Optical add/drop multiplexer

Details Optical add/drop multiplexer Optical add/drop multiplexer

2001-08-07

2004-01-13

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06678080

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to optical data transmission and especially to the optical add/drop multiplexing used in an optical data transmission system.
BACKGROUND OF THE INVENTION
The capacity of an optical fiber can be multiplied by increasing the number of different wavelengths to be conveyed in the fiber. This may be effectively implemented by Wavelength Division Multiplexing (WDM). In fiber optical systems, it is possible to use various WDM techniques, e.g. a unidirectional WDM technique or a bi-directional WDM technique. In the unidirectional technique, the WDM multiplexer at the transmitting end of the fiber combines the different wavelengths in the same fiber, while the WDM demultiplexer at the receiving end separates the different wavelengths from each other. In a bi-directional WDM system, information of different wavelengths is transmitted simultaneously in the same fiber in opposite directions. WDM systems using 4-20 different wavelengths for bit rates of 1-10 Gb/s a channel are available commercially, but in the near future systems of 32 and 40 wavelengths will also be commercially available. All different wavelengths travelling in the fiber can be amplified at the same time by using a linear optical fiber amplifier (OFA) in connection with the WDM technique. To use the resources as efficiently as possible, the amplifiers ought to be used at their full capacity in either direction.
Conventional telecommunications equipment uses only one optical signal, that is, at each end there are an optical transmitter and an optical receiver. In wavelength multiplexing, many such independent transmitter-receiver pairs use the same fiber, as was mentioned earlier, either in one direction or in two directions. On the output side of the system there is hereby an optical multiplexer, to the inputs of which are connected several optical conductors, in each of which a certain wavelength &lgr;
n
is transmitted.
FIG. 1
illustrates a system including N parallel transmitter-receiver pairs. Each source of information modulates one optical transmitter, of which each produces light at different wavelengths &lgr;
1
. . . &lgr;
N
(N>1). The modulation bandwidth of each source is smaller than the interval between wavelengths, so that the spectra of modulated signals will not overlap. The signals produced by the transmitter are combined into the same optical fiber OF in a WDM multiplexer WDM
1
, which is an entirely optical (and often passive) component. The node of the optical bus may be an add/drop multiplexer (one or more; not shown in the figure), which through an optical fiber is connected with an entirely optical (and often passive) WDM demultiplexer WDM
2
, wherein a reverse operation is done on the multiplexing, in other words, every incoming wavelength &lgr;
n
is separated to its own optical conductor. Thereupon, each signal is detected at its own receiver. A narrow wavelength window in a certain wavelength range is made available to the different signals. A typical system including e.g. four transmitter-receiver pairs in parallel could be such, wherein the signals are within a 1550 nm wavelength range, so that the first signal is in a wavelength range of 1544 nm, the second at a wavelength of 1548 nm, the third at a wavelength of 1552 nm and the fourth at a wavelength of 1556 nm. Nowadays a 100 GHz (about 0.8 nm) multiple is becoming the de facto standard for the distance between wavelengths, and this is also recommended by the ITU-T, so it has a strong official position.
Optical add/drop multiplexing (OADM) means that in an optical network different wavelengths are conveyed in the same fiber through the network, so that in a node of the network certain desired wavelengths are added and certain wavelengths are dropped. There are two kinds of OADM architecture: 1) all wavelengths are demultiplexed and combined again, whereby some wavelengths are dropped and added while some bypass the equipment transparently, or 2) only drop wavelengths are demultiplexed and only add wavelengths are multiplexed. Complete demultiplexing will cause unnecessary losses for all wavelengths, which of course is not desirable. The OADM structures may be either dynamic or static.
Central parameters for the functioning of the OADM are the following:
Attenuation of bypassing wavelengths (which means those wavelengths, which are directed through the optical add/drop multiplexer transparently from the input gate to the output gate) as they travel through the OADM multiplexer.
Cross-talk from bypassing wavelengths to drop outputs.
Attenuation of drop wavelengths as they pass through the OADM to the drop outputs.
Cross-talk from drop wavelengths to the outgoing fiber.
Attenuation of add wavelengths as they pass from the add inputs through the OADM.
Cross-talk from add wavelengths to the drop outputs.
In an optical loop network only a certain amount of attenuation is permissible between transmitter and receiver. The attenuation is formed by the a)-d) sum of the following items:
a) add attenuation of the transmitting node,
b) drop attenuation of the receiving node,
c) bypass attenuation of all nodes to be bypassed,
d) attenuation of all conveying fiber links.
When item d) is determined by the locations of the connections to be set up, the a)-d) sum must be made sufficiently small in the loop, so that it will fit into the remaining attenuation margin. Besides attenuation, another significant matter is to make cross-talks sufficiently small in the nodes of the loop, for the reason that they would not significantly reduce the signal quality in the drop outputs.
FIG. 2
a
) shows a simple optical drop device, which is based on the Fiber Bragg Grating and on a three-gate circulator. The Fiber Bragg Grating is a grating which is made into an optical fiber for a chosen wavelength and which reflects back the concerned wavelength in the opposite direction. To other wavelengths the grating is transparent, and the grating will let these waves through the grating without reflecting them. The circulator directs the light arriving from gate
1
out through gate
2
and the light arriving from gate
2
out through gate
3
. In the case shown in the figure, the grating reflects back the wavelength &lgr;
3
and then drops it through gate
3
. Other wavelengths &lgr;
1
, &lgr;
2
and &lgr;
4
will pass through the grating. It is possible on the same path to add the add/drop functionality, whereby the same wavelength, which was dropped, may again be added, e.g. by a coupler, as is shown in
FIG. 2
b
).
FIG. 2
c
) shows another simple add/drop device, the Mach-Zehnder interferometer (MZI), which typically includes two 3 dB 2×2 couplers and an optical path, which combine the output gates of the first coupler with the input gates of the second coupler. The first coupler divides the light wave arriving from either input gate (i
1
, i
2
) equally to two output gates of the concerned coupler. Of the sub-waves having a phase difference of &pgr;/2 after bypassing the coupler, each one propagates along its own path into the second coupler of the interferometer. With the aid of two couplers a complete cross-connection is formed, whereby the light wave connected from the top (bottom) input gate of the first coupler is obtained from the bottom (top) output gate of the second coupler. A complete cross-connection may also be implemented in such a way that on the optical paths combining the couplers there is a mirror or a grating reflecting the desired wavelength back in its direction of arrival, whereby the wave will pass twice through the same coupler (
FIG. 2
d
). Hereby the wavelength connected from the first input gate and divided equally onto two optical paths will be reflected back to the same coupler and it will be connected out through the second input gate of the said coupler.
The optical transmission technique is being constantly developed in order to implement the lower levels of broadband network architectures as entirely optical systems, which would allow entirely optical relaying of high-capac

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