JP2007158774A - Optical receiver and optical transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical receiver which can easily perform distortion compensation, be made compact and attain low power consumption and to provide an optical transmission system. <P>SOLUTION: A fixed high voltage generation circuit 16 applies fixed voltage to both ends of an APD (Avalanche Photo Diode) 11. A PIN diode 12 is connected as a load of the APD 11. When the APD 11 receives an optical signal, current is made to flow in a resistance 13 and the APD 11. A current detection circuit 17 detects the current flowing in the resistance 13 and supplies current based on the detected current to the PIN diode 12. Thus, the level (output voltage) of a high frequency signal of an output terminal NO is made to remain constant regardless of light receiving power. Since the optical signal is multiplied at a fixed multiplication factor M by the APD 11, the nonlinearity of distortion is also made to remain constant. Current made to flow in a distortion compensation diode 24 is adjusted by adjusting a variable resistance 23 in a distortion compensation circuit 22. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical receiver that receives an optical signal and an optical transmission system including the optical receiver.

  In an optical transmission system such as a system using a media converter or a G-PON (Gigabit Passive Optical Network) system, a signal to be transmitted and received is a complete PCM (pulse code modulation) signal (“0” or “1”). )) Data stream. It is desirable that the optical receiver in such an optical transmission system has a wide band and low noise characteristics. Therefore, an avalanche photodiode (hereinafter referred to as APD) having a low noise characteristic and a high multiplication factor is used in an optical receiver having a wide received light dynamic range (see, for example, Patent Document 1).

  FIG. 18 is a block diagram showing an example of an optical receiver in a conventional digital optical transmission system. The optical receiver of FIG. 18 is used as a termination device of a digital PCM (pulse code modulation) baseband system.

  18 includes an APD 71, an impedance matching circuit 72, amplifiers 73 and 74, a variable attenuation circuit 75, a distributor 76, a detection circuit 77, an automatic gain control circuit (hereinafter referred to as an AGC circuit) 78, and a high voltage control. The circuit 79 is configured.

  A reverse bias voltage is applied to the APD 71 by the high voltage control circuit 79. When the APD 71 receives a digitally modulated optical signal, a high-frequency photocurrent flows through the APD 71. The impedance matching circuit 72 performs impedance matching between the APD 71 and the amplifier 73. The amplifier 73 amplifies the high frequency signal output from the impedance matching circuit 72.

  The variable attenuation circuit 75 controls the level of the high frequency signal output from the amplifier 73. The amplifier 74 amplifies the high frequency signal output from the variable attenuation circuit 75. The distributor 76 distributes the high-frequency signal RF output from the amplifier 74 to the output terminal NO and the detection circuit 77.

  The detection circuit 77 detects the high frequency signal RF. The output signal of the detection circuit 77 is given to the AGC circuit 78 as a control voltage. The AGC circuit 78 controls the gain of the variable attenuation circuit 75 based on the control voltage and also controls the output voltage of the high voltage control circuit 79. Thereby, the multiplication factor of the APD 71 is controlled. In this way, the level of the high-frequency signal RF output from the output terminal NO is controlled to be constant.

  The high-frequency signal RF output from the output terminal NO is given to the subsequent clock recovery circuit and digital processing system.

  In an access network system, a conventional terminal network constructed with a local area network (LAN) cable, a coaxial cable for cable television, an ISDN (integrated service digital network) cable for telephone lines or an ADSL (asymmetric digital subscriber line) cable, etc. The movement that is replaced by optical fiber is accelerating. In the optical termination device used in such an optical transmission system, it is desired to reduce the price, the size, and the power consumption.

  On the other hand, the transition from analog broadcasting to terrestrial digital broadcasting eases the requirement for extremely severe distortion characteristics in transmission systems. In addition, in the video distribution of FTTH (Fiber to the Home) service, the demand for distortion characteristics is eased.

  However, even if there is a complete transition from analog broadcasting to digital broadcasting, an analog modulation method is used, so that a certain degree of good distortion characteristics is necessary as an absolute condition. This also requires good distortion characteristics in a system where analog signals and digital signals are mixed.

  In an optical receiver of a video distribution system that receives a multi-wave signal of AM (amplitude modulation) or QAM (quadrature amplitude modulation), distortion becomes a fatal problem, and a PIN having good distortion characteristics as a light receiving element. A photodiode (hereinafter referred to as PD) is used, and an APD having poor linearity is not used.

  FIG. 19 is a block diagram showing a first example of an optical receiver in the conventional analog optical transmission system, and FIG. 20 is a block diagram showing a second example of the optical receiver in the conventional analog optical transmission system. 19 and 20 is used as, for example, an optical termination device of an analog modulation SCM (video distribution) system.

  19 includes a PD 51, an impedance matching circuit 52, amplifiers 53 and 54, a variable attenuation circuit 55, a distributor 56, a DC / DC (direct current / direct current) converter 57, and a band-pass filter (hereinafter referred to as BPF). ) 58, a detection circuit 59 and an automatic gain control circuit (hereinafter referred to as an AGC circuit) 60. Each of the amplifiers 53 and 54 is made of GaAs or Si—Ge, and the variable attenuation circuit 55 includes 3 to 4 PIN diodes or FETs (field effect transistors).

  When the PD 51 receives an analog-modulated high-frequency optical signal, a high-frequency photocurrent flows through the PD 51. The impedance matching circuit 52 performs impedance matching between the PD 51 and the amplifier 53. The amplifier 53 amplifies the high-frequency photocurrent output from the impedance matching circuit 52 and converts it into a voltage. As a result, a high frequency signal is output from the amplifier 53. The variable attenuation circuit 55 controls the level of the high frequency signal output from the amplifier 53. The amplifier 54 amplifies the high frequency signal output from the variable attenuation circuit 55. The distributor 56 supplies the high frequency signal RF output from the amplifier 54 to the output terminal NO and distributes it to the BPF 58.

  The DC / DC converter 57 converts an external direct current voltage superimposed on an RF (high frequency) cable into a direct current voltage Vcc required by the optical receiver. The DC voltage Vcc is applied as a reverse bias voltage to the PD 51 or is applied to each circuit.

  On the other hand, the BPF 58 allows a frequency component in a predetermined band of the high frequency signal RF to pass. The detection circuit 59 detects the frequency component (pilot signal; usually determined between 40 and 70 MHz) that has passed through the BPF 58. The output signal of the detection circuit 59 is given to the AGC circuit 60 as a control voltage. The AGC circuit 60 controls the gain of the variable attenuation circuit 55 based on the control voltage.

  In this way, the level of the high-frequency signal RF output from the output terminal NO is feedback-controlled at a constant level.

  The optical receiver 6 in FIG. 20 differs from the optical receiver 5 in FIG. 19 in that the distributor 56, the BPF 58, and the detection circuit 59 in FIG. 19 are not provided, but the photocurrent detection circuit 61 is provided.

  In the optical receiver 6 of FIG. 20, the operations of the PD 51, the impedance matching circuit 52, the amplifier 53, the variable attenuation circuit 55, and the DC / DC converter 57 are the same as the operations of the corresponding parts of the optical receiver 5 of FIG. is there.

  The amplifier 54 amplifies the high frequency signal output from the variable attenuation circuit 55. The high frequency signal RF output from the amplifier 54 is applied to the output terminal NO.

  The photocurrent detection circuit 61 detects a high-frequency photocurrent flowing through the PD 51 and supplies a control voltage corresponding to the photocurrent to the AGC circuit 60. The AGC circuit 60 controls the gain of the variable attenuation circuit 55 based on the control voltage.

In this manner, the level of the high frequency signal RF output from the output terminal NO is feedforward controlled to be constant.
JP 2003-101356 A

  However, in the optical receivers 5 and 6, the amplifiers 53 and 54 having low distortion characteristics are used, so that relatively large power is consumed.

The optical receivers 5 and 6 are composed of a PD 51, amplifiers 53 and 54, and a low frequency IC (integrated circuit) constituting a power supply system and a control system. In this case, when the total power consumption of the optical receivers 5 and 6 is 3 to 5 W, the exclusive area is about 50 to 100 cm ² .

  Furthermore, in the optical receivers 5 and 6, each component must be arranged so as to have a certain heat dissipation effect from the power consumption and the circuit scale. Therefore, a large area is required.

  Further, the APD used in the digital optical receiver has a characteristic that the multiplication factor is not proportional to the applied voltage. As described above, since the analog optical receiver is required to have good distortion characteristics, it is difficult to use APD instead of the PD 51 of the conventional analog optical receivers 5 and 6.

  An object of the present invention is to provide an optical receiver and an optical transmission system that can easily perform distortion compensation and can be reduced in size and power consumption.

  (1) The optical receiver according to the first invention is supplied with an avalanche photodiode that receives an optical signal, a voltage application circuit that applies a constant reverse bias voltage to the avalanche photodiode, and a current that flows through the avalanche photodiode, A load element having a variable resistance value, an output terminal electrically connected to one end of the load element, and a current flowing through the avalanche photodiode are detected, and the output power of the output terminal becomes constant based on the detected current Thus, a control circuit for controlling the resistance value of the load element is provided.

  In the optical receiver according to the present invention, a constant reverse bias voltage is applied to the avalanche photodiode by the voltage application circuit. When the avalanche photodiode receives an optical signal, a photocurrent flows through the avalanche photodiode. The current flowing through the avalanche photodiode is supplied to a load element having a variable resistance value. Thereby, the voltage of the output terminal electrically connected to one end of the load element changes according to the change of the optical signal.

  Further, the current flowing through the avalanche photodiode is detected by the control circuit, and the resistance value of the load element is controlled based on the detected current so that the output power at the output terminal becomes constant. In this way, automatic gain control is performed in the optical receiver.

  In this case, since a constant reverse bias voltage is applied to the avalanche photodiode, the average multiplication factor of the avalanche photodiode is constant. Thereby, the nonlinearity between the applied voltage and the multiplication factor in the avalanche photodiode becomes constant. Therefore, it is possible to easily compensate for distortion of the output signal due to the non-linearity of the avalanche photodiode.

  In addition, since photoelectric conversion and automatic gain control can be performed with a small number of components, the optical receiver can be downsized. Furthermore, since a plurality of amplifiers having low distortion characteristics are not required, power consumption can be reduced.

  (2) The load element includes a variable resistance element having a characteristic that the resistance value decreases as the current increases, and the control circuit detects the current flowing through the avalanche photodiode and changes according to the detected change in the current. A current may be supplied to the variable resistance element.

  In this case, when the photocurrent flowing through the avalanche photodiode increases due to the increase in the received light power, the control circuit increases the current supplied to the variable resistance element. As a result, the voltage at the output terminal connected to one end of the variable resistance element decreases. Conversely, when the photocurrent flowing through the avalanche photodiode decreases due to the decrease in the received light power, the control circuit decreases the current supplied to the variable resistance element. Thereby, the voltage of the output terminal connected to one end of the variable resistance element rises. In this way, the average voltage at the output terminal is kept constant. As a result, even if the received light power changes, the output power is kept constant.

  (3) The variable resistance element is a PIN diode or a field effect transistor (FET), and the control circuit detects a current flowing through the avalanche photodiode, and changes the current that changes according to the detected change in the PIN diode or the electric field. You may supply to an effect transistor.

  A PIN diode or a field effect transistor has a characteristic that the resistance value decreases as the current increases. Therefore, when the photocurrent flowing through the avalanche photodiode increases due to the increase in the received light power, the control circuit increases the current supplied to the PIN diode or the field effect transistor. As a result, the voltage at the output terminal connected to one end of the PIN diode or field effect transistor decreases. Conversely, when the photocurrent flowing through the avalanche photodiode decreases due to the decrease in the received light power, the control circuit decreases the current supplied to the PIN diode or the field effect transistor. Thereby, the voltage of the output terminal connected to one end of the PIN diode or the field effect transistor increases. In this way, the average voltage at the output terminal is kept constant. As a result, even if the received light power changes, the output power is kept constant.

  (4) The avalanche photodiode has nonlinearity in which the multiplication factor is not proportional to the applied voltage, and the optical receiver compensates for distortion of the output signal at the output terminal by compensating for the nonlinearity of the avalanche photodiode. A distortion compensation circuit may be further provided.

  In this case, since a constant reverse bias voltage is applied to the avalanche photodiode, the average multiplication factor of the avalanche photodiode is constant. Thereby, the non-linear characteristic between the applied voltage and the multiplication factor in the avalanche photodiode becomes constant. Therefore, distortion of the output signal due to the non-linearity of the avalanche photodiode can be easily compensated by the distortion compensation circuit.

  (5) The avalanche photodiode has linearity in which the multiplication factor is substantially proportional to the applied voltage in a specific voltage range, and the voltage application circuit has a constant reverse bias voltage within the specific voltage range. May be applied.

  In this case, the multiplication factor is substantially proportional to the applied voltage in a specific voltage range. Therefore, distortion of the output signal is suppressed by applying a constant reverse bias voltage within a specific voltage range to the avalanche photodiode. Therefore, since the distortion compensation circuit is not necessary, the optical receiver can be further downsized.

  (6) The avalanche photodiode may include a multiplication layer having a superlattice structure.

  In this case, since the multiplication layer having the superlattice structure has low noise characteristics, the noise of the optical receiving device can be reduced.

  (7) The superlattice multiplication layer alternately includes a first semiconductor layer having a first band gap and a second semiconductor layer having a second band gap larger than the first band gap. The difference between the energy levels at the uppermost ends of the valence bands of the first and second semiconductor layers is smaller than the energy level difference between the lowermost ends of the conduction bands of the first and second semiconductor layers, and the first and second semiconductor layers And the voltage application circuit may apply a constant reverse bias voltage such that an electric field of 300 kV / cm or more is applied to the avalanche photodiode.

  In this case, the band discontinuity of the valence band of the superlattice multiplication layer is smaller than the band discontinuity of the conduction band. Thereby, the multiplication factor of electrons becomes larger than the multiplication factor of holes. As a result, the multiplication factor in the avalanche photodiode is substantially proportional to the electric field strength in a specific electric field strength range. Further, when the energy difference between the uppermost ends of the valence bands of the first and second semiconductor layers is 10 meV or less, the linearity of the multiplication factor is obtained in the range of a high electric field strength of 300 kV / cm or more.

  Therefore, an output signal with high gain and low distortion can be obtained in the optical receiver.

  (8) The optical receiver may further include an impedance matching circuit that matches the impedance of the avalanche photodiode and the impedance of the output terminal.

  In this case, gain reduction due to impedance mismatch between the avalanche photodiode and the output terminal is prevented.

  (9) The optical receiver further includes a solar battery, a secondary battery, and a charging circuit that charges the secondary battery with the power generated by the solar battery, and the voltage application circuit receives power from the secondary battery. May be.

  In this case, since the optical receiver can reduce power consumption, it can be operated by a secondary battery that is charged by the power of the solar battery. Therefore, a non-powered optical receiver is realized.

  (10) The optical signal may be an analog modulated high frequency signal. In this case, an analog-modulated high-frequency signal can be received by a small and low power consumption optical receiver, and distortion of the high-frequency signal can be easily compensated.

  (11) An optical transmission system according to a second invention includes an optical transmission device that transmits an optical signal, and an optical reception device that receives the optical signal transmitted by the optical transmission device. Receiving an avalanche photodiode, a voltage applying circuit for applying a constant reverse bias voltage to the avalanche photodiode, a current flowing through the avalanche photodiode is supplied via an output node, and a load element having a variable resistance value, and a load element A control that detects the current flowing through the output terminal electrically connected to one end of the output terminal and the avalanche photodiode, and controls the resistance value of the load element so that the output power of the output terminal becomes constant based on the detected current The avalanche photodiode has a non-linearity in which the multiplication factor is not proportional to the applied voltage. A signal source that generates distortion, a distortion component generation unit that generates a distortion component for compensation to compensate for distortion caused by the nonlinearity of the avalanche photodiode of the optical receiver, and a distortion in the electrical signal generated by the signal source It includes a combining unit that combines the compensation distortion components generated by the component generating unit, and a photoelectric conversion unit that converts an electrical signal output from the combining unit into an optical signal.

  In the optical transmission system according to the present invention, an optical signal is transmitted by the optical transmitter, and the transmitted optical signal is received by the optical receiver.

  In the optical receiver, a constant reverse bias voltage is applied to the avalanche photodiode by the voltage application circuit. When the avalanche photodiode receives an optical signal, a photocurrent flows through the avalanche photodiode. The current flowing through the avalanche photodiode is supplied to a load element having a variable resistance value. Thereby, the voltage of the output terminal electrically connected to one end of the load element changes according to the change of the optical signal.

  Further, the current flowing through the avalanche photodiode is detected by the control circuit, and the resistance value of the load element is controlled based on the detected current so that the output power at the output terminal becomes constant. In this way, automatic gain control is performed in the optical receiver.

  In this case, since a constant reverse bias voltage is applied to the avalanche photodiode, the average multiplication factor of the avalanche photodiode is constant. Thereby, the nonlinearity between the applied voltage and the multiplication factor in the avalanche photodiode becomes constant.

  On the other hand, in an optical transmission device, an electric signal is generated by a signal source. A distortion component for compensating for distortion caused by the non-linearity of the avalanche photodiode of the optical receiver is created by the distortion component creation unit. The compensation distortion component created by the distortion component creation unit is synthesized by the synthesis unit with the electrical signal generated by the signal source. Furthermore, the electrical signal output from the combining unit is converted into an optical signal by the photoelectric conversion unit.

  Therefore, when the optical signal transmitted by the optical transmitter is received by the optical receiver, the compensation distortion component combined with the optical signal is canceled by the distortion caused by the non-linearity of the avalanche photodiode. As a result, an output signal whose distortion is compensated for at the output terminal is obtained.

  In addition, since photoelectric conversion and automatic gain control can be performed with a small number of components, the optical receiver can be downsized. Furthermore, since a plurality of amplifiers having low distortion characteristics are not required, power consumption can be reduced.

  (12) The distortion component creation unit is obtained by a light emitting element that converts an electric signal from a signal source into an optical signal, an avalanche photodiode that converts an optical signal obtained by the light emitting element into an electric signal, and an avalanche photodiode. And a difference calculation unit that outputs a difference between the electric signal from the signal source and the electric signal from the signal source as a compensation distortion component, and the combining unit outputs a compensation distortion component output from the difference calculation unit to the electric signal from the signal source. May be synthesized.

  In this case, an electrical signal from the signal source is converted into an optical signal by the light emitting element, and the optical signal is converted into an electrical signal by the avalanche photodiode. This electrical signal includes distortion due to the non-linearity of the avalanche photodiode. Therefore, the difference between the electrical signal obtained by the avalanche photodiode and the electrical signal from the signal source is output as a compensation distortion component by the difference calculation unit. Further, the distortion component for compensation is synthesized with the electrical signal from the signal source by the synthesis unit.

  Thereby, when the optical signal transmitted by the optical transmission device is received by the optical reception device, the compensation distortion component combined with the optical signal is canceled by the distortion caused by the non-linearity of the avalanche photodiode.

  (13) The electrical signal may be an analog modulated high frequency signal.

  In this case, an analog-modulated optical signal including a compensation distortion component is transmitted from the optical transmission device. When the optical signal is received by the optical receiver, the compensation distortion component included in the optical signal is canceled by the distortion caused by the non-linearity of the avalanche photodiode. As a result, an analog-modulated high-frequency signal having no distortion at the output terminal is obtained.

  According to the present invention, the current flowing through the avalanche photodiode is detected by the control circuit, and the resistance value of the load element is controlled based on the detected current so that the output power of the output terminal becomes constant. In this way, automatic gain control is performed in the optical receiver.

  In this case, since a constant reverse bias voltage is applied to the avalanche photodiode, the average multiplication factor of the avalanche photodiode is constant. Thereby, the nonlinearity between the applied voltage and the multiplication factor in the avalanche photodiode becomes constant. Therefore, it is possible to easily compensate for distortion of the output signal due to the non-linearity of the avalanche photodiode.

  In addition, since photoelectric conversion and automatic gain control can be performed with a small number of components, the optical receiver can be downsized. Furthermore, since a plurality of amplifiers having low distortion characteristics are not required, power consumption can be reduced.

(1) First Embodiment (1-1) Configuration of Optical Receiver of First Embodiment FIG. 1 is a circuit diagram showing the configuration of an optical receiver according to the first embodiment of the present invention. is there.

  1 includes an avalanche photodiode (hereinafter referred to as APD) 11, a PIN diode 12, a resistor 13, an inductor 14, a transformer 15, a fixed high voltage generation circuit 16, a current detection circuit 17, and capacitors 18 to 21. And a distortion compensation circuit 22.

  The resistor 13 is connected between the node N1 and the node N2. The cathode of the APD 11 is connected to the node N2, and the anode of the APD 11 is connected to the node N3. The APD 11 receives the modulated optical signal. The APD 11 has a multiplication layer formed by, for example, a single layer of non-doped InGaAsP.

  The anode of the PIN diode 12 is connected to the node N3, and the cathode is connected to the ground terminal. The PIN diode 12 has a characteristic that the resistance changes depending on the current.

  The inductor 14 is connected between the node N3 and the node N4. This inductor 14 blocks the passage of high frequency components. The capacitor 18 is connected between the node N2 and the ground terminal, the capacitor 19 is connected between the node N3 and one end of the transformer 15, and the other end of the transformer 15 is connected to the ground terminal. The capacitor 20 is connected between the node N4 and the ground terminal.

  The fixed high voltage generation circuit 16 has output terminals 161, 162, and 163. The fixed high voltage generation circuit 16 has an output terminal 161 connected to the node N4, an output terminal 162 connected to the node N1, and an output terminal 163 connected to the node N2. The fixed high voltage generation circuit 16 controls the voltage of the output terminal 162 so that a constant voltage is applied between the nodes N2 and N4. As a result, a constant reverse bias voltage is applied to the APD 11.

  Current detection circuit 17 detects a current flowing through resistor 13 between nodes N1 and N2, and outputs a current based on the detected current to node N4.

  The intermediate tap of the transformer 15 is connected to the node N5, and the capacitor 21 is connected between the node N5 and the output terminal NO. A high frequency signal RF is output from the output terminal NO.

  The distortion compensation circuit 22 includes a variable resistor 23, a distortion compensation diode 24, and a capacitor 25. The variable resistor 23 is connected between the bias terminal NB that receives the bias voltage VB and the anode of the distortion compensation diode 24, and the cathode of the distortion compensation diode 24 is connected to the node N5. The capacitor 25 is connected between the bias terminal NB and the ground terminal. The distortion compensation diode 24 is composed of, for example, a Schottky barrier diode.

  In the optical receiver 1 of FIG. 1, the voltage level of the high-frequency signal RF can be controlled to be constant regardless of the level of the optical signal, and the distortion characteristics resulting from the non-linearity of the APD 11 can be kept constant. Accordingly, the distortion of the high frequency signal RF caused by the non-linearity of the APD 11 can be compensated by the distortion compensation circuit 22. Detailed operation will be described later.

(1-2) APD Characteristics Here, APD characteristics will be described. FIG. 2 is a diagram showing the relationship between the voltage applied to both ends of the APD and the multiplication factor of the APD.

  The horizontal axis in FIG. 2 indicates the voltage applied to both ends of the APD (hereinafter referred to as applied voltage VA), and the vertical axis indicates the multiplication factor M of the APD.

  As shown in FIG. 2, the APD multiplication factor M increases nonlinearly as the applied voltage VA increases. Thereby, when the multiplication factor M changes due to the change of the applied voltage VA, the level of distortion changes.

(1-3) Principle of distortion generation in APD FIG. 3 is a circuit diagram for explaining the principle of distortion generation when an APD receives an optical signal whose intensity is modulated.

  In FIG. 3, the cathode of APD 101 is connected to the positive electrode of DC power supply 103, and the anode of APD 101 is connected to node N30. The negative electrode of the DC power supply 103 is connected to the ground terminal. The resistor 102 is connected between the node N30 and the ground terminal. Node N30 is connected to output terminal N31 via capacitor 104.

  A DC voltage Vdc of the DC power supply 103 is applied to both ends of the APD 101 as a reverse bias voltage. When the APD 101 does not receive an optical signal, no current flows through the APD 101. When the APD 101 receives an optical signal, a photocurrent flows through the APD 11. In this case, the APD 101 generates a high-frequency voltage Vrf corresponding to the photocurrent. Thereby, a DC voltage Vdc- (R · Idc) on which the high-frequency voltage Vrf is superimposed is applied to both ends of the APD 101. Here, R represents a resistance value of the resistor 102, and Idc represents a direct current flowing through the APD 101 and the resistor 102. As a result, the multiplication factor M of the APD 101 changes, and the level of distortion that appears in the high-frequency signal RF at the output terminal N31 changes. Note that the average value of the voltages applied to both ends of the APD 101 is the DC voltage Vdc− (R · Idc).

  FIG. 4 is a waveform diagram for explaining distortion appearing in a sine wave in the circuit of FIG. 4A shows a waveform of a sinusoidal high-frequency voltage Vrf corresponding to the optical signal, and FIG. 4B shows a waveform of a high-frequency signal RF having distortion.

  As described above, a reverse bias voltage is applied to the APD 101 by the DC power supply 103. Therefore, when the level of the high-frequency voltage Vrf in FIG. 4A is high, the voltage applied to the APD 101 is low, and the multiplication factor M of the APD 101 is low. Thereby, as shown in FIG. 4B, the gain of the high-frequency signal RF is lowered. Conversely, when the level of the high-frequency voltage Vrf in FIG. 4A is low, the voltage applied to the APD 101 increases, and the multiplication factor M of the APD 101 increases. Thereby, as shown in FIG. 4B, the gain of the high-frequency signal RF is increased.

  In this way, amplitude modulation is applied to the high-frequency signal RF, and distortion is generated as can be seen from the characteristics of FIG. Since there is a large difference between the influence of the high frequency voltage Vrf when the voltage applied to the APD 101 is high and the influence of the high frequency voltage Vrf when the voltage applied to the APD 101 is low, when the voltage applied to the APD 101 changes, The distortion of the high-frequency signal RF cannot be uniquely compensated. In other words, by making the multiplication factor M of the APD 101 constant, the distortion of the high-frequency signal RF can be compensated.

(1-4) Measurement of distortion characteristics of APD FIG. 5 is a diagram showing a measurement result of the relationship between the applied voltage of the APD and the secondary distortion, and FIG. 6 shows a measurement result of the relationship between the applied voltage of the APD and the third distortion. FIG. FIG. 7 is a diagram showing the measurement result of the relationship between the current flowing through the APD and the secondary distortion, and FIG.

  5 and 6 indicate the APD applied voltage Vapd, and the vertical axes indicate the carrier / spurious ratios of the second-order distortion IM2 and the third-order distortion IM3, respectively. 7 and 8, the vertical axis represents the current Iapd flowing through the APD, and the vertical axis represents the carrier / spurious ratio of the second-order distortion IM2 and the third-order distortion IM3, respectively. 5 to 8, Pr represents the received light power of the APD.

  As shown in FIGS. 6 and 8, the range of the applied voltage Vapd and the range of the current Iapd that become substantially constant when the third-order distortion IM3 is −60 dBc or less are large.

  On the other hand, as shown in FIG. 5, the secondary distortion IM2 becomes −60 dBc or less when the applied voltage Vapd is within a certain range. Further, as shown in FIG. 7, the value of the current Iapd at which the secondary distortion IM2 becomes −60 dBc or less differs for each received power. That is, it can be seen that the magnitude of the secondary distortion IM2 is not related to the current flowing through the APD, but is determined by the applied voltage Vapd.

  From these measurement results, the level of distortion can be made substantially constant by always keeping the DC voltage, which is the average of the voltages applied to both ends of the APD, constant. It has been found that the characteristic of the APD applied voltage Vapd versus the multiplication factor M is an EXP curve (exponential curve) both theoretically and experimentally. Therefore, the distortion characteristics of the APD can be compensated for by a distortion compensation circuit using a Schottky barrier diode having a forward voltage vs. forward current characteristic having a LOG curve (logarithmic curve).

(1-5) Operation of Optical Receiver 1 In the optical receiver 1 of FIG. 1, as described above, the fixed high voltage generation circuit 16 has the output terminal 162 so that a constant voltage is applied between the nodes N2 and N4. To control the voltage. As a result, a constant reverse bias voltage is applied across the APD 11, and the APD 11 functions as a constant current source.

  Further, a PIN diode 12 is connected as a load of the APD 11. In this case, the voltage level of the node N3 changes depending on the resistance value of the PIN diode 12.

  When the APD 11 receives the optical signal, a photocurrent is generated in the APD 11 and a current flows in the resistor 13 and the APD 11. Current detection circuit 17 detects a current flowing through resistor 13 and outputs a current based on the detected current to node N4.

  Specifically, when the current flowing through resistor 13 and APD 11 increases, current detection circuit 17 increases the current flowing through inductor 14 via node N4. As a result, the level of the DC component (average current) of the current flowing through the PIN diode 12 increases, and the average resistance of the PIN diode 12 decreases. As a result, the RF voltage level (high frequency voltage level) of the node N3 is also lowered.

  On the contrary, when the current flowing through the resistor 13 and the APD 11 decreases, the current detection circuit 17 decreases the current flowing through the inductor 14 via the node N4. As a result, the level of the direct current component (average current) of the current flowing through the PIN diode 12 decreases, and the average resistance of the PIN diode 12 increases. As a result, the RF voltage level at the node N3 increases.

  In this way, the level (output power) of the high-frequency signal at the node N3 is kept constant regardless of the received light power. In this case, since the optical signal is multiplied at a constant multiplication factor M by the APD 11, the nonlinearity of the distortion is also kept constant.

  Therefore, the current flowing through the distortion compensation diode 24 is adjusted by adjusting the variable resistor 23 of the distortion compensation circuit 22. Thereby, the distortion of the high-frequency signal at the node N3 can be compensated. Therefore, the high-frequency signal RF at the output terminal NO has a certain level without depending on the received light power, and distortion is suppressed.

  In addition, the voltage of both ends of APD11 is 30-50V, for example, and the electric current which flows into APD11 is 0.1-0.3mA, for example. The voltage across the PIN diode 12 is, for example, 1 to 2 V, and the current flowing through the PIN diode 12 is, for example, 1 to 5 mA. The voltage across the distortion compensation diode 24 is, for example, 0.5 to 1.0 V, and the current flowing through the distortion compensation diode 24 is, for example, 0.1 to 1 mA. In this case, the power consumed by the optical receiver 1 is about 4 to 160 mW. By keeping the range of the received light power low, the power consumed by the optical receiver 1 can be further reduced.

(1-6) Voltage Level in Optical Receiver 1 Next, the RF voltage level of each part in the optical receiver 1 of FIG. 1 is compared with the RF voltage level of each part in the conventional optical receiver 5 of FIG. 19 as a comparative example. I will explain.

  FIG. 9 is a diagram showing the RF voltage level of each part in the optical receiver 5 of the comparative example. FIG. 10 is a diagram showing the RF voltage level of each part in the optical receiver 1 of the present embodiment.

  9 and 10 show the components of the main part of the optical receivers 1 and 5 of the comparative example and the embodiment. In the optical receivers of FIGS. 9 and 10, bandpass filters (hereinafter referred to as BPF) 61 and 29 are inserted in front of the output terminal NO.

  Consider a case where a 40 to 100 channel SCM (Sub Carrier Multiplexing) signal is transmitted in an optical transmission system. From the distortion characteristics and CNR (Carrier to Noise Ratio) characteristics, the range of received power is about 0 to -7 dBm, and the ratio of modulated waves per wave is about 2.5 to 4.0%. .

  On the other hand, the voltage level per wave of the high frequency signal required on the receiving side (for example, the voltage level required for the high frequency signal input to the home STB (set top box)) is 65 to 80 dBμV.

  As shown in FIG. 9, in the optical receiver 5 of the comparative example, the gain of the amplifier 53 at the front stage is 12 dB, the loss at the variable attenuating circuit 55 is a minimum of 3 dB, and the gain of the amplifier 54 at the rear stage is 12 dB. . Therefore, the total gain of the optical receiver 5 of the comparative example is about 21 dB.

  As shown in FIG. 10, in the optical receiver 1 of the present embodiment, when the output level equivalent to that of the optical receiver 5 of FIG. 9 is obtained, the multiplication factor M required for the APD 11 is only 10-12. Even considering the loss in the PIN diode 12 functioning as a variable load circuit and the loss in the BPF 29, a sufficient multiplication function can be obtained by operating the APD 11 at a multiplication factor M of 20-30.

  Therefore, in the optical receiver 1 of the present embodiment, a sufficient multiplication function can be realized without requiring the amplifiers 53 and 54 and the distributor 56 in the optical receiver 5 of the comparative example.

(1-7) Effect of Distortion Compensation in Optical Receiver 1 FIG. 11 is a diagram showing measurement results of secondary distortion before distortion compensation by the distortion compensation circuit 22 in the optical receiver 1, and FIG. It is a figure which shows the measurement result of the secondary distortion after the distortion compensation by the compensation circuit. In FIG. 11, the horizontal axis represents frequency, and the vertical axis represents power.

  11 and 12 show the carrier CR and the composite second order distortion (CSO) IM2. The measurement conditions were a received light power of −5 dBm and an applied voltage of APD 11 of 50V. The carrier / spurious ratio before distortion compensation by the distortion compensation circuit 22 was −44 dBc. In contrast, the carrier / spurious ratio after distortion compensation by the distortion compensation circuit 22 was improved to -54 dBc.

(1-8) Effects of the First Embodiment In the optical receiver 1 according to the present embodiment, when the photocurrent flowing through the APD 11 increases due to the increase in the received light power, the current detection circuit 17 includes the PIN diode 12. Increase the current supplied to the. As a result, the voltage at the node N3 decreases. On the contrary, when the photocurrent flowing through the APD 11 decreases due to the decrease in the received light power, the current detection circuit 17 decreases the current supplied to the PIN diode 12. As a result, the voltage at the node N3 increases. In this way, the average voltage of the node N3 is kept constant. As a result, even if the received light power changes, the output power is kept constant.

  In this case, since a constant reverse bias voltage is applied to the APD 11 by the fixed high voltage generation circuit 16, the average multiplication factor of the APD 11 becomes constant. Thereby, the non-linearity between the applied voltage and the multiplication factor in the APD 11 becomes constant. Therefore, the distortion compensation circuit 22 can easily compensate for the distortion of the high-frequency signal caused by the non-linearity of the APD 11.

  Further, since photoelectric conversion and automatic gain control can be performed with few components, the optical receiver 1 can be downsized. Furthermore, since a plurality of amplifiers having low distortion characteristics are not required, power consumption can be reduced.

(2) Second Embodiment Hereinafter, an optical receiver according to a second embodiment of the present invention will be described. The second embodiment differs from the first embodiment in the following points.

  In the optical receiver according to the second embodiment, the distortion compensation circuit 22 of FIG. 1 is not provided, and a superlattice APD having the structure of FIG. 13 described below is used instead of the APD 11.

(2-1) Structure of APD FIG. 13 is a schematic cross-sectional view showing the structure of a superlattice APD used in the optical receiver according to the second embodiment.

In the superlattice APD 11a of FIG. 13, an n ⁺ -InP buffer layer 502 having a thickness of 100 nm and a non-doped In _0.52 Al _0.48 As / In _0.8 Ga _0.2 As _0.6 P _0.4 superlattice having a thickness of 400 nm are formed on an n ⁺ -InP substrate 501. A multiplication layer 503 is formed in order. On the superlattice multiplication layer 503, a p-InP layer (sheet dope layer) 504 with an impurity density of 8 × 10 ¹⁷ cm ⁻³ and a thickness of 16 nm, and a p− with an impurity density of 2 × 10 ¹⁵ cm ⁻³ and a thickness of 1 μm. An In _0.47 Ga _0.53 As light absorption layer 505 and a p ⁺ -In _0.47 Ga _0.53 As layer 506 having an impurity density of 2 × 10 ¹⁷ cm ⁻³ and a thickness of 50 nm are sequentially formed. p ⁺ -In _0.47 Ga _0.53 on the As layer 506, a thickness of 100nm in impurity concentration 1 × 10 ¹⁸ cm ^-3 with a thickness of 100nm by p-InP window layer 507, and the impurity density of 1 × 10 ¹⁸ cm ^-3 p ^{A +} -In _0.47 Ga _0.53 As contact layer 508 is sequentially formed.

An AuZnNi electrode 509 is formed on the contact layer 508, and an AuGeNi electrode 510 is formed on the n ⁺ -InP substrate 501.

In such a configuration, light incident on the superlattice APD 11a from the n ⁺ -InP substrate 501 side is absorbed by the p-In _0.47 Ga _0.53 As light absorption layer 505, and a pair of electrons and holes is generated.

  Electrons travel toward the superlattice multiplication layer 503 by a reverse bias voltage applied between the AuZnNi electrode 509 and the AuGeNi electrode 510 and are injected into the superlattice multiplication layer 503.

  Since the film thickness of the superlattice multiplication layer 503 is so small that the ionization rate of holes can be ignored, the electrons injected into the superlattice multiplication layer 503 are multiplied by injection of pure electrons without increasing the multiplication noise. Is realized.

  In FIG. 13, the APD 11 a having a mesa structure is shown for ease of explanation, but an APD having a planar structure may be used. A diffusion guard ring is provided in the periphery of the planar structure APD. Thereby, the reliability of the APD is improved.

A non-doped In _0.47 Ga _0.53 As / In _0.8 Ga _0.2 As _0.6 P _0.4 superlattice multiplication layer is used instead of the non-doped In _0.52 Al _0.48 As / In _0.8 Ga _0.2 As _0.6 P _0.4 superlattice multiplication layer 503 of the APD 11a. May be.

(2-2) Energy Band and Characteristics of Superlattice APD FIG. 14A is an energy band diagram of a general superlattice APD (first superlattice APD), and FIG. It is an energy band figure of the superlattice APD (2nd superlattice APD) used with the form of.

  FIG. 15 shows the characteristics of the general superlattice APD (first superlattice APD) in FIG. 14A and the superlattice APD (second superlattice APD) used in the second embodiment. FIG. The horizontal axis of FIG. 15 shows the electric field E applied to the superlattice APD, and the vertical axis shows the multiplication factor M of the superlattice APD on an exponential scale.

Hereinafter, the superlattice APD in FIG. 14A is referred to as a first superlattice APD, and the superlattice APD in FIG. 14B is referred to as a second superlattice APD. In the first superlattice APD, the superlattice multiplication layer of FIG. 13 is composed of non-doped In _0.47 Ga _0.53 As / In _0.8 Ga _0.2 As _0.4 P _0.6 .

  In the first superlattice APD of FIG. 14A, a multiplication layer is formed by a superlattice of InGaAsP and InP. The energy level of the conduction band Ec of InGaAsP is indicated by Ec1, and the energy level of the conduction band Ec of InP is indicated by Ec2. In addition, the energy level of the valence band Ev of InGaAsP is indicated by Ev1, and the energy level of the valence band Ev of InP is indicated by Ev2.

In the first superlattice APD, large band discontinuities (| Ec1-Ec2 | and | Ev1-Ev2 |) are formed in both the conduction band Ec and the valence band Ev. As a result, electrons and holes are amplified at substantially the same ratio, and the multiplication factor M increases with exp (exp (−10 ⁶ / E)) as shown in FIG.

  On the other hand, in the second superlattice APD of FIG. 14B, a multiplication layer is formed by the superlattice of InGaAsP and InAlAs. The energy level of the conduction band Ec of InGaAsP is indicated by Ec3, and the energy level of the conduction band Ec of InAlAs is indicated by Ec4. Further, the energy level of the valence band Ev of InGaAsP is indicated by Ev3, and the energy level of the valence band Ev of InAlAs is indicated by Ev4.

  In the second superlattice APD, since the multiplication layer is formed by the superlattice of InGaAsP and InAlAs, the band discontinuity (| Ec3-Ec4 |) of the conduction band Ec is large, and the band discontinuity of the valence band Ev is large. (| Ev3-Ev4 |) is smaller. As a result, holes are hardly multiplied and only electrons are multiplied.

In general, APD multiplication noise becomes smaller as the ratio between the ionization rate α of electrons and the ionization rate β of holes, which is a value inherent to a semiconductor used in the multiplication layer, becomes farther from 1. In the first and second superlattice APDs of FIG. 14, a superlattice structure is used in the multiplication layer for the purpose of increasing the ratio of ionization rate α / β or β / α. In particular, the In _0.52 Al _0.48 As / In _0.8 Ga _0.2 As _0.6 P _0.4 layer lattice-matched to the InP substrate has a large conduction band discontinuity, whereas the valence band discontinuity is zero. The ionization rate α increases, which is effective for reducing noise.

  As shown in FIG. 15, in the second superlattice APD, the characteristic curve of the multiplication factor M with respect to the electric field E changes to an S shape in the middle. When the slope of the characteristic curve is expressed as a derivative, there is a point where the slope changes from increasing to decreasing. It is conceivable that good linearity exists in the vicinity of this point.

  According to the study of the present inventor, when a superlattice multiplication layer made of InAlAs / InGaAsP is used, by increasing the hole multiplication factor M to 1.2 or less, the electric field E is increased in a region of 300 kV / cm or more. It was found that the magnification M was proportional to the electric field E. In FIG. 15, a region (straight region) where the multiplication factor M is proportional to the electric field E is indicated by a dotted line. In FIG. 15, since the vertical axis is represented by an exponent scale, the linear region is curved. However, when the value on the vertical axis is represented by a scale proportional to the multiplication factor M, the dotted region is a straight line. become. It was also found that the hole multiplication factor M can be made 1.2 or less by setting the band discontinuity of the valence band Ev to 10 meV or less.

  Thus, in the second superlattice APD, the multiplication factor M is proportional to the electric field E in the region where the electric field E is 300 kV / cm or more, and a multiplication factor M of 10 or more is obtained. That is, the multiplication factor M is proportional to the electric field E in a region where a large multiplication factor M is obtained. Therefore, by applying a constant voltage in the linear region to the second superlattice APD, light reception characteristics with high gain M and low distortion can be obtained with respect to the analog-modulated optical signal. As a result, the distortion compensation circuit 22 of FIG. 1 becomes unnecessary.

(2-3) Effects of Second Embodiment In the optical receiver according to the second embodiment, the superlattice APD 11a is used instead of the APD 11 of FIG. It becomes unnecessary. As a result, a high gain M and low distortion light receiving characteristic can be obtained for an optical signal that is analog-modulated with a small number of components.

Even when a non-doped In _0.47 Ga _0.53 As / In _0.8 Ga _0.2 As _0.6 P _0.4 superlattice multiplication layer is used, the band discontinuity of the valence band Ev becomes 10 meV or less due to the effect of lattice distortion. Was found to be obtained. Note that the structure and the composition of each layer used in the embodiment are not limited to these as long as they are based on the idea of the present invention.

(3) Third Embodiment Hereinafter, an optical transmission system according to a third embodiment of the present invention will be described. The optical transmission system according to the third embodiment includes an optical transmission device and an optical reception device.

  In the optical transmission system according to the third embodiment, the optical receiver is not provided with the distortion compensation circuit 22 of FIG. 1, and the optical transmitter is provided with a distortion imparting function (predistortion). The configuration of the optical receiver in the third embodiment is the same as that of the optical receiver 1 in FIG. 1 except that the distortion compensation circuit 22 is not provided.

(3-1) Configuration and Operation of Optical Transmitting Device FIG. 16 is a block diagram showing the configuration of the optical transmitting device in the optical transmission system according to the third embodiment.

  16 includes a distributor 31, amplifiers 32, 33, and 34, a capacitor 35, a laser diode 36, an APD 37, a laser diode 38, phase control circuits 39 and 40, gain control circuits 41 and 42, and a subtractor 43. , 44 and optical fibers 45, 46.

  The cathode of the APD 37 is connected to the bias terminal Nb, and the anode is connected to the amplifier 33. A reverse bias voltage Va is applied to the bias terminal Nb. A capacitor 35 is connected between the bias terminal Nb and the ground terminal. The APD 37 has the same characteristics as the APD 11 of the optical receiver 1 of FIG.

  A high frequency signal RFin is input to the distributor 31. The distributor 31 distributes the high frequency signal RFin to the amplifier 32 and the phase control circuits 39 and 40. The amplifier 32 amplifies the high frequency signal supplied from the distributor 31. The laser diode 36 converts the high-frequency signal amplified by the amplifier 32 into an optical signal, and causes the optical signal to enter the optical fiber 45. The optical fiber 45 emits an optical signal to the APD 37. As a result, a high-frequency photocurrent flows through the APD 37. The photocurrent flowing through the APD 37 is amplified by the amplifier 33 and converted into a voltage. The high-frequency signal output from the amplifier 33 has distortion similar to the high-frequency voltage at the node N3 of the optical receiver 1 in FIG.

  On the other hand, the phase control circuit 39 controls the phase of the high-frequency signal given from the distributor 31. The gain control circuit 41 controls the gain of the high frequency signal output from the phase control circuit 39. The subtractor 43 performs subtraction between the high frequency signal having distortion output from the amplifier 33 and the high frequency signal output from the gain control circuit 41. Thereby, a distortion component is extracted.

  The phase control circuit 40 controls the phase of the high-frequency signal given from the distributor 31. The gain control circuit 42 controls the gain of the high frequency signal output from the phase control circuit 40. The subtractor 44 performs subtraction between the distortion component output from the subtracter 43 and the high frequency signal output from the gain control circuit 42. Thereby, a distortion component having a polarity opposite to that of the distortion component (hereinafter referred to as an inverse distortion component) is synthesized with the high frequency signal.

  The amplifier 34 amplifies the high frequency signal having the inverse distortion component output from the subtractor 44. The laser diode 38 converts a high-frequency signal having a reverse distortion component into an optical signal, and causes the optical signal to enter the optical fiber 46. The optical fiber 46 emits the optical signal OPT to the APD 11 of the optical receiver. This optical signal OPT has a reverse distortion component.

  When the APD 11 of the optical receiver receives the optical signal OPT, a photocurrent flows through the APD 11. This photocurrent is distorted due to the non-linearity of the APD 11. In this case, since the optical signal OPT has a reverse distortion component, the reverse distortion component of the photocurrent is canceled by the distortion caused by the non-linearity of the APD 11. As a result, the high-frequency signal RF output from the output terminal NO in FIG. 1 has no distortion.

(3-2) Effects of the Third Embodiment In the optical transmission system according to the third embodiment, the optical transmission device 3 combines the optical signal with the inverse distortion component so that the non-linearity of the APD 11 of the optical reception device. The reverse distortion component is canceled by the distortion caused by the property. This eliminates the need for the distortion compensation circuit 22 of FIG. 1 in the optical receiver. Therefore, a high gain M and low distortion light receiving characteristic can be obtained for an optical signal that is analog-modulated with a small number of parts.

  In particular, in the FTTH (Fiber to the Home) system, basically, one optical transmission device 3 is used, so that the above distortion compensation can be easily performed on the transmission side.

(4) Fourth Embodiment Hereinafter, an optical receiving apparatus according to a fourth embodiment of the present invention will be described. The fourth embodiment differs from the first embodiment in the following points.

(4-1) Configuration and Operation of Optical Receiving Device FIG. 17 is a circuit diagram showing the configuration of the optical receiving device according to the fourth embodiment of the present invention.

  The optical receiver 1a of FIG. 17 differs from the optical receiver 1 of FIG. 1 in that a secondary battery 26, a charging circuit 27, and a solar battery 28 are further provided. The solar cell 28 converts light into electric power. The charging circuit 27 charges the secondary battery 26 with electric power obtained from the solar battery 28. The fixed high voltage generation circuit 16 is driven by the secondary battery 26.

(4-2) Effect of Fourth Embodiment Since the power required by the optical receiver 1a is as small as 4 mW to 150 mW, by using the solar battery 28 and the secondary battery 26 having an output of about 300 mW, daytime Only outdoor charging enables the nighttime driving of the optical receiver 1a.

  When installing the optical receiver 1a in a building, the optical receiver 1a is installed outdoors, and the output terminal NO is connected to, for example, an existing coaxial cable for a television antenna. As a result, the high frequency signal RF can be supplied to the indoor terminal device without changing the wiring of the indoor terminal device.

(5) Correspondence between each constituent element of claim and each part of the embodiment Hereinafter, an example of correspondence between each constituent element of the claim and each part of the embodiment will be described, but the present invention is limited to the following example. Not.

  In the present embodiment, the fixed high voltage generation circuit 16 corresponds to a voltage application circuit, the PIN diode 12 corresponds to a load element, a variable resistance element, or a PIN diode, the output terminal NO corresponds to an output terminal, and the current detection circuit Reference numeral 17 corresponds to a control circuit.

  InGaAsP corresponds to the first semiconductor, InAlAs corresponds to the second semiconductor, and the superlattice multiplication layer 32 corresponds to the superlattice multiplication layer.

  Further, the difference between the energy level Ec3 and the energy level Ev3 corresponds to the first band gap, the difference between the energy level Ec4 and the energy level Ev4 corresponds to the second band gap, and the band discontinuity of the valence band Ev. (| Ev3-Ev4 |) corresponds to the energy level difference between the uppermost ends of the valence bands of the first and second semiconductor layers, and the band discontinuity (| Ec3-Ec4 |) of the conduction band Ec This corresponds to the energy level difference at the lowest end of the conduction band of the two semiconductor layers. The transformer 15 corresponds to an impedance matching circuit.

  The distributor 31 corresponds to a signal source, the amplifiers 32 and 33, the laser diode 36, the APD 37, the phase control circuit 39, the gain control circuit 41, and the subtractor 43 correspond to a distortion component creating unit, and the phase control circuit 40, The gain control circuit 42 and the subtractor 44 correspond to a combining unit, the laser diode 36 corresponds to a light emitting element, and the subtractor 43 corresponds to a difference calculation unit. The inverse distortion component corresponds to the compensation distortion component.

(6) Other Embodiments In the above embodiment, the PIN diode 12 is used as a load element or a variable resistance element. However, the present invention is not limited to this, and a field effect transistor whose resistance changes depending on current ( FET) may be used, or another element whose resistance value can be controlled by a control circuit may be used.

  The present invention can be used in an optical transmission system for transmitting various optical signals.

1 is a circuit diagram showing a configuration of an optical receiver according to a first embodiment of the present invention. The figure which shows the relationship between the voltage applied to the both ends of APD, and the multiplication factor of APD A circuit diagram for explaining the principle of distortion generation when an APD receives an optical signal whose intensity is modulated Waveform diagram for explaining distortion appearing in a sine wave in the circuit of FIG. The figure which shows the measurement result of the relationship between the applied voltage of APD and a secondary distortion The figure which shows the measurement result of the relationship between the applied voltage of APD and the third order distortion The figure which shows the measurement result of the relationship between the electric current which flows into APD, and a secondary distortion. The figure which shows the measurement result of the relationship between the electric current which flows into APD, and the third order distortion The figure which shows the RF voltage level of each part in the optical receiver of a comparative example The figure which shows the RF voltage level of each part in the optical receiver of this Embodiment. The figure which shows the measurement result of the secondary distortion before distortion compensation by a distortion compensation circuit in an optical receiver. The figure which shows the measurement result of the secondary distortion after distortion compensation by the distortion compensation circuit in the optical receiver. Typical sectional drawing which shows the structure of the superlattice APD used for the optical receiver which concerns on 2nd Embodiment (A) is an energy band diagram of a general superlattice APD (first superlattice APD), and (b) is an energy band of a superlattice APD (second superlattice APD) used in the second embodiment. Figure The figure which shows the characteristic of the general superlattice APD (1st superlattice APD) of Fig.14 (a), and the superlattice APD (2nd superlattice APD) used by 2nd Embodiment. The block diagram which shows the structure of the optical transmitter in the optical transmission system which concerns on 3rd Embodiment. The circuit diagram which shows the structure of the optical receiver which concerns on the 4th Embodiment of this invention Block diagram showing an example of an optical receiver in a conventional digital optical transmission system Block diagram showing a first example of an optical receiver in a conventional analog optical transmission system Block diagram showing a second example of an optical receiver in a conventional analog optical transmission system

Explanation of symbols

1, 1a Optical receiver 3 Optical transmitter 11, 11a APD
DESCRIPTION OF SYMBOLS 12 PIN diode 13 Resistance 14 Inductor 15 Transformer 16 Fixed high voltage generation circuit 17 Current detection circuit 18, 19, 20, 21 Capacitor 22 Distortion compensation circuit 23 Variable resistance 24 Distortion compensation diode 25 Capacitor 26 Secondary battery 27 Charging circuit 28 Solar battery 31 Distributor 32, 33, 34 Amplifier 35 Capacitor 36 Laser diode 37 APD
38 Laser diode 39, 40 Phase control circuit 41, 42 Gain control circuit 43, 44 Subtractor 45, 46 Optical fiber 101 APD
102 resistor 103 DC power supply 104 capacitor 161, 162, 163 output terminal 501 n ⁺ -InP substrate 502 n ⁺ -InP buffer layer 503 non-doped In _0.52 Al _0.48 As / In _0.8 Ga _0.2 As _0.6 P _0.4 superlattice multiplication layer 504 p -InP layer 505 p-In _0.47 Ga _0.53 As light absorption layer 506 p ⁺ -In _0.47 Ga _0.53 As layer 507 p-InP window layer 508 p ⁺ -In _0.47 Ga _0.53 As contact layer 509 AuZnNi electrode 510 AuGeNi electrode M multiplication factor N1, N2, N3, N4, N5, N30 Node N31 Output terminal Nb Bias terminal NB Bias terminal NO Output terminal OPT Optical signal RF High frequency signal RFin High frequency signal Va Reverse bias voltage VB Bias voltage Vdc DC voltage Vrf High frequency voltage

Claims (13)

An avalanche photodiode receiving an optical signal;
A voltage application circuit for applying a constant reverse bias voltage to the avalanche photodiode;
A current flowing through the avalanche photodiode is supplied, and a load element having a variable resistance value;
An output terminal electrically connected to one end of the load element;
And a control circuit that detects a current flowing through the avalanche photodiode and controls a resistance value of the load element so that an output power of the output terminal is constant based on the detected current. Optical receiver. The load element includes a variable resistance element having a characteristic that a resistance value decreases with an increase in current,
The optical receiver according to claim 1, wherein the control circuit detects a current flowing through the avalanche photodiode, and supplies a current that changes according to a change in the detected current to the variable resistance element. The variable resistance element is a PIN diode or a field effect transistor,
The said control circuit detects the electric current which flows into the said avalanche photodiode, and supplies the electric current which changes according to the change of the detected electric current to the said PIN diode or the said field effect transistor. Optical receiver. The avalanche photodiode has non-linearity in which the multiplication factor is not proportional to the applied voltage,
4. The optical receiver according to claim 1, further comprising a distortion compensation circuit that compensates for distortion of an output signal at the output terminal by compensating for nonlinearity of the avalanche photodiode. . The avalanche photodiode has linearity in which a multiplication factor is substantially proportional to an applied voltage in a specific voltage range,
The optical receiver according to claim 1, wherein the voltage application circuit applies a constant reverse bias voltage within the specific voltage range to the avalanche photodiode. 6. The optical receiver according to claim 5, wherein the avalanche photodiode includes a multiplication layer having a superlattice structure. The superlattice multiplication layer alternately includes a first semiconductor layer having a first band gap and a second semiconductor layer having a second band gap larger than the first band gap, The energy level difference between the uppermost ends of the valence bands of the first and second semiconductor layers is smaller than the energy level difference between the lowermost ends of the conduction bands of the first and second semiconductor layers, and the first and second The energy level difference at the uppermost end of the valence band of the semiconductor layer is 10 meV or less,
The optical receiver according to claim 6, wherein the voltage application circuit applies a constant reverse bias voltage so that an electric field of 300 kV / cm or more is applied to the avalanche photodiode. 8. The optical receiver according to claim 1, further comprising an impedance matching circuit that matches the impedance of the avalanche photodiode and the impedance of the output terminal. Solar cells,
A secondary battery,
A charging circuit for charging the secondary battery with electric power generated by the solar battery,
The optical receiver according to claim 1, wherein the voltage application circuit receives electric power from the secondary battery. The optical receiver according to claim 1, wherein the optical signal is an analog-modulated high-frequency signal. An optical transmitter for transmitting an optical signal;
An optical receiver that receives an optical signal transmitted by the optical transmitter;
The optical receiver is
An avalanche photodiode receiving an optical signal;
A voltage application circuit for applying a constant reverse bias voltage to the avalanche photodiode;
A load element in which a current flowing through the avalanche photodiode is supplied via an output node and has a variable resistance value;
An output terminal electrically connected to one end of the load element;
A control circuit that detects a current flowing through the avalanche photodiode and controls a resistance value of the load element so that an output power of the output terminal becomes constant based on the detected current;
The avalanche photodiode has non-linearity in which the multiplication factor is not proportional to the applied voltage,
The optical transmitter is
A signal source for generating electrical signals;
A distortion component creating unit for creating a distortion component for compensation for compensating for distortion caused by nonlinearity of the avalanche photodiode of the optical receiver;
A combining unit that combines the distortion component for compensation created by the distortion component creating unit with the electrical signal generated by the signal source;
An optical transmission system comprising: a photoelectric conversion unit that converts an electrical signal output from the combining unit into the optical signal. The distortion component creation unit
A light emitting element that converts an electrical signal from the signal source into an optical signal;
An avalanche photodiode for converting an optical signal obtained by the light emitting element into an electrical signal;
A difference calculation unit that outputs a difference between an electric signal obtained by the avalanche photodiode and an electric signal from the signal source as the compensation distortion component;
The optical transmission system according to claim 11, wherein the combining unit combines the compensation distortion component output from the difference calculation unit with an electric signal from the signal source. 13. The optical transmission system according to claim 11, wherein the electrical signal is an analog-modulated high frequency signal.

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