阅读说明：本技术 硅基调制深度可调双级联调制器及其微波光子链路高线性方法 (Silicon-based modulation depth adjustable double-cascade modulator and high linearity method of microwave photon link thereof ) 是由 余辉 张强 傅志磊 夏鹏辉 王肖飞 于 2019-10-09 设计创作，主要内容包括：本发明公开了一种硅基调制深度可调双级联调制器及其微波光子链路高线性方法,该方法通过两个基于载流子耗尽型的硅基MZM1、MZM2和一个TROPS；TROPS级联在MZM1和MZM2之间。激光器输出的光信号经过偏振控制器,通过光纤耦合器耦合到DSMZMs,调制小信号通过50/50的EPS加载DSMZMs上,在链路接收端用光电二极管直接检测。调整TROPS的分配比γ控制MZM2的调制深度,同时调控MZM1和MZM2直流反偏电压以及偏置工作点,使两个调制器中的IMD3信号相互抑制,从而实现了基于硅基调制器的高线性微波光子链路,并从理论上详细推导分析了DSMZMs的FH和IMD3信号变化与反偏直流电压以及γ之间的关系。(The invention discloses a silicon-based modulation depth adjustable double-cascade modulator and a high linearity method of a microwave photonic link thereof, wherein the method comprises the steps of using two silicon-based MZM1 and MZM2 based on carrier depletion modes and a TROPS; TROPS is cascaded between MZM1 and MZM 2. An optical signal output by the laser passes through the polarization controller and is coupled to the DSMMZMs through the optical fiber coupler, a small modulation signal is loaded on the DSMMZMs through EPS of 50/50, and the small modulation signal is directly detected by a photodiode at a link receiving end. The modulation depth of the MZM2 is controlled by adjusting the distribution ratio gamma of TROPS, and the direct current reverse bias voltage and the bias operating point of the MZM1 and the MZM2 are regulated at the same time, so that IMD3 signals in the two modulators are mutually inhibited, a high-linearity microwave optical sub-link based on the silicon-based modulator is realized, and the relationship between the FH and IMD3 signal changes of the DSMMZMs and the reverse bias direct current voltage and gamma is theoretically and specifically deduced and analyzed.)

1. A silicon-based modulation depth-adjustable dual cascade modulator, comprising: a silicon-based MZM1 based on carrier depletion mode, a silicon-based MZM2 based on carrier depletion mode, and a TROPS; and the TROPS is cascaded between MZM1 and MZM2, and the modulation depth of MZM2 is controlled by changing the splitting ratio gamma of TROPS.

2. The silicon-based modulation depth-adjustable double-cascade modulator according to claim 1, wherein the TROPS is a Mach-Zehnder interference structure and is regulated and controlled by a thermo-optic effect or an electro-optic effect.

3. The silicon-based modulation depth-adjustable double-cascade modulator of claim 1, wherein the MZM1 is a carrier depletion-based silicon-based mach-zehnder modulator, comprising an upper PN junction modulation arm, a lower PN junction modulation arm, and a thermal electrode for adjusting and controlling a bias operating point; MZM2 is based on the silicon-based Mach-Zehnder modulator of carrier depletion type, contains upper and lower two isometric PN junction modulation arms and is used for adjusting and controlling the hot electrode of bias operating point.

4. A microwave photonic link high linearity method based on the silicon-based modulation depth adjustable double cascade modulator of claim 1, characterized by comprising the following steps:

1) an output optical signal of the single-wavelength laser is coupled to MZM1 through a fiber coupler, and a double-tone modulation small signal is simultaneously loaded on MZM1 and MZM2 through EPS with a power distribution ratio of 50/50;

2) regulation of reverse bias voltage V of MZM1 and MZM2_DC1And V_DC2Enabling the metal oxide semiconductor wafer to work in a PN junction reverse bias region, and simultaneously adjusting thermodes HTR1 and HTR2 of MZM1 and MZM2 to enable MZM1 and MZM2 to work at pi/2 bias points and-pi/2 bias points respectively; at this time, the MZM1 and MZM2 both work at the first-order harmonic maximum value, so that the first-order harmonic signal in the microwave link is maximum, and the IMD3 signal generated by the double-tone modulation small signal modulation through the DSMZMs does not reach the minimum value;

3) adjusting the optical phase difference of the upper arm and the lower arm of the TROPS to regulate the distribution ratio gamma of the optical power of the upper arm and the lower arm of the MZM2, thereby controlling the modulation depth of the MZM 2; recording the power difference delta P of FH/IMD3 with the unit of dB on the premise that FH is greater than IMD 3;

4) circulating the steps 2) and 3) on the basis that the PN junction modulation arm works in a reverse bias region and the MZM1 and the MZM2 work at pi/2 and-pi/2 bias points_DC1And V_DC2And regulating and controlling the distribution ratio gamma of the TROPS until the delta P reaches the maximum, thereby realizing the high-linearity microwave photonic link based on the silicon-based modulator.

Technical Field

The invention relates to a silicon-based modulation depth adjustable double-cascade modulator and a microwave photonic link high linearity method based on the same, in particular to a microwave photonic link high linearity method of a double-cascade Mach-Zehnder modulator (DSMMZMs) comprising a tunable-ratio Optical Power Splitter (TROPS).

Background

In recent years, microwave photon technology combining microwave technology and light wave technology has been widely used in cable television, optical fiber wireless systems, and the like. It mainly studies how to realize the generation, transmission distribution, processing and the like of microwave/millimeter wave signals by using optoelectronic devices and methods. With the development of technology, microwave photonic technology has required devices and systems with small size, light weight, low power consumption and greater electromagnetic interference resistance while achieving higher speed, bandwidth, throughput and dynamic range.

In the analog optical link, a Modulator, such as a Mach-zehnder Modulator (MZM) or a micro-Ring Modulator (RM), is required to modulate a microwave signal onto an optical carrier for transmission. Although the transmission loss of the optical fiber link is already low, the electro-optical and electro-optical conversion thereof still introduces additional loss. Meanwhile, two important influence factors, namely link noise and nonlinear distortion, exist in the link in the process of electro-optical and photoelectric conversion. To achieve high fidelity transmission, the analog optical link needs to have high linearity and low noise. However, due to the inherent non-linear response of the modulator, higher harmonics and intermodulation products are generated, wherein the third order intermodulation (the 3) is located within the system passband^rdIMD3) has the greatest impact on the microwave link, greatly reducing the Dynamic Range of the microwave photonic link (SFDR). The spurious-free dynamic range is one of linearity, noise and gain of the microwave photon linkThe performance index may be defined as a range from an input rf power point where the input baseband signal power is equal to the noise power to an input rf signal power where the n-order nonlinear distortion power is equal to the noise power. Some definitions specify the n-order nonlinear distortion as n-order intermodulation distortion, and the definitions are actually applicable to n-order harmonic distortion.

At present, methods for improving the dynamic range of a microwave photonic link mainly focus on nonlinear suppression of a modulator, and the nonlinear suppression methods of the modulator mainly comprise two types. The first category is to increase the optical power or to reduce the noise and thus the signal-to-noise ratio, which generally improves the lower limit of the dynamic range by reducing the noise figure of the link. Another method is to improve the upper limit of the dynamic range by suppressing nonlinear distortion in the link. This type of method improves the upper limit of the dynamic range by suppressing nonlinear distortion, and improves the link performance, and is called a linearization method. The research of linearization technique is the leading research direction of the current hot spot. The electro-optical modulator for bearing photoelectric conversion is a key device of a microwave link of a system, and the nonlinearity of a transmission function of the electro-optical modulator can bring distortion to the link and influence the linearity of the microwave link.

In various silicon-based electro-optical modulators, a silicon-based MZM based on a Mach-Zehnder interference structure is most applied to a microwave photonic link due to the advantages of high speed, high extinction ratio, low insertion loss, simplicity in manufacturing and the like, and domestic and foreign researches propose various linearization schemes for inhibiting IMD3 for a high-linearity microwave photonic link of the silicon-based MZM, for example, the high-linearity link is realized by changing the overlapping area of an optical field and an electric field by changing the doping concentration of a modulation arm in the silicon-based modulator; optical nonlinear DC-Kerr effect is also used for compensating nonlinearity of carrier and sine modulation curve in silicon-based modulator, but the two methods change the process flow of chip foundry and increase the complexity and cost of chip processing.

In addition, by imitating the realization principle of the lithium niobate high-linearity modulator with the parallel MZM structure, the silicon-based high-linearity modulator can be realized by adjusting the optical power distribution ratio of the silicon-based parallel MZM, the bias working point and the optical power distribution ratio of the modulated small signal, but the method has higher operation complexity and more monitoring and control variables. At present, similar reports on cascaded silicon-based MZMs are not available, and particularly cascaded MZMs comprising adjustable splitting ratio optical power splitters are not available in other material modulators. Compared with a silicon-based MZM with a parallel structure, the structure does not need excessive monitoring points and control the distribution ratio of small modulation signals, and greatly reduces the control complexity.

Disclosure of Invention

The invention aims to provide a silicon-based modulation depth adjustable double-cascade modulator and a microwave photon link high-linearity method based on the silicon-based modulation depth adjustable double-cascade modulator.

The technical scheme of the invention is as follows:

a silicon-based modulation depth-tunable dual-cascade modulator comprises: a silicon-based MZM1 based on carrier depletion mode, a silicon-based MZM2 based on carrier depletion mode, and a TROPS; and the TROPS is cascaded between MZM1 and MZM 2.

As a preferable scheme of the invention, the TROPS is a Mach-Zehnder interference structure and is regulated and controlled through a thermo-optical effect or an electro-optical effect.

As a preferred scheme of the invention, the MZM1 is a carrier depletion type based silicon-based mach-zehnder modulator, and comprises an upper PN junction modulation arm, a lower PN junction modulation arm and a hot electrode for regulating and controlling a bias operating point; MZM2 is based on the silicon-based Mach-Zehnder modulator of carrier depletion type, contains upper and lower two isometric PN junction modulation arms and is used for adjusting and controlling the hot electrode of bias operating point.

The invention also discloses a high-linearity method of the microwave photon link based on the silicon-based modulation depth-adjustable double-cascade modulator, which comprises the following steps:

1) the single-wavelength laser output optical signal is coupled to MZM1 through the fiber coupler, the two-tone modulation small signal is loaded on MZM1 and MZM2 simultaneously through the Electric Power Splitter (EPS) with the Power splitting ratio of 50/50;

2) regulation of reverse bias voltage V of MZM1 and MZM2_DC1And V_DC2Enabling the metal oxide semiconductor wafer to work in a PN junction reverse bias region, and simultaneously adjusting thermodes HTR1 and HTR2 of MZM1 and MZM2 to enable MZM1 and MZM2 to work at pi/2 bias points and-pi/2 bias points respectively; at this time, the MZM1 and MZM2 both work at the first-order harmonic maximum value, so that the first-order harmonic signal in the microwave link is maximum, and the IMD3 signal generated by the double-tone modulation small signal modulation through the DSMZMs does not reach the minimum value;

3) adjusting the optical phase difference of the upper arm and the lower arm of the TROPS to control the distribution ratio gamma of the optical power of the upper arm and the lower arm of the MZM2 to regulate the modulation depth of the MZM2, and recording the power difference delta P of the FH/IMD3, wherein the unit is dB;

4) circulating the steps 2) and 3) on the basis that the PN junction modulation arm works in a reverse bias region and the MZM1 and the MZM2 work at pi/2 and-pi/2 bias points_DC1And V_DC2Next, the splitting ratio γ of the TROPS is regulated until Δ P is maximized, that is, the maximum linear point of the modulator is reached at this time, thereby realizing a high-linearity microwave photonic link based on a silicon-based modulator.

Compared with the method for improving the linearity of the silicon-based modulator through a processing technology or regulating the modulation signal distribution ratio, the method has the advantages that on the premise of not changing the technological parameters and the flow of a chip factory, the modulation depth of the MZM2 is continuously regulated and controlled by simply regulating and controlling the distribution ratio of the optical power divider in front of the second-stage modulator, the power distribution ratio of the modulation signal is not regulated and controlled, and on the premise of finding the optimal linear working point of the modulator, the link complexity is greatly reduced. The method does not need processing complexity and cost caused by process change or design complexity caused by regulating and controlling high-speed modulation signals, and can realize the high-linearity microwave optical sub-link based on the high-linearity silicon-based modulator only by changing the optical power distribution ratio of the upper arm and the lower arm of the second-stage modulator. 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 the modulator structure of the present invention.

FIG. 2 is a schematic cross-sectional view of a PN junction structure in the hot electrode and modulation arm.

FIG. 3 is the calculated power of the third order intermodulation IMD3 components at different modulation arm lengths L and different distribution ratios of optical power Based on the process simulation models in the literature (Q.Zhang, H.Yu, H.jin, T.Qi, Y.Li, J.Yang, and X.Jiang, "Linear company of Silicon Carrier-deletion-Based Single, Dual-Parallel, and Dual-Series Mach-Zehnder Modulators," J.Lightwave technol.36(16),3318-3331 (2018)).

FIG. 4 is the calculated power of a harmonic FH component at different modulation arm lengths L and different distribution ratios γ of optical power Based on process simulation models in the literature (Q.Zhang, H.Yu, H.jin, T.Qi, Y.Li, J.Yang, and X.Jiang, "Linear Complex of Silicon Carrier-deletion-Based Single, Dual-Parallel, and Dual-Series Mach-Zehnder Modulators," J.Lightwave technology.36 (16),3318-3331 (2018)).

Fig. 5 is a schematic diagram of a linearity test of the modulator of the present invention.

Detailed Description

The invention is further illustrated with reference to the following figures and examples.

As shown in FIG. 1, the structure of the invention is formed by cascading two stages of Mach-Zehnder modulators, and an optical power divider with adjustable division ratio is integrated at the front section of the modulator of the second stage. In the figure, "1" is a coupler of a modulator chip and an optical fiber, usually a grating coupler and an end face coupler, "2" is a first-stage modulator MZM1, "3" is a second-stage modulator MZM2, and "4" is an optical power splitter with adjustable splitting ratio in MZM 2. Both the modulator and the optical power splitter are constructed based on the structure "5" (direct coupler DC or multimode coupled interferometer MMI). In the figure, 6 is the thermal electrode control modulator or the optical phase difference of the upper arm and the lower arm of the optical power splitter to realize the regulation and control of the working point of the modulator and the power distribution ratio of the optical power splitter. The structure of "7" in the figure is a PN junction modulation arm.

Currently, two implementation structures of the optical power splitter used on a silicon optical chip are generally used, namely, a Direct Coupler (DC) or an optical power splitter formed based on a Multimode Interference (MMI). On the basis that the two-stage modulator works at pi/2 and-pi/2, the power distribution ratio of the optical power divider is regulated and controlled, so that three-order intermodulation signals generated by the two-stage modulator are mutually offset during demodulation of the high-speed detector, and the loss of fundamental frequency power is small.

According to the transmission matrix theory, the Mach-Zehnder linearity theory considering the third-order harmonic influence is deduced, and the method is used for theoretically analyzing the third-order intermodulation influence of the double-cascade Mach-Zehnder modulator. According to the electromagnetic field theory, the optical field input to the DSMZMs is given by:

E_in＝|E_in|e^jωt(1)

wherein E is_inIs the amplitude of the optical field input into the DSMZMs and ω is the frequency of the optical signal. According to the Mach-Zehnder interference transmission matrix theory, the output optical power of the DSMMZMs is obtained as follows:

Φ＝Φ_bias+θ(v) (3)

Δθ(v_rf,V_DC)＝θ(v)-θ(V_DC) (4)

where γ is the optical power division ratio of OPS, and V ═ V_DC+v_rf，V_DCAnd v_rfThe modulation arm can introduce different optical field loss coefficients α (v) under different driving voltages due to current-carrying absorption effect, α (0) represents the inherent loss coefficient when no driving signal exists, phi is the total phase difference of the upper arm and the lower arm of the modulator, and consists of two parts, namely a static bias phase phi caused by a device structure and a hot electrode_biasI.e. without bias voltageThe induced phase difference; the other is due to the addition of V_DCAnd v_rfResulting in a phase theta (v) of the PN modulation region. Substituting the above formula into formula (2), and calculating the modulated output light field I_out(v) First derivative I 'with respect to v'_out(v) And third derivative I'_out(v) According to the nonlinear theory [ Q.Zhang, H.Yu, H.jin, T.Qi, Y.Li, J.Yang, and X.Jiang, Linear company of Silicon Carrier-deletion-Based Single, Dual-Parallel, and Dual-Series Mach-Zehnder Modulators, "J.Lightwave technique.36 (16),3318-3331(2018) ], the sizes of the fundamental frequency signal and the third-order intermodulation signal corresponding to the transmitted light field can be calculated.

Based on the theoretical analysis, the high linearity method of the silicon-based microwave photonic link based on the double-stage-connection Mach-Zehnder modulator comprises the following steps:

1) as shown in fig. 5, an unmodulated optical signal output by the single-wavelength laser is coupled into the modulator chip through the structure "1" in fig. 1, and the polarization controller is adjusted to make the output optical signal reach a maximum value at this time;

2) as shown in fig. 5, the two-tone modulated small signal generated by the rf signal generator is coupled with the reverse bias voltage in biaste, and is loaded on the modulation arms of MZM1 and MZM2 on the modulator chip through 50/50 electric power couplers, respectively;

3) regulating DC bias voltage V loaded on two modulators_DC1And V_DC2The PN junction modulation arm works in a reverse bias region. As shown in fig. 2(b), "10" is the silicon dioxide material in the chip, and "11", "12" and "13" are all metallic copper for transmitting electrical signals. Because the PN junction of the high-speed silicon-based depletion mode modulator needs to work in a reverse bias region, namely, the PN junction is connected with the positive pole at '12' and connected with the negative pole or the ground at '13' in FIG. 2 (b);

4) the driving voltage loaded on the MZM1 and MZM2 thermodes is regulated, namely the left and right structures 6 in FIG. 1, the cross-sectional schematic diagram of the thermode structure is shown in FIG. 2(a), the structure 9 is a TiN material, the structure 8 is a waveguide, and the thermal phase modulation is realized by applying voltage to the structure 9, controlling the thermal power and changing the effective refractive index of the waveguide;

5) as shown in FIG. 5, using 1/99 coupler to input 1% of the modulator output optical power into the optical power meter, observing the change of optical power, MZM1 and MZM2 are operated at π/2 and- π/2;

6) as shown in fig. 5, the optical signal output from the end 99% of the 1/99 coupler is passed through an optical amplifier to compensate for coupling and insertion loss of the device itself, and an optical filter is used to filter out noise introduced by the optical amplifier. Inputting the electric signal demodulated by the high-speed photoelectric detector into a frequency spectrograph;

7) by controlling the applied voltage at "6" in "4" of fig. 1, the distribution ratio γ of the optical power splitter is controlled, and the power levels of the first order harmonic FH and the third order intermodulation signal IMD3 of the demodulated signal on the spectrometer are observed until the power difference between FH and IMD3 is maximum (while ensuring that the power of FH is greater than the power of IMD3), and FH does not change much from the maximum, where γ is the optical power distribution ratio required for the optimal linearity point.

8) Fig. 3 and 4 are graphs showing the power of the third-order intermodulation component 3 and the first-order harmonic component FH for IMD at different optical power distributions, when the modulator modulation arm is of length L, Based on PN junction simulation parameters in the literature (q.zhang, h.yu, h.jin, t.qi, y.li, j.yang, and x.jiang, "linear company of Silicon Carrier-repetition-Based Single, Dual-Parallel, and Dual-Series Mach-Zehnder Modulators," j.lightwave technique.36 (16),3318-3331 (2018)). As can be seen from fig. 3, an optical power splitting ratio γ can always be found so that IMD3 is minimized for a modulation arm length L of the modulator, and there is no large change in the fundamental frequency component from the maximum value for fig. 4.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.