Ria G. Krämer

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.

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.

Second generation OH suppression filters using multicore fibers

Ground based near-infrared observations have long been plagued by poor sensitivity when compared to visible observations as a result of the bright narrow line emission from atmospheric OH molecules. The GNOSIS instrument recently commissioned at the Australian Astronomical Observatory uses Photonic Lanterns in combination with individually printed single mode fibre Bragg gratings to filter out the brightest OH-emission lines between 1.47 and 1.70μm. GNOSIS, reported in a separate paper in this conference, demonstrates excellent OH-suppression, providing very “clean” filtering of the lines. It represents a major step forward in the goal to improve the sensitivity of ground based near-infrared observation to that possible at visible wavelengths, however, the filter units are relatively bulky and costly to produce. The 2nd generation fibre OH-Suppression filters based on multicore fibres are currently under development. The development aims to produce high quality, cost effective, compact and robust OH-Suppression units in a single optical fibre with numerous isolated single mode cores that replicate the function and performance of the current generation of “conventional” photonic lantern based devices. In this paper we present the early results from the multicore fibre development and multicore fibre Bragg grating imprinting process.

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.

Femtosecond pulse inscription of a selective mode filter in large mode area fibers

We present a selective mode filter inscribed with ultrashort pulses directly into a few mode large mode area (LMA) fiber. The mode filter consists of two refractive index modifications alongside the fiber core in the cladding. The refractive index modifications, which were of approximately the same order of magnitude as the refractive index difference between core and cladding have been inscribed by nonlinear absorption of femtosecond laser pulses (800 nm wavelength, 120 fs pulse duration). If light is guided in the core, it will interact with the inscribed modifications causing modes to be coupled out of the core. In order to characterize the mode filter, we used a femtosecond inscribed fiber Bragg grating (FBG), which acts as a wavelength and therefore mode selective element in the LMA fiber. Since each mode has different Bragg reflection wavelengths, an FBG in a multimode fiber will exhibit multiple Bragg reflection peaks. In our experiments, we first inscribed the FBG using the phase mask scanning technique. Then the mode filter was inscribed. The reflection spectrum of the FBG was measured in situ during the inscription process using a supercontinuum source. The reflectivities of the LP01 and LP11 modes show a dependency on the length of the mode filter. Two stages of the filter were obtained: one, in which the LP11 mode was reduced by 60% and one where the LP01 mode was reduced by 80%. The other mode respectively showed almost no losses. In conclusion, we could selectively filter either the fundamental or higher order modes.

Mitigation of stimulated Raman scattering in high power fiber lasers using transmission gratings

The average output power of fiber lasers have been scaled deep into the kW regime within the recent years. However a further scaling is limited due to nonlinear effects like stimulated Raman scattering (SRS). Using the special characteristics of femtosecond laser pulse written transmission fiber gratings, it is possible to realize a notch filter that mitigates efficiently this negative effect by coupling the Raman wavelength from the core into the cladding of the fiber. To the best of our knowledge, we realized for the first time highly efficient gratings in large mode area (LMA) fibers with cladding diameters up to 400 μm. The resonances show strong attenuation at design wavelength and simultaneously low out of band losses. A high power fiber amplifier with an implemented passive fiber grating is shown and its performance is carefully investigated.

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.

Femtosecond inscribed mode modulators in large mode area fibers: experimental and theoretical analysis

We present the experimental and theoretical analysis of a mode modulator in a few mode LMA fiber. The mode modulator consists of a section with a modified refractive index alongside the fiber core in the cladding, disturbing the guidance of the modes in the core. The extent of excitation of these disturbed modes depends on the overlap of the excited undisturbed and disturbed modes. At the end of the modulator, undisturbed modes will be excited again in the fiber core, in dependency of the spatial field distribution of the disturbed modes at the boundary. In the mode modulator disturbed higher order modes lead to modal interference, causing a dependency of the spatial distribution of the light in the mode modulator on the propagation length of the disturbed modes. Hence, the modal output field depends on the length of the mode modulator. For the experiments, the mode modulator was inscribed directly into the LMA fiber with ultrashort laser pulses. During the inscription process the modal content at the end of the fiber was measured using a computer generated hologram as a correlation filter. In dependency of the length of the modulator strong oscillations between the content of the fundamental and the higher order modes are observable. In the case of an initially excited fundamental mode, its content could be reduced to below 5%, whereat the content of the LP11 modes was up to 90%. While measurement and simulation show qualitative agreement, differences are caused by inhomogeneities of the refractive index modifications.