JP6353280B2 - Light modulator - Google Patents

Description

  The present invention relates to an optical modulation device for generating an optical signal, and more particularly to an optical modulation device in which a band of digital signal processing is compensated.

  With the rapid increase in communication demand, studies for increasing the capacity of communication networks are being energetically conducted. In order to increase the transmission capacity, there is a need for technological progress to increase the total capacity that can be transmitted to a single optical fiber by increasing frequency utilization efficiency and technical progress to increase the transmission capacity (channel rate) per channel. ing. In order to increase the channel rate, multi-carriers that handle multiple subcarriers as a single channel, increase the symbol rate (modulation code transmission speed) by increasing the bandwidth of the analog high-frequency circuit, and complex by optical signal phase and amplitude modulation. Quadrature Amplitude Modulation (QAM) in which symbol points (code points) are arranged on a plane has been actively studied.

  In recent transmissions of quadrature amplitude modulation signals, digital signal processing such as phase estimation, transmission path estimation, and dispersion compensation is used commercially, and the use of digital signal processing for optical communication is to improve the channel rate. It can be said that it was a breakthrough. In addition, digital signal processing in commercial transmission systems has been mainly used on the receiving side, but in next-generation transmission systems, studies are being made to use digital signal processing on the transmitting side as well. Processing technology is expected to continue to evolve. However, channel rate improvement by multi-valued quadrature amplitude modulation has a trade-off with the transmission distance as shown by Shannon's theorem, and when the transmission distance is fixed, there is a limit on the number of multi-values. In principle, it is difficult to improve the channel rate.

  The multi-carrier handling method is a method of improving the channel rate by separately modulating / demodulating carriers of different frequencies by using as many transmitters / receivers in parallel as the number of carriers and treating them as one channel. It is. This method has low technical hurdles because it simply handles the devices in parallel as a single transceiver system. However, in multicarrier transmission, devices are arranged in parallel by the number of carriers, so that the size and power consumption of the transmitter / receiver increase in proportion to the number of carriers, and further, there are problems related to cost and reliability. In order to improve the channel rate while avoiding the problems of the transmission distance limitation due to Shannon's theorem and the increase in the number of transceivers due to the increase in the number of carriers, it is necessary to improve the symbol rate.

JP 2014-33426 A

H. Yamazaki, T. Saida, T. Goh, S. Mino, M. Nagatani, H. Nosaka, K. Murata, “Dual-Carrier Dual-Polarization IQ Modulator Using a Complementary Frequency Shifter”, IEEE Journal of Selected Topics in Quantum Electronics, 2013, Vol.19, No 6

  However, it is considered difficult to increase the bandwidth of high-frequency devices because the high-frequency characteristics of high-frequency lines are a bottleneck. FIG. 1 shows an outline of a conventional light modulation device (see Patent Document 1 and Non-Patent Document 1). FIG. 1 shows a digital integrated circuit (IC) 101, a digital-analog converter 102 having first and second electric signal ports 103a and 103b, and first and second modulator drivers 104a and 104b. In addition, an optical modulation device including a laser light source 105 and an optical modulator 106 having an output optical port 107 is shown.

  In the optical modulation device shown in FIG. 1, the digital IC 101 generates a signal desired to be output from the optical modulation device, and the signal generated by the digital IC 101 is converted into an analog electric signal by the digital / analog converter 102. Thereafter, the analog electrical signals generated by the digital-analog converter 102 and output from the electrical signal ports 103 a and 103 b are amplified by the modulator drivers 104 a and 104 b and input to the optical modulator 106. The optical modulator 106 modulates the light input from the laser light source 105 with the electrical signal input from the modulator driver 104 to generate an optical signal. Since the high-frequency electrical signal passes through the digital-analog converter 102, the modulator driver 104, and the optical modulator 106, the symbol rate is limited by these bands.

  Some modulator drivers and optical modulators have a 3 dB band exceeding 100 GHz. However, in a digital-to-analog converter manufactured on the same substrate as the digital IC, the 3 dB band is about 20 GHz. It is a bottleneck. Further, even when considering the band on the receiving side of optical transmission, it has been reported that the 3 dB band of photodiodes and transimpedance amplifiers greatly exceeds that of digital-analog converters. Band limitation in digital-analog converters is a limitation due to the high-frequency characteristics of high-frequency lines on silicon that are widely used as materials for digital-analog converters. To improve the analog band, it is necessary to change the material, and compatibility with digital ICs. The nature will fade. Thus, in order to improve the symbol rate for improving the channel rate in the optical transmission / reception system, it is necessary to eliminate the bottleneck of the narrow analog band of the digital-analog converter.

  The present invention has been made in view of the prior art as described above, and an object thereof is to provide an optical modulation device in which an analog band of a digital-analog converter is compensated by digital signal processing and an external analog high-frequency integrated circuit. It is in.

In order to solve the above problems, an optical modulation device according to claim 1 of the present invention includes a digital IC, a digital-analog converter electrically connected to the digital IC, and an electrical connection with the digital-analog converter. An electrical signal synthesizer connected to the optical signal synthesizer, and an optical modulator that is electrically connected to the electrical signal synthesizer and modulates light input from a light source with an analog modulation signal output by the electrical signal synthesizer, The digital IC separates the digital modulation signal into a high frequency side component digital signal and a low frequency side component digital signal, down-converts the high frequency side component digital signal, and the low frequency side component digital signal The digital signal of the high frequency side component subjected to the frequency down conversion is output to the digital analog converter, and the digital analog converter A wave-side component digital signal is converted into a first analog signal, the low-frequency-side component digital signal is converted into a second analog signal, and the first and second analog signals are sent to the electric signal synthesis unit. outputting the electric signal synthesis unit generates a first analog signal frequency up conversion having the same frequency band as the digital signal of the first said high frequency side component analog signals with frequency up conversion of the The first analog signal that has been frequency-upconverted and the second analog signal are added, and the added signal is output to the optical modulator as an analog modulation signal.

Optical modulating device according to claim 2 of the present invention is an optical modulating device according to claim 1 of the present invention, the electrical signal combining unit, the first analog signal to frequency up conversion the a first frequency up conversion unit for generating an analog signal of the first analog signal and the second analog signal the frequency up conversion and which is frequency up conversion having the same frequency band as the digital signal of the high frequency side component And an adding unit that outputs the added signal as the analog modulation signal to the optical modulator.

Optical modulating device according to claim 3 of the present invention is an optical modulating device according to claim 1 of the present invention, the electrical signal combining unit, and the IQ modulator, and the periodic signal generator, the first And the second adder, wherein the IQ modulator includes a first 90-degree phase shifter and first and second multipliers, and the periodic signal generator includes the first adder. A signal having a frequency that is a frequency shift amount when the analog signal is frequency-upconverted is output to the first 90-degree phase shifter, and the first 90-degree phase shifter is input from the periodic signal generator. Based on the generated signal, a signal in phase with the signal of the periodic signal generator and a signal obtained by phase shifting the signal of the periodic signal generator by 90 degrees are output to the first and second multipliers, The first multiplier multiplies the in-phase signal by the first analog signal and performs the first addition. The second multiplier multiplies the signal phase-shifted by 90 degrees and the signal obtained by phase-shifting the first analog signal by 90 degrees and outputs the result to the first adder. The first adder adds the outputs from the first and second multipliers to generate the first analog signal that is frequency- upconverted and outputs the first analog signal to the second adder. The second adder adds the first analog signal that has been frequency- upconverted and the second analog signal, and outputs the added signal to the optical modulator as the analog modulation signal. It is characterized by.

Optical modulating device according to claim 4 of the present invention is an optical modulating device according to claim 3 of the present invention, the electrical signal combining unit further includes a second 90 degree phase shifter, the The second 90-degree phase shifter receives the first analog signal, outputs a signal in phase with the first analog signal to the first multiplier, and outputs the first analog signal by 90 degrees. The phase-shifted signal is output to the second multiplier.

Optical modulating device according to claim 5 of the present invention is an optical modulating device according to any one of claims 1 to 4 of the present invention, the digital IC, the bandwidth of the digital signal of the low frequency side component And the digital modulation signal is separated so that both of the band of the digital signal of the high frequency side component subjected to the frequency down conversion fall within the band of the digital-analog converter.

Optical modulating device according to claim 6 of the present invention is an optical modulating device according to any one of claims 1 to 5 of the present invention, the digital IC, the bandwidth of the digital signal of the low frequency side component However, the digital modulation signal is separated so as to be 52% or more of the band of the digital modulation signal.

Optical modulating device according to claim 7 of the present invention is an optical modulating device according to any one of claims 1 to 6 of the present invention, the electric signal synthesizer is integrated into one chip It is characterized by that.

Optical modulating device according to claim 8 of the present invention is an optical modulating device according to any one of claims 1 to 7 of the present invention, the electrical signal combining unit, an amplifier for amplifying the analog modulated signal Is further provided.

  Since the present invention can avoid the band limitation of the digital-analog converter by shifting the high-frequency component processed by the digital-analog converter to a low frequency, the optical signal of the modulation code transmission speed exceeding the band limitation of the digital-analog converter It produces the effect of generating.

It is a figure showing the conventional light modulation apparatus. It is a figure showing the light modulation apparatus which concerns on 1st Embodiment of this invention. It is a figure showing operation | movement of an electric signal synthetic | combination part. It is a figure showing the implementation example of the optical modulation apparatus of 1st Embodiment of this invention. It is a figure showing the input-output signal of a frequency up converter. It is a figure showing the electric signal characteristic of the light modulation apparatus which concerns on 1st Embodiment of this invention. It is a figure showing the output optical signal characteristic of the optical modulation apparatus which concerns on 1st Embodiment of this invention. It is a figure showing the light modulation apparatus which concerns on 2nd Embodiment of this invention. It is a figure explaining operation | movement of an IQ modulator. It is a figure explaining the signal processing in the optical modulation apparatus which concerns on 3rd Embodiment of this invention. It is a figure showing the periodic signal frequency dependence of output signal quality. It is a figure showing the light modulation apparatus which concerns on 4th Embodiment of this invention. It is a figure showing the result of having compared the symbol rate dependence of signal quality in the optical modulation apparatus which concerns on the 1st-4th embodiment. It is a figure showing the light modulation apparatus which concerns on 6th Embodiment of this invention. It is a figure showing the light modulation apparatus which concerns on 7th Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings and mathematical expressions. In the drawings, parts having the same function are given the same numbers to clarify the explanation.

(First embodiment)
FIG. 2 shows a configuration of the light modulation device according to the first embodiment of the present invention. As shown in FIG. 2, the light modulation device according to the first embodiment of the present invention is a digital IC 200 and a digital IC that is electrically connected to the digital IC 200 and provided with electrical signal ports 202a to 202d at the output unit. The analog converter 201, the frequency upward conversion unit 211a electrically connected to the electrical signal port 202a, the adder 212a electrically connected to the frequency upward conversion unit 211a and the electrical signal port 202b, and the electrical signal port 202c A frequency up-conversion unit 211b electrically connected to the frequency up-conversion unit 211b, an adder 212b electrically connected to the electrical signal port 202d, and adders 212a and 212b. 105, an optical modulator 106 optically connected to 105, and an output optically connected to the output of the optical modulator 106. It includes a optical port 107. The frequency up-conversion units 211a and 211b and the adders 212a and 212b constitute an electric signal synthesis unit 210.

  The digital IC 200 separates a digital modulation signal (hereinafter simply referred to as “digital modulation signal”) that is converted into an analog modulation signal by digital-analog conversion into a high-frequency component and a low-frequency component, and converts the digital signal of the high-frequency component into The frequency is down-converted by digital signal processing, and the low-frequency component digital signal and the high-frequency component digital signal down-converted are output to the digital-analog converter 201. Here, in the separation process of the digital modulation signal in the digital IC 200, the band of the low-frequency component after separation and the band of the high-frequency component after frequency down-conversion are both within the band of the digital-analog converter 201. To do.

  The digital-analog converter 201 converts the digital signal of the high frequency side component subjected to the frequency down conversion into a first analog signal, and outputs the first analog signal to the frequency up conversion units 211a and 211b from the electrical signal ports 202a and 202c, respectively. To do. The digital-analog converter 201 converts the low-frequency component digital signal into a second analog signal, and outputs the second analog signal from the electrical signal ports 202b and 202d to the adders 212a and 212b, respectively.

  The frequency up-conversion units 211a and 211b up-convert the input first analog signal into the frequency band of the original high-frequency component before frequency down-conversion, and output to the adders 223a and 223b. The adders 212a and 212b respectively add the first analog signal and the second analog signal that have been frequency-upconverted to generate an added signal, and drive the optical modulator 106 to output the added signal to the laser. The laser light output from the light source 105 is output to the optical modulator 106 as an analog modulation signal for modulating the laser light.

  With such a configuration, the optical modulation device according to the first embodiment can obtain a modulated optical signal with a high symbol rate while avoiding the band limitation of the digital-analog converter 201.

  FIG. 3 shows the operation of the electric signal synthesizer according to the present invention. In the following description of FIG. 3, a description will be given of the operation of a set of operations composed of the electrical signal ports 202a and 202b, the frequency up-conversion unit 211a, and the adder 212a, but the electrical signal ports 202c and 202d The same operation is performed for a set of operations including the frequency upward conversion unit 211b and the adder 212b.

The bands of the first and second analog signals output from the electric signal ports 202a and 202b are within the band of the digital-analog converter 201, and the maximum frequency component is f _lim . The frequency component distributed from 0 Hz to f _lim of the first analog signal output from the electrical signal port 202a is converted upward to the frequency component distributed from f _lim to 2f _lim by the frequency upward conversion unit 211a. Is done. After that, the first analog signal output from the frequency upward conversion unit 211a and the second analog signal output from the electrical signal port 202b are synthesized by the adder 212a, so that the frequency from 0 Hz to 2f _lim A signal having frequency components up to can be generated.

Thus, since it is sufficient that the band of the digital-analog converter 201 is about f _lim , when the band of the digital-analog converter 201 is f _lim , the electric signal synthesis unit 210 according to the present invention uses the adder 212 to increase the frequency. The frequency component up to 2f _lim corresponding to twice the band of the digital-analog converter 201 is obtained by adding the first analog signal that has been frequency-upconverted on the side and the second analog signal on the low-frequency side It is possible to generate an electrical signal (analog modulation signal) having That is, it is possible to generate a modulation signal with a high symbol rate while avoiding the band limitation of the digital-analog converter.

Hereinafter, the above operation will be described using mathematical expressions. The signal obtained as a result of the calculation according to the present invention, that is, the analog modulation signal s _out (t) which is the output of the electric signal synthesis unit 210 can be described as follows by Fourier series expansion.

First, a digital modulation signal s (t) input to the digital IC 200 is expressed by the following (Formula 1).
s (t) = S ₀ exp (2πf ₀ t) + S ₁ exp (2πf ₁ t) + ... + S _2N-1 exp (2πf _2N-1 t) (Formula 1)
Here, f _x (an integer from 0 x to 2N-1) frequency of each term, S _k (t) is (integer of k from 0 to 2N-1) the amplitude of each frequency component of the signal, 2N −1 is the maximum order of the Fourier series expansion. N is determined by the data length to be calculated and the oversampling coefficient. Through digital processing in the digital IC 200, the digital IC 200 divides the digital modulation signal s (t) as follows.
s _L (t) = S ₀ exp (2πf ₀ t) + S ₁ exp (2πf ₁ t) +… + S _N-1 exp (2πf _N-1 t) (Formula 2)
s _U (t) = S _N exp (2πf _N t) + S _{N + 1} exp (2πf _{N + 1} t) +... + S _2N-1 exp (2πf _2N-1 t) (Formula 3)
s _L (t) is a digital signal of a frequency component on the low frequency side of the digital modulation signal s (t), and s _U (t) is a digital signal of a frequency component on the high frequency side of the digital modulation signal s (t). . Here, the low-frequency side signal s _L (t) and the high-frequency side signal s _U (t) are separated at the Nth order. That is, the low frequency side signal s _L (t) and the high frequency side signal s _U (t) are separated at a frequency just in the middle of the frequency band of the original digital modulation signal s (t). It is not limited to. As mentioned earlier, the low-frequency side signal s _L (t) and the high frequency side signal s _U (t) frequency down converts the high-frequency-side signal s _U '(t), but none of the digital-to-analog converter 201 What is necessary is just to isolate | separate on the basis of the frequency which becomes in a band.

Next, the digital IC 200 down-converts the frequency of the high frequency side signal s _U (t) by digital processing. That is, the high frequency side signal s _U (t) is frequency down converted frequency side signal s _U '(t) is represented by the following (Equation 4).
s _U '(t) = S _N exp (2πf ₀ t) + S _{N + 1} exp (2πf ₁ t) +… + S _2N-1 exp (2πf _N t) (Formula 4)
The signals s _L (t) and s _U ′ (t) generated by performing the operations of (Equation 1) to (Equation 4) by the digital IC 200 are input to the digital-analog converter 201 and are converted into analog signals. Thereafter, the high-frequency side signal s _U ′ (t) subjected to frequency down-conversion and analog conversion is output from the electrical signal port 202a, and the low-frequency side signal s _L (t) subjected to analog conversion is output from the electrical signal port 202b. .

The frequency of the signal s _U ′ (t) output from the electrical signal port 202a is up-converted by the frequency up-conversion unit 211a. That is, the signal s _U '(t) signal s _U after being upconverted''(t) can be expressed as the following (Formula 5).
s _U ″ (t) = S _N exp (2πf _N t) + S _{N + 1} exp (2πf _{N + 1} t) +... + S _2N-1 exp (2πf _2N-1 t) (Formula 5)
Here, s _U ″ (t) shown in (Formula 5) is equal to the above (Formula 3). That is, s _U ″ (t) is expressed as the following (Formula 6).
s _U '' (t) = s _U (t) (Formula 6)
The output s _U ″ (t) of the frequency up-conversion unit 211a and the output s _L (t) of the electric signal port 202b are input to the adder 212a and added. That is, the output s _out (t) of the electrical signal synthesis unit 210, which is the output of the adder 212a, is expressed as the following (Equation 7).
s _out (t) = s _U '' (t) + s _L (t) = s _U (t) + s _L (t) = s (t) (Formula 7)
In this way, it can be seen that the analog modulation signal s _out (t) is obtained by analog modulation of the digital modulation signal s (t).

  Here, in the optical modulation device shown in FIG. 2, a set including the electrical signal ports 202a and 202b, the frequency upward conversion unit 211a, and the adder 212a, the electrical signal ports 202c and 202d, and the frequency upward conversion unit The configuration using two sets of electric signals using two sets of 211b and the set constituted by the adder 212b is illustrated, but as a configuration using only one set and using one set of electric signals Alternatively, a plurality of sets may be configured to use a plurality of systems of electrical signals.

  FIG. 4 shows an example of the light modulation device according to the first embodiment of the present invention. FIG. 4 shows a digital signal processor (DSP) 301, a digital-analog converter 302 electrically connected to the DSP 301 and provided with RF terminals 303a to 303d at the output unit, and an electric terminal connected to the RF terminal 303a. A frequency up-conversion unit 304a, a power combiner 305a electrically connected to the frequency up-conversion unit 304a and the RF terminal 303b, a frequency up-conversion unit 304b electrically connected to the RF terminal 303c, A power combiner 305b electrically connected to the frequency up converter 304b and the RF terminal 303d, an RF driver 306a electrically connected to the output of the power combiner 305a, and an output connected to the output of the power combiner 305b RF driver 306b and RF driver 306a Optical modulator 308 having RF terminals 307a and 307b electrically connected to the optical modulator 306b, an optical connector 310a optically connected to the output of the optical modulator 308, the optical modulator 308, and a narrow line width, respectively. An optical modulation device including optical connectors 310b and 310c for optically connecting a wavelength tunable laser 309 is shown.

The light modulation device shown in FIG. 4 realizes a configuration in which an optical signal generated by the light modulation device is generated from two systems of digital modulation signals s ₁ (t) and s ₂ (t). The symbol rate of the digital modulation signals s ₁ (t) and s ₂ (t) was 50 GBaud. As the optical modulator 308, a lithium niobate (LN) modulator was used.

The DSP 301 generates the digital modulation signals s ₁ (t) and s ₂ (t) according to (Equation 1), the signal division shown in (Equation 2) and (Equation 3), and the lower frequency shown in (Equation 4). Conversion is performed to generate and output signals s _1U ′ (t), s _1L (t), s _2U ′ (t), and s _2L (t). These digital signals s _1U ′ (t), s _1L (t), s _2U ′ (t) and s _2L (t) are converted into analog signals by the digital-analog converter 302, and are respectively sent from the RF terminals 303a to 303d. Is output. The signal s ₁ ′ (t) output from the RF terminal 303a and the signal s _2U ′ (t) output from the RF terminal 303b are input to the frequency up converters 304a and 304b. The frequency up converters 304a and 304b perform arithmetic processing according to (Equation 5) based on the input signals s ₁ ′ (t) and s _2U ′ (t), and the signals s _1U ″ (t) and s _{2U respectively.} '' (t) is generated and output. Thereafter, the signal s _1u ″ (t) output from the frequency up-converter 304a and the signal s _1L (t) output from the RF terminal 303b are input to the power combiner 305a and added, and (Formula 7) Thus, the analog modulation signal s _out1 (t) = s ₁ (t) is generated. Similarly, the signal s _2U ″ (t) output from the frequency up-converter 304b and the signal s _2L (t) output from the RF terminal 302d are input to the power combiner 305b and added, (Equation 7) Thus, the analog modulation signal s _out2 (t) = signal s ₂ (t) is generated.

The two analog modulation signals s ₁ (t) and s ₂ (t) generated in this way are amplified by the RF drivers 306a and 306b, respectively, and input to the optical modulator 308 from the RF terminals 307a and 307b. The optical modulator 308 converts unmodulated continuous wave (CW) light input from the narrow line width variable wavelength light source 309 via the optical connectors 310a and 310b into analog modulated signals s ₁ (t) and s ₂ (t ) To modulate. The output optical signal of the optical modulator 308 is output from the optical connector 310c to the outside of the optical modulator.

FIG. 5A shows the input signal spectrum of the frequency up converter 304a, and FIG. 5B shows the output signal spectrum of the frequency up converter 304a. The input signal is a signal s _1U ′ (t) before frequency up-conversion, and as shown in FIG. 5A, the power of the input signal is distributed in a frequency range of 0 to 12.5 GHz. On the other hand, the output signal is a signal s _1U ″ (t) after frequency up-conversion, and as shown in FIG. 5B, the power of the signal is distributed in a frequency range of 12.5 to 25 GHz. . Thus, it can be seen that the frequency of the signal s _1U ′ (t) is converted upward by the frequency upward converter 304a. In the above description, the input / output signals of the frequency upward conversion unit 304a in the process of generating the analog modulation signal s ₁ (t) have been described, but the input of the frequency upward conversion unit 304b in the process of generating the analog modulation signal s ₂ (t) is described. The same applies to the output signal.

FIG. 6 shows electric signal characteristics of the light modulation device according to the first embodiment of the present invention. FIG. 6A shows the spectrum of the analog modulated signal s ₁ (t) after the addition is performed by the power combiner 305a. From FIG. 6A, it can be seen that the power of the analog modulation signal s ₁ (t) is distributed in a frequency range of 0 to 25 GHz. Thus, it can be confirmed from the spectrum that the Nyquist waveform signal has a symbol rate of 50 GBaud and a roll-off rate α≈0. FIG. 6B shows a time waveform of the analog modulation signal s ₁ (t). From FIG. 6B, it can be seen that the time waveform is opened at 20 ps intervals, so that it is 50 GBaud, and the transition waveform is a Nyquist waveform. Although the analog modulation signal s ₁ (t) has been described above, the same applies to the analog modulation signal s ₂ (t).

FIG. 7 shows output optical signal characteristics of the optical modulation device according to the first embodiment of the present invention. FIG. 7A shows the spectrum of the output optical signal of the optical modulation device according to the present invention, because power is distributed in the region where the relative frequency of the output optical signal of the optical modulation device is −25 to 25 GHz. It can be seen that a Nyquist waveform optical signal having a roll-off rate α≈0 can be generated at a symbol rate of 50 GBaud. FIG. 7B shows a constellation as a result of demodulating the output optical signal of the optical modulation device according to the present invention. As shown in FIG. 7B, an error vector amplitude (EVM) indicating the magnitude of signal degradation is 10% or less, which corresponds to a bit error rate of 10 ⁻⁹ or less, which is good. It can be seen that excellent signal characteristics are obtained.

In the above-described embodiment, the case where two systems of electric signals s ₁ (t) and s ₂ (t) are input to the optical modulator to generate an optical signal has been described as an example. This is because optical IQ modulators that individually modulate the amplitude of the real axis and the imaginary axis of light are often used in generating optical phase modulation signals. However, the optical modulation device according to the present invention is not limited to this example, and the optical modulation device that generates an optical signal from only one electric signal, the one that generates an optical signal from four electric signals, and the like. Of course, an optical signal may be generated from a plurality of electrical signals.

  In the above embodiment, the case where a power combiner is used as an adder has been described as an example. This is because the impedance of the three ports of the power combiner is made 50Ω. However, the light modulation device according to the present invention is not limited to this example, and a power splitter having a 2-port impedance of 75Ω and a 1-port impedance of 50Ω may be used, such as a power splitter, or an integrated circuit. Of course, it is possible to use an adder having a configuration in which impedance matching is ignored.

  In the above embodiment, an optical IQ modulator made of lithium niobate is used as an example of the optical modulator. This is because the LN optical modulator has low loss and chirp, and has a high extinction ratio. This is because a signal can be generated. However, the light modulation device according to the present invention is not limited to this example. As a material for the light modulator, a semiconductor such as indium phosphide, gallium arsenide, gallium nitride, silicon, or the like may be used. Of course, any organic material may be used.

  In the above embodiment, a configuration using a narrow linewidth wavelength tunable light source as a laser light source has been described as an example. However, for coherent transmission using phase information of light, transmission with less phase noise is performed with a narrow linewidth. This is because the characteristics are good, and when the wavelength is variable, the entire optical communication wavelength band can be covered. However, the light modulation device according to the present invention is not limited to this example, and may be a laser that is not a narrow line width or a laser that is not variable in wavelength.

  In the above embodiment, the case where the format of the generated optical signal is phase shift keying (QPSK) has been described as an example, but this is the case where QPSK is generated by the optical IQ modulator. This is because the two systems of signals become binary signals and the phenomenon is simple. However, the optical modulation device according to the present invention is not limited to this example, and the multi-dimensional code modulation is possible regardless of whether the modulation format is on-off modulation (OOK) or QAM. Even so, of course.

  In the above embodiment, the configuration using the RF driver for amplifying the electric signal has been described as an example. This is because the output of the digital-analog converter is often smaller than the drive amplitude of the optical modulator. . However, the optical modulation device according to the present invention is not limited to the configuration using the RF driver, and may be configured so as not to use the RF driver by using an optical modulator having high modulation efficiency, or to a high output digital-analog conversion. Of course, a configuration that does not use an RF driver by using an amplifier, or a configuration that does not use an RF driver by using a frequency upward converter or an adder provided with an amplification function may be used.

  In the above embodiment, the case where the symbol rate of the generated optical signal is 50 GBaud has been described as an example. However, this is a symbol rate that cannot be generated in the band of the digital-analog converter used this time, and the calculation is simple. This is because it is easy to verify the principle. However, the light modulation device according to the present invention is not limited to this example, and the symbol rate may be 40 GBaud, 80 GBaud, or any other rate.

  In the above embodiment, the case where the generated optical signal has a Nyquist waveform with a roll-off rate α≈0 has been described as an example, but this is because the spectrum of the Nyquist waveform signal with α = 0 is rectangular. This is because it is easy to confirm the principle. However, the light modulation device according to the present invention is not limited to this example, and may be a Nyquist waveform with α = 0.2 or a signal with a rectangular time waveform with α = 1. Of course, other α may be used.

(Second Embodiment)
FIG. 8 shows an optical modulation apparatus according to the second embodiment of the present invention. In FIG. 8, the digital IC 200 is electrically connected to the digital IC 200, and is electrically connected to the digital-analog converter 201 provided with the electric signal ports 202 a to 202 d in the output unit. The electrical signal synthesizer 400, the optical modulator 106 electrically connected to the electrical signal synthesizer 400, optically connected to the laser light source 105, and the output optically connected to the output of the optical modulator 106 An optical modulator with an optical port 107 is shown.

  The electric signal synthesis unit 400 includes 90 ° phase shifters 401a and 401b, IQ modulators 410a and 410b, a periodic signal generator 402, and adders 403a and 403b. The IQ modulator 410a receives the 90 ° phase shifter 401c to which the output of the periodic signal generator 402 is input, one output of the 90 ° phase shifter 401a, and one output of the 90 ° phase shifter 401c. A multiplier 411a, a multiplier 411b to which the other output of the 90 ° phase shifter 401a and the other output of the 90 ° phase shifter 401c are input, and an adder 403c to which the outputs of the multipliers 411a and 411b are input including. The IQ modulator 410b receives the 90 ° phase shifter 401d to which the output of the periodic signal generator 402 is input, one output of the 90 ° phase shifter 401b, and one output of the 90 ° phase shifter 401d. A multiplier 411c, a multiplier 411d to which the other output of the 90 ° phase shifter 401b and the other output of the 90 ° phase shifter 401d are input, and an adder 403d to which the outputs of the multipliers 411c and 402d are input ,including.

  The 90 ° phase shifters 401a and 401b have a function of distributing the electric signals output from the electric signal ports 202a and 202c, respectively, to two signals having a 90 ° phase difference, that is, by performing a Hilbert transform. It corresponds to being. Since the signals subjected to the Hilbert transform in the 90 ° phase shifters 401a and 401b are input to the IQ modulators 410a and 410b, the IQ modulators 410a and 410b perform a single side band (SSB) modulation operation. To do.

  For example, when one of the two outputs of the 90 ° phase shifter 401a is 0 °, the other phase is 90 °. Similarly, when one of the two outputs of the 90 ° phase shifter 401b is 0 °, the other phase is 90 °. The multiplier 411a multiplies the 90 ° phase shifter 401a output of the 90 ° phase shifter 401a and the 90 ° phase shifter 401c output of the 90 ° phase shifter 401c, and the multiplier 411b outputs the 90 ° phase shifter 401a output of 90 ° and 90 °. When the phase 90 ° output of the phase shifter 401c is multiplied, the IQ modulator 410a operates in SSB modulation.

Even if the electrical signal synthesis unit has such a configuration, the effects of the present invention can be achieved. Hereinafter, the operations of the IQ modulators 410a and 410b will be described using mathematical expressions with reference to FIG. FIG. 9A shows the IQ modulator 410a. If the periodic signal generator 402 outputs a signal having a sine wave waveform and the angular frequency is ω (frequency f = ω / 2π), the input Vin _in the 90 ° phase shifter 401c is as follows: (Expression 8)

V _in = cos (ωt) (Formula 8)
The 90 ° phase shifter 401c receives cos (ωt) for the multiplier 411a and sin (ωt) for the multiplier 411b as two signals having a 90 ° phase difference with respect to the input of (Equation 8). Is output.

On the other hand, it is assumed that the 90 ° phase shifter 401a outputs a signal having a sinusoidal waveform, and its angular frequency is assumed to be Ω (frequency F = Ω / 2π). Since the 90 ° phase shifter 401a outputs two signals with a 90 ° phase difference, cos (Ωt) is output to the multiplier 411a and sin (Ωt) is output to the multiplier 411b. The multiplier 411a outputs cos (ωt) cos (Ωt) as a multiplication result, and the multiplier 411b outputs sin (ωt) sin (Ωt) as a multiplication result to the adder 403c. By adding the signals output from the multipliers 411a and 411b by the adder 403c, the output signal _Vout of the IQ modulator 410a is finally expressed by the following (Equation 9).

The spectrum of the output signal V _out is shown in FIG. As shown in FIG. 9B, the IQ modulator 410a up-converts the output angular frequency Ω of the 90 ° phase shifter 401a by the output angular frequency ω of the periodic signal generator 402 (ie, 90 ° phase shift). The output frequency F of the generator 401 a can be converted upward by the output frequency f of the periodic signal generator 402.

The output signal _Vout and the second analog signal are input to the adder 403a and added, and output to the optical modulator 106 as an analog modulation signal s ₁ (t).

In the above description, the output of the 90 ° phase shifter 401a has been described as a sinusoidal waveform signal that is a single frequency component, but in practice, the output of s _1U ′ (t) shown in FIG. Such a signal having frequency components distributed over a predetermined band. It goes without saying that even if the output of the 90 ° phase shifter 401a is such a signal, the IQ modulator 410a can upconvert the output signal of the 90 ° phase shifter 401a by the output frequency of the periodic signal generator 402. . Needless to say, the IQ modulator 410b operates in the same manner as the IQ modulator 410a.

  According to the optical modulation device according to the second embodiment of the present invention, when frequency up-conversion is performed by SSB modulation using the IQ modulator 410, generation of unnecessary spectral components at the time of frequency up-conversion calculation can be suppressed. If the frequency up-conversion unit is configured to generate unnecessary spectral components, an additional band-pass filter must be provided, increasing the number of components. In addition, when an additional bandpass filter is provided, it is generally difficult to realize a filter with a steep cutoff characteristic, so a filter with a gentle cutoff characteristic is used. In this case, the necessary signal is filtered. As a result, or unnecessary signals remain, the characteristics of the generated electric signal deteriorate. These characteristic deteriorations can be compensated in advance by digital processing in the digital IC 200, but in this case, the scale of the digital IC increases. From the above, it is an advantage of the light modulation device according to the second embodiment of the present invention that the effects of the present invention can be achieved while suppressing signal characteristic deterioration and digital IC scale increase.

  Further, in the optical modulation device according to the second embodiment of the present invention, the scale of the digital-analog converter can be suppressed by using a 90 ° phase shifter for generating the Hilbert transform pair necessary for performing the SSB modulation. it can. The generation of the Hilbert transform pair can be performed in the digital domain or the analog domain. However, when the Hilbert transform pair is generated in the digital domain, in order to perform signal processing similar to the configuration shown in FIG. Six RF terminals are required, and the number of electrical signal ports shown in FIG. 8 is increased by a factor of 1.5 compared to the four configuration. Thus, it is an advantage of the second embodiment of the present invention that the effects of the present invention can be achieved while suppressing the increase in the scale of the digital-analog converter.

(Third embodiment)
FIG. 10 shows an optical modulation apparatus according to the third embodiment of the present invention. As shown in FIG. 10, the light modulation device according to the third embodiment has the same configuration as the light modulation device according to the second embodiment, but the digital modulation signal s (t (t ) Is divided into the high frequency side signal s _U (t) and the low frequency side signal s _L (t), the division is asymmetric, and the signal band of the low frequency side signal s _L (t) is the high frequency side signal s. It differs from the second embodiment in that it is 52% or more with respect to the total of the signal band of _U (t) and the signal band of the low frequency side signal s _L (t).

FIG. 10 also shows an outline of signal processing in the light modulation device according to the third embodiment of the present invention. In the configuration of the optical modulation device according to the second embodiment of the present invention shown in FIG. 8, 90 ° phase shifters 401a and 401b are used as Hilbert converters. However, it is generally difficult to realize a 90 ° phase shifter that maintains 90 ° characteristics from a frequency band in which signal power is distributed to 0 Hz. That is, in the light modulation device according to the present invention, when a signal with a symbol rate of 50 GBaud is generated (the frequency band of the signal is 0 to 25 GHz), the signal s _U ′ obtained by down-converting the high frequency side signal s _U (t). Although a 90 ° phase shifter that maintains 90 ° property from 0 Hz to 12.5 GHz, which is the band of (t), is necessary, it is difficult to realize it. The reason is that, in principle, it is difficult to achieve impedance matching for frequencies that are different by digits.

Therefore, as in the optical modulation apparatus according to the third embodiment of the present invention, for example, the high frequency side signal s _U (t) is a frequency band of 13.5 to 25 GHz, and the signal s _U ′ obtained by frequency down-converting it. If the digital IC 200 performs signal processing so that (t) is a signal in the band of 1.0 to 12.5 GHz, it is possible to avoid a low-frequency band signal close to a DC component being input to the 90 ° phase shifter. The 90 ° property of the 90 ° phase shifter can be maintained. At this time, the signal band of the low frequency side signal s _L (t) is increased by the amount corresponding to the reduction of the signal band of the high frequency side signal s _U (t), and in this example, the digital IC 200 distributes from 0 Hz to 13.5 GHz. Signal processing may be performed. Then, similarly to the second embodiment, the periodic signal generator 402 outputs a signal having a sinusoidal waveform having a frequency (12.5 GHz in the above example) that is the center of the band of the digital modulation signal s (t). By setting as described above, an analog modulation signal having the same band as that of the original digital modulation signal s (t) can be obtained from the adders 403a and 403b as shown in FIG.

As described above, when digital processing is performed to asymmetrically divide the spectrum so as to avoid the low frequency band, the signal band (Δf _SU ) of the high frequency side signal s _U (t) and the low frequency side signal s _L (t When the signal band of the low frequency side signal s _L (t) is set to 52% or more with respect to the total of the signal band (Δf _SL ) of), an output optical signal with no deterioration in signal quality can be obtained. Here, “the sum of the signal band (Δf _SU ) of the high frequency side signal s _U (t) and the signal band (Δf _SL ) of the low frequency side signal s _L (t)” refers to the digital modulation signal s (t). This is the entire signal band.

Figure 11 shows the effect on the high frequency side signal s _U (t) and the output optical signal quality ratio of the low frequency side signal s _L low-frequency side signal s to the sum of (t) _L (t). The horizontal axis in FIG. 11 represents Δf _SL / (Δf _SL + Δf _SU ), and the vertical axis represents EVM representing the magnitude of signal degradation. As shown in FIG. 11, when Δf _SL / (Δf _SL + Δf _SU ) is 50% or less, EVM is 20% or more and signal deterioration is observed, but Δf _SL / (Δf _SL + Δf _SU ) is 52% or more. In this area, the EVM is 10% or less and no signal deterioration is observed.

As described above, in the configuration of the optical modulation device according to the third embodiment of the present invention, the low frequency side with respect to the sum of the high frequency side signal band (Δf _SU ) and the low frequency side signal band (Δf _SL ). It is desirable that the signal band of the above is 52% or more.

  As described above, in the optical modulation device according to the third embodiment of the present invention, the 90 ° configuration of the electric signal combining unit 410 without receiving the band limitation bottleneck caused by the narrow band digital-to-analog converter 201. A modulated optical signal with a high symbol rate can be obtained without being affected by the 90 ° phase shift characteristic in the phase shifter 401.

(Fourth embodiment)
FIG. 12 shows an optical modulation apparatus according to the fourth embodiment of the present invention. The light modulation device according to the present embodiment performs the Hilbert transform function in the electric signal synthesis unit 400 described in the second embodiment by digital processing in the digital IC 500 shown in FIG. The configuration other than the converter 501 and the electric signal ports 502a and 502f is the same as that of the second embodiment.

  Two electrical signal ports 502a and 502b of the digital-analog converter 501 are used as inputs to the IQ modulator 410a, and two electrical signal ports 502d and 502d of the digital-analog converter 501 are used as inputs to the IQ modulators 410a and 410b. 502e is used. Since the IQ modulators 410a and 410b are driven as SSB modulation, signals output from the electric signal ports 502a and 502b by digital processing in the digital IC 500 form a Hilbert transform pair, and similarly output from the electric signal ports 502d and 502e. The signal to be processed also forms a Hilbert transform pair. Even if it is such a structure, there can exist the effect of this invention.

In the optical modulation device according to the present embodiment, as in the third embodiment, in the digital IC 500, the band of the low frequency side signal s _L (t) is lower than the band of the modulation signal s (t). You may process so that it may divide | segment asymmetrically so that it may become a signal band of 52% or more.

  FIG. 13 shows a comparison of the symbol rate dependence of the signal quality in the light modulation devices according to the first to fourth embodiments. The horizontal axis of the graph shown in FIG. 13 is a symbol rate, and the vertical axis is EVM representing the magnitude of signal degradation. Here, a digital-to-analog converter having a 3 dB band of 20 GHz was used. The light modulation device described in any of the embodiments exhibits the same characteristics as those shown in FIG. As the symbol rate increases, EVM increases due to the band limitation of the digital-analog converter.

  As shown in FIG. 13, the output optical signal quality of the conventional optical modulator shown in FIG. 1 exceeds 10% when the symbol rate is 25 GBaud or more. On the other hand, the output optical signal of the light modulation device according to the present invention is lower than EVM 10% when the symbol rate is 50 GBaud or less. As described above, the optical modulation device according to the present invention has a symbol rate of 25 GBaud or more that would deteriorate the characteristics of the conventional optical modulation device when a digital-to-analog converter with a 3 dB band of 20 GHz is used. Also, it is possible to suppress the deterioration of the modulation characteristics. That is, in the optical modulation device according to the present invention, a modulated optical signal with a high symbol rate can be obtained without receiving a bottleneck of band limitation due to a narrow band digital-analog converter.

(Fifth embodiment)
FIG. 14 shows an optical modulation device according to the fifth embodiment of the present invention. In the optical modulation device according to the fifth embodiment of the present invention shown in FIG. 14, the frequency up-conversion units 202a and 202b and the adders 203a and 203b are integrated in an electric signal synthesis IC 600 of one chip.

  The output signal of the digital-analog converter 201 output from the electrical signal ports 202a to 202d is input to the electrical signal synthesis IC 600, and is output after frequency up-conversion and addition operations are performed within the electrical signal synthesis IC 600. With such a configuration, the effects of the present invention can be achieved using small components.

(Sixth embodiment)
FIG. 15 shows an optical modulation apparatus according to the sixth embodiment of the present invention. The optical modulation device according to the sixth embodiment of the present invention shown in FIG. 15 includes signal amplification units 701a and 701b for amplifying an analog modulation signal inside the electric signal synthesis IC 600. With such a configuration, even when an optical modulator with low modulation efficiency is used, the effects of the present invention can be achieved with a simple configuration without adding a modulator driver.

Digital IC 101, 200, 500
Digital-to-analog converter 102, 201, 302, 501
Electrical signal port 103, 202, 502
Modulator driver 104
Laser light source 105
Optical modulator 106, 308
Output optical port 107
Electric signal synthesizer 210, 400
Frequency upward conversion unit 211, 304
Adders 212 and 403
DSP 301
RF terminal 303, 307
Power combiner 305
RF driver 306
Narrow linewidth tunable laser 309
Optical connector 310
90 ° phase shifter 401
Periodic signal generator 402
IQ modulator 410
Multiplier 411
Electric signal synthesis IC 600
Signal amplification unit 701

Claims (8)

Digital IC,
A digital-to-analog converter electrically connected to the digital IC;
An electrical signal synthesis unit electrically connected to the digital-analog converter;
An optical modulator that is electrically connected to the electrical signal synthesis unit and modulates light input from a light source with an analog modulation signal output by the electrical signal synthesis unit;
The digital IC separates the digital modulation signal into a high frequency side component digital signal and a low frequency side component digital signal, down-converts the high frequency side component digital signal, and the low frequency side component digital signal Output the digital signal of the high frequency side component that has been down-converted to the frequency to the digital analog converter,
The digital-to-analog converter converts the digital signal of the high-frequency side component that has been down-converted into the frequency into a first analog signal, converts the digital signal of the low-frequency side component into a second analog signal, and Outputting a second analog signal to the electrical signal synthesizer;
The electrical signal synthesis unit frequency-upconverts the first analog signal to generate a frequency-upconverted first analog signal having the same frequency band as the digital signal of the high-frequency side component, and the frequency up-conversion An optical modulation device characterized by adding the first analog signal and the second analog signal, and outputting the added signal as an analog modulation signal to the optical modulator. The electrical signal synthesizer
A frequency up converter for generating a first analog signal frequency up conversion having the same frequency band as the digital signal of the first said high frequency side component and frequency up converts the analog signal, which is the frequency up conversion An adder that adds the first analog signal and the second analog signal and outputs the added signal to the optical modulator as the analog modulation signal;
The light modulation device according to claim 1, further comprising: The electrical signal synthesizer
An IQ modulator, a periodic signal generator, and first and second adders,
The IQ modulator includes a first 90 degree phase shifter, and first and second multipliers;
The periodic signal generator outputs a signal having a frequency that is a frequency shift amount when the first analog signal is frequency-upconverted to the first 90-degree phase shifter,
The first 90-degree phase shifter shifts the signal in phase with the signal of the periodic signal generator and the signal of the periodic signal generator by 90 degrees based on the signal input from the periodic signal generator. Generating a signal and outputting it to the first and second multipliers;
The first multiplier multiplies the in-phase signal and the first analog signal and outputs the result to the first adder.
The second multiplier multiplies the signal phase-shifted by 90 degrees and the signal obtained by phase-shifting the first analog signal by 90 degrees and outputs the result to the first adder;
The first adder adds the outputs from the first and second multipliers to generate a first analog signal whose frequency is up- converted and outputs the first analog signal to the second adder. ,
The second adder adds the first analog signal that has been frequency- upconverted and the second analog signal, and outputs the added signal to the optical modulator as the analog modulation signal. The light modulation device according to claim 1. The electrical signal synthesis unit further includes a second 90-degree phase shifter,
The second 90-degree phase shifter receives the first analog signal, outputs a signal in phase with the first analog signal to the first multiplier, and outputs the first analog signal to the first 90-degree phase shifter. 4. The optical modulation device according to claim 3, wherein the phase-shifted signal is output to the second multiplier. 5.   The digital IC includes the digital modulation signal so that both the low-frequency component digital signal band and the frequency down-converted high-frequency component digital signal band are within the digital-analog converter band. The light modulation device according to claim 1, wherein the light modulation device is separated.   6. The digital IC according to claim 1, wherein the digital IC separates the digital modulation signal so that a band of the digital signal of the low frequency side component is 52% or more of a band of the digital modulation signal. The light modulation device according to any one of the above.   The light modulation device according to claim 1, wherein the electric signal combining unit is integrated on one chip. The optical modulation device according to claim 1, wherein the electrical signal synthesis unit further includes an amplifier that amplifies the analog modulation signal.