werle phd thesis_mit_2002

8/10/2019 Werle PHD THESIS_MIT_2002

1/14

Optics and Lasers in Engineering 37 (2002) 101114

Near- and mid-infrared laser-optical sensors for

gas analysis

Peter Werlea,*, Franz Slemra, Karl Maurera, Robert Kormannb,

Robert M.ucke

c

, Bernd J.anker

d

aFraunhofer Institut, D-82467 Garmisch-Partenkirchen, GermanybMax-Planck Institut f.ur Chemie, D-55128 Mainz, Germany

c Infineon Technologies, D-93049 Regensburg, GermanydCarl Zeiss, D-73446 Oberkochen, Germany

Received 15 April 2001; accepted 12 July 2001

Abstract

Semiconductor diode lasers were first developed in the mid-1960s and found immediateapplication as much needed tunable sources for high-resolution laser spectroscopy commonly

referred to as tunable diode laser absorption spectroscopy (TDLAS). In this paper, currently

available semiconductor lasers for spectroscopy in the near- and mid-infrared spectral region

based upon gallium arsenide, indium phosphite, antimonides and lead-salt containing com-

pounds will be reviewed together with the main features of TDLAS. Room-temperature

measurements of atmospheric carbon dioxide near 2 mm will be discussed and recent results

obtained with a fast chemical sensor for methane flux measurements based on lead-salt diode

lasers operating near 7.8mm will be presented.r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Semiconductor lasers; Diode laser spectroscopy; Trace gas analysis; Carbon dioxide; Methane

1. Introduction

Laser-optical sensors are now at the threshold of routine applications in air

pollution monitoring and industrial process and gas analysis. The development of

this technology has been driven mainly by scientific questions, but increasingly these

sensors are transferred to industrial and other applications whenever sensitive,

selective and fast analysis is required. With the increasing complexity of processes,

*Corresponding author. Haidestr. 4, D-82438 Eschenlohe, Germany.

E-mail address: [email protected] (P. Werle).

0143-8166/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 0 9 2 - 6


2/14

online gas analysis is becoming a key issue in automated control of various industrial

applications, in combustion diagnostics, for investigations of aeroengines and

automobile exhaust measurements. Other challenges are online analysis of high-

purity process gases, medical diagnostics and monitoring of agricultural andindustrial emissions. The need to meet increasingly stringent environmental and

legislative requirements has also led to the development of analyzers to measure the

concentrations of a variety of gases [1] based on near- and mid-infrared absorption

spectroscopy. After a brief review of currently available diode lasers for different

spectral regions we will describe a near-infrared trace-gas sensor for carbon dioxide

based on a room-temperature diode laser and a fast chemical mid-infrared sensor for

methane based on a lead-salt diode laser.

2. Semiconductor lasers

Semiconductor diode lasers play an important role in telecommunications and

consumer electronics in applications such as fiber optical communications, laser

printers and CD players because of their small size, high reliability and ease of use.

As it can be seen from Fig. 1, the whole spectral range from the visible to the infrared

can be covered by semiconductor lasers of various compounds, which mainly are

gallium arsenide, indium phosphite, antimonides and lead salts. A drawback for

industrial applications of spectrometers based on semiconductor lasers is the lack of

high-quality, high-power laser diodes for many spectral regions of interest.Gallium arsenide and indium phosphite lasers are commercially made from the III

V group of semiconductor materials. These diode lasers emit from the visible to near-

infrared wavelengths from 0.63 to 1.55 mm including the InGaAsP/InP 1.31.55 mm

optical communication lasers, as well as the GaAs/AlGaAs 0.78 and 0.83 mm lasers

found in compact disk players. The technology of near-IR 1.3 and 1.55mm

InGaAsP/InP diode lasers developed for fiber-optic communication can be extended

to fabricate lasers that emit anywhere in the wavelength interval of about 1.22 mm.

Near-infrared multiple-quantum-well distributed-feedback (DFB) lasers have the

advantages of single-mode outputs at milliwatt power and near-room-temperature

operation [2]. A drawback of these devices is that they are available only in narrowspectral regions and only a limited number of molecular species have absorption

features in the spectral region covered by these lasers. Furthermore, near-IR

absorptions are overtone or combination bands that are typically one to several

orders of magnitude weaker than the IR-fundamental band, while many molecules

of interest have near-IR absorption bands that are strong enough for detection at

parts-per-million (ppm) and, in some cases, even parts-per-billion (ppb) levels [25].

Especially, the lasers for the visible/blue spectral range are of increasing importance

for element analysis based on atom absorption spectroscopy [6].

Antimonide lasers can be used for wavelengths longer than 1.8 mm and are based

on IIIV compounds such as AlGaAsSb, InGaAsSb, and InAsSbP. Roomtemperature lasing from 2 to 2.4mm has been reported from simple double

heterostructure antimonide diode lasers. As wavelength increases up to 3.7 mm, the

P. Werle et al. / Optics and Lasers in Engineering 37 (2002) 101114102


3/14

maximum operating temperature decreases as a result of increasing optical and

electrical losses. Laser devices based on InAsSb/ InAsSbP double heterostructure

devices were grown by liquid-phase epitaxy on InAs substrate and cover the spectral

range from 3 to 4 mm at LN2temperatures. These devices are well suited, e.g. for the

detection of HCHO at 3.6 mm and CH4 at 3.26 mm [7]. For this spectral range also

spectroscopic sources based on frequency mixing techniques have been designed,

which allow room-temperature operation [8].

Lead-salt diode lasers are made from IVVI semiconductor materials and operate

in the 330 mm spectral region [9]. Therefore, they cover the IR fundamental bands

with strong absorption for the most atmospheric trace gases [1]. They are usedalmost exclusively for spectroscopic applications [1014]. Since IVVI lasers and

their associated detectors operate at cryogenic temperatures, they are more expensive

Fig. 1. Spectral coverage of the visible to infrared region by different semiconductor laser materials: While

for pn-junction lasers room-temperature operation is only possible up to 23mm for IIIV materials,

Quantum cascade lasers become a promising alternative to the cryogenic cooled IVVI lead-salt diode

lasers. Also shown are absorption cross sections for methane and interfering water vapor.

P. Werle et al. / Optics and Lasers in Engineering 37 (2002) 101114 103


4/14

and more cumbersome to use than IIIV devices [15]. In trace gas monitoring

applications, lead-salt laser instruments have routinely achieved parts-per-billion

detection levels of a number of important molecular species [1]. For unattended

industrial routine applications, however, the use of lead-salt diode lasers is limited bythe need of cryogenic cooling by LN2or Stirling coolers (typ. 78120 K), occurrence

of multimode emission and power levels which are typically only several hundred

microwatts. Compared to GaAs lasers, lead-salt semiconductor lasers are at a

relatively early stage of their development due to a much smaller market.

Until recently, all semiconductor lasers, regardless of their operating wavelength,

relied upon direct band-to-band transitions in bulk material as shown in Fig. 1 (top

left). Rapid progress has been reported in mid-IR lasers based on new types of

transitions. The most notable example is the Quantum cascade (QC) lasers.

Although the basic concept was proposed as early as 1971, it took more than 20

years before an actual device was demonstrated in 1994 [16]. Quantum cascade lasers

are based on a completely different approach than the lasers described so far. In a

QC laser, electrons make transitions between bound states created by quantum

confinement in ultra-thin alternating layers of semiconductor materials. Since these

ultra-thin layers, called quantum wells, have a size comparable to the electrons de

Broglie wavelength, they restrict the electron motion perpendicular to the plane of

the layer. Due to this effect called quantum confinement, the electron can only jump

from one state to the other by discrete steps, emitting photons of light (Fig. 1 top

right). The spacing between the steps depends on the width of the well, and increases

as the well size is decreased. In a pictorial way, this laser is freed from bandgapslavery, i.e. the emission wavelength depends now on the layer thickness and not on

the bandgap of the constituent materials. This has allowed, using the same base

semiconductors (InGaAs and AlInAs grown on InP), the manufacture of lasers with

an emission wavelength from 3.5 to 11.5 mm (recent work in Bell labs at Lucent

Technologies has pushed this limit to 17 mm). Compared to conventional (interband)

lasers, the quantum cascade laser has the following advantages: The same

semiconductor material whose technology is very well mastered can be used to

manufacture lasers operating across the whole mid-infrared (and potentially even

farther in the far-infrared). It is based on a cascade of identical stages (typically 20

70), allowing one electron to emit many photons, emitting more optical power. It isintrinsically more robust (no interface recombination) and since the dominant non-

radiative recombination mechanism is optical phonon emission and not Auger effect

(as it is the case in narrow-gap materials), it allows intrinsically higher operating

temperature. While these new types of lasers are still in the experimental stage, they

appear to offer the prospect of more robust construction and higher temperature

operation than is possible with the materials which have been used up to this point.

These devices are operated in pulsed mode at room temperature and allow

continuous wave operation only at lower temperatures. In the 58 mm wavelength

region, continuous wave operation at temperatures above 120 K generates

remarkable output powers of 220 mW and about 200 mW at 80 K [17], while incomparison lead-salt diode lasers deliver only 200 mW at liquid nitrogen temperature.

With quantum cascade lasers already first spectroscopic measurements have been



5/14

reported [1820]. The commercial availability of these lasers might promote the

development of new operational systems that allow highly sensitive measurements

based on the strong fundamental IR transitions.

3. Diode laser spectroscopy

Semiconductor lasers were first introduced in the mid-1960s and found immediate

application as much needed tunable sources for high-resolution infrared laser

absorption spectroscopy. The principal setup of an optical absorption spectrometer

is shown in Fig. 2a. All designs contain a radiation source, a detector and the species

under investigation in a closed absorption cell for the determination of gas

concentrations according to Beers law. Most systems also provide a modulation unit

to generate an AC signal. As a prerequisite to obtain the required specific

wavelength, selective elements have to be inserted into the optical path. Gasfilter-

Correlator systems require a reference cell with a high concentration of the gas under

investigation. While for DOAS systems a monochromator is used, FTIR instruments

apply a Michelson interferometer as the selective element. Non-dispersive infrared

systems (NDIR) are based on gas selective devices. All instruments based on these

technologies meanwhile reached a high degree of complexity and sophistication. An

alternative approach is to insert the selective element directly into the radiation

sourceFthe laser. In principle, laser spectrometers allow less complex optomecha-

nical designs than the current analytical instrumentation and with semiconductor

Fig. 2. (a) Principle setup of optical absorption spectroscopy, (b) direct absorption spectroscopy using a

tunable diode laser, (c) conventional wavelength modulation (kHz) spectroscopy and (d) high-frequency

modulation spectroscopy (MHz).



6/14

lasers even the required modulation can be implemented electronically. Therefore,

diode laser spectroscopy is an attractive and promising technique for analytical

instrumentation. The most important research applications of TDLs in atmospheric

field measurements required long-path absorption cells to provide high-sensitivitylocal measurements. In spectroscopy a single narrow laser line usually scans over an

isolated absorption line of the species under investigation (Fig. 2b). To achieve the

highest selectivity, analysis is made at low pressure, where the absorption lines are

not substantially broadened by pressure. This type of measurements has developed

into a very sensitive and general technique for monitoring most atmospheric trace

species [1]. The main requirement is that the molecule should have an infrared line-

spectrum which is resolvable at the Doppler limit, which in practice includes most

molecules with up to five atoms together with some larger molecules. Since TDLAS

operates at reduced pressure it is not restricted in wavelength to the atmospheric

windows such as 3.45 and 813 mm.

Direct absorption measurements have to resolve small changes in a large signal. In

comparison with direct spectroscopy, the benefits of modulation spectroscopy in

TDLAS are twofold. Firstly, it produces a difference signal which is directly

proportional to the species concentration (zero baseline technique) and, secondly, it

allows the signal to be detected at a frequency at which the laser noise is significantly

reduced. Wavelength modulation spectroscopy (WMS) has been used with tunable

diode laser sources since the early 1970s. The earliest TDL systems used a

modulation frequency in the lower kHz regime and second harmonic detection.

Today 50 kHz modulation with 100 kHz detection is quite usual and, consequently, itis convenient to regard 100 kHz as a limit of conventional wavelength-modulated

TDLAS. Modulation spectroscopy is based on the ease with which diode lasers can

be modulated. In WMS with diode lasers, the injection current is modulated as the

laser wavelength is tuned repeatedly at about 100 Hz over the selected absorption

line and a computer-controlled signal-averager is used to accumulate the signal from

a lock-in amplifier (Fig. 2c or d). This produces a harmonic spectrum of the line,

with an amplitude proportional to the species concentration. Scanning over the line

gives increased confidence in the measurement because the characteristic feature of

the measured species is clearly seen and unwanted spectral features due to interfering

species or !etalon fringes can easily be identified.While mid-infrared lasers operated at cryogenic temperatures cover the funda-

mental absorption bands required for ultra-sensitive gas analysis, near-infrared

room-temperature diode lasers give access mainly to significantly weaker overtone

and combination bands. Therefore, the selection of the optimum operating

conditions for a gas analyzer is always a tradeoff between the required sensitivity

and an operational system. By averaging over periods longer than 1 s or by using

longer path length the detection limits can be improved. While the predicted

sensitivities are based on known line strengths and the performance of a typical

TDLAS system, it is in the nature of field measurements that optimum performance

is not always achieved due to drift effects and other interfering disturbances [21]. Tocope with these problems different approaches based upon signal processing [22] and

double modulation techniques [23] have been proposed and successfully applied.



7/14

To illustrate the performance and operation of state-of-the-art near- and mid-

infrared spectrometers based on tunable diode lasers in the next section an example

of a near-infrared trace gas sensor for carbon dioxide based on a room-temperature

2mm indium-phosphite-DFB-laser will be presented. Finally, a typical mid-infraredspectrometer for methane detection based on a lead-salt diode laser operating at

7.8mm at liquid nitrogen temperature will be described.

4. Near-infrared trace-gas sensor for carbon dioxide based on a room-temperature

diode laser

For many field measurements or industrial applications the use of liquid nitrogen

must be avoided, only temperature stabilization by thermoelectrical elements is

acceptable. Therefore, in a fast CO2 sensor (Fig. 3a) we implemented InP-DFB-

Fig. 3. (a) Optical layout of a near-infrared spectrometer using an optical multipass cell (Herriott), (b)

typical mode map of an indium-phosphite-DFB-laser at 2 mm with 12 mW power, (c) measurements of12CO2in ambient air at a pressure of 100 mbar and 100 m optical path length, (d) time series data recorded

from a 358 ppm 12CO2calibration gas mixture and (e) identification of 12CO2and

13CO2absorption lines

around 2001.3 nm.



8/14

lasers from Sensors Unlimited Inc. (Princeton, NJ) with room-temperature single-

mode emission at lE2mm as shown in Fig. 3b. The components are mounted on a

60cm 110 cm optical breadboard on top of a double 19 in rack. The DFB laser is

held inside a Peltier-cooled mount which is fixed on an xyz-stage. The laser beam iscollimated by an off-axis parabola (OAP) with 10 mm diameter and 12 mm focal

length. The beam is focused by a spherical mirror (f 1m) to the center of a

Herriott-type measurement cell. After 181 reflections, corresponding to 100 m, the

beam exits the cell and is focused onto a temperature-stabilized extended InGaAs

detector (sensors unlimited) by another OAP. About 8% of the laser beam is coupled

off by a beam-splitter and directed through the 28 cm reference cell onto another

InGaAs detector. The whole system is pre-aligned by a visible diode laser, coupled

into the setup by a pellicle beam-splitter which has to be removed during the

measurements. In order to provide static as well as flux measurements at defined cell

pressures, the measurement cell is equipped with a pressure sensor and on/off-valves

(at inlet and outlet) as well as with a needle valve at the inlet and a throttle valve at

the outlet. The reference cellFfilled with a high-concentration CO2 mixture and

sealed offFis connected to the measurement cell by a temperature bridge and a

differential pressure sensor. With adjusted laser power and the right concentration in

the reference cell the signals from both detectors have identical shape and amplitude

and after system calibration using certified gas mixtures the reference signal can be

used as a secondary calibration standard. The signal processing schemes for single-

tone and two-tone FM spectroscopy can be found in detail in a recent review of

advances in semiconductor laser-based gas monitors [1].The NIR system shown above has been applied for the detection of atmospheric

CO2. The DC current of the temperature-stabilized DFB laser is adjusted to the

selected absorption line. The laser is scanned over this line by a 1 kHz ramp and

additionally modulated by high frequency. In order to optimize the signal-to-noise

ratio and the system stability two different FM techniques (single-tone and two-tone)

have been implemented. Due to the limited detector bandwidth, frequencies up to

300 MHz are applied, corresponding to the line width of CO2 at a pressure of

100 mbar. Some FM signals from a near-infrared InP diode laser operating around

2mm are shown in Fig. 3c when outdoor air has been pumped through the Herriott

cell and the ambient CO2 signal of 350 ppm was recorded at 2.004 mm. From theweaker absorption lines of CO2and H2O under high amplification a precision of up

to 104 has been estimated for 256 averages in less than 1 s. It is obvious that this

spectral region has a significant advantage versus the 1.573 mm absorption band in

the NIR, where the line strength is about 2 orders of magnitude weaker. At 2 mm the

line strength is still weaker than in the fundamental band, but room-temperature

operation of diode lasers is still possible for cw applications [2]. Recording of

averaged spectra and calibration lead to time series data as shown in Fig. 3d for a

358 ppm CO2 calibration gas mixture. Besides ambient measurements another

possible application could be isotopic ratio measurements for medical diagnosis, e.g.

breath test.1 Therefore, 13CO2/12CO2 line pairs within the operation range of the

1 E.g. IRIS-infrared 13C stable isotope analyser, www.wagner-bremen.de.



9/14

tested 2mm laser have been analyzed in the vicinity of 2000.8 and 2001.3 nm. The

latter line pair is partly influenced by water vapor as displayed in Fig. 3e but one has

to consider that the CO2 absorption in expiration air is by a factor of 2030 higher

than the displayed ones. For medical applications the measuring instrument shouldbe able to determine the ratio of 13CO2 and

12CO2 with a resolution of at least 1%

[2], while the absolute CO2 concentration in human expiration air is 35%, with

100% relative humidity at body temperature. The natural ratio 13CO2 :12CO2 is

1.09570.003%. Therefore, the instrument must be able to detect both isotopes with

a resolution ofo103 unaffected by water vapor. The measurement of the 13CO2content in expiration air using the lines at 2000.8 and 2001.3 nm generates about the

same signal as 12CO2 in ambient atmospheric air masses at 2004 nm. Therefore,

equivalent detection limits are expected. For a typical required reproducibility of less

than 0.3 delta per mille for the determination of the 13C/12C ratio, the neighboring12CO2 lines, which are weaker by a factor of 4, dominate the sensitivity, but even at

absorption path lengths of less than 10 m clear signals are expected. The main

problem of accuracy is connected with temperature: the ratio of the line strength

must be kept constant, which means here that the temperature during the analysis

including calibration must be stable within 0.01 K, because the involved line

strengths show opposite temperature dependencies (at 2001.3 nm the ratio of the line

strengths changes from 5.4655 at 290 K to 4.081 at 300 K). A reliable measurement

of the 13CO2 :12CO2ratio using the 2000.8 and 2001.3 nm spectral regions is not easy

to perform, but feasible when using a small multipass cell. Nevertheless, the

utilization of line pairs with reduced sensitivity to temperature changes isrecommended. Therefore, the line pairs must be chosen in such a way that their

temperature coefficients are low but also as close as possible to each other. However,

this measurement illustrates the principle and feasibility of isotopic measurements

using tunable diode laser absorption spectroscopy, but especially for 13CO2/12CO2

isotopic ratio analysis already commercially available competitive systems based on

non-dispersive infrared analyzers exist, which meet the requirements in terms of

sensitivity and time resolution (see footnote 1).

5. Fast chemical mid-infrared sensor for methane based on a lead-salt diode laser

A fast laser-optical sensor is the key issue for trace gas flux measurements based

on the so-called eddy correlation technique [1214]. As high time resolution and high

chemical resolution were the prerequisites for the success of the measurements, high-

frequency modulation (FM) spectroscopy has been selected. In FM spectroscopy,

the laser is modulated at higher frequencies, typically in the radio frequency (rf)

region, thus allowing selective and fast scanning over an absorption line of a

molecule [1]. The layout of the detection electronics is shown in Fig. 4a. The system

can be run in either single-tone or two-tone mode and the optimum method can be

selected by the system operator from case to case depending on the experimentalconditions. The modulation section provides the required rf-modulation for the

laser. The laser beam is split into a sample and a reference path. Detector signals are



10/14

fed into a phase-sensitive detection electronics, which can be regarded as a high-

frequency lock-in amplifier. Both signals (sample and reference) are then digitized

and further processed by digital filters, line locking, normalization and calibration

procedures [22]. The reference beam goes through a reference cell, which provides a

high signal-to-noise ratio signal from the spectral feature under investigation. This

channel is used for line locking and online drift correction. A line locking proceduremonitors the deviation of the signal position from a given set-point and decides

whether a change in temperature or current has to be made to compensate for drifts.

Fig. 4. (a) Setup of a mid-infrared (7.8mm) high-frequency modulation spectrometer with a sample and a

reference cell, high-bandwidth HgCdTe detectors and subsequent phase-sensitive detection electronics, (b)

optical layout of the system with a small volume (0.3 l) Herriott cell, (c) photo of the fast chemical sensor

for eddy correlation trace gas flux measurements during a field campaign in Italy [14], (d) typical modemap of a lead-salt diode laser, (e) time series data of ambient methane concentrations with 10 Hz time

resolution to calculate the (f) methane flux from rice paddy fields over several days. Each point is based

upon 18,000 individual concentration measurements as shown in (e).



11/14

In order to compensate for fast and small fluctuations the signals are online shifted

prior to signal averaging. These corrected signals are then fed into a digital filter

routine which removes the background and references the signal to a previously

stored calibration signal. All algorithms and procedures were implemented in a real-time parallel multi-processor system. The optical layout together with the external

control units of the system based on the detection scheme described above is shown

in Fig. 4b/c. For typical line strengths an ambient concentration of 1 ppbv produces

an absorption of only 1 part in 107 over a 10 cm path length. Conventional

absorption spectroscopy would not be able to measure such small absorptions.

TDLAS overcomes this problem by using a (White or Herriott) multi-pass cell with

effective path lengths of 100 m or more. All optomechanical components of the

spectrometer are mounted on a 50 cm 90 cm optical breadboard. The lead-salt

diode laser is mounted on a cold head within an LN2 dewar (Laser Photonics, Inc.,

Analytics Division, Andover, MA, Model L5736 Laser Dewar). The diverging laser

beam is collimated to a nearly parallel beam (1) by an off-axis parabola (OAP) and

passes through the beam steering optics, which focuses the beam at the center of a

commercial astigmatic Herriott cell (New Focus, Inc., Santa Clara, CA, Model 5611

Multipass cell) with a total path length of 18 m. This cell has a very small internal

volume of 0.3 l and is especially designed for applications requiring high time

resolution. The outcoming thin and nearly parallel beam (3) is then focused onto a

broadband HgCdTe measurement detector using a BaF2 planoconvex lens. The

reflex of the cell inlet window is used as reference (2) after passing a small cell

containing pure methane gas. This reference beam is used, on the one hand, for thereliable identification of the absorption features of the trace gas of interest, and, on

the other hand, for the active stabilization (line locking) of the spectrometer. A

visible (680 nm) diode laser is used to align the system. The beam splitter, which sets

the alignment beam onto the main light path, is removed during the measurement. A

rotary vacuum pump (Leybold AG, K .oln, Germany, Model SOGEVAC SV 65)

provides the gas flow of about 18 slm through the Herriott cell at a pressure of about

50 hPa. A dust filter is mounted at the inlet point of the measurement head to protect

the gas system and especially the mirrors of the Herriott cell from pollution. A newly

developed calibration system allowed programmed sequences of measurements of

background signals (N2), calibration gas and ambient air. The calibration system isbased on a dilution system and high-concentration calibration gas from steel

cylinders is diluted to ambient concentrations.

The key element of the fast chemical sensor is the laser diode. In the lower part of

Fig. 1 the absorption spectrum of methane is plotted together with atmospheric

water absorption versus wavelength. The near-IR absorption consists of overtone or

combination bands that are typically one to several orders of magnitude weaker than

the IR-fundamental band. Antimonide lasers [7] can be used for the detection of CH4at 3.26mm (n3) and for the 7.8 mm (n4) spectral region lead-salt diode lasers are the

optimum choice, as they cover the IR fundamental bands with strong absorption for

the most atmospheric trace gases. In trace gas monitoring applications, lead-saltlaser instruments have routinely achieved parts-per-billion detection levels of a

number of important molecular species. When starting to select a laser, the first task



12/14

is to select from mode maps a combination of base temperature and drive current at

which the laser produces a strong, preferably single-mode emission, tuned to the

absorption line being monitored. After investigation of several antimonide and lead-

salt diode lasers, the lead-salt device with the characteristics shown in Fig. 4d hasbeen selected. For injection currents between 400 and 600 mA at temperatures

ranging from 85 to 95 K single-mode operation with an average power level of

200mW was ensured and isolated methane absorption lines could be reproducibly

selected for the measurements even after repetitive thermal cycling, which was an

important criterion for the planned field measurements.

With this spectrometer ambient methane concentrations around 2 ppm can be

detected with a precision of about 1% at a 10 Hz repetition rate and a typical data set

of 18,000 individual concentration values is shown in Fig 4e. The data have been

collected during about half an hour of sampling at 10 Hz. Each concentration value

has been obtained by averaging individual spectra followed by a background

correction. Such fast response measurements of state variables generate time series of

data that can be statistically analyzed. As a result a set of flux data has been

obtained, which is shown in Fig 4f. Each data point shown represents a 30 min

average obtained from time series data as shown in Fig. 4e. From an intercompar-

ison of these data with several other independent measurements based on gas

chromatography it was found that there is a bias of up to 70% between different flux

measurement methods [14]. This finding is important for atmospheric chemistry

research in the context of greenhouse gases. Such simultaneously fast and highly

sensitive measurements would not be possible with near-infrared systems due to thelack of sensitivity and the results shown here demonstrate that tunable diode laser

absorption spectroscopy can be a valuable tool for quality assurance and quality

control.

6. Conclusion

The features of diode laser spectroscopy rendering it such a valuable technique for

gas analysis are given below. It is specific and as a high resolution spectroscopic

technique it is virtually immune to interference by other speciesFa problem thatplagues most competing methods. This ability to provide unambiguous measure-

ments leads to the use of TDLAS as a reference technique against which other

methods are often compared. It is a technique universally applicable to all smaller

infrared active molecules and the same instrument can easily be converted from one

species to another by changing the laser and calibration gases. The time resolution of

TDLAS measurements can be traded off against sensitivity and this allows very fast

measurements with millisecond time resolution. In order to improve sensitivity

various types of modulation spectroscopy have been employed in which the diode

laser wavelength is modulated while being scanned across an absorption line. These

modulation techniques allow absorption as low as 1 part in 106 to be measuredwithin a 1 Hz bandwidth. In combination with optical multi-pass cells, this is

equivalent to detection limits of around 20 pptv for the most strongly absorbing



13/14

species and better than 1 ppbv for almost all species of interest [1]. TDLAS has made

the transition from a technique mainly of interest to instrument developers into one

which produces results of real value to trace gas analysis and atmospheric chemistry

studies. However, it has yet to become an instrument suited to routine use by non-expert operators, mainly due to the complex and sometimes unreproducible behavior

of the lasers. Further laser development is underway to remedy this and

semiconductor lasers are available meanwhile from the visible to the mid-infrared

and hopefully the spectral coverage is continuously increasing. Strong effort is

underway to further improve quantum cascade lasers as an alternative source of

infrared tunable radiation. Anyway, the near- and mid-infrared spectral regions will

provide complementary systems. For a certain limited number of species, where

ultrahigh sensitivity is not required, the near-infrared systems will provide

advantages of size, simplicity and cost. For other species requiring a more universal

and sensitive system, the mid-infrared diodes will continue to provide a superior and

highly specific measurement device.

References

[1] Werle P. A review of recent advances in semiconductor laser based gas monitors. Spectrochimica

Acta A 1998;54:197238 with 183 references.

[2] Werle P, M.ucke R, DAmato F, Lancia T. Near-infrared trace-gas sensors based on room-

temperature diode lasers. Appl Phys B 1995;67:30715.

[3] Modugno G, Corsi C, Gabrysch M, Marin F, Inguscio M. Fundamental noise sources in a high-sensitivity two-tone frequency modulation spectrometer and detection of CO2 at 1.6 mm and 2 mm.

Appl Phys B 1998;67:28996.

[4] Mihalcea RM, Webber ME, Baer DS, Hanson RK, Feller GS, Chapman WB. Diode-laser absorption

measurements of CO2, H2O, N2O, and NH3 near 2.0mm. Appl Phys B 1998;67:2838.

[5] Gianfrani L, De Natale P, De Natale G. Remote sensing of volcanic gases with a DFB-laser-based

fiber spectrometer. Appl Phys B 2000;70:46770.

[6] Koch J, Zybin A, Niemax K. Element selective trace detection of toxic species in environmental

samples using chromatographic techniques and derivative diode laser absorption spectrometry. Appl

Phys B 1998;67:47583.

[7] Werle P, Popov A. Application of antimonide lasers for gas sensing in the 34mm range. Appl Opt

1999;38:1494501.

[8] Lancaster DG, Richter D, Curl RF, Tittel FK. Real-time measurements of trace gases using a

compact difference-frequency based sensor operating at 3.5 mm. Appl Phys B 1998;67:33945.

[9] Tacke M. New developments and applications of tunable IR lead-salt lasers. Infrared Phys Technol

1995;36:44763.

[10] Werle P, Slemr F, Gehrtz M, Br.auchle Chr. Quantum-limited FM-spectroscopy with a lead-salt

diode-laser. Appl Phys B 1989;49:99108.

[11] Fried A, Henry B, Wert B, Sewell S, Drummond JR. Laboratory, ground-based, and airborne

tunable diode laser systems: performance characteristics and applications in atmospheric studies.

Appl Phys B 1998;67:31730.

[12] Zahniser MS, Nelson DD, McManus JB, Kebabian PL. Measurement of trace gas fluxes using

tunable diode laser spectroscopy. Philos Trans R Soc London 1995;351:37182.

[13] Kormann R, M.uller H, Werle P. Eddy flux measurements of methane over the fen Murnauer Moos,

111110E, 471390N, using a fast tunable diode laser spectrometer. Atmos Environ 2001;35:253344.

[14] Werle P, Kormann R. Fast chemical sensor for eddy correlation measurements of methane emissions

from rice paddy fields. Appl Opt 2001;40:84658.



14/14

[15] Werle P. Laser excess noise and interferometric effects in tunable diode-laser absorption spectro-

scopy. Appl Phys B 1995;60:499506.

[16] Faist J, Capasso F, Sivco DL, Sirtori C, Hutchinson AL, Cho AY. Quantum cascade laser. Science

1994;264:5535.[17] Gmachl C, Capasso F, Faist J, Hutchuinson AL, Tredicucci A, Sivco DL, Baillargeon JN, Chu SNG,

Cho AY. Continuous wave and high-power pulsed operation of index-coupled distributed feedback

quantum cascade lasers at lE8.5mm. Appl Phys Lett 1998;72:14302.

[18] Namjou K, Cai S, Whittaker EA, Faist J, Gmachl C, Capasso F, Sivco DL, Cho AY. Sensitive

absorption spectroscopy with a room-temperature distributed-feedback quantum-cascade laser. Opt

Lett 1998;23:21921.

[19] Kosterev AA, Tittel FK, Gmachl C, Capasso F, Sivco DL, Baillargeon JN, Hutchinson AL, Cho AY.

Trace gas detection in ambient air with a thermoelectric cooled, pulsed quantum cascade distributed

feedback laser. Appl Opt 2000;39(36):686672.

[20] Webster CR, Flesh GJ, Scott DC, Swanson JE, May RD, Woodward WS, Gmachl C, Capasso F,

Sivco DL, Baillargeon JN, Hutchinson AL, Cho AY. Quantum cascade laser measurements of

stratospheric methane and nitrous oxide. Appl Opt 2001;40(3):3216.[21] Werle P, M.ucke R, Slemr F. Limits of signal averaging in atmospheric trace gas monitoring by

tunable diode laser absorption spectroscopy. Appl Phy B 1993;57:1319.

[22] Werle P, Scheumann B, Schandl J. Real time signal-processing concepts for trace-gas analysis by

diode-laser spectroscopy. Opt Eng 1994;33:3093108.

[23] Werle P, Lechner S. Stark-modulation-enhanced FM-spectroscopy. Spectrochim Acta A

1999;55:194155.


werle phd thesis_mit_2002

Documents