Sarcoidosis is a rare disease with the number of patients in the European Union (28 member states), Norway, Iceland and Lichtenstein estimated as ∼102 000 [1, 2]. Some 40% of individuals experience sarcoidosis as a chronic condition, while >60% of individuals find that the disease resolves within 2–5 years [3]. However, it has been shown that patients can still have symptoms that result in a reduced quality of life (QoL) even after clinical signs of disease have disappeared. These symptoms include fatigue, pain, everyday cognitive failure, small fibre neuropathy, exercise limitation and depressive symptoms [4, 5]. Findings from a recent survey of patients and their partners in the Netherlands also showed that sarcoidosis has a significant impact on their lives [6].

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Two laser beams are focused onto a thin permalloy film. By changing the direction of the magnetization of the permalloy film the magneto-optic Kerr-effect is exploited either to rotate the plane of polarization or to modulate the amplitude of the reflected light. The domain wall behaviour is determined by the applied field and demagnetizing fields in order to compensate the inner field. Acting as a modulation aperture, the displacements can be controlled accurately by the amplitude or the frequency of the applied field. In a first step, the film is characterized for our application by measuring the velocity of the domain wall displacements by two focused beams with known distances. By measuring the delay between the modulation of the reflected beams it is possible to accurately determine the beam separation under the condition of reproducible wall movements. Therefore, a moving domain wall acts like a high-speed modulator. The method could be used for many applications, for instance refraction measurements by dispersion of the refractive index in the atmosphere or rapid quality control of magnetic materials.

Two laser beams are focused onto a thin permalloy film. By changing the direction of the magnetization of the permalloy film, the Kerr effect is exploited to either rotate the plane of polarization or modulate the amplitude of the reflected light. First, the film is characterized for our application by measuring the velocity of the domain wall displacements by two focused beams with known distances. By measuring the delay between the modulation of the reflected beams, it is possible to accurately determine the beam separation under the condition of reproducible wall movements. Therefore, a moving domain wall acts like a high-speed modulator. The method could be used for many applications, e.g., refraction measurements or rapid quality control of magnetic materials.