CN206594060U - A kind of index sensor for the Michelson structure that CNT is modified based on thin-core fibers - Google Patents

Abstract

The utility model patent provides a refractive index sensor based on a thin-core optical fiber modified CNT Michelson structure, which consists of a single-mode optical fiber (1), a thin-core optical fiber (2), a micro-arc top cladding (3), and a CNT deposition layer (4) Composition; the single-mode optical fiber (1) is connected to the thin-core optical fiber (2), the micro-arc top cladding (3) is connected to the thin-core optical fiber (2), and the CNT deposition layer (4) wraps the thin-core optical fiber (2) , the surface of the cladding (3) at the top of the micro-arc; the utility model has the advantages of high sensitivity, small structure and simple manufacture, and can be applied to various practical projects. The CNT with high refractive index will increase the effective refractive index of the cladding, so the external The effective refractive index difference between the cladding mode and the fiber core mode caused by the increase of the ambient refractive index will decrease, so that the wavelength shift of the sensor will be reduced compared with that of the unmodified CNT sensor. Appropriate control of the modification thickness of CNT will increase the drift of the sensor intensity, which can minimize the influence on the wavelength, and is used in the real-life refractive index monitoring.

Description

A Refractive Index Sensor Based on Michelson Structure of Thin Core Fiber Modified CNT

technical field

The invention discloses a refraction index sensor based on a Michelson structure of thin-core optical fiber modified CNT (Carbon Nanotube), which belongs to the technical field of optical fiber sensing.

Background technique

Optical fiber refractive index sensor is an important optical fiber sensor. Some common methods to improve the sensitivity of the sensor are to perform some special dissolution or chemical treatment on the optical fiber to make it more sensitive to the refractive index of the external environment. At the same time, the flexibility of the optical fiber itself is reduced, so that it is subject to some restrictions in practical applications. At the same time, most refractive index sensors will drift with the increase of the external refractive index, and the intensity will not fluctuate regularly or obviously. Such sensors are easily limited by FSR (Free Spectral Rang), and cannot perform continuous real-time monitoring in a relatively wide range of refractive index. In recent years, more and more people deposit a thin film on the surface of optical fiber to improve the sensing characteristics of the sensor. This type of sensor has the advantages of low cost, easy manufacture, and the measurement range of the refractive index of the structure changes with the change of the external environment.

The all-fiber Michelson interferometer is an important fiber optic interferometer. It has two structures. One is to splice a structure in the middle of a single-mode fiber, which can excite the cladding mode. At the same time, the end face of the fiber can be plated Silver with high reflectivity can reflect the excited cladding mode back into the fiber core to interfere with the light transmitted in the fiber core, or can weld some special structures on the end face of the fiber to excite higher-order cladding modes . The second structure is to fuse a section of single-mode fiber and special fiber together, and use the principle that the core diameters of single-mode fiber and special fiber do not match to excite the cladding mode. The structure is highly sensitive to the refractive index, enabling dual demodulation of wavelength and intensity.

CNT is a black material with high light absorption and high refractive index. On the other hand, CNT has good compatibility with silicon materials and can form a thin film on the surface of silicon. The important thing is that based on its sensing principle, the measurement range of the original structure's refractive index can be increased, and the intensity of the interference spectrum can be improved as the external refractive index changes.

Utility model content

The purpose of the utility model is to provide a refraction index sensor based on the Michelson structure of thin-core fiber-modified CNT. The device has low cost, is easy to manufacture, can increase the measurement range of the original structure's refractive index, and improves the intensity change of the interference wave spectrum as the external refractive index changes.

The utility model is realized by the following technologies:

A kind of refractive index sensor based on the Michelson structure of fine-core optical fiber modified CNT, it is characterized in that: by single-mode optical fiber (1), thin-core optical fiber (2), micro-arc tip cladding (3), CNT deposition layer (4 ) composition; the single-mode optical fiber (1) is connected with the thin-core optical fiber (2), the micro-arc top cladding (3) is connected with the thin-core optical fiber (2), and the CNT deposition layer (4) wraps the thin-core optical fiber (2), micro The surface of the cladding (3) at the top of the arc.

Described a kind of refractive index sensor based on the Michelson structure of fine-core optical fiber modified CNT is characterized in that: single-mode optical fiber (1), thin-core optical fiber (2), micro-arc top cladding (3) all can adopt G .652 single-mode optical fiber, the core and cladding diameters of the thin-core optical fiber (2) are 4 μm and 124.5 μm respectively, the length L of the thin-core optical fiber (2) is 2 mm, and the length l of the micro-arc top cladding (3) is 45.08 μm.

The Michelson structure based thin-core optical fiber modified CNT refractive index sensor is characterized in that: the CNT deposition layer (5) is uniformly deposited on the surface of the optical fiber using CNT.

The working principle of the present utility model is: according to the interference formula of Michelson interferometer is:

Where E ₁ and E ₂ are the size of the fundamental mode and the excited cladding mode respectively, Δn=n ₁ -n ₂ is the effective refractive index difference of the core of the cladding mode, l ₁ is twice of L, λ is the working wavelength, is the initial phase. Changes in the refractive index of the external environment affect the effective refractive index of the cladding mode, resulting in a change in phase.

This can explain that the wavelength and intensity of the Michelson structure based on the thin-core fiber change significantly with the change of the refractive index of the external environment. The Michelson structure modified by CNTs is shown in Figure 2. Since CNTs is a material with high refractive index and high light absorption rate, the high refractive index characteristic makes the effective refractive index of the cladding mode change, which will affect the intensity of the evanescent wave of the cladding mode on the surface of the thin-core fiber, resulting in Changes in wavelength and extinction ratio of interference peaks and troughs. When the refractive index of the external environment changes, the end face of the thin-core fiber modified with CNTs also changes, and the evanescent wave reflection intensity of the cladding mode can use the formula:

where a = 4πk/λ is the absorption coefficient of the CNT layer, n _clad and _nam are the refractive indices of the cladding and the external environment, respectively. r _x,y are reflection coefficients interfering at different layers. The corresponding four layers are fiber core, cladding, CNT film and external environment. It can be seen from the formula that the change of the refractive index of the external environment affects the change of R. When the refractive index of the external environment is lower than that of the cladding, an increase in the refractive index of the external environment will cause a decrease in R, which means that very little energy of the evanescent wave is reflected back to the core and the fundamental mode of the core Interference occurs, which eventually leads to a decrease in the extinction ratio. The change of R will be larger than that of the unmodified CNT sensor, because for the sensing of unmodified CNT, the intensity of the cladding mode only depends on the Fresnel reflection between the fiber cladding and the external environment . The CNT film will increase the reflection intensity of the cladding mode, so as the refractive index of the external environment changes, the intensity of the interference peaks and troughs will change more.

The beneficial effect of the utility model is that: a simple and effective deposition method is used in the experiment to deposit CNTs on the end face of the optical fiber. Based on its sensing principle, the measurement range of the refractive index of the original structure can be increased, and the intensity change of the interference spectrum can be improved as the external refractive index changes. Such thin-core fiber structures deposited with CNTs overcome the limitation of FSR. At the same time according to The CNT with high refractive index will increase the effective refractive index of the cladding, then the effective refractive index difference between the cladding mode and the fiber core mode caused by the increase of the external environment refractive index will decrease, so that the wavelength shift of the sensor is the same as that of the unmodified CNT Sensors will be reduced. Appropriate control of the modified thickness of CNT will increase the drift of sensor intensity, which can minimize the influence on wavelength.

Description of drawings

Fig. 1 is the structural refraction index sensor schematic diagram of the structure based on the Michelson junction of thin-core optical fiber modified CNT of the present invention

Fig. 2 is the change experimental diagram of the interference spectrum before and after the sensor of the present invention modifies CNT

Fig. 3 is the change diagram of the interference spectrum of the sensor of the present invention along with the change of the refractive index of the external environment

Fig. 4 is the fitting figure of external environment refractive index and interference peak wavelength of the present utility model

Fig. 5 is the fitting figure of external environment refractive index and interference peak intensity of the present utility model

detailed description

Experimental setup of fiber optic refractive index sensor:

A system of refractive index sensors based on thin-core fiber-modified CNT Michelson structure includes a broadband light source, fiber coupler and spectrometer. During the experiment, the Piranha solution-treated structure was connected to a spectrometer and a light source. Through the optical deposition method, in the process of depositing on the end face of the optical fiber, the broadband light source originally outputs 95dB of energy, which is adjusted to 120dB, which is about 20mW of energy. In this case, the end face of the structure is extended into the CNT solution for a continuous For 20 minutes, light and heat can slowly increase the temperature, so that CNTs are evenly deposited on the surface of the optical fiber, and then the structure is immersed in water for two hours to rinse off the unstably deposited CNTs. The thickness of the CNTs was controlled by spectrometer monitoring throughout the deposition process to ensure a good interference spectrum of the modified structure.

Below in conjunction with accompanying drawing and embodiment example, the utility model is further described:

Referring to accompanying drawing 1, a kind of refractive index sensor based on the Michelson structure of fine-core fiber modification CNT is characterized in that: by single-mode fiber (1), thin-core fiber (2), micro-arc top cladding (3), Composition of CNT deposition layer (4); single-mode optical fiber (1) is connected with thin-core optical fiber (2), micro-arc top cladding (3) is connected with thin-core optical fiber (2), and CNT deposition layer (4) wraps thin-core optical fiber (2), the surface of the micro-arc top cladding (3). Single-mode fiber (1), thin-core fiber (2), and micro-arc top cladding (3) can all use G.652 single-mode fiber, and the core and cladding diameters of the thin-core fiber (2) are 4 μm and 124.5 μm, the length L of the thin-core optical fiber (2) is 2mm, and the length l of the microarc tip cladding (3) is 45.08 μm. The CNT deposition layer (4) is uniformly deposited on the surface of the optical fiber using CNTs.

Figure 2 shows the reflection spectrum of the thin-core fiber structure in air before and after modification. It can be seen that the reflectance spectrum of the modified structure shifts to the long-wave direction. The high refractive index of CNTs causes a change in the effective refractive index of the cladding modes, which causes a red-shift in the interference spectrum. Measure the refractive index of the modified sensor.

Figure 3 shows the change of the reflection spectrum with the change of the external refractive index. The refractive index range of the unmodified thin-core fiber-based refractive index sensor is 1.33RIU (Refractive Index)-1.38RIU, the sensitivity after wavelength demodulation is 228.85nm/RIU, and the sensitivity after intensity demodulation is -158.75dB/RIU. Demodulation of the reflectance spectrum of the CNT-deposited sensor found that the wavelength shift has no obvious change with that of the unmodified one, but the intensity drift increases and the refractive index sensitivity after demodulation is -183.67dB/RIU,

Figure 4 shows the change of the external refractive index with wavelength.

Figure 5 shows the change in ambient refractive index with intensity.

Claims (3)

1. a kind of index sensor for the Michelson structure that CNT is modified based on thin-core fibers, it is characterised in that：By single-mode optics Fine (1), thin-core fibers (2), differential of the arc top covering (3), CNT sedimentaries (4) composition；Single-mode fiber (1) and thin-core fibers (2) phase Even, differential of the arc top covering (3) is connected with thin-core fibers (2), CNT sedimentaries (4) parcel thin-core fibers (2), differential of the arc top covering (3) surface.

2. a kind of index sensor of Michelson structure that CNT is modified based on thin-core fibers according to claim 1, It is characterized in that：Single-mode fiber (1), thin-core fibers (2), differential of the arc top covering (3) can be using G.652 single-mode fibers, thin core The fibre core and cladding diameter of optical fiber (2) are respectively 2 μm~5 μm and 120 μm~130 μm, the length L of thin-core fibers (2) for 2mm~ 5mm, the length l of differential of the arc top covering (3) is 40 μm~50 μm.

3. a kind of index sensor of Michelson structure that CNT is modified based on thin-core fibers according to claim 1, It is characterized in that：CNT sedimentaries (4) are uniformly deposited on the surface of optical fiber using CNT.