Prism Pairs | <<< | >>> | Feedback |
Definition: combinations of two prisms, mostly used for dispersion compensation
Pairs of (typically Brewster-angled) prisms can be used 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).
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, all wavelength components can later be spatially recombined.
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 lengths of such a dispersive delay line lead to anomalous dispersion [1], which may be partly offset by material dispersion in the prisms. The overall dispersion can be adjusted by varying the insertion of one or both crystals 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.
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 dispersion from prism pairs are a few thousand fs2. For larger amounts of dispersion, a pair of diffraction gratings may be required. The attraction of using a prism pair, however, is that anomalous dispersion can be provided without introducing significant losses into a laser resonator.
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 chirped mirrors.
Figure 3: 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. The insertion of the prisms at 800 nm is 2 mm in both cases. The SF10 prisms can generate more dispersion, but the higher-order dispersion is significantly higher.
Bibliography
| [1] | R. L. Fork et al., “Negative dispersion using pairs of prisms”, Opt. Lett. 9 (5), 150 (1984) |
See also: dispersion compensation
