Low Phase Noise Laser Synthesizer with Simple Configuration Adopting Phase Modulator and Fiber Bragg Gratings

We introduce a novel laser synthesizer with a simple configuration that contains no phase lock loop. This laser synthesizer generates an optical beat signal by modulating a CW light using a phase modulator and selecting two line spectra from modulation sidebands using serially-connected fiber Bragg gratings. With this configuration, the optical beat signal is obtained from a single laser source by converting CW laser light into an optical beat signal through a single optical path. This enables us to obtain a low phase noise of the optical beat signal which is comparable to that of the RF synthesizer that drives the phase modulator. In addition, polarization stability is also obtained between the two line spectra. The complete frequency range of the optical beat signals required in ALMA can be covered using this laser synthesizer.