Researchers compile radiation database
As the number of photonic systems used in nuclear, space and high-energy physics environments grows, radiation-induced performance-degradation of optical materials and devices becomes an increasingly important issue. Johan van der Linden discovers how it is to be tackled. From Opto & Laser Europe Testbed reactor Late last year, lasers were used for the first time to transmit data between orbiting satellites. But as the photonics revolution begins to extend into the harsh environments of the space and nuclear industries, there is an urgent need to assess the performance of optical components - both active and passive - under the influence of various forms of radiation. One organization that aims to do this is the Belgian nuclear research centre SCK-CEN. During the past decade it has investigated the radiation resistance of a number of photonic components, including optical fibres, semiconductor light sources and photodetectors, fibre-optic couplers and sensors, and liquid-crystal cells. Comparable effects Francis Berghmans, head of photonics at SCK-CEN, has led the work on the effects of radiation. "We found that, although the basic environmental conditions such as dose rate, total dose and radiation type may differ from one application to the next, the fundamental effects that influence devices often remain comparable," he said. However, the results of exposure can vary. Exposure to particle radiation, such as proton and neutron beams, can cause displacement damage, whereas exposure to electromagnetic radiation, such as gamma rays, will primarily induce defects resulting from ionization. This means that even though a particular component may be able to withstand large doses of gamma radiation, making it useful in civil nuclear facilities, it could be too sensitive to protons to be suitable for space applications. Lanzarote Passive devices, particularly optical fibres comprising Bragg gratings, are the most frequently studied components, due to their potential use as strain, temperature and multi-point structural integrity sensors in thermonuclear environments. High radiation doses generally create defects - known as colour centres - in optical glasses, which can lead to significant transmission losses and light generation from unwanted wavelength bands. This is a major obstacle to the efficient operation of optical communication systems. Berghmans has found that in standard germanium-doped fibres, high radiation doses can induce absorption losses of several hundred dB/km in the 1310 nm and 1550 nm telecom transmission windows. Pure silica fibres suffer about one tenth of the losses seen in germanium-doped fibres. However, the optical fibres required for data transmission in nuclear facilities are comparatively short in length, so standard fibre loss levels may be acceptable. In semiconductor-based active optical components, radiation-induced damage can introduce defect states into the crystal lattice and create new energy levels in the bandgap. These defects may act as generation-recombination centres, leading to increased threshold current and lower optical output from laser diodes. In photodiodes, increased dark current and lower responsivity are the likely hazards. VCSEL array "Our experiments have demonstrated that photodetectors are the most critical components in optical communication systems," said Berghmans. His findings show that, at low doses, III-V-based photodiodes are not as sensitive to radiation-induced degradation as silicon-based detectors. As far as sources are concerned, vertical-cavity surface-emitting lasers (VCSELs) seem to have more radiation tolerance than edge-emitting light sources. Berghmans puts this enhanced tolerance down to the VCSEL's thin active layer and initially brief carrier lifetime, which mean that a great many defects must be induced before they seriously affect the efficiency of the device. Optical components are increasingly used in space applications, ranging from teleobjective lenses to communication systems for use in spacecraft and between satellites. Most commonly-applied optical materials are prone to darkening - or solarization - in irradiation environments, so glass manufacturers supply radiation-hardened products (analogues of standard glasses that have been doped with cerium oxide) which exhibit improved end-of-life transmission properties. However, the performance of spaceborne optical systems rests on the reliability of refractive components. Cerium doping retains more than 90% transmittance in the visible spectrum, but it has been shown to have some negative effects on other system performance parameters. For instance, radiation has a substantial effect on the refractive-index profile of cerium-doped components. Glass sample a) Last November, the ESA's Research and Technology Centre (ESTEC) in Noordwijk, the Netherlands, presented the results of a study it had assigned to the France-based space company Astrium and SCK-CEN to assess the stability of physical properties in commercially available glass materials. Dominic Doyle, a technical officer at ESTEC, explained the need for such a study: "The main reason was the deficit of a reliable, usable and easily accessible database concerning the radiation characteristics of refractive optical materials. This [study] is a step towards establishing a comprehensive database to quantify radiation effects for use in the design and development of spaceborne optical systems." Glass sample b) Michel Fruit, manager of optical design and engineering at Astrium, and his colleagues place special emphasis on studying refractive index changes in proton and gamma radiation fields to simulate a range of different Earth orbits. "We found that cerium-doped specimens can show significant steps in the wavefront profile," he said. Depending on the base material, the refractive index change can be positive as well as negative, although it is generally rather small (less than 10-5). In optical systems that use a large number of lenses, however, the effect can be significant. Fortunately, says Fruit, it can be predicted. "The radiation-induced refractive index change and absorption-increase sensitivity is linear - particularly in proton environments - and this allows a dose-coefficient modelling approach to be used," he said. Huge task lies ahead Compiling the database is an enormous task and it will be several years before it is accessible, probably via the ESA's Web site. Once complete, the database could be of use in a range of related fields such as deep-ultraviolet lithography and pulsed high-power lasers, because such systems need high-performance refractive optical components. Since gamma rays are photons, any optical system that is exposed to high-energy photons could benefit from the radiation studies. This applies to deep-ultraviolet lithography in particular, since it would use many optical components and the long exposure times involved would result in significant radiation doses. Because such systems work to tight tolerances, an awareness of possible radiation effects is crucial. According to Doyle, standard methods must now be adopted. "Given the workload involved [in compiling the database], one of our most immediate goals is to concentrate on the standardization of the assessment methodology with industrial, institutional and agency partners," he said. "Such a methodology could eventually be approved by the ISO or the European Cooperation for Space Standardization."
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