Optical modulation device and control method

Abstract

Provided is an optical modulation device comprising: an optical modulator that generates an optical signal by modulating light on the basis of an inputted input signal; a bias control unit that determines an adjustment bias voltage for use in adjustment in order to control a bias voltage that is applied to the optical modulator; a tone signal generating unit that generates a low-frequency tone signal; an adding unit that adds the adjustment bias voltage and the tone signal to generate a bias control signal; a sample hold circuit that outputs the bias control signal generated by the adding unit in a time period in which the optical modulator is not used for communication, and holds the inputted adjustment bias voltage in a time period in which the optical modulator is used for communication; and a superposing unit that applies the bias voltage to the optical modulator by superposing the inputted input signal and the bias control signal and inputting the resultant signal to the optical modulator.　

Description

Technical Field

[0001] The present invention relates to an optical modulation device and a control method.

Background Art

[0002] Conventionally, as an optical modulator in optical communication, a Mach-Zehnder modulator has been widely used. The Mach-Zehnder modulator can achieve wideband and stable performance. On the other hand, since the operating point of the Mach-Zehnder modulator varies with time, bias control is required to always keep the operating point optimal. Therefore, as a bias control method during digital signal (NRZ (Non-Return-to-Zero) signal) transmission, a method has been proposed in which a low-frequency signal called a dither signal is superimposed on a bias control signal, and the bias is controlled by observing the change in output power (see, for example, Patent Document 1). In digital signal transmission, it is possible to suppress deterioration in quality by adjusting the dither signal to utilize the non-linear region (= limiter operation).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, when the technique described in Patent Document 1 is applied to analog signal transmission, there is a problem that since a dither signal is added to the bias control signal, the dither signal itself becomes noise and the transmission quality deteriorates.

[0005] In view of the above circumstances, an object of the present invention is to provide a technique capable of performing bias control while suppressing deterioration in transmission quality even during analog signal transmission.

Means for Solving the Problems

[0006] One aspect of the present invention is an optical modulation device comprising: an optical modulator that generates an optical signal by modulating light based on an input signal; a bias control unit that determines an adjustment bias voltage used for adjustment in order to control the bias voltage applied to the optical modulator; a tone signal generation unit that generates a low-frequency tone signal; an addition unit that generates a bias control signal by adding the adjustment bias voltage and the tone signal; a sample-and-hold circuit that outputs the bias control signal generated by the addition unit during times when the optical modulator is not used for communication, and holds the input adjustment bias voltage during times when the optical modulator is used for communication; and a superposition unit that applies the bias voltage to the optical modulator by superimposing the input signal and the bias control signal and inputting them to the optical modulator.

[0007] One aspect of the present invention is a control method that generates an optical signal by modulating light based on an input signal, determines an adjustment bias voltage to be used for adjustment in order to control the bias voltage applied to the optical modulator, generates a low-frequency tone signal, generates a bias control signal by adding the adjustment bias voltage and the tone signal, outputs the generated bias control signal during times when the optical modulator is not used for communication, holds the input adjustment bias voltage during times when the optical modulator is used for communication, and applies the bias voltage to the optical modulator by superimposing the input signal and the bias control signal and inputting them to the optical modulator. [Effects of the Invention]

[0008] This invention enables bias control while suppressing degradation of transmission quality even during analog signal transmission. [Brief explanation of the drawing]

[0009] [Figure 1] This figure shows an example of the configuration of a wireless system according to the present invention. [Figure 2] This figure shows an example configuration of the E / O converter in the embodiment. [Figure 3] This figure shows the relationship between the tone signal and the PD detection result in the embodiment. [Figure 4] This is a diagram illustrating the operation of the E / O converter in the embodiment. [Modes for carrying out the invention]

[0010] One embodiment of the present invention will be described below with reference to the drawings. Figure 1 shows an example configuration of the wireless system 100 according to the present invention. The wireless system 100 comprises a central station 10 and one or more branch stations 20. The central station 10 and the one or more branch stations 20 are connected via an optical transmission path 30. The optical transmission path 30 is an optical fiber. In the following description, the case where there is one branch station 20 will be used as an example.

[0011] Between the aggregation station 10 and the extension station 20, transmission is performed using analog RoF (Radio over Fiber), which transmits wireless signals using optical fiber. Here, the wireless signal transmitted through the optical fiber may be the radio frequency output by the wireless base station 110, or the radio frequency may be converted to an intermediate frequency lower than the radio frequency and then converted back to the original radio frequency at the extension station 20. Furthermore, different wavelengths are used for the uplink and downlink directions between the aggregation station 10 and the extension station 20. In the following explanation, the uplink direction is the direction from the extension station 20 to the aggregation station 10, and the downlink direction is the direction from the aggregation station 10 to the extension station 20. In the following explanation, a configuration in which the extension station 20 performs wireless communication with its subordinate wireless terminals using TDD (Time Division Duplex) is used as an example, but TDMA-FDD (Time Division Multiple Access - Frequency Division Duplex) may also be used as the wireless method.

[0012] Next, the specific configurations of the aggregation station 10 and the extension station 20 will be described. First, the specific configuration of the aggregation station 10 will be described. The aggregation station 10 comprises a radio base station 110, a TDD switch 120, an E / O converter 130, an E / O converter 140, a multiplexer / demultiplexer 150, and an O / E converter 160.

[0013] The wireless base station 110 outputs a modulated signal modulated with downlink transmission data to the TDD switch 120. Furthermore, the wireless base station 110 outputs a TDD signal for controlling the switching between transmission and reception to the TDD switch 120, the E / O converter 130, and the E / O converter 140. The TDD signal has alternating "transmission" time slots indicating the timing of transmission and "reception" time slots indicating the timing of reception, with a gap time inserted between the "transmission" time slots and the "reception" time slots.

[0014] The TDD switch 120 switches connections based on the TDD signal output from the radio base station 110. For example, during the period indicated by the "transmit" time slot in the TDD signal, the TDD switch 120 connects the radio base station 110 to the E / O converter 130. This allows the modulated signal output from the radio base station 110 to be input to the E / O converter 130. On the other hand, during the period indicated by the "receive" time slot in the TDD signal, the TDD switch 120 connects the radio base station 110 to the O / E converter 160. This allows the electrical signal output from the O / E converter 160 to be input to the radio base station 110.

[0015] The E / O converter 130 receives a modulation signal output from the TDD switch 120 and a TDD signal output from the wireless base station 110 through two input terminals. During the period indicated by the "transmit" time slot in the TDD signal, the E / O converter 130 does not update the bias control signal of the optical modulator (hold operation), and generates an optical signal of a certain wavelength by modulating continuous light with the optical modulator using the modulation signal. For example, the E / O converter 130 generates an optical signal of wavelength λ1. The E / O converter 130 outputs the generated optical signal of wavelength λ1 to the multiplexer / demultiplexer 150. The optical modulator is, for example, a Mach-Zehnder modulator.

[0016] The E / O converter 130 updates the bias control signal of the optical modulator during the period indicated by the "reception" time slot in the TDD signal. The specific processing of the optical modulator bias control will be described later. In this way, the E / O converter 130 updates the bias control signal during the time period when the optical modulator is not used for communication (for example, when receiving the TDD). Note that the E / O converter 130 is one embodiment of an optical modulation device.

[0017] To control the TDD switch 250 at the extension station 20, the TDD signal itself also needs to be transmitted via optical fiber. For this reason, the TDD signal output from the wireless base station 110 is branched and input to the E / O converter 140 from two input terminals. The E / O converter 140 does not perform bias control of the optical modulator during the period indicated by the "transmit" time slot for the TDD signal input from one input terminal, and modulates continuous light with the optical modulator using the TDD signal input from the other input terminal, thereby generating an optical signal of a different wavelength from that of the E / O converter 130. For example, the E / O converter 140 generates an optical signal of wavelength λ2. The E / O converter 140 outputs the generated optical signal of wavelength λ2 to the multiplexer / demultiplexer 150.

[0018] The E / O converter 140 does not update the bias control signal of the optical modulator during the period indicated by the "transmission" time slot in the TDD signal input from one input terminal (hold operation), and updates the bias control signal of the optical modulator during the period indicated by the "reception" time slot. In this way, the E / O converter 140 performs bias control during a time period when the optical modulator is not used for communication (for example, during reception in TDD). Note that the E / O converter 140 is an aspect of the optical modulation device.

[0019] The multiplexer / demultiplexer 150 multiplexes or demultiplexes the input optical signals. For example, the multiplexer / demultiplexer 150 multiplexes the optical signal with wavelength λ1 output from the E / O converter 130 and the optical signal with wavelength λ2 output from the E / O converter 140 and outputs the multiplexed signal to the optical transmission line 30. For example, the multiplexer / demultiplexer 150 demultiplexes the optical signal (for example, λ3) transmitted from the relay station 20 by wavelength and outputs it to the O / E converter 160.

[0020] The O / E converter 160 takes as input the optical signal with wavelength λ3 demultiplexed by the multiplexer / demultiplexer 150. The O / E converter 160 converts the input optical signal with wavelength λ3 into an electrical signal. The O / E converter 160 outputs the electrical signal to the TDD switch 120.

[0021] Next, the specific configuration of the relay station 20 will be described. The relay station 20 includes a multiplexer / demultiplexer 210, an O / E converter 220, a HPA 230, an O / E converter 240, a TDD switch 250, a LNA 260, an E / O converter 270, and an antenna 280. Note that the HPA 230 and the LNA 260 may be omitted depending on the design of the wireless signal strength transmitted and received by the antenna 280.

[0022] The multiplexer / demultiplexer unit 210 multiplexes or demultiplexes the input optical signals. For example, the multiplexer / demultiplexer unit 210 multiplexes the optical signals output from the E / O converter 270 and outputs them to the optical transmission line 30. For example, the multiplexer / demultiplexer unit 210 demultiplexes the optical signals transmitted from the concentration station 10 by wavelength and outputs them to the O / E converter 220 and the O / E converter 240. Here, it is assumed that the multiplexer / demultiplexer unit 210 outputs the optical signal of wavelength λ1 to the O / E converter 220 and the optical signal of wavelength λ2 to the O / E converter 240.

[0023] The O / E converter 220 takes the optical signal of wavelength λ1 demultiplexed by the multiplexer / demultiplexer unit 210 as an input. The O / E converter 220 converts the input optical signal of wavelength λ1 into an electrical signal. The O / E converter 220 outputs the electrical signal to the HPA 230. Note that the electrical signal output from the O / E converter 220 is a downstream transmission signal.

[0024] The HPA 230 is a high-power amplifier that amplifies the electrical signal (downstream transmission signal) output from the O / E converter 220. The HPA 230 outputs the amplified electrical signal to the TDD switch 250.

[0025] The O / E converter 240 takes the optical signal of wavelength λ2 demultiplexed by the multiplexer / demultiplexer unit 210 as an input. The O / E converter 240 converts the input optical signal of wavelength λ2 into an electrical signal. The O / E converter 240 outputs the electrical signal to the TDD switch 250 and the E / O converter 270. Note that the electrical signal output from the O / E converter 240 is a TDD signal.

[0026] The TDD switch 250 switches connections based on the electrical signal (TDD signal) output from the O / E converter 240. For example, the TDD switch 255 connects the HPA 230 and the antenna 280 during the period indicated by the "transmit" time slot in the TDD signal. As a result, the electrical signal output from the HPA 230 is transmitted as radio waves by the antenna 280. On the other hand, the TDD switch 250 connects the LNA 260 and the antenna 280 during the period indicated by the "receive" time slot in the electrical signal (TDD signal) output from the O / E converter 240. As a result, the electrical signal based on the radio waves received via the antenna 280 is input to the LNA 260.

[0027] The LNA260 is a low-noise amplifier that amplifies electrical signals based on radio waves received via the antenna 280. The LNA260 outputs the amplified electrical signal to the E / O converter 270.

[0028] The E / O converter 270 receives an electrical signal output from the LNA 260 and an electrical signal (TDD signal) output from the O / E converter 240 through two input terminals. During the period indicated by the "transmit" time slot in the TDD signal, the E / O converter 270 does not update the bias control signal of the optical modulator, and generates an optical signal of wavelength λ3 by modulating continuous light with the optical modulator using the electrical signal. The E / O converter 270 outputs the generated optical signal of wavelength λ3 to the multiplexer / demultiplexer 210.

[0029] The E / O converter 270 updates the bias control signal of the optical modulator during the period indicated by the "receive" time slot in the TDD signal. In this way, the E / O converter 270 performs bias control during times when the optical modulator is not used for communication (for example, when receiving the TDD). The E / O converter 270 is one embodiment of an optical modulation device.

[0030] Figure 2 shows an example of the configuration of the E / O converter 130 in the embodiment. In Figure 2, the E / O converter 130 is used as an example for explanation, but the E / O converter 140 provided by the aggregation station 10 and the E / O converter 270 provided by the extension station 20 have similar configurations. Note that, as described above, the update timing of the bias control signal of the optical modulator is reversed between the E / O converters 130 and 140 of the aggregation station 10 and the E / O converter 270 of the extension station 20. Therefore, in the following explanation, for the E / O converter 270 of the extension station 20, it is sufficient to reverse the "reception" and "transmission" settings.

[0031] The E / O converter 130 comprises a tone signal generation unit 131, a switch 132, a bias control unit 133, an addition unit 134, a sample-and-hold circuit 135, a superposition unit 136, a Mach-Zehnder modulation unit 137, a PD 138, and an LPF 139.

[0032] The tone signal generation unit 131 generates a tone signal having a constant low frequency. This signal corresponds to the dither signal shown in Patent Document 1. The tone signal generation unit 131 outputs the generated tone signal to the switch 132.

[0033] A TDD signal is input to switch 132. Based on the input TDD signal, switch 132 switches the connection or disconnection between the tone signal generator 131 and the adder 134. Specifically, during the period indicated by the "reception" time slot, switch 132 electrically connects the tone signal generator 131 and the adder 134. As a result, the tone signal generated by the tone signal generator 131 is output to the adder 134.

[0034] Switch 132 disconnects the electrical connection between the tone signal generator 131 and the adder 134 during the period indicated by the "transmit" time slot in the TDD signal. As a result, the tone signal generated by the tone signal generator 131 is not output to the adder 134. In this way, switch 132 disconnects the connection so that the tone signal is not output to the adder 134 when not transmitting.

[0035] The bias control unit 133 controls the bias voltage applied to the Mach-Zehnder modulation unit 137 during periods when the Mach-Zehnder modulation unit 137 is not used for communication. Specifically, the bias control unit 133 monitors the output power of the Mach-Zehnder modulation unit 137 based on the leaked radiation from the Y-coupler of the Mach-Zehnder modulation unit 137, which is detected by the PD 138 and filtered by the LPF 139. This makes it possible to understand the state of the applied bias voltage. Based on the monitored state of the bias voltage, the bias control unit 133 determines the bias voltage to be applied to the Mach-Zehnder modulation unit 137. For example, similar to Patent Document 1, the bias control unit 133 observes the change in the output power of the Mach-Zehnder modulation unit 137 and determines the adjustment bias voltage. The bias control unit 133 outputs the determined adjustment bias voltage to the adder 134.

[0036] During reception in TDD, the summer 134 receives the tone signal output from the switch 132 and the adjustment bias voltage output from the bias control unit 133. In this case, the summer 134 adds the input adjustment bias voltage and tone signal to generate a bias control signal.

[0037] During transmission in TDD mode, only the adjustment bias voltage is input to the summer 134. Therefore, during transmission in TDD mode, the summer 134 does not add the adjustment bias voltage and the tone signal. The summer 134 outputs either the bias control signal or the adjustment bias voltage to the sample-and-hold circuit 135.

[0038] The sample-and-hold circuit 135 receives the TDD signal and the bias control signal or adjustment bias voltage output from the summing unit 134. During the period indicated by the "transmit" time slot in the TDD signal, the sample-and-hold circuit 135 holds the input adjustment bias voltage. Therefore, the sample-and-hold circuit 135 does not output the adjustment bias voltage. As a result, bias control of the Mach-Zehnder modulation unit 137 is not performed. During the period indicated by the "receive" time slot in the TDD signal, the sample-and-hold circuit 135 outputs the input bias control signal to the superposition unit 136.

[0039] The superposition unit 136 superimposes (adds) the bias control signal output from the sample-and-hold circuit 135 onto the modulation signal output from the radio base station 110. The superposition unit 136 outputs the signal with the bias control signal superimposed to the Mach-Zehnder modulation unit 137. At this time, the superposition unit 136 applies the adjustment bias voltage included in the bias control signal to the Mach-Zehnder modulation unit 137. This performs bias control on the Mach-Zehnder modulation unit 137.

[0040] In the case of the superposition section 136 of the E / O converter 140 of the aggregation station 10, the signal output from the sample-and-hold circuit 135 is superimposed (added) to the electrical signal output from the radio base station 110. In the case of the superposition section 136 of the E / O converter 270 of the extension station 20, the signal output from the sample-and-hold circuit 135 is superimposed (added) to the electrical signal output from the LNA 260.

[0041] The Mach-Zehnder modulation unit 137 generates an optical signal by modulating continuous light using the input signal (e.g., the modulation signal) when transmitting an optical signal. The Mach-Zehnder modulation unit 137 is a Mach-Zehnder modulator.

[0042] PD138 is located near the output side of the Mach-Zehnder modulation unit 137. PD138 detects the stray emission from the Y-coupler of the Mach-Zehnder modulation unit 137. This allows monitoring of the bias state without reducing the power of the main optical signal. Alternatively, a portion of the output of the Mach-Zehnder modulation unit 137 could be branched and input to PD138 instead of the stray emission, but in this case, the power of the main optical signal would decrease. PD138 outputs the detection result of the stray emission (e.g., output power) to LPF139. PD138 is one embodiment of the photodetector.

[0043] The LPF139 is a low-pass filter that removes noise from the detection results of leaked synchrotron radiation output from the PD138.

[0044] Figure 3 shows the relationship between the tone signal and the detection result of PD138 in the embodiment. In the method of branching a portion of the output of the Mach-Zehnder modulation unit 137 described in paragraph 0041 above, the output power of PD138 changes in proportion to the amplitude of the tone signal. However, as shown in Figure 3, the operation is similar except that the phase of the tone signal and the detection result of PD138 are inverted, and the bias state can be monitored.

[0045] Figure 4 is a diagram illustrating the operation of the E / O converter in the embodiment. Here, the E / O converter 130 will be used as an example. As shown in Figure 4, the TDD signal input to the E / O converter 130 has a "reception" time slot that covers the period from time t1 to time t2 and from time t3 to time t4, and a "transmission" time slot that covers the period from time t2 to time t3.

[0046] In this case, from time t1 to time t2, which is the period indicated by the "reception" time slot, the tone signal generated by the tone signal generation unit 131 is output from the switch 132 of the E / O converter 130 to the adder 134 (switch output when the TDD signal is "received" in Figure 4). The tone signal output to the adder 134 is then used for bias control of the Mach-Zehnder modulation unit 137 after the adjustment bias voltage output from the bias control unit 133 is added to it. Along with the bias control, the optical signal output from the Mach-Zehnder modulation unit 137 reaches the output terminal of the O / E converter 220 on the extension station 20 side, but there is no effect because the TDD switch 250 on the extension station 20 side is off (in a state where the uplink signal can pass through). If there are any problems with the HPA230, etc. (for example, if the HPA230 output cannot be isolated from the surrounding circuitry), countermeasures such as turning off the power to the HPA230 when not transmitting can be taken.

[0047] PD138 detects the stray radiation from the Mach-Zehnder modulation unit 137 and notifies the bias control unit 133 of the detection result via LPF139. The bias control unit 133 controls the bias of the Mach-Zehnder modulation unit 137 based on the stray radiation detected by PD138 (DC bias control operation when the TDD signal is "received" in Figure 4). This process is performed for the period indicated by the "reception" time slot.

[0048] When the period from time t1 to time t2, which is indicated by the "reception" time slot, has elapsed, switch 132 disconnects the connection between the tone signal generator 131 and the summer 134 for the period from time t2 to time t3, which is indicated by the "transmission" time slot. As a result, no signal is output from switch 132 to the summer 134 (switch output during TDD signal "transmission" in Figure 4). The sample-and-hold circuit 135 performs a hold operation for the period from time t2 to time t3, which is indicated by the "transmission" time slot (DC bias control operation during TDD signal "transmission" in Figure 4).

[0049] Subsequently, after the period from time t2 to time t3, which is indicated by the "transmission" time slot, has elapsed, the switch 132 connects the tone signal generator 131 and the adder 134 for the period from time t3 to time t4, which is indicated by the "reception" time slot. As a result, the tone signal generated by the tone signal generator 131 is output from the switch 132 to the adder 134 (switch output during TDD signal "reception" in Figure 4). The E / O converter 130 switches and executes the above operations based on the TDD signal.

[0050] According to the wireless system 100 configured as described above, the E / O converters 130, 140, 270 include: a Mach-Zehnder modulation unit 137 that generates an optical signal by modulating light based on an input signal; a bias control unit 133 that determines an adjustment bias voltage used for adjustment in order to control the bias voltage applied to the Mach-Zehnder modulation unit 137; a tone signal generation unit 131 that generates a low-frequency tone signal; an adder unit 134 that generates a bias control signal by adding the adjustment bias voltage and the tone signal; a sample-and-hold circuit 135 that outputs the bias control signal generated by the adder unit 134 during times when the Mach-Zehnder modulation unit 137 is not used for communication, and holds the input adjustment bias voltage during times when the Mach-Zehnder modulation unit 137 is used for communication; and a superposition unit 136 that applies a bias voltage to the Mach-Zehnder modulation unit 137 by superimposing the input signal and the bias control signal and inputting them to the superposition unit 136. As a result, during the time period when the Mach-Zehnder modulation unit 137 is used for communication, no tone signal (dither signal) is superimposed on the Mach-Zehnder modulation unit 137. Therefore, degradation of signal transmission quality can be suppressed. Furthermore, it can be applied to analog wireless transmission. Thus, bias control becomes possible while suppressing degradation of transmission quality even during analog signal transmission.

[0051] Although embodiments of this invention have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments and includes designs and the like that do not depart from the spirit of this invention. [Industrial applicability]

[0052] This invention can be applied to technologies using analog RoF. [Explanation of symbols]

[0053] 10…Aggregation station, 20…Extended station, 30…Optical transmission line, 110…Wireless base station, 120, 250…TDD switch, 130, 140, 270…E / O converter, 150, 210…Multiplexer / demultiplexer, 160, 220, 240…O / E converter, 230…HPA, 260…LNA

Claims

1. An optical modulator that generates an optical signal by modulating light based on an input signal, A bias control unit that determines the adjustment bias voltage used for adjustment in order to control the bias voltage applied to the optical modulator, A tone signal generation unit that generates low-frequency tone signals, An adder unit that adds the adjustment bias voltage and the tone signal to generate a bias control signal, A sample-and-hold circuit that outputs the bias control signal generated by the summer during periods when the optical modulator is not used for communication, and holds the input adjustment bias voltage during periods when the optical modulator is used for communication, A superposition unit that superimposes the input signal and the bias control signal and inputs them to the optical modulator, thereby applying the bias voltage to the optical modulator. An optical modulation device equipped with the following features.

2. The system further includes a switch for switching between connecting or disconnecting the tone signal generation unit and the summing unit. The switch connects the tone signal generation unit and the summing unit during periods when the optical modulator is not used for communication. The optical modulation device according to claim 1.

3. The switch disconnects the connection between the tone signal generation unit and the summing unit during the time period when the optical modulator is used for communication. The optical modulation device according to claim 2.

4. The optical detection unit further comprises a light detection unit for detecting stray light from the optical modulator, The bias control unit determines the adjustment bias voltage based on the detection result of the photodetector. The optical modulation device according to any one of claims 1 to 3.

5. The optical modulator is a Mach-Zehnder modulator. The optical modulation device according to any one of claims 1 to 3.

6. An optical signal is generated by modulating light based on the input signal. To control the bias voltage applied to the optical modulator, the adjustment bias voltage used for adjustment is determined. It generates a low-frequency tone signal, The adjustment bias voltage and the tone signal are added together to generate a bias control signal. During periods when the optical modulator is not used for communication, the generated bias control signal is output. During the time period when the optical modulator is used for communication, the input adjustment bias voltage is maintained. A control method for applying the bias voltage to an optical modulator by superimposing the input signal and the bias control signal and inputting them to the optical modulator.

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