The ytterbium (Yb) and erbium (Er) fibers used in ultrafast fiber lasers are, like any other optical fibers, subject to chromatic dispersion, which results in pulse broadening. To reduce or eliminate dispersion in ultrafast fiber lasers, OFS (Norcross, GA) developed an all-fiber module called FemtoComp in its Brøndby, Denmark facility.
Dispersion is defined as “anomalous” when short wavelengths travel faster than long ones. Erbium fibers in the 1550 nm range are easily compensated because of the availability of conventional fibers with both normal and anomalous dispersion at this wavelength; however, the fundamental modes of conventional and ytterbium fibers have normal dispersion between 1000 and 1100 nm. Bulk-optic components often used to compensate dispersion reduce stability and increase loss in the fiber cavity. Conventional fibers can be designed, however, so that higher-order modes (HOMs) exhibit anomalous dispersion. The all-fiber module consists of a mode converter (long-period grating) on the input (converting the mode into an HOM), a length of HOM fiber where the mode experiences anomalous dispersion, and a mode converter on the output. Contact Poul Erik Schmidt at [email protected].