Prism compressor

Prism compressors are typically used to compensate for dispersion inside Ti:sapphire modelocked lasers.

A prism compressor inside the cavity can be designed such that it exactly compensates this intra-cavity dispersion.

[8] Almost all optical materials that are transparent for visible light have a normal, or positive, dispersion: the refractive index decreases with increasing wavelength.

In principle, the α angle can be varied to tune the dispersion properties of a prism compressor.

In practice, however, the geometry is chosen such that the incident and refracted beam have the same angle at the central wavelength of the spectrum to be compressed.

The refractive index of typical materials such as BK7 glass changes only a small amount (0.01 – 0.02) within the few tens of nanometers that are covered by an ultrashort pulse.

Within a practical size, a prism compressor can only compensate a few hundred μm of path length differences between the wavelength components.

It can be corrected with careful measurement of the ultrashort pulse and compensate the phase distortion.

MIIPS is one of the pulse shaping techniques which can measure and compensate high-order dispersion automatically.

As a muddled version of pulse shaping the end mirror is sometimes tilted or even deformed, accepting that the rays do not travel back the same path or become divergent.

In Figure 4, the characteristics of the dispersion orders of a prism-pair compressor made of fused silica are depicted as a function of the insertion depth of the first prism, denoted as

The assessment employs the Lah-Laguerre optical formalism — a generalized formulation of the high orders of dispersion.

Angular quantities are defined in the article for the multiple-prism dispersion theory and higher derivatives are given by Duarte.

A prism compressor with an appropriate anti-reflection coating can have less than 2% loss, which makes it a feasible option inside a laser cavity.

Chirped mirrors are difficult to manufacture; moreover the amount of dispersion is rather small, which means that the laser beam must be reflected a number of times in order to achieve the same amount of dispersion as with a single prism compressor.

Figure 1. A prism compressor. The red lines represent rays of longer wavelengths and the blue lines those of shorter wavelengths. The spacing of the red, green, and blue wavelength components after the compressor is drawn to scale. This setup has a positive dispersion.
Figure 2. Geometry of a prism compressor
Figure 3. Effective pathlength for a prism compressor with A = 100 mm, θ = 55°, and α = 10°. The colors correspond to different values of B , where B = 67.6 mm means that the beam barely hits the tips of both prisms at refractive index 1.6. (The colors do not correspond to those of the rays in Figure 1.)
Figure 4. Dispersion orders of a fused silica prism-pair compressor at 780nm. (p = 2 - GDD, p = 3 - TOD, p = 4 - FOD, p = 5 - FiOD, p = 6 - SiOD, p = 7 - SeOD, p = 8 - EOD, p = 9 - NOD, p = 10 - TeOD)