Optical Harmonic Vernier Effect: A New Tool for High Performance Interferometric Fiber Sensors

Abstract

The optical Vernier effect provides magnification of the sensing capabilities of an interferometer, allowing unprecedented sensitivities and resolutions to be achieved. Just like a calliper uses two different scales to achieve higher resolution measurements, the optical Vernier effect is based on the overlap between the responses of two interferometers with slightly detuned interference signals. Here we present in detail, as a novel approach, the generation of optical harmonics of the Vernier effect with Fabry-Perot interferometers, where the two interferometers can have very different frequencies in the interferometric pattern. We demonstrate not only a considerable enhancement, but also a better control of the sensitivity magnification factor, which scales up with the order of the harmonics, breaking the limits of the conventional Vernier effect as used today. In addition, this novel concept opens also new ways of dimensioning the sensing structures, together with improved fabrication tolerances.

Giant refractometric sensitivity by combining extreme optical Vernier effect and modal interference

“…1 a), the Vernier effect is generated in a parallel configuration, by means of a 3 dB fibre coupler 4 . It can be seen as a specific Michelson interferometer, however since the connecting parts are approximately invariant this approach for analysis of the parallel Vernier structure has not been used in literature 3 , 4 , 9 , 11 – 14 . The structure is illuminated with a supercontinuum source, and the reflected spectral response is measured with an optical spectrum analyser.…”

“…The reference FPI was fabricated to be in tune with the fundamental mode of the sensing FPI, while generating the first optical harmonic of the Vernier effect 9 . Apart from having a twice higher magnification factor, the first harmonic provides traceable internal envelopes, whose intersections are easier to monitor than the traditional Vernier envelope 9 (see “ Methods ”).…”

“…Initially, the cleaved end of a single mode fibre and a hollow capillary tube were spliced together with a splicing machine (Fitel S177). The electric arc was centred mainly in the single mode fibre by means of the manual mode of the fusion splicer; thereby we avoided the collapse of the capillary tube 9 . Two electric arcs were applied with an arc power of 30 arbitrary units and arc duration of 400 ms.…”

“…Since the reference interferometer is made of an air-filled cavity, the reference interferometer length is approximately half the optical path length, as the refractive index is about 1. The magnification factor for the first harmonic of the Vernier effect is then approximately given by 9 To obtain a negative magnification factor, the optical path length of the reference interferometer should be larger than the optical path length of the sensing interferometer, so that the detuning parameter ( ) in Eq. 9 becomes negative.…”

“…The detuning of the two interferometers, however, is quite a tricky problem. To achieve high magnification factors (typically in the order of tens [7][8][9][10] ), a small detuning value would be desirable. On the other hand, detuning by a very small amount (as an extreme optical Vernier effect) may result in a beating modulation with long period, which may become undetectable for a limited spectral range available.…”