GB2294372A - Optical network - Google Patents

Abstract

An optical network 3 allows communication between a receiving station 1 and a plurality of transmitting stations 2, or vice versa. Signals transmitted by Fabry Perot lasers 14 are modulated by subcarriors each of a different frequency. Receiving device 15 receives signals which are separated by a set of filters 8 each selecting one of the sub-carrier frequencies f1, f2 or f3. The signals 5 are recovered from the sub-carriers by means of demodulators 9. In order to ensure that the carrier frequencies of the lasers 14 are suitably separated, a further filter 10 selects energy in a frequency range which does not contain transmission signals and may be of a different band width from filters 8. Filter 10 transmits only noise rather than signal. The control means comprises a function 11 which measures the noise passed by the additional filter 10. The control means returns instructions which can alter one or more operating conditions of a light source 14 such as its temperature or power. <IMAGE>

Description

PASSIVE OPTICAL NETWORK This invention relates to a passive optical network and to a method of improving transmission over such a network.

Passive optical networks may be used in communication systems.

Such a system comprises a plurality of transmitting stations and one or more receiving stations. One example of such a system is a local network comprising a plurality of business subscribers connected to a local exchange. Such a system is generally bidirectional. In other words data may be transmitted from the business subscribers to the local exchange or from the local exchange to the business subscribers.

Transmission of data from the local exchange to one or more of the business subscribers is known as downstream transmission whereas the transmission of data from one or more of the business subscribers to the local exchange is known as upstream transmission.

During upstream transmission, the business subscribers may be considered to be transmitting stations and the local exchange may be regarded as a receiving station. There may be more than one receiving station in such a communications system.

The transmitting stations communicate with the one or more receiving stations via a network of optical fibres and couplers.

Each upstream transmitting station comprises a light source, typically a laser diode, which is used as an optical carrier, carrying data in analogue or digital form. The information, in the form of, for example, video, data or voice services, is impressed onto a radio frequency subcarrier, before modulating one or more optical carriers.

Each electrical subcarrier has a different frequency to each other electrical subcarrier, and thus the data is frequency multiplexed by the subcarriers. This is known as subcarrier multiplexing and is used in broad band direct detection optical systems. Such systems enable video, data and voice services to be provided.

An optical coupler combines the separate upstream data paths.

A wide band receiver in the receiving station detects all channels and then demultiplexes the channels by use of, for example, radio frequency filters.

In such a system, it is known that a problem of optical interference exists during upstream transmission where a plurality of optical sources are present in the system. This is due to mixing of optical fields which takes place at the shared wide band receiver which is typically photo detector, and is an inherent property of the detection process. It gives rise to interference known as optical interference noise.

This interference is described, for example, by Can Desem in IEEE Journal on Selected Areas in Communications, Vol 8 No 7 September 1990, and by Bates and Walker in the IEE Electronics Letters Vol 27 No 12 Page 1014.

The interference occurs due to the fact that the laser diodes used at the transmitting stations have a narrow but finite spectral content, and if two or more of the lasers have spectral contents within a few hundred of Mhz of each other, an optical beat frequency will be produced, leading to interference.

In a system having N light sources, up to N(N-1)/2 interference beats can be produced in the worst case situation when all the spectral lines are so close as to beat together. For example, a system having 32 transmitting stations could in the worst case situation result in optical beat noise 496 times that of a single beat.

In passive optical networks, it is known to use Fabry Perot type lasers for the optical sources. Such lasers have a wavelength which changes significantly with temperature.

Therefore the number of lasers with near coincident wavelength will vary as the temperature of their transmitting station varies. Worst case conditions where 496 optical beats occur in a system of 32 transmitting stations may be very unlikely, but could occur at some time after installing such a system. Noise at such a high level may obliterate the messages rendering the system temporarily inoperable.

According to the present invention there is provided a passive optical network comprising: a plurality of transmitters, each transmitter comprising an electrical source providing one or more electrical subcarriers, and a light source providing an optical carrier; and a receiver comprising a demultiplexer, the system further comprising control means for controlling the output of selected one or more of the optical sources.

Preferably, each light source comprises a laser.

Advantageously, each light source comprises a Fabry Perot laser.

Conveniently, the demultiplexer comprises a radio frequency demultiplexer.

Fabry Perot lasers have the advantage of being relatively cheap and are a good of source of light having a narrow spectral content. However, the wavelength of the Fabry Perot laser changes with various factors, particularly with the temperature of the transmitting station within which the laser is installed. As mentioned above this can lead to problems due to optical beat noise. However, by means of the present invention, the control means is able to control the output of selected one or more of the optical sources thus removing the coincidence of wavelengths of a particular optical source with another optical source in the system.

Preferably, the control means controls the output power or operating temperature of the selected one or more of the optical sources.

In a passive optical network according to the present invention, the transmitters may also be transceivers as may the receiver, in a case where the system is bidirectional.

Advantageously, the demultiplexer comprises a plurality of subcarrier filters each filter adapted to filter out signals within a particular band of radio frequencies.

Each electrical source in each transmitter operates at a particular frequency. Each filter corresponds to an operating frequency of one electrical source, and the multiplexed signal may be demultiplexed through use of suitable filters.

Preferably, the control means comprises an additional filter operating at a part of the radio frequency spectrum close to the frequency at which the sub-carriers operate, but not receiving any energy from any of the sub-carriers. Control means having a filter operating at such a frequency is able to monitor the level of noise due to optical beats.

The control means comprises logic means for comparing measured noise with a stored reference value or relative to the energy measured in other parts of the upstream receiver.

The control means comprises measuring means for measuring the noise energy selected by the filter.

Advantageously the logic means comprises a microprocessor and memory.

The control means further comprises means for allowing the control means to communicate with any one or more of the transmitters depending on the level of noise detected. These further means may comprise logic devices such as microprocessors.

On detection of noise due to optical beats, the control means will send a signal to one of the upstream transmitters. This will result in the operating parameters of that transmitter being altered. Depending on the change in noise, further alterations to different stations or to that initial station may be necessary.

The invention will now be further described with reference to the accompanying drawing in which: Figure 1 is a schematic representation of a system according to the present invention; and Figure 2 is a schematic representation of the operating frequencies of sub-carriers and filters in the system of Figure 1.

Referring to Figure 1 an embodiment of the invention as shown having one receiving station 1 receiving signals from three transmitting stations 2 via a network of optical fibres 3 and passive optical coupler 4. The network may contain more than one receiving station. The system may have more than three transmitting stations. The network may have more than one passive optical coupler.

Although stations 2 have been described as transmitting stations, and station 1 as a receiving station, it is to be understood that the system could equally well be bidirectional, and each station 1, 2 could therefore comprise a transceiver such that communication of data in both directions may occur.

The light from each transmitting station is modulated in amplitude in order to carry messages 5 from the transmitting stations to the receiving station. The method of carrying these signals shown in Figure 1 is the so called 'sub-carrier' method. Each signal modulates a radio frequency carrier 6, known as the 'sub-carrier' in a modulator 7 so that the modulated radio frequency subcarrier modules the light level from the transmitting station. These subcarriers are shown as having frequencies fl, f2 and f3. The method is well known to those skilled in the art of optical communications. The light source 14 used in each of the transmitting stations will normally be a laser diode.

The receiving device 15 will normally be a PIN photo-diode or avalanche photodiode. The signals it receives will be separated by a set of filters 8 each selecting one of the subcarrier frequencies fl, f2 or f3. The signals 5 are recovered from the subcarriers by means of demodulators 9.

In one transmitting station there may be more than one subcarrier modulating one light source. It is also possible for one of the set of signals to be carried without using a subcarrier, transmission without a subcarrier being known as 'baseband' transmission. This will now be further explained with reference to Figure 2.

Referring to Figure 2 there is shown a graph showing the frequency of operation of the sub-carriers f1, f2 and f3. The vertical axis represents energy. These frequencies are designated as 22, 24 and 25 respectively. These show the radio spectra of sub-carriers f1, f2 and f3 when modulated with information. Curve 21 is known as base band transmission. In theoretical terms it has a carrier of zero frequency. In practical terms it has no carrier. Dotted line 30 represents the level of noise.

Dotted curve 23 represents the spectral passband of filter 10 which selects energy in a frequency range indicated by curve 22. This range must not contain transmission signals at the time of measurement. It can be used for transmission of signals at other times. The filter 10 could be of different band width from filters 8. Whereas the filters 8 receive signal and noise, filter 10 receives only noise.

The allocation of band widths 21 to 24 is arbitrary. Noise measurement could be made at any of these frequencies by suitable allocation of the frequency of the filters 8 and 10.

Filter 10 could, for example, be a base-band filter so that frequency band 24 became the band used for noise measurement.

The allocation of frequency bands to upstream transmitters is arbitrary also. Some transmitter stations could have one channel, ie one filter 8 at each upstream receiving station.

In other systems there may be up to four or more frequency bands allocated to a transmitter station. The filters 8 may have differing band widths.

It is also envisaged that the noise measuring channel 25 will be used at other times by each of the upstream transmitter stations in turn in order to send small quantities of information such as alarm status to the receiving station.

None of the upstream transmitters will be allowed to send data in this frequency band while noise measurements are being made.

The control means comprises a further function 11 which measures the noise passed by the additional filter 10. This function 11 also contains logic devices such as a microprocessor and is able to communicate instructions to the transmitting stations by a return path 12. It returns instructions which can alter one or more operating condition of a light source 14 (eg laser diode) which is known to alter its optical wavelength.

The filter 10 selects energy from a band of radio frequencies where no modulated sub-carriers fl, f2, f3 etc are present. It is therefore able to select only noise which it passes to the control means 11. The control means 11 further comprises measuring means for measuring the noise energy selected by the filter 10.

The noise energy selected by filter 10 could be measured for example by means of a non-linear device such as the diode rectifier which converts radio frequency energy selected by filter 10 to an electrical current or voltage. This current or voltage may be passed to an analogue to digital converter so that the microprocessor or logic circuits can operate on digital instead of analogue numerical values. Other ways of measuring the noise energy could be used.

Additionally, the control means further comprises logic means which may take the form of, for example, a micro-processor and memory facility. The logic means is adapted to compare the measured noise with previous measurements, against a stored reference value or relative to the energy measured or inferred in other parts of the upstream receiver such as the level of sub-carriers (individual or average) or the level of light received.

The logic means is adapted to act in different ways according to the result of these measurements. Such actions may reside in logic or in a micro-processor programme memory.

The actions include the formulation of revised operating conditions to be applied to an upstream laser 14 such as a change in output power or operating temperature of a selected laser.

The logic means is adapted to store the noise measurements before and after such changes are made to a laser operating condition. For example, a memory device may have the information entered into a table recording noise measurements before and after the change of output power of each laser 14, or before and after the change of temperature of each laser 14.

The control means further comprises means for conveying the revised operating condition (temperature or output), for example, downstream to the controller 13 to which the operating condition is to be applied. This downstream transmission will normally be available as part of the capability of downstream equipment of a bidirectional system. It could alternatively be carried out by some external system such as a lower power radio control system.

The controller 13 receives instructions then sets up the new operating condition for the appropriate laser 14 which is under its control, and then maintains the laser 14 operating under those conditions until they are again changed by the control means 11. The laser controller 13 must be able to distinguish instructions it is to act on from those destined for a different controller which it is not to act on.

The controller 13 can distinguish a message it is to act on by means of information contained in the message. For example, a well known message format known as High Level Data Link Control (HDLC) places a message in a "packet" with a prefix indicating the recipient for the message. Other means of distinguishing messages are also well known in the art, and the method used will depend upon the nature of the downstream transmission path. For example, it may be convenient to allocate a separate downstream channel to each upstream controller 13 instead of using an addressed packet message.

The control means is also capable of disabling any transmissions originating in any upstream transmitter falling within the frequency band of filter 10. It may re-enable such transmissions when the noise level has been reduced to a predetermined level. The control means will, from time to time, disable any transmissions within the frequency band of the filter 10 and make noise measurements to determine whether the operating conditions of lasers 14 require revision to reduce the level of noise.

Filter 10 is a radio frequency filter. In a mass production situation it is likely that each filter 10 would be a "surface acoustic wave" type. Any radio frequency filter implementation could be used. For example, inductor-capacity filters, quartz crystal filters, helical resonators and microstrip filters may all be appropriate for use.

Each transmitting station is equipped with a controller 13 able to control such conditions. The controller receives instructions from the noise measuring function 11 and acts on instructions addressed to that transmitting station. All transmitting stations 2 include a controller 13 each of which receives individual instructions from the noise measuring and logic function 11 at the receiving station 1 by the return path 12.

The system is permitted to operate normally until a noise level corresponding, for example, to the presence of five optical beats is observed. When the measurement at 10 detects noise worse than this it first determines which lasers are interfering.

It does this by slightly altering an operating condition such as output power (or temperature) of one laser diode at a time while observing consequential changes in noise level. If noise is reduced by altering a laser's operating condition it indicates that the laser is one of those causing an optical beat and should be moved to a different wavelength, for example, by changing its temperature by a small amount.

It will be noted that some of the exploratory changes in laser operational condition may have no effect, probably because that laser's wavelength is sufficiently different from the others not to cause interference. It is also possible that the noise level could rise because the change actually causes additional optical beats by bringing more lasers into close spectral coincidence-incidence. Although this will raise the noise level further above the threshold (of five times a single beat), it will raise it only the amount that can be caused by one more laser causing optical beat interference.

From these tests the logic circuit 11 determines which optical source should be moved in wavelength and then applies new operating conditions to these optical sources. It repeats this process until the noise level is brought below the noise threshold which triggers this adjustment process, for example, a level five times that of a single beat.

In a practical system it is likely that the optical sources will be laser diodes of Fabry Perot type. It is also likely that exploratory changes in wavelength will be made by small changes in output power. It is likely that once an interfering laser has been identified in this way that its temperature will be altered to move it to a new wavelength. Very small temperature changes are needed to change the operating wavelength of laser diodes of Fabry Perot type.

Such small temperature changes could be made by means of a heater or cooler in thermal contact with the laser diode.

Claims (12)

1. A passive optical network comprising: a plurality of transmitters, each transmitter comprising an electrical source providing one or more electrical subcarriers, and a light source providing an optical carrier; and a receiver comprising a multiplexer, the system further comprising control means for controlling the output of selected one or more of the optical sources.

2. A network according to claim 1 wherein each light source comprises a laser.

3. A network according to claim 1 or claim 2 wherein each light source comprises a Fabry Perot laser.

4. A network according to any one of the preceding claims wherein the demultiplexer comprises a radio frequency demultiplexer.

5. A network according to any one of the preceding claims wherein the control means controls the output power or operating temperature of the selected one or more of the optical sources.

6. A network according to any one of the preceding claims wherein the demultiplexer comprises a plurality of sub-carrier filters each filter adapted to filter out signals within a particular band of radio frequencies.

7. A network according to any one of the preceding claims wherein the control means comprises an additional filter operating at a part of the radio frequency spectrum close to the frequency at which the sub-carriers operate, but not receiving any energy from any of the sub-carriers.

8. A network according to any one of the preceding claims wherein the control means comprises logic means for comparing measured noise with a stored reference value or relative to the energy measured in other parts of the receiver.

9. A network according to any one of the preceding claims wherein the control means comprises measuring means for measuring the noise energy selected by the filter.

10. A network according to claim 8 or claim 9 wherein the logic means comprises a microprocessor and memory.

11. A network according to any one of the preceding claims wherein the control means further comprises means for allowing the control means to communicate with any one or more of the transmitters depending on the level of noise detected.

11. A network according to claim 11 wherein the further means comprises logic devices such as microprocessors.

12. A passive optical network substantially as hereinbefore described with reference to the accompanying drawings.

12. A passive optical network substantially as hereinbefore described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A passive optical network comprising: a plurality of transmitters, each transmitter comprising an electrical source providing one or more electrical subcarriers, and a light source providing an optical carrier; and a receiver comprising a demultiplexer, the system further comprising control means for controlling the output of selected one or more of the optical sources.

2. A network according to claim 1 wherein each light source comprises a laser.

3. A network according to claim 1 or claim 2 wherein each light source comprises a Fabry Perot laser.

4. A network according to any one of the preceding claims wherein the demultiplexer comprises a radio frequency demultiplexer.

5. A network according to any one of the preceding claims wherein the control means controls the output power or operating temperature of the selected one or more of the optical sources.

6. A network according to any one of the preceding claims wherein the demultiplexer comprises a plurality of sub-carrier filters each filter adapted to filter out signals within a particular band of radio frequencies.

7. A network according to any one of the preceding claims wherein the control means comprises an additional filter operating at a part of the radio frequency spectrum close to the frequency at which the sub-carriers operate, but not receiving any energy from any of the sub-carriers.

8. A network according to any one of the preceding claims wherein the control means comprises logic means for comparing measured noise with a stored reference value or relative to the energy measured in other parts of the receiver.

9. A network according to any one of the preceding claims wherein the control means comprises measuring means for measuring the noise energy selected by the filter.

10. A network according to claim 8 or claim 9 wherein the logic means comprises a microprocessor and memory.

11. A network according to any one of the preceding claims wherein the control means further comprises means for allowing the control means to communicate with any one or more of the transmitters depending on the level of noise detected.

11. A network according to claim 11 wherein the further means comprises logic devices such as microprocessors.