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The current thesis aims at the development of a fiber optic relative humidity sensor with linear response over the widest possible dynamic range with optimal sensitivity by means of a simple fiber optic sensor configuration. Exhaustive Guided Mode Attenuation Based Optical Fiber Humidity Sensor: Improved sensitivity and wide linear dynamic range.

LIST OF TABLES

Introduction

Mitschke [66] has reported an optical fiber humidity sensor based on a porous thin film interferometer (Fabry-Perot) at the tip of the fiber. The dynamic range of the sensor is observed to be 40-98% RH with a response time of 150ms.

Figure 1.1: Total internal reflection and evanescent wave in an optical fiber.
Figure 1.1: Total internal reflection and evanescent wave in an optical fiber.

ZnO Nanoparticle based Simple Optical Fiber Relative Humidity Sensor: Realization of Wide

  • Introduction
  • Experimental .1 Sol-gel process
    • Humidity sensing probes preparation
    • Design and development of humidity chamber
    • Sensing probe characterization
  • Results and Discussion
  • Conclusion

To design an optical fiber sensor, the sol gel film containing the sensing agent can be realized on the bare core of the optical fiber. The optical fiber humidity sensing probe was fixed in the humidity chamber so that the sensing area with nanoparticle-immobilized sol-gel nanostructured film was in the center of the chamber. Increasing film thickness (from 2-dip to 3-dip) improves sensor sensitivity and dynamic range.

For the probe coated with 2 dips, the film thickness was much smaller than the penetration depth of the evanescent wave [30]. The sensing film thickness corresponding to the 3-dip coated probe was of the order of the penetration depth and therefore higher than that of the 2-dip film. The thickness of the sensing film for the probe coated with 4 dips was greater than the thickness of the evanescent wave.

The average response time of the optimized sensor is much less than the commercial sensor.

Figure 2.1: Schematic diagram of experimental setup for optical fiber humidity sensor characterization
Figure 2.1: Schematic diagram of experimental setup for optical fiber humidity sensor characterization

Exhaustive Guided Mode Attenuation based Optical Fiber Humidity Sensor: Enhanced Sensitivity and

Introduction

In general, the penetration depth of the evanescent wave in the cladding region is typically less than the wavelength of operation [30]. Consequently, the fraction of the total conducted power present in the cladding region is generally less than 1% (relative to the total conducted power) for poorly conducted multimode fiber. Regardless of the employed fiber geometries, most of such evanescent wave-based RH sensors reported in the literature show nonlinear response with piecewise limited linear sensitivity over the observed dynamic range.

In the previous chapter, an optical fiber RH sensor is reported with a full linear response over the dynamic range as high as 4-96% RH, using straight and uniform fiber geometry in conjugation with ZnO nanoparticle immobilized sol-gel sensing coating . Certainly, if the reagent can somehow be made accessible even in the core region of the fiber, the only way left is to increase the degree of interaction with the guided signal, the sensitivity will certainly increase a lot. However, this is not possible if the fiber used in the development of the sensor is a standard fiber with a solid silicon core.

The motivation for the work reported in this chapter is to develop an all-optical RH sensor with linear response over a wide dynamic range (closer to that reported in chapter 2) and to increase sensitivity.

Experimental

  • Fabrication of ZnO nanoparticles immobilized sol-gel nanoporous optical fiber
  • Designing of RH sensor
  • Sensing probe characterization

The total length of the bare fiber core was observed to be ∼14 mm, which can be easily tailored to other values. However, these pores together with the immobilized reagent also play the role of the scattering centers in the sol-gel fiber core; makes such structures highly loss-making. The diameter is also observed to be uniform throughout the length of the sol-gel fiber.

Furthermore, the end faces of the sol-gel fiber are observed to be almost flat and the fiber is free from any cracks. It is important to observe the central region (excluding the region near the surface) in the cross-section of the developed sol-gel fiber, the pore size is observed to be. Schematic of the method to realize an all-optical RH sensor using the sol-gel-derived nanoporous silica optical fiber core as the sensing element is shown in Fig.

There would be additional losses due to mechanical coupling and less-than-perfect end faces of the sol-gel fiber.

Figure 3.1: FE-SEM image of ZnO nanoparticle immobilized microstructured sol-gel silica fiber core
Figure 3.1: FE-SEM image of ZnO nanoparticle immobilized microstructured sol-gel silica fiber core

Results and Discussion

This is due to the fact that the nanoparticle-immobilized dry sol-gel fiber has almost negligible absorption at low humidity at 632.8 nm; It is worth noting that the performance of the sensor can be further improved by making a sol-gel fiber core whose diameter matches the core diameter of multimode fiber. Thus, the guided mode of the standard fiber is completely exposed to the reagent and analyte upon entry into the sol-gel fiber, allowing for an enormous degree of interaction that would be impossible to achieve with an identical structure as a sensor coating (rather than a feeling core).

A detailed comparison of this sensor response is performed in section 4.3 of the next chapter. In the next step, the dynamic performance and the repeatability characteristics of the ZnO nanoparticle immobilized nanoporous sol-gel fiber humidity sensor were investigated in an identical manner described in Chapter 2. Due to the porosity to the axis of the fiber in the detection area, later observation is expected .

Furthermore, the response time of the sensor should ideally have been identical regardless of the wetting or drying processes.

Figure 3.6: Experimentally observed sensor response of the ZnO nanoparticles immobilized nanoporous sol-gel fiber sensor
Figure 3.6: Experimentally observed sensor response of the ZnO nanoparticles immobilized nanoporous sol-gel fiber sensor

Conclusion

These were smaller than the average time response of a commercial sensor, but almost 10 times higher than that observed for the sensors in Chapter 2. A possible reason could be the fact that again during the reverse cycle (drying) humidity changes. Exposure to cyclic variations in RH changes again demonstrated that the response of the sol-gel fiber sensor is highly reversible and reproducible.

In addition, a maximum fiber yield variation during the repeatability and reversibility test over an interval of a few days is observed to be only in the order of 10 -4 . In addition, with an average time of 0.54 seconds for wetting and 0.68 seconds for dehydration, a maximum fiber output variation of the order of 10 -4 over a sufficiently long period, the developed sensor is best suited for real field applications.

Nanoparticle based Simple Optical Fiber Relative Humidity Sensor: Sensitivity Enhancement

  • Introduction
  • Experimental
    • Preparation of colloidal TiO 2 nanoparticles
    • Humidity sensing probes preparation
    • Sensing probe characterization
  • Results and Discussion
  • Conclusion

In the process of characterizing and optimizing the sensor's response, the effect of film composition and film thickness was once again investigated in detail. Thus, an increase in humidity increases the adsorption of water vapor and the condensation in the pores of the sensor film manifolds. Nevertheless, increasing the nanoparticle concentration in the sensing film improves the overall response of the sensor.

The sensing film thickness corresponding to the three-dip coated probe increases sufficiently, but remains within the penetration depth and is therefore higher than that of the two-dip film. The thickness of the sensing film corresponding to the probe coated with 4 dips was greater than the thickness of the evanescent wave. This is the thickness of the three-dip coated probe for a 9% (v/v) TiO2 nanoparticle colloidal solution concentration in a sol of the order of the penetration depth.

Further, comparing the response of the developed sensor with [76], the linear dynamic range of the developed sensor is much improved than [76].

Figure 4.2: Time variation of commercial humidity sensor and fiber optic humidity sensor (3- (3-dip coated probe of 9 %( v/v) nanoparticle concentration in sol)
Figure 4.2: Time variation of commercial humidity sensor and fiber optic humidity sensor (3- (3-dip coated probe of 9 %( v/v) nanoparticle concentration in sol)

Localized Surface Plasmon Resonance based U- shaped Optical Fiber Humidity Sensor

  • Introduction
  • Principle
    • Localized surface plasmon resonance in metal nanoparticles
  • Experimental
    • Preparation of U-shaped probe
    • Preparation of gold nanoparticles coated sensing probe
    • Preparation of silver nanoparticles coated sensing probe
    • Sensing probe characterization
  • Results and Discussion
  • Conclusion

The dependence of the resonance wavelength on the refractive index of the surrounding medium has been used in LSPR-based optical as well as optical fiber sensors [51–53] . In an optical fiber LSPR sensor, the clad part of the fiber is coated with a metal nanoparticle film. In addition, bending of the sensing probe causes a decrease in the incident angle of the propagated beam in the sensing area [103].

The developed touch probe was dried at 160ºC to remove ethanol and water from the pores of the sol-gel film. The fiber was placed in Tollen's reagent in such a way that it was almost equidistant from the walls of the beaker. Here, the evanescent wave associated with the guided modes of the fiber excites plasmons.

This leads to a decrease in the energy corresponding to the resonance wavelength in the transmitted (output) light.

Figure 5.1: Schematic diagram of plasmon oscillation in metal nanoparticles [49].
Figure 5.1: Schematic diagram of plasmon oscillation in metal nanoparticles [49].

Conclusion of Thesis

Almost identical linear dynamic range (5-95% RH) is observed with a multifold increase (~9 times) in sensitivity compared to the EW fiber sensor used by the ZnO nanoparticle and over 3.5 times increase in sensitivity compared to the sensor with the largest linear dynamic range reported in the literature. The maximum linear dynamic range decreased to 24-95% RH when the ZnO nanoparticles in the sol-gel sensor coating were replaced with TiO2 nanoparticles. The observed linear dynamic range of the TiO2 nanoparticle-based sensor is slightly lower than the largest reported in the literature; however, its sensitivity is ~102 times higher compared to the sensor with the largest linear dynamic range reported in the literature.

In the last sensing scheme (metal as well as metal-dielectric nanoparticles support LSPR), a linear dynamic range of 6-90%RH is observed for sensing probe with gold nanoparticle film. The observed linear dynamic range is better than the highest reported (23-97%RH, based on FBG) in the literature using the wavelength interrogation scheme. Importantly, the sensitivity of this probe is 3.8 times higher compared to the sensor with the highest linear dynamic range.

For the sensor based on the silver-TiO2 nanoparticle film, the linear dynamic range was reduced to 29–95% RH; however, the sensitivity improves.

Khijwania, “Fiber optical evanescent wave absorption sensor: effect of fiber parameters and probe geometry,” (PhD Thesis, Indian Institute of Technology, Delhi, India, 1999). Lopez-Amo, “Fiber-optic humidity sensor based on a conical fiber coated with agarose gel,” Sens. Claus, “Simultaneous measurement of humidity and temperature combining a reflective intensity-based fiber optic sensor and a fiber Bragg grating,” IEEE J .

Dubey, “Nano-like magnesium oxide films and its importance in optical fiber humidity sensing,” Sens. Arregui, “Optical fiber humidity sensor based on surface plasmon resonance in the infrared region”, J. Vainos, “Optical fiber long- Period grating humidity sensor with poly(ethylene oxide)/cobalt chloride coating,” Appl .

Gupta, “Fiber-optic evanescent field absorption sensor with high sensitivity and linear dynamic range,” Opt.

Gambar

Figure 1.1: Total internal reflection and evanescent wave in an optical fiber.
Figure 2.1: Schematic diagram of experimental setup for optical fiber humidity sensor characterization
Figure 2.2: Responses of commercial humidity sensor and fiber optic humidity sensor (3-dip coated probe of 0.2g ZnO nanoparticle concentration in sol solution)
Figure 2.3: Linear response of 2-dip, 3-dip and 4-dip coated probes corresponding to 0.4g ZnO nanoparticle concentration in sol solution
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Referensi

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Experimental .1 Sol-gel process Results and Discussion Introduction Fabrication of ZnO nanoparticles immobilized sol-gel nanoporous optical fiber Results and Discussion Results and Discussion Experimental Results and Discussion

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