Mode-locked fiber laser based on a small-period long-period fiber grating inscribed by femtosecond laser

Abstract

We demonstrate stable mode-locked pulses in an erbium-doped fiber laser (EDFL) using a femtosecond laser-inscribed small-period long-period grating (SP-LPG). The SP-LPG has a period of 25 µm and a length of 2.5 mm. The polarization dependent loss (PDL) of the SP-LPG reaches 20 dB at the wavelength of 1556 nm and 25 dB at the wavelength of 1607 nm, which is sufficient to trigger the mode-locking mechanism. In addition, a mode-locked fiber laser (MLFL) based on the SP-LPG has been demonstrated to generate 1.58-ps pulses at 1577 nm with a bandwidth of 4 nm and a repetition rate of 1.54 MHz. The signal-to-noise ratio (SNR) of 50 dB shows the high stability of this system. This work indicates various potential applications of the SP-LPG in ultra-fast laser technologies due to its simple fabrication, compact structure, and high damage threshold.

All-Fiber Narrow-Bandwidth Mode-Locked Laser Based on Polarization-Dependent Helical Long-Period Grating

“…The mode-locked threshold of the laser is approximately 110 mW. In comparison to other grating-based polarization devices [12,22], the PMF-HLPG-based laser exhibits a lower mode-locking threshold. By reducing the loss of the other device in the cavity, the mode-locking threshold of the laser can be further reduced.…”

“…Despite SA materials offering significant advantages, their implementation in fiber lasers is accompanied by certain limitations that affect the output performance due to their low damage threshold and degradation over time. In response to the growing demand for diverse applications, other fiber laser construction based on the optical Kerr effect has been proposed and adopted, including nonlinear optical loop mirrors [7,8], nonlinear amplification loop mirrors [9], nonlinear multimode interference [10], and nonlinear polarization rotation (NPR) [11,12], etc. Among these options, the NPR mechanism has emerged as a particularly effective and mature approach [13][14][15][16].…”

“…In response to the aforementioned challenges, a range of polarizationdependent all-fiber devices have been proposed and developed as a promising alternative. These devices include a segment of polarizing fiber [17], microfiber polarizer [18,19], singlemode-fiber coil [20], fiber grating [12,[21][22][23], and so on. Each of them has its associated advantages and drawbacks.…”

“…Nevertheless, the fabrication of a 45 • TFG requires the optical fiber to have strong photosensitivity and demands a high level of precision in the writing system. Then, Huang et al and Li et al [12,22] developed MLFLs utilizing a femtosecond laser inscribed in-fiber Brewster device and a small-period long-period grating (LPG), respectively. Nevertheless, complicated femtosecond laser inscription systems are not very affordable and the gratings suffer from a high insertion loss, with values exceeding 7 dB and 15 dB at a wavelength of 1560 nm, respectively.…”

“…However, it is important to note that the device exhibited significant insertion loss, resulting in a high mode-locking threshold exceeding 300 mW [24], while the polarization-maintaining fiber (PMF) helical LPG (HLPG) could enable effective polarization mode coupling, allowing it to function as an all-fiber polarizer [25]. In comparison to other types of gratings, such as the 45 • TFG [21,22], small-period LPG [12], and chiral LPG [23], the PMF-HLPG inscribed by a CO 2 -laser offers advantages in terms of simpler and faster manufacturing, a lower insertion loss, and a low cost. Moreover, adopted helical refractive index modulation could also remove the necessity of CO 2 -laser exposure alignment along the axis of PMFs during grating inscription, resulting in an improved grating writing efficiency.…”