keywords = {Instrumentation, measurement, and metrology; Metrological instrumentation; Electrical to optical converters; Optical directional couplers; Polarization maintaining fibers; Power spectral density; Traveling wave devices; Variable optical attenuators},
number = {11},
pages = {14650--14660},
publisher = {OSA},
title = {Measurement of optical to electrical and electrical to optical delays with ps-level uncertainty},
volume = {26},
month = {May},
year = {2018},
doi = {10.1364/OE.26.014650},
abstract = {We present a new measurement principle to determine the absolute time delay of a waveform from an optical reference plane to an electrical reference plane and vice versa. We demonstrate a method based on this principle with 2 ps uncertainty. This method can be used to perform accurate time delay determinations of optical transceivers used in fiber-optic time-dissemination equipment. As a result the time scales in optical and electrical domain can be related to each other with the same uncertainty. We expect this method will be a new breakthrough in high-accuracy time transfer and absolute calibration of time-transfer equipment.},
have shown that the performance of a WR switch currently commercially available can be
improved:
improved\textcolor{blue}{as follows}:
\begin{itemize}
\item ADEV clock stability (tau=1s) from 1e-11 to 1e-12,
\item Random jitter from 11 to 1.1~ps RMS\textcolor{red}{(integration bandwidth from} 1Hz to 100kHz\textcolor{red}{)}.
\item ADEV clock stability (tau=1s) \textbf{from 1e-11 to 1e-12},
\item Random jitter \textbf{from 11 to 1.1~ps RMS}\textcolor{red}{(integration bandwidth from} 1Hz to 100kHz\textcolor{red}{)}.
\end{itemize}
This prompted the development of the Low-Jitter Daughterboard
\cite{biblio:WR-LJD}\textcolor{red}{, which} improves the performance of the WR switch to 1e-12 without any
...
...
@@ -1172,7 +1200,7 @@ The improved WR Switches are now commercially available \cite{biblio:WR-LJD-swit
A high performance low-jitter WR node is developed for the SPS's RF transmission
achieving jitter of sub-100fs RMS from 100Hz to 20MHz \cite{biblio:SPS-WR-LLRF}.
A WR node \cite{biblio:SPEV7} to achieve stability of 1e-13 over 100 s is designed
within the WRITE project.
within the WRITE project\textcolor{blue}{\cite{biblio:WRITE-2}}.
\subsection{Temperature Compensation}
\label{sec:}
...
...
@@ -1193,7 +1221,7 @@ peak-to-peak variation from 700~ps to $<$150~ps with a standard deviation $<$50~
% This online compensation
% is used in LHAASO to ensure 500ps (rms)
% synchronization of 7000 WR nodes exposed to harsh environmental conditions
% synchronization of 7000 WR nodes exposed to harsh environmental conditions.
\subsection{Long-haul Link}
\label{sec:LongLinks}
...
...
@@ -1223,9 +1251,9 @@ parameter, is calibrated at room temperature and assumed constant.
However, the variation of fiber temperature results in changes of the actual
\textit{alpha} (e.g -0.12~ps/km/K for 1310/1490~nm)
while the variation of WR nodes/switches temperature result in laser wavelength
variation( e.g. 17~ps/nm km for 1550 nm). These and other
effects analyzed in \cite{biblio:SKA-80km} are significant on long links and
can amount to over 5~ns inaccuracy for bidirectional link using 1490/1550~nm
variation (e.g. 17~ps/nm km for 1550 nm). These and other
effects analyzed in \cite{biblio:SKA-80km} are significant on long links and
can amount to over \textcolor{blue}{3~ns} inaccuracy for \textcolor{blue}{80 km} bidirectional link using 1490/1550~nm
and exposed to 50 degrees Celsius temperature variation. The Square Kilometre Array (SKA) \cite{biblio:SKA}
radio telescope mitigates these effects to achieve $<$1~ns accuracy on 80~km links
by using DWDM SFP on ITU channels C21/C22 (1560.61/1558.98~nm) and combining them
...
...
@@ -1246,7 +1274,8 @@ on a single fiber via a simple DWDM channel filter, as described in \cite{biblio
\subsection{Absolute Calibration}
\label{sec:}
The accuracy of WR depends greatly on the calibration of hardware delays. WR uses
The accuracy of WR depends greatly on the calibration of hardware delays.
\textcolor{blue}{WR has been using}
procedures for relative calibration of these delays \cite{biblio:wrCalibration}.
With relative calibration, \textcolor{red}{sub-nanosecond} accuracy can be achieved provided that the
synchronized WR devices are calibrated against the same "golden calibrator".
...
...
@@ -1255,7 +1284,9 @@ synchronized WR devices are calibrated against the same "golden calibrator".
% cancels out only when WR devices calibrated to the same calibrator are connected.
% Relative calibration is performed for a complete WR device (e.g. a given version of WR switch and SFPs)
% and needs to be repeated each time a composing elements changes.
An ongoing work on absolute calibration \cite{biblio:WR-calibration} allows
\textcolor{blue}{The recently completed work on absolute calibration \cite{biblio:WR-calibration}\cite{biblio:WR-CALIB-ABSOLUTE}\cite{biblio:WR-CALIB-ABSOLUTE-2}} allows
\textcolor{red}{the precise measurement of the} actual value of hardware delays and their different contributors.
With such calibration, a "golden calibrator" will not be required and adding a new type of component
(e.g. SFP) to a WR network will not necessitate a time-consuming calibration of all
