WO2007141547A1

WO2007141547A1 - Optical fibre network for radio frequency and microwave signal distribution

Info

Publication number: WO2007141547A1
Application number: PCT/GB2007/002137
Authority: WO
Inventors: David William Smith
Original assignee: The Centre For Integrated Photonics Limited
Priority date: 2006-06-10
Filing date: 2007-06-11
Publication date: 2007-12-13
Also published as: GB0611483D0

Abstract

Apparatus for the distribution of communications signals of radio or microwave frequency to a plurality of receiving units (10). The apparatus comprises a first optical modulator (2) configured to receive an electrical communications signal of radio or microwave frequency and to generate a downstream optical signal modulated by the communications signal. A plurality of receiving units (10) are configured to receive and demodulate the downstream optical signal to generate an electrical signal corresponding to the communications signal. An optical distribution network (3, 4, 5) is arranged to distribute the downstream optical signal from the first optical modulator (2) to the receiving units (10). Each receiving unit comprises a second optical modulator (18) arranged to modulate light received from the optical network with a data signal to generate an upstream optical signal and to return the upstream optical signal to the optical distribution network for onward communication. The apparatus has the advantage that it provides a return communication path from the receiving units (10) that use the same optical network as the downstream optical signal without requiring a separate laser source at each receiving unit (10).

Description

Optical Fibre Network for Radio Frequency and Microwave Signal Distribution

Field of the Invention

This invention relates to apparatus for the distribution of communications signals of radio or microwave frequency to a plurality of receiving units.

Background to the Invention

Optical fibre with its huge bandwidth and low loss has significant advantages for a broadband communications network and can be applied to the distribution of satellite TV signals. Up until now, the economics for optical fibre distribution over the distances encountered within a building are not attractive compared to coaxial cable. However, the requirement to supply signals from several channel bands to multiple end user equipment leads to a complex network architecture involving remotely configurable switches to overcome the limited bandwidth of coaxial cable. Alternatively, by using optical fibre within the distribution network all signals can be broadcast to all users on the network through a simple passive optical network using optical power splitters.

The use of point-to-point optical fibre transmission to remotely connect satellite earth stations to a television receiver is well known (see for example: the SATLIGHT series of optical fibre inter-facility links available from Foxcom, Inc. of Princeton, New Jersey, USA). These systems offer the performance advantages of fibre in terms of high bandwidth and low loss but do not exploit the ability of a passive optical network as a method of distributing very wideband microwave signals to a large number of end users. Moreover, no consideration is given to methods for the end user to efficiently send data signals back to the service provider using a common shared infrastructure.

There are both economic and aesthetic advantages to be gained from sharing the received signal from a single satellite dish to a multiplicity of users who reside within the same building or adjacent buildings. This is especially the case when some users do not have a clear line of sight to the satellite. The use of a distribution network based on coaxial cable featuring splitters and amplifiers is a known solution in these situations but has limitations because of the restricted bandwidth, high attenuation and physical size of coaxial cable. These limitations of coaxial cable are likely to be multiplied as new services become available requiring more bandwidth and end users have equipment requiring multiple coaxial cable connections. Furthermore, it is now also usual for there to be a separate electrical connection from the end user's set top box to a telecommunications network to enable interactive services and for service configuration. Reducing the amount of cabling within a direct broadcast satellite (DBS) distribution system is therefore a worthwhile objective.

A unidirectional optical distribution network can be made at relatively low cost when compared with the more usual bidirectional networks employed within telecommunications networks because only a single laser transmitter is required for a complete distribution network and the end user's terminal equipment consists principally of a photodiode receiver. However, for many signal distribution applications it is desirable to provide a lower bandwidth return channel for messaging and control purposes.

The provision of a return channel within an optical fibre distribution system can have a significant negative impact on the economics of the network. Whilst a fibre signal distribution system can be relatively low cost even for high bandwidth signals, such as television and high definition television (HDTV), because PESF diode receivers can be relatively simple, the addition of the laser and diplexer needed for bidirectional single fibre working significantly increases the per terminal cost of the network. This invention, at least in its preferred embodiments, seeks, to provide a reverse signalling channel from each end user at modest incremental cost by modification of the end user's optical receiver.

Summary of the Invention

Accordingly, this invention provides apparatus for the distribution of communications signals of radio or microwave frequency to a plurality of receiving units. The apparatus comprises: a first optical modulator configured to receive an electrical communications signal of radio or microwave frequency and to generate a downstream optical signal modulated by the communications signal; a plurality of receiving units configured to receive and demodulate the downstream optical signal to generate an electrical signal corresponding to the communications signal; and an optical distribution network arranged to distribute the downstream optical signal from the first optical modulator to the receiving units

According to the invention, each receiving unit comprises a second optical modulator arranged to modulate light received from the optical network with a data signal to generate an upstream optical signal and to return the upstream optical signal to the optical distribution network for onward communication.

Thus, the invention, at least in its preferred embodiments, provides simultaneous distribution of radio frequency or microwave signals over an optical fibre network to two or more separate terminals with the additional capability for each terminal being able to independently signal back to the network operator or service provider over the same said distribution network. The modified optical receiver allows some of the incoming light to be modulated and returned back up the distribution network to a point where it can be detected in equipment that is shared by multiple end users. The invention is expected to find application, for example, in the distribution of television signals from a shared satellite dish to a multiplicity of television receivers and video recording devices. The - A - invention may also find application in the distribution of television signals received from terrestrial transmitters and cable television networks.

The first optical modulator may be connected to an antenna and the electrical communications signal may be received, in use, via the antenna. For example, the antenna may be part of a satellite receiver. Thus, the communications signal may be a direct broadcast satellite signal.

Viewed from a broad aspect, the invention provides a scheme to distribute signals from one or more radio frequency or microwave receiving antennas over optical fibre to a plurality of optical terminal units each of which can be connected to equipment designed to process signals received from the antenna. Data signals, including control messages, generated within the signal processing equipment or by the user of the signal processing equipment can be relayed back to either the network operator or some other third party through any suitable communications network.

Typically, radio frequency (RP) and microwave communications signals fall within the range 30 kHz to 300 GHz. For modern communications signals, the frequency band is generally above 30 MHz, and particularly above 300 MHz. Similarly, typical communications signals are often below 30 GHz.

Optionally, the electrical output signal from an RP antenna or satellite dish may be amplified and used to either directly modulate a semiconductor laser or drive an optical modulator, either of which may then be connected to an optical fibre transmission line which in turn is connected to an optical splitter network with a plurality of outputs which are connected to one or more optical reception units. Thus, in embodiments of the invention, the first optical modulator may comprise a directly-modulated semiconductor laser.

One or more optical amplifiers may be included in the distribution network to mitigate against the optical power lost from the splitter network and fibre transmission path. Data signals generated within the end user's equipment (receiving unit) may be transmitted back to a predetermined location within the distribution network, using optical fibre. In a deviation from the invention, metallic conductors embedded within the same cable sheaf as that containing the optical fibre used for distributing signals to the optical terminal units may be used to distribute the data signals directly, without a second optical modulator.

Data signals generated within the end user's equipment (receiving unit) may be transmitted back to a predetermined location within the distribution network over the same optical fibre waveguide that is used for connecting the same user's optical receiving unit (optical terminal unit).

An optical diplexer or circulator may be used to allow an optical transmitter to share a single fibre connection with a photodiode-based optical receiver. The optical transmitter may contain a laser, light emitting diode or an optical modulator.

A combined photo-detector and reflective modulator may be used to allow bidirectional transmission without the need for an optical diplexer or circulator. Thus, the second optical modulator may be arranged to return the upstream optical signal to the optical distribution network by reflection.

One or more additional optical sources may be provided for the purpose of providing a seeding signal for feeding a plurality of reflective modulators. Thus, the apparatus may comprise an optical source for generating the downstream optical signal and a further optical source arranged to provide a seeding signal to the second optical modulators via the optical distribution network. Optical splitter boxes within the distribution network may be provided with an additional input port so that it is possible to connect an optical source to energise all the modulators within optical terminals (receiving units) fed from the splitter box. The optical source required to energise the modulators may be located in the head end equipment associated with the receiving antenna. The optical source for seeding the upstream modulators can also be used to provide an additional downstream data transmission path to a plurality of optical terminals by the use of either a suitable time division multiplexing (TDM) protocol or by either sub-carrier electrical multiplexing or some other multiplexing technique.

An electro-absorption modulator may be used to achieve both optical modulation and optical detection in one device. The second optical modulator may comprise an electro- absorption modulator arranged to demodulate the downstream optical signal to generate an electrical signal corresponding to the communications signal and to modulate light received from the optical network with a data signal to generate an upstream optical signal. In general, the electro-absorption modulator operates, in use, in reflection mode. The electro-absorption modulator may be either an edge-coupled waveguide device or a surface normal device. The wavelength of light to be modulated may be at a wavelength close to the absorption band-edge. The light to be detected by the electro-absorption device operating as a receiver maybe shorter than the absorption band-edge. Thus, the wavelength of the downstream optical signal may be shorter than the absorption band- edge of the electro-absorption modulator.

A time division multiple access technique with associated ranging protocols where necessary may be used to avoid data collisions between signals transmitted from a plurality of network terminals. A time division multiple access technique may be used that avoids the requirement for a distance ranging protocol when the ratio of the maximum transit time difference between end terminals is much less than the burst length. A tuning reference signal may be distributed to all terminal equipment attached to the fibre distribution network so that each terminal can determine when to send data bursts.

Electrical sub-carrier multiplexing or code division multiplexing techniques may be used to avoid interference between signals transmitted from a plurality of network terminals. Optical wavelength multiplexing techniques may be used to avoid data collisions between signals transmitted from a plurality of network terminals.

The downstream optical signal and the upstream optical signal may have different wavebands. Signals transmitted from a plurality of optical terminals may be transmitted over a fibre network to an optical receiver which may be located in equipment at the head end of the network or associated with splitter boxes distributed throughout the network. Thus, the apparatus may comprise an optical receiver arranged to receive the upstream optical signal via the optical distribution network and retransmit the data signal via an external communications network. Data received at an optical receiver located within the distribution network or at the head end of the distribution network may retransmit the received data to one or more parties via a suitable external communication network which might typically compromise of one or more of the following: microwave satellite link; terrestrial microwave radio link; cellular radio network; twisted pair cable; coaxial cable; optical fibre cable.

The invention extends to a receiving unit adapted for use in the apparatus described above and to the use of an electro-absorption modulator in reflection mode in the second optical modulator of the apparatus.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows the layout of an optical distribution network for signals from a satellite receiving dish to multiple end users;

Figure 2 shows the layout of an optical distribution network with a reverse signalling channel according to an embodiment of the invention;

Figure 3 shows the configuration of the equipment used to extract the reverse signalling channel in the embodiment of Figure 2; and

Figure 4 shows the configuration of the end user's optical terminal equipment in the embodiment of Figure 2.

Detailed Description of Embodiments Figure 1 shows the layout of an optical fibre distribution scheme for signals from a satellite receiving dish. The satellite receiving dish 1 includes a low noise block converter (LNB) and is connected to a laser transmitter 2. The output of the laser transmitter 2 is coupled by single-mode optical fibre 3 to an optical splitter unit 4. Outputs from the splitter unit 4 are either coupled to optical terminal units 6 by optical fibre 5 or to additional splitter units 4. The electrical output of the optical terminal unit 6 is connected by electrical cable 7 to a set top box or television receiver 8.

The output from the satellite dish LNB 1 modulates an optical carrier wave through either direct modulation of laser injection current or by use of an external modulation device such as an electro-absorption modulator or Mach-Zehnder modulator within the optical transmitter 2. The modulated output is connected to a single mode fibre 3 which is in turn connected to an optical splitter 4 that distributes the optical power into a number of output ports. These output ports can be coupled to further fibres which in turn may be coupled to further splitter units 4 or optical receiver units 6 as required. The optical receiver units 6, only one of which is shown in Figure 1, are normally located either within the set top box associated with a television receiver 8 or in a unit close to the set top box. It is also possible that optical receivers 6 connected to the distribution network could be incorporated in other types of terminal equipment such as personal computers, video recording devices or within TV receivers.

Figure 2 shows the layout of an optical distribution scheme for signals from a satellite receiving dish according to an embodiment of the invention. In this embodiment, the optical distribution network includes a reverse signalling channel 11 to enable individual end users to pass signals to the network operator or service provider over an external communications network 13. To send data and control signals from the set top box back to the service provider it is possible to either use a completely separate physical network, such as the copper pair telephone network, or an optical channel sharing the same fibre connection as the optical receiver 6. The optical channel has a number of advantages as it shares the same physical network as the signals distributed from the satellite dish 1 and does not require an additional network connection to be provided to the set top box as is common in current direct broadcast satellite (DBS) set top boxes. For example, connection to the telephone network is required to provide a return data channel for the Sky® digital television service in the United Kingdom.

Figure 3 shows the configuration of the equipment 9 used to extract the reverse signalling channel at a central point within the optical network in the embodiment of Figure 2. A wavelength selective optical coupler 14 couples the signal from the optical fibre 3 into an optical circulator 17. Signal from the reverse signalling channel are converted to electrical signals by a photodiode receiver 16 for onward transmission to the communications network 13 via an data connection 12. A light source 15, typically a laser, provides light at a wavelength appropriate to reverse signalling channel.

Figure 4 shows the configuration of the end user's optical terminal equipment 10 in the embodiment of Figure 2. The optical connection from the network is through the optical fibre 5 and is made to a combined photodiode electro-absorption modulator device 18. The electrical signal to and from the combined modulator /photodiode 18 is connected to an electrical circuit 19 which can separate the two directions of transmission, typically an electrical filter with separate high pass and low pass connections. The use of the combined photodetector/ modulator 18 provides a return channel over the optical fibre distribution network without additional optical components within the end user's wall plate unit or set top box. The preferred embodiment for the device that provides the desired combination of photo-detection and modulation is an electro-absorption modulator operating in reflective mode. By varying the bias voltage to the device 18 it is possible to shift the position of the absorption band edge to shorter or longer wavelengths and hence modulate the transmission of the device 18 by altering the bias voltage. Light absorbed by the device 18 produces photocurrent which allows the same device to also be used as a photodiode. A preferred method of using this arrangement for modulation and detection within a single device is to ensure that the downstream wavelength conveying the signals received from the satellite dish 1 is at a shorter wavelength than the absorption edge of the device 18 and the light to be modulated for the return path is at a longer wavelength to coincide with a position where the slope of the absorption edge is steep.

A further refinement is to place a mirror stack or reflective coating on one facet of the device 18 (electro-absorption modulator), preferably the facet opposite the fibre connection 5, so that the device can operate in reflective mode enabling a single fibre connection to be used for both send and receive. The combined modulator/detector device 18 can be constructed either as an end-fire waveguide device or as a surface normal device. The former configuration allows a higher modulation index to be achieved for the return path but increases coupling losses for the downstream path whereas the surface normal approach can reduce coupling losses and ease alignment for the downstream path but at the expense of reduced modulation index.

Because of the asymmetric bandwidth required for the downstream distribution of TV signals (high bandwidth) compared to the upstream control and signalling data from the individual optical terminals connected to set top box the system is required to provide the distribution of comparatively broadband TV signals to a number of users whereas the reverse channel signalling from the optical terminal for the purposes of service and network control is usually a much lower bandwidth signal it is possible to design a system with asymmetrical power budget for the two directions. The asymmetric power budget allows optical modulators to be used for the return path with relatively high loss and lower modulation depth to be used. It is also possible for several reflective modulators to be fed from a common shared laser source located within the distribution network. To share the bandwidth of the return channel between multiple users a time-division multiple access (TDMA) protocol may be used as is known for passive optical networks used for telephony applications. However, because of the comparatively short distances involved it is possible to simplify aspects of the TDMA protocol used with passive optical networks for this new application. In particular, ranging may not be required when the transit time over the distribution network is much less than the time duration of the transmission bursts. In addition to the use of TDMA to multiplex the return signals from multiple users it is possible to use other schemes such as code division multiple access (CDMA) or electrical sub-carrier multiplexing.

To further increase the number of end users that can be connected to the optical network it is possible to increase the power budget for the downstream path through the use of optical amplifiers. Either Erbium doped fibre amplifiers or semiconductor optical amplifiers may be considered for this application. For a distribution network system where the downstream signal wavelength is within the 1550nm waveband an Erbium fibre amplifier is a suitable solution. Where it has been necessary to use the 1310 run wavelength region for downstream signals so that longer wavelengths can be reserved for the reverse signalling channel, a semiconductor optical amplifier with good linearity is the preferred solution. When an optical amplifier is required to increase the downstream power budget it may be included either immediately following the laser transmitter 2 or at some intermediate position within the distribution network.

In summary, apparatus for the distribution of communications signals of radio or microwave frequency to a plurality of receiving units 10. The apparatus comprises a first optical modulator 2 configured to receive an electrical communications signal of radio or microwave frequency and to generate a downstream optical signal modulated by the communications signal. A plurality of receiving units 10 are configured to receive and demodulate the downstream optical signal to generate an electrical signal corresponding to the communications signal. An optical distribution network 3, 4, 5 is arranged to distribute the downstream optical signal from the first optical modulator 2 to the receiving units 10. Each receiving unit comprises a second optical modulator 18 arranged to modulate light received from the optical network with a data signal to generate an upstream optical signal and to return the upstream optical signal to the optical distribution network for onward communication. The apparatus has the advantage that it provides. a return communication path from the receiving units 10 that use the same optical network as the downstream optical signal without requiring a separate laser source at each receiving unit 10.

Claims

1. Apparatus for the distribution of communications signals of radio or microwave frequency to a plurality of receiving units, the apparatus comprising: a first optical modulator configured to receive an electrical communications signal of radio or microwave frequency and to generate a downstream optical signal modulated by the communications signal; a plurality of receiving units configured to receive and demodulate the downstream optical signal to generate an electrical signal corresponding to the communications signal; and an optical distribution network arranged to distribute the downstream optical signal from the first optical modulator to the receiving units; wherein each receiving unit comprises a second optical modulator arranged to modulate light received from the optical network with a data signal to generate an upstream optical signal and to return the upstream optical signal to the optical distribution network for onward communication.

2. Apparatus as claimed in claim 1 , wherein the first optical modulator is connected to an antenna and the electrical communications signal is received, in use, via the antenna.

3. Apparatus as claimed in any preceding claim, wherein the communications signal is a direct broadcast satellite signal.

4. Apparatus as claimed in any preceding claim, wherein the first optical modulator comprises a directly-modulated semiconductor laser.

5. Apparatus as claimed in any preceding claim, wherein the second optical modulator is arranged to return the upstream optical signal to the optical distribution network by reflection.

6. Apparatus as claimed in any preceding claim, wherein the second optical modulator comprises an electro-absorption modulator arranged to demodulate the downstream optical signal to generate an electrical signal corresponding to the communications signal and to modulate light received from the optical network with a data signal to generate an upstream optical signal.

7. Apparatus as claimed in claim 6, wherein the electro-absorption modulator operates, in use, in reflection mode.

8. Apparatus as claimed in claim 6 or 7, wherein the wavelength of the downstream optical signal is shorter than the absorption band-edge of the electro-absorption modulator.

9. Apparatus as claimed in any preceding claim comprising an optical source for generating the downstream optical signal and a further optical source arranged to provide a seeding signal to the second optical modulators via the optical distribution network.-

10. Apparatus as claimed in any preceding claim, wherein the downstream optical signal and the upstream optical signal have different wavebands.

11. Apparatus as claimed in any preceding claim further comprising an optical receiver arranged to receive the upstream optical signal via the optical distribution network and retransmit the data signal via an external communications network.

12. A receiving unit adapted for use in the apparatus of any preceding claim.

13. Use of an electro-absorption modulator in reflection mode in the second optical modulator of the apparatus of any preceding claim.