JP2658843B2 - Single photon measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a measuring device for measuring light intensity, and more particularly to a single-photon measuring device for measuring weak light of one photon level.

[0002]

2. Description of the Related Art Generally, single photon measurement (single photon count) using a photomultiplier tube is used for detecting weak light. In this single photon measurement,
When weak light is incident on the photomultiplier tube, for example, even with one photon, which is the minimum unit, photoelectrons are emitted from the photocathode with a predetermined probability, and the photoelectrons are amplified to about 10 6 times. At a detectable level. Then, after reducing various disturbance noises, a detection level is set that does not operate at the disturbance noise level (the reduced noise level) but operates only when there is an optical input. As a result, light can be detected even with a weak light input of about one photon.

However, in the above single photon measurement, if there are two or more photons, these photons cannot be distinguished, and as a result, a plurality of photons may be recognized as one. In other words, at intervals less than the response time of the measurement circuit,
When the light intensity is such that two photons enter, it cannot be distinguished as two signals (two photons). For this reason, when the amount of incident light increases and two photons arrive at extremely short intervals, the number of measurements may decrease.

In the above description, the incidence of one photon causes one
Although it has been described that photoelectrons are emitted, actually,
Photoelectrons are emitted with a probability according to conditions such as the type (material) of the photocathode and the wavelength (wavelength of light). Generally, such a characteristic is called quantum efficiency. The photomultiplier has a high quantum efficiency of 2 to 30% and a low quantum efficiency of 0.1% or less. Therefore, even in a photomultiplier having a high quantum efficiency, one photoelectron is emitted when three or four photons are simultaneously incident. Therefore, the light input level at which two photons cannot be distinguished actually becomes higher, but there is a problem that the two photons cannot be distinguished at a predetermined light input level.

In a practical single photon measuring device, a photomultiplier tube is used, and when a signal (optical input) is given at regular intervals, several tens of megacounts (Mcount / sec) are given.
c) Capability of about 100 mega counts. By the way, at the single photon level, light arrives discretely and photoelectron emission is stochastic as described above, so that photoelectrons may be emitted at extremely short intervals when calculating the average light quantity. Therefore, the amount of light that can be reliably counted is about 10 mega counts, which is about 1/10.

On the other hand, in a single-photon measuring device using a solid-state detecting element, for example, an avalanche photodiode (hereinafter referred to as APD), even when photons are incident at regular intervals, the measurement limit is about 10 mega counts. Is said to be. The photocurrent amplification factor of the APD is much lower than that of the photomultiplier tube. Therefore, when used in a normal linear region, the noise level in the subsequent amplifier is stronger when a single photon is incident. The detection current is buried in the noise. For this reason, a technique is employed in which a single photon is measured by applying a voltage equal to or higher than the breakdown voltage to the APD so that a large current flows by breaking down even when one photon is incident.

The above-mentioned method is called Geiger mode (using a principle similar to Geiger counter). Although it is possible to detect a single photon, the following method is used unless a large current flowing due to breakdown is temporarily stopped. Measurement becomes impossible. Actually, when a large current flows and the electric charge stored in the stray capacitance or the like is discharged due to the internal resistance of the circuit for applying a voltage to the APD, the voltage applied to both ends thereof is sufficiently lower than the breakdown voltage. To voltage.
As a result, after a while, the current stops (stops flowing), and the next measurement becomes possible. Since it takes some time until the current stops naturally, the current is forcibly stopped (in this case, a method called quenching is used).

The above-mentioned quenching method includes a passive quenching that stops a current by using a voltage drop of a resistor inserted in series with an APD, and an active quenching that stops a current at a high speed by using an active element. Although the active quench has a complicated circuit configuration, the current can be stopped earlier, so that a quick measurement can be performed. The active quench is described in, for example, “Applied Optics, June 20, 1993, Vol. 32, No. 21, pp. 3894-3900.
Page (Applied Optics, Vol 132, N
o21, July 20, 1993) "and references cited therein.

[0009]

By the way, even in the case of the above-mentioned active quench, it is considered that about 10 mega counts / sec is the limit even if it is considered that a slight improvement is possible in the future. A photomultiplier tube capable of counting at a high speed of about 10 times has extremely low quantum efficiency at wavelengths of 1 μm or more. Therefore, when the input light level is extremely low, such as a laser radar mounted on an artificial satellite, Are not suitable for photomultiplier tubes. Further, a laser radar mounted on an artificial satellite compensates for a small amount of received light by using a telescope with a large aperture, but an image for the same viewing angle becomes large with a large telescope. This has the problem that not all light can be collected on the photocathode of the small light receiving element.

As described above, the solid-state photodetector has a higher quantum efficiency than the photomultiplier tube, but has a problem that it is difficult to collect light with a large telescope due to a low count rate and a small photocathode. There is a point.

An object of the present invention is to provide a single-photon measuring device having a high count rate using a solid-state photodetector.

Another object of the present invention is to provide a single-photon measuring device capable of condensing with a large telescope.

[0013]

According to the present invention, there is provided a light-collecting device having a predetermined light-collecting diameter for receiving light and condensing the light, and a light-detecting device having a light-collecting diameter smaller than the light-collecting diameter. A photodetector element section having a plurality of elements and transmitting a detection output in response to light from the light condensing device; applying a voltage to the photodetector element and performing quenching to perform a single photon with the photodetector element; A single-photon measuring apparatus is provided, comprising: first means for performing measurement; and counter means for receiving a detection output from the photodetector and counting the detection output.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments.

Referring to FIG. 1A, the illustrated single-photon measuring device includes a light collecting device 11 such as a telescope. Light (for example, laser light) from the target is scattered by the atmosphere, aerosol, or the like, is provided to the light collector 11 as scattered light A, and is collected here. At the focal point B of the light condensing device 11, a light detection element unit 12 is disposed.
Is divided into a plurality of elements as shown in FIG. In this embodiment, the light detection element section 12 is circular,
It is divided into a plurality of sector elements 12a to 12d. Each of the elements 12a to 12d is smaller than the light-collecting diameter (indicated by the symbol C), but the light-detecting element unit 12 itself is larger than the light-collecting diameter.

Although the scattered light A is generally at a very low level, it is unlikely that a large number of photons will arrive at the light concentrator 11 at the same time, but not without it. For this reason, if light is received by the large photodetector element, if the circuit is broken down by one photon, the current continues to flow until the quenching circuit operates, so that even if the next photon arrives, it cannot operate again. . However, in the single photon measuring device shown in FIG. 1A, the elements 12a to 12d are independent of each other. For example, even when the element 12a is in a breakdown state, the other elements 12b to 12d are Since it is not affected, it can be operated by the next photon. The position at which the photon arrives within the condensing diameter C is stochastic, and the next photon may arrive at the element 12a that has been broken down at a certain probability, but the next photon will break down at a large probability. Element 12b that is not
To 12d. As a result, the next photon can be detected by the elements 12a to 12d even before the broken element 12a is ready to wait for the next photon due to the quench. That is, by using the photodetector element section 12 divided into the elements 12a to 12d, it is possible to detect photons arriving at an early interval as a whole, and it is possible to improve an apparent count rate. In addition, a photon arriving at a position where one element deviates can be captured, and a light-collecting device having a large light-collecting diameter can be used.

In the illustrated single photon measuring device, each element 12a
To 12d are voltage application / quenching circuits 13a, respectively.
To 13d, and the voltage application / quenching circuits 13a to 13d are connected to the counter circuit 14, respectively.

Here, referring also to FIG.
When photons are detected in 2a to 12d, detection outputs are given to the counter circuit 14 via the voltage application / quench circuits 13a to 13d, respectively. Here, FIG.
As shown in the figure, three detection outputs are given to the counter circuit 14 from time T to two counter outputs from the time T to 2T, and two detection outputs are given to the counter circuit 14 from time T to 3T. If given (here, the time interval T is assumed to be a unit time), the counter circuit 14 stores the count value in the memory 15 as a count of the detection output for each unit time T.

In this way, by measuring photons, measurement having a time resolution can be performed. Also, as shown in FIG. 1 (c), by the time T, even if the detection output of the element 12a and the detection output of the element 12c are extremely close to each other, there is a sufficient interval between the individual elements, so that measurement is not possible. There is no hindrance. In particular, in a laser radar or the like, distance resolution or altitude resolution is performed by dividing the number of measurement pulses at regular intervals (unit time).
Therefore, it is possible to simultaneously measure the scattered light at each altitude with one laser pulse.

Although a photodetector element section having a plurality of elements may be commercially available, a voltage higher than the breakdown voltage should be taken into consideration because of its intended use in a normal linear region. Then, insulation between elements becomes a problem.
If the gap between the elements is increased in order to avoid the problem of insulation, the non-light-receiving area increases, so the configuration shown in FIG. 2 is used. That is, the photodetecting element unit 12 is a polygonal pyramid-shaped reflecting mirror 1.
6 and a plurality of single photodetectors 17 to 20. In the illustrated example, the reflecting mirror 16 has four reflecting surfaces,
Single photodetectors 17 to 2 corresponding to each reflection surface
0 (here, the reflecting mirror 1
Although the case where 6 has four reflecting surfaces is shown, the number is four.
The number is not limited to the number, and may be appropriately increased as needed.)

By the way, when the light collecting device 2 is large,
The condensing diameter may be larger than twice the photocathode of the single photodetector. In this case, as shown in FIG. 3, a reflecting mirror (polyhedral reflecting mirror) 21 having a number of surfaces having different inclinations due to diamond cutting or the like is prepared, and the number of single light detections corresponding to the number of these surfaces is prepared. An element (only the light detection elements 22 and 23 are shown in FIG. 3) is arranged. As a result, of course, the count rate can be improved.

In FIG. 1, for simplicity of explanation, elements irrelevant to the original function are omitted, but in reality, as shown in FIG. A field stop 24 for restricting light, a collimator lens 25 for converting the condensed light into parallel light once, an interference filter 26 for cutting unnecessary wavelengths, and a condensing lens 27 for condensing parallel light again are provided. But it is not an absolute component.

[0023]

As described above, in the present invention, a plurality of photodetectors are used to share and receive light having a diameter larger than the photocathode of the device. There is an effect that single photon measurement can be performed.

[Brief description of the drawings]

FIG. 1A is a block diagram showing an embodiment of a single-photon measuring device according to the present invention, and FIG. 1B shows an example of a configuration of a photodetector unit in the single-photon measuring device shown in FIG. Figure,
(C) is a figure for explaining operation of a single photon measuring device shown in (a).

FIG. 2 is a diagram showing another example of the light detection element unit, and FIG.
FIG. 4 is a diagram showing a polygonal pyramid-shaped reflecting mirror, and FIG. 4B is a diagram showing a positional relationship between a polygonal pyramid-shaped reflecting mirror and a photodetector.

FIG. 3 is a diagram showing another example of the photodetector element section, and (a)
FIG. 2 is a diagram showing a polyhedral reflecting mirror and a photodetector, and FIG. 2B is a diagram showing the polyhedral reflecting mirror from above.

FIG. 4 is a diagram illustrating an example of an apparatus used when performing single-photon measurement by a laser radar.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Condenser 12 Light detecting element part 12a-12d Light detecting element 13a-13d Voltage application / quench circuit 14 Counter circuit 15 Memory 16 Polygonal pyramid-type reflecting mirror 17-20,22,23 Single light detecting element 21 Polyhedral reflecting mirror 24 Field stop 25 Collimating lens 26 Interference filter 27 Condensing lens

Claims (4)

(57) [Claims] 1. A light-collecting device comprising: a light-collecting device having a predetermined light-collecting diameter and receiving light and condensing the light; and a plurality of photodetectors having a size smaller than the light-collecting diameter. A light detection element unit that sends out a detection output in response to the light of the light, a first unit that applies a voltage to the light detection element and performs quenching to perform single photon measurement with the light detection element; A single-photon measuring device, comprising: counter means for receiving a detection output from the photodetector and counting the detection output. 2. The single-photon measuring device according to claim 1, wherein the light-detecting element section is divided into a plurality of elements, and each element forms the light-detecting element. Single photon measurement device. 3. The single-photon measuring device according to claim 1, wherein the photodetector unit further includes a polygonal pyramid-shaped reflecting mirror, and the polygonal pyramid-shaped reflecting mirror is configured to emit light from the light collecting device. Receiving the reflected light from the polygonal pyramid-shaped reflecting mirror, wherein the photodetector is a single photocathode photodetector arranged at a position for receiving the reflected light from the polygonal pyramidal reflector. 4. The single photon measuring device according to claim 1, wherein the light detecting element further includes a polyhedral reflecting mirror, the polyhedral reflecting mirror receiving light from the light collecting device, and The single photon measuring device, wherein the photodetector is a single photocathode photodetector arranged at a position for receiving the reflected light from the polyhedral reflector.