OPTICAL LINKS

Abstract: An optical link (10) is described for transmitting a microwave signal (30) in modulated optical form over an optical fibre (24). The optical link (10) has a transmitter (12) and a receiver (18). The transmitter (12) splits and converts the microwave signal (30) into two separate optical signals (34 and 36), each of which represent either the positive varying part of the microwave signal (30) or the negative varying part of the microwave signal (30). The receiver (18) combines the two separate optical signals (34 and 36) and converts them back into the microwave signal (30).

Description: 1 OPTICAL LINKS This invention relates to optical links and is more especially, although not exclusively, concerned with high performance optical link configurations including wideband and/or high dynamic range optical links. An optical link comprises an optical transmitter for transmitting a light signal along an optical path and an optical receiver for receiving the light signal. Such links can be used to transmit signals originating in parts of the electromagnetic spectrum having frequencies lower than that of the light signal. They may be used to convert microwave signals in the range from RF to millimetre wave into light signals and then transmit them. Conventionally, wideband optical links are direct detection (DD) systems which use intensity modulated light sources. The performance of such (DD) links is usually limited by thermal noise in the optical receiver when low optical power levels are used and by optically generated noise (such as shot noise and relative intensity noise (RIN)) when high optical power levels are received. In high performance optical links, whether digital or analogue, output wideband noise is largely caused by optically generated noise. An optical carrier from a light source is intensity modulated with the signal which it is to carry. In many cases the modulated optical power is much less than mean optical power from the light source, in which case 2 there is a considerable level of standing optical power contributing to the noise and not to the signal. Alternative methods of reducing the noise contribution from the unmodulated optical signal, such as bias offset or carrier filtering, result in a reduced modulation efficiency and an increased second order distortion making the optical link unsuitable for wideband systems. In the known intensity modulated links, light from the source is incident on photodiodes in the optical receiver even if the optical transmitter is not receiving a modulating signal. In this event, a noise floor will always be present provided by optically generated noise. This restricts the dynamic range (DR) of the link. The operation of high dynamic range, wideband optical links is defined with certain parameters including compression dynamic range (CDR) and spurious free dynamic range (SFDR). CDR is defined as the signal-to-noise ratio (SNR) at the ldB compression point. For every ldB improvement in the noise level the CDR increases by ldB. SFDR is defined as the SNR at the driver power when the largest spurious tone equals the noise floor. This is generally specified for the two-tone third-order intermodulation signal. SFDR increases by 0.66dB for every ldB improvement in the noise level. Many potential applications for wideband optical links require a dynamic range of at 3 least lOdB better than can currently be realised. To achieve this performance it is necessary either to improve linearity of modulation which is applied to the light source or to reduce the noise floor. According to a first aspect of the invention there is provided an optical link comprising: a light source; modulating means for modulating light from the light source in response to an original electrical signal; an optical path for transmitting the modulated optical signal and demodulating means for converting the optical signal into an electrical signal which is representative of the original electrical signal; wherein the original electrical signal has an amplitude which varies from positive values to negative values; characterised in that the modulating means converts the original electrical signal into a first varying optical signal representing a positive varying component of the original electrical signal and a second varying optical signal representing a negative varying component of the original electrical signal and wherein the optical path carries the first and second varying optical signals to the demodulating means which converts them into a combined electrical signal which is representative of the original electrical signal. Preferably the first and second varying optical signals are transmitted as separate channels on a common optical path. Alternatively each is sent on its own individual optical path. Conveniently the or each optical path is an optical fibre. The first and second varying optical signals may be differentiated by having different wavelengths or polarisation. 4 Preferably the original electrical signal is a microwave signal. Preferably the original electrical signal is a communication signal. Alternatively it may be a radar signal. It may be a television signal. The original electrical signal may originate from cable based or antenna based systems used in communication systems or in radar systems. Preferably the modulating means comprises a first sub-modulator to produce the first varying optical signal and a second sub-modulator to produce the second varying optical signal. Preferably the first sub-modulator only produces a varying optical signal in response to positive varying electrical signals and the second sub-modulator only produces a varying optical signal in response to negative varying electrical signals. Preferably the demodulator comprises first and second photoreceivers. Preferably the first photoreceiver converts the first varying optical signal into the positive varying component. Preferably the second photoreceiver converts the second varying optical signal into the negative varying component. Conveniently the second photoreceiver extracts the negative varying component in anti-phase to the first photoreceiver extracting the positive varying component. Preferably the demodulator combines the positive and negative varying components to produce an output which is a reproduction of the original electrical signal. In one embodiment at least one of the sub-modulators comprises an electro-absorption modulator. In another embodiment at least one of the sub-modulators comprises an electro-optic device. It may be an interferometer. It may be a series of cascaded electro- 5 optic devices. According to a second aspect the invention provides a method of transmitting an original electrical signal as an optical signal over an optical link in accordance with the first aspect of the invention. An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows a schematic representation of an optical link; Figure 2 shows a schematic representation of a modulator; and Figure 3 shows operating characteristics of the modulator of Figure 2. Figure 1 shows a schematic representation of an optical link 10 which has an optical modulator 12 for modulating light from a light source 14 with an original varying electrical signal 16 and a demodulator 18 for converting a received optical signal 20 into an electrical signal 22 which is a reproduction of the original varying electrical signal 16. The original varying electrical signal 16 is an external signal which is to be transmitted over the link 10. It may be a communication signal or it may originate from a radar antenna. The link 10 may be being used to send the original varying electrical signal 16 to processing means which are remote from the source of the original varying electrical signal 16 for signal processing. The optical signal 20 is transmitted from the modulator 12 to the demodulator 18 via an optical fibre 24. 6 Referring now to Figure 2, this shows further detail of the modulator 12. The light source 14 comprises first and second light sources 14a and 14b which produce light at different wavelengths λ ! and λ 2 respectively. In one embodiment they are semiconductor lasers operating around 1.55μm. The first light source 14a supplies a first sub- modulator 26 and the second light source 14b supplies a second sub-modulator 28. The sub-modulators are each fed with a modulating signal 30 which is the original electrical varying signal. The voltage of the modulating signal 30 varies in magnitude and has positive and negative components. In this example it is shown as a sine wave although in a practical embodiment a more complicated signal would be present. The sub-modulator 26 is configured such that it modulates light from the light source 14a only when the modulating signal is varying in a range between zero and a positive value. The sub-modulator 28 is configured such that it modulates light from the light source 14b only when the modulating signal is varying in a range between zero and a negative value. Operation of the sub-modulators 26 and 28 is best explained with reference to Figure 3. This is a graph showing the operating characteristics of an electro-absorption modulator device which is an integral part of each of the sub-modulators 26 and 28. The graph is a plot of optical transmission against voltage. At low voltages most or all of the light is transmitted until a threshold voltage is reached at which no light is transmitted. In order to minimise distortion of the transmitted light, particularly if it is modulated with analogue signals, it is preferred that the device is operated in an operating region where 7 there is a linear relationship between transmission and voltage. As has been explained in the foregoing, each sub-modulator 26, 28 modulates light only when either a positive or a negative voltage is applied. Therefore operating region 30 is defined by biasing the modulator with a fixed DC voltage in order to obtain the desired operating characteristics. As an example, the sub-modulator 28 is biased such that its nominal 0V point is at point 32. As a result, it will only transmit light when a negative voltage is applied and will not transmit light when a positive voltage is applied. Since there is a modulated linear transmission/voltage relationship below point 32, the sub-modulator 28 produces a modulated light signal 36 which is a direct analogue of the negative part of the original varying electrical signal. It is straightforward to configure the sub-modulator 26 so that it only produces a response for positive applied voltages. In order to obtain what is effectively the mirror- image of the operating characteristics of Figure 3, the drive signal of the sub-modulator 26 is inverted. That is, the sub-modulators 26 and 28 are driven in anti -phase to each other. Suitable biasing provides a nominal 0V point below which the sub-modulator 26 will not transmit light and above which it will transmit light in proportion to the applied voltage. The level of second order distortion in the reproduced original varying electrical signal can be controlled and thus reduced to a negligible level provided that there is no phase or amplitude imbalance between the first and second varying optical signals when they are detected. 8 The sub-modulators 26 and 28 produce respective modulated optical signals 34 and 36 at different wavelengths λ. and λ 2 . In this example schematic waveforms are shown approximating to those which would be generated if the original varying electrical signal 16 is a sine wave. Obviously, since it is not possible to have "negative" light, both optical signals 34 and 36 have positive values. These signals are multiplexed onto the optical fibre 24 with a coupler 38. In this embodiment, the optical fibre 24 requires no special properties and so can be of a conventional type. The invention is therefore particularly suitable for use in upgrading existing optical links. At the demodulator, a wavelength dependent coupler 40 is used to separate, that is demultiplex, the two signals 34 and 36. The two optical signals are transported to the demodulator 18 which comprises a suitable photoreceiver. In a preferred embodiment the photoreceiver consists of two photodiodes 42 and 44 configured so as to produce a combined output 22 which is identical to the original varying electrical signal 16. As shown in Figure 2 both photodiodes are reverse biased and are connected back to back, that is their cathodes are connected together and the combined output 22 is derived from the common connection. In the embodiment shown in Figure 2 the sub-modulators 26 and 28 use different light sources 14a and 14b which supply light at different wavelengths. However, in another embodiment, a single light source is used to supply light to each of the sub-modulators. The single light source supplies an optical coupler which has output ports connected to 9 each of the sub-modulators. In this embodiment the two signals 34 and 36 are of the same wavelength and so need to be differentiated in some way. This can be done by placing them in different polarisation states such as orthogonal polarisation states. The signals 34 and 36 are then transmitted in a polarising maintaining (PM) fibre which maintains their polarisation states by each having a different propagation velocity. A typical PM fibre will produce a time delay error of less than 0.6ps if it is less than 0.5m long. This is equivalent to a phase error of 4° . It is therefore necessary to compensate for this effect. One technique is to cut the PM fibre in half and splice the halves together with a 90° rotation to swap the polarisation modes in the fibre. Demultiplexing at the demodulator 18 requires a polarisation splitter device such as a polarising beam splitter. Of course, the sub-modulators 26 and 28 can send their respective optical signals down respective optical fibres to avoid the need for multiplexing and demultiplexing. However, a single optical fibre 24 is preferred to reduce the likelihood of there being a path length imbalance. The SNR of such an optical link is greater than alternative approaches because there is substantially no unmodulated optical power arriving at either photodiode. Compared to a typical high performance optical link the invention can improve the SNR by more than lOdB. The mean optical power on either photodiode is relatively small, because it is dependent 10 only upon the magnitude of the modulating signal. This means that the noise power out of the links is not constant, but rather varies with drive power. The limiting noise power at low signal levels is now the thermal noise of the photoreceiver, which means that the dynamic range is increased relative to the known optical links. Optically generated noise (shot noise and RIN) is therefore only apparent at very high drive powers. The invention may provide an optical link in which the CDR can be increased due to reduced optical power being detected by the detector. At present the known direct detection links are limited since the mean optical power they detect is close to the compression limit of each photodiode. In contrast in the optical link of the present invention the mean optical power is reduced and therefore the source power can be increased thereby increasing the CDR by a corresponding amount. The invention provides an optical link configuration which minimises the level of standing optical power in a modulated optical signal and therefore minimises optically generated noise power.

Claims: 11 CLAIMS 1. An optical link comprising: a light source; modulating means for modulating light from the light source in response to an original electrical signal; an optical path for transmitting the modulated optical signal and demodulating means for converting the optical signal into an electrical signal which is representative of the original electrical signal; wherein the original electrical signal has an amplitude which varies from positive values to negative values; characterised in that the modulating means converts the original electrical signal into a first varying optical signal representing a positive varying component of the original electrical signal and a second varying optical signal representing a negative varying component of the original electrical signal and wherein the optical path carries the first and second varying optical signals to the demodulating means which converts them into a combined electrical signal which is representative of the original electrical signal. 2. An optical link according to claim 1 in which the first and second varying optical signals are transmitted as separate channels on a common optical path. 3. An optical link according to claim 1 or claim 2 in which the optical path is an optical fibre. 4. An optical link according to any preceding claim in which the first and second varying optical signals are differentiated by having different wavelengths. 12 5. An optical link according to any preceding claim in which the modulating means comprises a first sub-modulator to produce the first varying optical signal and a second sub-modulator to produce the second varying optical signal. 6. An optical link according to claim 5 in which the first sub-modulator only produces a varying optical signal in response to positive electrical signals and the second sub-modulator only produces a varying optical signal in response to negative electrical signals. 7. An optical link according to claim 5 or claim 6 in which at least one of the sub- modulators comprises an electro-absorption modulator. 8. An optical link according to any preceding claim in which the demodulator comprises first and second photoreceivers. 9. An optical link according to claim 8 in which the first photoreceiver converts the first varying optical signal into the positive varying component and the second photoreceiver coverts the second varying optical signal into the negative varying component. 10. An optical link according to claim 8 or claim 9 in which the second photoreceiver extracts the negative varying component in anti-phase to the first 13 photoreceiver which extracts the positive varying component. 11. An optical link according to Claim 9 or Claim 10 in which the demodulator combines the positive and negative varying components to produce an output which is a reproduction of the original electrical signal. 12. An optical signal according to any preceding claim in which the original electrical signal is a microwave signal. 13. An optical link according to any preceding claims in which the original electrical signal is a radar signal. 14. An optical link according to any of claims 1 to 12 in which the electrical signal is a communication signal. 15. An optical link according to any preceding claim for transmitting the original electrical signal from a signal source to signal processing means in which the signal source and signal processing means are in separate locations. 16. An optical link according to claim 15 in which the signal source is an antenna. 17. An optical link substantially as hereinbefore described or substantially as illustrated with reference to the figures of the accompany drawings. 14 18. A modulator for converting an original electrical signal into an optical signal for transmission over an optical path which converts the original electrical signal into a first varying optical signal representing a positive component of the original electrical signal and a second varying optical component representing a negative component of the original electrical signal. 19. A method of transmitting an original electrical signal as an optical signal over an optical link in accordance with any preceding claim. 20. A method of transmitting an original electrical signal as an optical signal over an optical link substantially as described herein with reference to the figures of the accompanying drawings.

Inventors: CARTER ANDREW CANNON, EDGE COLIN, CARTER, ANDREW, CANNON, EDGE, COLIN
Applicants: CARTER ANDREW CANNON, EDGE COLIN, MARCONI ELECTRONIC SYST LTD, CARTER, ANDREW, CANNON, EDGE, COLIN, MARCONI ELECTRONIC SYSTEMS LIMITED
Application year: 1999
Main classification: H04B10
IPC codes: A61K6, H04B10