Fiber optics. Part IV. A photorefractometer

A photorefractometer to determine slight changes in refractive index of a liquid is described. When a long dielectric cylinder is immersed in a specimen liquid and an axial light cone is condensed at one end, the energy transmitted by total internal reflections through the cylinder depends primarily on δ, the difference in refractive index of the rod and the surround. Thus change in index of the surround liquid is related to change in emergent flux from the immersed rod. If the photometric unit is capable of detecting 100 K% change in emergent energy, and if δ≲0.1, then changes in liquid index of K·δ can be detected. By proper choice of K and δ, a change in index of 10−6 is measurable.An instrument based on this principle was constructed and the experimental results show good agreement with theory. The effects of absorption, reflection losses at the two ends of the rod, and partial reflection within the rod on the sensitivity of the system are studied theoretically. It also seems possible to measure the low refractive index liquids by the use of a hollow rod refractometer. Various applications of this photorefractometer principle are indicated.