A NEW VARIANT OF AN INTERFERENCE LIGHT FILTER

In recent years, various nonabsorbing light filters have been successfully developed and have increasingly entered the practice of physical experiment—i.e., optical devices that isolate comparatively narrow spectral regions and are based not on the absorption of light, but on other optical phenomena (interference, total internal reflection, rotation of the plane of polarization, scattering, etc.). These devices, unlike absorption light filters, have the advantage that the isolation of a narrow spectral region occurs with considerably smaller losses of light within the transmission band. In addition, the position of the transmission band itself can, within known limits, be shifted over the spectrum, which makes it possible to adapt the filter much better to specific experimental requirements. On the other hand, unlike monochromators, the cross section of the filtered light beam and its angular aperture can be quite large, which brings them closer to absorption light filters and makes them suitable both for investigations of weak radiation and for obtaining optical images in monochromatic light.

Moreover, nonabsorbing light filters can be created for such spectral regions (for example, the infrared) where the selection of absorption light filters proves impossible. In the degree of monochromatization of light, filters of this type occupy an intermediate position between monochromators and absorption light filters.

Among the fairly numerous designs proposed at the present time, the so-called interference light filters have proved especially simple and convenient in use. They are a modification of the Fabry–Perot interferometer, namely: two semitransparent reflecting layers separated by a thin (of the order of a wavelength) layer of transparent dielectric.

With appropriate selection of the parameters, such a system, as a result of interference of the light waves reflected from the front and rear semitransparent layers, is transparent only for a narrow interval of wavelengths. The position and width of the transmission band depend on the thickness of the spacer, the transparency of the reflecting layers, and also on the angle of incidence of the light beam. If the semitransparent layers did not possess absorption, then the transparency of such a light filter at the transmission maximum could reach 100%. In reality, however, absorption by the silver (or platinum) used to make these layers reduces the transparency to 20–40%, and the more so the narrower the transmission band. This effect becomes especially significant in the infrared region of the spectrum, which substantially limits the possibilities of using light filters.

The authors of the paper under review*) have realized an original variant of an interference light filter, free from the latter drawback. To create semitransparent reflecting layers, instead of coating with thin metallic films they used the phenomenon of total internal reflection. As is known, under total internal reflection at the boundary with a medium having a lower refractive index, the light wave penetrates into the latter, and

) B. H. Billings and M. A. Pittman, JOSA 39*, 978 (1949).

sity of this wave decreases with distance from the boundary according to an exponential law. If a medium with a low refractive index forms a thin interlayer between two media with a high refractive index, then light incident on the boundary of the two media at the angle of total internal reflection partially penetrates through this interlayer, and to a greater extent the smaller its thickness. Thus, an interlayer of a substance with a low refractive index, in the case of light incident on it at the angle of total internal reflection, will play the role of a nonabsorbing semitransparent reflecting layer, the degree of its transparency being entirely determined by the thickness of the interlayer.

Then the interference light filter takes the form shown in the figure.

On the base of a 30-degree rectangular prism made of rock salt, a thin layer of sodium fluoride was deposited, having

[Figure: schematic of an interference light filter. The drawing is labeled with NaCl, NaF, AgCl, NaF; angles 30° and 60°; refractive indices n_0, n_1, n_2, n_1, n_0.]

a significantly smaller refractive index \((n_1<n_0)\); then a layer of a substance with a high refractive index \((n_2>n_1)\), close to the refractive index of rock salt (silver chloride was chosen as such a substance); and, finally, a second layer of sodium fluoride, covered by a second prism of rock salt. The position of the transmission band, as is easy to see, depends on the thickness of the middle layer (silver chloride), and the width of the transmission band depends on the transparency, i.e. on the thickness, of the framing layers (sodium fluoride). The choice of the named substances is determined by the requirements of transparency for the infrared radiation for which the filter was designed, as well as by the possibility of obtaining homogeneous transparent layers by evaporation in vacuum.

It should be noted that, owing to the difference observed under total internal reflection in the phase shifts of waves polarized parallel and perpendicular to the plane of incidence, the positions of the transmission bands corresponding to the two polarizations turn out to be

different and depending on the angle of incidence of the light. As a result, the transmission band of the filter is split, each half of it corresponding to a quite definite polarization and capable of being extinguished by a correspondingly oriented analyzer.

The filter constructed by the authors had dimensions of \(30 \times 30 \times 50\) mm. The sodium fluoride layers were each \(5.5\,\mu\) thick. The thickness of the middle layer (\(\mathrm{AgCl}\)) was \(3.8\,\mu\). The corresponding transmission bands were located in the region \(4.0\)–\(5.5\,\mu\), depending on the orientation of the prism. The width of the bands was about \(0.1\,\mu\) (with an aperture of the light beam of the order of \(3^\circ\)), and the distance between the bands corresponding to different polarizations was about \(0.5\,\mu\).

In conclusion, it should be noted that the width of the transmission band depends substantially on the degree of homogeneity and parallelism of the layers.

V. Yur’ev