Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
1999-12-30
2001-03-13
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S199200, C359S199200, C359S341430, C385S037000
Reexamination Certificate
active
06201636
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a gain control unit and an optical amplifier for compensating a gain wavelength characteristic for an optical signal, and more particularly to a gain control unit and an optical amplifier having a temperature compensating function which also compensates automatically a temperature dependency of the gain wavelength characteristic.
(2) Description of the Related Art
There has been conventionally known a gain equalizer (gain control unit) utilizing such as an optical filter which is provided within an optical amplifier to be used in an optical communication system, to compensate a gain wavelength characteristic of the optical amplifier. This conventional gain equalizer is designed in advance such that its loss wavelength characteristic compensates the gain wavelength characteristic of the optical amplifier to thereby obtain an overall flat wavelength characteristic. Generally, the design of loss wavelength characteristic of the gain equalizer is mostly optimized on the basis of the gain wavelength characteristic of the optical amplifier at a specific temperature. As such, it has been required that the temperature dependency of the loss wavelength characteristic of the gain equalizer is less. By adopting such a gain equalizer, it has become possible to realize an optical amplifier having a relatively flat gain wavelength characteristic in a predetermined wavelength band.
Meanwhile, it is known that a gain wavelength characteristic of an optical amplifier fluctuates depending on ambient temperature. For example, Erbium-doped fiber (EDF) typically used in an optical amplifier has such a gain wavelength characteristic as shown in
FIG. 16
, in which the gain deviation relative to the wavelength varies largely depending on the ambient temperature (0° C., 25° C., and 65° C., in this figure). Further, the difference between the gain at the ambient temperature of 0° C. and that at the ambient temperature 25° C. in the above mentioned EDF exhibits such a wavelength characteristic (A) as shown in
FIG. 17
, and the difference between the gain at the ambient temperature 65° C. and that at the ambient temperature 25° C. exhibits a wavelength characteristic (B) in FIG.
17
.
Since a gain wavelength characteristic of an EDF has a temperature dependency as noted above, it has been conventionally required to keep the EDF at a constant temperature. As such, it has been required to cover the EDF such as by a heat insulating material, and occasionally to adjust the temperature of the EDF by a heater or power source and the like, causing a disadvantage of increase in size of an optical amplifier.
Relatedly, there exists a technique such as disclosed in Japanese Unexamined Patent Publication 9-145941, as a long period fiber grating (“LPG”, which can be used to flatten a gain wavelength characteristic of EDF) aiming at improving stability against temperature variation. In this technique, there are newly designed a profile of cladding and a composition of fiber, in view of the fact that the refractive indices of core and cladding vary differently from each other relative to temperatures due to a temperature dependency of a long period fiber grating. Contrary, as described later herein, the present invention resides in a technique for realizing temperature compensation by appropriately combining fiber gratings having mutually different characteristics, in which the structures of the fiber gratings are common in themselves. Thus, the present invention is distinguished from the aforementioned known technique.
SUMMARY OF THE INVENTION
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide a gain control unit having a temperature compensating function capable of flexibly coping even with a gain wavelength characteristic which fluctuates depending on ambient temperature, to thereby provide an optical amplifier, of small size and simple constitution, capable of realizing a gain wavelength characteristic which is stable over a wide temperature range.
To this end, the present invention provides a gain control unit having a temperature compensating function, comprising: a plurality of grating portions provided in an optical transmission line through which an optical signal is propagated; wherein the plurality of grating portions have mutually different loss wavelength characteristics, and the mutually different loss wavelength characteristics have mutually different temperature dependencies.
According to such a constitution, it becomes possible to design freely a gain control unit in which a loss wavelength characteristic thereof fluctuates depending on ambient temperature change, by appropriately combining a plurality of grating portions constituted such that loss wavelength characteristics thereof are mutually different and the respective loss wavelength characteristics have temperature dependencies, to be used as an optical transmission line.
The present invention also provides an optical amplifier having a temperature compensating function, in which the optical amplifier includes an optical amplifying device for amplifying an optical signal to be propagated through an optical transmission line, and the optical amplifying device has a gain wavelength characteristic having a temperature dependency, the optical amplifier comprising: a plurality of grating portions provided within the optical amplifying device or on the optical transmission line connected to the optical amplifying device, wherein the plurality of grating portions have mutually different loss wavelength characteristics, and the mutually different loss wavelength characteristics have mutually different temperature dependencies; and wherein the loss wavelength characteristics of the grating portions and the temperature dependencies of the loss wavelength characteristics are set corresponding to the gain wavelength characteristic of the optical amplifying device and the temperature dependency of the gain wavelength characteristic of the optical amplifying device, respectively.
In such a constitution, an optical signal is amplified in the optical amplifying device corresponding to the gain wavelength characteristic having the temperature dependency. Nonetheless, there is automatically performed a gain compensation including the temperature characteristic since the optical signal is propagated through the plurality of grating portions in which the loss wavelength characteristics thereof and the temperature dependencies of the loss wavelength characteristics are set corresponding to the gain wavelength characteristic of the optical amplifying device and the temperature dependency of the gain wavelength characteristic of the optical amplifying device. In this way, it becomes possible to realize an optical amplifier, of small size and simple constitution, capable of obtaining a gain wavelength characteristic which is stable irrespectively of a temperature change.
For the gain control unit or the optical amplifier mentioned above, the plurality of grating portions may be long period fiber gratings which are formed along the optical transmission line, respectively. The long period fiber gratings preferably have respective grating pitches in a range of from 0.1 mm to 1 mm.
Further, for the optical amplifier, the optical amplifying device may be concretely a multistage amplification constitution having a plurality of optical amplifying portions, and the optical amplifying device may include an optical fiber amplifier adopting a rare earth element doped fiber.
Further objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiments when read in conjunction with the accompanying drawings.
REFERENCES:
patent: 5430817 (1995-07-01), Vengsarkar
patent: 5541766 (1996-07-01), Mizrahi et al.
patent: 5703978 (1997-12-01), DiGiovanni et al.
patent: 5764829 (1998-06-01), Judkins et al.
patent:
Fujitsu Limited
Moskowitz Nelson
Staas & Halsey , LLP
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