Abstract

One of the key components for improving positioning accuracy of the global navigation satellite system (GNSS) is the stability of the on-board microwave reference. Therefore, we have proposed a spaceborne pho-tonic microwave generator which is composed of a frequency-stabilized laser and an optical frequency comb whose frequency stability is much higher than that of currently-used rubidium atomic clocks. We use an iodine-stabilized laser as the optical frequency reference whose frequency is down-converted to the microwave region by using an optical frequency comb without degradation [1]. A nonlinear amplifying loop mirror (NALM)-type figure-8 mode-locked fiber laser has been selected as the oscillator of the optical frequency comb. The cavity of our figure-8 mode-locked fiber laser consists of all polarization-maintaining (PM) fibers, which shows less sensitivity against external disturbances such as mechanical vibrations and temperature fluctuations, and is expected to be robust and stable in long-term operation. However, the phase noise of conventional figure-8 mode-locked fiber lasers has been worse than that of the other-type fiber mode-locked laser (NALM figure-9 or nonlinear polarization rotation ) [2].

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