The present disclosure describes an electronic circuit for use as a receiver in digital fiber-optic systems. It is the nature of such systems that considerable amplification is required at the receiving terminus to bring to useful signal levels the electrical energy provided by photodetectors in response to the light incident thereupon. However, the photodetectors and amplifiers themselves contribute noise currents which vary with temperature and operational amplifiers have a characteristic initial dc offset. Accordingly, while receivers for fiber-optic systems have been complicated and expensive, the receiver of the present invention is neither of these. It amplifies the small signals from the photodetector to a suitable level, eliminates the effects of the aforementioned dc offset, rejects any long-term drift and provides output signals of any duration from a bistable stage, which signals correspond to the original digital information transmitted by the system.

BACKGROUND OF THE INVENTION 
In the transmission of signals via optical fibers, both analog and digital 
modulation of the light source is utilized in different applications. 
Recently, digital transmission has become more prominent due to its 
inherent error reducing capabilities. Considering the latter, the digital 
information to be transmitted modulates a light source, such as a light 
emitting diode (LED) or a solid-state laser diode (ILD). The light from 
such a source is propagated through the optical fiber or light pipe by 
total internal reflection. At the receiving terminus, the light is 
directed upon a photodetector. The latter may be, for example, either a 
PIN photodiode or an avalanche photodiode (APD). The small energy levels 
produced by the photodetector are thus amplified and converted back to 
digital form for further use. 
While the transmitters for use in such digital systems are easily designed 
and relatively low cost, the receivers involve complicated circuits and 
are expensive. The reason for this stems from the relatively small amount 
of light arriving at the receiving terminus and the limited sensitivity of 
the photodetectors. The former results from two major causes, namely 
attenuation within the light pipe itself and input coupling losses where 
only a fraction of the source's radiant power is actually coupled into the 
fiber and waveguided. Accordingly, it is apparent that a large amount of 
amplification is needed to bring the small signal input from the 
photodetector to a useful level. 
Several problems arise in the design of amplifiers for digital systems 
using photodetectors as their input sources. One of these involves a noise 
component within PIN photodiodes caused by fluctuations in dark current. 
The latter current flows through the diode-biasing circuit when no light 
is incident on the photodiode. An average dc value for dark current is 
usually specified by the manufacturer at a given temperature and bias 
voltage. However, it is known that dark current shot-noise power varies 
linearly with this average. Dark current increases with temperature and 
substantially doubles in amplitude for every 10 degrees Celsius increase 
in operating temperature. Another problem in amplifier design stems from 
the initial dc offset inherent in all operational amplifiers. When the 
aforementioned variables are in their worst-case direction, it is 
impossible to predict over a long period of time, the integrity of the 
signal levels exiting the receiver. 
In view of the foregoing, it is apparent that the need exists for a 
low-cost optical system for dc transmission of information. The receiver 
of the present invention, characterized by simplicity of design and 
economy, provides the required amplification of the photodetector signals, 
while rejecting any long-term drift. As such, it may be advantageously 
employed in the aforementioned system. 
BRIEF DESCRIPTION OF THE INVENTION 
In accordance with the invention, it may be assumed that a digital signal 
is being transmitted down an optical fiber or light pipe by simply turning 
the transmitter on and off. The very small voltages generated in response 
to light incident upon a photodetector are amplified to a predetermined 
amplitude. The amplification may be readily accomplished through the use 
of one or more operational amplifiers. The output signals from the latter, 
while being of the proper amplitude, may have a dc baseline which is not 
necessarily at ground potential. This condition results from the initial 
dark current of the photodetector and any dc offset in the amplifiers 
which are in turn multiplied by the amplifier gain. Accordingly, the 
square-wave output signals of the amplifier are ac coupled and referenced 
to ground potential by a differentiator network. The resulting positive 
and negative going edges of the waveforms are applied in common to the 
inputs of two comparators which are biased respectively to opposite 
polarities. The outputs of the comparators are then applied in common to 
one input terminal of a bistable device, such as a flip-flop. The latter 
is driven from one state to its opposite state in response to signals from 
the comparators, thereby providing an output corresponding to the original 
digital information transmitted down the light pipe. The flip-flop 
provides for true digital transmission in that it may remain in one state 
or the other indefinitely, and is therefore insensitive to the repetition 
rate of the transmitted data. 
From the foregoing brief description, it should be noted that the present 
receiver offers the following advantages, in addition to its simplicity 
and economy. It may be used in a low-cost system with a simple on-off 
transmitter design. No special component selection is required in the 
receiver design and since no drift is inherent therein, no drift 
compensation, such as might otherwise be performed by periodic 
potentiometer adjustments, is necessary. The receiver design is applicable 
to systems of any transmission speed. Other features and advantages of the 
present invention will become apparent in the detailed description 
appearing hereinafter.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a highly simplified data transmission system using 
fiber-optic techniques. Basically, the system is comprised of a 
transmitter 10 which includes a light source 12 and a source driver 14. 
The driver 14 is designed to modulate the light source 12 in accordance 
with the digital signals applied to its input terminal 16. The light 
source 12 may be of various types--the most commonly used being the light 
emitting diode (LED) or the solid-state laser diode (ILD). The modulated 
light from source 12 is transmitted down the fiber-optic cable 18 in the 
direction of the arrows to the receiver assembly 20, which comprises 
generally a photodetector 22 and the receiver circuits 24. The latter 
circuits provide on output terminal 26, digital signals corresponding to 
those applied to input terminal 16. The photodetector 22 may also be of 
various types depending upon application requirements. Two types presently 
being used in such systems are the PIN photodiode and the avalanche diode 
(APD). 
With reference to FIG. 2, the schematic diagram for receiver 20 of the 
present invention and FIG. 3, a timing diagram illustrating the waveforms 
appearing in FIG. 2, the operation of the receiver 20 will be described. 
It may be assumed that light modulated in accordance with input digital 
data has been transmitted down the fiber-optic cable 18 (FIG. 1) and is 
incident upon photodetector 22 at the receiving terminus of the system. 
Photodetector 22 is connected in series with resistor 28 and the 
combination is coupled to a positive voltage source, +V. Current flowing 
in the photodetector circuit causes a small signal voltage, as seen in 
FIG. 3A, to be developed across resistor 28. The last mentioned voltage, 
which may be of the order of 1 millivolt, is applied via line 30 to the 
non-inverting input of operational amplifier 32, and is amplified in 
accordance with the ratio of the resistance values of resistors 34 and 36. 
Assuming the gain of amplifier 32 to be ten a 10 millivolt signal as seen 
in FIG. 3B will appear at the output of amplifier 32 on line 38. The 
signal is then applied to the non-inverting input of the succeeding 
operational amplifier 42. Assuming that the gain of this last amplifier is 
fifteen as determined by the ratio of resistors 40 and 44, the output 
signal appearing on line 46 and shown in FIG. 3C has an amplitude of 1.5 
volts. The dc baseline of the signal in FIG. 3C is not necessarily at 
ground potential, because of the initial dark current of photodetector 22 
and the dc offsets of amplifiers 32 and 42 which are multiplied by the 
respective amplifier gains. In order to use the signals appearing in FIG. 
3C and reference them to ground potential, the signals are ac coupled to 
the succeeding stages by capacitor 48 and referenced to ground by resistor 
50. It should be noted that in true digital systems, the repetition rate 
of the transmitted data should be capable of having a range from dc to its 
maximum design value. Since at low frequencies, the value of capacitor 48 
would be prohibitively large, the network comprised of capacitor 48 and 
resistor 50 is designed to differentiate the square wave signals appearing 
on line 46. The values of capacitor 48 and resistor 50 may be chosen to 
permit recovery in half the time period for the highest frequency input 
data applied to the system. The output of the differentiator network 
appears on line 52, as seen in FIG. 3D. Only the respective positive and 
negative going leading edges of the waveforms in FIG. 3D are utilized in 
the succeeding stages. 
A pair of comparators 54 and 56 respectively are provided. Comparator 54 is 
biased positively by virtue of the potential appearing on its negative 
input terminal. This bias potential is derived from a divider network in 
which series resistors 58 and 60 are connected between a positive voltage 
source, +V and ground potential. Comparator 56, on the other hand is 
negatively biased. The bias potential is derived from the resistive 
network comprised of resistors 62 and 64 connected between a negative 
source, -V, and ground, and is coupled to the positive terminal of 
comparator 56. Both comparators are coupled via respective resistors 66 
and 68 to a common voltage source, +V. 
The differentiated signals (FIG. 3D) appearing on line 52 are applied via 
line 70 to the positive terminal of comparator 54 and via line 72 to the 
negative terminal of comparator 56. When a positive going pulse is seen on 
line 52, a positive pulse as illustrated in FIG. 3E appears at the output 
of comparator 54 on line 74. The pulse amplitude may be of the order of 
2.5 volts. When a negative going pulse appears at the output of the 
differentiation network on line 52, a positive pulse (FIG. 3F) is produced 
at the output of comparator 56 on line 76. Similarly, the pulse amplitude 
may be 2.5 volts. 
The output stage of receiver 20 is a flip-flop 78. The positive pulse on 
line 74 is applied to the "1" input terminal of flip-flop 78, thereby 
tending to set it in the "1" state. On the other hand, a positive pulse on 
line 76 is applied to the "0" input terminal, thereby placing flip-flop 78 
in its opposite, or "0" state. An output from flip-flop 78 appears on 
output terminal 26 which is coupled to the "1" side thereof. Reference to 
FIG. 3G, showing the receiver output on terminal 26, indicates that the 
original digital information represented by the small signal levels of 
FIG. 3A have been restored and amplified to usuable levels (approximately 
2.5 volts) in receiver 20. Flip-flop 78 retains its state indefinitely, 
and accordingly is not repetition-rate sensitive. A network comprised of a 
series connected capacitor 80 and resistor 82 coupled between the +V 
source and ground may be provided. The junction of capacitor 80 and 
resistor 82 is coupled via line 84 to a "0" input terminal of flip-flop 76 
to preset the latter to the "0" state when power is first turned on at the 
receiver. The last mentioned network may be eliminated at the expense of 
the possible loss of the first information pulse since the receiver will 
automatically lock-in to the subsequent signals. 
In conclusion, it is submitted that the fiber-optic system receiver taught 
by the present invention finds particular application in low-cost optical 
systems. It should be understood that the various circuit parameters 
mentioned in the course of the description of the receiver operation, have 
been included solely for purposes of example and are not limitative of the 
invention. Moreover, changes and modifications of the circuit organization 
presented herein may be needed to suit particular requirements. Such 
changes and modifications are well within the skill of the electronics 
circuit designer, and insofar as they are not departures from the true 
scope and spirit of the invention, are intended to be covered by the 
following claims.