Laser transition characterization by spectral and thermal dependences of the transient oscillations

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.