JPH08193882A - Optical signal detector - Google Patents

Abstract

PURPOSE: To detect fluctuation of a weak light without being influenced by an outer disturbance factor. CONSTITUTION: The title device comprises a first photodetector element PD1 , a second photodetector element PD2 and a current source CS1 . One of the terminals of each of the photodetector elements PD1 , PD2 is connected to the current source CS1 . A first output terminal OUT1 is provided to the other terminal of the first photodetector element PD1 and a second output terminal OUT2 is to the other terminal of photodetector element PD2 . First and second loads R1 , R2 are respectively connected to the other terminals of the photodetector elements PD1 , PD2 . It is preferable that the first output terminal OUT1 is provided to a position between the first photodetector element PD1 and first load R1 and the second output terminal OUT2 is to a position between the second photodetector element PD2 and second load R2 , respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal detecting device capable of detecting a weak light change in a stable operating state.

[0002]

2. Description of the Related Art Conventionally, in a device for processing an optical recording medium such as a compact disc device or a mini disc device (hereinafter referred to as "optical application device"), the light obtained by irradiating the optical recording medium with light is obtained. The signal is converted into an electric signal, and the optical application device itself is controlled by this electric signal.
Alternatively, recording / reproduction of information recorded on the optical recording medium is performed. Further, in such an optical application device, a change in intensity of reflected light or transmitted light obtained by irradiating an optical recording medium with light, a change in shape of a light spot, a change in relative position of a light spot, etc. are detected. Various information is obtained by this.

FIG. 8A shows a part of a general optical signal detection circuit of an optical pickup used in a conventional optical application apparatus. The operation of the optical application apparatus will be described below by taking as an example the case where the tracking error signal is detected in the configuration shown in FIG. The light reflected from the optical recording medium or the light transmitted through the optical recording medium is transferred onto a photodetector E and a photodetector F formed of semiconductor light receiving elements (for example, photodiodes and phototransistors) by an optical system (not shown). It is imaged. As a result, in the photodetectors E and F, a current according to the amount of light emitted is generated and output as a current signal. The current signals output from the photodetectors E and F are converted into voltage signals by a current-voltage converter (IV converter) and then supplied to the operational amplifier OP ₁ . And
The difference between the two voltage signals is taken in the operational amplifier OP ₁ and is supplied to the circuit of the next stage (not shown) as a tracking error signal. Since the tracking error signal has an offset value as shown in FIG. 8B, it is necessary to process the offset value in the circuit at the next stage.

[0004]

By the way, the recording density of optical recording media in recent years has dramatically increased, and along with this, the current signal output from the photodetector has become weak. When the current signal becomes weak, noise from the surrounding environment greatly affects the current signal. For example, the current signal flowing in the wiring from the photodetector to the IV converter is easily affected by the signal of the peripheral circuit, and the S / N ratio deteriorates. The same applies to the wiring from the IV converter to the operational amplifier.

Also, the IV converter and the operational amplifier itself are
Since there is a certain degree of variation in electrical characteristics, when the current signal obtained from the photodetector becomes weak, there arises the disadvantage that a normal signal cannot be obtained due to the difference in sensitivity. In particular, in a system using the magneto-optical effect (Magnetic Kerr Effect), since the change in the optical signal obtained from the optical system is weak, the current signal output from the photodetector is also small, and the signal processing in the circuit in the subsequent stage is small. There is a problem that it is difficult.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an optical signal detecting device capable of surely detecting a weak change in an optical signal in a stable operating state.

[0007]

In order to achieve the above object, the optical signal detecting device of the present invention has a first light receiving element and a second light receiving device, as shown in the block diagram of the basic structure of FIG. An element and a current source, one end of each of the first light receiving element and the second light receiving element is connected to the current source, and a first output terminal is provided at the other end of the first light receiving element. A second output terminal is provided at the other end of the second light receiving element.

In the optical signal detecting device of the present invention, the first
Alternatively, the load is connected to the other end of either one of the second light receiving elements, and the first or second output terminal is provided between the first or second light receiving element and the load. Is preferred.

Alternatively, as shown in FIG. 1, first and second loads are connected to the other end portions of both the first and second light receiving elements, respectively, and the first and second light receiving elements are connected. It is preferable that first and second output terminals are provided between the element and the first and second loads, respectively. That is, the first
A circuit in which the load and the first light receiving element are connected in series,
The second load and the second light receiving element are connected in series and the circuit is connected in parallel. A current source is connected to a connection point on the light receiving element side of these two circuits, and the first load and the first load are connected. It is preferable to provide a first voltage output terminal at a connection point with the light receiving element and a second voltage output terminal at a connection point with the second load and the second light receiving element.

The first light receiving element or the second light receiving element can be composed of a single light receiving element or a plurality of light receiving elements connected in series. Alternatively, the first light receiving element or the second light receiving element can be configured by connecting a plurality of light receiving elements in parallel. The light receiving element is a CdS cell, Ge cell, Si cell, PbS cell, P
bSe cell, InAs cell, InSb cell, HgCdT
It is preferably composed of a photoconductive type light receiving element such as an e-cell, or a photodiode or phototransistor which is a photovoltaic element. Examples of the photodiode include a pn junction photodiode, a pin junction photodiode, and an avalanche photodiode.

[0011]

In the optical signal detecting device of the present invention, the first light receiving element and the second light receiving element are differentially operated, and the current generated in the first light receiving element is adjusted in accordance with the amount of light emitted, The difference from the current generated by the second light receiving element is extracted as a voltage signal. Moreover, each of the first light receiving element and the second light receiving element generates a current according to the amount of light emitted. And
The respective currents generated in the first light receiving element and the second light receiving element are regulated by the current source and flow differentially.

When the current generated in the first light receiving element flows through the first load, for example, a voltage corresponding to the amount of light applied to the first light receiving element is generated across the first load, and this voltage is generated. Appears at the first output terminal. Similarly, when the current generated in the second light receiving element flows through the second load, for example, a voltage corresponding to the amount of light emitted to the second light receiving element is generated across the second load, and this voltage is generated. Appears at the second output terminal. Therefore, the voltage appearing between the first and second output terminals is a voltage that reflects the difference between the amount of light applied to the first light receiving element and the amount of light applied to the second light receiving element. The voltage appearing between the first and second output terminals is equivalent to the output of the conventional operational amplifier OP ₁ shown in FIG. If a load is provided on at least one of the light receiving elements, the voltage appearing between the first and second output terminals is equal to the amount of light applied to the first light receiving element and the amount of light applied to the second light receiving element. The voltage reflects the difference between

According to the optical signal detecting device of the present invention, the difference between the amounts of light emitted to the first and second light receiving elements can be obtained as a differential voltage output, so that it depends on the absolute value of the current. Even if the currents generated in the first and second light receiving elements are small, the influence of disturbance can be removed, and the weak optical signal difference can be reliably detected in a stable operation state.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The optical signal detecting device of the present invention will be described in detail below based on embodiments with reference to the drawings.

(Embodiment 1) FIG. 2A is a circuit diagram showing a configuration of an optical signal detection device of Embodiment 1. In the first embodiment, the first light receiving element is the first photodiode P.
D ₁ and the second light receiving element is a second photodiode PD ₂ . Furthermore, the first and second
The first and second loads are connected to the other ends of the photodiodes PD ₁ and PD ₂ , respectively. The first load is composed of the first resistor R ₁ and the second load is the second resistor R ₂
It consists of First and second output terminals OUT ₁ and OUT ₂ are provided between the first and _second photodiodes PD ₁ and PD ₂ and the first and second resistors R ₁ and R ₂ , respectively. There is. The current source CS ₁ can be composed of a diode DO, a resistor R ₃ , a resistor R ₄ and a power source E as shown in FIG. 3, for example. The details of the current source CS ₁ will be described later.

The first and second photodiodes PD ₁ ,
It is desirable that PD ₂ have the same electrical characteristics. The first and second photodiodes PD ₁ and PD ₂ each have a light-receiving surface that is physically divided into two, as shown in FIG. 2B, and such a light-receiving surface has an optical system (not shown). A light spot guided from is emitted. When the first photodiode PD ₁ is irradiated with light, an electromotive force corresponding to the amount of light is generated by the photovoltaic effect, and the first photodiode P 1
A current I ₁ flows from the cathode of D ₁ to the anode. Similarly, when the second photodiode PD ₂ is irradiated with light, a current I ₂ flows from the cathode to the anode of the _second photodiode PD ₂ . In the following, for simplification of description, the first and second resistors R ₁ and R ₂ are described as having the same resistance value, but they do not have to have the same resistance value.

One ends (upper side in the figure) of the first and second resistors R ₁ and R ₂ are commonly connected. The other end of the first resistor R ₁ is connected to the cathode of the _first photodiode PD ₁ , and the other end of the second resistor R ₂ is connected to the cathode of the _second photodiode PD ₂ . Then, the anodes of the first and second photodiodes PD ₁ and PD ₂ are commonly connected and further connected to the current source CS ₁ . The first resistor R ₁ and the first photodiode PD first output terminal OUT ₁ from the connection point of _1, the second resistor R ₂ and a second connection point of the photodiode PD ₂ of the second The output terminals OUT ₂ are drawn out, respectively. That is, the first and second output terminals OUT ₁ and OUT ₂ are respectively connected to the first and second photodiodes PD ₁ and PD ₁ .
It is provided on the opposite side of the current source CS ₁ with the PD ₂ in between.

In FIG. 2B, it is assumed that the light amount on the light receiving surface of the _first photodiode PD ₁ is P ₀₁ and the light amount on the light receiving surface of the second photodiode PD ₂ is P ₀₂ . Figure 2 now
As shown in (a) of (B), when the first and second photodiodes PD ₁ and PD ₂ are uniformly irradiated with light (P ₀₁ = P ₀₂ ), the first and second photodiodes Diode PD
_A current of I ₁ = I ₂ is generated in ₁ and PD ₂ . Therefore,
The voltage drops due to the first and second resistors R ₁ and R ₂ become equal, and the voltage V ₁ appearing at the first output terminal OUT ₁ becomes equal to the voltage V ₂ appearing at the second output terminal OUT ₂ . Therefore, the difference between the voltages appearing between the first output terminal OUT ₁ and the second output terminal OUT ₂ is V ₁ −V ₂ = 0 (V).
At this time, the first and second photodiodes PD ₁ , PD ₂
The dark currents of the two cancel each other out.

As shown in (b) of FIG. 2 (B), the first
And P on the light receiving surfaces of the _second photodiodes PD ₁ and PD _{2.
When the} light of ₀₁ > P ₀₂ is applied, a current of I ₁ > I ₂ is generated in the first and second photodiodes PD ₁ and PD ₂ . Due to these currents I ₁ and I ₂ , V 1 is applied to the first output terminal OUT ₁ and the second output terminal OUT ₂ , respectively.
A voltage having a relationship of ₁ <V ₂ appears.

Similarly, as shown in (c) of FIG. 2B, when the light receiving surfaces of the first and second photodiodes PD ₁ and PD ₂ are irradiated with light of P ₀₁ <P ₀₂ A current of I ₁ <I ₂ is generated in the first and second photodiodes PD ₁ and PD ₂ . Due to these currents I ₁ and I ₂ , the first output terminal OUT ₁ and the second output terminal OUT ₂ are
Voltages having a relationship of V ₁ > V ₂ respectively appear.

First and second photodiodes PD ₁ ,
A difference V ₁ − between the amount of light emitted to PD ₂ and the voltage appearing between the first output terminal OUT ₁ and the second output terminal OUT _2.
The relationship of V ₂ is shown in FIG.

The current source CS ₁ can be realized by the circuit shown in FIG. 3, for example. That is, the diode DO is connected to the connection point of the anodes of the photodiodes PD ₁ and PD _2.
Connect the anode of. And this diode DO
Is forward biased by the voltage from power supply E divided by resistors R ₃ and R ₄ . As a result, the diode DO always flows a constant current with forward bias, and thus the first photodiode PD ₁ and the second photodiode PD ₂ operate differentially. For example, in a state where current is flowing in the first photodiode PD ₁ and the second respectively 5mA to PD _2, I ₁ = 7 by the first irradiation light amount to the photodiode PD ₁ increases
If it becomes mA, the second photodiode PD ₂
Current changes to I ₂ = 3 mA. The current source CS ₁ is not limited to such a circuit, and various known circuits can be used.

As described above, according to the first embodiment, the difference between the amounts of light emitted to the _first photodiode PD ₁ and the second photodiode PD ₂ can be extracted as the differential voltage output voltage. Further, even if the current value is small, a weak optical signal can be reliably detected regardless of the absolute value of the current generated in the _second photodiodes PD ₁ and PD ₂ .
Further, the gain can be adjusted by adjusting the values of the resistors R ₁ and R ₂ . If the resistor is made of, for example, a thick film or thin film resistor, the gain can be adjusted easily and accurately by trimming the resistor.

The optical signal detecting device can be constructed by forming each circuit element of the circuit shown in FIG. 2A or FIG. 4 on the same semiconductor chip. With such a configuration, it is possible to suppress variations in the electrical characteristics of each circuit element and to shorten the wiring length, so that it is possible to reduce the influence of external noise, and An optical signal detecting device capable of processing a weak optical signal can be realized.

(Second Embodiment) In the first embodiment, the first and second
The resistors R ₁ and R ₂ are placed upstream of the first and second photodiodes PD ₁ and PD ₂ , respectively, so that a significant signal (active low) is taken out at a low level. On the other hand, in the second embodiment, as shown in FIG. 4, the first and second resistors R ₁ and R ₂ are arranged downstream of the first and second photodiodes PD ₁ and PD ₂ , respectively,
The current source CS _{1 is connected} to the first and second photodiodes PD ₁ ,
By arranging on the upstream side of PD ₂ , a significant signal (active high) at a high level can be taken out. Even with such a configuration, the same effects as the above can be obtained.

Further, in the first and second embodiments, each of the first light receiving element and the second light receiving element is composed of one photodiode, but a plurality of photodiodes connected in series or in parallel are used. It can also be configured using. For example, if the number of photodiodes forming the first light receiving element and the number of photodiodes forming the first light receiving element are changed, the intensity of light incident on the first light receiving element and the light incident on the second light receiving element are changed. Even when the intensities of the generated light greatly differ, a differential output can be obtained based on the appropriate output voltage from these light receiving elements. In some cases, the first load or the second load may be omitted. The optical signal detection device described in the first or second embodiment can be applied to, for example, the tracking error signal detection described above.

(Third Embodiment) FIG. 5A is a circuit diagram showing a configuration of an optical signal detecting device according to a third embodiment. In the third embodiment, the first phototransistor PT ₁ is used as the first light receiving element and the second phototransistor PT ₂ is used as the second light receiving element. Other components are the same as those in the first embodiment, and detailed description thereof will be omitted.

The phototransistors PT ₁ and PT ₂ are light receiving elements having an amplifying function. Phototransistor P
T ₁ and PT ₂ can generate a large current with respect to the same light amount as compared with a light receiving element having no amplifying action such as a photodiode.

The operation of the circuit of the optical signal detecting device of the third embodiment is the same as that when the photodiodes are used as the values of the currents I ₃ and I ₄ generated in the first and second phototransistors PT ₁ and PT _2. The operation is the same as that of the circuit of the optical signal detection device described in the first embodiment, except that it is large in comparison. Current I
_{If 3} and I ₄ are large, the voltage drop due to the resistors R ₁ and R ₂ is large. Therefore, a sufficiently large differential output (V ₃ −) is output from the first output terminal OUT ₁ and the second output terminal OUT _2.
V ₄ ) can be obtained. FIG. 5B shows the characteristic of the differential output with respect to the light amount difference. That is, the difference between a light receiving element that does not have an amplifying action (for example, a photodiode) and a light receiving element that has an amplifying action (for example, a phototransistor) is shown. The slope of each characteristic line changes slightly depending on the circuit constant.

As described above, when the phototransistor is used as the light receiving element, the phototransistors PT ₁ and PT ₂
Since a sufficiently large differential output can be obtained even if the difference in the amount of light received at is small, there is an advantage that the difference between the optical signals can be accurately detected and the signal is resistant to noise. Further, by using a phototransistor as the light receiving element, it is possible to obtain the currents I ₃ and I ₄ of appropriate magnitudes even with a weak light amount. Therefore, there is an advantage that a light emitting element for generating light for irradiating the optical recording medium, for example, a laser diode having a low output can be used.

(Fourth Embodiment) The circuit of the third embodiment can be modified as in the fourth embodiment shown in FIG. 6A, similarly to the generally known differential amplifier circuit. In Example 4 shown in FIG. 6A, the first light receiving element is
Two phototransistors PT ₁₁ and PT ₁₂ are connected in parallel. Similarly, the second light receiving element is composed of two phototransistors PT ₂₁ and PT ₂₂ connected in parallel.
Other configurations are similar to those of the third embodiment, and detailed description thereof will be omitted.

In the circuit configuration of the fourth embodiment, the current I ₅ flowing through the first resistor R ₁ is the phototransistors PT ₁₁ and P.
This is the total value of the currents generated at T ₁₂ , and the current I ₆ flowing through the second resistor R ₂ is the total value of the currents generated at the phototransistors PT ₂₁ and PT ₂₂ . Therefore, even when the phototransistors PT _{11 to} PT ₂₂ are irradiated with weak light, large currents I ₅ and I ₆ can be obtained. still,
The number of phototransistors connected in parallel is 2 each
The number is not limited to one and can be any number. Also,
Each of the phototransistors shown in FIG.
The optical signal detection device of the fourth embodiment, which is connected in parallel, can be applied to the detection of the focus error signal by the astigmatism method, the details of which will be described later.

(Embodiment 5) Embodiment 5 shown in FIG. 6 (B)
Is an example of configuring a logarithmic conversion circuit (log amplifier). In the fifth embodiment, a time constant circuit composed of a resistor R ₅ and a capacitor C is provided between the emitters of the _first phototransistor PT ₁ and the second phototransistor PT ₂ . In addition, the first phototransistor PT
_The current source CS ₂ is provided at the emitter of 1, and the current source CS ₃ is provided at the emitter of the _second phototransistor PT ₂ . With such a configuration, a logarithmic output can be obtained as a differential output. Further, with such a configuration, it is possible to eliminate the influence of noise such as blinking light such as a fluorescent lamp entering each phototransistor.

(Sixth Embodiment) Next, a sixth embodiment in which the optical signal detection device of the fourth embodiment shown in FIG. 6A is applied to a focus error signal detection circuit by the astigmatism method will be described with reference to FIG. Will be described with reference to. FIG. 7A is a circuit diagram showing the configuration of the focus error signal detection circuit. The phototransistors A, B, C and D have the same electrical characteristics, and are arranged so that the light receiving surface is divided into four, as shown in FIG. 7B. The phototransistors A, C, B, and D in the sixth embodiment shown in FIG. 7A are the same as the phototransistors PT ₁₁ , PT ₁₂ , and PT in the fourth embodiment shown in FIG. Equivalent to ₂₁ , PT ₂₂ .
Then, the light receiving surface is irradiated with, for example, a light spot guided from an optical system (not shown). The light spot is a substantially perfect circle (P ₀₃ ) if it is in focus. In this case, the phototransistors A, B, C and D are uniformly irradiated with light. Therefore, the voltage difference appearing between the first output terminal OUT ₁ and the second output terminal OUT ₂ becomes zero.

On the other hand, if the focal point is too close, an ellipse (for example, P ₀₄ ) inclined in a predetermined direction is formed, and phototransistors A and C
The current I ₅ generated in 1 increases and the current I ₆ generated in the phototransistors B and D decreases. Therefore, the first
The voltage appearing between the output terminal OUT ₁ and the second output terminal OUT ₂ becomes negative. On the other hand, if the focal point is too far away, an ellipse (for example, P ₀₅ ) inclined in the direction orthogonal to the above predetermined direction is generated, the current I ₅ generated in the phototransistors A and C becomes small, and the current I generated in the phototransistors B and D decreases.
₆ increases. Therefore, the first output terminal OUT ₁ and the second output terminal OUT ₁
The voltage appearing between the output terminal OUT ₂ and the output terminal OUT ₂ becomes positive. That is, a differential voltage output having the characteristic shown in FIG. 7C is obtained according to the focus position, and this is used as the focus error signal. This focus error signal is supplied to the actuator of the optical system and is feedback-controlled so that its value is zero, that is, the light spot on the light receiving surface becomes a substantially perfect circle.

Although the optical signal detecting device of the present invention has been described based on the preferred embodiments, the present invention is not limited to these embodiments. Applying the arrangement of the current source, the first and second light receiving elements, and the first and second loads described in the second embodiment to the optical signal detection device described in the third to sixth embodiments. You can The optical signal detection device of the present invention is applied not only to detection of a tracking error signal in an optical application device and detection of a focus error signal by an astigmatism method, but also to detection of a focus error signal by a knife-edge method. It can be applied to any field if it is necessary to detect the difference in the amount of light at one place.

[0037]

As described above in detail, according to the optical signal detecting device of the present invention, the difference between the amounts of light emitted to the first and second light receiving elements can be obtained as the differential output of the voltage. Even if the current values generated in the first and second light receiving elements are small, a weak change in the optical signal can be reliably detected without being influenced by the absolute value of the current. Also,
By performing the differential operation, unnecessary uniform light emitted from the outside can be canceled out, so that the environment resistance is improved. Further, since the dark current can be canceled by the differential operation, stable operation becomes possible.

Further, unlike the conventional case, the IV converter and the operational amplifier are not required, so that the number of parts constituting the optical signal detection device or the optical application device can be reduced, and the size and cost of the optical pickup can be reduced. Can achieve down. Further, since the number of parts is reduced, it is possible to prevent the deterioration of the S / N ratio due to the variation in the electrical characteristics of each part, and it is possible to improve the reliability of the optical signal detection device. Furthermore, the output signal in the optical signal detecting device of the present invention does not include an offset value like the output signal in the conventional technique shown in FIG. 8B. Therefore, the processing of the output signal becomes easier as compared with the conventional device.

If a phototransistor is used as the light receiving element, an electric current of an appropriate amount can be obtained even with a weak light quantity, so that it can be used as a light emitting element for generating light for irradiating an optical recording medium, for example, as a laser diode. An output type can be used, and low power consumption can be achieved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of an optical signal detection device of the present invention.

FIG. 2 is a diagram for explaining the configuration and operation of the optical signal detection device according to the first embodiment.

FIG. 3 is a diagram showing an example of a configuration of a current source in each example.

FIG. 4 is a diagram showing a configuration of an optical signal detection device according to a second embodiment.

FIG. 5 is a diagram for explaining the configuration and operation of the optical signal detection device according to the third embodiment.

FIG. 6 is a diagram showing a configuration of an optical signal detection device according to a fourth embodiment and a fifth embodiment.

FIG. 7 is a diagram for explaining the configuration and operation of an optical signal detection device according to a sixth embodiment in which the fourth embodiment is applied to a focus error signal detection circuit.

FIG. 8 is a diagram showing a part of a general optical signal detection circuit of an optical pickup used in a conventional optical application device.

[Explanation of symbols]

PD ₁ , PD ₂ photodiodes PT ₁ , PT ₁₁ , PT ₁₂ , PT ₂ , PT ₂₁ , PT ₂₂ , A,
B, C, D Phototransistor R ₁ , R ₂ Resistance (load) OUT ₁ First output terminal OUT ₂ Second output terminal CS ₁ , CS ₂ , CS ₃ Current source R ₃ , R ₄ , R ₅ resistance DO Diode E Power supply C Capacitor

Claims (6)

[Claims] (1) A first light receiving element, (b) a second light receiving element, and (c) a current source are provided, and one ends of the first light receiving element and the second light receiving element are Is connected to a current source, and a first output terminal is provided at the other end of the first light receiving element,
An optical signal detection device, wherein a second output terminal is provided at the other end of the second light receiving element. 2. A load is connected to the other end of either one of the first or second light receiving element, and the load is connected between the first or second light receiving element and the load. The optical signal detecting device according to claim 1, wherein the output terminal is provided. 3. A first load and a second load are connected to the other ends of both of the first and second light receiving elements, respectively.
The optical signal detection according to claim 1, wherein first and second output terminals are provided between the first and second light receiving elements and the first and second loads, respectively. apparatus. 4. The one or both of the first and second light receiving elements are composed of a plurality of light receiving elements connected in series. The optical signal detection device described in 1. 5. The one or both of the first or second light receiving element is composed of a plurality of light receiving elements connected in parallel. The optical signal detection device described in 1. 6. The optical signal detection device according to claim 1, wherein the light receiving element comprises a photodiode or a phototransistor.