A TELECOMMUNICATIONS NETWORK

Field of the Invention

The present invention relates to a telecommunications network including an optical

5 network.

Background

Many technologies require stable laser signals, such as the emerging set of Rydbergatom based technologies including atomic radio detectors, atomic clocks and atomic gravimeters. These Rydberg-atom based technologies often utilise a laser signal to excite an electron of an atomic medium, typically comprising Rubidium, Caesium or Strontium atoms, to a Rydberg state. For example, in an atomic radio detector based on an atomic medium of Rubidium-85 atoms, a first laser signal at 780nm may be used to excite electrons from the Rubidium atom’s ground state to a first excited state, and a

15 second laser signal may then be used to excite electrons from this first excited state to a Rydberg state. For long-term operation, these laser signals should be stabilised at these wavelengths. In an example, the stability of the laser signal required to transition an electron from the Rubidium-85 atom’s ground state to the first excited state (which has a linewidth of around 6MHz) should be less than 1 MHz (i.e. it should not deviate by

20 more than 1 MHz) over the time frame of operation to ensure efficient operation. This may be achieved using a saturation absorption spectroscopy technique, but this requires expensive and dedicated equipment. Accordingly, mass deployment of devices containing a Rydberg-atom based technology (such as in a wireless telecommunications network in which each wireless device includes a Rydberg-atom based radio frequency

25 detector) may be prohibitively expensive as each device requires locally applied saturation absorption spectroscopy to stabilise the laser signal.

Summary of the Invention

According to a first aspect of the invention, there is provided a method of operating a central node in a telecommunications network, the telecommunications network including an optical network and a plurality of distributed nodes each configured to use a first optical signal at a first wavelength, the method comprising the steps of: producing a first optical signal at a second wavelength; directing the first optical signal into a first path for the first optical signal and a second path for the first optical signal, wherein the

35 first path for the first optical signal is connected to a first optical stabiliser to stabilise the A35564 first optical signal produced by the central node, and the second path for the first optical signal provides the first optical signal to the optical network for distribution to each of the plurality of distributed nodes, wherein the second wavelength has a lower transmission loss than the first wavelength.

5

The first path for the first optical signal may include a first wavelength converter to convert the first optical signal from the second wavelength to the first wavelength before stabilisation by the optical stabiliser.

10 The second wavelength may be in a range from 1260nm to 1625nm.

Each of the plurality of distributed nodes may be configured to use a second optical signal at a third wavelength, and the method may further comprise the steps of: producing a second optical signal at a fourth wavelength; directing the second optical

15 signal into a first path for the second optical signal and a second path for the second optical signal, wherein the first path for the second optical signal is connected to a second optical stabiliser to stabilise the second optical signal produced by the central node, and the second path for the second optical signal provides the second optical signal to the optical network for distribution to each of the plurality of distributed nodes, wherein the

20 fourth wavelength has a lower transmission loss than the third wavelength.

The first path for the second optical signal may include a second wavelength converter to convert the second optical signal from the fourth wavelength to the third wavelength before stabilisation by the second optical stabiliser.

25

The fourth wavelength may be in a range from 1260nm to 1625nm.

The second wavelength converter may convert the second optical signal from the fourth wavelength to the third wavelength by mixing the second optical signal at the fourth

30 wavelength with the first optical signal at the first wavelength following conversion of the first optical signal by the first wavelength converter.

Each of the plurality of distributed nodes may utilise the first optical signal at the first wavelength to excite an electron of a Rydberg-atom from a first state to a second state.

35 Each of the plurality of distributed nodes may utilise the second optical signal at the third wavelength to excite an electron of a Rydberg-atom from the second state to a third state.

The Rydberg atom may be part of a Rydberg-atom based Radio Frequency, RF, receiver.

The optical network may distribute the first optical signal on a hollow core fibre.

The optical network may distribute the second optical signal on a hollow core fibre.

According to a second aspect of the invention, there is provided a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the first aspect of the invention. The computer program may be stored on a computer readable carrier medium.

According to a third aspect of the invention, there is provided a node for a telecommunications network, the telecommunications network including an optical network and a plurality of distributed nodes each configured to use a first optical signal at a first wavelength, the node comprising: a first optical source configured to produce a first optical signal at a second wavelength, wherein the second wavelength has a lower transmission loss than the first wavelength; a first optical stabiliser configured to stabilise the first optical signal; a communications interface connectable to an optical network; and a first splitting unit configured to direct the first optical signal into a first path for the first optical signal and second path for the first optical signal, wherein the first path for the first optical signal is connected to the first optical stabiliser and the second path for the first optical signal is connected to the communications interface so as to provide the first optical signal to the optical network for distribution to each of the plurality of distributed nodes. The node may be part of a telecommunications network, the telecommunications network further comprising an optical network and a plurality of distributed nodes.

Brief Description of the Figures In order that the present invention may be better understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of a cellular telecommunications network of a first embodiment of the present invention;

Figure 2 is a schematic diagram of a Rydberg-atom based radio frequency receiver of a base station of the network of Figure 1 ;

Figure 3 is a flow diagram of a first embodiment of a method of the present invention, illustrating a first process implemented by a core networking node of the network of Figure 1 ;

Figure 4 is a flow diagram of the first embodiment of the method of the present invention, illustrating a second process implemented by each base station of the network of Figure 1 ;

Figure 5 is a schematic diagram of a cellular telecommunications network of a second embodiment of the present invention; and

Figure 6 is a schematic diagram of a cellular telecommunications network of a third embodiment of the present invention.

Detailed Description of Embodiments

A first embodiment of a telecommunications network will now be described with reference to Figure 1. This embodiment is based on a cellular telecommunications network including a core network node 10 and a plurality of base stations 20, 30, 40, 50. The core network node 10 is connected to each base station of the plurality of base stations by an optical distribution network 60.

As shown in Figure 1 , the core network node 10 includes a master laser 12 configured to generate a laser signal at 1560nm. The core network 10 further includes a splitter 14, a wavelength converter 16 and a laser stabiliser 18. In this embodiment, the wavelength converter 16 is based on a Second Harmonic Generation (SHG) Periodically Poled Nonlinear (PPLN) optical waveguide in order to halve the wavelength of the 1560nm laser signal of the master laser 12 to 780nm. An example of this technique is described in European Patent number 0331303B1. Furthermore, in this embodiment, the laser stabiliser 18 is based on Modulation Transfer Spectroscopy (MTS), which stabilises the 780nm laser signal following the conversion by the wavelength converter to the required degree of stabilisation. An example of this technique is described in US Patent number 4590597.

Figure 1 also illustrates each base station of the plurality of base stations 20, 30, 40, 50 as including an amplifier 22, 32, 42, 52, a wavelength converter 24, 34, 36, 46, and a Rydberg-atom based Radio Frequency (RF) receiver 26, 36, 46, 56. The Rydberg-atom based RF receiver 26 of a first base station 20 of the plurality of base stations 20, 30, 40, 50 is shown in Figure 2 and operates as follows (and the skilled person will realise that the Rydberg-atom based RF receivers of the other base stations of the plurality of base stations may operate in the same manner). An atomic medium 26a is provided which, in this example, is a glass cell filled with a low density vapour of alkali atoms (such as Rubidium-85). Each Rubidium-85 atom has a number of electron states, including the ground state (|1>) and a plurality of excited states. The outer electron of the Rubidium-85 atom may be excited (e.g. by absorbing a photon of a particular wavelength) from the ground state (| 1 >) to an excited state. The electron may then decay from the excited state to a lower excited state (that is, an excited state at a lower energy level) or to the ground state (|1 >). However, some of these transitions are not allowed as they are dipole forbidden.

In the RF receiver, the laser signal (initially received from the core networking node 10) is passed through the atomic medium 26a and elevates the Rubidium-85 atom’s outer electron from its ground state (|1>) to a first excited state (|2>). This elevation occurs due to the 780nm wavelength of the laser signal corresponding to the energy required to elevate the Rubidium-85 atom’s outer electron from the ground state (|1>) to the first excited state (|2>). In this context, the laser signal may be referred to as a “probe” signal. A second “coupling” laser signal (generated by a coupling laser 26b) is also passed through the atomic medium 26a in an opposing direction at a relatively large power level (compared to the probe laser) and at a second wavelength which corresponds to the energy required to elevate the Rubidium-85 atom’s outer electron from the first excited state (|2>) to a Rydberg state (|3>). The transition from the Rydberg state (|3>) to the ground state (|1>) is forbidden so that the ground state (|1 >) becomes depopulated and so fewer atoms can absorb the 780nm laser signal. Accordingly, the atomic medium 26a becomes more transparent to the 780nm laser signal such that there is an increase in transmission of the 780nm laser signal through the atomic medium 26a, which is observable at an optical detector 26c. This phenomenon is known as Electromagnetically Induced Transparency (EIT) and the received signal is known as the EIT signal. Specifically, the above description is of a ladder scheme EIT effect, but the skilled person would understand that the EIT effect may be realised through alternative electron transitions, such as the Vee and Lambda schemes.

Once the atomic medium 26a has become transparent to the 780nm, then a further physical effect can be exploited to detect RF electric fields. As the Rubidium-85 atom’s outer electron is much further away from the atomic nucleus when in the Rydberg state compared to the ground state, a large dipole moment is created and it becomes responsive to incident RF electric fields. An incident RF electric field may cause a further transition of an electron from the Rydberg state to another Rydberg state. If the transition from the other Rydberg state to the ground state is not forbidden, then electrons may subsequently drop to the ground state so that the atomic medium 26a becomes less transparent to the 780nm laser signal, causing a drop in amplitude of the EIT signal. This drop in amplitude of the EIT signal is directly proportional to the incident RF electric field’s amplitude, thus creating a Rydberg-atom based AM RF receiver. A more detailed explanation of this effect can be found in the article, “A Multiple-Band Rydberg-Atom Based Receiver/Antenna: AM/FM Stereo Reception”, Holloway et al., National Institute of Standards and Technology).

Furthermore, a Rydberg-atom based FM RF receiver works in a similar manner. That is, when the RF electric field changes (or “detunes”) from its resonant RF transition frequency, the EIT signal splits into two non-symmetrical peaks. The separation of the two peaks increases with RF detuning. By locking the 780nm laser signal and coupling laser to particular frequencies, then the optical detector output is directly correlated to the FM RF electric field. A more detailed explanation of this effect can also be found in article, “A Multiple-Band Rydberg-Atom Based Receiver/Antenna: AM/FM Stereo Reception”, Holloway et al., National Institute of Standards and Technology) and in article, “Using frequency detuning to improve the sensitivity of electric field measurements via electromagnetically induced transparency and Autler-Townes splitting in Rydberg atoms” Appl. Phys. Lett. 108, 174101 (2016), Matt T. Simons.

Rydberg RF receivers may also be used to detect phase modulated RF fields, such as those of Binary Phase-Shift Keying (BPSK), Quadrature Phase-Shift Keying (QPSK), and Quadrature Amplitude Modulation (QAM) signals (used in many wireless and cellular communications protocols). In these modulation schemes, data is transmitted by modulating the phase of a carrier. To detect the carrier’s phase, a reference RF field being on-resonance with the transition to the Rydberg state is applied to the atomic medium, which acts as a local oscillator. The difference frequency, or “intermediate frequency”, is detected and the phase of the intermediate frequency signal corresponds directly to the relative phase between the local oscillator and the incident RF electric field.

Regardless of the modulation scheme used (amplitude, frequency or phase), the Rydberg atom based RF detector may be configured to detect RF fields of a specific frequency by selecting a particular second wavelength of the coupling laser 26b so that the electrons of the atomic medium 26a are elevated to a particular Rydberg state. This Rydberg state is selected so that photons at the specific frequency to be detected will elevate electrons from this Rydberg state to its adjacent Rydberg state, thus creating a detectable change in the EIT signal that may be observed at the optical detector 26c.

A first embodiment of a method of the present invention will now be described with reference to the cellular telecommunications network of Figure 1 and the flow diagrams of Figures 3 and 4. In a first step S101 , as shown in Figure 3, the master laser 12 produces a laser signal at 1560nm. In step S103, this laser signal is transmitted to the splitter 14, which splits the laser signal into a local path and a plurality of distribution paths. The local path directs the 1560nm laser signal towards the core network node’s wavelength converter 16, whilst each distribution path directs the 1560nm laser signal towards a particular base station of the plurality of base stations 20, 30, 40, 50. Following the local path, in step S105, the 1560nm laser signal is converted to a 780nm laser signal by the wavelength converter 16, and is then directed to the laser stabiliser 18 which, in step S107, generates an error signal. This error signal is fed back to the master laser 12 so as to stabilise its laser signal in step S109.

Following the distribution paths, the 1560nm laser signal is distributed to each base station of the plurality of base stations 20, 30, 40, 50 by a particular distribution path in the optical network 60. In step S201 , as shown in Figure 4, each base station of the plurality of base stations 20, 30, 40, 50 receives the 1560nm laser signal from the core networking node 10 at the amplifier 22, 32, 42, 52, which amplifies the 1560nm signal to compensate for any power reduction of the wavelength converter 24, 34, 44, 54. In step S203, the amplified 1560nm laser signal is directed to the wavelength converter 24, 34, 44, 54 which converts the 1560nm laser signal to a 780nm laser signal. In step S205, the 780nm laser signal is directed towards the Rydberg-atom based RF receiver 26, 36, 46, 56 and used to excite electrons of the atomic medium of Rubidium-85 atoms from a ground state to a first excited state (as described above).

The above embodiment enables distribution of a stable laser signal from a single node to a plurality of devices that each utilise a Rydberg-atom based RF receiver. In the prior art, each device that utilises a Rydberg-atom based RF receiver would require its own stabiliser. However, by distributing this laser signal, and implementing the stabiliser in the core networking node prior to distribution, the number of stabilisers required in the network is reduced from /V (where /V is a count of devices that utilise a Rydberg-atom based RF receiver) to 1.

Furthermore, as the wavelength converter of each base station 20, 30, 40, 50 applies the same conversion (i.e. to output a laser signal of the same wavelength) to the wavelength converter 16 of the core networking node, the laser signals as used in both the laser stabiliser 18 of the core networking node and the Rydberg-atom based RF receivers 26, 36, 46, 56 of the plurality of base stations 20, 30, 40, 50 have the same wavelength. This ensures that the error signal generated by the laser stabiliser 18 stabilises the master laser 12 to the accuracy required for operation of the Rydberg-atom based RF receivers in each base station of the plurality of base stations 20, 30, 40, 50. However, this is non-essential as the stabilisation unit may be based on alternative technologies that do not require the wavelength conversion, such as a frequency comb or stable cavity.

The above embodiment also distributes the stabilised laser signal at a wavelength of 1560nm, rather than at the 780nm wavelength required for the Rydberg-atom based RF receiver, as the laser signal will suffer far less attenuation as it is transmitted between the core networking node 10 and each base station of the plurality of base stations 20, 30, 40, 50 at this wavelength (<0.3db/km, compared to ~4db/km for a laser signal at 780nm). However, the skilled person will understand that it is non-essential that the stabilised laser signal is distributed at 1560nm. That is, the core networking node 10 may transmit the laser signal at any wavelength, but preferably one having a lower transmission loss compared to a laser signal having the wavelength required by the Rydberg-atom based RF receiver. The distributed laser signal may therefore have a wavelength between 1260nm to 1625nm, or more preferably one of the Original (O)- band (1260-1360nm), Extended (E)-band (1360-1460nm), Short (S)-band (1460- 1530nm), Conventional (C)-band (1530-1565nm) or Long (L)-band (1565-1625nm). Distributing the laser signal at 1560nm to a plurality of base stations 20, 30, 40, 50 utilising a Rydberg-atom based RF receiver requiring a 780nm laser signal is beneficial due to the relative ease of the wavelength conversion between 780nm and 1560nm.

In the above first embodiment, the RF receiver of each base station includes a coupling laser for producing a coupling signal. However, the telecommunications network may also be adapted to produce a signal in the core networking node that is used as a coupling signal in the RF receiver of each base station (alternatively or in addition to the signal that is used as a probe signal in each RF receiver). A second embodiment, illustrating the central networking node 10 producing both a probe signal and a coupling signal, will now be described with reference to Figure 5 (in which the same reference numerals as used in the first embodiment are used for like-for-like components). The master laser 12 (identified as the first master laser 12 in this second embodiment), splitter 14 (identified as the first splitter 14 in this second embodiment), SHG 16 (identified as the first SHG 16 in this second embodiment) and MTS 18 (identified as the first MTS 18 in this second embodiment) operate in the same manner as the first embodiment. In this second embodiment, the core networking node 10 includes a second master laser node 13, a second splitter 15, a second SHG 17 and a second MTS 19. The second master laser 13 produces a laser signal at 960nm and the second splitter 15, second SHG 17 and second MTS 19 operate to split and stabilise the 960nm signal (in the same way as the first splitter 14, first SHG 16 and first MTS 18 operate to split and stabilise the 1560nm signal). The core networking node 10 also includes a multiplexer 11 which multiplexes the 1560nm and 960nm signals for distribution by the optical network 60 to each of the plurality of base stations 20, 30, 40, 50. At each base station, the 1560nm and 960nm signals are amplified (by amplifiers 22, 32, 42, 52) and demultiplexed (by demultiplexers 23, 33, 43, 53). The 1560nm signal is directed to the 1 ^st SHG 24, 34, 44, 54 to be wavelength converted to 780nm, and the 960nm signal is directed to the 2 ^nd SHG 25, 35, 45, 55 to be wavelength converted to 480nm. The 780nm signal is then used as a probe signal in each RF receiver 26, 36, 46, 56, and the 480nm signal is then used as a coupling signal in each RF receiver 26, 36, 46, 56. This second embodiment therefore has the advantage that both the probe signal and coupling signal are centrally produced, stabilised and distributed to each base station 20, 30, 40, 50. As the 960nm signal would be more heavily attenuated than the 1560nm signal during its passage of each distribution path of the optical network 60, then one or more amplifiers (such as a Neodymium Doped Fibre Amplifier (NDFA)) may be provided on each distribution path to amplify the 960nm signal.

A third embodiment will now be described with reference to Figure 6. Figure 6 illustrates another telecommunications network in which the probe and coupling signals are centrally produced. However, in this third embodiment, the second master laser 13 produces a 1248nm signal and the second SHG is replaced with a mixer. The 1248nm signal is split by splitter 14 into a first path and a second path. The first path directs the 1248nm signal to the mixer, where it is mixed with the 780nm signal from SHG 16, thus producing a 480nm signal. This 480nm signal is then passed to the second MTS 19 to stabilise the 1248nm signal. The second path directs the 1248nm signal to the multiplexer 11 which multiplexes the 1560nm signal and 1248nm signal for distribution by the optical network 60 to each of the plurality of base stations 20, 30, 40, 50. At each base station 20, 30, 40, 50, the 1560nm and 1248nm signals are amplified (by amplifiers 22, 32, 42, 52) and demultiplexed (by demultiplexers 23, 33, 43, 53). The 1560nm is passed to an SHG to be wavelength converted to 780nm and is then passed to both the RF receiver 26, 36, 46, 56 (to act as a probe signal) and to a mixer 27, 37, 47, 57 where it is mixed with the demultiplexed 1248nm signal to produce a 480nm signal. The 480nm signal is then also passed to the RF receiver 26, 36, 46, 56 to act as a coupling signal. This third embodiment has an advantage over the second embodiment in that the 1248nm signal suffers less attenuation than the 960nm signal, reducing the need for amplifiers on the distribution paths.

In the above embodiments, the optical network 60 may use an optical fibre such as single-mode fibre or hollow-core fibre. Hollow-core fibre may be designed such that attenuation of the signals (e.g. at 1560nm, 1248nm, and/or 960nm) is less than what would be experienced with use of a single-mode fibre. This may also reduce the need for amplifiers on the distribution paths and/or at each distributed node. In the second and third embodiments above, a multiplexer/demultiplexer is used to distribute multiple signals. However, this is non-essential and each signal may be distributed using a dedicated optical fibre.

The skilled person will understand that it is non-essential that the present invention is realised in an optical network that is part of a cellular telecommunications network. That is, the present invention may be realised in any form of telecommunications network in which a laser signal is distributed from a central node to a plurality of distributed nodes via an optical network.

Furthermore, the skilled person will understand that it is non-essential that each distributed node of the plurality of distributed nodes implement a Rydberg-atom based RF receiver. That is, the benefit of distributing a stable laser signal may be realised in any telecommunications network in which a plurality of distributed nodes each require a stable laser signal. The plurality of distributed nodes may each implement an alternative form of Rydberg-atom based technology or any other technology requiring a stable laser signal. Furthermore, the skilled person will understand that it is non-essential for the stable laser to be used to transition an electron from the ground state to a first excited state (i.e. a probe laser signal), and may be used for any other electron transition. Furthermore, the central node may provide a plurality of stable laser signals to each of the plurality of distributed nodes using the above technique, such that a first stable laser signal may be used as a probe laser signal and a second stable laser signal may be used as a coupling laser signal.

The skilled person will also understand that the use of a splitter in the core networking node 10 is non-essential, as any other device capable of splitting (that is, routing or directing) the laser signal into a local path and a plurality of distribution paths may be used instead. This may include, for example, an optical coupler. Furthermore, the skilled person will understand that it is non-essential that a laser produces each optical signal in the embodiments above. That is, another optical transmitter (or coherent optical transmitter), such as a Light Emitting Diode, LED, may be used instead.

The skilled person will understand that any combination of features is possible within the scope of the invention, as claimed.