Transcript dseymour_48x36_glossy

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)
SHG Apparatus
Beam separation:
Prism, dichroic mirror
Nd:YAG 1064 nm
infrared 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.
Steering mirror:
Alignment must be
precise – the crystal is
only 0.5 mm thick
Steering mirror
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°.
Pump beam
(1064 nm)
Nonlinear
optical crystal
Second harmonic (532 nm)
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
Half-wave plate:
SHG is a polarizationsensitive process
Focusing lenses:
For maximum efficiency,
pump beam waist must
be precisely placed at
crystal center
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