Laser Communication Systems
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21-09-2008, 11:10 AM

Lasers have been considered for space communications since their realization in 1960. Specific advancements were needed in component performance and system engineering particularly for space qualified hardware. Advances in system architecture, data formatting and component technology over the past three decades have made laser communications in space not only viable but also an attractive approach into inter satellite link applications.

Information transfer is driving the requirements to higher data rates, laser cross -link technology explosions, global development activity, increased hardware, and design maturity. Most important in space laser communications has been the development of a reliable, high power, single mode laser diode as a directly modulable laser source. This technology advance offers the space laser communication system designer the flexibility to design very lightweight, high bandwidth, low-cost communication payloads for satellites whose launch costs are a very strong function of launch weigh. This feature substantially reduces blockage of fields of view of most desirable areas on satellites. The smaller antennas with diameter typically less than 30 centimeters create less momentum disturbance to any sensitive satellite sensors. Fewer on board consumables are required over the long lifetime because there are fewer disturbances to the satellite compared with heavier and larger RF systems. The narrow beam divergence affords interference free and secure operation.

Until recently, the United States government was funding the development of an operational space laser cross-link system employing solid-state laser technology. The NASA is developing technology and studying the applicability of space laser communication to NASA's tracking and data relay network both as cross-link and for user relay links. NASA's Jet Propulsion Laboratory is studying the development of large space and ground-base receiving stations and payload designs for optical data transfer from interplanetary spacecraft. Space laser communication is beginning to be accepted as a viable and reliable means of transferring data between satellites. Presently, ongoing hardware development efforts include ESA's Space satellite Link Experiment (SILEX) and the Japanese's Laser Communication Experiment (LCE). The United States development programs ended with the termination of both the production of the laser cross-link subsystem and the FEWS satellite program
Satellite use from space must be regulated and shared on a worldwide basis. For this reason, frequencies to be used by the satellite are established by a world body known as the International Telecommunications Union (ITU) with broadcast regulations controlled by a subgroup known as World Administrative Radio Conference (WARC). An international consultative technical committee (CCIR) provides specific recommendations on satellite frequencies under consideration by WARC. The basic objective is to allocate particular frequency bands for different types of satellite services, and also to provide international regulations in the areas of maximum radiation's level from space, co-ordination with terrestrial systems and the use of specific satellite locations in a given orbit. Within these allotments and regulations an individual country can make its own specific frequency selections based on intended uses and desired satellite services.
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14-06-2012, 05:35 PM

Laser Communication Systems


Laser communication systems are used to transfer information from one point to a distant point. The information may be an audio conversation, a stream of data from one computer to another, or several simultaneous television broadcasts. Optical communication systems in the past consisted of techniques such as fire signals, smoke signals, flash lanterns, reflected sunlight and signal flags. Such systems had limited bandwidth and were not competitive with electronic communications (like radio).
The invention of the laser provided a coherent optical source capable of transmitting information at extremely high data rates. However, limitations on transmission of light through the
atmosphere (such as turbulence, haze, fog, absorption and rain) limited the usefulness of l asers for transmission of information through the atmosphere. Modern optical communication systems use
semiconductor lasers that transmit light through optical fibers. Such systems have become widely used for telecommunications.
Here, we will learn about the components used in fiber-optic laser-based telecommunication systems, the trade-offs in the choice of components for systems and the applications of fiber-optic communications.

Laser communications may be divided into four general application classes:

• Terrestrial short range paths through the atmospheres
• Closed pipe systems for sending high data rate between and within major metropolitan centers
• Near space communications for relaying high data rates
• Deep space communications from the outer planets
This division is based on path length, data regimes, line of sight requirement for laser links and weather effects. Obviously, in space line of sight can be billion of miles, whereas on the earth, line of sight is restricted to less than 100 miles. Line of sight requirements and weather effects can be overcome over long distances on earth by use of relay stations, forward scatter techniques, and closed pipe systems. However, many applications over short earth links can be met without resorting to these additional complications.

Desirable Laser Characteristics

To attain highly information efficient modulation formats in the visible and near IR using direct detection, the laser transmitter must be capable of a unique set of characteristics. The laser transmitter should be capable of
• Being pulsed with accurate time control
• Generating short pulses
• Generating high peak powers
• Generating high repetition rates
• Being pulsed such that the interpulse spacing may vary considerably from pulse to pulse
• High power efficiency
• Diffraction limited operation for narrow beam width generation
• Operation without energy waste between short pulses

Continuous ware output type laser cannot meet these requirements because the combined requirements define a laser transmitter that is internally modulated to achieve efficient, very low duty cycle operation the pulsed capability is obtained through Q switching and time variable reflection techniques. Each type of laser may meet some requirements but it is lacking in others. This results in a different “best” laser choice depending on the application. Semiconductor lasers mighty meet all except (3) and (7), but these characteristics are critical to an efficient communication system. These laser that comes closest to meeting all requirements appears to be a mode locked YAG, but it is lacking in pulse spacing control and in high power efficiency.
The laser used in fiber-optic telecommunication systems is the semi-conductor laser. Semiconductor lasers are especially well-suited for use in this type of communication system. Semiconductor lasers have suitably small size and configuration for coupling into the small-diameter core of an optical fiber. Modern AlxGa1–xAs lasers operate continuously at milli-watt power levels sufficient for fiber-optic communications. They can be modulated easily, through modulation of an electric power supply, at frequencies up to the gigahertz range. This makes it possible to transmit information, by modulating a beam of light from a laser, through optical fibers. The semiconductor laser, with its small emitting area, is a natural choice as a source for fiber communications. But for a number of years, laser lifetime was too short and fiber losses too high to make laser-based fiber communications a success.

Modulation Techniques:

In order to communicate one must modulate the carrier. Therefore, one should establish a modulation format that is desirable, then find a method of modulation which can implement that format. In practice, a compromise between the two occurs because the requirements of most efficient format implementation are difficult to meet at the present state of laser system technology.
In general, for visible and near infrared laser systems it is desirable to utilize short pulse laser modulation techniques. Several system concepts have evolved that are in category. They are
• Pulse interval modulation (PIM)
• Pulse amplitude modulation (PAM)
• Pulse-width modulation (PWM)
• Pulse code modulation (PCM)
• Pulse gated binary modulation (PGBM)
• Pulse polarization binary modulation (PPBM)
• Pulse position modulation (PPM)

Pulse-interval modulation (PIM)

is an M-ary process in which one pulse conveys information representing many bits in an ordinary binary system. The normal interpulse time interval is divided into M discrete time slits. One and only one pulse is sent in the time interval of M slots. The specific time slot in which a pulse occurs ins representative of a code symbol. The number of bits transmitted per pulse is “log2 M”.

Pulse-amplitude modulation,

acronym PAM, is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses.
Example: A two-bit modulator (PAM-4) will take two bits at a time and will map the signal amplitude to one of four possible levels, for example −3 volts, −1 volt, 1 volt, and 3 volts.
Demodulation is performed by detecting the amplitude level of the carrier at every symbol period.
Pulse-amplitude modulation is widely used in baseband transmission of digital data, with non-baseband applications having been largely replaced by pulse-code modulation, and, more recently, by pulse-position modulation.
In particular, all telephone modems faster than 300 bit/s use quadrature amplitude modulation (QAM). (QAM uses a two-dimensional constellation).

Pulse-width modulation (PWM), or pulse-duration modulation (PDM),

is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches. The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied to the load is. The PWM switching frequency has to be much faster than what would affect the load, which is to say the device that uses the power. Typically switchings have to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.
The term duty cycle describes the proportion of 'on' time to the regular interval or 'period' of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on.

Fundamentally, the efficiency of a direct detection laser communication system depends upon
• Achieving system sensitivity with a high probability of signal detection
• Reducing the background noise level in the receiver to minimize the probability of false detection without significantly affecting the signal required to achieve a high probability of sing detection.
The noise in an optical communication system falls into two categories:
Internal system generated
The main internal noise sources are (a) the statistical fluctuation of the signal, (b) a finite extinction ratio in the optical modulator, © receiver noise

External background radiation

Three general types of noise discrimination are: spectral, spatial and temporal. The background noise level can be appreciably reduced by using filter at the receiver. Spatial filtering is used to minimize field of view. Temporal discrimination is achieved by optical receiver pulse gating techniques.

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27-06-2012, 06:09 PM


.ppt   laser comm.ppt (Size: 1.24 MB / Downloads: 58)

What Is Laser Communication

Laser communications systems are wireless connections through the atmosphere. They work similarly to fiber optic links, except the fact that, in lasers, beam is transmitted through free space.

What is the Transmitter?

The transmitter involves:
Signal processing electronics (analog/digital)
Laser modulator
Laser (visible, near visible wavelengths)

What is the Receiver?

The receiver involves:
Telescope (referred to as ‘antenna’)
Signal processor

Why not Fiber Optics?

Not always possible to lay fiber lines
Combat zones
Physically / Economically not practical

LC being incorporated into fiber optic networks when fiber is not practical.

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16-07-2012, 07:43 PM

advantages and dis advantages of laser communication
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17-07-2012, 10:16 AM

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19-07-2012, 11:41 AM

Introduction to laser communication

.pptx   lasser communication.pptx (Size: 970.07 KB / Downloads: 34)

Laser communications systems are wireless connections through the atmosphere. They work similarly to fiber optic links, except the beam is transmitted through free space.

Laser communication is now able to send information at data rates up to several Gbps and at distance of thousands of kilometers apart.

Intersatellite Links Why

Study for Military Satellite Communications (MILSATCOM)
High bandwidth advantage.
Frequency management
Secure communications
Weight and power requirements.

Laser Crosslinks ATP Assembly

Main features
Beacon Transmit Module
Generates the beacon laser for acquisition and tracking
Beacon Receive Module
Receives beacon laser inputs from target satellites in order to confirm link
ATP Processor
Calculates all ATP control commands to ensure link establishment and integrity


Transmit voice for miles line-of-sight
Use weak signal modes for “cloud scatter”
Transmit video
Transmit high speed data without WEP

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05-12-2012, 05:30 PM

Communication Systems

.ppt   Communication Systems.ppt (Size: 1.2 MB / Downloads: 35)

Design good algorithms used in synchronization, channel estimation and demodulation which are suitable for IC implementation for modern MIMO, OFDM and MIMO-OFDM based communication systems.
Space-time coding for MIMO-based communication systems.

Study the effect of circuit impairment on the performance of wireless communication systems.
Compensate the effect of circuit impairment on the performance of communication systems by using digital signal processing algorithm.
An important topic since some RF IC specifications are based on this study.

Error-Correction Codes

Design good error correction codes (ECC) with the constraint imposed by the real applications and IC implementation.
Apply some popular codes such as Turbo codes, LDPC codes on real applications such as 4G, DVD, optical communication and broadcasting with necessary modifications.

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