International Standard Electric Corporation

Other

Stc, Inc

David P Chown

Malcolm Spencer

An optical receiver, for use with optical fibre or for receiving a beam in free space, has its detector device, usually a PIN diode (D) connected in series with a resistor (R1) and to the base of a gallium arsenide field effect transistor (FET). Although the latter is satisfactory at most frequencies of operation, in certain load conditions, instability can occur at Gigahertz frequencies. To overcome this, the source-drain path of the FET is shunted by 50 ohm resistor (R2) in series with a capacitor (C) of about 5 nanofarads. In addition the FET is coupled to a bipolar transistor amplifier by a relatively long wire bond (L) which acts as an inductive impedance at the frequencies at which this instability occurs. These measures between them suppress this instability.

Optical fibre receiver

In an optical receiver for the reception of a modulated light beam, the beam falls on a PIN diode connected to the input of an amplifier which has a feedback resistor across it. This resistor is shunted by the series combination of a Schottky diode and of a PIN diode. The former diode ensures that a high magnitude pulse does not leave the receiver undesirably insensitive to a following smaller pulse. The latter diode, due to its low self-capacitance and low leakage, overcomes distortions due to the Schottky diode's higher self-capacitance and leakage. These effects are enhanced by providing the series-connected diodes with a low value (e.g. 0.2 volts) of reverse bias, due to the different connections of the diodes and the feedback resistor to a potential divider between the amplifier output and a point at a reference potential.

Optical receiver

An optical receiver of the so-called high impedance type has a photo-diode onto which the modulated light beam from an optical fiber, or received via free space, falls to produce a current in that diode. One end of the diode is coupled directly to the gate of a FET, the other end of the diode being coupled via a capacitor to the source of the FET. The FET's source and its bias resistor are on a source-connected pad. The source output of the FET is coupled via an emitter follower impedance buffer and an equalizing capacitor to a broad-band amplifier. Thus the receiver has a flat overall in-band response.

In an optical fibre receiver, the modulated light falls on a p-i-n diode (PD) connected to the input of an operational amplifier (A). This amplifier is shunted by a load resistor (R), which in turn is shunted by a non-linear Schottky diode (SD). This prevents a large pulse from spoiling the sensitivity of the receiver to a following smaller pulse. This also provides the receiver with a low pass response having a relatively long time constant. In addition to this, the a.c. coupling response for the connection to the next stage of the system has a relatively short time constant. These two time constants combine to give an overall characteristic with a relatively narrow bandwidth, which enhances the above effect. Finally, when the line muting function is needed, as between messages, the bit stream as detected includes a double amplitude pulse whose detection initiates the line muting function. This can be implemented by a double length pulse included in the bus bit stream which is so detected as to give a double amplitude pulse.

An optical receiver for responding to optical energy propagating in an optical fibre transmissions path (1) is detected by a reverse-biassed p-i-n diode (2) which is in series with a forward-biassed p-i-n diode (3). The junction between the two diodes is connected to the input of an amplifier (4), which feeds on auto-equalizer. The auto-equalizer includes another p-i-n diode (7) which is in series with the signal path, and is forward-biassed. Due to the associated components including resistors (9,10), the diode (7) passes the same average current as do the diodes (2,3) at the front end of the receiver. The overall effect of the third diode (7) is to provide the receiver with a substantial region in its characteristic whose frequency response is flat.

An optical receiver, for responding to optical energy propagating in an optical fibre transmission path (2) is detected by a reverse-biassed p-i-n diode (3) which is connected in series with a forward-biassed p-i-n diode (7). The junction between the two diodes is connected to the input of an amplifier (5), which is part of a two-stage amplifier (5,8), with automatic equalization (9,10,11). The forward biassed diode, by its characteristics leads to an improved dynamic range for the receiver as compared with conventional optical receivers.

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Great Dunmow, United Kingdom

David P Chown