Fig. 1. Strain-based target flow sensor structures with different structures. (a) Target FBG flow sensor for measuring rocket fuel flow in space^[30]; (b) FBG-based dual parametric sensors^[31]; (c) FBG sensors for multi-parameter monitoring^[32]; (d) CLFBG-based low-flow rate flowmeter^[33]

Fig. 2. Strain-based differential pressure FBG flow sensors with different structures. (a) Differential pressure FBG flow sensor physical drawing^［35］; (b) structure of high-precision differential pressure type FBG flow sensor^［36］

Fig. 3. Turbine type FBG flow sensor. (a) Cutaway view of FBG turbine flow rate/temperature sensor^[37]; (b) structure of the modified flowmeter^[38]

Fig. 4. Strain-based FBG Flow Sensors for other structures. (a) Probe-type FBG flow sensors for industrial monitoring^[39]; (b) flow temperature combined monitoring probe type FBG flow sensor^[41]; (c) fixed beam based FBG flow sensor^[42]

Fig. 5. Thermal FBG flow sensors with different structures based on temperature. (a) Fibre optic heated flow sensors^[45]; (b) schematic of temperature-based FBG flow sensor^[47]; (c) thermal flow sensors with temperature compensation^[49]; (d) FBG flow sensors for liquid measurement^[50]; (e) thermal FBG flow sensor^[51]