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Prism Pairs

Definition: combinations of two prisms, mostly used for dispersion compensation

More specific term: anamorphic prism pairs

German: Prismenpaare

Categories: general optics, light pulses, methods

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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 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).

prism pair
Figure 1: A prism pair for spatially dispersing different wavelength components and thus also introducing wavelength-dependent phase changes and chromatic dispersion.

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.
cavity of mode-locked laser
Figure 2: Resonator setup of a mode-locked laser. A prism pair is used for dispersion compensation. The overall anomalous chromatic dispersion allows for soliton mode locking, and can be adjusted via the prism insertion.

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 often 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 significant losses into a laser resonator, assuming operation with p-polarized beams close to Brewster's angle.

group delay dispersion of silica and SF10 prism pairs
Figure 3: Single-pass group delay dispersion of prism pairs: comparison of a setup with silica prisms, 50 cm spacing, and another one with SF10 prisms, 20 cm spacing. At 800 nm, the sum of the prism insertions is 4 mm and the beams are at Brewster's angle. The SF10 prisms can generate more dispersion, but the higher-order dispersion is significantly higher. The calculations have been done with the RP Fiber Power software.

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.

group delay dispersion of SF10 prism pairs
Figure 4: Single-pass group delay dispersion of SF10 prism pairs with different amounts of beam insertion from 2 mm to 6 mm in steps of 1 mm. Stronger insertion leads to the higher curves, which also extend more towards shorter wavelengths. In practice, the prisms cannot be operated close to the left end of such a curve due to beam clipping.

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.

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Bibliography

[1]O. E. Martinez, J. P. Gordon and R. L. Fork, “Negative group-velocity dispersion using refraction”, J. Opt. Soc. Am. A 1 (10), 1003 (1984), doi:10.1364/JOSAA.1.001003
[2]R. L. Fork et al., “Negative dispersion using pairs of prisms”, Opt. Lett. 9 (5), 150 (1984), doi:10.1364/OL.9.000150
[3]J. D. Kafka and T. Baer, “Prism-pair dispersive delay lines in optical pulse compression”, Opt. Lett. 12 (6), 401 (1987), doi:10.1364/OL.12.000401

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See also: prisms, dispersion compensation, anamorphic prism pairs, diffraction gratings
and other articles in the categories general optics, light pulses, methods

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