JPH0581588A - Preamplifier for otdr device - Google Patents

Abstract

PURPOSE:To suppress 'blind over edging' phenomena caused by frequency dispersion peculiar to a compound semiconductor. CONSTITUTION:The amplifier circuit 101 of the preamplifier consists of the compound semiconductor FET and it is the amplifier of a trans impedance type or a high impedance type. The feature of the preamplifier is that the equalization circuit 107 of RC constitution are provided between the light detecting element and the input of the amplifier circuit. 'VB' shows being connected with a power source terminal for a bias. The equalization circuit 107 consists of resistance R1 to R4 and a capacitor C4, e.g. When a frequency rises, the impedance of the capacitor C4 is decreased and current flowing resistance R3 is increased. Thus, the output v1 of the equalization circuit becomes larger and compensation is executed.

Description

Detailed Description of the Invention

[0001]

The present invention relates to an optical TDR (OTDR)
OT used for the front end of optical measuring instruments such as devices
The present invention relates to a preamplifier for a DR device, and particularly to a high-speed preamplifier using a compound semiconductor.

[0002]

2. Description of the Related Art An OTDR device is a measuring instrument for measuring various types of loss (transmission loss, connection loss, etc.) of an optical fiber cable and the position of a failure point (break point). The measurement is performed by injecting impulse-shaped light into the optical fiber cable and measuring the intensity of the returned light. As shown in FIG. 6, a typical measurement result shows a strong impulse (left side in FIG. 6) caused by end face reflection when light is incident on the optical fiber, and then weak due to scattering and reflection in the optical fiber cable. It is supposed to be followed by various signals.

Like the optical communication device, the preamplifier used in the OTDR device has a circuit configuration called a transimpedance type (see FIG. 7A) or a high impedance type (see FIG. 7B). Is used.
About these, for example, "K.Ogawa: Considerations
for Optical reciever design, IEEE Journal on Select
ed Areas in Comunications. Vol. SAC-1, No. 3, 1983 ". Further, FIG. 8 shows a circuit diagram in the case where the transimpedance type amplifier circuit of FIG. 7A is configured by a compound semiconductor FET.

[0004]

PROBLEM TO BE SOLVED BY THE INVENTION Compound semiconductor FET
Is extremely fast due to the high carrier mobility of the compound semiconductor, but has the drawback that its drain conductance has frequency dispersion as shown in FIG. The frequency range in which this frequency dispersion occurs is 1 kHz to 100 kHz.
It is about z, and it is said that this is an effect of the trap level existing on the interface between the channel substrates of the FET and the channel surface. In an amplifier circuit composed of such a compound semiconductor FET, frequency dispersion occurs in its voltage gain. Therefore, the transimpedance type amplifier circuit of FIG. 7A has a voltage gain characteristic as shown in FIG. Therefore, the frequency dispersion as shown in FIG. 11 also occurs in the transimpedance (output voltage V _OUT / input current I _PD ) indicating the conversion gain of this amplifier circuit.

When a step-like pulsed light as shown in FIG. 12 is input to the preamplifier constituted by such an amplifier circuit, the output voltage V _OUT of the preamplifier has a “tail” (plot in FIG. 13). .. This is due to frequency dispersion,
A good rise is shown when there is no frequency dispersion (Fig. 13).
Solid line). With the OTDR device, the measurement results are as shown in FIG. 6, but when light is incident on the optical fiber, a strong impulse (left side in FIG. 6) caused by Fresnel reflection of the incident end face causes “tailing”. This "Susukiki"
As shown in FIG. 13, since it lasts for several μS, it has a great influence on the waveform of the weak signal following the impulse, and there is a problem that measurement cannot be performed until the “tailing” stops.

[0006]

In order to solve the above problems, a preamplifier for an OTDR device of the present invention includes a photodetector for converting light from an optical fiber into an electric signal and a compound semiconductor FET. An amplification circuit configured to amplify and output an electric signal from the photodetection element and an equalization circuit having an RC configuration for compensating the frequency dispersion of the amplification circuit between the photodetection element and the input of the amplification circuit. It is characterized by having.

[0007]

Function: The compound semiconductor FET is very fast,
Has frequency dispersion. Therefore, the amplifier circuit composed of this FET has frequency dispersion.

In the preamplifier for the OTDR device of the present invention,
The equalization circuit between the photodetector and the input of the amplifier circuit compensates the electrical signal from the photodetector for the frequency dispersion of the amplifier circuit, and the amplified signal is amplified and output by the amplifier circuit. It

[0009]

Embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a basic configuration of a preamplifier for an OTDR device according to the present invention. Amplifier circuit 101 of this preamplifier
Is a transimpedance-type or high-impedance-type amplifier which is composed of a compound semiconductor FET. The preamplifier shown in FIG. 1 is characterized in that an equalizer circuit 107 having an RC structure is provided between the photodetector and the input of the amplifier circuit. Here, “V _B ” in the figure indicates that it is connected to the bias power supply terminal. Equalization circuit 107
Is, for example, resistors R ₁ , R ₂ , R ₃ and
It is composed of R ₄ and capacitor C _4, and is represented by the equivalent circuit of FIG. In the case of a high impedance type amplifier, the photocurrent I _{PD of the} photodiode and the output v of the equalization circuit
The relationship with ₁ is

[0010]

[Equation 1]

It is represented by the transfer function That is, as the frequency increases, the impedance of the capacitor C ₄ decreases and the current i ₃ flowing through the resistor R ₃ increases. Therefore, the output v _{1 of the} equalization circuit becomes large. This provides compensation.

By setting these resistors and capacitors to appropriate values, the light incident on the photodiode PD is converted into an electric signal (photocurrent i _PD ), and the frequency dispersion of the amplifier circuit 101 is converted by the equalization circuit 107. Compensated and amplified circuit 1
The signal is amplified by 01, the frequency characteristic is flattened, and output.

A transimpedance type amplifier circuit 101 is shown in FIG. 3, which is manufactured by the amplifier section 101A composed of the compound semiconductor FET shown in FIG. In this figure, the voltage gain | -A | of the amplifier 101A is 29 dB at direct current and 26 dB at 1 MHz, and the total input capacitance is 1 pF. Feedback resistance Rf is 30
kΩ, the resistance R ₁ of the equalization circuit 107 is 10 kΩ, the resistances R ₂ and R ₃ are 5 kΩ, the resistance R ₄ is 37 kΩ, and the capacitor C ₄ is 40 pF.

FIG. 4 shows the results of measuring the transimpedance of the amplifier circuit for the preamplifier of FIG. Further, FIG. 5 shows response characteristics to stepwise pulsed light similar to FIG. As is clear from the comparison of these figures, in the preamplifier of FIG. 3, the equalization circuit 107 compensates for the frequency dispersion and makes it substantially flat, and the pulse response characteristics are improved.

Even if light as shown in FIG. 6 is incident on the preamplifier from the optical fiber, since "tailing" is suppressed, there is no influence of a strong impulse generated when the light is incident on the optical fiber. Further, since the amplification unit 101A is composed of the compound semiconductor FET, it operates at high speed. Therefore, the waveform of the weak signal following the impulse can be measured well.

[0016]

As described above, according to the present invention, the frequency dispersion of the amplifier circuit is compensated by the equalization circuit, so that the influence of the frequency dispersion is suppressed and the "tailing" at the rise of the output is suppressed. be able to. Therefore, the influence of the impulse generated when light is incident on the optical fiber can be suppressed, the drawbacks of the compound semiconductor FET can be covered, its high speed can be used, and various measurements of the optical fiber by the OTDR device can be performed well. You can

[Brief description of drawings]

FIG. 1 is a basic configuration diagram of a preamplifier of the present invention.

FIG. 2 is a diagram equivalently showing an equalization circuit.

FIG. 3 is a configuration diagram when the preamplifier of the present invention is configured by the amplification unit of FIG. 8.

FIG. 4 is a transimpedance characteristic diagram.

FIG. 5 is a response characteristic diagram for a step pulse.

FIG. 6 is a diagram showing a typical measurement result of an OTDR device.

FIG. 7 is a configuration diagram of a conventional preamplifier.

FIG. 8 is a circuit diagram when a transimpedance type amplifier circuit is composed of compound semiconductor FETs.

FIG. 9 is a diagram showing frequency dispersion of drain conductance of a compound semiconductor FET.

FIG. 10 is a diagram showing frequency dispersion of voltage gain of an amplifier circuit composed of compound semiconductor FETs.

11 is a transimpedance characteristic diagram of the amplifier circuit of FIG.

FIG. 12 is a diagram showing input step pulses.

13 is a response characteristic diagram of the amplifier circuit of FIG. 11 with respect to a step pulse.

[Explanation of symbols]

PD ... Photodiode, 101 ... Amplification circuit, 107 ...
Equalization circuit

Claims (1)

[Claims] 1. A photodetector for converting light from an optical fiber into an electric signal; and an amplifier circuit including a compound semiconductor FET for amplifying and outputting the electric signal from the photodetector, A preamplifier for an OTDR device, comprising an RC-equalization equalizer circuit for compensating the frequency dispersion of the amplifier circuit between the photodetector and the input of the amplifier circuit.