Wavelength filter

Details Wavelength filter Wavelength filter

1999-07-26

2001-12-04

Ngo, Hung N. (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

C385S131000, C385S132000

Reexamination Certificate

active

06327404

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a wavelength filter, and more specifically, to a wavelength filter for selectively extracting a specific wavelength.
BACKGROUND OF THE INVENTION
A wavelength filter for extracting a specific wavelength can be realized for example using wavelength selectivity of diffraction gratings with various forms. FIGS.
11
(
a
)~
11
(
c
) show a conventional configuration in which a diffraction grating is formed and disposed to be adjacent to a waveguide and also its effective width (referred as coupling width hereinafter) to be constant in the light propagation direction z. FIG.
11
(
a
), FIG.
11
(
b
) and FIG.
11
(
c
) respectively show its plan view, front view, and right side view. Aplane diffraction grating
12
as shown in FIG.
11
(
a
) is formed on a substrate
10
, and then a waveguide
14
is formed on it. It is widely known that a cladding layer is sometimes formed on the side of the waveguide
14
. The width of the diffraction grating
12
is constant for the distance L of the light propagation direction z.
The diffraction grating
12
can be easily formed using conventional semiconductor crystal growth technology. For instance, after forming a film with a predetermined thickness on the substrate
10
, a periodic corrugation should be made using etching. Then the diffraction grating
12
is formed by crystal-growing material having different refractivity on the corrugation. There are several configurations for realizing the waveguide
14
itself. The waveguide
14
is logically illustrated in FIGS.
11
(
a
)~
11
(
c
) . The diffraction grating
12
is sometimes formed on the waveguide
14
.
FIG. 12
shows wavelength selective characteristics of a wavelength filter employing a diffraction grating with a constant coupling width as shown in FIGS.
11
(
a
)~
11
(
c
). The abscissa axis shows wavelengths, and the ordinate axis shows output intensity. A central wavelength is 1.55 &mgr;m. It is simulated for the structure that InGaAsP waveguides #1 and #2 (their band gap wavelength &lgr;g =1.24 &mgr;m) respectively having a rib with a width of 4 &mgr;m and a height of 0.2 &mgr;m are formed 0.8 &mgr;m apart from each other and a diffraction grating is disposed between the waveguides #1 and #2 as shown in FIG.
13
. The length L of the diffraction grating is preset to 3 mm. The InGaAsP waveguides #1, #2 and the diffraction grating are imbedded in InP.
FIG. 13
shows the structure in the surface orthogonal to the light propagation direction.
FIG. 12
shows an output optical spectrum when backward coupling between the waveguides #1 and #2 is employed. It is clear from
FIG. 12
that, in a conventional configuration in which the coupling width of the diffraction grating is constant in the light propagation direction, the wavelength selectivity is insufficient and the side mode is not effectively suppressed.
FIGS.
14
(
a
)
14
(
c
) show a conventional configuration in which a coupling width of a diffraction grating linearly widens and narrows in the light propagation direction z. Similarly to FIGS.
11
(
a
)~
11
(
c
), FIG.
14
(
a
), FIG.
14
(
b
) and FIG.
14
(
c
) respectively show its plan view, front view, and right side view. In this conventional configuration, also, a plane diffraction grating
22
shown in FIG.
14
(
a
) is formed on a substrate
20
, and then a waveguide
24
is formed on it. It is widely known that a cladding layer is sometimes formed on the side of the waveguide
24
.
The coupling way of the diffraction grating
22
with the waveguide
24
, namely the coupling width of the diffraction grating
22
varies in the light propagation direction. That insufficient.
Outputs of the conventional wavelength filters still contain many unnecessary components of other wavelength bands besides a selected wavelength, therefore the improvement of the wavelength filter has been expected.
SUMMARY OF THE INVENTION
An object of the invention is to provide a wavelength filter capable of greatly suppressing unnecessary components.
A wavelength filter according to the invention comprises a waveguide and a diffraction grating disposed adjacent to the waveguide which coupling width with the waveguide widens at a central part and narrows at both ends in a predetermined coupling area along the light propagation direction of the waveguide, and the wavelength filter is characterized in that the variation of the coupling width of the diffraction grating along the light propagation direction of the waveguide is large at the central part and decreases as approaching to both ends in the coupling area. Because of the above characteristics, the function for suppressing components of other wavelength bands excluding a specified wavelength becomes powerful and also the side mode characteristics improves. is, in the first half of the distance L in the light propagation direction z, the diffraction g rating
22
is gradually superimposed on the waveguide
24
along the light propagation direction z, and in the latter half, the diffraction grating
22
in reverse gradually separates from the waveguide
24
along the light propagation direction z. The variation of the coupling way of the diffraction grating
22
with the waveguide
24
, namely the variation of the ends of the diffraction grating
22
in the light propagation direction z is linear on both first and latter halves of the distance L.
FIG. 15
shows wavelength selectivity of a wavelength filter in which a coupling width of a diffraction grating linearly widens in the first half and narrows in the latter half as shown in FIG.
14
. The abscissa axis shows wavelengths, and the ordinate axis shows output intensity. A central wavelength is 1.55 &mgr;m. Similarly to the conventional configuration in FIGS.
11
(
a
)~
11
(
c
), it is simulated for a waveguide configuration which comprises the waveguide parameters shown in FIG.
13
. Similarly to
FIG. 12
,
FIG. 15
shows an output optical spectrum on condition that the backward coupling of the waveguides #1 and #2 is employed. Although the side mode characteristics are improved compared to the case in
FIG. 12
, it is still
For instance, the coupling width of the diffraction grating is set to vary at the n (n>1) power of the distance of the light propagation direction and become the largest at the central part of the coupling area.
The diffraction grating also can be disposed to stretch from one direction on the side of the waveguide to cover the waveguide. The diffraction grating may be a rhombus with its respective sides are incurved and disposed so that at least two vertexes of the rhombus are superimposed on the waveguide. The former configuration is easier to produce.
Furthermore, a second waveguide can be disposed adjacent to the forgoing waveguide. Accordingly, the invention can be applied to a wavelength selective filter due to coupling between two waveguides.


REFERENCES:
patent: Re. 37051 (2001-02-01), Welch et al.
patent: 6047096 (2000-04-01), Augustson

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