Abstract
When using a Photo-elastic modulator (PEM) in combination with a coherent light source, in addition to the modulation of the phase, Fabry-Perot interference in the PEM’s optical head induces large offsets in the 1ω and 2ω detector signals. A Jones matrix which describes both effects simultaneously, was derived for a single axis PEM and used to find an expression for the detector signal for two different MO Kerr setups. The effect of the PEM tilt angle, polarizer angle, analyzer angle, and retardation, on the detector signal offsets show that offsets can be zeroed by adjusting PEM tilt angle, polarizer angle, and retardation. This strategy will allow one to avoid large offset drifts due to the small retardation, intensity, and beam direction fluctuations caused by lab temperature fluctuations. In addition, it will enable one to measure in the most sensitive range of the lock-in amplifiers further improving the signal to noise ratio of the setup.
Highlights
The Jones matrix for coherent light was derived for a single axis Photo-elastic modulators (PEM)
The derived model shows how the offset signals depend on PEM tilt angle, polarizer angle and retardation depth
The offsets can be minimized at small PEM tilt angles by adjusting the PEM tilt angle not simultaneously for the 1ω and 2ω components
Summary
Laser intensity and wavelength, and laser beam direction fluctuate and can have a significant impact on the magnitude of the 1ω and 2ω detector signal offsets resulting in significant signal drifts These effects are similar to the effect of signal offsets observed when incorporating a PEM in a longitudinal Kerr measurement setup.[16,17,18]. Using the results of Polnau et al.[13] Oakberg[23] pointed out that the interference effect for double axis modulators disappears in the odd harmonics for 45 degrees incident linearly polarized light He suggested that the interference effect for the even harmonics can be suppressed by careful rotational positioning of the PEM head.
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