Frequency is one of the most important physical quantities of electromagnetic (EM) waves. With the development of terahertz (THz) technology, high-precision measurement of THz frequency is required in THz laser development, wireless communication and ultra fine spectrum measurement. The traditional Fabry-Perot (F-P) interferometry and heterodyne detection method are both difficult to achieve high-precision measurement of THz frequency. Within the range of light wave band, the femtosecond optical frequency comb has long been applied to the light wave frequency measurement due to its extremely high accuracy and stability. By using frequency comb method, measurement with accuracy in the order of 10−11 can also be achieved in THz band. To generate THz frequency combs with stable and controllable frequency, it is required to conduct precise stabilization control on repetition frequency of the femtosecond laser. As a result, some special designs are needed for the femtosecond laser in addition to repetition frequency control devices, including the reference signal source, servo-control module, HV drive module, temperature control module, etc., resulting in a rather complicated system. In this paper, a new method for THz frequency measurement by using an unstabilized femtosecond laser is introduced. The laser is free running and the repetition frequency continuously reduces approximately 8 kHz in 6 h, which is the result of a lengthened laser cavity due to the thermal expansion caused by temperature rise after the laser has been switched on. The repetition frequency and beat signal frequency are simultaneously and continuously measured by two frequency counters. The THz frequency can be calculated from the data with accuracy in the order of 10−10 . Although the measurement precision is reduced by one order compared with that obtained by using stabilized femtosecond laser, the system is greatly simplified. The femtosecond laser and complicated repetition frequency control devices no longer need to be specifically designed. This new method will greatly expand the applicable scope of the frequency comb method in measuring THz frequency.%频率是电磁波最重要的一个基本物理量，随着THz技术的发展，在光源研制、宽带通信、超精细光谱测量等领域都对THz频率的高精度测量提出了要求。传统的Fabry-Perot干涉法与外差探测法难以实现THz频率的高精度测量，频率梳方法虽然测量精度很高，但测量系统复杂。本文提出一种利用重复频率自由漂移的飞秒激光器实现太赫兹频率精密测量的新方法。通过对非锁定的飞秒激光器的重复频率和THz拍频频率进行同时连续采集与计算，得到被测THz频率，测量精度可以达到10−10量级。无需对飞秒激光重复频率进行复杂的锁定控制，测量系统大大简化。
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