The present disclosure is a photonic crystal fiber in which a plurality of holes are formed in a cladding, having a uniform light refractive index, capable of propagating three modes of a fundamental mode, a first higher-order mode, and a second higher-order mode, wherein the plurality of holes are disposed in a triangular lattice pattern so as to surround a center of the photonic crystal fiber with no hole disposed at the center of the photonic crystal fiber, and the photonic crystal fiber has a ratio d/A of a diameter d of each of the holes to a pitch A between the holes such that a confinement loss of a third higher-order mode at a minimum wavelength within a used wavelength range is 1.0 dB/m or more and a confinement loss at a maximum wavelength is 0.001 dB/km or less.

An optical apparatus comprising: a source and a loop. The source generates a pump. The resonating cavity of the source includes: a gain medium; and a tunable filter for selecting a wavelength. The loop comprises: an input coupler; a waveguide; and an output coupler. The input coupler receives the pump and a signal and outputs the pump and the signal into the waveguide In the waveguide, energy in the pump is transferred into energy in the signal while a relative center position of the signal is crossing a center position of the pump in a first direction while both are passing through the waveguide and into the output coupler. The output coupler r outputs a first portion of the signal and a second portion of the signal is fed into the input coupler as the signal, completing the loop.

There is provided a method to fabricate optical taps and waveguide devices in photonic crystal fibers and other fibers with hollow structures. The method involves a preparation step, where the hollow holes inside the fiber are collapsed or partially modified locally; and a waveguide fabrication step, where a femtosecond laser is focused inside the fiber and used to produce optical waveguides that interact in the region that was previously modified in the preparation step.

There is provided a photonic bandgap fiber used in a state in which at least a part of the photonic bandgap fiber is bent at radii of 15 cm or greater and 25 cm or less. A large number of high refractive index portions 57 are disposed in a nineteen-cell core type in three layers, and a V value is 1.5 or greater and 1.63 or less. In the high refractive index portions 57, conditions are defined that a relative refractive index difference is % and a lattice constant is m so as to remove light in a higher mode at the bent portion as described above.

A photonic crystal fiber-based surface plasmon resonance (PCF-SPR) sensor to detect a refractive index of an analyte includes a fiber core having a first scale-down (SCD) cavity having a first diameter, multiple second SCD cavities each having a second diameter, multiple third SCD cavities each having a third diameter, and a groove. A surface of the groove is coated with a metal having a first thickness. The sensor includes an analyte channel having a second thickness and is in contact with the metal. The analyte channel surrounds the fiber core and is configured to stream the analyte. The sensor further includes an outer layer surrounding the analyte channel. The refractive index of the analyte is detected based on an intensity lost by an incident light passing through the PCF-SPR sensor due to dissipation of plasmonic energy.

An optical fiber element wire includes: a bare wire part including a core and a cladding and extending in an axial direction of the bare wire part; a primary layer covering the bare wire part; and a secondary layer covering the primary layer. A Young's modulus of the primary layer is within a predetermined range that removes a void within the primary layer in response to the optical fiber element wire being heated at either 45 C. or 60 C. for 3 minutes or more.