A reflective optical electric field sensor based on lithium niobate thin film material

Abstract

The application relates to a reflective optical electric field sensor based on a lithium niobate thin film material, which adopts lithium niobate thin film material as a sensor base body, and forms a gradually-changing coupling structure, a polarization selection structure, a reflective MZ electro-optic modulator, a wideband high-gain electrode and an antenna structure on the lithium niobate thin film material in sequence along the light path propagation direction, wherein the reflective MZ electro-optic modulator only comprises a Y-branch optical waveguide, and a Bragg reflection grating is arranged on the parallel waveguide arms of the Y-branch optical waveguide; through input laser signals of the sensor, an electric field signal can be modulated on the laser, and the modulated laser is transmitted by using a long optical fiber; the rear end is demodulated by using a photoelectric detector and is sent into a receiver for measurement, and the measured electric field information can be obtained. The sensor has the advantages of simple structure, saved output optical fiber, small size, high sensitivity, small interference, accurate electric field measurement result and high measurement repeatability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of wideband fast electric field detection, and particularly relates to a reflective optical electric field sensor based on a lithium niobate thin film material. BACKGROUND

[0002] The development of electronic technology has given rise to the demand for electromagnetic field detection technology, from the initial antenna plus spectrum analyzer test mode to the current optical electric field sensor system test mode, the electric field test technology has been continuously deepened, and in this process, many problems have been exposed.

[0003] The optical electric field sensor is a sensor that modulates the to-be-detected electric field signal on the laser by using the electro-optic effect of the electro-optic crystal, and detects the laser intensity to inversely deduce the electric field information. The existing optical electric field sensor mostly uses a bulk lithium niobate crystal as a sensing crystal, and a channel waveguide sensing structure is made on the bulk lithium niobate to realize the sensing and measurement of the electric field. Since the bulk lithium niobate has a certain thickness and is not a thin film, the etching difficulty is high, and the bulk lithium niobate channel waveguide generally adopts titanium diffusion or proton exchange technology. 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 small, usually of the order of 10-4, so the waveguide width is large (usually a single-mode optical waveguide is 6-7 μm), and the beam light capacity 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 a longer structure to complete the modulation. The weak beam light capacity leads to a large turning radius of the branch structure in the waveguide, so a longer structure is also needed to complete the modulation. The overlong structure will exacerbate the light-electric velocity mismatch effect, thereby affecting the bandwidth of the probe. Therefore, the existing optical electric field sensor still has the problems of large size and narrow bandwidth. SUMMARY

[0004] Technical problems to be solved:

[0005] In view of the problems in the prior art, the application provides a reflective optical electric field sensor based on a lithium niobate thin film material, which etches a waveguide structure on the lithium niobate thin film material to form an integrated reflective electric field sensor. The sensor has a simple structure, small size, no need for an output optical fiber in the reflective structure, high sensitivity, small interference, accurate electric field measurement result, and high measurement repeatability.

[0006] The technical scheme adopted is as follows:

[0007] The application provides a reflective optical electric field sensor based on a lithium niobate thin film material, which can modulate the to-be-detected electric field signal on the laser by inputting a laser signal to the sensor, and transmit the laser by using a long optical fiber; the back end demodulates the laser by using a photoelectric detector and sends it into a receiver for measurement, so that the to-be-detected electric field information can be obtained.

[0008] The application discloses a reflective optical electric field sensor based on lithium niobate thin film material, which adopts lithium niobate thin film material as a sensor substrate, and etches a gradually changing coupling structure, a polarization selection structure, a reflective MZ electro-optic modulator, a wideband high-gain electrode and an antenna structure on the lithium niobate thin film material in sequence along the light path propagation direction; wherein the gradually changing coupling structure is arranged at the port of the sensor, and is used for coupling laser with a large light spot in the input laser to a single-mode optical waveguide with a small light spot, so as to realize single-mode transmission of the laser; the polarization selection structure is used for eliminating TM mode in the single-mode optical waveguide, so as to realize single-polarization transmission of the laser; the reflective MZ electro-optic modulator only comprises a Y branch optical waveguide, the Y branch optical waveguide comprises two parallel waveguide arms, and a Bragg reflection grating is arranged at the distal end of the two parallel waveguide arms; the reflective MZ electro-optic modulator is connected after the single-mode optical waveguide, and is used for modulating the measured electric field in space to the transmitted laser; the high-gain electrode is used for increasing the intensity of the measured electric field acting on the waveguide arm of the reflective MZ electro-optic modulator; and the Bragg reflection grating is used for reflecting waveguide light, and the reflected light is combined and outputted through the Y branch optical waveguide again.

[0009] 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.

[0010] Further, the gradually changing coupling structure is a tapered gradually changing transmission channel arranged at the port of the sensor, the tapered gradually changing transmission channel is wide at one end of the port and is used for connecting with an input optical fiber, is narrow at the other end and is connected with a single-mode optical waveguide, and is used for gradually coupling laser of the input optical fiber to the single-mode optical waveguide with a width of 1 mu m.

[0011] Further, the polarization selection structure is used for realizing a single-polarization working state of the sensor by arranging a metal coating layer on the single-mode optical waveguide, exciting a high-loss plasmonic surface mode, making a TE mode waveguide pass at low loss and a TM mode waveguide attenuate at high loss.

[0012] Further, the reflective MZ electro-optic modulator adopts a biased MZ electro-optic modulation structure, so that the sensor works at a linear working point of the MZ electro-optic modulator.

[0013] Further, the high-gain electrode adopts a wideband micro-nano high-gain electrode, comprises two electrodes with the same size, and the electrode spacing is within 2 mu m, and the high-gain electrode is integrated on the waveguide arm of the MZ electro-optic modulator.

[0014] The measurement system based on the lithium niobate thin film material optical electric field sensor for measurement comprises a laser source, a polarization maintaining optical fiber, a circulator, the lithium niobate thin film material reflective electric field sensor, a single-mode optical fiber, a light detector and a spectrum analyzer, laser is input to the optical electric field sensor through the polarization maintaining optical fiber and the circulator, an external electric field to be measured is applied to the sensor, output intensity-modulated laser is incident into the light detector through the circulator and the single-mode optical fiber, and is converted into an electric signal, finally input into the spectrum analyzer for detection, and the frequency and amplitude of the electric field to be measured are obtained.

[0015] The measurement method based on the measurement system is specifically as follows:

[0016] Step one, gradual mode spot conversion coupling: 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 to the 1-micron-wide single-mode optical waveguide through the gradual coupling structure, so that single-mode transmission of laser is realized;

[0017] Step two, polarization selection: when the single-mode transmitted laser passes through the single-mode optical waveguide coated with a metal layer, a plasmonic surface mode is excited, so that the TM mode in the waveguide is removed, and single-polarization transmission of laser is formed;

[0018] Step three, phase modulation: the transmitted laser in step two is evenly divided into two paths at the Y branch of the reflective MZ modulator, and enters the two waveguide arms of the MZ photoelectric modulator for interference, because the electric field to be measured exists in the space, the refractive index of the lithium niobate material changes, so that the phase of the waveguide light transmitted in the two arms changes, and the first phase modulation of the transmitted waveguide light is realized;

[0019] Step four, second phase modulation: the light in the two waveguide arms is transmitted forward to the Bragg reflection grating and is reflected; the waveguide light reflected by the Bragg reflection grating is phase-modulated by the electric field to be measured in the space for the second time;

[0020] Step five, intensity modulation: the twice-modulated light in the two waveguide arms passes through the Y branch again, is combined into one light, and then interferes, so that the intensity modulation of the spatial electric field on the laser is finally realized; the change rule is:

[0021] I out =I in αE

[0022] I out is the backward output power of the sensor, I in is the input power of the light source, and α is the modulation coefficient of the sensor; E is the electric field intensity to be measured.

[0023] Step six, the electric field intensity calculation: the intensity modulated laser output in step five is input into the optical detector to be converted into an electric signal, and the obtained electric signal is input into the frequency spectrometer to measure the signal amplitude V rf ; the electric field frequency of the measured electric field is the same as the signal frequency measured in the frequency spectrometer, and the electric field intensity amplitude of the measured electric field is E out = V rf + A F , wherein A F is the antenna coefficient of the measurement system after precision 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 etches the on-chip polarization selection structure directly 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.

[0029] 5. The present application adopts a reflective sensor structure, which is smaller in size and easier to hold and install; the present application designs an on-chip reflection structure, which avoids the use of external mirrors or reflective films and reduces the process steps. BRIEF DESCRIPTION OF DRAWINGS

[0030] 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 other drawings can also be obtained by those skilled in the art without creative labor.

[0031] Figure 1 is a schematic diagram of a reflective optical electric field sensor structure of a lithium niobate thin film material;

[0032] Figure 2 is a schematic diagram of a lithium niobate thin film material substrate;

[0033] Figure 3 Fig. 1 is a schematic diagram of a measurement system of optical electric field of lithium niobate thin film material;

[0034] Fig. 4(a) is an effect diagram of polarization selection of a TE polarized Gaussian beam by a polarization selection structure;

[0035] Fig. 4(b) is an effect diagram of polarization selection of a TM polarized Gaussian beam by a polarization selection structure;

[0036] Figure 5 Fig. 5 is a reflection effect diagram of a Bragg grating method.

[0037] Explanation of reference signs:

[0038] 1-polarization maintaining optical fiber, 2-tapered coupling structure, 3-polarization selection structure, 4-reflective MZ electro-optic modulator, 5-high-gain electrode, 6-Bragg reflection grating, 7-laser source, 8-optical electric field sensor of lithium niobate thin film material, 9-circulator, 10-single-mode optical fiber, 11-optical detector, 12-spectrometer, 13-radio frequency transmission line, 14-electric field to be measured, 15-lithium niobate single crystal thin film, 16-silicon dioxide, 17-silicon. DETAILED DESCRIPTION

[0039] 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.

[0040] Figure 1 Fig. 1 is a structural diagram of a reflective optical electric field sensor of lithium niobate thin film material. The sensor takes lithium niobate thin film as a substrate and etches, in the direction of an optical path, a tapered coupling structure 2, a polarization selection structure 3, an MZ electro-optic modulator 4, a high-gain electrode 5, an antenna structure (not shown) and a Bragg reflection grating 6 in sequence. A polarization maintaining optical fiber 1 with a tapered lens is arranged at the left end face of the sensor 8. Only one optical fiber is arranged to be directly connected with the sensor 8 to realize optical path connection.

[0041] The tapered coupling structure 2 is a tapered transmission channel arranged at the port of the sensor 8. The tapered transmission channel is wider at one end facing the port and is used to connect with the input polarization maintaining optical fiber 1. The other end is narrower and is connected with the single-mode optical waveguide inside the sensor 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.

[0042] The polarization selective structure 3 is achieved by setting a metal coating on the single-mode optical waveguide, exciting a high-loss plasmonic surface mode, so that the TE mode waveguide is low-loss while the TM mode waveguide is high-loss, and the sensor works in a single polarization state.

[0043] The reflective MZ electro-optical modulator 4 includes a Y-branch optical waveguide, which includes two parallel waveguide arms, and a Bragg reflection grating 6 is arranged at the distal end of the two parallel waveguide arms; the reflective MZ electro-optical modulator 4 adopts a biased MZ electro-optical modulation structure, so that the sensor works at a linear operating point of the MZ electro-optical modulator 4, thereby ensuring that the dynamic range of the sensor reaches the maximum; the Bragg reflection grating 6 is used to realize the reverse propagation of the waveguide light, without the need to set a mirror or a reflective film on the other end face of the waveguide, which simplifies the structure of the sensor, avoids the inconvenience of separation of the input and output ends during use of the sensor, and is more in line with ergonomics and more suitable for use in various measurement environments. Since no output optical fiber is needed, the reduction in the number of optical fibers is also conducive to the construction of a complex sensor system.

[0044] The high-gain electrode 5 adopts 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, and the high-gain electrode is integrated on the waveguide arm of the MZ electro-optical modulator. The antenna structure adopts a conventional antenna structure of an existing electric field sensor.

[0045] Figure 2 The lithium niobate thin film material substrate is achieved by attaching a lithium niobate single crystal thin film on a silicon substrate, specifically including a 0.5 mm silicon 17 support structure, a 2-5 μm thick silicon dioxide substrate 16 attached to the silicon 17 support structure, and a 300-600 nm lithium niobate single crystal thin film structure 15 attached to the silicon dioxide substrate 16, which can be an x-cut y-transmission lithium niobate single crystal thin film structure. The sensor structure is etched on the lithium niobate thin film material substrate by a micro-nano processing method.

[0046] Figure 3 The measurement system schematic diagram of the electric field sensor 8 is shown. The laser emitted by the laser source 7 is input from one end of the circulator 9 through the polarization maintaining optical fiber 1 and enters the sensor 8 from the other end; the laser is modulated by the electric field to be measured in the sensor and is reflected; the downlink laser enters the circulator 9 again and is separated from the uplink laser, and enters the optical detector 11; the optical detector 11 outputs an electrical signal carrying the electric field information to be measured, and inputs the electrical signal to the spectrum analyzer 12 through the radio frequency transmission line 13 for detection; the frequency and amplitude of the electric field to be measured are obtained. The specific measurement method includes the following steps:

[0047] 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 to the single-mode optical waveguide with a width of 1μm through the gradual coupling structure, so as to realize the single-mode transmission of the laser;

[0048] Step two, polarization selection: when the single-mode transmission 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 the single-polarization transmission of the laser is formed;

[0049] Step three, phase modulation: the transmitted laser in step two is evenly divided into two paths at the Y branch of the reflective MZ modulator, and enters the two optical waveguide arms of the MZ photoelectric modulator interference, respectively. Due to the existence of the electric field to be measured in space, the refractive index of lithium niobate material changes, so that the phase of the waveguide light transmitted in the two arms changes, realizing the first phase modulation of the transmitted waveguide light;

[0050] Step four, second phase modulation: the light in the two optical waveguide arms is transmitted forward to the Bragg reflection grating and is reflected; the waveguide light reflected by the Bragg reflection grating is phase-modulated by the space electric field to be measured for the second time;

[0051] Step five: intensity modulation: the twice-modulated light in the two optical waveguide arms passes through the Y branch again, is combined into one light, and then interferes, so that the intensity modulation of the spatial electric field on the laser is finally realized; the change rule is:

[0052] I out =I in αE

[0053] Wherein, I out is the backward output power of the sensor, I in is the input power of the light source, and α is the modulation coefficient of the sensor; E is the electric field intensity to be measured;

[0054] Step six, calculation of the electric field intensity to be measured: the intensity-modulated laser output in step five is input into the light detector to be converted into an electric signal, and the obtained electric signal is input into the spectrum analyzer, so that the signal amplitude V rf is measured in the spectrum analyzer, with the unit of dBμV; 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, which is a known parameter of the measurement system and a transfer coefficient of the electric field sensor, with the unit of dB / m.

[0055] Figure 4 shows the polarization selection structure 3 selected polarization effect diagram. In which, Figure 4(a) is in the end of the sensor input a TE polarized Gaussian beam of propagation field distribution diagram, Figure 4(b) is in the end of the sensor input a TM polarized Gaussian beam of propagation field distribution diagram. As can be seen from the figure, TM polarized light attenuation is larger, and TE polarized light can be lower attenuation through the selected polarization structure, thereby realizing single polarization transmission. In fact, its extinction ratio for TM polarization and TE polarization can reach more than 20dB.

[0056] Figure 5 For Bragg reflection grating structure reflection effect diagram. The left end input a Gaussian beam, and into the Bragg reflection grating. By Figure 5 It can be seen that the input beam is not transmitted to the right end, but is reflected back to the starting end, and the reflectivity can reach 85%.

[0057] Obviously, the above examples are only for the sake of clarity made by example, and not limited to the embodiments. For those of ordinary skill in the art, on the basis of the above description can also be made on the basis of other different forms of changes or variations. Here need not and can not be exhausted to all the embodiments. The obvious changes or variations derived from still within the scope of the present invention.

Claims

1. A reflective optical electric field sensor based on lithium niobate thin film material, characterized in that, The lithium niobate thin film material is used as a sensor substrate, and a gradually changing coupling structure, a polarization selection structure, a reflective 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 port of the sensor, and the laser with a large light spot is coupled into a single-mode optical waveguide with a small light spot, so that the single-mode transmission of the laser is realized. The polarization selection structure eliminates the TM mode in the single-mode optical waveguide, so that the single-polarization transmission of the laser is realized.

2. The reflective optical electric field sensor based on lithium niobate thin film material according to claim 1, characterized in that, The reflective MZ electro-optic modulator only includes a Y-branch optical waveguide, the Y-branch optical waveguide includes two parallel waveguide arms, and a Bragg reflection grating is arranged at the distal end of the two parallel waveguide arms.

3. The reflective optical electric field sensor based on lithium niobate thin film material according to claim 2, characterized in that, The reflective MZ electro-optic modulator is connected after the single-mode optical waveguide, and is used for modulating the measured electric field in space to the transmitted laser.

4. The reflective optical electric field sensor based on lithium niobate thin film material according to claim 3, characterized in that, The high-gain electrode increases the intensity of the measured electric field acting on the waveguide arm of the reflective MZ electro-optic modulator.

5. Measurement system for performing measurements with a reflective optical electric field sensor according to any one of claims 1 to 4, characterized in that The Bragg reflection grating reflects the waveguide light, and the reflected light is combined and output through the Y-branch optical waveguide of the reflective MZ electro-optic modulator. The gradually changing coupling structure is a tapered transmission channel arranged at the port of the sensor, the tapered transmission channel is wide at one end of the port for connecting with the input optical fiber, and the other end is narrow and connected with the single-mode optical waveguide, so that the laser of the input optical fiber is gradually coupled into the single-mode optical waveguide with a width of 1 um. The polarization selection structure is provided with a metal coating on the single-mode optical waveguide, and the TE mode waveguide passes through with low loss while the TM mode waveguide attenuates with high loss by exciting a high-loss plasmonic surface mode, so that the single-polarization working state of the sensor is realized. The lithium niobate thin film material substrate is supported by a silicon wafer, a 2-5 um 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. The reflective MZ electro-optic modulator adopts a biased MZ electro-optic modulation structure, so that the sensor works at the linear working point of the MZ electro-optic modulator. The high-gain electrode adopts a wideband micro-nano high-gain electrode, which includes two electrodes with the same size and a distance between the electrodes of 2 um or less, and the high-gain electrode is integrated on the waveguide arm of the MZ electro-optic modulator. The measurement system includes a laser source, a polarization maintaining optical fiber, a circulator, the lithium niobate thin film material reflective electric field sensor, a single-mode optical fiber, an optical detector and a spectrum analyzer.

6. A measurement method based on the measurement system of claim 5, specifically comprising the following steps: 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 to the single-mode optical waveguide with a width of 1 um through the gradual coupling structure, realizing single-mode transmission of the laser; 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, thereby removing the TM mode in the waveguide, forming single-polarization transmission of the laser; Step three, phase modulation: the transmitted laser in step two is evenly divided into two paths at the Y branch of the reflective MZ modulator and enters the two optical waveguide arms of the MZ photoelectric modulator interference, and due to the existence of the space electric field to be measured, the refractive index of lithium niobate material changes, thereby causing the phase of the waveguide light transmitted in the two arms to change, realizing the first phase modulation of the transmitted waveguide light; Step four, second phase modulation: the light in the two optical waveguide arms is transmitted forward to the Bragg reflection grating and is reflected; the waveguide light reflected by the Bragg reflection grating is phase-modulated by the space electric field to be measured for the second time; Step five: intensity modulation: the light in the two optical waveguide arms of the second phase modulation passes through the Y branch again, is combined into one light, and then interferes, finally realizing the intensity modulation of the laser by the space electric field; the change rule is: wherein, is the sensor backward output power, is the laser light source input power, is the sensor modulation coefficient, is the electric field intensity to be measured; Step six, the calculation of the electric field intensity to be measured: the intensity-modulated laser output in step five is input into the optical detector to be converted into an electric signal, and the obtained electric signal is input into the frequency spectrometer to measure the signal amplitude ; the frequency of the electric field 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 , wherein is the antenna coefficient of the measurement system after precision calibration.

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