Optical module for access networks to wide band...

Details Optical module for access networks to wide band... Optical module for access networks to wide band...

1999-03-16

2003-10-28

Chan, Jason (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C385S015000, C385S024000

Reexamination Certificate

active

06639702

ABSTRACT:

This application is the national phase under 35 U.S.C. §371 of prior PCT International Application No. PCT/EP97/03970 which has an International filing date of Jul. 23, 1997 which designated the United States of America.
FIELD OF THE INVENTION
This invention refers to an optical module for access networks to wide band communication systems composed of passive optical networks PON, in such a way as to permit the transportation in particularly efficient manner of flows of numeric and/or analog information, relating to different types of services such as:
telephone services in a broad sense, intended as traditional telephone services and data transmission services;
services of more strictly television nature such as the distribution of the CATV.
The access network is commonly identified by one or more connecting interfaces (of type V) in power-station and by various types of interfaces (of type S) for the user terminals, normally placed end to end to apparatuses positioned near the users.
The apparatuses which are used for the above purpose in the access network, and which form the so-called PON access system, are represented by a line termination (Optical Line Termination, or briefly OLT), generally placed or positioned near a switching station, and by a plurality of network units (Optical Network Units or briefly ONU), typically placed near the users, connected to the same OLT through a passive optical network, according to a layout like that illustrated in FIG.
1
.
The passive optical network, which forms the so-called access network infrastructure, has typically tree or point-multipoint structure, where the root is connected to the OLT and the terminal branches to each ONU, and is composed of pieces of optical fiber interlinked by passive optical components, called power splitter/combiners, which make it possible to split the optical power between the outputs or recombine the optical signals present at the inputs.
The operators of the above mentioned wide band communication systems have assumed a plurality of methods through which to link the single residential users to the PON network.
One of these methods provides, for example, that the passive optical network reaches a pavement or a building, and, after having made a conversion of the optical signals into electrical signals, the latter are sent to the single users, for example through an equal number of pieces of coaxial cable.
A second method provides instead that the passive optical network is extended until it reaches the single residential users and the invention is preferentially applied when this second method is adopted. The optical module in question is indicated in
FIG. 1
with MO and has the function of sending:
to a first termination unit NT_T the signals which transit on the PON relating to the above mentioned telephone services;
to a second termination unit NT_V the signals which transit on the PON relating to the above mentioned television services.
As known, the transmission systems on optical fiber predominantly use predetermined “windows” or bands of the optical spectrum through which the transmission of the signals along the fibers takes place with a minimum attentuation.
Carrying signals or communication channels, each with its own precisely defined wavelength, as produced by a relevant laser generator, included in one of these privileged windows, or bands modulable in intensity (commonly in digital or analog mode), may be transmitted along an optical fiber with extremely low losses:
The simultaneous transmission of various communication channels belonging to a certain band, or window, or channel on a same fiber, is made possible by operating in Wavelength Division Multiplexing (WDM).
Therefore, in the present context, the term “channel” indicates a certain band of the optical spectrum or “windows”, used for the transmission of wavelength division multiplexed (WDM) optical signals.
In other words, the term optical band indicates a continuous wavelength interval which may house various optical channels, for example one or more channels for data transmission, one or more channels of telephone type, one or more channels of television type with simple or high definition etc.
In relation to the above, communications of a certain type such as telephone communications by voice or by data, maybe defined between 1260 and 1350 nm. With the same logics, the so-called 3rd useful window, whose passband goes from approximately 1480 to 1580 nm, may be destined or reserved for video transmissions via cable.
Therefore a certain transmission system with optical fibers, operating with carrying signals with a wavelength included in the passband of a first channel or window, whose central or main wavelength is &lgr;1 (for example 1310 nm), may also support transmissions made in a second channel or window, whose central or main wavelength is &lgr;2 (for example 1550 nm).
At each user the problem arises of separating the signals transmitted in the 1310 nm band from the signals transmitted in the 1550 nm band.
When the operator provides the allocation of digital signals of telephonic type (for example for traditional and/or data telephony) in the 1330 nm band and the signals for diffusive television in the 1550 nm band, the installation is required on each user's permises of the above mentioned optical module MO which is illustrated in detail in FIG.
2
.
As required by some operators, this module includes:
a device named WDM (Wavelength Division Multiplexing) to whose input is connected an optical support
1
on which transit both the signals allocated in the optical band &lgr;1 and the signals allocated in the optical band &lgr;2 which is therefore suitable to
i) make available at a first output the optical signals allocated in the optical band &lgr;1, that is allocated in the 1310 nm band;
ii) make available at a second output the signals allocated in the optical band &lgr;2, that is allocated in the 1550 nm band;
an optical power splitter/combiner suitable to split in practically equal manner on two output branches the optical power associated to the input signal;
an RIV detector suitable to supply a level signal proportional to the optical power which crosses the fiber connected to one of the output branches of the optical power splitter/combiner
2
;
a laser diode LA_D at 1310 nm suitable to send on the other branch of the splitter/combiner optical signals derived from the digital signals of telephonic type, which, as known, need a transmission of bi-directional type;
a further optical support
3
connected to the second output of this WDM which presents such a radius that the output direction is at 180° with respect to that of the input in order to provide the presence of all the interlinking organs on the front side of the optical module MO.
Since, as mentioned above, there is one optical module for each user, its cost must be very limited in order not to be an obstacle for the diffusion of the multimedial services in question.
The most promising technology for the purpose of limiting the costs is the “glass on silicon” technology, but it should be borne in mind that the use of this technology is marked by the present of some steps of the production process which need rather long execution times, and in particular one of these steps needs a rather long stay inside special furnaces of the sublayer of silicium for the laying of the layers of glass forming the waveguides.
To permit a better appreciation of the advantages deriving from the precepts indicated in this invention, it is pointed out that the cost of these furnaces is quantifiable at approximately 1 million US dollars for each set of furnaces necessary for the purpose.
The limitation of the production costs of the optical module in question may therefore only be achieved if it is possible to limit its dimensions from which to be able to construct on a single wafer of silicium a very high number of optical modules.
As indication it is pointed out that the usual dimensions of this module are today approximately 5÷30 mm and therefore for production vo

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