A technique is described in which polarisation fading of the output of an interferometer is eliminated. The optical output of the interferometer is projected onto three (or more) polarisation states, detected and combined. The maximum variation in signal and in signal/noise ratio is 6 dB.

Technique for elimination of polarisation fading in fibre interferometers

A simple method has been developed for control and prevention of polarization fade on a single output lead from a remotely located fiber-optic interferometer. The method has been demonstrated using a fiber-optic Michelson interferometer. In contrast to a previously suggested method, only a single detector is required. The technique has application for passive, remotely located sensors. Extension to balanced homodyne detection schemes and coherent communications is discussed.

Remote polarization control for fiber-optic interferometers.

Fused biconical taper fibre couplers have been fabricated from single-polarisation single-mode fibres. A CVD technique is applied to permit maintenance of parallel alignment of the fibres in the coupler fabrication process. The couplers can hold linear polarisation along the principal axis of the fibre branches, showing an extinction ratio of about −15dB.

Fabrication of single-polarisation single-mode-fibre couplers

The key results obtained from experimental systems studies over monomode fiber and cable links are given for both direct detection and coherent optical systems. Direct detection techniques have been studied at 1300 nm using conventional lasers and at 1500 nm with narrow line transmitters of the injection-locked and external cavity types. In the case of coherent optical systems, developments of the same transmitters have been used in conjunction with amplitude and phase shift key (PSK) modulation techniques to obtain improved performance. Increases in receiver sensitivity of up to 14 and 17 dB have been obtained using, respectively, heterodyne and homodyne detection techniques. These results approach theoretical predictions and offer the possibility of much greater transmission capacity and extended regenerator spacings for monomode systems.

Progress in monomode transmission techniques in the United Kingdom

This invention provides apparatus and methods for converting a lightwave of an arbitrary polarization propagating in a single mode optical fiber into a wave having a single linear polarization state. A polarization couples light of an undesired polarization out of the fiber, and a photodetector produces an error signal responsive to light coupled from the fiber. Control circuitry processes the error signal to produce control signals which are input to a polarization of light input to the polarizer to null the error signal. With the error signal maintained at a minimum value the light input to the polarizer and, hence, the light output therefrom is a wave having only the desired linear polarization.

Fiber optic polarizer with error signal feedback

A simple active control system is described that maintains the output of a single-mode fiber at prescribed polarization and intensity, thus counteracting erratic birefringeance effects in the fiber that are due to temperature drift and mechanical disturbances. Whereas the system of Ulrich [Appl. Phys. Lett. 35, 840 (1979)] controls two polarization parameters by means of two interdependent control loops, ours controls only one parameter and requires only a single control loop. The resulting considerable reduction of optical and electronic system complexity is realized at the expense of about 30-50% of input light intensity.

Single-loop polarization stabilization for single-mode fiber.

Pressure induced changes in optical pathlength are being utilized to realize a hydro -phone consisting of glass fibers in a Mach - Zehnder interferometric arrangement. In our expe-rimental setup we use single -mode fiber optical power dividers and a simple control systemthat maintains the polarization of both light signals so that interference with constantcontrast is obtained after superposition. We use a modified phase- tracking detection systemto maintain maximum interferometer sensitivity and show a design for a sensor -body withhigh acoustical sensitivity.IntroductionOptical fiber sensors1 are being worked on in research and development since the pastfew years now because they offer a number of advantages over existing techniques. The mainfeatures are an increased sensitivity, light weight, more freedom in geometric shape design,insensitivity against electromagnetic field disturbances including a perfect electric insu-lation of the point of measurement and applications in high temperature or aggressive envi-ronments. Besides there is the possibility to use optical signal and data processing meth-

Feasibility Of Fiber Optical Hydrophone

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Single-loop polarization stabilization for single-mode fiber.

Feasibility Of Fiber Optical Hydrophone