ArticlePDF Available

Sensitive Absorption Spectroscopy with a Room-temperature Distributed-feedback Quantum-cascade Laser

Optics Letters

March 1998
23(3):219-21

DOI:10.1364/OL.23.000219

Source
PubMed

Authors:

Edward Anthony Whittaker

Stevens Institute of Technology

Jérôme Faist

ETH Zurich

Show all 8 authorsHide

We report what we believe are the first spectroscopic measurements to be made with a room-temperature quantum-cascade distributed-feedback laser. Using wavelength modulation spectroscopy, we detected N(2)O and CH(4) in the chemical fingerprint wavelength range near 8microm . The noise equivalent absorbance for our measurement was 5 parts in 10(5), limited by excess amplitude modulation on the laser output, which corresponds to a 1-Hz bandwidth detection limit of 250 parts N(2)O in 10(9) parts N(2) in a 1-m path length.

…

WM signal versus laser frequency for 10% diluted samples of N 2 O and CH 4 with corresponding HITRAN data plotted below.

…

Figures - uploaded by Jérôme Faist

Content may be subject to copyright.

Content uploaded by Jérôme Faist

Content may be subject to copyright.

Content uploaded by Edward Anthony Whittaker

Content may be subject to copyright.

February 1, 1998 / Vol. 23, No. 3 / OPTICS LETTERS 219

Sensitive absorption spectroscopy with a room-temperature

distributed-feedback quantum-cascade laser

K. Namjou, S. Cai, and E. A. Whittaker

Department of Physics and Engineering Physics, Stevens Institute of Technology, Hoboken, New Jersey 07030

J. Faist, C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho

Bell Laboratories, Lucent Technologies, 700 Mountain Avenue, Murray Hill, New Jersey 07974

Received September 18, 1997

We report what we believe are the first spectroscopic measurements to be made with a room-temperature

quantum-cascade distributed-feedback laser. Using wavelength modulation spectroscopy, we detected N2O

and CH4in the chemical f ingerprint wavelength range near 8 mm. The noise equivalent absorbance for our

measurement was 5 parts in 105, limited by excess amplitude modulation on the laser output, which corresponds

to a 1-Hz bandwidth detection limit of 250 parts N2Oin 109parts N2in a 1-m path length. 1998 Optical

Society of America

OCIS codes: 140.5960, 300.1030, 300.6340, 280.3420.

There is a wide range of measurement and control ap-

plications that require, in situ, chemically selective de-

tection of molecular species in the gas phase. Laser

absorption spectroscopy is ideally suited for this pur-

pose, and low detection limits have been reported.1–3

Because the strongest line strengths for molecules oc-

cur in the middle-infrared portion of the spectrum,

the lead salt family of tunable semiconductor diode

lasers has been the standard choice for use in this

spectral region. Effective methods for detecting weak

absorbances have been developed.3,4 However, as a

source in a practical sensing instrument these devices

have two limitations: Lead salt lasers must always be

operated cryogenically, and they are available commer-

cially only as Fabry – Perot cavity oscillators and hence

are subject to cavity mode hopping and tuning gaps.

The quantum-cascade (QC) laser5–8 overcomes these

two drawbacks, and we report here what is to our

knowledge the first application of the QC laser to the

detection of dilute samples of molecules through their

absorption spectrum. The QC laser derives its gain

from the transition of electrons between two excited

states in the conduction band of a coupled quantum-

well structure. Inversion is obtained by careful

engineering of the carrier lifetimes, determined by

longitudinal phonon scattering. The active region

and a matching electron injector are cascaded many

times to increase output power. The lasing wave-

length is determined by the thickness of the active

region’s quantum-well layer, essentially independently

of the semiconductor bandgap. Devices with wave-

lengths ranging from 4 to 11 mm have been fabricated

with an AlInAsyInGaAs lattice matched to InP.7This

spectral range overlaps the most important part of

the molecular fingerprint region and includes a

substantial fraction of the lead salt lasing range.

However, unlike the lead salt laser, the QC laser is

capable of operating at room temperature and above.9

Furthermore, room-temperature QC lasers with a

distributed-feedback (DFB) architecture have also

been demonstrated to have continuous, single-mode

tuning of .20 cm21.10

Figure 1 diagrams a sensitive detection measure-

ment that we implemented by using such a QC DFB

laser. The laser, designed to emit near 8 mm,10 is

mounted in a temperature controlled, evacuated cham-

ber. The laser is driven by a 2.6-A amplitude, 1-MHz

train of 11-ns-wide current pulses (,1% duty fac-

tor), resulting in 10-mW collected average laser out-

put power. The thermoelectric cooler temperature is

set to 4 ±C. This setup tunes the laser resonance close

to the N2O absorption line of interest. We fine tune

the laser by superimposing upon the pulsed waveform

a4-Hz current ramp, which alone would be insuffi-

cient in amplitude to bring the laser above thresh-

old but which causes a temperature modulation on

the laser suff icient to sweep the lasing frequency by

approximately one wave number. Finally, we com-

bine the ramp with a 1.8-kHz sinusoidal dither, which

adds a second, higher-frequency temperature modu-

lation, resulting in a concomitant wavelength modu-

lation (WM).11

Fig. 1. Experimental arrangement for wavelength modu-

lation spectroscopy with the quantum-cascade DFB laser.

0146-9592/98/030219-03$10.00/0 1998 Optical Society of America

220 OPTICS LETTERS / Vol. 23, No. 3 / February 1, 1998

The modulated laser beam is collimated and then

directed through a 10-cm-long gas cell equipped with

wedged CaF2windows. The transmitted light is fo-

cused onto a HgCdTe photoconductive detector, cooled

to 77 K. The detector is coupled to a matched pream-

plifier whose output is detected by a lock-in amplifier.

The wavelength-modulation dither serves as the lock-

in reference, and the lock-in output is recorded with

a14-bit digital signal-processor-equipped computer.

The computer records individual ramp scans and can

be set to average these scans. The data reported here

were recorded mostly by use of 16-scan averaging with

a lock-in bandwidth of 250 Hz, resulting in a net

detection bandwidth of 16 Hz. Because the dither fre-

quency is much lower than the underlying pulse rep-

etition frequency, this arrangement results in WM

absorption spectroscopy.

The gas cell was filled with commercially premixed

volumes of diluted gas, which were further diluted with

high-purity dry N2. For most of the measurements

reported here we used samples of 10%, 1%, and 0.1%

N2OinN

2diluted in steps of five as many as three

times with total pressures from 1 to 20 Torr.

By fixing the temperature of the thermoelectrically

cooled laser head, we obtainedsets of scans by using the

4-Hz current ramp, whose center frequency could be es-

timated from the wavelength calibration of the QC DFB

laser obtained independently with a Fourier-transform

infrared spectrometer.10 Figure 2 shows several such

scans overlaid to provide a continuous temperature

scan from ,217 ±Cto6±

C, corresponding to a 2.5-cm21

laser frequency scan starting at 1280 cm21. The spec-

tral data show the characteristic derivative line shape

of WM spectroscopy obtained from a 10%N2Oy90%N2

premixed sample with a total pressure of 1 Torr. We

checked the frequency calibration by comparing the

data with the known frequencies and line strengths of

the N2O10

0–0000 band obtained from the HITRAN

database.12 These are shown on the bottom of the fig-

ure. As an additional check on frequency calibration

we include a scan obtained from a 10%CH4y90%N2

sample at 10-Torr total pressure.

By switching the laser modulation from frequency

dither to mechanical chopping we were able to mea-

sure direct absorption and thus calibrate the WM

signal to absolute absorbance. Figure 3(a) shows the

resultant spectral scans obtained with 40 Torr of the

10,000-parts-in-106(ppm; 1%) N2O-in-N2sample.

The solid curve shows the absorption dip associated

with one of the strong N2O lines from Fig. 2. The

dotted curve shows a background trace with the gas

cell evacuated, and the dashed–dotted curve shows

the trace with the beam blocked. We obtained the

dashed trace by passing the beam through a 1.44-GHz

free-spectral-range solid germanium etalon.

Figure 3(b) shows the fractional attenuation of the

laser beam on line center as a function of N2O con-

centration inferred from the magnitude of the ab-

sorption dip for the 10,000-ppm sample and two 53

dilution steps (circles) and the undiluted 1000-ppm

sample (square). The straight line is a least-squares

fit to the data including the point (0 absorbance, 0

ppm). The slight deviation at low concentration is due

to accumulated error in dilution and the low signal-to-

noise ratio at low absorbance.

WM affords a substantial improvement over the di-

rect measurement in sensitivity to absorbance. Using

the two premixed samples of N2O and the dilution pro-

cedure outlined above, we measured the WM signal

amplitude for one of the strongest N2O lines as a func-

tion of N2O concentration. We calibrated the WM sig-

nal for absorbance by matching the WM signal to the

directly measured absorbance at the highest concentra-

tions. The result is plotted in Fig. 4. The solid line

is a linear least-squares fit from the data from dilu-

tions of both premixed samples, where we forced the

Fig. 2. WM signal versus laser frequency for 10% diluted

samples of N2Oand CH4with corresponding HITRAN data

plotted below.

Fig. 3. (a) Transmitted laser power versus frequency with

the N2Osample (solid curve), a calibration etalon (dashed

curve), or an opaque block (dotted– dashed line) in the

beam. (b) Absorbance versus concentration, obtained by

dilution of the samples as discussed in the text.

February 1, 1998 / Vol. 23, No. 3 / OPTICS LETTERS 221

Fig. 4. Plot of WM signal amplitude versus N2O concen-

tration for 20-Torr total pressure samples. The error bars

represent the amplitude of the background signal f luctua-

tions depicted in Fig. 5.

Fig. 5. (a) WM signal that is due to a 1000-ppm N2O

sample at 10-Torr total pressure. (b) Background trace for

an evacuated sample cell. (c) Background trace with the

laser beam blocked.

fit to pass through point s0, 0das before. Below a dilu-

tion of 20 ppm we were unable to distinguish the WM

signal from background fluctuations, which are indi-

cated by the error bars on the figure. To estimate

the noise equivalent absorbance we show in Fig. 5(a)

the WM signal from the 1000-ppm sample at 10 Torr

and in Fig. 5( b) the trace obtained with the gas cell

evacuated to less than 10 mTorr. The rms amplitude

of the random component of the signal was found to

be approximately 10 mVypHz. Comparing this sig-

nal level with the absorbance sensitivity deduced from

Fig. 4, we estimate the noise equivalent absorbance to

be 5 31025ypHz. With the strongest N2O line in this

spectral region, this absorbance corresponds to a detec-

tion limit of ,0.25 ppm-mypHz.

We show in Fig. 5(c) the background obtained with

the laser beam blocked. The 280-nVypHz amplitude

is consistent with that expected from 300-K back-

ground radiation. We conclude that the limiting back-

ground of our measurement is due to excess amplitude

modulation that appears on the output of the laser.

In conclusion, we have demonstrated sensitive detec-

tion of dilute samples of N2O, using a quantum-cascade

DFB laser operated near 4 ±C. The noise equivalent

absorbance of 5 31025ypHz is limited by two nonfun-

damental factors. Sensitivity to absorbance is reduced

because of excess laser line broadening.11 The appar-

ent laser linewidth of 720 MHz is due to the frequency

chirp that results from the pulsed operation and the

heating during the current pulse. This is in strong

contrast to the linewidth observed for continuous wave

operation.13 Furthermore, amplitude noise from the

drive electronics imparts an additional background

at the dither frequency. A factor-of-3–5 improvement

in both factors should be possible with improved de-

vice characteristics and optimized drive electronics.

Combined with the use of high-frequency-modulation

techniques, these improvements should bring the sen-

sitivity of the QC DFB laser to within an order of

magnitude of the 6 31028ypHz minimum detectable

absorbance reported for frequency-modulated lead salt

lasers.14 We conclude that, combined with the advan-

tage of room-temperature laser operation, the detection

sensitivity demonstrated here will make the QC DFB

laser an excellent choice for sensitive absorption spec-

troscopy detection of species with strong vibrational

features in the chemical fingerprint spectral region.

We acknowledge partial support from the Defense

Advanced Research Projects Agency and the U.S.

Army Research Office under contract DAAH04-96-C-

0026 (Bell Labs) and the National Science Foundation

(Stevens Institute) under grant DMI-9313320.

References

1. J. A. Silver, Appl. Opt. 6, 707 (1992).

2. C. B. Carlisle and D. E. Cooper, Appl. Phys. Lett. 56,

805 (1990).

3. P. Werle, Appl. Phys. B 60, 499 (1995).

4. H. C. Sun and E. A. Whittaker, Appl. Phys. Lett. 63,

1035 (1993).

5. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L.

Hutchinson, and A. Y. Cho, Science 264, 553 (1994).

6. J. Faist, F. Capasso, D. L. Sivco, A. L. Hutchinson,

C. Sirtori, and A. Y. Cho, Appl. Phys. Lett. 65, 2901

(1994).

7. F. Capasso, J. Faist, C. Sirtori, and A. Y. Cho, Solid

State Commun. 102, 231 (1997).

8. J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, J. N.

Baillargeon, A. L. Hutchinson, S. N. G. Chu, and

A. Y. Cho, Appl. Phys. Lett. 68, 3680 (1996).

9. C. Sirtori, J. Faist, F. Capasso, D. L. Sivco, A. L.

Hutchinson, and A. Y. Cho, IEEE Photon. Technol.

Lett. 9, 294 (1997).

10. J. Faist, C. Gmachl, F. Capasso, C. Sirtori, D. L. Sivco,

J. N. Baillargeon, and A. Y. Cho, Appl. Phys. Lett. 70,

2670 (1997).

11. J. M. Supplee, E. A. Whittaker, and W. Lenth, Appl.

Opt. 33, 6294 (1994).

12. L. S. Rothman, R. R. Gamache, A. Goldman, L. R.

Brown, R. A. Toth, H. M. Pickett, R. L. Poynter, J.

M. Flaud, C. Camy-Peyret, A. Barbe, N. Husson, C.

P. Rinsland, and M. A. H. Smith, Appl. Opt. 26, 4058

(1997).

13. F. Capasso, D. L. Sivco, A. L. Hutchinson, and A. Y.

Cho, Appl. Phys. Lett. 67, 3057 (1995).

14. P. Werle, F. Slemr, M. Gehrtz, and C. Br¨

auchle, Appl.

Phys. B 49, 99 (1989).

Stable single-mode operation of distributed feedback quantum cascade laser under high current via a grating reflector

Article

Full-text available

Mar 2023

We report on an index-coupled distributed feedback (DFB) quantum cascade lasers (QCLs) of high single-mode stability by using of a grating reflector (GR) to reflect effectively light of a specific wavelength. Fabrications of the QCLs were performed with λ ∕4 equivalent phase shift (EPS) or sampled Moiré grating (SMG) structure, which regulates the light intensity distribution in the cavity and coupling strength. Both EPS-GR and SMG-GR QCLs demonstrate a high emission wavelength precision and a side-mode-suppression-ratio (SMSR) of 24.5 dB at the injection current of 5 A and the pulse width of 5 μs, offering considerable output power even at the edge of the gain spectrum. Furthermore, we investigated the arrays of EPS-GR and SMG-GR QCLs with lower threshold current and higher output power by introducing semi-insulated InP (Fe) ranging from 8.25 to 8.67 μm and 8.16–8.63 μm, respectively. The present QCLs via a GR are prospective for applications requiring single-mode stability and wide tunability.

Real-time, chirped-pulse heterodyne detection at room-temperature with 100GHz 3dB-bandwidth mid-infrared quantum-well photodetectors

Preprint

Jun 2024

Thanks to intrinsically short electronic relaxation on the ps time scale, III-V semiconductor unipolar devices are ideal candidates for ultrahigh-speed operation at mid-infrared frequencies. In this work, antenna-coupled, GaAs-based multi quantum-well photodetectors operating in the 10-11um range are demonstrated, with a responsivity of 0.3A/W and a 3dB-cutoff bandwidth of 100GHz at room-temperature. The frequency response is measured up to 220GHz: beyond 100GHz we find a roll-off dominated by the 2.5 ps-long recombination time of the photo-excited electrons. The potential of the detectors is illustrated by setting up an experiment where the time dependent emission frequency of a quantum cascade laser operated in pulsed mode is measured electronically and in real-time, over a frequency range >60GHz. By exploiting broadband electronics, and thanks to its high signal-to-noise ratio, this technique allows the acquisition, in a single-shot, of frequency-calibrated, mid-infrared molecular spectra spanning up to 100GHz and beyond, which is particularly attractive for fast, active remote sensing applications in fields such as environmental or combustion monitoring.

Real-time, chirped-pulse heterodyne detection at room temperature with 100 GHz 3-dB-bandwidth mid-infrared quantum-well photodetectors

Article

Full-text available

Dec 2023

Thanks to intrinsically short electronic relaxation on the ps time scale, III-V semiconductor unipolar devices are ideal candidates for ultrahigh-speed operation at mid-infrared frequencies. In this work, antenna-coupled, GaAs-based multi-quantum-well photodetectors operating in the 10–11 µm range are demonstrated, with a responsivity of 0.3 A/W and a 3-dB-cutoff bandwidth of 100 GHz at room temperature. The frequency response is measured up to 220 GHz: beyond 100 GHz we find a roll-off dominated by the 2.5-ps-long recombination time of the photo-excited electrons. The potential of the detectors is illustrated by setting up an experiment where the time dependent emission frequency of a quantum cascade laser operated in pulsed mode is measured electronically and in real time, over a frequency range $\gt 60\; {\rm GHz}$ > 60 G H z . By exploiting broadband electronics, and thanks to its high signal-to-noise ratio, this technique allows the acquisition, in a single-shot, of frequency-calibrated, mid-infrared molecular spectra spanning up to 100 GHz and beyond, which is particularly attractive for fast, active remote sensing applications in fields such as environmental or combustion monitoring.

Anti-symmetric sampled grating quantum cascade laser for mode selection

Article

Full-text available

Feb 2023

For mode selection in a quantum cascade laser (QCL), we demonstrate an anti-symmetric sampled grating (ASG). The wavelength of the −1-th mode of this laser has been blue-shifted more than 75 nm (~10 cm ⁻¹ ) compared with that of an ordinary sampled grating laser with an emission wavelength of approximately 8.6 μm, when the periodicity within both the base grating and the sample grating are kept constant. Under this condition, an improvement in the continuous tuning capability of the QCL array is ensured. The ASG structure is fabricated in holographic exposure and optical photolithography, thereby enhancing its flexibility, repeatability, and cost-effectiveness. The wavelength modulation capability of the two channels of the grating is insensitive to variations in channel size, assuming that the overall waveguide width remains constant. The output wavelength can be tailored freely within a certain range by adjusting the width of the ridge and the material of the cladding layer.

CO2 Gas Temperature Sensing Near Room Temperature by a Quantum Cascade Laser in Inter Pulse Mode

Article

Full-text available

Jul 2022

A non-invasive CO2 gas temperature sensing technique at or close to the room temperature range based on quantum cascade laser absorption spectroscopy is presented. The method probes thermally populated ground state and hot-band rotational-vibrational transitions of CO2 in the frequency range from 2349 to 2351 cm−1 from which the gas temperature is obtained from Boltzmann statistics. Transmission spectra are recorded by injection-current driven frequency-scans, the so-called inter pulse mode, of a pulsed distributed feedback quantum cascade laser. The statistical uncertainty (1σ) in temperature for single frequency scans with time resolution of 10 ms is 4 K and can be further reduced down to ∼50 mK by long-time averaging of about 1 min. The technique is evaluated with particular emphasis on implementation, data acquisition, data analysis and potential improvements.

Stable single-mode 20-channel uniform buried grating DFB QCL array emitting at ~ 8.3 μm

Article

Full-text available

Apr 2022
OPT QUANT ELECTRON

A 20-channeldistributed feedback (DFB) quantum cascade laser (QCL) arrays based on uniform buried grating have been demonstrated. In pulsed mode, peak power reaches 80 mW and slope efficiency reaches 167 mW/A for 2.5 mm-long laser in the arrays at room temperature. The loss difference of two band-edge mode increases when reflectivity of the front facet becomes small, which prevents the mode hopping. The device shows linear tuning after the anti-reflectivity coating is deposited in the front facet, maintaining peak power of 64 mW. The whole chip covers a tuning range of 64 cm⁻¹, centering at 8.3 μm, with side-mode-suppression-ratio over 20 dB at room temperature.

Stable Single-mode 20-channel Uniform Buried Grating DFB QCL Array Emitting at ~8.3 μm

Preprint

Full-text available

Nov 2021

A 20-channel distributed feedback (DFB) quantum cascade laser (QCL) arrays based on uniform buried grating have been demonstrated. In pulsed mode, peak power reaches 80 mW and slope efficiency reaches 167 mW/A for 2.5-mm-long laser in the arrays at room temperature. The loss difference of two band-edge mode increases when reflectivity of the front facet becomes small, which prevents the mode hopping. The device shows linear tuning after the anti-reflectivity coating is deposited in the front facet, maintaining peak power of 64 mW. The whole chip covers a tuning range of 64 cm ⁻¹ , centering at 8.3 μm, with side-mode-suppression-ratio over 20 dB at room temperature.

A silicon micromechanical resonator with capacitive transduction for enhanced photoacoustic spectroscopy

Article

Full-text available

Nov 2021
SENSOR ACTUAT B-CHEM

This paper presents the design of a silicon micromechanical resonator with capacitive transduction for a gas sensor based on photoacoustic spectroscopy. It overcomes problems imposed by opposite physical trends originating from the sensor’s working principles. The capacitive transduction mechanism is considered incompatible with the photoacoustic gas detection as it reduces the mechanical displacement by a damping effect, thus decreasing the sensitivity. This occurs because the same part serves two functions: photoacoustic excitation and capacitive transduction. The suggested approach in this study focuses on the spatial separation of these two functions. We propose the first reported mechanical microresonator for photoacoustic gas sensing, which decouples photoacoustic excitation and capacitive transduction. The design has been modeled, fabricated, characterized, and compared with the on-beam Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) technique, a reference for compact and selective gas sensors. The performed photoacoustic measurement has been realized on calibrated concentrations of methane. For a 1s integration time in first harmonic detection, we obtained a limit of detection (LOD) of 667ppmv for silicon microresonator. In comparison to on-beam QEPAS technique, we obtained LOD of 163ppmv, which constitutes a factor 4 between QEPAS and MEMS. The conditions for both experiments were the same.

Quantum cascade lasers and their applications to spectral and kinetic investigations of reactive gaseous intermediate species

Chapter

Aug 2022

Infrared (IR) spectroscopy is considered to be among the most important analytical tools in laboratories, remote sensing, and industry. In addition to the commonly used Fourier-transform infrared (FTIR) spectrometers, spectral detection using IR laser sources plays important roles in varied applications. The quantum cascade laser (QCL) recently emerged as a novel, compact, tunable, and versatile IR laser source with narrow linewidth, high power, wide spectral coverage, and relatively straightforward operation. The scope of this chapter is limited primarily to mid-IR QCL and mainly their applications to spectroscopy and kinetics of reactive gaseous intermediate species. We introduce briefly the fundamental theories of QCL, their various types, advantages, and limitations. Implementations of several types are discussed, a detailed description of a continuous-wave external-cavity QCL coupled with a Herriott cell is presented. The high-resolution spectra of some stable molecules and reactive reaction intermediates, especially Criegee intermediates and other free radicals, are discussed. An account of kinetic investigations of Criegee intermediates and other free radicals in a flow reactor and of species in a shock tube is provided. Finally, we present some future perspective on research using the QCL.

Frequency Chirped Pulse-Based Methane Sensing with Inter-Pulse Calibration

Conference Paper

Jan 2021

A chirped pulse-based gas sensing technique was developed. With a short, 11.2 ns frequency chirped pulse generating differential absorption within the pulse duration, inter- pulse calibrated methane sensing up to 20.5% was demonstrated.

Quantum Cascade Laser

Article

Full-text available

Apr 1994

A semiconductor injection laser that differs in a fundamental way from diode lasers has been demonstrated. It is built out of quantum semiconductor structures that were grown by molecular beam epitaxy and designed by band structure engineering. Electrons streaming down a potential staircase sequentially emit photons at the steps. The steps consist of coupled quantum wells in which population inversion between discrete conduction band excited states is achieved by control of tunneling. A strong narrowing of the emission spectrum, above threshold, provides direct evidence of laser action at a wavelength of 4.2 micrometers with peak powers in excess of 8 milliwatts in pulsed operation. In quantum cascade lasers, the wavelength, entirely determined by quantum confinement, can be tailored from the mid-infrared to the submillimeter wave region in the same heterostructure material.

Quantum-limited FM-spectroscopy with a Lead-salt Diode Laser

Article

Full-text available

Aug 1989

Ultrasensitive absorption spectroscopy of NO2 was performed with a tunable lead-salt diode laser (TDL) using a single-tone high-frequency modulation (FM) technique. With a detection bandwidth of 200 kHz, an optical density of 2.7 10–5 was detectable at SNR of 1. The detectable optical density could be further improved by reducing the detection bandwidth in agreement with the f relationship, reaching 2.5 10–6 at a detection bandwidth of 1.56 kHz. Normalized to 1 Hz bandwidth, the demonstrated performance would then correspond to a detectable optical density of 5.9 10–8. This detection limit agrees well with the calculated quantum limited performance based on the measured laser power, modulation index, noise figure of the electronic components, and other parameters of the apparatus. These measurements and calculations show that by implementation of the FM technique, the sensitivity of the present TDL absorption spectrometers (TDLAS) can be improved by at least a factor of 10 and possibly even of 100. Such a sensitivity improvement would greatly extend the applicability of TDLAS for trace gas analysis, especially in atmospheric monitoring.

Infrared (4–11 μm) quantum cascade lasers

Article

Apr 1997

This paper reviews the state-of-the-art of quantum cascade lasers. Quantum cascade lasers are new unipolar light sources based on electron resonant tunneling and optical transitions between quantized conduction band states. Quantum engineering of the electronic energy levels, wavefunctions, matrix elements and scattering rates is used to design population inversion and optimize laser gain and overall performance. The laser wavelength can be selected over a broad range (4–12 μm) of the infrared spectrum using the same heterostructure material by tailoring the active layer thickness. Using MBE grown heterostructures, room temperature operation at 5 μm and 8.5 μm has been achieved and the operating wavelength has been extended to 11 μm.

High power mid‐infrared (λ∼5 μm) quantum cascade lasers operating above room temperature

Article

Jun 1996
APPL PHYS LETT

The high power operation of mid‐infrared quantum cascade lasers at temperatures up to T=320 K is reported. Gain at high temperature is optimized by a design combining low doping, a funnel injector, and a three‐well vertical transition active region. A molecular beam epitaxy grown InP top cladding layer is also used to optimize heat dissipation. A peak pulsed optical power of 200 mW and an average power of 6 mW are obtained at 300 K and at a wavelength λ=5.2 μm. The devices also operate in continuous wave up to 140 K. © 1996 American Institute of Physics.

Laser excess noise and interferometric effects in frequency-modulated diode-laser spectrometers

Article

Jun 1995

P. Werle

Tunable Diode-Laser Absorption Spectroscopy (TDLAS) is increasingly being used to measure trace-gas concentrations down to low part-per-billion levels (1 ppbv = 10–9 volume mixing ratio). Semiconductor lead-salt diode lasers give access to the mid-infrared spectral region and the application of high-Frequency Modulation (FM) schemes can further improve the sensitivity and detection speed of modern instrumentation. Several factors influence or even limit spectrometer performance. The central elements in such spectrometers are lead-salt diode lasers. Experimental data will be presented, which demonstrate that high-frequency excess-noise contributions above several MHz can be attributed to mode hopping and mode partition noise during multimode laser operation. Additionally it will be discussed how a FM-TDLAS spectrometer can be interpreted as an optimized Michelson interferometer for absolute distance measurements and, therefore, is extremely sensitive towards drift effects. The higher the modulation frequency, the higher is the drift sensitivity of the spectrometer due to interferometric effects. These drift effects are a second factor affecting ultrasensitive measurements. While wideband-laser noise characteristics call for high modulation frequencies, the aforementioned interferometric effects in the spectrometer require low modulation frequencies.

Real‐time in situ detection of SF6 in a plasma reactor

Article

Sep 1993
APPL PHYS LETT

We report on the real‐time in situ detection of SF 6 in a plasma etching reactor with μTorr sensitivity using a tunable diode laser absorption spectrometer. The spectrometer employs combined wavelength and frequency modulation of the laser diode, an approach which allows for sensitive, interference fringe‐free detection of the SF 6 . The dual modulation scheme also provides a feedback signal which enables the laser to be frequency locked to the absorption line.

Tunable diode laser frequency modulation spectroscopy through an optical fiber: High‐sensitivity detection of water vapor

Article

Mar 1990
APPL PHYS LETT

Frequency modulation spectroscopy through a single‐mode optical fiber has been demonstrated for monitoring H 2 O vapor at 7665 cm-1. Using a 2.5 mW distributed feedback laser and 100 m of optical fiber, a minimum detectable absorption of 5×10-7 was achieved. To obtain this sensitivity limit, very large spurious signals and technical noise were suppressed using a dual‐channel detection scheme.

Quantum cascade laser: Temperature dependence of the performance characteristics and high T 0 operation

Article

Jan 1995
APPL PHYS LETT

The design and temperature dependence of the performance characteristics of a quantum cascade intersubband laser operating pulsed in the midinfrared (λ≂4.3 μm) are reported. The threshold current density varies exponentially with temperature [exp(T/T 0 )] from ≊6.0 kA/cm2 at 50 K to ≊9.3 kA/cm2 up to the maximum operating temperature (125 K) with a T 0 ∼112 K. This weak temperature dependence, compared to interband lasers operating at similar wavelengths, is due to the intersubband nature of the laser transition, to the physics of optical phonons scattering, and to the negligible intersubband Auger transition rates. The measured peak optical power varies from 32 mW at 10 K to 18 mW at 80 K for a 1.2‐mm cavity length. The measured slope efficiency is 52 mW/A at 80 K which corresponds to an estimated differential quantum efficiency of ≂3.4×10-2 per facet per stage. © 1994 American Institute of Physics.

Continuous-wave operation of a vertical transition quantum cascade laser above T=80K

Article

Nov 1995
APPL PHYS LETT

The quantum cascade laser is a new semiconductor laser that is based on one type of carrier making transitions between energy levels created by quantum confinement. This work demonstrates that the design based on a vertical transition can be optimized to provide continuous wave operation. The InGaAs/AlInAs heterostructures consist of 25 stages, each comprising a coupled-well active region and a graded-gap superlattice injection/relaxation region. The samples are processed into mesa etched ridge waveguides by wet chemical etching and SiO2 insulation. The devices are also investigated in pulsed operation in which there is negligible heating.

Distributed Feedback Quantum Cascade Lasers

Article

May 1997

Pulsed single mode operation of distributed feedback quantum cascade lasers is reported above room temperature at both 5.3 and 8 mu m wavelengths, Peak optical powers up to 60 mW at 300 K are obtained with a tuning range of similar to 60 nm from 100 to similar to 320 K. The linewidth is limited by thermal drift during the pulse with a typical value of 0.3 cm(-1) for a 10 ns long pulse at 300 K. (C) 1997 American Institute of Physics.

Sensitive Absorption Spectroscopy with a Room-temperature Distributed-feedback Quantum-cascade Laser

Abstract and Figures

Recommended publications

Highly Sensitive Tunable Diode Laser Absorption Spectroscopy of CO 2 Around 1.57 μm

Fiber Ring Laser Intra-cavity Absorption Spectroscopy for Gas Sensing

Laser enhances spectroscopy technique's sensitivity

Laser absorption spectroscopy based on a broadband external cavity quantum cascade laser