JP2015106784A - Optical communication module, optical communication device and optical communication method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical communication module in which the multiplication factor of a light-receiving element can be adjusted well at a low cost, in a configuration including a light-receiving element having current amplification function, and to provide an optical communication device and an optical communication method.SOLUTION: An optical communication module includes a light-receiving element outputting a current depending on the intensity of an optical signal received, and capable of changing the multiplication factor, a reference generating section for generating a comparison reference from an electric signal based on the output current from the light-receiving element, an object generating section for generating a comparison object containing the frequency components of the electrical signal, in a region where the cut-off frequency of the electrical signal is in substantially inverse proportional to the multiplication factor of the light-receiving element, more than those of the comparison reference, an operating section for relatively comparing the size of the comparison reference and the size of the comparison object, and an adjusting section for adjusting the multiplication factor of the light-receiving element on the basis of the comparison results from the operating section.

Description

  The present invention relates to an optical communication module, an optical communication device, and an optical communication method, and more particularly, to an optical communication module, an optical communication device, and an optical communication method that include a light receiving element having a current amplification function.

  In recent years, the Internet has become widespread, and users can access various information on sites operated in various parts of the world and obtain the information. Accordingly, devices capable of broadband access such as ADSL (Asymmetric Digital Subscriber Line) and FTTH (Fiber To The Home) are rapidly spreading.

  In IEEE Std 802.3ah (registered trademark) -2004 (Non-patent Document 1), a plurality of home-side devices (ONU: Optical Network Unit) share an optical communication line and a station-side device (OLT: Optical Line Terminal). One method of a passive optical network (PON), which is a medium-sharing communication that performs data transmission with the network, is disclosed. That is, EPON (Ethernet (registered trademark) PON) in which all information is communicated in the form of an Ethernet (registered trademark) frame, including user information passing through the PON and control information for managing and operating the PON, and EPON An access control protocol (MPCP (Multi-Point Control Protocol)) and an OAM (Operations Administration and Maintenance) protocol are defined. By exchanging MPCP frames between the station side device and the home side device, the home side device joins and leaves, and uplink access multiplexing control is performed. Non-Patent Document 1 describes a registration method for a new home device, a report indicating a bandwidth allocation request, and a gate indicating a transmission instruction using an MPCP message.

  In addition, GE-PON (Giga Bit Ethernet (registered trademark) Passive Optical Network), which is an EPON realizing a communication speed of 1 gigabit / second, is standardized as IEEE 802.3av (registered trademark) -2009. Even in the 10 G-EPON, that is, the EPON corresponding to a communication speed of 10 gigabits / second, the access control protocol is premised on MPCP.

  By the way, in order to realize long-distance light transmission in a PON system or the like, for example, an avalanche photodiode (APD) having high light receiving sensitivity is used as a light receiving element. The APD is a light receiving element having a current amplifying function. In order to maintain a carrier multiplication coefficient having a high temperature dependency, that is, a multiplication factor with high accuracy, the APD has a reverse bias voltage applied to the APD at the temperature of the APD. It is necessary to adjust accordingly.

  As a technique for adjusting the multiplication factor of APD, for example, Japanese Patent Laying-Open No. 2005-135994 (Patent Document 1) discloses the following configuration. That is, the light receiving element in the optical receiver includes a substrate having a first surface and a second surface, a PIN photodiode portion having a light receiving layer and a diffusion region provided on the first surface of the substrate, An avalanche photodiode portion having a light receiving layer, a multiplication layer and a diffusion region provided on the second surface of the substrate, wherein the substrate transmits received signal light.

IEEE Std 802.3ah (registered trademark) -2004

JP 2005-135994 A

  However, in the configuration described in Patent Document 1, a PIN photodiode is required in addition to the APD, and it is necessary to provide a substrate through which signal light is transmitted, which increases the manufacturing cost.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to satisfactorily adjust the multiplication factor of the light receiving element at a low cost in a configuration including the light receiving element having a current amplification function. An optical communication module, an optical communication device, and an optical communication method are provided.

  In order to solve the above-described problems, an optical communication module according to an aspect of the present invention outputs a current according to the intensity of a received optical signal and can change a multiplication factor. A reference creation unit that creates a comparison reference from an electrical signal based on an output current, and includes more frequency components of the electrical signal in a region where a cutoff frequency of the electrical signal is approximately inversely proportional to a multiplication factor of the light receiving element. A target creation unit that creates a comparison target, a calculation unit that relatively compares the size of the comparison reference and the size of the comparison target, and an adjustment that adjusts the multiplication factor of the light receiving element based on a comparison result by the calculation unit A part.

  In order to solve the above problems, an optical communication apparatus according to an aspect of the present invention is an optical communication apparatus for transmitting / receiving an optical signal to / from another optical communication apparatus, and a current corresponding to the intensity of the received optical signal. A light-receiving element that can change the multiplication factor, a reference creation unit that creates a comparison reference from an electric signal based on an output current of the light-receiving element, and a cutoff frequency of the electric signal is increased by the light-receiving element. A target creation unit that creates a comparison target that includes more frequency components of the electrical signal than the comparison reference in a region that is approximately inversely proportional to the magnification; and a calculation unit that relatively compares the size of the comparison reference and the size of the comparison target; An adjustment unit that adjusts a multiplication factor of the light receiving element based on a comparison result by the calculation unit.

  In order to solve the above problems, an optical communication method according to an aspect of the present invention is an optical communication module including a light receiving element that outputs a current corresponding to the intensity of a received optical signal and can change a multiplication factor. An optical communication method comprising: creating a comparison reference from an electric signal based on an output current of the light receiving element; and a step of generating the electric signal in a region where a cutoff frequency of the electric signal is substantially inversely proportional to a multiplication factor of the light receiving element. A step of creating a comparison target including more frequency components than the comparison reference, a step of relatively comparing the size of the comparison reference and the size of the comparison target, and a multiplication factor of the light receiving element based on the comparison result Adjusting.

  The present invention can be realized not only as an optical communication module including such a characteristic processing unit, but also as a program for causing a computer to execute such characteristic processing steps. Further, it can be realized as a semiconductor integrated circuit that realizes part or all of the optical communication module.

  According to the present invention, in a configuration including a light receiving element having a current amplifying function, the multiplication factor of the light receiving element can be well adjusted at low cost.

FIG. 1 is a diagram showing a configuration of a PON system according to the first embodiment of the present invention. FIG. 2 is a diagram showing the configuration of the ONU in the PON system according to the first embodiment of the present invention. FIG. 3 is a diagram showing a configuration of an optical communication module in the ONU according to the first embodiment of the present invention. FIG. 4 is a diagram illustrating a configuration of a bias control unit in the optical communication module according to the first embodiment of the present invention. FIG. 5 is a diagram illustrating an example of a current multiplication characteristic of the light receiving element in the optical communication module according to the first embodiment of the present invention. FIG. 6 is a diagram showing an example of the relationship between the reverse bias voltage to the light receiving element and the communication quality in the optical communication module according to the first embodiment of the present invention. FIG. 7 is a diagram showing an example of the relationship between the multiplication factor of the light receiving element and the cutoff frequency in the optical communication module according to the first embodiment of the present invention. FIG. 8 is a diagram showing a configuration of a DC / DC converter in the bias control unit according to the first embodiment of the present invention. FIG. 9 is a flowchart showing a part of the procedure of the bias voltage adjustment method for the light receiving element in the optical communication module according to the first embodiment of the present invention. FIG. 10 is a flowchart showing a part of the procedure of the bias voltage adjusting method for the light receiving element in the optical communication module according to the first embodiment of the present invention. FIG. 11 is a flowchart showing the procedure of the gain adjustment method for the light receiving element in the optical communication module according to the first embodiment of the present invention. FIG. 12 is a diagram illustrating a downstream optical signal received by the optical communication module in the PON system according to the first embodiment of the present invention. FIG. 13 is a diagram showing a result of performing an FFT operation on the downstream optical signal shown in FIG. FIG. 14 is a diagram illustrating transfer characteristics of a received light signal in the optical communication module according to the first embodiment of the present invention. FIG. 15 is a diagram illustrating an example of a feedback limiting unit in the optical communication module according to the first embodiment of the present invention. FIG. 16 is a diagram illustrating a configuration of a bias control unit in the optical communication module according to the second embodiment of the present invention. FIG. 17 is a flowchart showing a part of the procedure of the method for adjusting the bias voltage of the light receiving element in the optical communication module according to the second embodiment of the present invention. FIG. 18 is a flowchart showing a part of the procedure of the bias voltage adjusting method for the light receiving element in the optical communication module according to the second embodiment of the present invention. FIG. 19 is a diagram illustrating a configuration of a bias control unit in a modification of the optical communication module according to the second embodiment of the present invention.

  First, the contents of the embodiment of the present invention will be listed and described.

  (1) An optical communication module according to an embodiment of the present invention outputs a current according to the intensity of a received optical signal and can change a multiplication factor, and is based on an output current of the light receiving element. A reference creation unit that creates a comparison reference from an electrical signal, and a comparison target that includes more frequency components of the electrical signal than the comparison reference in a region where the cutoff frequency of the electrical signal is substantially inversely proportional to the multiplication factor of the light receiving element. A target creation unit, a calculation unit that relatively compares the size of the comparison reference and the size of the comparison target, and an adjustment unit that adjusts the multiplication factor of the light receiving element based on a comparison result by the calculation unit. .

  As described above, the configuration for performing feedback control of the multiplication factor of the light receiving element can stabilize the multiplication factor and prevent deterioration in communication quality such as an increase in bit error rate even if temperature fluctuation occurs during normal operation. In addition, a monitor light receiving element such as a PIN photodiode is not required, and a peripheral circuit and structure for using the monitor light receiving element are not required. Therefore, it is possible to easily manufacture at a low cost while maintaining a certain level of communication quality. It becomes possible. Further, by integrating such a configuration as an optical communication module, the ease of realization can be improved. Therefore, in a configuration including a light receiving element having a current amplification function, the multiplication factor of the light receiving element can be adjusted well at low cost.

  (2) Preferably, the comparison criterion is that the frequency component of the electrical signal in a region where the frequency component of the electrical signal is substantially constant with respect to a change in the multiplication factor of the light receiving element, the DC component of the electrical signal, or , All frequency components of the electrical signal.

  With such a configuration, an appropriate value can be calculated as a comparison result using any one of the three components as a comparison reference.

  (3) Preferably, the said adjustment part adjusts the said multiplication factor so that the said comparison result may become a predetermined value, and the said predetermined value is set based on the target value of the multiplication factor of the said light receiving element.

  With such a configuration, the reverse bias voltage can be adjusted so that the multiplication factor of the light receiving element becomes the target value by a simple calculation process by comparing the comparison result with a predetermined value. That is, the multiplication factor can be controlled to a constant value regardless of the temperature change of the light receiving element.

  (4) Preferably, the optical communication module further includes a limiting unit that limits the adjustment of the multiplication factor by the adjusting unit.

  Such a configuration prevents the reverse bias voltage from being set to an incorrect value even when, for example, the input light to the optical communication module is minute and a certain level of measurement accuracy of the acquisition unit cannot be secured. be able to.

  (5) An optical communication device according to an embodiment of the present invention is an optical communication device for transmitting / receiving an optical signal to / from another optical communication device, and outputs a current corresponding to the intensity of the received optical signal, A light-receiving element capable of changing the multiplication factor, a reference creating unit that creates a comparison reference from an electric signal based on an output current of the light-receiving element, and a cutoff frequency of the electric signal is substantially inversely proportional to the multiplication factor of the light-receiving element An object creation unit that creates a comparison target that includes more frequency components of the electrical signal in the region to be compared than the comparison reference, a calculation unit that relatively compares the size of the comparison reference and the size of the comparison target, and the calculation unit And an adjustment unit that adjusts the multiplication factor of the light receiving element based on the comparison result.

  As described above, the configuration for performing feedback control of the multiplication factor of the light receiving element can stabilize the multiplication factor and prevent deterioration in communication quality such as an increase in bit error rate even if temperature fluctuation occurs during normal operation. In addition, a monitor light receiving element such as a PIN photodiode is not required, and a peripheral circuit and structure for using the monitor light receiving element are not required. Therefore, it is possible to easily manufacture at a low cost while maintaining a certain level of communication quality. It becomes possible. Further, by integrating such a configuration as an optical communication module, the ease of realization can be improved. Therefore, in a configuration including a light receiving element having a current amplification function, the multiplication factor of the light receiving element can be adjusted well at low cost.

  (6) An optical communication method according to an embodiment of the present invention is an optical communication method in an optical communication module that includes a light receiving element that outputs a current corresponding to the intensity of a received optical signal and that can change a multiplication factor. A step of creating a comparison reference from an electric signal based on an output current of the light receiving element, and a frequency component of the electric signal in a region where a cutoff frequency of the electric signal is approximately inversely proportional to a multiplication factor of the light receiving element. Creating a comparison object including more than the comparison reference; comparing the comparison reference size with the comparison target size; and adjusting the multiplication factor of the light receiving element based on the comparison result; including.

  Thus, by performing feedback control of the multiplication factor of the light receiving element, it is possible to stabilize the multiplication factor and prevent deterioration in communication quality such as an increase in bit error rate even if temperature fluctuation occurs during normal operation. In addition, a monitor light receiving element such as a PIN photodiode is not required, and a peripheral circuit and structure for using the monitor light receiving element are not required. Therefore, it is possible to easily manufacture at a low cost while maintaining a certain level of communication quality. It becomes possible. Further, by integrating such a configuration as an optical communication module, the ease of realization can be improved. Therefore, in a configuration including a light receiving element having a current amplification function, the multiplication factor of the light receiving element can be adjusted well at low cost.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated. Moreover, you may combine arbitrarily at least one part of embodiment described below.

<First Embodiment>
FIG. 1 is a diagram showing a configuration of a PON system according to the first embodiment of the present invention.

  Referring to FIG. 1, a PON system 301 is, for example, 10G-EPON, and includes ONUs 202A, 202B, and 202C, a station-side device 201 connected to an upper network, and a splitter SP. The ONUs 202A, 202B, 202C and the station side device 201 are connected via the splitter SP and the optical fiber OPTF, and transmit / receive optical signals to / from each other.

  FIG. 2 is a diagram showing the configuration of the ONU in the PON system according to the first embodiment of the present invention.

  2, the ONU 202 includes an optical communication module 101, a PON reception processing unit 92, a buffer memory 93, a UN transmission processing unit 94, a UNI (User Network Interface) port 95, and a UN reception processing unit 96. A buffer memory 97, a PON transmission processing unit 98, and a control unit 99.

  The optical communication module 101 is detachable from the ONU 202. The optical communication module 101 receives a downstream optical signal transmitted from the station-side device 201, converts it into an electrical signal, and outputs it.

  The PON reception processing unit 92 reconstructs a frame from the electrical signal received from the optical communication module 101 and distributes the frame to the control unit 99 or the UN transmission processing unit 94 according to the type of the frame. Specifically, the PON reception processing unit 92 outputs the data frame to the UN transmission processing unit 94 via the buffer memory 93 and outputs the control frame to the control unit 99.

  The control unit 99 generates a control frame including various control information and outputs it to the UN transmission processing unit 94.

  The UN transmission processing unit 94 transmits the data frame received from the PON reception processing unit 92 and the control frame received from the control unit 99 to a user terminal such as a personal computer (not shown) via the UNI port 95.

  The UN reception processing unit 96 outputs the data frame received from the user terminal via the UNI port 95 to the PON transmission processing unit 98 via the buffer memory 97, and the control frame 99 receives the control frame received from the user terminal via the UNI port 95. Output to.

  The control unit 99 performs home-side processing relating to control and management of the PON line between the station-side device 201 and the ONU 202, such as MPCP and OAM. That is, various controls such as access control are performed by exchanging MPCP messages and OAM messages with the station-side apparatus 201 connected to the PON line. The control unit 99 generates a control frame including various control information and outputs it to the PON transmission processing unit 98. The control unit 99 performs various setting processes for each unit in the ONU 202.

  The PON transmission processing unit 98 outputs the data frame received from the UN reception processing unit 96 and the control frame received from the control unit 99 to the optical communication module 101.

  The optical communication module 101 converts the data frame and the control frame, which are electrical signals received from the PON transmission processing unit 98, into optical signals and transmits them to the station-side apparatus 201.

  FIG. 3 is a diagram showing a configuration of an optical communication module in the ONU according to the first embodiment of the present invention.

  Referring to FIG. 3, optical communication module 101 includes a burst transmission unit 151 and a reception unit 152. The burst transmission unit 151 includes a preamplifier 86, an output buffer circuit (modulation current supply circuit) 87, a bias current supply circuit 88, and a light emitting circuit 89. The light emitting circuit 89 includes a light emitting element LD and inductors L1 and L2. The receiving unit 152 includes a light receiving element PD, a TIA (transimpedance amplifier) 81, an LIA (limit amplifier) 82, a bias control unit 83, and an output buffer 85.

  In the burst transmission unit 151, the preamplifier 86 receives the transmission data that is the data frame from the UN reception processing unit 96 and the control frame from the control unit 99, and amplifies and outputs the transmission data. For example, the preamplifier 86 receives the transmission data as a differential signal from the signal lines INP and INN.

  The output buffer circuit 87 supplies a modulation current to the light emitting circuit 89 based on the transmission data received from the preamplifier 86. This modulation current is a current having a magnitude corresponding to the logical value of data to be transmitted to the station side device 201.

  The light emitting circuit 89 transmits the upstream optical signal to the station side device 201. In the light emitting circuit 89, the light emitting element LD is connected to a power supply node supplied with a fixed voltage, for example, a power supply voltage Vcc, via an inductor L1, and is connected to a bias current supply circuit 88 via an inductor L2. The light emitting element LD emits light based on the bias current supplied from the bias current supply circuit 88 and the modulation current supplied from the output buffer circuit 87, and changes the light emission intensity.

  In the receiving unit 152, the light receiving element PD is, for example, an avalanche photodiode. The light receiving element PD converts the optical signal received from the station side device 201 into an electric signal, for example, an electric current and outputs it.

  The TIA 81 converts the current received from the light receiving element PD into a voltage, and outputs the converted voltage, that is, an electric signal to the LIA 82.

  The LIA 82 binarizes the voltage level received from the TIA 81 and outputs it as received data.

  The output buffer 85 amplifies the reception data received from the LIA 82 and outputs the amplified data to the PON reception processing unit 92. For example, the output buffer 85 outputs the received data from the signal lines OUTP and OUTN as a differential signal.

  Bias control unit 83 is connected to a power supply node supplied with a fixed voltage, for example, power supply voltage Vcc, and supplies a bias voltage to light receiving element PD. The bias controller 83 has a bias voltage adjustment function.

  Further, for example, the light emitting element LD is built in an assembled light emitting module (hereinafter also referred to as TOSA: Transmitter Optical Sub-Assembly). The light receiving elements PD and TIA 81 and a part of the bias control unit 83 are built in an assembled light receiving module (hereinafter also referred to as ROSA: Receiver Optical Sub-Assembly).

  FIG. 4 is a diagram illustrating a configuration of a bias control unit in the optical communication module according to the first embodiment of the present invention.

  Referring to FIG. 4, bias control unit 83 includes a DC / DC converter (reference creation unit) 11, a CPU (calculation unit and adjustment unit) 12, resistors 14 and 15, and a target creation unit 52. The object creation unit 52 includes an amplitude detection circuit 43 and a BPF (Band Pass Filter) 44. The resistance values of the resistors 14 and 15 are Ra and Rb, respectively. The resistors 14 and 15 may be included in the DC / DC converter 11. The receiving unit 152 in the optical communication module 101 further includes capacitors 45 and 46.

  The light receiving element PD has a cathode connected to the DC / DC converter 11 and an anode connected to the input end of the TIA 81. Resistor 14 has a first end connected to the output end of CPU 12 and the input end of DC / DC converter 11, and a second end connected to the ground node. Resistor 15 has a first end connected to the output end of DC / DC converter 11 and the input end of CPU 12, and a second end connected to the ground node. A power supply voltage Vcc is supplied to the DC / DC converter 11, the TIA 81, and the LIA 82.

  Capacitor 45 has a first end connected to one of the differential outputs of TIA 81 and a second end connected to one of the differential inputs of LIA 82. Capacitor 46 has a first end connected to the other differential output of TIA 81 and a second end connected to the other differential input of LIA 82.

  The light receiving element PD outputs a current Iapd corresponding to the intensity of the optical signal Pin received from the station side device 201 via the optical fiber OPTF to the TIA 81. The light receiving element PD can change the multiplication factor M by the reverse bias voltage Vapd.

  Capacitors 45 and 46 are provided for AC coupling between TIA 81 and LIA 82. Capacitors 45 and 46 block the DC component of the signal from TIA 81 to LIA 82. For example, the capacitance values of the capacitors 45 and 46 are 0.1 microfarad.

  The BPF 44 is connected to the second ends of the capacitors 45 and 46, and attenuates components outside the predetermined frequency band among the frequency components of the electric signal received from the TIA 81 via the capacitors 45 and 46.

  The amplitude detection circuit 43 detects the amplitude of the electrical signal that has passed through the BPF 44, and outputs a voltage Vf2 indicating the magnitude of the detected amplitude to the CPU 12.

  Here, the length of the path from the signal line between the TIA 81 and the LIA 82 to the amplitude detection circuit 43 is preferably as short as possible because the path may function as a stub.

  The CPU 12 obtains a digital value of the voltage Vf2 using a built-in A / D converter. Thus, the number of components in the optical communication module 101 can be reduced by using the built-in components of the CPU 12.

  The DC / DC converter 11 outputs a current Irssi corresponding to the output current Iapd of the light receiving element PD. This current Irssi is a current having a frequency band lower than that of the current Iapd.

  The current Irssi is converted into a voltage Vrssi by the resistor 15 and supplied to the CPU 12. The CPU 12 acquires a digital value of the voltage Vrssi using a built-in A / D converter. Thus, the number of components in the optical communication module 101 can be reduced by using the built-in components of the CPU 12.

  The CPU 12 adjusts the reverse bias voltage Vapd to the light receiving element PD based on the voltage Vrssi and the voltage Vf2.

  The DC / DC converter 11 supplies the reverse bias voltage Vapd to the light receiving element PD in accordance with the adjustment of the reverse bias voltage Vapd by the CPU 12.

  More specifically, the CPU 12 generates and outputs a current Ictrl based on the voltage Vrssi and the voltage Vf2. The current Ictrl is converted into a voltage Vctrl by the resistor 14 and supplied to the DC / DC converter 11.

  The DC / DC converter 11 supplies the reverse bias voltage Vapd to the light receiving element PD based on the voltage Vctrl received from the CPU 12.

Specifically, the reverse bias voltage Vapd is expressed by the following equation (B1) using coefficients K1 and K2 determined by the circuit configuration of the DC / DC converter 11 and the voltage Vctrl.
Vapd = K1 × Vctrl + K2 (B1)

When the resistance value of the resistor 14 is Ra, the voltage Vctrl is expressed by the following formula (B2) using the output current Ictrl of the CPU 12.
Vctrl = Ictrl × Ra (B2)

  From the equations (B1) and (B2), it can be seen that the reverse bias voltage Vapd to the light receiving element PD is changed by the output current Ictrl of the CPU 12.

  For example, the CPU 12 converts the control digital value into the current Ictrl by using a built-in D / A converter. Thus, the number of components in the optical communication module 101 can be reduced by using the built-in components of the CPU 12.

Further, if the multiplication factor of the light receiving element PD is M, the light receiving sensitivity of the light receiving element PD is RS [A / W], and the light receiving intensity of the light receiving element PD is Pin [W], the output current Iapd [A of the light receiving element PD ] Is represented by the following formula (C1).
Iapd = M × RS × Pin (C1)

  FIG. 5 is a diagram illustrating an example of a current multiplication characteristic of the light receiving element in the optical communication module according to the first embodiment of the present invention. In FIG. 5, the horizontal axis represents the reverse bias voltage Vapd, and the vertical axis represents the output current Iapd.

  Referring to FIG. 5, the output current Iapd increases as the reverse bias voltage Vapd to the light receiving element PD increases. That is, the multiplication factor M increases.

  More specifically, when the reverse bias voltage Vapd is small, the multiplication factor M of the light receiving element PD is constant without causing avalanche multiplication even if the reverse bias voltage Vapd is changed within a certain range. The multiplication factor M at this time is 1.

  When the reverse bias voltage Vapd rises from V1 included in the above range to V2, the output current Iapd becomes 10 times that when the reverse bias voltage Vapd is V1. That is, when the reverse bias voltage Vapd is V2, the multiplication factor M is 10.

  Further, the multiplication factor M of the light receiving element PD has a strong temperature characteristic. As shown in FIG. 5, the output current Iapd decreases as the temperature of the light receiving element PD increases, and the output current Iapd decreases as the temperature of the light receiving element PD decreases. Becomes larger. That is, when the temperature of the light receiving element PD increases, the multiplication factor M with respect to the reverse bias voltage Vapd decreases, and when the temperature of the light receiving element PD decreases, the multiplication factor M with respect to the reverse bias voltage Vapd increases.

  FIG. 6 is a diagram showing an example of the relationship between the reverse bias voltage to the light receiving element and the communication quality in the optical communication module according to the first embodiment of the present invention. In FIG. 6, the horizontal axis represents the reverse bias voltage Vapd, and the vertical axis represents the bit error rate.

  Referring to FIG. 6, when the reverse bias voltage Vapd is set so that the multiplication factor M becomes 10 and the multiplication factor M is increased by increasing the reverse bias voltage Vapd, the frequency band of the light receiving element PD is lowered. Further, as noise increases, the S / N (Signal to Noise Ratio) ratio deteriorates and the bit error rate increases.

  On the other hand, when the reverse bias voltage Vapd is set so that the multiplication factor M is 10 and the multiplication factor M is reduced by decreasing the reverse bias voltage Vapd, the light receiving sensitivity of the light receiving element PD is reduced and the intensity is lower. Since it becomes difficult to receive an optical signal, the S / N (Signal to Noise Ratio) ratio deteriorates and the bit error rate increases.

  Thus, there is an optimum value for the multiplication factor M, and for example, around 10 is the optimum value. Further, as described with reference to FIG. 5, the multiplication factor M has temperature characteristics. Therefore, it is necessary to adjust the multiplication factor M according to the temperature of the light receiving element PD and maintain the optimum value.

  FIG. 7 is a diagram showing an example of the relationship between the multiplication factor of the light receiving element and the cutoff frequency in the optical communication module according to the first embodiment of the present invention.

  In FIG. 7, the horizontal axis represents the multiplication factor M of the light receiving element PD, which is a log scale. The vertical axis is a cutoff frequency, that is, a frequency at which the gain of the electrical signal converted from the optical signal Pin by the light receiving element PD is reduced by a predetermined value, for example, 3 dB, compared to the low frequency. G1 is a graph plotting the measurement result of the cutoff frequency with respect to the multiplication factor of the light receiving element PD in a state where the intensity of the optical signal Pin is a constant value. G2 is a graph showing characteristics in which the GB product of the light receiving element PD, that is, the product of the multiplication factor and the cutoff frequency becomes a constant value. The graph G2 shows a GB product of 10 × 4 GHz = 40 GHz as a constant value.

  Referring to FIG. 7, the inventor of the present application performs various experiments to obtain a GB product under the condition of multiplication factor M at which the bit error rate of optical signal Pin received by optical communication module 101 becomes a good value. Was found to be almost constant. Specifically, the inventor of the present application has found that the GB product is substantially constant at about 40 GHz in the section of the multiplication factor M of about 8 to 13 where the bit error rate is a good value.

  Using this phenomenon, the optical communication module 101 monitors the cutoff frequency of the received light signal, which is an electrical signal based on the output current of the light receiving element PD, in a state where the multiplication factor M is between 8 and 13. When the off frequency decreases, it is determined that the multiplication factor M has increased, and the multiplication factor M is decreased. When the cutoff frequency increases, it is determined that the multiplication factor M has decreased, and the multiplication factor M is determined. increase.

  Specifically, the DC / DC converter 11 creates a direct current component of the received light signal that is an electric signal based on the output current of the light receiving element PD as a comparison reference, that is, creates the voltage Vrssi from the received light signal.

  The object creation unit 52 creates a comparison object that includes more frequency components of the received light signal than the comparison reference in a region where the cutoff frequency of the received light signal is approximately inversely proportional to the multiplication factor M of the light receiving element PD. That is, the comparison target mainly includes a frequency region in which the cutoff frequency of the light receiving signal is substantially inversely proportional to the multiplication factor M of the light receiving element PD, and may include components in other frequency regions.

  For example, the pass frequency band of the BPF 44 is set to 4 GHz which is a cutoff frequency corresponding to 10 which is the optimum value of the multiplication factor M in FIG. In this case, the output voltage Vf2 of the amplitude detection circuit 43 indicates the magnitude of the frequency component of 4 GHz in the light reception signal. When the ratio of the output voltage Vf2 of the amplitude detection circuit 43 to the intensity of the optical signal Pin becomes small, the reverse bias voltage Vapd is reduced to decrease the multiplication factor M, and the output voltage Vf2 with respect to the intensity of the optical signal Pin is reduced. When the ratio increases, the reverse bias voltage Vapd is increased to increase the multiplication factor M. In the optical communication module 101, for example, the voltage Vrssi that does not depend on the multiplication factor M is monitored as an index value of the intensity of the optical signal Pin.

  Note that the pass frequency band of the BPF 44 may be set to a value slightly around 4 GHz. Further, the pass frequency band of the BPF is generally an area having a certain width from the center frequency, but it is preferable that the pass frequency band of the BPF 44 is narrow.

  The bias control unit 83 in the optical communication module 101 monitors the voltage Vrssi and the voltage Vf2, and adjusts the output current Ictrl of the CPU 12 so that the multiplication factor M becomes a target value based on the monitoring result.

  More specifically, the CPU 12 calculates the ratio of the comparison reference created by the DC / DC converter 11 with respect to the comparison target created by the target creation unit 52, that is, the direct current component of the received light signal. Then, the CPU 12 adjusts the multiplication factor M of the light receiving element PD based on the calculated ratio.

  Specifically, for example, the CPU 12 calculates the ratio of the voltage Vrssi to the voltage Vf2, and adjusts the reverse bias voltage Vapd so that the ratio becomes a predetermined value. The predetermined value is set based on, for example, a target value of the multiplication factor M of the light receiving element PD.

  FIG. 8 is a diagram showing a configuration of a DC / DC converter in the bias control unit according to the first embodiment of the present invention.

  Referring to FIG. 8, a DC / DC converter 11 includes a differential amplifier 21, a comparator 22, a PWM control circuit 23, an N-channel MOSFET (Metal Oxide Field Effect Transistor) 24, an inductor 25, and a Schottky. A diode 26, a current mirror circuit 27, resistors 14 and 28 to 30, capacitors 31 to 33, and a filter circuit 35 are included. Filter circuit 35 includes a resistor 15 and a capacitor 34.

  The differential amplifier 21 has a non-inverting input terminal connected to the first end of the resistor 14 and the output end of the CPU 12, an inverting input terminal, and an output terminal connected to the first end of the resistor 28. Capacitor 31 has a first end connected to the second end of resistor 28 and a second end connected to the ground node. Comparator 22 has an inverting input terminal connected to the output terminal of differential amplifier 21 and the input terminal of PWM control circuit 23, a non-inverting input terminal connected to the output terminal of PWM control circuit 23, and an output terminal. . N-channel MOSFET 24 has a gate connected to the output terminal of comparator 22, a drain connected to the first end of inductor 25, and a source connected to the ground node. A second end of the inductor 25 is connected to a power supply node to which the power supply voltage Vcc is supplied. The Schottky diode 26 is connected to the anode connected to the drain of the N-channel MOSFET 24, the first end of the capacitor 32, the first end of the resistor 29, the first end of the capacitor 33, and the input end of the current mirror circuit 27. And a cathode. The second end of the resistor 29, the second end of the capacitor 33, and the first end of the resistor 30 are connected to the inverting input terminal of the differential amplifier 21. The mirror current output side of the current mirror circuit 27 is connected to the first end of the resistor 15, the first end of the capacitor 34 and the input end of the CPU 12. The reference current output side of the current mirror circuit 27 is connected to the light receiving element PD. Capacitors 32, 34 and second ends of resistors 14, 15, 30 are connected to the ground node.

  For example, the inductance of the inductor 25 is 4.7 microhenry, the resistance value of the resistor 29 is 1 megohm, the resistance value of the resistor 30 is 20 kohm, the resistance value of the resistor 15 is 2 kohm, and the capacitor 32 The capacitance value of .about.34 is 0.1 microfarad.

  The voltage at the connection node of the cathode of the Schottky diode 26 and the input end of the current mirror circuit 27 corresponds to the reverse bias voltage Vapd to the light receiving element PD.

  The capacitor 33 compensates the phase of the reverse bias voltage Vapd and suppresses oscillation.

  The differential amplifier 21 receives the voltage Vctrl obtained by converting the output current Ictrl of the CPU 12 at the non-inverting input terminal, and receives the feedback voltage FB based on the reverse bias voltage Vapd applied to the light receiving element PD at the inverting input terminal. Then, the differential amplifier 21 outputs a voltage according to the difference between the voltage Vctrl and the feedback voltage FB. The feedback voltage FB is Vapd / 50 depending on the resistance ratio of the resistor 29 and the resistor 30.

  The series circuit of the resistor 28 and the capacitor 31 compensates the phase of the output voltage of the differential amplifier 21 and suppresses oscillation.

  The comparator 22 compares the output voltage of the differential amplifier 21 received at the inverting input terminal with the ramp wave from the PWM control circuit 23 received at the inverting input terminal, and has a logic high level or a logic low level according to the comparison result. A signal is output to the gate of the N-channel MOSFET 24.

  The PWM control circuit 23 outputs the ramp wave to the non-inverting input terminal of the comparator 22. The PWM control circuit 23 adjusts the frequency of the output voltage of the comparator 22 by adjusting the ramp wave according to the output voltage of the differential amplifier 21.

  Specifically, when the feedback voltage FB is smaller than the voltage Vctrl, that is, when the reverse bias voltage Vapd is small, the differential amplifier 21 outputs a positive voltage. The PWM control circuit 23 receives this positive voltage and adjusts the ramp wave so that the frequency of the output signal of the comparator 22 is increased. As a result, the drain voltage of the N-channel MOSFET 24 increases and the reverse bias voltage Vapd increases.

  On the other hand, when the feedback voltage FB is larger than the voltage Vctrl, that is, when the reverse bias voltage Vapd is large, the differential amplifier 21 outputs a negative voltage. The PWM control circuit 23 receives this negative voltage and adjusts the ramp wave so that the frequency of the output signal of the comparator 22 is reduced. As a result, the drain voltage of the N-channel MOSFET 24 decreases and the reverse bias voltage Vapd decreases.

  Therefore, the reverse bias voltage Vapd can be adjusted by adjusting the voltage Vctrl. The reverse bias voltage Vapd is supplied to the light receiving element PD through the current mirror circuit 27.

  The current mirror circuit 27 is electrically connected to the light receiving element PD, uses the output current Iapd of the light receiving element PD as a reference current, and outputs a mirror current I2 corresponding to the reference current.

  Specifically, the current mirror circuit 27 outputs a mirror current I2, which is a current corresponding to the reference current I3 and having a magnitude corresponding to the mirror ratio. The reference current I3 is a current signal of 10 gigabits / second, for example, corresponding to the output current Iapd of the light receiving element PD.

When the input current to the current mirror circuit 27 is I1, the following equation is established.
I1 = I2 + I3 (D1)

  The filter circuit 35 attenuates a component equal to or higher than a predetermined frequency among the frequency components of the mirror current I2 output from the current mirror circuit 27, and outputs the attenuated component to the CPU 12 as a current Irssi.

  Specifically, the mirror current I2 is averaged by the capacitor 34 and the resistor 15, and the averaged mirror current I2 is converted into the voltage Vrssi and output to the CPU 12. The time constant of the capacitor 34 and the resistor 15 is, for example, 2 [kilo] × 0.1 [micro] = 0.2 milliseconds.

  Here, the speed at which the CPU 12 refers to the output value of the A / D converter that receives the voltage Vrssi is, for example, 100 Hz. That is, the sampling period of the voltage Vrssi in the CPU 12 is 10 milliseconds.

  That is, the time constant of the filter circuit 35 is larger than the reciprocal of the bit rate of the optical signal and smaller than the sampling period of the mirror current I2 in the CPU 12, that is, the voltage Vrssi.

  Thus, in the optical communication module 101, the time constant of the filter circuit that generates the current Irssi is set to be shorter than the period of the optical signal and longer than the sampling period of the CPU 12. As a result, the measurement of the voltage Vrssi by the CPU 12 can be made to follow the fluctuation of the intensity of the optical signal while averaging the output current Iapd and making the measurement easy and appropriate.

  Next, the adjustment process of the multiplication factor of the light receiving element in the optical communication module according to the first embodiment of the present invention will be described with reference to the drawings.

  Each device and its components in the PON system 301 include a computer, and an arithmetic processing unit such as a CPU in the computer reads out a program including a part or all of each step of the following flowchart and sequence from a memory (not shown). Run. Each of these plural devices and their component program can be installed from the outside. Each of the programs of the plurality of devices and their component parts is distributed while being stored in a recording medium.

  FIG. 9 is a flowchart showing a part of the procedure of the bias voltage adjustment method for the light receiving element in the optical communication module according to the first embodiment of the present invention. FIG. 9 shows an example of a method for setting a target value for feedback control in adjusting the bias voltage of the light receiving element PD.

  Referring to FIG. 9, first, in a state where the intensity of light to light receiving element PD is a constant value, for example, in a state where the average intensity of the optical signal to optical communication module 101 is a constant value, CPU 12 receives light receiving element PD. The output current Ictrl is adjusted and the reverse bias voltage Vapd is set so that the multiplication factor M becomes the target value (step S1).

  Next, in a state where the set reverse bias voltage Vapd is being supplied to the light receiving element PD, the CPU 12 measures the voltage Vrssi and the voltage Vf2 (step S2).

  Next, the CPU 12 calculates a target value of the ratio of the voltage Vrssi to the voltage Vf2. Specifically, the CPU 12 calculates Vrssi / Vf2 (step S3).

  Next, the CPU 12 sets and stores the calculated value as the target value A of Vrssi / Vf2 in the actual operation of the optical communication module 101 (step S4).

  Note that the CPU 12 is not limited to the configuration that measures the voltage Vrssi in a state where the multiplication factor M of the light receiving element PD is a target value, but measures the voltage Vrssi in the state where the multiplication factor M of the light receiving element PD is 1, for example. The target value A may be a ratio of a value obtained by multiplying the voltage Vrssi by the target value of the multiplication factor M with respect to the voltage Vf2 measured as described above. Here, when the multiplication factor of the light receiving element PD is 1, as described above, the avalanche multiplication is performed in the light receiving element PD even if the reverse bias voltage Vapd is small and the reverse bias voltage Vapd is changed within a certain range. It does not occur. However, the configuration using the voltage Vrssi in a state where the multiplication factor M of the light receiving element PD is the target value is preferable in the adjustment of the reverse bias voltage Vapd described later from the point that the measurement conditions for the voltage Vf2 are the same.

  Further, when manufacturing the optical communication module 101, the target value A may be set for each optical communication module 101, or the target value A is calculated for a plurality of optical communication modules 101, and the average value thereof is represented. A value may be set for each optical communication module 101.

  The target value of the multiplication factor M is not limited to 10 or the like, and for each optical communication module 101, the multiplication factor M that minimizes the bit error rate of the optical signal Pin received by the optical communication module 101 is set. It may be a target value.

  FIG. 10 is a flowchart showing a part of the procedure of the bias voltage adjusting method for the light receiving element in the optical communication module according to the first embodiment of the present invention. FIG. 10 shows a method of adjusting the bias voltage to the light receiving element PD for converging the multiplication factor M of the light receiving element PD to the target value in actual operation.

  Referring to FIG. 10, first, the CPU 12 performs the voltage Vrssi and the voltage Vf2 in an actual operation of the optical communication module 101, for example, in a state where the optical communication module 101 receives a downstream optical signal transmitted from the station side device 201. Measure. This measurement interval corresponds to the sampling period in the CPU 12 (step S11).

  Next, the CPU 12 calculates the ratio Vrssi / Vf2 of the voltage Vrssi with respect to the voltage Vf2, and compares it with the stored target value A (step S12).

  When Vrssi / Vf2 is larger than the target value A (YES in step S12), the CPU 12 determines that the multiplication factor M of the light receiving element PD is larger than the target value, and the current Ictrl so that the reverse bias voltage Vapd becomes smaller. Adjust. For example, the CPU 12 reduces the reverse bias voltage Vapd by reducing the switching frequency of the N-channel MOSFET 24 shown in FIG. 8 by reducing the current Ictrl (step S13).

  On the other hand, when Vrssi / Vf2 is smaller than the target value A (NO in step S12 and YES in step S14), the CPU 12 determines that the multiplication factor M of the light receiving element PD is smaller than the target value, and reverse bias voltage Vapd. The current Ictrl is adjusted so as to increase. For example, by increasing the current Ictrl, the CPU 12 increases the switching frequency of the N-channel MOSFET 24 shown in FIG. 8 and increases the reverse bias voltage Vapd (step S15).

  When Vrssi / Vf2 is equal to the target value A (NO in step S12 and NO in step S14), the CPU 12 determines that the multiplication factor M of the light receiving element PD is the target value, and sets the current Ictrl to the current value. Keep the value.

  In this manner, the CPU 12 repeats the measurement of each voltage, the calculation using the measurement value, and the adjustment of the current Ictrl so that the multiplication factor M of the light receiving element PD approaches the target value.

  Note that the CPU 12 may change the control width of the current Ictrl. Specifically, for example, the CPU 12 increases the change width of the current Ictrl when the difference between the Vrssi / Vf2 and the target value A is large, and the current Ictrl when the difference between the Vrssi / Vf2 and the target value A is small. Reduce the change width.

  In the optical communication module 101, Vrssi / Vf2, which is the ratio of the comparison reference to the comparison target, is calculated. However, the present invention is not limited to this. For example, the CPU 12 may be configured to calculate Vf2 / Vrssi, which is the ratio of the comparison target with respect to the comparison reference. The CPU 12 may be configured to perform a relative comparison between the size of the comparison reference and the size of the comparison target and adjust the multiplication factor M of the light receiving element PD based on the comparison result. That is, the comparison result is not limited to a ratio, and may be a ratio or a ratio, for example.

  FIG. 11 is a flowchart showing the procedure of the gain adjustment method for the light receiving element in the optical communication module according to the first embodiment of the present invention.

  Referring to FIG. 11, first, the CPU 12 measures the voltage Vrssi in a state where the optical communication module 101 receives a downstream optical signal transmitted from the station side device 201, for example, thereby outputting the light receiving element PD. A direct current component of the received light signal, which is an electrical signal based on the current Iapd, is detected as a comparison reference (step S21).

  Next, the object creation unit 52 measures the voltage Vf2 to measure the frequency component in which the GB product of the light receiving element PD is substantially constant, that is, the light reception signal in the region where the cutoff frequency of the light reception signal is approximately inversely proportional to the multiplication factor M. A comparison target including more frequency components than the comparison reference is created (step S22).

  Next, the CPU 12 performs a relative comparison between the size of the comparison target and the size of the comparison reference, for example, calculates Vrssi / Vf2, which is a ratio of the comparison reference to the comparison target (step S23).

  Next, the CPU 12 adjusts the multiplication factor M of the light receiving element PD so that the comparison result becomes a predetermined value, for example, the target value A. This predetermined value is set, for example, based on the target value of the multiplication factor M (step S24).

  FIG. 12 is a diagram illustrating a downstream optical signal received by the optical communication module in the PON system according to the first embodiment of the present invention. In FIG. 12, the horizontal axis represents time, and the vertical axis represents the level of the optical signal Pin.

  Referring to FIG. 12, the optical signal Pin received by the optical communication module 101 is, for example, a continuous signal whose intensity is modulated in the station-side apparatus 201. Ideally, a binary signal at a constant level as shown in FIG. With a digital waveform.

  If the signal levels when the logical value of the optical signal Pin is “0” and “1” are P0 and P1, respectively, the amplitude of the optical signal Pin is (P1−P0), which is a constant value.

The intensity of the optical signal Pin, that is, Pave which is RSSI (Received Signal Strength Indication) is expressed by the following equation.
Pave = (P0 + P1) / 2

  FIG. 13 is a diagram illustrating a result of performing FFT (Fast Fourier Transform) on the downstream optical signal illustrated in FIG. In FIG. 13, the horizontal axis represents frequency, and the vertical axis represents the level of the optical signal Pin.

  Referring to FIG. 13, optical signal Pin has, for example, a 10.3125 Gbps data pattern according to PRBS (Pseudo Random Bit Stream), and the level is zero at a frequency n times (n is a natural number) of 10.3125 GHz. Become.

  FIG. 14 is a diagram illustrating transfer characteristics of a received light signal in the optical communication module according to the first embodiment of the present invention. In FIG. 14, the horizontal axis is frequency and the vertical axis is gain. G1 to G5 are graphs showing transfer characteristics, and the multiplication factor M is increased in the order from the graph G5 to the graph G1. G10 is a graph showing characteristics in which the GB product of the light receiving element PD, that is, the product of the multiplication factor and the cutoff frequency becomes a constant value.

  Referring to FIG. 14, in graphs G1 to G4, when the multiplication factor M is small, the gain is small, while a gain of a certain level is maintained up to the high frequency region, and conversely, when the multiplication factor M is large, the gain is reduced. On the other hand, the gain decreases greatly in the high frequency region.

  Further, in the graphs G1 to G5, the gain in the direct current to low frequency region is reduced due to the influence of the capacitors 45 and 46, and the gain is substantially constant on the high frequency side from this range regardless of the magnitude of the frequency. There is a frequency range F1.

  In the graphs G1 to G4, the frequency f2 that overlaps the line X where the GB product of the light receiving element PD is a constant value exists on the higher frequency side than the range F1. On the other hand, in the graph G5, since the multiplication factor is small, the GB product of the light receiving element PD does not become a constant value.

  In the optical communication module according to the first embodiment of the present invention, in FIG. 14, a value indicating the cutoff frequency of the received light signal, specifically, a range F1 with respect to the voltage Vf2 indicating the magnitude of the received light signal at the frequency f2. Based on the ratio of the voltage Vrssi indicating the magnitude of the light receiving signal at the frequency f1, the multiplication factor M of the light receiving element PD is adjusted satisfactorily.

  As described above, in the optical communication module 101, feedback control of the reverse bias voltage Vapd, that is, the multiplication factor M is performed so that Vrssi / Vf2 becomes a constant value.

  However, in view of the linearity and offset characteristics of the A / D converter in the CPU 12, there is a possibility that correct feedback control cannot be performed depending on the amount of light input to the optical communication module 101.

  For example, when the input light to the optical communication module 101 is minute, the output value of the A / D converter in the CPU 12 becomes minute, and it becomes difficult to ensure a certain level regarding the measurement accuracy of the voltage Vrssi and the voltage Vf2.

  Further, when the optical connector for connecting the optical fiber OPTF and the optical communication module 101 is removed during the operation of the optical communication module 101, or when the optical connector is detached or attached during the operation of the optical communication module 101, Even when the amount of light input to the optical communication module 101 is in a transient state, it is difficult to ensure a certain level regarding the measurement accuracy of the voltage Vrssi and the voltage Vf2.

  If the measurement accuracy deteriorates due to such factors, unintended feedback control is performed and the multiplication factor M is significantly increased, that is, the reverse bias voltage Vapd is significantly increased, so that the light receiving element PD. May be damaged.

  Therefore, the optical communication module 101 further includes, for example, a feedback limiting unit that limits the adjustment of the multiplication factor M by the CPU 12.

  For example, in the optical communication module 101, the feedback limiting unit 36 that limits the multiplication factor M so as not to exceed a predetermined value is provided on the CPU 12 or the substrate. For example, the feedback limiting unit 36 limits the reverse bias voltage Vapd supplied to the light receiving element PD to, for example, a design allowable range related to the light receiving element PD.

  More specifically, for example, when the voltage Vrssi is equal to or lower than a predetermined threshold, the feedback limiting unit 36 fixes the reverse bias voltage Vapd to a predetermined value. As this predetermined value, for example, a bias voltage value typical for the light receiving element PD is set.

Specifically, a software limiting method for setting an upper limit on the voltage Vctrl, that is, the current Ictrl from the CPU 12 to the error amplifier, that is, the differential amplifier 21 in the DC / DC converter 11 can be considered. For example, when the voltage Vrssi is equal to or lower than a predetermined threshold, the CPU 12 fixes the current Ictrl to a predetermined value. This predetermined value is, for example, a value that provides a reverse bias voltage Vapd having a magnitude that is typical for the light receiving element PD.

  Alternatively, a hardware limiting method in which the DC / DC converter 11 is provided with a limiting circuit for the reverse bias voltage Vapd is conceivable.

  FIG. 15 is a diagram illustrating an example of a feedback limiting unit in the optical communication module according to the first embodiment of the present invention.

  Referring to FIG. 15, DC / DC converter 11 further includes a feedback limiting unit 36 as compared with the configuration shown in FIG. 8.

  The feedback limiting unit 36 is connected between the connection node of the CPU 12 and the resistor 14 and the non-inverting input terminal of the differential amplifier 21. When the magnitude of the voltage Vctrl exceeds a predetermined value, the feedback limiting unit 36 limits the voltage Vctrl to the predetermined value and outputs it to the differential amplifier 21. For example, the predetermined value is a value for obtaining a reverse bias voltage Vapd having a magnitude included in a design allowable range related to the light receiving element PD.

  If the measurement accuracy deteriorates due to the above factors, unintended feedback control is performed and the multiplication factor M is lowered, that is, the reverse bias voltage Vapd is lowered, so that the GB product of the light receiving element PD is decreased. May deviate from a certain range.

  Therefore, in the optical communication module 101, a feedback limiting unit 37 that limits the multiplication factor M so as not to fall below a predetermined value is provided on the CPU 12 or the substrate. For example, the feedback limiting unit 37 limits the reverse bias voltage Vapd supplied to the light receiving element PD to a range in which, for example, the GB product of the light receiving element PD is constant.

  More specifically, for example, when the voltage Vf2 is equal to or higher than a predetermined threshold, the feedback limiting unit 37 fixes the reverse bias voltage Vapd to a predetermined value. As this predetermined value, for example, a bias voltage value corresponding to the lower limit of the multiplication factor M in a range where the GB product of the light receiving element PD is constant is set.

Specifically, a software limiting method is conceivable in which a lower limit is set for the voltage Vctrl, that is, the current Ictrl from the CPU 12 to the error amplifier in the DC / DC converter 11, that is, the differential amplifier 21. For example, when the voltage Vf2 is equal to or higher than a predetermined threshold, the CPU 12 fixes the current Ictrl to a predetermined value. This predetermined value is, for example, a value for obtaining the reverse bias voltage Vapd having a magnitude corresponding to the lower limit of the multiplication factor M in a range where the GB product of the light receiving element PD is constant.

  Alternatively, a hardware limiting method in which the DC / DC converter 11 is provided with a limiting circuit for the reverse bias voltage Vapd is conceivable.

  That is, the DC / DC converter 11 further includes a feedback limiting unit 37 as compared with the configuration shown in FIG.

  Although not shown, like the feedback limiting unit 36 shown in FIG. 15, the feedback limiting unit 37 is connected between the connection node of the CPU 12 and the resistor 14 and the non-inverting input terminal of the differential amplifier 21. When the magnitude of the voltage Vctrl is lower than a predetermined value, the feedback limiting unit 37 limits the voltage Vctrl to the predetermined value and outputs it to the differential amplifier 21. For example, the predetermined value is a value for obtaining the reverse bias voltage Vapd having a magnitude corresponding to the lower limit of the multiplication factor M in a range where the GB product of the light receiving element PD is constant.

  By the way, in the structure described in Patent Document 1, a PIN photodiode is required in addition to the APD, and it is necessary to provide a substrate through which signal light is transmitted, which increases the manufacturing cost.

  On the other hand, in the optical communication module according to the first embodiment of the present invention, the DC / DC converter 11 creates a comparison reference from the received light signal. The object creating unit 52 includes more frequency components of the received light signal in the region where the cutoff frequency of the received light signal, which is an electrical signal based on the output current of the received light element PD, is approximately inversely proportional to the multiplication factor M of the received light element PD, compared to the comparison reference. Create a comparison target. The CPU 12 performs a relative comparison between the size of the comparison reference and the size of the comparison target. Then, the CPU 12 adjusts the multiplication factor M of the light receiving element PD based on the comparison result.

  As described above, the configuration for performing feedback control of the multiplication factor M of the light receiving element PD stabilizes the multiplication factor M even when temperature fluctuation occurs during normal operation, and prevents deterioration in communication quality such as an increase in bit error rate. Can do. In addition, a monitor light receiving element such as a PIN photodiode is not required, and a peripheral circuit and structure for using the monitor light receiving element are not required. Therefore, it is possible to easily manufacture at a low cost while maintaining a certain level of communication quality. It becomes possible. Further, by integrating such a configuration as an optical communication module, the ease of realization can be improved.

  Therefore, in the optical communication module according to the first embodiment of the present invention, in the configuration including the light receiving element having a current amplification function, the multiplication factor of the light receiving element can be well adjusted at low cost.

  In the optical communication module according to the first embodiment of the present invention, the DC / DC converter 11 creates a direct current component of the received light signal as a comparison reference.

  With such a configuration, an appropriate value can be calculated as a comparison result using the direct current component of the received light signal as a comparison reference.

  In the optical communication module according to the first embodiment of the present invention, the CPU 12 adjusts the multiplication factor M so that the comparison result becomes a predetermined value. The predetermined value is set based on the target value of the multiplication factor M of the light receiving element PD.

  With such a configuration, it is possible to adjust the reverse bias voltage Vapd so that the multiplication factor M of the light receiving element PD becomes the target value by a simple calculation process by comparing the comparison result with a predetermined value. That is, the multiplication factor M can be controlled to a constant value regardless of the temperature change of the light receiving element PD.

  In the optical communication module according to the first embodiment of the present invention, the feedback limiting units 36 and 37 limit the adjustment of the multiplication factor M by the CPU 12.

  With such a configuration, the reverse bias voltage Vapd is set to an incorrect value even when, for example, the input light to the optical communication module 101 is very small and a certain level of the measurement accuracy of the voltage Vrssi and the voltage Vf2 cannot be secured. Can be prevented.

  In the optical communication method in the optical communication module according to the first embodiment of the present invention, first, a comparison reference is created from a light reception signal that is an electrical signal based on the output current of the light receiving element PD. Next, a comparison target including a frequency component of the received light signal in a region where the cutoff frequency of the received light signal is substantially inversely proportional to the multiplication factor M of the light receiving element PD is created. Next, the comparison reference size is compared with the comparison target size. Next, the multiplication factor M of the light receiving element PD is adjusted based on the comparison result.

  In this way, by performing feedback control of the multiplication factor M of the light receiving element PD, the multiplication factor M is stabilized even when temperature fluctuation occurs during normal operation, and deterioration of communication quality such as an increase in bit error rate is prevented. Can do. In addition, a monitor light receiving element such as a PIN photodiode is not required, and a peripheral circuit and structure for using the monitor light receiving element are not required. Therefore, it is possible to easily manufacture at a low cost while maintaining a certain level of communication quality. It becomes possible. Further, by integrating such a configuration as an optical communication module, the ease of realization can be improved.

  Therefore, in the optical communication method according to the first embodiment of the present invention, it is possible to satisfactorily adjust the multiplication factor of the light receiving element at a low cost in the configuration including the light receiving element having a current amplification function.

  Next, another embodiment of the present invention will be described with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

<Second Embodiment>
The present embodiment relates to an optical communication module in which an index of the intensity of an optical signal is changed as compared with the optical communication module according to the first embodiment. The contents other than those described below are the same as those of the optical communication module according to the first embodiment.

  FIG. 16 is a diagram illustrating a configuration of a bias control unit in the optical communication module according to the second embodiment of the present invention.

  Referring to FIG. 16, the receiving unit 152 in the optical communication module 102 according to the second embodiment of the present invention is further compared to the optical communication module 101 according to the first embodiment of the present invention. A creation unit 51 is included. The reference creation unit 51 includes an amplitude detection circuit 41 and a BPF 42.

  The optical communication module 102 includes a bias control unit 73 instead of the bias control unit 83 as compared to the optical communication module 101. Compared with the bias control unit 83, the bias control unit 73 does not generate the voltage Vrssi, but includes the resistor 15, the current mirror circuit 27 in the DC / DC converter 11, the filter circuit 35, and the like for generating the voltage Vrssi. Absent.

  The reference creation unit 51 creates a frequency component of the light reception signal in a region where the frequency component of the light reception signal is substantially constant with respect to the change in the multiplication factor M of the light receiving element PD as a comparison reference.

  More specifically, in the reference creation unit 51, the BPF 42 is connected to the second ends of the capacitors 45 and 46, and out of a predetermined frequency band among the frequency components of the electrical signal received from the TIA 81 via the capacitors 45 and 46. Attenuate components.

  The amplitude detection circuit 41 detects the amplitude of the electrical signal that has passed through the BPF 42, and outputs a voltage Vf1 indicating the magnitude of the detected amplitude to the CPU 12.

  Here, the length of the path from the signal line between the TIA 81 and the LIA 82 to the amplitude detection circuit 43 is preferably as short as possible because the path may function as a stub.

  The CPU 12 obtains a digital value of the voltage Vf1 using a built-in A / D converter. Thus, by using the built-in components of the CPU 12, the number of components in the optical communication module 102 can be reduced.

  In the optical communication module 102, the voltage Vf1 is monitored as an index value of the intensity of the optical signal Pin instead of the voltage Vrssi.

  Specifically, the pass frequency band of the BPF 42 is set to a low frequency, for example, 100 MHz, at which the received light signal, which is an electrical signal based on the output current of the light receiving element PD, becomes substantially constant with respect to the change in the multiplication factor M. The output voltage Vf1 of the amplitude detection circuit 41 indicating the magnitude of the frequency component of 100 MHz is used as an index value of the intensity of the optical signal Pin.

  Note that the pass frequency band of the BPF 42 may be set to a value slightly around the frequency at which the light reception signal from the BPF 42 becomes substantially constant with respect to the change in the multiplication factor M.

  Further, the pass frequency band of the BPF 42 only needs to be substantially constant with respect to the change in the multiplication factor M of the output signal to the CPU 12. For example, an LPF may be used instead of the BPF 42.

  The CPU 12 adjusts the reverse bias voltage Vapd so that the ratio of the voltage Vf1 to the voltage Vf2 becomes a predetermined value. The predetermined value is set based on the target value of the multiplication factor M of the light receiving element PD.

  FIG. 17 is a flowchart showing a part of the procedure of the method for adjusting the bias voltage of the light receiving element in the optical communication module according to the second embodiment of the present invention. FIG. 17 shows an example of a method for setting a target value for feedback control in adjusting the bias voltage of the light receiving element PD.

  Referring to FIG. 17, first, in a state where the intensity of light to light receiving element PD is a constant value, for example, in a state where the average intensity of the optical signal to optical communication module 102 is a constant value, CPU 12 The output current Ictrl is adjusted and the reverse bias voltage Vapd is set so that the multiplication factor M becomes the target value (step S31).

  Next, in a state where the set reverse bias voltage Vapd is supplied to the light receiving element PD, the CPU 12 measures the voltage Vf1 and the voltage Vf2 (step S32).

  Next, the CPU 12 calculates a target value of the ratio of the voltage Vf1 to the voltage Vf2. Specifically, the CPU 12 calculates Vf1 / Vf2 (step S33).

  Next, the CPU 12 sets and stores the calculated value as the target value B of Vf1 / Vf2 in the actual operation of the optical communication module 102 (step S34).

  The CPU 12 is not limited to the configuration that measures the voltage Vf1 in a state where the multiplication factor M of the light receiving element PD is a target value. For example, the CPU 12 measures the voltage Vf1 in a state where the multiplication factor M of the light receiving element PD is 1. The ratio of the value obtained by multiplying the voltage Vf1 by the target value of the multiplication factor M to the voltage Vf2 measured as described above may be the target value B. However, the configuration using the voltage Vf1 in a state where the multiplication factor M of the light receiving element PD is the target value is preferable in the adjustment of the reverse bias voltage Vapd described later from the point that the measurement conditions of the voltage Vf2 are the same.

  Further, when manufacturing the optical communication module 102, the target value B may be set for each optical communication module 102, or the target value B is calculated for a plurality of optical communication modules 102, and the average value thereof is represented. A value may be set for each optical communication module 102.

  Further, the target value of the multiplication factor M is not limited to 10 or the like. For each optical communication module 102, the multiplication factor M that minimizes the bit error rate of the optical signal Pin received by the optical communication module 102 is set. It may be a target value.

  FIG. 18 is a flowchart showing a part of the procedure of the bias voltage adjusting method for the light receiving element in the optical communication module according to the second embodiment of the present invention. FIG. 18 shows a method of adjusting the bias voltage to the light receiving element PD for converging the multiplication factor M of the light receiving element PD to the target value in actual operation.

  Referring to FIG. 18, first, the CPU 12 performs the operation of the optical communication module 102, for example, the voltage Vf <b> 1 and the voltage Vf <b> 2 in a state where the optical communication module 102 receives a downstream optical signal transmitted from the station side device 201. Measure. This measurement interval corresponds to the sampling period in the CPU 12 (step S41).

  Next, the CPU 12 calculates the ratio Vf1 / Vf2 of the voltage Vf1 with respect to the voltage Vf2, and compares it with the stored target value B (step S42).

  When Vf1 / Vf2 is larger than the target value B (YES in step S42), the CPU 12 determines that the multiplication factor M of the light receiving element PD is larger than the target value, and the current Ictrl is set so that the reverse bias voltage Vapd becomes smaller. Adjust. For example, the CPU 12 reduces the reverse bias voltage Vapd by reducing the switching frequency of the N-channel MOSFET 24 shown in FIG. 8 by reducing the current Ictrl (step S43).

  On the other hand, when Vf1 / Vf2 is smaller than the target value B (NO in step S42 and YES in step S44), the CPU 12 determines that the multiplication factor M of the light receiving element PD is smaller than the target value, and reverse bias voltage Vapd. The current Ictrl is adjusted so as to increase. For example, by increasing the current Ictrl, the CPU 12 increases the switching frequency of the N-channel MOSFET 24 shown in FIG. 8 and increases the reverse bias voltage Vapd (step S45).

  When Vf1 / Vf2 is equal to the target value B (NO in step S42 and NO in step S44), the CPU 12 determines that the multiplication factor M of the light receiving element PD is the target value, and sets the current Ictrl to the current value. Keep the value.

  In this manner, the CPU 12 repeats the measurement of each voltage, the calculation using the measurement value, and the adjustment of the current Ictrl so that the multiplication factor M of the light receiving element PD approaches the target value.

  Note that the CPU 12 may change the control width of the current Ictrl. Specifically, for example, the CPU 12 increases the change range of the current Ictrl when the difference between Vf1 / Vf2 and the target value B is large, and increases the current Ictrl when the difference between Vf1 / Vf2 and the target value B is small. Reduce the change width.

  Further, the configuration is not limited to a configuration in which some frequency components in the light reception signal are created as a comparison reference, and a configuration in which all frequency components in the light reception signal are created as a comparison reference may be used.

  FIG. 19 is a diagram illustrating a configuration of a bias control unit in a modification of the optical communication module according to the second embodiment of the present invention.

  Referring to FIG. 19, the receiving unit 152 in the modification of the optical communication module 102 is different from the configuration shown in FIG. 16 in that an FFT processing unit (a reference creating unit and a target creating unit 52 instead of the reference creating unit 51 and the target creating unit 52). Creating section) 53.

  The FFT processing unit 53 obtains a frequency spectrum of the electric signal by performing an FFT operation on the electric signal received from the TIA 81 via the capacitors 45 and 46, and obtains, for example, a frequency at 4 GHz of the electric signal from the frequency spectrum. The component size is measured, and a voltage Vf2 indicating the measurement result is output to the CPU 12. Further, the FFT processing unit 53 measures the magnitude of the frequency component at 100 MHz of the electrical signal, for example, from the frequency spectrum, and outputs a voltage Vf1 indicating the measurement result to the CPU 12.

  The FFT processing unit 53 is not limited to the configuration in which the frequency component at 100 MHz of the electrical signal from the TIA 81 is monitored as an index value of the intensity of the optical signal Pin, but the magnitude of all the frequency components of the electrical signal from the frequency spectrum is determined. The above index value may be measured and the magnitude of all frequency components of the electrical signal in the time domain may be measured in the time domain by, for example, integrating the level of the electrical signal from the TIA 81 in the time domain according to Parseval's theorem. The index value may be used.

  Moreover, this modification can also be applied to the optical communication module according to the first embodiment of the present invention. In this case, the voltage Vrssi is used instead of the FFT processing unit 53 outputting the voltage Vf1.

  As described above, in the optical communication module according to the second embodiment of the present invention, the reference creating unit 51 is a region where the frequency component of the light reception signal is substantially constant with respect to the change in the multiplication factor M of the light receiving element PD. The frequency component of the received light signal is created as a reference for comparison. Alternatively, the FFT processing unit 53 creates all frequency components of the received light signal as a comparison reference.

  With such a configuration, an appropriate value can be calculated as a comparison result using either one of the two components as a comparison reference.

  Since other configurations and operations are the same as those of the optical communication module according to the first embodiment, detailed description thereof will not be repeated here.

  In FIGS. 4 and 16 in the above embodiments, at least one of BPF 42 and BPF 44 is connected to the first end of capacitors 45 and 46 and received from TIA 81 without passing through capacitors 45 and 46. A configuration in which a component outside a predetermined frequency band is attenuated among the frequency components of the electric signal may be employed. Further, in FIG. 19 in the second embodiment, the FFT processing unit 53 is connected to the first ends of the capacitors 45 and 46, and performs FFT on the electric signal received from the TIA 81 without passing through the capacitors 45 and 46. The structure which performs a calculation may be sufficient.

  Further, since the LIA 82 outputs the voltage level without restriction when the voltage level of the input signal is low, when the output voltage level of the TIA 81 is low in the optical communication module 101 or 102, at least one of the BPF 42 and the BPF 44 One of the components may be connected to the output terminal of the LIA 82 and attenuate a component outside a predetermined frequency band among the frequency components of the electrical signal received from the LIA 82.

  Further, at least one of the BPF 42 and the BPF 44 may be configured to receive only one of the differential signals from the TIA 81 or the LIA 82.

  Further, at least one of the BPF 42 and the BPF 44 may be configured to change the setting of the pass frequency band. This eliminates the need to replace the BPF even when using light receiving elements having different GB products.

  In each of the above embodiments, the ONU 202 including the optical communication module 101 has been exemplified. However, the present invention is widely applicable to optical communication apparatuses. Moreover, the present invention is suitable for use in an optical communication apparatus that receives continuous signals. In particular, it is more effective when used in a home-side device in a PON system, in which many are manufactured according to the same specifications at the time of manufacture, and in actual use, the multiplication factor of the light receiving element will be different depending on the environment of the installation location. It is.

  The present invention can also be used in a communication device that receives a burst signal. In this case, for example, the CPU 12 validates the feedback control of the multiplication factor M by the voltage Vf2 or the like during the burst signal reception period, and invalidates the feedback control outside the reception period.

  The above embodiment should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  The above description includes the following features.

[Appendix 1]
A light receiving element that outputs a current according to the intensity of the received optical signal and can change the multiplication factor;
A reference creation unit for creating a comparison reference from an electrical signal based on an output current of the light receiving element;
A target creation unit for creating a comparison target that includes more frequency components of the electrical signal than the comparison reference in a region where the cutoff frequency of the electrical signal is approximately inversely proportional to the multiplication factor of the light receiving element;
An operation unit that relatively compares the size of the comparison reference and the size of the comparison target;
An adjustment unit for adjusting a multiplication factor of the light receiving element based on a comparison result by the calculation unit;
The light receiving element is an avalanche photodiode;
An optical communication module used in a PON system.

[Appendix 2]
An optical communication device for transmitting / receiving an optical signal to / from another optical communication device,
A light receiving element that outputs a current according to the intensity of the received optical signal and can change the multiplication factor;
A reference creation unit for creating a comparison reference from an electrical signal based on an output current of the light receiving element;
A target creation unit for creating a comparison target that includes more frequency components of the electrical signal than the comparison reference in a region where the cutoff frequency of the electrical signal is approximately inversely proportional to the multiplication factor of the light receiving element;
An operation unit that relatively compares the size of the comparison reference and the size of the comparison target;
An adjustment unit for adjusting a multiplication factor of the light receiving element based on a comparison result by the calculation unit;
The light receiving element is an avalanche photodiode;
An optical communication device used in a PON system.

[Appendix 3]
An optical communication method in an optical communication module that outputs a current according to the intensity of a received optical signal and includes a light receiving element capable of changing a multiplication factor,
Creating a comparison reference from an electrical signal based on the output current of the light receiving element;
Creating a comparison target that includes more frequency components of the electrical signal than the comparison reference in a region where the cutoff frequency of the electrical signal is approximately inversely proportional to the multiplication factor of the light receiving element;
Relatively comparing the size of the comparison reference with the size of the comparison target;
Adjusting the multiplication factor of the light receiving element based on the comparison result,
The light receiving element is an avalanche photodiode;
An optical communication method used in a PON system.

11 DC / DC converter (reference creation part)
12 CPU (calculation unit and adjustment unit)
14, 15, 28-30 Resistor 21 Differential amplifier 22 Comparator 23 PWM control circuit 24 N-channel MOSFET
25 Inductor 26 Schottky Diode 27 Current Mirror Circuit 31-34 Capacitor 35 Filter Circuit 36 Feedback Limiting Unit 41, 43 Amplitude Detection Circuit 42, 44 BPF
45, 46 Capacitor 51 Reference creation unit 52 Target creation unit 53 FFT processing unit (reference creation unit and target creation unit)
81 TIA
82 LIA
73, 83 Bias control unit 85 Output buffer 86 Preamplifier 87 Output buffer circuit (modulation current supply circuit)
88 Bias current supply circuit 89 Light emission circuit 92 PON reception processing unit 93 Buffer memory 94 UN transmission processing unit 95 UNI port 96 UN reception processing unit 97 Buffer memory 98 PON transmission processing unit 99 Control unit 101, 102 Optical communication module 151 Burst transmission unit 152 Receiver 201 Station side device 202A, 202B, 202C ONU
301 PON system SP splitter OPTF optical fiber LD light emitting element L1, L2 inductor PD light receiving element

Claims (6)

A light receiving element that outputs a current according to the intensity of the received optical signal and can change the multiplication factor;
A reference creation unit for creating a comparison reference from an electrical signal based on an output current of the light receiving element;
A target creation unit for creating a comparison target that includes more frequency components of the electrical signal than the comparison reference in a region where the cutoff frequency of the electrical signal is approximately inversely proportional to the multiplication factor of the light receiving element;
An operation unit that relatively compares the size of the comparison reference and the size of the comparison target;
An optical communication module comprising: an adjustment unit that adjusts a multiplication factor of the light receiving element based on a comparison result by the arithmetic unit. The comparison criteria are:
The frequency component of the electrical signal in a region where the frequency component of the electrical signal is substantially constant with respect to a change in the multiplication factor of the light receiving element,
DC component of the electrical signal, or
The optical communication module according to claim 1, wherein all frequency components of the electrical signal are present. The adjustment unit adjusts the multiplication factor so that the comparison result becomes a predetermined value,
The optical communication module according to claim 1, wherein the predetermined value is set based on a target value of a multiplication factor of the light receiving element. The optical communication module further includes:
The optical communication module according to claim 1, further comprising a limiting unit that limits the adjustment of the multiplication factor by the adjusting unit. An optical communication device for transmitting / receiving an optical signal to / from another optical communication device,
A light receiving element that outputs a current according to the intensity of the received optical signal and can change the multiplication factor;
A reference creation unit for creating a comparison reference from an electrical signal based on an output current of the light receiving element;
A target creation unit for creating a comparison target that includes more frequency components of the electrical signal than the comparison reference in a region where the cutoff frequency of the electrical signal is approximately inversely proportional to the multiplication factor of the light receiving element;
An operation unit that relatively compares the size of the comparison reference and the size of the comparison target;
An optical communication device comprising: an adjustment unit that adjusts a multiplication factor of the light receiving element based on a comparison result by the arithmetic unit. An optical communication method in an optical communication module that outputs a current according to the intensity of a received optical signal and includes a light receiving element capable of changing a multiplication factor,
Creating a comparison reference from an electrical signal based on the output current of the light receiving element;
Creating a comparison target that includes more frequency components of the electrical signal than the comparison reference in a region where the cutoff frequency of the electrical signal is approximately inversely proportional to the multiplication factor of the light receiving element;
Relatively comparing the size of the comparison reference with the size of the comparison target;
Adjusting the multiplication factor of the light receiving element based on the comparison result.

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