The Application Of CO₂ Laser-Induced Fiber Deformation In Strain Sensing

Strain measurement is of great significance in practical engineering.There are large demands of accurate strain measurement in fields of structural health monitoring,artificial intelligence,and aerospace.Based on the advantages of high sensitivity,good multiplexing capability,small size,and anti-electromagnetic interference,optical fiber strain sensors have developed rapidly in recent years.This article analyzes and studies the effect of CO₂laser on the fiber.This effect mainly includes the effects of removing the surface material of the fiber,causing the fiber to melt and deform,and releasing the residual stress in the fiber.The first two effects are collectively referred to CO₂laser-induced fiber deformation.Utilizing this deformation effect,we have proposed and developed two fiber strain sensors,first of which is the bent structures Mach-Zehnder Interferometer（BS-MZI）.The BS-MZI consists of two identical bent structures,the bent structures are fabricated by releasing the pulse of the CO₂laser along the specific designed paths.One of which excites the cladding modes,and the other couples them back to the core.Experimental results show that the strain sensitivity of the proposed sensor is inversely proportional to the interference length L and proportional to the modulated length L_M.Based on this feature,both the sensitivity and the compactness can be improved by increasing L_Mand decreasing L.However,the decrease in interference length L induces the degradation of spectral quality,and the excessive increase in L_Mreduces the mechanical strength.Hence,a BS-MZI that has an appropriate L and L_Mof 8 mm and 0.8 mm,respectively,is selected for strain measurement.It exhibits an ultrahigh strain sensitivity of 165 pm/μεin the dynamic range of 0–200με.In the dynamic range of 30–130℃,it shows a low temperature-strain cross-sensitivity of 0.21με/℃.The other fiber strain sensor fabricated by the CO₂laser-induced fiber deformation is jagged long period fiber grating（J-LPFG）.J-LPFG is manufactured by performing offset etching on the opposite sides of the SMF so that the segment SMF is changed from a smooth cylindrical surface to a jagged shape.The damage of each zigzag structure modulation to the optical fiber is relatively small,and the etched area is evenly distributed on the upper and lower surfaces of the fiber.Therefore,the modulated fiber still has high mechanical strength.The experimental results show that the strain sensitivity and measurement range of J-LPFG are related to the number of periods.The J-LPFG with eight periods has the highest sensitivity and the largest measurement range.The strain sensitivity of this J-LPFG is 5.4 pm/μεin the measurable range of 0–7000με.As the periodic number increases,the strain sensitivity and measurable range of J-LPFG decreases.The fiber shaping method based on CO₂laser polishing is simple and flexible,and the spectrum of the completed manufacturing process can be monitored in real time,which is conducive to the large-scale production of fiber sensors.In addition,through structural design,it is possible to fabricate a sensing structure with ultra-high strain sensitivity or a sensing structure with an ultra-large strain measurement range.Therefore,CO₂laser-induced fiber deformation has a good prospect in the design and manufacture of strain sensors.