JPS61290806A - Photodetection circuit for optical communication - Google Patents

Abstract

PURPOSE:To receive stably an optical signal from a minute input to a large input even under communication environment of external light by connecting a load impedance circuit where a photodetector element and an active element whose impedance is increased at a modulation frequency region are connected in series to a prescribed potential point. CONSTITUTION:In making signal light subjected to intensity modulation including external disturbing light due to space propagation incident in a photodiode 1 and a photoconductive element 2, with respect to a high frequency component of the photo current subjected to photoelectric conversion by the photodiode (photodetector) 1, that is, the current of the modulation frequency component of the signal light, since the impedance of capacitors 4, 5 is sufficiently low, the current flows to the capacitor 5 almost through the photoconductive element (impedance element)2, the capacitor 4 and a resistor 3. On the other hand, the low frequency component of the light current subjected to photoelectric conversion by the diode 1, that is, the current of noise component of the external disturbing light becomes partly a base current of a transistor 6 via a resistor 3, because the impedance of the capacitors 4, 5 is sufficiently high conversely for the high frequency component current.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an optical receiver for optical communication, and particularly to a light receiving circuit for optical communication suitable for receiving an optical signal whose intensity is modulated by spatial propagation.

[Background of the invention]

2. Description of the Related Art Short-distance optical space communications using spatial propagation have been widely used for controlling various types of equipment in homes, factories, and the like.

In these types of optical space communications, the distance between the transmitter and receiver is generally not constant, and the influence of disturbance light such as sunlight and illumination light is large. In addition to stably receiving light, it is necessary to remove disturbance light and extract only necessary optical signal components.

Conventionally, as such a light receiving circuit, for example,
As shown in Japanese Patent No. 10835, a trap using a photoelectric element as a load resistance of a light receiving element is known. In this light receiving circuit, the photoconductive element, which is the load resistance of the light receiving element, changes its resistance value in proportion to the intensity of the optical input level.
When the optical input level of signal light or disturbance light is high, the value of the load resistance is lowered to reduce the sensitivity of the light receiving element, which allows the light receiving element to receive optical signals without becoming saturated. .

However, in this light receiving circuit, the load resistance value changes at the same level not only due to the original signal light but also due to external light disturbance.
There is a problem in that the reception sensitivity of signal light changes with the intensity of external light disturbance, and the communicable distance becomes short in a communication environment with strong external light disturbance. In addition, it is generally known that most of the noise components of disturbance light in short-distance optical space communications are in the low frequency band including direct current near the commercial frequency, and therefore the modulation frequency of the optical signal is improved by 8/N. Although a frequency as high as possible is used, in this photodetector circuit, even if the receiving sensitivity changes in proportion to the optical input level, the frequency characteristics of photoelectric conversion depend on the photodetector, and the filter function that removes low frequency component noise cannot be performed. I don't have it. Therefore, in order to remove unnecessary low-frequency noise and extract high-frequency signal components, a filter circuit is required in the subsequent amplifier circuit, which causes problems such as making the optical receiver's circuit configuration complicated and expensive. there were.

[Purpose of the invention]

The purpose of the present invention is to solve the problems of the prior art described above, to realize both a filter function and an AGC function in the load circuit of a light receiving element with a simple and inexpensive circuit configuration, and to achieve micro An object of the present invention is to provide a light receiving circuit for optical communication that can stably receive optical signals from input to large input.

[Summary of the invention]

The present invention provides a light receiving circuit for optical communication, particularly optical space communication, in which the load impedance circuit of the light receiving element is replaced with an impedance element (such as a photoconductive element) whose impedance decreases in proportion to an increase in the level of an optical input signal necessary for communication. By connecting in series an active element circuit whose impedance is high in the modulation frequency region of the optical input signal, the connection point between the light receiving element and the load impedance circuit responds only to the modulation frequency region of the optical input signal. The present invention is a light-receiving circuit for optical communication that realizes both a high-frequency filter function and an AGO function for obtaining an electric signal according to the level of an optical input signal with a simple and inexpensive circuit configuration.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

The figure is a circuit diagram showing an embodiment of an optical communication receiving circuit according to the present invention. In the figure, 1 is a photodiode (light receiving element) for signal reception, 2 is a photoconductive element (impedance element) whose resistance (impedance) decreases as the intensity of incident light increases, 3 is a resistor, and 4 and 5 are each capacitor,
6 is a transistor (active element), 7 is an AC bypass capacitor, 8 is a signal amplifier, 9 is a DC power supply, and 10 is an output terminal. The cathode side of this photodiode 1 is connected to the anode of a DC power supply 9, and the anode side is also connected to one end of each of the photoconductive element 2, the resistor 3, the capacitor 7, and the collector of the transistor 6. The end is connected to one end of the capacitor 4. Also transistor 6
The 0 pace is connected to the capacitor 9-4, each other end of the resistor 3, and one end of the capacitor 5, and the emitter of the transistor 6, the other end of the capacitor 5, and the shade of the DC 1 source 9 are commonly grounded. The other end of the capacitor 7 is connected to an output terminal 10 via a signal amplifier 8.

Note that the photoconductive element 2, resistor 3, and capacitors 4 and 5 shown in the figure
and transistor 6 constitute a load impedance circuit for photodiode 1.

Here, the capacitance values of the capacitors 4 and 5 are selected so that the impedance is sufficiently low in the high frequency region of the optical signal component, and the impedance is sufficiently high in the low frequency region, which is the main noise component of the disturbance light. Since the signal light input to the photoconductive element 2 is weak, the resistance value of the resistor 3 is determined by the parallel impedance of the resistor 3, the photoconductive element 2, and the capacitor 4, which is a combination of the internal junction capacitance of the photodiode 1 and the modulation frequency of the signal light. The impedance should be selected to have as high an impedance as possible.

Next, with the above configuration and selected values, the operation when intensity-modulated signal light including disturbance light due to spatial propagation etc. is incident on the photodiode 1 and the photoconductive element 2 is explained as follows: A detailed explanation will be given focusing on the changes in beadance. First, the photodiode (
Regarding the high-frequency component of the photocurrent photoelectrically converted by the photodetector (light receiving element) 1, that is, the current (electrical signal) of the modulation frequency component of the signal light, the impedance of the capacitors 4 and 5 is sufficiently low, so it is mostly photoconductive. Capacitor f5￥C passes through element (impedance element) 2, capacitor 4 and resistor 3
flows.

Therefore, almost no base current flows through the transistor (active element) 60, and the collector-emitter of the transistor 6 is substantially in a cut-off state. It becomes impedance. Here, photoconductive element 2
Since the resistance value of the photodiode 1 decreases in proportion to the increase in the intensity of the signal light, the load impedance of the photodiode 1 similarly decreases in proportion to the increase in the intensity of the signal light. This means that when the current of the photodiode 1 increases due to an increase in the intensity of the signal light, the output voltage from the anode terminal of the photodiode 1 is kept constant by lowering the load impedance of the load impedance circuit. Indicates that it operates as an AGO function.

On the other hand, regarding the low frequency component of the photocurrent photoelectrically converted by the photodiode 1, that is, the current of the noise component of the disturbance light, a part of it is This becomes a pace current for the transistor 6 via the resistor 3. At this time, transistor 6
The pace current is applied to the collector of the current amplification factor h F ! + times the current flows. Here, since the current amplification factor hyz of the transistor 6 is generally easily available with a value of several hundred or more, the collector-emitter of the transistor 6 can be considered to be in a very conductive state, and at this time, the current amplification factor hyz of the photodiode 1 The load impedance is sufficiently low compared to the impedance for high frequency components. This means that for disturbance light whose main noise component is in the low frequency region,
By sufficiently lowering the load impedance compared to the signal light in the high frequency region, no voltage is output to the anode terminal of the photodiode 1, and the light receiving circuit does not react. That is, the load impedance circuit of the photodiode 1 removes the current of the disturbance light in the low frequency range from the connection point to the output terminal 10, and passes only the current of the signal light in the high frequency range. ! ! This indicates that it operates as a function. Regarding the influence of disturbance light on the AGC function of the signal light, since the capacitor 4 has a sufficiently high impedance with respect to the noise component 9 in the low frequency region, the photoconductive element 2. Even if the resistance value of the photoconductive element 2 changes due to disturbance light being incident on the photoconductive element 2, the load impedance of the photodiode 1 for the signal light component in the high frequency region does not substantially change.
Does not affect C functions.

Next, the change in the load impedance of the photodiode 1 due to the frequency range of the incident light will be determined and shown using a specific mathematical formula.

Now, the load impedance of the load impedance circuit of photodiode 1 is determined by 21 photoconductive element 2 and conductor f4.
The parallel impedance of R21 and resistor 3 is R21 capacitor 5
If the impedance of the photodiode 1 is R, the base resistance of the transistor 6 is rB, and the current amplification factor is hFK, then the impedance 2 of the load impedance circuit of the photodiode 1 is
is approximately given by the following equation.

Therefore, the load impedance 2 in the high frequency region of the signal light is z
(H) and the load impedance Z in the low frequency region of the main noise component of the disturbance light is determined by equation (1) as Z(L). Here, the impedance RIc of the capacitor is Rc (IK) = 0 in the high frequency region and R1' (t,) in the low frequency region.
Since the selection is made so that = ■, the following equation can be approximately obtained.

From equations (2) and +3), the load impedance Z(i) in the high frequency region and the load impedance Z(L
) is calculated as follows.

In equation (4), hν of the normal transistor 6 is expressed by the formula 1
00 or more, and since the pace resistance rB has a value of the same order as the parallel impedance Rz, the load impedance ratio Z of the high frequency region and the low frequency region of the photodiode 1 is
(1/Z(b) generally has a value of 100 (40 dB) or more. Therefore, in order to extract the current of the signal light including the disturbance light from the anode terminal of the photodiode 1, the load impedance circuit of the photodiode 1 is the main source of the disturbance light. This shows that the filter functions as a high-frequency filter that removes the current in the low-frequency region of the noise component and passes only the current in the high-frequency region of the signal light.

Also, from equation (2), it can be seen that the load impedance Z(1) in the high frequency region of the signal light is approximately equal to the parallel impedance R,z IC of the photoconductive element 2, the capacitor 4, and the resistor 3. The change in parallel impedance Rz with respect to intensity is determined using a specific formula as follows. That is, now the resistance value of photoconductive element 2 is rP, the impedance of contact f4 is rc, and the resistance value of resistor 3 is rP.
If Rl is Rl, the parallel impedance Rz is given by the following equation.

Here, the impedance r of the contact +4 is selected to be 7"a(11):O in the high frequency region, so
If the parallel impedance Rz in the high frequency region is Rg(ir), the following approximate expression is obtained from equation (5).
Since the resistance value ra decreases as the intensity of the signal light increases, the parallel impedance R2(H) similarly decreases the resistance value J''p.
It can be seen that it decreases in proportion to the change in . That is, the fact that the load impedance Z(i), that is, the parallel impedance FLz (!I) of the load impedance circuit of the photodiode 1 changes with respect to the intensity of the signal light, indicates that it functions as an AGC function of the signal light. Regarding the influence of disturbance light in the low frequency region on the parallel impedance Rz, in equation (5), the impedance rc of the contact f4 is selected so that 7''c(L) = ω in the low frequency region. , parallel impedance R in the low frequency region
When rz is RZ(I,), the following approximate expression is obtained from equation (5).

Rz(b), rR(7) In other words, is the parallel impedance in the low frequency region &(1,) the resistance value? ”H, and the resistance value r of the photoconductive element 2
Not affected by p. This shows that the main noise component in the low frequency region of the disturbance light has no effect on the change in the load impedance Z (H>) of the load impedance circuit with respect to the intensity of the signal light, that is, on the AGC function of the signal light.

As described above, in the light receiving circuit for optical communication of this embodiment, the load impedance circuit of the photodiode (light receiving element) 1 is composed of the photoconductive element (impedance element) 2, the resistor 3, the contact f4゜5, and the transistor (lfI dynamic element). By connecting the photodiode 1 and the active element circuit including 6 in series, an electrical signal that responds only to the modulation frequency region of the optical input signal and that corresponds to the level of the optical input signal is generated from the connection point between the photodiode 1 and the load impedance circuit. Both the high frequency filter function and the AGC full function can be realized with a simple and inexpensive circuit configuration.

〔Effect of the invention〕

As described above, according to the present invention, a light receiving circuit for optical communication has a filter function that removes the influence of noise caused by disturbance light with a simple and inexpensive circuit configuration, and a filter function that can be applied to a light receiving circuit for optical communication using a simple and inexpensive circuit configuration. Because it realizes the AGC function that stably receives light up to the input, it is generally possible to improve the S/N ratio by 20 dB or more compared to conventional circuits, even in communication environments with a lot of external noise light, and the effect of expanding the communication distance. There is. In particular, the effect is remarkable when used in spatial optical communication using a diffusion method in which an unspecified number of light receiving circuits are provided.

[Brief explanation of drawings]

FIG. 11 is a circuit diagram showing an embodiment of a light receiving circuit for optical communication according to the present invention. 1... Light receiving element, 2... Photoconductive element (impedance element), 3... Resistor, 4, 5... Capacitor, 6
. . . Transistor (active element), 7. AC bypass conductor, 8. Signal amplifier, 9. Direct current, 10. Output terminal.

Claims (1)

[Claims] A load in which a photodetector that receives a modulated optical signal, an impedance element whose impedance decreases as the level of the optical input signal increases, and an active element circuit whose impedance increases in the modulation frequency region of the optical input signal are connected in series. An impedance circuit is connected in series between a predetermined potential point, and an electric current that responds only to the modulation frequency region of the optical input signal and responds to the level of the optical input signal from the connection point of the light receiving element and the load impedance circuit is connected in series between predetermined potential points. A light receiving circuit for optical communication that obtains signals.