To produce such systems, weakly guiding single-mode fibers are generally used, to obtain a unimodal propagation and a large core radius.
Any optical source required to deliver power (or energy) over a distance must have the highest radiance possible so as to provide the strongest irradiance (or fluence) on a target.
Fiber lasers are, at the present time, the sources that enable the highest radiances to be obtained. The radiance of a source is given by the formula:
      Rad    ⁡          (                        W          /                      m            2                          ·        sr            )        =                    P        L            ⁡              (        W        )                            S        ⁡                  (                      m            2                    )                    ⁢              Ω        ⁡                  (          sr          )                    PL being the radiant flux delivered by the source, S is the emissive area of the source, and Ω represents the solid angle of the beam. The productS×Ωis called the throughput, which is an optical invariant.
For an optical fiber in a single-mode (or unimodal) operating regime the relationship between the core radius a and the relative index difference
  Δ  ≈                    n        1            -              n        2                    n      1      (n1 corresponding to the refractive index of the core material and n2 to the refractive index of the optical cladding) is given by the following equation, published in the article “Weakly guiding fibers” by D. Gloge, Appl. Opt. 10 (10), October 1973, p. 2252:V=ka√{square root over (n12−n22)}≈kan1√{square root over (2Δ)}≤2.4048where V is a parameter called the normalized frequency andk=2π/λ,with λ the wavelength. This relationship is applicable to step-index fibers. FIG. 1 shows the various optical and geometrical parameters that characterize a standard optical fiber.
So as to reduce the appearance of nonlinearity effects, waveguiding structures having the largest mode area possible are preferably used.
To this end, the following problem should be solved: a large mode-dimension requires a very weakly-guiding fiber. A very weakly-waveguided propagation makes the handling of the fiber very critical because the fiber is very sensitive to any bending or microbending. However, a strongly-guiding fiber implies a small core radius, which is unsatisfactory due to the associated nonlinearity effects.