Wavelength-stabilized tunable mode-locked thulium-doped fiber laser beyond 2 µm.

We report the development of a widely tunable mode-locked thulium-doped fiber laser based on a robust chirped fiber Bragg grating (CFBG). By applying mechanical tension and compression to the CFBG, an overall tunability of 20.1 nm, spanning from 2022.1 nm to 2042.2 nm, was achieved. The observed mode-locked pulse train from this fiber laser has a repetition rate of 9.4 MHz with an average power of 12.6 dBm and a pulse duration between 9.0 ps and 12.8 ps, depending on the central wavelength. To the best of our knowledge, this is the first demonstration of a tunable mode-locked thulium-doped fiber laser operating beyond 2 µm using a CFBG as a wavelength-selective element.

Long wavelength operation of a dysprosium fiber laser for polymer processing.

We demonstrate operation of a mid-infrared dysprosium-doped fiber laser with emission at 3388 nm, representing the longest wavelength yet achieved from this class of laser, to the best of our knowledge. Oscillation far removed from the Dy3+ gain peak around 3 µm is achieved through the design of a high feedback optical cavity employing a directly inscribed fiber Bragg grating as the output coupler. Laser performance is characterized by a slope efficiency with respect to injected pump power of 38% and maximum output power of 134 mW, an improvement of at least three orders of magnitude over prior attempts at long wavelength Dy3+ fiber laser operation. This wavelength coincides with a maximum in the absorption coefficient of PMMA, which we exploit for preliminary demonstration of the utility of this source in polymer processing.

Femtosecond laser direct-written fiber Bragg gratings with high reflectivity and low loss at wavelengths beyond 4 µm.

We report on the fabrication of, to the best of our knowledge, the first highly reflective fiber Bragg gratings for the 4 µm wavelength range. A second-order grating with a coupling coefficient (κ) of 230m-1, losses <0.25dB/cm, and a bandwidth of approximately 3 nm was inscribed into the core of a passive indium fluoride (InF3) fiber using a femtosecond (fs) laser. Thermal annealing of this grating at a temperature of 150°C for 90 min resulted in the enhancement of κ to 275m-1. Further, we show that InF3 fibers respond very differently to irradiation with fs laser pulses as compared to ZBLAN fibers and that this difference manifests itself in a significantly larger process window for inscription and in the formation of a more complex refractive index profile that is believed to be caused by the larger nonlinearity of InF3. This Letter paves the way to the development of new wavelength stabilized all-fiber mid-infrared lasers beyond 4 µm.

Optimized laser-written ZBLAN fiber Bragg gratings with high reflectivity and low loss.

We report the direct femtosecond laser inscription of type-I fiber Bragg gratings (FBGs) into the core of soft-glass ZBLAN fibers. We investigate and compare various fabrication methods such as single pass (line by line), double pass, and stacking (plane by plane) to create the highest reflectivity FBGs (99.98%) for mid-infrared (mid-IR) applications. In addition, we experimentally demonstrate how the parameters that influence the coupling coefficient, i.e., refractive index modulation and overlap factor, can be controlled in these gratings to specifically tailor the FBG properties. The performance of the direct-written type-I gratings after 6 h of annealing is further analyzed, and the reflectivity increases by approximately 10 dB. To the best of our knowledge, this is the first demonstration of temperature-stable mid-IR FBGs with highest coupling coefficient (464 m-1) and lowest loss (<0.5 dB/cm) without the use of an expensive phase mask.

Realization of aperiodic fiber Bragg gratings with ultrashort laser pulses and the line-by-line technique.

We demonstrate the fabrication of aperiodic fiber Bragg gratings (AFBGs) for their application as filter elements. Direct inscription was performed by focusing ultrashort laser pulses with an oil-immersion objective into the fiber core and utilizing the line-by-line technique for flexible period adaptation. The AFBGs inscribed allow for the suppression of 10 lines in a single grating and are in excellent agreement with simulations based on the specific design. Applications in astronomy for the suppression of hydroxyl emission lines are discussed.

Direct inscription of Bragg gratings into coated fluoride fibers for widely tunable and robust mid-infrared lasers.

We report the development of a widely tunable all-fiber mid-infrared laser system based on a mechanically robust fiber Bragg grating (FBG) which was inscribed through the polymer coating of a Ho3+-Pr3+ co-doped double clad ZBLAN fluoride fiber by focusing femtosecond laser pulses into the core of the fiber without the use of a phase mask. By applying mechanical tension and compression to the FBG while pumping the fiber with an 1150 nm laser diode, a continuous wave (CW) all-fiber laser with a tuning range of 37 nm, centered at 2870 nm, was demonstrated with up to 0.29 W output power. These results pave the way for the realization of compact and robust mid-infrared fiber laser systems for real-world applications in spectroscopy and medicine.