Phase noise reduction by optical phase-locked loop for a coherent bichromatic laser based on the injection-locking technique

Abstract

We present a coherent bichromatic laser system with low phase noise. An optical injection process is used to generate coherent laser beams with a frequency difference of 9.192 631 77 GHz using an electro-optical modulator. An optical phase-locked loop is then applied to reduce the phase noise. The phase noise of the beat note is -41, -81, -98, -83, and -95 dBrad/Hz at the offset frequencies of 1 Hz, 100 Hz, 1 kHz, 10 kHz, and 1 MHz, respectively. Compared to a system that uses optical injection alone, the phase noise is reduced by up to 20-30 dB in the low-frequency range, and the intermodulation effect on the continuous atomic clock is reduced by an order of magnitude. This configuration can adjust the intensities and polarizations of the laser beams independently and reduce the phase noise caused by environmental disturbances and optical injection, which may be useful for application to atomic coherence experiments.

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“…Both signals need to be coherent preserving the phase information of the RF signals and allowing the transmission of complex modulation formats [25]. These optical waves may be provided by low linewidth lasers driven by an optical phase locked loop or dual-mode lasers [26,27,28]. Furthermore, instead of employing multiple laser sources, a feasible and low cost solution is the use of a frequency comb generator in the OLTs transmitting multiple coherent frequency lines [29,30].…”