**3. Fabrication of POF grating devices**

POF FBG devices are normally obtained with direct writing technology [33], and phase mask technology [34]. Direct writing shown the advantage such as flexibility performance in terms of structure and wavelength, but the high cost femtosecond laser system is required for irradiation, and the resolution imposes limitations for the low wavelengths. LPG are normally obtained by heat imprinting [35], direct writing [36], and amplitude mask [37]. Here, we present several typical achievements obtained recently in the optimization of the fabrication process, grating devices inscription in a different kind of POF.

The fabrication process of POF grating devices has been reported by using different kind of lasers system such as 800 nm Ti:sapphire femto-second laser system [38], 532 nm Nd:YVO4 laser system [35], 387 nm Ti:sapphire femto-second laser system [39], 355 nm Nd:YAG laser system [40], 325 nm He-Cd laser [41], 266 nm Nd-YAG laser system [42] and 248 nm KrF excimer laser system [43]. Although 325 nm was the first irradiation wavelength for POF reported by the researchers in The University of New South Wales [44] and initially 248 nm and 266 nm wavelength were not considered suitable for POF grating writing due to high absorption of POF material at such wavelength, the first POF Bragg grating devices successful inscription less than one minutes with low flow and repetition rate 248 nm UV pulse opened a novel field of interest for POF grating irradiation [43]. Since then, the research about POF grating devices fabrication use 248 nm and 266 nm wavelength UV pulse has been continuously growing. A typical POF Bragg grating devices fabrication system is shown in **Figure 4**, where the pulse repetition rate and power can be optimized to modify the fabrication process, as shown in **Figure 4**.

Pure PMMA POF with low photosensitivity performance make it is difficlut to obtain strong POF grating device, doped POFs have attracted the researchers' interest to enhance photosensitivity performance. Researchers in The University of New South Wales employed a step-index multimode PMMA POF doped with organic dye in the core for grating devices irradiation with 325 nm wavelgnth UV

**33**

*Fabrication and Application of Polymer Optical Fiber Grating Devices*

under 325 nm wavelength UV beam irradiation [49].

face and the reflected spectrum shown in **Figure 5**.

beam. The same group investigated benzyl methacrylate doped step-index multimode POF which lead a − 28-dB transmission spectrum with 85 mins exposure [45]. Researchers in The Hong Kong Polytechnic University investigated a step-index PMMA POF doped with TS (1% w.t.) and diphenyl sulfide (DPS) (5% mole) in the core and a pure 150 μm diameter PMMA cladding, obtained the FBG spectrum with −10 dB in transmission after 10 mins [46]. The researcher at Aston University reported a highly photosensitive mPOF doped with BDK in the core, FBG with −23 dB in the transmission was achieved after 13 mins [47]. The researchers in Belgium improved the optical fiber drawing technology with selected center hole doped with BDK in mPOF, a rapidly growing process of FBG with 83% reflectivity with 40 s [48]. Recently, the researcher in Hongkong investigated a new dopant material, named as diphenyl disulphide, which enables a fast and positive refractive index change with a low ultraviolet dose and leads to FBG fabrication with 7 ms

The researchers optimized the fabrication process of FBGs in BDK doped POF with 248 nm and 266 nm wavelength, obtained strong grating with a single short pulse (15 and 8 ns of duration), the short time is even suitable for the fiber drawing process [42]. R. Min et al. also reported two, three, and five rings structure undoped PMMA POF fabricated with 266 nm Nd:YAG laser in the 850 nm region and using commercial ferrule connectors for POF connecting with silica fiber [50], the rend

POF special grating devices such as tilted FBG and Chirped FBG are under research due to attractive applications. The first tilted POF FBG was investigated by the researcher's in Belgium, use of TS-doped photosensitive step-index PMMA POF with scanning phase mask technique, the transmitted amplitude spectrum evolution of a 3° angle is analyzed for the surrounding refractive index varies [51]. The first chirped POF FBG was irradiated with a KrF excimer laser operating at 248 nm wavelength and a 25-mm long chirped phase mask which customized for telecomband FBG inscription in 2017 [52]. The laser pulse rate was 1 Hz and several shots were used for the grating response with a 1.2 nm/cm chirp and 3.9 nm bandwidth. The chirped phase mask technology offers good stability with the high cost and no flexibility as main drawbacks. Since then, different kind of techniques have been demonstrated for obtain chirped gratings in POF. The researchers in Cyprus used the femtosecond direct writing method for fabricate chirped FBG in commercial CYTOP POF [53], obtained 2000 periods with 10 nm bandwidth and a total length of around 4.5 mm. Femtosecond laser direct writing used for flexible chirped grating writing, although with the disadvantage as limit for low wavelengths. The first tunable chirped FBG was obtained with a tapered BDK doped mPOF by using a

*DOI: http://dx.doi.org/10.5772/intechopen.94351*

**Figure 4.**

*Typical fabrication setup for POF FBG.*

*Fabrication and Application of Polymer Optical Fiber Grating Devices DOI: http://dx.doi.org/10.5772/intechopen.94351*

**Figure 4.** *Typical fabrication setup for POF FBG.*

*Application of Optical Fiber in Engineering*

**Figure 3.**

*LPG transmission graph.*

Where Δ*n* =- shown the difference between the effective refractive indices of the fundamental mode in core and the *i* th mode in cladding. As can be seen from the relation of the resonance wavelength, which is determined by the effective refractive indices of the core and cladding modes, so that any photo-induced, thermal-induced, geometrical, or mechanically induced periodic change will modify the position of the resonance wavelength. Which makes LPFGs are useful

POF FBG devices are normally obtained with direct writing technology [33], and phase mask technology [34]. Direct writing shown the advantage such as flexibility performance in terms of structure and wavelength, but the high cost femtosecond laser system is required for irradiation, and the resolution imposes limitations for the low wavelengths. LPG are normally obtained by heat imprinting [35], direct writing [36], and amplitude mask [37]. Here, we present several typical achievements obtained recently in the optimization of the fabrication process,

The fabrication process of POF grating devices has been reported by using different kind of lasers system such as 800 nm Ti:sapphire femto-second laser system [38], 532 nm Nd:YVO4 laser system [35], 387 nm Ti:sapphire femto-second laser system [39], 355 nm Nd:YAG laser system [40], 325 nm He-Cd laser [41], 266 nm Nd-YAG laser system [42] and 248 nm KrF excimer laser system [43]. Although 325 nm was the first irradiation wavelength for POF reported by the researchers in The University of New South Wales [44] and initially 248 nm and 266 nm wavelength were not considered suitable for POF grating writing due to high absorption of POF material at such wavelength, the first POF Bragg grating devices successful inscription less than one minutes with low flow and repetition rate 248 nm UV pulse opened a novel field of interest for POF grating irradiation [43]. Since then, the research about POF grating devices fabrication use 248 nm and 266 nm wavelength UV pulse has been continuously growing. A typical POF Bragg grating devices fabrication system is shown in **Figure 4**, where the pulse repetition rate and power can be optimized to modify the fabrication process, as shown in **Figure 4**. Pure PMMA POF with low photosensitivity performance make it is difficlut to obtain strong POF grating device, doped POFs have attracted the researchers' interest to enhance photosensitivity performance. Researchers in The University of New South Wales employed a step-index multimode PMMA POF doped with organic dye in the core for grating devices irradiation with 325 nm wavelgnth UV

for the applications in fields as biological, chemical and optical sensing.

**3. Fabrication of POF grating devices**

grating devices inscription in a different kind of POF.

**32**

beam. The same group investigated benzyl methacrylate doped step-index multimode POF which lead a − 28-dB transmission spectrum with 85 mins exposure [45]. Researchers in The Hong Kong Polytechnic University investigated a step-index PMMA POF doped with TS (1% w.t.) and diphenyl sulfide (DPS) (5% mole) in the core and a pure 150 μm diameter PMMA cladding, obtained the FBG spectrum with −10 dB in transmission after 10 mins [46]. The researcher at Aston University reported a highly photosensitive mPOF doped with BDK in the core, FBG with −23 dB in the transmission was achieved after 13 mins [47]. The researchers in Belgium improved the optical fiber drawing technology with selected center hole doped with BDK in mPOF, a rapidly growing process of FBG with 83% reflectivity with 40 s [48]. Recently, the researcher in Hongkong investigated a new dopant material, named as diphenyl disulphide, which enables a fast and positive refractive index change with a low ultraviolet dose and leads to FBG fabrication with 7 ms under 325 nm wavelength UV beam irradiation [49].

The researchers optimized the fabrication process of FBGs in BDK doped POF with 248 nm and 266 nm wavelength, obtained strong grating with a single short pulse (15 and 8 ns of duration), the short time is even suitable for the fiber drawing process [42]. R. Min et al. also reported two, three, and five rings structure undoped PMMA POF fabricated with 266 nm Nd:YAG laser in the 850 nm region and using commercial ferrule connectors for POF connecting with silica fiber [50], the rend face and the reflected spectrum shown in **Figure 5**.

POF special grating devices such as tilted FBG and Chirped FBG are under research due to attractive applications. The first tilted POF FBG was investigated by the researcher's in Belgium, use of TS-doped photosensitive step-index PMMA POF with scanning phase mask technique, the transmitted amplitude spectrum evolution of a 3° angle is analyzed for the surrounding refractive index varies [51]. The first chirped POF FBG was irradiated with a KrF excimer laser operating at 248 nm wavelength and a 25-mm long chirped phase mask which customized for telecomband FBG inscription in 2017 [52]. The laser pulse rate was 1 Hz and several shots were used for the grating response with a 1.2 nm/cm chirp and 3.9 nm bandwidth. The chirped phase mask technology offers good stability with the high cost and no flexibility as main drawbacks. Since then, different kind of techniques have been demonstrated for obtain chirped gratings in POF. The researchers in Cyprus used the femtosecond direct writing method for fabricate chirped FBG in commercial CYTOP POF [53], obtained 2000 periods with 10 nm bandwidth and a total length of around 4.5 mm. Femtosecond laser direct writing used for flexible chirped grating writing, although with the disadvantage as limit for low wavelengths. The first tunable chirped FBG was obtained with a tapered BDK doped mPOF by using a

#### *Application of Optical Fiber in Engineering*

uniform phase mask under strain performance [54]. The spectral reflected power of a 10 mm bandwidth with a chirp of ~0.26 nm/mm under 1.6% strain, and the strain and temperature sensitivity obtained with 0.71 ± 0.02 pm/με and 56.7 pm/°C. Then, Chirped POFBGs have been also obtained by hot water-assisted gradient thermal annealing, where one grating device with around 1.1 nm/mm chirp performance was obtained as **Figure 6** shown [55]. The simplicity of this method is one of the main advantages since no special phase mask or additional etching is needed, and enables easy control of the chirp characteristics and the central wavelength.

Finally, regarding LPG in POF, the extensive literatures with different methods and mechanisms appeared in the last years. Recently, the researcher in Spain [56]

**Figure 5.** *The end face and the reflected spectrum of three types mPOF.*

**Figure 6.**

*The spectrum and the bandwidth varies of the chirped POF FBG fabrication with strong gradient annealing.*

**35**

**Figure 7.**

*Schematic of the bench test rig [58].*

*Fabrication and Application of Polymer Optical Fiber Grating Devices*

demonstrated a − 20-dB transmission LPG in mPOF with point by point method use a slit width of 0.2 mm; the beam was shifted 1 mm for inscribing each point and 25 steps were implanted. And the researchers in Cyprus reported an LPG in a CYTOP POF using a femtosecond laser inscription method. The LPG was inscribed directly in the center of the optical fiber core, tailored for operation at 1560 nm, which was characterized in transmission, and the response for relative humidity

POF FBG already goes to industry measurement such as used for water content

POF FBG can be also used to monitor the strain of human arteries with pulse wave signals. A variety of different vital signs including blood pressure can be derived from the signals, which show a higher signal to noise than silica FBG [59],

POF FBG can be used as health equipment for dynamic monitoring of gait. Five FBGs inscribed in CYTOP POF was embedded in a cork insole, as shown in **Figure 9**. The advantages of POF such as higher flexibility and robustness enabled monitoring patients with higher body mass, compared the results obtained with similar systems based on silica fiber, a mean sensitivity of ~8.14 PM/kPa was obtained, which is

Consider the special POF grating applications, due to polymer special characteristics, strain sensing is the most attractive and reliable applications. There is a lot of literature reported POF FBG for strain sensing [61–63]. However, strain sensing under variable humidity and temperature conditions is always an issue for POF sensing technology go to real applications. The researchers in Spain demonstrated

detection in aviation fuel as shown in the **Figure 7**, the water content in Jet-A1 was measured by using POF FBG sensing technology which calibrated with both coulometric titration and environmental chamber. The results indicate a better

*DOI: http://dx.doi.org/10.5772/intechopen.94351*

and temperature was also measured [57].

**4. Applications of POF grating devices**

performance compare with coulometric titration [58].

the experimental measurement as **Figure 8** shown.

much higher compared with FBGs in silica optical fiber [60].

*Fabrication and Application of Polymer Optical Fiber Grating Devices DOI: http://dx.doi.org/10.5772/intechopen.94351*

demonstrated a − 20-dB transmission LPG in mPOF with point by point method use a slit width of 0.2 mm; the beam was shifted 1 mm for inscribing each point and 25 steps were implanted. And the researchers in Cyprus reported an LPG in a CYTOP POF using a femtosecond laser inscription method. The LPG was inscribed directly in the center of the optical fiber core, tailored for operation at 1560 nm, which was characterized in transmission, and the response for relative humidity and temperature was also measured [57].
