Spurious-Free THz Frequency Measurement using Modulator-Based Optical Comb Source

Abstract

Recently, frequency measurement of THz waves using optical combs has been actively studied, because it is important for application to wireless communication or spectroscopic measurement and so forth. Previous techniques demonstrated by other groups were performed using mode-locked lasers [1,2]. By locking the repetition frequency of the mode-locked laser to the frequency standard, high precision measurement can be carried out. An issue in this technique is that the detection bandwidth of the THz frequency measurement system is limited to the repetition frequency. The typical value of the repetition frequency is on the order of MHz (typically 50–100 MHz). This limitation hinders broadband measurement in the applications mentioned above. We have proposed THz frequency measurement using a modulator-based optical comb source, because it has high frequency accuracy and flexibilities [3]. The repetition frequency (or comb spacing) of this type of source is on the order of GHz, so that the detection bandwidth in the THz frequency measurement is significantly extended. Moreover, optical combs with a GHz-order spacing enable to carry out line-by-line spectral synthesis, so that optical two-tone signals can be generated. In principle, by using the optical two-tone, the detection bandwidth is not limited by the comb spacing. In this paper, we report on comparison of detection bandwidths in the THz frequency measurement between two cases: one is using a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) and the other is using a mode-locked fiber laser.