Optical: systems and elements – Light interference – Electrically or mechanically variable
Reexamination Certificate
2003-06-03
2004-12-21
Assaf, Fayez (Department: 2872)
Optical: systems and elements
Light interference
Electrically or mechanically variable
C359S589000, C359S590000, C359S587000, C359S588000, C356S454000
Reexamination Certificate
active
06833957
ABSTRACT:
This application is based on Japanese patent application NO. 2002-163645, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tunable filter to be used when selecting from, eliminating out of, or adding to a wavelength division multiplex optical signal a particular wavelength, mainly in an optical communication network in which wavelength division multiplexing is employed, and manufacturing method thereof, and further to an optical switching device comprising such tunable filter.
2. Description of the Related Art
Recently, wavelength division multiplexing has come to be popularly used in a field of optical communication, making it more and more important to select and pick up a particular wavelength out of a wavelength division multiplex (hereinafter referred to as “WDM”) optical signal. An optical add/drop device and an optical cross-connect device, which are currently available in the market, are generally designed to select a fixed wavelength out of a WDM optical signal. In such fixed wavelength filters, generally a Fabry-Perot etalon filter comprising a dielectric multi-layer film is incorporated because of its excellent cost performance.
Also, the U.S. Pat. No. 4,825,262 discloses a variable wavelength type Fabry-Perot etalon filter that can vary a wavelength by controlling a voltage. Further, the U.S. Pat. No. 5,739,945 discloses a Fabry-Perot etalon filter having a semi conductor/air constitution based on MEMS (Micro Electro-Mechanical System) technology, for increasing a resolution for selecting a wavelength.
Further, Japanese Patent Laid Open Publication (JP-A) H11-142752 discloses an interference filter of a variable transmitted wavelength type. In this variable transmitted wavelength type interference filter, a pair of substrates is disposed in parallel and a multi-layer film is formed on the respective confronting surfaces of the substrates with a certain clearance therebetween, which clearance is variable by an external force.
FIG. 11
shows a conventional tunable filter similar to the variable wavelength interference filter set forth in the foregoing literature. An optical multilayer film
1003
is formed on a substrate
1001
, which is a silicon substrate. The optical multilayer film
1003
is composed of films of a high refractive index and films of a low refractive index, alternately layered. The high index film consists of a Ta
2
O
5
film while the low index film consists of an SiO
2
film, both of which are layered by ion beam deposition. Further an Au film is formed as an electrode
1004
a
on the optical multilayer film
1003
by sputtering.
In a manufacturing method of such tunable filter, a polyimide film (not shown) is first deposited to form a sacrificial layer, so that a cavity gap
1006
is formed by sacrificial layer etching. An Au film is then formed as an electrode
1004
b
on the polyimide film by sputtering. Then an Si
3
N
4
film is formed by sputtering, which serves as a spring portion
1005
. Further, another optical multilayer film
1002
is formed in a similar way to the optical multilayer film
1003
, i.e. by forming alternate layers of Ta
2
O
5
films used as high index film and SiO
2
films used as low index film utilizing ion beam deposition method. Also, a portion on the substrate
1001
where a transmitted light beam
1009
is to go through is selectively eliminated by silicon crystal anisotropic etching utilizing KOH. And then the sacrificial layer is removed by oxygen plasma ashing, so as to form the cavity gap
1006
.
In a tunable filter as constituted above, a clearance between the optical multilayer film
1002
and the optical multilayer film
1003
(cavity gap
1006
) can be electrostatically varied by applying a voltage between the electrodes
1004
a
and
1004
b
. By controlling the cavity gap
1006
in this way, a wavelength that matches the cavity gap out of the WDM light
1007
can be selectively transmitted, so that a transmitted light beam
1009
is obtained. The remaining portion of the WDM light is reflected and turns into reflected light
1008
. Consequently, only light of a desired wavelength passes through the filter, and the wavelength to be transmitted can be varied by controlling the cavity gap
1006
.
As a result of performance measurement of a tunable filter of the conventional constitution as above, bandwidth for −1 dB was 0.20 nm, adjacent channel cross talk value was −10.5 dB, tuning range was 1520 to 1630 nm, and insertion loss was 2.1 dB.
SUMMARY OF THE INVENTION
The foregoing conventional tunable filter may be reasonably applicable to a particular condition of use within a certain restriction, however in view of the expected increase of extent of wavelength division multiplication and demand for a more extensive flexibility of wavelength to be used, its performance is not sufficient. Specifically, for example, the adjacent channel cross talk must be smaller in order to increase a number of available wavelengths. Also it is essential to quickly change a wavelength to be added or dropped in the optical add/drop device.
In a conventional system in which a fixed wavelength filter is used, flexibility of wavelength to be used is virtually nonexistent and the filter has to be replaced as a whole in order to change a wavelength to be used, which is a great disadvantage as it requires considerable time and cost.
Also, the tunable filters disclosed in the U.S. Pat. No. 4,825,262 and U.S. Pat. No. 5,739,945 have a large adjacent channel cross talk value mainly because the Fabry-Perot etalon has only one cavity, therefore it is impossible to increase the number of available wavelengths, which is another serious disadvantage.
The present invention has been achieved in view of the foregoing problems, with an object to provide a tunable filter that can minimize the adjacent channel cross talk despite an increase of a number of available wavelengths and quickly switch a wavelength to be used, and manufacturing method thereof, and also an optical switching device comprising such tunable filter.
A tunable filter according to a first aspect of the present invention comprises a first substrate; a second substrate; a first and a fourth optical multilayer films respectively formed on confronting surfaces of the first substrate and the second substrate; a second optical multilayer film disposed with a clearance from the first optical multilayer film and supported by the first substrate through an elastically deformable spring portion; a third optical multilayer film disposed between the second optical multilayer film and the fourth optical multilayer film with a clearance from both of them and supported by the second substrate through an elastically deformable spring portion; a supporting member for mutually supporting said first substrate and said second in such a manner that a clearance between said first substrate and said second substrate can be changed; and a first, a second, and a third driving devices for respectively changing the clearance between the first and the second optical multilayer films, the clearance between the third and the fourth optical multilayer films and the clearance between the first substrate and the second substrate.
In such tunable filter, the first through the third driving devices may, for example, be electrostatic driving devices comprising a first pair of electrodes respectively provided on confronting surfaces of the first and the second optical multilayer films; a second pair of electrodes respectively provided on confronting surfaces of the third and the fourth optical multilayer films; and a third pair of electrodes respectively provided on the confronting surfaces of the first and the second substrates; for controlling the clearance between the first and the second optical multilayer films, the clearance between the third and the fourth optical multilayer films, and the clearance between the first substrate and the second substrate by changing a voltage to be applied to the respe
Assaf Fayez
NEC Corporation
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