JPH0644196Y2 - Optical receiver - Google Patents

Description

[Detailed Description of the Invention] [Industrial field of application] The present invention relates to an optical receiver for receiving and demodulating an optical signal, and more particularly to an avalanche photodiode (hereinafter, AP) which is a light receiving element.
The optical receiver is configured such that the output voltage of an APD bias generation circuit that supplies a bias voltage to D) is fixed.

[Conventional technology]

In a conventional optical receiver of this type, as shown in FIG. 3, an optical signal p directly intensity-modulated by a digital signal is transmitted to an AP.
Receive on D2. The APD 2 is applied with a reverse bias voltage (a positive voltage in the illustrated example) by the APD bias generation circuit 1, and photoelectrically converts the optical signal p with a conversion ratio proportional to the multiplication factor M determined by the bias voltage. , Preamplifier 3
Send to. The preamplifier 3 amplifies the output current of the APD 2 while keeping the deterioration of the signal-to-noise ratio small, and sends it to the limiter amplifier 4. The limiter amplifier 4 amplifies the output level so as to be constant regardless of the signal level sent from the preamplifier 3 and sends it to the output terminal 5. In this way, at the output terminal 5, an output signal whose amplitude is constant regardless of the level of the optical signal p and whose waveform is similar to that of the optical signal p is obtained.

In this conventional optical receiver, in order to improve the signal-to-noise ratio and reduce the error rate of the digital signal when a low received power is input, APD2 is used as the light receiving element, and the bias voltage applied to APD2 is set appropriately. The configuration is such that the multiplication factor M is optimized (maximum signal-to-noise ratio) when low received light power is input.

[Problems to be solved by the invention]

However, the multiplication factor M of the APD2 is the same as when the low received power is input even when the high received power is input. Therefore, when a high received light power is input, the output level of the preamplifier 3 becomes too large and waveform distortion occurs, making it difficult to transmit a high-quality signal. As a result, the maximum received light power cannot be made too high.

Therefore, if the configuration of the preamplifier 3 is set so that the waveform is not distorted even when the output level is large, an increase in power consumption and deterioration of noise characteristics generally occur, which is not preferable. Also, even if
Even if the output waveform of the preamplifier 3 is configured not to be distorted, similarly high quality signal transmission becomes difficult due to the characteristic deterioration of the limiter amplifier 4 when a large amplitude is input.

An object of the present invention is to provide an optical receiver capable of obtaining a high-quality output signal waveform not only when a low received power is input but also when a high received power is input.

[Means for solving problems]

According to the present invention, a series connection circuit in which an avalanche photodiode for receiving an optical signal and a constant current diode are connected in series, and an avalanche photodiode for applying a reverse bias voltage to the avalanche photodiode via the constant current diode. A bias generation circuit and an amplification means for amplifying an output signal of the series connection circuit,
By making the output signal of the amplifying means a receiver output signal and applying the reverse bias voltage to the avalanche photodiode through the constant current diode,
When the light receiving power of the avalanche photodiode is higher than a predetermined power, the multiplication factor of the avalanche photodiode is adjusted so that a current equal to or higher than the saturation current of the constant current diode does not flow through the avalanche photodiode. An optical receiver characterized in that it is smaller than the multiplication factor of the avalanche photodiode when the power is lower than the predetermined power.

〔Example〕

 Next, an embodiment of the present invention will be described with reference to the drawings.

Referring to FIG. 1, an optical receiver according to an embodiment of the present invention includes a series connection circuit in which an APD 2 and a constant current diode 6 are connected in series, and a reverse bias voltage applied to the APD 2 via a constant current diode 6. APD that gives (a positive voltage in the example shown)
It has a bias generation circuit 1 and amplification means (including a preamplifier 3 and a limiter amplifier 4) for amplifying the output signal of the series connection circuit, and the output signal of the amplification means is used as a receiver output signal. The receiver output signal is available at output terminal 5.

The relationship between the terminal voltage V _D of the constant current diode 6 and the passing current I in FIG. 1 is shown in FIG. As is clear from FIG. 2, when the terminal voltage V _D is V _P or less, the passing current I is the terminal voltage.
It gradually increases as V _D increases. When V _D reaches V _P , I
I _P (that is, the saturation current of the constant current diode 6) is reached.
Furthermore, even if V _D is increased, the passing current I remains almost I _P until V _B.

In the embodiment of the present invention shown in FIG. 1, the terminal voltage V _D of the constant current diode 6 is used below V _B , and I _P is APD.
2 without distortion output waveform before the signal current flows end amplifier 3 at the constant level relationship between I _P becomes direct current and I _P, and the limiter amplifier 4, which is pre-input the output waveform of the preamplifier 3 A value that does not cause distortion in the output waveform of is used. In FIG. 1, an optical signal p directly intensity-modulated by a digital signal is input to APD2. APD2
Is applied with a bias voltage V _A through the constant current diode 6 by the APD bias generation circuit 1 and photoelectrically converts the optical signal p with a conversion ratio proportional to the multiplication factor M determined by the bias voltage V _A. , To the preamplifier 3.

The capacitor 7 is a bypass capacitor that prevents V _A from changing by the signal current flowing through the APD2, and its value is selected so that the impedance becomes sufficiently small at the lowest frequency of the signal current flowing through the APD2.

APD bias V _A at low received power input as in the conventional example
Is chosen to give the optimum multiplication factor M. Since the direct current flowing through the APD2 is small when the low received power is input, the current I flowing through the constant current diode 6, which is equal to the direct current flowing through the APD2, is naturally small, and as can be seen from FIG. Is less than V _P.
Next, as the received light power of APD2 increases, the direct current flowing through APD2, that is, the current I flowing through the constant current diode increases to reach I _P , and similarly the terminal voltage V _D of the constant current diode 6 becomes V _P Reach When the light receiving power of APD2 further increases, the current I flowing through the constant current diode 6, that is, the direct current flowing through APD2 does not increase from I _P due to the property of the constant current diode shown in FIG. V _A becomes smaller (that is, the multiplication factor M becomes smaller) so that the current becomes I _P. On the other hand, the voltage between the terminals of the constant current diode V _D
Becomes stable by increasing the voltage at which V _A becomes smaller.

In this way, the DC current flowing through APD2 does not exceed I _P even when the high received power of APD2 is input, that is, the signal current flowing through APD2 that has a constant level relationship with I _P also does not exceed a certain level. The output waveform of the preamplifier 3 is not distorted, and high quality signal transmission becomes possible.

It should be noted that the APD bias generation circuit 1 may be any one that applies a reverse bias voltage to the APD 2 via the constant current diode 6, and the APD bias generation circuit may supply a negative bias to the anode of the APD via the constant current diode. May be applied. In this case, the constant current diode shall be connected in reverse to the APD bias circuit and APD.

[Effect of device]

As explained above, the present invention can optimize the multiplication factor of the avalanche photodiode by inputting the bias voltage to the avalanche photodiode via the constant current diode, at the time of low received power input. Moreover, even when a high received power is input, distortion of the output waveform of the preamplifier that amplifies the output current of the avalanche photodiode can be reduced. As a result, high quality signal transmission can be performed.

[Brief description of drawings]

FIG. 1 is a block diagram of an optical receiver according to an embodiment of the present invention, FIG. 2 is a diagram showing a relationship between a terminal voltage of a constant current diode and a passing current, and FIG. 3 is a block of a conventional optical receiver. It is a figure. 1 ... APD bias generation circuit, 2 ... Avalanche photodiode, 3 ... Preamplifier, 4 ... Limiter amplifier, 5 ... Output terminal, 6 ... Constant current diode, 7 ... Capacitor.

Claims (1)

[Scope of utility model registration request] 1. A series connection circuit in which an avalanche photodiode for receiving an optical signal and a constant current diode are connected in series, and an avalanche photodiode bias for applying a reverse bias voltage to the avalanche photodiode via the constant current diode. A generating circuit and an amplifying means for amplifying an output signal of the series connection circuit, wherein the output signal of the amplifying means is a receiver output signal, and the reverse current is supplied to the avalanche photodiode via the constant current diode. By applying the bias voltage, when the light receiving power of the avalanche photodiode is higher than a predetermined power, the avalanche photodiode is increased so that a current equal to or higher than the saturation current of the constant current diode does not flow through the avalanche photodiode. Magnification of the avalanche Optical receiver, wherein the received optical power of the photodiode than the multiplication factor of the avalanche photodiode is lower than the predetermined power, reduced.