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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

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Solid-state Raman lasers: a tutorial

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    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 Electronics27 (2003) 3-56

    3. 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 (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 Electronics6 (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 hmS2wS3G dW in units cm/GW, and the integral Raman scattering cross-section is introduced as ds = wS4mS . h . da2 dW c4mL 2mwRdq Here da/dq is the derivature of the normal-mode polarisability (the square is proportional to c3), Gis 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, gRscales 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: IS(0) = hwS2mS3DW (2p)3c2 In practice to reach threshold i.e. for 1% depletion of the pump, the exponent gRIPz typicallymust 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!

    6. SRS theory (cont.) In the transient Raman regime, wheretP << TRthe 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.

    7. Common Raman crystals* *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. Crystal Raman spectra KGW Raman spectrum* c 768 High gain for pump propagation aligned along the crystal b-axis Access two high gain Stokes shifts: 901cm-1 768cm-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 negativefor many key Raman crystals * HM Pask et al, OSA TOPS: Advanced Solid State Lasers 50(2001) 441-444.

    10. Thermal properties of Raman crystals * 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-cavity 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 >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) * 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. . # 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 ) > 1 R1 , R2reflectances 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 > 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 Nd:YAG 35W 3.2mm 77% R, pump 55% T 1st-3rd Stokes 85%T 1064nm HR 1st-3rd Stokes High energy BaWO4 95mm 50 x 380mJ, 50ns 20 kHz at 1062nm Nd:GGG 19J 3.2mm 85%T 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 90%T 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 Photonics2006, MC3 *also Mildren et al, Opt. Express12 (2004) 785; Pask et al, Opt. Lett. 28 (2003) 435. KGW 50mm 2.4W at 532nm 10ns, 5kHz 90%T 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 Nd:YAG thermal lens only

    20. All-solid-state intracavity Raman lasers Nd:YAG Raman crystal diode pump HR pump/ fund Stokes coupling HT pump HR fund/Stokes Q-switch

    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 JQE25 (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 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

    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) 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. Nd:YAG Raman crystal LBO HR end mirror input mirror Q-switch Nd:YAG 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 Nd:YAG Raman crystal Q-switch M2 flat LBO M3 (R=300mm) 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 SFG SHG SFG SHG SFG :768cm-1 532 555 579 606 636 nm :901cm-1 KGW 532 559 589 622 658 nm

    26. LBO 2 =90, =11.6 Discretely tunable visible all-solid-state laser TEMPERATURE-TUNING ANGLE-TUNING LBO 1 resonator axis  =90, =0 TEC TEC • 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. 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 2.4W, 808nm Nd:KGW, l =40mm 9(54)mW, 1181nm ( ~2.5% diode-1st Stokes) 1067nm

    28. 800 600 1176nm power (mW) 400 200 L =total non output coupling losses at the Stokes wavelength (1%) R2= reflectivity of mirror M2 (0.25%) 0 0 10 20 30 diode input power (W) CW crystalline Raman lasers Pask, Opt. Lett.30 (2005) 2454 recently calculated pump (fundamental) power threshold for CW intracavity KGW Raman laser: Nd:YAG 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: Nd:YVO4 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 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:GdVO4 self-Raman laser has already demonstrated >100mW cw yellow for a 3W diode pump!

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