Elsevier

Optical Materials

Volume 32, Issue 10, August 2010, Pages 1368-1371
Optical Materials

New silicate bonding technique for composite laser materials

https://doi.org/10.1016/j.optmat.2010.03.026Get rights and content

Abstract

We report a new low loss silicate bonding method for the assembly of laser materials. Original heterogeneous composite laser crystals have been obtained thanks to this sol–gel method: Er/Yb phosphate glass||sapphire and Nd:YVO4||sapphire. Sol composition containing additives enables to bond chemically and thermo-mechanically different materials. Composite materials made with KH2PO4 rich sol–gel demonstrated the best temperature resistance. Potassium and phosphate ions add extra flexibility and chemical affinity. The bond is resistant to temperatures higher than 200 °C and laser actions have been demonstrated in both composite materials for the first time.

Introduction

In the field of solid state lasers, thermal management has always been an important issue. Numerous works have been carried out from our research groups to find new crystals with very good thermal properties [1], [2], [3], [4]. On the other hand, other workers try to reduce thermal loading using specific configurations such as thin disk [5] or material with low dopant content in crystalline fiber materials [6] or by crystals assembly [7]. In the present work, a very thin film made by sol–gel is used as an interface layer which can glue two different samples. Then, with this composite material one can improve the thermal properties of well developed compounds. For instance, in the 1.5 μm range, Er/Yb phosphate glass presents outstanding laser results in the eye safe range in comparison to Yb/Er crystals [8]. Phosphate glass material exhibits favorable optical spectroscopy parameters in term of energy level lifetimes, but has quite low thermo-mechanical properties [8]. The thermal conductivity value is around 1 W m−1 K−1 while for comparison, the thermal conductivity value of the YAG is ∼10 W m−1 K−1. Therefore, thermal loading generated at the center of the material is not removed efficiently by lateral heat sink. It appeared interesting to cool the phosphate glass directly from the hottest face with a material with high heat conductivity value. Sapphire for instance could be a very nice material as the thermal conductivity of the sapphire reaches 30 W m−1 K−1. One other demonstration of the interest of the method could be done in the yttrium vanadate laser Nd:YVO4. In that case, even if the thermo-mechanical properties of the crystal is quite high around 8.2 W m−1 K−1 for the two orientations for 1.1% Nd [9], [10], this material still requires good thermal load extraction as in the case of neodymium pumped at 808 nm, the quantum defect is 23%. This means that 23% of the pump power is converted into heat. Diffusion bonding technique [11] is an efficient way to decrease thermal loading, and is used with success in numerous applications. This technique enables efficient crystal cooling but the two media have to be chemically and thermo-mechanically quite close. An alternative method presented in the following part of the paper is the silicate bonding method which can gather two chemically quite different materials. In order to obtain good material assembly, one should fulfil the following requirements:

  • (i)

    Transparency of the interlayer at pump and laser wavelengths (∼1 μm).

  • (ii)

    Submicron interlayer thickness and good interlayer homogeneity to prevent optical losses.

  • (iii)

    Capability to bond a wide range of oxide materials.

  • (iv)

    High flexibility to absorb stresses induced by thermal expansion difference of bonded materials.

  • (v)

    Thermal resistance to at least 200 °C.

All these requirements lead to the use of inorganic materials as interlayer material. Silica based thin solid films are well-known and easy to synthesize. In order to have thin enough films, the sol–gel method has been chosen. For the sake of homogeneity (requirement ii) and good density of the silicate film, an acid catalyst has been chosen [12]. The acidic catalyst leads to gels with different reticulation properties than the basic procedure [13]. Density is also an important parameter in order to have better heat conduction.

Section snippets

Sol–gel preparation and materials assembly

The first step is the preparation of the sol–gel solution. This solution is prepared in two steps. First, a pre-hydrolyzed silica (PHS) solution is realized at room temperature by mixing ethanol, tetraethylorthosilicate (TEOS) and diluted HCl (2 × 10−3 M). This PHS solution is stable during several months below 4 °C. Then, sol solution is obtained by mixing PHS, water, ethanol, HCl (2 M) and potassium salt concentrations according to Table 1. Several solutions have been evaluated: #1 corresponds to

Film characterization

Firstly, the thickness of the films is estimated from the SEM pictures presented in Fig. 2. A rough indication in the range of 200 nm can be made as clear separation between the two substrates is visible but interlayer thickness cannot be accurately determined. However this estimation is in good agreement with previous sol–gel results as for instance open surface film thickness prepared by dip coating has been measured to be 190 nm [14], and this value is also in good agreement with calculation

Conclusions

The sol–gel assembly method presented in this work enables to achieve composite laser materials with chemically different compounds. These composite materials are composed of an active media and one heat sink material. This method demonstrated to be very efficient to reduce thermal effects: thermal strains are lowered and the maximal pumping power is significantly increased in the case of Er:Yb phosphate glass||sapphire composite while the laser tests are similar in the case of Nd:YVO4

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