Presentation on theme: "Solid-state Raman lasers: a tutorial"— Presentation transcript:
1Solid-state Raman lasers: a tutorial Jim PiperProfessor of PhysicsCentre for Lasers and Applications, Macquarie University, Sydney(Carnegie Centenary Professor, Heriot-Watt University, Edinburgh)Acknowledgements: H Pask, R Mildren, H Ogilvy, P DekkerAustralian Research Council, DSTO Australia
2Overview 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) SSRLsIntracavity frequency-doubled SSRLs for visible outputsCW external-cavity and intracavity SSRLsNote 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)Pask Progress in Quantum Electronics 27 (2003) 3-56
3Stimulated 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)wPwS1wS1wS2wS2wS3wRSRS does not require phase matching.
4SRS theory** Penzkofer et al Progress in Quantum Electronics 6 (1979)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 gaincoefficient isgR = 8pc2 N . dshmS2wS3 G dWin units cm/GW,and the integral Raman scattering cross-section is introduced asds = wS4mS . h da 2dW c4 mL 2mwR dqHere 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.
5SRS 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:IS(0) = hwS2mS3 DW(2p)3c2In 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!
6SRS 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.
7Common Raman crystals* Raman shift cm-1Raman linewidth cm-1IntegralX-section (cfdiamond=100)Raman gaincm/GWDamage threshold GW/cm2LiIO3 (LI)8227705.0544.8~ 0.1Ba(NO3)2 (BN)10470.42111~ 0.4CaWO4 (CW)9087.0523.0~ 0.5KGd(WO4)2(KGW)7689015.97.859504.43.3~ 10BaWO4 (BW)9241.68.5~ 5SrWO4 (SW)9222.7YVO4 (YV)8904.5~ 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.
8Crystal Raman spectra c b a KGW Raman spectrum* 768 901 901 768 901 High gain for pump propagation aligned along the crystal b-axisAccess two high gain Stokes shifts: cm cm-1 which are pump polarisation dependent.901901768b901*IV Mochalov Opt. Eng. 36 (1997) 1660; for thermal properties see also S Biswal et al, Appl. Opt. 44 (2005) 3093.a901
9Thermal 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)
10Thermal properties of Raman crystals LiIO3CaWO4Ba(NO3)2KGd(WO4)2BaWO4thermal conductivity kc at 25oC Wm-1K-1161.173.0thermal expansion amK-1 (x10-6)136thermo-optic dn/dTK-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.
12Pulsed 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)2KGd(WO4)2BaWO4steady-state 532nm47 cm/GW11.8 cm/GW40 cm/GWtransient 532nm4.711.814.3steady-state 1064nm1148.5transient 1064nm1.133.8* Cerny et al, Prog. Quantum Electron. 28 (2004) 113.
13Pulsed Raman generators Reported first-Stokes conversion efficiencies for single-pass Raman generators**After Basiev & Powell Handbook of Laser Technology and Applications B1.7 (2003) 1and Cerny et al, Prog. Quantum Electron. 28 (2004) 113..spectral/temporal regimeBa(NO3)2KGd(WO4)2BaWO4532nm, 5-20 ns, mJ26%30%45%532nm, ps, ~0.1mJ25%50%40%#1064nm, 5-20 ns, mJ35-40%1064nm, 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
14External-resonator Raman lasers Raman crystal length lThe pump is usually double-passed.Raman threshold is reached when:R1R2 exp (2gRIP l ) > 1R1 , R2 reflectances at first-Stokeshigh intensitypulsed pumpoutputmirror 2inputmirror 1Resonating 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.
15External-cavity (resonator) Raman lasers Basiev et al, OSA Advanced Solid-State Photonics 2004, TuB11High average power8 x 145mJ, 50ns, 50ms30 Hz at 1064nmBaWO4 95mmNd:YAG 35W3.2mm85%T 1064nmHR 1st-3rd Stokes77% R, pump55% T 1st-3rd StokesHigh energy50 x 380mJ, 50ns20 kHz at 1062nmBaWO4 95mmNd:GGG 19J3.2mm85%T 1064nmHR 1st-3rd Stokes77% R, pump55% T 1st-3rd Stokes
16External-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 281nm180mJ, 20ns10 Hz at 532nmBa(NO3)2 70mm90%T 532nmHR 1st-StokesHR, pump70% T 1st-Stokes5mm176mm unstableTakei et al, Appl. Phys B74 (2002) 521.11mJ, 20Hz 3rd-Stokes at 1598nm (eyesafe region) after compensation for strong thermal lensing in BN140mJ, 20ns20 Hz at 1064nmBa(NO3)2 58mmHR pumpHR 1st-2ndStokes71% T 3rd-StokesHT 1064nmHR 1st-3rd Stokes5mm200mm
17External-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 532nm10ns, 5kHzKGW 50mm90%T 532nmHR 1st-2nd Stokes160mmHR pump, 1st-Stokes50-60% 2nd-Stokes52mm mode-matchedKGW E//Nm (588nm)KGW E//Ng (579nm)Conversion efficiency into 2nd-Stokesat 588nm: 64% (slope eff. 78%);at 579nm: 58% (slope eff. 68%).
18Intracavity 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+ lasercrystal*Intracavity Raman*including coupled-cavityRaman crystaldiodepumpMirror 1HT pumpHR fundamental/StokesQ-switchMirror 2HR pump/ fundamentalStokes couplingNd3+ laser/Raman crystalIntracavity self-RamanMirror 1HT pumpHR fund/StokesMirror 2HR pump/fundStokes couplingQ-switch
19Intracavity crystalline Raman lasers Effects of thermal lenses on resonator designPask & Piper, IEEE J. Quantum Electron. 36 (2000) 949.*also Pask, Prog. Quantum Electron. 27 (2003) 3.Resonator mode size taking account of LIO3 thermal lensinstabilitypump mode sizeMode size taking account of Nd:YAG thermal lens only
20All-solid-state intracavity Raman lasers Nd:YAGRaman crystaldiodepumpHR pump/ fundStokes couplingHT pumpHR fund/StokesQ-switchDiode powerRaman crystall firstStokest pulse/prfpower/effReference5WCaWO41178nm6ns/10kHz0.5W/9%Murray et al, OSA TOPS 19 (1998) 12930WBa(NO3)21197nm15ns/10kHz3W/10%Pask & Piper, IEEE JQE 36 (2000) 949LiIO31156nm20ns/10kHz2.6W/9%23WKGd(WO4)21158nm30ns/15kHz4W/17%Mildren et al, Opt.Lett. 30 (2005) 150010WBaWO41181nm24ns/20kHz1.6W/17%Chen et al, Opt. Lett. 30 (2005) 3335
21Intracavity 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 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
23Intracavity 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:YAGRaman crystalLBOHR endmirrorinputmirrorQ-switchNd:YAGLBOdichroicturning/outputmirror
24Intracavity 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 NoNd:YAGRaman crystalQ-switchM2 flatLBOM3 (R=300mm)M1flat250mm overall resonator length
25Discretely 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 Stokes2nd StokesFundSHGSFGSHGSFGSHGSFG532555579606636 nm:768cm-1532559589622658 nm:901cm-1KGW
26Discretely tunable visible all-solid-state laser ANGLE-TUNINGTEMPERATURE-TUNINGLBO 1=90, =11.6LBO 2resonator axis=90, =0TECTECLBO1AngleWavelength(nm)Output power (W)0 5791.811 5550.9517 5321.7TempLBO1LBO2Wavelength(nm)Output power (W)19 C(52 C)6060.2548 C5790.5795 C5550.52-25 C5321.5beam displacementphase-matching limits possible wavelengthstemperature range too big for single stage TEClow powers due to insertion loss of 2nd crystaldual crystals reduce switching times
27CW 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+ pump5W, 514nmBN, l =68mm164mW, 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 pump2.4W, 808nmNd:KGW, l =40mm9(54)mW, 1181nm ( ~2.5% diode-1st Stokes)1067nm
28CW crystalline Raman lasers Pask, Opt. Lett. 30 (2005) 2454 recently calculated pump (fundamental) power threshold for CW intracavity KGW Raman laser:L =total non output coupling losses at the Stokes wavelength (1%)R2 = reflectivity of mirror M2 (0.25%)diode input power (W)1020302004006008001176nm power (mW)Nd:YAGKGW800mW1176nmdiodepumpunstableMaximum stable CW Raman output power was 800mW for 20W diode pump power at diode-1st Stokes (1176nm) efficiency ~4%*
29A 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:YVO4KGWLBO22W diodeDekker, 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!
30Solid-state Raman lasers: a tutorial Thank you for your attention!