Single-frequency Sb-based distributed-feedback lasers emitting at 2.3 μm above room temperature for...

Single-frequency Sb-based distributed-feedback lasersemitting at 2.3 �m above room temperature for applicationin tunable diode laser absorption spectroscopy

Abdelmagid Salhi, David Barat, Daniele Romanini, Yves Rouillard, Aimeric Ouvrard,Ralph Werner, Jochen Seufert, Johannes Koeth, Aurore Vicet, and Arnaud Garnache

GaInAsSb�GaAlAsSb�GaSb distributed-feedback (DFB) laser diodes based on a type I active region werefabricated by molecular beam epitaxy at the Centre d’Electronique et de Micro-Optoélectronique deMontpellier (CEM2). The DFB processing was done by Nanoplus Nanosystems and Technologies GmbH.The devices work in the continuous-wave regime above room temperature around an emission wave-length of 2.3 �m with a side-mode suppression ratio greater than 25 dB and as great as 10 mW of outputpower. The laser devices are fully characterized in terms of optical and electrical properties. Their tuningproperties made them adaptable to tunable diode laser absorption spectroscopy because they exhibit morethan 220 GHz of continuous tuning by temperature or current. The direct absorption of CH4 is demon-strated to be possible with high spectral selectivity. © 2006 Optical Society of America

OCIS codes: 140.3070, 120.0280.

1. Introduction

Applications to trace-gas measurement by tunablediode laser absorption spectroscopy (TDLAS) requirereliable devices, preferably operating in the cw re-gime and at room temperature. The 2.3 �m atmo-spheric window gives access to many gaseous speciesof industrial interest, such as CH4, NH3, CO, andHF, without water or carbon dioxide interference.Fabry–Perot edge-emitting InGaAsSb�AlGaAsSb�GaSb laser diodes1–4 have been used in trace-gasspectroscopy5–8 but exhibit several mode hops or arein multimode operation. Thus there is a real need todevelop cw distributed-feedback (DFB) lasers that

are tunable in this spectral range in order to accessthe wavelength of the targeted absorption lines. Inaddition, stable and reproducible spectral emissionproperties are evidently needed.

We present InGaAsSb�AlGaAsSb�GaSb type Iquantum-well DFB lasers emitting at a wavelengtharound 2.3 �m and operating in cw mode above roomtemperature. They are based on samples grown bysolid-source molecular beam epitaxy (MBE) and areprocessed for ridge waveguide lasers. A metal gratingis then processed along both sides of the ridge toobtain DFB for single-frequency operation.

The lasers show a wide continuous tuning rangewith temperature and current and a good side-modesuppression ratio with a narrow linewidth, renderingthem well adaptable to trace-gas absorption spectros-copy.

2. Device Design and Fabrication

The laser structure was grown by solid-source MBEin a Varian Gen II machine on an n-type (100) GaSbsubstrate. The active region is made of three 11 nmthick 1.6% compressively strained quantum wellsof Ga0.65In0.35As0.06Sb0.94 separated by two 30 nmthick Al0.35Ga0.65As0.03Sb0.97 electronic barriers latticematched to a GaSb substrate.1 Outside the activeregion the waveguide layers consist of 350 nmAl0.35Ga0.65As0.03Sb0.97. The thickness of p �Be�- andn �Te�-Al0.9Ga0.1As0.07Sb0.93 cladding layers is 1.5 �m.

A. Salhi, D. Barat, Y. Rouillard, A. Ouvrard, A. Vicet([email protected]), and A. Garnache are with the Cen-tre d’Electronique et de Micro-Optoélectronique de Montpellier,Centre National de la Recherche Scientifique, Unité Mixte de Re-cherche 5507, Place Eugène Bataillon, CC 67, Université Montpel-lier 2, 34095 Montpellier Cedex 05, France. D. Romanini is withthe Laboratoire de Spectrométrie Physique, Centre National de laRecherche Scientifique, Unité Mixte de Recherche 5588, Univer-sité Joseph Fourier de Grenoble, France. R. Werner, J. Seufert,and J. Koeth are with Nanoplus Nanosystems and TechnologiesGmbH, Oberer Kirschberg 4, 97218 Gerbrunn, Germany.

Received 18 October 2005; accepted 19 November 2005; posted20 January 2006 (Doc. ID 65350).

0003-6935/06/204957-09$15.00/0© 2006 Optical Society of America

10 July 2006 � Vol. 45, No. 20 � APPLIED OPTICS 4957

The electron–photon confinement factor is � � 0.034.The active region was nominally undoped. The dop-ing in the n-cladding layer was 2 � 1018 cm�3. Thep-cladding layer was doped to 5 � 1018 cm�3 exceptfor the first 0.2 �m in which the doping level wasdecreased to 5 � 1017 cm�3 to reduce free-carrier ab-sorption.2 The 250 nm thick p� GaSb contact layerwas doped to more than 1 � 1019 cm�3. The 150 nmthick graded layers, n- and p-type doped, were in-serted, respectively, before the n-cladding layer andafter the p-cladding layer. These layers were insertedto improve the injection of carriers inside the activeregion.

Ridge waveguide structures were defined after thecomplete growth of the laser structure was finished.The ridge structures were transferred into the semi-conductor by using a dry etching process. Figure 1shows a scanning electron microscopy (SEM) photo-graph of the ridge after the etching process. For thisa chlorine�argon-based electron cyclotron resonance,a dry etching process was used. Ridge waveguideswith different widths were processed from 5 to 3 �m.The depth of the ridges was optimized for a strongcoupling between the light wave and the metal grat-ing for stable single-longitudinal-mode emission.Note that the laser waveguide supports only the TE00mode because the TM00 mode has a much lower gainbecause of the selection rules. The distance betweenthe two gain-guided filaments in laser action is�16 �m for our device.9 Thus only one comb of lon-gitudinal modes can oscillate in the laser cavity.

First-order lateral DFB gratings were exposed byelectron-beam lithography beside the ridge. Accord-ing to the effective refractive index of the structureand the wavelength emission, a period of 334 nm wasrealized. After the development of the gratings ametal layer was evaporated and lifted off. Figure 2shows a SEM photograph of the lateral gratings.

The grating was fabricated on both sides of theridge. Because the coupling between the light waveand the metal grating was highest in the region rightnext to the ridge,10 it is important that the gratings

be well defined, especially adjacent to the ridge. Thecoupling coefficient is estimated to be 10 cm�1.11 Inthe SEM photograph of the gratings it can be clearlyseen that this is well fulfilled with the technologyused.

After fabrication of the gratings, a polymer is spincoated for planarization of the ridge structure. Ap-contact metallization followed by thinning andn-contact metallization finish the laser process. Thechip is cleaved into single lasers with a length of800 �m and a width of 400 �m. Individual chips aremounted on standard TO 5.6 or TO 8 headers byusing a tin–lead solder and tested. In the device con-sidered here the free spectral range (FSR) betweenthe two modes is 1.08 nm. The maximum modulationloss is less than 3 cm�1 between two neighboringmodes near the minima.

3. Electro-optical Characterizations

Figure 3 shows the voltage versus drive current atroom temperature and the light versus drive currentin the cw regime with increasing temperature from

Fig. 1. SEM picture of a ridge waveguide structure. The ridge is4.7 �m wide and 1.55 �m high.

Fig. 2. SEM picture of the lateral metal grating.

Fig. 3. Continuous-wave regime L–I–V characteristic of diode 4–5(L from the front facet). The L–I characteristic is given for differentdevice temperatures from 20 °C to 60 °C, giving T0 � 120 K (inset).

4958 APPLIED OPTICS � Vol. 45, No. 20 � 10 July 2006

20 °C to 60 °C. In this paper, to help understand theDFB laser properties reported and the effect of thepostgrowth processing, some of its features are com-pared with those obtained with a broad-area laserwith the same structure design.

The threshold current is Ith � 43 mA (or Jth �1.25 kA�cm2) at room temperature for this three-quantum-well structure. Note that this thresholddensity value is relatively high compared with thevalue obtained for broad-area diode lasers processedfrom the same wafer (Jth � 220 A�cm2 from Ref. 2).This is mainly due to the current spreading in thequantum wells, typically in the narrow ridge wave-guide. The maximum emitted power reaches 10 mWat room temperature at 300 mA with an externalefficiency of 11% �0.051 W�A�. This leads to an inter-nal quantum efficiency in laser action of �i � 0.3deduced from the optical losses measured below(Table 1) instead of typically �i � 0.70 for the broad-area diode laser processed from the same wafer,2which is consistent with the high threshold currentdensity obtained. The temperature dependence of thethreshold current in the cw regime is plotted in theinset, giving a high characteristic temperaturein the T0 � 120–70K range, depending on the mis-match between the gain peak wavelength and the

DFB design wavelength. In addition the T0 in thenarrow-ridge device is generally higher comparedwith a broad-area diode laser processed from thesame wafer (T0 � 70 K from Ref. 2) because it is alsoa characteristic of the current spreading in the quan-tum wells, which is less sensitive to temperature.

Figure 4 shows the monomode transverse beam far-field intensity distribution along the growth axis (ver-tical) and in the plane of the quantum well, with astrongly elliptical and divergent beam �FWHM,49 � 11.5°�. The beam-quality factor M2 along thehorizontal and the vertical axis is characterized (seeFig. 5) by a Xenics extended InGaAs photodiode lin-ear array (512 pixels). The output beam is focused ona 70 �m � 45 �m spot (along the vertical and thehorizontal axis) with a commercial aspheric lens�8 mm, 0.5 numerical aperture (NA)] at 50 cm fromthe chip. By measuring the beam distribution at dif-ferent positions, we can deduce the beam waist in thenear field and the divergence angle in the far field(Fig. 5). The M2 factor is obtained by

M2 � ��w,

where w is the beam waist (at 1�e2) in the near field,� is the far-field angle �at 1�e2�, and � is the laserwavelength. We measured M2 � 1.15 0.05 alongthe horizontal axis. This value is a typical value of M2

for a single-mode laser diode. Along the vertical axiswe measured M2 � 3.1 0.2. Owing to the higher

Fig. 4. Vertical (growth axis) and horizontal far-field beam inten-sity distribution recorded with a Xenics linear photodiode array:I � 100 mA, T � 298 K. The optical pattern on the vertical inten-sity distribution is stable and is due to interference in the laserwindow and diffraction on the copper block.

Fig. 5. Beam waist (1�e2) (millimeters) along the horizontal andthe vertical axis as a function of the distance (centimeters) of thelinear array from the near field.

Table 1. Distributed Feedback Diode Laser Parameters Measured and Computeda

Parameter Value Parameter Value Parameter Value

R1 0.3 Jtr�QWb (A�cm2) 300 � Factor 3–4R2 0.8 Jth�QW (A�cm2) 410 � 2Lc (�m) 745 Rth (K�W) 130 � 10 vc (GHz) 31�i (cm1) 13.5 � 0.5 �i 0.3 vL (MHz) 2.2� 0.043 �o 0.35 P0 (mW) 3

�r 0.1 sp 7 � 105

aSee text for a description.bQW, quantum well.


divergence angle (49° at 1�e2), the beam is truncatedby the lens, which increases the beam-quality factor.

The laser shows single-frequency operation witha high side-mode suppression ratio (SMSR) of 35 dBat I � 120 mA and 300 K (Fig. 6; see below for detailsconcerning the apparatus). The theoretical SMSR forthe single-transverse-mode ridge waveguide laser inthe cw regime (for modulation frequencies well belowthe megahertz level), limited by spontaneous emis-sion, is given by12

SMSR�dB� � 10 log�� g�g 1�

with

�g � �i �ln�R1R2�

2Lc�sp�r

Ith

I � Ith,

where I is the driving current, R1 is the front facetreflectivity, R2 is the back facet reflectivity, Lc is thecavity length, �� is the loss margin produced by thegrating between the central mode and its neighbor,�g is the modal gain margin between the centralmode and its neighbor, �r is the radiative efficiencybelow threshold, and �sp is the spontaneous emissionfactor. With the parameters of our DFB laser (seeTable 1) we found that �g � 0.05 cm�1, �sp � 7� 10�5, and we took �r � 0.1 to be in agreement withthe threshold current density value. First, withoutthe DFB grating in the device, one obtains a theoret-ical SMSR of 25 dB at I � 120 mA. Note, however,that this ideal single-mode ridge waveguide laser be-havior rarely occurs with real devices operating farabove threshold and without a DFB grating becausethe homogeneous nature of the gain is disturbed bynonlinear mode coupling, such as spatial-hole burn-ing. Thus one needs a spectral filter, such as a DFBgrating, to avoid any multimode behavior. Now withthe grating and taking the maximum value, ��

� 3 cm�1, one gets a SMSR of 43 dB. These values arein relatively good agreement with the 35 dB mea-sured at I � 120 mA (Fig. 6).

The variation in the emitted wavelength with anincreasing temperature in the cw regime is shown forthree different lasers in Fig. 6 for an injected currentof 100 mA. Devices for which the DFB grating is op-timized show no mode hopping over the whole tem-perature and current range investigated here, and agood SMSR ��20 dB with the apparatus functionlimited) is observed from 19 °C to 35 °C.

The continuous wavelength tuning range (with ashift in the longitudinal mode) reaches almost 7 nm�400 GHz� for laser 15-08 with a temperature tuningrate of 0.22 nm�K, which is in good agreement withother measurements on similar antimonide-based de-vices.13

The current tuning is shown in Fig. 7 at differenttemperatures for three lasers. The tuning rate wasmeasured through a germanium Fabry–Perot etalon�length, 16 mm; FSR, 2.34 GHz]. The laser devicesshow a continuous current tuning from 60 to 150 mA.As can be seen in the inset, the tuning rate varieslinearly with the injected current, whereas the emit-ted wavelength shows a quadratic evolution. Contin-uous tuning of 4 nm for laser 15-08 represents220 GHz of continuous tuning.

We have measured the evolution of the emittedwavelength as a function of the drive current. Thetemperature tuning then gives access to the temper-ature of the active region of the device versus theinjected current (Fig. 8). The thermal resistance Rth isthen deduced from

Tdevice�I� � Tsupport�I� RthPth�I�,

where Tdevice is the device temperature, Tsupport is thetemperature of the heat sink where the device ismounted, and Pth is the thermal dissipated power.

Using these tuning properties, we deduced a ther-mal resistance value of 130 10 K�W for these de-

Fig. 6. (Color online) Continuous temperature tuning of threelasers at a fixed drive current, I � 100 mA, from 19 °C to 45 °C. Anemission spectra is in the inset, corresponding to laser 13-15 atT � 25 °C. The SMSR reaches 35 dB.

Fig. 7. (Color online) Continuous current tuning of three devicesat different temperatures. The tuning rate deduced from the trans-mitted intensity through a Ge Fabry–Perot etalon is plotted in theinset for laser 15-08.


vices. This high value implies the good tuningproperties of the devices related to the heating of theactive region.14

4. Fine Characterizations of the Distributed FeedbackSpectral Properties (Continuous Wave, T � 300 K)

A. Amplified Spontaneous Emission Spectra belowThreshold and Gain–Loss Measurement by theMinimum–Maximum Method

Experimental setup5: To record the laser cavity modespectrum below threshold, we used a high-resolutiongrating monochromator �150 cm focal length). The di-ode beam was collimated with an aspheric lens�0.68 NA�, showing 70% of collection–transmissionaround a wavelength of 2.3 �m. The beam was fo-cused on the entrance slit with a CaF2 lens with lightpolarization parallel to the slits. At the exit slit theoutput light was imaged on a 2.6 �m extended In-GaAs photodiode. The input optical signal was mod-ulated with a mechanical chopper (with 3 kHzfrequency) by using a lock-in technique to obtain agood signal-to-noise ratio. To obtain high resolutionand sharp tails for the apparatus function, the slitwidth and the focused beam diameter on the entranceslit are optimized to produce a near-Gaussian appa-ratus function with a width of 6 GHz—or0.11 nm—�FWHM� at 2.35 �m, which is �10 timesthe laser cavity FSR (see Fig. 6). The number of spec-tral points within the apparatus function width is 1.5.

1. Low-Resolution Distributed FeedbackSpectra below IthFrom the spectra in Fig. 9 one can deduce the trans-parency current density Itr. We found that Itr� 31–32 mA, leading to a current density of Jtr

� 900 A�cm2 for three quantum wells, which is �10times higher compared with the broad-area diode la-

ser.2 Above this current value, amplification is ob-served at the electroluminescence (EL) spectrumpeak around 2.35 �m, the laser wavelength atthreshold.

2. High-Resolution Distributed FeedbackSpectra below ThresholdFigure 10 shows an amplified spontaneous emissionspectrum of the DFB cavity at high spectral resolu-tion above quantum-well transparency and belowthreshold. On this plot, no strong amplification mod-ulation on the EL spectrum is observed between twoadjacent modes near the EL peak, where the gratingis supposed to modulate the losses, showing that forthis particular device the grating period is not locatedat the same wavelength as the gain maximum. Thus

Fig. 8. Laser (active region) temperature against the drive cur-rent, deduced from spectra (squares) and thermal resistance (fittedcurve) when Eq. (1) is used. One can deduce a value ofRth � 130 � 10 K�W.

Fig. 9. (Color online) Low-resolution amplified spontaneous emis-sion spectra from the front diode facet (uncoated) recorded as afunction of the excitation current below threshold. Dotted lines,Fermi distribution function for energies above the Fermi quasi-levels. The monochromator resolution is 1 nm, T� 300 K, cwregime.

Fig. 10. (Color online) High-resolution amplified spontaneousemission spectra from the front diode facet (uncoated) recorded asa function of the excitation current below threshold. Monochroma-tor resolution, 0.11 nm. Note the little kinks induced by the gratingat 2.35 and 2.364 �m, corresponding to the loss modulation of �1cm1. Inset, enlargement showing Fabry–Perot-mode oscillations.T � 300 K, cw regime.


the modulation losses introduced by this DFB gratingseem to be �1 cm�1 maximum here (noise limited).Newly processed devices allowed us to improve themode-hop-free tuning range from the threshold to ashigh as the maximum current (see Fig. 7), thus indi-cating a higher modulation loss value.

From the measured data we can deduce the netmodal gain spectra. These spectra are plotted in Fig.11 versus the injected current. Here, to quantify thefinesse of the DFB cavity versus wavelength, we usedthe ratio between the maxima and the minima of thelongitudinal Fabry–Perot modes, obtaining the netmodal gain value.15 Note that this technique is suit-able here, because the laser waveguide design of ourdevice supports only one comb of longitudinal modes.However, this technique does not allow measuringnet gain modulation with a period faster than twomode spacing because the measured value is aver-aged over this interval. Thus the DFB grating-induced modulation loss could not be clearlyobserved. We calculate an underestimation of �20%of the net modal gain value at �5 cm�1 (and �5% at�15 cm) because of the limited resolution of our ap-paratus.15 The broadband optical losses, deducedfrom the net modal gain measurement, are found tobe �I � 13.5 0.5 cm�1, a value slightly higher com-pared with that measured for a broad-area diode la-ser with the same structure design.2

The quantum-well material gain peak (the modalgain peak divided by the confinement factor �) versuscurrent density is plotted in Fig. 12. Note that thedifferential gain at the peak is constant across thefull current window investigated here. This contra-dicts the common quasi-logarithmic dependence ofthe quantum-well gain with the excitation for thebroad-area diode laser but is consistent with thestrong influence of the current spreading in the quan-tum wells together with a shortening of the quantum-well radiative lifetime (an increase in the internal

quantum efficiency) while increasing the carrier den-sity. This observation is in agreement with the veryhigh threshold density observed (5.7 times higher)and a moderate decrease in the laser internal quan-tum efficiency compared with the broad-area diodelaser (2.3 times lower; see Section 3). We also show inFig. 12 the material gain spectral width ��g �FWHM�as the excitation density is increased above transpar-ency. This value is directly linked to the conduction tovalence-band Fermi quasi-level energy separation�EF, which is plotted in the inset together with thegain peak energy Epeak.

B. Laser Linewidth Measurement at 3 mW Output

The observed width of the resonances of a high-finesse scanning interferometer was used to estimatethe laser linewidth. The interferometer was formedby two dielectric mirrors (Layertec) with high reflec-tivity �R � 99.99%� in the 1.95–2.4 �m range. One ofthese was mounted on a piezoelectric tube, allowingmodulation of the cavity length. We used one flatmirror and one with a 10 cm curvature radius with acavity length of 8.1 cm, giving a cavity FSR of�1.85 GHz. (These choices were not dictated for anyspecial reason.)

The piezoelectrically mounted mirror allowed scan-ning of the cavity length by slightly more than oneFSR. The cavity output was focused by a short focallens over an extended 2.6 �m InGaAs photodiode�0.5 mm in diameter) equipped with a transimped-ance amplifier with a gain of �100 k� and a responsetime of �1 �s. The cavity transmission patternshown in Fig. 13 was easily observed, where thepeaks correspond to different transverse cavitymodes excited at different times during a cavitylength scan. A periodic peak pattern with approxi-mately seven peaks, corresponding to the cavity FSRindicated in Fig. 13, was easily recognized by tuningthe laser wavelength. This almost sevenfold pattern

Fig. 11. (Color online) Processed high-resolution amplified sponta-neous emission spectra, showing the net modal gain (averaged overthree adjacent modes) of the device active region as a function of theexcitation current. T � 300 K, cw regime. Dotted line, optical cavityloss at transparency; short vertical curves, position of the Fermiquasi-level separation.

Fig. 12. Measured material gain peak per quantum well (QW)and gain spectral bandwidth ��g as a function of the currentdensity per quantum well. T � 300 K, cw regime. The ��g value isobtained by fitting the top gain spectrum by a parabola centered atthe peak value. The gain peak value has been corrected to take intoaccount the resolution limitation. Inset, energy separation �EF

between the quasi-Fermi levels and the gain peak energy Epeak asa function of current density.


depends on the cavity geometry, whereas the inten-sity distribution of the peaks depends on the trans-verse overlap of the incident laser beam with thedifferent transverse modes. Identification of the cav-ity FSR is clearly necessary for converting the timescale of the cavity length scanning to a frequencyscale. The exact assignment of each peak to a specifictransverse mode is not relevant to this measurementas long as each of these peaks is much narrower thanthe laser linewidth.

A tunable optical isolator (by Isowave) was placedbefore the cavity entrance. After optimization at thelaser wavelength its isolation was measured to be�1.4�4 (with a transmission of 75%). This was suffi-cient to reduce optical feedback since we could notdetect any of its typical effects, in particular fre-quency locking to cavity modes, which causes an ap-parent broadening and large intensity fluctuations onthe injection of the modes, depending on the fluctu-ations in the laser–cavity distance (since this affectsthe phase of the optical feedback).

The laser was installed on a Peltier cooler inside ahomemade package, including an aspheric collimat-ing lens (with a 0.68 NA), adjusted to the optimalcollimated beam. (An IR thermal camera allowed thegood quality of the laser beam to be checked.) Thelaser temperature was stabilized by a homemade pro-portional, integral, and differential controller to bet-ter than 0.01 °C. A low-noise (by ILX) current sourcewas used, which was operated in battery mode for thebest performance.

In Fig. 13 the signal is recorded from a digital oscil-loscope during a single-cavity scan. The intensity of thepeaks is noisy, and the zooming around the peak re-veals the spiky character of this noise, which is a man-ifestation of the laser phase noise being directlydependent on the laser linewidth.16 The larger theemission linewidth, the faster the phase fluctuations ofthe laser field, which in turn give a shorter laser co-herence time. A destructive interference in the fieldresonating in the cavity with the incoming field may

occur if the fluctuations get out of phase, resulting inan inversion or interruption of the buildup process andin a jagged cavity injection. The envelope of the noisypassage through resonance still gives an estimate ofthe laser linewidth. The presence of this noise alsoguarantees that the cavity modes are much narrowerthan the laser, which is confirmed in the present caseby an independent estimation by the ring-down time,which can be observed when one is tuning the laseracross the cavity resonances at high speed. The ring-down time was found to be close to 3 �s, which corre-sponds to a mirror reflectivity of 0.99990, a finesse of30,000, and a mode width of 50 kHz.

To determine the laser linewidth, a Lorentzian anda Gaussian fit of the profile of one of the peaks wasperformed. The best fit is given by a Gaussian, whichis shown in the inset of Fig. 13. However, the FWHMobtained in the two cases is exactly the same. Actuallythe natural laser line shape should be close to Lorent-zian. The fact that a Gaussian works best implies thatthe observed linewidth is limited by instrumentalnoise (e.g., injection current noise) and is not the in-trinsic laser linewidth. On the other hand, the valuefound here is not much larger than the estimate (givenbelow) of the theoretical linewidth for this laser. Toconvert the FWHM from the cavity scan time to thefrequency units, we use the cavity FSR readily de-duced in time units in Fig. 13. To partially compensatefor the nonlinearity of the piezoelectric translator,which gives the nonuniform spacing of the transversecavity mode in Fig. 13, we chose a peak at approxi-mately the center of the cavity scanning range. Wefinally obtain a laser linewidth (FWHM) of �2.2 MHz(to within �20%). This is the short-time linewidthsince the analysis of the laser spectrum by the cavityoccurs on the time scale of the passage through res-onance, which is �50 �s.

C. Schawlow–Townes Fundamental LaserLinewidth Limit

The theoretical DFB laser linewidth in the cw regime(for modulation frequencies well below the megahertzlevel), limited by the spontaneous emission noise andthe carrier noise, is given by the modified Schawlow–Townes expression12:

�vc �2�hc�0

�P0��vc�2�1 ��factor

2��,

where � �� 2.365 �m� is the laser wavelength, h isPlanck’s constant, c is the velocity of light, �vc is thecold cavity mode width (FWHM), �factor is the line-width enhancement factor, � is the ratio between thespontaneous emission rate and the stimulated emis-sion rate (reduced to one photon), P0 is the poweremitted from the output facet, and �0 is the outputfacet external efficiency given by

�0 ��ln�R1�

2Lc�i � ln�R1R2�.

Fig. 13. Distributed feedback laser linewidth obtained by estimat-ing the FWHM of the resonances of a high-finesse interferometer(see text for details). The diode laser driving current and its operat-ing temperature are kept constant (I � 120 mA, T � 300 K).


In Table 1 is a list of the DFB laser cavity para-meters. From the measured gain spectra we deduced� at the laser threshold. We found from Ref. 17 the�factor for our device. We obtained a theoretical line-width in the 1.5 � 0.4 MHz range, slightly smallerthan the measured value.

5. Gas Measurements

In the emission range of the devices many gaseousspecies of industrial interest can be detected by Tun-able diode laser absorption spectroscopy. Gas detectionwith one of these devices was realized by photoacousticspectroscopy.18 We show in Fig. 14 the direct absorp-tion measurements of CH4 in a large continuous range.This measurement was performed with a cali-brated 18 mm long cell filled with pure methane atatmospheric pressure. The lines strengths were10�22–10�21 cm�1�molecule cm�2, which gave a max-imum absorption of 20%–40%. Good agreement isobserved between the experiment and HITRAN 96simulations.19 With the same laser at two differenttemperatures more than 7 nm of continuous currenttuning has been achieved.

The large current tuning properties of these de-vices allows the simultaneous detection of many spe-cies showing absorption lines in this spectral rangewith a high selectivity because of the very low line-width and the large tuning properties. Using high-sensitivity spectroscopy, traces could be detected inthis spectral range of the following species: CO,CH4, HF, NH3, and weak H2O lines.

We now plan to reach other spectral ranges withthese DFB devices, for example, to take isotopic ratiomeasurements in the 2.65 �m water absorption bandand CO2.

6. Conclusion

Distributed feedback laser diodes emitting at 2.3 �min the cw regime above room temperature, operatingat single frequency with more than 5 mW of outputpower, have been demonstrated. These devices aregrown on GaSb-n substrates by molecular beam epi-taxy. They are based on InGaAsSb�AlGaAsSb three-quantum-well type I active zones. The wafers wereprocessed into narrow ridge laser diodes, and a metalgrating was deposed on both sides of the ridge. Thelaser device has been fully characterized, optically andelectrically. The lasers show a high SMSR and broadcontinuous frequency tuning over 220 GHz, combinedwith a narrow linewidth of �2.2 MHz under a con-stant driving current, close to the Schawlow–Townesfundamental limit, which make them good candi-dates for high-resolution spectroscopy. Some of thefirst measurements of the CH4 absorption lines wereperformed in large spectral ranges that demonstratethe feasibilty of these laser devices for applicationsrelated to tunable diode laser spectroscopy.

This study was supported in part by RégionLanguedoc Roussillon and by the GLADIS (Gas Anal-ysis by Infrared Spectroscopy) European Unionproject (IST 2001-35178).

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Fig. 14. (Color online) An 18 mm long cell containing pure CH4

used to perform direct absorption measurements: The theoreticaltransmission of the cell is computed with the HITRAN 96 data-base. Inset, experimental transmission of the cell measured with asingle laser. The device was tuned with the current and tempera-ture to reach a very large tuning range, as large as 7 nm.


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