A semiconductor saturable absorber mirror (SESAM) has the advantages of self-starting, easy integration, wide wavelength coverage, support for all-solid-state laser technology, fast saturation, compact structure, and flexible design. It has become a Q-switched and mode-locked element for various types of lasers such as solid-state, fiber, and semiconductor lasers. Recently, the rapid development of picosecond Yb-doped fiber lasers and their wide application in industrial processing have heightened interest in SESAM applied to Yb-doped fiber lasers. The technology of designing and epitaxial growing SESAM has been relatively mature abroad, and the development of SESAM has been carried out in China in recent years, but the research on SESAM devices in China mainly focuses on solid-state lasers, and there are few reports on the development and characterization of SESAM for fiber lasers. In the present study, we report the effects of the quantum well period numbers in the absorption region on the field distribution, modulation depth, and reflection spectrum of SESAM, and the key characteristic parameters of SESAM are characterized, which has important reference value for the further study of SESAM.

In order to improve the characteristic parameters of multi-quantum well semiconductor saturable absorption mirror (SESAM) for fiber lasers, the effects of different quantum well period numbers on the field distribution, modulation depth, and reflection spectrum of the device were analyzed. The epitaxial growth of three kinds of quantum well structures with different period numbers of 7, 15, and 30 quantum wells was carried out by metal-organic compound vapor deposition (MOCVD) method. The reflectance spectra of the samples were measured by spectrophotometer, and the nonlinear test and mode-locking experiments were carried out on the developed three kinds of SESAM structures. The dynamic response of SESAM structures was tested by pump detection technology.

The simulation calculates the electric field distribution of the semiconductor saturable absorption mirror at 1064 nm (Fig. 2). When the complete electric field wave is present in the saturable absorption region, there are always peaks and troughs in the absorption region. Reflectance is calculated for saturable absorption mirrors of different quantum well structures (Fig. 3). The results show that the lowest reflectivity of the three structures is at 1064 nm, and more periods of quantum wells indicates lower reflectivity of SESAM at 1064 nm and higher modulation depth. Nonlinear tests and mode locking experiments are performed on epitaxial sheets of the three structures after growth (Fig. 7). The test results show that the SESAM of the three structures realizes self-starting mode locking, and the pump interval of stable mode locking is 150-200 mW. Pump detection of the SESAM of 15 quantum well structures yields a recovery time of 5 ps (Fig. 8).

By simulating the calculation of the light field distribution of SESAM of different periods, it is found that when the number of quantum wells is large enough, there is a complete standing wave in the absorption zone generated by the incident light field, and the number of standing waves increases with the thickness of the absorption layer. The reflectance of the saturable mirror of the subtrap structure with different number of periods is calculated. The results show that the reflectance of SESAM decreases gradually at 1064 nm with the increase in the number of periods of the absorption layer quantum well, and the bandwidth at low reflectance becomes narrower, which also means that the tolerance of the growth error of SESAM is also smaller. By using MOCVD technology, epitaxial growth of three SESAM structure samples with different quantum well period numbers is carried out, and nonlinear testing and mode-locking experiments are carried out on the grown samples. The results show that the three SESAM structures tested all realize self-starting mode-locking, and the pump range of stable mode-locking is 150-200 mW. When the pump power is less than 150 mW, stable mode-locking cannot occur. When the pump power is more than 200 mW, the mode-locking pulse appears double pulse phenomenon. For resonant SESAM, although increasing the number of quantum wells can increase the modulation depth of the SESAM, too many quantum wells are more likely to deviate from the design value in the epitaxial growth process. The number of quantum wells has little effect on the saturation flux, and the improvement of saturation reflectance is very limited. The narrowest mode-locking pulse width of 7 quantum well structure samples is about 20 ps; the narrowest mode-locking pulse width of 15 quantum well structure samples is about 11 ps, and the narrowest mode-locking pulse width of 30 quantum well structure samples is about 8 ps. The dynamic response of 15 quantum-well SESAM structures is tested using pump detection technology, and the response recovery time is measured to be 5 ps.