Optical multiplexer/demultiplexer

Details Optical multiplexer/demultiplexer Optical multiplexer/demultiplexer

2001-02-15

2003-10-07

Healy, Brian (Department: 2874)

Optical waveguides

With optical coupler

Plural

C385S031000, C385S039000, C385S042000, C359S199200, C359S199200

Reexamination Certificate

active

06631223

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to an optical multiplexer/demultiplexer, and particularly to an optical multiplexer/demultiplexer which has excellent wavelength flatness in passband, has a wide rejection band, and can function over a wide waveband.
BACKGROUND OF THE INVENTION
An interleave system, which is one form of advanced wavelength multiplexing communications, requires an optical multiplexer/demultiplexer having a function such that a signal with certain channel wavelength spacings is demultiplexed to two signals with doubled channel wavelength spacings, or conversely, two signals are multiplexed to one signal.
FIG. 11
is an explanatory view showing one example of a prior art technique for coping with this demand.
FIG. 11A
shows a quartz-based plane optical wave circuit provided on a quartz substrate. This quartz-based plane optical wave circuit comprises four optical couplers
24
,
25
,
26
,
27
and waveguide pairs each comprising two waveguides with different lengths (
28
,
29
), (
30
,
31
), (
32
,
33
), for connecting the optical couplers to each other, that is, a pair of waveguides with different lengths (
28
,
29
) for connecting the optical coupler
24
to the optical coupler
25
, a pair of waveguides with different lengths (
30
,
31
) for connecting the optical coupler
25
to the optical coupler
26
, and a pair of waveguides with different lengths (
32
,
33
) for connecting the optical coupler
26
to the optical coupler
27
. Here due to a difference in optical path length, a phase difference occurs between light, which passes through the waveguide
28
, and light which passes through the waveguide
29
. The quartz-based plane optical wave circuit is designed so that, when the phase difference caused in the waveguide pair (
28
,
29
) is &phgr;, the phase difference caused in the waveguide pair (
30
,
31
) is 2&phgr; while the phase difference caused in the waveguide pair (
32
,
33
) is 4&phgr;. There are eight optical paths for allowing light to be input through an input port
20
and to be output through output ports
22
,
23
. Among them, the shortest optical path is such that light is passed through the waveguide
29
, the waveguide
31
, and the waveguide
33
in that order. The next shortest optical path is such that light is passed through the waveguide
28
, the waveguide
31
, and the waveguide
33
in that order. The phase difference in the shortest optical path and the next shortest optical path is &phgr;. Likewise, in the case of third, fourth, fifth, sixth and seventh shortest optical paths and the longest optical path, the phase differences are 3&phgr;, 4&phgr;, 5&phgr;, 6&phgr;, and 7&phgr;, respectively. Here a rectangular, periodic spectral response can be achieved by suitably determining the percentage coupling of optical couplers
24
,
25
,
26
, and
27
. Specifically, each term of Fourier series is expressed in terms of a phase difference in each optical path in such a manner that, when a rectangular periodic function is subjected to Fourier series development, the first term is expressed in terms of a component having a phase difference of &phgr; and the next term is expressed in terms of a component having a phase difference of 2&phgr;. In this case, for the optical couplers, the percentage coupling is determined according to the Fourier coefficient. Thus, when the components for the optical paths are added, a spectral response close to a rectangular shape is provided.
In the case of optical multiplexer/demultiplexers for use in interleave, it is ideal that passband and rejection band are periodically realized as a rectangular spectrum. In short, what is important for the prior art technique is to provide a rectangular spectral response by determining this period through the phase difference &phgr;, taking the phase difference created in the second-stage waveguide pair (
30
,
31
) as 2&phgr;, and taking the phase difference created in the third-stage waveguide pair (
32
,
33
) as 4&phgr; to properly determine the percentage coupling of the optical couplers.
The conventional multiplexer/demultiplexer is designed so as to function in the best state at a wavelength of 1.545 &mgr;m. The optical couplers
24
,
25
are directional couplers having a percentage coupling of about 50% at a wavelength of 1.545 &mgr;m, the optical coupler
26
is a directional coupler having a percentage coupling of about 98% at a wavelength of 1.545 &mgr;m, and the optical coupler
27
is a directional coupler having a percentage coupling of about 2% at a wavelength of 1.545 &mgr;m. The waveguide
28
is identical to the waveguide
29
in refractive index and shape of waveguide and is longer by about 2,033 &mgr;m than the waveguide
29
. Similarly, the waveguide
30
is longer by 4,066 &mgr;m than the waveguide
31
, and the waveguide
32
is longer by 8,132 &mgr;m than the waveguide
33
. The waveguides each have a core width of 6 &mgr;m and a core height of 6 &mgr;m. The difference in specific refractive index between the core and the cladding, &Dgr;, is 0.8%.
FIGS. 12 and 13
are diagrams showing wavelength loss characteristics for the conventional optical multiplexer/demultiplexer, wherein
FIG. 12
shows wavelength loss characteristics for the output port
22
in the case where light is introduced through the input port
20
, while
FIG. 13
shows wavelength loss characteristics for the output port
23
in the wavelength range of 1.546 &mgr;m to 1.550 &mgr;m. As can be seen from the drawings, a wavelength-flat passband and a wide rejection band are realized at the designed wavelength around 1.545 &mgr;m.
Further, the passband and the rejection band are repeated at periods of about 0.8 nm. This is determined according to the optical path length difference of the waveguides
28
,
29
.
In the construction wherein the differences in phase between the optical couplers are &phgr;, 2&phgr;, 4&phgr;, however, the use of optical couplers having a high percentage coupling of not less than 50% is unavoidable. That is, this construction should comprise directional couplers having a high percentage coupling. This construction, when applied to practical use, poses the following problems.
FIG. 14
shows spectral characteristics at wavelengths shorter than the designed wavelength, and
FIG. 15
spectral characteristics at wavelengths longer than the designed wavelength. As can be seen from the drawings, the level of the rejection band is lowered, and, as compared with the isolation characteristics around the wavelength 1.545 &mgr;m, a deterioration in isolation characteristics is significant. The worst isolation value at an ITU-grid wavelength ±0.08 nm in the wavelength range of 1.53 &mgr;m to 1.56 &mgr;m is 17 dB which cannot be said to be satisfactory for practical use. This is attributable mainly to the dependency of the percentage coupling of the optical couplers
24
,
25
,
26
upon the wavelength. For the optical coupler
27
, the percentage coupling is so low that the coupling length is short and the wavelength dependency is small.
FIGS. 16 and 17
are diagrams showing the influence of a dimensional error of the gap between waveguides (Gap in
FIG. 11C
) in a directional coupler caused in the preparation of the directional coupler, wherein
FIG. 16
shows wavelength characteristics in the case where the gap has been narrowed by 0.3 &mgr;m, and
FIG. 17
wavelength characteristics in the case where the gap has been widened by 0.3 &mgr;m. As shown in the drawings, since the isolation characteristics are significantly deteriorated, these directional couplers cannot be used in the optical multiplexer/demultiplexer. This is attributable to the fact that the percentage coupling of the directional coupler is likely to be influenced by the dimensional error. In the case of the optical coupler
27
, however, since the coupling length is small, the influence of the error is likely to be relatively small, whereas, for the other optical couplers
24
,
25
,
26
, the isolation characteristics are significantly influenced and deteriorated.

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