We describe a novel type of optical oscillator whose gain element is located outside the resonator. Although this is a common technique for microwave oscillators, separated-gain-schemes are rarely used in the optical regime. However, the Q-factor of an empty resonator may be orders of magnitude higher than its counterpart with internal gain element, leading to substantial reduction of the quantum-noise limited emission line width. Thus, oscillators of this kind are highly suited for demanding applications like interrogation oscillators for optical frequency standards or flywheel oscillators in synthesis chains. As a further advantage of our approach, its gain can be chosen almost arbitrarily high since it is provided by electronic amplifiers. We demonstrate, that the oscillator is capable of generating highly coherent light, starting from incoherent pump light. The incoherent optical pump field used during these initial experiments has been prepared by filtering the coherent emission of a laser diode. However, estimations indicate, that commercially available light emitting diodes have the potential to act as pump sources as well.
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