Active bandwidth control of external-cavity system for improving mode-hop-free synchronous tuning characteristics of an external-cavity diode laser

Abstract

The mode-hop-free (MHF) tuning range and wavelength tuning rate are two critical characteristics for an external-cavity diode laser (ECDL). On the premise of stable dynamic response of the external-cavity system inherent in the ECDL, a wide MHF tuning range can be guaranteed by using the synchronous tuning principle. However, when high-frequency tuning, the mechanical vibration of the external-cavity system is excited due to limited system bandwidth, which will break the mode-matching of the internal and external-cavity and greatly decrease the MHF tuning range. Therefore, the system bandwidth of the external-cavity limits the tuning frequency and thus the wavelength tuning rate. In order to obtain a wide MHF range at high-frequency tuning, we used an active bandwidth control method with a resistance-capacitance (RC) filter to increase the system bandwidth and suppress the dynamic vibration of the external-cavity. With the RC filter, the system bandwidth of the external-cavity was increased from 290 rad/s to 5012 rad/s, an improvement of 17 times. The experimental results indicated that the MHF range at high-frequency tuning was significantly improved and a maximum wavelength tuning rate of 18.6 THz/s was obtained at tuning frequency of 100 Hz.