Optical feedback interferometry (OFI) sensors are experiencing a consistent increase in their applications to biosensing due to their contactless nature, low cost and compactness, features that fit very well with current biophotonics research and market trends. The present paper is a review of the work in progress at UPC-CD6 and LAAS-CNRS related to the application of OFI to different aspects of biosensing, both in vivo and ex vivo. This work is intended to present the variety of opportunities and potential applications related to OFI that are available in the field. The activities presented are divided into two main sensing strategies: The measurement of optical path changes and the monitoring of flows, which correspond to sensing strategies linked to the reconstruction of changes of amplitude from the interferometric signal, and to classical Doppler frequency measurements, respectively. For optical path change measurements, measurements of transient pulses, usual in biosensing, together with the measurement of large displacements applied to designing palliative care instrumentation for Parkinson disease are discussed. Regarding the Doppler-based approach, progress in flow-related signal processing and applications in real-time monitoring of non-steady flows, human blood flow monitoring and OFI pressure myograph sensing will be presented. In all cases, experimental setups are discussed and results presented, showing the versatility of the technique. The described applications show the wide capabilities in biosensing of the OFI sensor, showing it as an enabler of low-cost, all-optical, high accuracy biomedical applications.

This paper reports the application of the motion history image (MHI) method for biospeckle processing of a bacterial growth. The method avoids the complexity as well as the large computation in sequence-matching-based methods and detects whether the speckle structure has changed or not. Encouraging experimental results on the real-time evolution of the growing bacteria during 12 h demonstrate the effectiveness of the proposed method. The MHI presented an online result without loss of resolution and definition. In turn, the MHI also presented the ability to provide a close answer to the traditional offline method of generalized differences.

Optical feedback interferometry (OFI) is a compact sensing technique with recent implementation for flow measurements in microchannels. We propose implementing OFI for the analysis at the microscale of multiphase flows starting with the case of parallel flows of two immiscible fluids. The velocity profiles in each phase were measured and the interface location estimated for several operating conditions. To the authors knowledge, this sensing technique is applied here for the first time to multiphase flows. Theoretical profiles issued from a model based on the Couette viscous flow approximation reproduce fairly well the experimental results. The sensing system and the analysis presented here provide a new tool for studying more complex interactions between immiscible fluids (such as liquid droplets flowing in a microchannel).

In this paper, we show that laser speckle analysis (LSA) can provide valuable information about the structure of crumpled thin sheets. Crumpling and folding of slender objects are present in several phenomena and in various ranges of size, e.g., paper compaction, cortical folding in brains, DNA packing in viral capsids, and flower buds, to name a few. The analysis of laser speckles, both numerical and graphical, is a source of information about the activity of biological or non-biological materials, and the development of digital electronics, which brought the ease of image processing, has opened new perspectives for a spectrum of LSA applications. LSA is applied on randomly crumpled and one-, two-, and three-times folded papers, and appreciable differences in LSA parameters are observed. The methodology can be applied for easy-to-implement quantitative assessment of similar phenomena and samples.