Essay: Optical fibers

Abstract:
Optical fibers have been extensively utilized in both the communications and sensors industries. In addition to their use in transmission, they have been used to realize many fiber devices, including sensors. This project focuses on the investigation of simple optical fiber sensors based on wave interference in multimode fibers with possible use in strain sensing on industrial pipelines.
The sensors are composed of optical fiber sections, where modal interference takes place in the multimode sections, which are sensitive to applied disturbances. Optical fibers and semiconductor optical sources and detectors have been studied. Multimode interference in different waveguide structures is to be studied and numerically simulated. The proposed sensors are then to be experimentally realized, characterized and investigated for possible application in bending and strain sensing.
Introduction:
Allah created sun, and made it to be the first light source on the planet. Without sun, there’ll be no life nor creatures. After man inhabited earth, he tried to find other sources of light to be able to see at night, so he found a method to light fire by igniting sparks using flints. With the passage of time, he created light by oil, and then electrical lamps. With every discovery, man was utilizing light in different ways; he used it in lightning, warming, communication, etc…
Light was used in communication since the old times; man used it to lead ships to land with lighthouses built on coasts. The first lighthouse was built in 270 B.C. in Alexandria. With the advancement of human technology, the reflection of light on mirrors was used to send messages via light signals.
Man was fascinated by the speed of light, so he tried sending voice messages through it. At 1880, Alexander Graham created a device he called “Light-Phone”, which he was able to use in sending his voice for a distance of 200m, but it wasn’t put in use due to its distortion by weather conditions, and the possibility of information transferred to get leaked. Light-Phone didn’t get past test stages due to the unavailability of a high efficiency light source and low-loss medium, the sun source and light medium weren’t satisfactory. Those experiments waited 80 years before they reached a vital step, which is the invention of laser in 1960. LASER (Light Amplification by Stimulated Imation Radiation) provides a tight-band high-energy light beam source feeding on an electrical source which was a suitable method for information transfer.
However, the experiments of using laser beam in communication via air wasn’t applicable in practice, and in the commercial field due to the danger of blinding human vision when the laser light band contacts the eye.
But the invention of laser devices motivated researchers to utilize it in communication by using glass as a carry-medium. However, the purity of the available glass at the time was a big barrier in front of valid long-distance communications.
In 1970, a company specialized in glass technology invented a 4db\\km optical fiber cable, which meant the energy of the light signal transferred through the cable gets halved after around 800m. Although this is a bad value by today standards, it was a major forward step at the time.Next to laser diodes, Light Emitting Diode (LED) was invented and used in small networks, for example: Connecting between personal computers and local information networks, also airplanes control systems.
In this project, we’re going to study optical fiber and make a sensor based on it.
The subjects covered in this report are:
Optical Fibers, Fiber Cables, Fiber Advantages, Loss , Propagation, Connectors, Splitters, Optical Detectors, Optical Spectrum Analyzer, and Sources.
And we’re talked about :
Optical Sensors, Multimode Interference Sensors, and general sensor characteristics.
And we’re going to perform experiments on:
Optical fibers, optical sensors, and making a Multimode Interference Sensor.
Optical Fibers:
3.1- Introduction:
An optical fiber is a single, thin filament drawn from molten silica glass. These fibers are used as the transmission medium in high-speed, high-capacity communications systems that convert information into light, which is then transmitted via Optical Fiber cable. Currently, American telephone companies represent the largest users of Optical Fiber cables, but the technology is also used for power lines, local access computer networks, and video transmission.
In an Optical Fiber communications system, cables made of optical fibers connect data links that contain lasers and light detectors. To transmit information, a data link converts an analog electronic signal—a telephone conversation or the output of a video camera—into digital pulses of laser light. These travel through the optical fiber to another data link, where a light detector reconverts them into an electronic signal.
3.2- Optical Fiber Cable Components:
Figure 3.1- Optical Fiber Cable Components
Core – Transparent plastic or glass through which light travels
Cladding – Glass covering surrounding the core that acts as a mirror to reflect light back into the core. This is called total internal reflection
Buffer coating – Coats and protects the fiber
Aramid yarn strength member – Reinforces the integrity of data transmission through the optical fibers in the cable
Protective outer jacket – Extruded PVC is typical
3.3- Propagation Modes In Optical Fibers:
Fiber-optical cable has two propagation modes:
1- Multimode
2- Single mode.
They perform differently with respect to both attenuation and time dispersion. The single-mode fiber-optical cable provides much better performance with lower attenuation. To understand the difference between these types, we first must understand the meaning of “mode of propagation.”
Light has a dual nature and can be viewed as either a wave phenomenon or a particle phenomenon that includes photons and solitons. Solitons are special localized waves that exhibit particle-like behavior. For this discussion, we consider the wave mechanics of light. When the light wave is guided down a fiber-optical cable, it exhibits certain modes. These are variations in the intensity of the light, both over the cable cross section and down the cable length. These modes are actually numbered from lowest to highest. In a very simple sense, each of these modes can be thought of as a ray of light. For a given fiber-optical cable, the number of modes that exist depends on the dimensions of the cable and the variation of the indices of refraction of both core and cladding across the cross section. The various modes include multimode step index, single-mode step index, single-mode dual-step index, and multimode graded index.
Single Mode:
Single Mode Optical Fiber cable has a small diameter core that allows only one mode of light to propagate. Because of this, the number of light reflections created as the light passes through the core decreases, lowering attenuation and creating the ability for the signal to travel faster.
Figure 3.2- Single Mode fiber is usually 9/125 in construction.
Classification:
1- Single-Mode Step Index:
Single-mode propagation is illustrated in below Figure 3.3. This diagram corresponds to single-mode propagation with a refractive index profile that is called step index. As the figure 3.3 shows, the diameter of the core is fairly small relative to the cladding. Because of this, when light enters the optical fiber cable on the left, it propagates down toward the right in just a single ray, a single mode, which is the lowest-order mode. In extremely simple terms, this lowest-order mode is confined to a thin cylinder around the axis of the core. The higher-order modes are absent.
2- Single-Mode Dual-Step Index:
These fibers are single-mode and have a dual cladding. Depressed-clad fiber is also known as doubly clad fiber. Figure 3.4 below corresponds to single-mode propagation with a refractive index profile that is called dual-step index. A depressed-clad fiber has the advantage of very low macro bending losses. It also has two zero-dispersion points and low dispersion over a much wider wavelength range than a singly clad fiber. SMF depressed-clad fibers are manufactured using the inside vapor deposition (IVD) process. The IVD or modified chemical vapor deposition (MCVD) process produces what is called depressed-clad fiber because of the shape of its refractive index profile, with the index of the glass adjacent to the core depressed. Each cladding has a refractive index that is lower than that of the core. The inner cladding the lower refractive index than the outer cladding.
Multimode:
Multimode Optical Fiber cable has a large diameter core that allows multiple modes of light to propagate. Because of this, the number of light reflections created as the light passes through the core increases, creating the ability for more data to pass through at a given time. Because of the high dispersion and attenuation rate with this type of fiber, the quality of the signal is reduced over long distances. This application is typically used for short distance, data and audio/video applications in LANs. RF broadband signals, such as what cable companies commonly use, cannot be transmitted over multimode fiber.
Classification:
1- Step-Index Multimode Fiber
Due to its large core, some of the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they bounce off the cladding. These alternate paths cause the different groups of light rays, referred to as modes, to arrive separately at the receiving point. The pulse, an aggregate of different modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses to prevent overlapping limits the amount of information that can be sent. This type of fiber is best suited for transmission over short distances.
Figure 3.6- Multimode Step Index
2-Graded-Index Multimode Fiber:
Contains a core in which the refractive index diminishes gradually from the center axis out toward the cladding. The higher refractive index at the center makes the light rays moving down the axis advance more slowly than those near the cladding. Due to the graded index, light in the core curves helically rather than zigzag off the cladding, reducing its travel distance. The shortened path and the higher speed allow light at the periphery to arrive at a receiver at about the same time as the slow but straight rays in the core axis. The result: digital pulse suffers less dispersion. This type of fiber is best suited for local-area networks.
Figure 3.7- Multimode Graded Index
Fiber Cables :
4.1- Types of cable:
There are two basic types of cable. Simplex and Duplex. Both types of cable come in: Singlemode and Multimode. Singlemode is for long distance cable runs and Multimode is for shorter cable runs.
1- Simplex Cable:
Simplex fiber optical cable consists of a single fiber and is typically used in applications that only require one-way data transfer.
2- Duplex Cable:
Duplex Optical Fiber cable consists of two fibers and is typically used for applications that require simultaneous, bi-directional data transfer.
4.2- Fiber Cables Designs:
There are two basic types of cable design. They are: Loose Tube (typically used for “OSP” outside plant installations) and Tight Buffered (typically used for inside installations).
1- Loose tube fiber cable consists of:
Multiple 250µm coated fibers
One or more loose tubes holding those fibers
Gel-fill to block moisture and protect movement of the fibers
Central strength member
Aramid yarn strength member
Outer jacket
2- Tight Buffered fiber cable consists of:
900µm tight buffer around a 250µm fiber
Central Strength member
Aramid yarn strength member
Outer jacket
Fiber Optical Advantages:
1- Fiber optical cables have a much greater bandwidth than metal cables. This means that they can carry more data.
2-Fiber optical cables are less susceptible than metal cables to interference.
3- Fiber optical cables are much thinner and lighter than metal wires.
4-Data can be transmitted digitally (the natural form for computer data) rather than in analog format.
Loss in Optical Fibers:
Loss or attenuation and pulse dispersion represent the two characteristics of an optical
fiber most important in determining the information-carrying capacity of a fiber optical communication system. In digital communication systems, information to be sent is first coded in the form of pulses, and these pulses of light are then transmitted from the transmitter to the receiver, where the information is decoded. A typical fiber optical communication system (Fig. 6.1) consists of a transmitter, which could be either a laser diode or a light-emitting diode, whose light is modulated by the signal and coupled into an optical fiber. Along the path of the optical fiber, there are splices,
which are permanent joints between sections of fibers, and repeaters, which boost
the signal and correct any distortion that may have accumulated along the path of the
fiber. At the end of the link, the light is detected by a photo detector, which converts
the optical signals to electrical signals, which are then processed electronically to
retrieve the signal. The greater the number of optical pulses that can be sent per
unit time and still be detectable and resolvable at the receiver end, the larger will
be the transmission capacity of the system. A pulse of light sent into a fiber gets
attenuated as it propagates through the fiber, and if the loss is large, there would not
be enough light for the detector to separate the signal from the noise, and thus it cannot detect individual pulses. In addition to the attenuation, the pulse broadens in time as it propagates through the fiber. This phenomenon is called pulse dispersion,
The lower the attenuation (and similarly, the lower the dispersion), the greater will be the required repeater spacing and therefore the lower will be the cost of the system.
Figure 6.1- Typical Optical Fiber communication system. C, connector; S, splice; R, repeater, T, transmitter, D, detector.
The decibel is related to the ratio of two power levels, the loss in decibels is given by
Thus, if the output power is only half of the input power, the loss is 10 log 2 ≈3dB.Similarly, 20- and 10-dB losses will correspond to power reductions by factors of100 and 10, respectively; every multiplication by a factor of 10 increases the value in decibels by 10. Thus, a loss of 60 dB would correspond to an output power that is only one millionth of the input power. On the other hand, if 96% of the light is transmitted through the fiber, the loss is given by
6.1- The loss mechanism:
When light propagates through any medium, even the purest materials, it suffers loss,
due to various mechanisms: scattering and absorption, among others, caused by the
atoms and molecules that form the material. in 1966 the most transparent glass available had a loss of 1000 dB/km, due primarily to trace amounts of impurities
present in the glass. A loss of about 1000 dB/km (or equivalently, 1 dB/m) implies that for every 10 m the power will fall by a factor of 10. Thus, after propagating through1 km of such a fiber, the output power will be 〖10〗^(-100)of the input power; this value is, for all practical purposes, zero (i.e., almost no light will emerge from the output).
6.2- Rayleigh scattering:
Rayleigh scattering is a basic mechanism by which light gets scattered by very small in homogeneities as it propagates through any medium. Rayleigh scattering loss is wavelength dependent and is such that shorter wavelengths scatter more than longer wavelengths with the loss proportional to λ−4, where λ is the optical wavelength. It is this phenomenon that is responsible for the blue color of the sky. As sunlight passes through the atmosphere, the component corresponding to the blue color gets scattered more than the component corresponding to the red color (since blue wavelengths are shorter than red wavelengths). Thus, more blue than red reaches our eye from the sky and the sky looks blue. This is also the reason that both a rising and a setting sun appear red to our eyes.
Rayleigh scattering is an important component of the scattering of optical signals in optical fibers. Silica fibers are glasses, disordered materials with microscopic variations of density and refractive index. These give rise to energy losses due to the scattered light, with the following coefficient.
n is the refraction index
p is the photoelastic coefficient of the glass
k is the Boltzmann constant
β is the isothermal compressibility
Tf is a fictive temperature, representing the temperature at which the density fluctuations are “frozen” in the material.
Figure 6.2 – Rayleigh scattering in optical fiber
Figure 6.3- Typical wavelength dependence of attenuation for a silica fiber.
The lowest attenuation occurs at 1550 nm.
Rayleigh scattering causes attenuation of optical signals as they propagate through
an optical fiber. Very small in homogeneities present in the fiber scatter light out of
the fiber, leading to loss.
Figure 6.3 shows the attenuation spectrum of a typical optical fiber. The primary
reason that the loss coefficient decreases up to about 1550 nm is the Rayleigh scattering loss. The two absorption peaks around 1240 and 1380 nm are due primarily to traces of water and traces of metallic ions. If these impurities are removed completely, the two absorption peaks will disappear (see Fig. 6.4) and we will have very low loss in the entire range of wavelength from 1250 to 1650 nm. Such fibers are now available commercially (Fig. 6.5). For wavelengths longer than about 1650 nm, the loss increases, due to the absorption of infrared light by the silica molecules themselves. As this is an intrinsic property of silica, no amount of purification can remove this infrared absorption tail.
As we can see from Fig. 6.3 , there are two windows at which loss attains its
minimum value. The first window is around 1300 nm (with a typical loss coefficient
of less than 1 dB/km), where the material dispersion is negligible. However, the loss attains its absolute minimum value of about 0.16 dB/km when the wavelength is around 1550 nm, as a consequence of which the distance between two consecutive repeaters (used for amplifying and reshaping the attenuated signals) could be increased significantly. Furthermore, the 1550-nm window has become extremely important in view of the availability of erbium-doped fiber amplifiers.
Figure 6.4- Loss spectrum of an ultimately low water content in an optical fiber.
The low-loss window extends from 1250 to 1650 nm (about 50 THz), and such fibers are now available commercially.
Figure 6.5- Loss spectrum of a low-water-peak single-mode fiber from Corning.
Figure 6.6- Loss spectrum of LEAF fiber from Corning. The low-loss spectrum is divided
into various bands, which are given in Table 6.1.
TABLE 6.1: Wavelength Bands Used in Optical Fiber Communication Systems
Wavelength Band Wavelength Region (nm)
O-band (old band) 1260–1360
E-band (extended band) 1360–1460
S-band (short band) 1460–1530
C-band (conventional band) 1530–1565
L-band (long band) 1565–1625
Figure 6.6 shows the attenuation spectrum of a commercial fiber (called
LEAF, for “large effective area fiber”) from Corning superimposed on the various
wavelength bands of operation that can be used with such low-loss fibers.
The wavelength ranges corresponding to various wavelength bands are listed in
Table 6.1. Current Optical Fiber communication systems use primarily the C- and the
L-bands. The coarse wavelength-division-multiplexing (CWDM) scheme utilizes
wavelength channels spaced 20 nm apart. Due to the large channel spacing, the wavelength of a transmitter need not be very precise, and thus uncooled laser diodes cane used. Such a system was developed for metropolitan applications in which cost is a very important factor.
Optical Propagation:
7.1- Ray Theory of Propagation:
The transmission of light along an optical fiber where light is described as a simple ray is known as the ray theory.
The advantage of the ray approach is getting a clearer picture of the propagation of light along a fiber. The ray theory is used to approximate the light acceptance and guiding properties of optical fibers.
Two types of rays can propagate along an optical fiber.
A- Meridional rays: Rays that pass through the axis of the optical fiber. Meridional rays are used to illustrate the basic transmission properties of optical fibers.
B- Skew rays: Skew rays are rays that travel through an optical fiber without passing through its axis.
When a light ray encounters the interface of a medium, the light ray is refracted and its direction of propagation changes according to Snell’s law of refraction.
n1 sin⁡〖(∅1)〗=n2 sin⁡〖(∅2)〗
Figure 7.1- For angles of incidence less than the critical angle, the rays are reflected with leakage refraction.
Figure 7.2- For angles of incidence equal to the critical angle, the rays are reflected with the refracted wave propagating parallel to the interface and the angle of refraction reaching 90°
Figure 7.3- For angles of incidence greater than the critical angle, the rays is Totally Reflected. – Total internal reflection occurs.
The core (the interior layer) with refractive index n1 serves as the medium for light propagation, while the cladding (the exterior layer) has a lower refractive index n2 where n1 >n2 assuring that light rays are reflected back to the core.
Figure 7.4- Representation of the acceptance angle
Figure 7.5- How a light ray enters an optical fiber.
7.2- Electromagnetic Theory of Wave Propagation:
Here we use electromagnetic wave behavior to describe the propagation of light along a fiber. We call set of guided electromagnetic waves the modes of the fiber.
Maxwell’s equations provide the basis for the study of electromagnetic wave propagation.
‘
Electric field E , Magnetic field H , Electric flux density D , Magnetic flux density B.
The divergence conditions:
No free charges
No free poles
∇ is vector operator.
The four field vectors are related by the relations:
ε: Dielectric permittivity.
μ: Magnetic permeability of the medium
a light wave can be represented as a plane wave. A plane wave is described by its direction, amplitude, and wavelength of propagation. A plane wave is a wave whose surfaces of constant phase are infinite parallel planes normal to the direction of propagation.
Figure 7.6- propagation along an optical fiber.
The change in the propagation constant for different wavelengths is called chromatic dispersion. The change in propagation constant for different modes is called modal dispersion.
These dispersions cause the light pulse to spread as it goes down the fiber. Some dispersion occurs in all types of fibers.
Maxwell’s equations describe electromagnetic waves or as having two components:
1-Electric field, E(x, y, z),
2-magnetic field, H(x, y, z).
The electric field, E, and the magnetic field, H, are at right angles to each other. Modes traveling in an optical fiber are said to be transverse. The transverse modes, propagate along the axis of the fiber. The mode field patterns are said to be transverse electric (TE). In TE modes, the electric field is perpendicular to the direction of propagation.
Optical Fiber Connectors:
A non-permanent device for connecting two fibers or fibers to equipment where they are expected to be disconnected occasionally for testing or rerouting. It also provides protection to both fibers.
An optical fiber connector terminates the end of an optical fiber, and enables quicker connection and disconnection than splicing. The connectors mechanically couple and align the cores of fibers so that light can pass. Connectors are available in APC (Angle Polished Connector) and UPC (Ultra Polished Connector) varieties.
8.1- Types of Connectors:
SC Connectors:
SC stands for Subscriber Connector. A general purpose push/pull style connector. SC has an advantage in keyed duplexibility to support send/receive channels. Mostly used for CATV applications.
Figure 8.1- SC Connectors
FC Connectors:
FC stands for Fixed Connection. It is fixed by way of threaded barrel housing. FC connectors are typical in test environments and for singlemode applications. FC connectors were designed for use in high-vibration environments
Figure 8.2- FC Connectors
ST Connectors:
ST stands for Straight Tip. A quick release bayonet style connector with long ferrule. STs were predominant in the late 80s and early 90s. Common connector for multimode fibers.
Figure 8.3- ST Connectors
LC Connectors:
LC stands for Lucent Connector. The LC is a small form-factor connector much like the SC connector but with a ferrule that is half the size. (Duplex shown right)
Figure 8.4- LC Connectors
8.2- Connectors Properties:
LC, SC, and ST Connectors
Fiber Size/Type Insertion Loss
Typical/Maximum (dB) Reflectance (dB) Ferrule
62.5 µm MM 0.1/0.5 ≤ -20 Zirconia
50 µm MM 0.1/0.5 ≤ -20 Zirconia
50 µm LOMMF 0.1/0.5 ≤ -26 Zirconia
Figure 8.5- FC, SC, and LC Connectors, SMF and MMF.
Optical Fiber Splitters, Combiners and Isolators:
9.1- Splitters:
An optical fiber splitter combines light signals and splits them out over single or multiple outputs. Multicom splitters are immune to electro-magnetic interference (EMI), consume no electrical power, and do not add noise to system design.
Multicom splitters can be fabricated in custom fiber lengths and with any type of connector.
Optical Fiber splitters take an optical signal and supply two outputs. They can further be described as either Y-couplers or T-couplers.
Y-couplers have equal power distribution, meaning that the two output signal each receive half of the transmitted power.
T-couplers have an uneven power distribution. The signal outputs still carrier the same signal, however the power of one output is greater than the second output.
Figure 9.1 – Optical splitter
9.2- Combiners:
Optical Fiber combiners receive two signals and provide a single output. The output signal is typically comprised of multiple wavelengths, due to the amount of interference that occurs when attempting to combine two signals that share the same wavelength.
Figure 9.2 – Optical combiner
9.3- Isolators:
Optical isolators allow light to pass in only one direction. This prevents scattered or reflected light from traveling in the reverse direction.
E.g., can keep backward-traveling light from entering a laser diode and possibly causing instabilities in the optical output.
Operation of laser diodes (LD) and optical amplifiers (EDFA) become unstable and generate noise when returned light enters.
Optical isolator utilize Faraday effect to cut off the returned beam and stabilize the operation of lasers and amplifiers.
Key parameters are insertion loss and excess loss.
Example of circulators:
Figure 9.3 – circulators.
Figure 9.4 – Isolators
10- Optical Detectors:
Optical detectors are the components that convert the light wave energy of Optical Fiber communications into electrical signals for recovery of data.
The most common detector is the semiconductor photodiode, which produces current in response to incident light. Detectors operate based on the principle of the p-n junction. An incident photon striking the diode gives an electron in the valence band sufficient energy to move to the conduction band, creating a free electron and a hole. If the creation of these carriers occurs in a depleted region, the carriers will quickly separate and create a current. As they reach the edge of the depleted area, the electrical forces diminish and current ceases. While the p-n diodes are insufficient detectors for Optical Fiber systems, both PIN photodiodes and avalanche photodiode (APDs) are designed to compensate for the drawbacks of the p-n diode.
10.1- Optical Detector Properties:
Optical Fiber communications systems require that optical detectors meet specific performance and compatibility requirements. Many of the requirements are similar to those of an optical source. Optical Fiber systems require that optical detectors:
Be compatible in size to low-loss optical fibers to allow for efficient coupling.
Have a high sensitivity at the operating wavelength of the optical source.
Have a sufficiently short response to handle the system’s data rate.
Produces low amounts of noise in the system.
Maintain stable operation in changing environmental conditions, such as temperature.
Optical detectors that meet many of these requirements and are suitable for Optical Fiber systems are semiconductor photodiodes. The principal optical detectors used in Optical Fiber systems include semiconductor positive-intrinsic-negative (PIN) photodiodes and avalanche photodiodes (APDs).
10.2- Types of Photodiode Detectors:
1- PIN Photodiode:
The conversion of light into electric current takes place in the I-region of a PIN diode
structure. The detectors are typically integrated circuits that also include an Optical Fiber interface, coplanar waveguide interconnections to support high frequency signals extracted from the light wave input. III-V semiconductor material is used, such a InGaAs/InP or InGaAs/InAlAs, with predictable photoelectric properties. The doping, thickness, and other properties are controlled to obtain the optical response, sensitivity, noise and other desired performance properties. Like integrated transistors, thinner, smaller area I-layers result in higher speed (frequency response) of the detector. The series resistance of the device is also critical for high-speed response. InGaAs/InAlAs detectors can operate to beyond 50 GHz.
Figure 10.1- (A) PIN Photodiode
Figure 10.2- (B) PIN Photodiode
2- Avalanche Photodiode (APD):
The avalanche photodiode (APD) operates as the primary carriers, the free electrons and holes created by absorbed photons, accelerate, gaining several electron Volts of kinetic energy. A collision of these fast carriers with neutral atoms causes the accelerated carriers to use some of their own energy to help the bound electrons break out of the valence shell. Free electron-hole pairs, called secondary carriers, appear. Collision ionization is the name for the process that creates these secondary carriers. As primary carriers create secondary carriers, the secondary carriers themselves accelerate and create new carriers. Collectively, this process is known as photo multiplication. Typical multiplication ranges in the tens and hundreds. APDs require high-voltage power supplies for their operation. The voltage can range from 30 or 70 Volts for InGaAs APDs to over 300 Volts for Si APDs. This adds circuit complexity. Also, APDs are very temperature sensitive, further complicating circuit requirements. In general, APDs are only useful for digital systems because they possess very poor linearity. Because of the added circuit complexity and the high voltages that the parts are subjected to, APDs are always less reliable than PIN detectors. This, added to the fact that at lower data rates, PIN detector-based receivers can almost match the performance of APD-based receivers, makes PIN detectors the first choice for most deployed low-speed systems. At multi gigabit data rates,
however, APDs rule supreme.
Figure 10.3 – Avalanche Photodiode (APD)
Table 10.1— Comparison of PIN Photodiodes and APDs
Parameter PIN Photodiodes APDs
Construction Materials Si, Ge, InGaAs
Si, Ge, InGaAs
Bandwidth DC to 40+ GHz DC to 40+ GHz
Wavelength 0.6 to 1.8 µm 0.6 to 1.8 µm
Conversion Efficiency 0.5 to 1.0 Amps/Watt 0.5 to 100 Amps/Watt
Support Circuitry Required None High Voltage, Temperature Stabilization
Cost (Fiber Ready) $1 to $500 $100to $2,000
11-Optical Spectrum Analyzer (OSA):
Optical spectrum analyzer (OSA) is an embedded, integrated monitor that delivers precise measurements and powerful processing capabilities to coarse wavelength division multiplexing applications compliant.
An optical spectrum analyzer uses reflective and/or refractive techniques to separate out the wavelengths of light. An electro-optical detector is used to measure the intensity of the light.
The input to an optical spectrum analyzer may be simply via an aperture in the instrument’s case, an optical fiber or an optical connector to which a fiber-optic cable can be attached.
12- Optical Sources:
The heart of a Optical Fiber data system. It’s a hybrid device that converts electrical signals into optical signals and launches these optical signals into an optical fiber for data transmission. The device consists of an interface circuit, drive circuit, and components for optical source. (LEDs, ELEDs, SLEDs, LDs, etc.)
12.1- Laser:
A laser is a device that generates light by a process called stimulated emission. The acronym laser stands for Light Amplification by Stimulated Emission of Radiation. It’s commonly used in fiber optical.
12.1.1- Laser Types:
Gas lasers.
Chemical lasers.
Excimer lasers.
Solid-state lasers.
Fiber lasers .
Photonic crystal lasers.
Semiconductor lasers .
Dye lasers.
12.1.2 – Laser in Optical Fiber:
Light travels down an Optical Fiber glass at a speed = c/n, where n = refractive index. Light carries with it information. Different wavelength travels at different speed. This induces dispersion and at the receiving end the light is observed to be spread. This is associated with data or information lost.
The greater the spread of information, the more loss.
Figure 12.1 – Laser in optical fiber cable
12.1.3- Advantages of Fiber Lasers:
1-Superior Performance: Fiber lasers provide high beam quality over the entire power range.
2-Ease of Use: Many features of fiber lasers make them easy to operate, maintain and integrate into laser-based systems.
3-Compact Size and Portability: Fiber lasers are typically smaller and lighter in weight than traditional lasers, saving valuable space.
4-Choice of Wavelengths and Precise Control of Beam: The design of fiber lasers generally provides a broad range of wavelength choices.
12.1.4 – Laser Characteristics:
Lasing threshold is minimum current that must occur for stimulated emission.
Any current produced below threshold will result in spontaneous emission only.
At currents below threshold LDs operate as Entangled LEDs.
LDs need more current to operate and more current means more complex drive circuitry with higher heat dissipation.
Laser diodes are much more temperature sensitive than LEDs.
12.2- Light Emitting Diodes (LED) :
LED converts input electrical energy into output optical radiation in the visible portion of the spectrum.
LEDs are very similar to laser diodes, when operating below threshold current, all laser diodes act as LEDs. They’re used for multimode systems with 100-200 Mb/s rates.
12.2.1- LED types:
a- Surface Emitting LEDs:
Primary active region is a small circular area located below the surface of the semiconductor substrate. A well is etched in the substrate to allow the direct coupling of emitted light to the optical fiber.
b- Edge Emitting LEDs:
Primary active region is a narrow strip that lies beneath the semiconductor substrate. And they emit light at narrower angle which allows for better coupling and efficiency than SLEDs.
12.2.2- LED Advantages:
High efficiency light source and low power consumption.
Instantaneous switch-on.
Relatively low cost.
Long lifetime and low maintenance need.
Environmental friendly. It doesn’t depend on filaments that’ll burn out, become heated, or spread toxic gas.
Designability. LED open up many new design options, some of which weren’t possible before.
12.2.3- LED Characteristics:
When an LED is forward biased to the threshold of conduction, its current increases rapidly and must be controlled to prevent destruction of the device.
The light output is quite linearly proportional to the current within its active region.
Unlike ordinary diodes, the reverse voltage cannot by controlled by changing the concentration of the PN junction.
The life of the LED lengthens if it is used at a dissipation(junction temperature) below a certain level.
12.3- Superluminescent Diodes:
Superluminescent means amplified spontaneous emission: the emission of fluorescence which experiences significant optical gain within the emitting device.
SLDs are similar to laser diodes, in that they contain an electrically-driven p-n junction and an optical waveguide, but lack optical feedback so no laser action can occur. The optical output of an SLD is more powerful and more sharply confined than a standard LED.
The typical output power range for SLDs is a few microwatts to a few tens of mill watts, similar to that of a single-mode laser diode. Radiation emitted by SLDs has a much shorter coherence length than that produced by lasers, thus they can produce speckle-free beams. This a feature that is of use in telecommunications, sensor technologies, and some imaging applications.
SLD Advantages:
Emitters are fully compatible with the wavelengths used in the optical communications systems.
Efficient coupling of output into single-mode optical fibers due to its optical waveguide.
Cost-effective and reliable.
SLD Characteristics:
Total optical power emitted by an SLED depends on the injected current (bias). Unlike laser diodes, the output intensity does not exhibit a sharp threshold but it gradually increases with current.
The optical power emitted by SLEDs is distributed over a wide spectral range.
Superluminescent light emitting diodes are based on the generation and on the amplification of spontaneous emission in a semiconductor waveguide.
The optical power emitted by semiconductor active devices is always affected by fluctuations (intensity noise) that are induced by the spontaneous emission.
Intensity modulation of SLEDs can be easily achieved through direct modulation of the bias current.
13- Optical Sensors:
Optical sensors are sensors using optical fibers as the sensing element, or as a means of relaying signals from a remote sensor to the electronics that process the signals.
Fiber optical sensor technology offers different parameter measurements such as strain, pressure, temperature, current and many more. For that, different type of sensors are used and these sensors converts these parameters to optical parameters like light intensity or phase or polarization of light. These converted parameters are transmitted using an optical link over a long distance. Optical Fiber for sensing application is used to communicate with sensors through optical channel or fiber as optical sensor. Optical sensors itself are efficient to monitor physical, biological, chemical changes in the object or over a process.
13.1- Introduction:
Over the past few years there have been revolutionary inventions done in field of fiber optical. This is due to the advantages of optical transmission over electrical transmission. Optical fiber provides terrific large bandwidth which gives rise to very high data rate and now a days, high data rate is the key factor for data transmission. There are many more advantages of optical fiber communication like data security, data multiplexing, ease to make optical source and optical detectors.
In the field of optical fibers, lot of research has been done, which focuses on suitable design of fibers. Very high demand of optical fiber in telecommunication lowered the cost of fiber within countable years. As a result, the possibility to replace ordinary sensors like acceleration, electric and magnetic field measurement, temperature, pressure, acoustics, vibration, linear and angular position, strain, humidity, viscosity, chemical measurements and many others, should be replaced by optical sensors. There are other fascinating reasons, such as small size, light weight, immunity to electromagnetic interference, high temperature performance, large bandwidth etc.. which give rise to implementation of optical sensor in regular life.
13.2- Advantages of Optical Fiber Sensors:
1- Completely passive: can be used in explosive environment.
2- Immune to electromagnetic interference: ideal for microwave environment.
3- Resistant to high temperatures and chemically reactive environment: ideal for harsh and hostile environment.
4- Small size: ideal for embedding and surface mounting.
5- High degree of biocompatibility, deal for medical applications like
intra-aortic balloon pumping.
6- Can monitor a wide range of physical and chemical parameters.
7- Potential for very high sensitivity, range and resolution.
8- Complete electrical insulation from high electrostatic potential.
9- Remote operation over several km lengths without any lead
sensitivity: ideal for deployment in boreholes or measurements
in hazardous environment.
10- Multiplexed and distributed sensors are unique in that they provide measurements at a large number of points along a single optical cable: ideal for minimizing cable deployment and cable weight, or for monitoring extended structures like pipelines, dams.
Optical sensors also provide many advantages over conventional electronics sensors. Some of them as listed below:
They are easy to implement in any structure due to their small size and cylindrical geometry.
No moving mechanical part.
Robust to environment.
Compact in size and light in weight.
High sensitivity.
High integration possibility.
Low cost components
Remote sensing capability.
Immune to electromagnetic interference and radio .
Frequency interface.
13.3- Optical Fiber Sensor Principles:
The general block diagram of Optical Fiber sensor is shown in figure 13.1 . It consists of optical source (Laser, Laser diode, LED, etc.), optical fiber as transmission channel, sensing element, optical detector and end processed devices (oscilloscope, optical spectrum analyzer, etc.