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Photonics crystal ball

A new crystal doping technique could enable more powerful lasers

By doping alumina crystals with neodymium ions, engineers at the University of California San Diego have developed a new laser material that is capable of emitting ultra-short, high-power pulses – a combination that could yield smaller, more powerful lasers with improved thermal shock resistance, broad tunability and high-duty cycles.

To achieve this advance, engineers devised new materials processing strategies to dissolve high concentrations of neodymium ions into alumina crystals. The result, a neodymium-alumina laser gain medium, is the first in the field of laser materials research. It has 24 times higher thermal shock resistance than one of the leading solid-state laser gain materials.

The research was published in July in the journal Light: Science and Applications. The team presented their work at the SPIE Optics and Photonics conference, which took place from 19 to 23 August in San Diego.

Neodymium and alumina are two of the most widely used materials in today’s solid-state laser components. Neodymium ions, a type of light-emitting atoms, are used to make high-power lasers. Alumina crystals, a type of host material for light-emitting ions, can yield lasers with ultra-short pulses. Alumina crystals also have the advantage of high thermal shock resistance, meaning they can withstand rapid changes in temperature.

Combining neodymium and alumina to make a lasing medium is challenging. The problem is that they are incompatible in size. Alumina crystals typically host small ions like titanium or chromium, and neodymium ions are too large – they are normally hosted inside a crystal called yttrium aluminium garnet (YAG).

‘Until now, it has been impossible to dope sufficient amounts of neodymium into an alumina matrix. We figured out a way to create a neodymium-alumina laser material that combines the best of both worlds: high power density, ultra-short pulses and superior thermal shock resistance,’ said Javier Garay, a mechanical engineering professor at the UC San Diego Jacobs School of Engineering.

Cramming more neodymium into alumina

The key to making the neodymium-alumina hybrid was to rapidly heat and cool the two solids together. Traditionally, researchers dope alumina by melting it with another material and then cooling the mixture slowly so that it crystallises. However, this process is too slow to work with neodymium ions as the dopant, as ‘they would essentially get kicked out of the alumina host as it crystallises,’ explained first author Elias Penilla, a postdoctoral researcher in Garay’s research group. The solution was to speed up the heating and cooling steps fast enough to prevent neodymium ions from escaping.

The process involves rapidly heating a pressurised mixture of alumina and neodymium powders at 300°C per minute until it reaches 1,260°C. This is hot enough to ‘dissolve’ a high concentration of neodymium into the alumina lattice. The solid solution is at that temperature for five minutes, then cooled.

The researchers characterised the atomic structure of the neodymium-alumina crystals using x-ray diffraction and electron microscopy. To demonstrate lasing capability, the team optically pumped the crystals with infrared light (806nm). The material emitted amplified light (gain) at a lower frequency infrared light at 1,064 nm.

Researchers also showed that neodymium-alumina has 24 times higher thermal shock resistance than one of the leading solid-state laser gain materials, neodymium-YAG. ‘This means we can pump this material with more energy before it cracks, which is why we can use it to make a more powerful laser,’ said Garay.

The team is working on building a laser with the new material. ‘That will take more engineering work. Our experiments show the material will work as a laser and the fundamental physics is there,’ said Garay.

 

Commerical products

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Heatpoint from Eksma Optics is a compact round oven designed for heating nonlinear crystals and keeping them at their optimal operation temperature. It is also suitable for thermo-stabilisation of crystals with low thermal acceptance. The oven could be used for heating LBO, KTP, DKDP or other crystals.

Heatpoint is also useful to prevent moisture condensation on humidity sensitive crystal’s faces to extend their lifetime or for thermo-stabilisation of the crystals with low thermal acceptance.

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Heatpoint oven has a special crystal adapter to fit crystals of sizes up to 6 x 6 x 30mm. The adapter is made exactly for particular crystal sizes and it cannot be used for a crystal of a different size.

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Laser Components offers a range of non-linear optical crystals, including lithium niobate. LiNbO3 crystals are used most commonly as frequency doublers for laser applications. This artificially created crystal has a large electro-optic and acousto-optic coefficient, and as such it is a favoured material for a number of non-linear applications; Q-switches, Pockels cells, phase modulators and waveguides.

Laser Components can provide large lithium niobate crystals, with apertures up to 15 x 15mm and lengths up to 50mm, and has the capability to fulfil high volume orders. Varying grades are available, from commercial qualities optimised for more favourable pricing and higher volumes to precision grades with more accurate orientation and perpendicularity, and better surface quality. Custom sizes and specifications are available upon request.

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OptoSigma provides a range of laser single crystals. This product line is for use as a gain medium for solid-state lasers and amplifiers. The host matrix for the company’s crystals is YAG and lutetium aluminium garnet (LuAG). For some applications, mixed host matrix may be produced, for example yttrium-lutetium aluminium garnet (YLuAG).

YAG is well known as an excellent host material thanks to its good thermal and mechanical characteristics.

It is the most used host material for high power lasers, because of its low thermal expansion, high optical transparency, low acoustic losses and rather high threshold for optical damage, hardness and stability to chemical and mechanical changes.

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