Solid-state Raman lasers: a tutorial
Jim Piper
Professor of Physics
Centre for Lasers and Applications, Macquarie University, Sydney
(Carnegie Centenary Professor, Heriot-Watt University, Edinburgh)
Acknowledgements: H Pask, R Mildren, H Ogilvy, P Dekker
Australian Research Council, DSTO Australia
Solid-state Raman lasers
Overview of presentation
• Introduction to Stimulated Raman Scattering (SRS),
crystalline Raman materials, and solid-state Raman
lasers (SSRL)
• Raman generators (picosecond pulse conversion)
• External-cavity SSRLs (nanosecond pulse conversion)
• Intracavity (including self-Raman) SSRLs
• Intracavity frequency-doubled SSRLs for visible outputs
• CW external-cavity and intracavity SSRLs
Note excellent recent reviews of solid-state Raman lasers are given by:
Basiev & Powell Handbook of Laser Techn. & Applns B1.7 (2003) 1-29
Cerny et al Progress in Quantum Electronics 28 (2004) 113-143
Pask Progress in Quantum Electronics 27 (2003) 3-56
Solid-state Raman lasers
Stimulated Raman Scattering
Spontaneous Raman scattering was first reported by Raman and
Krishnan (also Landsberg and Mandel’shtam) in1928.
Stimulated Raman Scattering (SRS) arises from the third order
nonlinear polarisability P3 = eoc3E3, which gives rise to various
nonlinear optical phenomena, including also two-photon absorption,
stimulated Brillouin scattering and self-focussing.
Photons passing through a Raman-active
medium are inelastically scattered, leaving the
molecules of the medium in an excited
wP wS1
(usually ro-vibrational) state:
wS1 wS2
wS1 = wP - wR (first-Stokes generation)
wS2 wS3
wS2 = wS1 - wR (second-Stokes generation)
wS3 = wS2 - wR (third-Stokes generation)
wR
SRS does not require phase matching.
Solid-state Raman lasers
SRS theory*
* Penzkofer et al Progress in Quantum
Electronics 6 (1979) 55-140.
In the “steady-state” regime, where the pump duration tP is long
compared to the Raman dephasing time TR, the Stokes intensity
IS(z) grows as:
IS(z) = IS(0) exp (gR IP z)
where IP is the pump intensity, the steady-state Raman gain
coefficient is gR = 8pc2 N . ds in units cm/GW,
2w 3 G dW
hmS S
and the integral Raman scattering cross-section is introduced as
ds = wS4mS . h . da 2
dW c4 mL 2mwR dq
Here da/dq is the derivature of the normal-mode polarisability (the
square is proportional to c3), G is the Raman linewidth, the inverse of
the dephasing time i.e. G = TR-1, and small-signal conditions are
assumed. Typically TR ~ 10ps , G ~ 1011 s-1 or DnR ~ 5 cm-1.
Solid-state Raman lasers
SRS theory (cont.)
In the steady-state regime, gR scales with the Raman (Stokes)
frequency wS and the integral Raman scattering cross-section
ds/dW , and inversely as the Raman linewidth G = cDnR .
Raman media of choice for this regime have small Raman linewidth
(30. Thus for a high gain
crystal with gP ~10 cm/GW, and a crystal length 30mm, the pump
intensity needs to be IP >1GW/cm2. This is above the damage
threshold of many materials!
Solid-state Raman lasers
SRS theory (cont.)
In the transient Raman regime, where tP 10 ps.
Raman crystals are chosen for high Raman gain and damage threshold (e.g.
BN, KGW, BW). First-Stokes pump thresholds are typically ~0.5-1GW/cm2.
For ultra-short pulses 1
input output
R1 , R2 reflectances at first-Stokes
mirror 1 mirror 2
Resonating the first- and higher-order-Stokes fields effectively reduces the
Raman threshold: for a 50mm-long BN crystal the calculated threshold for
first-Stokes from a 1064nm, nanosecond pump is ~10 MW/cm2 compared
with ~300 MW/cm2 for single-pass Raman generation*.
Achieving high conversion efficiency requires matching of the pump
mode to the Raman Stokes mode in the resonator. At (Stokes) average
powers > 1W this is likely to require consideration of thermal lensing in
the Raman crystal due to heat deposition by the Raman process itself.
* HM Pask Prog. Quantum Electron. 27 (2003) 3-56.
Solid-state Raman lasers
External-cavity (resonator)
Raman lasers
Basiev et al, OSA Advanced Solid-State Photonics 2004, TuB11
High average power
8 x 145mJ, 50ns, 50ms BaWO4 95mm
30 Hz at 1064nm
Nd:YAG 35W
85%T 1064nm 3.2mm 77% R, pump
HR 1st-3rd Stokes 55% T 1st-3rd Stokes
High energy
50 x 380mJ, 50ns BaWO4 95mm
20 kHz at 1062nm
Nd:GGG 19J
85%T 1064nm 3.2mm 77% R, pump
HR 1st-3rd Stokes 55% T 1st-3rd Stokes
Solid-state Raman lasers
External-cavity (resonator)
Raman lasers
180mJ, 20ns Ba(NO3)2 70mm Ermolenkov et al, J. Opt.
10 Hz at 532nm Technol. 72 (2005) 32.
35mJ, 10Hz 1st-Stokes at
90%T 532nm HR, pump
HR 1st-Stokes
5mm
70% T 1st-Stokes
563nm (20% eff.) external
SHG 4.2mJ at 281nm
176mm unstable
Takei et al, Appl. Phys B
140mJ, 20ns Ba(NO3)2 58mm
20 Hz at 1064nm 74 (2002) 521.
11mJ, 20Hz 3rd-Stokes at
HR pump 1598nm (eyesafe region) after
HT 1064nm 5mm
HR 1st-3rd Stokes HR 1st-2ndStokes compensation for strong
71% T 3rd-Stokes thermal lensing in BN
200mm
Solid-state Raman lasers
External-cavity Raman lasers
Mildren et al, OSA Adv. Solid-State Photonics 2006, MC3
*also Mildren et al, Opt. Express 12 (2004) 785; Pask et al, Opt. Lett. 28 (2003) 435.
2.4W at 532nm KGW 50mm
10ns, 5kHz 100 100
Output Coupler Transmission (%)
Fraction of Output (%)
80 80
90%T 532nm HR pump, 1st-Stokes
160mm 60 60
HR 1st-2nd Stokes 50-60% 2nd-Stokes
40 40
52mm mode-matched
1.6 20 20
1.4 KGW E//Nm
0 0
1.2 (588nm)
OUtput Power (W)
560 580 600 620
1.0 Wavelength (nm)
0.8
KGW E//Ng
0.6 (579nm) Conversion efficiency into 2nd-Stokes
0.4 at 588nm: 64% (slope eff. 78%);
0.2 at 579nm: 58% (slope eff. 68%).
0.0
0.0 0.5 1.0 1.5 2.0 2.5
Input Power (W)
Solid-state Raman lasers
Intracavity Raman lasers
Intracavity Raman lasers allow for both the pump and the
Stokes wavelength(s) to be resonated, substantially reducing
the effective Raman threshold (~MW/cm2)
Nd3+ laser
Intracavity Raman
diode
crystal
* Raman crystal
*including coupled-cavity
pump
Mirror 1 Q-switch Mirror 2
HT pump HR pump/ fundamental
HR fundamental/Stokes Stokes coupling
Nd3+ laser/
Raman crystal
Intracavity self-Raman
Mirror 1
Q-switch Mirror 2
HT pump HR pump/fund
HR fund/Stokes Stokes coupling
Solid-state Raman lasers
Intracavity crystalline
Raman lasers
Effects of thermal lenses on resonator design
Pask & Piper, IEEE J. Quantum Electron. 36 (2000) 949.
*also Pask, Prog. Quantum Electron. 27 (2003) 3.
Resonator mode 1.0
stability parameter g2
1. I=0 4. I=32A unstable
size taking account 0.8 2. I=11A 5. I=38A region
of LIO3 thermal lens 3. I=14A 6. I=40A 6
Plane HR mirror
Nd:YAG LiIO3
0.6
Plane OC mirror 45
1000 0.4
900
instability
1
800 0.2 2
3
unstable region stable region
beam waist (µm)
700
0.0
600
-3 -2 -1 0 1 2 3
500
400 stability parameter g1
300
200
100
0
0 5 10 15 20 25 30 35 40 45 50 55
Mode size taking pump mode size
position (cm)
account of Nd:YAG
thermal lens only
Solid-state Raman lasers
All-solid-state intracavity
Raman lasers
Nd:YAG Raman crystal
diode
pump
HT pump Q-switch HR pump/ fund
HR fund/Stokes Stokes coupling
Diode Raman l first t pulse/prf Stokes Reference
power crystal Stokes power/eff
5W CaWO4 1178nm 6ns/10kHz 0.5W/9% Murray et al, OSA TOPS 19
(1998) 129
30W Ba(NO3)2 1197nm 15ns/10kHz 3W/10% Pask & Piper, IEEE JQE 36
(2000) 949
30W LiIO3 1156nm 20ns/10kHz 2.6W/9% Pask & Piper, IEEE JQE 36
(2000) 949
23W KGd(WO4)2 1158nm 30ns/15kHz 4W/17% Mildren et al, Opt.Lett. 30
(2005) 1500
10W BaWO4 1181nm 24ns/20kHz 1.6W/17% Chen et al, Opt. Lett. 30 (2005)
3335
Solid-state Raman lasers
Intracavity Raman lasers
Spatial and temporal characteristics
Raman beam clean-up is
observed for intracavity
Raman lasers. Despite
poor mode quality on the
fundamental, the Stokes
field grows in the lowest
order (TEM00) mode*#.
* Murray et al, Opt. Mater. 11 (1999) 353, #Band et al, IEEE JQE 25 (1989) 208.
The Stokes output is commonly observed
to be strongly modulated at the cavity
round-trip time. This is due to self-
modelocking, which arises from the
dynamics of energy transfer between
fundamental and Stokes fields (analogous
to synchronous pumping)#.
Solid-state Raman lasers
(Intracavity) self-Raman lasers
Andryunas et al, JETP Lett, 42 (1985) 410 first reported self-Raman
conversion in Nd3+ doped tungstates. Grabtchikov et al, Appl. Phys. Lett. 75
(1999) 3742 a self-Raman laser operation based on a 1W-diode-pumped
Nd:YVO4 / Cr4+:YAG microchip, giving 15mW 1st -Stokes at 1181nm in sub-ns
pulses at 20kHz. Subsequently there have been numerous reports of diode-
pumped, Q-switched self-Raman lasers based on Nd:SrWO4, Nd:BaWO4,
Nd:PbMoO4, and Yb:KLu(WO4)2.
Chen, Opt. Lett. 29 (2004) 1915 has
demonstrated a diode-pumped, Q-
switched Nd:YVO4 self-Raman laser
giving 1.5W on first-Stokes at 1176nm
(20kHz) from 10.8W pump (13.9%).
Using mirrors coated for 1342nm
fundamental and1525nm first-Stokes,
1.2W is obtained in the eyesafe region
from 13.5W pump (at 9% diode-S1)
Chen, Opt. Lett. 29 (2004) 2172
Solid-state Raman lasers
Intracavity frequency-doubled
Raman lasers
The high intracavity fluences which can be achieved if the fundamental
and Stokes wavelengths are resonating in high-Q cavities are well-
matched to intracavity sum-frequency/second harmonic generation.
Nd:YAG
Pask & Piper, Opt.Lett. 24
Raman crystal LBO
(1999) 1492 reported 1.2W
at 578nm from an
intracavity frequency-
HR end doubled, crystalline LI laser
input
mirror Q-switch mirror based on Q-switched
(10kHz) Nd:YAG laser.
Nd:YAG LBO dichroic 1.7W at 579nm has been
turning/
output
reported subsequently for
mirror KGW at diode-yellow
efficiencies ~ 9.5%*
*Mildren et al, OSA Adv. Solid-
State Photonics 2004, TuC6.
Solid-state Raman lasers
Intracavity frequency-doubled
Raman lasers
At the design operating point, the laser resonator must be optically stable
and give the optimum mode sizes at the fundamental laser crystal, Raman
crystal and SHG crystal, to give maximum extracted power and avoid
optical damage to the components*.
* Design of intracavity frequency-doubled cyrstalline
Raman lasers subject to USA Patent No. 6901084
Nd:YAG Raman crystal Q-switch M2 flat LBO M3 (R=300mm)
M1
flat
250mm overall resonator length
Solid-state Raman lasers
Discretely tunable visible
all-solid-state laser
Mildren et al, Opt. Lett. 30 (2005) 1500 demonstrated that intracavity
SFG/SHG can be used in combination with intracavity SRS in crystalline
materials to select one of a wide range of visible outputs from the
second-harmonic of the fundamental, to various combinations of sum-
frequency and second-harmonic of the various cascading Stokes orders.
1st 2nd
Using angle- or temperature-tuning of Fund Stokes Stokes
the nonlinear SFG/SHG crystal, the
fundamental or Stokes field can be
dumped by way of the nonlinear
coupling through a dichroic cavity
optic. To avoid cavity mis-alignment
issues with angle tuning, or large
temperature ranges in tuning a single SHG SFG SHG SFG SHG SFG
532 555 579 606 636 nm :768cm-1
NL crystal, a second temperature- 532 559 589 622 658 nm :901cm-1
tuned NL crystal can be introduced. KGW
Solid-state Raman lasers
Discretely tunable visible
all-solid-state laser
ANGLE-TUNING TEMPERATURE-TUNING
resonator LBO 1 LBO 2
a axis =90, =90,
=0 =11.6
TEC TEC
LBO1 Wavelength Output Temp Temp Wavelength Output
Angle (nm) power (W) LBO1 LBO2 (nm) power
(W)
0 579 1.8
19 C (52 C) 606 0.25
11 555 0.95
48 C (52 C) 579 0.57
17 532 1.7
95 C (52 C) 555 0.52
- 25 C 532 1.5
• beam displacement
• phase-matching limits possible •temperature range too big for single stage TEC
wavelengths •low powers due to insertion loss of 2nd crystal
•dual crystals reduce switching times
Solid-state Raman lasers
CW crystalline Raman lasers
Reaching threshold for CW operation of Raman lasers requires small mode
sizes to achieve pump intensities high enough for sufficient Raman gain, and
low-loss (high-Q) resonators.
Grabtchikov et al, Opt. Lett. 29 (2004) 2524 reported the first CW crystalline
Raman laser using BN in an external-resonator pumped by an argon ion laser.
Ar+ pump BN, l =68mm
5W, 514nm 164mW, 543nm ( ~3% pump-1st Stokes)
Demidovich et al, Opt. Lett. 30 (2005) 1701 subsequently demonstrated a
(long-pulse) CW Raman laser at 1181nm based on self-Raman conversion in a
diode-pumped Nd:KGW laser (intracavity self-Raman gives reduced losses).
diode pump Nd:KGW, l =40mm
2.4W, 808nm 9(54)mW, 1181nm ( ~2.5% diode-1st Stokes)
1067nm
Solid-state Raman lasers
CW crystalline Raman lasers
Pask, Opt. Lett. 30 (2005) 2454 recently 2000
TEM22
Threshold power (W)
calculated pump (fundamental) power 1500
237µm
threshold for CW intracavity KGW Raman
laser: 1000 TEM00
R2 (1 L) exp( 2 g R I L l ) 1 500
136µm
L =total non output coupling losses at the
0
Stokes wavelength (1%)
0 1 2 3 4 5
R2 = reflectivity of mirror M2 (0.25%)
total cavity loss (%)
Nd:YAG KGW 800mW
1176nm power (mW)
800
diode 1176nm
600
pump 400
unstable
200
Maximum stable CW Raman output power
was 800mW for 20W diode pump power at 0
0 10 20 30
diode-1st Stokes (1176nm) efficiency ~4%*
diode input power (W)
Solid-state Raman lasers
A CW intracavity frequency-doubled
crystalline Raman laser?
Efficient, high-power CW operation of intracavity crystalline Raman lasers
offers the prospect of using intracavity SFG/SHG to make simple, compact
and efficient CW visible sources:
Instantaneous 588 nm power ( mW )
Nd:YVO4 KGW LBO 1600
CW (100% duty cycle)
22W diode 1400 Modulated (50% duty cycle)
1200
1000
800
Dekker, Pask and Piper (submitted to 600
Optics Letters) report 700mW CW output 400
at 588nm by intracavity SHG of 1196nm 200
1st -Stokes of KGW pumped intracavity by 0
1064nm from diode-pumped Nd:YAG, at 2 4 6 8 10 12 14 16 18 20 22 24 26
diode-yellow efficiency ~5%. Instantaneous incident pump power ( W )
Improved resonator design and thermal management are expected to
result in ~2W cw yellow output at ~8% diode-yellow. A miniature
(25mm) intracavity frequency-doubled Nd:GdVO4 self-Raman laser has
already demonstrated >100mW cw yellow for a 3W diode pump!
Solid-state Raman lasers
Solid-state Raman lasers: a tutorial
Thank you for your attention!
jim.piper@vc.mq.edu.au
Solid-state Raman lasers