MICROWAVE OVER FIBER Applications and Performance

Transcription

1 MICROWAVE OVER FIBER Applications and Performance IEEE North Jersey LEOS November 12, 2012 John A. MacDonald Vice President of Engineering, LLC Dr. Allen Katz President, Linearizer Technology, Inc. Professor, The College of New Jersey 1

2 OUTLINE 1. MICROWAVE LINKS: ANALOG / DIGITAL 2. INTENSITY MODULATION & DETECTION 3. PRACTICAL LINK STRUCTURES DIRECT MODULATION EXTERNAL MODULATION PHOTORECEIVERS TRANSMISSION MEDIUM 4. PERFORMANCE LIMITATIONS LINEAR EFFECTS NONLINEAR EFFECTS 5. NONLINEAR PERFORMANCE IMPROVEMENT/LINEARIZATION 6. SUMMARY 2

3 Analog / Digital The bulk of fiber optic communications uses digital modulation Fast switching and low pulse distortion determine link fidelity Certain applications not suited to digital: Bandwidth too high to be digitized System complexity favors a broad pipe Primary distinction between digital and analog is linearity Analog/Microwave links depend upon low distortionto achieve high fidelity 3

4 Microwave Fiber Optic Link RF Input E to O transducer Optical Fiber O to E transducer RF Output Ideally, the output is a linearcopy of the input E/O transducer modulates the RF information onto an optical carrier O/E transducer reverses the operation Transducers must effectively transfer RF power (information) 50 Ω impedance to RF environment Modern Microwave Link technology is dominated by: Intensity modulation of semiconductor lasers Envelope detection using PIN or APD photodiodes 4

5 Microwave Link Applications Radar Low weight, complexity Beamsteering and Direction-finding Antenna and Signal Remoting Direct-RF over longer distances (many km) Reliable alternative to wireless in fixed services Electronic Warfare / SIGINT / ELINT Secure Comms(EMI hard) Towed Decoys Space-based Mass advantage Deployed fiber: can be radiation hard; less thermally sensitive Precise Time and Frequency Distribution High RF and Magnetic EMI Environments Fiber is almost completely EMI-proof, and non-metallic High Voltage Environments Fiber will not conduct 5

6 Practical Intensity Modulation Direct Laser diode is modulated directly External Laser source drives a separate optical component 6

7 Direct Modulation 12 Bias I L P 10 8 Diode Laser Power vs. Current Modulating Signal i m 6 4 Slope Efficiency η L Semiconductor 2 Laser Optical Intensity Mean Offset + Modulation I TH I L Laser Modulation Current DC Bias + RF P ( im ) =ηl ( I L ITH + im) 7

8 External Modulation A CW optical signal is intensity modulated via a field-dependent optical medium µwave / mm-wavemodulation speeds can be achieved with 2 major methods: Electro-Optic Modulation Field-dependent change in optical index (electooptic effect) Electro-Absorption Modulation Field-dependent change in optical attenuation 8

9 Electro-Optic Modulation Mach-Zehnder Interferometer Index of refraction is dependent on applied field (modulating signal) Electro-Optic effect can be realized in Lithium Niobate(LiNbO 3 ), InP, and other crystal structures, i.e. KDP (KH 2 PO 4 ) 2 4 Optical Vector Diffused Optical Waveguide on LiNbO 3 substrate Vm (RF + Bias) Propagation constant of the beam in the lower leg is retarded due to applied electric field experiences less phase shift than upper leg. Optical intensity (power) is modulated by the applied RF signal due to summation of out-of-phase vectors. 9

10 Mach-Zehnder 1 Intensity 0.5 Optical Intensity Mean Offset + Modulation Vm Modulation Voltage DC Bias + RF Optical Output power follows cos 2 function o Vector phase summation V π is DC voltage that causes 180 phase rotation o Depends on crystal physics and electrode length o Corresponds to min and max output power o Digital Modulation: variation between min and max Analog Modulation: Bias at Quadrature (shown) o Results in linear intensity modulation o Slope = 1 at quadrature point o Even order distortions are balanced (zero) 10

11 Electro-Absorption Absorption of optical signal dependent on applied bias Transmission follows exponential relationship with applied field Generally not as linear as MZM V m P in P transmitted P out P( V m ) = e f ( Vm) 0.5 N i a absorbed 0.25 Transmitted Optical Intensity Mean Offset + Modulation Modulation Voltage DC Bias + RF 11

12 Photodetection P-I-N diodes are most common VDC Iave iout I ( P) = R P R = responsivity (amps/watt) = ηq / hω Intrinsic bandwidth limited by diode capacitance Package and launch considerations may also limit performance ~30 GHz bandwidth from lateral PIN ~100 GHz from waveguide PIN 12

13 Transmission Medium Fiber is Optical Waveguide n core > n clad Fiber has very low loss < 0.25 db/km at 1550 nm Singlemode High-fidelity Microwave Links require Single-Mode fiber 13

14 Qualitative Link Measures E to O transducer Optical Fiber O to E transducer RF Input RF Output Gain (Loss) Added Noise Gain Broadband links are lossy Gain Slope / Ripple important to system design Noise Figure Generally higher than the link loss Third-Order Intercept Intercept of fundamental and 3 rd -order IMD curves Spur-Free Dynamic Range (SFDR) Power Range over which the intermodulation distortion is below the noise floor Useful Quality Factor incorporates Noise and Third-order Distortion Nonlinear Distortion, LLC 14

15 Performance Limits Linear Factors affect the Gain and Noise of the output signal Microwave Launch Inherent Bandwidth Limited by device and package reactances Noise Fiber Medium Absorptive Loss Dispersion (Chromatic, Polarization Mode) Nonlinear Factors affect the shape of the output signal Distortion (Harmonic, Intermodulation, ) Fiber Medium Stimulated Brillouin Scattering Raman Scattering Four-wave Mixing 15

16 Linear Impacts: Microwave Launch Primary source of microwave loss is input/output matching 10 GHz DC R S C L R L R S C J Forward Biased Laser Diode Small real resistance R L Junction Capacitance and Ohmic Resistance Narrow band links can be reactively tuned Limited Bandwdith Fano s rule: (BW)(Reflection Coefficient) < c However: Many or Most links require Broadband Must use lossy matching affects link gain Reverse Biased Photo Diode Large reactive impedance dominated by junction capacitance Ohmic Resistance adds dissipative loss 16

17 Linear Impacts: Noise Laser Relative Intensity Noise (RIN) Caused by spontaneous emission Laser is not a perfect oscillator 100 THz carrier is spread over 100 s of GHz Receiver detects as microwave noise Noise Power follows detection square-law: N o ~ I 2 RIN Receiver Shot Noise Random arrival of photon quanta Output Noise power is white and follows the optical power: N o ~ I Thermal Noise Ubiquitous Johnson Noise Output Noise Power is constant and white Noise power delivered to the RF load is the sum of 3 independent sources 17

18 Noise Figure Output Noise of F/O Link Gain of F/O Link Noise Figure of F/O Link Noise Power Density (dbm/hz) Thermal Shot RIN Total Gain (db) Noise Figure (db) Optical Receive Power (dbmo) Optical Receive Power (dbmo) Optical Receive Power (dbmo) Output noise depends on optical power 2:1 in RIN region 1:1 in shot region constant in thermal region Gain also depends on optical power Always 2:1 Noise Figure decreases with optical power Asymptotic Link Noise Figure and Dynamic Range vary with optical power defined in conjunction with the operational system 18

19 Linear Impacts: Fiber Medium SM Fiber Attenuation Due primarily to Rayleigh (elastic) Scattering 0.25 db/km (1550 nm) 0.5 db/km (1310 nm) Chromatic Dispersion Wavelength dependent propagation velocity Sidebands arrive out-of-phase: gain nulling Polarization mode Dispersion Orthogonal Polarization Modes different propagation velocities Non-uniform through fiber length concentricity defects mechanical and thermal perturbations laser spontaneous emission Nondeterministic Affects pulsed (digital) systems Affects Time/Freq Distribution systems SLOW AXIS FAST AXIS Chromatic Dispersion Polarization Mode Dispersion 19

20 Nonlinear Impacts: Modulator Transfer Function Optical Power Bias Current Direct Modulation Gain Compression Even and Odd order amplitude distortion Phase distortion due to laser chirp (FM to PM) Laser wavelength = function(drive level, temperature) Intensity Vm MZM External Modulation Gain Compression Primarily Odd order amplitude distortion Very little phase distortion 20

21 Linearity Measures Third-Order Intercept (IP3) (Imaginary) point where two-tone third-order intermodulation products (IMD3) are equal to the fundamental Spur-Free Dynamic Range Range of power over which the fundamental is above the noise floor and the IMD3 are below the noise floor SFDR 3( db Hz ) = + db 3 [ 174 NF( db) + IIP3( dbm) BW ( )] P out Output Noise Floor SFDR-3 IIP3 P in 21

22 Nonlinear Impacts: Fiber Stimulated Brilluoin Scattering (SBS) Vibrational/Acoustic oscillations generated by high energy photons Forward wave energy is converted to acoustic backward wave (phonons) Threshold Effect Reduced forward gain; Increased noise floor Stimulated Raman Scattering (SRS) Inelastic photon scattering Nonlinear fiber effects must be considered during link design phase. Wavelength translation Reduction in gain Self-Phase Modulation AM-PM conversion of a single signal Cross-Phase Modulation AM-PM Transfer from one signal to another (WDM systems) 4-Wave Mixing Intermodulation Distortion (WDM systems) 22

23 Intensity Modulation Summary TYPE COMPLEXITY SIZE DIRECT Low: one optical component (laser) WEIGHT POWER PRACTICAL MODULATION FREQUENCY LINEARITY Lowest 15 GHz Poor 2 nd and 3 rd -order performance COST Lowest ELECTRO- Moderate: Similar to 40 GHz Poorest Higher, ABSORPTION requires separate direct mod comparable source laser and to EOM small modulator ELECTRO- OPTIC (MZM) Highest: requires source laser, large modulator, plus optical and electrical controls for bias locking Highest > 60 GHz Well-defined (sin curve). Operation at quadrature provides 2 nd - order null. Highest 23

24 Closing the Link LINK Directly Modulated Laser / PIN Receiver / Postamplifier Reactively Tuned GHz LINK E-O Modulator (MZM) / Waveguide PIN Receiver Broadband Response to 40 GHz PHOTORECEIVER Broadband O/E Response to 20 GHz and Output Return Loss Link Type 4 GHz Direct Mod 20 GHz MZM 30 GHz EAM 40 GHz MZM Centerband Gain Input IP3 Noise Fig SFDR3 db dbm db db Hz^2/ Typical Link Gain, Noise, Dynamic Range 24

25 Trade-Offs, or Why Use Fiber? What are the System Engineer s Tradeoffs? Should I consider fiber? Tradeoff performance, cost, weight vs. other options If fiber, then what do I need to know? Direct Mod vs. Ex-Mod (Decision Needed) Direct Mod (usually < 12 GHz) Weight, Cost, Loss, Dynamic Range, DC Power Often Comparing directly to Coax Ex-Mod Must evaluate system impact for linearity, G/T, etc 25

26 Direct Mod Comparison to Coax At Direct Mod frequencies, choice of Fiber vs. Copper often made on basis of performance as a function of link length Primary System Trades include: Attenuation Noise Figure Weight Cost 26

27 Direct Mod vs. Coax: Attenuation Attenu uation (db) Direct Mod Link Distance (meters) 2000 MHz 27

28 Direct Mod vs. Coax: Noise Figure Noise Figure (db) Direct Mod Link Distance (meters) 2000 MHz 28

29 Direct Mod vs. Coax: Weight and Cost WEIGHT (pounds) 1 m 100 m 1000m RG LDF Direct Mod Link COST 1 m 100 m 1000m RG8 $3 $300 $3,000 LDF6 $38 $3,800 $38,000 Direct Mod Link $5,000 $5,100 $6,000 29

30 Direct Mod Comparison to Coax Crossover Length when DM performance exceeds that of coax Attenuation Noise Figure Weight Cost RG8 110 m 180 m 30 cm 1800 m Heliax LDF6 560 m 900 m 50 cm 125 m Example: Crossover Lengths at 2 GHz Other factors may influence choice of Fiber vs. Copper: Linearity (link dynamic range is less than coax) EMI immunity (may trump performance) Safety, Reliability, etc. 30

31 Performance Improvement Linearization Techniques Linearization improves nonlinear distortion, increases dynamic range Major techniques under study include optical, electrical, and combinatorial approaches Electrical: aim is to cancel distortion products Feedforward/Feedback: Inject out-of-phase distortion products to cancel Predistortion: Nonlinear circuits with opposing distortion characteristics Optical: generally more complex Operates in optical domain inherently wide-band 31

32 Electrical Predistortion Predistortion Linearization has long history in Broadcast Power Amplifiers; SSPAs, TWTAs, Space and Ground Station equipment Generally less complex than optical or combinatorial systems Does not rely on sampled waveforms Bandwidth is the major challenge The aim is to compensate for the gain and phase compression of the nonlinear system by providing a cascaded element function that has the opposite gain and phase characteristic: gain and phase expansion

33 Electrical Predistortion PERFORMANCE OF LINK IS PRIMARILY LIMITED BY THE DISTORTION INTRODUCED BY OPTICAL MODULATION. PREDISTORTION (PD) LINEARIZATION ELIMINATES THIS DISTORTION BY GENERATING A FUNCTION WITH OPPOSITE MAGNITUDE AND PHASE OF THE MODULATOR

34 Predistortion Linearization 3 2 Output Power Input Power Phase Input -3 Power Nonlinear Device exhibits Gain and Phase Compression 34

35 Predistortion Linearization Output Power Output Power Input Power Input Power Phase 4 3 Phase Input -3 Power Input Power Nonlinear Device exhibits Gain and Phase Compression Precede it with another nonlinear device that exhibits gain and phase expansion, in conjugate with the device to be linearized (the linearizer) 35

36 Predistortion Linearization Output Power Output Power Output Power Input -3 Power -2-1 Input -3 Power -2-1 Input -3 Power Phase Phase Phase Input -3 Power Input -3 Power Input -3-2Power Nonlinear Device exhibits Gain and Phase Compression Precede it with another nonlinear device that exhibits gain and phase expansion, in conjugate with the device to be linearized (the linearizer) The desired outcome is an ideal limiter The linearity of an ideal limiter cannot be improved 36

37 Wideband Predistorter Functions from 1.5 to > 20 GHz Target: ΔGain = 2.5 db ΔФ< 5 degrees (Ach. to 13 GHz) Flatness ±0.5 db (Ach. to 12 GHz) Feel can achieve <1GHz to <30GHz LPL generic predistorter is very broadband

38 Linearized Microwave Link 1550 nm Source Laser RF IN Preamp Predistorter Postamp/ Equalizer MPR0020 Photoreceiver RF OUT LINEARIZER MZM NONLINEARIZED LINK LINEARIZED LINK Nonlinearized MZM Link: Commercial modulator biased at quadrature 20 GHz flat receiver driven at 0 dbmo Linearizer: Includes broadband gain stages Predistorteris single-chip GaAscircuit (proprietary design) Signal levels adjusted to match gain expansion of predistorter to gain compression of MZM Postamp stage includes slope equalizer to match levels over frequency 38

39 MZM Linearization Demonstration of IMD improvement from predistorting an MZM link Third-Order IMD (dbc) db IMD improvement (yields 5 db SFDR 3 ) MZM Microwave Link Linearized IMD Products Non Linearized Linearized Results at 8 GHz Measured improvement >14 db (minimum) from 4 to 12 GHz Optical Modulation Index

40 Predistortion Linearizer Performance Linearization Results of EAM Link at 14 GHz Pout Gain Pout Gain -12 Input Power Backoff (IPBO) in db 0-20 Input Power Backoff (IPBO) in db 0 Non-Linearized 4 db Gain Compression at Ref. Input Power Linearized Predistortion linearizer effectively compensates the gain compression

41 Predistortion Linearizer Performance Intermodulation Distortion Improvement EAM Measured at 6 db IPBO Non-Linearized Linearized 15 db improvement in IMD equates to 5 db improvement in SFDR3

42 Multi-Octave Problem FOR WB OPERATION (> OCTAVE BW) - EVEN & ODD ORDER DISTORTION MUST BE CONSIDERED THIRD ORDER AM COMPRESSION TERMS F1 F2 EMISSION LIMIT F2-F1 2F1-F2 2F2-F1 2F1 FREQUENCY IM AND HARMONIC DISTORTION A PROBLEM 2F1, F2-F1, F1, 2F2-F1 F1 AND 2F1-F2 F2 PRODUCTS OF MOST CONCERN MOST PREDISTORTERS CORRECT ONLY ODD ORDER DISTORTION

43 Multi-Octave Linearization MZMs produce minimal 2 nd harmonic distortion with bias voltage control Predistorters can generate 2 nd in addition to 3 rd order nonlinearities 2 nd order terms may worsen performance for > octave bandwidth Even terms not in the proper phase to cancel Developed predistorters that generate only 3 rd -order components and operate over multi-octave bandwidth 43

44 Multi-Octave Linearizer A multi-octave broadband with even order cancelation -operating from 1 to 20 GHz -with single linearizer providing both IM and harmonic distortion correction using push-pull NLG Pre-Distortion Linearizer Circuitry NLG NLG Balun Atten Balun NLG NLG

45 Optical & Electro-Optical Linearization Dual Series MZM Modulators Proper biasing of series MZMs may result in linearization of sinusoidal transfer Has been shown to approximate ideal limiter response Inherently wideband Difficult to tune/align Feedforward Nonlinear response is sampled (electrically) and reinjected to fundamental path in order to cancel undesired frequency products Limited bandwidth due to need for electrical delay in feedforward path

46 Optical Linearization Example Optical Feedforward Coupled linearization of Mach-Zehnder modulator Third-order cancellation OMI 0.2(OMI)^3 MZM biased at Vpi/2-0.5sin(piOMI/2) optical output Vrf s p l i t a2 AC coupled MZM biased at Vpi/2 delay a1 0.2(a2)(OMI)^3 Potential for even greater cancelation (> SFDR)

47 Description First MZM generates distortion products Amplitudes of distorted detected outputs are: = 1 π sin OMI 3 V V 0.2 OMI fund 2 2 IMD = 2-tone 3 rd -order amplitudes (IMDs) were found by eval. Fourier Series of the output Note that Fundamental and IMD products are always out of phase RF signal is delayed and added to the distorted output Level is set to just cancel the carriers of the detected signal, leaving just the distortion Distortion products are re-modulated, and summed with the first modulator output. Summation must be noncoherent Dual lasers or sufficient delay

48 Summary Fiber Optic Links are increasingly being deployed for linear microwave transport applications Radar/Antenna remoting where weight and loss are critical Sensor Systems Precise Time and Frequency Distribution High EMI environments Photonic microwave links beneficial alternative to coax or wireless in many applications Size, Weight, Cost, Performance, EMI, Safety, Practical intensity modulation links were presented Typical Performance, Limitations, and Methods of Improvement Linearization can offer dramatic improvement in dynamic range 48