Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
1998-10-26
2001-04-24
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Particular coupling structure
C385S037000, C385S050000
Reexamination Certificate
active
06222963
ABSTRACT:
DESCRIPTION
1. Technical Domain
This invention relates to a phased array device, or PHASAR, and a process for manufacturing this device.
This type of device is also called AWG (for Arrayed-Waveguide Grating) and is applicable particularly to:
the domain of telecommunications by optical fibers that use wave length multiplexing and demultiplexing, and
the domain of optic spectrometry.
The device according to the invention will be called a PHASAR throughout the rest of this description.
2. State of Prior Art
A PHASAR is an integrated optical device based on a particular type of dispersive grating.
It comprises an array of optical microguides which create a periodic phase shift.
Further information on this subject is given in documents (1), (2) and (12) which, like the other documents referenced later, are mentioned at the end of the description.
This type of dispersive grating does not use a facets manufacturing technique as is the case in diffraction gratings used in conventional optics and integrated optical devices forming etched diffraction gratings.
Refer to document (3) for further information on this subject.
FIG. 1
is a diagrammatic view of a known PHASAR with a conventional double S shape.
The PHASAR in
FIG. 1
comprises a central array of light guides or microguides
2
and two planar areas
4
and
6
.
One side of the planar area
4
is optically coupled to one side of the microguides array
2
.
Similarly, one side of the planar area
6
is optically coupled to the other side of the microguides array
2
.
In the example shown, the other side of area
4
is optically coupled to means of inputting a light wave composed of set of light guides or microguides
8
.
Similarly, the other side of area
6
is optically coupled to light wave output means composed of a set of light guides or microguides
10
.
Thus, the microguides
8
form the PHASAR input microguides in FIG.
1
and the microguides
10
form the output microguides from this PHASAR.
There may be a single input microguide and several output microguides (for example for wave length demultiplexing) or several input microguides and a single output microguide (for example for wave length multiplexing).
There could also be optical components other than the PHASAR input and/or output microguides.
A PHASAR may also be connected to input and/or output optical fibers.
It can also be monolithically integrated into input light sources and possibly connected to output optical fibers.
A PHASAR may also be monolithically integrated into output photodetectors and possibly connected to input optical fibers.
FIG. 1
illustrates the case of demultiplexing for a PHASAR with E inputs and N outputs.
A polychromatic light wave with wave lengths denoted &lgr;i
1
, &lgr;i
2
, . . ., &lgr;iN, is injected into one
i
of the input channels
8
, where i varies from 1 to E, the channels being microguides in this example.
The PHASAR is designed to provide light with wave lengths &lgr;i
1
, &lgr;i
2
, . . ., &lgr;iN respectively on the N output microguides
10
.
The planar areas
4
and
6
are regions in which light can propagate freely.
The planar area
4
enables the width of the polychromatic light wave to expand, in order to light up the entire width of the central microguides array
2
.
A combined interference and focusing effect occurs along the planar area
6
, located at the output from the microguides array
2
, which makes it possible to separate wave lengths &lgr;i
1
, &lgr;i
2
, . . ., &lgr;iN from each other, and to obtain light with wave lengths &lgr;i
1
, &lgr;i
2
, . . . , &lgr;iN, respectively at the output from the N microguides
10
.
Therefore the central array, associated with area
6
, forms the PHASAR dispersing element.
In general, the particular feature of a PHASAR lies in the process of creating the periodic phase shift necessary to separate wave lengths.
This phase shift is obtained by means of the microguides array
2
, the number of microguides being denoted M.
The optical paths corresponding to these M microguides are different from each other.
The difference D between two consecutive optical paths, called the optical step difference, is equal to a constant.
This optical step difference D satisfies the following equation:
D=∫r
k
n
k
ds
k
−∫r
k+1
n
k+1
ds
k+1
=2&pgr;
p&lgr;
m
(1)
in which
p is the order of the diffraction grating formed by the PHASAR
&lgr;
m
represents the average wave length of the PHASAR, also called the central wave length of the PHASAR
k is the number of a microguide in the PHASAR central array, the index k varying from 1 to M
n
k
is the effective index at a point on the curved abscissa S
k
along microguide number k
S
k
is the curved abscissa along microguide number k.
The first curved integral is calculated along the path &Ggr;
k
varying from input point A
k
to output point B
k
of microguide number k.
Similarly, the second curved integral is calculated along the path &Ggr;
k+1
from input point A
k+1
to output point B
k+1
of microguide number k+1.
For example,
FIG. 1
shows path &Ggr;
1
varying from point A
1
to point B
1
on microguide number
1
, and path &Ggr;
M
from point A
M
to point B
M
on microguide number M.
Note that if necessary, the PHASAR in FIG.
1
and also in other figures can also be used in the opposite direction, based on the principle of inverse return of light.
In this case, the input becomes an output and the output becomes an input.
Thus in this description, the words “input” and “output” are used for simplification, but it would be more accurate to talk about input/output.
There are many advantages of a PHASAR compared with devices using etched diffraction gratings made using integrated optics techniques (see document (3)).
Some of the main advantages are:
production of a PHASAR with a single masking level,
the possibility of achieving a low level of losses (2 to 3 dB) instead of 5 to 7 dB in case of devices using etched diffraction gratings,
for a PHASAR, the possibility of working with several inputs and several outputs,
with a PHASAR, the possibility of increasing the spectral density (in other words reducing the difference between dispersed wave lengths when demultiplexing) up to values of the order of 0.008 nm (frequency 10 GHz)—see document (4)—due the use of operation with very high orders p of the diffraction grating (p typically being equal to 100), without introducing any additional losses.
Known PHASARs have disadvantages.
Firstly, for applications with a high spectral density, it is absolutely essential to adjust the wave length of a PHASAR as a function of light sources.
Since light sources themselves have a perfectly defined wave length, the objective is to very precisely fix the central wave length or the average wave length V&lgr;
m
m of the PHASAR.
The problem is the same in the spectrometry domain.
Dispersed wave lengths have to be measured in absolute terms.
The error on the wave length is related to the error on the optical step difference D.
There are three additional main sources of uncertainty:
the difference between the required index and the obtained index
an average index gradient on the width of the central microguides array
a systematic error on the length.
Consider an example for which an experimental set up was made (see document (12)).
In this example, the difference between the multiplexed/demultiplexed wave lengths &Dgr;&lgr; is equal to 1.6 nm, the pitch (p) of the PHASAR diffraction grating considered is equal to 60 and the required central wave length is equal to 1.55 &mgr;m.
An uncertainty calculation carried out using approximate data on the three causes of errors mentioned above and starting from equation (1) gives a relative uncertainty on the optical step difference equal to:
&Dgr;D/D very close to 1.6×10
−3
.
This thus gives an uncertainty &Dgr;&lgr;
m
on the central wave length of the PHASAR &lgr;
m
which is very close to 2.4 nm.
Consequently, the uncertainty on &lgr;
m
may be greater than the spectral difference &Dgr;&
Delisle Vincent
Grand Gilles
Pouteau Patrick
Anderson Kill & Olick P.C.
Commissariat A l'Energie Atomique
Font Frank G.
Lauchman Layla G.
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