On-chip temperature-drift-free microring resonator optical sensor based on composite slot waveguide

Abstract

The present invention discloses an on-chip temperature drift-free micro-ring resonant cavity optical sensor based on a composite material slit waveguide. The micro-ring resonant cavity comprises a composite material slit waveguide-based micro-ring resonant cavity and a straight waveguide mutually coupled with the micro-ring resonant cavity. The micro-ring resonant cavity is divided into a first annular waveguide composed of a positive thermo-optic coefficient material and a second annular waveguide composed of a negative thermo-optic coefficient material; the first annular waveguide and the second annular waveguide are arranged concentrically, and there is an annular slit between them. The present invention makes full use of the advantages of high sensitivity of the optical sensor based on the slit waveguide, and solves the disadvantage of high temperature drift of the traditional on-chip annular resonant cavity based on the temperature drift-free composite material structure, that is, there is no temperature drift and it is only sensitive to the covered analyte. At the same time, it has the characteristics of simple processing and compact structure, and can meet the actual needs of on-chip optical sensors.

Description

Technical Field

[0001] The invention belongs to the field of sensors and relates to an optical sensor, in particular to an on-chip temperature-drift-free micro-ring resonant cavity optical sensor based on a composite material slit waveguide. Background Art

[0002] The rapid development of 5G communication and Internet of Things technology has put forward higher and higher requirements for information perception technology. In order to meet the requirements of small size, low energy consumption and high performance of sensors, on-chip integrated photonic sensors (On-Chip Integrated Photonic Sensor) have attracted more and more attention. In particular, silicon-based optical waveguide sensors that are compatible with mature complementary metal-oxide-semiconductor (CMOS) processes have attracted much attention due to their potential for large-scale production and thus greatly reducing costs. The current mainstream silicon-based optical waveguide sensors can be divided into strip waveguide type, surface plasmon waveguide type, slit waveguide type and ridge waveguide type in terms of waveguide structure; they can be divided into Fabry–Pérot Cavity type, Mach-Zehnder Interferometer (MZI) type, Microring Resonator (MRR) type and Photonic Crystal (PhC) type in terms of sensor structure. Among them, microring resonator has become one of the mainstream structures of integrated optical sensors on silicon substrates due to its simple structure, rich functions and easy expansion. At the same time, slit waveguide is favored by researchers of integrated optical sensors on chip because it can strongly localize the light field in the slit, thereby greatly improving the interaction with the substance to be measured.

[0003] However, regardless of the type of waveguide structure based on the microring resonator sensor - slot waveguide based is no exception - its main constituent materials are mostly silicon or silicon nitride. The thermo-optic coefficient (TOC) of the former is about 1.8×10 -4 / ℃, while the latter is about 2×10 -5 / ℃. This will make the microring resonator greatly affected by temperature. For example, the temperature drift of the microring resonator based on silicon waveguide is about 80pm / ℃, while that based on silicon nitride is about 20pm / ℃. The interference caused by such a large temperature drift makes the practical application of on-chip optical sensors face great challenges, especially when analyzing trace changes in substances.

[0004] There are currently three main methods to solve the temperature drift of the microring resonant cavity. One is to use an additional negative feedback temperature control system to ensure that the chip is kept at a constant temperature. Obviously, this method increases the complexity of the sensing system and will also greatly increase the energy consumption of the system. The second method is to cover the silicon or silicon nitride waveguide with a negative thermo-optic coefficient material to compensate for the influence of temperature on the light field. Obviously, this method is not suitable for sensors because the light field cannot pass through the cladding layer to contact the object to be measured. The third method is to use a sensor array, that is, a part of the sensor does not contact the object to be measured to specifically characterize the temperature change, thereby removing the influence of temperature changes during back-end data processing. This method will increase the size of the entire sensor chip, and the temperature change obtained is not measured in situ, which will introduce errors. All of the above methods are difficult to meet the needs of practical applications. Summary of the invention

[0005] In order to solve the problems existing in the background technology, the purpose of the present invention is to propose a composite material based slot waveguide, which uses the combined effect of positive and negative thermo-optic coefficient materials separated on both sides of the slot to offset the regulation of the light field phase by temperature changes. Therefore, the micro-ring resonant cavity formed by the slot waveguide can maintain a high sensitivity response to the object under test while being unaffected by temperature changes, thereby realizing a high-sensitivity on-chip optical sensor without temperature drift.

[0006] The technical solution adopted by the present invention is as follows:

[0007] The invention comprises a micro-ring resonant cavity based on a composite material slot waveguide and a straight waveguide mutually coupled with the micro-ring resonant cavity. The micro-ring resonant cavity is divided into a first annular waveguide composed of a positive thermo-optic coefficient material and a second annular waveguide composed of a negative thermo-optic coefficient material.

[0008] Normally, temperature changes will change the light wave mode in the microring resonant cavity, and thus change the resonant wavelength. The resonant wavelength of the microring resonant cavity composed of two circles of materials with different thermo-optic coefficients provided by the present invention does not change with temperature changes, is not affected by temperature, and maintains the same resonant characteristics at different temperatures.

[0009] The first annular waveguide and the second annular waveguide are arranged concentrically with an annular slit therebetween.

[0010] The straight waveguide is made of the same material as the first ring waveguide's positive thermo-optic coefficient material.

[0011] The micro-ring resonant cavity and the straight waveguide are both located on a silicon chip with an optical isolation layer.

[0012] In the present invention, since the energy of the light mode in the slit waveguide is distributed in both the positive thermo-optic coefficient material and the negative thermo-optic coefficient material, its total phase does not change with temperature, that is, the resonant wavelength of the microring resonant cavity it constitutes has no temperature drift and is only sensitive to the covered analyte.

[0013] The beneficial effects of the present invention are:

[0014] (1) The present invention uses a composite of materials having positive and negative thermo-optic coefficients to form a slot waveguide, so that the optical mode phase in the slot waveguide does not change with temperature.

[0015] (2) The microring resonant cavity sensor based on the slot waveguide can not only maintain the high sensitivity advantage of the slot waveguide, but also avoid the interference of temperature drift.

[0016] (3) The proposed drift-free sensor is based on on-chip photonic integration technology and is easy to fabricate and expand.

[0017] The on-chip temperature-drift-free micro-ring resonant cavity optical sensor based on a composite material slit waveguide of the present invention fully utilizes the advantage of high sensitivity of the optical sensor based on the slit waveguide, and solves the disadvantage of high temperature drift of the traditional on-chip ring resonant cavity based on the temperature-drift-free composite material structure. At the same time, it has the characteristics of simple processing and compact structure, which can not only meet the actual needs of on-chip optical sensors, but also has the ability to expand to other fields (such as optical communications). BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic structural diagram of an on-chip temperature-drift-free micro-ring resonant cavity optical sensor based on a composite material slot waveguide according to the present invention;

[0019] Figure 2 is a cross-sectional view of a composite material-based slot waveguide;

[0020] Figure 3 The simulation result of the micro-ring resonator wavelength drifting with temperature in the embodiment is shown in FIG. 1 , which is specifically based on silicon-rich nitride (SRN) as the positive thermo-optic coefficient material and titanium dioxide (TiO 2 ) The change of the resonant wavelength of the micro-ring resonator with temperature as an example of a slit waveguide composed of a negative thermo-optic coefficient material;

[0021] In the figure: the circle represents the resonant wavelength of the microring resonator at a certain temperature.

[0022] Figure 4The simulation result of the micro-ring resonator of the embodiment is that the resonance wavelength of the micro-ring resonator drifts with the change of the concentration of the sodium chloride solution in the upper cladding layer, specifically, silicon-rich nitride (SRN) is used as the positive thermo-optic coefficient material and titanium dioxide (TiO 2 ) The change of the resonant wavelength of the micro-ring resonator with the concentration of the sodium chloride solution in the cladding layer as an example of a slit waveguide composed of a negative thermo-optical coefficient material;

[0023] In the figure: the triangle represents the resonant wavelength of the microring resonator at a certain sodium chloride solution concentration; the dotted line represents the linear fit to these points, and the fitting equation is marked next to it.

[0024] Figure 1 and Figure 2 In the figure: I is a straight waveguide coupled with a microring resonant cavity, II is a microring resonant cavity based on a composite material slit waveguide, 1 is a ring waveguide made of a material with a positive thermo-optic coefficient, 2 is a ring waveguide made of a material with a negative thermo-optic coefficient, 3 is an area covered by a measured substance such as a gas or liquid, 4 is an area where the substrate silicon oxide optical isolation layer is located, and 5 is substrate silicon. DETAILED DESCRIPTION

[0025] The present invention will be further described below in conjunction with the accompanying drawings and an embodiment of an on-chip temperature drift-free micro-ring resonant cavity optical sensor based on a composite material slot waveguide.

[0026] like Figure 1 As shown, it includes a micro-ring resonant cavity II based on a composite material slit waveguide, and a strip straight waveguide I coupled to the micro-ring resonant cavity II. The straight waveguide I is placed beside the micro-ring resonant cavity II. An optical signal is input from one end of the straight waveguide I, and the optical signal of the resonant wavelength is filtered out through coupling with the micro-ring resonant cavity II and then coupled back to the straight waveguide I, and finally output from the other end of the straight waveguide I.

[0027] The micro-ring resonator II is divided into a first ring waveguide 1 made of a positive thermo-optic coefficient material and a second ring waveguide 2 made of a negative thermo-optic coefficient material. The straight waveguide I is made of the same material as the first ring waveguide 1, and is a strip structure.

[0028] The first annular waveguide 1 and the second annular waveguide 2 are arranged concentrically with an annular slit therebetween. Thus, the first annular waveguide 1 and the second annular waveguide 2 form a slit waveguide, which is made of a composite material, and the waveguide materials on both sides of the slit have a positive thermo-optic coefficient and a negative thermo-optic coefficient, respectively. The slit size is very small and can support the slit mode of light.

[0029] like Figure 2As shown, both the straight waveguide 1 and the slit waveguide are located on a silicon chip 5 with an optical isolation layer 4. The area above the optical isolation layer 4 except the waveguide is the material coverage area 3 of the on-chip temperature drift-free micro-ring resonant cavity optical sensor based on the composite material slit waveguide to be tested. The slit also belongs to the area 3, that is, the material to be tested will be in full contact with the enhanced light field in the slit.

[0030] In the present invention, the broadband light is inputted from the coupled straight waveguide I, passes through the micro-ring resonator II, and is coupled back to the straight waveguide I to obtain the output spectrum.

[0031] Light that meets the resonance condition of micro-ring resonator II will leave a series of troughs on the output spectrum, which are the positions of the resonant wavelengths. The positions of these resonant wavelengths are affected by the change in the refractive index of the material being measured. At the same time, since the refractive index of the material on both sides of the slit is much larger than the refractive index of the material being measured in the slit, there is a strong light field in the slit, which can strongly interact with the material being measured, making the resonant wavelength very sensitive to the changes in the material being measured, achieving the effect of high-sensitivity detection.

[0032] Furthermore, the slot waveguide of the present invention is composed of a composite material, and the light field of the light mode in the slot waveguide is distributed in the positive thermo-optic coefficient material and the negative thermo-optic coefficient material at the same time. When the temperature changes, the total equivalent refractive index of the slot light mode remains unchanged, that is, its phase does not change with the change of temperature, so that the resonant wavelength of the microring resonant cavity based on the slot waveguide does not change and does not change with the change of temperature, thereby achieving the effect of no temperature drift.

[0033] The specific embodiments of the present invention and their implementation are as follows:

[0034] First, ordinary silicon wafers are used and oxidized to obtain a silicon dioxide light isolation layer about 2 microns thick. The refractive index of silicon dioxide at room temperature is 1.444, and the thermo-optical coefficient is 0.8×10 -5 / ℃. Silicon-rich silicon nitride is used as the material for the coupling waveguide and the positive thermo-optic coefficient. By adjusting the silicon content, the refractive index of the material at room temperature can be obtained to be 2.29 and the thermo-optic coefficient is 2×10 -5 / ℃. Titanium dioxide is used as the negative thermo-optical coefficient material, which has a refractive index of 2.29 at room temperature and a thermo-optical coefficient of -1×10 -4 In this embodiment, the height of all waveguides is 300 nanometers. Considering the wavelength of 1550 nanometers, the fundamental mode (Fundamental Transverse-Electric Mode, TE 0 ).

[0035] The structure was simulated and designed using the Finite-Difference Time-Domain (FDTD) method. When the width of the silicon-rich silicon nitride part in the slot waveguide is 453 nanometers, the width of the titanium dioxide is 300 nanometers, and the distance between the two, i.e. the slot width, is 100 nanometers, the response of the resonant ring to temperature is approximately 0 (±4×10 -5 nm / ℃).

[0036] At this time, the material covered above is pure water with a refractive index of 1.33. When pure water is changed to sodium chloride solution with concentrations of 1%, 2%, 3%, 4%, and 5%, the resonant wavelength changes with concentration up to 577 picometers per 1% concentration change. Figure 4 shown.

[0037] At the above concentrations, the temperature-induced shift of the resonant wavelength is as follows: Figure 3 As shown, within the range of 10 degrees, it is no more than 0.4 picometers, which is negligible compared with the wavelength drift caused by the concentration of the measured substance, so it can be considered as a sensor without temperature drift.

[0038] The above embodiments are used to illustrate the present invention rather than to limit the present invention. Any modification and change made to the present invention within the spirit of the present invention and the protection scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A temperature drift-free on-chip optical sensor based on a composite material slot waveguide, characterized in that: The invention comprises a micro-ring resonant cavity (II) based on a composite material slit waveguide and a straight waveguide (I) coupled to the micro-ring resonant cavity (II). The micro-ring resonant cavity (II) is divided into a first ring waveguide (1) composed of a material with a positive thermo-optic coefficient and a second ring waveguide (2) composed of a material with a negative thermo-optic coefficient. The first annular waveguide (1) and the second annular waveguide (2) are arranged concentrically, and an annular slit is provided between them; The straight waveguide (I) is made of the same material as the positive thermo-optic coefficient material of the first ring waveguide (1), which is silicon-rich silicon nitride; The negative thermo-optic coefficient material is titanium dioxide.

2. The temperature drift-free on-chip optical sensor based on a composite material slot waveguide according to claim 1, characterized in that: The micro-ring resonant cavity (II) and the straight waveguide (I) are both located on a silicon chip (5) with an optical isolation layer (4).

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