Single-frequency InAsSb lasers emitting at 3.4 μm

SPECTROCHIMICA ACTA PART A

ELSEVIER

Spectrochimica Acta Part A 52 (1996) 863-870

Single-frequency InAsSb lasers emitting at 3.4 pm Andrei Popov ~'*, Victor Sherstnev a, Yury Yakovlev a, Robert M/.icke b, Peter Werle b aloffe PhysicoTechnical Institute, 194021 St. Petersburg, Russia' bFraunhofer Institut fffr Atmosphdrische Umwelff'orschung, Kreuzeckbahnstr. 19, D-82467 Garmisch-Partenkirchen, Germany 2

Accepted 23 October 1995

Abstract

Single frequency InAsSb lasers operating from 77 to ll0 K in CW mode have been tested for their suitability for trace gas analysis. The threshold current l,h was as low as 25 mA at 78 K with a characteristic temperature TO of 25 K. Selected lasers show single-frequency operation over a large range of currents and temperatures with a side mode suppression ratio of 25 dB. The FWHM of the far-field pattern was 30 and 50 degrees in lateral and transverse directions, respectively. The beam elliptical factor was 1.7. The maximum CW optical power was up to 1.2 mW per facet to single frequency at 80 K. The lasers allowed tuning by the injection current without mode hopping of about 1.2 cm i with a tuning rate of 0.032 cm i mA 1. The average temperature tuning was 1.3 c m - J K i Relative intensity noise in normal operation was as low as 4 × 10 16 at 1 Hz. Therefore, a theoretical detection limit in terms of optical density of 2 × 10-8 has been estimated for measurements by tunable diode laser spectroscopy at 3.4 pm. Keywords: Diode laser; Laser spectroscopy; Single mode lasers

I. I n t r o d u c t i o n

Tunable diode lasers are very attractive for m a n y applications in spectrochemistry, process analysis and atmospheric trace gas monitoring. These applications pose challenging requirements to analytical techniques: sensitive, selective and often simultaneous measurements with a high time resolution are needed using fast, accurate, rugged and operational instruments [1,2]. Tunable diode laser absorption spectroscopy using high * Corresponding author. Fax: + 7 812 247 0006. 2 Fax: + 49 8821 73573.

frequency m o d u l a t i o n ( F M - T D L A S ) satisfies most o f these requirements and is being frequently used for measurements o f trace gas pollutants in the atmosphere. One limiting factor for ultra-sensitive measurements is the performance o f the available lasers. Lasers for spectroscopic applications should provide single m o d e operation over a wide tuning range, high optical power output with low optical noise, and a Gaussian beam profile. The single m o d e operation is required to minimize m o d e partition noise and to prevent absorption signals from other modes interfering with the desired signal from the spectral feature o f interest. Since the detection limit in F M - s p e c t r o s c o p y under q u a n t u m limited conditions is proportional to

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A. Popov et al. / Spectrochimica Acta Part A 52 (1996) 863 870

the square root of the laser power on the detector, the laser output power should be as high as possible. Due to the losses in multipass cells and the noise floor of middle-infrared photodetectors the lasers should have a minimal power level of at least 1 mW [3]. Semiconductor diode lasers are available in the mid-infrared spectral region (3 10 itm), where most atmospheric species have strong rotational vibrational absorption bands. For wavelengths above 3.3 /~m, lead-salt diode lasers are used. However, in the range 2.7-4/~m, there are absorption lines of molecules of interest, where low detection limits might be obtained (H2CO at 3.596 /lm, C H 4 at 3.26 /~m, HCN at 2.997 /Lm, or C2H2 at 3.067 /lm) [3], which can now be used for spectroscopic applications by novel semiconductor lasers based on III V-materials (InAsSb). Such lasers were investigated with respect to their radiative and thermal properties in comparison to the lead-salts [4]. Optical power outputs up to 1 W (pulse) and 16 mW (CW) have been achieved from 4/~m InAsSb multimode optical pumped lasers [5]. From 3.2 /lm quasi-CW injection lasers optical powers up to 8 mW at 82 K have been reported [6] with threshold currents substantially smaller than in lead-salt devices [7]. In this paper we report the characteristics of III V indium arsenide antimonide lasers which were designed for single-frequency emission near 3.4/~m. The InAsSb diode lasers were grown by liquid phase epitaxy at Ioffe Physico Technical Institute, Russian Academy of Science in St. Petersburg. The lasers were investigated in detail with a computer-controlled calibrated laser test setup at the Fraunhofer Institute of Atmospheric Environmental Research, FhG IFU in GarmischPartenkirchen, Germany.

2. Experimental setup and test laser devices An automated test system was set up which was equipped to record the laser threshold current, the optical output power, the longitudinal mode structure, the tuning response, wideband optical noise and the beam profile over a broad range of current and temperature [4,8]. The system was mounted on a vibration isolated optical table.

Electrically actuated mirrors were used to pass the beam through the various elements, pre-aligned by a HeNe laser. The laser samples were mounted in a LN2-cooled dewar typically working at heatsink temperatures in the range between 78 and 110 K. An off-axis parabolic mirror (OAP) with 25 mm diameter was used to collimate the laser output beam. Where possible, reflective optics were used to minimize back reflection into the laser cavity. The output power was measured by a electronically calibrated power meter. A 0.5 m Czerny Turner monochromator and a HgCdTedetector combined with a lock-in amplifier was used for recording the longitudinal mode structure. Due to the high dynamic range of the data acquisition system (about 5 orders of magnitude), weak side modes could be identified. A 40 mm long germanium Fabry Perot Etalon was used to calibrate the tuning rate. For measurements of the total noise of all modes as well as noise of selected modes, a second HgCdTe high frequency detector (up to 1 GHz) was used in combination with a radio frequency spectrum analyzer. The laser farfield pattern was visualized by a pyroelectric vidicon placed directly in front of the laser facet. The whole experimental setup was computer controlled via an IEEE-488 bus. The raw data were stored for final analysis using standard software packages. The lasers consisted of an InAsSb/InAsSbP double heterostructure (DH) which was lattice matched to an InAs substrate (Fig. 1). The investigated heterostructure devices were grown by liquid-phase epitaxy (LPE). They consisted of a 1 /~m thick active layer (calculated band gap energy Eg of 365 meV at 77 K) which was enclosed between two 2.5 ,um thick InAsSbP cladding layers (Eb was 550 meV). The cap was a 1 /zm thick InAs layer. The 14/~m width mesa-stripe geometry chips were fabricated by standard photolithography and wet chemical etching. The substrate was lapped to 100 /tm thickness. The 280 /~m cavity length lasers with uncoated facets were mounted junction side up on the special copper heatsink (Fig. 1). The substrate and stripe contact were metal-coated with AuAg providing a series resistance as low as 2 fL The direct voltage was typically 0.45 0.6 V (inset Fig. 1).

A. Popov et al. / Spectrochimica Acta Part A 52 (1996) 863-870

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Fig. 1. Schematic diagram of the double heterostructure diode lasers, grown by liquid phase epitaxy on InAs substrate. The InAsSb is the active layer, InAsSbP are the cladding layers. The overall physical dimensions of the chip are 280 x 500/~m 2. the uncoated facet laser chips are mounted on a special copper heatsink (photo). The graph shows the voltage-current characteristics of the diode at 78 K in a semi-log scale. 3. Results

Several lasers in the 3 /~m spectral range have been tested; first data were presented in [4]. The lasers have been tested from 25 to 250 m A with heatsink temperatures in the range of 78 110 K. Here, the characteristic data and noise measurements of a selected laser will be presented as an example. The current-voltage dependence is presented in Fig. 1. The direct voltage is typically 0.45-0.6 V. Taking into account the voltage on the interfaces

and the serial resistance, this value was close to the bandgap energy of the active layer. Therefore the radiative characteristics of this laser are defined by recombination in the bulk of the active layer. As it can be seen from Fig. 1, the U - I curve shows no kinks, a prerequisite for singlemode operation. The optical output power has been recorded for different heatsink temperatures. Fig. 2 indicates the power per one facet in a temperature range from 80 to 96 K. Since the laser facets were uncoated, the full optical power is a factor of two

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A. Popov et al. ! Spectrochimica Acta Part A 52 (1996) 863-870

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Current [mA] Fig. 2. Output power vs. current at different temperatures. The optical output power characteristics per facet were presented for heatsink temperatures from 80 to 96 K. The inset shows the measured exponential temperature dependence of the threshold current in a semi-log scale.

higher. By these measurements, 1.2 mW per facet CW power was obtained at 200 mA drive current at a temperature of 80 K. All characteristics show a tendency to saturation with increasing current drive. The differential quantum efficiency qext is 5% (corresponding to 11 /~W mA ~ initial slope efficiency) near threshold and decreases to 3% (7 /~W m A - ~ ) at 3 times the threshold current. For temperatures up to 110 K (not shown in the figure) the efficiency drops to 2% (5/~W mA ~). For semiconductors with an InAsSb/InAsSbP interface this behaviour can be explained by temperature dependent non-radiative losses like carrier leakage through small valence band offsets. The inset in Fig. 2 shows the temperature dependence of the threshold current which varies approximately exponentially with heatsink temperature. This dependence is empirically described by Ith = I, ho exp(T/To), where a characteristic temperature of TO= 25 K can be calculated from our data. The laser has been tested only up to 110 K. No measurements were made above 110 K since the

temperature range of the dewar was limited and the threshold current was already as high as 150 mA. It should be mentioned that values of Ith, To and ~/ext are close to the values reported in [6] for quasi-CW mode InAsSb(P) lasers. The spatial power distribution of the laser beam was monitored by a pyroelectric vidicon placed at the distance of 30 mm from the laser facet [4]. From this measurement we can conclude that in the active layer volume only the lowest order transverse and lateral modes are supported by the waveguide. From these data the quantity of practical interest is the angular distribution of the laser beam which is important for the design of the laser optics. The distribution of the beam has been calculated as a gaussian beam far from the facet. The values for full width at half maximum (FWHM) were estimated to be 30 and 50 deg in transverse and lateral directions, respectively. The elliptic factor is 1.7. From the equation for the divergence of the fundamental TE mode of a symmetric three-layer slab waveguide [9]

867

A. Popov et al. / Spectrochimica Acta Part A 52 (1996) 863 870

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Current [mA] Fig. 3. Wavelength and noise characteristics of laser at 95 K heatsink temperature. The spectral purity is close to 100% demonstrating single-frequency operation without significant spontaneous emission.

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nl, n2 are the refractive indices of the active and cladding layer, 2 is the wavelength, we can calculate the refractive index step between the active and the cladding layer as 0.2. This is sufficient for index-guiding and in good agreement with experimental measurements. If the refractive index of I n A s S b P cladding layer, n2, is taken as 3.54 [9], the refractive index of the active layer, n~, can be estimated as 3.34. Detailed m e a s u r e m e n t s of the m o d e structure show that lasing occurs in a single longitudinal

868

A. Popov et al. / Spectrochimica Acta Part A 52 (1996) 863-870

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Fig. 4. Noise characteristics of the laser at 95 K at two selected operating points: excess noise can only be observed at the mode hop position, whereas for single mode operation an almost white spectrum can be observed. mode over a wide temperature and current range. As an example the spectra of the laser at one temperature are presented here (Fig. 3). For the spectral characterization of the lasers the "spectral purity" (SP), was introduced

noise (mode competition, mode hopping noise) can be found, whereas in the region of single mode operation the detector noise floor is dominating [3,10]. Two traces of the p l o t - - o n e at the mode hop and one in the single mode r e g i o n - have been investigated in more detail. They are shown in the Fig. 4, where the "relative intensity noise" (RIN) [10] is used for the characterization

where Pi is the power of a isolated mode and P is the overall power. Since this value takes into account all detected side modes, it reflects--in our o p i n i o n - - t h e spectral purity of the laser better than the commonly used side mode suppression ratio, defined as the ratio of the main-mode power to the power of the most intensive side mode [9]. For the laser under investigation this value was at least 25 dB (limited by the noise floor of the measurement). F r o m Fig. 3 it can be seen that the laser has a broad current tuning range (about 1 cm 1) in a single longitudinal mode with a tuning rate of 0.032 c m - ~ mA ], which is very attractive for spectroscopic applications; the average temperature tuning rate was found to be 1.3 cm * K [4]. In the upper part of Fig. 3, the high frequency noise spectra of the laser, which were measured after each m o n o c h r o m a t o r scan, are shown in a contour plot. In the vicinity of the mode hop at 90 mA driving current, very strong laser excess

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where • f i s the spectrum analyzer bandwidth, and P is the optical power. These measurements indicate a detection limit in terms of R I N of about 10 ,6. At the mode hop position, strong 1/f-noise contributions are present up to frequencies of more than 100 MHz. In single mode operation, no significant 1/f-dependence can be found down to 5 MHz, and the RIN is always smaller than 5 × 1 0 ,Sat 1 Hz, reaching a value of 4 × 1 0 - *° at 100 MHz. Therefore, for a measurement of trace gases, the theoretical detection limit in optical density could be as small as 2 × 10 s (square root of RIN), only if the laser dominates as a limiting factor. This laser is well-suited for twotone or low-frequency modulation techniques where, due to the slightly higher noise contributions, a detection limit of 7 × 10 -8 can be expected.

A. Popov et al. / Spectrochimica Acta Part A 52 (1996) 863-870

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In order to evaluate the performance for spectroscopic field applications the laser has been used in a FM-spectrometer [ll]. The multi-path absorption cell (White-cell) in the measurement channel has been filled with calibration gas of H2CO and NO2 [12], and in the reference channel a small absorption cell filled with C H 4 has been placed. The laser was continuously tuned over 0.75 cm-1 by variation of the injection current. Fig. 5 shows spectra of different gases together with calculated absorption lines from the spectroscopic database HITRAN 92 [13]. From the variance of the two-tone-signal in the measurement channel and a direct absorption signal, measured with 100 ppbv calibration gas concentration, the detection limit has been estimated to be 5 × 10-6 in terms of optical density (OD). The difference

between the theoretical limit (7 x 10- 8, calculated from the RIN of the laser at a frequency of 20 MHz) and the measured value can be explained by drift effects and interferences in the spectrometer used [3,11].

4. Summary and conclusions We investigated mid-infrared I I I - V lasers emitting at 3.4 pm in the viewpoint of applications for molecular spectroscopy. The lasers are InAsSb double heterostructure grown by LPE. They were tested between 77 and 110 K. The threshold current was as low as 25 mA at 78 K. The lasers have been operated up to 6 8 times threshold current. They showed single-frequency lasing with optical

870

A. Popov et al. / Spectrochimica Acta Part A 52 (1996) 863-870

power o u t p u t as high as 1.2 m W per facet and exhibited relative intensity noise below 10 15 at 1 Hz. The temperature and current t u n i n g rates were 1.3 c m - 1 K i and 0.032 cm 1 m A ~ respectively. The laser has been used in a F M - T D LAS spectrometer for trace measurements, where a detection limit d o w n to 5 x 10 6 0 D has been predicted. The investigated I n A s S b lasers have d e m o n s t r a t e d significantly higher optical o u t p u t power t h a n typical lead-salt lasers. Since the laser showed no 1/J-noise at frequencies down to 5 M H z this laser is well suited for low-frequency m o d u l a t i o n techniques, because lower m o d u l a t i o n frequencies are required for high stable instruments [8].

Acknowledgement The a u t h o r s t h a n k the E u r o p e a n C o m m u n i t y for further support o f this work in the frame o f the " C o p e r n i c u s " project.

References [1] D.J. Brassington, Tunable diode laser absorption spec-

troscopy for the measurement of atmospheric species in R.E. Hester and R.J. Clark, (Eds.), Advances in Spectroscopy, Vol. 24, Spectroscopy in Environmental Science, Wiley, New York, 1994. [2] H.I. Schiff, G.I. Mackay and J. Bechara, The use of Tunable diode laser absorption spectroscopy for atmospheric measurements in M.W. Sigrist (Ed.), Air Monitoring by Spectroscopic Techniques, Wiley, New York, 1994. [3] P. Werle, Proc. SPIE, 2092 (1993) 4. [4] A. Popov, B. Scheumann, R. Mficke, A. Baranov, V. Sherstnev, Y. Yakovlev and P. Werle, Infrared Physics, 37 (1996). [5] H.Q. Le, G,W. Turner, J.R. Ochoa and A. Sanchez, Electron. Lett., 30 (1994) 1944. [6] A.N. Baranov, A.N. lmenkov, V.V. Shersmev and Y.P. Yakovlev, Appl. Phys. Lett.. 64 (1994) 2480. [7] J. Xu, A. Lambrecht and M. Tacke, Electron. Lett., 30 (1994) 571. [8] P. Werle, Appl. Phys. B, 60 (1995) 499. [9] G.P. Agrawal and N.K. Dutta, Long-wavelength Semiconductor Laser, Van Nostrand, New York, 1986. [10] K. Peterman, Laser Diode Modulation and Noise, Kluwer, Dordrecht, 1988. [11] P. Werle, R. Miicke and F. Slemr, Appl. Phys. B, 57 (1993) 131. [12] R. Mficke, B. Scheumann, F. Slemr and P. Werle, Proc. SPIE, 2112 (1994) 87. [13] HITRAN 92: L.S. Rothman, R.R. Gamache, R.H. Tipping, C.P. Rinsland, M.A.H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S.T. Massie, L.R. Brown, and R.A. Toth, J. Quant. Spectrosc. Radiat. Transfer, 48 (1992) 469.