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Abstract
We demonstrate a passively Q-switched Ho,Pr:LiLuF4 laser at 2.95 μm using a WS2 saturable absorber (SA). A shortest pulse width of 654 ns was achieved with an average output power of 82 mW and the repetition rate of 90.4 kHz at T = 2%. Our result reveals that WS2 -SAs have great potential to be the modulator in mid-IR wavelength range.
© 2017 Optical Society of America
1. Introduction
Diode pumped mid-IR pulsed laser sources operating at 3 μm are of great importance, due to wide applications in various fields, such as laser surgery [1], high quality pump sources for longer-wavelength oscillators [2] and atmospheric monitoring [3]. Compared with Er3+ laser around 2.8 μm, Ho3+ often emits a longer wavelength at 2.9 μm, which better overlaps with the peak of the fundamental OH absorption [4, 5]. However, it is difficult to obtain an efficient laser operation in Ho3+ lasers, due to the self-terminal phenomenon induced by the considerably longer fluorescence lifetime of the lower level (5I7) compared to the upper level (5I6) [6]. An effective method to depopulate the 5I7 multiplet is the co-doping with the Pr3+-ions [7], which has been verified to greatly facilitate the 3 μm laser emission in Ho3+-doped fluoride fiber lasers. In 2003, a Ho3+, Pr3+ -doped fluoride fiber laser at 2.84 μm was firstly demonstrated which was pumped by a Yb3+-doped silica fiber, resulting in a continuous wave (cw) output power of 210 mW, and its slope efficiency was only 3.2% [8]. More significantly, a diode laser emitting at 1150 nm which was more suitable for the absorption center of Ho3+ was adopted in 2009, leading to the high slope efficiency up to 32% [9]. Until recently, 172 mw cw 2.95 μm laser was gained from solid state Ho,Pr:LiLuF4 laser, and a pulse width of 937.5 ns was also achieved with the monolayer graphene SA [10].
Until now, several kinds of saturable absorbers (SAs) have been employed to realize passively Q-switched 3 μm laser operation [11–13]. Among of them, the two dimension (2D) transition metal dichalcogenide: WS2 has attracted more attention in recent years, due to its excellent optical and electrical properties [14]. The natural WS2 is far from applications in infrared lasers, because of the huge band gap of 1.3 eV (~0.95 μm) for bulk crystal [14]. However, by the introduction of suitable W defects, it can possess the property of broadband saturable absorption [15]. Up to now, WS2-SAs have been successfully applied in pulsed lasers at 0.64, 1.06, 1.5 and 2 μm [16–19]. In addition, a Ho3+, Pr3+ co-doped fluoride fiber laser with the pulse duration of 1.73 μs at 2865.7 nm was demonstrated using a WS2-SA in 2016 [13], while there is no report about the WS2-SA applied in 3 μm solid state laser up to present.
In this paper, we report a passively Q-switched Ho,Pr:LiLuF4 laser using a WS2-SA for the first time. The morphology of the thin film and the saturable absorption characteristics of the WS2-SA near 3 μm were investigated. The narrowest pulse width of 654 ns at 2954.7 nm was generated with a maximum output power of 82 mW and a repetition rate of 90.4 kHz, corresponding to the single pulse energy of 0.9 μJ.
2. Characterization of WS2-SA
The WS2 dispersions were prepared by liquid-phase exfoliation method [20], and the WS2-SA was consequently fabricated by drop-casting diluted dispersions on a YAG substrate. Figure 1(a) and (b) show the atomic force microscopy (AFM) morphology of the as-prepared WS2-SA and the corresponding heights of section A and B. The thickness of the WS2 thin film was estimated to be between 5 to 10 nm and the layers number can be calculated to be about 7 to 14, assuming that the thickness of mono-layer WS2 is 0.7 nm [21]
To investigate the absorption properties of the WS2 film, the optical transmission spectrum was measured by an UV/VIS/NIR spectrophotometer (U-4100, Hitachi, Japan). As described in Fig. 2, the smoothly linear transmission curve varied with the wavelength increasing from 500 nm to 3000 nm, which demonstrated the broadband absorption characteristics of our sample. Also, it can be seen that the transmission at 2950 nm was measured to be 94.7%. Figure 2(inset) was the saturable absorption curve of the WS2-SA, which was measured by a Q-switched Er:Lu2O3 laser with a pulse width of ~100 ns at 2.84 μm. With the increase of the input pulse fluence, the transmission trends to saturation with the initial value of ~90.5%. And the modulation depth can be estimated to be 3.5%. Considering the loss of the YAG substrate which was measured to be ~4.5%, the non-saturable loss of the MoS2-SA was according calculated to be ~1.5%.
Fig. 2 Optical transmission spectrum of the WS2 film on a YAG substrate; inset: saturable absorption curve of the WS2-SA near 3 μm.
3. Experiment setup and results
The experimental setup was shown in Fig. 3. A 3-cm-long plane-concave linear cavity was utilized. A uncoated Ho3+ (2at.%), Pr3+ (0.6at.%) co-doped LiLuF4 crystal with a size of 2 mm × 5 mm × 10 mm was employed as the gain medium. The crystal was wrapped in indium foil and tightly mounted in a water-cooled copper heat sink. The temperature of cooling water was kept at 10°C all the time in our experiment. The pump source was a commercially available diode laser (LD) centered at 1150 nm with a core diameter of 400 μm and a numerical aperture (NA) of 0.22. The pump light was focused into the laser crystal through a 1:1 lens system. The input mirror M1 was a flat mirror anti-reflection (AR) coated for 1150 nm (T>95%) and high-reflection (HR) coated in the range of 2.8-3 μm (R>99%). Two concave mirrors of 100 mm curvature radius with different transmissions of 1% and 2% were used as output coupling (OC) mirrors. The beam radius of the oscillating mode inside the laser crystal was calculated by using the ABCD matrix propagation law, where the thermal effect of the laser crystal was also considered. The thermally induced focal length of the crystal was calculated by fT = 2*π*Kc*w2p /[(dn/dT + αT*n)*ε*Pabs] [22], where Kc = 5 W/(m*K) is the thermal conductivity, dn/dT = −6.6*10−6/K is the temperature coefficient of refractive index; αT = 108*10−6/K is the coefficient of thermal expansion; n = 1.488 is the refractive index; ε = 0.6 is the heat transfer coefficient; Pabs is the absorbed pump power; wp is the pump beam radius [23]. Considering the pump beam radius of 200 μm, absorbed pump power of 3.77 W, the mode radius in the crystal was calculated to be about 180 μm. To block the residual pump light, a filter was placed behind M2, and the average output power was measured by a PM100D power meter with a S314C power head (Thorlabs Inc., USA). The laser spectra of cw and Q-switching operations were recorded by an optical spectrum analyzer with a spectral resolution of 0.1 nm (MS3504i, made in Belarus) under the maximum absorbed power.
The cw output performance was investigated. The single-pass absorption of the pump light in Ho,Pr:LiLuF4 crystal was studied under non lasing condition. The small-signal absorption efficiency was measured to be 16%-39%. The thresholds of the cw lasers with different transmissions of 1% and 2% were almost the same with an approximately value of 1 W. Under the absorbed pump power of 3.77 W, the maximum cw output powers of 90 and 120 mW were obtained, respectively. The output powers increased with a lower slope efficiency when the absorbed pump power exceeded 1.5 W. We attributed this phenomenon to the mismatching ratio between Ho3+ and Pr3+ (3.3:1 in our work) with the optimal value of 10:1, as described in Ref [9]. Therefore, optimizing the concentration ratio may be helpful for solving this dilemma. As shown in the inset of Fig. 4, the center wavelength of cw laser was located at 2954.9 nm with a full width at half-maximum (FWHM) of 1.1 nm.
Inserting the WS2-SA into the laser cavity, the Q-switched laser operation was achieved when the absorbed pump powers surpassed 1 W and 1.2 W for the transmissions of 1% and 2%, respectively. The relationship between the output power and the absorbed pump power is depicted in Fig. 5. Consistent with the cw regime, the decrease in efficiency was observed at an absorber pump power of ~1.5 W. The average output power increased with absorber pump power up to 82 mW (T = 2%) with a slope efficiency of ~1.8%. The inset shows the output spectrum of the PQS laser, which is centered at 2954.7 nm with a narrower FWHM of 0.8 nm due to insert losses of the MoS2-SA.
A fast HgCdTe IR detector with a response time of 20 ns (PVI-2TE-5, Vigo System S. A.) was used to detect the laser pulses. The pulse trains were recorded by a Tektronix DPO 7104C digital phosphor oscilloscope (1 GHz bandwidth and 20 GS/s sampling rate). As can be shown in Fig. 6(a) and 6(b), the repetition rate increased with absorbed pump power, while the pulse duration had an opposite tendency. Figure 6(c) depicts the changes of single pulse energy versus the absorbed pump power. It is positively correlated with absorbed pump power [24], while the saturation phenomenon appeared for the single pulse energies near 0.7 (T = 1%) and 0.9 (T = 2%) μJ. Under the maximum absorbed pump power of 3.77 W, a stable Q-switched operation, as illustrated in Fig. 6(d), was realized with a repetition rate of 90.4 kHz and a pulse width of 654 ns. The corresponding peak power and the single pulse energy were calculated to be 1.4 W and 0.9 μJ, respectively. Until now, the narrowest pulse duration generated by the passively Q-switched Ho3+-doped lasers at 3 μm was 820 ns by using Fe2+:ZnSe as saturable absorber [11]. Recently, a passively Q-switched Ho,Pr:LiLuF4 laser based on a monolayer grapheme-SA was realized with the shortest pulse duration of 937.5 ns [10]. In this work, a pulse duration as short as 654 ns was obtained, demonstrating that the WS2-SA is a potential optical modulator at 3 μm and Ho,Pr:LiLuF4 crystal is a promising host material for Mid-IR lasers.
Fig. 6 Performance of the Q-switched Ho,Pr:LiLuF4: (a) repetition rate; (b) pulse duration; (c) single pulse energy; (d) the typical pulse train and a single pulse envelope at T = 2%.
4. Conclusion
In conclusion, we have demonstrated a stable passively Q-switched Ho,Pr:LiLuF4 laser at 2.95 μm by utilizing the WS2-SA. Under the absorbed pump power of 3.77 W, the narrowest pulse width of 654 ns was obtained with the repetition rate of 90.4 kHz, corresponding to the single pulse energy of 0.9 μJ.
Funding
National Key Research and Development Program of China (2016YFB1102201); National Natural Science Foundation of China (NSFC) (61405213); National Natural Science Foundation of China (NSFC) (61405101) and the Science and Technology Project of Qingdao (No.16-5-1-9-jch).
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