An object of the present disclosure is to allow gain compensation with a simple configuration without adding a new device to the outside. The present disclosure discloses a rare earth doped fiber including a core doped with a rare earth and a cladding region around the core, wherein one or more cavities are provided inside the core, and at least one of the cavities are provided along a longitudinal direction of the rare earth doped fiber.

An optical fiber configured to improve the pump conversion efficiency of an L-band fiber amplifier which uses the multimode pump source. By directly absorbing multimode light including 915 nm, an active fiber core region co-doped with both erbium and ytterbium can provide gain to the L-band signals via stimulated emission. The unwanted C-band amplified spontaneous emission (ASE) light generate from this active fiber core region can be absorbed by another active fiber core region doped with erbium, then provides additional gain to the L-band signals. Active regions and cladding can be configured to match a given spatial mode of the optical signal. Signal-pump combiners with end-coupling or side coupling can be used.

In some implementations, a waveguide may comprise an inner core to receive a first beam and an outer core surrounding the inner core to receive a second beam that is displaced from the first beam by an offset. The outer core may comprise a beam guiding region that rotationally expands over a length of the waveguide into an annulus that concentrically surrounds the inner core or a partial annulus that partially surrounds the inner core. For example, the beam guiding region may be defined by one or more low refractive index features that have a varied orientation and/or a varied shape over the length of the waveguide such that the second beam enters the waveguide as an offset beam and exits from the waveguide as a ring-shaped beam or a partial ring-shaped beam.

Various example embodiments relate to active optical fibers and devices using active optical fibers. An active optical fiber may comprise a central part surrounded by an annular active core. The fiber may have a tapered longitudinal profile such that the fiber comprises a single-mode portion and a multimode portion. The annular core may have low birefringence, obtained for example by rotating (spinning) the fiber preform during manufacture of the fiber. Refractive index of the annular core may be higher than the refractive indices of the central part and cladding layer(s) surrounding the annular core. The active optical fiber enables selective generation or amplification of light modes with orbital angular momentum (OAM). Furthermore, the fiber has a large mode field diameter (MFD) and it is not sensitive to internal heating or environmental influences.

A stimulated Raman scattering (SRS) spectrometer for real-time, high-resolution molecular analysis of gases is based on two hollow-core fibres illuminated by a single high-power, short-pulse laser pump. The first fibre is prefilled with high-concentration target gases. Interaction of each target gas inside the first fibre, with the laser pump, generates Raman signals corresponding to the target gases. The combined beam of the Raman signals and the pump laser beam is directed into the second fibre containing the measured target gases. Interaction of each target gas with the combined beam generates the Stimulated Raman Growth (SRG), i.e., amplification of the Raman signal, which is proportional to the corresponding target gas concentration. A receiver subsystem receives the beam from the second fibre, spectrally separates it to wavelengths corresponding to each target gas, extracts the SRG value corresponding to each target gas and calculates the concentration of each target gas.

The present disclosure discloses an optical device structure including an optical fiber including a core part, a clad part, and a three-dimensional micro hole structure in the clad part, wherein a surface of the three-dimensional micro hole structure is provided with at least a non-flat surface, and a conformal graphene layer is formed on the surface of the three-dimensional micro hole structure, and a method of manufacturing the same.

Incoherently combining light from different lasers while maintaining high brightness is challenging using conventional fiber bundling techniques, where fibers from different lasers are bundled adjacently in a tight-packed arrangement. The brightness can be increased by tapering the tips of the bundled fibers to match a single, multi-mode output fiber, e.g., one whose core that is just wide enough to fit the input cores. This increases the brightness of the beam combining. In addition, reducing the outer diameters of the signal fiber claddings allows the signal fibers to be bundled closer together, making it possible to couple more signal fiber cores to the core of a multi-mode output fiber. Similarly, reducing the outer diameter of the pump fiber cladding and/or etching away corresponding portions of the signal fiber cladding in a pump/signal combiner makes it possible to couple more pump light into the signal fiber cladding, again increasing brightness.

An optical fiber may include a core in which core-guided light generated by one or more light sources propagates along a length of the at least one optical fiber, one or more claddings, surrounding the core, to guide cladding-guided light generated by the one or more light sources along the length of the at least one optical fiber, and a reflector structure machined into the at least one optical fiber. The reflector structure may include multiple angled facets arranged at one or more respective angles relative to an axis of the optical fiber to reflect at least a portion of the core-guided light and/or the cladding-guided light passing through the optical fiber.

A fiber amplifier is provided, including a pump laser (202), a pump and signal combiner (203), and a few-mode doped fiber (204). The pump laser (202) is configured to output pump light. The pump and signal combiner (203) is configured to couple input few-mode signal light and the pump light into the few-mode doped fiber (204). Refractive indexes of a fiber core of the few-mode doped fiber (204) are distributed to be gradient along a radial direction of a cross section, the fiber core is etched with periodic gratings along an axial direction, and periods of the gratings satisfy a phase matching condition. The fiber amplifier achieves strong coupling and co-amplification between optical signal modes, thereby reducing a differential gain between mode groups.

An object of the present disclosure is to implement a clad-excitation rare-earth-added optical fiber amplifier with a high light-to-light conversion efficiency. The present disclosure is an optical fiber amplifier having, in a longitudinal direction of a rare-earth-added optical fiber, a light collection structure that collets an excitation light, which propagates through a clad portion, into a core portion.