Abstract: A method for gain equalization. In this method, an optical transmission line including an optical amplifier for giving a gain to WDM (wavelength division multiplexed) signal light is first provided. Gain equalization is next performed on the optical transmission line so as to obtain a gain changing substantially monotonously with respect to wavelength. Gain equalization is further performed on the optical transmission line so as to obtain a gain substantially fixed with respect to wavelength. A wavelength characteristic of gain changing monotonously can be easily gain-equalized by an active gain equalizer having a simple configuration. Accordingly, even when the optical amplifier has a nonmonotonous wavelength characteristic of gain, a gain tilt in the optical amplifier can be easily flattened.

Abstract: A light source apparatus suitable for use for evaluation of an optical device and for production of pump light for an optical amplifier. The apparatus includes a light emitting element having a gain band and a band reflection filter optically connected to the light emitting element. The light emitting element outputs a light beam having a spectrum which is determined by the gain band. The filter produces, from the light beam outputted from the light emitting element, a transmission beam and a reflection beam which returns to the light emitting element. The filter has a reflection band included in the gain band of the light emitting element and narrower than the gain band, and the transmission beam of the filter has a maximum power higher than the maximum power of the light beam to be outputted from the light emitting element. With the construction, pump light having a maximum power higher than the maximum power of the light to be outputted from the light emitting element can be obtained.

Abstract: A wavelength characteristic control device capable of variably controlling a wavelength characteristic in a satisfactory manner. A polarized light wavelength characteristic changing element has a wavelength characteristic such that the transmittances or reflectances of P- and S-polarized rays vary differently with respect to wavelength. Polarization variable control means subjects the plane of polarization of the polarized light incident on the polarized light wavelength characteristic changing element to rotatory control to change the ratio of the P-polarized ray to the S-polarized ray, thereby variably controlling the wavelength characteristic.

Abstract: A wavelength characteristic control device capable of variably controlling a wavelength characteristic in a satisfactory manner. A polarized light wavelength characteristic changing element has a wavelength characteristic such that the transmittances or reflectances of P- and S-polarized rays vary differently with respect to wavelength. Polarization variable control means subjects the plane of polarization of the polarized light incident on the polarized light wavelength characteristic changing element to rotatory control to change the ratio of the P-polarized ray to the S-polarized ray, thereby variably controlling the wavelength characteristic.

Abstract: Disclosed herein is a method for gain equalization. In this method, an optical transmission line including an optical amplifier for giving a gain to WDM (wavelength division multiplexed) signal light is first provided. Gain equalization is next performed on the optical transmission line so as to obtain a gain changing substantially monotonously with respect to wavelength. Gain equalization is further performed on the optical transmission line so as to obtain a gain substantially fixed with respect to wavelength. A wavelength characteristic of gain changing monotonously can be easily gain-equalized by an active gain equalizer having a simple configuration. Accordingly, even when the optical amplifier has a nonmonotonous wavelength characteristic of gain, a gain tilt in the optical amplifier can be easily flattened.

Abstract: Disclosed herein is an optical amplifier reduced in gain deviation. The optical amplifier includes a nonsilica fiber doped with a rare earth, and a fiber holder in which the nonsilica fiber is inserted and fixed. The fiber holder is optically arranged so that signal light to be amplified propagates in the nonsilica fiber. The nonsilica fiber is pumped so that the nonsilica fiber provides a gain band including the wavelength of the signal light. Opposite ends of the fiber holder are hermetically sealed to thereby easily cut off the nonsilica fiber from the outside air, so that the nonsilica fiber is stabilized physically or chemically. A fluoride glass fiber is suitable for the nonsilica fiber to reduce the gain deviation in a 1.55 &mgr;m band.

Abstract: A polarized light synthesizing apparatus has a lens assembly including a securing member for securing ends of a plurality of optical fibers which can propagate light having different polarization components and a lens for converting the plural kinds of polarization component light emitted from the optical fibers into collimated beams, and a birefringent member for polarization-synthesizing the plural kinds of polarization component light as the same optical axis using a difference in refractive index characteristic between the plural kinds of polarization component light emitted from the lens, thereby largely decreasing a packaging size of the apparatus while keeping functions of a known optical circuit device, a required resistance to light, reliability and readiness of fabrication.

Abstract: The invention provides an optical device suitable for use with an optical amplifier, for example, in an optical communication system and advantageous in that different functions of a plurality of optical elements are achieved using an optical element of a simple construction and the optical device can be produced in a reduced size at a reduced cost. The optical device includes a tapered prism constructed such that a first face and a second face thereof opposing to each other do not extend in parallel to each other. A dielectric multi-layer film is formed on the first face of the tapered prism positioned on an optical signal inputting side. A reflection film is formed on the second face for reflecting the optical signal inputted to the tapered prism through the first face at least once toward the first face.

Abstract: An optical repeater having a first optical amplifier for a first optical fiber transmission line and a second optical amplifier for a second optical fiber transmission line. Each of the first and second optical amplifiers has a doped fiber doped with a rare earth element, a pumping light source for outputting pumping light, a supplying optical coupler for supplying the pumping light to the doped fiber, and an extracting optical coupler for extracting the pumping light. Each extracting optical coupler has an exchange port for outputting a part of the pumping light remaining in the corresponding doped fiber after pumping. The optical repeater further has an optical fiber for connecting the exchange ports in the first and second optical amplifiers to exchange the pumping light. With this configuration, even if one of the pumping light sources has failed, the pumping light from the other pumping light source is supplied to both doped fibers, thereby making the optical repeater have redundancy.

Abstract: A wavelength detecting device is used for a wavelength multiplexing optical communication system, is simple and inexpensive, and can strictly control the wavelengths of signal beams that are close to one another. The device has a path splitter and a wavelength selector. The path splitter splits a beam into at least three beams having different path angles. The wavelength selector filters the split beams into beams having predetermined wavelengths. One of the filtered beams is provided as a signal beam and the other beams are provided as monitor beams used to detect the wavelength of the signal beam.

Abstract: A wavelength division multiplexing module capable of multiplexing or demultiplexing a plurality of optical signals having close wavelengths with less crosstalk. The wavelength division multiplexing module includes a housing having a bottom wall, a first side wall, and a second side wall inclined a predetermined angle with respect to the first side wall, and a common port assembly fixed to the bottom wall of the housing so as to emit a collimated beam substantially perpendicular to the first side wall. One or more filter port assemblies are fixed to the first side wall in the condition that the pass wavelength characteristic and optical axis of each assembly have been adjusted. Similarly, one or more filter port assemblies are fixed to the second side wall in the condition that the pass wavelength characteristic and optical axis of each assembly have been adjusted. Each filter port assembly includes an optical film holder rotatable in a hole having a center axis inclined a predetermined angle .theta.

Abstract: An optical device consists of integrated parts and is used for an optical amplifier.The optical device has first to third assemblies. The first assembly includes two optical fibers each for transmitting a beam and a lens for receiving the beams and providing collimated beams oriented in different directions at given angles. The second assembly includes an optical fiber and a lens to receive one of the collimated beams. The third assembly includes an optical fiber and a lens to receive the other collimated beam. At least one of the second and third assemblies has a second optical fiber. An optical film is arranged in front of the lens of the assembly having the second optical fiber. The optical film reflects an optical signal whose wavelength is different from that of the collimated beam. The two optical fibers of the assembly having the second optical fiber are coupled together through the lens and optical film, to pass the optical signal between the two optical fibers.

Abstract: An optical device having a collimating mechanism for converting light emitted from an excitation port into a parallel beam and outputting the parallel beam onto an optical path, an optical film provided so as to pass the parallel beam output from the collimating mechanism, and a supporting mechanism for supporting the optical film so that the optical film is rotatable about an axis inclined with respect to the optical path. The optical film is inclined with respect to a plane perpendicular to the axis. An incident angle of the parallel beam upon the optical film can be changed by rotating the optical film about the axis. Thus, the optical device having the optical film that can vary the incident angle thereupon can be provided with a simple structure.

Abstract: An optical device including a lens and an optical thin film for coupling and separating two light components. First and second optical fibers are arranged on one side of the lens at symmetrical positions with respect to the optical axis of the lens, and the optical thin film and a third optical fiber are arranged on the other side of the lens. An isosceles triangular block is arranged to simultaneously move the first and second optical fibers to adjust an incident angle and an emerging angle of the light made incident to the optical thin film. Alternatively, a movable isosceles triangular prism having two oblique reflecting surfaces is arranged.

Abstract: A polarization independent optical device capable of eliminating the apparent polarization dependence of a dielectric multi-layer film and permitting the design of an optical path at an optional angle of incidence of light beam. The polarization independent optical device comprises a tapered double-refractive crystal disposed so that an optic axis thereof is perpendicular to an incident optical path, and a dielectric multi-layer film disposed on an exit side of light beam with respect to the tapered double-refractive crystal.

Abstract: A polarization independent optical device capable of eliminating the apparent polarization dependence of a dielectric multi-layer film and permitting the design of an optical path at an optional angle of incidence of light beam. The polarization independent optical device comprises a tapered double-refractive crystal disposed so that an optic axis thereof is perpendicular to an incident optical path, and a dielectric multi-layer film disposed on an exit side of light beam with respect to the tapered double-refractive crystal.

Abstract: A polarization beam coupler adapted to produce an output light beam for monitoring is disclosed. The polarization beam coupler includes a polarization beam coupling cube for coupling two incident light beams having polarization planes perpendicular to each other and projecting the coupled light beams on the same optic axis, a beam splitting cube having a beam splitting film formed from a dielectric multi-layer or single-layer film for extracting a monitor beam, and a quarter-wave plate interposed between the polarization beam coupling cube and the beam splitting cube such that incident light beams to the beam splitting cube may be circularly polarized light. Due to the construction, the output level of a monitor light beam can be prevented from depending upon the polarization planes of the incident light.

Abstract: An optical coupler is disclosed which couples a signal beam and a pumping beam having proximate wavelengths to each other. The optical coupler includes a first prism having an included angle of 45 degrees, and a triangular prism having an apex angle of 22.5 degrees and adhered to a face of the first prism on which an optical wavelength multiplexing film is formed. The signal beam and the pumping beam multiplexed by the optical coupled is introduced into an Er doped fiber, in which the signal beam is amplified directly.

Abstract: A polarization coupler in which a polarization maintaining fiber is employed and a process of producing such polarization coupler are disclosed. Where the polarization coupler is used, beams of light from two laser diodes can be introduced into a single optical fiber and light for the monitoring can be split from a main signal output. Since minimum components required to realize the function are a rotator, a birefringent plate and two or three polarization maintaining fibers, the polarization coupler is superior in operability in production and is suitable for miniaturization. The branching ratio of light for monitoring can be adjusted by adjustment of the angle of rotation of light by the rotator.