except for the inclusion of defect 1202. (It is also noted
that the longitudinal thickness and longitudinal pitch of
cylindrical index-altered portions 104 in one dimensional photonic
crystal structure 1200 are desirably significantly less than those
in long period Bragg grating structure 106, although this is not
clear in the scaled Figures.)
[0088] FIG. 12B illustrates an exemplary multimode optical fiber
with three dimensional photonic crystal structure 1204 formed
within multimode fiber core 100. Exemplary three dimensional
photonic crystal structure 1204 is formed of large number of
regularly spaced spherical index-altered portions 1206. The lattice
formed by spherical index-altered portions 1206 is interrupted by
defects 1208, which occur at regular intervals.
[0089] It is noted that, although both defect 1202 in one
dimensional photonic crystal structure 1200 and defects 1208 in
three dimensional photonic crystal structure 1204 result from a
missing index-altered portion, other types of defects may be formed
in these exemplary photonic crystal structures, such as an
additional index-altered portion, an index-altered portion having a
different shape, or change in the period structure of the photonic
crystal. It is also noted that exemplary two dimensional photonic
crystal structures may be formed in multimode optical fiber cores
according to this exemplary embodiment of the present
invention.
[0090] A further exemplary embodiment of the present invention is
an optical fiber with integral diffractive coupling optics. These
integral diffractive coupling optics structures may also be formed
using an ultrafast laser machining system to alter portions of the
optical fiber core near the input and output surfaces of the fiber.
The inclusion of integral diffractive coupling optics within the
core of optical fibers may greatly improve coupling efficiencies
between optical fibers and other optical components. They may also
allow for space saving solutions in fiber optics systems by
reducing, or eliminating, the need for free space coupling optics
within these systems. As in the exemplary embodiments of FIGS. 12A
and 12B, multimode optical fibers have been selected in FIGS. 13A-F
for illustrative purposes.
[0091] FIGS. 13A and 13B illustrate one exemplary multimode optical
fiber with integral diffractive coupling optics. In this example,
multimode optical fiber core 100 includes a coupling section
adjacent to the substantially planar end surface. This coupling
section is formed by concentric annular index-altered portions 1300
and 1302 and cylindrical index-altered portion 1304 which have an
altered index of refraction different from the initial index of
refraction of the non-photosensitive material of multimode fiber
core 100. Concentric annular index-altered portions 1300 and 1302
and cylindrical index-altered portion 1304 are arranged to form a
circular two dimensional diffractive optical lens. This lens may be
spherical or aspherical depending on the radii of the index-altered
portions. It is noted that the focal length of this exemplary
circular two dimensional diffractive optical lens is wavelength
dependent. Thus, such lenses may not be desirable for broad
bandwidth applications.
[0092] FIGS. 13C and 13D illustrate another exemplary multimode
optical fiber with an integral elliptical two dimensional
diffractive optical lens. In this example, the coupling section is
formed by concentric elliptical annular index-altered portions 1306
and 1308 and elliptical index-altered portion 1310 which have an
altered index of refraction different from the initial index of
refraction of the non-photosensitive material of multimode fiber
core 100. This lens may be designed to have a small ellipticity of
a large ellipticity depending on the desired ratio of the cone
angles in the X and Y directions.
[0093] FIGS. 13E and 13F illustrate a further exemplary multimode
optical fiber with an integral one dimensional diffractive optical
lens. This exemplary integral coupling optics section may function
as a cylindrical lens. Such lens may be particularly desirable for
coupling light from semiconductor lasers into the multimode optical
fiber. The coupling section of FIGS. 13E and 13F is formed in
multimode fiber core 100 by parallel linear index-altered portions
1312, 1314, and 1316. Parallel linear index-altered portions 1312,
1314, and 1316 are sized and arranged such that the integral
diffractive coupling optics formed in the coupling section of
multimode optical fiber core 100 is a one dimensional diffractive
optical lens. It is noted that parallel linear index-altered
portions 1312, 1314, and 1316 may also form a transmission grating,
if equally sized and spaced, allowing various wavelengths of light
propagating in the multimode optical fiber to by diffracted in
separate directions.
[0094] The various multimode optical fiber structures described
above may be used to design a number of exemplary optical devices,
one example of which is a wavelength stabilized, high power,
uncooled laser source. Operating a laser in an uncooled mode may be
desirable to reduce power consumption used to cool the laser, as
well as to reduce the feedback circuitry used to control the
laser's temperature. Unfortunately, such uncooled operation may
cause difficulties with maintaining a constant output wavelength of
the laser. This is due to the thermal dependence of the output
wavelength of the laser. These difficulties may be magnified in
high power applications where large quantities of heat are
generated by the laser and the temperature may vary over a large
range.
[0095] One method of overcoming these difficulties is the use of an
external optical cavity to lock the output wavelength of the laser
by coupling light resonant with the external cavity back into the
laser. Optically coupling the laser and the external cavity may
necessitate additional optics, leading to added complexity and
increased power loss. Such external cavities also are desirably
thermally isolated or are designed to have low temperature
dependence.
[0096] FIG. 14 illustrates an exemplary wavelength stabilized, high
power, uncooled laser source, which uses exemplary multimode LPFBG
1408 to lock the laser output wavelength. An exemplary type of high
power laser for which the exemplary embodiment of FIG. 14 may be
particularly desirable is a continuous wave semiconductor laser.
This exemplary wavelength stabilized, high power, uncooled laser
source includes four high power lasers 1400. These four lasers are
optically coupled into four coupling multimode optical fibers 1404,
which are optically coupled to a single multimode optical fiber at
fiber coupler 1406. Fiber coupler 1406 is desirably a low loss
fiber coupler, such as a star coupler or a spliced fiber coupler,
as shown in FIG. 14. It is noted that multimode optical fibers are
desirable in this application for their high power handling
capabilities.
[0097] The single multimode optical fiber desirably includes a low
loss multimode core formed of a non-photosensitive material in
which a plurality of index-altered portions, having an altered
index of refraction, have been formed using an ultrafast laser
machining system. The index-altered portions are arranged within
the non-photosensitive material of the multimode core to form long
period Bragg grating structure 1408. This long period Bragg grating
structure is desirably adapted to reflect a predetermined fraction
of light in the desired wavelength band back along the optical
fibers and into high power lasers 1400, thereby locking the output
wavelength band of the wavelength stabilized, high power, uncooled
laser source to the desired wavelength band. Long period Bragg
grating structure 1408 may desirably reflect up to 99.9%,
preferably 3% to 20%, of the light provided by the laser in the
desired wavelength band back into the laser.
[0098] It is noted that it may be desirable for high power lasers
1400 and the multimode optical fibers of the exemplary system to be
substantially thermally uncoupled, or, alternatively, for the
non-photosensitive material of the multimode core of the single
multimode optical fiber to have a coefficient of thermal expansion
low enough to prevent an undesirable shift in the desired
wavelength band reflected by long period Bragg grating structure
1408 during operation. Another approach to reduce heating of long
period Bragg grating structure 1408 during operation of high power
lasers 1400 is to provide thermal buffering section 1412 between
the laser coupling surface and long period Bragg grating section
1408.
[0099] This exemplary external cavity wavelength locker includes
only a small number of relatively simple optical components. Also,
by utilizing low loss multimode optical fibers with multimode cores
formed of non-photosensitive materials, power loss in the system is
kept low. Additionally, coupling losses may be reduced further by
forming additional exemplary diffractive structures in the
multimode optical fiber cores, such as coupling sections 1402,
similar to those shown in FIGS. 13A-F, adjacent to the laser
coupling surfaces of coupling multimode optical fibers 1404 and
output section 1410 adjacent to the output surface of the single
multimode optical fiber. Because these diffractive structures are
formed within the multimode optical fibers, they may have lower
losses than the free standing optical elements that they may
replace.
[0100] It is noted that the exemplary wavelength stabilized, high
power, uncooled laser source shown in FIG. 14 includes four high
power lasers 1400. The choice of four lasers is only exemplary and
one skilled in the art may understand that other numbers of high
power lasers may be used in an exemplary wavelength stabilized,
high power, uncooled laser source according to the present
invention. This may include a system with a single high power
laser, in which case, coupling multimode optical fibers 1404 and
fiber coupler 1406 may be omitted from the laser source without
affecting its operation.
[0101] Further, long period Bragg grating structure 1408 may
include any of the alternative embodiments described above with
reference to FIGS. 1A-1B. In particular, long period Bragg grating
structure 1408 may include multiple subsets of index-altered
portions preferentially coupled to different subband of wavelengths
of the predetermined wavelength band and/or different transverse
modes of the laser light propagating in the multimode fiber in
which long period Bragg grating structure 1408 is formed.
[0102] Although many exemplary embodiments of the invention are
described in terms of forming structures in circular optical
fibers, it is contemplated that the exemplary structures described
herein may be formed in optical waveguides of different
cross-sectional shaped, including elliptical
polarization-maintaining optical fibers.
[0103] Although illustrated and described above with reference to
certain specific embodiments, the present invention is nevertheless
not intended to be limited to the details shown. Rather, various
modifications may be made in the details within the scope and range
of equivalents of the claims and without departing from the
invention.
* * * * *