Nd:YAG Laser Crystal
Neodymium-Doped: Yttrium Aluminium Garnet
Neodymium-doped yttrium aluminium garnet laser crystal (Nd:YAG) provides the laser system designer with the most versatile solid state laser source in use today. With thousands of systems in daily use, Nd:YAG continues as the best of the rare earth garnet laser materials characterised by four level laser operation; permitting low threshold pulse or CW operation. Nd:YAG laser rods produce efficient laser output at 1.064 µm. The systems designer can select from several levels of dopant concentration to optimise laser performance.
For mobile applications, size and weight of the laser systems are important. The laser must be able to produce Q- switched operation and high brightness from a limited input power. Rods with a high dopant concentration, 1.0 to 1.2 atomic percent Nd, are normally specified to achieve the best pump efficiency.
On the factory floor and other industrial settings, where input power is more available, Nd:YAG laser rods with dopant levels of 0.7 to 1.0 atomic percent Nd are very effective. Rods at this dopant level are an excellent choice for pulse and multimode CW laser systems to provide high output power coupled with balanced pumping uniformity and excellent beam quality.
When single mode (TEM00) operation is required, the selection of the proper dopant concentration is critical to the system's operation. Generally, the optimum laser rods for such systems have Nd doping concentrations in the range of 0.5 to 0.8 atomic percent. The final selection is based on many specific system design factors such as lamp versus diode pumping, mode stability, polarisation, beam divergence, etc.
In any design calculation or evaluation of the Nd:YAG laser, the thermal loading of the rod must be considered. The objective is a careful balance of output power requirements versus the tolerance for thermal effects. Years of experience in Nd:YAG and other laser materials enables us to offer useful assistance with the material selection to create a successful beam profile and optimise the performance of your laser.
Manufacture
Laser rod "blanks" are extracted from completed boules using a diamond core drill; for slabs, a slicing saw is used tocut out the rough rectangular shape. In either case, the rough finished blanks are then sent out for the finishing operations of precision grinding to final size, polishing, and anti-reflection coating. There is the option of purchasing finished rods directly, or purchasing unfinished "blanks" and using the fabricator of your choice.
When single mode (TEM00) operation is required, the selection of the proper dopant concentration is critical to the system's operation. Generally, the optimum laser rods for such systems have Nd doping concentrations in the range of 0.5 to 0.8 atomic percent. The final selection is based on many specific system design factors such as lamp versus diode pumping, mode stability, polarisation, beam divergence, etc.
In any design calculation or evaluation of the Nd:YAG laser, the thermal loading of the rod must be considered. The objective is a careful balance of output power requirements versus the tolerance for thermal effects. Years of experience in Nd:YAG and other laser materials enables us to offer useful assistance with the material selection to create a successful beam profile and optimise the performance of your laser.
Growth of neodymium doped yttrium aluminum garnet (Nd:YAG) crystals by the Czochralski technique is the method of choice for virtually all commercially available Nd:YAG. This is a time consuming process requiring careful control of the growth environment over a period of 4 to 5 weeks just to produce one crystal boule. Still, the Czochralski method has proven to be the only acceptable way to produce Nd:YAG with sufficient optical clarity and homogeneity for use in a laser system.
Crystal Quality and Boule size
One of the most important advances in Nd:YAG production in recent years has been the trend toward larger diameter boules.Internal strain in the grown boule is the principle cause of optical distortion in finished laser rods more than a few tens ofmillimeters in length (in shorter rods, the quality of the end finish is more important). Larger boules have significantly lower strain levels over much of their cross section resulting in significantly lower optical distortion in finished rods.An additional advantage of larger boules is reduced cost. The cross-sectional area is increased while the linear growth rateremains comparable, resulting in an increased rate of material growth.
At large diameters, Nd:YAG is more sensitive to process parameter fluctuations and obtaining high yields of good product is more difficult. But as a result of improved control electronics and computerization of the growth process, growth rate fluctuations can be maintained well within tolerance to provide high yields of good product at large diameter.
Neodymium Concentration
In neodymium doped yttrium aluminum garnet, neodymium substitutes for yttrium in the crystal lattice. However, because neodymium is larger than yttrium, this substitution does not occur readily. In fact, the concentration of neodymium in thecrystal is only a small fraction of its concentration in the melt. Since the growing crystal is continually rejecting neodymium, the concentration of the melt (and hence the crystal) increases as the growth progresses. To minimize this effect it is necessary to use a large crucible and to pull only a small fraction (typically20-30%) of the total material available.
The upper graph shows the concentration of neodymium increases as a function ofmelt fraction pulled. Most boules are grown with an average Nd concentrations of 0.80%Nd and 1.10%Nd. However, other average concentrations are available and we are able to offer material for most specialist applications
The Nd concentration profile for each is illustrated in the lower graph. The composition is engineered to provide the specified average concentrationin 200 mm lengths. Lengths up to 250 mm can be provided with slightly higher average Nd concentrations. For material less than 200 mm, the average concentration will vary depending on from where in the boule the material is cut.
Each individual laser rod shipped is supplied with data including the average Nd concentration and the change in concentrationover the rod's length. Standard tolerances for various rod lengths are published.
It should be noted that the absolute accuracy of the neodymiumconcentration must take into how accurately the distribution coefficient (ratio of dopant concentration in the crystal to that in the melt) is known. Usually formulations are based on a value of 0.18, which is consistent with industry practice and most determinations in the literature
Applications
- Materials Processing, Welding, Cutting
- Medical Laser Systems
- Pulse and CW Operation
- Slab Technology
Optical and laser Properties
| Laser Action | 4 Level Laser | Output Wavelength | 1.064 µm |
| Radiative Lifetime | (4F3/2 4I11/2) 550ms | Emission Cross Section | Σ 21 = 2.7 -8.8 x 10-19 cm2 |
| Spontaneous Fluorescence Lifetime | 230 µs | Refractive Index | 1.8 at 1.0 µm |
| Absorption bands | Flashlamp Pumped | Chemical Formula | Y3Al5O12:Nd |
ND:YAGLaser Rod Standard Fabrication Specifications
| Feature | Standard | Special Order |
| Crystal Orientation | <111> within 5° | Available |
| Diameter | 3 mm to 10mm | Available |
| Diameter Tolerance | +0.0 mm/- 0.05 mm | Available |
| Length (plano/plano) | 30 mm to 150 mm | Available |
| Length Tolerance | ± 0.75 mm | ± 0.50 mm |
| Perpendicularity of End Faces (plano/plano) | 5 arc-minutes | 2 arc-minutes |
| Parallelism of End Faces (plano/plano) | 10 arc-seconds | |
| Flatness | 0.1 wave maximum | 0.10 wave maximum |
| (plano/plano) | over 90% of aperture | over full aperture |
| Surface Finish at 5X | 20-10 (scratch & dig) | 10-5 (scratch & dig) |
| Barrel Finish | 400 grit | Polished approx. 80-50 |
| End Face Bevel | 0.075 mm to 0.12 mm at 45° angle | |
| Chips | No chips allowed on end face of rod; chip having maximum length of 0.3 mm permitted to lie in the area of bevel and barrel surfaces. | |
| End Configurations | plano/plano, plano/wedge, wedge/wedge or Brewster cut. Plano ends can be finished flat, concave or convex | |
| Coatings | Standard coating is AR at 1.064 µm with R < 0.25% each face. Other coatings available. |