Abstract: A beam splitter formed by laminating a plurality of optical films on a glass substrate having a refractive index of 1.51.+-.0.10. The optical films are first, second, third, fourth, and fifth layers laminated in this order from the glass substrate. The first layer has a refractive index of 1.44.+-.0.10 and an optical film thickness of 0.05-0.15; the second layer has a refractive index of 1.68.+-.0.10 and an optical film thickness of 0.01-0.10; the third layer has a refractive index of 1.44.+-.0.10 and an optical film thickness of 0.50-0.60; the fourth layer has a refractive index of 2.23.+-.0.10 and an optical film thickness of 0.25-0.35; and the fifth layer has a refractive index of 1.44.+-.0.10 and an optical film thickness of 0.25-0.35. The optical film thickness means a relative value represented by n.times.d/.lambda. where n denotes a refractive index; d denotes an actual film thickness, and .lambda. denotes a wavelength deciding the center of a wavelength band.

Abstract: An optical device including a lens located between a first port and a second port, and a photodetector having a photodetecting surface located in the vicinity of the second port. The first port and the second port have a first aperture and a second aperture, respectively. The lens has a converging portion for converting a light beam so as to couple the first aperture and the second aperture, and a deflecting portion for deflecting a part of a light beam from the first port to make the part incident on the photodetecting surface. The converging portion and the deflecting portion are integral with each other, for example. By using this optical device, light power can be monitored stably with no polarization dependence.

Abstract: A beam splitter formed by laminating a plurality of optical films on a glass substrate having a refractive index of 1.51.+-.0.10. The optical films are first, second, third, fourth, and fifth layers laminated in this order from the glass substrate. The first layer has a refractive index of 1.44.+-.0.10 and an optical film thickness of 0.05-0.15; the second layer has a refractive index of 1.68.+-.0.10 and an optical film thickness of 0.01-0.10; the third layer has a refractive index of 1.44.+-.0.10 and an optical film thickness of 0.50-0.60; the fourth layer has a refractive index of 2.23.+-.0.10 and an optical film thickness of 0.25-0.35; and the fifth layer has a refractive index of 1.44.+-.0.10 and an optical film thickness of 0.25-0.35. The optical film thickness means a relative value represented by n.times.d/.lambda. where n denotes a refractive index; d denotes an actual film thickness, and .lambda. denotes a wavelength deciding the center of a wavelength band.

Abstract: A beam splitter formed by laminating a plurality of optical films on a glass substrate having a refractive index of 1.51.+-.0.10. The optical films are first, second, third, fourth, and fifth layers laminated in this order from the glass substrate. The first layer has a refractive index of 1.44.+-.0.10 and an optical film thickness of 0.05-0.15; the second layer has a refractive index of 1.68.+-.0.10 and an optical film thickness of 0.01-0.10; the third layer has a refractive index of 1.44.+-.0.10 and an optical film thickness of 0.50-0.60; the fourth layer has a refractive index of 2.23.+-.0.10 and an optical film thickness of 0.25-0.35; and the fifth layer has a refractive index of 1.44.+-.0.10 and an optical film thickness of 0.25-0.35. The optical film thickness means a relative value represented by n x d/.lambda. where n denotes a refractive index; d denotes an actual film thickness, and .lambda.denotes a wavelength deciding the center of a wavelength band.

Abstract: A method and apparatus for controlling gain differences between optical signals, and hence an output of each individual optical signal, in an optical amplifier including a rare-earth-doped fiber for optically amplifying a wavelength-division multiplexed signal. Pumping light is injected into an erbium-doped fiber that is also supplied with additional pumping light that gives a different gain characteristic to the erbium-doped fiber. The output of each signal light is detected by a photodetector and, based on the detected output, injection power is controlled individually for each pumping light.

Abstract: Disclosed is a light quantity measurement device that can accurately monitor light intensity regardless of the polarized condition of incident light. A light quantity measuring device comprises: a birefringent optical device; transmission means for passing incident light from a light source; and a light-receiving device for receiving an ordinary light beam and an extraordinary light beam from the transmission means. The light-receiving device has at the least two electrically separated light-receiving areas from which currents produced by received light are independently output, and includes first and second variable gain amplification means for amplifying currents, which are produced by received light, from the two electrically separated light-receiving areas, and an addition circuit for adding together those currents that are amplified by the first and the second amplification means.

Abstract: An optical coupler divides signal light beam in optical transmission path at predetermined ratio. The optical coupler includes single-layer dividing film evaporated on substrate with optical film thickness being of predetermined value, the film having refractive index which is equal to or smaller than square root of refractive index of substrate, and coupler holder holding single-layer dividing film in optical transmission path to apply signal light beam to single-layer dividing film at incident angle of about 45 degrees. Alternatively, optical coupler includes dividing film made of three films having respective refractive indexes of 1.46.+-.0.10, 1.65.+-.0.10, and 2.30.+-.0.10 and formed in four, five, or seven layers on substrate having refractive index of 1.51.+-.0.10 or 3.50.+-.0.

Abstract: An optical transmitter provided by the present invention comprises: a light source; a band-pass filter for filtering a light generated by the light source; a light splitting means for splitting a light output by the band-pass filter into at least two split lights; an optical receiving unit for converting one of the split lights into an electrical signal having a level proportional to the intensity of the split light; and a control means for receiving the electrical signal for use in controlling the wavelength of the light source so as to sustain the intensity of the split light at a fixed value. With the configuration described above, fluctuations in wavelength occurring on the transmitter side can be reduced. On the top of that, by applying this optical transmitter to a WDM system, a system cold start can be done with ease.

Abstract: A compact, inexpensive laser diode module having a depolarizer function of converting linearly polarized light into unpolarized light. The laser diode module includes a laser diode; a polarization-maintaining optical fiber arranged so that an axis of polarization of the fiber forms an angle of 45.degree. with respect to a plane of polarization of incident light, the fiber having a length set so that an optical path difference between two polarization modes of transmitted light is greater than a coherence length of the incident light; and an optical coupling element for coupling emitting light from the laser diode to the polarization-maintaining optical fiber.

Abstract: A beam splitter formed by laminating a plurality of optical films on a glass substrate having a refractive index of 1.51.+-.0.10. The optical films are first, second, third, fourth, and fifth layers laminated in this order from the glass substrate. The first layer has a refractive index of 1.44.+-.0.10 and an optical film thickness of 0.05-0.15; the second layer has a refractive index of 1.68.+-.0.10 and an optical film thickness of 0.01-0.10; the third layer has a refractive index of 1.44.+-.0.10 and an optical film thickness of 0.50-0.60; the fourth layer has a refractive index of 2.23.+-.0.10 and an optical film thickness of 0.25-0.35; and the fifth layer has a refractive index of 1.44.+-.0.10 and an optical film thickness of 0.25-0.35. The optical film thickness means a relative value represented by n.times.d/.lambda. where n denotes a refractive index; d denotes an actual film thickness, and .lambda. denotes a wavelength deciding the center of a wavelength band.

Abstract: A tunable filter module which can automatically make the center wavelength in the passband coincident with the center wavelength of a signal light beam according to variations in wavelength of the signal light beam. The tunable filter module includes a beam splitting mechanism for splitting an incident signal light beam into a first beam undeviated, a second beam deflected to one side of the first beam; and a third beam deflected to another side of the first beam, and a multilayer film filter located so as to transmit the first, second, and third beams. A first photodetector is located so as to detect the second beam, and a second photodetector is located so as to detect the third beam. A comparator is connected to the first and second photodetectors to compare outputs from the first and second photodetectors and output a differential signal indicative of the difference between these outputs.

Abstract: An optical band-pass filter adapted for enabling mutually independent fine adjustments of two pass bands. The optical band-pass filter comprises a first filter having a first pass band, first and second reflective films for respectively reflecting the transmitted light and the reflected light obtained from the first filter film, and a second filter film having a second pass band different from the first pass band and serving to combine the light out of the first and second reflective films with each other.

Abstract: An optical filter including first, second, and third birefringent elements arranged in this order on an optical path leading from an input port to an output port, and a device for changing a vector of birefringence in the second birefringent element. The first birefringent element splits an input beam into first and second beams having planes of polarization perpendicular to each other to output the first and second beams. The second birefringent element receives the first and second beams to output third and fourth beams converted in polarized condition from the first and second beams, respectively. The third birefringent element receives the third and fourth beams, splits the third beam into fifth and sixth beams having planes of polarization perpendicular to each other to output the fifth and sixth beams, and splits the fourth beam into seventh and eighth beams having planes of polarization perpendicular to each other to output the seventh and eighth beams.

Abstract: A light amplifier includes an optical fiber doped with a rare-earth ion, a light source emitting an exciting light and thereby exciting the optical fiber, and an optical system which applies a given light component to the optical fiber in a first direction opposite to a second direction in which a signal light is propagated through the optical fiber when a light exceeding a threshold level is propagated through the optical fiber, so that a gain of the optical fiber can be reduced.

Abstract: An optical device which optically connects a first and a second optical fiber with an optical path provided therebetween and has two or more functions including the function of an optical isolator. A beam of parallel rays obtained by having a beam from an excitation end of a first optical fiber collimated by a lens is passed through a double refraction element, a magnetooptic element, and a double refraction element in order of mention and converged by a lens to be introduced to a second optical fiber through its excitation end, while a beam from the second optical fiber is not coupled to the first optical fiber. Meanwhile, a beam from an excitation port is coupled to the first optical fiber. The optical device is suitable for use in an optical amplification system and a two-way optical transmission system.

Abstract: Disclosed herein is a duplicated light source module constructed so that rays of light emitted from two light sources pass through one lens and then enter one optical fiber through a birefringent element. This module is useful when a light source in an optical transmitter is intended to be duplicated or when a high-power pumping source in an optical fiber amplifier is required.

Abstract: A magneto-optic disk reading head system including an external semiconductor laser resonator. A magneto-optic disk forms a part of the external resonator. In the external resonator, orthogonal two mode oscillations are established to produce a beat signal used for reading data stored in the magneto-optic disk. The magneto-optic disk reading head system is a floating type reading head.