Definition: devices for reducing the intensity noise of an optical beam by automatically adjusting the degree of power attenuation
Categories: photonic devices, fluctuations and noise
Author: Dr. Rüdiger Paschotta
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A noise eater is a device made for reducing the intensity noise in a laser beam. The principle of operation is that the optical power is reduced with an electrically controllable attenuator, and the control signal is derived from the input power (feedforward scheme) or the output power (feedback scheme) as measured with, e.g., a fast photodiode. Essentially, the attenuation is increased when the power is measured to be too high, and vice versa. This makes it possible to stabilize the laser power, i.e., to decrease intensity noise.
The most common approach is that based on an electronic feedback loop, e.g. of a PID type. An electro-optic modulator can be used to control the power throughput with a high servo bandwidth. Proper design of the feedback electronics is vital for achieving effective noise suppression over a large bandwidth. Alternatively, it is possible to use a feedforward method.
An alternative method is to directly influence the laser output power with some technique of laser stabilization. However, a noise eater can be a good solution if the laser is not suitable for such an intervention.
The RP Photonics Buyer's Guide contains two suppliers for noise eaters.
See also: laser noise, intensity noise, relative intensity noise, stabilization of lasers, optical attenuators
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