PPLN Frequency-Doubling Project

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PPLN Frequency-Doubling Project

• Diana Parno
• Hall A Parity Collaboration Meeting
• May 17, 2007

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Green Laser Upgrade

• The 100 mW commercial green laser is problematic
• Not enough power
• May be unreliable over time (it spent the fall
  with the manufacturer for extended repairs)
• Possible solution Use nonlinear optics to build
  a higher-power, more reliable green laser.

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Second Harmonic Generation

• The pump wave generates a polarization inside a
  nonlinear optical crystal oscillating at twice
  the pump frequency.
• The nonlinear polarization radiates an EM wave
  with twice the pump frequency. This second
  harmonic propagates in the same direction.
• With advances in nonlinear optics (periodic
  poling, new crystal types), we can efficiently
  convert a reliable infrared laser to a reliable
  green one.

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Periodic Poling

• Second harmonic generation (SHG) depends on the
  phase difference f
• flt180 Energy transfers from pump to 2nd
  harmonic
• fgt180 Energy transfers from 2nd harmonic to
  pump

• Without phase matching, SHG intensity oscillates
  with a low amplitude over the crystal length

• Periodic poling induces a 180 phase shift in the
  2nd harmonic at every domain reversal, so that
  SHG is efficient over the entire crystal length

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Single-Pass SHG

• Why not use a powerful (several Watt) commercial
  green laser?
• NdYAG lasers are converted to 532 nm through SHG
• These lasers lock to secondary cavities for
  multiple passes through the crystal
• Our fast feedback scheme for the Fabry-Perot
  (based on PZTs) is thus impossible for these
  lasers
• Single-pass SHG allows us to achieve efficient
  locking to the Fabry-Perot cavity for Compton
  polarimetry

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SHG Apparatus

• The pump infrared beam must be carefully steered
  and focused into the SHG crystal (periodically
  poled lithium niobate PPLN)

Prism
Dichroic mirror
Infrared laser (1064 nm, 700 mW)
SHG crystal (inside oven)
Steering mirror
Steering mirror
Half-wave plate
Lenses
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SHG Achievements

• We have achieved 10-15 mW of green power with a
  700-mW infrared input
• Optimal phase-matching temperature is 62C
• Changes in alignment, polarization and lasing
  temperature may also improve efficiency

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Crystal Temperature Scan

• We expect a well-defined temperature response
  symmetrical sidebands about a sharp peak

Gregory Miller, Stanford PhD thesis, 1998

• For our crystal, poor temperature stability and
  resolution obscure the structure

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Pump Power Scan

• We expect a quadratic increase in SHG power as a
  function of pump power

• The structure we see is significantly different
• Possible temperature effects?

• Scans taken 15 hours apart show a substantial
  difference our setup has clear stability
  problems

Possible peak
Turn-on
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SHG Future Work

• Design a more stable oven/temperature controller
  for the PPLN crystal
• Improve separation of fundamental and
  second-harmonic beams
• Fully characterize crystal response to changes in
  pump power and polarization, crystal temperature
  
• Consider techniques for power amplification
• Test a 5-W fiber amplifier with our seed laser
  this summer

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Thank you!