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We report on the growth, fabrication, experimental study and application in an absorption gas setup of distributed feed-back antimonide diode lasers with buried grating. First, half laser structures were grown by molecular beam epitaxy on GaSb substrates and stopped at the top of the waveguide. A second order Bragg grating was then defined by interferometric lithography on the top of the structure and dry etched by Reactive Ion Etching. The grating was, afterwards, buried thanks to an epitaxial regrowth of the top cladding layer. Finally, the wafer was processed using standard photolithography and wet etched into 10 µm-wide laser ridges. A single frequency laser emission around 2.3 µm was recorded, a maximum output power of 25 mW and a total continuous tuning range reaching 4.2 nm at fixed temperature. A device has been used to detect methane gas and shows strong potential for gas spectroscopy. This process was also replicated for a target of 3 µm laser emission. These devices showed an output power of 2.5 mW and a SMSR of at least 23 dB, with a 2.5 nm continuous tuning range at fixed temperature.
© 2015 Optical Society of America
1. Introduction
The development of sensitive and accurate measurements of trace gases using laser-based diagnostics is important for both the understanding and monitoring of a wide variety of applications, including environmental studies (i.e. characterization of global climate change due to anthropogenic and biogenic effects [1]), combustion processes and other industrial processes [2], semiconductor manufacturing processes [3] and medical diagnostics [4].
Over the last two decades, Tunable Diode Laser (TDL) absorption techniques have proven extremely useful in providing fast-response and precise measurements of many key species such as CH4, CO2, NH3 [5–7 ]. Diode lasers play a fundamental role in these applications due to their tunability, high reliability, rapid modulation characteristics and relative simplicity. They provide a sensitive and highly selective method for trace gas detection [8]. However, efforts still remain to improve the diode laser design and develop more sensitive techniques.
We focus on the development of compact, robust, simple and sensitive gas sensors of environmental and agronomic interest [9, 10 ]. We are particularly interested on methane since it is a very important greenhouse gas originating both from anthropogenic and natural sources.
This paper details the design of new diode lasers targeting the spectral range from 2 - 3 µm, where many gaseous species exhibit narrow and strong absorption lines [11]. In particular, the atmospheric transparency window around 2.3 µm is important for the development of methane sensors because methane absorption lines are numerous and intense there, while water vapor and CO2 have scarce and weak lines [12].
In this wavelength range, lasers structures commonly using GaInAsSb/AlGaAsSb/GaSb quantum wells lasers, are now mature [13, 14 ]. Application to TDLS (Tunable Diode Laser Spectroscopy) requires the development of distributed feedback lasers (DFB) having a tuning range of a few nanometers, enough to identify gas line absorption with a width of ̴ 1 Å. During the last decade, numerous studies have been conducted to obtain the distributed feed-back effect on these structures. The first approach made use of a loss coupling distributed feedback, with a 1st order metallic grating deposited on both sides of the laser ridge [15]. This type of filter, however, induces radiation losses, which reduces the emitted power of the laser. More recently index coupling DFBs were made by etching a lateral grating on both sides of the mesa [16]. We propose here a new approach, inspired by previous work on InP-based DFBs (mainly used for telecom applications) [17], in which a diffraction grating is implemented between the top of the wave guide and the upper cladding. This approach involves a molecular beam epitaxy (MBE) regrowth step, to bury the grating. This is the main innovative aspect of this study. Only one prototype has been made this way so far, emitting at 2.2 µm [18]. The lasers presented here exhibit higher performances, due to a better control of the fabrication process and show that this process is repeatable and adaptable to longer wavelength.
Two different fabrication processes were carried out to produce 2.3 and 3 µm wavelength active structures. The processed devices where then used in a methane detection setup to demonstrate that these sources are well adapted for gas detection.
2. Modelling buried grating
The principle of index-coupled DFB is to introduce along the laser ridge an index grating to increase the amplification of the optical mode having the same wavelength as the mode selected by the period of the grating, thus leading to single-frequency emission.
In our component, the index variation is due to the buried grating. This Bragg grating is placed at the interface between the waveguide and the upper cladding. It is etched on the top of the waveguide. Figure 1(a) presents a scheme of the realized structure.
Fig. 1 (a) Schematic structure of the device, showing the grating etched in the top waveguide layer and covered by a regrown AlGaAsSb cladding layer, (b) magnification of the output facet of a 3 µm laser., (c) SEM image of a cleaved section of the regrown part of a 3µm laser. The white line corresponds to the QW position.
2D finite element numerical calculations, perpendicular to the propagation, were used using calculated indexes [19] to determine the electric field distribution in the laser section and the modal effective index, neff. The average neff between sections with and without etching of the grating is 3.38 with Δneff = 0.02.
The Bragg condition links the laser wavelength λ and the grating period Λ for the different orders of diffraction (1st, 2nd, 3rd, …). In these structures, the QWs were designed to emit at 2.3 µm and a 3 µm respectively. By increasing the grating order, the coupling coefficient decreases and the diffraction losses increase. As a consequence it is better to use a 1st order grating if at all possible, even if the 2nd order can be interesting to discriminate more easily the 2nd DFB mode, thank to diffraction effects [20]. Due to our experimental holographic setup, a first order grating was not reachable below 3 µm, thus for the 2.3 µm devices we have used 2nd order (690 nm period). For the 3 µm lasers we processed a 1st order grating with a 485 nm period.
Based on coupled waves theory [21], a perturbation analysis is used to obtain an expression of the coupling coefficient κ produced by the grating [22]. We have used an analytic simplification [23] to evaluate κ, taking into account the order, the duty cycle, and the etching depth (ED). As we use a dry etching process, we suppose vertical sidewalls. In this model was used the previously calculated indexes.
To optimize both modal discrimination and optical power, we target a κl value around 1.25 [24], with the length of the diode (l) fixed at 1 mm to minimize the threshold current. For the 2.3µm 2nd order DFB, we target a grating depth of 25 nm, and a 75% duty cycle. For the 3 µm 1st order DFB, we consider a grating depth of 40 nm, and a 50% duty cycle.
3. Technological process
The growth of the laser structure is performed by molecular beam epitaxy in a reactor equipped with valved cracker cells for Sb and As. The first part of the growth is realized on a (100)-oriented, n-type GaSb substrate at a temperature of 450°C. After growth of an n-type GaSb buffer layer, a 100 nm graded AlGaAsSb layer is grown to make the transition from GaSb to the 1.3µm n-Al0.90Ga0.10As0.07Sb0.93 cladding layer. The active region (2.3 µm laser) is made of two 10 nm-thick Ga0.62In0.38As0.10Sb0.90 quantum wells (QW) separated by 30 nm of Al0.25Ga0.75As0.02Sb0.98 barriers. These QWs are embedded between two 400 nm Al0.25Ga0.75As0.02Sb0.98 waveguide layers. The waveguide is capped with GaSb to prevent the oxidation of the Al containing layer.
For 3 µm lasers, the active region is made of three 10 nm Ga0.41In0.59As0.28Sb0.72 QW separated by two 40 nm of Al0.20Ga0.60In0.20As0.19Sb0.81 barriers.
After MBE growth, a 50 nm SiO2 deposition by plasma-enhanced chemical vapor deposition (PECVD) is done to protect the surface from contamination. The DFBs are processed on the half of the wafer. The other unprocessed half is used to control the in situ deoxidation before the regrowth and also serves as a reference, and Fabry-Pérot diode lasers are made from this part. The DFB grating is first realized by holographic lithography insolation with a 404 nm laser diode. The pattern is transferred in the dielectric material with CHF3/O2 dry etching by inductively coupled plasma reactive-ion etching (ICP RIE), followed with a O2 plasma cleaning to remove the holographic photoresist. The structure is patterned with Argon sputtering. Finally the dielectric mask is removed by HF wet etching. This step allows suppressing the surface oxide layer. Using a diluted solution with 30 s etching time, AFM measurements do not show any surface degradation. A 25 nm deep pattern was achieved, with a 78% duty cycle for the 2.3 µm fabrication process, and a 35 nm–deep pattern, with a 50% duty cycle for the 3 µm one. The etched samples exhibit a sub-nanometer surface roughness.
After grating formation and wet deoxidation of the surfaces, the sample is reloaded in the SS-MBE system to complete the structure with the growth of a 1.3 µm-thick p- Al0.90Ga0.10As0.07Sb0.93 cladding layer, followed by a 100 nm-thick AlGaAsSb graded layer, and a 300-nm highly p-doped GaSb contact layer. While AFM measurements are not relevant at this stage of the process because of the surface smoothing due to the epitaxial regrowth, we have checked the DFB grating geometry by scanning electron microscopy (SEM) (Fig. 1(c)) on a cleaved section of the wafer. As can be observed, at the interface between the top of the waveguide and the upper cladding, the period and etching depth have been preserved.
The fabrication is ended using standard technological process. 10 µm-wide and 1.5 µm-deep ridges were defined using contact optical lithography and fluoro-chromic acid wet etching. The electrical insulation is realized with a 200 nm Si3N4 layer deposited by PECVD. The top contact is made by Ti/Au (20 nm/300 nm) sputtering deposited through a 3.5 µm-wide aperture, which ensures a good ohmic contact. We do not carry out metallization of the back contact.
Then a 1500 µm-long device is cleaved. No facet coating or passivation is added. The device is indium soldered and down-mounted on a copper heat-sink. In Fig. 1(b), we present a SEM picture of the laser edge after the whole technological process.
5. Characterizations of 2.3 µm devices
Regarding a 10 µm-wide and 1500 µm-long device, the light-current-voltage plots and the characteristics temperatures are reported in Fig. 2 . It shows a 57 mA threshold current (Fig. 2(a)) and a 380 A/cm2 current density at 15°C with a T0 of 90 K. The quantum efficiency (ηd) is 25% at 15°C with a T1 of 225 K (Fig. 2(b)). The series resistance is 2.3 Ω and the built-in potential Vd is 0.9 V. The optical power per facet reaches 25 mW. Mono-mode emission is obtained from threshold to 350 mA.
Fig. 2 (a) Light-current-voltage characteristic of a 1500 µm-long DFB laser diode emitting at 2.3 µm recorded at different heat-sink temperatures and (b) evolution of the threshold current and quantum efficiency and determination of the T0 and T1 parameters of the diode.
Improved control of the technological process yields better results than our previous devices [18], with lower threshold current density and twice the power. These performances and the ones of a Fabry Pérot laser processed on the same wafer are similar, which confirms that this kind of DFB filter does not degrade the laser performances. The variation of the emission with temperature and current was recorded with a FTIR having a spectral resolution of 0.5 nm. We report the spectral variation with temperature at 150 mA with an increment of 2°C (Fig. 3(a) ) and the variation with current at 48°C with an increment of 25 mA (Fig. 3(b)).
As far as variation with temperature is concerned, the DFB mode moved on 4.2 nm between 48°C and 68°C from 2373.2 nm to 2377.4 nm with a tuning rate of 0.21nm/K. In regards to the variation with current, the mode moved on 3.6 nm between 125 mA and 350 mA from 2372.9nm to 2376.5nm. For injected currents up to 375 mA, the device switched on Fabry-Perot emission around 2.4 µm. For lower injected currents, below 150 mA, the TE10 lateral mode appeared. This mode has been identified by far field measurements [18] and predicted by rigorous coupled-wave analysis modeling [25]. The tuning rate is 0.17 nm/mA, which corresponds to a thermal resistance of 75K/W.
The SMSR measurement is shown on Fig. 4 at 48°C and 200 mA. From this figure, we measure a 22 dB SMSR.
6. Gas spectroscopy
As a first step of demonstration of single-frequency operation and wide mode-hop-free tuning range of our laser, we have carried out direct absorption spectroscopy in a static cell containing methane gas. A very simple optical setup (see Fig. 5 ) was used in which two beam splitters divided the laser beam in three parts: a reference way, a way with a Germanium Fabry-Pérot étalon (length = 16 mm, from which the free-spectral range is FSR = 2.34 GHz <-> 0.078 cm−1) and a 2 cm-long path with a saturated calibrated methane cell. The transmitted signal through the étalon, the absorption cell and the reference beam signal were recorded by a 2.6 µm InGaAs photodiode. We controlled the acquired signals by using an USB digital-to-analog and analog-to digital conversion card from National Instruments. Standard Labview I/O routines were used to sample signals from the photodiodes.
Fig. 5 Scheme of experimental setup. (LD) laser diode, (L) lens, (BS) beam splitter, (PD) photodiode. The CH4 cell is a reference cell filled with pure methane.
We recorded several étalon and methane cell transmitted signals by applying a ~100 mA current ramp to the laser (whose DC component is about 117 mA) at several temperatures. The temperature was fixed during each current ramp. Both étalon and methane cell signals were normalized with respect to the reference beam. An example of these signals is reported in Fig. 6 . Monitoring étalon yielded a sufficiently good fringe contrast.
Fig. 6 Transmitted signal through the CH4 cell and Étalon signal obtained by applying a 100 mA current ramp to the laser while fixing its temperature at 48.1°C.
Frequency scale calibration was carried out by quadratic polynomial fit of the étalon peak positions (time to relative frequency conversion) and comparison with a HITRAN simulation [11] of the methane absorption spectrum (in order to fix the absolute frequency). Several scans are shown in Fig. 7 . The obtained mode-hop free range varies from 1 to 1.5 cm−1.
Fig. 7 Comparison between CH4 direct absorption spectra (obtained from a 2 cm-long absorption cell at atmospheric pressure) and the HITRAN spectrum corresponding to the conditions used for the experiment. The numbers between brackets indicate the following laser temperatures: (1) 64.6°C, (2) 63.2°C, (3) 60.4°C, (4) 56.8°C, (5) 54.7°C, (6) 50.2°C, (7) 48.1°C.
No significant mode hop was observed as the laser temperature was increased. Therefore, a continuous single-mode tuning about 7 cm−1 could be envisaged by simultaneous application of a current and a temperature ramp to the laser. A fine experimental study of the position of the mode hops resulting from the laser temperature variation at fixed current and vice versa will allow determination of a map of mode hops [26] as a function of these two parameters, allowing determination of the the best conditions for a mode-hop-free continuous tuning.
In the next stage of this research, lasers will be coupled to a sensitive, simple and relatively low cost technique - the Quartz-Enhanced Photoacoustic Spectroscopy - to detect very low concentration of gases [6, 7, 9, 10 ]. The final aim is to develop a very compact and robust sensor for multi-gas detection with a suitable design to meet industrial needs in particular those related to environmental and agronomic monitoring.
7. Characterization of 3 µm devices
Here we report on a 10µm-wide and 900 µm-long device. It exhibits a threshold current of 220 mA and reaches a power of 2.5mW at 10°C as shown in Fig. 8 . The series resistance is 2.2 Ω and the built-in potential Vd is 1.2 V. The thermal roll over appears at 400 mA and beyond.
Fig. 8 Light-current-voltage characteristics of a 900 µm-long DFB laser diode emitting at 3 µm recorded at different heat-sink temperatures.
The variation of the emission with current is shown from 360 mA to 440 mA at 15°C with a 10 mA increment on Fig. 9(a) . The DFB mode moves on 2.6 nm from 3041.8nm to 3044.2nm with a tuning rate of 0.03nm/mA. In regard to the variation with current, the mode moves with a tuning rate of 0.3 nm/mA which corresponds to a thermal resistance of 75 K/W.
Fig. 9 (a) Evolution of the emission spectrum with current at 15°C, (b) log-scale spectrum of the single mode emission at 15°C and 330 mA CW injected current.
The log-scale spectrum, shown in Fig. 9(b) under 330mA and 15°C, exhibits a SMSR of 23 dB.
8. Conclusion
We have fabricated GaSb-based laser diodes with index-coupled buried DFB grating. The first run of fabrication was carried out on 2.3µm emitting devices. A 10 µm-wide and 1500 µm-long component was presented, showing stable single-mode emission over 4.2 nm at fixed temperature, a threshold current of 57 mA and a power reaching 25 mW at 15°C. The measured SMSR reaches at least 22 dB. The second run of fabrication was carried out on 3µm emitting devices. The performances of a 10 µm-wide and 900 µm-long device show stable single-mode emission over 2.6 nm at fixed temperature, with threshold current of 220 mA and a power reaching 2.5 mW at room temperature. The measured SMSR reaches at least 23dB. We attribute the increase of the threshold current with wavelength to the Auger effect. These performances show that the use of a buried Bragg filter does not induce any degradation of the quality of the laser. As such, these lasers are suitable light sources for next-generation tunable laser spectrometers applied to gas sensing.
Acknowledgments
This work is supported by the ANR NexCILAS international project (2011 NS09-002-01), MIDAS project (2011 NANO 028-01) and by NUMEV labex (Laboratory of excellence). The authors acknowledge support from the European Union through FEDER grant n° 47851. This work was partially supported by the French “Investment for the Future” program (EquipEx EXTRA, ANR-11-EQPX-0016).
References and links
1. G. Durry, T. Danguy, and I. Pouchet, “Open multipass absorption cell for in situ monitoring of stratospheric trace gas with telecommunication laser diodes,” Appl. Opt. 41(3), 424–433 (2002). [CrossRef] [PubMed]
2. R. M. Mihalcea, D. S. Baer, and R. K. Hanson, “A diode-laser absorption sensor system for combustion emission measurements,” Meas. Sci. Technol. 9(3), 327–338 (1998). [CrossRef]
3. S. I. Chou, D. S. Baer, R. K. Hanson, W. Z. Collison, and T. Q. Ni, “HBr concentration and temperature measurements in a plasma etch reactor using diode laser absorption spectroscopy,” J. Vac. Sci. Technol. A 19(2), 477–484 (2001). [CrossRef]
4. J. H. Shorter, D. D. Nelson, J. B. McManus, M. S. Zahniser, and D. K. Milton, “Multicomponent breath analysis with infrared absorption using room-temperature quantum cascade lasers,” IEEE Sens. J. 10(1), 76–84 (2010). [CrossRef] [PubMed]
5. D. Romanini, M. Chenevier, S. Kassi, M. Schmidt, C. Valant, M. Ramonet, J. Lopez, and H.-J. Jost, “Optical-feedback cavity-enhanced absorption: a compact spectrometer for real-time measurement of atmospheric methane,” Appl. Phys. B 83(4), 659–667 (2006). [CrossRef]
6. M. Ghysels, G. Durry, N. Amarouche, J. Cousin, L. Joly, E. D. Riviere, and L. Beaumont, “A light weight balloon-borne laser diode sensor for the in-situ measurement of CO2 at 2.68 micron in the upper troposphere and the lower stratosphere,” Appl. Phys. B 107(1), 213–220 (2012). [CrossRef]
7. R. Lewicki, G. Wysocki, A. A. Kosterev, and F. K. Tittel, “Carbon dioxide and ammonia detection using 2 µm diode laser based quartz-enhanced photoacoustic spectroscopy,” Appl. Phys. B 87(1), 157–162 (2007). [CrossRef]
8. J. Hodgkinson and R. P. Tatam, “Optical gas sensing: a review,” Meas. Sci. Technol. 24(1), 012004 (2013). [CrossRef]
9. M. Triki, T. Nguyen Ba, and A. Vicet, “Compact sensor for methane detection in the mid infrared region based on Quartz Enhanced Photoacoustic Spectroscopy,” Infrared Phys. Technol. 69, 74–80 (2015). [CrossRef]
10. T. Nguyen Ba, M. Triki, G. Desbrosses, and A. Vicet, “Quartz-enhanced photoacoustic spectroscopy sensor for ethylene detection with a 3.32 μm distributed feedback laser diode,” Rev. Sci. Instrum. 86(2), 023111 (2015). [CrossRef] [PubMed]
11. L. S. Rothman, I. E. Gordon, Y. Babikov, A. Barbe, D. Chris Benner, P. F. Bernath, M. Birk, L. Bizzocchi, V. Boudon, L. R. Brown, A. Campargue, K. Chance, E. A. Cohen, L. H. Coudert, V. M. Devi, B. J. Drouin, A. Fayt, J.-M. Flaud, R. R. Gamache, J. J. Harrison, J.-M. Hartmann, C. Hill, J. T. Hodges, D. Jacquemart, A. Jolly, J. Lamouroux, R. J. Le Roy, G. Li, D. A. Long, O. M. Lyulin, C. J. Mackie, S. T. Massie, S. Mikhailenko, H. S. P. Müller, O. V. Naumenko, A. V. Nikitin, J. Orphal, V. Perevalov, A. Perrin, E. R. Polovtseva, C. Richard, M. A. H. Smith, E. Starikova, K. Sung, S. Tashkun, J. Tennyson, G. C. Toon, V. G. Tyuterev, and G. Wagner, “The HITRAN2012 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 130, 4–50 (2013). [CrossRef]
12. M. Jahjah, A. Vicet, and Y. Rouillard, “A QEPAS based methane sensor with a 2.35 μm antimonide laser,” Appl. Phys. B 106(2), 483–489 (2012). [CrossRef]
13. J. Angellier, D. Barat, G. Boissier, F. Chevier, P. Grech, and Y. Rouillard, “Toward an AlGaAsSb/GaInAsSb/GaSb laser emitting beyond 3 µm,” Proc. SPIE 6485, 64850B (2007).
14. T. Lehnhardt, M. Hümmer, K. Rößner, M. Müller, S. Höfling, and A. Forchel, “Continuous wave single mode operation of GaInAsSb/GaSb quantum well lasers emitting beyond 3µm,” Appl. Phys. Lett. 92(18), 183508 (2008). [CrossRef]
15. L. Naehle, S. Belahsene, M. Edlinger, M. Fischer, G. Boissier, P. Grech, G. Narcy, A. Vicet, Y. Rouillard, J. Koeth, and L. Worschech, “Continuous-wave operation of type-I quantum well DFB laser diodes emitting in 3.4 [micro sign]m wavelength range around room temperature,” Electron. Lett. 47(1), 46 (2011).
16. S. Forouhar, R. M. Briggs, C. Frez, K. J. Franz, and A. Ksendzov, “High-power laterally coupled distributed-feedback GaSb-based diode lasers at 2µm wavelength,” Appl. Phys. Lett. 100(3), 031107 (2012). [CrossRef]
17. J. L. Zilko, L. J. P. Ketelsen, Y. Twu, D. P. Wilt, S. G. Napoltz, J. P. Blaha, K. E. Strege, V. G. Riggs, D. L. Van Haren, S. Y. Leung, P. M. Nitzche, J. A. Long, C. B. Roxlo, G. Przyblek, J. Lopata, M. W. Focht, and J. Koszi, “Growth and characterization of high yield, reliable, high-power, high-speed, inp/ingaasp capped mesa buried heterostructure distributed feedback (CMBH-DFB) lasers,” IEEE J. Quantum Electron. 25(10), 2091–2095 (1989). [CrossRef]
18. Q. Gaimard, L. Cerutti, R. Teissier, and A. Vicet, “Distributed feedback GaSb based laser diodes with buried grating,” Appl. Phys. Lett. 104(16), 161111 (2014). [CrossRef]
19. M. Afromowitz, “Refractive index of Ga1−xAlxAs,” Solid State Commun. 15(1), 59–63 (1974). [CrossRef]
20. R. F. Kazarinov and C. H. Henry, “Second-order distributed feedback lasers with mode selection provided by first-order radiation losses,” IEEE J. Quantum Electron. 21(2), 144–150 (1985). [CrossRef]
21. H. Kogelnik and C. V. Shank, “Coupled-Wave theory of distributed feedback lasers,” J. Appl. Phys. 43(5), 2327 (1972). [CrossRef]
22. W. Streifer, D. Scifres, and R. Burnham, “Coupling coefficients for distributed feedback single and double heterostructure diode lasers,” IEEE J. Quantum Electron. 11(11), 867–873 (1975). [CrossRef]
23. P. Correc, “Coupling coefficient for trapezoidal gratings,” IEEE J. Quantum Electron. 24(1), 8–10 (1988). [CrossRef]
24. J. Carroll, J. Whiteaway, and D. Plump, Distributed Feedback Semiconductor Lasers (IET, 1998).
25. Q. Gaimard, A. Larrue, M. Triki, B. Adelin, T. Nguyen-Ba, Y. Rouillard, O. Gauthier-Lafaye, R. Teissier, and A. Vicet, “2.2 µm to 2.7 µm Side wall corrugated index coupled distributed feedback GaSb based laser diodes,” Semicond. Sci. Technol. 30(6), 065015 (2015). [CrossRef]
26. M. Triki, P. Cermak, L. Cerutti, A. Garnache, and D. Romanini, “Extended continuous tuning of a single-frequency diode-pumped vertical-external-cavity surface-emitting laser at 2.3 μm,” IEEE Photonics Technol. Lett. 20(23), 1947–1949 (2008). [CrossRef]
