An active element-doped optical fiber includes a core that includes a first region and a second region. The first region satisfies 0≤r≤0.65d, and the second region surrounds the first region and satisfies 0.65d<r≤d, where d is a radius of the core and r is a distance from a central axis of the core in a radial direction. At least a part of the first region is doped with an active element excited by excitation light, the second region is not doped with the active element, and a shape index is 0.99 or more and less than 1.

Methods for synthesizing fibers having nanoparticles therein are provided, as well as preforms and fibers incorporating nanoparticles. The nanoparticles may include one or more rare earth ions selected based on fluorescence at eye-safer wavelengths, surrounded by a low-phonon energy host. Nanoparticles that are not doped with rare earth ions may also be included as a co-dopant to help increase solubility of nanoparticles doped with rare earth ions in the silica matrix. The nanoparticles may be incorporated into a preform, which is then drawn to form fiber. The fibers may beneficially be incorporated into lasers and amplifiers that operate at eye safer wavelengths. Lasers and amplifiers incorporating the fibers may also beneficially exhibit reduced Stimulated Brillouin Scattering.

Laser waveguides, methods and systems for forming a laser waveguide are provided. The waveguide includes an inner cladding layer surrounding a central axis and a glass core surrounding and located outside of the inner cladding layer. The glass core includes a laser-active material. The waveguide includes an outer cladding layer surrounding and located outside of the glass core. The inner cladding, outer cladding and/or core may surround a hollow central channel or bore and may be annular in shape.

A photodarkening-resistant ytterbium-doped quartz optical fiber and a method for preparing such a fiber are provided. Glass of a photodarkening-resistant ytterbium-doped quartz optical fiber core rod includes at least Yb.sub.2O.sub.3, Al.sub.2O.sub.3, P.sub.2O.sub.5, SiO.sub.2. The proportions of Yb.sub.2O.sub.3, Al.sub.2O.sub.3, and P.sub.2O.sub.5 in the entire substance are Yb.sub.2O.sub.3: 0.05-0.3 mol %, Al.sub.2O.sub.3: 1-3 mol %, and P.sub.2O.sub.5: 1-5 mol %, respectively. In the preparation method for the photodarkening-resistant ytterbium-doped quartz optical fiber, a sol-gel method and an improved chemical vapor deposition method are combined. By using the molecular-level doping uniformity and the low preparation loss thereof respectively, ytterbium ions, aluminum ions and phosphorus ions are effectively doped in a quartz matrix, thereby effectively solving the problems in the optical fiber of high loss, photodarkening caused by cluster or the like, and a central refractive index dip.

An amplification optical fiber includes: a core; an inner cladding having a refractive index lower than a refractive index of the core, wherein an active element pumped by pumping light is entirely doped to the core, and a relative effective refractive index difference of light in an LP01 mode is greater than or equal to 0.05% and a relative effective refractive index difference of light in an LP21 mode is less than 0.05% in light propagating through the core.

Methods for synthesizing fibers having nanoparticles therein are provided, as well as preforms and fibers incorporating nanoparticles. The nanoparticles may include one or more rare earth ions selected based on fluorescence at eye-safer wavelengths, surrounded by a low-phonon energy host. Nanoparticles that are not doped with rare earth ions may also be included as a co-dopant to help increase solubility of nanoparticles doped with rare earth ions in the silica matrix. The nanoparticles may be incorporated into a preform, which is then drawn to form fiber. The fibers may beneficially be incorporated into lasers and amplifiers that operate at eye safer wavelengths. Lasers and amplifiers incorporating the fibers may also beneficially exhibit reduced Stimulated Brillouin Scattering.

An amplification fiber which can generate a laser beam in a visible region even when a silica glass is used as a base material of a core of the amplification fiber is realized. An amplification fiber according to an embodiment of the present disclosure includes a core configured to generate a laser beam from an excitation beam in a visible region, and a cladding surrounding the core. The core is composed of a core material including Dy, one or more elements selected from Al, Ge, and P, and a silica glass.

A Raman fiber laser source is configured with a feeding fiber delivering MM pump radiation to an inner cladding of double-clad MM Raman fiber laser. The MM pump beam radiation has a sufficient power to produce Raman scattering in the MM Raman fiber converting the pump radiation to a MM signal radiation at a Raman-shifted wavelength λram which is longer than a wavelength λpump of the pump radiation. The Raman laser source further has a pair of spaced reflectors defining therebetween a resonator for the signal radiation at a 1.sup.st Stokes wavelength and flanking at least part of the MM core of the Raman fiber which is provided with a central core region which is doped with impurities for enhancing Raman process. The reflectors and central core region are dimensioned to correspond to the fundamental mode of the MM signal radiation which is output from the Raman fiber with an M.sup.2 factor ≤1.1 and in a power range between a few kW and tens of kW.

An optical element is provided. The optical element may comprise a material, the material being a matrix and a set of particles included in the matrix, the material having a molar fraction of SiO.sub.2 higher than or equal to 65 percent, each particle having a dimension smaller than or equal to 80 nanometers.

A multi-clad optical fiber design is described in order to provide low optical loss, a high numerical aperture (NA), and high optical gain for the fundamental propagating mode, the linearly polarized (LP) 01 mode in the UV and visible portion of the optical spectrum. The optical fiber design may contain dopants in order to simultaneously increase the optical gain in the core region while avoiding additional losses during the fiber fabrication process. The optical fiber design may incorporate rare-earth dopants for efficient lasing. Additionally, the modal characteristics of the propagating modes in the optical core promote highly efficient nonlinear mixing, providing for a high beam quality (M.sup.2<1.5) output of the emitted light.