Abstract:
A monitoring system useful for monitoring the state of a fiber communication channel in which a portion of the fiber signal is tapped and detected by a photodiode. The dark current of the photodiode and other DC offsets of the system may be compensated by passing a known signal through the photodiode and extracting the component due to the known signal. The known signal may be a locally generated optical signal having a predetermined harmonic signature. Optionally, the fiber signal may be selectively attenuated before being combined with the known signal so only the known signal is detected. The known signal may also be a dark signal induced by selectively attenuating the fiber signal. The system may include a temperature sensor which allows the compensation to be performed against stored temperature dependent compensation values while the fiber signal is not attenuated.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to optical detectors. In particular, the invention relates to optical detection systems used in digital optical communication systems. 
     BACKGROUND ART 
     Communication networks, particularly those extending over long distances, are increasingly being implemented in optical fiber. At the transmitting end, a digital signal is impressed upon a laser light source typically outputting an optical signal in the 1.3 or 1.7 μm wavelength band. The optical fiber, for example, a single-mode silica fiber waveguide, carries the digitally modulated optical carrier over long distances with little distortion. At the receiving end, an optical detector such as a photodiode receives the optical signal and outputs an electrical signal corresponding to data signal impressed on the transmitting end. Data rates are limited primarily by the electronics at either end. Data rates of 40 gigabits per second (Gbs) are possible with a single optical carrier although some fielded systems operate at no more than about 50 or 100 megabits per second (Mbs). 
     Although the optical loss in the optical fiber is relatively low, allowing transmission over tens to even hundreds of kilometers, eventually the signal attenuates to the point that it requires some sort of amplification. Amplification is also required because optical signals are sometimes split between multiple paths or the amplitude of the optical signal needs to be increased for detection or other purposes. One possible type of amplifier is a regenerator which detects the optical signal and uses the resultant electrical signal to control an optical transmitter which brings the optical signal back to an unattenuated level. However, in a more convenient and typically less expensive approach, an optical amplifier amplifies the optical carrier signal including any data modulation impressed upon it. Optical amplifiers are particularly advantageous in wavelength-division multiplexed systems in which a single fiber carries multiple optical carrier signals of slightly different respective wavelengths modulated with multiple respective data signals. A single well designed optical amplifier can amplify all of the wavelength channels while a separate regenerator needs to be provided for each wavelength channel. 
     The most prevalent optical amplifier is an erbium-doped fiber amplifier (EDFA), which has a fairly flat gain bandwidth of about 40 nm, which is sufficient for many WDM networks. However, optical amplifiers tend to be sensitive to variations in the average power levels on the fiber and to noise spikes. For these reasons, a robust network often includes a monitoring system located at an optical amplifier that determines the average power level and other physical parameters of the optical signals. Such a monitor taps a small amount of optical power from the signal on the cable and measures its intensity at a sampling rate of typically kilohertz to megahertz, far below the gigahertz rate of data signals. 
     Typically, the light is detected by a photodiode included in the monitoring system. However, photodiodes present some difficulties in measuring optical intensity to the significant resolution desired for the monitoring system. A photodiode is a light-sensitive electrical device, specifically a semiconductor diode which is negatively biased so that the photodiode passes very little current in the absence of light. The current in the absence of light is called the dark current. However, when the reversed biased photodiode is irradiated with light, such as that received from the optical fiber, the light generates electron-hole pairs, which are detected by the electronic circuitry as a current signal. In the presence of light, the signal includes both the photocurrent resulting from the incident light and dark current, of which the dark current is independent of the data signal. Because the photocurrent is being used to monitor the fiber and its value is not fixed, the dark current represents noise signal detracting from the accuracy of the measurement even though the noise signal varies only slowly with time. 
     Often the dark current can be reduced by the use of better components. However, better components may be excessively expensive, or the system performance is pushed even further so that dark current again becomes a problem. An alternative approach attempts to compensate for dark current and other similar sources of noise by determining their magnitudes and removing their contributions from the measured signal. Of course, in a commercial system, compensation must be accomplished without unduly complicating the system or excessively increasing its cost. Accurate power measurements when the dark current is of the same order of magnitude as the photocurrent requires compensation for the dark current. The typical tradeoffs are usually between the magnitude of the dark current, sensitivity of the photodiode, and the overall speed of the circuit. 
     Dark current is known to strongly depend upon the temperature of the photodiode. For this and other reasons, the photodiode is usually maintained at a fairly constant temperature. But temperature can be controlled only to a limited degree in a fielded commercial environment, and some temperature variations must be accepted. Furthermore, the photodiode is subject to some uncontrollable aging effects which may affect the dark current. Particularly the temperature variations limit the sensitivity of the system. 
     For these reasons, it is desirable to provide dynamic means for compensating the dark current of a photodiode, that is, to periodically, over periods of minutes to hours, readjust the detection circuitry to account for the change in dark current. 
     SUMMARY OF THE INVENTION 
     A system and associated method determine the effect of photodiode dark current or other DC offsets in a monitoring system associated with an optical communication channel which uses a photodiode detecting a signal tapped from the channel to determine characteristics of the channel and its signal, for example, average signal power or noise spikes. A known signal is passed through the photodiode, and its detected value is extracted to allow the photodiode or other electrical circuitry to be characterized. 
     The known signal may be a locally generated repetitive signal with a known harmonic signature or characteristic and is combined with the tapped signal. Signal processing including harmonic analysis allows the detected signal to be separated into the channel component and the component arising from the known signal. 
     Optionally, the tapped signal may be selectively attenuated so that during the attenuation period mostly the known signal is being detected. 
     Alternatively, the tapped signal is nearly completely attenuated during a system characterization period so that the known signal corresponds to a dark signal being received. No locally generated harmonic signal is required. 
     Advantageously, the system includes a temperature sensor and storage means for storing system response values at a number of temperatures while the tapped signal is attenuated. When the tapped signal is not so attenuated, the temperature is measured and the one or more of the stored system response values is used to correct the system response value according to the measured temperature. The system can be calibrated while the tapped signal is attenuated by controllably heating the system and recording the system response associated with the temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an optical detection circuit. 
     FIG. 2 is a schematic diagram of a first embodiment of a monitoring system of the invention including digitally synthesized characterizing signal enabling the extraction of photodiode dark currents. 
     FIG. 3 is a schematic diagram of a second embodiment of the invention additionally including a controllable optical absorber. 
     FIG. 4 is a schematic diagram of a third embodiment of the invention which is simplified from that of FIG.  3 . 
     FIG. 5 is a schematic diagram of a fourth embodiment of the invention which additionally includes a temperature sensor enabling a temperature calibration of the system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A typical photodetection system is illustrated in the schematic diagram of FIG.  1 . Light  10  received, for example, from an optical fiber, is incident upon a photodiode  12 , which produces a detector current I PD . A transimpedance amplifier (TZA)  14  receives the detector current I PD  and converts it to a voltage signal V TZA , which is processed by the detection circuitry including that of the invention. The transimpedance amplifier  14  also applies a DC bias voltage to the photodiode  12  to keep it negatively biased. The detector current I PD  includes two components 
     
       
           I   PD   =I   photo   +I   dark   (1) 
       
     
     where I photo  is the photocurrent induced by the light  10  incident on the photodiode  14  and I dark  is the dark current resulting from the I-V characteristics of the biased diode  14 . These two terms may in turn be respectively represented by                I   photo     =         P   recvd          (   ω   )            e   ℏω          (           η   DC     -     η   AC           1   +       (     ω   /     ω   D       )     2           +     η   AC       )               (   2   )                                
     and                I   dark     =         I   S          (                                 V   D       kT       -   1     )       +     I   leakage               (   3   )                                
     where P recvd  is the received optical power, ω is the radial frequency of the light, ω D  is the 3dB power point of the device dictated by the carrier diffusion frequency, e is the electronic charge, h is Planck&#39;s constant, k is Boltzmann&#39;s constant, η DC  is the DC electro-optic quantum efficiency, ω AC  is the AC electro-optic quantum efficiency, and V D  is the bias voltage applied to the photodiode. 
     The dark current I dark  is always present even if no light is irradiating the photodiode. Although the dark current is substantially a DC signal, its value is uncertain and tends to vary slowing with time, for example, as the photodiode temperature changes. On the other hand, the photocurrent I photo  in a well designed system is proportional to the intensity of the light incident on the photodiode, which is itself proportional to the received optical power P recvd . 
     A first embodiment of the invention illustrated in the schematic diagram of FIG. 2 extends the usable range of a photodiode by continuously measuring the dark current of the photodiode. In a typical optical communications network, digitally coded optical signal propagates on a main optical fiber  20 . A tap  22  coupled to the fiber  20  taps a portion, typically a small percentage, of the optical energy onto an optical fiber  24  inputting to a monitoring system  26  of the invention. An optical isolator  28  prevents signals from the monitoring system  26  from propagating back onto the main fiber  20  and thereby introducing perturbations on the propagating signal. An optical coupler  30  combines the optical signal input into the receiver with an internally generated local characterizing signal. A photodiode  32  converts the combined optical signal to an electrical current. A transimpedance amplifier  34  both amplifies the electrical signal and converts the current signal to an analog voltage signal. An A/D converter  36  samples the analog electrical signal output from the amplifier  34  and converts it to a multi-bit digital signal, the value of which represents an amplitude of the detected optical signal. The number of bits in the digital signal determines the dynamic range of the monitoring system  26 . The photodiode  32 , transimpedance amplifier  34 , and A/D converter  36  are all part of a conventional monitoring system. The sampling rate of the A/D converter  36  is substantially lower than the data rate on the fiber  20 , for example, by a factor of at least 100 and preferably at least 1000, so that the data signal is lost upon A/D conversion if not before. 
     A signal processing engine  38  receives the digitized signal from the A/D converter  36 . The signal processing engine  38  is typically a microprocessor-based system conventionally used to perform measurements of the tapped optical signal, as well as to control the timing of the measurements and sequencing of the measurement devices. Its output is supplied to an interface to a system controller at the communications node. The system controller uses the information provided by the monitoring system to adjust the operation of the communication node, to communicate system performance to other parts of the communications network, and to notify the need for maintenance if necessary. 
     According to the invention, the capabilities of the signal processing engine  38  are expanded. It controls a direct digital synthesis (DDS) generator  40  that generates a continuous, repetitive train of a predetermined sequence of digital signals. This sequence is chosen to have an easily recognizable harmonic signature. In particular, the DDS signal has a controlled amplitude and frequency so that it acts as a known reference. The harmonic signature of the DDS signal should be unique and not be a signature that would occur during normal operation of the monitoring system. The output of the DDS generator  40  modulates a laser or light-emitting diode  42  emitting in the optical band to which the photodiode detector  32  is sensitive. The optical signal emitted by the laser  42 , which is the aforementioned local characterizing signal, passes through another optical isolator  44  before the optical coupler  30  combines local characterizing signal with the communication signal received from the main fiber  20 . 
     The signal processing engine  38  receives both the communication signal and the locally generated characterizing signal with its known harmonic signature. It establishes which part of signal is the local characterizing signal arising from the DDS based upon the known harmonic signature. For example, it may use a fast Fourier transform (FFT) to extract the reference frequency and amplitude. The signal processing engine  38  may use the extracted information to calculate a signal-to-noise ratio from which the dark current magnitude may be determined. The signal processing engine  38  may then remove the dark current component from the detected signal to determine the monitoring signal tapped from the fiber. Because the known local characterizing signal has been detected by the photodiode  32  having the uncertain dark current, the measurement of the detected local characterizing signal allows the dark current of the photodiode to be determined. 
     The local characterizing signal generated by the DDS generator has an easily recognizable harmonic signature and may be as simple as an alternating binary signal, that is, a square wave. Preferably, the data rate for the DDS signal is significantly lower than that of the data signal being carried on the fiber, for example, by a factor of 100, so that the local characterization signal is easily separated from the data signal. 
     The system of FIG. 2, however, has the disadvantage that it cannot distinguish the photodiode dark current from a DC optical signal propagating on the main fiber  20 , that is, an unmodulated optical signal at the carrier frequency. The photodiode converts a DC optical signal to a DC voltage indistinguishable from a photodiode dark current. 
     A second embodiment illustrated in the schematic diagram of FIG. 3 allows the DC optical signal to be separated from the photodiode dark current. A monitoring system  50 , otherwise similar to the monitoring system  26  of FIG. 2, additionally includes a controllable optical absorber  52  placed in the path of the received communication signal between the isolator  28  and the optical coupler  30 . The signal processing engine  38  periodically sets the optical absorber  52  to completely absorb all of the optical communication signal including its DC component. Since the optical absorber  52  operates between only two states and then only infrequently, it can be implemented as a switch, for example, a fiber physically movable between a coupling and non-coupling state with another fiber. However, other forms of selective optical absorbers are available, for example, an electro-optic absorber. While the communication signal is blocked, only the local characterizing signal from the DDS generator  40  is detected, thereby allowing accurate determination of the dark current. 
     If the optical absorber  28  is effective at absorbing or otherwise blocking all of the light received from the main fiber  20 , then the system can be substantially simplified to a measurement system  56  illustrated in the schematic diagram of FIG.  4 . The simplified measurement system  56  eliminates the local characterizing signal and the associated DDS generator and laser of FIG.  3 . During the period in which the optical absorber  52  blocks the communication signal as well as any optical DC signal on the fiber, the monitoring system  56  is measuring only the DC signal associated with the photodiode dark current as well as any DC offset introduced by the electronic circuitry of the measurement system  56 . A sufficient number of values are sampled during the dark period to provide an accurate background level corresponding to the dark current and electronic DC offsets. Once this value has been determined, the absorber  52  is set to its transmissive state, and operation returns to normal data signal reception during which the experimentally determined background level is subtracted from all signal values that the signal processing engine  38  receives from the A/D converter  36 . If the system is recalibrated every 1 to 10 seconds by measuring a new background level, any changes introduced by temperature variations will be tracked. However, recalibration every few minutes or up to an hour may be sufficient in a controlled telecommunications environment. 
     A fourth embodiment, illustrated in the schematic diagram of FIG. 5, includes a monitoring system  60  that does not require frequent interruption of the communication signal. A temperature sensor  62  measures the temperature either of the photodiode  32  only or an ambient temperature of the entire monitoring system  60  and provides that temperature to the signal processing engine  38 . During a calibration period, the optical absorber  52  is set to its blocking state, and the photodiode  32  or the entire monitoring system  60  is controllably heated, for example, by a resistive heater  64  which is driven by an unillustrated power source controlled by the signal processing engine  38  and which is thermally coupled to at least the photodiode  32 . If the other electronics are also heated to the same temperature, their DC offsets may also be compensated. The range of temperatures generated by the heating means need to cover the anticipated temperature range to which the system will experience during operation. During the calibration phase, the signal processing engine  38  measures the detected signal values corresponding to the dark state on the fiber and which include both the photodiode dark current and the electronic DC offsets. The signal processing engine  38  then stores in a table the measured values and the corresponding temperature sensed during the measurement. The table is stored in a memory  64 , which is typically already included in the signal processing engine  38 . Of course, it is possible that the table is directly indexed by temperature so that the temperature is not directly stored. Instead, the measured intensity is associated with a temperature by its location in the table. 
     After completion of calibration, the heater  64  is turned off, the optical absorber  52  is set to the transparent state, and the system returns to normal communication operation. During normal operation, the signal processing engine  38  monitors the temperature sensed by the temperature sensor  62  and consults the stored table in the memory  64  to determine the proper dark current and electronic DC offset for that temperature and accordingly compensates the detected signals, for example, by subtracting the stored calibration value for that temperature from the measured value during normal operation. 
     The calibration needs to be performed only infrequently, for example, during scheduled maintenance of the optical receiver during which the communication signal is interrupted anyway. 
     The technique can be extended to compensate for a number of other electro-optical devices ranging from optical amplifiers to optical channel monitors and to thereby extend their dynamic range. 
     Although the invention is easily implemented with a direct digital synthesis generator, alternatively a harmonic generator or some other analog method may be used to generate the reference signal. 
     The invention thus allows the monitoring system to achieve a higher dynamic range with the addition of relatively simple components.