Open Access
Add to BibSonomyAbstract
In this paper, a multiple folded resonator is presented which consists of a multiple optical folding setup, a flat total reflector, a flat output coupler, a Q-switch crystal and a polarizer. By this technique, the output energy of 32.6mJ and pulse width of 13.4ns with a repetition rate of 5Hz was obtained, which is three times higher than that reported in the past publications by the use of the currently existing technique of the Q-switched slab gain lasers with the unstable resonator. The output beam with a quality of M2 = 1.55 in the slow axis and M2 = 1.40 in the fast axis was also obtained.
© 2014 Optical Society of America
1. Introduction
The Yb:YAG Q-switched lasers are more suitably pumped by laser diode array, because the strong pump absorption coefficient of Yb:YAG is around 940nm [1,2]. In 1993, the first diode-pumped Q-switched Yb:YAG laser was reported by T.Y.Fan [3], which can generate 72μJ/pulse at 1.03μm with a pulse width of 11ns. As we know, there are three possible geometries of gain medium for diode-pumped solid state lasers: rod, disk, and slab geometries. Compared to rod and disk geometries, slab geometry is more suitable to solid state lasers owing to its large area surface for cooling, which guarantees high beam quality output laser. In the past twenty years, several researchers investigated the Q-switched Yb:YAG slab laser. So far, the maximum output energy is 60mJ with the pulse width of 60ns, which is reported by A.Mandl et al. [4].
In the currently existing technique of the Q-switched slab gain lasers with unstable resonator, the aperture of electro-optic crystal should be larger than the width size of gain medium, and the laser beam in the unstable resonator passes through the electro-optic crystal in a multiple folded way, which leads to the high inserting loss, therefore the lower energy extraction. Besides, in the existing Q-switched slab gain lasers, the large aperture of the electro-optic crystal leads to the high cost for the lasers. In this paper, a new multiple folded resonator with a single end LD pumped Q-switched Yb:YAG slab laser is presented. In this resonator, the laser beam will pass through the electro-optic crystal in a way as in the ordinary stable resonator laser, no multiple folded beam passes through the electro-optic crystal, the inserting loss will be much lower than that as in the existing Q-switched slab gain lasers with unstable resonator. By the use of the presented technique, the pulse energy of 32.6mJ with a good beam quality has been obtained, which is three times higher than that reported in the past publications by the use of the existing technique of the Q-switched slab gain lasers with unstable resonator.
2. Experimental setup of a multiple folded resonator for a slab gain medium
The multiple folded resonator for the LD pulse end pumped Q-switched Yb:YAG slab laser is shown in Fig. 1. In this multiple folded resonator, an optical folding setup is constructed by two curved mirrors (M3 and M4), which is positioned between a flat total reflector M5 and a flat output coupler M6, a slab Yb:YAG crystal with a size of 10mm in width × 1mm in height × 10mm in length is put inside the optical folded setup. The RTP Q-switch crystal with the aperture of 4mm × 4mm is put outside the optical folding setup and in the front of the total flat reflector. The polarizer is set in the front of the Q-switch crystal. When a quarter-wave electrical voltage is applied on the RTP Q-switch crystal, the multiple folded optical resonator is at a low Q-state, no lasing is produced in the resonator, and when the quarter-wave electrical voltage applied to the RTP Q-switch is removed, the linear polarized lasing is produced in the resonator.
Fig. 1 Schematic diagram of experiment setup of the single end LD pumped Q-switched Yb:YAG slab laser.
In the presented technique, the aperture of RTP crystal is only needed to fit the output aperture size of the optical folding setup, no need to fit the width of the slab laser crystal as that in the slab lasers with the unstable resonator [5,6]. In the experiments, a QBU-BT-3012 Pockels cell driver (OEM Tech) is used to drive the RTP Q-switch. The DG535 Digital Delay and Pulse Generator (Stanford Research System, INC.) is used to synchronize a Laser Diode driver and the Pockels cell driver. In order to increase the output energy of the pulse laser beam, the opening time of RTP Q-switch is at falling edge of the pump light.
In the experiments, the pump source is a laser diode array incorporated with four bars, the angles of divergence are 10 × 0.4 deg (FWHM) along the slow axis (x direction) and fast axis (y direction) respectively. The pitch between two adjacent bars is 1.8mm. The dimension of the diode array is around 10mm along the slow axis and 7.2mm along fast axis. The center wavelength was fixed at around 940nm for Yb:YAG gain medium by maintaining the temperature of the cooling water at 23°C. Its peak output power is 1000W when operating at a repetition rate of 10Hz, a pulse width of 500μs and a DC drive current of 270A. Then, the pump light was focused into a rectangular waveguide for homogenization by a cylindrical lens (L1) in slow axis and the dimension of the waveguide is 10mm (x) × 8mm (y) × 100mm (z). The focal length of L1 is fixed at 50mm with consideration to the numerical aperture of the waveguide and the distance between L1 and the waveguide is 45mm. Only the two side surfaces of the waveguide, which are perpendicular to z, are coated with high transmission at 940nm. The focal length of L2 is identical to L1, and so does the distance between the waveguide and L2. L1 and L2 compose a cylindrical telescope in slow axis. In order to acquire a homogeneous line pump light inside Yb:YAG gain medium, a spherical lens (L3) with focal length of 50mm is used and the distance between L2 and L3 is 15mm. The dimension of the pumping line is about 10mm (x) × 0.4mm (y). The loss of the transforming optical system is 20%. In the experiments, two plane dichroic mirrors (M1 and M2) placed at 45° to the pump light. M1 and M2 are coated for high transmission at 940nm and high reflection at 1030nm for coupling the pump light and folding the optical cavity.
The dimension of Yb:YAG is 10mm × 1mm × 10mm with 2.5-at.% doping concentration. The two large area surfaces (10mm × 10mm) are coated with gold for cooling. Both of large area surfaces are tightly mounted with water-cooled copper heat sinks. Only the other two surface (10mm × 1mm) are polished and coated for passing the pump light (940nm) and the laser beam (1030nm).
The folding optical setup consists of two curved mirrors with the curvature radius of R1 = −1226mm for M3 and R2 = 1406mm for M4. The distance between the two folding curved mirrors is L = 90mm. The aperture size of one end of the folding optical setup is 2.9mm in width and the aperture size of the other end is 3.8mm in width. The Q-switch crystal is put at the side of 3.8mm aperture end of the folding optical setup. By the folding optical setup, the laser beam is five paths folded in the Yb:YAG slab crystal. The flat total reflector (M5) is coated for high reflection at 1030nm and the flat coupler M6 is used as output mirror with the transmission of 80% at 1030nm. The mirror M7 is placed at 45° for guiding the laser beam.
3. Experimental results and discussion
Figure 2 shows the output energy per pulse with respect to pump width in Q-switch and free-run modes, at 5Hz repetition rate and 270A DC driving current. In the free run mode, no electrical filed is applied on the RTP Q-switch crystal, under the condition of pulse pumping, a free run laser pulse can be produced. In the Q-switch mode, a quarter-wave electrical voltage is applied on the RTP Q-switch crystal during the pulse pumping time, and following the end of the pumping pulse, as the quarter-wave electrical voltage is removed, a linear polarized laser pulse is produced. In the experiments, the output pulse energy was measured by a LE-3B laser energy meter (Beijing Physcience Opto-Electronics Co., Ltd). In the experiments, the highest pulse energy of 32.6mJ with the pulse width of 13.4ns was obtained in the Q-switch mode at a pump width of 1300μs, and the highest pulse energy of 40.4mJ is obtained in the free run mode at a pump width of 1300μs. In the experiments, the phenomenon of the green light of the frequency doubled effect was observed when the laser energy is over 20mJ, which is due to the characteristics of RTP crystal. The higher the laser pulse energy is, the stronger the scattered green light is from the RTP crystal. It has been found that the output pulse energy of Q-switch mode is higher than that of free run mode when the pump width is less than 1100μs as shown in Fig. 2. And, it can also be seen that the output pulse energy in the Q-switch mode is lower than that in the free run mode when the pump width is more than 1100μs. The reason for the difference between the energy extraction efficiency in the Q-switch and the free-run modes is the difference of optimum output coupling for the Q-switch mode and the free-run mode.
The corresponding output pulse width and output intensity (Beam spot size is 3.1 × 0.7mm2 at a distance of 200mm from the output mirror M6) are shown in Fig. 3. The pulse width is measured by a DET02AFC Si biased Detector (Thorlabs, Ltd) and a 500MHz-bandwidth LeCroy Wavepro 7000 digital oscilloscope.
A pulse width of 13.4ns and the output pulse intensity of 108.8MW/cm2 were obtained at a pump width of 1300μs. The pulse profile is shown in Fig. 4.
Figure 5 shows the near-field intensity distribution of the output pulse laser with output energy of 32.6mJ and pulse width of 13.4ns.
Fig. 5 (a) 3D intensity distributions of Near-field output pulse laser beam. (b) Beam transverse profile. (c) Intensity distribution in slow axis: experimental data (*) and Super-Gaussian Fitting (red curve). (d) Intensity distribution in fast axis: experiment data (*) and Gaussian Fitting (red curve).
The corresponding far-field intensity distribution in the fast axis direction and the slow axis direction is obtained at the focal point by the cylindrical lens of 150mm focal length respectively, which are shown in Fig. 6. It can be seen that the far filed intensity distributions are well fitted with the Gaussian fundamental mode. By measuring the beam widths at different positions along the beam propagation direction after the cylindrical focusing lens, the M2 factor of the output beam is obtained, which is 1.55 in the slow axis and 1.40 in the fast axis.
Fig. 6 (a) 3D intensity distribution of Far-field output pulse laser beam. (b) Beam transverse profile. (c) Intensity distribution in slow axis: experimental data (*) and Gaussian Fitting (red curve). (d) Intensity distribution in fast axis: experimental data (*) and Gaussian Fitting (red curve).
4. Conclusion
In this paper, a new technique of a multiple folded resonator for LD pumped Q-switched Yb:YAG slab laser has been presented. By the use of the presented technique, the pulse energy of 32.6mJ with the pulse width of 13.4ns and the beam quality of M2 = 1.55 in the slow axis and M2 = 1.40 in the fast axis were obtained from a slab gain with the size of 10mm × 1mm × 10mm, which is three time higher than that by the use of the currently existing technique of the Q-switched slab gain lasers with the unstable resonator from the same size of a slab gain reported in the past publications.
References and links
1. F. D. Patel, E. C. Honea, J. Speth, S. A. Payne, R. Hutcheson, and R. Equall, “Laser demonstration of Yb3Al5O12 (YbAG) and materials properties of highly doped Yb:YAG,” IEEE J. Quantum Electron. 37(1), 135–144 (2001). [CrossRef]
2. C. Bibeau, R. J. Beach, S. C. Mitchell, M. Emanuel, J. Skidmore, C. A. Ebbers, S. B. Sutton, and K. S. Jancaitis, “High-average-power 1-μm performance and frequency conversion of a diode-end-pumped Yb:YAG laser,” IEEE J. Quantum Electron. 34(10), 2010–2019 (1998). [CrossRef]
3. T. Y. Fan, S. Klunk, and G. Henein, “Diode-pumped Q-switched Yb:YAG laser,” Opt. Lett. 18(6), 423–425 (1993). [CrossRef] [PubMed]
4. A. Mandl and D. E. Klimek, “Compact diode-pumped Yb:YAG slab laser,” SPIE 4267, 1–8 (2001).
5. H. Zhang, P. Shi, D. Li, and K. Du, “Diode-End-Pumped, Electro-Optically Q-Switched Nd:YVO4 Slab Laser and Its Second-Harmonic Generation,” Appl. Opt. 42(9), 1681–1684 (2003). [CrossRef] [PubMed]
6. D. Li, Z. Ma, R. Haas, A. Schell, P. Zhu, P. Shi, and K. Du, “Diode-end-pumped double Nd:YLF slab laser with high energy, short pulse width, and diffraction-limited quality,” Opt. Lett. 33(15), 1708–1710 (2008). [CrossRef] [PubMed]
