On-chip polarizing lithium niobate optical waveguide electric field sensor and its measurement system

Abstract

The present application relates to a kind of on-chip polarized lithium niobate optical waveguide electric field sensors and its measuring system, adopt lithium niobate thin film material as sensor matrix, and gradually etching to form gradually coupled structure, polarization selection structure, MZ electro-optic modulator and broadband high gain electrode, antenna structure on the lithium niobate thin film material along the direction of light path propagation;Through the input laser signal of sensor, electric field signal can be modulated on laser, and the modulated laser is transmitted using long optical fiber;Back end is demodulated by photoelectric detector and sent into receiver to measure, and the electric field information to be measured can be obtained.The sensor is simple in structure, small in size, high in sensitivity, small in interference, accurate in electric field measurement result, and high in measurement repeatability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of wideband electric field detection, and particularly relates to a chip-on-polarization lithium niobate optical waveguide electric field sensor and a measuring system thereof. BACKGROUND

[0002] With the development of electronic technology, the electromagnetic environment is more and more complex, and the electromagnetic interference problem is more and more serious. Electromagnetic compatibility design has become one of the key links in the design and development of electronic equipment. Electromagnetic compatibility design requires a large amount of input data, and in many cases, these input data need to be obtained through testing and measurement. Therefore, electric field sensors play an important role in the design link of electromagnetic compatibility. The traditional electric field measurement adopts the form of an antenna plus a receiver (and a spectrum analyzer) for measurement. Although the measurement principle is simple, it has the following defects: (1) The bandwidth of a single antenna is narrow, and multiple antennas are needed to cover the entire wide frequency band, resulting in a complex measurement system structure. (2) The metal structure of the antenna and the transmission line has a great influence on the measured electric field, resulting in low accuracy and poor repeatability of the measurement results. (3) The antenna is large in size, which is not conducive to the measurement of narrow spaces inside the equipment. (4) The antenna plus receiver (spectrum analyzer) system has high sensitivity and is easily affected by strong electromagnetic interference.

[0003] An optical electric field sensor system based on the electro-optic effect can avoid these problems. The optical electric field sensor modulates the electric field signal on the laser through the electro-optic effect of the sensing medium material, and uses low-loss optical fiber for long-distance transmission. The sensor probe uses a medium material, has a wide operating frequency band, and can cover a wide frequency band with a single probe; the medium structure has little interference with the measured electric field, the measurement results are accurate; the volume is small, and it is suitable for measuring electric fields in various spaces; the front and rear ends are optoelectronically isolated, and have strong anti-interference and anti-damage capabilities.

[0004] Based on the above advantages, the optical electric field sensor has become an important measurement means in the field of electric field measurement. However, existing optical electric field sensors mostly adopt the form of preparing an optical waveguide structure on a bulk lithium niobate material. Since the bulk lithium niobate has a certain thickness and is not a thin film, it is difficult to etch, and usually needs to use titanium diffusion and proton exchange technology to prepare a channel type optical waveguide in the bulk lithium niobate. The refractive index difference between the core layer and the cladding layer of the optical waveguide structure prepared on the bulk lithium niobate material based on this technology is relatively small, usually 10 -2 orders of magnitude, so the waveguide width is large (usually 6-7 μm for a single-mode optical waveguide), and the beam light ability is weak. The large waveguide width leads to a large electrode spacing, resulting in a small electric field between the electrodes, low modulation efficiency, and the need for longer structures to complete the modulation. The weak beam light ability leads to a large turning radius of the branch structure in the waveguide, so longer structures are also needed to complete the modulation. The overlong structure exacerbates the light-electric speed mismatch effect, which further affects the bandwidth of the probe. Therefore, the existing optical electric field sensor still has the problems of large volume and narrow bandwidth. SUMMARY

[0005] Technical problems to be solved:

[0006] In view of the defects in the prior art, the present application adopts a thin-film lithium niobate crystal material to prepare an optical electric field sensor, and etches a waveguide structure and other sensor structures on the lithium niobate thin film material to form an integrated electric field sensor. The sensor structure is simple in structure, small in size, high in sensitivity, small in interference, accurate in electric field measurement result, and high in measurement repeatability.

[0007] The technical scheme adopted is as follows:

[0008] The present application discloses a chip-on-polarized lithium niobate optical waveguide electric field sensor. By inputting a laser signal into the sensor, an electric field signal can be modulated on the laser, and the modulated laser is transmitted by a long optical fiber. The back end demodulates the laser by an optoelectronic detector and sends it into a receiver for measurement, so that the information of the electric field to be measured can be obtained.

[0009] A chip-on-polarized lithium niobate optical waveguide electric field sensor adopts a lithium niobate thin film material as a sensor substrate, and a gradually changing coupling structure, a polarization selection structure, an MZ electro-optic modulator, a wideband high-gain electrode, and an antenna structure are etched on the lithium niobate thin film material in sequence along the direction of light propagation. The gradually changing coupling structure is arranged at the laser input port of the sensor, and the laser with a larger light spot is coupled into a single-mode optical waveguide with a smaller light spot, so as to realize single-mode transmission of the laser. The polarization selection structure eliminates the TM mode in the single-mode optical waveguide, so as to realize single-polarization transmission of the laser. The MZ electro-optic modulator is connected after the single-mode optical waveguide, and modulates the electric field to be measured in space onto the input laser. The high-gain electrode increases the intensity of the electric field to be measured acting on the waveguide arm of the MZ electro-optic modulator.

[0010] Further, the lithium niobate thin film material substrate is supported by a silicon wafer, a 2-5 mu m thick silicon dioxide substrate is attached to the silicon wafer, and a 300-600 nm thick lithium niobate single crystal thin film is attached to the silicon dioxide substrate.

[0011] Further, the gradually changing coupling structure is a tapered gradually changing transmission channel arranged at the sensor port. The tapered gradually changing transmission channel is wider at one end facing the port for connecting with an input optical fiber, and narrower at the other end for connecting with an internal single-mode optical waveguide, so as to gradually couple the laser of the input optical fiber into a single-mode optical waveguide with a width of 1 mu m.

[0012] Further, the polarization selection structure is provided with a metal cladding layer on the single-mode optical waveguide. By exciting a high-loss plasmonic surface mode, the TE mode guided wave passes with low loss, while the TM mode guided wave is attenuated with high loss, so as to realize a single-polarization working state of the sensor.

[0013] Further, the MZ electro-optical modulator comprises two Y-branch optical waveguides, each of which comprises two parallel waveguide arms, and adopts a biased MZ electro-optical modulation structure, so that the sensor works at a linear operating point of the MZ electro-optical modulator.

[0014] Further, the high-gain electrode adopts a broadband micro-nano high-gain electrode, comprises two electrodes with the same size, and the electrode spacing is within 2 μm, and the high-gain electrode is integrated on the waveguide arm of the MZ electro-optical modulator.

[0015] Further, another tapered coupling structure is further included, and a tapered transmission channel is connected to the output end of the MZ electro-optical modulator at the port at the laser outlet of the sensor.

[0016] The measurement system based on the on-chip polarized lithium niobate optical waveguide electric field sensor for measurement comprises a laser source, a polarization maintaining optical fiber, the lithium niobate thin film material optical electric field sensor, a single-mode optical fiber, an optical detector, and a spectrum analyzer. The laser is input to the optical electric field sensor through the polarization maintaining optical fiber, the external electric field to be measured is applied to the sensor, the output intensity-modulated laser is incident into the optical detector through the single-mode optical fiber and is converted into an electric signal, and finally is input into the spectrum analyzer for detection to obtain the frequency and amplitude of the electric field to be measured.

[0017] The measurement method based on the sensor measurement system for measurement is specifically as follows:

[0018] Step one, tapered mode spot conversion coupling: the laser emitted by the laser source is transmitted to the input end of the optical electric field sensor through the polarization maintaining optical fiber, and is transmitted into the 1 μm wide single-mode optical waveguide through the tapered coupling structure, so as to realize single-mode transmission of the laser;

[0019] Step two, polarization selection: when the single-mode transmitted laser passes through the single-mode optical waveguide coated with a metal layer, the plasmonic surface mode is excited, so that the TM mode in the waveguide is removed, and single-polarization transmission of the laser is formed;

[0020] Step three, intensity modulation: the laser transmitted in step two is evenly divided into two paths at the first Y branch of the MZ optical modulator, and enters the two arms of the interference optical waveguide of the MZ optical modulator, respectively. Due to the existence of the electric field to be measured in space, the refractive index of the lithium niobate material changes, so that the phase of the laser transmitted in the two arms changes, and finally the intensity of the laser output at the second Y branch of the MZ optical modulator changes, and the change rule is:

[0021] I out =I in αE

[0022] wherein, Iout I is the output power of the sensor in α is the input power of the laser light source, E is the electric field intensity to be measured

[0023] Step four, the electric field intensity to be measured is calculated: the intensity-modulated laser output in step three is input into the optical detector to be converted into an electric signal, and the obtained electric signal is input into the spectrum analyzer, and the signal V is measured in the spectrum analyzer rf ; the electric field frequency to be measured is the same as the signal frequency measured in the spectrum analyzer, and the electric field intensity amplitude of the electric field to be measured is E out = V rf + A F , wherein A F is the antenna coefficient of the measurement system after accuracy calibration.

[0024] Compared with the prior art, the present application has the beneficial effects that:

[0025] 1. The core layer and the cladding layer of the single-mode optical waveguide formed by the lithium niobate thin film have a refractive index difference of about 0.7, which is much higher than that of the traditional process (titanium diffusion, proton exchange, about 6-7 μm), so that the single-mode optical waveguide width is within 1 μm, the beam light capacity is strong, and the sensor size is reduced.

[0026] 2. The high-gain wideband electrode spacing of the present application is within 2 μm, the inter-electrode electric field is larger than that of the traditional process, the modulation efficiency is improved, the electrode size and the optical path size are reduced, and the bandwidth is improved.

[0027] 3. The present application directly etches the on-chip polarization selection structure on the lithium niobate thin film, without the need for an external polarizer, which reduces the complexity of the sensor, saves cost, and reduces size.

[0028] 4. The present application adopts a high-gain wideband electrode-antenna structure, which increases the sensitivity and improves the bandwidth. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or the prior art description. Obviously, the drawings described below are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0030] Figure 1 is a schematic diagram of an on-chip polarized lithium niobate optical waveguide electric field sensor structure;

[0031] Figure 2 is a schematic diagram of a lithium niobate thin film material matrix;

[0032] Figure 3 Figure 1 is a schematic diagram of a measurement system of optical electric field of lithium niobate thin film material;

[0033] Figure 4(a) is an effect diagram of polarization selection of a TE polarized Gaussian beam by a polarization selection structure;

[0034] Figure 4(b) is an effect diagram of polarization selection of a TM polarized Gaussian beam by a polarization selection structure.

[0035] Explanation of reference signs:

[0036] 1-polarization maintaining optical fiber, 2-polarization selection structure, 3-gradual coupling structure, 4-high gain electrode structure, 5-MZ electro-optic modulator, 6-single mode optical fiber, 7-laser source, 8-optical electric field sensor of lithium niobate thin film material, 9-optical detector, 10-spectrometer, 11-radio frequency transmission line, 12-electric field to be measured, 13-lithium niobate single crystal thin film, 14-silicon dioxide, 15-silicon. DETAILED DESCRIPTION

[0037] To make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0038] Figure 1 Figure 1 is a structure diagram of a polarization maintaining lithium niobate optical waveguide electric field sensor 8. The sensor takes lithium niobate thin film as a substrate and etches a gradual coupling structure 3, a polarization selection structure 2, an MZ electro-optic modulator 5, a high gain electrode 4 and an antenna structure (not shown) in the direction of the optical path in sequence. The optical path communication of a polarization maintaining optical fiber 1, the sensor 8 and a single mode optical fiber 6 is realized by directly connecting a tapered lens fiber at both end faces of the sensor 8.

[0039] The gradual coupling structure 3 is a tapered gradual transmission channel arranged at the laser input port of the sensor 8. The tapered gradual transmission channel is wider at one end facing the port for connecting the input polarization maintaining optical fiber 1 and narrower at the other end for connecting the internal single mode optical waveguide, so as to gradually couple the laser of the input polarization maintaining optical fiber 1 into the single mode optical waveguide with a width of 1 μm. In addition, the same tapered gradual transmission channel can also be arranged at the laser output port of the sensor 8 for connecting after the MZ electro-optic modulator.

[0040] The polarization selective structure 2 is a metal coating arranged on the single-mode optical waveguide, which excites a high-loss plasmonic surface mode to make the TE mode waveguide low-loss pass through while the TM mode waveguide high-loss attenuate, so as to realize the single-polarization working state of the sensor.

[0041] The MZ electro-optic modulator 5 is connected with the single-mode optical waveguide provided with the polarization selective structure 2, and includes two Y-branch optical waveguides, each of which includes two parallel waveguide arms. The MZ electro-optic modulator structure with bias makes the sensor work at a linear working point of the MZ electro-optic modulator, and the linear detection range ensures the accuracy of the electric field detection.

[0042] The high-gain electrode 4 is a broadband micro-nano high-gain electrode, which includes two electrodes with the same size and a distance between the electrodes of less than 2 μm. The high-gain electrode is integrated on the parallel waveguide arms of the MZ electro-optic modulator and can be arranged on the upper waveguide arm or the lower waveguide arm. The high-gain electrode enhances the intensity of the detected electric field to improve the sensitivity of the detection. The antenna adopts a conventional antenna structure of the existing electric field sensor.

[0043] Figure 2 The lithium niobate thin film material substrate is realized by attaching a lithium niobate single crystal thin film on a silicon substrate, and specifically includes a 0.5 mm silicon support structure, a 2-5 μm thick silicon dioxide substrate 14 attached on the silicon support structure, and a 300-600 nm lithium niobate single crystal thin film structure 13 attached on the silicon dioxide substrate. The lithium niobate single crystal thin film structure can be an x-cut y-transmit d lithium niobate single crystal thin film structure. Figure 1 The sensor structure is etched on the lithium niobate thin film structure by a micro-nano processing method.

[0044] Figure 3 The measurement system schematic diagram of the on-chip polarization lithium niobate optical waveguide electric field sensor is shown. The laser emitted by the laser source 7 is input to the sensor 8 through the polarization maintaining optical fiber 1, the detected electric field 12 is applied to the sensor 8, and the intensity of the input laser is modulated by the detected electric field 12. Then the intensity modulated laser is incident into the optical detector 9 through the single-mode optical fiber 6, and is converted into an electric signal. Finally, the electric signal is input into the spectrum analyzer 10 through the radio frequency transmission line 11 for detection. The spectrum analyzer reads out the signal to obtain the frequency and amplitude of the detected electric field intensity. The specific measurement method includes the following steps:

[0045] Step one, gradual mode spot conversion coupling: the laser emitted by the laser source is transmitted to the input end of the optical electric field sensor through the polarization maintaining optical fiber, and is transmitted into the 1 μm wide single-mode optical waveguide through the gradual coupling structure, so as to realize the single-mode transmission of the laser;

[0046] Step two, polarization selection: the single-mode transmission laser passes through the single-mode optical waveguide coated with a metal cladding, exciting plasmonic surface modes, thereby removing the TM mode in the waveguide and forming single-polarization transmission of the laser.

[0047] Step three: intensity modulation: the laser transmitted in step two is evenly divided into two paths at the first Y branch of the MZ photoelectric modulator and enters the two arms of the MZ photoelectric modulator interference optical waveguide. Due to the presence of the electric field to be measured in space, the refractive index of lithium niobate material changes, causing the phase of the laser transmitted in the two arms to change, and finally the intensity of the laser output at the second Y branch of the MZ photoelectric modulator changes, and the change rule is:

[0048] I out =I in αE

[0049] I out is the output power of the sensor, I in is the input power of the light source, and α is the measurable modulation coefficient of the sensor, and E is the electric field intensity to be measured.

[0050] Step four, calculation of the electric field intensity to be measured: the intensity-modulated laser output in step three is input into the optical detector to convert it into an electrical signal, and the obtained electrical signal is input into the frequency spectrometer to measure the signal amplitude V rf in dBμV. Then the electric field frequency to be measured is the same as the signal frequency measured in the frequency spectrometer, and the electric field intensity amplitude of the electric field to be measured is E out =V rf +A F , where A F is the antenna coefficient of the measurement system after calibration, which is a known parameter of the measurement system and is the transfer coefficient of the electric field sensor, with a unit of dB / m.

[0051] Figure 4 shows the polarization selection structure selection effect diagram. In figure 4(a), a TE polarized Gaussian beam is input into the sensor at one end, and the propagation field distribution diagram is shown in figure 4(b). As can be seen from the figure, the TM polarized light attenuates greatly, while the TE polarized light can pass through the selection structure with low attenuation, thereby realizing single-polarization transmission. In fact, the extinction ratio of TM polarization and TE polarization can reach more than 20 dB.

[0052] Obviously, the above embodiments are merely example for clearly illustrating but not limitation to the embodiments. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, all the embodiments need not and can not be enumerated. The obvious changes or variations derived from the above description are still within the protection scope of the present application.

Claims

1. An on-chip lithium niobate photoelectric conductive field sensor, characterized in that, A lithium niobate thin film is used as the sensor substrate, and a gradient coupling structure, a polarization selection structure, an MZ electro-optic modulator, a broadband high-gain electrode, and an antenna structure are sequentially etched onto the lithium niobate thin film along the optical path propagation direction. The gradient coupling structure is located at the laser input port of the sensor, coupling a larger laser spot to a smaller single-mode waveguide to achieve single-mode laser transmission. The polarization selection structure eliminates the TM mode in the single-mode waveguide to achieve single-polarization laser transmission. The MZ electro-optic modulator is connected after the single-mode waveguide and modulates the electric field to be measured in space onto the input laser. The high-gain electrode increases the intensity of the electric field to be measured acting on the waveguide arm of the MZ electro-optic modulator. The tapered coupling structure is a tapered tapered transmission channel set at the sensor port. The tapered tapered transmission channel is wider at one end facing the port for connecting to the input optical fiber, and narrower at the other end for connecting to the single-mode optical waveguide inside the sensor, so as to gradually couple the laser from the input optical fiber into the 1μm wide single-mode optical waveguide. The polarization selection structure achieves single-polarization operation of the sensor by setting a metal cladding on the single-mode optical waveguide and exciting a high-loss plasma surface mode, allowing the TE mode guided wave to pass through with low loss while the TM mode guided wave attenuates with high loss.

2. The on-chip lithium niobate photoconductive field sensor according to claim 1, characterized in that, The lithium niobate thin film material substrate is supported by a silicon wafer, on which a silicon dioxide substrate with a thickness of 2μm-5μm is attached, and on the silicon dioxide substrate a lithium niobate single crystal thin film with a thickness of 300nm-600nm is attached.

3. The on-chip lithium niobate photoelectric conductive field sensor according to claim 2, characterized in that, The MZ electro-optic modulator includes two Y-branch optical waveguides, each of which includes two parallel waveguide arms. The MZ electro-optic modulator adopts a biased MZ electro-optic modulation structure, which enables the sensor to operate at the linear operating point of the MZ electro-optic modulator.

4. The on-chip lithium niobate photoelectric conductive field sensor according to claim 3, characterized in that, The high-gain electrode is a broadband micro-nano high-gain electrode, comprising two electrodes of the same size with a spacing of less than 2 μm. The high-gain electrode is integrated on the parallel waveguide arm of the MZ electro-optic modulator.

5. The on-chip lithium niobate photoelectric conductive field sensor according to claim 4, characterized in that, It also includes another gradient coupling structure, in which a tapered gradient transmission channel is connected at the port of the sensor laser exit, which is the output end of the MZ electro-optic modulator.

6. A measurement system based on the on-chip lithium niobate photoconductive field sensor according to any one of claims 1-5, characterized in that, include: The system includes a laser source, a polarization-maintaining fiber, the aforementioned sensor, a single-mode fiber, a photodetector, and a spectrum analyzer. The laser is input to the sensor through the polarization-maintaining fiber. An external electric field to be measured is applied to the sensor. The output laser, after intensity modulation, is incident on the photodetector through the single-mode fiber and converted into an electrical signal. Finally, the signal is input to the spectrum analyzer for detection to obtain the frequency and amplitude of the electric field intensity to be measured.

7. A measurement method based on the measurement system of claim 6, characterized in that, The specific steps are as follows: Step 1, Gradient Mode Conversion Coupling: The laser emitted by the laser source is transmitted to the input end of the sensor through a polarization-maintaining fiber, and then transmitted to a 1μm wide single-mode optical waveguide through a gradient coupling structure to achieve single-mode transmission of the laser. Step 2, Polarization Selection: When the single-mode transmitted laser passes through the single-mode optical waveguide coated with a metal layer, it excites the plasma surface mode, thereby removing the TM mode in the waveguide and forming single-polarization transmission of the laser. Step 3: Intensity Modulation: The laser transmitted in Step 2 is split into two equal paths at the first Y-branch of the MZ photoelectric modulator, and these paths enter the two arms of the interference waveguide of the MZ photoelectric modulator. Due to the presence of the electric field to be measured in space, the refractive index of the lithium niobate material changes, thereby changing the phase of the laser transmitted in the two arms. Finally, the laser intensity output at the second Y-branch of the MZ photoelectric modulator changes, and the change pattern is as follows: in, For sensor output power, The input power of the laser source. The sensor modulation coefficient is a measurable value. The electric field strength to be measured; Step 4: Calculation of the electric field strength to be measured: The intensity-modulated laser output from Step 3 is input into a photodetector and converted into an electrical signal. The resulting electrical signal is then input into a spectrum analyzer, where the signal amplitude is measured. Then the frequency of the electric field to be measured is the same as the frequency of the signal measured in the spectrum analyzer, and the amplitude of the electric field intensity of the electric field to be measured is... ,in This refers to the antenna coefficient of the measurement system after accuracy calibration.

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