Dario G. Falquier

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 transitions 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.

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.

A depolarized Er-doped superfluorescent fiber source with improved long-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 /spl plusmn/2.5 ppm and a long-term drift of /spl plusmn/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 /spl sim/0.1/spl deg/C.

Polarized superfluorescent fiber source.

We report the demonstration of a polarized superfluorescent fiber source (SFS) that increases the power output in the desired linearly polarized component by a factor of 1.76 over that of a standard, unpolarized SFS that uses the same pump power. This increase in efficiency is accomplished by insertion of a low-loss polarizer at an optimum point in the erbium-doped fiber of a standard SFS.

Basis for a polarized superfluorescent fiber source with increased efficiency

We propose to produce a superfluorescent fiber source (SFS) with a linearly polarized output by inserting a discrete polarizer near the middle of a polarization-maintaining Er-doped fiber. We show that with a relatively low-loss polarizer (<0.5 dB) the output power of the polarized SFS is predicted to approach that of a standard, unpolarized SFS, i.e., the power of the desired polarization mode is nearly doubled. The efficiency of this source depends weakly on the polarizer location and extinction ratio, although it depends strongly on its insertion loss.

Measurements and modeling of the output polarization of Er-doped fiber lasers

We report a new theoretical study of the effects of anisotropic erbium dipoles on the polarization of Er-doped fiber lasers (EDFLs), as well as measurements of the polarization of EDFLs made of a polarization-maintaining fiber. Convenient closed-form expressions are presented for the gain of a signal polarized either parallel or perpendicular to the pump. These results are used to interpret the polarization behavior of our EDFLs, stressing the effects of pump orientation, fiber length, and cavity Q. We identify in particular operating conditions that produce a fiber laser with either a linearly polarized output or equal power in both polarizations.

Improved polarization stability of the output mean wavelength in an Er-doped superfluorescent fiber source incorporating a Faraday rotator mirror

Erbium-doped superfluorescent fiber sources (SFS) in the standard forward, backward, and double-pass configurations exhibit significant dependencies of the output mean wavelength on the pump state of polarization. This is due largely to polarization-dependent gain (PDG) and gives rise to long-term drifts of the source output mean wavelength through changes in the fiber birefringence. By replacing the mirror in a double- pass source with a Faraday rotator mirror (FRM), the magnitude of the SFS mean wavelength variations due to pump polarization changes is reduced by a factor of 5 - 40, depending on source design parameters. Simulation results as well as experimental demonstration of the Faraday double-pass SFS are discussed in this paper. We measured 10 parts per million for the polarization-related mean-wavelength variations in this source, approaching the requirement for high-accuracy fiber gyros.

Polarization dependence of the mean wavelength of Er-doped superfluorescent fiber sources

Changes in the pump polarization state are shown to have a large effect on the mean wavelength of erbium-doped superfluorescent fiber sources due to polarization-dependent gain. This effect causes long-term drift as the fiber birefringence in the source changes over time. By depolarizing the pump, this effect is reduced to under 3 ppm, the noise level of our instrument. A second depolarizer is used to compensate for polarization-dependent transmission of the other source components, namely the WDM coupler and the isolator. The resulting source is insensitive to drifts in the fiber birefringence due to environmental effects.

Highly efficient polarized Er-doped superfluorescent fiber source

We report the experimental demonstration of a polarized superfluorescent fiber source (SFS) made by inserting a low- loss polarizer at an optimum position along the erbium-doped fiber of a standards SFS. This source produces a strongly polarized output with nearly the same output power as a standard, unpolarized SFS. We show good agreement between the behavior of the source and numerical simulations.