Apparatus and method for reducing distortion of an optical signal

Abstract

An RF signal to be carried by an optical link is modulated onto two optical beams. The modulators are tuned differently so that the distortion products carried on one beam are relatively larger compared to the fundamental compared with other beam. One of the beams is optically upconverted by the appropriate Brillouin shift frequency and the two beams counter-propagated through an optical waveguide in order to create a Brillouin grating. The grating acts to separate the distortion products from the fundamental so as to provide at an output of the link a signal in which the distortion products are insignificant is not absent.

Claims

1. Apparatus for reducing distortion in an optical signal, the apparatus comprising: an optical waveguide; an input for receiving a signal; means for generating a first coherent light beam modulated to carry the signal received at the input, a means for generating a second coherent light beam modulated to carry the signal received at the input; wherein the optical frequencies of the first coherent light beam and second coherent light beam are selected to differ to satisfy a Brillouin condition; and wherein the first and second coherent light beams are selected to have different fundamental to distortion product ratios; the apparatus being arranged to cause the first coherent light beam to propagate along the optical waveguide in a first direction, and the second coherent light beam to propagate along the optical waveguide in an opposite direction to generate a dynamic Brillouin grating in the optical waveguide through stimulated Brillouin scattering.

2. Apparatus according to claim 1, wherein the second coherent light beam is selected to have a lower fundamental to distortion product ratio than the first coherent light beam.

3. Apparatus according to claim 2, wherein the means for generating the first coherent light beam comprises: a first optical modulator tuned so as to be of relatively high efficiency and relatively highly linear compared with the means for generating the modulated second coherent light beam, such that the first coherent light beam will have a relatively high fundamental to distortion product ratio compared with the second coherent light beam.

4. Apparatus according to claim 1, wherein the means for generating the first coherent light beam comprises: a single sideband (SSB) modulator.

5. Apparatus according to claim 1, wherein the means for generating a second coherent light beam comprises: a detuned modulator compared with the first modulator such that the second coherent light beam will have a relatively low fundamental to distortion product ratio relative to the first coherent light beam.

6. A radio receiver according to claim 5, wherein the detuned modulator is biased substantially at pi ().

7. Apparatus according to claim 1, wherein the means for generating the second coherent light beam comprises: a single sideband (SSB) modulator.

8. Apparatus according to claim 1, comprising: means to shift the frequency of the second coherent light beam relative to the first optical beam to meet the Brillouin condition.

9. Apparatus according to claim 2, comprising: an optical receiver arranged to receive a portion of the first coherent light beam that has passed through the dynamic Brillouin grating.

10. Apparatus according to claim 9, comprising: an analogue-to-digital converter arranged to receive an output from the optical receiver.

11. Apparatus according to claim 9, comprising: means for generating a third coherent light beam modulated with a frequency shifted from a frequency of interest by a beat frequency; a photonic mixer arranged to combine an output optical signal that has passed through the Brillouin grating with the third coherent light beam, and wherein the optical receiver is arranged to receive the output from the photonic mixer in order to output an electrical signal carrying the beat frequency.

12. Apparatus according to claim 1, comprising: an amplifier arranged and configured to amplify the modulated second coherent light beam before it is propagated through the optical waveguide.

13. Apparatus according to claim 12, wherein the amplifier is arranged and configured to amplify the modulated second coherent light beam to a power to cause the grating to scatter optical distortion products of the first coherent light beam to a sufficient extent that said distortion in a portion of the light beam that has passed through the Brillouin grating will be below the noise floor.

14. A radio receiver system including: a radio frequency receiver and an optical link, the optical link comprising: an optical waveguide; an input for receiving a signal from the radio frequency receiver; means for generating a first coherent light beam modulated to carry a signal received at the input; a means for generating a second coherent light beam modulated to carry the signal received at the input; wherein the optical frequencies of the first coherent light beam and second coherent light beam are selected to differ to satisfy the Brillouin condition; and wherein the first and second coherent light beams are selected to have different fundamental to distortion product ratios; the apparatus being arranged such that the first coherent light beam propagates along the optical waveguide in a first direction and the second coherent light beam propagates along the optical waveguide in an opposite direction to generate a dynamic Brillouin grating in the optical waveguide through stimulated Brillouin scattering.

15. A method for reducing distortion in an optical signal, the method comprising: modulating a first coherent light beam to carry a signal and propagating the modulated coherent light beam along an optical waveguide; modulating a second coherent light beam to carry the signal and propagating the modulated coherent light beam along an optical waveguide; and generating a dynamic Brillouin grating in the optical waveguide using stimulated Brillouin scattering by propagating the second coherent light beam through the optical waveguide in an opposite direction to the first coherent light beam; wherein the first and second coherent light beams are modulated so as to have different fundamental to distortion product ratios.

16. A method according to claim 15, wherein the second coherent light beam is modulated to have a lower fundamental to distortion product ratio than the first coherent light beam, and wherein an optical-electro transducer is arranged to receive a portion of the first coherent light beam that has passed through the Brillouin grating.

17. A method according to claim 15, wherein the second coherent light beam is modulated by a detuned optical modulator.

18. A method according to claim 17, wherein the second coherent light beam is modulated by an optical modulator detuned substantially to .

19. A method according to claim 15, comprising: upconverting the second coherent light beam by a frequency substantially equal to an anti-stokes shift for Brillouin scattering.

20. A method according to claim 15, comprising: amplifying the modulated second coherent light beam before being propagated through the optical waveguide.

21. A method according to claim 15, comprising: amplifying the modulated second coherent light beam to a power such that distortion products caused from optical distortion are below a noise floor.

22. A method according to claim 15, wherein the first coherent light beam is modulated by a single sideband (SSB) modulator.

23. A method according to claim 15, comprising: splitting a coherent light beam to create the first and second coherent light beams.

24. A method according to claim 15, wherein an analogue to digital converter is arranged to receive an output of the optical-electro transducer.

25. A method according to claim 24, comprising: modulating a third light beam so as to impose thereon a frequency that is spaced from a frequency of interest by a separation frequency that lies within a operational bandwidth of the analogue to digital converter, the third light beam being mixed with a portion of the first coherent light beam that has passed through the Brillion grating to form a mixed beam that is received by the optical-electro transducer.

26. A method for reducing distortion in an optical signal of a photonic link connected to a RF receiver or RF transmitter, the method comprising: modulating a first coherent light beam to carry a signal received by the RF receiver or transmitted by the RF transmitter, and propagating the modulated first coherent light beam along an optical waveguide; modulating a second coherent light beam to carry the signal received by the RF receiver or transmitted by the RF transmitter and propagating the modulated second coherent light beam along an optical waveguide; and generating a dynamic Brillouin grating in the optical waveguide using stimulated Brillouin scattering by propagating the second coherent light beam through the optical waveguide in an opposite direction to the first coherent light beam; wherein an optical frequency of the first coherent light beam and second coherent light beam are selected to differ by an amount to satisfy a Brillouin condition; and wherein the first and second coherent light beams are modulated so as to have different fundamental to distortion product ratios.

Description

(1) The invention will now be described by way of example with reference to the following figures in which:

(2) FIG. 1 is functional schematic of optical distortion removal mechanism;

(3) FIG. 2 is a schematic of a wide band RF photonic receiver comprising an optical link incorporating an optical distortion removal mechanism;

(4) FIG. 3 is a schematic of a further variant wide band RF photonic receiver;

(5) FIG. 4 is a schematic of a variant to the wide band RF photonic receiver of FIG. 2.

(6) FIG. 5 is a schematic of a photonic link used to remove distortion from the output of an electrical mixer.

(7) FIG. 1 illustrates functional schematic of distortion removal apparatus being part of an optical link for use with a RF receiver, e.g. a wide band receiver adapted to receive EW signals between 1 Khz-100 GHz.

(8) A first light beam, imposed with both the fundamental and unwanted distortion products, is propagated through an optical waveguide in a first direction. A second light beam, imposed with both the fundamental and unwanted distortion products, is propagated through an optical waveguide in an opposite direction to the first beam. The ratio of the amplitude of fundamental to amplitude of largest distortion product is lower for the second beam than the equivalent ratio for the first light beam. In other words the amplitude of the distortion products relative to the fundamental are relatively larger in the second beam compared with the first beam. The second light beam is frequency shifted with respect that of the first beam by the antistoke frequency (Brillouin shift) for the waveguide material.

(9) The second beam is of sufficient optical power to generate a dynamic SBS grating in the waveguide that acts to partially reflect the first beam back in the opposite direction.

(10) Because the second beam has a lower ratio of fundamental to distortion products compared with the first beam, the grating preferentially reflects distortion products within the first beam over fundamental. As such the portion of the first light beam that passes through the grating has distortion products of reduced amplitude relative to fundamental compared with the first beam before incidence with the grating. In other words the output has a higher fundamental to distortion product ratio that than the first beam. The apparatus may include an optical-electric transducer arranged to receive the portion of the first beam transmitted through the grating.

(11) If the second beam is of sufficient optical power the amplitude of all distortion products can be removed to below the noise floor such as to provide the output with a very high spectrally free dynamic range.

(12) The optical power of the beam needed to generate a SBS grating will depend on the optical medium used, which can be straightforwardly determined through empirical experimentation. The polarization of first and second beams need to be suitable controlled to generate the grating. Such control is taught in Nikolay Primerov thesis mentioned above, but nevertheless will be known to those skilled in the art.

(13) FIG. 2 is a schematic of an EW RF receiver and optical linkage that utilises the distortion removal apparatus of FIG. 1.

(14) There is shown a laser 1, typically a distributed feedback laser, a first splitter 2, a polarisation controller 3, a single sideband carrier suppressed (SSB(C)) modulator 4, an optical medium 5 (typically a length of optical fibree.g. single mode fibre) in which a stimulated Brillion dynamic grating is produced; an optical combiner 6, a high frequency photodiode 9, an analogue digital converter 10 having an output for receipt by a digital EW system; a detuned modulator 12; a photonic upconverter 13 tuned to upconvert an input by an anti-stokes frequency; an optical amplifier (e.g. a EDFA); and a second polarisation controller 15.

(15) The apparatus further includes an EW receiver 18 comprising a RF antenna and amplifier, and a wide band RF coupler 19.

(16) An EW signal received by receiver 18 is combined through the wide band RF coupler 19, (e.g. a Wilkinson coupler) with a local oscillator (lo) tuned to ensure the photodiode 9 beats at a frequency that the ADC 10 can resolve. The output from the coupler 19 is used to drive modulators 4 and 12.

(17) The laser 1 provides a coherent light beam, e.g. of a near-IR frequency, that is split by first splitter 2 into a first light beam L1 and a second light beam L2.

(18) The first light beam L1 modulated by the SSB(C) modulator 4 and the second light beam L2 is modulated by the detuned modulator 12 such that both the first and third light beams L1 L2 carry EW signals fed from receiver 18.

(19) Both the first and second modulators are optionally Mach Zehnder modulators. The first modulator is biased for high efficiently, e.g. to /2.

(20) The detuned modulator L2 is biased at 7E such that L2 has a smaller ratio of the amplitude of the fundamental:amplitude of largest distortion products is than L1.

(21) The amplitude of the distortion products in L2 may still be smaller than the amplitude of the fundamental (though an amplitude larger than the fundamental would be preferable).

(22) Modulated beam L2 is up converted by the antistokes frequency (typically 11 GHz depending on the material(s) used to construct the optical fibre) and then amplified by optical amplifier (e.g. of EDFA type or other) 14.

(23) The first beam L1 and second beam L2 are counter propagated through optical fibre 5. The second beam L2 is of sufficient power to incite a Brillouin grating within the optical fibre 5.

(24) Polarisation controllers 3 and 15 are used to control the polarisation of L1 and L2 respectively such that Brillion grating occurs within the fibre 5 at the correct orientation to provide the desired reflection of L1. This technique will be familiar to those skilled in the art.

(25) Because the ratio of the amplitude of the fundamental:distortion products in the second beam L2 is lower than the ratio of the amplitude of the fundamental to distortion products in first beam L1, the grating preferentially reflects the distortion products within L1 over the fundamental, meaning that output beam L3, being that part of L1 transmitted through grating in fibre 5, has a greater ratio of fundamental:distortion products than L1. L3 thus has a higher spectral free dynamic range than L1.

(26) It will be appreciated that the amplitude of the distortions products in L2 need to be above a threshold level for the Brillouin grating to be effective at reflecting the distortion products carried by L1.

(27) The optical power of L2 can be adjusted through control of amplifier 14. Favourably L2 is made to be sufficiently optically powerful to reduce all distortion products within L3 to below the noise floor.

(28) By increasing the optical power of L2, the distortion products will be more strongly reflected which allows shortening of the fibre 5 and thus reducing latency of the signal of L3 with respect to L1. Depending on the power of L2, and thus the efficiency of the grating, the fibre 5 may be between tens of metres to several kilometres in length.

(29) Output beam L3 is received by photodiode 9 (optionally being amplified beforehand by a further amplifier such as a EDFA or equivalent) which outputs a corresponding electrical signal. The beat in the output of the photodiode 9 is detected by ADC 10. In some embodiments an amplifier may be positioned to amplify the output from photodiode 9 inputted to ADC 10. The output of the ADC may be fed to a digital EW system for analysis.

(30) It will be appreciated that the detuned modulator 12 may be biased other than at 7E, so long as the fundamental:distortion products ratio of the modulated second beam L2 is less than the modulated first beam L1.

(31) Rather than using a detuned modulator 12, a directly modulated laser may be used at low frequency (less than 10 GHz). This arrangement can naturally produce large distortion products. Where a directly modulated laser is used, it should of the same wavelength as laser 1 and have a line width that is the same or greater than laser 1.

(32) It may be possible to dispense with the amplifier 14 if L2 is already sufficiently powerful.

(33) The paths of the counter propagating beams should be arranged so that the counter propagating beams pass through the fibre 5 at the same time in order to create the grating. This requires making the optical path lengths substantially the same taking into account and delays caused by optical components and/or the laser's 1 pulse length.

(34) FIG. 3 is a schematic of a variant EW RF receiver and optical linkage that utilises the distortion removal apparatus of FIG. 1 and provides photonic mixing. Parts shared with the embodiment of FIG. 2 are given like numbering.

(35) There is shown a laser 1, typically a distributed feedback laser, a first splitter 2, a polarisation controller 3, a single sideband carrier suppressed (SSB(C)) modulator 4, an optical medium 5 (typically a length of optical fibree.g. single mode fibre) in which a stimulated Brillion dynamic grating is formed; an optical combiner 6, a second polarisation control 7, a polarisation combiner 8, a high frequency photodiode 9, analogue digital converter 10 having an output to a digital EW system; a second optical splitter 11, a detuned modulator 12; a photonic upconverter 13 tuned to upconvert an input by an anti-stokes frequency; an optical amplifier (e.g. a EDFA); and a third polarisation controller 15.

(36) The apparatus further comprises a tuned carrier suppressed SSB converter 16 with appropriate local oscillator and a fourth polarisation controller 17.

(37) The apparatus further includes an EW receiver 18 comprising a RF antenna and amplifier that drives the SSB(C) modulator 4 and detuned modulator 12.

(38) The laser 1 generates a coherent light beam, e.g. of a near-IR frequency, that is split by first splitter 2 into a first light beam B1 and a second light beam B2. The second light beams B2 is further split into a third light beam B3 and fourth light beam B4.

(39) The first light beam B1 is modulated by the SSB(C) modulator 4 and the third light beam B3 is modulated by the detuned modulator 12 such that both the first and third light beams B1 B3 carry EW signals fed from receiver 18.

(40) The detuned modulator 12 could be a standard Mach Zehnder modulator (though other modulators could be used) biased at 7E such that the ratio of the amplitude of the fundamental:amplitude of largest distortion products in B3 is lower than the ratio of the amplitude of the fundamental:amplitude of largest distortion products in B1. The amplitude of the distortion products in B3 may still be smaller than the amplitude of the fundamental (though larger than the fundamental would be preferable).

(41) Modulated third beam B3 is up converted by the antistokes frequency and amplified by optical amplifier 14.

(42) The first beam B1 and third beam B3 are transmitted through optical fibre 5 in opposition to one another. The third beam B3 is of sufficient power as to form a Brillouin grating within the optical fibre 5.

(43) Polarisation controllers 3 and 15 are used to control the polarisation of B1 and B3 respectively such that Brillion grating occurs within the fibre 5 at the correct orientation to provide the desired reflection of B1.

(44) As before, because the ratio of the amplitude of the fundamental:distortion products in B3 is lower than the ratio of the amplitude of the fundamental:distortion products in B1, the grating preferentially reflects the distortion products over the fundamental meaning that output beam B5 that has transmitted through grating in fibre 5 has a greater ratio of fundamental:distortion product than B1.

(45) The optical power of B3 is preferably selected to ensure distortion products are suppressed down to below the noise floor.

(46) Fourth beam B4 is frequency converted by tuned carrier suppressed SSB converter 16 by a frequency using a local oscillator tuned to be shifted from a frequency of interest; the size of the shift being within the resolution bandwidth of the ADC 10.

(47) Up converted fourth beam B4, and output beam B5 are combined in polarisation combiner 8 and the output (optionally amplified by a further amplifier) fed to photodiode 9. Combined beams B4 B5 generate a beat in the output of the photodiode 9 that is within the resolution bandwidth of ADC 10. In some embodiments an amplifier may be positioned to amply the output from photodiode for input to ADC. The output of the ADC may be fed to a digital EW system for analysis.

(48) Polarisation controllers 7 and 17 are used to control the polarisation of L5 and L4 into the polarisation combiner 8 to provide efficient combination. The arrangement of which will be familiar to those skilled in the art.

(49) It will be appreciated that the detuned modulator L2 may be biased elsewhere so long as the fundamental:distortion products ratio is less than in L1. Again, a directly modulated laser may be used instead of a detuned modulator 12.

(50) Rather than using SSB converter 16 it may be possible instead to redirect Brillouin scattered light from the stimulated emission in the waveguide, e.g. by using a suitable optical fibre.

(51) Amplifier 14 may be dispensed with if B3 is sufficiently powerful.

(52) The embodiments described above may optionally include a further optical amplifier (not shown) positioned in front of the photodiode to amplify the optical signal.

(53) FIG. 4 is a variant of FIG. 2 in which the output of the lo is fed into an I channel of the modulator 4 and the output of the receiver 18 is fed into a Q channel of the modulator 4. This allows the RF coupler of FIG. 2 to be omitted. For further details of this implementation see Erwin H. W. Chan and Robert A. Minasian, High conversion efficiency microwave photonic mixer based on stimulated Brillouin scattering carrier suppression technique, Opt. Lett. 38, 5292-5295 (2013).

(54) FIG. 5 illustrates application of the invention used to reduce distortion of the output of an electronic mixer.

(55) The optical link has the same configuration as that of FIG. 2, with the difference that the first modulator 4 is arranged to modulate the first light beam to carry a signal using the output from a first electrical mixer 20A arranged to run at high power and low distortion. The second modulator 12 is arranged to modulate the second light beam to carry the signal using the output of a second mixer 20B configured to run at high power and create high distortion. In this way the beam output from modulator 12 will have a lower fundamental to distortion product ratio than the beam outputted from modulator 4 such that the dynamic Brillouin grating formed in fibre 5 will act to preferentially reflect (and thus suppress) distortion products from the output of modulator 4.

(56) The optical link is conceived to have applications other than for connecting an RF receiver to processing system. For example, the optical link may be used to connect a RF transmitter to an antenna. In such an arrangement it may be favourable to up-convert the output from the grating before transmission to the antenna, e.g. using a electronic or photonic mixer. In such an arrangement the ADC could be dispensed with (as it could in any application were only an analogue output is required).

(57) The optical link may also have use in commercial telecommunication applications such as with a RF transmitter and/or RF receiver forming part of a broadband cellular network.