OPTICAL COMMUNICATION SYSTEM

Abstract: An optical distribution system is used for a signal to be transmitted at radio frequencies, for example to mobile telephone users. High frequency (LO) and Intermediate Frequence (IF) signals are modulated onto optical supercarriers. The optical signals are distributed via an optical network to one or more remote sites. An integral mixer/detector at the remote site both mixes the signals and converts them to the electrical domain, thereby generating the RF electrical signal required for transmission. In a preferred embodiment, the mixer/detector is a Heterojunction Bipolar Transistor.

Description: OPTICAL COMMUNICATION SYSTEM The present invention relates to an optical communications system, and in particular to a system suitable for distributing signals to be transmitted at radio frequencies to mobile or cordless users. It has been proposed to use millimetre-wave radio as a short-range "final- drop" access medium for future telecom services requiring both medium to high data capacity and the use of mobile or cordless terminals by the end-user. Such a system might be used to extend services such as video telephony, or other multimedia services beyond the fixed network to mobile users. Current proposals suggest the use of radio frequencies above 30 GHz, and particularly in the region of 60 GHz in a cellular network comprising small cells having a diameter of, e.g. , 1 00 to 200 metres. The small size of the cell implies a need for a large number of cell transmitter sites and accordingly it is important that the cost of the cell transmitter sites, and of the associated signal distribution systems, are kept as low as possible. Fibre-fed systems in which signals at the carrier frequency are distributed optically from a central location potentially offer significant capital cost savings in terms of infrastructure, as well as simplified management and control. There are many different ways of configuring the optical link in these systems, ranging from conventional approaches using external optical modulators and PIN photodiodes, to more exotic approaches, for example using harmonic techniques for optical carrier generation, [ 1 ] . Conventionally, data modulation of the carrier has been carried out at the source located in the central station, prior to the transmission of the signal over the optical link to a remote site in a given cell. There has also been a proposal for the use of a system in which the carrier and the modulated IF signal are transmitted separately across the optical link and mixed at the remote site, after the conversion of one or other of the signals to the electrical domain. This approach however, has resulted in a significant increase in the cost and complexity of the remote site, and so has not found commercial acceptance. According to a first aspect of the present invention, there is provided an optical communications system for the distribution of a signal comprising: a central station including an optical source and arranged to output carrier frequency and intermediate frequency signals modulated on respective optical carriers; an optical network ; at least one remote station connected to the central station by the optical network, the at least one remote station including an integral mixer/detector arranged to receive at an input the carrier frequency and intermediate frequency signals in the optical domain and to output an electrical RF signal produced by the mixing of the carrier and intermediate frequency signals. The present inventor found that a great saving in the cost and complexity of the remote site can be achieved by using an integral mixer/detector which receives signals in the optical domain, and outputs in the electrical domain an RF signal. The signals received in the optical domain typically comprise RF signals which have been modulated onto optical supercarriers. At the same time, the approach adopted in the present invention is found to minimise problems with dispersion which, in prior art systems, have led to signal fading and other forms of signal degradation. The term "optical network" as used herein encompasses a simple point-to-point link, as well as more complex topologies Preferably the optical mixer/detector is a phototransistor, and more preferably a heterojunction bipolar phototransistor (HBT). The use of a phototransistor is preferred as being able to provide a significant level of gain, together with the functions of mixing the signals and converting to the electrical domain. The use of an HBT is found to give a particularly good performance and can be constructed to facilitate optical access to the device. The present invention is not however limited to the use of phototransistors, and other devices may be used. For example, an avalanche photodiode (APD) may be used as the mixer/detector. Preferably the network comprises a passive optical network (PON) linking a plurality of remote stations to the central station. Where a PON and optical splitting are used to take signals from a single central station to a number of remote stations then the PON will in general use erbium-doped fibre amplifiers (EDFA) . However, since these have a fixed wavelength of operation, at around 1 550nm, the transmission wavelength cannot then be freely chosen to minimise dispersion in the fibre. Accordingly, it is particularly advantageous in this context to be able to transmit the carrier and IF signal separately and thereby substantially eliminate problems due to dispersion. Preferably the optical source for the carrier signal comprises a dual-mode semiconductor laser. The use of a signal from a dual-mode semiconductor laser, as described in the paper by C R Lima, D Wake and P A Davies, Electronics Letters, 2nd March 1 995, Vol 31 No. 5pp364-365, further optimises the dispersion performance of the system. According to a second aspect of the present invention there is provided a remote station for use in an optical communications system comprising a) an optical input for connection to an optical network, b) an electrical output for outputting an RF signal, and c) an integral mixer/detector arranged to receive from the optical input carrier frequency and intermediate frequency signals in the optical domain and to supply to the electrical output an electrical RF signal produced by the mixing of the carrier and intermediate frequency signals. According to a third aspect of the present invention, there is provided a method of operating an optical communications system comprising a central station including an optical source, at least one remote station, and an optical network connecting the central station the or each remote station, the method comprising- outputting from the central station onto the network a carrier frequency signal and intermediate frequency signal both modulated onto respective optical carriers; and at the remote station feeding both the said carrier and intermediate frequency signals in the optical domain to an integral mixer/detector and outputting from the integral mixer/detector an RF electrical signal corresponding to the mixing product of the intermediate and carrier frequency signals. Systems embodying the present invention will now be described in further detail, by way of example only, and contrasted with the prior art, with reference to the accompanying drawings in which:- Figures 1 A and 1 B are schematics of a prior art system and a system embodying the present invention respectively; Figure 2 is a plot of frequency of response curves for a photo HBT and a photodiode; Figures 3A and 3B are charts showing signal and noise levels for a prior art system and for a system embodying the present invention; Figure 4 is a diagram showing a system embodying the present invention and incorporating a multiple access PON; Figure 5 is a schematic cross section of an HBT for use in systems embodying the present invention; Figures 6A and 6B are graphs illustrating the relative performance of a photoHBT and a photodiode; and Figure 7 shows a dual mode semiconductor laser. As shown in figure 4, an optical communication system comprises a central station 1 linked by a fibre network 2 to a number of remote sites 3. Each remote site 3 includes an antenna 31 and transmits RF signals to mobile terminals 4 within a respective cell. In the present example, the RF signals are transmitted in the 62-63 GHz or 65-66 GHz frequency bands. The mobile terminals may include, for example, audio/video transceivers and input/output devices for use with video telephony or interactive multimedia services (IMS) . The central station includes a carrier source 1 1 and a modulated IF source 1 2, the IF source being modulated with the data which is to be transmitted by the remote station to the users. For the carrier frequency source, a dual-mode semiconductor laser is used (Figure 7) which includes a continuous Bragg grating 73. As described in the above-cited paper by Lima, Wake and Davies, this is a specially modified DFB device in which oscillation occurs simultaneously on both sides of the Bragg frequency. The 2mm-long laser operates in the region of 1 560nm and is divided into four sections of lengths 85,610,610 and 730 μm respectively. It has four electrical contacts 70a-d. The sections are independently biased at 20- 1 10mA and modulation is applied via the shortest section in order to provide the highest current density for a given level of drive power. The optical output is taken from the laser via a lens ended fibre. The facets 71 , 72of the laser have anti-reflection coatings. As shown in figure 1 B, the remote station in this example uses a photo HBT as a combined mixer/detector. The HBT is shown in figure 5 and is a two terminal, edge-coupled InGaAs/lnP device developed at BT Labs and designed with low parasitics and efficient optical access (2). The HBT has a dc responsivity of around 200 A/W. The device and its fabrication are described in further detail in (2) . Alternative mixer/detector devices include APDs, and FET phototransistors. For example, the mixer/detector may be a Germanium or InGaAs APH. The technique of the present invention was verified experimentally, and compared to a more conventional approach using two photodiodes and an electrical mixer. Fig. 1 shows these configurations; A is the prior art two photodiode plus mixer approach, and B is the single phototransistor alternative. The remote end of configuration B embodying the present invention is much simpler than for A, requiring no mixer or WDM coupler, and only requiring one photodetector and one amplifier. In each configuration, the carrier (LO) and data (IF) modulate separate lasers, at wavelengths of, e.g., 1 552 and 1 554 nm, whose outputs are combined optically using a WDM coupler. In this example using conventional, rather than dual-mode, laser sources, the output from each laser has a spectrum with three main peaks. In the case of the LO laser, the central peak is at the free-running frequency of the laser, and is accompanied by sidebands separated from the central peak by the modulation frequency. In this example the modulation is applied as a sine wave at 4GHz. Similarly, the output from the IF laser has a central peak and sidebands at a separation of, e.g., 1 40MHz. In configuration A, the signals are separated at the remote site using another WDM coupler, and detected using photodiodes. Low noise amplifiers (LNA) are used to bring the signals to a power level required by the electrical mixer. In configuration B, the photoHBT is used to detect both signals, and the internal nonlineaπties are used to provide the mixing function in the electrical domain. The six peaks in the optical spectra of the received optical signals first mix in the HBT in the optical domain. The difference products of this optical mixing comprise two signals in the electrical domain at the LO frequency of 4GHz and two signals in the electrical domain at the IF frequency of 1 40MHz. These pairs add constructively, provided SUBSTITUTE SHEET (RULE 28) that the dispersion-related phase distortion is low. The other mixing products fall outside the bandwidth of the HBT. Following this optical mixing, the resulting electrical signals undergo a second mixing process inside the HBT to produce further sum and difference frequencies - f surn = f L0 + f, F , f dlff = f L0 . f, F - as a result of the phototransistor's electrical non-linearities. The phototransistor also amplifiers the electrical RF signals produced by the electrical mixing. A low noise amplifier is used to further amplify the signal to the same level as in configuration A. To compare these approaches, a link similar to configuration B was constructed, and measurements of signal and noise levels were performed using a spectrum analyser with a low noise preamplifier. An optical attenuator was used in place of a long span of optical fibre to reproduce the effects of losses due to fibre attenuation and potential splitting losses. A photodiode was also used in place of the photoHBT for measurements relevant to configuration A. The photoHBT used was developed at BT Labs, and has a dc responsivity of 0.75A/W and a 3dB bandwidth of around 20GHz (3) . The frequency response curves of these devices are shown in fig. 2. The vertical scale in this figure shows the response in dB (electrical) referred to a responsivity of 1 A/W. The photoHBT response has a gain (compared to the photodiode) of around 45dB at 1 40MHz, and 25dB at 4GHz. These frequencies were used in this experiment (IF of 1 40MHz and LO of 4GHz) in order to demonstrate the concept. This technique, however, is equally applicable and particularly relevant for use at mm-wave frequencies. Substitution of a dual mode laser in place of a conventional semiconductor laser in the configuration described above, has the result that the spectrum of the LO laser has just two peaks, and so produces just a single peak when self-mixed in the optical domain in the HBT. This eliminates the sensitivity of the system to phase distortion. Fig. 3 shows the signal and noise levels (in 1 Hz bandwidth) obtained for each configuration as the signal passes through from IF at the source to RF at the remote site. These levels were obtained from measurement of the experimental optical link and adjusted downstream for typical amplifier and mixer performance. The noise level at the photodiode for configuration A was too low to measure, and was assumed to be negligible compared to the noise generated by the LNA (since the photocurrent was only 7.5μA) . The noise at the source was dominated by phase noise, and the level at 1 MHz offset from the carrier was used (-1 30dBm). An RF signal level of OdBm was chosen as the final output power. At each position in the link, the signal to noise ratio is also shown in the figure. For the photodiode configuration (A), the signal level drops by 73dB across the optical link, and 83dB of amplification must be used to provide sufficient output power if a mixer gain of - 10dB is assumed. A S/N ratio of 92dB is obtained; most of the degradation being due to the optical link. For the photoHBT configuration (B), the signal level drops by only 28dB due to the high internal gain of the device. However, this gain is also accompanied by higher noise (-1 35dBm) which limits the S/N ratio to only 1 3dB more than in the case of the photodiode. After the internal mixing process the S/N ratio becomes almost identical that of the photodiode configuration, but only 65dB of amplification is required (compared to 83dB for configuration A) . The table below lists the output powers obtained for the IF signal, the carrier signal (LO) and the RF signal at different bias currents and voltages. In the table, Iph is the photocurrent. These results are further illustrated in the graphs of Figures 6A and 6B. Although for experimental convenience, a relatively low radio frequency of 4GHz is used, in commercial implementations the higher frequencies of around 60GHz referred to above would be used. REFERENCES 1 . D.Wake, I .C. Smith, N.G .Walker, I .D.Henπing, and R. D. Carver, 'Video transmission over a 40GHz radio-fibre link', Electron. Lett. , vol.28, pp.2024-2025, 1.992. 2. D.Wake, D.J.Newson, M.J. Harlow, and I.D. Henning, 'Optically-biased edge-coupled InP/lnGaAs heterojunction phototransistors' , Electron. Lett. , vol.29, pp.221 7-221 9, 1 993. 3. D.Wake, R.H. Walling, I.D.Henning, and D.G.Parker, 'Planar junction, top- illuminated GaAs/lnP pin photodiode with bandwidth of 25GHz' , Electron. Lett., vol.25, pp.967-968, 1 989. TABLE - PERFORMANCE DATA HPT MIXING EXPTS 5 Set-up: LO LASER ATTN OUT Analyser IF LASER LO Laser S/N 00811 H =90mA l2 = 22mA f = 4GHz P=14dBm IF Laser S/N 00637 l1 =80mA l2 = 22mA f = 140MHz P=15dBm LO Osc.83620A IF Osc. 8341 B Sp. Analyser HP71000 OUT HPT *33732 (AR10992) HSP 1628/2 Device Attn.dB Iph.mA Bias.V P.dBm IF LO RF 0.14 4 4.14 HSP 0 0.52 -5 -36.3 -26.4 -81.3 15 0.016 -5 -67.2 -55.8 -111.3 18 0.008 -5 -73.2 -61.8 -117.3 HPT 18 0.67 -0.25 -34.5 -52.5 -75 1 -0.5 -30.2 -46.3 -68.8 1.3 -0.75 -28.5 -43.4 -66.5 1.6 -1 -27.9 -42.3 -64.9

Claims: CLAIMS 1 . An optical communications system for the distribution of a signal comprising: a central station including an optical source and arranged to output carrier frequency and intermediate frequency signals modulated on respective optical carriers; an optical network ; at least one remote station connected to the central station by the optical network, the at least one remote station including an integral mixer/detector arranged to receive at an input the carrier frequency and intermediate frequency signals in the optical domain and to output an electrical RF signal produced by the mixing of the carrier and intermediate frequency signals. 2. A system according to claim 1 , in which the integral mixer/detector is a phototransistor. 3. A system according to claim 2, in which the integral mixer/detector is a heterojunction bipolar phototransistor (HBT) . 4. A system according to any one of the preceding claims, in which the optical network comprises a passive optical network (PON) which links a plurality of remote stations to the central station. 5. A system according to any one of the preceding claims, in which an optical source for the carrier signal comprises a dual-mode semiconductor laser. 6. A remote station for use in an optical communications system comprising: a) an optical input for connection to an optical network; b) an electrical output for outputting an RF signal; and c) an integral mixer/detector arranged to receive from the optical input carrier frequency and intermediate frequency signals in the optical domain and to supply to the electrical output an electrical RF signal produced by the mixing of the carrier and intermediate frequency signals. 7. A remote station according to claim 6, in which the integral mixer/detector is a phototransistor. 8. A remote station according to claim 7, in which the integral mixer/detector is a phototransistor. 9. A method of operating an optical communications system comprising a central station including an optical source, at least one remote station, and an optical network connecting the central station the or each remote station, the method comprising: outputting from the central station onto the network a carrier frequency signal and intermediate frequency signal both modulated onto respective optical carriers; and at the remote station feeding both the said carrier and intermediate frequency signals in the optical domain to an integral mixer/detector and outputting from the integral mixer/detector an RF electrical signal corresponding to the mixing product of the intermediate and carrier frequency signals 1 0. A method according to claim 9, in which in step b) the signals in the optical domain are fed to the optical input of a phototransistor 1 1 . A method according to claim 1 0, in which the phototransistor is a heterojunction bipolar transistor (HBT) . 1 2. An optical communications system for the distribution of a signal comprising: a central station including an optical source and arranged to output carrier frequency and intermediate frequency signals modulated on respective optical carriers; an optical network ; a multiplicity of remote stations connected to the central station by the optical network, each remote station including an integral mixer/detector comprising a phototransistor arranged to receive at an input the carrier frequency and intermediate frequency signals in the optical domain and to output an electrical RF signal produced by the mixing of the carrier and intermediate frequency signals. 1 3. A cellular mobile radio system including an optical communications system according to any one of claims 1 to 5, or according to claim 1 2.

Inventors: WAKE DAVID, WAKE, DAVID
Applicants: BRITISH TELECOMM, WAKE DAVID, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY, WAKE, DAVID
Application year: 1996
Main classification: H04B10
IPC codes: H04B10