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Intensity Noise

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Ask RP Photonics for advice on any aspect of intensity noise, e.g. its reliable measurement, its proper specification, its simulation, its suppression with feedback systems, its coupling to phase noise, etc.

Definition: noise of the optical intensity or power of a laser beam

German: Intensitätsrauschen

Categories: fluctuations and noise, lasers

How to cite the article

An important type of noise in a light beam is noise of its intensity. Strictly, the noise of the optical power, rather than of the optical intensity, is usually considered, but the common term is intensity noise rather than power noise.

Specifications for Intensity Noise

Intensity noise is usually quantified as relative intensity noise (RIN), i.e. as noise of the power divided by the average power. Common specifications are based on

Sometimes specifications encountered such as “±0.1%” are not valid, since statistical aspects are not properly described, and it is not clear which range of noise frequencies is considered.

More details are found in the article on noise specifications.

Measurement of Intensity Noise

Intensity noise is normally measured by detecting the intensity (or power) with a fast photodetector (e.g. with a p–i–n photodiode) and evaluating the noise spectrum with an electronic spectrum analyzer. Although this appears simple in principle, there can be various technical challenges:

For the characterization of low-frequency noise, it may be required to record the power variations in the time domain and process them numerically.

Origins of Intensity Noise

Intensity noise of a laser results partly from quantum noise (associated with laser gain and resonator losses) and partly from technical noise sources such as excess noise of the pump source, vibrations of resonator mirrors, thermal fluctuations in the gain medium, etc. The resulting intensity noise also depends on the operation conditions; in particular, it often becomes weaker at high pump powers, where relaxation oscillations are strongly damped. There are methods to reduce the noise further by using a feedback system (→ stabilization of lasers).

In most cases, the lowest possible intensity noise level for laser beams results from shot noise. At least at high noise frequencies, well above the relaxation oscillation frequency, this noise level is approached by many lasers. However, for so-called squeezed states of light, the intensity noise can be below the shot noise, at the cost of increased phase noise.

intensity noise spectrum

Figure 1: Intensity noise spectrum of a solid-state laser. The noise level is given in decibels above the shot noise limit. There is a peak at 74 kHz, resulting from relaxation oscillations. Increased low-frequency noise is caused by excess noise of the pump source.

Noise Reduction

Intensity noise of a laser can be reduced in various ways:

The intensity noise in a laser beam (external to the laser cavity) may be reduced with a noise eater.

Bibliography

[1]R. Paschotta, "Noise in Laser Technology – Part 1: Intensity and Phase Noise"

See also: noise specifications, relative intensity noise, phase noise, laser noise, shot noise, quantum noise, noise eaters

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