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Electronics Letters, 46, 17, pp. 1218-1223, 2010-08-19
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External cavity tunable type-I diode laser with continuous-wave
singlemode operation at 3.24 um
Gupta, J.A.; Ventrudo, B.F.; Waldron, P.; Barrios, P.J.
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External cavity tunable type-I diode laser
with continuous-wave singlemode
operation at 3.24 mm
J.A. Gupta, B.F. Ventrudo, P. Waldron and P.J. Barrios
A tunable external cavity laser (ECL) near 3.24 mm was developed using semiconductor laser gain chips based on GaSb. The type-1 inter-band laser diodes were grown by molecular beam epitaxy using 17 nm InGaAsSb compressively-strained quantum wells with 30 nm AlInGaAsSb quinary barriers for improved hole confinement. In con-tinuous-wave operation with a diode temperature of 108C, the ECLs produce up to 1.8 mW of singlemode output power with a tuning range of 60 nm. The devices are tunable through the fundamental n3
vibrational absorption features of methane gas, providing a platform for highly-sensitive detection of trace hydrocarbons.
Introduction: Gas detection systems are currently in demand for
indus-trial safety, process monitoring and environmental studies. Singlemode lasers are being developed very actively for these applications because of the high sensitivity possible with tunable diode laser absorption spec-troscopy, in which the laser emission wavelength is scanned through a spectral region having prominent absorption features of the target species. The 3 – 4 mm range is particularly interesting as it contains strong absorption features of methane and more complex hydrocarbons. Methane detection, monitoring and isotope analysis are important for applications in agriculture, atmospheric science and planetary science. There is also growing interest in the detection of volatile organic pounds such as benzene, toluene, ethylbenzene and xylene (BTEX com-pounds) which are precursors of atmospheric nanoaerosols and can contribute to poor indoor air quality.
In this reported work, a tunable external cavity laser (ECL) was devel-oped using GaSb-based diode lasers with emission near 3.24 mm. The long type-I emission wavelength was achieved using InGaAsSb quantum wells (QWs) with AlInGaAsSb barriers. This materials system has shown great promise recently for the development of type-I lasers up to 3.36 mm [l – 4] and for gas detection in this wavelength range[5]. The wide tuning provided by the ECL is ideal for characteris-ing hydrocarbons, especially BTEX compounds, which have broad absorption features.
Fabrication: The laser structure was grown on a (100) GaSb:Te
sub-strate by molecular beam epitaxy[3] and was designed to limit the far-field divergence [6] by using a narrow waveguide and relatively low Al cladding composition of Al0.6Ga0.4As0.05Sb0.95. The active
region contains four 17 nm In0.55Ga0.45As0.21Sb0.79QWs separated by
30 nm Al0.2In0.2Ga0.6As0.19Sb0.81barriers. With waveguide thickness
of 992 nm, the calculated fast-axis divergence is 508 (full width at half maximum). The reduced divergence is helpful for coupling out-going laser light through the collimating lenses into the external cavity[6, 7].
Ridge waveguide lasers were fabricated by inductively-coupled plasma reactive ion etching with Ni-Ge-Au and Ti-Pt-Au n- and p-contact metallisations, respectively. The laser used herein (width 8 mm, cavity length 2 mm) was mounted epi-side-up onto a Au-coated Cu submount using Ag epoxy and wirebonded to facilitate electrical connections. We emphasise that the device was tested in the ECL with both facets as-cleaved. Broader wavelength tuning should be expected with optimised facet coatings. The laser submount was placed on a thermoelectrically cooled Cu heatsink designed to accom-modate the two short-working-distance (,0.7 mm) lenses required for the external cavity coupling (Fig. 1). The aspheric chalcogenide glass lenses (Lightpath 390037IR3) have antireflection coatings optimised for the 3 – 4 mm range and very high numerical aperture for high collec-tion efficiency (specified as NA ≏ 0.85 atl¼ 7 mm). The intra-cavity
lens directed the laser light onto the diffraction grating (Optometrics ML-502, blazed at 3.1 mm, 450 grooves/mm, S-polarisation reflectivity of ≏0.87 at 3.2 mm) which was mounted on a precision, piezo-electrically-stabilised New Focus External Cavity control assembly in a modified Littrow configuration [8]. The light output power was measured with a calibrated thermopile detector and spectral measure-ments were made with a Bruker Vertex 80 Fourier transform infrared spectrometer with a CaF2 beamsplitter and a liquid-nitrogen-cooled
InSb detector. grating laser diode pivot aspheric lenses output beam
Fig. 1 Schematic of external cavity laser in modified Littrow configuration
Results: Fig. 2shows the variation of the ECL threshold current with grating tuning (emission wavenumber) in continuous-wave (CW) opera-tion at 108C. The free-running laser (FRL) threshold current was found to be 299 mA at 3138 cm21. The threshold current was reduced
signifi-cantly by the presence of the grating [7]. As the grating angle was adjusted to scan across the range of available laser gain, the threshold current was reduced according to the gain spectrum, having a minimum of 268 mA at the wavenumber corresponding to the gain maximum. At either limit of the tuning range the threshold approached the value obtained in FRL operation. These characteristics are similar to those reported in[7]for an ECL operating at shorter wavelengths.
3170 265 threshold current, mA 270 275 280 285 290 295 300 3160 3150 3140 10°C, CW 3130 free-running laser wavenumber, cm–1 3120 3110 3100
Fig. 2 Variation in ECL threshold current with emission wavenumber in CW operation at 108C (squares); threshold current of free-running laser (no grating) shown for comparison (circle)
The maximum front facet output power was similarly affected by the presence of the grating (Fig. 3). When the grating was tuned to the FRL wavenumber, the ECL with grating produced a front facet output power of 1.8 mW, compared with the FRL output power of 1.15 mW. This increase is mainly due to the high grating reflectivity which forces most of the light to be emitted from the front facet, as opposed to the FRL which has equal light output from both facets. At both the short-and long-wavelength (l) limits of the ECL tuning range the output power was close to that obtained for the FRL.
0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 100 peak no grating long l short l 200 300 current, mA
front facet light output, mW
400 500 10°C, CW
Fig. 3 Front facet light output with current in CW operation at 108C Measurements made for free-running laser and for gratings positions as indicated
The ECL tuning characteristics with grating position above threshold are shown inFig. 4. At a current of 320 mA, the wavenumber could be tuned over a range 3160 – 3102 cm21. At higher currents, the
heating-induced redshift of the laser gain spectrum shifted the tuning range to shorter wavenumbers. For a current of 450 mA, corresponding to the maximum output power, singlemode emission was obtained over a range from 3145 – 3085 cm21 (3180 – 3241 nm). Longer wavelengths
were achieved at a current of 530 mA, which is beyond the thermal roll-over of the light output with current curve (Fig. 3).
intensity , a. u. 3160 0 500 1000 1500 2000 2500 3000 3500 530mA 450mA 400mA 320mA 3140 wavenumber, cm–1 3120 3100 3080
Fig. 4 ECL emission spectra at several grating positions in CW operation at 108C and 320 mA drive current (lines); variation in singlemode laser intens-ity with emission wavenumber at several drive currents (symbols)
Additional fine-tuning was achieved using current and temperature tuning.Fig. 5shows the lasing spectrum at a temperature of 10.98C and current of 450 mA, with a peak wavenumber of 3086.2 cm21and
27 dB sidemode suppression ratio. The wavenumber decreased linearly at a rate of 0.26 cm21/K with increasing temperature, to 3085.5 cm21at
138C. Adjustment of the laser current shifted the emission at a rate of 20.0165 cm21/mA. 3090 –50 –40 –30 –20 –10 0 3089 3088 3087 3086 temperature (°C) smsr 27 dB 10 3086.4 3086.2 3086.0 3085.8 3085.6 3085.4 11 12 13 3085 3084 3083 10.9°C 450mA wavenumber, cm–1 w a ven umber , cm –1 intensity , dB
Fig. 5 ECL emission spectrum in CW operation at 10.98C and 450 mA drive current
Inset: Variation in singlemode laser wavenumber with temperature
Conclusions: An external cavity tunable diode laser has been
demon-strated with continuous-wave singlemode output near the fundamental n3vibrational absorption features of methane gas. The devices employ
GaSb-based type-I active regions and produce up to 1.8 mW of single-mode CW output power near 3.24 mm when operated at 108C. The devices can be tuned with adjustment of the grating angle, laser tempera-ture or laser drive current to allow absorption spectroscopy of trace hydrocarbons.
Acknowledgment: The authors are grateful to J. Weber for the design of
the laser submount and lens assembly. #National Research Council Canada 2010
13 July 2010
doi: 10.1049/el.2010.1790
One or more of the Figures in this Letter are available in colour online. J.A. Gupta, B.F. Ventrudo, P. Waldron and P.J. Barrios (Institute for
Microstructural Sciences, National Research Council of Canada, Ottawa K1A 0R6, Canada)
E-mail: james.gupta@nrc. ca References
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