Optical fiber acceleration sensor principle

The principle of a direct optical fiber sensor is to use an optical fiber as a transmission channel for optical signals. A direct optical fiber emits light, a receiving optical fiber receives the optical signal, and the receiving optical fiber is located in the propagation direction of the outgoing optical fiber. The outgoing light signal propagates in space and enters the receiving fiber after passing through a medium.

Fiber acceleration sensor advantages

Compared with traditional piezoelectric, ultrasonic and other sensing technologies, fiber optic sensing technology has significant advantages, including: large amount of sensing and transmission information; small size and light weight; operating frequency bandwidth; higher than other sensing technologies1 Sensitivity and resolution of 3 orders of magnitude; diversity of structure and geometry of the sensing part; universally applicable to sensing of various physical phenomena, such as sound field, magnetic field, temperature, rotation, etc. of hot air aging test chamber; The influence of electromagnetic interference; it can be applied to harsh environments such as high temperature and pressure, flammable and explosive; easy to reuse and form a sensor network; easy to realize real-time, online, distributed sensing, etc.

Optical fiber acceleration sensor application

Fiber optic sensors are divided into light-transmitting type and sensor type in application. As the name suggests, the former is to transmit light, and the sensing element must be connected to the optical fiber; the latter is to have both the function of transmitting light and the function of sensing. Now the research hotspots are almost the latter, so I will briefly introduce the latter, because fiber optic sensors have many applications as sensors, such as anti-corrosion, anti-electromagnetic interference, etc., and can be used in complex and harsh environments. As a sensing fiber, in principle, it is a device that performs sensing by modulating the polarization, intensity, phase, wavelength, period, frequency, etc. of the transmitted light, and obtaining the modulation result by the detector. Because when the external environment changes, such as temperature, stress, magnetism, sound, pressure, temperature, acceleration, etc., it will have a slight impact on some structures such as the refractive index distribution of the optical fiber, resulting in changes in the characteristics of transmitted light. By detecting these Change to get external changes and play a sensing role.

Fiber Acceleration Sensor Classification

Intensity modulation

The intensity-modulated acceleration sensor refers to the purpose of measuring acceleration by modulating the intensity of the transmitted light in the optical fiber. It mainly includes transmission, reflection, and polarization. Its advantages are relatively simple structure, easy signal demodulation, and relatively low cost. The disadvantage is that the accuracy is not high.

(1) Transmissive fiber acceleration sensor.

The structural feature of this type of sensor is that the fiber itself is used as a mobile unit. The acceleration causes the output fiber to vibrate and changes the amount of light coupled into the output fiber. The intensity of the light detected from the receiving end can reflect the magnitude of the acceleration.

(2) Reflective fiber-optic acceleration sensor.

This type of sensor is structurally different from the transmissive type in that it has an additional mirror, and both the optical fiber and the mirror may be used as moving elements. An optical fiber whose axis is perpendicular to the reflecting surface is called a positive mirror type, and an optical fiber whose axis is not perpendicular to the reflecting surface is called an oblique mirror type.

(3) Polarized fiber-optic acceleration sensor.

This type of sensor uses the optical fiber itself to directly sense the inertia of the mass to produce a change in polarization state, which causes a change in the output light intensity to measure acceleration. Four types of mechanical energy conversion structures based on optical fiber birefringence proposed by Tihon Pierre and others in 2012 are U-shaped aluminum beams that produce bending, squeezing, stretching, and twisting effects on optical fibers. When polarized light is input from one end of a single-mode fiber of this structure, acceleration causes deformation of the fiber and thus changes the laser polarization state. The polarized light output from the other end is detected by a photodiode after passing through the analyzer. Different accelerations correspond to different polarization states, that is, different received light intensities [1].

2 phase modulation type

Phase modulation acceleration sensor refers to the purpose of measuring acceleration by modulating the phase of the transmitted light in the optical fiber. It mainly includes Michelson interference type, Mach-Zennder interference type, FP interference type, etc. Its advantages are flexible and diverse geometric structure, resolution The performance indicators such as sensitivity and sensitivity are very high, and they are widely studied.

(1) Michelson interference fiber optic acceleration sensor.

F Peng et al. Designed a compact Michelson interferometer accelerometer in 2012, which takes advantage of the inherent advantages of the fiber itself, so that the size and weight of the sensor can be made very small. In the middle of two single-mode optical fibers that have been glued together, the upper and lower ends of the optical fiber are fixed with metal tubes and solid frames. The change in acceleration will cause the optical path difference between the two arms of the interferometer to change. The corresponding acceleration can be obtained by demodulating the phase change. The sensitivity and frequency response of this acceleration sensor can achieve 0.42 rad / g and 600 Hz.

(2) M-Z interference optical fiber acceleration sensor.

Chen Liuhua et al. Proposed an optical acceleration sensing scheme based on grating 1st-order interference and phase carrier (PGC) modulation and demodulation in 2010. Unlike traditional MZ interferometers, this structure does not rely on MZ- One arm is used as a sensing arm, but a sinusoidal amplitude grating with perpendicular incident laser light is used as a sensing element. When an acceleration in the direction of the vertical grid line acts on the grating in the grating plane, the grating generates a corresponding displacement, which in turn causes a change in the PD phase interference phase difference. One arm of M-Z uses PZT to generate a carrier to improve the phase demodulation accuracy. The experimental system error is [2].

(3) F-P interference optical fiber acceleration sensor.

A high-resolution acceleration sensor structure proposed by QLin et al. In 2011, a single-mode fiber end face is plated with a semi-reflective film, which is used as both a transmitting fiber and a receiving fiber, and is fixed on a V-shaped groove. The end face of the optical fiber is formed with a silicon micro-mirror fixed on a 0.8 mm thick mass in the center of a 0.1 mm thick stainless steel ring spring net-an F-P resonant cavity. A PZT is installed between the V-shaped groove and the optical fiber fixing bracket, and the phase length (PGC) modulation is generated by applying an audio signal to the cavity length. The change of the cavity length has a linear relationship with the acceleration of the fiber in the axial direction. The sensitivity of this structure is 36dB per 1 rad / g, the resonance frequency is 160 Hz, and the lateral sensitivity is -1.8dB per 1rad / g [3].

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