A vertical-external-cavity surface-emitting laser (VECSEL) offers unique advantages such as high power, good beam quality, and designable emitting wavelength. Additionally, the external-cavity structure allows for the convenient insertion of other optical components, thus enabling the VECSEL to operate in mode-locking, single-frequency running, or wavelength-tuning mode. These characteristics render the VESCEL a desirable candidate for various applications not realizable by conventional lasers (e.g., solid-state lasers, gas lasers, fiber lasers, or laser diodes). In particular, the peak power pulses generated by a mode-locked VECSEL can be used in diverse fields such as multiphoton imaging, high-resolution time-domain terahertz spectroscopy, and supercontinuum generation. However, obtaining high peak-power pulses from a mode-locked VECSEL is not trivial. To increase the peak power of mode-locked pulses, one can increase the average output power of the laser, reduce the pulse time width, or reduce the pulse repetition rate. However, these three methods are mutually restrictive; thus, the pulse peak power in a mode-locked VECSEL can only be improved to a certain extent. In particular, the carrier lifetime of the semiconductor gain media used in a VECSEL is short, i.e., in the nanosecond level, which significantly limits the further reduction of the repetition rate of mode-locked laser pulses. Consequently, the increase in the peak power of pulses generated from the mode-locked VECSEL is restricted considerably.

In this study, a custom-designed saturable Bragg reflector (SBR) was used as a saturable absorber, where a moderate saturation fluence can effectively balance between the cycling power and multi-pulse generation in the resonant cavity, thereby maintaining a sufficiently high average output power at low repetition rates and significantly improving the peak power of the mode-locked laser pulses. The epitaxial structure of the gain chip used in the experiment (Fig. 1) is a reverse-order structure of the active region, followed by a distributed Bragg reflector (DBR). First, an Al_0.86GaAs etching stop layer was deposited on the GaAs substrate. Subsequently, a GaAs protective layer, a high-barrier Al_0.6GaAl window layer, an active region, and a DBR were deposited. Finally, the entire structure was terminated with an oxygen-resistant GaAs layer. Unlike gain chips achieved via reverse growth, the SBR exhibits an epitaxial structure (Fig. 1) that is consistent with the normal order of a DBR followed by an absorption region. The saturable absorber is a single InGaAs quantum well located in the final quarter-wavelength layer of the DBR, with a thickness of 10 nm. The quantum well is intentionally set at the peak position away from the laser standing wave to obtain a large saturation fluence. Additionally, because the saturation fluence of the SBR should not be excessively high, we implemented a single quantum well with a commonly used thickness of 10 nm. The laser resonant cavity used in the experiment (Fig. 2) comprises six mirror cavities, including a DBR each at the bottom of both the gain chip and SBR. Except for the output coupler, which presents a certain transmittance at the laser wavelength, all other mirrors demonstrate high reflectivity at the laser wavelength.

Based on the experimental result, the temporal waveform of the pulses exhibits double or triple pulses connected to each other (Fig. 3). We believe that this may be due to the low intensity of the intra-cavity pulses (corresponding to a relatively high saturation fluence of the SBR) and the inadequateness of one pulse in fully saturating the SBR. Therefore, before the SBR is fully recovered, absorption continues to occur on one or two following pulses, thus resulting in double or triple pulses. Stable continuous-wave (CW) fundamental mode-locked pulse trains, steady second-harmonic mode-locked pulses, and higher-order unstable harmonic mode-locked pulses are obtained (Fig. 5). In our opinion, the high-order harmonics occurred because as the pump power increases, the pulse intensity inside the cavity increases, thus resulting in a low saturation fluence of the SBR. After a pulse saturates the SBR, owing to the long resonant cavity length, the SBR has sufficient time to recover. Therefore, one or more subsequent pulses can saturate the SBR again, thus resulting in multiple pulses in the cavity, which eventually evolve into higher-order harmonic mode locking. The interconnected double or triple pulses mentioned above, as well as the occurrence of high-order harmonic mode locking can be described by numerically solving the delay differential equations for passive mode locking, and the evolution of intra-cavity pulses over time can be obtained (Fig. 4). The measured fundamental CW mode-locked pulse has a period of approximately 14.92 ns, a repetition rate of 67 MHz, and a pulse width of 2.08 ps (Fig. 6). When the absorbed pump power increases to 18.9 W, an output power of 0.325 W can be obtained. When the absorbed pump power exceeds 21.7 W, the maximum output power of the second harmonic mode locking is 0.836 W. The maximum output power of the fourth-harmonic mode locking is 0.683 W (Fig. 7). Under fundamental, second- and fourth-harmonic mode locking, the maximum peak powers of the pulses are 2.33, 3.00, and 1.23 kW, respectively.

The short carrier lifetime (of nanosecond level) of a VECSEL significantly limits the further reduction of the pulse repetition rate under passive mode locking, thereby limiting the improvement to the pulse peak power. For saturable absorbers used to commence mode locking, a relatively high saturation fluence may generate interconnected double or triple pulses, whereas a relatively low saturation fluence may result in multiple pulses in the resonant cavity. This study utilizes a custom-designed SBR with a moderate saturation fluence to achieve both low repetition rates and high average output power levels in a passive mode-locked VECSEL. We experimentally achieved a repetition rate of 67 MHz. In the case of fundamental, second- and fourth-harmonic mode locking, the corresponding peak powers of the pulses are 2.33, 3.00, and 1.23 kW, respectively, which demonstrate a passive mode-locked VECSEL with a repetition rate below 100 MHz and a peak power above 1 kW simultaneously.