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Solid-state Raman lasers Solid-state Raman lasers: a tutorial Jim Piper Professor of Physics Centre for Lasers and Applications, Macquarie University,

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Presentation on theme: "Solid-state Raman lasers Solid-state Raman lasers: a tutorial Jim Piper Professor of Physics Centre for Lasers and Applications, Macquarie University,"— Presentation transcript:

1 Solid-state Raman lasers 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 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) Pask Progress in Quantum Electronics 27 (2003) 3-56

3 Solid-state Raman lasers 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 P 3 =     E 3, 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P  S1 RR SRS does not require phase matching.  S1  S2  S3

4 Solid-state Raman lasers SRS theory* * Penzkofer et al Progress in Quantum Electronics 6 (1979) In the “steady-state” regime, where the pump duration  P is long compared to the Raman dephasing time T R, the Stokes intensity I S (z) grows as: I S (z) = I S (0) exp (g R I P z) where I P is the pump intensity, the steady-state Raman gain coefficient is g R = 8  c 2 N.   h  S 2  S 3   =  S 4  S. h.  2  c 4  L 2m  R  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.  = T R -1, and small-signal conditions are assumed. Typically T R ~ 10ps,  ~ s -1 or  R ~ 5 cm -1.

5 Solid-state Raman lasers 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: I S (0) = h  S 2  S 3   (2  ) 3 c 2 In the steady-state regime, g R scales with the Raman (Stokes) frequency  S and the integral Raman scattering cross-section , and inversely as the Raman linewidth  c  R. In practice to reach threshold i.e. for 1% depletion of the pump, the exponent g R I P z typically must be >30. Thus for a high gain crystal with g P ~10 cm/GW, and a crystal length 30mm, the pump intensity needs to be I P >1GW/cm 2. This is above the damage threshold of many materials!

6 Solid-state Raman lasers In the transient Raman regime, where  P << T R the Stokes signal grows as: I S (z) = I S (0) exp (–  P /T R ) exp [2 (  P g R I P z/T R ) 1/2 ]. Since  T R = 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 I P as in steady-state, in the transient regime the dependence is on the square root of  P I P 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.)

7 Solid-state Raman lasers Common Raman crystals* CrystalRaman shift cm -1 Raman linewidth cm -1 Integral X-section (cf diamond=100) Raman gain g cm/GW Damage threshold GW/cm 2 LiIO 3 (LI) ~ 0.1 Ba(NO 3 ) 2 (BN) ~ 0.4 CaWO 4 (CW) ~ 0.5 KGd(WO 4 ) 2 (KGW) ~ 10 BaWO 4 (BW) ~ 5 SrWO 4 (SW) ~ 5 YVO 4 (YV) ~ 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 Solid-state Raman lasers KGW Raman spectrum* b c a High gain for pump propagation aligned along the crystal b-axis Access two high gain Stokes shifts: 901 cm 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) Crystal Raman spectra

9 Solid-state Raman lasers  Heat deposited in the crystal by the (first-Stokes) SRS process is: P heat = P S1 [( S1 / P ) – 1]  Assuming TEM 00 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*. 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)

10 Solid-state Raman lasers Thermal properties of Raman crystals LiIO 3 CaWO 4 Ba(NO 3 ) 2 KGd(WO 4 ) 2 BaWO 4 thermal conductivity k c at 25 o C Wm -1 K thermal expansion  mK -1 (x10 -6 ) thermo-optic dn/dT K -1 (x10 -6 ) -85 (o) -69 (e) -7.1 (o) (e) (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.

11 Solid-state Raman lasers Raman laser configurations Raman generator (picosecond pumps) external-cavity Raman laser (nanosecond pumps) intracavity Raman laser (CW diode end- or side-pump; flashlamp) high intensity pulsed pump input mirror Raman crystal output mirror diode pump input mirror laser crystal Q-switch Raman crystal output mirror high intensity pulsed pump

12 Solid-state Raman lasers Pulsed Raman generators high intensity pulsed pump I S (z) = I S (0) exp (g R I P 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/cm 2. 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(NO 3 ) 2 KGd(WO 4 ) 2 BaWO 4 steady-state 532nm47 cm/GW11.8 cm/GW40 cm/GW transient 532nm steady-state 1064nm transient 1064nm * Cerny et al, Prog. Quantum Electron. 28 (2004) 113.

13 Solid-state Raman lasers Pulsed Raman generators spectral/temporal regimeBa(NO 3 ) 2 KGd(WO 4 ) 2 BaWO 4 532nm, 5-20 ns, mJ26%30%45% 532nm, ps, ~0.1mJ25%50% 40% # 1064nm, 5-20 ns, mJ35-40%50%30% 1064nm, ps, ~1mJ25% 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) # 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 Solid-state Raman lasers External-resonator Raman lasers high intensity pulsed pump input mirror 1 output mirror 2 The pump is usually double-passed. Raman threshold is reached when: R 1 R 2 exp (2g R I P l ) > 1 R 1, R 2 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/cm 2 compared with ~300 MW/cm 2 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.

15 Solid-state Raman lasers External-cavity (resonator) Raman lasers 50 x 380mJ, 50ns 20 kHz at 1062nm 85%T 1064nm HR 1 st -3 rd Stokes 77% R, pump 55% T 1 st -3 rd Stokes BaWO 4 95mm 3.2mm Nd:GGG 19J High average power High energy Basiev et al, OSA Advanced Solid-State Photonics 2004, TuB11 8 x 145mJ, 50ns, 50  s 30 Hz at 1064nm 85%T 1064nm HR 1 st -3 rd Stokes 77% R, pump 55% T 1 st -3 rd Stokes BaWO 4 95mm 3.2mm Nd:YAG 35W

16 Solid-state Raman lasers External-cavity (resonator) Raman lasers 180mJ, 20ns 10 Hz at 532nm 90%T 532nm HR 1 st -Stokes HR, pump 70% T 1 st -Stokes Ba(NO 3 ) 2 70mm 5mm 176mm unstable Ermolenkov et al, J. Opt. Technol. 72 (2005) mJ, 10Hz 1 st -Stokes at 563nm (20% eff.) external SHG 4.2mJ at 281nm 140mJ, 20ns 20 Hz at 1064nm HT 1064nm HR 1 st -3 rd Stokes HR pump HR 1 st -2 nd Stokes 71% T 3 rd -Stokes Ba(NO 3 ) 2 58mm 5mm 200mm Takei et al, Appl. Phys B 74 (2002) mJ, 20Hz 3 rd -Stokes at 1598nm (eyesafe region) after compensation for strong thermal lensing in BN

17 Solid-state Raman lasers External-cavity Raman lasers KGW E//N m (588nm) KGW E//N g (579nm) 2.4W at 532nm 10ns, 5kHz 90%T 532nm HR 1 st -2 nd Stokes HR pump, 1 st -Stokes 50-60% 2 nd -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 2 nd -Stokes at 588nm: 64% (slope eff. 78%); at 579nm: 58% (slope eff. 68%).

18 Solid-state Raman lasers diode pump Mirror 1 HT pump HR fundamental/Stokes Q-switchMirror 2 HR pump/ fundamental Stokes coupling Mirror 1 HT pump HR fund/Stokes Nd 3+ laser/ Raman crystal Q-switch Raman crystal Mirror 2 HR pump/fund Stokes coupling Nd 3+ 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/cm 2 )

19 Solid-state Raman lasers Intracavity crystalline Raman lasers Resonator mode size taking account of LIO 3 thermal lens Mode size taking account of Nd:YAG thermal lens only pump mode size instability Effects of thermal lenses on resonator design Pask & Piper, IEEE J. Quantum Electron. 36 (2000) 949. *also Pask, Prog. Quantum Electron. 27 (2003) 3.

20 Solid-state Raman lasers All-solid-state intracavity Raman lasers diode pump HT pump HR fund/Stokes Q-switch HR pump/ fund Stokes coupling Raman crystal Nd:YAG Diode power Raman crystal  first Stokes  pulse/prf Stokes power/eff Reference 5WCaWO nm6ns/10kHz0.5W/9% Murray et al, OSA TOPS 19 (1998) WBa(NO 3 ) nm15ns/10kHz3W/10% Pask & Piper, IEEE JQE 36 (2000) WLiIO nm20ns/10kHz2.6W/9% Pask & Piper, IEEE JQE 36 (2000) WKGd(WO 4 ) nm30ns/15kHz4W/17% Mildren et al, Opt.Lett. 30 (2005) WBaWO nm24ns/20kHz1.6W/17% Chen et al, Opt. Lett. 30 (2005) 3335

21 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 (TEM 00 ) 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.

22 Solid-state Raman lasers (Intracavity) self-Raman lasers Andryunas et al, JETP Lett, 42 (1985) 410 first reported self-Raman conversion in Nd 3+ doped tungstates. Grabtchikov et al, Appl. Phys. Lett. 75 (1999) 3742 a self-Raman laser operation based on a 1W-diode-pumped Nd:YVO 4 / Cr 4+ :YAG microchip, giving 15mW 1 st -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:SrWO 4, Nd:BaWO 4, Nd:PbMoO4, and Yb:KLu(WO 4 ) 2. Chen, Opt. Lett. 29 (2004) 1915 has demonstrated a diode-pumped, Q- switched Nd:YVO 4 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-S 1 ) Chen, Opt. Lett. 29 (2004) 2172

23 Solid-state Raman lasers Intracavity frequency-doubled Raman lasers Nd:YAG Q-switch Raman crystal LBO input mirror Nd:YAGLBO dichroic turning/ output mirror HR end mirror 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.

24 Solid-state Raman lasers Intracavity frequency-doubled Raman lasers 250mm overall resonator length Nd:YAGRaman crystalQ-switchLBOM2 flat M1 flat 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

25 Solid-state Raman lasers Discretely tunable visible all-solid-state laser Fund 1st Stokes 2nd Stokes SHG SFG nm nm :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.

26 Solid-state Raman lasers LBO1 Angle Wavelength (nm) Output power (W) 0    beam displacement phase-matching limits possible wavelengths Temp LBO1 Temp LBO2 Wavelength (nm) Output power (W) 19  C(52  C)  C(52  C)  C(52  C)  C 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 2 nd crystal dual crystals reduce switching times  =90 ,  =11.6  LBO 2  ANGLE-TUNING Discretely tunable visible all-solid-state laser

27 Solid-state Raman lasers CW crystalline Raman lasers BN, l =68mm Ar + pump 5W, 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 =40mm diode pump 2.4W, 808nm 164mW, 543nm ( ~3% pump-1 st 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-1 st 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

28 Solid-state Raman lasers CW crystalline Raman lasers diode input power (W) nm power (mW) Nd:YAGKGW diode pump L =total non output coupling losses at the Stokes wavelength (1%) R 2 = 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-1 st Stokes (1176nm) efficiency ~4%* 800mW 1176nm unstable

29 Solid-state Raman lasers A CW intracavity frequency-doubled crystalline Raman laser? Nd:YVO 4 KGW 22W 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 1 st -Stokes of KGW pumped intracavity by 1064nm from diode-pumped Nd:YAG, 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 Nd:GdVO 4 self-Raman laser has already demonstrated >100mW cw yellow for a 3W diode pump!

30 Solid-state Raman lasers Thank you for your attention! Solid-state Raman lasers: a tutorial


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