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Building a Green Laser Source via Second Harmonic Generation Diana Parno ([email protected])

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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:

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slide1

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 ([email protected])

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
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