A novel signal processing technique using sinusoidal optical frequency modulation of an inexpensive continuous-wave laser diode source is proposed that allows highly linear interferometric phase measurements in a simple, self-referencing setup. Here, the use of a smooth window function is key to suppress unwanted signal components in the demodulation process. Signals from several interferometers with unequal optical path differences can be multiplexed, and, in contrast to prior work, the optical path differences are continuously variable, greatly increasing the practicality of the scheme. In this paper, the theory of the technique is presented, an experimental implementation using three multiplexed interferometers is demonstrated, and detailed investigations quantifying issues such as linearity and robustness against instrument drift are performed.

Using a novel range-resolved interferometric signal processing technique based on the sinusoidal optical frequency modulation of a cost-effective laser diode, a fiber sensing approach termed fiber segment interferometry (FSI) is described. In FSI, a chain of long-gauge length fiber optic strain sensors are separated by identical in-fiber partial reflectors. Targeted at dynamic strain analysis and ultrasound detection for structural health monitoring, this approach allows integrated strain measurements along fiber segments, removing the sensing gaps and sensitivity to inhomogeneities found with localized fiber sensors. In this paper, the multiplexing of six fiber segments, each of length 12.5 cm, is demonstrated. The sensor array can be interrogated at 98 kHz data rate, achieving dynamic strain noise levels ≤ 0.14 n • Hz −0 .5. The reflector fabrication is discussed, an analysis of linearity and noise performance is carried out and results from an exemplar experiment to determine the speed-of-sound of a stainless steel rod are shown.

This paper describes the development of a Mach-Zehnder interferometric filter based planar Doppler velocimetry (MZI-PDV) flow measurement technique. The technique uses an unbalanced Mach-Zehnder interferometer (MZI) to convert Doppler frequency shifts into intensity variations. The free spectral range of the interferometric filter can be selected by adjusting the optical path difference of the MZI. This allows the velocity measurement range and resolution to be varied. In contrast to molecular filter based PDV any laser source with single-frequency operation and a narrow linewidth can be used as the requirement for a suitable absorption line is no longer necessary. The processing methods used to extract the velocity information are described and discussed. The construction of a MZI-PDV system that incorporates a phase-locking system designed to stabilize the filter is described and example measurements made on the velocity field of a rotating disc and an axis-symmetric air jet are presented.

A planar Doppler velocimetry (PDV) system has been designed which is able to generate two beams from a single source separated in frequency by 690 MHz. This allows a common-path imaging head to be constructed, using a single imaging camera instead of the usual camera pair. Both illumination beams can be derived from a single laser and a set of acousto-optic modulators used to affect the frequency shifts. One illumination frequency lies on an absorption line of gaseous iodine, and the other in a region of zero absorption. The beams sequentially illuminate a plane within a seeded flow and Doppler-shifted scattered light passes through an iodine vapor cell onto the camera. The reference beam that lies in a zero absorption region is unaffected by passage through the cell, and provides a reference image. The signal beam, the frequency of which coincides with an absorption line, encodes the velocity information as a variation in transmission dependent upon the Doppler shift. Images of the flow under both illumination frequencies are formed on the same camera, ensuring registration of the reference and signal images. This removes a major problem of a two-camera imaging head, and cost efficiency is also improved by the simplification of the system. The dual illumination technique has been shown to operate successfully with a spinning disc as a test object and is currently achieving a velocity resolution of about +/−2 ms−1, limited by the quality of the light sheet generated from the multimode fiber. Automatic superposition of the signal and reference images is achieved, and polarization errors caused by the beam splitter in the conventional system are eliminated. Measurements have also been made on an axisymmetric air jet, seeded with a commercial smoke generator, which has maximum velocities of ∼100 ms−1. A comparison with data obtained simultaneously, using a conventional two camera PDV arrangement has been made and the difference between the measurements found to be within a few m/s.

Interferometric filter-based planar Doppler velocimetry is used in conjunction with imaging fibre bundles to make time-averaged three-component velocity measurements using a single imaging head. The Doppler frequency shifts of light scattered by particles entrained into the flow to be measured are transduced to intensity variations using a Mach–Zehnder interferometer. The free spectral range of the filter can be selected by adjusting the optical path difference of the interferometer. This allows the velocity measurement range, sensitivity and resolution to be varied. Three-component measurements are made possible by porting different views of the measurement plane to a single imaging head using the imaging fibre bundles. A comparison of three different image-processing techniques is presented and analysed with the aid of modelled images. Results are presented here for time-averaged measurements of a rotating disc with maximum velocities of ∼ ±34 m s−1 in the field of view with the computed measurement error in the orthogonal velocity components being (0.89, 0.68, 1.42) m s−1 for the measurement geometry used. Three-component velocity measurements were also made on a seeded air jet with a nozzle diameter of 20 mm and an exit velocity of ∼85 m s−1.