Active optical fiber and method for fabricating an active optical fiber

1. A section of active optical fiber comprising

a core with a first index of refraction, said core being active,

an inner cladding layer with a second index of refraction for propagating pump radiation, said second index of refraction being smaller than said first index of refraction and

an outer cladding layer with a third index of refraction around said inner cladding layer, said third index of refraction being smaller than said second index of refraction,

wherein the diameter of said core and the thickness of said inner cladding change gradually along the length of said section of active optical fiber forming a tapered longitudinal profile enabling a continuous mode conversion process along the length of the section of fiber and the tapered core supporting multimode operation at the thicker end of the section of fiber, wherein said tapered longitudinal profile of said section of active optical fiber is selected from a group of profiles including a power law profile, an exponential profile and a combination of these profiles.

2. The section of active optical fiber of claim 1 wherein said core of said section of active optical fiber supports single mode operation at the thinner end of the section of fiber.

3. The section of active optical fiber of claim 1 , wherein the outer boundary of said inner cladding layer has a non-circular cross section.

4. The section of active optical fiber of claim 1 , wherein said core of said section of active optical fiber is offset from the middle of the fiber.

5. The section of active optical fiber of claim 1 , wherein said section of active optical fiber comprises a third cladding layer with a fourth index of refraction around said outer cladding layer, said fourth index of refraction being smaller than said third index of refraction.

6. The section of active optical fiber of claim 1 , wherein said core of said section of active optical fiber is strongly birefringent, the core having a refractive index difference of greater than 5×10 −5 between the ordinary and extraordinary polarizations.

7. The section of active optical fiber of claim 1 , further comprising pump-light coupled to said section of fiber at a angle lower than a maximum angle dictated by numerical aperture of the fiber.

8. A method for fabricating a section of active optical fiber, said method comprising

fabricating a preform for drawing active optical fiber from said preform in a drawing tower,

installing said preform into a drawing tower,

drawing optical fiber in said drawing tower and

altering at least one of the two parameters including the take-off preform speed and the take-up fiber speed during drawing of the optical fiber to synthesize a tapered longitudinal profile for said section of active optical fiber, wherein said tapered longitudinal profile of said section of active optical fiber is selected from a group of profiles including a power law profile, an exponential profile and a combination of these profiles.

9. The method of claim 8 wherein said method, further comprises

coating the optical fiber with a polymer.

10. The method of claim 8 wherein said method further comprises

altering the temperature of said preform to synthesize a tapered profile for said section of active optical fiber.

11. The method of any one of claim 8 wherein said method further comprises the step of

pre-drawing the preform before drawing said active optical fiber.

12. A sequence of sections of active optical fiber wherein said sequence comprises more than one of the sections of active optical fiber according to claim 1 residing in sequence to form a section of fiber.

13. The sequence of sections of active optical fiber of claim 12 wherein a pump-light is coupled to said sequence of sections of active optical fiber from both ends of said sequence.