The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

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Optical gas sensing: a review

Semiconductor lasers are the most efficient man-made narrow-band light sources and convert up to three-quarters of electric energy into light High-power diode lasers are characterized by very high internal power densities in their small cavity, resulting in local heating and sometimes device degradation Catastrophic optical damage (COD) of diode lasers is a relevant degradation mechanism and limit for reaching ultra-high optical powers An overview is given on research activities targeting the mechanisms being relevant for the COD process in GaAs-based diode lasers emitting in the 630―1100 nm range The discussion of experiments, where COD is artificially provoked, represents the main topic The sequence of events and fast kinetics taking place on a nanosecond to microsecond time scale are addressed A particular emphasis is laid on recent experimental work performed in the authors' laboratories Paving the way for knowledge-based solutions towards more robust diode lasers represents the ultimate goal of this work COD diagram determined for a batch of broad-area AlGaAs diode lasers The time to COD within a single current pulse is plotted versus the actual average optical power in the moment when the COD takes place Full circles stand for clearly identified COD events (right ordinate), whereas open circles (left ordinate) represent the pulse duration in experiments, where no COD has been detected A borderline (gray) exists between two regions, i e, parameter sets, of presence (orange) and absence of COD (blue) This borderline is somewhat blurred because of the randomness in filamentation of the laser nearfield and scatter in properties of the involved individual devices

Mechanisms and fast kinetics of the catastrophic optical damage (COD) in GaAs-based diode lasers

We present an optical gas sensor based on the classical nondispersive infrared technique using ultracompact photonic crystal gas cells. The ultracompact device is conceptually based on low group velocities inside a photonic crystal gas cell and low-reflectivity antireflection layers coupling light into the device. Experimentally, an enhancement of the CO2 infrared absorption by a factor of 2.6 to 3.5 as compared to an empty cell, due to slow light inside a 2D silicon photonic crystal gas cell, was observed; this is in excellent agreement with numerical simulations. We show that, theoretically, for an optimal design enhancement factors of up to 60 are possible in the region of slow light. However, the overall transmission of bulk photonic crystals, and thus the performance of the device, is limited by fluctuations of the pore diameter. Numerical estimates suggest that the positional variations and pore diameter fluctuations have to be well below 0.5% to allow for a reasonable transmission of a 1 mm device.

Miniature infrared gas sensors using photonic crystals

We report on detailed experimental investigation of thermal properties of AlGaAs/GaAs quantum cascade lasers (QCLs) emitting at wavelength of 9.4 μm. Different mounting options and device geometries are compared in terms of their influence on the relative increase of the active region temperature. High resolution, spatially resolved thermoreflectance is used for mapping temperature distribution over the facet of pulse operated QCLs. The devices’ thermal resistances are derived from experimental data. We also develop a numerical thermal model of QC lasers, solving heat transport equation in 2D and 3D, which includes anisotropy of thermal conductivity. By combining experimental and numerical results, an insight into thermal management in QCLs is gained. Thermal optimization of the design focuses on improving heat dissipation in the device, which is essential to increase the maximal operation temperature of the devices.

Investigation of thermal properties of mid-infrared AlGaAs/GaAs quantum cascade lasers

The transient thermal properties of high-power diode laser bars with active and passive cooling are analyzed. Both thermal imaging and the analysis of the thermal wavelength tuning behavior are employed to extract the device temperature as a function of time. A steady-state thermal situation is established with rise times of about 10 and 60ms for active and passive cooling, respectively. The latter number, however, is substantially increased by the particular properties of the external heat sink. Such results are confirmed by model calculations based on the finite element method.

Transient thermal properties of high-power diode laser bars

Imaging thermography in the 3–5μm wavelength range is applied to the analysis of thermal properties of high-power diode lasers We investigate these devices by inspecting their front facets as well as their active regions along the resonator The latter is done through top windows within the substrate Raw data are found to be mostly interfered by thermal radiation traveling through the substrate, which is transparent for infrared light Substracting this contribution and recalibration allows for obtaining realistic temperature profiles along laser structures Facet heating is analyzed complementary by micro-Raman spectroscopy We show how hot spots at the front facet, in the substrate, or even in the active region within the substrate are discovered Our approach paves the way for an advanced methodology of device screening

Analysis of thermal images from diode lasers: Temperature profiling and reliability screening

Broadband imaging thermography is applied to investigate the infrared (IR) emission from high-power diode laser bars. We demonstrate the capabilities of a multispectral imaging system that operates in two optical channels, the near IR at 1.5–2μm and the mid IR at 2.4–5.5μm. In the near-IR region, deep level luminescence of the device contributes significantly to the thermo-images. A complementary measurement of the IR emission spectrum shows a broad defect band with maxima at E1=0.94eV and E2=1.05eV. Analysis of IR transients in both spectral channels makes a distinction of thermal radiation and deep level emission possible. In the mid IR, thermal radiation dominates, allowing for an analysis of thermal properties of the device with measurement times of fractions of a second only. This makes imaging thermography highly attractive for device screening.

Deep level emission from high-power diode laser bars detected by multispectral infrared imaging

Thermal tuning rates of single emitters in “cm-bar” high-power diode laser arrays are analyzed We find these tuning rates to consist of purely thermal and mechanical pressure contributions, of −048 and −008 meV(K)−1, respectively We estimate the mechanical deformation such a device experiences during pulsed operation to be 007%, and then apply an adequate external force to single segments of cm bars These single segments model the central emitters within the array Effects that arise due to gradual aging, such as nonequilibrium carrier lifetime decrease, sheet carrier concentration increase, and defect concentration rise are monitored and analyzed over up to 2×106 deformation cycles These experiments provide the basis for a type of accelerated aging experiment for device testing, especially of devices designed for low-frequency pulsed operation

Device deformation during low-frequency pulsed operation of high-power diode bars

The thermal properties and the degradation behavior of high-power broad-area diode lasers emitting at 650nm are analyzed. Imaging thermography is applied to assess the bulk temperature while the facet temperature is measured by micro-Raman spectroscopy. Although no visible facet alteration is observed, power degradation is found to be accompanied by increased temperatures at the facets. The immediate vicinity of them also turns out to be the starting point for the creation of defect networks within the quantum well seen in cathodoluminescence images. The observed behavior is compared to that known for near-infrared emitting devices.

Fritz Weik

Papers

Transient thermal properties of high-power diode laser bars

Analysis of thermal images from diode lasers: Temperature profiling and reliability screening

Deep level emission from high-power diode laser bars detected by multispectral infrared imaging

Device deformation during low-frequency pulsed operation of high-power diode bars

Thermal properties and degradation behavior of red-emitting high-power diode lasers