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The Impact of Optical Impairments on Low-Noise Optical Links Curtis R. Menyuk 1 and Olukayode Okusaga 2

The Impact of Optical Impairments on Low-Noise Optical Links Curtis R. Menyuk 1 and Olukayode Okusaga 2 1 Department of Computer Science and Electrical Engineering University of Maryland Baltimore County

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The Impact of Optical Impairments on Low-Noise Optical Links Curtis R. Menyuk 1 and Olukayode Okusaga 2

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  1. The Impact of Optical Impairments on Low-Noise Optical Links Curtis R. Menyuk1 and Olukayode Okusaga2 1Department of Computer Science and Electrical Engineering University of Maryland Baltimore County 2Army Research Laboratory, Adelphi, MDMicrophotonics Branch, Sensors and Electronic Devices Division

  2. The Impact of Optical Impairments on Low-Noise Optical Links Other participants: Weimin Zhou; Army Research Laboratory Eric Adles,*† Andrew Docherty, James Cahill,* and Gary Carter; UMBC Moshe Horowitz, Etgar Levy, and Asaf David; Technion, Israel Sponsors: DARPA (S. Pappert and R. Esman) Coalition Warfare Program Maryland Proof of Concept Alliance Fund *working at ARL; †now at APL

  3. Motivation • The best free-running X-band DROs have Sf < -145dBc/Hzat 1 kHz (Howe and Hati, 2005) • The best CW optical oscillators have Sf > -80dBc/Hz at 1 kHz (inferred from the measured Sf> -20dBc/Hz at 1 kHz with a 1.5 Hz linewidth laser; H. Stoehr, et al., 2006) • Free-running optical oscillators are not competitive with microwave oscillators! • BUT • Optics-based oscillators have become competitive within the last decade!

  4. Motivation Free-running optical oscillators are not competitive with microwave oscillators! BUT Optics-based oscillators have become competitive within the last decade! AND What we have learned about these oscillators has important implications for time transfer over fibers

  5. Optics-Based Oscillators Optoelectronic oscillators Basic idea: Use a long length of optical fiber to make a high-Q microwave cavity Optical Fiber Optical Laser X. S. Yao and L. Maleki, Opt. Lett., vol. 21, pp. 483–485, 1996; X. S. Yao and L. Maleki, J. Opt. Soc. Am. B, vol. 13, pp. 1725–1735,1996 Modulator RF Output RF Coupler RF RF Photodetector Filter Amplifier optical domain electronic domain

  6. Optics-Based Oscillators • Carrier-envelope phase-locked lasers • Basic idea: • Create a frequency comb using a short-pulse laser • Lock the frequency separation ( frep) using a resonant cavity* • Lock the frequency offset ( fceo) using an octave of bandwidth and a frequency-doubling comparison *For time or frequency generation, you would use a microwave reference

  7. Optics-Based Oscillators Carrier-envelope phase-locked lasers • J. J. McFerran, et al.,” Electron. Lett., vol. 41, no. 11, pp. 650–651, 2005.

  8. Optics-Based Oscillators Carrier-envelope phase-locked lasers S. Diddams, JOSA B, vol. 27, no. 11, pp. B51–B62, 2010 A revolutionary new technology witha broad range of applications!!

  9. Optics-Based Oscillators Microresonators • P. Del’Haye, et al., • Phys. Rev. Lett., vol. 101, • 053903, 2008. (< 5 mm)

  10. X-Band Oscillator Comparison • D. A. Howe and A. Hati, • Proc. 2005 IFCS • and PTTI Mtg. CEPL lasers arethe big winnerbelow 1 kHz

  11. X-Band Oscillator Comparison • D. A. Howe and A. Hati, • Proc. 2005 IFCS • and PTTI Mtg. Spurs were themajor issue Length-dependentnoise is now themajor issue

  12. Dual Optoelectronic Oscillator Short Long Optical Fiber Optical Fiber Optical Laser Modulator RF Output Photodetector RF Optical Coupler Laser Modulator RF Amplifier RF RF Master OEO Combiner RF RF Phase- Coupler Filter Shifter RF RF Photodetector Slave OEO Amplifier Filter Strong back-injection is critical optical domain electronic domain W. Zhou and G. Blasche, IEEE MTT, vol. 53, pp. 929–933, 2005

  13. Dual Optoelectronic Oscillator • O. Okusaga, et al., • Optics Express, vol. 19, • no. 7, pp. 5839–5854, 2011. • THEORY: • E. C. Levi, et al., Optics • Express, vol. 18, no. 20, • pp. 21461–21476, 2010. Complete first spur suppression is achieved!

  14. Length-Dependent Noise Length-dependent noise is an important limiting factor - 20 E. Adles, et al., 2010 International Frequency Control Symposium 40 m - 40 500 m 6 km - 60 - 80 Phase Noise (dBc/Hz) - 100 - 120 - 140 - 160 0.01 0.1 1 10 Frequency (kHz)

  15. Length-Dependent Noise Length-dependent noise is an important limiting factor - 40 5.46 km E. Adles, et al., 2010 International Frequency Control Symposium 9 km - 60 - 80 Phase Noise (dBc/Hz) - 100 - 120 - 140 - 160 0.01 0.1 1 10 Frequency (kHz)

  16. Modeling the Fiber Effects Long Optical Fiber Laser Optical Photodetector Modulator fopt fopt fopt– fRF fopt+ fRF DC Sf Sf Sf Sf Sf Sf Sf fRF 2fRF f f f f f f f RF filter fRF Each RF tone must be transported separately through the fiber

  17. Time-Frequency Transfer 251 Km Fiber Link Experimental setup • P. A. Williams, et al., • J. Opt. Soc. Am. B, vol. 25, • no. 8, pp. 1284–1293, 2008

  18. Time-Frequency Transfer • P. A. Williams, et al., • J. Opt. Soc. Am. B, • vol. 25, no. 8, • pp. 1284–1293, 2008 There is a substantial penalty for duplex transport [Attributed by the authors to polarization mode dispersion (PMD)]

  19. Time-Frequency Transfer • Two-way frequeny transport through dark fiber: • O. Lopez, et al., IFCS 2010: Allan deviation of a few 10–16 at 1 s and a few 10–19 at 104 s over 108 km. • G. Grosche, et al., IFCS 2010: Accuracy of 10–19 for a 148 km link. Instability (ADEV) of 2 10–14 /  (s–1) for a 480 km bi-directional link with Brillouin amplifiers. • H. Schnatz, et al., IFCS 2011: 900 km long link between the Physikalisch-TechnischeBudesanstalt (PTB) in Braunschweig and the Max-Planck Institute of Quantum Optics (MPQ) near Munich enabling precise clock comparisons between the two institutes.

  20. Time-Frequency Transfer One-way frequency-transfer experiment • J. L. Hanssen, et al., Proc. IFCS 2011

  21. Time-Frequency Transfer One-way frequency-transfer experiment • J. L. Hanssen, et al.,Proc. IFCS 2011 Effect of length-dependent noise is visible

  22. Length-Dependent Noise • Possible Sources: • Polarization mode dispersion: Suggested by Williams, et al. • Double-scattering processes:— Brillouin, connectorsSuggested by Nelson, et al.; consistent with their observation that modulation reduces noise • Laser frequency noise + chromatic dispersion: There is strong experimental evidence in the OEO experiments of Volyanskiy, et al.

  23. Length-Dependent Noise • Possible Sources: • Guided Entropy Mode Rayleigh Scattering (GEMRS) + RF-Domain Amplitude to Phase Noise Conversion: This effect appears to dominate our own experiments — and is very fundamental! • The key effects are not environmental • …but there is some recent evidence that vibrations may also play a role.

  24. Length-Dependent Noise Modulation reduces noise:* C. Nelson, et al., Proc. 2007 International Frequency Control Symposium *This was part of a larger study Looking at the effect of RIN reduction. For that to work, themodulation was necessary. This results suggests that scattering in the fiber is occurring!

  25. Length-Dependent Noise Frequency to phase noise conversion via chromatic dispersion: K. Volyanskiy, et al., J. Lightwave Technol., vol. 28,no. 18, pp. 2730–2735, 2010 We do not see this behavior…better laser drivers?

  26. Length-Dependent Noise • A scenario that fits our facts: • Guided (transverse) entropy modes are generated by the transverse intensity gradient (electrostrictive force) • These modes generate longitudinal density fluctuations • RIN from the laser is multiplied by a factor ~200by scattering from the density fluctuations • The output laser RIN is converted to RF amplitude noise • RF amplitude noise is converted to phase noise in the photodetector and RF amplifiers

  27. Length-Dependent Noise The evidence for RIN enhancement:

  28. Length-Dependent Noise The Rayleigh gain spectrum in a 6 km Spool: This bandwidthis consistent witha Rayleigh band-width that isgoverned by thefiber radius Gain Spectrum (dB)

  29. Length-Dependent Noise Optical Power Dependence in a 6 km Spool: Gain Spectrum (dB)

  30. Length-Dependent Noise Fiber Length Dependence: Gain Spectrum (dB)

  31. Length-Dependent Noise • The gain has the following properties: • At 10 Hz offset and 6 km of fiber, it increases from 15 dB to 25 dB when the input power increases from –10 dBm to +16 dBm • At 1 kHz offset and 6 km of fiber, it increases from 33 dB at to 40 dB when the input power increases from –10 dBm to +16 dBm • The gain increases with the fiber length roughly linearly • The gain peaks at 50 kHz • A peak at 50 kHz is the signature of a transverseprocess!

  32. Length-Dependent Noise A peak at 50 kHz is the signature of a transverseprocess! GEMRS = Guided entropy mode Rayleigh scattering is analogous to GAWBS = Guided acoustic wave Brillouin scattering QUESTION: Why is is this potentially important for time and frequency transport? ANSWER: The longitudinal density fluctuations will be the same in both directions of one fiber, but different for different fibers • R. M. Shelby, et al., Phys. Rev. B, vol. 31, no. 8, pp. 5244–5252, 1985

  33. Length-Dependent Noise The transmitted gain is similar up to the Brillouin threshold BUT: The gain is ~200-2000 at low frequencies due to interference 6 km fiber spool (transmitted power) O. Okusaga, et al., Proc.2011 International Frequency Control Symposium

  34. Length-Dependent Noise The evidence for amplitude to phase noise conversion in the photodetector:

  35. Length-Dependent Noise The evidence for amplitude to phase noise conversion in the RF amplifiers:

  36. Conclusions • Fiber impairments have a large impact on close-in phase noise • The causes of these impairments are not well-known BUT: they are NOT just environmental AND: they are NOT necessarily the same as in fiber comms • ISSUES TO INVESTIGATE: • A full theory of GEMRS • A careful look at PMD • A careful look at environmental effects (vibration, in particular)

  37. Noise Sources1,2 LEGEND: A = additive,1 M = multiplicative,2 F = fundamental,3 NF = non-fundamental or environmental,4 L = fiber-length dependent,5 NL = non-fiber length dependent

  38. Noise Sources1,2 LEGEND: A = additive,1 M = multiplicative,2 F = fundamental,3 NF = non-fundamental or environmental,4 L = fiber-length dependent,5 NL = non-fiber length dependent

  39. Noise Sources • This table misses the critical effects for • our system • The limiting effects are different from those in communications systems or CW lasers • These effects are not primarily environmental!

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