Littrow incidence

Fig. 2.27. Top view of the femtosecond pulse stretcher (Quantronix model 4822) and laser beam tracing. The pulse from the seed laser is stretched about 5000 times with the use of a diffractive grating mounted at Littrow incidence to reduce aberrations. To prevent optical feedback to the seed laser a Faraday rotator (FR) is used for optical isolation. A aperture TFP thin-film polarizer (by courtesy of Quantronix)...

Apart from using the grating in zero order so that it works like a mirror, maintaining a (compact) round beam at the camera (A = 1) requires that the spectrograph adopt the Littrow configuration for which Tf = 0 and the incident and diffracted rays are parallel. From equations 6 and 9, the resolving power at blaze in the Littrow configuration is... 

As the beam leaves the prism predisperser, it is focused on the entrance slit of the grating monochromator. The slit is curved, has variable width, and opens symmetrically about the chief ray (optical center line of system). The monochromator itself is of the off-axis Littrow variety (James and Sternberg, 1969 Stewart, 1970 Jennings, 1974) and uses a double-pass system described by McCubbin (1961). The double-pass aspect of the system doubles the optical retardation of the incident wave front and theoretically doubles the resolution of the instrument. The principal collimating mirror is a 5-m-focal-length, 102-cm-diam parabola. 

The following definitions are required 6g is the angle between a ray parallel to the collimator axis and the grating normal, the angle of incidence 8 = 0g + Si9 and the diffraction angle 9d = 9g — 5d. In the symmetrical Littrow case 5 = — <5d, and Eq. (3) follows from the general grating equation... 

The important conclusion is that the angular dispersion of a grating depends only on the angles of incidence and diffraction. If it is arranged so that i = 0 and they are both on the same side of the normal (the Littrow mount) then... 

In laser-spectroscopic applications the case a = often occurs, which means that the light is reflected back into the direction of the incident light. For such an arrangement, called a Littrow-grating mount (shown in Fig. 4.20), the grating equation (4.21) for constructive interference reduces to... 

Maximum reflectivity of the Littrow grating is achieved for / = r = 0 = or (Fig. 4.20b). The Littrow grating acts as a wavelength-selective reflector because light is only reflected if the incident wavelength satisfies the condition (4.21a). 

Calculate the number of grooves/mm for a Littrow grating for a 25° incidence at X = 488nm (i.e., the first diffraction order is being reflected back into the incident beam at an angle a = 25° to the grating normal). 

Fig. 5.85. Short dye laser cavity with grazing incidence grating. Wavelength tuning is accomplished by turning the end mirror, which may also be replaced by a Littrow grating...

Fig. 5.86. Littman laser with grazing incidence grating and Littrow grating using longitudinal pumping...

F.J. Duarte, R.W. Conrad Multiple-prism Littrow and grazing incidence pulsed CO2 lasers. Appl.Opt. 24, 1244 (1985)... 

In case of multiline lasers (e.g., argon or krypton lasers), line selection and mode selection can be simultaneously achieved by a combination of prism and Michelson interferometers. Figure 5.41 illustrates two possible realizations. The first replaces mirror M2 in Fig. 5.39 by a Littrow prism reflector (Fig. 5.41a). In Fig. 5.41b, the front surface of the prism acts as beam splitter, and the two coated back surfaces replace the mirrors M2 and M3 in Fig. 5.39. The incident wave is split into the... 

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