Optical frequency comb transmission method

Abstract

There is herein described a method of optical transmission from a first optical terminal to a second optical terminal, the first optical terminal being connected to the second optical terminal by a free-space optical link, the method comprising transmitting a first master optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link receiving at the first optical terminal, a return signal transmitted by the second optical terminal over the free-space optical link, the return signal being derived from the transmitted master optical frequency comb, transmitting a second master optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link, the second master optical frequency comb having been modified using the return signal.

Description

[0001] Optical Frequency Comb Transmission Method

[0002] High-precision time dissemination is a requirement in multiple industries. Examples include, high-frequency trading, high-throughput data services in telecomms and broadcasting, and Positioning, Navigation and Timing (PNT) services.

[0003] PNT services, for example, enable user devices to determine their position, to navigate or track and to determine the local time with high accuracy. Currently, satellite-based systems (GNSS) are used for this purpose. In these systems, multiple satellites transmit line-of-sight radio signals to each user device. The user devices use the satellite signals in geo-positioning. For these systems to function effectively, the internal clocks of the satellites and the user devices must be synchronised with extremely high accuracy. This is challenging.

[0004] It would be desirable to overcome and / or substantially mitigate some or all of the above- mentioned and / or other disadvantages of the prior art.

[0005] According to a first aspect of the invention there is provided a method of optical transmission from a first optical terminal to a second optical terminal, the first optical terminal being connected to the second optical terminal by a free-space optical link, the method comprising:

[0006] Transmitting one or more component frequencies of a first master optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link;

[0007] Receiving at the first optical terminal, a return signal transmitted by the second optical terminal over the free-space optical link, the return signal may be derived from the master optical frequency comb;

[0008] Transmitting one or more component frequencies of a second master optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link, the second master optical frequency comb having been modified using the return signal.

[0009] The method may further comprise transmitting the return signal from the second optical terminal to the first optical terminal over the free-space optical link, the return signal being derived from the transmitted master optical frequency comb. According to a second aspect of the invention there is provided a method of optical transmission from a first optical terminal to a second optical terminal, the first optical terminal being connected to the second optical terminal by a free-space optical link, the method comprising:

[0010] Receiving at the second optical terminal, one or more component frequencies of a first master optical frequency comb transmitted by the first optical terminal over the free- space optical link;

[0011] Transmitting a return signal from the second optical terminal to the first optical terminal over the free-space optical link, the return signal being derived from the transmitted master optical frequency comb;

[0012] Receiving at the second optical terminal, one or more component frequencies of a second master optical frequency comb transmitted by the first optical terminal over the free-space optical link, the second master optical frequency comb having been modified using the return signal.

[0013] The method may further comprise generating a slave optical frequency comb using the transmitted second master optical frequency comb. The method may further comprise transmitting the first master optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link. The method may further comprise transmitting the second master optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link, the second master optical frequency comb having been modified using the return signal.

[0014] According to a third aspect of the invention there is provided a method of optical transmission from a first optical terminal to a second optical terminal, the first optical terminal being connected to the second optical terminal by a free-space optical link, the method comprising:

[0015] Transmitting one or more component frequencies of a first master optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link;

[0016] Transmitting a return signal over the free-space optical link from the second optical terminal to the first optical terminal, the return signal being derived from the transmitted master optical frequency comb; Transmitting one or more component frequencies of a second master optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link, the second master optical frequency comb having been modified using the return signal;

[0017] Generating a slave optical frequency comb using the transmitted second master optical frequency comb.

[0018] The following features are applicable to each of the first, second and third aspects of the invention.

[0019] The method may comprise distributing timing information from a master communications device to a slave communications device. The first optical terminal may be a component of the master communications device. The second optical terminal may be a component of the slave communications device. The method may comprise generating the first master optical frequency comb and / or the second master optical frequency comb. The method may further comprise setting a central frequency of the master optical comb using a master clock which may be a component of the master communications device. The method may further comprise setting the timing of a slave optical clock using timing information extracted from the slave optical frequency comb. The slave clock may be a component of the slave communications device.

[0020] A path taken by the return signal over the free space optical link may have the same length as a path taken by the first master optical frequency comb and / or the second master optical frequency comb over the free space optical link.

[0021] The free-space optical link may have a communication medium of air. The method may comprise generating a preliminary slave optical frequency comb using the transmitted first master optical frequency comb. The step of generating a slave optical frequency comb using the transmitted second master optical frequency comb may comprise modifying the preliminary slave optical frequency comb using the transmitted second master optical frequency comb.

[0022] The first master optical frequency comb and the second master optical frequency comb may be generated by a single master optical frequency comb generator. The master optical frequency comb generator may continuously generate a single continuous optical frequency comb. The first master optical frequency comb may comprise the single continuous comb at a first point in time and the second comb may comprise the single continuous comb at a second point in time which is later than the first point in time.

[0023] The method may comprise modifying the second master optical frequency comb using the return signal. This may comprise modifying the second master optical frequency comb using the central frequency signal of the transmitted master optical frequency comb. Alternatively, this may comprise modifying the second master optical frequency comb using any of the component frequency signals of the transmitted master optical frequency comb. The nature of optical frequency combs is such that once the interference on one of the component frequency signal has been determined and compensated for, the same compensation can be used for all of the frequency signals in the optical comb. Thus the whole optical comb can be stabilised simultaneously.

[0024] The step of modifying the second master optical frequency comb using the return signal may comprise comparing the return signal with the transmitted master optical frequency comb to produce a correction factor. The return signal may comprise a central frequency of the transmitted master optical frequency comb. The purpose of modifying the second master optical frequency comb may be to compensate for the effects of atmospheric conditions in the free-space link.

[0025] The method may further comprise modifying the one or more further master optical frequency combs using the correction factor. This may comprise modulating the one or more further master optical frequency combs using an acousto-optic modulator. The correction factor may modify the frequency and / or phase and / or power of the one or more subsequent master optical frequency combs. The method may further comprise transmitting one or more further master optical frequency combs that have been modified using the correction factor. The method may further comprise generating a slave optical frequency comb using the one or more subsequent master optical frequency combs.

[0026] The method of the invention may be performed iteratively and / or continuously.

[0027] According to a fourth aspect of the invention there is provided a method of optical transmission from a first optical terminal to one or more second optical terminals, the first optical terminal being connected to each of the one or more second optical terminals by a respective free-space optical link, the method comprising:

[0028] Transmitting one or more component frequencies of a first master optical frequency comb from the first optical terminal to the one or more second optical terminals over the free-space optical links;

[0029] Transmitting a return signal over the free-space optical links from each of the one or more second optical terminals to the first optical terminal, the return signal being derived from the transmitted master optical frequency comb;

[0030] Transmitting one or more component frequencies of a second master optical frequency comb from the first optical terminal to each of the one or more second optical terminals over the free-space optical links, the second master optical frequency comb having been modified using the return signal;

[0031] Generating a slave optical frequency comb using each of the transmitted second master optical frequency combs.

[0032] According to a fifth aspect of the invention there is provided an apparatus for distributing information, the apparatus comprising a first optical terminal and a second optical terminal, the first optical terminal being connected to the second optical terminal by a free-space optical link, the first optical terminal comprising a transmitter adapted to transmit one or more component frequencies of a first master optical frequency comb to the second optical terminal over the free-space optical link; the second optical terminal comprising a transmitter adapted to transmit a return signal over the free-space optical link to the first optical terminal, the return signal being derived from the transmitted master optical frequency comb; the transmitter at the first optical terminal being adapted to transmit one or more component frequencies of a second master optical frequency comb to the second optical terminal over the free-space optical link, the second master optical frequency comb having been modified using the return signal; wherein the apparatus is further adapted to generate a slave optical frequency comb using the transmitted second master optical frequency comb.

[0033] An embodiment of the invention will now be described in detail, for illustration only, with reference to the appended drawings, in which: Fig 1 is a schematic view of an arrangement according to the prior art;

[0034] Fig 2 is a schematic view of an arrangement according to an embodiment of the invention;

[0035] Fig 3 is a flow chart showing the operation of the arrangement of Fig 2.

[0036] Fig 1 shows an arrangement according to the prior art. The arrangement comprises a transmitter side 200, containing a master clock 1 , and a receiver side 201 , containing a slave clock 10. The aim of the arrangement is to synchronise the slave clock 10 to the master clock 1. In particular, the master clock 1 transmits a time signal, i.e. a signal indicative of the timing of the master clock 1 , to master clock laser 2. Master clock laser 2 uses the time signal to produce an optical signal of frequency fs. The signal fsis transmitted to master comb generator 3, which uses the signal fsto generate a selfreferenced optical frequency comb, in respect of which fsis the central frequency. The master comb is then passed through a phase-locked loop 4 and acousto-optic modulator 11 to stabilise the frequency of the comb.

[0037] The master comb is then transmitted to the transmitter’s transfer system 5 where it is transmitted over optical fibre to the receiver’s transfer system 6. The received master comb is passed through a phase-locked loop 7 and acousto-optic modulator 12 to stabilise the signal. It then passes into a slave comb generator 8 where it is used to set the central frequency of a slave comb. The slave comb is a self-referenced optical frequency comb. The slave comb is then passed to a slave clock laser 9, which extracts the central frequency fsof the slave comb. This central frequency fsis transmitted to the slave clock 10, where it is used to set the time of the slave clock 10.

[0038] In order to compensate for noise and loss in the optical fibre link between the transmitter side and the receiver side, part of the received master comb is transmitted back from the receiver side transfer system 6 to the transmitter side transfer system 5 over the same fibre. The part of the master comb is transmitted to the phase-locked loop 4, where it is used in the path stabilisation process. In particular, the frequency of the master comb is modified in order to correct for the loss and noise introduced in the transmission from transmitter side to receiver side. Fig 2 shows an arrangement according to the invention. In particular, the arrangement comprises a transmitter side 300, containing a master clock 21, and a receiver side 301 , containing a slave clock 32. The purpose of the arrangement is to synchronise the slave clock 32 to the master clock 21. In particular, the master clock 21 transmits a time signal, i.e. a signal indicative of the timing of the master clock 21 , to master clock laser 22. Master clock laser 22 uses the time signal to produce an optical signal of frequency fs. The signal fsis transmitted to master comb generator 23, which uses the signal fsto generate a self-referenced optical frequency comb with fsas its central frequency. The master comb is then passed through a path stabilisation system 24.

[0039] The path stabilisation system comprises a circulator 25, a Proportional-lntegral- Derivative controller (PID) 26 and an acousto-optic modulator 27. The master comb is passed from the Master Comb Generator 23 to the circulator 25. The circulator 25 directs the master comb through the acousto-optic modulator 27 to a transmitter-side transfer system 28. It is then transmitted over a free-space link 33 to a receiver-side transfer system 29. From there, the master comb passes to a slave comb generator 30, where it is used to produce a slave comb, the central frequency of which is the same as the received master comb. The slave comb is also a self-referenced optical frequency comb. The slave comb is then passed to a slave clock laser 31 , which extracts the central frequency of the slave comb. This central frequency is transmitted to the slave clock 32, where it is used to set the time of the slave clock 32. In this way, timing has been distributed from the master clock 21 to the slave clock 32.

[0040] The transmission of the master comb over the free-space link introduces some noise and signal loss to the master comb. Therefore, although the central frequency of the master comb when it is transmitted is fs, the central frequency of the master comb when it is received is slightly different. I will call this frequency fsn. A path stabilisation process is used to correct for this difference. This will now be described.

[0041] Path Stabilisation Process

[0042] After the master comb has been received at the receiver-side transfer system 29, the receiver-side transfer system transmits the central frequency fsnof the master comb it has received, back to the transmitter side transfer system 28. The transmitter-side transfer system 28 then transmits that received central comb frequency fsn, along with the original central comb frequency fs, through the acousto-optic modulator 27 to the circulator 25. From there the two signals pass to the Proportional-lntegral-Derivative controller (PID) 26.

[0043] The PID 26 calculates the difference between fsand fsnand uses that information to compute the error that has been introduced by the free-space link. This error will contain information such as changes in phase, frequency, and power caused by the atmospheric conditions of the link. As the signals are transmitted over a free space link, the return path taken by the central frequency will be exactly the same length as the outbound path taken by the master comb. The fact that these two paths are the same length improves the accuracy of the path stabilisation system. This is because, for example, the exact phase of the signal when it reaches the receiver will be determinable from the return signal.

[0044] The PID 26 uses the computed error to generate a correction factor. The PID 26 provides an input to the acousto-optic modulator that is proportional to the correction factor. The acousto-optic modulator 27 modulates the outgoing master comb signal using the input it has received from the PID 26. In this way the outgoing master comb is modulated to correct for the noise and loss caused by the atmospheric conditions in the free-space link 33. This modulation results in the central frequency of the resulting slave comb being brought closer to the original central frequency of the master comb.

[0045] This path stabilisation process occurs continuously such that changes in atmospheric conditions over the link are continuously compensated for.

[0046] Fig 3 is a flow chart showing the steps of the invention. In particular, at step 101, at a transmitter, a master comb is continuously generated from a master clock and transmitted to a receiver over a free-space link. At step 102, the receiver uses the received master comb to generate a slave comb and uses the slave comb to set the timing of a slave clock. At step 103, the receiver transmits the central frequency of the received master comb back to the transmitter. At step 104, at the transmitter, the central frequency of the received master comb is compared to the central frequency of the transmitted master comb and a correction factor is calculated. At step 105, using an acousto-optic modulator, the correction factor is used to modulate the outgoing master comb. At step 106, at the receiver, the received modulated master comb is used to generate a modified slave comb which is used to update the timing of the slave clock. The process then returns to step 101 and operates continuously.

Claims

Claims1.A method of optical transmission from a first optical terminal to a second optical terminal, the first optical terminal being connected to the second optical terminal by a free-space optical link, the method comprising:Transmitting one or more component frequencies of a first master optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link;Receiving, at the first optical terminal, a return signal transmitted by the second optical terminal over the free-space optical link, the return signal being derived from the transmitted master optical frequency comb;Transmitting one or more component frequencies of a second master optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link, the second master optical frequency comb having been modified using the return signal.

2. A method of optical transmission from a first optical terminal to a second optical terminal, the first optical terminal being connected to the second optical terminal by a free-space optical link, the method comprising:Receiving, at the second optical terminal, one or more component frequencies of a first master optical frequency comb transmitted by the first optical terminal over the free- space optical link;Transmitting a return signal from the second optical terminal to the first optical terminal over the free-space optical link, the return signal being derived from the transmitted master optical frequency comb;Receiving at the second optical terminal, one or more component frequencies of a second master optical frequency comb transmitted by the first optical terminal over the free-space optical link, the second master optical frequency comb having been modified using the return signal.

3. A method as claimed in any preceding claim, the method further comprising generating a slave optical frequency comb using the transmitted second master optical frequency comb.

4. A method of optical transmission from a first optical terminal to a second optical terminal, the first optical terminal being connected to the second optical terminal by a free-space optical link, the method comprising:Transmitting one or more component frequencies of a first master optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link;Transmitting a return signal over the free-space optical link from the second optical terminal to the first optical terminal, the return signal being derived from the transmitted master optical frequency comb;Transmitting one or more component frequencies of a second master optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link, the second master optical frequency comb having been modified using the return signal;Generating a slave optical frequency comb using the transmitted second master optical frequency comb.

5. A method as claimed in any preceding claim, the method being adapted to distribute timing information from the first optical terminal to the second optical terminal.

6. A method as claimed in any preceding claim, the method further comprising generating the first master optical frequency comb and the second master optical frequency comb.

7. A method as claimed in any preceding claim, wherein free-space optical link has a communication medium of air.

8. A method as claimed in any preceding claim, wherein the return signal comprises a central frequency of the transmitted first master optical frequency comb.

9. A method as claimed in any preceding claim, the method further comprising modifying the second master optical frequency comb using the return signal.

10. A method as claimed in claim 9, wherein the step of modifying the second master optical frequency comb using the return signal comprises comparing the return signal with the transmitted master optical frequency comb to produce a correction factor.

11. A method as claimed in claim 10, the method further comprising modifying, using an acousto-optic modulator, one or more further master optical frequency combs using the correction factor.

12. A method as claimed in claim 11 , the method further comprising transmitting one or more further master optical frequency combs that have been modified using the correction factor.

13. An apparatus for distributing information, the apparatus comprising a first optical terminal and a second optical terminal, the first optical terminal being connected to the second optical terminal by a free-space optical link, the first optical terminal comprising a transmitter adapted to transmit one or more component frequencies of a first master optical frequency comb to the second optical terminal over the free-space optical link; the second optical terminal comprising a transmitter adapted to transmit a return signal over the free-space optical link to the first optical terminal, the return signal being derived from the transmitted master optical frequency comb; the transmitter at the first optical terminal being adapted to transmit one or more component frequencies of a second master optical frequency comb to the second optical terminal over the free-space optical link, the second master optical frequency comb having been modified using the return signal; wherein the apparatus is further adapted to generate a slave optical frequency comb using the transmitted second master optical frequency comb.

14. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of any of claims 1 to 12.

15. A computer-readable carrier medium comprising the computer program of claim 14.

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