GB2030020A

GB2030020A - Optical receiver

Info

Publication number: GB2030020A
Application number: GB7836871A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1978-09-14
Filing date: 1978-09-14
Publication date: 1980-03-26
Also published as: AU5069679A; AU535779B2; GB2030020B

Abstract

A receiver for responding to a light beam, especially from an optical fibre light guide, includes a photo-diode which responds to the light of the beam, and which is connected to the input of an amplifier. The output current of the amplifier flows through a light-emitting diode and a resistor in series, with the output taken across the resistor. The light emitted by the light-emitting diode is received by a second photo-diode connected to the amplifier's input. Thus the light-emitting diode - photo-diode combination forms an optical isolator in the amplifier's feedback loop. This relatively simple circuit avoids difficulties inherent in known devices where, for instance resistors and capacitors may be in the feedback loop. <IMAGE>

Description

SPECIFICATION Optical receiver This invention relates to an optical receiver, for receiving light either in the form of a light beam in free space or light arriving along an optical fibre light guide. In either case the light beam may be modulated.

Existing optical receivers normally use one of two design approaches, either the so-called transimpedance design or the so-called high impedance design. Both of these, which are schematically rep resented in Figs. 1 and 2 respectively, of the accompanying drawing, use a high input impedance amplifier.

In the circuit of Fig. 1, which uses the transimpedance design a photo-diode PD1 is connected to one input of the amplifier, which has a feedback network RfCF. To achieve best sensitivity, the feedback resistor RF must be made as large as possible, but its maximum value is limited by feedback capacitance.

The resistor RF also contributes thermal noise, which tends to dominate amplifier noise at low frequencies and to limit the sensitivity. CF iS stray capacitance.

We now consider Fig. 2, which relates to the high impedance design, in which the photo-diode PD2 is connected to one input of the amplifier, which input is connected to a reference potential via a resistor R1. Here the load resistor R1 is made as large as possible for maximum sensitivity, and the photodiode and amplifier capacitances are allowed to integrate the signal. A post-equalisation circuit PEC connected to the output of the amplifier is used to restore the bandwidth of the receiver. However, because of the integration and equalisation processes, the dynamic range of the receiver is restricted.

An object of the invention is to provide an amplifier circuit which overcomes the above disadvantages, which amplifier circuit, though specifically intended for use in the optical context, has other applications.

Accordingly, the invention provides an electrical amplifier circuit, in which current feedback from the output of the circuit to its input is provided via a feedback loop which includes an optical isolator.

We have mentioned above that the specific application of such a circuit is for use in optical systems: hence the present invention further provides an opto-electrical receiver circuit, which includes an opto-electronic transducer such as a photo-diode or a photo-transistor on which the light to be received falls, an amplifier to an input of which the transducer is connected so that the amplifier receives an input current proportional to the light, and a feedback loop between the output of the amplifier and its input, said feedback loop including an optical isolator formed by a light-emitting diode via which there flows the output current of the amplifier or a current proportional thereto, and a further opto-electronic transducer such as a photo-diode or a phototransistor, the current flowing in said transducer being applied to the input of the amplifier.

An example of a circuit which embodies the present invention, and which is for use in an optical system will now be described with reference to Fig. 3 of the accompanying drawing.

In Fig. 3, the photo-diode PD3 which responds to the light to be received in connection to one input of an amplifier A, e.g. to the positive input of an integrated circuit amplifier. In the present case it is assumed that the light to be received has reached the receiver via an optical fibre light guide. However, the receiver can also be used where the light is received in the form of a light beam in free space.

The output of the amplifier is connected to reference via a light-emitting LED and a resistor R2, the output being taken from across the resistor R2. The light emitting-diode's emission is "aimed at" another photo-diode PD4. Thus we have a feedback loop which includes an optical isolator formed by the two diodes LED and PD4. Note that in some case the photo-diodes may be replaced by photo-transistors.

Thus the LED-PD4 combinations perform a currentto-current conversion, with the ratio of the current through the LED to the current generated by the photo-diode (or photo-transistor) determined by the optical attenuation between them. Note that instead of exciting the diode LED by the entire output current of the amplifierA, it would be possible to excite it by a fixed proportion of that current.

In the feedback loop of the receiver, which includes the optical isolator LED-PE4, the feedback photo-diode PE4 produces a current equal to the signal current produced by the signal receiving photo-diode PD3.

The output of the receiver can be obtained by monitoring the current through the LED, or the optical output of the LED can be used as the receiver's output. In the latter case the circuit would act in effect as an optical amplifier.

In such an arrangement, the use of optical feedback means that the problem of feedback capacitance can be virtually eliminated. The optical feedback system need not work at the same optical wavelength as the receiving photo-diode PD3.

The receiver sensitivity is limited only by the amplifier noise, and is therefore better than in either of the two known arrangements described above. The dynamic range of the receiver is comparable with that of a "transimpedance" type circuit, and is superior to that of a "high-impedance" type. By automatically reducing the optical attenuation in the feedback loop, a much greater dynamic range may be attained. The improved sensitivity attainable by the use of such a circuit is of especial importance at long wavelengths were suitable avalanche photodioes may not be available.

Furthermore, at high frequencies the transimpedance design can also be limited by amplifier noise.

The optical feedback design is likely to have better sensitivity at low frequencies. (At low frequencies the transimpedance design is limited by thermal noise in the feedback resistor, the transition to being limited by amplifier noise occurs at a fairly indeterminate frequency depending on photodiode and stray capacitances.

1. An electrical amplifier circuit, in which current feedback from the output of the circuit to its input is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Optical receiver This invention relates to an optical receiver, for receiving light either in the form of a light beam in free space or light arriving along an optical fibre light guide. In either case the light beam may be modulated. Existing optical receivers normally use one of two design approaches, either the so-called transimpedance design or the so-called high impedance design. Both of these, which are schematically rep resented in Figs. 1 and 2 respectively, of the accompanying drawing, use a high input impedance amplifier. In the circuit of Fig. 1, which uses the transimpedance design a photo-diode PD1 is connected to one input of the amplifier, which has a feedback network RfCF. To achieve best sensitivity, the feedback resistor RF must be made as large as possible, but its maximum value is limited by feedback capacitance. The resistor RF also contributes thermal noise, which tends to dominate amplifier noise at low frequencies and to limit the sensitivity. CF iS stray capacitance. We now consider Fig. 2, which relates to the high impedance design, in which the photo-diode PD2 is connected to one input of the amplifier, which input is connected to a reference potential via a resistor R1. Here the load resistor R1 is made as large as possible for maximum sensitivity, and the photodiode and amplifier capacitances are allowed to integrate the signal. A post-equalisation circuit PEC connected to the output of the amplifier is used to restore the bandwidth of the receiver. However, because of the integration and equalisation processes, the dynamic range of the receiver is restricted. An object of the invention is to provide an amplifier circuit which overcomes the above disadvantages, which amplifier circuit, though specifically intended for use in the optical context, has other applications. Accordingly, the invention provides an electrical amplifier circuit, in which current feedback from the output of the circuit to its input is provided via a feedback loop which includes an optical isolator. We have mentioned above that the specific application of such a circuit is for use in optical systems: hence the present invention further provides an opto-electrical receiver circuit, which includes an opto-electronic transducer such as a photo-diode or a photo-transistor on which the light to be received falls, an amplifier to an input of which the transducer is connected so that the amplifier receives an input current proportional to the light, and a feedback loop between the output of the amplifier and its input, said feedback loop including an optical isolator formed by a light-emitting diode via which there flows the output current of the amplifier or a current proportional thereto, and a further opto-electronic transducer such as a photo-diode or a phototransistor, the current flowing in said transducer being applied to the input of the amplifier. An example of a circuit which embodies the present invention, and which is for use in an optical system will now be described with reference to Fig. 3 of the accompanying drawing. In Fig. 3, the photo-diode PD3 which responds to the light to be received in connection to one input of an amplifier A, e.g. to the positive input of an integrated circuit amplifier. In the present case it is assumed that the light to be received has reached the receiver via an optical fibre light guide. However, the receiver can also be used where the light is received in the form of a light beam in free space. The output of the amplifier is connected to reference via a light-emitting LED and a resistor R2, the output being taken from across the resistor R2. The light emitting-diode's emission is "aimed at" another photo-diode PD4. Thus we have a feedback loop which includes an optical isolator formed by the two diodes LED and PD4. Note that in some case the photo-diodes may be replaced by photo-transistors. Thus the LED-PD4 combinations perform a currentto-current conversion, with the ratio of the current through the LED to the current generated by the photo-diode (or photo-transistor) determined by the optical attenuation between them. Note that instead of exciting the diode LED by the entire output current of the amplifierA, it would be possible to excite it by a fixed proportion of that current. In the feedback loop of the receiver, which includes the optical isolator LED-PE4, the feedback photo-diode PE4 produces a current equal to the signal current produced by the signal receiving photo-diode PD3. The output of the receiver can be obtained by monitoring the current through the LED, or the optical output of the LED can be used as the receiver's output. In the latter case the circuit would act in effect as an optical amplifier. In such an arrangement, the use of optical feedback means that the problem of feedback capacitance can be virtually eliminated. The optical feedback system need not work at the same optical wavelength as the receiving photo-diode PD3. The receiver sensitivity is limited only by the amplifier noise, and is therefore better than in either of the two known arrangements described above. The dynamic range of the receiver is comparable with that of a "transimpedance" type circuit, and is superior to that of a "high-impedance" type. By automatically reducing the optical attenuation in the feedback loop, a much greater dynamic range may be attained. The improved sensitivity attainable by the use of such a circuit is of especial importance at long wavelengths were suitable avalanche photodioes may not be available. Furthermore, at high frequencies the transimpedance design can also be limited by amplifier noise. The optical feedback design is likely to have better sensitivity at low frequencies. (At low frequencies the transimpedance design is limited by thermal noise in the feedback resistor, the transition to being limited by amplifier noise occurs at a fairly indeterminate frequency depending on photodiode and stray capacitances. CLAIMS

1. An electrical amplifier circuit, in which current feedback from the output of the circuit to its input is provided via a feedback loop which includes an opti cal isolator.

2. A circuit as claimed in claim 1, and in which said optical isolator includes a light-emitting diode at the putput side of the amplifier circuit via which the output current flows and a photo-diode at the input side, which photo-diode is connected to the input of the amplifier ascent.

3. An opto-electrical receiver circuit, which inc ludes an opto-electronic transducer such as a photo-diode or a photo-transistor on which the light to be received falls, an amplifier to an input of which the transducer is connected so that the amplifier receives an input current proportional to the light, and a feedback loop between the output of the amplifier and its input, said feedback loop including an optical isolator formed by a light-emitting diode via which there flows the output current of the amplifier or a current proportional thereto, and a further opto-electronic transducer such as a photo-diode or a photo-transistor, the current flowing in said transducer being applied to the input of the amplifier.

4. An opto-electrical receiver circuit substantially as described with reference to Fig. 3 of the accompanying drawings.

4. A circuit as claimed in claim 3, and in which the light-emitting diode is connected in series with a resistor from across which an output may be obtained.

5. An opto-electrical receiver circuit, substantially as described with reference to Fig. 3 of the accompanying drawing.

New claims filed on 22 March 1979 Superseded claims 1-5 New claims 1-4:- CLAIMS

1. An opto-electrical receiver circuit, which includes an opto-electronictransducer such as a photo-diode or a photo-transistor to which the light to be received is applied, an electronic amplifier to an input of which the transducer is connected so that the amplifier receives an input current proportional to the intensity of the light, and a feedback loop between the output of the amplifier and the input thereof, said feedback loop including an optical isolator formed by a light-emitting diode via which there flows the output current of the amplifier or a current proportional thereto and a further optoelectronic transducer such as a photo-diode or a photo-transducer, the current caused to flow in said further transducer being applied to the input of the amplifier.

2. A circuit as claimed in claim 3, and in which light emitting diode is connected in series with a resistor from across which an output may be obtained.

3. An opto-electrical receiver circuit, which includes an opto-electrical transducer such as a photodiode or a photo-transistor to which is applied light to be received as a light beam in free space or as light from an optical fibre transmission line, an electronic amplifier to an input of which the transducer is connected so that the amplifier receives an input current whose value is determined by the intensity of the light which falls on the transducer, and a connection from the output of the amplifier to a reference potential, which connection extends via the series combination of a light emitting diode of an optical isolator and a resistive impedance, the output of the receiver circuit being derived from across the resistive impedance, and in which the photo-diode or the photo-transistor of the optical isolator is connected to the input of the amplifier, the arrangement being such that the opto-isolator provides a negative feedback loop for the amplifier.