Design guidelines for ultrashort pulse generation by a Mamyshev regenerator.

We study numerically the possibility of using various gain-switched seed laser pulse parameters and fibers for a low-cost, all-fiber Mamyshev regenerator scheme. We find that for increasing pulse durations, sufficient spectral broadening will be difficult to achieve in practice and careful design of the system parameters is required for the regenerator to function. Furthermore, an optimal input peak power level can be defined for a given fiber and pulse duration that results from a balance of competing Kerr effect and stimulated Raman scattering. We also demonstrate experimental results of 3 ps pulse generation seeded by an 80 ps gain-switched diode. Our results pave the way for designing pulse-on-demand picosecond scale fiber sources for applications.

Fiber Vector Bend Sensor Based on Multimode Interference and Image Tapping.

A grating-less fiber vector bend sensor is demonstrated using a standard single mode fiber spliced to a multimode fiber as a multimode interference device. The ring-shaped light intensity distribution at the end of the multimode fiber is subject to a vector transition in response to the fiber bend. Instead of comprehensive imaging processing for the analysis, the image can be tapped out by a seven-core fiber spliced to the other end of the multimode fiber. The seven-core fiber is further guided to seven single mode fibers via a commercial fan-out device. By comparing the relative light intensities received at the seven outputs, both the bend radius and its direction can be determined. Experiment has shown that a slight bend displacement of 10 µm over a 1.2-cm-long multimode fiber in the X direction (bend angle of 0.382°) causes a distinctive power imbalance of 4.6 dB between two chosen outputs (numbered C4 and C7). For the same displacement in the Y direction, the power ratio between the previous two outputs C4 and C7 remains constant, while the imbalance between another pair (C3 and C4) rises significantly to 7.0 dB.

Evidence of Tm impact in low-photodarkening Yb-doped fibers.

In contrast to Yb/Al-doped fibers, the influence of very low Tm(2)O(3) concentrations (≥ 0.1 mol-ppm) on photodarkening (PD) is clearly detectable in Yb/P-doped fibers that are known to show little degradation effects. For Tm(2)O(3) additions of more than 50 mol-ppm, the measured PD loss is even similar to Yb/Al-doped fibers with comparable rare earth concentrations. Our work reveals the risk of color center generation by pumping at wavelengths of 915 nm or 976 nm even in Al-free Yb-doped fibers and emphasizes the importance of high purity of raw materials for the preparation of Yb laser fibers with expected very low PD.

In situ FBG inscription during fiber laser operation.

We demonstrate the inscription of a 266 nm UV femtosecond pulse-induced fiber Bragg grating (FBG) in an Yb-doped fiber during optical pumping at 976 nm and the initiation of lasing with increasing grating reflectivity. Output spectra show the emission of the pumped fiber changing from the broad-ranged amplified spontaneous emission in the nonlasing case to the narrow-range laser operation due to the enhancement of FBG reflectivity during inscription. The proposed technique enables the direct characterization and control of FBG performance in fiber lasers. After FBG fabrication, we investigate the spectral characteristics of the fiber laser for different laser powers and study the influence of a thermal treatment of the FBG.

Bending insensitivity of fiber Bragg gratings in suspended-core optical fibers.

This Letter presents simulation and experimental results that explore bending insensitivity of fiber Bragg gratings in suspended-core optical fibers. The implementation of thin silica bridge in the fibers enhances index contrast of the fiber core and reduces bending-induced strain transfer to the fiber core. This fiber design lead to a reduction of over 7 times in strain-induced fiber Bragg grating resonant peak shifts in the suspended-core fiber compared with that in standard telecommunication fiber, and an 0.14 dB bending loss at a bending radius of 6.35 mm.

Suspended-core fiber Bragg grating sensor for directional-dependent transverse stress monitoring.

This Letter presents simulation and experimental results of orientation-dependent transverse stress fiber sensors using fiber Bragg gratings (FBGs) inscribed in four-hole suspended-core fibers. Resonant peak shifts and splitting of FBGs were studied as functions of the applied transverse load and fiber orientation. Both simulation and experimental results revealed that the response of FBGs in suspended-core fibers is sensitive to both the orientation and magnitude of an applied transverse stress.