A polarization-stable Er-doped superfluorescent fiber source including a Faraday rotator mirror

We report a tenfold stability improvement in the mean wavelength variations induced by polarization fluctuations in an Er-doped superfluorescent fiber source. This was accomplished by replacing the mirror in a double-pass source with a Faraday rotator mirror, thereby nearly eliminating polarization dependent gain. The resulting mean wavelength variations of less than 3.5 parts-per-million (limited by the detection noise floor), approach the requirement for inertial-grade fiber optic gyroscopes.     

Fundamental limitations of the McCumber relation applied to Er-doped silica and other amorphous-host lasers

The approximate McCumber procedure is often used to predict the emission cross-section spectrum of the 1.5-/spl mu/m transition of Er-doped glass fibers from the transition's measured absorption spectrum. By applying this procedure to a large number of published Er-doped fiber absorption spectra, we demonstrate that its accuracy is actually statistically quite low: it tends to overestimate the peak cross-section (by up to 75%) and predicts an emission spectrum that is erroneously depressed in the S band (below /spl sim/1530 nm) and inflated in the C and L bands. Error levels are substantial and yield unacceptably large errors when modeling Er-doped fiber devices. We provide analytic evidence that this failure is rooted in part in the approximations inherent to the procedure, and in part in a fundamental limitation of the underlying McCumber relation. Specifically, when applied to broad optical transitions, the McCumber relation yields poor predictions of the emission cross-section spectral shape, the error worsening in the L and S bands, with increasing homogeneous broadening, and with increasing bandwidth. The McCumber relation should be avoided for broad laser transitions, which includes most rare-earth transitions in many amorphous hosts.     

Polarized Superfluorescent Fiber Sources

We report the theoretical and experimental performance of new, linearly polarized superfluorescent fiber sources (SFS). Internal polarization is produced either by splicing a fiber polarizer at an optimum location along a standard Er-doped SFS or by using a single-polarization Er-doped fiber. Numerical simulations predict that when operated in the backward, forward, or double-pass configuration, these SFSs produce nearly twice the power in the desired linear polarization as a standard, unpolarized SFS. This efficiency figure depends weakly on the polarizer location and extinction ratio, but requires a low polarizer insertion loss ( < 0.5 dB). Laboratory prototypes of a backward and a forward polarized SFS are presented that exhibit a power output in the desired linear polarization about 1.75 times larger than that of an unpolarized SFS, and an extinction ratio in excess of 17.5 dB.     

A Mueller matrix formalism for modeling polarization effects inerbium-doped fiber

The effects of erbium anisotropy in erbium-doped fiber lasers, sources, and amplifiers are examined. Starting from basic ion properties, inversion and gain equations are derived analytically to describe polarization dependencies. A novel matrix form of the Er³⁺ rate equations is presented to propagate powers and polarization states. These equations are then numerically integrated and compared to experimentally observed polarization hole burning and polarization dependent gain. The theoretical predictions agree strongly with experiment in all cases     

A depolarized Er-doped superfluorescent fiber source with improvedlong-term polarization stability

In a backward Er-doped superfluorescent fiber source (SFS), we report the observation of large mean wavelength variations (>100 ppm) induced by external perturbations of the fiber birefringence. These previously unreported variations, which need to be reduced to the ppm level for high-accuracy fiber-optic gyroscope applications, are shown to originate first from polarization-dependent gain induced by the polarized pump source, and second from a slight polarization dependence of the SFS wavelength division multiplexing (WDM) coupler and fiber isolator. We demonstrate that these effects can be substantially reduced by incorporating two Lyot fiber depolarizers in the source. The new depolarized SFS exhibits short-term mean wavelength stability of ±2.5 ppm and a long-term drift of ±3 ppm. The latter is probably due mostly to slow variations in the Er-doped fiber temperature, which can be eliminated with a simple temperature control to ~0.1°C