The generation of high-frequency signals including those in the terahertz region has become indispensable in state-of-the-art communication systems. A synthesizer as a measurement device generates signals of desired frequencies with high-frequency resolution. Electric signals (microwaves or millimeter waves) can be generated by a photomixer performing optical-electrical energy conversion based on the phase difference between two coherent optical signals. Recently, the frequency bandwidth for photomixers has reached the terahertz region and photomixers are now more readily available. The main purpose of this letter is to provide an optical synthesizer with a high-frequency resolution (better than 1 Hz) and with seamless frequency coverage from the microwave to the terahertz-wave regions using one apparatus. The system is designed in such a way that the mean of the resolution frequency includes the frequency instability of the generated signal.
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