Thomas Kissinger

Range-resolved interferometric signal processing using sinusoidal optical frequency modulation

A novel signal processing technique using sinusoidal optical frequency modulation of an inexpensive continuous-wave laser diode source is proposed that allows highly linear interferometric phase measurements in a simple, self-referencing setup. Here, the use of a smooth window function is key to suppress unwanted signal components in the demodulation process. Signals from several interferometers with unequal optical path differences can be multiplexed, and, in contrast to prior work, the optical path differences are continuously variable, greatly increasing the practicality of the scheme. In this paper, the theory of the technique is presented, an experimental implementation using three multiplexed interferometers is demonstrated, and detailed investigations quantifying issues such as linearity and robustness against instrument drift are performed.

Fibre segment interferometry using code-division multiplexed optical signal processing for strain sensing applications

A novel optical signal processing scheme for multiplexing fibre segment interferometers is proposed. The continuous-wave, homodyne technique combines code-division multiplexing with single-sideband modulation. It uses only one electro-optic phase modulator to achieve both range separation and quadrature interferometric phase measurement. This scheme is applied to fibre segment interferometry, where a number of long-gauge length interferometric fibre sensors are formed by subtracting pairs of signals from equidistantly placed, weak back reflectors. In this work we give a detailed account of the signal processing involved and, in particular, explore aspects such as electronic bandwidth requirements, noise, crosstalk and linearity, which are important design considerations. A signal bandwidth of ?20?kHz permits the resolution of phase change rates of 2.5 ? 104?rad?s?1 for each of the four 16.5?m long segments in our setup. We show that dynamic strain resolutions below 0.2?nanostrain?Hz?0.5 at 2?m sensor gauge length are achievable, even with an inexpensive diode laser. When used in applications that require only relative strain change measurements, this scheme compares well to more established techniques and can provide high-fidelity yet cost-effective measurements.

Fiber Segment Interferometry for Dynamic Strain Measurements

Using a novel range-resolved interferometric signal processing technique based on the sinusoidal optical frequency modulation of a cost-effective laser diode, a fiber sensing approach termed fiber segment interferometry (FSI) is described. In FSI, a chain of long-gauge length fiber optic strain sensors are separated by identical in-fiber partial reflectors. Targeted at dynamic strain analysis and ultrasound detection for structural health monitoring, this approach allows integrated strain measurements along fiber segments, removing the sensing gaps and sensitivity to inhomogeneities found with localized fiber sensors. In this paper, the multiplexing of six fiber segments, each of length 12.5 cm, is demonstrated. The sensor array can be interrogated at 98 kHz data rate, achieving dynamic strain noise levels ≤ 0.14 nϵ · Hz-0.5. The reflector fabrication is discussed, an analysis of linearity and noise performance is carried out and results from an exemplar experiment to determine the speed-of-sound of a stainless steel rod are shown.

Simultaneous laser vibrometry on multiple surfaces with a single beam system using range-resolved interferometry

A novel range-resolved interferometric signal processing technique that uses sinusoidal optical frequency modulation is applied to multi-surface vibrometry, demonstrating simultaneous optical measurements of vibrations on two surfaces using a single, collimated laser beam, with a minimum permissible distance of 3.5 cm between surfaces. The current system, using a cost-effective laser diode and a fibre-coupled, downlead insensitive setup, allows an interferometric fringe rate of up to 180 kHz to be resolved with typical displacement noise levels of 8 pm Hz-0.5. In this paper, the system is applied to vibrometry measurements of a table-top cryostat, with concurrent measurements of the optical widow and the sample holder inside. This allows the separation of common-mode vibrations of the whole cryostat from differential vibrations between the window and the sample holder.

Range-resolved signal processing for fibre segment interferometry applied to dynamic long-gauge length strain sensing

A range-resolved interferometric signal processing technique using sinusoidal optical frequency modulation is applied to fibre segment interferometry. Here, six optical fibre segments of gauge length 12.5 cm are used as interferometric strain sensors and are formed between seven weak, broadband fibre Bragg gratings, acting as in-fibre partial reflectors. In a very simple and cost-effective optical setup using injection current modulation of a laser diode source, interferometric measurement of acoustic wave propagation in a metal rod is used to demonstrate the capabilities of the technique.

Multiplexing curvature sensors using fibre segment interferometry for lateral vibration measurements

Dynamic fibre-optic curvature sensing is demonstrated by interrogating chains of fibre segments, separated by broadband Bragg grating reflectors, using range-resolved interferometry (RRI). Four fibre strings, containing four fibre segments each of gauge length 20 cm, are attached to the opposing sides of a support structure and the resulting differential strain measurements allow inference of lateral displacements of a cantilever test object. Dynamic tip displacement resolutions in the micrometre range at an interferometric bandwidth of 21 kHz demonstrate the suitability of this approach for highly sensitive and cost-effective fibre-optic directional vibration measurements of smart structures.

Simultaneous optical interrogation of multiple tuning fork resonators using range-resolved interferometry

In a proof-of-principle experiment, the vibrations of a set of three tuning folk resonators are simultaneously interrogated along a single optical beam using range-resolved interferometry. Measurements with noise standard deviations in the nm levels over bandwidths of many kHz are possible. In addition, a high degree of environmental noise rejection is inherently achievable because only differential measurements between the vibrating tuning folk prong pairs are evaluated. Employing only highly coherent, robust and cost-effective diode lasers, this approach could be useful for a wide range of mechanical sensor interrogation tasks.

Interferometric fibre-optic curvature sensing for structural, directional vibration measurements

Dynamic fibre-optic curvature sensing using fibre segment interferometry is demonstrated using a cost-effective rangeresolved interferometry interrogation system. Differential strain measurements from four fibre strings, each containing four fibre segments of gauge length 20 cm, allow the inference of lateral vibrations as well as the direction of the vibration of a cantilever test object. Dynamic tip displacement resolutions in the micrometre range over a 21 kHz interferometric bandwidth demonstrate the suitability of this approach for highly sensitive fibre-optic directional vibration measurements, complementing existing laser vibrometry techniques by removing the need for side access to the structure under test.

Characterisation of a cryostat using simultaneous, single-beam multiple-surface laser vibrometry

A novel range-resolved interferometric signal processing technique that uses sinusoidal optical frequency modulation is applied to multi-surface vibrometry, demonstrating simultaneous optical measurements of vibrations on two surfaces using a single, collimated laser beam, with a minimum permissible distance of 3.5 cm between surfaces. The current system, using a cost-effective laser diode and a fibre-coupled, downlead insensitive setup, allows an interferometric fringe rate of up to 180 kHz to be resolved with typical displacement noise levels of 8 pm · Hz−0 5. In this paper, the system is applied to vibrometry measurements of a table-top cryostat, with concurrent measurements of the optical widow and the sample holder target inside. This allows the separation of common-mode vibrations of the whole cryostat from differential vibrations between the window and the target, allowing any resonances to be identified.

Range-resolved single-sideband optical fibre interferometry for quasi-distributed dynamic strain sensing

A novel optical signal processing scheme for fibre sensors is proposed, which combines interferometric phase measurements with range multiplexing. The scheme is based on single-sideband signal processing and uses continuous-wave pseudo-random range encoding. The potential of the technique for cost-effective dynamic quasi-distributed strain sensing is explored by applying it to an array of fibre segments. In its current implementation dynamic strains in segments of 3.4 m gauge lengths can be measured with a resolution well below microstrains at a bandwidth of 100 kHz.