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We demonstrate a diode-side-pumped continuous wave (CW) Nd:YAG laser at 1123 nm with over 100 W’s output power and good beam quality. The resonator adopts convex-convex structure working in a thermally near unstable cavity. By precise coating design a single 1123 nm wavelength is delivered. Under the pumped power of 870 W, an output power of 106.5 W is obtained, corresponding to an optical-optical conversion efficiency of 12.2%. The beam quality of M2 factor is measured to be 5.6. To the best of our knowledge, this is the highest output power with good beam quality for 1123 nm CW Nd:YAG laser.
©2010 Optical Society of America
1. Introduction
The continuous wave (CW) lasers with output wavelength around 1.1 μm are widely studied in recent years due to its promising applications in laser display, biology, laser guide star etc. The wavelength region from 550 to 600 nm can be covered through frequency doubling technology from 1.1x μm solid-state lasers [1–4]. For example, frequency doubling 556 nm laser from 1112 nm Nd:YAG laser is suitable for laser display and lighting because it is proximate to 555 nm which is most sensitive to our eyes [4]. An 1120 nm Raman fiber laser was used as a pump source for Raman fiber amplifier at 1178 nm, the latter can be frequency doubled to 589 nm for astronomy guide star [5]. The 1123nm line of the Nd:YAG crystal can be used as a pump source for thulium up conversion fiber lasers to generate blue light emission [6,7]. Till now, different techniques have been developed to obtain 1.1x μm lasers, mainly including: solid-state lasers [1–4,6–13], fiber Raman lasers [5] and frequency conversion by stimulated Raman scattering effect [14,15]. In fact, there exist abundant laser lines around 1.1 μm in Nd:YAG materials which correspond to the Stark components of the 4F3/2—4I11/2 transition, such as 1112 nm,1116 nm and 1123 nm [16].
Concerning the 1123 nm Nd:YAG lasers, J. Marling [17] investigated 1123 nm CW laser using a 5 kW krypton arc-lamp as pump source in 1978, where about 30 W’s output power was obtained. In 1999, N. Moore et al. [18] reported a 1.7 W 1123 nm Nd:YAG laser by diode-end-pump. E.J.Zang et al. [11] developed a single frequency 1.25 W 1123 nm laser for stable coherent light sources study. Recently, S.S.Zhang et al. [12,13] demonstrated the diode-end-pumped CW 1123 nm laser both for ceramic Nd:YAG and composite Nd:YAG, the output power was 10.8 W and 9.3 W, respectively. Till now, the output power of diode-end-pumped 1123 nm Nd:YAG lasers was several Watts level.
Compared with the diode-end-pumped solid state laser, side-pumped-method has its advantages in obtaining both high output power and compact configuration. In present work, we report a diode-side-pumped CW 1123 nm Nd:YAG laser with high output power and high beam quality based on a simple convex-convex symmetrical cavity with precise coating design. The laser contains two laser modules with a five-fold symmetrical structure. Under the incident pump power of 870 W, the laser output power is up to 106.5 W, which corresponds to an optical-to-optical conversion efficiency of 12.2%. The beam quality factor M2 is further measured to be 5.6 at the highest output power. This is the first time to demonstrate the 1123 nm laser with over 100 W output power and beam quality factor less than 10.
2. Theoretical analysis
As it is well known, several dozen laser transitions of Nd:YAG have been achieved up to the present. Most of the laser operation comes from the 4F3/2→4I9/2, 4I11/2, and 4I13/2 manifolds. Figure 1 shows the typical energy level diagram of Nd:YAG for these transitions. From the stimulated emission cross section data of Nd:YAG in Ref [16], the 1123 nm transition has upper and lower laser levels within the same manifolds as the more familiar 1064 nm line, but it has the problem of a less peak stimulated emission cross section that is approximately 15 times smaller than for the 1064 nm. Compared with the other two strong transitions such as 1319 nm and 946 nm, the relative performance of the 1123 nm is about 3 and 2 times smaller, respectively. Hence, in order to obtain the laser oscillation at 1123 nm, not only 1064 nm oscillation, but also the 1319 nm and 946 nm must be restrained by means of high transmission output coupler.
Another point should be emphasized that there exist three lines in a narrow region that are 1112 nm, 1116 nm and 1123 nm, respectively. They have relatively equal laser performance [16]. However, the 1112 nm and 1116 nm lines could be suppressed through precise coating and the single 1123 nm Nd:YAG laser oscillation can be performed [13].
3. Experimental
The experimental setup was shown in Fig. 2 . The laser system has two laser modules, eachmodule contains a Nd:YAG rod with 4 mm diameter and 120 mm length, the doping concentration of Nd3+ is 0.6at%. For each laser module, a home-made 30 LD-bars array arranged in a five-fold symmetry around the laser crystal is served as the CW pump source operated at central wavelength of 808.5 nm. The total pump power for each module is 540 W. In order to get higher beam quality, the laser cavity adopts convex-convex symmetrical structure. Here, L1 is the distance from the mirror M1 to the left Nd:YAG rod end surface,L2 is the distance from the mirror M2 to the right Nd:YAG rod end surface. Both of them are set to be 150 mm, which form a symmetric resonator. L3 is the distance between the Nd:YAG rods to be 8 mm. The radius of curvature is 500 mm both for the rear mirror M1 and the output mirror M2. In our experiment, we took the method of coating technique to obtain 1123 nm output. M1 is coated with 99.8% high-reflection ratio at 1122.7 nm and antireflection at 1064 nm, 946 nm and 1319 nm. M2 is coated with 5% transmission ratio at 1122.7 nm and 85% for 1064 nm. In order to restrain the 1112 nm and 1116 nm oscillations, we measured the transmissions in the band range from 1050 to 1150 nm. The transmission ratio for 1112, 1116, and 1123 nm is 7.8%, 6.5% and 5%, respectively. For spectrum measurements, the laser was split through an attenuator mirror M3 (5% transmission ratio, incidence angle of 45°) into the spectrometer via an optical fiber. The output power was monitored by Ophir F300A power meter.
4. Results and discussions
The primary consideration of resonator design is how to obtain a high output power while keeping a good beam quality. Firstly we used Gaussian light ABCD propagation matrix formulism of fundamental mode to optimize the cavity parameters. The components and the gain medium can be regarded as the separate elements in the function. Owing to the thermal effect, the laser material may be simplified to a lens with various focal length that shortens with the increase of absorbed pump power. The ABCD light propagation matrix for our laser structure is given as following:
Where, ft is the thermal focal length of the gain medium. L1’, L2’ and L3′ is the optical path after considering the refractive index of Nd:YAG material, respectively. R1 and R2 is the curvature radium of the mirror M1 and M2. The calculated beam size at the center of the second laser rod is: Figure 3 shows the simulation result for the calculated fundamental mode radius at the center of the laser rod as a function of thermal focal length. According to the theory in Ref [19], a good light beam quality can be obtained without output power reducing when laser operates in thermally-near unstable zone, i.e., the border of the stable region.For the diode-side-pumped solid-state laser, the laser material absorbs the pump laser energy, and the thermal loading leads to a lens behavior in the gain medium. To determine the thermal focal length of the single laser module, we constructed a short plano-plano resonator and used the method suggested by D.G.Lancaster [20]. Figure 4 shows the dependence of measured thermal focal length for single laser module on the pump power. As an example, the thermal focal length is 231mm at the pump power of 435W where the maximum output power is acquired in our experiment. Then the cavity parameters could be designed and optimized by the simulation results.
We measured the average output power of the 1123 nm Nd:YAG laser as a function of the incident pump power, the result is shown in Fig. 5 . It can be seen that the threshold isabout 330W. Following,the output power increases approximate linearly with the incident pump power. We observed a roll-over effect, i.e. the output power increases up to a maximum then falls with the increase of the pump power. While the laser operated with thermally near unstable zone at pump power of 870W, the maximum output power is 106.5 W corresponding to an optical-optical efficiency of 12.2%. The drop in the output power is contributed to the thermal lens effect that causes the laser working into the unstable zone. We also measured the power stability at the highest output power in every 5 minutes. The fluctuation of laser output power was ± 2.1% over 30 minutes.
The laser spectrum was determined by an Anritsu laser spectrometer (Anritsu MS9710B). Under the output power of 106.5W, the measured spectrum is shown in Fig. 6 . It can be seenthat only 1123 nm line exists. The 1112 nm and 1116 nm wavelength nearby does not appear, which indicates the wavelength selectivity is achieved through precise coating.
We tested the beam quality of 1112 nm laser by a Spiricon M2-200 laser beam analyzer using the second moment method as shown in Fig. 7 . The inserted image is the 3D intensity distribution of the beam. The M2 value was measured to be 5.6 at the output power of 106.5W.
The upper right is its 3D intensity distribution From the beam spatial intensity distribution it is clearly seen that the laser operates in a Quasi-Gaussian mode. This beam quality is enough for most applications.
5. Conclusions
In conclusion, we demonstrate a diode-side-pumped CW 1123 nm laser with high output power and high beam quality based on Nd:YAG crystal. The cavity adopts convex-convex configuration. Through precise coating the laser delivers single 1123nm wavelength. Under the pump power of 870 W, an output power of 106.5 W with M2=5.6 is obtained, corresponding to an optical-optical conversion of 12.2%. As far as we know, this is the first time to report over 100W and beam quality of 5.6 for 1123nm Nd:YAG laser by diode-side-pumped method.
Acknowledgements
The authors thank the support of the National High Technology Research and Development Program (“863”Program) of China under contract No.2006AA030103 and Major Program of the National Natural Science Foundation of China with No.50990304.
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