JP6523046B2 - Optical sensor - Google Patents

Description

  The present invention relates to an optical sensor with improved disturbance light resistance.

  Conventionally, in the optical communication and time-of-flight measurement (TOF), an avalanche photodiode using an avalanche amplification effect of a photodiode is used as a light receiving element for detecting weak light at high speed. The avalanche photodiode operates in a linear mode when a reverse bias voltage less than the breakdown voltage is applied, and the output current fluctuates so as to have a positive correlation with the amount of light received. On the other hand, the avalanche photodiode operates in Geiger mode when a reverse bias voltage higher than the breakdown voltage is applied. A Geiger mode avalanche photodiode causes an avalanche phenomenon even when single photons are incident, so a large output current can be obtained. For this reason, Geiger mode avalanche photodiodes are called single photon avalanche diodes (SPADs).

JP, 2014-81254, A (May 8, 2014 publication) Unexamined-Japanese-Patent No. 2010-28340 (February 4, 2010 publication)

  The avalanche photodiode can detect weak light in the above Geiger mode, but when disturbance light components other than the signal light enter the light receiving section, it can not distinguish between the signal light and the disturbance light, and the signal light can be accurately detected. It can not be detected.

  Therefore, in order to improve the resistance to disturbance light (hereinafter referred to as disturbance light resistance), Patent Document 1 describes that the light reception sensitivity is controlled in accordance with the amount of light received by the reference light reception means. In the same document, it is shown that the light reception sensitivity is controlled by changing the bias voltage to the avalanche photodiode.

  However, when the reverse bias voltage is in the vicinity of the breakdown voltage, the light reception sensitivity largely changes with respect to a minute change in the reverse bias voltage, which makes sensitivity control difficult. Further, in the Geiger mode, since the light reception sensitivity does not fluctuate linearly with respect to the light reception amount, it is not possible to set an appropriate light reception sensitivity for disturbance light. Therefore, with the technique of Patent Document 1, sufficient disturbance light resistance can not be obtained.

  Further, Patent Document 2 describes that in an optical receiver, an AC voltage component of an electrical signal converted from an optical input signal by an optical-electrical conversion circuit is converted into a DC voltage component. The document further indicates that a voltage corresponding to the DC voltage component is supplied to the photoelectric conversion circuit using a current extracting circuit. Thus, although the electrical signal can be adjusted according to the alternating current component of the disturbance light, it is difficult to adjust the electrical signal according to the direct current component of the disturbance light. Therefore, even if the technique of Patent Document 2 is used, there is a problem that sufficient disturbance light resistance can not be obtained.

  The present invention has been made in view of the above problems, and an object thereof is to provide a technique for improving the disturbance light resistance of an optical sensor.

  In order to solve the above problems, an optical sensor according to an aspect of the present invention includes a light receiving element, a resistor connected in series to the light receiving element, and a series connected to the light receiving element and the resistor. And a control unit configured to control the resistance value of the resistor according to the alternating current component of the disturbance light and control the current value of the current source according to the direct current component of the disturbance light.

  According to one aspect of the present invention, the disturbance light tolerance of the optical sensor can be improved.

It is a circuit diagram showing composition of an optical sensor concerning Embodiment 1 of the present invention. It is a circuit diagram showing composition of two photosensors concerning Embodiment 2 of the present invention. It is a graph which shows the Example which the control part detects disturbance light and raises detection accuracy of signal light based on Embodiment 2 of this invention. (A) is a graph which shows the waveform of the light source signal of a light emitting element. (B) is a graph which shows the waveform of the Geiger mode output signal in an optical sensor. (C) is a graph which shows the waveform of the linear mode output signal in an optical sensor. It is a layout which shows arrangement | positioning of a photodiode based on Embodiment 3 of this invention. (A) is a layout diagram when the upper left photodiode is set to the linear mode. (B) is an arrangement | positioning figure at the time of setting the photodiode on the upper left to Geiger mode. It is sectional drawing which shows the structure of the photodiode based on Embodiment 1 of this invention. It is a graph which shows the Example which a control part detects signal light correctly based on Embodiment 1 of this invention. (A) is a graph which shows the waveform of a light source signal. (B) is a graph which shows the waveform of the output signal to the transistor 15 before correct | amending electric potential V1. (C) is a graph which shows the waveform of the output signal to the transistor 15 after correct | amending electric potential V1.

  Hereinafter, embodiments of the present invention will be described in detail. However, unless otherwise specified, the configuration described in this embodiment is not intended to limit the scope of the present invention to that only, and is merely an illustrative example.

Embodiment 1
<Configuration of optical sensor>
First, an optical sensor 1 according to Embodiment 1 of the present invention will be described with reference to FIG. FIG. 1 is a circuit diagram showing a configuration of an optical sensor 1 according to the present embodiment. The optical sensor 1 is a device that outputs a voltage in response to reception of signal light, and as shown in FIG. 1, the photodiode 11, the quenching resistor 12, the switch 13, the variable current source 14, the transistor 15, and A control unit 16 is provided.

  The photodiode 11 is a light receiving element, and an avalanche photodiode is used. In the optical sensor 1, the photodiode 11 operates in Geiger mode by applying a reverse bias voltage equal to or higher than the breakdown voltage across its both ends. The reverse bias voltage is determined as the potential difference between the high potential HV and the potential V1 minus the voltage drop due to the quenching resistor 12.

  The configuration of the photodiode 11 according to the first embodiment of the present invention will be described with reference to FIG. FIG. 5 is a cross-sectional view showing the configuration of the photodiode 11 according to the present embodiment. As shown in FIG. 5, the photodiode 11 is formed of a PN junction. That is, P layer 111 is formed on N layer 112, and N layer 112 is formed on P substrate 113. The P layer 111 is a layer of a P-type semiconductor and is connected to the anode electrode AN. The N layer 112 is a layer of an N type semiconductor and is connected to the cathode electrode CA. N + indicates a portion of the N layer 112 where the concentration of the N-type impurity is high. The P substrate 113 is a silicon substrate and is connected to the ground electrode GND.

  The photodiode 21 according to the second embodiment of the present invention and the photodiode 31 according to the third embodiment of the present invention have the same structure.

  The quenching resistor 12 is a resistor that operates to stop the current from the photodiode 11. The quenching resistor 12 is configured by connecting a plurality of resistors connected in parallel to one another in series with the photodiode 11. The switch 13 is an on / off changeover switch, is interposed between the photodiode 11 and each quenching resistor 12, and connects each quenching resistor 12 to the photodiode 11 in accordance with an instruction from the control unit 16. Switch whether or not. Therefore, the resistance value of the quenching resistor 12 as a whole is determined depending on the on or off state of each switch 13. Thus, the quenching resistor 12 functions as a variable resistor. The configuration in which the switch 13 is attached to each quenching resistor 12 is an example of a variable resistor, and the quenching resistor 12 may be any other configuration as long as it is a variable resistor that can be controlled by the control unit 16 .

  The variable current source 14 is a current source capable of changing the current value, and is connected in series to the photodiode 11 and the quenching resistor 12. The variable current source 14 changes the value of the current flowing therethrough in accordance with an instruction from the control unit 16. The transistor 15 is a field effect transistor, and performs switching operation according to the potential difference between the potential V1 of the output current of the photodiode 11 operating in Geiger mode and the ground potential V0. Hereinafter, an output voltage that performs switching operation by the transistor 15 and performs digital driving is referred to as Geiger mode output signal # 1.

  The controller 16 is connected to the switch 13 and controls the resistance value of the quenching resistor 12 according to the AC component of the disturbance light according to the instruction to the switch 13. The control unit 16 is also connected to the variable current source 14 and controls the current value output from the variable current source 14 according to the direct current component of the disturbance light according to an instruction to the variable current source 14.

<Operation of optical sensor>
When light is incident on the photodiode 11 to which a reverse bias voltage is applied, avalanche amplification occurs and a current is output from the photodiode 11 to the quenching resistor 12. As a result, a potential difference is generated between the potential V1 and the ground potential V0. This potential difference may fluctuate depending on the difference between the amount of current from the photodiode 11 and the value of current from the variable current source 14 in the quenching resistor 12. When the potential difference exceeds the threshold voltage of the transistor 15, it is converted to a digital signal and output as a Geiger mode output signal # 1. However, when disturbance light is incident on the photodiode 11, the light reception sensitivity of the photodiode 11 fluctuates, the threshold voltage of the transistor 15 is exceeded, and the switching operation can not be performed on the signal light.

  Therefore, in the present embodiment, as shown in FIG. 1, the control unit 16 changes the resistance value of the quenching resistor 12 and the current value of the variable current source 14 according to the amount of received disturbance light. The output voltage of the photodiode 11 (Geiger mode output signal # 1) is stabilized.

  More specifically, the alternating current component of the disturbance light can be reduced from the output voltage of the photodiode 11 by changing the resistance value of the quenching resistor 12 according to the peak value of the alternating current component of the disturbance light.

  In addition, since the variable current source 14 has less variation in resistance value with respect to the current value, the current value of the variable current source 14 is changed according to the DC component of disturbance light, and the current of the current value is output to the quenching resistor 12 Do. As a result, since the current corresponding to the DC component of the disturbance light is subtracted from the output current of the photodiode 11, the DC component of the disturbance light can be reduced from the output voltage of the photodiode 11.

The fact that the variable current source 14 has less variation in resistance value with respect to the current value will be described below. That is, when the variable current source 14 is configured by a CMOS transistor, the resistance component Rs of the transistor is Rs = 1 / {2√ (kI)} (k = 15.5 × 10 ⁻⁶ [wherein, it varies depending on the transistor size]), I = current value). For example, Rs = 12.7 kΩ at I = 0.1 mA, and Rs = 4 kΩ at I = 1 mA. That is, even if the current value increases 10 times, the resistance value decreases about 1/3 times. Therefore, the influence of the variable current source 14 on the quenching effect when the light reception signal is input to the quenching resistor 12 is reduced.

<Example of signal light detection>
Next, an example of a method of accurately detecting signal light according to the first embodiment of the present invention will be described with reference to FIG. FIG. 6A is a graph showing the waveform of the light source signal. FIG. 6B is a graph showing the waveform of the output signal to the transistor 15 before the potential V1 is corrected. FIG. 6C is a graph showing the waveform of the output signal to the transistor 15 after the potential V1 has been corrected. In this method, the signal detection level of the light reception signal is adjusted to the threshold voltage of the transistor 15.

  As shown in FIG. 6B, the signal before the potential V1 correction is detected not only in the signal light component but also in the AC component of the disturbance light.

  Therefore, at timings other than when the light source signal is input, the control unit 16 monitors the level of the DC component of the disturbance light and the level of the AC component. When the DC component of the disturbance light is detected (when the DC component of the disturbance light is determined to be larger than a predetermined threshold), the control unit 16 increases the amount of current of the variable current source 14 as follows: As shown in FIG. 6C, the potential V1, that is, the output voltage to the transistor 15 is reduced. Thereby, detection of the DC component of disturbance light can be suppressed.

  Further, when the AC component of the disturbance light is detected (when it is determined that the AC component of the disturbance light is larger than a certain predetermined threshold), the control unit 16 reduces the resistance value of the quenching resistor 12 as follows. As shown in FIG. 6C, the gain is improved, the amplitude difference between the signal light and the noise light is increased due to the optical design difference, and the potential V1, that is, the output voltage to the transistor 15 is generated. Thereby, the signal light component and the alternating current component of the disturbance light can be determined. If a plurality of output voltages of the photodiodes 11 shown in FIG. 1 are added, the simultaneous detection probability of the signal light is increased, so that the amplitude difference between the signal light and the noise light can be further increased. However, when the resistance value of the quenching resistor 12 is reduced, the time for quenching completion (pulse off) is increased, so the resistance value is set to an optimum value from the viewpoint of trade-off.

<effect>
Not only a direct current component but also an alternating current component by inverter light such as a fluorescent lamp in a room exists in disturbance light. On the other hand, by removing both the DC component and the AC component of the disturbance light from the output voltage of the photodiode 11, the output voltage can be stabilized and the signal light can be accurately detected. That is, the disturbance light tolerance of the light sensor 1 can be improved.

  In addition, by improving the disturbance light tolerance of the optical sensor, it becomes possible to facilitate the design of the lens and the package filter and to widen the use range of the optical sensor outside the room or the like.

Second Embodiment
<Configuration of optical sensor>
The light sensor according to the second embodiment of the present invention includes the light sensors 1a and 2 shown in FIG. In the following, the light sensors 1a and 2 will be described with reference to FIG. FIG. 2 is a circuit diagram showing the configuration of the light sensors 1a and 2 according to the present embodiment. As shown in FIG. 2, the optical sensor 1 a is a device that receives signal light and outputs a voltage according to the amount of received light, and the photodiode 11, the quenching resistor 12, the switch 13, the variable current source 14, and the transistor 15. , A control unit 16a, and a light emitting element 17. The functions and connections of the photodiode 11, the quenching resistor 12, the switch 13, the variable current source 14, and the transistor 15 are the same as in the first embodiment. The control unit 16a will be described later.

  The light emitting element 17 is an element that emits light to the detection target, and is disposed adjacent to the photodiode 11 of the light receiving element. On the other hand, the photodiode 11 reflects the light emitted from the light source of the light emitting element 17 to the object to be detected, and receives the returned light as signal light. The light emitting element 17 is used, for example, when measuring the delay time of the reflected light from the detection target.

  The optical sensor 2 is a device that receives disturbance light and outputs a voltage according to the amount of received light, and includes a photodiode 21, a resistor 22, and a variable current source 23. The photodiode 21 is a light receiving element, and an avalanche photodiode is used. In the light sensor 2, the photodiode 21 operates in the linear mode by applying a reverse bias voltage less than the breakdown voltage across the both ends thereof. The reverse bias voltage is determined as the potential difference between the low potential LV and the potential V2 minus the voltage drop across the resistor 22.

  The resistor 22 is connected in series to the photodiode 21 and has a fixed resistance value. The variable current source 23 is a current source whose current value can be changed, and is connected in series to the photodiode 21 and the resistor 22. The variable current source 23 changes the value of the current flowing therethrough in accordance with an instruction from the control unit 16a.

<Operation of optical sensor>
When light is incident on the photodiode 21 to which a reverse bias voltage is applied, a current following the amount of light received is output from the photodiode 21 to the resistor 22. Thereby, a potential difference is generated between the potential V2 and the ground potential V0. This potential difference may fluctuate according to the difference between the current value from the photodiode 21 and the current value from the variable current source 23 in the resistor 22. The potential difference, that is, the output voltage of the photodiode 21 is output as the linear mode output signal # 2.

  The control unit 16a acquires the output voltage of the photodiode 11 of the light sensor 1 amplified by the transistor 15 as the Geiger mode output signal # 1. Further, the control unit 16a acquires the output voltage of the photodiode 21 of the light sensor 2 as the linear mode output signal # 2. Then, the control unit 16a refers to the linear mode output signal # 2 to detect the ratio of the alternating current component to the direct current component of the disturbance light. The controller 16a calculates a target resistance value of the quenching resistor 12 based on the value of the ratio, and adjusts the resistance value of the quenching resistor 12 so as to be the target resistance value. Further, the control unit 16a calculates a target current value of the variable current source 14 based on the value of the ratio, and adjusts the current value of the variable current source 14 so as to be the target current value.

  Furthermore, when the linear mode output signal # 2 includes the DC component of disturbance light, the control unit 16a increases the amount of current of the variable current source 23 to increase the potential V2, ie, the linear mode output signal # 2. Reduce the voltage of Thereby, detection of the DC component of disturbance light can be suppressed.

<effect>
In the optical sensor 1a, when the photodiode 11 is used in Geiger mode, the reception sensitivity of signal light may fluctuate due to disturbance light, an AC component of electrical noise, or the like. Therefore, the ratio of the alternating current component and the direct current component of the disturbance light is detected, and the resistance value of the quenching resistor 12 and the current value of the variable current source 14 are calculated, or the resistance value and the current value are calculated. give feedback. Thus, by removing the disturbance light component from the output voltage of the photodiode 11, the signal light can be accurately detected. That is, the disturbance light tolerance of the light sensor 1a can be improved.

<Example>
Next, an example according to the second embodiment of the present invention in which the control unit 16a detects disturbance light to increase the detection accuracy of signal light will be described with reference to FIG. FIG. 3A is a graph showing the waveform of the light source signal of the light emitting element 17. FIG. 3B is a graph showing the waveform of the Geiger mode output signal # 1 in the optical sensor 1a. FIG. 3C is a graph showing the waveform of the linear mode output signal # 2 in the light sensor 2.

  In the graph of FIG. 3B, for convenience, the waveform of the Geiger mode output signal # 1 is illustrated as a pulse waveform. However, the waveform of the Geiger mode output signal # 1 has a shape as shown in the frame FR, in which waveforms having a narrow width are arranged. Such a waveform is generated by the voltage output of the photodiode 11 and the quenching of the quenching resistor 12. The number of the narrow-width waveforms differs depending on the detection time of the photodiode 11.

  As shown in FIG. 3A, the light emitting element 17 emits light from the light source to the object to be detected at time t1. Next, as shown in FIG. 3B, the control unit 16a detects a Geiger mode output signal # 1 according to the signal light which is the reflected light at a time t2 with a predetermined time delay from the time t1. Then, as shown in FIG. 3C, the control unit 16a detects a linear mode output signal # 2 according to the signal light at time t3 with a predetermined time delay from time t2.

  Similarly, as shown in FIG. 3A, the light emitting element 17 emits light from the light source to the detection target at time t4. Next, as shown in FIG. 3B, the control unit 16a detects a Geiger mode output signal # 1 according to the signal light which is the reflected light at a time t5 with a predetermined time delay from the time t4. Then, as shown in FIG. 3C, the control unit 16a detects a linear mode output signal # 2 corresponding to the signal light at time t6, which is delayed by a predetermined time from time t5.

  Here, as shown in FIG. 3 (c), when the controller 16a detects an AC component of disturbance light at time t2 in the linear mode output signal # 2, as shown in FIG. 3 (b), Geiger mode It is determined that the signal at time t2 in the output signal # 1 is due to noise.

  On the other hand, as shown in FIG. 3 (c), when the controller 16a does not detect the AC component of the disturbance light at time t5 in the linear mode output signal # 2, as shown in FIG. 3 (b), It is determined that the signal at time t5 in the mode output signal # 1 is the signal light to be detected.

  Furthermore, the control unit 16a detects the direct current component of the disturbance light in the linear mode output signal # 2, and detects the signal light accurately by adjusting the current value of the variable current source 14 according to the direct current component. Can.

Third Embodiment
<Configuration and operation of optical sensor>
The optical sensor according to the third embodiment of the present invention includes a plurality of photodiodes arranged in an array, in which Geiger mode photodiodes and linear mode photodiodes are alternately arranged.

  The arrangement of the photodiodes 31 according to the third embodiment of the present invention will be described with reference to FIG. FIG. 4A is a layout view in the case where the upper left photodiode 31a is set to the linear mode. FIG. 4B is a layout diagram when the upper left photodiode 31a is set to the Geiger mode. In each of the drawings, a total of 36 photodiodes 31 are arranged, 6 in length × 6 in width.

  The linear mode is set by applying a reverse bias voltage less than the breakdown voltage to both ends of the photodiode 31. The Geiger mode is set by applying a reverse bias voltage higher than the breakdown voltage to both ends of the photodiode 31. The application of the reverse bias voltage to each photodiode 31 is controlled by a control unit 16 b (not shown).

  As shown in FIG. 4A, the upper left photodiode 31a is set to the linear mode. On the other hand, the photodiode 31b adjacent to the right side of the photodiode 31a is set to Geiger mode. The photodiode 31c adjacent to the lower side of the photodiode 31a is set to Geiger mode. Then, as shown in FIG. 4A, as a whole, the Geiger mode and the linear mode are alternately set in the longitudinal direction and the lateral direction.

  On the other hand, as shown in FIG. 4B, the photodiode 31a on the upper left is set to Geiger mode. On the other hand, the photodiode 31b adjacent to the right of the photodiode 31a is set to the linear mode. The photodiode 31c adjacent to the lower side of the photodiode 31a is set to the linear mode. Then, as shown in FIG. 4B, as a whole, the Geiger mode and the linear mode are alternately set in the longitudinal direction and the lateral direction.

  The control unit 16b alternately switches the condition of the reverse bias voltage applied to both ends of each photodiode 31 (that is, less than the breakdown voltage or more than the breakdown voltage) at each detection timing of the signal light. As a result, the arrangement shown in FIG. 4A and the arrangement shown in FIG. 4B are switched at every detection timing of the signal light.

  The control unit 16b adjusts the resistance value of the quenching resistor connected to the Geiger mode photodiode and the current value of the variable current source in accordance with the light reception amount of the linear mode photodiode adjacent to the photodiode. . Then, the control unit 16 b averages the output voltage of each photodiode in Geiger mode, and sets the average value as an output voltage of signal light detection. In addition, not only the average value of the output voltage of each photodiode but an average value by a weighted average may be used, and another calculated value may be used as the output voltage of the signal light detection.

<effect>
The light is not uniformly distributed in the light receiving surface due to refraction or the like. Therefore, as described above, Geiger mode photodiodes and linear mode photodiodes are alternately arranged, and the output voltage of signal light detection is calculated using the output voltage of each photodiode to obtain signal light Variations in detection can be reduced. And, in Geiger mode photodiodes, a crosstalk phenomenon may occur that causes another avalanche by falsely reacting with an electron avalanche in an adjacent pixel, but by arranging every other Geiger mode photodiode, The occurrence of the crosstalk phenomenon can be reduced.

[Summary]
An optical sensor (1) according to aspect 1 of the present invention includes a light receiving element (11), a resistor connected in series to the light receiving element, and a current connected in series to the light receiving element and the resistor. And a controller configured to control a resistance value of the resistor according to an alternating current component of disturbance light, and control a current value of the current source according to a direct current component of disturbance light.

  According to the above configuration, the control unit of the optical sensor controls the resistance value of the resistance according to the alternating current component of the disturbance light, and controls the current value of the current source according to the direct current component of the disturbance light. Thereby, the disturbance light component contained in the output voltage of the light receiving element can be reduced. Therefore, the disturbance light tolerance of the light sensor can be improved.

  In the light sensor according to aspect 2 of the present invention, in the above aspect 1, the light receiving element may be a photodiode operating in Geiger mode by applying a reverse bias voltage.

  According to the above configuration, the light receiving element of the light sensor is a photodiode operating in Geiger mode. Therefore, a large current can be obtained by the light entering the light receiving element.

  The optical sensor according to aspect 3 of the present invention further includes another light receiving element different from the light receiving element in the above aspects 1 and 2, and the disturbance light is received by the other light receiving element. You may do it.

  According to the above configuration, the control unit of the optical sensor controls the resistance value of the resistor and the current value of the current source according to the disturbance light received by the other light receiving element. Therefore, the disturbance light tolerance of the optical sensor can be improved by the light reception sensitivity of the disturbance light in the other light receiving elements.

  In the light sensor according to aspect 4 of the present invention, in the above aspect 3, the other light receiving element may be a photodiode operating in a linear mode by applying a reverse bias voltage.

  According to the above configuration, the other light receiving element of the light sensor is a photodiode operating in the linear mode. Therefore, since the current can be obtained following the amount of received disturbance light, the disturbance light resistance of the optical sensor can be further improved.

  The photosensor according to aspect 5 of the present invention includes a plurality of photodiodes arranged in an array, and at a certain point of time, some of the plurality of photodiodes operate in Geiger mode, Other photodiodes other than the some photodiodes operate in the linear mode.

  According to the above configuration, when light is detected, it is possible to selectively use some of the photodiodes operating in the Geiger mode and other photodiodes operating in the linear mode.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and embodiments obtained by appropriately combining the technical means disclosed in the different embodiments. Is also included in the technical scope of the present invention. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

  The present invention can be used for an optical sensor.

1 light sensor 1a light sensor 2 light sensor 11 photo diode (light receiving element)
12 Quenching resistance (resistance)
14 Variable current source (current source)
21 Photodiode (other light receiving element)
31 Photo diodes (some photodiodes, other photodiodes)

Claims (4)

A light receiving element,
A resistor connected in series to the light receiving element;
A current source connected in series to the light receiving element and the resistor;
A control unit that controls a resistance value of the resistor according to an alternating current component of disturbance light and controls a current value of the current source according to a direct current component of disturbance light;
An optical sensor characterized by comprising. The optical sensor according to claim 1, wherein the light receiving element is a photodiode operating in Geiger mode by applying a reverse bias voltage. It further comprises another light receiving element different from the above light receiving element,
The optical sensor according to claim 1 or 2, wherein the disturbance light is received by the other light receiving element. 4. The light sensor according to claim 3, wherein the other light receiving element is a photodiode which operates in a linear mode by applying a reverse bias voltage .

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