Interferometric current sensors using optical fibres

Two types of fibre-optic current sensor have been developed and successfully tested in the laboratory: magnetostrictive jacketing materials and a heating effect, to produce a phase shift in the optical fibre. A ruggedised all-fibre interferometer powered by a diode laser was used to detect the phase changes induced by the electrical current.     

Theory of internal upconversion and frequency doubling in optical parametric oscillators

A uniform plane-wave theory of the type previously employed for the optical parametric oscillator (OPO) is extended to include internal upconversion and frequency doubling. All the experimental configurations for internal upconversion and doubling in the three basic OPO types are treated. Generated intensities are calculated for representative system parameters, and both design parameters and quantum upconversion and doubling efficiencies are obtained for optimum operation in each configuration. The position of the upconverter in the OPO cavity relative to the oscillator crystal and input-pump direction is shown to affect efficiency and output characteristics drastically. The detrimental effects of internal up-conversion on threshold are shown to be an important design consideration. Applicability of the results to pulsed OPO internal upconversion and frequency doubling is determined, and design criteria for selecting the best mode of operation are discussed.     

Fibre optic accelerometer

A sensitivity of less than 1 ?g for a fibre optic accelerometer is demonstrated. The accelerometer measures the strain in the fibre when a mass is accelerated. An all-fibre Mach-Zehnder interferometer was used to detect the change in optical path length produced by the strain in the fibre.     

Multiuser ML sequence estimator for convolutionally codedasynchronous DS-CDMA systems

The optimal multiuser sequence estimator is formulated for an asynchronous direct-sequence code-division multiple-access (DS-CDMA) system where each user employs convolutional coding to improve its performance on a nondispersive additive white Gaussian noise (AWGN) channel. It is shown that the decoder may be implemented efficiently using a Viterbi algorithm which operates on a time-varying trellis with a number of states which is exponential in the product of the number of users in the system and the constraint length of the codes used (for the rate 1/2 code case). The asymptotic efficiency of this receiver relative to an uncoded coherent binary phase shift keying (BPSK) receiver (termed asymptotic multiuser coding gain, or AMCG) is then upper and lower bounded. The AMCG parameter unifies the asymptotic coding gain parameter and the asymptotic multiuser efficiency parameter which are traditional figure of merit parameters for single-user coded systems and multiuser uncoded systems, respectively. Finally, some simulations are presented to illustrate the performance of the maximum likelihood sequence estimator (MLSE) at moderate and low bit error rates     

Noncoherent demodulation techniques for trellis coded M-DPSKsignals

Considers noncoherent decoding of trellis coded M-DPSK transmission, and attempts to unify and contrast the approaches to this problem which have been previously reported. In the second part of the paper, the maximum likelihood block decoders for the AWGN channel are outlined, first assuming that the decoder has no knowledge of the state of the encoder at the beginning of the block, and then assuming that it knows the state. Because these exhaustive decoders suffer from an exponential growth in complexity with block length, two simplified decoders are proposed that approach the performance of the exhaustive decoders with a much more manageable complexity. Through a union bound analysis and computer simulations, it is shown that it is possible to achieve large gains over uncoded systems with a very reasonable decoder complexity     

Single-mode fibre Fourier transform spectrometer

A single-mode all-fibre Fourier transform spectrometer is described. A resolution of ?1 cm?1 (?0.7 ?) is demonstrated at 820 nm. The device can be configured as a remote gas sensor.     

Single-mode-fibre point and extended temperature sensors

The response of single-mode fibres to time-varying thermal perturbations has been characterised by using a Mach-Zehnder interferometer. It is shown that single-mode fibres offer the possibility of high-speed, high-sensitivity remote temperature sensing with a minimum disturbance of the thermal environment and the capability to operate in an electro-magnetically active environment. In addition, fibre temperature sensors can be configured to measure not only the specific temperature at any given point, but also offer the unique feature of extended temperature measurements over the length of a fibre segment in which the device optically integrates any thermally induced fluctuations.