Demultiplexer and demultiplexer-receiver

Optical waveguides – With optical coupler – Plural

Reexamination Certificate

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C385S015000, C385S018000, C385S014000, C385S037000, C385S130000, C385S131000, C372S050121, C372S096000, C372S097000, C372S099000, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06404947

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a demultiplexer for separating signal light, which has been densely wavelength-multiplexed within a relatively narrow wavelength range, into multiple optical signals corresponding to their respective wavelengths and outputting those demultiplexed signals. The present invention also relates to a demultiplexer-receiver for receiving and demultiplexing wavelength-multiplexed signal light and then converting the resultant optical signals into electrical signals.
In the field of fiber-optics communications, a technique of increasing the information-carrying capacity by utilizing wavelength division multiplexing (WDM), by which a plurality of optical signals corresponding to mutually different wavelengths are combined into a single signal, is well known. Especially in recent years, a system for multiplexing four waves with respective wavelengths around 1.55 &mgr;m (each pair of which are different from each other by 3.2 nm) or even eight waves (each pair of which are different from each other by 1.6 nm) is on the verge of being implemented. And yet research and development is vigorously carried on to realize a super-high-density fiber-optics WDM network in the near future by reducing the wavelength difference to as small as 0.8 nm. Generally speaking, though, in a WDM telecommunications network, an optical signal, which has once been multiplexed on the transmitting end, should be demultiplexed on the receiving end. Accordingly, to realize a super-high-density WDM network like this, demultiplexing must be performed at a very high resolution. It is not impossible to realize that high-resolution demultiplexing using a spectroscope including a diffraction grating. However, a more cost-effective alternative would be constructing a system including either small-sized demultiplexers or an optical receiver module with those demultiplexers integrated on a semiconductor substrate, for example.
A known optical receiver module of this type, i.e., a module with demultiplexers integrated on a semiconductor substrate, is disclosed in Japanese Laid-Open Publication No. 8-46593, for example. Hereinafter, the optical receiver module will be described with reference to FIG.
8
.
FIG. 8
illustrates a cross-sectional structure for a demultiplexing and light-receiving portion of an optical receiver module as disclosed in the Japanese Laid-Open Publication No. 8-46593 identified above. As shown in
FIG. 8
, a vertical cavity filter
105
is formed as a stack of lower and upper reflectors
102
and
104
and spacer layer
103
on the principal surface of a semiconductor substrate
101
. Each of the lower and upper reflectors
102
and
104
and spacer layer
103
is formed out of a semiconductor layer. On the filter
105
, multiple receivers
106
are formed just like the same number of islands. And a reflective film
107
is formed on the backside of the substrate
101
opposite to the principal surface thereof.
The filter
105
shows a transmittance of 100% against an incoming light beam with the same wavelength as one of the resonant wavelengths of the filter
105
. In this structure, each of the resonant wavelengths is determined by the uneven thickness of the spacer layer
103
. In addition, high-reflectance wavelength bands, termed “stop bands”, exist around each resonant wavelength. An incoming wavelength-multiplexed light beam
109
, which has traveled through an optical fiber bundle
108
, impinges onto the backside of the substrate
101
. And only a part of the light beam
109
, of which the wavelength is equal to one of the resonant wavelengths, can be transmitted through the filter
105
and incident onto associated one of the receivers
106
. The remaining part of the light beam
109
, which has been reflected off from the filter
105
, is reflected by the reflective film
107
and then incident onto the filter
105
again. The thickness of the spacer layer
103
is not constant but changes horizontally, i.e., relative to the principal surface of the substrate
101
. Thus, the filter
105
has multiple resonant wavelengths for the respective receivers
106
. As a result, optical signals corresponding to mutually different wavelengths are received one after another.
A resonant wavelength of the vertical cavity filter
105
is given by
2
nL
·cos &thgr;/m
where n is a refractive index of the spacer layer
103
, L is the thickness of the spacer layer
103
at a given point of incidence, &thgr; is an angle of incidence of the incoming light beam
109
(i.e., an angle formed by the light beam
109
with a normal of incidence perpendicular to the principal surface of the substrate
101
) and m is a natural number. Accordingly, if the angle &thgr; of incidence of around 20 degrees changes by 1 degree, then the resonant wavelength of the filter
105
will change by about 0.63%. For example, when the resonant wavelength is around 1.55 &mgr;m, the change in wavelength will be about 10 nm. Stated otherwise, if the absolute value of the resonant wavelength should have a precision of 1 nm or less, then the shift in the angle &thgr; of incidence should be 0.1 degrees or less.
The known optical receiver module, however, has the following drawbacks.
Firstly, the above-identified publication does not particularly point out a method of securing the optical fiber bundle
108
onto the semiconductor substrate
101
at a predetermined angle. Thus, it is difficult even for a skilled artisan to precisely define the angle &thgr; of incidence of the incoming light beam
109
.
Secondly, according to the technique disclosed in the above-identified publication, the thickness of the spacer layer
103
is not controllable accurately enough. In general, a crystal-growing method for making the thickness variable relative to the surface of a substrate is already well known in the art (see U.S. Pat. No. 5,029,176, for example). However, this method is not precise enough to determine the absolute value of the resonant wavelength just as desired. In addition, it is usually hard to apply normal semiconductor device processing, in which a great number of devices are formed at a time on a single semiconductor wafer and then divided into respective chips after a wafer process, to the fabrication of the optical receiver modules. This is because a crystal-growing method allowing for a periodic thickness change of the wafer is needed in that case. But crystals can be grown in that manner just by a few methods among the numerous ones cited in the U.S. Patent identified above.
SUMMARY OF THE INVENTION
A first object of the present invention is allowing an incoming light beam to be incident onto the cavity filter of a demultiplexer or demultiplexer-receiver at a much more accurate angle.
A second object of the present invention is controlling the horizontal thickness change of at least one layer in the cavity filter precisely enough and thereby providing multiple selectable wavelengths for the filter through normal semiconductor device processing.
To achieve the first object, a first inventive demultiplexer includes: a semiconductor substrate; and a vertical cavity filter, which is formed on the principal surface of the substrate and transmits an incoming light beam with a predetermined wavelength. A resonant wavelength of the filter changes depending on at which point on the principal surface the light beam is incident. And the substrate has a recess with a slope that reflects or refracts the light beam and thereby makes the light beam incident onto the filter.
In the first demultiplexer, a slope that will make an incoming light beam incident onto a cavity filter is formed in a semiconductor substrate. The slope can be formed easily in the backside of the substrate, opposite to its principal surface, by a wet etching process, for example, so that an exactly predetermined angle is formed between the slope and the principal surface. In such a structure, if incoming signal light is incident onto the slope of the substrate, the light will be input to the vertical cavity filter whi

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