Most optical sensors rely on the detection of small wavelength changes in response to changes in temperature, strain, adhesion of bio-particles, or change in the chemical environment for the system under test. Examples include Fiber Bragg Grating (FBG)-based sensors, laser cavity, micro-sphere, or micro-ring based sensors, and surface-plasmon-resonance or photonic-crystal sensors. Despite their advantages including electromagnetic-interference (EMI) immunity and use in remote and distributed-sensing applications, optical sensors typical require bulky and expensive interrogation systems (such as a broadband light source/grating-based spectrum analyzer setup or a tunable light source with photodetector) capable of resolving small wavelength shifts to sub-nanometer or even picometer resolution. But a new technique developed by researchers at Palo Alto Research Center (PARC; Palo Alto, CA) incorporates a position-sensitive detector to create a miniature read-out interrogator unit that can resolve wavelength changes as small as 50 fm.
Kiesel, P. Miniature read-out sensor resolves wavelength to 50 femtometers. Laser Focus World. 2011 January; 47 (1): 21-22.