ESA SAT ’N LIGHT PROJECT Optical Handling of Microwave and

ESA SAT ’N LIGHT PROJECT

Optical Handling of Microwave and Digital Signals

SUMMARY REPORT

Deliverable n° 10

ESTEC contract n°15695/01/NL/ND

ESA STUDY CONTRACT REPORT

ESA Contract No 15695/01/NL/ND

SUBJECT Optical Handling of Microwave and Digital Signals

CONTRACTOR THALES ALENIA SPACE France (formerly ALCATEL SPACE)

* ESA CR( )No

* STAR CODE

No of volumes : 1

CONTRACTOR'S REFERENCE

ABSTRACT The study entitled Optical Handling of Microwave and Digital Signals and nicknamed as SAT ‘N

LIGHT, was concerned with the development of critical optical technologies for the handling of microwave and digital electronic signals on -board future telec om and RF sensing satellites. The activity started with a review of future payloads and their functional needs for on -board signal handling. The baseline telecom scenario was a broadband communication GEO satellite mission in Ka -band, with flexible-connectivity repeaters and multiple -beam advanced antennas. RF sensors ( synthetic-aperture radar, altimeters and meteorology radar) were found to have in common, complex antenna designs, specific means for generation/processing of high -frequency signals, as well as digital processing capabilities.

Photonic satellite payload sub -system concepts were preliminarily investigated: optical TMTC networking, high-throughput optical interconnects for digital processors, optical sub-systems for local oscillator (LO) and sig nal distributions, and beam -forming in active antennas, opto-microwave re -configurable repeaters for analogue payloads, optical generation and control of microwave signals. Novel photonic repeater and antenna system architectures were elaborated for provid ing multi-beam, telecom satellites in Ka -band, with enhanced reconfiguration capabilities. Transparent, opto -microwave re-configurable repeaters (ORR) based on microwave photonic and optical MEMS technologies , were found attractive, as they can support fle xible channel cross -connection at attractive mass and consumption, and can grow up to large system scales . A multi-beam optically -controlled active antenna (OCA) concept was investigated based on a coherent beam -forming architecture, enabling to improve amplitude/phase control, and to provide fine tuning at low mass and volume, with application to a receive focal -array-fed reflector (FAFR) antenna for a GEO -based satellite.

Breadboards were developed in order to prove the concepts, assess the RF performan ce, and demonstrate optical building blocks in representative environment. The ORR concept making use of optical technology to distribute LO’s, perform frequency -conversion and RF channel cross -connection, was successfully proven. It enables to cross -connect RF channels with beam and frequency interchange. The RF performance are compatible with the implementation of such repeaters with a large number of beams. The basics of the OCA concept were proven in a 2 -channel demonstrator at low frequency. A dual -frequency LO laser, a single -polarisation optical modulator, a SLM -based amplitude/phase control module, and an optical summation device, were assessed as optical building blocks. Electro -optical modulators and optical MEMS switches were dedicated specific de velopment efforts. High -frequency modulators with low loss and reduced drive voltage, and highly -integrated 8x8 optical switches were prototyped. It was concluded that optical and microwave photonic technologies are critical not only for improving mass and volume, EMI/EMC, etc , but also for enabling the implementation of advanced payloads with enhanced or new functionality and excellent scalability. The work described in this report was done under ESA Contract. Responsibility for the contents resides in the author or organisation that prepared it.

Names of main authors: M. Sotom, B. Benazet, N. Vodjdani, T. Merlet, S. Blanc, J. Lopez, Ch. Voland, H. Porte, A. Seeds, P. Herbst, C. Marxer.

ESA STUDY MANAGER: J.M. Perdigues Armengol DIV: MM DIRECTORATE: TE C

** ESA BUDGET HEADING B1/100.061/600.510/VM+/RD104/01M33

ESA SAT ‘N LIGHT Project Summary report – Del. N°10

REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 1 / 26

Tous droits réservés © Alcatel Alenia Space All rights reserved

ESA SAT ’N LIGHT PROJECT Optical Handling of Microwave and Digital Signals

ESTEC contract n°15695/01/NL/ND

SUMMARY REPORT Deliverable n° 10

Written by Company Date Signature

M. SOTOM Alcatel Alenia Space – Research Department 24 / 05 / 07


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 2 / 26


TABLE OF CONTENTS

1 INTRODUCTION..................................................................................................................................................3

2 SCOPE OF THE PROJECT ................................................................................................................................3

3 PHOTONIC PAYLOAD SUB-SYSTEM CONCEPTS..........................................................................................4

4 OPTO-MICROWAVE RECONFIGURABLE REPEATER ...................................................................................8

5 OPTICALLY-CONTROLLED ACTIVE ANTENNA ...........................................................................................14

6 ADVANCED OPTICAL TECHNOLOGY DEVELOPMENTS ............................................................................19 6.1 ELECTRO-OPTICAL MODULATORS .....................................................................................................19 6.2 INTEGRATED OPTICAL MEMS SWITCHES ..........................................................................................20

7 CONCLUSIONS.................................................................................................................................................22

8 PUBLICATIONS ................................................................................................................................................24

9 GLOSSARY .......................................................................................................................................................25


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 3 / 26


1 INTRODUCTION

The present document constitutes the Summary Report - Deliverable n° 10 of the ESA Contract

N°15695/01/NL/ND - Optical Handling of Microwave and Digital Signals, also known as SAT ‘N LIGHT. The Project was concerned with the identification and the development of critical optical and opto-electronic technologies for the handling of microwave and digital electronic signals on-board future telecom and remote sensing satellites.

2 SCOPE OF THE PROJECT

In the past decade, optics has emerged as the enabling technology for efficient transmission and handling of

broadband, digital and microwave, signals in telecommunication networks. Terrestrial lightwave systems today benefit from a wide range of optical technologies (e.g. lasers, modulators, amplifiers, switches, detectors, etc) and offer performance that surpass ground-based RF/microwave communications systems.

Modern and future satellite payloads will have to receive, process, route and retransmit a large number of electronic signals. Optical technologies are thus expected to play an important role, for the handling of microwave and digital signals on board. They may contribute to the advent of new payload designs by enhancing or complementing conventional electronic functions, or by implementing new functions for which there is no satisfactory electronic solution. Critical issues such as EMC/EMI, RF isolation, mass, volume and power consumption figures could be improved significantly as well.

The development of optical technologies and techniques is of high interest for the satellite industry since the benefits both in technical and economic terms are considered substantial. ESA has been working consistently on applications of optical technologies in satellite payloads over the past 20 years, with special emphasis on laser inter-satellite links. However, anticipated relevant applications also encompass intra-satellite digital links (backplane interconnects, high-speed buses…), high data-rate routing and switching, generation, distribution and routing of RF and microwave signals, optical processing of microwave signals (frequency-conversion, analogue-to-digital conversion), complex antenna beam forming and steering networks, Telemetry/Telecommand networking (optical fibre harness, optical wireless communications).

Having identified the potential benefits and challenges, and the importance of maintaining Europe’s strategic independence and competitiveness, ESA initiated the activity “Optical Handling of Microwave and Digital Signals”. The contract was awarded to a consortium led by ALCATEL ALENIA Space, including THALES, CONTRAVES Space, PHOTLINE Technologies, University College of London, AVALON Photonics and SERCALO Microtechnology, and was nicknamed SAT ‘N LIGHT.

The main objective was to identify and to develop critical optical and opto-electronic technologies and

techniques for the handling of microwave and digital electronic signals on-board future telecom and remote sensing satellites. SAT ‘N LIGHT was split into two phases.

• Phase 1 was a conceptual design phase dedicated to explore and assess the potential of optical technologies and techniques for all applications of handling of electronic signals on board satellites, to elaborate new payload concepts and architectures, and to design payload sub-systems making best use of optical and hybrid opto-electronic technologies.

• Phase 2 consisted in demonstrating their feasibility and assessing their performance through proof-of-concept demonstrators (TRL4), i.e. breadboards of a selected number of such sub-systems and functional blocks.


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 4 / 26


3 PHOTONIC PAYLOAD SUB-SYSTEM CONCEPTS

The activity started with an overall review of future satellite payloads including telecom and RF remote

sensing, and a system-driven definition of typical functional requirements to signal handing on-board. The baseline scenario considered was a broadband telecom GEO (geostationary Earth orbit) satellite

mission. Future telecom repeaters for multimedia applications will make use of the higher bandwidth available in Ka and higher-frequency bands. Large platforms in GEO orbit will be capable of accommodating large antennas with high directivity and multi-spot beam coverage. Payload mass and DC power consumption are typically larger than 1000 kg and 15 kW respectively. Full use of the available bandwidth is possible by standard frequency reuse techniques. In order to thoroughly exploit the satellite resources and reduce user terminal complexity, on-board repeater and antennas must provide flexible and reconfigurable connectivity between the different channels and beams.

Digital repeaters remain a target for telecom payloads. However, the associated complexity and power consumption make analogue repeaters an attractive alternative/complementary option. Therefore, it was considered that both types of telecom repeaters, (regenerative and transparent) digital and transparent analogue, will co-exist in the future telecom satellite market. Furthermore, novel architectures based on high-throughput transparent digital repeaters are being studied to further reduce mass and power consumption, while improving connectivity performance and granularity, maximizing the use of available on-board resources and being fully transparent to future modulation formats and coding schemes.

Modern RF remote sensing missions and payloads were also reviewed in terms of functionality, architectures

and typical performance requirements. This application domain now embraces a wide range of various instruments namely imaging synthetic-aperture radar, altimeters, rain, cloud and other meteorology radar’s. RF sensing applications usually call for various and mission-specific requirements, but they have in common, complex antenna designs with up to one thousand radiating elements, specific means for generation and processing of high-frequency microwave signals (including local oscillator and chirped-frequency signals) as well as digital processing capabilities (e.g. for digital beam-forming) to handle the data output.

A bottom-up analysis of major optical technologies and techniques was also conducted with a view on their potential application on board satellites. This review included in particular :

- Datacom low-cost optical technologies (multimode and single mode fibres, 850 & 1300 nm transceivers, bi-directional transceivers, infra-red wireless devices), with potential application for TMTC networking and other low bit rate-rate digital communications, typically below 10 Mbps);

- Vertical-cavity surface-emitting lasers (VCSEL’s) and optical interconnect technologies, for serial or parallel optical data links with throughput well above 10 Gbps ;

- Micro-electro-mechanical-systems (MEMS) and optical switching technologies, with potential application in telecom repeaters;

- Analogue opto-electronic active devices (including lasers, photo-receivers, and optical amplifiers), for distributing microwave LO signals and handling multi-carrier signals in telecom repeaters and antennas;

- High-frequency optical modulators with application as electrical-to-optical interfaces in microwave analogue sub-systems;

- WDM optical technologies (fixed and tuneable wavelength lasers, as well as wavelength multiplexers and filters) for multiplexing and possibly routing applications in telecom repeaters and/or antennas.

From the identification of future system applications and requirements, and the review of optical

technologies, a number of photonic payload concepts, i.e. photonic sub-systems of satellite payloads that could advantageously be implemented by means of optical technologies, were formulated and preliminarily investigated.

• Optical TMTC networking

Concepts for optical TMTC harness and other on-board data handling (OBDH) communications (see Fig. 1), based on existing standards, were reviewed in terms of protocols, topologies and implementation options, and compared to a metal wire harness.


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 5 / 26


Figure 1 : Generic architecture for optical TMTC networking

Optical TM/TC networking concepts were found to allow for mass reduction and improved EMI immunity; one critical aspect might be the increased failure rate. A trade-off between reliability and mass may be looked for depending on the application and the design drivers. For low data rates, a MIL-STD-1773 (or equivalent) with light-emitting diode (LED) bus would offer a good reliability, power consumption and mass compromise. At higher data rates, an optical space wire bus would have to be implemented with VCSEL for the sake of better reliability. As the database is today very poor, it was also recommended to further investigate on reliability aspects.

• High-throughput optical interconnects

Optical interconnect concepts and architectures were paid a special attention and extensively reviewed. They were found quite attractive as they find application into many digital equipment and processors, in both telecom payloads (e.g. transparent digital processor, digital beam-formers) and remote sensors (mass memory, data handling units), either for intra-equipment back-planes supporting board-to-board connections (see Fig. 2) or for equipment-to-equipment connections as well.

Such optical interconnects bring fundamental advantages over electrical ones, namely much higher bandwidth, longer distance, lower power, and compact packaging, that should result in much larger and higher performance interconnection networks. They are mostly based on 850nm-VCSEL-array technology with flexible optical multimode fibre circuits or wave-guide interconnects for high bandwidth links, or with free-space interconnects for widely connected topologies and lower bit rates.

Figure 2 : Example of optical inter-board interconnects based on fibre circuits


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 6 / 26


Larger optical networks with flexible connectivity might also be designed. Three major options were

reviewed, respectively based on optical time-division-multiplexing (OTDM), wavelength-division-multiplexing (WDM) and optical code-division-multiplexing (OCDM). WDM was perceived as an attractive option, but further work was recommended to find out which concept and architectures would take best advantage of the performance provided by optical technologies.

• Optical antenna sub-systems

Optical distribution and collection of digital signals, distribution of microwave LO (local oscillator) signals and

optical beam forming to support active antenna command and telemetry and enable re-configurable beam forming have all been considered for future multi-beam active antennas in Ka-band telecom GEO-based satellites.

A number of optical architectures have been assessed corresponding to various steps of introduction of optical functions. Optical distribution and collection of digital signals enable to support active antenna command and telemetry with lower mass and volume. Two optical architecture options are found to be particularly attractive; they are respectively based on fibre-optics tree-like network architectures using WDM (wavelength-division-multiplexing) for bi-directional propagation, and optical wireless architectures. In the long term, and provided that there is no obstacle between the antenna control unit and the antenna transmit/receiver pair, free-space architectures will further simplify the integration and the connections between the different elements.

The distribution of microwave LO signals to the antenna can also be achieved through fibre-optics network, the maximum splitting factor being a function of the carrier-to-noise ratio (CNR) and phase noise level requirements. For a 130 dB CNR requirement, the antenna analogue beam-forming network (BFN) can be entirely fed through an optical distribution of a master local oscillator.

In all architectures, optical technologies were found to enable to achieve substantial gains in terms of volume and mass savings while they are expected to provide isolation and immunity against electromagnetic interference (EMI).

The multi-beam optically-controlled active antenna (OCA) was formulated by THALES as an advanced coherent optical beam-forming architecture enabling to improve amplitude/phase control accuracy, and provide fine tuning at low mass and volume. Receive focal-array-fed reflector (FAFR) antenna for telecom GEO-based satellites was taken as target. This concept was selected for further study and is described in detail in Section 5.

• Opto-microwave repeaters

Opto-microwave reconfigurable repeater (ORR) concepts were proposed and elaborated by ALCATEL

ALENIA SPACE, for supporting broadband, transparent, and flexible cross-connectivity in multiple spot-beam, GEO-based, telecom payloads in Ka-band or above. SAT ’N LIGHT typically targeted simple and flexible analogue repeaters with coarse (~ 10’s MHz) switching granularity and connectivity over 10’s of beams.

Some conservative concepts basically rely upon the replacement and/or enhancement of RF and microwave distribution harnesses, interconnects and switches, within relatively conventional RF architectures. Optical technologies may be used for the distribution of master local oscillators as well as the distribution and switching of RF signals at intermediate frequency.

More advanced repeater concepts were proposed that incorporate optical mixing functionality as well, with broader impact on both functional architectures and practical implementations. These particular optical architectures turned to be very attractive in terms of RF performance and size and mass/power consumption for future broadband telecom payloads in comparison with a full RF/microwave implementation. This concept was also selected for further study and is described in detail in Section 4.

• Optical generation and control of microwave signals

Advanced concepts for the optical generation and mixing of signals were also studied by University College of London. The project focus on millimetre-wave missions greatly increased the attractiveness of heterodyne approaches for LO generation. Optical heterodyning is based on the interference of two optical carriers onto a


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 7 / 26


photodiode to generate a microwave signal. For example, two optical carriers at 1550 nm with a wavelength spacing of 0.2 nm that fall on a high-bandwidth photo-detector generate a beat frequency of about 25 GHz.

Figure 3 : Principle of LO generation based on an Optical Phase Locked Loop

Using two separate laser diodes requires an optical phase-locked loop (OPLL) in order to lock the phase of one laser onto the phase of the other, and to drastically reduce the phase noise resulting from the beat linewidth. Both optical phase-lock loop (OPLL) heterodyne and optical injection phase-lock loop (OIPLL) techniques were found capable of meeting most of system requirements.

Figure 4 : Receive antenna beam-former based on electro-optical modulator mixer

Frequency-conversion in electro-optical modulators with heterodyne optical inputs was also identified as an

interesting technique, offering unique advantages, such as infinite LO to RF port isolation. An attractive application is the receive antenna beam former. A comparative study of electronic mixer and optical modulator-based receiver down-conversion was recommended.


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 8 / 26


4 OPTO-MICROWAVE RECONFIGURABLE REPEATER

ALCATEL ALENIA Space elaborated innovative re-configurable (ORR) repeater concepts for supporting

broadband, transparent, and flexible cross-connectivity in multiple spot-beam, GEO-based, telecom payloads in Ka-band.

A prospective broadband backbone telecom mission (see Fig. 5), supported by a Geo-stationary Earth orbit (GEO) satellite system, shall consist in offering semi-permanent access and mesh connectivity services to a number of Earth stations called gateways. Such a backbone system shall be used by Internet Service Providers to connect isolated nodes to their core network, or by different providers to interconnect their respective networks. Communications in Ka-band (30/20 GHz) making larger bandwidth available will constitute the baseline for such broadband satellite applications. The high directivity of Ka-band antennas enables to ensure the link budget from/to GEO systems. Global reach is achieved by implementing complex multiple spot-beam coverage based on frequency reuse (see Fig. 6).

Figure 5 : Prospective broadband backbone connectivity mission

Figure 6 : Example of multi-spot beam regional coverage with frequency reuse

Such future broadband telecom satellites will receive and re-transmit hundreds of radio-frequency (RF)

channels over tens of antenna beams; they will require flexible payloads that outperform the conventional “bent pipe” repeater in order to cross-connect or switch signals on demand.

Due to the long lifetime of a satellite mission (15 years) compared to the fast moving telecommunication market, flexible, versatile and future-proof solutions are mandatory. While digital processors shall offer advanced capabilities like fine granularity channelizing, packet routing and beam-forming, transparent analogue repeaters will make sense, especially for these backbone missions, provided that re-configurability is provided at moderate complexity, mass and volume. So far, this has not been offered by conventional RF technologies, so that photonic technologies may bring major benefits in the development of such payloads with broader bandwidth, wider connectivity, and enhanced routing flexibility at low mass and small size.

The architectures (see Fig. 7) rely upon conventional microwave low-noise receive and high-power transmit sections, and incorporate a photonic core to distribute microwave local oscillators (LO), perform frequency down-conversion, and achieve channel routing. All the LO’s are generated and transferred on optical carriers within a centralised unit, and delivered to electro-optical mixers with one microwave and one optical input, and one optical output. Each microwave telecom signal received from an up-link antenna beam is transferred onto an optical carrier at the electro-optical mixer. This mixer is fed by an optical local oscillator, and converts the input RF frequency down to an intermediate frequency (IF). Once under optical form, the signals are amplified and routed through an optical cross-connect made of passive splitters and switching matrices. Opto-microwave receivers convert the signals back into microwave ones at IF, so that RF channel filtering is achieved by conventional microwave means.


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 9 / 26


Figure 7 : Opto-microwave re-configurable repeater (ORR) by AAS architectural concept (left, and cross-connection functionality (right)

Such a flexible repeater enables to cross-connect a large number of channels with antenna access and

frequency-band interchange. It compares favourably with microwave implementations in that it may bring drastic mass savings (more than 40%) at identical system functionality and scale.

Even more important is that scalability considerations show that, based on optical MEMS technologies, these architectures could grow up to larger connectivity (10’s of beams). Additional benefits should arise from transparency to RF frequency bands, full RF isolation, suppression of EMC/EMI issues, that, at the end, may shorten the design-to-integration cycle.

More advanced architectural variants that make additional use of wavelength-division multiplexing (WDM), have also been proposed where simultaneously multiple frequency down-conversion is performed based on WDM. These repeater architectures could support more flexible cross-connection capabilities such as frequency-slot interchange, which definitely outperforms the capabilities of microwave implementations. Figure 8 : Opto-microwave re-configurable repeater breadboard demonstrator (1) Microwave photonic LO (2) Electro-optical mixer (3) Optical cross-connect (4) Microwave optical receivers

The proof-of-concept demonstrator assembled in Phase 2 aimed at demonstrating the system functionality, assessing the feasibility of the RF performance, and proving optical building blocks in system environment. The validation of the performance calculation tools developed to provide system guidance was also an objective. The repeater architecture selected for demonstration enables to cross-connect a large number of channels with access and frequency sub-band interchange. The demonstrator (see Fig. 8) was designed as a sub-populated breadboard, representative of an end-to-end opto-microwave path in Ka-band. It was split into the following four building blocks:

Frequency

Bea

m

Frequency

Bea

m

Frequency

Bea

m


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 10 / 26


microwave photonic LO source (1), microwave electro-optical mixer (2), optical cross-connection block (3), microwave optical receivers (4).

Key photonic payload building blocks and functionalities were thus demonstrated. All were implemented at

1550nm wavelength window to take benefit from the wide range of optical technologies for ground communications.

The microwave photonic LO source was developed based on double-side band modulation with carrier suppression (DSB-CS), which allows for doubling the frequency of the reference oscillator. The microwave photonic LO source delivered an optical LO from 20 to 40 GHz, with up to +18 dBm optical power and relative intensity noise as low as –155 dB/Hz.

Optical LO distribution was demonstrated in the range of 20 GHz with phase noise floor lower than –130 dBc/Hz (see Fig. 10). This is applicable in a very broad frequency range (at least, from 10 to 60 GHz), and enables distribution to 100’s of RF equipment with excellent performance, with the advantages of an optical fibre harnesses, namely loss mass (< 1g/m), distance-independent loss, EMI immunity and RF isolation …

Figure 9 : Microwave photonic LO source

breadboard (left), and optical spectrum (right)

Figure 10 : Phase noise performance of the optical LO distribution (at 20 GHz)

Optical frequency-conversion of microwave and millimetre signals can be achieved based on the mixing properties of electro-optical modulators. The arrangement developed within the project is shown in Fig. 11 and consisted in feeding a Mach-Zehnder intensity modulator with the LO signal from a microwave photonic oscillator.

Microwave inputOptical output

Control I/OOptical output

-160

-140

-120

-100

-80

-60

-40

-20

0

1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6 1E+7

Offset frequency (Hz)

Pha

se n

oise

(dB

c/H

z)

oscillator @ 10 GHz 20 dB optical loss 25 dB optical loss 30 dB optical loss 35 dB optical loss


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 11 / 26


Figure 11 : Optical frequency conversion based on electro-optical modulator

The microwave signal to be down-converted is applied to the modulator RF input, and superimposing a RF

modulation of the optical intensity. Direct optoelectronic detection generates both the LO and RF frequencies as well as the beat products, i.e. the frequency sum and the frequency difference. By designing the photo-detector response appropriately (see Fig. 13), the LO, RF and the sum frequencies can be cancelled out so that the IF frequency only is made available at the output.

The electro-optical modulator was a commercial modulator, with broadband response (see Fig. 12), drive voltage of 12 V at 30 GHz, and 4 dB optical loss. The C-band optical receiver was customised so as to provide with flat response within the 3-5 GHz band (see Fig. 13).

Figure 12 : Broadband Frequency response of the electro-optical mixer

Figure 13 : Band-pass frequency response of the

optoelectronic receiver in C band

Optical frequency-conversion was effectively demonstrated from Ka (~ 30 GHz) to C-band (~ 4 GHz). High

RF and LO frequency rejection (see Fig. 14), flat frequency response (see Fig. 14), and good linearity were obtained. No increase of the converted signal phase noise was observed. The RF gain & NF performance that are usual limitations of optical analogue links were considerably improved, so that they can now fit in with an overall system design. This function is also applicable to a range of frequencies and configurations much broader than those demonstrated.

-10

-5

0

5

0 1 2 3 4 5 6 7 8

Frequency (GHz)

Pow

er re

spon

se (d

B)

-15

-10

-5

0

5

10

0 10 20 30 40

Frequency (GHz)

Res

pons

e (d

B)

ω IF = ω RF -ω LOEOM EOM ω RF

ω LO

O/E O/E

PhotonicLO EOM : Electro-optical mixer

O/E : Optoelectronic detector


ω LO

O/E O/E




ω LO

O/E O/E




REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 12 / 26


Figure 14 : Optical frequency down-conversion demo in Ka band : RF output spectrum without RF filtering (left) and frequency response (right)

Demonstration and tests of the ORR system concept were then conducted through the insertion of an optical

cross-connect (see Fig. 15) built by CONTRAVES Space, and based on a MOEMS switch from SERCALO. The optical switch was a strictly non-blocking 4x4 matrix from SERCALO realized by cascading discrete 1x2 and 2x2 devices. The main features of the switch are optical insertion loss lower than 2 dB, optical crosstalk below –60 dB, and return loss higher than 50 dB. With these excellent optical performance, flexible repeater system operation was demonstrated with pretty good overall performance.

Optical frequency-conversion and optical cross-connection of RF channels through the MOEMS switch were tested together. The optical MEMS switch was operated under PC control so that microwave signals could be routed according to demand, with limited variations of the RF output power (see Fig. 15).

Figure 15 : MOEMS-based optical cross-connection of RF channels View of the optical 4x4 cross-connect with detail of the MOEMS switch (left) and RF output level matrix (right)

The ORR system functionality, i.e. channel cross-connection with access & frequency interchange was

successfully proven. The MEMS-based optical cross-connect (OXC) was shown to guarantee lower than – 80 dB RF crosstalk (see Fig. 17). Another important results is that MEMS-based optical cross-connect (OXC) was shown not to introduce any increase of the signal phase noise level (see Fig. 16).


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 13 / 26


Figure 16 : Phase noise of IF signal without and through

the MEMS OXC in the opto-microwave path Figure 17 : RF linearity and crosstalk performance

performance of the opto-microwave path

The feasibility of RF performance was extensively assessed in relation to system parameters and component specifications. RF gain, noise figure, crosstalk, linearity and phase noise performance were measured. The gain and NF performance of the optical section were considerably improved. Typically, RF gain about -22 dB and NF about 47dB can be obtained with up 17 dB optical loss in the link (see Fig. 18). These performance are considered compatible with the fulfilment of higher-level system requirements in terms of RF gain, NF penalty and multi-tone C/I linearity. As the optical section is to be preceded by, and is compatible with a microwave low-noise amplifier chain with high gain (e.g. 55 to 60 dB), this will make the overall gain and noise figure performance compliant.

Figure 18 : RF performance of the opto-microwave down-converting link vs. optical losses

RF gain (left) and noise figure (right)

The test results and the performance predicted by the models were also compared, and very good agreement was found. This enabled to extrapolate system performance to other conditions and scales with good confidence.

A very important outcome of the study is that high optical link loss budgets were shown feasible, and are available for growing these opto-microwave repeater architectures to much larger scales, i.e. which can support up to 10’s - 100 beams.

-160

-140

-120

-100

-80

-60

-40

-20

0

1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6 1E+7

Offset frequency (Hz)

Pha

se n

oise

(dB

c/H

z)

17 dB optical loss 17 dB loss with OXC (route 1-1) 17 dB loss with OXC (route 1-1 / all inputs conn.) 17 dB loss with OXC (route 4-1 / all inputs conn.)

-50

-40

-30

-20

-10

0

5 10 15 20 25

Optical loss (dB)

RF

gain

(dB

)

without MOEMS switch

through MOEMS switch

20

30

40

50

60

70

5 10 15 20 25

Optical loss (dB)

Noi

se fi

gure

(dB

)

without MOEMS switch

through MOEMS switch


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 14 / 26


5 OPTICALLY-CONTROLLED ACTIVE ANTENNA

THALES investigated the applicability of optical technologies to future multi-beam active antennas for Ka-

band telecom GEO-based satellites. Optical beam-forming (BF) was considered as the ultimate step in the introduction of optical technology in

active antennas. Optical BF in receive focal-array-fed reflector (FAFR) antennas for Ka-band (see Fig. 19) was selected as study case, first, because this would meet the needs of multimedia telecom GEO-based satellites, second because it was also considered more realistic to start with an FAFR antenna where each beam is formed with less than 12 feeds, than with a DRA antenna where about 300 feeds are necessary to form 1 beam. The target was to demonstrate that optics can improve amplitude/phase control accuracy, reduce mass-consumption-volume budgets and provide fine-tuning and /or re-configurability.

Figure 19 : Schamtic architecture (left) and view (right of a FAFR antenna (spots are formed through 7 to 12 feeds of the 170 elements retina)

The optically-controlled active antenna (OCA) selected within SAT ’N LIGHT project, was elaborated by

THALES as an advanced coherent optical BF architecture enabling to improve amplitude/phase control accuracy, and provide fine tuning at low mass and volume in future multi-beam phased-array antennas.

The OCA concept (see Fig. 20) gathered several innovative principles, in particular smart encoding of RF phase weightings through time delays implemented in the optical domain. The RF signals at Ka-band from the retina feeds are converted into optical signals by electro-optical modulators, fed optically with an heterodyne LO source, namely a dual-frequency laser (DFL) with two cross-polarized modes with a frequency separation exactly locked at the LO frequency. To guarantee stability, the two crossed polarizations propagate along the same optical path. Ideally, the modulator should modulate only one polarization and transparently transmit the orthogonal polarization. Down-conversion takes place and IF signal is recovered through photo-detection. Amplitude and phase weightings are performed by two matrices of spatial light modulators (SLM). Such a configuration shall offer good performance in terms of flexibility and scalability. Summation of the signals from different retina feeds to generate a beam is performed optically in an optical summation module.


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 15 / 26


Figure 20 : Schematic of the optically controlled phased-array antenna architecture

detectordetector

detectordetector

heterodyne source

modulator

signal RF 1

LO

Beam 1

Beam p

Phase weights : 34x12 Amplitude weights : 34x12

modulatormodulator

modulatormodulator

modulator

heterodyne source 7

Wafer 1

Wafer 7

signal RF 170

signal RF 145

signal RF 25

Optical fiber

The impact of optical beam-forming on radiating patterns was studied. The accuracy required as much on

phase as amplitude weightings put some constraints to the optical architecture. The fibre length has to be controlled with fine accuracy. As well, the power of the laser sources shall be monitored and the overall optical BFN shall hold its own calibration network with SLM pixels as feedback items. With limited pixels on SLM modules, it is already possible to finely tilt some beams. Increasing the number of pixels would permit full reallocation but will drastically increase the mass, volume, and consumption of the architecture. The consumption critically depends on the drive voltage of the input electro-optical modulator.

Figure 21 : Optically-controlled antenna (OCA) demonstrator by THALES Schematic (left) and view of the 2-channel demonstrator (right)

It was targeted to validate this new coherent optical BFN concept, by demonstrating first the basic principles,

and by extending it later to Ka-band. The OCA demonstration was implemented at 1064 nm wavelength and using L and C band as intermediate frequencies to be representative of satellite applications in S-band and Ka-band. The demonstrator (see Fig. 21) consisted in a 2-channel OBFN based in particular on an existing dual-frequency LO laser breadboard (TRL 4) in S-band. Two single-polarization modulators were realised so that, by using additional


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 16 / 26


1x2 splitters, up to 4 phase and amplitude SLM channels can be addressed, and recombined in a 4 into 1 summation module. The different building blocks were tested separately before integration in the demonstrator.

The dual-frequency laser LO (see Fig. 22) was based on a Nd:YAG cavity at 1064 nm pumped by laser

diode. It included an etalon holder and an electro-optic element (LiTO3) to generate and control two cross-polarized optical modes separated by the LO frequency. Part of the optical power was detected and used to feed the phase lock loops. The design of the mechanical sub-mount was paid special care for optimum stability. A stable free-running beating, an output power of up to 20 mW, and a locking range around 1.5 GHz were obtained. Though, long-term phase locking could not be obtained in a reproducible manner, due to remaining thermal effects.

Figure 22 : Dual-frequency LO laser source

- Schematic of the laser cavity (left) - View of the breadboard assembly (right)

Two single-polarisation phase modulators (see Fig. 23) were implemented at 1064 nm wavelength by hybrid assembly of the following discrete components : an input polarisation splitter, in one arm a phase modulator and in the other arm an optical attenuator, and an output optical polarisation combiner. This arrangement enabled to modulate the phase of only one polarization state, the other being kept un-changed. A dual-channel single polarisation modulator module was thus realised for OCA demonstration. The principle of the single-polarisation modulator was demonstrated but this hybrid assembly still suffered from instabilities. The concept of an integrated version of the single-polarisation modulator was proposed but not demonstrated.

Figure 23 : Single-polarisation phase modulator : schematic (left) and dual-channel module (right)


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 17 / 26


The amplitude and phase weighting module was based on two matrices of spatial light modulator (SLM) in between input and output fibre-pigtailed collimators (see Fig. 25). The SLM’s was mounted on a board with 8 trimmers for voltage tuning. Polarisation maintaining input collimators were been aligned with output collimators and fixed. The maximum phase shift was 250°, exceeding the 180° shift target. The amplitude control range was superior to 15 dB. Phase and amplitude stability were measured and shown compliant with target system requirements.

Figure 24 : Optical summation module (left) and microwave optical summation demo (right)

The optical summation module (see Fig. 24) consisted of an 8-channel optical fibre bundle held in a ferule

and precisely positioned in front of a PIN photodiode thanks to a micro-mechanical holder. The optical summation module was operated as a 4 into 1 microwave combiner, with a bandwidth of 6 GHz, and demonstration was carried out at 2 GHz. Each time the number of channel is doubled (i.e. the optical power rises by +3 dB), the microwave power shows a +6 dB increase. This enables to efficiently combine RF signals and improve dynamic range performance in microwave analogue optical links.

Figure 25 : Optical beam-former with amplitude/phase

control based on spatial light modulators

Figure 26 : 2-channel beam-forming demo : gain vs relative phase shift

Tests and demonstration of the optically-controlled active antenna (OCA) were carried out by THALES. The

principle and overall functionality of optical heterodyne beam-forming based on this coherent optical architecture

4 channels2 channels1 channel

4 channels2 channels1 channel


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 18 / 26


were demonstrated. Frequency down-conversion based on single-polarisation phase modulator, dynamic amplitude and phase control based on SLM matrices, and optical summation of RF beam channels were demonstrated. The accuracy and stability of phase and amplitude control were shown compliant with requirements. The advantage of optical summation at IF frequency was effectively proven.

At the end, the principle of coherent optical beam-forming was demonstrated in various configurations with 2 and 4 channels with contrast in excess of 20 dB (see Fig. 26). However, the RF performance feasibility and stability were, at this stage, still pending on the effective demonstration and availability of a stable dual-frequency laser LO source, and an integrated single-polarisation modulator.


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 19 / 26


6 ADVANCED OPTICAL TECHNOLOGY DEVELOPMENTS

This activity has been driving the development of a number of optical building blocks and technologies.

Optical devices and building blocks, other than those directly developed for, and included in the system demonstrations, were developed and demonstrated in parallel, in particular electro-optical (mixer) modulators and 2D-MOEMS switches.

6.1 ELECTRO-OPTICAL MODULATORS

The major objective was to develop electro-optical modulators with low driving voltage in the Ka microwave

band, i.e. around 30 GHz. A new concept of a band-pass Mach-Zehnder modulator on Lithium Niobate based on crossing waveguides

and a phase-reversal electrode (see Fig. 27), was designed and prototyped by PHOTLINE. The principle was considered as proven, as band-pass electro-optical response centred around 30 GHz was measured, and some reduction of the drive voltage at 30 GHz was obtained. However, the sample exhibited high optical losses, and the design was found extremely sensitive to technology process deviations.

Figure 27 : Bandpass electro-optical modulator by PHOTLINE

- modulator structure (left with crossing waveguides and phase-reversal electrodes - band-pass electro-optical frequency response (right)

Then, most of work was concentrated on broadband Mach-Zehnder intensity modulators on Lithium Niobate

(see Fig. 28) and successful results were obtained. A broadband optical modulator optimised for microwave applications was demonstrated with substantial improvements.

The new broadband modulator design allowed to get a reduction by about 20% of the half-wave voltage at

DC compared to former design, this with the same electrode design and RF performance. A new design of the Y junctions of the Mach-Zehnder modulator was studied in order to optimize the insertion loss, since part of the former insertion losses was identified to be introduced by a non-optimized Y-junction in the former design. The new design was patented by PHOTLINE. At the end, broadband electro-optical modulator with loss ~2.7 dB and 8.7 V drive voltage @ 30GHz was obtained. The modulator figure of merit, that accounts for both the drive voltage and the optical loss, and that critically impacts on RF performance, was improved by a factor of 4. Possible improvements and perspectives of further developments were also identified.

-80

-70

-60

-50

0 10 20 30 40

Frequency (GHz)

EO re

spon

se (d

B)


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 20 / 26


Figure 28 : Broadband electro-optical modulator - schematic of Mach-Zehnder modulator (top left) - packaged device (bootom left) - frequency response (right)

6.2 INTEGRATED OPTICAL MEMS SWITCHES

An integrated 2D-MOEMS switch (see Fig. 29) was developed by SERCALO, with potential for low optical

loss and extremely low crosstalk. An integrated 8x8 switch in a cross-bar architecture was designed and fabricated. The switch consists of 64 vertical mirrors sitting on a torsional platform. By electrostatic actuation, each mirror can individually be switched out of the optical path. Figure 29 : Schematic of the monolithic integrated 8x8 MEMS optical switch based on crossbar architecture, by SERCALO

Prototypes showing the functionality of the switch function were successfully fabricated (see Fig. 30). Very

significant and promising results were obtained. In particular, optical insertion losses lower than 3 dB, optical crosstalk below -65 dB, switching time in the millisecond range (see Fig. 31) were measured.

Nevertheless none of these switches could demonstrate 100 % operation, i.e. on all prototypes at least one mirror was broken. This bad yield was mainly due to strong non-homogeneity in the spring diameters. However, the concept could prove the feasibility of the operation of the micro-mirrors as well as of the self aligned optical collimation technique.

Once packaged, the mirrors showed to be very reliable. In an endurance test of over 9 million actuation cycles, no degradation could be observed.

-30

-25

-20

-15

-10

-5

0

0 5 10 15 20 25 30 35 40

Frequency (GHz)

S21

& S1

1 pa

ram

. (dB

)

Mirror

Torsion beam

Electrode

Flip mirror

AnchorInput fibers

Output fibers

Collimators

250µm


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 21 / 26


Figure 30 : Monolithic integrated 8x8 MEMS optical switch by SERCALO - SEM view of the MEMS chip (left) - view of packaged and fibre-pigtailed device (right)

Figure 31 : Dynamic response the 8x8 MEMS optical switch

Upper trace is the electrical command Lower trace is the optical response. (horizontal scale : 4 ms /division)

Optical loss uniformity and actuation voltage homogeneity are still to be improved. Further research activities will improve this non-homogeneity issue as well as the optical performance of the switch. Solutions are at hand to bring the insertion loss below the a commercial barrier which is around 2 dB. This 2D-MEMS switch design is well-suited to small-to-medium port counts (up to 10x10), but will not grow well above.

For extension to larger port counts, typically 10’s of input/output ports, it was recommended to go to 3D-MEMS optical switch architectures.


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 22 / 26


7 CONCLUSIONS

The opportunities and benefits of introducing optical technologies and techniques for the handling of

microwave and digital electronic signals on-board future telecom and remote sensing satellites were investigated within the SAT‘N LIGHT project. Novel photonic payload concepts and architectures were elaborated, in particular for repeater and antennas supporting broadband, transparent, multiple spot-beam, GEO-based, telecommunication satellites in Ka-band and higher frequencies, with enhanced flexible reconfiguration capabilities.

On the one hand, transparent, opto-microwave re-configurable repeaters (ORR), based on microwave

photonic and optical technologies, were found attractive with respect to the broad bandwidth, on-board re-configurability, versatility and future-proofness requirements of future telecom payloads in Ka-band. Under the assumption that the appropriate optical technologies are made available, estimates of the system complexity show that flexible cross-connection capabilities could be provided at attractive mass and power consumption budgets.

Whereas early applications may include flexible modern analogue repeaters in Ku/C bands, the most advanced repeater architectures with larger scale, and enhanced functionality (frequency-slot interchange), will apply for Ka-band. Optical building blocks and technologies find applications in other payload sub-systems, in particular optically-assisted, advanced antenna concepts.

On the other hand, optical beam-forming based on a coherent optical architecture (OCA) was considered as

the ultimate step of introduction of optical technologies in active antennas, The selected OCA architecture gathered several innovative principles, in particular the use of a heterodyne LO source with two coherent crossed-polarisation optical carriers. It was expected to improve amplitude/phase control accuracy, reduce mass-consumption-volume budgets, and enable fine-tuning of the beam pointing, especially in multi-beam active antenna (OCA) for Ka-band telecom GEO-based satellites. A receive focal-array-fed reflector (FAFR) antenna where each beam is formed with less than 12 feeds, was taken as a reference application.

From the system level definition of these two applications, representative breadboards were developed and

tested in order to demonstrate the feasibility of these concepts and validate the performances of the major optical building blocks and the overall sub-system. The major goals were to demonstrate the soundness of the concepts, by proving the basic functionality and assessing the RF performance feasibility, and to demonstrate key optical building blocks in system representative environments.

The repeater architecture selected for demonstration incorporated optical technologies to achieve local

oscillator distribution, frequency down-conversion and channel cross-connection. It enables to cross-connect a large number of channels with full access port and frequency sub-band interchange. As it already includes most of their optical building blocks, the selected architecture was also to some extend representative of more advanced repeaters that would take benefit from WDM technology to perform enhanced-flexibility cross-connection function. The concept of a microwave photonic cross-connect repeater for telecom satellites, making full use of optical technologies to distribute LO ’s, perform frequency-conversion and RF channel cross-connection, was successfully proven. Test results showed that the RF performance can be compatible with the implementation of such repeaters at large scale with an attractive number of beams.

The basic principles of the OCA architecture were proven in a 2-channel demonstrator at lower frequency.

The heterodyne LO laser source based on a dual-frequency laser, the single-polarisation optical phase modulator, the SLM-based amplitude and phase control module, and the optical summation device, were also assessed as key optical building blocks.

Electro-optical modulators on Lithium Niobate and MEMS-based optical switching matrices were dedicated

specific development efforts respectively by PHOTLINE Technologies and SERCALO Microtechnology. Outstanding results were obtained, in particular on high-frequency electro-optical modulators with improved figure of merit, and highly-integrated 8x8 optical switching matrices. Possible improvements and perspectives of further developments were also highlighted.


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 23 / 26


The introduction of optical technology in satellite payloads was also put in perspective. Recommendations for future Research & Development were expressed, at both sub-system and technology levels, so as to support the development, or to increase the maturity level of critical optical technologies, out of which electro-optical modulators with high figure of merit, MOEMS switches with medium to large port counts, low-consumption optical amplifiers, microwave photonic LO sources, and opto-microwave receivers.

Optical and microwave photonic technologies were concluded to emerge as enabling technologies in space,

not only for the improvement of mass and size figures with respect to all-microwave implementations, but also for the practical implementation of advanced satellite payload system concepts, with enhanced enhanced/new functionalities (e.g. optical down-conversion, routing flexibility, etc.), and excellent scalability properties.


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 24 / 26


8 PUBLICATIONS

M. Sotom, B. Benazet, M. Maignan, “Microwave photonics for future telecommunication satellite payloads : applications and technologies“, International Topical Meeting on Microwave Photonics 2006, Grenoble, France, 3-6 October, 2006. M. Sotom, B. Benazet, M. Maignan, “A Flexible Telecom Satellite Repeater based on Microwave Photonic Technologies”, 6th International Conference on Space Optics (ICSO 2006), Noordwijk, The Netherlands, 27-30 June, 2006. B. Bénazet, M. Sotom, M. Maignan, J. Perdigues, “Microwave Photonic Cross-connect Repeater for Telecommunication Satellites”, Proceedings of SPIE Vol. 6194, Photonics Europe ’06, Strasbourg, France, 3-7 April 2006, paper 6194-03. P. Herbst, C. Marxer, M. Sotom, C. Voland, M. Zickar, W. Noell and N. de Rooij, “Micro-optical switches for future telecommunication payloads : achievements of the SAT 'N LIGHT Project”, ESA 5th Round Table on Micro/Nano Technologies for Space, Noordwijk, The Netherlands, Oct. 3-5, 2005. M. Sotom, B. Bénazet, M. Maignan, J-M. Perdigues Armengol, “Optical technologies for on-board processing of microwave signals”, Symposium on Disruption in Space, Marseille, France, July 4-6, 2005 J.M. Perdigues Armengol, M. Sotom, B. Benazet, M. Maignan, T. Merlet, S. Blanc, N. Vodjdani, C. Voland, C. Marxer, H. Porte, “An overview of the SAT ‘N LIGHT ESA project“, OPTRO 2005, Paris, France, 2005, May 9-12. S. Blanc, J. Lopez, L. Menager, T. Merlet, “A new coherent optical multibeam former for the receive mode in 2D antennas”, SPIE, 2004.


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 25 / 26


9 GLOSSARY

BB Broad Band BF (N) Beam-Forming (Network) C/I Carrier to Intermodulation (product) CNR Carrier-to-Noise Ratio CW Continuous Wave DFB Distributed Feed-Back (laser) DRA Direct Radiating Array DSB-CS Double Side-Band with Carrier Suppression E/O Electrical-to-Optical, or Electro-Optical FAFR Focal-Array-Fed Reflector GEO Geostationary Earth Orbit IF Intermediate Frequency IMP Inter-Modulation Product I/O Input/Output LO Local Oscillator MEMS Micro-Electro-Mechanical System MOEMS Micro-Opto-Electro-Mechanical System MZ(M) Mach-Zehnder (Modulator) NB Narrow Band NF Noise Figure OBF (N) Optical Beam-Forming (Network) OCA Optically-Controlled Antenna O/E Optical-to-Electrical OFA Optical Fibre Amplifier ORR Opto-microwave Re-configurable Repeater OXC Optical cross(X)-Connect PD Photo-Detector PMF Polarisation Maintaining Fibre RF Radio Frequency RIN Relative Intensity Noise SLM Spatial Light Modulator SMF Single Mode Fibre TMTC TeleMetry / TeleCommand TOI Third-Order Intercept TRL Technology Readiness Level WDM Wavelength-Division Multiplexing


REFERENCE :

DATE :

100148043R

24/05/2007 ISSUE : 1.1 PAGE : 26 / 26


END OF DOCUMENT

Documents

ESA SAT ’N LIGHT PROJECT Optical Handling of Microwave and