Optical fiber for Coherent Anti-Stokes Raman scattering endoscopes

摘要

An optical fiber for use in a Coherent Anti-Stokes Raman Scattering (CARS) endoscope, comprising a core guiding lightwaves at a pump wavelength and at a Stokes wavelength, the core being single-mode at both wavelengths. The core is surrounded by cladding layers, including an inner cladding layer, a trench cladding layer, an intermediate cladding layer and an outer cladding layer. The refractive index of the trench cladding layer is lower than those of both neighboring cladding layers so as to define a trench in the radial refractive-index profile. The bending losses of the fundamental LP01 mode of the fiber at the Stokes wavelength are limited while maintaining high confinement losses for the higher-order LP11 mode of the fiber at the pump wavelength. The combination of the intermediate and outer cladding layers forms a multimode waveguide for guiding a collected CARS signal generated by an object or medium probed with the endoscope.

主权项

1. An optical fiber for use in a Coherent Anti-Stokes Raman Scattering (CARS) endoscope, the optical fiber comprising:
a core having a radius a1 and a refractive index n1, the core extending along a longitudinal axis, the core being adapted to guide a lightwave at a Stokes wavelength and a lightwave at a pump wavelength along a first propagation direction parallel to the longitudinal axis, said core being single-mode at the Stokes and pump wavelengths, each of the lightwaves at the Stokes and pump wavelengths having a direction of polarization; an inner cladding layer surrounding the core, the inner cladding layer having a refractive index n2, the refractive index n1 being higher than the refractive index n2, the refractive index n1 and the refractive index n2 being chosen to obtain a predetermined value of the numerical aperture of the core; a trench cladding layer surrounding the inner cladding layer and having a trench inner radius a2, a width W, and a refractive index n3, the refractive index n3 being lower than the refractive index n2 of the inner cladding layer; an intermediate cladding layer surrounding the trench cladding layer and having a refractive index n4, the refractive index n4 being higher than the refractive index n3 of the trench cladding layer by a depth δn2 equal to a difference n4- n3; two stress-applying parts enclosed within the intermediate cladding layer and disposed symmetrically on either side of the longitudinal axis, each stress-applying part being disposed substantially parallel to the longitudinal axis, the stress-applying parts maintaining the directions of polarization of the lightwaves at the Stokes and pump wavelengths as the lightwaves are guided within the core; an outer cladding layer surrounding the intermediate cladding layer and having a refractive index n5, the refractive index n5 being lower than the refractive index n4 of the intermediate cladding layer; wherein a combination of the intermediate and outer cladding layers is adapted to allow the optical fiber to act as a multimode optical waveguide for guiding a lightwave at a CARS wavelength along a second propagation direction parallel to the longitudinal axis and opposite to the first propagation direction; and wherein the width W and depth δn2 of the trench cladding layer are selected to limit bending losses of a fundamental LP01 mode of the optical fiber at the Stokes wavelength below 0.5 dB/turn for a fiber curved with a radius of about 2 cm while maintaining confinement losses above 5 dB/m for a higher-order LP11 mode of the optical fiber at the pump wavelength.