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Beam Pointing Fluctuations

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Ask RP Photonics for advice on the origin of beam pointing fluctuations and their reduction with suitable laser designs or with feedback stabilization.

Definition: fluctuations of the propagation direction of a laser beam

German: Strahlrichtungsfluktuationen

The direction of the output beam of a laser is subject to some beam pointing fluctuations, which can in some cases cause significant problems – e.g., when the beam must be coupled into a single-mode fiber, or when the beam must precisely hit a target at a large distance. For such reasons, a quantitative measure for the beam pointing stability (see below) can be of importance.

Physical Origins of Beam Pointing Fluctuations

Beam pointing fluctuations in a bulk laser can have different origins:

It is important to note that a certain tilt of a resonator mirror does not necessarily translate into a tilt of equal size of the output beam. Instead, it generally leads to some combination of a (larger or smaller) tilt with some horizontal shift (offset) of the beam. The type of that influence depends on the whole resonator design (as discussed in the article on alignment sensitivity). For a linear resonator, the alignment sensitivity can be very different in the two stability zones, and can even diverge near the edge of such a zone. The alignment of different resonator mirrors can also very much differ in terms of sensitivity. Such issues have important implications for the optimization of pointing stability (see below).

Quantification of Beam Pointing Stability

The beam pointing stability of commercial laser products is often quantitatively specified. Unfortunately, such specifications are often not precise or even meaningless. A useful specification of angular fluctuations has to observe a number of important issues:

The magnitude of angular fluctuations alone is actually often not sufficient to calculate the effect of beam pointing fluctuations in an application; it can also be important how large parallel beam offsets occur, and how these are correlated with angular fluctuations.

Influence of External Optics on the Beam Pointing Stability

If a laser beam is sent through some optical setup, this will in general modify the magnitude and type of beam pointing fluctuations, even if the optical components are absolutely stable. The following two examples illustrate this:

Such behavior can be understood with a purely geometric reasoning, based e.g. on the ABCD matrix algorithm.

For judging the angular beam stability of a laser, not only the magnitude of angular fluctuations, but also the beam radius has to be taken into account. It is instructive to compare the angular fluctuations with the diffraction-limited beam divergence, i.e., the beam divergence of a Gaussian beam with the given size. The larger the radius of such a beam is, the smaller is its divergence angle, and the more severe is the influence of pointing fluctuations with a given angular spread.

Of course, vibrations of optical elements can further increase the magnitude of pointing fluctuations.

Optimization of Beam Pointing Stability

A laser design for optimum beam pointing stability must take into account various aspects:

With a good laser design, the angular beam pointing fluctuations of a laser can be a tiny fraction of the beam divergence. This corresponds to phase changes across the beam profile which are much smaller than one radian.

A further reduction in pointing fluctuations may be achieved with an active stabilization scheme. It is possible e.g. to monitor the beam position at some point with a four-quadrant photodiode and correct it via piezo-actuated mirrors.

For an existing laser, pointing fluctuations are often minimized by careful alignment for maximum output power.

Bibliography

[1]ISO Standard 11670, “Lasers and laser-related equipment – Test methods for laser beam parameters – Beam positional stability”
[2]R. Paschotta, "Noise in Laser Technology – Part 3: Beam Pointing Fluctuations"

See also: laser noise, lasers, laser resonators, noise specifications

Category: lasers

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