Many ion species commonly used for laser-cooled ion trapping studies have alow-lying metastable 2D3/2 state that can become populated due to spontaneousemission from the 2P1/2 excited state. This requires a repumper laser tomaintain the ion in the Doppler cooling cycle. Typically the 2D3/2 state, orsome of its hyperfine components if the ion has nuclear spin, has a highermultiplicity than the upper state of the repumping transition. This can lead todark states, which have to be destabilized by an external magnetic field or bymodulating the polarization of the repumper laser. We propose usingunpolarized, incoherent amplified spontaneous emission (ASE) to drive therepumping transition. An ASE source offers several advantages compared to alaser. It prevents the buildup of dark states without external polarizationmodulation even in zero magnetic field, it can drive multiple hyperfinetransitions simultaneously, and it requires no frequency stabilization. Thesefeatures make it very compact and robust, which is essential for thedevelopment of practical, transportable optical ion clocks. We construct atheoretical model for the ASE radiation, including the possibility of thesource being partially polarized. Using 88Sr+ as an example, the performance ofthe ASE source compared to a single-mode laser is analyzed by numericallysolving the eight-level density matrix equations for the involved energylevels. Finally a reduced three-level system is derived, yielding a simpleformula for the excited state population and scattering rate, which can be usedto optimize the experimental parameters. The required ASE power spectraldensity can be obtained with current technology.
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