U.S. patent application number 10/287109 was filed with the patent office on 2003-03-13 for multi-channel optical transceiver.
Invention is credited to Alwan, James J., Bloom, Scott H., Chan, Victor J..
Application Number | 20030048513 10/287109 |
Document ID | / |
Family ID | 22757601 |
Filed Date | 2003-03-13 |
United States Patent
Application |
20030048513 |
Kind Code |
A1 |
Bloom, Scott H. ; et
al. |
March 13, 2003 |
Multi-channel optical transceiver
Abstract
An optical transceiver such as used, for example, in a wireless
optical network (WON), includes multiple laser sources including a
first laser source configured to transmit a first output channel
beam having a first optical characteristic and at least a second
laser source configured to transmit a second output channel beam
having a second optical characteristic; multiple detectors
including a first detector configured to detect a first input
channel beam having the first optical characteristic and at least a
second detector configured to detect a second input channel beam
having the second optical characteristic; and multiple apertures
including a first aperture through which the first output channel
beam and the second input channel beam pass and a second aperture
through which the second output channel beam and the first input
channel beam pass.
Inventors: |
Bloom, Scott H.; (Encinitas,
CA) ; Chan, Victor J.; (San Diego, CA) ;
Alwan, James J.; (Ramona, CA) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET
FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
22757601 |
Appl. No.: |
10/287109 |
Filed: |
November 1, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10287109 |
Nov 1, 2002 |
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09860078 |
May 16, 2001 |
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6490067 |
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60204360 |
May 16, 2000 |
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Current U.S.
Class: |
398/139 |
Current CPC
Class: |
H04B 10/1125 20130101;
H04B 10/2587 20130101 |
Class at
Publication: |
359/152 ;
359/114; 359/122 |
International
Class: |
H04B 010/24; H04J
014/06 |
Claims
What is claimed is:
1. An optical transceiver comprising: a plurality of laser sources
including a first laser source configured to transmit a first
output channel beam having a first polarization and at least a
second laser source configured to transmit a second output channel
beam having a second polarization different from the first
polarization; a plurality of detectors including a first detector
configured to detect a first input channel beam having the first
polarization and at least a second detector configured to detect a
second input channel beam of the second polarization; and a
plurality of apertures including a first aperture through which the
first output channel beam and the second input channel beam pass
and a second aperture through which the second output channel beam
and the first input channel beam pass.
2. The transceiver of claim 1 wherein the first polarization
comprises transverse electric polarization and the second
polarization comprises transverse magnetic polarization.
3. The transceiver of claim 1 wherein at least one of the laser
sources comprises a laser diode that emits an output field that is
one of substantially transverse electric and substantially
transverse magnetic.
4. The transceiver of claim 1 wherein at least one of the detectors
comprises an avalanche photodiode and a polarizer.
5. The transceiver of claim 4 wherein the polarizer comprises one
of a transverse electric polarizer and a transverse magnetic
polarizer.
6. The transceiver of claim 1 further comprising a plurality of
beamsplitters including a first beamsplitter associated with the
first aperture and a second beamsplitter associated with the second
beamsplitter.
7. The transceiver of claim 6 wherein at least one of the
beamsplitters comprises a polarizing beamsplitter.
8. An optical transceiver comprising: a laser source configured to
transmit an output channel beam having a first polarization; a
photodetector configured to detect an input channel beam having a
second polarization different from the first polarization; an
aperture through which the output channel beam and the input
channel beam pass; and a beamsplitter, arranged in an optical path
of the aperture and the laser source, and configured to pass the
output channel beam from the laser source to the aperture and to
reflect the input channel beam from the aperture to the
photodetector.
9. A wireless optical communication method comprising: using a
first aperture to transmit a first output channel beam having a
first polarization and to receive a first input channel beam of a
second polarization different from the first polarization; and
using a second aperture to transmit a second output channel beam
having the second polarization and to receive a second input
channel beam of the first polarization.
10. An optical transceiver comprising: a plurality of laser sources
including a first laser source configured to transmit a first
output channel beam having a first polarization and at least a
second laser source configured to transmit a second output channel
beam having a second polarization; a plurality of detectors
including a first detector configured to detect a first input
channel beam having the first polarization and at least a second
detector configured to detect a second input channel beam of the
second polarization; and a plurality of apertures including a first
aperture through which the first and second output channel beams
pass and a second aperture through which the first and second input
channel beams pass.
Description
RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 09/860,078, filed May 16, 2001, which claims the benefit of
U.S. Provisional Application No. 60/204,360, filed May 16, 2000,
the disclosures of which are incorporated herein by reference in
their entireties.
FIELD OF THE INVENTION
[0002] The present application relates to optical transceiver
technology, for example, as used in a wireless optical network
(WON).
BACKGROUND
[0003] Wireless optical networks (WONs) are becoming increasingly
popular in the telecommunications market as a strategy to meet
last-mile demand, enabling reliable high-bandwidth connectivity
previous available only to customers directly connected to fiber or
cable. An example of a WON is Airfiber's OptiMesh system, which is
described generally in U.S. Pat. No. 6,049,593, and co-pending U.S.
patent application Ser. No. 09/181,043, entitled "Wireless
Communication Network."
[0004] Historically used by military and aerospace industry, WON
technology has evolved into systems with backup and redundant
optical links, providing high reliability and fiber-like bandwidth
to customers located up to a kilometer away from buried fiber. Such
systems are being deployed to commercial buildings in urban area,
breaking the so-called "last-mile" bottleneck. These WONs provide
higher bandwidth than Radio Frequency (RF) wireless systems and are
considerably less expensive to deploy than laying fiber.
[0005] FIG. 1 illustrates an example of a WON application. As shown
therein, facilities 104 (e.g., commercial office buildings) can be
linked to a high bandwidth network 102 (e.g., a fiber-based
network) by means of optical transceivers 106 and 107, which use
"open-air" or "free-space" laser beams to maintain wireless, high
bandwith communication links 108 among each other. The central, or
main, optical transceiver 107 can have a communication link 110
(e.g., either wired or wireless) to the network 102, and thereby
serve as a hub for the other optical transceivers 106.
[0006] FIG. 2 shows an example of a conventional wireless optical
transceiver 106. As shown therein, the transceiver 106 is composed
of two basic elements: an output channel 200 for transmitting a
laser beam (modulated or otherwise impressed with data) to another
transceiver in the WON, and input channel 202 for receiving a
modulated laser beam from another transceiver in the WON. Each of
the input and output channels is composed of three basic
components. The output channel 200 includes a laser diode (LD),
which emits a laser beam of a predetermined wavelength (in this
example, 785 nanometers) that passes through a diffuser 206 and
which is focused by optics 204 (e.g., a plano aspheric lens). An
incoming beam, for example, from another transceiver in the WON, is
received by optics 204 of the input channel 202, passed through a
bandpass filter 210 and ultimately received by a photodetector
(PD), e.g., an avalanche photodiode.
[0007] The open-air laser beams used by WONs to transmit and
receive data pose a potential threat to human eye safety. The
collimated, beam-like quality of a laser results in very high
irradiance (also known as "power density" or "flux"), which can
damage tissues in the human eye causing serious conditions such as
photokeratitus ("welder's flash") and cataracts.
[0008] Accordingly, several laser safety standards have come into
existence that specify and regulate the parameters of lasers
operating in environments that may expose the human eye to laser
radiation. In general, three main aspects of regulations exist for
lasers and their usage: Class definitions, Accessible Emission
Limits (AEL), and Maximum Permissible Exposure (MPE). The class
definitions provide non-technical descriptions understandable to
lay-persons, AELs define the classification breakpoints, and MPEs
are based on biophysical data and indicate actual tissue damage
thresholds.
[0009] Class definitions--for example, Class 1, 2, 3, or 4 provide
an abbreviated way to readily communicate a hazard level to a user.
Class 1 represents lasers that are safe under reasonably
foreseeable conditions, including the possibility of a human eye
being exposed, either aided (e.g., through binoculars) or unaided,
to a laser beam. At the other end of the spectrum, a Class 4 laser
is capable of producing hazardous diffuse reflections that may pose
skin and fire hazards. As an example, to meet the most stringent
standard--class 1--a laser operating at 785 nm must be limited in
power density such that the power collected by a human eye exposed
to the laser is no greater than 0.56 milliwatts (the class 1 AEL
for 785 nm lasers). Various factors such as the distance from the
eye to the laser during exposure, and whether the viewing is aided
or not, have a significant impact on how much power is collected by
the eye.
[0010] The present inventors recognized that, while increased
demand for WON bandwidth and link range generally would require the
power densities of lasers used in WON transceivers to be increased,
eye safety standards and concerns for human ocular safety represent
strict limits on increasing such power densities. For example, the
optical transceiver shown in FIG. 2 uses 622 megabits per second in
both directions. However, beyond some level--for example, 1.2
gigabits per second--more power would be required to sustain the
data rate. Accordingly, the present inventors developed systems and
techniques that, among other advantages, enable laser output
devices such as WON transceivers to transmit and receive data at
increased bandwidths but without exceeding existing safety
standards and without increasing risks to humans.
SUMMARY
[0011] Implementations of the systems and techniques described here
may include various combinations of the following features.
[0012] In one aspect, an optical transceiver such as used, for
example, in a wireless optical network (WON), may include multiple
laser sources including a first laser source configured to transmit
a first output channel beam having a first optical characteristic
and at least a second laser source configured to transmit a second
output channel beam having a second optical characteristic;
multiple detectors including a first detector configured to detect
a first input channel beam having the first optical characteristic
and at least a second detector configured to detect a second input
channel beam having the second optical characteristic; and multiple
apertures including a first aperture through which the first output
channel beam and the second input channel beam pass and a second
aperture through which the second output channel beam and the first
input channel beam pass.
[0013] In an embodiment, the first optical characteristic may be a
first wavelength (e.g., 830 nm) and the second optical
characteristic may be a second wavelength different from the first
wavelength (e.g., 785 nm). A difference between the first
wavelength and the second wavelength is about 50 nanometers or
greater. One or more of the wavelengths may be between 1530 and
1570 nanometers.
[0014] In another embodiment, the first optical characteristic may
be a first polarization (e.g., transverse electric polarization)
and the second optical characteristic may be a second polarization
different from the first polarization (e.g., transverse magnetic
polarization).
[0015] Laser sources that may be used include laser diodes, gas
lasers, fiber lasers, and/or diode-pumped solid state (DPSS)
lasers. In an embodiment, a laser diode is used that emits an
output field that is either substantially transverse electric or
substantially transverse magnetic.
[0016] Detectors that may be used includes an avalanche photodiode
with a bandpass filter or an avalanche diode with a polarizer, for
example, a transverse electric polarizer or a transverse magnetic
polarizer.
[0017] The aperatures may include a lens, for example, a plano
aspheric lens having a diameter of about 75 mm.
[0018] In an embodiment, the transceiver further may include
multiple beamsplitters, including a first beamsplitter associated
with the first aperture and a second beamsplitter associated with
the second beamsplitter, which differentiate between the first and
second optical characteristics. At least one of the beamsplitters
may be an optical highpass filter such as a dichroic mirror. At
least one of the beamsplitters may be a polarizing beamsplitter.
One or more of the beamsplitters may pass beams of the first
optical characteristic and reflect beams of the second optical
characteristic. Alternatively, or in addition, one or more of the
beamsplitters may pass beams of the second optical characteristic
and reflect beams of the first optical characteristic.
[0019] In an embodiment, the first output channel beam passes
through the first beamsplitter to the first aperture, the second
input channel beam is reflected by the first beamsplitter to the
second detector, the second output channel beam is reflected by the
second beamsplitter to the second aperture, and the first input
channel beam passes through the second beamsplitter to the first
detector.
[0020] The transceiver further may include a third laser source
configured to transmit a third output channel beam having a third
optical characteristic, and a third detector configured to detect a
third input channel beam of the third optical characteristic.
[0021] The transceiver may include at least two beamsplitters each
configured to differentiate between the first and second optical
characteristic. In that case, the laser sources, detectors and
beamsplitters are arranged relative to each other such that, when
the transceiver is operating, the first output channel beam will be
passed and the second input channel beam will be reflected by a
first beamsplitter, and such that the first input channel beam will
be passed and the second output channel beam will be reflected by a
second beamsplitter.
[0022] In another aspect, an optical transceiver includes a
plurality of dichroic mirrors, each of which is configured to pass
a beam of a first wavelength and reflect a beam of a second
wavelength. The optical transceiver further includes multiple laser
sources including a first laser source arranged to transmit a first
output channel beam of the first wavelength through a first
dichroic mirror and a second laser source arranged to transmit a
second output channel beam of the second wavelength that is
reflected by a second dichroic mirror. The transceiver also
includes multiple photodetectors, including a first photodetector
configured to detect a first input channel beam of the second
wavelength reflected by the first dichroic mirror and a second
photodetector configured to detect a second input channel beam of
the first wavelength passed by the second dichroic mirror. The
transceiver also includes multiple lenses including a first lens
arranged to focus the first output channel beam and the first input
channel beam and a second lens arranged to focus the second output
channel beam and the second input channel beam.
[0023] The first and second lenses may be physically separated
(e.g., by about 25 millimeters or greater) to increase eye safety.
Further, the physical dimension of the lens (e.g., about 75 mm
diameter) may be selected to increase eye safety.
[0024] In another aspect, an optical transceiver includes a laser
source configured to transmit an output channel beam having a first
optical characteristic; a photodetector configured to detect an
input channel beam having a second optical characteristic different
from the first optical characteristic; an aperture through which
the output channel beam and the input channel beam pass; and a
beamsplitter, arranged in an optical path of the aperture and the
laser source, and configured to pass the output channel beam from
the laser source to the aperture and to reflect the input channel
beam from the aperture to the photodetector. The first and second
optical characteristics may be different wavelengths and/or
different polarizations.
[0025] In another aspect, performing wireless optical communication
may be performed by using a first aperture to transmit a first
output channel beam having a first optical characteristic and to
receive a first input channel beam of a second optical
characteristic different from the first optical characteristic; and
using a second aperture to transmit a second output channel beam
having the second optical characteristic and to receive a second
input channel beam of the first optical characteristic. Further, at
least one beamsplitter may be used to differentiate between the
first and second optical characteristics, which may be different
wavelengths and/or different polarizations. Data may be impressed
upon the either or both of the first and second output channel
beams using one or more of the following techniques: on/off keying,
phase-shift keying, pulse-position modulation, and/or
frequency-shift keying.
[0026] In another aspect, an optical transceiver may include
multiple laser sources including a first laser source configured to
transmit a first output channel beam having a first optical
characteristic and at least a second laser source configured to
transmit a second output channel beam having a second optical
characteristic; multiple detectors including a first detector
configured to detect a first input channel beam having the first
optical characteristic and at least a second detector configured to
detect a second input channel beam of the second optical
characteristic; and multiple apertures including a first aperture
through which the first and second output channel beams pass and a
second aperture through which the first and second input channel
beams pass.
[0027] In another aspect, a wireless optical network may include
multiple optical transceivers, each of which is in communication
with at least one other optical transceiver. Each of at least two
of the optical transceivers may include the following: multiple
laser sources including a first laser source configured to transmit
a first output channel beam having a first optical characteristic
and at least a second laser source configured to transmit a second
output channel beam having a second optical characteristic;
multiple detectors including a first detector configured to detect
a first input channel beam having the first optical characteristic
and at least a second detector configured to detect a second input
channel beam of the second optical characteristic; and multiple
apertures including a first aperture through which the first output
channel beam and the second input channel beam pass and a second
aperture through which the second output channel beam and the first
input channel beam pass.
[0028] One or more of the following advantages may be provided. The
techniques and methods described here result in an optical
transceiver that provides dramatically increased bandwidth compared
with a conventional transceiver but without any corresponding
increase in ocular safety-risks. By separating the output beams,
the potential power collected by an observer's eye can be
maintained at safe levels, while at the same time, providing
roughly twice or more the total power output for the transceiver as
a whole. Enabling the use of different techniques to differentiate
the beams (e.g., based on wavelength or polarization) provides
design and implementation flexibility. Moreover, the systems and
techniques described here enable the total bandwidth of an optical
transceiver to be scalable to a high degree.
[0029] The details of one or more embodiments are set forth in the
accompanying drawings and the description below. Other features,
objects, and advantages of the invention will be apparent from the
description and drawings, and from the claims.
DRAWING DESCRIPTIONS
[0030] FIG. 1 shows an example of a WON application.
[0031] FIG. 2 is a block diagram of a conventional WON
transceiver.
[0032] FIG. 3 is a block diagram of dual channel WON
transceiver.
[0033] FIG. 4 is a diagram to illustrate the effect on viewers of
the dual channel WON transceiver shown in FIG. 3.
[0034] FIG. 5 shows an embodiment of a dual channel WON
transceiver.
[0035] FIG. 6 is a table showing parameters of the dual channel WON
transceiver shown in FIG. 5.
[0036] FIG. 7 is a graph showing filter characteristics of a
dichroic mirror used in one embodiment.
[0037] FIGS. 8A and 8B are plan views, respectively, of the housing
and components forming a dual channel WON transceiver according to
one embodiment.
[0038] FIG. 9 shows an alternative embodiment of a dual channel WON
transceiver.
[0039] FIG. 10 shows a polarization-based embodiment of a dual
channel WON transceiver.
DETAILED DESCRIPTION
[0040] FIG. 3 shows a block diagram of a wavelength-based
embodiment of an optical transceiver having two bi-directional
channels. As shown in FIG. 3, the optical transceiver 300 has
circuitry and other components therein, as described in more detail
below, that enable it to transmit and receive data-carrying laser
beams over two separate output channels .lambda..sub.1o,
.lambda..sub.2o and two separate input channels .lambda..sub.1i and
.lambda..sub.2i. Data can be impressed on the output laser beams
using any of a number of methods including (1) on/off keying (OOK),
which involves modulating the amplitude of laser emission; (2)
phase-shift keying (PSK), which involves shifting the instantaneous
phase of the modulated wave among predetermined discrete values;
(3) pulse-position modulation (PPM), which involves varying the
position in time of a pulse without modifying the pulse duration;
or (4) frequency-shift keying (FSK), which involves shifting the
output frequency of the laser between predetermined values. The
optical transceiver 300 would transmit its two output beams to, and
receive its two input beams from, another similar optical
transceiver in the WON. Accordingly, the bandwidth of the
transceiver 300 essentially can be double that of the conventional
single-frequency transceiver shown in FIG. 1.
[0041] To enhance the eye safety of the transceiver, the two output
channels .lambda..sub.1o, .lambda..sub.2o are transmitted through
different apertures 302 and 304, having a diameter D1 and D2,
respectively, and separated by distance S. The values of D1, D2 and
S can be chosen such that the MPE for the overall transceiver 300
satisfies the desired classification level. In particular, the
values of D1, D2 and S can be chosen such that the eye of the human
viewer essentially cannot be exposed to an aggregate irradiance in
excess of the AEL.
[0042] FIG. 4 helps to illustrate the significance of values D1, D2
and S with respect to eye safety standards. As shown therein, the
aperture sizes D1, D2 and the separation S between the apertures
are set such that a human eye, whether aided or not, generally is
not exposed radiation from both sources .lambda..sub.1o,
.lambda..sub.2o. For example, an eye either at position 401, 403 or
407 could not simultaneously be exposed to radiation from both
apertures 302 and 304. Accordingly, the power density of each of
.lambda..sub.1o and .lambda..sub.2o can be set to the maximum
allowable value for the classification of interest without
violating standards or risking human sight. This is true even
though the total output power density of the transceiver 300 in
FIG. 3 would be roughly twice that of the total power density of
the conventional transceiver shown in FIG. 1.
[0043] As shown in FIG. 4, the output beams 409, 411 from the two
apertures 302, 304 in the near range remain relatively collimated
but begin to spread out at far ranges. In region 413, for example,
the two beams 409, 411 have spread and in fact overlap.
Consequently, an eye at position 405 potentially would be exposed
to radiation from both output beams .lambda..sub.1o and
.lambda..sub.2o. For that reason, parameters of the laser sources
and optics, and/or values of D1, D2, and S, must be chosen such
that an entity in region 413 would collect power no greater than
the AEL under consideration. Generally, overlap region 413 would
occur only at a range sufficiently far from the laser sources such
that a considerable amount of the laser beam's power would be
dissipated and the exposure would be well within the desired
AEL.
[0044] In a typical Class 1 application, an 830 nm laser diode
would be used as the source of .lambda..sub.1o, a 785 nm laser
diode would be used as the source of .lambda..sub.2o, a 75
millimeter lens would be used for each of the apertures 302, 304
(thus D1 and D2 would have the same value (75 mm) and S would be
set to be greater than or equal to 25 mm. As a result, an aided or
unaided viewer in regions 409, 411 and/or 413 would receive no
greater than the MPE of 0.56 milliwatts. At the same time, the
transceiver 300 is able to transmit and receive data at roughly
twice the bandwidth of conventional systems.
[0045] In alternative embodiments, a single optical transceiver
could have three or more output channels (and a corresponding
number of input channels), for example, by using three or more
laser sources of different wavelengths, thereby increasing the
bandwidth of the transceiver by a corresponding amount. In that
case, however, if eye safety was a concern, care would have to be
taken to ensure that the total power density emitted through any of
the apertures did not exceed the AEL for the desired application.
This three-or-more channel embodiment could be particularly
advantageous, however, if the transceiver was to be used in an
environment in which eye safety was not a concern (e.g., in an area
that humans and/or other animals could not enter).
[0046] FIG. 5 is a block diagram of a dual wavelength optical
transceiver 500 that-provides a data rate of 2.5 Gigabits per
second (2 channels.times.1.25 Gbps/channel). This embodiment uses
two dichroic mirrors 516, 517 as beam splitters to differentiate
between the different wavelength laser beams. The transceiver 500
also includes two sets of optics 514, 515, each of which focuses
one output beam and receives one input beam; two laser diodes (LDs)
502, 504--one for each of the different wavelengths emitted; two
photodetectors (PDs) 506, 508--one for each of the different
wavelengths received; and two bandpass filters 510, 512 for
filtering each of the two received beams.
[0047] In FIG. 5, one output channel is provided by LD 502 which is
driven at 1.25 Gbps and which emits a laser beam having a
wavelength of 830 nm. Other laser sources could be used in place of
LD 502, for example, fiber lasers, gas lasers, diode-pumped solid
state (DPSS) lasers and the like. The optical filter
characteristics of the dichroic mirror 516, described in more
detail below, are such that radiation having a wavelength of 830 nm
is passed (i.e., transmitted) by the mirror 516 to the optics 514,
thereby emitting an output beam. The optical filter characteristics
of the mirror 516 also are such that input channel beams of 785 nm
(e.g., transmitted by another transceiver at a remote location)
received by the optics 514 are reflected by the mirror 516 to the
filter 510 and PD 506.
[0048] The other output and input channels of the transceiver 500
are provided in a similar manner by LD 504, PD 508, filter 512,
optics 515, and mirror 517. Specifically, the second output channel
laser beam of wavelength 785 nm emitted by LD 504 (also driven at
1.25 Gbps) is reflected by the mirror 517 and focused by optics
515. An input laser beam of wavelength 830 nm passes through mirror
517 to filter 512 and PD 508, thereby forming the second input
channel.
[0049] By using the arrangement of components shown in FIG. 5, the
transceiver 500 has two output. channels of different wavelengths
and two input channels of different wavelengths. Collectively,
these two bidirectional channels provide roughly twice the
bandwidth of the conventional optical transceiver shown in FIG. 1.
At the same time, by separating the two output beams, the
transceiver 500 is capable of operating in an eye safe manner that
complies with eye safety regulations.
[0050] FIG. 6 is a table showing parameters of the components used
in the embodiment of FIG. 5.
[0051] FIG. 7 shows a graph (the vertical axis is Transmittance, T,
where T=1-Reflectance, R; the horizontal axis is wavelength) of the
optical filter characteristics of the dichroic mirrors used in the
embodiment of FIG. 5. Each dichroic mirror effectively acts as an
optical high pass filter that passes radiation of wavelengths
longer than a predetermined threshold and reflects radiation of
wavelengths shorter than a predetermined threshold. As shown in
FIG. 7, beams of wavelength 785 nm are reflected by the dichroic
mirror (i.e., T is at or near zero and R is at or near one) while
beams of wavelength 830 nm are passed by the dichroic mirror (i.e.,
T is at or near one and R is at or near zero). Other wavelength
pairs could be used instead of 785 nm and 830 nm. For example, one
of the wavelengths could be a value between 1530 nm and 1570 nm
(e.g., 1550 nm). If the difference between the two wavelengths used
becomes too small, however, then it generally becomes more
expensive and/or complicated to design an optical bandpass filter
that is capable of differentiating between the beams. In the
embodiment of FIG. 5, the difference between the wavelengths of the
two beams is 50 nm, which represents an adequate delta for purposes
of differentiation using a dichroic mirror of readily available
commercial quality.
[0052] Although a pair of dichroic mirrors is used in the
embodiment of FIG. 5, other optical bandpass filter devices could
be used to differentiate between the beams of different
wavelengths. Moreover, although two dichroic mirrors having
identical filter characteristics are used in the embodiment of FIG.
5, mirrors having different characteristics could be used to
differentiate between different wavelength beams. For example, to
facilitate a different layout of components in the transceiver, the
mirrors could have inverse characteristics (e.g., one mirror passes
830 nm and reflects 785 nm while the other mirror passes 785 nm and
reflects 830).
[0053] FIGS. 8A and 8B are plan views, respectively, of the housing
and components forming a dual channel WON transceiver according to
one embodiment. As shown therein, a housing 800 is adapted to hold
two lenses 802, 804 and the various other components of the
transceiver including an 830 nm laser diode 806, a 785 nm laser
diode 808, an 830 nm photo diode 810, a 785 nm photo diode 812, a
785 nm bandpass filter 814, an 830 nm bandpass filter, two dichroic
mirrors 820 and six lenses 818. These components are arranged and
interact in the manner described above with reference to FIG. 5.
The six lenses 818 are placed at various locations in the beam
paths as shown in FIG. 8B and serve to collimate the beams that
pass through.
[0054] FIG. 9 shows an alternative embodiment of the dual channel
transceiver. In this configuration, one of the lenses 1002 is used
to focus both of the output beams and the other lens 1004 is used
to receive both of the input beams. Although this embodiment
realizes the same increased bandwidth advantages as the embodiment
of FIG. 5, it does not necessarily provide eye safety benefits.
Because both of the output beams are transmitted through the same
aperture--namely lens 102--the total power density of the beam
emanating from lens 102 is roughly twice that of the conventional
transceiver. Accordingly, depending on the particular application
and environment in which the transceiver was to operate, the power
densities of the two output beams might have to be reduced to
comply with eye safety requirements.
[0055] FIG. 10 shows a polarization-based embodiment of a dual
channel optical transceiver. In this embodiment, laser beams of
different polarizations are used as the data transmission media and
differentiation of the beams is performed by a polarizing beam
splitter (PBS). More specifically, one output channel. is provided
by transverse magnetic (TM) laser diode 1122, which emits a laser
beam having an output field that is substantially transverse
magnetic. The optical filter characteristics of the PBS 1110 are
such that TM radiation passes through PBS 1110 to the quarter wave
plate 1106, which converts the TM radiation into a right-hand
circularly polarized (RCP) beam, which is focused by optics 1102 to
emit an output beam. A left-hand circularly polarized (LCP) input
beam received by optics 1102 is converted by quarter. wave plate
1110 into a transverse electric (TE) beam, which due to the optical
filter characteristics of the PBS 1110, is reflected by the PBS
1110 to the TE polarizer 1114 and ultimately to PD 1116 (e.g., an
avalanche photodiode).
[0056] The other output and input channels of the transceiver 1100
are provided in a similar manner by LD 1118, PD 1120, TM polarizer
1115, optics 1104, quarter wave plate 1108, and PBS 1112.
Specifically, the second output channel TE laser beam emitted by LD
1118 (i.e., having an output field that is substantially transverse
electric) is reflected by PBS 1112, converted into a LCP beam by
quarter wave plate 1108 and focused by optics 1104 to emit a second
output beam. An input RCP laser beam received by optics 1104 is
converted into a TM beam by quarter wave plate 1108, and then
passes through PBS 1112 to TM polarizer 1114 and ultimately to PD
1120, thereby forming the second input channel.
[0057] Various implementations of the systems and techniques
described here may be realized in digital electronic circuitry,
integrated circuitry, specially designed ASICs (application
specific integrated circuits) or in computer hardware, firmware,
software, or combinations thereof.
[0058] A number of embodiments of the present invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. Accordingly, other embodiments are within
the scope of the following claims.
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