solid-state raman lasers solid-state raman lasers: a tutorial jim piper professor of physics centre...

Solid-state Raman lasers

Solid-state Raman lasers: a tutorial

Jim PiperProfessor 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


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) 1-29Cerny et al Progress in Quantum Electronics 28 (2004) 113-143Pask Progress in Quantum Electronics 27 (2003) 3-56


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 = E3, which gives rise to various nonlinear optical phenomena, including also two-photon absorption, stimulated Brillouin scattering and self-focussing.

Stimulated Raman Scattering

Photons passing through a Raman-active medium are inelastically scattered, leaving the molecules of the medium in an excited (usually ro-vibrational) state:

S1 = P - R (first-Stokes generation)

S2 = S1 - R (second-Stokes generation)

S3 = S2 - R (third-Stokes generation)

P S1

R

SRS does not require phase matching.

S1 S2

S2 S3


SRS theory** Penzkofer et al Progress in Quantum Electronics 6 (1979) 55-140.

In the “steady-state” regime, where the pump duration P 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 = 8c2 N .

hS

2S3

= S4S . h . 2

c4 L 2mR q

in units cm/GW,

and the integral Raman scattering cross-section is introduced as

Here /q is the derivature of the normal-mode polarisability (the square is proportional to 3), is the Raman linewidth, the inverse of the dephasing time i.e. = TR

-1, and small-signal conditions are assumed. Typically TR ~ 10ps , ~ 1011 s-1 or R ~ 5 cm-1.


SRS theory (cont.)

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:

IS(0) = hS2S

3 (2)3c2

In the steady-state regime, gR scales with the Raman (Stokes)

frequency S and the integral Raman scattering cross-section

, and inversely as the Raman linewidth cR .

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!


In the transient Raman regime, where P << TR the Stokes signal grows as:

IS(z) = IS(0) exp (–P/TR) exp [2 (PgRIP z/TR)1/2] .

Since 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 PIP that is, of the pulse energy.

Raman media of choice for the transient regime (<<10 ps) have large integral Raman scattering cross-section.

SRS theory (cont.)


Common Raman crystals*

Crystal Raman shift cm-1

Raman linewidth cm-1

Integral

X-section (cf

diamond=100)

Raman gain gL @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.


KGW Raman spectrum*

b

c

a

901

768

901

901

901

768 High gain for pump propagation aligned along the crystal b-axis

Access two high gain Stokes shifts: 901 cm-1

768 cm-1

which are pump polarisation dependent.

*IV Mochalov Opt. Eng. 36 (1997) 1660; for thermal properties see also S Biswal et al, Appl. Opt. 44 (2005) 3093.

Crystal Raman spectra


Power density (Wm-2)

0 1e+6 2e+6 3e+6 4e+6 5e+6 6e+6 7e+6 8e+6 9e+6 1e+7

The

rma

l len

s p

ow

er

(m-1

)

0

1

2

3

4

5

6

7

8

9

10

Ba(NO3)2

LiIO3

Heat deposited in the crystal by the (first-Stokes) SRS process is:

Pheat = PS1 [(S1/P) – 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*.

)1(1

)(1

21

1 P

S

s

S

cthermal

P

kdT

dn

f

Thermal lensing in Raman crystals

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.


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 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 (p[gg]p)*-5.5 (p[mm]p)

* 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.


Raman laser configurations

Raman generator(picosecond pumps)

external-cavityRaman laser(nanosecond pumps)

intracavity Ramanlaser (CW diode end- or side-pump; flashlamp)

high intensity

pulsed pump

input mirror

Raman crystal

outputmirror

diode

pump

input mirror

lasercrystal

Q-switch

Raman crystal

outputmirror

high intensity pulsed pump


Pulsed Raman generators

high intensity pulsed pumpIS(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.


Pulsed Raman generators

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%

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..

# 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


External-resonator Raman lasers

high intensity

pulsed pump

input mirror 1

outputmirror 2

The pump is usually double-passed.Raman threshold is reached when:

R1R2 exp (2gRIP l ) > 1

R1 , R2 reflectances at first-Stokes

Raman crystal length l

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.


External-cavity (resonator) Raman lasers

50 x 380mJ, 50ns20 kHz at 1062nm

85%T 1064nmHR 1st-3rd Stokes

77% R, pump55% T 1st-3rd Stokes

BaWO4 95mm

3.2mmNd:GGG 19J

High average power

High energy

Basiev et al, OSA Advanced Solid-State Photonics 2004, TuB11

8 x 145mJ, 50ns, 50s30 Hz at 1064nm

85%T 1064nmHR 1st-3rd Stokes

77% R, pump55% T 1st-3rd Stokes

BaWO4 95mm

3.2mmNd:YAG 35W


External-cavity (resonator) Raman lasers

180mJ, 20ns10 Hz at 532nm

90%T 532nmHR 1st-Stokes

HR, pump70% T 1st-Stokes

Ba(NO3)2 70mm

5mm

176mm unstable

Ermolenkov et al, J. Opt. Technol. 72 (2005) 32.

35mJ, 10Hz 1st-Stokes at 563nm (20% eff.) external SHG 4.2mJ at 281nm

140mJ, 20ns20 Hz at 1064nm

HT 1064nmHR 1st-3rd Stokes

HR pump HR 1st-2ndStokes71% T 3rd-Stokes

Ba(NO3)2 58mm

5mm

200mm

Takei et al, Appl. Phys B74 (2002) 521.

11mJ, 20Hz 3rd-Stokes at 1598nm (eyesafe region) after compensation for strong thermal lensing in BN


External-cavity Raman lasers

KGW E//Nm

(588nm)

KGW E//Ng

(579nm)

2.4W at 532nm 10ns, 5kHz

90%T 532nmHR 1st-2nd Stokes

HR pump, 1st-Stokes50-60% 2nd-Stokes

KGW 50mm

m

52mm mode-matched

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.

Conversion efficiency into 2nd-Stokes at 588nm: 64% (slope eff. 78%);at 579nm: 58% (slope eff. 68%).


diodepump

Mirror 1HT pump HR fundamental/Stokes

Q-switch Mirror 2HR pump/ fundamentalStokes coupling

Mirror 1HT pumpHR fund/Stokes

Nd3+ laser/Raman crystal

Q-switch

Raman crystal

Mirror 2HR pump/fundStokes coupling

Nd3+ laser crystal

Intracavity Raman lasers

Intracavity Raman*including coupled-cavity

Intracavity self-Raman

*

Intracavity Raman lasers allow for both the pump and the Stokes wavelength(s) to be resonated, substantially reducing the effective Raman threshold (~MW/cm2)


Intracavity crystalline Raman lasers

position (cm)

0 5 10 15 20 25 30 35 40 45 50 55

be

am

wa

ist (µ

m)

0

100

200

300

400

500

600

700

800

900

1000

Nd:YAG LiIO3Plane HR mirror Plane OC mirror

Resonator mode size taking account of LIO3 thermal lens

Mode size taking account of Nd:YAG thermal lens only

pump mode size

instability

Effects of thermal lenses on resonator designPask & Piper, IEEE J. Quantum Electron. 36 (2000) 949.*also Pask, Prog. Quantum Electron. 27 (2003) 3.

stability parameter g1

-3 -2 -1 0 1 2 3

stab

ility

par

amet

er g

2

0.0

0.2

0.4

0.6

0.8

1.0

54 6

123

1. I=02. I=11A3. I=14A

unstable region stable region

unstableregion

4. I=32A5. I=38A6. I=40A


All-solid-state intracavity Raman lasers

diodepump

HT pump HR fund/Stokes

Q-switch HR pump/ fundStokes coupling

Raman crystalNd:YAG

Diode power

Raman crystal

firstStokes

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


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*#.

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)#.

* Murray et al, Opt. Mater. 11 (1999) 353, #Band et al, IEEE JQE 25 (1989) 208.


(Intracavity) self-Raman lasersAndryunas 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


Intracavity frequency-doubled Raman lasers

Nd:YAG

Q-switch

Raman crystal LBO

input mirror

Nd:YAG LBO dichroicturning/outputmirror

HR endmirror

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) Nd:YAG 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.


Intracavity frequency-doubled Raman lasers

250mm overall resonator length

Nd:YAG Raman crystal Q-switch LBOM2 flat

M1flat

M3 (R=300mm)

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


Discretely tunable visible all-solid-state laser

Fund1st

Stokes2nd

Stokes

SHG SHG SHGSFGSFG SFG532 579 636 nm606555532 589 658 nm622559

:768cm-1

:901cm-1

KGW

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.


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

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

resonator axis

LBO 1

TEC

=90, =0

TEC

TEMPERATURE-TUNING

•temperature range too big for single stage TEC•low powers due to insertion loss of 2nd crystal•dual crystals reduce switching times

=90, =11.6

LBO 2

ANGLE-TUNING

Discretely tunable visible all-solid-state laser


CW crystalline Raman lasers

BN, l =68mmAr+ pump5W, 514nm

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.

Nd:KGW, l =40mmdiode pump2.4W, 808nm

164mW, 543nm ( ~3% pump-1st Stokes)

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.

9(54)mW, 1181nm ( ~2.5% diode-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).

1067nm


CW crystalline Raman lasers

diode input power (W)

0 10 20 300

200

400

600

800

1176

nm p

ower

(m

W)Nd:YAG KGW

diode

pump

total cavity loss (%)

0 1 2 3 4 5

Thr

esho

ld p

ower

(W

)

0

500

1000

1500

2000TEM22237µm

TEM00136µm1)2exp()1(2 lIgLR LR

L =total non output coupling losses at the Stokes wavelength (1%) R2 = reflectivity of mirror M2 (0.25%)

Pask, Opt. Lett. 30 (2005) 2454 recently calculated pump (fundamental) power threshold for CW intracavity KGW Raman laser:

Maximum stable CW Raman output power was 800mW for 20W diode pump power at diode-1st Stokes (1176nm) efficiency ~4%*

800mW 1176nm

unstable


A CW intracavity frequency-doubled crystalline Raman laser?

Nd:YVO4 KGW22W diode

LBO

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:

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%.

2 4 6 8 10 12 14 16 18 20 22 24 26

0

200

400

600

800

1000

1200

1400

1600

Inst

an

tan

eo

us

58

8 n

m p

ow

er

( m

W )

Instantaneous incident pump power ( W )

CW (100% duty cycle) Modulated (50% duty cycle)

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!


[email protected]

Thank you for your attention!

Solid-state Raman lasers: a tutorial

solid-state raman lasers solid-state raman lasers: a tutorial jim piper professor of physics centre...

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