2.1

Optical path design

The Stokes vector S = [S₀, S₁, S₂, S₃]^T contains four components. S₀ represents the total intensity of scattered light. S₁ represents the difference between the vertical and parallel components of the scattered light. S₂ represents the difference between ±45°. S₃ represents the difference between left and right circularly polarized component. The optical path design is shown in Figure 1. The semiconductor laser with wavelength 650nm is chosen as the light source, and it has good coherence and high stability. It is a continuous laser containing an expanding beam and a collimating lens. The polarizing film can further improve the polarization degree of the emitting laser. Daheng Photoelectric GCL-05 artificial polarizing film is adopted. It’s pass-through aperture is 25.4mm. The extinction ratio is 500:1, and the applicable band range is 400-700nm. The polarization direction of GCL-05 polarizer is not exactly marked, so it needs to be calibrated again. A quarter wave plate is used to adjust the polarization of the incident light. By adjusting the fast axis, the linearly polarized light passing through a quarter wave plate can become circularly polarized light. By rotating the fast axis, the half wave plate can adjust the vibration direction of the incident light. Four groups of incident completely polarized light can be modulated by the above devices. They are I(0°,0), I(90°,0), I(45°,0), I(45°, π / 2). The I(0°,0) is the intensity of light scattered parallel to the plane of incidence. The I(90°,0) is the intensity of light scattered perpendicular to the plane of incidence. The I(45°,0) is the intensity of the scattered light at an angle of 45º from the plane of incidence. The I(45°, π / 2) is right-handed circularly polarized light.

Figure 1.

The schematic diagram of optical path design

The measurement formula of Stokes vector is

2.2

Mechanical structure design

In order to study the laser scattering characteristics of cement road surface with different roughness, a high-precision laser scattering measurement system is designed, as shown in Figure 2. The system is based on virtual instrument technology, and the Labview software is used to design flexible and expandable instrument panel relying on the computer hardware platform. Labview realizes serial port communication with micro controller unit (MCU) through graphical programming, and issues control commands to MCU. The single chip microcomputer controls the stepper motor 1 to realize accurate adjustment of the angle of the turntable, and controls the stepper motor 2 to rotate the screw drive mechanism to drive the sample to be tested to move. The average value of the scattered light intensity can be obtained by multiple measurements.

Figure 2.

Schematic diagram of scattering measurement mechanical structure

2.3

Control system

A high-precision digital light intensity detection module with BH1750FVI as the core was made. The BH1750FVI is a 16-bit digital output integrated circuit. It uses IIC bus to communicate with single chip microcomputer to upload measurement data. Its internal photoelectric sensor has optical efficiency similar to that of human eyes, with a peak sensitive wavelength of 560nm, which can achieve a wide range of optical intensity measurement within 65535lx. In order to avoid the situation that scattered light intensity is weak and difficult to measure, LD-650Y2 high-power semiconductor laser is adopted for laser emission, with output power up to 250mW and working voltage of 3-4.5V. Output power can be adjusted with working voltage. The transmitter module contains focusing and collimating lens to realize adjustable laser speckle diameter.

The schematic diagram of control system as shown in Figure 3. The Labview running on the upper computer issues the control command of stepper motor 1 to the MCU through serial port communication, so that the turntable rotates to the specified angle within the range of 0°-180°. The Labview issues the stepper motor 2 control command to the MCU to make the screw drive mechanism move the sample to be tested. The MCU issues the data acquisition command. The MCU communicates with the BH1750FVI module through IIC bus, and obtains the measurement data. It uploates the measured data to the upper computer LABVIEW. Cycle for the next data acquisition.

Figure 3.

Schematic diagram of control system.

3.1

The Stokes vector of the scattered light

The experiment results of scattered light Stokes vector are shown in Figure 4 and Figure 5, and the results are normalized by the maximum scattering light intensity of the A cement sample with P light incident. It can be seen from the figure that, with the increase of roughness, the scattered light intensity is gradually broadened outside the incident plane. When P light is incident, the broadening rate is higher than that of S light.The Stokes vector V component is almost zero, indicating that the scattered light contains almost no circularly polarized component. At the same time, with the increase of roughness, the difference between Q component and I component becomes larger, indicating that the degree of depolarization of scattered light increases. When P light is incident, the U component has an anti-symmetric peak with respect to the peak position outside the incident plane.When S light is incident, U component has an anti-symmetric peak with respect to peak position in the incident plane.The movement direction of the peak position of Q and U components is basically consistent with that of I components.

Figure 4.

Stokes vector of scattered light from different cement samples under P polarized light is incident.

Figure 5.

Stokes vector of scattered light from different cement samples under S polarized light is incident.

The scattered light intensity distribution characteristics can be obtained from Stokes vector I component as shown in Table 1. With the increase of roughness, the intensity of scattered light attenuates and the attenuation rate gradually slows down when the incident P light and incident S light occur. The intensity distribution of scattered light is gradually broadened, and the amplitude of incident S light is higher than that of P light. The scattering light intensity peak position moves, P light incident position moves to the direction less than the scattering angle, and S light incident position moves to the position greater than the scattering angle. The shift amplitude of the incident peak position of S light is larger than that of P light. When S light is incident, the intensity of scattered light is higher than P light, which is about 2 times. The angle difference at 0.5 times of the scattered light intensity is used to characterize its width in the incident plane.

Table 1

Scattering intensity distribution of dielectric rough surface

3.2

Discussion

In order to study the position movement of the scattered light intensity peak, the scattering in the incident plane is selected for analysis. In order to study the position movement of the scattered light intensity peak, the scattering in the incident plane is selected for analysis. As shown in Figure 6, when the incident angle of the local ground element is θ=(θ_i+θ_s)/2, then the surface element makes the greatest contribution to the scattering field. At this point, the slope of the surface element is k = tan α=tan[(θ_i – θ_s)/2], which satisfies the normal distribution, and its probability density function is , where the average slope μ_s=0 and the root mean square of the slope . As shown in Figure. 6, when the scattering angle varies from 0° to 90°, the reflectivity of the metal surface is significantly higher than that of the dielectric. The metal surface has little difference in the reflectivity of S light and P light, while the dielectric surface has a large difference in the reflectivity of S light and P light.

Figure 6.

Analysis on the mechanism of incident-plane scattering.

(a) Schematic diagram of incident-plane scattering; (b) Reflectance of dielectric and metal rough surface

The product of the reflectivity and the slope probability density function of the microplane element is shown in Figure 7, which is directly proportional to the scattered light intensity.It can be seen that the scattering intensity on the rough surface S light is greater than that of P light, which is about 2 times.When P light is incident, the position of the scattered light intensity peak shifts to the left. When S light is incident, the peak position moves to the right. The scattered light intensity of the metal rough surface is much higher than that of the dielectric rough surface, which is about 20 times that of the incident dielectric rough surface, while the scattered light intensity of the metal rough surface S is slightly higher than that of the P light, but the difference is not significant. P light incident on the rough surface of the metal, the position of the scattered light peak hardly moves. When S light is incident on the rough surface of metal, the position of the scattered light peak shifts to the right, but the shift amplitude is less than that of S light incident on the rough surface of dielectric.

Figure 7.

The intensity of scattered light from rough surfaces.

(a) P light incident dielectric rough surface, (b) S light incident dielectric rough surface, (c)P light incident metal rough surface, (d) S light incident metal rough surface