U.S. patent application number 12/854552 was filed with the patent office on 2011-08-25 for seed source for high power optical fiber amplifier.
This patent application is currently assigned to PyroPhotonics Lasers, Inc.. Invention is credited to Reynald Boula-Picard, Richard Murison, Tullio Panarello, Benoit Reid.
Application Number | 20110206076 12/854552 |
Document ID | / |
Family ID | 39635614 |
Filed Date | 2011-08-25 |
United States Patent
Application |
20110206076 |
Kind Code |
A1 |
Murison; Richard ; et
al. |
August 25, 2011 |
SEED SOURCE FOR HIGH POWER OPTICAL FIBER AMPLIFIER
Abstract
Methods and systems are provided to reduce stimulated Brillouin
scattering in high power optical fiber amplifiers. In an
embodiment, a seed source includes a narrow linewidth semiconductor
laser driven with a current ramp that simultaneously sweeps the
optical power and the lasing frequency at a rate fast enough to
reduce stimulated Brillouin scattering.
Inventors: |
Murison; Richard;
(St-Lazare, CA) ; Panarello; Tullio; (St-Lazare,
CA) ; Reid; Benoit; (Laval, CA) ;
Boula-Picard; Reynald; (Montreal, CA) |
Assignee: |
PyroPhotonics Lasers, Inc.
Dollard-des-Ormeaux
CA
|
Family ID: |
39635614 |
Appl. No.: |
12/854552 |
Filed: |
August 11, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12015427 |
Jan 16, 2008 |
7796654 |
|
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12854552 |
|
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60885604 |
Jan 18, 2007 |
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Current U.S.
Class: |
372/28 |
Current CPC
Class: |
B23K 26/0622 20151001;
G02F 2201/17 20130101; H01S 3/06754 20130101; H01S 5/0085 20130101;
H01S 3/2308 20130101; G02F 1/212 20210101; H01S 5/0057 20130101;
H01S 2301/03 20130101 |
Class at
Publication: |
372/28 |
International
Class: |
H01S 3/10 20060101
H01S003/10 |
Claims
1. A method of generating laser pulses, the method comprising:
directly modulating a semiconductor laser to generate a plurality
of optical pulses, each of the plurality of pulses having a
frequency chirp substantially larger than a Brillouin gain
bandwidth associated with an optical fiber; and externally
modulating each of the plurality of optical pulses to reduce a
duration of each of the plurality of optical pulses.
2. The method of claim 1 wherein externally modulating each of the
plurality of optical pulses is performed at a first time as each of
the plurality of optical pulses passes through a modulator a first
time and at a second time as each of the plurality of optical
pulses passes through a modulator a second time.
3. The method of claim 1 wherein each of the plurality of optical
pulses are characterized by a single frequency.
4. The method of claim 1 wherein each of the plurality of optical
pulses are characterized by a plurality of frequencies.
5. The method of claim 4 wherein a linewidth of each of the
plurality of frequencies is less than a Brillouin gain bandwidth of
about 30 MHz.
6. The method of claim 1 wherein the frequency chirp is larger than
1 GHz.
7. The method of claim 6 wherein the frequency chirp is larger than
about 10 GHz.
8. A method of generating laser pulses, the method comprising:
directly modulating a semiconductor laser to generate a plurality
of optical pulses, each of the plurality of pulses having a
frequency chirp substantially larger than a Brillouin gain
bandwidth associated with an optical fiber; amplifying the
plurality of optical pulses to generate a plurality of amplified
optical pulses; and externally modulating the plurality of
amplified optical pulses to reduce a temporal length of each of the
plurality of amplified optical pulses.
9. The method of claim 8 wherein each of the plurality of optical
pulses are characterized by a single frequency.
10. The method of claim 8 wherein each of the plurality of optical
pulses are characterized by a plurality of frequencies.
11. The method of claim 10 wherein a linewidth of each of the
plurality of frequencies is less than a Brillouin gain
bandwidth.
12. The method of claim 8 wherein externally modulating each of the
plurality of optical pulses is performed at a first time as each of
the plurality of optical pulses passes through a modulator a first
time and at a second time as each of the plurality of optical
pulses passes through a modulator a second time.
13. The method of claim 8 wherein the frequency chirp is larger
than about 1 GHz.
14. The method of claim 13 wherein the frequency chirp is larger
than about 10 GHz.
15. The method of claim 8 wherein amplifying the plurality of
optical pulses comprises passing the plurality of optical pulses
through an optically pumped optical fiber amplifier including a
length of rare-earth-doped optical fiber.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] The present application is a division of U.S. application
Ser. No. 12/015,427, filed Jan. 16, 2008; which claims benefit
under 35 U.S.C. .sctn.119(e) of U.S. Provisional Patent Application
No. 60/885,604, filed Jan. 18, 2007, the disclosures of which are
hereby incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to the field of
optical amplifiers and lasers. More particularly, the present
invention relates to a method and apparatus for providing high
power pulsed laser sources useful for various applications
including industrial applications such as trimming, marking,
cutting, and welding. Merely by way of example, the methods and
systems of the present invention have been applied to broaden the
apparent laser linewidth in order to reduce Stimulated Brillouin
Scattering. But it would be recognized that the invention has a
much broader range of applicability.
[0003] Conventional laser-based material processing has generally
used high peak power pulsed lasers, for example, Q-switched Nd:YAG
lasers operating at 1064 nm, for marking, engraving,
micro-machining, and cutting applications. More recently, laser
systems based on fiber gain media have been developed. In some of
these fiber-based laser systems, fiber amplifiers are utilized.
[0004] Some optical amplifiers and lasers utilizing a fiber gain
medium are optically pumped, often by using semiconductor lasers
pumps. The fiber gain medium is typically made of silica glass
doped with rare-earth elements. The choice of the rare-earth
elements and the composition of the fiber gain medium depend on the
particular application. One such rare-earth element is ytterbium,
which is used for optical amplifiers and lasers emitting in the
1020 nm-1100 nm range. Another rare-earth element used in some
fiber gain media is erbium, which is used for optical amplifiers
and lasers emitting in the 1530 nm-1560 nm range.
[0005] The wavelength of the optical pump source used for
ytterbium-doped fiber amplifiers and lasers is typically in the
wavelength range of 910 nm to 980 nm. The wavelength of the optical
pump source used for erbium-doped fiber amplifiers and lasers is
typically in a wavelength range centered at about 980 nm or about
1480 nm.
[0006] Because optical fibers usually have small diameters, they
tend to be prone to optical nonlinear effects degrading the
amplifier performance, especially in high peak power pulsed
operation. The nonlinear effect threshold depends on the optical
intensity, which is given by the ratio of the peak optical power to
the optical beam cross-section. Since, in a number of optical
fibers, the beam cross-section is small, the optical peak power to
reach the nonlinear threshold can also be small. Optical nonlinear
effects can also be present in bulk optical amplifiers using, for
example, rods as the gain media, but since the beam diameter is
much larger than in optical fibers, the optical peak power to reach
the nonlinear threshold is likewise much larger in bulk optical
amplifiers than in optical fibers.
[0007] One such optical nonlinear effect that has been observed to
limit the output power in an optical fiber amplifier is Stimulated
Brillouin Scattering (SBS). High intensity pulses in optical fibers
generate high frequency sound waves, which periodically modify the
index of refraction of the fiber, hence creating a moving Bragg
grating. Bragg gratings can be used to couple different beams of
light, one to another. In the particular case of SBS, the effect of
the moving Bragg grating is to couple the incident signal light to
a counter-propagating wave having a frequency down-shifted by the
frequency of the sound wave, which is typically .about.10 GHz. Wave
coupling results in energy being exchanged between the two waves.
If the intensity of the incident wave is high enough, SBS in effect
enables it to provide a very substantial amount of gain to the
counter-propagating wave. In real-world high-power fiber lasers,
the SBS mechanism can transfer large amounts of energy to a
counter-propagating wave seeded only by Rayleigh scattering or
amplified spontaneous emission, limiting the amount of
amplification in the signal wave that can be extracted.
[0008] Thus, there is a need in the art for improved methods and
systems related to high peak power fiber-based amplifiers.
SUMMARY OF THE INVENTION
[0009] According to the present invention, techniques related
generally to the field of optical amplifiers and lasers are
provided. More particularly, the present invention relates to a
method and apparatus for amplifying laser pulses to high power for
various applications including industrial applications such as
trimming, marking, cutting, and welding. Merely by way of example,
the invention has been applied to high peak power ytterbium-doped
fiber laser amplifiers. However, the present invention has broader
applicability and can be applied to other sources.
[0010] According to an embodiment of the present invention, a laser
source is provided. The laser source includes a seed source adapted
to generate a seed signal and an optical circulator having a first
port coupled to the seed source, a second port, and a third port.
The see source may emit a single frequency or a plurality of
frequencies. In a particular embodiment, the linewidth of each of
the plurality of frequencies is less than a Brillouin gain
bandwidth (e.g., less than about 30 MHz). The seed source may be a
semiconductor laser, a fiber laser, or a solid-state laser.
[0011] The laser source also includes an amplitude modulator
characterized by a first side and a second side. The first side of
the amplitude modulator is coupled to the second port of the
optical circulator. The laser source further includes a first
optical amplifier characterized by an input end coupled to the
second side of the amplitude modulator and a reflective end having
a spectral-domain reflectance filter and a second optical amplifier
coupled to the third port of the optical circulator. In a specific
embodiment, the laser source is adapted to provide a tunable,
pulsed output.
[0012] According to another embodiment of the present invention, a
method of providing a series of laser pulses is provided. The
method includes providing a seed signal at a first port of an
optical circulator, transmitting the seed signal to a first side of
an amplitude modulator, and time-domain filtering the seed signal.
The method also includes amplifying the pulse using a first optical
amplifier, frequency-domain filtering the amplified pulse, and
amplifying the frequency-domain filtered pulse using a second
optical amplifier. In a particular embodiment, the seed signal is
provided by a directly modulated semiconductor laser and includes
one or more optical pulses having a frequency chirp substantially
larger than a Brillouin gain bandwidth associated with an optical
fiber. In an embodiment, the method also includes externally
modulating the seed signal to generate one or more optical pulses,
each of the optical pulses having a frequency chirp substantially
larger than a Brillouin gain bandwidth associated with an optical
fiber.
[0013] According to yet another embodiment of the present
invention, a method of providing a series of laser pulses is
provided. The method includes directly modulating a semiconductor
laser to generate a seed signal including a plurality of optical
pulses. Each of the plurality of optical pulses has a frequency
chirp substantially larger than a Brillouin gain bandwidth
associated with an optical fiber. The method also includes
providing the seed signal to a first port of an optical circulator
and time-domain filtering the seed signal a first time with an
amplitude modulator optically coupled to the optical circulator.
The method further includes amplifying the time-domain filtered
seed signal in a double-pass optical amplifier optically coupled to
the modulator to provide a time-domain filtered seed signal and
time-domain filtering the amplified time-domain filtered seed
signal a second time with the amplitude modulator.
[0014] According to a specific embodiment of the present invention,
a method of generating laser pulses is provided. The method
includes directly modulating a semiconductor laser to generate a
plurality of optical pulses. Each of the plurality of pulses has a
frequency chirp substantially larger than a Brillouin gain
bandwidth associated with an optical fiber. The method also
includes externally modulating each of the plurality of optical
pulses to reduce a temporal length of each of the plurality of
optical pulses. The plurality of optical pulses may include a train
of pulses at a repetition rate of between about 0 and 100 kHz. Each
of the plurality of optical pulses may be characterized by a single
frequency or by a plurality of frequencies. In a particular
embodiment, a linewidth of each of the plurality of frequencies is
less than a Brillouin gain bandwidth of about 30 MHz.
[0015] According to another specific embodiment of the present
invention, a method of generating laser pulses is provided. The
method includes directly modulating a semiconductor laser to
generate a plurality of optical pulses. Each of the plurality of
pulses has a frequency chirp substantially larger than a Brillouin
gain bandwidth associated with an optical fiber. The method also
includes amplifying the plurality of optical pulses to generate a
plurality of amplified optical pulses and externally modulating the
plurality of amplified optical pulses to reduce a duration (e.g., a
temporal length) of each of the plurality of amplified optical
pulses. In a specific embodiment, optically pumping the optical
fiber amplifier includes receiving pump energy from one or a
plurality of semiconductor lasers. In another specific embodiment,
the rare-earth doped optical fiber includes at least one of
ytterbium, erbium, holmium, neodymium, thulium, or
praseodymium.
[0016] Numerous benefits are achieved using the present invention
over conventional techniques. For example, in an embodiment
according to the present invention, high peak power, pulsed lasers
suitable for laser-based processing are provided that utilize a
compact architecture that is inexpensive in comparison to lasers
with comparable performance characteristics. Moreover, in
embodiments of the present invention, short pulses are generated
with pulse characteristics that are tunable in real-time while
maintaining pulse-to-pulse stability. Furthermore, in another
embodiment according to the present invention, optical pulses can
be shaped to optimize the temporal pulse profile for the particular
application, or to maximize energy extraction efficiency in the
laser system. Depending upon the embodiment, one or more of these
benefits may exist. These and other benefits have been described
throughout the present specification and more particularly below.
Various additional objects, features and advantages of the present
invention can be more fully appreciated with reference to the
detailed description and accompanying drawings that follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a simplified schematic illustration of a high
power pulsed laser with tunable pulse characteristics using optical
fiber amplifiers according to an embodiment of the present
invention;
[0018] FIG. 2 is a simplified timing diagram illustrating
electrical and optical pulses at different locations in a high
power pulsed laser according to an embodiment of the present
invention;
[0019] FIG. 3 illustrates the optical pulse shape and chirp for a
conventional directly-modulated semiconductor laser seed;
[0020] FIG. 4 is a simplified schematic illustration of pulse
shapes generated using a seed source using a laser and an external
modulator according to an embodiment of the present invention;
[0021] FIG. 5 is a simplified schematic illustration of pulse
shapes generated using a seed source using a directly-modulated
semiconductor laser according to another embodiment of the present
invention;
[0022] FIG. 6 is a simplified schematic illustration of pulse
shapes generated using a seed source using a directly-modulated
semiconductor laser and an external modulator according to yet
another embodiment of the present invention;
[0023] FIG. 7 is a simplified schematic illustration of pulse
shapes generated using a seed source using an amplified
directly-modulated semiconductor laser and an external modulator
according to another embodiment of the present invention; and
[0024] FIGS. 8A and 8BB are simplified schematic illustrations of
high power pulsed lasers according to embodiments of the present
invention.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0025] FIG. 1 is a simplified schematic illustration of a high
power pulsed laser with tunable pulse characteristics using optical
fiber amplifiers according to an embodiment of the present
invention. The laser is configured to generate optical pulses. High
power pulsed laser 100 includes a continuous wave (CW) seed source
110 that generates a seed signal that is injected into a first port
114 of an optical circulator 120. According to an embodiment of the
present invention, the optical seed signal is generated by using a
seed source 110 that is a continuous wave (CW) semiconductor laser.
In a particular embodiment, the CW semiconductor laser is a fiber
Bragg grating (FBG) stabilized semiconductor diode laser operating
at a wavelength of 1032 nm with an output power of 20 mW. In
another particular embodiment, the CW semiconductor laser is an
external cavity semiconductor diode laser operating at a wavelength
of 1064 nm with an output power of 100 mW. In alternative
embodiments, the seed signal is generated by a compact solid-state
laser or a fiber laser. One of ordinary skill in the art would
recognize many variations, modifications, and alternatives.
[0026] After passing through the optical circulator 120, the seed
signal exits from a second port 122 of the circulator 120 and
impinges on a first side 132 of an optical amplitude modulator 130.
Circulators are well known in the art and are available from
several suppliers, for example, model OC-3-1064-PM from OFR, Inc.
of Caldwell, N.J.
[0027] The optical amplitude modulator 130 is normally held in an
"off" or low transmission state, in which the signal impinging on
the modulator is substantially not transmitted. According to
embodiments of the present invention, optical amplitude modulator
provides amplitude modulation and time-domain filtering of the seed
signal as well as filtering of the CW amplified spontaneous
emission (ASE). In a particular embodiment, the length of the
optical pulse is determined by the operation of the optical
amplitude modulator 130, which may be an APE-type Lithium Niobate
Mach-Zehnder modulator having a bandwidth >3 GHz at 1064 nm.
[0028] According to embodiments of the present invention, the
optical amplitude modulator 130 is an electro-optic Mach-Zehnder
type modulator, which provides the bandwidth necessary for
generating short optical pulses. In other embodiments, the optical
amplitude modulator 130 is a phase or frequency modulator with a
suitable phase or frequency to amplitude converter, such as an edge
optical filter, an extinction modulator, or an acousto-optic
modulator.
[0029] In order to pass the seed signal, the optical amplitude
modulator 130 is pulsed to the "on" state for a first time to
generate an optical pulse along optical path 136. The pulse width
and pulse shape of the optical pulse generated by the optical
amplitude modulator 130 are controlled via by the modulator drive
signal applied to the optical amplitude modulator 130. The optical
pulse then passes for a first time through a first optical
amplifier 150, where it is amplified. According to embodiments of
the present invention, the amplitude modulator, driven by a time
varying drive signal, provides time-domain filtering of the seed
signal, thereby generating a laser pulse with predetermined pulse
characteristics, including pulse width, pulse shape, and pulse
repetition rate.
[0030] According to an embodiment of the present invention, the
optical amplifier 150 is an optical fiber amplifier. Fiber
amplifiers utilized in embodiments of the present invention
include, but are not limited to rare-earth-doped single-clad,
double-clad, or even multiple-clad optical fibers. The rare-earth
dopants used in such fiber amplifiers include ytterbium, erbium,
holmium, praseodymium, thulium, or neodymium. In a particular
embodiment, all of the fiber-optic based components utilized in
constructing optical amplifier 150 utilize polarization-maintaining
single-mode fiber.
[0031] Referring to FIG. 1, in embodiments utilizing fiber
amplifiers, a pump 142 is coupled to a rare-earth-doped fiber loop
144 through optical coupler 140. Generally, a semiconductor pump
laser is used as pump 142. One of ordinary skill in the art would
recognize many variations, modifications, and alternatives. In
alternative embodiments, the optical amplifier 150 is a solid-state
amplifier including, but not limited to, a solid-state rod
amplifier, a solid-state disk amplifier or gaseous gain media.
[0032] In a particular embodiment, the optical amplifier 150
includes a 5 meter length of rare-earth doped fiber 144, having a
core diameter of approximately 4.1 .mu.m, and doped with ytterbium
to a doping density of approximately 4.times.10.sup.24
ions/m.sup.3. The amplifier 150 also includes a pump 142, which is
an FBG-stabilized semiconductor laser diode operating at a
wavelength of 976 nm, and having an output power of 100 mW in an
embodiment. In another particular embodiment, the pump 142 is a
semiconductor laser diode operating at a wavelength of about 915
nm. In yet another particular embodiment, the pump 142 is a
semiconductor laser diode operating at an output power of 450 mW or
more. In a specific embodiment, the amplifier 150 also includes a
pump to fiber coupler 140, which is a WDM pump combiner. In some
embodiments, the double-pass optical amplifier 150 is constructed
using rare-earth-doped optical fibers. Generally, the
rare-earth-doped optical fibers are excited by one or more
semiconductor pump lasers.
[0033] The signal emerging from optical amplifier 150 along optical
path 148 then impinges on a reflecting structure 146, and is
reflected back into optical amplifier 150. The signal passes for a
second time through optical amplifier 150, wherein the signal is
amplified. The reflecting structure 146 performs spectral domain
filtering of the laser pulse and of the amplified spontaneous
emission (ASE) propagating past optical path 148. Thus, the seed
signal experiences both amplitude and time-domain modulation
passing through amplitude modulator 130, and spectral-domain
filtering upon reflection from reflecting structure 146.
[0034] In an embodiment, the reflecting structure 146 is a fiber
Bragg grating (FBG) that is written directly in the fiber used as
the optical amplifier 150. The periodicity and grating
characteristics of the FBG are selected to provide desired
reflectance coefficients as is well known in the art. Merely by way
of example in a particular embodiment, the reflecting structure 146
is a FBG having a peak reflectance greater than 90%, and a center
wavelength and spectral width closely matched to the output of the
seed source 110.
[0035] The signal emerging from optical amplifier 150 along optical
path 136 impinges on the second side 134 of the optical amplitude
modulator 130, which is then pulsed to the "on" state a second time
to allow the incident pulse to pass through. According to
embodiments of the present invention, the timing of the second "on"
pulse of the optical amplitude modulator 130 is synchronized with
the first opening of the modulator 130 (first "on" pulse) to take
account of the transit time of the signal through the amplifier 150
and the reflecting structure 146. After emerging from the first
side of the optical amplitude modulator 130, the amplified pulse
then enters the second port 122 of optical circulator 120, and
exits from the third port 116 of optical circulator 120 along
optical path 148.
[0036] The signal is then amplified as it passes through a second
optical amplifier 160. As discussed in relation to FIG. 1,
embodiments of the present invention utilize a fiber amplifier as
optical amplifier 160, including a pump 154 that is coupled to a
rare-earth-doped fiber loop 156 through an optical coupler 152.
Generally, a semiconductor pump laser is used as pump 154, although
pumping of optical amplifiers can be achieved by other means as
will be evident to one of skill in the art. In a particular
embodiment, the second optical amplifier 160 includes a 5 meter
length of rare-earth doped fiber 156, having a core diameter of
approximately 4.8 .mu.m, and is doped with ytterbium to a doping
density of approximately 6.times.10.sup.24 ions/m.sup.3. The
amplifier 160 also includes a pump 154, which is an FBG-stabilized
semiconductor laser diode operating at a wavelength of 976 nm, and
having an output power of 500 mW. In another particular embodiment,
the second optical amplifier 160 includes a 2 meter length of
rare-earth doped fiber 156, having a core diameter of approximately
10 .mu.m, and is doped with ytterbium to a doping density of
approximately 1.times.10.sup.26 ions/m.sup.3. The amplifier 160 can
also includes a pump 154, which is a semiconductor laser diode
having an output power of 5 W.
[0037] In another particular embodiment, in order to pass the seed
signal, the optical amplitude modulator 130 is pulsed once instead
of twice. The optical amplitude modulator 130 is turned to the "on"
state to generate the rising edge of the pulse propagating along
optical path 136. This signal is then amplified a first time
through optical amplifier 150. The signal then impinges on the
reflecting structure 146 and is amplified a second time through
optical amplifier 150. Now the signal emerging from optical
amplifier 150 along optical path 136 impinges on the second side
134 of the optical amplitude modulator 130, which is subsequently
turned to the "off" state. The pulse width is therefore given by
the time duration during which the optical amplitude modulator 130
is held in the "on" state subtracted by the transit time of the
signal through the amplifier 150 and the reflecting structure
146.
[0038] Although FIG. 1 illustrates the use of a single optical
amplifier 160 coupled to the third port of the optical circulator
120, this is not required by the present invention. In alternative
embodiments, multiple optical amplifiers are utilized downstream of
the optical circulator 120 as appropriate to the particular
applications. One of ordinary skill in the art would recognize many
variations, modifications, and alternatives.
[0039] FIGS. 8A and 8B are simplified schematic illustrations of
high power pulsed lasers according to embodiments of the present
invention. In FIG. 8A, a high power pulsed laser 800 includes a
pulsed seed source 810 that generates a seed signal, which is
amplified by optical amplifier 820 The optical output pulse width
is determined by the seed source pulse width. In FIG. 8B, a high
power pulsed laser 850 includes a CW seed source 860 that generates
a seed signal and a first optical amplifier 862 to amplify the seed
signal. The high power pulsed laser 850 further comprises an
optical amplitude modulator 870 to generate optical pulses, which
are then amplified in a second optical amplifier 880. The optical
output pulse width is determined by the optical modulator, that is,
the optical output pulse width is determined by the electrical
pulse width applied to the modulator and is independent of the seed
signal. In some applications for the high power pulse laser
illustrated in FIG. 8B, the first optical amplifier is not
required, for example, if the seed power is high enough.
[0040] FIG. 2 is a simplified timing diagram illustrating
electrical and optical pulses at different locations in a high
power pulsed laser according to an embodiment of the present
invention. Merely by way of example, FIG. 2 illustrates the timing
of repetitive electrical drive signals to the amplitude modulator
and optical pulses propagating through an embodiment of the
invention as described in FIG. 1. Following an electrical trigger
210, a first electrical drive signal 220 is applied to the
amplitude modulator to generate an optical pulse 240. After some
propagation delay, the optical signal 250 passes through the
optical amplifier a first time. The optical signal 260 then
impinges on the reflecting structure and passes through the optical
amplifier a second time 250. The optical pulses 240 are transmitted
through the amplitude modulator a second time, which is driven
electrically a second time 220 with the optical pulses 240. Finally
the optical pulses 230 exit port 3 of the circulator after some
propagation delay.
[0041] Utilizing embodiments of the present invention, high power
pulsed laser sources are provided that generate streams of optical
pulses with independently adjustable pulse characteristics
including pulse width, peak power and energy, pulse shape, and
pulse repetition rate. Merely by way of example, a particular
embodiment of the present invention delivers output pulses at the
output 170 of second optical amplifier 160 of more than 5 .mu.J per
pulse at a pulse width of 10 ns and at a repetition rate of 10 kHz.
Of course, other pulse characteristics are provided by alternative
embodiments.
[0042] In the embodiments described above, the seed signal is
modulated to provide a pulsed seed signal. Providing a pulsed seed
signal minimizes ASE build-up caused by seed leakage and enables
the operating power range of the seed source to be increased. The
pulsed seed signal may be of a pulse width equal to or longer than
the desired pulse width of overall pulsed laser source. Pulsing the
seed can also increase the effective linewidth of the seed laser to
reduce Stimulated Brillouin Scattering (SBS). Utilizing
architecture similar to those presented in FIG. 1 and FIGS. 8A and
8B, streams of high power optical pulses can be generated. In high
power lasers constructed with doped optical fibers, the peak power
or energy per pulse is often limited by SBS. SBS strongly depends
on the signal linewidth propagating in the optical fiber. The SBS
gain is maximized for linewidth less than about 30-60 MHz and
decreases progressively (e.g., approximately linearly) with the
further widening of the linewidth. For example, for a laser
linewidth of about 1-30 MHz, the SBS gain is at or near a maximum
value, but for a linewidth of 30 GHz, the SBS gain is approximately
1000 times smaller. Therefore, to minimize SBS, embodiments of the
present invention utilize an optical signal with an effective broad
linewidth. Embodiments of the present invention are used to
generate optical frequency chirp during the optical pulses as a
means to obtain an effective broad linewidth.
[0043] In a semiconductor laser, a driving current waveform is
substantially transformed into an amplitude modulated optical
output waveform. Also, since the intrinsic optical properties of
semiconductor lasers depend on the injection current and the
instantaneous optical power, the optical carrier frequency is
modulated as well, which is called chirp, according to:
v ( t ) = .alpha. 4 .pi. [ 1 P ( t ) P ( t ) t + kP ( t ) ] . ( 1 )
##EQU00001##
The first term in equation (1) is called the transient chirp and is
determined by the rate of change of the optical power (e.g., the
rate of temporal variation of the optical power). The second term
in equation (1) is called adiabatic chirp and is determined by the
optical power. Equation (1) provides that the optical carrier
frequency will be modified ("chirped") by any time dependency
present in the optical power. Transient chirp may also be generated
in external optical modulators, for example, Mach-Zehnder
modulators, or electro-absorption modulators. Generally, adiabatic
chirp is not generated in external optical modulators. The unitless
parameter .alpha. is called the "chirp parameter" or the linewidth
enhancement factor and can usually have a value in the range -1 to
+5. The parameter k can typically have a value in the range of 2-6
GHz/mW.
[0044] Reference is now made to FIG. 3, illustrating the optical
pulse shape and chirp for a conventional directly-modulated
semiconductor laser. Curve 11 illustrates the output optical power
waveform and is very similar to the current driving waveform. Curve
12 illustrates the total chirp given by the sum of the transient
chirp 13 and the adiabatic chirp 14. In the case of an external
optical modulator used to modulate the output power of a laser with
a waveform similar to curve 11, only the transient chirp 13 is
present.
[0045] The Continuous Wave (CW) output spectrum of the laser source
can consist of a single carrier frequency, as found in Distributed
Feedback (DFB) lasers, or a plurality of carrier frequencies, as
found in Fabry-Perot or ring lasers. When a plurality of carriers
is present, the chirp modulates each of them individually.
[0046] A chirped or modulated carrier frequency means that the
numerical value of the carrier frequency varies during the pulse.
For example, referencing once again curve 12 of FIG. 3, it may be
possible that the carrier frequency between the center and the edge
of the pulse varies by 10 GHz. In this particular example, it is
noted that the carrier frequency in the center of the pulse does
not substantially change. Therefore, most of the pulse energy is
characterized by a fixed carrier frequency and the carrier
frequency variations at the rising and falling edges of the pulse
affect a substantially small fraction of the total energy of the
pulse.
[0047] To minimize Stimulated Brillouin Scattering (SBS) in optical
fibers, especially when signals are amplified to high peak power in
optical fiber amplifiers, it would be advantageous if the carrier
frequency would change during that part of the optical pulse
wherein most of the energy is contained. In other words, a useful
source characterized by reduced SBS would have substantially all of
its energy distributed at different carrier frequencies at
different times.
[0048] A high peak power signal in an optical fiber can lose energy
through SBS to a back traveling parasitic signal at a carrier
frequency approximately 10 GHz away. This back traveling parasitic
signal can originate from spontaneous emission in an excited doped
fiber or from spontaneous Brillouin scattering. The gain bandwidth
of this process is about 30 MHz. In other words, each frequency in
the forward propagating input signal can lose energy to a
back-traveling wave at a carrier frequency of 10 GHz+/-15 MHz. If
the input signal's optical carrier frequency is fixed most of the
time, the back-traveling signal may be amplified during the whole
pulse duration, which can be expressed as an amplification length.
For example, if the pulse has a 20 ns duration, the SBS signal
would be amplified and rob energy from the input signal during
about a 20 ns interaction, or approximately a 4 m length of the
fiber.
[0049] However, if the input signal's optical carrier frequency
varies with time because of chirping, then any given frequency band
of the parasitic SBS signal will only be amplified during that time
over which the input signal's carrier frequency is about 10 GHz
away. The total amplification may therefore be significantly
smaller than in the fixed carrier frequency case. Each frequency of
the input signal only provides SBS gain for a corresponding
frequency in a back-traveling signal. For example, if the carrier
frequency were chirped at a rate of 1 GHz/ns, then each 30 MHz band
of the input signal will only amplify the SBS signal during a 30 ps
window, or a 6 mm interaction length. The same 20 ns pulse as in
the previous example would only amplify the SBS signal for 6 mm
instead of 4 m.
[0050] Consider an input signal whose spectrum had an instantaneous
spectral width larger than about 30 MHz, and which was chirped. In
this case a parasitic signal with a spectral width of 30 MHz or
less could be amplified by more than one 30 MHz spectral band of
the input signal as the chirping process brings other 30 MHz
portions of the spectrum into alignment (offset by 10 GHz) with the
spectrum of the parasitic signal. If the parasitic signal had a
spectral width greater than 30 MHz, the above argument would apply
equally to each 30 MHz-wide component of the parasitic signal.
However this effect can be canceled out by the lower spectral power
density of a signal having a larger spectral bandwidth for a given
total power, since Brillouin gain coefficient is proportional to
the product of the spectral power densities of the signal and
parasitic beams, measured over two 30 MHz linewidths separated by 1
GHz. Nonetheless to minimize SBS, it is advantageous to limit the
input signal spectral width to a value of the order of the SBS gain
bandwidth or less. This could be achieved with DFB lasers.
Alternatively, the signal power can also be spread over a plurality
of carrier frequencies, as generated by Fabry-Perot lasers, and
each carrier frequency would advantageously have a width less than
the SBS gain bandwidth.
[0051] Consequently, embodiments of the present invention provide a
method to minimize the effects of SBS on high peak power pulses in
optical fibers. The method includes the generation of an optical
signal having a frequency chirp during the portion of the pulses
containing the highest energy instead of having the frequency chirp
located during times with low energy. The inventors have done
numerous simulations and have discovered that a frequency chirp
rate higher than 30 MHz/ns provides significant benefits.
Accordingly, some embodiments include a frequency chirp rate higher
than 1 GHz/ns. Additionally, the inventors have discovered that
having a total chirp of at least 1 GHz and preferably 10 GHz
provides advantages not available using other conventional
techniques. The examples that follow are not intended to limit the
scope of embodiments of the present invention, but to merely
describe various methods and systems in accordance with embodiments
of the present invention.
[0052] Reference is now made to FIG. 4, which illustrates a first
embodiment of a seed source using a CW laser 21 and an external
amplitude modulator 22 receiving CW light from the laser. The CW
laser can be a fiber laser, a semiconductor laser, a solid-state
laser, or the like. The external modulator can be a Lithium Niobate
March-Zehnder modulator with a chirp parameter different than zero.
The modulator is used both for generating the optical pulse and for
chirping the optical carrier frequency. The modulator's chirp
parameter and the drive waveform 23 are chosen appropriately such
that the optical pulse 24 with a peak power of 100 mW has a chirp
25, comprising the transient chirp in this case, varying at the
pulse peak with a slope of about 0.1 GHz/ns. Also in this example,
the chirp is more than 1 GHz over a period of time containing most
of the pulse energy. The appropriate optical waveform resulting in
the targeted chirp could be calculated with the help of equation
(1). Then the drive waveform can be determined using the known
modulator's transfer function.
[0053] Reference is now made to FIG. 5, which illustrates a second
embodiment of a seed source using a directly-modulated
semiconductor laser 31. In contrast with external modulators, a
semiconductor laser also possesses an adiabatic chirp component
that can be exploited to generate a chirp waveform that varies
almost linearly across an optical pulse. The drive current waveform
32 is chosen appropriately to generate an optical pulse 33 and a
chirp 34 that varies almost linearly across the pulse. In
semiconductor lasers, the output optical pulse and therefore the
adiabatic chirp follow closely the drive current waveform. Other
variations are also possible, but the illustrated embodiment
provides chirp that varies at a fast rate, approximately 9 GHz/ns
in this example, around the pulse peak power of 100 mW. Also, the
chirp varies by several tens of GHz across the pulse. A further
advantage of such a pulse shape is that it can compensate in part
for pulse distortion happening in high power fiber amplifiers. Such
a distortion can happen when a fast leading edge pulse extracts the
stored energy in an amplifier too rapidly. A slower leading edge
can be employed to ameliorate this effect.
[0054] Reference is now made to FIG. 6, which illustrates a third
embodiment of a seed source using a directly-modulated
semiconductor laser 41 and a modulator to generate optical pulses.
The drive current waveform 46 is chosen appropriately to generate
an optical pulse 48 and a chirp 47 that varies almost linearly
across the pulse as described in FIG. 5. Other variations are also
possible, but the illustrated embodiment provides chirp that varies
at a fast rate, approximately 9 GHz/ns in this example, around the
pulse peak power of 100 mW. Also, the chirp varies by several tens
of GHz across the pulse. The pulse generated by the
directly-modulated semiconductor laser has a width larger than the
intended output pulse width.
[0055] An external modulator 42, driven by voltage waveform 43,
receives this pulse and slices a narrower pulse out of it as
illustrated by pulse 45. In this example, the semiconductor laser
pulse width is approximately 40 ns and the modulator slices it to
about 10 ns. The external modulator thus serves as a gate to the
initially generated pulse from the directly-modulated laser. As
explained in reference to FIG. 4, most external modulators would
also generate chirping on the optical carrier frequency. This
frequency chirp would be added to the chirp generated by the
directly-modulator semiconductor laser. This effect can be used to
increase or decrease the total chirp in the output signal according
to the type of modulator used. In other embodiments, the external
modulator is advantageously chirpless to minimize the impact of the
modulator on the output frequency chirp. A chirpless modulator can
be produced for example by using a Mach-Zehnder modulator in a
push-pull modulation technique, such that only the chirp produced
by the directly-modulated semiconductor laser is gated at the
output of the seed source 44.
[0056] Methods and systems for generating a pulsed seed source are
especially attractive when the directly-modulated semiconductor
laser suffers from significant ringing at the rising or falling
edges of the optical pulses. The ringing depends on the rate of
change of the rising or falling edges and also on the semiconductor
laser design. While the ringing may be associated with advantageous
amounts of chirp, the associated amplitude variations may be
difficult or impossible to compensate for elsewhere in the system.
The external modulator may therefore be used to remove the ringing
at the edges and to produce cleaner pulses. For example, the
directly-modulated semiconductor laser pulse width can be larger
than 100 ns, or larger than 1000 ns, even though the desired output
pulse width is much smaller, of the order of 5 to 50 ns. The larger
1000 ns width could be used advantageously for applications
requiring multiple closely spaces pulses. In this example, the
multiple pulses could be gated with the modulator during a single
directly-modulated semiconductor laser pulse. As an example, an
application could require that three 10 ns pulses be generated with
100 ns spacing between them for a total pulse train of 230 ns.
Advantageously, a semiconductor laser pulse width larger than 230
ns, for example 1000 ns or more, could be used.
[0057] Reference is now made to FIG. 7, which illustrates a fourth
embodiment of a seed source using a directly-modulated
semiconductor laser 51 and an external modulator to generate
optical pulses, which could have higher power than illustrates in
FIG. 6. The drive current waveform 56 is chosen appropriately to
generate an optical pulse 58 and a chirp 57 that varies almost
linearly across the pulse. Other variations are also possible, but
the illustrated embodiment provides a chirp that varies at a fast
rate, approximately 9 GHz/ns in this example, around the pulse peak
power of 100 mW. Also, the chirp varies by several tens of GHz
across the pulse. The pulse generated by the directly-modulated
semiconductor laser has a width larger than the intended seed
source pulse width. An optical amplifier 59 receives the pulse and
amplifies it to high peak power. The optical amplifier can be an
optical fiber amplifier. An external modulator 52, driven by
voltage waveform 53, receives this pulse and slices a narrower
pulse out of it as illustrated by pulse 55. The external modulator
thus serves as a gate to the initially generated pulse from the
directly-modulated laser. In a specific embodiment, the external
modulator is chirpless, as can be produced for example by using a
Mach-Zehnder modulator in a push-pull modulation technique, such
that only the chirp produced by the directly-modulated
semiconductor laser is gated at the output of the seed source
54.
[0058] According to various embodiments of the present invention,
methods and systems adapted to reduce or minimize SBS in optical
fiber are provided. The illustrated embodiments may be used to
inject trains of optical pulses in one or a plurality of optical
amplifiers to amplify these pulse trains to high peak power. The
optical amplifiers can include lengths of rare-earth-doped optical
fiber and optical pumping structures. Such a configuration is
sometimes denoted a Master Oscillator Power Amplifier (MOPA).
Furthermore, embodiments have been illustrated for convenience
without limitation in the context of the MOPA described in FIGS. 2,
but are readily applicable to any other MOPA configurations
especially as described by FIG. 1. In a double-pass configuration
as illustrated in FIG. 1, chirping and gating could be achieved in
any of the first or second pass as those experienced in the art
would recognize.
[0059] According to some embodiments of the present invention,
methods and systems are provided that result in the generation of
sequences of optical pulses, which may not be equally separated in
time. Moreover, the pulse widths and pulse energies are
individually tailored in a predetermined manner from pulse to
pulse. Furthermore, it will be recognized that although the above
description discussed the generation of a single optical pulse,
embodiments of the present invention provide for the generation of
multiple pulses by repeating the single pulse a multiplicity of
times. These multiple pulses may include an arbitrary train of
optical pulse sequences.
[0060] While the present invention has been described with respect
to particular embodiments and specific examples thereof, it should
be understood that other embodiments may fall within the spirit and
scope of the invention. The scope of the invention should,
therefore, be determined with reference to the appended claims
along with their full scope of equivalents.
* * * * *