U-shaped cascaded waveguide nested micro-ring resonator cavity type electric field sensor and preparation method thereof

Abstract

The application belongs to the technical field of optical electric field sensing, and discloses a U-shaped cascade waveguide nested micro-ring resonant cavity type electric field sensor and a preparation method thereof. The electric field sensor comprises an input coupling unit, an optical sensitive unit and a spectrum detection output interface. The optical sensitive unit comprises an incident grating, a U-shaped cascade waveguide structure, a nested micro-ring resonant cavity and an electro-optic polymer film. The nested micro-ring resonant cavity comprises a reference resonant cavity and a sensing resonant cavity. The electro-optic polymer film covers the cladding layer of the sensing resonant cavity. The application expands the equivalent free spectral range of a single resonant unit through a U-shaped feedback structure, utilizes the free spectral range difference between the two resonant units to form a vernier effect to amplify the sensing response, and simultaneously adopts a heterogeneous integrated vertical coupling mode to improve the system integration and packaging consistency, and improves the sensing sensitivity and measurement range.

Description

Technical Field

[0001] This invention relates to the field of optical electric field sensing technology, and in particular to a U-shaped cascaded waveguide nested microring resonant cavity electric field sensor and its fabrication method. Background Technology

[0002] As power systems evolve towards digitalization and intelligence, the demand for online monitoring of key electrical parameters such as electric field and voltage continues to grow. Measurements in complex electromagnetic environments often require sensors with strong anti-interference capabilities, wide measurement bandwidth, and high reliability, thus driving the development of non-contact electric field sensing technology based on optical principles.

[0003] Optical electric field sensors typically utilize the property of electro-optic materials to change their refractive index under the influence of an external electric field. This converts the electric field information into changes in the phase or spectral characteristics of an on-chip optical waveguide, which is then measured via optical demodulation. Compared to traditional electrical measurement methods, this approach offers advantages in terms of electromagnetic interference resistance, insulation safety, and wideband response.

[0004] Currently, most on-chip silicon-based optical electric field sensors employ a Mach-Zehnder interferometer (MZI) structure as the core sensing unit. A resonant microring is coupled to the sensing arm of the MZI, utilizing the superlinear phase response of the microring under specific coupling conditions to compensate for the inherent third-order intermodulation distortion of the MZI structure. This achieves a linear conversion of phase information to light intensity information, improving the sensor's linearity. However, in this approach, the microring operates in a non-resonant state, and the sensitivity is limited by the inherent optical path difference of the MZI interferometer arm, making it difficult to achieve both high sensitivity and a wide measurement range while maintaining linearity.

[0005] Another approach employs a cascaded microring resonator structure, utilizing two microring resonators with slightly different free spectral ranges to create a vernier effect, which can significantly improve sensing sensitivity. However, traditional dual-microring vernier sensors are mostly used for refractive index detection, and there is a mutual constraint between the expansion of the free spectral range of the sensing unit and the vernier amplification factor, making it difficult to simultaneously improve sensitivity and detection range. Directly transplanting such a structure into the field of electric field sensing also faces systematic perturbations to the original optical field mode, coupling conditions, and interference phase relationships caused by the introduction of electro-optic functional materials, as well as engineering challenges in terms of packaging volume, assembly complexity, and long-term stability between the external light source and the on-chip waveguide structure.

[0006] Therefore, it is necessary to propose an electric field sensor that combines a highly integrated coupling method with a high-performance sensitive structure, so as to achieve device miniaturization and integration while taking into account high sensing sensitivity and wide measurement range. Summary of the Invention

[0007] The technical problem this invention aims to solve is: existing on-chip optical electric field sensors suffer from significant dependence on precise assembly and adjustment of packaging space and angle in input coupling methods, insufficient system integration and long-term stability, and the difficulty of simultaneously achieving high sensing sensitivity and wide measurement range with a single micro-ring or single interference structure. This invention provides a U-shaped cascaded waveguide nested micro-ring resonant cavity electric field sensor and its fabrication method, which achieves device miniaturization and integration while simultaneously achieving high sensing sensitivity and wide measurement range.

[0008] The technical solution adopted by the present invention to solve its technical problem is: a heterogeneous integrated U-shaped cascaded waveguide nested micro-ring resonant cavity electric field sensor, comprising: an input coupling unit, an optical sensing unit, and a spectral detection output interface integrated on an SOI (Silicon-On-Insulator) chip from left to right; The input coupling unit includes an SLD (Superluminescent Diode) laser source, a lens group, and a prism. The beam emitted from the SLD laser source is collimated and focused by the lens group, and then the prism alters the optical path, coupling the beam to the optical sensing unit in a direction perpendicular or approximately perpendicular to the SOI chip surface. The lens group shapes the beam, ensuring that the spot size, divergence angle, and energy distribution at the incident prism and incident grating match the on-chip coupling structure, thus improving input coupling efficiency and stability. The prism refracts the laterally propagating beam into a vertically downward incident beam, reducing the reliance on high-precision angle adjustment and spatial alignment required by traditional oblique incidence or end-face coupling methods, which helps reduce package size and improve system integration. The lens group includes a first collimating lens and a second focusing lens arranged sequentially along the output optical path of the SLD laser source. The first collimating lens is used to collimate the diverging beam emitted from the SLD laser source, and its thickness is preferably 0.73 mm. The second focusing lens is used to focus and shape the collimated beam onto the prism and the incident grating, and its thickness is preferably 0.69 mm. The center wavelength of the SLD laser source is 1550 nm, and the horizontal divergence angle and vertical divergence angle are 20°~30° and 25°~35°, respectively. The distance between the first collimating lens and the SLD laser source is 0.10~0.30 mm; the distance between the second focusing lens and the prism is 0.30~0.60 mm; and the target spot diameter at the incident grating is 6~12 μm.

[0009] The optical sensing unit includes an incident grating, a U-shaped cascaded waveguide structure, a nested micro-ring resonator, an electro-optic polymer film, and an exit grating. The incident grating receives a beam of light coupled vertically downwards through the prism and couples it to the U-shaped cascaded waveguide structure. The nested micro-ring resonator includes a reference resonator and a sensing resonator, each being a racetrack-shaped micro-ring resonator. The U-shaped cascaded waveguide structure includes a first U-shaped waveguide, a second U-shaped waveguide, and an intermediate straight waveguide. The first U-shaped waveguide connects the Through port of the reference resonator to the Add port, and the second U-shaped waveguide connects the Through port of the reference resonator to the Add port of the sensor. The through port of the sensing resonant cavity is connected to the add port, and the drop port of the reference resonant cavity is cascaded with the input port of the sensing resonant cavity through an intermediate straight waveguide; the electro-optic polymer film covers at least the upper cladding of the region where the sensing resonant cavity is located, and is used to generate a refractive index change under the action of an external electric field, and to cause the output resonance spectrum of the nested micro-ring resonant cavity to drift or change its spectral shape through the evanescent field effect; the incident grating is connected to the input port of the reference resonant cavity, and the emitted grating is connected to the drop port of the sensing resonant cavity, and is used to couple the output light to the spectral detection output interface.

[0010] As a preferred embodiment, the first U-shaped waveguide and the reference resonant cavity, and the second U-shaped waveguide and the sensing resonant cavity, respectively satisfy the equivalent phase matching condition. This ensures that the first U-shaped waveguide, the reference resonant cavity, the second U-shaped waveguide, and the sensing resonant cavity together constitute a transmission path that extends the free spectral range, thus ensuring that the propagation phase difference between the U-shaped waveguide and the racetrack-shaped microring satisfies the interference enhancement condition. Therefore, spectral line reconstruction can be achieved within each U-shaped feedback racetrack microring resonant unit, effectively extending the equivalent free spectral range compared to traditional Add-Drop microrings.

[0011] Based on satisfying U-shaped feedback phase matching, a preset difference exists between the equivalent free spectral range of the reference resonant cavity and the equivalent free spectral range of the sensing resonant cavity. This preset difference is less than 10% of the smaller of the equivalent free spectral ranges of the reference and sensing resonant cavities, preferably 0.02~0.20 nm, and more preferably 0.03~0.08 nm, so that a vernier effect is formed between their output spectra. Through the vernier effect, the resonant wavelength drift of the sensing resonant cavity caused by the external electric field is further amplified, thereby significantly improving the electric field sensing sensitivity.

[0012] As a preferred embodiment, the reference resonant cavity is coupled to the intermediate straight waveguide and the first U-shaped waveguide via two straight waveguides, respectively, and the sensing resonant cavity is coupled to the intermediate straight waveguide and the second U-shaped waveguide via two other straight waveguides, respectively. The coupling coefficient between the reference resonant cavity and its coupled two straight waveguides is asymmetrically designed with respect to the coupling coefficient between the sensing resonant cavity and its coupled two straight waveguides. Specific implementations of the asymmetrical coupling design include: using different coupling spacings for the reference resonant cavity and the sensing resonant cavity; locally adjusting the waveguide width in the coupling region of the sensing resonant cavity; primarily covering the sensing waveguide region with a local avoidance area in the coupling region using an electro-optic polymer; or re-optimizing the coupling length or coupling gap of the sensing resonant cavity based on the mode field distribution after electro-optic polymer coverage, so that both resonant units maintain good coupling matching in their respective operating states, ensuring the extinction ratio and peak identification accuracy of the vernier spectral envelope. By using asymmetric coupling design, the mismatch in mode overlap and coupling state between the reference resonator and the sensing resonator after electro-optic polymer coverage is compensated, thus maintaining the extinction ratio of the vernier spectral envelope and the accuracy of spectral peak identification.

[0013] As a preferred embodiment, the incident grating is a binary blazed grating (BBGC), comprising a periodic structure consisting of multiple sub-grating segments with different duty cycles to approximate the blazed phase distribution. By introducing multiple sub-grating segments within a single grating period and non-uniformly modulating the duty cycle of each sub-grating segment, the linear phase distribution of an ideal blazed grating is approximated in a discrete manner. This achieves high coupling efficiency and bandwidth under vertical or small-angle incident conditions, while also ensuring simple manufacturing processes and good consistency.

[0014] As a preferred embodiment, the sensor is composed of a silicon substrate layer, a silicon dioxide buried oxide layer and a top silicon waveguide layer from bottom to top; the top silicon waveguide layer includes the waveguide structure of the U-shaped cascaded waveguide structure and the nested micro-ring resonant cavity.

[0015] As a preferred embodiment, the SLD laser source, the lens group, and the prism are heterogeneously integrated and positioned above the SOI chip. The beam emitted from the SLD laser source is shaped by the lens group, then deflected 90° by the prism and incident vertically downwards into the incident grating, where it is coupled into the on-chip waveguide structure. This heterogeneous integration coupling method reduces the dependence on precise packaging space and angle adjustment, improving the long-term stability of the device.

[0016] As a preferred embodiment, the thickness of the electro-optic polymer film is 600~1000nm. Under the action of an external electric field, the electro-optic polymer film undergoes a change in refractive index, which causes a change in the effective refractive index of the on-chip waveguide mode through the evanescent field effect, thereby causing the output resonance spectrum of the nested micro-ring resonator to drift or change in spectral shape.

[0017] As a preferred embodiment, the spectral detection output interface includes an output single-mode optical fiber, and the emission grating couples the output light to the output single-mode optical fiber at a set angle. The emission grating couples the modulated output light to an off-chip spectral detection device, and by detecting the drift or spectral shape change of the resonance spectrum, establishes the correspondence between the output spectral characteristic quantities and the electric field to be measured, thereby realizing the measurement of the electric field intensity.

[0018] This invention provides an electric field measurement system, comprising: a heterogeneously integrated U-shaped cascaded waveguide nested microring resonant cavity type electric field sensor; and a spectral detection device for receiving the output light of the electric field sensor and detecting the drift or spectral shape change of the resonance spectrum to calculate the intensity of the electric field to be measured. The emitted light from the SLD laser source is coupled to the optical sensing unit via an input coupling unit. Under the action of an applied electric field, the refractive index of the electro-optic polymer film covering the sensing area changes, causing a drift or spectral shape change in the resonance spectrum. The output light is coupled to the spectral detection device via an emission grating to realize the measurement of the electric field to be measured.

[0019] This invention provides a fabrication method comprising: fabricating an incident grating, a U-shaped cascaded waveguide structure, a nested microring resonator, and an exit grating on an SOI substrate; fabricating an electro-optic polymer film in at least the region where the sensing resonator is located within the nested microring resonator; heterogeneously integrating an SLD laser source, a lens group, and a prism above the SOI substrate, such that the beam emitted from the SLD laser source is collimated and focused by the lens group, and after being refracted by the prism, is incident vertically downward onto the incident grating. The sensor is integrated using a silicon-on-insulator platform and utilizes mature microelectronic CMOS fabrication technology, making this electric field sensor easy to mass-produce and thus reducing costs.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. A heterogeneous vertical coupling method integrating the SLD laser source, lens group, and prism significantly improves system integration and packaging consistency. The horizontally emitted beam from the SLD laser source is collimated and focused by the lens group, then refracted 90° by the prism to propagate vertically downwards. It is then coupled into the on-chip waveguide structure via an incident grating, replacing the traditional method of using external optical fibers aligned to the chip end face or relying on tilted grating coupling. This coupling architecture reduces the dependence on precise packaging space and angle adjustment, lowers coupling efficiency fluctuations caused by angle deviations, vibrations, and packaging stress, and improves the long-term stability of the device. Simultaneously, heterogeneous integration of the light source and chip in the vertical direction avoids dependence on chip edges, facilitating high-density integration of sensitive on-chip structures within a compact area.

[0021] 2. A U-shaped cascaded waveguide nested microring resonator structure is employed to simultaneously achieve a significant improvement in sensing sensitivity and an expansion of the measurement range. A first U-shaped waveguide connects the Through port and Add port of the reference resonator, and a second U-shaped waveguide connects the Through port and Add port of the sensing resonator, forming U-shaped feedback interference structures. When the equivalent phase matching condition is met, this structure expands the free spectral range of a single microring resonator, thereby increasing the sensor's measurable range. Furthermore, the Drop port of the reference resonator and the Input port of the sensing resonator are cascaded through a straight waveguide. Utilizing the slight difference in the free spectral ranges of the two resonators to create a vernier effect, the resonant wavelength drift of the sensing ring is further amplified, resulting in a sensing sensitivity far exceeding that of a single microring. This structure achieves compatibility between free spectral range expansion and vernier sensitivity amplification on a single chip, overcoming the technical contradiction in existing sensors where detection range and sensitivity are difficult to balance.

[0022] 3. Through systematic collaborative design following the introduction of electro-optic polymers, the effectiveness of the U-shaped feedback interference condition and vernier spectral envelope under actual operating conditions is ensured. The perturbation of the U-shaped feedback interference condition and dual-ring vernier coupling matching caused by the changes in the effective refractive index and group refractive index of the waveguide modes after the electro-optic polymer film covers the cladding of the sensing resonator is fully considered. An equivalent phase-matching condition replaces the traditional U-shaped waveguide design rule of integer multiples of geometric length. By compensating for the effects of phase coverage on the electro-optic polymer refractive index, process deviations, and dispersion, it is ensured that the U-shaped feedback structure can still achieve spectral line reconstruction and free spectral range expansion under actual operating conditions. Simultaneously, to address the difference in mode distribution between the reference resonator and the sensing resonator due to the different cladding materials, an asymmetric coupling design is employed to maintain the extinction ratio and peak identification accuracy of the vernier spectral envelope.

[0023] 4. The incident grating employs a binary blazed grating, achieving high coupling efficiency and a wide operating bandwidth under perpendicular incidence conditions. The binary blazed grating, through non-uniform modulation of the sub-period duty cycle, discretizes the linear phase distribution of an ideal blazed grating, more effectively concentrating incident light energy onto the target diffraction order, improving perpendicular coupling efficiency and suppressing back reflection. Compared to traditional uniform rectangular gratings, the binary blazed grating achieves higher coupling performance under perpendicular or small-angle incidence conditions, reducing the grating's dependence on incident angle accuracy and improving process tolerance and packaging compatibility. Furthermore, the binary etching process is compatible with CMOS standard processes, facilitating large-scale mass production of sensors.

[0024] 5. An electro-optic polymer material with a high electro-optic coefficient is used as the electric field-sensitive layer to achieve efficient electro-optic energy coupling through the evanescent field effect. The electro-optic polymer film covers the upper cladding of the sensing resonant cavity. Under the action of an applied electric field, the refractive index of the electro-optic polymer changes linearly with the electric field strength. This refractive index change is coupled to the waveguide guided mode through the evanescent field effect, causing a change in the effective refractive index of the waveguide, which in turn manifests as a drift or spectral shape change in the output resonance spectrum of the nested micro-ring resonant cavity. Compared with the solution using inorganic electro-optic crystals, electro-optic polymers have a higher electro-optic coefficient, a lower dielectric constant, and can be fabricated on the chip at low temperatures using spin-coating, making them compatible with CMOS back-end processes. This facilitates on-chip optoelectronic integration and reduces manufacturing costs.

[0025] 6. The sensor is based on a silicon-on-insulator (SOI) platform, a mature technology that facilitates on-chip integration and mass production. The sensor is fabricated using a standard SOI substrate. The high refractive index difference between silicon and silicon dioxide effectively confines the light field within the top silicon waveguide, laying the foundation for a high-Q resonant cavity and efficient modulation. The SOI platform is compatible with CMOS processes, enabling the sensor to be standardized and mass-produced using mature microelectronic fabrication techniques, thus reducing device costs and promoting engineering applications. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the on-chip structure of the heterogeneously integrated U-shaped cascaded waveguide nested micro-ring resonant cavity electric field sensor designed in this invention. Figure 2 This is a schematic diagram of the cross-section of the sensitive area of ​​the electric field sensor; Figure 3 This is a schematic diagram of the optical path structure of the input coupling unit of the present invention.

[0027] In the figure: 1. Incident grating, 2. Reference resonant cavity, 3. Nested micro-ring resonant cavity, 4. U-shaped cascaded waveguide structure, 5. Electro-optic polymer film, 6. Sensing resonant cavity, 7. Emission grating, 8. Prism, 9. Lens group, 10. SLD laser source, 11. Output single-mode fiber, 12. Base plate. Detailed Implementation

[0028] The invention will now be described in further detail with reference to the accompanying drawings. It should be emphasized that the following description is merely exemplary and not intended to limit the scope or application of the invention.

[0029] Example 1

[0030] This embodiment provides a heterogeneously integrated U-shaped cascaded waveguide nested microring resonant cavity type electric field sensor. For example... Figures 1 to 3As shown, the sensor includes, from left to right, an input coupling unit, an optical sensing unit, and a spectral detection output interface integrated on the SOI chip. The electric field sensor structure of this invention utilizes a silicon-on-insulator (SOI) platform for integration. The working structure of the SOI substrate is a typical two-dimensional planar waveguide structure, consisting of a silicon substrate layer, a silicon dioxide buried oxide layer, and a top silicon waveguide layer from bottom to top. The top silicon waveguide layer comprises a U-shaped cascaded waveguide structure and a waveguide structure with nested micro-ring resonators. In this embodiment, the preferred thickness of the top silicon waveguide layer is 300 nm, the thickness of the silicon dioxide buried oxide layer is 3 μm, and the thickness of the silicon substrate is approximately 500 μm.

[0031] The input coupling unit includes an SLD laser source 10, a lens group 9, and a prism 8. The SLD laser source 10, lens group 9, and prism 8 are heterogeneously integrated and positioned above the SOI chip. The SLD laser source 10, lens group 9, and prism 8 are fixed above the SOI chip via a packaging substrate with positioning grooves or steps. The SLD laser source 10 is fixed to the packaging substrate by soldering, conductive silver paste, or epoxy adhesive. The lens group 9 and prism 8 are fixed to the positioning grooves or steps using UV-curable optical adhesive, epoxy adhesive, or benzocyclobutene adhesive, and cured after active optical alignment. By introducing a synergistic optical path folding structure between the laterally arranged independent laser source and the on-chip incident grating of the SOI chip, efficient input coupling between the external light source and the on-chip waveguide structure is achieved through heterogeneous integration without altering the main structure of the on-chip silicon-based optical sensing unit. Specifically, the beam emitted from the SLD laser source 10 propagates horizontally, is collimated and focused by the lens group 9, and then enters the 90° prism 8. The prism 8 then folds the beam into a vertical downward direction, and it is incident on the on-chip incident grating 1 in a direction perpendicular or approximately perpendicular to the SOI chip surface, and finally coupled into the on-chip waveguide.

[0032] In a preferred embodiment, the lens group 9 collimates and focuses the light beam to adapt the spot size, divergence angle, and energy distribution incident on the prism 8 and the incident grating 1 to the on-chip coupling structure, thereby improving input coupling efficiency and stability. The lens group 9 includes a first collimating lens and a second focusing lens arranged sequentially along the output optical path of the SLD laser source 10. The first collimating lens is used to collimate the diverging beam emitted from the SLD laser source 10, reducing the divergence angle of the emitted beam. The thickness of the first collimating lens is preferably 0.73 mm. The second focusing lens is used to focus and shape the collimated beam onto the prism 8 and the incident grating 1, compressing the beam into the effective coupling aperture of the incident grating 1. The thickness of the second focusing lens is preferably 0.69 mm. In this embodiment, the center wavelength of the SLD laser source 10 is preferably 1550 nm, and the horizontal and vertical divergence angles are 20°~30° and 25°~35°, respectively, preferably 3° and 28°. The distance between the first collimating lens and the SLD laser source 10 is 0.10~0.30mm, preferably 0.162mm; the distance between the second focusing lens and the prism 8 is 0.30~0.60mm, preferably 0.44mm; and the diameter of the target spot at the incident grating 1 is 6~12μm.

[0033] By folding the optical path with a 90° prism 8, the beam, which originally propagated laterally, is transformed into a vertically downward incident beam. This makes it easier to achieve a stable match with the on-chip incident grating 1, reducing the reliance on high-precision angle adjustment and spatial alignment required by traditional oblique incidence or end-face coupling methods. Compared to traditional end-face coupling, this method does not require strict alignment between the input fiber or laser and the chip end face, nor does it require reserving large space at the chip edge for end-face coupling, alignment, and fixing. Therefore, this coupling method avoids the occupation of chip edge areas and the reliance on precise alignment required by traditional end-face coupling, allowing the on-chip U-shaped feedback waveguide, reference UNMRR unit, and sensing UNMRR unit to be integrated in a more compact area.

[0034] The optical sensing unit includes an incident grating 1, a U-shaped cascaded waveguide structure 4, a nested micro-ring resonant cavity 3, an electro-optic polymer film 5, and an exit grating 7. To achieve accurate measurement of the external electric field, this invention constructs an optical sensing structure on-chip, primarily based on a U-shaped cascaded waveguide, and forms an electric field-sensitive region within the sensing area by setting an electro-optic polymer cladding. The incident grating 1 receives the beam of light coupled vertically downwards through the prism 8 and couples it to the U-shaped cascaded waveguide structure 4.

[0035] In a preferred embodiment, the incident grating 1 is a binary blazed grating, used to receive the beam coupled vertically downwards by the prism 8 and couple it to the U-shaped cascaded waveguide structure 4. The binary blazed grating comprises a periodic structure consisting of multiple sub-grating segments, each with a different duty cycle to approximate the blazed phase distribution. In this embodiment, the center operating wavelength of the incident grating 1 is 1550 nm, the operating band is 1500~1600 nm, the target diffraction order is first-order diffraction, and the grating coupling angle is 0° or approximately 0° relative to the surface normal of the SOI chip. A single grating period T of the binary blazed grating comprises N=2 sub-periods, each sub-period Λ=T / 2, where T is preferably 588 nm and Λ is preferably 294 nm. The shallow etching depth H3 of the binary blazed grating is 70~85 nm, preferably 77 nm. The two sub-grating ridge widths are W1 = 47~67nm and W2 = 215~235nm, preferably W1 = 57nm and W2 = 225nm, corresponding to duty cycles of approximately 19.4% and 76.5%, respectively, to form an asymmetric blazed phase distribution in the binary etching structure. While maintaining the feasibility of the "binary etching / non-etching" process, multiple sub-grating segments are introduced within a single grating period, and the duty cycle of each sub-grating segment is non-uniformly modulated to discretely approximate the linear phase slope distribution of an ideal blazed grating, thereby replacing the single duty cycle structure of a traditional uniform rectangular grating. Its advantages are as follows: First, it can significantly improve coupling directionality and efficiency. The binary blazed grating constructs an equivalent "blazed phase" through sub-period duty cycle modulation, enabling better coherent superposition of the radiation field in the target direction, concentrating more energy at the desired emission angle, thereby improving the fiber-chip coupling efficiency and reducing back reflection. Second, the binary blazed structure, through multi-sub-period equivalent refractive index gradient and optimized coupling strength distribution, can suppress in-band ripple and improve the stability of broadband coupling, making the output spectral baseline smoother and facilitating subsequent spectrometer demodulation. Finally, it improves assembly tolerance and packaging adaptability. Compared with traditional grating schemes that rely on large tilt angles to avoid reflection, the binary blazed grating can achieve higher coupling performance under vertical or small tilt angle conditions, making fiber alignment more intuitive and array arrangement easier. At the same time, it reduces coupling efficiency fluctuations caused by angular deviations, and can be achieved with only binary etching, which is simple in process, has better consistency, and ensures the reliability of the sensing system in engineering applications.

[0036] The optical sensing unit employs a U-shaped cascaded waveguide nested microring resonator (CUNMRR) structure. Its core innovation lies in: using a U-shaped waveguide to connect the Through port and the Add port of the microring resonator, so that the output light from the Through port is reloaded to the Add port and interferes with the light from the Drop port, thereby forming alternating constructive / destructive interference at the resonance point and improving the free spectral range (FSR); through the cascaded structure, the vernier effect of the cascaded MRR is used to improve the refractive index sensing sensitivity.

[0037] Specifically, the nested micro-ring resonator 3 includes a reference resonator 2 and a sensing resonator 6, each employing a racetrack-shaped micro-ring resonator structure. The U-shaped cascaded waveguide structure 4 includes a first U-shaped waveguide, a second U-shaped waveguide, and an intermediate straight waveguide. The first U-shaped waveguide connects the Through port and the Add port of the reference resonator 2, and the second U-shaped waveguide connects the Through port and the Add port of the sensing resonator 6. The Drop port of the reference resonator 2 and the Input port of the sensing resonator 6 are cascaded through the intermediate straight waveguide. The incident grating is connected to the Input port of the reference resonator.

[0038] In a preferred embodiment, the first U-shaped waveguide and the reference resonant cavity 2, and the second U-shaped waveguide and the sensing resonant cavity 6, respectively satisfy the equivalent phase matching condition. After the electro-optic polymer covers the sensing area, it changes the refractive index of the waveguide cladding, causing changes in the effective refractive index and group refractive index of the waveguide mode. Therefore, the propagation phase in the U-shaped feedback waveguide is not only determined by its geometric length but also influenced by the refractive index, thickness, coverage area, material dispersion, and process uniformity of the electro-optic polymer. The U-shaped feedback condition should not be simply stated as "the length of the U-shaped waveguide is an integer multiple of the microring's circumference," but rather as satisfying the equivalent phase matching condition. Specifically, for a racetrack-shaped microring, its equivalent circumference can be expressed as... Where R is the bending radius of the runway-shaped micro-ring, Let be the length of the straight waveguide segment on one side of the runway-shaped microring. The phase matching condition between the U-shaped feedback waveguide and the runway-shaped microring can be expressed as: , and These represent the propagation constant and length of the U-shaped feedback waveguide, respectively. and Let be the propagation constant and equivalent perimeter of the runway-shaped microring, respectively, where q is an integer. The compensation phase is caused by phase delay in the coupling region, electro-optic polymer coverage, process deviations, and dispersion. Preferably, q=0, which satisfies the phase matching condition and can increase the equivalent FSR. However, considering the pursuit of smaller size and lower transmission loss, the smallest non-negative integer q=0 is usually selected that can achieve phase matching and make the U-shaped feedback waveguide length shorter.

[0039] The determination of the equivalent phase matching condition can be made through the transfer matrix method, FDTD simulation, or experimental calibration. Specifically, the phase residual at the working center wavelength of 1550 nm is used as the criterion, i.e., calculating whether the phase difference between the U-shaped feedback waveguide and the racetrack-shaped microring is close to the equivalent phase matching condition. When the phase residual is no greater than 0.1π, preferably no greater than 0.05π, the equivalent phase matching condition can be considered met. Simultaneously, the output spectrum should show clear resonance peaks and valleys without obvious valley splitting, and the equivalent FSR should deviate from the design target by no more than 5%. For example, with a working center wavelength of 1550 nm, an effective refractive index of approximately 2.40, and q=0, if the reference resonant cavity adopts a racetrack-shaped structure with a bending radius of 40 μm and a single-sided straight waveguide section length of approximately 188.5 μm, its equivalent perimeter is approximately 628.3 μm. In this case, the length of the U-shaped feedback waveguide corresponding to the reference resonant cavity can be designed to be approximately 628.3 μm, and the phase delay in the coupling region can be fine-tuned through simulation. If the sensing resonant cavity adopts a racetrack-shaped structure with a bending radius of 40 μm and a single-sided straight waveguide section length of approximately 177.5 μm, its equivalent perimeter is approximately 606.3 μm. Therefore, the corresponding U-shaped feedback waveguide length can be designed to be approximately 606.3 μm. After fine-tuning, if the phase residual is controlled within 0.05π, the equivalent phase matching condition can be considered satisfied.

[0040] Therefore, the U-shaped feedback waveguide in this invention is not simply designed according to an integer multiple of its geometric length, but requires compensation based on the effective refractive index change after electro-optic polymer coverage. In the basic design, the initial geometric dimensions can be determined by making the U-shaped waveguide length an integer multiple of the equivalent perimeter of the racetrack-shaped microring. Based on this, the length of the U-shaped waveguide is fine-tuned and compensated according to the actual change in the effective refractive index of the waveguide mode after electro-optic polymer coverage, so that the final structure satisfies the equivalent phase-matching condition. This design avoids disrupting the U-shaped feedback interference condition after the introduction of the electro-optic polymer, thus ensuring that the U-shaped feedback structure can still achieve spectral reconstruction and equivalent FSR extension. For a single U-shaped feedback racetrack microring resonator (UNMRR), its normalized transmission model... T :

[0041] In the formula, E 1, E 2 represents the optical field of the micro-ring Input port and Drop port, respectively. t 1 and t 2 are the self-coupling coefficients of the first coupling region a1 and the second coupling region a2, respectively. k 1 and k 2 are the mutual coupling coefficients of the first coupling region a1 and the second coupling region a2, respectively. and These are the propagation coefficients of the runway microring and the U-shaped waveguide, respectively. This is the amplitude loss coefficient of the waveguide, with units equal to the reciprocal of its length. It characterizes the degree of amplitude attenuation of the optical field along the propagation direction within the waveguide; for example... Figure 2 As shown, for a racetrack-shaped microring resonator, the coupling region between its Input port and Through port is defined as the first coupling region a1, and the coupling region between the Add port and Drop port is defined as the second coupling region a2. When the optical field propagates in a waveguide of length L, its field amplitude attenuation factor can be expressed as exp(-αL); This refers to the phase difference generated during half a cycle of optical signal transmission in the runway microring. Let be the phase difference of light propagating within the U-shaped waveguide. The radius of curvature of the runway-shaped micro-ring. The length of the straight waveguide section on one side of the runway-shaped microring. The length of the U-shaped waveguide between the upper and lower coupling points is denoted by α. In this invention, both the reference resonator and the sensing resonator are uniformly represented by α, which is a simplified representation when the losses of the reference resonator and the sensing resonator are similar. Considering that the sensing resonator is covered with an electro-optic polymer film, which may introduce additional absorption and interface scattering losses, under strict conditions, the loss coefficients of the reference resonator and the sensing resonator should be defined as α respectively. r and α s .

[0042] When the equivalent phase matching condition is met, the U-shaped feedback interference mechanism can effectively improve the free spectral range (FSR) of the output spectrum relative to the traditional Add-Drop microring, thus expanding the sensor's measurement range.

[0043] With the introduction of U-shaped feedback, the equivalent free spectral range of a single UNMRR unit can be expressed as: ,in, The free spectral range of traditional runway-shaped microrings. The FSR extension coefficient introduced for U-shaped feedback. Under ideal phase-matching conditions, This can approach 2, effectively extending the equivalent free spectral range compared to traditional Add-Drop microrings. With the introduction of U-shaped feedback, the equivalent free spectral range of a single UNMRR unit can be expressed as FSR. UN =η·FSR MRR FSR MRRHere, η represents the free spectral range of a traditional racetrack-shaped Add-Drop microring, and η is the FSR expansion coefficient introduced by the U-shaped feedback. For the single U-shaped feedback waveguide used in this invention, the U-shaped feedback light and the Drop port output light alternately engage in constructive and destructive phases between adjacent resonance orders, thereby primarily suppressing spectral lines separated by one resonance peak / valley, making the interval between identifiable resonance peaks / valleys approximately twice the original free spectral range. Therefore, the theoretical upper limit of η is 2. If the length of the U-shaped feedback waveguide satisfies L1≈2mπR or the equivalent length of an even-numbered half-cycle of the corresponding racetrack-shaped microring, η can approach 2, where m is a positive integer; if L1≈(2m-1)πR, the spectral suppression relationship does not form peak-spacing selection, and η is typically close to 1. Considering electro-optic polymer coverage, coupling region phase, propagation loss, dispersion, and process error, the typical value range of η is 1.6~2.0, preferably 1.8~2.0. In the U-shaped waveguide nested micro-ring simulation structure mentioned, when L1 is 2πR or 4πR, the FSR is doubled compared to the traditional Add-Drop micro-ring, while when L1 is πR or 3πR, the FSR does not expand much. Therefore, it is preferable to use the U-shaped feedback length corresponding to an even number of half-cycles.

[0044] Based on satisfying the U-shaped feedback phase matching condition, the equivalent free spectral range of reference resonator 2 is... Equivalent free spectral range of sensing resonator 6 A preset difference is set between the two, which is less than 10% of the smaller of the equivalent free spectral ranges of the reference resonator and the sensing resonator, preferably 0.02~0.20 nm, more preferably 0.03~0.08 nm, so as to form a vernier effect between their output spectra, thereby improving the electric field sensing sensitivity. T r , T s , T t Given the normalized models for the reference ring, sensing ring, and CUNMRR respectively, then:

[0045] During sensor operation, the electro-optic polymer in the sensing region converts changes in the electric field into changes in the effective refractive index, altering the resonant wavelength and resulting in a change in the output spectrum. By altering the radii of the two micro-rings, a slight difference in their FSR is created, causing a misalignment of the resonant wavelength and generating a vernier effect. This vernier effect enhances the sensor's sensitivity. The wavelength interval between the two single resonant peaks and valleys with the highest extinction ratios after the vernier effect is generated represents the equivalent free spectral range of this structure. FSR t express:

[0046] In the formula, FSRr and FSR s These are the equivalent free spectral ranges of the reference ring and the sensing ring, respectively.

[0047] The sensitivity amplification factor F can be obtained as follows:

[0048] In the formula and These represent the changes in the refractive index of the sensing area. The resonant wavelength offset at that time.

[0049] Therefore, the U-shaped cascaded waveguide nested microring resonator structure, through the vernier effect, can increase the sensing sensitivity by a factor of F compared to a single U-shaped waveguide nested microring structure. For example, the SOI top silicon waveguide has a cross-section of 220nm × 450nm, a center operating wavelength of 1550nm, and a reference resonator bending radius R. r Take 40μm, and set the bending radius R of the sensing resonant cavity. s The reference resonant cavity mutual coupling coefficient is set to 36.5 μm; 1r and k 2r A value of approximately 0.22 is preferred, corresponding to a self-coupling coefficient t. 1r and t 2r Approximately 0.9755; mutual coupling coefficient k of the sensing resonant cavity 1s and k 2s A value of approximately 0.51 is preferred, corresponding to a self-coupling coefficient t. 1s and t 2s Approximately 0.8602; α approximately 3~5 dB / cm, β approximately 9.54 μm -1 The U-shaped feedback waveguide can be initially designed according to the corresponding micro-ring perimeter or the equivalent perimeter of the racetrack-shaped micro-ring, and fine-tuned by combining the phase of the coupling region and the phase deviation caused by the electro-optic polymer coverage. In one layout example, the straight waveguide section length of the reference ring U-shaped waveguide is about 61.567 μm and the radius of curvature is about 40.805 μm, while the straight waveguide section length of the sensing ring U-shaped waveguide is about 56.326 μm and the radius of curvature is about 37.142 μm.

[0050] In this invention, instead of first forming a conventional dual-micro-ring vernier effect and then simply superimposing a U-shaped feedback structure, an UNMRR unit with equivalent FSR extension is first formed through U-shaped feedback, and then vernier amplification is generated by utilizing the FSR difference between the two UNMRR units. The design process requires satisfying the following two conditions: first, each UNMRR unit must satisfy the U-shaped feedback phase matching condition; second, the reference UNMRR unit and the sensing UNMRR unit must satisfy a preset FSR difference condition. In this invention, the reference resonant unit and the sensing resonant unit are not simply replicated structures, but are designed based on their racetrack-shaped micro-ring equivalent perimeter, electro-optic polymer coverage state, and U-shaped feedback phase condition, ensuring that each UNMRR unit satisfies feedback phase matching while maintaining a preset equivalent FSR difference between the two resonant units, thus achieving compatibility between U-shaped feedback FSR extension and vernier effect sensitivity amplification. Taking a racetrack-shaped micro-ring structure for both the reference and sensing resonant cavities as an example, both have a bending radius of 40 μm. The equivalent perimeter of the racetrack-shaped microring is determined by two curved waveguides and two straight waveguides. Therefore, the difference in equivalent perimeter between the two resonators can be controlled by adjusting the length of the straight waveguide section on one side. Specifically, the length of the straight waveguide section on one side of the reference resonator can be approximately 188.5 μm, corresponding to an equivalent perimeter of approximately 628.3 μm; the length of the straight waveguide section on one side of the sensing resonator can be approximately 177.5 μm, corresponding to an equivalent perimeter of approximately 606.3 μm. After U-shaped feedback waveguide expansion, the equivalent free spectral ranges of the reference UNMRR unit and the sensing UNMRR unit can be designed to be in the range of approximately 1.7–1.9 nm, and the preferred difference between their preset free spectral ranges is 0.03–0.08 nm. Through the above design, a vernier amplification factor of approximately 40–60 times can be obtained, and the vernier envelope free spectral range falls within the range of approximately 80–110 nm.

[0051] Through the vernier effect, the resonant wavelength drift of the sensing resonant cavity 6 caused by the external electric field is further amplified, thereby significantly improving the electric field sensing sensitivity. The vernier amplification factor can be expressed as... .

[0052] After the electro-optic polymer is applied to the sensing ring, it alters the mode distribution of the sensing ring waveguide, changing the degree of mode overlap between the sensing ring and the bus waveguide and U-shaped feedback waveguide, thus affecting the coupling coefficient. If the reference resonant unit and the sensing resonant unit still use the exact same coupling spacing, coupling length, and waveguide width, it may lead to a mismatch in their coupling states, thereby affecting the extinction ratio, spectral contrast, and peak-valley identification accuracy of the vernier spectral envelope. Therefore, this invention preferably employs an asymmetric coupling design between the reference resonant unit and the sensing resonant unit. Specifically, the reference resonant cavity 2 is coupled to the intermediate straight waveguide and the first U-shaped waveguide through two straight waveguides, respectively, and the sensing resonant cavity 6 is coupled to the intermediate straight waveguide and the second U-shaped waveguide through two other straight waveguides, respectively; the coupling coefficient between the reference resonant cavity 2 and its coupled two straight waveguides is asymmetric with the coupling coefficient between the sensing resonant cavity 6 and its coupled two straight waveguides. During optimization, the adjustment direction of the coupling coefficient can be determined according to the following principles: compensate for the mode distribution changes and additional losses introduced by the electro-optic polymer, so that the sensing resonant cavity operates in a non-critical, moderately coupled state; at the same time, ensure that the resonant peaks and valleys have sufficient contrast and a narrow resonant linewidth, and avoid valley splitting, spectral line broadening, or vernier envelope distortion caused by excessive coupling, thereby ensuring that the cascaded output spectrum has clear, stable, and identifiable vernier envelope characteristics. When the sensing ring is in a critically coupled state, the extinction ratio is usually too small.

[0053] Specific implementations of the asymmetric coupling design may include: using different coupling spacings for the reference resonator and the sensing resonator; adjusting the waveguide width locally in the coupling region of the sensing resonator; primarily covering the sensing waveguide region with an electro-optic polymer and setting a local avoidance region in the coupling region; or re-optimizing the coupling length or coupling gap of the sensing resonator based on the mode field distribution after electro-optic polymer coverage. The asymmetric coupling design can compensate for the mismatch in mode overlap and coupling state between the reference resonator and the sensing resonator after electro-optic polymer coverage, ensuring the extinction ratio and peak identification accuracy of the vernier spectrum envelope. In a preferred embodiment, different coupling spacings are used, with the electro-optic polymer locally avoiding a region. The coupling gap for the reference resonator is 355~365nm, the coupling gap for the sensing resonator is 190~200nm, and the electro-optic polymer reserves a 2~10μm avoidance distance outside the coupling region.

[0054] The electro-optic polymer film 5 covers at least the upper cladding of the region where the sensing resonant cavity 6 is located, preferably only the sensing waveguide region, and a local avoidance region is set in the coupling region to ensure the stability of the coupling state. Figure 2The electro-optic polymer sensitive layer is the electro-optic polymer film 5. The thickness of the electro-optic polymer film 5 is preferably 600-1000 nm, and it can be fabricated on the chip at low temperature using a spin-coating process, compatible with CMOS back-end processes. Under the action of an applied electric field, the electro-optic polymer film 5 undergoes a refractive index change, which couples to the waveguide guided mode through the evanescent field effect, thus changing the effective refractive index and causing a drift or spectral shape change in the output spectrum of the cascaded structure. This invention uses an electro-optic polymer material with a high electro-optic coefficient, utilizing the evanescent field effect for electro-optic energy coupling. Examples include the DR1 / PMMA system, the AJ-CKL1 / APC system, SEO series materials, or organic electro-optic polymers with equivalent performance. After polarization treatment, the typical electro-optic coefficient of the electro-optic polymer film is 30-150 pm / V, preferably 80-120 pm / V.

[0055] The emission grating 7 is connected to the Drop port of the sensing resonant cavity 6, and the spectral detection output interface includes an output single-mode fiber 11. The emission grating 7 couples the output light from the on-chip waveguide to the output single-mode fiber 11 at a set angle. The emission grating 7 is a uniform grating, and the set angle is the angle between the direction of the emitted beam from the emission grating 7 and the normal to the SOI chip surface, with a set angle of 0° to 12°, preferably 6° to 10°.

[0056] The working process of the electric field sensor of the present invention is as follows: The beam emitted from the SLD laser source 10 is collimated and focused by the lens group 9, then folded 90° by the prism 8 and coupled vertically into the SOI on-chip waveguide structure via the incident grating 1. The optical signal sequentially passes through the reference resonant cavity 2 and the sensing resonant cavity 6, and is finally coupled to the output single-mode fiber 11 via the exit grating 7. The reference resonant cavity 2 includes a first U-shaped waveguide, and the sensing resonant cavity 6 includes a second U-shaped waveguide and an electro-optic polymer thin film 5.

[0057] Under the influence of an applied electric field, the refractive index of the electro-optic polymer film 5 cladding the sensing resonant cavity 6 changes. This change in the effective refractive index of the sensing resonant cavity 6, through the evanescent field effect, causes a shift in the resonant wavelength of the sensing UNMRR unit. This shift is further amplified by the vernier effect between the reference UNMRR unit and the sensing UNMRR unit, resulting in a significant shift in the envelope of the output vernier spectrum. The output light is coupled to an off-chip spectral detection device via the emission grating 7 to measure the electric field under test.

[0058] The technical effect of this invention cannot be achieved by using U-shaped feedback structure, vernier effect or electro-optic polymer sensitive layer alone. Instead, it achieves parameter coupling and synergistic design by forming a combination of electro-optic polymer electric field response, U-shaped feedback FSR extension, dual resonant unit vernier amplification and heterogeneous integrated vertical coupling.

[0059] This invention not only utilizes a U-shaped feedback structure to extend the equivalent FSR of a single resonant unit, but also further utilizes the slight difference in equivalent FSR between two UNMRR units to form a vernier effect. The basic unit participating in the vernier effect in this invention is not a traditional racetrack micro-ring, but an equivalent resonant unit modulated by U-shaped feedback.

[0060] The following comparison of four different schemes illustrates the synergistic mechanism of this invention:

[0061] Wherein, the subscript RR represents the traditional Add-Drop runway microring, the subscript UN represents the U-shaped feedback runway microring unit, and the subscript V represents the equivalent free spectral range under the vernier effect; the subscripts r and s represent the reference resonator and the sensing resonator, respectively; the envelope drift sensitivity of scheme D can be expressed as... .

[0062] Compared to scheme B, scheme D not only utilizes the U-shaped feedback structure to extend the equivalent FSR of a single resonant unit, but also further utilizes the slight difference in equivalent FSR between two UNMRR units to form a vernier effect. Compared to scheme C, the basic unit participating in the vernier effect in this invention is not the traditional racetrack microring, but the equivalent resonant unit modulated by the U-shaped feedback. Therefore, this invention can simultaneously achieve measurement range extension and spectral drift amplification in the same on-chip sensitive structure. This technical effect stems from the synergistic design between U-shaped feedback FSR extension, electro-optic polymer electric field response, and dual UNMRR vernier amplification, rather than a simple parallel arrangement of functional modules. For example, under the conditions of a working wavelength of 1550 nm, a group refractive index of approximately 4.2, and a bending radius of 40 μm for both the reference and sensing resonant cavities, by adjusting the length of the straight waveguide section on one side of the racetrack-shaped microring, the free spectral ranges of the traditional reference microring and the traditional sensing microring can be approximately 0.88 nm and 0.90 nm, respectively, with a difference of approximately 0.02 nm. At this point, the free spectral range of the vernier envelope in the traditional dual-micro-ring structure is approximately 39.6 nm, and the sensitivity amplification factor is approximately 45. After introducing a U-shaped feedback waveguide, if the equivalent free spectral range is expanded by approximately two times, the equivalent free spectral ranges of the reference UNMRR unit and the sensing UNMRR unit are approximately 1.76 nm and 1.80 nm, respectively, with a difference of approximately 0.04 nm. At this point, the free spectral range of the vernier envelope in the dual UNMRR vernier structure is approximately 79.2 nm, and the sensitivity amplification factor remains approximately 45. Therefore, this invention can approximately double the free spectral range of the vernier envelope while essentially maintaining the vernier's sensitivity amplification capability, thereby simultaneously improving both the measurement range and the spectral drift amplification effect.

[0063] Example 2

[0064] This embodiment provides an electric field measurement system, including the heterogeneously integrated U-shaped cascaded waveguide nested microring resonant cavity type electric field sensor described in Embodiment 1, and a spectral detection device.

[0065] The spectral detection device is used to receive the output light of the electric field sensor and detect the drift or spectral shape change of the resonance spectrum in order to calculate the intensity of the electric field to be measured. The spectral detection device can be a spectrometer or other photoelectric detection equipment with spectral analysis function.

[0066] In actual measurements, the emitted light from the SLD laser source 10 is coupled to the optical sensing unit via the input coupling unit. Under the action of an applied electric field, the refractive index of the electro-optic polymer film 5 covering the sensing area changes, causing a change in the effective refractive index of the sensing UNMRR unit through the evanescent field effect, thereby causing a shift in the resonant wavelength. This shift is amplified by the vernier effect between the reference UNMRR unit and the sensing UNMRR unit, manifesting as a significant shift in the output vernier spectrum envelope. After the output light is coupled to the output single-mode fiber 11 via the emission grating 7, it is received and analyzed by the spectral detection device to determine the shift in the peak wavelength of the vernier spectrum envelope. Based on the correspondence between the shift and the electric field intensity established in advance through calibration experiments, the measured electric field intensity value is calculated.

[0067] The spectral detection device can demodulate the electric field in various ways, such as monitoring the peak wavelength drift, envelope shape changes, or specific resonance peak position changes of the vernier spectral envelope. Before actual measurement, the output spectrum is acquired under known electric field intensities E1, E2, ..., En. The peak wavelength of the vernier spectral envelope or the wavelength of the highest extinction ratio resonance valley λ1, λ2, ..., λn is extracted, and a calibration relationship of λ=aE+b or E=(λ-b) / a is obtained by fitting the data. λ represents the peak wavelength of the vernier spectral envelope in the output spectrum, or the wavelength of the highest extinction ratio resonance valley, which is a characteristic quantity extracted from the sensor output light by the spectral detection device; E represents the electric field intensity to be measured; a represents the slope of the fitted line, i.e., the sensor sensitivity; and b represents the intercept of the fitted line. When measuring an unknown electric field, the spectral detection device extracts the corresponding wavelength from the real-time output spectrum and substitutes it into the calibration relationship to obtain the electric field intensity to be measured. If temperature drift exists within the working range, it can be compensated by adding a reference spectrum or the drift of the reference resonant cavity spectral line under no-electric-field conditions.

[0068] Example 3

[0069] This embodiment provides a method for fabricating a heterogeneously integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor as described in Embodiment 1, including the following steps: Step S1: On an SOI substrate, an incident grating 1, a U-shaped cascaded waveguide structure 4, a nested micro-ring resonator 3, and an exit grating 7 are fabricated using CMOS-compatible micro / nano fabrication processes. The nested micro-ring resonator 3 includes a reference resonator 2 and a sensing resonator 6. The top silicon waveguide layer of the SOI substrate is typically 300 nm thick, the silicon dioxide buried oxide layer is typically 3 μm thick, and the silicon substrate is typically approximately 500 μm thick. Utilizing mature microelectronic CMOS fabrication processes makes this electric field sensor easy to mass-produce, which helps reduce costs.

[0070] The incident grating 1 adopts a binary blazed grating structure, which is achieved by introducing multiple sub-grating segments within a single grating period and non-uniformly modulating the duty cycle of each sub-grating segment. The U-shaped cascaded waveguide structure 4 includes a first U-shaped waveguide, a second U-shaped waveguide, and an intermediate straight waveguide. The nested micro-ring resonator 3 includes a reference resonator 2 and a sensing resonator 6, each of which adopts a racetrack-shaped micro-ring resonator structure. During fabrication, the reference resonator and the sensing resonator adopt an asymmetric coupling design, which can be achieved by using different coupling spacings, locally adjusting the waveguide width in the coupling region of the sensing resonator, or setting a local avoidance area in the coupling region.

[0071] Step S2: Prepare an electro-optic polymer film 5 in at least the region where the sensing resonant cavity 6 is located within the nested micro-ring resonant cavity 3. The thickness of the electro-optic polymer film 5 is preferably 600-1000 nm, and it can be prepared on the chip at low temperature using a spin-coating process, compatible with CMOS back-end processes. Depending on actual needs, photolithography or etching processes can be used to pattern the coverage area of ​​the electro-optic polymer film, ensuring that the electro-optic polymer mainly covers the sensing waveguide region, with local avoidance areas set in the coupling region to maintain the stability of the coupling state. The electro-optic polymer can be selected from DR1 / PMMA system, AJ-CKL1 / APC system, SEO series materials, or organic electro-optic polymers with equivalent performance. Specifically, the electro-optic polymer film is formed through spin coating, soft baking, patterning, and electric field polarization processes. The spin coating speed is 1000~4000 r / min and the spin coating time is 30~90 s. The soft baking temperature is 80~120℃, the polarization electric field is 50~150 V / μm, and the polarization temperature is 100~160℃. After polarization, the electric field is maintained to cool down and solidify the orientation of the chromophores.

[0072] Step S3: The SLD laser source 10, lens group 9, and prism 8 are assembled on top of the SOI substrate using a heterogeneous integration method. The beam emitted from the SLD laser source 10 is collimated and focused by the lens group 9, and then refracted by the prism 8 before being incident vertically downwards onto the incident grating 1. Precision mechanical positioning and alignment processes ensure the optical path alignment accuracy between the SLD laser source, lens group, prism, and incident grating, guaranteeing input coupling efficiency and stability. Specifically, the SLD laser source 10, lens group 9, and prism 8 are heterogeneously integrated on top of the SOI substrate using positioning grooves, active optical alignment, and low-temperature bonding processes. The bonding material for the lens group 9 and prism 8 is UV-curable optical adhesive, epoxy adhesive, or benzocyclobutene adhesive, with a curing or bonding temperature of 25~120℃.

[0073] The above-mentioned fabrication method is based on the silicon-on-insulator platform and utilizes mature microelectronic CMOS processing technology, which makes the electric field sensor easy to mass-produce and helps to reduce costs.

[0074] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A heterogeneously integrated U-shaped cascaded waveguide nested microring resonant cavity type electric field sensor, characterized in that, include: From left to right: the input coupling unit, the optical sensing unit, and the spectral detection output interface integrated on the SOI chip; The input coupling unit includes an SLD laser source, a lens group, and a prism. The beam emitted from the SLD laser source is collimated and focused by the lens group, and then the prism changes the optical path so that it is coupled to the optical sensing unit in a direction perpendicular or approximately perpendicular to the SOI chip surface. The optical sensing unit includes an incident grating, a U-shaped cascaded waveguide structure, a nested microring resonant cavity, an electro-optic polymer film, and an exit grating. The incident grating receives a beam of light coupled vertically downwards through the prism and couples it to the U-shaped cascaded waveguide structure. The nested microring resonant cavity includes a reference resonant cavity and a sensing resonant cavity, each of which is a racetrack-shaped microring resonant cavity. The U-shaped cascaded waveguide structure includes a first U-shaped waveguide, a second U-shaped waveguide, and an intermediate straight waveguide. The first U-shaped waveguide transmits the Thr wave from the reference resonant cavity to the exit grating. The through port is connected to the add port, and the second U-shaped waveguide connects the through port of the sensing resonant cavity to the add port. The drop port of the reference resonant cavity and the input port of the sensing resonant cavity are cascaded through an intermediate straight waveguide. The electro-optic polymer film covers at least the upper cladding of the region where the sensing resonant cavity is located. The incident grating is connected to the input port of the reference resonant cavity, and the emitted grating is connected to the drop port of the sensing resonant cavity, for coupling the output light to the spectral detection output interface.

2. The heterogeneously integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor as described in claim 1, characterized in that: The lens group is used to collimate and focus the beam emitted from the SLD laser source, so that the spot size, divergence angle and energy distribution at the incident prism and the incident grating are adapted. The lens group includes a first collimating lens and a second focusing lens arranged sequentially along the emitted light path of the SLD laser source. The first collimating lens is used to collimate the diverging beam emitted from the SLD laser source, and the second focusing lens is used to focus and shape the collimated beam onto the prism and the incident grating.

3. The heterogeneously integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor as described in claim 2, characterized in that: The SLD laser source has a center wavelength of 1550nm, a horizontal divergence angle of 20°~30° and a vertical divergence angle of 25°~35°, and a distance of 0.10~0.30mm between the first collimating lens and the SLD laser source; a distance of 0.30~0.60mm between the second focusing lens and the prism; and a target spot diameter of 6~12μm at the incident grating.

4. The heterogeneously integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor as described in claim 1, characterized in that: The first U-shaped waveguide and the reference resonant cavity, and the second U-shaped waveguide and the sensing resonant cavity, respectively satisfy the equivalent phase matching condition, so that the first U-shaped waveguide, the reference resonant cavity, the second U-shaped waveguide and the sensing resonant cavity together constitute a transmission path that extends the free spectral range.

5. The heterogeneously integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor as described in claim 4, characterized in that: There is a preset difference between the equivalent free spectral range of the reference resonant cavity and the equivalent free spectral range of the sensing resonant cavity. The preset difference is less than 10% of the smaller value of the equivalent free spectral range of the reference resonant cavity and the sensing resonant cavity, so as to form a vernier effect.

6. The heterogeneously integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor as described in claim 4, characterized in that: The reference resonant cavity is coupled to the intermediate straight waveguide and the first U-shaped waveguide through two straight waveguides, respectively. The sensing resonant cavity is coupled to the intermediate straight waveguide and the second U-shaped waveguide through two other straight waveguides, respectively. The coupling coefficient between the reference resonant cavity and the two straight waveguides it is coupled to is asymmetrically designed with respect to the coupling coefficient between the sensing resonant cavity and the two straight waveguides it is coupled to.

7. The heterogeneously integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor as described in claim 1, characterized in that: The incident grating is a binary blazed grating, which consists of a periodic structure composed of multiple sub-grating segments. Each sub-grating segment has a different duty cycle to approximate the blazed phase distribution.

8. The heterogeneously integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor as described in claim 1, characterized in that: The SLD laser source, the lens group, and the prism are disposed above the SOI chip via heterogeneous integration.

9. The heterogeneously integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor as described in claim 1, characterized in that: The thickness of the electro-optic polymer film is 600~1000nm.

10. The heterogeneously integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor as described in claim 1, characterized in that: The sensor is composed of a silicon substrate, a silicon dioxide buried oxide layer, and a top silicon waveguide layer from bottom to top; the top silicon waveguide layer includes the waveguide structure of the U-shaped cascaded waveguide structure and the nested micro-ring resonant cavity.

11. The heterogeneously integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor as described in claim 1, characterized in that: The spectral detection output interface includes an output single-mode fiber, and the emission grating couples the output light to the output single-mode fiber at a set angle.

12. An electric field measurement system, characterized in that, include: The heterogeneous integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor according to any one of claims 1 to 11; A spectral detection device is used to receive the output light of the electric field sensor and detect the drift or spectral shape change of the resonance spectrum in order to calculate the electric field strength to be measured.

13. A method for fabricating a heterogeneously integrated U-shaped cascaded waveguide nested microring resonator type electric field sensor as described in any one of claims 1 to 11, characterized in that, include: An incident grating, a U-shaped cascaded waveguide structure, a nested micro-ring resonant cavity, and an exit grating are fabricated on an SOI substrate. An electro-optic polymer film is prepared in at least the region where the sensing resonant cavity is located within the nested micro-ring resonant cavity; The SLD laser source, lens group, and prism are heterogeneously integrated on the SOI substrate, so that the beam emitted from the SLD laser source is collimated and focused by the lens group, and after being refracted by the prism, it is incident vertically downward onto the incident grating.

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