Self-started figure-8 mode-locked fiber laser for space borne optical frequency comb

Global navigation satellite system (GNSS) has extensive applications in autonomous driving, mapping, remote sensing and metrology. In order to improve the accuracy of GNSS, we have proposed an optical-based high-precision microwave generation system for the next-generation Japanese GNSS. Our high-precision microwave generation system consists of an optical frequency reference (iodine-stabilized laser) whose frequency is down-converted to the microwave region by using an optical frequency comb, and its frequency stability is expected to be three orders of magnitude higher than that of the current microwave generator (rubidium atomic clock) [1] . Though other research groups have already demonstrated a space-qualified mode-locked laser [2] and the operation of the optical frequency combs in sounding rocket [3] , stable long-term operation of the optical frequency comb in space has never reported before. The nonlinear polarization rotation (NPR) mode-lock fiber laser has widely utilized for the light source of the optical frequency comb due to its low intrinsic phase noise. However, the NPR operation is very sensitive to the external disturbances, which becomes a serious problem for a space borne laser. Therefore, we have developed an all polarization-maintaining (PM) figure-8 type mode-locked fiber laser by using a nonlinear amplifying loop mirror (NALM) ( Fig. 1 ). In recent years, a phase bias component is introduced in the oscillator which gives a larger modulation depth and results in the higher repetition rate to more than 50 MHz. Instead of its robustness, the NALM mode-locked laser has the following disadvantages that the phase noise is relatively high and the active trigger is necessary for starting mode-lock operation. We try to solve these problems by optimizing the design of the oscillator. We design the dispersion of the oscillator with the net-dispersion of 0.005 ps 2 in the stretched-pulse region. Our mode-locked laser obtains high-energy pulses with a stretched-pulse spectrum without Kelly-sideband or modulation instability configuration ( Fig. 2 ). The average power and center wavelength of our mode-locked laser are 0.8 mW and 1560 nm, respectively, and the repetition frequency is 48.4 MHz which is close to the target frequency of our project (50.12 MHz). The spectral bandwidth is 45.1 nm which is, to our knowledge, wider than any other figure-8 type NALM mode-locked erbium-doped fiber (EDF) laser. Fourier transform-limited pulse is 62 fs in the case of sech 2 pulses. After replacing the gain fiber with the higher concentration EDF (80 dB/m) and optimizing the phase bias, we observe the self-starting of its mode-locking at the pump power around 200 mW. It is considered that the higher concentration fiber enhances the phase difference between clockwise and counterclockwise rotating modes in the fiber loop, which would result in the self-starting of mode-locking. Self-stating is one of the advantages for space borne mode-locked laser. For further understanding of the oscillation mechanism, we investigate transient response of self-starting. Thanks to all PM fiber configuration, continuous mode-locking operation has been kept for more than a week in a laboratory environment. Our mode-locked fiber laser in the stretched-pulse region is expected to have very low phase noise whose characteristics have currently been investigated.