Passive Q-switching of a diode-pumped Yb:LYSO laser at 1060 nm with a Yb3+ ions-doped CaF2 crystal without the excited-state absorption (ESA) was demonstrated. An average output power of 174 mW with pulse duration of 5.6 μs and repetition rate of 27 kHz have been obtained under the unoptimized conditions. And the Q-switching conversion efficiency was as high as 51.7%.
© 2007 Optical Society of America
Passive Q-switching is a simple, low-cost and reliable method to produce the giant laser pulses. Several solid-state saturable absorbers have been extensively used as passive Q-switches, such as Cr4+:YAG,1,2 Cr2+:ZnSe,3 V3+:YAG,4 Co2+:MgAl2O4,5 Er3+:CaF2,6 and U:CaF2.7 A common disadvantage of these absorbers is the innegligible excited state absorption (ESA), which will result in a residual loss in the laser resonator when the ground state absorption (GSA) has been saturated. The residual absorption results in that the transmission could never reach 100% in practical saturable absorbers. So, the FOM (figure of merit) of a passive Q-switch was defined as the ratio between the ground-state absorption and ESA cross sections, i.e., FOM=σgs/σes, which is a measure of the quality of the saturable absorber.8 Yb3+ is well-known as a laser-active ion, with the simplest energy level scheme of only two electronic multiplets for the f→f transition, without detrimental processes such as excited state absorption or energy transfer. It is very straightforward for us to use the Yb3+-doped crystals with an infinite FOM value as passive Q-switches, without any residual absorption resulted from the excited state absorption. In this paper, we for the first time utilized Yb3+-doped CaF2 crystal as a passive Q-switch for the diode-pumped Yb3+ laser.
The experimental arrangement is shown in Fig.1. The resonator consisted of one dichroic input coupler M1 (HT@977 nm and HR@1020–1070 nm), one flat mirror M2 (HT@977 nm and HR@1020–1070 nm), and a concave output coupler (OC) with the curvature radius of 300 mm, which was used to produce two beams of light. In order to realize the laser operation in TEM00 mode and result in high conversion efficiency, the length of two arms were configured to keep the mode matching in crystal between the pump beam and the fundamental resonant mode. A fiber-coupled diode laser with a core-diameter of 200 μm and a numerical aperture of 0.22, emitting at 977 nm was used as the pump source. The 5×5×3 mm3 5.0 at.% Yb:LYSO (LuYSiO5) laser crystal (uncoated) was selected as the active element owing to its relatively low emission absorption cross section of 0.16×10-20 cm2 at 1060 nm, whose room-temperature absorption and emission spectra are displayed in Fig.2. 3.0 at% Na+, 2.0 at% Yb3+-codoped CaF2 crystal (uncoated, 7 mm thickness) with a small-signal transmission of 90.6% at 1060nm was selected as the passive Q-switch.
3. Results and Discussion
Figure 3 shows a typical sequence of Yb:LYSO laser pulses at 1060nm Q-switched by Na,Yb:CaF2 at a certain pump level. The repetition rate and width of passively Q-switched pulses are shown in Fig.4 as a function of incident pump power. At the pump level up to 3.7 W, the repetition rate is 27 kHz and the pulse width begins to be steady at the level of 5.6μs. All the characteristics of the passive Q-switched Yb:LYSO laser pulses are similar to those of self-Q-switched Na,Yb:CaF2,9 except the larger steady pulse width in this experiment resulted from the longer laser cavity of 695mm. For a fully saturated absorber, the FWHM pulse width (τp) can be expresses as10
where TR is te cavity round-trip time, ΔR the maximum modulation depth of 3% according to the determined bleaching data of the Na,Yb:CaF2 absorber at 1060nm, L the optical length of the resonator of 695 mm, c light velocity. Thus, we can estimate τp with a value of 0.54 μs, which is one order of magnitude smaller than that of the experimental data of 5.6 μs. The disagreement between the predicted and observed values might indicate that the absorption cross section of Na,Yb:CaF2 should be not much greater than the cross section of the lasing transition, because the equation (1) comes into existence only under the case that the cross section of the saturable absorber is much greater than the cross section of the lasing transition11.
The average output powers of Yb:LYSO laser versus absorber pump powers in cw and Q-switched operating regimes are plotted in Fig.5. Under the current laser cavity far from the optimal conditions, the slop efficiency of passive Q-switched laser was 9.0% while that of cw laser 17.4%, i.e., the conversion efficiency of free running into Q-switched mode is 51.7%. Under absorbed pump power of 3 W, the Q-switched average output power was 174 mW. Such a high extraction efficiency must be owing to the infinite FOM value of the absorber Na,Yb:CaF2. The peak power of laser pulses was 1.15 W, which could be expected to enhance several magnitudes of order through extremely shortening the length of laser cavity, and/or increasing the output coupler transmission and/or modulation depth of Na,Yb:CaF2 absorber according to the formulas (1) and (2).
In a previous paper,9 we reported the self-Q-switched laser operating of Yb3+, Na+-codoped CaF2 single crystal, where possible color centers or Yb3+ ions in a certain kind of sites were tentatively supposed to act as the saturable absorber. Further experiments have been carried out to determine whether color centers existed in the crystal. Such as additional absorption after annealed at temperatures up to 600°C or γ-irradiated with doses up to 2×105 Gy were measured to observe the changes of absorption spectra around 1.06 μm. Moreover, Yb3+, Na+:CaF2 was excited by 1064 nm Nd:YAG laser to measure the possible emission in the wavelength range of 1100 to 1700 nm. However, neither additional absorption nor emission belonging to color centers was observed. So, the proposal that Yb3+ in some special sites acting as the saturable absorber has to carry more weight upon the mechanism for passive Q-switching than the other.
The cw Yb:LYSO laser at 1060 nm was used as the pumping source to investigate the dependence of the transmission of Na,Yb:CaF2 Q-switch on the incident pump intensity. Experimental results are shown in Fig.6. The maximal transmission is close to 93.8%, which corresponds to 100% in consideration of the total surface-reflection losses (about 6.2%) of the uncoated sample. Under a cw pump condition, the fast absorber solution of the rate equations should be adopted as the following equation:12,13
where T is intensity transmission, T 0 is the small-signal transmission, I is incident pump power intensity, I s is the saturation intensity. A least-squares fit to the experimental data yielded that the saturation intensity I s was 4.26 kW/cm2. The absorption cross section (σa) can be obtained using σa = hv/(2I sτa),11 where τa is the upper level lifetime of Yb3+ ions in Na,Yb:CaF2, σa is inverse proportional to τa. The maximal lifetime of all kinds of Yb3+ sites in Na,Yb:CaF2 crystal is 7.2 ms of Na+-Yb3+ pairs.14 So, the minimal value of σa is 0.3×10-20 cm2, which is two times the emission cross section (σg) of Yb:LYSO (0.16×10-20 cm2).
For the quasi-four-level laser of Yb:LYSO, the passive Q-switching criterion can be written as the following expression:15
where Ag is effective laser beam area, and Aa is the beam area on saturable absorber. In our experiment arrangement, Aa is approximately equal to Ag. With the minimal ratio σa/σg of 1.8, the criterion is satisfied. It should be noted that the absorption cross sections were not equal to the values calculated as the absorption coefficients divided by the total atomic concentration of Yb3+ for a multi-center system as the Na,Yb:CaF2 crystal. For example, the absorption cross section of the Na,Yb:CaF2 crystal at 1060 nm was calculated to be 0.007×10-20 cm2 by the normal method, which is only 1/43 times the minimal value of σa estimated from the bleaching curve in Fig.6. That is to say, the actual amount of the Yb3+ ions contributing to the absorption intensity at 1060 nm must be only or less than 1/43 of the total ions.
With an Yb:CaF2 crystal replacing Na,Yb:CaF2 as a passive Q-switch, Yb:LYSO also operated in a Q-switching regime. In all the various host crystals, the absorption cross sections of Yb3+ exhibit a significant range of 0.1×10-20 cm2 to 7.0×10-20 cm2, and the upper lifetimes of 0.3 to 11 ms.16 So, it can be expected that Yb3+ doped in different host crystals as passive Q-switches could satisfy the Q-switching criterion for diverse laser crystals to produce interesting and useful ultra-short laser pulses.
We have firstly demonstrated that Yb3+ ions doped CaF2 crystal can be utilized as a passive Q-switch for diode-pumped Yb:LYSO laser. Bleaching experiment showed that the maximal transmission is close to 93.8%, which corresponds to 100% considering the total surface-reflection losses (about 6.2%) of the uncoated sample. In the primary experiments, we have obtained the passive Q-switched laser with the pulse-width of 5.6 μs and repetition rate of 27 kHz. The Q-switching conversion efficiency was as high as 51.7%. It can be expected that Yb3+-doped materials as saturable absorbers without ESA and with broad absorption bands in the region of 900–1100nm have more and more applications in the compact and practical solid state lasers.
One of the authors L. B. Su thanks Prof. R. Moncorgé of Centre Interdisciplinaire de Recherches Ions et Lasers (CIRIL), Université de Caen for the discussion. This work was sponsored by the National Science Foundation of China (Grant no. 60508016), Shanghai Rising-Star Program (A) (Grant no. 06QA14054) and National Outstanding Youth Foundation (Grant no. 60425516).
References and links
1. Y. Kalisky, “Cr4+-doped crystals: their use as lasers and passive Q-switches,” Prog. in Quant. Electron. 28, 249–303 (2004). [CrossRef]
2. Y. X. Bai, N. L. Wu, J. Zhang, J. Q. Li, S. Q. Li, J. Xu, and P. Z. Deng, “Passively Q-switched Nd:YVO4 laser with a Cr4+:YAG crystal saturable absorber,” Appl. Opt. 36, 2468–2472 (1997). [CrossRef] [PubMed]
3. R. D. Stultz, V. Leyva, and K. Spariosu, “Short pulse, high-repetition rate, passively Q-switched Er:yttrium-aluminium-garnet laser at 1.6 microns,” Appl. Phys. Lett. 87, 241118-1–241118-2 (2005). [CrossRef]
4. A. M. Malyarevich, I. A. Denisov, K. V. Yumashev, V. P. Mikhailov, R. S. Conroy, and B. D. Sinclair, “V:YAG - a new passive Q-switch diode-pimped solid-state lasers,” Appl. Phys. B 67, 555~558 (1998). [CrossRef]
5. K. V. Yumashev, I. A. Denisov, N. N. Posnov, N. V. Kuleshov, and R. Moncorge, “Excited state absorption and passive Q-switch performance of Co2+-doped oxide crystals,” J. Alloys Compound. 341, 366 (2002). [CrossRef]
6. M. B. Camargo, R. D. Stultz, M. Birnbaum, and K. Spariosu, “Novel erbium-doped crystal saturable absorber Q-switches for the Er:glass laser,” Conference on Lasers and Electro-Optics Society Annual Meeting 8, 115–116 (1994).
7. R. D. Stultz, M. B. Camargo, and M. Birnbaum, “Passive Q-switch at 1.53μm using divalent uranium ions in calcium fluoride,” J. Appl. Phys. 78, 2959–2961(1995). [CrossRef]
8. G. H. Xiao and M. Bass, “A generalized model for passively Q-switched lasers including excieted state absorption in the saturable absorber,” IEEE J. Quant. Electron. 33, 41–44 (1997). [CrossRef]
11. A. Szabo and R. A. Stein, “Theory of laser giant pulsing by a saturable absorber,” J. Appl. Phys. 35, 1562–1566 (1965) [CrossRef]
12. W. Rudolph and H. Weber, “Analysis of saturable absorbers, interacting with gaussian pulses,” Opt. Commun. 34, 491–496 (1980). [CrossRef]
13. A. Sennaroglu, U. Demirbas, S. Ozharar, and F. Yaman, “Accurate determination of saturation parameters for Cr4+-doped solid-state saturable absorbers,” J. Opt. Soc. Am. B 23, 241–249 (2006). [CrossRef]
14. L. B. Su, J. Xu, H. J. Li, W. Q. Yang, Z. W. Zhao, J. L. Si, Y. J. Dong, and G. Q. Zhou, “Codoping Na+ to modulate the spectroscopy and photoluminescence properties of Yb3+ in CaF2 laser crystal,” Opt. Lett. 30, 1003–1005 (2005). [CrossRef] [PubMed]
16. L. D. Deloach, S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway, and W. F. Krupke, “Evalution of absorption and emission properties of Yb3+ doped crystals for laser application,” IEEE J. Quant. Electron. 29, 1179–1191 (1993). [CrossRef]