Optical filter

Details Optical filter Optical filter

2002-08-07

2004-04-20

Glick, Edward J. (Department: 2882)

Optical waveguides

With optical coupler

Particular coupling function

C385S040000, C385S042000, C398S053000, C398S082000, C398S085000

Reexamination Certificate

active

06724957

ABSTRACT:

This application is based on Patent Application No. 2001-241367 filed Aug. 8, 2001 in Japan, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical filter, and more specifically, to an optical filter that splits or combines wavelength multiplexed optical signals.
2. Description of the Related Art
An optical filter that splits or combines optical signals according to wavelengths is a key device in a wavelength multiplexing optical communication system. In particular, an interleaver that can extract or insert every other of a plurality of optical signals spaced at optical frequency intervals of &Dgr;f can be combined with an arrayed wavelength grating or a dielectric multilayer optical filter to easily and economically provide a high-density wavelength multiplexing optical communication system.
Conventional interleavers are implemented in a lattice type configuration having a large number of Mach-Zehnder interferometers connected together. Such an interleaver is described in, for example, Oguma et al., “Flat-passband interleave filter with 200 GHz channel spacing based on planar lightwave circuit-type lattice structure”, Electronics Letters, Vol. 36, No. 15, pp. 1299 to 1300.
FIG. 1
shows the configuration of a conventional interleaver. The interleaver is composed of a substrate
100
and the following components formed on the substrate
100
and connected together in the following order: input optical waveguides
101
a
and
101
b
, a first optical coupler
102
, a Mach-Zehnder interferometer consisting of a pair of optical waveguide delay lines
103
a
and
103
b
having an optical path difference 2&Dgr;&tgr;, a second optical coupler
104
, a pair of optical wavelength delay lines
105
a
and
105
b
having an optical path difference &Dgr;&tgr;, a third optical coupler
106
, and output optical waveguides
107
a
and
107
b.
The splitting ratio of the three optical couplers constituting the interleaver is set so that the first optical coupler is 10%, the second optical coupler is 70%, and the third optical coupler is 50%. Further, the optical path difference &Dgr;&tgr; is set at 10 psec so that the free spectral range (period on an optical frequency axis) of transmission characteristics is 100 GHz.
FIG. 2
shows the transmission characteristics of the interleaver. These transmission characteristics are observed at a first and second output waveguides when an optical signal is input to a first input waveguide. Both transparencies form the first input waveguide to the first and second output waveguides have stop bands where the optical signal is prohibited and pass bands where the optical signals is allowed to go through in an alternative manner. Further, when light at a frequency is allowed to go to the first waveguide (the pass band), the light is not allowed to go to the second waveguide (the stop band), and vice versa.
As shown in
FIG. 3
, an optical multi/demultiplexer of 50 GHz channel spacing can be implemented by combining the interleaver
110
with optical multi/demultiplexers
111
and
112
of 100 GHz channel spacing. In general, it is more difficult to produce an optical multi/demultiplexer with a narrower channel spacing. However, an optical multi/demultiplexer with a substantially narrow channel spacing can be provided by using an optical multi/demultiplexer of a relatively wide channel spacing which can be easily produced and the interleaver
110
. To avoid degrading the transmission characteristics of an optical multi/demultiplexer of a narrow channel spacing, the transmission characteristics of the interleaver must be such that for a pass band, transmittance is close to 1 over a relatively wide range of optical frequencies, and for a stop band, transmittance is close to 0 over a relatively wide range of optical frequencies.
However, the conventional interleaver has the following problems: first, the conventional interleaver has only two outputs but has more than two parameters to be set. Accordingly, adjustment of the parameters is very complicated. In the example shown in
FIG. 1
, coupler splitting ratio must be set for three locations, and phase must be set for two locations. Thus, a total of five parameters must be set.
Second, the conventional interleaver has delay lines arranged in series, so that if half wave plates are used to compensate for birefringence in the optical waveguides, a half wave plate must be inserted into each stage. As a result, excess losses increase. Further, to compensate for the dependence of the optical waveguides on temperature, material (hereinafter referred to as “temperature compensating material”) whose refractive index change with a temperature has an opposite sign to that of the optical waveguides must be inserted into each stage. As a result, excess losses increase.
Third, instead of realizing the flat characteristics of pass bands using a smaller number of stages, the conventional interleaver has non-linear phase characteristics. As a result, the pass bands have chromatic dispersion. Such chromatic dispersion may cause signal quality to be degraded when interleavers are used in a long-distance high-speed optical transmission system.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical filter which allows parameters to be easily adjusted, which allows birefringence and temperature dependence to be easily compensated for, and which is essentially free from dispersion.
To attain this objects, an optical filter comprises an input optical waveguide, a first optical coupler that splits an optical signal guided by the input optical waveguide into two parts, at least one optical splitting means connected to an output of the first optical coupler, two groups of optical waveguide delay lines (the minimum number of constituents of the group is 1) connected to outputs of the optical splitting means, or an output of the optical coupler and an output of the optical splitting means, at least one optical combining means for combining any lights from the two groups of optical waveguide delay lines, a second optical coupler having a symmetric power splitting ratio and connected to outputs of the optical combining means, or an output of the optical combining means and the optical waveguide delay line, and an output optical waveguide connected to an output of the second optical coupler, all of said components being formed on a substrate, and wherein delay time provided by one of the two groups of optical waveguide delay lines is set to be &tgr;
0
+2n&Dgr;&tgr;+&agr;
n
(&tgr;
0
and &Dgr;&tgr; are positive real numbers, n is an integer that varies with optical waveguide delay line, |&agr;n|≦&lgr;/u, &lgr; is a wavelength, and u is the speed of light propagating through the waveguides), and delay time provided by the other group of optical waveguide delay lines is set to be &tgr;
0
+(2 m+1)&Dgr;&tgr;+&bgr;
m
(m is an integer that varies with optical waveguide delay line, |&bgr;m|≦&lgr;/u).
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.


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patent: 5351317 (1994-09-01), Weber
patent: 5930414 (1999-07-01), Fishman et al.
patent: 6212315 (2001-04-01), Doerr
patent: 6351581 (2002-02-01), Doerr et al.
patent: 6546158 (2003-04-01), Fondeur et al.
patent: WO 00/70402 (2000-11-01), None
Sasayama et al., “Coherent Optical Transversal Filter Using Silica-Based Single-Mode Waveguides”, Electronic Letters, IEE Stevenage, vol. 25, No. 22, pp. 1508-1509, (Oct. 26, 1989).
Jinguji et al., “Two-part Optical Wavelength Circuits Composed of Cascaded Mach-Zehnder Interferometers With Point-Symmetrical Configurations”, Journal of Lightwave Technology, IEEE, vol. 14, No. 10, pp. 2301-2310, (Oct. 1996).
Oguma et al., “Flat-top and Low-Loss WDM Filter Composed of Lattice-form Interleave Fi

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