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.

An analysis of speckle effects and the techniques to overcome them in self-mixing interferometry signals is presented. We characterize the effect of surface roughness and laser spot size on the speckle modulation of the signal, and then propose two simple experimental approaches to overcome the amplitude fading induced by the speckle effect. Unlike the techniques proposed until now, our first approach uses an adaptive optical element in the form of a voltage-programmable liquid lens, which adaptively changes its focal length to modify the speckle pattern. Our second approach combines two laser signals which present different performance parameters. By using any of these simple methods, the introduction of inaccuracies in the measurement process due to speckle is avoided.

We propose differential optical feedback interferometry, a technique able to measure nanometer-size amplitude displacements by comparing the optical power of two lasers subject to optical feedback. In this letter, the principles of the technique are explained in detail, and its limits are explored by simulation. Theoretical results are presented showing that the technique can measure nanometer scale displacements with resolution within the angstrom scale. An experimental setup for validation has been built, and a series of experimental tests were performed using a capacitive sensor as a reference. Results show good agreement between theory and experiment with a reasonable reduction in performance due to mechanical coupling and signal noise. The proposed technique, thus, provides measurements of a very high resolution using an extremely simple and robust experimental setup.

In this paper we present a technique based on self-mixing interferometry as a method for the acquisition and reconstruction of the arterial pulse wave. A modification of the classic fringe counting reconstruction algorithm is proposed to deal with some of the problems caused by biological tissue surface roughness, therefore allowing a reconstruction of the arterial displacement with a resolution of 400 nm. The traits of the arterial pulse wave have been retrieved with high detail, allowing their interpretation by a skilled practitioner. The heart beat measurements show a good agreement when compared to the readings of a commercial pulse-meter, therefore proving the versatility and the viability of the technique for the measurement of other cardiovascular signals.Postprint (published version

An analysis of speckle effects in self-mixing interferometry signals has been performed. We will characterize the effect of surface roughness and laser spot size on the speckle modulation of the signal, and we will propose two simple experimental approaches to overcome the amplitude fading induced by speckle effect. Differently to the techniques proposed up to the moment, our first approach uses an adaptive optical element in the form of a voltage programmable liquid lens, which adaptively changes its focal length to modify the speckle pattern. Our second approach combines two laser signals which present different performance parameters. By using any of these simple methods, the introduction of inaccuracies in the measurement process due to speckle is avoided.