Transcript PPLN Frequency Doubling

PPLN FrequencyDoubling Project
Diana Parno
Hall A Parity Collaboration Meeting
May 17, 2007
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.
Second Harmonic Generation
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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.
Periodic Poling

Second harmonic generation (SHG) depends on
the phase difference φ:
 φ<180°:
Energy transfers from pump to 2nd harmonic
 φ>180°: 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
Single-Pass SHG

Why not use a powerful (several Watt)
commercial green laser?
 Nd:YAG
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
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
SHG Achievements
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We have achieved 10-15 mW of green power
with a 700-mW infrared input
Optimal phase-matching temperature is ~62°C
Changes in alignment, polarization and lasing
temperature may also improve efficiency
Crystal Temperature Scan

We expect a well-defined
temperature response:
symmetrical sidebands
about a sharp peak
Gregory Miller, Stanford PhD thesis, 1998
Approximate
Efficiency
of Green Scan
Power Production
Crystal
Temperature
Sharper peak expected

For our crystal, poor
temperature stability
and resolution
obscure the structure
Percentage Efficiency (Green
Power/Laser Power)
2.5
2
1.5
1
Possible sideband?
0.5
0
35
40
45
50
55
60
-0.5
Crystal Temperature (degrees Celsius)
65
70
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?
April 30 (afternoon)
Pump Power Scan
Measured
MeasuredGreen
GreenOutput
Output (<50%
(<50%
actual)
actual)
May 1 (morning)
Possible peak
4
3.5
3
3
2.5
2
1.5
1.5
Turn-on
1
1
0.5
0.5
0
0
0
0
100
100
200
200
300
400
500
300
400
500
Nominal
Nominal Laser
Laser Output
Output
600
600
700
700
800
800
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Scans taken ~15 hours
apart show a
substantial difference:
our setup has clear
stability problems
SHG Future Work
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Design a more stable oven/temperature controller for
the PPLN crystal
Improve separation of fundamental and secondharmonic 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
Thank you!