Introduction osc requirements for 2 micron opa
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Introduction: OSC requirements for 2 micron OPA. Radiator emits 1 ns long photon bunch containing momentum information of proton beam Bunch peak power ~ 1 W (E=1 nJ) Repetition rate 10 MHz Need ≥ 10 ^4 phase-coherent gain Need ~ 10% bandwidth

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Introduction osc requirements for 2 micron opa

MIT Bates OSC Workshop

M. Babzien

Introduction: OSC requirements for 2 micron OPA

Radiator emits 1 ns long photon bunch containing momentum information of proton beam

Bunch peak power ~ 1 W (E=1 nJ)

Repetition rate 10 MHz

Need ≥ 10^4 phase-coherent gain

Need ~ 10% bandwidth

Need ≤ several cm optical path length increase

Optical considerations
Optical Considerations

  • Basic problem: high duty factor (1-2 ns pulsewidth @ 10 MHz repetition rate), so peak power is low for very efficient OPA, but average power is high relative to material stress limits

  • E.g. KNbO3 (deff~6 pm/V) ~ highest visible/NIR nonlinearity

  • Compare FOM for KNbO3 & cadmium germanium arsenide (CGA) proposed for 10 mm OSC: 0.8 versus 10, respectively.

  • Is there an appropriate nonlinear material ?!

Material choices bpm vs qpm
Material choices: BPM vs. QPM

  • Birefringent phasematching (BPM) allows very precise matching of phase velocities, and in most materials more than one configuration (type I & II) exist; some flexibility in choosing parameters

  • Using Non-collinear phasematching, group velocities can be matched for signal & idler, thereby leading to extremely broad bandwidths

  • However, spatial walkoff in many crystals is on the order of 10’s mm/mm, so cm length crystals require walk-off compensation schemes (gratings, inverting alternating crystal segments)

  • Also, 10’s cm crystal lengths impose constraints on bandwidth

  • Quasi-phasematching (QPM) gives access to highest deff in material leading to shorter crystals, can avoid walkoff problem, and adds additional degrees of freedom

  • However, QPM crystals tend to have lower damage thresholds (at 2% duty cycle, average power is limiting factor)

Ppln details
PPLN Details

  • Periodically poled LiNbO3 has one of the highest deff of crystals transparent in visible - NIR region (FOM=6)

  • Low absorption (<3%/cm) below 2 microns

  • Damage threshold is ~100 MW/cm2 for 10 ns pulses, few hundred kW/cm2 CW (without parasitic pump SHG inducing photorefractive damage)

  • One of first ferroelectrics to be demonstrated - widely developed processing techniques developed over past decade

D.C. Hanna, University of Southampton

Ppln gain calculation
PPLN gain calculation

  • Input parameters:

  • Pump l=1.05 mm Signal l=2.0 mm Idler l=2.2 mm

  • Epump=150 mJ

  • tpump =2 ns FWHM

  • Crystal length=30 mm

  • Poling period 32 mm

  • Øpump,FWHM=520 mm

Ppln bandwidth
PPLN bandwidth

  • Using SNLO QPM module, calculate FWHM bandwidth of ~8% @ 2 micron signal

  • Approximate expression

where Dng= |ng,signal – ng,idler|, yields a smaller bandwidth; more accurate calculations needed.

  • Non-collinear geometry may possibly be used to extend bandwidth

Ppln challenges
PPLN challenges

  • Pump peak intensity = 20 MW/cm2

    average intensity = 400 kW/cm2

  • Common photorefractive damage (grey-tracking) not likely to be a concern, unless significant SHG of pump occurs - may need suppression technique (multiple crystal segments)

  • Thermal expansion 15 ppm/K, 4 ppm/K

  • Poor conversion efficiency from pump to signal:use spatially & temporally square pump, not gaussian

  • Temperature bandwidth and beam heating?Depends on ∂(ns-ni)/∂t, absorption, specific heat (684 J/kg*K)

Pump laser requirements
Pump laser requirements

  • Need ~1500 W average power at 1 micron in 10 MHz pulsetrain synchronized to proton beam

  • Need about 2 ns pulsewidth (or square pulse) to flatten temporal variation of gain through 1 ns long signal pulse

  • Need “well-behaved” laser, i.e. reasonable transverse mode (flat-top), good pointing stability, minimal downtime, turnkey operation

Pump laser scheme
Pump laser scheme

  • Yb:YAG lases at 1030 nm, is directly diode pumpable, has good optical & mechanical properties, and has been extensively characterized

  • Thin disk geometry allows easy scalability to higher average power

  • Can use modelocked oscillator precisely synchronized to harmonic of 10 MHz proton beam frequency

  • Optically chop oscillator pulse train down to 10 MHz

  • Stretch pulses to 2 ns (without minimal chirp, or build ns oscillator)

  • Use multipass amplifier to get up to 150 mJ /pulse