Knowledge Gained In Pre Project Preparation Stage

Published Date: 02 Nov 2017

Bio/chemical Sensing Platform

Using fiber gratings FBG`s & LPG`s

Muhammad Arif Iftikhar

BEng Electrical and Electronic Engineering

School of Engineering and Applied Science

Aston University

Supervisor: Professor Lin Zhang

April 2013

ACKNOWLEDGEMENT

SYNOPSIS

The aim of the project was to research and investigate a bio/chemical optical sensing platform using Fibre Bragg Gratings (FBGs) and Long Period Gratings.

The project involved the design of a temperature compensation scheme measuring responses of each type of gratings treated for a series of varying temperature values. Also the project involved refractive index sensing of Long Period Gratings to evaluate the best possible bio/chemical sensing platform. The obtained experimental results were analysed and evaluated with regards to established theory about temperature and refractive index sensing applications of FBGs and LPGs.

Experimental results along with an evaluation of the project are included in the report. Proposed improvements and some key ideas for work in future in the field are also included in the report presented.

INTRODUCTION

Fibre Optics have not only revolutionised the world of telecommunications but also they have given a new parameters to the field of sensing. Compact size & weight, Immunity to radio frequency & electromagnetic interference are the key properties of an optical fibre giving it a boost in sensing. Fibre Optic Sensors can measure/sense light intensity, magnetic field and electric field, strain, pressure, temperature and rotation, liquid flow and chemical analysis etc. One of the latest emerging aspects of optical fibre sensing is Bio/chemical sensing, this recent leap is a result of struggle to develop cheap and more reliable alternatives to conventional bio/chemical sensors.

Thorough research on fibre optic bio/chemical sensing and various grating properties was conducted for the project. The main focus persisted to be temperature and refractive index sensing using FBG and LPG respectively, so as to propose a temperature compensated bio/chemical sensor.

AIM OF THE PROJECT

"We value information almost instinctively, like desert nomads think water is a "good thing", but faced with an overwhelming abundance, we are as lost as Bedouin by the sea. We have long dreamt of making the desert bloom, but we don’t know how." [ [1] ]

The field of telecommunications have entirely been revolutionised by the advent of fibre optics and its recent progress. However it’s not only the telecom sector which has been affected, one of the latest area undergoing considerable changes is sensing. Fibre optic sensors have proven to be best alternatives to the conventional sensors due to their distinctive properties such as compact size, electromagnetic radiation immunity and large information carrying capacity. Still there is a lot of research to be done in bio/chemical sensing using optical fibre sensors and to exploit various types of grating structures of fibre optics to identify best possible sensing platform. In a nutshell this is the aim of this project. Bio/chemical sensing can benefit a lot from the use of fibre optics and the efficiency could be further enhanced by exploring the ways to use various structured grating based sensing systems. So it is important to keep researching about optical fibre sensors and their applications in bio/chemical environment.

MOTIVATION

Primary motive in pursuing this project was to undertake a challenging and research oriented project in a vast growing field of photonics to open up possibilities of higher education at postgraduate level. This projected is aimed to make contribution towards the research work of Photonics Research Department at Aston University. The photonics group of Aston University is considered to be one of the largest photonics research group in the United Kingdom. The group has continuously and extensively contributed in the field of photonics along with its work cited in literature published internationally.

REPORT STRUCTURE

The first chapter outlines background knowledge gained in pre-project preparation stage. It holds immense and crucial importance as it outlines optical theory fundamentals which are required to understand later chapters. Chapter 2 and 3 gives basic theory of FBG`s and LPG`s respectively including their fabrication and sensing principles. Chapter 4 presents experimental procedures carried out in the lab and analysis on the results. Chapter 5 is project evaluation and finally chapter 6 highlights research in the direction of project scope to be carried out in future.

BACKGROUND

To understand functionality of an optical fibre, basic principles of optic theory were studied. This chapter briefly highlights the information which plays a vital role in understanding more complex ideas prevailing over next chapters of the report.

Basics of Optical Fibres

An optical fibre is made from transparent material as glass and it is used to transmit light signals through the process of total internal reflection [ [2] ]. Optical fibres are small in size and weight and can transmit signals without any electromagnetic interference as they are insensitive to any electrical or magnetic disturbance of surroundings. High signal carrying bandwidths and higher carrier frequencies also make an optical fibre an ideal signal transmission medium [ [3] ].

Figure 1.1 The Basic Principle of an Optical Fibre.

To understand the principle behind the working of an optical fibre, consider that the fibre is two layers of mirror which traps the light in and the light is being reflected from both layers with a forward propagation mode as shown in figure 1.1. Optical fibre structures can vary from very basic to more complex ones depending upon their light coupling properties.

http://upload.wikimedia.org/wikipedia/commons/7/7d/Singlemode_fibre_structure.png

Figure 1.2 Three dimensional cross section of a basic optical fibre [ [4] ]

The main components of an optical fibre are core, cladding, buffer and jacket as shown in figure 1.2. Core is the central part of fibre and is responsible for carrying the light signal. The diameter of core varies from 8µm to 100 microns in the most common optical fibres. Surrounding the core is cladding region and it is responsible for confining the light in the core. Typical diameter of cladding region is 125 microns. Refractive index of the core is always higher than the refractive index of cladding. And both core and the cladding are mostly doped glass materials. Buffer is the region surrounding the cladding and is mainly responsible for protecting and preserving the strength of the fibre [ [5] ]. The outer most region of an optical fibre is called jacket. It does two main tasks, absorb all light not guided properly from the cladding and also to protect the fibre from outside atmosphere and its interactions [ [6] ].

Analysing light propagation properties of fibre makes it easy to categorise them into two main types’ i.e. single mode and multimode fibres. Mode is generally the guided waveform of the fibre [ [7] ]. Fibre supporting only one modes is termed as single mode fibre and holds very important position in communications where very high bandwidth is essential. Multimode fibres support very large number of modes and are normally used for short distance transmissions where high power is an important factor. Further categorising of multimode fibre gives step index and graded index multimode fibres [ [8] ].

Step index multimode fibres have a consistent core of a uniform refractive index and the cladding refractive index is lower than that of core as shown in figure 1.3 [ [9] ]. This property results in guiding the light entering at an angle of less than the critical angle, towards forward propagating mode and the light exceeding the critical angle is lost as it is refracted into the cladding. The inconsistency of the arrival times of different light rays is generally known as dispersion and in case of multimode step index fibre high dispersion is a major phenomenal characteristic which is inevitable [ [10] ].

C:\Users\Arif\Desktop\lfib-msm.jpg

Figure 1.3 Multimode step index fibre [ [11] ]

Graded index multimode fibre was developed to compensate the dispersion effect of step index fibre. In graded index multimode fibre, there is a gradual decrease in the refractive index of the core when moving farther from centre of the core. The augmented refractive index of the core at centre is to slow down some light rays so as to enable almost all of the rays to reach at the same time, hence minimising dispersion effect [10].

C:\Users\Arif\Desktop\lfib-msm.jpg

Figure 1.4 Multimode Graded index fibre [11]

Single mode fibre have a very small core diameter hence it allows only one ray of light to pass through [9]. Low attenuation and low dispersion are the key characteristics of Single mode index fibre [5].

C:\Users\Arif\Desktop\lfib-msm.jpg

Figure 1.5 Single mode index fibre [11]

Refractive Index

Refractive index can be defines by a ratio of the speed of light in vacuum to the speed of light in a medium/material:

(1)

Where n is the refractive index, c is the speed of light in vacuum and v is the speed of light in the material.

Refractive index is the measure of how much light beam slows when entering a medium. Hence when light travels at a slow speed in optically denser material it can be deduced that the dense medium has a higher refractive index value [ [12] ].

Willebrord Snell, an astronomer, contributed a lot to the basics of optic theory by discovering that refractive index and the sine of incident and refracted angles are related to each other by:

(2)

Where and are the refractive indices of the two materials light propagating through, is angle of incidence and is angle of refraction. When light ray enters a dense medium at an angle, a part of it is refracted at an angle of and remaining is reflected. As the angle of incidence of the first material is increased gradually angle of refraction will reach 90°and the light is refracted along the margin between the two mediums, the angle of incidence which results in this phenomena is known as critical angle. However if the angle of incidence is larger than the critical angle then the light is reflected back from the boundary into the first medium. Hence the light is trapped inside the first medium and this process is called total internal reflection.

Total internal reflection is the main principle behind working of an optical fibre and is already discussed in figure 1.1 earlier. Refractive index of core of the fibre is greater than that of cladding, hence confining the light signal in the core by total internal reflection. However diffraction effect can also be used instead or in conjunction with total internal reflection in order to confine the light signal in an optical fibre [ [13] ].

Numerical Aperture

Numerical aperture (NA) is yet another important element of optics theory and it plays a vital role in an optical fibre.

C:\Users\Arif\Desktop\Optic_fibre-numerical_aperture_diagram.bmp

Figure 1.7 Acceptance angle as the light enters the face of the fibre [ [14] ]

The light collecting ability of an optical fibre is determined by its numerical aperture. The light in an optical fibre is only propagated if and only if it is entering within a specified range which is generally known as the acceptance cone. Acceptance angle is the half angle of this cone as shown in figure 1.7.

(3)

(4)

Where in equation (3) the refractive index of the surrounding at the entrance of the fibre is referred as. The most commonly used and the most relevant is equation (4) as it can be applied to any type of fibre [14].

Optical Fibre Sensors

The recent advancements in the field of telecommunications and sensing has paved a way for fibre optics. Optical fibre has been used in transmission systems for decades however the recent years have yielded it to be a very good, efficient and viable means of sensing and is considered to be a better alternative to traditional electrolytic sensors. Ideal properties of an optical fibre responsible for its success in sensing are:

Compact size & weight, remote-able and high precision accuracy,

Secure and efficient data transmission,

High bandwidth and signal carrying capacity,

Immune to radio frequency & electromagnetic interference,

Fibre optic sensors can measure/sense:

Light intensity, magnetic field and electric field,

Strain, pressure, temperature and rotation,

Liquid flow and chemical analysis etc.

Optical fibre sensors can be categorised into two main groups according to transduction nature:

Intrinsic: An optical sensor in which intrinsic characteristics of transmission/reflection of the optical waveguide, are modified accordingly with the magnitude of the measurand. Two main examples of this type are sensors based on curvatures, and Bragg gratings fabricated inside an optical fibre.

Extrinsic: In this type of sensor the interaction of the measurand and light, takes place in an external optical device and incoming or other waveguides receive the modulated light. The light modulation processes take place outside the optical waveguide. Sensors based on mirrors or confronted fibres are examples of this type [ [15] ].

FIBRE BRAGG GRATINGS

Introduction

Fibre Bragg grating is a periodic modulation of the refractive index of the core of the fibre, achieved by exposing the core to an intense optical interference pattern of ultra violet (UV) light [ [16] ]. Hill et al, was the first one to demonstrate the formation of permanent gratings in an optical fibre following the discovery of photosensitivity, at Canadian Communications Research Centre (CRC), Ottawa, Canada in 1978 [16][ [17] ][ [18] ]. Photosensitivity is the process in which the core of the fibre is exposed to ultraviolet light in order to change the refractive index of the core permanently. The amount of change in the index of refraction ∆n obtained through photosensitivity depends on various factors such as the irradiation conditions (for example intensity and wavelength), the composition of the glassy material of the core and any processing conducted prior to irradiance [16].

Figure 2.1 Bragg grating structure and operation [16]

Fibre Bragg gratings (FBGs) have a specific period of gratings which is responsible for coupling of light in the core from forward to backward propagating mode. This light coupling occurs at a specific wavelength known as Bragg wavelength and is given by:

λB = 2 neff Λ (5)

Where λB is the Bragg wavelength, neff is effective refractive index of the core and Λ is period of the gratings [ [19] ].

The reflectivity at the Bragg wavelength can be determined by:

(6)

Where,

(7)

And V ≥ 2.4. It can be observed using equations (6) and (7) that R is directly proportional to L and the index modulation [ [20] ].

Fabrication Techniques

Fabrication techniques of FBGs hold an important position as to obtain high quality and low cost gratings. The essential characteristics of an ideal FBG fabrication technique are listed below [20].

Flexibility,

Capability of economic mass production,

Good physical and optical characteristics,

Repeatability should be upright.

A few of the existent FBG fabrication techniques are:

Bulk Interferometer: This method of writing the gratings was first practically conducted by Meltz et al, [ [21] ] as the figure 2.2 shows.

C:\Users\Arif\Pictures\My Scans\2013-04 (Apr)\scan0001.jpg

Figure 2.2 UV Bulk Interferometer fabrication of FBGs [ [22] ]

The interferometer consists of a UV beam split into two separate beams using a beam splitter and later are brought together at an angle by reflection from two UV mirrors. The Bragg wavelength can be chosen independent of the UV wavelength as:

(8)

Where,

λB is the Bragg wavelength, neff is effective refractive index of the core, nuv is the refractive index of silica in the UV, λuv is the wavelength of UV radiation and θ is the angle between the UV beams.

The fibre to be written with the gratings is held at the intersecting point of the UV beams. This technique is an ideal when writing single pulse of short gratings [22].

Phase-mask technique: This technique have known to be a modified version for the fabrication of FBGs (Hill et al 1993, Anderson et al 1993). Lateral formation of interference pattern is done by a diffractive element called phase mask as shown in figure 2.3.

Figure 2.3 Phase mask writing technique [20]

Refractive index modulation is photo-imprinted in the fibre by using the interference pattern. Bragg wavelength of the gratings produced by this technique is given by:

(9)

(10)

Where,

λB is the Bragg wavelength, neff is effective refractive index of the core, is the period of gratings and is the period of phase mask.

This technique is the most preferable as is conducted in a computer controlled imprinting process using a phase mask so the process can be repeatable at low cost and carries mass production capability [20].

Sensing Principles

Sensitivity to external thermal or mechanical perturbations, of both grating period within the fibre and the refractive index of the optical fibre is the key to sensing functionality of the Bragg Gratings.

Strain: The dependency of the reflected light from FBG is upon the refractive index neff and the spacing of the index modulation ΛG , makes the strain field to directly affect FBG response, through the expansion and compression changes of ΛG and also by the strain-optic effect, i.e., the strain-induced change in the refractive index of glass as shown in figure 2.4 [ [23] ].

Figure 2.4 Basic FBG based sensor system [23]

The Bragg wavelength shift if a longitudinal strain is applied, is given by:

(11)

Where,

∆λBS is the wavelength shift, λB is the Bragg wavelength and ∆ε is the strain applied [20]. Typical wavelength shift response of FBG to applied strain is shown in the figure 2.5 [ [24] ].

C:\Users\Arif\Desktop\fbg strain.jpg

Figure 2.5 Typical wavelength shift of FBG to strain []

Temperature: Thermo-optic effect is mainly responsible for temperature sensitivity of the FBG. Thermo-optic effect: Change in glass refractive index and lesser but yet also on the thermal coefficient of the fibre, induced by temperature. Thus, λB shifts by an amount ∆λB in response to strain ɛ and temperature change ∆T by:

(12)

Where,

Pe is the strain-optic coefficient, αs and αf are the thermal expansion coefficients of any fibre bonding material and of the fibre itself, respectively and is the thermo-optic coefficient. Typical wavelength shift response of FBG to temperature is shown in the figure 2.6 [24]

C:\Users\Arif\Desktop\fbg temp.jpg

Figure 2.6 Typical wavelength shift of FBG to temperature [24]

Refractive Index: To sense the change in refractive index of a material, fiber’s core must be exposed to the medium under test. Due to this very factor FBG in its normal form is insensitive to refractive index change of material under test, however in order to make FBG based refractive index sensor some modifications could be done to the fiber. Etching is the process by which cladding diameter is reduced of FBG so that the interaction of the material to be tested becomes in mere contact with the core and light is can be coupled into the core. However the shift in the Bragg wavelength is not linear function of the change in index of refraction for any given ∆n. The sensitivity of this type of FBG refractive index sensor is ∼ 200nm-RIU-1 around ∆n = 0.01 [ [25] ].

LONG PERIOD GRATINGS

Introduction