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All-Fiber Frequency-Agile Dual Comb Spectroscopy

with High Sensitivity

Debanuj Chatterjee,

PhLAM, University of Lille, France

​

Vanguard Seminar, Rabat, Morocco

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All-Fiber Frequency-Agile Dual Comb Spectroscopy

with High Sensitivity

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Motivation : Optical Frequency Comb

What is an optical frequency comb?

  • Equally spaced frequencies
  • Ruler of light
  • Pulse train in time

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How can we use it?

  • Detect chemicals
  • Chemical composition of astronomical objects
  • Multiplexing of data for optical communication
  • Analog signal distribution (radar signals)

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Is it a big deal?

  • Theodor Hӓnsch and John Hall received half of the Nobel Prize in Physics in 2005
  • Contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique

(source : nobelprize.org)

(source : Victor Torres-Company et al, Laser Photonics Rev. 2014)

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All-Fiber Frequency-Agile Dual Comb Spectroscopy

with High Sensitivity

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Spectroscopy : Principle

(Nathalie Picqué et al, Nature Photonics, 2019)

Dual comb spectroscopy

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All-Fiber Frequency-Agile Dual Comb Spectroscopy

with High Sensitivity

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Dual Comb Spectroscopy : Principle

Time domain

(Takuro Ideguchi, Optics & Photonics News, 2017)

Frequency domain

Vernier effect

f1

f2=f1+Δf

Δf

fAOM

1/f1

1/f2

Optical

RF

Optical

RF

Pay attention to scales!

  • fc ~ 192 THz (optical carrier)
  • f1, f2 ~ 500 MHz
  • Δf ~ 50 kHz
  • fAOM ~ 100 MHz

fc

fAOM

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All-Fiber Frequency-Agile Dual Comb Spectroscopy

with High Sensitivity

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All Fiber vs Free Space

  • Mirror alignment is time consuming
  • Susceptible to vibrations
  • Bulky

Free space

All fiber

  • Limited to near infrared
  • Compact
  • No alignment

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All-Fiber Frequency-Agile Dual Comb Spectroscopy

with High Sensitivity

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Electro-Optic Frequency Combs

Agile

Input

Output

frequency

frequency

Mach-Zehnder modulator

Electro-optic crystal (Pockel’s effect)

Agile EO comb!

Time

RF voltage

T

1/T

MZM transfer function

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All-Fiber Frequency-Agile Dual Comb Spectroscopy

with High Sensitivity

  • Configuration (symmetric/asymmetric)
  • Broadband
  • Something else

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Asymmetric vs Symmetric Dual Comb Spectroscopy

Asymmetric/Dispersive Configuration

Symmetric/Collinear Configuration

  • Only one comb passes through the sample
  • Both amplitude and phase is recovered
  • Amplitude sensitivity weaker by 3 dB
  • Both combs pass through the sample
  • Only amplitude is recovered
  • Amplitude sensitivity is stronger by 3 dB

We focus on symmetric/collinear configuration

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All-Fiber Frequency-Agile Dual Comb Spectroscopy

with High Sensitivity

  • Configuration (symmetric/asymmetric)
  • Broadband
  • Something else

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Spectral Broadening

  • Frequency spectrum broadened with a nonlinear medium
  • Thanks to four wave mixing

Sample’s absorption

Cascaded four wave mixing

Nonlinear medium

Nonlinear medium

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Partial Summary

Basics of Dual Comb Spectroscopy

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All-fiber dual comb source

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Symmetric configuration (more sensitive amplitude detection)

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EOM based comb with frequency agility

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What can we do better?

Agile

Time

RF voltage

In

Out

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All-Fiber Frequency-Agile Dual Comb Spectroscopy

with High Sensitivity

  • Configuration (symmetric/asymmetric)
  • Broadband
  • Signal to noise ratio (SNR)

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Signal-to-Noise Ratio

SNR = Signal power/noise floor level

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Frequency

Power (dBm)

Signal

Noise floor

SNR

How to increase the SNR?

Increase the number of ifg pulses in the time domain

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SNR Improvement Strategy

Unused space

(Yu Zhang et al, Optics Letters, 2021)

Strategy : Increase Δf

(to pack more ifg pulses)

1/Δf

Time domain interferogram

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SNR Improvement Strategy

Strategy : Increase Δf

1/Δf

Time domain interferogram

But, there is a problem…

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AOM Shift

Frequency domain

f1

f2=f1+Δf

Δf

fAOM

Pay attention to scales!

  • fc ~ 192 THz (optical carrier)
  • f1, f2 ~ 500 MHz
  • fAOM ~ 100 MHz (~f1/4)
  • Δf ~ 50 kHz

zoom

zoom

fc

fAOM

 

fAOM

Optical

RF

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Problem of RF Spectral Aliasing

f1

Strategy

f1

f

f

RF spectrum

f1/2

f1/2

f1/4

f1/4

Available spectral width

aliasing

fAOM

[Constant number of comb lines in both case]

LPF

0

0

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SNR Improvement Strategy

Strategy : Spectral filtering

(to shape the comb and increase effective Δf)

Standard

Spectral filtering

(Akiko Nishiyama, Optics Express, 2017)

(Nazanin Hoghooghi, Applied Physics B, 2021)

  • Avoids aliasing
  • Filter induces losses
  • Complicated to implement

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SNR Improvement Strategy

Numerical simulation of time multiplexing with EOM combs

Comb 1

Comb 2

(standard)

Comb 2

(multiplexed)

standard

multiplexed

Phase shift

Strategy : Time domain shaping of optical pulses (time multiplexing)

Δf=1 MHz

f1=500 MHz

f2=499 MHz

CW source

EOM

EOM comb

RF pulse

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SNR Improvement Strategy

Numerical simulation of time multiplexing

SNR increase

  • SNR improved / Faster acquisition speed (for given SNR)
  • Aliasing avoided
  • Spectral resolution is reduced

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Experimental Scheme

50%

50%

Arm1

Arm 2

100 MHz

f1 = 500 MHz

f2 = 500 MHz – 50 kHz

Laser

AOM

IM

IM

Common clock

AWG

AWG

PD

PD

DCF

50%

50%

Channel 1

Channel 2

Oscilloscope

AWG : arbitrary waveform generator, AOM : acousto optic modulator, EDFA : Erbium doped fiber amplifier, DCF : dual core fiber (nonlinear), IM : intensity modulator, PD : phorodetector, WS : waveshaper, VOA : variable optical attenuator

EDFA

EDFA

VOA

WS

Multiplexing

Lorentzian filter

(50 GHz FWHM)

Time domain ifg (1.4 multiplexed)

Spectral broadening

50%

50%

Reference

multiplexed

standard

20 μs

14.3 μs

80 ps pulsewidth

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SNR improvement / Faster Acquisition

  • Multiplexed case provides a larger SNR (for same acquisition time)
  • Multiplexed case provides a faster acquisition (for same SNR)
  • Multiplexed case show extra spiking features due to a sinc envelope

Time domain ifg (mul=1.4)

Frequency domain ifg (mul=1.4)

standard

multiplexed

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Theoretical Analysis

 

0

t

3T

6T

-3T

-6T

 

 

0

t

3T

6T

-3T

-6T

S(t)=P(t)

P(t)

G(t)

w

0

t

3T

6T

-3T

-6T

L(t)=P(t).G(t)

0

t

3T

-3T

S(t)

 

 

Standard

Multiplexed

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Theoretical Analysis

  • For the simplest case of w=T, the multiplexed case gives the Fourier domain signal for the standard case
  • For the multiplexed case, we see a dense Dirac comb, with a frequency spacing of 1/w (smaller than 1/T), which is further shaped by a sinc like envelope

0

t

3T

-3T

S(t)

 

 

Multiplexed

 

For w=T,

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11.5 dB

SNR improvement with Scan of Multiplexing

  • Multiplexed case provides a larger SNR
  • For multiplexing rate 3.3, about 11.5 dB increase in SNR
  • Experiment matches theory

Standard

Part of frequency domain interferogram spectrum

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Proof of Concept for Time Multiplexed DCS

f1=500 MHz

Δf=50 kHz

Lorentzian filter

No filter

Frequency domain ifg

Comb 1

Comb 2

PD

PD

Filter

VOA

50%

50%

50%

50%

Reference

FT

FT

Attenuation

Frequency

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Proof of Concept for Time Multiplexed DCS

  • f1=500 MHz, Δf=50 kHz
  • Filter shape reconstructed for standard and multiplexed case
  • Residue (Experiment-Truth) is lower for multiplexed case
  • Larger SNR / faster acquisition for multiplexed case

Lorentzian filter

No filter

Frequency domain ifg

Waveshaper filter reconstruction

Time domain ifg

multiplexed

standard

residue

Data

processing

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Single Shot Measurements with Multiplexing

Multiplexed

Standard

Waveshaper reconstruction

Ifg freq (no absorption)

Ifg freq (absorption)

Ifg time (no absorption)

Ifg time (absorption)

Ifg peak missed for std case

  • Standard case, ifg peak (time domain) is missed, we cannot reconstruct the waveshaper function
  • Multiplexed case, we get the waveshaper function since there is always an ifg peak
  • Delta frep=2.5 kHz (ifg beating period for standard case=400 micro sec)
  • Multiplexing = 3.33 (ifg beating period for multiplexed case=121 micro sec)
  • Comparison : 1. standard case (blue) 2. multiplexed case (mul=3.3) (red)
  • Waveshaper : Lorentzian profile with 10 dB attenuation, 50 GHz FWHM
  • Oscilloscope : 1 G samples/s, 200 k pts, total recording time : 200 micro sec (less than standard beating period, but more than multiplexed beating period)

Lorentzian shape

detected

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Conclusion

Conclusion

  • EOM-based, all-fiber, frequency-agile DCS
  • Temporal folding or time multiplexing for larger SNR
  • Demonstration of proof-of-concept of the time multiplexed DCS
  • Striking improvement for single shot measurement

All-Fiber Frequency-Agile Dual Comb Spectroscopy

with High Sensitivity

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Perspectives

Multidimensional spectroscopy with multicore fibers

Compressive sensing : hybrid approach

Fast sensing in engines

(Eve-Line Bancel et al, Nature Communications, 2023)

(Anthony Draper et al, Optics Express, 2019)

(Akira Kawai et al, Scientific Reports, 2021)

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Acknowledgements

Nonlinear optics group at PhLAM

Matteo Conforti

(faculty)

Arnaud Mussot

(group leader)

Francesco Tani

(faculty)

Siddharth Sivankutty

(faculty)

Thomas Bunel

(PhD student)

Eve-line Bancel

(PhD student)

Stefano Negrini

(PhD student)

Debanuj Chatterjee

(Postdoc)

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Thank You!