Optical fiber for coherent anti-stokes raman scattering endoscopes

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 LP10 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.