Putting opticalfiber frequency links to work Jeroen Koelemeij

Putting optical-fiber frequency links to work Jeroen Koelemeij Laser. La. B VU University j. c. j. koelemeij@vu. nl Partners Funding

Atomic clocks & network synchronization • Atomic clocks: – – Define International Atomic Time (TAI) and UTC Stratum 1 frequency reference in telecom networks Global Positioning System (GPS) Scientific clocks • Important: network synchronization – Telecom, power grid, electronic financial transactions, …

Outline 1. Atomic clock – Principle of operation – State of the art atomic clocks 2. (Optical) time and frequency transfer – Clock comparisons through optical networks – Applications Optical network synchronization Geodesy Positioning

Atomic clock: principle of operation • Traditional (cesium) atomic clocks: based on microwaves • Accuracy 15 -16 digits (commercially 10 -14 digits) • NEW: ‘optical’ clocks based on lasers Oscillator Servo loop 1. Stable oscillator (microwave source) 2. Atoms (fixed oscillation frequency) Counter 3. Counter (convert oscillator frequency to useful quantity)

Optical clock: principle of operation Atoms: single-ion trap (NIST) Oscillator: ultrastable laser (<1 Hz linewidth) Servo loop Oscillator Image courtesy W. Oskay (NIST) ~10 mm *NIST Al+ clock: 17+ digits accuracy C. W. Chou, D. B. Hume, J. C. J. Koelemeij, D. J. Wineland, T. Rosenband, PRL 104, 070802 (2010) intensity Counter: Counterfrequency comb laser frep d clock laser f 0 frequency CCD camera image

How accurate are 17 digits accuracy? • Relativistic effects at ‘human scale’ are ~10 -17 = Time dilation running @ 2 m/s = Gravitational redshift over 10 cm height difference • Effects visible when moving/lifting two Al+ clocks!* Optical clocks = ‘Einstein sensors’ 2 m/s 10 cm * C. W. Chou, D. B. Hume, T. Rosenband, D. J. Wineland, Science 329, 1630 (2010)

Clock comparisons: scientific value • Atomic clock frequency depends on value of fundamental constants • Example: fine structure constant (electromagnetic coupling strength) • Atomic clocks based on different elements depend differently on a • If value of a is changing over time(!), clocks based on different elements drift apart over time

Current best test of variation a • Comparison of Al+ and Hg+ optical clocks at NIST Boulder, CO (USA) – – 27 Al+ : 199 Hg+ : little dependence on a large dependence on a Groningen (223 Ra+) • Constraint on variation a : (-1. 6 ± 2. 3) × 10 -17/year T. Rosenband et al. , Science 319 1808 (2008) • Europe: number of optical clocks (under development), but located in laboratories >>100 km apart • Example: Netherlands Amsterdam (27 Al+) 0 15 km

Comparing clocks at >100 km distance ‘Long-haul time and frequency transfer’: • GPS (traditionally state of the art) • Works well for traditional atomic clocks, but orders of magnitude insufficient for optical clocks… ? Development of atomic clock accuracy over time Digits of accuracy ‘traditional’ atomic clocks ‘optical’ atomic clocks GPS accuracy Year

Comparing clocks at >100 km distance ‘Long-haul time and frequency transfer’: Digits of accuracy • GPS (traditionally state of the art) • Works well for traditional atomic clocks, but orders of magnitude insufficient for optical clocks… ? Development of atomic clock accuracy over time Limitations GPS accuracy: • Based traditional atomic clocks ‘traditional’ atomic clocks ‘optical’ atomiconclocks • Atmospheric disturbances • Relatively weak signals • Fixed signal format GPS accuracy Year

Optical clock through optical fiber? • ‘Ticking’ signal of optical clock is a laser • Use optical fiber for T&F transfer?

Telecom-wavelength optical clock Frequency comb laser spectrum intensity 1 GHz Telecom wavelengths (1. 5 mm) Clock laser 1 121 015 123… frequency • This example: fclock laser = 1 121 015 × (1 GHz) • ‘Copy’ optical clock laser to frequency comb (PLL) all comb teeth become replicas of the clock laser!

Clock comparisons through fiber Location B Clock laser B Telecom wavelengths (1. 5 mm) B Location A frequency Telecom wavelengths (1. 5 mm) Clock laser A A frequency But what about optical path length fluctuations in the fiber? (acoustic, thermal)

Test: SURFnet optical fiber link VU - KVI Frequency comb • • Link part of SURFnet DWDM network Length 317 km, round trip 635 km Single l-channel (1559. 79 nm) Fiber carrying live data traffic

Optical link frequency stability test 1 KVI Groningen SURFnet optical fiber link Clock laser B Telecom wavelengths (1. 5 mm) B VU Amsterdam frequency Telecom wavelengths (1. 5 mm) Clock laser GPS comparison A frequency GPS comparison

Optical link frequency stability test 1 Result: • Identical laser frequencies measured in Amsterdam and Groningen (to within GPS accuracy, 12 digits) Stability: Allan deviation vs. averaging time t Combined inaccuracy GPS links

Optical link frequency stability test 2 • Bypass GPS altogether: compare optical frequency before and after 635 km round trip • Signal: optical beat note send and return light

Optical link frequency stability test 2 • Resulting frequency stability (Allan deviation) 6× 10 -13 @ 1 s 1× 10 -14 @ 1 200 s (20 min) 2× 10 -15 @ 20 000 s (5. 5 hr) Performance better than state-of-the-art GPS! Stability (ADEV) Limiting factor: fiber optical path length fluctuations (likely) Averaging time

Ultrastable frequency links • Standard network fiber links provide 14 -15 digits accuracy in frequency transfer • BUT optical clocks require 17+ digits! • Solution: active fiber length compensation

Optical path length stabilization Compensation of frequency fluctuations due to length fluctuations*: *L. -S. Ma, P. Jungner, J. Ye, J. L. Hall, Opt. Lett. 19, 1777(1994) 1. 5 mm clock laser roundtrip contains 2× noise! Partial reflector Clock laser + noise power Noise detection PLL +compensation Optical fiber (~ 100 km) Laser frequency in Laser frequency out

Ultrastable fiber frequency transfer • Does this work in, say, urban dark telecom fiber? • YES - tests on links 50 – 900 km in Germany, France, USA, UK *: • w/o compensation: • compensated link: 14 -15 digits accuracy 19 digits accuracy! • Does this work in, say, live telecom networks? • YES - France, Paris metro area** • w/o compensation: 13 -14 digits accuracy • compensated link: 19 digits accuracy! • Daisy-chaining of ~100 km segments demonstrated GPS accuracy: 12 • Can we get rf/microwave instead of optical carrier too? -15 digits • YES – Use AM, 14 digits w/o compensation, 18 digits with* * 2008 -2011 PTB, NIST/JILA, NPL ** 2010 Observatoire de Paris/U. Paris Nord

Optical T&F transfer activities Europe EMRP Joint Research Program NEAT-FT: “Accurate time/frequency comparison and dissemination through optical telecommunication networks” Coordinator: Harald Schnatz, PTB Braunschweig (D) General issue: Getting access to (dark) fiber is difficult and/or expensive

Applications • Fundamental scientific research • ‘Relativistic geodesy’

Clocks & fibers for height measurement • • Compare clocks through fiber to sense height differences Clocks ‘measure’ geoid (sea level) GPS position w. r. t. to ellipsoid Complementary method to GPS geodesy – Leveling & geodesy: geoid/sea level is relevant – frame transformation ellipsoid geoid: ~2 -3 cm inaccuracy – This decade: clocks < 1 cm accuracy Model of the Earth

Digging for (black) gold • Gravimeters (accelerometers): – Measure g ~1/R 2 (R = distance to center of the Earth) – Sensitive to density variations in Earth crust (oil, mineral reserves) AND to height variations R + h • Fiber-coupled clocks: – Measure gravitational potential U ~1/R – Barely sensitive to density or g variations, measure primarily R + h GPS data: inaccuracy 2 -3 cm • Combination: improve gravimeter sensitivity to hidden oil and mineral reserves

Applications • • Fundamental scientific research ‘Relativistic geodesy’ Surveying and oil/mineral exploration With accurate time and frequency available everywhere in the network you can do whatever GPS can do – and better!

A vision of ‘Super. GPS’ through optical networks Time & Frequency transfer through optical networks: • • Digits of accuracy • Almost all building blocks available Essential timing backup for vital systems in case of GPS outages (mobile telecom network, power grid, …) Next-generation network synchronization… Development of atomic clock accuracy over time ‘Super. GPS’ accuracy GPS accuracy Year

A vision of ‘Super. GPS’ through optical networks • Enhanced Terrestrial Positioning Systems (ETPS) Very high-accuracy positioning, indoors and on busy highways • Verify timing in ‘superluminal neutrino’ experiment CERN-LNGS • ‘Einstein sensors’ for relativistic geodesy (monitor sea level, dike conditions at cm level) • ‘Innovation beyond imagination’

Thanks! … and special thanks to: Roeland Nuijts, Bram Peeters (SURFnet) Tjeerd Pinkert, Kjeld Eikema, Wim Ubachs (VU) Oliver Böll, Lorenz Willmann, Elwin Dijck, Klaus Jungmann (KVI)