JPH0531111B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention measures a measured quantity by measuring the intensity of light modulated according to an analog measured quantity such as a measured voltage or a magnetic field. The present invention relates to an optical measurement device configured to do the following.

[Conventional technology]

FIG. 4 is a connection diagram of an example of a conventional optical measuring device of this type. In the figure, APC is an auto power control circuit, AGC is an auto gain control circuit, FI is an optical fiber, and S
is a voltage (or magnetic field) sensor, and this sensor is installed in the middle of the optical fiber FI. An alternating voltage to be measured Vx is applied to the sensor S.
Optical fiber FI is provided with optical connectors CN ₁ and CN ₂ .

In the auto power control circuit APC, a part of the light emitted by a semiconductor light receiving element Ld such as a laser diode or a light emitting diode is irradiated onto a photo diode Pd for rear monitoring, and a photocurrent Ip is sent to a detection resistor _R1. and compares the voltage drop R ₁ ·Ip with the reference voltage ES ₁ in the amplifier A ₁ to increase or decrease the current ILd flowing through the light emitting element Ld,
Ip is controlled to be constant. in this case,
Since the optical output P ₁ of the light emitting element Ld is proportional to the photocurrent Ip of the monitor photo diode Pd, the optical output P ₁
will be kept at a constant value. This light output P ₁
is input to the avalanche photo diode APd in the auto gain control circuit AGC via the optical connector CN ₁ , optical fiber FI, voltage sensor S, and optical connector CN ₂ . The light output _P1 of the light emitting element Ld is modulated by the analog voltage to be measured Vx in the voltage sensor S,
As a result, the light from Ld changes as shown by the solid line in FIG. Therefore, if we measure the input light P ₂ of APd,
The value of Vx can be measured.

By the way, in a device with this configuration, _{the input light P 2 to APd} is Fifth
As shown by the dotted line in the figure, it fluctuates at a constant rate. This variation is compensated for in the auto gain control circuit AGC as follows.

In other words, the input light P ₂ to APd is modulated by this APd into a current Ia, and a voltage drop R ₂・Ia occurs across the detection resistor R ₂ .
cause This voltage drop passes through the low-pass filter LPF and its DC component (the average value of R ₂ Ia
_R2 .) and the reference voltage _ES2 are compared, and the difference voltage is amplified by the amplifier _A2 . Its amplified output is
Feedback control is applied to the cathode of APd to change the multiplication factor M of APd so that R ₂ · becomes approximately equal to ES ₂ . In this way, even if the amount of light changes at the optical connectors CN ₁ , CN _{2 ,} etc., R ₂ · can be controlled to a constant value by changing the multiplication factor M of APd.

On the other hand, the DC component R ₂ of R ₂ Ia is a capacitor
Cc, amplified by amplifier _A3 , and taken out from output terminal OUT as output voltage Vout. The voltage sensor S is
In the vicinity of Vx=0, (ΔP ₂ / ₂ )∝Vx (1) (∵ΔP ₂ is the change in P ₂ , ₂ is the DC component of P ₂ ).

The above formula (1) means the following. That is,
If the optical power input to the voltage sensor S is constant, and the value of the voltage to be measured Vx changes from, for example, 0V to 0.1V, the power of the optical output of the voltage sensor S increases from, for example, 1mW to 1.1mW. , _P2
The change in ΔP ₂ is 0.1 mW. Similarly, when the voltage to be measured Vx changes from 0V to 0.2V, the power of the optical output of the voltage sensor S changes, for example.
If the power increases from 1 mW to 1.2 mW, the change in P ₂ ΔP ₂ will be 0.2 mW. In other words, Vx in Figure 5 =
Since it can be regarded as almost a straight line near 0, the change in ΔP ₂ is proportional to the voltage to be measured Vx.

Next, the connection of optical connectors CN ₁ , CN ₂ , etc. is not complete, and the connection is not complete as shown from the solid line to the dotted line in Figure 5.
If the input light P ₂ to APd is reduced, for example, if P ₂ decreases to 70% when the measured voltage Vx is 0V, then when Vx is 0.1V, P ₂ also decreases to the same 70%. Therefore, the value of (ΔP ₂ /P ₂ ) does not change. Therefore, in equation (1), the value of (ΔP ₂ / ₂ ) is always the measured voltage.
This means that it is determined only by the value of Vx, and the relationship becomes a proportional relationship near Vx=0 in FIG. 5, as described above.

On the other hand, the photocurrent Ia of APd is the input light P ₂ of this APd
Since it is proportional to, (ΔIa/)=(ΔP ₂ / ₂ )...(2) (ΔIa is the change in Ia). Since Vout is R ₂ · ΔIa amplified by amplifier A ₃ , Vout∝R ₂ · ΔIa ... (3) From equations (1), (2), and (3), Vout∝R ₂ · (ΔP ₂ / ₂ ) ∝R ₂ ...Vx...(4). As mentioned above, R ₂ is always constant even if the light intensity changes, so the output voltage shown by equation (4)
Vout is not affected by light intensity fluctuations, and the measured voltage Vx
It will be proportional to.

The optical measurement device having such a configuration is known as described above, but as a means of transmitting analog information using optical fiber, it uses a dual control circuit, that is, an APC circuit and an AGC circuit, as its constituent elements. I had it. This necessity is due to the circumstances of the light emitting element Ld. That is, the maximum current and maximum light emission output are specified for the light emitting element Ld,
Even if the light is emitted at an output within the maximum light output, it is desirable to reduce the output as much as possible in consideration of the lifetime. However, in order to make the signal as large as possible compared to the noise of the entire system including the noise of the laser diode itself, it is desirable to emit light with as large an optical output as possible. Therefore, it was natural to control the output of the laser diode Ld to an optimal constant value. On the other hand, if the semiconductor light emitting device Ld is a light emitting diode, the light output value is small to begin with, and the light emitting area is also larger than that of a laser diode. only a certain amount is incident.
Therefore, although the noise of the light emitting diode itself is small, in order to improve the S/N ratio of the entire system, the light emitting output needs to be as large as possible and at a constant value.

If the light emitting output of the semiconductor light emitting device Ld is controlled constant for the above reasons, the optical connector
The optical output P ₂ changes by the amount of change in the amount of light caused by CN ₁ and CN ₂ , the optical fiber FI, the voltage sensor S, and the like. To compensate for this, an avalanche photo diode APd is used in the device shown in Figure 4.
It was necessary to use an AGC circuit including an AGC circuit, which caused the problem that the overall configuration became complicated. In addition, the fourth
The circuit shown in the figure has an APC circuit, so even if optical connectors CN ₁ and CN ₂ become disconnected while the system is operating, the semiconductor light emitting device Ld will continue to emit a constant amount of light, so there is no risk of damage. There are some features that don't exist.

[Problem that the invention seeks to solve]

The present invention has been made to solve the above-mentioned problems, and includes a photo diode Pd for rear monitoring in an APC circuit or an avalanche photo diode Pd in an AGC circuit to compensate for sensitivity fluctuations due to fluctuations in light. The purpose of this invention is to provide an optical measurement device with a simplified overall configuration that does not require APd or the like.

[Means for solving problems]

In order to achieve the above object, the present invention inputs the light emission output of a semiconductor light emitting element through an optical fiber to an optical modulation element that undergoes modulation according to an analog measured quantity, and the modulated output light of this optical modulation element is input to the semiconductor photodetector through an optical fiber, the voltage drop caused by the detection resistor that detects the output current of the semiconductor photodetector is compared with a reference voltage via a low-pass filter, and the output of the difference is outputted to the semiconductor photodetector. The automatic power control circuit is equipped with an auto power control circuit that controls the voltage drop to a constant value by applying the voltage drop to It is composed of Examples will be described below.

〔Example〕

FIG. 1 is a circuit diagram of an embodiment of an optical measuring device according to the present invention. In the figure, Ld is a semiconductor light emitting device such as a laser diode or a light emitting diode, and A is an amplifier. The cathode of the light emitting element Ld is connected to the output terminal of the amplifier A via the resistive element _R3 . D is a light receiving element such as a photo diode. Here, the avalanche photodiode APd used in the circuit of Figure 4 is:
As is well known, light is input while applying a voltage close to the breakdown voltage (corresponding to the breakdown voltage, which is referred to as Vbr) at the time of reverse bias, causing an avalanche of electrons and doubling the photoelectrons. let me,
This provides a large output current. In order to use the multiplication factor M for doubling the photoelectrons at this time, for example, M = 50, an avalanche photodiode APd
and apply a reverse bias of about 0.93 times its breakdown voltage Vbr. This Vbr is normally 100V to 200V,
Delicate control of such a high voltage, for example about 0.93 times Vbr, requires complex circuitry and advanced technology. Furthermore, since Vbr itself fluctuates depending on the temperature, measures to avoid this are also required.

In contrast, the photo diode used in the circuit of Figure 1, which does not use the avalanche effect,
For example, in the case of a photodetector D such as a pin photo diode, it is only necessary to apply a constant DC voltage with a low reverse bias of about 10V, so the circuit is simple and easy to use. be. The bias power supply Vb is connected to the cathode of this diode D, and the anode is connected to a common potential point through a resistor _R2 , and through a series circuit of a DC component cutting capacitor Cc and an operational amplifier _A3 . is connected to the output terminal OUT, and further connected to the (+) of amplifier A via a low-pass filter LPF.
connected to the input terminal. A reference voltage ES ₁ is connected to the (-) input terminal of the amplifier A. Dm
is a diode, Vm is a power supply, and the cathode of diode Dm is connected to the connection point between amplifier A and resistive element _R3 . FI is a fiber, S is a voltage sensor (or magnetic field sensor), and this sensor is provided in the middle of the optical fiber FI as in FIG. 4, and the AC voltage Vx to be measured is applied.

CN ₁ and CN ₂ are optical connectors installed on the optical fiber FI. One end of the optical fiber FI faces the semiconductor light emitting element Ld, and the other end faces the semiconductor light receiving element D. The operation of the device having such a configuration will be explained as follows.

A current ILd flows through the semiconductor light emitting device Ld via the resistor _R3 due to the output of the amplifier A, and the light emitting output _P1 of the light emitting device Ld corresponding to the current ILd is output to the optical connector.
The signal is input to the light receiving element D via CN ₁ , fiber FI, voltage sensor S, and optical connector CN ₂ .

As explained in Fig. 4, the light output P ₁ of the light emitting element Ld
is the AC measured voltage Vx at the voltage sensor S
As a result, the light from Ld changes as shown by the solid line in FIG. Therefore, by measuring the input light _P2 of the light receiving element D, the value of Vx can be measured.

Input light P ₂ to the light-receiving element D is converted into a photocurrent Ia in this element D, causing a voltage drop R ₂ ·Ia in the detection resistor R ₂ . This voltage drop is applied to a low pass filter LPF, and its DC component is extracted. This DC component is applied to the amplifier A and compared with the reference voltage _ES1 , increasing or decreasing the current ILd flowing through the light emitting element Ld, and as a result, the average value of the voltage drop _R2 ·Ia generated across the detection resistor _R2 remains constant. controlled so that This voltage drop R ₂・I ₂ is the capacitor
Cc, its DC component is cut, and the AC component is amplified by amplifier A and sent from the output terminal OUT.
Extracted as Vout.

In the present invention having such a configuration, even if any of the optical connectors CN ₁ , CN ₂ , the voltage sensor S, or the optical fiber FI changes and the light is attenuated as shown by the dotted line in FIG. The system is controlled so that the voltage drop across the detection _{resistor R2} of the photocurrent Ia, which is proportional to the input light of the photodetector D, is constant.The conventional example shown in Fig _. 4 Equations (1) to (4) hold exactly as in the case of . As a result, Vx can be accurately measured without being affected by changes in the degree of coupling between the optical connectors CN ₁ and CN ₂ or the attenuation of the voltage sensor S or the optical fiber FI.

In addition, in the circuit shown in Fig. 1, if optical connectors CN ₁ and CN ₂ are disconnected while the system is operating, the light receiving element D
Since the input light becomes zero, the light emitting element Ld operates to generate a large current, which may damage the light emitting element Ld. In the present invention, in order to prevent the life of the light emitting element Ld from being shortened even in such a case, a current limiting circuit is provided to prevent ILd from becoming larger than the current corresponding to the optical output _P2 .
This is a circuit consisting of a resistive element _R3 , a diode Dm, and a power supply Vm. In this circuit, when the output ILd of the light emitting element Ld increases, the voltage drop R ₃・ILd
This causes current to flow through the diode Dm. As a result, the current ILd does not increase beyond a predetermined value, and the light emitting element Ld will not be destroyed or its life will be shortened.

FIGS. 2 and 3 show other embodiments of the apparatus of the present invention, respectively. The circuit shown in Figure 2 uses _an amplifier to detect the photocurrent Ia of the photodetector D in the circuit shown in Figure 1.
The photocurrent Ia is obtained by combining _A4 as the output voltage of the amplifier. The circuit shown in FIG. 3 is a combination of a low pass filter LPF and an amplifier A, the output of which is applied to the amplifier _A5 and its output current is fed to the light emitting element Ld.
The basic operations of the apparatuses shown in FIGS. 2 and 3 are exactly the same as those shown in FIG. 1.

〔Effect of the invention〕

In the conventional device shown in Fig. 4, as described above, the light emitting element Ld is controlled by the auto power control circuit APC including the rear monitor diode Pd.
Since the output P ₁ of the sensor is controlled to a constant value, sensitivity fluctuations occur. Therefore, an auto gain control circuit containing a special diode such as an avalanche photo diode was used to compensate for the sensitivity variation. Therefore, the overall configuration becomes complicated. On the contrary,
In the present invention, the amount of light passing through the entire system including the optical fiber, the light modulation element, etc. is detected, and the amount of light is controlled to be a constant value.
The overall configuration eliminates the need for a rear monitor diode, which was an essential component in conventional equipment, and also eliminates the need for an avalanche photo diode with a gain control function as a light receiving element, allowing freedom in selecting the light receiving element. A simplified optical measurement device can be obtained. Furthermore, in the present invention, a current limiting circuit consisting of a resistive element R ₃ , a diode Dm, and a DC power source Vm is provided, so if the connectors CN ₁ and CN ₂ become disconnected, the action of the current limiting circuit will The current ILd flowing through the light emitting element Ld does not increase beyond a predetermined value,
It is possible to obtain an optical measurement device in which the light emitting element Ld is not destroyed or its life is not shortened.

[Brief explanation of the drawing]

1 to 3 are connection diagrams of embodiments of the optical measurement device according to the present invention, FIG. 4 is a connection diagram of a conventional device, and FIG. 5 is for explaining the output characteristics of this type of optical measurement device. This is a diagram. Ld...Semiconductor light emitting element, D...Semiconductor light receiving element, _R2 ...Detection resistor, LPF...Low pass filter, S...Voltage sensor, FI...Fiber, ES ₁ ...Reference voltage element, Cc... Capacitor for DC cut.

Claims (1)

[Claims] 1. A current limiting circuit consisting of a semiconductor light emitting element Ld whose cathode is connected to one end of the resistive element _R3 , and a diode Dm whose anode is connected to the negative side of the DC power supply and whose cathode is connected to the other end of the resistive element _R3 . and connects the light emission output of the semiconductor light emitting device Ld to an optical fiber having a connector CN ₁ in the middle.
The light is input to a light modulation element S that undergoes modulation according to the analog measured quantity through the FI, and the modulated output light of the light modulation element S is sent to a semiconductor light receiving element D via an optical fiber FI provided with a connector _CN2 . The voltage drop caused by the detection resistor R2 which detects the output current of the semiconductor light receiving element D is passed through a low pass filter LPF to an amplifier A whose output terminal is connected to the connection point of the resistor _R3 and the diode Dm. By comparing with the reference voltage ES ₁ and giving the output of the difference to the semiconductor light receiving element Ld,
An optical measurement device equipped with an auto power control circuit that controls the voltage drop to a constant value, and extracts the voltage drop as an output _Vput via a capacitor Cc for cutting off a DC component. .