Optical receivers

An optical receiver incorporating a voltage dependant impedance arranged to shunt excess AC photodetector signal away from the receiver amplifier in response to light level.

FIELD OF THE INVENTION 
This invention relates to optical receivers used in optical fibre 
telecommunications systems. 
BACKGROUND TO THE INVENTION 
The three basic components to any optical fibre communications based system 
are: 
i. a transmitter, which converts an electrical signal to be transmitted 
into an optical form, 
ii. the optical fibre which acts as a waveguide for the transmitted optical 
signal, and the 
iii. optical receiver which first detects the optical signal transmitted 
and converts it to electrical form. 
A typical optical receiver comprises a photodetector, e.g. a reverse 
biassed avalanche photodiode (APD) or PIN diode, coupled across the inputs 
of a high impedance amplifier, commonly a transimpedance amplifier. A 
feedback circuit from the amplifier output is commonly used to provide 
automatic gain control of the bias applied to the photodetector. 
In practice optical receivers have to deal with very large variation of 
signal strengths. When strong optical signals are detected they usually 
lead to overloading of the frontend amplifier. The range between the 
maximum sensitivity and minimum overload point of an optical receiver is 
called the dynamic range of the receiver. 
The present invention seeks to provide a means for increasing the dynamic 
range of an optical receiver by removing the overload criteria within the 
operating range without affecting the sensitivity of the receiver. 
Examination of the time averaged D.C. photocurrent generated from the 
received optical signal shows that a linear relationship exists. However, 
when the optical power received is expressed in dBm relative to the 
photocurrent generated, an exponential curve is obtained, as shown in FIG. 
1. 
This curve is fundamental to an understanding of the overload problem since 
it clearly indicates the kind of current variation the frontend amplifier 
has to deal with at its input. 
Examination of the characteristics of a semiconductor diode shows that the 
forward bias current variation through the diode with respect to the 
voltage developed across it is also nonlinear and follows an exponential 
form, as shown in FIG. 2. 
SUMMARY OF THE INVENTION 
According to the invention there is provided an optical receiver having a 
reverse biassed photodetector including a voltage dependant impedance 
arranged to shunt the photodetector photocurrent in response to increasing 
voltage drop in the photodetector bias.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The invention essentially provides a solution to the overload problem 
since, by placement of a diode D.sub.o in series with the photodiode 
D.sub.d as shown in FIGS. 3a and 3b, and by applying correct biassing the 
exponential variation of the photocurrent can be followed. 
Placement of a resistor R.sub.1 in parallel with the diode D.sub.o as in 
FIG. 4 enables control of the biassing of the diode D.sub.o to be achieved 
since the photocurrent generated by the photodiode D.sub.d demands a bias 
current through the resistor R.sub.1 and diode D.sub.o. 
At very low light levels the photocurrent generated is very small, of the 
order of uAmps. At this stage the bias current is supplied through the 
resistor R.sub.1 but as light level increases the bias current I.sub.1 
flowing through R.sub.1 leads to a voltage drop across it. 
Looking at FIG. 2 it can be seen that this voltage drop acts as a bias 
voltage V.sub.b across the diode D.sub.o which then gradually turns the 
diode on according to the time averaged level of the optical signal. 
Another advantage of this resistor is that at sensitivity the majority of 
the bias current to the photodiode D.sub.d is supplied through the 
resistor R.sub.l. This minimises the noise contribution of the diode 
D.sub.o and also its junction capacitance C.sub.t, which affects the AC 
component of the signal. 
So far only the time averaged DC effect of the optical signal has been 
considered. However, there is also an AC component which increases with 
the increased optical signal level and leads to overloading of the 
frontend amplifier. 
Therefore a practical solution should also include means of limiting the 
maximum AC signal level at the input of the front end amplifier. However, 
the solution should not affect the AC signal at very low signal levels. 
From what was said earlier it can be seen that maximum sensitivity is 
obtained when minimum loading of the photodiode occurs. 
So to provide sensitivity as far as the photodiode is concerned, the diode 
D.sub.o must represent a very high impedance . For AC signals this means 
that the diode D.sub.o must have a very small device capacitance. This 
factor becomes more critical to the receiver sensitivity as the frequency 
of operation is increased. 
Examination of semiconductor diodes reveals that Schottky diodes present 
very low junction capacitances when reverse biassed or at zero bias, so a 
Schottky diode can be used. It has been found that with forward biassing 
of the diode the capacitance of the diode increases exponentially, as 
shown in FIG. 5. This is exactly what is required since as the 
photoelectric signal amplitude generated by the diode increases it is 
required to direct some of the AC signal away from the frontend amplifier. 
The diode D.sub.o also provides such a path. A grounded capacitor 
connected to the other end of the diode provides an AC shunting path to 
the ground such that the impedance presented to the AC signal of the 
photodiode is only that of the dynamic impedance of the diode D.sub.o at 
its point of operation. 
In the diagram of FIG. 6 a photodetector D.sub.d is connected across the 
inputs to a receiver front end amplifier A.sub.r. A feedback circuit from 
the amplifier includes a signal level detector S.sub.1d which provides a 
gain control signal to a bias voltage circuit B.sub.v. Bias voltage Vs is 
applied to the photodetector via resistor R.sub.l. The photodetector 
D.sub.d is shunted by diode D.sub.o in series with capacitance C and 
resistance R.sub.c. Diode D.sub.o is connected to the bias source through 
resistance R.sub.s and diode D.sub.s. 
At low light levels the photocurrent I.sub.p generated by the photodetector 
D.sub.d is very small, therefore the bias current I.sub.1 required to flow 
through R.sub.1 and D.sub.d is also quite small. At this stage most of the 
bias current is supplied through R.sub.1 and there is only a very small 
current flow through D.sub.o, the impedance of D.sub.o being very high. As 
the optical signal level applied to D.sub.d increases so the photocurrent 
I.sub.p increases. This leads to a larger current I.sub.l flowing through 
R.sub.1 causing the voltage V.sub.1 across R.sub.1 to rise. Therefore, as 
V.sub.1 tends towards V.sub.s (the voltage across D.sub.s and R.sub.s) 
plus V.sub.f (the voltage across D.sub.o) then diode D.sub.o is gradually 
turned on. When V.sub.l =V.sub.s +V.sub.f the diode D.sub.o is forward 
biassed and current I.sub.d, through D.sub.s and R.sub.s, starts to become 
the dominant bias current supply for the photodetector D.sub.d. At this 
stage the impedance of D.sub.o is lowered and some of the photocurrent 
generated by D.sub.d is shunted via capacitance C and resistance R.sub.c 
to ground, thus limiting the amplitude of the input signal applied to the 
amplifier A.sub.r. The reduction in impedance is due to the non linear 
behaviour of the diode D.sub.o. In general a Schottky device is chosen for 
the low junction capacitance, low leakage and a suitable variation of 
dynamic impedance with current. The circuit has an inbuilt feedback 
control whereby with increasing signal level the bias voltage V.sub.1 
stays equal to V.sub.s +V.sub.f, therefore controlling the bias current 
flow through D.sub.o and also its impedance to the signal current. 
It is to be noted that the circuit of FIG. 6 would still operate is R.sub.1 
is removed. The sensitivity is reduced slightly but, since the current 
through D.sub.o is very small it still represents a high impedance to the 
photodetector current. 
There are two main reasons for receiver overload. One reason is the lack of 
sufficient bias current applied to the photodetector at light optical 
signal levels. The other reason is that the front end amplifier A.sub.r 
becomes saturated with a large input signal level. If the second reason is 
not a limiting criteria the capacitor C can be removed. 
The principle of operation of the invention has been explained by way of 
example using the particular circuit of FIG. 6. However, there are many 
other arrangements of circuits which would become apparent to those 
skilled in the art which would achieve the same results. For example, the 
voltage dependant impedance, exemplified by the diode D.sub.o in FIG. 6, 
could be implemented instead by a transistor and controlled by a signal 
from the signal level detector. 
In the alternative circuit shown in FIG. 7, the photodetector D.sub.d is 
connected in a balanced configuration for amplifier A.sub.r, the fixed 
bias being applied via R.sub.11 and R.sub.12. Shunting of the photocurrent 
is now effected via voltage dependent diodes D.sub.01 and D.sub.02 to 
provide for both polarities of signal current. (Note that in practice 
series pairs of diodes are used to reduce capacitance effects.). Diode 
D.sub.1 in the fixed bias supply provides dynamic impedance which improves 
the sensitivity of the photodetector. Diode D.sub.s1 is included to 
compensate for changes in D.sub.1 due to temperature fluctuation. Diodes 
D.sub.s1 and D.sub.s2 are switching diodes. The forward bias voltage 
V.sub.f of D.sub.s1 and D.sub.s2 controls the point at which the overload 
diodes switch on.