Abstract:
An optical receiver, for use for instance in an analog optical communications system such as a return path in cable television, operates over a wide range of input power to its photodetector. This is accomplished by providing in the optical receiver a series of RF amplifier stages where at least one of the stages includes a fixed attenuator and a parallel amplifier with switches suitably connected so that, at any one time, only one of the fixed attenuator or the amplifier is in the signal path. The switches are controlled so that the dynamic range of the optical receiver is improved in order to make gain reduction more accurate. Hence by redirecting the RF signal path to either the attenuator or amplifier of the amplifier stage, excessive noise and distortion by the amplifier stage are eliminated or reduced.

Description:
FIELD OF THE INVENTION 
   This disclosure relates to optical communications and especially to RF amplification in analog optical communications. 
   BACKGROUND OF THE INVENTION 
   The disclosure is directed to extending the dynamic range of analog optical receivers of the type used in Hybrid (optical) Fiber/Coax (HFC) return path systems such as used in cable television. Due to system characteristics, the optical receivers in such systems often face a large variation in the optical input power requirement, that results in an even larger variation in the RF gain requirement. The conventional solution is to add attenuation to reduce the excessive gain. This is a simple and straightforward approach and works well when the needed attenuation is relatively small. However, when the amount of needed attenuation is close to the gain of the amplifier, the added attenuation produces unwanted noise and distortion. 
   In a HFC system such as a cable television or other communications network, the “forward path” (head end to subscriber) media (optical fiber and coaxial cable) carries modulated video, data, and cable telephony signals. In the return path (subscriber to head end), the media carries mainly data signals in the QPSK or QAM modulation format and frequency division multiplexed within the frequency bandwidth of interest, e.g., 5 to 42 MHz. The return path data signals originate as electrical signals from cable modems at various subscriber locations, are combined at an optical node, and transmitted therefrom by an optical transmitter. The corresponding optical receiver converts the transmitted optical signal into an electrical RF (Radio Frequency) signal and then amplifies the RF signal to the desired output level. The RF signal is then combined with other optical receiver RF output signals and transmitted to a CMTS (Cable Modem Termination System) input and ultimately to the system head end. 
   Because of the nature of a HFC system, the optical receiver often faces a large variation of optical input power depending on the optical transmitter optical output power, link loss, the number of receivers combining signals at the CMTS input, and the CMTS input power requirement. The optical power received by an optical receiver can be as low as −15 dBm and as high as +2 dBm. For every 1 dB of optical power variation, the corresponding RF signal varies by two dB due to the optical receiver&#39;s optical to electrical conversion process in its photodetector. For optical power varying 17 dB (from −15 dBm to +2 dBm) the RF signal will vary by 34 dB. This is a very large gain variation. Therefore the optical receiver must be capable of providing RF signal gain over a broad range. In addition, sometimes a certain degree of optical AGC (Automatic Gain Control) is desirable in order to maintain the signal stability and additional gain must be preserved to allow the AGC to take effect. This adds further gain variations. 
   In a HFC network return path, a key system performance parameter is the NPR (Noise Power Ratio) dynamic range. NPR is the ratio of carrier magnitude to unwanted noise magnitude. For a return path to operate seamlessly, the system NPR must meet a minimum requirement, otherwise errors occur and system transmission speed slows down. If the NPR is plotted on a y-axis of a graph and the system input or output power is plotted on the graph x-axis, the resulting curve usually has a reverse V shape. The dynamic range is the dB difference between the two input or output power points at which the NPR is identical. 
   Dynamic range thus is essentially the “headroom” at which the return path operates. The headroom is required because in the return path there are various unpredictable sources of noise interference such as impulse noise and ingress (external) noise. The larger the dynamic range, the better is the system performance in dealing with unpredictable noise interference. The dynamic range defined here is that in which the low end of carrier performance is limited by CNR (carrier to noise ratio) and the high end is limited by second or third order distortion. On NPR plots, the noise contribution to the left side of the reverse V shape curve is thermal noise and the noise contribution to the right side of the curve is distortion noise. The distortion noise is due to system or device non-linearity. If the carrier signal is a CW (continuous wave) tone, the distortions are manifested as second and third order harmonics. Since the return path signal carriers are in the form of QPSK and QAM, their power spectral densities are alike in terms of thermal noise and therefore its harmonics are also alike in terms of thermal noise. 
   In a typical system, the NPR is predominately the optical transmitter NPR in the middle of the optical power range received by the associated optical receiver. However, at very low and very high optical power, the optical receiver&#39;s NPR dominates the system&#39;s NPR. At very low input optical power, the optical receiver thermal noise contributes the majority of the noise, and at very high optical input power the optical receiver distortion noise contributes majority of the noise. In the optical receiver, noise contributions in terms of components are from the photo-detector and RF amplifiers. Commercially available analog photodetectors are well behaved in both respects such that additional improvement in this area requires substantial component cost increase with little or no improvement of overall system performance. RF amplifiers, on the other hand, are available with various performance tradeoffs between gain, noise figure, distortion and power consumption. Generally speaking, one skilled in this field will have little difficult choosing the amplifiers having the best tradeoffs. What remains is the gain and attenuation approach to obtain optimum performance for both extreme (high and low) power conditions. 
   There are known approaches to solve this in the field of digital communications where the signals are base band signals operating between two (digital) logic states. These approaches are not applicable to analog optical receivers as in cable television systems, because many performance requirements critical to analog optical receivers are not critical to digital optical receivers. For example, transimpedance amplifiers used in digital communications inherently have low performance in terms of distortion. Also, impedance matching and frequency response are not critical in digital communications but important in an analog receiver. Therefore in analog systems in order to support large power variations of the gain portions for an optical receiver, matched attenuators are connected between the RF amplifier stages, see  FIG. 1 . 
     FIG. 1  shows a receiver the primary optical element of which is photodetector (phototransistor or photodiode) D 1   12 . This is conventionally arranged so that it receives light, as indicated, from e.g. an optical fiber, via a lens (not shown). Diode  12  is coupled to a voltage source V with filter capacitor C 1   14  and the diode  12  output current is coupled to a transformer T 1   16 . Blocking capacitor C 2   18  couples the output RF signal from the transformer T 1  (but blocks any D.C. signal) to the first of a series of amplifier stages which in this case includes an RF amplifier A 1   20  and a variable attenuator P 1   22 . Each of the two subsequent stages respectively also includes an amplifier and variable attenuator  24 ,  26 ;  28 ,  30 . The attenuators provide a constant impedance (at any one setting). The final stage includes only the amplifier A 4   32  providing the RF output signal. The variable attenuators  22 ,  26 ,  30  are each controlled, via its control terminal, by a control circuit  36  which senses the level of output power from photo-detector D 1  across resistor R 1   34 . 
   The number of amplifiers used depends on the gain required and the gain of each amplifier. In this configuration, the noise performance of amplifier A 1  makes the most noise contribution while amplifier A 4  makes the most distortion contribution. The operating points for amplifiers A 1  and A 4  are set by the system requirements and after the particular amplifiers are chosen, a base line for the achievable maximum dynamic range is set. The dynamic range performance further degrades when the inter-stage amplifiers&#39; (A 2 , A 3 ) noise and distortion contribution become significant. Tradeoffs must be considered in order to minimize the contribution by the inter-stage amplifiers. When the RF signals vary in strength as much as 30 dB, the inter-stage amplifier contribution will unavoidably be significant. Generally speaking, more stages allow better thermal noise and distortion performance tradeoffs. In the case of low optical input power, the attenuation is allocated to attenuators P 2  and P 3  with zero attenuation at attenuator P 1  in order to minimize the noise contribution by amplifier A 2 . In the case of high optical input power, the attenuation is distributed among the three attenuators P 1 , P 2 , P 3  where attenuator P 3  has the lowest attenuation in order to minimize the distortion contribution by amplifier A 3 . 
   In order to illustrate this, assume the amplifiers in  FIG. 1  are all identical and each has a gain of 15 dB. Further assume that for a given set of conditions, when optical input power is −15 dBm, all attenuators P 1 , P 2 , P 3  must be set to 0 dB attenuation to achieve required RF output power. If the same RF output power must be maintained when the optical input power is changed to 0 dBm, then 30 dB attenuation is required in the signal path. If the 30 dB attenuation is evenly distributed among attenuators P 1  and P 3  (that is, 15 dB each) while the attenuation of attenuator P 2  is set to zero, then amplifier A 2  will make exactly the same thermal noise contribution as does amplifier A 1 , since the input power to these two amplifiers is identical. 
   By the same token, amplifier A 3  will make exactly the same distortion contribution as amplifier A 4 , since the output power of these two amplifiers is identical. When this occurs, the thermal noise degradation is 3 dB while second order distortion degradation is 3 dB and third order distortion degradation is 6 dB. These are significant degradations for receiver performance and when the receiver is used in a system, the overall system NPR performance will be degraded. In this example, a 3 dB receiver thermal noise degradation may not be a significant degradation for the system since the transmitter thermal noise is still dominant, but 6 dB receiver distortion degradation can be significant. If the trade off is made to reduce the distortion but increase thermal noise, then the thermal noise contribution by the receiver will take effect. Of course, if the attenuation is distributed among all three attenuators, the degradation will be reduced but this will not be significant, and so this requires a more elaborate attenuation scheme. 
   SUMMARY 
   This disclosure is directed to improving the dynamic range of an optical receiver, of the type used in analog optical communications, by redirecting the RF signal path to reduce the gain instead of adding attenuation, and thereby switchably leaving an unused RF amplifier in the signal path. The redirection is accomplished in one embodiment by connecting a pair of single pole double throw (SPDT) switches at both the input and output terminals of the amplifier. The RF signal path is redirected when the reduction of gain is close to the gain of the amplifier. By directing the RF signal path, excessive noise and distortion due to unwanted gain and loss are eliminated and consequently, the NPR (Noise Power Ratio) dynamic range is increased. The switches are implemented in the form of e.g., a relay, or PIN or Schottky diode pairs, or other types of switches. 
   The signal path redirection can be done dynamically, when the return path is in normal operation. The redirection is more commonly used when the cable TV system is first set up, and is especially useful then since it allows use of one type of amplifier in the return path at various system locations, in spite of system-caused variations in optical power. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows schematically an optical receiver with variable attenuators. 
       FIG. 2  shows schematically an optical receiver in accordance with the invention with switched stages. 
       FIG. 3  shows schematically detail of one of the switches of  FIG. 2 . 
       FIG. 4  is a graph of performance of the  FIGS. 1 and 2  receivers. 
       FIGS. 5 and 6  show variations of the  FIG. 2  optical receiver. 
   

   DETAILED DESCRIPTION 
     FIG. 2  illustrates the present optical receiver that includes elements mostly similar to those of  FIG. 1 , and which are similarly labeled. However, in  FIG. 2  the second amplifier stage includes a parallel arranged fixed attenuator  40  which can be coupled to the output terminal of attenuator  22  via switch S 1   42 . In its upper position, switch S 1  couples fixed attenuator  40  into the signal path; in its lower position switch S 1  couples in amplifier A 2 . Attenuator  40  is, e.g., conventionally a “T” of three resistors or an equalizer (R, L, C circuit) providing a constant impedance or a similar circuit providing attenuation. Similarly, switch S 2   44  operates in conjunction with switch S 1  to couple either attenuator  40  or amplifier A 2  in the signal path. Thus at any one time either attenuator  40  or amplifier A 2  is connected in the signal path. The second amplifier stage including switches S 3 , S 4  (respectively  48  and  50 ) operates similarly. As shown, switches S 1  and S 2  are single pole double throw switches as are switches S 3  and S 4  and all the switches are controlled by the control circuit  54  as described further below. The redirected signal path thereby is simply a straight through connection or one with a fixed attenuation to make gain reduction more accurate. For example, if the RF amplifier stages have an actual gain of 14.7 dB while 15 dB is more desirable, a 0.3 dB fixed attenuator  40  is provided in the redirected signal path to achieve the gain reduction of 15 dB. The optical receiver is constructed on, e.g., a printed circuit board using conventional microstrip connections to carry the RF signals. 
   In one variation of the  FIG. 2  receiver, terminations are connected to amplifiers A 2 , A 3  when they are switched out of the signal path. In another variation, amplifiers A 2 , A 3  are turned off (powered down) when they are switched out of the signal path. 
     FIG. 3  shows in detail an exemplary circuit for, for instance, any one of switches S 1 , S 2 , S 3 , and S 4 .  FIG. 3  illustrates a toggle switch. A common terminal (COM) is common to terminals A and B. Terminal Com corresponds in  FIG. 2  to the connection between switch S 1  and attenuator P 1 . Terminals A and B correspond respectively in  FIG. 2  to the connections between switch S 1  and attenuator  40  and switch S 1  and amplifier A 2 . As shown in  FIG. 3 , two parallel connected PIN diodes  31 ,  33  are respectively connected to terminals A and B via filter capacitors  37  and  39 . A bipolar transistor  41  has its base (control) electrode connected via resistor  43  to the control terminal CTL which is connected to the control circuit  54 , its emitter connected to ground, and its collector connected (via resistor  51 ) to a voltage source VCC. The transistor  41  collector in turn is connected (via inductor  49 ) to one terminal of diode  33 . Filter capacitors  45  and  47  are also provided, as are inductors  49  and  53  and resistor  55 . This is merely one embodiment of a suitable switch, for instance S 1 , of  FIG. 2 . The switch works as follows: when a logic high voltage is applied to the control terminal CTL, the connection between terminals Corn and A is closed but the switch connection between terminals Corn and B is open. When a logic low voltage is applied to terminal CTL, the switch reverses, connecting terminals Corn and B. 
   Thus in addition to controlling variable attenuators  22 ,  30  as in  FIG. 1 , control circuit  54  controls switches S 1 , S 2  and S 3 , S 4  responsive to the voltage drop sensed across resistor R 1 , which voltage is proportional to the input optical power to photodiode D 1 . In one embodiment, control circuit  54  includes two comparators each with one input terminal connected to resistor R 1 ; one comparator is set to compare the input signal to a voltage equivalent to 15 dB and the second to 30 dB, as disclosed above. The output signal of the first comparator controls (via suitable drivers) switches S 1 , S 2  and the second comparator controls switches S 3 , S 4 . 
   By thereby redirecting the signal path using the switches, the excessive noise and distortion contribution of amplifiers A 2  and A 3  of  FIG. 2  are eliminated. In contrast, in  FIG. 1  the large attenuation eliminates the function of the inter-stage amplifiers but undesirably leaves their unwanted noise and distortion in the signal path and thereby degrades the NPR dynamic range performance. 
   In the corresponding plot of  FIG. 4 , the horizontal axis shows output power density (dBM V/Hz) with the vertical axis showing noise power ratio (NPR) in dB. This plot compares the NPR comparison for the  FIG. 1  approach at curves  1  and  2  (at two different attenuation setting points) and that of the  FIG. 2  approach at curve  3 . The receivers are in the operating condition as in  FIGS. 1 and 2  where all the amplifiers have a gain of 15 dB. The photodetector and optical transmitter are the same for all curves. Finally, the receivers are all set to the same system gain. Curve  1  is the NPR of the  FIG. 1  receiver where attenuators P 1  and P 3  are set to 15 dB attenuation and attenuator P 2  is set to 0 dB. Curve  2  is also the NPR of the  FIG. 1  implementation but where attenuator P 1  is set to 30 dB and attenuator P 2  and P 3  are set to 0 dB. Curve  3  is for the  FIG. 2  receiver. It can be seen that curve  1  has the same NPR at its left side as curve  3 , but the right side of the NPR is much worse than that of the curve  3 . The dynamic range is about 7 dB worse at an NPR of 42 dB NPR. This is the condition as discussed above. The 3 dB receiver thermal noise degradation has not yet degraded the system performance since the transmitter thermal noise is still dominant. However, the distortion degradation, 3 dB by the second order and 6 dB by the third order, have degraded system performance significantly. For example, the NPR is degraded in such a way that the dynamic range is 7 dB worse at an NPR of 42 dB. Curve  2  is the other side of the extreme where receiver distortion degradation has no impact on the system but receiver thermal noise degradation becomes dominant. It can be seen that the dynamic range is about 3 dB worse at 42 dB NPR. This comparison illustrates that no matter how well one arranges the attenuation in the  FIG. 1  receiver, the  FIG. 2  receiver always can be controlled to give better performance. 
     FIG. 2  is one possible implementation of the present optical receiver where optical AGC is required. The control circuit  54  performs the following functions; 1) detecting the input optical power, 2) comparing the detected optical power to one or two sets of thresholds and determining if redirection of the signal path is required, 3) providing a set of driving circuits that driving the variable attenuators, and 4) providing logic to make the tradeoff between the attenuation and signal redirection. The control circuit has a set of logic elements that has two inputs connected to the outputs of two comparators corresponding to 15 dB and 30 dB respectively as described above and two outputs that once the signal is redirected (the switches have changed positions) reset the attenuator to 0 dB. For example, the optical power is initially at the 14.5 dB point. The first comparator is activated since the threshold is the 15 dB point. The attenuation driving circuit sets the total attenuation at 14.5 dB. Now the optical power is increased to the 15 dB point, the first comparator is activated and causes the switch to change its position and also activate the logic circuit to reset the attenuator to have the total attenuation to be 0 dB. If the optical signal continues to increase, the attenuator driving circuit will drive the attenuators accordingly until the 30 dB point is reached and the second comparator is activated and changes the second switch&#39;s position and causes the logic circuit to reset the attenuator to 0 dB again, giving a total of 30 dB of less gain, not through the attenuation but by the signal redirection. 
     FIG. 5  is another embodiment of the present optical receiver where AGC is not required but the attenuation is set manually by the user. In the  FIG. 5  optical receiver, most elements are identical to those of  FIG. 2  and are similarly labeled except that in  FIG. 5  the control circuit  56  is controlled (trimmed) by a user interface, including for instance variable resistor R 1   58 , rather than electrically sensing the power output by the photodiode  12 . The user here typically would observe (using suitable instrumentation) the output RF signal from amplifier A 4  and thereby set the desired output power level. 
     FIG. 6  is an embodiment of the present optical receiver where a combination of optical AGC and user set up is provided.  FIG. 6  is largely similar to  FIG. 2  and has most of the same elements identically labeled except that here substituted for the control circuit  54  of  FIG. 2  there is microcontroller  62  controlled via a user interface  64 . The program (firmware) executed by microcontroller  62  carries out the functions described above for control circuit  54  and the firmware is readily coded in light of this disclosure. In this case a suitable analog to digital converter (not shown) is connected between resistor R 1  and the input terminals of the microcontroller, and suitable digital to analog converters and drivers (not shown) are connected between each of the output terminals of the microcontroller and the respective control terminals of attenuators P 1 , P 2  and switches S 1 , S 2  and S 3 , S 4 . Is it to be understood that microcontroller  62  and user interface  64  need not be dedicated to control of the optical receiver but can also accomplish other tasks, in terms of controlling other circuitry associated with the optical receiver. 
   This disclosure is illustrative but not limiting; further modifications will be apparent to one skilled in the art in light of this disclosure and are intended to fall within the scope of the appended claims.