CN114137743A - High-linearity modulator chip based on cascade silicon-based micro-ring modulator and modulation method - Google Patents
High-linearity modulator chip based on cascade silicon-based micro-ring modulator and modulation method Download PDFInfo
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Abstract
The invention discloses a high-linearity modulator chip based on a cascade silicon-based micro-ring modulator and a modulation method. Two lasers with different wavelengths output optical carrier signals and are combined by an on-chip beam combiner or a wavelength division multiplexer, and because the micro-ring modulator has a wavelength selection function, two paths of optical carriers are respectively coupled into two silicon-based micro-ring modulators through bus waveguides. The electric modulation signal is divided into two paths by an electric power divider with adjustable power division ratio and the two micro-ring modulators are respectively modulated. By regulating and controlling the working points of the two micro-ring modulators, one of the micro-ring modulators works at the maximum point of the first-order harmonic component, and the other micro-ring modulator works at the maximum point of the third-order nonlinearity. And meanwhile, the distribution ratio of the power divider is regulated and controlled, so that three-order nonlinearity in the two micro-rings can be mutually offset through the operation when the photoelectric detector end is used for demodulation, and the change of the first-order harmonic component is ensured to be small.
Description
Technical Field
The invention relates to a high-linearity modulator chip based on a cascade silicon-based micro-ring modulator, in particular to a high-linearity modulator chip based on a cascade silicon-based micro-ring modulator and a modulation method.
Background
In recent years, microwave photon technology combining microwave technology and light wave technology has been widely used in wireless communication, optical fiber radio frequency transmission systems, radar electronic warfare and other fields. The method mainly researches how to realize generation, transmission, processing, detection and the like of microwave/millimeter wave signals by using a photoelectric device and an optical method. With the increasing of the complexity of the functions of the microwave photonic system and the urgent need for cost reduction, the size, weight, and power consumption of the system from the device to the system are required to be reduced, and the miniaturization, weight, and cost reduction of the system are required.
In the microwave photonic link, the performance of the modulator for converting the microwave signal from the electrical domain to the optical domain is important because the transmission function of the modulator has certain nonlinearity, and high-order harmonics and intermodulation signals other than the first-order harmonic signal, particularly the third-order harmonic HD3(the 3) are generated in the modulation processrdHarmonic Distortion, HD3) and third-order intermodulation signal IMD3(the 3)rdInter-Modulation discrimination, IMD3) reduces the Spurious-Free Dynamic Range, SFDR, of microwave photonic systems. The linearity of the modulator is a very important criterion for achieving high fidelity transmission of microwave signals in analog optical links and for achieving high dynamic link performance.
At present, most of microwave photonic links use lithium niobate modulators, but the lithium niobate modulators have the defects of large volume, large insertion loss, high power consumption and the like. In contrast, a silicon optical integration platform having advantages of small size, low power consumption, CMOS (Complementary Metal Oxide Semiconductor) compatibility, and capability of optoelectronic monolithic integration has a great potential in the aspects of miniaturization and low cost of microwave photonic systems. Silicon-based modulators as important silicon optical devices have a significant impact on link performance. Silicon-based modulators may be classified into Silicon-based Mach-Zehnder modulators (Si-MZMs) and Silicon-based micro Ring modulators (Si-MRMs) according to their structures. Compared with Si-MZM, Si-MRM has advantages such as super small-size, super low power consumption and super large bandwidth, has very big potentiality in microwave photonic system miniaturization. However, the Si-MRM is affected by the lorentz modulation curve and the carrier dispersion effect, and has very strong electro-optic modulation nonlinearity, that is, very strong third-order harmonic and intermodulation nonlinear signals can be generated in the modulation process, thereby reducing the dynamic range of the microwave photonic link. Therefore, how to improve the linearity of the Si-MRM is a scientific issue that is of great interest to the current academic and industrial fields.
The current methods for improving the linearity of Si-MRM can be divided into two categories: the first type is the integration of linear electro-optic material films on silicon optical chips, such as silicon-based lithium niobate film micro-ring modulators. The method uses the linear electro-optic material, so that the linearity is improved to a certain extent compared with Si-MRM based on the carrier dispersion effect. However, this method introduces a very complicated process and would introduce very high costs. And the method can not realize the monolithic integration of the photoelectric device, and is not beneficial to the further integration and miniaturization of the microwave photon system. The second method is to use the nonlinearity of the DC-Kerr effect to counteract the nonlinearity of the carrier dispersion effect, thereby realizing the high linear modulation of Si-MRM. However, this method changes the process flow of the chip foundry, and increases the complexity and cost of chip processing.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a high-linearity modulation chip based on a cascade silicon-based micro-ring modulator and a modulation method, the method respectively couples two optical carrier signals with the same power into corresponding Si-MRMs to regulate and control the working points of the two Si-MRMs, so that one Si-MRM works at the maximum working point relative to a first-order harmonic HD1, the other Si-MRM works at the maximum working point relative to a third-order harmonic HD3, and the phase difference pi of the two working points is ensured. By regulating the power ratio of the modulation signals loaded on the two Si-MRMs, complete cancellation of the HD3 component can be realized, and the power of the HD1 component is kept basically unchanged.
The technical scheme of the invention is as follows:
the invention firstly provides a high linearity modulator chip based on a cascade silicon-based micro-ring modulator, which comprises: the optical fiber Modulator comprises two input end couplers for coupling optical carrier signals output by a laser into a Modulator chip, a beam combiner for combining optical signals of the two input end couplers into one path, a bus waveguide connected with the beam combiner and used for transmitting the optical signals, two cascaded Silicon-based Micro-Ring modulators (Silicon-based Micro-Ring modulators 1, Si-MRM1) and an output end coupler connected with the bus waveguide;
the two cascaded silicon-based micro-ring modulators comprise a first silicon-based micro-ring modulator and a second silicon-based micro-ring modulator, and the two silicon-based micro-ring modulators are completely the same and are coupled on the same bus waveguide.
As a preferred scheme of the present invention, the input end coupler is a grating coupler or an end face coupler; the output end coupler is a grating coupler or an end face coupler.
As a preferred embodiment of the present invention, the beam combiner for combining the optical signals of the two input end couplers into one path is an optical beam combiner or a wavelength division multiplexer.
As a preferred scheme of the invention, the silicon-based micro-ring modulator is a straight-through micro-ring modulator and comprises a micro-ring resonant cavity embedded with a PN junction phase shifter, and the silicon-based micro-ring modulator is integrated with a thermo-optic phase shifter and used for regulating and controlling a working point; the resonant cavities of the first silicon-based micro-ring modulator and the second silicon-based micro-ring modulator are distributed on one side of the same bus waveguide.
The invention also discloses a modulation method of the high-linearity modulator chip, which comprises the following steps:
1) two wavelengths are respectively lambda1And λ2The optical carrier signals output by the peripheral laser are respectively coupled into the modulator chip through the input end coupler;
2) two paths of optical carrier signals with different wavelengths of the two input end couplers are combined into one path of optical signal through the beam combiner and transmitted in the bus waveguide;
3) regulating and controlling reverse bias voltage V of PN junction phase shifter on two silicon-based micro-ring modulatorsDC1And VDC2Making PN junction modulation arms on the two silicon-based micro-ring modulators work in a reverse bias region;
4) regulating and controlling thermo-optic phase shifters on the two silicon-based micro-ring modulators to enable the first silicon-based micro-ring modulator to work at the HD1 relative maximum point, the second silicon-based micro-ring modulator to work at the HD3 relative maximum point, and the phase difference between the two working points is pi;
5) a radio frequency signal generator is used for generating a double-tone modulation signal and the double-tone modulation signal is divided into two paths through a radio frequency power divider with adjustable power division ratio; the two paths of double-tone signals and two paths of direct current reverse Bias voltage generated by a voltage source are loaded on the corresponding two silicon-based micro-ring modulators through two Bias-tee;
6) by regulating the power dividing ratio of the radio frequency power divider, the three-order nonlinear HD3 generated by the two silicon-based micro-ring modulators are mutually offset at the end of the photoelectric detector, the power change of the HD1 component is within 3dB, and the dynamic range of a link is improved.
As a preferred embodiment of the present invention, before the optical signal modulated by the high linearity modulator chip is connected to the photodetector for demodulation, the method further includes a step of amplifying the optical signal modulated by the high linearity modulator chip by using an optical fiber amplifier to compensate for the coupling loss of the chip.
As a preferred embodiment of the present invention, the step 6) specifically comprises: the optical signal is accessed into a photoelectric detector for demodulation, the demodulated signal is introduced into a frequency spectrograph, the power division ratio of a radio frequency power divider is regulated to regulate the power ratio of the double-tone modulation signals loaded on two silicon-based micro-ring modulators, the power of first-order harmonic HD1 and third-order intermodulation signals IMD3 on the frequency spectrograph is observed at the same time until the power difference between HD1 and IMD3 is maximum, the power of HD1 is ensured to be greater than that of IMD3, and the power change of HD1 component is within 3dB, and the optimal linear working point of the modulator chip is obtained at the moment.
As a preferred embodiment of the present invention, the step 4) specifically comprises:
4.1) adjusting and controlling thermo-optic phase shifters on two silicon-based micro-ring modulators to make the wavelength be lambda1And λ2The optical carrier waves are respectively coupled into a first silicon-based micro-ring modulator and a second silicon-based micro-ring modulator;
4.2) regulating and controlling the thermo-optic phase shifter of the first silicon-based micro-ring modulator to enable the thermo-optic phase shifter to work at the maximum slope of the transfer function of the modulator, wherein the maximum point corresponds to the first-order harmonic component HD 1;
4.3) regulating and controlling the thermo-optic phase shifter of the second silicon-based micro-ring modulator to enable the thermo-optic phase shifter to work at the third-order nonlinear maximum point, and simultaneously ensuring that the direction of the thermo-optic phase shifter is opposite to that of the working point of the first silicon-based micro-ring modulator, and correspondingly, the third-order harmonic component HD3 is maximum.
Compared with the method for improving the linearity of the Si-MRM by changing the chip processing technology, the method has the advantages that the minimum working point of the third-order intermodulation IMD3 can be found by simply regulating and controlling the distribution ratio of the modulation signals loaded on the two Si-MRMs on the premise of not changing the technological parameters and the flow of a chip foundry, and the first-order harmonic HD1 basically keeps the maximum value under the condition of finding the minimum third-order intermodulation IMD 3. The method does not bring high processing complexity and cost due to process change, and the high-linearity microwave photonic link based on the high-linearity silicon-based modulator can be realized only by changing the power division ratio of the modulation signals loaded on the two Si-MRMs. The chip manufacturing process is based on the CMOS process, namely, the photonic chip and the electronic chip can be manufactured on the same chip, and the peripheral control chip can be integrated on the same chip, so that the size and the power consumption of the whole system are greatly reduced, and the production cost is also saved.
Drawings
Fig. 1 is a schematic diagram of a modulator structure and a test flow of the present invention.
Fig. 2 is a schematic diagram of the modulation principle of the micro-ring modulator.
Fig. 3 is a simulation result of the variation of the components of the micro-ring modulators HD1 and HD3 with wavelength. The abscissa represents the wavelength Detuning amount (Frequency Detuning) with respect to the resonance wavelength, and "0" represents that the operating point is the resonance wavelength point.
Detailed Description
The invention is further illustrated with reference to the following figures and examples.
As shown in fig. 1, the high linearity modulator chip based on the cascaded silicon-based micro-ring modulator includes: the optical fiber modulator comprises two input end couplers for coupling optical carrier signals output by a laser into a modulator chip, a beam combiner for combining optical signals of the two input end couplers into one path, a bus waveguide connected with the beam combiner and used for transmitting the optical signals, two cascaded silicon-based micro-ring modulators (Si-MRM1) and an output end coupler connected with the bus waveguide;
the two cascaded silicon-based micro-ring modulators comprise a first silicon-based micro-ring modulator and a second silicon-based micro-ring modulator, and the two silicon-based micro-ring modulators are completely the same and are coupled on the same bus waveguide. In the figure, "1" is a coupler of a modulator chip and an optical fiber, typically a grating coupler or an end-face coupler. "2" is an on-chip beam combiner or wavelength division multiplexer, and "3" is a bus waveguide.
As shown in FIG. 1, the invention cascades two Si-MRMs based on carrier depletion type, and the resonant cavities of the two Si-MRMs are distributed on one side of the same bus waveguide. Two paths of optical carriers with different wavelengths are synthesized by an on-chip optical beam combiner or a wavelength division multiplexer, transmitted by the same bus waveguide and respectively coupled into corresponding Si-MRM. By regulating thermo-optical phase shifters on the two Si-MRMs, the two Si-MRMs respectively work at a relative maximum point HD1 and a relative maximum point HD3, and the phase difference of the two working points is pi. On a secondary basis, the power distribution ratio of the radio frequency power divider is regulated, so that HD3 components generated by the two-stage modulator are mutually offset during demodulation of the high-speed detector, and the power of the output HD1 component is changed slightly.
The high-linearity modulation method based on the cascade silicon-based micro-ring modulator comprises the following steps:
1) two laser devices with different wavelengths output two paths of unmodulated optical carrier signals, and the two paths of unmodulated optical carrier signals are coupled into a modulator chip through a structure 1 in the graph 1;
2) as shown in fig. 1, two optical signals are synthesized into one optical signal by a structure "2" and transmitted by a bus waveguide "3";
3) as shown in fig. 1, two dc power supplies generate two reverse Bias voltages (generally-2V), the rf dual tone modulation signal is divided into two paths by the rf power divider with adjustable power division ratio, and the two paths of rf electrical signals and the corresponding reverse Bias voltages are loaded on the PN junction phase shifters of Si-MRM1 and Si-MRM2, respectively, after being combined by two Bias-tee circuits. After the modulation signal is loaded on the micro-ring modulator, the resonance wavelength of the micro-ring modulator can be changed, so that the output optical power of the bus waveguide is changed, and the modulation function is realized. FIG. 2 is a schematic diagram of the principle of the Lorentz transfer curve, the modulation signal and the modulated optical signal including the micro-ring modulator;
4) thermo-optical phase shifters TOPS1 and TOPS2 on Si-MRM1 and Si-MRM2 are adjusted to couple two optical carrier signals corresponding to wavelengths λ 1 and λ 2 into Si-MRM1 and Si-MRM2, respectively, and to operate the operating points of the two modulators at the relative maximum points of HD1 and IMD3, and the relationship between HD1 and IMD3 and the wavelength of the micro-ring modulator, respectively. It can be seen that the relative sizes of the HD1 and IMD3 components at the off-channel wavelengths differ;
5) the modulated optical signal is output through a structure 1, passes through coupling loss of an EDFA amplification compensation chip, and is finally input into a high-speed photoelectric detector for demodulation;
6) inputting the demodulated electric signal into an electric frequency spectrograph, regulating and controlling a power division ratio gamma of the radio frequency power divider, and observing relative sizes of HD1 and IMD3 components through the electric frequency spectrograph, namely a carrier to Distortion ratio (CDR);
7) step "6" is repeated until the CDR reaches the maximum and the HD1 component does not change much, which is the optimal linear modulation point. The method specifically comprises the following steps: and introducing the demodulated signal into a frequency spectrograph, regulating and controlling the power division ratio of the radio frequency power divider to regulate and control the power ratio of the two-tone modulation signals loaded on the two Si-MRMs, and simultaneously observing the power of a first-order harmonic HD1 and a third-order intermodulation signal IMD3 on the frequency spectrograph until the power difference between HD1 and IMD3 is maximum (simultaneously ensuring that the power of HD1 is greater than that of IMD3) and HD1 basically keeps unchanged, and at the moment, the optimal linear working point of the modulator chip is obtained.
According to the time domain coupling mode theory, the linearity theory of a single micro-ring modulator considering the influence of third-order harmonic waves is deduced:
suppose angular frequency is ωRFAmplitude v0RF modulation signal v0cosωRFt is loaded on the micro-ring modulator, and the generated AC alternating current signal is as follows:
in the formula f (omega)RF) And
respectively representThe mode and phase of the frequency response.
According to the time-domain coupling mode theory, the energy amplitude α (t) stored in the micro-ring satisfies:
wherein, ω and AmAngular frequency and amplitude, omega, respectively representing the amplitude of the incident light field0Representing the micro-ring modulator resonance angular frequency. 1/tau represents the amplitude decay rate of the optical field in the whole micro-ring resonant cavity. Omega can be adjusted by controlling PN junction design parameters0And 1/tau, so that the designed and fabricated micro-ring modulator has a fixed omega0And 1/τ (Haifeng Shao, Hui Yu, Xia Li, Yan Li, Jianfei Jiang, Huan Wei, Gencheng Wang, Tingge Dai, Qimei Chen, Jianyi Yang, and Xiaoqiing Jiang].Optics Letters,2015,40(14):3360-3363)。
For a single silicon-based micro-ring modulator, the transmission optical field S (t) Ae of the bus waveguidejωt-j μ a (t). To obtain third order nonlinearity of the micro-ring modulator and simplify the calculation, for ω0And 1/τ Taylor expansion to the fourth term, yielding:
wherein, ω isDCAnd 1/tauDCRespectively representing the resonance frequency and the amplitude decay rate at the dc bias point. k is a radical of1、k2、k3Is omega0Coefficients of corresponding first, second and third order terms, r, during expansion of the DC bias point1、r2、r3Is the coefficient of the corresponding first order term, second order term and third order term when 1/tau expands at the DC bias point. These coefficients are formed by the micro-ring coupling states andthe waveguide mode decision can be obtained by fitting experimental test results.
Substituting equations (3) - (4) into equation (2) can result:
an expression of α (t) can be obtained from the differential equation in equation (5):
wherein, the expression of A, B, C and D is as follows:
the transmission light field S (t) under the time domain can be obtained by substituting the formula (6) into S (t), and the corresponding light power is | S (t)2. Because the receiving end uses the square-law detection high-speed photoelectric detector to demodulate the modulated optical signal, the first-order harmonic component HD1 and the second-order harmonic HD (the 2) can be calculated according to the formulas (2) - (4)ndHarmonic discrimination, HD2) and third Harmonic HD3 have optical powers of:
HD1＝∫|S(t)|2cos(ωRFt)dt+∫|S(t)|2sin(ωRFt)dt (8)
HD2＝∫|S(t)|2cos(2ωRFt)dt+∫|S(t)|2sin(2ωRFt)dt (9)
HD3＝∫|S(t)|2cos(3ωRFt)dt+∫|S(t)|2sin(3ωRFt)dt (10)
according to the literature and the square-law detection characteristic of the photodetector, the demodulated third-order intermodulation nonlinear IMD3(the 3)rdInter-modulated Distortion, IMD3) is 9.5dB higher than the electrical power of HD3, i.e., PIMD3＝PHD3+9.5 dB. Therefore, finding the HD3 minimum operating point is alsoA minimum operating point for IMD3 is found.
Based on the above nonlinear theory, a normalized relationship between the relative sizes of HD1 and HD3 in the micro-ring modulator and the operating wavelength can be obtained, as shown in fig. 3. Based on the simulation result, TOPS1 and TOPS2 on Si-MRM1 and Si-MRM2 are regulated so that Si-MRM1 works at the maximum point of HD1 relative to HD3 and Si-MRM2 works at the maximum point of HD3 relative to HD 1. The optical powers of HD1 and HD3 outputted by Si-MRM1 at this time are respectively PHD1-MRM1And PHD3-MRM1The optical powers of HD1 and HD3 output by Si-MRM2 are respectively PHD1-MRM2And PHD3-MRM2. By regulating and controlling the power ratio gamma of the modulation signals loaded on the Si-MRM1 and the Si-MRM2, the optical power of HD1 and HD3 components output by the two silicon-based micro-ring modulators can be flexibly controlled. Due to the phase difference of pi between the operating points of the two modulators, it is achieved that HD3 cancels each other out while HD1 is substantially unchanged.
Claims (8)
1. A high linearity modulator chip based on a cascade silicon-based micro-ring modulator is characterized by comprising:
the modulator comprises two input end couplers for coupling optical carrier signals output by a laser into a modulator chip, a beam combiner for combining optical signals of the two input end couplers into one path, a bus waveguide connected with the beam combiner and used for transmitting the optical signals, two cascaded silicon-based micro-ring modulators and an output end coupler connected with the bus waveguide;
the two cascaded silicon-based micro-ring modulators comprise a first silicon-based micro-ring modulator and a second silicon-based micro-ring modulator, and the two silicon-based micro-ring modulators are completely the same and are coupled on the same bus waveguide.
2. The cascaded silicon-based micro-ring modulator-based high linearity modulator chip of claim 1, wherein the input end coupler is a grating coupler or an end-face coupler; the output end coupler is a grating coupler or an end face coupler.
3. The high linearity modulator chip based on the cascaded silicon-based micro-ring modulator of claim 1, wherein the beam combiner for combining the optical signals of the two input end couplers into one path is an optical beam combiner or a wavelength division multiplexer.
4. The high linearity modulator chip based on the cascaded silicon-based micro ring modulator of claim 1, wherein the silicon-based micro ring modulator is a straight-through micro ring modulator, and comprises a micro ring resonant cavity embedded with a PN junction phase shifter, and the silicon-based micro ring modulator is integrated with a thermo-optic phase shifter for adjusting and controlling a working point; the resonant cavities of the first silicon-based micro-ring modulator and the second silicon-based micro-ring modulator are distributed on one side of the same bus waveguide.
5. A modulation method based on the high linearity modulator chip of claim 1, characterized by comprising the steps of:
1) two wavelengths are respectively lambda1And λ2The optical carrier signals output by the peripheral laser are respectively coupled into the modulator chip through the input end coupler;
2) two paths of optical carrier signals with different wavelengths of the two input end couplers are combined into one path of optical signal through the beam combiner and transmitted in the bus waveguide;
3) regulating and controlling reverse bias voltage V of PN junction phase shifter on two silicon-based micro-ring modulatorsDC1And VDC2Making PN junction modulation arms on the two silicon-based micro-ring modulators work in a reverse bias region;
4) regulating and controlling thermo-optic phase shifters on the two silicon-based micro-ring modulators to enable the first silicon-based micro-ring modulator to work at the HD1 relative maximum point, the second silicon-based micro-ring modulator to work at the HD3 relative maximum point, and the phase difference between the two working points is pi;
5) a radio frequency signal generator is used for generating a double-tone modulation signal and the double-tone modulation signal is divided into two paths through a radio frequency power divider with adjustable power division ratio; the two paths of double-tone signals and two paths of direct current reverse Bias voltage generated by a voltage source are loaded on the corresponding two silicon-based micro-ring modulators through two Bias-tee;
6) by regulating the power dividing ratio of the radio frequency power divider, the three-order nonlinear HD3 generated by the two silicon-based micro-ring modulators are mutually offset at the end of the photoelectric detector, the power change of the HD1 component is within 3dB, and the dynamic range of a link is improved.
6. The modulation method according to claim 5, wherein the optical signal modulated by the high linearity modulator chip is further amplified by an optical fiber amplifier before being connected to a photodetector for demodulation, so as to compensate for the coupling loss of the chip.
7. The modulation method according to claim 5, wherein the step 6) is specifically: the optical signal is accessed into a photoelectric detector for demodulation, the demodulated signal is introduced into a frequency spectrograph, the power division ratio of a radio frequency power divider is regulated to regulate the power ratio of the double-tone modulation signals loaded on two silicon-based micro-ring modulators, the power of first-order harmonic HD1 and third-order intermodulation signals IMD3 on the frequency spectrograph is observed at the same time until the power difference between HD1 and IMD3 is maximum, the power of HD1 is ensured to be greater than that of IMD3, and the power change of HD1 component is within 3dB, and the optimal linear working point of the modulator chip is obtained at the moment.
8. The modulation method according to claim 5, wherein the step 4) is specifically:
4.1) adjusting and controlling thermo-optic phase shifters on two silicon-based micro-ring modulators to make the wavelength be lambda1And λ2The optical carrier waves are respectively coupled into a first silicon-based micro-ring modulator and a second silicon-based micro-ring modulator;
4.2) regulating and controlling the thermo-optic phase shifter of the first silicon-based micro-ring modulator to enable the thermo-optic phase shifter to work at the maximum slope of the transfer function of the modulator, wherein the maximum point corresponds to the first-order harmonic component HD 1;
4.3) regulating and controlling the thermo-optic phase shifter of the second silicon-based micro-ring modulator to enable the thermo-optic phase shifter to work at the third-order nonlinear maximum point, and simultaneously ensuring that the direction of the thermo-optic phase shifter is opposite to that of the working point of the first silicon-based micro-ring modulator, and correspondingly, the third-order harmonic component HD3 is maximum.
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