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Description  |
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INTRODUCTION
The present invention is directed to an optical multiplexing device which
spatially disburses collimated multi-wavelength light from a fiber optic
waveguide into individual wavelength bands, each of which can be directed
to an individual fiber optic waveguide output line, light detector, etc.,
or multiplexes such individual wavelength bands to a common fiber optic
waveguide or other destination. In certain preferred embodiments, the
improved multiplexing devices of the present invention are particularly
well suited for dense channel wavelength division multiplexing (DWDM)
systems for fiber optic telecommunications systems.
BACKGROUND
While fiber optic cable is finding widespread use for data transmission and
other telecommunication applications, the cost of new installed fiber
optic cable presents a barrier to increased carrying capacity. Wavelength
division multiplexing (WDM) would allow different wavelengths to be
carried over a common fiber optic waveguide. Presently preferred
wavelength bands for fiber optic transmission media include those centered
at 1.3.mu.and 1.55.mu.. The latter is especially preferred because of its
minimal absorption and the commercial availability of erbium doped fiber
amplifiers. It has a useful band width of approximately 10 to 40 nm,
depending on application. Wavelength division multiplexing can separate
this band width into multiple channels. Ideally, the 1.55.mu.wavelength
band, for example, would be divided into multiple discreet channels, such
as 8, 16 or even as many as 32 channels, through a technique referred to
as dense channel wavelength division multiplexing (DWDM), as a low cost
method of substantially increasing long-haul telecommunication capacity
over existing fiber optic transmission lines. Wavelength division
multiplexing may be used to supply video-on-demand and other existing or
planned multimedia, interactive services. Techniques and devices are
required, however, for multiplexing the different discreet carrier
wavelengths. That is, the individual optic signals must be combined onto a
common fiber optic waveguide and then later separated again into the
individual signals or channels at the opposite end of the fiber optic
cable. Thus, the ability to effectively combine and then separate
individual wavelengths (or wavelength bands) from a broad spectral source
is of growing importance to the fiber optic telecommunications field and
other fields employing optical instruments.
Optical multiplexers are known for use in spectroscopic analysis equipment
and for the combination or separation of optical signals in wavelength
division multiplexed fiber optic telecommunications systems. Known devices
for this purpose have employed, for example, diffraction gratings, prisms
and various types of fixed or tunable filters. Gratings and prisms
typically require complicated and bulky alignment systems and have been
found to provide poor efficiency and poor stability under changing ambient
conditions. Fixed wavelength filters, such as interference coatings, can
be made substantially more stable, but transmit only a single wavelength
or wavelength band. In this regard, highly improved interference coatings
of metal oxide materials, such as niobia and silica, can be produced by
commercially known plasma deposition techniques, such as ion assisted
electron beam evaporation, ion beam spattering, reactive magnetron
sputtering, e.g., as disclosed in U.S. Pat. No. 4,851,095 to Scobey et al.
Such coating methods can produce interference cavity filters formed of
stacked dielectric optical coatings which are advantageously dense and
stable, with low film scatter and low absorption, as well as low
sensitivity to temperature changes and ambient humidity. The theoretical
spectral performance of a stable, three-cavity filter (tilted 12.degree.)
produced using any of such advanced, deposition methods is shown in FIG. 1
of the appended drawings. The spectral profile is seen to be suitable to
meet stringent application specifications.
To overcome the aforesaid deficiency of such interference filters, that is,
that they transmit only a single wavelength or range of wavelengths, it
has been suggested to gang or join together multiple filter units to a
common parallelogram prism or other common substrate. Optical filters are
joined together, for example, in the multiplexing device of U.K. patent
application GB 2,014,752A to separate light of different wavelengths
transmitted down a common optical waveguide. At least two transmission
filters, each of which transmits light of a different predetermined
wavelength and reflects light of other wavelengths, are attached adjacent
each other to a transparent dielectric substrate. The optical filters are
arranged so that an optical beam is partially transmitted and partially
reflected by each optical filter in turn, producing a zigzag light path.
Light of a particular wavelength is subtracted or added at each filter
(depending upon whether the element is being used as a multiplexer or
demultiplexer). Similarly, in the device of European patent application
No. 85102054.5 by Oki Electric Industry Co., Ltd., a so-called hybrid
optical wavelength division multiplexer-demultiplexer is suggested,
wherein multiple separate interference filters of different
transmittivities are applied to the side surfaces of a glass block. A
somewhat related approach is suggested in U.S. Pat. No. 5,005,935 to
Kunikani et al, wherein a wavelength division multiplexing optical
transmission system for use in bi-directional optical fiber communications
between a central telephone exchange and a subscriber employs multiple
separate filter elements applied to various surfaces of a parallelogram
prism. Alternative approaches for tapping selective wavelengths from a
main trunk line carrying optical signals on a plurality of wavelength
bands is suggested, for example, in U.S. Pat. No. 4,768,849 to Hicks, Jr.
In that patent, multiple filter taps, each employing dielectric mirrors
and lenses for directing optical signals, are shown in an arrangement for
removing a series of wavelength bands or channels from a main trunk line.
Applying multiple separate filter elements to the surface of a prism or
other optical substrate involves significant disadvantages in assembly
cost and complexity. In addition, a significant problem associated with
wavelength division multiplexing devices and the like employing multiple
discreet interference filter elements, arises from uncertainty as to the
precise wavelength of a filter element as it is manufactured. That is, in
the manufacture of multiplexing devices, wherein bandpass filter elements
are produced separately, a device employing eight individual bandpass
filters, for example, typically will require considerably more than eight
coating lots to produce the necessary eight suitable filter elements.
Bandpass filters (particularly in the infrared range) are extremely thick
and require complicated and expensive vacuum deposition equipment and
techniques. Accordingly, each coating lot can be expensive and difficult
to produce. For this reason, devices employing, for example, eight
separate interference filter elements to produce an eight channel WDM
device, have been relatively costly and have not enjoyed full commercial
acceptance.
Another problem associated with optical multiplexing devices employing
multiple individual bandpass filter elements, involves the need to mount
the elements in nearly perfect parallelism on an optical substrate. The
filter elements are quite small, typically being on the order of 1 to 5 mm
in diameter, and are, accordingly, difficult to handle with precision.
Improper mounting of the filter elements can significantly decrease the
optical accuracy and thermal stability of the device. A related problem is
the necessity of an adhesive medium between the filter element and the
surface of the optical substrate. The optical signal path travels through
the adhesive, with consequent system degradation. In optical multiplexing
devices intended for the telecommunications industry, preferably there is
as little as possible epoxy adhesive in the optical signal path.
It is an object of the present invention to provide improved optical
multiplexing devices which reduce or wholly overcome some or all of the
aforesaid difficulties inherent in prior known devices. Particular objects
and advantages of the invention will be apparent to those skilled in the
art, that is, those who are knowledgeable and experienced in this field of
technology, in view of the following disclosure of the invention and
detailed description of certain preferred embodiments.
SUMMARY OF THE INVENTION
In accordance with a first aspect, an optical multiplexing device comprises
an optical block which may be either a solid optical substrate, such as
glass or fused silica or the like, or an enclosed chamber which is hollow,
meaning either evacuated or filled with air or other optically transparent
medium. The optical block has an optical port for passing multiple
wavelength collimated light. Depending upon the application of the optical
multiplexing device, such multiple wavelength collimated light may be
passed through the optical port into the optical block to be
demultiplexed, or from the optical block as a multiplexed signal to a
fiber optic transmission line or other destination. Multiple ports are
arrayed in spaced relation to each other along a multiport surface of the
optical block. As illustrated below in connection with certain preferred
embodiments, the optical block may have more than one such multiport
surface. Each of these multiple ports is transparent to the optical signal
of one channel. Thus, each transmits a wavelength sub-range of the
multiple wavelength collimated light passed by the optical port. In an
application of the optical multiplexing device in a multi-channel
telecommunication system, each of the multiple ports typically would pass
a single discreet channel and, in combination, the channels form the
aforesaid multiple wavelength collimated light transmitted by the optical
port. A continuous, variable thickness interference filter, preferably a
multi-cavity interference filter, is carried on the multiport surface of
the optical block to provide the aforesaid multiple ports. Because this
continuous interference filter extending over the multiport surface has a
different optical thickness at each of the multiple ports, the wavelength
(or wavelength range) passed by the filter at each such port will differ
from that passed at the other ports. Thus, a single film, preferably
deposited directly onto the surface of the optical block, separately
passes optical signals for each of a number of channels at separate
locations, while reflecting other wavelengths. As noted above, the optical
block may be either solid or a hollow chamber. In the case of a solid
optical block, the multiport surface carrying the continuous, variable
thickness interference filter would typically be an exterior surface of
the block. As discussed further below, the individual ports of the
multiport surface may be bandpass filters, preferably narrow bandpass
filters transparent to a wavelength sub-range separated from the sub-range
of the next adjacent port(s) by as little as 2 nm or even less for DWDM.
Alternatively, some or all of the multiple ports could be dichroic, i.e.,
a long wavepass or short wavepass edge filter, preferably with a very
sharp transition point. The transition point of each port would be set at
a slightly (e.g., 2 nm) longer (or shorter) boundary wavelength. In a
demultiplexing operation, each port, in turn, would pass or transmit only
optic signals in the incremental range beyond the boundary wavelength of
the previous port, since all light at shorter (or longer) wavelengths
would already have been removed. Light beyond the boundary wavelength of
the new port would be reflected, in accordance with the above described
principles of operation.
The optical multiplexing device further comprises means for cascading light
within the optical block along a multi-point travel path from one to
another of the multiple ports. In a demultiplexing operation, the optical
signals would enter the optical block at the aforesaid optical port and
travel to the multiple ports (acting in this case as output ports) along
the aforesaid multi-point travel path. The signal for each individual
channel is transmitted out of the optical block at its corresponding port;
other wavelength are reflected, or bounced, back to cascade further along
the optical travel path within the optical block. It will be understood
that at the last output port(s) there may be no remainder light to be
reflected. It will also be understood from this disclosure, that the
optical multiplexing device can operate in the reverse or both directions.
The cascading means preferably comprises a reflective film carried on a
second surface of the optical block, either as a continuous coating
spanning the multi-point travel path of the cascading light signals, or as
multiple discreet reflector elements. The optical block would most
typically be rectilinear, having the reflective film on or at a second
surface of the optical block opposite and parallel to the multiport
surface carrying the aforesaid continuous interference filter. This second
film can be a broadband high reflector, that is, a film coating which is
highly reflective of all wavelengths of the channels which combine to form
the multiple wavelength collimated light, or can act as a second
interference filter transparent at spaced locations (i.e., at some or each
of the bounce points) to the optical signal of one or more of the
channels. In either case, the interference filter and reflective film on
spaced surfaces of the optical block operate to cascade optical signals
through the optical block in a multiple-bounce sequence starting at (or
finishing at) the optical port through which the multiple wavelength
collimated light passes. This multiple-bounce cascading will be further
described below in connection with certain preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
Certain preferred embodiments of the invention are discussed below with
reference to the accompanying drawings in which:
FIG. 1 is a graph showing the theoretical performance of a high quality
multi-cavity, dielectric, optical interference filter.
FIG. 2 is a schematic illustration of a first preferred embodiment of an
optical multiplexing device, specifically, a dense channel wavelength
division multiplexing device for an eight channel fiber optic
telecommunications system or like application;
FIG. 3 is a schematic illustration of an alternative preferred embodiment
of an optical multiplexing device in accordance with the invention,
specifically, a dense channel wavelength division multiplexing device for
an eight channel fiber optic telecommunications system or like
application;
FIG. 4 is a schematic illustration of another alternative preferred
embodiment of an optical multiplexing device in accordance with the
invention, specifically, a dense channel wavelength division multiplexing
device for an eight channel fiber optic telecommunications system or like
application; and
FIGS. 5 and 6 are schematic illustrations, in cross-section, of the
continuous, variable thickness, three cavity interference filter of the
optical multiplexing device of FIG. 2.
It should be understood that the optical multiplexing devices and
interference filter illustrated in the drawings are not necessarily to
scale, either in their various dimensions or angular relationships. It
will be well within the ability of those skilled in the art to select
suitable dimensions and angular relationships for such devices in view of
the foregoing disclosure and the following detailed description of
preferred embodiments.
DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
The optical multiplexing device, as disclosed above, has numerous
applications including, for example, in fiber optic telecommunication
systems. Optical multiplexing devices of this design are particularly
useful, for example, in test equipment and the like, as well as laboratory
instrumentation. For purposes of illustration, the preferred embodiments
described below in detail are dense channel wavelength division
multiplexing devices which can solve or reduce the above described
problems of individually mounting multiple filter elements to an optical
substrate for each individual signal channel, the problems of cost and
complexity involved in mu | | |
