Interferenční multi-vrstvy pro lasery a interferometrii

Pavel Pokorný

Optical interference coatings are used for desired forming the spectral curves of reflectance and transmittance or phases of the complex reflection or transmission coefficient. Most common applications of thin films are narrowband or achromatic antireflection coatings, non-absorbing mirrors, neutral or one-wavelength beamsplitters, monochromatic filters, additive or subtractive dichroic colour filters, cold mirrors, heat filters. In cases, which require an oblique incidence, the spectra become the function of light polarization. Examples of such applications of thin films are polarizers, depolarizers or various defined beam splitters. The task of our laboratory is to design and produce optical coatings for lasers and interferometry.

One of the most important sorts of elements is that of mirrors forming the resonator of He-Ne lasers operating at the 543 and 633 nm wavelengths. The required parameters are: maximum reflectance and specified transmittance at the functional wavelength, minimum reflectance at the wavelengths corresponding to the undesired concurrent quantum transitions, low absorption and light scattering, mechanical, chemical and thermal resistance. Such a laser mirror consists of 20-30 alternating layers with high and low refractive indeces, respectively. The design of their thickness is complicated especially by requirements regarding suppression of reflection for concurrent wavelengths. Demands on minimisation of the scattering and absorption necessitate searching for corresponding layer materials and technology of their deposition.

An example of the so called minus filter is the laser goggles that protect eyes of people working with lasers. Transmittance of the laser wavelength must be suppressed (proportional to power of laser), but ability of sufficient seeing must be retained.

Antireflection coatings are used to reduce unwanted surface reflectance of optical components and to transluminate them. Each type of glass, each wavelength, polarization and angle of incidence of light needs some other coating. The simplest case of one wavelength and normal incidence requires a double-layer with their thickness matched to the type of glass and wavelength. A difficult case is simultaneous suppression of reflectance in oblique incidence for both polarizations.

Thin film edge filters can serve in multi-colour interferometry as beam separators. For example, the beam from the green laser can be transmitted while that from the red one is reflected. Problems can arise in oblique incidence, when the position of the edge is dependent on the polarization of light.

Polarizing beam splitters are used for simultaneously polarizing 543 and 633 nm laser light for reflection and transmission. The extinction ratio of both polarizations should be at least 500. An additional requirement is that the reflected and the transmitted beam must form the right angle. The refractive indices of both layer-forming materials and glass must respect the technological possibilities. From this it follows that it must be a cemented cube. It is possible to choose from two compromises: a cube of perfect optical glass BK7 with a coating containing unstable cryolith layers, or worse heavy optical glass with a good quality layers-technology TiO2/SiO2. Such a polarizer needs 10-15 layers. Attention should be paid to the optical effect of the cement. Outer walls of the cube must be provided with antireflection coatings for both wavelengths and for the chosen type of glass.

On the contrary, some interferometers make use of nonpolarizing beam splitters, in which case both polarizing components are required to show 50% reflectance and transmittance for an angle of incidence of 45°, and phase differences p-s are required to be zero for both reflection and transmission. For a single wavelength, these conditions can be fulfilled by using a simple alternating system, e.g. 15 quarter-wave layers of TiO2/ZrO2. For more wavelengths, it is difficult to fulfil the phase condition. All these systems are however very sensitive to usual production deviations and it is therefore better to be content with compromise depolarization that can be achieved using for example a system of 6 quarter-wave layers TiO2/Al2O3. The residual polarization of reflectance/transmittance amounts to about 7%, the phases are ideally depolarized, spectral dependence is weaker and deposition of such a system is reproducible.

Another application of coatings is compensation of the polarization effect of the cube-corner retro-reflector. Phase differences that arise in the cube-corner at total reflections on its three faces are usually undesirable in interferometry. For total reflection, reflectance (100%) and transmittance (0%) cannot be changed but the values of phases of the reflection coefficient can be changed by deposition of a suitable coating. So unwanted phase changes can be compensated by using correcting dielectric multilayers deposited on each face and matched to the type of glass and angle of incidence. In the most often case of normal incidence on the base of the cube-corner, the same coating consisting of 2-5 layers is deposited on all faces. More layers in the coating bring less spectral dependence. The base of the cube-corner must be provided with an antireflection coating.