100 W, tunable in-band thulium fiber amplifier pumped by incoherently combined 1.9 µm fiber lasers.

We detail the design and performance of a high efficiency in-band pumped thulium fiber amplifier operating at the 100 W level. Using a novel pumping architecture based on three incoherently combined thulium fiber oscillators at 1904 nm and a seed laser tunable from 1970-1990 nm, efficient amplification is demonstrated in a high dopant concentration 25/65/250 µm thulium fiber. Here we use the 65 µm pedestal surrounding the core as a pump cladding to increase the cladding to core overlap and improve the overall pump absorption. Up to 89% slope efficiency is obtained with ∼100 W output power at 1990 nm. These results indicate that in-band pumping is a viable route to circumvent the thermal limitations associated with 793 nm diode pumping and provide a pathway for development of multi-kW laser sources in the 2 µm spectral window.

First steps in the development of next-generation chirped volume Bragg gratings by means of fs laser inscription in fused silica.

The first steps towards the development and characterization of next-generation chirped volume Bragg gratings (CVBGs) by means of fs laser inscription were made. Based on the phase mask inscription technique we realized CVBGs in fused silica with a 3 × 3 mm2 aperture and a length of almost 12 mm with a chirp rate of ∼190 ps/nm around a central wavelength of 1030.5 nm. Strong mechanical stresses induced serious polarization and phase distortions of the radiation. We show a possible approach to solution of this problem. The change in the linear absorption coefficient associated with local modification of fused silica is quite small, enabling utilization of this type of gratings in high average power lasers.

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.

Dispersion tailoring of femtosecond laser written chirped fiber Bragg gratings by selective femtosecond laser post-processing.

We present the tuning of the dispersion properties of a femtosecond (fs) laser inscribed chirped fiber Bragg grating (CFBG), realized by selectively modifying the refractive index of the already inscribed CFBG by fs laser post-processing. This Letter demonstrates for the first time, to the best of our knowledge, a flexible approach for tailoring higher-order dispersion terms of a fs inscribed CFBG via fs post-processing of selected grating regions, thus paving the way, e.g., for applications in dispersion management of ultrashort pulse fiber lasers.

Extremely robust femtosecond written fiber Bragg gratings for an ytterbium-doped fiber oscillator with 5 kW output power.

We present highly robust fiber Bragg gratings (FBGs) in passive large-mode-area fibers for kilowatt fiber laser systems. The gratings were inscribed directly through the fiber coating using near-infrared femtosecond laser pulses and then implemented in an all-fiber ytterbium-doped single-mode oscillator setup reaching up to 5 kW signal output power. The untreated cooled FBGs showed thermal coefficients as low as ${1}\;{\rm K}\;{{\rm kW}^{ - 1}}$1KkW-1, proving excellent qualification for the implementation into robust high-power fiber laser setups.

Efficient long period fiber gratings inscribed with femtosecond pulses and an amplitude mask.

We present efficient long period fiber gratings written with femtosecond laser pulses at 800 nm and an amplitude mask, to the best of our knowledge, for the first time. The measured transmission spectra depict strong resonances, while the total grating length and polarization-dependent loss could be significantly reduced compared to previous results. Two gratings are exemplarily shown-one in a standard single mode, and one in a large-mode-area fiber revealing a predictable spectrum without intermediate peaks due to the suppression of coupling to asymmetric higher-order cladding modes.

Control of higher-order cladding mode excitation with tailored femtosecond-written long period fiber gratings.

We report on the detailed investigation of the core to cladding mode coupling in femtosecond-written long period fiber gratings (LPFG). It is shown that the excitation of higher-order cladding modes with strong selectivity and high precision is possible. The coupling behavior of several gratings, as well as its dependence on the modified core cross-section, is determined theoretically and confirmed experimentally by its spectral response. The presented tool paves the way for a completely new class of tailored LPFGs for different fiber integrated devices.

Femtosecond written fiber Bragg gratings in ytterbium-doped fibers for fiber lasers in the kilowatt regime.

We investigate the high-power durability of fiber Bragg gratings written directly into an ytterbium-doped large mode area fiber using ultrashort laser pulses. The gratings were successfully integrated as a high reflector into an oscillator setup reaching up to 1.9 kW signal output power with an efficiency of 87%. Defect states induced during the inscription process could be drastically reduced by a self-annealing process resulting in a stable laser performance.

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.

Flexible femtosecond inscription of fiber Bragg gratings by an optimized deformable mirror.

The period of fiber Bragg gratings is adapted by shaping the wavefronts of ultrashort laser pulses applied in a phase mask inscription technique. A specially designed deformable mirror, based on a dielectric substrate to withstand high peak powers, is utilized to deform the wavefront. A shift of about 11 nm is demonstrated for a Bragg wavelength around 1550 nm.

Minimizing residual spectral drift in laser diode bars using femtosecond-written volume Bragg gratings in fused silica.

Ultrashort laser pulses are used to inscribe volume Bragg gratings (VBGs) into fused silica. These VBGs demonstrate excellent performance for the external stabilization of laser diode bars. The stabilized system emits at a wavelength of 969 nm with a signal width (FWHM) of 100 pm and shows a spectral drift as low as 24 pm for a change in output power of 45 W for a grating surface area of 10 mm2.

Variable wavefront tuning with a SLM for tailored femtosecond fiber Bragg grating inscription.

We report on the inscription of fiber Bragg gratings using femtosecond laser pulses and the phase-mask technique. The wavefront of the inscription laser is variably tuned with a spatial light modulator (SLM). By applying Fresnel lenses with different focal lengths, the period of the fiber Bragg gratings could be shifted. A linear change of the grating period for a FBG inscribed with a third-order deformed wavefront and a quadratic-period behavior for a fourth-order wavefront could be verified experimentally for the first time.

Discrete nonplanar reflections from an ultrashort pulse written volume Bragg grating.

In this Letter, we present a direct writing technique for two-dimensional periodic volume Bragg gratings (VBGs) in fused silica based on the phase mask technology, ultrashort laser pulses, and three-beam interference. An algorithm to predict the grating pattern and its diffraction behavior under collimated, spectral broad illumination is developed. The predicted data are in good agreement with the measurements.

Detuning in apodized point-by-point fiber Bragg gratings: insights into the grating morphology.

Point-by-point (PbP) inscription of fiber Bragg gratings using femtosecond laser pulses is a versatile technique that is currently experiencing significant research interest for fiber laser and sensing applications. The recent demonstration of apodized gratings using this technique provides a new avenue of investigation into the nature of the refractive index perturbation induced by the PbP modifications, as apodized gratings are sensitive to variation in the average background index along the grating. In this work we compare experimental results for Gaussian- and sinc-apodized PbP gratings to a coupled-mode theory model, demonstrating that the refractive index perturbation induced by the PbP modifications has a negative contribution to the average background index which is small, despite the presence of strong reflective coupling. By employing Fourier analysis to a simplified model of an individual modification, we show that the presence of a densified shell around a central void can produce strong reflective coupling with near-zero change in the average background index. This result has important implications for the experimental implementation of apodized PbP gratings, which are of interest for a range of fiber laser and fiber sensing technologies.

Femtosecond direct-writing of low-loss fiber Bragg gratings using a continuous core-scanning technique.

We report the inscription of low-loss fiber Bragg gratings using focused femtosecond (fs) pulses and a continuous core-scanning technique. This direct-write technique produces high-fidelity Type I-IR gratings that share the inherent advantages of other direct-write methods, such as the point-by-point (PbP) method, for which the grating period is a free parameter. However, here we demonstrate an order of magnitude improvement in scattering loss compared to PbP gratings, to a level comparable with that of phase-mask-based fs inscription. A first-order grating was inscribed in standard telecommunications fiber with -49 dB transmission at the Bragg wavelength and 0.1 dB broadband scattering loss. Potential application of these gratings to large-mode-area fibers and chirped grating fabrication are highlighted.

Highly polarized all-fiber thulium laser with femtosecond-laser-written fiber Bragg gratings.

We demonstrate and characterize a highly linearly polarized (18.8 dB) narrow spectral emission (<80 pm) from an all-fiber Tm laser utilizing femtosecond-laser-written fiber Bragg gratings. Thermally-dependent anisotropic birefringence is observed in the FBG transmission, the effects of which enable both the generation and elimination of highly linearly polarized output. To our knowledge, this is the first detailed study of such thermal anisotropic birefringence in femtosecond-written FBGs.

Cladding mode coupling in highly localized fiber Bragg gratings II: complete vectorial analysis.

Highly localized fiber Bragg gratings can be inscribed point-by-point with focused ultrashort pulses. The transverse localization of the resonant grating causes strong coupling to cladding modes of high azimuthal and radial order. In this paper, we show how the reflected cladding modes can be fully analyzed, taking their vectorial nature, orientation and degeneracies into account. The observed modes' polarization and intensity distributions are directly tied to the dispersive properties and show abrupt transitions in nature, strongly correlated with changes in the coupling strengths.