The Researches and Investigations of Pulsed Laser in NIR Eye-safe Wavelength ?????????????????

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The Researches and Investigations of Pulsed Laser
in NIR Eye-safe Wavelength?????????????????

• ???
• ???????
• Department of Electrophysics
• National Chiao Tung University , TAIWAN

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Outline

• Introduction, Background, and Motivation
• Intracavity Optical Parametric Oscillator (OPO)
  Pumped by Nddoped Laser
• Self-Stimulated Raman Scattering (Self-SRS)
• Passively Q-switched Erbium/Ytterbium Fiber Laser
• PCF Laser pumped OPO
• Optically Pumped Semiconductor Laser (OPSL)
• ConclusionContribution and Future Work

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Eye-safe wavelength
VIS
UV
100-280nm
400-700nm
315-400nm
280-315nm
IR
NIR lt 1400nm
NIR gt 1400 nm
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Background and Motivation
High pulse energy
High rep rate
Wavelength tunable
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Conventional methods for generating NIR Eye-safe
laser

• Nonlinear conversion process
• Optical Parametric Oscillator
• Optical Parametric Amplification
• Stimulated Raman Scattering
• Rare-earth-ion-doped materials
• Er3, Tm3, Ho3
• Semiconductor Laser
• InGaAsP, AlGaInAs, GaInAsSb

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Outline

• Introduction, Background, and Motivation
• Intracavity Optical Parametric Oscillator (OPO)
  Pumped by Nddoped Laser
• Self-Stimulated Raman Scattering (Self-SRS)
• Passively Q-switched Erbium/Ytterbium Fiber Laser
• PCF Laser pumped OPO
• Optically Pumped Semiconductor Laser (OPSL)
• ConclusionContribution and Future Work

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Optical Parametric Oscillator
External cavity
Intra-cavity
NLC
pump
signal
Idler
OPO Cavity

• Lower threshold
• Higher efficiency
• Dynamic pulse

• Higher threshold
• Lower cavity stability requirement

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NIR Eye-safe Laser with Intra-cavity OPO
Requirement
Application Laser Range Finder

• High pulse energy up to mJ
• High peak power
• 0.x x10 Hz Rep rate

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Passively Q-switched Intra-cavity OPO
OPO
3-bars

• x-cut KTP
• Temp. insensitive
• Large nonlinear coeff.
• Non-walk-off
• High damage threshold

ASAP simulation
High power QCW LD
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• Output coupler with 40 60 reflectivity is
  commonly used to optimize energy conversion
  efficiency.

Appl. Phys. B 79, 823-825 (2004)
Appl. Opt. V45 N25, 6007-6015 (2006)

• How to optimize peak power?
• Loss vs. threshold

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Threshold of IOPO
Photon density of threshold in a passively
Q-switched laser
JJ Degan (1995)
?f,max 1.5 x 1017 cm-3
Photon density of threshold in a SRO
Brosnan and Byer (1979)
?f,th 6 x 1015 cm-3 6 x 1016 cm-3
The threshold of an intracavity OPO is determined
by the bleach of the saturable absorber.
ff,max gtgt ff,th
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Experimental Result

• Lasing threshold is nearly constant for different
  output coupler

• There is an individual optimum value of pulse
  energy and peak power.

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Pulse profile
Eopo3.7 mJ Peak power0.5 MW
Eopo4 mJ Peak power0.7MW
Eopo3.3 mJ Peak power1.5MW
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Experimental Result
Enlarge cross section to enable higher pump power
Opt. Express 15, 4902-4908 (2007)
Output energy 10.8 mJ
? OPO back conversion
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Thermally induced birefringence effect
Depolarization
NdYAG

• Portion of electric field transfer into another
  orthogonal axis.? Energy feedback

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Theoretical analysis of Passively Q-switched IOPO
Depolarization term
OPO loss term
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Pulse profile
0.9 MW
0.68 MW
0.62 MW
0.56 MW

• Good coincidence between theory and experimental
  result.
• Output coupler with higher reflectivity gives
  higher feedback of energy and results in more
  fluctuation in pulse dynamics.
• Optimized peak power still holds in lower
  reflectivity.

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Summary in IOPO

• Up to 10-mJ pulse energy eye-safe laser is
  achieved with IOPO.
• The threshold of passively Q-switched IOPO is
  dominated by the bleach of saturable absorber.
• The birefringence effect induced from thermal
  effect in NdYAG gives rise to parasitic pulse in
  time domain.

Y. P. Huang, H. L. Chang, et al. Subnanosecond
mJ eye-safe with an intracavity optical
parametric oscillator in a shared resonator Opt.
Express 17, 1551-1556 (2009)
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Outline

• Introduction, Background, and Motivation
• Optical Parametric Oscillator (OPO)
• Self-Stimulated Raman Scattering (SRS)
• Passively Q-switched Erbium/Ytterbium Fiber Laser
• PCF Laser pumped OPO
• Optically Pumped Semiconductor Laser (OPSL)
• ConclusionContribution and Future Work

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Raman Scattering

• First discovered by C. V. Raman in 1928.
• Third-order nonlinear process. (Four-wave mixing)
• Inelastic collision
• No consideration of phase-matching

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Conventional crystals for stimulated Raman
scattering
Material Raman shift (cm-1) Raman linewidth (cm-1) Cross section (arb. Units) Raman gain (cm/GW) Damage threshold (GW/cm2)
Ba(NO3)2 1047 0.4 21 11 0.4
BaWO4 924 1.6 52 8.5 5
KGd(WO4)2 768 901 6.7 5.7 59 54 4.4 3.3 10
YVO4 890 2.6 92 gt 4.5 1
GdVO4 882 3 92 gt 4.5 1
A. A Kaminskii et. al. Opt. Com. (2001)
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NdYVO4 used as a self-SRS crystal
NdYVO4 can be used to serve as a gain medium
and Raman medium simultaneously.
Self-stimulated Raman scattering crystal

• Heat generation resulting from quantum defect
  restricts the available output power

Diode-pumped actively Q-switched c-cut NdYVO4
self-Raman laser, Y. F. Chen, V 29. No. 11 Opt.
Lett. 1251 (2004) Compact efficient
all-solid-state eye-safe laser with
self-frequency Raman conversion in a NdYVO4
crystal, Y. F. Chen, V 29. No. 18 Opt Lett
2172-2174 (2004)
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Thermal effect

• The influence of thermal effect
• Thermal lens
• Mode matching
• Cavity stability
• Conversion efficiency
• Beam quality

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Thermal effect
W. Webber et. al. IEEE J. Quantum Electron 34
(1998)
Uniform doped NdYAG
Undoped YAG-bundled
15W
15W
Temp dist.
Stress dist.
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Thermal lens in fundamental cavity

• Thermal diffuser
• Raman gain medium

0.3- doped NdYVO4
20W
R92

• The thermal lens effect is reduced by 1.6 times

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Actively Q-switched intra-cavity Self-SRS

• Lasing threshold decreased.
• Maximum power of 2.23 W was obtained.
• Conversion efficiency was enhanced from 8.9 to
  13 with double-end crystal.

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40 kHz
20 kHz
Pulse energy 56 uJ Peak power 17 kW
Pulse energy 86 uJ Peak power 22 kW
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Summary in Self-SRS

• A double-end diffusion bond NdYVO4 crystal was
  firstly used to be a self-stimulated Raman
  scattering crystal in eye-safe wavelength.
• Thermal effect ? (1.6X)
• Critical pump power ?? Raman power ?
• Conversion efficiency? (9? 13)

Y. T. Chang, K. W. Su, H. L. Chang, et al.
Compact efficient Q-switched eye-safe laser at
1525 nm with a double-end diffusion-bonded
NdYVO4 crystal as a self-Raman medium Opt.
Express 17, 4330-4335 (2009)
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Outline

• Introduction, Background, and Motivation
• Optical Parametric Oscillator (OPO)
• Self-Stimulated Raman Scattering (Self-SRS)
• Passively Q-switched Erbium/Ytterbium Fiber Laser
• PCF Laser pumped OPO
• Optically Pumped Semiconductor Laser (OPSL)
• ConclusionContribution and Future Work

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Double-Clad Fiber

• Double clad fiber
• Low NA 0.07 (Single mode )
• Large mode area
• High pump absorption 3dB/m

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Er/Yb codoped material

• Broad Yb Absorption relaxes pump wavelength
  constraints
• 100X increase in practical pump absorption

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Passively Q-switched Er/Yb Fiber Laser
Fiber-coupled LD _at_976 nm
cavity
20W
Er/Yb doped double-clad fiber clad/core Dia.
300/25 µm NA gt0.46 /lt0.07
HT_at_976 nm HR_at_15301600 nm
HR _at_ 15301600 nm
SA
R4
7m
Laser output
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• Conventional saturable absorber at 1.5 um
• Co2MgAl3O4, Cr2ZnSe, Co2ZnS, Co2ZnSe

• Semiconductor saturable absorber at 1.5 um
• Quantum wells InGaAsP, AlGaInAs

• AlGaInAs SA was firstly proposed in eye-safe
  fiber laser

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Semiconductor saturable absorber at 1.5 µm
InGaAsP v.s. AlGaInAs

• AlGaInAs vs. InGaAsP
• Higher conduction band-offset ? better
  confinement
• Higher modulation depth
• Larger absorption cross section

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Semiconductor saturable absorber

• Periodic structure with half wavelength of
  lasing mode ? Damage avoided.

• High modulation depth results in high pulse energy

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Laser Performance of Er/Yb Fiber Laser
Q-switching efficiency gt 85
Pulse energy 105 µJ
Higher pulse energy than InGaAsP1
Higher efficiency than bulk SA
1 Passively Q-switched 0.1-mJ fiber laser
system at 1.53 um, R. Pasbotta, et. al. Opt.
Lett V24. 388 (1999)
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Summary in EYDFL

• An AlGaInAs/InP semiconductor absorber was
  firstly used in eye-safe laser region.
• No active cooling.
• High pulse energy up to 100-µJ.
• Good beam quality was obtained with M2 lt 1.5

J. Y. Huang, S.C. Huang, H. L. Chang, et. al
Passive Q-switching of Er-Yb fiber laser with
semiconductor saturable absorber Opt. Express,
V16, 3002-3007 (2008)
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Outline

• Introduction, Background, and Motivation
• Intracavity Optical Parametric Oscillator (OPO)
  Pumped by Nddoped Laser
• Self-Stimulated Raman Scattering (Self-SRS)
• Passively Q-switched Erbium/Ytterbium Fiber Laser
• Widely tunable eye-safe laser by a PCF laser and
  OPO
• Optically Pumped Semiconductor Laser (OPSL)
• ConclusionContribution and Future Work

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Pump Source of External-cavity OPO
Yb doped Photonic Crystal Fiber
AlGaInAs/InP

• Single mode
• Extreme large mode area
• Polarization maintain
• High pump absorption 30 dB/m

• Periodic structure with half wavelength of
  lasing mode
• High modulation depth
• High absorption cross section

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External cavity OPO pump source
Yb-doped PCF Passively Q-switched Laser
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Performance of PCF Laser
Low power pumping

• Central wavelength 10291031 nm
• Optical-to-optical conversion efficiency 37
• Rep rate 1k 6 kHz
• Maximum output peak power 170 kW

High power pumping
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External-cavity OPO

• PPLN
• High nonlinear coefficient 15pm/V (5 x of KTP)
• Large phase matching wavelength coefficient (0.5
  nm/?)

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Performance of external-cavity OPO
Conversion efficiency 35
Maximum pulse energy 138 uJ Maximum peak power
19 kW
Round trip loss
Pth0.12W
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Wavelength vs. Temperature
Wavelength tuning range 1513 to 1593 nm
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Summary in PCF pumped OPO

• A 750 uJ PCF fiber with AlGaInAs semiconductor
  saturable absorber was used to be a 1030-nm OPO
  pump source.
• Wavelength tuning range up to 80 nm and pulse
  energy of 138 uJ in eye-safe region from 1513 to
  1593 nm was obtained by a PPLN-OPO.

H. L. Chang, W. Z. Zhuang, W. C. Huang, et. al
Widely tunable eye-safe laser by a passively
Q-switched Photonic crystal fiber laser and an
external-cavity optical parametric oscillator
Laser Phys. Lett. To be published.
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Outline

• Introduction, Background, and Motivation
• Optical Parametric Oscillator (OPO)
• Self-Stimulated Raman Scattering (SRS)
• Passively Q-switched Erbium/Ytterbium Fiber Laser
• PCF Laser pumped OPO
• Optically Pumped Semiconductor Laser (OPSL)
• ConclusionContribution and Future Work

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Optically Pumped Semiconductor Laser

• Advantages
• Uniform distribution of pump power over large
  active region
• Excellent beam quality
• Wide wavelength range

• Concerning issue
• Thermal management

• Heat sink/spreader
• Pumping style

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Optically Pumped Semiconductor Laser
Barrier pumping and in-well pumping
Transmission spectrum

• The quantum defect of in-well pumping is reduced
  from 32 to 14 compared with barrier-pumping

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Barrier-pumping
Fluorescence spectrum
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The laser performance
30kHz
10C

• The performance of output power depends on the
  temperature

• Output power saturates at 135 mW at a repetition
  rate of 30 kHz

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In-well pumping
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In-well v.s. Barrier Pumping

• Comparison of single chip
• Higher conversion efficiency in in-well pumping
• Lower absorption

Double chips were used to increase absorption
efficiency
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Laser performance Double chips
10C

• Conversion efficiency 30 _at_ 9C

• Optimum rep rate 40kHz 60 kHz

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Laser performance
M2 lt1.3
Barrier-pumping
In-well pumping

• The maximum output peak power 290 W at a pump
  peak power of 2.3 kW

• The maximum output peak power 520 W at a
  pump peak power of 3.7 kW.

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Summary in OPSL (1/2)

• A periodic AlGaInAs QW/barrier structure was
  firstly developed to be a gain medium in a
  high-peak -power nanosecond laser at 1.57µm .
• With barrier pumping, a maximum average output
  power of 135 mW was obtained under 1.25 W pump
  power. The maximum peak power was up to 290 W at
  a peak pump power of 2.3 kW.

S. C. Huang, H. L. Chang, et. al AlGaInAs/InP
eye-safe laser pumped by a Q-switched NdGdVO4
laser Appl. Phys. B, 94, 483-487 (2009)
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Summary in OPSL (2/2)

• Barrier-pumping scheme and in-well pumping were
  compared in the performance of thermal
  improvement.
• The optical-to-optical conversion efficiency of
  in-well pumping scheme was up to 30. The maximum
  output peak power was up to 0.52 kW under 3.7 kW
  pump peak power

H. L. Chang, S. C. Huang, et. al Efficient
high-peak-power AlGaInAs eye-safe wavelength disk
laser with optical in-well pumping Opt.
Express. V17, 1409-11414 (2009)
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Conclusion Contribution (1/2)

• We achieved eye-safe lasers by means of
  structures including OPO, self-SRS, Er/Yb fiber
  laser, PCF laser and OPSL.
• Theoretical analyses of lasing threshold and
  dynamic pulse behavior in the passively
  Q-switched intracavtiy OPO were proposed and
  investigated.
• A double-end diffusion bond self-SRS crystal were
  used to diminish the thermal effect.

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Conclusion Contribution (2/2)

• A semiconductor saturable absorber, AlGaInAs, was
  used in a passively Q-switched Er/Yb laser to
  obtain a high pulse energy laser.
• An 80-nm tuning range eye-safe laser was achieved
  by an AlGaInAs passively Q-switched PCF laser and
  an external-cavity OPO.
• An optically pumped disk laser with AlGaInAs/InP
  quantum well/barrier structure as gain medium was
  demonstrated. In-well pump scheme was proposed to
  reduce the thermal effect.

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Conclusion Future work
Tunable eye-safe laser with narrow linewidth
pump
HR_at_10301080nm
10301080nm AR HR_at_1550nm
Grating
signal
PPLN
Beam combiner
2.5cm
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Thanks for you attention
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APPENDIX
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Rep Rate and Peak Power vs. Range
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Cr4YAG (Cr4Y3A15O12) SpecificationsHigh
absorption cross section of 4.3 x 10-18
cm2 Wavelength range 900-1200 nm Damage
threshold gt500 MW/cm2 Initial transmittance
10-99 Recovery time 8.5µs
V3YAG Specifications High ground state
absorption (GSA) cross section of 7x10-18 cm2
near 1.3µ m . Wavelength range, 1000-1450 nm , in
particular, for 1.3 m Nd-lasers Damage threshold
7-10 J/cm2 Initial transmittance 50-99
The ratio of initial (small signal) to saturated
absorption is higher than 10 Excellent optical,
mechanical, and thermal properties.
Co2MgAl2O4 SpecificationsHigh absorption
cross section of 3.5 x 10-19 cm2 Wavelength
range, 1200-1600 nm , in particular, for eye-safe
1.54 µm Erglass laser Damage threshold, 10
J/cm2 Initial transmittance, 30-99 The ratio
of initial (small signal) to saturated absorption
is higher than 10.which results in high contrast
of Q-switch
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Semiconductor saturable absorber (SESA)
AlGaInAs quantum well as a saturable absorber in
diode-pumped solid state (DPSS) laser with
proper parameters for passively Q-switched (PQS)
and mode-locked laser ? For 1.06µm PQS ,
SESA would be designed with large
cross section , large modulation depth
and high damage threshold
Optically-pumped semiconductor laser (OPSL)
By use of AlGaInAs quantum well as a gain medium
in OPSL ? Design for 1.1-1.6µm
wavelength spectral range ? Power
scaling Different wavelength spectral
(red) with different semiconductor materials
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Saturable absorber fluence intensity
FP filter
Cr4YAG
?O
SESA
FP filter
Fsat,Cr4YAG
Fsat,SESA
13 ?O

• Easier for cavity setup
• Lower optical intensity on SA

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• Conventional saturable absorber at 1.5 um
• Co2MgAl3O4, Cr2ZnSe, Co2ZnS, Co2ZnSe

• Semiconductor saturable absorber at 1.5 um
• Quantum wells InGaAsP, AlGaInAs

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Conventional saturable absorber at 1.5 um
SA Lifetime(µs) ses/ sgs (10-19cm2) Results Fiber parameters Active medium
Co2MgAl3O4 1 0.34 0/2.4 22µJ, 370 ns 60W_at_60kHz 12µm, NA0.22 Er/Yb
Cr2ZnSe1 8 0.2/3.4 18µJ, 380 ns 45W_at_70kHz 12µm, NA0.22 Er/Yb
Co2ZnS2 200 1.1/10 60µJ, 3.5 ns gt10 kW_at_6kHz 11 µm, NA 0.21 Er/Yb
Co2ZnSe3 290 1.1/11.5 15 nJ, 43 mW, 350 ns _at_235kHz 2.7 µm, NA 0.27 Er
SA
1 Philippov, V et al., In, XI International
Conference on Lasers Optics (LO'2003), St
Petersburg, Russia, 30 Jun - 4 Jul 2003. 2 M.
Laroche et al., Opt. Lett. 27, 1980-1982
(2002).3 V. Filippm. et al., Opt. Lett 26,
,343-345 (2001).
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The laser performance
30kHz

• The performance of output power depends on the
  temperature
• Output power saturates at 135 mW at a repetition
  rate of 30 kHz

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Laser Performance

• The transmittance of the gain material exceeds
  85 at the excitation intensity higher than 3.0
  MW/cm2.

• The maximum output peak power 290 W at a pump
  peak power of 2.3 kW

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Time domain of in-well pumping
20ns/div
20ns/div
1064nm
1342nm
1565nm
1570nm
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• Tuning range gt 60 nm

Narrow-linewidth widely tunable hybrid
Q-switched double-clad fiber laser Y. X. Fan et
al. Opt. Lett. V28, No. 7 (2003)
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