WO2022208086A1

WO2022208086A1 - Optical apparatus for use in an optical network

Info

Publication number: WO2022208086A1
Application number: PCT/GB2022/050802
Authority: WO
Inventors: Alistair POUSTIE; James Harrison
Original assignee: Rushmere Technology Limited
Priority date: 2021-03-31
Filing date: 2022-03-30
Publication date: 2022-10-06
Also published as: GB202104651D0

Abstract

There is presented an apparatus for use in an optical line terminal of an optical network and for: A) transmitting first optical signals to one or more end user terminals via an optical fibre; B) receiving second optical signals from the one or more end user terminals via the optical fibre. The apparatus comprises I) a transmission system comprising one or more optical transmitters and configured to transmit the first optical signals to the one or more end user terminals; the first optical signals being time division multiplexed; II) a receiving system comprising a plurality of optical receivers and for receiving the second optical signals transmitted from the end user terminals; the receiving system comprising a wavelength division multiplexed system.

Description

Optical apparatus for use in an optical network

Field

The present application is in the field of optical networks, In particular, but not limited to Passive Optical Networks (PONs) and apparatus for use in an Optical Line Terminal (OLT)

Background

Passive optical network (PON) systems are increasingly used to connect home users to the internet.

In these systems, an optical line terminal (OLT) positioned several kilometres from the home users is connected by optical fibre and a passive optical power splitter to each home. Each home has an optical network unit (ONU) that receives the downstream optical signals and transmits optical signals upstream to the OLT. Figure 1 shows an example of an existing time division multiplexed (TDM) PON system 102 of the prior art, such as Ethernet PON (EPON) or Gigabit PON (GPON). In this example, a single optical transmitter 107 at the OLT (100) is broadcast over a feeder optical fibre 103 to the ONUs (104) in a downstream direction 'D' for each user on the PON 102 via an optical power splitter (105). In the upstream direction, labelled 'U' in figure 1, each user is allocated a specific time slot to send data, and there is a single burst mode optical receiver 106 at the OLT 100. An optical device 101, such as an optical filter or circulator, separates the upstream data from the downstream data. A number of optical power splits 105 can be used with the PON and typically a 32-way power splitter is used to share the data sent from the OLT 100 along the feeder fibre 103 with a plurality of ONU's 104. In hybrid fibre cable networks for example, the OLT 100 is often housed remotely in a weather sealed box, not requiring a building to maintain temperature and a stable environment. This remote OLT box is typically restricted in physical volume to less than 0.1m³ to mechanically site the OLT on a cable. This in turn restricts the number of optical transceivers that can be sited at the remote OLT 100 to generate the downstream 'D' PON optical signals. In telecom fibre networks for example, the OLT 100 can be housed in an exchange or central office building and may take the form of a line card or pluggable module.

Any time-division multiple access (TDMA) PON 102 has a restriction on the upstream 'U' bandwidth since it is shared amongst all the end users through the use of time division multiplexing (TDM), and each user ONU 104 needs a laser source capable of transmitting data at the full upstream data capacity of the PON.

Summary In a first aspect there is presented an apparatus for use in an optical line terminal of an optical network and for: transmitting first optical signals to one or more end user terminals via an optical fibre; receiving second optical signals from the one or more end user terminals via the optical fibre; the apparatus comprising: A) a transmission system comprising one or more optical transmitters and configured to transmit the first optical signals to the one or more end user terminals; the first optical signals being time division multiplexed; B) a receiving system comprising a plurality of optical receivers and for receiving the second optical signals transmitted from the end user terminals; the receiving system comprising a wavelength division multiplexed system. The first aspect may be adapted accordingly to any feature or configuration disclosed herein including, but not limited to, any of the optional features provided in this summary section.

In a second aspect there is presented an apparatus for use in an optical network and for transmitting optical signals down an optical fibre to a plurality of end user terminals; the apparatus comprising: I) a transmission system comprising one or more optical transmitters; the transmission system configured to transmit first optical signals to the end users via the optical fibre; the first optical signals centred on a first set of one or more different wavelengths; II) a receiving system for receiving second optical signals transmitted from the end user terminals; the receiving system comprising a plurality of optical receivers configured to detect the second optical signals; the second optical signals centred on a second set of different wavelengths; wherein the second set of wavelengths comprises more wavelengths than the first set of wavelengths. The second aspect may be adapted accordingly to any feature or configuration disclosed herein including, but not limited to, any of the optional features provided in this summary section.

The apparatus may be configured such that the transmission system transmits time division multiplexed optical signals.

The apparatus may be configured such that the receiving system comprises a wavelength demultiplexer configured to: receive wavelength multiplexed optical signals from the end users; output wavelength demultiplexed optical signals to the plurality of optical receivers.

The apparatus may be configured such that at least one optical receiver is configured to receive continuous second optical signals.

Preferably all optical receivers of the apparatus are configured to receive continuous second optical signals. Any electrical circuitry, such as processors and controller that receive electrical signals from the plurality of detectors detecting the second optical signals may be configured to process the electrical signals in accordance with rules or protocols on processing multiple continuous transmission signals. For example, the processor may not need to treat the detector signals as those arising from burst mode data. The processor may thus process the data in accordance with continuous transmission characteristics. The processed data may be used to generate one or more electrical signals to control further upstream transmission to other parts of the network, for example providing an electrical control signal to drive a further optical source, such as a laser (optionally combined with an optical modulator) to transmit further optical signals upstream, for example to other networks outside of the local PON.

The apparatus may further comprise a routing component configured to: I) receive the first optical signals from the one or more optical transmitters and route the received first optical signals into the said optical fibre for transmission to the end users; II) receive the second optical signals as a wavelength multiplexed set of optical signals transmitted via the said optical fibre; and route the wavelength multiplexed optical signals to the receiving system.

The apparatus may be configured such that the transmission system comprises a plurality of optical transmitters wherein at least a first optical transmitter outputs optical signals at a different central wavelength than a second optical transmitter.

The apparatus may further comprise a wavelength multiplexer configured to receive the optical signals from the first and second optical transmitters and wavelength multiplex them into a combined light path, for transmission down the said optical fibre towards the end users.

The apparatus may be configured such that the routing component is configured to receive the wavelength multiplexed first optical signals and route them along the said optical fibre towards the end users.

The apparatus may be configured such that the number of optical receivers is greater than the number of optical transmitters.

There is further presented an optical line terminal comprising the apparatus of the first or second aspects.

There is further presented an end user terminal for use in an optical network and for receiving optical signals down an optical fibre; the end user terminal comprising: I) an optical receiver configured to receive first optical signals from the network and use time division demultiplexing to output third data based on the detection of the first data; II) an optical transmitter configured to output second optical signals for upstream transmission along the optical fibre. The end user terminal may be adapted according to any feature of configuration disclosed herein, including but not limited to the following optional feature. The end user terminal may be configured such that the optical transmitter is configured to output second optical signals in a continuous transmission mode.

There is further presented a collection of optical end user terminals comprising a plurality of the end user terminals as described above, and for use in an optical network, wherein the optical transmitter of a first of the end user terminals outputs second optical signals at a different central wavelength than the optical transmitter of a second of the end user terminals.

The collection may be configured such that each of the end user terminals of the plurality of end user terminals comprises an optical transmitter configured to output light at a different central wavelength than the optical transmitters of any of the other said end user terminals.

The collection may be configured such that the end user terminals comprise a wavelength tuneable optical transmitter.

There is further presented a system comprising: I) an apparatus as described above in any of the first or second aspects; and, II) the collection of end user terminals as described above; and, optionally,

III) the optical fibre as described in the first aspect.

The system may further comprise a wavelength demultiplexer configured to: A) receive the first optical signals from the optical fibre; B) route a first set of the first optical signals to a first set of one or more end user terminals; C) route a second set of the first optical signals to a second set of one or more end user terminals; wherein the first set of optical signals comprises a different central wavelength that the second set of optical signals.

Brief description of the drawings

Figure 1 shows an optical network of the prior art;

Figure 2 shows an optical network having a first example of an apparatus described herein; Figure 3 shows an optical network having a second example of an apparatus described herein; Figure 4 shows an optical network having a third example of an apparatus described herein.

Description

In a first aspect as there is presented an apparatus for use in an optical line terminal of an optical network 1102. Figure 2 shows an example of the apparatus 1100. The apparatus 1100 may be for transmitting first optical signals to one or more end user terminals 1104, such as an ONU, via an optical fibre 1103. The apparatus 1100 may also be configured for receiving second optical signals from the one or more end user terminals via the optical fibre 1103. The apparatus 1100 comprises an optical time division multiplexed (TDM) system 1108 configured to transmit the first optical signals to the one or more end user terminals 1104. The apparatus 1100 further comprises a wavelength division multiplexed (WDM) system 1110 comprising a plurality of optical receivers 1106 for receiving the second optical signals from the one or more end users 1104. Whilst the present apparatus 1100 may be used in an OLT, it not necessarily limited to be part of an OLT, however for purpose of discussion underneath the apparatus 1100 may be referenced as an OLT. Similarly, an end user terminal 1104 may not necessarily be an ONU, but for purposes of discussion will be referred to an ONU. Furthermore, the optical network 1102 described below for the examples is a PON, however other optical networks may be used. For example, the network may be a local network feeding very high data rate satellite signals to multiple end user terminal manned by scientists wherein each user terminal uses all of the incoming downstream 'D' data.

Using a WDM system for upstream data allows different ONUs to transmit data upstream on different wavelengths. These wavelengths are typically multiplexed onto a common optical fibre that guides the light to the OLT. The end user terminals may transmit optical signals upstream to the OLT using either burst mode transmission or continuous transmission. In continuous transmission the ONU transmitters, when operating, do not output burst of optical data signals but instead continuously output optical data signals. This may be the same optical fibre as the OLT used to transmit the downstream optical signals or a different optical fibre. At the OLT the different ONU wavelengths are detected by different receivers. This may be accomplished in different ways including a wavelength demultiplexer 1111 that feeds light into filtered or unfiltered detectors 1106 or a power splitter that divides the upstream signals and inputs the divided light into detectors with front end optical wavelength filters that reject all but the desired wavelength for the particular detector (each detector having a different optical filter).

By using a TDM transmission system 1108 for providing downstream transmission and WDM system 1110 for receiving upstream transmission the OLT increases the number of optical receivers 1106 over the prior art in figure 1 for receiving upstream data. However, these OLT receivers may be simpler, hence less expensive, because each receiver may receive data at a lower data rate than the prior art TDM receiver 106 shown in figure 1. Furthermore, in a WDM system 1110 these receivers 1106 may be identical and/or not require any front-end optical filtering because a WDM demultiplexer (DEMUX) 1111 may be used to split the incoming wavelength multiplexed upstream signals into a plurality of different physical output channels. Each DEMUX output channel carries optical signals centred at a different wavelength to other output channels of the DEMUX and inputs light into a separate receiver 1106 to the other output channels of the DEMUX. The receivers 1106 may be formed into an integrated array, for example on a single integrated component or a small number of integrated components, which in turn reduces fabrication cost and packaging complexity. This array may be integrated with the wavelength demultiplexer 1111. The use of an OLT that uses WDM for upstream signals in turn means that the ONUs do not require expensive burst mode transmitters that need to operate (in bursts) at the full data rate of the PON. Instead, each ONU 1104 may use a simpler and cheaper transmitter that outputs optical signals upstream towards the OLT at a different central wavelength than the transmitters of the other ONUs. These ONU transmitters may have a fixed wavelength or may be tuneable. If the ONU transmitters are tuneable then a standard ONU device may be supplied to each end user and the device may then be tuned to the desired upstream wavelength either by an engineer or automatically by ONU driver software and/or hardware.

In some examples however, the apparatus 1100 may not require using TDM to transmit optical signals to the end users 1104. For example, the apparatus may transmit the same optical data to all end users wherein all end users use all of the incoming data.

There is further presented in a second aspect an apparatus 1100 for use in an optical network 1102 and for transmitting optical signals down an optical fibre 1103 to a plurality of end user terminals 1104. The apparatus 1100 comprises a transmission system comprising one or more optical transmitters 1107; the transmission system configured to transmit first optical signals to the end users via the optical fibre 1103; the first optical signals centred on a first set of one or more different wavelengths. The apparatus 1100 further comprises a receiving system 1110 for receiving second optical signals transmitted from the end user terminals 1104. The receiving system comprising a plurality of optical receivers 1106 configured to detect the second optical signals; the second optical signals centred on a second set of wavelengths. The second set of wavelengths comprises more wavelengths than the first set of wavelengths.

The apparatus of the second aspect may therefore use different wavelengths to transmit optical data signals to different end users and also receive different wavelengths from the end users, however at least some of the end users receive the same wavelength optical signals from the OLT. This may be advantageous for a number of reasons. Firstly, some end users may share common data, for example in a 32 end-user system: end users 1 and 2 need certain data, so get wavelength 1 whilst end users 3 and 4 need a different set of common data so get wavelength 2, whilst each of end users 5-32 each need different data to any other user so get a different wavelength each. Secondly, for example, all end users may need different data so the OLT transmission system uses TDM but users 1-16 receive their downstream data on wavelength 1 whilst users 17-32 receive the data on wavelength 2. Because half the amount of data is required to transmit downstream per wavelength, the bit rate per transmission laser is less which allows for larger pulse width, cheaper (possibly more reliable) modulation equipment, hence less dispersion along the optical fibre, hence possible longer fibre span lengths between OLT and ONUs. In the above example 32 ONU users are described but the network may include fewer or more ONUs.

The apparatus of the first and second aspects may be adapted according to any feature or configuration described herein, for example the transmission system of the second aspect may use TDM whilst the apparatus of the first aspect may use multiple wavelengths to transmit optical data signals downstream.

Figures 2-4 are further discussed underneath. The apparatus, in any of the aspects and examples described herein, may be configured to output and receive optical signals of any wavelength, for example between 700-1700 nm. For telecommunications and other applications this may be in any one or more of the following bands: the O-band (original band: 1260-1360 nm); the C-band (conventional band: 1530-1565 nm), the L-band (long-wavelength band: 1565-1625 nm); the S-band (short-wavelength band: 1460-1530 nm); the E-band (extended-wavelength band: 1360-1460 nm). Any of the optical transmitters may be wavelength tuneable or have a fixed operating wavelength.

The pulse repetition frequency of any of the optical sources may be any frequency including any of the following ranges: lOOMFIz to 25GFIz. The pulse time widths of the sources may be any width, including but not limited to any of the following: 10ns to 25ps.

In one example implementation of the apparatus 1100, there is presented an optical PON system which may retain the compact OLT physical space and also provides independent upstream bandwidth for each user. Figure 2 shows and example of the apparatus 1100 in this example. The apparatus 1100 may be used in an OLT and ONU of a PON 1102, but can also be used in other parts of an optical network. The apparatus is physically connectable to one or more optical fibres 1103. The optical fibre can be any type of optical fibre but will typically be a single mode optical fibre carrying a single spatial mode in the TE, TM or both TE and TM modes. This optical fibre 1103 and optionally other network components such as optical amplifiers and other lengths of optical fibre, may be used to optically link the apparatus 1103 to one or more user end terminals 1104 via an optical power splitter 1105 that splits the incoming downstream optical signals into a plurality of output channels, each channel feeding a different ONU 1104. These user end terminals may be ONUs. Thus, the apparatus 1100 may be part of an OLT having a wired connection optically linking with single or multiple ONUs 1104 of a PON 1102. The apparatus 1100 may be used with other types of network. The apparatus may receive data for transmitting to the user end terminals via one or more input electrical signals and/or optical signals coming from upstream in the network (not shown in the figures).

Figure 2 shows an example using single high data rate optical source (1107) operating at 25Gbps at wavelength 1. The optical source is typically a laser and may be used to broadcast data to all the users (1104) on the PON 1102. The high-rate optical source can have any wavelength, for example selected such that the downstream signal has the advantage of being minimally affected by dispersion during transmission over the optical fibre 1103. This also allows longer distance PONs to be deployed than the maximum distance of 20km defined by international standards. The OLT transmitter 1107 consist of said single mode optical source that broadcasts data and optically via an optical fibre 1103 to the PON. The optical source 1107 forms at least part of the transmission system 1108 and is typically, but not limited to, a single mode source of known wavelength. The transmission system 1108 may comprise other components such as optical amplifiers, optical modulators and polarisation controllers and any associated electronics to control and provide power to such components, for example controllers or processors configured to allow the transmission system to output TDM optical signals.

In this example, the downstream broadcast data comprises of single or multiple combined data sets that are transmitted to all users simultaneously (known as broadcast data). Each users ONU 1104 receives all of the data set transmitted down the optical fibre 1103 but selects which time slot to interrogate in order to get that specific users' data. The data is received by an optical receiver whose bandwidth may allow the detection of the full data rate of the full transmitted data set. The ONU can either be connected to the PON via two separate optical fibres 1103, one for downstream and one for upstream transmission, or via a single optical fibre which is used for bi-directional transmission. In the latter case the downstream and upstream data is separated by an optical filter, or an optical coupler or an optical circulator.

The upstream data is provided by a distinct optical source within each ONU (1104). Each user's optical source generates and transmits data at a unique optical wavelength. Typically, optical sources are separated by uniform frequency spacing but they could also be separated by a non- uniform frequency spacing so as to reduce unwanted detrimental non-linear effects during transmission over the optical fibre. The ONU 1104 also contains an optical receiver to detect downstream data. Where the PON 1102 uses the same optical fibre 1103 to deliver upstream and downstream data the optical receiver at the ONU may be combined with any one or more of an optical filter or a band-splitter filter to separate the downstream and upstream optical wavelengths. In this approach, each user's upstream data is distinguished by a different wavelength of light and the data is transmitted continuously, not in temporal bursts. Each user 1104 can optionally have their own data rate, being independent from every other user data rate on that PON. In the OLT1100, there is an optical receiver 1106 for each different wavelength of light corresponding to each user on the PON's own unique wavelength. The receivers 1108 are labelled Rx lΐ -n in figure 2 and form at least part of the receiving system 1110. Therefore, each receiver can only receive data from one user. The optical signal transmitted upstream 'U' by the different ONU's 1104 propagate along the fibre, optionally amplified by optical amplifiers, and received at the OLT 1100. An optical routing component 1101, such as an optical circulator or optical filter, is optically linked to the optical fibre 1103 and directs the received upstream data to the demultiplexer 1111 via a further optical fibre, although other optical transmission means may be used such as integrated optics or free space. The same optical routing component also receives downstream transmission data from the OLT transmission system 1108 and route it onto the feeder fibre 1103 towards the ONU's 1104. The received data is separated by using a wavelength demultiplexer device 1111, such as an arrayed waveguide grating (AWG), a diffraction grating, optical thin-film filter or Echelle grating. This demultiplexer is also part of the receiving system but may in principle be separate. The demultiplexer 1111 has a common input which is connected to the upstream fibre and separates the data into each individual wavelength channel. The wavelength demultiplexer can have a common frequency difference between the upstream channels, or a non-common frequency difference. The receivers can be integrated together to minimise the physical space required at the remote OLT or can be separate units.

This apparatus has the advantages that continuous wave optical receivers 1106 can be used at the OLT 1108, which have higher optical sensitivity than burst mode receivers, and also operate at a data rate lower than the total upstream system data capacity, which again increases optical sensitivity and makes the optical signals less affected by dispersion when transmitted upstream over the optical fibre.

The OLT optionally has an electrical multiplexer (1112) to combine all the upstream signals electrically, process them and transmit them to a headend via a separate optical transmission link (not shown).

In another approach, more than one optical transmitter 1107 can be used at the OLT 1100 in order to increase the total downstream data capacity of the PON 1102. Figure 3 shows an example similar to that of figure 2 with like reference numerals representing like components and any of the variations and options in figure 2 being equally applicable to figure 3. As shown in figure 3, the output of two OLT transmitters (1013, 1014) in the transmission system 1108, with different wavelengths lΐ, l2, are optically combined (1015) (such as by optical power couplers, thin-film filters, AWGs or polarisation multiplexer). The OLT transmitter wavelengths lΐ, etc may be the same or different to the OLT receiver wavelength channels output from the demux. 1111.

The combined signal is output to the routing component 1101 and broadcast to all the users concurrently on the PON 1102. The combined optical signal is split into two using an optical filter such that each ONU only receives the data from one of the two OLT transmitters. The optical filter could be located at the ONU (1016) or elsewhere on the PON 1102 such as at the before, after or in replacement of the splitter 1105. There is also the option to increase the data rate per user by connecting less than half of the PON users 1104 to one of the downstream PON wavelengths. The optical filter can be tuneable (whether wavelength or polarization, as appropriate) such that the user 1104 can select which of the OLT transmitters it connects with at any one time.

Figure 4 shows an example similar to that of figures 2 and 3 with like reference numerals representing like components and any of the variations and options in figures 2 and 3 being equally applicable to figure 4. As an extension to this approach, figure 4 shows M OLT transmitters (1017) can be combined to increase the OLT data capacity for broadcast, such that M<N, where N is the number of users 1104 on the PON 1102. This can be used to initially increase the total capacity of the PON or can be used as a later upgrade to add additional capacity to the system.

There now follows some general teachings that may optionally apply to any of the aspects and examples herein.

Any of the components shown for the apparatus 1100 may be separately manufactured and/or separately packaged and housed in an overall package forming the OLT or part of the OLT.

The apparatus may combine the outputs of the optical sources using any suitable means including, but not limited to any of the following: optical fibre combiners, integrated optic combiners; beam splitter/combiners.

The apparatus may be, or comprise, integrated optic apparatus. Any optical components described herein that may form the optical apparatus may be integrated together. This integration may take the form of monolithic integration, hybrid integration or both. Additionally, or alternatively any one or more of the components may be separate but co-located in the same housing and optically linked where required. For example, certain components of the apparatus may be monolithically integrated together wherein the monolithic device may be further hybrid integrated with other portions of the apparatus wherein several integrated modules are packaged separately and located in the OLT housing.

The optical pathways optically linking different components together in the apparatus may be any appropriate optical component or configuration including, but not limited to, any one or more of: free space optical pathways; bulk optical components; integrated optic waveguides; optical fibres.

It is appreciated that any of the components described herein that require an electrical signal to operate, such as a laser or detector, may be electrically coupled to appropriate driving electronics including electrical lines and electrical apparatus outputting electrical signals via the electrical lines to the said components.

The apparatus and any one or more components associated with the apparatus may be part of an optical system. Any one or more components of the optical system may be integrated into an integrated optical device. Such associated components may be, but not limited to: optical filters, the optical sources, optical modulators, one or more optical detectors to detect any of the light output from the apparatus; one or more optical fibres to input light into the apparatus or receive light output from the apparatus; any electrical apparatus to drive any of the components requiring or outputting electrical signals.

End user optical source

An example of an optical source that may be used at the ONU's 1104 is now described. It should be appreciated that other optical sources may be used in addition or in replacement of the optical source described below. The optical source may be substantially similar to that described in any of EP3028352 and/or WO2019122877, the entire contents of both are incorporated by reference herein. The summary of an example of such an optical source is set out below.

The example optical source comprises a laser having a laser cavity. The laser cavity is disposed between a first optical reflector and a second optical reflector. The laser cavity may include an optical gain section and an optical phase control section in optical communication with the gain section. The optical phase control section is configured to be able to change the longitudinal mode frequency of the laser. This is typically done by controllably changing the amount of electrical drive current input into the phase control section. The optical source may also utilise a laser without a multi-section design.

The optical source further comprises an optical filter (for example a thin film transmission filter) that is external to the laser cavity and is configured to receive and filter light that is output from the laser cavity. The optical filter may comprise a passband filter response. At least one of the optical reflectors is a partial optical reflector configured to receive filtered light from the filter and to input filtered light back into the laser. The optical source is configured to change the central wavelength of the passband response of the optical filter by changing the angle of incidence that the output laser light subtends with the optical filter. The optical source may also comprise at least one base member and a temperature control element thermally connected to the base member. The base member/s may be used to mount the above optical source components upon. The base member/s may be a substrate formed of extremely low expansion glass ceramic such as lithium-aluminosilicate glass-ceramic material. The extremely low expansion glass ceramic which may have a coefficient of linear thermal expansion (CTE) of 0 ± 0.007 x 10-⁶/°K in the temperature range 0°C to 50°C. It is possible for other materials to be used for the base member. For example, the base member may comprise a substrate having a different thermal expansion coefficient, for example a Nickel-iron alloy such as Fe-33Ni-4.5Co or FeNi36. Such materials may have a CTE of 0.55 x 10-⁶/°C in the temperature range 20°C to 100°C.

Claims

1. An apparatus for use in an optical line terminal of an optical network and for: transmitting first optical signals to one or more end user terminals via an optical fibre; receiving second optical signals from the one or more end user terminals via the optical fibre; the apparatus comprising:

A) a transmission system comprising one or more optical transmitters and configured to transmit the first optical signals to the one or more end user terminals; the first optical signals being time division multiplexed;

B) a receiving system comprising a plurality of optical receivers and for receiving the second optical signals transmitted from the end user terminals; the receiving system comprising a wavelength division multiplexed system.

2. An apparatus for use in an optical network and for transmitting optical signals down an optical fibre to a plurality of end user terminals; the apparatus comprising:

I) a transmission system comprising one or more optical transmitters; the transmission system configured to transmit first optical signals to the end users via the optical fibre; the first optical signals centred on a first set of one or more different wavelengths;

II) a receiving system for receiving second optical signals transmitted from the end user terminals; the receiving system comprising a plurality of optical receivers configured to detect the second optical signals; the second optical signals centred on a second set of different wavelengths; wherein the second set of wavelengths comprises more wavelengths than the first set of wavelengths.

3. An apparatus as claimed in claim 2 wherein the transmission system transmits time division multiplexed optical signals.

4. An apparatus as claimed in any preceding claim wherein the receiving system comprises a wavelength demultiplexer configured to: receive wavelength multiplexed optical signals from the end users; output wavelength demultiplexed optical signals to the plurality of optical receivers.

5. An apparatus as claimed in any preceding claim wherein, at least one optical receiver is configured to receive continuous second optical signals.

6. An apparatus as claimed in any preceding claim further comprising a routing component configured to:

I) receive the first optical signals from the one or more optical transmitters and route the received first optical signals into the said optical fibre for transmission to the end users;

II) receive the second optical signals as a wavelength multiplexed set of optical signals transmitted via the said optical fibre; and route the wavelength multiplexed optical signals to the receiving system.

7. An apparatus as claimed in any preceding claim wherein the transmission system comprises a plurality of optical transmitters wherein at least a first optical transmitter outputs optical signals at a different central wavelength than a second optical transmitter.

8. An apparatus as claimed in claim 7 wherein the apparatus comprises a wavelength multiplexer configured to receive the optical signals from the first and second optical transmitters and wavelength multiplex them into a combined light path, for transmission down the said optical fibre towards the end users.

9. An apparatus as claimed in claim 8 as dependent upon claim 6 wherein the routing component is configured to receive the wavelength multiplexed first optical signals and route them along the said optical fibre towards the end users.

10. An apparatus as claimed in any preceding claim wherein the number of optical receivers is greater than the number of optical transmitters.

11. An optical line terminal comprising the apparatus as claimed in any preceding claim.

12. An end user terminal for use in an optical network and for receiving optical signals down an optical fibre; the end user terminal comprising:

I) an optical receiver configured to receive first optical signals from the network and use time division demultiplexing to output third data based on the detection of the first data;

II) an optical transmitter configured to output second optical signals for upstream transmission along the optical fibre.

13. An end user terminal as claimed in claim 12 wherein the optical transmitter is configured to output second optical signals in a continuous transmission mode.

14. A collection of optical end user terminals comprising a plurality of the end user terminals as claimed in claims 12 or 13, and for use in an optical network, wherein the optical transmitter of a first of the end user terminals outputs second optical signals at a different central wavelength than the optical transmitter of a second of the end user terminals.

15. A collection as claimed in claim 14 wherein each of the end user terminals of the plurality of end user terminals comprises an optical transmitter configured to output light at a different central wavelength than the optical transmitters of any of the other said end user terminals.

16. A collection as claimed in any of claims 14 and 15 wherein the end user terminals comprise a wavelength tuneable optical transmitter.

17. A system comprising: I) the apparatus as claimed in any of claims 1-10; and,

II) the collection of end user terminals as claimed in any of claims 14-16; and, optionally,

III) the optical fibre as described in claim 1.

18. A system as claimed in claim 17 further comprising a wavelength demultiplexer configured to:

A) receive the first optical signals from the optical fibre

B) route a first set of the first optical signals to a first set of one or more end user terminals

C) route a second set of the first optical signals to a second set of one or more end user terminals; wherein the first set of optical signals comprises a different central wavelength that the second set of optical signals.