TWI633324B - Active Polarized Laser Radar System - Google Patents

Abstract

This case discloses an active polarized laser radar system, including: a signal processing unit that sends a control signal; a laser transmitting unit that receives the control signal to emit a first laser light to a test object, Wherein, the laser emitting unit includes a liquid crystal polarizing element group and a liquid crystal polarizing driver, the liquid crystal polarizing driver controls a phase delay of the liquid crystal polarizing element group to change a polarization state of the first laser light; and a laser receiving The unit is for receiving a second laser light reflected by the object to be tested, and analyzing the polarization information of the second laser light by the signal processing unit to obtain the surface characteristics of the object to be measured.

Description

Active Polarized Laser Radar System

The present invention relates to a laser radar system with active polarization, and more particularly to a laser radar system that changes the polarization state of a light source by a liquid crystal polarization element.

In a general laser radar system, the main measurable information is time / distance information. For example, the time-of-flight (TOF) method is used to determine the distance of distant objects. More advanced laser radar systems can roughly estimate the effective reflectivity characteristics of distant objects based on the size of the signal obtained at the receiving end; multi-wavelength systems can further provide relatively limited spectral information for distant objects. However, this method is useful for Information for judging distant objects is quite limited, because there is no direct way to judge the surface roughness, tilt angle, or even whether the object is artificial, unless a higher spatial resolution is used, for example, 3D images supplemented by Complex algorithms can be used to determine whether the object is a vehicle, a pedestrian, or other objects based on its appearance. However, in this way, not only will it need to collect a large amount of data, but it will also require a large amount of computing power for automated judgment. Laser radar systems can hardly make this judgment.

In the case of a laser radar or laser rangefinder, the size of the optical signal provides information on the effective reflectivity of the reflected laser object. The effective reflectivity is caused by the surface characteristics of the object, such as roughness, scattering, and the surface of the object and the incident light. Angle. The polarization information can provide a possible opportunity to analyze the physical characteristics of the aforementioned object surface. Using the polarization information can even distinguish the object as a natural or artificial substance. As shown in Figure 1, it can be clearly seen that the polarization information can obtain a high degree of smoothness. Contrast between surface and surrounding rough surface.

Generally speaking, the source of light used to measure polarization information is randomly polarized, and the detection end needs to change the direction of polarization detection. For the concept of flash imaging, it needs to be in the detector. The front end of the device is equipped with a variable polarization direction component. However, this method is easy to confuse the external ambient light with the signal light signal. In addition, if the laser radar system uses multiple detectors, it is necessary to Installing variable polarization direction elements will increase cost and system complexity.

In view of the shortcomings of the prior art, the present invention provides an active polarization laser radar system, which uses liquid crystal as a phase retarder optical element at the transmitting end to change the polarization state of the light source. Laser radar system with one or more receivers to achieve the effect of no moving parts, non-scanning medium and low resolution distance and spatial distribution of polarization information, and reduce the manufacturing cost and complexity of the system.

To achieve the above and other objectives, the present invention proposes an active polarization laser radar system including: a signal processing unit that sends out a control signal; and a laser transmitting unit that receives the control signal to transmit a first Laser light to a test object, wherein the laser emitting unit includes: a liquid crystal polarization driver and a liquid crystal polarization element group, the liquid crystal polarization driver controls a phase delay of the liquid crystal polarization element group to change the first laser light Polarization state; and a laser receiving unit, which receives a second laser light reflected by the object to be tested, and analyzes the polarization information of the second laser light by the signal processing unit to obtain the surface characteristics of the object to be measured .

In the above laser radar system, the laser transmitting unit further includes: a laser diode; and a laser diode driver for receiving the control signal to drive the laser diode to emit the first laser. Shoot light.

In the above laser radar system, the liquid crystal polarizing element group includes: a first liquid crystal polarizing element, which is disposed at the front end of the laser diode, and a second liquid crystal polarizing element, which is disposed on the first liquid crystal. The front end of the polarization element; wherein the liquid crystal polarization driver is electrically connected to the first liquid crystal polarization element and the second liquid crystal polarization element to control the phase delay of the first liquid crystal polarization element and the second liquid crystal polarization element.

In the above laser radar system, the laser emitting unit further includes: a first lens group, which is disposed at the front end of the second liquid crystal polarizing element, so that the first laser light is directly projected on the object to be measured.

In the above-mentioned laser radar system, the signal processing unit has a coefficient signal processor (Digital Signal Processor, DSP) or a field effect programmable gate array (FPGA).

In the above laser radar system, the laser receiving unit includes: a second lens group for receiving and converging the second laser light reflected by the detection object; and a light detector disposed on the second lens. The rear end of the group to detect the second laser light and convert the second laser light into a current signal; and an amplifier module is disposed at the rear end of the light detector, the amplifier module includes a front Amplifying circuit and a filtering and main amplifying circuit. The pre-amplifying circuit, the filtering and the main amplifying circuit convert the current signal into an amplified voltage signal and filter out noise to output a second voltage signal to the signal processing. unit.

In the above laser radar system, the first laser light is a single high-power pulsed laser light.

In the above laser radar system, the photodetector is a photodiode.

In this way, the present invention uses a liquid crystal polarizing element to change the polarization state of the laser light source, which can achieve the effect of simplifying the judgment of the characteristics of distant objects. For laser radar systems with low and medium resolution, only a few polarizing elements can be added. Under the condition of its driving circuit, and the situation that does not require complex algorithms or high calculation requirements, it is possible to judge whether the distant object is a vehicle, a pedestrian, other artificial or natural objects, and even have the opportunity to distinguish the difference between camouflage, camouflage and natural objects. In addition, because the ice crystal has a specific angle on the surface, it will also have a significant effect on the polarization state. Therefore, the active polarization laser radar system of the present invention can also be installed on an aircraft, which will have the opportunity to detect ice crystal rainwater in space. The distribution situation in China helps to provide early warning of icing on the surface of the aircraft.

In order to fully understand the purpose, features and effects of the present invention, the following specific embodiments are described in detail with the accompanying drawings to explain the present invention in detail, as described below:

In the following, the term "front end" used in this manual refers to the end near the object under test; and the term "back end" refers to the end far from the object under test.

Please refer to FIG. 1, the active polarization laser radar system 1 of the present invention includes a signal processing unit 10, a laser transmitting unit 20 and a laser receiving unit 30.

The laser emitting unit 20 includes a liquid crystal polarization element group 21, a liquid crystal polarization driver 22, a laser diode 23, a laser diode driver 24, and a first lens group 25.

First, the signal processing unit 10 is electrically connected to the laser diode driver 24 and sends a control signal to the laser diode driver 24. The laser diode driver 24 drives the laser diode driver after receiving the control signal The laser diode 23 emits a first laser light S _L1 , wherein the first laser light S _L1 is a single high-power pulsed laser light.

The liquid crystal polarizing element group 21 includes a first liquid crystal polarizing element 211 and a second liquid crystal polarizing element 212. The first liquid crystal polarization element 211 is disposed at the front end of the laser diode 23 and the second liquid crystal polarization element 212 is disposed at the front end of the first liquid crystal polarization element 211. The liquid crystal polarization driver 22 is electrically connected to the first liquid crystal polarization element 211 and the second liquid crystal polarization element 212 to control a phase delay of the first liquid crystal polarization element 211 and the second liquid crystal polarization element 212.

The first lens group 25 is disposed at the front end of the second liquid crystal polarizing element 212, so that the first laser light S _{L1 is} projected straight on a test object 40 (not shown). The first lens group 25 may include a collimator.

The laser receiving unit 30 includes a second lens group 31, a light detector 32 and an amplifier module 33. The second lens group 31 receives and focuses a second laser light S _L2 reflected by the detection object 40. The light detector 32 is disposed at the rear end of the second lens group 31 to detect the second laser light S _L2 and convert the second laser light S _L2 into a current signal S _I , wherein the light detection The device 32 is a photodiode.

In one embodiment of the present invention, the laser receiving unit 30 may be a multi-receiving laser receiving unit, that is, the laser receiving unit 30 is composed of multiple laser receiving units. The polarized laser radar system 1 uses the liquid crystal polarizing element group 21 at the laser transmitting end to change the polarization state of the laser light source, instead of setting the polarizing element at the laser receiving end, so the manufacturing cost of the overall system can be reduced because the liquid crystal The polarizing element group 21 is fixed at the laser receiving end, so its number is not affected by the number of multi-receiving laser receiving units (the conventional polarizing element is provided at the laser receiving end, so if there are multiple laser receiving Unit, you need multiple polarizing elements).

Please refer to FIG. 2, the amplifier module 33 includes a preamplifier circuit 331 and a filter and main amplifier circuit 332. The preamplifier circuit 331 the current signal S _I can be converted into a first voltage signal S _V1, the filtering and amplifying circuit 332 main line of the first voltage signal S _V1 and filter noise amplified output to a second voltage The signal S _{V2 goes} to the signal processing unit 10.

The following describes in detail how the signal processing unit 10 analyzes the polarization information of the second laser light S _L2 to obtain the surface characteristics of the DUT 40, that is, receives the second voltage signal S _V2 and performs signal processing. The signal The processing unit 10 may be a digital signal processor or a field effect programmable logic gate array.

Because semiconductor lasers are both linearly polarized and monochromatic, they can change the phase delay by liquid crystal to achieve linearly polarized light in different directions, and change the polarization state of the first laser light S _L1 for flight. The time signal of time-of-flight (TOF) will not change, but the signal size caused by different polarization states will be different, so the surface characteristics of the polarization state and the object 40 can be calculated by the signal processing unit 10 Stokes parameters and Mueller matrix.

(1)

Equation (1) is the relationship between the basic Stokes parameters and the Mueller matrix, where Is the Stokes parameter of the laser light source, For the Stokes parameter of the light signal received by the light detector, It is the characteristic Mueller matrix of the object to be measured. By changing the polarization state and Stokes parameter of the laser light source and measuring the Stokes parameter on the corresponding light detector, the The matrix parameters in the Mueller matrix can be used to derive the surface characteristics of the object.

The laser light source can change the Stokes parameter of its output. In general, four different linear polarization states are required: 0 °, 45 °, 90 °, and 135 °. They are:

(2)

For the light detector, only the S ₀ parameter can be measured, so the system operation can be written as equation (3).

(3)

In formula (3), R is the effective reflectivity of the object. The Mueller matrix can be written as equation (4).

(4)

In formula (4), M _{11 is} defined as 1. In addition, the Mueller matrix is an oblique symmetric matrix. In other words, M _ij = M _ji , so there are only 9 variables. By changing the Stokes of the laser light source Parameters, the corresponding S _{0_detector} will be able to solve the important elements of the Muller matrix of the object and the effective reflectivity R. Taking S _{source_0 °} and S _{source_90 °} as examples, the value measured by the light detector is Equations (5) and (6).

(5)

(6)

Therefore, the effective reflectivity R and M _{12 of the} object can be obtained by simple calculations, and other parameters can also be derived in a similar manner. After obtaining the elements of the Mueller matrix, the roughness of the object can be calculated. For the calculation of the roughness of the object, please refer to [Reference 1], which will not be described in detail here. In addition, from the Mueller matrix of a general linearly polarized object (Eq. (7)):

(7)

The polarization angle q can be deduced, and the angle between the object and the laser emitting end can be known. Then, the Fresnel equation can be used to estimate the refractive index of the surface of the object to the air, such as As shown in Figure 4, the reflectance and transmittance of the TE polarized wave and TM polarized wave obtained from the Fresnel equation versus the angle of incidence ( ), Where q _i is the angle of incidence (radian), q _p is the Brewster angle, R ₁ (n, q _i ) is the change in the reflectivity of the TE polarized wave as the angle of incidence increases, and R ₂ ( n, q _i ) is the change in the reflectivity of the TM polarized wave as the incident angle increases, and T ₁ (n, q _i ) is the change in the transmittance of the TE polarized wave as the incident angle increases, T ₂ (n, q _i ) is the change in the transmittance of TM polarized waves with increasing incident angle.

Based on the above, the laser radar system of the present invention can obtain more detailed information about distant objects through polarization measurement, so it is more likely to determine the properties of distant objects (roughness, scattering, the angle between the surface of the object and the incident light, Natural or artificial substances ... etc.). For the generation of various polarization states with liquid crystal, refer to the structure proposed by [Document 2] using dual liquid crystal elements to achieve arbitrary polarization states to generate various polarization states, which will not be described in detail here. It is a schematic diagram of a complete polarization state generator with a fixed polarizer and two liquid crystal phase variable retarders in Reference 2, where Sc is a monochromatic light source, P is a fixed linear polarizer, and LC1 is a first liquid crystal phase variable retarder , LC2 is the second liquid crystal phase variable retarder, QWF is a quarter deferred wave module, the X axis corresponds to the slow axis of LC2, the slow axis of LC1 and the X axis form an angle q, and LC1 and LC2 introduce phases The displacement g and phase displacement a, P are parallel to the main axis of QWF and have an azimuth angle of −45 ° with respect to the slow axis of LC1.

Literature 1: W. Yang, GH Gu, XJ Zhou, FY Xu, and K. Ren, "The estimation of surface roughness with the utilization of Mueller matrix," Infrared Physics & Technology, vol. 76, pp. 748-755, May 2016.

Document 2: M. Shribak, "Complete polarization state generator with one variable retarder and its application for fast and sensitive measuring of two-dimensional birefringence distribution," Journal of the Optical Society of America A, vol. 28, p. 9, 2011 .

In this way, the active polarization laser radar system of the present invention provides the surface information of distant objects by changing the polarization state of the laser transmitting end, and achieves the judgment of the characteristics and attributes of distant objects with a lower amount of calculation. It can also be used to judge vehicles, pedestrians, guardrails, and other objects while driving, and even military camouflage, camouflage, and natural objects. Therefore, the possible application fields are in addition to general smart vehicle detection systems. Opportunities are used for military ranging and identification. In addition, since the polarization signal can distinguish the difference between water droplets and ice crystals in the air, installing the active polarization laser radar system of the present invention on an aircraft will provide an early warning of icing on the surface of the aircraft.

The present invention has been disclosed in the foregoing with a preferred embodiment, but those skilled in the art should understand that this embodiment is only for describing the present invention, and should not be interpreted as limiting the scope of the present invention. It should be noted that all changes and substitutions equivalent to this embodiment should be included in the scope of the present invention. Therefore, the scope of protection of the present invention shall be defined by the scope of the patent application.

1 Laser Radar System with Active Polarization 10 Signal Processing Unit 20 Laser Transmitting Unit 21 Liquid Crystal Polarization Element Group 211 First Liquid Crystal Polarization Element 212 Second Liquid Crystal Polarization Element 22 Liquid Crystal Polarization Driver 23 Laser Diode 24 Laser Diode Body driver 25 First lens group 30 Laser receiving unit 31 Second lens group 32 Photodetector 33 Amplifier module 331 Preamplifier circuit 332 Filter and main amplifier circuit 40 DUT LC1 First liquid crystal phase variable retarder LC2 Second liquid crystal phase variable retarder P Fixed linear polarizer QWF Quarter deferred wave module Sc Monochromatic light source S _L1 First laser light S _L2 Second laser light S _I Current signal S _V1 First voltage signal S _V2 Second voltage signal g, a Phase shift q Angle q _i Incident angle q _p Brewster angle

[Figure 1] (a) general visible light image; (b) polarization image. [Fig. 2] is a structural diagram of an active polarization laser radar system according to an embodiment of the present invention. [Fig. 3] is a schematic diagram of photoelectric conversion processing of a laser receiving unit according to an embodiment of the present invention. [Fig. 4] It is a relationship diagram between the reflectance and transmittance of TE polarized wave and TM polarized wave obtained by Fresnel equation according to an embodiment of the present invention, and the angle of incidence with respect to the incident angle. [Fig. 5] is a schematic diagram of a complete polarization state generator with a fixed polarizer and two liquid crystal phase variable retarders in Document 2.

Claims (6)

An active polarized laser radar system includes: a signal processing unit that sends a control signal; and a laser transmitting unit that receives the control signal to emit a first laser light to a test object, wherein the The laser emitting unit includes: a liquid crystal polarization driver and a liquid crystal polarization element group, the liquid crystal polarization driver controls a phase delay of the liquid crystal polarization element group to change a polarization state of the first laser light; and a laser receiving unit, Receives a second laser light reflected by the object under test; wherein the laser emitting unit further includes: a laser diode and a laser diode driver, and receives the control signal to drive the laser diode The body emits the first laser light, and a first lens group is disposed at the front end of the second liquid crystal polarizing element, so that the first laser light is directly projected on the object to be measured; wherein the liquid crystal polarizing element group includes: A first liquid crystal polarizing element is disposed at the front end of the laser diode and a second liquid crystal polarizing element is disposed at the front end of the first liquid crystal polarizing element, wherein The liquid crystal polarization drive train is electrically connected to the first liquid and the second liquid crystal polarizer polarizing element, to control the phase of the first liquid and the second liquid crystal polarizer of the polarizing element delay. The laser radar system according to claim 1, wherein the signal processing unit is a digital signal processor (DSP) or a field programmable gate array (FPGA). The laser radar system according to claim 1, wherein the laser receiving unit includes: a second lens group for receiving and converging the second laser light reflected by the detection object; and a light detector for setting At the rear end of the second lens group to detect the second laser light and convert the second laser light into a current signal; and an amplifier module is disposed at the rear end of the light detector, the amplifier module The set includes a preamplifier circuit and a filter and main amplifier circuit. The preamplifier circuit, the filter and the main amplifier circuit convert the current signal into an amplified voltage signal and filter out noise to output a second voltage. Signal to the signal processing unit. The laser radar system according to claim 3, wherein the photodetector is a photodiode. The laser radar system according to claim 1, wherein the first laser light is a single high-power pulsed laser light. The laser radar system according to claim 1, wherein the laser receiving unit is a multi-receiving laser receiving unit.