Fibre To The Premises fixed access networks comprise a Passive Optical Network (PON) defined by an Optical Line Terminal (OLT) at a central exchange, which connects by optical fibre to multiple local exchanges. The local exchanges in turn connect to multiple Optical Network Units (ONUs) located in customer premises.

For at least reasons of cost, it may be desirable to eliminate local exchanges from the PON. However, removing a local exchange means that the optical connections from the central exchange to the ONUs extend over a much larger distance than if a local exchange is used. This results in the attenuation of the signals between the central exchange and the ONUs.

It would be useful to find a way to reduce the attenuation suffered by the signals and so improve the quality of the service.

The present invention addresses and/or substantially mitigates the above- mentioned and/or other disadvantages of the prior art.

According to a first aspect of the invention there is provided an optical access network comprising:

An optical line terminal and an optical network unit, the optical line terminal being connected to the optical network unit by an optical path, the optical path being capable of carrying data signals;

An optical amplifier located in the optical path, the optical amplifier comprising gain material;

A pump signal source;

A hollow-core fibre arranged to carry a pump signal from the pump signal source to the gain material of the optical amplifier, the wavelength of the pump signal being suitable for pumping the gain material such that it is capable of performing stimulated emission for amplifying the data signals; Wherein the hollow-core fibre provides low signal attenuation over an operative range of wavelengths and the pump signal has a wavelength that is within the operative range of wavelengths.

A hollow-core fibre is a fibre in which the outer portion is formed in a solid material and the inner core is hollow. An optical signal passing along the fibre propagates predominantly through the hollow core. This can mean that the signal suffers less attenuation than in a fully-solid fibre. For a given hollow-core fibre, the attenuation suffered by the signal depends on the wavelength of the signal. The inventor has realised that by choosing a hollow-core fibre having a spectral low attenuation region which includes the wavelength of the pump signal, attenuation of the pump signal can be minimised.

The data signals and/or the pump signal may be in the 0 band, the C band, the S band or the L band.

The optical line terminal may be located at a head end of the optical access network and may be located at a central exchange. The optical network unit may be located at a remote end of the optical access network and may be located at a customer premises. The optical data signals may be broadband signals. The optical path may comprise optical fibre. The optical data signals may be upstream signals that travel from the optical network unit to the optical line terminal. Alternatively or in addition, the optical data signals may be downstream signals that travel from the optical line terminal to the optical network unit.

In particular the data signals may be transmitted between the optical line terminal and the optical amplifier over optical fibre. The data signals may be transmitted between the optical amplifier and the optical network unit over optical fibre.

The network may comprise a plurality of optical network units, and each may be arranged to receive data signals from the optical line terminal that have been amplified by the optical amplifier. The optical amplifier may be located remotely from the optical line terminal. The optical amplifier may be a doped fibre amplifier. A dopant in the doped fibre amplifier may be one of the following: Erbium, Thulium, Ytterbium, Praesodynium and Bismuth. The optical amplifier may be an Erbium Doped Fibre Amplifier (EDFA).

The pump signal source may be a laser source and may be located at the head end and may be located in the central exchange.

The hollow-core fibre may be of a photonic bandgap type or may be of antiresonant type. The hollow-core fibre may be separate from the optical path. The hollow-core fibre may be a dedicated supply line for the pump signal. The hollow-core fibre and the fibre carrying the optical data signal may lie in the same cable.

According to a further aspect of the invention there is provided a method of amplifying an optical data signal in an optical access network, the method comprising:

Transmitting a pump signal over a hollow-core fibre from a pump signal source to an optical amplifier so as to pump gain material in the optical amplifier, thereby causing the optical amplifier to enter an active condition in which the optical amplifier is capable of amplifying an incoming data signal;

Transmitting an optical data signal from an optical line terminal or an optical network unit to the optical amplifier;

Amplifying the optical data signal by passing the optical data signal through the pumped gain material;

Transmitting the amplified optical data signal to the optical network unit or the optical line terminal,

Wherein the hollow-core fibre provides low signal attenuation over an operative range of wavelengths and the pump signal has a wavelength that is within the operative range of wavelengths. A specific embodiment of the invention will now be described, for illustration only and with reference to the appended drawings, in which:

Fig 1 is a schematic view of a known customer access network PON;

Fig 2 is a schematic view of an optical amplifier of a PON configured to perform the method of the invention;

Fig 3 is a schematic view of a hollow core fibre;

Fig 4 a more detailed view of the optical amplifier of Fig 2;

Fig 5 is a schematic view of the components of a metro node of a PON configured to perform the method of the invention.

Fig 1 shows a known PON 1 as part of an access network. Multiple ONlls 2 located in customer premises are connected by optical fibre to a passive splitter 3, which is connected by optical fibre to a local exchange 5. The local exchange 5 is connected to a central exchange, labelled as metro node 4. The local exchange 5 provides broadband services to the ONlls 2 over the optical fibres.

It may be desirable to dispense with the local exchange 5 and instead, connect the ON Us directly to the central exchange 4. This would remove the costs of setting up and running the local exchange, but has the disadvantage that the signals from the ONUs 2 have to travel a greater distance. This results in attenuation of those signals. The present invention addresses this.

Fig 2 shows a PON in accordance with an embodiment of the invention. The ONUs (not shown) connect to the central exchange (referred to as the metro node 4) via an optical amplifier 7. The upstream signals (i.e. the signals transmitted by the ONUs to the metro node 4) have wavelengths in the 0 band. On arrival at the optical amplifier 7 they pass through a demultiplexer 8 which separates them from the downstream signals travelling in the opposite direction (i.e. the signals transmitted by the metro node 4 to the ON Us). The upstream signals then pass through an 0 band amplifier 9. The 0 band amplifier 9 receives pump power from a first pump laser 10 located in the metro node 4 over a first hollow core fibre 31 . The pump power pumps the gain material in the 0 band amplifier 9 so that the arrival of the upstream signals stimulate light emission from the gain material in the 0 band amplifier, amplifying the upstream signals. The amplified upstream signals then pass through a multiplexer 11 to enable them to share a fibre with downstream signals coming in the opposite direction. The upstream signals then arrive at an OLT 6 at the metro node 4.

The downstream signals (i.e. the signals transmitted by the metro node 4 to the ONUs) have wavelengths in the S band. On arrival at the optical amplifier 7 they pass through a demultiplexer 11 which separates them from the upstream signals travelling in the opposite direction. The downstream signals then pass through an S band amplifier 12. The S band amplifier receives pump power from a second pump laser 13 located in the metro node 4 over a second hollow core fibre 32. The pump power pumps the gain material in the S band amplifier 12 so that the arrival of the downstream signals stimulate light emission from the gain material in the S band amplifier, amplifying those downstream signals. The amplified downstream signals then pass through a multiplexer 8 to enable them to share a fibre with upstream signals coming in the opposite direction. The downstream signals then pass through a splitter (not shown) and arrive at the ONUs (not shown).

The first and second hollow core fibres are “tuned” to give low attenuation at the wavelength of the pump light. Hollow core fibres are known in the art and so their structure and function will not be described in detail here. However, in essence, a hollow core fibre provides light propagation with minimal attenuation for certain wavelengths, those wavelengths depending on the precise dimensions of the fibre. The hollow core fibres 31 and 32 can be of either the photonic bandgap type or the anti-resonant type. A schematic view of a cross section through an example of an anti-resonant hollow core fibre is shown in Fig 3. The fibre comprises a central hollow (i.e. airfilled core 33) surrounded by cladding. The cladding comprises an outer glass layer 34 and an inner layer of eight silica rods 35.

Fig 4 shows the optical amplifier in more detail. In particular, Fig 4 shows that the pump power received from the first pump laser (not shown) passes through the first hollow core fibre 31 and a multiplexer 14 to combine the pump light with the 0 band (upstream) signal. The multiplexer 14 outputs the pump light and upstream signal to the gain material 15 of the 0 band amplifier. Together the multiplexer 14 and gain material 15 define the 0 band amplifier 9 shown in Fig 2. The pump power received from the second pump laser (not shown) passes through the second hollow core fibre 32 and a multiplexer 16 to combine the pump light with the downstream signal. The multiplexer 16 outputs the pump light and downstream signal to the gain material 17 of the S band amplifier. Together the multiplexer 16 and gain material 17 define the S band amplifier 12 shown in Fig 2

In use, the ONlls transmit signals to the metro node according to a schedule. A copy of this schedule is contained in a lookup table. When the schedule indicates that a particular ONU is about to transmit a signal, the first pump laser transmits a burst of pump light (a pump burst) to the 0 band amplifier. This pumps the gain material in the 0 band amplifier. The ONU then transmits the scheduled signal, which passes through the 0 band amplifier. The now amplified signal has sufficient power to reach the metro node.

Fig 5 is a schematic representation of the components for controlling the pump power within the metro node 2. In particular, the metro node 2 contains an intensity determiner 18, a lookup table 19, a first pump controller 20 and a second pump controller 21. As noted, the lookup table 19 contains a schedule of forthcoming transmissions from the ONUs. This includes the time at which each ONU will transmit scheduled signals and the amount of data that each ONU will transmit in the scheduled transmission. The lookup table also contains the length of the fibre connecting each ONU to the optical amplifier.

The first 20 and second 21 pump controllers are operable to control the output intensity of the first 10 and second 13 pump lasers respectively. The remote node is able to control the output intensity of the first 10 and second 13 pump lasers depending on information contained in the lookup table 19 concerning the transmitting ONU.

In use, the intensity determiner 18 reads from the lookup table 19 to determine which ONU will be the next to transmit an upstream signal. The intensity determiner 18 also reads the length of the fibre to which the next ONU to transmit is connected. Using this length, the intensity determiner determines the gain required to be applied to the signal by the O band amplifier 9 to overcome the attenuation that the signal will suffer. The intensity determiner then determines the amount of laser pump power required to provide the determined gain and instructs the first pump laser 10 to transmit pump light to the O band amplifier 9 at the determined power. The first pump laser 10 then does so, and the upstream signal is transmitted and then amplified to the determined extent.

Take the example in which the lookup table indicates that one of the most remote ONUs will be the next ONU to transmit. This means that the signal transmitted by the ONU will likely be fairly heavily attenuated by the time it reaches the optical amplifier 7. The instructs the first pump controller 20 to transmit a pump burst of larger than average intensity to the O band amplifier 9. This high intensity pump burst provides the O band amplifier a gain which is larger than that provided by a pump burst of average intensity. The resulting output signal is more heavily amplified than the average signal.

Conversely, if the lookup table 19 indicates that one of the least remote ONUs will be the next to transmit, the signal transmitted by the ONU will likely be only lightly attenuated by the time it reaches the optical amplifier 9. The processor 18 instructs the first pump controller 20 to transmit a pump burst of smaller than average intensity to the 0 band amplifier. This lower-intensity pump burst provides the 0 band amplifier 9 a gain which is smaller than that provided by a pump burst of average intensity. The resulting output signal is less heavily amplified than the average signal.

This method of adjusting the intensity of the pump signal in accordance with the degree of attenuation suffered by the upstream signal causes the signals output by the 0 band amplifier 9 to be of more uniform intensity. This reduces the harmful transients suffered by the 0 band amplifier and reduces the “dead time” that must be allowed between successive transmissions.

A similar process is followed in relation to the downstream signals. In particular, the intensity determiner 18 reads from the lookup table 19 to determine which ONU will be the recipient of the next downstream signal to be sent from the metro node 4. The intensity determiner 18 also reads the length of the fibre to which that ONU is connected. Using this length, the intensity determiner determines the gain required to be applied to the signal by the S band amplifier 12 to overcome the attenuation that the signal will suffer. The intensity determiner 18 then determines the amount of laser pump power required to provide the determined gain and instructs the second pump laser 13 to transmit pump light to the S band amplifier 12 at the determined power. The second pump laser 13 then does so, and the downstream signal is transmitted and then amplified to the determined extent.