A high-linear sweep laser source to interrogate sub-terahertz range fiber sensors for dynamic strain sensing applications

摘要

Sub-terahertz range fiber sensors have been well investigated for distributed stain sensing applications. Due to the use to sub-millimeter range structures, high accuracy measurement using relative small interrogation bandwidth (~ 100 GHz) can be achieved. The interrogation system is based on optical frequency domain reflectometry (OFDR), where the key component is the high-linear frequency sweep laser source. Previously the external cavity lasers have been employed as the frequency sweep sources. The external cavity lasers are capable to sweep over large interrogation bandwidth (> 3 THz). However, compared with the 100 GHz resonation period of sub-terahertz range fiber sensors with a pitch length of 1mm, this broad sweep bandwidth is unnecessary. Besides, the external cavity lasers require the use of moving mechanical components, which limits the system update rate and increases the system complexity. This paper presents a design of a high linear sweep laser source suitable for sub-terahertz range fiber sensors. A distributed feedback laser is employed as the frequency sweep source based on the injection current modulation technique, and the sweep velocity is locked at a constant value (14.2 GHz/ms) using a semi-digital feedback control system. A high-linear sweep bandwidth of 117.69 GHz with a system update rate of 50 Hz has been demonstrated. In addition, a dynamic experiment was conducted to demonstrate the system distributed strain sensing capability. The proposed system holds the potential for dynamic structural health monitoring.