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… Show more
“…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.…”
Section: Resultsmentioning
confidence: 99%
“…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 ”).…”
Section: Resultsmentioning
confidence: 99%
“…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.…”
Section: Methodsmentioning
confidence: 99%
“…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.…”
Section: Methodsmentioning
confidence: 99%
“…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.…”
Section: Giant Refractometric Sensitivity By Combining Extreme Opticamentioning
The optical Vernier effect consists of overlapping responses of a sensing and a reference interferometer with slightly shifted interferometric frequencies. The beating modulation thus generated presents high magnified sensitivity and resolution compared to the sensing interferometer, if the two interferometers are slightly out of tune with each other. However, the outcome of such a condition is a large beating modulation, immeasurable by conventional detection systems due to practical limitations of the usable spectral range. We propose a method to surpass this limitation by using a few-mode sensing interferometer instead of a single-mode one. The overlap response of the different modes produces a measurable envelope, whilst preserving an extremely high magnification factor, an order of magnification higher than current state-of-the-art performances. Furthermore, we demonstrate the application of that method in the development of a giant sensitivity fibre refractometer with a sensitivity of around 500 µm/RIU (refractive index unit) and with a magnification factor over 850.
“…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.…”
Section: Resultsmentioning
confidence: 99%
“…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 ”).…”
Section: Resultsmentioning
confidence: 99%
“…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.…”
Section: Methodsmentioning
confidence: 99%
“…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.…”
Section: Methodsmentioning
confidence: 99%
“…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.…”
Section: Giant Refractometric Sensitivity By Combining Extreme Opticamentioning
The optical Vernier effect consists of overlapping responses of a sensing and a reference interferometer with slightly shifted interferometric frequencies. The beating modulation thus generated presents high magnified sensitivity and resolution compared to the sensing interferometer, if the two interferometers are slightly out of tune with each other. However, the outcome of such a condition is a large beating modulation, immeasurable by conventional detection systems due to practical limitations of the usable spectral range. We propose a method to surpass this limitation by using a few-mode sensing interferometer instead of a single-mode one. The overlap response of the different modes produces a measurable envelope, whilst preserving an extremely high magnification factor, an order of magnification higher than current state-of-the-art performances. Furthermore, we demonstrate the application of that method in the development of a giant sensitivity fibre refractometer with a sensitivity of around 500 µm/RIU (refractive index unit) and with a magnification factor over 850.
The optical analog of the Vernier effect applied to fiber interferometers is a recent tool to enhance the sensitivity and resolution of optical fiber sensors. This effect relies on the overlap between the signals of two interferometers with slightly detuned interference frequencies. The Vernier envelope modulation generated at the output spectrum presents magnified sensing capabilities (i.e., magnified wavelength shift) compared to that of the individual sensing interferometers that constitute the system, leading to a new generation of highly sensitive fiber sensing devices. This review analyses the recent advances and developments of the optical Vernier effect from a fiber sensing point-of-view. Initially, the fundamentals of the effect are introduced, followed by an extensive review on the state-of-the-art, presenting all the different configurations and types of fiber sensing interferometers used to introduce the optical Vernier effect. This paper also includes an overview of the complex case of enhanced Vernier effect and the introduction of harmonics to the effect.
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