Contents I. Introduction
Fiber laser sensors (FLSs) have attracted extensive research attentions due to their light weight, compact size, high signal-to-noise ratio, narrow linewidth and being immune to electromagnetic interference. They have been successfully used in different sensing applications to measure many physical parameters, such as temperature [1], [2], strain [2]–[4], pressure [5]–[8], acoustic wave [9]–[11] and etc. Especially in some fields, such as acoustic detection, where the sensitivity is the primary consideration, the FLS will be a preferred choice due to its high sensitivity. Moreover, the multiplexing capability of FLS is also an attractive advantage, which makes large-scale measurements possible in many areas, such as aerospace engineering systems, civil engineering infrastructure, tunnel safety monitoring. Conventionally, tens of FLSs can be multiplexed and simultaneously interrogated by wavelength division multiplexing [12] , [13], time division multiplexing [14], spatial/wavelength division multiplexing (WDM) [15]. However, the cross coupling and laser intensity instability limit the maximum number of multiplexed FLSs. For solving these issues, some complex Fiber Bragg grating (FBG) apodization technologies are proposed. By apodization profile to the grating phase and amplitude, the external back reflections are suppressed [16] –[19]. Even so, due to the limit of pump power budget and optical bandwidth, the reported maximum number of FLS array is still 16 [13]. Although the FLSs have attracted extensive attention for acoustic detection due to their higher sensitivity, the complex apodization technology and the limited multiplexing number hinder the promotion of FLSs. In most application fields, practical advantages such as reduction in system complexity, powerful multiplexing capability and ease of fabrication have become the main drivers, and a sensor system with simpler laser structure and the ability to multiplex more sensors is expected.
