Broadband mirrors

There are two basic types of mirror, those for a single wavelength and those for a fairly broad spectral band. The single wavelength mirrors are simple quarterwave multilayers. The broadband mirrors are designed using methods as already described in Section 9.4.5. 

Quarter-wave interference films have seen numerous uses in the field of optical security, due to the strength of reflection and the ability to select numerous colors, and in particular for their observable color shift. Decorative nanolayer quarter-wave polymeric material with more than 100 layers have been known for twenty years, but was not used in the security sector because of their weak iridescent appearance. The 3 M Corporation uses PEN to manufacture reflectors with good band-edge control which are extremely efficient broadband mirrors for communication. These devices are easily noticed by the typical observer and are machine-readable. The PEN quarter-wave mirror films may be fine-line embossed and thinly layered which enliances their appearance [76]. 

LInknown broadband influences due to atmospheric aerosols, lamp fluctu ations, and dust on the mirrors are minimized by dividing the spectrum by a fitted fifth-order polynomial. The construction of the measuring instrumentation IS shown in Fig. 13.50. 

The laser light travels through the epifluorescence or side port of the microscope. A dichroic mirror reflects the laser light and passes the green fluorescence to either of the detectors. Detectors are positioned on the bottom port of the inverted microscope or the top port of the upright microscope. The choice of detector is discussed in more detail below. Broadband and band-pass filters placed in the detection path prevent residual IR from reaching either of the detectors. 

Transflection is essentially a cross between transmission and reflection. When light is shined onto a reflective surface covered by an optically clear sample, two processes occur - reflection off the top surface of the sample and transmission and reflection off the mirror, followed by a second transmission. As for most materials the surface reflection is low in comparison to the reflection off the mirror, the total collected radiation corresponds to a transmission measurement with double pathlength. Instead of a transparent support, transflection systems require only a spectrally neutral, or at least constant, broadband reflector under the sample. 

The c.w. dye laser can also be passively mode-locked and two different arrangements have been used. The first employed two free flowing dye streams, one for the laser dye and the other for the absorber (see Fig. 4) [18, 19]. In the alternative arrangement, the saturable absorber dye flows in a narrow channel of variable thickness (0.2—0.5mm) and in contact with a 100% broadband reflectivity mirror. With an absorber thickness of 0.5 mm, output pulses of 1 ps duration have been obtained [20]. Pulses as short as 0.3ps were produced when the DODCI cell length was shortened to 0.2 mm. The subpicosecond pulses produced in this arrangement were transform-limited in bandwidth. 

A commercial high-resolution FTS is depicted in Fig. 4.2. The output of the broadband source is focused on a circular aperture (entrance iris). As in the dispersive set-up, the optical beam is made parallel by a collimating mirror, and it intercepts a beam splitter at a non-normal incidence (usually 45 or 60°). One part of the beam is transmitted towards a fixed plane mirror while the other part towards a plane mirror, which can be translated continuously or in steps at a given distance (scan mirror). The beams reflected back by... 

Dielectric colour-separating mirrors made of extremely hard and chemically resistant oxide multilayer films enable nearly losss-free separation or mixture of the primary colours. Such colour splitters can be made for different spectral ranges of the reflection band, broadband and narrowband. Fig. 16 shows an example of a red reflector at 45° incidence. 

Figure 27 shows the spectral reflectance curve of a broadband laser mirror. 

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