Design of wide field of view and broad spectrum objective system for terahertz imaging

Terahertz (THz) wave has great potential applications in the fields of security, material, biomedicine and nondestructive testing, due to its low energy, high penetrability, nondestructive property and fingerprint characteristics. In this paper, a high-performance objective system was presented and designed for THz camera. The THz objective system has an effective focal length of 39.5 mm, a F-number of 0.9, a full field angle of 24°, and a working band covering 70 - 200 μm (frequency range from 1.5 to 4.28 THz). In the system, a coaxial structure consisted of three spherical lens and a piece of aspherical lens was used. The design result shown that the root mean square radius (RMS Radius) of the sport diagrams were basically within the airy disk radius. At a cut-off frequency of 5 lp/mm, the modulation transfer function (MTF) values in the full field of view were above 0.22, which was close to the diffraction limited curve. In summary, the designed objective system has good imaging quality and high resolution, which can meet the performance requirements of large-field THz imaging systems, and has good application prospects in THz microscopic imaging.