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| United States Patent |
6,639,702
|
|
Chiaretti
, et al.
|
October 28, 2003
|
Optical module for access networks to wide band communication systems and
relevant production method
Abstract
An optical module for access networks to wideband 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, and services of more strictly
television nature such as the distribution of the CATV.
| Inventors:
|
Chiaretti; Guido (Novate Milanese, IT);
Fincato; Antonio (Cameri, IT)
|
| Assignee:
|
Italtel SpA (Milan, IT)
|
| Appl. No.:
|
230484 |
| Filed:
|
March 16, 1999 |
| PCT Filed:
|
July 23, 1997
|
| PCT NO:
|
PCT/EP97/03970
|
| PCT PUB.NO.:
|
WO98/04943 |
| PCT PUB. Date:
|
February 5, 1998 |
Foreign Application Priority Data
| Jul 26, 1996[IT] | MI96A1588 |
| Current U.S. Class: |
385/15; 385/24 |
| Intern'l Class: |
H04J 014/02; G02B 006/26; G02B 006/42 |
| Field of Search: |
385/15,24
359/130
|
References Cited [Referenced By]
U.S. Patent Documents
| 4441181 | Apr., 1984 | Winzer et al. | 359/130.
|
| 5042895 | Aug., 1991 | Chouinard et al. | 385/2.
|
| 5267336 | Nov., 1993 | Sriram et al. | 385/2.
|
| 5361157 | Nov., 1994 | Ishikawa et al. | 359/168.
|
| 5457760 | Oct., 1995 | Mizrahi | 385/37.
|
| 5572615 | Nov., 1996 | Emori | 385/92.
|
| 5737104 | Apr., 1998 | Lee et al. | 359/124.
|
| 5764820 | Jun., 1998 | De Dobbelaere et al. | 385/14.
|
| 5764825 | Jun., 1998 | Mugino et al. | 385/24.
|
| Foreign Patent Documents |
| 0713109 | May., 1996 | EP.
| |
| 9325014 | Dec., 1993 | WO.
| |
Primary Examiner: Chan; Jason
Assistant Examiner: Payne; David C.
Attorney, Agent or Firm: Birch, Stewart, Kolasch & Birch, LLP
Claims
What is claimed is:
1. An optical module comprising:
a first multiplexer receiving a signal input to said optical module along a
first waveguide and having at least a first signal component, in a first
optical band, and a second signal component, in a second optical band,
said first multiplexer separating said first and second signal components
and outputting said first and second signal components via separate
outputs; and
a second multiplexer operatively connected to said first multiplexer to
receive said first signal component along a second waveguide, said second
multiplexer containing a reflective element and operating to output a
signal, along a third waveguide, in the opposite direction of said signal
input to said optical module, wherein said reflecting element is
integrated into said second multiplexer, and said first, second, and third
waveguides are separate waveguides and are substantially parallel.
2. An optical module according to claim 1, wherein
at least one of said first multiplexer and said second multiplexer is a
Mach-Zehnder interferometer.
3. An optical module according to claim 1, wherein
at least one of said first multiplexer and said second multiplexer is a
directional coupler.
4. A method for producing an optical module comprising the steps of:
depositing a first multiplexer on a single semiconductor wafer, said first
multiplexer having an input on one side and two outputs on the opposite
side of the input;
depositing a second multiplexer on said single semiconductor wafer, said
second multiplexer having an input and an output on one side and a
reflective element on the opposite side;
depositing connections on said single semiconductor wafer, such that a
connection to the input of said first multiplexer is deposited, a
connection to one output of said first multiplexer is deposited, a
connection from the second output of said first multiplexer to the input
of said second multiplexer is deposited, and a connection to the output of
said second multiplexer is deposited;
defining an etch area at the second multiplexer ends by means of a mask;
chemically etching the area outside said mask by means of an oxide etch
technique in such a way to verticalize the front face of said etch area of
said second multiplexer; and
depositing on said front face a layer of light reflecting material in order
to define a reflecting element integrated in said second multiplexer.
5. The method according to claim 4, wherein
said light reflecting material comprises gold.
6. The method according to claim 4, wherein
at least one of the deposited first multiplexer and the deposited second
multiplexer is a Mach-Zehnder interferometer.
7. The method according to claim 4, wherein
at least one of the deposited first multiplexer and the deposited second
multiplexer is a directional coupler.
8. An optical module comprising:
a first multiplexer receiving a signal input to said optical module and
having at least a first signal component, in a first optical band, and a
second signal component, in a second optical band, said first multiplexer
separating said first and second signal components and outputting said
first and second signal components via separate outputs; and
a second multiplexer operatively connected to said first multiplexer to
receive said first signal component, said second multiplexer containing a
reflective element and operating to output a signal in the opposite
direction of said signal input to said optical module, wherein said
reflecting element is integrated into said second multiplexer, wherein
said reflecting element is integrated on the front face of an etched and
verticalized surface of said second multiplexer.
9. An optical module according to claim 8, wherein at least one of said
first multiplexer and said second multiplexer is a Mach-Zehnder
interferometer.
10. An optical module according to claim 8, wherein at least one of said
first multiplexer and said second multiplexer is a directional coupler.
Description
This application is the national phase under 35 U.S.C. .sctn.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 .lambda.1 (for
example 1310 nm), may also support transmissions made in a second channel
or window, whose central or main wavelength is .lambda.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 .lambda.1 and the signals allocated in the optical
band .lambda.2 which is therefore suitable to
i) make available at a first output the optical signals allocated in the
optical band .lambda.1, that is allocated in the 1310 nm band;
ii) make available at a second output the signals allocated in the optical
band .lambda.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.degree.
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.div.30 mm and therefore for production volumes
up to several millions of modules/year large investments are necessary, in
particular correlated to the cost of these furnaces.
Since, however, as mentioned above, the above mentioned piece of waveguide
has a radius of around 180.degree., making use of usual phosphor-doped
glass to construct the above mentioned optical support or wave guide 3, if
the radius is lower than a predetermined amount, the optical losses become
unacceptable.
For example, the compliance with the above mentioned width of 55 mm imposes
that the radius is not over 2 mm, while, making use of the normal
phosphor-doped glass, the minimum obtainable radius is equal to
approximately 10 mm. Making use of this type of glass, reducing the curve
with respect to this value, the losses become unacceptable.
BACKGROUND ART
In order to obtain the limitation of the above mentioned dimensions, it has
been suggested to abandon the approach aimed at the reduction of the above
mentioned radius and to achieve this objective by making use of a
particular WDM*, inside which, in suitable position, is placed a dichroic
interferential filter 4, that is a reflecting element as illustrated in
FIG. 3 relating to the .lambda.2 at 1550 nm.
As known, this interferential filter is composed of a glass on which are
laid plurality of layers of oxides which give it the property of being
transparent to the optical band .lambda.1 and of reflecting instead the
optical band .lambda.2, which is then coupled to the second output of the
WDM* unit.
This WDM* unit is composed of a section of waveguide which runs parallelly
to the wave guide of input 1 for a length L/2, where L is the length of
the coupling necessary to determine the transfer of all the optical power
from the waveguide 1 to the waveguide 2.
The waveguide constructing this section of length L/2 is in practice
continued to construct said piece of wave guide which transports the
optical band .lambda.2 on the side of the module provided to support the
connectors.
In reality, the waveguide which corresponds to the output of the WDM* unit
is cut transversally, usually through an operation of cutting or incision
or of excavation, then the above mentioned interferential filter is
positioned in the furrow thus obtained.
The optical module constructed according to the known art fully achieves
the objective of the above mentioned dimensional limitation, but the
relevant production process presents the following problems.
The cutting operation may be carried out with fairly modest tolerances
(.+-.100.div.50 .mu.m) which involve considerable processing waste
correlated to the non-compliance with the above mentioned length L/2 of
the WDM unit.
The cutting operation must be followed by a costly lapping operation to
polish the end of the interrupted waveguide.
The positioning of this interferential filter must be made by hand, module
by module, using expensive equipment for the automatic positioning which
annuls the advantage of reduced cost deriving from the reduced dimensions
and of the resulting low investment in equipment for the manufacture of
the optical guide.
To sum up, the module constructed according to the invention is a module of
hybrid type constructed in part with integrated optics (waveguide) and in
part with traditional microoptics (interferential filter).
The use of particular glass doped with germanium which has the intrinsic
power of strongly guiding the light is also described in literature.
The use of this technology involves, however, high investments in the
purchase of furnaces which deposit glass then destined to be submitted to
a doping process using the germanium as doping element.
The invention necessary for the purchase of the relevant equipment for the
production of glass doped with germanium is, however, similar to that for
the production of phosphor-doped glass and estimable at around one million
US dollars.
OBJECT OF THE INVENTION
The object of this invention is to identify an optical module which does
not need the use of microoptical component of any type, such as the above
mentioned interferential filter.
Another object is to identify a production method which makes it possible
to obtain the above mentioned dimensional limitation using only integrated
optical components.
A further object is to construct the above mentioned optical module without
the use of special glass, such as the glass doped with germanium mentioned
above.
DISCLOSURE OF THE INVENTION
This object is achieved by means of the optical module for access networks
to wide band communication systems of the type including at least:
a first multiplexer in wavelength suitable to receive in input an optical
support, coming from a predetermined side of the module, in which transit
signals allocated in a first and in a second optical band and suitable to
make available on a first output the signals allocated in this first
optical band and on a second output the signals allocated in this second
optical band, and
a second optical support, one end of which is adapted to correspond to a
predetermined side of the optical module;
a reflecting element placed downstream from one of the optical branches of
the first multiplexer in wavelength,
characterized by the fact that
to the second output of the first multiplexer in wavelength is connected a
second multiplexer in wavelength of the propagating and counterpropagating
type incorporating the reflecting element, the other end of the second
optical support being connected to the output of the second multiplexer in
wavelength.
An additional object of the present invention is a method for the
production of an optical module for access networks to wide band
communications systems of the type including at least
a first multiplexer in wavelength suitable to receive in input a first
optical support, coming from a predetermined side of the module, in which
transit signals allocated in a first and in a second optical band and to
make available on a first output the signals allocated in the first
optical band and on a second output the signals allocated in the second
optical band, and
a second optical support, one end of which is adapted to correspond to the
predetermined side of the optical module;
a reflecting element placed downstream from one of the optical branches of
the first multiplexer in wavelength,
characterized by the fact that it provides the steps of:
laying on a single water of silicon optical supports forming the first
multiplexer in wavelength, the optical support and a second multiplexer in
wavelength of the propagating and counterpropagating type;
performing a structure of definition of an area of attack of the ends of
the optical supports forming the second multiplexer in wavelength defining
it photolithographically by means of a masking process;
chemically attacking the area not involved in the masking with an attack
technique of the glass used to construct these optical supports until they
are totally removed in such a way as to verticalize their front face;
laying on said front face of the optical supports a layer of material
reflecting the light.
BRIEF DESCRIPTION OF THE DRAWINGS
The feature of the present invention which are believed to be novel are set
fort with particularity in the appended claims.
The invention, together with further objects and advantages thereof, may be
understood with reference to the following description taken in
conjunction with the accompanying drawings, and the several figures of
which like referenced numerals identify like elements, and in which:
FIG. 1, already described, shows the architecture of the access network to
a wide band communication system;
FIG. 2, already described, shows the general architecture of the optical
module in question;
FIG. 3, already described, shows an optical module constructed in
conformance with the known art;
FIG. 4 shows a first embodiment of the optical module constructed according
to the invention;
FIG. 5 shows the conformation of a Mach-Zehnder interferometer;
FIG. 6 shows a section of the module in FIG. 4 made according to the plane
A--A;
FIG. 7 shows a second embodiment of the optical module constructed
according to the invention.
DETAILED DESCRIPTION OF A FIRST EMBODIMENT OF THE OPTICAL MODULE
CONSTRUCTED ACCORDING TO THE INVENTION
In FIG. 4 is illustrated a first embodiment of the optical model indicated
in the invention which includes a first WDM, here countermarked with WDM1,
preferentially constructed by means of a Mach-Zehnder interferometer
which, as known, is a device suitable to realize the insertion or
extraction of a signal or of a certain optical channel of a certain
wavelength on an optical fiber carrying another optical signal or optical
channel.
With reference to the diagram in FIG. 5, a Mach-Zehnder interferometer is
essentially composed of a first directional input coupler whose structure
is essentially that of two optical paths (for example two waveguides)
brought near o one another for a certain "coupling" length L1 and of a
second directional output coupler with coupling length L2. Unlike a
manufacturing technique with "merged fibers", in a form of integrated
manufacture the two optical paths in the two directional couplers of input
and output are not merged but are defined in such a way as to develop
parallely one to the other at a certain distance of separation (not shown
in figure for graphic needs).
The intermediate stage of the device is essentially a phase shifting stage
suitable to determine a certain difference .DELTA.L of the optical path on
the two branches of the device.
The characteristic attentuation curve of a common Mach-Zehnder
interferometer is substantially of periodic type and characterized by
relatively selective peaks which are exploited to inject a certain
frequency (wavelength centered to one of these peaks) in fiber and/or to
extract it.
Similar to everything described with reference to the basic layout of FIG.
2, to the first output of WDM1 unit is connected the optical power
splitter/combiner 2 on which transit the luminous radiations allocated in
the optical band .lambda.1 which therefore reach the detector RIV, the
laser LA_D, similarly to everything described with reference to FIG. 2,
sends luminous radiations on the second branch of the splitter/combiner.
The luminous radiations allocated in the optical band .lambda.2 correspond
instead to the second output of the unit WDM1 and reach a particular
Mach-Zehnder interferometer WDM2, here subsequently named propagating and
counterpropagating interferometer.
This interferometer includes, in fact, a first directional coupler with a
predetermined length A/2 and a portion of this intermediate stage .DELTA.L
composed of two pieces of wave guide of different length and such that
their difference is equal to .DELTA.L/2, that is half of the difference L
between the lengths of the two guide sections of the interferometer WDM1.
Preferentially the end portion of the interferometer WDM2 is rectilinear
for the reasons indicated below.
This rectilinear portion is protected by a mask and then submitted to
chemical attack in such a way as to verticalize the front surfaces of the
two waveguides.
On these vertical surfaces is then laid in known manner a metal layer such
as a layer of gold as illustrated in the section of FIG. 6 which shows the
wafer of silicium Si sectioned to show the presence of the support in
silicium and of the waveguide composed of the lower cladding, of the core
and of the upper cladding.
The layer of metal acts as reflecting element which reflects the luminous
radiations allocated in the optical band .lambda.2 towards the piece of
the waveguide 3 which ends on the front face of the module.
The advantages of the invention are evident. The module is entirely
constructed in integrated optics not incorporating any component in
microoptics. Moreover, it does not present any critical aspect since the
masks may be positioned with a decidedly higher precision that obtainable
with said cutting operation.
Finally, if the chemical attack is carried out on said rectilinear sections
placed at the end of the unit WDM2, no degree of precision of the
positioning of the masks is required with priority.
In other words, the invention makes it possible to construct a passive chip
of very limited dimensions and equal to approximately 2.div.20 mm, even
lower than those of similar chips of known type which present dimensions
equal to approximately 5.div.30 mm and as mentioned above make use of
components in microoptics.
DETAILED DESCRIPTION OF A SECOND EMBODIMENT OF THE INVENTION
FIG. 7 details a second embodiment of the invention which differs from the
previous one for the fact that the unit WDM1 is preferentially constructed
by means of a directional coupler and also the unit WDM2 is constructed by
means of a particular type of directional coupler.
As known, a directional coupler may be constructed by means of two optical
paths (for example two wave guides) brought near to each other for a
certain length of "coupling" L. In particular, if this length is suitably
dimensioned, the luminous radiations which transit on a first optical path
and which present wavelength coinciding with that for which the coupler
has been sized, "couple" at the second optical path with negligible
losses.
The first coupler WDM1 is of traditional type and has therefore a coupling
length L, while the second coupler WDM 2 is of particular type and has a
coupling length L/2.
Similarly to everything described with reference to the first embodiment,
the terminal portion of this coupler is protected by a mask then submitted
to chemical attack in such a way as to verticalize the front surfaces of
the two waveguides.
On these vertical surfaces is then deposited in known manner a layer of
metal as illustrated previously with reference to FIG. 6.
The layer of metal acts also in this case as reflecting element with the
same functionalities illustrated above.
The second embodiment makes it possible to contain, in equal manner to said
first embodiment, the width of the module. As regards the length of the
module, this second embodiment makes it possible to obtain a further
dimensional limitation, to the advantage of the number of devices
producible in the unit of time and of the number of the wafers of silicium
required to produce them, since it presents a width equal to 2.div.3 mm
and a length equal to approximately 15 mm.
While a particular embodiment of the present invention has been shown and
described, it should be understood that the present invention is not
limited thereto since other embodiments may be made by those skilled in
the art without departing from the scope thereof.
It is thus contemplated that the present invention encompasses any and all
such embodiments covered by the following claims.
* * * * *
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