Mid-infrared multiheterodyne spectroscopy using quantum and interband cascade lasers

Jonas Westberg1*, Lukasz Sterczewski1, Link Patrick1, and Gerard Wysocki11Department of Electrical Engineering, Princeton University, Princeton N.J. 08544

*email: westberg@princeton.edu

PrincetonUniversity

Laser

Sensing

Laboratorypulse.princeton.edu

October, 2016

Motivation

Acknowledgments

Future work

Results

Experimental methods and procedures

Conclusions

• Broadband, non-invasive trace gas sensing with multimode Fabry-Pérot Quantum Cascade Lasers (QCLs) or Interband Cascade Lasers (ICLs).

• Direct down-conversion of optical modes to multiheterodyne beat notes.

• Separate access to individual radio-frequency (RF) beat notes for broadband measurement.

• Conventional (wavelength modulation) and advanced modulation techniques (e.g. Faraday modulation) to increase sensitivity of multiheterodyne beat note spectroscopy.

3060 3070 3080 3090 3100 3110 3120

LO

Sig. sample cell+

-

FSRsig

νopt.

-1

-2+1

+2

0

-3

νopt.

0

-1-2

-3

+1

+2

FSRLO

sig. det.

νopt.

-1

-2+1

+2

0

-3

νRF

νRF

absorption dispersion

+

-

50 mA 3V

νopt.

-1

-2+1

+2

0

-3

20 GS/s

ICL1

ICL2

FTIR

50 mA 3V

a

*

cm-1

*

b

Locking procedure

Frequency discriminator

log PID

28-46 MHz

RFToptica

mFALC 110LO

RF

40 MHz

OPLL

Sam

ple

sig

nal

LO

Sig.sample cell

absorption dispersion

QCL1

QCL2

Reference signal

Dis

crim

inat

or

volt

age

(V)

70 mV/MHz

Locking stability

Figure a: Experimental schematic of the multiheterodyne setup. The light sources in this particular example are a pair of ICLs operating around 3.2 μm. The setup is based on the conventional dual-comb spectroscopy configuration.

Figure b: FITR spectra of the signal and local oscillator multi-mode lasers.

115 120 125

-80

-60

-40

-20

2 MHz

2 kHz†

720 725 730 735

-70

-60

-50

-40 *

Frequency (MHz)

0 200 400 600 800

-80

-70

-60

-50

-40

-30

-20

*†

Pow

er

(dB

)

Frequency (MHz)

Offset and repetition rate corrected

-80

-70

-60

-50

-40

-30

-20

Pow

er

(dB

)

Offset-corrected†

*

-80

-70

-60

-50

-40

-30

-20

Pow

er

(dB

)

Raw spectrum†

*

3108 3110 3112 3114 3116

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Tra

nsm

issio

n

Wavenumber (cm-1)

MH Measurement

HITRAN fit

3110 3112 3114 3116

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Tra

nsm

issio

n

Wavenumber (cm-1)

MH Measurement

HITRAN fit

-0.2 -0.1 0.0 0.1 0.2 0.340

80

120

160

200

Am

plit

ude (

a.u

.)

Time (ms)

MH DAS

Ref.-corr.

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

1 s averaging

Detuning (GHz)

0.2

0.4

0.6

0.8

1.0

Tra

nsm

issio

n

-0.2 -0.1 0.0 0.1 0.2 0.3-40

-20

0

20

40 1 s averaging

Phase (

°)

Time (ms)

MH DS meas.

Neighbor-corr.

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

Detuning (GHz)

20 μs acq. time

Frequency [MHz]

Dete

cto

r sig

nal [V

] 0 -1+1 -2

+2-3 +3 -4 +4

-5 +5 -6 +6-7

+7-8 +8

-9 +9-10 +10 -11

FWHM 1.9 MHz

QCL RF beat note spectrum

• Multiheterodyne spectroscopy using FP-QCLs and ICLs

• Intermutual frequency stabilization/locking• Frequency discriminator (low cost)• OPLL

• Single mode detection• swept direct absorption (ramp rate 1 kHz)• wavelength modulation (fm = 10 kHz, 2f lock-in

det.)• dispersion spectroscopy (ramp rate 1 kHz)

• Multimode (broadband) detection• 25 cm-1 coverage• Rapid response time (10 μs)

• Fully electronically controlled• Seamless switching between detection modes

• Opto-mechanically simple setup• Complexity on the detection/signal

processing side

• Phase and timing correction.

• Post-processing algorithm to improve SNR and enable coherent averaging.

• ICL multiheterodyne spectroscopy of ethylene and methane.

• Swept absorption and dispersion measurements at 1 kHz ramp rate.

• Short acquisition times (20 μs) for broadband measurements.

• QCL multiheterodyne measurements of N2O at low pressure

• absorption

• dispersion

• QCL multiheterodyne measurement of N2O at low pressure using 2fwavelength modulation.

• Broadband QCL multiheterodyne measurement of isobutane.

• 10 μs acquisition time.

• 25 cm-1 spectral coverage.

ref. det.

νRF

νRF

RSA

RSA

40 MHz

40 MHz

phase det.

P(t

)

t

P(t

)

oscilloscope

t

sig. det.

• Dispersion engineering of devices to expand stable bias and temperature regions.

• Explore THz QCL multiheterodyne spectroscopy.

• Coherent averaging to increase sensitivity for longer acquisition times.

• Broadband measurements ( 100 cm-1)• Rapid response time ( μs)• High resolution ( MHz)

Challenges:

The authors gratefully acknowledge F. Capasso, L. Diehl, M. Troccoli for providing FP-QCLs, and J. Meyerfrom the Naval Research Laboratory for providing the FP-ICLs used in this study.

The work is supported by • DARPA SCOUT program (W31P4Q161001), • U.S. Environmental Protection Agency (U.S.

EPA), RD-83513701-0 • National Science Foundation (NSF ERC

MIRTHE), EEC-0540832.