Discriminating photo counts and dark counts in an avalanche photodiode

The output of an avalanche photodiode (APD) comprises a “photocurrent” component comprising photon initiated events resulting from the interaction of photons with the APD and a “dark current” component comprising dark carrier events arising in the APD even when the APD is not exposed to light. Differences in the pulse height distributions of photon initiated events and dark carrier initiated events are used to statistically discriminate between photocurrent and dark current components of APD output.

BACKGROUND OF THE INVENTION

The present invention relates to a photoreceiver and, more particularly, to a method and apparatus for discriminating between photon initiated signals and dark signals arising in the absence of interaction with a photon in an avalanche photodiode of a photoreceiver.

A photodiode is a semiconductor device which absorbs and transforms light into an electric current. Detection of an electrical event, an electrical pulse or electric current, at the output of the photodiode evidences the interaction of light with the photodiode. The electrical current generated by the absorption of light in the photodiode is called “photocurrent” and the ratio of the magnitude of the photocurrent, in amperes, to the incident luminous power, in watts, is the photodiode's “responsivity.” Amplification electronic circuits are often used to boost the amplitude of the photocurrent above system noise sources to improve detection. The amplification process itself contributes noise, causing output to fluctuate about its mean value, even in the absence of light. Ideally, one would want to have gain in the photodetection process, such that the current created in the photodiode could be increased in magnitude above noise contribution of the amplification electronics. An avalanche photodiode (APD) is a photodiode exhibiting increased responsivity due to internal amplification of the photocurrent through impact-ionization in which “charge carriers,” electrons or holes, with sufficient kinetic energy can knock a bound electron out of its bound state in the valence band of a semiconductor and promote it to a state in the conduction band, creating an electron-hole pair. APDs are particularly useful for detecting weak luminous signals because their high responsivity boosts the photocurrent signal relative to noise produced by sources in the detection system downstream of the photodiode. However, the benefit of avalanche multiplication comes at the expense of an increase in “shot noise” by APD excess noise factor which is a measure of gain uncertainty.

Moreover, electric current flows in a photodiode or APD even in the absence of illumination. This “dark current” is a spurious output signal which itself has a temperature dependent increase in shot noise by an excess noise factor, a result of the quantization of the electric current's constituent charge, which causes the output of the APD to fluctuate about its mean value in the absence of light. Since individual charge carriers of either polarity, electrons and holes, are indistinguishable, the current resulting from photon generated electrons or holes, “photocarriers,” cannot be distinguished from the current resulting from electrons or holes generated by other processes, such as “dark carriers,” and the total current flowing in the APD cannot be segregated into photocurrent and dark current by inspection.

What is desired, therefore, is a method of discriminating photon induced current events and dark current events produced by an APD.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring in detail to the drawings where similar parts are identified by like reference numerals, and, more particularly toFIGS. 1 and 2, in an exemplary photoreceiver20, such as is used in a laser range finder or a direct detection laser detection and ranging device (LADAR), incident light, one or more photons22, impinges on the device's optics24and is focused on an avalanche photodiode (APD)26. In the APD, one or more photons are converted to an electrical signal, a photogenerated current comprising photon induced events or pulses, which is transmitted to a detector decision circuit28which detects the electrical signal and, thereby, the interaction of the photon or photons with the APD. The popularity of APDs in high speed photoreceivers is attributable to the APD's high internal optoelectronic gain which enables the photogenerated current to dominate the thermal noise of sources in the photoreceiver circuitry without the need to amplify the incident light. The optoelectronic gain of the APD is the result of a cascade of charge carrier impact ionizations in a strong electric field in an intrinsic multiplication layer of the APD.

The exemplary separate absorption, charge and multiplication avalanche photodiode (SACM APD)26comprises generally an absorption region30, a charge region32and a multiplication region34(indicated by a bracket) arranged between an anode36and a cathode38which are interconnected by a biasing circuit40. The biasing circuit40comprises a voltage source42which exerts an electrical potential between the anode36and the cathode38producing an electric field in the APD. The strength of the electric field can be varied by adjusting the magnitude of the electrical potential, the bias, exerted by the biasing circuit40. Current does not flow freely between the anode and the cathode because a positive voltage is applied to the cathode and a negative voltage is applied to the anode so the diode junction is reverse biased by the resulting electric field.

A photon22, entering the absorption region30of the APD26, generates an electron-hole pair comprising a positively charged hole43which under the influence of the electric field drifts toward the anode36and a photoelectron44which drifts toward the cathode38into the charge region36and then into the multiplication region34of the APD. Preferably, the multiplication region34of the APD comprises plural discrete heterostructured gain stages46,48(indicated by brackets) each comprising an ordered sequence of layers preferably including a first field up layer50, a first intrinsic layer52, a second field up layer54, a second intrinsic layer or ionization layer56, a field down layer58and a relaxation layer60. The charge region32functions as the first field up layer and the first intrinsic layer of the gain stage46nearest the anode. U.S. Pat. No. 7,432,537, incorporated herein by reference, discloses in detail the construction and linear mode operation of an exemplary SACM APD with multiple heterostructured gain stages.

In an APD having separate absorption and multiplication regions, avalanche multiplication of photocurrent is initiated by the carrier type that drifts from the absorption region to the multiplication region and in the reverse biased exemplary SACM APD26, the absorption and multiplication regions are ordered such that photoelectrons are injected into the multiplication region from the absorption region. Referring also toFIG. 3, when an electron44drifts into the multiplication region34from the absorption region36, it accelerates in a strengthening portion82of the electrical field80of a first portion of the charge region36in the case of the first gain stage46or the first field up layer50of subsequent gain stages48. The electron continues to drift through the charge region toward the cathode and into a second portion of the charge region where the electric field is not increased84. Similarly, in subsequent gain stages48, the electron drifts out of the first field up layer50into the first intrinsic layer52where the electric field remains constant84and the electron can lose energy due to collisions in the undoped layer. The electric field is again increased86in a second field up layer54accelerating the electron above the saturation velocity so that when electrons enter the second intrinsic or ionization layer56a portion of the electrons has sufficient kinetic energy to impact ionize. The strength of the electric field is maximized88in the ionization layer and the material of the ionization layer is selected to have a lower band gap and, consequently, a lower ionization threshold than the material of the other layers of the multiplication region. A portion of the charge carrier population accumulates enough kinetic energy between scattering events to induce creation of new electron-hole pairs90pushing an electron from the valence band into the conduction band while leaving a hole behind.

Electrons pass from the ionization layer to a field down layer58where the electric field strength is rapidly reduced92below the level required to sustain impact ionization. From the field down layer, the electrons drift to a relaxation layer60where a weak field94is maintained and the energy of the electrons is reduced by normal scattering so that the electrons arrive at the first field up layer50of the next gain stage48with relatively uniform, lower energy. As the electrons44drift toward the cathode38the impact ionization process is repeated in each gain stage48and the number of electrons increases substantially linearly90in an APD biased below the breakdown voltage so as to operate in the linear mode. During the time of the transport of the initial photoelectron and its progeny through the APD junction, assuming single carrier ionization, the optoelectronic gain for the APD is the gain for each gain stage raised to a power equal to the number of stages. For example, for an APD with stage gain of m=1.8, a five stage cascaded multiplication region can achieve a device gain of M=18.9 (1.86) multiplication region can achieve a device gain M=357 (1.810), and a ten stage cascaded.

On the other hand, the ionization layer56is sized so that secondary holes96created in the ionization layer by impact ionization cannot gain sufficient energy to cause further ionization before they drift out of the high-field region and while a hole96created by impact ionization in the ionization layer56will tend to accelerate toward the anode36it will pass out of the ionization layer to the second field up layer54on the anode side of the ionization layer before sufficient energy is gathered to cause further ionization. Although the hole may continue to accelerate, the reduced strength of the electric field and the greater bandgap of the material making up the second field up layer reduce the probability that the hole will ionize. When the hole passes into the first intrinsic layer52it will lose energy due to collisions so that when the hole enters the ionization layer of the next gain stage it will have too little energy to impact ionize. As illustrated inFIG. 3, the number of electron-initiated ionization events90grows with each gain stage48as the initial photoelectron44passes through the multiplication region34and creates more progeny electron-hole pairs, but the probability of hole-initiated ionization is minimal. In this fashion, the portion of the APD's responsivity related to electron-initiated ionization events is enhanced while that of holes is suppressed and feedback is minimized.

The exemplary photoreceiver20includes a decision circuit28. The exemplary decision circuit28comprises a transimpedance amplifier62which converts the current output by the APD26to a voltage and a threshold comparator64which outputs68a “count” when the input voltage from the transimpedance amplifier exceeds a threshold voltage66. On other hand, a decision circuit of photoreceiver might comprise a transimpedance amplifier having an output current and a threshold comparator comparing the output of the transimpedance amplifier to a threshold current. The transimpedance amplifier is by nature noisy and by setting the threshold voltage high enough to account for an estimate the amplifiers noise, the output of the decision circuit reflects the output of the APD. However, the output of the APD comprises both photocurrent, arising from the interaction of light with the APD, and dark current comprising dark counts or events arising within the APD even in the absence of the APD's exposure to light. Since photo generated charge carriers, “photocarriers,” of either polarity, electrons and holes, are indistinguishable from “dark carriers,” electrons or holes generated by other processes, the total current flowing from the APD cannot be segregated into photocurrent and dark current.

A dark count is an avalanche event which is not induced by a carrier generated by a photon and an electric current known as “dark current” flows in an APD even in the absence of illumination. The APD26is represented schematically as a diode26′ and, in parallel with the diode, a dark current generator26″ which is the source of dark events or pulses. In SACM APDs, non-photon generated dark counts or events arise from the injection of charge carriers into the semiconductor junction, primarily as a result of thermal excitation, tunneling across the semiconductor's bandgap, possibly mediating mid-gap states. Thermal excitation can provide a source of dark current by causing charge carriers to transfer from the valence band to the conduction band, either directly or by way of a midgap defect. In addition, the strong electric field required to drive impact-ionization can also cause electrons to quantum tunnel through the potential energy barrier separating the valence band and the vacant conduction band states in the narrow bandgap semiconductor alloys used in the ionization layers of the gain stages. Tunnel leakage in the multiplication region of an APD can be the dominant source of dark events and this is particularly true of APDs which are cooled during operation and APDs, such as SACM APDs, which are designed for low excess noise, a statistical noise inherent in the multiplication process. Chemical impurities and lattice defects in the multiplier also create mid-bandgap trap states which reduce the effective energy barrier to quantum tunneling. The mid-bandgap trap states divide the dark current generation process into sequential steps each requiring penetration of lower potential energy barrier than the full bandgap of the semiconductor material. The trap assisted tunneling process is relatively insensitive to device temperature because carrier generation is via quantum tunneling through the potential energy barrier rather than thermal promotion of carriers over the barrier. In addition, impurities and crystal defects can result in charge traps and the high current in the junction results in a probability that the charge traps will be filled with a carrier which is released later initiating a second pulse or “after pulse.” Many types of APDs, including APDs manufactured from InP, InGaAs and In0.52Alo.4sAs, exhibit a dark current generation rate at high bias which scales linearly with trap concentration in the multiplier, exponentially with the applied reverse bias and which has a weaker exponential dependence upon temperature.

While both photocarriers and dark carriers are subject to avalanche multiplication, the respective types of carriers are generated by different mechanisms and, as result, exhibit differing pulse height distributions, that is, the variation of the magnitude of the output pulses at a constant applied voltage. While the portions of the total output current of the APD attributable respectively to photon induced events and dark events cannot be determined, the inventors concluded that the differences in pulse height distribution for dark current and photocurrent could be exploited to statistically distinguish between photon induced pulses and dark pulses, that is, pulses not induced by interaction of a photon with the APD.

All primary photocarriers, photoelectrons44in the exemplary photoreceiver20, are generated in the absorption region30and are injected into multiplication region34at one end of the multiplication region and pass through the multiplication region on the maximum possible path length. When a secondary electron-hole pair is generated in one of the ionization layers of the multiplication region, the electron travels toward the cathode and the hole travels toward the anode but neither secondary carrier, hole nor electron, traverses the maximum path length through the multiplication region. A population of dark current carriers is generated within the absorption region30and, like the photoelectrons44pass through the multiplication region34on the maximum possible path length. Another population of dark current carriers are generated within the multiplication region and, like the secondary carriers, the path through the multiplication region34traversed by a dark carrier, either a dark electron or a dark hole, will depend on where the dark carrier originated. Since gain occurs in the spatially discrete gain stages of the multiplication region, the contribution of each dark carrier to the dark count or current will depend on where the dark carrier arose. For example, in the simplified case of single carrier ionization, in a 10 stage APD with a gain 1.8 per stage, one tenth of the dark carriers can be expected to arise in the tenth stage and experience no gain. A second tenth of the dark carriers can be expected to arise in the ninth, the second to last, stage and experience a gain of 1.8 and so forth. Since generation of dark carriers by tunneling will be localized in the respective multiplication layers56where the electric field is strongest and the band gap the narrowest and will have too little energy to impact ionize in the gain stage in which they are generated, a primary dark electron generated in the ith stage of the multiplication region will experience an average gain of approximately:
Mi=mi-1(1)
where:
Mi=average gain of a carrier at the i'th gain stage and
mj=effective average gain of the of the jth multiplication stage.
The average gain (Mi) is an approximation because equation (1) does not account for the counter propagating holes which are generated with the electrons and which have a finite chance of triggering impact ionization in the earlier gain stages of the multiplication region.

Assuming a uniform probability of primary dark carrier generation in each stage, in the case of largely single carrier ionization, the pulse height distribution of the dark current from all stages of the multiplication region is approximated by the weighted average of the pulse height distributions of dark electrons generated in each stage:

P⁢⁢H⁢⁢Ddark=1j⁢∑j⁢⁢P⁢⁢H⁢⁢D⁡(i)(2)
where:
PHDdark=pulse height distribution of all stages, and
PHD(i)=normalized pulse height distribution for primary carrier generation in the ithstage.

The pulse height distribution for each gain stage can be approximated by the McIntyre distribution:

Γ⁡(z)=∫0∞⁢⁢d⁢⁢t0⁢⁢tz-1⁢exp⁡(-t)
While equation (3) does not account for hole feedback into earlier stages of the multiplication region and the resulting probability of hole initiated impact ionization in earlier gain stages, k is small for an SACM APD and the approximation is reasonable.

Referring also toFIG. 4, since primary photocarriers are generated by interaction between photons22and the material of the absorption region30of the APD and are injected into the multiplication region34to traverse the maximum path length in the APD, the pulse height distribution of the photon induced pulses or events110is different than the pulse height distribution of dark carrier pulses or events112which are not initiated by photon interaction with the APD but which arise by way of thermal and tunneling events distributed throughout the spatially separated ionization layers56. The inventors reasoned that the number of correctly detected photon induced events114could be maximized while the number of dark current generated false alarms, “false counts”118, arising from incorrectly discriminated dark events could be minimized by setting the decision circuit's threshold voltage66to the voltage maximizing the difference116(indicated by a bracket) between the pulse height distribution for dark current112and the pulse distribution for photocurrent110. Similarly, the number of correctly detected pulse events could be maximized if a central tendency, for example, the mean voltage of a plurality of pulses115or a current, exceeded the threshold66. Since dark carrier generation and dark carrier pulse height distribution varies with temperature and the bias of the APD, the exemplary photoreceiver20includes a temperature sensing element70, for example a thermocouple, communicatively connected to a logic unit72, such as a data processing unit operating according program instructions, to vary the threshold voltage66of the decision circuit68. Likewise, the pulse height distributions of the photocurrent110and the dark current112vary with the bias applied to the APD and the logic unit72is connected to the variable voltage source42of the bias circuit40to control the bias voltage and to adjust the threshold voltage66to maximize the difference between the photocurrent and dark current pulse height distributions in response to a change in the bias voltage.

A false count rate (FCR) for a photoreceiver can be modeled as a sum of the electronic noise count rate originating solely from circuit noise in the transimpedance amplifier connected to the APD and a dark count rate determined by convolution of the transimpedance amplifier noise with the amplified dark current in the output of the APD:
FCR=ENC+DCR  (4)
where:
FCR is the false count rate,
ENC is the electronic noise count of the amplifier, and
DCR is the dark current count rate.
And, the electronic noise count of the amplifier is:
ENC=rate×∫n=threshold∞PHDTIA(n)dn
where:
rate is the minimum separation between consecutive pulses that can be resolved and counted separately, and
PHD is the pulse height distribution of the transimpedance amplifier which is often modeled as a Gaussian distribution centered at n=O. The dark count rate (DCR) from the APD dark current is determined in a similar manner where the amplitude distribution of darks counts from the APD is determined by deconvolving the amplifiers noise with the mean pulse height distribution of the APD's dark generated carriers as might be determined in equation (2):
PHD=PHDTIA*PHDAPD(6)

Referring also toFIG. 5, a laser detection system120includes a laser122arranged to illuminate a surface124to be ranged with a pulse of light. Photons22reflected from the surface are detected by a photoreceiver and analyzed to determine the distance from and/or speed of the surface124relative to the photoreceiver. The inventors concluded that the accuracy of the laser detection system could be improved if the threshold voltage of the system's detector maximized the difference between the pulse height distribution of photocurrent events and dark current events. In the laser ranging system120, the laser122transmits pulses of light which are reflected from the surface124to a detection surface126. The area of the detection surface126is divided into a plurality of subareas128each of which focuses one or more photons22on a respective one of a plurality of photoreceivers130. In one embodiment, the plurality of photoreceivers are smaller than the diffraction limited resolution limit of the optical system, such that signals are spread over multiple of the plurality of the photoreceivers130. For instance the airy disk can overlap 2, 3, 4 or more of the plurality of photoreceivers130. The photo receivers include respective decision circuits132each having a threshold voltage134which maximizes the difference between the pulse height distributions of the photocurrent and the dark current of the photoreceiver. Electrical events detected by the plural photoreceivers are summed136and output to a second decision circuit138which detects the sum of outputs exceeding a threshold. In another embodiment the binary output or amplitude of the signals are summed and output. Since dark current events occurring in the respective receivers are not time correlated and photocurrent events arising from the interaction of coherent photons of the reflected light pulse with the ones of the plural receivers are time correlated, summed events exceeding a threshold in the second decision circuit are more likely correctly detected photocurrent events and less likely false alarms.

Referring also toFIGS. 6 and 7, another embodiment of a photoreceiver150comprises a photodiode26and a transimpedance amplifier62to convert the current output by the photodiode to a voltage. The output of the transimpedance amplifier is transmitted to plural threshold comparators152,154,156each biased to a different threshold voltage;Vthn158,Vthn+1160,Vthn+2162. Each of the threshold voltages,Vthn158,Vthn+1160,Vthn+2162, corresponds to a pair of photocurrent and dark current probability values where the respective threshold voltage intersects the photocurrent pulse height distribution180and the dark current pulse height distribution182. The probability of detecting the events is given by the area under the curve for all threshold values below the setting, see an inverse cumulative probability distribution function,FIG. 13A, and related discussion further hereinbelow. Preferably, one of the threshold voltages158is selected to minimize the difference between dark pulse height distribution182and the photocurrent pulse height distribution180, a second threshold voltage160is selected to maximize the difference between the dark pulse height distribution and the photocurrent pulse height distribution and a third threshold voltage162is a voltage exceeding the maximum voltage of dark pulses. The outputs of the threshold comparators164,162,168are input to a secondary decision circuit170which applies a second decision criterion, for example, designating a pulse having a value exceeding a central tendency, such an average or mean, of said three thresholds as a photon induced pulse. In general the dark pulse height is a summation of the thermal amplifier noise and the photodiode noise. Here the thermal amplifier noise contribution181provides a practical limit, where signal averaging must be implemented in order to better discriminate between photon induced pulses and dark induced pulses.

Unlike photocarriers, dark carriers do not experience gain in all of the gain stages of the multiplication region and, therefore, the average gain of dark carriers is different than that of photocarriers. Since the pulse height distributions comprise, respectively, the probabilistic occurrence of the amplitudes of the photocurrent count events and the probabilistic occurrence of the amplitudes of the dark count events, the pulse height distribution of the photocurrent will be different than the pulse height distribution of the dark current and the difference can be used to statistically discriminate between photon induced events and dark events.

Referring toFIG. 8describing another embodiment of the present disclosure, a photoreceiver200has the photodiode28and the decision circuit28as that shown inFIG. 2, further comprising a timing unit202. The timing unit202provides temporal reference of said photon induced pulses emitted by the photodiode28upon. The timing unit can implement a time-to-digital converter (TDC) or a time-amplitude converter (TAC) and combinations thereof. The temporal reference can be used to measure the time interval between a plurality of events. In aforementioned applications, laser rangefinding (LRF), LADAR, use the plurality of events to determine the distances to an object. In much the same ware the receiver can be used to detect target information recorded in the laser pulse in a LADAR system, in another application, optical communications, information is transmitted in optical signals recorded in a time relative to one another in a temporal reference frame. The photoreceiver can be used to discriminate dark events from photon events in a communications data stream. Also, the embodiments herein can be used for general digital low light level imaging, wherein the APDs are used to detect single, or low photon count signals, and the images are built up by the photon counts in each pixel.

The timing unit can provide for time-of-flight and amplitude information for a series of photon induced pulses. For instance a first optical pulse can be emitted and return pulses detected and indexed temporally according to their TOA. Subsequent optical pulses can be detected, time stamped and the TOF can be calculated and recorded. The time stamps can be accumulated in a time-of-arrival histogram or their arrival time can otherwise be correlated with varying levels of temporal accuracy. In this way the timing information from correlated photon induced pulses can be discriminated from the uncorrelated dark induced pulses or false alarm events. Further, processing of the events can be used to increase the uncertainty of the time-of-arrival and amplitude of the signal.

Alternatively, recording both the TOA and amplitude of multiple pulse echoes it is possible to create a histogram of the signal events, or otherwise correlate the recorded pulse events with respect to time and use both the temporal correlation and the pulse amplitude information to discriminate photon induced pulses from dark induced pulses. Using statistical analyses and signal processing, with knowledge of the pulse height distribution of the photon and dark pulse events it is possible to discriminate photon induced signals from dark induced signals and further possible to determine the TOA and pulse amplitude magnitude with greater accuracy.

Referring toFIGS. 9 and 10, a LRF or LADAR application210, a photoreceiver212has an optical source214, such as a laser, and a photoreceiver216. An optical pulse emission214P from the optical source214is one reference, and receipt of a first echo218by the photodiode216from a first target220, here a tree, generates a first return pulse220R. If multiple objects are illuminated, and multiple returns detected, the distance to each can be determined. Here the pulse emission214P from the source214is divergent and another target is in the path. A second target222, also a tree, provides a second return pulse222R. The distance (D) to the first target is given by D=(c×t)/2, where D is distance, c is the speed of light, and t is time between the optical pulse emission214P and the receipt of the first echo218. In laser ranging applications the range of each target and the distances between each target can be resolved, in LADAR application multiple angle-angle-range measurements in a scene can be resolved. The photoreceiver can detect and capture the time of arrival (TOA), pulse amplitude, or both, of a portion of the optically induced pulse, including the leading edge of the return pulse, the peak of the return pulse, or the last return, and combinations thereof. The photoreceiver can also capture the timing of the leading and edges of each pulse return. The photoreceiver can capture an analog or digital value signal representative of the entire of the laser pulse echo. During the time of flight of the laser pulse, to and from the reflecting target, there is also some probability of detecting a dark event occurring. The interval of time in consideration can be shortened so that it includes return pulses within a certain time window, corresponding to echoes created from objects at a particular distances, and the time window can be shortened to reduce the number of echoed pulse returns, reduce the probability of recording a dark event, or from detecting the outgoing laser pulse. The inventors concluded that by setting the threshold value where it maximizes the difference between the pulse height distributions of the photo-induced signal and the dark current induced signal of the photoreceiver, the area under the receiver operating characteristic (ROC) curve is maximized.

Referring toFIG. 11A, a binary accumulation scheme240is provided in a LRF or LADAR application. In this binary accumulation scheme, events or pulses generated are counted when above the voltage threshold. Multiple optical pulses are emitted by an optical source in order to aggregate return echoes, and the resulting photodiode pulses. Photodiode pulses are aggregated when above the voltage threshold and indexed according to their respective time-of-arrival (TOA) or distance. Photodiode output pulses are segregated in time from a first pulse BP1, a second pulse BP2, a third pulse BP3, and a fourth pulse BP4. A pulse sum BPT of the detected pulses provides the accumulated pulses and a binary outcome BPO provides the resultant pulses distinguished in the process. A voltage threshold VBis common to the pulses BP1, BP2, BP3, and BP4. Pulses resulting from an optical event are illustrated with diamond tipped markers whereas pulses from dark current events are illustrated with dot tipped markers.

In the first pulse BP1, a dark pulse242and a dark pulse244are generated, both below the voltage threshold VB. An optical pulse246and an optical pulse248are generated, both above the threshold and therefore each counted, or accumulated, indexed by their TOA. In the second pulse BP2a dark pulse252and a dark pulse254are generated, both below the voltage threshold VBand not corresponding with the previous dark pulses. An optical pulse256and an optical pulse258are generated, above the threshold voltage VBand correspond to the two previous detected optical pulses. In the third pulse BP3, a dark pulse262and a dark pulse264are generated. Dark pulse262is above the voltage threshold VB, and therefore accumulated and indexed. Dark pulse264is below the voltage threshold. An optical pulse266is above the threshold and indexed, whereas an optical pulse268is below the threshold, effectively no detected. In the fourth pulse BP4a dark pulse272and a dark pulse276are both below the threshold. An optical pulse276and278are both above the threshold and indexed. The pulse sum BPT provides the total temporally indexed pulses. A pulse accumulation280corresponds with the dark pulse262has a single occurrence within the four pulses. A second pulse accumulation286corresponds with optical pulses246,256,266, and276having four occurrences within the four pulses. A third pulse accumulation288corresponds with optical pulses248,258, and278. Accumulation pulses280,286, and288have a normalized binary accumulation of 0.25, 1, and 0.75. Here, a normalized binary accumulation threshold of 0.5 eliminates the optical pulse optical pulse280and the resultant binary accumulation output BPO has a binary optical pulse296corresponding with optical pulses246,256,266, and276, and a binary optical pulse298corresponding with optical pulses248,258,268, and278. In addition to increasing discrimination between optical and dark pulses, distribution of grouped binary pulses can be used to approximate pulse shape, return waveform and increase accuracy in TOF measurements.

Referring toFIG. 11B, a binary accumulation histogram241provides a temporal indexing of photon induced pulses243from a laser ranging application. Here the distribution of the binary return pulses can be used to determine an approximate pulse shape245of the return pulse, wherein each of the photon induced pulses243are detected at low-light levels, i.e. single photon detection. In this example the pulse shape is approximated by weighing the temporal relation of each of the photon induced pulses, assuming an equal amplitude. The temporal aggregation of the binary pulses provide an approximately gaussian return pulse. In practice, the calculation of the return pulse would depend on the emission pulse of the optical source used, which in turn would inform the extent of an temporal window247. For instance, the temporal window for the pulse shape detection may be on the order of a few nanoseconds. Longer more complex waveforms can also be approximated. For instance when emissions reflect and backscatter through foliage such as trees, or other semi-transparent matter which have multiple reflecting surfaces, the waveform will be a continuous train with varying amplitude. Again, the single photon binary distribution can be used to approximate the complex waveform shape.

Referring toFIG. 12Aa pulse probability histogram300A. The histogram300illustrates a preferred carrier probability curve302and a non-preferred carrier probability curve304. Graphically illustrated and corresponding with the carrier curves are pulses events vs voltage, a graphic illustration of the histogram as a series of pulse events. The preferred carrier curve304and the non-preferred carrier curve302show the difference in probability of inducing a peak voltage. A preferred carrier induced pulse curve302has a lower probability of inducing a pulse with low voltage than a non-preferred carrier curve304. The preferred carrier induce pulse curve302shows a higher probability of inducing a high voltage pulse than the non-preferred carrier curve304. As described above, the voltage threshold at a value to discriminate between preferred and non-preferred carriers, here the voltage threshold VT.

Here a pulse event310,312, and314, are above the voltage threshold VTwith peak voltage increasing respectively. Collection of amplitude information for each pulse allows increased discrimination and accuracy in determining TOF measurements. Additionally information can be used to weight return pulses. For instance pulse event314, taken alone, is more likely to be induced optical return and not noise. Accumulation of amplitude information allows discrimination based on the accumulated amplitude mode, amplitude mean, or other statistical discriminator. For instance, repeated measurements at a target can be recorded, the amplitude of each added, then divided by the number of measurements taken. Alternatively, each value can be assigned the mean value anticipated by the pulse height distribution before processing. Additionally, pulse shapes can be approximated based on various resolution time bins and amplitude information. To increase accuracy of TOF, the errors in the time measurements introduced by the amplitude of the signal can be reduced with constant-fraction discrimination or zero-crossing schemes in the receiver circuitry. Time-walk can be compensated with a look-up table, which calibrates the timing of the decision circuits over all pulse amplitude, including if necessary, the APD gain setting.

Referring toFIG. 12B, an induced current signal graph300B shows a current signal320,322and324corresponding with the pulse events310,312, and314respectively. Amplitude information can be collected by implementing peak hold circuitry, integrating the signal, and other such methods. TOF can be determined by rising edge detection or other more accurate techniques. Assuming each pulse event is corresponds with the same TOF, or distance, the rising edge detection can create error based on temporal difference in rising edge discrimination, called time-walk. Here the amplitude and thus difference in rate of the rising edge of the current signal320,322, and324result in a time-walk error326. One method of reducing time-walk error is derivative or other operations can be performed in order to increase accuracy.

Referring toFIG. 12Cthe zero-crossing scheme300C shows a second order derivative330and334of the accumulated current signals320and324respectively (first order derivative of current signals). The accumulated signals coincide at a zero crossing336. The zero crossing336corresponds temporally with the current amplitude peak of each current signal320,322, and324. Since the amplitude of each of the current signals are aligned, the zero crossing provides a more accurate reference between the pulses and reduces error associated with time-walk.

Referring toFIG. 13A, a complementary cumulative distribution function (CCDF)400A displays probability of detection PDvs threshold voltage at low light levels. An optical induced pulse probability402and a dark induced pulse probability404are described by: PD=1−∫VminVth∫PV, where Vminis the minimum voltage, Vthis the threshold voltage, and PVis the pulse height distribution curve the pulse height distribution of preferred carriers and non-preferred carriers, respectively. Both the induced pulse probability402and the dark induced pulse probability404have a high (100%) probability of detection at very low threshold values. As the threshold voltage increases the probability of detection between the optical induced pulse and dark induced pulse separates slowly at first, then more rapidly as the dark pulse detection quickly falls. The dark pulse probability404reaches a very low probability (≈0) before the optical induced pulse probability402.

Referring toFIG. 13B, a receiver operating characteristic (ROC) graph400B, for single photon detection, provides the probability of a true positive P(H) vs a false alarm (P(F), or ROC curve, for a typical APD410, an non-accumulating APD412of the present disclosure, and an accumulating APD412of the present disclosure. The ROC graph provides a comprehensive description of an APD photoreceiver performance for a given set of conditions, here single photon detection. In general, photoreceivers with a greater area under the curve, have better performance. For reference, the top left starting point of the CCDF curve ofFIG. 13roughly corresponds with an end point418of the ROC curve. Similarly the end point of the CCDF roughly corresponds with a start point420of the ROC curve.

For the typical APD410, at single photon detection, the impulse function and probability of detection for optically induced and dark induced pulses is roughly equal, resulting in a linear ROC curve. For the non-accumulating APD412and the accumulating APD414, of the present disclosure, the ROC curve rises rapidly since at higher threshold values, the probability of detecting dark induced pulses is low. The non-accumulating APD412and accumulating APD414separate, corresponding with lower threshold values. The accumulating APD has better discrimination true positive and false alarms and thus has an increased area under the curve than the non-accumulating APD412. All the APDs terminate at the end point418corresponding with 100% probability of detecting both true positive and false positives. At very low threshold values, the probability of detecting either optical induced pulses or dark induced pulses is very high, since there is no discriminating between the two.

The above describe embodiments can include temperature sensors arranged to sense the temperature of the photodiode. For instance a thermistor can be placed on the photodiode. The device logic can include provide feedback and change operational characteristics based on the photodiode temperature. For instance the threshold can change based on the temperature.

The detailed description, above, sets forth numerous specific details to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid obscuring the present invention.

All the references cited herein are incorporated by reference.