GB2194111A

GB2194111A - Optical signal receiver circuits

Info

Publication number: GB2194111A
Application number: GB8717080A
Authority: GB
Inventors: Christopher James Dakin; Robert David Gwynn
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1986-07-18
Filing date: 1987-07-20
Publication date: 1988-02-24
Also published as: GB8717080D0

Abstract

In an optical signal receiver circuit utilising a PIN diode receiver device PIN, a high-value resistor R or equivalent device is connected in series with the diode across a bias voltage source so as to reduce the reverse bias actually applied to the diode as the received optical signal level, and thereby the current flow through the diode, increases, thereby reducing its sensitivity and avoiding overloading of the following amplifier. <IMAGE>

Description

SPECIFICATION Optical signal receiver circuits The present invention relates to optical signal receiver circuits.

Electric circuits are known which utilise devices such as PIN diodes for receiving optical signals and converting them into electric signals. In such circuits a problem arises where the level of the received optical signal may vary over a wide range, since if a receiver circuit is designed to be sensitive to very low level optical signals that receiver circuit, or the following circuits, or both, may be overloaded if relatively high level optical signals are received.

According to the present invention in an optical signal receiver circuit in which an optical signal receiver device is connected across a bias voltage source, a high-value resistive impedance means is arranged to be connected in series with said device across said source such that for high levels of received optical signals the bias voltage available to said receiver device is substantially reduced compared with that available with low levels of received optical signals.

Two different forms of optical receiver circuit in accordance with the present invention will now be described by way of example with reference to the accompanying drawing, in which Figs. 1 and 2 show the two forms of circuit diagrammatically.

In the form of circuit shown in Fig. 1 a PIN diode optical signal receiver device is connected in series with a load resistor RL, electric voltage signals developed across RL in response to received optical signals being applied to one input of an amplifier A. An auxiliary amplifier S is arranged in an integrating circuit arrangement effectively to develop across a capacitor C1 a voltage which is of equal magnitude and opposite sense to the mean value set up across the load resistor RL such that the junction between the PIN diode and the load resistor RL is maintained at the earth reference potential applied to the other input of the amplifier A.

In the form of circuit shown in Fig. 2 one terminal of a PIN diode optical receiver device is connected directly to the input of a transimpedance amplifier circuit.

In both forms of circuit the bias voltage source by means of which the required, usually reverse, voltage bias is applied to the PIN diode is connected by way of a high resistive impedance, which is shown as a resistor R but which could be constituted by a field effect transistor or other device. The value of this resistor may for example be 5 Megohms.

Stray capacitance to earth may be augmented by a decoupling capacitor C2.

In the form of circuit shown in Fig. 1 the load resistor RL may also have a value of 5 Megohms, while in the circuit of Fig. 2 the feedback resistor RF may have a value of 470 Kilohms.

When the PIN diode is receiving low-level, but usable, values of optical signal the current flow through the device may be of the order of, or even less than, 1 microamp, and the diode may be subject to a reverse bias of, say, 5 to 7 volts. For high level optical signals on the other hand the current flowing through the PIN diode, given an unrestricted bias source, could be of the order of 100 microamps. Clearly with the high value of series resistance given, however, the rise in current flow for high level optical signals will be restricted and the reverse bias voltage across the PIN diode will be reduced, or in some cases even changed to a forward bias. This situation could occur, for example, if the resistor or other device R were of sufficiently high impednace, inlcuding the case of an open circuit. This change in bias voltage serves to reduce the sensitivity of the device and to restrict the levels of electrical signals pased to the amplifier A and the foilowing circuits, thereby avoiding overload.

The optical signals may for example be conveyed to the PIN diode by way of an optical fibre path forming part of a telecommunication system, and may be of a free space wavelength of, say between 400 and 2600 nanometres.

Claims

1. An optical signal receiver circuit in which an optical signal receiver device is connected across a bias voltage source, a highvalue resistive impedance means is arranged to be connected in series with said device across said source such that for high levels of received optical signals the bias voltage available to said receiver device is substantially reduced compared with that available for low levels of received optical signals.

2. An optical signal receiver circuit comprising an optical signal receiver device, a high-value resistive impedance means, a bias voltage source and means effectively connecting said high-value resistive impedance means in series with said optical signal receiver device across said bias voltage source.

3. An optical signal receiver circuit in accordance with Claim 2 including an amplifier circuit arrangement and means connecting said receiver device for current flow into an input circuit of said amplifier arrangement.

4. An optical receiver circuit substantially as hereinbefore described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.