Using GNSS for optical frequency and wavelength measurements Stephen Lea, Guilong Huang, Helen Margolis, and Patrick Gill National Physical Laboratory Teddington, Middlesex TW11 0LW, UK
outline of talk introduction to femtosecond combs optical frequency metrology revolutionised enabling technology for optical clocks brief summary of work at NPL wide variety of optical standards measured H-maser/caesium fountain reference world-leading optical frequency measurement GPS referenced transportable comb compact femtosecond laser reference frequency from GPS-disciplined LO
mode-locked laser comb δ - function FT t f train of δ - fns τ R FT f R t f
carrier envelope offset frequency time domain E(t) φ 2 φ t frequency domain I(f) f 0 1/ f R = τ R 2πf 0 = φ f R f R f f n =nf R + f 0
measuring the offset frequency f 0 SHG f beat f mode m 1 mode m 2 =2m1 m 1 -th mode: f 1 = m 1 f R + f 0 SHG: 2f 1 = 2m 1 f R + 2f 0 m 2 -th mode: f 2 = m 2 f R + f 0 m 2 = 2m 1 : f beat = f 0
measuring an optical frequency f 0 stabilised laser ν opt mode m f f R Cs primary standard (SI Hertz) H-maser (short-term stability) f rf synthesisers and counters ν opt =mf R + f 0 +f
femtosecond laser mode-locked Ti:sapphire laser tilt dispersion compensation prisms pump light translation power/au wavelength domain: 30 nm FWHM time domain: 30 fs pulse 770 820 870 wavelength/nm
repetition rate stabilisation mode-locked Ti:sapphire laser tilt dispersion compensation prisms pump light translation 10 db Σpf R 0 1 2 rf frequency/ghz
octave span comb mode-locked Ti:sapphire laser microstructure fibre tilt dispersion compensation prisms pump light translation f R
carrier offset frequency stabilisation mode-locked Ti:sapphire laser tilt dispersion compensation prisms translation microstructure fibre pump light f R f 0 SHG ~1060 nm ~530 nm carrier envelope offset frequency f0 (not servoed) delay line grating λ/2 PBS
optical frequency measurement mode-locked Ti:sapphire laser tilt dispersion compensation prisms translation f R microstructure fibre f 0 SHG ~1060 nm ~530 nm pump light PBS λ/2 from optical standard grating λ/2 PBS delay line grating f
Ti:sapphire-based comb the most intensively used femtosecond comb in the world? measurements of over half-a-dozen stabilized laser systems ranging from optical clock transitions in single cold trapped ions Sr + and Yb + to stabilized 88 MHz repetition rate yellow HeNe comb span ~ 500 1100 nm H-maser/Cs fountain reference breadboard system; water cooling
frequency calibrations in addition to measuring in-house standards, we have calibrated laser systems for users in academia and industry scientific laser system iodine-stabilised diode laser at 656 nm from the University of Sussex, for high-precision spectroscopy of He + industrial laser stabilised yellow HeNe at 594 nm
transportable fs comb off-site fs comb measurements lower accuracy often adequate (cf. HeNe) possibility of comb comparisons mode-locked Cr:LiSAF laser comb spectrum centred ~ 850 nm diode-pumped at 670 nm, 500 mw compact configuration possible (laser under development) GPS referencing 10 MHz reference frequency from GPS-disciplined LO in lieu of H-maser
GPS-referenced measurements tests with Ti:sapphire-based comb referenced to 10 MHz local oscillator (LO) steered by GPS compare optical frequency measurements using a GPS-disciplined standard quartz LO and a GPS-disciplined rubidium oscillator steering a low-phasenoise quartz LO
GPS-referenced measurements fractional frequency offset/10 10 2 1 0-1 -2 frequency measurement 10 khz 674 nm ECDL stabilised to ULE cavity log[σy(τ)] -11-12 -13 Allan deviation standard quartz rubidium + low-phase-noise quartz rubidium 0 3600 7200 10800 time/s 1 2 3 4 5 log[τ/s]
GPS-referenced measurements fractional frequency offset/10 11 2 1 0-1 -2 frequency measurement 1 khz 674 nm ECDL stabilised to ULE cavity log[σy(τ)] -11-12 -13 Allan deviation standard quartz rubidium + low-phase-noise quartz rubidium 0 3600 7200 10800 time/s 1 2 3 4 5 log[τ/s]
GPS-referenced measurements fractional frequency offset/10 11 2 1 0-1 -2 frequency measurement 1 khz 674 nm ECDL stabilised to ULE cavity 0 3600 7200 10800 time/s low-phase-noise quartz LO gives adequate short-term stability for comb measurements 1 x 10-12 stability @ 1000 s offset [GPS - H-maser] < 5 x 10-12 over 1 hour averaging time compare 1 x 10-10 (50 khz) limit of interferometric measurements
Summary H-maser/Cs referenced comb optical frequency measurement revolutionized enabling technology for representation of the SI second using optical frequency standards future redefinition of the second? GPS referenced transportable comb demonstrated: u ~ few khz with GPS referenced comb outlook: compact, diode-pumped Cr:LiSAF fs laser acknowledgements: thanks to Philip Baker and Alan Turner (Rapco Electronics Ltd); this work is funded by the DTI as part of the NMS Length Programme