We present power stable fiber Bragg gratings for linear fiber lasers in the 2 µm wavelength range. The FBG were inscribed through the coating material using infrared femtosecond laser pulses. By improved inscription parameters signal light scattering at the induced structure resulting in heating and shifting of the resonance laser was minimized. A stable linear polarized fiber laser at 2 µm with an output power of 50 W was implemented.
We report on the investigation of realization limits of ultrashort pulse written volume-Bragg-gratings with respect to the laser pulse repetition rate. The VBGs are directly inscribed in fused silica by applying ultrashort laser pulses and the phase mask scanning technique. In order to investigate the scaling potential of the realization process of such gratings by means of higher repetition rates we investigate the fundamentally given material constraints. These investigations will help to pave the way of this realization scheme for VBGs beyond the prototype regime.
We present an investigation of birefringence in ultrashort pulse and phase mask written fiber Bragg gratings (FBGs). Polarization dependent loss and shift of the resonance wavelength as well as the polarization extinction ratio of the FBGs was determined. As expected, we could find a significant dependency connecting the grating strengths and the birefringence . For high reflector gratings, birefringence of up to 10^-5, corresponding to a resonance wavelength shift of up to 10 pm was measured. The results show excellent suitability for usage of the FBGs in linear polarized fiber lasers.
We report on the optimization of ultrashort pulse laser written VBGs for UV applications. The VBGs are directly inscribed in fused silica by applying ultrashort laser pulses and the phase mask scanning technique and exhibit geometries and reflectivities comparable to conventional gratings. For applications below 300nm, absorption in the VBG is a critical issue. Therefore, we investigate the influence of absorption on writing parameters and thermal annealing. The optimized gratings in fused silica show a significantly reduced residual absorption, making these optimized gratings well suited for low wavelength applications especially in the Near-UV and below.
We present a novel technique for the control of the spectral profiles of fiber Bragg gratings written with ultrashort laser pulses and the phase mask technique. Instead of controlling the modification strength along the grating, the apodisation is realized by controlling the cross-sectional overlap of modification and fiber core by a custom designed shaping aperture. As a proof of concept, gratings were realized with super-Gaussian shaped profiles with different orders, producing the expected spectral profiles, which was verified by simulations.
We report on the first inscription of chirped VBG in fused silica for high power applications such as CPA systems. The cVBG are inscribed into bulk fused silica using ultrashort laser pulses and the phase mask scanning technique. The influence of inscription parameters on the grating properties as well as the dispersion characteristics of the cVBG are investigated. Further steps towards scaling the dimensions of the fs-written cVBG towards applications inside CPA systems are discussed.
Recently, we proposed using embedded nanogratings to change the polarization state in fused silica femtosecond laser direct written optical circuits. Full control over the elements’ birefringence properties can be attained by changing the inscription parameters and using a suitable writing geometry. Therefore, these structures can be used to arbitrarily transform the polarization state on an optical chip. Due to the intrinsic birefringence of these structures, the required length of the functionalized section is only a few hundred micrometers. We demonstrated four single qubit quantum gates based on these structures (Hadamard, Pauli-x, Pauli-z and Pi-8th). However, the overall losses of these structures are still rather high. We present our endeavour to reduce the losses by using adapted beam shaping. The improved performance and their potential for optical quantum computing will be presented.
The ongoing increase of peak and average power of diode lasers makes the consideration of thermo-optical effects due to absorption in the optical system necessary. Otherwise, this local increase in temperature might lead to a significant reduction of the entire functionality or a complete failure. Here, we consider this issue in volume Bragg gratings (VBG) as they are typically used for wavelength stabilization of the laser diodes. Conventionally, these gratings are fabricated very effectively by inscription using ultraviolet light into special photosensitive glasses. However, the use of these special glasses might compromise the transmission and, thus, limit the application.
As an alternative, we inscribe VBGs in pure fused silica using ultrashort laser pulses and the phase mask scanning technique. Applying these gratings for external stabilization of high power diode lasers already demonstrated an outstanding performance in sense of spectral drift due to load change compared to conventional VBGs. Here, we present investigations to further optimize the residual absorption of the gratings especially in the NIR. Apart from analyzing the influence of the processing parameters on the residual absorption, we also studied the possibility for a thermal post-processing to anneal absorbing defects. The resulting gratings exhibit a residual absorption close to the intrinsic absorption of fused silica, making them ideally suited for high power applications. Further applications in ultrashort pulse laser systems for pulse stretching and compression are discussed.
We report on the selective modal control of tailored femtosecond-written long period fiber gratings (LPFG). It is shown that the excitation of higher order cladding modes is possible with strong selectivity and high precision. The coupling behavior of several gratings dependent on the modified core cross section is determined theoretically and confirmed by experiments. Additional mode field measurements proof our concept. The presented tool could pave the way for a completely new branch of fiber integrated devices such as highly efficient transmission gratings or mode converters.
We demonstrate the direct inscription of aperiodic fiber Bragg gratings (AFBGs) for their use as in-fiber filter elements. The modifications are induced by focusing ultrashort laser pulses with an oil-immersion objective into the fiber core. We apply an advanced point-by-point inscription technique for flexible period adaptation. The fabricated AFBGs are targeted on the suppression of 10 lines in a single grating and simulations based on the specific design show excellent agreement. Furthermore, we discuss the application in astronomy as filters for the suppression of OH emission lines.
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