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Description  |
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TECHNICAL FIELD
This invention relates to optical transmission systems.
BACKGROUND OF THE INVENTION
In the rapidly-expanding market for office communications, computer
interconnections, local area networks, and factory automation--to name a
few examples--fiber-optic point-to-point data links offer a cost-effective
and functionally superior alternative to copper interconnections.
Multimode fiber-optic data links are intended for applications in which
distances are relatively short (up to a few kilometers in length), and
where cost, size, and reliability are paramount. Such requirements are
characteristic of local area networks or campus computing facilities,
where moderate amounts of information are transmitted among several
locations. This is in contrast to the long-haul transmission market, which
involves extremely high capacity and long distances. As a result of the
different requirements, short-haul data links generally use low-cost
light-emitting diode (LED) sources, multimode fiber, and simple junction
photodetectors. In contrast, long-haul applications use semiconductor
lasers, single-mode fiber, and more complex receivers.
Single-mode fiber transmission systems often use one fiber and some form of
optical or electronic bidirectionality to achieve bidirectional
communications. In contrast, bidirectional communications via short-haul
data links are presently effected through the use of a pair of multimode
fibers, one for each direction of transmission. Most commonly, identical
LED transmitters are used with both fibers, so that both fibers transport
light of the same wavelength. The wavelengths that are commonly used are
either the 0.87 .mu.m wavelength radiation emitted by gallium aluminum
arsenide (GaAlAs) diodes or the 1.31 .mu.m wavelength radiation emitted by
indium gallium arsenide phosphide (InGaAsP) diodes.
The components and the operation of single-mode fiber transmission systems
are expensive; in contrast, those of the multi-mode data links are
relatively inexpensive. However, some of this cost benefit of short-haul
data links is offset by the need for two data links to construct a
bidirectional communications system. It would be advantageous if this
benefit offset could be avoided.
SUMMARY OF THE INVENTION
This invention is directed to meeting these and other needs and overcoming
disadvantages of the prior art. Generally according to the invention, an
optical bidirectional data link is constructed using a single
wavelength-division multiplexed multi-mode fiber. Such a link comprises a
multimode fiber, and a first and a second optical transceiver. The first
optical transceiver includes a first light-emitting diode (LED) generating
light at a first wavelength (e.g., 0.85 .mu.m) which is optically coupled
to a first end of the fiber, and a first optical detector detecting light
at a second wavelength (e.g., 1.3 .mu.m) different from the first
wavelength and which has the first end of the fiber optically coupled
thereto. The second optical transceiver includes a second LED generating
light at the second wavelength which is optically coupled to a second end
of the fiber, and a second optical detector detecting light at the first
wavelength and which has the second end of the fiber optically coupled
thereto.
Because it uses only a single fiber, the optical bidirectional data link
constructed according to the invention avoids the benefit offset vis-a-vis
single-mode transmission systems experienced by conventional short-haul
data links. And because it can use the inexpensive components that are
already employed by multi-mode transmission technology, the invention
reinforces the cost advantage that multi-mode systems have over
single-mode systems. Since it uses conventional multimode fiber, the
invention can be retrofitted into existing multimode transmission systems
to immediately double their data-carrying capacity. And in new data-link
installations, the invention can be used to cut the cost of each data link
substantially in half.
According to another aspect of the invention, an optical transceiver for an
optical bidirectional data link having a single multi-mode fiber comprises
an LED for generating light having a first wavelength and transmitting the
generated light into the fiber, and an optical detector for detecting
light received from the fiber and having a second wavelength different
from the first wavelength. This transceiver is preferably manufactured as
a single integrated-circuit device, thereby providing a small and
inexpensive transceiver that is easy to install in a circuit pack.
According to a further aspect of the invention, an optical coupler is used
for coupling an optical transceiver to a multi-mode fiber in an optical
bidirectional data link, and this optical coupler comprises a light pipe
that defines a first surface along its length for positioning adjacent to
an emitting portion of an LED and a detecting portion of an optical
detector of the optical transceiver, defines at one end a reflective
second surface for positioning in front of the emitting portion of the LED
and the detecting portion of the optical detector and forming a
substantially 45.degree. angle with the first surface, and defines at the
other end a lens for optically coupling the emitted/detected light to/from
an end of the multi-mode fiber. A simple single-lens optical coupler that
is relatively inexpensive and easy to make and install is thus employed to
couple both the transmitter and the receiver of the optical transceiver to
the multi-mode fiber.
These and other advantages and features of the invention will become more
apparent from the following description of an illustrative embodiment of
the invention taken together with the drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a cross-section view of an optical bidirectional data link that
embodies an illustrative implementation of the invention;
FIG. 2 is a top view of an optical coupler and a transceiver of the data
link of FIG. 1; and
FIG. 3 is a front view of the optical coupler and transceiver of FIG. 2.
DETAILED DESCRIPTION
FIG. 1 shows a single-fiber bidirectional data link in cross-section. The
data link comprises a single multimode optical fiber 130, a pair of
optical transceivers 100, 150, and a pair of optical couplers 111, 161
each one of which couples one of the transceivers 100, 150 to a different
end of fiber 130. Optical fiber 130 conventionally comprises a core 132
surrounded by a cladding 131. Transceiver 100 comprises an optical
transmitter in the form of a surface-emitting light-emitting diode (LED)
101 that emits radiation having wavelengths around 0.85 .mu.m, and an
optical receiver in the form of a large-scale (e.g., at least about 1
mm.sup.2) optical detector diode 102 and a dielectric optical filter 103
that has an optical pass band at wavelengths around 1.3 .mu.m and blocks
wavelengths around 0.85 .mu.m. Conversely, transceiver 150 comprises an
optical transmitter in the form of a surface-emitting LED 151 that emits
radiation having wavelengths around 1.3 .mu.m, and an optical receiver in
the form of a large-scale optical detector diode 152 and a dielectric
optical filter 153 that has an optical passband at wavelengths around 0.85
.mu.m and blocks wavelengths around 1.3 .mu.m. Consequently, multimode
fiber 130 is wavelength-division multiplexed and its core 132 carries
light having wavelengths around 0.85 .mu.m and around 1.3 .mu.m in
opposite directions. The range of wavelengths emitted by LED 101 is about
100 nm, and the range of wavelengths emitted by LED 151 is about 110 nm.
This results in about 350 nm of separation between the two lightwave
spectra being conducted by fiber 130 in opposite directions, which ensures
that the two spectra do not interfere with each other.
LEDs 101, 151, detectors 102, 152, and filters 103, 153 may be separate
discrete components. Preferably, however, for ease and low cost of
manufacturing and assembly and for minimizing circuit pack "real estate"
occupied by the transceivers, transceivers 100 and 150 are each fabricated
as an integrated device on a common substrate 104, 154, respectively, by
conventional integrated circuit (IC) fabrication processes. Electrical
connections to transceivers 100 and 150 are made via leads 105-106 and
155-156, respectively. These leads also serve to mount transceivers 100
and 150 onto printed-circuit wiring boards. Both surface mounting and
through-hole mounting is envisioned.
Each transceiver 100, 150 is optically coupled to multimode fiber 130 by
its own optical coupler 110, 160. Optical coupler 110 is a glass rod that
is substantially semi-circular along its length (i.e., in cross-section),
as can be seen in FIG. 3. Its flat side 114 is positioned over (e.g.,
glued to) filter 103 and the emitting surface of LED 101. The end of the
glass rod that lies above filter 103, and LED 101 is cut flat at an angle
of 45.degree. from flat side 114, as can be seen in FIG. 2, to form a
planar light-reflective surface 111 that reflects light 122 emitted by LED
101 into the body of the glass rod which acts as a light pipe 112, and
that reflects light 121 from light pipe 112 to filter 103 and therethrough
to detector 102. Any light 120 emitted by LED 101 that is reflected by
surface 111 to filter 103 is blocked by filter 103 from reaching detector
102, thereby ensuring that emissions of the local transmitter will not
interfere with local detection of remote transmissions. The other end of
the glass rod of optical coupler 110 forms a convex lens 113 that faces an
end of multimode fiber 130 which is positioned at the focal point 115 of
the lens 113. Illustratively, lens 113 is formed by the conventional
process of arcing (melting) the end of the glass rod. Lens 113 focuses
light 122 emitted by LED 101 and arriving through light pipe 112 onto core
131 of fiber 130, and collimates light 121 emanating from fiber 130 into
light pipe 112 for transmission to its detector 102.
Optical coupler 160 is identical to optical coupler 110 and comprises a
semi-circular glass rod having a flat surface 164, a reflective surface
161, a light pipe 162, and a convex lens 163 with a focal point 165 at
which is positioned the other end of fiber 130.
Present-day state-of-the-art LEDs are high-powered devices that can
generate about 14 mW of power, or about 11.5 dBm. Coupling losses between
transceivers 100 and 150 and fiber 130 are about 6 dB at each end. Losses
in fiber 130 are about 2 dB/km in the worst case (at 0.85 .mu.m
wavelength). Losses due to only a fraction of the received light being
projected by a surface 111, 161 onto a detector 102, 152 are estimated to
be about 12 dB (assuming that the detecting surface area of a detector
102, 152 is about 1/16th of the surface area of reflecting surface 111,
161). Assuming a length of 3 km for fiber 130, detector 102, 152 sees an
optical power of (11.5 dBm - 6 dB - 6 dB - 6 dB - 12 dB), or -18.5 dBm.
Since typical photodetector sensitivity is about -30 dBm, this provides a
more-than-adequate safety margin of about 11.5 dB for effective signal
detection by detectors 102 and 152.
Of course, various changes and modifications to the described illustrative
embodiment will be apparent to those skilled in the art. For example,
light of wavelength other than 0.85 .mu.m or 1.3 .mu.m may be used--most
notably wavelengths of 1.55 .mu.m, which have the benefit of low
attenuation during fiber transmission, although 1.5 .mu.m LEDs are
presently relatively expensive. Also, the multimode fiber can be either
step-index or graded-index type of fiber, with the step-index fiber being
used for very short distances up to about 2 km, and the graded-index fiber
being used for longer distances up to about 20 km. Furthermore, for
extremely short distances, up to about 100 m, plastic fiber can be used,
with optical sources operating at wavelength of 650 nm and 850 nm. Such
changes and modifications can be made without departing from the spirit
and the scope of the invention and without diminishing its attendant
advantages. It is therefore intended that such changes and modifications
be covered by the following claims.
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