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Applied Physics Letters

Spectroscopy of the transient oscillations in a Nd3+-doped fiber laser for the four-level 4F3/2-4I11/2 (1060 nm) and three-level 4F3/2-4I9/2 (900-nm) transitions

Oleg G. Okhotnikov, José R. Salcedo

Published May 16, 1994• 24 citations

Abstract

We performed spectroscopic measurements of the relaxation oscillations in a tunable Nd3+-doped fiber laser for the four-level (4F3/2-4I11/2) and for the three-level (4F3/2-4I9/2) transitions. We observed that the relaxation oscillation frequency for the three-level transition exhibited a strong wavelength dependence; in contrast, the relaxation oscillation frequency for the four-level transition did not exhibit any wavelength dependence. A simple theory to explain this remarkable feature was developed, confirming that the wavelength dependence is related to the thermal laser level population, which is significant in three-level systems but negligible in four-level systems. This should be taken into account for a careful description of laser systems, and is also instrumental for predicting oscillator or amplifier performance.

DOI: 10.1063/1.111523

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We performed spectroscopic measurements of the relaxation oscillations in a tunable Nd3+-doped fiber laser for the four-level (4F3/2-4I11/2) and for the three-level (4F3/2-4I9/2) transitions. We observed that the relaxation oscillation frequency for the three-level transition exhibited a strong wavelength dependence; in contrast, the relaxation oscillation frequency for the four-level transition did not exhibit any wavelength dependence. A simple theory to explain this remarkable feature was developed, confirming that the wavelength dependence is related to the thermal laser level population, which is significant in three-level systems but negligible in four-level systems. This should be taken into account for a careful description of laser systems, and is also instrumental for predicting oscillator or amplifier performance.

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.

An experimental method was developed for the determination of the rate of relaxation of the 4I11/2 state and of the absorption cross section of the 4I11/2–4F3/2 transition. These parameters were deduced from the dependences of the intensity of the λ = 0.88 μ luminescence, excited as a result of the 4I11/2–4F3/2 transition by laser pulses of λ = 1.06 μ wavelength, on the energy density and duration of the laser pulses. Estimates of the relaxation time obtained for the investigated silicate (GLS-1) and phosphate (LGS-40, LGS-I) glasses, and also for a liquid based on POCl3-SnCl4, gave τ21 < 2 nsec. A considerable increase in the population of the 4I11/2 level occurred as a result of the Raman scattering of high-power coherent radiation by vibrations of the PO4 group.

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.>

To explain the high third-order nonlinearity observed in rare earth doped fibers, we examine the role of the various transitions of the dopants. We present experimental evidence that in fibers doped with Er3+, Nd3+, or Yb3+, most of the nonlinearity is caused by the dopant's UV transitions, i.e., it is non-resonant in nature. The magnitude of the nonlinearity is shown to be in excellent quantitatively agreement with a model. Fiber modulators and switches based on this widespread mechanisms are predicted to operate over a large range of signal wavelengths and with negligible signal loss.