Experimental

Introduction

Chapter 6 Fiber Bragg Grating Based Humidity Sensor Employing

6.3 Experimental

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surrounding RI variations. Hence, neff versus nout is also plotted, as an example, for b = 6μm in the inset of Fig. 6.1. If the fundamental mode is confined within the fiber with its evanescent tail dying-off sufficiently well below the clad-outer medium interface, the effective-index (neff) seen by the mode will have no influence of the surrounding medium refractive-indices. This is exactly the case of unetched fiber in weekly-guiding approximation. However, when the fiber is sufficiently etched in its cladding region down towards the core, average RI in the cladding region gets significantly affected. Lower the average RI in the cladding region (as is the case with the lowest value of nout = 1), the tighter is the confinement of the fundamental mode to the fiber core; which in turn, leads to a week influence of the surrounding RI on the effective-index (neff) seen by the mode. As nout

increases, average RI also increases. This leads to a lesser confinement of the fundamental mode to the fiber core and a greater interaction with the outer medium in the thinned cladding region, and that way, a greater influence of nout on neff seen by the mode. Any influence of nout

on neff gets accordingly imprinted on the reflected Bragg wavelength, B. The observed theoretical response of the FBG that exists in an etched fiber region is used to analyze the experimental findings in the subsequent section.

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standard, but appropriately controlled, chemical etching procedure that employed HF (Hydrofluoric) acid solution. To do this, an etching chamber was designed with an inner dimension of 8 mm. This chamber was initially filled with HF solution at 40%wt.

concentration. Fiber was fixed inside the etching chamber in such a way that only the length of the fiber that carries first FBG could get exposed to the HF solution. Effective etching of the fiber requires a suitable control over the etching time, especially when one desires to reduce the total fiber diameter to a few microns only. Hence, fiber in the FBG region was first exposed to this 40% HF for 48 minutes. Etching was stopped by removing the fiber from the etching chamber. Etched region of the fiber was then cleaned with NaOH solution.

Afterwards, fiber diameter in the etched portion was examined by a field-emission scanning- electron-microscope (FE-SEM) (Sigma FE-SEM, Carl Zeiss NTS, USA, resolution 2.5 nm).

A uniform fiber diameter with a reduced value of ~18µm was observed for the etched portion of the fiber. To develop the proposed FBG based RH sensor with fiber diameter in the FBG

Figure 6.2: Scanning electron microscopic image of the etched FBG region of the optical fiber.

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region reduced to the lowest possible working limit, this etched region of ~18m fiber diameter was again fixed in the etching chamber to further expose it to the HF solution. For this second phase of final etching, the concentration of HF solution was suitably diluted to 15% wt. in order to slow down and to have a fine control on the etching processes. Exposure of the etched fiber to this 15% wt. HF solution was suitably controlled in order to estimate the critical exposure time required for the fiber to get completely etched out. This critical exposure time was observed to be 34.5 minutes. Etching of the FBG in this phase was stopped a few seconds before the observed critical exposure time by draining the HF solution out of the etching chamber and cleaning the etched fiber with NaOH solution. Entire etching process was carried out at room temperature (19.8^oC). Fiber diameter in the etched region at the end of this final etching process was again analyzed with FE-SEM. Figure 6.2 shows the FE-SEM image of the fiber in the finally etched FBG region. A uniform fiber diameter with a reduced value of ~13.5µm is observed for the etched portion of the fiber. Fiber with a further lower cladding diameter could not be achieved. It’s worth mentioning that etching is a nonlinear process where etching rate increases extremely sharply within a few seconds just before the critical exposure time. One requires an automated system to stop etching process a few microseconds before the critical exposure time to achieve lowest possible fiber diameter within the working limit. Stopping the etching process a few seconds before the critical exposure time, as was the case in the present work, will result in a little higher cladding diameter. Finally, the Bragg wavelength of the FBG that exists in this finally etched fiber,

1E, is observed to be 1541.311nm.

6.3.2 Sensor characterization

T

o investigate sensor characteristics against the applied RH perturbations, a humidity

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inside the chamber was varied by appropriately mixing the humid and dry airs. Two sophisticated flow meters were used to suitably control the flow rate as well as the mixing of dry and humid airs. A commercially available RH sensor (capacitive type), interfaced to a computer via DAQ card and LABview programming, was fixed to this humidity chamber.

This commercial RH sensor monitored instantaneous humidity as well as temperature inside the chamber. Fiber carrying etched FBG at 1E (RH sensor) and unetched FBG at 2

(temperature sensor) was carefully fixed inside this in-house developed humidity chamber.

Figure 6.3: Schematic diagram of experimental setup for sensor characterization.

Fiber was fixed in such a way that both the gratings remained parallel to each other in the same horizontal plane and at the center of the chamber, as depicted in the schematics of the experimental setup (Fig. 6.3). While fixing the fiber, special attention was paid in order to keep Bragg wavelengths for both the FBGs to their natural values (1E& 2). This ensured that the gratings were not strained and preserved their normal length. In this configuration, unetched FBG can only respond to the temperature variations, if any, inside the humidity chamber. Thus, it acted as the temperature sensor. One end of this fiber was connected to a FBG interrogator (Micron Optics, SM130-700, resolution 0.05pm), which was configured

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with a computer. To carry out the experiment, humidity level was first increased to the maximum possible value by injecting humid air into the chamber. Subsequently, RH was slowly decreased in suitable steps to the minimum possible value. RH changes in steps were imposed only after a proper stabilization of humidity inside the chamber at a particular value, that is, after a sufficiently large time gap. Entire experiment was carried out at a fixed room temperature of 19.8^oC. During the experiment, commercial sensor’s output was monitored as a function of time with a resolution of 1s. Simultaneously, responses of both the FBG sensors were also monitored through the FBG interrogator in real-time basis. Wavelength response of the unetched FBG (acting as temperature sensor) was subtracted from the wavelength response of the etched FBG (acting as RH sensor) in order to compensate the effect of temperature variations during a particular experiment. It is this temperature insensitive response for the etched FBG based RH sensor which was subsequently analyzed.