A harmonic frequency conversion module is disclosed including a polarization maintaining (PM) fiber optical link for providing an output stabilized from power fluctuation by the inclusion of one or more polarizers in the PM fiber optical link. Removing polarization distortions removes noise which has a significant negative effect on the output of harmonic frequency conversion elements. It has been found that the noise in frequency converted light has additional components, caused by mode interaction during conversion. In accordance with the present invention, we are able to remove the spikes in this noise, making it more stable and less dependent on external conditions. If the PM optical fiber route consists of multiple elements creating polarization distortions, a polarizer should be inserted between the most distorting element and the output of the fiber system. If many elements contribute to polarization distortions, several polarizers can be inserted into the system.