Pulse-repetition-frequency-variable mode-locked fiber laser

Abstract

Mode locking is the most effective technique for generating pulses shorter than a sub nanosecond, and is used in practical applications such as spectroscopy and micro-fabrication. The pulse repetition frequency (PRF) of a mode-locked laser is determined by the cavity length, which cannot be changed without degrading its mechanical stability. In our previous work, we invented an electronically tunable laser that uses an acousto-optic tunable filter (AOTF) [1]. It provides fast wavelength tuning and mechanically stable operation in the gain-switching regime of a Ti:sapphire laser. Such laser was also realized in a CW regime [2]. In this scheme, mode-lock operation was achieved under appropriate conditions. Using a fiber laser, it is easy to construct multiple laser cavities using fiber Bragg gratings (FBGs) for different wavelengths with a specific cavity length coaxially. Therefore, the PRF of the mode-lock pulse train can be changed by varying the AOTF diffraction wavelength in the fiber laser. Figure 1 shows the experimental setup. The laser cavity was constructed with a FBG train which consists of three FBGs for different reflection wavelength, λ 1 =1056 nm, λ 2 =1060 nm, and λ 3 =1064 nm, and an output coupling mirror (OC) located in free-space. Yb-doped fiber was used as gain medium and pumped by a single mode fiber-coupled laser diode. A collimator, a polarization controller, and an AOTF were inserted between an anti-reflective coated FC connector and the OC. The total optical length of the cavities for λ 1 , λ 2 , and λ 3 were 9960, 8600, and 6900 mm, respectively. The laser diode pump power was 100 mW. Changing the RF frequency applied to the AOTF varied the output power and the wavelength, as shown in Fig. 2(a). The dot, rectangle, and triangle indicate the RF frequency regions in which the laser is emitted at λ 1 , λ 2 , and λ 3 , respectively. The wavelength was locked by the reflection spectrum of each FBG, while the broad diffraction bandwidth of the AOTF allowed laser emission other than the RF frequency of the diffraction peak of each FBG. Near each peak, we found a pulse train caused by mode locking; the pulse trains are shown in Fig. 2 (b). No other frequency components were observed at λ 2 , while low frequency components were observed for the λ 1 and λ 3 pulses. The PRF of the pulse train corresponded to the photon roundtrip time for each cavity length. The wavelength could be switched among the three wavelengths by applying an appropriate RF frequency.