seminar laser communication

LASER COMMUNICATION 2011

ABSTRACT

Laser communications offer a viable alternative to RF communications for inter satellite links

and other applications where high-performance links are a necessity. High data rate, small

antenna size, narrow beam divergence, and a narrow field of view are characteristics of laser

communications that offer a number of potential advantages for system design.

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.

Laser communication systems offer many advantages over radio frequency (RF) systems.

Most of the differences between laser communication and RF arise from the very large

difference in the wavelengths. RF wavelengths are thousands of times longer than those at

optical frequencies are. This high ratio of wavelengths leads to some interesting differences

in the two systems. First, the beam-width attainable with the laser communication system is

narrower than that of the RF system by the same ratio at the same antenna diameters (the

telescope of the laser communication system is frequently referred as an antenna). For a

given transmitter power level, the laser beam is brighter at the receiver by the square of this

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ratio due to the very narrow beam that exits the transmit telescope. Taking advantage of this

brighter beam or higher gain, permits the laser communication designer to come up with a

system that has a much smaller antenna than the RF system and further, need transmit much

less power than the RF system for the same receiver power. However since it is much harder

to point, acquisition of the other satellite terminal is more difficult. Some advantages of laser

communications over RF are smaller antenna size, lower weight, lower power and minimal

integration impact on the satellite. Laser communication is capable of much higher data rates

than RF.

The laser beam width can be made as narrow as the diffraction limit of the optic allows. This

is given by beam width = 1.22 times the wavelength of light divided by the radius of the

output beam aperture. The antennae gain is proportional to the reciprocal of the beam width

squared. To achieve the potential diffraction limited beam width a single mode high beam

quality laser source is required; together with very high quality optical components

throughout the transmitting sub system. The possible antennae gain is restricted not only by

the laser source but also by the any of the optical elements. In order to communicate,

adequate power must be received by the detector, to distinguish the signal from the noise.

Laser power, transmitter, optical system losses, pointing system imperfections, transmitter

and receiver antennae gains, receiver losses, receiver tracking losses are factors in

establishing receiver power. The required optical power is determined by data rate, detector

sensitivity, modulation format ,noise and detection methods.

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INTRODUCTION

Lasers have been considered for space communications since their realization in 1960.

However, it was soon recognized that, although the laser had potential for the transfer of data

at extremely high rates, specific advancements were needed in component performance and

systems 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 a viable but also a attractive approach to

intersatellite link applications.

The high data rate and large information throughput available with laser communications are

many times greater than in radio frequency (RF) systems. The small antenna size requires

only a small increase in the weight and volume of host vehicle. In addition,this feature

substantially reduces blockage of fields of view of the most desirable areas on satellites. The

smaller antennas, with diameters typically less than 30cm, create less momentum disturbance

to any sensitive satellite sensors. Fewer onboard consumables are required over the long

lifetime because there is less disturbance to the satellite compared with larger and heavier RF

systems. The narrow beam divergence of affords interference-free and secure operation.

FEATURES OF LASER COMMUNICATIONS SYSTEM

A block diagram of typical terminal is illustrated in Fig 1. Information,typically in the form

of digital data, is input to data electronics that modulates the transmitting laser source. Direct

or indirect modulation techniques may be employed depending on the type of laser employed.

The source output passes through an optical system into the channel. The optical system

typically includes transfer, beam shaping, and telescope optics. The receiver beam comes in

through the optical system and is passed along to detectors and signal processing electronics.

There are also terminal control electronics that must control the gimbals and other steering

mechanisms, and servos, to keep the acquisition and tracking system operating in the

designed modes of operation.

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The extremely high antenna gain made possible by the narrow beams enables small telescope

apertures to be used. Plots of aperture diameter Vs. data rate for millimeter and optical waves

are shown in Fig 2. A laser communications system operating at 1 Gb/s requires an aperture

of approximately 30 cm. In contrast, a 1 Gb/s millimeter wave system requires a significantly

larger aperture, 2-2.75m.

The laser beamwidth can be made as narrow as the diffraction limit of the optics allows. This

is given by the beamwidth equal to 1.22 times the wavelength of the light, divided by the

radius of the output beam aperture. This antenna gain is proportional to the reciprocal of the

beamwidth squared. The most important point here is that to achieve the potential diffraction-

limited beamwidth given by the telescope diameter, a single-mode high-beam-quality laser

source is required, together with very high-quality optical components throughout the

transmitting subsystem. The beam quality cannot be better than the worst element in the

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optical chain, so the possible antenna gain will be restricted not only by the laser source itself,

but also by any of the optical elements, including the final mirror or telescope primary.

Because of the requirement for both high efficiency and high beam quality, many lasers that

are suitable in other applications are unsuitable for long distance free-space communication.

In order to communicate, adequate power must be received by the detector to distinguish

signal from noise. Laser power, transmitter optical system losses, pointing system

imperfections, transmitter and receiver antenna gains, receiver losses, and receiver tracking

losses are all factors in establishing receiver power. The required optical power is determined

by data rate, detector sensitivity, modulation formats, noise, and detection methods.

When the receiver is detecting signals, it is actually making decisions as to the nature of the

signal (when digital signal are being sent it distinguishes between ones and zeros). Fig 3.

shows the probability of detection vs. measured photocurrent in a decision time. There are

two distributions: one when a signal is present (including the amount of photocurrent due to

background and dark current in the detector),and one when there is no signal present

(including only the nonsignal current sources). A threshold must be set that maximizes the

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success rate and minimizes the error rate. One can see that different types of errors will occur.

Even when there is no signal present, the fluctuation of the nonsignal sources will

periodically cause the threshold to be exceeded. This is the error of stating that a signal is

present when there is no signal present. The signal distribution may also fall on the other side

of the threshold, so errors stating that no signal is present will occur even when a signal is

present. For laser communication systems in general, one wants to equalize these two error

types. In the acquisition mode, however, no attempt is made to equalize these errors since this

would increase acquisition time.

OPERATION

Free space laser communications systems are wireless connections through the atmosphere.

They work similar to fiber optic cable systems except the beam is transmitted through open

space. The carrier used for the transmission of this signal is generated by either a high power

LED or a laser diode. The laser systems operate in the near infrared region of the spectrum.

The laser light across the link is at a wavelength of between 780 – 920 nm. Two parallel

beams are used, one for transmission and one for reception.

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ACQUISITION AND TRACKING

There are three basic steps to laser communication: acquisition,tracking, and ommunications.

Of the three, acquisition is generally the most difficult; angular tracking is usually the easiest.

Communications depends on bandwidth or data rate, but is generally easier than acquisition

unless very high data rates are required.Acquisition is the most difficult because laser beams

are typically much smaller than the area of uncertainty. Satellites do not know exactly where

they are or where the other platform is located, and since everything moves with some degree

of uncertainty, they cannot take very long to search or the reference is lost. Instability of the

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platforms also causes uncertainty in time. In the ideal acquisition method, shown in Figure 4,

the beamwidth of the source is greater than the angle of uncertainty in the location of

receiver. The receiver field of includes the location uncertainty of

thetransmitter.Unfortunately, this ideal method requires a significant amount of laser power.

It is possible to operate a number of laser types at high peak power and low duty cycle to

make acquisition easier. This is because a lower pulse rate is needed for acquisition than for

tracking and communications. High peak power pulses more easily overcome the receiver set

threshold and keep the false alarm rate low. A low duty cycle transmitter gives high peak

power, yet requires less average power, and is thus a suitable source for acquisition. As the

uncertainty area becomes less, it becomes more feasible to use a continues source of

acquisition, especially if the acquisition time does not have to be short.

OPTICAL NOISE

Noise characteristics play an important role in laser communication systems. At optical

frequencies noise characteristics are significantly different than those at radio frequencies. In

the RF domain, quantum noise is quite low, while thermal noise predominates and does not

vary with frequency in the microwave region. However, as the wavelength gets shorter,

quantum noise increases linearly, and in the laser regime thermal noise drops off very rapidly,

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becoming insignificant at optical wavelengths. Because there is so little energy in a photon at

radio frequencies, it takes many problems to equal the thermal noise. The quantum noise is

actually the statistical fluctuations of the photons, which is the limiting sensitivity at optical

frequencies. However, in optical receivers employing direct detection and avalanche

photodiodes, the detection process does not approach the quantum limit performance. For this

type of optical receiver, the thermal noise due to the preamplifier is usually a significant

contributor to the total noise power.Free space optical communication links, atmospheric

turbulence causes fluctuations in both the intensity and the phase of the received light signal,

impairing link performance. Atmospheric turbulence can degrade the performance of free-

space optical links,particularly over ranges of the order of 1 km or longer.Inhomogeneities in

the temperature and pressure of the atmosphere lead to variations of the refractive index

along the transmission path.These index inhomogeneities can deteriorate the quality of the

received image and can cause fluctuations in both the intensity and the phase of the received

signal. The primary background noise is the sun. The solar spectral radiance extends from the

ultraviolet to the infrared, with the peak in the visible portion of the spectrum. Atmospheric

scattered sunlight,sunlit clouds, the planets, the moon, and the Earth background have similar

radiances; the sun’s radiance is much higher and a star field’s much lower. A star field is an

area of the sky that includes a number of stars. If one were able to look only at an individual

star, one would find a brightness similar to that of the sun; but a star field as a whole is

composed of small point sources of light, the stars in the field,against a dark area having no

background level. The background is reduced by making both the field of view and the

spectral width as narrow as possible. For direct detection systems, narrow field of view

spectral filters on the order of 20A*(2 nm) are typical. Heterodyne systems will enable

further reduction, but with a increase in terminal complexity. However, some systems can be

signal-quantum-noiselimited,rather than background-limited, without having to resort to

heterodyne detection.

SYSTEM CHARACTERISTICS AND DESCRIPTON

Here we discuss specific key system characteristics which, which when quantified, together

give a detailed description of a typical laser communication system. Key system

characteristics are identified and subsequently quantified for a particular application. In the

first part of this section we identify the key parameters that make up a link table listing. In the

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second part, we will describe how a link analysis is used to provide a description of a laser

communications cross-link operating at 10 Mb /s. This low data rate is only used as an

example and gives a point of reference for RF systems of similar performance. Key system

characteristics or parameters must be identified and quantified to fully describe the system.

Critical parameters can be grouped in to five major categories: link, transmitter,

channel,receiver, and detector parameters. Free-space laser communications is a very flexible

means to connect end users to a high-bandwidth data network via ground-based terminals on

top of buildings or to bring a variety of data services to remote locations via satellite

terminals in space. External influences on the optical link due to atmospheric turbulence and

vibrations in the transmitter's environment require some method of beam control to stabilize

the optical link and maintain a high transmission rate. Liquid crystal (LC) optics can provide

a compact and low-power solution to beam control in laser communications systems.

LINK PARAMETERS

The link parameters are the type of laser, wavelength, type of link, and required signal

criteria. Although virtually every laser type has been considered at one time of another, today

the lasers typically used in free space laser communications system are either semiconductor

laser diodes, solid state lasers, or fiber amplifiers/lasers. Laser sources are typically described

as operating in either single or multiple longitudinal modes. In single longitudinal mode

operation the laser emits radiation at a single frequency, while in multiple longitudinal mode

operation multiple frequencies are emitted. Single-mode sources are required in coherent

detection systems and typically have spectral widths of the order of 10 kHz- 10MHz.

Multimode sources are employed in direct detection systems and typically have spectral

widths from 1.5 to 10 nm. Semiconductor lasers have been in development for the three

decades and have only recently (within last five years) demonstrated the levels of

performance needed for reliable operation as direct sources. Typically operating in the 800-

900 nm range (gallium arsenide/gallium aluminum arsenide, GaAs/GaAlAs, material

system), their inherently high efficiency (approaching 50%) and small size made this

technology attractive. However key issues have been the lifetimes, asymmetric beam shape,

and output power. Research into integrated phased arrays proved to be more challenging than

first anticipated, forcing the use of single emitters and output powers in the 100-150mW

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range. Inherent beam combiners employing wavelength-division multiplex or other

techniques were employed for those application requiring greater power.

Solid state lasers have offered higher power levels and the ability to operate in high peak

power modes for acquisition. When diode lasers are used to optically pump the lasing media

graceful degradation and higher overall reliability (compared to lamp pumped systems) is

achieved. A variety of materials have been proposed for laser transmitters; however,

neodymium doped yttrium aluminium garnet (Nd:YAG) is the most widely developed.

Operating at 1064nm,these lasers require an external modulator, leading to a slight increase

in complexity and reliability. The modulator must be capable of operating at required pulse

rates as well as handling the power levels from the laser.

TRANSMITTER PARAMETERS

The transmission parameters consist of certain key laser characteristics, losses incurred in the

transmit optical path,transmit antenna gain, and transmit pointing loss. The key laser

characteristics include peak and average optical power, pulse rate,and pulse width. In a

pulsed configuration the peak laser power and duty cycles are specified, while in continues-

wave applications the average power is specified. In a pulsed application the pulse rate and

width describe the laser’s temporal performance. In continues-wave applications, such as

coherent communication employing frequency shift keying (FSK) or phase shift keying

(PSK), the pulse rate and width describe the symbol rate and symbol duration of the data

impressed on the laser carrier.

Transmit optical path loss is made up of optical transmission losses and loss due to the wave-

front quality of the transmitting optics, degrading the theoretical far-field on-axis gain.The

wave front error loss is analogous to the surface roughness loss associated with RF antennas.

The optical transmit antenna gain is exactly analogous to the antenna gain in RF systems, and

describes the on-axis gain relative to an isotropic radiator with the distribution of the

transmitted laser radiation defining the transmit antenna gain.

The laser sources suitable to the free-space laser communications trend to exhibit a Gaussian

intensity distribution in the main lobe. The reduction in the far-field signal strength due to

transmitter mispointing is the transmitter pointing loss. For each link in a laser system, a

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pointing budget must be determined. The pointing budget is typically composed of bias

(slowly varying) and random (more rapidly varying) components. The bias components are

the alignment and detector gain mismatch errors; the random components are the NEA and

residual error due to base motion disturbances.

For a system employing a Gaussian beam, where the pointing loss is predominantly a bias,

the on-axis transmitted gain-pointing loss product is maximized when the1/e2 beamwidth is

set equal to approximately 2.8 times the pointing error. Increasing antenna diameter further

(decreasing the 1/e beamwidth) will degrade performance. When pointing error is a

combination of bias and random terms, a somewhat more complex expression must be

evaluated. The point to stress here is that once the pointing error is determined, the system

beamwidth must be sized appropriately.

CHANNEL PARAMETERS

The channel parameters for an optical intersatellite link (ISI) consist of the range and

associated loss, background spectral radiance, and spectral irradiance. Since this article deals

with ISLs,losses due to the atmosphere are not considered. These losses can be quite large

and mitigation of the effects complex. The range loss is simply RL = (l/(4pR))2, where R is

the separation between the two platforms in meters, and l is the wavelength. The background

level depends on the relative altitudes of the platforms, the time of the year, and the

wavelength selected.

RECEIVER PARAMETERS

The receiver parameters are the receiver antenna gain, the receiver optical path loss, the

optical filter bandwidth and the receiver field of view. The receiver antenna gain is given by

GR = (pDR/l)2 where D is the effective receiver diameter diameters in meters. The receiver

optical path loss is simply the optical transmission loss for systems employing direct

detection techniques.However, for laser systems employing coherent optical detection (either

homodyne or heterodyne) there is an additional loss due to wavefront error. The preservation

of the wavefront quality is essential for optical mixing of the received signal and local

oscillator fields on the detector surface. To first order, the loss expression is the same as that

previously defined for the transmit wavefront error.The optical filter bandwidth specifies the

spectral width of the narrow-bandpass filter employed in optical intersatellite links. Optical

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filter reduce the amount of unwanted background entering the system. The optical width of

the filter must be compatible with the spectral width of the laser source. In addition to source

considerations, the minimum width also be determined by the acceptable transmission level

of the filter; typically the transmission of the filter decreases with spectral width.

DETECTOR PARAMETERS

The detector parameters are the type of detector, gain of the detector (if any), quantum efficiency, heterodyne mixing efficiency (for coherent detection only), noise due to the detector, noise due to the following preamplifier, and (for track links) angular sensitivity orslope factor of the detector.For optical ISLs based on semiconductor laser diodes or Nd: YAG lasers, the detector of choice is a p-type-intrinsic-n-type (PIN) or an avalanche photodiode (APD). A PIN photodiode can be operated in the photovoltaic or photoconductive mode, and has no internal gain mechanism. An APD is always operated in the photoconductive mode and has internal gain by virtue of the avalanche multiplication process. At shorter wavelengths (810-900 nm) PINs and APDs made of silicon show the best response, but at longer wavelengths (1300- 1550 nm) InGaAs and Ge APDs have significantly more excess noise than comparable silicon devices. For application requiring gain and operating at Nd: YAG wavelengths, a silicon APD is typically preferred because of its internal gain. However, if gain is not required an InGaAs PIN would be preferred because of the higher quantum efficiency. The quantum efficiency, h, of the detector is the efficiencywith which the detector converts incident photons to electrons.

The mean output current for both PINs and APDs is proportional to the quantum efficiency. By definition, quantum efficiencies are always less than unity. For silicon detectors operating at GaAlAs wavelengths, h = 0.85-0.9, while at the Nd: YAG wavelength h may be only 0.4. For InGaAs detectors, operated at InGaAsP and Nd: YAG wavelengths, h is about 0.8. Another detector parameter to consider is the noise due to the detector alone. Typically, in detector there is a DC current even in the absence of signal or background. This DC “dark”current, as it is commonly called, produces a shot-noise current just as the signal and background currents do. In an APD there are two contributors to the total dark current: an unmultiplied current and a multiplied current. The multiplication is provided by the avalanche gain mechanism and, as expected, for typical operating gains (>50) the multiplied term is dominant. In a PIN photodiode there is only the unmultiplied term.

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The output of the detector is input to a preamplifier that converts the detector signal current into a voltage and amplifies it to a workable level for further processing. Being the first element past the detector, the noise due to the preamplifier have a significant effect on the system’s sensitivity. The selection of preamplifier design (transimpedance or high impedance), internal transistor design (bipolar or FET), and device material (GaAs or silicon) depends on a number of factors. Transimpedance designs have greater dynamic range, but are nominally less sensitive than high-impedance designs. Silicon bipolar transistors may come from a more mature technology,but GaAs FETs have a higher bandwidth capability and are inherently radiation resistant.

APPLICATIONS

Depending on the climatic zone where the free space laser communications systems are used,

they can span distances up to 15 km at low bitrates or provide bitrates up to 622 Mbps at

shorter distances. The systems are protocol transparent allowing transmission of digital

computer data (LAN interconnect), video, voice over IP, multiplexed data, or ATM. They are

suitable for temporary connectivity needs such as at conventions, sporting events, corporate

and university campuses, disaster scenes or military operations.

With the rapid development of terrestrial fiber communications,a wide array of components

are available for potential application in space. These include detectors, lasers, multiplexers,

amplifiers, drive electronics, optical preamplifiers, and others. Operating at 1500 nm,erbium

doped fiber amplifiers (EDFA) have been developed for commercial optical fiber

communications that offer levels of performance consistent with many free-space laser

communications applications (500mW range).

ADVANTAGES AND DISADVANTAGES

Free space laser communications links eliminate the need for securing right of ways, and

buried cable installations. As the equipments operate within the near infrared spectrum, they

are not subject to government licensing and no spectrum fees have to be paid (according to

Art. 7 in [3] requires only the use of the frequency spectrum below 3’000 GHz a licence).

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Additionally, since no radio interference studies are necessary, the systems are quickly

deployable. The narrow laser beamwidth precludes interference with other communication

systems of this type. Free space laser communications systems provide only interconnection

between points that have direct line-of-sight. They can transmit through glass,however, for

each glass surface the light intensity is reduced, due to a mixture of absorption and refraction,

thus reducing the operational distance of a sys-tem. Occasionally, short interruptions or

unavailability events lasting from some hours up to a few days can occur.

CONCLUSIONS

The system and component technology necessary for successful intersatellite laser

communication link exist today. The growing requirements for efficient and secure

communications has led to increased interest in the operational deployment of laser crosslinks

for commercial and military satellite systems in both low earth and geosynchronous orbits.

With the dramatic increase in the data handling requirement for satellite communication

services, laser intersatellite links offer an attractive alternative to RF with virtually unlimited

growth potential and an unregulated spectrum. The demonstration programs underway in the

United States, Europe, and Japan will show the way for future large-scale applications of

laser communications to satellite cross-links.

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REFERENCES

1. IEEE communications Magazine. August 2000, free space laser communications :Laser

cross-link systems and technology by: David L. Begley, Ball Aerospace & technologies

corporation

2. Chaotic Free-Space Laser Communication over a Turbulent Channel By: N. F.

Rulkov,1 M. A. Vorontsov, and L. Illing institute for Nonlinear Science, University of

California,San Diego, La Jolla, California 92093 Army Research Laboratory, Adelphi,

Maryland 20783

3. Free Space Optics or Laser Communication through the Air BY: Dennis Killinger

Optics & Photonics News ■ October 2002

4. High data-rate laser transmitters for free-space laser Communications. BY:A. Biswas,

H. Hemmati and J. R. Lesh Optical Communications Group Jet Propulsion Laboratory,

California Institute of Technology.

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seminar laser communication

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