Abstract: A Peltier cooler (12) according to the present invention has a structure in which a Peltier device (12c) is put between a low temperature side substrate part (12a) and a high temperature side substrate part (12b) both made of insulating material. A first metal frame (22a) and a second metal frame (22b) are solder-fixed to the edge portions of the insulating substrates, respectively. The first and second metal frames (22a) and (22b) can be fixed highly precisely and stably by means of laser welding. By so doing, no solder creep occurs on the semiconductor laser module, thereby ensuring the stable arrangement of the optical system for a long time.

Abstract: A Peltier cooler includes a plurality of Peltier elements, first and second ceramic substrates that are disposed to hold the Peltier elements through metallized electrodes formed inside the ceramic substrates and to electrically connect the Peltier elements in series, and first and second metal substrates respectively fixed to the first and second ceramic substrates by brazing, so as to hold them. The metal substrates are fixable to a substrate on which optical components are mounted and a semiconductor laser package, by yttrium-aluminum-garnet (YAG) laser welding. The YAG laser welding and the brazing operation eliminate the use of low-temperature solder, where creeps may occur.

Abstract: In a variable wavelength optical filter, a glass plate has a curved surface on one or both sides thereof, and a dielectric multi-layer thin film formed on the curved surface. Light from the outside of the filter is transmitted through the film. The glass plate is movable perpendicularly to an optical axis by being driven by a piezoelectric actuator accommodated in a housing. By moving the glass plate, it is possible to control the angle of the film relative to the incident light and, therefore, the pass wavelength. The actuator positions the glass plate with accuracy, promoting extremely accurate wavelength control. The glass plate may be provided with a convexity on one side and a concavity on the other side which is complementary in configuration to the convexity. This reduces the wavelength dependency of the insertion loss by maintaining the optical axis of input light and that of output light in coindicence.

Abstract: The temperature control of a device incorporated in a module is performed using a device temperature and a surroundings temperature of the device. The device temperature is detected by a device temperature sensor disposed near the device and the surroundings temperature is detected by a surroundings temperature sensor disposed on the module. Using the surroundings temperature when the device is operated, an actual temperature difference between the device temperature and a predetermined control target temperature is estimated based on a predetermined relation between the surroundings temperature and the device temperature detected when desired characteristics of the device are obtained. The device temperature detected when the device is operated is compensated with the estimated temperature difference to adjust the temperature of the device on the predetermined control target temperature.

Abstract: An optical circuit according to the present invention includes a first beam splitter for splitting a signal light to two polarization lights, first and second optical couplers for coupling the split signal lights and local lights in each, and second and third beam splitters for coupling corresponding polarization lights supplied from the first and second optical couplers. The second and third beam splitters supply output signals to one double-balanced receiver of a polarization diversity receiver.

Abstract: Frequency division multiplex signal lights are propagated through an optical fiber, and polarizations of the lights are collectively controlled by a polarization controller. The polarization-controlled signal lights are received separately in receiving systems including optical heterodyne or homodyne receivers. In the collective polarization control, a step is selected from steps of controlling relative polarization states of local oscillation lights to coincide to each other, receiving a control signal from one of the receiving systems which receives a signal light of a frequency to be allocated in the center of a frequency band of the signal lights or the vicinity thereof, and dividing the frequency division multiplex signal lights into plural groups, thereby controlling polarizations collectively in each of the plural groups.

Abstract: First to fifth birefringences are generated in series along a light propagating medium. The first to fifth birefringences have main axes of 0.degree., 45.degree., 0.degree., 45.degree. and 0.degree. relative to an arbitrary direction orthogonal to a light propagating direction of the medium. The magnitude of the birefringences are changed to change first to fifth phase differences. In an ordinary polarization control, the second to fourth phase differences are changed. However, one or both of the first and fifth phase differences are changed in a resetting operation for one of the second to fourth phase differences. Consequently, the phase differences are reset without the dependency on polarizations of an input light supplied to a polarization controller and an output light supplied from the polarization controller.

Abstract: A polarization controller comprises a plurality of devices connected in series to each other. Each device includes an optical channel waveguide which is common to the plurality of the devices, and an electrode positioned on the optical channel waveguide and two electrodes positioned on both sides of the optical channel waveguide. A retardation induced by electrooptic effect in each device is adjusted by voltages applied to the electrode and the two electrodes. The voltages are not kept increasing or decreasing, but changed periodically so that there is no limitation in an operating range for controlling a polarization of light which is propagated through the optical channel waveguide.

Abstract: For first and second polarization controlling elements (181, 182) driven by first and second driving voltages in a polarization controlling device, a beam splitter (15) has a dividing ratio controllable between 1:0 and 0:1 and divides an input polarized beam having an input polarization state. An output polarized beam is derived from first and second polarization controlled beams to have an output polarization state. While the dividing ratio is kept at 1:0 so that the second polarization controlled beam is null, the first driving voltage may approach either of a positive and a negative limit. In this event, a control unit (25) changes the dividing ratio to 0:1. Even while the second polarization controlled beam is null, the control unit controls the second driving voltage so that the both driving voltages are congruent modulo a unit voltage difference which makes each element carry out equivalent polarization control. The first driving voltage is likewise controlled.