DFB Lasers Between 760 nm and 16 μm for Sensing Applications

Recent years have shown the importance of tunable semiconductor lasers in optical sensing. We describe the status quo concerning DFB laser diodes between 760 nm and 3,000 nm as well as new developments aiming for up to 80 nm tuning range in this spectral region. Furthermore we report on QCL between 3 μm and 16 μm and present new developments. An overview of the most interesting applications using such devices is given at the end of this paper

A Sensitive and Reliable Carbon Monoxide Monitor for Safety-Focused Applications in Coal Mine Using a 2.33-μ\mu m Laser Diode

In this paper, a stable and reliable carbon monoxide (CO) monitoring system with high sensitivity (at sub-ppm level) was designed and demonstrated with particular reference to use in the mining industry, tailoring the design specifically for forecasting spontaneous combustion, a major hazard to miners. An appropriate strong CO absorption line was used to minimize the interferences expected from gases present in ambient air, with several preferred CO absorption lines selected and investigated, therefore choosing a distributed feedback (DFB) laser operating at a wavelength of 2330.18 nm as the excitation source. Through a detailed investigation, a minimum detection limit of ~0.2 ppm and a measurement precision of <50 ppb were achieved with a 1 s averaging time. Further in tests, a long-term continuous monitoring evaluation was carried out, demonstrated the excellent stability and reliability of the developed CO monitor. The results obtained have validated the potential of this design of a CO monitoring system for practical monitoring applications underground to enhance safety in the mining industry

III–V-on-silicon photonic integrated circuits for spectroscopic sensing in the 2–4 μm wavelength range

The availability of silicon photonic integrated circuits (ICs) in the 2-4 mu m wavelength range enables miniature optical sensors for trace gas and bio-molecule detection. In this paper, we review our recent work on III-V-on-silicon waveguide circuits for spectroscopic sensing in this wavelength range. We first present results on the heterogeneous integration of 2.3 mu m wavelength III-V laser sources and photodetectors on silicon photonic ICs for fully integrated optical sensors. Then a compact 2 mu m wavelength widely tunable external cavity laser using a silicon photonic IC for the wavelength selective feedback is shown. High-performance silicon arrayed waveguide grating spectrometers are also presented. Further we show an on-chip photothermal transducer using a suspended silicon-on-insulator microring resonator used for mid-infrared photothermal spectroscopy

Light Propagation and Gas Absorption Studies in Turbid Media Using Tunable Diode Llaser Techniques

Optical absorption spectroscopy is a widely used analytical tool for constituent analysis in many applications. According to the Beer-Lambert law, the transmitted light intensity through a homogeneous medium is an exponential function of the product of the concentration, the total pathlength, and the absorption cross-section of the absorbing substance. By studying the intensity loss at the unique absorption band of the absorbing substance, its concentration can be retrieved. However, this method will encounter some difficulties if the light is not only absorbed but also strongly scattered in the material, e.g., in a turbid medium (biological tissues, porous ceramics, wood), which results in an unknown absorption pathlength. Such a problem can be solved by studying light propagation with different theoretical models, and the scattering and absorption properties are then retrieved. One aim of the present thesis work is to develop a new experimental approach to study light propagation in turbid media – frequency-modulated light scattering interferometry (FMLSI), originating from the well-known frequency-modulated continuous-wave technique in telecommunication field. This method provides new possibilities to study optical properties and Brownian motion simultaneously, which is particularly useful in biomedical applications, food science, and for colloidal suspensions in general. Another important application of absorption spectroscopy is to monitor gas concentration in turbid media, where the gas absorption pathlength is a priori unknown due to heavy light scattering in the porous medium. The technique is referred to as gas in scattering media absorption spectroscopy (GASMAS), and is based on the principle that the absorption spectrum of gases is much narrower than that for the solid- or liquid-phase host materials. By linearly scanning the wavelength of the light source across an absorption line of the gas and examining the absorption imprint superimposed on the transmitted light signal, the very weak intensity loss due to the gas of interest can be measured for gas concentration assessment. In order to obtain the absolute gas concentration, a focus in the present thesis work is to determine the gas absorption pathlength in turbid media. The FMLSI technique is proposed to obtain the mean optical pathlength – the total pathlength through both the pores and the matrix material. The combined method of FMLSI and GASMAS techniques is then developed to study porous media, where an average gas concentration in the porous media can be obtained. A conventional method for pathlength or optical properties determination – frequency domain photon migration – is also combined with the GASMAS technique to study the total gas absorption pathlength and the porosities of ceramics, which, as a result, also contributes to further understanding of light propagation in porous media. Another method is also proposed to get the absolute gas concentration without knowing the optical pathlength. It is based on absorption line shape analysis – relying on the fact that the line shape depends upon the concentration of the buffer gas. This method is found to be very useful for, e.g., gas concentration monitoring in food packaging

Development of laser spectroscopy for scattering media applications

Laser spectroscopy for both large and small spatial scales has been developed and used in various applications ranging from remote monitoring of atmospheric mercury in Spain to investigation of oxygen contents in wood, human sinuses, fruit, and pharmaceutical solids. Historically, the lidar group in Lund has performed many differential absorption lidar (DIAL) measurements with a mobile lidar system that was first described in 1987. During the years the lidar group has focused on fluorescence imaging and mercury measurements in the troposphere. Five lidar projects are described in this thesis: fluorescence imaging measurement outside Avignon, France, a unique lidar project at a mercury mine in Almadén, Spain, a SO2 flux measurement at a paper mill in Nymölla, Sweden, and two fluorescence imaging projects related to remote monitoring of vegetation and building facades characterization. A new method to measure wind speed remotely in combination with DIAL measurements is presented in this thesis. The wind sensor technique is called videography and is based on that images of plumes are grabbed continuously and the speed is estimated by the use of image processing. A technique that makes it possible to measure a gas in solids and turbid media, non-intrusively, is presented in this thesis. The technique is called gas in scattering media absorption spectroscopy (GASMAS) and has been used since 2001. The GASMAS concept means that a traditional spectroscopy instrument, based on tunable diode lasers, is used but the gas cell or optical path is replaced by a material that strongly scatters light. Mostly, wavelength modulation spectroscopy has been utilized. Four projects using the GASMAS technique to measure gases in fruit, wood, pharmaceutical solids, and human tissue are presented. Two applications have shown a great potential so far; to be able to diagnose the health of human sinuses and gas ventilation in sinuses, and to measure gas inside pharmaceutical solids. A performance analysis of the GASMAS technique is included. This thesis also presents a technique to suppress optical noise in fiber lasers and how to construct a compact tunable diode laser spectroscopy system based on plug-in boards for a standard computer

InP based lasers and optical amplifiers with wire-/dot-like active regions

Long wavelength lasers and semiconductor optical amplifiers based on InAs quantum wire-/dot-like active regions were developed on InP substrates dedicated to cover the extended telecommunication wavelength range between 1.4 and 1.65 mu m. In a brief overview different technological approaches will be discussed, while in the main part the current status and recent results of quantum-dash lasers are reported. This includes topics like dash formation and material growth, device performance of lasers and optical amplifiers, static and dynamic properties and fundamental material and device modelin

High-brightness lasers with spectral beam combining on silicon

Modern implementations of absorption spectroscopy and infrared-countermeasures demand advanced performance and integration of high-brightness lasers, especially in the molecular fingerprint spectral region. These applications, along with others in communication, remote-sensing, and medicine, benefit from the light source comprising a multitude of frequencies. To realize this technology, a single multi-spectral optical beam of near-diffraction-limited divergence is created by combining the outputs from an array of laser sources. Full integration of such a laser is possible with direct bonding of several epitaxially-grown chips to a single silicon (Si) substrate. In this platform, an array of lasers is defined with each gain material, creating a densely spaced set of wavelengths similar to wavelength division multiplexing used in communications.Scaling the brightness of a laser typically involves increasing the active volume to produce more output power. In the direction transverse to the light propagation, larger geometries compromise the beam quality. Lengthening the cavity provides only limited scaling of the output power due to the internal losses. Individual integrated lasers have low brightness due to combination of thermal effects and high optical intensities. With heterogeneous integration, many lasers can be spectrally combined on a single integrated chip to scale brightness in a compact platform. Recent demonstrations of 2.0-µm diode and 4.8-µm quantum cascade lasers on Si have extended this heterogeneous platform beyond the telecommunications band to the mid-infrared.In this work, low-loss beam combining elements spanning the visible to the mid-infrared are developed and a high-brightness multi-spectral laser is demonstrated in the range of 4.6–4.7-µm wavelengths. An architecture is presented where light is combined in multiple stages: first within the gain-bandwidth of each laser material and then coarsely between each spectral band to a single output waveguide. All components are demonstrated on a common material platform with a Si substrate, which lends feasibility to the complete system integration. Particular attention is focused on improving the efficiency of arrayed waveguide gratings (AWGs), used in the dense wavelength combining stage. This requires development of a refined characterization technique involving AWGs in a ring-resonator configuration to reduce measurement uncertainty. New levels of low-loss are achieved for visible, near-infrared, and mid-infrared multiplexing devices. Also, a multi-spectral laser in the mid-infrared is demonstrated by integrating an array of quantum cascade lasers and an AWG with Si waveguides. The output power and spectra are measured, demonstrating efficient beam combining and power scaling. Thus, a bright laser source in the mid-infrared has been demonstrated, along with an architecture and the components for incorporating visible and near-infrared optical bands

Gas traces measurement by photoacoustic spectroscopy using Helmholtz resonator-based sensors

Photoacoustic spectroscopy is a well-established gas traces optical detection technique, which consists in the generation of an acoustic wave in the investigated gas compound excited by a modulated laser beam, and in the detection of this sound wave with a sensitive microphone. The sensitivity of this technique can be greatly enhanced through the use of acoustic resonators. A wide range of resonant configurations has been developed and reported in the literature in the last decades. Among these, Helmholtz resonators are known for their simplicity of implementation, though with a reduced efficiency. The main goal of this work is to demonstrate that Helmholtz resonators can be successfully applied to photoacoustic spectroscopy, delivering sensitivities in the same order of magnitude as other more common configurations, as well as offering a powerful tool to perform differential measurements. Two Helmholtz-based sensors are presented within this thesis. The first sensor has been developed for ammonia sensing, taking benefit from the properties of antimonide-based lasers; the final design has been the result of a carefully optimised compromise between high sensitivity, noise reduction, technical constraints, compact size and straightforward use. After several implemented improvements, an ultimate sub-ppm concentration detection limit has been achieved. The second sensor exploits the intrinsic phase shift existing between the two volumes of a Helmholtz resonator to perform differential measurements. Each of the two volumes of the resonator is filled with a different concentration of the same gas, whereas the exciting laser beam is split in two arms that separately illuminate the volumes, the resulting photoacoustic signal being proportional to the difference between the respective concentrations of the probed gas in the volumes. A particular detection scheme has been implemented to guarantee a linear measurement. The achieved sensitivity is not as high as obtained with the first sensor; nonetheless, the developed sensor offers the possibility to perform measurements that would otherwise require two different sensing devices, resulting in a clear gain in terms of cost and of detection complexity