Microstructured cladding elements to enhance performance and flexibility of large mode area leakage channel fibers

Large mode area fibers are imperative for scaling up the average power of fiber lasers. Single-mode behavior and low FM loss are the crucial functionalities for these fibers. However, for key applications such as picosecond pulsed lasers, the device length needs to at least a few meters. This makes a certain degree of bend tolerance a prerequisite in the fiber design. While rod-type PCFs have been very successful in offering large mode areas, their rigid configuration limits their application domain. Alternatively, leakage channel fibers (LCFs) have shown a great potential for offering substantial bend tolerance along with large mode areas. However, the proposed use of Fluorine-doped rods in the all-solid version limits their practical design space. Here, we propose a novel design concept to attain single-material, large mode area fibers (mode area >~ 1000µm2) with effectively single mode operation coupled with bending characteristics comparable to all-solid LCFs and greater design flexibility and easier splicing that is comparable to rod-type PCFs

Study and Control of Nonlinearity in Large-Mode-Area Fibers.

Practical advantages and high power of fiber lasers make them important in many scientific and industrial applications. However, relatively small mode-area and long propagation-length in an optical fiber also enhances the nonlinear interactions, posing certain limits on achievable average and peak powers in fiber lasers. In this dissertation, we explore such nonlinear effects and their control in CCC fibers, a practically important type of large-core effectively-single-mode fibers. Many applications require short wavelengths. We study use of four-wave-mixing (FWM) for wavelength conversion in CCC fibers. Our theoretical analysis shows that under proper conditions CCC fibers can be used for efficient and high-power wavelength conversion from ~1µm to yellow-red visible wavelengths. We study use of spectral filtering properties of CCC fibers for suppressing stimulated Raman scattering (SRS). SRS suppression has been experimentally achieved in two types of spectrally-tailored CCC fibers, demonstrating an additional degree of design freedom, combining core-size scalability and SRS suppression. Average powers in large-core amplifying fibers are limited by the thermally induced transverse mode instability (TMI). We show that TMI is essentially a two-beam coupling process, causing stimulated scattering from the fundamental to higher-order modes. We show that increasing higher-order mode suppression in CCC fibers increases TMI threshold power. CCC fibers are low-birefringence fibers, in which fiber coiling and twisting produces externally induced linear and circular birefringence. Presence of the later complicates nonlinear polarization evolution (NPE) at high peak powers, which can degrade polarization preservation at the amplifier or laser output. Our experimental and theoretical analysis shows that with proper signal excitation and fiber packaging conditions linear output polarization can be maintained under a wide range of output peak powers. Additionally, this dissertation also includes a study of some design aspects of large-core polygonal-CCC fibers, directly related to fiber modal properties used in controlling nonlinear interactions. Results of this work are important for using CCC, as well as other types of flexible (i.e. non-rod type) effectively-single-mode fibers, in high power and energy fiber lasers.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116668/1/andrehu_1.pd

Towards Large-Mode-Area fibers fabricated by the full vapor-phase SPCVD process

International audienceLarge-Mode-Area (LMA) fibers are key elements in modern high power fiber lasers operating at 1 µm. LMA fibers are highly ytterbium-doped and require a fine control of the core refractive index (RI) close to the silica level. These low RI have been achieved with multi-component materials elaborated using a full-vapor phase Surface Plasma Chemical Vapor Deposition (SPCVD) process, enabling the fabrication of large core diameter preforms (up to 4 millimeters). Following the technology demonstration, presented in Photonics West 2017, with results on 10/130 (core-to-clad diameters (in µm) ratio) fibers, this paper aims to present updated results obtained for double-clad 11/130, 20/130 and 20/400 LMA fibers, with numerical apertures at, respectively, 0.08 and 0.065. The study is based on aluminosilicate core material co-doped either with fluorine or phosphorus to achieve optimal radial RI tailoring. The fiber produced exhibit low background losses (<20dB/km at 1100nm) and high power conversion efficiencies, up to 74% for output powers of 100W limited by our test setup. The Gaussian beam quality has been evaluated using the M² measurement. Photodarkening behavior will be discussed for both fluorine and phosphorus-doped aluminosilicate materials and particularly the use of cerium as co-dopant. The SPCVD technology can indeed be used for the production of Yb-doped LMA fibers. Current development is now focused on other rare-earth doped fibers

The radiated fields of the fundamental mode of photonic crystal fibers

The six-fold rotational symmetry of photonic crystal fibers has important manifestations in the radiated fields in terms of i) a focusing phenomena at a finite distance from the end-facet and ii) the formation of low-intensity satellite peaks in the asymptotic far field. For our study, we employ a surface equivalence principle which allows us to rigorously calculate radiated fields starting from fully-vectorial simulations of the near field. Our simulations show that the focusing is maximal at a characteristic distance from the end-facet. For large-mode area fibers the typical distance is of the order 10 Lambda with Lambda being the pitch of the triangular air-hole lattice of the photonic crystal fiber.Comment: 6 pages including 4 figures. Accepted for Opt. Expres

Ultra large mode area fibers with aperiodic cladding structure for high power single mode lasers

International audienceThis communication presents the latest designs, fabrication steps and first results of large mode area fibres with aperiodic cladding structure for high power singlemode emission. Pre-compensation of thermal loading and first laser emission are detailed

Large mode area fibers of multicore type and their modal properties

Since the 1960s optical fibers gained evermore importance and are today well established in telecommunication, metrology and high power lasers. The content of transverse modes in such fibers determines the fundamental properties of the emerging beam. Therefore the work comprises the modal decomposition at a multicore fiber using computer-generated holograms, including different approaches for the calculation of modes in this special type of microstructured fiber. The modal power and modal polarization as a function of external disturbances, such as fiber bending or vertical pressure, is investigated, as well as the transmission characteristics of various input polarizations. Further subjects include the measurement of the beam quality and the demonstration of the Talbot effect

Radiation-hardened Erbium doped LMA fiber with AlP composition from solution doping process

We report on Erbium doped large-mode-area fibers based on the phosphoalumino-silicates. The radiation induced attenuation are reduced compared to standard highly doped fibers. We measured 22% power conversion efficiency for core pumping at 1532nm

All-solid aperiodic Large Pitch Fibers for operation in high power regime

International audienceThis communication intends to summarize the recent strides carried out by the study of original Very Large Mode Area fibers (VLMA) that offer remarkable beam qualities. This work also aims in pushing farther the threshold of appearance for non-linear processes as well as thermal induced beam degradation hindering the power scaling in optical fibers. So as to fulfil this objective, thorough investigations of fibers modal content has been performed, leading to the evidencing of primordial statements for exacerbation of the beam quality and its robustness. Theoretical principles driven toward the conception of aperiodic Large Pitch Fibers (LPFs) will be reported together with experimental validation into passive fiber designs. The relevance of these original structures will then been discussed in regard to the power scaling

Measuring photodarkening from Yb doped fibers

Yb doped fibers are used widely in applications requiring high beam quality and efficiency. A detrimental phenomenon, photodarkening, is known to reduce the efficiency of fiber devices under some operating conditions, and in extreme cases to prevent lasing altogether. Photodarkening manifests as an incrementally increasing spectrally broad transmission loss in the Yb doped core of a fiber, and it is caused by the exposure to pump- or signal photons. This work focused on development of methods to benchmark fibers for photodarkening, and to develop methods to study the temporal and spatial characteristics of photodarkening. A measurement method for conducting benchmarking and spectral studies on single-mode fibers was developed, and a similar alternative method was developed for large-mode-area fibers. Both approaches were studied experimentally and by simulations. Additionally, an experimental setup was built to study the spatial photodarkening differences in large-mode-area fibers. Observations agreed with the simulated results. Photodarkening rate was found to have a repeatable spectral response with fibers of similar but varying compositions, and the photodarkening induced temporal transmission loss was found to follow a stretched-exponential decay rate, and a bi-exponential decay rate. Photodarkening was found to be proportional to the inversion of a fiber sample; and also to have a 7th power dependency to inversion; and more generally, to have a 7th power dependency to the excited state Yb ion density. The methodology obtained through this work enable benchmarking of Yb doped single-mode and large-mode-area fibers. The observed excited state ion density dependency to the photodarkening rate has strong implications to Yb fiber devices, as a given fiber may photodarken with a high rate in one application (having a higher inversion), and non-measurably in another application (having a lower inversion)