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Once the device's initial exposure is complete, the device can be tested and then slightly modified with additional laser exposure to "tune" it. The process of index of refraction modification is somewhat cumulative, with longer exposures resulting in larger index of refraction change. This allows for the fine-tuning of the index of refraction, or "trimming." The ensuing yield advantage is best illustrated when manufacturing interferometric devices such as the 50-GHz interleaver shown in the popup windows to the right.
Most interleavers are unbalanced Mach-Zehnder interferometers. In order to be centered on the ITU grid, the phase difference (i.e. the optical path difference) between the two arms must be controlled to lambda/20 or better. This means that the optical path-lengths must be accurate to a few tens of nanometers over the full length of the device. This is a very stringent requirement, which drastically affects the manufacturing yield of good devices.
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