Enhanced short temporal coherence length measurement using Newton’s rings interference

In this work, we used a simple accurate method to measure the short temporal coherence length of a light emitting diode. This method is based on accurate visibility determination of Newton’s ring fringes. The visibility of Newton’s rings has its maximum value around the position of contact point between the spherical and the plane surfaces while it decreases, until vanishing, with moving towards the outer rings. This is due to the gradually increasing thickness of the air gap separating the two surfaces; starting from zero at the contact point. When the lowest detectable value of visibility tends to zero, there are only two interfered rays which are temporarily separated by the value of the temporal coherence length of the used light source. By detecting these two rays and calculating their optical paths, it was easy to evaluate the temporal coherence length of the used light source. For the purpose of accuracy enhancement, both intensity and visibility calculations of each detected interference order were automatically determined by employing an algorithm prepared using MATLAB software environment. Moreover, tracing the real optical paths of the two interfered rays forming the interference enabled us to calculate the spatial distance which is separating them without any approximation. This distance was too short (<20 nm) which means that we almost dealt with a pure temporal coherency. In this work, three commercial light emitting diode sources with different wavelengths were investigated and their temporal coherence length values were determined. Investigation of tolerance of temporal coherence length measurement was discussed.