U.S. patent application number 15/017661 was filed with the patent office on 2016-10-06 for optical pulse conditioning apparatus and methods for conditioning optical pulses.
The applicant listed for this patent is Fianium Ltd.. Invention is credited to Paulo Almeida, John Redvers Clowes, Christophe Codemard, Pascal Dupriez, Anatoly Borisovich Grudinin.
Application Number | 20160294147 15/017661 |
Document ID | / |
Family ID | 47883610 |
Filed Date | 2016-10-06 |
United States Patent
Application |
20160294147 |
Kind Code |
A1 |
Clowes; John Redvers ; et
al. |
October 6, 2016 |
Optical Pulse Conditioning Apparatus and Methods for Conditioning
Optical Pulses
Abstract
A pulse conditioning apparatus can comprise a signal splitter
for splitting a pulsed input signal into first and second pulsed
signals; a spectrally dispersive element configured for subjecting
the first pulsed signal to a spectral dispersion for compressing or
stretching the first pulsed signal, the first and second pulsed
signals being subjected to substantially different spectral
dispersion; and a combiner for combining the first and second
pulsed signals to an output pulsed signal, the output pulsed signal
output comprising pulses having a surge pulse portion and a base
pulse portion, at least one of the portions derived at least in
part from the spectral dispersion and attendant compression or
stretching of the first pulsed signal.
Inventors: |
Clowes; John Redvers; (NEW
MILTON, GB) ; Almeida; Paulo; (SOUTHAMPTON, GB)
; Grudinin; Anatoly Borisovich; (SOUTHAMPTON, GB)
; Dupriez; Pascal; (Leognan, FR) ; Codemard;
Christophe; (EASTLEIGH, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fianium Ltd. |
Southampton |
|
GB |
|
|
Family ID: |
47883610 |
Appl. No.: |
15/017661 |
Filed: |
February 7, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14186763 |
Feb 21, 2014 |
9300105 |
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15017661 |
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PCT/US2012/052176 |
Aug 24, 2012 |
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14186763 |
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61644424 |
May 8, 2012 |
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61527042 |
Sep 14, 2011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 3/0057 20130101;
H01S 3/0078 20130101; H01S 3/06745 20130101; H01S 3/06725 20130101;
H01S 3/094007 20130101; H01S 3/0675 20130101; H01S 3/067 20130101;
G02B 27/283 20130101; H01S 3/09415 20130101; H01S 3/005 20130101;
B23K 26/0624 20151001; H01S 3/06754 20130101; H01S 3/08013
20130101; B23K 26/36 20130101; H01S 3/094003 20130101; H01S 3/1618
20130101; H01S 3/1118 20130101; H01S 3/094011 20130101; H01S 3/2308
20130101 |
International
Class: |
H01S 3/00 20060101
H01S003/00; H01S 3/094 20060101 H01S003/094; G02B 27/28 20060101
G02B027/28; H01S 3/11 20060101 H01S003/11; H01S 3/16 20060101
H01S003/16; H01S 3/067 20060101 H01S003/067; H01S 3/0941 20060101
H01S003/0941 |
Claims
1. A pulse conditioning apparatus, comprising: a signal splitter
for splitting a pulsed input signal into first and second pulsed
signals; a spectrally dispersive element configured for subjecting
the first pulsed signal to a spectral dispersion for compressing or
stretching the first pulsed signal, the first and second pulsed
signals being subjected to substantially different spectral
dispersion; a combiner for combining the first and second pulsed
signals to an output pulsed signal, the output pulsed signal
comprising pulses having a surge pulse portion and a base pulse
portion, at least one of the portions derived at least in part from
the spectral dispersion and attendant compression or stretching of
the first pulsed signal.
2. The pulse conditioning apparatus of claim 1 comprising a
polarization beam splitter, said signal splitter and said beam
combiner each comprising said polarization beam splitter.
3. The pulse conditioning apparatus of claim 1 comprising a
polarization controller optically upstream of the signal splitter
for allowing the polarization state of the pulsed input signal to
be changed by a user.
4. The pulse conditioning apparatus of claim 1 comprising a time
delay apparatus for introducing a time delay to the second pulsed
optical signal.
5. The pulse conditioning apparatus of claim 1 wherein said
spectrally dispersive element is configured for compressing the
first pulsed signal.
6. A method of conditioning pulses, comprising: splitting an input
signal comprising input pulses into first and second pulsed
signals; subjecting the first and second one of the first and
second pulsed signals to spectral dispersion so as to temporally
compress or stretch pulses of the one of the first and second
signals, the first and second pulsed signals undergoing
substantially different spectral dispersion; and combining the
first and second pulsed signals to form an output signal comprising
pulses having a surge pulse portion and a base pulse portion, at
least one of the portions derived at least in part from the
spectral dispersion and attendant compression or stretching of the
one of the first and second pulsed signals.
7. The method of claim 6 comprising changing the state of
polarization of pulses of the input signal.
8. The method of claim 6 comprising varying the polarization state
of the input signal so as to vary the relative optical energies of
the surge and base pulse portions of the output pulses.
9. The method of claim 6 comprising varying a time delay of one of
the first and second pulsed signals so as to change the temporal
relationship of the surge and base pulse portions of the output
pulses.
10. The method of claim 6 comprising varying the optical energy of
pulses of at least one of the first and second pulsed signals so as
to vary the optical energy of at least one of the surge and base
pulse portions.
11. The method of claim 6 wherein at least one of the splitting and
combining comprises using polarization discrimination.
12. The method of claim 6 wherein both of the splitting and
combining comprise using polarization discrimination.
13. The method of claim 6 comprising wherein splitting comprises
splitting the input signal into at least two beam paths with a
polarization beam splitter.
14. The method of claim 13 comprising changing the state of
polarization of the input signal prior to splitting the input
signal with the polarization beam splitter.
15. The method of claim 6 comprising refraining from subjecting one
of the first and second signals to any substantial spectral
dispersion.
16. The method of claim 6 wherein subjecting one of the first and
second signals to spectral dispersion comprises compressing pulses
of the one of the first and second signals.
17. The method of claim 6 wherein subjecting one of the first and
second signals to spectral dispersion comprises stretching pulses
of one of the signals.
18. The method of claim 6 wherein the surge pulse portion comprises
a substantially different state of polarization than a state of
polarization comprised by the base pulse portion.
19. The method of claim 6 wherein the state of polarization of the
surge pulse portion is substantially the same as the state of
polarization of the base pulse portion.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is continuation of U.S. patent
application Ser. No. 14/186,763, accorded a filing date of 24 Aug.
2012 which is a continuation of International Patent Application
No. PCT/US2012/052176, bearing an International Filing Date of 24
Aug. 2012, which in turn claims priority to U.S. Provisional Patent
Application No. 61/527,042, accorded a filing date of 14 Sep. 2011,
and to U.S. Provisional Patent Application No. 61/644,424, accorded
a filing date of 8 May 2012. Each of the foregoing applications is
incorporated by reference herein.
FIELD OF THE DISCLOSURE
[0002] The present disclosure relates to optical apparatus, such as
apparatus for providing optical pulses, and methods for providing
optical pulses.
BACKGROUND OF THE DISCLOSURE
[0003] Lasers based on optical fiber, whether in whole or in part,
can provide more flexible, rugged and relatively simple sources of
optical energy. For example, an optical fiber laser can have a
smaller footprint, or can be more efficient, or can require less
sophisticated cooling arrangements as compared to, for example, a
gas based laser, particularly where gain media, whether amplifiers
or oscillators, use fiber as the gain medium. Considerable
technical effort and development has been focused on the production
and use of nanosecond ("ns") and femtosecond ("fs") and laser
pulses for materials processing, and particularly on fiber lasers
for producing such pulses for materials processing.
[0004] Q-switched lasers, and in particular Q-switched fiber
lasers, readily produce ns pulses with technology that is becoming
relatively well understood and that can be relatively inexpensively
manufactured so as to be reliable and dependable, even in a harsh
production environment. However, the pulses can be longer than
desired, and result in more energy than necessary being delivered
to a work piece, which can cause an undesirable "heat affected
zone" (HAZ) and damage.
[0005] Fs pulses can feature high peak power and a short time
duration that can facilitate removing, such as by ablation, a
material without creating as much, or any, HAZ that can damage the
material. Fs pulses, however, can be considerably more difficult to
produce and can require considerably more complex technology. For
example, amplifying such fs pulses directly can induce undesirable
nonlinear effects, particularly in a fiber amplifier where the peak
power is high due to the short time duration of the pulse and where
the optical intensity, which triggers the nonlinear effects, is
also high due to the relatively small cross sectional area of the
typical optical fiber. Certain nonlinear effects, such as
Stimulated Brillouin Scattering (SBS) or Stimulated Raman
Scattering (SRS), simply prevent providing higher output powers at
the desired wavelengths. Accordingly, fs fiber lasers often use
Chirped Pulse Amplification (CPA) to avoid triggering nonlinear
effects such as SBS and SRS. In a typical CPA system a fs seed
pulse is stretched via a pulse stretcher that provides linear
chirp, and then the stretched, linearly chirped pulse, which has
lowered peak power and optical intensity, is linearly amplified and
then compressed using a linearly chirped pulse compressor to a fs
pulse having high peak power. The compressed pulse can be
substantially transform limited. The CPA technique works well, but
its implementation can be technically complex and typically uses
bulky, free space components (e.g., a bulk grating pair stretcher
as well as a similar compressor). A fs CPA system can be large
(several times larger than a ns system) as well as costly (e.g.,
more than 10 times the price of a ns system). Though less prevalent
than CPA systems, it is also known to produce fs output pulses
starting from picoseconds (e.g., 10 ps) seed pulses that are
directly generated from a mode locked fiber laser and subsequently
amplified by a fiber amplifier that adds spectrum via self phase
modulation while producing an amplified pulse of nJ or greater
pulse energy. A low spectral dispersion diffraction grating pair
compressor (e.g., 0.5 ps/nm-1 ps/nm) compresses the amplified
pulses to about 300 fs.
[0006] Picosecond ("ps") pulsed laser systems are also known, and
can provide for certain materials processing an attractive
compromise between the too long ns pulses and the short, but often
complicated to produce, fs pulses. Ps pulses can be directly
produced by a ps seed oscillator, which can be a fiber laser, and
amplified directly by fiber laser amplifiers to provide ps output
pulses without resorting to a complicated CPA stretcher--compressor
arrangement. This simplicity can be seen as a desirable feature in
comparison to fs systems, and helps reduce the size, price or
complexity of a ps laser system as opposed to a fs laser
system.
[0007] Temporally shaped pulses can also be of interest,
particularly in the case of ns pulses, where having too high a
pulse power for too long can create heating issues, as indicated
above. Temporally shaping ns pulses often involves control
circuitry controlling an external modulator, which adds cost and
complexity to the system.
[0008] Applicants, however, have found that there can be drawbacks
associated with the direct production of ps output pulses,
particularly when using lasers using optical fiber. Applicants are
also aware that shaped ps pulses can be of interest and that
drawbacks associated with certain methods of producing shaped
pulses should be minimized or avoided. Accordingly, it is an object
of the present disclosure to provide improved methods and apparatus
for making and using ps pulses.
[0009] Other objects will be apparent from a study of the remainder
of the present disclosure, including the drawings and claims.
SUMMARY OF THE DISCLOSURE
[0010] The present disclosure teaches, in certain practices,
methods and apparatus pertaining to a pulsed fiber laser that uses
a relatively high spectral dispersion for compression in
combination with a relatively low compression ratio. Systems known
in the art, such as conventional fs CPA architectures, teach that
an opposite approach is advantageous, namely, relatively low
spectral dispersion and relatively high compression ratio.
Furthermore, in the ps regime, a ps pulse can often simply be
directly produced, such as by fiber delivered pulsed seed laser
(FDPSL), or other seed laser, and amplified by a fiber amplifier or
fiber amplifier cascade, that can include LMA fibers for reducing
optical intensity and hence SPM and its attendant spectral
broadening, without the use of one or more of stretching,
compression, significant spectral broadening, etc. LMA fibers can
have a V-number greater than 2.405 at the wavelength of operation
(and hence be multimode), which results in ever lower intensity for
the fundamental mode. However, Applicants have found that the
methods and apparatus taught herein, which can be more complex than
some of the prior art teachings and which run counter to
conventional wisdom, can have advantages.
[0011] For example, following the above more conventional approach
can result in a ps pulsed fiber laser wherein the output power
degrades over time, particularly in the instance of higher output
power (e.g., higher peak power) pulsed fiber lasers. This
phenomenon is not well understood, and may involve photodarkening
in one or more gain fibers, which itself can be a complex
phenomenon whose dynamics are different in different regimes (i.e.,
continuous wave operation, ns pulse operation, ps pulse operation,
fs operation, etc.). Applicants have found that a ps pulsed fiber
laser as taught in embodiments herein can reduce a drop in output
power over time, even when the gain fiber has higher intensity,
such as by being single mode. The single mode fiber need not
necessarily comprise an LMA fiber, and can have a smaller core
diameter and/or a higher NA. If LMA fiber is used, cores can have
smaller diameters and/or higher NAs. The use of single mode fiber,
including single mode LMA fiber (or less multimode LMA fiber) can
improve beam quality (usually specified in terms of the M.sup.2
parameter of an output beam) or reduce or avoid the need for
techniques such as coiling fiber to selectively introduce higher
bend loss for higher order modes in multimode gain fibers, such as
multimode LMA gain fibers. A pulsed fiber laser of the invention
can include nonlinear amplification prior to compression, where the
nonlinear amplification results in spectral broadening. The
spectral broadening can be useful in compressing the pulse.
[0012] Also taught herein are methods and apparatus concerning the
generation and use of pulses, such as ps pulses, having a temporal
power profile having a base pulse portion and a surge pulse
portion. Such pulses can, in some practices of the disclosure, be
generated at least in whole or in part via what according to the
prior art would be considered a "flawed" compression process (e.g.,
incomplete compensation during compression of non-linear chirp), as
it produces "flawed" pulses-pulse that not "clean" (e.g., are not
transform or very near transformed limited). Nevertheless,
according to some practices of the disclosure, such "flawed"
production process and pulses, particularly in the case of ps
pulses, are considered to be of beneficial use, such as, for
example, in materials processes. Pulses having surge and base pulse
portion can be generated via "incomplete" compression, such as due
to non-compensated non-linear chirp generated, which can be
generated via nonlinear amplification, or via other processes, such
as, for example, diving a pulse into portions, applying different
amounts of spectral dispersion to the portion prior to
recombination into an output path.
[0013] Method and apparatus pertaining to pulse conditioning, which
can produce pulses having base and surge pulse portions, are also
taught herein and are described in more detail below.
[0014] More particularity, in one aspect, the present disclosure
teaches a pulsed fiber laser apparatus for outputting optical
pulses, such as picosecond (ps) optical pulses. The pulsed fiber
laser can comprise a pulsed seed laser, such as a fiber delivered
pulse seed laser (FDPSL), for providing optical seed pulses (e.g.,
ps optical seed pulses), at least one optical amplifier (e.g., a
fiber amplifier having a gain optical fiber) in optical
communication with the pulsed seed laser, and a pulse compressor
apparatus in optical communication with the at least one optical
amplifier (and hence with the FDPSL). The at least one optical
amplifier can operate in a nonlinear regime wherein the optical
pulses are spectrally broadened during amplification thereof, such
as by, at least in part, self phase modulation. The optical pulses
can be broadened by at least selected broadening factor (e.g., by a
factor of at least 8). The pulse compressor apparatus can be
configured so as to provide a spectral dispersion of at least 50
ps/nm and a compression ratio of no greater than about 50, where
the compression ratio refers to the ratio of the time duration of
the optical pulses (e.g., ps optical pulses) received by the pulse
compressor apparatus to the time duration of the compressed optical
pulses (e.g., compressed pulses having ps time duration).
[0015] The optical amplifier can receive picosecond optical pulses
having a time duration that is at least about 20 ps, or in another
example, at least about 40 ps. The optical amplifier can be
configured such that the time duration of the optical pulses does
not substantially change during optical amplification, such as
during optical amplification by the gain optical fiber of an
optical fiber amplifier. The pulse compressor apparatus can provide
compressed ps optical pulses having a time duration of no less than
2 ps. The pulse compressor apparatus can provide compressed ps
optical pulses having a pulse energy of at least 5 .mu.J.
[0016] The optical seed pulses can have a bandwidth of not greater
than about 0.5 nm. The time duration of the optical pulses received
by the optical amplifier can be substantially the same as the time
duration of the optical seed pulses. The time durations can also be
different, such as by the pulsed fiber laser apparatus comprising a
pulse stretcher optically downstream of the seed laser (e.g., a
FDPSL) and the fiber amplifier for providing temporal stretching.
For example, the pulse stretcher can be located optically upstream
of an amplifier, including being located between amplifiers (e.g.,
downstream of one and upstream of another). The seed laser can be
configured and arranged such that the optical seed pulses have a
wavelength of about 1 .mu.m
[0017] In some practices of the disclosure, the pulsed fiber laser
apparatus does not include any pulse stretching that increases the
duration of a pulse by more than a factor of 20, or by more than a
factor 10, or by more than factor of 5, or more than by 50%, 20% or
10%. Percentage change can be measured by subtracting the time
duration of the shorter pulse from that of the longer pulse and
dividing by the time duration of the shorter pulse.
[0018] The FDPSL can comprise a fiber-based master oscillator, the
master oscillator being fiber based at least in that it comprises a
laser cavity comprising a length of rare earth doped (RED) optical
fiber. The RED optical fiber can be operated in a normal dispersion
regime (i.e., provide normal dispersion at the wavelength of the ps
optical seed pulses). The RED optical fiber can be doped with
ytterbium and provide ps optical seed pulses in the 1000 nm range
(e.g., 1060 nm), and typically in this case the RED fiber has
normal dispersion. The RED optical fiber can operate in the
anomalous dispersion range, and typically in this case the
wavelength of the ps seed pulses will be different (e.g., in the
1550 nm range) and the RED optical fiber is doped with a different
rare earth (e.g., erbium), perhaps in conjunction with the
ytterbium dopant. The laser cavity can comprise a mode locked laser
cavity. The mode locking can comprise passive mode locking. The
mode locked laser cavity can comprise a SESAM mode locking element.
As other examples, the seed laser can comprise a gain switched or
externally modulated laser diode or a diode pumped solid state
(DPSS) laser, any of which can comprise a FDPSL.
[0019] In one practice of the invention, the optical fiber
amplifier comprises substantially only single mode gain fiber. The
pulsed fiber laser apparatus can be configured and arranged such
that the fiber amplifier comprises an optical gain fiber having a
fundamental mode having a mode field diameter (MFD) of not greater
than about 14 .mu.m. The pulsed fiber laser apparatus can be
configured and arranged such that the fiber amplifier comprises an
optical gain fiber comprising a MFD of less than about 25 microns
for at least a location along the gain optical fiber. The pulsed
fiber laser apparatus can be configured and arranged such that for
any optical fiber amplifier optically upstream of the compressor
the MFD does not exceed about 14 .mu.m, and/or can be configured
and arranged such that the pulsed fiber laser apparatus does not
comprise a fiber amplifier having a gain optical fiber that
amplifies at a wavelength wherein the gain optical fiber is
multimoded.
[0020] The pulsed fiber laser apparatus can be configured and
arranged such that the fiber amplifier comprises an optical gain
fiber comprising a taper wherein the MFD increases along a length
of the optical gain fiber. The taper can comprise a taper ratio of
at least 1.5.
[0021] The pulsed fiber laser apparatus can comprise an optical
output in optical communication with the pulse compressor apparatus
and configured for delivering compressed ps optical output pulses
for material processing, where the compressed ps optical pulses
comprise a temporal power profile having a base pulse portion and
surge pulse portion within the base pulse portion, the peak power
of the surge pulse portion being the peak power of the compressed
ps optical pulse, and wherein the surge pulse portion of the
compressed optical output pulse comprises, for example, no more
than about 75% (or no more than about 65% or about 50%) of the
total energy of the compressed optical output pulse.
[0022] In various practices of the disclosure the pulsed fiber
laser apparatus can be configured and arranged to comprise only
single pass amplification. The pulsed fiber laser apparatus can
comprise an all fiber construction. The pulsed fiber laser
apparatus can comprise a bulk optical amplifier located optical
downstream of the optical fiber amplifier. The pulsed fiber laser
apparatus can comprise a spectral filter, located optically
downstream of the optical fiber amplifier, for spectrally filtering
the spectrally broadened optical pulses. The spectral filter can
comprise the bulk optical amplifier.
[0023] In yet another aspect, the disclosure teaches a pulsed fiber
laser apparatus for outputting ps optical pulses, where the
apparatus comprises a fiber delivered pulsed seed laser (FDPSL)
(e.g., a fiber based master oscillator for providing ps optical
pulses), at least one optical fiber amplifier in optical
communication with the FDPSL, and a pulse compressor apparatus in
optical communication with at least one optical fiber amplifier.
The pulsed fiber laser apparatus can be configured and arranged
such that the at least one optical fiber amplifier receives ps
optical pluses having a time duration of no less than about 20 ps.
The at least one optical fiber amplifier can be configured to
amplify in a nonlinear regime wherein the received ps optical
pulses are spectrally broadened by self phase modulation. The pulse
compressor apparatus can provide compressed ps optical pulses
having a temporal power profile having a base pulse portion and
surge pulse portion within the base pulse portion and wherein the
peak of the surge pulse portion is also the peak of the compressed
ps pulse temporal power profile. The pulsed fiber laser apparatus
can include an optical output in optical communication with the
pulse compressor and can be configured for delivering output pulses
(e.g., configured for delivering output pulses for materials
processing, such as material removal), where the surge pulse
portion of the temporal power profile of the output pulse comprises
no more than 75% (or no more than about 65% or about 50%) of the
total energy of the output pulse.
[0024] In some practices of the disclosure, the optical fiber
amplifier receives ps optical pulses having spectral bandwidth of
no greater than about 0.5 nm. The time duration of the surge pulse
portion is, in some practices, no more that 20% of the time
duration of the output pulse. The output pulses can have, in some
practices of the invention, a time duration of no less than 2 ps.
The FDPSL can comprise a fiber based master oscillator having a
laser cavity that includes a length of rare earth doped optical
fiber.
[0025] As noted above, a pulse compressor apparatus of an
embodiment of the present invention can provide a spectral
dispersion of at least about 50 ps/nm, at least about and/or a
pulse compression ratio of not greater than about 50.
[0026] The pulse compressor apparatus can comprise at least one
chirped volume Bragg grating (CVBG). The at least one CVBG can
comprise first and second CVBGs configured and arranged such that
the first CVBG receives optical energy having a different
polarization state than the optical energy received by the second
CVBG. The pulsed fiber laser apparatus can comprise a spectral
filter for spectrally filtering the spectrally broadened pulse so
as to change the relative optical energies or the relative temporal
durations of the surge and base pulse portions. The spectral filter
can be tunable, (e.g., one or more of the center wavelength or
bandwidth may be tunable). Tuning of the spectral filter can tune
the relative time durations or optical energies.
[0027] The present disclosure also teaches methods.
[0028] In one aspect, the disclosure teaches a method of providing
output optical pulses (e.g., picoseconds output optical pulses)
with a laser system including an optical fiber amplifier,
comprising inputting optical pulses to an optical amplifier (e.g.,
an optical fiber amplifier having a length of gain-providing
optical fiber), amplifying the optical pulses with the optical
amplifier (e.g., the gain-providing optical fiber of the optical
fiber amplifier) wherein the optical amplifier operates in a
nonlinear regime and spectrally broadens the optical pulses by at
least a selected broadening factor (e.g., at least a factor of 8);
and compressing optical pulses with a pulse compressor apparatus in
optical communication with the optical amplifier so as to provide
ps compressed optical pulses (e.g., ps compressed optical pulses),
the pulse compressor providing a dispersion of at least about 50
ps/nm and a compression ratio of the time duration of the optical
pulses received by the compressor apparatus to the time duration of
the compressed picosecond optical pulses of no greater than about
50. The amplifier can be configured so as to not substantially
change the time duration of the optical pulses during the
amplification thereof.
[0029] In various practices of the invention, the optical input
pulses can have a spectral bandwidth not in excess of about 0.5 nm
and/or a time duration of at least about 20 ps. In various
practices of the invention the ps compressed optical pulses can
have a time duration of no less than about 2 ps.
[0030] The optical fiber amplifier can be operated as a single mode
optical fiber amplifier. Operating as a single mode fiber amplifier
can be accomplished (in accordance with the teachings of the
various embodiments herein, including apparatus and systems), such
as by, for example, an amplifier comprising only single mode gain
fiber or, alternatively, an amplifier comprising multimode gain
fiber and being operated single mode. Accordingly, in one practice,
the gain optical fiber operates at a wavelength of operation and
wherein the gain providing optical fiber is single mode at the
wavelength of operation.
[0031] The gain providing optical fiber can comprise a taper
wherein the mode field diameter of the fundamental mode increases
along a length of the gain providing optical fiber. The taper can
have a taper ration of at least 1.5.
[0032] Inputting optical pulses can comprise providing a pulsed
fiber laser comprising a mode locked laser cavity comprising a
length of ytterbium doped fiber operating in the normal dispersion
regime, the fiber laser comprising a saturable semiconductor
absorber mirror (SESAM) mode locking element. The ps compressed
optical pulses can have a pulse energy of at least 5 .mu.J.
Inputting optical pulses can comprise providing optical seed pulses
having a first temporal duration and temporally stretching the
optical seed pulses to increase the time duration thereof.
[0033] The method can comprise spectrally filtering the spectrally
broadened pulse, and spectrally filtering can comprise optically
amplifying with a bulk optical amplifier.
[0034] The method can also include providing an optical output in
optical communication with the pulse compressor apparatus and
configured for delivering ps optical output pulses for material
processing. The ps optical output pulses, responsive to the
compression, can comprise a temporal power profile having a base
pulse portion and a surge pulse portion within the base pulse
portion, wherein the peak power of the surge pulse portion is the
peak power of the temporal power profile of the ps optical output
pulse, and the surge pulse portion comprises no more than 85% of
the total energy of the ps optical output pulse. The surge pulse
portion can comprise no more than 75%, or no more than 70%, or no
more than 65% of the total energy of the ps optical output
pulse.
[0035] The method can comprise processing a material with the
picosecond optical output pulses.
[0036] In another aspect, the present disclosure teaches a method
of providing ps laser pulses for processing a material, comprising
nonlinearly amplifying optical pulses (e.g., ps optical pulses)
such that the optical pulses are spectrally broadened and include
nonlinear chirp; directing compressed optical output pulse (e.g., a
ps compressed optical output pulse) to a material for the
processing thereof, including compressing the nonlinearly amplified
optical pulse to provide a compressed optical pulse having a
temporal power profile having a base pulse portion and a surge
pulse portion within the base pulse portion, wherein the peak of
the surge pulse portion is the peak of the compressed optical pulse
temporal profile, and wherein the surge pulse portion of the
compressed optical output pulse has a peak power of at least 500 kW
and wherein the optical energy of the surge pulse portion is no
more than 75% of the optical energy of the compressed optical
output pulse including the surge and base pulse portions thereof.
Compressing the nonlinearly amplified ps optical pulses can
comprise providing a dispersion of at least about 50 ps/nm and/or a
compression ratio of no greater than about 50. The optical pulses
being nonlinearly amplified can comprise ps optical pulses having a
bandwidth of no greater than about 0.5 nm and/or pulse duration of
at least about 20 ps.
[0037] In various practices of the disclosure, optical seed pulses
have a spectral bandwidth of no greater than 5 nm, no greater than
3 nm, no greater than 2 nm, or no greater than 1 nm. In various
practices of the invention, the seed pulses have a temporal
bandwidth no less than 1 ps and no greater than 100 ps and, by way
of example and not limitation, in conjunction with the foregoing
the seed pulses typically have a spectral bandwidth of no greater
than 2 nm, or, as may be more likely, no greater than 1 nm.
[0038] The method can comprise spectrally filtering the spectrally
broadened pulses so as to change the relative optical energies or
the relative temporal durations of the surge and base pulse
portions.
[0039] An optical seed laser of the embodiments of the present
disclosure, including those described in the methods of apparatus
noted herein, can comprise any type of pulsed ps laser that can be
fiber delivered (i.e., has an optical fiber output) and that is
commensurate for use in the ps pulsed fiber lasers as described in
the disclosure herein. For example, the FDPSL can comprise a fiber
delivered diode pumped solid state (DPSS) laser; a gain switched
diode laser; a diode laser in combination with an external
modulator (such as, for example, a Mach Zhender modulator); a
microchip laser; or a fiber laser.
[0040] In various practices of the disclosure, including any of the
apparatus and methods described above, compression can be achieved
with a pulse compressor apparatus that provides a dispersion of at
least about 100 ps/nm; at least about 150 ps/nm; at least about 200
ps/nm; at least about 250 ps/nm; at least about 300 ps/nm; at least
about 350 ps/nm or at least about 500 ps/nm. The compression ratio
can be, in various practices of the disclosure as taught herein,
such as the methods and apparatus described above, not greater than
about 50, not greater than about 30, not greater than about 25, or
not greater than about 15. In the many practices of the invention,
any of the methods and apparatus embodiments taught herein can use
any combination of the foregoing recited spectral dispersions and
compression ratios.
[0041] Dispersion for compression can be provided by a chirped
volume Bragg grating (CVBG), and two or more CVBGs can be arranged
to provide even higher dispersion than one of the CVBGs can provide
alone. The pulse compressor can comprise at least one CVBG, and can
comprise, for example, first and second CVBGs configured and
arranged such the first CVBG receives optical energy having a
substantially different polarization state than the optical energy
received by the second CVBG. The pulse compressor can comprise more
than two CVBGs (e.g. three, four or five CVBGs), wherein a first
pair of the CVBGs receives optical energy having substantially
different polarization states and a second pair receives optical
energy having substantially the same polarization state (one of the
CVBGs of the first pair can be the same as one of the second
pair).
[0042] Time duration and spectral bandwidth, unless otherwise
specifically defined in a particular instance herein, refers to
full width--half maximum (FWHM) duration and bandwidth. The terms
"spectral dispersion" and "dispersion" are used
interchangeably.
[0043] The present disclosure also teaches pulse conditioning
methods and apparatus.
[0044] In one aspect, the present disclosure teaches a pulse
conditioning apparatus comprising a signal splitter for splitting a
pulsed input signal into first and second pulsed signals; a
spectrally dispersive element configured for subjecting the first
pulsed signal to a spectral dispersion for compressing or
stretching the first pulsed signal, the first and second pulsed
signals being subjected to substantially different spectral
dispersion; and a combiner for combining the first and second
pulsed signals to an output pulsed signal, the output pulsed signal
output comprising pulses having a surge pulse portion and a base
pulse portion, at least one of the portions derived at least in
part from the spectral dispersion and attendant compression or
stretching of the first pulsed signal.
[0045] The pulse conditioning apparatus can comprise a polarization
beam splitter, where one or both of the signal splitter and the
beam combiner comprise the polarization beam splitter. The pulse
conditioning apparatus can comprise a polarization controller
optically upstream of the signal splitter for allowing the
polarization state of the pulsed input signal to be changed. The
pulse conditioning apparatus can comprise a time delay arrangement
for introducing a time delay to the second pulsed optical signal.
The spectrally dispersive element can be configured for compressing
(i.e., temporally compressing) the first pulsed signal. The
spectrally dispersive element can comprise a grating. The grating
can comprise a chirped volume Bragg grating. The grating can have a
dispersion of, in one practice o the disclosure, at least 50 ps/nm.
The grating can provide a compression ratio of, in one practice of
the disclosure, no greater than 50
[0046] In another aspect, the disclosure teaches a method of
conditioning pulses, comprising splitting an input signal into
first and second pulsed signals; subjecting one of the first and
second pulsed signals to spectral dispersion so as to temporally
compress or stretch pulses of the signal; and combining the first
and second pulsed signals to form an output signal comprising
pulses having a surge pulse portion and a base pulse portion, at
least one of the portions derived at least in part from the
spectral dispersion and attendant compression or stretching of at
least one of the first and second pulsed signals. The first and
second pulsed signal can be subjected to substantially different
spectral dispersion. The method can include refraining from
subjecting one of the pulses to substantial spectral dispersion as
compared to the other of the pulses.
[0047] In various practices the method can comprise one or more of
the following: changing the state of polarization of pulses of the
input signal; varying the polarization state of the input signal so
as to vary the optical energies of at least one of the surge and
base pulse portions of the output pulses; varying a time delay of
one of the first and second pulsed signals so as to change the
temporal relationship of the surge and base pulse portions of the
output pulses; or varying the optical energy of pulses of at least
one of the first and second pulsed signals so as to vary the
optical energy of at least one of the surge and base pulse
portions.
[0048] At least one of the splitting and combining steps can
comprise using polarization discrimination. Both of the splitting
and combining can comprise using polarization discrimination.
Splitting can comprise splitting the input signal into at least two
beam paths with a polarization beam splitter. The method can
comprise changing the state of polarization of the input signal
prior to splitting the input signal with the polarization beam
splitter.
[0049] In various practices of the disclosure subjecting one of the
first and second signals to spectral dispersion can comprise
compressing pulses of one of the first and second pulsed signals or
stretching pulses of one of the first and second signals. In one
practice, the method can include refraining from subjecting one of
the first and second pulsed signals to any substantial spectral
dispersion. The surge pulse portion can comprises a substantially
different state of polarization than a state of polarization
comprised by the base pulse portion. In another practice of the
invention, the state of polarization of the surge pulse portion is
substantially the same as the state of polarization of the base
pulse portion.
[0050] Various features and aspects of the invention are described
herein. The features, aspects and practices described herein may be
arranged in any combination with any of the other features, aspects
or practices described herein, regardless of the particular
exemplary embodiment in which such a feature, aspect or practice is
described, except where clearly mutually exclusive or a statement
is explicitly made herein that such a combination is unworkable. To
avoid undue repetition and length of the disclosure, every possible
combination is not explicitly recited as a separate embodiment. The
various embodiments of the invention considered disclosed as within
the scope of the invention are at least as described in the
multiply dependent claims appended hereto.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] FIG. 1 schematically illustrates one embodiment according to
the present disclosure of a pulsed fiber laser apparatus for
outputting laser pulses;
[0052] FIG. 2 schematically illustrates one example of an optical
fiber amplifier of the pulsed fiber laser apparatus of FIG. 1;
[0053] FIG. 3A schematically illustrates one example of a pulse
compression apparatus of the pulsed fiber laser apparatus of FIG.
1;
[0054] FIG. 3B schematically illustrates another example of a pulse
compression apparatus of the pulsed fiber laser apparatus of FIG.
1;
[0055] FIG. 3C schematically illustrates yet a further example of a
pulse compression apparatus of the pulsed fiber laser apparatus of
FIG. 1;
[0056] FIG. 4A schematically illustrates an example of a pulse
conditioning apparatus;
[0057] FIG. 4B schematically illustrates one example of an output
pulse having surge and base pulse portions;
[0058] FIG. 4C schematically illustrates another example of an
output pulse having surge and base pulse portions;
[0059] FIGS. 5A, 5B and 5C schematically illustrate, respectively,
the pulse duration, pulse spectrum and chirp of an example of a
seed pulse provided by a seed source according to the apparatus and
methods of the present disclosure;
[0060] FIGS. 6A, 6B and 6C schematically illustrate, respectfully,
an example of the temporal power profile, spectral power profile
and chirp of pulses after nonlinear amplification according to the
methods and apparatus disclosed herein;
[0061] FIGS. 7A, 7B, and 7C schematically illustrate, respectfully,
an example of the temporal power profile, spectral profile and
chirp of pulses after compression according to the methods and
apparatus disclosed herein;
[0062] FIG. 8 schematically plots normalized peak power, pulse
fraction energy, and derivative power for the compressed pulse
represented by FIGS. 7A, 7B and 7C;
[0063] FIG. 9 is a flow chart schematically illustrating steps that
can be included according to one method of the present disclosure
for providing picoseconds laser pulses;
[0064] FIG. 10 is a flow chart schematically illustrating steps
that can be included according to another method of the present
disclosure for providing picoseconds laser pulses;
[0065] FIG. 11 schematically illustrates another embodiment of a
pulsed laser according to the present disclosure.
[0066] Not every component is labeled in every one of the foregoing
FIGURES, nor is every component of each embodiment of the invention
shown where illustration is not considered necessary to allow those
of ordinary skill in the art to understand the invention. The
FIGURES are schematic and not necessarily to scale.
[0067] When considered in conjunction with the foregoing FIGURES,
further features of the invention will become apparent from the
following detailed description of non-limiting embodiments of the
invention.
DETAILED DESCRIPTION
[0068] FIG. 1 schematically illustrates one embodiment according to
the present disclosure of a pulsed fiber laser apparatus 12 for
outputting picosecond ("ps") laser pulses. The pulsed fiber laser
apparatus 12 can comprise a pulsed seed laser 20, at least one
optical fiber amplifier 24 in optical communication with the pulsed
seed laser 20, a pulse compressor apparatus 28 in optical
communication with the at least one optical fiber amplifier 24, and
an optical output 30 in optical communication with the pulse
compressor apparatus 28. Optical path 34A provides optical
communication between the pulsed seed laser 20 and the least one
optical fiber amplifier 24, which communicates with the pulse
compressor apparatus 28 via optical path 34B. Optical path 34C
provides optical communication between the pulse compressor
apparatus 28 and the optical output 30. The at least one optical
fiber amplifier 24 is considered optically "downstream" of the
pulsed seed laser 20, the pulse compressor apparatus 28 optically
downstream of the at least one amplifier 28, and the output 30
optically downstream of the pulse compressor apparatus 28.
[0069] The pulsed fiber laser apparatus 12 can optionally include a
pulse stretcher in optical communication with the pulsed seed laser
20 and the at least one optical fiber amplifier 24. For example,
see the pulse stretcher 40 shown in dotted lines as interposed in
optical path 34A, optically downstream of the pulsed seed laser 20.
The pulse stretcher 40 can be interposed between the pulsed seed
laser 20 and the at least one optical amplifier 24, or interposed
between optical amplifiers of the at least one optical fiber
amplifier 24, or located optically downstream of the at least one
optical amplifier 24. Generally speaking, a pulse stretcher can be
located optically upstream of the output 30. The pulsed fiber laser
apparatus 12 can also optionally include a spectral filter, such as
for example, the spectral filter 44 shown in FIG. 1 as interposed
in the optical path 34C between the compressor apparatus 28 and the
output 30. The pulsed fiber laser apparatus 12 can include both the
optional spectral filter along with the optional pulse stretcher
40. The optional pulse stretcher 40 and spectral filter 44 are
discussed in more detail later in this disclosure.
[0070] The pulsed seed laser 20 outputs ps pulses, wherein "ps
pulses", as that term is used herein, means pulses having time
duration of no less than 1 ps and no greater than 500 ps. In
various practices of the invention, the pulsed seed laser 20 can
provide ps pulses having a time duration of no less than about 25
ps; no less than about 40 ps, or no less than about 80 ps. In other
practices of the invention, the pulsed seed laser 20 can provide ps
pulses having a spectral bandwidth of no greater than about 0.1 nm,
no greater than about 0.5 nm, no greater than about 1 nm, or no
greater than about 5 nm. Any of the foregoing recitations regarding
spectral bandwidth can be combined with any of the foregoing
recitations regarding time duration in the many practices of the
invention. The seed pulses provided by the seed laser 20 may, in
certain practices of the invention, be substantially transform
limited. The seed pulses may, in certain practices of the
invention, be substantially unchirped, substantially nonlinearly
chirped, or substantially linearly chirped. Typically the pulses
are substantially unchirped. The time duration (or spectral
bandwidth) of a pulse can be determined by the full width half
maximum (FWHM) approach, unless otherwise specified herein.
[0071] The output 30 can be configured for outputting ps pulses for
use in, for example, processing a material. "Materials processing"
or "processing a material", as those terms are used herein, are
intended to be broadly construed. For example, processing can
include both processing that modifies a material via deliberate
material removal (e.g., machining, cutting, drilling, ablating,
vaporizing, etc.); processing that modifies with no or much less
removal but that includes perhaps some transformation of the
material (e.g., marking, printing or internal structural
modification, such as heat treating, hardening or annealing, etc.);
or processing that characterizes a material with in most instances
no permanent transformation, such as by facilitating inspecting,
measuring, or characterizing the material (e.g., imaging,
illuminating, measuring temperature, causing fluorescence, such as
in a fluorescence lifetime imaging measurement, etc.).
[0072] The pulsed fiber laser apparatus 12 can comprise an all
fiber construction, that is, all components can be entirely
realized in fiber or at least fiber pigtailed.
[0073] The pulsed seed laser 20 preferably comprises a fiber
delivered pulsed seed laser ("FSDPSL). FDPSL can comprise any type
of pulsed ps laser that can be fiber delivered (i.e., has an
optical fiber output) and that is commensurate for use in the ps
pulsed fiber lasers as described in the disclosure herein. By way
of example, the FDPSL 20 can comprise a fiber delivered diode
pumped solid state (DPSS) laser; a gain switched diode laser; a
diode laser in combination with an external modulator (such as, for
example, a Mach Zehnder modulator); a microchip laser; or a fiber
laser. Of course a pulsed seed laser according to the disclosure
can comprise any one of the foregoing lasers absent the fiber
delivered feature (e.g., free space output).
[0074] The pulsed seed laser 20 can advantageously comprise a mode
locked fiber laser, such a fiber delivered mode locked fiber laser.
A suitable mode locked fiber laser can comprise passive mode locked
laser cavity. The laser cavity can have at one end a fiber coupled
semiconductor saturable absorber (SESAM) mirror (SESAM) as a mode
locking element and a second reflector (e.g., a fiber Bragg grating
or "FBG") at the other end. The laser cavity can also comprise a
length of active fiber, such as, for example, a length of rare
earth doped (RED) optical fiber. The rare earth dopant can comprise
ytterbium. The fiber laser can output ps pulses having a center
wavelength of 1064 nm at a pulse repetition frequency (PRF) from
between about 20 MHz to about 100 MHz, with the pulses having a
pulse energy in the pJ range. The average power of the optical
fiber laser can be in the range of a few to a few tens of
milliwatts.
[0075] FIG. 2 schematically illustrates one example of an optical
fiber amplifier 70 of the at least one optical fiber amplifier 24
shown in FIG. 1. The optical fiber amplifier 70 of FIG. 2 can
include a pair of pump sources 72A and 72B, which can each comprise
a fiber pigtailed laser diode, that provide pump optical energy via
optical couplers 74A and 74B, respectively, to the length of gain
optical fiber 76, which comprises a conventional optical fiber. The
length gain fiber typically comprises RED optical fiber. A
controller 78 is also shown in FIG. 2, and can control the gain of
the amplifier 70 via control of the optical power emitted by the
pump sources 72A and 72B, such as by controlling the current
supplied to the laser diodes comprised by the pump sources 72A and
72B. Similar control is also possible of the pulsed seed laser 20
of FIG. 1, by the same or a separate controller. The length of gain
RED fiber 76 can comprise single clad fiber and the pump sources
72A and 72B can each comprise a single-mode fiber delivered laser
diode delivering, for example, 200 mW of 976 nm optical energy. The
optical couplers 74A and 74B can each comprise a WDM optical fiber
coupler that couples the 976 nm optical pumping energy to the core
of the length of RED fiber 76 while providing low insertion loss to
the optical energy being amplified by the length of RED fiber 76.
The length of RED fiber 76 can include a Yb-doped core having a
diameter of approximately 6 microns and a cladding having a
diameter of approximately 125 microns. The core can have a
numerical aperture (NA) of approximately 0.14. The core can include
a concentration of Yb sufficient, for example, for providing pump
absorption greater than 200 dB per meter at 976 nm. One suitable
fiber is the YB 500 fiber available from specialty optical fiber
manufacturer CorActive High-Tec Inc., having coordinates of 2700
Jean-Perrin, Suite 121, Quebec City, QC, Canada G2C 1S9. As the
artisan of ordinary skill can ascertain, the forgoing fiber is of
conventional construction.
[0076] The length of gain fiber 76 can comprise a length of
double-clad fiber, where the fiber comprises RED optical core,
surrounded by an inner cladding region which is surrounded by a
second, outer cladding region of lower refractive index than the
inner cladding. The inner-cladding and outer-cladding index
contrast forms a multi-mode waveguide in the inner cladding, into
which multimode pump light can be launched. The overlap of the pump
light with the RED core of the fiber results in absorption of the
pump light by the RED ions and lasing or amplification through
stimulated emission of light. Where the length of gain fiber
comprises a double clad fiber the pump sources 72A and 72B can be
fiber-coupled multimode laser diodes delivering up to 10 Watts
power at 915 nm or could equally be pump modules comprising
multiple laser diodes, the output of which are combined into a
multi-mode optical fiber. In the latter case, powers in excess of
40 Watts can be achieved from each of the pump sources for the
delivery of very high average power optical amplifiers. The length
of gain fiber can comprises a numerical aperture greater than
0.04.
[0077] In one practice, the RED amplifier operates single mode,
where the gain fiber is entirely single mode and has for example, a
core diameter of less than 6 microns and a core NA of approximately
0.14. However, large mode area "LMA" RED fibers and even fibers
supporting more than a single mode can be used. Although there is
no strict definition in the art as to exactly what constitutes LMA
fiber, such fibers are typically characterized by a core having a
reduced NA and an increased diameter. For example, an LMA fiber can
have a Yb doped core (Yb doping sufficient for one to a few dB
absorption in the 915 nm band) having a 20 .mu.m (or greater)
diameter and a NA of about 0.07 to 0.09. Such a fiber can provide
the amplifier with an output pulses having a pulse energy of, for
example, 7 .mu.J and a peak power (PP) of 150 kW. It can be
desirable to avoid operation that would trigger Raman phenomena.
Often the peak power of pulses in the power amplifier should not
exceed about 150 kW as the threshold value for the onset of Raman
scattering. Larger diameter fiber need not always be used, and in
some embodiments of the pulsed fiber laser apparatus 12 of FIG. 1
the mode field diameter of the fundamental mode does not exceed
about 14 microns for any optical fiber amplifier optically upstream
of the compressor.
[0078] In one approach, the pulsed fiber laser apparatus 12 does
not comprise a fiber amplifier having a gain optical fiber that
amplifies at a wavelength wherein the gain optical fiber is
multimoded.
[0079] The optical fiber amplifier 70 can comprise a tapered fiber
amplifier wherein a feature of the gain fiber 76 (e.g., mode field
diameter of the fundamental mode or "MFD") changes (e.g.,
increases) along at least a selected length of the length of gain
fiber 76. The taper profile (plot of magnitude of feature as
function of length along fiber) can be substantially linear. A
taper can equally have a nonlinear or arbitrary taper profile. For
example, the magnitude of the rate of change of a feature with
respect to longitudinal length at a first location ("input
location) optically upstream of a second location ("output
location") can be greater than the magnitude of the rate of change
of the feature with respect to longitudinal length at the second
location. In another practice found to be useful the opposite is
true: the magnitude of rate of change of MFD with respect to
longitudinal length along the taper is less at the input location
than at the output location. MFD is typically a strong function of
the diameter of the core of the gain fiber, and the MFD can be
increased along a taper by tapering the core to increase its
diameter. Thus the core can have a smaller diameter (and/or fiber
can have smaller MFD) at the input location than at the output
location. The feature can taper so as to increase substantially
exponentially along the length, or can behave according to a power
law, or any combination of linear, exponential and power law
behavior. The taper profile along the length of the fiber can be
designed for improving, including optimizing, the performance of
the amplifier in terms of nonlinearity and gain. For example, the
effective nonlinear length of an amplifier is defined by both the
core-size and the gain profile along the fiber length. By having a
non-uniform taper profile the effective nonlinear length of the
amplifier can be very short, since the highest gain of the
amplifier occurs at a region of the fiber where the MFD is largest.
The rate at which the MFD evolves along the length of the fiber can
also affect beam properties of the amplified signal.
[0080] Preferably the taper is configured such that the core is
substantially single mode at the first location. The core can be
"large mode area" (LMA) fiber at the first or input location (and
along the length of the taper). LMA, for the purpose of this
disclosure, can mean a core numerical aperture (NA) of about 0.11
or less at 1060 nm and a core diameter of at least 10 .mu.m.
[0081] The gain fiber can be single mode at the first location,
having V-value less than 2.405, and at the second location, where,
for example the MFD and/or core diameter is increased, the fiber
can have a V-value of more than 2.405 (can support more than one
transverse mode). The gain fiber can have a tapered section
followed by a uniform (substantially untapered) section along which
the feature does not substantially change. The uniform section of
the fiber can have a larger mode field area than that at the first
location. The uniform section can have a V-value greater than
2.405. For example, in one practice a gain fiber can have a core
diameter of approximately 10 um and numerical aperture of 0.08
(V-value at 1064 nm<2.405) at a first or input location and
hence be single mode and have a core diameter of 30 um or greater
(or 40 um or greater) with a waveguide numerical aperture of 0.08
at second or output location (V-value at 1064 nm>2.405). The MFD
at the second location can be greater than, for example, 25 um.
[0082] As should be clear, the first and second locations need not
be transitions to free space. For example, one or both of the first
and second location can be locations selected along a length of
fiber according to a criterion or criteria noted herein.
[0083] The optical amplifier can be configured such that the input
beam quality does not substantially degrade along the length of the
amplifier, even though the core of the fiber tapers up in diameter
such that the core can support higher order modes. That is,
although the taper is such that core transitions from substantially
single mode to supporting higher order modes, little or no optical
energy is transferred into higher order modes. Accordingly, the
length of gain fiber can provide an output that is substantially in
a single transverse mode and that accordingly has good output beam
quality. Beam quality can be measured and quantified according to
the "M-squared" or "M2" parameter. Thus even in instances where the
gain fiber includes sections where the V-value is greater than
2.405, the optical fiber amplifier can be configured such that
substantially only the fundamental mode of the waveguide is excited
and propagates as the signal propagates through the device.
[0084] In the simplest form of manufacture, not only does the core
diameter vary, but so too does the outer diameter of the fiber
along the taper. Some or all of the length of the gain fiber, at
least a majority of its length, or substantially all of its length
can be tapered.
[0085] More particularly, a taper can have at the first location a
core having a diameter D.sub.core-in. The core can have the larger
diameter D.sub.core-out at the second location. The diameter of the
core can increase along at least some the length between the
locations. The fiber can have a cladding (which can be the first
cladding after the core, where cladding refers to a region having
an optical function of tending to confine light to a region the
cladding surrounds) that also tapers. The cladding can have a
diameter at the first location of D.sub.clad-in which can increase
to a diameter at the second location of D.sub.clad-out.
[0086] By way of example, a tapered gain fiber according to the
present disclosure can have a taper ratio (ratio of a feature such
as MFD or core diameter at the output or second location to the
diameter of the same feature at the input or first location) of at
least 1.5, at least 2, at least 2.5, or at least 3. In various
practices the taper ratio can be between (inclusive of endpoints of
the stated ranges) about 1.5 and about 2, between about 2 and about
3, or between about 3 and about 5. In various practices of the
invention, the foregoing recitations regarding taper ratios can
apply to D.sub.core-out/D.sub.core-in, or to the ratio
D.sub.clad-out/D.sub.clad-in, or to both of the foregoing ratios.
In terms of actual diameters of fibers, a tapered fiber can have a
core having a diameter that tapers from, for example (again
including endpoints), about 10 .mu.m to about 20 .mu.m, from about
10 .mu.m to about 30 .mu.m, from about 10 .mu.m to about 40 .mu.m,
or from about 10 .mu.m to > about 50 .mu.m. The length of the
gain fiber can be, in various practices of the disclosure, no
greater than about 500 cm, no greater than about 250 cm, no greater
than about 150 cm, no greater than about 100 cm, no greater than
about 75 cm, no greater than about 50 cm, no greater than about 30
cm, or no greater than about 25 cm.
[0087] In some practices of the invention the length of the gain
fiber can be no longer than 2 metres or no longer than 4 metres
and, independent of the foregoing considerations or in conjunction
with either of the them, the length along which the gain fiber is
tapered is, in certain practices, is no longer than 1 metre. The
gain fiber can include an MFD that exceeds 30 um, or exceeds 50 um,
or even exceeds 75 um.
[0088] The core diameter of a tapered optical fiber can be selected
such that pulse-energy handling is not limited. However, equally,
if an application does not require substantially single-transverse
mode operation, the taper and/or the diameter can be such that the
beam quality is not maintained throughout the amplifier.
[0089] The tapered gain fiber can have a taper profile arranged
such that the tapered gain fiber delivers substantially single mode
pulses and wherein the signal intensity therealong remains high
enough such that the amplified spontaneous emission (ASE) power is
no greater than 10% of the total optical power output and also
wherein the optical signal intensity remains low enough such the
optical power generated at the first Raman stoke shift is no
greater than 10% of the total optical power delivered.
[0090] Furthermore, the optical fiber amplifier may be arranged
such that amplified spontaneous emission (ASE) power is no greater
than 10% of the total optical power propagated along the tapered
optical gain fiber. Alone or in combination with the foregoing, the
optical fiber amplifier may be arranged such that any optical power
generated at the first Raman stoke shift is no greater than 10% of
the total optical power provided by the tapered gain fiber.
Finally, along or in combination with either of the foregoing, the
optical pulses provided by the tapered gain fiber may have a
spectrum with a fundamental wavelength and wherein the tapered gain
fiber is arranged such that no more than 10% of the total optical
power provided by the tapered gain fiber is outside of a 30 nm
bandwidth centered about the fundamental wavelength.
[0091] In some instances where the length of gain fiber 76
comprises a taper (and particularly where higher pump absorption is
desired), the length of gain fiber is preferably substantially core
pumped (as opposed to substantially cladding pumped), such as, for
example, with substantially single mode pump light from one or both
of the pump sources 72A and 72B. However, the tapered length of
gain fiber 76 can be cladding pumped. In either case the gain fiber
can be substantially single mode or few moded or MM along all or at
least part of its length. Regarding higher pump absorptions, the
optical fiber amplifier 70 may be configured such that the tapered
gain fiber has an absorption rate of pump light of at least 2.5
dB/meter (or at least 5 dB/meter, or at least 9 dB/meter) and/or
propagates pulses having a time duration of less than 500 ns and a
peak power of at least 100 KW. The higher pump absorptions can
advantageously be achieved in many practices with the limits on the
length of the gain fiber or taper length noted above.
[0092] Although "up" tapers are often described above, a gain fiber
can include one or both of "up" tapers (feature increasing in
magnitude in a direction optically downstream along the length of
gain fiber) and "down" tapers (magnitude of feature decreasing in a
direction optically downstream along the length of gain fiber). The
foregoing description regarding up tapers can apply to down tapers
as well (with feature such as core diameter noted for first
location now pertaining to second location and vice versa). Thus
although the diameter or other feature is typically smaller at the
first location than at the second location, the invention as noted
in the embodiments can also be practiced where the opposite is true
and the instead of increasing along the taper the aforementioned
features can decrease, with the larger and smaller limits above now
applying to the down taper.
[0093] Also, in regard to tapers useful with the embodiments
disclosed herein, see, for example, U.S. Provisional Patent
Application 61/644,424, entitled "Amplifying Optical Device Having
Tapered Gain Element", filed 8 May 2012, which is incorporated by
reference herein.
[0094] Typically the optical fiber amplifier 70 is configured as
single pass. The pulsed fiber laser apparatus can be configured
such that it comprises only single pass amplification and/or not
include any regenerative amplification. The time duration of the ps
optical pulses received by a gain optical fiber of an amplifier can
be, in certain practices of the disclosure, substantially the same
as the time duration of the ps optical seed pulses.
[0095] As part of what is often referred to as the Kerr effect,
high optical pulse intensity in a medium can cause a nonlinear
change in the refractive index of the medium, which can in turn
lead to a nonlinear phase delay that depends on the optical
intensity of the pulse. Stated in other words, when a medium
propagates a high peak power optical pulse, the Kerr effect can
cause a time dependent phase shift that varies according to the
time dependent pulse intensity. The pulse acquires a so-called
chirp, that is, a temporally varying instantaneous frequency.
Intensity and hence time dependent phase shift is often referred to
as self-phase modulation, or SPM, and in propagation in optical
waveguides is usually primarily due to the Kerr effect, though
other phenomena can also contribute. Although SPM can cause
spectral broadening of a pulse, in some circumstances pulses can
retain substantially the same bandwidth despite SPM, or SPM can
cause spectral compression of a pulse. As one example, where an
input pulse having a peak power sufficient to induce SPM in a
particular waveguide is substantially unchirped or is up-chirped,
the SPM can lead to spectral broadening. However, when an input
pulse is down-chirped, the SPM can cause spectral compression
(assuming, in the foregoing examples, a positive nonlinear index of
refraction).
[0096] Other factors can be of importance in determining the effect
of SPM. For example, in optical fibers having anomalous dispersion,
the dispersion of the fiber can compensate for the chirp added by
SPM, and this phenomenon is often employed to lead to the formation
of solitons, where the spectral width of a pulse remains constant
during propagation, despite the SPM effect. Mode locked fiber
lasers operating at, for example, about 1.5 nm (a wavelength at
which silica fibers can have anomalous dispersion) can make use of
the interplay between anomalous dispersion of the fiber and SPM to
form soliton output pulses. Mode locked fiber lasers can also form
soliton-like pulses, with little or no overall spectral broadening
of the pulse, where the fiber has normal dispersion at the
wavelength of operation but a spectrally dispersive element is
added to the laser cavity such that the overall cavity dispersion
is anomalous. In such cases, increasing the output power of a pulse
propagating in or emanating from a laser or amplifier does not
necessarily result in spectral broadening of the pulse, even where
power levels are such that SPM is occurring.
[0097] The length of RED gain fiber 76 in any of the embodiments
disclosed herein can be configured to can serve as a gain optical
fiber that optically amplifies by operating in a nonlinear regime
wherein ps optical pulses are spectrally broadened during
amplification thereof, at least in part by SPM, by a selected
factor (e.g., by a factor of at least 8). The spectral broadening
adds spectral components that contribute, typically substantially,
to the subsequent compression of the pulse. The spectral broadening
provides new or enhanced (e.g., in terms of spectral power
distribution) spectral components on which the spectral dispersion
of the pulse compression process acts in compressing the pulse. The
length of RED fiber 76 is typically also normally dispersive. The
pulsed apparatus 12 can be configured such that the length of RED
fiber 76 receives ps optical pulses having a time duration that is
at least about 40 ps, at least about 60 ps, at least about 75 ps,
or at least about 100 ps. The pulsed seed laser 20 (e.g., FDPSL)
can provide such pulses or, as another possibility, the pulsed seed
laser 20 can generate shorter time duration pulses which can be
stretched by the optional pulse stretcher 44 before being received
by the length of RED optical fiber 76. Typically the optical fiber
amplifier 70 is configured such that the time duration does not
substantially change during the optical amplification by the length
of RED optical fiber 76.
[0098] The nonlinear propagation in an optical fiber amplifier is
well described by the nonlinear Schrodinger equation and can be
solved numerically by a split-step Fourier method. The modeling
procedure can be programmed on a computer following the equations
and numerical solutions described in Chapter 2 of Nonlinear Fiber
Optics by Govind P. Agrawal. Some assumptions can be in order, as
is understood by the skilled worker. For example, the gain in the
fiber amplifier can be modeled as constant along the amplifier as a
helpful initial simplification when modeling a counter-pumping
scheme.
[0099] FIG. 3A schematically illustrates one embodiment of a pulse
compression apparatus 28 of FIG. 1. The pulse compression apparatus
80 can include a spectrally dispersive element 84, a .lamda./4 wave
plate 86 and a polarization beam splitter (PBS) 88. The spectrally
dispersive element 84 can comprise, for example, a chirped fiber
Bragg grating (CFBG), a chirped volume Bragg grating (CVBG) or a
diffraction grating pair. CVBGs are advantageous because they can
provide a fairly large amount of spectral dispersion for a rather
compact volume and are typically preferred. CVBGs are understood to
be available from a number of sources, including, for example,
OptiGrate Corporation, 3267 Progress Drive, Orlando, Fla. 32826,
USA. (www.optigrate.com). Accordingly, the spectrally dispersive
element 84 of the pulse compression apparatus 80 is shown as
comprising the CVBG 89 in FIG. 3A.
[0100] With reference to FIG. 3A, consider the input beam 92, which
can arrive along optical path 34B of FIG. 1, to have linear
polarization designated by the letter "S". Assume that the PBS 88
directs S polarized beams orthogonally and passes beams having P
(linear but orthogonal to S) polarization. The PBS 88 can redirect
the S polarized beam 92 as beam 92A. The .lamda./4 plate 86 can be
oriented to transform beam 92A to beam 92B having circular
polarization, which upon reflection as beam 94A from the CVGB 89 is
now compressed. Reflection from the CVBG 89 also reverses the
"handedness" of the circular polarization of beam 94A relative to
beam 92B. That is, if beam 92B was right hand circular polarized
(RHCP), beam 94A is left hand circularly polarized (LHCP), and vice
versa. The .lamda./4 plate 86 now transforms beam 94A to linearly
polarized beam 94B having polarization "P" that is orthogonally
polarized relative to S polarized beam 92A. The PBS 88 passes the P
polarized beam 94B, with some loss, as the output beam 98, which
can be provided, with or without additional conditioning, to the
optical output 30 of FIG. 1.
[0101] FIG. 3B schematically illustrates another embodiment of a
pulse compression apparatus 28. The pulse compression apparatus 150
of FIG. 3B comprises the PBS 154 and first and second spectrally
dispersive elements, shown in FIG. 3B as first and second CVBGs
156A and 156B, respectively. Each of the CVBGs provides spectral
dispersion, such that the pulse compression apparatus 150 can
provide higher dispersion than the embodiment of FIG. 3A that uses
a single CVBG. The PBS 154 redirects the input beam 160 having S
polarization to the S polarized beam 162A. The .lamda./4 wave plate
166 is oriented so as to transform the linearly polarized S beam
162A to the circularly polarized beam 162B, which reflects from the
first CVBG 156A as beam 168A, and is compressed relative to beam
162B. Because beams 162A and 168A each pass through the .lamda./4
plate 166 and because of the reflection from the CVBG 156A, the
.lamda./4 plate 166 transforms beam 168A to P polarized beam 168B,
which passes through the PBS 154 to the second CVBG 156B, which
provides further dispersion and further compression. A .lamda./4
plate is also associated with the second CVBG 156B, as shown in
FIG. 3B, such that the beam 170 is S polarized and is redirected by
the PBS 154, with some loss, of course, to provide the output beam
174, which can be provided, with or without further conditioning,
to the output 30.
[0102] FIG. 3C schematically illustrates another embodiment of a
pulse compression apparatus 28. The pulse compression apparatus 200
includes an optical circulator (labeled "Faraday Rotator System" in
FIG. 3C) 202, three spectrally dispersive elements, which, as shown
in FIG. 3C, can comprise the first, second and third CVBGs 204A,
204B and 204C, respectively, each having an associated .lamda./4
wave plate, and the PBS 206. The optical circulator 202 redirects
the input beam 210 to S-polarized beam 218, which the PBS redirects
to beam 228. Beam 236, which is now compressed due to prior
reflection from first CVBG 204A, now has P polarization and is
passed through the PBS to emerge as beam 238. After compression by
second CVBG 204B, S-polarized beam 246 is directed by PBS 206 to
third CVBG 204C, and beam 260, now P polarized and compressed by
all three CVBGs, is passed by PBS 206 as beam 266 to the optical
circulator 202, and exits the circulator as beam 270.
[0103] It is noted that the embodiments of the pulse compressor
apparatus shown in FIGS. 3A, 3B and 3C can be used in combination
to provide increased spectral dispersion. For example, the output
174 of the pulse compressor apparatus 150 of FIG. 3B can be
directed to the input 210 of the pulse compressor 200 of FIG. 3C,
such that five spectrally dispersive elements act to provide pulse
compression. Other combinations of the pulse compressor apparatus
shown in FIGS. 3A, 3B and 3C are possible and within the scope of
the invention, as the skilled worker, cognizant of the present
disclosure, will ascertain upon reflection. Furthermore, any of the
pulse compressor apparatus of FIGS. 3A, 3B and 3C can become a
pulse stretching apparatus by reversal and of the red and blue ends
of the spectrally dispersive elements shown in the FIGURES.
[0104] FIG. 4A schematically illustrates an example of a pulse
conditioning apparatus 300 according to the present disclosure. In
the embodiment shown in FIG. 4A, the pulse conditioning apparatus
300 includes a spectrally dispersive element 302 arranged for
compression and accordingly the pulse conditioning apparatus 300 as
Illustrated in FIG. 4A is considered to comprises a pulse
compression apparatus. However, as explained in more detail below,
the pulse conditioning apparatus 300 of FIG. 4A has independent
application and could include spectrally dispersive elements
configured for pulse stretching as opposed to pulse compression. A
pulse conditioning apparatus 300 can, in some of its embodiments,
shape a pulse without compression. Also, in a general case the
spectrally dispersive element 302 can include any of the pulse
compressor (or pulse stretcher apparatus, as noted above) shown in
FIG. 3A, 3B or 3C.
[0105] The pulse conditioning apparatus 300 of FIG. 4A can include
polarization controller 305 that can change the state of input beam
or optical energy 310 such that beam 318 has a polarization state
that is selectively different than that of beam 310. In the
embodiment of FIG. 4A the polarization controller 305 comprises a
rotatable .lamda./4 wave plate 319. The pulse conditioning
apparatus 300 also includes the spectrally dispersive element 302,
which in the embodiment shown in FIG. 4A comprises a CVBG 324
arranged for compression, a PBS 326, a highly reflective element
338, and two .lamda./4 waveplates (not indicated by reference
numerals).
[0106] To understand one mode of operation of the pulse
conditioning apparatus 300, consider that the input optical energy
310 comprises linearly polarized (e.g., s-polarized) light, and
that rotatable wave plate 319 is configured such that beam 318 is
circularly polarized and can be considered, as the skilled worker
understands, to comprise equal intensity s and p polarizations
having an appropriate phase relationship. The PBS 326 redirects the
s polarized part of beam 318 to the left. After passing through the
.lamda./4 plate the beam is incident on the CVBG 324 as circularly
polarized beam 328, which upon reflection from the CVBG 324 is re
incident on the .lamda./4 plate as the opposite handed circularly
polarized and compressed beam 336. The .lamda./4 plate converts
beam 336 to p polarized light, which is passed to the right by the
PBS 326 to the output 335. The operation of the .lamda./4 plate in
conjunction with a reflector, such as the CVBG 324, has been
described more than once at this point and is familiar to the
reader.
[0107] The p polarized portion of light beam 318 passes through the
PBS 326 as beam 337 and after the usual polarization transformation
and reflection from reflective element 338, shown as a simple
highly reflective (HR) mirror. The beam 340 is redirected by the
PBS 326 in the s-polarized state to the output 335 with a time
delay 355 that can be adjusted via the position of the reflective
element 338 relative to the PBS 326.
[0108] The pulse conditioning apparatus 300 therefore form an
output 335 from two pulsed beams where the pulses of one of the
beams can have different time duration than the pulses of the other
beam. The two beams can be formed by splitting an input beam into
two beams and subjecting them to different spectral dispersion so
as to, for example, compress one of the beams. Spectral dispersion
can be different in a number of ways, including quantitatively
(absolute value of ps/nm) as well as qualitatively (compression or
stretching, which can be indicated by the sign of the ps/nm). The
pulse conditioning apparatus can also provide for time delay of one
of the beams, which can provide a relative time delay between the
two beams. In one practice of the disclosure, the dividing and
recombining can be based on polarization discrimination.
[0109] With reference to FIG. 4B, the pulse conditioning apparatus
300 can output a pulse 412 comprising a surge pulse portion 414
(derived at least part from compression by the CVBG 324) and a base
pulse portion represented by reference numerals 420A and 420B
(derived at least in part from uncompressed beam 340 reflected from
the HR mirror). The pulse conditioning apparatus 300 can allow
temporal relationship of the surge and base pulses to be varied
(such as, for example, via adjustment of the optical delay 355) to
shift their relative position in time. For example, the time delay
355 can be changed such the surge pulse moves relative to the base
pulse to the position indicated by reference numeral 425. Thus the
surge pulse can lead the base pulse, be largely centered within the
base pulse, or can trail the base pulse. The surge pulse can even
be temporally spaced from the base pulse such that they are in
effect different pulses.
[0110] Rotating the wave plate 319 can provide polarization control
that can change the relative proportion of s polarized to p
polarized light of the beam 318 and hence the amount of energy in
the surge pulse portion relative to the amount of energy in
comprised by the base pulse portion. For example, with reference to
FIG. 4C showing output pulse 432, increasing the p polarization
content of the beam 318 can increase the energy in the base pulse,
represented by reference numerals 440A and 440B of FIG. 4C, and
decrease the energy in the surge pulse 434, as can been seen by
comparison of FIGS. 4B and 4C. Adjustment of the time delay can
move the surge pulse to the location indicated by reference numeral
445, as in the case of the FIG. 4B.
[0111] In the embodiment shown in FIG. 4A the reflective element
338 is shown as highly reflective mirror. However, the reflective
element 338 can comprise a spectrally dispersive element that can
provide compression or stretching, such as providing a spectral
dispersion that is substantially the same as, or different than or
substantially different than, the spectral dispersion provided by
the spectrally dispersive element 324. Generally speaking, the
pulse conditioning apparatus provides different amounts of spectral
dispersion to two signals or beams and then combines them to form
an output. One of the signals can be more or less compressed than
the other, where "less compressed" can include stretching.
[0112] Typically the compressed and less compressed portions (e.g.,
the surge and base pulse portions) of the output 355 will comprise
different polarizations. In many applications this may be
advantageous, or at least of little or no detriment. In other
cases, however, it may be desirable to condition the polarization
of the one or both the compressed and less compressed pulse
portions so as to change the polarization difference between the
two. For example, the polarization of one of the pulse portions
could be modified to be substantially the same as the other of the
polarizations, such as by use of appropriate beam splitting and
Faraday rotation elements, a fast polarization controller, etc.
[0113] The pulse conditioning apparatus 300 can optionally include
the spectral filter 365 which can provide spectral filtering
according to any of the teachings herein regarding such filtering.
Spectral filtering is described elsewhere in conjunction other
aspects and embodiments of then invention, and those teachings are
not repeated here as the skilled worker will appreciate that they
are applicable to the pulse conditioning apparatus 300 and/or its
use in a pulse laser apparatus.
[0114] The pulse conditioning apparatus 300 is exemplary. As one of
ordinary skill informed of the present disclosure will appreciate,
a pulse conditioning apparatus can have a different structure than
the particular embodiment shown in FIG. 4A. For example, a pulse
conditioning apparatus can include a fiber based beam splitter
having first and second outputs and a spectrally dispersive element
in optical communication with one of the outputs for providing, for
example, compression or stretching. The spectrally dispersive
element can be tunable (e.g., a piezo electrically tunable FBG).
The other output can be (optionally) directed through a variable or
fixed time delay arrangement. A second combiner (e.g., a fiber
combiner) can combine the spectrally dispersed signal with the
other signal into an output path. The pulse compressor 44 can
comprise the pulse compressor of a pulse conditional apparatus
described above.
[0115] FIGS. 5A, 5B and 5C schematically illustrate, respectively,
the pulse duration, spectral power profile and chirp of an example
of a seed pulse provided by a seed source according to the
apparatus and methods of the present disclosure. With reference to
FIG. 5A, which shows the pulse temporal power profile, the seed
pulse can have a time duration of approximately 80 ps. With
reference to FIG. 5B, the seed pulse can have a spectral bandwidth
of approximately 0.015 nm. As indicated by FIG. 5C, the seed pulse
can be substantially unchirped. The seed pulse can have a peak
power of, for example, less than about 1 kW and a pulse energy of,
for example, less than 100 nJ (about 550 W and 50 nj, respectively,
for the pulse shown in FIGS. 6A-6C).
[0116] FIGS. 6A, 6B and 6C schematically illustrate, respectfully,
an example of the temporal power profile, spectral power profile
and chirp of pulses after nonlinear amplification according to the
methods and apparatus disclosed herein. With reference to FIG. 6A,
the nonlinearly amplified pulse has time duration that is
substantially the same as that of the seed pulse of FIG. 5A,
namely, 80 ps. However, with reference to FIG. 6B, the spectral
profile is substantially different from the spectral profile of the
pulse before nonlinear amplification shown in FIG. 5B. The spectral
width of the nonlinearly amplified pulse of FIG. 6B is about 0.776
nm, and includes many undulations and attendant local minima and
maxima, as well as a pair of outer "wings", representing, for the
spectrum shown in FIG. 6B, a pair of substantially equal maxima.
With reference to FIG. 6C, the chirp of the nonlinearly amplified
pulse can also be substantially different than the chirp of a pulse
prior to nonlinear amplification shown in FIG. 5C. The pulse of
FIGS. 6A-6C is substantially chirped, including being substantially
nonlinearly chirped.
[0117] The nonlinearly amplified pulse of FIGS. 6A-6C has a peak
power of approximately 110 W and a pulse energy of 9.1 .mu.J. Thus
the nonlinearly amplified pulses can have a peak power of at least,
for example, 100 kW and a pulse energy of at least, for example, 8
.mu.J.
[0118] FIGS. 7A, 7B, and 7C schematically illustrate, respectfully,
an example of the temporal power profile, spectral power profile
and chirp of pulses after compression according to the methods and
apparatus disclosed herein. The pulse shown in FIGS. 7A-7C is
modeled as compressed with a chirped volume Bragg grating having a
spectral dispersion of 55 ps/nm. The compressed pulse has a FWHM
time duration of about 3.6 ps (for a compression ratio 80/3.6 of
approximately 22.2), a peak power of about 1.2 MW and a pulse
energy of about 9.1 .mu.J. The peak power is less than a simple
calculation of the time duration of the compressed pulse divided by
the overall pulse energy because, for this particular embodiment,
some energy remains in a base portion of the pulse and the base
portion that is does not contribute to in FWHM determination of the
pulse width (see additional discussion below). Also, the modeling
does not include any loss incurred during the pulse compression
process. With reference to FIG. 7B, the compressed pulse comprises
a spectral bandwidth of about 0.774 nm and can also include a pair
of outer wings, representing, for the spectrum shown in FIG. 7B, a
pair of substantially equal maxima. As indicated by FIG. 7C, the
compressed pulse comprises a nonlinear chirp.
[0119] FIG. 8 schematically plots normalized peak power, pulse
fraction energy, and derivative power for the compressed pulse
represented by FIGS. 7A, 7B and 7C. The curve labeled "Normalized
Peak Power" is the right hand half of the plot shown in FIG. 6A,
with the vertical power scale now plotted as normalized to one. The
vertical line 515 separates the pulse into surge pulse (indicated
by reference numeral 520A) and base pulse (indicated by reference
numeral 520B) portions. A similar vertical line, not shown, can be
drawn for the left hand part of the pulse, with surge pulse portion
being located between the right hand (line 515) and left hand (not
shown) lines. The vertical line 515 crosses the Pulse Fraction
Energy curve at a pulse fraction energy of about 45%, indicating
that the surge pulse comprises about 45% percent of the energy of
the overall pulse. The percentage of the energy included in the
surge pulse portion can be a function of factors including the
spectrum of the pulse and the degree of nonlinearity of the chirp.
The degree of nonlinearity can be different for different parts of
the spectral profile. For example, the aforementioned spectral
"wings" shown in FIG. 6B correspond to the more highly nonlinearly
chirped portions of the pulse and are understood to contribute to
formation of the base pulse portion of the overall pulse. The
derivate power curve is the derivative of the normalized peak power
curve.
[0120] In certain practices of the disclosure, pulses compressed by
the pulse compression apparatus described herein (e.g., such as in
any of the embodiments described above) can have a time duration of
no less than about 2 ps and/or a pulse energy of at least 5 .mu.J.
Typically the pulse compression apparatus provides a dispersion of
at least 50 ps/nm and/or provides a compression ratio (ratio of the
time duration of the ps optical pulses received by the pulse
compressor apparatus to the time duration of the compressed ps
optical pulses) of no greater than about 50. However many
variations are possible, as noted below.
[0121] A pulse compressor apparatus of a pulsed fiber laser as
disclosed herein, such as in, for example, the specific embodiments
of methods and embodiments described above (and below), can provide
a spectral dispersion of at least about 50 ps/nm, at least about
100 ps/nm, at least about 150 ps/nm, at least about 200 ps/nm, at
least about 250 ps/nm, at least about 300 ps/nm, at least about 350
ps/nm, at least about 400 ps/nm, or at least about 500 ps/nm. In
any combination with any of the foregoing, the pulse compression
apparatus can provide different compression ratios, such as, for
example, a pulse compression of no greater than about 200, no
greater than about 150, no greater than about 125, no greater than
about 100, no greater than about 75, no greater than about 30, no
greater than about 25 or no greater than about 15.
[0122] A fiber amplifier of the present application is typically
configured to operate in a regime where the length of the gain
fiber is less or much less than the dispersion length (Zd) of the
pulse being amplified. This is not typically understood to be
desirable, as in such a regime the nonlinear chirp associated with
spectral broadening in the nonlinear fiber amplifier will affect
the pulse compression, such as by causing the compressed pulse to
include an undesirable "pedestal". Pedestal-free or reduced
pedestal pulses are typically understood to result from the
condition opposite to the above--amplifier operation in the regime
wherein the length of the gain fiber is much greater than the pulse
dispersion length.
[0123] Without wishing to be bound by theory (except in a claim
where specifically recited) some discussion may be helpful. In
pulse compression it is usually desirable that the chirp across the
pulse to be compressed is as linear as possible. Unfortunately,
nonlinear amplification typically introduces significant nonlinear
chirp. However, even in the presence of such nonlinear chirp, the
chirp of the pulse can be linearized, at least to some extent, if
the nonlinear gain medium has an optical length that is much longer
than the dispersion length Zd of the pulse. Consider equations (1)
and (2) below. Equation (1) simply states the length of the gain
fiber to be much greater than the dispersion length of the
pulse:
Z.sub.a>>Z.sub.d where Z.sub.d=.tau..sup.2/k'' (1)
[0124] In expression (1) .tau.=.tau..sub.exp/1.76 and
k''=(.lamda..sup.2/2.pi.c)D, (2)
where .tau..sub.exp is the measured pulse width and D is the fiber
dispersion in ps/nm km.
[0125] If .lamda.=1064 nm and D=-40 ps/nm km, then
k''=7.5-10.sup.-28 sec.sup.2/cm and for .tau..sub.exp=10 ps we have
Z.sub.d=460 m.
[0126] Thus for high quality (pedestal free) compression of 10 ps
pulses it is preferable to use an amplifier (gain fiber) length
much longer than 460 m. This is completely impractical in most
instances.
[0127] However, if 1 ps pulses are used instead of 10 ps pulses,
then the dispersion length Zd becomes just 4.6 m; for a 0.5 ps or
500 fs pulse, the dispersion length Zd is 1.1 m. 4.6 m and 1.1 m
are much closer to practical lengths of gain fiber used in an
amplifier.
TABLE-US-00001 Pulsewidth 0.5 ps 10 ps 100 ps Fiber Dispersion 40
ps/nm km Dispersion Length Z.sub.d 1.1 m 460 m 46 km
[0128] Generally speaking the shorter the pulse width, the closer
the approximation to the condition Za>>Zd. Nevertheless, in
many practices of the present disclosure, longer ps pulse widths
are used such that the gain fiber length is much shorter than the
dispersion length Zd. However, Applicants consider that this
condition, typically considered to be a disadvantage, can in fact
allow for a simplified method of providing temporally shaped pulses
useful for material processing, such as the pulses described herein
as having a surge and base pulse portions. Accordingly, the
apparatus and methods described herein as producing pulses have
surge and base pulse portions can include a nonlinear optical
amplifier, or involve nonlinear amplification, wherein the length
optical gain medium (e.g., length of optical gain fiber) is not
substantially greater than the dispersion length. The length of the
optical gain medium can be on the order of, or less than, the
dispersion length.
[0129] Accordingly, in certain practices of the disclosure, a
compressed pulse is formed having a temporal power profile having a
base pulse portion and surge pulse portion within the base pulse
portion is used for materials processing. Temporal pulse profiles
other than a Gaussian temporal profile have been found useful in ns
systems for material removal. See, for example, U.S. Pat. No.
7,348,516, entitled "Methods of and Laser Systems for Link
Processing Using Tailored Pulses with Specially Tailored Power
Profiles", issued Mar. 25, 2008 to Sun et al. Applicants consider
that non-Gaussian temporal pulse profiles can be of use for ps
materials processing, and in particular pulses formed from what may
normally be considered flawed pulse compression.
[0130] FIG. 9 is a chart schematically illustrating steps that can
be followed in one practice of a method of the present disclosure.
As indicated by reference numeral 720, the method can comprise
inputting optical pulses to an optical amplifier. As indicated by
reference numeral 730, the method can further comprise amplifying
optical pulses with the optical amplifier operating in a nonlinear
regime such that it spectrally broadens the optical pulses by at
least a first selected broadening factor. Referring to reference
numeral 740, the method can include compressing the optical pulses
with a pulse compressor apparatus to provide compressed optical
pulses, where the pulse compressor apparatus provides at least
about a first amount of dispersion and has a compression ratio that
is less than or equal to about a first selected compression ratio.
Proceeding to consideration of reference numeral 750, the method
can include providing an optical output in optical communication
with the pulse compressor apparatus, where the optical output is
configured for delivering optical output pulses, such as to a
material for the processing thereof.
[0131] FIG. 10 is a chart schematically illustrating steps of
another method that can be practiced in accordance with the present
disclosure. As indicated by reference numeral 820, the method can
comprise nonlinearly amplifying optical pulses with an optical
fiber amplifier such that the optical pulses are spectrally
broadened and include nonlinear chirp. With reference to reference
numeral 830, the method can include compressing the nonlinearly
amplified optical pulses to provide compressed optical pulses
having a temporal power profile having a base pulse portion and a
surge pulse portion within the base pulse portion, wherein the peak
of the surge pulse portion is the peak of the temporal power
profile. As indicated by reference numeral 840, the method can
comprise directing compressed optical output pulses to a material
for the processing thereof, where the surge pulse portion of the
compressed optical output pulse has a peak power of at least 500 kW
and an optical energy that is no more than 75% of the optical
energy of the compressed optical output pulse including the surge
and base pulse portions thereof.
[0132] The methods and apparatus described herein, such as the
foregoing methods, can be practiced in accordance with the various
aspects and practices of the disclosure herein. That is, for
example, as the skilled artisan will readily appreciate from a
reading of the present disclosure, the first selected compression
ratio can be any of the compression ratios described herein (such
as, for example, the compression ratios described above a latter
portion of the Summary). Similarly, the first selected amount of
dispersion can be any of the amounts of dispersion described herein
in conjunction with compression or a compression apparatus (e.g.,
in the same paragraph with the compressions ratios described in the
latter portion of the Summary), alone or in any combination with
any of the disclosed compression ratios (as noted in the Summary).
The foregoing consideration applies to the other parameters
described herein, such as, for example, the spectral bandwidth of
the input pulses, which, for example, can have any of the values
noted in paragraph 30 above. By way of example, and not limitation,
in one practice of the method, the input optical pulses can have a
spectral bandwidth of less than or equal to 0.5 nm and/or a time
duration of less than or equal to 40 ps. The first selected
broadening factor can be, for example, 8, or 10, or 15, or 20.
[0133] As noted above, a pulsed fiber laser apparatus according to
the present disclosure can include a spectral filter, such as the
optional spectral filter 44 shown in FIG. 1. A spectral filter need
not be optically downstream of the pulse compressor apparatus 28,
as shown in FIG. 1, but can be incorporated elsewhere with the
fiber laser apparatus 12, such as, for example, with the pulse
compressor apparatus 28. For example, the pulse compressor
apparatus 300 shown in FIG. 4 can include the optional spectral
filter 350. The pulse fiber laser apparatus can include a tunable
spectral filter. For example, the spectral filter 350 can be
tunable so as to provide a band (e.g., a stop band or pass band)
that is tunable in bandwidth and/or center wavelength.
[0134] Spectral filtering before and/or after pulse compression can
adjust the parameters of the surge pulse relative to the base
pulse. For example, the spectral filter can be configured to filter
out part or all of the aforementioned spectral wings shown in FIG.
6B. Because the wings represent nonlinear chirp, which in turn
contributes to the formation of the base pulse portion, filtering
out or otherwise modifying the wavelengths of the wings can affect
the shape of the pulse by modifying the base portion relative to
the surge portion. Generally speaking, a spectral filter can be
optically located upstream or downstream of the pulse compressor
apparatus and otherwise configured for changing, for example, the
ratio of the temporal duration and/or the energy of the surge pulse
portion to the overall temporal duration of the output pulse.
[0135] FIG. 11 schematically illustrates another embodiment of a
fiber laser according to the present disclosure, which can include
a bulk optical amplifier. The embodiment of FIG. 11 includes a seed
source, which can comprise the fiber delivered pulsed seed laser
920, a nonlinear amplifier 924, which can comprise a nonlinear
fiber amplifier as described above (e.g., a described in
conjunction with FIG. 2), a bulk optical amplifier 950, and a pulse
compressor 930. Also shown in FIG. 11 are a pre amplifier 936 and
pulse picker 942. As the skilled worker familiar with the present
disclosure will readily appreciate, the embodiment shown in FIG. 1
can also include a pulse picker and/or a preamplifier, and the
embodiment of FIG. 11 an include a pulses stretcher, such as the
pulse stretcher 40 as described in conjunction with the discussion
of FIG. 1.
[0136] The bulk optical amplifier 950, which can comprise, for
example, diode pumped solid state (DPSS) amplifier, can have a
spectral bandwidth that is less than the spectral bandwidth of the
nonlinearly amplified pulse, and can filter the pulse so as to, for
example, remove in whole or in part the aforementioned spectral
"wings". The filtering action of the bulk amplifier can result in
pulse have a reduced or eliminated base portion, allowing some
conditioning of the output pulse. Selection or control of the
spectral bandwidth of the filter (e.g., a solid state amplifier
having spectral bandwidth that is substantially narrower than that
of the nonlinearly amplified pulse) can allow selection or control
of the shape of the compressed, such as, for example, the relative
time duration or energy content of the surge and base pulses, and
hence of the surge pulse to the overall pulse.
[0137] FIG. 11 schematically illustrates the spectral power
profiles 953, 956 and 960. Spectral power profiles 953 and 956
represent, respectively, pulse spectral power profiles before and
after nonlinear amplification. Spectral power profile 960 represent
the pulse spectral profile after spectral filtering, such as
spectral filtering the bulk optical amplifier 950, thought more
generally it can represent spectral filtering by any spectral
filter, such as a dedicated spectral filter (i.e., a optical
element that does not have a substantial function other than
filtering) or, as another example, by a grating, such as a VBG or
CVBG (e.g., a CVBG used for pulse compression) having a spectral
bandwidth that is less than that of the optical pulse downstream of
the nonlinear amplification process. Note that the filtered
spectral power profile 960 does not include the "wings" at the
outer most portions of the spectral power profile 956. In the
example shown in FIG. 11, the spectral filtering process filters
out substantially all of the spectrum that is responsible for base
portion of the compressed pulse, such that both temporal power
profiles 965 and 967, which schematically represent, respectively,
temporal pulse profile before nonlinear amplification and after
compression, represent "clean" pulses.
[0138] In various practices of the present disclosure a spectral
filter, and hence the spectral filtering process of a method
practiced in accordance with the teachings herein, has a spectral
bandwidth that is at least 5%, at least 15%, at least 25%, or at
least 35% or at least 45% less than that of the pulse received by
filter, which received pulse will typically include spectral
components added or enhanced during the nonlinear amplification.
The percentage by which the spectral bandwidth is less can be
determined by taking the difference between the spectral bandwidth
of the received pulse and that of the spectral filter and dividing
by the spectral bandwidth of the received pulse.
[0139] As the ordinarily skilled worker, apprised of the present
disclosure, will appreciate, a spectral filtering process as
described above in conjunction with FIG. 11, or in any other method
or apparatus embodiment described herein, can be tunable such
relative powers or temporal durations of the surge and base
portions can be adjusted. For example, the bandwidth of the
spectral filter can be tuned, and or the degree of suppression of
particular wavelengths. In a variation of the embodiment shown in
FIG. 11, spectrally filtering with a larger pass band can provide
that the compressed pulse includes a base and surge pulse
portions.
[0140] One of ordinary skill in the art will recognize, based on
the disclosure herein, that in many instances structures
alternative to those shown in the appended FIGURES can be used to
achieve the benefits of the inventions disclosed herein.
[0141] Several embodiments of the invention have been described and
illustrated herein. Those of ordinary skill in the art will readily
envisage a variety of other means and structures for performing the
functions and/or obtaining the results or advantages described
herein and each of such variations or modifications is deemed to be
within the scope of the present invention. More generally, those
skilled in the art would readily appreciate that all parameters,
dimensions, materials and configurations described herein are meant
to be exemplary and that actual parameters, dimensions, materials
and configurations will depend on specific applications for which
the teaching of the present disclosure is used.
[0142] Those skilled in the art will recognize or be able to
ascertain using no more than routine experimentation many
equivalents to the specific embodiments of the invention described
herein. It is therefore to be understood that the foregoing
embodiments are presented by way of example only and that within
the scope of the appended claims and equivalents thereto, the
invention may be practiced otherwise than as specifically
described. The present disclosure is directed to each individual
feature, system, material and/or method described herein. In
addition, any combination of two or more such features, systems,
materials and/or methods, if such features, systems, materials
and/or methods are not mutually inconsistent, is included within
the scope of the present invention.
[0143] In the claims as well as in the specification above all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving" and the like are understood to
be open-ended. Only the transitional phrases "consisting of" and
"consisting essentially of" shall be closed or semi-closed
transitional phrases, respectively, as set forth in the U.S. Patent
Office Manual of Patent Examining Procedure .sctn.2111.03, 7th
Edition, Revision.
[0144] The phrase "A or B" as in "one of A or B" is generally meant
to express the inclusive "or" function, meaning that all three of
the possibilities of A, B or both A and B are included, unless the
context clearly indicates that the exclusive "or" is appropriate
(i.e., A and B are mutually exclusive and cannot be present at the
same time). "At least one of A, B or C" (as well as "at least one
of A, B and C") reads on any combination of one or more of A, B and
C, including, for example the following: A; B; C; A & B; A
& C; B & C; A & B; as well as on A, B & C.
[0145] It is generally well accepted in patent law that "a" means
"at least one" or "one or more." Nevertheless, there are
occasionally holdings to the contrary. For clarity, as used herein
"a" and the like mean "at least one" or "one or more." The phrase
"at least one" may at times be explicitly used to emphasize this
point. Use of the phrase "at least one" in one claim recitation is
not to be taken to mean that the absence of such a term in another
recitation (e.g., simply using "a") is somehow more limiting.
Furthermore, later reference to the term "at least one" as in "said
at least one" should not be taken to introduce additional
limitations absent express recitation of such limitations. For
example, recitation that an apparatus includes "at least one
widget" and subsequent recitation that "said at least one widget is
colored red" does not mean that the claim requires all widgets of
an apparatus that has more than one widget to be red. The claim
shall read on an apparatus having one or more widgets provided
simply that at least one of the widgets is colored red. Similarly,
the recitation that "each of a plurality" of widgets is colored red
shall also not mean that all widgets of an apparatus that has more
than two red widgets must be red; plurality means two or more and
the limitation reads on two or more widgets being red, regardless
of whether a third is included that is not red, absent more
limiting explicit language (e.g., a recitation to the effect that
each and every widget of a plurality of widgets is red).
* * * * *