U.S. patent application number 14/967504 was filed with the patent office on 2016-04-07 for optical pulse source with increased peak power.
This patent application is currently assigned to IMRA AMERICA, INC.. The applicant listed for this patent is IMRA AMERICA, INC.. Invention is credited to Gyu CHO, Jingzhou XU.
Application Number | 20160099541 14/967504 |
Document ID | / |
Family ID | 46798697 |
Filed Date | 2016-04-07 |
United States Patent
Application |
20160099541 |
Kind Code |
A1 |
XU; Jingzhou ; et
al. |
April 7, 2016 |
OPTICAL PULSE SOURCE WITH INCREASED PEAK POWER
Abstract
In at least one embodiment time separated pulse pairs are
generated, followed by amplification to increase the available peak
and/or average power. The pulses are characterized by a time
separation that exceeds the input pulse width and with distinct
polarization states. The time and polarization discrimination
allows easy extraction of the pulses after amplification. In some
embodiments polarization maintaining (PM) fibers and/or amplifiers
are utilized which provides a compact arrangement. At least one
implementation provides for seeding of a solid state amplifier or
large core fiber amplifier with time delayed, polarization split
pulses, with capability for recombining the time separated pulses
at an amplifier output. In various implementations suitable
combinations of bulk optics and fibers may be utilized. In some
implementations wavelength converted pulse trains are generated. A
method and system of the present invention can be used in time
domain applications utilizing multiple beam paths, for example
spectroscopy.
Inventors: |
XU; Jingzhou; (Ann Arbor,
MI) ; CHO; Gyu; (Ann Arbor, MI) |
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Applicant: |
Name |
City |
State |
Country |
Type |
IMRA AMERICA, INC. |
Ann Arbor |
MI |
US |
|
|
Assignee: |
IMRA AMERICA, INC.
Ann Arbor
MI
|
Family ID: |
46798697 |
Appl. No.: |
14/967504 |
Filed: |
December 14, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13413304 |
Mar 6, 2012 |
9240670 |
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14967504 |
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61449955 |
Mar 7, 2011 |
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Current U.S.
Class: |
372/6 ;
372/25 |
Current CPC
Class: |
H01S 3/0057 20130101;
H01S 3/302 20130101; H01S 3/10061 20130101; H01S 3/2383 20130101;
H01S 3/0092 20130101; H01S 3/2308 20130101; H01S 3/06712 20130101;
H01S 3/11 20130101 |
International
Class: |
H01S 3/11 20060101
H01S003/11; H01S 3/067 20060101 H01S003/067; H01S 3/10 20060101
H01S003/10; H01S 3/23 20060101 H01S003/23 |
Claims
1. A pulsed laser system, comprising: an input providing time
separated pulses having a temporal delay therebetween, each pulse
having a different polarization state; and a single medium in which
said time separated pulses propagates, wherein a peak power and
energy of each pulse is sufficiently low to avoid substantial
distortion of a pulse output from said medium, wherein the power of
the time separated pulses, if combined in said medium without said
temporal delay, would exceed a non-linear threshold of said medium,
wherein said system is configured such that the time separated
pulses output from said medium propagate in separate optical paths
providing for synchronization of time separated pulses.
2. The pulsed laser system of claim 1, comprising a wavelength
converter disposed in a first optical path and receiving a first
pulse from said medium, and converting the wavelength of said first
pulse from said medium to a first converted wavelength.
3. The pulsed laser system claim 2, wherein said system comprises a
second wavelength converter in a second optical path and receiving
a second pulse from said medium, and converting the wavelength of
said second pulse from said medium to a second converted
wavelength.
4. The pulsed laser system of claim 1, wherein said input comprises
a gain medium comprising substantially all-fiber and no bulk
optical components in said gain medium.
5. The pulsed laser system of claim 4, wherein said input comprises
a delay generator, and at least a portion of said delay generator
comprises bulk optics.
6. A pulsed laser system, comprising: a seed source generating a
pulse; a polarization splitter to split said pulse into different
polarization states, thereby forming polarization split pulses; a
delay generator which receives said polarization split pulses and
generates time separated pulses, said delay generator configured to
control a temporal relationship between said polarization split
pulses and to provide a temporal delay therebetween, each pulse
having a different polarization state; a single medium in which
said time separated pulses having said different polarization
states propagate, wherein a peak power and energy of each of said
time separated pulses is sufficiently low to avoid substantial
distortion of a pulse output from said medium, wherein the power of
the time separated pulses, if combined in said medium without said
temporal delay, would exceed a non-linear threshold of said medium;
and a combiner that receives said time separated pulses from said
medium and substantially re-combines the time separated pulses and
compensates said temporal delay to form an output pulse having
increased peak power, wherein said polarization splitter and said
delay generator introduce delay larger than duration of said pulse
generated by said seed source.
7. The pulse laser system of claim 6, wherein said polarization
splitter comprises PM fiber operably arranged such that an input
beam to said PM fiber is coupled into both the fast and slow axes
of said PM fiber, via fiber splicing, and said polarization
splitting is controlled by angular offset in splicing.
8. A pulse laser system, comprising, a seed source generating an
optical pulse; an input providing polarization split optical
pulses, each polarization split pulse having a different
polarization state; and one, or a series, of optical fibers at
least one of which comprises a gain fiber having a gain medium,
said time separated pulses propagating in said one or series of
optical fibers and exhibiting variable and increasing temporal
separation in at least one portion therein, said time separated
pulses having said different polarization states, wherein a peak
power and energy of each of said time separated pulses is
sufficiently low to avoid substantial distortion of a pulse output
from said one or series of optical fibers, wherein the power of the
time separated pulses, if combined in said gain medium without said
temporal separation, would produce substantial degradation of
output pulse quality; and a combiner that receives said time
separated pulses from said one or series of optical fibers and
substantially re-combines the time separated pulses and compensates
said temporal delay to form an output pulse having increased peak
power.
9. The pulse laser system of claim 8, wherein said input is
integral with said one or series of optical fibers and said
polarization split pulses are generated therein.
10. The pulse laser system of claim 8, wherein pulses provided by
seed source are linearly polarized.
11. The pulse laser system of claim 8, wherein pulses provided by
seed source are circularly polarized, elliptically polarized, or
non-polarized, depolarized or partially polarized.
12. The pulse laser system of claim 8, wherein said input comprises
a PM fiber pigtail optically connected to said seed source and
arranged to receive said pulse, and operably arranged such that
power is distributed in each of the fast and slow axes of said one
or more fibers disposed downstream from said input.
13. The pulse laser system of claim 8, wherein said input is
disposed downstream from said seed source.
14. The pulse laser system of claim 8, wherein said input comprises
a polarization splitter and a delay generator.
15. The pulse laser system of claim 8, wherein said input comprises
a linearly polarized beam, and wherein at least one fiber is a PM
fiber, said input and said one or series of optical fibers operably
arranged such power is distributed in each of the fast and slow
axes of said PM fiber.
16. The pulse laser system of claim 8, wherein said gain fiber is a
PM amplifier fiber.
17. The pulse laser system of claim 8, wherein at least one fiber
supports multiple modes.
18. The pulse laser system of claim 8, wherein at least one fiber
is single mode.
19. The pulse laser system of claim 8, wherein said one or series
of fibers comprises: a large mode multimode amplifier fiber (MMFA)
capable of providing substantially fundamental mode output, large
core leakage channel amplifier fiber (LCF design), photonic crystal
amplifier fiber (PCF design),
20. The pulse laser system of claim 8, wherein if the power of said
time separated pulses were combined in said medium without said
temporal delay, a non-linear threshold of said medium would be
exceeded, and one or more nonlinear effects would substantially
degrade output pulse quality.
21. The pulse laser system of claim 8, wherein if the power of said
time separated pulses were combined in said medium without said
temporal delay, output pulse quality would be substantially
degraded by pulse breakup or noise.
22. The pulse laser system of claim 8, wherein one or both of
polarization splitting or delay generation are implemented in an
all-fiber arrangement, including splices between fibers.
23. The pulse laser system of claim 8, wherein said series of
fibers are joined by fiber splices.
24. A pulse laser system, comprising, a seed source generating a
linearly polarized optical pulse; one or more optical fibers,
including at least one polarization maintaining (PM) optical fiber
optically connected to said seed source and operably arranged to
generate polarization split optical pulses having different
polarization states and a temporal delay therebetween; and a
combiner that receives said time separated pulses as output from
said one or more optical fibers, and substantially re-combines the
time separated pulses and compensates said temporal delay to form
an output pulse having increased peak power, wherein a peak power
and energy of each of said time separated pulses is sufficiently
low to avoid substantial distortion of a pulse output from said one
or more optical fibers, wherein the power of the time separated
pulses, if combined in said one or more fibers without said
temporal delay, would substantially degrade output pulse
quality.
25. The pulse laser system of claim 24, wherein said seed source
comprises a passively mode locked laser.
26. The pulse laser system of claim 25, wherein said passively mode
locked laser includes a fiber gain medium.
27. The pulse laser system of claim 24, wherein passively mode
locked fiber laser comprises PM fiber.
28. The pulse laser system of claim 24, wherein said at least one
PM fiber comprises a PM fiber amplifier.
29. The pulse laser system of claim 24, wherein at least one of
said one or more optical fibers comprises a gain medium.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation application of U.S.
application Ser. No. 13/413,304 filed Mar. 6, 2012, which claims
benefit of Provisional Application No. 61/449,955, filed Mar. 7,
2011. The above-noted applications are incorporated herein by
reference in their entirety.
FIELD OF THE INVENTION
[0002] The invention relates to methods and systems for generating
laser pulses with high average power or high peak power, and is
particularly applicable to time-domain spectroscopy such as
pump-probe or terahertz measurement, where multiple beams carrying
pulse trains are to be utilized, or where average power and/or
pulse energy can be provided without undesirable nonlinear
effects.
BACKGROUND
[0003] Utilization of pulsed laser sources has increased in
industrial and scientific applications. In particular, applications
of ultrashort laser technology have increased over the last few
years in metrology, imaging and material processing applications.
Fiber-based ultrashort systems are now well established for
numerous applications, and are particularly well suited for high
repetition rate applications at low-medium pulse energy. However,
in either passive or gain fiber, the peak power of the amplified
pulse is constrained because of the pulse distortion and signal
shifting out of the gain spectrum caused by nonlinear effects, for
example Raman shifting. Chirped pulse amplification is often used
to greatly extend the capability of fiber systems. Pulses are
temporally stretched, thereby lowering the peak power, then
amplified and recompressed. Such constraints also apply to other
optical media as the pulse energy scales up, for example Nd: based
bulk optical amplifiers.
[0004] The following patents, published patent applications, and
publications relate, at least in part, to fiber lasers and
amplifiers, ultrashort laser material processing, optical
measurement techniques, and/or various arrangements for generating
groups of laser pulses: U.S. Pat. No. 6,339,602; U.S. Pat. No.
6,664,498; U.S. Pat. No. 6,954,575; U.S. Pat. No. 7,088,878; U.S.
Pat. No. 7,580,432; U.S. Patent Application Pub. No. 2002/0167581;
U.S. Patent Application Pub. No. 2003/0151053; U.S. Patent
Application Pub. No. 2005/0218122; U.S Patent Application Pub. No.
2010/0272137; WIPO Pub. No. 2009146671; Strickland and G. Mourou,
Opt. Commun. 56, 219 (1985). H. Hofer et. al., Opt. Lett. 23, 1840
(1998); M. E. Fermann et al., Phys. Rev. Lett., 84, 2000
(2010).
[0005] Various applications require multiple beams or pulse trains.
In such applications, pulses in the multiple beams may have a
well-defined relative time interval, requiring some level of
synchronization. Time domain measurements are an example. More
specifically, with optical time gating or correlation techniques, a
first beam is used for optical interaction with a sample, and a
second beam is used for a time gating or correlation function.
Specifically, for an ultrashort measurement, synchronization is
needed to obtain the desired time resolution. Terahertz
spectroscopy, optical pump-probe spectroscopy and other time gated
imaging processes utilizing an ultrashort pulse laser fall into
this application category.
[0006] Conventional laser-based systems used for such applications
are often designed to create pulses with sufficiently high energy,
and to subsequently divide the beam into multiple beam paths in the
application system.
[0007] Amplification of high intensity optical pulses in an optical
fiber and other gain media, for example regenerative amplifiers,
ultimately requires consideration of nonlinearity. Often the pulse
energy constraint results in limited average power. Increasing the
average power without loss of pulse energy would be a useful
improvement for high peak power pulse laser systems.
[0008] Therefore, a need exists to extend the peak power capability
of pulsed laser sources, including fiber based systems,
regenerative amplifiers, thin disk lasers, and the like.
SUMMARY OF THE INVENTION
[0009] In one aspect the present invention features a method to
reduce nonlinear pulse distortion and to increase the available
average power of a pulsed laser source.
[0010] Amplifying pulses distributed in the time domain reduces
nonlinear effects which would otherwise be induced by high peak
power. If the time distributed pulses have different polarization
states, the pulses can be easily separated.
[0011] In various embodiments a pulse is split into distinct
polarization states prior to or during amplification. The resulting
split pulse portions may have orthogonally linear polarizations.
The distinct polarization states provide for easy extraction of the
synchronized pulse trains into multiple beams utilizing relatively
simple polarization sensitive devices. A time separation between
pulse pairs can be introduced during propagation in an optical
medium, with time separation being greater than a pulse width.
[0012] In some embodiments a second splitting unit may be used to
recombine two polarization split, time separated pulses by
propagating a beam in an opposite direction to compensate the time
delay. In some implementations the second splitting unit may
include the same optical components as the first unit.
[0013] In various embodiments, polarization split, time separated
pulses are generated in an active medium before the pulses are
amplified to a threshold at which unwanted nonlinear effects
occur.
[0014] In at least one embodiment pulses are temporally split prior
to amplifying with one or more amplifier stages.
[0015] In at least one embodiment both the polarization splitting
and delay generation are implemented in an all-fiber arrangement,
which may include active or passive polarization maintaining (PM)
fiber. For example, with PM optical fiber, a polarization splitter
and delay generator may be integral with an active/passive PM fiber
medium and not require separate components.
[0016] In a fiber laser configuration the oscillator output can be
separated into two or more beams, with an optional delay stage for
each beam path, before being combined and injected into the
amplifier fiber. If sufficient delay is provided beyond the pulse
width of the pulse then the pulses are amplified without
interference, thereby preserving the duration of each pulse.
[0017] In various embodiments utilizing linearly polarized pulses,
the split beams may be manipulated so that the polarization is
linear but with orthogonal polarization states. After amplification
in a gain medium, a polarization sensitive device can easily
separate the pulses for subsequent operations. In various
embodiments utilizing fiber lasers with polarization maintaining
fibers, the splitting in time and polarization can be further
simplified by utilizing the group velocity difference of the two
orthogonally linear polarized pulses in the fiber. In this example,
the input polarization to the PM fiber is set so that slow and fast
axis polarizations are simultaneously excited. After a sufficient
propagation length in the fiber the pulses in the two polarization
components will be separated, preferably by more than the pulse
width of the input pulse. At least two amplified pulse trains can
then be extracted by a polarization component.
[0018] In at least one embodiment, the two laser pulses can be
coherently or incoherently combined to generate a single laser
pulse with higher peak power output than a non-linear threshold of
at least one medium.
[0019] In at least one embodiment, laser pulse trains in both
polarization states, or in one polarization state, can be input to
an optical nonlinear device to convert a first wavelength to a
second wavelength.
[0020] In one application the laser output comprises two physical
beams, and the corresponding pulse trains are synchronized.
Non-limiting examples of applications include polarization based
material modification and processing, time domain spectroscopy and
imaging based on the time domain information, and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1A schematically illustrates conventional pulse
propagation in a medium which results in pulse distortion. FIG. 1B
schematically illustrates time separated pulses with different
polarization states in the medium and pulses being combined into a
single output beam with higher power or pulse energy, and
negligible distortion.
[0022] FIG. 2 schematically illustrates an example of a pulsed
laser source utilizing PM fibers.
[0023] FIG. 3A schematically illustrates an example of a
polarization splitting unit or combining unit.
[0024] FIG. 3B schematically illustrates an example of a delay
generator with controllable delay.
[0025] FIG. 4A schematically illustrates another example of a
polarization splitter or reciprocal combiner.
[0026] FIG. 4B schematically illustrates two examples of delay
generators.
[0027] FIG. 5 is a plot showing a measured autocorrelation function
(ACF) corresponding to two time separated and amplified pulses.
[0028] FIG. 6 illustrates an exemplary system for an application
where two beams and two separate pulses are used to obtain time
domain information.
[0029] FIG. 7 schematically illustrates time separated,
polarization split pulses in a plurality of optical paths and
wavelength conversion of one or more pulses. A pre-determined time
separation provides for synchronization.
DETAILED DESCRIPTION
[0030] In at least one embodiment the available average output
power of an amplifier is increased without substantial increase of
pulse energy.
[0031] In at least one embodiment one pulse is split into a pair of
pulses, each having a different polarization state. A relative
delay between the polarization split pulses is generated and
temporally separates the pulses during propagation in a medium, for
example a passive optical material or an active, amplifying gain
medium. In some embodiments the pulses may separate, at least in
part, during amplification in a gain medium.
[0032] Some pulsed laser sources, such as fiber laser amplifiers
utilizing PM fibers, preserve the polarization states. When
excitation is sufficient, each polarization state can independently
propagate in the laser source.
[0033] By way of example, FIGS. 1a and 1b compare pulse propagation
110 in a medium of a conventional laser system with propagation 120
of time separated pulses in accordance with one implementation of
the present invention. FIG. 1a shows a laser pulse 105a propagating
in a portion of medium 101, which can be a gain medium or passive
medium. This example illustrates output pulse distortion 105-b
induced by nonlinear effects within the medium. Such effects may be
manifested by pulse breakup, noise, and other distortion. As
discussed above, distortion levels caused by non-linear effect(s)
constrain the achievable peak power for various laser-based
applications. It is known that Raman shifting and/or self-phase
modulation can significantly transform a Gaussian-like input pulse
to a substantially distorted temporal pulse shape similar to 105-b,
for example. Non-linear effects have been exploited to improve
pulse quality, for example as disclosed in U.S. Pat. No. 7,414,780,
where output pulse quality was improved with an increase in
self-phase modulation. Nevertheless, further increase in the
available peak power is beneficial in such a system, at a power
level when substantial degradation of output pulse quality is
observed.
[0034] FIG. 1b schematically illustrates how laser pulses with
orthogonal polarizations 115-a can be used to double the available
output maximum pulse energy in a pulsed laser system. As
schematically illustrated in FIGS. 1a and 1b, an output pulse 115-b
having increased peak power may have a pulse temporal shape similar
to the temporal shape of a pulse 105-a generated with the seed
source, e.g.: prior to a time and location at which the input
(seed) pulse becomes distorted. If two laser pulses with orthogonal
polarization are temporally separated prior to or during
propagation in the medium 101, each pulse can separately reach the
maximum pulse energy supported by the medium before onset of
non-linear effects. With pre-determined beam profile and pulse
shape, the maximum pulse energy can be determined. An optional
combiner after the medium 101 compensates the temporal delay
between these two pulses, and combines the two pulses into a single
pulse. A resultant pulse 115-b has twice the maximum energy of a
single pulse (before separation into the pulse pair).
[0035] FIG. 2 illustrates one example of the polarization splitting
and combining operation in an exemplary fiber-based laser system.
In this example, the laser source includes a seed laser, which may
include a mode locked laser oscillator coupled to PM fiber. In this
configuration the seed has a pure linear polarized output, depicted
with the vertical arrow. The seed laser is configured to provide
laser pulse trains with ultrashort pulses. The pulse trains are
amplified with a laser amplifier 220 having a doped gain fiber as a
gain medium, also based on PM fiber
[0036] One efficient way to split each seed pulse into two
orthogonal polarization states is to arrange PM fiber by splicing
two sections of PM fiber 210, 215 between the seed source and the
amplifier 220 with an angular shift. Schematic cross sectional
views 210-a, 215-a illustrate the relative angular displacement of
the PM fiber polarization axes. The axes are determined, at least
in part, by the birefringent material disposed in the fiber
cladding which partially surrounds the fiber core. Such splicing
may be carried out automatically with commercially available
splicing machines and software.
[0037] Assume polarization of the seed laser is parallel to the
slow axis of the input fiber. Then, with an angular shift of
.theta., the power in the fast axis and the slow axis of the output
fiber are I.sub.0 cos.sup.2.theta., and I.sub.0 sin.sup.2 .theta.,
respectively. In this particular example, the temporal delay
between these two polarization states is applied by the PM fiber in
an amplifier portion of the pulsed laser system. The PM fiber has
birefringence .delta.n on the order of 10.sup.-4 between its fast
and slow axes. This results a temporal delay from a few hundreds of
femsosecond (fs) to about a picosecond (ps) in each meter of PM
fiber. If the amplifier portion has a PM fiber length of a few
meters, several ps of temporal delay results. This temporal delay
is usually sufficient to separate two ultrashort pulses.
[0038] Notably, in this example, the temporal delay between the
pulses of a pulse pair varies in the laser amplifier 220 and two
pulses completely separate near the end portion of the amplifier.
This variable delay is not detrimental because laser power
increases during the propagation along with the separation. Thus,
although the pulse is not temporally separated immediately upon
injection to the amplifier, the total power of each pulse is still
below the maximum peak power because both pulses have reduced pulse
energy, and the pulses separate before the sum of the pulse powers
exceeds a threshold for non-linear effects. Maximum pulse
separation is to be obtained at or near the output end of the
amplifier, where the polarization split, time delayed amplified
pulses 230 have maximum energy.
[0039] It is to be understood that FIG. 2 illustrates one of many
possible implementations. For example, the seed laser may not be
linearly polarized, it can be in other polarization states, such as
circularly polarized, elliptically polarized, or even
non-polarized, depolarized or partially polarized. In various
embodiments a seed pulse in a pure polarization state is more
desirable for controlling the ratio between two orthogonal
polarization states in the amplifier, and well suited for coherent
combining after amplification. The required temporal separation
need not occur in a laser amplifier. The separation can also be
applied in a passive medium, laser oscillator, and/or
amplifier,
[0040] In various implementations the polarization splitting can be
done using free space coupling rather than fiber splicing. A
polarization splitter may include a mechanism for relative rotation
of a fiber or waveplate to further control and align the
polarization. A delay can also be set using bulk optical components
as will be discussed below with respect to FIG. 3 and FIG. 4.
Furthermore, a delay can be variable rather than fixed.
[0041] Moreover, the amplifier need not be a fiber amplifier. The
medium can be any suitable medium, passive or active, which limits
the maximum pulse energy, and may be disposed outside of the laser
source itself.
[0042] FIG. 3a illustrates an exemplary polarization splitting unit
(or reciprocal combining unit with a reversed beam). A slab of
birefringent crystal 310, which has different refractive indices
(n.sub.s, n.sub.p) for the two orthogonal polarization states, can
be used to split a pulse or combine polarization split, time
separated pulse pairs. LiNbO.sub.3, for example, has .delta.n=0.085
between the ordinary and extraordinary polarization states. A 3.5
mm thick portion of LiNbO.sub.3 can provide about 1 ps temporal
delay between the pulse pair. As illustrated in FIG. 3b, crystals
330-a, 330-b, each having a prism shape, can change the optical
path length, and thus control the temporal delay by translating one
or both crystals.
[0043] FIG. 4a illustrates yet another example of a polarization
splitting unit 400 (or reciprocal combining unit). A polarized beam
splitter PBS is used to separate the laser pulse into different
polarization states and direct them along different paths (arms).
Delay generating units 410-a, 410-b are configured to control the
temporal relationship between the pair of pulses.
[0044] FIG. 4b illustrates two examples of devices suitable for
delay generation. One is a linear delay line 420 and the other one
comprises two prisms 430-a, 430-b, similar to the components
illustrated in FIG. 3b, which can be used to control the optical
path length. Such devices may be used alone or in combination.
Also, although a two-fold increase in peak power is available with
two pulses, additional paths may be used to form more than two time
separated pulses in a desired time sequence, thereby providing for
further increased output peak power.
[0045] The pulses may then be recombined in a combiner, which may
comprise any suitable combination of bulk and fiber optics, for
example as schematically illustrated in FIGS. 3-4. As also
discussed above, a reversed path may be used to combine the pulses
in FIG. 4a.
[0046] By way of example, the output pulses may be used with
suitable beam conditioning optics as an input to one or more of a
downstream bulk solid state gain medium, a large mode multimode
amplifier fiber (MMFA) capable of providing substantially
fundamental mode output, large core leakage channel amplifier fiber
(LCF design), photonic crystal amplifier fiber (PCF design), a high
power coherent amplifier array, and/or other high peak power gain
media. A combiner comprising bulk optics may be implemented at an
output of the downstream gain medium to form a single pulse with
increased peak power (not separately shown). Similarly, in various
embodiments, the MMFA, LCF, or PCF may comprise PM fiber and
provide at least a portion of the splitting and delay
generation.
[0047] Moreover, various combinations of the above components and
configurations may be utilized for any application where
polarization split, time delayed pulse pairs can be advantageous to
improve the peak and/average output power capability of an optical
medium.
[0048] Different beams and pulse trains can be used in a time
domain measurement technique such as terahertz spectroscopy or pump
and probe spectroscopy. A schematic illustration of a time domain
measurement system 600 is shown in FIG. 6. A laser source 605 as
discussed above delivers two beams and pulse trains, where one
pulse train can be further delayed in time relative to the other
pulse train via delay line 610. Sample 1 620 interacts with a first
pulse train while the other pulse train interacts with the first
pulse train in Sample 2 630.
[0049] By way of example, Sample 1 can be a terahertz emitter and
Sample 2 the time gating element of the terahertz wave interacting
with the gating pulse which is time delayed. Pump and probe or
similar optical correlation techniques share a similar principle of
operation, in which the time gating of the optical signal is
induced in a sample.
[0050] In some embodiments one or both pulse trains may be
wavelength converted. By way of example, a harmonic converter, a
Raman shifter, or optical parametric amplifier (OPA) may be
utilized for wavelength conversion. The resultant output may be
combined and time synchronized, or processed separately. The
wavelength converter may be disposed either before or after a beam
combiner.
[0051] FIG. 7 schematically illustrates a further application of
two polarization split, time separated pulses. In this example, an
input provides polarization split, time separated pulses as
discussed above. The pulses are directed to separate optical paths.
In a first optical path a pulse is frequency doubled with second
harmonic generator SHG. The second pulse, propagating in a second
optical path, may have a pre-determined delay and therefore can be
time-synchronized with the frequency converted pulse. Many
variations are possible, for example wavelength converting in the
second optical path, combining, and the like.
[0052] In one experiment a Raman soliton laser amplifier was used
in a configuration similar to that illustrated in FIG. 2. A mode
locked fiber laser oscillator generated a central wavelength about
1560 nm, and was configured with a PM fiber pigtail. The output
laser polarization was aligned with the slow axis. The seed pulse
was injected into a laser amplifier by splicing the output PM
pigtail to a PM gain fiber. 45 degree shifting was applied during
splicing. Thus, equal power was split into the fast and slow axis
in the laser amplifier. The gain fiber was pumped with a laser
diode. In this example, a Raman soliton was formed during
amplification. The output Raman soliton had pulse duration of about
100 fs. With only one polarization seed, the Raman soliton pulse
energy saturated to maximum pulse energy. With two polarization
seed inputs, each polarization produced a Raman soliton and
saturated to the maximum pulse energy. As a result, the total Raman
soliton power was doubled.
[0053] FIG. 5 shows the measured autocorrelation function (ACF) of
the amplified laser output. The ACF clearly shows a double pulse
structure with 1.7 ps separation, which completely separates the
pulse pair. It is possible if the Raman soliton were not used in
the experiment, the time separation might have been slightly
different due to the difference in group velocity dispersion of
each polarization axis associated with the Raman generation
process. However, such variation is not significant in
demonstrating time separation of the pulses during the propagation
in a PM fiber.
[0054] Thus, the invention has been described in several
embodiments. It is to be understood that the embodiments are not
mutually exclusive, and elements described in connection with one
embodiment may be combined with, or eliminated from, other
embodiments in suitable ways to accomplish desired design
objectives.
[0055] At least one embodiment includes a pulsed laser system. The
system includes a seed source for generating a pulse. A
polarization splitter splits a pulse from the seed source into
different polarization states, thereby forming polarization split
pulses. A delay generator receives the polarization split pulses
and generates time separated pulses, each pulse having a different
polarization state. The system includes a medium in which the time
separated pulses having the different polarization states
propagate, wherein a peak power and energy of each of the time
separated pulses are individually sufficiently low to avoid
substantial distortion of a pulse output from the medium. The power
of the time separated pulses, if combined in the medium, would
exceed a non-linear threshold of the medium. A combiner receives
the time separated pulses from the medium and substantially
re-combines the time separated pulses to form an output pulse
having increased peak power.
[0056] In any or all embodiments a medium may include a
polarization splitter.
[0057] In any or all embodiments a medium may include a fiber gain
medium.
[0058] In any or all embodiments a medium may include a
polarization maintaining (PM) amplifier fiber.
[0059] In any or all embodiments a seed source may generate
linearly polarized pulses.
[0060] In any or all embodiments a seed source may include a mode
locked fiber oscillator.
[0061] In any or all embodiments at least a portion of a
polarization splitter may include a polarization sensitive, bulk
optic.
[0062] In any or all embodiments a delay generator and a
polarization splitter may be coupled with optical fiber.
[0063] In any or all embodiments at least a portion of a delay
generator may include an active or passive PM fiber.
[0064] In any or all embodiments a seed source may include a
mode-locked fiber oscillator having at least one polarization
maintaining (PM) fiber.
[0065] In any or all embodiments a polarization splitter or
combiner may include at least one polarized beam splitter, which
splits laser pulses with each polarization state into separate
arms, and a delay generator disposed in at least one arm.
[0066] In any or all embodiments a pulse width generated by the
seed source may be shorter than the time spacing between adjacent,
time separated pulses.
[0067] In any or all embodiments a medium may include a plurality
of PM fibers, including at least one active PM fiber.
[0068] In any or all embodiments an active fiber may include a
multimode amplifier fiber capable of providing a substantially
fundamental mode output, a leakage channel amplifier fiber, a
photonic crystal amplifier fiber, or a combination thereof.
[0069] In any or all embodiments an active fiber may be capable of
Raman soliton generation with multiple polarization states.
[0070] In any or all embodiments a wavelength of a seed source or
Raman soliton wavelength may be in an anomalous dispersion
regime.
[0071] In any or all embodiments a medium may include a bulk, solid
state or a regenerative amplifier gain medium.
[0072] In any or all embodiments a polarization splitter may
include PM fiber configured such that an input beam to the
polarization splitter is coupled into both the fast and slow axes
of the PM fiber.
[0073] In any or all embodiments an input beam may be coupled to
the PM fiber via fiber splicing, and the polarization splitting may
be controlled by angular offset in the splicing.
[0074] In any or all embodiments a polarization splitter may be
controllable with relative rotation of a fiber or waveplate.
[0075] In any or all embodiments a seed pulse or output pulse may
have a pulse width in the fs-ps regime.
[0076] In any or all embodiments a medium may include an amplifier
fiber, and a seed beam may be coupled into the amplifier fiber with
at least one bulk optical element and free space coupling.
[0077] In any or all embodiments a medium may include an amplifier
fiber, and a seed beam may be coupled into the amplifier fiber with
fusion splicing, and polarization splitting may be controlled by
angle offset in splicing.
[0078] In any or all embodiments the medium may include a gain
medium, and a delay between pulses in different polarization states
may be comparable or larger than a pulse duration in the gain
medium.
[0079] In any or all embodiments a medium may include an amplifier
fiber, and a delay between laser pulses may be longer than the
laser pulse width in at least in one portion of the laser
amplifier.
[0080] In any or all embodiments a PM fiber may be configured as a
delay generator, the PM fiber comprising one or both of active and
passive fiber.
[0081] In any or all embodiments a medium may be capable of
amplifying laser pulse trains with orthogonal polarization states,
and capable of generating a Raman shift with the polarization
states.
[0082] In any or all embodiments a Raman soliton may optionally be
generated with orthogonal polarization states, in a laser
amplifier.
[0083] In any or all embodiments a seed laser may produce a pulse
width in the range from about 100 fs to a few ps, with a
corresponding spectral bandwidth of at least a few nm.
[0084] In any or all embodiments at least one portion of a medium
may include large mode area fiber comprising Yb/Er co-doped double
cladding fiber with a core diameter of at least about 15 .mu.m.
[0085] In any or all embodiments a polarization splitter and a
delay generator may introduce delay larger than a pulse duration in
the medium.
[0086] In any or all embodiments at least a portion of a combiner
may be configured with identical components of a polarization
splitter and a delay generator for reciprocal operation.
[0087] In any or all embodiments an output pulse having increased
peak power may have a pulse temporal shape similar to the temporal
shape of a pulse generated with the seed source.
[0088] In any or all embodiments a peak power and energy of each of
the time separated pulses may be sufficiently low to avoid
substantial distortion of a pulse during propagation in the medium
and when output from the medium.
[0089] In any or all embodiments a pair of time separated pulses
may be generated with a delay generator.
[0090] At least one embodiment includes a pulsed laser system. The
system includes a seed source for generating a pulse. A
polarization splitter splits a pulse from the seed source into
different polarization states, thereby forming polarization split
pulses. A delay generator receives the polarization split pulses
and generates time separated pulses, each pulse having a different
polarization state. The system includes a medium in which the time
separated pulses having the different polarization states
propagates, wherein a peak power and energy of each of the time
separated pulses is sufficiently low to avoid substantial
distortion of a pulse output from the medium. The power of the time
separated pulses, if combined in the medium, would exceed a
non-linear threshold of the medium. A bulk, solid state amplifier
disposed downstream from the medium receives the time separated
pulses therefrom, and generates amplified time separated pulses.
The system includes a combiner that receives the amplified time
separated pulses from the bulk, solid state amplifier and
substantially re-combines the amplified time separated pulses to
form an output pulse having increased peak power.
[0091] In any or all embodiments both of a polarization splitter
and a delay generator may be configured with PM maintaining
fibers.
[0092] In any or all embodiments a medium may include PM optical
fiber, and a polarization splitter and a delay generator may be
integral with the medium.
[0093] In any or all embodiments a medium may include at least one
amplifier fiber.
[0094] In any or all embodiments at least one amplifier fiber may
include single mode, polarization preserving fiber.
[0095] In any or all embodiments at least one amplifier fiber may
include a multimode amplifier fiber capable of providing a
substantially fundamental mode output, leakage channel amplifier
fiber, a photonic crystal amplifier fiber, or a combination
thereof.
[0096] In any or all embodiments at least one amplifier fiber may
be capable of Raman soliton generation.
[0097] In any or all embodiments a polarization splitter, a delay
generator, and a medium may include optical fiber and no bulk
optical components.
[0098] In any or all embodiments a pair of time separated pulses
may be generated with a delay generator.
[0099] At least one embodiment includes a pulsed laser system. The
system includes a seed source for generating a pulse. A
polarization splitter splits a pulse from the seed source into
different polarization states, thereby forming polarization split
pulses. A delay generator receives the polarization split pulses
and generates time separated pulses, each pulse having a different
polarization state. The system includes an optical amplifier in
which the time separated pulses having the different polarization
states propagates, wherein a peak power and energy of each of the
time separated pulses is sufficiently low to avoid substantial
distortion of a pulse output from the optical amplifier. The power
of the time separated pulses, if combined in the optical amplifier,
would exceed a non-linear threshold of a gain medium of the optical
amplifier. The optical amplifier generates amplified time separated
pulses as an amplifier output. The system includes a combiner that
receives the amplified time separated pulses from the optical
amplifier and substantially re-combines the amplified time
separated pulses to form an output pulse having increased peak
power.
[0100] In any or all embodiments an optical amplifier may include
at least one fiber amplifier.
[0101] In any or all embodiments an optical amplifier may include a
large core amplifier configured as one or more of a multimode fiber
amplifier (MMFA) capable of providing a substantially fundamental
mode output, a leakage channel fiber amplifier (LCF), a photonic
crystal fiber amplifier (PCF), or a combination thereof.
[0102] In any or all embodiments a bulk solid state amplifier may
be disposed between the optical amplifier and the combiner.
[0103] In any or all embodiments an output pulse having increased
peak power may have a pulse temporal shape similar to the temporal
shape of a pulse generated with the seed source.
[0104] In any or all embodiments a peak power and energy of each
time separated pulse may be sufficiently low to avoid substantial
distortion of a pulse during propagation in the medium and when
output from the medium.
[0105] In any or all embodiments a pair of time separated pulses
may be generated with a delay generator.
[0106] At least one embodiment includes a pulsed laser system. The
system includes an input providing time separated pulses, each
pulse having a different polarization state. The system includes a
medium in which the time separated pulses propagates, wherein a
peak power and energy of each of the time separated pulses is
sufficiently low to avoid substantial distortion of a pulse output
from the medium. The system is further configured such that the
time separated pulses propagate in separate optical paths, thereby
providing for synchronization of time separated pulses.
[0107] In any or all embodiments a wavelength converter may be
disposed in a first optical path and receives a first pulse from
the medium, and converts the wavelength of the first pulse from the
medium to a first converted wavelength.
[0108] In any or all embodiments a second wavelength converter may
be disposed in a second optical path and receives a second pulse
from the medium, and converts the wavelength of the second pulse
from the medium to a second converted wavelength.
[0109] In any or all embodiments an input may include a gain medium
comprising substantially all-fiber and no bulk optical components
in the gain medium.
[0110] In any or all embodiments an input may include a delay
generator, and at least a portion of a delay generator may include
bulk optics.
[0111] In any or all embodiments bulk optics may include a pair of
prism-shaped birefringent crystals, wherein the thickness and/or
relative position of one or both prism-shaped crystals provides for
adjustable delay.
[0112] At least one embodiment includes a pulsed laser system. The
system includes a means for generating time separated pulses, each
pulse having a different polarization state. The system includes a
medium in which the plurality of time separated pulses propagates,
wherein a peak power and energy of each of the time separated
pulses is sufficiently low to avoid substantial distortion of a
pulse output from the medium, wherein the system is configured such
that the time separated pulses propagate in separate optical paths,
thereby providing for synchronization of time separated pulses.
[0113] In any or all embodiments the means for generating and at
least a portion of the medium may include optical fiber and no bulk
optical components.
[0114] In any or all embodiments the power of time separated
pulses, if combined in the medium, may exceed a non-linear
threshold of the medium.
[0115] In any or all embodiments an output pulse having increased
peak power may have a pulse temporal shape similar to the temporal
shape of a pulse generated with the seed source.
[0116] In any or all embodiments a peak power and energy of each
time separated pulse may be sufficiently low to avoid substantial
distortion of pulses during propagation in the medium and when
output from the medium.
[0117] Thus, while only certain embodiments have been specifically
described herein, it will be apparent that numerous modifications
may be made thereto without departing from the spirit and scope of
the invention. Further, acronyms are used merely to enhance the
readability of the specification and claims. It should be noted
that these acronyms are not intended to lessen the generality of
the terms used and they should not be construed to restrict the
scope of the claims to the embodiments described therein.
* * * * *