Optical switch

Details Optical switch Optical switch

1999-11-16

2001-04-10

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S326000

Reexamination Certificate

active

06215570

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to the handling of optical telecommunications signals in digital form. More particularly it is concerned with optical signals in a time division multiplex format and on the separation of channels for supply to different terminal equipment. In many cases, one channel is removed and replaced by a new signal.
Optical telecommunication is particularly attractive because of the high speed of optical systems. In fact, optical systems have developed to the stage where it is difficult, sometimes impossible, to design electrical or electronic circuitry which is capable of matching the operating speeds of the fastest optical systems. In such fast systems it is appropriate or necessary that the signal processing be carried out by all optical equipment.
As mentioned above, this invention is particularly concerned with optical signals in time division multiplexed format. In such a format it is possible that each individual channel may be slow enough for processing in high speed electronic equipment but the multiplex may be too fast. For example, if four channels are multiplexed the bit rate of the multiplex will be four times the bit rate of each individual channel. Thus if each channel is operating at only 75% of the maximum speed available electronically the multiplex will be operating at 3 times the limit. In such a system it is clearly necessary that the multiplex be handled optically whereas the individual channels can be processed electronically.
It is appropriate to distinguish between two versions of time division multiplex format. These two versions are conveniently designated as “byte interleaved” and “bit interleaved”. The byte interleaved format is more familiar than the bit interleaved. Each “byte” comprises a plurality of bits, usually 8, and the byte represents a unit of transmission. In the case of digitised analogue signals, e.g. digitised audio or digitised video, each byte represents a single sample of the analogue signal. In the case of data transmission each byte usually represents a single symbol of the data, e.g. an alphanumeric character. In the byte interleaved version of time division multiplex format each slot of the multiplex relates to its own channel and it contains one byte relating to that channel.
The bit interleaved version of the time division multiplexed format is less familiar and each slot contains only one bit. The signals will normally consist of bytes as described above but each byte is spread over a plurality of slots (instead of the more usual version wherein the whole byte is contained in one slot). As mentioned above, a byte usually consists of 8 bits and, therefore, in bit interleaved format such a byte is spread over 8 slots.
“Electronic Letters” 30 (1994) 3rd February 1994 at pages 255 and 256 describes a laboratory experiment which demonstrated an all-optical time division multiplex to wavelength division multiplex conversion using four wave mixing in a semiconductor optical amplifier. The discussion is limited to demultiplexing and nothing is said about the removal and replacement of a channel.
SUMMARY OF THE INVENTION
This invention relates to techniques for the handling of high speed optical telecommunications signals.
This invention, which is more fully defined in the claims, relates to
(a) optical switching means for separating channels from optical signals in time division multiplex format, and replacing the removed channels by new signals modulated with local data,
(b) telecommunications stations which include the switching means, and
(c) telecommunications systems which include the stations.
The invention also includes methods of handling optical telecommunications signals in time divisional multiplex format.
The invention is based upon applying wavelength modulation to optical signals which are already modulated with data in a time division multiplex format. The modulation applies characteristic wavelengths to different channels of the multiplex. For example, to achieve the separation a primary wavelength is applied to all channels except selected channels and a complementary wavelength is applied to the selected channels. Having applied the wavelength modulation, the channels are separated by a suitable network of wavelength selective splitters so that signals having the primary wavelength go to one port and signals with the complementary wavelength go to a different port. This achieves the separation and the separated signals can, if desired, be converted into electrical form for further processing. The replacement is achieved by generating new signals at the primary wavelength in synchronisation with received time division multiplex. The wavelength selective splitters provide the new signals to the correct output terminal with appropriate synchronisation.
In preferred embodiments of the invention the wavelength modulation is achieved utilising clock signals generated in synchronisation with the received time division multiplex. The clock signals includes the wavelength modulation which defines the intended separation of the time division multiplex signals. The clock signals and the time division multiplex are combined preferably using an optical AND-gate. (An optical AND-gate has two input terminals, i.e. one for the time division multiplex and one for the clocks signals. The AND-gate produces an output signal when both of its inputs receive a signal. When an output is produced the output has the same wavelength as the clock signal. It will be appreciated that an AND-gate of this nature makes the appropriate combination of wavelength and data modulation).
This invention is particularly suitable for use in conjunction with signals which have a pulsed waveform. That is to say each timeslot potentially (depending on the data modulation) contains a signal pulse which has a low, preferably zero, intensity at the beginning of the slot. The intensity rises to a maximum within the slot, preferably at the middle of the slot, and then the intensity becomes low, preferably zero, at the end of the slot. It is emphasised that, in real transmissions, timing is unlikely to be prefect and the timing imperfections are often designated as “jitter”. It is emphasised that while it is desirable to make the timing as accurate as possible satisfactory communication is maintained provided that the intensities at the beginning and end of slots are sufficiently low and the intensities in the middle of the slots are sufficiently high.
Both the time division multiplex signals and the clock signals have the same basic pulsed waveform but the nature of the modulation is different in each case. In the case of the traffic signals all the pulses have the same wavelength and the modulation takes the form of the presence and absence of pulses. The presence of a pulses usually indicates a logical “one” and in that case the slot contains a pulse as described. Other slots relate to a logical “zero” and in this case there is no pulse in the relevant slot, e.g. the intensity remains low, (preferably zero) throughout the slot. In the case of clock signals there is a pulse as described in every time slot but the pulses have different wavelengths to define the destination of signals in that particular slot. Because a clock signal is separately generated at each location the clock signals should be subject to less jitter than the traffic signals.
With pulsed signals as described the function of the AND-gate can be defined as follows. When a pulse is received at both terminals the clock pulse is transmitted so that the output has the same wavelength as the clock pulse. When a clock pulse is received in the absence of a traffic pulse the AND-gate has no output and nothing is transmitted. (The possibility that there is no clock pulse need not be considered since there is a clock pulse in every timeslot).
The AND-gate is conveniently implemented as a loop mirror which contains a semi-conductor amplifier located therein preferably symmetrically. A loop mirror involves a waveguide which is fed from both ends simultaneously so t

Affiliated with

Also associated with

Rating

Optical switch does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Optical switch, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical switch will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2459219