Objective Fourier-domain mode locking (FDML) is a new type of optical spectrum modulation technique that outputs a narrow linewidth continuous frequency-swept ( or wavelength-swept) laser, which differs from the traditional intensity modulation mode-locking technique. By appropriately controlling the relationship between the scanning speed of the tunable optical filter and the length of the laser cavity, an FDML frequency-swept laser can achieve stable simultaneous oscillation of all longitudinal modes inside the cavity within the filter scanning range, consequently achieving an ultrahigh-speed frequency sweep. Most reported FDML frequency-swept fiber lasers (FSFLs) are fabricated using semiconductor optical amplifiers, which can provide a broad wavelength-swept range but exhibit a relatively wide instantaneous linewidth. The long energy-level lifetime and homogeneous broadening gain effect of rare-earth ions doped silica glass achieve strong wavelength-dependent characteristic, which are expected to realize the narrow instantaneous linewidth of an FDML FSFL. However, FDML FSFLs based on the rare-earth-doped fibers have rarely been reported. Additionally, although many reports can be found in the literatures, no study comprehensively evaluates the theory and mechanism analysis, design, construction, and performance characterization of FDML FSFLs. Therefore, proposing a set of comprehensive study methods on FDML FSFLs is crucial. Considering FDML FSFLs based on erbium-doped fibers (EDF) as an example, we systematically demonstrated the theoretical and experimental research processes on FDML FSFLs and achieved a high-quality frequency-swept laser output using the EDF laser (EDFL). To the best of our knowledge, this is the first such study to date. Methods The basic theoretical operating principle of the FDML technique and the influence of laser cavity length matching and dispersion management on the performance of FDML FSFLs were analyzed. The operating characteristics of several common optical filters were introduced, and the high-frequency operating capability of the fiber Fabry-Perot tunable filter (FFP-TF) was studied (Fig. 2). Moreover, the advantages of EDFs as the gain medium of FDML FSFLs are examined. An FDML frequency-swept EDFL based on a ring cavity configuration was designed and fabricated (Fig. 3) with an FFP-TF as the FDML scanning optical filter, and the electro-optic modulator-based time-gating technique was used to characterize the frequency-swept laser output. Results and Discussions The characteristics of the proposed FDML FSFL were experimentally studied in detail, including the mode-locking wavelength range, single-direction frequency sweeping, dispersion, filter driving frequency deviation, and laser instantaneous linewidth. We found that the gain level of EDF directly affects the wavelength sweeping range ( Fig. 4). Further, the overall spectral power distributions for forward and backward frequency sweeps differ owing to the nonlinearity of the delay fiber (Fig. 5). A greater pure dispersion in the laser cavity induces a higher sensitivity of the output power to the FFP-TF 's driving frequency deviation ( Fig. 6). Moreover, a larger FFP-TF 's driving frequency deviation induces a higher broadening effect of the swept laser' s instantaneous linewidth (Fig. 8). Therefore, to obtain the best operating condition of the proposed FDML FSFL, an EDF with high luminous efficiency should be selected as the gain medium, either a delay fiber with low nonlinearity and zero dispersion should be selected or a laser cavity with zero dispersion should be designed, and the driving frequency of the FFP-TF should be fixed exactly at the base oscillating frequency of the fiber laser. Conclusions Considering the EDFL as an example, the theoretical and experimental research on FDML FSFLs is systematically demonstrated for the first time. On the one hand, the theory, principle, and method for matching the length of the laser cavity and the scanning rate of the tunable filter, intracavity dispersion management, performance characterization of tunable filter, analysis of laser gain medium, and system design of the FDML FSFL are studied in detail. On the other hand, the maximum wavelength sweeping range of the FDML mechanism using EDF as the gain, the performance of single-direction frequency sweeping, the influence of different delay fibers and FFP-TF' s driving frequency deviation on the laser output power, and the effect of the driving frequency deviation on the laser' s instantaneous linewidth for different sweeping directions of the FDML swept laser are experimentally studied and discussed. Consequently, for the first time, an FDML frequency-swept EDFL is realized with the scanning range, optical signal-to-noise ratio, scanning rate, and instantaneous linewidth of 3.072 nm, 57.31 dB, 62.918 kHz, and 4.28 GHz, respectively. In the future, we will focus on the development of large-mode-area high-gain fibers and ultranarrow-band high-speed scanning filters, as well as on the study of more efficient scanning mechanisms. Our work emphasizes the research method and combines the theoretical and experimental findings. This work is expected to provide guidance, particularly to researchers initially studying FDML frequency/wavelength-swept lasers.
