Photonics

Microwave Photonics

forSpace Applications

Dr. S. Pal & V.S.RaoISRO Satellite Centre Bangalore

August 27, 2000 Dr. S.Pal & V. S. Rao, ISRO Satellite Centre, Bangalore.

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Microwave Photonics for Space Applications

• Hardly two decades back, it was almost impossible to convince an RF engineer that

– direct detection using photo detectors with internal gain control could be of value and that one didn’t need a heterodyne receiver at visible wavelengths to achieve good sensitivity.

– that micro radiation tracking was possible

• It was not well understood that the noise limitations for optical communication systems were significantly different than for RF systems and that the quantum noise which was of unconcern to RF engineer, could dominate the S/N issues.


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• Who would have imagined in 1970s – that the major highways of the terrestrial communication

network would be optical fiber systems instead of analog co-axial cable systems and microwave radio relay systems.

– that fiber systems would give an option of Gbits/sec system operation

– that one could have repeaterless system in thousands of kilometers

– that fiber systems would challenge satellite communication systems

– that fiber systems would surpass the traffic density handled by satellite systems


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• Rapid development over the last two decades have led to many novel applications of optics.

– In the home, for example, we regularly use infrared remote controls for televisions and videos

– We listen to compact discs on laser based CD players

– Many of our telephone conversations are carried on fiber optic cables.

– Within the military context, modern optical techniques are used for imaging, targeting and range finding.

• In principle all of these systems can be described using classical optics.


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• In microwave/RF communication links, the limiting noise process is thermal noise, which is a result of random motion of charge carriers in a conductor.

• The thermal noise power is given by equation: N = kTB

• Noise floor for microwave systems is temperature dependent.


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– Photonic systems, in contrast, are governed by the quantum nature of photon which has an associated energy ‘h’ where ‘h’ is Planks constant and ‘’ is the optical frequency.

• The spontaneous emission of photons, common to all light sources, will cause a random variation in the average light intensity. This variation results in a random generation of electron-hole pairs in the optical detector which follows Poisson statistics and is called shot noise. Shot noise is the limiting noise in photonic systems.


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• The dynamic range of an RF link is limited at the low power end by the link noise floor – system noise

– atmosphere/galactic noise

• For the RF/optical link, a noise floor varying from -150 to -165 dBm/Hz was observed over a 100 MHz to 1500 MHz band.

• Optical directional isolators inserted immediately after the laser diode could further improve the noise floor.


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• Where microwave and optics meet– Although digital transmission over fiber optic links is

commonplace today, the ability to handle analog signals with arbitrary modulation in photonic systems is relatively recent development

– The technology makes it possible to bring the advantages of fiber optics to radio frequency (RF) and microwave transmission systems, including systems that use Amplitude Modulation.

– With the devices that are now available, RF or microwave applications now can make use of fiber optic links, whether the transmission distance is 10 meters or 20 kilometers


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Where microwave and optics meet (contd.)

• The recent availability of ultra fast laser diodes with matching fast photo detectors that can be directly modulated at Radio frequencies (RF) offer an alternative interconnection technology in air/space borne systems.

• These diodes when used with optical fibers, can replace RF cables/wave guides– Can save substantially on weight and cost

– provides superb Electro Magnetic Interference performance


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• Where microwave and optics meet (contd.)

– RF/microwave applications have required the development of a new generation photonic devices, which differ significantly from their digital counter parts.

• Semiconductor laser transmitters and photo diode receivers now have been engineered to meet the wide bandwidths needed for RF and microwave systems.

• Ruggedized packaging to withstand harsh environments

• compatibility with existing electronic equipment


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• Analog fiber optic links:– Compared with digital signal transmission,

analog RF & microwave transmissions cover a much wider spectrum

– The signal to noise (S/N) ratio for analog systems - particularly AM systems such as CATV-must be much higher than in digital systems.

• Calls for fiber optic laser transmitters with low noise– With specially engineered devices, a number of

important RF and microwave frequency bands are now well within the range of current analog fiber optic technology.


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• Analog fiber optic links (contd.):– Semiconductor lasers used as analog photonic

transmitters are based on ultra high speed gallium arsenide diodes.

– The design of these lasers is Fabry-Perot (FP). In an FP laser, the diode is a resonant cavity structure that emits coherent light when biased above threshold.

– FP lasers are available with up to 10 GHz modulation bandwidths.

– The transfer of electrical RF signal to modulated optical light power is highly linear and free of distortion.


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• Analog fiber optic links (contd.):

– Analog photonic receivers also stem from gallium arsenide technology.

• Like laser modules, photo diodes are also available in 3, 10 & 12 GHz bandwidths.

– A persistent problem in fiber optics is that optical connectors and the receiving photo diode tend to reflect a portion of the incoming signal back down the fiber.Reflections back into the laser cause additional light intensity noise at the ends of the link. Analog links, which typically are less and may be as short as few meters, can be particularly susceptible to reflections.


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• Analog fiber optic links (contd.):– better transmission medium than coaxial cable

• As its application in digital communications has already shown, Optical fiber is a nearly ideal medium

• Lower signal loss,

• immunity to electromagnetic interference (EMI)

• huge saving in cross sectional area and weight

• inherent security - lack of radiation and resistant to tampering

– Interception of beam is very difficult

– No scattering in space

• exceptionally wide bandwidth

• Jamming is extremely difficult due to space orbital dynamics & narrow beams.


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• Analog fiber optic applications:– Fiber optic technology is now being used in a variety of

demanding analog applications

• Replaces heavy coaxial and spline cable links

• Protect receivers, down converters etc. from harsh environments without costly repeaters

• absolute signal security

• inter facility links for satellite ground stations at intermediate frequency (IF), C-band or Ku-band.

• In Telemetry tracking systems, fiber optic links make it possible to locate receivers at a convenient and safe distance from the antenna pedestal while maintaining secure link between antenna and the receiver.


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• Analog fiber optic applications (contd.):

• microwave antenna remoting• upgrading of cable TV distribution systems• localized extension of cellular telephone

service• Delay lines for radar calibration• feeds for phased array radar &

communication antennas• spacecrafts


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Microwave Photonics for Space Applications• Fiber optics for satellites?

– Unprecedented number of earth observation and communication spacecrafts are deployed and more will be flown in coming years making much greater demands on space communication infrastructure.

• Direct communication links between spacecrafts will be an important element in coping with this demand

• Optical inter satellite links offer a route to low mass and power consumption transceivers with important advantages over conventional microwave links.

– Optical technology for space communications has been under development since 1970s and has now reached the stage where space qualified transceivers are feasible.


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• Satellites and fiber optics:– Satellites and fiber optic cable systems are evolving

towards network architectures which are radially different in concept and application.

– Satellites are usually at their best when they are directly linked to the end user or at least to an urban gateway earth station located not far away from the customer. This kind of distributed system whether for mobile services, direct broadcasting or interactive business networks, are horizontally linked via the satellite and requires no great concentration of traffic.

– Fiber optic cables, however, because of their characteristics as high capacity pipes need vertical concentration to achieve maximum efficiency.


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Microwave Photonics for Space Applications• Photonic application in Satelllites:

– From LNA/Down converter to the central processors for large spacecrafts

– Driver signal to antenna / TR modules

– Beam forming network/phased arrays

– Onboard switching matrix

– Onboard LAN

– Inter-satellite links / air to satellite links for strategic data exchanges

– Docking

– Optical tracking

– Laser inertial systems


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• Photonic application in Rockets and Misiles

– Onboard LAN– Inter space LAN & inter stage interface connection– Umbilical chord


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Some examples of photonics use in satellites


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Baseband data processing systems

Data I

Clock

Data Q

QPSK

Modulator

@ X-band

Power amplifier

X-band carrier source Antenna

Schematic of typical Data transmitting system of IRS satellitesSchematic of typical Data transmitting system of IRS satellites

Co-axial cables (To be replaced by fiber optics)


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• Why fiber optics within the satellite?– For digital high bit rate data interconnection:

Latch threshold


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• Why fiber optics within the satellite (contd.)?

– For analog signals• To replace the co-axial RF cables interconnecting

remote antennas to basic hardware– Size, mass reduction

– EMI reduction


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LEO S/C

GEO S/C

Optical link

19GHz link

ESA’s SILEX ecperiment

E/S


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• SILEX ( Semiconductor laser Intersatellite Link Experiment)

• ESA’s Optical inter-orbit communication system– Link between LEO (SPOT-4) and a

GEO(ARTEMIS) spacecrafts– 50 MBPS data - image data of SPOT-4 or

Psedudo-random noise (PN) code for BER measurement combined with TM data

– 4 pulse position modulation ( 4-PPM)• higher sensitivity compared to NRZ system

– The 4-PPM data stream modulates directly the intensity of the laser source.


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• Both GEO and LEO S/Cs are equipped with a 25 cm dia telescope, mounted on top of a pointing mechanism with more than hemispherical pointing range.

• Laser diode operating at a wave length of 830 nanometer with optical power of 100mw.

• GEO terminal is provided with an optical beacon source of almost 10 watt optical power for acquisition.

• LEO terminal in turn equipped with as beacon receiver a CCD matrix of 384 x 288 pixels for acquisition.

• Another 14 x 14 CCD matrix acts as tracking receiver


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• Most challenging task within a free space optical communication system is to maintain accurate pointing of the two terminals.

• Another critical issue in a narrow beam communication system is the spatial acquisition between the two terminals- the process of correctly aligning the two beams before the automatic tracking algorithm can become effective.– Modestly sized optical source of about 10 watts optical power on

GEO S/C and to select a beam divergence of 700 microrad. This requires a spatial scan of the beacon signal until the LEO terminal reports to the GEO terminal that the light ahs been detected.


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• Beam forming networks for phased array antennas– The signal distribution in phased array antennas, until

recent times, has been performed using conventional electrical techniques.

• Inherently creates an ultimate limitation to the array size and to the instantaneous bandwidth that can be obtained.

– Photonics based beam forming offers a number of unique properties that open the way towards the realization of massively dense multibeam architectures operating over multiple microwave bands.

• There is a strong motivation for introducing optical interconnect techniques for processing array signals.


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• High performance multifunction phased arrays– may have in excess of 1000 elements

– require large bandwidths

• To satisfy the wide bandwidth requirements, true time delay interconnects must be instrumented to set the antenna element feed lengths, which define the beam direction, independent of frequency.

• Active phased arrays which generate as many simultaneous and independent multiple beams as required to fill the required field of view. This implies a large number of delay elements in the Beam Forming Network (BFN), and the array realization becomes primarily an interconnect problem.


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• High performance multifunction phased arrays (contd.)

– Optical techniques offer some unique and intrinsic advantages

• wide bandwidth

• advantages in size

• freedom from EMI & Cross talk effects

– enables the signal distribution in large true time delay phased arrays to be realized.

– Use of Erbium Doped Fiber Amplifiers(EDFA) to provide gain in to provide gain in the optical domain and use of external modulation (EOM)techniques in the signal distribution network help in practical implementation of dense multibeam BFNs possible


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EOM

EDFA

EOM

EDFA

EOM

EDFA

Photo DiodePhoto Diode Photo Diode

Beam outputs 1 2 M

N:1 combiners

Optical fiber delay lines

1:M splitters

Electro optic modules

MW preamplifiers

Antenna elements

• Optical beam forming interconnect network1 2 N

Erbium Doped Fiber Amplifier


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96 way power Divider

X-band spherical active phased array antenna for IRS Data Transmission

5(or 6)bit Phase shifter Amplifiers

X-band QPSK Modulated carriers (2 nos)

+20 dBm

( 96 o/ps )

+30 dBm*

0 dBm

Driver Amplifier

Driver Amplifier

Control/Processor circuits

Control/Processor circuits

-0 dBm


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• Future:– Commercial applications will follow on from pre operational

demonstrations of SILEX and other ISL systems as the technology base is consolidated and space qualified components with the required reliability become available.

– Two areas of application will be in high data rate single access links and low data rate multiple access for data relay applications.

– larger ‘bit rate x distance’ product at economically attractive mass and power consumption levels.

• It is expected that laser diode outputs will continue to increase

• reduced mass and increase in bit rate capacity

– Applications are envisaged in data relay,interconnection of GEO/GEO systems and deep space probes.

– Other applications may be in LEO satellite constellations such as Iridium.


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• Future R&D in the field of Laser Intersatellite Communication Systems– high data rate systems for s/c applications– Low mass, low cost of user terminals capable of

covering the data rate range up to several 100 MBPS.– High degree of modularity and flexibility to be

compatible with broad range of mission requirements– Sufficiently small telescope diameters which do not put

prohibitive high requirements on the pointing system.– Communication performance

• high power optical transmitters - up to 1 watt• Coherent receivers• pointing, acquisition and tracking systems which minimize

mass, complexity, cost as well as pointing loss


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Thank You


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