Abstract:
An optical homodyne detection system employs the phenomenon of stimulated Brillouin scattering to amplify the carrier component of an incoming coherent modulated optical wave. The composite wave formed of the amplified carrier component and unamplified information component is locked in phase, frequency and polarization with the incoming modulated wave. Using this composite wave, a homodyne detection system is formed to detect the data in the information component.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to the homodyne detection of modulated optical signals. In homodyne detection, the carrier component of an optical wave which has been modulated in response to data is amplified prior to feeding the modulated wave to detection apparatus such as a photodiode. If an independent local oscillator such as a laser is used to provide the amplification power it is necessary to phase lock this to the incoming optical carrier. This requires that the local signal must have the same frequency as the carrier signal. Optical phase locking of two lasers using an optical analogue of the electronic phase lock loop has proved to be possible, but difficult. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, an optical homodyne detection system comprises a source of optical radiation; means for injecting radiation from the source in one direction into an optical waveguide in which, in use, a modulated optical wave having carrier and information components travels in the opposite direction, the characteristics of the radiation from the source and the form of the waveguide being such that scattering occurs whereby the carrier component of the modulated optical carrier wave is amplified; and demodulating means to which the optical wave with the amplified carrier component is fed. 
     In accordance with a second aspect of the present invention, an optical homodyne detection method comprises injecting optical radiation in one direction into an optical waveguide in which a modulated optical wave having carrier and information components travels in the opposite direction, the characteristics of the injected radiation and the form of the waveguide being such that scattering occurs whereby the carrier component of the modulated optical carrier wave is amplified; and demodulating the optical wave with the amplified carrier component. 
     Preferably, the characteristics of the injected radiation and the form of the waveguide are such that stimulated Brillouin scattering occurs. 
     Stimulated Brillouin scattering (SBS) has until now been considered a limitation on the transmission of radiation through waveguides. It is particularly apparent in the transmission of optical wavelengths through dielectric waveguides and occurs in low loss optical fibres if narrow linewidth laser light of above a certain power level threshold is injected. 
     The principle of SBS will now be described in connection with optical radiation. SBS can be described essentially as a coupled three-wave interaction involving the incident light wave (pump), a generated acoustic wave, and the scattered light wave (Stokes). The pump creates a pressure wave in the medium due to electrostriction and the resultant variation in density changes the optical susceptibility. Thus the incident light wave pumps the acoustic wave which scatters it and the scattering creates the Stokes wave. 
     The three waves obey the energy conservation law which relates the three frequencies by: 
     
         f.sub.A =f.sub.L -f.sub.S 
    
     where the subscripts L, S, A refer to the laser (pump), Stokes and acoustic frequencies respectively. Maximum power transfer occurs when the wave-vector mismatch is zero: 
     
         k.sub.A =k.sub.L -k.sub.S 
    
     There are two important consequencies of these two equations. Firstly, the Stokes wave experiences maximum gain when the pump and Stokes wave vectors are parallel and counter-directional. Thus in a monomode fibre SBS generates a backward-travelling Stokes wave. Secondly, the Stokes wave is shifted to a lower frequency with respect to the pump by an amount equal to the acoustic frequency. 
     In homodyne detection a local oscillator must be locked in frequency, phase and polarisation to the carrier component of the incoming signal. This is achieved very simply with the invention since the SBS process results in a narrow linewidth wave travelling in the opposite direction to the injected light i.e. in the same direction as the incoming optical wave and with the same polarisation. Effectively, therefore, providing the injected radiation has the correct frequency, the carrier component of the incoming optical wave will be amplified. 
     Furthermore, accurate tuning of the local oscillator is not required. Typically this only needs to be tuned to an accuracy of the order of 1 MHz, the SBS process generating the correct returning wave. 
     As has been explained above, the scattered wave will have a frequency offset from the injected wave and this offset is independent of ambient conditions and depends essentially on the medium defining the optical waveguide and in particular its refractive index, the acoustic velocity in the medium and the wavelength of the injected radiation. In the case of fused silica in which the acoustic velocity is 5960 m/s, refractive index=1.44 and the wavelength of the injected light is 1.55 μm, the frequency offset will be 11.1 GHz. 
     The demodulation means can comprise a direct detection optical receiver. 
     The detection system may be tuned by controlling the frequency of radiation from the local source with offset control means being provided to offset the radiation frequency by the required amount, for example 11 GHz. A suitable offset control means would be an AFC circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE shows one embodiment of an optical homodyne detection system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The source of optical radiation could be an external cavity diode laser or possibly a distributed feedback (DFB) laser. 
     The means for injecting radiation may comprise a semi-silvered mirror but preferably comprises a directional optical coupler whereby radiation from the source may pass along a first path to the coupler for injection into the optical waveguide, radiation in the optical waveguide being prevented by the coupler from passing in an opposite direction along the first path. 
     Although the invention is primarily concerned with the detection of ASK signals it may also be possible to detect PSK signals if it can be arranged that a small pilot carrier is provided at the transmitter. Preferably, this is in phase with the incoming signal but if not will simply result in a residual pilot carrier in quadrature with the signal sidebands; the amplified signal in this case can be corrected by detuning the pump laser away from the centre of SBS resonance. 
     We believe that a homodyne detection system based on the invention could be less demanding to develop than a heterodyne receiver and yet offer the higher performance of homodyne detection both in terms of receiver sensitivity and Gbit capability. 
     In this specification, the term optical is intended to refer to that part of the electro-magnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet region at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres. Typically the radiation has a wavelength in the range of 0.5 μm-10 μm. 
     An example of part of a communications network including an optical homodyne detection system according to the invention will now be described with reference to the accompanying drawing which is a schematic block diagram. 
     The drawing illustrates a single transmitter and receiver but it should be understood that these may form part of a much larger network and are not necessarily directly connected together. A transmitting station 1 comprises a transmission laser 2 for generating a narrow linewidth optical signal which is fed to an optical modulator 3. The optical modulator is controlled via a data input 4 to amplitude modulate the incoming signal. The resultant modulated optical wave having a carrier component with a frequency f L  is fed along a monomode optical fibre 5 (possibly via switching circuits not shown) to a receiving station 6. 
     The receiving station 6 comprises a local oscillator pump laser 7 which generates a narrow linewidth optical signal, having a frequency offset by a relatively small amount f A  from the carrier component frequency f L , which is fed to a directional coupler 8. The directional coupler 8 is also connected to the optical fibre 5 so that the optical wave from the laser 7 is coupled with the optical fibre 5 in a direction opposite to that of the optical wave from the transmitting station 1. The directional coupler 8 is such that a wave travelling along the optical fibre 5 to the receiving station 6 is not coupled with an optical fibre 9 connecting the coupler with the laser 7. 
     The frequency and power of the optical wave injected into the optical fibre 5 from the laser 7 is chosen so that stimulated Brillouin scattering takes place in the optical fibre 5. Power levels of the order of 500 μW-several mW are possible. In particular, significant Brillouin gain can, for example, be achieved at power levels of the order of milliwatts for fibre length of &gt;10 km in the wavelength region of 1.5  micrometers. In view of the choice of frequency for the injected wave, the scattered wave will have the same frequency as the carrier component of the incoming wave. Essentially, this means that the carrier component is amplified. It should also be noted that the scattered wave will automatically lock in phase, frequency and polarisation with the carrier component. 
     The incoming wave with the amplified carrier component is then fed to a direct detection element 10 such as a photodiode which provides an electrical output corresponding to the original data. 
     In some cases the optical fibre 5 may be too short to enable SBS to occur. In that event additional optical fibre may be inserted. 
     In a practical experiment, a HeNe laser operating at a wavelength of 1.52 μm was employed as transmitter laser 1. At the receiver the optical output of a Burleigh KCl:Tl colour laser 7 was coupled back into 30 km of optical fibre 5 via a fused fibre coupler 8. Using a dither control technique, the laser 7 was locked to a frequency 11 GHz greater than the carrier transmitted from the laser 1. The composite signal, containing the amplified carrier and modulation sidebands was then detected on a transimpedance PIN-FET receiver 10. 
     Practical measurements have shown amplification of the carrier by up to 40 dB. The principle of homodyne detection was then demonstrated by modulating the transmitter with a 125 MHz sine wave and measuring the signal photo current from the receiver with and without the pump. With approximately 5 mW of pump power the detected 125 MHz signal was increased by 25 dB. 
     It should be noted that detuning the pump laser will reduce the available gain and, more significantly, produce a phase shift in the amplified carrier; it is calculated that a frequency change of 370 kHz will result in a phase shift of 0.1 rads. Consequently, frequency fluctuations of the pump are converted into carrier phase noise which in turn will degrade system performance.