ISMS 2017 MK02 High-resolution dual-comb spectroscopy with ultra-low noise frequency combs W. Hänsel1, Michele Giunta1,2 , K. Beha1, A. Perry1, R. Holzwarth 1,2 Menlo Systems GmbH, Munich, Germany Max-Planck Institute for Quantum Optics, Garching, Germany Max Planck Institute of Quantum Optics

Outline: Dual-Comb Spectroscopy Motivation Principle of DCS ULN frequency comb DCS with ULN frequency comb Conclusion ν

Ted Hänsch 1978 - 1998 Paradigm shift! Frequency combs Ted Hänsch 1978 - 1998 Paradigm shift! wopt=N wrep + w0

Comb: Swiss Knife for metrology Frequency Comb Time (as) Length (fm) Frequency (PHz)

Dual Combs: Principle Optical spectum mapped to RF I(n) frep frep+d I(n) Optical frequency n Down-conversion factor d/frep I(f) Radio frequency d 2d f

Principle of dual-comb spectroscopy 𝑇 𝑠𝑖𝑛𝑔𝑙𝑒 =1/Dfrep FFT Optical unambiguity range: ∆𝑓 𝑜𝑝𝑡 = 𝑓 𝑟𝑒𝑝 2 /(2 ∆𝑓 𝑟𝑒𝑝 ) Speed: 𝑇 𝑠𝑐𝑎𝑛,𝑠𝑖𝑛𝑔𝑙𝑒 =1/ ∆𝑓 𝑟𝑒𝑝 Resolution: ∆𝑓 𝑟𝑒𝑠 = 𝑓 𝑟𝑒𝑝 1/ 𝑇 𝑠𝑐𝑎𝑛 = 𝑛 𝑠𝑐𝑎𝑛 ×∆𝑓 𝑟𝑒𝑝 single-shot scanning Three important parameters for DCS: Optical unambiguity, scanning speed, resolution. A high repetition rate helps to increase the optical unambiguity range at reasonable Df_rep (due to the square of f_rep). This outweighs the fact that several scans with different comb teeth positions need to be performed at higher repetition rate. Technical challenge: low dfRep. Example: fRep = 250 MHz, dfRep=321.5 Hz gives 95 THz (3100 wavenumbers) optical coverage. You can use adaptive sampling (Picquet-Hänsch-Paper), however, a stable comb is better, otherwise the comb jitter may smear out the features of interest.

Classic FTIR vs. dual combs Frequency Comb 1 Down-conversion factor d/frep Frequency Comb 2 Detector Down-conversion factor v/c Light source Detector

Dual combs in 2004 800nm 30 THz Ti:S laser GaSe 125 MHz Ti:S laser 125 MHz + D sample First demonstration in 2004: F. Keilmann, C. Gohle, R. Holzwarth, Opt. Lett. 29, 1542 (2004)

Today: fiber lasers comb fs laser fs lasers: nothing to be afraid of

Ultra-low noise frequency comb Hänsel et al, Appl. Phys. B, 123:41 (2016) Pump Beam splitter Er:NALM CEO-actuator new CEO actuator  ultra-low noise fCEO Bandwidth: Dl = 40 nm Repetition Rate: frep = 250 MHz Single-tooth line width: < 1Hz Instability: < 10-16 @ 1s Phase noise: < 50 mrad (100 Hz – 3 MHz)

Full optical comb-stabilization Sub Hz line width reference laser Act. 1 Act. 2 ν fCEO lock Optical lock Hz line width for Offset

Optical reference for Dfrep 194 THz FCs share same frequency at two spectral locations Dfrep ~ 321.5 Hz n1 Amplitude comb 1 Amplitude comb 2 n1-1 The two frequency combs share the same offset frequency (fCEO) and two modes of different mode number coincide inthe optical domain. This makes the interferogram of the DCS strictly repetitive.

Optical reference for Dfrep 194 THz FCs share same offset at two spectral locations Dfrep ~ 321.5 Hz n1 Amplitude comb 1 Amplitude comb 2 n1-1 It is not useful, to have the unamiguity range end where the optical lock is placed. Therefore, we add an offset to one of the frequency coms. (We have chosen different offsets, but illustration is better like this)

Ultra-low noise CEO frequency Beat notes are resolution limited: RF-spectrum of fCEO (RBW = 1Hz) For those who like it: spectrum with resolution bandwidth of

Schematic setup sample BPD: balanced photo detection Frequency Comb 1 AD-converter (14 bit) BPD 50/50 Frequency Comb 2 PBS Acetylen cell (20 torr) Faraday- Mirror sample References for ULN comb 1) Delay display of branch for update clock; 2) Add cw-laser for optical lock BPD: balanced photo detection PBS: polarizing beam splitter

Visualization of the comb structure FFT of time trace covering 5 interferograms Each RF-peak represents a pair of optical comb lines

Comb spectrum in the RF domain absorption dips This trace shows the peaks of the RF comb as connected points. The reference (cyan) has been optained by Fourier-filtering the signal trace. There may be slight artefacts from this. Also, broad signals will be eliminated by this process. One trace with and w/o spectroscopic cell would be more elegant. Only the comb peaks are visualized

Absorption spectrum of acetylene P-branch R-branch P(9) Overview over acetylene absorption. The red data is taken from spectraplot.com with p=20 torr, T=20°C, L=11cm (back and forth through cell). Cell is from „wavelengthreferences.com“ Avg. 64 inteferograms, T = 200 ms

Absorption spectrum of acetylene The close-ups show the highest peak of the P branch and a low side peak, to viusalize sensitivity. P(9) 3.5 0.1

Phase spectrum of acetylene P-branch P(9) R-branch Optical phase delay from the acetylene absorption. Again, the reference phase has been optained by Fourier-filtering the signal trace. There are artefacts from this. Close-up Avg. 64 inteferograms, T = 200 ms

Phase spectrum of acetylene Optical phase delay from the acetylene absorption. Again, the reference phase has been optained by Fourier-filtering the signal trace. There are artefacts from this.

Conclusion Dual comb spectroscopy with ultra-low noise combs High signal-to-noise ratio Well defined peaks in the optical domain Low complexity in signal analysis (no adaptive sampling necessary) High flexibility in choice of sampling parameters

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TO do: Integriertes Phasenrauschen hinzufügen, Zusammenfassung, Bild ELMO-Seeder, Bild ULN