WO2022195684A1

WO2022195684A1 - Optical receiver and station-side device

Info

Publication number: WO2022195684A1
Application number: PCT/JP2021/010409
Authority: WO
Inventors: 啓敬川中; 聡吉間
Original assignee: 三菱電機株式会社
Priority date: 2021-03-15
Filing date: 2021-03-15
Publication date: 2022-09-22

Abstract

An optical receiver (100) includes: a variable bias circuit (14) that generates a detection differential signal by applying a bias voltage to a differential signal which is based on a current signal output by a light receiving element for receiving an optical signal and is capable of adjusting a voltage offset between a positive phase and a reverse phase of the detection differential signal; a comparator (17) that compares the number of crossings per unit time of the detection differential signal with a first threshold and a second threshold smaller than the first threshold; a signal detection circuit (18) that detects an input of an optical signal on the basis of the result of comparison between the number of crossings and the first threshold; and a computing unit (19) that adjusts a setting regarding the value of the voltage offset of the variable bias circuit on the basis of the result of comparison between the number of crossings and the second threshold.

Description

Optical receiver and station equipment

The present disclosure relates to an optical receiver and a station-side device that receive burst signals.

In recent years, a one-to-many access optical communication system called a PON (Passive Optical Network) system that allows multiple users to share a single optical fiber has been widely used.

A PON system consists of one OLT (Optical Line Terminal), which is a station-side device, and multiple subscribers connected to the OLT via an optical star coupler, which is a passive element that does not require a power supply. It is composed of an ONU (Optical Network Unit), which is a terminal device on the user side.

Due to the increase in the number of units accommodated in the PON system, it is required to lengthen the maximum connection distance between OLT and ONU and increase the number of ONU branches. To realize this requirement, each ONU will be installed at a different distance from the OLT. That is, the distance between the OLT and each ONU is not constant. Since the strength of the signal reaching the OLT from each ONU depends on the distance from the ONU to the OLT, the OLT must be configured to receive packet signals with large signal strength variations.

Patent Document 1 describes a receiving circuit for receiving an optical burst signal in a PON system. The receiving circuit described in Patent Document 1 converts an optical signal input from an optical fiber into a current signal with a photodiode, amplifies this current signal with a preamplifier that is a differential amplifier, and then converts it into a constant current signal in a main amplifier circuit. voltage amplitude signal and output. The output amplitude of the preamplifier depends on the input optical power and varies from several mV to several hundred mV. That is, a signal having a fluctuation range of several mV to several hundred mV is input to the main amplifier circuit. On the other hand, several hundred mV is required for the output amplitude value of the main amplifier circuit. Generation of a constant voltage amplitude signal that does not depend on the input optical power is an essential process for stable signal identification in the subsequent clock data recovery circuit.

Further, the main amplifier circuit constituting the receiving circuit described in Patent Document 1 counts the number of times the input signal becomes high level, and when the count number reaches a predetermined number, the main signal, that is, the optical burst signal is received. It has a signal detection function (Signal Detect function, hereinafter sometimes referred to as SD function) that determines that Reception of the main signal is determined based on the number of times the input signal becomes high level, thereby suppressing erroneous detection of the main signal.

JP 2016-63345 A

As described above, when the number of PON systems accommodated is increased, the change in signal strength received by the OLT increases. However, in the conventional technique as described in Patent Document 1, the case where the amount of change in the strength of the received signal is large is not considered, and the signal detection accuracy deteriorates when the strength is greatly changed. there is a possibility. In the main amplifier circuit constituting the receiving circuit described in Patent Document 1, the presence or absence of burst signal reception is determined based on the signal after the input signal from the preamplifier is amplified by the main amplifier. Since the amplitude of the output signal of the main amplifier has a level corresponding to the amplitude of the input signal, the amplitude of the signal used to determine whether or not the burst signal has been received may fluctuate greatly, resulting in a determination error. In addition, the electrical characteristics of devices such as photodiodes, preamplifiers, and amplifiers that make up the receiving circuit vary greatly depending on operating conditions such as ambient temperature and operating voltage. In addition, the electrical characteristics of each device also change due to aged deterioration. Therefore, it is required to realize a configuration that can stably operate even when the electrical characteristics of the device fluctuate.

The present disclosure has been made in view of the above, and an object thereof is to obtain an optical receiver capable of stably detecting a signal even when the electrical characteristics of the devices constituting the circuit change. .

In order to solve the above-described problems and achieve an object, an optical receiver according to the present disclosure provides a differential signal for detection by applying a bias voltage to a differential signal based on a current signal output by a light receiving element that receives an optical signal. A variable bias circuit that generates a signal and is capable of adjusting a voltage offset between the positive phase and the negative phase of the differential signal for detection; a comparator that compares with a second threshold that is smaller than the first threshold; a signal detection circuit that detects input of an optical signal based on the comparison result between the number of crossings and the first threshold; and a computing unit that adjusts the setting of the voltage offset value of the variable bias circuit based on the result of the comparison with the threshold.

The optical receiver according to the present disclosure has the effect of being able to stably detect signals even when the electrical characteristics of the devices that make up the circuit change.

1 is a diagram showing a configuration example of an optical receiver according to an embodiment; FIG. FIG. 11 is a diagram showing an example of a detection differential signal input to the crossing detection unit in the main amplifier circuit; Timing chart for explaining the operation of the optical receiver FIG. 10 is a diagram showing the relationship between the number of crossing detections of the detection differential signal per unit time and the sensitivity characteristic of the SD function; Diagram for explaining voltage offset adjustment operation of differential signal 3 is a flow chart showing an example of voltage offset adjustment operation of a differential signal in a main amplifier circuit of an optical receiver;

The optical receiver and station-side device according to the embodiments of the present disclosure will be described below in detail based on the drawings.

Embodiment.
FIG. 1 is a diagram showing a configuration example of an optical receiver 100 according to an embodiment. The optical receiver 100 is mounted, for example, in an OLT, which is a station-side device of a PON system, and receives optical burst signals from each of ONUs, which are a plurality of subscriber-side terminal devices, via an optical transmission line composed of an optical fiber or the like. to receive The optical receiver 100 includes a photodiode 2, which is a light receiving element that outputs a current signal corresponding to an optical signal received via an optical transmission line, and a preamplifier that converts the current signal output by the photodiode 2 into a voltage signal. (TIA: Trans-Impedance-Amplifier) 3, and a main amplifier circuit 1 that amplifies the voltage signal output from the preamplifier 3 and outputs it to a transmission control section (not shown) of an external circuit. The preamplifier 3 is a differential amplifier that amplifies an input signal and outputs it from a non-inverting output terminal, and also inverts the amplified signal and outputs it from an inverting output terminal. The signal output by the preamplifier 3 is a differential signal.

The main amplifier circuit 1 includes a main amplifier 11 that differentially amplifies the differential signal input from the preamplifier 3, and a limiting amplifier 12 that adjusts the output of the main amplifier 11 to a differential signal of constant amplitude.

Further, the main amplifier circuit 1 receives a signal obtained by branching the output of the main amplifier 11, and an AC coupling capacitor 13 that removes the DC voltage from the input differential signal. and a variable bias circuit 14 that applies a bias voltage of a predetermined value to the dynamic signal to generate a detection differential signal.

The main amplifier 11 is a low-noise, high-frequency differential amplifier. AC coupling capacitance 13 is composed of

capacitors

131 and 132 that pass only high frequency components. The variable bias circuit 14 is composed of a power supply and a plurality of resistors. In variable bias circuit 14, resistors R4 and R5 form a voltage dividing circuit. Resistors R4 and R5 are variable resistors, and their resistance values can be set externally. Resistors R6 and R7 also constitute a voltage dividing circuit, and by dividing the power supply voltage, a predetermined bias voltage is applied to the signal after the DC component has been removed.

The main amplifier circuit 1 further includes a crossing detection unit 15 that detects a crossing of the detection differential signals output from the variable bias circuit 14, and a crossing detection unit 15 that is input from a calculation unit 19 described later. A comparator 17 that compares the detected number of crossings with a threshold value, and a signal detection circuit 18 that outputs a detection signal indicating the presence or absence of an optical signal based on the comparison result of the comparator 17 . Comparator 17 compares the number of crossings input from crossing detector 15 with a predetermined first threshold and a second threshold selected by counter threshold selection circuit 5 . The comparator 17 outputs a comparison result between the number of crossings and the first threshold to the signal detection circuit 18 , and outputs a comparison result between the number of crossings and the second threshold to the calculation unit 19 .

The crossing detection unit 15 outputs a short pulse when crossing of the detection differential signals occurs. The signal detection circuit 18 outputs a detection signal indicating "optical signal present" when the comparator 17 determines that the number of crossings measured by the crossing detection unit 15 is greater than the first threshold. In the following description, the detection signal output by the signal detection circuit 18 will be referred to as an SD output signal. The SD output signal is, for example, a signal that changes from Low level to High level when an optical signal is detected. When the signal detection circuit 18 detects the optical signal, it changes the SD output signal to a state indicating that effect (for example, High level), and maintains this state until a reset signal is input from the reset signal generation circuit 4 . The reset signal generation circuit 4 is configured to generate a reset signal when the end of input of the main signal is detected in the transmission processing section that processes the signal output from the optical receiver 100 . That is, the reset signal generation circuit 4 generates a reset signal when the state where the main signal is input to the optical receiver 100 changes to a no-signal state where there is no main signal input. returns to the state before the optical signal was detected.

Here, the input signal to the intersection detection unit 15 will be described. FIG. 2 is a diagram showing an example of a detection differential signal input to the crossing detection section 15 in the main amplifier circuit 1. As shown in FIG. The waveform on the left side of FIG. 2 shows the waveform of the differential signal for detection input to the crossing detection section 15 when there is no signal, and the waveform on the right side shows the waveform of the differential signal for detection input to the crossing detection section 15 when the minimum received power is received. indicates The waveform on the right side of FIG. 2 is the waveform of the detection differential signal that is input to the crossing detector 15 when the optical receiver 100 receives an optical signal of the prescribed lowest level.

As shown on the left side of Fig. 2, the noise when there is no signal follows a normal distribution and spreads with a standard deviation of A (mV). When detecting an optical signal based on the number of crossing detections of the differential signal for detection, in order to prevent the judgment that "there is an optical signal" when there is no signal, the positive It is necessary to determine the voltage offset (the DC voltage difference between positive phase and negative phase) so that the phase signal (eg, the signal shown in the upper part of FIG. 2) and the negative phase signal (eg, the signal shown in the lower part of FIG. 2) do not cross. There is As is clear from the waveforms shown on the right side of FIG. 2, the greater the voltage offset, the lower the probability that the positive and negative phases intersect. In other words, the area where the distributions of positive-phase noise and negative-phase noise overlap becomes smaller. In other words, it is possible to avoid erroneous signal detection due to detection of crossing of the positive phase and the negative phase when there is no signal. On the other hand, if the voltage offset is increased, it becomes a factor that inhibits the occurrence of the crossing of the positive phase and the negative phase when there is an optical signal, increasing the possibility that the reception of the optical signal cannot be detected. Also, the noise distribution during no signal changes depending on the ambient temperature, operating voltage, and the like of the optical receiver 100 .

In consideration of such circumstances, the main amplifier circuit 1 has a function of dynamically changing the voltage offset described above, and stabilizes the operation by changing the voltage offset. The voltage offset can be changed by adjusting the values of resistors R4 and R5 of variable bias circuit 14. FIG.

Returning to the description of FIG. 1 , in the main amplifier circuit 1 , the crossing detector 15 is periodically reset based on the output signal of the local oscillator 16 . The output signal of the local oscillator 16 is also input to the calculation section 19 . The calculation unit 19 calculates the number of crossing detections B per unit time by measuring the output signal of the local oscillator 16 and the output signal of the crossing detection unit 15 . The calculation unit 19 outputs the calculated number of crossing detections B per unit time to the comparator 17 . Here, the arithmetic unit 19 is, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit), an FPGA (Field Programmable Gate Array), or a combination of these implemented in a circuit. Further, the arithmetic unit 19 may be realized by a control circuit configured by a memory and a processor that executes a program stored in the memory. The memories that make up this control circuit are, for example, RAM (Random Access Memory), ROM (Read Only Memory), flash memory, EPROM (Erasable Programmable Read Only Memory), EEPROM (registered trademark) (Electrically Erasable Programmable Read Only Memory) etc.

FIG. 3 is a timing chart for explaining the operation of the optical receiver 100. FIG. In FIG. 3, optical input indicates an optical signal input to the photodiode 2 . The crossing detector input voltage offset indicates the differential voltage difference between the output of the variable bias circuit 14 , that is, the input of the crossing detector 15 . Local Oscillator Output indicates the output waveform of local oscillator 16 . The intersection detector output indicates the output of the intersection detector 15 . The number of crossings detected per unit time indicates the number of crossings of differential signals detected by the crossing detector 15 per unit time (corresponding to the number of crossings detected B described above). The result of comparison with X ₂ is the threshold X ₂ for determining the characteristic change of the optical receiver 100 and the number of crossings of the differential signal detected by the crossing detector 15 per unit time (crossing detection count B, hereinafter referred to as This is the result of comparison with the number of detections B in some cases). FIG. 3 shows an example in which X ₂ =4, and the comparison result is logic level '1' when the detection count B is less than four.

Here, the value of the detection count B fluctuates according to the following formula (1).

In other words, the larger the voltage offset V _offset , the smaller the number of detections B, and the larger the noise variation σ, the larger the number of detections B. From the above, it can be said that the calculation result of the detection count B is the result of measuring the variation due to the noise variation σ.

SD output indicates the signal detection result by the signal detection circuit 18 . FIG. 3 shows an example in which the threshold used for signal detection is X ₁ =16, and the SD output becomes logic level '1' when the number of crossing detections per unit time is greater than 16. In FIG.

Next, we will explain the sensitivity characteristics of the SD function when the number of times of detection B is changed. FIG. 4 is a diagram showing the relationship between the number of cross detections of the detection differential signal per unit time and the sensitivity characteristic of the SD function. In FIG. 4, the horizontal axis indicates the voltage offset (mV), and the vertical axis indicates the average false detection time (until the signal detection circuit 18 falsely detects the noise input). =T/B, T indicates the oscillation period of the local oscillator 16 (unit: seconds). When the accumulated number setting corresponding to the detection number is small, for example, "OUT1 (accumulated number setting: 2 times)" in FIG. On the other hand, when the number of times of detection is large, for example, "OUT4 (accumulation number setting: 16 times)" in FIG. What is important is that the characteristics when the cumulative count setting is small and the characteristics when the cumulative count setting is large can be uniquely described as shown in FIG. FIG. 5 is a diagram for explaining voltage offset adjustment operation of a differential signal. In FIG. 5 , the horizontal axis represents the average detection time when the cumulative number of times is set to 4, that is, the average value of the time required until the number of crossing detections of the differential signal by the crossing detector 15 reaches 4 times. The vertical axis represents the average detection time when the cumulative number of times is set to 16, that is, the average value of the time required until the number of crossing detections of the differential signal by the crossing detection unit 15 reaches 16 times.

Considering the characteristics required for the SD function, it is desirable to set the above cumulative count to a large value. This is because the voltage offset is as small as possible and the average false detection time can be increased. On the other hand, if the number of accumulated times is large when the characteristics vary, the average erroneous detection time is long, making it difficult to detect the variation. Therefore, in the optical receiver 100, not only the threshold X1, which is the _first threshold used for signal detection, but also the threshold X2, which is the _second threshold used for detecting variations in the SD function, is set. .

_A characteristic variation detection and a voltage offset adjustment operation using the threshold value X2 will be described using a specific example. The calculation unit 19 performs characteristic variation detection and voltage offset adjustment. In the optical receiver 100, the calculation unit 19 first calculates the average detection time T _ave when the cumulative count setting shown in FIG. 4 is small, and then uses the average detection time T _ave as shown in FIG. Then, the characteristic T _error for signal detection ( _average erroneous detection time when threshold X1 is set) is calculated (T _error =f(T _ave )). The computing unit 19 then compares the signal detection characteristic T _error with the target value T _target of the erroneous detection average time, and controls the voltage offset, that is, the values of the resistors R4 and R5 of the variable bias circuit 14 based on the comparison result. do. In the example shown in FIG. 5, T _error calculated from T _ave is longer than T _target . From this, the calculation unit 19 determines that the sensitivity of the SD function fluctuates (for example, the noise variation σ decreases) and the voltage offset can be reduced. The timing chart of FIG. 3 also shows the control when a similar variation is caught, and when the number of crossings detected per unit time is 3, it is smaller than X ₂ (=4), so the crossing detection unit input voltage offset is changed to a smaller value. Conversely, if the number of crossings detected per unit time is greater _than X2, the crossing detector input voltage offset is changed to a larger value.

The _next important item in the optical receiver 100 according to the present embodiment is the definition of the time interval for performing the comparison calculation with X2 and the measurement result used for the calculation. As can be seen from the crossing detection unit output shown in FIG. 3, the number of pulses output by the crossing detection unit 15 per unit time increases from the time the optical signal is received. In the example shown in FIG. 3, the number of crossing detections per unit time is 24 when the light input starts. Since this value is larger than the cumulative count threshold value X ₁ (=16) for signal detection determination, the SD output signal becomes a value indicating signal detection (High: logic level '1'). On the other hand, the crossing detection number of 24 is not suitable for use in the above calculation for detecting variations in the characteristics of the optical receiver 100 . Therefore, when the signal detection circuit 18 detects a signal, the number of crossing detections affected by the signal is not used in calculations for detecting variations in characteristics. That is, the variation in characteristics is detected using the number of crossings detected by the crossing detection unit 15 in the no-signal state.

A flow chart of the above voltage offset adjustment operation is shown in FIG. FIG. 6 is a flow chart showing an example of the voltage offset adjustment operation of the differential signal in the main amplifier circuit 1 of the optical receiver 100. As shown in FIG.

As shown in FIG. 6, first, the initial value of the voltage offset, the function f(x) for signal detection characteristic calculation, and the target value T _target of the signal detection characteristic are set (step S1). The function f(x) is set for _each candidate threshold value X2 for characteristic change detection. For example, when the candidates for the threshold value X ₂ are 2 times, 4 times, and 8 times, the function f(x) used when X ₂ =2, the function f(x) used when X ₂ =4, and , X ₂ =8, the function f(x) is set.

Next, the calculation unit 19 confirms whether or not it is a non-signal section (step S2). That is, the calculation unit 19 confirms the state of the SD output signal input from the signal detection circuit 18 . If it is not a no-signal section (step S2: No), the calculation unit 19 continues the checking operation of step S2. If it is a no-signal section (step S2: Yes), the calculation unit 19 measures the number of intersection detection times M per unit time (step S3). The calculation unit 19 then calculates an average detection time T _ave that is the average value of the time required for the number of crossing detections to reach the threshold value X2 ₍ step S4). That is, the calculation unit 19 measures the time required for the _number of intersection detections to reach the threshold value X2 a plurality of times, and calculates the average detection time T _ave .

After calculating the average detection time T _ave , the calculator 19 calculates the signal detection characteristic f(T _ave ) (step S5). Calculation of the signal detection characteristic f(T _ave ) is performed using a function corresponding to the set value of the threshold value X ₂ . Note that the threshold X ₂ is set by the counter threshold selection circuit 5 . The counter threshold selection circuit 5 receives, for example, a user's selection operation of the threshold X ₂ and sets the selected threshold X ₂ in the comparator 17 .

Returning to the description of the operation shown in FIG. 6, the calculator 19 next compares the signal detection characteristic f(T _ave ) with the target value T _target (step S6). If the signal detection characteristic f(T _ave ) is smaller than the target value T _target (step S6: Yes), the calculator 19 increases the voltage offset (step S8). Note that the voltage offset is changed step by step by a predetermined change amount, for example, and the calculation unit 19 changes the voltage offset to a value obtained by adding the change amount to the current value. After executing step S8, the process returns to step S2.

When the signal detection characteristic f(T _ave ) is equal to or greater than the target value T _target (step S6: No), the calculation unit 19 adds the signal detection characteristic f(T _ave ) to the target value T _target and the offset value α. value (step S7). If the signal detection characteristic f(T _ave ) is greater than T _target +α (step S7: Yes), the calculator 19 reduces the voltage offset (step S9). In step S9, the voltage offset is changed in the same manner as in step S8 described above. Specifically, the calculator 19 changes the voltage offset to a value obtained by subtracting the change amount from the current value. After executing step S9, the process returns to step S2. If the signal detection characteristic f(T _ave ) is equal to or less than T _target +α (step S7: No), the process returns to step S2.

As described above, in the optical receiver 100 according to the present embodiment, the main amplifier circuit 1 that amplifies the differential signal output from the preamplifier 3 measures the number of crossings of the differential signal per unit time and the number of crossings per unit time. The input of the optical signal is detected based on the result of the comparison with the threshold value of 1. Further, when no optical signal is input, the main amplifier circuit 1 outputs a positive phase signal of the differential signal based on the result of comparison between the number of crossings of the differential signal per unit time and the second threshold. Adjust the voltage offset, which is the DC voltage difference between the and the inverse signal. As a result, even if the characteristics of the optical receiver 100 change due to aging of components, etc., the voltage offset can be adjusted in accordance with the change in characteristics, and stable signal detection becomes possible.

The configuration shown in the above embodiment is an example, and can be combined with another known technique, and part of the configuration can be omitted or changed without departing from the scope of the invention. It is possible.

1 main amplifier circuit, 2 photodiode, 3 preamplifier, 4 reset signal generation circuit, 5 counter threshold selection circuit, 11 main amplifier, 12 limiting amplifier, 13 AC coupling capacitance, 14 variable bias circuit, 15 crossing detector, 16 Local oscillator, 17 comparator, 18 signal detection circuit, 19 arithmetic unit, 100 optical receiver.

Claims

A differential signal for detection is generated by applying a bias voltage to a differential signal based on a current signal output by a light receiving element that receives an optical signal, and a voltage between the positive phase and the negative phase of the differential signal for detection. a variable bias circuit with adjustable offset;
a comparator that compares the number of crossings per unit time of the differential signal for detection with a first threshold and a second threshold smaller than the first threshold;
a signal detection circuit that detects input of an optical signal based on a comparison result between the number of crossings and the first threshold;
a computing unit that adjusts the setting of the voltage offset value of the variable bias circuit based on the comparison result between the number of crossings and the second threshold;
An optical receiver comprising:
The computing unit adjusts the setting based on a result of comparison between the number of crossings and the second threshold by the comparator when the optical receiver is in a no-signal state in which no optical signal is input.
2. The optical receiver according to claim 1, wherein:
The computing unit adjusts the setting so that the voltage offset is reduced when the number of crossings is less than the second threshold, and adjusts the setting to reduce the voltage offset when the number of crossings is greater than the second threshold. adjusting the settings to be larger;
3. The optical receiver according to claim 1, wherein:
The calculation unit calculates an average value of the time required from the start of counting the number of crossings to the number of crossings reaching the second threshold in a no-signal state in which no optical signal is input to the optical receiver. Then, based on the calculated average value, the required detection time, which is the time required from the start of the input of the optical signal to the optical receiver until the signal detection circuit detects the input of the optical signal, is calculated. adjusting the setting to reduce the voltage offset if the detected required time is greater than a predetermined target value; and adjusting the setting to increase the voltage offset if the detected required time is smaller than the target value. adjust settings,
4. The optical receiver according to any one of claims 1 to 3, characterized in that:
The optical receiver according to any one of claims 1 to 4, wherein the second threshold is changeable.
A station-side device comprising the optical receiver according to any one of claims 1 to 5.