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High power operation of highly-doped ceramic Nd:YAG laser is demonstrated using a laser diode transversely-pumped bounce geometry. Using a 2% doped ceramic Nd:YAG amplifier, an output power of 45W was achieved with 158W diode pumping. The cavity configuration was optimized for TEM00 operation and a high spatial quality output (M2 <1.5) was obtained at an output power of 27W. A ceramic Nd:YAG laser with 4% doping was operated at over 1 watt of output power. These are the highest power levels achieved in ceramic Nd:YAG lasers at >1% doping level.
©2005 Optical Society of America
1. Introduction
Polycrystalline ceramic laser materials, such as neodymium (Nd) doped ceramic Y3Al12O5 (YAG), have received considerable attention because of their attractive features. These include excellent thermal properties, high Nd doping, no limitation of shape and size, ease of power scaling, and low cost [1–3]. Of these the high Nd doping offers potential advantage over conventional single-crystal Nd:YAG, since the high doping capability results in strong diode absorption and hence relatively shallow absorption depth which makes ceramic Nd:YAG ideally suited for use in end-pumped microchip laser geometry with increased efficiency.
Until now, very high output powers of over 100-watt continuous-wave (cw) has been obtained from ceramic Nd:YAG rod with low Nd doping (<1%). However, whilst several researchers have demonstrated efficient end-pumped 1µm lasers based on Nd:YAG ceramic rod with high Nd doping (>1%) their output power has been extremely limited. For 2% Nd doping output power has been limited to <1W [4], whilst in the case of >4% Nd doped ceramic YAG, the maximum output power was just 250mW [5]. Issues that need to be considered in power scaling highly doped ceramic Nd:YAG are increased thermal loading caused by enhanced energy transfer up-conversion effects and impaired lasing properties caused by decreased lifetime of the upper laser level. The end-pumped geometry is severely limited in power scaling due to the strong thermal lens induced and material fracture in this configuration.
Highly doped ceramic Nd:YAG lends itself naturally for use in a side-pumped bounce amplifier configuration [6]. In this configuration the strong diode absorption is advantageous with intense inversion density occurring in a shallow absorption depth below the pump face. This laser geometry has been successfully demonstrated to produce high powers at ultrahigh efficiency and high beam quality output in diode-pumped Nd:YVO4 [7,8] and Nd:GdVO4 [9]. Recently, we have demonstrated a >10-watt cw and Q-switched ceramic Nd:YAG laser with high Nd doping (2%) using a side-pumped bounce geometry [10].
In this paper, we present power-scaling results of highly Nd-doped ceramic YAG slab laser with a bounce amplifier geometry. The aim of the work was to investigate how well the power of highly-doped Nd:doped ceramic YAG could be increased by use of this geometry. The average power of as much as 45W was obtained with a 2% Nd doped ceramic YAG slab. This value is the highest, to the best of our knowledge, obtained by ceramic Nd:YAG laser with high doping (>1%). The corresponding optical-to-optical conversion efficiency of 28% was observed. We also optimized the spatial quality of the output from the ceramic Nd:YAG laser and M2 factor of <1.5 was measured. With the use of 4% Nd doped ceramic YAG, an output power of over 1W was also obtained, which is also the highest power obtained at this doping level to the best of our knowledge. The further power scaling at this highly doped level of 4% is shown to be limited by the strong thermally-induced lensing.
2. Experiments
2.1 2% Nd doped ceramic YAG
A schematic diagram of experimental laser system is shown in Fig.1. A Nd:YAG ceramic slab with 2% Nd doping and dimensions of 2 × 5 × 20mm3 was used. The end faces were AR-coated for 1064 nm and the pump face was AR-coated for 808 nm. To prevent self-lasing within the slab amplifier, the end faces were cut at 3° relative to the normal of the pump face. The top and bottom faces of the slab were conduction cooled for heat removal from the intensely pumped laser material.
The Nd:YAG ceramic slab amplifier with a bounce geometry was transversely pumped by a fast-axis lensed 40W continuous-wave (CW) diode array. The diode output was focused by a 12.7mm cylindrical lens to be a line with dimensions of ~0.2mm × 10mm on the AR-coated pump face of the slab. The laser cavity was formed by a high reflectivity (HR) plane mirror, M1, for 1064 nm and a partially reflecting plane output coupler, M2, (reflectivity 80% or 90% for 1064 nm). The cavity was arranged so that the laser mode reflected at an internal incidence angle of 80° with respect to the pump face normal. Two cylindrical lenses, CL1 and CL2, (f=50mm) in the vertical direction were used to yield a good spatial overlap between the laser mode and the small gain region in the vertical dimension. The total cavity length was ~13cm (L1=L2~6.5cm).
Figure 2 shows the experimental output power as a function of pump diode power for the bounce amplifier laser cavity with the use of the 90% output coupler. The output power reached up to 13W at the maximum pump level of 37W. The slope efficiency of 44% was obtained, and the lasing pump threshold power was ~8W.
For the further power scaling, a three-bar diode array stack replaced the single pump diode bar. The diode stack had nominal maximum output power of 180W, with optics for fast-axis collimation. The results of output power against stack diode pump power are shown in Fig. 3. With the use of an 80% reflectivity output coupler, the laser produced up to 45W of output power with 158W of diode pumping power. To the best of our knowledge, this is the highest output power achieved in a >1% Nd-doped ceramic YAG laser. The lasing threshold was ~16W, and the slope efficiency was >30% with respect to the output from the pump diode. Operating with a 90% reflectivity output coupler the threshold was reduced to ~10W, however, a lower output power of approximately 40W was achieved at 160W diode pumping. The output had a spatial form with lower M2 in the vertical (y-axis) but was highly multimode along the horizontal axis (x-axis). A typical spatial output profile of the laser is shown in Fig. 4(a). Figure 4(b) shows the beam propagation factors and as a function of the pump power. The increased at higher values of the pump power, and it reached ~50 at the maximum pump level, while the factor was typically ~2 but reached ~ 5 at maximum pump level. The diode array stack used in these experiments exhibited a relatively broadband spectrum, which would result in a reduced absorption and hence longer absorption depth. This factor was a major contributing factor to the decrease in the optical efficiency and to some extent to a reduced beam quality compared to that of the output pumped by the 40W diode array.
Fig. 3. Output power as a function of diode pump power with the use of three-bar diode array stack for different output coupler reflectivities and for single mode (TEM00) configuration.
In order to optimize the beam quality of the output, we extended the cavity length and made it asymmetric with respect to the position of the Nd:YAG amplifier (L1=16cm, L2=25cm, total cavity length ~41cm). As related in previous papers, this approach can be used to make an operation with a TEM00 mode with the gain medium forming the limiting aperture by utilising the power-dependent thermal lensing of the laser material [8,11]. Such an asymmetric cavity with an intracavity thermal lens is known to have two stability regions [11,12]. The results of output power against pump power are shown in Fig. 3 for TEM00 operation. An output power of 27W at the pump power of 158W was achieved. As shown in Fig.3, a kink was seen in the power curve around the pump power of 100W. Near this power level the thermal lensing in the laser amplifier made the laser cavity unstable. At higher pump level, the thermal lensing became stronger, and the laser cavity came back to the second stable region again. The beam quality was optimized for operation in the second stable region. The output exhibited a TEM00 profile as shown in Fig.5. The corresponding and factors were measured to be < 1.5.
2.2. 4% Nd doped ceramic YAG
Use of the ceramic YAG slab with the higher Nd doping can make the absorption depth shallower. In principle, this should lead to the further improvement of efficiency and beam quality in the bounce amplifier geometry. A 4% Nd doped ceramic YAG slab with dimensions of 2 × 5 × 5mm3 was used. The end faces were AR-coated for 1064 nm, and they were cut at 3° relative to the normal of the pump face. The pump face was AR-coated for 808 nm.
The Nd:YAG ceramic slab was transversely pumped by the fast-axis lensed 40W CW diode array. The diode output was reshaped by a 50mm horizontal cylindrical lens and a 12.7mm vertical cylindrical lenses to produce pump dimensions of ~0.2mm × 5mm on the AR-coated pump face of the slab. The bounce laser cavity was formed by a plane mirror, M1, with HR at 1064 nm and a 90% reflectivity plane output coupler at 1064nm, M2. The cavity was arranged so that the laser mode reflected at an internal incidence angle of 80° with respect to the pump face normal. To minimize the diffraction loss in the cavity, the cavity length was made as compact as possible and cylindrical lenses were not used. The total cavity length (L1 + L2) was ~6cm.
Figure 6 shows the output power against diode pump power for the 4% Nd-doped ceramic YAG laser system. A maximum output power of 1.2W was obtained at the pump power of 11.6W. This is the highest value, to the best of our knowledge, obtained by a 4% Nd doped ceramic YAG laser. The corresponding optical efficiency was 10%. The and factors of the output were 4.1 and 1.6, respectively. Above the pump power of 12W, the cavity became unstable.
We also used a 4% Nd doped ceramic YAG slab with dimensions of 2mm × 5mm × 20mm. In this longer length of slab we could not obtain any lasing at all. Though the whole physical reason cannot be understood at this stage, a serious concentration quenching due to highly doping might induce serious heat generation in the slab. There may also be scattering losses occurring in the material. The poorer efficiency and power levels achieved in the 4% Nd doped material compared to the 2% doping indicates that although the 4% material has
The ceramics with <3% Nd doping show a relatively longer fluorescence lifetime of >120μs [13]. In the view of concentration quenching as well as diode absorption, the optimal Nd concentration for the side-pumped bounce amplifier should be around 2.5–3%.
3. Conclusion
We have demonstrated power-scaling of CW highly Nd doped YAG ceramic slab lasers with a bounce geometry. With the use of 2% Nd doped ceramic YAG slab, the maximum output power of 45W was obtained at the pump power of 158W. This is the highest value, to the best of our knowledge, obtained by ceramic YAG lasers with a high Nd doping of 2%. We also optimized the beam quality of the output from the ceramic Nd:YAG laser by use of asymmetric cavity configuration and achieved a beam propagation parameter (M2) of <1.5 at an output power of 27W. Further power scaling of the high Nd-doped ceramic YAG should be possible by use of a master oscillator and power amplifier system. With the use of 4% Nd doped ceramic YAG slab, output power of over 1W was achieved. This is the highest power achieved at this high doping level. It was observed that a strong thermally-induced lensing was incurred at this doping level and this limited its further power scaling.
Acknowledgments
The authors acknowledge support from the Joint Research Project of the Japan Society for the Promotion of Science, and from The Engineering and Physical Sciences Research Council (UK) under grant number GR/T08555/01. T. Omatsu’s e-mail address is omatsu@faculty.chiba-u.jp.
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