U.S. patent number RE35,962 [Application Number 08/693,689] was granted by the patent office on 1998-11-17 for single longitudinal mode pumped optical waveguide laser arrangement.
This patent grant is currently assigned to United Technologies Corporation. Invention is credited to Gary A. Ball, William H. Glenn.
United States Patent |
RE35,962 |
Ball , et al. |
November 17, 1998 |
Single longitudinal mode pumped optical waveguide laser
arrangement
Abstract
An optical waveguide laser arrangement includes a solid optical
waveguide having a waveguiding portion that extends along an axis
between two spaced regions of the optical waveguide and is of a
material capable of emitting stimulated light upon excitation by
pumping light that is launched into the optical waveguide for axial
propagation through the axially spaced regions of the optical
waveguide. Two reflectors are provided, each being disposed at one
of the axially spaced regions of the optical waveguide to delimit a
laser resonator. At least one of the reflectors is constituted by a
Bragg grating consisting of a multitude of axially consecutive
periodic perturbations in the refractive index of the respective
one of the axially spaced regions that cumulatively reflect the
stimulated light at a reflectivity that decreases for adjacent
longitudinal modes of the stimulated light with increasing
deviation from a central wavelength. The axial length of the laser
resonator, the reflectivity of the Bragg grating and the gain of
the excitable material are so coordinated with one another as to
sustain lasing in only a single longitudinal mode.
Inventors: |
Ball; Gary A. (Simsbury,
CT), Glenn; William H. (Vernon, CT) |
Assignee: |
United Technologies Corporation
(Hartford, CT)
|
Family
ID: |
27486255 |
Appl.
No.: |
08/693,689 |
Filed: |
August 5, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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13117 |
Feb 1, 1993 |
5317576 |
|
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|
732552 |
Jul 19, 1991 |
|
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457118 |
Dec 26, 1989 |
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Reissue of: |
017069 |
Feb 12, 1993 |
05305335 |
Apr 19, 1994 |
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Current U.S.
Class: |
372/6; 372/102;
372/92 |
Current CPC
Class: |
H01S
3/0675 (20130101); H01S 3/08031 (20130101); H01S
3/2383 (20130101); H01S 3/094019 (20130101); H01S
3/094007 (20130101); H01S 3/06737 (20130101) |
Current International
Class: |
H01S
3/067 (20060101); H01S 3/06 (20060101); H01S
3/08 (20060101); H01S 3/23 (20060101); H01S
003/30 () |
Field of
Search: |
;372/6,102,99,92
;385/37 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Scott, Jr.; Leon
Attorney, Agent or Firm: DePardo; Gerald L.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a continuation-in-part of copending U.S.
patent application .Iadd.Ser. No. 08/013,117, filed Feb. 1, 1993,
now U.S. Pat. No. 5,317,576 which is a continuation-in-part of Ser.
No. 07/732,552, filed Jul. 19, 1991 .Iadd.which .Iaddend.was a
continuation-in-part of U.S. patent application Ser. No.
07/457,118, filed Dec. 26, 1989, .[.and Ser. No. 07/659,952, filed
Feb. 26, 1991, both now.]. abandoned.
Claims
We claim:
1. In an optical waveguide laser arrangement including a solid
optical waveguide having a gain portion that extends along an axis
between two axially spaced reflectors disposed along the optical
waveguide, said gain portion being of an excitable material which
emits stimulated light upon excitation by pumping light, means for
launching the pumping light into the optical waveguide for axial
propagation along the optical waveguide, the two reflectors axially
delimiting said gain portion thereby forming a laser resonator in
said gain portion, the improvement wherein at least one of the
reflectors is constituted by a Bragg grating consisting of a
plurality of axially consecutive periodic perturbations in the
refractive index of the optical waveguide, said Bragg grating
reflecting the stimulated light with a reflectivity profile this is
a maximum at a central wavelength corresponding to a central
longitudinal mode of the laser resonator, said reflectivity profile
decreasing for longitudinal modes adjacent to said central
longitudinal mode of the laser resonator, and wherein the axial
length of the laser resonator, the reflectivity of the one Bragg
grating, and the gain of the excitable material are such that
lasting occurs only at said central longitudinal mode of the laser
resonator.
2. The improvement as defined in claim 1, wherein both of said two
reflectors are constituted by said Bragg gratings.
Description
TECHNICAL FIELD
The present invention relates to pumped optical waveguide lasers in
general, and more particularly to lasers of the above type which
utilize reflective elements constituted by Bragg gratings to
achieve lasing in a single longitudinal mode.
BACKGROUND ART
There are already known various constructions of pumped optical
waveguide lasers, among them such utilizing fully or partially
reflective coatings on the respective end faces delimiting the
laser resonance cavities in solid material optical waveguides, and
such using external reflectors to delimit the laser cavities. Even
though these solutions have achieved quite a degree of success
and/or acceptance in the laser manufacturing and utilization
fields, they still suffer of certain drawbacks, one of which is
that, because reflecting elements of the above type are reflective
over a relatively wide frequency bandwidth, lasing often occurs in
more than one mode. This is true even if the optical waveguide,
such as an optical fiber, is of the so-called single mode type in
which all transverse modes but one are below the cutoff. As a
matter of fact, experience has shown that even such a single-mode
optical fiber laser typically lases in a great number of
longitudinal modes. This, of course, significantly degrades the
properties of the laser radiation issued by the optical fiber laser
due to mode beating and other optical effects, thus considerably
reducing the utility of the issued laser beam. This multiple
longitudinal mode operation diminishes if not defeats utility of
such lasers for many applications in the telecommunications, sensor
and spectroscopic evaluation fields.
Narrow-linewidth single-frequency semiconductor laser diodes are
currently the source of choice in low to moderate power coherent
systems. In laser diodes, single-frequency narrow-linewidth
operation is typically achieved through a distributed feedback or a
distributed Bragg reflector structure. Moreover, miniature,
diode-pumped, single-frequency, narrow-linewidth solid state lasers
are currently being manufactured in a variety of configurations.
Both of these laser types, however, suffer of certain drawbacks, an
important one of which is that, when they are to be used in optical
fiber systems, they require integration into such systems that is
alignment insensitive, and hence expensive. Therefore, for these
applications, it would be preferable to use optical fiber lasers.
However, experience has shown that it is not at all easy to achieve
the desired single longitudinal mode operation in linear optical
fiber lasers, especially when they are to have substantial lengths.
This is so because, even when the reflectors axially delimiting the
laser cavity are of the relatively narrow passband type, still the
number of longitudinal modes falling within this passband and thus
in competition for lasing is substantial when the length of the
laser cavity is in excess of a few millimeters or centimeters.
Because of this, it was heretofore very difficult if not impossible
to provide a linear solid material optical waveguide, especially an
optical fiber, laser that would lase exclusively in a single
longitudinal mode.
Accordingly, it is a general object of the present invention to
avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to
provide pumped optical fiber laser arrangements which do not
possess the disadvantages of the known arrangements of this
kind.
Still another object of the present invention is so to develop the
arrangements of the type here under consideration as to achieve
lasing in only a single longitudinal mode.
It is yet another object of the present invention to devise
arrangements of the above type which are able to achieve such
single mode operation at various cavity lengths and gains of the
optically excitable material of the optical waveguide.
A concomitant object of the present invention is to design the
optical waveguide lasers of the above type in such a manner as to
be relatively simple in construction, inexpensive to manufacture,
easy to use and integrate into optical fiber systems, and yet
reliable in operation.
DISCLOSURE OF THE INVENTION
In keeping with these objects and others which will become apparent
hereafter, one feature of the present invention resides in an
optical waveguide laser arrangement that includes a solid optical
waveguide having a waveguiding portion that extends along an axis
between two spaced regions of the optical waveguide and is of a
material capable of emitting stimulated light upon excitation by
pumping light, means for launching the pumping light into the
optical waveguide for axial propagation through the axially spaced
regions of the optical waveguide, and two reflectors each disposed
at one of the axially spaced regions of the optical waveguide to
delimit a laser resonator. According to the invention, at least one
of the reflectors is constituted by a Bragg grating consisting of a
multitude of axially consecutive periodic perturbations in the
refractive index of the respective one of the axially spaced
regions that cumulatively reflect the stimulated light at a
reflectivity that decreases for adjacent longitudinal modes of the
stimulated light with increasing deviation from a central
wavelength, and the axial length of the laser resonator, the
reflectivity of the one Bragg grating and the gain of the excitable
material are so coordinated with one another as to sustain lasing
in only a single one of the longitudinal modes of the stimulated
light.
BRIEF DESCRIPTION OF THE DRAWING
The present invention will be described in more detail below with
reference to the accompanying drawing in which:
FIG. 1 is a somewhat simplified side elevational view of a laser
arrangement of the present invention incorporating an optically
pumped optical fiber axially delimited by slanted end faces;
FIG. 2 is a graphic representation of the dependence of the
reflectivity of the respective Bragg grating employed in the
arrangement of FIG. 1 on frequency;
FIG. 3 is a view corresponding to that of FIG. 2 but showing a
reflectivity curve obtained when at least one of the end faces of
the optical waveguide of FIG. 1 extends normal to the optical fiber
axis;
FIG. 4 is a simplified three-level energy diagram for erbium that
may be used as an optically excitable material in the waveguiding
portion of the optical fiber of FIG. 1;
FIGS. 5a to 5c are graphic representations of single frequency
operation domains in dependence on the laser cavity length and
grating reflectivity at different gains and using gratings of
different lengths;
FIGS. 6a and 6b are graphic representations of normalized slope
efficiencies as functions of grating reflectivities for fibers
having different gains and for different grating lengths; and
FIGS. 7a and 7b are graphic representations of the dependence of
the maximum permissible dominant longitudinal mode shaft from the
grating central wavelength before transitioning from single mode to
multimode operation on grating reflectivity for different grating
lengths.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring now to the drawings in detail, and first to FIG. 1
thereof, it may be seen that the reference numeral 10 has been used
therein to identify a pumped optical fiber laser arrangement in its
entirety. The arrangement 10 includes, as its main components, an
optical fiber section 11 and a laser pump 12 which is of any known
construction and advantageously is constituted by or includes a
diode laser. The laser pump 12 issues light of a wavelength
.lambda..sub.1 that is launched, in any well known manner, into the
optical fiber section 11 for propagation longitudinally thereof. As
is well known to those practicing in this field, the optical fiber
section 11 may include a fiber core and a cladding surrounding the
core, with the stimulating light from the laser pump 12 being
launched into and guided in the core. As shown, the optical fiber
section 11 is delimited, as respective ends thereof, by respective
end faces 11a and 11b that are inclined at the Bragg angle with
respect to the axis of the section 11 to minimize or eliminate
undesired reflections therefrom. It is assumed that optical
feedback from other sources is negligible.
The optical fiber section 11 is shown to be provided, in the core
thereof, with two gratings 13 and 14 which are spaced from one
another in the axial or longitudinal direction of the section 11
and are shown to be situated close to the respective ends of the
section 11. As shown in FIG. 2 of the drawing, the gratings 13 and
14 are reflective, to a greater or lesser degree, but only to light
within a very narrow band around a central wavelength
.lambda..sub.2, being substantially transparent to light at all
other wavelengths, preferably including, at least as far as the
grating 13 is concerned, the pumping wavelength .lambda..sub.1. The
gratings 13 and 14 act as reflectors longitudinally delimiting the
laser cavity in which the lasing takes place.
It is proposed in accordance with the present invention for the
gratings 13 and 14 to be of the type, and to be made in accordance
with the method, disclosed in the commonly owned U.S. Pat. No.
4,807,805, the disclosure of which is incorporated herein by
reference to the extent necessary to provide additional explanation
of the type of the Bragg gratings that is particularly suited for
use in the optical fiber laser arrangement 10. As more fully
explained in the above patent, gratings of this type act as
wavelength selective reflectors. An embedded grating of specified
periodicity will partially or fully reflect a specific wavelength
of an optical signal carried by the fiber section 11. This type of
grating, which has been developed for sensing stress and/or
temperature changes in structures, is incorporated or embedded in
the core of an optical fiber by a method disclosed in the above
patent. As discussed there, permanent periodic gratings of this
kind can be provided or impressed in the core of an optical fiber
by exposing the core through the cladding to the interference
pattern of two compatible ultraviolet light beams that are directed
against the optical fiber at two angles relative to the fiber axis
that supplement each other to 180.degree.. This results in a
situation where the material of the fiber core has permanent
periodic variations in its refractive index impressed therein by
the action of the interfering ultraviolet light beams thereon, with
the individual grating elements (i.e. the periodically repetitive
regions of the core exhibiting the same refractive index behavior)
being oriented normal to the fiber axis so as to constitute a Bragg
grating. The embedded Bragg grating of this kind reflects, of the
light launched into or otherwise guidedly propagating in the fiber
core in a propagation direction that brings it to the respective
grating 13 or 14, only that having a wavelength within a very
narrow range around a central wavelength .lambda..sub.2 that is
dependent on the grating element periodicity, back along the fiber
axis opposite to the original propagation direction while being
substantially transparent to light at wavelengths outside the
aforementioned narrow band so that it does not adversely affect the
passage of such other light therethrough. In effect, this type of
grating creates a narrow notch in the transmission spectrum and, by
the same token, a similarly narrow peak in the reflection spectrum.
This reflection spectrum peak is being utilized in accordance with
the present invention to determine the lasing wavelength
.lambda..sub.2, and the axial length of each of the gratings 13 and
14 is selected in such a manner as to obtain the desired
reflectivities at the gratings 13 and 14, respectively.
For the optical fiber section 11 to operate as an optically pumped
laser, the material of at least the core thereof is to contain at
least one dopant, for instance a rare-earth element, such as
neodymium or erbium. On the other hand, the above patent teaches
the making of the gratings of the type here under consideration in
germanium doped optical fiber cores. To obtain an optically pumped
laser in accordance with the present invention, it is currently
preferred to impress the gratings 13 and 14 directly into the
(rare-earth doped) active material of the optical fiber section 11
(or its core), in which case at least the regions of the active
material that are to be provided with the gratings 13 and 14 are
doped with germanium as well. However, it would also be possible to
form such gratings 13 and 14 in optical fiber pieces (or cores
thereof) of a germanium-doped passive material, and to splice the
same with an optical fiber piece made of, or having a core of, the
rare-earth doped active material, to thus obtain the optical fiber
section 11. In either event, the optical fiber section 11 is
unitary or integral, so that no problems arising from the presence
of interfaces or gaps are encountered.
It will be appreciated that the most useful output of the laser
arrangement 10 is that in which the issued laser beam exhibits only
a single longitudinal mode. Therefore, it is beneficial to
determine the design constraints required to achieve single
longitudinal mode operation from a standing wave, homogeneously
broadened, three-level fiber laser of the above type which utilizes
intracore Bragg reflectors for cavity feedback. In the following,
there will be presented a closed-form solution to the laser rate
equations which bounds the region in which only single longitudinal
mode operation is supported under different conditions.
Especially when a communications grade fiber is used for the
section 11, the output coupling through the gratings 13 and 14
results in the only significant cavity losses as both reflector and
fiber propagation losses are considered to be negligible. As
alluded to before, each Bragg reflector 13 and 14 is characterized
by a reflectivity range which has a peak reflectivity and an
associated bandwidth. In a typical grating having a length of
approximately 1 to 2 cm, the reflectivity bandwidth is
approximately 0.1 to 0.2 nm. Since lasing will only take place
within this bandwidth, and the grating bandwidth is much less than
the homogeneous gain linewidth in rare-earth, such as erbium, doped
fibers, spatial hole burning is the primary source of
multi-longitudinal mode operation in the linear fiber laser. Single
longitudinal mode operation will therefore be maintained when the
residual gain from spatial hole burning is insufficient for any
mode other than the dominant mode to lase.
The gain in an erbium fiber may be found by solving the appropriate
atomic rate equations. For instance, the three-level laser
transition (.sup.4 I.sub.13/2 and .sup.4 I.sub.15/2) in an erbium
doped germanosilicate fiber may be modeled according to the
simplified four-level energy diagram shown in FIG. 4. A
germanosilicate fiber composition was chosen since the
germanosilicate core permits convenient noninvasive holographic
formation of intracore Bragg reflectors. However this may not be
the only material exhibiting such properties.
In the lasing process, pump photon ground state absorption excites
electrons from level 1 to level 3. This absorption is followed by a
rapid decay to level 2, which is the upper laser level. From level
2, the electron may decay to the ground level through either
spontaneous emission or stimulated emission, at the signal
wavelength. In this analysis, pump excited state absorption has
been ignored since it does not pertain to the commonly used 980 nm
and 1480 nm pump bands. Finally, as ion can be excited from the
ground state to level 2 through the absorption of a signal
photon.
Given the transition rates (R.sub.13, W.sub.a, and W.sub.e) shown
in FIG. 4, the spontaneous lifetimes (.tau..sub.1-3), the simulated
pump absorption cross-sections for ground state absorption
(.sigma..sub.p), the pump and signal photon intensities in the
fiber (I.sub.p and I.sub.s), and the pump and signal photon
energies (h.nu..sub.p and h.nu..sub.s), the rate equations
governing steady state ion population density (N.sub.i) in the four
level system may be written. In the case where the spontaneous
lifetime .tau..sub.3 is much shorter than .tau..sub.2, the ion
population densities are given by ##EQU1## where the total
population satisfies N.sub.o .apprxeq.N.sub.1 +N.sub.2, since
N.sub.3 is negligible. In these rate equations, the transition
rates are given by R.sub.13 =.zeta..sub.p I.sub.p /h.nu..sub.p,
W.sub..alpha. =.zeta..sub..alpha. I.sub.s /h.nu..sub.s and W.sub.e
=.zeta..sub.e I.sub.s /h.nu..sub.s.
The gain is defined, in terms of the steady state ion population
densities, as
which may be rewritten as ##EQU2##
Given the gain and loss mechanisms within the resonator, the
required design constraints to achieve single mode operation within
a linear-cavity fiber laser which utilizes Bragg reflectors for
cavity feedback can be found. In modeling the laser, the transient
energy density per mode within the resonator will be considered. In
general, this can be written as ##EQU3## where .alpha. is the model
gain, .gamma. is the modal loss, and n is the index of the lasing
mode. For fiber lasers in which both fiber propagation and Bragg
grating losses are negligible, .gamma. is given by
where R.sub.1 (.lambda.) and R.sub.2 (.lambda.) the reflectivity
spectra of the Bragg gratings. In determining the conditions for
single mode operation, two potential longitudinal modes, E.sub.1
and E.sub.2, will be considered.
When considering the saturable gain presented in Eq. (4), only a
solution in which pure standing waves are formed within the
resonator will be considered. This assumption is justified when
practical low-gain fiber lasers are considered. In general, erbium
doped fibers with low doping concentrations yield relatively low
small signal gains, 2-4 dB/m. For short resonator lengths, a
relatively low loss cavity is therefore required to achieve lasing.
This requirement leads to the use of relatively high reflectivity
reflectors which produce a high contrast standing wave. Since in
the low gain regime the pure standing wave will yield the maximum
spatial hole burning, which is assumed to be the sole source of
mode competition in the homogeneously broadened fiber laser, this
assumption provides for a conservative design.
When considering the potential lasing of two modes within the
resonator the gain, Eq. (4) may be rewritten and linearized as
##EQU4## Lc is the cavity length and both M and N are integers. In
expanding Eq. (9), the strong pump regime which results in a high
saturation intensity, as presented in Eq. (6), will be considered.
In the strong pump regime, the lasing power within the resonator is
much less than the saturation intensity.
Introduction of the linearized gain, Eq. (9), into Eq. (7) and
integration over the length of fiber result in the rate equation
for the energy buildup for each mode: ##EQU5##
At steady state, the intensity of each mode is then found to be
##EQU6##
Finally, when the laser is operating in a single mode, the slope
efficiency, as given by Eq. (12), with respect to lasing power
versus pump power, is determined to be ##EQU7## wherein A.sub.laser
and A.sub.pump are the laser and pump mode areas, respectively. For
comparison purposes, the mode areas will be presented as a function
of the mode field diameter which is defined as
2.6.lambda./(.pi.NA).
Stable single-mode operation will occur when the laser is operated
above threshold so that mode 1 is operating at steady state with
intensity I.sub.1 and the losses incurred by mode 2 are greater
than the gain so that the rate of energy buildup in mode 2 is
negative. In this case,
and when the steady state value of I.sub.1, Eq. (12), is
substituted into the energy density rate equation for mode 2, Eq.
(11), we find
which is the condition for single mode operation.
The condition for stable single mode operation is dependent on both
the gain and the loss for the two lowest loss cavity modes. Since
the gain medium is considered to be homogeneous and constant over
the narrow grating reflector bandwidth, the gain is assumed to be
identical for both modes. Mode discrimination must, therefore,
result from the cavity loss mechanism. When narrow-band intracore
Bragg reflectors are used for cavity feedback, a differential loss
as a function of wavelength is naturally imposed on the cavity.
Therefore, two neighboring longitudinal modes will encounter
different cavity losses.
The reflectivity and transmission of rectangular Bragg gatings
which are holographically written in a single mode fiber can be
determined by using coupled mode theory. When weak guiding,
negligible absorption, and negligible coupling of propagating modes
to radiation modes are assumed, the grating reflectivity as a
function of wavelength and grating length is given by ##EQU8##
where .lambda. is the wavelength, L.sub.8 is the grating length,
.OMEGA. is the coupling coefficient,
.DELTA..beta.=.beta.-(.pi./.LAMBDA.), .beta. is the eigen
propagation constant, .LAMBDA. is the grating period,
S=(.OMEGA..sup.2 -.DELTA..beta..sup.2).sup.1/2, and
Q=(.DELTA..beta..sup.2 -.OMEGA..sup.2).sup.1/2. The coupling
co-efficient .OMEGA. is dependent on the induced index perturbation
which is a function of the fiber composition and grating exposure.
For the purposes of this simplified basic fiber laser analysis,
this treatment of the grating reflectors will be employed and the
gain from the doped fiber grating region will be neglected.
Gratings with reflectivities ranging between 0-100%, and bandwidths
ranging between 0.1-0.2 nm regime have been fabricated in erbium
doped germanosilicate fibers. In the case of germanosilicate
fibers, n is generally on the order of 5.times.10.sup.-4. For a 1
cm, 95% reflection grating, the theoretical reflectivity as a
function of wavelength is shown in FIG. 2. It is noted that the
predicted bandwidth of the reflector is approximately 0.12 nm,
which is in good agreement with experimentally obtained
results.
When evaluating the condition for single mode operation, Eq. (14),
the case where the dominant lasing mode is at line center and the
gain within the grating region is negligible will be considered
initially. The next mode will then be detuned one free spectral
range from the line center. This corresponds to a .DELTA..beta. of
##EQU9## where L.sub.c is the fiber laser cavity length.
In addition to the mode discrimination from a pair of Bragg
reflectors 13 and 14 that are used to define a linear cavity in the
laser arrangement 10 illustrated in FIG. 1, the case in which a
Bragg reflector and a complex reflector formed by two closely
spaced Braggs gratings or one Bragg grating and a planar reflector,
such as a multilayer dielectric coating or the 4% reflection from
the end of a normally cleaved fiber, should be discussed since this
will result in enhanced mode selectivity. To evaluate the
reflectivity from such a structure, the discussion will be limited
to the case where the fiber between the reflectors of the reflector
pair has negligible gain and the reflector pair forms a complex
coupler instead of a coupled resonator.
Analysis of the reflectance from multiple gratings, and/or a single
grating with a broadband reflector is treated according to the
cascaded T matrix approach. The reflectivity spectrum from a
complex reflector consisting of a Bragg grating with an 80%
reflectivity and a 1 cm length and a normally cleaved fiber end
face (instead of the slanted end face 11a or 11b of FIG. 1)
yielding a 4% reflection located 10 cm from the Braggs grating is
shown in FIG. 3. In this case, a considerably narrower effective
bandwidth than before is realized, which results in further
improved mode discrimination. This allows for the use of both
higher gain fibers and/or increased fiber lengths. This is
desirable when increased laser powers are required.
Under the model described above, the bounds of single-frequency
operation can be determined as a function of grating reflectivity
and resonator length when the grating length, grating wavelength,
fiber small signal gain, and effective fiber refractive index are
known. While we are not limited to these variables, they are
convenient when using commercially available fiber. The bounds of
single mode operation will then range between a lower limit, which
is defined by the lasing threshold, and an upper limit in which the
laser will transition from single to multimode operation due to
spatial hole burning as given by Eq. (14). In all calculations
discussed here, we will assume a laser having a pair of identical
gratings of peak reflectivity R.sub.max.
The single-frequency domain, defined between the lasing threshold
and the solution to Eq. (14), is shown in FIG. 5 as a function of
grating reflectivity and resonator length, for six fiber laser
designs. All plots assume a laser wavelength of 1.53 .mu.m, and a
fiber having an effective refractive index of 1.46 of FIG. 5a
defines the single frequency domain for a fiber having a small
signal gain of 2.5 dB/m. In FIG. 5a, the darkly shaded lower region
represents the use of 1.25 cm length gratings. FIGS. 5b and 5c are
to be interpreted similarly, but the fiber small signal gains are
3.0 dB/m and 4.0 dB/m, respectively. Again, it is noted that the
gratings having a gain of unity in this simplified analysis.
The laser slope efficiency is found to be maximum at the
single/multimode boundary. The normalized slope efficiency, as
given by Eq. (13) is shown in FIG. 6 as a function of grating
reflectivity for fiber gains of 2.5 dB/m, 3.0 dB/m and 4.0 dB/m.
FIG. 6a represents the use of 1.25 cm gratings and FIG. 6b
represents the use of 2.5 cm gratings.
When considering single-frequency fiber laser which utilize
intracore Bragg reflectors, a factor of importance is the stability
of the single mode solution. That is, it is to be determined to
what extent the single lasing mode can be detuned from the grating
resonance peak before the cavity loss of the neighboring mode
decreases relative to the increasing cavity loss of the dominant
mode to the point that multimode operation exists, or Eq. (14) is
violated. The stability of the solution is important since the
fiber optical path length is strongly effected by thermal,
acoustic, and strain variation perturbations, making the cavity
length difficult to fix, and a relatively small optical path length
variation, .lambda./2n, will shift the resonance by one
free-spectral-range.
Up to this point, it has been assumed that the dominant lasing mode
resides at the Bragg grating line center. Under small resonator
optical path length perturbations, the longitudinal modes of the
resonator will shift under the Bragg grating spectrum and the free
spectral range will not significantly vary. The shift can therefore
be normalized with respect to the laser free spectral range. If the
fraction of a free spectral range by which the dominant mode is
shifted from the grating line center is given by q, where q=0
indicates that the dominant lasing mode is on the center of grating
resonance, the maximum resonance shift, q.sub.max, at which point
the laser will transition from single mode to multimode operation
can be determined. The maximum resonance shift q.sub.max may then
be plotted as a function of grating pair reflectivity.
The single frequency stability for a laser length which lies half
way between laser threshold and the single-mode/multimode boundary
is shown in FIG. 7. Once again, FIG. 7 is plotted as a function of
grating reflectivity of gains of 2.5 dB/m, 3.0 dB/m, and 4.0 dB/m.
FIG. 7a represents the use of 1.25 cm gratings and FIG. 7b
represents the use of 2.5 cm gratings. It is noted that q is zero
when the reflectivity and length are chosen to lie on either the
single frequency operation boundary or the threshold boundary.
Although the slope efficiency of the fiber laser, when operating
half way between the single mode/multimode boundaries, is
approximately 50% of that when the laser is designed to operate on
the single-mode/multimode boundary, the solution is significantly
more stable.
In order to partially assess the validity of the design criteria
required to achieve single mode operation, a fiber laser was
designed, constructed, and tested. The 1.53 .mu.m fiber laser was
designed in erbium doped germanosilicate fiber. The fiber was doped
at approximately 500 ppm, has a 1.02 .mu.m cutoff wavelength, and a
NA of 0.24. The small signal gain was approximately 3 dB/m, in the
strong pump regime, when the fiber was pumped at 980 nm with a
Ti-Sapphire laser. The computed single-frequency design bounds for
these conditions are shown in FIG. 5a.
The fiber laser design consisted of a pair of 95% Bragg reflectors
separated by a 9 cm cavity. This design was chosen to achieve a
relatively high slope efficiency and reasonable stability with
respect to frequency drift.
The Bragg reflectors were written holographically through the fiber
cladding. The optics used to fabricate the gratings limited the
grating length to approximately 1.25 cm, which permitted gratings
with linewidths of approximately 0.1 nm to be fabricated. Since the
grating wavelength typically shifts slightly as a function of
increased exposure, due to a slight increase in the average
refractive index, both Bragg reflectors where chosen to have the
same reflectivity in an effort to maximize mode discrimination
through optimum grating overlap.
Single mode operation was confirmed with a scanning Fabry-Perot
interferometer. The fiber laser threshold was extremely low since
it took little pump power to nearly completely invert the fiber
laser. Experimental limitations in separating the pump and laser
power at the laser output did not permit an accurate measurement of
the fiber laser threshold; however, the slope efficiency of the
laser with respect to launched pump power was measured to be
approximately 0.13%. This value is in good agreement with the
0.151% which is computed from Eq. (13) and may be increased more
than two orders of magnitude when used in an integrated master
oscillator power amplifier configuration which utilizes all pump
radiation.
Thus, it may be seen that a technique has been developed for
determining the design parameters relative to achieving
single-frequency operation in a fiber laser which utilizes
intracore Bragg reflectors for mode discrimination. Given the
rare-earth doped fiber and Bragg reflector characteristics, the
single-frequency operation regime, relative slope efficiency, and
relative fiber laser stability can be determined.
While the present invention has been illustrated and described as
embodied in a particular construction of an embedded grating laser
arrangement, it will be appreciated that such invention is not
limited to this particular example; rather, the scope of protection
thereof is to be determined solely from the attached claims.
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