Abstract:
A differential optical signal receiver includes an interference-rejecting circuit to provide enhanced interference signal rejection. The differential optical signal receiver includes a differential optical signal detector for detecting a received differential optical signal and converting it to a differential electrical signal. An interference-rejecting differential electrical circuit, including a common source load, processes the differential electrical signal so as to reject any electrical interference signal other than said differential electrical signal.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to optical signal receivers and, more particularly, to a differential optical signal receiver having an interference-rejecting circuit. 
     BACKGROUND OF THE INVENTION 
     Optical signals are increasingly being used to communicate between electronic processing elements. In certain applications, large amounts of information need to be processed together, thereby creating a need to process many optical signals. If such processing is to be done electronically, it is necessary to convert large numbers of optical signals to electrical signals, and then to process the resulting electrical information. Particularly if this processing is done with integrated optical receiver arrays, the receiver is subject to interference from a wide range of potential sources. What is desired is an optical receiver having improved interference-rejection capabilities so as to reject the interference arising from various electronic sources. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to solving the prior art interference problems using an a differential optical signal receiver having an interference-rejecting circuit. 
     More particularly, a differential optical signal receiver is disclosed comprising (1) a differential optical signal detector for detecting a received differential optical signal and converting it to a differential electrical signal and (2) a differential electrical circuit, including a common source load, for processing the differential electrical signal so as to reject any electrical interference signal other than the differential electrical signal. According to another aspect of the invention, the differential electrical circuit includes a differential transimpedance preamplifier connected to receive and amplify the differential electrical signal. In another aspect the differential electrical circuit further includes a differential amplifier circuit connected to receive output signals from the differential transimpedance preamplifier. 
     In one embodiment of the differential optical signal receiver, the differential electrical circuit and the differential amplifier circuit are integrated together on a common chip. The devices utilized in the chip may be Field Effect Transistors (FETs) which may be fabricated using Complementary Metal Oxide Semiconductor (CMOS) technology or other comparable Very Large Scale Integration (VLSI) technology. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing, 
     FIG. 1 shows a block diagram of a prior art optical signal receiver; 
     FIG. 2 shows an illustrative block diagram of our differential optical signal receiver; 
     FIG. 3 shows a first illustrative arrangement of our interference-rejecting differential amplifier; 
     FIG.  4 . shows an illustrative cascode-based interference-rejecting preamplifier; and 
     FIG.  5 . shows a fully differential circuit for detecting and processing of the incoming optical signals. 
    
    
     DETAILED DESCRIPTION 
     In the following description, each item or block of each figure has a reference designation associated therewith, the first number of which refers to the figure in which that item is first located (e.g.,  101  is located in FIG.  1 ). When the description references the prior work of the inventors and others, such references will be designated by a bracketed number [2] which indicates the reference citation location in the Appendix. 
     Optical receivers typically contain several distinct and identifiable elements. These are described in many references.[6,7] Such a prior-art receiver is shown the block diagram in FIG.  1 . The light signal is received and converted to an electrical signal in detector  101 , amplified in pre-amplifier  102 , filtered in channel filter  103 , and a converted to a digital output signal in decision circuit  104 . On occasion, a post-amplifier, not shown, may be inserted after the Pre-Amp  102 . 
     The integration of large numbers of optical inputs and outputs to CMOS VLSI circuits is well known.[2,3] Such integration is attractive for various reasons, not the least of which is the ability to bring large amounts of information onto and off of the VLSI chip—a task that becomes increasingly difficult to do electronically as the complexity and speed of the CMOS VLSI increases. In CMOS VLSI, there are large numbers of digital processing elements, and these elements are fabricated in a conducting substrate, being isolated from one another by p-n junctions.[5] During the course of their operation, these digital elements generate spurious signals. The CMOS VLSI chip has a plethora of these signals and represents a very ‘noisy’ environment. Some of the effects are summarized below, and more may be found in various references[5,1] 
     injection of minority and majority carriers into the substrate 
     transient deviations from the power supply potential on wires connecting the elements to external supplies 
     transient deviations from ground potential on wires connecting the elements to ground potential. 
     radiation from rapidly moving charges 
     inductive coupling of currents flowing in signal, ground, and power lines on the chip. 
     capacitive coupling of voltage signals between wires on the chip. 
     In particular, supply and ground noise are important sources of interference because they can grow by coupling through linear amplifiers and cause signal corruption. These sources of interference can be tolerated by the digital logic because of the thresholding and regenerative properties of the logic elements.[5] 
     On the other hand, an optical receiver is intended to convert relatively small optical signals into full-logic-level signals, suitable for further processing by the digital logic that may be surrounding it. Necessarily then, the optical receiver would also be sensitive to these interfering nearby digital logic sources. Such interference may corrupt the received signal and it is therefore desirable to shield the receiver, as much as possible, from these sources. Even in the absence of digital processing circuitry, the receiver may be subject to interference from neighboring receivers, particularly if such receivers are integrated together on the same semiconductor substrate. Such interference from neighboring receiver elements is often referred to as ‘crosstalk’. 
     In accordance with the present invention, we have recognized that if a received optical signal could be made available as a complementary optical signal, a differential optical signal receiver can be designed to reject the interference arising from the various electronic sources. In particular, our differential optical signal receiver includes an interference-rejecting element, which greatly reduces the interference arising from electrical sources present nearby the receiver. This receiver is resistant to interference from digital logic sources and crosstalk from adjacent receivers, particularly noise coupled from power supply and ground lines. While the interference-rejecting stage of our receiver performs this function, to fully take advantage of this stage, it was necessary to re-design other stages of the receiver. 
     With reference to FIG. 2 we describe a block diagram of our differential optical signal receiver. As shown, complementary optical input signals are applied to detectors  201  and  202 . The resulting differential electrical signals are amplified by pre-amplifiers  203  and  204 , respectively and applied to the interference rejecting element or stage  205 . The pre-amplifiers  203  and  204  are each single-ended transimpedance amplifiers which convert an input current from the respective detector diode,  301  and  302 , to a voltage signal for input to Q 1  and Q 3 , respectively. If required a post-amplifier  206  and channel filter  207  are utilized. The differential signal is then digitized in decision circuit  208  to obtain the differential digital outputs. 
     Our interference/crosstalk rejecting optical receiver is based on the principles of differential amplifiers, which reject common-mode signals. These principles are well described in various textbooks.[4] To reject signal variations imposed by supply noise, it is therefore desirable to cause those variations to appear in the common-mode of a differential amplifier, and thereby be rejected in the signal output of the amplifier, when that output is taken as the difference between the complementary outputs of the amplifier. For common-mode rejection, it is particularly important that the response of the two sides of the differential amplifier be properly balanced. Thus, for example, the commonly used practice of biasing the load elements of a CMOS differential amplifier with a current mirror technique is inappropriate here and results in poor interference rejection in simulation.[4] 
     To take advantage of the differential amplifier, it is necessary to provide signals to both sides of the amplifier. It is further necessary to properly bias the amplifier. If the receiver is used for processing digital information without special coding, it is desirable that this biasing be obtained without ac-coupling between stages of the receiver. 
     With reference to FIG. 3, the above goals are achieved by feeding the inputs of the differential amplifier  305  from two (optional) pre-amplifiers,  303  and  304 , each of which is driven by an input photodiode,  301  and  302 , that is, in turn, driven by a differential pair of optical input data signals. If the input preamplifiers  303  and  304  are located nearby one another, they will experience similar levels of electrical interference, which will be similarly amplified and rejected by the interference-rejecting differential amplifier stage  305 . This crosstalk-reduction stage  305  gives rise to a differential electrical signal, which is known to be more robust against interference than a single-ended electrical signal. 
     The operation of crosstalk-reduction stage  305  is as follows. The input Field Effect Transistors (FETS) Q 1  and Q 3  share a common source impedance, FET Q 5 . Note a FET is assumed to have a negative gate and a positive gated FET is denoted PFET. The FETs may be fabricated using Complementary Metal Oxide Semiconductor (CMOS) technology or using other comparable Very Large Scale Integrated (VLSI) circuit technology. 
     In response to differential input signals applied to the gates of Q 1  and Q 3 , the FET Q 5  exhibits a very low small signal impedance. As a result a significant differential signal current flows in Q 1  and Q 3 , producing a substantial differential output voltage (across the OUT and OUT bar leads) at their respective load impedances. Since it is difficult to form resistors in CMOS technology, a diode impedance (formed by the PFETs Q 2  and Q 5  connected as diodes) is utilized. Thus, crosstalk reduction stage  305  produces a substantial gain to differential input signals. 
     However, crosstalk-reduction stage  305  also provides significant attenuation to common mode input signals to Q 1  and Q 3 . Significant attenuation in stage  305  also occurs to any commonly inducted or coupled signals caused by supply voltage or ground lead based noise or interference signals. This reduction occurs because when a common mode signal is applied to Q 1  and Q 3 , the common source impedance, FET Q 5 , is substantial, resulting in a greatly diminished common mode current flow in Q 1  and Q 3 . The result is that almost no output common mode signal develops across the load diodes Q 2  and Q 4 . Moreover, since the output is taken as a differential voltage (across the OUT and OUT bar leads), there is almost no differential output signal from stage  305  caused by a common mode input signal or any other commonly induced or coupled signal. 
     For the same reason, it is desirable to maintain differential electrical signals and circuits for the remainder of the receiver processing, e.g., in the post-amplifier, channel filter and decision circuits of FIG.  2 . The implementation of these circuits may utilize the same differential circuit techniques described above. 
     As already noted, the interference-rejecting properties of this receiver are most effective when the pre-amplifiers are physically close e.g., integrated together) to one another, so that they may experience substantially the same interference signals. This is because the interference rejection is perfect only if the interference experienced by the two photo-diode/preamp combinations,  301 / 303  and  302 / 304 , are exactly the same. Moreover, if the preamplifiers,  303  and  304 , are combined with the differential amplifier,  305 , the rejection property may be expected to further improve. Note, if photodiodes  301  and  302  are implemented using a incompatible technology (e.g., Gallium Arsenide GaAs), they cannot be readily integrated together with the CMOS technology of the preamplifier  303  and  304  and differential amplifier  305  circuits. However, a photodiodes  301  and  302  chip can be mounted (e.g.,by flip-chip bonding) on a hybrid chip together with a CMOS chip. 
     An alternative crosstalk-reducing element is a cascode amplifier configuration although it is has been found to be less effective than the differential amplifier element. Such a cascode amplifier configuration is shown in FIG.  4 . The particular type of interference that is rejected by this cascode element  401  are spurious signals on the supply and ground leads of the amplifier. The interference rejecting properties of the cascode element  401  are not immediately apparent, and derive primarily from its biasing technique. Some bias must be applied to the gate of the cascode FET (Vc in the FIG. 4 ). If this bias is derived from the same supply, VDD, and ground voltage used by the rest of the circuit, it would contains the same variations (e.g., interference signals) as those present on supply and ground. As a result this type of biasing would provide some interference cancellation in the circuit. As a result it provides a ‘screening’ effect on the output signal. Similar supply noise rejection effects are obtained when the load element (the PFET biased with Vb as shown in FIG. 4) is considered. The cascode circuit  401  may be substituted for both of the diode load impedances Q 2  and Q 4  of differential amplifier,  305 , of FIG. 3 to provide further rejection of spurious signals that exist on the supply, VDD, and ground, GND, voltages. 
     With reference to FIG. 5 there is shown a preferred embodiment of the present invention where the separate transimpedance pre-amplifiers,  303  and  304 , and the differential electrical amplifier,  305 , of FIG. 3 are integrated together into one fully differential optical signal receiver circuit. Mismatches in the performance of these two single-ended pre-amplifiers,  303  and  304 , can degrade the common mode rejection properties of the differential amplifier  305 . For example, supply noise might be amplified unequally by the two single-ended pre-amplifiers,  303  and  304 , and thereby give rise to an interference signal present in the differential outputs. By combining the transimpedance amplifiers,  303  and  304 , with the differential amplifier  305 , better circuit matching characteristics are obtained and the susceptibility to noise and interference is further reduced. 
     Differential optical signals  501  and  502  are incident on a differential optical detector formed by the two photodetectors  503  and  504 , respectively, which are connected to +Vdet. The FETs Q 1 -Q 5  define a differential amplifier, with Q 1  and Q 2  as the input circuits, Q 3  and Q 4  the respective load impedances and Q 5  as the common source impedance (current source). The FETs Q 9  and Q 10  acting as feedback elements to form a differential transimpedance receiver out of differential amplifier Q 1  and Q 2  which receives the input differential photocurrent signals from photodetectors  503  and  504 . The voltages Vtune 1  and Vtune 2  are used to adjust the transimpedance levels of Q 9  and Q 10 . The photocurrent signals are amplified by the differential transimpedance receiver (also referred to as a differential electric circuit) and differential voltage outputs are provided at Out and Out(bar). To provide stable biasing of the amplifier load elements, Q 6 -Q 8  form a replica biasing network for loads Q 3  and Q 4 . This insures that noise and interference signals on the supply and ground leads are further canceled in the circuits. 
     It should be noted that our differential optical signal receiver of  5  may operate with either a received asynchronous or synchronous differential input optical signal. Our illustrative examples employ asynchronous amplifiers, that is, they will amplify signals regardless of their timing relationship (i.e., they can be synchronous or asynchronous) without requiring a clock signal. Clocked amplifiers can also be employed by clocking various elements of the amplifier in a well known manner. 
     Thus, what has been described is merely illustrative of the application of the principles of the present invention. Other arrangements and methods can be implemented by those skilled in the art without departing from the spirit and scope of the present invention. 
     Appendix 
     References 
     [1] Bakoglu, H. B. Circuits,  Interconnections, and Packaging for VLSI . Reading MA: Addison-Wesley; 1990; ISBN: 0-201-06008-6. 
     [2] Krishnamoorthy, A. V, , Miller, D. A. B, “Scaling Optoelectronic-VLSI Circuits into the 21st Century: A Technology Roadmap”,  IEEE J. Spec. Topics in Quant. Electr ., v.2, no.1, pp. 55-76, 1996. 
     [3] Lentine, A. L., et. al., “Optoelectronic VLSI Switching Chip with Greater than 1 Terabit per second potential I/O Bandwidth”  Electronics Left . v. 33 pp 894-895, 1997. 
     [4] Sedra, A. S.; Smith, K. C.;  Microelectronic Circuits : Harcourt Brace, 1991 ISBN 0-03-05-1648-X. 
     [5] Weste, Neil H. E.; Eshraghian, Kamran.  Principles of CMOS VLSI Design: A Systems Perspective , 2 nd. Edition . Reading, MA: Addison-Wesley; 1993; ISBN: 0-201-53376-6. 
     [6] Williams, G. Lightwave Receivers.  Topics in Lightwave Systems . Li, Tingye ed. : Academic Press; 1991: 79-148. ISBN: 0-12-447302-4. 
     [7] Woodward, T. K., U.S. Pat. No. 5,644,418 “Smart Pixel Optical Receiver Employing Sense Amplifier and Method of Operation Thereof”.