This year's Mid-Infrared Optoelectronic Materials and Devices conference featured developments that are enabling mid-infrared lasers and detectors to operate both at higher temperatures and further into the infrared, reports Yves Rouillard.

Mid-infrared lasers and detectors have made tremendous progress over the past decade, with room temperature laser emission now possible in the 1.712.0 m wavelength range. The most recent advances were described at this year s Mid-Infrared Optoelectronic Materials and Devices (MIOMD) conference on April 14, in Montpellier, France. 120 delegates met to discuss advances in frequency converters, quantum cascade lasers, MQW lasers, VCSELs, LEDs, detectors and thermophotovoltaic devices. Detectors track industrial processes According to S Smith of Edinburgh Instruments, Scotland, gas sensors are required for industrial processes with detection limits of the order of 1% for food packing to less than 1 ppm for trace gas detection. Examples of wavelengths required for analysis include 3.4 m for CH4 and 4.2 m for CO2 gases, 4.6 m for CO, and 5.5 m for the explosive Semtex. Beyond the 4.5 m wavelength, however, low-cost filament lamps (with product costs of $100500) are no longer favored as infrared emitters, and Smith described advances in InSb diode detectors operating at 300 K for use as potential replacements. Using immersion optics, devices with a detectivity D* of 2.6 109 cmHz0.5 W1 were possible at 5 m. Leak detection is another important area for industrial process monitoring, particularly for pipelines carrying CH4. To date, specially adapted vehicles have been used to traverse pipelines, according to F Bournazaud of French company GDF. The "car" is equipped with a flame ionization detector. However, the low response time of the detector limits the speed of the car to 20 km/h. Bournazaud revealed that laser diode detectors, developed in collaboration with gas detector manufacturer Oldham, are allowing the measurement collection rate to be considerably increased. Fiber for infrared applications New kinds of fiber are needed to counteract the lack of transmission beyond 2.5 m using silica fiber. A Katzir of the University of Tel Aviv, Israel, discussed developments in extruded single crystal silver halide (AgClxBr1x) fibers adapted for transmission in the 330 m spectral range. Using such fibers, CO2 laser power densities could exceed 15 kW/cm2, which is sufficient for material heating and cutting, or endoscopic laser surgery. The Tel Aviv group has produced fibers with diameters of 0.51.0 mm and lengths up to 10 m. Minimum losses are still as high as 0.2 dB/m at a wavelength of 10 m, largely as a result of the relative lack of maturity of the approach. However, once fully developed, this new fiber could address fiber transmission limitations. Telecom lasers aid spectroscopy Absorption spectroscopy techniques that use near-infrared vibrational overtones and combination bands play an important role in detection schemes for gas sensors in the 13 m, and 325 m spectral regions using fundamental bands. Different sources are envisaged depending on the exact wavelength and detection limits required, and F Tittel (Rice University) described a number of variations. Candidates include telecommunications lasers, difference frequency generation and quantum cascade (QC) lasers. An example of a gas sensor configuration for NH3 detection that makes use of a 1.5 m DFB telecom laser is shown in . The schematic shows the set up required to detect NH3 employed at the NASA Johnson Space Center on a bio-reactor developed for water reprocessing. According to Tittel, fiber pigtailed telecom DFBs operating in CW mode are highly suitable. Where appropriate, Yb and Er/Yb fiber amplifiers can also be used to boost power and provide fiber-beam delivery. Quantum cascade lasers have also been employed, operating in pulsed mode at room temperature, or alternatively CW mode lasers operating at liquid nitrogen temperature. Finally, optical parametric oscillators (OPOs) can be used as frequency converters. These devices employ LiNbO3 crystals to pump the near-infrared telecom laser diodes, and can be integrated into a portable NH3 sensor. Sensitivities in the three approaches were found to range from 106 to 108 cm1. Cavity ring-down spectroscopy Cavity ring-down spectroscopy (CRDS) was introduced in the 1980s as a way to enable absorption measurements using pulsed laser sources, which, according to D Romanini (University of Grenoble), would otherwise have remained unsuitable for absorption spectroscopy. CRDS uses a short light pulse coupled into a stable optical cavity formed by two highly reflecting mirrors. The light entering the cavity on one side oscillates or "rings" back and forth many times between the two mirrors, and the time behavior of the light inside this cavity is monitored. Being dependent on the rate of absorption rather than the magnitude, the advantage compared to conventional absorption spectroscopy lies in the intrinsic insensitivity to light source fluctuations, and the very long effective path lengths (many kilometers) in a sample with a volume of a few cubic centimeters. Two CRDS set ups were described: one allows the cavity length to be altered using a piezo element (a swept-cavity scheme), and the second takes advantage of optical feedback in the cavity. The detection sensitivity is 3 1010 cmHz0.5 W1 for the swept-cavity scheme, and trace concentrations of CH4 as low as 10 ppb can be detected using this approach. A Vicet presented a tunable diode laser absorption spectroscopy sensor developed at the University of Montpellier. The sensor is based on a GaInAsSb/AlGaAsSb QW laser diode. The diode is grown on GaSb substrates by MBE and fabricated into a narrow (5 m) ridge laser to provide single-mode, CW operation up to 140C. Tunability from 2.2 to 2.4 m permits the detection of CH4, CO, H2O, HF and NH3 in ambient air. For an optical path of 13 m, the detection limit for CH4 was 0.17 ppm. VECSELs Diode-pumped tunable vertical external cavity surface emitting lasers (VECSELs) show great potential for mid-infrared devices operating up to 3 m in CW mode. In particular, the external cavity maintains a diffraction-limited beam output at high power levels, reducing the line width in single-mode operation and rendering the system insensitive to feedback. A Garnache of the University of Southampton described a VECSEL. The device is a monolithic structure consisting of a bottom Bragg mirror, QW active region and an external cavity comprised of a void surmounted by a dielectric mirror. For spectroscopy applications, the cavity void is filled with the gas to be analyzed. The device is pumped by an 830 nm GaAs-based diode. The VECSEL emission is centered on 1 m using a GaAs structure, and 2.2 m if a GaSb structure is used for the VECSEL active region. The principal advantage is that the effective optical path inside the external cavity can be as high as 18 km, which allows for high sensitivity. In addition, a broad tunability of up to 60 nm allows for a much wider detection range. The potential sensitivity of this new kind of compact spectrometer is dI/I < 108. Micro-cavity light emitters manufactured using an MBE-grown CdHgTe active region and two evaporated YF3/ ZnS Bragg mirrors were presented by E Hadji (CEA-Grenoble). Evaporated mirrors allow the emission wavelength to be tailored and the line width to be fitted to the absorption band of a particular gas. These devices feature a total micro-cavity region of around 5 m, and are pumped using an uncooled infrared GaAs laser diode to produce emission ranging from 2 to 6 m. An important wavelength lies at 3.3 m, used primarily in the detection of CH4. By employing a piezoelectric actuator to control the movement of the mirror, a tuning range of 300 nm was possible, while the sensitivity to CH4 was of the order of 1 ppm. The cavity volume of such a device also lends itself to scaling, and a 1 cm3 cavity would make a laser sufficiently compact to allow integration into a gas detection micro-system. II-VI devices Edge-emitting lasers based on II-VI materials (i.e. lead-salts) are typically used for chemical gas analysis and atmospheric pollution monitoring. This is a result of the ability to access emission wavelengths out to 30 m, and achieve high CW output powers at temperatures up to 223 K. II-VI surface emitters are also now beginning to emerge, and T Schwarzl (University of Linz) described a 3.1 m VCSEL with laser emission up to 65C. The lasers were based on two PbEuTe/ EuTe Bragg mirrors and nine PbTe QWs in the active region, optically pumped using a 1.97 m source. Substrates In terms of world sales volumes of compound semiconductor substrates, the quantity of wafers used for mid-infrared devices is small compared to GaAs and InP for wireless and optoelectronics applications). According to I Grant of Wafer Technology, sales are led by GaAs with $300 million, followed by InP with $50 million. The remaining $5 million of the market can be broken down as 50% for InSb, 36% for InAs and 14% for GaSb. Frequency converters Optical parametric oscillators are non-linear optical devices that convert a fraction of the output of a laser (i.e. the pump) into two outputs the signal and idler at longer wavelengths. V Berger and colleagues (Thomson CSF) discussed the feasibility of a parametric oscillator made from GaAs/oxidized Al and integrated onto a GaAs chip. Berger s device uses frequency conversion of the laser emission into an idler portion in the 1.82.3 m range, which is achieved by tuning the pump laser emission from 1.008 to 1.012 m. M van Herpen and colleagues from the Nijmegen Catholic University have employed OPOs in the detection of ethane, butane and pentane. The set up uses a YAG pump laser emitting at 1.064 m, a signal laser at 1.5 m and a periodically poled lithium niobate crystal. Quantum cascade lasers J Cockburn (University of Sheffield) proposed a novel GaAs/AlAs structure that achieved lasing at 7.2 m. By inserting monolayers of InAs into the active region, the maximum lasing temperature could be increased to 250 K, in this case at the longer wavelength of 8.3 m. The best quantum cascade lasers re-corded threshold current densities of 3.2 kA/cm2 and peak power of 800 mW at 10 K. This was achieved with a structure based on InGaAs/InAlAs for the active region. Cockburn noted that further optimization is required for room temperature operation. A Mller from Alpes Lasers in Switzerland reported excellent results for a quantum cascade laser with an output power of 0.06 mW at 9.3 m. The device emits under CW operating conditions at room temperature. 700 mW pulsed power could be achieved at ambient temperature, while 90 mW was available up to 450 K. Mid-infrared lasers J Meyer (Naval Research Laboratory) described a method of achieving high-quality beam profiles for devices employing wide pump-stripes. This is based on an angled-grating distributed feedback device (a-DFB). In this structure, a diffraction grating is etched into the top GaSb separate confinement region at an angle of 16 with respect to the facets. For an optically-pumped laser emitting at 4.46 m with a 50 m wide stripe, the result is a reduction in beam divergence from 25 for a conventional FabryPerot cavity to 1.4 FWHM for the a-DFB. The use of ion bombardment to fabricate a-DFBs with "virtual mesas" and more restricted regions (called "spoilers") has led to considerable improvements in beam quality in devices with active stripes as wide as 300 m (). Princeton Lightwave s D Garbuzov summarized his group s work on MBE-grown GaInAsSb/AlGaAsSb lasers in the 2.02.7 m range. For diodes operating around 2 m (indium content of 20%), emphasis was laid on broadening the waveguide region out to 0.8 m and reducing the number of quantum wells to 2. This leads to a decrease in internal losses to 2 cm1. Ridge diodes 200 m in width exhibited a differential efficiency of 53% (for a cavity length L = 2 mm), a threshold current density of 115 A/cm2 and a record CW output power of 1.9 W. By increasing the indium content to 40%, CW lasing has been obtained up to 2.7 m at room temperature. In the 2.32.6 m range, laser diodes exhibit threshold current densities around 300 A/cm2 and CW output powers of more than 100 mW. Ridge diodes with a width of 5 m were processed for spectroscopy applications. At 2.33 m, these devices exhibit a CW output power of 20 mW, with 1 to 2 mW available in a single longitudinal mode. D Yarekha from the University of Montpellier discussed laser diodes based on GaInAsSb/AlGaAsSb. The devices were grown by MBE on n-GaSb (Te) substrates. The undoped active region comprises three 10 nm Ga0.65In0.35Sb0.84As0.16 QWs and four 30 nm Ga0.65Al0.35SbAs barriers. Laser structures emitting at 2.26 m exhibit differential efficiencies that change from 80% to 33% for cavity lengths (L) in the 0.452.25 mm range, and a threshold current density of 180 A/cm2 (L = 2.25 mm). The CW performance of a device with a cavity length of 0.95 mm is shown in as a function of temperature. With a ridge width of 5 m, the CW output power reaches 40 mW at ambient temperature. The important feature is the high-temperature CW operation, which reaches 140C, and up to 180C in pulsed mode. Yarekha attributes the performance to the incorporation of high concentrations of indium and arsenic, which provides a higher conduction band offset and improves the electron confinement. M Rattunde (Fraunhofer Institute) has also investigated GaInAsSb/AlGaAsSb laser diodes emitting in the 1.732.35 m range in CW mode. Rattunde detailed lasers that emit at 1.94 m and feature an internal quantum efficiency of 77% and threshold current density of 121 A/cm2 (for an infinite cavity length). Mid-infrared LEDs and detectors B Matveev from the Ioffe Institute in Russia spoke of the possibility of operating LEDs in the range 57 m at up to 80C. In particular, the superior features of negative luminescence over electroluminescence for temperatures over 60C were elucidated. D Smith from Edinburgh Instruments, Scotland reported a detectivity D* of 4.5 109 cm Hz1/2 W1 for an uncooled InSb diode, which employed immersion optics and operated at a spectral wavelength of 6 m (the best performance at this wavelength). Northwestern University s M Razheghi related improvements obtained using submicron lithography for detectors used out to 16 m. The processed devices were type II InAsSb superlattices exhibiting 108 cm Hz1/2 W1 at room temperature (). The R0A value is two orders of magnitude better than that found in CdHgTe devices. Uncooled InAsSb detectors were also covered in G Marre s presentation. Marre noted that the main advantage of this detector is an ability to grow structures that are lattice matched to GaSb substrates. Conclusions shows a summary of the maximum operating temperatures achieved by laser devices discussed at the conference. It highlights three regions of the spectrum corresponding to a specific kind of device. In the 23 m range, GaInAsSb/ AlGaAsSb MQW laser diodes are the most interesting devices, featuring CW operation out to 2.7 m (e.g. Garbuzov) for temperatures up to 140C (Yarekha). For the 34 m range, and when portability is not an issue, the frequency conversion approach achieved by OPOs appears to be the solution (e.g. van Herpen). Optically pumped II-VI VCSELs can also be considered at this wavelength, as they are now able to operate above room temperature at 3.1 m (Schwarzl). Above 4 m and up to 24 m, it is now clear that a quantum cascade laser with an embedded DFB grating provides a promising new mid-infrared coherent radiation source. A very significant breakthrough was announced during the meeting: the first demonstration of a QC laser emitting at 9.3 m in CW mode at room temperature (Mller). For detectors, cooled CdHgTe still dominates the market for military applications, although research laboratories point to the superior properties of InAsSb in the field with the possibility of room temperature operation (Marre, Razhegi).