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IEEE Journal of Quantum Electronics

Fluorescence and superfluorescence line narrowing and tunability of Nd/sup 3+/ doped fibers

Gérard Monnom, Bernard Dussardier, Eric Maurice, A. Saı̈ssy, Daniel B. Ostrowsky

Published Jan 1, 1994• 5 citations

Abstract

The inhomogeneous behavior of the 1.06 /spl mu/m-neodymium transitions in doped optical fibers have been investigated, using the fluorescence line narrowing technique, pumping on the /sup 4/F/sub 3/2/ and /sup 4/F/sub 5/2/ sublevels at 4 K. Each observed transition has been identified. As the pump wavelength varies, the shift of the main fluorescence line is 40 nm, with the two pumping levels. We have studied the spectral behavior of the superfluorescence as a function of the pump wavelength, the temperature, and the absorbed power. The spectral evolution depends on /spl lambda//sub p/ with 19 nm-tuning range at low temperature. At 300 K, the quasihomogeneous behavior of the transition decreases the tunability to 14 nm. Based on these results, we present a simple technique permitting precise prediction of gain and spectral line shapes of superfluorescent Nd-doped fiber sources.>

DOI: 10.1109/3.328588

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The inhomogeneous behavior of the 1.06 /spl mu/m-neodymium transitions in doped optical fibers have been investigated, using the fluorescence line narrowing technique, pumping on the /sup 4/F/sub 3/2/ and /sup 4/F/sub 5/2/ sublevels at 4 K. Each observed transition has been identified. As the pump wavelength varies, the shift of the main fluorescence line is 40 nm, with the two pumping levels. We have studied the spectral behavior of the superfluorescence as a function of the pump wavelength, the temperature, and the absorbed power. The spectral evolution depends on /spl lambda//sub p/ with 19 nm-tuning range at low temperature. At 300 K, the quasihomogeneous behavior of the transition decreases the tunability to 14 nm. Based on these results, we present a simple technique permitting precise prediction of gain and spectral line shapes of superfluorescent Nd-doped fiber sources.>

A detailed investigation is made of the spectral properties of several superfluorescence operations in Nd-doped and Er-doped silica fibres pumped by a 514.5 nm Ar ion laser. The study demonstrates the spectral narrowing processes in the operations of superfluorescence. In Nd-doped fibre, the higher gain at 1080 nm suppresses the emission at about 910 nm, but the authors have still realized, for the first time, superfluorescence in this wavelength band, and a 0.4 mW output was achieved in a double-pass configuration. Also, the process of change from superfluorescence to lasing was recorded and analysed.

A study of the complex atomic susceptibility of erbium-doped fiber amplifiers operating near the 1.53- mu m transition (/sup 4/I/sub 13/2/-/sup 4/I/sub 15/2/) developed from a semiclassical theoretical description is presented. Expressions for the emission cross section sigma /sub e/( lambda ) and absorption cross section sigma /sub a/( lambda ) of erbium:glass as a quasi-three-level laser system with Stark-split sublevel manifolds are derived. The results are used to calculate the cross sections of aluminosilicate erbium-doped fibers at low temperatures (T=4.2, 77 K) and at room temperature. It is demonstrated that an expression for the gain coefficient of the fiber amplifier can be derived from the expression of the susceptibility. Using Kramers-Kronig relations, an expression for the refractive index change in the fiber is derived. The theoretical spectral gain profile of the fiber amplifier and concurrent refractive index changes are analyzed for different pumping regimes.>

Spectroscopic measurements on (4)F(3/2) - (4)I(11/2) and (4)F(3/2) - (4)I(9/2) transitions for a Nd(3+)-doped fiber laser demonstrated that the wavelength dependence of the relaxation oscillations provides a simple method to distinguish threeand four-level transition. Spectral analysis of relaxation oscillations in a tunable Er(3+)-doped fiber laser was performed at liquid-nitrogen temperature, and results were compared with measurements at room temperature. It was found that the relaxation oscillation frequency depends on the occupation of the terminal level and thus is directly related to the absorption cross section. Using this method, we show that laser transitions at lambda > 1555 nm become four-level at T = 77K.

Summary form only given. Recently it has been shown that small changes in pump wavelength within the 980-nm pump band change the shape of the gain spectrum of erbium-doped fiber amplifiers (EDFAs). These gain variations can be as large as 3 dB in a typical line amplifier and cannot be explained by the well known homogeneous model for EDFAs. Pump-induced gain changes must be accounted for in multiwavelength amplifiers and systems operating over the pump-wavelength-sensitive short-wavelength portion of the erbium gain band. In this paper, we compare the impact of pump-mediated inhomogeneities on the gain spectra of EDFAs pumped in the 1480- and 980-nm pump bands. We find that the shape of the EDFA gain spectrum is less affected by changes in pump wavelength in the 1480-nm band, and show that in both cases the dynamic gain tilt function (DGTF) is relatively insensitive to changes in pump wavelength. The latter indicates that inversion-induced gain changes can be predicted from a (measured) DGTF despite pump band and some signal band inhomogeneities. We present fluorescence line narrowing measurements that explain these results and suggest a physical mechanism for 980-nm band pump-mediated inhomogeneities.