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Figure 1.9.2 Schematic diagram of a laser. Brewster windows used to produce polarized output would be placed between the medium and the mirrors; a Littrow prism or wedge used for wavelength selection is typically placed between the medium and the high reflector mirror.

Lasers for Flow Cytometry dispersive element other than a prism, e.g., a grating, etalon, or optical filter, can also be used for wavelength selection.

The resonator can be thought of as analogous to an organ pipe; the length of the pipe and the effective distance between the mirrors of the resonator determine the frequency of the standing wave sustained by the structure. In the case of the resonator, this characterizes what is described as the longitudinal mode of the laser.

The energy profile of the beam itself, or the transverse mode of the laser, is determined by the geometry of the medium as well as by the geometric optics of the mirrors. If stimulated emission is confined to a volume close to the axis of the resonator, the laser will operate in what is called transverse electromagnetic mode zero zero (TEM00), and the intensity profile will be Gaussian, as described in detail in unit 1.6. As the effective cross-section of the medium increases, other transverse electromagnetic modes are superimposed on TEM00. These are individually undesirable in lasers designed for use in cytometry because they are, in general, not radially symmetric, but rather, multilobed, and are likely to produce nonuniform illumination.

In many types of lasers, polarization is introduced into the beam by putting special windows between the medium and the end mirrors. The windows are placed at Brewster's angle to the axis of the system and usually oriented such that light reflected from the surface of the window is directed upward (vertically). At Brew-ster's angle (~57° for glass), reflection from the window surface is minimized for vertically polarized light, while a small percentage of horizontally polarized light is reflected out of the cavity. The slight difference in transmissions of the two polarizations is magnified many-fold by the feedback characteristic of the optical resonator structure, with the result that the laser output in a system with such Brewster windows is highly polarized (usually vertically), and typically in a ratio of at least 500:1.

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