U.S. patent application number 15/364946 was filed with the patent office on 2017-03-23 for compact ultra-short pulse source amplifiers.
This patent application is currently assigned to IMRA AMERICA, INC.. The applicant listed for this patent is IMRA AMERICA, INC.. Invention is credited to Martin FERMANN.
Application Number | 20170085053 15/364946 |
Document ID | / |
Family ID | 50488886 |
Filed Date | 2017-03-23 |
United States Patent
Application |
20170085053 |
Kind Code |
A1 |
FERMANN; Martin |
March 23, 2017 |
COMPACT ULTRA-SHORT PULSE SOURCE AMPLIFIERS
Abstract
The present invention relates to compact, low noise, ultra-short
pulse sources based on fiber amplifiers, and various applications
thereof. At least one implementation includes an optical
amplification system having a fiber laser seed source producing
seed pulses at a repetition rate corresponding to the fiber laser
cavity round trip time. A nonlinear pulse transformer, comprising a
fiber length greater than about 10 m, receives a seed pulse at its
input and produces a spectrally broadened output pulse at its
output, the output pulse having a spectral bandwidth which is more
than 1.5 times a spectral bandwidth of a seed pulse. A fiber power
amplifier receives and amplifies spectrally broadened output
pulses. A pulse compressor is configured to temporally compress
spectrally broadened pulses amplified by said power amplifier.
Applications include micro-machining, ophthalmology, molecular
desorption or ionization, mass-spectroscopy, and/or laser-based,
biological tissue processing.
Inventors: |
FERMANN; Martin; (Dexter,
MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
IMRA AMERICA, INC. |
Ann Arbor |
MI |
US |
|
|
Assignee: |
IMRA AMERICA, INC.
Ann Arbor
MI
|
Family ID: |
50488886 |
Appl. No.: |
15/364946 |
Filed: |
November 30, 2016 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14571367 |
Dec 16, 2014 |
9553421 |
|
|
PCT/US2013/065169 |
Oct 16, 2013 |
|
|
|
15364946 |
|
|
|
|
61714344 |
Oct 16, 2012 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 3/08013 20130101;
H01S 3/094026 20130101; H01S 3/109 20130101; B23K 26/0624 20151001;
H01S 3/1106 20130101; H01S 3/06741 20130101; H01J 49/161 20130101;
H01S 3/0057 20130101; H01S 3/10046 20130101; H01S 3/1115 20130101;
H01S 3/06725 20130101; H01S 3/0675 20130101; H01S 3/06754 20130101;
H01S 3/094061 20130101; H01S 3/06708 20130101; H01S 3/094003
20130101; H01S 3/0941 20130101; H01S 3/094042 20130101 |
International
Class: |
H01S 3/11 20060101
H01S003/11; H01S 3/0941 20060101 H01S003/0941; H01S 3/109 20060101
H01S003/109; H01S 3/094 20060101 H01S003/094; H01S 3/067 20060101
H01S003/067 |
Claims
1. An optical amplification system, comprising: a fiber laser seed
source producing seed pulses at a repetition rate corresponding to
the fiber laser cavity round trip time, said seed source producing
seed pulses having a pulse width greater than about 5 picoseconds
(ps); a nonlinear pulse transformer comprising a fiber length
greater than about 10 m and less than 3000 m, said pulse
transformer receiving a ps seed pulse at its input and producing a
spectrally broadened ps output pulse at its output; a fiber power
amplifier receiving and amplifying said spectrally broadened ps
output pulse; and a pulse compressor configured to temporally
compress spectrally broadened pulses amplified by said power
amplifier.
2. The optical amplification system according to claim 1, wherein
said fiber power amplifier receives and amplifies said spectrally
broadened pulse to produce a pulse energy greater than about 5
.mu.J.
3. The optical amplification system according to claim 1, wherein
said seed source generates a pulse width>10 ps.
4. The optical amplification system according to claim 1, wherein a
pulse width generated with said ps seed source is compressible to a
pulse width less than 5 ps.
5. An optical amplification system according to claim 1, wherein
said nonlinear pulse transformer spectrally broadens a pulse
injected to its input by a factor>1.5.
6. The optical amplification system according to claim 1, wherein
said nonlinear pulse transformer spectrally broadens a pulse
injected to its input by a factor>4.
7. The optical amplification system according to claim 1, said
amplification system further comprising a down counter.
8. The optical amplification system according to claim 1, wherein
said nonlinear pulse transformer comprises lengths of amplifier
fiber and passive undoped fiber.
9. The optical amplification system according to claim 1, wherein
said fiber seed source comprises a modelocked fiber laser.
10. The optical amplification system according to claim 1, wherein
said amplification system is further configured to substantially
optimize the pulse quality of the pulses coupled out of the pulse
compressor by insertion of elements with optimized values of third
or fourth order dispersion.
11. The optical amplification system according to claim 1, further
comprising: a nonlinear frequency conversion stage.
12. An optical amplification system according to claim 1, wherein
said amplification system is configured as an element of a system
for micro-machining, ophthalmology, molecular desorption,
ionization, mass spectroscopy, or laser-based biological tissue
processing.
13. The amplification system according to claim 1, comprising a
nonlinear fiber power amplifier disposed between said seed source
and said compressor.
14. The amplification system of claim 1, wherein said seed source
comprises: a fiber-based modelocked laser, a semiconductor mode
locked laser, a Fourier domain mode locked laser, or a
gain-switched laser diode.
15. The amplification system according to claim 1, comprising: a
fiber oscillator generating pulses at its cavity round trip time; a
down-counter, said down-counter disposed between said fiber
oscillator and said fiber power amplifier; and a pump source
configured to optically pump both said fiber oscillator and said
fiber power amplifier.
16. A modelocked fiber oscillator generating ps pulses, said
modelocked fiber oscillator comprising: a fiber grating; and a
saturable absorber, wherein said modelocked fiber oscillator is
characterized by having a low susceptibility to Q-switching
instabilities, said low susceptibility to Q-switching instabilities
arising from selecting a grating bandwidth .DELTA..lamda.[nm] and
negative grating dispersion D.sub.grat[ps.sup.2], where the grating
bandwidth .DELTA..lamda.[nm] is selected to be smaller than 5/
(D.sub.grat[ps.sup.2]).
17. A modelocked fiber oscillator generating ps pulses according to
claim 16, configured as a Fabry-Perot cavity.
18. A modelocked fiber oscillator generating ps pulses according to
claim 16, configured as a ring cavity.
19. A modelocked fiber oscillator generating ps pulses according to
claim 16, configured as a twisted cavity.
20. A modelocked fiber oscillator generating ps pulses according to
claim 16, further comprising photonic crystal or Kagome fiber.
21. A system for amplifying pulses, comprising: a picosecond (ps)
seed source, said seed source producing pulses with a pulse width
greater than about 5 ps; a nonlinear pulse transformer, said pulse
transformer receiving a ps pulse at its input and producing a
spectrally broadened, near linearly chirped and near parabolic ps
output pulse, said ps output pulse having a temporal waveform with
substantially smaller pulse wings compared to a seed input pulse
waveform; a nonlinear fiber power amplifier; and a pulse compressor
configured to temporally compress pulses amplified by said
nonlinear power amplifier.
22. A system for amplifying pulses, comprising: a picosecond (ps)
seed source, said seed source producing pulses with a pulse width
greater than about 5 ps; a nonlinear pulse transformer, said pulse
transformer receiving a ps pulse at its input and producing a
spectrally broadened ps pulse at its output; a nonlinear fiber
power amplifier; and a pulse compressor configured to temporally
compress pulses amplified by said power amplifier, wherein said
nonlinear pulse transformer is configured to substantially optimize
the pulse quality obtained after the compressor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional application of application
Ser. No. 14/571,367, which is a National Stage of international
application No. PCT/US2013/065169, filed Oct. 16, 2013, which
claims priority to U.S. Provisional Application No. 61/714,344,
filed Oct. 16, 2012. The contents of the prior applications are
incorporated herein by reference in their entirety.
FIELD OF THE INVENTION
[0002] The present teachings relate to compact, low noise,
ultra-short pulse sources based on fiber amplifiers, and various
applications thereof.
BACKGROUND
[0003] Ultra-short pulse sources have had a major impact on
laser-based technology during the last decade. Applications include
imaging, micro-machining and ophthalmology. Because of their unique
stability, compactness and ease of construction, fiber laser based
ultra-short pulse sources have started to dominate the ultra-short
pulse source market segment. Exemplary ultra-short pulse sources
are described in U.S. Pat. Nos. 8,031,396; 7,688,499; 7,167,300;
6,885,683 and 5,696,782. In order to be able to address many
industrial applications, pulse sources producing pulse widths
ranging from sub picosecond to 10 ns are highly desirable, where in
order to minimize cost, preferably, these systems are based on the
same technology platform.
[0004] Modelocked fiber lasers are an attractive source for
producing pulses in the fs to ns range, where further amplification
in fiber amplifiers can be implemented to reach pulse energies up
to the mJ level. Thus, an all-fiber system construction can be
achieved.
[0005] One limitation of all-fiber systems is the relatively low
pulse energies of mode-locked fiber oscillators. Several fiber
amplification stages may be required to reach high pulse energies.
Another limitation arises from the requirement of linear
amplification stages as, for example, encountered when implementing
chirped pulse amplification systems, which limit the achievable
peak power from fiber amplifiers. As the pulse energy and/or peak
power is increased, chirped pulse amplification further requires
complex schemes for pulse stretching and compression with precisely
matched values of dispersion. On the other hand, when nonlinear
fiber amplifiers are employed, the pulse quality can be
detrimentally affected when implementing pulse compression stages
after the nonlinear fiber amplifiers.
[0006] Gain-switched diode-based laser systems or micro-chip lasers
have been implemented as front ends to fiber amplifiers to
circumvent the limitation of all fiber systems, but so far with
limited success. For example, it is generally very difficult to
generate bandwidth-limited pulses from gain-switched diode lasers
in the pulse width range from 10 ps-1 ns, and also the generation
of pulse width tunable systems with pulse widths around 100 ps is
relatively complex. As other examples, continuous wave emitting
diode lasers have been suggested as a solution to pulse width
tunable short pulse laser systems (U.S. Pat. No. 7,330,301 to D. J.
Harter et al.). Arrangements of pulse shortening stages implemented
in conjunction with micro-chip lasers can require relatively
complex schemes to minimize pulse jitter. For example, see A.
Steinmetz et al., Sub-5-ps, multimegawatt peak-power pulses from a
fiber-amplified and optically compressed passively Q-switched
microchip laser, Optics Letters, Vol. 37, Issue 13, pp. 2550
(2012).
SUMMARY
[0007] In one aspect, the present invention provides a method and
system for generating high energy pulses from modelocked fiber
oscillators based on the implementation of high energy picosecond
(ps) fiber oscillators. The coherence of the fiber oscillators and
their stability is ensured by selecting an appropriate value of
dispersion and oscillator bandwidth.
[0008] In another aspect, a fiber-based pulse transformer increases
the available peak power output of a pulsed laser system while
improving pulse quality.
[0009] In another aspect, the present invention provides for the
generation of coherent or partially coherent pulses over a wide
range of pulse widths, energies, and/or repetition rates.
[0010] In one or more implementations, oscillators with pulse
widths adjustable from the ps to ns range can be constructed using
oscillator pulse characteristic(s) with reduced values of
coherence.
[0011] Examples herein demonstrate that the use of ps-ns
oscillators enables the generation of high energy pulses directly
from fiber amplifiers without a requirement for complicated pulse
stretching schemes.
[0012] With the use of ps-ns oscillator pulses, and by appropriate
design of the amplification stages, the bandwidth and the chirp of
the amplified high energy pulses can be manipulated. Such an
implementation provides for the generation of pulses with
relatively small bandwidth for subsequent nonlinear frequency
conversion while allowing for the generation of high quality
compressed pulses, with a pulse width much shorter than the pulses
generated by the ps-ns oscillators.
[0013] Embodiments of the present invention are compatible with
fiber lasers based on rare-earth dopants, such as Nd, Er, Yb, Tm,
or Ho, for example.
[0014] Certain embodiments of the present invention may utilize an
optical amplification system in which an output of a power
amplifier provides an input to a frequency converter. An optional
pulse transformer and/or pulse compressor may be included prior to
frequency conversion to provide spectrally broadened pulses and/or
compressed pulses to the frequency converter.
[0015] Ps and ns pulse sources are suitable for use in a vast
number of industrial and medical applications, for example. Reduced
cost of such ps and ns pulse sources can provide for efficient
implementation of frequency conversion schemes to both the UV
(up-conversion) and the IR (down-conversion), further expanding the
application potential. In addition the low cost of the pulse
sources further expands the capability for applications that were
traditionally covered by ns solid state lasers. For example, the
ionization of gases, liquids and solids can be performed with a
very high ionization efficiency, allowing the implementation of
such pulse sources in spectroscopy applications, such as mass
spectroscopy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 schematically illustrates a high energy ps fiber
pulse oscillator with low susceptibility to Q-switching
instabilities.
[0017] FIG. 2 schematically illustrates high energy ps fiber pulse
oscillator according to FIG. 1 with an added amplification stage,
providing a conventional MOPA configuration.
[0018] FIG. 3 schematically illustrates a high energy MOPA
configuration in accordance with an embodiment of the present
invention.
[0019] FIG. 4 schematically illustrates an example of an optical
amplification system which includes a high energy ps fiber pulse
oscillator, and a fiber-based pulse transformer which produces
spectrally broadened pulses prior to power amplification and
compression. The amplification system generates output pulse widths
shorter than the oscillator pulse width.
[0020] FIGS. 4a and 4b are plots of simulation results that show
the improvement in pulse quality obtainable with an appropriately
designed pulse transformer; FIG. 4a illustrates obtainable
autocorrelation traces of compressed 1 ps pulses obtained with
(solid line) and without the pulse transformer (dashed line); FIG.
4b illustrates the corresponding optical spectra at the output of
the power amplifier.
[0021] FIG. 5 schematically illustrates a fiber ps pulse
amplification system with frequency conversion of the amplification
system output.
[0022] FIG. 6a schematically illustrates an amplification system
and frequency conversion arrangement of FIG. 5 configured for
ionization of a material.
[0023] FIG. 6b schematically illustrates an amplification system
and frequency conversion arrangement of FIG. 5 configured for
tissue cutting.
DETAILED DESCRIPTION
[0024] FIG. 1 shows a mode-locked, dispersion compensated fiber
oscillator 100. The gain element of the laser includes a rare-earth
doped amplifier fiber 110. Also, an optional length of undoped
fiber 120 can be spliced to the gain fiber in order to reduce the
repetition rate of the oscillator. The undoped fiber can further be
a photonic crystal fiber or a Kagome fiber to minimize the amount
of self-phase modulation in the undoped fiber 120 and to maximize
the possible pulse energy. Preferably the oscillator is configured
with overall negative dispersion at the signal wavelength within
the gain bandwidth of the gain fiber. Examples of rare-earth doping
materials include but are not limited to Er, Er/Yb, Nd, Yb, Ho, Pr,
Yb/Tm or Tm, for example.
[0025] The laser cavity includes a saturable absorber mirror 130,
which has high reflectivity (HR) at the signal wavelength. The
cavity includes an output coupling element (OC) 105 which has
partial transmittance for the signal wavelength. In this example a
chirped fiber Bragg grating is illustrated as the output coupling
element.
[0026] The chirped fiber Bragg grating 105 can be designed with
negative dispersion in order to compensate for the overall positive
dispersion of the intra-cavity fiber(s) 110 and 120. The chirped
fiber Bragg grating can be written in standard single mode silica
fiber fusion spliced to the gain fiber 110. Fiber splicing
techniques for silica fibers are well known and are not further
discussed here. The use of chirped fiber Bragg gratings in
mode-locked fiber oscillators is described in U.S. patents U.S.
Pat. No. 5,450,427 ('427) and U.S. Pat. No. 7,088,756 ('756) to
Fermann et al. The '427 and '756 patents are hereby incorporated by
reference in their entirety. Alternatively, the chirped fiber Bragg
grating can be written directly into the rare-earth doped fiber.
Such an implementation is not separately shown. The saturable
absorber mirror 130 is preferably mounted onto a heat sink.
[0027] Mode-locking operation of the oscillator is initiated and
stabilized by the saturable absorber mirror 130. Details about
suitable saturable absorber mirrors are disclosed in the '756
patent to Fermann et al. The end of the intra-cavity fiber 110
which is not spliced to the fiber Bragg grating is preferably
anti-reflection coated with a reflectivity of less than 0.1%.
Alternatively the fiber end can be angle cleaved. The optical
output from this end is imaged via relay optics 175 onto the
saturable absorber mirror 130. The relay optics allows adjustment
of the spot size and therefore the fluence on the saturable
absorber, independent from the mode field diameter in the gain
medium. Alternatively the saturable absorber mirror can be butt
coupled onto the AR coated or cleaved fiber end. A polarization
beam splitter (PBS) 170 is included in the cavity to provide
polarization selection.
[0028] For the case of Er or Yb oscillators, the oscillator can be
pumped with telecom grade single mode pump diodes 160 at
wavelengths about 980 nm. The pump light can be coupled into the
gain fiber via a WDM coupler as shown in FIG. 1 and additional
fiber sections or pig-tails (not shown).
[0029] For low noise operation of the laser 100, the use of low
noise pump lasers is preferred because noise of the pump laser
leads to excess noise in the modelocked oscillator. This is
especially important for noise at noise spectral frequencies below
the lifetime of the active laser transition in the modelocked
oscillator where no filtering action of the laser medium is
present. Pump laser noise may be reduced either by active noise
cancellation via acousto-optical modulators or passively by diode
pumping another cw pump laser which is used as pump laser for the
modelocked fiber oscillator. For example a cw Er fiber laser can be
used for pumping a Tm fiber laser.
[0030] An isolator for the signal light (not shown) can be further
integrated into the WDM coupler for improved oscillator stability.
The pump light counter-propagates relative to the signal light in
the output pigtail and passes through fiber Bragg grating 105 into
the cavity. To enhance the available pump power two pump diodes can
be coupled into a polarization maintaining fiber and can be
polarization multiplexed by a polarizing beam combiner.
Alternatively, two pump diodes operating at different wavelengths
can be used in series to provide high levels of pump power to the
oscillator. Such configurations are well known in the state of the
art and are not further discussed here.
[0031] In one implementation the arrangement shown in FIG. 1 can be
constructed from polarization maintaining (PM) fiber to provide a
high degree of environmental stability. Alternatively, only some
fiber sections can be PM and other sections can be non PM. In this
case additional polarization controllers can also ensure stable
modelocked operation.
[0032] The use of intra-cavity chirped fiber Bragg gratings allows
the incorporation of large values of negative dispersion into the
cavity which can be configured to restrict the oscillating pulse
bandwidth. As a result, highly coherent, high power soliton pulses
with ps pulse widths can be generated inside such oscillators. In
certain preferred embodiments the bandwidth of the fiber grating is
carefully selected to minimize Q-switching instabilities and to
maximize the coherence of the oscillating pulses. Any Q-switching
instability in a fiber laser tends to increase the bandwidth of the
Q-switch pulse via self-phase modulation. In the presence of a
bandwidth restriction, this bandwidth increase produces an increase
in loss, which tends to suppress the Q-switching instability
resulting in stable modelocked operation. Such a mechanism was for
example described in C. Hoenninger et al., Q-switching stability
limits of continuous-wave passive mode locking, JOSA B, Vol. 16,
Issue 1, pp. 46-56 (1999).
[0033] Two variations of FIG. 1 were implemented as working
examples. A first implementation generated highly coherent pulses
in the ps-ns range. A second implementation was used to generate
pulses with reduced coherence, referred to herein as "noise
pulses":
Working Example #1
Coherent Pulse Generation
[0034] A working example was constructed following the specific
design example as shown in FIG. 1, in which a semiconductor
saturable absorber for mirror 130 was used for modelocking. In the
working example, all intra-cavity fibers 110,120 were polarization
maintaining (PM). A Yb fiber with an absorption of about 500 dB/m
was used as the gain fiber 110 to construct a high power soliton
laser. The total intra-cavity fiber length, which included doped
fiber 110 and undoped fiber 120, was selected as 2 m to allow
oscillation at repetition rates of 50 MHz. The oscillator was
pumped with a SM diode laser with a power up to 750 mW at 980 nm.
To generate 5 ps pulses at 1030 nm, a grating 105 having a
dispersion of -5 ps.sup.2 was used, where the grating bandwidth was
0.5 nm and the grating reflectivity about 50%.
[0035] The generated temporal pulse shapes are influenced by
stability conditions as typically found with respect to soliton
lasers, namely, the grating bandwidth has to be sufficiently small
in order to prevent the formation of incoherent or partially
coherent pulse forms or to prevent Q-switching. Generally, the
generation of longer ps pulses requires an increase in grating
dispersion (in absolute terms) and a reduction in grating
bandwidth. However, longer ps pulses can also be generated by using
incoherent or partially coherent pulse characteristics. Such pulses
generally have much higher pulse energies than soliton-like pulses
and further allow for tuning of the oscillator pulse width by
varying the oscillator pump power.
[0036] For the generation of near bandwidth-limited pulses the use
of grating reflectivities<90% is advantageous, otherwise the
grating "carves out" (reflects) the central part of the generated
spectrum, producing a pulse much broader than the bandwidth limit.
However, such spectrally broadened oscillator pulses can be of some
benefit in conjunction with additional pulse shortening stages. As
a guideline, the generation of coherent soliton like oscillator
pulses requires a grating bandwidth in nm<5/ {square root over
(D.sub.grat[ps.sup.2])}, where D.sub.grat[ps.sup.2] is the grating
dispersion in ps.sup.2 to facilitate the suppression of Q-switching
instabilities and to ensure a high level of coherence for the
generated pulse forms. Therefore, for a grating dispersion of 5
ps.sup.2, the grating bandwidth should be <2 nm.
[0037] Using appropriate values for grating dispersion, highly
coherent pulses with widths from 1 ps-1 ns, greater than about 5
ps, greater than about 10 ps, and/or up to about 10 ns can be
generated. Pulse repetition rates can be pre-selected from 1 MHz to
1 GHz determined by the intra-cavity fiber length. In various
implementations for amplification to high pulse energies, the
output obtained at the output of a fiber amplifier downstream of
the oscillator may, for example, be in the range from 1-1000 .mu.J.
In the oscillator, pulse repetition rates<50 MHz may be
utilized, or <25 MHz with some increase in available pulse
energy. Pulse energy may be further increased with oscillator pulse
repetition rates<25 MHz. Depending on the desired system
configuration, oscillator repetition rates<10 MHz can increase
the available oscillator pulse energy, for example up to several
.mu.J with a pulse width in the range from about 10 ps-10 ns based
on the design illustrated in FIG. 1. Generally, appropriate
saturable absorbers (SAs) need to be also selected in order to
ensure stable pulse oscillation, for example the SAs should enable
some pulse forming function for the desired pulse width. The SAs
preferably have at least one decay component with a life-time in
the vicinity of the desired pulse width. Such a lifetime is not
critical, however, indeed, it was verified that even saturable
absorbers with a life time ten times shorter than the obtained
pulse width enabled the oscillation of stable coherent pulses.
Working Example #2
Partially Coherent Pulse Generation
[0038] The cavity design shown in FIG. 1 is also adaptable for the
generation of partially coherent or incoherent pulses, sometimes
also referred to as noise bursts. Such `noise` pulses can be
distinguished from highly coherent pulses by their phase
properties. By way of example, gain-switched laser diodes or
Fourier domain mode locked lasers are known to generate pulses with
reduced temporal coherence. Just as modelocked lasers, noise
sources produce pulses at the cavity round trip time. However,
whereas the temporal phase of highly coherent modelocked pulses is
approximately time-invariant from pulse to pulse and well defined,
the phase of `noise` pulses generally fluctuates from pulse to
pulse or at least has a substantial level of random noise. Another
useful distinguishing feature is compressibility: highly coherent
modelocked pulses can be essentially compressed to near their
transform limit, which is not the case for noise pulses. This is
true even if precision pulse shapers are used for pulse compression
that can compensate for higher order phase modulations in the
temporal waveform of the pulses.
[0039] In an exemplary embodiment of a noise pulse source the fiber
grating was chirped with a dispersion of -1 ps.sup.2. Note,
however, that the dispersion and the chirp of the fiber grating was
not critical and incoherent pulses can also be obtained with other
grating dispersion values. Whereas in the above Example #1 the
grating 105 bandwidth was 2 nm, the grating bandwidth in the
present example was 10 nm and the grating center wavelength was
1045 nm. The grating reflectivity was 10%. A conventional SA mirror
was also used to enable modelocking. A single polarization in the
cavity was selected with a polarizer and an additional pump
blocking filter was used to prevent pump light from hitting the
saturable absorber mirror. Two additional lenses 175 were used
inside a saturable absorber module to collimate the light from the
PM undoped fiber 120 and to focus it onto the saturable absorber
130. Such focusing arrangements are well known in saturable
absorber designs and are not further discussed here.
[0040] When pumping with a pump power up to 500 mW at 980 nm the
laser generated pulses at a repetition rate of 1 MHz with an
average power between 40-100 mW. The pulse width varied in a range
from 200 ps-1.3 ns for average power values between 40-100 mW. The
pulse energies thus varied in a range from 40-100 nJ depending on
output power. The pulse spectral width was 3 nm and depended only
weakly on pump power. The spectral width of the pulses as well as
the pulse width was further changeable by changing the grating
bandwidth, generally, the broader the grating bandwidth, the
broader the pulses. The pulse bandwidth can further be controlled
with additional intra-cavity bandpass filters (not shown). The
pulse width can further be adjusted by adjusting the cavity
dispersion; generally a larger absolute value of dispersion
produces longer pulses.
[0041] The RF noise of pulse trains comprising incoherent pulses
can indeed be very low, indicating a high degree of suppression of
Q-switching instabilities. In this example the RF power density
near the relaxation oscillation peak of the laser located at about
1050 kHz was suppressed by more than 90 dB compared to the spectral
power density of the RF peak at around the repetition rate of
.apprxeq.1 MHz. Instead of a grating, only the 4% Fresnel
reflection of the fiber can also be used as a cavity mirror. In
this case the RF spectrum showed some increased amplitude noise of
the laser. Generally, the amplitude noise of the pulse train was
very low and maximum short term fluctuations of the pulse energy of
.+-.5% were observed.
[0042] To extract the largest pulse energy from such fiber lasers,
the use of large mode gain fibers and large-mode undoped fibers is
most advantageous. This also applies to the generation of highly
coherent ps pulses. Here we classify large mode fibers as fibers
that have core-diameters>10 .mu.m and preferably >15 .mu.m.
Pulse energies of several .mu.J can be generated at repetition
rates of 1 MHz. Moreover, the repetition rate of the pulse train
can simply be selected by changing the intra-cavity fiber length.
High pulse energies can further be extracted by inserting photonic
crystal fibers or photonic crystal fibers with central air holes
such as Kagome fibers in place of the PM undoped fiber. Photonic
crystal fibers can be spliced to standard fiber or lens
arrangements can be used to optically couple photonic crystal
fibers to standard fibers. Moreover, PM photonic crystal fibers are
also well known in the state of the art, so the replacement of PM
undoped fiber with photonic crystal fibers is straight forward and
is not shown separately.
[0043] Since the pulses generated by the modelocked oscillators are
generally affected by self-phase modulation and stimulated Raman
scattering, the maximum pulse energy is limited. This limitation is
most severe for highly coherent pulses, whereas for incoherent or
partially coherent pulses, the presence of self-phase modulation
(SPM) is not as restricting. Particularly, for the generation of
soliton-like pulse forms, the maximum tolerable value of
intra-cavity self-phase modulation is of the order of .pi., or at
most several .pi.. On the other hand, for partially coherent pulse
forms or chirped pulse forms, intra-cavity self-phase modulation
values can exceed 10.pi.. Therefore, further amplification of the
pulses in additional amplifiers is highly desirable.
[0044] The examples above demonstrate flexibility for generating
pulses with a certain degree of coherence with an arrangement
according to FIG. 1, which includes a mode-locked fiber oscillator.
It is apparent that various design options and tradeoffs are
possible. For example, although compressibility of partially
coherent pulses is limited a stable pulse train can be generated
with low q-switching instability. In cases where high oscillator
pulse energy is important a reduced degree of coherence can be
advantageous. On the other hand, coherent pulses are well suited
for minimizing compressed pulse widths.
[0045] Regarding implementations for further amplification, most
straight-forward are master-oscillator-power-amplifier (MOPA)
configurations as shown in FIG. 2 and also discussed, for example,
in U.S. Pat. No. 7,190,705 ('705). In an exemplary MOPA
configuration in FIG. 2 of the present application a fiber
amplifier 250, which corresponds to a power amplifier in the MOPA
configuration, is disposed between the intra-cavity fiber grating
105 and the wavelength-division output coupler as also discussed in
the '705 patent.
[0046] A limitation of MOPA designs is the relatively inefficient
use of pump power, particularly when it is desired to reduce the
repetition rate of the system via the implementation of a down
counter to simplify the generation of high pulse energies.
Repetition rate reduction or down counting is conveniently
performed with the use of optical modulators as discussed in U.S.
Pat. No. 8,072,678; optical down counters are well known in the
state of the art and are not further discussed here. Optical down
counters are generally implemented to reduce the pulse rate from a
high repetition rate oscillator to facilitate the amplification of
the oscillator pulses to high pulse energies.
[0047] Improved MOPA configurations may be utilized in various
embodiments of the present invention. A configuration providing
highly-efficient use of pump power is shown in FIG. 3. In this
example the pump from the laser diode (LD) 160 is split by a
coupler into an oscillator arm (at the bottom) and an amplifier arm
(top). The oscillator output is then directed through an optional
isolator (ISO) 370 and an acousto-optic down-counter (AOM) 375 to
an amplifier 350. The pump light is coupled to the oscillator via
the grating coupler 305 and amplifier 350 respectively via the
shown wavelength division-multiplexing couplers (WDM). All
components were polarization maintaining and single-mode.
[0048] In a working example this split pumping scheme allowed the
generation of more than 10 times higher pulse energies compared to
a MOPA design. For example, an Yb oscillator that generated 50 ps
pulses with a pulse energy of 4 nJ at 20 MHz, allowed for
amplification up to a pulse energy of only 12 nJ in a MOPA
configuration. In contrast, the split pumping scheme allowed for
amplification up to pulse energy of 100 nJ with simultaneous
down-counting to a pulse repetition rate of 1 MHz. The pump diode
generated 600 mW at 980 nm in both cases. The split pumping scheme
further enables down-counting to repetition rates as a low as 100
kHz with relatively small levels of amplified spontaneous emission
in the amplifier.
[0049] Although the examples discussed above were based on standing
wave cavities, fiber ring cavities can also be used in conjunction
with fiber gratings, as already disclosed in U.S. Pat. No.
5,450,427. Also, twisted cavity designs can be implemented as for
example disclosed in U.S. Patent Appl. Pub. No. 2011/0069723,
entitled HIGHLY RARE-EARTH-DOPED OPTICAL FIBERS FOR FIBER LASERS
AND AMPLIFIERS to Dong et al. The use of twisted cavities or ring
lasers allows a further increase in oscillating pulse energies
compared to standing wave cavities by a factor of 3-30, which is
highly advantageous for amplification of the oscillator pulses to
high pulse energies. Split pumping schemes can further be easily
adapted to such cavities designs and are not separately explained
here. Equally, instead of core pumped fiber oscillator and
amplifiers as discussed here, cladding pumped oscillators and
amplifiers can also be used. Cladding pumping is well known in the
state of the art and not further explained here.
[0050] For the generation of high average powers, the seed systems
discussed with respect to FIGS. 1-3 are further amplified in high
power fiber or solid state amplifiers. In the case of fiber
amplifiers, cladding pumping can be conveniently implemented and
average powers in the range from 1 W to 1 kW can be readily
obtained. The obtainable pulse energies and pulse peak powers are
only limited by the self-focusing limit which can be >10 MW for
example in Ho or Tm fiber amplifiers. Cladding pumped fiber
amplifiers and solid-state amplifiers are well known in the state
of the art and not further described here.
[0051] Particularly, when amplifying high energy pulses in positive
dispersion amplifiers, self-phase modulation can lead to spectral
broadening and therefore provides an opportunity for pulse
compression after the amplification stage. Such schemes were for
example discussed in U.S. Pat. No. 6,885,683 and U.S. Pat. No.
7,330,301 and also more recently in A. Steinmetz et al. However, as
discussed by A. Steinmetz et al, nonlinear amplification of ps
pulses with a width of around 100 ps typically produces relatively
poor pulse quality after pulse compression.
[0052] A solution to this pulse quality problem is shown in FIG. 4.
In accordance with an embodiment of the present invention FIG. 4
shows a high power fiber amplification system 400. The system
comprises a seed module 410 as for example described with respect
to FIGS. 1-3, delivering seed pulses. The seed module may be
arranged in oscillator-only configuration as illustrated in FIG. 1,
or include amplifier stage(s) as illustrated in FIGS. 2 and/or 3.
In addition to fiber laser based seed sources, a seed source based
on a solid-state laser, a micro-chip laser, a gain-switched, an
externally modulated cw semiconductor laser or a modelocked
semiconductor laser can also be implemented. For example the seed
module can generate near bandwidth-limited or chirped 50 ps pulses
with pulse energy of a few nJ.
[0053] In certain preferred embodiments the pulse transformer 420
comprises a relatively long length of amplifier fiber.
Alternatively, a linear succession of short amplifier fiber
sections with longer lengths of dispersive fibers located
in-between the amplifier fiber sections may be utilized (not
shown). Preferably the amplifier fiber sections are all spliced
together to make a single integral pulse transformer unit. And also
preferably, the fibers in the pulse transformer unit have overall
positive dispersion. For example, such a pulse transformer unit can
have an overall length of 30 m-3000 m.
[0054] The pulse transformer unit transforms a Gaussian-like pulse
output from the seed module into a near parabolic, near linearly
chirped pulse with a spectral bandwidth broader than that of the
seed pulse. Generally, near parabolic pulses, which are produced by
the pulse transformer unit, have lower pulse wings compared to
Gaussian pulses. The near parabolic pulses are then injected into a
fiber power amplifier 430, comprising large core fibers, such as
fiber rods, leakage channel fibers, photonic crystal fiber or large
pitch fibers.
[0055] At the output of the power amplifier a pulse compressor 450
temporally compresses the pulses to near the bandwidth limit. The
pulse compressor can comprise a bulk grating arrangement, an
optical arrangement using grisms, prisms, fiber Bragg gratings,
bulk Bragg gratings, a length of photonic crystal fibers with a
central air-hole or Kagome fibers. All these temporal pulse
compressors are well known in the state of the art and not further
explained here.
[0056] The addition of optimized pulse transformers enables a
notable improvement in pulse quality obtainable after a final
compressor stage. The improvement in pulse quality over the prior
art here takes advantage of parabolic pulse forming processes in
amplifier stages prior to final power amplification to improve
pulse quality from nonlinear waveguide amplification systems that
incorporate pulse compressors. At the same time the final power
amplifier can be operated at high levels of self-phase modulation
in order to maximize the obtainable pulse energy. Since power
amplifiers are generally as short as possible to enable the
amplification of the highest energy pulses, they are oftentimes too
short to transform a ps pulse into a near parabolic pulse shape and
hence it is desirable to inject a near parabolic pulse into a power
amplifier in order to avoid the generation of non-compressible
non-linear chirp during power amplification.
[0057] U.S. Pat. No. 6,885,683 is hereby incorporated by reference
in its entirety. As expressly pointed out in '683, parabolic
amplifiers obey simple scaling laws and allow for the generation of
parabolic pulses. For example, a parabolic pulse with a spectral
bandwidth of around 2 nm can be generated using a parabolic
amplifier length of around 100 m. Specifically, ps-ns pulses can be
transformed to near parabolic pulses by selecting appropriate fiber
lengths, where the length of the pulse transformer needs to
increase with an increase in pulse width. In other words, a much
shorter pulse transformer can be used in conjunction with 10 and 20
ps seed pulses compared to 50 ps seed pulses. An exact pulse
transformation is not required for improvement in compressed pulse
quality; typically, an appropriate pulse transformer can be
designed using nonlinear pulse propagation routines in a computer
and can further be experimentally determined.
[0058] As an illustration of the dramatic improvement in pulse
quality obtainable with an appropriately designed pulse
transformer, numerical propagation routines were applied to a
specific example. In the specific example a pulse transformer
length of 440 m was assumed for 40 ps pulse(s) amplified in a large
core fiber to a pulse energy of >10 .mu.J, allowing for pulse
compression to a pulse width of less than 1 ps in a grating
compressor. The pulse transformer length and the input pulse energy
of 500 pJ to the pulse transformer were further designed to stay
just below the onset of stimulated Raman scattering, i.e. any
increase in input pulse energy compared to the presently used
value, would result in the onset of stimulated Raman scattering,
which is typically undesirable. The output energy of the pulse
transformer was further designed to be 10 nJ to provide enough seed
pulse energy to seed a power amplifier allowing for the
amplification of the pulses to pulse energies>10 .mu.J, without
the onset of stimulated Raman scattering in the power amplifier.
Here most of the amplification in the pulse transformer was
confined to the end of the pulse transformer to prevent the onset
of stimulated Raman scattering in the pulse transformer.
[0059] The resulting approximations are shown in FIGS. 4a and 4b).
Here FIG. 4a) illustrates the obtainable autocorrelation traces of
compressed 1 ps pulses obtained with (solid line) and without the
pulse transformer (dashed line). FIG. 4b) illustrates the
corresponding optical spectra at the output of the power amplifier.
Here the data without the pulse transformer were generated by
assuming a short 3 m length pre-amplifier, instead of the pulse
transformer. The great improvement in achievable pulse quality with
the pulse transformer is evident from FIG. 4a. Although FIGS. 4a)
and 4b) show an ideal case, it is to be understood that a
substantial improvement in pulse quality can also be obtained by
using shorter pulse transformer lengths of the order of 30-200
m.
[0060] Here the achievable pulse energy and pulse peak power
depends on the doping level and core diameter of the implemented
large core fiber. The ultimate peak power limit of fiber amplifiers
is the self-focusing limit of around 5 MW at a wavelength near 1000
nm and around 10 MW near 2000 nm. Hence, in the 1000 nm wavelength
region, 50 ps pulses can in principle be amplified to pulse
energies up to 250 .mu.J. For some applications, pulse
energies<10 .mu.J may also be of interest, even for these
smaller pulse energies the implementation of pulse transformers as
discussed with respect to FIG. 4 are highly beneficial.
[0061] It is instructive to distinguish the present nonlinear
amplification technique from conventional chirped pulse
amplification systems. In chirped pulse amplification, a short
pulse (sub ps to a few ps) from an oscillator is temporally
stretched in a pulse stretcher, amplified and subsequently
compressed in a pulse compressor, where the pulse stretching and
amplification stages are preferably only slightly nonlinear. The
small nonlinearity manifests itself in limited spectral broadening
in either the stretcher or amplifier stages, i.e. the amplifier
output typically has a bandwidth which is smaller than the
oscillator bandwidth. Alternatively, the amplifier output typically
has a bandwidth which is smaller than the input to the amplifier.
Typically, the pulse stretcher stretches the pulses by more than a
factor of ten.
[0062] In the present nonlinear amplification system, a long, >5
ps pulse, and up to about 1 ns, is generated by a seed system,
subsequently this ps-ns pulse is transformed in a pulse transformer
420 into a near parabolic pulse, linearly or non-linearly amplified
in a power amplifier 430 and finally compressed in a pulse
compressor 450. Here the pulse transformer provides only a small
amount of pulse stretching, for example the pulses are preferably
stretched by less than a factor of ten in the pulse transformer. In
the calculation example described with respect to FIGS. 4a and 4b,
the pulse stretching factor was around 1.75. The high level of
nonlinearity in the pulse transformer or power amplifier generally
manifests itself in a pulse transformer output spectral bandwidth
which is significantly larger than the input bandwidth to the pulse
transformer, say by a factor of 1.5. In the calculation example
described with respect to FIGS. 4a and 4b, the spectral broadening
factor was in the range from 10-40. Alternatively, the high level
nonlinearity in the amplifier manifests itself in an amplifier
output bandwidth that can be larger than the amplifier input
bandwidth. Such highly nonlinear power amplifiers are further
susceptible to multi-pass interference from fiber or optical
components up-stream of the power amplifier. Fortunately,
multi-pass interference from optical components upstream of the
pulse transformer have only a very limited effect on compressible
pulse quality, since the pulses inserted into the pulse transformer
are typically only slightly chirped. Most detrimental is any
multi-pass interference arising from optical components that are
traversed when the pulses are significantly spectrally broadened or
chirped. For example detrimental multi-pass interference can arise
from birefringent optical components located between the output of
the pulse transformer and the input to the power amplifier. One
viable option for minimization of such multi-pass interference at
this stage is to couple the output of the pulse transformer
directly into the power amplifier without any intervening
additional birefringent fibers. The use of non-birefringent or
polarizing power amplifiers as well known in the state of the art
can also help to suppress multi-pass interference.
[0063] In other nonlinear amplification schemes the need for pulse
transformation to a near parabolic near linearly chirped waveform
has not been realized. Moreover, the use of modelocked fiber
oscillators generating pulses>5 ps as input to such systems has
not been suggested.
[0064] A useful way of characterizing the action of a pulse
transformer is the obtained reduction in pulse pedestal after the
final compression stage 450 by the insertion of undoped fiber into
the pulse transformer. For example, if the pulse transformer 450 is
configured as pre-amplifier containing lengths of undoped and doped
fibers, the elimination of most of the undoped fiber in the pulse
transformer would decrease the obtainable pulse quality after the
final compression stage, as evident from FIG. 4b.
[0065] Pulse transformers that do not provide any gain can also
improve the pulse quality at the output of the pulse compressor;
preferably, the pulse transformer imparts non-negligible nonlinear
self-phase modulation onto the injected pulse, for example a level
of self-phase modulation>.pi., and up to about 100.pi., limited
by the onset of stimulated Raman scattering in the pulse
transformer. Pulse transformers can also be designed using sections
of fibers with decreasing mode size so as to increase self-phase
modulation in order to improve pulse quality.
[0066] Compared to prior art chirped pulse amplification systems,
the final power amplifiers can be operated at larger levels of
self-phase modulation, allowing for the extraction of increased
pulse energy from short pulse fiber systems.
[0067] In addition to only one power amplifier, more than one power
amplifier, for example a fiber power amplifier and a fiber rod
amplifier can be implemented.
[0068] From the known power limitations of fiber amplifiers, the
achievable pulse energies from such systems scale approximately
linearly with the implemented seed pulse width. Therefore, the
nonlinear amplification system described is preferably implemented
using pulses with a width>5 ps and more preferably with pulses
with a width>10 ps, and up to about 1000 ps, whereas for chirped
pulse amplification systems, seed pulses with a width<5 ps are
preferred. The use of chirped pulses in the pulse transformer is
further advantageous, as it allows a reduction in required pulse
transformer length. Particularly for pulses>100 ps, the onset of
stimulated Raman scattering limits the amount of spectral
broadening one can obtain in a pulse transformer.
[0069] In contrast to chirped pulse amplification, nonlinear
amplification systems further allow the use of partially coherent
or incoherent seed sources. A long pulse transformer length can
also transform such pulses into near parabolic pulses, allowing for
some degree of pulse compression in the pulse compressor. The basic
configuration of such a system is similar to the one shown in FIG.
4 and is not further described here.
[0070] In various embodiments solid-state amplifiers can further be
incorporated after the fiber power amplifier in FIG. 4 to enable
the generation of higher pulse energies.
[0071] The nonlinear amplification scheme here is applicable to any
type of fiber amplifier; for example high power Er amplifiers can
also be constructed as shown in FIG. 4. In this case Er fiber
amplifiers with positive dispersion can be implemented. Due to the
bandwidth-limitation of Er amplifiers, undoped positive or negative
dispersion fibers can be spliced onto the end of the Er power
amplifier to further increase the bandwidth of the amplified
pulses. This is not separately shown. For final pulse compression,
a separate pulse compression element 450 is still required. When
using undoped negative dispersion fiber, spectral broadening can be
combined with pulse compression. Element 450 can combine spectral
broadening and pulse compression. The insertion of an undoped fiber
section after a power amplifier for pulse broadening is indeed
useful for any rare-earth amplifier and not separately discussed
here. High energy pulses with pulse widths<200 fs can thus be
generated with such nonlinear amplification schemes using ps Er
amplifiers in the seed module. To achieve the shortest possible
pulses from such amplifiers, the control of 3.sup.rd and 4.sup.th
order dispersion also become significant, i.e. the third order
dispersion of the overall system needs to be adjusted to enable
optimum pulse quality at the final output of the system. For
example, third and fourth order dispersion can be manipulated by
using pulse transformer fibers with appropriate values of third
order dispersion. Such fibers are well known in the state of the
art and not further described here.
[0072] When using Tm amplifiers or large core Er amplifiers, the
scheme as shown in FIG. 4 can also be implemented. In this case,
the pulse transformer preferably comprises long lengths of positive
dispersion fiber and short lengths of Tm or Er fiber, selected in
such a way that the amount of self-phase modulation dominates in
the lengths of positive dispersion fiber. Because large core Tm or
Er power amplifiers generally have large values of negative
dispersion, the Tm or Er amplifier will produce pulse compression,
however, the presence of a near parabolic seed pulse minimizes
pulse distortions during pulse compression. More generally, with a
near parabolic seed pulse, in the presence of an undoped negative
dispersion fiber, or fiber amplifier, and self-phase modulation,
the pulse spectral bandwidth increases and the pulse width
decreases, while the parabolic pulse shape is also preserved.
Numerical simulations verified that inverted parabolic pulse
amplifiers can sustain self-phase modulation values of several
without detrimental pulse distortions. Since spectral broadening in
inverted parabolic amplifiers increases with a reduction in pulse
width, the parabolic pulses are preferably coupled out of the
inverted parabolic amplifier while still being positively chirped.
A subsequent conventional pulse compressor can then be used to
compress the pulses close to the bandwidth limit.
[0073] In various embodiments the laser systems shown in FIGS. 1-3
can be configured for further amplification. Amplification systems
as shown in FIG. 4 can also be implemented and further combined
with nonlinear frequency conversion techniques, such as frequency
doubling, tripling, quadrupling or even quintupling. Optical
parametric generation or amplification can be used for frequency
down conversion of the amplifier output. Optical schemes for
frequency conversion are well known in the state of the art and not
further described here. A generic scheme for frequency conversion
500 is shown in FIG. 5. When implementing nonlinear frequency
down-conversion, optical parametric amplification can also be
implemented; such schemes were discussed for example in U.S. Pat.
No. 8,040,929 and are not further described here.
[0074] The laser systems as described here are suitable for
micro-machining applications or for the ionization of solid,
liquids or gases as well as desorption of molecules from target
materials. A generic system implementation 600 is shown in FIG. 6a.
Typically an optical focusing arrangement (not shown) is further
included to attain a desired beam size on the target 610. For
specific applications, an x-y scanner can be inserted upstream of
the target material to move the laser beam to the desired target
site. Alternatively, the target 610 can be manipulated in three
dimensions using appropriate opto-mechanical stages, as well known
in the state of the art. A scanning stage is not separately shown
here.
[0075] Specifically, the laser systems as described here are
suitable for mass spectroscopy applications as a means of desorbing
molecules. Target molecules in a condensed phase must be desorbed
from the surface and ionized in order to be carried by the
spectrometer electric field. In the typical method of
matrix-assisted laser desorption/ionization (MALDI), strong
absorption of ultraviolet laser light by a matrix is used to desorb
and ionize embedded target molecules. The ultraviolet light can be
replaced with infrared light from laser systems as described here.
The new wavelength can be chosen to be resonant with a more
desirable matrix, while reducing direct interaction with the
target. Alternatively, off-resonant infrared light is advantageous
due to the greater penetration depth, accessing material further
down in the matrix, as in Rezenom et al. in `Infrared
laser-assisted desorption electrospray ionization mass
spectrometry` published on Nov. 29, 2007, in Volume 133 of Analyst,
pp. 226-232 (2008). The non-resonant interaction is also nonlinear,
which is advantageous for the greater control over interaction
volume size by controlling the focal volume.
[0076] Moreover, the short and intense infrared pulses from laser
systems as described can further be used as a means of ionizing
target molecules either during desorption from the condensed phase,
or directly from the gas phase. This type of ionization is a softer
ionization source that can cause less undesired fragmentation than
electrospray, the typical ionization method in the gas phase. For
such applications, infrared wavelengths are desirable for their
non-resonant tunnel ionization that works for all types of
molecules without relying on a particular resonance. The high
repetition rates of fiber lasers are desirable for increasing
detection efficiency. The benefits of tunnel ionization by infrared
laser pulses for mass spectroscopy are described in Peng et al.,
High-Pressure Gas Phase Femtosecond Laser Ionization Mass
Spectrometry in the Jul. 3, 2012, Volume 84 issue of Analytical
Chemistry. Pulse widths from 100 fs-10 ns can be readily
implemented for desorption or ionization applications and
wavelengths can be readily selected from the 1 .mu.m to 8 .mu.m
range.
[0077] Finally, the laser systems as described here are also
suitable for medical processing applications, such as surgery or
tissue cutting. The benefits of ps lasers for applications in
surgery were for example described in Saeid Amini-Nik, Ultrafast
Mid-IR Laser Scalpel: Protein Signals of the Fundamental Limits to
Minimally Invasive Surgery in the September 2010, Volume 5 issue of
PLoS ONE. For such applications frequency down-converted fiber
lasers are desirable, where the frequency is down-converted to the
2.6-6 .mu.m wavelength range depending on the target medical
material. A generic system implementation 650 is shown in FIG. 6b
in which biological material 660 is to be heated, cut, ablated or
otherwise modified. Optical focusing arrangements and
opto-mechanical arrangements for two or three dimensional scanning
as discussed with respect to FIG. 6 are not separately shown
here.
[0078] The above description of the preferred embodiments has been
given by way of example. From the disclosure given, those skilled
in the art will not only understand the present invention and its
attendant advantages, but will also find apparent various changes
and modifications to the structures and methods disclosed. It is
sought, therefore, to cover all such changes and modifications as
fall within the spirit and scope of the invention, as defined by
the appended claims, and equivalents thereof.
[0079] As used herein, the term "or" is intended to mean an
inclusive "or" rather than an exclusive "or", unless specified. In
addition, the articles "a" and "an" as used in this application and
the appended claims are to be construed to mean "one or more" or
"at least one" unless specified otherwise.
[0080] For purposes of summarizing the present invention, certain
aspects, advantages and novel features of the present invention are
described herein. It is to be understood, however, that not
necessarily all such advantages may be achieved in accordance with
any particular embodiment Thus, the present invention may be
embodied or carried out in a manner that achieves one or more
advantages without necessarily achieving other advantages as may be
taught or suggested herein.
[0081] 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.
* * * * *