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
wS2 wS3
wR
wS1 = wP - wR (first-Stokes generation)
wS2 = wS1 - wR (second-Stokes generation)
wS3 = wS2 - wR (third-Stokes generation)
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,
2
3
hmS wS G dW
and the integral Raman scattering cross-section is introduced as
ds = wS4mS . h . da
dW c4 mL 2mwR dq
2
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
(< 3 cm-1) and large scattering cross-section.
In the absence of an injected Stokes signal, SRS grows from
spontaneous Stokes noise:
2
3
IS(0) = hwS mS DW
(2p)3c2
In practice to reach threshold i.e. for 1% depletion of the pump,
the exponent gRIPz typically must be >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 << TR the Stokes signal
grows as:
IS(z) = IS(0) exp (–tP/TR) exp [2 (tPgRIP z/TR)1/2] .
Since G TR= 1 , we see that Stokes growth is independent of
Raman linewidth, and the exponent has a slower (square root)
dependence on the propagation distance z in the Raman medium
and the integral Raman cross-section. Moreover instead of the
exponent depending on IP as in steady-state, in the transient
regime the dependence is on the square root of tPIP that is, of
the pulse energy.
Raman media of choice for the transient regime (<<10 ps) have
large integral Raman scattering cross-section.
Solid-state Raman lasers
Common Raman crystals*
Crystal
Raman
shift cm-1
Raman
linewidth
cm-1
Integral
Raman gain
gL @1064nm
X-section (cf
diamond=100) cm/GW
Damage
threshold
GW/cm2
LiIO3 (LI)
822
770
5.0
54
4.8
~ 0.1
Ba(NO3)2 (BN)
1047
0.4
21
11
~ 0.4
CaWO4 (CW)
908
7.0
52
3.0
~ 0.5
KGd(WO4)2
(KGW)
768
901
5.9
7.8
59
50
4.4
3.3
~ 10
BaWO4 (BW)
924
1.6
52
8.5
~5
SrWO4 (SW)
922
2.7
50
5.0
~5
YVO4 (YV)
890
3.0
4.5
~1
*Extensive lists of properties of Raman-active crystals are given by Basiev & Powell, Handbook
of Laser Technology and Applications B1.7 (2003) 1; and e.g. Kaminskii et al, Appl. Opt. 38
(1999) 4553.
Solid-state Raman lasers
Crystal Raman spectra
KGW Raman spectrum*
c
High gain for pump
propagation aligned along
the crystal b-axis
768
901
901
Access two high gain
Stokes shifts:
901 cm-1
768 cm-1
which are pump
polarisation dependent.
768
b
901
a
Solid-state Raman lasers
901
*IV Mochalov Opt. Eng. 36
(1997) 1660; for thermal
properties see also S Biswal et
al, Appl. Opt. 44 (2005) 3093.
Thermal lensing in
Raman crystals


Direct measurement of thermal lens
power undertaken using lateral shear
interferometry has demonstrated
good agreement with theory*.
* HM Pask et al, OSA TOPS: Advanced Solid
State Lasers 50 (2001) 441-444.
Solid-state Raman lasers
10
Ba(NO3)2
9
8
-1
Heat deposited in the crystal by the
(first-Stokes) SRS process is:
Pheat = PS1 [(lS1/lP) – 1]
Assuming TEM00 mode the thermal
lens arising from the thermo-optic
effect is:
1
dn 1 PS1 lS
=( )
(  1)
2
f thermal
dT kc pws1 lP
Thermal lens power (m )

7
6
5
4
LiIO3
3
2
1
0
0
1e+6
2e+6
3e+6
4e+6
5e+6
6e+6
7e+6
8e+6
9e+6
Power density (Wm-2)
Note dn/dT and thus the
thermal lens is negative for
many key Raman crystals
1e+7
Thermal properties of Raman
crystals
LiIO3
thermal conductivity kc
at 25oC Wm-1K-1
CaWO4
Ba(NO3)2
16
1.17
2.5-3.4
3.0
13
1.6-8.5
6
-20
-0.8 (p[gg]p)*
-5.5 (p[mm]p)
thermal expansion a
mK-1 (x10-6)
thermo-optic dn/dT
K-1 (x10-6)
-85 (o)
-69 (e)
-7.1 (o)
-10.2 (e)
KGd(WO4)2
BaWO4
* An athermal orientation (dn/dT = 0) for KGW has been identified by Mochalov,
Opt. Eng. 36 (1997) 1660; see also Biswal et al, Appl. Opt. 44 (2005) 3093.
Solid-state Raman lasers
Raman laser configurations
Raman generator
(picosecond pumps)
high intensity pulsed pump
Raman crystal
high intensity
pulsed pump
input
mirror
laser
crystal
diode
output
mirror
Raman crystal
pump
input
mirror
Q-switch
Solid-state Raman lasers
output
mirror
external-cavity
Raman laser
(nanosecond pumps)
intracavity Raman
laser (CW diode endor side-pump; flashlamp)
Pulsed Raman generators
high intensity pulsed pump
IS(z) = IS(0) exp (gR IP z)
For most crystals the steady-state regime applies for pulse durations >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 < 10 ps, the transient regime applies and Raman
crystals with high integral scattering cross-section (and high damage
threshold) are favoured (e.g. tungstates)
Raman gain*
Ba(NO3)2
KGd(WO4)2
BaWO4
steady-state 532nm
47 cm/GW
11.8 cm/GW
40 cm/GW
transient 532nm
4.7
11.8
14.3
steady-state 1064nm
11
4
8.5
transient 1064nm
1.1
3
3.8
* Cerny et al, Prog. Quantum Electron. 28 (2004) 113.
Solid-state Raman lasers
Pulsed Raman generators
Reported first-Stokes conversion efficiencies for
single-pass Raman generators*
*After Basiev & Powell Handbook of Laser Technology and Applications B1.7 (2003) 1
and Cerny et al, Prog. Quantum Electron. 28 (2004) 113.
.
spectral/temporal regime
Ba(NO3)2
KGd(WO4)2
BaWO4
532nm, 5-20 ns, 10-100 mJ
26%
30%
45%
532nm, 20-50 ps, ~0.1mJ
25%
50%
40%#
35-40%
50%
30%
25%
25%
1064nm, 5-20 ns, 10-100 mJ
1064nm, 20-50 ps, ~1mJ
#
Near quantum-limited efficiency (85%) in double-pass Cerny et al, Opt. Lett. 27 (2002) 360.
In general, direct optical damage and self-focussing impose practical
limitations to power and efficiency of crystalline Raman generators
Solid-state Raman lasers
External-resonator Raman
lasers
Raman crystal length l
high intensity
pulsed pump
input
mirror 1
output
mirror 2
The pump is usually double-passed.
Raman threshold is reached when:
R1R2 exp (2gRIP l ) > 1
R1 , R2 reflectances at first-Stokes
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
30 Hz at 1064nm
BaWO4 95mm
Nd:YAG 35W
3.2mm
85%T 1064nm
HR 1st-3rd Stokes
77% R, pump
55% T 1st-3rd Stokes
High energy
50 x 380mJ, 50ns
20 kHz at 1062nm
BaWO4 95mm
Nd:GGG 19J
3.2mm 77% R, pump
85%T 1064nm
HR 1st-3rd Stokes
55% T 1st-3rd Stokes
Solid-state Raman lasers
External-cavity (resonator)
Raman lasers
180mJ, 20ns
10 Hz at 532nm
Ba(NO3)2 70mm
90%T 532nm
HR 1st-Stokes
5mm
HR, pump
70% T 1st-Stokes
176mm unstable
140mJ, 20ns
20 Hz at 1064nm
Ba(NO3)2 58mm
HT 1064nm
HR 1st-3rd Stokes
5mm
200mm
Solid-state Raman lasers
HR pump
HR 1st-2ndStokes
71% T 3rd-Stokes
Ermolenkov et al, J. Opt.
Technol. 72 (2005) 32.
35mJ, 10Hz 1st-Stokes at
563nm (20% eff.) external
SHG 4.2mJ at 281nm
Takei et al, Appl. Phys B
74 (2002) 521.
11mJ, 20Hz 3rd-Stokes at
1598nm (eyesafe region) after
compensation for strong
thermal lensing in BN
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.
HR pump, 1st-Stokes
50-60% 2nd-Stokes
160mm
52mm mode-matched
1.6
KGW E//Nm
(588nm)
OUtput Power (W)
1.4
1.2
100
100
80
80
60
60
40
40
20
20
0
0
560
1.0
580
600
620
Wavelength (nm)
0.8
KGW E//Ng
(579nm)
0.6
Conversion efficiency into 2nd-Stokes
at 588nm: 64% (slope eff. 78%);
at 579nm: 58% (slope eff. 68%).
0.4
0.2
0.0
0.0
0.5
1.0
1.5
Input Power (W)
Solid-state Raman lasers
2.0
2.5
Output Coupler Transmission (%)
90%T 532nm
HR 1st-2nd Stokes
Fraction of Output (%)
KGW 50mm
2.4W at 532nm
10ns, 5kHz
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
crystal
diode
pump
*
Raman crystal
Mirror 1
Q-switch
HT pump
HR fundamental/Stokes
Intracavity Raman
*including coupled-cavity
Mirror 2
HR pump/ fundamental
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.
Resonator mode
size taking account
of LIO3 thermal lens
1000
instability
900
800
beam waist (µm)
1.0
4. I=32A
1. I=0
unstable
region
0.8 2. I=11A 5. I=38A
3. I=14A 6. I=40A 6
45
Plane OC
mirror
0.6
LiIO3
Nd:YAG
Plane HR mirror
stability parameter g2
*also Pask, Prog. Quantum Electron. 27 (2003) 3.
700
600
0.4
0.2
0.0
unstable region
-3
-2
-1
0
1
2
3
stable region
1
2
500
stability parameter g1
400
300
200
100
0
0
5
10
15
Mode size taking
account of Nd:YAG
thermal lens only
Solid-state Raman lasers
20
25
30
35
pump mode size
position (cm)
40
45
50
55
3
All-solid-state intracavity
Raman lasers
Nd:YAG
Raman crystal
diode
pump
HT pump Q-switch
HR fund/Stokes
HR pump/ fund
Stokes coupling
Diode
power
Raman
crystal
l first
Stokes
t pulse/prf
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%
Solid-state Raman lasers
Reference
Chen et al, Opt. Lett. 30 (2005)
3335
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 selfmodelocking, 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 diodepumped, 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, Qswitched 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 wellmatched to intracavity sum-frequency/second harmonic generation.
Nd:YAG
input
mirror
Raman crystal
LBO
HR end
mirror
Q-switch
Nd:YAG
LBO
dichroic
turning/
output
mirror
Pask & Piper, Opt.Lett. 24
(1999) 1492 reported 1.2W
at 578nm from an
intracavity frequencydoubled, crystalline LI laser
based on Q-switched
(10kHz) Nd:YAG laser.
1.7W at 579nm has been
reported subsequently for
KGW at diode-yellow
efficiencies ~ 9.5%*
*Mildren et al, OSA Adv. SolidState 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
M1
flat
250mm overall resonator length
Solid-state Raman lasers
LBO
M3 (R=300mm)
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 sumfrequency and second-harmonic of the various cascading Stokes orders.
Using angle- or temperature-tuning of
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
NL crystal, a second temperaturetuned NL crystal can be introduced.
Solid-state Raman lasers
1st
Stokes
Fund
SHG
532
532
SFG
555
559
SHG
579
589
2nd
Stokes
SFG
606
622
SHG
SFG
:768cm-1
636 nm
:901cm-1
658 nm
KGW
Discretely tunable visible
all-solid-state laser
TEMPERATURE-TUNING
ANGLE-TUNING
resonator
axis
a
LBO 1
=90,
=0
TEC
LBO1
Angle
Wavelength
(nm)
Output
power (W)
0
579
1.8
11 
555
0.95
17 
532
1.7
• beam displacement
• phase-matching limits possible
wavelengths
Solid-state Raman lasers
LBO 2
=90,
=11.6
TEC
Temp
LBO1
Temp
LBO2
Wavelength
(nm)
Output
power
(W)
19 C
(52 C)
606
0.25
48 C
(52 C)
579
0.57
95 C
(52 C)
555
0.52
-
25 C
532
1.5
•temperature range too big for single stage TEC
•low powers due to insertion loss of 2nd crystal
•dual crystals reduce switching times
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
5W, 514nm
BN, l =68mm
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
1067nm
Solid-state Raman lasers
9(54)mW, 1181nm ( ~2.5% diode-1st Stokes)
CW crystalline Raman lasers
L =total non output coupling losses at the
Stokes wavelength (1%)
R2 = reflectivity of mirror M2 (0.25%)
Nd:YAG
diode
KGW
800mW
1176nm
pump
Maximum stable CW Raman output power
was 800mW for 20W diode pump power at
diode-1st Stokes (1176nm) efficiency ~4%*
Threshold power (W)
R2 (1  L) exp( 2g R I L l )  1
2000
TEM22
237µm
1500
1000
TEM00
136µm
500
0
0
1
2
3
4
5
total cavity loss (%)
1176nm power (mW)
Pask, Opt. Lett. 30 (2005) 2454 recently
calculated pump (fundamental) power
threshold for CW intracavity KGW Raman
laser:
800
600
400
unstable
200
0
0
10
20
30
diode input power (W)
Solid-state Raman lasers
A CW intracavity frequency-doubled
crystalline Raman laser?
22W diode
Nd:YVO4
KGW
LBO
Dekker, Pask and Piper (submitted to
Optics Letters) report 700mW CW output
at 588nm by intracavity SHG of 1196nm
1st -Stokes of KGW pumped intracavity by
1064nm from diode-pumped Nd:YAG, at
diode-yellow efficiency ~5%.
Instantaneous 588 nm power ( mW )
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:
1600
CW (100% duty cycle)
Modulated (50% duty cycle)
1400
1200
1000
800
600
400
200
0
2
4
6
8
10 12
14 16 18
20 22 24 26
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