Analysis of color-center-related contribution to Bragg grating formation in Ge:SiO 2 fiber based on a local Kramers -Kronig transformation of excess loss spectra.

UV-induced excess losses have been measured at various pulse energy densities and exposure times in germanosilicate optical fiber preform cores. The corresponding refractive-index changes have been determined through a Kramers -Kronig analysis. Because of the nonlinear behavior of the excess losses as a function of both exposure time and fluence per pulse, one should be careful when comparing the refractive-index modulation deduced from such measurements with that obtained from Bragg grating reflectivity. Indeed nonlinear effects such as saturation imply that it is necessary to take into account the local character of the change in absorption to calculate the evolution of the refractive-index modulation accurately as a function of the exposure time. Implications of these results are discussed.

250-fs optical pulse generation by simultaneous soliton compression and shaping in a nonlinear optical loop mirror including a weak attenuation.

We report the generation of 30-W peak-power 250-fs-wide pedestal-free optical pulses at 1.54 microm by use of a chirpcompensated optically amplified semiconductor laser followed by simultaneous soliton compression and shaping within a nonlinear optical loop mirror. A weak controllable attenuation is inserted within the loop to permit precise control of the pulse shape. Pedestal-free pulses are obtained after optimization of both the fiber length and the power balance between the counterpropagating pulses within the loop. The corresponding spectrum reveals a twofold split structure that is due to a residual chirp from self-phase modulation.

Ultraviolet-induced permanent Bragg gratings in cerium-doped ZBLAN glasses or optical fibers.

A holographic method was used to write refractive-index gratings in ZBLAN fluoroziroconate glasses or fibers doped by a Ce concentration of 5000 or 10,000 parts-in-10(6) weight. Direct pumping of the (2)F(5/2)-5d transitions of Ce(3+) ions near 245 nm resulted in a change in the refractive index. The photoinduced change partly recovered on a time scale of several hours at room temperature. The remaining change in the refractive index looked stable on a time scale of a month. This permanent change reaches 2 x 10(-5) at 1560 nm.

Channeled spectrum of a fiber laser.

We describe the polarization properties of a long Nd(3+)-doped optical fiber laser. An intracavity prism acts as a dichroic element and fixes the polarization of the laser beam essentially in its plane of incidence. The slight birefringence of the fiber makes a wavelength-dependent phase plate that modifies the state of the polarization. In the laser output spectrum, the azimuth for the elliptically polarized field can thus rotate by several times pi/2 if the fiber is long enough. This gives a channeled spectrum when observed through a polarizer.

Analytic modeling of high-gain erbium-doped fiber amplifiers.

We describe an analytic method that calculates accurately (within a 1.5-dB discrepancy with numerical models) the gain of an erbium-doped fiber amplifier. Amplified spontaneous emission (ASE) is taken into account so that the gain of ASE-saturated erbium-doped fiber amplifiers is calculated properly. It is effective for wavelength multiplexing (several signals) and for different pumping schemes (copropagating or counterpropagating or both).