JPS62105541A - Optical receiving circuit - Google Patents

Abstract

PURPOSE:To omit an equalizing circuit composes of L and C by adjusting a bias voltage applied to a PIN photodiode, and varying frequency characteristics corresponding to the width of depletion layer and thus equalizing waveform. CONSTITUTION:An electric signal converted by the PIN photodiode 11 is passed through amplifiers 14 and 15 and extracted by a tank circuit 16, whose output is amplified 17, so that received data from a level discriminator 18 is outputted as a timing signal for level discrimination. The depletion layer width of the diode 11 varies with the bias voltage and the electrostatic capacity between terminals varies, so that the output of a high frequency area decreases as the bias voltage becomes lower. For the purpose, the bias voltage is adjusted to adjust the frequency characteristics of the photodiode and perform desired equalization, so the equalizing circuit as a conventional example is omitted.

Description

[Detailed Description of the Invention] (Summary) By adjusting the bias voltage applied to the PIN photodiode, the frequency characteristics corresponding to the depletion layer width of the PIN photodiode are changed to equalize the waveform, and the coil, capacitor, etc. This allows the equalization circuit consisting of the following to be omitted.

[Industrial application field]

The present invention relates to an optical receiving circuit that converts an optical signal into an electrical signal and outputs received data based on level identification.

In an optical transmission system, an optical signal transmitted through an optical transmission line is subject to attenuation and waveform distortion, and in an optical receiving circuit, the received optical signal is converted into an electrical signal by a light receiving element. After conversion, amplification, and waveform equalization, the received data is output based on level identification. It is desirable for such an optical receiving circuit to obtain desired characteristics with a simple configuration.

[Conventional technology]

A conventional optical receiving circuit has, for example, a configuration shown in FIG. 4, in which 21 is a light receiving element, 22 is a resistor, 23 is a preamplifier,
4 is the main amplifier, 25 is an equalization circuit, 26 is a tank circuit, 2
7 is a timing signal amplifier, and 28 is a level discriminator. A predetermined bias voltage VCC is applied to the series circuit of the light receiving element 21 and the resistor 22, and an optical signal is input to the light receiving element 21 via the optical transmission path.

The optical signal is converted into an electrical signal by the light receiving element 21, amplified by the preamplifier 23, the amplified output is applied to the main amplifier 24, amplified to a predetermined level by an AGC function, etc., and applied to the equalization circuit 25. The equalization circuit 25 is composed of a combination circuit of a coil, a capacitor, etc., and forms a kind of low-pass filter. The signal whose waveform has been equalized by the equalization circuit 25 is applied to a level discriminator 28 and also to a tank circuit 26. This tank circuit 26
is a tank circuit whose resonant frequency is the frequency of the clock signal component of the transmission signal, in which the clock signal component is extracted and amplified by the timing signal amplifier 27,
8 as a timing signal for level identification.

The level discriminator 28 discriminates the level of the waveform-equalized signal at the timing according to the timing signal, and outputs received data.

[Problem that the invention seeks to solve]

The equalization circuit 25 in the conventional optical receiving circuit is composed of a combination circuit of coils, capacitors, etc., so it is difficult to miniaturize it, and the adjustment for equalizing the waveform of the received signal is complicated. There was a drawback.

The present invention provides an equalization circuit 25 consisting of a coil, a capacitor, etc.
The object of the present invention is to provide an optical receiving circuit that can omit the above steps.

[Means for solving problems]

The optical receiving circuit of the present invention uses a PIN photodiode as a light receiving element, and performs waveform equalization using the frequency characteristic corresponding to the depletion layer width of the PIN photodiode. To explain, a PIN photodiode 1 as a light receiving element is provided, and a bias circuit 2 is provided which changes the depletion layer width by applying a bias voltage to the PIN photodiode 1, changes its frequency characteristics, and thereby equalizes the waveform. , PIN photodiode 1
The electrical signal converted by the above is amplified by an amplifying section 3, and according to the timing signal reproduced by a timing reproducing section 4, the level is discriminated by a level discriminating section 5, and received data is output.

[Effect]

The depletion layer width of the PIN photodiode 1 increases as the bias voltage increases, and the width of the depletion layer of the PIN photodiode 1 increases.
The capacitance between the terminals becomes smaller, and the frequency characteristics extend to higher frequencies.

Therefore, by adjusting the bias voltage applied to the PIN photodiode 1 by the bias circuit 2, a desired frequency characteristic can be obtained, thereby performing waveform equalization.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 2 is a block diagram of an embodiment of the present invention, in which 11 is a PIN photodiode as a light receiving element, 12 is a variable resistor constituting a bias circuit, 13 is a series resistor, 14 is a preamplifier, and 15 is a main amplifier. , 16 is a tank circuit, 17 is a timing signal amplifier, and 18 is a level discriminator.

The electrical signal converted by the PIN photodiode 11 is amplified by a preamplifier 14 and a main amplifier 15,
The timing signal added to the tank circuit 16 and the level discriminator 18, extracted by the tank circuit 16, and amplified by the timing signal amplifier 17 is added to the level discriminator n18 as a timing signal for level discrimination, and the timing signal is added to the level discriminator 18.
The received data is output from.

In addition, a bias voltage is applied to the PIN photodiode 11 by adjusting the power supply voltage VCC using a variable resistor 12, and the depletion layer width of the PIN photodiode 11 changes in accordance with the bias voltage, thereby reducing the static voltage between the terminals. Capacity changes.

The frequency characteristics of the PIN photodiode 11 are: (1) CR time constant due to junction capacitance and load resistance, (2) transit time of photocarriers in the depletion layer, (3) diffusion current in the P layer and the N layer. It is determined by the ingredients. When the bias voltage of the PIN photodiode 11 is lowered, since the depletion layer width is narrow, the diffusion current (3) becomes dominant, and the response speed becomes slow. If the bias voltage is increased until the depletion layer width is widened to the extent that the diffusion current is negligible, the frequency characteristic will be as shown in (2), and the response speed will be determined by the transit time of the photocarriers. The response speed in this case is faster than the response speed in (3).

In the case of (2), it takes time for carriers to travel through the depletion layer, so for light that changes at high speed, the conversion efficiency decreases in proportion to the speed. If the frequency at which this conversion efficiency decreases by 3 dB is the high cutoff frequency fc, it is inversely proportional to the travel time t1. When the depletion layer width is W and the carrier velocity is V, tr = W / v ・-
(Since 11, the cutoff frequency fc is generally expressed as follows.

f c = 0.44 v/W −
(2) That is, the bias voltage is increased (then the depletion layer width W becomes thicker), but the carrier velocity V becomes further larger and the cutoff frequency number fc becomes higher, so the frequency of the PIN photodiode 11 The characteristics improve. Conversely, when the bias voltage is lowered, the depletion layer width W becomes smaller, and the carrier velocity V becomes even smaller, reducing the cutoff frequency fC.
becomes lower.

FIG. 3 is a frequency characteristic curve diagram of the PIN photodiode, which shows the frequency characteristics when the output light of the AβGaAs semiconductor laser is received by the PIN photodiode and its bias voltage is changed. In the same figure, the song TJa shows the case where the bias voltage is 5■, the curve is IOV, the curve C is 20V, and the curve d is 50V.When the bias voltage is 5V, the depletion layer width is 9μm and 50V. The depletion layer width at this time was 125 μm. As can be seen from the curve a w d, as the bias voltage is lowered, the high frequency overhead output decreases. Therefore, by adjusting the bias voltage, the frequency characteristics of the PIN photodiode 11 can be adjusted to achieve desired equalization. This makes it possible to omit the equalization circuit in the conventional example.

〔Effect of the invention〕

As explained above, the present invention uses PIN as a light receiving element.
It uses a photodiode 1.11 and adjusts its bias voltage to adjust the frequency characteristics that correspond to changes in the depletion layer width, thereby equalizing the waveform of the received signal. The equalization circuit can be omitted, and the optical receiving circuit can be made smaller and more economical. Furthermore, it is possible to perform optimal equalization corresponding to the transmission frequency simply by adjusting the bias voltage, which has the advantage of easy adjustment.

[Brief explanation of drawings]

FIG. 1 is a block diagram of the principle of the present invention, FIG. 2 is a block diagram of an embodiment of the present invention, FIG. 3 is a frequency characteristic curve diagram, and FIG. 4 is a block diagram of a conventional example. 1 and 11 are PIN photodiodes, 2 is a bias circuit, 3 is an amplifier section, 4 is a timing recovery section, 5 is a level discrimination section, I2 is a variable resistor, 13 is a series resistor, 14 is a preamplifier, and 15 is a main amplifier , 16 is a tank circuit, 17 is a timing signal amplifier, and 18 is a level discriminator.

Claims (1)

[Scope of Claims] In an optical receiving circuit that converts an optical signal into an electrical signal by a light receiving element, amplifies the electrical signal, equalizes the waveform, and outputs received data by level discrimination, a PI as the light receiving element
Waveform equalization is performed by adjusting the bias voltages to the N photodiode (1) and the PIN photodiode (1), and adjusting the frequency characteristics corresponding to changes in the depletion layer width of the PIN photodiode (1). An optical receiving circuit comprising a bias circuit (2).