Fiber Optic Displacement Sensor Simulation 2

The reader (signal conditioner) demodulates the optical signal sent back by the sensor and calculates the displacement signal. The above process can be represented by Figure 4.
The reader is equipped with a white light source. The light returned from the multimode optical fiber is converted into parallel light through a cylindrical lens and projected onto the inclined surface of the TFFI interferometer. The lower surface of the TFFI is closely attached to a CCD sensor that is sensitive to light intensity. As shown in Figure 5, assuming that monochromatic light is evenly irradiated on the upper surface of the optical wedge, at each point in the x direction, the reflected light from the upper and lower surfaces of the optical wedge will form interference, and the light transmitted from the lower surface will be detected by the CCD.

Here, it is assumed that the structure of the demodulated TFFI interferometer is exactly the same as that in the sensor, that is, it is taken from the same batch of products, so that the influence of the optical wedge position tolerance on the measurement results can be eliminated.
The modulated light signal shown in Figure 3 is input to the demodulation interferometer. For simplicity, only the wavelength corresponding to the maximum light intensity is considered here. The interference results formed by these wavelengths are vector superimposed in the length direction of the CCD. Since it is white light interference, the more times it is superimposed, the finer and sharper the interference fringes obtained on the CCD. The simulation results under Matlab are shown in the figure.

According to the simulation results, the light intensity value of the CCD at the position of 12.5 mm in length is exactly the maximum, which corresponds exactly to the situation when the optical fiber in the sensor is at the center of the optical wedge (x=12.5 mm).

When the sensor displacement is S, the light wave with the largest interference intensity also has the largest interference on the Fizeau interferometer of the reader. Therefore, by analyzing the coordinate position x=Smax of the point with the maximum light intensity on the CCD, the absolute position S=Smax of the sensor can be obtained.

2. Performance characteristics

According to the previous analysis and relevant information, the white light displacement sensor can measure absolute position, and it has the following characteristics:

(1) Using a white light diode light source instead of a laser light source, there is no need for the preheating time and constant temperature control required by a laser diode, which reduces the requirements for light source stability. In addition, the life of a white light LED is much longer than that of a laser diode LD.

(2) The sensor and the reader use a wedge-shaped thin film interferometer TFFI with the same structure, which can compensate for the measurement error caused by the manufacturing error of TFFI. Usually, the maximum linear error obtained without any compensation is 0.15% of the full scale.

(3) The manufacturing process of TFFI is complex. Currently, it can only provide displacement sensors with a range of 20 mm. It is difficult to manufacture TFFI of larger size, which limits the improvement of the range of this sensor.

(4) This sensor essentially works by using the optical path difference between the upper and lower surfaces of the optical wedge, so it is insensitive to environmental vibrations and changes in optical fiber parameters. The optical wedge (TFFI) is generally made of materials that are insensitive to temperature. There is no lens in the sensor, and the installation of the optical fiber does not require strict alignment, so it can work in harsh environments;

(5) A CCD or PSD light detector can be used in the reader. The light intensity distribution received by the CCD can have multiple extreme points, but through reasonable structural design, it can be guaranteed that there is only one maximum point. The signal processing uses an algorithm for finding the maximum value.

The main performance indicators of this sensor are shown in the table:

3. Conclusion and Outlook

Fiber optic position sensors that use the principle of white light interference can measure absolute linear position and angular displacement. They have the characteristics of simple structure, high accuracy, wide operating temperature range and insensitivity to vibration, so they are expected to be used in optical transmission systems.