Cladding member for optical fibers and optical fibers formed with the cladding member

An optical fiber suitable for use in fiber lasers and amplifiers, comprising:

• a center core member;
• a first cladding member surrounding the center core member and arranged for receiving pump light energy to be transferred to the core member, the first cladding member being substantially circular in cross-section and having a predetermined refractive index (nc);
• one or more index modified regions formed in the cross section of the first cladding member wherein each index modified region contains one or more dopants selected from a group comprising Ge, Al, P, B and F and is operative to direct light energy to the core member, and a stress field portion with a predetermined refractive index (ns) is formed in an area surrounding each index modified region within the first cladding member;
• and
• a second cladding member surrounding the first cladding member;
• wherein a difference between the refractive index of the first cladding member and that of the stress field portions (nc−ns) is set within such a range that the stress field portions do not substantially affect polarization properties of light traveling in the core member.