The measurement of pressure by using distributed opticalﬁber sensors has represented a challenge for many years. While single-point opticalﬁber pressure sensors have reached a solid level of technology maturity, showing to be very good candidates in replacing conventional electrical sensors due to their numerous advantages, distributed sensors are still a matter of an intense research activity aimed at determining the most proper and robust pressure-sensitivity enhancement mechanism. This paper reviews early and recent works on distributed pressure sensors, classifying the sensors according to the sensing mechanism. For each type of mechanism, the issues and potentials are analyzed and discussed. Keywords:opticalﬁber sensor, distributed opticalﬁber sensor, pressure measurement, distributed opticalﬁber pressure sensor 1. Introduction The measurement of pressure has always been pivotal in many diﬀerent applications [1, 2, 3], often characterized by the needs of high accuracy, robustness, and dense spa- tial coverage. As a result, many types of sensor technology 5 have been developed to address the needs of these appli- cations. Among the many, conventional electrical sensors such as load cells, piezometers, and capacitive sensors are the most popular, especially for pressure sensing in liquid. However, these electrical sensors are unsuitable for long- 10 range monitoring, as they are sensitive to electromagnetic interference (EMI), and cannot be easily multiplexed in a large sensor network. Single-pointﬁber optic sensor technologies have been proposed as solutions for pressure sensing for many years. 15 At present, some single-pointﬁber pressure sensors, e.g., Fiber Bragg gratings (FBGs) [4, 5, 6, 7] and interferometric- based ones [8, 9], have already been released as commercial products due to the many features, such as minimal in- vasiveness, remote powering and interrogation, multiplex 20 capability, and roughness. In particular, they can also work in high-temperature environments [10, 11], also due to the technology improvement in FBG writing [12, 13, 14]. Nonetheless, there are still applications for which the spatial resolution attainable with single-pointﬁber 25 optic sensors is not enough, such as in the geo-hydrological sector [15] and oil&gas industry [16]. Indeed, single-point ﬁber sensors, even when densely multiplexed into quasi- ∗Corresponding author ∗∗Co-corresponding author Email addresses:luca.schenato@cnr.it(Luca Schenato), andrea.galtarossa@unipd.it(Andrea Galtarossa)distributedﬁber sensors, cannot provide the wealth of in- formation that may come from distributed opticalﬁber 30 sensors (DFOSs). Although the pressure has been one of theﬁrst physi- calﬁeld addressed by the researchers’ eﬀorts towards the development of speciﬁc distributed opticalﬁber sensors, to date, these have not yet delivered the required reliability 35 for the commercialization. Nowadays, there is an increased and renovated interest in the development of distributed pressure sensors (DPSs), boosted by the potential appli- cations and by the growing availability and reliability of high resolution distributed strain sensing schemes. 40 This review paper aims at discussing the key concepts of distributedﬁber optic pressure sensors and reviews the more recent results. The diﬀerent approaches adopted so far to enhance the pressure sensitivity are outlined, empha- sizing the sensing mechanism. Each solution is compared 45 in terms of pressure sensitivity, resolution, and, when pos- sible, temperature cross-talk. Finally, open issues and future perspectives are analyzed. Guide to the reading The response of an opticalﬁber to a pressureﬁeld 50 can be divide into three diﬀerent regimes depending on the frequency of the applied pressure [17], as shown in Fig. 1. For frequencies below about 10 kHz, we have the hydrostaticregime, in which both axial and radial strains contribute to the pressure sensitivity of theﬁber. Above 55 this threshold and up to about 1 MHz, the pressure has only a radial eﬀect, and theﬁber length is almost constant. Finally, for higher frequencies, the wavelength of the pres- sureﬁeld becomes comparable to theﬁber diameter, and the induced strain depends on the direction of propaga- 60 tion of the pressure wave. As a result, the induces stress Preprint submitted to Optical Fiber Technology July 14, 2020 103104105106107ISOTROPIC FIBER ANISOTROPIC FIBERHYDROSTATIC AXIALLY CONSTRAINED Frequency (Hz)Acoustic Pressure Sensitivity ≈ Figure 1: Pressure response of a single modeﬁber at diﬀerent frequency regimes. Data from [17]. ﬁeld is asymmetric, and theﬁber becomes anisotropic and birefringent. In this work, we will focus on the hydrostatic regime. Unfortunately, uncoated opticalﬁbers are almost in- 65 sensitive to hydrostatic pressure [18, 19] and, therefore, its direct measurement by merely exposing theﬁber to the pressureﬁeld cannot achieve high sensitivity. Thus, over the years, the research has focused on developing trans- ducing mechanisms that can eﬀectively convert pressure 70 into another physical parameter–typically strain or bire- fringence. This approach, originally conceived for single- point or integral opticalﬁber sensors, is nowadays adopted also for distributed opticalﬁber sensors. Among the dif- ferent standpoints that can be chosen to explore DPSs, 75 we have opted to classify them on the basis of the sensing mechanisms. A simpler classiﬁcation could be based on the acquisition technique, but this would not be informative given that what impacts the performance of the sensor is the transducing mechanism, rather than the interrogation 80 technique. Therefore, we can conveniently distinguish the following three main categories: �measurement of pressure-induced strain in compliant coated opticalﬁbers and engineered cables; �measurement of pressure-induced birefringence in spe- 85 cial opticalﬁbers; �measurement of pressure-induced losses in special opticalﬁbers; while keeping the classiﬁcation based on the acquisition technique, within theﬁrst two categories. 90 In the following, each category will be extensively de- scribed and discussed; a great eﬀort has been spent toinclude as many signiﬁcant