The advent of phased arrays of fiber lasers has sparked interest in traceable beam quality metrics for use in arrays. Any metric that is valid for a single emitter can be used for an array; however, the metric might not mean the same as it would for the single emitter. Any proposed metric for arrays should pass the following criteria:
The metric does not show better beam quality upon the loss of an emitter. If the metric uses a near-field aperture, the aperture must be chosen to fit the application, not the array. Themetricmust be traceable to the performance of the system with respect to its intended application, not create an arbitrary comparison with some aspect of propagation behavior.
It is also important to carefully consider what question the intended metric is supposed to answer. The most relevant questions for arrays are:
What fraction of the total energy is within the central lobe? What is the width of the central lobe compared to a filled aperture?
This chapter examines some of the sources of beam quality degradation common to arrays and how they impact beam quality metrics. It then examines some common beam quality metrics in light of an array of emitters.
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