Using Laser Diode Instabilities for Chip-Scale Stable Frequency References

摘要

Semiconductor lasers are known to undergo significant changes in their output characteristics when subjected to external optical perturbations such as near-resonant injection from an external source or optical feedback. Over a range of operating conditions, the perturbations can induce a periodic pulsating output where the pulsation frequency can be controlled by the bias point of the laser(s), and amplitude (and frequency offset) of the injection. The output optical spectrum can be adjusted to be dominated by two strong frequency components with a controllable offset. Adding a weak microwave modulation to the bias can lock the pulsation frequency to this reference. Such a spectrum is nearly ideal for the excitation of Coherent Population Trapping (CPT) resonances of gas-phase atomic media such as cesium (Cs) and rubidium. We describe the double locking of a laser diode to the CPT optical (852 nm) and microwave (9.2 GHz) resonances in Cs gas in a cell containing Cs and a buffer gas. The microwave power required for the modulation reference is a small fraction of the dc-bias power, unlike a directly modulated laser diode. The combination of all-optical excitation of the Cs gas and reduced microwave electronics specifications is very useful for the fabrication of ultrasmall frequency references.