Contents I. Introduction
Laser based sensors are widely used in surface scanning and profilometry [1] –[3], particle detection [4] , vehicular monitoring [5], navigation [6], [7], security based applications [8], [9] and infrastructure monitoring [10]. Laser based vibration sensors have atomic level resolution and are integral aspects of precision instrumentation and high fidelity manufacturing [10]. Use of lasers in physical monitoring of structures like bridges have also been studied [11], [12]. In recent years, as the role of wireless sensor network in real time health monitoring of existing assets and infrastructure is increasing [13] –[15], the prospective role of laser based sensors in these avenues become even more important. Currently, Micro-Electro-Mechanical Systems (MEMS) based sensors comprising a control unit and a transmitter form the key elements of smart sensor systems for environmental and civil infrastructure monitoring [16]. An issue of special interest is use of MEMS sensors for monitoring railway tracks [17]– [19]. Such a technology has also been used exclusively for obstacle detection on a railway track which is of special interest for tracks in distant and mountainous region which are prone to accidents cause by events like landslide and rock fall under extreme weather [20] . An important challenge with this technology, is the requirement of physical contact of the sensors with the target. It increases the level of acoustic noise contributed by different sources which can become difficult to filter out leading to high false alarm. Other challenges are obtaining high rate of data and maintaining high fidelity [21]. These challenges have posed fundamental constraints on large scale deployment of MEMS based sensors for environmental monitoring.
