Coherent characteristics of broadband and extended light sources based on the Twyman–Green interferometer

The Twyman–Green interferometer, as a representative type of interferometric structure, possesses unique advantages in the field of interferometry due to its adjustable single optical path characteristic. However, using a laser as the light source for the Twyman–Green interferometer, with its long coherence length, can result in noisy fringes when measuring planar elements, including multiple surface interference fringes and speckle noise. To address these issues, this paper proposes the use of broadband extended light source as the coherent light source in the Twyman–Green interferometer to achieve short coherent source illumination, thereby eliminating interference fringes and coherent noise. This paper theoretically derives the coherence characteristics of broadband extended light sources and, in particular, quantitatively analyzes the influence of the thickness difference of glass in the Twyman–Green dual optical paths on the contrast of the interference fringe. The corresponding theoretical expressions for interference intensity are derived, and the validity of these theoretical findings is demonstrated through simulation analysis and experimental verification. This innovative research, to our knowledge, significantly supplements the existing coherence theory of light sources, offering substantial theoretical research insights and practical engineering applications.