Optical system design suitable for high-precision laser ranging

Abstract: With the development of lidar technology and the increasing demand for ranging accuracy, new requirements have been put forward for the transmitting and receiving optical systems, which need to have the characteristics of adjustable beam, small measurement spot, and high echo efficiency. Design an integrated transceiver optical system that works in the 1550nm optical communication band. The transmitting and receiving modules share part of the optical path to reduce the blind area of the receiving field of view and facilitate the miniaturization of the structure. In order to solve the problem of echo energy differences caused by different measurement distances and different surface inclination angles, the beam expansion component of the optical system is designed as a continuously adjustable structure with a magnification of 2x~3.5x; two sets of double cemented lenses are used for chromatic aberration correction. Reduce the impact of spectral width on system propagation distance. After design optimization, the laser divergence angle after collimation of the system is less than 0.3mrad, and the exit spot diameter is continuously adjustable from 6.26mm to 10.20mm. For measurement targets within 50m, the irradiation spot diameter is less than 20mm, and the divergence angle is at different zoom positions. and spot diameter all meet the design requirements.

Keywords: optical design; laser ranging; integrated transceiver system; zoom system

Introduction

Since the world's first laser rangefinder was successfully developed in the United States in the 1960s, lidar has become increasingly important in the field of non-contact measurement. LiDAR actively emits laser light onto the surface of the detected target, and measures the distance to the target by collecting echo signals. Compared with traditional infrared ranging, ultrasonic ranging, millimeter wave ranging and other methods, laser ranging has a longer detection distance and higher measurement accuracy. In recent years, lidar has developed rapidly in both the military and civilian fields, the application demand continues to increase, and the advantages of laser ranging technology have also been fully utilized. At the level of high-precision technologies, such as aerospace, satellite remote sensing, and debris detection, high-precision lidar detection technology has become the focus of various countries' efforts to develop it.

With the advancement of laser and chip technology, laser ranging is developing in the direction of long range, high precision, and miniaturization, which also puts forward higher requirements for optical systems. In addition, if the ranging accuracy is below the millimeter level, the system error caused by non-coaxiality needs to be considered. However, in most existing lidar optical systems, the transmitting system and the receiving system use different optical paths, which are independent of each other and off-axis, and there is a blind spot in the receiving field of view. In order to improve the ranging accuracy and ensure the miniaturization of the system, it is urgent to develop a compact lidar with integrated transceiver and receiver.

This article designs a laser ranging optical system that integrates transceiver and receiver. The optical communication band of 1550nm is used in the laser wavelength selection. This wavelength not only has better atmospheric transmittance, but also has the advantage of human eye safety and can be used in densely populated areas. occasion. At the same time, we make full use of the low background noise advantage of the optical fiber interface and use single-mode fiber as the emission port of the laser beam. In order to effectively solve the problem of blind spots in the receiving field caused by the non-coaxial nature of traditional systems, the transmitting system and the receiving system share an expanded beam optical path. Finally, in order to adapt to the measurement of different distances and take into account the adjustability of the system, the expanded beam optical path is made into a variable magnification structure. The optical system designed and optimized in this article will provide a theoretical and experimental basis for the development of subsequent engineering prototypes.

1. Working principle of integrated transceiver and laser ranging

The working principle of the integrated transceiver laser ranging system is shown in Figure 1. The optical part consists of four parts: a collimation module, a beam splitter, a beam expansion module and a focusing module (the lenses in the figure are all model illustrations). The laser signal is emitted from the optical fiber port and is first shaped into a parallel beam by the collimation module, then passes through the beam splitter (BS), amplifies the beam diameter through the beam expander module, and finally irradiates the target surface to be measured. After the laser beam is diffusely reflected on the surface to be measured, part of the echo signal is collected again by the optical system and is received and amplified by the avalanche photodiode (APD). In order to calculate the time interval from laser emission to collection, the system is equipped with a reference mirror, which can compare the difference in collection time between the two pulsed lights and indirectly calculate the relative distance to the target to be measured.

Figure 1. Schematic diagram of the integrated transceiver laser radar

From the perspective of the optical path, the collimation module is the first to participate in laser shaping, which directly affects the propagation effect of subsequent beams; the beam expansion module participates in both transmitting and receiving, which is the focus of the design of the integrated transceiver system. Therefore, the design quality of the collimation module and beam expansion module will directly affect the system transceiver efficiency and measurement accuracy.

2. Theoretical model of system design

Since the design process of the system's collimation module and beam expansion module is relatively complex and involves the principles of laser shaping and continuous zooming, theoretical models need to be established separately to guide the design of the optical system.

2.1 Alignment module design

The laser beam emitted through the optical fiber has the properties of a Gaussian beam. During the transmission process, its center of curvature and radius of curvature continuously change, but the amplitude and intensity always maintain Gaussian distribution characteristics within the cross-section. Therefore, when shaping a Gaussian beam, we cannot simply use geometric optics to simulate calculations. We need to consider its beam waist, divergence angle, Rayleigh range and other physical optical propagation parameters.

The schematic diagram of the collimation optical system is shown in Figure 2. The laser beam is emitted from the fiber end face, with an initial beam waist radius of ω0, a divergence angle of θ, and a distance l from the shaping lens. After being shaped by the lens, the beam still has the properties of a Gaussian beam. The beam waist radius of the new beam is ω′0, and its spot radius ω′(z) is a function of the propagation distance z.

Figure 2. Schematic diagram of collimation optical system

Figure 3. Optical path diagram of collimation optical system

Figure 4. Optimized variable magnification beam expansion system structure diagram

Figure 5. Overall optical path structure diagram

3  Conclusion

This article designs an optical system suitable for high-precision ranging, which not only integrates transceiver and receiver, but also uses a continuous zoom structure, which has the advantage of adjustable beam. Only 11 lenses are used, which reduces processing costs. A modular design method is used to decompose the optical path into a collimation module, a beam expansion module and a focusing module. Then the principles of each module are analyzed and designed and optimized respectively. From the emission point of view of the final optical system, the far-field divergence angle of the shaped outgoing beam under each variable magnification configuration is less than 0.3 mrad, and the outgoing spot diameter is continuously adjustable from 6.26 mm to 10.20 mm. For measurements within 50 m The diameter of the target and system illumination spot is less than 20 mm; from the reception point of view, the echo reception efficiency of each configuration in the 1° field of view is higher than 90%. The biggest feature of this optical system is the coaxial transceiver, which structurally eliminates the non-coaxial error between the transmitter and the receiver, which is beneficial to improving ranging accuracy and can provide a reference for the design of a laser radar system that integrates transceiver and transceiver.