AN OPTICAL DETECTOR

Abstract: An optical detector for use in detecting digital optical signals has an optical fibre coupler (2) which splits a received optical signal into two signal portions. The coupler (2) has unequal length output fibres (8, 10) so that one signal portion impinges an associated photodetector (12, 14) delayed relative to the other. The photodetectors (12, 14) are connected in series non-opposed. When the delay is equal to the bit period of the optical signal the output of the detector is the differential code (dicode) of the received signal which can be recovered by a high input impedance, i.e. integrating, amplifier. By forming the dicode in the optical domain in this manner large DC and AC dynamic ranges are obtained without linearity constraints on the optical transmission source that apply when the optical signal is transmitted as a dicode.

Description: AN OPTICAL DETECTOR This invention relates to optical detectors. Optical detectors are used in digital optical fibre communications to receive and then convert incident optical signals into corresponding electrical signals which are then amplified and processed by electronic means. High input impedance preamplifiers are commonly used in preference to low impedance or transimpedance configurations when the sensitivity of the receiver is of primary importance because of their lower thermal noise. In high impedance preamplifiers the value of the input bias resistance, used to bias the input transistor, is made arbitrarily high to reduce its thermal noise. There are, however, three disadvantages associated with high impedance amplifiers. Firstly the high input resistance combined with the input capacitance of the transistor severely restricts the bandwidth available to process the incoming digital signal. The receiver acts as a leaky integrator and requires subsequent equalisation to restore bandwidth. Pulse shapes therefore become increasingly difficult to achieve as data rates increase when the equalisation reduces the output signal swing. This can lead to degradation of the signal-to-noise ratio on its output. Secondly the ramping effect caused by the integration of pulses introduces an AC dynamic range limitation when binary sequences are used which depends on the coding scheme used to construct the transmitted data sequence. The dynamic range is most severely restricted when long consecutive runs of one digital state are allowed. Thirdly, the mean photocurrent flows through an input bias resistor introducing a large voltage drop across the resistor which limits the DC dynamic range of the receiver. This is commonly alleviated by the introduction of a control loop which adjusts the voltage at the end of the resistor to compensate for the voltage dropped across it. However, the DC dynamic range is still limited by the power supply limitation of the control circuit. Reductions in the value of this resistor to increase the DC dynamic range decrease the sensitivity of the receiver. One solution to the problem of extending the D.C. dynamic range of optical detectors has been proposed in an article by V. A. Druchevskii et al., "Extension of dynamic range of a photoreceiver based on a photodiode", in "INSTRUMENTS AND EXPERIMENTAL TECHNIQUES", vol. 23, no. 3, part 2, May - June 1980, pp. 758-760. The circuit described in this article increases the dynamic range of a photodiode detector by supplying the bias voltage to the photodiode from the output of an operational amplifier downstream of the photodiode, the bias voltage being varied once the output voltage of the operational amplifier exceeds a preset threshold level. One known solution of the problem of AC dynamic range limitation is to employ a differential code (dicode) scheme to encode the optically transmitted information to prevent ramping in the amplifier output. A means of achieving optical dicoding is described in the United Kingdom patent specification GB2,135,551. It comprises encoding a two-level, input electrical signal at a transmitter representative of digital data as a three level optical signal such that a transition in the input signal from low to high is encoded as a first light intensity level, a transition from high to low as a second intensity level, and an absence of a transition as a third intensity level intermediate the first and second light intensity levels. A method of obtaining the electrical dicode of binary data to be used to modulate an optical source at the transmitter is to introduce a bit time delay and subtract the delayed electrical waveform from the undelayed electrical waveform. Since the three-level code is a differentiating code it may be directly decoded by the integrating behaviour of the high input impedance receiver. The three level code however imposes a linearity constraint on both the optical source and its drive circuitry as the three-level coder has to produce pulses which are symmetrical about the mid-level which itself should remain independent of the input binary state. According to a first aspect of the present invention an optical detector comprises two photodetectors connected in series non-opposed, and an optical splitter for splitting a received optical signal into two portions such that each signal portion impinges on an associated one of the photodetectors, one signal portion being delayed relative to the other. When such an optical detector is used to detect binary encoded optical signals having a data rate such that the difference in optical path lengths between the optical splitter and each photodetector corresponds to one bit period, the combined photocurrent at an output line connected to a point between the photodetectors is the dicode corresponding to the received binary digital data. This photocurrent can be connected to a high impedance preamplifier to perform the integration to decode the dicode. The detector therefore provides the advantages of improved AC dynamic range associated with the known method of transmitting an optical dicode signal but because the dicoding is achieved at the detector the problems of linearity constraints on the transmission optical source and its drive circuitry are obviated. Further, the use of non-opposed photodetectors to achieve the optical dicoding simultaneously provides a large DC dynamic range without recourse to feedback control to adjust the photodetector bias voltage. Ideally the responsivity of the photodetectors will be accurately matched and each well optically coupled to a 50:50 splitter but a mismatch in either of the first two conditions aspect can be compensated by employing a non-50:50 coupler. A mismatch should however be avoided because of a consequent reduction in DC dynamic range and the introduction of low frequence pulse distortion. The optical splitter is preferably an optical fibre coupler as it is relatively cheap and convenient to manufacture and the delaying of one signal portion relative to the other readily achieved by simply making one of the output fibres of the splitter longer than the other. It will be appreciated other forms of splitter, for example 1/2 mirrors or Y-splitters, and other means of achieving a difference in optical path length, for example placing one detector further from the splitter than the other, can be employed to obtain the advantages of the present invention which in no manner depends an the particular means used to achieve such optical splitting and relative delays of the signal portions. The choice of photodetector will depend on the user's requirements. For example, an avalanche photodiode (APD) may be used in preference to a PIN diode if greater sensitivity is required but this introduces greater constraints on the stabilising of the power supply to ensure the gains are accurately controlled and matched. The use of two photodetectors results in a slight reduction in sensitivity compared to previously other known direct detectors but not as much as would be incurred in using a low input impedance amplifier or transimpedance amplifier to avoid AC dynamic range limitations. According to a second aspect of the present invention a method of detecting an optical signal comprises optically splitting the optical signal into two signal portions and causing each portion to impinge on an associated one of two photodetectors connected in series non-opposed one signal portion being delayed relative to the other. Embodiments of the invention and its method of operation will now be further described by way of examples only with reference to the accompanying drawings of which - Figure 1 is diagrammatic representation of an optical detector according to the present invention; Figures 2(a) to 2(d) illustrate a typical received non-return to zero (NRZ) optical binary signal, the two optical signal portions impinging on the photodetectors, and the corresponding dicode electrical output from the embodiment of Figure 1, respectively; Figures 3(a) and 3(b) illustrate the integration of the electrical dicode output of Figure 2(b) and the electrical output from a threshold detector fed with the output of 3(a), respectively; Figures 4 and 5 are diagrammatic representation of further embodiments of the present invention which use a half mirror as an optical splitter; and Figure 6 is a diagrammatic representation of a tunable optical detector according to the present invention. Referring to Figure 1, an optical detector 2 includes a conventional 50:50 optical fibre coupler 4, manufactured for example by fusion tapering a pair of optical fibres together, having an input fibre portion 6 and first and second fibre output portions 8 and 10 respectively. The optical fibre portion 10 is longer than the portion 8. Two matched photodetectors 12 and 14 are connected in series, non-opposed, between electrical connections 16 and 18 across which a supply voltage (not shown) can be applied. An electrical output 20 is connected between the photodetectors 12 and 14. The ends of fibre portions 8 and 10 are located equidistantly from, and optically coupled to, the photodetectors 12 and 14 respectively. A digital optical signal may be coupled into the fibre portion 6 by any convenient technique, for example by splicing the portion 6 to a communications fibre on which the optical signal is propagated. The received optical signal is split into two equal intensity portions which propagate along the respective fibre portions 8 and 10 to impinge on the photodetectors 12 and 14. The photodetectors 12 and 14 provide an electrical signal at the output 20 proportional to the difference in intensities of the two optical signals at the photodetectors. Consider now a digital optical signal having a bit rate such that the delay of the signal portion caused by the longer length of fibre portion 10 relative to the signal portion propagating along fibre portion 8 causes the photodetector 14 to receive a signal delayed by 1 bit relative to the signal portion received by the photodetector 12. Assuming, for the sake of example, the received optical signal is the digital signal as illustrated in Figure 2(a), the signal portions detected by the photodetector 13 and 14 will be as shown in Figures 2(b) and 2(c) respectively. The electrical output current from the output 20 will then be as illustrated in Figure 2(d), that is, the output will the dicode of the optical input signal 2(a). Referring now to Figures 3(a) and 3(b) there is illustrated the recovery of the electrical equivalent to original data signal. In particular, Figure 3(a) • illustrates the output of an integrating, high input amplifier (not shown) for input signal 2(d) with a mid-level of 0 volts. The uniform amplitude signal may be subsequently restored to a binary signal by means of a threshold detector (not shown). Referring now to Figures 4 and 5, there are shown further embodiments of the present invention each using a half-mirror 22 to derive two equal intensity signal portions 24 and 26 from an input optical signal 28, the portion 24 travelling further via the full mirror 28 to reach the corresponding photodetector 12 than the portion 26 and so its detection is delayed relative to it. The input optical signal is, in this embodiment, the output from a communications fibre 30 collimated by a lens 32. Lenses 31 focus the two portions of light from the beam splitter 22 onto the photodetectors 12 and 14. The embodiment of Figure 5 is the same as that of Figure 4 except that the mirror 28 is dispensed with, the photodetector being located to intercept portion 24 directly from the half mirror 22 and further from it than photodetector 14 to obtain the required relative delay in detection of the signal portions 24 and 26. It will be clear that any means for splitting the received optical signal into two portions and causing one to be delayed relative to the other may be employed in the present invention. If the data rate of the optical signal to be detected is known and fixed, an optical detector according to the present invention may have the optical splitter and photodetectors fixed to provide the corresponding delay in signal portions. If the data rate is not known or variable, the photodetector may be made tunable to the data rate by providing means for altering the difference in optical path length between the optical splitter and each photodiode. An example of such a tunable optical detector according to the present invention is shown in Figure 6. This is similar to the apparatus of Figure 1, the same elements being similarly numbered, but with the fibre 10 fixed relative to a collimating lens 32 by a movable support 34. The collimated light from the fibre 10 is focussed on the photodetector 14 by a lens 36. The fibre 10 and lens 32 when moved relative to the photodetector 14 and lens 36 adjusts the optical path length from the coupler 4 to the photodetector 14 thereby introducing a degree of tunability. Clearly alternative means for providing a variable delay may be employed which does not significantly alter the intensity of light impinging on the associated photodetector, for example switched optical delays on a lithium niobate substrate. It may be advantageous to arrange the relative delay between arrival of the signal portions at their associated photodetectors to be greater than 1 bit so that trapezoidally shaped pulses are output on detection of square optical input pulses if this would aid discrimination.

Claims: CLAIMS 1. An optical detector comprising two photodetectors connected in series non-opposed and an optical splitter for splitting a received optical signal into two portions such that each signal portion impinges on an associated one of the photodetectors, one signal portion being delayed relative to the other. 2. An optical detector as claimed in claim 1 in which the optical splitter comprises an optical fibre coupler having two output fibres one being longer than the other. 3. An optical detector as claimed in claim 1 in which the optical splitter comprises an optical beam splitter. 4. An optical detector as claimed in any one of claims 1 to 3 in which the delay of one signal portion relative to the other can be adjusted. 5. An optical detector as claimed in claim 1 in which the distance from one output fibre to the associated photodetector can be adjusted and including collimating means for collimating a signal portion exiting the fibre portion and focusing means for focusing a signal portion collimated by the collimating means onto the associated photodetector. 6. An optical receiver comprising an optical detector as claimed in any one of claims 1 to 5 electrically coupled to a high impedance amplifier. 7. A method of detecting an optical digital signal comprising optically splitting the optical signal into two signal portions and causing each portion to impinge on an associated one of two photodetectors connected in series non-opposed one signal portion being delayed relative to the other. 8. A method as claimed in claim 7 in which the delay is equal to the bit rate of the optical signal.

Inventors: SMYTH PETER PAUL, WHITE BRIAN ROBERT, SMYTH, PETER, PAUL, WHITE, BRIAN, ROBERT
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: H04B10
IPC codes: G01J1, G02B6, H04B10