Carlo Molardi

Thermally resilient Tm-doped large mode area photonic crystal fiber with symmetry-free cladding

The design of a Tm-doped photonic crystal fiber with ∼ 80 μm core diameter and robust single-mode guiding is proposed. State-of-art modal discrimination is obtained through the suppression of the inner cladding C(6ν) symmetry, which fosters the delocalization of the LP(11)-like mode. The effects of thermally-induced refractive index change are investigated by means of a computationally-efficient thermal model, and the possibility to obtain wide-band single-mode propagation and effective area exceeding 2500 μm(2) under a heat load of over 300 W/m is demonstrated.

Symmetry-Free Tm-Doped Photonic Crystal Fiber With Enhanced Mode Area

Thulium-doped fiber lasers have recently attracted a growing interest because of the possibility to combine emission around 2 μm with the high beam quality and compactness provided by the fiber medium. A key factor for power scaling of these devices, especially if operation in the short- and ultrashort-pulse regime is desired, is the availability of active fibers capable of joining large effective area and robust single-mode guiding. The strong thermo-optic effect originating from the large quantum defect of Tm ions hinders the possibility to easily find a design with such features, strongly improving the confinement of high-order modes and shrinking the mode area of the fundamental one. In this paper, a rod-type double-cladding photonic crystal fiber with inner cladding properly designed without any mirror symmetry in the cross-section is presented, and its guiding properties are thoroughly analyzed by means of numerical simulations, taking into account the effects of thermally-induced refractive index change. The results have shown that, with a careful choice of the structural parameters, the proposed fiber is capable of ensuring effective high-order mode suppression and effective area up to 3800 μm 2 when operating under heat load of about 300 W/m.

Modal analysis in 2D media with variable disorder

Modal properties of 2D disordered optical structures have been numerically analyzed, in the Mid-IR region, varying the amount of scattering and the disorder level. The modal properties study has been carried out through the use of Finite Element Method, highlighting the localized regime transition and investigating the quality factor. The results have been interpreted in a statistical fashion, investigating light diffusion in these structures with the help of Monte Carlo Method. An alternative measure of randomness weight has been proposed to support the numerical results.

Polarization-Maintaining Large Mode Area Fiber Design for 2-

Large mode area polarizing maintaining (PM) photonic crystal fiber (PCF) designs, for 2-μm operation, are studied. A commercial PM-PCF optimized for 1-μm operation has been scaled to obtain an 80-μm-diameter core, and then numerically analyzed with the help of a finite-element method-based software to investigate its behavior at different values of bending radius. From the problems emerged by this design, a new optimized design has been proposed and numerically simulated. The new fiber consists in a thulium-doped 19-cell core with a diameter of 80 μm. The holes lattice, which follows the stack-and-draw scheme, has been modified to include two boron-doped stress applying parts, in order to induce a consistent birefringence. The results show that it is possible to achieve a single mode, single polarization operation, efficiently suppressing one of the fundamental mode (FM) polarization and the most significant higher order modes, in a range of bending radius from 33 to 40 cm. A remarkable value of 2600 μm2 has been registered for the effective area of the survived FM polarization.

\mu \text{m}

Lasing behaviour of 2-dimensional active random structures, designed to work in the Mid-IR region, has been investigated at different input powers by varying the amount of scattering intensity. A Monte Carlo based simulation tool has been developed including a model to manage the optical amplification. The analysis of photon travel distance has been considered to show the random lasing behaviour with particular attention on lasing threshold at different scattering intensity. The simulated results are in agreement with experiments.

Operation

The mode discrimination criterion for single mode operation, usually considered in fiber amplifiers designed for high power operation, has been investigated and tested on three different fiber designs, a large pitch fiber and two symmetry free photonic crystal fibers. To have a significant collection of results, parameters like pump configuration, pump power, and amplifier length have been varied. The analysis has been carried out through the use of a custom numerical tool provided with efficient thermal and spatial amplification models. From the obtained results, it is possible to observe that the mode discrimination criterion is helpful but not strictly necessary to pledge an effective single mode operation through differential amplification. This fact extends the possibility for the study, as well as for the optimization, of different fiber designs. The use of advanced numerical analysis, which takes into consideration amplification along with thermally influenced modes guidance, becomes extremely useful for an effective fiber design.

Analysis of mid-infrared lasing in active random media

The development of low loss, small size and flexible waveguides is one of the most challenging issues of THz research due to the poor characteristics of both metal and dielectrics in this frequency range. Hollow core tube lattice fibers (HCTLFs) have been recently proposed and experimentally demonstrated to overcome this problem. However, they require very large hollow core size leading to big and hardly flexible fibers. Scaling law analysis plays an important role in determining the best trade-off between low loss and small fiber diameter. The dependence of the confinement on frequency and core radius are here numerically investigated. Results show that confinement loss exhibits a stronger dependence on core size and frequency with respect to other hollow core fibers proposed for THz waveguiding, such as Bragg, Tube, and Kagome fibers.

Mode discrimination criterion for effective differential amplification in Yb-doped fiber design for high power operation

Designs of Tm-doped photonic crystal fibers for laser operation must take in account the strong thermo-optical effects due to the Tm quantum defect and the consequent corruption of the single mode guiding properties. A new fiber design with a ∼ 80 μm core diameter, based on the cladding mirror symmetry reduction is proposed and analyzed using a full-vector FEM-based modal solver. The thermal effects are investigated using a computationally efficient model. A large pitch fiber with similar core diameter, which represents the actual state-of-art of Tm-doped laser technology, has been investigated in order to have a basis of comparison. Optimizing some key parameters of the new symmetry free fiber, the possibility to achieve a wide band single mode operation under an heavy heat load of over 300 W/m is demonstrated. In particular a very high modal discrimination value larger than 0.5 is obtained.

Confinement loss scaling law analysis in tube lattice fibers for terahertz applications

The dental industry without lasers is inconceivable right now. This captivating technology has outlasted other possible alternative technologies applied in dentistry in the past due to its precision, accuracy, minimal invasive effect as well as faster operating time. Other alternatives such as soldering, resistance (spot) welding, plasma (torch) welding, and single pulse tungsten inert gas welding have their pros and cons; nevertheless, laser welding remains the most suitable option so far for dental application. This paper attempts to give an insight into the laser principle and types of lasers used for dental purposes, types of dental alloys used by the dentist, and effect of laser parameters on prosthesis/implants. It is apparent from the literature review that laser assisted dental welding will continue to grow and will become an unparalleled technology for dental arena.

Thermo-optical effects in Tm-doped large mode area photonic crystal fibers

Yb-doped Photonic Crystal Fibers (PCFs) have triggered a significant power scaling into fiber-based lasers. However thermally-induced effects, like mode instability, can compromise the output beam quality. PCF design with improved Higher Order Mode (HOM) delocalization and effective thermal resilience can contain the problem. In particular, Fully- Aperiodic Large-Pitch Fibers (FA-LPFs) have shown interesting properties in terms of resilience to thermal effects. In this paper the performances of a Yb-doped FA-LPF amplifier are experimentally and numerically investigated. Modal properties and gain competition between Fundamental Mode (FM) and first HOM have been calculated, in presence of thermal effects. The main doped fiber characteristics have been derived by comparison between experimental and numerical results.