A new concept for the detection of very small changes in the refractive index of a small sample of transparent material is given. The concept is based on measuring the frequency difference between two modes of a laser (possibly a twin- laser), where the evanescent field of one mode is affected by small refractive index changes. Intracavity sensing allows for orders of magnitude greater sensitivity than with external sensing. The frequency difference is obtained by light beating of the two modes. An imbedded diffractive element ensures proper modematching for the light beating. The relative frequency change is equal to the relative change in refractive index properly averaged over the waveguide. The performance of the intracavity system is compared with a system based on a Mach-Zehnder interferometer. The intracavity system may achieve a resolution that is 10⁴ - 10⁶ higher than the sensitivity of a system based on an external interferometer. The effect of thermal instability is investigated and it is discussed how the required very low thermal off-set can be maintained. Injection locking can be a problem. The problem may be solved by either introducing a fixed frequency off- set or by proper design of the cavity structure. An implementation based on III-V materials with a waveguide configuration and Bragg-mirrors is possible with existing technologies. A concept based on a polymer configuration is proposed.

Date Published: 15 March 1998
PDF: 8 pages
Proc. SPIE 3276, Miniaturized Systems with Micro-Optics and Micromechanics III, (15 March 1998); doi: 10.1117/12.302399

Published in SPIE Proceedings Vol. 3276:
Miniaturized Systems with Micro-Optics and Micromechanics III
M. Edward Motamedi; Rolf Goering, Editor(s)