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Statistical Process Monitoring

noisy. Laser power wanders over time by several hundredths of a decibel (dB) due to changes in ambient conditions, degradation of the laser components and other factors. The EWMA statistic in the lower panel of Figurę 1 often moves outside the ‘3-sigma” control limits confirming that the power level does indeed wander. But these smali changes in laser power have almost no effect

27.7^J

i ...... ■ i    ■    , u— — ....    ,    i    _t----— — — i

0    100    200    300    400    500

Time Order

Time Order

Figurę 1. The top plot shows 500 measurements of laser power entering successive pieces of optical fiber. The power level wanders over time but the fluctuations are so smali that they have virtually no effect on the ąuality of measurements produced by the equipment. The bottom plot is an EWMA chart of the power measurements. The EWMA statistic freąuently plots outside the control limits showing that power measurements do indeed wander. The chart is too sensitive to be useful because power changes of less than about 0.5 dB have little effect on the performance of the measuring device. An EWMA chart with wider limits would be much morę useful.