Lens-less fiber coupling of a 1550-nm mode-locked fiber laser light on a low-temperature-grown GaAs photoconductive antenna

Abstract

A fiber-coupled photoconductive antenna (PCA) is a powerful tool for portable terahertz (THz) systems using a compact 1550-nm mode-locked Er:fiber laser with a fiber output port. However, a low-temperature-grown GaAs (LTG-GaAs) PCA could not be used for this purpose due to the need for wavelength conversion of the 1550-nm light, regardless of the good characteristics for PCA. In this article, we achieved the fiber coupling of the 1550-nm mode-locked fiber laser light on a bowtie-shaped LTG-GaAs PCA detector without the need for wavelength conversion. While the two-step photo-absorption mediated by midgap states in the LTG-GaAs PCA makes it possible to use the 1550-nm light, the similarity of the size between the PCA gap spacing and the fiber core diameter enables the direct contact coupling between the fiber output tip and the PCA gap without any optical components. The developed lens-less fiber-coupled LTG-GaAs PCA detector was effectively applied for the absolute frequency measurement of continuous-wave THz radiation based on the photo-carrier THz frequency comb. The combination of the lens-less fiber-coupled LTG-GaAs PCA with the compact 1550-nm fiber laser will be useful for the portable apparatus for the absolute frequency measurement of practical CW-THz sources and other applications.