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We report, for the first time, on the passive Q-switching laser performance of Yb:YSGG disordered garnet crystal. An average output power of 2.6 W at 1025.8 nm was produced at a pulse repetition rate of 11 kHz with a slope efficiency of 47%; the resulting pulse energy, duration, and peak power were respectively 236 μJ, 3.6 ns and 65.6 kW. The laser performance in continuous-wave mode under 935-nm diode pumping was also investigated, with output coupling changed over a wide range from 0.5% to 60%.
© 2014 Optical Society of America
1. Introduction
Garnet crystals of cubic structure, with Y3Al5O12 (YAG) as their typical representative, have great importance in making laser materials based on trivalent lanthanide rare-earth active ions such as Nd3+, Yb3+, Er3+, Tm3+, and Ho3+. With the rapid development of Yb ion lasers, gallium garnets as host medium for this active ion have attracted much attention in recent years. These include Yb:Y3Ga5O12 (Yb:YGG) [1–4], Yb:Lu3Ga5O12 (Yb:LuGG) [5,6], and another relatively early developed Yb:Gd3Ga5O12 (Yb:GGG) [7–11]. Apart from these ordered gallium garnet crystals, some kind of disordered ones, for instance, yttrium scandium gallium garnets (Y3ScxGa5−xO12), can be formed with scandium replacing some fraction of gallium in YGG. In 2013 one such Yb doped disordered gallium garnet, Yb:Y3Sc1.5Ga3.5O12 (Yb:YSGG), was first developed; its structural, thermal, and spectroscopic properties, as well as the influences of the Yb concentration on these properties, have been studied [12, 13]. Continuous-wave (cw) laser action was achieved with 970-nm diode pumping, showing very promising laser performance [13]. Possessing a smaller peak emission cross section (1.5 × 10−20 cm2) and a longer fluorescence lifetime (1.22 ms) compared to Yb:YAG or other ordered gallium garnets, Yb:YSGG seems to be advantageous for Q-switched laser operation. Indeed, actively Q-switched operation has been demonstrated with a compact Yb:YSGG laser, producing laser pulses in milli-joule level [14].
In spite of the work mentioned above, studies on this disordered Yb:YSGG garnet are still insufficient; some important laser properties of mode-locking, passive Q-switching, and wavelength tuning, still remain unknown, even for cw laser operation the results reported previously were also very preliminary. More work therefore needs to be conducted, in order to fully characterize the laser properties of this disordered Yb doped garnet. In this paper we report on the passive Q-switching laser performance of Yb:YSGG crystal achieved under different operational conditions, with Cr4+:YAG crystal as saturable absorbers. Laser action in cw mode has also been studied in the present work, employing a high-power 935-nm diode as pump source, with output coupling changed in a wide range from T = 0.5% to T = 60%.
2. Description of experiment
The experimental laser setup, utilized in the studies of cw and passive Q-switching laser performance of Yb:YSGG crystal, was built with a simple plano-concave resonator. The plane mirror was coated for high reflectance at 1030 nm and high transmittance at 935 nm. A group of concave mirrors having radius of curvature of 25 mm, with transmissions at 1030 nm ranging from T = 0.5% to T = 60%, served as the output coupler. The crystal sample of Yb:YSGG tested in the experiment was uncoated, 5 mm long, with a square aperture of 3 mm × 3 mm, the Yb ion concentration in the crystal was 5 at. % (6.23 × 1020 cm−3). The Yb:YSGG sample, held in a copper block that was kept at 8 °C by cooling water, was placed close to the plane mirror in the resonator. As saturable absorber for passive Q-switching, several Cr4+:YAG crystal plates with initial transmissions of T0 = 94.4%, 89.7%, 85.4% (determined at 1.06 μm) were used, all of which were coated for antireflection at 1.06 μm on their surfaces. To generate efficient Q-switched laser action, the saturable absorber was inserted between the laser crystal and the output coupler, leaving a distance of about 2 mm from the rear surface of the laser crystal. The cavity length for Q-switched operation was 14 mm, it was increased to 23 mm in the case of cw operation to optimize the laser efficiency. The pump source employed was a fiber-coupled diode laser emitting at a center wavelength of 935 nm with a bandwidth of about 3.5 nm, the fiber core diameter was 200 μm and the NA was 0.22. The pump radiation was focused by a re-imaging unit and coupled into the Yb:YSGG crystal with a pump spot radius of approximately 100 μm.
3. Results and discussion
In the early preliminary study on the cw laser properties of Yb:YSGG crystal, only very limited results for a specific low output coupling of T = 3% were presented, which were obtained under 970-nm diode pumping [13]. It is evident, from the absorption spectrum [13], that the main absorption band around about 930 nm with a bandwidth amounting to more than 30 nm proves to be more desirable for diode pumping, compared to the weaker and narrower zero-phonon line absorption band located at about 970 nm. The unsaturated or small-signal absorption of the 5-mm long crystal sample (5 at. % of Yb concentration) for the 935-nm pump radiation was measured to be 0.80, very close to the calculated one (0.78) obtained using the absorption spectrum that gives an absorption coefficient of 3.0 cm−1 at 935 nm.
In the current experiment, the cw laser performance of Yb:YSGG crystal was studied thoroughly under 935-nm diode laser pumping, with output coupling varied in a wide range of 0.5%−60%. It was found that for the resonator configuration utilized in our experiment, the laser performance (output power and efficiency) achieved with output couplings ranging from 3% to 20% turned out to be very close, with the optimum output coupling being T = 10%.
Figure 1 shows the cw output power as a function of absorbed pump power (Pabs) measured in the cases of T = 10%, 40%, and 60%. For the sake of clarity, the results for other output couplings are not presented. In the case of T = 10%, lasing threshold was reached at Pabs = 1.45 W. Above threshold the output power increased with pump power, reaching 7.9 W at Pabs = 15.6 W, the highest pump level available in the experiment, the corresponding optical-to-optical efficiency was 51%, while the slope efficiency, determined for the high pump power region exceeding Pabs ≈7 W, was 64%. It is worth noting that even under an output coupling as high as T = 60% that was far from the optimum value (T = 10%), very efficient oscillation could still be obtained with a slope efficiency of 60%, generating an output power of 6.5 W at the highest pump power applied. With very low output couplings, T = 0.5%, 1%, laser oscillation arrived at threshold at Pabs = 0.45, 0.60 W, much lower than in other cases with higher output couplings, which was due to the very low overall losses of the laser resonator. However, with such low output coupling, the output power tended to get saturated at a moderate pump level, the maximum output produced in the cases of T = 0.5% and T = 1% was measured to be 2.40 W (at Pabs = 8.5 W) and 3.73 W (at Pabs = 10.6 W), respectively.
Depicted in Fig. 2 are laser emission spectra for various output couplings measured at an intermediate pump power of Pabs = 8.1 W, revealing the dependence of oscillation wavelengths upon the output coupling of the laser resonator. For laser operation with a fixed output coupling, the emission spectrum changed only slightly with pump power. One sees for the lowest output coupling utilized, T = 0.5%, the laser oscillation occurred in a wavelength range 1067−1075 nm, which was far beyond the wide main emission band peaked at 1025.4 nm [13]. With the output coupling increased to a slightly higher T = 1%, the laser oscillation shifted largely toward short-wavelength side to 1046.3−1048.9 nm, which were within the main emission band edge [13]; Increasing the output coupling further would force the laser action to occur at still shorter wavelengths, and eventually for T = 60%, the laser oscillated at 1025.8−1027.0 nm, very near the emission peak. This evolution behavior of oscillation wavelengths with output coupling can be understood qualitatively from the calculation of σg(λ), the effective gain cross section versus wavelength [13], where the excitation level (β) is roughly in direct proportion to the output coupling. The free-running laser action usually occurs at or around the peak wavelength of σg(λ), which depends critically on the value of β. For a very low excitation level of β = 0.03, the gain maximum is found to appear at about 1072 nm; it shifts to 1046 nm when the value of β is increased to 0.04. With the magnitude of β further increased to 0.06 and 0.15, the gain maximum moves, respectively, to 1040 and 1026 nm. In a qualitative sense, this provides a rough prediction of the measured emission spectra for T = 0.5%, 1%, 10%, and 60%. It is worth noting that for λ = 1072 nm, the minimum excitation necessary for achieving laser action is determined to be βmin ≈0.01, for the limiting case where the overall losses of the resonator are assumed to be zero.
Fig. 2 Variation of laser emission spectrum with the output coupling, measured at an intermediate pump power of Pabs = 8.1 W.
For passively Q-switched operation the cavity length was shortened to 14 mm, to reduce the cavity photon lifetime and hence the duration of laser pulses generated. To avoid optical damage to the laser crystal and the saturable absorber, the output coupling utilized in Q-switched operation was limited to T ≥ 30%. For this resonator configuration, the fundamental mode radius at the position of the laser crystal is calculated to be 65 μm, which is smaller than the pump beam spot radius of 100 μm. Multiple transverse modes oscillation is therefore expected for the Yb:YSGG laser operating in either cw or Q-switching mode. A slightly larger mode radius of about 70 μm is estimated at the position of the saturable absorber. It is worth mentioning that the thermal lensing effect, which usually occurs inside the laser crystal, particularly under high pumping power conditions, turns out to have little influence on these mode sizes for the current plano-concave resonator, provided the thermal focal length is limited to fT ≥ 200 mm.
Passively Q-switched laser action was realized under different operational conditions (T and T0). Figure 3 shows the output power versus Pabs for the cases of T = 30%, T0 = 94.4%; T = 50%, T0 = 89.7%; and T = 60%, T0 = 85.4%. Under conditions of T = 30%, T0 = 94.4%, Q-switched oscillation reached threshold at Pabs = 2.2 W, above which the average output power increased with pump power. At Pabs = 9.43 W, an average output power of 3.22 W was measured, resulting in an optical-to-optical efficiency of 34.1%, while the slope efficiency for Pabs > 5.5 W was 55%, which was slightly lower than the corresponding cw value (58%). At this output level, the pulse repetition frequency (PRF), which increased progressively with pump power, was measured to be 28 kHz. The energy contained in a single laser pulse was estimated, from the average output power and the corresponding PRF, to be 115 μJ. Above this output level, a second pulse was observed after the intense main laser pulse. In the case of T = 50%, T0 = 89.7%, a maximum output power of 3.1 W could be obtained at Pabs = 9.8 W before the onset of second small pulse, the PRF measured at this output level was 20 kHz, leading to a pulse energy of 155 μJ. With the output coupling increased to T = 60% and the initial transmission of the saturable absorber decreased to T0 = 85.4%, efficient Q-switched laser action was also achieved, with the lasing threshold pump power increased to Pabs = 3.3 W. In this case, the highest average output power, 2.6 W, was generated at a pump level of Pabs = 9.8 W with a slope efficiency of 47%, the corresponding PRF was measured to be 11 kHz, giving rise to an estimated pulse energy of 236 μJ. Increasing the pump power in excess of this level, optical damage was likely to occur on the end faces of the laser crystal.
Fig. 3 Output power as a function of Pabs, measured for the Yb:YSGG laser under different Q-switching operational conditions.
Shown as the inset to Fig. 3 is a typical beam pattern of the Yb:YSGG laser, which was recorded at an intermediate pump power of Pabs = 6.5 W for the case of T = 30%. The corresponding beam quality factor (M2) was measured to be 2.8 and 2.4 for the horizontal and vertical directions, respectively, implying the presence of higher-order transverse modes, which was a result of the smaller TEM00 mode size (65 μm) than the pump spot size (100 μm). Due to the multimode nature, the spot radius of the laser beam will increase to M times its corresponding TEM00 mode size [15]. Accordingly, the beam radius in the saturable absorber is estimated to be 113 μm (an average value of 2.6 is taken for M2), from which the maximum energy fluence reached in the saturable absorber during the passive Q-switching process is determined to be 0.59 J/cm2 (pulse energy of 236 μJ).
In passively Q-switched laser operation obtained with the Yb:YSGG crystal, the amplitude fluctuations from pulse to pulse were estimated to be less than 15%, while the timing jitters were no more than 20%. Illustrated in Fig. 4(a) is an oscilloscope trace showing a laser pulse train, which was recorded at the highest output level (Pabs = 9.8 W) for the case of T = 60%, T0 = 85.4%, an individual pulse profile is shown in Fig. 4(b), the pulse duration (FWHM) was 3.6 ns. Given the pulse energy of 236 μJ, the peak power reached in the Q-switched operation was 65.6 kW.
Fig. 4 Oscilloscope trace showing a pulse train (a) and an individual laser pulse profile (b) measured at Pabs = 9.8 W in the case of T = 60%, T0 = 85.4%.
Illustrated in Fig. 5 are laser emission spectra measured at Pabs = 7.8 W under different operational conditions, giving a direct comparison of oscillation wavelengths between Q-switched and cw laser operation. In the case of T = 30%, laser oscillation in Q-switching and cw modes occurred over a nearly identical wavelength range; whereas in Q-switched operation under conditions of T = 60%, T0 = 85.4%, the pulsed laser oscillated at a single main wavelength of 1025.8 nm, with the second line at 1025.2 nm, which was present in cw oscillation, almost suppressed completely during the Q-switching process, owing to the much higher saturable and unsaturable losses that greatly enhanced the effect of axial mode discrimination.
Fig. 5 Laser emission spectra measured under different Q-switching and cw operational conditions at Pabs = 7.8 W.
One notes in Fig. 5 that the emission spectrum for T = 60% differs to some extent from that shown in Fig. 2 for the same output coupling. This difference originated mainly from the change in the resonator configuration (cavity length shortened to 14 mm from 23 mm), rather than from the small variation of the pumping power at which the spectrum was measured. In fact, the laser emission spectrum evolved merely slightly with pumping power, as long as the output coupling remained unchanged. This was particularly the case for high output couplings. For example, the emission wavelengths changed only from 1025.8−1027.0 to 1025.6−1027.3 nm, when the pump power increased from 8.1 to 13.7 W, in the cw laser operation for T = 60%. Physically, with the pump power being raised, the thermally induced resonant re-absorption losses would increase by some small amount, forcing the oscillation wavelengths to shift slightly toward the short-wavelength side where higher gain is available.
Table 1 lists the primary parameters describing the passively Q-switched laser action achieved with the Yb:YSGG disordered garnet crystal under different operational conditions, in which Pavr is the average output power; Ep, tp, and Pp are respectively the pulse energy, duration, and peak power; ηopt is the optical-to-optical efficiency; ηs is the slope efficiency; and λc is the center lasing wavelength. It may be of some sense to compare the results presented here with those previously reported for a very compact passively Q-switched Yb:YSAG laser [16], where 0.4 W of output power was generated at a pulse repetition rate of 12.7 kHz with a slope efficiency of 13%, under conditions of T = 10%, T0 = 95%, with the resulting pulse energy, duration, and peak power being 31 μJ, 2.5 ns, and 12.4 kW, respectively.
Table 1. Parameters Characterizing the Passive Q-switching Laser Parameters of Yb:YSGG Garnet Crystal
4. Conclusion
In summary, the cw laser performance of Yb:YSGG disordered garnet crystal was investigated under 935-nm diode pumping, with output coupling changed over a wide range from 0.5% to 60%. An output power of 7.9 W was generated with optical-to-optical and slope efficiencies being 51% and 64%. Efficient passively Q-switched laser operation was realized under different operational conditions. In the case of T = 60%, T0 = 85.4%, an average output power of 2.6 W at 1025.8 nm was produced at a pulse repetition rate of 11 kHz, with a slope efficiency of 47%; the resulting pulse energy, duration, and peak power were 236 μJ, 3.6 ns, and 65.6 kW, respectively.
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