Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer...

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Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber 1 , J. Kodet 1 , J. Eckl 2 , St. Riepl 2 and Ch. Buerkel 2 1 Technical University of Munich, Geodetic Observatory Wettzell, Germany 2 Federal Agency for Cartography and Geodesy, Geodetic Observatory Wettzell, Germany The Optical Time Transfer between a H-maser on the ground and the atomic clock ensemble in space (ACES) via the ELT interface, requires two concatenated highly stable two way laser links. While the use of satellite laser ranging provides both the range information and the reading (epoch) of the remote clock by a free space laser link relative to the geometrical reference points on the ground and the satellites, another actively stabilized fiber based ground link provides the accurate time reference between the H-maser on the ground and the ranging system. Once the local reference target at the ranging site is also equipped with similar two-way compensated timing reference, it is possible to identify and remove systematic errors to within 1 ps caused by otherwise not detectable system delays. We report on a lossless time and frequency distribution system that delivers this functionality.

Transcript of Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer...

Page 1: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

Accurate Optical Time Transfer between a Clock on Ground and in Space

U. Schreiber1, J. Kodet1, J. Eckl2, St. Riepl2 and Ch. Buerkel2

1Technical University of Munich, Geodetic Observatory Wettzell, Germany2Federal Agency for Cartography and Geodesy, Geodetic Observatory Wettzell, Germany

The Optical Time Transfer between a H-maser on the ground and the atomic clock ensemble in space (ACES) via the ELT interface, requires two concatenated highly stable two way laser links. While the use of satellite laser ranging provides both the rangeinformation and the reading (epoch) of the remote clock by a free space laser link relative to the geometrical reference points on the ground and the satellites, another actively stabilized fiber based ground link provides the accurate time reference between the H-maser on the ground and the ranging system. Once the local reference target at the ranging site is also equipped with similartwo-way compensated timing reference, it is possible to identify and remove systematic errors to within 1 ps caused by otherwisenot detectable system delays. We report on a lossless time and frequency distribution system that delivers this functionality.

Page 2: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

https://imgur.com/RaWqYkV

The value of optical time transfer

• Clocks probe the local physics (gravity and velocity)

• Clock oscillator performance demonstrated to 1 part in 1018

• Frequency transfer over fiber links theoretically stable to 1 part 1019

• Optical time transfer (ground to satellite on T2L2 ≈ 7 ps @ 30s)

• Coherent optical round trip time transfer (2 km free space on ground: 1 fs @ 1000 s)

• 2-way optical time transfer on a compensated fiber link (600 m) achieved stability of 1 ps over a week.

https://imgur.com
Page 3: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

master

laser link

fs pulse laser(local)

long distance fiber link

exploitation or challengeof relativity

local clock

sync. clock

How to make time an observable

• An ensemble of slave clocks are sync’ed to a master (per compensated fiber link)

• Against a local clock this can be used to exploit or challenge relativity

• Local networks provide time and frequency

Page 4: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

master

laser link

fs pulse laser(local)

long distance fiber link

exploitation or challengeof relativity

local clock

sync. clock

SLR

time transfer(ACES)

How to make time an observable

• An ensemble of slave clocks are sync’ed to a master (per compensated fiber link)

• Against a local clock this can be used to exploit or challenge relativity

• Local networks provide time and frequency

• Wide area networks rely on optical time transfer (SLR, other?)

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master

fs-laser

Probing System Performance utilizing Time

Page 6: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

master

fs laser

SLRVLBIGNSS

master

fs laser

Probing System Performance utilizing Time

Page 7: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

Highly accurate clocks exploit frequency to utilize GR for a height system:

Sr

JILA 2015

“clock geodesy” -> Comparison with the PTB traveling Sr Clock and INRIM

Building Block 1

Page 8: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

Accurate Comparison of remote optical oscillators by fiber link

Lossless Fiber Links for frequency transfer -> Clock comparison between NMI’s (PTB - Syrte - NPL)

Building Block 2

Page 9: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

How to compare two remote clocks… Einstein Synchronization!

Annalen der Physik. 17, 1905, S. 891–921

Isotropy of the speed of light!

A B

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D

SLR is the practical Realization of the Einstein Synchronization…

D

… including the unavoidable System- Delays

Game changer: We go from a narrow band transmission channel to an ultra-wide band technology

Page 11: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

Building Block 3

geodetic reference point

(space time)

satellite clock(interpolator)

Accurate distribution of time (Einstein Synchronization)

• Time is associated with a well defined geodetic coordinate

• The satellite is in fast motion and under changing gravity

• In order to preserve time, the orbit must be known

• Internal delays have to be knocked down

Page 12: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

satellite time scale

clock

seconddetected

laser pulse

transmitted

laser pulse

clock

secondreturned

laser pulse

ground time scale

ground station PPS

Connecting a “local clock” to the world - The ACES mission

time

range

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Inter Continental Time Transfer via ACES

Page 14: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

Observation: Variable delays due to electronics

asymmetry of clock channels

in the optical domain we are good!

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Corr.t, f

fs-modelocked

laser

closed loop

delay line

Schreiber et al.: Space Science Reviews, 214 (1), p. 1371, (2017)

Page 16: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

Campus Distribution for accurate Time

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Maser fs-pulses

Correlator

VLBI

SLR

GNSS

StationFiducial

f

pps

f, t, ∆

2-way

Single Point of Reference in Time and Space

Interpolator

lossless distribution

Geodetic Techniques

Page 18: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

Maser fs-pulses

Correlator

VLBI

SLR

GNSS

StationFiducial

f

pps

f, t, ∆

2-way

Single Point of Reference in Time and Space

Interpolator

lossless distribution

Geodetic Techniques

Opt. Ref.Cavities

fiber links

Comp.Clock

Page 19: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

Link Verification

WLRS

T&F lab

Page 20: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

Common Clock for Space Geodetic Techniques

Electrical timing signals Electrical timing signals

“electrical” Timing stability ~1ps

LaserFront-end

LaserBack-end

Ele

ctr

onic

Ele

ctr

onic

Two way optical link stabilization10 - 100 fs

Kim et al. Nature Photon, 2(12), 733–736, (2008)

6.4 fs r.m.s.

optical cross- correlation of2 fiber lines (300 m)

Page 21: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

Common Clock for Space Geodetic Techniques

Electrical timing signals

LaserFront-end

Ele

ctr

onic

LaserBack-end

Ele

ctr

on

ic

LaserBack-end

Ele

ctr

on

ic

WLRS

T&F

TWIN

TW

OT

T

Page 22: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

The distribution of the broadband PPS time signal over cable and electronic devices shows variability at the level of several hundred ps - and next to none over a compensated fiber link

… over a longer period: ∆t ≤ 5 ns

cable based fiber based

?

Page 23: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

Error Signal for the closed loop fiber stretcher

Most of the excursions appear to be caused by the air conditioning

Page 24: Accurate Optical Time Transfer between a Clock on Ground ...€¦ · Accurate Optical Time Transfer between a Clock on Ground and in Space U. Schreiber1, J. Kodet1, J. Eckl2, St.

Error Signal for the closed loop fiber stretcher


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