Building a Green Laser Source via Second Harmonic GenerationBuilding a Green Laser Source via Second Harmonic GenerationDiana Parno (dparno@cmu.edu)

for the Hall A Compton Polarimetry Group

Motivation: Polarimeter Upgrade

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)

Nd:YAG 1064 nminfrared laser:

Narrow linewidth, frequencytuning via PZT or lasing

temperature

Periodically poled lithium niobate crystal for SHG:

(in oven)Crystal is temperature tuned to achieve QPM.

Half-wave plate: SHG is a polarization-sensitive process

Focusing lenses:For maximum efficiency, pump beam waist must be precisely placed at crystal center

Steering mirror

Steering mirror:Alignment must be precise – the crystal is only 0.5 mm thick

SHG Apparatus

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

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°.

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

Nonlinear optical crystal

Pump beam (1064 nm) Second harmonic (532 nm)

Beam separation: Prism, dichroic mirror