For you to share: 《Working Principle and Simulation of Optical Fiber Displacement Sensor》

 1. Introduction

　　fiber optic sensorCompared with traditional sensors, it has a series of unique advantages, such as high sensitivity, anti-electromagnetic interference, corrosion resistance, good electrical insulation, explosion-proof, flexible optical path, simple structure, small size and light weight. Therefore, the optical fiber sensor has become the inevitable development trend of airborne optical sensor.

Roctest Canada has produced a commercial fiber optic displacement sensor (Fiber-Optic Linear Position & Displacement Sensor, FO-LPDS) that uses the Fizeau interferometer demodulation patent.Technology(US patent #5202939/#5392117), which has the advantages of simple structure, high accuracy and fast response, and has been successfully applied in the field of civil engineering. This article will introduce the principle and use of this kind of sensor in detail.

　　2. composition structure and working principle

1. Sensor structure

The simple structure of the sensor is shown in Figure 1. Its connecting rod can move in the horizontal direction. The thin film Fizeau interferometer (TFFI) is fixed on the connecting rod. Its detailed structure is shown in Figure 2.
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2. Working principle

(1) Optical signal modulation

In actual use, the sensor is connected to the reader (Demodulator), and the light emitted by the white light diode light source in the reader is incident from one end of the optical fiber connected to the reader, transmitted to the Fabry- Perot sensor, and then emitted by the multimode optical fiber to illuminate the surface of the TFFI interferometer (optical wedge). When the TFFI moves horizontally, the position of the illumination point will also be different. The upper and lower surfaces of the optical wedge are coated with a semi-reflective film, thus forming a Fabry-Perot cavity. After a part of the white light emitted by the reader is reflected by the first half mirror, the rest of the white light passes through the Fabry-Perot cavity and is again reflected by the second half mirror, and the two reflected lights interfere with each other, so that the spectrum of the originally incident white light is modulated.

Assuming that the material of the optical wedge is glass, its refractive index is n = 1.6, and the wavelength range of the incident white light diode is 600nm ~ 1750nm according to the literature [1]. According to fig. 2, the optical path difference of reflected light from the upper and lower surfaces of the optical wedge is 2nh, assuming that the amplitudes of all frequency light waves in the light source spectrum are a, the phase difference between the two beams when interfering at the meeting point is d, the reflectivity of the optical wedge surface is r, and the transmittance is 1-R, then the composite amplitude y is: y = a aRe-iδ (1)

According to Euler's formula e-iδ = cosδ-isinδ, we can get: y(t)= a(1 Rcosδ-iRsinδ) (2)

The light intensity is proportional to the square of the amplitude of the light wave. If the light intensity at the point where the light wave meets is I, then:

I = y(t)×y(t)* = a2(1 R2 2 Rcosδ) (3)

For a certain position of TFFI, the height of the optical wedge surface is h, and the interference phase difference δ corresponding to light with different wavelengths l is:

δ =(2nh/l)× 2p = 4pnh/l (4)

The extreme value of light intensity I is:

　　I＝a2（1 R2 2R）                              （5）

In the TFFI interferometer, in order to form a reflective surface of light, a film needs to be plated on the upper and lower surfaces of the optical wedge respectively, and the coating film has a certain thickness, so the reflected light on the upper and lower surfaces of the coating film will form interference, which will affect the measurement result. Therefore, the thickness of the coating should be controlled at 1/4 of the center wavelength of the light source. For example, if the wavelength of the light source is 600nm to 1000nm, the thickness of the coating is 800nm (assuming that the refractive index of the coating material is 1), so that the phase difference of most reflected light on the upper and lower surfaces of the coating is 180 °, and the intensity is attenuated.

In the coordinate system shown in fig. 2, the distance between the incident point and the origin of the coordinate is x, the inclination angle of the optical wedge is a, and the height of the corresponding optical wedge surface is h:

H = 7 + xtga (mm) (6)

Tga=18/25000=7.2′10 4

Here take x = 12.5mm = 12500mm to calculateSensorModulate the intensity distribution of light, substitute the value of x into formula (6) to obtain h = 16mm, substitute into formula (4) to obtain d, and then substitute d into formula (3) to obtain light intensity I. Taking the wavelength range of the light source from 0.6mm to 1.75mm and the reflectivity of the optical wedge coating R = 0.5, the light intensity distribution diagram shown in fig. 3 can be obtained.

It can be seen that a finite number of interference maxima are generated at some wavelengths in the spectral range of the light source. Obviously, at different positions where the sensor is located, the TFFI modulates the light source differently, that is, the wavelength value corresponding to the interference maximum value changes. Where the wavelength l is small, the peak of the interference maximum is also dense.
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