1. 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

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

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

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

5. 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

6. 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.

7. 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

8. 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)

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

10. 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: < 1s
Phase noise: < 50 mrad (100 Hz – 3 MHz)

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

12. Optical reference for Dfrep
194 THz
FCs share same frequency at two spectral locations
Dfrep ~ 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.

13. Optical reference for Dfrep
194 THz
FCs share same offset at two spectral locations
Dfrep ~ 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)

14. 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

15. 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

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

17. 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

18. 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

19. 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

20. 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

21. 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.

22. 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

23. Thank you …
… for your attention
for your support

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