Expected Life-Time of an Optical Silica Fiber Intended for Open Space Applications: Probabilistic Predictive Model

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Acta Astronautica

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The recently suggested probabilistic design-for-reliability (PDfR) concept in microelectronics and photonics reliability engineering enables quantifying the static fatigue (delayed fracture) lifetime and the corresponding (in effect, never-zero) probability of failure of optical silica fibers intended for open space (outside the spacecraft) applications. In the author's opinion, an adequate operational reliability of space photonics cannot be assured, if it is not quantified, and, because of numerous inevitable intervening uncertainties, such a quantification should be done on the probabilistic basis. Multi-parametric Boltzmann–Arrhenius–Zhurkov (BAZ) constitutive equation is used to design and interpret the results of a highly-cost-effective and highly focused failure-oriented accelerated testing (FOAT). Such testing is both the experimental basis of the PDfR and a physically meaningful proof-test for optical fibers. The combined action of a moderately low temperature, tensile loading, ionizing radiation and random vibrations is considered. Although random vibrations might not be an actual stressor in the application in question, it is nonetheless included in the analysis as a critical stimulus: fracture mechanics studies indicate that fatigue and brittle cracks propagate particularly rapidly, when the material under test is subjected to a low-temperature/vibration bias. The general concept is illustrated by a detailed numerical example. It should be pointed out, however, that the calculated data, although confirm the viability of the suggested methodology, use hypothetical input information and should be viewed just as a suitable illustration, not more. The developed approach is rather broad, and could be applied to any type of optical fibers, including laser fibers, and, perhaps, with some modifications, also to other types of optical, opto-electronic and even non-optical materials and vulnerable structural elements (such as, e.g., solder joint interconnections) intended for highly demanding space applications. The approach could be applied also, as has been indicated by one of the reviewers, in other astrophysics related conditions and locations, where optical fiber instrumentation is intended to be employed in low temperature conditions, such as, e.g., those that take place at the bottom of the Shackleton impact crater at the lunar south pole. The important restriction of our model has to do with the fact that the BAZ equation, which is based on classical thermodynamics, assumes that no failure could possibly occur at absolute zero. Because of that, the suggested model should be used at temperatures, sufficiently remote from the absolute zero. Future work should focus, first of all, on the experimental verification of the suggested approach and model.


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