Christian Voigtländer

Point-by-point inscription of apodized fiber Bragg gratings.

We demonstrate apodized fiber Bragg gratings (FBGs) inscribed with a point-by-point (PbP) technique. We tailor the grating phase and coupling amplitude through precise control over the longitudinal and transverse positions of each laser-inscribed modification. This method of apodization is facilitated by the highly localized, high-contrast modifications generated by focused IR femtosecond laser inscription. Our technique provides a simple method for the design and implementation of PbP FBGs with complex apodization profiles.

550-mW Output Power From a Narrow Linewidth All-Phosphate Fiber Laser

We present a compact monolithic all-phosphate glass fiber laser with up to 550 mW of output power operating on a single longitudinal mode. We measured a linewidth of less than 60 kHz and relaxation oscillation peak amplitudes below -100 dB/Hz without active RIN-supression. The laser cavity has been formed by inscribing fiber Bragg gratings (FBG) directly into heavily Er3+ Yb3+ doped phosphate glass fiber using femtosecond laser pulses and a phasemask. The compact form factor and higher output power combined with the low noise and narrow line width characteristic make this laser an ideal candidate for ranging, interferometry and sensing applications.

Ultrashort pulse inscription of tailored fiber Bragg gratings with a phase mask and a deformed wavefront [Invited]

We report on the inscription of chirped fiber Bragg Gratings (FBGs) with a phase mask and a deformed wavefront using a femtosecond laser. A qualitative model is developed to predict the behavior of the resulting grating period for a deformed wavefront. In addition the quantitative change of the period was simulated based on a ray optical solution of the diffraction behind the phase mask. For deforming the wavefront experimentally a cylindrical tuning lens was used. Tilting of the lens increased the higher order aberrations like coma and spherical aberration, which leads to chirped FBGs. A chirped FBG with a FWHM bandwidth of 2.5 nm could be realized. The change of the resulting fiber Bragg grating period was measured using a side diffraction setup yielding good agreement with the measured spectra.

Fiber based polarization filter for radially and azimuthally polarized light.

We demonstrate a new fiber based concept to filter azimuthally or radially polarized light. This concept is based on the lifting of the modal degeneracy that takes place in high numerical aperture fibers. In such fibers, the radially and azimuthally polarized modes can be spectrally separated using a fiber Bragg grating. As a proof of principle, we filter azimuthally polarized light in a commercially available fiber in which a fiber Bragg grating has been written by a femtosecond pulsed laser.

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.

Chirped fiber Bragg gratings written with ultrashort pulses and a tunable phase mask.

We report a fabrication technique for chirped fiber Bragg gratings (CFBGs) using a flexible setup based on a poly(methyl-methacrylate) phase mask. The period of the phase mask can be thermally tuned during the inscription process, allowing the grating period of uniform fiber Bragg gratings to be shifted about 7 nm by a temperature change of 74 K. In addition, CFBGs with bandwidths up to 2 nm are demonstrated in non-photosensitive fibers by IR femtosecond inscription.

Continuously chirped fiber Bragg gratings by femtosecond laser structuring

We report on what we believe to be the first successful inscription of continuously chirped fiber Bragg gratings (CCFBG) into a nonphotosensitive single-mode fiber (SMF) with near-IR femtosecond laser pulses. The continuous chirp was achieved by inscribing into a bent fiber using a phase mask scanning technique. We fabricated a 20 mm long CCFBG with a bandwidth of 6 nm and a maximal reflectivity of 50%. The second-order dispersion of the CCFBG of up to 28 ps(2) was measured with spectral interferometry.

Mode selective fiber Bragg gratings

Focussing ultrashort laser pulses allows for inscribing fiber Bragg gratings (FBGs) directly into rare earth doped fiber cores - without prior photosensitivity treatment. High reflective FBGs can be written into active Large Mode Area (LMA) Fibers with 20 micron core diameter using a phase mask scanning technique. Here, we demonstrate fiber Bragg gratings (FBGs), which cover only a fraction of the core. With this additional degree of freedom it is possible to taylor the reflectivity of individual modes. We show for example how those FBGs can be used in few mode LMA fibers to suppress reflections into higher order modes.

Passively stabilized 215-W monolithic CW LMA-fiber laser with innovative transversal mode filter

We report on the development of a high power monolithic CW fiber oscillator with an output power of 215 W in a 20μm core diameter few-mode Large Mode Area fiber (LMA). The key parameters for stable operation are reviewed. With these optimizations the root mean square of the output power fluctuations can be reduced to less than 0.5 % on a timescale of 20 s, which represents an improvement of more than a factor 5 over a non-optimized fiber laser. With a real-time measurement of the mode content of the fiber laser it can be shown that the few-mode nature of LMA fibers is the main factor for the residual instability of our optimized fiber laser. The root of the problem is that Fiber Bragg Gratings (FBGs) written in multimode fibers exhibit a multi-peak reflexion spectrum in which each resonance corresponds to a different transversal mode. This reflectivity spectrum stimulates multimode laser operation, which results in power and pointing instabilities due to gain competition between the different transversal modes . To stabilize the temporal and spatial behavior of the laser output, we propose an innovative passive in-fiber transversal mode filter based on modified FBG-Fabry Perot structure. This structure provides different reflectivities to the different transversal modes according to the transversal distribution of their intensity profile. Furthermore, this structure can be completely written into the active fiber using fs-laser pulses. Moreover, this concept scales very well with the fiber core diameter, which implies that there is no performance loss in fibers with even larger cores. In consequence this structure is inherently power scalable and can, therefore, be used in kW-level fiber laser systems.

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.