ELECTRICAL PULSE TRANFORMATION USING OPTICAL DELAY LINES

Abstract: A recirculating optical delay line (30) has a laser (32) controlled by wavelength control (33) so as to vary the wavelength of radiation over time to provide a first input (34) to an optical modulator (35). The modulator (35) modulates the intensity of the first input (34) with a pulsed electromagnetic frequency signal (36) to produce a pulsed modulated optical signal (39). The signal (39) passes through an optical coupler (40) into a delay loop (41) having a delay fibre (44) arranged to delay the signal (39) for a predetermined duration. The wavelength control (33) is arranged to vary the first input (35) so as to ensure that overlapping pulses of the signal (39) in the delay loop (41) are at different wavelengths, thereby inhibiting optical mixing effects between the overlapping pulses.

Description: ELECTRICAL PULSE TRANFORMATION USING OPTICAL DELAY LINES The present invention relates to an optical delay line and to a method of generating a delayed optical signal. Optical fibre technology is gradually making an impact on electronic systems through their ability to implement various signal processing functions, for example producing multiple delays and microwave filtering. Optical fibres also offer a low loss, compact solution to the generation of long delays for optical signals. The low dispersion properties of optical fibres effectively provide modulation frequency independent delay lines that minimise degradation of an input radio frequency signal. Currently such delays are introduced using recirculating optical delay lines which offer a number of advantages over alternative serial or parallel type delay line architectures and provide a low cost solution for the generation of a range of delays of a pulsed signal. From Figure 1 , a prior art recirculating optical delay line 10 comprises a fixed wavelength carrier wave optical source 11 providing a first input 12 to an external optical modulator 13 arranged to modulate the light from the optical source 11 with a pulsed radio frequency signal 14 which passes through a radio frequency amplifier 15 to act as a second input 16 to the external optical modulator 13. A modulated optical signal 17 is generated by the external optical modulator 13 which then passes through a two-by-two optical coupler 18 that is arranged to allow 50% of the modulated signal 17 to enter a delay loop 19 and the other 50% of the modulated signal 17 to bypass the delay loop 19 and to proceed to an output of the optional coupler 18. The delay loop 19 comprises an optical amplifier 20 in series with a band pass optical filter 21 and a delay fibre 22. It will be understood that the modulated signal 17 is a series of radio frequency modulated pulses having a pulse length determined by the pulsed radio frequency signal 14. In this delay line 10 the modulated signal 17 enters the delay loop 19 through the two-by- two optical coupler 18 and circulates through the amplifier 20, filter 21 and delay fibre 22 to achieve the desired delay duration of each pulse of the modulated signal 17. It is important to note that the pulse length of each pulse of the modulated signal 17 must be equal to or less than the overall delay duration of the delay loop 19 in order to prevent coherent optical mixing affects between overlapping sections of the same pulse of the modulated optical signal 17. The coupler 18 is also arranged to extract delayed optical pulses 23 from the delay loop 19 after each circulation of a pulse of the modulated signal 17 around the delay loop 19. Each delayed optical pulse 23 is detected by a photodiode 24 which serves to convert each delayed optical pulse 23 into an electrical signal 25 which passes through a radio frequency amplifier 26 so as to produce a delayed pulsed radio frequency output 27. However, prior art recirculating optical delay lines 10 are constrained by the requirement that the pulses of the radio frequency modulated signal 17 must have a shorter duration than the recirculating duration of the pulse around the delay loop 19 in order to prevent coherent optical mixing affects between overlapping sections of the same pulse of the modulated signal 23 when they are detected on the photodiode 24. In some applications the pulse duration of the modulated signal 17 may be unknown or uncontrolled. It is an object of the present invention to obviate or mitigate the problems associated with the prior art, especially to inhibit coherent optical mixing of pulses of light in a delay line. According to a first aspect of the invention an optical delay line, comprising an optical source arranged to generate intensity modulated pulses of light, a delay fibre arranged to carry the pulses of light, an optical coupler arranged to allow the pulses of light to enter and to exit the delay fibre and an optical source controller arranged to vary the wavelength of the pulses of light with respect to time. In this manner, the optical source controller ensures that overlapping pulses of intensity modulated light within the optical delay line are at different optical wavelengths and hence coherent optical mixing effects between the overlapping pulses of light is inhibited. Therefore, input pulses of light to the delay fibre can have a longer duration than the delay fibre delay duration without distortion of the pulses of light due to coherent optical mixing effects between overlapping pulses of light and the optical delay line can be optimised for shorter delay times. The optical source controller may be arranged to vary the wavelength of each pulse of light such that each pulse of light within the delay fibre is segregated by wavelength. In this manner, the optical source controller ensures that the overlapping pulses of light within the delay fibre are at different optical wavelengths. In one embodiment of the invention a delayed pulse combiner may be arranged to construct a delayed electiical signal from the delayed pulses of light after they have exited the delay fibre in accordance with the wavelength of each delayed pulse of light. The delayed pulse combiner may comprise one or more Bragg fibre gratings arranged to segregate the pulses of light after they have exited the delay fibre in accordance with their wavelength. At least one of the Bragg fibre grating may have an associated photodiode arranged to convert impinging pulses of light into an electrical sub-signal. An electrical combiner may be arranged to construct the delayed electrical signal from the electrical sub-signals produced by photodiode associated with each Bragg grating. In this manner, pulses of light of a given wavelength are detected prior to being combined so as to reconstruct the delayed electrical signal. The optical coupler may be a 2 by 2 optical coupler. In another embodiment of the invention a tuneable bandpass optical filter may be arranged to construct a delayed electiical signal from the delayed pulses of light after they have exited the delay fibre in accordance with the wavelength of each delayed pulse of light. The tuneable bandpass optical filter may be arranged to allow pulses of light to exit the delay fibre in accordance with their wavelength. The tuneable bandpass optical filter may be controlled by the optical source controller so as to vary the wavelength of the tuneable bandpass optical filter in time with the variation in the wavelength of the pulses of light. Preferably, the optical coupler may comprise a 2 by 1 optical coupler and an optical circulator. An optical modulator may be arranged to generate the pulses of light by modulating a light source with a pulsed electromagnetic frequency input. For example, the electromagnetic frequency input may be a pulsed radio frequency input, in this manner, a delayed pulsed radio frequency output from the optical delay line is achieved. The optical source may be a distributed feedback semiconductor laser. According to another aspect of the invention a method of generating a delayed electrical signal includes generating modulated pulses of light, passing the pulses of light through a delay fibre, varying the wavelength of the pulses of light with respect to time, converting delayed pulses of light into electrical sub-signals after they have exited the delay fibre and according to the wavelength of each delayed pulse of light, and constructing the delayed electrical signal from the electrical sub-signals. The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates a prior art recirculating optical delay line; Figure 2 illustrates an embodiment of a recirculating optical delay line according to the present invention; Figure 3 illustrates the relationship between an input pulse of a given duration and a recirculating duration of an optical delay line according to the invention; Figure 4 illustrates the relationship between laser wavelength and duration of an optical delay line according to one embodiment of the invention; Figure 5 illustrates the relationship between laser wavelength and duration of an optical delay line according to an alternative embodiment of the invention, and Figure 6 illustrates an alternative embodiment of a recirculating optical delay line according to the present invention. Referring to Figure 2, a recirculating optical delay line 30 comprises a variable wavelength carrier wave optical source 31 comprising a laser 32 having a wavelength control 33 to vary the wavelength of radiation generated by the laser 32 with respect to time so as to provide a first input 34 to an external optical modulator 35. The external optical modulator 35 is arranged to modulate the intensity of the first input 34 from the optical source 31 with a pulsed electromagnetic frequency signal 36, for example a pulsed radio frequency signal, which passes through an amplifier 37 to act as a second input 38 to the external optical modulator 35. A modulated optical signal 39 is generated by the external optical modulator 35 which then passes through a two-by-two optical coupler 40 that is arranged to allow 50% of the modulated signal 39 to enter a delay loop 41 and the other 50% of the modulated signal 17 to bypass the delay loop 41 and proceed to an output of the optical coupler 40. The delay loop 41 comprises an optical amplifier 42 in series with a bandpass optical filter 43 and a delay fibre 44. It will be understood that the modulated signal 39 comprises a series of intensity modulated optical pulses having a pulse length determined by the pulsed signal 36 and an intervening period having a reduced intensity, that is a continuous signal during the interpulse period. The optical coupler 40 is also arranged to extract delayed optical pulses 45 from the delay loop 41 after each circulation of a pulse of the modulated signal 39 has circulated around the delay loop 41. The optical pulses 45 pass to a delayed signal combiner 46 arranged to construct a delayed electrical signal 47 from the delayed optical pulses 45 extracted from the delay loop 41 in accordance with the wavelength of each delayed optical pulse 45. The operation of the delayed signal combiner 46 is described in greater detail below. Each delayed electrical signal 47 is amplified in a signal amplifier 48 so as to produce a delayed pulsed output 49. The delayed signal combiner 46 comprises a number of sub-units 50 arranged in series with one another, each comprising a optical circulator 51 arranged to allow the delayed optical pulses 45 to pass to a Bragg fibre grating 52 arranged to either reflect delayed optical pulses 45 having a wavelength corresponding to that written into the Bragg fibre grating 52 or to allow delayed optical pulses 45 of an alternative wavelength to pass onto the next sub-unit 50. It will be understood that the delayed optical pulses 45 will pass through the optical circulators 51 of the sub-units 50 until they are reflected by a Bragg fibre grating 52 having a corresponding wavelength. A reflected delayed optical pulse 45 will return to the optical circulator 51 associated with the Bragg fibre grating 52 from which it was reflected, where it will then be diverted by the optical circulator 51 down an associated arm 53 to a photodiode 54 which serves to convert the delayed optical signal 45 diverted to that arm 53 into an electrical signal 55 which then passes through a signal amplifier 56 so as to produce a delayed electrical sub-signal 57. An electrical combiner 58 serves to combine the electrical sub-signals from each arm 53 so as to produce a single output, that is electrical signal 47. The delayed signal combiner 46 ensures that each sub-signal 57 of a different wavelength is detected separately before being summed coherently at the electrical combiner 58. Figure 3 indicates the relationship between the pulse length T of the pulsed signal 36, and the delay recirculation time Tj of a modulated signal 39 through the delay loop 41 described with reference to Figure 2. It will be noted that the recirculation optical delay line 30 has been arranged to accommodate pulsed signals 36 whose pulse length is up to four times the. duration provided by the recirculating delay line 41. That is in Figure 3 the pulse length T is over three times the length of the duration Ti provided by the delay line. It will be understood, that more sub-units 52 can be added to the delay compensator 46 if a longer pulsed signal 36 pulse length is required to be accommodated by the delay loop 41. From Figure 3, it will be noted that the input pulse length T does not need to be an integer multiple of T}. This is indicated by the time period of pulse length T 2 , where T 2 <T,. The laser 32 of Figure 2 can be a distributed feedback semi-conductor laser having a line width of between 1 to 4 MHz and its wavelength can be adjusted by controlling a combination of its input bias current (typically 1.1 GHz per mA) and its temperature (typically O.lnm per C° which is approximately equivalent to 12.5GHz per C°). The bias current provides a fast wavelength control response while the temperature control of the laser gives a large tuning range at a slower rate. Figure 4 indicates the relationship between laser wavelength shown along the ordinate 60 and the delay line recirculating time Ti given as time t along the abscissa 61. It is the objective of the variation in laser wavelength, illustrated as graph line 63, to ensure that delayed optical pulses within a delay loop at substantially the same time operate at different wavelengths. The wavelength variation between recirculations can be controlled, as indicated in Figure 4, using a liner control waveform, that is graph line 63. Alternatively, as indicated in Figure 5 in which like references have been used to indicate similar integers to those shown in Figure 4 the wavelength variation can be controlled using a step wavelength control waveform, illustrated as graph line 64. An advantage of the stepped wavelength control waveform is that the laser is not required to sweep back to an initial starting wavelength at some point during the wavelength variation. For example, if the line width of the laser is 4 MHz, the linewidth will broaden to approximately 0.46nm (that is equivalent to 57.4 GHz) at a level of approximately 30dBc. For recirculation delay times Ti of 10 microseconds, this requires a wavelength tuning rate of 0.046nm per microsecond (that is equivalent 5.7GHz per microsecond). It will be understood that different laser wavelength tuning rates will be required for use with alternative recirculating delay times Ti. The 0.046nm per microsecond wavelength tuning rate will require a laser tuning control algorithm which combines both laser temperature and bias current control, as is known from the prior art. Tuneable semi-conductor laser diodes can offer a wider electronically controlled tuning range and may be suitable for application requiring a larger tuning range. In Figure 6, in which like references have been used to indicate similar integers to those shown in Figure 2, the two by one optical coupler 70 allows the modulated signal 39 to enter the delay loop 41 which also includes, in series with the optical amplifier 42, the bandpass filter 43 and the delay fibre 44, an optical circulator 71 which allows delayed optical pulses 45 to pass to an optical tuneable bandpass filter 72 that allows a delayed optical pulse 45 of a given wavelength to pass to a single photodiode 73. The optical tuneable bandpass filter 72 can be controlled by a wavelength control 74 arranged to allow delayed optical pulses 45 of the correct wavelength to pass through the optical tuneable bandpass filter 72 or the wavelength control 74 can be substantially the same as wavelength control 33 to the laser 32. Delayed optical pulses 45 which do not pass through the optical tuneable bandpass filter 72 are reflected and recirculate around the delay loop 41 until such time as the wavelength control 74 allows the optical tuneable bandpass filter 72 to pass delayed optical pulse 45 of that given wavelength. It will be understood that it is the control of the optical tuneable bandpass filter 72 which allows the construction of the delayed electrical signal 47. The tuneable bandpass filter 72 of Figure 6 enables a single recirculated pulse 45 to be selected from multiple recirculated pulses within the delay loop 41 whereas the optical delay line 30, as illustrated in Figure 2, outputs all delayed pulses in the delay loop 41.

Claims: CLAIMS 1. An optical delay line, comprising an optical source arranged to generate intensity modulated pulses of light, a delay fibre arranged to carry the pulses of light, an optical coupler arranged to allow the pulses of light to enter and to exit the delay fibre, and an optical source controller arranged to vary the wavelength of the pulses of light with respect to time. 2. An optical delay line, as in Claim 1, wherein the optical source controller is arranged to vary the wavelength of each pulse of light such that each pulse of light within the delay fibre is segregated by wavelength. 3. An optical delay line, as in Claims 1 or 2, wherein a delayed pulsed combiner is arranged to construct a delayed electrical signal from the delayed pulses of light after they have exited the delay fibre in accordance with the wavelength of each delayed pulse of light. 4. An optical delay line, as in Claim 3, wherein the delayed pulse combiner comprises one or more Bragg fibre gratings arranged to segragate the pulses of light after they have exited the delay fibre in accordance with their wavelength. 5. An optical delay line, as in Claim 4, wherein at least one of the Bragg fibre grating has an associated photodiode arranged to convert impinging pulses of light into an electrical sub-signal. 6. An optical delay line, as in Claim 5, wherein an electiical combiner is arranged to construct the delayed electrical signal from the electrical sub-signals produced by each photodiode associated with each Bragg grating. 7. An optical delay line, as in any preceding claim, wherein the optical coupler is a 2 by 2 optical coupler. 8. An optical delay line, as in Claims 1 or 2, wherein a tuneable bandpass optical filter is arranged to construct a delayed electrical signal from the delayed pulses of light after they have exited the delay fibre in accordance with the wavelength of each delayed pulse of light. 9. An optical delay line, as in Claim 8, wherein the tuneable bandpass optical filter is arranged to allow pulses of light to exit the delay fibre in accordance with their wavelength. 10. An optical delay line, as in Claims 8 or 9, wherein the tuneable bandpass optical filter is controlled by the optical source controller so as to vary the wavelength of the tuneable bandpass optical filter in time with the variation in the wavelength of the pulses of light. 11. An optical delay line, as in any of Claims 8 to 10, wherein the optical coupler comprises a 2 by 1 optical coupler and an optical circulator. 12. An optical delay line, as in any preceding claim, wherein an optical modulator is arranged to generate the pulses of light by modulating a light source with a pulsed electromagnetic frequency input. 13. An optical delay line, as in any preceding claims, wherein the optical source includes a distributed feedback semiconductor laser. 14. An optical delay line substantially as illustrated in and/or described with reference to Figures 2 to 6 of the accompanying drawings. 15. A method of generating a delayed electrical signal, including generating modulated pulses of light, passing the pulses of light through a delay fibre, varying the wavelength of the pulses of light with respect to time, converting the delayed pulses of light into electrical sub-signals after they have exited the delay fibre and according to the wavelength of each delayed pulse of light, and constructing the delayed electrical signal from the electrical sub-signals. 16. A method of generating a delayed optical signal substantially as illustrated in and/or described with reference to Figures 2 to 6 of the accompanying drawings.

Inventors: PESCOD CHRISTOPHER RALPH, PESCOD, CHRISTOPHER, RALPH
Applicants: BAE SYS ELECTRONICS LTD, PESCOD CHRISTOPHER RALPH, BAE SYSTEMS ELECTRONICS LIMITED, PESCOD, CHRISTOPHER, RALPH
Application year: 2001
Main classification: G02B6
IPC codes: G02B6, H01Q3