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Schematic of measurement system. Photoluminescence is excited by a probe laser at one end of the sample, modulated at . The PL output, as a function of wavelength, is measured at lock-in 1 referenced to this frequency. The PL passes through the waveguide region optically pumped by the pump laser, modulated at . The change in the probe PL, , is measured by lock-in 2 referenced to the pump modulation frequency.
(a) Curve 1 (●) shows the PL due to the probe laser only, taken from the output of lock-in 1. The region between 1500 and has also been expanded vertically to better reveal the structure on the spectrum in this region. Curve 2 (▴), coincident with the origin, is the output from lock-in 2 with the pump laser on and probe laser off. Curve 3 (∎) is the change in PL, , with both the probe and pump lasers on and gives the change in the PL due to the probe laser induced by the pump laser. The curves have been displaced for clarity; in all cases the dashed line is the zero axis. (b) Data of (a) converted to , where with the pumped length.
(a) Normalized as a function of temperature, measured at , for samples S1 (▴), S2 (+), S3 (◆), and S4 (◻). The solid lines are guides to the eye. Inset shows spectra of for sample S2 for three different temperatures: (solid line), (dashed line), and (crossed line). (b) measured at for the same samples as a function of pump power density. The solid lines are theoretical fits to the data. (c) A plot of , obtained from the theoretical fits to the data in (b) against the effective Er concentration. The dashed line is calculated using the expected value for the optical cross section of .
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