**6. Conclusion**

270

to be 22 mHz. A recent experiment extends this concept to Yb-fiber laser and achieves 1.39-

As reported in Ref. [74], a commercial available pump current supply can provide a minimum resolution of pump power as 1.5 μW and thus achieve a controlling accuracy of 0.05 Hz, which is more than two orders of magnitude than PZT-based method. Therefore, an interesting experiment worth to do is to use RIC-method to achieve high-precision *f*rep stabilization. So far, the RIC method has been fully investigated in NPR mode-locked lasers, which applied non-PM fibers and components [73–75], and the locking accuracy limited to ~mHz. Considering the environmental perturbation on non-PM fiber, a straightforward idea is to implement RIC method in a PM fiber laser. Therefore, the following part will discuss high-precision repetition

The laser setup shown in **Figure 13** is same as **Figure 4(a)**, except the net dispersion of laser cavity. In the current experiment setup for all-optical repetition stabilization, a 56-cm-long Er3+-doped fiber (EDF2) is spliced asymmetrically in the NALM to act as a frequency controller, while the LD3, which is controlled by the error signal from frequency mixer, provides the feedback modulating pump power via WDM3 on EDF2. Besides, a segment of DCF38 is used to compensate the anomalous dispersion of PM1550 fiber. The dispersion of linear loop and

dispersion for the whole cavity. Self-started mode-locking in multiple-pulse regime can be achieved by over-pumping method, and stable single-pulse operation can be obtained by decreasing the pump power of LD1 and LD2. At fundamental repetition rate of 11.9 MHz, the

The repetition rate was detected by PD3 and successively compared with standard reference (Rb clock) in a frequency mixer to produce the error signal. Subsequently, the error signal was filtered and amplified by low-noise voltage preamplifier with frequency cutoff at 1 MHz and

The long-term stabilization was depicted in **Figure 14**. As low as 27-μHz accuracy is achieved within 16-h measurement. The inset of **Figure 14** magnifies the measured dates from 30,000 to 31,000 s and shows fluctuation range within ±0.1 mHz. Typically, thermal effect, Kerr nonlinear effect, pump-induced nonlinear effect, and random acoustic perturbations contribute to the precision of *f*rep stabilization. For our experiment, a temperature-controlled incubator with a ripple of 0.2°C was used to take the laser cavity to isolate environmental perturbation. As for Kerr-nonlinearity, the RIC is proportional to the traveling power of resonant laser. Assuming 5-mW traveling power in NALM, the Kerr-induced RIC is estimated as 1.2 × 10–7/mW, having the same order of magnitude of the pump-induced RIC (2.1 × 10−7/mW). However, when the pump power of LD3 increased from 30 to 205 mW, only 1.6% of output power change was observed, which means little change on the dynamic process of pulse evolution in NALM. Thus, the Kerr-induced RIC is near ~1% of the RIC by pump-induced nonlinearity. Therefore, we postulate that the nonlinearity on the RIC of fibers owes to pump-induced nonlinear effect

and further processed by a proportional-integral-derivative

, producing −0.183 ps2 net

mHz SD of residual fluctuation in an hour measurement [73].

rate stabilization by using RIC method in a PM figure-eight laser cavity.

the NALM was estimated numerically to be −0.208 and 0.025 ps2

figure-eight laser cavity delivers 1.5-mW average power via CP2.

a maximum voltage gain of 5 × 104

and thermal effect rather than Kerr effect.

(PID) controller.

In this chapter, we first present several types of mode-locked fiber lasers, as well as their derivatives for SC generation. Second, an effective method named DPA was applied in Erdoped fiber laser system allowing simultaneous pulse amplification and compression so that additional pulse compressor is no longer needed. With ×8 replicas in DPA, as high as 500-mW average power was achieved and the highest SHG conversion efficiency was measured to be 56.3%. Third, an all-optical method, named as pump-induced nonlinearity, is applied to stabilize the repetition rate of a figure-eight Er-doped fiber laser, achieving as low as 27-μHz accuracy within 16-h measurement.
