JP2012178858A - Cross-point detection circuit of receiving optical signal and optical receiving apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cross-point detection circuit for easily detecting a cross-point of a receiving optical signal, and to provide an optical receiving apparatus for optimum control of an electrical waveform equalizing circuit and an optical variable dispersion compensating circuit of the optical receiving apparatus using a detected cross-point value.SOLUTION: Bias voltage is superimposed to a signal in which a DC element is cut off through AC coupling of an electrical signal photoelectrically converted from a receiving optical signal. The signal to which the bias voltage is superimposed is amplified with an automatic gain control amplifying circuit to detect an average voltage of an output signal of the circuit. Moreover, a tap coefficient of an electrical waveform equalizing circuit is optimum-controlled using the detected average voltage value.

Description

  The present invention relates to a cross point detection circuit for a received optical signal and an optical receiver using the same. Here, the cross point refers to the intersection of the rise and fall of the optical signal that forms an eye pattern in which the waveforms at the time of signal identification are superimposed.

  Up to now, the speed of optical fiber transmission has been increased by time division multiplexing, but the problem that the transmission distance is limited by the chromatic dispersion of the optical fiber has become apparent. As one means for solving this problem, it is conceivable to use a dispersion compensation device such as a dispersion compensation fiber. However, it is desirable to avoid the use if possible from the viewpoint of device size, device cost, and additional loss. As one of the solutions, an optical duobinary transmission system has been proposed, and the effect has been widely demonstrated (for example, see Non-Patent Document 1).

  As a method for improving the chromatic dispersion tolerance performance of an optical duobinary signal, a method using an electrical waveform equalization circuit has been proposed (see, for example, Non-Patent Document 2). FIG. 25 shows a configuration example of an optical receiver using an electrical waveform equalization circuit. The received optical signal is converted into a photocurrent signal by an avalanche photodiode (APD) 1. The photocurrent signal is input to a transimpedance amplifier (TIA) 2 connected to the subsequent stage of APD1 and converted into a voltage signal. This voltage signal has its DC (direct current) component cut off by the AC coupling circuit 4-1, and is input to the electrical waveform equalization circuit (EQ) 5 for waveform shaping.

  The waveform-shaped electric signal passes through an AC coupling circuit 4-2 at the subsequent stage, and is then identified and reproduced by a clock / data reproduction circuit (CDR) 6 and output as a demodulated data signal. Here, the tap coefficient of the electrical waveform equalization circuit 5 is controlled by the tap voltage control circuit 50. At this time, the tap coefficient controlled by the tap voltage control circuit 50 is controlled by an external control signal, or is semi-fixed after initial setting.

On the other hand, a waveform monitor circuit for detecting a changing transmission line characteristic has been proposed (see, for example, Non-Patent Document 3). A configuration example of the waveform monitor circuit is shown in FIG. The received optical signal is converted into an electric signal (D _in ) by a photodetector (PD) 60 and sampled by an asynchronous clock by a D flip-flop (DFF) 61. The output (D _out ) of the DFF 61 is smoothed by a low-pass filter (LPF) 63, converted into a digital signal by an analog / digital converter (ADC) 64, and input to a personal computer (PC) 66. The PC 66 supplies a reference voltage (V _ref ) to the DFF 61 via a digital / analog converter (DAC) 65 and calculates a histogram from input data to monitor the waveform quality.

K. Yonenaga and S. Kuwano, “Dispersion-tolerant optical transmission system using duobinary transmitter and binary receiver”, IEEE J. Lightwave Technol., Vol. 15, No. 18, pp. 1530-1537, 1997. C. Xia and W. RosenKranz, “Performance Enhancement for Duobinary Modulation Through Nonlinear Electrical Equalization”, ECOC2005, PaperTu 4.2.3, Glasgow, UK, 2005. N. Kikuchi, S. Hayase, K. Sekine and S. Sasaki, “performance of Chromatic Dispersion Monitoring Using Statistical Moments of Asynchronously Sampled Waveform Histograms”, IEEE Photonics Technology Letters, Vol. 17, No. 5, pp. 1103-1105, 2005.

  However, when the tap coefficient of the electrical waveform equalization circuit is used at a semi-fixed level, it is difficult to cope with the case where the signal path is switched and the chromatic dispersion of the transmission path is changed. Also, when externally controlling the tap coefficient of the electrical waveform equalization circuit, it is necessary to acquire information such as chromatic dispersion of the transmission line by another means in order to determine the set value. It was difficult to reduce the cost and cost.

  In addition, when using a waveform monitor that uses histogram calculation by sampling and A / D (analog / digital) conversion, a high-speed clock source, a high-speed DFF, and a high-speed arithmetic circuit are required, simplifying the system, reducing the size, and reducing the cost. It was difficult to convert.

  The present invention has been made in view of such a problem, and an object of the present invention is to provide a crosspoint detection circuit that can easily detect a change in a crosspoint of a received optical signal mainly due to wavelength dispersion in an optical transmission line. Another object of the present invention is to provide an optical receiver that optimally and easily controls an electrical waveform equalization circuit or optical waveform dispersion compensation means based on the crosspoint value detected by the crosspoint detection circuit.

  The present invention includes a photoelectric conversion unit that converts a received optical signal into an electrical signal, an AC coupling unit that blocks a DC component of the electrical signal output from the photoelectric conversion unit, and an electrical signal output from the AC coupling unit. Bias voltage superimposing means for superimposing the bias voltage, an automatic gain control circuit for controlling the gain of the output signal of the bias voltage superimposing means, and an average voltage detecting means for receiving the DC-coupled output signal of the automatic gain control circuit And an output terminal for outputting the output value of the average voltage detecting means.

  Alternatively, the present invention provides a photoelectric conversion unit that converts a received optical signal into an electrical signal, an optical average power detection unit that detects an average power of the received optical signal, and a DC component of the electrical signal output from the photoelectric conversion unit. AC coupling means for cutting off the bias, bias voltage superimposing means for superimposing a bias voltage on the electric signal output from the AC coupling means, an automatic gain control circuit for controlling the gain of the output signal of the bias voltage superimposing means, and An average voltage detecting means for detecting an average voltage of the DC-coupled output signal of the automatic gain control circuit, an optical average power detected by the optical average power detecting means, and an average voltage detected by the average voltage detecting means. A calculating means for inputting, and an output terminal for outputting an output value of the calculating means, wherein the calculating means sets a value proportional to the average voltage to the optical average power. Thus the cross point detection circuit of the received optical signal and correcting.

  Alternatively, the present invention provides a photoelectric conversion unit that converts a received optical signal into an electrical signal, an AC coupling unit that blocks a DC component of the electrical signal output from the photoelectric conversion unit, and a gain of an output signal of the AC coupling unit. An automatic gain control circuit for controlling the automatic gain control circuit, an average voltage detection means for receiving a DC coupled output signal of the automatic gain control circuit, and an output terminal for outputting an output value of the average voltage detection means. A cross-point detection circuit for a received optical signal, wherein a relationship between an input amplitude and an output amplitude of a gain control circuit is nonlinear.

  Alternatively, the present invention provides a photoelectric conversion unit that converts a received optical signal into an electrical signal, an optical average power detection unit that detects an average power of the received optical signal, and a DC component of the electrical signal output from the photoelectric conversion unit. AC coupling means for cutting off, an automatic gain control circuit having non-linear input / output amplitude characteristics for controlling the gain of the output signal of the AC coupling means, and a DC-coupled output signal of the automatic gain control circuit as an input Average voltage detecting means, calculating means for inputting the optical average power detected by the optical average power detecting means and the average voltage detected by the average voltage detecting means, and outputting an output value of the calculating means A cross-point detection circuit for a received optical signal, wherein the arithmetic means corrects a value proportional to the average voltage by the optical average power. That.

  Alternatively, the present invention provides a photoelectric conversion unit that converts a received optical signal into an electrical signal, an AC coupling unit that blocks a DC component of the electrical signal output from the photoelectric conversion unit, and an electrical output that is output from the AC coupling unit. A limiter amplifier circuit for amplifying a signal, an average voltage detection means for receiving a DC-coupled output signal of the limiter amplifier circuit, and an output terminal for outputting an output value of the average voltage detection means It is a cross point detection circuit of the received optical signal.

  Further, when the present invention is viewed from the viewpoint of an optical receiver, the present invention provides an electrical waveform that equalizes the received signal crosspoint detection circuit of the present invention and the electrical signal waveform output from the photoelectric conversion means. An equalization means and an identification reproduction means for identifying and reproducing the output signal of the electrical waveform equalization means, and branching the output signal of the automatic gain control circuit constituting the crosspoint detection circuit into at least two; One of them is used as an input to the electrical waveform equalization means, and the equalization characteristic of the electrical waveform equalization means is optimally controlled using the output value of the crosspoint detection circuit. Device.

  Alternatively, the present invention provides a cross-point detection circuit for a received optical signal according to the present invention, an electrical waveform equalizing unit for equalizing an electrical signal waveform output from the photoelectric conversion unit, and an electrical waveform equalizing unit. And an identification reproducing unit for identifying and reproducing the output signal, wherein the equalization characteristic of the electrical waveform equalizing unit is optimally controlled using the output value of the crosspoint detection circuit. .

  Alternatively, the present invention provides a cross-point detection circuit for a received optical signal according to the present invention, an optical chromatic dispersion compensation unit for giving chromatic dispersion to a received optical signal input to the photoelectric conversion unit, and an output from the photoelectric conversion unit. Electrical waveform equalization means for equalizing the electrical signal waveform and identification reproduction means for identifying and reproducing the output signal of the electrical waveform equalization means, and using the output value of the crosspoint detection circuit, An optical receiver characterized by optimally controlling at least one of a chromatic dispersion value of an optical chromatic dispersion compensation means and an equalization characteristic of an electrical waveform equalization means.

  For example, the electrical waveform equalization means is a feed forward equalizer (FFE), a decision feedback equalizer (DFE), or FFE + DFE, and using the output value of the crosspoint detection circuit, It is characterized by optimally controlling the tap coefficient of FFE, DFE, or FFE + DFE. Alternatively, the electrical waveform equalization means includes a waveform equalization mode for equalizing the input signal waveform and a through mode for allowing the input signal waveform to pass through as it is, and using the output value of the crosspoint detection circuit, The waveform equalization mode and the through mode of the electrical signal equalization means can be switched.

  Alternatively, the present invention provides a cross-point detection circuit for a received optical signal according to the present invention, an optical chromatic dispersion compensation unit for giving chromatic dispersion to a received optical signal input to the photoelectric conversion unit, and an output from the photoelectric conversion unit. An optical reproducing apparatus comprising: an identification reproduction means for identifying and reproducing an electrical signal, wherein the chromatic dispersion value of the optical chromatic dispersion compensation means is optimally controlled using an output value of the crosspoint detection circuit. is there.

  According to the present invention, it is possible to easily realize a cross-point detection of a received optical signal mainly on an optical transmission path, and thereby to easily provide an optical receiving apparatus that can optimally cope with the state of the optical transmission path. .

It is a figure which shows the structural example of 1st embodiment of the optical receiver of this invention. It is a figure which shows the structural example of a feed-forward equalizer. It is a figure which shows the structural example of a decision feedback equalizer. It is a figure which shows the example of a combination structure of a feed-forward equalizer and a decision feedback equalizer. It is a figure which shows the structural example of the feed-forward equalizer with a through mode. It is a figure which shows an optical duobinary signal waveform. It is a figure which shows the structural example of 1st embodiment of the crosspoint detection circuit of this invention. It is a figure which shows operation | movement of 1st embodiment of the crosspoint detection circuit of this invention. It is a figure which shows operation | movement of 1st embodiment of the crosspoint detection circuit of this invention when an input signal amplitude changes. It is a figure which shows the structural example of 2nd embodiment of the crosspoint detection circuit of this invention. It is a figure which shows the structural example of 2nd embodiment of the optical receiver of this invention. It is a figure which shows the structural example of 3rd embodiment of the crosspoint detection circuit of this invention. It is a figure which shows operation | movement of 4th embodiment of the crosspoint detection circuit of this invention. It is a figure which shows operation | movement of 4th embodiment of the crosspoint detection circuit of this invention. The measured value of the relationship between the chromatic dispersion (horizontal axis) and the crosspoint detection voltage (vertical axis) when the optical duobinary signal of the configuration example of the third embodiment of the crosspoint detection circuit of the present invention is input is plotted. FIG. It is a figure which shows the structural example of 5th embodiment of the crosspoint detection circuit of this invention. It is a figure which shows the operation | movement by the different principle of 1st-5th embodiment of the crosspoint detection circuit of this invention. It is a figure which shows the structural example of 6th embodiment of the crosspoint detection circuit of this invention. It is a figure which shows operation | movement of 6th embodiment of the crosspoint detection circuit of this invention. It is a figure which shows the structural example of 7th embodiment of the crosspoint detection circuit of this invention. It is a figure which shows operation | movement of 7th embodiment of the crosspoint detection circuit of this invention. It is a figure which shows the structural example of 3rd embodiment of the optical receiver of this invention. It is a figure which shows the structural example of 4th embodiment of the optical receiver of this invention. It is a figure which shows the structural example of 5th embodiment of the optical receiver of this invention. It is a figure which shows the structural example of the optical receiver using the conventional electrical waveform equalization circuit. It is a figure which shows the structural example of the waveform monitor circuit of the conventional reception signal.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment of optical receiver)
An optical receiver according to a first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a diagram illustrating a configuration example of the optical receiving apparatus according to the first embodiment. The received optical signal is converted into a photocurrent signal by an avalanche photodiode (APD) 1. The photocurrent signal is input to a transimpedance amplifier (TIA) 2 connected to the subsequent stage of APD1 and converted into a voltage signal. Up to this point, it is usually connected by DC coupling.

  That is, a photocurrent flows at a moment when the optical signal power is present, a TIA output potential is generated with respect to the ground level, and a photocurrent does not flow at a moment when there is no optical signal power, and therefore, the TIA output voltage is the ground level (0 V). It becomes. This electrical signal is input to the cross point (XP) detection circuit 3-1. The XP detection circuit 3-1 detects a cross point of the electric signal and outputs a corresponding voltage, and outputs the electric signal itself as a main signal.

  The XP detection circuit 3-1 may or may not adjust the amplitude of the main signal to be output to a value suitable for the input of the electrical waveform equalization circuit (EQ) 5 connected to the subsequent stage. May be. If not adjusted, an automatic gain adjustment circuit (AGC) provided in the electrical waveform equalization circuit 5 is adjusted to an optimum amplitude. Note that the DC component of the output of the XP detection circuit 3-1 is blocked by the AC coupling circuit 4-1.

The electric waveform equalization circuit 5 includes, for example, a feed-forward equalizer (FFE) as shown in FIG. 2, a decision feedback equalizer (DFE) as shown in FIG. 3, and an FFE + DFE as shown in FIG. Realized with a circuit. 2 to 4, “Delay” is a delay circuit, “CDR” is a clock / data recovery circuit, “×” is a multiplier, and “+” is an adder. The characteristics of these electrical waveform equalization circuits can be adjusted by adjusting the tap coefficients (b ₀ , c ₀ , c _l , c ₂ ) shown in FIGS. The electrical waveform equalization circuit may have a through mode that does not perform waveform equalization.

FIG. 5 shows an example of an FFE circuit having a through mode. “Selector” is a selection circuit, and can switch between a through mode and a waveform equalization execution mode. Depending on the state of the optical transmission line, there may be a case where it is better not to perform waveform equalization. In that case, it is effective to use the through mode to let the electrical waveform equalization circuit pass through. For example, the waveform equalization function can be turned off by setting any one of the tap coefficients (c ₀ , c _l , c ₂ ) to “1” and the rest to “0”. By adopting it, it is possible to avoid deterioration when passing through the electrical waveform equalization circuit.

  The XP detection circuit 3-1 detects a voltage value corresponding to the cross point position of the signal waveform and inputs it to the optimum control circuit 8. The optimum control circuit 8 instructs the tap voltage control circuit 7 to provide the electrical waveform equalization circuit 5 with a tap voltage corresponding to the optimum tap coefficient at the position of the cross point of the electrical signal measured in advance. put out. The signal output from the electrical waveform equalization circuit 5 passes through the subsequent AC coupling circuit 4-2, and is then identified and reproduced by the clock and data reproduction circuit (CDR) 6, and is output as a demodulated data signal.

The cross point of the optical reception waveform mainly depends on the chromatic dispersion of the optical transmission line. In particular, the tendency appears remarkably in an optical duobinary signal having a high chromatic dispersion tolerance. This is shown in FIG. The horizontal axis in FIG. 6 is time, and the vertical axis is the intensity of chromatic dispersion. FIG. 6A shows the case where the chromatic dispersion is 0 ps / nm. FIG. 6B shows the case where the chromatic dispersion is 800 ps / nm. FIG. 6C shows the case where the chromatic dispersion is 1590 ps / nm. FIG. 6D shows the case where the chromatic dispersion is 2360 ps / nm. It can be seen that when the chromatic dispersion is small, the cross point is above 50%, the cross point gradually decreases as the chromatic dispersion increases, and when the chromatic dispersion is 2360 ps / nm, the cross point decreases to near zero level.
(First Embodiment of Crosspoint Detection Circuit)

A crosspoint detection circuit according to a first embodiment of the present invention will be described with reference to FIG. FIG. 7 is a diagram illustrating a configuration example of the crosspoint detection circuit according to the first embodiment. In this configuration example, a DC-coupled received electrical signal is branched into two, and one of them is output to be supplied to the electrical waveform equalization circuit 5, and the other is blocked by the AC coupling circuit 10. A bias voltage V _bias is applied by the bias application circuit 11. The electric signal to which the bias voltage is applied is input to the automatic gain control (AGC) amplifier circuit 12. In the AGC amplifier circuit 12, the gain is controlled and output so that the peak voltage of the output signal is constant. The average voltage of the output signal is detected by the average voltage detecting means 13, and this becomes a value corresponding to the cross point.

  The operation will be described in detail with reference to the drawings. FIG. 8 schematically shows input / output waveforms of the AGC amplifier circuit 12. FIG. 8A shows a case where the crosspoint value is 50% (XP = 50%), and FIG. 8B shows a case where the crosspoint value is smaller than 50% (XP <50%). The left waveform is the AGC amplifier circuit input waveform, and the right waveform is the AGC amplifier circuit output waveform. For simplicity, the gain of the AGC amplifier circuit 12 when the cross point is 50% is set to “1”. In the case of the same average received signal power, the peak value of the input signal of the AGC amplifier circuit 12 increases as the cross point value decreases. This can be understood in the same way that an RZ (Return to Zero) signal having the same average power has a higher peak power than an NRZ (Non Return to Zero) signal.

Since feedback control is applied so that the peak voltage of the output waveform is kept constant in the AGC amplifier circuit 12, when the crosspoint value is small, that is, when the peak value of the AGC amplifier circuit input signal waveform is high, the AGC amplifier circuit 12 The gain of becomes smaller. At this time, since the bias voltage V _bias applied to the input signal of the AGC amplifier circuit 12 is also amplified simultaneously with the signal, it is affected by a change in gain. Assuming that the output DC level when the gain G = 1 is V _dc and the output DC level when the gain G <1 is V _dc ′, V _dc > V _dc ′
It becomes. Therefore, the voltage corresponding to the cross point value can be detected by detecting the average voltage of the output signal of the AGC amplifier circuit 12. In the present embodiment, the detected voltage decreases as the cross point value decreases.

However, the value detected as the crosspoint value changes even if the crosspoint does not change when the level of the input signal to the AGC amplifier circuit 12 changes. This is schematically shown in FIG. FIG. 9A shows the case where the amplitude of the AGC amplifier circuit input signal is small, and FIG. 9B shows the case where the amplitude of the AGC amplifier circuit input signal is large. The left waveform is the AGC amplifier circuit input waveform, and the right waveform is the AGC amplifier circuit output waveform. For simplicity, the gain of the AGC amplifier circuit 12 when the amplitude of the input signal of the AGC amplifier circuit is small is “1”. Even if the cross point does not change at 50%, it can be seen that when the amplitude of the input signal of the AGC amplifier circuit 12 changes, the voltage value detected as the cross point value changes (V _dc > V _dc ′). A method for solving this problem will be described below as a second embodiment of the cross-point detection circuit.
(Second Embodiment of Crosspoint Detection Circuit)

  A crosspoint detection circuit according to a second embodiment of the present invention will be described with reference to FIG. FIG. 10 is a diagram illustrating a configuration example of the cross point detection circuit according to the second embodiment. In this configuration example, the above-described problem of the AGC amplifier circuit input signal amplitude dependency can be solved, and a crosspoint detection circuit that operates in a wide dynamic range of the received signal level can be provided.

This configuration example is different from the configuration example of FIG. 7 in that the received electrical signal is once branched into two and then branched again before being input to the AC coupling circuit 10, and the average voltage (V _ave ) of the branched signal is calculated. The detection is performed by the average voltage detection circuit 14, and the arithmetic circuit 15 obtains the product of the average voltage (V _ave ) and the voltage value (V _dc ) corresponding to the DC level of the AGC amplifier circuit output and outputs the product as a cross point value. is there. When the power of the received optical signal increases and the amplitude of the input signal of the AGC amplifier circuit 12 increases, the gain of the AGC amplifier circuit 12 decreases in inverse proportion, so that the voltage value corresponding to the amplitude of the input signal of the AGC amplifier circuit 12 By correcting the DC level (V _dc ) of the AGC amplifier circuit output with (V _ave ), the amplitude dependency of the input signal of the AGC amplifier circuit 12 can be eliminated.
(Second embodiment of optical receiver)

An optical receiving apparatus according to the second embodiment of the present invention will be described with reference to FIG. FIG. 11 is a diagram illustrating a configuration example of the optical receiver according to the second embodiment. This configuration example is different from the configuration example shown in FIG. 1 in that an average photocurrent flowing through the APD1 is input to the XP detection circuit 3-2. The XP detection circuit 3-2 corrects the cross point value using this average photocurrent.
(Third embodiment of cross-point detection circuit)

A crosspoint detection circuit according to a third embodiment of the present invention will be described with reference to FIG. FIG. 12 is a diagram illustrating a configuration example of the cross point detection circuit according to the third embodiment. This configuration example differs from the configuration example of FIG. 10 in that the received electric signal is branched and the average voltage is not detected by the average voltage detection circuit, but the average photocurrent from the APDL is detected by the average photocurrent detection circuit 16. It is a place to detect directly. However, the output of the average photocurrent detection circuit 16 is output as a voltage value (V _ave ).

The arithmetic circuit 15 obtains the product of the voltage value (V _ave ) corresponding to the average photocurrent value detected by the average photocurrent detection circuit 16 and the voltage value (V _dc ) corresponding to the DC level of the AGC amplifier circuit output. Output as a crosspoint value. When the power of the received optical signal increases and the amplitude of the input signal of the AGC amplifier circuit 12 increases, the gain of the AGC amplifier circuit 12 decreases in inverse proportion, so that the voltage value corresponding to the amplitude of the input signal of the AGC amplifier circuit 12 By correcting the DC level (V _dc ) of the AGC amplifier circuit output with (V _ave ), the amplitude dependency of the input signal of the AGC amplifier circuit 12 can be eliminated.
(Fourth Embodiment of Crosspoint Detection Circuit)

A crosspoint detection circuit according to a fourth embodiment of the present invention will be described with reference to FIG. FIG. 13 is a diagram illustrating a configuration example of the cross point detection circuit according to the fourth embodiment. In this configuration example, the part after the AC coupling circuit of the configuration example according to the first embodiment shown in FIG. 7 is described with a more specific circuit, and a differential configuration generally used is taken. Yes. The received electrical signal is described as a differential signal (V _IP , V _IN ) and is input to the AGC amplifier circuit 12 via the AC coupling circuit 10 and the bias application circuit 11, respectively.

An offset ΔV _OFF between differential signals is given to the AGC input signal by the bias application circuit 11. V _cc is the power supply voltage of the circuit. The AGC amplifier circuit 12 includes a gain control amplifier 20, an averaging circuit 22, a top hold circuit 23, and a differential amplifier 21. The averaging circuit 22 detects the average voltage (V _A ) from the differential output of the gain control amplifier 20 and inputs it to the differential amplifier 21. The top hold circuit 23 detects the peak voltage (V _T ) from the differential output of the gain control amplifier 20 and inputs it to the differential amplifier 21.

The differential amplifier 21 amplifies the difference between V _A and V _T and outputs the gain control voltage (V _AGC ) of the gain control amplifier 20. When the peak voltage V _T increases, the gain control amplifier 20 operates in a direction to decrease the gain, so that the peak voltage of the output signal of the AGC amplifier circuit 12 is kept constant. The output signal from the AGC amplifier circuit 12 is blocked by a low-pass filter (LPF), which is an average voltage detection means 13, and a high-frequency component is blocked, and a DC voltage component is detected differentially (E _OP , E _ON ).

The operation of the configuration example shown in FIG. 13 will be described in detail with reference to FIG. FIG. 14A shows the operation of the cross point detection circuit when there is no differential offset ΔV _OFF . In this description, the difference of the cross points is expressed by the difference of the duty ratio. The duty ratio and the cross point value are observed on the positive phase (V _IP ) side of the differential signal, and are naturally opposite on the negative phase (V _IN ) side.

When the cross point value is 50%, the duty ratio is 100%, and when the cross point value is small (<50%), the duty ratio is also small (<100%). The upper row is when the duty ratio is 100% (cross point value 50%), and the lower row is when the duty ratio is less than 100% (cross point value less than 50%). As shown in FIG. 14A, when the duty ratio is less than 100%, the AC coupling circuit output has a high peak voltage. When this signal is input to the AGC amplifier circuit 12, the gain control voltage V _AGC ′ of the gain control amplifier 20 is generated when the feedback loop is open.

Since V _AGC ′ becomes larger than the gain control voltage target value V _{AGC of} the gain control amplifier 20, the gain is reduced when the feedback loop of the AGC amplifier circuit 12 is closed, and the gain control voltage of the gain control amplifier 20 is V _AGC. To stabilize. On the other hand, FIG. 14B shows the operation of the cross point detection circuit when there is a differential offset ΔV _OFF . The diagram on the left shows the output waveform of the AGC amplifier circuit when the feedback loop is closed, and the diagram on the right shows the LPF output voltage at that time. It can be seen that a differential voltage having a gain multiple of the differential offset (ΔV _OFF × G) is generated in the LPF output. FIG. 14C schematically shows the relationship between the dispersion amount of the optical transmission line and the cross-point detection voltage (E _OP −E _ON ). As shown in FIG. 6, since there is a fixed relationship between the chromatic dispersion and the cross point of the received signal waveform, it is possible to estimate the dispersion value of the transmission line by detecting the cross point value.

FIG. 15 is a plot of measured values of the relationship between chromatic dispersion (horizontal axis) and crosspoint detection voltage (vertical axis) when an optical duobinary signal is input to an actual crosspoint detection circuit. It can be confirmed that the cross-point value (E _OP −E _ON ) decreases as the chromatic dispersion increases.
(Fifth Embodiment of Crosspoint Detection Circuit)

  A crosspoint detection circuit according to a fifth embodiment of the present invention will be described with reference to FIG. FIG. 16 is a diagram illustrating a configuration example of a crosspoint detection circuit according to the fifth embodiment. This configuration example is different from the configuration example shown in FIG. 7 in that a signal for detecting a cross point and a signal output from the XP detection circuit 3-1 or 3-2 and supplied to the electrical waveform equalization circuit 5 Is performed after the AGC amplifier circuit 12 instead of before the AC coupling circuit 10. By adopting such a configuration, the AGC amplifier circuit 12 used in the XP detection circuit 3-1 or 3-2 is used to optimally adjust the amplitude of the signal supplied to the electrical waveform equalization circuit 5. Therefore, the function of adjusting the amplitude inside the electrical waveform equalization circuit can be omitted, which can contribute to the reduction in size, power consumption, and cost of the entire optical receiver.

  The XP detection circuit 3-1 or 3-2 including the AGC amplifier circuit 12 shown in FIGS. 7, 10, 12, 13, and 16 has the nonlinearity of the AGC amplifier circuit 12 as shown in FIG. 17. By using it, it is possible to operate on the principle different from the operation principle described so far. The non-linearity of the AGC amplifier circuit 12 is a property that even if the input amplitude is doubled, the output amplitude is smaller than twice or larger than twice.

FIG. 17A shows the case where the actual gain is smaller than the ideal linear gain, and FIG. 17B shows the opposite case. In this description, a differential signal model is used and a difference in cross points is expressed by a difference in duty ratio. In the case of FIG. 17A, the duty ratio on the positive electrode side is smaller than 100% and the duty ratio on the negative electrode side is larger than 100%. In the case of FIG. A high pulse) has a smaller gain than a wide pulse (a pulse with a low peak). Therefore, when the average voltage of the output signal is monitored, the positive side (E _OP ) decreases and the negative side (E _ON ) increases.

On the other hand, in the case of FIG. 17B, due to the reverse saturation shape of the actual gain curve, a narrow pulse (high peak pulse) has a gain compared to a wide pulse (low peak pulse). growing. Therefore, when the average voltage of the output signal is monitored, the positive side (E _OP ) increases and the negative side (E _ON ) decreases. Since these E _OP and E _ON change depending on the duty ratio of the input signal, the cross point of the input signal can be detected. When the gain characteristic of the AGC amplifier circuit 12 changes depending on the amplitude of the input signal to the AGC amplifier circuit 12, as shown in FIG. 10 or FIG. By correcting, a cross point detection circuit with a wide dynamic range of input amplitude can be provided.
(Sixth embodiment of cross-point detection circuit)

  A crosspoint detection circuit according to a sixth embodiment of the present invention will be described with reference to FIG. FIG. 18 is a diagram illustrating a configuration example of the cross point detection circuit according to the sixth embodiment. This configuration includes a limiter amplifier circuit 17 in place of the bias application circuit 11 and the AGC amplifier circuit 12 in the configuration example shown in FIG. The operation will be described in detail with reference to FIG. Similarly to FIG. 14A, the upper part shows a case where the duty ratio is 100% (cross point value 50%), and the lower part shows a case where the duty ratio is less than 100% (cross point value less than 50%).

As shown in FIG. 19, when the duty ratio is less than 100%, the AC coupling circuit output has a high peak voltage. When this signal is input to the limiter amplifier circuit 17, the peak voltage of the entire waveform becomes a constant value at the output of the limiter amplifier circuit because the high peak is limited. Since this signal has a DC component corresponding to the amount whose amplitude is limited, a voltage (E _OP −E _ON ) corresponding to the duty ratio (cross point value) of the input signal is detected in the LPF output.
(Seventh Embodiment of Crosspoint Detection Circuit)

  A crosspoint detection circuit according to a seventh embodiment of the present invention will be described with reference to FIG. FIG. 20 is a diagram illustrating a configuration example of the cross point detection circuit according to the seventh embodiment. This configuration example includes a peak voltage detection means 18 instead of the bias application circuit 11 and the AGC amplification circuit 12 of the configuration example shown in FIG. The peak voltage detection means 18 can be realized by, for example, the top hold circuit 23 shown in FIG.

  The operation will be described in detail with reference to FIG. FIG. 21A shows a case where the crosspoint value is 50%, and FIG. 21B shows a case where the crosspoint value is less than 50%. The left waveform is the average voltage detection means input signal waveform, and the right waveform is the peak voltage detection means input signal waveform. The average voltage detection means input signal is a DC coupled signal, and the peak voltage detection means input signal is an AC coupled signal.

As shown in FIG. 21B, when the cross point value is small, the peak voltage becomes high and the peak voltage detection means output becomes large (Vpeak ′> Vpeak). On the other hand, when the amplitude of the input signal increases, the peak voltage detection means output increases even if the crosspoint value does not change. Therefore, by correcting the calculation means 19 using the value of the average voltage detection means output, the cross point value can be detected more accurately.
(Third embodiment of optical receiver)

  An optical receiver according to a third embodiment of the present invention will be described with reference to FIG. FIG. 22 is a diagram illustrating a configuration example of the optical receiver according to the third embodiment. This configuration example is different from the configuration example shown in FIG. 1 in that an optical variable attenuation means (VOA) 30 is provided in front of APD1.

The possibility that the cross-point value detected by the conventional cross-point detection circuit depends on the optical reception signal power and the means for avoiding it have been described. In the configuration examples shown in FIGS. 10, 12, and 20, the input signal power dependency is avoided by using the arithmetic circuit 15 and the arithmetic means 19, but in this configuration example, the APD input power is kept constant by using the VOA 30. As a result, it is possible to achieve a wide dynamic range while avoiding dependence on the input signal power.
(Fourth embodiment of optical receiver)

  An optical receiving apparatus according to the fourth embodiment of the present invention will be described with reference to FIG. FIG. 23 is a diagram illustrating a configuration example of the optical receiving apparatus according to the fourth embodiment. This configuration example is different from the configuration example shown in FIG. 1 in that an optical variable dispersion compensation means (TDC) 42 is provided instead of the electrical waveform equalization circuit (EQ) 5.

In this configuration example, the chromatic dispersion value of the optical variable dispersion compensator 42 is optimally controlled using the crosspoint value detected by the crosspoint detection circuit. The optical variable dispersion compensation means 42 includes, for example, a planar lightwave circuit (PLC) type, a spatial optical system type, and a fiber type. Any configuration is possible as long as the chromatic dispersion value can be controlled by an external control signal. It doesn't matter.
(Fifth embodiment of optical receiver)

  An optical receiving apparatus according to the fifth embodiment of the present invention will be described with reference to FIG. FIG. 24 is a diagram illustrating a configuration example of the optical receiving apparatus according to the fifth embodiment. This configuration example is a configuration in which the configuration example shown in FIG. 1 and the configuration example shown in FIG. 23 are combined, and an electrical waveform equalization circuit (EQ) 5 and an optical variable dispersion compensation means (TDC). 42 and both.

  The cross-point value detected by the cross-point detection circuit optimally controls at least one of the tap coefficient of the electrical waveform equalization circuit 5 and the chromatic dispersion value of the optical variable dispersion compensation means 42. Other operations are the same as those in the configuration example shown in FIG. 1 or the configuration example shown in FIG.

  According to the present invention, a simple optical receiver can be provided at low cost.

1 Avalanche photodiode (APD)
2 Transimpedance amplifier (TIA)
3-1, 3-2 XP detection circuit 4-1, 4-2, 10 AC coupling circuit 5 Electrical waveform equalization circuit (EQ)
6 Clock and data recovery circuit (CDR)
7, 50 Tap voltage control circuit 8, 40 Optimal control circuit 11 Bias application circuit 12 AGC amplifier circuit 13 Average voltage detection means 14 Average voltage detection circuit 15 Arithmetic circuit (V _dc × V _ave )
16 Average photocurrent detection circuit 17 Limiter amplifier circuit 18 Peak voltage detection means 19 Calculation means (Vpeak / V _ave )
20 gain control amplifier 21 differential amplifier 22 averaging circuit 23 top hold circuit 30 optical variable attenuation means (VOA)
41 dispersion amount control circuit 42 optical variable dispersion compensation means (TDC)
60 Photodetector (PD)
61 D flip-flop (DFF)
62 Clock source 63 Low pass filter (LPF)
64 Analog / Digital Converter (ADC)
65 Digital / analog converter (DAC)
66 Personal Computer (PC)

Claims (11)

Photoelectric conversion means for converting the received optical signal into an electrical signal;
AC coupling means for blocking the DC component of the electrical signal output from the photoelectric conversion means;
Bias voltage superimposing means for superimposing a bias voltage on the electric signal output from the AC coupling means;
An automatic gain control circuit for controlling the gain of the output signal of the bias voltage superimposing means;
Average voltage detection means for receiving the DC-coupled output signal of the automatic gain control circuit;
An output terminal for outputting an output value of the average voltage detection means. A cross point detection circuit for a received optical signal. Photoelectric conversion means for converting the received optical signal into an electrical signal;
Optical average power detection means for detecting the average power of the received optical signal,
AC coupling means for blocking the DC component of the electrical signal output from the photoelectric conversion means;
Bias voltage superimposing means for superimposing a bias voltage on the electric signal output from the AC coupling means;
An automatic gain control circuit for controlling the gain of the output signal of the bias voltage superimposing means;
An average voltage detecting means for detecting an average voltage of the DC-coupled output signal of the automatic gain control circuit;
Calculation means for inputting the light average power detected by the light average power detection means and the average voltage detected by the average voltage detection means,
An output terminal for outputting the output value of the computing means, and
The calculation means corrects a value proportional to the average voltage by the optical average power. A cross-point detection circuit for a received optical signal. Photoelectric conversion means for converting the received optical signal into an electrical signal;
AC coupling means for blocking the DC component of the electrical signal output from the photoelectric conversion means;
An automatic gain control circuit for controlling the gain of the output signal of the AC coupling means;
An average voltage detecting means for receiving the DC-coupled output signal of the automatic gain control circuit;
An output terminal for outputting the output value of the average voltage detection means,
A cross-point detection circuit for a received optical signal, wherein the relationship between the input amplitude and the output amplitude of the automatic gain control circuit is non-linear. Photoelectric conversion means for converting the received optical signal into an electrical signal;
Optical average power detection means for detecting the average power of the received optical signal,
AC coupling means for blocking the DC component of the electrical signal output from the photoelectric conversion means;
An automatic gain control circuit having nonlinear input / output amplitude characteristics for controlling the gain of the output signal of the AC coupling means;
Average voltage detection means for receiving the DC-coupled output signal of the automatic gain control circuit;
An arithmetic means for inputting the optical average power detected by the optical average power detection means and the average voltage detected by the average voltage detection means, and
An output terminal for outputting the output value of the computing means, and
The calculation means corrects a value proportional to the average voltage by the optical average power. A cross-point detection circuit for a received optical signal. Photoelectric conversion means for converting the received optical signal into an electrical signal;
AC coupling means for blocking the DC component of the electrical signal output from the photoelectric conversion means;
A limiter amplifier circuit for amplifying the electric signal output from the AC coupling means;
An average voltage detecting means for receiving the DC coupled output signal of the limiter amplifier circuit;
An output terminal for outputting an output value of the average voltage detection means. A cross-point detection circuit for a received optical signal, comprising: A cross-point detection circuit for a received optical signal according to any one of claims 1 to 4,
Electrical waveform equalization means for equalizing the electrical signal waveform output from the photoelectric conversion means;
Discriminating and reproducing means for discriminating and reproducing the output signal of the electrical waveform equalizing means,
The output signal of the automatic gain control circuit constituting the cross point detection circuit is branched into at least two, one of which is used as an input to the electrical waveform equalization means,
An optical receiver that optimally controls equalization characteristics of the electrical waveform equalization means using an output value of the crosspoint detection circuit. A cross-point detection circuit for a received optical signal according to any one of claims 1 to 5,
Electrical waveform equalization means for equalizing the electrical signal waveform output from the photoelectric conversion means;
Discriminating and reproducing means for discriminating and reproducing the output signal of the electrical waveform equalizing means,
An optical receiver that optimally controls equalization characteristics of the electrical waveform equalization means using an output value of the crosspoint detection circuit. A cross-point detection circuit for a received optical signal according to any one of claims 1 to 5,
Optical chromatic dispersion compensation means for providing chromatic dispersion to the received optical signal input to the photoelectric conversion means;
Electrical waveform equalization means for equalizing the electrical signal waveform output from the photoelectric conversion means;
Discriminating and reproducing means for discriminating and reproducing the output signal of the electrical waveform equalizing means,
An optical receiver characterized by optimally controlling at least one of the chromatic dispersion value of the optical chromatic dispersion compensation means and the equalization characteristic of the electrical waveform equalization means using the output value of the cross point detection circuit. The electrical waveform equalization means is a feed forward equalizer (FFE), a decision feedback equalizer (DFE), or FFE + DFE,
9. The optical receiver according to claim 6, wherein the FFE, DFE, or FFE + DFE tap coefficient is optimally controlled using an output value of the crosspoint detection circuit. The electrical waveform equalization means includes a waveform equalization mode for equalizing the input signal waveform and a through mode for allowing the input signal waveform to pass through as it is,
9. The optical reception according to claim 6, wherein the waveform equalization mode and the through mode of the electrical signal equalization means are switched using an output value of the crosspoint detection circuit. apparatus. A cross-point detection circuit for a received optical signal according to any one of claims 1 to 5,
Optical chromatic dispersion compensation means for providing chromatic dispersion to the received optical signal input to the photoelectric conversion means;
And an identification reproducing means for identifying and reproducing the electric signal output from the photoelectric conversion means,
An optical receiver that optimally controls the chromatic dispersion value of the optical chromatic dispersion compensation means using the output value of the cross point detection circuit.

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