Optics: measuring and testing – By light interference – Having partially reflecting plates in series
Reexamination Certificate
2000-10-27
2004-12-07
Bruce, David V. (Department: 2882)
Optics: measuring and testing
By light interference
Having partially reflecting plates in series
C356S454000, C359S588000, C359S584000, C359S578000, C359S337100, C359S346000, C359S238000, C359S288000, C359S290000
Reexamination Certificate
active
06829053
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an airgap type etalon as a Fabry-Perot interferometer, and more particularly to an airgap type etalon capable of freely adjusting temperature dependency of a wavelength characteristic (hereinafter called wavelength-temperature characteristic) and an apparatus utilizing such an etalon.
2. Related Art
Recently, increase of a transmission capacity in optical communications has resulted in adoption of a wavelength division multiplexing (WDM) transmission method. In such a WDM transmission method, wavelengths of different channels have been brought closer to one another for further increase of a transmission capacity. Correct transmission and reception of signal light including channels having such mutually closer wavelengths requires that a wavelength characteristic of an optical filter such as an etalon to be used in a transmission apparatus is stable relative to a change of environmental conditions such as temperature. As such, there is demanded an optical filter having an extremely low wavelength-temperature characteristic.
Further, in long-distance optical transmissions, an erbium (Er)-doped optical fiber amplifier (hereinafter called “EDFA”), for example, has been widely adopted as an optical repeater so as to increase a transmission distance. A gain characteristic of such an EDFA generally has a wavelength characteristic such as based on a composition of an optical fiber and the like. Thus, a technique for flattening such a gain wavelength characteristic of an EDFA has been put to practical use by utilizing, as a gain-equalizer (hereinafter called “GEQ”), an optical filter such as an etalon having a loss wavelength characteristic opposite to the gain wavelength characteristic of the EDFA.
However, since the gain wavelength characteristic of the EDFA is also changed according to the temperature of an erbium-doped optical fiber (EOF), this characteristic is affected by the change of the environmental temperature. As such, deviations of optical SNR's (signal
oise ratios) of optical signals at respective wavelengths have been problematically caused in a WDM transmission apparatus. To deal with this problem, there is desired a gain-equalizer capable of passively flattening the gain wavelength characteristic of the EDFA even when this characteristic is changed due to a temperature change. Unfortunately, realization of such a gain-equalizer requires an optical filter having a higher wavelength-temperature characteristic, contrary to the aforementioned demand for an optical filter.
Further, EDFA's are being widely used in a superlong-distance transmission for undersea repeating. In such usage, a change of an input level of an EDFA due to repair of an optical fiber transmission path and/or a change with time lapse has problematically resulted in a change of the gain of the EDFA when the output level of the EDFA is fixedly controlled. Since the gain change of the EDFA causes an inclination in the wavelength characteristics of the EDFA, there is desired a wavelength characteristic varying apparatus capable of compensating for the inclination of the wavelength characteristic. However, realization of such a wavelength characteristic varying apparatus requires two kinds of optical filters such as having high wavelength-temperature characteristics, respectively, and the respective wavelength temperature characteristics having shift directions opposite to each other.
Concerning such optical filters, various demands are presented not only for transmission wavelength characteristics of the filters but also for the temperature dependencies of such characteristics, corresponding to usage and purpose of the filters. Therefore, such optical filters are needed to be designed in response to these demands. As the aforementioned optical filters, there have been generally used: an etalon (Fabry-Perot interferometer) of a solid type or airgap type; a multilayered dielectric film filter; a fiber Bragg grating (FBG); and the like.
Concrete constitutions of conventional etalons include those such as known from Japanese Unexamined Patent Publication Nos. 7-86673, 3-185402, 7-27943, and 9-257567.
Concerning a multilayered dielectric film filter or an FBG among the aforementioned optical filters, however, materials usable in the manufacturing process are limited to silica (SiO
2
) and the like, and wavelength-temperature characteristic is determined corresponding to the physical properties (an expansion coefficient; and a temperature coefficient of a refractive index), resulting in a defect that the degree of design freedom is extremely low.
Further, concerning a solid type etalon, although various kinds of transparent multicomponent glass materials can be used, the wavelength-temperature characteristic of the etalon is also determined corresponding to the physical properties (an expansion coefficient; and a temperature coefficient of a refractive index) of the etalons, resulting in a limited degree of design freedom. In this respect, synthetic resins such as acrylic resins and polycarbonate resins are conceivable as transparent materials. However, it is difficult to adopt such synthetic resins as materials of etalons, due to the properties of these resins such as: water absorptivity leading to susceptibility to humidity; poor flatness even after machining; and lower light resistance leading to occurrence of material deterioration upon entrance of light at a higher intensity.
Concerning an airgap type etalon, there has been conventionally used a constitution in which a gap material is interposed between two transparent parallel flat plates such as glass as shown in FIG.
21
. Each of the transparent parallel flat plates is formed with an antireflection coating on an outer flat surface and a reflection augmenting coating on an inner flat surface. In such a constitution, the gap material can be provided not only by a transparent material but also by an opaque material, leading to a slightly higher degree of freedom for selecting a material as compared to a solid type etalon. However, gap materials are actually restricted to a glass material and/or some metals in view of stability for the environment and machinability into a flat surface. Accordingly, in the present state, a sufficient degree of design freedom for wavelength-temperature characteristic is hardly obtained.
As described above, it has been rather difficult to actually obtain a desired wavelength-temperature characteristic in conventional optical filters. Also, since suitable optical filters have not been obtained in various devices such as wavelength detecting devices, gain-equalizers, and wavelength characteristic varying devices, which devices need optical filters having predetermined wavelength-temperature characteristics, there has been a problem in that a desired performance can not be achieved.
There will be now concretely explained problems of conventional etalons described in the aforementioned publications.
Firstly, the etalon described in Japanese Unexamined Patent Publication No. 7-86673 includes a gap material constituted by mutually joining a material having a positive expansion coefficient and another material having a negative expansion coefficient, for the purpose of eliminating a temperature dependent change of a gap length of the airgap type etalon to thereby eliminate wavelength-temperature characteristic. In such a constitution, it is possible to realize an etalon having a lower wavelength-temperature characteristic, but it will be difficult to also provide an etalon having a higher wavelength-temperature characteristic. Further, in constituting an etalon utilizing this conventional technique, it is required to adopt a specific ceramic material such as having a negative expansion coefficient, leading to a defect that the degree of design freedom for wavelength-temperature characteristics is restricted. Moreover, the ratio of thickness between two gap materials is determined by the expansion coefficients of respective materials. Thu
Mitamura Nobuaki
Nagaeda Hiroshi
Bruce David V.
Fujitsu Limited
Staas & Halsey , LLP
Suchecki Krystyna
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