Temperature-stable lithium niobate electro-optic Q-switch for improved cold performance. Dieter Jundt Gooch & Housego, Palo Alto, CA, USA

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1 Temperature-stable lithium niobate electro-optic Q-switch for improved cold performance Dieter Jundt Gooch & Housego, Palo Alto, CA, USA PAGE Temperature-stable LN Q-switch September 25, 2014

2 Gooch & Housego Locations Cleveland HQ Boston Palo Alto Sales by region North America 48% Continental Europe 22% Asia Pacific 15% United Kingdom 15% Manufacturing Sales Offices PAGE Temperature-stable LN Q-switch September 25, 2014

3 Cleveland Crystal growth and Electro-optics Crystals grown Lithium Niobate (LN) KDP and KD*P BBO TeO2 AgGaS 2, AgGaSe 2, CdTe PAGE Temperature-stable LN Q-switch September 25, 2014

4 Boston Semiconductor lasers and fiber optics Hermetically sealed fiber packages EM750 Fiber-coupled acousto-optic Q-switches Analog RF over fiber 100mW external modulation 1310nm or nmDFB lasers Line-width<10kHz >50mW fiber output 150GHz hysteresis-free tuning PAGE Temperature-stable LN Q-switch September 25, 2014

5 Outline Temperature-stable Q-switch Background Electro-optic Q-switch Pyro-electricity Fabrication Results Chemical reduction 2 alternate techniques Conductivity Absorption Performance PAGE Temperature-stable LN Q-switch September 25, 2014

6 Background Q-switching Q-switched Nd:YAG lasers Continuous optical pumping Electro-optic Q-switch triggers laser pulse release when shorting voltage V QWP Low Q High Q Laser pulse Imperfect quarter-wave plate insufficient extinction pre-lasing PAGE Temperature-stable LN Q-switch September 25, 2014

7 Background Pyro-electric effect LN is ferro-electric O 2- Li + Nb 5+ Spontaneous polarization Ps Cations displaced P s drops with increasing T Surface charges appear on faces Pyro-electric effect p dp dt s μc 95 m 2 K Example 9x9x25mm Q-switch 20 C change in T T>T c : P s =0 T=25 C : P s =0.75C/m µC per surface 185 kv voltage along Z PAGE Temperature-stable LN Q-switch September 25, 2014

8 Dissipation of pyro-charges LN intrinsic conductivity is very small Problem severe at low T Dissipation not adequate At room temp, takes days When cold, even longer E act~ 1eV Need to enhance dissipation Extrinsic means Increase LN conductivity Decay time (h) 1E8 1E Experiment on wafer model prediction ( adjusted) Temperature ( o C) PAGE Temperature-stable LN Q-switch September 25, 2014

9 Possible Solutions Ionized air Americium (alpha-emitter) Most common approach Radioactivity creates headache Discharge electrodes - complex Conductive coatings ITO, but slightly absorbing Lowers damage threshold (LIDT) Make LN more conductive UV illumination Cole, Goldberg 2010 Chemical reduction Brickeen et al, 2010 We continue and expand on this work PAGE Temperature-stable LN Q-switch September 25, 2014

10 Chemical Reduction method 1 Anneal LN in reducing atmosphere C for 1-2 hours Atmosphere 4%H 2 in N 2 Process finished, regular devices Need to protect optical windows Physics of process Surface reaction Oxygen loss at all surface Sides reduce more because rougher Electrons get liberated Charge diffusion Electrons diffuse as polarons Li + also diffuses (charge compensation) Non-uniform absorption 4% at 1064 at center Aperture restricted to ~4mm Ø Loss (%) 25mm 325 C 480 C Position (mm) PAGE Temperature-stable LN Q-switch September 25, 2014

11 Chemical Reduction method 2 Process slab before polishing Can control surface condition (lapped) Reduced only where pyro charges appear Typical process not aggressive enough (surface/volume small) Activate Surface Solution of Li 2 C 2 O 4 : NaC 12 H 25 SO 4 : H 2 O (3.6% : 2.2% : 94.2%) Oxalate provides Li + charges SDS is surfactant to help wetting surface Spin-on at 1000rpm let dry Anneal as before Finish Q-switch Polish both sides Cut into final shape Metal coat X-faces for electrodes AR coat surfaces may need second, low Temperature anneal PAGE Temperature-stable LN Q-switch September 25, 2014

12 Properties of reduced LN Optical Polarons Broad absorption Impurities Brownish color, Fe 2+ dominates Does not help with conductivity Conductivity Only polarons contribute Impurities reduce more easily Measure heavily reduced sample Ohmic behavior G to T resistance yes, slightly conductive Less so at colder temperatures Trade-off Absorption & conductivity linked Choose level of reduction Gain of laser cavity Temperature range for operation PAGE Temperature-stable LN Q-switch September 25, 2014 Absorption (%/cm) ln(t/r) - units of T/R=K/ polaron 0.5ppm Fe Wavelength (nm) Activation energy = 0.672eV /T (1000/K) 4.0

13 Slab reduction profile Diffusion depth Measured using polished cross section Depends on anneal T ~0.3mm removed in polish Absorption at 1064nm is 6x smaller Absorption at 633nm (cm -1 ) Absorption even across aperture Target 1-2% loss 450 C 100 minutes o C a = 330 m 460 o C a = 260 m 430 o C a = 130 m Depth from Z-surface ( m) PAGE Temperature-stable LN Q-switch September 25, 2014

14 Cold temperature measurement Samples Setup 3 different treatments 2-stage TE cooler no reduction method 1 method 2 Flow nitrogen No voltage applied main beam Laser 1064nm Polarizer 2 nd 1 st stage TE Wollaston prism depolarized Start at 60 C Drop temperature in steps 20 C every 2 hours Track extinction ratio PAGE Temperature-stable LN Q-switch September 25, 2014

15 Cold temp results no reduction Temperature ( o C) Extinction ratio (db) no reduction detail time (min) PAGE Temperature-stable LN Q-switch September 25, 2014

16 Cold temp results method 1 (anneal finished device) Temperature ( o C) method 1 Extinction ratio (db) Loss at center 4% time (min) PAGE Temperature-stable LN Q-switch September 25, 2014

17 Cold temp results method 2 (anneal slab) Temperature ( o C) method 2 Extinction ratio (db) Loss <2% time (min) PAGE Temperature-stable LN Q-switch September 25, 2014

18 Summary LN remains good choice for Q-switch of 1µm lasers Cost-effective Our treatment helps dissipating pyro-charges Drop-in replacement 2 production methods available Both provide charge dissipation Slab method produces makes available full aperture Reduction can be tailored Trade-off conductivity vs. absorption Degree of reduction needs to be matched to laser cavity gain PAGE Temperature-stable LN Q-switch September 25, 2014