Spectral and modulation properties of a largely tunable MEMS-VCSEL in view of gas phase spectroscopy applications

An extensive characterization of the spectral properties of a largely tunable laser in the 1.56-μm spectral range is reported. This device combines a vertical-cavity surface-emitting laser (VCSEL) with a micro-machined (MEMS) Bragg mirror in a very compact arrangement. The large tunability obtained by an electro-thermal actuation of the MEMS mirror makes this device very attractive for high-resolution spectroscopy. Relevant laser parameters for the implementation of wavelength modulation spectroscopy techniques in gas sensing, such as tuning and modulation properties, are presented. A preliminary gas spectroscopy experiment performed with this laser is also shown.     

Determination of Thermal Parameters of Vanadium Oxide Uncooled Microbolometer Infrared Detector

Vanadium oxide thin films are the potential candidates for uncooled microbolometers due to their high temperature coefficient of resistance (TCR) at room temperature. A 2D array of 10-element test microbolometer without air-gap thermal isolation structure was fabricated with pulsed laser deposited vanadium oxide as IR sensing layer for the first time. Infrared responsivity of the uncooled microbolometer was evaluated in the spectral region 8-15 m. The device exhibits responsivity of about 12 V/W at 30 Hz chopper frequency for 20 A bias current. Thermal time constant (), Thermal conductance (G) and thermal capacitance (C) are the thermal parameters characterize the performance of the uncooled microbolometer infrared detectors are determined as 15 ms, 10^-3 W/K and 3.5 × 10^-5 J/K respectively. The influence of the thermal parameters on the performance of the microbolometer is discussed.     

A novel infrared absorbing structure for uncooled infrared detector

In this paper, we proposed a novel infrared absorbing structure for uncooled infrared detectors. The infrared absorber makes use of a quarter-wavelength structure composed of a dielectric layer, a protecting layer, an active layer, a supporting layer and a reflecting layer. Sputtered amorphous silicon is used as a dielectric layer because of its high refractive index. We fabricated the uncooled microbolometer with the proposed infrared absorbing structure by surface micromachining method. Then we characterized various bolometric properties such as thermal conductance, thermal time constant, responsivity and infrared absorptance. The fabricated bolometer showed the thermal conductance of 6.72 × 10⁻⁷ W/K, the thermal mass of 4.43 × 10⁻⁹ J/K, the thermal time constant of 6.6 ms and the responsivity of 7.76 × 10³ V/W at 10 Hz chopper frequency and 9.22 μA bias current. From the results, the estimated absorptance is about 80%. We expect that the proposed absorbing structure shows high infrared absorption and high performance of uncooled microbolometer.     

微反射镜阵列器件技术及其应用

概述了微反射镜阵列器件的分类、其主要驱动方式，以及现有商用阵列器件的情况。就4种驱动方式，介绍了其机电特性和工艺特点，并进行比较分析，指出选择驱动方式应依据应用的性质、工艺集成难度和成本效益原则。总结了微反射镜阵列器件在5个方向的应用。最后，阐述了微反射镜阵列器件技术展望。     

A New CMOS Read-out IC for Uncooled Microbolometer Infrared Image Sensor

An uncooled microbolometer image sensor, used in an IR image sensor, is made by a micro electro mechanical systems (MEMS) process, so the value of the microbolometer resistor has a process variation. Also, the reference resistor, which is used to connect to the microbolometer, is fabricated by a standard CMOS process, and the difference between the values of the microbolometer resistor and the reference resistor generates an unwanted output signal for the same input from the sensor array. In order to minimize this problem, a new CMOS read-out integrated circuit (ROIC) was designed. Instead of a single input mode, a differential input mode scheme and a simple method to compensate the resistor value are proposed. Using results from a computer simulation, it is observed that the output characteristic of the ROIC was improved and the effect of the process variation was decreased without using complex compensation circuits. Based on the simulation results, a prototype device including an ROIC that was fabricated by a standard 0.25um CMOS process and a microbolometer with a 16 x 16 sensor array was fabricated and characterized.     

Thermally tunable narrow-bandpass filter based on a linearly chirped fiber Bragg grating

We propose a new method for the development of a tunable optical bandpass filter (TOBF) based on a linearly chirped fiber Bragg grating (LCFBG). A NiCr wire heater is used to heat the LCFBG at a small point to introduce a narrow passband within the stop band of the LCFBG. The central wavelength of the passband is tuned by scanning the wire heater along the LCFBG. As an example demonstrating the effectiveness of the proposed method, we demonstrate a TOBF with a very small 3-dB bandwidth of approximately 7 pm, a tuning range of 16.4 nm, and a rejection ratio of more than 25 dB. Compared with previously reported tunable-fiber-based bandpass filters, this method provides the advantages of a large tuning range, continuous tunability, a switchable passband, a simple tuning mechanism, low cost, and narrow bandwidth.     

Response model of resistance-type microbolometer

There are certain limitations in the application of uncooled focal plane array (FPA) detectors owing to the shortage of the response model that transforms bias voltage to analog output voltage at a certain constant radiation power; Moreover, bias voltage affects the input radiation gain. In this study, we established a response model of a microbolometer through examining the detection theory of a microbolometer and the heat balance equation under the condition of the pulse voltage bias. In the establishment process, we simplified the heat balance equation in order to acquire a simple answer. The experimental data show that, in the full variable range of bias voltages, the biggest difference between the model data and the experiment data is about 0.7 K. This model can reflect the real responses of microbolometers with only small differences, which are acceptable in engineering applications.     

Fabricating Microbolometer Array on Unplanar Readout Integrated Circuit

In this paper, the integration of an experimental 32 × 32 uncooled IR microbolometer array with an unplanar CMOS Readout Integrated Circuit (ROIC) is presented. A vanadium oxide film fabricated by low temperature reactive ion beam sputtering is utilized as thermal-sensitive material in the bolometric detectors Before the integration, the unplanar ROIC for commercial use is first planarized by bisbenzocyclobutene film, then a electroless nickel-plating on ohmic contact areas is accomplished. Finally the bolometer array is fabricated using a micromachining process, which is completely compatible with CMOS technology. Measurements and calculations for the as-fabricated samples show that the responsivity of 1.4 × 10⁴ V/W and the detectivity of 2.1 × 10⁸cmHz^1/2W^–1 and a thermal response time of 10ms are obtained at a pulse bias of IV.     

Spatial light modulator as a reconfigurable intracavity dispersive element for tunable lasers

An improved approach for narrow-band wavelength selection in tunable lasers is described. To provide the tunability, a reconfigurable diffractive optical element (DOE) based on a programmable spatial light modulator (PSLM) is applied. With a proper choice of the phase transfer function of the PSLM, the device can be used as a dispersive intra-cavity component for precise tuning within the lasing spectral band of a solid-state dye laser. The suggested design allows avoiding the mechanical movement of any cavity components. The tunability performance and simulation are demonstrated using the Fourier optics method.     

Operation of a high power,broadly tunable infrared dye laser

We discuss the operation and performance of a ruby pumped infrared dye laser with a wide tuning range and a high out-put power. The longitudinally pumped infrared laser was operated in the Littrow configuration without an intracavity beam expander. The dye, IR 125, exhibited a tunability range that extended from 8325 Å to 9375 Å. The dye laser had a pulse duration of 20 ns and an output power of 12.5 MW with a 2.3 joule ruby pump. The maximum output power of the infrared laser was limited by the damage threshold of the tuning element.     

A Novel Method for the Detection of Trace Gases Based on Photo-Acoustic Spectroscopy

We study quartz tuning fork (QTF) characteristics using a 532 nm semiconductor laser with a power of 39 mW and calculate QTF vibrations caused by thermal noise and disturbance of the air using the equipartition theorem; the vibration value is about 1.152 Pm. The signal-to-noise ratio and QTF resonance amplitude acquired experimentally are 104.56 and 214.75 Pm, respectively. In addition, we develop a new photo-acoustic spectroscopic system for detection of trace acetylene using a CW diode laser source with distributed feedback operating near 1,532 nm, measure the absorption spectrum of acetylene employing this system, and show that the method elaborated is more sensitive than photoelectric detectors that provides new directions for research in photo-acoustic spectroscopy.     

Effect of pump power on the tuning range of a filterless erbium-doped fiber ring laser

In this paper, we establish the influence of pump power on the tuning characteristics of a filterless erbium-doped fiber laser, operating in the L band. Tunable action is achieved with the control of intra cavity loss. The explicit dependence of tunability on the average inversion levels is brought out. The shift in the lower limit of tuning range and the non-linear dependence of lasing wavelength on the intra cavity loss are analytically deduced using the gain spectra and experimentally interpreted using amplified spontaneous emission spectra. Continuous tunability is achieved with the careful control of pump power. PACS 42.55.Wb; 42.60.Da; 42.55.-f     

Current-controlled curvature of coated micromirrors

Precise control of micromirror curvature is critical in many optical microsystems. Micromirrors with current-controlled curvature are demonstrated. The working principle is that resistive heating changes the temperature of the micromirrors and thermal expansion induces a controlled curvature whose magnitude is determined by coating design. For example, for wide focal-length tuning, the radius of curvature of a gold-coated mirror was tuned from 2.5 to 8.2 mm over a current-induced temperature range from 22 degrees to 72 degrees C. For fine focal-length tuning, the radius of curvature of a dielectric-coated (SiO2/Y2O3 lambda/4 pairs) mirror was tuned from -0.68 to -0.64 mm over a current-induced temperature range from 22 to 84 degrees C. These results should be readily extendable to mirror flattening or real-time adaptive shape control.     

320×240长波非致冷微测辐射热计红外热象仪的研制

介绍了基于微测辐射热计320×240长波非致冷红外热象仪的研制,详细说明了其电气系统的设计方法及技术关键,并对其光学系统进行了简要说明.采用高象质的双反射光学系统以减少象差;红外探测器采用320×240长波非致冷集成微测辐射热计红外焦平面阵列(FPA);电气系统采用复杂可编程逻辑器件(CPLD)完成红外焦平面阵列的驱动,应用数字信号处理(DSP)技术实现红外焦平面阵列的非均匀性校正、图象增强及红外图象实时显示等.     

Phonon scattering and thermal conductance properties in two coupled graphene nanoribbons modulated with bridge atoms

The phonon scattering and thermal conductance properties have been studied in two coupled graphene nanoribbons connected by different bridge atoms by using density functional theory in combination with non-equilibrium Green's function approach. The results show that a wide range of thermal conductance tuning can be realized by changing the chemical bond strength and atom mass of the bridge atoms. It is found that the chemical bond strength (bridge atom mass) plays the main role in phonon scattering at low (high) temperature. A simple equation is presented to describe the relationship among the thermal conductance, bridge atom, and temperature.     

The optical responsivity in IR-photodetector based on armchair graphene nanoribbons with p–i–n structure

Armchair graphene nanoribbons (A-GNRs) are an alternative material to use in novel infrared photodetectors, because of their tunable energy gap in the infrared spectrum, and their high quantum efficiency. In this paper, an A-GNR p–i–n structure with all three structural families, different width, and different number of layers to use in IR detectors have been investigated. With calculating the band structure and energy gap using the tight-binding model and by including the edge deformation, the optical absorption in the single electron approximation has been obtained by calculating the optical conductance. Finally, we have calculated the quantum efficiency and the optical responsivity of A-GNR based IR photodetector as a function of incident photon energy, temperature, nanoribbon width and the number of layers. Results show that the responsivity of the A-GNR based IR photodetector increase by increasing the width and number of layers and decrease by increasing the temperature.     

QWIP or other alternative for third generation infrared systems

Choosing the right detector technology for third generation thermal imaging systems is directly derived from the requirements of these new generation infrared imaging systems.

It is now evident that third generation thermal imager will still need the higher resolution capabilities as well as capabilities in multispectral detection and polarization sensitivity. Four technologies candidates are analyzed; the field-proved HgCdTe (MCT), uncooled microbolometer technology, antimonide based materials and quantum well infrared photodetectors (QWIP). Taking into account the risks, maturity and technologies barrier of each technology, we claim that for non-strategic applications (not low background conditions), QWIP technology is the most favorite. The ternary and superlattice antimonide based materials group seems to be theoretically the best alternative, but are not recommended due to its immaturity and the high risk involved in this technology (passivation, doping control, etc.). We anticipate large penetration of the uncooled detectors to the low-end and medium-end market. The HgCdTe will still be in progress due to the inertia of the large funding and the strategic importance of this detectors technology.     

Test and evaluation of reference-based nonuniformity correction methods for microbolometer infrared detectors

In the paper, reference-based nonuniformity correction methods for microbolometer infrared detectors are discussed and tested. In order to evaluate their effectiveness, a complete readout circuit for amorphous silicon microbolometer focal plane array has been designed. The tests were carried out on a developed stand including several extended blackbodies. Some modification of standard two-point correction algorithm incorporating detectors response at external shutter to compensate offset drift is also proposed. The obtained results are presented.     

Polycrystalline lead selenide: the resurgence of an old infrared detector

The existing technology for uncooled MWIR photon detectors based on polycrystalline lead salts is stigmatized for being a 50-year-old technology. It has been traditionally relegated to single-element detectors and relatively small linear arrays due to the limitations imposed by its standard manufacture process based on a chemical bath deposition technique (CBD) developed more than 40 years ago. Recently, an innovative method for processing detectors, based on a vapour phase deposition (VPD) technique, has allowed manufacturing the first 2D array of polycrystalline PbSe with good electro optical characteristics. The new method of processing PbSe is an all silicon technology and it is compatible with standard CMOS circuitry. In addition to its affordability, VPD PbSe constitutes a perfect candidate to fill the existing gap in the photonic and uncooled IR imaging detectors sensitive to the MWIR photons. The perspectives opened are numerous and very important, converting the old PbSe detector in a serious alternative to others uncooled technologies in the low cost IR detection market. The number of potential applications is huge, some of them with high commercial impact such as personal IR imagers, enhanced vision systems for automotive applications and other not less important in the security/defence domain such as sensors for active protection systems (APS) or low cost seekers. Despite the fact, unanimously accepted, that uncooled will dominate the majority of the future IR detection applications, today, thermal detectors are the unique plausible alternative. There is plenty of room for photonic uncooled and complementary alternatives are needed. This work allocates polycrystalline PbSe in the current panorama of the uncooled IR detectors, underlining its potentiality in two areas of interest, i.e., very low cost imaging IR detectors and MWIR fast uncooled detectors for security and defence applications. The new method of processing again converts PbSe into an emerging technology.     

Broadly tunable second-harmonic generation in periodically poled silica fibers

Broadband tunability of the second-harmonic wavelength is achieved in periodically poled silica fibers. A wavelength tuning range of almost 45 nm of the fundamental wave is demonstrated through mechanical compression tuning of the quasi-phase-matched periodic structure. The uniform strain applied along the entire periodic structure enables the spectral profile and the conversion efficiency of the generated second harmonic to be preserved for the full tuning range. To our knowledge, the achieved tuning range realized through this technique is far greater than that possible with uniform periodically poled crystals.