A 100-GHz-tunable photonic millimeter wave synthesizer for the Atacama Large Millimeter Array radiotelescope

We propose a broadband photonic millimeter wave synthesizer based on the optical phase locking of telecom-grade DFB semiconductor lasers. A mathematical model of the OPLL shows that the frequency noise profile of these lasers is such that millimeter wave beat notes with residual phase noise levels of 2 x 10^-5 rad² can be achieved with loop path lengths as long as 60 cm. Experiments on a simplified proof-of-concept unit validate this model. The synthesizer is tunable over more than 100 GHz by changing the laser junction temperature. The photonic millimeter wave synthesizer will be used to generate and distribute the LO reference signal of the Atacama Large Millimeter Array radiotelescope.