Optical Gas Detection with
Vertical Cavity Surface
Emitting Lasers
The vertical cavity surface emitting laser (VCSEL), a new
generation of laser sensors, is poised to play a major
role in optical gas detection.
Hans P. Zappe, Centre
Suisse d'Electronique
et de Microtechnique
(CSEM)
Surgical patients must be administered a precisely controlled gas mixture during and after a procedure. Incineration plant managers are required to verify that the plant's smokestack emissions are not environmentally hazardous. Theatergoers need to know that airborne vapors in the building do not exceed safe levels.
Optical gas detection techniques are increasingly common in these types of measurement applications, and a new generation of semiconductor laser--the vertical cavity surface emitting laser, or VCSEL--is assuming a leading role in reducing both the cost and the size of optical gas sensing systems.
Semiconductor Lasers
CSEM's vertical cavity surface emitting laser, with its low-divergence, circular beam, facilitates the integration of lasers with silicon. Among its capabilities is the detection of trace gases.
Since their invention almost 40 years ago, semiconductor lasers have revolutionized many industrial fields. The devices have become firmly established in areas as diverse as telecommunications, data storage, medicine, and sensors [1].
Among their benefits:
- Very small size (<0.1 mm3)
- Wide range of available wavelengths (400 nm to >5 mm)
- High output power (>5 kW for arrays)
The type of semiconductor laser in general use to date has been the so-called edge emitter in which light is emitted parallel to the wafer surface from a cleaved or etched semiconductor facet. While edge-emitting semiconductor lasers are technologically well established, their use often entails significant technical and economic tradeoffs. For low-cost applications, the end user can choose a relatively inexpensive, cleaved-facet Fabry-Perot laser such as those found in CD drives, with a poor single-mode spectrum and very limited wavelength tuning without incurring mode hops.
Alternatively, high-performance distributed feedback lasers, which use a grating reflector to form the laser cavity, offer excellent spectral characteristics but at a price only high-end telecommunications systems can usually afford. Many developers of optical sensors have therefore been restricted in their inclusion of semiconductor lasers in new sensor systems.
Vertical Cavity Surface Emitting Lasers
Figure 1. The upper and lower Bragg mirrors and the very short active region can be seen in this cross section of a VCSEL. The total height of the structure is typically <10 mm.
In the VCSEL, the cavity is formed vertically on the wafer surface. Epitaxially grown Bragg mirrors serve as distributed reflectors above and below the laser's very short active region (see Figure 1). The mirrors are fabricated from dozens of pairs of two different semiconductor materials, each with precisely controlled thickness and composition, laid out in alternating layers. With proper design and well-controlled epitaxial growth, Bragg mirrors can be made to have a very high reflectance (>99.99%) in a narrow wavelength band, key to good VCSEL spectral performance.
Because the VCSEL emits vertically from the wafer surface, the formation of 1D or 2D laser arrays is straightforward (see Figure 2). The use of a circular laser aperture leads to a circularly symmetric beam with small divergence, simplifying collimation and fiber pigtailing. In addition, the planar laser arrangement is closely analogous to that of semiconductor ICs, implying that semiconductor mass production and, perhaps even more important, mass testing techniques are applicable to VCSEL manufacture--factors that strongly influence fabrication costs.
VCSELs always operate with only a single longitudinal mode. Because the short cavity does not support more than one mode, wavelength tuning is completely free of mode hops. The small volume of the active region also leads to very low laser threshold operating currents, in some cases <1 mA, so that power dissipation in VCSELs is considerably lower than for many other types of semiconductor laser. These properties are coupled with VCSEL spectral performance, which features low noise, good wavelength stability, and narrow spectral linewidth.
Trace Gas Sensing with VCSELs
The use of laser emission wavelengths that overlap the optical transitions of particular gases of interest has demonstrated VCSELs' effectiveness in detecting trace gases by optical absorption [2]. The recent availability of single-mode, narrow-linewidth VCSELs in the NIR wavelength range (750–950 nm) will lead to the manufacture of a new generation of compact, low-power gas sensors based on this approach.
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| Figure 2. The planar, surface-emitting structure of VCSELs makes these lasers well suited to the fabrication of 1D and 2D laser arrays. |
CSEM has developed an O2 sensor designed for a center wavelength around 762 nm, where absorption due to magnetic dipole transitions in the gas molecule is present without cross-interference from other gases. The laser design criteria were single transverse-mode emission with a high side-mode suppression ratio, narrow spectral linewidth, and an adequate wavelength tuning range. The emission spectrum is shown in Figure 3.
With a threshold current of 2.5 mA, the lasers were designed to have a wavelength tuning range of 2 nm free of mode hops, accomplished by varying the laser current or the temperature of the device. The narrow spectral linewidth, <30 MHz, accompanied by a low noise level, RIN <5 * 10–11/Hz, was essential for good sensor performance. The long-term frequency drift, <1 GHz/month, corresponds to a wavelength stability of 0.002 nm/month under constant operation.
The VCSEL was tested in a simplified wavelength spectroscopy system built from off-the-shelf electronics and optics (see Figure 4). The sensor was mounted on a temperature-controlled chuck, and current was supplied by a low-noise laser driver that was externally modulated by two function generators. The optical beam was directed through a beam splitter from which a portion was directly focused onto a silicon reference photodetector placed 0.03 m from the laser. The rest of the beam was directed through a 1 m ambient air (20% O2 concentration) path onto a second silicon photodiode.
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| Figure 3. A 763 nm VCSEL designed for O2 detection produced this optical spectrum. |
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| Figure 4. The O2 dectector was demonstrated with a test setup made of off-the-shelf components. During testing, the VCSEL emission was directed along a 1 m path through ambient air. The signal and reference photodiode responses were evaluated by a second harmonic detection scheme. |
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| Figure 5. These signals from a scan across four O2 absorption lines around 762 nm are the second derivative of the absorption spectrum and the reference signal. The triangular feature in both plots is the laser threshold. |
The VCSEL wavelength was varied by ramping the laser drive current about a bias point, with a superimposed sinusoidal signal permitting second-harmonic (2f) detection at the output. With a current ramp from 2.0 nA to 4.6 mA, the wavelength was swept from 762.9 nm to 763.5 nm, scanning over the O2 absorption lines at 762.915 nm, 763.032 nm, 763.312 nm, and 763.427 nm.
Figure 5 clearly reveals the 2f peaks corresponding to the second derivative of the absorption lines of ambient O2. The amplitude increases with laser current due to the increase in optical absorption by the lines of longer wavelength covered during the sweep. A measurement resolution of 0.2% O2/meter path length was determined for second harmonic detection using this simple demonstrator system. Improvements in sensitivity, with constant path length, may be expected through reduction of thermal gradients, air currents, and power supply noise sources.
Industrially deployed [3], these O2 sensor VCSELs have been incorporated into systems with a minimum detection limit of 0.01% volume over the same path length. The advantages of optical measurement, in this case deriving from the high temperature of the measurement medium coupled with the advantages of VCSELs, particularly the low long-term drift, were proof of principle in this measurement application.
Future Directions
A large number of laser manufacturers are developing VCSELs and products based on them. The market potential in short-range optical communications, for instance, has spurred a significant effort in the fabrication of low-cost, medium-performance VCSELs for this application.
Considerable development work is now focused on extending the VCSELs' available wavelength range. Given the success of GaAs/AlGaAs-based VCSELs in the NIR range, extending the available wavelengths to the visible as well as to longer (1.3–1.5 mm and longer) wavelengths is an industrial priority. Optical sensors will continue to benefit from successes in this area as the available VCSEL wavelength range expands to cover an increasing number of trace gas absorption peaks.
References
1. T. Whitaker. Apr. 1999. "Commercial VCSELs," Compound Semiconductor:16-27.
2. I. Linnerud et al. 1998. "Gas Monitoring in the process industry using laser diode spectroscopy," App Physics B:297-305.
3. H.P. Zappe et al. 1999. "Laser-Based Remote Sensing System for Gas Detection," Proc Sensors Expo Baltimore.
Acknowledgments
The author wishes to thank a dynamic and hard-working team: M. Brunner, S. Eitel, H.-P. Gauggel, B. Graf, K. Gulden, M. Hess, R. Hoevel, D. Jeggle, F. Monti di Sopra, M. Moser, H.P. Schweizer, and A. Vonlanthen for their essential contributions to the success of CSEM VCSELs.
CSEM's VCSELs were awarded the Grand Prize in the Best of Sensors Expo awards program at Sensors Expo Baltimore, May 1999.
Hans P. Zappe is Senior Staff Scientist, Centre Suisse d'Electronique et de Microtechnique (CSEM), Badenerstrasse 569, 8048 Zurich, Switzerland; 41-1-497-1499, fax 41-497-1400, hans.zappe@csem.ch
