PPLN Frequency-Doubling Project

1. PPLN Frequency-Doubling Project Diana Parno Hall A Parity Collaboration Meeting May 17, 2007

2. Green Laser Upgrade • The 100 mW commercial green laser is problematic: • Not enough power • May be unreliable over time (it spent the fall with the manufacturer for extended repairs) • Possible solution: Use nonlinear optics to build a higher-power, more reliable green laser.

3. Second Harmonic Generation • The pump wave generates a polarization inside a nonlinear optical crystal oscillating at twice the pump frequency. • The nonlinear polarization radiates an EM wave with twice the pump frequency. This second harmonic propagates in the same direction. • With advances in nonlinear optics (periodic poling, new crystal types), we can efficiently convert a reliable infrared laser to a reliable green one.

4. Periodic Poling • Second harmonic generation (SHG) depends on the phase difference φ: • φ<180°: Energy transfers from pump to 2nd harmonic • φ>180°: Energy transfers from 2nd harmonic to pump • Without phase matching, SHG intensity oscillates with a low amplitude over the crystal length • Periodic poling induces a 180° phase shift in the 2nd harmonic at every domain reversal, so that SHG is efficient over the entire crystal length

5. Single-Pass SHG • Why not use a powerful (several Watt) commercial green laser? • Nd:YAG lasers are converted to 532 nm through SHG • These lasers lock to secondary cavities for multiple passes through the crystal • Our fast feedback scheme for the Fabry-Perot (based on PZTs) is thus impossible for these lasers • Single-pass SHG allows us to achieve efficient locking to the Fabry-Perot cavity for Compton polarimetry

6. SHG Apparatus • The pump infrared beam must be carefully steered and focused into the SHG crystal (periodically poled lithium niobate – PPLN) Prism Dichroic mirror Infrared laser (1064 nm, 700 mW) SHG crystal (inside oven) Steering mirror Steering mirror Half-wave plate Lenses

7. SHG Achievements • We have achieved 10-15 mW of green power with a 700-mW infrared input • Optimal phase-matching temperature is ~62°C • Changes in alignment, polarization and lasing temperature may also improve efficiency

8. Crystal Temperature Scan Sharper peak expected Possible sideband? Crystal Temperature Scan • We expect a well-defined temperature response: symmetrical sidebands about a sharp peak • For our crystal, poor temperature stability and resolution obscure the structure Gregory Miller, Stanford PhD thesis, 1998

9. Pump Power Scan • We expect a quadratic increase in SHG power as a function of pump power • The structure we see is significantly different • Possible temperature effects? • Scans taken ~15 hours apart show a substantial difference: our setup has clear stability problems Possible peak Turn-on

10. SHG Future Work • Design a more stable oven/temperature controller for the PPLN crystal • Improve separation of fundamental and second-harmonic beams • Fully characterize crystal response to changes in pump power and polarization, crystal temperature … • Consider techniques for power amplification • Test a 5-W fiber amplifier with our seed laser this summer

11. Thank you!