The Birefringence Modulator

The Birefringence Modulator is a device that modulates the polarisation of light at a high frequency, like a modulated quarter wave plate. The peak phase shift, or retardation amplitude, can be adjusted to any desired value.

In high sensitivity applications it is often advantageous to work with ac modulated light signals. Lock-in amplification at the modulation frequency greatly reduces signal noise. For ellipsometry, modulation of the elliptical polarisation of the incident light beam leads to simple relationships between the ac signals and the real and imaginary parts of the complex reflectivity ratio.

Our Birefringence Modulator modifies the polarisation of a light beam at a frequency of 50 kHz without moving parts. This is achieved through the photolastic effect: A transparent isotropic glass slab is set into longitudinal mechanical resonance. The optical thickness then varies as the crystal oscillates, and an oscillating phase difference between light polarised parallel and transverse to the axis of vibration is generated. If the incident light is linearly polarised at say 45° to the vibration axis, the light will be elliptically polarised with a time-varying phase shift between the components parallel and perpendicular to the vibrational axis. Using low-loss elements with a thickness of about 1 cm, vibrational amplitudes can be obtained such that the light has a maximum phase shift of more than p (l/2) for optical wavelengths of up to 900 nm. For longer wavelengths, thicker elements can be used. The standard optical element is fused silica, which is transparent from 200 nm to 2 µm.

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