The current state of the art of optical fiber sensors is reviewed. The principles of operation are detailed and the various types of fiber sensors are outlined. Achievable performance and limitations are discussed and a description of technology used to fabricate the sensor is presented. The characteristics of acoustic, magnetic, gyro, laser diode, and other sensors are described. Trends in the development of this sensor technology and expected application areas are briefly outlined.

Optical fiber sensor technology

We have measured the effects of feedback on the spectra of 1.5-μm DFB lasers from feedback power ratios as low as -80 dB up to -8 dB. Five distinct regimes of effects are observed with well defined transitions between them. The dependence of these effects on the distance to the reflection is also investigated.

Regimes of feedback effects in 1.5-&#181;m distributed feedback lasers

A distributed sensor system for detecting and locating intruders based on the phase-sensitive optical-time-domain reflectometer (/spl phi/-OTDR) is described. The sensing element is a cabled single-mode telecommunications fiber buried along the monitored perimeter. Light pulses from a continuous-wave Er:fiber Fabry-Pe/spl acute/rot laser with a narrow (/spl ap/3 kHz) instantaneous linewidth and low (few kilohertz per second) frequency drift are injected into one end of the fiber, and the backscattered light is monitored with a photodetector. The effect of phase changes resulting from the pressure of the intruder on the ground immediately above the buried fiber are sensed by subtracting a /spl phi/-OTDR trace from an earlier stored trace. In laboratory tests with fiber on reels, the effects of localized phase perturbations induced by a piezoelectric fiber stretcher on /spl phi/-OTDR traces were observed. In field tests, people walking on the ground above a buried fiber cable induced phase shifts of several-/spl pi/ radians.

Distributed fiber-optic intrusion sensor system

The objective of this paper is to present an overview of topics related to one of the fundamental parameters for semiconductor lasers-the linewidth broadening factor α that describes the coupling between carrier-concentration-induced variations of real and imaginary parts of susceptibility. After introducing the definition of α and discussing its dependence on carrier concentration, photon energy, and temperature, we give a short historical summary on how the concept of α evolved over the past two decades. This is followed by a discussion of α dependence on device structure in gain-guided and subdimensional lasers (quantum wells and quantum wires). The bulk of the paper is devoted to a detailed review of laser properties influenced by α and of associated methods of estimating the value of α. Results of measurements reported to date are collected and the most reliable methods are indicated.

Linewidth broadening factor in semiconductor lasers--An overview

The recently developed digital coherent receiver enables us to employ a variety of spectrally efficient modulation formats such as $M$ -ary phase-shift keying and quadrature-amplitude modulation. Moreover, in the digital domain, we can equalize all linear transmission impairments such as group-velocity dispersion and polarization-mode dispersion of transmission fibers, because coherent detection preserves the phase information of the optical signal. This paper reviews the history of research and development related to coherent optical communications and describes the principle of coherent detection, including its quantum-noise characteristics. In addition, it discusses the role of digital signal processing in mitigating linear transmission impairments, estimating the carrier phase, and tracking the state of polarization of the signal in coherent receivers.

Fundamentals of Coherent Optical Fiber Communications

The spectral spread of the best stabilised semiconductor lasers has been reduced to several megahertz. Conventional spectroscopy techniques cannot offer a spectral resolution fine enough for measuring such a sharp spectrum. The letter proposes a novel method by which 50 kHz resolution can be obtained. The principle, experimental set-up and results are described.

Novel method for high resolution measurement of laser output spectrum

In heterodyne or homodyne optical fiber communications, the fluctuation of the polarization state in the fiber may deteriorate the receiver sensitivity. This paper reviews the principles and features of six polarization-state control schemes so far proposed, including two new attempts, one using Faraday rotators and the other using rotatable fiber cranks (RFC's), both of which have been proposed very recently. Experiments simulating actual optical heterodyne receivers are also described.

Polarization-state control schemes for heterodyne or homodyne optical fiber communications

In coherent optical communication schemes, the FM noises of the transmitter and/or the local oscillator seriously deteriorate the receiver sensitivity. A theory is developed describing the effect of the FM noise on the bit-error rate (BER) performance of various heterodyne and homodyne schemes. To verify the theory, the BER of a phase-shift keying (PSK) heterodyne system is measured. The feasibility of the PSK scheme using semiconductor lasers is also discussed on the basis of analysis and experimentation.

Degradation of bit-error rate in coherent optical communications due to spectral spread of the transmitter and the local oscillator

The primary purpose of this paper is to propose a comprehensive optimum-design theory of lenticular-sheet three-dimensional pictures. The proposed theory features the use of depth resolution of a 3-D image as the measure of the 3-D picture quality. The optimum parameters in the picture taking process, the optimum lens pitch and the depth-resolution limitation, are discussed. The obtained results are also applicable to a specific type of integral photography and to the projection-type 3-D display including projection-type holography. It is found that the optimum pitch of the lens sheet or the lens-type direction selective screen ranges between 0.1 mm and 0.5 mm in most cases, whereas it ranges between 0.2 mm and 1 mm for the triple-mirror screen.

Optimum design and depth resolution of lens-sheet and projection-type three-dimensional displays.

The receiver sensitivity in optical communication is improved by heterodyne and/or coherent communication schemes, as compared with an ordinary intensity-modulation/direct-detection system. This paper presents the computation of the minimum signal level for achieving a prescribed bit-error rate in various heterodyne and coherent-type optical communication systems, as functions of the information-transmission rate and mixerdiode parameters. It is shown that in long-wavelength (1.3 um ~ 1.6 urn) region, the improvement in the signal level by the heterodyne/coherent schemes is relatively high (10 dB-20 dB). improvement of the receiver sensitivity especially at the longer wavelengths, because these schemes bring forth a sensitivity close to the shot-noise limit, regardless the mixer-noise performance [1]. The improvement of the sensitivity by these schemes was first computed by Yamamoto [2], who gave the BER vs. signal-power relations for various heterodyne/coherent schemes.

T. Okoshi

Papers

Novel method for high resolution measurement of laser output spectrum

Polarization-state control schemes for heterodyne or homodyne optical fiber communications

Degradation of bit-error rate in coherent optical communications due to spectral spread of the transmitter and the local oscillator

Optimum design and depth resolution of lens-sheet and projection-type three-dimensional displays.

Computation of Bit-Error Rate of Various Heterodyne and Coherent-Type Optical Communication Schemes