Optical communications – Multiplex – Optical switching
Reexamination Certificate
2000-06-16
2004-08-03
Sedighian, M. R. (Department: 2633)
Optical communications
Multiplex
Optical switching
C398S045000, C398S048000, C398S051000, C398S055000, C398S056000
Reexamination Certificate
active
06771906
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical line distribution frame which can be used in a telecommunication exchange, in particular when information bit rates are very high. The distribution frame of the invention is principally an all-optical distribution frame but some functions can be implemented by conversion to an electronic mode followed by return to an optical mode.
2. Description of the Prior Art
In the field of distribution of lines, also referred to as switching or cross-connection, the function of a distribution frame is to enable a signal conveyed by one of N input lines of the distribution frame to be routed to one of N′ output lines of the distribution frame. To simplify the description it is assumed that N′=N, firstly because a call normally requires the same number of calling and called parties and secondly because it can be shown that any other organization can be reduced to an N by N type distribution.
In an all-optical distribution frame the N input lines are optical lines, i.e. individual optical fibers. As an alternative to this, one fiber can convey a plurality of signals simultaneously using wavelength division multiplexing. The signals conveyed by the individual optical fibers can be amplitude-modulated onto carriers with the some wavelength for all of them or with different wavelengths.
A distribution frame core normally includes frequency domain cross-connection modules. To this end, all the separate optical signals terminating at a cross-connection module modulate carriers with different wavelengths. The signals are therefore “colored” by the different wavelengths. The input of a cross-connection module amalgamates or mixes all the signals to be distributed at the same time and distributes the whole of this combination each time between a plurality of output channels. Frequency domain filters in each channel select a single wavelength, i.e. a single optical signal. The combination of the coloring function, the cross-connection function and the filter function achieves the required selective routing.
However, frequency domain cross-connection means that the energy distributed between the channels is shared, and therefore reduced in each channel, simply by virtue of the fact that all the signals are present in all the output channels.
Frequency domain cross-connection is complemented by spatial switching using space switch modules to complement frequency domain cross-connection, in particular to prevent too great a loss of energy if the number of output channels is too high. A space switch can include a mirror which reflects an optical signal emanating from a termination of an optical fiber to one of K terminations of receiving optical fibers. A K by K switch would therefore include K mirrors. It is equally feasible to connect optical fiber ferrules directly to each other. A space switch module is normally opto-mechanical whereas a frequency domain module is all-optical or opto-electronic. All-optical solutions, i.e. solutions with no mechanical moving parts, can be envisaged for spatial cross-connection.
An architecture of the above kind gives rise to two problems. Firstly, frequency domain cross-connection leads to high losses and requires the optical signal to be regenerated before subsequent routing. Secondly, frequency domain cross-connection requires coloring devices whose function is to convert a signal conveyed by a wave at a wavelength &lgr;i into a signal conveyed by a wave at a wavelength &lgr;i. All-optical converters, or more generally opto-electronic converters, of this type are known in the art. These converters are the least reliable components in a distribution frame. They break down. To prevent the harmful consequences of these breakdowns, the circuits of a normal distribution frame include converters which are redundant compared to the number of optical signals to be processed.
For example,
FIG. 1
shows a prior art distribution frame in which an input block receiving P optical signals includes P converters IWT&lgr;
1
n°
1
to IWT&lgr;
1
n°
8
(for simplicity P=8 in this example). To enable the addition of a redundant converter IWT&lgr;
1
n°p (“p” signifying “protection”), the converters must be preceded by a P to P+1 switch (here an 8 to 9 switch) and followed by a P+1 to P switch. In the solution shown, an input block therefore provides P (8) signals at a wavelength &lgr;
1
. Other input blocks among the M available blocks (M=16 in this example) produce signals with wavelengths from &lgr;
2
to &lgr;M. Each of the P outputs of an input block is assigned one of P ranks i. The N=P×M outputs of the M input blocks are connected to the inputs of P star couplers each of which has M inputs of a distribution core C. However, there can be a greater number of star couplers if each of them has fewer inputs. A coupler has the same number of outputs and inputs. The assigned outputs are of rank i.
For example, a first star coupler receives signals from all outputs of rank
1
of the output switches of the input blocks. A final star coupler, coupler number P (number 8), receives signals from the inputs of rank P of the output switches of the input blocks. In other words, each star coupler receives at its inputs signals with different wavelengths. A coupler of this kind therefore mixes all the signals and distributes them to all its outputs. The mixing involves no risk of degrading the quality of the signals since their colors (wavelengths) are different. Nevertheless, and due entirely to the fact that the signals are distributed between a large number of outputs, the energy that can be distributed is inevitably reduced in proportion to the number of outputs.
It follows from what has already been stated that cross-connection can advantageously be complemented by space switching. In this example, all the output channels of rank i of the P star couplers are connected to P inputs of a space switch of rank i. In practice a space switch of this kind therefore receives at its input P mixes of signals colored by wavelengths &lgr;
1
to &lgr;M. In an architecture of the above kind a space switch therefore switches groups of signals, i.e. the mixes, rather than individual signals.
The outputs of the space switches are connected to filters for extracting a single wavelength in each mix. The filters and the space switches are controlled in accordance with orientation commands OR processed by a central control unit G.
Output blocks take the signals from the filter outputs and color them with a wavelength suited to their subsequent routing. Like the input blocks, the output blocks include converters. Theses converters suffer from the same lack of reliability as the input converters. They are also complemented by redundant converters.
From the practical point of view, for reasons of reliability, even the distribution core C is duplicated. Thus all the output switches of the input blocks, the star couplers, the space switches, the filters and the input switches of the output blocks are present twice over.
Various technologies are feasible for these various units. If the technology of the output switches of the input blocks and the input switches of the output blocks is a switching technology, energy losses are incurred of the order of 4 dB for each signal. If the technology is a broadcast technology (of the kind used in a star coupler) the losses are higher. The losses depend on the number of outputs and therefore on the number of inputs of the switch. The loss is 6 dB if this number is equal to eight, as shown here.
A star coupler has the same disadvantages and, especially if it is a 16 by 16 coupler, its transmission loss for each signal transmitted on each line is 12 dB. The space switch has a loss of 9 dB. Simplifying, it can therefore be assumed that a distribution core like that shown in
FIG. 1
causes a loss of 29 dB on each signal. This loss can be compensated, in particular in the converters of the input blocks and the output blocks, by ampl
Bouette Claude Le
Ollivier François-Xavier
Sotom Michel
Alcatel
Sedighian M. R.
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