Pellicle beamsplitters are ultra-lightweight components that virtually eliminate multiple reflections commonly associated with thicker glass beamsplitters. Pellicles are manufactured by stretching a 5 µm thick polymer membrane over a flat metal frame. The extreme thinness eliminates secondary reflections by making them coincident with the original beam. Consequently, the optic does not require AR coatings on the backside as in a traditional beamsplitting optic. Multiple wavelength versatility is an added benefit of an ultra-thin reflective membrane, making it ideal for applications where chromatic dispersion needs to be minimized.
Newport's Pellicle beamsplitters are coated and uncoated to be used across an extremely wide wavelength range from 375-2400nm. The average transmittance and reflectance across a range of wavelengths and incidence angles are shown. The coated pellicle provides equal reflectance and transmission over the visible range by means of a neutral, semi-absorbing coating. The uncoated pellicle yields an average reflectivity of 8% over 375–2400 nm and is well suited for beam sampling applications requiring high transmission and low reflectance.
Interference effects due to thin membranes should be considered when using pellicles, which act as low finesse interferometers. While the thin membranes are ideal for eliminating ghost images in incoherent white light applications, in monochromatic applications interference between the front and back surfaces can introduce unwanted fringes into a transmitted or reflected beam. Over any wavelength range and for a fixed incidence angle, a ±5% sinusoidal oscillation is superimposed on the spectral transmission and reflection graphs shown above. The oscillation varies rapidly with small changes in the incidence angle. When using pellicles to split beams spanning a range of angles, the oscillations will average out.
The extreme thinness of pellicles provides many advantages in reducing losses and ghost images. However, the polymer membrane can resonate due to vibrations in the environment. Stiff posts or riser blocks should be used in mounting pellicles to an optical table. Applications subject to air currents or acoustical noise should use glass beamsplitters.
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