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!