J.D.C. Jones

Coherence multiplexing of remote fibre optic fabry-perot sensing system

Abstract A monomode fibre optic coherence multiplexing sensor system incorporating passive miniature fibre optic probes is demonstrated using a coherence tuned local interferometer to sequentially interrogate the individual sensors. An example of its use as a high resolution relative point temperature measurement system is described with a range to resolution in excess of 10 4 .

Single-mode fibre-optic holography

A fibre-optic holographic system is described in which the functions of all the optical components conventionally used to produce holograms-such as beam splitters, mirrors and beam expanding spatial filters-are performed with single-mode optical fibres. Design considerations are discussed, and an electronic servo system which provides compensation for thermally induced phase shifts is described.

Optical polarisation state control schemes using fibre optics or Bragg cells

The ability to create an optical beam of a controlled or modulated state of polarisation is an important requirement in a range of optical instruments, such as polarimeters and ellipsometers. The authors describe a range of novel techniques by which this may be achieved. The schemes described are of an interferometric type, and are the macroscopic analogues of extrinsically induced circular birefringence. They are based on the use of fibre-optic or acousto-optic modulators. A pseudoheterodyne electronic signal processing scheme for the measurement of circular birefringence is described, and results of demonstration experiments for the measurement of optical activity and particle size by light scattering are presented.

All single-mode fibre optic holographic system with active fringe stabilisation

The recording of reflection holograms through single-mode optical fibres has been demonstrated. An all fibre Michelson interferometer formed by the cleaved ends of the object and reference fibres was utilised to provide phase compensation in the fibre arms via an electronic servo in conjunction with a piezoelectric phase modulator.

Optical-fibre flammable gas sensor

A flammable gas sensor is demonstrated, in which the sensing element is a 100 mm monomode fibre, coated with platinum metal. The rate of the exothermic reaction of the hydrocarbon gases with oxygen is increased in the presence of a platinum catalyst. The resultant heat of the chemical reaction is transduced to a phase retardance in a fibre-guided light beam, and which is recovered interferometrically. The experiments have been performed in an oxygen (or air) atmosphere at laboratory temperature.

Interferometric fibre-optic hydrogen sensor

A hydrogen sensor has been developed in which the sensing element is a palladium wire, mechanically attached to a monomode optical fibre. In the presence of hydrogen, the palladium is converted to palladium hydride, with consequent changes in the physical dimensions of the wire. In turn, these dimensional changes produce a longitudinal strain in the optical fibre. The strain is transduced to a phase retardance in a light beam guided by the fibre, and which is recovered interferometrically. The sensor has been demonstrated in a nitrogen atmosphere at laboratory temperature. The phase retardance is proportional to the square root of the hydrogen partial pressure, and has been demonstrated in the partial pressure range 0-100 Pa, with an accuracy of +or-2 Pa.

A high-power fiber optic laser Doppler velocimeter

A fibre optic Doppler difference type laser velocimeter for use with high optical powers has been devised and constructed. The system provides a flexible monomode optical fibre link between the argon ion laser used and the launching optical system, which illuminates the measurement volume. A multimode optical fibre links the receiving optical system and the photodetector. Optical powers of up to 500 mW were delivered to the measurement volume.

Two-dimensional fibre-optic laser velocimetry using polarisation state control

A two-dimensional fibre-based laser Doppler velocimeter has been constructed where the orthogonal velocity distributions can be recovered either sequentially or simultaneously using a novel fibre-optic polarisation state control scheme. The scheme is described and the benefits of the two approaches are discussed. The instrument is demonstrated in a simple fluid flow experiment.

All-fiber-optic polarization state azimuth control: application to Faraday rotation

An all fiber-optic system for the control and modulation of the azimuth of a linearly polarized beam is described. The experimental configuration is basically that of a Mach-Zehnder interferometer but with the birefringence of the two fiber arms controlled to produce orthogonal circular states of polarization which recombine in the final directional coupler. Azimuth control is then achieved by relative phase modulation using a piezo-electric element. The system has required the use of monomode fiber directional couplers of accurately characterized polarization properties, and the development of fiber linear retarders. The application of the system in the measurement of electric currents via Faraday rotation is described, although extensions to other types of optical instrument are equally feasible. >

Alternating-current measurement and noninvasive data ring utilizing the faraday effect in a closed-loop fiber magnetometer

A novel fiber-optic polarimetric configuration for a magnetometer utilizing the Faraday effect is described. The system is operated in a closed-loop mode using magneto-optic feedback through a simple current-driven solenoid to maintain the azimuth of the optical output from the polarimeter constant. In this mode of operation scale-factor errors due to changes in the Verdet constant of the fiber are effectively eliminated. The minimum detectable current was less than 1 mA, with linear operation up to several amperes. A multiplexed system was also demonstrated, with potential application as a multiaccess noninvasive data ring.