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With as saturable absorber, passive -switching and -switched mode-locking operations of a Tm-doped calcium lithium niobium gallium garnet (Tm:CLNGG) laser were experimentally demonstrated. The -switched laser emitted a maximum average output power of 62 mW and highest pulse energy of 0.72 μJ. -switched mode locking was also obtained in the experiment. The research results will open up applications of at the mid-infrared wavelength.
© 2015 Chinese Laser Press
1. INTRODUCTION
-switched solid-state lasers have been attractive, as they can provide high peak power and pulse energy which can be used in material processing, medical treatment, scientific research, etc. Eye-safe 2 μm -switched solid-state lasers will be especially promising in surgery, ranging, and nonlinear optical frequency conversion. Among the -switching techniques, passive switching with a saturable absorber (SA) is a convenient and low-cost way to achieve -switching operation. Ultra-broadband SAs in particular can extend -switch operation wavelength to the mid-infrared band. So far, SAs such as semiconductor saturable absorber mirrors (SESAMs) [1–3], carbon nanotubes [4–7], graphene [8–17], and topological insulators [18,19] have been adopted for passive -switching and mode-locking operations. An excellent SA should possess the properties of moderate saturation intensity, high damage threshold, ultrafast recovery time, and broadband saturable absorption. However, SESAMs have the drawback of narrow operation bandwidth and complex fabrication process. Thus, there is great motivation to explore SAs with broadband saturable absorption and low-cost fabrication. Typically graphene, a two-dimensional zero-bandgap material, has been confirmed as an excellent broadband SA, but its weak absorption limits its modulation ability for light. Recently, a new type of two-dimensional material, transition metal dichalcogenides [20,21] have attracted extensive attention due to their characteristics of large modulation depth, broadband saturable absorption, and high nonlinear effects.
Monolayer or few-layer is one of the representative two-dimensional transition metal dichalcogenides, with a hexagonal structure of molybdenum atoms sandwiched between two layers of chalcogen atoms (S) [22]. has a thickness-dependent electronic band structure, which endows it with some new optical properties such as strong enhancement of photoluminescence in monolayer [23–25].
Few-layer as a broadband SA has been reported. First, the saturable absorption of dispersions with large population of single- and few-layer were measured at 800 nm [26]. Then, a mode-locked Yb-fiber laser with was realized by Zhang et al. [27]. Luo et al. demonstrated passively -switched fiber lasers at 1, 1.5, and 2 μm, respectively, by exploiting a few-layer polymer composite as SA [28]. Inspired by the introduction of quantum dots in semiconductor, Wang et al. developed a broadband SA by introducing suitable defects in the process of few-layer fabrication [29], which has broadband saturable absorption extending to 2.4 μm wavelength.
Tm-doped calcium lithium niobium gallium garnet (Tm:CLNGG) is a type of disordered crystal with significant inhomogeneous spectrum line broadening. Continuous wave (CW) mode-locking operations of Tm:CLNGG lasers with SESAM [2] and graphene [30] have been realized before. In this letter, we demonstrate the diode-pumped passive -switching and -switched mode-locking operations of a Tm:CLNGG laser by adopting a golden mirror as saturable absorber mirror (SAM). In the stable -switching state, the maximum pulse energy reached 720 nJ. In the -switched mode-locking state, the laser operated in harmonic mode locking with a repetition rate of 187 MHz.
2. EXPERIMENT SETUP
The schematic of a passively -switched and -switched mode-locked Tm:CLNGG laser was shown in Fig. 1. An X-folded cavity was adopted in the experiment. The Tm:CLNGG crystal employed had a length of 9 mm and a cross-section of , with a Tm doping of 6%. The crystal was wrapped with indium foil and mounted in a water-cooled copper block. During the laser operation, the circulating water temperature was kept at 13°C. A single-emitter AlGaAs laser diode at about 790 nm was used as the pump source. The pump light was collimated by a doublet lens (L1) and then focused into the Tm:CLNGG crystal by another doublet lens (L2). Both lenses have the same focal length of 100 mm. The three plano-concave mirrors M1, M2, and M3 have the same radius of curvature of , and were all highly reflectively coated for laser wavelength and antireflectively coated for pumping wavelength. A plano–plano mirror with 2% transmission was used as the output coupler. The SA was grown on a golden mirror with the pulsed laser deposition method as described in Ref. [23]. The laser mode size was in radius on the SA mirror. The inset of Fig. 1 shows the picture of the golden mirror. The large-area, few-layer can be clearly seen in the middle part of the golden mirror, and the modulation depth and nonsaturable loss were measured at 2 μm wavelength to be 7.2% and 10.0%, respectively.
3. RESULTS AND DISCUSSION
While the absorbed pumped power rose up to 2.19 W, the switching was initiated with a gold mirror as an end mirror. In the -switching operation, the maximum average output power was 62 mW with an absorbed pump power of 2.57 W. This low output power was attributed to a large linear loss of the mirror. The output laser had a round mode. With a high-speed detector (EOT, ET-5000) and a 500 MHz bandwidth oscilloscope (Tektronix, DPO3054), the typical -switched pulse train was recorded in 400 μs and 10 ms time scales, as shown in Fig. 2. It shows a clear and stable -switching operation. The optical spectrum of the -switched laser was measured with a mid-infrared optical spectrum analyzer (Ocean Optics, SIR5000) with a resolution of 0.22 nm, as shown in Fig. 3. The spectrum was centered at 1979 nm with a bandwidth of .
In the stable -switching operation, the dependence of repetition rate, pulse duration, average output power, and pulse energy of the laser on absorbed pump power was shown in Fig. 4. The maximum average output power was 62 mW and the highest repetition rate was 110 kHz under the maximum absorbed pump power of 2.57 W. While the absorbed pump power was beyond 2.42 W, the output laser tended to saturate [Fig. 4(b)]. However, the repetition rate continuously increased with pump power, and thus the pulse energy began to decrease beyond 2.42 W of absorber pump power. The maximum single-pulse energy was 720 nJ at the absorbed pump power of 2.38 W, and the shortest pulse duration was 4.84 μs. When the pump power was beyond 2.57 W, the -switching state disappeared. However, after the pump power decreased to 2.57 W, the -switching state recovered. During the experiment, no damage of was observed.
Fig. 4. (a) -switched pulse repetition rate and pulse duration versus the absorbed pump power. (b) Average output power and pulse energy versus the absorbed pump power.
By carefully optimizing the laser spot position on the SAM and aligning the laser cavity, -switched mode-locking operation was realized at the average output power of . The -switched envelope and the mode-locked pulse train within the envelope were shown in Figs. 5(a) and 5(b), respectively. The mode-locked pulses have a period of 5.35 ns, which is exactly half of the cavity roundtrip time of 10.7 ns. This means there was harmonic mode-locking operation, induced by the high nonlinear effects of , as reported in mode-locked fiber lasers with [31].
Fig. 5. (a) Envelope of -switched mode locking in 100 μs time scale. (b) Mode-locking pulse train within the -switching envelope in 200 ns time scale.
The output spectrum of the -switched mode-locked laser was shown in Fig. 6. The spectrum was centered at 1977 nm with a width of . Compared to the spectrum in the -switching state in Fig. 4, the spectrum width in the -switched mode-locking state became broader. It can be understood that the -switched mode-locked pulses have higher peak power and thus induced larger nonlinear phase, which results in spectrum broadening. However, CW mode locking in the laser was not obtained due to too large modulation depth of .
4. CONCLUSION
In conclusion, we have experimentally demonstrated passive -switching and -switched mode-locking operations in a Tm:CLNGG laser with a golden mirror as the SAM. In the stable -switching operation, the maximum pulse energy reached 720 nJ. In the -switched mode-locking state, harmonic mode-locked pulses with a repetition rate of 187 MHz were observed. These results indicate that few-layer is a potential SA at the mid-infrared wavelength.
ACKNOWLEDGMENTS
This work was partially supported by the National Natural Science Foundation of China (Grant Nos. 61008018 and 11421064) and the National Basic Research Program of China (Grant No. 2013CBA01505).
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