Optical communication system

Details Optical communication system Optical communication system

1998-06-19

2001-12-04

Chan, Jason (Department: 2632)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

Reexamination Certificate

active

06327062

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a WDM optical communications system wherein uncooled lasers are utilized.
BACKGROUND OF THE INVENTION
Conventional Two-Fibre Transmission
FIG. 1
depicts a conventional two-fibre transmission link where blocks
101
and
102
can represent regeneration or central office sites. Connecting the two sites together is a fibre optic cable. Within the cable there are multiple strands of fibre
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 (&lgr;1) and one strand of an optical cable. Communication in the opposite direction uses a different strand of the optical cable and the same, or different, wavelength (&lgr;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 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 terminating transmission equipment is reached.
Depending upon the wavelength chosen for transmission, the strand of optical fibre
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 fibre 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.
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 fibre. For example, if the system is operating within a 1550 nm window, two signaling wavelengths of 1533 and 1557 nm might be used. For standard single mode fibre, the two main transmission “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 fibre
103
. Bi-directional transmission is achieved through the utilization of wavelength-division multiplexing (WDM) filters,
201
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 fibre. In the example shown, WDM filters
201
and
203
couple the two wavelengths 1557 and 1533 nm on and off a single fibre
103
of a fibre 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, 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 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 transmissive and reflective properties for a given—target—wavelength. For example, if &lgr;1 is 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 &lgr;1 and (2) the necessary isolation requirements for all other transmitted wavelengths, for example, a wavelength, &lgr;2, transmitted by a second transmitter.
The dichroic, or WDM, filter is constructed by placing self-focusing lenses, such as “SELFOC” lenses
310
, on either side of the dielectric substrate
305
. “SELFOC” lens
310
focuses incoming light (&lgr;1 and &lgr;2) to a particular location on the dielectric substrate.
Attached to the “SELFOC” lenses through an adhesive bonding process are, typically, 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 of the fight (comprised of &lgr;1 and &lgr;2) passing through fiber
335
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 &lgr;1, virtually all of the light at &lgr;1 passes through the dielectric substrate
305
and, via the second “SELFOC” lens, is 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 &lgr;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.
This technology could be used to, for instance, implement WDM filter
201
shown in FIG.
2
.
FIG. 4
is a variation of the system shown in
FIG. 1
, a two-fiber design where one wavelength (&lgr;1) is transmitted on one fiber in one direction, and another (or possibly the same) wavelength (&lgr;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: 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 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 such as that shown in FIG.
4
.
Illustrative Systems
FIG. 5
depicts one configuration for a dual wavelength, bi-directional narrow-band WDM optical amplifier module,
901
. Components used to construct the amplifier module
901
include: two WDMs,
201
and
203
(input and output ports of the 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 transmission utilizing a single-fibre strand of the cable facility
103
.
It

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