JP2011226922A - Photon detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photon detector with an improved detection speed by a simpler structure than conventional technologies.SOLUTION: A voltage signal from an APD 701 and a signal indicating a gate voltage application time from a pulse power supply 703 are input into photon incidence discrimination means 704. In the photon incidence discrimination means, a differential waveform voltage of the gate voltage and a photon incidence signal in response to a photon incidence to the APD 701 included in the voltage signals from the APD 701 are discriminated by using the gate voltage application time and a signal indicating only a photon detection time without the gate voltage application time is output. The photon incidence discrimination means 704 has a threshold circuit which outputs a signal pulse when an input signal exceeds a predetermined threshold. The threshold is set to be lower than peak values of the differential waveform voltage of the gate voltage and the photon incidence signal. Even with such a small threshold value, the photon incidence can be properly detected by the configuration described above.

Description

  The present invention relates to a photon detector, and more particularly to a photon detector for detecting a single photon.

  In recent years, research on quantum information communication systems using the quantum properties of light has been advanced. In the quantum information communication system, since information is transferred using a single photon, a photon detector that detects a single photon is a key device for mounting. As device performance, detection efficiency, noise characteristics, high speed, and the like are important. Several types of photon detectors have been studied. Among them, those using avalanche photo diodes (APDs) are excellent in practicality because they have a simpler configuration than other photon detectors. .

  In the APD photon detector, when a single photon is incident, electrons are excited thereby, and an avalanche phenomenon occurs in which excited electrons are multiplied in the device to become a macroscopic current. Using this, the incidence of a single photon is detected. Here, in order to generate a macroscopic current from one excited electron by an avalanche phenomenon, it is necessary to apply a high bias voltage to the APD. However, if a high bias voltage is continuously applied, malfunction occurs due to a phenomenon called “after-pulse”.

  Afterpulse is a phenomenon in which some of the excited electrons generated by the avalanche phenomenon are trapped by impurity levels in the APD, and the electrons are thermally excited again to cause the next electron avalanche. Following the first avalanche current pulse, a current pulse that should be called a ghost is output, so it is called an after pulse. The after-pulse is a phenomenon in which a current pulse is output even though no photon is originally incident, and causes a malfunction that a photon detection signal is output even though the photon is not input.

  In order to avoid such a malfunction due to the after-pulse, it is usually performed that the voltage applied to the APD is pulsed. That is, a high bias voltage is applied only at a time when photon incidence is expected, and a low bias state is set during other time periods. Since no avalanche phenomenon occurs when the bias is low, no after pulse is generated. If the low bias state continues for a while, electrons trapped in the impurity level during that time relax to the valence band, and no after-pulse occurs even if a high bias voltage is applied next. In this way, malfunctions due to afterpulses are avoided. A high bias voltage pulse applied at a predetermined time is referred to as a “gate voltage”.

  After-pulse can be avoided by the gate voltage driving method, but the high speed of the device is limited. APD detects photons only when a gate voltage is applied. In order to detect many photons within a unit time, the gate voltage is repeatedly applied. The time interval of the applied gate voltage is the time for the electrons trapped in the impurity level to relax to the valence band. It must be longer. That is, there is an upper limit to the repetition frequency at which the gate voltage is applied. In the case of normal InGaAs-APD, this upper limit is about several MHz. Faster repeated photon incidence cannot be detected. The high speed of a single photon detector using APD is limited by the repetition frequency of this gate voltage.

  In view of the above circumstances, a technique for increasing the gate voltage frequency by devising the peripheral circuit of the APD has been devised. FIG. 1 shows an example of the configuration (see Non-Patent Document 1). In general, in an APD, an avalanche current generated by photon incidence is passed through a resistor, and an output voltage signal is obtained from the resistance voltage. Normally, this output voltage is input to the threshold circuit as it is, and when the set threshold is exceeded, it is determined that a photon has been detected. However, in the configuration of FIG. A synthesis circuit is inserted. That is, the APD output signal is branched into two, a time delay is given to one, and then differential synthesis is performed again before inputting to the threshold circuit. Hereinafter, a situation where the gate voltage repetition frequency can be increased by inserting such a circuit will be described.

  Even if a gate voltage is applied to the APD, an avalanche current does not flow if no photons are incident. In this case, the APD acts on the applied voltage in the same way as a capacitor. In other words, it acts as a high-pass filter, and its differential waveform voltage is output in response to rectangular voltage application (see FIG. 2). Normally, this output voltage is directly input to the threshold circuit. In this case, in order to correctly determine the presence or absence of photon incidence, the threshold value of the threshold circuit must be set higher than the peak value of the differential waveform voltage when no photon is incident. Furthermore, the output voltage generated from the avalanche current at the time of photon incidence must be higher than this threshold. Specifically, the output voltage at the time of photon incidence is increased by increasing the avalanche current generated by applying a high gate voltage.

  The above is the operation of a normal APD photon detector. In addition to this, a branch / delay / combination circuit is inserted in FIG. This is to suppress the differential waveform voltage when no photon is incident. The differential waveform voltage from the APD is bifurcated, and one is given a time delay. Here, the delay time is set equal to the time interval between the gate voltage pulses. Then, in the synthesis circuit, the differential waveform voltage generated from the adjacent gate voltages is synthesized. This synthesis is performed differentially. That is, a signal voltage obtained by subtracting the other differential waveform voltage from one differential waveform voltage is output. Since the gate voltage waveforms are the same, the two differential waveform voltages are the same. Therefore, the differential synthesis circuit outputs a signal voltage in which the differential waveform voltage is canceled.

  When the differential waveform voltage when no photon is input is zero, the threshold value of the threshold circuit can be set low. When the threshold is low, photon detection can be performed even if the avalanche current at the time of photon incidence is small. Specifically, photons can be detected with a low gate voltage.

  A small avalanche current means that the number of electrons excited in an avalanche is small. If the number of excited electrons is small, the number of electrons trapped in the impurity level is also reduced, and as a result, afterpulses are less likely to occur.

  If the after-pulse probability is low, it is not necessary to increase the gate voltage application time interval, and thus the repetition frequency of the gate voltage can be increased.

  Based on the above principle, an APD photon detector with improved high speed is realized using the circuit configuration of FIG.

Z. L. Yuan, B. E. Kardynal, A. W. Sharpe, and A. J. Shields, ‘‘ High speed single photon detection in the near infrared, ’Appl. Phys. Lett. 91, 041114 (2007).

  However, in order for the above prior art to be effective, it is necessary to differentially synthesize the branched signals with exactly the same waveform at the same timing. If the delay time is shifted or the waveform is distorted or the amplitude is changed in the branch / delay path, the differential waveform voltage that has passed through the two paths in the differential synthesis circuit does not completely cancel each other. In other words, precise circuit setting is required for the implementation of the above-described prior art.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a photon detector with improved high-speed performance by a simpler configuration than the prior art.

  In order to achieve such an object, according to a first aspect of the present invention, there is provided an avalanche photodiode (APD), bias voltage applying means for constantly applying a constant bias voltage to the APD, A gate voltage applying means for applying a gate voltage which is a rectangular pulse voltage to the APD in addition to the bias voltage, a voltage signal from the APD, and a signal indicating a gate voltage application time from the gate voltage applying means And a differential waveform voltage of the gate voltage in the voltage signal and a photon incident signal due to photon incidence on the APD are discriminated using the gate voltage application time, and a signal indicating the photon detection time is output. A photon incidence discrimination means, and the photon incidence discrimination means is a threshold circuit that outputs a signal pulse when an input signal exceeds a predetermined threshold. I, the predetermined threshold is a photon detector, characterized in that it comprises a threshold circuit is smaller than the peak value of the peak value and the photons incident signal of the differential waveform voltage of the gate voltage.

  According to a second aspect of the present invention, in the first aspect, the photon incidence determination means includes the threshold circuit to which the voltage signal from the APD is input, the signal pulse from the threshold circuit, and the A signal that receives the signal indicating the gate voltage application time from the gate voltage application means, removes the input signal corresponding to the gate voltage application time from the signal pulse, and outputs a signal indicating the photon detection time And a processing circuit.

  According to a third aspect of the present invention, in the first aspect, the photon incidence determination means receives a signal indicating a gate voltage application time from the gate voltage application means and is synchronized with the gate voltage application time. A pulse signal source that outputs a pulse signal; a differential synthesis circuit that differentially synthesizes the voltage signal from the APD and the pulse signal from the pulse signal source to output a differential synthesized signal; and And a threshold circuit to which the differential combined signal from the differential combining circuit is input, and the signal pulse output from the threshold circuit is a signal indicating the photon detection time.

  According to the present invention, it is possible to realize a photon detector that avoids malfunction due to the differential waveform voltage of the gate voltage output from the APD without requiring precise circuit setting, and as a result, excellent in high speed. A photon detector can be provided.

It is a figure which shows the example of a conventional structure which devises the peripheral circuit of APD and raises a gate voltage frequency. It is a figure for demonstrating the effect | action of APD with respect to rectangular-shaped voltage application. It is a figure which shows the structure of the 1st Embodiment of this invention. It is a figure for demonstrating the photon detection by the photon detector of 1st Embodiment. It is a figure which shows the structure of the 2nd Embodiment of this invention. It is a figure for demonstrating the photon detection by the photon detector of 2nd Embodiment. It is a figure which shows the structure of the 3rd Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 3 is a diagram showing a configuration of the first exemplary embodiment of the present invention. As in the prior art, a bias voltage is applied from the DC power supply 302 to the APD 301, and in addition, a gate voltage is applied from the pulse power supply 303 at a time when photon incidence is expected. A voltage signal is output from the APD 301. In this embodiment, the voltage signal from the APD 301 is input to the threshold circuit 304 as it is. The threshold circuit 304 outputs a signal pulse when the input signal exceeds a set threshold. A signal pulse from the threshold circuit 304 is input to the signal processing circuit 305. A signal indicating a gate voltage application time is also input to the signal processing circuit 305 from a pulse power supply 303 that applies a gate voltage. Then, a signal indicating the photon detection time is output from the signal processing circuit 305. The operation of the signal processing circuit 305 will be described later.

  In the above configuration, the pulse width of the gate voltage applied to the APD 301 is longer than the pulse width of the differential waveform at the rising portion (see FIG. 4A). In this way, when a photon is incident while the gate voltage is being applied, the APD 301 first outputs a differential waveform voltage at the rising portion of the gate voltage, and then outputs an avalanche signal due to the incident photon (FIG. 4). (See (b)).

  For such an output voltage of the APD 301, the discrimination threshold of the threshold circuit 304 is smaller than the peak value of the differential waveform voltage of the gate voltage and the peak value of the photon incident signal (see FIG. 4B). Then, the threshold circuit 304 outputs a signal pulse when the gate voltage rises and when a photon is incident.

  A signal pulse is input to the signal processing circuit 305 at the gate voltage application time and the photon incidence time. Here, the signal processing circuit 305 also receives a signal indicating the gate voltage application time from the pulse power supply 303. Using this time information, it can be seen which of the signal pulses input from the threshold circuit 304 is the signal at the rise of the gate voltage. Therefore, the signal processing circuit 305 removes the signal when the gate voltage rises. Then, what remains is a signal at the time of photon incidence, and this is output as the photon detection time.

  When the circuit is configured and operated as described above, photon incidence can be detected correctly even if the threshold value is small. Therefore, the avalanche current can be reduced by reducing the gate voltage. Then, the number of electrons excited in an avalanche is reduced, and the number of electrons trapped in the impurity level is reduced. As a result, the after pulse rate is reduced, and the repetition frequency of the gate voltage can be increased.

  This embodiment is not configured so that two signals cancel each other as in the conventional example. Therefore, precise circuit setting as in the conventional example is not necessary.

  Furthermore, since the time width of the gate voltage is wide, the time period in which photons can be detected is long, which is an advantage over the conventional example.

  In addition, if the time width of the gate voltage is wide, an after pulse may occur within one gate voltage application time. However, once an avalanche occurs, all the electrons in the APD are discharged for a moment, so the APD It becomes insensitive (this is called “quenching”). Therefore, after-pulse does not occur within a certain time.

(Second Embodiment)
In the first embodiment, the signal processing circuit 305 removes the signal at the time corresponding to the gate voltage application time, but the same effect can be obtained by analog circuit processing.

  FIG. 5 is a diagram showing the configuration of the second exemplary embodiment of the present invention. The gate voltage having a wide pulse width is applied to the APD 501 from the pulse power source 503, and the voltage signal is output from the APD 501 as in the first embodiment. In the present embodiment, the voltage signal from the APD 501 is differentially synthesized with the pulse signal from the pulse signal source 504 by the differential synthesis circuit 505. This pulse signal is synchronized with the gate voltage application time. Here, the pulse width of the pulse signal is preferably approximately the same as the pulse width at the rise of the differential waveform voltage of the gate voltage from the APD 501.

  When such a pulse signal from the pulse signal source 504 and a voltage signal from the APD 501 are differentially synthesized by the differential synthesis circuit 505, the output is as shown in FIG. That is, since the pulse signal from the pulse signal source 504 is subtracted with respect to the differential waveform voltage of the gate voltage, the peak value of this combined signal becomes zero level or less. On the other hand, the photon incident signal is output as it is.

  The differential composite signal is input to the threshold circuit 506. The threshold circuit 506 outputs a signal pulse when the threshold value exceeds the zero level and below the peak value of the photon incident signal. Thereby, a signal pulse is output from the threshold circuit 506 only when a photon is incident.

  When the circuit is configured and operated as described above, photon incidence can be correctly detected with a low threshold, and thus the gate voltage can be reduced and the avalanche current can be reduced. Then, the number of electrons excited in an avalanche is reduced, and the number of electrons trapped in the impurity level is reduced. As a result, the after pulse rate is reduced, and the repetition frequency of the gate voltage can be increased.

(Third embodiment)
FIG. 7 is a diagram showing the configuration of the third exemplary embodiment of the present invention. Although specific photon detectors using threshold circuits have been described in the first and second embodiments, the present invention is not limited to these configurations. As shown in FIG. 7, in the present invention, the voltage signal from the APD 701 and the signal indicating the gate voltage application time from the pulse power supply 703 are input to the photon incidence determining means 704, and the photon incidence determining means 704 The differential voltage of the gate voltage and the photon incidence signal due to the photon incidence on the APD 701 are determined using the gate voltage application time. Then, a signal indicating only the photon detection time not including the gate voltage application time is output.

  The photon incidence determination means 704 has a threshold circuit that outputs a signal pulse when the input signal exceeds a predetermined threshold. Here, the predetermined threshold is set smaller than the peak value of the differential waveform voltage of the gate voltage and the peak value of the photon incident signal. Thus, even if the threshold value is small, according to the above configuration, photon incidence can be detected correctly. Therefore, the avalanche current can be reduced by reducing the gate voltage. Then, the number of electrons excited in an avalanche is reduced, and the number of electrons trapped in the impurity level is reduced. As a result, the after pulse rate is reduced, and the repetition frequency of the gate voltage can be increased.

  3, 5 and 7, a resistor is arranged between the APD and the ground, and a voltage signal from the APD is obtained by the resistance voltage, but it is not intended to be limited to such a configuration.

301 APD
302 DC power supply (corresponding to "bias voltage application means")
303 Pulse power supply (corresponding to "gate voltage application means")
304 threshold circuit 305 signal processing circuit 501 APD
502 DC power supply (corresponding to “bias voltage application means”)
503 Pulse power supply (corresponding to "gate voltage application means")
504 Pulse signal source 505 Differential synthesis circuit 506 Threshold circuit 701 APD
702 DC power supply (corresponding to "bias voltage application means")
703 Pulse power supply (corresponding to "gate voltage application means")
704 Photon incidence discrimination means

Claims (3)

An avalanche photodiode (APD),
Bias voltage applying means for constantly applying a constant bias voltage to the APD;
Gate voltage application means for applying a gate voltage, which is a rectangular pulse voltage, to the APD in addition to the bias voltage;
A voltage signal from the APD and a signal indicating a gate voltage application time from the gate voltage application means are input, and a differential waveform voltage of the gate voltage in the voltage signal and a photon incidence signal due to photon incidence on the APD And photon incidence determining means for outputting a signal indicating the photon detection time.
The photon incidence determining means is a threshold circuit that outputs a signal pulse when an input signal exceeds a predetermined threshold, and the predetermined threshold is a peak value of the differential waveform voltage of the gate voltage and the threshold voltage. A photon detector comprising a threshold circuit that is smaller than a peak value of a photon incident signal. The photon incidence determination means is
The threshold circuit to which the voltage signal from the APD is input;
The signal pulse from the threshold circuit and a signal indicating the gate voltage application time from the gate voltage application means are input, the input signal at the time corresponding to the gate voltage application time is removed from the signal pulse, The photon detector according to claim 1, further comprising a signal processing circuit that outputs a signal indicating a photon detection time. The photon incidence determination means is
A pulse signal source that receives a signal indicating the gate voltage application time from the gate voltage application means, and outputs a pulse signal synchronized with the gate voltage application time;
A differential synthesis circuit that differentially synthesizes the voltage signal from the APD and the pulse signal from the pulse signal source to output a differential synthesis signal;
A threshold circuit to which the differential composite signal from the differential composite circuit is input,
The photon detector according to claim 1, wherein the signal pulse output from the threshold circuit is a signal indicating the photon detection time.

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