JPS6142977B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an APD bias control optical receiving circuit that uses an avalanche photodiode (hereinafter referred to as APD) as a light receiving element and has an AGC loop that controls the optical signal DC to be constant. More specifically, the present invention relates to an optical reception squelch circuit that has a squelch function for optical signals and can cut off unnecessary shot noise.

FIG. 1 is a circuit diagram showing the configuration of a conventional optical receiving circuit of this type.

In Figure 1, APD1 has one end a connected to DC-
The output side of the DC converter 2 is connected and a bias voltage is supplied. On the other hand, the other end b of the APD 1 is connected to the input part of an inverting amplifier 5 that detects the optical signal by comparing it with a load resistor 3 and a constant voltage of a reference DC voltage source 4, and the output part of the inverting amplifier 5 is
Connected to the input side of DC-DC converter 2. A DC type AGC circuit composed of a reference DC voltage source 4, an inverting amplifier 5, etc. controls the bias voltage of the APD and works to keep the optical signal DC voltage constant. A capacitor 7 is connected to the optical signal load resistor 3 as a means for extracting a signal output.

When the DC input level of the optical signal entering the APD 1 becomes low, the output voltage of the inverting amplifier 5 of the AGC circuit 6 increases, and the output voltage of the CD-CD converter 2 driven by this output voltage also increases. do
APD1 operates so that a high voltage is applied to it. Therefore, if the DC input level of the optical signal to APD1 disappears due to interruption in the optical transmission line system or switching of the optical transmission line system, and the APD bias voltage becomes extremely high and exceeds the APD breakdown voltage V _B. , there was a risk of damaging APD1. In addition, even if APD 1 is not damaged at this time, the APD bias voltage is high, so as shown in Miller's experimental equation (1), However, V: APD bias voltage V _B : APD breakdown voltage n: APD-specific constant (n = 3 to 6) As the APD multiplication factor M increases, shot noise due to background light noise and dark current noise, etc.
NS has the disadvantage that it undergoes avalanche multiplication as shown in equation (2) and rapidly increases, resulting in a considerably high noise level.

Ns = 2qB (Ip + Ib + Id) M ² F ... (12) where q: electron charge B: frequency bandwidth I _p ; photocurrent at M = 1 I _b ; background optical noise I _d ; dark current noise F: excess noise Exponential This noise level reaches, for example, about -10 dB in terms of S/N ratio.

An object of the present invention is to provide an optical reception squelch circuit that can eliminate these conventional drawbacks.

In order to achieve the above object, an optical reception squelch circuit according to the present invention includes an APD for detecting an optical signal, and an APD for detecting an optical signal;
A load resistor connected to one end of the APD, an AGC circuit that compares the voltage applied to both ends of the load resistor and a reference voltage, and outputs a DC voltage corresponding to the comparison value; and an AGC circuit that boosts the DC voltage of the AGC circuit. , a DC-DC converter that applies the boosted DC voltage to the APD, a detection means for detecting that the bias voltage of the APD has exceeded a certain voltage, and when the detection means detects a voltage above the certain voltage. supply means for supplying a DC voltage to the load resistor; when an optical signal in a certain range is incident on the APD, bias control is performed so that the optical signal DC voltage of the APD is constant; When the optical signal deviates from a certain range and becomes an optical signal below a certain range and the detection means detects a voltage above a certain voltage, the bias voltage of the APD is maintained at a certain voltage or below.

According to the above configuration, if the DC input level of the optical signal to the APD is lost due to interruption or switching of the optical transmission line system, damage to the APD or reduction in the S/N ratio, etc. can be prevented, and the present invention The goal can be fully achieved.

Hereinafter, the present invention will be explained in more detail with reference to the drawings.

FIG. 2 is a circuit diagram showing the configuration of an optical reception squelch circuit according to the present invention. In the figure, the same components as in FIG. 1 are designated by the same reference numerals.
10 is an APD bias voltage detection means connected to one end a of the APD 1, that is, the output side of the DC-DC converter 2, and 11 is an external DC voltage supply means connected to the output of the APD bias voltage detection means. The output of the external DC voltage supply means is the other end b of the APD1,
That is, it is connected to the load resistor 3 of the optical signal.

When the optical signal is incident on the APD 1 within a certain level range, the AGC loop is activated to control the optical signal DC voltage of the APD 1 to be constant, as explained in the conventional technology section. That is, in this case, a bias voltage is supplied to one end a of the APD 1 from the output side of the DC-DC converter 2,
An optical signal current flows through the other end b of the APD, and a load DC voltage is generated across the load resistor 3. This DC voltage is compared and detected by the inverting amplifier 5 with the constant voltage of the reference DC voltage source 4 of the AGC circuit 6, and the inverting and amplified output voltage is sent via the DC-DC converter 2.
Controls the bias voltage of APD1.

On the other hand, if no optical signal is incident on APD1,
The bias voltage of APD1 becomes considerably high. This bias voltage is detected by the APD bias voltage detection means 10 when it exceeds a constant voltage Ec that is slightly higher than the operating range of the AGC. Detection means 10
operates by outputting a constant output voltage by detecting this Ec voltage, turning on the external DC voltage supply means 11, and supplying the DC voltage to the load resistor 3. Due to this operation, even if no optical signal current is actually flowing through APD1, the AGC circuit 6 operates as if an optical signal current is flowing, and the bias voltage of APD1 is prevented from rising above a certain voltage Ec. to control. In addition, load resistance 3
A capacitor 7 is connected as a means for taking out a signal output.

Figure 3 shows the APD bias voltage detection means 10.
It is a circuit diagram showing an example of.

In the figure, 12 is an input terminal, 13 is a resistor, 1
4 is a variable resistor, 15 is a reference DC voltage source, 16 is an amplifier, 17 is a feedback resistor, 18 is an output terminal,
The aforementioned bias voltage and reference DC voltage source 15 are compared. Now, when a constant voltage Ec or more is applied to the input terminal 12, the divided voltage divided by the resistor 13 and the variable resistor 14 and the reference voltage of the reference voltage source 15 are input to the amplifier 16 and compared, and the divided voltage is is determined to be larger, and an output voltage is generated at the output terminal 17 via the feedback resistor 17.

FIG. 4 shows an embodiment of the external voltage supply means.
In the figure, 19 is an input terminal, 20 is a diode,
21 is an output terminal, and when the output voltage from the APD bias voltage detection means 10 is applied to the input terminal 19, the diode 20 is turned on and the output terminal 2 is turned on.
An output voltage is generated at 1.

FIG. 5 is a circuit diagram showing another embodiment of the external voltage supply means. Unlike the previous embodiment, this example includes a transistor 22, a relay 23, and a DC voltage source 24.
It is a relay circuit consisting of
When the output voltage from the APD bias voltage detection means 10 is applied, the base potential of the transistor 22 rises, and the collector of the transistor 22 is driven in series by the relay 23 and the DC voltage source 24, and is turned on, and the contact of the relay 23 is turned on. Output terminal 21
An output voltage is generated.

Above, we have explained the embodiment in which all the output voltages of the DC-DC converter 2 are positive voltages.
Even in the case of negative voltage output, the same configuration can be achieved by simply reversing the polarity of the APD, transistor, and DC power supply.

As explained above in detail, the optical reception squelch circuit according to the present invention uses a normal DC voltage detection type when a certain range of optical signals is incident on the APD.
Utilizing the AGC function, if an optical signal does not enter the APD, the APD bias voltage is detected by blocking the optical transmission line system or switching the optical transmission line system, and supplies a DC voltage from the outside to the load resistance of the optical signal. By doing so, the APD bias voltage can be lowered. Therefore, avalanche multiplication of shot noise due to background light noise and dark current noise is prevented, and unnecessary noise is cut off by approximately 30 dB or more.
The reliability of the signal-to-noise ratio of the signal transmission system can be improved.

It also has the effect of preventing damage to the APD due to high bias voltage application.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a conventional optical reception circuit, FIG. 2 is a configuration diagram of an optical reception squelch circuit according to the present invention, and FIG.
4 is a circuit diagram showing an embodiment of the APD bias voltage detection means shown in FIG. 2, FIG. 4 is a circuit diagram showing an embodiment of the external DC voltage supply means shown in FIG. 2, and FIG.
This figure is a circuit diagram showing another embodiment of the external DC voltage supply means of FIG. 2. 1...Avalanche photodiode (APD), 2
...DC-DC converter, 3. Load resistance, 4. Reference DC voltage source, 5. Inverting amplifier, 6. AGC circuit,
7... Capacitor, 10... APD bias voltage detection means, 11... External DC voltage supply means, 12, 19
...Input terminal, 13...Resistor, 14...Variable resistor, 15
...Reference DC voltage source, 16...Amplifier, 17...Feedback resistor, 18, 21...Output terminal, 20...Diode,
22...Transistor, 23...Relay, 24...DC voltage source.

Claims (1)

[Claims] 1 An avalanche photodiode that detects an optical signal, a load resistor connected to one end of the avalanche photodiode, and a voltage applied to both ends of the load resistor and a reference voltage are compared, and a DC voltage corresponding to the comparison value is output. The AGC circuit and the AGC circuit
A DC device that boosts the DC voltage of the circuit and applies the boosted DC voltage to the avalanche photodiode.
- A DC converter, a detection means for detecting that the bias voltage of the avalanche photodiode has exceeded a certain voltage, and a supply that supplies a DC voltage to the load resistor when the detection means detects a voltage above the certain voltage. When an optical signal in a certain range is incident on the avalanche photodiode, bias control is performed so that the optical signal DC voltage of the avalanche photodiode is constant; An optical reception squelch circuit configured to maintain a bias voltage of the avalanche photodiode below a certain voltage when the detection means detects a voltage above a certain voltage due to an optical signal below a certain range.