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Quasi-phasematching A versatile tool for coherent source development

Explore the historical perspective, basic concepts, energy considerations, and conversion efficiency of Quasi-Phasematching (QPM) as a versatile tool for coherent source development. Learn about birefringent phasematching, modern QPM OPO, and interesting adaptations, along with insights into the present and future applications of this technology. Discover how QPM allows for wavelength selection, periodic reversal of the electric field, and unique domain structures for optimal phasematching. Gain knowledge on the limitations and advancements in modern QPM OPO devices and their benefits in ultrafast optics research.

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Quasi-phasematching A versatile tool for coherent source development

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  1. Quasi-phasematchingA versatile tool for coherent source development LUMOS Group K-State University Kansas, U.S.A Karl Tillman Ultrafast Optics Group Heriot-Watt University Edinburgh, Scotland (U.K)

  2. Outline • A Historical Perspective • The Basic Concepts • Energy, momentum and the phase condition • Conversion efficiency • Birefringent phasematching and Quasi-phasematching • The Modern QPM OPO • Interesting Adaptations • The Present • The Future

  3. A Brief History • 1960 – Theodore Maiman builds the Ruby laser • Visible spectral region quickly populated by range of different gain mediums • Important problems - Atomic / Molecular spectroscopy • Infrared region is where all the good stuff happens! • Absorption region for common hydrocarbons (C-H, N-H, O-H) • Biological fingerprint region (amino acids, proteins, etc) • Nonlinear materials offered a solution • 2 – Parametric conversion process

  4. The Motivation • Parametric frequency conversion offers advantages over normal laser action • Generated output wavelength determined by energy and momentum conservation rather than an atomic energy level structure • Level of tuneability not available from traditional laser sources (based on phasematching conditions) • Output profiles generally determined by pump pulse characteristics • Predetermined and predictable spectral and temporal characteristics

  5. The Basics • Energy Conservation • Momentum conservation • Phase condition

  6. The Basics • Parametric conversion efficiency:

  7. The Basics • Efficient parametric conversion requires: • Appropriate nonlinear medium (i.e. birefringence) • Interacting waves maintain temporal overlap • Interacting waves remain phasematched (k ≈ 0) • Main problems: • Group velocity dispersion • Pulse walk-away causes loss of phase condition • Phase mismatch (Dk) increases with crystal length • Limited length  limited gain  high threshold

  8. Birefringent Phasematching • Initial solution – Birefringence phasematching • Reduces effect of GVD, increasing interaction length • Relatively high parametric gain due to crystal length • Issues: • Gain medium must be sufficiently birefringent • Crystals require exact phase-matching angle • Relatively low damage thresholds (initially at least) • Rarely access highest nonlinear coefficient, deff

  9. An Alternative • Birefringence not a perfect solution • Alternative originally suggested in 19621 • Concept of quasi-phasematching (QPM) is born • Removes need for a birefringent material • Phasematching condition becomes an issue of material engineering • Also not perfect but better in most situations • Solves some of the key drawbacks of BPM • Introduces a few different problems 1 J. A. Armstrong et al, Phys. Rev. A, 127 (6), p:1918-1939, 1962

  10. Quasi-phasematching

  11. More Basics • QPM allows wavelength selection • Crystal makes momentum contribution

  12. QPM favours collinear phasematching More Basics • Periodic reversal of electric field • Regular domain structure with period: • k3, k2, k1co-propagate • Simple expt. set-ups

  13. Summary • QPM enables phasematching to become a characteristic of the crystal structure • Can select phasematched wavelengths by design • Original concept used stacked single crystals • High boundary losses  poor conversion efficiency • Not technologically feasible initially • 1993 lithographic approach used to periodically reorient electric field in lithium niobate crystal1 • Introduction of the periodically poling technique 1 M. Yamada et al, Appl. Phys. Lett, 62 (5), p: 435-436, 1993

  14. Summary • Periodic poling requires a ferroelectric medium • Larger choice of materials than BPM • Access to highest nonlinear coefficient (i.e. d33) • Tuning capabilities limited mainly by material transparency rather than Poynting vector walk-off • Common materials include: • Lithium Niobate (LN) • Potassium Titanyl Phosphate (KTP) • Associated isomorphs (KTA, RTA, CTA)

  15. The Modern QPM OPO • The Optical Parametric Oscillator (OPO) • First successfully demonstrated in 19651 • Versatile parametric device • Very broad tuning range • Low operational threshold • Self-seeding • QPM allows larger choice of nonlinear materials • Choose material with highest nonlinearity • Design the phasematched wavelength • First successful QPM OPO built in 19952 1. J.A. Giordmaine et al, Phys. Rev. Lett. 14 (24) p. 973-976, 1965 2. L. E. Myers et al, Opt. Lett. 20 (1) p.52-54, 1995

  16. The Modern QPM OPO • Q-Switched OPO • High repetition frequencies (kHz) • Fast pulse durations (ns) • Compact, robust devices • Very high pulse energies (~µJ) – damage risk!

  17. The Modern QPM OPO • Synchronously pumped (SPOPO) • Ultrafast devices (fs/ps) • Very high repetition frequencies (~MHz) • High peak powers (~kW), modest average powers (~W) • Operate well below crystal damage thresholds

  18. The Modern QPM OPO Limitations still exist: • Temporal overlap still an issue • GVD more pronounced in QPM OPOs • Limited useful crystal length, restricted gain • Limitation on appropriate materials • Crystal dimensions restricted by fabrication capabilities • Only pole Lithium Niobate if wafer ≤500µm • Restricts aperture size (limits power scaling) • Could use other materials but fabrication technology not as mature • Increases device costs

  19. Adaptations Ability to control phasematching condition by engineerable methods allows limitations to be addressed and other ideas tested • Increase tuning capabilities • Compression techniques • Improve efficiency • Designer pulses • Non-ferroelectric materials – e.g. semiconductors

  20. Adaptations • Increasing tuning capabilities Lithography allows the design of several gratings to be ‘written’ into the structure of a single crystal. • Different grating different PM  • Grating tuning • Temperature tuning • Successfully demonstrated in 19951 1. L. E. Myers et al, Opt. Lett. 20 (1) p.52-54, 1995

  21. Adaptations • Increasing tuning capabilities Lithium niobate is the most common material used to date in QPM devices. Suffers from photorefractive effects below Tc~80°C so requires heating. • Impurities reduce onset of photorefractive damage • MgO doping allows lithium niobate to operate at room temperature • Increased temperature tuning range

  22. Adaptations • Compression techniques • Lithography able to produce aperiodic structure • Chirped structures used to generate a compressed SHG output1 1. M. A. Arbore et al, Opt. Lett. 22 (12) p.865-867, 1997

  23. Adaptations • Compression techniques • Spatial localisation of conversion uses GVD as a temporal control • Shorter wavelengths generated earlier • Travel faster than longer wavelengths generated later in crystal resulting in a pulse compression 1. T. Beddard et al, Opt. Lett. 25 (14), p.1052-1054, 2000; 2. P. Alverez et al, JOSA B, 16 (9), p. 1553-1560, 1999

  24. Adaptations • Improving Efficiency Conversion efficiency dependant on available gain determined by crystal length • Longer crystals have a narrower conversion bandwidth • GVD  pump/signal pulse walk-away increased • Both reduce conversion efficiency Chirped grating could maintain conversion bandwidth as crystal length is increased

  25. Adaptations • Improving Efficiency • Allows the use of longer crystals

  26. Adaptations • Improving Efficiency • Longer crystals  More parametric gain  Better conversion efficiency  Lower threshold[1,2]  Smaller OPOs possible as less pump power needed • Longer pump pulses reduce effect of GVD • K. A. Tillman et al, Opt. Lett. 28 (7) p.543-545, 2003 • K. A. Tillman et al, J. Opt. Soc Am B. 20 (6) P.1309-1316, 2003

  27. Adaptations • Improving Efficiency • Photon recycling • Generate two identical pulses using one pump • Cascaded process • 3 tuneable outputs • Improves quantum efficiency 1. K. A. Tillman et al, J. Opt. Soc.Am. B. 21 (8) p.1551-1558, 2004

  28. Adaptations • Designer Pulses • Each grating has well defined phase response • Possible to design arbitrary aperiodic grating structures based on the overall phase response • Careful design can lead to generation of pulses with desirable temporal profile • Double and triple pulses • Triangle pulses • square pulses • stepped profiles 1. U. K. Sapaev et al, Opt. Exp. 13 p.3264-3276, 2005

  29. Adaptations • New Materials Semiconductors can have very high nonlinearities in comparison to current QPM materials e.g. • LN: d33 = 27pm/V • KTP: d33 = 18pm/V • GaAs: d14=170pm/V • InSb: d14=307pm/V Organic molecules can be even higher (~105)

  30. The Present • New Materials Semiconductors not ferroelectric so alternative poling method needed • Growth techniques (MBE & HPVE)1 Stanford group (M. Fejer & co) OP - GaAs • Ion implantation2 St Andrews group (W. Sibbett & co) PNS - GaAs • L. Eyres et al, Appl. Phys. Lett. 79 (7) p. 904-906, 2001 • D. Artigas et al, IEEE J. Quan. Opt. 40 (8) p.1122-1130, 2004

  31. The Future • Directly diode pumped QPM OPOs • Dual colour OPOs for phase stabilized operation 1 • Heriot-Watt, Edinburgh • Compact semiconductor QPM devices for operation at GHz repetition rates 2 • St Andrews • QPM devices for THz frequency generation 3 • Stanford • Organic QPM OPOs ?? • J. H. Sun et al, Opt. Lett. 31 (13) p.2021-2023, 2006 • T. C. Brown et al, New Journal of Physics, 6 Art.175, 2004 • K. L. Vodopyanov et al, Appl. Phys. Lett. 89 (14) 141119, 2006

  32. PPLN CEO phase-locking of Cr:Forsterite • Joined LUMOS group 9/25 • Project: CEO phase stabilization of Cr:Forsterite laser • In the process of locking frep and fceo • Next task – Lock laser to GPS signal then use it to make absolute spectral measurement of acetylene P(11) – P(16) absorption lines at ~ 1.53mm

  33. The End

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