Off-axis parabolic mirrors (OAPMs) are widely used in the THz and mm-wave communities for spectroscopy and imaging applications, as a result of their broadband, low-loss operation and high numerical apertures. However, the aspherical shape of an OAPM creates significant geometric aberrations: these make achieving diffraction-limited performance a challenge, and lower the peak electric field strength in the focal plane. Here, we quantify the impact of geometric aberrations on the performance of the most widely used spectrometer designs, by using ray tracing and physical optics calculations to investigate whether diffraction-limited performance can be achieved in both the sample and the detector plane. We identify simple rules, based on marginal ray propagation, that allow spectrometers to be designed that are more robust to misalignment errors, and which have minimal aberrations for THz beams. For a given source, this allows the design of optical paths that give the smallest THz beam focal spot, with the highest THz electric field strength possible. This is desirable for improved THz imaging, for better signal-to-noise ratios in linear THz spectroscopy and optical-pump THz-probe spectroscopy, and to achieve higher electric field strengths in non-linear THz spectroscopy.

离轴抛物面镜 (OAPM) 因其宽带、低损耗运行和高数值孔径而广泛应用于太赫兹和毫米波领域的光谱和成像应用。然而，OAPM 的非球面形状会产生显着的几何像差：这使得实现衍射极限性能成为一项挑战，并降低了焦平面中的峰值电场强度。在这里，我们通过使用光线追踪和物理光学计算来研究是否可以在样品和探测器平面中实现衍射极限性能，从而量化几何像差对最广泛使用的光谱仪设计性能的影响。我们基于边缘射线传播确定了简单的规则，允许设计出对未对准误差更稳健的光谱仪，并且对太赫兹光束的像差最小。对于给定的光源，这使得光路设计能够提供最小的太赫兹光束焦斑，并具有尽可能高的太赫兹电场强度。这对于改进太赫兹成像、线性太赫兹光谱和光泵太赫兹探针光谱中更好的信噪比以及在非线性太赫兹光谱中实现更高的电场强度是理想的。