Frequency comb spectroscopy Frequency combs evolutionary tree Overview

Frequency comb spectroscopy

Frequency combs – evolutionary tree

Overview • Basic Features: Frequency Combs • Applications: – Frequency Metrology • Measuring Frequency Gaps • Frequency Combs as Optical Synthesizers – Time Domain Applicatons • Stabilizing the Carrier Envelope Phase – Comb Spectroscopies

1. Basic Features: Frequency Combs 4

Ultra short pulse train(I) L T=2 L/vg fr=1/T S. T. Cundiff , J. Phys. D 35, R 43 (2002)

Ultra short pulse train(II) Time domain E(t) Δφ 2Δφ t T

Frequency comb(I) For a coherent pulse train including N pulses Fourier transform

Frequency comb(II)

Frequency comb(III) Time domain E(t) Δφ 2Δφ t T Frequency domain I(f) fo fr=1/T f

Self reference I(f) 2 n n nfr+fo x 2 f 2(nfr+fo ) 2 nfr+fo fo = 2(nfr + fo ) – (2 nfr + fo ) Phys. Rev. Lett. 84, 5102 (2000) & Phys. Rev. Lett. 84, 3232 (2000)

Generating an Octave Spanning Comb Photonic crystal fiber: William Wadsworth Jonathan Knight Tim Birks Phillip Russell U. of Bath England Note: if fiber does the same for all the pulses, the field stays stricly periodic. This property is the only one necessarily to derive fn=nfr+fo

f to 2 f interferometer Grating fo SHG dichroic beam splitter fr λ/2 PBS λ/2 Photonic fiber λ/2 Ti: Sapphire fs laser

f to 2 f interferometer

Monolithic carrier-envelope phase-stabilization scheme T. Fuji, et al, Opt. Lett. 30, 332 (2005).

Offset Frequency and Repetition Rate Carrier envelope offset frequency Repetition rate

2. Applications: Frequency Metrology Time Domain Applicatons Comb Spectroscopies

Optical Frequency Synthesizer locked to a RF frequency standard ω n = n ω r + ω CE every mode can be used for optical frequency measurement a million stabilized lasers in a single beam!

The Comb as a Ruler in Frequency Space

Time Domain application Time domain E(t) Δφ 2Δφ t T Oscillator CE phase stabilization How about amplifier?

Frequency comb spectroscopies • Broadband, multi-targeting • Fast • Resolution • Single comb power low ( n. W~ µW range) - Dual frequency comb spectroscopy - Freqency comb vernier spectroscopy

Dual frequency comb spectroscopy Time domain For two frequency combs Frequency domain The down conversion factor is

Near-IR Dual comb spectroscopy Fiber comb 2 Mirror(fr 2 ) RF out PZT HV amplifier λ/4 λ/2 Lowpass filter Gas cell PI controller Variable Local Oscillator 370 Hz ~5 k. Hz(δ) Rb Standard GPS Lowpass filter Grating Mixer λ/2 Mixer 250 MHz Synthesizer PI controller HV amplifier PBS PD Mixer Lowpass filter PZT λ/4 λ/2 Mirror(fr 1) RF out Fiber comb 1 Lowpass filter Data acquisition and Fourier transform

Single Interferogram (Yellow) and FFT spectra (Red) of Reference and absorption spectra in the interest spectral region.

Mid-IR Dual comb spectroscopy with a multipass cell Data acquisition and Fourier transform MIR comb 2 MCT BS Multipass cell BS CH 4 gas cell Red laser MIR comb 1 Experimental setup includes two DFG MIR frequency comb lasers, optics, multipass chamber, a MCT detector with electronics.

Frequency comb vernier spectroscopy C. Gohle, et al PRL 99, 263902 (2007)

Frequency comb vernier spectroscopy M-fiber (Menlo Systems) 250 MHz Repetition rate ~400 m. W 1. 5~1. 7 μm ~90 fs PZT Grating Galvo

Frequency comb vernier spectroscopy: image processing