[SOLVED] Description Index Anti Grading in laser?

in document and ebook laser diode i saw this idiom "Index Anti Grading "?
i didn't know about it and i read but i can not understand it.
i will search and share my conclusion here !
too i will be glad to anyone help me.


best regards stackprogramer

Re: Description Index Anti Guiding ?

i was wrong ,i wanted Description Index Anti Guiding

In the past decade and a half, fiber lasers with very high multimode output powers have been developed by doping the core of such fibers with ions that can lase. These fiber lasers are pumped by light from diode-laser arrays that is coupled into both the core and the cladding, exciting ions over the entire length of the fiber.

An inherent problem with these powerful lasers is that they cannot readily be made to oscillate in a single transverse mode, so the beam quality is not suitable for some applications. Single-mode oscillation requires a small core, typically 8 to 10 μm in diameter. Larger cores allow high-order modes to oscillate. But the small core leads to high intensities, and high intensities propagating over long distances lead to deleterious nonlinear effects such as stimulated Raman and Brillouin scattering. Many researchers have proposed clever ways of avoiding the nonlinearities, but not one of these to date has provided the robustness and reliability suitable for all applications.

Index-antiguided fiber

An alternative type of fiber laser has been demonstrated in our lab at CREOL at the University of Central Florida’s College of Optics and Photonics in Orlando. This laser oscillates only on the lowest-order transverse mode, and because the core can be large, the optical intensity in the core will be limited, so the laser can be scaled to very high powers.

When you think of an optical fiber, you automatically assume that the core has an index of refraction that is greater than that of the cladding. Light traveling in the core is totally internally reflected from the core-cladding interface and can travel for great distances with minimal loss. Such fibers are the basic transmission mechanism of our modern telecommunications systems. But unlike conventional optical fibers, the fiber we demonstrated does not rely on total internal reflection to confine light to the core. Indeed, the refractive index of the cladding is greater than that of the core, so that the fiber is index-antiguided. Light generated in the core of the fiber can leak out into the cladding, though it experiences some guiding by grazing incidence Fresnel reflection at the core-cladding interface. However, there is loss at each reflection, so the fiber cannot sustain propagating modes the same way a traditional index-guided fiber does.

Now consider what happens when the core of an index-antiguided fi-ber is doped with lasing ions and optically pumped. If pumped hard enough, a point is reached where the gain between core-cladding reflections is equal to the loss at each reflection. This point is the threshold for what is called “gain-guiding,” where propagation through a length of fiber takes place with no net loss. This threshold is different from the threshold for laser oscillation: Laser threshold occurs when the total gain during a round-trip of the laser resonator becomes equal to the round-trip loss.
more info
https://www.photonics.com/Article.aspx?AID=30100