Nobuaki Iehisa

1.1-kW high-efficiency LD-pumped Nd:YAG laser

Recently, the development of multi-kW all solid-state lasers for industrial applications has been evaluated for the energy saving characteristics. We report the laser oscillator using the LD pumped slab Nd:YAG producing the highest efficiency on the cooling condition of room temperature. The two-side pumping is generally used for realizing symmetrically distributed excitation and temperature in the crystal. However, there exist two demerits coming from the useless LD pump light which is not absorbed in the slab. The first demerit is the fact that this light does not contribute to laser oscillation and the second one that the same light degrades the performances of LDs locating on the opposite side. We have eliminated these demerits by employing the one- side pumping method in which LDs are located on one side of the slab and a highly reflecting mirror is located on the other side. Based on this concept, the LD pumped slab Nd:YAG laser has been developed using a 6 X 25 X 206 mm3 slab crystal. The average output power of 1109 W and the peak output power of 4900 W have been obtained with the optical efficiency of 40.6% and the electrical efficiency of 15.3% on the cooling condition of 299 K.

Thermal analysis of high-power LD-pumped slab Nd:YAG laser

We have developed a high power LD pumped Nd:YAG laser using one zig-zag slab crystal, and obtained the average output power of 3.3 kW with the optical efficiency of more than 35% under the cooling condition of 12 degrees Celsius. Our method is to mount the LD stacks on both the sides of the 6 mm X 25 mm X 206 mm slab crystal in which the LD stacks mounted on one side of the slab pump the upper part of it while the ones on the other side pump the lower part. The LD pump light transmitted through the crystal is reflected by Au mirror and is introduced back into the slab. These LD stacks are arranged so that the thermally induced birefringence can be eliminated by maintaining uniform pumping in the width direction of the slab (non-zig-zag direction). The pumping distribution in the width direction was made constant by ray tracing simulation. The small signal gain distribution was measured constant in this direction, which indicates that the pumping distribution in the width direction has become uniform. The total heat generated in the slab was calculated to be less than 2.3 kW and the temperature distribution and thermal stress distribution were also simulated. According to this calculation, the maximum thermal stress of 40 MPa occurs in the surface of the slab, which is one-fourth of the fracture limit of YAG crystal.

1.7kW high efficiency LD pumped Nd:YAG laser

Material processing kilowatt-class LD pumped solid-state lasers have been actively developed to be used in such industries as automotive, aerospace, electronic and so on. In this development, the key technologies are the following two, i.e. development of high power LD possessing high efficiency and thermal management within the laser crystal.

LD-pumped slab Yb:YAG laser

In recent years, the high power Yb:YAG lasers have been actively investigated due to the advantage of the high quantum efficiency of 91% which reduces the thermal loading in the Yb:YAG crystal. So far, the Yb:YAG laser with the output power higher than several hundreds watts has been developed using the crystal configurations of rod and thin disk. We have developed the Yb:YAG laser by employing the rectangular slab crystal in order to examine the possibility of realizing the high power slab Yb:YAG laser. The dimension of the Yb:YAG crystal used is 1 mm X 5 mm X 10 mm and its configuration is a rectangular parallelepiped, and the density of Yb is 1.1 atom%. The LD (Laser Diode) pump light focused with plano-convex lens is introduced through the 1 mm X 10 mm plane of this slab which is AR-coated at 940 nm while the opposite 1 mm X 10 mm plane is HR-coated at the same wavelength. The Yb:YAG laser cavity axis is in the direction perpendicular to the 1 mm X 5 mm planes which are AR-coated at 1030 nm. The two 5 mm X 10 mm planes are cooled by being contacted with the copper heat sinks which are cooled by the water at the temperature of 18 degrees Celsius. The CW output of 35 W was obtained when the power of LD pump light was 496 W. The optical efficiency was 7.1% with the optical slop efficiency of 12.2%.

2.5-kW slab Nd:YAG laser using a 400-mm-long crystal bonded by optical contact method

The optical contact method has been developed to obtain a large volume slab Nd:YAG crystal for achieving a multi-kW output power. The bonded crystal with the dimension of 6 X 25 X 402 mm3 (tip to tip) is mounted in a single laser head and is pumped with four krypton arc lamps. In continuous wave (CW) operation, the maximum output power of 2480 W, the overall efficiency (eta) E-O of 4.0% and the slope efficiency of 5.5% have been obtained. A beam quality which is defined as the product of full divergence angle and diameter of the beam waist is better than 10 mm (DOT) mrad and 80 mm (DOT) mrad in the directions of the thickness and the width of the slab, respectively. The output laser beam was transmitted through a GI type 600 micrometer-core optical fiber completely. This configuration has realized a simple resonator laser producing high processing performance.