Solid-state Raman lasers: a tutorial - PowerPoint PPT Presentation

About This Presentation
Title:

Solid-state Raman lasers: a tutorial

Description:

... the laser resonator must be optically stable and give the optimum mode sizes at the fundamental laser crystal, Raman crystal and SHG crystal, ... – PowerPoint PPT presentation

Number of Views:813
Avg rating:3.0/5.0
Slides: 31
Provided by: HelenP154
Category:

less

Transcript and Presenter's Notes

Title: Solid-state Raman lasers: a tutorial


1
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

2
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
3
Stimulated Raman Scattering
  • Spontaneous Raman scattering was first reported
    by Raman and Krishnan (also Landsberg and
    Mandelshtam) 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 (usually ro-vibrational)
state wS1 wP - wR (first-Stokes
generation) wS2 wS1 - wR (second-Stokes
generation) wS3 wS2 - wR (third-Stokes
generation)
wP
wS1
wS1
wS2
wS2
wS3
wR
SRS does not require phase matching.
4
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 hmS2wS3 G dW
in units cm/GW,
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.
5
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 (lt 3 cm-1) and large scattering
cross-section. In the absence of an injected
Stokes signal, SRS grows from spontaneous Stokes
noise
IS(0) hwS2mS3 DW (2p)3c2
In practice to reach threshold i.e. for 1
depletion of the pump, the exponent gRIPz
typically must be gt30. Thus for a high gain
crystal with gP 10 cm/GW, and a crystal length
30mm, the pump intensity needs to be IP gt1GW/cm2.
This is above the damage threshold of many
materials!
6
SRS theory (cont.)
In the transient Raman regime, where tP ltlt 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 (ltlt10 ps) have large integral Raman
scattering cross-section.
7
Common Raman crystals
Crystal Raman shift cm-1 Raman linewidth cm-1 Integral X-section (cf diamond100) Raman gain gL _at_1064nm 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.
8
KGW Raman spectrum
Crystal Raman spectra
c
768
High gain for pump propagation aligned along the
crystal b-axis Access two high gain Stokes
shifts 901 cm-1 768 cm-1which are pump
polarisation dependent.
901
901
768
b
IV Mochalov Opt. Eng. 36 (1997) 1660 for
thermal properties see also S Biswal et al, Appl.
Opt. 44 (2005) 3093.
901
a
901
9
Thermal lensing in Raman crystals
  • 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
  • Direct measurement of thermal lens power
    undertaken using lateral shear interferometry has
    demonstrated good agreement with theory.

Note dn/dT and thus the thermal lens is negative
for many key Raman crystals
HM Pask et al, OSA TOPS Advanced Solid State
Lasers 50 (2001) 441-444.
10
Thermal properties of Raman crystals
LiIO3 CaWO4 Ba(NO3)2 KGd(WO4)2 BaWO4
thermal conductivity kc at 25oC Wm-1K-1 16 1.17 2.5-3.4 3.0
thermal expansion a mK-1 (x10-6) 13 1.6-8.5 6
thermo-optic dn/dT K-1 (x10-6) -85 (o) -69 (e) -7.1 (o) -10.2 (e) -20 -0.8 (pggp) -5.5 (pmmp)
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.
11
Raman laser configurations
Raman generator (picosecond pumps) external-cav
ity Raman laser (nanosecond pumps) intracavity
Raman laser (CW diode end- or side-pump
flashlamp)
high intensity pulsed pump
Raman crystal
high intensity pulsed pump
output mirror
input mirror
laser crystal
Raman crystal
diode pump
input mirror
Q-switch
output mirror
12
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 gt10 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 lt 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.
13
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
1064nm, 5-20 ns, 10-100 mJ 35-40 50 30
1064nm, 20-50 ps, 1mJ 25 25
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
14
External-resonator Raman lasers
Raman crystal length l
The pump is usually double-passed. Raman
threshold is reached when R1R2 exp (2gRIP l
) gt 1 R1 , R2 reflectances at first-Stokes
high intensity pulsed pump
output mirror 2
input mirror 1
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
gt 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.
15
External-cavity (resonator) Raman lasers
Basiev et al, OSA Advanced Solid-State Photonics
2004, TuB11
High average power
BaWO4 95mm
8 x 145mJ, 50ns, 50ms 30 Hz at 1064nm
NdYAG 35W
3.2mm
77 R, pump 55 T 1st-3rd Stokes
85T 1064nm HR 1st-3rd Stokes
High energy
BaWO4 95mm
50 x 380mJ, 50ns 20 kHz at 1062nm
NdGGG 19J
3.2mm
85T 1064nm HR 1st-3rd Stokes
77 R, pump 55 T 1st-3rd Stokes
16
External-cavity (resonator) Raman lasers
Ermolenkov et al, J. Opt. Technol. 72 (2005)
32. 35mJ, 10Hz 1st-Stokes at 563nm (20 eff.)
external SHG 4.2mJ at 281nm
Ba(NO3)2 70mm
180mJ, 20ns 10 Hz at 532nm
90T 532nm HR 1st-Stokes
HR, pump 70 T 1st-Stokes
5mm
176mm unstable
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
Ba(NO3)2 58mm
140mJ, 20ns 20 Hz at 1064nm
HR pump HR 1st-2ndStokes 71 T 3rd-Stokes
HT 1064nm HR 1st-3rd Stokes
5mm
200mm
17
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 10ns, 5kHz
KGW 50mm
90T 532nm HR 1st-2nd Stokes
HR pump, 1st-Stokes 50-60 2nd-Stokes
160mm
52mm mode-matched
KGW E//Nm(588nm)
KGW E//Ng(579nm)
Conversion efficiency into 2nd-Stokes at 588nm
64 (slope eff. 78) at 579nm 58 (slope eff.
68).
18
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

Intracavity Raman including coupled-cavity
Raman crystal
diode pump
Mirror 1 HT pump HR fundamental/Stokes
Q-switch
Mirror 2 HR pump/ fundamental Stokes coupling
Nd3 laser/ Raman crystal
Intracavity self-Raman
Mirror 1 HT pump HR fund/Stokes
Mirror 2 HR pump/fund Stokes coupling
Q-switch
19
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 size taking account of LIO3
thermal lens
instability
pump mode size
Mode size taking account of NdYAG thermal lens
only
20
All-solid-state intracavity Raman lasers
NdYAG
Raman crystal
diode pump
HR pump/ fund Stokes coupling
HT pump HR fund/Stokes
Q-switch
Diode power Raman crystal l first Stokes t pulse/prf Stokes power/eff Reference
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
21
Intracavity Raman lasersSpatial 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).
22
(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 NdYVO4 / Cr4YAG
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 NdSrWO4,
NdBaWO4, NdPbMoO4, and YbKLu(WO4)2.
Chen, Opt. Lett. 29 (2004) 1915 has demonstrated
a diode-pumped, Q-switched NdYVO4 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
23
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.
Pask Piper, Opt.Lett. 24 (1999) 1492 reported
1.2W at 578nm from an intracavity
frequency-doubled, crystalline LI laser based on
Q-switched (10kHz) NdYAG laser. 1.7W at 579nm
has been reported subsequently for KGW at
diode-yellow efficiencies 9.5 Mildren et al,
OSA Adv. Solid-State Photonics 2004, TuC6.
NdYAG
Raman crystal
LBO
HR end mirror
input mirror
Q-switch
NdYAG
LBO
dichroic turning/ output mirror
24
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
NdYAG
Raman crystal
Q-switch
LBO
M2 flat
M3 (R300mm)
M1 flat
250mm overall resonator length
25
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.
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
temperature-tuned NL crystal can be introduced.
1st Stokes
2nd Stokes
Fund
SHG
SHG
SHG
SFG
SFG
SFG
532
579
636 nm
606
555
768cm-1
901cm-1 KGW
532
589
658 nm
622
559
26
Discretely tunable visible all-solid-state laser
TEMPERATURE-TUNING
ANGLE-TUNING
LBO 1
resonator axis
?
?90?, ?0?
TEC
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
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
  • temperature range too big for single stage TEC
  • low powers due to insertion loss of 2nd crystal
  • dual crystals reduce switching times

27
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.
BN, l 68mm
Ar pump 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 NdKGW laser
(intracavity self-Raman gives reduced losses).
NdKGW, l 40mm
diode pump 2.4W, 808nm
9(54)mW, 1181nm ( 2.5 diode-1st Stokes)
1067nm
28
CW crystalline Raman lasers
Pask, Opt. Lett. 30 (2005) 2454 recently
calculated pump (fundamental) power threshold for
CW intracavity KGW Raman laser
NdYAG
KGW
800mW 1176nm
diode pump
unstable
Maximum stable CW Raman output power was 800mW
for 20W diode pump power at diode-1st Stokes
(1176nm) efficiency 4
29
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
NdYVO4
KGW
LBO
22W diode
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
NdYAG, at diode-yellow efficiency 5.
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 NdGdVO4 self-Raman laser has
already demonstrated gt100mW cw yellow for a 3W
diode pump!
30
Solid-state Raman lasers a tutorial
Thank you for your attention!
jim.piper_at_vc.mq.edu.au
Write a Comment
User Comments (0)
About PowerShow.com