Determination of the coherence length and beat frequency length using a p-emf sensor

The coherence of light is essential for understanding interference phenomena, which are pivotal in a wide range of applications. However, due to the technical challenges of conventional methods, quantifying coherence is difficult to achieve in undergraduate optics laboratories. In this work, we present a modification of a method and experimental setup that can enhance the understanding of coherence concepts employing a sensor based on the non-steady-state photo-electromotive force (p-emf) effect. This p-emf sensor generates an electrical current proportional to the square of the interference pattern’s visibility, eliminating the need for image processing and high-quality optical elements, and allowing for real-time measurements. We demonstrated the method by measuring the coherence length of a He-Ne laser and the corresponding length associated with the beat frequency of the laser cavity’s longitudinal modes. This approach is robust, straightforward and simple, making it suitable for implementation in undergraduate optics laboratories.