GB2225684A - Circuits for optical receivers - Google Patents

Abstract

An optical detector circuit has a constant current device 11, 12 in parallel with the primary optical detector 10. This has the effect of increasing the apparent dark current of the primary detector but reducing the variations whereby the performance of the detector circuit is improved. The constant current device is preferably an optical detector positioned to respond to the constant optical output of an optical source. The circuit output 15 is taken through a conventional op-amp 20 with feedback 22, 23. <IMAGE>

Description

CIRCUITS FOR OPTICAL RECEIVERS BT CASE A23895 BOTH (0914P) This invention relates to circuits for optical receivers.

Optical receivers are utilised at the ends of optical telecommunications links to convert optical signals into electrical form. The electrical signal may be used as output, e.g. to drive a speaker, or for processing (including amplification). The primary detector, ie. the device upon what the radiation actually impinges, is connected into an electronic circuit. This circuit drives the primary detector and processes its output. Ideally the primary detector should provide no output when no optical signal is received but the random movement of electrons ensures that there will always be some transitions even in the complete absence of light.

Furthermore, the random movement increases rapidly with temperature and therefore the number of transitions also increase with temperature. All transitions, whether wanted or not, give rise to an electrical output and all primary detectors produce an output even in the dark.

This output is called the "dark current". For a well designed detector the dark current is low but, in any case, it increases rapidly with temperature.

The phenomenon of "dark current" causes problems in the design of detector circuits and their performance is adversely affected. It is an object of this invention to reduce the effect of dark current on the performance of the detector circuit.

According to this invention, a constant current means is connected in parallel with the primary optical detector. The constant current means is preferably adjusted to provide, during use, a constant bias current which is many times, e.g. 10 to 100 times, the highest dark current to be expected. The configuration just described is equivalent to a primary detector which has a relatively high but constant dark current and we have found that these operational conditions give rise to an improved performance. A lower dark current is preferred to a higher dark current but constancy is more important than absolute value.

The constant current means preferably takes the form of a second optical detector positioned so that it is able to respond to a light source adapted to provide a constant actuating signal whereby the second optical detector functions as a constant current device. The light source is conveniently semiconductor device, e.g. an LED or a laser, located in a constant current drive circuit.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 illustrates a generalised version of a circuit according to the invention, and Figure 2 is a more specific version of Figure 1.

The optical receiver circuit shown in Figure 1 comprises a photo-diode 10, i.e. the primary detector of the circuit, which is located in branch 16 of the receiver circuit. The circuit also includes a second photo-diode 11 located in branch 17 of the circuit and an LED 12 in series with a resistor 13. Branches 16 and 17 are in parallel and they combine into branch 18. Conventional processing circuitry 14 receives its input from branch 18 and its output is provided at terminal 15.

The primary detector 10 is adapted to receive telecommunications traffic signals, eg. it adapted for connection to an optical fibre. It is positioned so that it is not able to receive the light output of the LED 12.

The photo-diode 11 and the LED 12 are positioned so that the photo-diode is adapted to receive the light output of the LED 12. The photo-diode is not able to receive the telecommunication traffic.

During use, the LED 12 is driven by a constant current through resistor 13 and it provides a constant lightsignal to the photo-diode 11 which therefore provides a constant bias current through branches 17 and 18. (The combination of LED 12/photo-diode 11 gives a constant current with very low noise, eg substantially below the noise level which can be obtained with a resistor. The primary detector 10 receives a traffic signal which is, of course, variable. Sometimes the optical traffic signal is substantially zero and the primary detector passes its dark current through branches 16 and 18. When the traffic signal is zero, the current in branch 18 is the sum of the dark current of primary detector 10 and the constant bias current of the photo-diode 11.

The dark current rises substantially with temperature and its maximum values may be 100 times its.minimum values. The value of resistor 13 is selected so that the bias current is many times the highest anticipated dark current. That is, the constant bias current is adjusted so that even the highest dark currents are small, and preferably negligible, in comparison with the constant current. Under these circumstance the "sum" current in branch 18 is substantially equal to the constant bias current. If the constant current is set at 50 times the maximum dark current then the current in branch 18 varies over about the ratio 50:51 when the telecommunications input is zero.

The overall effect in branch 18 of the two photo-diodes 10 and 11 is that of a single detector having a high but constant dark current. Conventional circuitry 14 processes the signal in branch 18 giving the output of the whole circuit at terminal 15.

Two indications, (A) and (B), of numerical values are given in Table 1 below.

TABLE 1 (A) (B) Nin dark current lpA lOfA Nax dark current 100pA lpA Constant current (photo-diode 11) lnA 100pA (Constant current):(Nax dark current) 10 100 Min:Hax dark current 1:100 1:100 The component whose performance is indicated as (B) can be regarded as better quality than the component whose performance is indicated as (A) because the dark current is lower, ie 100 times lower. This means that the "constant current can be set lower, ie 100pA instead of lnA and, even more important, the variation of total current, in branch 18 is lower, ie about 100:101 for (B) and about 100:110 for (A).

A conventional circuit lacks diode 11 and LED 12 and circuitry 14 responds to a primary detector, ie. 10 alone, which has a low dark current but, as table 1 shows, this current varies over a wide range. According to the invention the circuitry 14 responds to a primary detector, ie. 10 and 11 combined, which has a much higher dark current but a smaller percentage variation. It has been observed that this second set of conditions gives more consistant performance. Thus the circuit stability, sensitivity and overload are more accurately defined.

It is emphasised that the combination of two photo-diodes (10 and 11) is advantageous with all forms of conventional circuitry 14. Figure 2, in which components shown in Figure 1 have the same numerals, shows a specific version of the conventional circuitry 14. This comprises an operational amplifier 20 connected to receive its input from branch 18 and having its output connected to reference via an LED 22 and a resistor 24. The output is of the circuit as a whole is taken from the operational amplifier 20. Branch 18 also includes a photo-diode 23 which receives its light input from LED 22. This arrangement constitutes feedback across the amplifier 20.

The light sources mentioned in this specification are all semiconductor devices, eg. an LED (light emitting diode) or a laser. The optical detectors are also semicondutor devices, eg. photo-diodes such as a PIN diode or an avalanche photo-detector.

Claims (6)

CLAINS

1. An optical detector circuit which comprises a constant current device in parallel with the primary optical detector whereby the current of the constant current device is effectively the dark current of the primary detector.

2. An optical detector circuit according to claim 1, wherein the primary optical detector is a photo-diode.

3. An optical detector according to either claim 1 or claim 2, wherein the constant current device takes the form of a second optical detector operatively associated with light source.

4. An optical detector circuit which comprises (a) amplifying means having an input circuit and an output circuit, said output circuit including means for providing the output of the circuit; (b) first and second optical detectors connected in parallel with one another said first and second optical detectors being comprised in said input circuit of the amplifier, (c) feedback means connected so us to provide feedback from the output circuit of the amplifier to the input circuit of the amplifier.

(d) an optical source adapted to provide a substantially constant optical output, wherein said first optical detector is adapted for connection to a source of telecommunications optical signals whereby it constitutes the primary optical input of the circuit and said second optical detector is operatively associated with the optical source whereby it is adapted to receive said constant optical output and function as a constant current device.

5. A circuit according to claim 4, wherein the optical source includes drive means for proving a constant drive current.

6. A circuit according to either claim 4 and claim 5, wherein the feedback means comprises an optical source in the output circuit and an optical detector in the input circuit said optical detector being positioned to respond to the optical output of said optical source.