K. Weir

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. >

The optimized wavelength combinations of two broadband sources for white light interferometry

Two broadband sources can be used in combination in white light interferometry to simplify the problem of central fringe identification. In this paper, a method for selecting the optimum wavelength combinations of such sources is presented. The results are obtained from a theoretical analysis and computer simulations, verified by experiment. It has been shown that the minimum signal-to-noise ratio required by the system to identify the central fringe position can be greatly reduced by using the optimum wavelength combination. >

An all fibre white light interferometric strain measurement system

Abstract We report on an all fibre implementation of a white light interferometric fibre strain sensor. Utilising white light tandem interferometry, two all fibre Michelson interferometers are used, one to form the sensor element and the other the optical path scanning interferometer. Results are presented which demonstrate the performance of the system and the measurement of applied strain up to 10 000 μe.

Three-wavelength combination source for white-light interferometry

A three-wavelength combination source in white-light interferometry (WLI) is used to simplify the central fringe identification. It is found that, by using this technique, the minimum signal-to-noise ratio required by the system to identify the central fringe position can be reduced when compared with the use of two-wavelength combination sources. This technique is simple and effective and has potential uses in coherence-multiplexed interferometric sensor systems.<<ETX>>

Optimized multiwavelength combination sources for interferometric use

We present the use of multiwavelength combination sources in a direct method for improved central fringe identification in a white-light interferometric system. The optimum wavelength combinations of such sources can be obtained by the use of the results of a simple analysis. We find that this multiwavelength technique can greatly reduce the minimum signal-to-noise ratio required by the systemwhen used to identify the central fringe, and thus it offers an increased signal resolution. As a result, it is suitable for high-precision measurement purposes as well as for applications in coherence multiplexed interferometric sensor systems.

Characteristics of synthesized light sources for white-light interferometric systems

Results of a study on the use of synthesized light sources in white-light interferometry are presented. The optimum wavelength combination with a pair of multimode laser diodes used to generate a synthetic wavelength source was simulated theoretically and verified experimentally. Using the best wavelength combination, we found that the lowest signal-to-noise ratio required by the system was 18.1 dB in theory and 22.1 dB from experiment. The relationships between the wavelengths of the two diodes used, their coherence lengths, and the signal-to-noise ratio required by the system are shown and discussed.

Relation between the coherence length and modal noise in a graded-index multimode fiber for white-light interferometric systems.

The results of a comparative experimental study on the effect of modal noise induced by the modal coupling effect in a graded-index multimode fiber that is illuminated by a light source with a tunable coherence length are reported. It has been shown that, in the coherence-length region of 30-80 microm, the value of the signal-to-noise ratio in an interferometric system could be reduced by the perturbation-induced modal noise, and, as the coherence length increases, the more the signal-to-noise ratio decreases. However, when the value of the coherence length is in the region of 80 microm and upward, the value of the corresponding signal-to-noise ratio reduction is seen to vary in only a very small range (<2 dBV). Under this condition the modal noise induced in the fiber cannot be suppressed by the incoherent nature of the light source, thus showing the practical limitation of the use of multimode fibers in an interferometric system with a low-coherence light source.

An optical scanning technique in a white light interferometric system

A white light interferometric system in sensor applications is often configured with a sensing interferometer to respond to the measurand and a receiving interferometer to recover the signal by means of a scanning system. Mechanical and electronic scans are the two techniques commonly used at present. In this paper, an optical scanning technique is presented as an alternative for sensor use with comparable performance. This technique offers the possibility of eliminating the receiving interferometer and the potential of a simple, stable and compact system.<<ETX>>

Measurement of up- and down-lead fiber sensitivity caused by the lead in a multimode fiber in an interferometric system.

The results of a comparative experimental study on the effect of the modal noise induced by lead-in fibers in an extrinsic interferometric system, illuminated by high- and low-coherence light sources, respectively, are reported. When the up-lead fiber was subject to a perturbation, the sensitivity of the system was reduced by 20.9 dB through the use of a high-coherence source, and by 1.8 dB through the use of a low-coherence source. When the down-lead fiber was perturbed, the sensitivity dropped by 30.3 dB and 4.9 dB for high- and low-coherence sources, respectively. The results from the experimental analysis supported qualitatively by simple theory show that the use of a low-coherence light source can greatly suppress the modal noise induced in both the up- and down-lead fibers, if the coherence length of the light source used is less than the optical path difference between two adjacent fiber modes. This shows the practicality of the use of niultimode fibers in an interferometric system with a suitable lig t source.

A novel optical heterodyne vibration sensor scheme, preserving directional information and using a short coherence length source

Abstract A novel optical heterodyne interferometer vibration sensor which utilises optical fibres is proposed, in which a simplified optical configuration is employed, and a low coherence “CD”-type multimode laser diode source is used. Using this arrangement, directional information is preserved and the results thus obtained are contrasted with those obtained using a single mode gas laser source.