Building a Green Laser Source via Second Harmonic Generation Diana Parno (dparno@cmu)

1. Beam separation: Prism, dichroic mirror Periodically poled lithium niobate crystal for SHG: (in oven) Crystal is temperature tuned to achieve QPM. Nd:YAG 1064 nm infrared laser: Narrow linewidth, frequencytuning via PZT or lasing temperature Steering mirror: Alignment must be precise – the crystal is only 0.5 mm thick Steering mirror Focusing lenses: For maximum efficiency, pump beam waist must be precisely placed at crystal center Half-wave plate: SHG is a polarization-sensitive process Pump beam (1064 nm) Nonlinear optical crystal Second harmonic (532 nm) Building a Green Laser Source via Second Harmonic Generation Diana Parno (dparno@cmu.edu) for the Hall A Compton Polarimetry Group Motivation: Polarimeter Upgrade SHG Apparatus • The upgrade of the Hall A Compton Polarimeter, which will double its analyzing power and allow 1% accuracy in an hour of continuous electron beam polarization measurements, requires a 532 nm laser with: • Narrow linewidth • PZT-driven fast-feedback ability for locking to a Fabry-Perot cavity • Temperature tuning • We propose to construct a green laser via single-pass second harmonic generation (SHG), the nonlinear optical process at the heart of green laser technology. Our advantages: • Reliable infrared seed laser • New, more efficient crystals (e.g. lithium niobate, LiNbO3) • Better available crystal structures (periodic poling) Second Harmonic Generation • The nonlinear optical process of second harmonic generation (SHG) occurs inside a crystal for a pump wave of frequency ν: • The pump wave stimulates a polarization that oscillates at 2ν. • This polarization radiates an EM wave with frequency 2ν. • Energy is transferred from the pump to the second harmonic while the phase difference between the two EM waves is less than 180°. Results and Future Work • Results: • We have achieved a green output of about 15 mW with a 700-mW continuous-wave infrared input • We have found the optimal crystal temperature range • Future Work: • Power instabilities are likely caused by temperature problems, so we are seeking new temperature control solutions • Better beam separation (a chicane of four dichroic mirrors) will improve quality of green output • Coupling the infrared laser to a fiber amplifier will allow us to achieve several hundred mW of green power • How do we ensure that energy transfer always goes the right way? • Birefringent phase matching (BPM): Prevent phase mismatch by controlling incident angle: both waves see the same refraction index. Not possible for LiNbO3! • Quasi-phase matching (QPM): Introduce periodic domain reversals (periodic poling) to regularly induce a 180° phase shift to compensate for phase mismatch