Abstract: An optical fiber cable in which at least those metallic members of the optical fiber cable which are of high chemical reactivity and electrochemical reactivity are each precoated with a film for preventing them from making direct contact with water when it diffuses into the cable. The film may be a nonmetallic film or a metallic film of low chemical reactivity and electrochemical reactivity.

Abstract: An Er-doped optical fiber laser device adapted for pumping by light from a pumping light source in an erbium (Er)-doped optical fiber which enters a single mode at the wavelength of amplified light. In accordance with the present invention, the Er-doped optical fiber is further doped with at least one of holmium (Ho), thulium (Tm) and dysprosium (Dy). The pumping light source emits light in a 1.1 to 1.4 .mu.m band. The output light from the pumping light source and signal light incident to the Er-doped optical fiber are combined by a combining optical fiber coupler.

Abstract: A multi-point optical amplification repeating system is disclosed in which an output of a unidirectional amplifier inserted in a unidirectional repeating optical transmission system is branched and coupled to an input of a unidirectional amplifier inserted in another unidirectional repeating optical transmission system, so that bidirectional optical communication can be performed between many points connected to a plurality of first optical transmission lines. If a bidirectional amplifier is employed in a bidirectional repeating optical transmission system, the above bidirectional optical communication can be also realized by fed back the first port of the bidirectional amplifier to the second port thereof through an optical isolator.

Abstract: An optical amplification method or system for an optical communication system which transmits signal light from the transmitting side to the receiving side via an optical transmission medium formed by an optical fiber or space. In accordance with the present invention, the signal light entered a certain state of polarization in at least one part of the transmitting side, the receiving side and the optical transmission medium is separated into two orthogonal polarized waves by a polarizing beam coupler/splitter. The two polarized waves are amplified by optical amplifiers disposed in correspondence to their state of polarization, respectively. The amplified orthogonal polarized waves are combined together by another polarizing beam coupler/splitter.

Abstract: A fault monitoring system for optical fiber communication system having a bidirectional optical amplifier between first and second optical fiber transmission lines. An optical monitoring signal of a first frequency different from frequencies of communication signal light of the optical fiber communication system is applied to the first input/output port of the bidirectional optical amplifier through the first optical fiber transmission line and then branched through a first optical filter passing therethrough of the first frequency to apply the branched optical monitoring signal of the first frequency to the second input/output port of the bidirectional amplifier.

Abstract: A polarization diversity light receiving system using base combining is disclosed, which is arranged so that the control signal for the local oscillator laser is the sum of squared values of received signals.

Abstract: A high power pump light source for optical direct amplifier using the stimulated Raman effect is obtained by using a plurality of semiconductor lasers each of which provides low output power. More than two beams of the same wavelength generated by semiconductor lasers are combined (polarization multiplex) by using a polarization prism block, which provides sufficient high power pump light for the stimulated Raman effect amplifier. The output of said polarization prism block is wavelength division multiplexed with signal light to be amplified by using an interference filter block, the output of which is applied to a transmission optical fiber in which amplification of said signal light is effected.

Abstract: A system for measuring laser spectrum is disclosed in which the laser light to be measured is branched into one branched light and the other branched light so that the laser spectrum of the light is measured from a beat output resulting from mixing a delayed light obtained by delaying the one branched light for a certain period of time with the other branched light undelayed. In accordance with the present invention, the delay time of the delayed light undergoes such variations as to assume a value equal to an integral multiple of a fixed delay time by the use of an optical directional coupler.

Abstract: An optical coupler is formed of at least one polarization maintaining single mode fiber, a polarizing beam splitter connected to the polarization maintaining single mode fiber, and a main optical fiber which is connected with the polarizing beam splitter at one end thereof. This optical coupler is useful as a changeover switch for a light source.

Abstract: Elongation distortion at any location of an optical fiber cable is measured by using the principle of the stimulated Raman effect in which a stokes beam with a little frequency difference from that of a pumping beam is generated when a pumping beam and a seed beam with the same frequency as that of the stokes beam meet in a Raman material like an optical fiber cable. Due to the fact that the group refractive index of an optical fiber cable for a pumping beam is different from that for a stokes beam, elongation of a cable is measured by observing time difference of the arrival of pumping beam and stokes beam. The location that the pumping beam meets with the stokes beam can be adjusted merely by shifting the pumping pulse, then, elongation distortion at any location can be measured. According to the invention, the pumping pulse is applied to the cable to be tested from both the ends of the cable with one of the pumping pulse delayed according to the desired location for meeting of two pulses.

Abstract: An optical de-polarizer is implemented by using an optical fiber, and/or a beam splitter coupled with output of said optical fiber. An optical fiber is longer than a predetermined length so that equation .vertline.t.sub.1 -t.sub.2 .vertline.>t.sub.c is satified where t.sub.1 is polarization mode dispersion in a first optical fiber, t.sub.2 is a polarization mode dispersion in a second optical fiber which is coupled with output of the first optical fiber, and t.sub.c is coherence time of a beam defined by an optical source. According to the present invention, one of the conditions (1) an angle between polarization axis of the first optical fiber and polarization axis of the second optical fiber is 45.degree., and (2) an angle between polarization axis of the optical fiber and polarization axis of the beam splitter is 45.degree., is satisfied.

Abstract: A light dispersion measuring apparatus, provided with a light source for generating reference light pulses of short duration and of a constant frequency and also variable-frequency light pulses in synchronization with the reference light pulses but at a frequency different therefrom. A variable optical delay line delays the reference light pulses. An optical transmission medium, to be detected and to which the variable-frequency light pulses are applied is provided. An optical non-linear effect element receiver the output light pulses from the variable optical delay line and the optical transmission medium and is disposed so that when the two light pulses coincide emitted from the light source with each other, the sum component of the frequencies of the two types of light pulses produces a maximum output. A photo detector detects the sum component.

Abstract: An optical repeater system for optical communication that comprises a light source for providing a light of frequency .nu..sub.0, and a beam splitter for synthesizing said light from said light source and optical communication signal pulses including Stokes light corresponding to said frequency .nu..sub.0 introduced by Raman effect in the optical fibers, the optical repeater system being characterized in that the signal pulses are amplified with induced Raman effect in the optical fiber caused by the light and the dispersion characteristic of the optical fiber.