FP (Fabry-Perot) cavity sensor without free spectral range based on staggered multimode Bragg grating

Abstract

The invention discloses an FP (Fabry-Perot) cavity sensor without a free spectral range based on a staggered multimode Bragg grating. The sensor is combined with a multimode sub-wavelength grating (SWG) cavity through a pair of multimode Bragg gratings (MBG) with staggered reflection bandwidths, free spectrum range limitation of a traditional FP cavity is eliminated, a mode multiplexer is used for separating a reflection mode, and wide spectrum sensing of a single harmonic peak is achieved. Wherein the sensitivity of the SWG chamber is remarkably improved by enhancing the interaction area of a light field and a substance; the working bandwidth of the MBG designed in a staggered mode is expanded to be two times of the Bragg bandwidth, the range of 88 nm is covered, and only a single harmonic peak is stored. Experiments show that the temperature sensitivity of the sensor reaches 91.17 pm / DEG C, and the refractive index sensitivity is 305.83 nm / RIU. In addition, cascade connection of multiple sensing units is supported through the mode multiplexer, and high expansibility is achieved. The sensor is compact in structure, is suitable for multi-parameter sensing scenes such as environment monitoring and biomedical diagnosis, is compatible with various material platforms, and has the advantages of simplicity in processing, quick response and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of integrated photon sensors, and particularly to a free spectral range (FSR)-free Fabry-Perot (FP) cavity sensor based on multimode Bragg gratings (MBGs) and subwavelength gratings (SWGs) and a manufacturing method thereof, which is applicable to multi-parameter sensing scenarios such as environmental monitoring, biomedical diagnosis, and chemical analysis. Background Art

[0002] The spectral bandwidth and the number of channels of traditional FP cavity sensors are severely restricted due to the limitation of the free spectral range (FSR). For example, although the whispering gallery cavities based on micro-rings or micro-disks have high sensitivity, their FSR limitations result in a limited number of channels and poor system scalability. In the prior art, the FSR is extended by cascading micro-rings using the Vernier effect, but there are problems such as difficult wavelength alignment and complex coupling coefficient control. Arrayed waveguide gratings (AWGs) support multiple channels, but they are bulky and lack independent control capabilities. Although multimode FP cavities can separate the reflected light modes, the number of their channels is still limited by the reflection bandwidth of the Bragg grating. In addition, the light-matter interaction of traditional FP cavities is weak, and the refractive index sensitivity is generally lower than 200 nm / RIU. Therefore, there is an urgent need for a new sensor design that can achieve high sensitivity, broadband width, and multi-channel integration while eliminating the FSR limitation. Summary of the Invention

[0003] Traditional FP cavity sensors are limited by the free spectral range. The present invention proposes an FSR-free multimode FP cavity sensor, which eliminates the FSR limitation and achieves high sensitivity by using misaligned multimode Bragg gratings and SWG cavities, aiming to overcome the limitations and defects of the prior art and improve the sensitivity and detection performance of optical waveguide sensors.

[0004] To achieve the above object, the technical solution of the present invention is as follows:

[0005] An FSR-free FP cavity sensor based on misaligned multimode Bragg gratings includes a pair of multimode Bragg gratings (MBGs) with a certain misalignment in the working bandwidth, a multimode subwavelength grating waveguide (MSWG) cavity, and a mode (de)multiplexer (mode MUX). A single resonance peak is formed by the misalignment of the reflection bandwidth, the FSR limitation is eliminated, and the working bandwidth can be extended to twice the Bragg bandwidth.

[0006] Furthermore, the multimode Bragg grating reduces mode mismatch and transmission loss through adiabatic evolution design. The light field is incident in the TE0 mode. When the Bragg condition n eff0 +n eff1 =λ bragg / Λ bragg is satisfied near the wavelength, the light will be converted into the TE1 mode and reflected and propagated along the incident direction.

[0007] Furthermore, the multimode subwavelength grating waveguide chamber adopts a 3-core SWG waveguide to achieve low-loss transmission of the TE0 and TE1 modes (transmission loss < 1 dB), enhancing the interaction between light and matter.

[0008] Furthermore, the mode (de)multiplexer is based on an adiabatic dual-core tapered structure, which satisfies the mode matching condition during the propagation of light waves, enabling the reflected TE1 mode to be downloaded through the mode multiplexer and converted into the TE0 mode, thus separating the reflected TE1 mode and the incident TE0 mode.

[0009] Optionally, multiple other sensing units can be cascaded at the backend of the mode (de)multiplexer to improve the scalability of the system, thereby realizing the cascaded expansion of multiple unit devices to form a sensing array.

[0010] The technical advantages of the present invention are as follows:

[0011] High sensitivity: Since the SWG is used to replace the traditional straight waveguide cavity, the interaction area between the light field and matter is increased, and a higher refractive index sensitivity is obtained compared to the traditional FP cavity. In a specific implementation case, the temperature sensitivity is measured to be 91.17 pm / °C and the refractive index sensitivity is 305.83 nm / RIU through experiments.

[0012] Wide-spectrum single-peak spectral characteristics: Through the misalignment of the MBG reflection bandwidth, only the resonance in the overlapping region is retained, eliminating the periodic resonance limitation of the traditional FP cavity. In a specific implementation case, the working bandwidth reaches 88 nm (the FSR of the traditional FP cavity is only 15 nm at the same cavity length).

[0013] Cascadable scalability: By separating the reflected light through the mode MUX, multiple sensing units can be cascaded on a single optical link, suitable for distributed sensing networks.

[0014] Generally speaking, the present invention can be processed and manufactured on various material platforms (such as silicon-on-insulator, silicon nitride, polymer platform), and is easy to process, with a simple structure, small volume, large extinction ratio, high sensing sensitivity, and fast response speed. Description of the Drawings

[0015] Figure 1 : Schematic diagram of the overall structure of the sensor.

[0016] In the figure, a1 and a2 are a pair of multimode Bragg gratings with different periods, b is a sub-wavelength grating chamber, and c is a pair of mode (de)multiplexers.

[0017] Figure 2 : Schematic diagram of an FSR-FREE multimode FP cavity sensor enhanced by an SWG chamber.

[0018] Subfigure (a) overall device schematic diagram, (b) multimode Bragg grating schematic diagram, (c) multimode SWG chamber structure schematic diagram, (d) mode MUX structure schematic diagram, (e) misaligned FP cavity schematic diagram, (f) cross-sectional schematic diagram of the waveguide structure

[0019] Figure 3 : Low-loss transmission spectra of the sub-wavelength grating for TE0 and TE1 modes.

[0020] Figure 4 : FSR-free FP cavity spectrum.

[0021] Figure 5 : Experimental results of temperature sensitivity and refractive index sensitivity.

[0022] Subfigure (a) temperature sensitivity calibration and (b) linear fitting of temperature sensitivity, (c) refractive index sensitivity calibration and (d) linear fitting of refractive index sensitivity. Specific implementation manner

[0023] The technical solution of the present invention will be further described below in conjunction with the accompanying drawings and examples.

[0024] The present invention relates to the technical field of integrated photon sensors, and discloses a free spectral range (FSR-free) Fabry-Perot (FP) cavity sensor based on misaligned multimode Bragg gratings. The sensor combines a pair of multimode Bragg gratings (MBGs) with misaligned reflection bandwidths with a multimode sub-wavelength grating (SWG) chamber to eliminate the free spectral range limitation of the traditional FP cavity, and uses a mode multiplexer to separate the reflected modes to achieve wide-spectrum sensing of a single resonance peak. Among them, the SWG chamber significantly improves the sensitivity by enhancing the interaction area between the optical field and the substance; the misaligned design of the MBG expands the working bandwidth to twice the Bragg bandwidth, covering a range of 88 nm with only a single resonance peak. Experiments show that the temperature sensitivity of the sensor reaches 91.17 pm / °C, and the refractive index sensitivity is 305.83 nm / RIU. In addition, the mode multiplexer supports the cascading of multiple sensing units, with high scalability. The structure of the present invention is compact, suitable for multi-parameter sensing scenarios such as environmental monitoring and biomedical diagnosis, compatible with various material platforms (such as silicon-based and silicon nitride), and has the advantages of simple processing and fast response.

[0025] As Figure 1As shown in the figure, the free-spectral-range-free FP cavity sensor based on a misaligned multimode Bragg grating of the present invention includes a1 / a2 - a pair of multimode Bragg grating groups with different periods, b - a subwavelength grating cavity, and c - a pair of mode (de)multiplexer groups. One end of the multimode Bragg grating a1 is connected to a mode (de)multiplexer, the other end of the multimode Bragg grating a1 is connected to one end of the subwavelength grating cavity b, the other end of the subwavelength grating cavity b is connected to one end of the multimode Bragg grating a2, and the other end of the multimode Bragg grating a2 is connected to another mode (de)multiplexer.

[0026] Further, as Figure 2 shown in the subfigure (a), the optical field is incident from the INPUT port in the TE0 mode. When the Bragg condition n eff0 +n eff1 =λ bragg / Λ bragg is satisfied, the wavelength near it will be converted into the TE1 mode and reflected and propagated along the incident direction and downloaded and converted via the mode MUX and output from the DROP port. Where n eff0 and n eff0 represent the effective refractive indices of the TE0 and TE1 modes respectively, λ bragg is the Bragg wavelength, and Λ bragg is the grating period. The wavelength that satisfies the FP cavity resonance condition will be output from the THROUGH port through the entire device.

[0027] Further, as Figure 2 shown in the subfigure (b), both sides of a single multimode Bragg grating have a linear tooth depth change with N tp periods to reduce the transmission loss caused by mode matching.

[0028] Further, as Figure 2 shown in the subfigure (e), Figure 1 a1 and a2 in it should have different Bragg periods Λ bragg1 and Λ bragg2 , and the Bragg bandwidths overlap. For example, the working bandwidth of a1 is 1520 nm - 1550 nm, and the working bandwidth of a2 is 1545 nm - 1575 nm. An FP resonance can be formed in the range of 1545 nm - 1550 nm. And because the overlapping bandwidth is smaller than the FSR of a normal FP cavity (for example, Figure 4 the FP cavity formed by a pair of identical Bragg gratings shown in the subfigure (a) has an FSR of 15 nm), there will be only one resonance peak in the entire working bandwidth, and the remaining resonance peaks will not be excited, thus eliminating the FSR of the FP cavity.

[0029] Further, as Figure 2As shown in subfigure (c), a sub-wavelength grating cavity b is used to replace the traditional straight waveguide cavity, and by increasing the interaction contact area between light and matter, the refractive index sensitivity is improved.

[0030] Furthermore, as Figure 2 shown in subfigure (d), a pair of c-mode (de)multiplexers are placed at both ends, enabling the reflected TE1 mode to be downloaded through the mode multiplexer and converted into the TE0 mode. The mode (de)multiplexer is based on an adiabatic dual-core tapered structure, which satisfies the mode matching condition during the propagation of light waves, allowing the reflected TE1 mode to be downloaded through the mode multiplexer and converted into the TE0 mode, thus separating the reflected TE1 mode from the incident TE0 mode.

[0031] Optionally, other sensing units can be cascaded at the drop end after the mode (de)multiplexer to enhance the system scalability, thereby realizing the cascaded expansion of multiple unit devices to form a sensing array.

[0032] Specific examples of the present invention are as follows:

[0033] In terms of manufacturing, as Figure 2 shown in subfigure (f), a 220nm SOI wafer on a traditional 2um SiO2 insulating substrate is selected as the preparation material for the Fano micro-ring sensor: its core layer is silicon material with a thickness of 220nm and a refractive index of 3.4744; its substrate material is SiO2 with a thickness of 2μm and a refractive index of 1.4404. The device pattern is defined by electron beam lithography (EBL). The structure preparation is completed in a single step by ICP dry etching.

[0034] As Figure 2 shown in subfigure (b), the basic parameters of the MBG are: grating width W = 1.1um, grating depth δ = 290nm, number of periods N = 40, and number of periods N tp of the linear tapered evolution = 20.

[0035] As Figure 2 shown in subfigure (e), the two-sided MBGs have different periods Λ bragg1 = 352nm, Λ bragg2 = 365nm.

[0036] As shown in subfigure Figure 2 (c), the specific parameters of the SWG cavity are N SWG = 100, W tip = 0.1μm, Λ SWG = 0.17μm. The length L = 17μm (L = NSWG × ΛSWG). As Figure 3 shown, it demonstrates the low-loss and high-efficiency transmission of the TE0 and TE1 modes in the multimode SWG cavity under these design parameters.

[0037] As shown in the figure of neutrons Figure 2 As shown in (d), the design parameters of the mode MUX are as follows: the input / output widths of the waveguides on the left and right sides are (wa1, wa2) = (0.5, 0.74) μm and (wb1, wb2) = (0.28, 0.18) μm respectively, the taper lengths (L1, L2, L3) = (20, 40, 20) μm, and the gap widths (wg1, wg2, wg3) = (1.2, 0.2, 0.5) μm.

[0038] The specific implementation cases for the fabricated sensor device to detect refractive index and ambient temperature are as follows:

[0039] Figure 4 The actual spectral results of this example are shown, and the spectrum and peak position drift are monitored by an ASE light source and a spectrometer. Figure 4 (a) is a general FP spectral diagram with the same MBG period of 356 nm on both sides and the same cavity length as the FSR-free FP cavity. As Figure 4 shown in (a): The FSR of the general FP cavity is only 15 nm within a working bandwidth of 45 nm. Figure 4 (b) is the FSR-free FP cavity with the structure described in the specific example. As Figure 4 shown in (b): The FSR-free FP cavity has only a single resonance peak within an 88-nm bandwidth, with a 3-dB bandwidth of 0.46 nm and a quality factor Q ≈ 1700.

[0040] Figure 5 The detection effects of temperature sensitivity and refractive index sensitivity are shown. During the calibration process of temperature sensitivity, a thermoelectric controller (TEC, accuracy 0.01 °C) is used to regulate the chip temperature, and the resonance peak drift is recorded. Figure 5 Subfigure a of the neutrons shows the resonance peak spectra at different temperatures. Figure 5 Subfigure b of the neutrons shows the linear fitting result of temperature sensitivity. The goodness of linear fitting R2 > 0.99, and the temperature sensitivity reaches 91.17 pm / °C.

[0041] Dipping the device into NaCl solutions with different concentrations (0–8%) will cause the resonance peak to drift, and thus the refractive index sensitivity of the device is calibrated. Figure 5 Subfigure c of the neutrons shows the resonance peak spectra under the background of NaCl solutions with different concentrations. Figure 5 Subfigure d of the neutrons shows the linear fitting result of refractive index sensitivity. The goodness of linear fitting R2 > 0.99, and the refractive index sensitivity reaches 305.83 nm / RIU.

[0042] Compared with a general FP cavity formed by the same MBG fabricated on a 220nm silicon-on-insulator (220nm SOI) wafer, the temperature sensitivity of a general multi-mode Bragg FP cavity is typically about 70pm / °C, and the refractive index sensitivity is typically about 80nm / RIU. This shows that the FSR-free FP cavity breaks the traditional performance limitations in terms of FSR, working bandwidth, temperature sensitivity, and refractive index sensitivity.

Claims

1. A non-free spectral range FP cavity sensor based on a dislocated multimode Bragg grating, characterized in that: The sensor comprises a pair of multimode Bragg grating groups with different periods, a subwavelength grating cavity and a pair of mode multiplexer groups; wherein one end of the multimode Bragg grating (a1) is connected to a mode multiplexer, the other end of the multimode Bragg grating (a1) is connected to one end of the subwavelength grating cavity (b), the other end of the subwavelength grating cavity (b) is connected to one end of the multimode Bragg grating (a2), and the other end of the multimode Bragg grating (a2) is connected to another mode multiplexer; a single resonance peak is formed by reflection bandwidth misalignment, the FSR limitation is eliminated, and the working bandwidth can be extended to twice the Bragg bandwidth.

2. The non-free spectral range FP cavity sensor based on the offset multi-mode Bragg grating according to claim 1, characterized in that: The light field is incident from the INPUT port in TE0 mode. When the Bragg condition n is met, eff0 +n eff1 =λ bragg / Λ bragg The wavelength within the specified range will be converted into TE1 mode and reflected and propagated along the incident direction and then transmitted from the DROP end after being downloaded and converted by the mode MUX; where n eff0 and n eff0 denote the equivalent refractive index of TE0 and TE1 modes, λ bragg is the Bragg wavelength, Λ bragg is the grating period; the wavelength that meets the FP cavity resonance condition will pass through the entire device and be emitted from the THROUGH port.

3. The non-free spectral range FP cavity sensor based on the offset multi-mode Bragg grating according to claim 1, characterized in that: A single multimode Bragg grating has N on both sides. tp Linear tooth depth variation over a period of one cycle.

4. The non-free spectral range FP cavity sensor based on the offset multi-mode Bragg grating according to claim 1 or 2, characterized in that: The multimode Bragg grating groups a1 and a2 should have different Bragg periods Λ bragg1 and Λ bragg2 , and the Bragg bandwidths overlap; FP resonance can be formed within the bandwidth overlap range, and because the overlapping bandwidth is smaller than the FSR of the normal FP cavity, there will be only one resonance peak in the entire working bandwidth, and the remaining resonance peaks will not be excited, thereby eliminating the FSR of the FP cavity.

5. The non-free spectral range FP cavity sensor based on the offset multi-mode Bragg grating according to claim 1, characterized in that: The multimode sub-wavelength grating waveguide cavity adopts 3-core SWG waveguide to achieve low-loss transmission of TE0 and TE1 modes.

6. The non-free spectral range FP cavity sensor based on the offset multi-mode Bragg grating according to claim 1, characterized in that: A pair of mode multiplexers are placed at both ends so that the reflected TE1 mode can be downloaded through the mode multiplexer and converted into the TE0 mode; the mode multiplexer is based on an adiabatic dual-core conical structure, which meets the mode matching conditions during the propagation of the light wave, so that the reflected TE1 mode can be downloaded through the mode multiplexer and converted into the TE0 mode, thereby separating the reflected TE1 mode from the incident TE0 mode.

7. The non-free spectral range FP cavity sensor based on the offset multi-mode Bragg grating according to claim 1, characterized in that: Other multiple sensor units are cascaded at the back end of the mode multiplexer to improve the scalability of the system, thereby realizing the cascade expansion of multiple unit devices to form a sensor array.

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