JPS6020655A - Optical detecting circuit - Google Patents

Abstract

PURPOSE:To improve the minimum measuring level of incident light power and to obtain a wide dynamic range by impressing reverse bias voltage which is zero or minute at a region having small incident light power and large at a region having large power to a photo diode. CONSTITUTION:When the incident light is given to the photo diode 1, a light detecting current I0 flows, a voltage VL corresponding to the current I0 is generated across a load resistance RL and a positive output voltage V0 corresponding thereto is obtained. The output voltage V0 is inputted also to a voltage generating circuit 5 for reverse bias and an output voltage V1 for reverse bias is obtained at the output terminal of an operational amplifier 6 for reverse bias but the diode 8 is kept turned off until the output voltage V0 rises to a prescribed value. Thus, the output voltage V1 is kept nearly zero and the reverse bias voltage of the photo diode 1 is kept also nearly zero. The forward series resistance value of the diode 8 is decreased with the increase in the incident light power, the output voltage V1 is increased and the reverse bias of the photo diode 1 is made deep.

Description

Detailed Description of the Invention Technical Field The present invention relates to a photodetection circuit used in optical power meters and the like, and more particularly, to a photodetection circuit that can improve the minimum measurement level of light received by a photodiode and obtain a wide dynamic range. Regarding circuits.

Conventional technology Through the practical application of optical application technology centered on optical communication technology,
Devices (optical power meters) that measure optical power (level or amount of light) have become increasingly important.

Photodetectors for optical power meters used in the field of optical communications can be broadly classified into thermal conversion type photodetectors and photoelectric conversion type photodetectors. A thermopile detector is a thermal conversion type photodetector of the former type, and is used in a standard optical power meter. However, it has the disadvantage that it has low light detection sensitivity and cannot provide a wide dynamic range. On the other hand, there is a photodetector using a photodiode as the latter photoelectric conversion type photodetector. This photodiode photodetector can easily measure weak optical power and can perform photodetection with excellent linearity over a relatively wide dynamic range.

FIG. 1 shows a conventional photodetection circuit capable of measuring weak optical power. In this photodetection circuit, one end of the photodiode (1) is one input terminal (inverting input terminal) of a high input impedance operational amplifier (2) of FET input type.
The other end of the photodiode (1) and the other input terminal (non-inverting input terminal) of the operational amplifier (2) are each grounded. Rf is a feedback resistor, which is connected between one input terminal and the output terminal of the operational amplifier (2). (3) is a variable resistor for adjusting the zero point, and when the incident light of the photodiode il+ is made zero, the output voltage VD becomes zero.
This is a resistance that can be adjusted to

In this circuit, when the photodetection current corresponding to the incident light is generated from the photodiode (1), the operational amplifier (2
) has a high input impedance, so the photodetection current δ does not flow there, but flows through the feedback resistor Rf. On the other hand, the operational amplifier (2) operates so that the potential of the inverting input terminal becomes the same as the potential (OV) of the non-inverting input terminal, so there is a virtual short circuit between the inverting input terminal and the non-inverting input terminal. becomes. Therefore, a photodetection voltage of VO=l0Rf is obtained at the output terminal.

Incidentally, the element that determines the maximum measurement level of light received by a photodiode photodetector is the series resistance determined by the material and light receiving area of the photodiode.

Furthermore, the element that determines the minimum measurement level of light reception is the parallel resistance of the photodiode. Therefore, the use of photodiodes with low series impedance and high parallel impedance is a prerequisite for obtaining a wide dynamic range. To explain this point in detail, FIG. 2 shows an equivalent circuit when a load and other components are connected to the photodiode.

Here, Ip is the current generated by the incident light (photodetection current), or diode current, ■' is the parallel resistance current, cj is the junction capacitance, RIIR is the parallel resistance, Rs is the series resistance, Io is the output current, and VO is the output. It is voltage. From this equivalent circuit
By subtracting the output voltage when =00, that is, the open circuit voltage vop, the following equation is obtained.

Here, s is the saturation current of the photodiode, q is the electron charge, K is Boltzmann's constant, and T is the absolute temperature of the element.

Also, RL=0. Current when VO=0, that is, short circuit current Ia
Subtracting w gives the following formula.

・・・・・・・・・・・・・・・・・・(2) The open circuit voltage vop is proportional to p when IP is large, but in general, as seen from equation (1), VOP Since Vop is not linearly proportional to Ip and is also affected by temperature, it is not appropriate to measure the level of incident light based on the open circuit voltage Vop.

For this reason, it is desirable to perform optical detection using the short-circuit current generator gm. The linearity of short-circuit current IIH is determined by the second equation (2).
and the third term are related.

Generally, R8 is several Ω to several Ω, and 1 (si is 10
'Ω to 1011Ω, and the second and third terms can be ignored in a fairly wide range. For example, in a silicon (Si) photodiode that is sensitive to visible light to near-infrared light, a linear proportional relationship is established between short circuit current IIB and photodetection current Ip. However, in germanium (Ge) photodiodes, which are sensitive to so-called long wavelength light (wavelength λ = 1.0 μm% - 1.7 μm) used in optical communications, as the photodetection current p increases ( 2) The second term of the equation can no longer be ignored, and the short circuit current sH becomes saturated.

The nonlinearity due to the saturation of the short circuit current gg increases as the series resistance R1 of the photodiode increases.

In order to suppress the linearity of the short circuit current Isa to, for example, a deviation of 0.3% or less, it is necessary to set the series resistance R8 to about 50Ω or less. However, the series resistance Ra of common Ge photodiodes currently on the market is in the range of 150Ω to several orders of magnitude, and sufficient linearity cannot be obtained in a region where the incident light power is large. If the resistance of the photodiode substrate is lowered to lower the series resistance R8, the nonlinearity due to saturation of the short circuit current gft can be improved. However, the parallel resistance 1 (sn
It is difficult to significantly reduce only the series resistance R8 while keeping R8 large. Ordinary photodiodes are designed to reduce junction capacitance Cj and increase response speed by increasing the resistance of the substrate or by adopting a PIN structure.

The above-mentioned saturation of the short circuit current VT in the region of high incident optical power can be improved by applying a reverse bias voltage to the photodiode. FIG. 3 shows a conventional reverse bias type photodetector circuit. In this circuit, the photodiode (1)
Connect a load resistor R in series with the photodiode (1).
Apply a reverse bias of ■1 to the cathode of the load resistance R
Photodetection voltage corresponding to voltage VL obtained across L
is obtained at the output terminal of the operational amplifier (4). However, this method has the following drawbacks. First of all, by reverse-biasing the photodiode (11), the dark current Id of the photodiode +11 increases, making it difficult to achieve high sensitivity.Especially with ordinary photodiodes, the increase in dark current is significant in high-temperature environments, causing measurement errors. In addition, the dark current of a Ge photodiode, which is generally used in a long optical wavelength region, is about 3 to 4 orders of magnitude larger than that of a Si photodiode, which is used in a short optical wavelength region. Practical measurements in the above range are extremely difficult.

Second, when applying a constant reverse bias voltage ψ“,
If the voltage VL across the load resistor RL increases in response to an increase in the photodetection current, the reverse bias voltage applied to the photodiode (1) actually decreases. Therefore, there is a drawback that the photodetection sensitivity fluctuates due to changes in the incident light power. As a result, it is difficult to accurately measure weak input optical power using the circuit shown in FIG.

OBJECTS OF THE INVENTION Therefore, an object of the present invention is to provide a photodiode photodetection circuit that can improve the minimum measurement level of incident light power and obtain a wide dynamic range.

Structure of the Invention To achieve the above object, the present invention provides a photodiode that is connected between a photodiode that generates a current corresponding to the power of incident light and a common potential applying means that provides a ground level or a constant voltage level. an amplifier connected between the photodiode and the current detection resistor; a reverse bias voltage generating circuit which is formed to generate a voltage Vl) and generate a large reverse bias output voltage in a region where the power of the incident light is large, and whose output terminal is connected to the other end of the photodiode. This relates to a photodetection circuit consisting of.

Effects of the Invention According to the above invention, application of an excessive reverse bias voltage in a region where the power of incident light is low is restricted, and dark current is restricted. Therefore, the minimum measurement level is reduced and the dynamic range is expanded.

First Embodiment Next, a photodetection circuit according to a first embodiment of the present invention will be described with reference to FIG. The basic parts of this photodetection circuit are the same as those shown in FIG. 3, and include a Ge photodiode 111, a current detection load resistor RL connected between one end (anode) of the photodiode 111, and the ground as a common potential applying means.
It consists of an FET input type operational amplifier (4) whose non-inverting input terminal is connected between the resistor RL and the photodiode tl+. The inverting input terminal of the operational amplifier (4) is connected to its output terminal, and a variable resistor Rz is provided to adjust the output voltage VO to zero when the incident light of the photodiode Fi+ is zero. ing.

(5) is a reverse bias voltage generation circuit, which includes a reverse bias operational amplifier (6) whose non-inverting input terminal is connected to the output terminal of the previous stage operational amplifier (4), and It consists of a feedback resistor Rf connected between the inverting input terminal and the output terminal, and a series circuit of a variable resistor (7) and a rectifier diode (8) connected between the inverting input terminal and the ground. By connecting it to the other end (cathode) of the photodiode +1, which is the output terminal of the amplifier (6), the photodiode (1) is reverse biased with the output voltage 1.

Next, the operation of the photodetection circuit shown in FIG. 4 will be explained.

When the incident light power of the photodiode 11) is zero,
Output voltage V of operational amplifier (4) adjusted to zero offset
O is also zero. That is, the photodetection current Io of the photodiode 11+ becomes almost zero, the voltage VL across the resistor RL also becomes almost zero, and the output voltage Vo becomes zero. Further, the output voltage (1) of the reverse bias operational amplifier (6) also becomes zero, and no reverse bias voltage is applied to the photodiode 11).

When incident light is applied to the photodiode (1), a photodetection current Io flows, a voltage VL corresponding to the current IO is generated in the load resistor RL, and a positive output voltage V corresponding to this voltage VL is generated.
o is obtained. The output voltage Vo is then converted into a digital signal by, for example, an analog-to-digital converter (not shown). Output voltage VO is reverse bias voltage generation circuit (5)
A reverse bias output voltage is obtained, which is applied to the cathode of the photodiode +11. However, R1 in the above formula is the sum of the resistance value of the force and the resistance value of the diode (8).

By the way, the diode (8) is kept off until the photodetection output voltage Vo rises to a certain value, and then turned on. Therefore, in the region where the incident light power is low, the resistance value of the diode (8) is small, and the reverse bias operational amplifier (
6) The output voltage ■1 is kept at approximately zero. Therefore, the reverse bias voltage of the photodiode (1) is also kept almost zero,
Dark current does not flow. As a result, the minimum measurement level of the incident optical power becomes smaller. Furthermore, it becomes possible to accurately measure low incident optical power.

When the diode (8) becomes conductive due to an increase in the incident light power, the forward series resistance value of the diode (8) decreases, the output voltage (1) increases, and the reverse bias of the photodiode (1) becomes deeper. The reverse bias voltage VD applied to both ends of the photodiode (11) becomes equal to the output voltage v1 of the reverse bias voltage generating circuit (5) to the load resistor RL. Since the output voltage Vl of the generating circuit (5) increases, the reverse bias voltage required by the photodiode (1) can be reliably obtained.

By the way, when we investigated the dark current of a Ge-P I N photodiode, we found that when the temperature of the photodiode is constant, the dark current is almost constant when the reverse bias voltage is 0.5 V or more, and when the reverse bias voltage is 1.0 to 10 V. .0 mV
It was found that in the range of , the dark current is minute and the change in sensitivity is almost negligible. Therefore, in FIG. 4, the reverse bias voltage VD is set at 0.0 to 3.0 μW until the incident light power is about 400 μW.
0 mW, and the incident light power at which the linearity deteriorates as the photodetection current of the photodiode (1) approaches saturation is 600 μ.
For example, set the reverse bias voltage VD to 540 from before and after exceeding W.
11 mV at μW, 20 mV at 630 μW,
71 mV at 1 mW.

Apply a deep voltage such as 155 mV at 2 mW.

Therefore, the dark current caused by the reverse bias voltage is reduced as much as possible.

As is clear from the above, this embodiment has the following effects.

(a) Since the reverse bias voltage is kept at zero or very small in the region where the incident light power is low, the reduction in measurement accuracy and linearity due to dark current is virtually eliminated, and the minimum measurement level can be improved. .

That is, it becomes possible to measure very small optical power, and the dynamic range is expanded.

(b) In a region where the incident light power is high, that is, in a region where the photodetection current would be saturated if no reverse bias voltage is applied, the photodiodes f and 11 are set so that the reverse bias voltage increases as the incident light power increases. Since a reverse bias voltage is applied, it is possible to increase the maximum measurement level and improve the linearity between the incident optical power and the photodetection voltage.

Therefore, it is possible to measure optical power in a wide range from extremely small optical power to several mW or more.

(C) Output voltage V1 of reverse bias operational amplifier (6)
Since it is configured to control using the forward characteristics of the rectifier diode (8), it is possible to easily generate a reverse bias voltage that reduces dark current as much as possible.

Modifications The present invention is not limited to the above-described embodiments, but includes various modifications, such as the following.

(4) As shown in Fig. 5, even if it is directly connected to the anode of the photodiode il+, which is the non-inverting input terminal of the operational amplifier (6),
The same effect as in FIG. 4 can be obtained.

Φ) In FIGS. 4 and 5, the anode and cathode of the photodiode (1) may be connected in the opposite way to the diagram.

However, in this case, since the voltages Vo and Vl also have opposite polarities, the diode (8) is also connected in reverse.

(Q Instead of the diode (8), another non-linear element may be connected. Also, by combining the diode (8) and a resistor or another non-linear element, a reverse bias that matches the characteristics of the photodiode (1) The diode (8) may be omitted if the reverse bias voltage is increased in accordance with the increase in the photodetection current in the entire range from the low optical power region to the high optical power region. Good too.

0 The photodetection current of the photodiode ill is thick (and if the saturation of the operational amplifier (6) has an adverse effect, a pair of transistors Ql, Q2, a pair of diodes D1, D2, a pair of diodes D1, D2, A complementary emitter follower output circuit consisting of resistors R11 and R12 may be added.

■ Si photodiodes other than Ge photodiodes, I
It is also applicable when using an nP photodiode or the like.

(g) The reverse bias voltage generating circuit (5) is composed of a transistor Q3 and its pace drive circuit (9) as shown in FIG.
), whose input terminal is connected to the positive power supply V+, whose output terminal is connected to the cathode of the photodiode (1), and whose base drive circuit is connected to the output terminal of the operational amplifier (4). It's okay. In this case,
The output voltage v1 of the transistor Q3 is controlled so as to increase as the photodetection voltage (2)0 increases. Photodiode (
1) The reverse bias voltage VD applied to both ends is VD:
Vl - VR2, and since Vl is controlled based on VR, the reverse bias voltage VD can be kept constant. That is, the operation is such that the voltage drop caused by the resistor RL is compensated for by the reverse bias voltage generating circuit (5). This circuit uses a relatively small resistance such as 20 to 200Ω for the resistor RL, and a relatively large optical power (for example, 1mW to 200mW).
This circuit is effective when measuring W). In FIG. 7, dark current flows through the photodiode (1) even in the low incident light region, but it does not flow more than in the high incident light region.

Therefore, compared to the fixed bias method shown in FIG. 3, higher precision and a wider dynamic range can be obtained. In order to obtain a reverse bias voltage similar to that shown in FIG. 7, a configuration as shown in FIG. 8 may be used. This reverse bias voltage generation circuit (
5) is the FET connected between the V+ power supply and the ■- power supply.
It consists of a QOI and a resistor (11). Since the control gate of the FET (lα) is connected to one end of the load resistor RL,
The conduction state corresponds to the voltage VL across the load, and a reverse bias voltage Vl corresponding to the voltage VL can be obtained at the source. As a result, V is applied to both ends of the photodiode (1).
A substantially constant reverse bias voltage determined by l -VL is applied. The method of applying a constant reverse bias voltage to the photodiode (1) despite the fluctuation of the incident light power as described above can also be achieved by the circuit shown in FIG. In this circuit, the output terminal of the operational amplifier (4) and the photodiode fi
Since a constant voltage source E is connected between the positive cathode and the positive cathode, the voltage VD across the photodiode (1) is Vo
= E + V.

-VL, and if the gain G of the operational amplifier (4) is set to 1 so that Vo = vL, the reverse bias voltage V
D is always a constant voltage E.

■] A control photodiode having substantially the same characteristics as the photodetection photodiode (1) is provided, and the same light as the photodetection photodiode fi+ is applied to this photodiode,
The reverse bias voltage (1) may be generated based on the current of this control photodiode.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing a conventional photodetection circuit, Figure 2 is an equivalent circuit diagram when a load is connected to a photodiode, Figure 3 is a circuit diagram showing a conventional reverse bias application type photodetection circuit, and Figure 2 is an equivalent circuit diagram when a load is connected to a photodiode. FIG. 4 is a circuit diagram showing a photodetection circuit according to an embodiment of the present invention, and FIGS. 5, 6, 7, 8, and 9 are circuit diagrams showing modified photodetection circuits. . lit... photodiode, (4)... operational amplifier, (5)... reverse bias voltage generation circuit, (6)...
・Reverse bias operational amplifier, (8)...diode,
RL...Load resistance. Agent Noriyuki Takano Figure 4 Figure 5

Claims (1)

[Claims] 111: a photodiode that generates a current corresponding to the power of incident light; a current detection resistor connected between one end of the photodiode and a common potential applying means that provides a ground level or a constant voltage level; , an amplifier connected between the photodiode and the current detection resistor, which generates a zero or very small reverse bias output voltage in a region where the power of the incident light is small and a large output voltage in a region where the power of the incident light is large; and a reverse bias voltage generating circuit formed to generate a reverse bias output voltage, the output terminal of which is connected to the other end of the photodiode. (2) The photodetection circuit according to claim 1, wherein the reverse bias voltage generation circuit is a circuit that generates a reverse bias voltage corresponding to the output voltage of the amplifier. 13) The photodetection circuit according to claim 1, wherein the reverse bias voltage generation circuit is a circuit that generates a reverse bias voltage in response to the voltage of the current detection resistor. (4) The reverse bias voltage generation circuit generates a reverse bias voltage corresponding to the power of the incident light in a region where the photodetection current of the photodiode is saturated when no reverse bias voltage is applied, and the circuit generates a reverse bias voltage corresponding to the power of the incident light. 2. The photodetector circuit according to claim 1, which is a circuit that generates a minute reverse bias voltage that is zero or substantially constant in a current region lower than the current region. (5) The reverse bias voltage generating circuit is a circuit that generates a voltage such that the reverse bias voltage across the photodiode is kept substantially constant regardless of changes in the power of the incident light. The photodetection circuit according to item 1.