Laser-based Sensor Research Articles

Active control of lean blowout in a swirl-stabilized combustor using a tunable diode laser

A novel tunable diode laser temperature sensor is used to prevent lean blowout (LBO) in a swirl-stabilized, partially premixed dump combustor which serves as a model of gas turbine combustors. The fast-response temperature sensor is based on rapid, repetitive (2 kHz) wavelength-scanned laser absorption of two vibrational overtone transitions of water vapor near 1.4 μm using a single fiber-coupled diode laser. Although the gas composition and temperature are not uniform along the line-of-sight, low-frequency temperature fluctuations are identified in the FFT power spectrum of the real-time temperature measurements at an optimized location. We found that the fraction of the FFT power at low-frequencies (0–50 Hz) increases exponentially as the flame approaches LBO. The intensity of these low-frequency temperature fluctuations is used to detect the proximity to LBO and as a control variable for the active LBO control system. Successful prevention of LBO during power reduction and transient fuel variation is demonstrated even without knowledge of the LBO fuel/air ratio limit for the specific operating conditions of the combustor. The active control system also can maintain the flame at very lean conditions (0.03 above the LBO equivalence ratio), and thus reduce the LBO margin. To our knowledge, this is the first application of laser-based sensors in active LBO control of swirl-stabilized flames. The results demonstrate the potential of tunable diode laser sensors for closed-loop combustion control applications.

Fibre length-dependent fluorescence spectral characteristics in high erbium concentration fibres for the optimization of FBG-based fibre sensor systems

This paper reports the behavioural characteristics of the fluorescence spectrum in high concentration 5000 ppm erbium-doped fibre (EDF), as a function of the EDF length, showing their potential for optical sensor applications to increase the potential number of multiplexed ‘all fibre’ sensors on a single channel. Experiments were conducted to analyze the fluorescence peak wavelength shift, the bandwidth and output power at a convenient fixed low pump power of 37 mW, using excitation from two different laser diodes wavelengths, at 980 and 1480 nm. It was found that the fibre length of the high concentration EDF has a direct effect on the fluorescence peak shift. The fluorescence spectrum tends to show different spectral profiles with its maximum position shifting towards longer wavelengths as the length of the EDF increases, with a bandwidth at full width half maximum (FWHM) covering portions of the C-, L-, and U-band. It has also been shown that pumping with the 980 nm LD contributes to a higher fluorescence shift compared to the 1480 nm LD, at the expense of output power. The use of the fluorescence peak shift is important in the design and optimization of efficient fibre Bragg grating (FBG)-based optical fibre and fibre laser-based sensor systems.

Quantum cascade laser-based integrated cavity output spectroscopy of exhaled nitric oxide

A nitric oxide (NO) sensor employing a thermoelectrically cooled, continuous-wave, distributed feedback quantum cascade laser operating at 5.47 μm (1828 cm-1) and off-axis integrated cavity output spectroscopy was used to measure NO concentrations in exhaled breath. A minimum measurable concentration (3σ) of 3.6 parts-per-billion by volume (ppbv) of NO with a data-acquisition time of 4 s was demonstrated. Five prepared gas mixtures and 15 exhaled breath samples were measured with both the NO sensor and for intercomparison with a chemiluminescence-based NO analyzer and were found to be in agreement within 0.6 ppbv. Exhaled NO flow-independent parameters, which may provide diagnostic and therapeutic information in respiratory diseases where single-breath measurements are equivocal, were estimated from end-tidal NO concentration measurements collected at various flow rates. The results of this work indicate that a laser-based exhaled NO sensor can be used to measure exhaled nitric oxide at a range of exhalation flow rates to determine flow-independent parameters in human clinical trials.

Experimental Detection and Reception Performance for Uplink Underwater Acoustic Communication Using a Remote, In-Air, Acousto-Optic Sensor

Covert communications between underwater and aerial platforms would increase the flexibility of surface and air vehicles engaged in undersea warfare by providing a new netcentric warfare communications capability and could have a variety of commercial and oceanographic applications. Research into an acousto-optic sensor shows promise as a means for detecting acoustic data projected toward the water surface from a submerged platform. The laser-based sensor probes the water surface to detect perturbations caused by an impinging acoustic pressure field. Experimental studies were conducted to demonstrate acousto-optic sensor feasibility for obtaining accurate phase preserved recordings of communication signals across the air-water interface. The recorded surface velocity signals were transferred to an acoustic communications receiver that used conventional acoustic telemetry algorithms such as adaptive equalization to decode the signal. The detected, equalized, and decoded bit error rate performance is presented for hydrostatic and more realistic, hydrodynamic water surface conditions

Static Shape and Vibration Control of Flexible Payloads With Applications to Robotic Assembly

This paper addresses the simultaneous control of vibration and static shape deformation of arbitrarily shaped thin-walled flexible payloads. During robotic assembly of these thin-walled parts, gravity and inertial forces acting on the parts may induce both static shape deformation and vibrations in the part sufficiently large that accurate and high speed assembly of these parts is hindered. Static deformations, which arise due to deformation of the part caused by its own weight under the influence of gravity, lead to misalignment of mating points of the parts. Unwanted vibrations, arising from inertial forces acting on the thin-walled parts as they are positioned for assembly, must damp out before parts can be mated, further hindering the process. In this work, a smart gripper with actuated contact points to grasp the flexible thin-walled parts is proposed to solve this problem. The smart gripper is capable of both part reshaping and active damping of unwanted vibrations of the part. It is fixed to a robotic manipulator and is comprised of multiple linearly actuated fingers with laser-based noncontact proximity sensors, and associated signal processing and controllers. In this paper, a simultaneous vibration and static shape controller is developed. The proposed controller is a composite modal controller in conjunction with a quasi-static modal filter and a bias Kalman filter, which is synthesized based on the reduced-order dynamic model of the flexible payload. A near industrial practice demonstration of the feasibility of the proposed approach is carried out using a proof-of-concept smart gripper to manipulate an automotive fender. Experimental results indicate that unwanted vibrations are successfully damped out, allowing faster cycle times for an assembly process, and static shape deformations are corrected, allowing accurate positioning of parts for assembly.

Carbon isotopomers measurement using mid-IR tunable laser sources

Recent developments of two mid-infrared tunable laser spectrometers dedicated to carbon isotope ratio determination are presented. First, a field deployable quantum cascade laser-based sensor is described, along with line selection strategy for 13/12CO2 ratio measurements. Secondly, an instrument architecture based on difference frequency generation is presented. The analyses of fundamental limitations, specifically temperature and pressure stability, and water vapor collision broadening, are detailed.

Measuring Pavement Friction Characteristics at Variable Speeds for Added Safety

Abstract Pavement friction testing is frequently conducted in accordance with the provisions outlined in ASTM E 274, “Standard Test Method for Skid Resistance of Paved Surfaces Using a Full-Scale Tire.” The standard speed of testing in Florida is 40 mph (64.4 km/h). However, due to safety concerns related to testing on high-speed facilities, considerable attention has been focused in recent years on height-sensor based (non-contact) technology. It is potentially well suited for surveying the surface texture characteristics of pavement sections while operating at highway speed. Although the height-sensor based technology has been available since the 1960s, it continues to mature. A considerable amount of research has been conducted to gain further understanding on the factors affecting high-speed pavement surface surveying from both the analytical and experimental points of view. Still some problems have not fully been resolved, particularly in the interpretation of the measured data and selection of adequate sensing technology (or sensor designs). The Florida Department of Transportation (FDOT) initiated the present study to assess the feasibility of using high-speed, laser-based sensors to quantify the texture and friction characteristics of asphalt pavements. The main objective of this study is to provide for a safer, faster and more appropriate method of estimating pavement friction characteristics on high-speed facilities, ramps, and at other potentially hazardous sites. Further, it is also intended to provide for a means to obtain a measure of International Friction Index (IFI) in accordance with ASTM E 1960. This paper presents a description of the FDOT testing program, the data collection effort, as well as the subsequent analyses and findings.

Methane monitoring by near infrared photoacoustic spectroscopy: The importance of relaxation phenomena

The importance of molecular relaxation in photoacoustic spectroscopy is discussed. The particular case of methane monitoring in dry oxygen using a 1.65-m laser-based photoacoustic sensor is reported. The slow vibration-to-translation energy transfer occurring in this gas mixture results in a drastic reduction of the detection sensitivity. A quadratic response of the sensor to the methane concentration is also reported and explained by molecular relaxation effects. 1. INFLUENCE OF MOLECULAR RELAXATION IN PHOTOACOUSTICS Photoacoustic (PA) spectroscopy basically consists in exciting an absorbing gas sample with a modulated laser beam at proper wavelength and detecting the generated acoustic wave using a microphone. The acoustic wave results from molecular absorption of photons and subsequent deactivation of the excited vibrational state via inelastic collisions with the nearby molecules (collisional relaxation). When the excited vibrational energy is transferred into kinetic energy of the surrounding molecules, a periodic heating occurs in the sample and induces the pressure wave. In order to efficiently generate a PA signal, the typical timescale of the vibration-to-translation (V-T) energy transfer should be much shorter than the period of the laser modulation (generally in the millisecond range). This condition is most often fulfilled, as the collisional relaxation time of most molecules ranges from nanosecond to hundreds of microseconds at atmospheric pressure. However, the relaxation time of some particular molecules may be orders of magnitude longer. In particular, this is the case for a few diatomic molecules, such as N2 or O2 (see Table 1), and for collisions with some particular partners (N2 and O2, too). When the timescale of the V-T processes is comparable or larger than the modulation period, the PA signal is poorly excited.

Fiber laser-based temperature sensor systems using uniform wavelength-matched Bragg grating reflectors

A detailed investigation has been carried out into the output characteristics of a fiber laser forming, as it does the basis of an optical sensor system, in terms of its wavelength, signal intensity and linewidth, and the output spectrum when each individual fiber Bragg grating, acting as the reflectors of the laser system, has been allowed to respond to temperature in a wavelength-matched normal Bragg grating-based laser configuration. An analytical model has been created reflecting the relationship between the grating bandwidth and the temperature under each operational condition and the results obtained have significant implication for the optimum design of this type of fiber laser-based sensors. Additionally, experiments were performed to investigate the output characteristics of a fiber laser-based sensor when used for temperature measurement.

Evaluation of pulse detonation engine modeling using laser-based temperature and OH concentration measurements

Two novel, laser-based sensors employing ultraviolet (UV) wavelengths are developed and applied to measure time-resolved temperature and OH concentration in a pulse detonation tube. These results, along with pressure data, are employed to evaluate two computational simulations utilizing different chemistry and heat transfer models to predict pulse detonation engine (PDE) flowfields. The first sensor applies 266 and 306 nm UV laser absorption to infer temperature from 2000 to 4000 K because of broadband CO 2 absorption. These results represent the first thermometry based on line-of-sight, cw UV laser absorption by CO 2. The second sensor utilizes a single 306 nm UV laser to probe an individual OH absorption feature. Time-resolved OH concentration is inferred from the measured absorption, temperature, and pressure. These two sensors provide microsecond time-resolved measurements, over a dynamic range of temperatures and pressures (2000–4000 K and 0.5–30 atm), of two important parameters needed for evaluation of PDE computational models. The first computational model evaluated utilizes a frozen gas composition assumption; the second model incorporates finite-rate chemistry and includes losses due to heat transfer and friction. The results show that the frozen gas composition simulation can successfully predict temperature and pressure profiles, although the agreement is artificial in that inclusion of heat transfer effects would negate this agreement. The simulation including finite-rate chemistry and no heat transfer overpredicts the measured temperature and OH concentration. However, with the proper inclusion of losses due to heat transfer, the finite-rate chemistry code accurately predicts all three measured parameters. We conclude that both finite-rate chemistry and heat transfer are important for correct modeling of PDE flowfields.

A Novel Lane Detection Algorithm Based on Support Vector Machine

In this paper, a new lane detection algorithm based on support vector machine (SVM) is presented. This algorithm can overcome the flaws when applying traditional lane algorithms which are only applicable to some special situations. The main steps of this algorithm: road surface extraction by using SVM pattern recognition, image morphology operation, transforming the image into a bird-view image by using the relationship of image coordinate system (ICS) and world coordinate system (WCS), getting the center points from the road’s midline, regressing the road shape function by using SVM. Relative to the traditional laser-based sensors and millimeterwave radar which are classified as active sensors, vision-based passive sensor has an advantage that it acquires data in a noninvasive mode and will not alter the environment.

Pulsed quantum-cascade laser-based sensor for trace-gas detection of carbonyl sulfide

Simultaneous exhaled carbonyl sulfide (OCS) and carbon dioxide concentration measurements in human breath are demonstrated with a compact pulsed quantum-cascade laser-based gas sensor. We achieved a noise-equivalent sensitivity (1sigma) of 1.2 parts per billion by measuring a well-isolated OCS P(11) absorption line in the v3 band at 2057.6 cm(-1) using an astigmatic Herriott cell of 36-m optical path length and 0.4-s acquisition time.

Near-infrared diode laser hydrogen fluoride monitor for dielectric etch

A hydrogen fluoride (HF) monitor, using a tunable diode laser, is designed and used to detect the etch endpoints for dielectric film etching in a commercial plasma reactor. The reactor plasma contains HF, a reaction product of feedstock gas CF4 and the hydrogen-containing films (photoresist, SiOCH) on the substrate. A near-infrared diode laser is used to scan the P(3) transition in the first overtone of HF near 1.31μm to monitor changes in the level of HF concentration in the plasma. Using 200ms averaging and a signal modulation technique, we estimate a minimum detectable HF absorbance of 6×10−5 in the etch plasma, corresponding to an HF partial pressure of 0.03mTorr. The sensor could indicate, in situ, the SiOCH over tetraethoxysilane oxide (TEOS) trench endpoint, which was not readily discerned by optical emission. These measurements demonstrate the feasibility of a real-time diode laser-based sensor for etch endpoint monitoring and a potential for process control.

Bragg grating tuned fiber laser system for measurement of wider range temperature and strain

A Bragg grating-tuned fiber laser-based sensor system has been developed to measure over a wide range of temperatures (22–500 °C) and strain (0–1200 με) using both a normal grating and a chirped grating as optical feedback elements and Er3+-doped fiber as the active gain medium. A high reflectivity normal Bragg grating was written into a specially fabricated high temperature sustaining Sb–Ge co-doped photosensitive fiber and a chirped grating was inscribed into a commercial B-Ge co-doped fiber, due to its higher photosensitivity. The shift in the laser wavelength was monitored in this system when the normal grating was temperature or strain-tuned, with it also forming the active element of the sensor head of the system. The root mean square (RMS) error values of the active sensor system were found to be 2.6 °C and 28.3 με over the above measured ranges of temperature and strain, respectively.

Evaluation of Lateral Mechanical Tension in Thin-Film Tissue Constructs

Fibroblast-populated collagen matrices provide a simplified tissue model for wound healing and development processes. A technology (CELLDRUM Technology) evaluating lateral mechanical tension in fibroblast-populated collagen matrices (tissue constructs) with a thickness of 1 mm was introduced. Defined mechanical boundary conditions together with the known number and orientation of the cells revealed precise data on the average tension exerted by a single cell. Circular cell-populated collagen gels were manufactured inside the CELLDRUM on top of a flexible membrane. The collagen matrix was then excited by a sound pulse. The resulting resonance oscillation was monitored by a laser-based deflection sensor and frequency and damping were analyzed giving information on mechanical properties of the tissue construct. Several evaluation experiments were performed. Calf serum enhanced contractile forces of fibroblasts dose dependently. After the gels were treated with cytochalasin D for 24 h, the cell forces were reduced by 42% of control. The remaining tension was attributed to the extracellular matrix remodeling occurring during cell growth and to other cytoskeletal structures like microtubules and intermediate filaments. We also found that only after a few hours of culture fibroblast-seeded collagen gels began developing significant mechanical tension. A mechanical tension profile of proliferating fibroblasts in collagen gels over culture time was obtained.

Laser visual sensing for seam tracking in robotic arc welding of titanium alloys

Sensor errors arose from detecting the weld-seam profile of titanium alloys with laser-based vision sensors because the surface of such a material is highly reflective to a laser light. In this study, the correct weld-seam profile was established from the distorted raw data by signal processing. Furthermore, a fully automatic seam-tracking system was developed. This system featured training and saving of the template, autofinding of the starting welding point, auto–calibration, detection of seam position and groove dimensions and tracking control. The system was capable of tracking V-groove, fillet, lap and butt joints of titanium alloys with high accuracy of less than 0.4 mm.

A wide temperature tunable fibre laser using a chirped grating and a type IIA fibre Bragg grating

A fibre laser sensor has been developed to operate over a wide temperature range from room temperature to 440 °C, where the laser cavity has been formed using a combination of a chirped grating and a type IIA fibre Bragg grating (FBG), enclosing a length of erbium doped fibre as the active gain medium. A FBG stabilized 1480 nm laser diode was employed as the pump source to achieve laser oscillation associated with the type IIA Bragg grating wavelength on the wavelength band around 1550 nm. The broadband-chirped grating was used as one of the cavity end reflectors to achieve temperature-tunable laser action for sensing applications over a wide range. The sensitivity of the sensor was found to be 13.94 pm °C−1 with a root mean square error of 8.85 °C. The primary advantage of this laser-based sensor over passive optical fibre sensors is the significant improvement in both the signal-to-noise ratio and the narrow linewidth of the laser signal, making it especially well suited to multiplexing situations.

In situ measurements of HCl during plasma etching of poly-silicon using a diodelaser absorption sensor

Tunable diode laser absorption spectroscopy is used to monitor hydrogen chloride(HCl) concentration in a commercial, high-density, low-pressure plasma reactorduring plasma etching. A near-infrared diode laser is used to scan the P(4)transition in the first overtone of HCl near 1.79 µmto measure changes in HCl levels. A variety of HBr and Cl2feedstock recipes are investigated at a process pressure of 10 mTorr asa function of rf power transformer coupled plasma, bias power andthe total flow rate. Using 50 ms averaging and a signal modulationtechnique, we estimate a minimum detectivity of 4 × 10−6in peak absorbance, which corresponds to an HCl numberdensity of ∼2 × 1011 cm−3.The diode-laser based HCl sensor is sufficiently sensitive to detect smallconcentration variations and HCl concentration correlates with poly-Si etch ratefor the conditions studied. These measurements demonstrate the feasibility of areal-time diode laser-based sensor for etch rate monitoring and the potential forprocess control.

Preliminary Experiments in the Development of a Laser-Based Imaging Sensor for AUV Navigation

The Cranfield University DMT5 torpedo-shaped underwater vehicle has been modified to accommodate a laser stripe illumination system. As well as providing enhanced viewing capabilities, this system will derive real-time navigational data during the mission and gather images to produce a post mission enhanced optical waterfall image of a surveyed area. This paper describes a preliminary set of constrained motion trials at the IFREMER wave basin in Brest, where thel system was towed through the 50 metre test tank at different altitudes and orientations whilst the true trajectory was measured.

Role of laser sensor systems in automation and flexible manufacturing

New industries and production plants require a flexible system, which is capable of picking up objects of various shapes, weights, and colors with arbitrary position and orientation. Such a system also needs recognition and guiding sub-systems. The recognition system includes target function for the recognition sub-system and relation between object characteristics and recognition target. The laser sensor system can be used for such object recognition. Wire-based telemetry and control systems can cause many problems in shop floors and factories, and so there has been a strong growth of interest in wireless guidance like vehicles equipped with laser guiding and navigation systems. For the continuous measurements of parameters such as temperature, etc. optical laser sensor technology seems to become more applicable at this stage. This article describes the operational principles and the use of the most advanced laser sensor systems for quantity measurements, guiding, navigation, pattern recognition, and vision systems for inspection purposes. A variety of laser-based sensors, which can be used as sensing devices in manufacturing, and production technology, are described in this study. Adaptive cruise control systems that can be used in automobile industry to monitor distance and speed are described in this report. As a typical example, the principal operation of a laser guided mobile robot using a laser navigation system is also described.