By using quite uniformly nine-stacks side-around arranged compact pumping system, a high power Nd:YAG ceramic quasi-CW laser with high slope efficiency of 62% has been demonstrated. With 450 W quasi-CW stacked laser diode bars pumping at 808 nm, performance of the Nd:YAG ceramic laser with different output coupling mirrors has been investigated. Optimum output power of 236 W at 1064 nm was obtained and corresponding optical-to-optical conversion efficiency was as high as 52.5%. The laser system operated quite stably and no saturation phenomena have been observed, which means higher output laser power could be obtained if injecting higher pumping power. The still-evolving Nd:YAG ceramics are potential super excellent media for high power practical laser applications.
©2005 Optical Society of America
High power lasers are widely used in a variety of applications, including materials processing, remote sensing, free-space communications, laser particle acceleration, gravitational wave interferometers, and even inertial confinement fusion (ICF) . The optical gain media of the system is the key factor for efficient laser oscillation. Since Maiman discovered the first ruby laser in 1960, numerous materials have been developed and improved to achieve high efficiency and high power for all-solid-state lasers. There are three primary groups of solid-state host materials: single crystals, glasses and ceramics, and among them Nd:YAG single crystal may be the most widely used laser media. But Nd:YAG single crystal grown by conventional Czochralski method has its own insurmountable disadvantages such as expensive, time-consuming, small size and low concentration , which has limited its applications in high power lasers. And for Nd-doped glass material, though it is very easy to get large size and high concentration, but its thermal conductivity and gain are quite low and the laser efficiencies were not satisfying when comparing with single crystals. Fortunately, with the breakthrough preparation, novel ceramic media combines the predominance of single crystals and glasses. It not only contains good thermal, mechanical and spectra properties as fine as single crystal but also can be made with large size (1 m×1 m×0.02 m) and high concentration (up to 4 at. % with no gradient). Further more, mass manufacture is possible .
Since 1960’s, a number of researchers had speculated that a theoretically dense polycrystal of an isotropic, pure material would be optically indistinguishable from a single crystal of the same material. In 1966, Hot-pressed CaF2 doped dysprosium appears to be the first reported polycrystalline material which established laser oscillation . Then several decades passed, no remarkable development had been acquired because the scattering losses of ceramic hosts were too high to gain effective laser output. Until in 1995, Ikesu et. al. fabricated transparent polycrystalline YAG by a solid-state reaction method. When pumping with a laser diode, laser output of about 70 mW was attained with the slope efficiency of 28% and this was the first laser demonstrated on Nd:YAG ceramics . Later in 1999, Konoshima chemical Co. Ltd. improved Nd:YAG ceramics successfully with NTVS method (nano crystalline technology and the vacuum sintering method) [6–7]. High quality, high transparent Nd:YAG ceramics with much low scattering losses have been fabricated. Optical absorption, fluorescence and emission spectra, physical and laser properties of Nd: YAG ceramics have been measured and compared with those of Nd: YAG single crystals, and almost identical superiority features have been obtained in qualitative analysis [8–10]. It shows that Nd:YAG ceramics are indeed potential superexcellent gain media for high efficient and high power lasers. Using these Nd:YAG ceramic samples, high slope efficiency of 60.9 % was achieved under end-pumping disk laser . And for middle and high power laser oscillation, slope-efficiencies from 18.8% to 36.3% were reported one by one [12–15]. In fact, it’s hard to get high efficiency in side-pumped high power laser system because of the thermal effects. In this paper, we designed a quite uniformly side-around arranged compact pumping system and thus demonstrated a high efficiency high power quasi-CW laser with a Nd:YAG ceramic rod. With 450 W quasi-CW stacked laser diode bars pumping at 1064 nm, 236 W optimum output laser at 1064 nm was obtained. The optical-to-optical conversion efficiency was 52.5% and corresponding slope efficiency was 62%. This is up to now the highest slope-efficiency acquired in high power Nd:YAG ceramic laser.
2. Experiment setup
A schematic diagram of the laser setup is shown in Fig. 1. The Nd:YAG ceramic rod used in the experiment was 75 mm in length and 5 mm in diameter with neodymium doping level of 1 at.%. Both the end facets of the rod were flat and antireflection coated at 1064 nm in order to reduce the intra-cavity losses, and the lateral surface was frosted. The rear mirror of the laser cavity was high-reflection mirror at 1064 nm and a series of output coupling mirrors were prepared with reflectivity from 30% to 84% at 1064 nm. Thus we could find the optimized output in experiment. The cavity length was about 195 mm. The pump source was operated at 808 nm. Liquid cooling was employed to remove heat from the ceramic rod and diode heat sink. The operation temperature was kept at about 16 °C.
In order to optimize the uniformity and radial profile of the pump distribution within the gain medium and decrease the coupling losses, we designed a compact side-around arranged direct radial-pumping head, of which cross-section configuration was illustrated in Fig. 2. The optical pump head consisted of nine LD stacked arrays mounted around the rod from 9 directions with proportional angle. The ceramic rod was mounted inside a flow-tube. The side-face of the ceramic rod and the emitting surface of the laser diodes were close proximity, and no coupling optics was employed between them. The coupling efficiency was by far the most desirable. Each LD stacked array consisted of five quasi-CW types LD bars, which were placed along the length of the laser rod and pumped perpendicularly to the direction of propagation of the laser radiation. Each bar generated 60 W peak powers. The arrays operating at 20% duty cycle were pulsed at a repetition rate of 1 kHz with a pulse width of 200 μs. The design of 9 LD arrays arranged around the ceramic rod symmetrical allowed optimizing the uniformity and radial profile of the pump distribution within the gain medium with good spatial overlap between pump radiation and low-order modes in the resonator, which in turn leads to a high-brightness laser output. Figure 3 showed the 2D contour plot of pump intensity distribution simulated by computer with ray tracing method.
3. Results and discussion
By changing the rear mirror with different reflectivity of 30%, 50%, 62.5%, 78%, and 83.4%, we get a relationship laser output power as a function of the average pumping power, which was shown in Fig. 4. The output power increased almost linearly with the pumping power, and the optimum output appeared with the coupling mirror of the reflectivity near 78%. When the pump current rose to 60 A, the total average pump power was about 450 W, and the maximum average power of 236 W multi-mode laser output was obtained by using optimum output coupling mirror. The optical-to-optical conversion efficiency was as high as 52.5% and corresponding slope efficiency was 62%. No obvious evidence of saturation was observed from the output curve, which means higher output power is possible if higher pump power is available. It also indicated that the laser cavity is stable enough.
Referring to the former experimental record of a Nd:YAG single crystal with the same concentration and size using in this system with an output coupling mirror of T=70%, we made a comparison between ceramic and crystal, which was shown in Fig. 5. The optical to optical efficiencies were 29% and 27% for the ceramic laser and for the single crystal laser, respectively. The corresponding slope efficiency was 46% for ceramic laser, and 44% for single crystal laser. It showed that these two kinds of laser materials share extraordinary the same laser output properties in quasi-CW operating.
Figure 6 showed the two and three-dimensional beam profiles of the Nd:YAG ceramic laser from CCD. Some interference stripes could be seen because the cavity length was fixed and the pass length differences between the transmitted beams were multiple numbers of the laser wavelength. It can be eliminated just by adjusting the cavity length slightly. The divergence angle of laser beam was measured about 12 mrad. For high power rod Nd:YAG lasers, thermal lensing and thermal stress-induced birefringence play very important roles. They would result a distortion of the laser beam and cause a significant decrease in beam quality and optical efficiencies. The detailed study will be explored later.
In conclusion, a high efficiency high power quasi-CW Nd:YAG ceramic rod laser operating at 1064 nm was demonstrated by using compact quasi-CW LD stacked arrays side-pumping system. High average output power of 236 W was achieved under 450 W pumping, corresponding to an optical-to-optical efficiency of 52.5% and slope-efficiency of 62%. The experiment showed that Nd:YAG ceramic has excellent performance with hundreds of watts laser output. To improve the beam quality of Nd:YAG ceramic and try to get higher output power is our next step. Multi-kW-level laser output with high optical efficiencies can be expected. Apart from having the equal laser efficiencies, ceramics have remarkable fabrication advantages, such as ease of fabrication, low cost, and large size, as well as multilayer and multifunctional structures. There is no doubt that Nd:YAG ceramics are very good alternative to Nd:YAG single crystals for high efficiency and high power practical laser applications in the near future.
References and links
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