Lecture 3 Ultra-short pulse parametric devices

1. Lecture 3Ultra-short pulse parametric devices David Hanna Optoelectronics Research Centre University of Southampton Lectures at Friedrich Schiller University, Jena July/August 2006

2. Lecture Outline General features and attractions of ultrashort pulse parametric devices Synchronously Pumped OPOs (SPOPOs): general considerations Specific examples of SPOPO performance Optical Parametric Amplifiers (OPA), Optical Parametric Chirped Pulse Amplifiers (OPCPA) & Optical Parametric Generators (OPG) Carrier Envelope Phase considerations

3. Attractions of parametric processes in the ultrashort pulse regime High gain; damage intensity behaves ~1/(pulse duration)� Broad gain bandwidth Wavelength flexibility (eg different from Ti:Sapphire!) Reduced ASE, reduced background, good contrast High Quantum efficiency Low thermal effects Good beam quality Scalability

4. Some disadvantages of parametric processes Small aperture dimensions available No energy storage Synchronisation requirements High pump brightness required

5. Some general features of ultra-short pulse parametric devices High gain and wide bandwidth can be obtained in a single pass of a parametric amplifier: lasers require regenerative amplification For the shortest pulses, ensure a large enough gain-bandwidth + good temporal overlap between the interacting waves over the NL medium Short crystal length can ensure the above, but places limits on the achievable gain Alternative ways to increase the gain bandwidth include: near-degenerate operation non-collinear phase-matching Double refraction effects are reduced for shorter crystals Non-collinear phase-matching can contribute to group-velocity-matching

6. Dependence of double-refraction effects on crystal length For a given double-refraction walk-off angle ?, and beam diameter D, the effect of walk-off in a crystal of length is insignificant if ?L/D << 1 For confocal focussing, 2pw02n/? = L, i.e., D = 2w0 = [2L?/np]� so; ?L/D = ?[pnL/2?]� Hence, for shorter crystals, as required for shorter pulses, confocal focussing is less compromised by double refraction 10x shorter pulse ?10x shorter Xtal ? toleratev10x greater ? value

7. Synchronously-pumped OPO

8. SPOPO pump requirement versus crystal length If length L is determined by the allowable Group Delay Difference, then, L ? T and if confocal focussing is used, then, gain ? LP = LE/T ? E Hence, threshold is specified by an energy, independent of pulse duration, & for a given repetition rate, threshold average power is then independent of pulse duration. But Self Phase Modulation is more problematic for shorter pulses, since effect of SPM ( fractional spectral broadening) ? IL ? PL/L ? E/T (T,P,E,I are, respectively, pump pulse duration, power, energy, intensity)

9. Some Attractions of SPOPOs Low threshold average power (amenable to diode pumping) Power scalable, eg via fibre-pumped SPOPOs Very wide tuning Synchronised outputs at two wavelengths (e.g. for CARS) Very high gain possible, can oscillate even with very high idler loss Very high efficiency, e.g. makes the tandem OPO practical

10. SPOPO facts and figures

11. Crystal length constraint for a SPOPO

12. Typical resonator arrangement for SPOPO

13. How to tune a QPM OPO

14. SPOPO slope efficiency of > 100%

15. Order of magnitude pulse compression in a PPLN SPOPO

16. Other features of SPOPO Cavity length change can change signal wavelength: not a good technique for tuning as pulse characteristics will change Oscillation tolerates cavity length changes of many pulse widths. Stabilise cavity length via stabilising the output frequency Tuning through the gain profile can lead to higher order transverse modes of the signal Tuning elements involving angular dispersion, eg grating, produce tilted pulses In QPM materials, many additional outputs may be seen (2?s, 2?i, ?s+?p, ?i+?p).

17. PPLN SPOPO with feedback via diffraction grating

18. Tilted pulses produced by diffraction grating

19. CdSe tandem-pumped SPOPO

20. CdSe SPOPO

21. Infrared absorption edge of Lithium Niobate

22. SPOPO with idler absorption (1)

23. SPOPO with idler absorption (2) Photon conversion efficiency to idler output:

24. SPOPO with idler absorption (3)

25. SPOPO pumped by femtosecond mode-locked fibre laser

26. High power femtosecond fibre feedback SPOPO

27. Fibre feedback SPOPO: insensitivity of output power to resonator length changes

28. Femtosecond (down to 13fs) visible OPOvia non-collinear phase-matching in BBO

30. Non-collinearly phase-matched femtosecond OPA with a 2000cm-1 bandwidth

31. Matching of group velocities by spatial walk-off in collinear three-wave interaction with tilted pulses

32. Pulse-front matched OPA for sub-10-fs pulse generation�

33. Visible pulse compression to 4fs by OPA +programmable dispersion control

34. Visible compression to 4fs by OPA+ programmable dispersion control

35. Yet more OPA designs� OPCPA +multiple pumps, at different wavelengths, to increase the gain bandwidth. Wang et al., Opt Commun., 237,169, (2004) Use of chirped broadband pump + operation near degeneracy. Limpert et al., Opt. Express, 13, 19, 7386, (2005) Ultrabroadband (octave-spanning) OPCPA, using angularly dispersed signal Arisholm et al., Opt. Express, 12, 518, (2004)

36. Efficiency-enhanced soliton OPA Pump, signal and idler are mutually trapped in a spatial soliton This requires a phase-mismatch whose ideal value depends on the mix of pump, signal and idler powers These powers evolve through the amplifier, hence ideally one needs a longitudinally varying phase-mismatch through the medium SOLUTION: Use aperiodic QPM medium

37. Tandem-chirped OPA grating design for simultaneous control of group delay and gain control

38. Cavity-enhanced OPCPA

39. Generation of few cycle terawatt light pulses via OPCPA

40. Carrier Envelope Phase (CEP) Carrier phase offset between carrier peak and envelope peak can vary from pulse to pulse This has significant effects in high field experiments using few-cycle pulses

41. Self-stabilisation of CEP via parametric processes In an OPA, with signal only as input, the phase relation, fp-fs-fi = -p/2 , applies through the medium if ?k = 0 If the signal is derived from the pump, eg as in generation of supercontinuum, signal and pump have the same phase behaviour. So, using the pump to amplify this signal in an OPA leads to a CEP stable idler even if the pump is not CEP stable. If this CEP stable idler does not have the desired power it can be used as the input signal to a second amplifier, OPA2 Since this amplified signal has its phase preserved in OPA2 one now has a high power pulse that is CEP stable

42. Generation of high energy self-phase-stabilised pulses via DFG and OPA

43. Concluding remarks OPAs are widely seen as a preferred alternative to TiS for amplification of ultrashort pulses to high powers Much needs to be done to establish power-scaling limits of OPOs, and OPAs. Designs for OPAs are numerous and new proposals keep appearing. Not yet a mature field; work is in progress. Different circumstances, e.g. pulse energy, duration, wavelength, call for different designs. Not a case of �one size fits all� Numerical calculations need to include transverse effects. Plane-wave models are ignoring vital aspects