Femtosecond optical parametric oscillator frequency combs for high-resolution spectroscopy in the mid-infrared

Transcription

1 Femtosecond optical parametric oscillator frequency combs for high-resolution spectroscopy in the mid-infrared Zhaowei Zhang, Karolis Balskus, Richard A. McCracken, Derryck T. Reid Institute of Photonics and Quantum Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK ultrafast.hw.ac.uk

2 Outline Introduction to OPO frequency comb technology Concepts and operating principles Comb-offset control in a fs OPO METROCOMB project Recent results Frequency combs tunable across one octave of bandwidth OPO frequency combs at 1 GHz and above Precision metrology and stabilisation

3 Outline Introduction to OPO frequency comb technology Concepts and operating principles Comb-offset control in a fs OPO METROCOMB project Recent results Frequency combs tunable across one octave of bandwidth OPO frequency combs at 1 GHz and above Precision metrology

4 Operating Principles An optical parametric oscillator is effectively a "photon splitter" Every converted pump photon ( ), yields signal ( ) and idler ( ) photons femtosecond laser femtosecond OPO Wavelength coverage from the visible to the mid-infrared is readily available by adjusting: crystal phasematching OPO cavity length (tens of µm adjustment)

5 Synchronously pumped OPOs have near-ir and mid-ir frequency combs with spacing equal to the pump. Comb-offset control in a fs OPO Offsets of pump, signal and idler combs described by: Femtosecond laser Power OPO comb offsets are easily tuned by small (nm) changes to the cavity length idler (i) signal (s) pump (p) f

6 Detecting the OPO comb offset frequency Teresa I. Ferreiro, Jinghua Sun and Derryck T. Reid, Opt. Lett. 35, 1668 (2010) The carrier envelope offset can be measured by using interference between the pump supercontinuum and the pump + signal, or pump + idler sum-frequency mixing light e.g. idler CEO: idler (i) signal (s) pump (p) f f i = nf rep +d i f p = mf rep +d p f SFM = (m + n) f rep + d p +d i pump super-continuum (s/c) sumfrequencymixing (SFM) The beat signal between f S/C and f SFM contains the frequency d i, and harmonics of the laser repetition frequency, f rep. f S/C = lf rep +d p

7 Outline Introduction to OPO frequency comb technology Concepts and operating principles Comb-offset control in a fs OPO METROCOMB project Recent results Frequency combs tunable across one octave of bandwidth OPO frequency combs at 1 GHz and above Precision metrology

8 The METROCOMB consortium Development of high-repetition-rate OPO frequency combs for mid-ir spectroscopy Bring together 5 SME partners from across the supply chain Research carried out by 3 RTD performers with extensive comb experience

9 The METROCOMB consortium Development of high-repetition-rate OPO frequency combs for mid-ir spectroscopy Bring together 5 SME partners from across the supply chain Research carried out by 3 RTD performers with extensive comb experience xc xc xc xc

10 The METROCOMB consortium Project led to increased collaborations between suppliers and researchers Strong benefits for both parties Patent applications Engineering support IP and knowledge transfer Rewarding research Sales! Papers!

11 Outline Introduction to OPO frequency comb technology Concepts and operating principles Comb-offset control in a fs OPO METROCOMB project Recent results Frequency combs tunable across one octave of bandwidth OPO frequency combs at 1 GHz and above Precision metrology

12 Quasi-phasematching: designer wavelength conversion devices Balskus et al, CLEO 2015, Paper STh1N.7 Quasi-phasematched crystals (top) are tuned for different processes by changing the domain pattern, unlike conventional angle-tuned crystals (bottom) We utilise QPM PPKTP, engineering it to produce multiple colours in the visible / IR l 1 l 2 PPLN f L l 1 l 2 PPKTP d 33 coefficient 27 pm/v 14 pm/v Refractive 1.3 µm µm 160 fs 2 mm fs 2 mm -1

13 Quasi-phasematching: designer wavelength conversion devices Balskus et al, CLEO 2015, Paper STh1N.7 Quasi-phasematched crystals (top) are tuned for different processes by changing the domain pattern, unlike conventional angle-tuned crystals (bottom) We utilise QPM PPKTP, engineering it to produce multiple colours in the visible / IR Pump+idler Signal SHG SFG OPO section Unpoled L 1 L 2 L 3 L 4 (µm) A B C D E F G H I J

14 Quasi-phasematching: designer wavelength conversion devices Balskus et al, CLEO 2015, Paper STh1N.7 L 1 L 2 L 3 L 4 (µm) A B C D E F G H Phase-matching SFM section G in the PPKTP crystal design I J

15 OPO frequency-comb stabilisation Balskus et al, CLEO 2015, Paper STh1N.7 Pump laser (333-MHz Gigajet) repetition rate stabilisation achieved via PZT1 placed in the laser cavity 2-GHz 6 th harmonic) OPO (CEO) stabilisation: via PZT2

16 OPO frequency-comb stabilisation Balskus et al, CLEO 2015, Paper STh1N.7 Near-continuous coverage from µm Supercontinuum p+i SFG idler) p+i SFG idler) tune fceo frep-fceo frep

17 Comb offset beat (f CEO ) Balskus et al, CLEO 2015, Paper STh1N.7 Stabilised to a 10-MHz reference (synthesizer) Sidebands present due to 30-kHz noise on green pump laser for Ti:Sapphire 10 -3dB Note narrow (instrument limited) linewidth of the CEO beat frequency

18 Stability (phase-noise) measurement Balskus et al, CLEO 2015, Paper STh1N rad integrated phase noise Strong peak at 30 khz originating from the pump has a high contribution to the f CEO integrated phase noise (>0.6 rad)

19 Mode filtering to 10-GHz in a Fabry-Perot cavity Zhang et al, "Mode-resolved 10-GHz frequency comb from a femtosecond OPO," Opt. Lett. 40, 2692 (2015) 10.3 GHz filtered comb mode spacing Pulse repetition period = 97 ps 830-MHz resolution (data window 36 cm) 1.0-GHz linewidth after apodization Modes resolved with high contrast

20 Summary Multi-section PPKTP design enhanced SFG process needed for f CEO locking over broad range of wavelengths Demonstrated continuously tunable frequency comb operation across >2000 nm in the mid-ir region The f CEO locking quality is preserved across all idler tuning range the locking is as good at 2000 nm as it is at 4000 nm Signal tuning range can also be stabilised as a comb µm Modes can be subsequently filtered in a high-finesse filter cavity 10

21 Outline Introduction to OPO frequency comb technology Concepts and operating principles Comb-offset control in a fs OPO METROCOMB project Recent results Frequency combs tunable across one octave of bandwidth OPO frequency combs at 1 GHz and above Precision metrology

22 OPO comb concepts Singly-resonant 1-GHz optical parametric oscillator frequency comb 1-GHz Ti:sapphire laser f-to-2f reference Heterodyne locking OPO SHG SFM Doubly-resonant 1-GHz optical parametric oscillator frequency comb 1-GHz Ti:sapphire laser f-to-2f reference Dither locking + heterodyne test OPO SHG SFM Doubly-resonant 10-GHz optical parametric oscillator frequency comb 10-GHz Ti:sapphire laser Rb-ECDL reference Dither locking OPO SHG SFM

23 Towards higher mode spacings: fundamentally-pumped 1-GHz OPO comb Operation at 1-GHz requires some compromises The peak power of the pump laser is lower......so more average power is needed to generate the super-continuum used for locking...leaving less power to pump the OPO...restricting its tuning range But it can be done! Spectra of composite comb formed by pumping OPO directly at 1-GHz (Ti:sapphire pump):

24 p, i + SFM Self-referenced 1-GHz OPO comb McCracken et al, CLEO 2015, Paper JTh2A.75 CEO interferometer 1 GHz 1.3 W 30 fs Mirror on PZT silica plate as OC l signal 1160 nm 1230 nm 1280 nm 1350 nm 85 fs 87 fs 83 fs 71 fs

25 Self-referenced 1-GHz OPO comb McCracken et al, CLEO 2015, Paper JTh2A.75 Ti:sapphire stabilisation achieved in a standard f-2f nonlinear interferometer The detected CEO frequency was used in a feedback loop that modulated the diode current in the 532-nm pump laser. For OPO CEO control, SHG signal pulses were heterodyned against a portion of the pump supercontinuum to detect a beat frequency Associated feedback loop modulated the OPO cavity length

26 Self-referenced 1-GHz OPO comb McCracken et al, CLEO 2015, Paper JTh2A.75 CEO stabilisation to 400 mrad (pump) and 1.5 rad (OPO) Milli-radian stability of repetition rate Sufficient stability for applications but limited tuning range

27 Atomically-referenced 1-GHz OPO comb McCracken et al, "Atomically referenced 1-GHz optical parametric oscillator frequency comb," Opt. Express (in press) Direct comb stabilisation to an atomically-referenced cw laser demands Needs much less average power, allowing OPO to tune further CEO stabilisation to 1.6 rad (pump) and 3.4 rad (OPO) i.e. noisier than self-referencing nm lock

28 OPO comb concepts Singly-resonant 1-GHz optical parametric oscillator frequency comb 1-GHz Ti:sapphire laser f-to-2f reference Heterodyne locking OPO SHG SFM Doubly-resonant 1-GHz optical parametric oscillator frequency comb 1-GHz Ti:sapphire laser f-to-2f reference Dither locking + heterodyne test OPO SHG SFM Doubly-resonant 10-GHz optical parametric oscillator frequency comb 10-GHz Ti:sapphire laser Rb-ECDL reference Dither locking OPO SHG SFM

29 Degenerate 1-GHz OPO for astronomy OPO behaves like an optical frequency divider one pump mode Maintains frequency comb structure of high quality Ti:sapphire 1-GHz pump comb Two outputs, with the signal typically resonant w idler signal w/2 29

30 Degenerate 1-GHz OPO for astronomy OPO behaves like an optical frequency divider one pump mode Maintains frequency comb structure of high quality Ti:sapphire 1-GHz pump comb Two outputs, with the signal typically resonant w idler signal w/2 30

31 Degenerate 1-GHz OPO for astronomy Degenerate OPO mirror coating is centered at half the pump frequency one pump mode Signal and idler merge into one pulse Intrinsically broadband and smooth spectrum w Low oscillation threshold and high efficiency High starting mode spacing (1 GHz) relaxes performance needed from Fabry-Pérot filter cavity Requires active stabilisation of OPO cavity idler signal Δτ=0 w/2 1-GHz Ti:sapphire laser f-to-2f locking Dither locking + heterodyne test OPO Max bandwidth is limited by nonlinear group delay l dispersion conversion Edges of signal/idler pulse must stay in sync to achieve gain 31

32 Doubly-resonant degenerate 1-GHz OPO comb Five-cycle optical pulse generated at 1.6 µm Duration 27 fs and bandwidth 145 nm (full coverage from µm) Self-referenced locking of the pump laser leaves plenty power for OPO Still requires filtering for comb modes to be resolved 50 mm

33 OPO comb concepts Singly-resonant 1-GHz optical parametric oscillator frequency comb 1-GHz Ti:sapphire laser f-to-2f reference Heterodyne locking OPO SHG SFM Doubly-resonant 1-GHz optical parametric oscillator frequency comb 1-GHz Ti:sapphire laser f-to-2f reference Dither locking + heterodyne test OPO SHG SFM Doubly-resonant 10-GHz optical parametric oscillator frequency comb 10-GHz Ti:sapphire laser Rb-ECDL reference Dither locking OPO SHG SFM

34 Summary Degenerate OPO combs provide: Low pump thresholds, compatible with direct 10-GHz pumping Removes (or at least reduces) the dependence on filter cavities Eliminates sidebands Broadband outputs Short pulses, compatible with implementing further coherent nonlinear broadening Locked OPO comb is produced when pumped by another locked comb Limitations? Locked linewidth remains to be fully investigated Not readily tunable

35 Outline Motivation HIRES calibration requirements Introduction to OPO frequency comb technology Concepts and operating principles Comb-offset control in a fs OPO Recent results Frequency combs tunable across one octave of bandwidth OPO frequency combs at 1 GHz and above Precision metrology and stabilisation

36 Benchmarked the stability and metrology performance of a 333-MHz OPO comb against a 250-MHz Menlo Systems fibre comb at 1.5 µm Institut fédéral de métrologie METAS Characterized noise, stability and linewidth for f rep, f CEO and n opt Metrology demonstration (absolute optical frequency measurement) Two campaigns, in March and April 2015 HWU OPO frequency comb UniNE ultra-stable optical cavity laser Frequency metrology with an OPO comb UniNE Thomas Südmeyer Stéphane Schilt Valentin Wittwer Pierre Brochard Nayara Jornod HWU Karolis Balskus Laser Quantum Albrecht Bartels Tobias Ploetzing

37 Metrology results Parameter OPO measurement Parameter 1 OPO measurement 2 Menlo comb measurement f rep [Hz] f rep [Hz] ± ± ± ± f ceo [Hz] f ceo [Hz] ± ± ± ± f beat [Hz] f beat [Hz] ± ± ± ± N (calculated) N (calculated) ( ) ( ) ( ) ( ) Measured frequency [Hz] Measured frequency ± [Hz] ± ± ± Theoretical value [Hz] Theoretical value [Hz] Frequency offset [Hz] Frequency offset [Hz] OPO and Menlo Er:fibre comb measurements in agreement within uncertainty margin Difference between the measured frequencies arises from different settings in the lock of the cw laser to the Rb transition and from etalon fringes occurring in the laser setup Short-term noise on the measured optical frequency at the khz level, mainly limited by the MHz linewidth of the cw laser OPO measurement 2 conducted under conditions of better cw laser stability

38 Summary Performance characterisation of 333-MHz OPO comb showed it to be very comparable to near-ir Menlo Systems fibre comb in its stability Equivalent accuracy observed in an absolute metrology measurement Limitations of the measurement ultimately determined by residual noise in synthesizers noise on the 532-nm laser used to pump the Ti:sapphire laser bandwidth of the OPO f CEO lock

39 Conclusions OPO combs provide wavelengths which are (at best) marginal from existing laser combs can reach 1 GHz and potentially up to 10 GHz directly, without F-P filtering, with potential advantages of lower noise and better sideband suppression Ongoing research attempting to demonstrate multi-ghz comb operation over an extended wavelength range Doubly-resonant OPO architecture expected to enable 10-GHz operation Direct stabilisation to a Rb optical transition will ensure traceability of comb line frequencies, without f-to-2f stabilisation Applications beyond spectrograph calibration such as dual-comb spectroscopy Acknowledgements Dr. Zhaowei Zhang, Dr. Karolis Balskus, Stuart Leitch.