Pre-compensation of thermally induced refractive index changes in a depressed core fully aperiodic large-pitch fiber for high average power operation.

To prevent the thermally induced spatial beam degradation occurring in high-power fiber lasers and amplifiers, index-depressed core "fully aperiodic large-pitch fibers" (FA-LPFs) have been designed and fabricated. In contrast to previous experimental works performed on FA-LPFs, in which the active core and the surrounding cladding material are quasi-index-matched, the core refractive index is in slight depression compared to the surrounding material (Δn≈-3×10-5). Thus, the index-depressed fiber core tends first to behave as an anti-guide, preventing light from being properly guided into it. However, by increasing the absorbed pump power, the thermal load induces a parabolic refractive index change sufficient to compensate for the -3×10-5 index depression in the core, enabling a robust single-mode amplification at high average power. As a proof of concept, using a 110 µm depressed core FA-LPF, M2 values of 1.3 were demonstrated in amplifier configuration from 60 W to a maximal value of 170 W of emitted average power only limited by the available pump power.

Widely tunable Q-switched dual-wavelength synchronous-pulsed Tm-doped fiber laser emitting in the 2 µm region.

We demonstrate a widely tunable Q-switched dual-wavelength fiber laser emitting synchronized pulses in the 2 µm spectral range. Owing to the use of a Tm-doped rod-type fully aperiodic large pitch fiber, together with an acousto-optic modulator and two volume Bragg gratings (VBGs), the wavelength separation was shown to be continuously tunable from 1 to 120 nm (∼0.1-10 THz). A peak power higher than 8 kW was demonstrated over the whole tuning range for a repetition rate (RR) of 1 KHz and a 26 ns pulse duration. The RR was modulated from 1 to 30 kHz, and the laser pulse duration measured between 23 ns and 130 ns, depending on the RR and the wavelength separation.

Polarization-maintaining Yb-doped large-mode-area fully aperiodic large-pitch fibers.

Based on a special large-pitch architecture that has already proved its single-mode single-polarization behavior in a passive configuration, two ytterbium-doped versions of such large-mode-area fibers have been fabricated and tested in both laser and amplification configurations for high-power laser source applications. Due to the high sensitivity of large-pitch fiber design to the active-core-to-passive-cladding index mismatch, the realization of a single-polarization structure is highly challenging. However, we report on the preservation of a polarization-maintaining feature. A linear polarization with an extinction ratio of 17 dB is demonstrated for mode field diameters reaching up to 58 µm as long as the single-modeness of the emitted signal is preserved.

Polarization evolution in single-ring antiresonant hollow-core fibers.

Understanding polarization in waveguides is of fundamental importance for any photonic device and is particularly relevant within the scope of fiber optics. Here, we investigate the dependence of the geometry-induced polarization behavior of single-ring antiresonant hollow-core fibers on various parameters from the experimental perspective, showing that structural deviations from an ideal polygonal shape impose birefringence and polarization-dependent loss, confirmed by a toy model. The minimal output ellipticity was found at the wavelength of lowest loss near the center of the transmission band, whereas birefringence substantially increases toward the resonances. The analysis that qualitatively also applies to other kinds of hollow-core fibers showed that maximizing the amount of linearly polarized light at the fiber output demands both operating at the wavelength of lowest loss, as well as carefully choosing the relative orientation of input polarization. This should correspond to the situation in which the difference of the core extent along the two corresponding orthogonal polarization directions is minimal. Due to their practical relevance, we expect our findings to be very important in fields such as nonlinear photonics or metrology.

Noninvasive characterization of optical fibers.

Capillary optical fibers with hole diameters of several micrometers are important for novel plasmonic applications and medical diagnosis. In order to ensure the optical functionality of these fibers, the diameter of the capillary hole needs to be realized with high accuracy. Here, we introduce a novel and noninvasive methodology to characterize optical fibers and discuss it for the assessment of capillaries. In this method, the fiber is side-illuminated by a coherent beam, and the resulting diffraction pattern is analyzed. This corresponds to an in-line holographic measurement in the presence of strong scattering. A numerical parameter retrieval allows us to characterize the capillary hole diameter with an accuracy of approximately 100 nm for radii between several hundreds of nanometers and several tens of micrometers.

Role of Ce in Yb/Al laser fibers: prevention of photodarkening and thermal effects.

We report on detailed investigations of ytterbium (Yb) and aluminum (Al) doped silica fiber and preform samples co-doped with cerium (Ce). The prevention of pump-induced photodarkening (PD) by temporary oxidation of Ce3+ to Ce4+ (or rather Ce3++) was proved by observed modifications in the ultraviolet (UV) spectra of transient absorption during near-infrared (NIR) pumping of thin preform slices. Only a small part of available Ce3+ ions (< 4%) was found to be involved in this process despite Yb inversions of up to 0.28. The modifications in the UV absorption spectra disappeared completely when the pump power was switched-off. From these observations we conclude that the recombination to Ce3+ takes place very fast thereby enabling these ions to capture liberated holes h+ perpetually during further pumping. We found a concentration ratio of Ce/Yb ≈0.5 to be sufficient to reduce PD loss to 10% in comparison to Ce-free fibers. Thus, the thermal load caused by absorption of PD color centers at pump (and laser) wavelength is expected to be also reduced. Unfortunately, new heat sources arise with the presence of Ce which cannot be explained by the absorption of Ce ions at the pump wavelength but must be attributed to the interaction with excited Yb ions. Fiber temperature increase of more than 200 K was observed if both, Yb2O3 and Ce2O3 concentration exceed 0.4 mol%.

Enhanced sensitivity in single-mode silicon nitride stadium resonators at visible wavelengths.

The marker-free and noninvasive detection of small traces of analytes in aqueous solution using integrated optical resonators is an emerging technique within bioanalytics. Here, we present a single-mode silicon-nitride stadium resonator operating at the red edge of the visible spectrum, showing sensitivities larger than 200 nm/RIU and transmission dips with extinction ratios of more than 15 dB. We introduce a mathematical model that allows analyzing of the resonator sensitivity using the properties of the guided mode only. Large geometric parameter scans using finite element simulations show that optimal sensing conditions are achieved for TM-polarized modes close to the modal cutoff. Due to its compactness and the short operation wavelength, we anticipate applications of our resonator for integrated bioanalytics.

50.4% slope efficiency thulium-doped large-mode-area fiber laser fabricated by powder technology.

We report on a triple clad large-mode-area Tm-doped fiber laser with 18 µm core diameter manufactured for the first time by an alternative manufacturing process named REPUSIL. This reactive powder sinter material enables similar properties compared to conventional CVD-made fiber lasers, while offering the potential of producing larger and more uniform material. The fiber characterization in a laser configuration provides a slope efficiency of 47.7% at 20°C, and 50.4% at 0°C with 8 W output power, with a laser peak emission at 1970 nm. Finally, a beam quality near the diffraction-limit (M(x,y)2<1.1) is proved.

Origins of modal loss of antiresonant hollow-core optical fibers in the ultraviolet.

Recently, a novel antiresonant hollow core fiber was introduced having promising UV guiding properties. Accompanying simulations predicted ten times lower loss than observed experimentally. Increasing loss is observed in many antiresonant fibers with the origin being unknown. Here, two possible reasons for the enhanced loss are discussed: strand thickness variation and surface roughness scattering. Our analysis shows that the attenuation is sensitive to thickness variations of the strands surrounding the hollow-core which strongly increase loss at short wavelengths. The contribution of surface roughness stays below the dB/km level and can be neglected. Thus, preventing structural irregularities by improved fabrication approaches is essential for decreasing loss.

Low-loss single-mode guidance in large-core antiresonant hollow-core fibers.

We present an approach how to combine large-mode field diameters with effective single-mode guidance in a hollow-core antiresonant optical fiber. We demonstrate experimentally and in simulations that single-mode guidance is achieved in a simplified hollow-core fiber design with a core diameter of 30 µm by shifting the effective indices of the first cladding modes close to those of higher order core modes. Our fiber shows low loss propagation and effective single-mode operation from the near infrared to deep ultraviolet wavelengths down to 270 nm on a loss level of approximately 3 dB/m.

Double antiresonant hollow core fiber--guidance in the deep ultraviolet by modified tunneling leaky modes.

Guiding light inside the hollow cores of microstructured optical fibers is a major research field within fiber optics. However, most of current fibers reveal limited spectral operation ranges between the mid-visible and the infrared and rely on complicated microstructures. Here we report on a new type of hollow-core fiber, showing for the first time distinct transmission windows between the deep ultraviolet and the near infrared. The fiber, guiding in a single mode, operates by the central core mode being anti-resonant to adjacent modes, leading to a novel modified tunneling leaky mode. The fiber design is straightforward to implement and reveals beneficial features such as preselecting the lowest loss mode (Gaussian-like or donut-shaped mode). Fibers with such a unique combination of attributes allow accessing the extremely important deep-UV range with Gaussian-like mode quality and may pave the way for new discoveries in biophotonics, multispectral spectroscopy, photo-initiated chemistry or ultrashort pulse delivery.

High temperature sensing with fiber Bragg gratings in sapphire-derived all-glass optical fibers.

A structured sapphire-derived all-glass optical fiber with an aluminum content in the core of up to 50 mol% was used for fiber Bragg grating inscription. The fiber provided a parabolic refractive index profile. Fiber Bragg gratings were inscribed by means of femtosecond-laser pulses with a wavelength of 400 nm in combination with a two-beam phase mask interferometer. Heating experiments demonstrated the stability of the gratings for temperatures up to 950°C for more than 24 h without degradation in reflectivity.

Advanced attenuation and fluorescence measurement methods in the investigation of photodarkening and related properties of ytterbium-doped fibers.

Considering the continuous development of ytterbium (Yb)-doped fibers for fiber lasers and amplifiers, thorough characterization of the features and quality of such fibers is necessary now more than ever. In particular, the evaluation of the very strong core attenuation that results from Yb absorption and the losses accompanying photodarkening (PD) processes in Yb-doped optical fibers is still of immense interest. Keeping that in mind, the potential of a method using pinhole fibers will be demonstrated here. Moreover, the investigation of fluorescence properties of Yb3+ is also important; it can be affected by phenomena such as reabsorption, inhomogeneous pump intensity distribution, amplified spontaneous emission, lasing, and, lastly, PD. As these phenomena occur while measuring the fluorescence properties of Yb3+ in optical fibers, they can distort the fluorescence spectra and fluorescence decay significantly. We suggest a twice-perpendicular measurement method and strict time management to avoid these interfering phenomena during the fluorescence measurements.

Pr3+-doped YPO4 nanocrystal embedded into an optical fiber.

Optical fiber with YPO4:Pr3+ nanocrystals (NCs) is presented for the first time using the glass powder-NCs doping method. The method's advantage is separate preparation of NCs and glass to preserve luminescent and optical properties of NCs once they are incorporated into optical fiber. The YPO4:Pr3+ nanocrystals were synthesized by the co-precipitation and hydrothermal methods, optimized for size (< 100 nm), shape, Pr3+ ions concentration (0.2 mol%), and emission lifetime. The core glass was selected from the non-silica P2O5-containing system with refractive index (n = 1.788) close to the NCs (no = 1.657, ne = 1.838). Optical fiber was drawn by modified powder-in-tube method after pre-sintering of glass powder-YPO4:Pr3+ (wt 3%) mixture to form optical fiber preform. Luminescent properties of YPO4:Pr3+ and optical fiber showed their excellent agreement, including sharp Pr3+ emission at 600 nm (1D2-3H4) and 1D2 level lifetime (τ = 156 ± 5 µs) under 488 nm excitation. The distribution of the YPO4:Pr3+ NCs in optical fiber were analyzed by TEM-EDS in the core region (FIB-SEM-prepared). The successful usage of glass powder-NCs doping method was discussed in the aspect of promising properties of the first YPO4:Pr3+ doped optical fiber as a new way to develop active materials for lasing applications, among others.

Evidence of Tm impact in low-photodarkening Yb-doped fibers.

In contrast to Yb/Al-doped fibers, the influence of very low Tm(2)O(3) concentrations (≥ 0.1 mol-ppm) on photodarkening (PD) is clearly detectable in Yb/P-doped fibers that are known to show little degradation effects. For Tm(2)O(3) additions of more than 50 mol-ppm, the measured PD loss is even similar to Yb/Al-doped fibers with comparable rare earth concentrations. Our work reveals the risk of color center generation by pumping at wavelengths of 915 nm or 976 nm even in Al-free Yb-doped fibers and emphasizes the importance of high purity of raw materials for the preparation of Yb laser fibers with expected very low PD.

Broadband, diode pumped Yb:SiO2 multicomponent glass laser.

Fabrication, spectroscopic properties, and laser performance of a Yb:SiO(2) multicomponent glass have been investigated in this paper. The glass system composed of SiO(2), Al(2)O(3), and La(2)O(3) excels in terms of a high thermal stress resistance compared to other laser glasses. The laser experiments were conducted with a 3.4 mm thick and 0.9 mol. % Y(2)O(3) doped sample. A maximum slope efficiency of 51%, a maximum optical to optical efficiency of 42%, and a tuning range from 1010-1090 nm was realized. Due to the promising laser properties and a straightforward fabrication technique it may well qualify as an alternative gain medium in high-energy, ultrashort pulse laser systems.

Influence of Tm- or Er-codoping on the photodarkening kinetics in Yb fibers.

We investigated photodarkening (PD) parameters of Yb/Al-doped silica fibers as a function of the concentration of additional rare earth ions like Tm or Er. It was found that both Tm and Er cause a decrease in Yb inversion followed by a reduction of PD in the case of Er, whereas Tm-codoping with more than 10 mol-ppm can strongly accelerate the process and also increase the PD loss. However, contrary to [1], we conclude that the typical PD behavior of Yb/Al fibers is an intrinsic feature of this fiber type and not caused by trace impurities of Tm (< 1 mol-ppm) unintentionally incorporated by the raw materials during fiber preparation.

Multimodal nonlinear endomicroscopic imaging probe using a double-core double-clad fiber and focus-combining micro-optical concept.

Multimodal non-linear microscopy combining coherent anti-Stokes Raman scattering, second harmonic generation, and two-photon excited fluorescence has proved to be a versatile and powerful tool enabling the label-free investigation of tissue structure, molecular composition, and correlation with function and disease status. For a routine medical application, the implementation of this approach into an in vivo imaging endoscope is required. However, this is a difficult task due to the requirements of a multicolour ultrashort laser delivery from a compact and robust laser source through a fiber with low losses and temporal synchronization, the efficient signal collection in epi-direction, the need for small-diameter but highly corrected endomicroobjectives of high numerical aperture and compact scanners. Here, we introduce an ultra-compact fiber-scanning endoscope platform for multimodal non-linear endomicroscopy in combination with a compact four-wave mixing based fiber laser. The heart of this fiber-scanning endoscope is an in-house custom-designed, single mode, double clad, double core pure silica fiber in combination with a 2.4 mm diameter NIR-dual-waveband corrected endomicroscopic objective of 0.55 numerical aperture and 180 µm field of view for non-linear imaging, allowing a background free, low-loss, high peak power laser delivery, and an efficient signal collection in backward direction. A linear diffractive optical grating overlays pump and Stokes laser foci across the full field of view, such that diffraction-limited performance is demonstrated for tissue imaging at one frame per second with sub-micron spatial resolution and at a high transmission of 65% from the laser to the specimen using a distal resonant fiber scanner.

Nanoparticle layer deposition for plasmonic tuning of microstructured optical fibers.

Plasmonic nanoparticles with spectral properties in the UV-to-near-IR range have a large potential for the development of innovative optical devices. Similarly, microstructured optical fibers (MOFs) represent a promising platform technology for fully integrated, next-generation plasmonic devices; therefore, the combination of MOFs and plasmonic nanoparticles would open the way for novel applications, especially in sensing applications. In this Full Paper, a cost-effective, innovative nanoparticle layer deposition (NLD) technique is demonstrated for the preparation of well-defined plasmonic layers of selected particles inside the channels of MOFs. This dynamic chemical deposition method utilizes a combination of microfluidics and self-assembled monolayer (SAM) techniques, leading to a longitudinal homogeneous particle density as long as several meters. By using particles with predefined plasmonic properties, such as the resonance wavelength, fibers with particle-adequate spectral characteristics can be prepared. The application of such fibers for refractive-index sensing yields a sensitivity of about 78 nm per refractive index unit (RIU). These novel, plasmonically tuned optical fibers with freely selected, application-tailored optical properties present extensive possibilities for applications in localized surface plasmon resonance (LSPR) sensing.

Non-phase-matched tunable band rejection in an all-solid photonic bandgap fiber with high-index rods on graded-index pedestals.

A new type of an all-solid photonic bandgap fiber for a non-phase-matched tunable band-rejection filter was proposed and fabricated by introducing a hexagonal array of high-index rods surrounded by graded-index pedestals in silica cladding. Due to the graded index and subsequent weak confinement of light, the proposed fiber showed two contrasting transmission spectra: flat transmission for a long fiber segment of ~1 m in contrast to typical bandgap transmission in a short fiber segment of ~10 cm. For the 120-cm-long fiber, we observed unique band-rejection transmission without any requirement of phase-matching conditions, whose rejection strength was tunable by mechanical perturbations such as bending and twisting. Detailed device principles, fiber design, fabrication, and transmission characteristics are discussed in both theory and experiment.