U.S. patent application number 11/493140 was filed with the patent office on 2010-09-09 for gain-switched fiber laser system.
Invention is credited to Min Jiang, Daniel Mahgerefteh.
Application Number | 20100226395 11/493140 |
Document ID | / |
Family ID | 42646691 |
Filed Date | 2010-09-09 |
United States Patent
Application |
20100226395 |
Kind Code |
A1 |
Jiang; Min ; et al. |
September 9, 2010 |
GAIN-SWITCHED FIBER LASER SYSTEM
Abstract
This invention discloses a method to control laser dynamics in a
gain-switched fiber laser so as to generate stable, clean pulses in
an all-fiber format. The gain-switched fiber laser is suitable as a
standalone laser source, and as a pump source for harmonic
generation and an optical-parametric-oscillator.
Inventors: |
Jiang; Min; (Acton, MA)
; Mahgerefteh; Daniel; (Somerville, MA) |
Correspondence
Address: |
Coherent, Inc. c/o Morrison & Forester
425 Market Street
San Francisco
CA
94105-2482
US
|
Family ID: |
42646691 |
Appl. No.: |
11/493140 |
Filed: |
July 26, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60702492 |
Jul 26, 2005 |
|
|
|
Current U.S.
Class: |
372/6 |
Current CPC
Class: |
H01S 3/094076 20130101;
H01S 3/06791 20130101; H01S 3/0675 20130101; H01S 3/067 20130101;
H01S 3/094003 20130101; G02F 1/37 20130101; H01S 3/0092 20130101;
H01S 3/1024 20130101 |
Class at
Publication: |
372/6 |
International
Class: |
H01S 3/30 20060101
H01S003/30 |
Claims
1-11. (canceled)
12. A pulsed, gain switched fiber laser comprising: a resonant
cavity; a thulium doped gain fiber located within the cavity, said
thulium doped gain fiber having an emission band which overlaps
with an absorption band; and a radiation pump source with the
output thereof directed into the cavity to pump the thulium doped
gain fiber, with the wavelength of the pump source being resonant
with the lasing wavelength so that the relaxation from the pump
energy level to the lasing energy level is essentially
instantaneous and wherein the wavelength, intensity and duration of
the pump pulses are selected so that the excitation population
generated in the gain fiber by the pump pulses is depleted to a
level below the lasing threshold in a time duration less than the
relaxation oscillation period of the fiber laser in manner to
generate one and only one output laser pulse for each pump pulse
and wherein said output laser pulses are stable in amplitude and
time.
13. The laser as recited in claim 12, wherein the gain fiber
includes a thulium doped core and an outer cladding and wherein the
fiber is cladding pumped.
14. The laser as recited in claim 12, wherein the laser is
configured to generate pulses as short as 10 nanoseconds.
Description
REFERENCE TO PENDING PRIOR PATENT APPLICATION
[0001] This patent application claims benefit of pending prior U.S.
Provisional Patent Application Ser. No. 60/702,492, filed Jul. 26,
2005 by Min Jiang for DOUBLE MODULATED FIBER MOPA, GAIN-SWITCHED
MID-IR FIBER LASER WITH FLEXIBLE WAVEFORM (Attorney's Docket No.
TAYE-58 PROV), which patent application is hereby incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates to pulsed fiber lasers in general,
and more particularly to in-line pulse generation achieved through
gain-switching.
BACKGROUND OF THE INVENTION
[0003] Fiber lasers have the significant advantages of high
efficiency, easy thermal management and inherent robustness. With
recent advances in high quality, double-clad fibers, fiber lasers
have been demonstrated to achieve power conversion efficiencies of
over 90%, and power levels approaching 1 kW in continuous wave (cw)
operation. For pulsed operation, however, fiber lasers often need
free-space devices (such as acousto-optical or electro-optical
switches) to initiate and maintain stable operation. However, the
use of such free-space devices generally diminishes the inherent
advantages of the confined structured optical fiber, and typically
leads to problems in system complexity and reliability.
[0004] A gain-switched fiber laser generally has the following
advantages:
[0005] (i) All-fiber, in-line pulse generation--no free-space
optics are required.
[0006] (ii) Flexible repetition rate--gain-switched fiber lasers
can produce a stable pulse train at any modulation rate up to the
relaxation oscillation frequency, which is typically in the range
of MHz.
[0007] (iii) Wide range of pulse widths are possible, from ns to
.mu.s--the pulse width of gain-switched lasers depends on gain and
the "cavity lifetime", which can be varied over a wide range in
fiber lasers.
[0008] Gain-switching is a pulse generation method which generates
pulsed output by varying the gain of a laser cavity. In fiber
lasers, the gain-switching is achieved within the gain fiber, which
typically comprises an optical fiber doped with a rare-earth
element which serve as active ions, which makes it possible to
achieve a potentially in-line pulse generation.
[0009] However, the challenge of generating a stable pulse chain in
gain-switched fiber lasers lies in the control of the relaxation
oscillation, which typically results in chaotic pulsation.
[0010] In principle, laser dynamics can be described by using rate
equations for population inversion, N(t), and the cavity photon
number P(t):
dN(t)/dt=R(t)-N(t)/.tau..sub.E-KN(t)P(t) (1)
dP(t)/dt=KN(t)P(t)-P(t)/.tau..sub.P+KN(t) P.sub.ASE (2)
where R(t) is the effective pump rate for population buildup of
N(t), K is the coupling coefficient, P.sub.ASE are the spontaneous
emission photons coupled into the laser mode, .tau..sub.E is the
excitation lifetime and .sub.C is the cavity (photon) lifetime. The
cavity lifetime .tau..sub.C=T.sub.C/.delta..sub.C is determined by
the cavity roundtrip time T.sub.C and cavity loss .delta..sub.C.
For fiber lasers which typically have a large output coupling, the
cavity lifetime is essentially the same as the cavity roundtrip
time, namely, .tau..sub.C.apprxeq.T.sub.C=L/2C, with L being the
effective cavity length and C the speed of light.
[0011] Under continuous wave (cw) operation, the laser output has a
relatively stable amplitude, except for the short time interval
when the pump source is first turned on. For this short time
interval, the excitation population builds up initially according
to Eqs. 1 and 2, to a level near the lasing threshold
(N.sub.th=1/K.tau..sub.c) and the photon numbers begin to grow
exponentially. The growth rate of the photons is essentially given
by (KN(t)-1/.tau..sub.C), which grows larger as the N(t) increases
with continuous pumping. As the growth rate of the photons exceeds
the pumping rate, the excitation population eventually decreases
below the lasing threshold and the lasing stops. This photon
buildup and lasing action is repeated until the laser operation
reaches a steady state with constant laser output (dP/dt=0), when
the excitation population supplied by the pump is equal to that
consumed by the photons. The oscillation buildup time
(.tau..sub.B), the period (.omega..sub.Re) and amplitude damping
rate (.gamma..sub.Re) of the relaxation oscillation can be
approximated as:
.tau..sub.B=C.sub.S.tau..sub.C/(.gamma.-1) (3)
.omega..sub.Re=[(.gamma.-1)/.tau..sub.E.tau..sub.C].sup.0.5 (4)
.gamma..sub.Re=.gamma./2.tau..sub.E (5)
where .gamma.=RT.sub.E/N.sub.th is the inversion ratio, and
C.sub.S.apprxeq.20-30 is a constant for a given laser representing
the log ratio of the steady-state light intensity and that at
initial noise level. The buildup time, relaxation oscillation
period and damping rate all depend on the pumping rate, the cavity
lifetime and the lifetime of the excited state.
[0012] Under gain-switched operation, using a modulated pump with a
sufficiently large amplitude contrast and time period, the
multi-peak relaxation oscillation pattern may be repeated following
each pump pulse. The laser buildup, oscillation period and
amplitude damping are, in principle, similar as that in Eqs. 3-5,
with the population inversion ratio adjusted according to the
effective pump rate R(t). The effective pumping rate in a typical
laser system, however, is a function of the modulation pattern of
the pump laser and, more importantly, a function of the relaxation
dynamics from the pump absorption energy level
(E.sub.P=hc/.lamda..sub.P) to the meta-stable energy level
E.sub.S=hc/.lamda..sub.S where the excitation population N(t) is
accumulated. The relaxation oscillation pattern may vary
significantly depending on the energy levels and time scales
involved in the excitation population buildup process.
[0013] Relaxation oscillation is a characteristic of most solid
state lasers in which the recovery time of the excited state
population is substantially longer than the laser cavity decay
time. The phenomenon was observed as early as the first generation
of ruby lasers. Because of the long lifetime of the gain medium,
relaxation oscillation is particularly severe in fiber lasers.
Chaotic pulsation has been commonly observed. For example, in a
gain-switched thulium-doped fiber laser pumped by a Ti:Sapphire
laser at 790 nm, the output pulse train is somewhat periodic but
the pulse width and frequencies are different than those of the
pump pulses: the pulse width is .about.0.3 .mu.s and separated by 1
.mu.s for the thulium-doped fiber laser (TDFL) output while the
pump pulses are separated by 0.67 .mu.s. Moreover, the laser
dynamics in fiber lasers can be particularly complicated when the
laser dynamics involve more than the typical 3-level or 4-level
laser system. One outstanding example is the excited state
absorption in thulium-doped fiber lasers. When the pump wavelength
is near 1 .mu.m, the absorption cross-sections of the excited
states are on the same order as the ground state absorption. The
decay time of the excited state absorption upper energy level is
also on the same order as the laser population. Under this
condition, the spiking phenomenon in a thulium-doped fiber laser
becomes totally chaotic.
SUMMARY OF THE INVENTION
[0014] As a result, one object of the invention is to provide
amplitude-stable, regular repetition rate and short duration pulses
from fiber lasers through gain-switching within the gain fiber. A
significant advantage of this method is the provision of an
all-fiber pulse laser source which is simple, reliable, compact and
maintenance-free even in harsh environments.
[0015] A further object of the invention is to extend the
gain-switched fiber laser for high efficiency wavelength conversion
through nonlinear optical effects.
[0016] These and other objects of the invention are achieved by
providing a gain-switched fiber laser comprising a gain fiber laser
and cavity optics, a modulated pump laser, and means for coupling
the pump laser into the gain fiber and for switching the gain of
the fiber laser.
[0017] This invention describes a method for gain-switching a fiber
laser so as to provide pulses with stable amplitude and regular
repetition and duration. This is to be contrasted with the prior
art, in which a gain-switched fiber laser produces pulses with
fluctuating amplitude and irregular or chaotic repetition and pulse
duration. The invention described in this disclosure is exemplified
by thulium-doped fiber lasers, which are widely considered to be
among the most significant candidates for the gain-switched fiber
laser technology. These thulium doped fiber lasers have emission
spectra between the 1.8-2.2 .mu.m spectral region. Gain-switched
thulium doped fiber laser with stable and clean short pulse output
could enable a wide range of applications such as remote sensing,
mid-infrared spectroscopy, and optical countermeasures for missile
defense. The gain-switching fiber laser technology of the present
invention, however, is also readily applicable to other fiber
lasers, such as Ytterbium (Yb), Erbium (Er), Neodymium (Nd) and
Praseodymium (Pr) doped fiber lasers.
[0018] In one form of the present invention, there is provided a
gain-switched fiber laser system for producing a short duration,
amplitude stable, and regulated pulse period, comprising:
[0019] a fiber laser comprising a gain fiber in a cavity, and an
output coupling; and
[0020] an optical pump providing regulating pulses for driving the
fiber laser, the regulating pulses having an intensity, duration,
and wavelength such that the excitation population generated by the
pump is depleted to a level below the lasing threshold in a time
duration less than the relaxation oscillation period of the fiber
laser.
[0021] In another form of the present invention, there is provided
a high efficiency optical pulse generator comprising:
[0022] a gain switched fiber laser system for producing a short
duration, amplitude stable, and regulated pulse period comprising:
[0023] a fiber laser comprising a gain fiber in a cavity, and an
output coupling; and [0024] an optical pump providing regulating
pulses for driving the fiber laser, the regulating pulses having an
intensity, duration, and wavelength such that the excitation
population generated by the pump is depleted to a level below the
lasing threshold in a time duration less than the relaxation
oscillation period of the fiber laser; and
[0025] a nonlinear optical crystal for receiving the output of the
fiber laser and converting the output of the fiber laser to a
different wavelength.
[0026] In another form of the present invention, there is provided
a method for producing a short duration, amplitude stable, and
regulated pulse period, comprising:
[0027] providing a fiber laser and an optical pump for driving the
fiber laser; and
[0028] operating the optical pump so as to generate regulating
pulses, the regulating pulses having an intensity, duration, and
wavelength such that the excitation population generated by the
optical pump is depleted to a level below the lasing threshold in a
time duration less than the relaxation oscillation period of the
fiber laser.
[0029] Other features and advantages of the invention will be made
clear by the following detailed description, which is to be
considered with the corresponding drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1A illustrates a gain-switched fiber laser in a
preferred embodiment of the invention;
[0031] FIG. 1B illustrates the absorption and emission spectrum of
a thulium-doped fiber laser;
[0032] FIG. 2A illustrates a gain-switched fiber laser in the
multi-pulsing operation;
[0033] FIG. 2B illustrates a gain-switched fiber laser in the
single pulsing operation of the present invention;
[0034] FIG. 3 illustrates another preferred embodiment wherein a
wavelength converter is pumped by a gain-switched fiber laser;
and
[0035] FIG. 4 illustrates a carrier pulse modulation scheme.
[0036] In the drawings, like components are identified by like
numbers.
DETAILED DESCRIPTION OF THE INVENTION
[0037] A primary aspect of the present invention is the proper
choice of the effective pump rate so as to establish a desired
population buildup at a selected energy level. The effective pump
rate should be chosen such that the accumulated excitation
population is depleted to, and maintained at, a level below the
lasing threshold in a time duration less than the relaxation
oscillation period of the fiber laser so as to ensure the
suppression of secondary pulses. This desired manner of operation
is achieved by properly selecting the wavelength and the modulation
characteristics of the pump laser.
[0038] Fiber lasers can generally be pumped by a number of
absorption bands. One preferable choice for the pump wavelength
.lamda..sub.P is to be "resonant" with the lasing wavelength
.lamda..sub.S. This is particularly feasible for fiber lasers which
have broad and overlapping absorption and emission bands. Under
this condition, the pump rate for excitation buildup at
.lamda..sub.S is essentially the same as the pump modulation at
.lamda..sub.P. The gain of the fiber laser can then be accurately
established and time-controlled (i.e., so as to create consistent,
well-defined pulses) directly through the modulation of the pump
pulse.
[0039] With the selected pump rate, relaxation oscillation is
determined by the cavity parameters as described by Eqs. 3-5. In
fiber lasers, these parameters can be adjusted over a wide range so
as to achieve regular stable pulses with various parameters such as
pulse width, repetition rate, and power level.
[0040] FIGS. 1A and 1B illustrate a gain-switched fiber laser 1
which emits light at wavelength .lamda..sub.S, and a pump laser 2
which emits light at wavelength .lamda..sub.P. The output of the
pump laser 2 is coupled into an optical fiber 21 which directs the
pump beam into the rare-earth doped gain fiber 11 through a power
coupler 22. Partially-reflective cavity mirrors 12, 13 are added at
the ends of the fiber so as to form a laser cavity. Alternatively,
optical fiber 21 may be configured in other ways well known in the
art so as to form a laser cavity, e.g., optical fiber 21 can be
looped back on itself. The pump laser 2 (emitting light at
wavelength .lamda..sub.P) is preferably operated in pulsed mode at
repetition frequency f.sub.P, and with pulse width .tau..sub.P, as
determined in accordance with the present invention.
[0041] Looking next at FIGS. 1A and 1B, operation of the laser
system is as follows. The pump laser 2 is modulated, with the
amplitude and the temporal waveform controlled by an electric
current driver. The pump laser pulses at wavelength .lamda..sub.P,
within the absorption band 24, and is injected into the gain fiber
11 and is absorbed by the doping ions. The excited ions are quickly
relaxed to an energy level at the corresponding laser wavelength
.lamda..sub.S in the emission band 14. The excitation population of
excited states is proportional to the total energy of the pump
pulse. The photons of spontaneous emission at wavelength
.lamda..sub.S propagate along the gain fiber 11 and are amplified
according to Eq. 2; this in turn consumes the population of the
excited states. With sufficiently high population inversion, laser
emission at .lamda..sub.S is produced when the roundtrip gain
exceeds the cavity loss.
[0042] With the condition that the relaxation from energy level
.lamda..sub.P to energy level .lamda..sub.S is essentially
"instantaneous", the output of the laser emission at .lamda..sub.S
depends on the duration and amplitude of the pump pulse and the
fiber laser cavity lifetime. With an arbitrarily broad pump pulse,
the fiber laser output at wavelength .lamda..sub.S shows relaxation
oscillation, with multiple pulses of damping amplitude over time,
as shown in FIG. 2A. The fiber laser output pulsing can also be
irregular or chaotic in time. This multiple pulsing is an
undesirable feature of fiber lasers in the prior art.
[0043] To generate single, short pulses with stable amplitude from
the fiber laser, the excitation population has to be significantly
depleted through the first cycle of relaxation oscillation of the
laser. This condition can be fulfilled by using a properly
modulated pump with appropriate large pulse energy and appropriate
short duration. The pulse energy should be large enough to generate
a sufficient excitation population to ensure laser action. The
duration of the pump pulse should be sufficiently short such that
the excitation population buildup essentially ceases when the fiber
laser output pulse is generated.
[0044] The parameters on the appropriate pump rate (i.e., a pump
rate which will create a fiber laser output having single, short
pulses with stable amplitude) is a function of the cavity design of
the fiber laser. This is shown in Eqs. 3-5. It should be
appreciated that the population inversion ratio and the cavity
lifetime are two most important parameters in determining the pulse
dynamics in a gain-switched laser. The population inversion ratio
is inversely proportional to the laser threshold, which is
primarily determined by the output coupling in a fiber laser. The
cavity lifetime is primarily determined by the cavity roundtrip
time, which is easy to vary by increasing or decreasing fiber
length. Due to the confined waveguide design, fiber lasers can have
a very high gain, e.g. 20 dB, when sufficient pump power is
provided. This high cavity gain allows substantial flexibility in
setting the output coupling, thereby allowing easy optimization of
the population inversion ratio for optimum gain-switching
operation.
[0045] For pulsed lasers, pulse generation with the shortest
possible pulse width is often of most interest. Under the condition
that the population inversion is built up under fast pumping, the
pulse width of the gain-switched fiber laser (.tau..sub.GS) is
essentially determined by the number of roundtrips needed to
consume all the generated excitation population. For gain-switched
fiber lasers, the high cavity gain allows fast photon density
buildup, and hence large excitation population consumption within
relatively few roundtrips, which is a condition necessary for short
pulse generation. The pulse width of the gain-switched fiber laser
decreases with increasing pump level as the population buildup and
depletion is sped up. Using Eq. 3, the pulse width of the
gain-switched fiber laser can be estimated to be 10-100 times of
the cavity roundtrip time. With large pump energies, pulse widths
as short as 10 ns may be generated from a gain-switched fiber laser
with cavity length of 10 cm. For such fiber lasers, the gain fiber
is preferably of a high doping level, and the pump laser is
injected into the doped core to achieve effective pump
absorption.
[0046] Various forms of gain fiber may be used in this invention.
The gain fiber may be single clad with various core sizes and
doping levels, or the gain fiber may be double clad with various
core sizes and doping level. The non-doped, clad pump fiber may be
of various sizes. Using single clad gain fiber, the pump pulse is
injected directly into the doped core and the absorption is high. A
short length of gain fiber, e.g. 10 cm, is needed to build a
gain-switched fiber laser capable of generating <10 ns pulses.
Double clad gain fiber has the advantage of allowing effective
coupling of high-power, multi-mode pump lasers, such as broad-area
diode lasers or multi-mode solidstate lasers. High power output can
be generated from a gain-switched fiber laser. The cavity length of
double clad fiber lasers, however, is generally relatively long
(e.g., a few meters) leading to a relatively long pulse width
(e.g., on the order of a .mu.s).
[0047] A gain-switched fiber laser constructed in accordance with
the present invention preferably further comprises optical feedback
means and output coupling means to form a laser cavity and to
couple the output of the fiber laser. The optical feedback means
and the output coupling means may be of various forms such as fiber
Bragg gratings, dielectric or metallic-coated reflectors, fiber
reflectors, or any combination of these.
[0048] The pump laser for the gain-switched fiber laser may be of
various forms. The pump laser may be a semiconductor laser,
solid-state laser, fiber laser, or a fiber or solid-state amplifier
seeded with a pulsed oscillator. The pump laser is preferably
coupled into an optical fiber which is compatible with the geometry
of the gain fiber. For short pulse generation from the gain-witched
fiber laser, the pump laser pulse is preferably of short duration
and high amplitude.
[0049] The pump coupler 22 in FIG. 1A directs the pump laser into
the gain fiber. The pump coupler may be of various forms, based on
fiber or dichroic reflector/transmitter, single mode or multi-mode,
depending on the geometry of the gain fiber.
[0050] The laser cavity geometry may be linear, ring-shaped,
sigma-shaped or other configuration that will be obvious to those
skilled in the art of fiber lasers.
[0051] It should be appreciated that the gain-switched fiber laser
of the present invention may be used as a standalone laser source,
or as a seed oscillator for a laser amplifier, or for other uses
which will be obvious to those skilled in the art in view of the
present disclosure.
[0052] A further important application for the gain-switched fiber
laser of the present invention is for high-efficiency wavelength
conversion to generate fiber-delivered laser sources at wavelengths
not directly attainable from fiber lasers.
[0053] Referring now to FIG. 3, there is shown a gain-switched
fiber laser 31. Gain-switched fiber laser 31 comprises the gain
fiber 1 and pump laser 2, etc. of FIG. 1A, with the pump laser
being pulsed at appropriate large pulse energy and appropriate
short duration so as to produce single, short pulses with stable
amplitude at wavelength .lamda..sub.S. The pulse output from
gain-switched fiber laser 31 is directed into an optical fiber 33
and then is directed into the wavelength converter 34 so as to
generate laser emission at wavelength .lamda..sub.C. The wavelength
converter 34 preferably comprises any focusing optics needed to
direct the laser output into a nonlinear optical crystal. The
wavelength converter 34 preferably comprises a nonlinear laser
crystal such as PPLIN, KDP, KTP, BBO, ZGP, etc.
[0054] A fiber power amplifier 32 may be inserted between the
gain-switched fiber laser 31 and the wavelength converter 34 so as
to increase the power level of the laser pulse.
[0055] The wavelength converter 34 may be a second-harmonic
generator which generates laser emission with wavelength
.lamda..sub.S=.lamda..sub.GS/2. An important application for this
embodiment is a gain-switched ytterbium-doped fiber laser producing
laser pulse at a wavelength near 1.06 micrometer, with the second
harmonic generation at wavelength 530 nm, which effectively
produces a high power, fiber-based green laser.
[0056] The wavelength converter 34 may be an
optical-parametric-oscillator receiving the signal laser beam at
wavelength .lamda..sub.S, and an idler beam at wavelength
.lamda..sub.ID correlated with
1/.lamda..sub.S+1/.lamda..sub.id=.lamda..sub.GS. An important
application for this embodiment is a gain-switched thulium-doped
fiber laser generating laser pulses at a wavelength near 2
micrometers. The signal and idler wavelengths may be in the 2-10
micrometer region, providing a fiber-based mid-infrared laser
source.
[0057] An important application for fiber-based mid-infrared lasers
is in infrared optical countermeasures (IRCMs), which have become
powerful defense tools against IR-guided missiles. A fiber-based
IRCM source provides a transformative path to fulfill the needed
advancement in efficiency, compactness, robustness and transmission
flexibility. It is also frequently desirable for an IRCM laser to
have the capability for generating flexible waveforms including the
capability to mix and sweep the waveform modulation frequency. A
gain-switched fiber laser can fulfill this requirement using a
carrier pulse modulation scheme.
[0058] Referring to next to FIG. 4, through amplitude modulation,
the data waveform 42 is coded as an envelope function on the
carrier pulse train 41, resulting in a modulated carrier pulse
stream 43. The advantage of this scheme is that the parameters of
the carrier pulse and those of the data waveform are decoupled.
Using this modulation scheme as the pump pulse to gain-switch the
fiber laser, the data waveform is faithfully preserved; hence, this
approach can be used to directly encode the information data.
[0059] It is to be understood that the present invention is by no
means limited to the particular constructions herein disclosed
and/or shown in the drawings, but also comprises any modifications
or equivalents within the scope of the invention.
* * * * *