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Spaceborne precision frequency reference is one of the essential technologies for extensive applications in metrology, astronomy, remote sensing and global navigation satellite system, and we have researched and developed a high-precision microwave generation system which is based on an optical frequency reference. In our system, a highly-stable optical frequency is generated from an iodine-stabilized laser whose frequency is down-converted to the microwave region without degradation by using an optical frequency comb. The frequency stability of the iodine-stabilized laser would reach 10−15 level. In this conference, we will mainly present the development of the optical frequency comb. Nonlinear amplifying loop mirror (NALM)-type mode-locked fiber lasers have two kinds of configurations, figure-8 and figure-9. The figure-9 laser is widely used as an oscillator for the optical frequency comb due to its excellent low phase noise, and there are few reports of the figure-8 mode-locked fiber laser-based optical frequency comb. On the other hand, the operation of the figure-8 would be very stable against external perturbations because of its all polarization-maintaining fiber configurations. Therefore, we have developed low-phase-noise figure-8 lasers for our optical frequency comb because its robustness is suitable for the spaceborne system. One of the significant points of our figure-8 laser is that two types of erbium-doped fibers with different concentrations are used as the gain media of the oscillator. The repetition rate, the center wavelength and the width of the optical spectrum of our figure-8 laser are 51 MHz, 1560 nm and 58 nm, respectively. Though any NALM type mode-locked fiber laser needs initial impulse for starting mode-locking operation, our figure-8 laser can start mode-locking automatically without special procedure, which is called self-starting. We tried the operation test repeatedly for about 1600 times whose success rate reaches 99.94%. The disturbance tests and the thermal vacuum tests have been applied to our figure-8 laser, and under the conditions of both tests, our figure-8 laser shows the stable mode-locked operation without any break of mode-locking. In the radiation exposure test, we have confirmed the stable operation of our figure-8 laser till 30 krad radiation dose, and at higher radiation doses the mode-locking operation stopped.
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