Telecentric f-theta scanning lens design for terahertz imaging systems

Beam scanning system using f-theta lenses which are telecentric in the image space offers a linear beam displacement with the incident beam deflection angle and a focused beam has a normal incidence angle on the target plane. Therefore, the beam from the source reflects and travels back through the system to the source. The reflection coefficient (S11) measurement can lead to the inspection of the target plane reflectivity. In this work, a telecentric f-theta lens is designed using ray tracing optimization and later, to verify ray tracing results, physical-optics simulations are performed on the system at 500 GHz. The designed lens has a thickness of 37 mm and shows acceptable f-theta distortion (0.5% for deflection angle of 10° with 22mm beam displacement) and telecentricity.

I. Introduction

F-theta lenses have diverse applications in beam scanning systems, including terahertz imaging systems where it is required to scan and image a planar target plane [1] –[3]. The beam displacement in f-theta lenses is a linear function of its deflection angle, resulting in a constant scan rate on the target plane. Furthermore, for the terahertz beam to be normally incident on the target plane, the lens is designed to be telecentric in the image space. The proposed optics to realize the imaging system is illustrated in figure 1. In this work, the shape of the scanning lens is optimized using ray tracing simulations, where the minimized merit function consists of the telecentric condition, beam spot size on the target plane, and the f-theta distortion. Later, using physical-optics simulations the beam profile on the target plane is calculated and verified. Fig. 1.

Scanning system geometry.