Analog RF-over-Fiber Technology by ghkgkyyt


									Analog RF-over-
Fiber Technology

Association of Old Crows
January 19, 2008
Bruce G. Montgomery, Syntonics LLC

 Components for analog photonics
 RF-over-fiber link design
 Applications of RF photonics
 Who is Syntonics LLC?
1. Introduction

What is RF-over-fiber?

What is RF-over-Fiber?

RF-over-fiber = Electro-optic components and light used
to transport RF signals over optical fibers [all analog]
   Radical change from what Mr. Marconi had in mind
RF-over-fiber has certain advantages over coax cable
   Long, secure, lightweight, easily routed
But RF-over-fiber isn’t an engineering silver bullet:
   Practical links need gain / attenuation / filtering / closed-loop control
   Links add thermal and spurious noise and limit SFDR
   Link doesn’t does not “know” everything the radio “knows”

How does RF-over-fiber work?

        Transmitter                                        Receiver

                         Amplitude-modulated light
                        propagates over optical fiber
          Laser diode                                       PIN diode
                                                             Bias tee
  RF Source
  RF Source                                                             RF Out
                                                                        RF Out
                        DC bias                  DC bias

Any RF signal can be transmitted,             The RF Out must be amplified to
up to many GHz depending on the                  provide a useable signal
 laser and its modulation scheme.

Generic Link Topologies
Different ways to combine lasers, modulators, fibers, detectors …
Directly Modulated        @ Antenna                     AM light               @ Receiver
Laser at Antenna
                                    Laser                                            Detector

                                                                                                       RF Out

Externally Modulated      @ Antenna                     AM or PM               @ Receiver
Laser at Antenna                                                         Detector (Several designs
                            Laser                                            to be considered)
                                                External AM or
                                light           PM Modulator

                                                                                                       RF Out

Externally Modulated
                          @ Antenna                          AM or PM
                                                                 light         @ Receiver
Laser at Receiver           External AM or PM Modulator                  Detector (Several designs
                                                                             to be considered)


                                                                                                       RF Out

Optical Communications
             1870—John Tyndall
             1880—William Wheeling
             1880—Alexander Graham Bell

Mid-Twentieth Century
 Early 1950s: Fiberscope & optical fibers with
   Snell’s law: The angle at which light is reflected depends on
   the refractive indices of the two materials
   Used to inspect inaccessible welds, for laparoscopic
   Optical fiber loss ~1,000 dB/km

                                                          1962: First visible laser diode

Late ’50s to early ’70s                                   photographed in its own light

 1957—Gordon Gould at Columbia University
 conceives of laser as an intense light source
    Supported by Charles Townes, Arthur Schawlow at
    Bell Labs
 1960—Ruby laser and helium-neon (HeNe)
                                                        1962: IBM scientists observe their
 1962—Semiconductor lasers                               new GaAs direct injection laser.

 1966—Charles Kao and Charles Hockham at
 Standard Telecommunication Laboratory,
    Posit optical fiber communications possible if
    attenuation < 20dB/km (1% of light at 1 km)
 1970: Robert Maurer, Donald Keck, Peter
 Schultz at Corning
    Achieve glass fiber with < 20 dB/km loss — purest
    glass ever made
2. Components for
Analog Photonics

EO Components

                External Modulators

  Communication Lasers
                          Butterfly module includes
                          a Peltier thermoelectric
                          cooler/heater to maintain
                          temperature (wave length

                           TOSA (Transmit Optical
                           Sub-Assembly) lasers
                           are simpler devices

Metrics: Optical Power & Relative Intensity Noise (RIN)
RIN describes the instability in the power level of a laser; usually
presented as relative noise power in db/Hz

External Modulators
 Electro-absorption (EA) devices
 are intensity modulators
   Uses Franz-Keldysh effect, i.e., a
   change of the absorption spectrum
   caused by an applied electric field         Oki Semiconductor ( 40 Gb/s
                                                EA modulators using GaInAsP semiconductor
 Electro-Optic (EO) devices are
 phase or intensity modulators
   Uses Pockels effect, which is small
     Most commonly used electro-optic                n3
     crystal, lithium niobate (LiNbO3), has    ∆n ≅ rE
     r = 34x10-12 m/V.                                2
     An electric field of 106 V/m (1V across   Where :
     electrode gap of 1 µm) produces a         n = index of refraction
     fractional index change of ~ 0.01%.
                                               r = electro optic coefficien
   Most effective with polarized light
                                               E = applied electric field

    Mach-Zehnder Modulator (MZM)
        Mach-Zehnder interferometer invented ~1900
             Measures phase shift (∝ index of refraction) using two optical
        A MZM is an intensity modulator

Modulation of an electrical data stream using an   Schematic cross section through the two arms of a push-pull
                external MZM.                         MZM (not to scale). Light to be modulated propagates
                                                    through core layer along the length of the device (into the
                                                    page). Dipoles are aligned anti-parallel in the two arms of
                                                                             the MZ.

  Optical Detectors
      Single-ended PIN diode photoreceiver (e.g., ROSA)
         Absorbed photons generate a mobile electron and electron hole
         Carriers are swept from the junction by the built-in field of the
         depletion region, producing a photocurrent
      Used with reverse bias.                              Anode         Cathode

         Reverse-biased diode has extremely high resistance
         Can be used as a photon detector by monitoring the current
         running through it

Metrics: Responsivity, Leakage, NEP
Responsivity — Ratio of generated photocurrent to incident light power, typically
                                                    incident light
expressed in A/W when used in photoconductive mode
Dark current (“Leakage”) — Current through the photodiode in the absence of any
input optical signal, when it used in photoconductive mode
Noise-equivalent power (NEP) — Minimum input optical power to generate
photocurrent equal to the RMS noise current in 1-Hz bandwidth

Optical fibers & connectors

Four Wavelength Regions of
Optical Fibers

Multi-Mode Optical Fibers
 Multimode ~ numerous simultaneous wave modes
 related to acceptance angle
 Two types:
   Step-index (left)
   Graded-index (right)
      Light waves follow serpentine path

Single-Mode Optical Fibers
 Small diameter core precludes
 dispersion caused by multiple
 mode and achieves lower attenuation losses
   Early single-mode fiber generally had step-index cladding
   Modern single-mode fibers have matched clad, depressed
   clad, and other exotic structures

 Single-mode fiber presents incremental difficulties
   Smaller core diameter makes coupling light into core more
   Tolerances for connectors and splices are more demanding

Optical Connectors
 Names reflect “1. connector type”/“2. end-face polish”
   1. ST, SC, FC, etc.
   2. End polish applied to connector’s (i.e., fiber’s) end face
      PC = Physical Contact, end face polished and usually convex
      APC = Angled Physical Contact at 8° angle
         Can also find defined as “Angled Polished Contact)

 Typically, xx/PC connectors have lowest insertion loss;
 xx/APC connectors have lowest return loss

Wave Division Multiplexing

Coarse Wave Division
Multiplexing (CWDM)
  18 wavelengths specified, 10 used (1430-1610 nm)

Dense WDM
 Dense Wave Division Multiplexing DWDM)
   64 wavelengths specified
 RF-over-fiber link

Loss & Gain & Noise (Noise Figure)
& Linearity ⇒ SFDR

RF-over-fiber versus Copper
 Advantages                            Disadvantages
   {Distance x Bandwidth}                {Distance x Bandwidth}
   product give better RF                product give less RF
   performance for “relatively
                                         performance for “relatively
   long” links
                                         short” links
   Easier to route cabling
                                            Noise figure and SFDR
       Reduced weight, diameter,
       bend radius                       Requires TX amplifier at
   Signals are isolated                  antenna
       No cross talk                        Redundant with HPA in radio
       EMP/EMI isolation of radio        Size-Weight-and-Power
       Security regulations              (SWAP) for transmitter
   Relocates                             components
   expensive/sensitive radios               Lasers, current and TEC
   to the user                              control circuits, modulator,
       Can maintain or change               bias controller
       receiver without going to the        Higher power LNAs
       antenna or changing the fiber
               Loss versus frequency for coax cables

          50          750                                       2.5
                                                                                                                                              Cable outside
          40          600                                         2                                                                             diameter
                                                                                                                                                 0.15" OD
loss dB

                                          loss (dB/m)
          30                                                                                                                                     0.2" OD
                      450                                       1.5
                                                                                                                                                 0.3" OD
                                                                                                                                                 0.3" OD
                      300                                         1
                                                                                                                                                 0.375" OD
          10          150
                                                                                                                                                 0.5" OD

          20 m         300 m
          Cable run    Length of
                                                                       0               10               20                     30        40
                       aircraft carrier                                                       frequency (GHz)
                                                                                               frequecy (GHz)

                                                                           3rd order dynamic range of typical LNAs

                                                                                                                           20 dB gain
                                              SFDR (dB-Hz2/3)
                                               SFDR (dB-Hz )

                                                                                                                           30 dB gain

                                                                                                                           40 dB gain



                                                                      15       20            25           30              35        40
                                                                                    1 dB output compression point (dBm)

Optical link loss calculations
Optical Component                                 Typical Loss
Connector (SC/APC or FC/APC)                          0.25 dBo
Splitter                                               3.5 dBo
Switch                                                 0.5 dBo
Fusion Splice                                          0.1 dBo
SM Optical Fiber                                     0.2 dBo / km

                   0.25 + 0.5 + 0.5 + 0.25 + 0.2 ≈ 1.7 dBo
           ~0.25 dBo         ~0.5 dBo             ~0.5 dBo
                                                      ~0.25 dBo

                                          ~0.2 dBo

Optical loss ≠ RF loss
 RF power loss = 2 x optical power loss
 With electrons, “RF Power” ∝ I2 (∝ V2)
   “Half (RF) Power” = 10•log(power ratio) = 10•log(2÷1)
   = 3 dB down
   = 70.7% of current (or voltage)

 With light, “Optical Power” ∝ number of photons
   “Half (optical) Power” = 1/2 number of photons
   = 1/2 of current in PIN diode detector
   RF output Power ∝ I2 = 1/4 of RF input power = 6 dB down

 Gain is required to:
   Overcome losses in the optical path (TX and RX paths)
     2X RF gain to overcome 1X optical loss
   Work above the noise floor of the laser (TX and RX paths)
     Typically requires 30-40 dB gain in RX path
     Reduces SFDR
   Power the antenna (HPA, TX path)
     Coax losses much reduced so less power may be necessary
     Many military waveforms require linear amplification
     Some military waveforms require fast T/R switching

 Attenuation is required to:
   Survive radio TX power
   Implement ALC control of HPA

 Noise results from RIN, Shot Noise, Thermal Noise
    RIN results from laser’s spurious optical emissions (spurious electron
    Shot noise results from quantum physics of discrete photons striking PIN
    diode detector
    Thermal noise results from resistance in laser diode and PIN diode
    detector + noise due to RF amplification

            At low optical power,
            thermal noise dominates
            At high optical power, RIN

Noise Figure
 Link output noise can be computed directly from specified component
 noise constants and optical loss (OL):
    Nthermal = cthermal (dBm/Hz)
    Nshot = cshot – OL (dBm/Hz)
    NRIN = cRIN – 2OL (dBm/Hz)

 Total output noise,

 Noise figure (dB) =

 where GL is the total link gain,

 Linearity refers to second-order, third-order, and
 higher order distortion terms
 “Linearity” required by system is function of:
   Bandwidth, dynamic range, modulation type, number of
 Sources of third-order non-linearities include:
   Laser diode, fiber, PIN diode detector, RF amps
 Sources of second-order non-linearities include:
   Nonlinear spectral components of sum & difference freqs.
   and harmonic freqs.
   Not an issue if system bandwidth < 1 octave
 Typical RF-over-fiber links operate at low power
   Only laser diode and RF circuits create non-linearities

Spur-free dynamic range
 SFDR is a consequence of gain, noise, linearity
 Usually dominated by third-order SFDR:

 Where third-order intercept, IIP3 (dBm) is:

                                     Excellent reference:
                                             Application Note 106-1,
                                             “System Design Using
                                             Direct Modulation Fiber
                                             Optic Links”
                                             J. A. MacDonald, Linear
                                             Photonics, Hamilton, NJ

     Noise figure and SFDR for MZ quadrature biased link
                       NF(Vπ,Ip), balanced link                            NF(Vπ,Ip), single detector,
                                                                                            RIN limited


NF=18                                 NF=29
Po=150mW (fiber laser)                Po=40mW, RIN=-160dB/Hz (laser
Dual output modulator, Vπ=3           diode)
Modulator + link loss = 3.8 dB        Dual output modulator, Vπ=5
Balanced detector, rd=0.8, Ip=25 mA   Modulator + link loss = 5 dB
                                      Balanced detector, rd=0.8, Ip=5 mA
Military Comms
Applications for RF-

Mobile Communications
           Central antenna   Exposed antennas with
                                 good lines of sight

                                fiber cable        Antennas
                                                  on railings
Perimeter Security
   Radio Port, typ.                                  Radios
                                                below decks

 Mobile command
 center in defilade

Fixed-Site Communications
                                            Encrypted, opto-isolated
                                            radio located with operator
                                                                                               Antenna Interface Unit (AIU)

Use existing fiber optic cables     Secure signal
to connect radio in basement                                                                   Secure signal on fiber cable
to antennas on the roof                                                                        • No emissions
                                                                                               • No susceptibility
                                      Radio Interface Unit (RIU)                               • Carry multiple radios on
                                                                                                  single cable

      Radios in                              QuickTime™ and a
                                         TIFF (LZW) decompressor
                                     are needed to see this picture.

                                                                              Geographic diversity cable #2
 command center

          Geographic diversity cable #1

                              Remote antenna                                  Remote antenna
                              farm #1                                                farm #2

ATC, Ranges

Moves radios to central
control location … yet
                               Air Traffic Control
antennas can be distant
  Lowers maintenance costs
  Reduces maintenance
  reaction times

                             Test & Training Ranges
Syntonics LLC

Syntonics LLC
 Specializes in RF        DCAA-approved accounting
 communications           system
 technology for DoD:      ISO 9001:2000 registered
   FORAX communications   Quality Management System
   Unique antennas        DSS-supervised facility
 Founded 1999             clearance (SECRET)

Syntonics Customers
        Air Force ACOMS,& Comms Sqdrns (multiple units)
        Air Force NORTHCOM
        Air Force Office of Scientific Research (AFOSR)
        Air Force Pentagon Comms Agency (AFPCA)
        Army Brigade Combat Teams (multiple units)
        Army Research Laboratory (ARL)
        Army Tobyhanna Depot
        DoD Technical Support Working Group (TSWG)
        Dwyer Hill Training Center, Ontario
        Institute for Defense Analysis
        Joint Air Defense Operations Center (JADOC)
        Maryland Procurement Office
        Missile Defense Agency (MDA)
        Naval Air Systems Command, Pax River (NAVAIR)
        Naval Air Warfare Center Weapons Division
        Naval Special Warfare Groups (multiple units)
        Naval Surface Warfare Command Carderock
        Naval Undersea Warfare Center Division Newport
        Space and Naval Warfare System Command
        Special Operations Command (USSOCOM)
        General Dynamics, Lockheed Martin, BAE,
        Brown International, TSE, others
FORAX RF-over-Fiber

What Is FORAX?
FORAX connects communication       RF-over-fiber is well suited for:
radios to remote antennas using       Fixed and mobile command centers
optical fibers                        Air Traffic Control
FORAX can replace multiple            Test & Training Ranges
coaxial cables with single fiber   Developed for USSOCOM
optic cable

Why Use FORAX? (Warfighters)
FORAX enables a dramatic change in “Radio
  Removes the radios and crypto equipment to the defilade
  safety of the command post
  Replaces heavy, short, power wasting coaxial cables — one
  per radio — with a single long, lightweight, secure fiber optic

FORAX will change tactical communications
doctrine as it relates to radio/antenna separation:
  Decrease risk to personnel and costly radio/crypto equipment
  Decrease time of CP set-up
  Lower maintenance response time

Why Use FORAX?
(Radio Engineers)

Feature            Benefit
Long Connections   Radio and its antenna can be located up to 10 km apart using single mode
                   fiber (2-3 km with multi-mode fiber)
Signal Security    Entire connection from radio to antenna can be encrypted signal
                   Fiber optic link is extremely difficult to intercept
                   RF signals can penetrate SCIF walls using optical fibers
EMP/EMI            Electromagnetic pulses or interference cannot propagate over, or
Immunity           influence the signals on, optical fiber cables
                   Radio equipment is opto-isolated
Easy Routing       RF signals are carried on lightweight, flexible, rugged, optical cables
                   Multiple radios can be carried on a single fiber optic cable
                   Geographic diversity in RF signal routing becomes easy
All Frequencies,   FORAX tactical radio products cover 0.5-512 MHz @ all modulations
All Modulations      •HF,   SINCGARS, VHF, UHF, UHF TACSAT
                   EPLRS coming early 2008

  FORAX RF-over-fiber Users
                User                 Radio(s)                    Application                Use       Since
DoD/Ft. Bragg                         PRC-117               VHF & UHF LOS, TACSAT         Mobile C2   2004
Canadian Forces, DHTC                 PRC-117               VHF & UHF LOS, TACSAT         Mobile C2   2005
Pentagon Renovation/GD              PSC-5, USC-61                   TACSAT                Fixed C2    2005
Army Research Lab                  PRC-117, PSC-5                   TACSAT                Mobile C2   2006
56th ACOMS Hickam AFB                 PRC-117                       TACSAT                Fixed C2    2006

347th CS Moody AFB                     PSC-5                    VHF & UHF LOS             Fixed C2    2006
Pentagon USAFPCA                   PRC-117, PSC-5                   TACSAT                Fixed C2    2006
Army JADOC Bolling AFB          VRC91, PRC-117, PSC-5   SINCGARS, VHF & UHF LOS, TACSAT   Fixed C2    2006
603th ACOMS Ramstein AFB              PRC-117                       TACSAT                Fixed C2    2006
612th ACOMS Davis-Monthan AFB         PRC-117                       TACSAT                Fixed C2    2006
Pentagon USAFPCA                      PRC-117                  UHF LOS, TACSAT            Fixed C2    2007
GD/Electric Boat                       PSC-5D                       TACSAT                Fixed C2    2007
Tobyhanna Army Depot                  PRC-117                       TACSAT                Mobile C2   2007
Army Research Lab                     PRC-117                  UHF LOS, TACSAT            Fixed C2    2007
NORTHCOM                              PRC-117                       TACSAT                Fixed C2    2007
CENTCOM                            VRC93, PRC-117                  SINCGARS               Fixed C2    2007
612th ACOMS Davis-Monthan AFB           GPS                        GPS L1/L2              Fixed C2    2007
                                  RT-1523, PRC-117,
CENTCOM                                                        SINCGARS, EPLRS            Fixed C2    2008
Al Udeid Air Base               PRC-117, AFTRS, CTII         TACSAT, GPS, UHF LOS         Fixed C2    2008
(Pending)                              Various               HF, LOS, TACSAT, EPLRS       F & M C2    2008

Existing aerostats equipped with
FORAX-HARC (“High Antennas for
Radio Communications”) can extend
radio communications both over the
horizon and into “urban canyons.”

HARC Concept
High antennas improve a CP’s line-of-sight
(LOS) comms
  Example: Over-the-horizon comms to distant radios
  Example: Local comms into “urban canyons”
  Useful for SINCGARS, VHF, UHF, EPLRS, etc.
Aerostats can carry high antennas
FORAX can connect a CP’s radios
to multiple high antennas using
the aerostat’s tether         Rx /Tx

                                                      Rx /Tx

FORAX-HARC: Many radios
sharing a 5000’ antenna tower
         FORAX-HARC AIU (not visible)
         • Antennas connected via coax cables
         • Packaged to meet platform’s requirements

Optical fiber
• 1 @ single-mode                            Four high antennas (not visible)
                                             • Separate TX and RX elements
                                             • Two types
Coax cables

                                                Aerostat mooring point & FORJs

                                                FORAX-HARC RIU
                                                • Radios connected via coax cables
                                                  to radios’ antenna ports

                                                               User Radios

To buy from Syntonics …
 Bruce Montgomery
   410-884-0500 x201

 Ray Madonna
   410-490-2680 —or— 410-884-0500 x206


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