Note: Descriptions are shown in the official language in which they were submitted.
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WAVELENGTH DEPENDENT CROSSOVER SYSTEM FOR BI-DIRECTIONAL
TRANSMISSION
Field of the Invention
This invention relates generally to the transmission of signals through optical fibers.
More specifically the invention utilizes multi-port optical circulators in combination with
transmitting/reflecting optical elements such as Bragg optical fiber gratings and an amplifier,
for bi-directional communications through a single optical fiber.
Background of the Invention
Conventional Two-Fiber Transmission
FIG. 1 depicts a conventional two-fiber transmission link where blocks 101 and 102
can represent regeneration or central office sites. Connecting the two sites together is a fiber
optic cable. Within the cable there are multiple strands of fiber 103, of which two have been
shown. In this type of transmission system, communication from a transmitter (TX) at site A
to a receiver (RX) at site B utilizes one signal wavelength ~1 ) and one strand of an optical
20 cable. Communication in the opposite direction uses a different strand of the optical cable
and the same, or different, wavelength ~2) to carry the signal.
Referring again to FIG. 1, sites A and B (101 and 102) can represent different site
configurations. In one configuration, one terminal site might communicate directly to another
25 terminal site in a complete end-to-end, communication system. Alternatively, FIG. 1 could
represent a single link in a longer chain of transmission stations. In other words, sites A and
B might be representative of a site C and a site D and a site E and so on, until a final site
containing termin~ting transmission equipment is reached.
Depending upon the wavelength chosen for tr~n~mi~ion, the strand of optical fiber
103 used may exhibit different attenuation characteristics which may limit the possible
sparing of regenerator sites, e.g., sites A and B. Attenuation in a typical single-mode optical
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.
fiber is about 0.35 dB/kilometer at 1310 nanometer (nm) and about 0.25 dB/kilometer at
1550 nm. Thus, for systems operating at data rates of a few gigabits per second, regenerator
sites could be spaced anywhere from about 35 to 45 kilometers when operating at 1310 nm
and into the 70 to 80 kilometer range when operating at 1510 nm.
s
Wavelength-Division Multiplexer (WDM) Filters FIG. 2 depict a conventional
narrow-band wavelength-division multiplexing communication system. Here, the term
"narrow-band" is used to mean that more than one wavelength is utilized within the same
transmission "window" of the optical fiber. For example, if the system is operating within a
0 1550 nm window, two signaling wavelengths of 1533 and 1557 nm might be used. For
standard single mode fiber, the two main tr~n.smi ssion "windows" of interest are 1310 nm
and 1550 nm. Unlike the configuration shown in FIG. 1, communication between site A and
site B in FIG. 2 is provided by a single strand of optical fiber 103. Bi-directional transmission
is achieved through the utilization of wavelength-division multiplexing (WDM) filters, 201
5 and 203. (The devices 201 and 203 can be the same or slightly different devices, depending
upon the manufacturing technique used to create them.) The purpose of WDM filters is to
couple multiple wavelengths into (hereafter referred to as 'on') and out of (hereafter referred
to as 'off ) the transmission fiber. In the example shown, WDM filters 201 and 203 couple
the two wavelengths 1557 and 1533 nm on and off a single fiber 103 of a fiber optic cable.
WDM Technology
There are several technologies that can be used to construct WDM filters. For
example, etalon technology, defraction grading technology, fused biconic taper technology,
25 and holographic filter technology. One technology that has proven to be widely useful in the
telecommunications industry is dichroic filter technology. This technology offers wide
channel passbands, flat channel passbands, low insertion loss, moderate isolation, low cost,
high reliability and field ruggedness, high thermal stability, and moderate filter roll-off
characteristics .
An illustrative example of a conventional three-port dichroic filter 300 is shown in
FIG. 3. A dichroic filter is comprised of one or more layers of dielectric material coated onto
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a, for example, glass substrate 305 with lenses 310 to focus the incoming and outgoing
optical signals. The choice of dielectric material, the number of dielectric layers coated onto
the substrate, and the spacing of these layers are chosen to provide the appropriate
tr~n.~mis~ive and reflective properties for a given--target--wavelength. For example, if_~1 is
5 the target wavelength to be transmitted through the filter, the number and spacing of the
dielectric layers on the substrate 305 would be chosen to provide (1) a specified passband
tolerance around _~1 and (2) the necessary isolation requirements for all other transmitted
wavelengths, for example, a wavelength, _~2, transmitted by a second transmitter.
0 The dichroic, or WDM, filter is constructed by placing self-focusing lenses, such as
"SELFOC" lenses 310, on either side ofthe dielectric substrate 305. "SELFOC" lens 310
focuses incoming light ~1 and _~2) to a particular location on the dielectric substrate.
Attached to the "SELFOC" lenses through an adhesive bonding process are, typically,
5 single-mode optical fibers. For convenience, the locations at which optical fibers attach to the
"SELFOC" lenses 310 are called ports: port 1 320, port 2 325, and port 3 330. Connected to
the ports are optical fibers 335, 340, and 345 respectively.
For example, all ofthe fight (comprised of_~1 and_~2) passing through fiber 335
20 connected to port 1 320 is focused by lens 310 to a single location on the dielectric substrate
305.
Since the substrate is coated to pass wavelengths around _~1, virtually all of the light
at _~1 passes through the dielectric substrate 305 and, via the second "SELFOC" lens, is
25 collimated into port 3 330, and passes away from the filter on optical fiber 345.
Any other wavelength incident on the filter through port 1 320 (e.g., light of wavelength _~2)
is reflected off the multilayer substrate, focused back through the first "SELFOC" lens to port
2 325, and passes away from the filter on optical fiber 340. Likewise, the
filter performs the same function for light traveling in the opposite direction.30 This technology could be used to, for instance, implement WDM filter 201 shown in FIG. 2.
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FIG. 4 is a variation of the system shown in FIG. 1, a two-fiber design where one
wavelength ~1) is transmitted on one fiber in one direction, and another (or possibly the
same) wavelength ~2) is transmitted on the other fiber in the opposite direction. Erbium-
doped fiber amplifiers (EDFAs) can be deployed along such a link in multiple locations:
5 immediately following the transmitter (TX), making them post-amplifiers; immediately
preceding a receiver (RX), making them pre-amplifiers; or between a transmitter and
receiver, as shown in FIG. 4, making them line-amplifiers. Commercially available EDFA
devices only operate in the 1550 nm window. Typically, in the line-amplifier configuration,
regenerator spacing can be almost doubled, from approximately 70 to 80 kilometers to
o approximately 140 to 160 kilometers. (This analysis assumes typical filter attenuation and
that at 80 kilometers the system is attenuation limited and not dispersion limited for distances
less than 160 kilometers). Hence, if the cost of two EDFAs is less than the cost of a
conventional fiber optics transmission system regenerator, the two EDFAs 401 and 403 can
be used to reduce equipment deployment costs when constructing a transmission network
15 such as that shown in FIG. 4.
Erbium-Doped Fiber Amplifier (EDFA) Technology
FIG. 5 shows a conventional design for an EDFA such as that shown in FIG. 4,
20 blocks 401 and 403. In a typical dual-pumped amplifier there are either two or three optical
isolators 501, two WDM filters 505 and 511, two laser pump sources 503 and 509, and a
length of erbium-doped single mode fiber 507. If the amplifier is single-pumped, one of the
pump sources 503 or 509 is removed. If a pump source is removed, its corresponding WDM
filter is likewise removed: if pump source 503 is removed, WDM filter 505 is also removed;
25 if pump source 509 is removed, WDM filter 511 is also removed.
WDM filters perform the function of coupling the pump source laser wavelength into
the erbium-doped fiber. Pump energy is used to elevate the erbium ions concentrated in the
erbium-doped fiber to a higher-than-normal energy level. These ions will stay excited until
30 they decay on their own accord or are stimulated to decay by the arrival of a signal
wavelength photon arriving from the transmission link 103. It is through the process of
"stimulated decay" that an optical signal is amplified in an EDFA.
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Isolators function as one-way conduits for optical signals. In other words, isolator
elements 501 allows an optical signal to pass in a single direction, e.g., from left to right, but
not from fight to left.
Consider the case where a signal photon enters the amplifier of FIG. 5 at the point
labeled IN. The photon passes through isolator 501 and enters the WDM filter 505, where it
is routed into the length of erbium-doped fiber 507. Both during and preceding the arrival of
the signal photon, laser pumps 503 and 509 have been providing energy to the erbium-doped
0 fiber via the WDMs 505 and 511, exciting the fiber's erbium ions. Upon entering the erbium
fiber, the signal photon will cause decay of some of the excited erbium ions, releasing their
energy in the form of (stimulated) photons. The original signal photon plus the stimulated
photons then pass out of the WDM 511, through the output isolator 501, and back onto the
transmission fiber 103.
Several aspects of amplifier design and utilization are well-known to those of
ordinary skill. Of great importance in network applications is the configuration of the optical
amplifier. If optical isolators are used internal to the amplifier, then they make the amplifier
an inherently unidirectional device. In FIG. 5 for example, the isolators 501 prevent a signal
20 from propagating from right-to-left (OUT toward IN). These isolators are important for
elimin~ting the amplification of unwanted back reflections that could degrade system
stability. Another characteristic that must be considered when deploying an amplifier is what
signal wavelength to use in conjunction with the amplifier's pump(s) wavelength. Because
amplifier gain is not perfectly flat for all incoming wavelengths (different wavelengths
25 exhibit different gain characteristics), the precise wavelengths to use are a function of the
gain variations of the different available pump wavelengths.
EDFA Based Amplifier Systems
Two prior art communication links lltili7ing EDFAs and conventional WDMs are
shown in FIGS. 6 and 7. In FIG. 6 a single-fiber tr~n.~mi.~ion link is shown with one EDFA
401 configured as a line amplifier. As previously stated, if the EDFA 401 of FIG. 6 were a
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typical amplifier (built as described in FIG. 5 for example) this communication link would
not provide bi-directional transmission; tr~nsmis.sion would occur from site A to site B, but
not from site B to site A. (it is possible to build an optical amplifier without the optical
isolators but this creates inherent instability problems that make it difficult to maintain a safe
5 operating environment and is, therefore, not recommended by existing industry standards).
In FIG. 7 EDFA amplifiers 401 and 403 are deployed as post-amplifiers, immediately
following the transmitters (TX) and immediately before the WDM filters 201 and 203. It is
possible to obtain bi-directional transmission over the single fiber link 103 in this
o configuration. There are, however, at least two disadvantages to this implementation. First, in
this design the high power signal leaving a transmitter is physically collocated with an optical
receiver (RX). In such cases, care must be taken to avoid near-end optical loop backs. In
other words, at site A 101 with a high power signal leaving EDFA 401, any signal reflection
from the WDM filter 203 could return to site A's receiver and cause an optical feedback
5 problem. The same is true of site B's configuration. Another drawback to this configuration is
in the economics of deploying post amplifiers versus line amplifiers. (Line amplifiers provide
a larger gain margin than do post amplifiers). If line amplifiers could be used to extend the
distance between sites, while maintaining the ability to provide bi-directional transmission,
the cost of the system's hardware could be significantly reduced.
Conventional unidirectional amplifier systems (e.g., FIG. 4) use two fibers per link;
one fiber carrying data in one direction and the other fiber carrying data in the opposite
direction. If two signal channels are needed in such a system, four fibers are required.
Likewise, conventional bi-directional amplifier systems (e.g., FIG. 2) use one fiber per link.
25 If two signal channels are needed in such a system, two fibers are required. The reduction in
fiber count of a bi-directional WDM design could also be achieved in a unidirectional WDM
design by employing multiple transmitters on a single fiber in one direction and multiple
transmitters on a single fiber in the opposite direction. An example of the latter system
design, using two transmitters and two receivers at each site, is depicted in FIG. 8. In this
30 design, transmitter one (TXI) and transmitter two (TX2), located at site A 801 and operating
at wavelength 1 and wavelength 2 respectively, are coupled onto a single fiber 103 through
the WDM filter 203. Both of these wavelengths are amplified by the EDFA 401 during signal
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tr~n.~mi~ion. WDM filter 201, located before the two receivers RX, and RX2, is used to
separate the two wavelengths and route each signal to the correct receiver.
The design of FIG. 8 could be built using conventional EDFAs, including internal5 isolators, because only unidirectional transmission through the amplifiers are required. The
primary disadvantage of this design lies in the difficulty of protecting such a system. With
multiple systems on a single fiber, if that fiber is lost due to a cable cut or some other
disaster, then multiple systems would be down at the same time. By convention, transmission
systems employ a 1-by-N protection scheme, meaning that one backup system is used to
o protect multiple (N) transmission channels. If a single channel fails, that channel's traffic is
rerouted to the backup channel and no traffic is lost. The failed channel is said to have been
"switched to protect." In a 1-by-N scheme if multiple systems (transmitters or receivers) fail,
only one system can switch to protect. In order to protect the configuration shown in FIG. 8
beyond a 1-by-1 system, multiple protect systems would be required, since there are multiple
5 systems on a single fiber. This is a costly endeavor and one which the invention addresses.
Illustrative Systems
FIG. 9 depicts one configuration for a dual wavelength, bi-directional narrow-band
20 WDM optical amplifier module, 901. Components used to construct the amplifier module
901 include: two WDMs, 201 and 203 (input and output ports ofthe amplifier module), and
two EDFAs, 903 and 905, which can be either single-pumped or dual-pumped depending
upon the communication system's power constraints/requirements. This line-amplifier
configuration extends the regenerator spacing while providing bi-directional tr~n.smi~.~ion
25 lltili~ing a single-fiber strand ofthe cable facility 103.
It should be noted that the amplifier module 901 can be cascaded to extend even
farther the distance between site A and site B. (The number of amplifiers that can be
cascaded, between sites A and B, is limited by the dispersion characteristics of the
30 transmission equipment deployed at sites A and B.)
Referring now to prior art FIG. 10, United States patent number 5,452,124 describes a bi-
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directional amplifier module design that can be constructed l]tili7ing a single EDFA. In this
configuration, bi-directional tr~n~mis~ion over a single optical fiber is achieved using four
WDM filters. All signal wavelengths must pass unidirectionally through the EDFA 401 due
to the constraint of using optical isolators in the EDFA 401 (refer to FIG. 5). Therefore, the
5 two tr~n.~mis~ion wavelengths traveling in opposite directions, must be broken apart and
recombined through WDM filters to pass unidirectionally through the EDFA. Similarly, the
two amplified wavelengths must be broken apart and recombined through WDM filters to
continue propagating toward their respective receiver sites. WDM filter 203 is constructed to
bandpass 1557 nm and WDM filter 201 is constructed to bandpass 1553 nm.
Assuming a typical 1550 nm EDFA operational band, then going through FIG. 10 in a
left-to-right direction we see a 1557 nm signal is transmitted from site A 101, through the
east WDM filter 203, and onto the fiber cable 103. As the signal enters the amplifier module
it is separated by the west WDM filter 201. (Each WDM filter in FIG. 10 has its external
5 connection points labeled either 33 or 57. Connections labeled 33 carry optical signals at the
1533 nm wavelength. Connections labeled 57 carry optical signals at the 1557 nm
wavelength.) The signal then travels to the east WDM filter 203 where it is routed into the
EDFA amplifier 401. Upon leaving the EDFA, the 1557 nm signal is routed by another west
WDM filter 201 to the amplifier module's output east WDM filter 203 where it is placed onto
20 the fiber optic transmission cable 103. Finally, the signal leaves the transmission cable 103,
enters the west WDM filter 201 at site B 102, and is routed to that site's receiver equipment.
Signals transmitted from site B, at 1533 nm, take a different path through the WDM filters
201 and 203 and EDFA 401 on their way to site A's receiver. An advantage of this prior art
embodiment over the configuration described in the earlier prior art of FIG. 9 is that only a
25 single erbium-doped fiber amplifier is required. Because multiple wavelengths are being
amplified by a single amplifier, it is sometimes preferable that the EDFA 401 in FIG. 10 use
a dual-pumped amplifier rather than a single-pumped amplifier. The additional gain provided
by a dual-pumped EDFA could compensate for the signal strength lost by virtue of passing it
through a number of additional elements.
As noted above, bi-directional amplification is important in adhering to the protection
philosophy of a single fiber failure only resulting in outage to a single transmission system.
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Given this constraint, there are two basic ways to provide bi-directional amplification. One
method, shown in FIG. 9, utilizes two separate EDFA sources--one EDFA to amplify one
signal in one direction and the other EDFA to amplify another signal in the opposite
direction. The system of FIG. 10, has the advantage of using only a single amplifier, but
5 requires four WDM filters in order to route the different (signal) wavelengths so that they
pass unidirectionally through the single amplifier.
In an attempt to overcome the limitations of the aforementioned prior art systems,
United States patent number 5,452,124 issued September 19, 1995 in the name of Baker,
0 discloses a system that utilizes a four-port wavelength-division multiplexing (WDM) filter
and a single erbium-doped optical amplifier (EDFA) to implement a dual wavelength bi-
directional (single fiber) optical amplifier module.
The optical amplifier module described by Baker conveniently provides bi-directional
15 signal transmission using a single EDFA and a single four-port WDM.
Prior art FIG. 11 depicts Baker's system incorporating a single fiber bi-directional amplifier
module 1100. At site A, a WDM 203 is used to combine two wavelengths of light ~1 and
_~2) onto a single fiber 103. The transmitter at site A is transmitting light at wavelength _~1.
20 The receiver at site A is receiving light from site B at wavelength _~2. Hence, _~l travels
from site A to site B or from west to east on fiber 103, and _~2 travels from site B to site A
in an east to west direction on the fiber 103.
Incorporated within the amplifier module 1100 is a four-port WDM filter 1105. As25 shown in FIG. 11, port 1 connects to the west fiber link 103, port 2 connects to the east fiber
link 103, port 3 is connected to the input of the amplifier module's EDFA via an optical fiber
link 1110, and port 4 is connected to the output of the amplifier module's EDFA via an
optical fiber link 1110. Site A's 101 WDM filter 203 is a dichroic filter designed to pass a
center wavelength _~2. Site B's 102 WDM filter 201 is also a dichroic filter, but is designed
30 to pass a center wavelength _~1. The amplifier module's WDM filter 1105 can be constructed
from either WDM filter 201 or 203 with the addition of an extra port. The functionality of a
four-port WDM will be described below.
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FIG. 12 of the Baker patent depicts a four-port WDM filter 1105. West fiber link 103,
coming from site A, is connected to port 1 1200. East fiber link 103, coming from site B, is
connected to port 2 1205. In this example, let WDM filter 1100 (i.e., the multilayer dichroic
5 substrate 305) have a designed pass--center--wavelength of_~2. This means that signals
having a wavelength _~2 will pass through the WDM filter (i.e., the multilayer dielectric
substrate) while signals of all other wavelengths will be reflected.
Light traveling into port 1 1200 on "west" fiber 103 having wavelength _~l will, after
o being focused onto the filter's substrate by "west" lens 310, be reflected back to port 3 1210
through west lens 310 (recall, only light having a wavelength of _~2 will pass through the
filter's substrate). In a similar manner, light traveling into port 2 1205 on "east" fiber 103
having wavelength _~2 will, after being focused onto the filter's substrate by "east" lens 310,
be passed through the filter's substrate material 305, recollimated by "west" lens 310, and
collected at port 3 1210. Hence, port 3 1210 collects light having both wavelengths _~l and
_~2. As shown, light leaving port 3 1210 is routed via fiber link 1110 to the input port of a
conventional EDFA 401. (Fiber links 1110 can be conventional single-mode optical fiber.) In
this manner light traveling from site A to site B as well as light traveling in the opposite
direction, from site B to site A, is passed unidirectionally through the EDFA 401.
After amplification, both wavelengths _~1 and _~2 exit the amplifier 401 and arerouted to port 4 1215 where they are focused by the "east" lens 310 onto the filter's substrate
305. Light of wavelength _~1 is reflected back through the "east" lens into port 2 1205 where
it exits the filter on its way to site B. Light of wavelength _~2 is passed through the substrate
25 and focused by the "west" lens 310 into port 1 1200 where it exits the filter on its way to site
A.
Although the Baker patent appears to adequately provide its intended function, there
is a need for unidirectional amplification for bi-directional tr~n~mi~ion that is particularly
30 tolerant of unwanted back-reflections from any high reflecting device, poor or faulty
connectors and the like.
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Furthermore, there is a need for a device that offers the ability to transmit a first
narrow range of wavelengths in a first direction, and a second narrow range of wavelengths
in an opposite direction, wherein the transition between the first and second range of
5 wavelengths is extremely abrupt.
Conventional dichroic interference filters such as bandpass filters described heretofore
designed to allow wavelengths within a predetermined range of the desired pass-band to be
transmitted, while a range of wavelengths on either side of the pass band are highly reflected,
o are in some instances inadequate in their response. Ideally a bandpass filter should be square
in its response; thus, the transition from the rejection regions to the passband should be as
rapid as possible, or expressed differently, the slope or transition region should be as steep as
possible, while obtaining a pass band region that is uniform having little or no ripple.
In a preferred embodiment of this invention this inherent problems associated with
using conventional WDM interference dichroic filters is somewhat remedied by lltili7ing
Bragg optical fiber gratings. For example dichroic filters offer reasonably high isolation on
transmission but, lower and often unacceptable levels of isolation on reflection. In an
amplification system such as Baker's, this can critically effect the performance of the device,
20 where unwanted non-isolated signals become amplified. By using Bragg diffraction gratings,
that have substantially symmetrically high isolation in reflection and in transmission, these
potential problems are substantially obviated.
Furthermore, a unique design having two four-port optical circulators in combination
25 with these Bragg optical fiber gratings provide a means of steering oppositely propagating
optical signals through a single unidirectional device such as an EDFA, and ensure that any
unwanted back reflections are minimi7ed by being substantially extinguished.
In addition to these advantages, the arrangement of this invention offers a further
30 advantage. It is known and practiced in the art, to provide an isolator at an input and an
output end of an optical amplifier. The device in accordance with this invention, does not
require this additional isolation as sufficient isolation is provided by the two optical
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circulators that the amplifier is coupled to.
Summary of the Invention
s In accordance with the invention there is provided, an amplifying device comprising;
a first circulator having at least 4 ports;
a second circulator having at least 4 ports;
first wavelength selective means disposed between a first and a second port of the first and
second circulator respectively;
o second wavelength selective means disposed between a third and fourth port of the first and
second circulator respectively, the first and second wavelength selective means substantially
transmitting light of a first wavelength and substantially reflecting light of a second
predetermined wavelength;
amplifying means, disposed between two other ports of the first and second circulators, said
5 amplifying means being disposed along a path that is provided to carry light of the first and
second wavelength, the first and second circulators being arranged to receive two
wavelengths of oppositely propagating light of at least the first and second wavelength and in
cooperation with the first and second wavelength selective means, direct the first and second
oppositely propagating light in a same direction through the amplifying means.
In accordance with the invention there is provided, a device for ch~nging a
characteristic of a first optical signal having a first wavelength, that is propagating in a first
direction in a first optical fiber and for ch~nging a characteristic of a second optical signal,
having a second wavelength that is propagating in a second direction along a second optical
25 fiber, comprising:
a first multi-port optical circulator coupled to the first optical fiber to receive the first optical
signal;
a second multi-port optical circulator coupled to the second optical fiber to receive the second
optical signal;
30 wavelength selective means for substantially transmitting optical signals of the first
wavelength and for substantially reflecting signals of the second wavelength, said wavelength
selective means disposed between the first and second optical circulators;
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means for ch:~nging a characteristic of an optical signal passing therethough, said means
disposed between and for communicating with said first and second optical circulator, the
first and second multi-port optical circulators in combination with the wavelength selective
means, cooperating to steer the first optical signal so that a characteristic of the first optical
5 signal is changed, and for later steering the first incoming signal onto the second optical
fiber, and for steering a second oppositely prop~g~ting optical signal so that a same
characteristic of the second optical signal is changed, and for later steering the second signal
onto the first optical fiber.
o In accordance with another aspect of the invention there is provided, a method of
amplifying a first optical signal having a first wavelength, propagating in a first direction and
amplifying a second optical signal, having a second wavelength propagating in a second
direction utilizing a same optical amplifier, comprising the steps of:
(a) routing the first and second optical signals to an input port of the optical amplifier
through a first router;
(b) amplifying the first and second optical signals;
(c) rerouting the first optical signal in the first direction through a second router; and
(d) retrouting the second optical signal in the second direction through the second router.
In accordance with another aspect of the invention there is provided, an opticalamplifier arrangement for amplifying a first optical signal having a first wavelength,
propagating in a first direction and for amplifying a second optical signal, having a second
wavelength propagating in a second direction, comprising:
means for directing the first and second optical signals in predetermined and opposite
directions in dependence upon their wavelength.
amplifying means for amplifying the first and second optical signals;
a first router coupled to and in combination with said means for directing for routing the first
and second optical signals to an input port of the optical amplifier; and,
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a second router coupled to and in combination with said means for directing the first and
second optical signals for routing the first optical signal in the first direction and for routing
the second optical signal in the second direction.
In accordance with the invention, there is further provided a device for ch~nging a
characteristic of a first optical signal having a first wavelength, that is propagating in a first
direction in a first optical fiber and for ch~nging a same characteristic of a second optical
signal, having a second wavelength that is propagating in a second direction along a second
o optical fiber, comprising:
a first multi-port optical circulator coupled to the first optical fiber to receive the first optical
signal;
a second multi-port optical circulator coupled to the second optical fiber to receive the second
optical signal;
5 wavelength selective means for substantially transmitting optical signals of the first
wavelength and for substantially reflecting signals of the second wavelength, said wavelength
selective means disposed between the first and second optical circulators;
terminals for connection to means for ch~nging a characteristic of an optical signal passing
therethough, said means when connected to said terminals, being disposed between and for
20 communicating with said first and second optical circulator, the first and second multi-port
optical circulators in combination with the wavelength selective means, cooperating to steer
the first optical signal so that a characteristic of the first optical signal is changed, and for
later steering the first incoming signal onto the second optical fiber, and for steering a second
oppositely propagating optical signal so that a same characteristic of the second optical signal
25 iS changed, and for later steering the second signal onto the first optical fiber.
In accordance with another aspect of the invention device is provided for use with an
amplifier comprising;
a first circulator having at least 4 ports;
30 a second circulator having at least 4 ports;
first wavelength selective means disposed between a first and a second port of the first and
second circulator respectively;
14
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second wavelength selective means disposed between a third and fourth port of the first and
second circulator respectively, the first and second wavelength selective means substantially
transmitting light of a first wavelength and substantially reflecting light of a second
predetermined wavelength;
s terminals disposed between two other ports of the first and second circulators for connection
to amplifying means, said amplifying means when connected to said terminals being
disposed along a path that is provided to carry light of the first and second wavelength, the
first and second circulators being arranged to receive two wavelengths of oppositely
prop~g~ting light of at least the first and second wavelength and in cooperation with the first
o and second wavelength selective means, direct the first and second oppositely propagating
light in a same direction through the amplifying means.
15 Brief Description of the Drawings
Exemplary embodiments of the invention will now be described in conjunction with some of
the following figures.
FIGS. 1, 2 and 4 through 8 are block-diagram representations of some
20 conventional fiber optic communication systems as discussed in more
detail above.
FIG. 3 is a block diagram representation of a conventional three-port
wavelength-division multiplexer filter.
FIG. 9 is a block diagram representation of a prior art single-module amplifier for
25 bi-directional transmission employing wavelength-division multiplexing
and erbium-doped fiber amplifier technology.
FIG. 10 is a block diagram representation of a prior art bi-directional optical
amplifier module comprising a single erbium-doped fiber amplifier and
four conventional three-port wavelength-division multiplexers.
30 FIG. 11 is a block diagram representation of a prior system of a bi-directional amplifier
module comprising a single four-port wavelength-division multiplexer filter and a single
erbium-doped fiber amplifier.
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FIG. 12 is an expanded block diagram of the prior art bi-directional communication
system of FIG. 11.
FIG. 13 is a block diagram of a bi-directional amplifier module comprising a single erbium-
doped fiber amplifier, in accordance with this invention.
5 FIG. 14 is a plot of the tr~n~mi~sive/reflective characteristics of a first Bragg optical fiber
grating.
FIG. 15 is a detailed block diagram of a alternative embodiment of an optical system in
accordance with this invention utilizing the bi-directional amplifier module shown in FIG.
13.
0 FIG. 16 is a plot of the transmissive/reflective characteristics of a second Bragg optical fiber
grating.
FIG. 17 is a plot of the transmissive/reflective characteristics of a third Bragg optical fiber
grating.
15 FIG. 18 is a block diagram of a bi-directional module comprising terminals for connection to
a single erbium-doped fiber amplifier, in accordance with this invention.
Detailed Description of a Specific Embodiment
One illustrative embodiment of the invention is described below as it might be
implemented using a Bragg optical fiber grating and EDFA technology. In the interest of
clarity, not all features of an actual implementation are described in this specification. It will
of course be appreciated that in the development of any such actual implementation (as in
any hardware development project), numerous implementation-specific decisions must be
25 made to achieve the developers' specific goals and subgoals, such as compliance with system-
and business-related constraints, which will vary from one implementation to another.
Moreover, it will be appreciated that such a development effort might be complex and time-
consuming, but would nevertheless be a routine undertaking of device engineering for those
of ordinary skill having the benefit of this disclosure.
Introduction
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Single mode optical fiber communication systems have matured in a remarkably short
time since the proposal by Kao and Hockham for using dielectric waveguides as a low-loss
transmission medium. The application of photosensitivity of germanium doped optical fibers
is another milestone. Photosensitivity of optical fibers remained dormant for several years
5 after it was first reported by Hill et al. in a paper entitled "Photosensitivity in optical
waveguides: Application to reflection filter fabrication," published in Appl. Phys. Lett., vol.
32, no. 10, 647, (1978). Since that time optical fiber reflection gratings have become more
prevalent. A history of the development and description relating to the current state of the art
is found in a paper by Raman Kashyap entitled Photosensitive Optical Fibers: Devices and
10 Applications, published in Optical Fiber Technology 1, 17-34 (1994).
Chirped Bragg optical fiber gratings are now becoming available and have
characteristics that are well suited to WDM applications. For example, it possible to design
and write a Bragg grating into an optical fiber that is substantially square in response and
5 achieving desired tr~n~mis~ive and reflective characteristics; thus, the transition from the
rejection regions to the passband are as rapid as possible obtaining a pass band region that is
uniform having little or no ripple. An embodiment of the invention described hereafter
utilizes chirped Bragg optical fiber gratings.
Referring now to FIGs 13 and 14, the amplification module 5 in accordance with this
invention comprises two input optical fibers 8 and 9 on which optical signals (propagating
from the left) having a wavelength 11 and 12, and optical signals (propagating from the right)
having a wavelength 13 and 14 are carried, respectively. An end of optical fiber 8 is coupled to
a port 2 of a first four-port optical circulator 12, and an end of optical fiber 9 is coupled to a
25 port 3 of a second four-port optical circulator 14. An optical fiber having a Bragg diffraction
grating therein is disposed between and coupled to the two circulators 12 and 14 in such a
manner has to have one end optically coupled to port 3 of the first circulator 12 and to have
the other end optically coupled to port 4 of the second optical circulator 14. In a somewhat
similar fashion a second Bragg diffraction grating is disposed between the two circulators,
30 however ends of the optical fiber containing the second grating are coupled to ports 1 and 2
of the first and second grating respectively. A single unidirectional path comprising an
erbium doped optical amplifier 16 having optical fibers at each end is provided wherein the
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optical fiber ends are coupled to ports 4 and 1 of the first and second optical circulators 12
and 14 respectively. Thus, in operation light from port 4 of the first circulator propagates
through the amplifier 16 to port 1 at the second circulator.
s The Bragg fiber gratings 1 8a and 1 8b are both designed to reflect light of wavelengths
11 and 12, and to transmit light of wavelengths 13 and 14. A more detailed explanation of the
input/output characteristics of the Bragg gratings is understood with reference to the plot
shown in FIG. 14 depicting transmission versus wavelength. In this embodiment the
requirementsaresuchthatll= 1533~4nm,12= 1541~2nm,13= 1549~2nmandl4=
o 1557 ~ 4 nm. Thus as the plot illustrates, wavelengths in the range of to 1529 to 1543 are
substantially, and preferably, totally reflected, and wavelengths in the range of 1547 nm to
1551 nm are substantially, and preferably totally transmitted with no loss. Thus the Bragg
grating (referred to hereafter as a Bragg filter) must be capable of reflecting a first signal
having a first wavelength of light and transmitting a second optical signal that is within a 4
5 nm difference from the first signal, in wavelength.
In an attempt to simplify and clearly describe the operation of the device in the
absence of unnecessary detail, reference will be made to only a first optical signal and a
second optical signal. Of course the first optical signal referred to can be of wavelength 11 or
20 12; and, the second optical signal can be of wavelength 13 or 14.
The first optical signal is launched into optical fiber 8 via an optical connector 1 9a.
This signal enters the device 5 via port 2 of circulator 12 circulates to and exits port 3
directed toward the Bragg filter 1 8a. This first signal is reflected from the filter 1 8a
25 backwards to port 3 and subsequently circulates to port 4 of the circulator 12. From port 4,
the first signal propagates through the amplifier 16 and onward to port 1 of the second
circulator 14. The amplified first signal then circulates to port 2 of 14, and is reflected
backward (again to port 2) by the Bragg filter 1 8b. The amplified first signal then circulates
from port 2 to port 3 and out of the second circulator 14 onto the optical fiber 9. Thus the first
30 signal launched into the optical fiber 8, becomes amplified and is launched outward in a same
direction onto optical fiber 9.
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The second optical signal is launched into optical fiber 9 (in an opposite direction
from the first signal) via an optical connector 1 9b. This second signal enters the device 5 via
port 3 of circulator 14 and exits port 4 directed toward the Bragg filter 1 8a. Since the second
signal is of wavelength 13 or 14, it is transmitted through the Bragg filter 1 8a and enters port 3
5 of the circulator 12. After passing from port 3 to port 4 of 12 the second signal is amplified
by the amplifier 16 and passes from port 1 to port 2 of the second circulator 14. The second
signal now amplified, is transmitted through the Bragg filter 1 8b and passes through the first
circulator from port 1 to port 2 and out onto optical fiber 8 in its original direction, now as an
amplified signal.
0
Advantageously, this implementation lltilizing two optical circulators has additional
and unexpected advantages. For example, if the second optical signal exiting port 2 of
circulator 12 encounters a break in the fiber 8, or a poor connector 1 9a, unwanted reflections
will result, and some light of the first signal will reflect backwards into the device 5. In this
5 instance, the unwanted reflected light enters port 2 of 12 circulates to port 3, propagates
through 1 8a and circulates to port 4 of circulator 14, where the light energy is extinguished.
Similarly, if some of the amplified light energy of second signal reflects from 1 9b backward
into the device, the light circulates from port 3 to port 4 of circulator 14; the light then
propagates to the Bragg filter 1 8a where it is reflected back to port 4 of circulator 14. Once
20 again, the reflections are extinguished at port 4.
Preferred embodiments of this invention require the use of optical fiber gratings,
however, other elements having similar characteristics may be envisaged. For example,
however less preferably, optical filters such as dichroic filters may be utilized in place of the
25 Bragg filters, however the less than ideal isolation characteristics may be prohibitive.
Although the description heretofore relates primarily to an amplifying optical
element, other unidirectional optical elements may replace the amplifying optical element
and similar advantages may result in implementing this basic structure.
FIG. 18 illustrates an embodiment of the invention having connection terminals (or
optical connectors) 1 7a and 1 7b for connection to an amplifier or other optical device. Thus?
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conveniently and advantageously, by the provision of these terminals 1 7a and 1 7b, one or
more amplifiers or optical elements can be interchanged.
Turning now to FIGs. 15, 16 and 17 an alternative and more complex embodiment of5 this invention will be described, that includes the basic device 5. This embodiment provides
input and output terminals efficiently coupled to the device 5, for providing 4 separate optical
signals of different wavelengths to be amplified by the single erbium-doped amplifier 16. The
reflective/tr~n~mi.s~ive characteristics versus wavelength for the first group of Bragg optical
fiber gratings referred to bearing the reference numerals 1 8c or 1 8d, are illustrated in FIG. 14.
o The reflective/transmissive characteristics versus wavelength for the second group of Bragg
optical fiber gratings referred to bearing the reference numerals 28a or 28b, are illustrated in
FIG. 16; and, The reflective/transmissive characteristics versus wavelength for the third
group Bragg optical fiber gratings referred to bearing the reference numerals 38a or 38b, are
illustrated in FIG. 17.
Three 3-port optical circulators 22, 24, and 26 are coupled to the left side of module 5
for directing input signals of wavelength 11 and 12 into the amplifying module 5 for
amplification and for directing already amplified optical signals of wavelength 13 and 14
outward. A 4-port and two 3-port optical circulators are coupled to the right side of the
20 amplifying module 5 via a connector 1 9b for directing amplified optical signals of
wavelength 11 and 12 outward and for directing optical signals of wavelength 13 and 14 into
the module 5.
The operation of the four wavelength amplifying optical system will now be
25 described. An optical signal of wavelength 11 is launched into a port 1 of the circulator 26
and circulates to port 2. The optical filter 28a reflects the signal of wavelength 11 and it
circulates from port 2 to port 3 of 26. The signal then circulates from port one to port 2 of
circulator 22 and then propagates into the amplifying module 5. After being amplified the
optical signal circulates from port 2 to port 3 of circulator 32 and is reflected backward by the
30 optical filter 1 8d from port 3 to port 4. The signal then circulates from port 1 to port 2 of
circulator 36 and is then reflected backward by 28b from port 2 to port 3 to reach its
destination. Of course, Bragg fiber gratings 28a and 28b are designed to pass wavelength 12
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and to reflect light of wavelength 11.
When light of wavelength 12 is launched into the system it circulates from port 2 to
port 3 of optical circulator 26. The light then circulates from port 1 to port 2 of 22 and then
5 enters the amplifying module 5 via connector 1 9a. After being amplified, the light circulates
from port 2 to port 3 of 32 and is reflected by the filter 1 8d back from port 3 to port 4 of 32.
The amplified light of wavelength 12 then circulates from port 1 to port 2 of circulator 36 and
through grating 28b to reach its destination.
I o In the other direction (from right to left) an optical signal of wavelength 13 is
launched into port one of circulator 34 and passes to port 2 of the same device. The light is
then reflected backward by Bragg grating 38b from port 2 to port 3 of 34. The light
subsequently enters port 1 of 32 and then passes to port 2 and into the arnplifying module 5.
After being amplified, the optical signal passes outward from 5 through circulator 22 from
5 port 2 to port 3. The light then passes from port 1 to port 2 of circulator 24 and is reflected
back to port 2 by Bragg grating 3 8a and circulates from port 2 to port 3 of 24, and outward.
Light of wavelength 14 essentially follows a similar route, however it is launched into port 2
of 34 and circulates directly to port 3 following a same path to 5. After being amplified, this
light follows a same path as the light of wavelength 13, however is not reflected backward by
20 38a after circulating from port 1 to port 2 of 24. After propagating through 38a, the amplified
light of wavelength 14 propagates outward.
The Bragg grating 1 8c advantageously provides a means of preventing any unwanted light of
wavelength 11 or 12 reflected backward (i.e. from l9a) from reaching the output ports of 13 or
14.
Thus, the system shown in FIG 15, provides a means for launching 4 separate signals
of 4 different wavelengths, into the amplifying module 5, and provides 4 separate light
conduits for receiving each of the 4 amplified signals.
Of course, numerous other embodiments may be envisaged without departing from
the spirit and scope of the invention.
