Frequency quadrupling for tunable microwave and sub-terahertz generation using a single polarization modulator (PolM) in a Sagnac loop without using an optical filter or a wideband microwave phase shifter is proposed and experimentally demonstrated. In the proposed system, a linearly polarized continuous wave from a tunable laser source (TLS) is split into two orthogonally polarized optical waves by a polarization beam splitter (PBS) and sent to the Sagnac loop traveling along the clockwise and counter-clockwise directions. A PolM to which a reference microwave signal is applied is incorporated in the loop. The PolM is a traveling-wave modulator, due to the velocity mismatch only the clockwise light wave is effectively modulated by the reference microwave signal, and the counter-clockwise light wave is not modulated. This is the key point that ensures the cancelation of the optical carrier without the need of an optical filter. Along the clockwise direction, the joint operation of the PolM, a polarization controller (PC), and a polarizer corresponds to a Mach–Zehnder modulator (MZM) with the bias point controlled to suppress the odd-order sidebands. The optical carrier is then suppressed by the counter-clockwise light wave at the polarizer. As a result, only two ±2nd-order sidebands are generated, which are applied to a photodetector (PD) to generate a microwave signal with a frequency that is four times that of the reference microwave signal. A theoretical analysis is developed, which is validated by an experiment. A frequency-quadrupled electrical signal with a large tunable range from 2.04 to 100 GHz is generated. The performance of the proposed system in terms of stability and phase noise is also evaluated.
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