Improvements to wavelength-division multiplexing

Abstract

There is herein described a method of wavelength division multiplexed (WDM) transmission from a first optical terminal to a second optical terminal, the first optical terminal being communicatively connected to the second optical terminal by a free-space optical link, the method comprising, at the first optical terminal, receiving a correction signal from the second optical terminal over the free space optical link, transmitting one or more component frequencies of an optical frequency comb to the second optical terminal over the free space optical link using WDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data, where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal

Description

[0001] Improvements to Wavelength-Division Multiplexing

[0002] Wavelength-division multiplexing (WDM) is a known technique for multi-channel optical data transmission. It enables multiple signals to be sent over the same channel.

[0003] In typical arrangements, multiple optical carrier signals of different wavelengths are produced by respective laser sources. Each carrier signal is then encoded with transmission data. The resulting modulated carrier signals are then multiplexed onto a single communications channel and transmitted to a receiver end. There, the signal is demultiplexed and the demultiplexed signals are demodulated to extract the transmission data.

[0004] Such systems require a significant number of components and are therefore costly to set up. Furthermore, such systems suffer from noise and signal loss.

[0005] 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.

[0006] According to a first aspect of the invention there is provided a method of wavelength division multiplexed (WDM) transmission from a first optical terminal to a second optical terminal, the first optical terminal being communicatively connected to the second optical terminal by a free-space optical link, the method comprising, at the first optical terminal: Receiving a correction signal, the correction signal having passed from the second optical terminal over a free space optical link;

[0007] Transmitting one or more component frequencies of an optical frequency comb to the second optical terminal over the free space optical link using WDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data;

[0008] Where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal.

[0009] The method may further comprise transmitting one or more component frequencies of a preliminary optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link using WDM, the one or more component frequencies of the first optical frequency comb having been modulated with preliminary transmission data.

[0010] According to a second aspect of the invention there is provided a method of wavelength division multiplexed (WDM) transmission from a first optical terminal to a second optical terminal, the first optical terminal being communicatively connected to the second optical terminal by a free-space optical link, the method comprising at the second optical terminal:

[0011] Transmitting a correction signal to the first optical terminal over the free space link;

[0012] Receiving one or more component frequencies of an optical frequency comb from the first optical terminal over the free-space optical link using WDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data;

[0013] Where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal.

[0014] The method may further comprise receiving one or more component frequencies of a preliminary optical frequency comb transmitted by the first optical terminal over the free- space optical link using WDM, the one or more component frequencies of the preliminary optical frequency comb having been modulated with preliminary transmission data.

[0015] According to a third aspect of the invention there is provided a method of wavelength division multiplexed (WDM) transmission from a first optical terminal to a second optical terminal, the first optical terminal being communicatively connected to the second optical terminal by a free-space optical link, the method comprising:

[0016] Transmitting a correction signal from the second optical terminal to the first optical terminal over the free space link;

[0017] Transmitting a plurality of component frequencies of an optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link using WDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data;

[0018] Where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal. The method may further comprise transmitting one or more component frequencies of a preliminary optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link using WDM, the one or more component frequencies of the preliminary optical frequency comb having been modulated with preliminary transmission data.

[0019] The method may further comprise modifying the preliminary optical frequency comb with the preliminary transmission data. This may take place at the first optical terminal. The method may further comprise modifying the optical frequency comb with the transmission data. This may take place at the first optical terminal. The received correction signal may have been transmitted from the first optical terminal and reflected from a beam splitter at the second optical terminal. The correction signal may comprise one or more component frequencies of the preliminary optical comb and may comprise the maximum and minimum frequencies of the preliminary optical comb.

[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 method may further comprise generating a slave optical frequency comb at the second optical terminal. The method may further comprise using one or more component frequencies of the preliminary optical comb transmitted from the first optical terminal to the second optical to set the frequency of the slave optical frequency comb. The one or more component frequencies of the preliminary optical comb may comprise the correction signal. The one or more component frequencies of the preliminary optical comb may be transmitted to from the first optical terminal to the second optical terminal via a beamsplitter. The method may further comprise extracting the transmission data from the one or more component frequencies of the preliminary optical frequency comb. The slave optical comb may be used to extract transmission data from the transmitted preliminary optical frequency comb.

[0022] The method may comprise generating the preliminary optical frequency comb and / or the second optical frequency comb. The free-space optical link may have a communication medium of air. The preliminary optical frequency comb and the second optical frequency comb may be generated by a single optical frequency comb generator. The master optical frequency comb generator may continuously generate a single continuous optical frequency comb. The preliminary optical frequency comb may comprise the single continuous comb at a first point in time and the optical frequency comb may comprise the single continuous comb at a second point in time which is later than the first point in time.

[0023] The step of modifying the optical frequency comb using the correction signal may comprise determining the difference in frequency and / or phase and / or power between two component signals of the correction signal. This difference may be used to produce the noise-correction factor.

[0024] The method may comprise modifying the optical frequency comb using the noisecorrection factor. This may take place at the first optical terminal and may comprise modulating the one or more further 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 optical frequency combs.

[0025] The method of the invention may be performed iteratively and / or continuously.

[0026] According to a fourth aspect of the invention there is provided a first optical terminal adapted to perform wavelength division multiplexed (WDM) transmission with a second optical terminal, the first optical terminal being communicatively connected to the second optical terminal by a free-space optical link, the first optical terminal comprising:

[0027] A receiver adapted to receive a correction signal, the correction signal having passed from the second optical terminal over a free space optical link;

[0028] A transmitter adapted to transmit one or more component frequencies of an optical frequency comb to the second optical terminal over the free space optical link using WDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data;

[0029] Where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal.

[0030] According to a fifth aspect of the invention there is provided a second optical terminal adapted to perform wavelength division multiplexed (WDM) transmission with a first optical terminal, the second optical terminal being communicatively connected to the first optical terminal by a free-space optical link, the second optical terminal comprising:

[0031] A transmitter adapted to transmit a correction signal to the first optical terminal over the free space link;

[0032] A receiver adapted to receive one or more component frequencies of an optical frequency comb from the first optical terminal over the free-space optical link using WDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data;

[0033] Where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal.

[0034] According to a sixth aspect of the invention there is provided a system for performing wavelength division multiplexed (WDM) transmission from a first optical terminal to a second optical terminal, the first optical terminal being communicatively connected to the second optical terminal by a free-space optical link, the system comprising:

[0035] A transmitter adapted to transmit a correction signal from the second optical terminal to the first optical terminal over the free space link;

[0036] The transmitter being further adapted to transmit a plurality of component frequencies of an optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link using WDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data;

[0037] Where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal.

[0038] An embodiment of the invention will now be described in detail, for illustration only, with reference to the appended drawings, in which:

[0039] Fig 1 is a schematic view of a system according to the prior art;

[0040] Fig 2 is a schematic view of an embodiment according to the invention;

[0041] Fig 3 is a schematic view of comb unit 21 of Fig 2;

[0042] Fig 4 is a schematic view of comb unit 28 of Fig 2; Fig 5 is a flow chart of an embodiment according to the invention.

[0043] Fig 1 shows an arrangement for data transmission using WDM according to the prior art. In particular, there is a transmitting apparatus 10 and a receiving apparatus 11. The transmitting apparatus 10 contains multiple laser sources 1. Each laser source 1 outputs a signal to a respective modulator 2 which modulates the signal with data for transmission. The modulated signals are then passed to a multiplexer 3, where they are multiplexed onto an optical fibre and transmitted to the receiving apparatus 11. At the receiving apparatus 11 , they are received at a demultiplexer 4, where they are demultiplexed into individual signals. Each individual signal is input into a respective coherent receiver 5. The transmission data is then extracted from the signals detected at the coherent receivers 5.

[0044] Fig 2 shows an arrangement for data transmission using WDM according to an embodiment of the invention. The arrangement comprises a transmitter end 200 and a receiver end 201. In use, comb unit 21 at the transmitter end produces an optical frequency comb. I will refer to this comb as the master comb. As the person skilled in the art would understand, an optical frequency comb is a plurality of optical signals of different frequencies with a constant frequency interval therebetween.

[0045] Fig 3 shows the comb unit 21 in more detail. In use, comb generator 50 generates the master comb, the frequency of which is locked by a laser (not shown). The master comb is transmitted to circulator 51 from where it passes to acousto-optic modulator 53. From there it passes to transfer system 54 and out of the comb unit 21.

[0046] Returning to Fig 2, the master comb arrives at a filter 22 which extracts the highest (f1) and lowest (f2) frequency signals from it. Signal f1 is transmitted over free space optical link 29 to a beam splitter 30 at the receiver end. A portion of the f1 signal is reflected by the beam splitter 30 back to the filter 22 at the transmitter end 200. The remainder of the f1 signal passes through the beam splitter 30 to a filter 27 at the receiver end 201. Signal f2 is transmitted over free space optical link 31 which carries signals to a beam splitter 32 at the receiver end. A portion of the f2 signal is reflected by the beam splitter 32 back to the filter 22 at the transmitter end 200. The remainder of the f2 signal passes through the beam splitter 32 to the filter 27. The remaining frequencies of the master comb are passed from filter 22 to respective modulators 23 where transmission data is encoded onto each. These modulated signals are each transmitted to a multiplexer 24 where they are multiplexed. This multiplexed signal is then transmitted to a demultiplexer 25 at the receiver end 201 over a free space optical channel 99.

[0047] At the demultiplexer 25, the multiplexed signal is demultiplexed into its component signals which are input into respective coherent receivers 26. Each coherent receiver 26 detects the incoming signal and outputs a corresponding signal to the filter 27.

[0048] Comb generating unit 28 generates a ‘slave’ optical frequency comb, which is centred at the same frequency as the master comb. Fig 4 shows the comb unit 28 in more detail. In use, comb generator 40 generates the slave comb, the frequency of which is locked by a laser (not shown). The slave comb is transmitted to circulator 41 from where it passes to acousto-optic modulator 43. From there it passes to transfer system 44 and out of the comb unit 28.

[0049] From there it arrives at the filter 27 shown in Fig 2. The filter 27 uses the slave comb to ‘remove’ the master comb signal from the signals received by the coherent receivers 26, thereby extracting the transmission data.

[0050] Due to atmospheric conditions, transmission over free space introduces noise and loss. For the system to work effectively this noise and loss must be compensated for. For this reason, a path stabilisation process is performed. The path stabilisation process has two parts which are performed simultaneously.

[0051] Path stabilisation process - transmitter end

[0052] The first part takes place generally at the transmitter end. In particular, the portion of the signals f1 and f2 that are reflected back to the filter 22 from the respective beam splitters 30 and 32 are passed into comb unit 21, shown in Fig 3. They pass into the acousto-optic modulator 53 and travel via the circulator 51 to a Proportional-lntegral- Derivative Controller (PID) 52. PID 52 determines the difference in phase, frequency, and power between f1 and f2 and 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 path taken by f1 will be exactly the same length as the path taken by f2. 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.

[0053] The PID 52 uses the computed error to generate a correction factor. The PID 52 provides an input to the acousto-optic modulator that is proportional to the correction factor. The acousto-optic modulator 53 modulates the outgoing master comb signal using the input it has received from the PID 52. 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. This path stabilisation process occurs continuously such that changes in atmospheric conditions over the link are continuously compensated for.

[0054] Path stabilisation process - receiver end

[0055] The second part of the path stabilisation process concerns the portion of the signals f1 and f2 that pass through beam splitters 30 and 32 and arrive at filter 27. In particular, there signals pass from filter 27 into the acousto-optic modulator 43 of comb unit 28. From there they pass through transfer system 44 to the circulator 41 and on to the Proportional-lntegral-Derivative Controller (PID) 42. PID 42 determines the difference in phase, frequency, and power between the two signals and 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 path taken by f1 will be exactly the same length as the path taken by f2. 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.

[0056] The PID 42 uses the computed error to generate a correction factor. The PID 42 provides an input to the acousto-optic modulator 43 that is proportional to the correction factor. The acousto-optic modulator 43 modulates the slave comb signal using the input it has received from the PID 42. In this way the slave comb is modulated to correct for the noise and loss in the transmitted master comb caused by the atmospheric conditions in the tree-space link. This path stabilisation process occurs continuously such that changes in atmospheric conditions over the link are continuously compensated for.

[0057] The flow chart of Fig 5 shows, at step 101 , at a transmitter end 200, an optical comb is continuously generated and transmission data is modulated onto the component frequencies of the optical comb. At step 102, the component frequencies are multiplexed together and transmitted over free space to a receiver end 201. At step 103, the receiver end 201 demultiplexes the frequencies and extracts the transmission data using coherent receivers 26 and a slave optical comb. At step 104, two frequencies from the optical comb are transmitted from the transmitter end 200 to the receiver end 201 where they are reflected back to the transmitter end 200 from beam splitters 30,32. At step 105, the two frequencies are compared to generate a noisecorrection factor. At step 106, the noise-correction factor is used to calculate a modulation which is applied to the outgoing optical comb using an acousto-optic modulator. The modulated comb is transmitted to the receiver end 201.

Claims

Claims1 . A method of wavelength division multiplexed (WDM) transmission from a first optical terminal to a second optical terminal, the first optical terminal being communicatively connected to the second optical terminal by a free-space optical link, the method comprising, at the first optical terminal:Receiving a correction signal from the second optical terminal over the free space optical link;Transmitting one or more component frequencies of an optical frequency comb to the second optical terminal over the free space optical link using WDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data;Where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal.

2. A method of wavelength division multiplexed (WDM) transmission from a first optical terminal to a second optical terminal, the first optical terminal being communicatively connected to the second optical terminal by a free-space optical link, the method comprising, at the second optical terminal:Transmitting a correction signal to the first optical terminal over the free space link;Receiving one or more component frequencies of an optical frequency comb from the first optical terminal over the free-space optical link using WDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data;Where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal.

3. A method of wavelength division multiplexed (WDM) transmission from a first optical terminal to a second optical terminal, the first optical terminal being communicatively connected to the second optical terminal by a free-space optical link, the method comprising:Transmitting a correction signal from the second optical terminal to the first optical terminal over the free space link;Transmitting a plurality of component frequencies of an optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link usingWDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data;Where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal.

4. A method as claimed in any preceding claim, wherein the method further comprises modulating the optical frequency comb with the transmission data.

5. A method as claimed in any preceding claim, wherein the correction signal has been transmitted from the first optical terminal and reflected back to the first optical terminal from a beam splitter at the second optical terminal.

6. A method as claimed in any preceding claim, wherein the correction signal comprises maximum and minimum frequencies of the optical frequency comb.

7. A method as claimed in any preceding claim, wherein the method further comprises generating a slave optical frequency comb at the second optical terminal and using one or more component frequencies of the optical comb transmitted from the first optical terminal to the second optical terminal to set a frequency of the slave optical frequency comb.

8. A first optical terminal adapted to perform wavelength division multiplexed (WDM) transmission with a second optical terminal, the first optical terminal being communicatively connected to the second optical terminal by a free-space optical link, the first optical terminal comprising:A receiver adapted to receive a correction signal, the correction signal having passed from the second optical terminal over a free space optical link;A transmitter adapted to transmit one or more component frequencies of an optical frequency comb to the second optical terminal over the free space optical link using WDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data;Where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal.

9. A second optical terminal adapted to perform wavelength division multiplexed (WDM) transmission with a first optical terminal, the second optical terminal being communicatively connected to the first optical terminal by a free-space optical link, the second optical terminal comprising:A transmitter adapted to transmit a correction signal to the first optical terminal over the free space link;A receiver adapted to receive one or more component frequencies of an optical frequency comb from the first optical terminal over the free-space optical link using WDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data;Where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal.

10. A system for performing wavelength division multiplexed (WDM) transmission from a first optical terminal to a second optical terminal, the first optical terminal being communicatively connected to the second optical terminal by a free-space optical link, the system comprising:A transmitter adapted to transmit a correction signal from the second optical terminal to the first optical terminal over the free space link;The transmitter being further adapted to transmit a plurality of component frequencies of an optical frequency comb from the first optical terminal to the second optical terminal over the free-space optical link using WDM, the one or more component frequencies of the optical frequency comb having been modulated with transmission data;Where the optical frequency comb has been modified using a noise-correction factor derived from the correction signal.

11. 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 7.

12. A computer-readable carrier medium comprising the computer program of claim 11 .

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