mid-infrared tunable quantum cascade lasers for gas-sensing applications

Mid-Infrared Tunable Quantum Cascade Lasers for Gas-Sensing Applications

emiconductor lasers are familiartomost peopleas lightsources in laser pointers, laserprinters, or CD players, aswell as optical communications. In the latter application they have reached avery high standardofsophistication, withultrafine con- trol of wavelength (typically around 1.3 and 1.55 wm], linewidth, and output power, and they are mostly built from S InP-based semiconductor heterostructure materials. They represent, however, only a small fraction of the semiconductor

lasers that are available. While optical data-storage evolves toward blue lasers, and optical communications around near-infrared wavelengths, another

wavelength range-the mid-infrared, ranging from - 3.5 to 12 pm-is attracting a lot of attention. Most trace gases of importance, from byproducts of burning fossil fuel to constituents of human breath, have telltale absorption features in this wavelength range

w IO 8751-3996/00/$10.00 02000 ItEE CIRCUITS 8 DEVICES m MAY 2000 -.--,.a- . . .__-__ -..--,. - _l-~______________I

(known as the molecular "finger-prinv region of the spectrum) as a result of molecular rotational-vibrational transitions. Nar- row linewidth, lunable semiconductor lasers in this wavelength range are used to speclrally map out and qualitativelylquantita- tively detect these trace gases through a measurement tech- n ique called tunab le inf ra red laser-diode absorp t ion spectroscopy (TILDAS) I I]. The advantages of TILDAS include its high sensitivity and specificity, its noninvasive and real-time nature typical of optical methods, and the compactness and ro- bustness of a packaged semiconductor laser device.

The fundamentals of conventional semiconductor lasers dic- tate that the material bandgap determines to a large degree the emission wavelength. Light is generated when an electron makes a transition from the conduction band into the valence band,whileemittingaphotonofenergyclose to the bandgapen- ergy. The latter is defined as the smallest energy difference between the conduction andvalence hands. Therefore, in order to reach ever-longer wavelengths, different materials have to be used. I h r the wavelength range around 3-4 bin, antimony-based materials are the materials of choice 12, 31, while for longer wavelengths up to 20 bm lead-salt combinations are indicated 141. However, under many circumstances, and particularly for longer wavelengths, these materials are less developed and less reliable than the well-known and mature InP- or GaAs-based semiconductor heterostructure materials of the near-infrared. Therefore, semiconductor lasers have, despite their promises, only been used sparsely in trace gas-sensing applications, and predictions of their market dominance were less optimistic.

This situation may have changed with the invention of the quantum cascade (QC) laser in 1994 15, 61. This fundamentally different laser does not involve the material handgap for the generation of light. Therefore, InP- and Gds-based 111-V semiconductor materials can now he used for the generation of long-wavelength, mid-infrared light. These materials are also straightforward to process and pattern. This is essential for the more sophisticated device geometries such as distributed feedback lasers (DFBs), which require a periodic modulation (i.e., a grating) ofamaterial parameter (e.g.,therefractiveindexor loss coefficient) to he integrated into the device.

DliB lasers provide a very elegant and reliable method to achieve a well-defined single-wavelength emission (called single-mode operation) as opposed to the usually multiple-mode emission of free-running Fabry-Perot resonators. Repeated scattering from the grating favors one wavelength (the Bragg wavelength) and it is the grating period rather than the peak position ofthe emission spectrum that delermines thesingle-modeemis- sion wavelength. QC-DFB lasers were first demonstrated in 1996 17, X].They have evolved very rapidly and have already shown great promise in many different gas-sensing applications [9-161.

Ouantum Cascade lasers Light is generated when an electron makes a transition from an upper energy state to a lower energy state, emitting a photon of exactly the energy difference ofthe two states. In QC lasers these energy levels (the upper and lower laser level) are produced in

in many different gas-sensing applications.

the conductionbandofamultiple quantum-well structure. Thin layers of InGaAs (the wells) are alternated with thin layers of AlInAs (barriers). The layers are so thin that the allowed energy levels of the electrons are noticeably separated (quantized) so that their separation exceeds the natural broadening of the individual states. Electrons traversing the structure can do so only through transitions between these discrete energy levels, one oi which provides for the laser transition. Two major consequences arise from this way of generating light. First, the energy separation of the energy levels is primarily a function of the thickness of the layers and to a much lesser extent of the material itself. Second, after having made the optical transition, the electron is still in the conduction band, althoughatalower energy. Bygain- ing again energy (e.g., through an applied electric field), the electron can he reused in another adjacent active region if many arestackedontopofeachother. Thisway, manyphotonsarecre- ated per electron, giving rise to high optical power for a large number of stages (N - 30 ~ 75). This "cascading scheme" also leads to an external quantum efficiency (i.e., increase of light intensity per current increase) larger than one, which is unthink- able for conventional semiconductor lasers. In the latter, the electron makes the optical transition, from the conduction band into the valence band, just once. Only recently, specially designed interband semiconductor lasers have been configured as cascaded structures with electron recycling L171.

QC lasers are an excellent example of a"materia1-by-design." By solving the Schroedinger equation of quantum mechanics for the "particle in the box" problem adapted to the electron in the quantum well, QC lasers are designed to emit at particularwavelengths. In addition, their material parameters, such as electron scattering rates and optical dipole matrix elements, as well as the optimum conditions for the electrons tunneling resonantly into the upper Ia- ser level, are optimized by manipulating layer thicknesses to achieve a high-performance laser. Unconventional features, suchas

(IRIUITS B OFVIES MAY 2000 I1

multiple-wavelength emission or a bias-polarity-dependent emission wavelength, can ais0 be designed into the lasers [18, 191.

Figure 1 shows an example of a QC laser emitting a t a - 9.5 p m wavelength [20]. The electron traverses the structure from left to right. After tunneling through the injection barrier (I) into the upper laser state (energy level 3), it makes a transition into the lower levels (2 or 1). Two competing mechanisms are possible: emission of a longitudinal optical (LO) phonon-a lat- tice vibration-or emission of a photon. The lifetimes associated with LO-phonon emission, which is the dominant process below laser threshold, between levels 3,2, and 1 can be calculated as TJZ

= 2.3 ps, 231 = 2.5 ps, and 721 = 0.3 ps, respectively. The latter is much shorter than ~ 3 2 because levels 2 and 1 are designed to have an energy separation approximately resonant with the LI phonon (- 34 meV). This helps in the fast emptying of level and, therefore, leads to a strong population inversion betwee levels 3 and 2 (i.e., fewer electrons scatter from 3 into 2 than 01

of2 in the same time interval). If the optical dipole matrix elc ment is also designed to be large, so that the optical gain ca overcome the losses at reasonable current densities, this allou for laser emission between 3 and 2.After the rapid transition be tween levels 2 and 1 by LO-phonon emission, the electron tur nels into the injector through the exit barrier. The injectc region is designed to allow for electron transport out of levels

i Active 1

rC Reaion i

3

2 1

1. Conduc f ion -band~~"~1~ oftwo actiue regions with the infermedi- ate injector region under an applied electric field of50 kVkm of 11 41

laxer emitting around 9.5pm. The modulisquared ofthe relevant wauefinctions are labeled L2, 5 andg for the ground state of the in jector. I indicates the injection harrier. The wau.q arrow den0te.i the 1a.w transition. The actual lager 1hicknesse.s in nanometer,? for one period ofadive region and injector are from lelt to right, starting

from the first injection harriers: 3.6/2.7/1.2/7.8/1.0/6.1/2.8/4.8/1.8/e2/1.4/~1.2/3.4/0.9/3.3. The

Alo.rdna& lagers (energg barriers, 520 mevhigh) are in hold and are alternated with the Ca~.r~In~,s lAs wells. The underlined 1ci.qers urc doped to n = 2 x lo" on"; italics indicates the injector re.qion l2Ol.

__-_____(^~. jji ~L-~---------- .. 12

and 2, and it also provides a region of low density of states for en- ergiesat thevalueoflevel3. This helpstoprevent electronsfrom tunneling out of state 3 directly into the injector and therefore improves the yield for transitions of the type 3 + 2.

Asaresultoftheappliedelectricfield, theelectron traversing the injector regionacquires enough energy to be re-injected into state 3 of the following downstream active region, where it can create the next laser photon. In a more extensive work on 8 pm-wavelength QC lasers [ZI], we have demonstrated working lasers containing only a single active region and lasers containing up to 75 stages of alternating active regions and injectors (standard lasers typically contain approximately 25-30 stages). We were able to demonstrate unit)) "cascade efficiency": i.e., that, within the experimental errors, in the best lasers each electron (injected above laser threshold) indeed emits one photon per stage.

Using the same approach demonstrated for the 9.5 pm-wavelength laser, also called the "three-well vertical" active region [22],QClaserswithwavelengthsasshortas3.5pmandup to 13 p m have been demonstrated. Figure 2 shows a transmission-electron-microscope (TEM) image of such a laser emitting a t 5.2 pm, Many different approaches to various types of active regions have been used as well, containing one, two, three, four, or more quantum wells. Here we will only briefly mention the so-called superlattice (SL) active region [23], as it is frequently used to cover the longer wavelength range from 7 to 17 pm, In SL QC lasers, the optical transition takes place between the low- est state ofan upper miniband and the highest state ofthe lower miniband. Minibands are manifolds of electronic states that are delocalized over several (typically 6-R for QC-lasers) quantum wellsand barriers (the "superlattice") with the individual energy levels broadened into a band. The energy range oflow-density of states in the SL between the minibands is called "minigap." Minibands allow for high current density, as tunneling between distinct states is replaced by band-like carrier transport. A large current density is essential in overcoming the increasingly higher waveguide losses at longer wavelengths. Furthermore, the attainable optical dipole matrix element is higher for the SL lasers than for other active region designs. Finally, the higher current-carrying capabilities also allow for higher peak optical-power levels.

QC lasers are routinely operated up to room temperature and above in pulsed mode (with 50 ns pulse width and less than 100 ktlz repetition rate) in the wavelength range from - 4 to 11.5 pm, Record peak power levels of 0.5 Ware achieved around the 8 p m wavelength, either by employing a very high number of stages or by using large aperture (broad area) device geometries. At lower temperatures (e.g., liquid nitrogen (LN2) temperature -ROK),peakpowerlevelsarehigher,closeto1.5W[21].Contin- uous wave (cw) operation is presently limited to temperatures below 175 K 1241. However, a t LN2 temperature, maximum power levels of 200 mW per facet have been measuted at 5 and 8 km wavelengths.

CIR[UIIS a DEW . MAY zooo

Single-Mode Tunable OC Distributed-Feedback laser

Typical QC lasers are processed as deep-etched-ridge waveguide lasers several micrometers wide and 1-3 mm long (see Pig. 2). While the narrow ridge provides good current confinement and also lateral optical confinement, the extended length of the device is needed to reduce the effect of the outcoupling losses from the cleaved laser facets that serve as the cavity mirrors, each with only - 30% reflection. The alternative approach of depositing a high-reflection coating onto at least one facet is more difficult for the longer wavelengths and sometimes contends with un- wanted absorption in the coating material. The longer cavity leads to a spacing of the Fabry-Perot modes that is small (- 0.7 cm? fora2.25mmlonglaser) comparedto thewidthofthegain spectrum (greater than50 cm'). This is responsible for the lasers operating in multiple modes under usual conditions, certainly in pulsed but also in cw at high current levels.

AtypicalFabry-PerotspectrumofaQClaseremittingat8~m

., .<, I I .(, .* A-.: '. . . -5. c. .; P,'. i"

.%~ . . ,

* s. :

M o s t t race gases of importance have

characteristic absorption features

ji

in t he mid-infrared, between the 3 and

17 pm wavelengths.

is shown in Fig. 3. Such a spectrum, while acceptable for some spectroscopy applications targeting liquidsand solids, where the 1 characteristic absorption features are broad, continuous bands, is not useful for sensing applications requiring high sensitivity and wavelength selectivity, such as trace gas analysis to sub parts-per-million in volume (ppm-v) resolution. Rather, a single-mode output with good side-mode suppression ratio (SMSR), which can be controlled and tuned over a certain wavelength range, is needed. To that aim, QC lasers are fabricated as DFB lasers incorporating a first-order Bragg-grating.

In order to vertically confine the mid-infrared wavelength to a structure less than I 8 Fm thick (for a reduced growth time and efficient heat transfer out ofthe structure), the top cladding ofthe conventional QC laser waveguide is usually composed of several layers: low-doped inner layers capped by a highly doped layer 1251. Taking advantage of the anomalous dispersion of a material close to its plasma frequency, the refractive index of this highly doped layercanbemadeclosetoone (while keepingitsabsorptioncoeffi-

This Characteristic QC laser waveguide 1251 provides several opportunities to produce agrating modulationstrong enough to provide the necessary wavelength-selective feedback. In the first type, the grating is etched into the surface (the highly doped top-most layer) ofthe waveguide with three consequences; Pig. 2 shows apartialphotograph ofsuch adevice. First, the waveguide loss in the position of the grating grooves is higher than in the position of the grating ridges (the unetched portion of the grating), leading to a loss-modulation of the waveguide. Second, the metal layer in the grating grooves-now closer to the low-doped section of the waveguide-pulls the mode toward itself. As the mode is pulled into the top-cladding layer, its effective refractive index (roughly defined as theaverage refractive indexofthevari- ous waveguide layers, weighted by their overlap with the optical mode) changes. Third, this displacement of the mode also results in a modulation of the overlan ofthe guided mode with the

cient still comparatively low).

infrared radiation. In direct (a) (b) with the high

tive index semiconductor material, the metal layer also

s'l'porLs interface plasmon modes [26].

\. (a) Scanning-electrun~microscope image of the clwmed end-facet rqion o f a QC-DFn laser with top grot- ing 171. For m m t o f the length ofthe deepetched ridge, the 850 nm period Bragg-gmtiny is covered with metal (upper edge ofphnlngraph), onlg the end region< are lee open for better c1eavin.g. 01) Transmis-

sion-eleclron~miLrosCoPL. VEM) image tif three periods o f actirje reqion m d injector of a RC laser emitting around 52ym /ZZl. The thinnmt InCoAs lager (hnght) i.5 1 nm thick; one period of active region and injec-

tor ut 45 nm. Allmk luger.? are dark. PEM6g S.N. George Chu, Bell Lnhurutories, Lucent Technologies.)

(IRCUIIS 8 DEVICES MAY 2000 13.

1

active material, inducing a modulation ofthe modal gain, whir has the same character as a loss modulation and usually is d signed to increase the effect of the latter. Depending on the a tual etch depth compared to the thickness of the cladding layer one component of waveguide modulation may win over tt other ones (the modulation of the gain via the confinement fa tor is usually the weakest component). All our gratings were fa ricated by optical-contact lithography and wet chemical etchir in aged HBr : HN03 : Hz0 = 1 : 1 : 10. The obtainable etch depth for practical reasons limited (e.g., to - 350 nm for a grating p riod of ~ 1 pm). The single-mode emission wavelength is ve close to the Bragg wavelength h~ = Z.ndi.A, where n,ff is the e fective refractive index of the waveguide and A the grating p riod. Grating periods vary from 800 nm for h - 5 Fm emissic wavelength to 1.25 p m for h ~ 8 pm.

The first QC-DFB lasers were "complex-coupled" structure i.e., they had a modulation of both the loss and the refractive i i

dex, with both being comparable in strength. The plasmon-lay, thickness was greater than 0.6 Fm and was only partly etched t the grating. Single-mode emission was achieved in pulsed ope ation for two different QC-DFB lasers (5.3 and 7.8 Fm) [7].1'1

8.55 8.6 8.65 Wavelength (pm)

(a)

I 4 '

8.55 8.6 8.65 Wavelength (pm)

(b)

3. (ui Single-mode emission spectrum ofu QC-DFFH laser with buried grutiny operated of room temperature in pulsed mode (50 ns pulse

length, 80 kHz repetition rutei 1291. Tlle deuices are single mode up ti high current levels. (6) Multiple-mode emission spectrum of the corre sponding Fabrg~Perot-type luser. The spectrum has been taken close t

threshoid; us the current is increused. more modes are aci ted

best lasers operated single-mode in a temperature range from - 100 K up to room temperature (300 K) and displayed single-mode tuning ranges of70 and 150 nm, respectively. As the temperature is increased, both the Bragg resonance via the temperature dependence of the refractive index and the gain spectrum via the temperature dependence of the intersubband structure are shifted to longer wavelengths. The red-shift of the peak gain is approximately twice as strong as the shift of the Bragg resonance. The interplay of this temperature-induced de- tuning with the strength of the Uragg-grating is ultimately responsible for the extent of the single-mode tuning range (an initial blue-detuningofgain peakand Bragg resonance can be fa- vorably used to increase the tuning range and single-mode yield). The grating "strength" can be roughly approximated by the coupling coefficient K [27]. For our early devices we esti- mated a coupling coefficient of I K ~ - 2-3 cm'.

Recent QC-DFB laser designs [20,28] improved the top-grating approach by using a thinner, highly doped, top-most waveguide layer, which could be entirely removed in the grating grooves by etching. This resulted in a greater grating strength with a coupling coefficient of I K / t 15 cm-', which is noM1 clearly dominated by the modulation ofthe effective refractive index (by afactorof-4).Thelasersweredesignedfor -4.Gpmwavelength and also for the wavelength range of 9.5-10.5 pm. The single-mode tuning ranges with heat-sink temperature and the lasers operated in pulsed mode were 65 and 150 nm, respectively. Figure 4 shows the tuning curves of some representative lasers (including the 4.6 and 9.5-10.5 Fm lasers) operated in pulsed mode. The devices at 4.6 Fm were also the first QC-DSB lasers with a top grating to operate in cw; 20 nm ofsingle-mode tuning range was achieved at LN2 temperature by increasing the current through the device from threshold current to approximately twice that value. The heat dissipated in the device by the current is responsible for the temperature increase that leads lo the red-tuning of the wavelength.

Although the top-grating approach yields quite good results and provides for a straightforward time-saving processing technology, it suffers the fundamental drawback that the Bragg-grating is located only in the exponentially decaying wing of the waveguide mode. This ultimately limits the maximum achiev- able strength of the grating.

Therefore, early on we took another, parallel approach lo QC-DFB lasers, which positions the grating close to the active waveguide core, where the mode intensity is high. In a first growth cycle by MBE, the active waveguide core [a several-hundred nanometer-thick InGaAs layer followed hy the stack of many (- 30) periods of alternated active regions and injectors and capped by another - 500 nm-thick I n G d s layer] is grown. The wafer is then removed from the growth chamber, and the Bragg-grating is fabricated into the upper InCaAs layer using the conventional technique. The wafer is then transferred back into another growth chamber, where an InP top cladding is grown on top of the Bragg-grating using solid-source MBE. The refractive indexcontrast between InGaAs (- 3.48) and InP (- 3.1) and the strong overlap ofthe grating with the mode provide for a strong

I 4

modulation ofthe effective refractive index of the waveguide and a large coupling coefficient of I K I - 30-80 cm-'. Although the procedure of two growth cycles increases the fabrication efforts, QC-DFB lasers with buried gratings made for the best single-mode laser devices. We have worked extensively in the 5 and 8 p m wavelength range with this type of device [8, 29,301.

Figure 3 shows the single-mode spectrum of such a device at R.6 pm wavelength operated in pulsed mode at room temperature. The SMSR for QC-DFB lasers readily reaches 30 dB, which qualifies them as "single-mode" lasers. However. a precise deter- mination of this value is difficult due to the intrinsic noise floor and lineshape obtained from spectral measurements using a Fou- rier transform spectrometer and varions mid-infrared detectors. In Fig. 4, the single-mode tuning range (-150 nm) for this device is displayed. QC-DFB lasers with buried gratings also operate withoutdifficultyassingle-mode in cw. Figure 5shows thecwsin- gle-mode spectraand tuning ranges (with heat-sink temperature and drive current) of a similar laser around 7.95 pm. An equivalent performance was demonstrated for lasers arouml 5.2 wm. Their cw light output versus current characteristics are shown in Fig, 6 . Very high single-mode output power of - 150 mW at LN2 temperalure is achieved for these lasers, and - 120 mW is achieved for the lasers at 7.95 pm wavelength. The latter is a record value for semiconductor lasers in this wavelength range.

AthirdmethodofincorporatingaBragg-gratingintoaQC laser waveguide has recently been discussed and demonstrated by us [XI. Very long wavelength ( h - 17 pm) QC lasers are difficult to fabricate with dielectric waveguides (which would have to be very thick to accommodate the long wavelength while keeping the loss low). Furthermore, Bragg-gratings for this long wavelength would have to be very deep in order to result in an effective modulation, which makes regrowth on top of such a grating quite difficult.

Afundamentally different waveguide has been found to work well for these long-wavelength lasers; the surface plasmon [%I, pinned a t the metal-semiconductor interface, functions as the laser mode, with its intensity maximum right a t this interface. The waveguide only consists of the stack of active regions and injectors and the overlying metal layer (which at the same time may act as the top contact). The waveguide parameters strongly depend on the metal's dielectric function. Therefore, a Bragg-grating fabricated from alternating stripes of two different metals can have sufficient strength for single-mode operation a t the very long wavelengths. We demonstrated a pulsed single-mode QC-DFB laser based on this principle using a Bragg-grating of alternating titanium and gold (resulting in K - 7 cm-') with a periodicity of 2.0 pm. The lasers emitted a t 16.2 gm with 50 nm tuning range (between 10 and 100 K heat-sink temperature-which was also the temperature range of laser ac- tion for this device); see the bottom-most panel of Pig. 4. !

gle-mode DFR devices for QC lasers in the GaAs/AIGaAs material system [341. Many more groups have reported QC lasers, including Kazeghi at Northwestern University, IL; Li at the Shanghai Institute of Metallurgy, China: Ironside at Glasgow University, UK and Cockburn a t Sheffield University, UK. Sirtori a t Thomson CSF, France, and coworkers have demonstrated the first QC laser based on GaAdAIGaAs multiple quantum wells. Even more groups are working theoretically or experimentally on QC lasers and light-emitting devices. Furthermore, it should be noted that several other recent and novel approaches for mid-infrared semiconductor lasers exist, all with very impres- sive results, such as interband cascade devices (originally pro- posed and demonstrated by Yang e t al.; optically pumped intersubband lasers demonstrated by Julien a t the University of Paris-Sud, France, and coworkers; sophisticated multiple quan-

(a) ,!?mi.ssion wuuelength us ( I function ofthe heat-sink temperature as measured (circles) for uurious QC-DFIj laser devices operated in

ming approach [32] and also surface.emitting QC-DFB lasers (331 using a higher-order Bragg-grating. Schrenk et al. from the

Optlcul trunsmission of11 few hundred meters ofstandard atmosphere at sea level. The white regions indicate the so-called "otmmphwic

windows."These are sepam(e(/bg wide regions ofsfrong &orp[ion

tum-well structures as demonstrated by Meyer and coworkers a the Naval Research Laboratory, Washington D.C.; and others However, to our knowledge, single-mode DFB lasers have no been reported so far for these devices.

Gas-Sensing Applications Using aC lasers Figure 4(a) shows several single-mode tuning ranges of ou QC-DFB lasers operated in pulsed mode. Figure 4(b) shows th, transmission of several-hundred meters of standard atmosphere at sea level. Two large regions of good transparency are founi around 5 pm and from 7.5 to 13 Fm, the hvo so-called "atmo sphericwindows." Furthermore, most trace gases of importanci

1 .o

0.5

0 7.93 7.95 7.97 7.99

Wavelength (pm) I

5. Emission spectra at various cw currenl leuels between 420 ond 1000 mA at a constant heul-sink temperature of80 K of a QC-DFE la

ser with buriedgrating. The apparent linewidth is set hy the spectrometer re.wlution (> 0.25cm3. Signol levels ore druwn tu scule;

individuul speclra are, however, ofhe! vertically by 0.01. Insel: Zmis sion waudength as a function o f the cw current at various constant heat-sink temperatures. The symbols represent the actual data; the

lines are quadratic functions fitled to the duta I301

0 t 300

I " ' I " ' I " ' A // in Ambient Air , /

I i O K

500 700 900 1100

,;Ambient Air

Current (mA)

6. Light output ver.sus current characteristics ut various constant heat-sink temperatures obtained for a QC-DFD laser with buriedgm

inq emitting around5.2Mm. The dip in the traces marked bq an arrow is due to a water-nbso?ption feature in ambient air 1301

have characteristic absorption features in the mid-infrared, he- tween the 3 and 17 Fm wavelengths. As discussed earlier, QC-DFB lasers can he designed to emit at any wavelength in this wavelength range. Therefore, it was straightforward to examine the lasers' potential in demanding gas-sensing applications. To that aim, many fruitful collaborations with expert spectrosco- pists were established; some of the results are presented in the following.

Whittaker et al. [SI at Stevens Institute of Technology, NJ, used pulsed QC-DFB lasers with a top-grating a t near- room-temperature conditions to measure mid-infrared ( h - 7.8 bm) absorption spectra of NzO and CI 14 diluted in Nz (prepared in a 10-cm-long single-pass gas cell). They employed a measurement technique known as wdvelength-modulation spectroscopy. The noise-equivalent sensitivity limit ofthe measurement was 50 ppm.

Pulsed QC-DFB lasers show a wavelength chirp caused by heating during the current pulse, which results in an integrated linewidth ofseveral hundred megahertz.This is sufficiently narrow to measurewith confidence the pressure-broadened absorption features of trace gases dispersed in standard atmosphere. It is too broad, however, for gases at low pressure, when the ah- sorption width (around 10 MHzj is predominanlly determined hy the motion of the molecules, an effect called "Doppler-broadening." To achieve the necessary narrow linewidth, the lasers have to be operated in cw. Therefore, the following measurements all used cw-operated buried-grating QC-DFB devices at LN2 temperature. The tuning of the single-mode output over the absorption feature(s) of the target gas is accomplished hy varying the current through the device. A rough glimpse of such an experiment can be seen in Fig. &The dips in the light output versus current characteristics (indicated hy arro\vs) are due to absorption of the laser light by water vapoy in the few centime- ters of room air hatween the laser cryostat and the detector. In a real gas-sensing experiment, the emission wavelength as a function of the heat-sink temperature and laser drive current would be calibrated (inset of Fig. 5), resulting in the position of the water absorption line, and the depth of the dip would correspond to the amount ofwater in the air. (The feature shown in Fig. 6 is ac- tually adoublet ofabsorption lines, pressure-broadened into one broader feature.)

Sharpe et al. I101 at the Pacific Northwest National Labora- tory (PNNL), WA, conducted high-resolution, Doppler-limited, direct-absorption measurements of NO and NH3 using QC-DFB lasers at 5.2 and 8.5 bm, respectively. The laser drive current was asaw-tooth ratchet with a 6-11 kHz repetition rate: with the ris- ing current the laser would tune - 2.5 cm?, covering, for example, 11 absorption features of NHj. The noise equivalent sensitivity limit in these measurements was3x 10-6absorbance.

Williams et al. (111 at PNNL in collaboration with M. Taubman and J. Hall at JILA, CO, measured the intrinsic linewidth of several of our QC-DFB 8 pm lasers by observing fluctuation ofthe collected optical intensity when the laser light was passing through a sample cell containing a gas (NzO) with well-known absorption features and with the laser being tuned

to (he side of one such absorption line. This resulted in a linewidth of - 1 MHz over - 1 ms integration time. The laser could furthermore be electronically stabilized (and locked to the wavelength of the NzO absorplion feature) by using the detector signal as feedback to the drive current. The so-stabilized lasers had linewidths of less than 20 kHz, which is very narrow for any as-cleaved semiconductor-DFB laser.

Paldus et al. 1121 at Stanford University, CA, together with Oomens el al. a t the University of Nijmepen, The Netherlands, reported photoacoustic spectroscopy on NH:l and Ha0 diluted in Nz using a cw QC-DFB laser emitting at 8.5 pm. The noise-limited minimum detectable concentration of Nlln was 100 ppb-v for 1 s integralion time. Paldus and coworkers at Informed Diag- nostics have also demonstrated sub-ppb-v (NHd sensilivity measurements using cavity-ring-down spectroscopy [ 191.

Kosterev el al. (14, 151 at Rice University, TX, reported on measurements of the concentration of '2C114; its natural iso- topes "CH4 and "CH<D, HaO, NzO; and CzHsOH diluted in standard air using a direct-absorption technique around 7.95 pm. Wilh a 100 m multipass gas cell and employing a new back- ground subtraction method, they were able to demonstrate a sensitivity limit in the ppb-v concentration range; e.g., 125 ppb-v of CzHsOIl.

Finally, Webster and coworkers 1161 at JPL, California Insti- tute of Technology, conducted measurements of the concentration of CI Iq and NzO in the earth's atmosphere from ground level tothestratosphere (- 70,000ft)usingacw-operated7.95pmQC laser and a wavelength-modulation technique. The laser in a 1,NZ dewarwasplaced on boarda hi~h-altitudeairplane. Thesur- rounding air was sucked into a multipass gas cell aboard the plane as the plane made eight-hour-long flights to map the atmosphere for the trace gases. The noise-equivalent sensitivity limit was ~ 2 ppb-v.

Several more projects are underway to test and demonstrate the QC-DFB lasers' qualification for demanding gas-sensing applications.

The Future Despile being quite-recent invenlions, QC and QC-DllB lasers have already achieved a high level of maturity, demonstrated by the many laboratories and research groups who study, fabricate, or use the lasers. Still, some questions remain unsolved thus far. First of all, for QC-DFB lasers there is the quest for the higher cw operating temperature (preferably close to 300 Ki. So far, Ihis has been miide impossible by the large dissipated power and the limited options for efficient thermal management of the large devices. Other fields of research are QC-DFB lasers fabricated from multiple-wavelenglh QC-lasers, which would emit corre- spondingly multiple single-modes, and the extension of lhe ac- cessible wavelength range into the near-infrared as well as inlo

Acknowledgments We acknowledge the fruitful collaborationwith J. Faist in the be- ginningofthiswork.l'hismateria1 is baseduponworksupported in part by DARPAIUS Army Research Office under Contract No. DAA65.5-I) 8 - C- 0 0 5 0. R , K . a c k n o w I e dge s s u p p o r t by Studienstiftung des Deutschen Volkes, Deutschland.

Cluire Gmachl (e-mail: [email protected]), Federko Capasso. Rudeger Kohler, Alessandro Tredicucci, ilIbert L. Hiitchinson, Deborah L. Sivco, James N. Haillurgeon, and Alfred % Cho are with Bell Laboratories, Lucent Technologies, in Murray Hill, New Jersey.

References 1.lI.I. Schiff, G.I. Miidcty, and J. Bec11;ira. "l-he use of tunablediode laser ab^

sorption spectroscopy for almarphcric measurements," in Air Monitoring bg Sneclrorcopic Tecliniqoer, M.W. Sigr is l , cd. NEIV York: \ W e y Intcrrcicnce, pp. 239-344, 1994.

2. D. Garhnmnv. M. Miliorav, H Lee, V. Khallin, IC. Martinelli, and 1. Cnnnolly. "Temperatore dclmdcnce of continunus wave threshold cur ren t lor 2.3~2.C u m InGaAsShlAl(:dsSh seiinrnt~ confinement heleroitruclurc i l u m l u m w c l i semicorrduclor diode l~~sers,"Appl. Phys /,ell., vol. 74. PI'. 29903892, 1999: WW. B w l ~ y , C.L. Felix, 1. Vurgaftman, D.W. Stolres, K.H. i\ifei, L J . Olafsen, J.R. Mcyer, M.J. Yang, R.V. Shanahrool~, H. Lee, R.U. Martinelii, i d A.I<. Sugg, "lligh lcmpcrature cml in i ious wave 3~6.1 uin'W' I r l~erswit l i diamand-pressure-band lieat ainking:Appl, i ""I. 74, 1075-1077,1999.

3. H.K Choi, G.W:rumcr, J.N.\Val~iolc. M.J. Manfr;i, Mli. Cnnnors, and L J . M i s s a g g i a , " l ,ow-threshold, h i g h - p o w e r , h i g h - h r i g h l n e s s GalnArShlAlGahSb quantum-well laseis emitting at 2.05 um," Pmr.

3284, ~ i p . 2fiX-273, IYBX: R. Lane, Z. \Vu, A. Slein, J. D i m and M. InAsSbllnAsl'strained-layer superlattice injeclion lasers operat-

ing at 4.0 um grown by melal-organic cliemical vapor deposilinn."Apjil.

U.W. Schiessl, "Lead sal1 tunable diode IISEIS: Key devices far high sensitivity gas analysis," Proc. SPIE, vol. 3628,pi). 113-121, 1999.

5. J. Paist, F. Capasso. U.L. Sivco, C. Sirtori, A.I.. Hutchinson, and A.Y. Cha,

6. F. Capasso, C. Cmachl, 11.1,. Sivca, and A.Y. Cho, "Quantum cascade lasers," Ph(is1cs World vol. 12, pp. 27-33, June 1999; F. Capasso, C. Gmachl, A. Trcdicucci, A.I.. Hulchinson, D.I.. Sivca, and A.Y. Cho, "High performance quantum cilscade i>tseis,ii Opt. ondPhaton.Nero.~, vol. 10, pp. 31~37.1

7. J. Faist,C. Gmachl, I?. Capasso, C. Sirtoii, D.I..Sivco, J.N. Raillargeon,A.L. Hutchinsan, and A.Y. Cho, "Dislrihuted feedback quantum cascade lilsers,'' Appl. l'hgs. Lett., vol. 70, pp. 2G70-2672, 1997.

8. C. Cmachl, J. lbist, J.N. Baillargeon, F. Capasso, C. Sirtori, D.L. Sivco, Complex-Couplcil Quantum Cascade D is t r ib~

"Quantum Cascade Laaer,"Science, vol. 264, pp, 553-556, 1994.

Photon. Tdtnoi . Lett., "01. 9, pp, 1omin92,

9. K. Namjou, S. Cai,and E A Whittaker, 1. Wlst, C. Cmachl, P. Capasso, D.L. Siveo, and A.Y. Cho, ''Sensitive absorplion ipcclroscopy wilh a r o o m ~ t e i n ~ peiatuie rlistrihutcil-feedback quantum~cascadc iimi," Opt. Lett., vol. 23, pp. 219-221, 1998.

10. S.W. Sharpe, J.li. Kelly, J.S. Hartman, C. Gmachl, F. Capasso, D.I.. Sivco, J.N. Baillargeon, and A.Y. Cho, "Higbresalution (Doppler-limilcil) spectroscopy using quantum~cascade distributed feedhack laseis," Opt. 1,ctt.. vol. 23, pp, 1396-l39R, 1998,

11. I 1 . M Williams, J.F. Kelly, J.S. Iliirtmiln, S.W. Sharpe, M.S.Taubman, J.I.. Hall, F. Capasso, C. Gmachl, D.L. Sivca, J.N. Haillargean, and A.Y. Cho, "Kilo-Hertz llncwidlh from ircquency stabilised mid-infrared quantum cascade lilsers.II Opt. Lett.. vol. 24, pp. 1844-1846. 1999.

12. B.A. Paldus, T.G. Spence, R.N. Zare; 1. Oomens, I%J. Harrcn, D.H. I'arker; C. Cmilchl, P. Capasso, D.I.. Sivco, J.N. Ba~llargeon,A.L. Hutchin- son, and A.Y. Cho, "Photuacousticspectroscopy using quantum~cascadc 1 a ~

13. R.A. Paldus, C.C. llarh, T.G. Spence, R.N. &re, C. Gmachl, F. Capasso, D.L. Sivca, J.N. Raillargeon, A.I.. Hutchinson, and A.Y. Cho, "Cavity ring-down spectroscopy using mid-infrared quantum cascade lasers,ii suh- mitted for publication.

14. A. Kusterev, R.F. Curl, F.K'Citte1; C. Cmachl, I?. Capasso, D.1,. Sivco, J.N. Haillargean, A.1.. Ilutchinson, and A.Y. Cho, "Methane concenlralion and isotopic composition measurements wsitli a mid-lnfrilrcil qunnlum cascade laser."Opt. Lett., vol. 24, pp. 1762-1764, 1999.

J.N. Baillargeon, A.L. Hutchinsan, and A.Y. Cha, "Dele~t ion olsimple and complex molecules in ambient air with B quantum cascade distributed iecdhack laser," siibmitted for publication.

16. C.R. Wehster, G.J. Ylescli, I1.C. Scolt, 1. Swansan, R.D. May, W.S. Wood- ward. C. Gmachl, F. Capasso,D.L. Sivcu, J.N. Baillargeon, A.I.. Hutchinsan, and A. V. Cbo, "Quantum cascade laser mcasurements o i stratospheric methane (CII,) and nitrous oxide (N,O)."submitted lor puhlicalion.

Rosencher, I 'h Collol, N. ILaurenl, J.I.. Guyaun, B.Vinter, ilaxinliy stacked lasers with Esaki junct ions: A lhipolar cas^

cade iaser,"Appl. PIzg.5. Lett., vol. 71, pp. 3752-3754, 1997: W. Sehmid, D. Wiedenmann, M. Grabherr, It. Jagcr, It. Michalzilt, K.J. Ebeling, "Cw opera- t ion o fa diode cascade InGaAs quantum well VCSEL,"Electron. Lett., vol. 34, pp. 553-555, 1998.

18. A. Tredicucci, C. Gmilchl, 1). Capasso, D.1.. Sivco, A.L. Hutchinson, anil A.Y. Cho, "A multiwavelength semiconductor laser,"Nolore, vol. 396, pp. 350-353, 1998.

sers,''opt. Let t . , ""~. 24, pI,. 17~~180 , 1999.

15. A.A. Kostel'ev, R.F. Curl, FSi. Tittel, C. Cmnchl, I?. Capasso, D

19, C. Gmacbl, A. Trcdicucci. D.I.. Sivca, A.I.. Hutchinson, P. Capasso, and A.Y. Cho, "Bidireclional semiconductor I ~ s c r , " Science, vol. 286, pp. 749-752.1999.

20. R. Kiihler, C. Gmachl,l'. Capasso, A. Tredicucci, D.I.. Sivco, andA.Y. Cho, "Singlc~matle, lunable cluantum cascade lasers in lhe spectral range 01 the CO? laser at h = 9.5-10.5 pm," submilled for publication.

21. C. Cmachl, I?, Capassn, A. Tredicucci, D.L. Sivca, It. Kohler, A.L. Hutchin- son, anil A.Y. Chi), "llcpendcncc a1 lhe device perlormsncc on t h e number olslages in quantum cascade lascrs,"l J. Select. Topics Qunnlum Mec~ Iran., vol. 5, pp. 808-816, MayiJune 1

22.1. Faist, 1. Caparro,C. Sirlori, 1).1,.Sivco, J.N. Bail1argcon.A.I.. H u l c h i n ~ son, S.N.G. Chu, andA.Y. Clra, "Higlwoiuer mid-infrared (h - 5 trm) quan~ tum cascade laseis operating above room temperature, Alqil, Php. Lelt., vol. 68, pp. 3680-3682, 1996.

23. F. C~passo, A. Tredicucci, C. Cmachl, D.L. Sivco, A.L. Hulchinsa A.Y. Cho,"HIgb performance superlattice quantum cascade lasers,"l Select. Topic.< Ouantrrm Electron., vol. 5, pp. 792-807, MayIJune 1999.

24. C. Gmachl, A. Michael Sergent, A. Tredicucci, I?. Capasso, A, L. Hutchin- son, l1.L Sivca, J.N. Baillargean, S.N.C. Chu, andA.Y. Cho, "Improvedcw

l ion ai quantum cascade imn with epitaxial-side heat-sinking," Photon. Techn. Lett., vol. 11, pp. 1369~1371, 1

25. C. Sirtori, J. Fuisl, I:. Capasso, D.l,.Sivco,A.L. Hutcl~inian,andA.Y.Cl~o, '"Quantum cascade laser wilh I l lasmon-enl~an~e~l waveguide operaling at 8.4 iim wnvrlmgtli,";ippl. Phg.~. Lelt., vol. 86, pp. 3242-3244, 1995.

26. C. Sirlori, C. Cmachl, P. Gipasso, J. Paist, 0.1.. Sivco, A.L. Ilutehinaon, and A.Y. Cha, "Lonpwauelenptl, (h = 8-11.5 pm) semiconductor lasers with waveguides hased on surface plasmoiis," Opt. Lett.. vol. 23, lip. 1366~1388, 19'18.

27. 11. Kogelnik anti C.V. Shank, "Coupledh;we theory of distributed ieetl- bsck lasera,"J. Appl. Phgs., vol. 43, pp, 2327 -2335, 1972; G . Marlhicr, P. Vankwinkelbcrge, Handbook of Ulstrlbuted Feedback I.oser fliodes. Boston, M A Artech Iloose, 1997.

28. R. Kohler, C. Cmachl,I'. Capasso, A. Tredicucci. D.L. Sivco, S.N.G. Chu, and A.Y. Cha, "Single-mode tunahle. puiscd and conlinuous wave quantum~cascade distrihuted feedback lasers at h z 4.6-4.7 pm," Anpl, Ph!/,s. Lett., l o be published.

2% C. Gmachl, F. Capasso, J. I'aisl, A.L. I lutchinsun, A.Tredicucci, 11.1,. Sivca, J.N. naillargeon, S.N.G. Chu, and A.Y. Cho, "Continuowwave and high-power polscd operalion of index-coupled distributed lcedback q m n ~ lum cascade laser at h = 8.5 pm,"A,>pl. Phgs /.elf., vol. 72, pp. 1430-1432, 1998.

30. C. Gmachl. I?. Capasso, A. Tredicucci, D.L. Slvcn, J.N. Raillargeon. A.1.. Hutchinson, and A.Y. Cho, "High 1power. continuous wave, currenl tunable, single-mode qiiantum cascade distrihuted feedback ls~ers at h z 5.2 and 7.95 tim. Opt. /,et/., l o be published.

:I l . A. Tredicucci, C. Cmachl, IF. Capasso, A.L. Ilutchinson, l1.L Sivca, and AY. Cha."Single~mode surface idismon hser,"rnhmilled far pubicetion.

32.0. Hofstcttcr, 1. Ibist,M. Beck, A, MliIler,ilndU. OeslerlE,"Demonstr~tion of high-performance 10.16 pm quantum cascade distribuled feedback lasers without epilaxial regmuth,"AppI. t'lz<i,s I.ett., wi. 75, pp, 6G5-6G7, 1999.

33. D. Hiifstetter. J. l%ist, M. Reck. and U. Oesterle. "Surface-emiltine 10.1 pm quantum-cascade distribuled feedback lasers) Appl, Phgs. Let;, vol. 75, PII. 3769-3771, 1999.

34. W. Schrenk, N. I h g e r , S. Cianordoli, 1.. Ilvosdsra, 6. Strasser, and E. Gornik, "GaAsIAlCaAr dislrihuled feedback quantum cascade lascrs,"Appl, Phw. Lett., vol. 76, pp. 253-255, 2000. COW

mid-infrared tunable quantum cascade lasers for gas-sensing applications

Documents