Wavelength multiplexer and optical unit

Details Wavelength multiplexer and optical unit Wavelength multiplexer and optical unit

2001-03-16

2002-09-03

Palmer, Phan T. H. (Department: 2874)

Optical waveguides

With optical coupler

Plural

C385S014000, C385S045000

Reexamination Certificate

active

06445849

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical unit having optical waveguides, particularly an optical unit having a so-called MMI (Multi-Mode Interference) type optical waveguide. According to the present invention it is possible to provide a wavelength multiplexer of an extremely good quality.
With the recent development of multi-media communication, including the Internet, studies are becoming more and more extensive about WDM (Wavelength Division Multiplexing) technique which is intended to attain a high-speed large-capacity communication. As one of optical parts which are important for building a WDM communication system there is known a wavelength multiplexer which combines or branches wavelengths of light having plural wavelengths. Above all, what is attracting attention of many concerns from the standpoint of attaining the reduction of cost and size and a high function is a method wherein optical waveguides using quartz (glass) or a polymer for example and a wavelength multiplexer are formed on a substrate and optical transmitter and receiver are mounted thereon to achieve integration.
As the wavelength multiplexer there is known, for example, a directional coupler type or a Mach-Zehnder interferometer type. Further, in connection with the technique advantageous to the reduction in size of a module, there is known, for example, such a technique as is disclosed in Japanese Patent Laid Open No. Hei 8-190026 (Article 1).
FIG. 1
shows a filter type wavelength multiplexer described in the above Article 1. In this optical wavelength multiplexer, linear optical waveguides
1
and
2
are crossed and an optical filter
4
is disposed in a cross point of the two. In the illustrated example, a WDM signal is divided into reflected light and transmitted light by utilizing wavelength transmitting and reflecting characteristics of the optical filter
4
. According to this structure it is necessary to make design in such a manner that an intersecting point
3
of the axes of the two optical waveguides
1
and
2
which intersect each other at an angle of 2&thgr; lies on an equivalent reflection center plane
5
of the optical filter
4
. In
FIG. 1
, central axes of the optical waveguides
1
, and
2
are indicated at
6
, and
7
respectively.
SUMMARY OF THE INVENTION
It is the first object of the present invention to enlarge the tolerance for a deviation of an installed position of reflecting means typical of which is an optical filter installed within an optical guide in an optical unit utilizing light reflected by the reflecting means, the optical unit being typified by a wavelength multiplexer. In other words, it is the first object of the invention to minimize the increase of loss in the optical unit based on a deviation of the installed position of the reflecting means.
It is the second object of the present invention to diminish an optical crosstalk in the above optical unit, especially a wavelength multiplexer.
A typical form for achieving the above first object of the invention can take the form of, for example, a wavelength multiplexer which is formed on a plane substrate and which functions to combine or branch wavelengths of a signal light having plural wavelengths. According to the present invention it is possible to achieve the above first and second objects together.
A typical mode of the present invention resides in an optical unit having optical waveguides, including at least first, second and third optical waveguides, a fourth optical waveguide capable of propagating light in a multi-mode, and an optical filter disposed perpendicularly to a traveling direction of light in the fourth optical waveguide, the first optical waveguide being connected to a first end face of the fourth optical waveguide, the second and third optical waveguides being connected to predetermined individual positions of a second end face opposed to the first end face of the fourth optical waveguide, the first and second end faces of the fourth optical waveguide being end faces intersecting the traveling direction of light in the fourth optical waveguide, and the fourth optical waveguide being an optical waveguide capable of propagating light in a multi-mode such that upon input of light having a first wavelength from either the second or the third optical waveguide, light corresponding to the light input of the first wavelength can be propagated into the first optical waveguide after passing through the optical filter by the propagation of light in the fourth optical waveguide, and upon input of light having a second wavelength from either the second or the third optical waveguide, light corresponding to the light input of the second wavelength can be propagated into a light input-free optical waveguide out of the second and third optical waveguides through reflection by the optical filter.
The principle of the present invention will now be described.
FIG. 7
is schematically illustrates in what manner light of a wavelength passing through a filter
15
in wavelength multiplexer according to the present invention is incident from a waveguide
12
(distance from a center line: D), then is propagated through a multi-mode interference type waveguide
10
and is emitted to a waveguide
11
. In the multi-mode interference type waveguide
10
, for example, a signal light incident from the second optical waveguide
12
excites various modes of lights in the multi-mode interference type optical waveguide
10
. Although in
FIG. 7
show excited states of zero-, first- and second-order modes, there may be excited higher modes. Since the propagation velocities of the excited modes are different from one another, there occur phase differences from one another with propagation through the multi-mode interference type waveguide(as shown FIG.
7
), and a light intensity distribution, which is the sum of the modes, changes as light is propagated through the multi-mode interference type waveguide. This change is periodical and it is known that at a cycle Lp light intensity distribution is reproduced again into the same shape as an input section (self-imaging of the multi-mode waveguide). The light intensity distribution at the position of Lp/2 which is half of the said period corresponds to a reflected image obtained by folding back the light intensity distribution of the input section symmetrically with respect to the center line of the multi-mode optical waveguide. In the present invention, the length (L) of the multi-mode waveguide is set at the above Lp/2 and the center of the first optical waveguide
11
is placed at the center of peak (i.e., the position of distance D from the center line) of a symmetric optical intensity distribution which occurs at an outlet of the multi-mode optical waveguide, whereby the incident light from the second optical waveguide can be guided again through the optical waveguide
11
with scarcely any loss.
Also as to the case where light of a wavelength reflected by the filter
15
is incident on the above wavelength multiplexer, the same argument as above also applies except that the light is reflected by the filter
15
. More particularly, if the filter is installed at a position of L/2=(Lp/2)/2 from the inlet, the light incident from the waveguide
12
is reflected by the filter
15
and thereafter reproduces, at the inlet of the multi-mode waveguide, a light intensity distribution which is symmetric with respect to the center line in comparison with that obtained at the time of incidence. Therefore, if a waveguide
13
is installed at a position (the position of distance D from the center line) symmetric with the waveguide
12
relative to the center line, the light incident from the optical waveguide
12
can be guided again through the optical waveguide
13
with little loss.
In this case, part of the reflected light is propagated reverse through the optical waveguide
12
, creating a reflected return light (resulting in poor optical directivity). However, by arranging the optical waveguides
12
and
13
so that the spacing D between b

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