LINEAR MICROWAVE FIBER OPTIC LINK SYSTEM DESIGN

Transcription

1 LINEAR MICROWAVE FIBER OPTIC LINK SYSTEM DESIGN John A. MacDonald and Allen Katz Linear Photonics, LLC Nami Lane, Suite 7C, Hamilton, NJ LINEAR PHOTONICS, LLC Bringing Performance to Light!

2 Microwave Fiber Optic (F/O) Link Analysis Microwave (linear) F/O Links used for point-to-point communications Antenna remoting Sensors Radio-over-Fiber. We present a new method for quickly determining the system impact of a F/O Link GAIN NOISE LINEARITY. We demonstrate a linearization method for improved F/O Link linearity

3 Brief Overview of DM Links Directly-Modulated (DM) Fiber Optic Links Transmitter: RF pre-amplification, biasing, and matching to low-impedance laser diode Output is Intensity Modulated Modulation Efficiency = ratio of peak output modulation envelope to peak microwave input current G F RF IN Laser Diode OPTICAL OUT P out Link Length OPTICAL IN P in Receiver: PIN photodiode, matching from highimpedance diode, RF post-amplification Responsivity = ratio of generated current to incident light intensity PIN diode performs direct envelope detection G F RF OUT Photodiode

4 Brief Overview: Fiber and Optical Medium Propagation Loss in Optical Medium Fiber is (almost) lossless, compared to coax 9 µm single-mode ~.5 db/km at 55 or.4 at Connectors, splices, switches, splitters add loss Chromatic Dispersion Upper and Lower sidebands arrive at different times Generally not a concern for < km and < 4 GHz Fiber Nonlinearities Items to be aware of, but generally not a concern for single-wavelength with < mw optical power: Stimulated Brillouin Scattering (SBS) Backscattering effect leading to high noise and nonlinear power transfer Stimulated Raman Scattering (SRS) Can lead to degenerative optical gain when optical power is very high Four-Wave Mixing (FWM) and Cross-Phase Modulation (XPM) Intermodulation distortion and cross-talk in WDM systems (multiple optical wavelengths) Self-Phase Modulation (SPM) Leads to nonlinear group delay Lower Sideband Optical Carrier Upper Sideband Dispersion causes phase delay between detected optical (AM) sidebands 4

5 System Parameters using DiLink End-to-End Gain DiLink gain is typically or 5 db Fully temperature-compensated, referenced to db OL End-to-End gain is lower by twice the optical loss Noise Three primary Sources: Laser (RIN), Shot, Thermal Output noise level depends on optical power at the receiver Third-order Linearity IP is intersection of st and rd order transfer lines Contributors: Laser, Receiver, RF Amplifiers Transmitter: Fixed IP Receiver: IP decreases with higher optical receive power (photoreceiver and amps driven harder) Overall: depends on optical loss Noise Density (dbm/hz) Output Power (dbm) thermal Output Noise Density Total Noise shot Received Optical Power (dbm) Output noise density is the result of primary sources: RIN, Shot, Thermal Linear Photonics, LLC. Non-linearized Pout vs Pin - 8 GHz Input Power (dbm) Fundamental Pout rd Order Pout IP Third-order Intercept is the (imaginary) intersection of st and rd order output RIN IP = 8 dbm By By defining the the link link properly, the the primary system parameters depend only only on on optical loss loss 5

6 System Equations Standard Link Definitions (available for DiLink) G ref End-to-End Link Gain with no optical loss (db) C th Thermal noise constant (dbm/hz) C shot Shot noise constant (dbm/hz) C rin RIN noise constant (dbm/hz) IIP eq Equivalent Input Intercept (dbm) OIP eq Equivalent Output Intercept (dbm) From these, the Black Box System Parameters: Link Gain, G L G L ( db) = G OL ref Link Noise Figure, F and Output Noise Power Density, N out F( db) = N ( dbm / Hz) + 74 G ( db) N out out = log cth cshot OL c + + rin L OL Link Input Intercept, IIP IIP ( dbm) log + IIP = OIP IIP + G eq OIP eq + eq eq L 6

7 Example DiLink Broadband Direct Mod Tx 55 nm Optical Splitter.5 km 5 km DiLink Receiver DiLink Receiver RF Out (local building) RF Gain ~ +5.6 db RF Out (distant site) RF Gain ~ +.5 db DiLink DLT5N8 / DLR5N8 standard definitions G ref C th C shot C rin 5 db -9.9 dbm/hz -44 dbm/hz -9 dbm/hz RF Input ( 4 MHz) IIP eq OIP eq.6 dbm 7. dbm SYSTEM-LEVEL END-TO-END LINK PARAMETERS PARAMETER PATH PATH Optical Loss 4.7 db 6.5 db Link Gain +5.6 db +.5 db Link Noise Figure 9.5 db. db Link Input rd order Intercept.9 dbm. dbm Other Parameters Derived from above: Link Noise Temperature 6 K 6 K Third-order SFDR 4.9 db Hz /. db Hz / 7

8 Linearity Improvement Linearization: Methods of improving the linearity of a nonlinear network Predistortion Linearization is one technique Employs a nonlinear element in the microwave signal path Operates at instantaneous microwave rate Not limited by delay as in feedback or feedforward approach Not limited by overly complicated component-count Limited primarily by microwave matching, preamplifiers, etc. Linearizer Technology, Inc. (Linear Photonics sister company) has been manufacturing linearizers and linearized networks for > 5 yrs Technology is readily applied to fiber optic networks Linearizer Technology, Inc. 8

9 Predistortion Linearization Output Power Input Power Phase - - Input Power Nonlinear Device exhibits Gain and Phase Compression 9

10 Predistortion Linearization 5 4 Output Power Output Power Input Power Input Power Phase 4 Phase Input 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)

11 Predistortion Linearization Output Power Output Power Output Power Input Power Input Power Input Power Phase Phase Phase Input Power Input 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) The desired outcome is an ideal limiter The linearity of an ideal limiter cannot be improved

12 Predistortion Linearization Result is reduction in IMD: SFDR is impacted : (db) with IMD

13 Microwave Link Linearization 55 nm Source Laser RF IN 6- GHz MPR Photoreceiver RF OUT MZM NONLINEARIZED LINK Nonlinearized MZM Link: Commercial Mach-Zehnder Modulator biased at quadrature GHz flat receiver driven at dbmo

14 Microwave Link Linearization 55 nm Source Laser RF IN 6- GHz Preamp Predistorter Postamp/ Equalizer MPR Photoreceiver RF OUT LINEARIZER MZM NONLINEARIZED LINK LINEARIZED LINK Nonlinearized MZM Link: Commercial modulator biased at quadrature GHz flat receiver driven at dbmo Linearizer: Includes broadband gain stages Predistorter is single-chip GaAs circuit (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 4

15 Results: Gain and Phase Transfer 5.7 db.5 db Non 8 GHz P db is 5.7 db from saturation Phase compression rapidly above sat 8 GHz P db is.5 db from saturation Phase nonlinearity held to < past sat 5

16 IMD Improvement 7 linearized 6 8 GHz 6 GHz GHz 4 GHz 5 IMD (dbc) 4 GHz nonlinearized 4, 6, 8,, GHz Input Power Backoff IPBO (db) Broadband correction for input drive below ~ 5 db from saturation 5 db IPBO is about 65% Optical Modulation Index (OMI) 6

17 IMD / IIP / SFDR Frequency (GHz) IMD Improvement (db) IIP Improvement (dbm) SFDR Improvement (db Hz / )

18 Correction Bandwidth Two-tone IMD measured with span of MHz and GHz Centered at 9 GHz Pout (dbm) Linearized MZM Third-Order Products with Varying Carrier Spacing MHz spacing: & 9. GHz GHz spacing: 7.5 &.5 GHz P low ( MHz) P low ( MHz) P hi (MHz) P hi (MHz) Plo ( GHz) P lo ( GHz) P hi ( GHz) P hi ( GHz) Pin / tone (dbm) Linearizer is is effective over full octave instantaneous bandwidth 8

19 Summary A proper set of link definitions allows rapid calculation of system-level performance, i.e. Gain, Noise, Linearity. LPL DiLink-series Fiber Optic Links provide fully defined operational parameters intended for System Engineers A broadband linearized link has been demonstrated with improved SFDR Improvement of 7.9 db at 8 GHz Instantaneous correction over multioctave bandwidth THANK YOU! Copies of of this this presentation are are available at at Booth