Abstract
The effect of the PEM tilt angle and incident polarization on the PEM interference is studied for a single axis photo-elastic modulator. The dc, 1ω, and 2ω components of the detector signal vary periodically as a function of PEM tilt angle. Although it is possible to adjust the PEM tilt angle to minimize the 1ω or 2ω detector signal at small tilt angles, it is not possible to null both of them simultaneously. For the case where no analyzer is used, the ac detector signals can be minimized simultaneously by adjusting the polarization angle of the light incident on the PEM and the PEM tilt angle. Direct observations of the detector signal indicate that the effects of refraction index and thickness variations are opposite consistent with a lower polarizability for compressive strain of the modulator.
Highlights
When using the modulator with a coherent light source this effect causes a time dependent interference of the laser beam in the crystal which can result in intensity variations several orders of magnitude larger than the intensity variations caused by the MO Kerr effect of the sample.[7]
In this paper we investigate in more detail the Photo-Elastic modulators (PEMs) interference effect for a single axis modulator in the dependence on PEM tilt angle and polarizer angles are investigated and the consequences for the MO Kerr technique are discussed
Note that the intensity variations decrease with the PEM tilt angle similar to the result reported by Oakberg.[8]
Summary
Photo-Elastic modulators (PEMs)[1] are often used to measure the magneto-optical Kerr effects of thin films and multilayers.[2,3] The incident or reflected beam’s state of polarization are modulated by a standing sound wave in the optical head[4,5,6] of the PEM and converted to an intensity variation using polarizers. This allows for a determination of the Kerr rotation and ellipticity with a S/N ratio limited by the shot noise of the light source. For single axis modulators the resonance condition is only fulfilled for one axis resulting in a time dependent refraction index for light polarized parallel to this axis (p-axis). Because of Poisson’s ratio, periodic strain variations are also expected perpendicular to this modulation direction resulting in a weak modulation of the refraction index for light linearly polarized perpendicular to the modulation axis (s-axis). In addition the modulation of the strain parallel to the optical axis will result in a time dependent thickness of the crystal, resulting in a time dependent of the optical path length. When using the modulator with a coherent light source this effect causes a time dependent interference of the laser beam in the crystal which can result in intensity variations several orders of magnitude larger than the intensity variations caused by the MO Kerr effect of the sample.[7] These large signal offsets are undesirable as it forces one to use a higher range setting on the lock-in amplifiers losing measurement sensitivity. The interference effect can be avoided or suppressed by using incoherent light, by coating the PEM with anti-reflection coatings, by tilting the PEM with respect to the optical axis of the setup, or by using a special optical head design so input and output surface of the modulator are no longer exactly parallel.[7,8] Polnau et al showed that the interference effects also takes place in double axis modulators although those modulators do not have a time varying thickness. The modulation of the refraction index is sufficient to induce intensity variations. They concluded this from measurements of the 1ω component versus the polarizer angle and from the time dependence of the detector signal.[9] In this paper we investigate in more detail the PEM interference effect for a single axis modulator in particularly the dependence on PEM tilt angle and polarizer angles are investigated and the consequences for the MO Kerr technique are discussed.
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