Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
2000-06-01
2002-08-13
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
C385S011000, C385S024000, C385S037000, C385S042000, C385S014000, C359S199200, C359S199200
Reexamination Certificate
active
06434296
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is situated in the general field of optoelectronics, and relates to more precisely an optical multiplexer/demultiplexer.
The present invention relates to a multiplexer/demultiplexer having at least three waveguides, each pair of waveguides constituting distinct grating-assisted couplers so as to couple respectively at least two predetermined wavelengths.
The multiplexer/demultiplexer according to the invention can be used in an application for multiplexing and/or demultiplexing at least three wavelengths propagating in three different transmission windows whatever the numerical values of the latter.
Such a multiplexer/demultiplexer can also be used in an optical filtering application, the first coupler filtering a given wavelength and the second coupler, a so-called dummy coupler, serving as a rejection exit for another wavelength, close to the first, so as to refine the spectral response (or transfer function) of the first coupler.
The present invention particularly finds an application in the field of optical fibre distribution networks with direct access at the subscriber. In fact, in the field of optical telecommunications, the concept of FTTH (Fibre to the Home) has become an essential point in development for operators wishing to meet the ever increasing requirements of their customers.
Such distribution networks are already widely used and principally utilise optical fibres in which optical signals propagate in different transmission windows.
The optical signals are received and transmitted, and multiplexed and demultiplexed by optical modules.
Generally, the most usual case is to use two transmission windows, a first 1.3 &mgr;m window for voice communications, and another 1.5 &mgr;m window for video distribution.
FIG. 1
is a schematic diagram illustrating the principle of optical telecommunication by FTTH.
An Optical Line Terminal (OLT) provides communication between the different customers through an optical fibre distributor.
Each customer is equipped with an Optical Network Unit (ONU).
The two optical modules, the OLT and the ONU, are advantageously identical through their design. Laser diodes (LD) are used for sending an optical signal at a given wavelength, such as 1.3 &mgr;m or 1.5 &mgr;m, and photodiodes (PD) allow reception of the said optical signals.
The different wavelengths are multiplexed or demultiplexed in two steps. First of all, a filter separates the two wavelengths used and transmitted by optical fibres, and then a coupler separates the inputs and outputs of one and the same transmission window.
In the example illustrated, the 1.3 &mgr;m window is used as an uplink channel and downlink channel for a so-called half-duplex voice communication, that is to say interference can occur between the signals propagating from the OLT to the ONU, and from the ONU to the OLT, the 1.5 &mgr;m window being reserved for video distribution in a downlink channel only.
There exist other embodiments for obtaining a full-duplex voice communication, that is to say one without interference, using for example the 1.3 &mgr;m window for the uplink channel and the 1.5 &mgr;m window for the downlink channel. This embodiment must however abandon video distribution.
The present invention seeks to produce an optical transmitter which allows full-duplex voice communication, that is to say one on two different wavelengths for the uplink and downlink channels, whilst maintaining downlink video distribution.
To this end, the invention proposes to use a first 1.3 &mgr;m transmission window allowing simultaneous bidirectional communication on two different wavelengths, at 1.3− &mgr;m and 1.3+ &mgr;m, and another high transmission rate 1.5 &mgr;m transmission window for video distribution simultaneous with voice communication.
Up to the present time, the 1.3 &mgr;m and 1.5 &mgr;m wavelengths were, in general, separated by a filter on two waveguides. The filtering function could be provided either by hybrid components provided with an adapted dielectric mirror, or by integrated optical components such as a Mach-Zehnder interferometer.
The 1.3 &mgr;m waveguide was then separated into two ports consisting of an input and an output, either by a conventional Y junction or by a 3 dB coupler.
Such a transmission method nevertheless has many drawbacks.
This is because the separation between the input and the output of the 1.3 &mgr;m wave routinely introduces a loss of 3 dB.
In addition, a “ping-pong” effect is introduced into the voice communication channel because the uplink and downlink transmissions use the same 1.3 &mgr;m window.
In addition, transmission on the 1.3 &mgr;m channel is of low throughput, typically a few tens of megabytes per second.
The present invention thus seeks to produce a bidirectional multiplexer/demultiplexer with three wavelengths which allows on the one hand simultaneous bidirectional communication on two different wavelengths and on the other hand distribution on another wavelength.
In the context of an application to direct-access networks, the present invention proposes to use two different wavelengths for a bidirectional communication in the same 1.3 &mgr;m window, for example 1.28 &mgr;m and 1.32 &mgr;m, and to separate them by means of an isotropic filter with a single passband in order not to interfere with the transmission of the optical wave at 1.5 &mgr;m.
Thus another problem which the invention seeks to resolve is to manage to produce an optical filter making it possible to separate signals propagating at wavelengths close to each other (for example 1.28 &mgr;m and 1.32 &mgr;m).
French patent N
O
2 732 478 describes a method of filtering two wavelengths by means of a codirectional coupler. Such a method is illustrated in FIG.
2
.
This patent describes a structure with two optical waveguides having a bottom confinement layer
2
, a core
3
for guiding the light and two strips
4
and
5
for loading the core and forming the optical guides. Such a structure is suitable for producing a filter, a coupling grating being etched on one of the strips
5
.
With such a filter, when the light propagates in a waveguide, all the wavelengths except that of the filter pass through the guide in the direct channel, whilst the chosen wavelength is transferred into the lateral channel in the parallel coupled waveguide.
According to a particularity of the invention described in this patent, the thicknesses of the core and strips are defined so that the two optical guides have the same modal birefringence. The core and strips have in fact a succession of alternating thin layers respectively made from binary material and quaternary material.
Thus a codirectional asymmetric coupler makes it possible to separate a given wavelength &lgr;
0
, fixed by the periodic interference &Lgr; etched on one of the strips, the other wavelengths propagating in the other waveguide whatever the state of polarisation of the signals.
The present invention seeks to produce an optical filter which makes it possible to extract a given wavelength &lgr;
0
, the coupler constituting this filter having a spectral response with a high rejection ratio and a narrow passband. This is because the efficiency of an optical filter is generally limited by the size of the secondary lobes of its transfer function.
FIGS. 3
a
and
3
b
illustrate the spectral responses respectively on the lateral channel and on the direct channel of a conventional optical filter consisting of an optical coupler such as the one described with reference to FIG.
2
. It can be seen that the &lgr;
0
passband &Dgr;&lgr; is relatively broad and that the rejection ratio &tgr; is low. Such a coupler can therefore not be used for filtering a signal at a given wavelength &lgr;
0
propagating with other signals at close wavelengths.
Different solutions have been proposed in the prior art for eliminating or reducing the secondary lobes of the spectral response of an optical filter. Such an operation is known as “apodisation” of the filter transfer function.
One particular solution
Carenco Alain
Lupu Anatolie
Alcatel
Healy Brian
LandOfFree
Optical multiplexer/demultiplexer with three waveguides does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical multiplexer/demultiplexer with three waveguides, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical multiplexer/demultiplexer with three waveguides will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2942746