Littrow mirror

Fig. 1.1. Typical arrangement of a double-beam recording prism spectrophotometer. M is the motor driving the Littrow mirror and the chart drum. (Bladon and Eglinton, 1964.)...

Let us consider the potential for wavelength modulation. In 1953 Hamond and Price [4] proposed the wavelength modulation principle based on a vibrating Littrow mirror. One year later the photobiologists French, Church and Eppley [5] adapted it to study first-order derivative spectra of photosynthetic systems in the visible region [6-10]. In the years that followed, various modulation techniques were employed to generate derivatives of different spectra a review is given in Table 3-1. 

The Littrow prism is also widely used. It is a 30° prism with a mirror back face the light passes through the prism to the mirror face and is reflected back through the prism, the total path being equal to a 60° prism. The prism mounting results in a fairly compact instrument (Section 17.7). 

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. 

Figure 14.12 —Schematic of an instrument showing deuterium lamp background correction. Perkin Elmer, model 3300 with a Littrow-type monochromator. This double beam assembly includes a deuterium lamp whose continuum spectrum is superimposed, with the aid of semitransparent mirrors, on the lines emitted by the hollow cathode lamp. One beam path goes through the flame while the other is a reference path. The instrument measures the ratio of transmitted intensities from both beams. The correction domain is limited to the spectral range of the deuterium lamp, which is from 200-350 nm. (Reproduced by permission of Perkin Elmer.)...

Most available infrared instruments use the Littrow mount for the prism, the beam being reflected from a plane mirror behind the pnsm and thus returning it through the prism a second time. This doubles the dispersion produced. Actually, a double-pass system is also used so that the beam goes through the pnsm four times. Other design modifications include those with single beam and double monochromator, double beam and double monochromator, and related combinations, See also Infrared Radiation. 

The diverged infrared radiation from the input slit is directed to a parabolic mirror and returned toward the splitting system (prism or gird). Depending on the type of optical principle, the parallel reflected infrared light passes through the prism or split by the diffraction gird. It is then reflected back by a plane mirror at the same parabolic reflector for the Littrow monochromator or at a second parabolic reflector for the Ebert monochromator. After this, the monochromic infrared radiation is directed to the output slit. 

Fig.3.9 Experimental setup for a line tunable liquid nitrogen cooled CO-laser. The reflection grating acts as wavelength selective end mirror and as coupling device at the same time (Littrow mount). ...

Fig. 3.5 Experimental arrangement for intracavity Raman spectroscopy with an argon laser CM, multiple reflection four-mirror system for efficient collection of scattered light LM, laser-resonator mirror DP, Dove prism, which turns the image of the horizontal interaction plane by 90° in order to match it to the vertical entrance slit S of the spectrograph FPE, Fabry-Perot etalon to enforce single-mode operation of the argon laser LP, Littrow prism for line selection [315]...

Littrow Simple arrangement 1 mirror 3 reflections Slit positions allow compact system and minimize astigmatism Susceptible to coma aberration Needs paraboloid mirror Slits are close together The cunrature of the slit image is wavelength dependent... 

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...

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... 

Commercial instruments frequently use a so-called Littrow moimt. Here a Littrow prism with a 30° angle and one surface silvered, as shown in Fig. 6.20, is used for increased dispersion. Arrangements with lenses or mirrors can be used for coUimating the incoming light onto the prism and for focusing the refracted beams towards the photographic plate or the exit slit. 

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