Exploring, tailoring, and traversing the solution landscape of a phase-shaped CARS process

Pulse shaping techniques are used to improve the selectivity of broadband CARS experiments, and to reject the overwhelming background. Knowledge about the fitness landscape and the capability of tailoring it is crucial for both fundamental insight and performing an efficient optimization of phase shapes. We use an evolutionary algorithm to find the optimal spectral phase of the broadband pump and probe beams in a background-suppressed shaped CARS process. We then investigate the shapes, symmetries, and topologies of the landscape contour lines around the optimal solution and also around the point corresponding to zero phase. We demonstrate the significance of the employed phase bases in achieving convex contour lines, suppressed local optima, and high optimization fitness with a few (and even a single) optimization parameter

Implementation of vibrational phase contrast coherent anti-Stokes Raman scattering microscopy

Detection of molecules using vibrational resonances in the fingerprint region for narrowband coherent anti-Stokes Raman scattering (CARS) is challenging. The spectrum is highly congested resulting in a large background and a reduced specificity. Recently we introduced vibrational phase contrast CARS (VPC-CARS) microscopy as a technique capable of detecting both the amplitude and phase of the CARS signal, providing background-free images and high specificity. In this paper we present a new implementation of VPC-CARS based on a third-order cascaded phase-preserving chain, where the CARS signal is generated at a single (constant) wavelength independent of the vibrational frequency that is addressed. This implementation will simplify the detection side considerably

A route to sub-diffraction-limited CARS Microscopy

We theoretically investigate a scheme to obtain sub-diffraction-limited resolution in coherent anti-Stokes Raman scattering (CARS) microscopy. We find using density matrix calculations that the rise of vibrational (Raman) coherence can be strongly suppressed, and thereby the emission of CARS signals can be significantly reduced, when pre-populating the corresponding vibrational state through an incoherent process. The effectiveness of pre-populating the vibrational state of interest is investigated by considering the excitation of a neighbouring vibrational (control) state through an intense, mid-infrared control laser. We observe that, similar to the processes employed in stimulated emission depletion microscopy, the CARS signal exhibits saturation behaviour if the transition rate between the vibrational and the control state is large. Our approach opens up the possibility of achieving chemically selectivity sub-diffraction-limited spatially resolved imaging

Hyperspectral imaging in biomedical applications

No abstract available

Large bandwidth, highly efficient optical gratings through high index materials

We analyze the diffraction characteristics of dielectric gratings that feature a high index grating layer, and devise, through rigorous numerical calculations, large bandwidth, highly efficient, high dispersion dielectric gratings in reflection, transmission, and immersed transmission geometry. A dielectric TIR grating is suggested, whose -1dB spectral bandwidth is doubled as compared to its all-glass equivalent. The short wavelength diffraction efficiency is additionally improved by allowing for slanted lamella. The grating surpasses a blazed gold grating over the full octave. An immersed transmission grating is devised, whose -1dB bandwidth is tripled as compared to its all-glass equivalent, and that surpasses an equivalent classical transmission grating over nearly the full octave. A transmission grating in the classical scattering geometry is suggested, that features a buried high index layer. This grating provides effectively 100% diffraction efficiency at its design wavelegth, and surpasses an equivalent fused silica grating over the full octave.Comment: 15 pages, 7 figure

Possibilities for Groundwater Flow Sensing with Fiber Bragg Grating Sensors

An understanding of groundwater flow near drinking water extraction wells is crucial when it comes to avoiding well clogging and pollution. A promising new approach to groundwater flow monitoring is the deployment of a network of optical fibers with fiber Bragg grating (FBG) sensors. In preparation for a field experiment, a laboratory scale aquifer was constructed to investigate the feasibility of FBG sensors for this application. Multiparameter FBG sensors were able to detect changes in temperature, pressure, and fiber shape with sensitivities influenced by the packaging. The first results showed that, in a simulated environment with a flow velocity of 2.9 m/d, FBG strain effects were more pronounced than initially expected. FBG sensors of a pressure-induced strain implemented in a spatial array could form a multiplexed sensor for the groundwater flow direction and magnitude. Within the scope of this research, key technical specifications of FBG interrogators for groundwater flow sensing were also identified

Dynamic consolidation measurements in a well field using fiber bragg grating sensors

Currently available groundwater flow prediction tools and methods are limited by insufficient spatial resolution of subsurface data and the unknown local heterogeneity. In this field study, fiber Bragg grating (FBG) sensors were installed in an extraction well field to investigate its potential to measure groundwater flow velocity. Reference in-situ pore pressure and temperature measurements were used to identify possible sources of FBG responses. FBG strain sensors were able to detect soil consolidation caused by groundwater extraction from 250 m distance. The results show that FBG responses were influenced by interface friction between soil and FBG packaging. FBG packaging slipped in soil and the effect was more pronounced during higher groundwater flow around a nearby well. These FBG fibers could be applied for indirect flow monitoring that does not require any tracer and provide real-time and long-term data during regular operation of extraction wells