Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-04-19
2001-08-28
Henry, Jon (Department: 2872)
Optical waveguides
With optical coupler
Input/output coupler
C372S006000, C372S020000, C359S199200, C359S199200
Reexamination Certificate
active
06282340
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light wavelength tuning device for tuning the wavelength of light emitted from an optical device, and a light source, optical demultiplexer and wavelength division multiplexed optical communication system using the tuning device.
2. Description of the Related Art
Recently, there has been a demand for the development of a system for high-density wavelength optical communication to meet the requirement for increase in communication capacity in the optical communication technique. In a conventional wavelength division multiplexed (WDM) optical communication system, for example, light signals from a plurality of light sources are multiplexed by means of an optical coupler or a WDM coupler and transmitted by means of a single-core optical fiber. In this optical communication system, the light signals are transmitted separately for individual wavelengths on the transmission side, and light signals with specific wavelengths, branched by means of an optical demultiplexer, are received on the reception side.
According to this system, however, if the lasing wavelength of light emitted from any of the light sources shifts, for example, the light may possibly interfere with light from any other light source. Therefore, the respective lasing wavelengths of the light sources must be accurately adjusted to a constant value.
As a measure to attain this, there is a light wavelength tuning device that is described in Jpn. Pat. Appln. Publication No. 2546151.
Conventionally, however, an optical filter used in the light wavelength tuning device of this type is formed of a dielectric multilayered film, of which the wavelength characteristic depends much on temperature, so that wavelength tuning cannot be accurate enough.
In a conventional optical demultiplexer, however, the light wavelength tuning device serves only to keep the device temperature constant, and never to adjust the passing wavelength to a fixed value. If the wavelength of light transmitted through the optical demultiplexer shifts in the aforesaid WDM optical communication system of the high-density wavelength type, however, light with a desired wavelength cannot be transmitted, or light with any other wavelength may be transmitted. Possibly, therefore, degradation of transmission quality or transmission failure may occur.
Thus, if the control wavelength for the light wavelength tuning device for each optical device is subject to a substantial temperature error in the WDM optical communication system, degradation in transmission quality, such as a transmission error, is caused in the optical communication system.
A novel lasing wavelength fixing device is described in “Laser wavelength stabilization using holographic filters” (IEEE/LEOS meeting on optical networks and their enabling technologies, Jul. 11-13, 1994, Lake Tahoe, Nev.) by C. Malo, et al. In this device, the lasing wavelength of a laser is fixed by means of a crystal of lithium niobate (LiNbO
3
) on which a grating is formed.
It is known, however, that the Bragg wavelength of the grating in this device has temperature dependence of 0.01 nm/° C. The crystal of lithium niobate is placed on a Peltier cooler, and is fitted with a thermistor for detecting temperature. Further, the device is expected to contain therein a lens for collimating light emitted from an optical fiber, beam splitter, condenser, and two photodiodes, thus entailing a complicated optical system. If this device is used, therefore, the device configuration is complicated and large-sized, so that the manufacturing cost is too high.
SUMMARY OF THE INVENTION
The present invention has been contrived in consideration of these circumstances, and its object is to provide a light wavelength tuning device capable of wavelength tuning with low temperature dependence.
Another object of the invention is to provide a light source having a simple construction and subject to less fluctuation in lasing wavelength.
Still another object of the invention is to provide an optical demultiplexer having a simple construction and subject to less fluctuation in passing wavelength.
A further object of the invention is to provide a WDM optical communication system subject to less degradation in transmission quality.
In order to achieve the first object, a light wavelength tuning device according to the present invention comprises: an optical filter for outputting part of light emitted from an optical device as wavelength tuning light by transmission or reflection, the output level of the wavelength tuning light varying with the change of the wavelength of the tuning light; and a wavelength tuner for tuning the wavelength of the light emitted from the optical device so that the output level of the wavelength tuning light delivered from the optical filter is approximate to a given value. The optical filter includes a fiber Bragg grating section provided with a wavelength stabilizer.
Thus, the passing wavelength characteristic of a fiber grating, which is believed originally to depend little on temperature, is prevented from further changing by means of the wavelength stabilizer. The wavelength of the optical device is tuned by means of the wavelength stabilizer, whereby the tuned wavelength is less liable to suffer an error that is attributable to the change of environmental temperature.
Further, the optical filter is formed by arranging two fiber Bragg grating sections with different Bragg wavelengths in series. The wavelength tuner tunes the wavelength of the light from the optical device so that the output level of the wavelength tuning light from the optical filter is approximate to a peak value between the respective Bragg wavelengths of the two fiber Bragg grating sections. Thus, the two fiber Bragg grating sections can be arranged in series by only forming two sets of gratings on one fiber, so that the construction is simple.
Furthermore, the optical filter is a resonator formed by arranging fiber Bragg grating sections in series. The wavelength tuner tunes the wavelength of the light from the optical device so that the output level of the wavelength tuning light from the optical filter is approximate to a peak value corresponding to the Bragg wavelength of the fiber Bragg grating sections. Thus, the two fiber Bragg grating sections can be arranged in series by only forming two sets of gratings on one fiber, so that the construction is simple.
The wavelength tuner tunes the wavelength of the light from the optical device so that the output level of the wavelength tuning light from the optical filter is approximate to a given level on a slope of a wavelength dependence curve of the fiber Bragg grating section. Thus, the light wavelength tuning device requires use of only one fiber Bragg grating section, so that its construction is simple. Since variation of voltage values corresponds to variation of wavelength in the wavelength tuner, moreover, tuning operation is easy.
Further, the optical filter causes one of branch beams of the light from the optical device to be transmitted or reflected as the wavelength tuning light by the fiber Bragg grating section having a given Bragg wavelength, and causes the other beam to be transmitted or reflected as reference light by a reference light pass section. The wavelength tuner tunes the wavelength of the light from the optical device so that a tuning voltage value, based on the difference between the output level of the wavelength tuning light delivered from the fiber Bragg grating section and the output level of the reference light delivered from the reference light pass section, is approximate to a given set voltage value. Thus, the branch light beam is used for the reference voltage value, so that an error that is attributable to variation of optical output can be canceled.
According to the invention, moreover, there is provided a light source including an automatic output controller, in which output light is partially used as wavelength tuning light and the temperature of an optical device is cont
Kato Tomoyuki
Kogure Kazuo
Nasu Hideyuki
Omura Hideyuki
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Henry Jon
The Furukawa Electric Co. Ltd.
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