Spaceborn Precision Photonic Microwave Generator for Japanese Global Navigation Satellite System

Highly stable microwave frequency generation is a key technology for precision metrology, remote sensing, and global navigation satellite systems[1]. We have developed a spaceborne photonic microwave generation system that consists of an iodine-stabilized laser as an optical frequency reference and an optical frequency comb as an optical-to-microwave convertor. The optical frequency comb, generated using an Er-doped all-polarization-maintained (PM) figure-8 mode-locked fiber laser (MLFL) with a nonlinear amplifying loop mirror (NALM), ensures robustness against external perturbations [2]. The iodine-stabilized laser, locked to the molecular absorption line of iodine, utilizes a 515 nm wavelength generated via third harmonic generation (THG) from a 1545 nm planar lightwave circuit (PLC)- ECLD through fiber-coupled PPLN for saturated absorption spectroscopy. Compared to conventional modulation transfer methods [3], our light source applies phase modulation using an inline electro-optic modulator (EOM) and frequency shift using an acousto-optic modulator (AOM) at 1545 nm. This approach enables all-fiber integration of both the light source and modulation components, allowing spectroscopy with minimal free-space optical systems, contributing to long-term stable operation. Residual amplitude modulation (RAM) and intensity noise of both signal and pump beam are also suppressed for improving frequency stability. Allan deviation measurements, comparing the iodine-stabilized laser with a highly-stable hydrogen maser at NICT using a phase-locked optical frequency comb, confirm its frequency stability higher than 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−14</sup>. We have improved the cavity length adjustment technique of all-fiber mode-locked laser within 0.3 mm which corresponds to repetition rate frequency of 3 kHz. In this conference, we will present detailed evaluations of optical and microwave frequency stability of our photonic microwave generation system.