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1 University of Southern California, Los Angeles, CA 90089 USA

True Time Delays using Conversion / Dispersion with Flat Magnitude Response for Wideband Analog RF Signals. Ömer Faruk Yilmaz 1 , Lior Yaron 2 , Salman Khaleghi 1 , M. Reza Chitgarha 1 , Moshe Tur 2 , Alan Willner 1 Presented by Hao Huang 1.

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1 University of Southern California, Los Angeles, CA 90089 USA

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  1. True Time Delays using Conversion / Dispersion with Flat Magnitude Response for Wideband Analog RF Signals Ömer Faruk Yilmaz1, Lior Yaron2, Salman Khaleghi1, M. Reza Chitgarha1, Moshe Tur2, Alan Willner1 Presented by Hao Huang1 1University of Southern California, Los Angeles, CA 90089 USA 2Tel Aviv University, Ramat Aviv 69978 ISRAEL oyilmaz@usc.edu Thanks to DARPA, Prof. Fejer and Dr. Langrock, Prof. Jalali and Mr. Fard O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  2. Outline • Motivation • Delays based on Conversion/Dispersion • Concept of the Flattened Magnitude Response • Experimental Details • Results • Transfer Function Measurements • Impulse Response to Linear Frequency Modulated Pulses • Summary and Conclusions O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  3. Tunable Optical Delays • Tunable optical delays may be preferred in some future applications • Optical equalization • Optical signal processing • Optical sampling • Contention resolution in optical routers • Packet / bit synchronization • Time slot interchange • Phased array antennas • Coherence tomography • Microwave delays Tunable Delays • Compensate system drift • Temperature fluctuations • Laser drift • Acoustic scattering • Changes in data rate • Add/Subtract FEC • Format Change • Other QoS All Optical Buffer O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  4. Tunable Optical Delays for Analog Applications • True-time delay is preferred over phase shifters for some RF systems: • support wideband signals, • have high dynamic ranges, • avoid beam squint… • (e.g. radar beam steering in phased-array antennas, UWB signals) • Tunable optical delays might be useful for these applications. • linear optical transfer functions, • lightweight, and compact, • immunity to electromagnetic interference … MIM-104 Patriot Radar unit (JASDF Iruma Airbase) http://en.wikipedia.org/wiki/MIM-104_Patriot O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  5. Tunable Optical Delays for Analog Applications • A wide-range of photonic approaches are demonstrated for true-time delays (TTD), these include: • Fiber-optic prism (R. D. Esman, et al., PTL 1993) • Switching between fixed fiber delay lines (W. Ng, et al. PTL 1994) • Chirped fiber Bragg gratings (J. L. Cruz, et al., EL 1997) • Higher-order mode dispersive fibers (O. Raz et al., PTL 2004) • SOA + FBG (S. Sales, et al, PTL 2007) • Photonic crystal fibers (M. Y. Chen, et al., PTL 2008) • Opto-VLSI processors (B. Juswardi, et al., OE 2009) • Photonic crysral waveguides (S. Combrié, et al., SPIE 2010) • SBS based (J. Sancho, et al., PTL 2010) • In this work: • We investigate the performance of conversion/dispersion delays for wideband RF applications: • Effects of wave-mixing, • Dispersion, • System design parameters O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  6. RelativeGroup Delay (ns) Δ t 2 λ Δ Conversion-Dispersion Based Delay Wavelength-Dependent Delay via Dispersion: Δ ≈ D x Δλ λin λ1 λin λin λ1 Dispersion Compensator Dispersive Element W/C2 W/C1 Δτ Δτ Δτ λ2 t λ2 λin λin • High chromatic dispersions are used to achieve >microsecond delays • Dispersion is usually compensated for delays longer than 1 symbol time: 1 O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  7. Concept of TTD using Conv. Dispersion RF in RF out λSIG λOUT λOUT λC O/E E/O Wavelength Conversion (PPLN) Wavelength Conversion (PPLN) Dispersive Element Dispersion Compensation λPUMP(s) λPUMP(s) O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  8. Concept – Wavelength Conversion RF in RF out λSIG λOUT λOUT λC O/E E/O Wavelength Conversion (PPLN) Wavelength Conversion (PPLN) Dispersive Element Dispersion Compensation λPUMP(s) λPUMP(s) Wavelength Conversion via Cascaded SFG:DFG in a PPLN Waveguide Sum Frequency Generation (SFG) Difference Freq. Gen. (DFG) λ λSIG λC λP λQPM λD PPLN: Periodically-Polled Lithium Niobate O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  9. Concept – Wavelength Conversion RF in RF out λSIG λOUT λOUT λC O/E E/O Wavelength Conversion (PPLN) Wavelength Conversion (PPLN) Dispersive Element Dispersion Compensation λPUMP(s) λPUMP(s) Wavelength Conversion via Cascaded SFG:DFG in a PPLN Waveguide L: WG length, Δk: Phase mismatch Λ: Polling period Sum Frequency Generation (SFG) phase matching (SFG) window (~100s of pm) Difference Freq. Gen. (DFG) λ λSIG λC λP λQPM λD O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  10. Sidelobe Suppression Phase Matching Optimized for Optical Carrier (fSIG) λC λSIG reduced side-tones Wavelength Conversion (PPLN) Optical Spectrum Optical Spectrum λPUMP(s) fSIG+ fRF fopt fSIG- fRF fSIG fSIG+ fRF fopt fSIG- fRF fSIG Higher attenuation at higher frequencies  Non-flat magnitude response O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  11. Concept of Flattening the Magnitude Response Phase Matching Optimized for Optical Carrier (fSIG) λC λSIG reduced side-tones Wavelength Conversion (PPLN) Optical Spectrum Optical Spectrum λPUMP(s) fSIG+ fRF fopt fSIG- fRF fSIG fSIG+ fRF fopt fSIG- fRF fSIG Reduced side-tones at higher frequencies  Non-flat magnitude response Phase Matching with a SFG Pump Frequency Offset Δfoffset λC λSIG improved sidetone power Wavelength Conversion (PPLN) Optical Spectrum Optical Spectrum λPUMP(s) fSIG+ fRF fopt fSIG- fRF fSIG fSIG+ fRF fopt fSIG- fRF fSIG Increased side-tone power  Can flatten the magnitude response

  12. Experimental Setup - Spectra After PPLN-1 After PPLN-2 2 1 SIG OUT P P D D C C 1551.8 1564.3 1551.8 1539.3 1539.3 1564.3 Wavelength (nm) Wavelength (nm)

  13. Magnitude Response Double W/Cs result in a ~13 dB RF power drop over 40 GHz. By using a ~0.27 nm pump offset, it is < 1 dB fluctuations over 40 GHz. The pump offset causes a ~ 2 dB reduction RF power (@ 1 GHz) when compared to optimized W/C. O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  14. Single W/C Phase Response • Wavelength conversion phase response does not show a significant change in the phase response. The phase response with a wavelength offset deviates from the “carrier optimal wavelength conversion” by ~| 0.1o | over 9 GHz.

  15. Delay Results • The converted signal laser is tuned in the range 1543 – 1559 nm by tuning the DFG laser in the PPLN-1. • A delay of ~6.5 ns is shown for the ~16 nm bandwidth.

  16. Actual System Transfer Function: HTTD (f) • Actual transfer function of the system is measured in 1.25 kHz steps (12 hours) • Actual transfer function of the system is measured without the SMF (5 mins) • We believe the drift in the system parameters (temperature, polarization, bias, etc.) caused the significantly larger deviations. O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  17. Calculated System Impulse Response to LFM Pulses Matched Filter HTTD(f) Linear Frequency Modulated Pulses T = 10 µs, B = 1 GHz, fc = 10.5 GHz Impulse Response 1 GHz BW, 10-11 GHz Peak-to-Side Lobe-Ratio (PSLR) • > 30 dB PSLR is maintained with the TTD system.

  18. Summary • Transfer function of a True-Time Delay system using conversion/dispersion based delays is measured. • The 3 dB bandwidth of the TTD system is ~18 GHz when carrier phase matching is maintained, and can be increased to > 40 GHz using offset pumps in the wavelength conversion. • Phase response of the TTD system is not significantly distorted by the pump-offset. • Calculated impulse response to LFM pulses show good performance maintaining > 30 dB PSLR. O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

  19. Thank You! oyilmaz@usc.edu O. F. Yilmaz et al., ECOC 2011, Mo.1.A.6

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