The performance of a diode-end-pumped passively Q-switched dual-wavelength Nd:GGG laser operating at 932.9 and 936.5nm with V3+:YAG as the saturable absorber was demonstrated for the first time to the best of our knowledge. The maximum dual-wavelength average output power of 150mW was achieved with a T = 2% output coupler under the absorbed pump power of 2.55W, corresponding to the optical-to-optical conversion and slope efficiency of 5.9% and 8.0%, respectively. The minimum pulse width was 395ns with the pulse repetition frequency of 140kHz, which was attained with a T = 5% output coupler under the absorbed pump power of 2.55W.
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Yttrium-Aluminum Garnet doped with three-valence vanadium (V3+:YAG) has attracted great interest due to the excellent optical, mechanical and thermal properties, which give an opportunity to design compact and reliable pulsed laser sources. The V3+ ions can occupy both tetrahedrally and octahedrally coordinated positions, which make V3+:YAG have different absorption peaks [1,2]. The tetrahedrally coordinated V3+ ions contribute to the peaks at 800, 1140 and 1320nm, corresponding to the 3A2-3T1 (3F), 3A2-1E (1D) and 3A2-3T2 (3F) transitions, respectively. Therefore it is this co-ordination of the ions that is important for the action of the passive Q-switching at 1.0 and 1.3μm. The peak at 425nm is assigned to the 3T1 (3F)-3T2 (3P) transition of V3+ ion in octahedral position of garnet lattice . Due to the extremely high ground state absorption cross-section (3.0 × 10−18cm2 at 1064nm, 7.2 × 10−18cm2 at 1342nm) and nearly negligible excite state absorption , V3+:YAG has been successfully exploited in the efficient Q-switching of Nd3+-doped lasers emitting at 1.0 and 1.3μm [5–7]. Q-switching and passive mode-locking for the solid state lasers with wavelengths at 747nm, 780nm have also been obtained with V3+:YAG saturable absorber . However, to the best of our knowledge, there is no report on the pulsed laser at 0.9μm using V3+:YAG as the saturable absorber. According to the absorption spectrum of V3+:YAG presented in Ref , the broad absorption band near infrared covers the water vapor absorption region ranging from 930 to 960nm [9,10]. Therefore, the pulsed laser at 940nm region by exploiting V3+:YAG as the saturable absorber can be a promising source for the differential absorption lidar in water vapor detection.
Nd3+-doped Gadolinium gallium garnet (Nd:GGG) has many advantages suitable for the diode pumping, such as good thermal conductivity, good mechanical properties, and wide absorption band near 808nm. Due to that Nd3+ substituting Gd3+ has only weak concentration quenching; the concentration of Nd3+ in GGG can reach to 4% or even higher. Furthermore, the Nd:GGG crystal can be grown core-free in larger size with superior optical quality [11,12]. The diode-pumped Nd:GGG lasers emitting at 1.06 and 1.33μm have been studied widely [13,14].The quasi-three-level operation of Nd:GGG induced by the Nd3+ transition of 4F3/2-4I9/2 emits at 940nm region, overlapping the absorption peaks of water vapor. Zhang Chun-Yu et.al have reported the continuous-wave laser operation of a diode pumped 1.6 at. % Nd:GGG laser at 938nm, with 620mW output power obtained at the incident pump power of 5.0W .
In this paper, a diode-end-pumped passively Q-switched dual-wavelength Nd:GGG laser operating at 932.9 and 936.5nm with V3+:YAG as the saturable absorber was realized in a simple linear cavity. The maximum dual-wavelength average output power of 150mW was achieved with a T = 2% output coupler under the absorbed pump power of 2.55W, corresponding to the optical-to-optical conversion and slope efficiency of 5.9% and 8.0%, respectively. The minimum pulse width was 395ns with the pulse repetition frequency of 140kHz, which was attained with a T = 5% output coupler under the absorbed pump power of 2.55W.
2. Experimental setup
The experimental arrangement of the diode-pumped passively Q-switched Nd:GGG laser was shown schematically in Fig. 1 . The pump source was a fiber-coupled 808nm diode laser with a core diameter of 0.6mm and numerical aperture of 0.22. Its radiation was coupled into the laser crystal by a focusing optical system with a 15mm focal length. The radius of the pump beam within the laser crystal was around 160μm. The input mirror M1 was a planar mirror with antireflection coatings at 808nm on one surface, high-reflection coatings at 0.93μm (R>99.8%) and high-transmission coatings at 808nm (T>99.8%) on the other surface. The plano-concave mirrors M2 with the radius of curvature of 100 mm and coated partial transmission at 0.93μm (T = 2.0% and T = 5.0%, correspondingly) were used as the output couplers. In addition, both the input and output mirrors were high-transmission coated at 1061 nm (T>95%) and 1331 nm (T>90%) to suppress the more efficient four-level transitions. The Nd:GGG crystal was with the dimensions of 4 × 4 × 1.2mm3. It was <111>-cut and had a Nd-doping concentration of 1.0 at.%. It was wrapped with indium foil and mounted in copper block cooled by water at a temperature of 18°C. The V3+:YAG absorber, with the dimensions of 5 × 5 × 0.5mm3 was placed next to Nd:GGG to realize the Q-switching operation. Its initial transmission at 0.93μm was measured to be about 98%. The geometric cavity length was ~30mm.The laser pulse was recorded by a Tektronix DPO7104 digital oscilloscope (1GHz bandwidth, 5Gs/s sampling rate) and a photodetector (New focus, model 1611).
3. Experimental results and analysis
In the quasi-three-level operation, a thin laser crystal could reduce the loss induced by the reabsorption of the lower laser state. However, this would inevitably lower the absorption of laser crystal at the pump radiation. For the 1.2mm long and 1.0at.% Nd:GGG exploited in our experiment, the absorption efficiency at the 808nm was measured to be about 40%, corresponding to the absorption coefficient of 4.3cm−1. To make an accurate evaluation of the performance of Nd:GGG laser, the absorbed pump power was introduced in the following discussion. The continuous-wave operation of the Nd:GGG laser was performed firstly. The dependence of output power on the absorbed pump power for different output couplers was given in Fig. 2 . As could be seen, when the output couplers T = 2% and T = 5% were exploited, the maximum output powers of 430 and 360mW were respectively obtained under the absorbed pump power of 3.3W, with the threshold pump powers of 0.54 and 0.85W, respectively. The corresponding optical-to-optical conversion efficiency for the said two situations were estimated to be respectively 13.0% and 10.9%, with the slope efficiency estimated to be 15.6% and 14.7%, respectively. According to the measured threshold pump power under different output couplers, the Findlay–Clay analysis could be performed to deduce the intracavity passive losses in the current resonator, with the value estimated to be ~1.8%. The high quality Nd:GGG as well as optimized resonator (to enhance the overlap efficiency between the lasing mode and the pump volume) were expected to be used to improve the CW quasi-three-level laser performance.
By inserting the V3+:YAG into the cavity adopted in the continuous-wave operation, the passive Q-switching operation of the Nd:GGG laser was realized, with the relationship between the average output power and absorbed pump power also shown in Fig. 2. The best performance was obtained with the T = 2% output coupler. The maximum average output power of 150mW was achieved under the absorbed pump power of 2.55W, corresponding to optical-to-optical conversion and slope efficiency of 5.9% and 8.0%, respectively. The fluctuation at the average output power 150mW over hours-long operation was found to be less than 1.5%. For the other situation, 100mW average output power was obtained under the same pump power, with the threshold pump power of 1.24W. In our experiment, the efficient and stable passive Q-switching operation of Nd:GGG/V3+:YAG laser could be readily achieved in a short plano-concave resonator, which could be attributed to three factors: the smaller stimulated emission cross-section of Nd:GGG at 0.93μm, the relatively larger ground state absorption cross-section and nearly negligible excited state absorption cross-section of V3+:YAG. This looses control on the ratio of the effective mode area in the gain medium to that in the absorber.
A fiber-coupled spectrometer with a resolution of 0.75nm (HR4000CG-UV-NTR Ocean Optics) was used for the spectral measurements. Figure 3 presented the laser emission spectrum for the pulsed Nd:GGG laser with 150mW output power. As could be seen, the laser emitted at dual-wavelength locating at 932.9 and 936.5nm. It was further found that the two modes had almost the same thresholds. Ref  has reported that two fluorescence emission peaks at 934 and 938nm was obtained with a 1.6at.% Nd:GGG, which make the Nd:GGG have the potential to operate at dual-wavelength. However, only one wavelength at 936.5nm was observed in our continuous-wave Nd:GGG laser. Once one mode oscillated in the continuous-wave operation, the residual inversion population density would be too low to sustain the other mode reaching the threshold. The greatly increased inversion population density induced by the Q-switching operation of V3+:YAG gave the other wavelength (932.9nm) an opportunity to oscillate, which consequently result in the simultaneously dual-wavelength operation. The passively Q-switched simultaneously dual-wavelength laser should be possible to be used as a source for the generation of terahertz radiation. Considering the little difference in the spectral response of spectrometer at the two neighboring wavelengths, the output power ratio of the two wavelengths could be estimated by comparing the spectral intensity of the two wavelengths. It was found that the spectral intensity ratio of the two wavelengths was nearly unchanged with increasing the pump power. The corresponding value was 0.72:1 for 932.9:936.5nm. So the maximum output power at 932.9 and 936.5nm were estimated to be 63 and 87mW, respectively. The beam spatial profile at the average output power 150mW was also measured by using a laser beam analyzer (Spiricon), with the image displayed in the inset of Fig. 3.
The relationship between the pulse repetition frequency (PRF) and the absorbed pump power was displayed in Fig. 4(a) . It was found that the PRF increased with the pump powers. When T = 2% output coupler was used, the PRF presented the variations of 61-136 kHz with the pump power increasing from 1.24 to 2.55W. For the other situation, the PRF increased from 80 to 140kHz with the increase of pump power from 1.6 to 2.55W. The high PRF induced by the V3+:YAG has also been observed in the passively Q-switched lasers at 1.34μm [6,7], which should be attributed to the short absorption recovery time and low saturable energy intensity of V3+:YAG.
Figure 4(b) depicted the pulse widths versus the absorbed pump powers for the two output couplers. As could be seen, the pulse width for the T = 2% output coupler firstly decreased to 522ns with increasing the pump power to 2.23W, and then slightly increased with the pump power. For the Q-switched lasers, when the PRF achieved such a high value that the initial inversion population could not be fully consumed during the interval when the Q-switch was turned on, the rise time of a pulse would be extended, resulting in the generation of stretched pulses. So the slight increase of the pulse widths was observed in our experiment. The minimum pulse width of 395ns with the PRF of 140kHz was attained with the T = 5% output coupler under the absorbed pump power of 2.55W. The temporal pulse profiles for the above situation, along with the corresponding train of pulses, were shown in Fig. 5(a) and 5(b), respectively. Compared with the previously published results on the passively Q-switched lasers at 0.9μm obtained with Nd,Cr:YAG, Cr4+:YAG or SESAM [16–18], the 395ns pulse width was obviously large. This should be attributed to the low pump power available and over 100kHz PRF generated by the pulsed Nd:GGG/V3+:YAG laser.
In summary, a diode-pumped quasi-three-level dual-wavelength Nd:GGG laser with V3+:YAG as the saturable absorber was demonstrated in this paper. The maximum continuous-wave output power of 430mW at 936.5nm was achieved with the T = 2% output coupler under the absorbed pump power of 3.3W. As for the passive Q-switching operation, the simultaneously dual-wavelength operation at 932.9 and 936.5nm was realized. The maximum dual-wavelength average output power of 150mW was obtained with the T = 2% output coupler, corresponding to the optical-to-optical conversion and slope efficiency of 5.9% and 8.0%, respectively. The minimum pulse width was 395ns with the pulse repetition frequency of 140kHz, which was attained with a T = 5% output coupler under the absorbed pump power of 2.55W.
This work was supported by the National Natural Science Foundation of China (No: 60878012, and 50721002), the Grander Independent Innovation Project of Shandong Province Grant No: 2006GG1103047, and Program for Taishan Scholars. He Jing-Liang’s e-mail address is firstname.lastname@example.org.
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