JP2009117752A - Weak light detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a weak light detector which is not easily affected by the sticking of dust and moisture or the like and can obtain multiplied light. <P>SOLUTION: By feeding back signals from the output side to the input side using a photocoupler, the insulation of the input side and the output side is improved. A photodetector for emitting light to be detected is a first photodetector; and the photodetector of the signal feedback is a second photodetector. A circuit configuration for that is as follows. The first photodetector and the second photodetector are serially connected, and a predetermined potential difference is impressed to both ends. The potential of the node of the first photodetector and the second photodetector is input to the inverted input terminal side of an operational amplifier, and a non-inverted input terminal is grounded. The output voltage of the operational amplifier is applied to a light emitting element to emit light. The emitted light is branched in a light branching device, one is input to the second photodetector, and the other is output. The output voltage of the operational amplifier is detected or the intensity of the light output from the light branching means is detected. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a weak light detector configured to use photoelectric conversion elements and detect weak light with high sensitivity by using a photocoupler in a feedback circuit.

  Up to now, for example, an avalanche photodiode (APD), a photomultiplier tube (PMT), or a CCD camera has been used as a photodetector for detecting weak light. However, for example, in the field of nanotechnology and the leading-edge fields of biology and chemistry dealing with DNA, environmental hormones, etc., more sensitive light detection is required. The above-described photodetector also has several problems that limit its use.

  The weak light detector of the present invention not only realizes light detection with higher sensitivity than the above-described light detector, but also has an excellent characteristic that it can output light with increased intensity. have.

  The inventor of the present invention has already disclosed an invention relating to a very weak light detector and a very weak light imaging apparatus in Japanese Patent Application Laid-Open No. 2006-203050. This disclosure is shown in FIG. This uses an APD multiplication element using low-speed electrons. In this circuit, a feedback capacitance element for accumulating carriers generated and multiplied inside the APD is connected to an avalanche photodiode (APD) whose bias voltage is adjusted so that the multiplication factor is 30 or less, The voltage of the feedback capacitance element is input to the gate electrode of the reading transistor, and the output of the transistor is periodically read. The voltage of the feedback capacitance element is set to a predetermined voltage each time it is read through the reset semiconductor diode. By resetting, the intensity of the light applied to the APD is detected.

  That is, instead of using electrons accelerated at such a high speed as to cause a significant avalanche effect as in the past, low-speed electrons are used in the region of the acceleration voltage where the excitation cross section becomes high. In general, the excitation scattering cross section increases as the electrons become slower, but decreases rapidly when the excitation energy is below a certain level of excitation energy necessary for generating new carrier electrons. Therefore, the scattering cross section as a function of the electron velocity has a peak at a velocity where the kinetic energy is several times the excitation energy. If the electron velocity comes close to this, the electrons will be excited with a high probability, and the photoelectrons will surely generate the next carrier electrons. If the electrons generated in this way are also slow, further carrier electrons are surely generated, and the fluctuation of the multiplication factor is reduced.

  Patent Document 1 describes that when the total excess noise coefficient is a multiplication factor of 30 or less, the measured value is lower than the calculated value. For this purpose, it is described that the bias voltage applied to the APD is adjusted so that the multiplication factor is 30 or less.

  The main difference between the present invention and the disclosure of Patent Document 1 is an operational amplifier (op-amp) feedback circuit. In the disclosure in Patent Document 1, a feedback capacitive element constituting an integrating circuit is used, but a photocoupler is used in the present invention. Due to the difference in these configurations, in the case of Patent Document 1, the feedback capacitance element is periodically reset, but in the case of the present invention, continuous measurement can be performed.

  Japanese Patent Application Laid-Open No. 59-181680 discloses a circuit for outputting a current equal to an input current using a feedback circuit using a photocoupler. This is shown in FIG. Further, the emitter current of the transistor Qc in FIG. 6 is a current proportional to the input current.

  The main difference between the disclosure of Patent Document 2 and the present invention is that in the present invention, the input current is the current from the photodetector, the output of the operational amplifier is used as the detection output, and the feedback amount in the photocoupler is For example, it is variable.

JP 2006-203050 A JP 59-181680 A

  In the conventional circuit system, a resistance feedback type circuit that is easy to handle but has low sensitivity or a capacitive feedback type circuit that has high sensitivity but is complicated to manufacture and handle is selected as necessary. In the present invention, a light generator and a light detector are introduced into a feedback circuit, and the same effect as that obtained by using light as a result of current feedback is obtained. By using light, the insulation of the input and output of the feedback circuit is made extremely high.

  As described above, by making the input and output insulation of the feedback circuit extremely high, for example, thermal noise can be suppressed. In addition, it is possible to manufacture a weak light detector that is not easily affected by adhesion of dust or moisture and is easy to handle. In addition, the multiplied light can be obtained.

  The present invention is a weak light detector using an operational amplifier, which improves the insulation between the input side and the output side by performing signal feedback from the output side to the input side using a photocoupler. In general, the photodetector that irradiates the light to be detected is a first photodetector, and the signal feedback photodetector is a second photodetector. At this time, the circuit configuration is as follows. That is, the first photodetector and the second photodetector are connected in series, and a predetermined potential difference is applied to both ends thereof. The potential at the node between the first photodetector and the second photodetector is input to the inverting input terminal side of the operational amplifier, and the non-inverting input terminal is grounded. The output voltage of the operational amplifier is applied to the light emitting element to emit light. The emitted light is branched by an optical branching device, one of which is input to the second photodetector and the other is output. With this configuration, as described above, whether the input light to be detected is input to the first photodetector and the output voltage of the operational amplifier is detected or the intensity of the light output from the optical branching means is detected. Then, the intensity of the input light is detected.

  It is possible to increase the sensitivity by connecting a plurality of the weak light detectors in series and using the output light of the preceding stage as the input light of the subsequent stage for two consecutive stages.

  A light intensity detecting means is further provided, and the light intensity of the light output from the light branching means can be measured using the light intensity detecting means.

  As the optical branching unit, a spatial type, an optical fiber type, or a planar waveguide type optical branching unit can be used. It is desirable that the branching ratio in this branching can be adjusted from the outside. The branching ratio may be adjusted by switching to an optical branching unit having a different branching ratio.

  For example, a semiconductor diode to which a reverse bias voltage is applied can be used for the first photodetector and the second photodetector.

  The first photodetector is, for example, a photodiode detection element, an avalanche photodiode, a photoconductive diode, or a pyroelectric detection element.

  The light emitting element is a light emitting diode connected in series with a resistance element, and is connected between the operational amplifier and a predetermined potential terminal.

  In the following description, devices having the same function or similar functions are denoted by the same reference numerals unless there is a special reason.

  FIG. 1 shows a circuit diagram of the weak light detector 101 of the present invention. This is a weak light detector using an operational amplifier, and improves the insulation between the input side and the output side by performing signal feedback from the output side to the input side using a photocoupler. The photodetector for irradiating the weak light to be detected is the first photodetector 6, and the signal feedback photodetector is the second photodetector 7. The first photodetector 6 and the second photodetector 7 are connected in series, and a predetermined potential difference is applied to both ends thereof. In FIG. 1, one is grounded and a negative bias potential is applied to the other. The potential at the node between the first photodetector 6 and the second photodetector 7 is input to the inverting (or non-inverting) input terminal side of the operational amplifier 2, and the non-inverting (or inverting) input terminal is grounded. The output voltage of the operational amplifier 2 is applied to the light emitting element 3 to emit light. When the input terminal of the operational amplifier 2 is set in the above parentheses, the direction of the light emitting element 3 is reversed. The light emitted from the light emitting element 3 is input to the second photodetector 7.

  With this configuration, as described above, when the input light to be detected is input to the first photodetector 6, the light emitted by the light emitting element 3 passes through the waveguide 10, and the amount of light is reduced by the optical splitter 13. By entering the second photodetector 7 through 11, a current flows through the second photodetector 7, and the current from the first photodetector 6 and the current from the second photodetector 7 are balanced.

  The photo-induced current flowing through the third photodetector 9 can be read with an ammeter connected to the terminal T4, for example. As a matter of course, the intensity of incident light can also be known by reading the output voltage of the operational amplifier 2.

  The optical branching unit 13 guides the light from the light emitting element 3 to the optical branching unit 13 through the waveguide 10, and the waveguide 11 connected to the second photodetector 7 and the waveguide 12 connected to the third photodetector 9. It branches to. In this branching, it is desirable to increase the number of branches to the third photodetector 9. For example, if there is no other loss by setting one to 100, the input light to the first photodetector 6 Light emission of almost 100 times can be obtained from the light emitting element 3. Moreover, it is desirable that the light intensity division ratio of the optical branching device 13 can be arbitrarily set.

  Here, as the optical branching device, the spatial type (bulk type) shown in FIG. 5A, 5B or 5C can be used. FIG. 5A adjusts the amount of light incident on the waveguides 11 and 12 by moving the lens. 5B and 5C are respectively adjusted by the transmittance of the beam splitter (half prism) and the partial transmission mirror (half mirror). Further, an optical fiber type optical branching device shown in FIG. The branching ratio is adjusted by the distance between the cores and the close distance (coupling length). Further, the planar waveguide type shown in FIG.

  Further, as can be easily understood, by changing the resistance value of the variable resistor 4, the light emission intensity of the light emitting element 3 can be changed, so that the sensitivity to the input light, the dynamic range, and the like can be changed.

  Similarly, it can be seen that the ratio of the output voltage of the operational amplifier 2 to the input light to the first photodetector 6, that is, the sensitivity can be changed also by changing the branching ratio of the optical splitter 13. The optical branching device 13 is commercially available, and the branching ratio may be variable, but if it is not variable, the branching ratio is changed by replacing it.

  The first photodetector 6 and the second photodetector 7 are, for example, a photodiode detection element, an avalanche photodiode, a photoconductive diode, or a pyroelectric detection element.

  FIG. 2 shows a weak light detector 102 in which the third photodetector 9 and the wiring around the third photodetector 9 in the configuration of FIG. 1 are removed, and instead the waveguide 12 is extended to the outside. This weak light detector 102 can be cascaded. For example, as shown in FIG. 3, light to be measured is incident on the first-stage weak light detector 102, and output light from the waveguide 12 is incident on the first photodetector 6 in the next stage. The weak light detector 101 detects the output light at this stage. This last stage may be the weak light detector 101 or the light detector shown in FIG. As described above, since the light intensity is sequentially increased by the cascade connection, the sensitivity can be easily increased.

  For example, the present invention can be used as a radiation detector by using a semiconductor radiation detector as the first photodetector. In this case, the radiation intensity is converted into voltage, current or light intensity.

It is a circuit diagram of the weak light detector of the present invention using an optical branching device. It is a circuit diagram of the weak light detector of the present invention capable of cascade connection. It is a block diagram which shows the example of a cascade connection. It is a figure which shows the example of an optical splitter. FIG. 10 is a circuit diagram of a very weak photodetector disclosed in Patent Document 1. 10 is a circuit diagram of a current source circuit disclosed in Patent Document 2. FIG.

Explanation of symbols

2 Operational Amplifier 3 Light-Emitting Element 4 Variable Resistance 6 First Photodetector 7 Second Photodetector 8 Waveguide 9 Third Photodetector 10, 11, 12 Waveguide 13 Amplifier 101, 102 Weak Photodetector T1-T4 Terminal

Claims (7)

A series connection circuit of a first photodetector and a second photodetector to which a predetermined potential difference is applied;
An operational amplifier in which the potential of the node between the first photodetector and the second photodetector is input to the inverting input (or non-inverting) terminal, and the non-inverting (or inverting) input terminal is grounded;
A light emitting element that emits light upon application of an output voltage of the operational amplifier;
A light branching means for branching the light of the light emitting element;
Input means for inputting the light branched by the light branching means to the second photodetector;
Means for outputting the light branched by the light branching means,
The input light to be detected is input to the first photodetector and the intensity of the input light is detected by detecting the output voltage of the operational amplifier or the intensity of the light output from the optical branching means. A weak light detector. A weak light detector, wherein a plurality of the weak light detectors according to claim 1 are connected in series, and the output light of the preceding stage is used as the input light of the subsequent stage for two consecutive stages. A light intensity detecting means;
3. The feeble light detector according to claim 1, wherein the light intensity of the light output from the light branching means is measured using the light intensity detecting means. The weak light detector according to any one of claims 1 to 3, wherein the optical branching unit is a spatial, optical fiber, or planar waveguide type optical branching unit. The weak photodetector according to any one of claims 1 to 4, wherein the first photodetector and the second photodetector are semiconductor diodes to which a reverse bias voltage is applied. 5. The weak light according to claim 1, wherein the first photodetector is a photodiode detection element, an avalanche photodiode, a photoconductive diode, or a pyroelectric detection element. Detector. 7. The light emitting device according to claim 1, wherein the light emitting device is a light emitting diode connected in series with a resistance device, and is connected between the operational amplifier and a predetermined potential terminal. The weak light detector described.