Microlasers and Nonlinear Optics

1. Microlasers and Nonlinear Optics R. Schmitt, D. Armstrong, A. Smith, B. Do, and Greg Hebner Sandia National Laboratories Laser, Remote Sensing, Plasma Physics and Complex Systems Department 1128 Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. Optics, Lasers, and Remote Sensing Department 1128

2. 0.6 THz image Emerging and enabling technologies require investment • Long history of atomic and molecular physics • Spectroscopy, optical surface diagnostics • Fiber lasers • Emerging compact high power sources. • Frequency extension, nonlinear optics • Generate wavelength(s) matched to the mission requirements. • Recent advances in nonlinear optics understanding are enabling new system designs. • Exploiting the THz region of the EM spectra • New source and detector technology is opening up the THz spectral region. • High chemical specificity, unique and not widely published spectral signatures, low probability of intercept, broad area imaging using SAR like processing. • Compact, robust sources • Micro lasers, high power in a small size. • Solid-state and semiconductor laser systems • UV solid state lasers are relatively well developed at 400 nm with 290 nm sources on the horizon. Wavelengths are a good fit to some remote sensing opportunities. Optics, Lasers, and Remote Sensing Department 1128

3. Basic design for passively Q-switched microlaser 808-nm pump light lens Fiber Cr:Nd:GSGG Cr4+:YAG 1.06 mm output pulse • fiber-coupled diode laser pump (electrical isolation) • passively Q-switched (electrical isolation) • Cr:Nd:GSGG active material (rad hard) • Cr4+:YAG Q-switch (rad hard) • simple cavity with mirror coatings directly on crystal faces • crystals bonded together to form rugged, monolithic laser • thermal lensing and gain guiding stabilize flat-flat cavity Optics, Lasers, and Remote Sensing Department 1128

4. The Cr:Nd:GSGG microlaser produces ~1.6-ns-wide pulses • Eout = 57 μJ / pulse • ~1.6 ns (FWHM) pulse width • raw beam Imean ~ 150 MW/cm2 • after-pulse is common • ~15 - 25% total energy in second pulse typical • affected by details of pump focus, pump beam quality • linearly polarized (>100:1) near field beam intensity (MW/cm2) • output energy is scalable from μJ to 100’s of μJ Optics, Lasers, and Remote Sensing Department 1128

5. The Cr:Nd:GSGG microlaser produces excellent beam quality scan through focus • The near-field beam diameter (1/e2) is 150 μm (h) x 144 μm (v). • The far-field divergence (1/e2 full angle) is 10.2 mRad (h) x 10.6 mRad (v). • M2≈ 1.05 (fitted second moment beam diameter to propagation equation) • Eout = 57 μJ / pulse  Fpeak = 0.67 J / cm2  Imax ~ 300 MW/cm2 (raw beam) Optics, Lasers, and Remote Sensing Department 1128

6. 2 1 3 laser Waveguides written directly into bulk material can be used for optical interconnects • Bulk optical waveguide elements provide functional “circuits” while fibers provides the “wires”. • Waveguide structures are a building block for buried optical computing. • All optical interconnects are a significant safety improvement. • Alignment and materials are very robust but if broken, can be time consuming to repair. • Waveguides in bulk material fail based upon well understood material properties (heat, water). Phase contrast image of a waveguide written in borosilicate glass Near field mode profile of a laser written waveguide is near Gaussian. Writing custom optical elements directly into bulk glass Ring coupling structures have been manufactured using femtosecond laser machining. Optics, Lasers, and Remote Sensing Department 1128

7. OPO signal far-field fluence F 200 Critical direction  Depleted pump Signal Tunable high pulse energy UV: A difficult problem • Typical method: SFG using Nd:YAG-pumped ns OPO signal + Nd:YAG 2 • 803 nm + 532 nm  320 nm • Many problems to overcome: • High-energy ns OPO beam quality is poor • Nanosecond OPO’s start late and back-convert • Q-switched Nd:YAG beam quality is poor Optics, Lasers, and Remote Sensing Department 1128 SPIE 5887-3

8. 1.975" Mechanically robust. Long-term stabilityNo mirror adjustments Image rotating nonplanar ring “RISTRA” OPO OPO: xz-cut KTP,  = 58.4803(e) + 1576(o)  532(o)10 × 10 × 15 mm3 Pump out Pulsed “self seed” beam l/2 UV out Pump in Signal out l/2 SFG crystal Type-II BBO,  = 48.2803(e) + 532(o)  320(e) “RISTRA” cavity: Rotated Image Singly-Resonant Twisted RectAngle JOSA B 19, 1801–1814 (2002) Optics, Lasers, and Remote Sensing Department 1128 SPIE 5887-3

9. Near- and far-field  = 803 nm signal fluence profiles Near field: Image of OPO output coupler Far field: Lens with effective f/# 77 Fresnel # D2 / L> 450 for Signal = 803 nm Optics, Lasers, and Remote Sensing Department 1128 SPIE 5887-3

10. Pump depletion for seeded and unseeded oscillation Free-running oscillation in two-crystal RISTRA~37% pump depletion Self-seeded oscillation in two-crystal RISTRA~85% pump depletion Optics, Lasers, and Remote Sensing Department 1128 SPIE 5887-3

11. Are flat-top beam profiles important? Flat-top pump 100 mJ flat-top seed 10 mJ Pump depletion = 93% Flat-top pumpFlat-top seed~ 85 % pump depletion Flat-top pump 100 mJGaussian seed 10 mJPump depletion = 92% 2nd-order Gaussian pump0th-order Gaussian seed~52% pump depletion Optics, Lasers, and Remote Sensing Department 1128 SPIE 5887-3

12. Depleted SFG Pulses67 % depletion of 80350 % depletion of 532UV energy ~ 180 mJ Detector: Scientech 380101 absorberCalibration: 1 mJ/mV @ 10 HzTransmission loss:~5%Efficiency: 1064 nm to 320 nm > 21% Scientech 380101 Maximum extra-cavity UV energy ~ 190 mJ Optics, Lasers, and Remote Sensing Department 1128 SPIE 5887-3

13. End Optics, Lasers, and Remote Sensing Department 1128