**4. Applications**

For an LPG to function at its optimum sensitivity when exposed to an external perturbation, its period should be chosen such that it is able to operate at a turning point. Optical LPGs operating at the turning point provide the potential for low cost sensors with fast response time [21, 55, 56] and can provide a simpler detection method as some are able to work as intensity-based sensors [15, 17, 55].

LPGs operating at the PMTP have been used for temperature, strain, refractive index sensing [11, 35] and as filters. PMTP LPGs, when modified with a functional film can be adapted for potential uses as enhanced gas and chemical sensors [28].

### **4.1 Filters**

By employing the broadband characteristics, PMTP LPGs can make successful bandpass and rejection filters [57, 58]. A coated PMTP LPG with a π phase shift is simulated to provide tuneable broadband characteristics for rejection filtering applications [58]. By introducing multiple π phase shifts, it is possible to adjust the separation between the dual resonant bands. On the other hand, by partially coating a phase shifted PMTP LPG, bandgaps appear over a narrow wavelength band which could be useful for designing spectral filters [59].

### **4.2 Temperature sensing**

By careful choice of the grating period to allow coupling close to or at the turning point, it is possible to improve the temperature sensitivity of an LPG [14, 15, 33, 60]. Shu et al. [14] showed that for an LPG with a 175 μm period, the dual resonance band had a temperature sensitivity of 3.2 nm/°C whereas the single band away from the PMTP had a much lower sensitivity of -0.31 nm/°C. The response of the dual resonant bands to changing temperature is non-linear, with a reduction in the rate of separation as the resonance moves away from the phase matching turning point. A band operating away from the turning point has a linear response to changing temperatures [60], which may make it easier to characterise temperature sensitivity. When comparing the sensitivities of LPGs with periods of 110.8, 111 and 111.5 μm the highest sensitivity was seen when the LPG was chosen to operate near the PMTP (111.5 μm period), just as the single broad resonance band would begin to appear. A sensitivity of 0.99 nm/°C was achieved for the sensor at turning point, which was more than five times greater when compared to the sensitivity of a band away from turning point (0.17 nm/°C). As the temperature response changes depending on the surrounding environment, it may also be possible for the thermo-optic coefficients of a surrounding medium to be characterised [60].

### **4.3 Strain sensing**

Previous studies have shown that the dual resonance bands will move together, when increasing strain is applied, with a near linear trend [14]. The separation of the dual bands was calculated to be -33.6 nm/1000 μm whereas the sensitivity of an LPG can be more than an order of magnitude less [16]. Using the single broad band resonant mode at PMTP, Grubsky et al. [15] were able to obtain a sensor resolution of 1 μm by changing the coupling strength using different strengths of strain; the band would show an appreciable decrease in amplitude as strain increased, whilst the wavelength remained fixed as shown in **Figure 7**. The fixed wavelength allows for a simpler detection method as spectrometers or post processing can be bypassed for a simple photodetector [15].

### **Figure 7.**

*Transmission spectrum of a 50.1 μm period LPG with increasing strain. The wavelength of the band remains fixed at 1420 nm but the coupling efficiency decreases. Reprinted with permission from Ref. [15], OSA.*

**125**

**Figure 8.**

*Optical Fibre Long-Period Grating Sensors Operating at and around the Phase Matching…*

The optical intensities of the guided cladding modes will likely dissipate out of the fibre after a short distance. This could be due the fibre being bent, of from scattering or absorption due to the protective jacket of the fibre. By removing the jacket, the cladding mode evanescent field extends into the immediate surroundings, which will influence the fibre mode properties. The refractive index of the local environment will affect the effective refractive indices of the cladding modes propagating in the fibre. These effective refractive indices determine the sensitivity

A refractive index PMTP LPG sensor based on intensity as opposed to wavelength shift provides high sensitivity with a linear response for different refractive indices. A linear correlation coefficient, of more than 0.98 and sensitivity of 59.88/ RIU for the refractive index range of 1.410–1.420 was achieved using a PMTP LPG with a period of 231.5 μm [17]. This allows for simple calibration and linear interpolation to determine the sensitivity of the sensor within this refractive index range. By coating a PMTP LPG, such that it coincides with the mode transition region, it can be possible to enhance the refractive index sensitivity of a sensor [18, 31]. Mode transition describes the reorganisation of cladding modes caused when a higher refractive index material of a certain optical thickness surrounds the LPG [61–63]. After a certain thickness, the surrounding material is able to guide the outer most cladding mode, causing large shifts in the resonance wavelengths. The sensitivity of the LPG can also be optimised by controlling the optical thickness of the overlay so that the turning point coincides with the mode transition region, and has been proven theoretically and experimentally [28, 64]. Pilla et al. [18] were able to achieve a sensitivity exceeding 9000 nm/RIU in solutions with RIs similar to water, using an LPG with a single resonance band close to 1.55 μm. By increasing the refractive index, the dual bands appear and eventually split as shown in **Figure 8**. A mesoporous coating consisting of silica nanospheres was able to improve the refractive index sensitivity of a 100 μm period LPG operating near turning point, with a maximum sensitivity of 1927 ± 59 nm/RIU, as well as increase the detection range of the LPG [19]. The refractive index sensitivity of the first electric arc induced LPG at turning point was increased from 400 to 700 nm/RIU to 887–2146 nm/RIU

*Transmission spectra showing the response to different refractive indices of ethanol solutions. SRI is surrounding* 

*refractive index. Reprinted with permission from Ref. [18], OSA.*

*DOI: http://dx.doi.org/10.5772/intechopen.81179*

**4.4 Refractive index sensing**

of the PMTP LPG.

*Optical Fibre Long-Period Grating Sensors Operating at and around the Phase Matching… DOI: http://dx.doi.org/10.5772/intechopen.81179*

### **4.4 Refractive index sensing**

*Applications of Optical Fibers for Sensing*

surrounding medium to be characterised [60].

By careful choice of the grating period to allow coupling close to or at the turn-


Previous studies have shown that the dual resonance bands will move together, when increasing strain is applied, with a near linear trend [14]. The separation of the dual bands was calculated to be -33.6 nm/1000 μm whereas the sensitivity of an LPG can be more than an order of magnitude less [16]. Using the single broad band resonant mode at PMTP, Grubsky et al. [15] were able to obtain a sensor resolution of 1 μm by changing the coupling strength using different strengths of strain; the band would show an appreciable decrease in amplitude as strain increased, whilst the wavelength remained fixed as shown in **Figure 7**. The fixed wavelength allows for a simpler detection method as spectrometers or post processing can be bypassed

*Transmission spectrum of a 50.1 μm period LPG with increasing strain. The wavelength of the band remains fixed at 1420 nm but the coupling efficiency decreases. Reprinted with permission from Ref. [15], OSA.*

ing point, it is possible to improve the temperature sensitivity of an LPG [14, 15, 33, 60]. Shu et al. [14] showed that for an LPG with a 175 μm period, the dual resonance band had a temperature sensitivity of 3.2 nm/°C whereas

the single band away from the PMTP had a much lower sensitivity of

**4.2 Temperature sensing**

**4.3 Strain sensing**

for a simple photodetector [15].

**124**

**Figure 7.**

The optical intensities of the guided cladding modes will likely dissipate out of the fibre after a short distance. This could be due the fibre being bent, of from scattering or absorption due to the protective jacket of the fibre. By removing the jacket, the cladding mode evanescent field extends into the immediate surroundings, which will influence the fibre mode properties. The refractive index of the local environment will affect the effective refractive indices of the cladding modes propagating in the fibre. These effective refractive indices determine the sensitivity of the PMTP LPG.

A refractive index PMTP LPG sensor based on intensity as opposed to wavelength shift provides high sensitivity with a linear response for different refractive indices. A linear correlation coefficient, of more than 0.98 and sensitivity of 59.88/ RIU for the refractive index range of 1.410–1.420 was achieved using a PMTP LPG with a period of 231.5 μm [17]. This allows for simple calibration and linear interpolation to determine the sensitivity of the sensor within this refractive index range.

By coating a PMTP LPG, such that it coincides with the mode transition region, it can be possible to enhance the refractive index sensitivity of a sensor [18, 31]. Mode transition describes the reorganisation of cladding modes caused when a higher refractive index material of a certain optical thickness surrounds the LPG [61–63]. After a certain thickness, the surrounding material is able to guide the outer most cladding mode, causing large shifts in the resonance wavelengths. The sensitivity of the LPG can also be optimised by controlling the optical thickness of the overlay so that the turning point coincides with the mode transition region, and has been proven theoretically and experimentally [28, 64]. Pilla et al. [18] were able to achieve a sensitivity exceeding 9000 nm/RIU in solutions with RIs similar to water, using an LPG with a single resonance band close to 1.55 μm. By increasing the refractive index, the dual bands appear and eventually split as shown in **Figure 8**. A mesoporous coating consisting of silica nanospheres was able to improve the refractive index sensitivity of a 100 μm period LPG operating near turning point, with a maximum sensitivity of 1927 ± 59 nm/RIU, as well as increase the detection range of the LPG [19]. The refractive index sensitivity of the first electric arc induced LPG at turning point was increased from 400 to 700 nm/RIU to 887–2146 nm/RIU

### **Figure 8.**

*Transmission spectra showing the response to different refractive indices of ethanol solutions. SRI is surrounding refractive index. Reprinted with permission from Ref. [18], OSA.*

by operating close to the turning point and by coating a thin film overlay of silicon nitride [20]. By combining these phenomena with a reduced diameter fibre, it can be possible to further enhance the sensitivity of the LPG, which can help improve the resolution of biochemical sensing applications.

### **4.5 Chemical and gas sensing**

Their small dimensions, suitability in harsh environments and versatility make fibres ideal sensing platforms. The high sensitivity property of the PMTP allows for detection of low quantities and concentrations of different chemicals. For example, a PMTP is able to detect a 0.01% aqueous solution of cane sugar [65].

Optical fibre sensors with functional thin film coatings have become of interest due to the large pool of possible applications, especially in the chemical and biosensing fields. These sensors have the potential to measure concentrations of chemicals or for detecting gaseous species. These thin films can improve the sensing ability of the fibre and allow them to have different responses to different stimuli, such as concentrations of chemicals in the surrounding environment [19, 66, 67]. Functional materials can also be used to enhance the sensitivity for detection of a particular analyte. The thickness of the film on the LPG sensor is usually in the region of a few 100 nm as the transmission spectrum can be greatly affected [28, 61, 68, 69].

The following techniques allow for nanoscale thickness deposition control of the coating. These include the Langmuir–Blodgett deposition [28, 59, 70], self-assembly [21, 55, 71], layer-by-layer deposition [19], atomic layer deposition [72], sol-gel [73] and liquid phase deposition [67].

Functionalised LPGs operating around the PMTP have been tested for volatile organic compound (VOC) detection [70, 74, 75]. VOCs can be generated from a variety of processes. These include, but are not limited to, fuels and combustion processes, petroleum products, paints, and in nature and farming [76]. PMTP LPGs have been used for toluene [70, 74] and benzene [74] detection. By applying a functional overlay, particular compounds will affect the refractive index of the overlay and therefore influence the fibre modes, which will be shown as changes in transmission spectrum [74]. Providing clean water is an integral part of life, therefore monitoring water quality is critical. Partridge et al. demonstrated a proof-of-concept sensor for detecting toluene contamination in water. A 97 μm period LPG at turning point coated with calix [4]res C11 and was shown to be specific to toluene when compared to another potential contaminant, ethanol as shown in **Figure 9**(**a**). The sensor was able to achieve a minimum detection limit of 100 ppm (see **Figure 9**(**b**)) which is the approximate limit of oil weep sampling and leaking oil plumes [70]. Some gases, such as hydrogen, are odourless and colourless, and have a low ignition energy. Means of detecting leaks in small quantities are therefore an important safety tool. A sensor coated with a 70 nm thick palladium overlay, when exposed to 4% hydrogen, experienced a dual band wavelength shift apart of 7.5 nm [77]. A thin film PMTP LPG with a functional material of poly(acrylic acid) PAA was successfully used to selectively bind to ammonia with lower detection levels when compared to other devices such as colorimetric and absorption spectroscopic devices [71].

Optical sensors have become more popular and valuable in the biomedical field. They have the potential to be used for diagnosis and monitoring and can be cost effective, portable and easy to use. This has also contributed to the increase in interest in label-free sensing using LPGs, especially at the PMTP where there is high RI sensitivity, rapid response and adaptability by choice of overlay [18, 72, 78, 79]. LPGs at PMTP have been used for real time monitoring of phage-bacteria interactions [80, 81] where a 1.3 nm wavelength shift was detected as bacteria binding

**127**

*Optical Fibre Long-Period Grating Sensors Operating at and around the Phase Matching…*

occurred [80], and target-probe DNA hybridisation [82, 83]. The well-known properties of the streptavidin-biotin interaction, as well as its use for studying biological processes, have encouraged its use as a means of understanding the characteristics of thin film PMTP sensors [21, 55]. Korposh et al. showed a mesoporous SiO2 film coated sensor with an additional functional material could detect a specific chemical species. In this case the species was a porphyrin compound, with a 10 μM

*(a) Plots showing the response of a calix [4]red C11 coated LPG sensor to toluene and ethanol concentrations. Exposure to ethanol shows negligible response compared to toluene. (b) Transmission spectra showing the response of the dual resonance bands of a calix [4]red C11 coated LPG to different concentrations of toluene. Partridge et al.,. Reprinted from [70]; originally published under CC BY 3.0 licence. Available from: 10.1016/j.*

Selectivity is an important indicator for a sensor as it can potentially prevent false readings, which is especially helpful at the highly sensitive turning point region [66, 67, 84]. Molecular imprinting provides a versatile platform as the properties of the receptor can be modified to detect a desired molecular compound [66, 67]. An LPG coated with a molecularly imprinted polymer (MIP) was prepared, for the detection of antibiotics [66]. In the presence of different commonly prescribed antibiotics, the sensor showed selectivity to the target antibiotic vancomycin. The target compound can also be removed and the sensor reused. Removal methods used include organic solvents, and photodecomposition have also been investigated [67]. Reusing an LPG or sensor also increases the versatility of a biosensor [81, 82, 84] and can therefore be

Choosing a particular fibre type can also contribute to the final characteristics of the fabricated sensor. For instance, PMTP LPGs written in boron co-doped fibres have been demonstrated as radiation dose sensors [36], pressure sensors [34] (boron co-doping can increase the pressure-optic coefficient of a material [85]),

The inherent high sensitivity of the PMTP LPG also leads to its limitations. For instance, cross-talk or unwanted interference, such as from varying temperature, have to be limited in order to ensure the shift in the wavelengths is only due to the desired parameter. By fabricating cascaded PMTP LPGs, based on a Mach-Zehnder interferometer, it is possible to eliminate interference [86] and make simultaneous measurements for parameter compensated sensing [87]. James et al. demonstrated that coating a cascaded PMTP LPG device with mesoporous silica nanoparticles, and subsequently infusing a functional material to the central of the region (between the two gratings) enables measuring of only the desired analyte [86].

*DOI: http://dx.doi.org/10.5772/intechopen.81179*

concentration being detected in under 10s [56].

more time and cost effective.

**Figure 9.**

*snb.2014.06.121*

**4.6 Sensor limitations**

and for fuel adulteration detection [39].

*Optical Fibre Long-Period Grating Sensors Operating at and around the Phase Matching… DOI: http://dx.doi.org/10.5772/intechopen.81179*

**Figure 9.**

*Applications of Optical Fibers for Sensing*

**4.5 Chemical and gas sensing**

and liquid phase deposition [67].

colorimetric and absorption spectroscopic devices [71].

Optical sensors have become more popular and valuable in the biomedical field. They have the potential to be used for diagnosis and monitoring and can be cost effective, portable and easy to use. This has also contributed to the increase in interest in label-free sensing using LPGs, especially at the PMTP where there is high RI sensitivity, rapid response and adaptability by choice of overlay [18, 72, 78, 79]. LPGs at PMTP have been used for real time monitoring of phage-bacteria interactions [80, 81] where a 1.3 nm wavelength shift was detected as bacteria binding

the resolution of biochemical sensing applications.

by operating close to the turning point and by coating a thin film overlay of silicon nitride [20]. By combining these phenomena with a reduced diameter fibre, it can be possible to further enhance the sensitivity of the LPG, which can help improve

Their small dimensions, suitability in harsh environments and versatility make fibres ideal sensing platforms. The high sensitivity property of the PMTP allows for detection of low quantities and concentrations of different chemicals. For example,

Optical fibre sensors with functional thin film coatings have become of interest due to the large pool of possible applications, especially in the chemical and biosensing fields. These sensors have the potential to measure concentrations of chemicals or for detecting gaseous species. These thin films can improve the sensing ability of the fibre and allow them to have different responses to different stimuli, such as concentrations of chemicals in the surrounding environment [19, 66, 67]. Functional materials can also be used to enhance the sensitivity for detection of a particular analyte. The thickness of the film on the LPG sensor is usually in the region of a few 100 nm as the transmission spectrum can be greatly affected [28, 61, 68, 69].

The following techniques allow for nanoscale thickness deposition control of the coating. These include the Langmuir–Blodgett deposition [28, 59, 70], self-assembly [21, 55, 71], layer-by-layer deposition [19], atomic layer deposition [72], sol-gel [73]

Functionalised LPGs operating around the PMTP have been tested for volatile organic compound (VOC) detection [70, 74, 75]. VOCs can be generated from a variety of processes. These include, but are not limited to, fuels and combustion processes, petroleum products, paints, and in nature and farming [76]. PMTP LPGs have been used for toluene [70, 74] and benzene [74] detection. By applying a functional overlay, particular compounds will affect the refractive index of the overlay and therefore influence the fibre modes, which will be shown as changes in transmission spectrum [74]. Providing clean water is an integral part of life, therefore monitoring water quality is critical. Partridge et al. demonstrated a proof-of-concept sensor for detecting toluene contamination in water. A 97 μm period LPG at turning point coated with calix [4]res C11 and was shown to be specific to toluene when compared to another potential contaminant, ethanol as shown in **Figure 9**(**a**). The sensor was able to achieve a minimum detection limit of 100 ppm (see **Figure 9**(**b**)) which is the approximate limit of oil weep sampling and leaking oil plumes [70]. Some gases, such as hydrogen, are odourless and colourless, and have a low ignition energy. Means of detecting leaks in small quantities are therefore an important safety tool. A sensor coated with a 70 nm thick palladium overlay, when exposed to 4% hydrogen, experienced a dual band wavelength shift apart of 7.5 nm [77]. A thin film PMTP LPG with a functional material of poly(acrylic acid) PAA was successfully used to selectively bind to ammonia with lower detection levels when compared to other devices such as

a PMTP is able to detect a 0.01% aqueous solution of cane sugar [65].

**126**

*(a) Plots showing the response of a calix [4]red C11 coated LPG sensor to toluene and ethanol concentrations. Exposure to ethanol shows negligible response compared to toluene. (b) Transmission spectra showing the response of the dual resonance bands of a calix [4]red C11 coated LPG to different concentrations of toluene. Partridge et al.,. Reprinted from [70]; originally published under CC BY 3.0 licence. Available from: 10.1016/j. snb.2014.06.121*

occurred [80], and target-probe DNA hybridisation [82, 83]. The well-known properties of the streptavidin-biotin interaction, as well as its use for studying biological processes, have encouraged its use as a means of understanding the characteristics of thin film PMTP sensors [21, 55]. Korposh et al. showed a mesoporous SiO2 film coated sensor with an additional functional material could detect a specific chemical species. In this case the species was a porphyrin compound, with a 10 μM concentration being detected in under 10s [56].

Selectivity is an important indicator for a sensor as it can potentially prevent false readings, which is especially helpful at the highly sensitive turning point region [66, 67, 84]. Molecular imprinting provides a versatile platform as the properties of the receptor can be modified to detect a desired molecular compound [66, 67]. An LPG coated with a molecularly imprinted polymer (MIP) was prepared, for the detection of antibiotics [66]. In the presence of different commonly prescribed antibiotics, the sensor showed selectivity to the target antibiotic vancomycin. The target compound can also be removed and the sensor reused. Removal methods used include organic solvents, and photodecomposition have also been investigated [67]. Reusing an LPG or sensor also increases the versatility of a biosensor [81, 82, 84] and can therefore be more time and cost effective.

Choosing a particular fibre type can also contribute to the final characteristics of the fabricated sensor. For instance, PMTP LPGs written in boron co-doped fibres have been demonstrated as radiation dose sensors [36], pressure sensors [34] (boron co-doping can increase the pressure-optic coefficient of a material [85]), and for fuel adulteration detection [39].

### **4.6 Sensor limitations**

The inherent high sensitivity of the PMTP LPG also leads to its limitations. For instance, cross-talk or unwanted interference, such as from varying temperature, have to be limited in order to ensure the shift in the wavelengths is only due to the desired parameter. By fabricating cascaded PMTP LPGs, based on a Mach-Zehnder interferometer, it is possible to eliminate interference [86] and make simultaneous measurements for parameter compensated sensing [87]. James et al. demonstrated that coating a cascaded PMTP LPG device with mesoporous silica nanoparticles, and subsequently infusing a functional material to the central of the region (between the two gratings) enables measuring of only the desired analyte [86].

The broad spectral width of the resonance bands can also limit the multiplexing capabilities of the PMTP LPG. By utilising the double resonance bands, simultaneous measurements of surrounding refractive index and temperature were carried out for temperature ranges limited to ±3°C if the refractive index range is ±0.004 RIU [87]. This information enables temperature calibrated sensing.
