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Computer Controlled Variable Attenuator for Tunable Lasers

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Computer Controlled Variable Attenuator for Tunable Lasers

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This application note describes a closed loop, computercontrolled, variable attenuator for the control of output power from a tunable laser. It is based on the combination of a 1 /2 wave-plate and a polarizer. The algorithm used to control power operates in a closed loop with Proportional Integral Derivative (PID) feedback, compensating for slow changes in output power.It is based on Malus’s law. The intensity, I, of linearly polarized light that passes through the polarizer is given by where Io is the input intensity and θ is the angle between the beam’s initial direction of polarization and the axis of the polarizer. Rotation of a 1 /2 wave-plate mounted in a rotational stage allows for continuous rotation of the polarization direction of the linearly polarized light. Thus, combined with a polarizer, it provides variable attenuation according to (1).

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