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Optical Fibres and Sensors and History of Optical Fibres

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ABSTRACT

In this term paper report I am starting with the of optical fibres and sensors, history of optical fibres. Further explaining how we can construct sensors and how optical fibres are used in it.

I have also discussed how the known facts points out the problems which were faced before the concept of optical fibers arrived. Further how these problems seem to fade out with the help of fibers in detail.

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Finally I have discussed all the applications of sensors that makes our life so easy, and technology that optical fibers provides to human life.

Introduction

Optical fibres are of great used in optoelectronic communication.

What is optoelectronic communication

The process of converting and transferring electronic signal to optical signal from source to receiver is called optoelectronic communication

Many of the communication system have been appeared in the past, main motivation behind this is to either increase the transition rate or to improve the data rate. Initially reflecting Mirrors were used for transmission of shorter ranges, then electronic devices have dominated modern processing systems. This device passes with high speed and large capacity example telephone but the electronic circuits have some limitations which was overcome by Discovery of fibre optics and lasers in 1960.

Advantages of optical fibres

Optical fibres are of great used to us some of the advantages are as follows

  • Bandwidth
  • Small size
  • Lightweight
  • Electrical insulation
  • Immunity to interference
  • Signal security
  • Low transmission loss
  • Flexible
  • System reliability
  • Ease of maintenance
  • Potential low cost
  • Noise free

Basic Optical Fibre

A normal fibre consists of three basic parts : core, cladding and jacket

Core:-it is the part where optical signal travels with a refractive index n¹

Cladding:- outside the core another layer exist called cladding with refractive index n² where n¹>n²

Jacket:- it is the outermost part of Optical Fibre which act as a covering material.

Fibre optic sensors

A fibre optic sensor is a sensor that uses Optical Fibre either as a sensing material or as a mean of relay in signal from a remote sensor to the Electronic device that process the signal.

Optical Fibre sensing mechanism

Sensor is a device that converts any physical phenomenon into electrical quantity essentially , where as transducer convert physical phenomenon to another not essentially electrical.

so we can say that every sensor is a transducer but not every transducer is a sensor

Transmission measurement

It is a type of Apparatus used in sensors, in this the light source sensor and detective all lie in the same plane and signal is transmitted in one direction.

Reflection measurement

It is a type of operators used in sensors, in this type source of light is connected with the photodetector by the use of coupler that binds or connects sensor photodetector and light source.

  • Power
  • Wavelength
  • Polarization
  • Phase (optical path)

Working of Optical Fibre sensors

  • Transmit light from a source along an optical fibres to a sensor which sense only change in desired parameters such as strain temperature chemical composition and so on
  • The sensor modulates the characteristics such as intensity wavelength at sector
  • The modulated light is transmitted from sensor to signal processor and converted into a signal that process in control system
  • Properties of light involved in fibre optic sensors such as reflection refraction interference and grating

Types of sensors

Sensors are of many types but there are mainly to intrinsic and extrinsic Although we will discuss about many types such as intrinsic extrinsic. Sensors distributed sensor and Multiplex sensor in brief.

Intrinsic sensors are those sensors in which the modulation of signal occurs inside the optical fibre and extrinsic are those in which modulation goes into a separated equipment through Optical Fibre we should also know about. Sensors in which Optical Fibre detect at only a single point whereas in distributed sensors the sensing is distributed along a length of fibre there is an another type of sensor which is known as multiplexed sensors in Multiplex sensor the sensing is distributed along the fibre but at certain places of multiple localised sensors.

What can be measured using Optical Fibre sensors

Total number of parameters that can be measured sensed using Optical Fibre sensors are still And counting there are numerous things that we can measure through Optical Fibre sensors temperature strain pressure and song here we discuss about several number of sensors in which optical fibres plays an important role out of which following are

  • Temperature sensor
  • pressure sensor (intensity based sensors and FBG based sensors)
  • Humidity sensor based on FBG
  • Fluid sensors based on SPR

Temperature sensors

Temperature sensing is a very basic need in today’s Lifestyle we use temperature sensors for weather forecasting humidity check in industries and machineries etc.

Hair we discuss about fibre optic sensors gallium arsenide based temperature sensors its principle material construction and working

Principle

Basic principle behind optic fibre temperature sensors is that at measurement and is a gallium arsenide crystal as the Crystal temperature increases transmission spectrum shifts to higher wavelength.

Material and construction

Material of gallium arsenide Crystal consists of gallium arsenide Crystal and a dielectric Mirror on one end of Optical Fibre and a stainless steel connector the entire system is covered with the teflon coating.

As the outer jacket is covered with teflon then we can say that the sensor is completely known conductor and the sensor it in nude towards radio frequency electromagnetic wave and microwave.

Working

Working of GaAs based temperature sensor can we analyse by using the following block diagram

Light from light source falls on the sensor using optic fibre cable which carries the charge photons towards the sensor, then the photon falls on crystal and electron gets charged up due to energy of photon and jump from balance band to conduction band depending on energy provided and 13 month of wavelength is transmitted from the sensor. Energy gap of GaAs – 1.423eV

The wavelength from the sensor is captured by spectrometer with help of optical fibre linked with apparatus using coupler.

Finally characteristics of our drone using wavelength which determine the unknown temperature.

Pressure Sensor

A pressure sensor is a gadget for pressure estimation of gases or fluids. Pressure is a declaration of the power required to prevent a liquid from extending, and is generally expressed as far as power per unit region. A pressure sensor normally goes about as a transducer; it produces a flag as an element of the weight forced.

We mainly discuss about two types of pressure sensors namely intensity based sensor and FBG based sensors.

Intensity based pressure sensors

When any kind of pressure is applied on the fibre optic it gets deformed this deformation results and leakage or loss of light passing from the optic cable which is detected by the detector.

Working:- this is a conventional type of sensor in which an optical fibre is placed between a suppressor and the light coming from source is deformed by the pressure applied and with the help of amount of deformation taking place at the detector the pressure is detected.

This is to be noted that the container should be properly insulated from heat and any other conduction its work is to detect the change in light and correlate with change in temperature, pressure, strain etc.

Limitations although they are easy and simple and also inexpensive but they have a poor noise shielding so a pressure sensor with advanced technology is the demand of today’s world this demand is fulfilled by fbg based sensor.

FBG BASED SENSORS

Term FBG stands for fiber braggs grating

Before knowing about pressure sensor based on this technique we must know that what fiber bragg grating is-:

Simple concept behind bragg grating is that electromagnetic wave of wavelength Lambda false onto object separated by a distance Lambda so their terms of phase difference between them ,if the waves interfere constructively then there exist reflection and if they interfere destructively there will be antireflection.

Fiber braggs grating

Fibre bragg grating are zones modified with refraction refractive index in optical fibre with constant periodicity.

Why does it reflects certain wavelength of light.

  • This reflects due to fresnel reflection
  • Fresnel reflection is reflection of energy at any mismatch interference
  • Grating can be as thin as 1mm and as long as 10 cm
  • 10 centimetre gratings are very expensive

Working of FBG pressure sensor

Fbg based pressure sensor are known as intelligent sensors which includes Digital circuit and other parts for linearization of its characteristic.

Pressure is proportional to light deformed and this mechanical value is converted into digital value with help of computers according to the wavelength reflected back.

Advantages of FBG

  • Small size
  • Ideal for harsh environment
  • Immune to electromagnetic interference and radio frequency
  • Multiplexing capability
  • Pressure monitoring temperature monitoring and power transition monitoring

Applications

  • Optical Sensors
  • fibre lasers
  • fibre bragg filters
  • humidity sensors

Humidity sensor

Humidity is the relative amount of water vapour present in the air to maximum amount of water vapour that can be trapped by air.

Device used to measure humidity is known as hygrometer

Applications

  • Forecasting humidity
  • used as human breath rate
  • humidity control in textile applications

Optic fibre plays a very important role in building a humidity sensor humidity sensors uses of optical fibres as an optical absorption of materials bragg grating based of optical fibres long period fibre grating optical fibres and many other techniques are used for measuring humidity in many sectors of modern technology here we discuss about different types by which humidity sensors are based on optical fibres namely first we discuss how optical absorption of material are used to make humidity sensors and then fibre Bragg grating technique.

Optical absorption of material

  • In this type of sensor Optical Fibre with certain absorbing chemical substances is coated
  • The substances are melting or quick vanishing field that are used for sensing material these materials are tungsten disulphate(WS2) and zinc oxide(ZNO)and reduced graphene oxide(rGO)
  • Preferably tungsten disulfide is used as water molecules make adsorption with these materials and produce some electric charge which are quick in response
  • Reduced graphene oxide is used in high humidity conditions that is when humidity is about 70 to 95%

FBG BASED HUMIDITY SENSORS

FIBER BRAGG’S GRATING-: Fibre bragg grating are zones modified with refraction refractive index in optical fibre with constant periodicity.

Why does it reflects certain wavelength of light.

  • This reflects due to fresnel reflection
  • Fresnel reflection is reflection of energy at any mismatch interference
  • Grating can be as thin as 1mm and as long as 10 cm
  • 10 centimetre gratings are very expensive

The expansion of water molecule in Air Works as a strain for fibre bragg grating and gas changes bragg’s wavelength dynamically this change in the wavelength due to the strain produced by humidity is detected by detector and the mechanical strain is converted into a digital value using computers.

Fluid sensors

Fluid sensors are the devices that are used to sense fluids on basis of their viscosity density and other parameters.

For example fluid sensors can be used to measure cholesterol bye examine the blood samples or can also be used for detection of diabetes pregnancies and other hazardous gases in fluids chemicals blood urine etc.

There are many techniques which are used for development and building of fluid sensors hair we discuss about one such technique namely SPR which stands for surface plasmon resonance

What is SPR

SPR stands for surface plasmon resonance in this technique light is exposed on a certain metal depending on its excitation wavelength certain electrons gets excited and leaves their valence States then this metal plate is exposed with fluid to be used as sample depending on the characteristics of fluid the exciting electron gets in contact with them and produce resonance in the fluid this resonance is detected by the detector and characteristic curves are made which determine the fluid property with respect to that of Ideal conditions.

A new use of SPR

Researchers at the University of Hannover Germany have developed a self-contained fibre optic sensor for smartphone with the potential for use in wide variety of biomolecular test including dose for pregnancy or monitoring diabetes the readings of the sensor can run through an application on smartphone which provide real time results when properly province the smartphone user has ability to monitor multiple types of body Fluids that is blood urine saliva sweat breath in case of Medical Application the sensor reading can be combined with GPS sensing of smartphone and user can be guided to next drugstore hospital or the ambulance the sensor uses the optical phenomenon of surface plasmon resonance to detect composition of liquid or presence for the particular biomolecules or traces of gas surface plasmon resonance occurs when a fixed beam of light strikes a metallic film most of the light is reflected but a small band is absorbed by the film causing the electrons to resonate it is a phenomenon commonly used for biosensing but typically requires a bulky lab for light detection and light sources as we know that our smartphones already have both of these allowing minimalist u shaped device the researchers designed to consists only r400 micrometre diameter core multimode fibre with silver coated sensing region.

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