W.J.O. Boyle

Self-mixing interference inside a single-mode diode laser for optical sensing applications

This paper presents a theoretical analysis and a comparison with experimental results on self-mixing interference inside a single-longitudinal-mode diode laser. A theoretical model, based on the steady-state equations of the lasing condition in a Fabry-Perot type laser cavity, is described, and through it a satisfactory analysis of self-mixing interference for optical sensing applications is given. In this work, the self-mixing interference produced by an external optical feedback is found to be due to the variations in the threshold gain and in the spectral distribution of the laser output. The gain variation results in an optical intensity modulation, and the spectral variation determines both the modulation waveform and the coherence properties of the interference. The theoretical analysis of the self-mixing interference is seen to yield a simulation of the laser power modulation which is in good agreement with the experiment results reported. >

Self-mixing interference in a diode laser: experimental observations and theoretical analysis

The experimental results of an investigation of self-mixing effects or backscatter modulation in diode lasers coupled with a simple theoretical analysis are presented. The laser is used to send light, either in free space or through an optical fiber, to a movable target from which the optical backscatter is detected and fed back into the laser. In the experiment three significant conclusions are drawn: (1) self-mixing interference is not dependent on the coherence length of the laser, (2) the interference is not dependent on the use of a single-mode or multimode laser as the source, and (3) the interference is independent of the type of fiber employed, i.e., whether it is single mode or multimode. A comparison of this kind of interference with that in a conventional interferometer shows that self-mixing interference has the same phase sensitivity as that of the conventional arrangement, the modulation depth of the interference is comparable with that of a conventional interferometer, and the direction of the phase movement can be obtained from the interference signal. The above factors have implications for the optical sensing of a wide range of physical parameters. Several applications of the method are discussed that highlight the significant advantages of simplicity, compactness, and robustness as well as the self-aligning and self-detecting abilities of fiber-based self-mixing interferometry when compared with the use of conventional interference methods.

A field deployable, multiplexed Bragg grating sensor system used in an extensive highway bridge monitoring evaluation tests

A multiplexed optical fiber Bragg grating sensor system with a measurement bandwidth of up to 200 Hz enabling dynamic loading events, e.g., road traffic, to be observed has been designed, installed, and tested over an 18-month period on a 346-m road bridge in Norway, for design verification and structural integrity monitoring purposes. A network of 32 fiber Bragg sensors was surface bonded along with a corresponding set of resistive strain gauges for comparative tests to be made. The wavelength data were calibrated against two thermally stabilized (/spl sim/0.15 pm) reference gratings, which rejected common mode noise and provided absolute wavelength scaling. These data provides independent strain and temperature information. Long-term test results showed good linearity and repeatability of <10 /spl mu//spl epsiv/ over the test period with a precision of /spl plusmn/5 /spl mu//spl epsiv/ and a resolution of /spl plusmn/1 /spl mu//spl epsiv/. The readings from the FBG sensors were comparable to those from the foil gauge sensors to within /spl plusmn/4 /spl mu//spl epsiv/.

A novel adaptation of the Michelson interferometer for the measurement of vibration

An adaptation of the Michelson interferometer for vibration measurement is described. Two outputs are provided, the phases of which can be independently controlled. The device provides a simple low cost solution to the problem of vibration measurement where the direction of the surface velocity is required. The relative phase of the two outputs is controlled without using any moving parts or electronic elements. The ability of the system to track vibration is demonstrated. >

Liquid core waveguide with fiber optic coupling for remote pollution monitoring in the deep ultraviolet

A new combination of silica fibres, highly transparent in the ultraviolet (UV) and showing long term stability, combined with a teflon-coated liquid-core waveguide (LCW) is presented for remote sensing in the deep UV, for monitoring one of the major pollutants, nitrates, in water, but with potentially wider applicability. The arrangement exhibits low spectral loss in the range between wavelengths of 200 nm and 400 nm and can be used for analytical investigations to determine small concentrations of such impurities in water. The operation of the optical system to achieve guidance of UV light below a wavelength of 250 nm for fibre optic sensors is discussed. With an optical pathlength of 203 mm, nitrate concentrations as low as 22 μg/l could be detected.

Fiber-optic Doppler velocimeter that incorporates active optical feedback from a diode laser.

A fiber-optic Doppler velocimeter that incorporates the effect of self-mixing in a diode laser is described. A theoretical model, based on self-mixing interference theory, is presented, and a simple experimental arrangement is constructed. The results of the experimental research are found to be in good agreement with the theoretical analysis. A Doppler velocity of as much as 3 m/s was measured directly, and a good linear relationship between the Doppler velocity and the Doppler-shifted frequency was obtained, which can be used to determine the speed of a moving object.

Smart-sensor approach for a fibre-optic-based residual chlorine monitor

Developments of an optical-fibre-based sensor system for monitoring residual chlorine in water are discussed. The system, based on differential absorption spectroscopy, utilizes a novel miniature monolithic diode array spectrometer operating in the ultraviolet and visible (UV-Vis) region of the spectrum in combination with an optical flow-through cell of length 430 mm and a computer-controlled deuterium lamp source. The sensor, having a detection limit of 0.2 mg l−1 of free chlorine in water, relies on the fact that the OCl− ion, in which form dissolved chlorine exists at high pH (>9), strongly absorbs light at 290 nm. This paper describes the systematic approach that is used in the modelling and design of this sensor system. It also outlines the construction of the device and gives an evaluation of the performance in the laboratory environment.

Active optical feedback in a dual-diode laser configuration applied to displacement measurements with a wide dynamic range

The principles, experimental apparatus, and advantages of the use of an optical feedback technique for extended displacement measurements based on the use of a dual-diode laser configuration are described. This device is capable of creating a synthetic wavelength from the two lasers simultaneously through the frequency selectivity of the individual lasers, which respond only to their own wavelength. Theoretical analysis and experimental evidence are presented to show the feasibility of the measurement method and the simplicity of its operation.

Strain Measurement Using Embedded Fiber Bragg Grating Sensors Inside an Anchored Carbon Fiber Polymer Reinforcement Prestressing Rod for Structural Monitoring

Results are reported from a study carried out using a series of Bragg grating-based optical fiber sensors written into a very short length (60 mm) optical fiber network and integrated into carbon fiber polymer reinforcement (CFPR) rod. Such rods are used as reinforcements in concrete structures and in tests were subjected to strain through a series of cycles of pulling tests, with applied forces of up to 30 kN. The results show that effective strain measurements can be obtained from the different sensors mounted along the rod. Additionally, the tests show that close agreement with the results obtained from the calibrated force applied by the pulling machine and from a conventional resistive strain gauge mounted on the rod itself is obtained. Calculations from strain to shear stress show a relatively uniform stress distribution along the bar anchor used. The results give confidence to results from various methods of in situ monitoring of strains on such CFRP rods when used in different engineering structures.

UV-stabilized silica-based fibre for applications around 200 nm wavelength

New optical fibres having a nearly constant UV transmission at around a wavelength of 200 nm have been fabricated for the first time. Due to their improved UV properties when compared to those commercially available, these new fibres are tested as a potential transmission medium for the broadband spectrum of a deuterium lamp. In addition, a complete sensor system, including the lamp itself and the associated optical system, will be described in light of its intended application to spectroscopic chemical measurements. Details of the spectral intensities at the output of such a fibre, of 1 m length appropriate for such systems, are given in this work.