Thin Film Polarizers

In order to answer frequently asked questions and to help LAYERTEC’s customers to specify thin film polarizers, definitions of the most important technical terms are given here.

Light is a transversal wave; the vector of the electric field oscillates perpendicular with respect to the propagation direction of the light. Natural light (from the sun or from a lamp) is mostly “unpolarized”. This means that the oscillation planes of the electric field vectors of the single light waves are randomly distributed, but always transversal with respect to the direction of propagation. In contrast, the term “linearly polarized light” signifies that there is only one plane of oscillation. There are different optics which can polarize light. An example of this would be crystal polarizers which split light into an unpolarized “ordinary beam” and a polarized “extraordinary beam” or thin film polarizers.

To explain the meaning of the terms “s-polarization” and “p-polarization”, first a reference plane must be determined (see Fig. 1). This plane is spanned by the incident beam and by the surface normal of the mirror (or polarizer). “S-polarized light” is the part of the light which oscillates perpendicularly to this reference plane (“s” comes from the German word “senkrecht” = perpendicular). “P-polarized light” is the part which oscillates parallel to the reference plane. Light waves with a plane of oscillation inclined to these directions can be described as having a p-polarized and an s-polarized part.

The upper part of Fig. 1 shows the reflectance of an uncoated glass surface vs. AOI for s- and p-polarized light. The reflectance for s-polarized light increases with rising angle of incidence. In contrast, the reflectance of p-polarized light decreases until reaching R = 0 at the “Brewster’s angle”, then increases for angles of incidence beyond the Brewster’s angle. In principle, the same is true for dielectric mirrors. Thin film polarizers separate the s-polarized component of the light from the p-polarized component using the effect that s-polarized light posseses a higher reflectance and broader reflection band than p-polarized light.

There always is a wavelength range, where Rs is close to 100 % while Rp is close to zero. Special coating designs are used to make this wavelength range as broad as possible and to maximize the polarization ratio Tp/Ts. Very high values of Tp (> 99.5 %) can be measured very precisely using a special Cavity ring-down set-up. The TFP is inserted into a cavity thus introducing additional losses equal to 100 %-Tp.

Utilizing this method, the most beneficial AOI for each TFP can be determined. Thin film polarizers (TFPs) are key components in a wide variety of applications, e.g. in regenerative amplifiers. LAYERTEC produces thin film polarizers on plane substrates (dimensions according to customer specifications) for wavelengths between 260 nm and 2500 nm. All TFPs are optimized for high laser-induced damage thresholds. Although there are no certified measurements available, LAYERTEC has learned from several customers that the LIDT of a TFP is approximately one third of the LIDT of a highly reflecting mirror for the same wavelength coated using the same technology.

Standard Thin Film Polarizers

• TFPs can be produced for AOI > 40°. Please note that thin film polarizers working at the Brewster’s angle exhibit a considerably broader bandwidth and a higher Tp/Ts ratio than those working at AOI = 45°.
• Typical polarization ratios Tp/Ts standard: > 500 (AOI = 45° or 55°)
• An extended wavelength range with a limited polarization ratio can be obtained by choosing AOI beyond the Brewster’s angle
• Special designs with a polarization ratio of Tp/Ts up to 10 000 are possible
• High laser-induced damage thresholds (useful for intra cavity applications)
• It is beneficial to design the laser in a way that the polarizers can be tilted by ±2° to adjust the polarizer to its best performance
• The standard design can be used for wavelengths between 260 nm and 2500 nm

Special Thin Film Polarizers

This special design provides an extremely broad polarizing wavelength range (±10 % of the center wavelength) with Tp/Ts = 300 to 1000.