Fibre Optical Sources & Detectors

Fiber Optical Sources & Detectors

GENERAL OPTICAL FIBER COMMUNICATION SYSTEM The major elements of general fiber optic communication system are shown in fig. A. The fiber optic system can be described in one sentence as a transmission system employing a light emitting source, turned on off very rapidly by electrical impulses, whose emission are sent through an optical fiber to light sensitive receiver to convert the changing light intensities back into electrical impulses.

The information source provides an electrical signal to a transmitter. The main function of the transmitter section is to convert an electrical signal into optical signal. The transmitter consist of a light source and its drive circuitry. The light source may be either semiconductor LASER or light emitting diode (LED) depending on application and requirement of optical fiber communication system. The transmission medium is optical fiber cable. The cable offers mechanical strength and environmental protection to the optical fiber contained inside. The cable may also contain copper wires for powering repeaters which are needed for periodically amplifying and reshaping the signal when the link spans long distances.

The receivers consists of photo detector, pulse amplifier and the signal restoring circuitry. The main function of the receiver is to convert optical signal into electrical signal. Photo diodes (p-n, p-i-n or avalanche photo diode) and in some

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Fiber Optical Sources & Detectors

instances photo transistor and photo conductors are utilised for the detection of the optical signal and optical electrical to conversion.

Additional components includes optical connectors, splices, couplers or beam splitters etc. The connectors and splices are required for joining fiber pieces together to achieve ling distance communication. The optical couplers are required to coupled light source to fiber at transmitter side and from fiber to photo-detector at receiver side. Optical amplifier the optical signal without changing it into electrical form.

The optical fiber generally contain several cylindrical hair thin glass fibers each of which is independent communication channel.

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Fiber Optical Sources & Detectors

LIGHT SOURCES Essentially, there are two devices commonly used to generate light for fiber optic communication system.

1. Light emitting diode (LED) 2. Injection LASER diode (ILD)

Light Emitting Diode (LED): It is simply a P-N junction diode. It is usually made from semiconductor material such as aluminium gallium arsenide (AIGaAs) or gallium arsenide phosphide (GaAsP). LED emits light by spontaneous emission, light is emitted as result of the recombination of electrons and holes. When LED is forward biased, minority charge carriers are injected across the P-N junction, these minority charge carriers are injected across the P-N junction, these minority carriers recombine with the majority carriers and give up energy in the form of light. This process is same in the conventional diode expect that the process is radiative, a photon is produced. A photon is a quantum of electromagnetic wave energy. The energy gap of the material used to construct the LED determined whether the light emitted by it is visible or invisible and of what colour.

The simplest LED structure are homojunction, epitaxially grown or signal diffused devices.

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Fiber Optical Sources & Detectors

Epitaxially Grown LED Epitaxially grown LEDs are generally constructed of silicon doped gallium arsenide. This is shown in fig. B. A typical wavelength of light emitted from this construction is 940 nm, and a typical output power approximately 3m W at

100

mA of forward current.

Planer Diffused Homojunction LED Planer diffused homojunction LED is shown in fig. C. The typical out put power from this structure is 500 micro watts at wavelength of 900nm. The primary disadvantage of homo junction LED is the non directionality of their light emission, which makes them a poor choice as a light source for fiber optic system.

Planer Hetrojunction LED The planer hetero junction LED is quite similar to the epitaxially grown LED except that the geometry is designed such that the forward current is concentrated from six layer of semiconductor materials as shown in fig. D.

The planer hetero junction LED has several advantages over the homo junction type. They are : 1. The increase in current density generates a more brilliant light spot. (Higher directivity). 2. The smaller emitting area makes it easier to couple its emitted light into a fiber. Govt.Poly.Washim.

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Fiber Optical Sources & Detectors

3. The small effective area has smaller capacitance which allows the planer hetero junction LED to be used at higher speeds.

The radiant light power emitted from the LED is a linear as function of the forward current passing through the LED.

Edge Emitting Double Hetrojunction LED Edge emitting double hetero junction LED gives highly directive light beam.

It consists of two different alloy layers having different band gap and

refractive index on each side of the active region which is the source of incoherent light source. The construction of edge emitting bouble hetero junction LED is shown in fig.E.

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Fiber Optical Sources & Detectors

CHARACTERISTICS OF LED i) Radiance (Brightness) : Radiance is defined as the optical power radiated into a unit solid angle per unit area of the emitting surface. It is measured in watts/cm2. High radiance is necessary to couple sufficiently high optical power levels into a fiber.

ii) Response Time : The emission response time is the time delay between the application of current pulse and the onset of optical emission. This time delay is the factor, limiting the bandwidth with which the source can be modulated directly by varying the injected current.

iii) Quantum Efficiency : The quantum efficiency is related to the fration of injected electron hole pairs that recombine radiatively.

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Fiber Optical Sources & Detectors

INJECTION LASER DIODE The word LASER is an acronym for light amplification by stimulated emission of radiation.

LASERs are constructed from many different materials,

including gases, liquids and solids. Although, the type of LASER used most often for fiber optic communication is the semiconductor LASER.LED emits the light having combinations of various wavelengths (ultimately various frequencies) where as LASER emits light a of signal frequency.

Therefore LED is called non

monochromatic source and LASER is monochromatic source.

The injection LASER diode (ILD) is similar to the LED. In fact below a certain threshold current an ILD acts as LED. Above the threshold current, an ILD oscillates; Lasing action occurs. The construction of an ILD is similar to that of LED except that the ends are highly polished. The mirror like end surfaces traps the photons in the active region, as they reflect back & forth, stimulate free electrons to recombine with the holes. The two larger sides are deliberately roughened in the cutting process to discourage the light emission.

When this heterojunction diode is forward biased with a DC voltage, both ends of the LASER chip emit light. When one polished end is gold plated, the other end will emit light. The construction of ILD is shown in fig. F.

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Fiber Optical Sources & Detectors

The three key transition processes involved in LASER action are : 1. Photon absorption. 2. Spoteneous emission. 3. Stimulated emission.

These three processes are represented by the simple two energy level diagrams in fig. G. Where E1 is the ground state energy and E2 is excited state energy. The open circle represents the initial state of electron & the heavy dot represents the final state. According to Plank’s law, a transition between these two stats involves the absorption or emission of photon energy, hv12 = E2 – E1.

Normally the system is in the ground state. When photon of energy hv12 impinges on the system an electron in state E1 can absorb the photon energy and be excited to state E2 as shown in fig. H. Since this is an unstable state, the electron will shortly return to the ground state, there by emitting a photon of energy.

hv12 = E2 – E1. This occurs without any external stimulation and is called spontaneous emission. As shown in fig. 1

The electron can also be induced to make a downward transition from the excited level to the ground level by external stimulation, as shown in fig. J. if a

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Fiber Optical Sources & Detectors

photon of energy hv12. This emitted photon is in phase with the incident photon, and the resultant emission is known as stimulated emission.

In a thermal equilibrium, the density of excited electrons is very small. Most of photons incident on the system will therefore be absorbed, so that stimulated emission is essentially negligible.

Stimulated emission will exceed

absorption only if the population of the excited state is greater than that of the ground state.

This condition is known as population inversion.

Since this is not an

equilibrium condition, population inversion is achieved by various ‘Pumping” techniques.

In semiconductor laser, population inversion is accomplished by

injecting electrons into the material at the device contacts to fill the lower energy states of the conduction band.

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Fiber Optical Sources & Detectors

ADVANTAGES OF INJECTION LASER DIODE (ILD) 1. But to highly directional pattern (i.e.LASER emits light which is concentrated in very narrow region in one direction), it is easier to couple their light into an optical fiber. This reduces the coupling losses and allows smaller fibers to be used. 2. The optical power output from ILD is greater that for an LED. The output optical power from ILD is 1 to 100 mW where as the output optical power from LED is 1 to 10 mW. 3. ILD can be used at higher bit rates (>200mb/s) and for longer distance communication than LED. 4. ILD generate monochromatic light, which reduces chromatic or wavelength dispersion.

Disadvantages of ILD : 1. ILDs are highly expensive than LED. 2. Because ILD operates at higher powers, they have much shorter life time than LEDs. 3. ILDs are more temperature dependant than LEDs.

Thermal stabilisation is

essential for ILD where LED does not require such thermal stabilization. Due to this LASER has complex drive circuitry than LED.

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Fiber Optical Sources & Detectors

LIGHT DETECTORS There are two devices that are commonly used to detect light energy in fiber optic communication receivers, PIN (Positive – intrinsic – Negative) diodes and APD (Avalanche Photo Diodes).

1) PIN Photo Diode : PIN photo diode is the most common device used as the light detector in fiber optic communication system.

A very lightly doped (almost pure or intrinsic) layer of n-type semiconductor material is sand witched between the junction of the two heavily doped n and p- type contact areas as shown in fig. K. Light enters the device through a very small windows and falls on the carriers void intrinsic material. The intrinsic material is made thick enough so that most of the photons that enter the device are absorbed by this layer. The PIN photo diode operates just the opposite of an LED. Most of the photons are absorbed by electrons in the valence band of the intrinsic material. When the photon are absorbed, they add sufficient energy to generate carriers in the depletion region and allow the current to flow through the device.

The PIN photo diodes has responsivity of the order of 0.5 microampere/microwatt. It has rise time about 1 ns and a very good frequency

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Fiber Optical Sources & Detectors

response upto 1 GHz. The required bias voltage lies between 5 to 10 volts. The disadvantages of this diode is its poor sensitivity and poor signal to noise ratio.

2) Avalanche Photo Diodes : An APD is p-i-p-n structure as shown in fig. L. Light enters the diode and is absorbed by the thin, heavily doped p-layer. This causes a high electric field intensity to be developed across the i-p-n junction. The high reversed biased field intensity causes impact ionization to occur near breakdown voltage of the junction. During impact ionization, the carrier can gain sufficient energy to ionize other bound electrons. These ionized carriers, in turn causes more ionization to occur. The process continues like an avalanche and is, effectively, equivalent to an internal gain or carrier multiplication. APDs are more sensitive than PIN diodes and require the less additional amplification. The responsivity and rise time of APDs are of the order of 15 micro ampere/micro watt and 2 ns respectively. The disadvantage of APDs are relatively long transit times, additional internally generated noise due to the avalanche multiplication factor and its temperature sensitiveness which requires compensating networks.

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Fiber Optical Sources & Detectors

PARAMETERS OF PHOTO DETECTORS i) Quantum efficiency (n) : Quantum efficiency is defined as the ratio of number of electrons-hole pairs generated to the number of incident photons on the surface of photo detector.

ii) Responsivity (R): The performance of photo detector is always characterised by reponsivity. It specifies the photo detector is always characterised by reponsivity. It specifies the photo current generated per unit optical power. It is given by,

R – IP/Po µA/µW Where Po

-

Optical power incident on photo diode.

Ip

-

current flowing through the photo diode due to optical power.

Responsivity is function of wavelength and material of photo diode.

iii)

Dark Current : Dark current is the current flowing through the photo diode without

light input.

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Fiber Optical Sources & Detectors

APPLICATION OF FIBER OPTIC COMMUNICATION SYSTEM The application of optical fiber communication systems extend in all facts of communication fields such as : 1. Metropolitan telephone exchanges. 2. Long haul commercial trunking systems. 3. Under sea transmission systems. 4. Local Area network (LAN) like intrabuilding communications, computer networking, cable T.V. etc. 5. Communication and control in hazardous situations such as coal mines, fuel mines etc. 6. Railway communication. 7. On board communication in aeroplane, shop, train etc. 8. Military applications including long distance communication, tractical field application, missile guidance systems, night vision system etc. 9. As sensors having very high sensitivity and large dynamic range. 10. Optical fiber have their use in medical application for obtaining cold light illumination in the fields of opthalmology, gynaecology, ENT, general surgery etc. 11. A light source of one end of a bundle of optical fiber illuminates whatever is at the other end. This can make a decorative lamp, a flexible illuminator for hard to reach places illuminated signboards. Govt.Poly.Washim.

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Fiber Optical Sources & Detectors

CONCLUSION While studying, delivering and adopting the knowledge of advance system or advance techniques we have to develop our basic knowledge and keep it fresh. As my seminar topic on Fiber Optical Sources and detectors, it is cleared somewhat basic about Fiber Optical Sources & Detectors.

The Fiber Optical System has several advantages over other communication system so this is the most efficient and accurate system in point to point and hard reach places.

REFERENCES 1. Communication System 2. Advance Communication System -

Vrinda Publication.

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Fiber Optical Sources & Detectors

Electrical Form Transmitter Input Signal

Drive Circuit

Light Source

Electrical Optical splice

Fiber flylead

Connector Optical Fiber Electrical signal Optical signal

Repeater

Optical coupler or Beam splitter

Optical Receiver Electronics To other equipment Optical Transmitter

Receiver Fiber flylead Electrical Optical Amplifier

Photodetector

Signal restorer

Signal Output

Figure: Major elements of an optical fibre transmission link. The basic components and the transmitter, cable, receiver. Additional elements include fibre and cable splices, repeaters, beam splitters and optical amplifiers.

Fig. A.

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Fiber Optical Sources & Detectors

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