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Over the past decade a revolution has taken place in the methods of short pulse generation from lasers. The development of tunable, solid-state materials has resulted in the appearance of ever more powerful and efficient ultrashort-pulse laser sources. An excellent example is titanium-doped sapphire laser. The advantages of the Ti:sapphire laser over the dye laser are great convenience, more stable operation and tunability. The Ti:sapphire laser can operate with changes of optics to cover the spectral region from 680 nm to 1050 nm, however, dye lasers typically need four different dyes in this region. Ti:sapphire femtosecond laser pulse duration is limited by the group-velocity dispersion (GVD) of the laser cavity with the main contribution coming from the laser rod itself. The GVD of Ti:sapphire crystal is positive throughout its tuning range thus, to minimize the effect one has to include into laser cavity a negative GVD delay line. Several designs based on Brewster cut prisms made out of different materials have been reported so far. In this paper the delay lines consisting of two- and four- prisms sequence, both in double pass configuration are considered. The four-prism sequence is an attractive solution since use of two pairs of prisms made from different materials gives one more control over the dispersive properties of the delay line. We use the method of the ray tracing to estimate wavelength dependent group velocity delay for two- and four-prism sequences. Results of our computer calculations indicate clearly, that a four-prism delay line is superior to that of a two-prism, providing very similar cancellation of third-order dispersion, as two-prism sequence with considerably longer interprism separation.
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