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ADAPTOOL Report Summary

Project ID: HPRI-CT-1999-50012
Country: France

Large aperture SLM

Laser resistance for OA-SLMs

Laser damage resistance of OASLM based on both polymer (PVK) and crystalline (BSO) photoconductive materials has been evaluated during the course of this project. They presented for the different types of photoconductors roughly the same damage level of 0.5 J/cm2 @ 1.06µm - 8.5ns pulse duration

It has been found that the following routes can be implemented in order to improve this key figure:
- by using either 100nm thick PVK layers or 1mm thick BSO with optimised polishing quality, quite similar damage threshold levels > 1J/cm2 were measured, which corresponds to an improvement factor of 2,
- improved ITO coating led to a factor of 3,
- changing the materials for alignment layers provided a more reduced gain of 50%.

Therefore, by implementing these changes in the assembly process of OASLM it can be stated that the damage threshold should be found in the range of 1 J/cm2

Improved damage threshold ITO coating

It has been demonstrated that one of the most critical interfaces in OASLM device is the transparent electrode, which are based on Indium -tin oxide (ITO). Because ITO coatings have to satisfy criteria of both high near infrared optical transmission and low electrical sheet resistance (R < 200 ohms) one have to manage the trade-off between free carriers' mobility, tin doping level and thickness.

This project has demonstrated the feasibility of improving the laser threshold of ITO electrodes by a factor of 3, with respect to the state of the art for a laser damage threshold Eth > 1.8 J/cm2 @ 1.06µm - 8.5ns pulse duration. ITO coatings on the BSO single crystal used for optical addressing present a slightly lower damage threshold of 0.9 J/cm2 which has been demonstrated to be related to optical polishing quality of the crystal.

Optical performance comparison of PVK-OASLM vs BSO-OASLM:

This project enabled a comparison of OASLM optical characteristics vs photo-conductor types in terms of sensitivity, dynamic range spatial resolution¿:
- PVK photo-conductive polymer needs a writing power density of 15mW/ cm2 @ 514nm which is 3 times higher than required by a 1mm BSO crystal.
- PVK-OASLM offers the advantage in a higher spatial frequency range corresponding to nearly 1µm correction amplitude at 60lp/mm. But due to its band pass behaviour (50 to 120lp/mm) such devices are mostly interesting for intra-cavity phase correction and cannot be applied for correcting main distortions of high-energy laser chains, which are below 30lp/mm.
- In comparison, a BSO-OASLM will process phase distortion at lower spatial frequency. The same 1µm correction amplitude is reached at 20lp/mm and the device operates as only a high frequency cut-off.
- BSO-SLM provides 300% more phase dynamic range than PVK-SLM if devices are driven at their optimum driving conditions for a 10µm liquid crystal layer. This difference is related to the very low dark conductivity of BSO crystal preventing without illumination, the voltage to be carried forward onto the liquid crystal layer. Both materials enable AC voltage driving as required for the OA-SLM reliability and lifetime and lead to similar optical transmission values at 1.06µm: 73% for BSO-SLM and 70% for PVK-SLM.

For both materials and for large aperture sizes, the static phase distortion of the OASLM (>1.2µm on 35x35mm aperture) remains the most critical parameter thus requiring thicker liquid crystal layers as it reduces the dynamic range of the adaptive correction which can be provided by the liquid crystal layer of the device: a maximum 2.2µm phase correction dynamic range being provided by a 14µm layer.

Large scale BSO / OA-SLM

During the course of the project a larger scale BSO-OASLM was realized using the largest BSO crystals available on the market (40x40mm), expected to be sufficient for achieving a 34mm useful aperture suitable for evaluation in the LULI TW laser chain. Thanks to assembly process optimisations we were able to provide the useful aperture at the price of increasing the static phase distortion amplitude up to 1.2µm, which has to be compared to the 0.35µm usually obtained with 25x25mm crystals for a 15mm useful aperture.

The final characteristics of the large scale OASLM supplied at the end of the project are:
- useful aperture :35x35mm achieved with a 40x40x2mm BSO crystal,
- 1.06µm optical transmission: 73%,
- static phase distortion amplitude : 1.2µm,
- 1µm effective dynamic range measured with 14µm liquid crystal layer,
- driving conditions: 80V at f= 300Hz.

Finally, another supplier of large scale (>60mm) BSO crystal has been recently identified during the very last stage of the project.

Related information


Brigitte LOISEAUX, (Group manager)
Tel.: +33-1-69415501
Fax: +33-1-69415552
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