We report a semiconductor saturable absorber mirror (SESAM) with phase shift compensation. The SESAM shows a uniform low-intensity reflectivity and a modulation depth up to 12% for the wavelength from 1000 to 1100 nm. A linear cavity all-normal dispersion Yb-doped fiber laser has proved its mode locking performance for the wavelength of both 1030 nm and 1064 nm. As the demand for highly reliable passively mode-locked fiber lasers is increasing, highly reliable SESAMs become important. Unlike in solid-state lasers, mode locking in fiber lasers requires a high modulation depth. A simple solution to the high modulation depth is to increase the absorption layer thickness. However, there are two issues in the SESAMs in doing such way: the large variation in the reflectivity and the possible optical damage on the SESAMs. A thick absorption layer introduces a large phase shift that results in a modulation over the reflection band of the Bragg mirror [4,5], due to the imaginary part of the refractive index of the absorption material. The high absorption may also cause a high saturation fluence, which is the origin of the optical damage.
YbTo avoid the optical damage on the SESAM, Grange et al. [6] realized that a low the saturation fluence is the key solution for the damage since the low saturation fluence *Corresponding author (email: zhgzhang@pku.edu.cn) ensures the laser mode locking at relatively lower fluence, so that it never exceeds the damage threshold. They demonstrated that a thin layer can also show a relatively high modulation depth but with a saturation fluence as low as 1.1 μJ/cm 2 [7]. The key point is to locate the absorption layer in the maximum electric field, as well as to apply an anti-reflection coating to enhance the field.However, there has not been a solution reported for the thick absorption layer. The commercially available SESAMs with high modulation depth basically have a non-uniform low-intensity reflectivity with respect to wavelength, and a high saturation fluence (>50 μJ/cm 2 ). In this paper, we propose a phase shift compensated high modulation depth design of SESAM and demonstrate the uniform low intensity reflectivity and low saturation fluence. This SESAM has been verified in mode locking of a linear cavity all-normal dispersion Yb-doped fiber laser both in 1030 nm and 1064 nm, respectively [8,9].