Fiber Optic Sensors (Humidity Sensors)

Introduction

Optic fiber is normally made of plastic or glass of a considerable length, which allows light rays to pass through. Regulation of losses is achieved through the principle of total internal reflection. Fiber sensors are very receptive to restrictions that are capable of modifying either the frequency, intensity, or the segment of light that travels through the fiber. The normal optic fiber consists of two major parts, which include the cladding and the core. Rays of light should pass through the length of the fiber without deviation. The process that makes this possible is total internal reflection achieved by the use of materials of different refractive indexes in both the core and the cladding.

Fiber Bragg Grating is a concept applicable in fiber optic sensors. Maximum reflectivity can be achieved only when the wavelength reaches a specific point referred to as the Bragg wavelength (Grattan & Sun, 2000). Fiber Bragg grating depends on some factors of the immediate environment such as temperature. As compared to other ordinary sensors, optical fiber sensors are corrosion and water resistant. This paper will discuss the history and the principles under which optic fiber works especially regarding humidity and fiber Bragg grating.

History of the Fiber Optics

A single fiber can hold several sensors with manageable guides linked to the central points. Optic fiber can transmit large data over long distances at a relatively high speed as compared to other forms of communication. They can also be able to work more efficiently and  with less losses than the normal metal wires. Relative humidity sensing technology requires a combination of thin film optics and the optical fiber sensor (Weijing et.al, 2014). The application of this combination has produced instruments that could not have been imagined.

Modern optical fiber sensors were developed based on the most significant scientific discoveries in the 1960s. There were endoscopes made of small lengths which had low transparency. They were used in industrial processes and medicine. In the early 1970s, optical fibers with low-loss capacity were used as sensors not only in the sphere of telecommunication. The main reason behind the development of optical fibers was their application in telecommunication. However, with time, there appeared other development concepts which were used in a variety of areas such as medicine and sensitive industrial activities. Research groups were formed so that they could formulate mechanisms that could be used to develop the upcoming technology in measurement and sensing. Some of the common conventional sensors are not adapted to be used in particular environments as opposed to fiber sensors which have various applications regardless of the environment (Grattan & Sun, 2000).

Due to the escalating challenges in the modern society, there has been a dire need to improve the technological advancements to counter these challenges. Scientists have been carrying out extensive research in the field of fiber optics in the last decades to try improve certain departments such as aerospace, military, health, heritage, and culture among other engineering fields (Annamdas & Gopal, 2011). The application of optic operated electronics has also been prevalent in several electronics such as laser printers, laser pointers, CD players, and bar code scanners. Mutual development of both the fiber optic communication and the optic-operated electronics have resulted in the optimization and development of products of higher quality that are capable of replacing the traditional sensors for humidity, temperature, pressure, and rotation among others.

Working Principles of Fiber Optics in Humidity and FBG

Any sensory monitoring system is desired to have ‘A to E’ characteristics. A refers to Accuracy. In this case, the sensory technology should be reliable in terms of its accuracy. B refers to Benefit; the technology should be reasonable in terms of its cost. C stands for Compact. This means that the technology should be small in size and be preferably embedded. D implies Durable, which means that the technology should provide the consumer with a long-lasting service. E refers to Easy; the sensor should be easy to operate, and time that it needs to retrieve data should be minimal. Fiber optic sensor has been found to meet these requirements (Annamdas & Gopal, 2011). The technology uses optic fiber either as the sensing element or a means of relaying signals to the machine that will be used to process these signals.

The performance of the sensors in detecting humidity requires the control of the environment in which the sensors are located. A stainless steel chamber is designed to achieve this. Inside the chamber, there is a circuit that regulates and stabilizes the amount of heat and therefore the temperatures. Divided-flow volumetric mix ratio technique is used to make sure that there are controlled levels of ambient humidity. This process demands dividing the section of the dry part of the air into two different sections. There is a saturated chamber where one of the divided sections passes so that it can be used to form the wet stream. The remaining section is required to be dry and therefore does not need to be subjected to another process. It remains as it is so that it can form the dry stream. Humidity can be estimated by calculating the relative ratios of the dry and saturated streams respectively (Consales et al, 2011). The formula for calculating the relative humidity (RH) is %=  100.  Fw and Fd represents the rate of flow of the wet stream and the dry stream. Vp (dry) and Vp (wet) are the vapor pressure of the dry and wet streams.

Humidity is a physical quantity. Different reasons make it possible to measure humidity due to the diverse applications. Some of the applications include air conditioning to keep human body in comfort, prevent bacterial infection, and thus increase the quality of products. The requirements used in the monitoring of relative humidity may vary depending on the intended application. In this respect, humidity is measured using various techniques. It can specifically be defined as the amount of water vapor present in the environment in gaseous form. Water vapor in the atmosphere is expected to obey the gas laws. From Dalton’s law, the vapor pressure can be estimated through estimating the partial pressures of the parts of the vapor in the air mixture (Yeo, Sun, & Grattan, 2008). The amount of pressure in the saturated water vapor and the pressure in the dry vapor can be used to estimate the partial pressure.

Pressure of the saturated vapor is usually affected by the temperature in the environment; therefore, it is necessary to check it. Estimated pressure under the ordinary circumstances when the air is fully saturated can be used to estimate the relative humidity. Humidity can be scientifically determined by the use of fiber optic. The sensor used inside the fiber applies fiber Bragg grating. The changes in the refractive index and temperature help it to evaluate humidity. This kind of application can be used to detect chemicals (Yeo, Sun, & Grattan, 2008).

Optic sensors for humidity are confirmed to respond faster than the ordinary instruments. Electromagnetic waves have minimal interference in the operation when fiber optic sensors are incorporated. What makes the fiber optic sensors even better is the fact that they can be operated via remote control. Optical fibers are usually designed to function in such a way that there is a total internal reflection to minimize the loss of light rays. In addition to this concept, the fiber optic sensor used to measure humidity has in addition a hygroscopic material covering the optic fiber. The purpose of the hygroscopic material in this case is to adjust the amount of light passing through the length of the optical fiber. The efficient fiber optic humidity sensors are complex in design that makes light rays able to interact with the water vapor. This can be done in processes such as removal of the cladding, fabrication of gratings, and polishing the sides among others (Mathew, Semenova, & Farrell, 2012).

The Fiber Bragg Grating-based humidity sensor used in the detection of cracks in sewerage systems works according to a specialized technique. For instance, there is a bare and a polyimide-coated FBG. Both FBGs are connected in series along the fibre. Polyimide in this case acts as a hygroscopic material. The material swells in presence of vapor and therefore a strain is caused on the FBG. The extent of the strain depends on the amount of relative humidity that is present. Monitoring the changes in the Bragg wavelength and comparing it with the known values of a calibrated scale allows determining the ambient Relative Humidity value (Bremer et al, 2014).

There are certain fiber optic sensors that are used to measure humidity and apply the concept of an enlarged core. Cladding is not carried out in this type of equipment. A moisture sensitive film is applied on the unclad area as a coating. The refractive index of the core is made less relative to the one on the unclad region. Since the film is made of a material with a higher refractive index, parts of the light entering the fiber optic are forced to come into contact with the section which is very sensitive in the presence of moisture. One of the common moisture sensitive materials is the phenol red in the film. The interaction between the moisture sensitive material in the film and light happens at this point. To make the system more sensitive by increasing the interaction of the two, the sensitive region is designed in such a way that it resembles letter U (Gupta & Ratnanjali, 2001). This works to decrease the angle of incidence at the interface. This increases the section of the light going into the film and works to increase the sensitivity of the system.

Another fiber optic sensor measuring the humidity uses the silica xerogel film. From the apparatus, when the relative humidity changes, water molecules spread through the film. The amount of time required for the molecules of water to evenly distribute throughout the film depends on the thickness of the film and the coefficient of diffusion of water inside the xerogel.  Xerogel swells consequently making the sensitive layer change in its thickness as well as the refractive index. There is an adjustment in the interface of the film and the fiber. As a result, there is an output signal (Juncal, Vicente, Echeverría, & Garrido, 2010).

Another type of a fiber optic sensor for humidity uses the silica sol-gel film. The principle behind an effective operation is based on evanescent scattering and absorption of wave. The optical fiber is U bend. Under given conditions, the ordinary absorbance of the sensor corresponds to the relative humidity; therefore, it is easier to determine the relative humidity (Zhongjun & Duan, 2011).

A Fiber Bragg Grating works to vary the values of the refractive index of the core of the fiber. This facilitates the reflection of light during specific intervals of wavelength called the Brag wavelength. However, this factor is usually dependent upon the refractive index of the fiber. Temperature is among the factors that affect the refractive index of the system (Berruti et al, 2013).

Fiber optic interferometric humidity sensors use hygroscopic substances that are capable of changing their thickness or refractive index because of the changes in the environment humidity level. The changes are caused by bulging and de-bulging. In this kind of a set-up, the fiber cladding is reduced. In addition, the chitosan film is oxidized. A mirror is also fitted in the system to facilitate reflection. Humidity can be measured using this simple set-up in a wide range of applications (Pengbing et al, 2014).

Conclusion

Fiber optic sensors are among the most efficient technologies, and scientists have been working to manufacture fiber sensors that are smaller than the size of the human hair. The application of the idea of total internal reflection has made the fiber optic be relied upon in several key areas including the human health, agriculture, sewerage systems, and telecommunication among other sectors. They are efficient since they minimize losses. Fiber optics has been effectively used in the estimation of relative humidity in the atmosphere especially with the improved technique of Fiber Bragg Gratting (FBG). In the fabrication of instruments to estimate the amount of relative humidity, different approaches are used with regard to the materials making the core and the cladding. However, it is worth noting that the principle behind their successful application in the field is total internal reflection. This is a concept that is used to achieve the desired different reflective indices of the core and the cladding. When this is achieved, light rays entering fiber optic can be transmitted in desired directions.

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