1 Introduction

High-stability passively mode-locked picosecond solid-state lasers have received considerable attention in material processing, medical treatment, spectroscopy, and telecommunication. In such lasers, semiconductor saturable absorber mirrors (SESAMs) are the most used mode-locking element. To generate picosecond and sub-picosecond pulses, SESAMs have been used as effective devices for mode-locked (ML) lasers. The combination of semiconductor saturable absorber and Bragg reflector makes it possible to control loss, saturation fluence, modulation depth, and other parameters precisely. In addition, with the rapid development of the growth technology of semiconductor material and energy band project, the adjustment of the operating wavelength and relaxation time become much easier. To date, SESAMs have been successfully applied to generate ultrashort pulses experimentally in various ML lasers in the past years [1–7].

In the SESAM-assisted ML lasers, single SESAM was usually applied as reported, under which regime the characteristics of the ML lasers have much dependence on the property of SESAM. Generally, SESAMs are quite sensitive to heat, vibration, and other changes to the working circumstance. These factors will absolutely degrade the ML laser performance, especially the stability, which may even block the mode-locking. The current dominant technology to ensure high long-term stability is through complicated cavity design, intelligent electrical feedback control system together with optical modulators or with microactuators [8, 9]. Though it is an efficacious way, the laser system becomes complex and expensive. Therefore, a simple, low-cost, reliable, and high-stability ultrafast laser source is expected to better fulfill the scientific and industrial needs.

In this letter, inspired by doubly Q-switched laser results [10–12], dual SESAMs were first introduced in a ML laser system to achieve long-term ML stability. In this case, with two SESAMs in the cavity, both of them can generate stable ML pulses, even if one of the SESAMs could not perfectly work when the situation in the cavity changes, the other one can still generate sufficient pulse energy and restart the former one. Then, ML pulses will be sustained and the system stability will be consolidated. In fact, the self-adjustment process described above is instantaneous and the two SESAMs work corporately, one of them could be analogous to the feedback to the other. It is an easier and simpler way to improve system stability, compared to electronic feedback control system. In this novel dual-SESAM ML laser system, the continuous-wave mode-locked (CWML) laser operation exhibited high stability as shown from waveform, output power, and spectrum during 50-hour continuous monitoring. Our results indicate that the dual-SESAM mode-locking is a simple, low-cost, and reliable technique to increase the long-term laser stability.

2 Experimental setup

Figure 1 shows the laser setup, a five-mirror W-shape resonator for ML operation. The pump source was a commercial, fiber-coupled AlGaAs diode laser (NA = 0.22, 200 μm core diameter), delivered up to 30 W at 808 nm. A 6-mm-long a-cut Nd:YVO4 crystal was selected as the gain medium, with an aperture of 3 × 3 mm2. To remove heat, the crystal was wrapped with indium foil and held in a copper block that was cooled at 20 °C. The pump beam was focused into 200 μm in diameter onto the crystal by a laser coupling system with a focal length of 25 mm. M1 was a plane input mirror, its first side was antireflection coated at 808 nm and second side was high transmission coated at 808 nm, and HR coated at 1,064 nm. M2 and M3 were folded mirrors, whose curvature radii were 500 and 200 mm, with the same coating as M1. We used two commercial SESAMs in this letter, one (SESAM1) was reflectivity type and the other was a partly transmission SESAM (SESAM2) with T = 3.2 %, which also functioned as the output coupler simultaneously. Both of them were placed as end mirrors of the resonator. The parameters of SESAM1 and SESAM2 are listed in Table 1.

Fig. 1
figure 1

Experiment setup of the dual-SESAM ML laser

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Table 1 Parameters of the saturable absorbers used in our experiment
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3 Results and discussion

The distances between each mirror are the key points for bleaching the two SESAMs (SESAM1 and SESAM2) simultaneously as the intracavity beam radii on the SESAMs varied greatly if the distances changed. In fact, a couple of different resonators were tried before CWML pulse was obtained. When the distances were not optimized, two overlapping Q-switched mode-locked envelopes with different pulse widths could be observed by the oscilloscope (1 GHz bandwidth, 5 Gs/s sampling rate, Tektronics DPO 7104). Figure 2 shows the typical dual-SESAM Q-switched ML pulse trains in different timescale. In Fig. 2a, unstable Q-switched ML pulses were embedded in different envelops with duration of about 80 ns, which were actually in another 1 μs envelope in duration. Observed from a large timescale, the Q-switched pulse train is shown in Fig. 2b. The overlapped envelopes demonstrated that both the SESAMs in the cavity functioned to modulate the intensity due to their respective saturable absorption process, but unsuitable cavity parameters lowered the power intensity on both the SESAMs; thus, the pulse formation process was influenced.

Fig. 2
figure 2

Q-switched ML pulse envelopes with different pulse width

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To achieve CWML in this dual-SESAM laser system, fluence on the two SESAMs should be sufficient to saturate the SESAMs and suppress Q-switched ML tendency, simultaneously. After carefully simulating the laser beam radii in the cavity, the cavity parameters were optimized. The beam radii were determined to be 390 and 270 μm on SESAM1 and SESAM2, and 210 μm in laser gain medium, respectively. CWML operation was achieved under the pump powers ranging from 7.81 to 13.5 W, with the corresponding output powers from 1.78 to 2.98 W (see Fig. 3, solid circles and line). The repetition rate of the ML pulses was 127 MHz, corresponding to total cavity length of 1.18 m.

Fig. 3
figure 3

The output power versus pump power and CWML power range in dual and single-SESAM laser operations

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To investigate the stability of the laser system, the CWML laser pulse and output power were monitored continuously. Figure 4a shows the average output power recorded automatically by a power meter (Coherent Inc. FieldMaxII-TO), with a sampling rate of ten points per second. The average output power within 50 h was 2.4 W, and the power instability level was determined to be ±3 %. The CWML operation shows high stability in waveform, output power, and spectrum during 50-hour continuous monitoring. Figure 4c shows the pulse train with the timescale of 20 ns and 1 ms.

Fig. 4
figure 4

a 50-h measurement of the output power of the dual-SESAM laser, b 50-h measurement of the output power of the single-SESAM laser, c the corresponding pulse train of the dual-SESAM laser, d the corresponding pulse train of the single-SESAM laser

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For comparison, the CWML laser operations with single SESAM were also demonstrated. SESAM1 was replaced by a plane mirror with high reflection coated at 1.06 μm. The cavity parameters were not changed to ensure that the beam radius on SESAM2 was the same as that of dual-SESAM system. CWML was also realized under the pump power of 4.71–10.42 W, and the corresponding output power was 1.15–2.79 W (see Fig. 3, empty circles and line). The threshold pump power of CWML in dual-SESAM system was higher than that of single-SESAM system. It is attributed to two aspects, additional loss introduced by the second SESAM and sufficient fluence required to bleach both of them.

The output power of this single-SESAM laser system was also monitored under the pump power of 8.05 W, and Fig. 4b shows the results recorded in 50 h. The CWML laser sustained 35.8 h with a stable output power of 2.2 W, and then the output power dropped down. Several factors were contributed to this phenomenon. First, accumulated heat could affect the output power; second, inevitable vibration of the cavity devices increased the loss; finally, the performance of the SESAM was degraded after hours of continuous work. In dual-SESAM system, the extra SESAM introduced additional mode-locked modulation to the laser intensity. If one SESAM could not work when intracavity situation changes, it works as a normal cavity mirror and the other one can still sustain the ML pulses. This helps to prolong the mode-locked operation lifetime (50 h compared with 35.8 h). The stable operation lifetime of the dual-SESAM mode-locked laser was increased by 40 %, compared with the single-SESAM case. Figure 4d shows the CWML pulse trains of this single-SESAM system with same timescale of dual-SESAM laser.

From single-SESAM ML laser, the recorded autocorrelation trace gave a 10.44-ps pulse duration if a sech2 profile was assumed (Fig. 5a). The emission wavelength was centered at 1,064.46 nm (Fig. 5b) with a FWHM of 0.19 nm and a time-bandwidth product of 0.525. From dual-SESAM ML laser, the measured autocorrelation trace gave a 9.97-ps (Fig. 5c) pulse duration by assuming the same sech2 fitting. The emission wavelength was centered at 1,064.48 nm with a FWHM of 0.18 nm (Fig. 5d) and a time-bandwidth product of 0.475. It is noted that the additional SESAM had a slight impact on pulse duration and emission wavelength. The explanation is as follows: Though dual-SESAM laser had a higher modulation depth, which would lead to shorter pulse width [13], but the additional SESAM introduced +GDD to the cavity at the same time. Then, the pulse width was barely changed as the modulation depth and GDD interact with each other.

Fig. 5
figure 5

a Pulse width of the single-SESAM laser with SESAM2, b emitted spectra of the single-SESAM laser, c pulse width of the dual-SESAM laser, d emitted spectra of the dual-SESAM laser

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In addition, it is confirmed that no double pulse operation happened in dual-SESAM CWML laser. The spectrum was smooth and exhibited good Gaussian shape (see Fig. 5d). And no sub-pulse was observed in the autocorrelation trace (see Fig. 5c). Furthermore, the RF spectrum of the dual-SESAM laser was measured, as presented in Fig. 6, to verify that it was a normal mode-locking laser. With a bandwidth of 1.7 GHz, we obtained a typical RF spectrum of normal mode-locking laser and did not observe any suppression of harmonics of longitudinal beating, which is the sign of multiple pulsing mode-locking.

Fig. 6
figure 6

RF spectrum of the dual-SESAM laser

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The results of the experiments are gathered in Table 2 for clear comparison.

Table 2 Results of the experiments
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4 Conclusion

In conclusion, we demonstrated a novel high-stability ML laser realized by dual SESAMs. In comparison with single-SESAM ML laser, dual-SESAM ML laser ensured a long-term stable CWML operation. Meanwhile, the pulse duration was compressed with the aids of two SESAMs. Parameters such as output power, pulse energy, and peak power were also improved in certain degrees. It is proved that the dual-SESAM mode-locking is an easy and reliable way to improve the long-term working stability of a ML laser system.