Prism Pairs

Author: the photonics expert Dr. Rüdiger Paschotta

Pairs of (typically Brewster-angled) prisms can be used as beam expanders – see the article on anamorphic prism pairs. Another application, discussed in this article, is either for optical filtering or for introducing anomalous chromatic dispersion e.g. into a laser resonator without introducing significant power losses. A first prism refracts different wavelength components to slightly different angles. A second prism then refracts all components again to let them propagate in parallel directions after that prism (see Figure 1), but with a wavelength-dependent position (which is sometimes called a spatial chirp).

With a second prism pair, or simply by reflecting the beams back through the original prism pair (possibly with a small vertical offset for easier separation from the input beam), all wavelength components can later be spatially recombined; the total chromatic dispersion (see below) is then twice that for a single pass through the prism pair.

The spatial separation of different wavelength (or frequency) components can be utilized in different ways:

• An optical filter can be realized e.g. by inserting a knife edge from one side, attenuating primarily the short- or the long-wavelength components. This can be used e.g. for wavelength tuning of lasers by placing such a prism pair within the laser resonator.
• The wavelength-dependent optical path length of such a dispersive delay line leads to anomalous chromatic dispersion [2], which may be partly offset by material dispersion in the prisms. The overall chromatic dispersion can be adjusted by varying the insertion of one or both prisms into the beam. This technique is often used to provide adjustable dispersion compensation in mode-locked lasers (Figure 2) (e.g. for soliton mode locking) and for dispersive compression (or stretching) of optical pulses. For calculating the obtained amount of chromatic dispersion (second and higher orders), one may use Ref. [2] or some suitable software.

Typical amounts of anomalous group delay dispersion from prism pairs are up to a few thousand fs². This is often sufficient for dispersion compensation in mode-locked bulk lasers, but usually not for chirped-pulse amplification, for example. For larger amounts of dispersion, a pair of diffraction gratings may be required; these exhibit far greater angular dispersion and thus also greater chromatic dispersion. The attraction of using a prism pair, however, is that anomalous dispersion can be provided without introducing substantial losses into a laser resonator, assuming operation with p-polarized beams close to Brewster's angle.

Fig. 4 shows the chromatic dispersion of a pair of SF10 prisms for different values of the prism insertion; this demonstrates how the dispersion can be adjusted simply by translating a prism.

For the compression of ultrashort pulses in the few-cycle region, prisms with a fairly small apex angle (and anti-reflection coatings) are sometimes used. Such configurations can achieve a lower residual chirp from higher-order dispersion. However, it is often necessary to compensate the higher-order dispersion with other means, e.g. with additional dispersive mirrors.

More to Learn

Encyclopedia articles:

• prisms
• dispersion compensation
• anamorphic prism pairs
• diffraction gratings