Prism, dichroic mirror
Periodically poled lithium niobate crystal for SHG:
Crystal is temperature tuned to achieve QPM.
Nd:YAG 1064 nm
Narrow linewidth, frequencytuning via PZT or lasing temperature
Alignment must be precise – the crystal is only 0.5 mm thick
For maximum efficiency, pump beam waist must be precisely placed at crystal center
SHG is a polarization-sensitive process
Nonlinear optical crystal
Second harmonic (532 nm)
Building a Green Laser Source via Second Harmonic Generation
Diana Parno ([email protected])
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)
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
- 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