2019

show Content T. Luehder, J. Plentz, J. Kobelke, K. Wondraczek, and M.A. Schmidt, "All-Fiber Integrated In-Line Semiconductor Photoconductor," J. Lightwave Technol. 37, 3244 (2019).

Abstract

Here, we present a fully fiber-integrated semiconductor wire based photoconductor for the photodetection of light at visible and near-infrared wavelengths. The device consisting of an electrically connected micrometer sized germanium wire running parallel to a single-mode core of a gradient index fiber shows features such as temporal responses shorter than 20 ns and a linear response between 10 nW and 20 μW in the wavelength domain between 600 nm and 1300 nm, while light powers up to 2 mW can be measured. A sophisticated polarization dependency has been revealed, showing a non-intuitive dependence on wavelength and light power that has been explained by a developed model. Due to its design flexibility our electrically connected semiconductor enhanced fiber concept principally allows to detect a defined fraction of light, while the remaining part is still available for further diagnostics, thus defining a novel type of detection scheme for all-in-fiber optoelectronic light detection.

link to the article

show Content M. Heck, G. Schwartz, R. G. Krämer, D. Richter, T. A. Goebel, C. Matzdorf, A. Tünnermann, and S. Nolte, "Control of higher-order cladding mode excitation with tailored femtosecond-written long period fiber gratings," Opt. Express 27, 4292 (2019).

Abstract

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.

doi: 10.1364/OE.27.004292

show Content H. Kämmer, G. Matthäus, K. A. Lammers, C. Vetter, M. Chambonneau, and S. Nolte, "Origin of Waveguiding in Ultrashort Pulse Structured Silicon," Laser Photonics Rev. 13, 1800268 (2019).

Abstract

The origin of waveguiding in the bulk of silicon after sub‐ps laser inscription is investigated. Locally resolved Raman measurements of waveguide cross sections and along the propagation axis reveal highly localized crystal deformations. These modifications consist of highly confined regions of silicon with a disturbed crystal structure accompanied with strain. This transformation is responsible for a local increase of the refractive index allowing localized waveguiding. On the basis of near‐field measurements at an excitation wavelength of 1550 nm, the absolute value of the refractive index change is estimated to be in the range of 10−3. The origin of waveguiding after sub‐picosecond laser inscription in the bulk of crystalline silicon is examined. Micro‐Raman spectroscopy analyses of the waveguide cross sections reveal highly localized disturbed crystal structures. These material transformations result in a local increase of the refractive index responsible for guiding light.

doi: 10.1002/lpor.201800268

show Content R. G. Krämer et al., "Femtosecond written fiber Bragg gratings in Ytterbium-doped fibers for fiber lasers in the kilowatt regime," Opt. Lett. 44, 723-726 (2019).

Abstract

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.

 doi: 10.1364/OL.44.000723

show Content S. C. Warren-Smith, K. Schaarschmidt, M. Chemnitz, E. Schartner, H. Schneidewind, H. Ebendorff-Heidepriem, and M. Schmidt, "Tunable multi-wavelength third-harmonic generation using exposed-core microstructured optical fiber," Opt. Lett. 44, 626 (2019).

Abstract

We demonstrate that exposed-core microstructured optical fibers offer multiple degrees of freedom for tailoring third-harmonic generation through the core diameter, input polarization, and nanofilm deposition. Varying these parameters allows control of the phase-matching position between an infrared pump wavelength and the generated visible wavelengths. In this Letter, we show how increasing the core diameter over previous experiments (2.57 μm compared to 1.85 μm) allows the generation of multiple wavelengths, which can be further controlled by rotating the input pump polarization and the deposition of dielectric nanofilms. This can lead to highly tailorable light sources for applications such as spectroscopy or nonlinear microscopy.

doi: 10.1364/OL.44.000626

show Content F. Maes, C. Stihler, L.-P. Pleau, V. Fortin, J. Limpert, M. Bernier, and R. Vallée, "3.42 µm lasing in heavily-erbium-doped fluoride fibers," Opt. Express 27, 2170 (2019).

Abstract

In this paper, we investigate laser emission at 3.4μm in heavily-erbium-doped fluoride fibers using dual-wavelength pumping. To this extent, a monolithic 7 mol% erbium-doped fluoride fiber laser bounded by intracore fiber Bragg gratings at 3.42 μm is used to demonstrate a record efficiency of 38.6 % with respect to the 1976 nm pump. Through numerical modeling, we show that similar laser performances at 3.4 μm can be expected in fluoride fibers with erbium concentrations ranging between 1 – 7 mol%, although power scaling should rely on lightly-doped fibers to mitigate the heat load. Moreover, this work studies transverse mode-beating of the 1976 nm core pump and its role in the generation of a periodic luminescent grating and in the trapping of excitation in the metastable energy levels of the erbium system. Finally, we also report on the bistability of the 3.42 μm output power of the 7 mol% erbium-doped fluoride fiber laser.

doi: 10.1364/OE.27.002170

show Content G. K. Tadesse et al., “Wavelength-scale ptychographic coherent diffractive imaging using a high-order harmonic source”, Sci. Rep. 9(1), 1735 (2019).

Abstract

Ptychography enables coherent diffractive imaging (CDI) of extended samples by raster scanning across the illuminating XUV/X-ray beam thereby generalizing the unique advantages of CDI techniques. Table-top realizations of this method are urgently needed for many applications in sciences and industry. Previously, it was only possible to image features much larger than the illuminating wavelength with table-top ptychography although knife-edge tests suggested sub-wavelength resolution. However, most real-world imaging applications require resolving of the smallest and closely-spaced features of a sample in an extended field of view. In this work, we resolve features as small as 2.5 \lambda (45 nm) by using a table-top ptychography setup and a high-order harmonic XUV source. For the first time, a Rayleigh-type criterion is used as a direct and unambiguous resolution metric for high-resolution table-top setup. This reliably qualifies this imaging system for real-world applications e.g. in biological sciences, material sciences, imaging integrated circuits and semiconductor mask inspection.

doi:10.1017/S1431927618012515

show Content F. Löchner, R. Mupparapu, M. Steinert, A. George, Z. Tang, A. Turchanin, T. Pertsch, I. Staude, and F. Setzpfandt, “Controlling second-harmonic diffraction by nano-patterning MoS2 monolayers,” Opt. Express 27, 35472 (2019).

Abstract

Monolayers of transition metal dichalcogenides have a strong second-order nonlinear response enabling second-harmonic generation. Here, we control the spatial radiation properties of the generated second harmonic by patterning MoS2 monolayers using focused ion beam milling. We observe diffraction of the second harmonic into the zero and first diffraction orders via an inscribed one-dimensional grating. Additionally, we included a fork-like singularity into the grating to create a vortex beam in the first diffraction order.

doi.org/10.1364/OE.27.035475

show Content C. P. Jisha, J. Beeckman, F. Van Acker, K. Neyts, S. Nolte and A. Alberucci, "Generation of multiple solitons using competing nonlocal nonlinearities," Opt. Lett. 44, 1162 (2019).

Abstract

We discuss the dynamics of fundamental Gaussian beams launched in saturable and nonlocal nonlinear media. Solely in the presence of a self-focusing saturable nonlinearity, the breathing solitons undergo strong deformation. The addition of a defocusing nonlinearity leads to the generation of couples of solitons. Experimentally, we demonstrate in nematic liquid crystals the formation of multiple spatial solitons starting from a bell-shaped input, with both direction and the number of filaments depending on the input power, confirming the theoretical predictions.

doi: 10.1364/OL.44.001162

show Content C. P. Jisha, A. Alberucci, J. Beeckman and S. Nolte, "Nonlinear localization of light using Pancharatnam-Berry phase," Phys. Rev. X 9, 021051 (2019).

Abstract

Since its introduction by Sir Michael Berry in 1984, geometric phase became of fundamental importance in physics, with applications ranging from solid state physics to optics. In optics, Pancharatnam-Berry phase allows the tailoring of optical beams by a local control of their polarization. Here we discuss light propagation in the presence of an intensity-dependent local modulation of the Pancharatnam-Berry phase. The corresponding self-modulation of the wavefront counteracts the natural spreading due to diffraction, i.e., self-focusing takes place. No refractive index variation is associated with the self-focusing: the confinement is uniquely due to a nonlinear spin-orbit interaction. The phenomenon is investigated, both theoretically and experimentally, considering the reorientational nonlinearity in liquid crystals, where light is able to rotate the local optical axis through an intensity-dependent optical torque. Our discoveries pave the way to the investigation of a new family of nonlinear waves featuring a strong interaction between the spin and the orbital degrees of freedom.

doi: 10.1103/PhysRevX.9.021051

show Content C.P. Jisha, S. Nolte, and A. Alberucci, "Polarization-insensitive wavefront shaping using the Pancharatnam-Berry phase," Opt. Lett. 44, 5517 (2019).

Abstract

We discuss a method to achieve a polarization-independent modulation of the electromagnetic wavefront based upon the Pancharatnam–Berry phase. When the length of the twisted anisotropic material is equal to the birefringence length (i.e., full-wave plate length), a phase delay proportional to the squared transverse derivative of the twisting angle appears. Physically, the phase delay is associated with the Kapitza effect applied to the Pancharatnam–Berry phase. Our theoretical results are confirmed by finite-difference time-domain (FDTD)-based numerical simulations.

doi.org/10.1364/OL.44.005517

show Content M. Heck, R. G. Krämer, T. Ullsperger, T. A. Goebel, D. Richter, A. Tünnermann, and S. Nolte, "Efficient long period fiber gratings inscribed with femtosecond pulses and an amplitude mask," Opt. Lett. 44, 3980-3983 (2019).

Abstract

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.

doi: 10.1364/OL.44.003980

show Content A. Alberucci, C. P. Jisha, U. Peschel and S. Nolte, Effective breaking of the actionreaction principle using spatial solitons, Phys. Rev. A 100, 011802(R) (2019).

Abstract

We discuss a class of interactions between self-confined optical beams breaking the action-reaction principle. The effective force intertwining the beams does not satisfy momentum conservation, paving the way to the potential existence of situations where both beams are pushed in the same direction, in turn leading to the so-called diametric drive. In our theoretical proposal the interaction between the two light beams is enabled by optical nonlinearity. The nonlinearity is assumed to change sign with the light polarization, in turn allowing the two light beams to be attracted or repelled by an inhomogeneous region according to the photon polarization. We demonstrate that this exotic type of nonlinear spin-orbit-like interaction can be achieved in nematic liquid crystals (NLCs). In fact, depending on the input polarization, in NLCs a change in temperature corresponds to a focusing or defocusing index change. For the polarization seeing a thermal defocusing response (corresponding to the extraordinary component), the self-confinement is ensured by the simultaneous action of reorientational nonlinearity.

doi: 10.1103/PhysRevA.100.011802

show Content M. Heck, J.-C. Gauthier, A. Tünnermann, R. Vallée, S. Nolte and M. Bernier, “Long period fiber gratings for the mitigation of parasitic laser effects in mid-infrared fiber amplifiers,” Opt. Express 27, 21347-21357 (2019).

Abstract

A concept to mitigate parasitic lasing in mid-IR fiber amplifiers using a single long period fiber grating is shown. Using tightly confined ultrashort laser pulses at 800 nm, a grating was directly inscribed into the core of an erbium doped fluoride glass fiber showing a strong attenuation down to −27 dB at desired wavelength. The concept reveals great potential to improve the average output power and attainable spectral range of low repetition rate in-amplifier supercontinuum generation.

doi: 10.1364/OE.27.021347

show Content A. L. M. Muniz, M. Wimmer, A. Bisianov, R. Morandotti, U. Peschel, “Collapse on the line – how synthetic dimensions influence nonlinear effects,” Scientific Reports 9, 9518 (2019).

Abstract

Power induced wave collapse is one of the most fascinating phenomena in optics as it provides extremely high intensities, thus stimulating a range of nonlinear processes. For low power levels, propagation of beams in bulk media is dominated by diffraction, while above a certain threshold self-focusing is steadily enhanced by the action of a positive nonlinearity. An autocatalytic blow-up occurs, which is only stopped by saturation of the nonlinearity, material damage or the inherent medium discreteness. In the latter case, this leads to energy localization on a single site. It is commonly believed that for cubic nonlinearities, this intriguing effect requires at least two transverse dimensions to occur and is thus out of reach in fiber optics. Following the concept of synthetic dimensions, we demonstrate that mixing short and long-range interaction resembles a two-dimensional mesh lattice and features wave collapse at mW-power levels in a genuine 1D system formed by coupled fiber loops.

doi: 10.1038/s41598-019-46060-8.

show Content K. Lammers, M. Ehrhardt, T. Malendevych, X. Xu, C. Vetter, A. Alberucci, A. Szameit and S. Nolte, Embedded nanogratings-based waveplates for polarization control in integrated photonic circuits, Opt. Mat. Express 9, 2560-2572 (2019).

Abstract

Femtosecond laser direct writing (FLDW) enables precise three-dimensional structuring of transparent host materials such as fused silica. With this technique, reliable integrated optical circuits can be written, which are also a possible candidate for future quantum technologies. We demonstrate the manufacturing of integrated waveplates with arbitrary orientations and various phase delays by combining embedded birefringent nanograting structures and FLDW waveguides in fused silica glass. These waveplates can be used both for classical applications and for quantum gates.

doi: 10.1364/OME.9.002560

show Content A. Reupert, M. Heck, S. Nolte, and L. Wondraczek, "Side-emission properties of femtosecond laser induced scattering centers in optical fibers," Opt. Mater. Express 9, 2497-2510 (2019).

Abstract

Fiber optical light diffusers that enable interstitial light delivery have become a useful tool for various illumination tasks, such as in photodynamic therapy. However, existing methods based on light diffusing fiber tips are not applicable for spatially selective light delivery in more complex structures. Here, we employ femtosecond laser induced scattering centers without mechanical manipulation and removal of the outer coatings for generating customized emission patterns. Tailoring of the cumulative emission profile is achieved through controlling the step-width between modification spots. An in-depth evaluation shows that the side-emission pattern is the result of an interplay of several scattering mechanisms that interact with cladding and core modes.

doi: 10.1364/OME.9.002497

show Content K. Schaarschmidt, H. Xuan, J. Kobelke, M. Chemnitz, I. Hartl, and M.A. Schmidt, "Long-term stable supercontinuum generation and watt-level transmission in liquid-core optical fibers," Opt. Lett. 44, 2236-2239 (2019).

Abstract

Due to their unique properties such as transparency, tunability, nonlinearity, and dispersion flexibility, liquid-core fibers represent an important approach for future coherent mid-infrared light sources. However, the damage thresholds of these fibers are largely unexplored. Here we report on the generation of soliton-based supercontinua in carbon disulfide (CS2) liquid-core fibers at average power levels as high as 0.5 W operating stably for a long term (>70h) without any kind of degradation or damage. Additionally, we also show stable high-power pulse transmission through liquid-core fibers exceeding 1 W of output average power for both CS2 and tetrachloroethylene as core materials.

doi: 10.1364/OL.44.002236

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