1 / 9

Introduction: OSC requirements for 2 micron OPA

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

cybil
Download Presentation

Introduction: OSC requirements for 2 micron OPA

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. 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 ?!

  3. 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)

  4. 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

  5. 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

  6. 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

  7. 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)

  8. 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

  9. 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

More Related