U.S. patent application number 12/459405 was filed with the patent office on 2010-02-11 for narrow linewidth injection seeded q-switched fiber ring laser based on a low-sbs fiber.
Invention is credited to Peter D. Dragic.
Application Number | 20100034221 12/459405 |
Document ID | / |
Family ID | 39641169 |
Filed Date | 2010-02-11 |
United States Patent
Application |
20100034221 |
Kind Code |
A1 |
Dragic; Peter D. |
February 11, 2010 |
Narrow linewidth injection seeded Q-Switched fiber ring laser based
on a low-SBS fiber
Abstract
A narrow linewidth injection-seeded Q-switched fiber ring laser
based on a low-SBS optical fiber. High peak powers are achieved
through the use of a single-clad erbium doped fiber with an
acoustic waveguide. 12.5 .mu.J per pulse (250 ns pulse width) is
achieved before a weakened form of stimulated Brillouin scattering
appears. This laser has the potential to scale to very high power
in a low-SBS dual clad fiber.
Inventors: |
Dragic; Peter D.; (Chicago,
IL) |
Correspondence
Address: |
Carmen Patti Law Group , LLC
ONE N. LASALLE STREET, 44TH FLOOR
CHICAGO
IL
60602
US
|
Family ID: |
39641169 |
Appl. No.: |
12/459405 |
Filed: |
July 1, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11656812 |
Jan 23, 2007 |
7577178 |
|
|
12459405 |
|
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Current U.S.
Class: |
372/6 ; 372/10;
385/123 |
Current CPC
Class: |
H01S 3/06708 20130101;
H01S 3/06791 20130101; H01S 2301/03 20130101; H01S 3/117 20130101;
H01S 3/10092 20130101; H01S 3/1608 20130101 |
Class at
Publication: |
372/6 ; 385/123;
372/10 |
International
Class: |
H01S 3/30 20060101
H01S003/30; G02B 6/02 20060101 G02B006/02 |
Claims
1. A narrow-linewidth injection seeded Q-switched fiber ring laser
comprising; a low-SBS Er doped fiber.
2. The laser of claim 1 wherein the laser provides 12.5 .mu.J per
pulse (250 ns pulse width).
3. The laser of claim 1 wherein the laser is utilized as a
saturating master oscillator for a high power low-SBS dual-clad
fiber amplifier or stand-alone transmitter for micropulse lidar
applications.
Description
BACKGROUND OF THE INVENTION
[0001] Fiber lasers have become very attractive for use in lidar
applications. This is due to a number of superior parameters that
are characteristic of these lasers, namely high efficiency, small
size, and low weight, making them especially suitable for space
applications. Many lidar applications, such as differential
absorption (DIAL) and resonance fluorescence, require narrow
linewidth operation of the fiber laser. For example, the remote
detection of CO.sub.2 could be facilitated by narrow linewidth
erbium-doped fiber lasers due to the presence of a strong
absorption feature near 1572 nm that resides in the Er L-Band.
[0002] In pulsed mode however, these systems are ravaged by
Stimulated Brillouin Scattering (SBS), which substantially limits
the peak power available for narrow linewidth systems. Considering
the low duty cycles required for a traditional pulsed lidar
transmitter (.about. 1/1000), SBS substantially limits total
average power resulting in degraded system signal-to-noise ratio
(SNR) leading to the requirement of long and usually impractical
integration times. As a result, the suppression of SBS in fiber
lasers would enable a number of new lidar configurations and
applications.
SUMMARY OF THE INVENTION
[0003] The invention comprises a narrow linewidth, 12.5
.mu.J-per-pulse (250 ns pulse width), injection seeded, Q-switched
fiber ring laser based on a low-SBS fiber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] Reference will be made to the drawings, wherein,
[0005] FIG. 1 of the drawings is a cross-sectional view of the low
SBS optical fiber. An acoustic waveguide surrounds the core to
cause a decrease in the effective Brillouin gain coefficient. Both
the optical and acoustic index profiles are provided;
[0006] FIG. 2 of the drawings are plots of the fundamental optical
mode at 1550 nm and excited L01 acoustic mode demonstrating the low
spatial overlap;
[0007] FIG. 3 of the drawings is a basic laser block diagram. An
output coupler with a split ratio of K is employed. Laser direction
is clockwise;
[0008] FIG. 4 of the drawings is a L-1 curve for the ring laser.
The laser was seeded with 6 mW of power at 1531 nm. The Q-switch
operated at 500 Hz and was open for 1 .mu.s;
[0009] FIG. 5 of the drawings is an optical spectrum of the
Q-switched ring laser at 75 mW of pump power. ASE noise is very
low;
[0010] FIG. 6 of the drawings are output pulses at 65 mW, 75 mW,
and 85 mW of pump power. SBS appears in Region B, beyond about 77
mW of pump power;
[0011] FIG. 7 of the drawings are secondary-SBS pulse shape from
[15], and three random pulse shapes from the low-SBS ring laser at
100 mW of pump power; and
[0012] FIG. 8 of the drawings are laser power distribution in the
EDF multiplied by the coupler ration (0.9) using the theory in
[17].
DETAILED DESCRIPTION OF THE DRAWINGS
[0013] While this invention is susceptible of embodiment in many
different forms, there is shown in the drawings and described
herein in detail specific embodiments with the understanding that
the present disclosure is to be considered as an exemplification of
the principles of the invention and is not intended to limit the
invention to the embodiments illustrated.
[0014] It will be understood that like or analogous elements and/or
components, referred to herein, may be identified throughout the
drawings by like reference characters. In addition, it will be
understood that the drawings are merely schematic representations
of the invention, and some of the components may have been
distorted from actual scale for purposes of pictorial clarity.
[0015] SBS is a well-known interaction between an acoustic wave and
the optical field in fiber. In general, the scattering amplitude
can be found from a volume integral
D p , q = .intg. Vol E _ p * .delta. _ p , q E _ iq V
##EQU00001##
[0016] where p,q=r,.phi.,z are the cylindrical coordinates, is the
electric field and .delta. .epsilon. are the dielectric
perturbations resulting from the acoustic strain. Taking the
optical field component E.sub.z to be zero in the fiber, the
non-zero contributions to the scattering amplitude are D.sub.rr,
D.sub..phi..phi., and D.sub.r.phi. (=D.sub..phi.r). The relevant
dielectric perturbations (.delta..epsilon..sub.p,q) are functions
of the acoustic strain fields (S.sub.p,q) and are expressed as
.delta..epsilon..sub.rr=.epsilon..sub.0n.sup.4(p.sub.11S.sub.rr+p.sub.12-
S.sub..phi..phi.+p.sub.12S.sub.zz)
.delta..epsilon..sub..phi..phi.=-.epsilon..sub.0n.sup.4(p.sub.11S.sub..p-
hi..phi.+p.sub.12S.sub.rr+p.sub.12S.sub.zz)
.delta..epsilon..sub..phi.r=-.epsilon..sub.0n.sup.4.sub.2p.sub.44S.sub..-
phi.r,
where n is the index of refraction and .epsilon..sub.0 is the
permittivity of free space. The photoelastic constants, p, for
vitreous silica at .lamda.=632.8 nm as p.sub.11=0.121,
p.sub.12=0.271, and p.sub.44=-0.075. The generalized form of the
acoustic strain field can be written in Cartesian coordinates as a
function of the displacement vector
S ij = 1 2 ( .differential. u j .differential. r i + .differential.
u i .differential. r j ) . ##EQU00002##
[0017] In general, the components of are coupled and can be found
from a generalized damped acoustic wave equation,
.rho. u - .gradient. _ [ c _ .gradient. _ u + .eta. _ .gradient. _
u . ] = - 1 2 .gradient. _ [ .gamma. _ _ E k E l ] ##EQU00003##
[0018] where .rho. is the mass density, the electrostrictive
coefficients are given by a fourth rank tensor in .gamma., and the
damping term .eta. is a tensor of rank four. Finally, c is the
rank-four elastic modulus tensor. A damped wave equation is needed
since at the acoustic frequencies involved in SBS (.about.10 GHz),
the acoustic wave is heavily damped. To determine the acoustic
eigenmodes of an acoustically guiding fiber, sixth equation is
solved subject to the typical boundary conditions: both the force
and displacement, both normal and tangential to the fiber
interfaces are continuous.
[0019] However, it has been shown that in the SBS interaction, the
dominant displacement vector component is u.sub.z. This makes sense
since SBS is known to result from a longitudinally varying
acoustically-induced Bragg grating. Furthermore, the analysis in
showed that D.sub.rr is the dominant scattering amplitude.
[0020] As a result, several assumptions can be made to simplify the
mathematical analysis; 1) the components of are de-coupled from
each other and from the optical field; 2) the acoustic wave is
un-damped; and 3) the shear velocity and mass density are constant
in the radial direction
(V.sup.s.sub.core.apprxeq.V.sup.s.sub.cladding and
.rho..sub.core.apprxeq..rho..sub.cladding). For an acoustically
guiding optical fiber, this leads to a set of solutions for known
as `Leaky` longitudinal modes, designated L.sub.nm, with u.sub.z
being the dominant component.
[0021] A traditional single mode fiber (i.e. Ge-doped core and pure
silica cladding) is an acoustic waveguiding fiber. Interestingly,
the dispersion relationship for the leaky acoustic modes in this
fiber is identical to that of the guided optical modes.
Furthermore, the L.sub.01 leaky mode dominates the SBS process due
to a large scattering integral in the first equation. This is a
direct consequence of a high spatial overlap with the optical mode.
This makes sense since both the fundamental optical and acoustic
modes are defined by a Bessel function of the first kind (J) in the
core and modified Bessel function of the second kind (K) in the
cladding.
[0022] Therefore, our first order approach to the suppression of
SBS in optical fiber is the removal of the high spatial overlap
with the fundamental mode, thereby introducing a significant
decrease in the effective Brillouin gain coefficient. Our approach
is the inclusion of an additional acoustic waveguide layer to
remove the Bessel-J functional form at the center of the fiber. A
cross-sectional view of the optical fiber is provided in FIG.
1.
[0023] In this fiber, the L.sub.0m acoustic eigenmodes are
therefore represented by the usual Bessel functions
u z = { A a I 0 ( u 1 r a ) r .ltoreq. a A 1 J 0 ( u 2 r b ) + A 2
Y 0 ( u 2 r b ) a < r .ltoreq. b A 3 K 0 ( u 3 r b ) r > b
with { u 1 = 4 .pi. a n opt .lamda. opt V a ( 1 V a 2 - 1 V 1 2 ) 1
/ 2 u 2 = 4 .pi. b n opt .lamda. opt V a ( 1 V 2 2 - 1 V a 2 ) 1 /
2 u 3 = 4 .pi. b n opt .lamda. opt V a ( 1 V a 2 - 1 V 3 2 ) 1 / 2
##EQU00004##
where n.sub.opt is the index of refraction of the optical mode and
.lamda..sub.opt is the vacuum optical wavelength. V.sub.1, V.sub.2,
and V.sub.3 are the acoustic velocities (km/s) in the core,
acoustic waveguide layer, and cladding, respectively. It is clear
from the seventh equation that the overlap with the L.sub.01
fundamental leaky acoustic mode can be substantially degraded.
[0024] To determine the acoustic eigenmodes V.sub.a, we match the
boundary conditions and solve the following determinant
expression
I 0 ( u 1 ) - J 0 ( u 2 a b ) - Y 0 ( u 2 a b ) 0 u 1 a I 1 ( u 1 )
u 2 b J 1 ( u 2 a b ) u 2 b Y 1 ( u 2 a b ) 0 0 J 0 ( u 2 ) Y 0 ( u
2 ) - K 0 ( u 3 ) 0 - u 2 b J 1 ( u 2 ) - u 2 b Y 1 ( u 2 ) u 3 b K
1 ( u 3 ) = 0. ##EQU00005##
The eigenfrequencies are then found by taking V.sub.a/.lamda..sub.a
where the acoustic wavelength is determined by the Bragg condition
.lamda..sub.opt/2n.sub.opt. To determine the scattering amplitude
in the first equation, the normalized acoustic mode is needed, and
therefore the coefficients A in the seventh equation are similarly
found by matching the boundary conditions.
[0025] For silica glass, it is well known that Ge and P dopants
both act to decrease the acoustic velocity and increase the index
of refraction. B and F also decrease the acoustic velocity, but act
to decrease the index of refraction. Al increases the acoustic
velocity while increasing the index. As a result, there are
sufficient degrees of freedom with the most common fiber dopants to
achieve the profiles shown in FIG. 1.
[0026] A single-clad Er-doped optical fiber was produced (Neolight
Labs model 111-001 low-SBS fiber) doped with 6.0 mol %
Al.sub.2O.sub.3, 0.1 mol % P.sub.2O.sub.5 and GeO.sub.2, and 0.2
mol % F in the core, 4.0 mol % GeO.sub.2 and 1.9 mol % F in the
acoustic layer, and a pure silica cladding. These values are
provided for the center of each layer, since the profiles were
super-Gaussian in shape, as opposed to the ideal profiles shown in
FIG. 1. The fiber has the dimensions a=2.2 .mu.m and b=6.0 .mu.m
with single-mode cutoff wavelength around 950 nm. The Er doping
concentration is 1000 ppm/wt. The acoustic layer has an index of
refraction matched to that of the cladding, although this is not a
necessary condition.
[0027] The resulting acoustic velocities were estimated to be 6073
m/s, 5322 m/s, and 5933 m/s in the core, acoustic layer, and
cladding, respectively. The resulting L.sub.01 leaky longitudinal
acoustic mode is calculated to have an acoustic velocity of 5.33
km/s. FIG. 2 provides a plot of the normalized acoustic mode
(amplitude) and optical mode (intensity) demonstrating the
successful removal of the L.sub.01 acoustic mode as a dominant
mode.
[0028] Interestingly, as a result of the super-Gaussian doping
profile resulting from the fiber manufacture, SBS interacts most
significantly with the wide guided-acoustic mode tails of high
order modes that extend deeply into the core from the acoustic
layer.
[0029] FIG. 3 shows the basic laser setup in which a 20 m length of
the Er-doped fiber (EDF) is employed. This fiber has a measured
effective Brillouin gain coefficient (g.sub.B) of less than
0.5.times.10.sup.-11 m/W. Two isolators are used to ensure
unidirectional laser operation (clockwise in FIG. 2). Pump light
(976 nm) is coupled into the ring using a wavelength division
multiplexer (WDM). The 90% arm of a 90/10 2.times.2 coupler is used
as the laser output. The coupler input forms part of the ring. Seed
power is launched into the other input arm of the coupler,
providing 90% injection efficiency into the ring. We used a fiber
coupled pulse modulated acoustooptic modulator (AOM) [Brimrose
Corp.] as the Q-switch and an isolated Agilent 81682A external
cavity diode laser (ECDL) provided seeding. The AOM had a maximum
diffraction efficiency of about 50% and insertion loss of about -3
dB. The linewidth of the ECDL was measured using a self-heterodyne
technique to be about 85 kHz. The output power of the ring was
measured using a broad area Ge detector, and the pulses were
analyzed using a fast InGaAs APD.
[0030] The peak passband wavelength of the AOM was .about.1531 nm
(with .about.25 nm width) when operated at an acoustic frequency of
106 MHz. Thus, we employed a seed wavelength of 1531 nm. To achieve
other wavelengths, such as 1572 nm for the CO.sub.2 application,
the AOM should be optimized for the desired wavelength range.
Furthermore, intracavity filters would aid in L-band laser
operation.
[0031] Injection seeding is normally considered to be a narrow
linewidth operation and can be a very difficult process. For
effective seeding to take place, the seed wavelength should be
well-aligned to a cavity mode and have a linewidth less than that
of a ring mode. However, in the case of a low-finesse cavity, this
requirement is substantially relaxed since the cavity modes overlap
forming a quasi-continuum. The finesse of the ring cavity of FIG. 3
is estimated to be .about.2.
[0032] FIG. 4 provides an L-1 curve for the injection seeded ring
laser. The ring was operated at 500 Hz and the Q-switch was open
for 1 .mu.s, with P.sub.seed=6 mW. When no pump power is present,
some seeder leakage is present at the output, which we subtracted
from the L-I curve. We see that over 6 mW of average output power
was produced by the ring laser. This is an order-of-magnitude
improvement in average output power relative to a fiber with an
identical length and mode field diameter (MFD) with no SBS
suppression. Furthermore, ASE noise from the ring was found to be
very low. FIG. 5 shows a pulsed-mode optical spectrum of the
laser.
[0033] The L-I curve has two distinct operating regions, labeled in
FIG. 4 as A and B. In region A, normal seeded operation of the
laser is observed. A sharp transition into region B is observed at
about 77 mW of pump power, where SBS first appears in the
laser.
[0034] To explain laser operation, FIG. 6 shows the output pulses
at 65 mW, 75 mW, and 85 mW of pump power. Characteristic of all of
the pulses is the comb-like shape due to the round trip time in the
ring cavity (150 ns, 30 meter total ring length). It also is
observed that the pulses narrow and the peaks sharpen as the pump
power is increased.
[0035] A unique feature regarding the performance of this laser is
the effect when SBS appears. SBS appears as the laser transitions
from operating zone A to zone B. In a previously known laser, this
resulted in an initial drop in forward output power. In this case,
however, it causes an immediate increase in forward power. It is
conjectured that since the peak power is so high in the last few
meters of the Er-doped fiber, secondary SBS is immediately excited.
This results in increased energy extraction efficiency in the
forward direction, thereby giving rise to the increase in power at
the onset of SBS.
[0036] We can justify that SBS is causing the transition between
zones B and C by the observation that at around 77 mW of pump
power, the backside of the output pulse (third peak from the left
at about 2.6 .mu.s) `flickers` in-and-out, consistent with an SBS
back-reflection. There also appears to be a small second-order SBS
signal that appears in the forward direction. Both of these
phenomena can be seen in FIG. 7 where three random pulse shapes
were taken from the ring at 100 mW of pump power. However, unlike
the prior art, the laser does not lock into an SBS-dominated pulse
shape, represented in FIG. 7 as "high SBS fiber." Instead, the
forward SBS component simply flickers weakly and the output pulse
shape seen in FIG. 6 is the same for all pump powers up to a
maximum available 150 mW. This suggests that besides an increased
SBS threshold, the SBS interaction itself has been substantially
weakened, such that most of the available energy is extracted from
the laser before SBS can dominate laser operation.
[0037] The SBS threshold of the fiber ring laser can be estimated.
The distribution of average laser power along the fiber length
(multiplied by the output coupler ratio 0.9) is modeled in FIG. 8,
using the spectroscopic parameters found in W. J. Miniscalco,
"Erbium-Doped Glasses for Fiber Amplifiers at 1500 nm," J.
Lightwave Technol., vol. 9, no. 2, pp. 234-250, February 1991, for
a pumping wavelength at the peak of the 980 nm (75 mW) band and
laser wavelength of 1531 nm. It is clear from the graph that 20 m
of fiber is not an optimal length, but it enables a comparison with
prior art employing an identical fiber with no SBS suppression.
Further improvements in power can be achieved by shortening the
Er-fiber length, both in raw power (albeit only slightly) and SBS
threshold.
[0038] Assuming an effective fiber length of 10 m (z=10 to 20
meters in FIG. 8), and using g.sub.B=5.0.times.10.sup.-11 m/W for
bulk fused silica (polarized value) and a MFD of about 7 .mu.m at
1530 nm, we can estimate the SBS threshold to be about 2 W, peak,
in a fiber with no SBS suppression. This is consistent with the
maximum output power measured in the prior art for an identical
fiber length and MFD, and no SBS suppression. The use of the
polarized value for the gain coefficient is justified here since
the seeder is linearly polarized and the signal and Stokes' waves'
polarizations remain well-correlated throughout the short fiber
length.
[0039] The FWHM of the envelope of the output pulse at 75 mW of
pump power is roughly 250 ns. This corresponds to a peak power of
about 50 Watts. Much of this high peak power can be attributed to
the low effective Brillouin gain coefficient of the Er-doped fiber,
at more than an order-of-magnitude lower than the bulk value.
However, the laser also gains in SBS threshold from two additional
advantages to the configuration in FIG. 3.
[0040] First, the comb-like peaks sharpen and narrow as the pump
power is increased, so that the overall pulse shape consists of a
few sub-pulses (.about.100 Watts peak). The width (FWHM) of each of
these sub-pulses is .about.40 ns at 75 mW of pump power,
representing an interaction length of about 4 m, or substantially
less than the 20 m of Er-doped fiber used. The sub-pulses also give
rise to a slight spectral broadening (.about.9.4 MHz if
transform-limited Gaussian sub-pulses are assumed), offering a
slight further increase in the SBS threshold. Second, since the AOM
in the cavity imparts a frequency shift of 106 MHz each time around
the ring, each of the sub-pulses are separated by 106 MHz. Since
this shift falls outside the Brillouin gain spectrum, there is no
SBS averaging effect observed from sub-pulse to sub-pulse as they
make a round trip in the ring. The total shift between the
sub-pulses can be controlled by the AOM frequency, but the
instantaneous linewidth (of each sub-pulse) remains narrow.
[0041] The foregoing description merely explains and illustrates
the invention and the invention is not limited thereto except
insofar as the appended claims are so limited, as those skilled in
the art who have the disclosure before them will be able to make
modifications without departing from the scope of the
invention.
* * * * *