A differential mode attenuation compensator includes a first multi-mode optical fiber and a third multi-mode optical fiber in which a plurality of propagation modes propagate in a wavelength of a propagating optical signal; and a second multi-mode optical fiber including a core and a clad and arranged with a central axis aligned between the first multi-mode optical fiber and the third multi-mode optical fiber, in which each loss in the plurality of propagation modes is different in the first multi-mode optical fiber and the third multi-mode optical fiber.

An object of the present invention is to provide a structure of an optical fiber capable of satisfying desired requirements of an output power, a propagation distance, and a beam quality. In the design of the PCF of the present invention, the PCF has air holes having diameters d and intervals in an overlapping region where a region of A.sub.eff of a desired value or more and a cutoff region in a desired higher-order mode overlap each other on a graph where the horizontal axis represents d/ and the vertical axis represents , so that it is possible to sufficiently cut off the mode which is the desired higher-order mode or more, and thus, it is possible to select a region where the A.sub.eff is large.

A multi-core fiber performs communication using light up to an xth-order LP mode (where x is an integer of 1 or more) in a communication band. The multi-core fiber includes: a plurality of signal light propagation cores that propagate light up to an (x+1)th-order LP mode; and at least one high-loss core that has a higher loss of propagated light than the signal light propagation cores. Crosstalk occurs between light of the (x+1)th-order LP mode propagated through at least one signal light propagation core and light of a primary LP mode propagated through at least one high-loss core.

The present disclosure provides a few mode optical fiber. The few mode optical fiber includes a core region. A core region defined by a region around a central longitudinal axis of the few mode optical fiber. In addition, the core region has a first annular region extended from central longitudinal axis to radius r.sub.1, a second annular region extended from radius r.sub.1 to radius r.sub.2, a third annular region extended from radius r.sub.2 to radius r.sub.3, a fourth annular region extended from radius r.sub.3 to radius r.sub.4 and a fifth annular region extended from radius r.sub.4 to radius r.sub.5. Also, the few mode optical fiber has a cladding defined by the sixth annular region extended from radius r.sub.5 to radius r.sub.6.

Disclosed is an optical fiber which includes a core including silica with a core diameter and having at least two dopants, a maximum relative refractive index delta of at least 0.7% and an alpha value in the range of 1.9-2.2. The core has a refractive index profile configured to transmit light in a multimode propagation at a first wavelength .sub.1 in the range of 800-1100 nm and to propagate light in a LP01 mode at a second wavelength .sub.2. The second wavelength .sub.2 is greater than 1200 nm. The optical fiber is structured to have a LP01 mode field diameter in the range of 8.5-12.5 m at 1310 nm.

A higher-order mode (HOM) fiber is configured as a polarization-maintaining fiber by including a pair of stress rods at a location within the cladding layer that provides for a sufficient degree of birefringence without unduly comprising the spatial mode profile of the propagating higher-order modes. Long-period gratings are used as mode couplers at the input and output of the PM-HOM fiber, where the gratings are formed by exposing areas of the core region orthogonal to the position of the stress rods. The diameter of the stress rods (D) and displacement of the rods from the center of the core region (R1) are controlled to yield a configuration with an acceptable birefringence and polarization extinction ratio (PER) within the HOM fiber, even in situations where the fiber is bent (a bend radius less than 50 cm).

A multicore fiber includes: a first core configured to propagate an LP.sub.01 mode, an LP.sub.11 mode, and an LP.sub.21 mode light beam; and a second core configured to propagate an LP.sub.01 mode light beam, wherein a different mode interaction section is provided in which a propagation constant of the LP.sub.21 mode light beam propagated through the first core is matched with a propagation constant of the LP.sub.01 mode light beam propagated through the second core, a different mode non-interaction section is provided in which propagation constants of the LP mode light beams propagated through the first core are not matched with propagation constants of the LP mode light beams propagated through the second core, and the first core includes an inner core and an outer core surrounding the inner core with no gap and having a refractive index higher than a refractive index of the inner core.

The present disclosure relates more to mode mixing optical fibers useful, for example in providing optical fiber laser outputs having a desired beam product parameter and beam profile. In one aspect, the disclosure provides a mode mixing optical fiber that includes a core having a refractive index profile; and a cladding disposed about the core. The core of the mode mixing optical fiber supports at least two (e.g., at least five) guided modes at the wavelength. The mode mixing optical fiber is configured to substantially distribute optical radiation having the wavelength propagating therein (e.g., input at its input end or generated or amplified within the core) among a plurality of the guided modes (e.g., to distribute a substantial fraction of the optical radiation having the wavelength propagating therein from its lower-order guided modes to its higher-order guided modes).

A multicore fiber communicates using light up to an xth-order LP mode (where x is an integer of 1 or more) in a communication band. The multicore fiber includes: a plurality of cores; a clad that surrounds the plurality of cores and has a refractive index lower than refractive indexes of the plurality of cores; and a cover layer that covers the clad and has a refractive index higher than the refractive index of the clad. Each of the plurality of cores propagates light up to an (x+1)th-order LP mode. A core pitch is set to a distance where crosstalk of the light up to the xth-order LP mode becomes less than or equal to 40 dB/km and crosstalk of light of the (x+1)th-order LP mode becomes greater than or equal to 30 dB/km.

A multicore fiber includes: a center core that propagates four LP mode light beams including an LP.sub.02 mode light beam; and a first to a fifth cores disposed on a first line to a fifth line segments extend from the center of the center core in the radial direction at predetermined angles. The multicore fiber includes a different mode interaction section in which the propagation constants of each mode light beam propagated through the center core are matched with the propagation constants of LP.sub.01 mode light beams propagated through the first to fifth cores.