Abstract: A beam of light to be evaluated for coherence length is projected through an optical cavity having non-parallel reflecting surfaces to form a Fizeau fringe pattern. A multiple detector array senses the light output of the cavity at various locations along the fringe pattern. The sensed values of the light are compared with one another and the polarity of their difference is used to determine whether the coherence length of the light beam is above a predetermined threshold value.

Abstract: A double-layer photodiode is created in an integral structure to form two diodes, the upper diode having a relatively thin active region and the lower diode a relatively thick active region. Light whose wavelength is to be measured is directed onto the upper diode. The thickness of the first diode is chosen so that, in the spectrum of light wavelengths being measured, the energy of the shortest wavelength is entirely absorbed therein. As the radiation wavelength increases, the absorption by the upper diode decreases exponentially, and the unabsorbed light is transmitted through the thin active region into the thick active region of the lower diode where it is absorbed. The thickness of the active region of the lower diode is chosen so that it absorbs substantially all of the energy of longest wavelength in the spectrum being measured.