Post on 27-Mar-2015
PPLN Frequency-Doubling 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 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
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
Without phase matching, SHG intensity oscillates with a low amplitude over the 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)
Infrared laser(1064 nm, 700 mW)
Steering mirror Steering mirror
Half-wave plate Lenses
SHG crystal(inside oven)
Dichroic mirror
Prism
SHG Achievements 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
For our crystal, poor temperature stability and resolution obscure the structure
Approximate Efficiency of Green Power Production
-0.5
0
0.5
1
1.5
2
2.5
35 40 45 50 55 60 65 70
Crystal Temperature (degrees Celsius)
Pe
rce
nta
ge
Eff
icie
nc
y (
Gre
en
P
ow
er/
La
se
r P
ow
er)
Possible sideband?
Sharper peak expected
Crystal Temperature Scan
Gregory Miller, Stanford PhD thesis, 1998
We expect a well-defined temperature response: symmetrical sidebands about a sharp peak
Pump Power Scan
0
0.5
1
1.5
2
2.5
3
3.5
4
0 100 200 300 400 500 600 700 800
Nominal Laser Output
Mea
sure
d G
reen
Out
put (
<50%
ac
tual
)
Pump Power Scan
0
0.5
1
1.5
2
2.5
3
3.5
4
0 100 200 300 400 500 600 700 800
Nominal Laser Output
Mea
sure
d G
reen
Out
put (
<50%
ac
tual
)
April 30 (afternoon)
May 1 (morning)
Pump Power Scan We expect a quadratic increase in SHG power
as a function of pump power
Turn-on
Possible peak Scans taken ~15 hours apart show a substantial difference: our setup has clear stability problems
The structure we see is significantly different Possible temperature effects?
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
Thank you!