Over last decade, ultrafast lasers became industrially viable tool for high precision material processing. Ability to modify in the bulk of transparent materials is one of unique attributes of this technology. This have been successfully used for glass cutting, implementation of photonic circuits and microfluidic chips, local engineering of optical fibre properties.
This talk will explore how ultrafast lasers can be used to engineer optical scattering systems. The exploitation of this process for developing low loss distributed sensing systems and compact optical spectrometer will be discussed and demonstrated.
We demonstrate a polarization analyser based on processing of speckle patterns generated by a scattering medium. Each speckle pattern at a given wavelength and polarization state is unique and deterministic, and thus the polarization angle alters the speckle pattern motif. The polarization state of a given input light is obtained using reconstructive linear algebra methods. The system consists of a femtosecond laser written scattering chip and a CMOS sensor and contains no moving parts, making the proposed solution is low-cost and compact. The linear polarization angle was accurately reconstructed over a 0-20° test range, with 6 arcminutes (1/10° ) standard error. To demonstrate an application as a polarimeter, we used the system to measure Faraday rotation in a SF59 lead silicate glass within an electromagnet. The magnetic field was successfully traced by determining the induced changes in the input beam’s linear polarization angle in the range 0-80 mT with 10 mT standard error.
Fiber Bragg gratings are the most popular type of optical fiber sensor. However, its commercial use is frequently limited by high cost and complexity of the interrogation unit. Here, an interrogator based on a femtosecond laser written silica scattering chip is designed and implemented. Such device can directly reconstruct strain, from the scattering speckle patterns, with a resolution of 70 μϵ (microstrain) within the range of 180-700 μϵ, limited by the slippage of the fiber coating, with the potential to be reduced with the system improvements.
The common challenge for reconstructive spectrometers is achieving high spectral resolution without sacrificing device stability, size and costs. Here a fully integrated scattering chip-based spectrometer build on Raspberry Pi platform is designed and implemented. It exhibits no dependence on temperature and humidity (22.7-23.8 °C and 39.5-41 %), is confined in small space (box 50x35x35 mm) and it can reconstruct spectra with resolution up to 0.05 nm (50 pm). The only instability – gravitational mechanical micromovements were compensated by applying pixel binning and device can still reconstruct spectra from binned pictures as small as 32 x 24 pixels.
Femtosecond laser irradiation allows to modify the optical properties of transparent materials with high accuracy. In many applications, optical scattering produced by the laser irradiation is one of the major limiting factors. However, there are situations when the scattering is responsible for the basic principle of operation of the optical element. This report reviews two research directions where laser-induced scattering can be successfully exploited. First, spectroscopic measurements can be performed by analyzing the speckle patterns created by the scattering medium. The measurements are made possible by the strong dependence of the speckle pattern on the wavelength of light. A scattering chip created thanks to a femtosecond laser makes allows addressing the stability problem faced by many scattering spectrometers. The volumetric scattering centers are induced in silica substrate via micro-explosions caused by the focused laser beam. Such a spectrometer can be successfully used for interrogating fiber Bragg gratings or interferometers. Second example is found in optical reflectometry. This technology allows turning an optical fiber into a distributed microphone or thermometer. A single optical fiber can monitor a stretch of several tens of kilometers with an accuracy of several meters. Such systems have wide range of applications in civil engineering, geosciences and other fields. Reflectometry measurements are performed by observing the back-scattered light produced by the glass medium of the optical fiber. Femtosecond laser writing allows effectively increase backscattered light whilst introducing minimal additional losses. In this way, the sensitivity of reflectometric systems can be increased or their range can be extended.
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