A material that exhibits birefringence, or different refractive indices for different input polarizations, is said to be anisotropic. This anisotropy affects the transmission and absorption properties of light and is the primary mechanism used in polarizers and waveplates. However, even isotropic materials (same index for different polarizations) can enable polarization selection via reflection. For a linearly polarized beam, both S- and P-polarizations exhibit different changes in reflectivity versus incident angle. There is an incident angle known as Brewster’s angle (θ_B) at which P-polarized light is transmitted without loss, or exhibits zero reflectance, while S-polarized light is partially reflected. This angle can be determined from Snell’s law to be θ_B = arctan(n₂/n₁). The accompanying figure shows this response when light is incident from air onto a dielectric material where θ_B ≈ 56°.

In a beam sampler, the percentage of reflected light is determined by the Fresnel reflection from a single surface. By reflecting approximately 10% of S-polarization and 1% of P-polarization at the nominal 45° AOI, the beam sampler picks off 1-10% of an incident beam, depending on its polarization upon incidence. Reflectance from a single uncoated surface for both S- and P- polarizations is shown here. The graph exhibits reflectance of the plate as a function of incident angle, showing the minimum in the P-polarization at Brewster’s angle, 55° 34’.

N-BK7 is an excellent lens material for general use such as most visible and near infrared applications. It is the most common borosilicate crown optical glass, and it provides great performance at a good value. Its high homogeneity, low bubble and inclusion content, and straightforward manufacturability make it a good choice for transmissive optics.

UV Grade Fused Silica is synthetic amorphous silicon dioxide of extremely high purity providing maximum transmission from 195 to 2100 nm. This non-crystalline, colorless silica glass combines a very low thermal expansion coefficient with good optical qualities, and excellent transmittance in the ultraviolet region. Transmission and homogeneity exceed those of crystalline quartz without the problems of orientation and temperature instability inherent in the crystalline form. It will not fluoresce under UV light and is resistant to radiation. For high-energy applications, the extreme purity of fused silica eliminates microscopic defect sites that could lead to laser damage.

Optics with perfectly parallel faces are unsuited for use as reference elements in laser setups as reflections from both surfaces would be collinear resulting in undesired large period fringes. The introduction of a slight wedge causes unwanted reflections from the second surface to be deflected from the optical path. The back surface (S₂) of these beam samplers is slightly wedged. The laser beam should enter the non-wedged surface (S₁) where an arrow on the side points to.

An antireflection coating of high damage threshold (500 W/cm² CW, 1.0 J/cm² with 10 nsec pulses, typical) is applied on the back surface (S2) to minimize ghost reflections. Four broadband AR coatings are offered and optimized for 45° angle of incidence beams with <0.75% reflectance. NC.1 covers the 440-700 nm spectral range, NC.2 covers the 660-1000 nm spectral range, NC.3 covers the 1010-1550 nm spectral range and NC.UV covers the 255-440 nm spectral range. Uncoated version is also available, used as beam pick-off optics, to monitor the laser beam, independent of wavelength.

Polarization-selective reflectivity is exploited in laser cavities to produce strongly polarized light, as well as for fine tuning of the output laser wavelength. A Brewster’s window is used in the laser cavity, providing polarized light output with minimal power loss. It follows the formula θ_B = arctan(n), where θ_B is Brewster’s angle and n is the index of refraction of the material. Often, fixed beam polarization is necessary in order that optical components can consistently perform as they were designed.

A common application for beam samplers is picking off a portion of a beam in order to track changes in beam power vs time using a detector. This information may be used as part of a feedback loop to regulate power, or could simply be used to log power in order to normalize experimental results at a later time. For measuring beam power, the beam sampler should be oriented such that the laser is p-polarized. This choice will maximize transmitted power and attenuate power incident on the detector.