Adaptive image normalisation based on block processing for enhancement of fingerprint image - Electronics Letters

To measure temperature, the device was placed on top of a heating element of a thermoelectric heat controller. The arrangement was enclosed in a plastic box to maintain a constant temperature across the length of the device. Again, both ends of the fibre were fixed with a tape to maintain a constant strain on the device. Temperature around the device was varied from 22 to 102°C and the corresponding spectral changes were captured using the OSA. When the cascaded-LPG device was tested against temperature, a different behaviour emerged, with peaks ‘f’ and ‘g’ of the cascaded LPG device red-shifting with increasing temperature. Typically, peak ‘f‘ of LPGl shifts 19.4 nm whilst peak ‘g’ of LPG2 shifts 5 nm for a temperature rise from 22 to lOo”C, as shown in Fig. 2b. The temperature cross-sensitivity that could affect the SRI measurement using LPG2, therefore, can be compensated using the information extracted from LPG1. We have evaluated the temperature sensitivity of peak ‘f‘ of the index-insensitive LPGl of the cascaded device, the value obtained is AALPG,,,/AT= 194 pm/”C, which is 16 times greater than that of the index-insensitive component of the SFBG we reported earlier [5]. We also note that since peak ‘f’ of LPGl is insensitive to SRI and it is in the 1500 nm region, by embedding the grating in a relevant material its temperature sensitivity can be reduced, paving the way for the application of such types of grating for gain-flattening applications.