Fiber Optic System TOPIC: INTRODUCTION TO: Basic concept of block diagram Characteristics of Optical fibre circuit devices such as LED,ILD, PIN photodiode and APD BY : Mohd Nasir bin Said Telecommunications Department ADTEC Kulim

Fiber optic System Block Diagram Light pulses Coder Fiber Repeater (long distance) Light Detector Decoder Light Source

The three primary building blocks of the link are the transmitter, the receiver, and the fiber guide. The transmitter consists of an analog to digital converter (coder), and a light source. The A/D converter is used to convert continuous analog signals such as voice or video (TV) signals into a series of digital pulses.

The digital pulses are then used to flash a powerful light source off and on very rapidly. The light source is either a light-emitting diode (LED) or an injection laser diode (ILD).The light-beam pulses are then fed into a fiber-optic cable where they are transmitted over long distances.

The optical fiber consists of a glass or plastic fiber core, a cladding, and a protective jacket ( transmission medium) Repeaters are used to ensure the signals can be transmitted efficiently when the two stations are separated far enough from each other. The receiver includes a light detector or photocell and a decoder. The light detector is very often either a PIN (p-type-intrinsic-n-type) diode or an APD (avalanche photodiode). The light detector, acting as the receiving element, converts the received light pulses back to pulses of electrical current. The electrical pulses are amplified and reshaped back into digital form that is fed to a decoder such as a D/A converter, where the original voice or video is recovered

Light Sources Generally, a light source must meet the following requirements: It must be able to turn on and off several tens of millions, or even billions, of times per second. It must be able to emit a wavelength that is transparent to the fiber. It must be able to couple light energy into the fiber. The optical power emitted must be sufficient enough to transmit through optical fibers. The performance of the fiber-optic should not be affected by the temperature variation. The manufacturing cost of the light source must be relatively inexpensive

LED LEDs are complex semiconductors that convert an electrical current into light. The conversion process is fairly efficient in that it generates little heat compared to incandescent lights. LEDs are of interest for fiber optics .

Light-emitting diodes use GaAlAs (gallium aluminum arsenide) for short-wavelength devices. Long-wavelength devices generally incorporate InGaAsP (indium gallium arsenide phosphide).

LED characteristics: 1. They are small. 2. They possess high radiance (i.e., They emit lots of light in a small area). 3. The emitting area is small, comparable to the dimensions of optical fibers. 4. They have a very long life, offering high reliability. 5. They can be modulated (turned off and on) at high speeds

LED is an incoherent light source that emits light in a disorderly way as compared to ILD, which is a coherent light source that emits light in a very orderly way (see Figure1.0). Incoherent radiation (a) Coherent radiation (b) Figure 1.0 Radiation patterns for (a) LED ; (b) ILD

LEDs are economical and are common for short distance,low data rate applications. They are available for all three wavelengths but are most common at 850 and 1310 nm ( 850 nm LEDs are usually the least expensive )

Light power from an LED covers a broad spectrum, from 20 to over 80 nm Light power from an LED covers a broad spectrum, from 20 to over 80 nm . The LED is more stable and reliable than a laser in most environments.

ILD The word 'LASER' is an acronym standing for 'Light Amplification by Stimulated Emission of Radiation' The diode laser is the most compact among all commercially available laser products.

Injection Laser Diodes are more expensive. The advantagesof using a laser diode are in the high modulation bandwidth ( over 2 GHz ), with high optical output power and narrow spectral width. Their application is in long distance, high data rate requirements. Lasers are common in single mode optical fiber applications and their light power covers a very narrow spectrum, usually less than 3 nm. This results in a low chromatic dispersion value and hence high fiber bandwidth. Their life span is shorter than that of an LED. Lasers are sensitive to the environment (especially to temperature variation).

The diode's construction begins with a three-leaded header The diode's construction begins with a three-leaded header. Mounted on the header resides a monitoring photodiode designed to detect light. Above the photodiode is the laser diode chip where the laser beam is emitted. The laser chip has three layers, two clad layers with an active layer in between.

The light beam is emitted from the active layer, also known as the Multiple Quantum Well (MQW). The glass window on a diode laser not only transmits the light beam, but also forms part of the hermetically sealed package. Once the case is sealed its contents are protected from hazardous elements, unless it is physically destroyed. The glass envelope comes in two different styles, flat window and ball lens

Laser Applications Biomedical, photodynamic therapy, entertainment & display, graphic arts, holography, inspection, instrumentation, laser pumping, optical data storage, sensing, pollution monitoring, trace element sensing, spectroscopy, high resolution spectroscopy, Raman spectroscopy, fluorescence spectroscopy, molecular spectroscopy, and telecommunications.

Here we list the commonly available diode laser wavelengths and their typical applications: 635   Pointing, holography, replacement of HeNe laser, DVD, CD 650   Pointing, holography, DVD, CD 660   Pointing, holography, DVD, CD 760   Gas sensing 780   CD ROM 808   Solid state laser pumping 850   Communications 980   Erbium-doped fiber pumping 1060 Replacement of low-power Nd:YAG 1310 Communications 1321 Gas sensing 1460 Raman amplifier 1480 Erbium-doped fiber pumping 1540 Gas sensing 1550 Communications, range finding 1578 Gas sensing 1625 Telecommunications testing 1640 Gas sensing 1790 Gas sensing

Light Detector Optical detection occurs at the light wave receiver’s circuitry. The photo detector is the device that receives the optical fiber signal and converts it back into an electrical signal.

The most important characteristics of light detectors are : i) Responsitivity: Responsitivity is a measure of the conversion efficiency of a photodetector. ii ) Dark current: Dark current is the leakage current that flows through a photodiode with no light input. iii ) Transit time: Transit time is the time it takes a light-induced carrier to travel across the depletion region. iv ) Spectral response: Spectral response is the range of wavelength values that can be used for a given photodiode. v ) Light sensitivity: Light sensitivity is the minimum optical power a light detector can receive and still produce a usable electrical output signal.

The most common types of photo detectors are the positive intrinsic negative photodiode ( PIN ) and the avalanche photodiode (APD ).

PIN photodiodes are inexpensive, but they require a higher optical signal power to generate an electrical signal. They are more common in short distance communication applications.

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