Time and frequency transfer over Optical fiber links

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Two way Satellite Time and Frequency transfer (TWSTFT)

One of the most precise and accurate techniques for comparison of frequency standards located at remote sites is the Two way Satellite Time and Frequency transfer [16, 17]. This is an implementation of the principle described above with signal transmission as follows [11]:
1. The metrology signal is carried at high frequencies in free space. The method utilizes a modem to generate a pseudo random code synchronized to the local clock and this code is used to modulate a microwave signal of about 14 GHz. This modulated signal is transmitted to a geostationary telecommunication satellite from the Earth station and is referred to as the uplink.
2. The satellite re-transmits this modulation on a downlink at a frequency of 12 GHz which is received at the remote site. The signal is demodulated by a modem and the received pseudo random code is cross correlated with the locally generated pseudo random code.
The scheme for TWSTFT is illustrated in Fig. 1.7 [18]. At Clock A station, a time interval counter measures the time difference (TA) between the Clock A (CA) signal and the received signal from Clock B (delayed by the propagation and instrumental delays). Similarly, at the Clock B station, a time interval counter measures the time difference (TB) between the Clock B (CB) signal and the received signal from Clock A. The time difference between the clock stations ( DT ) is given by the following equation [18]: TA = CA 􀀀 (CB + dTB + dBS + dSBA + dSA + dRA + 2wA/c2).

Time and frequency transfer over Optical fiber links

Over the last fifteen years, optical fiber links have been intensively studied for frequency dissemination. They have demonstrated frequency transfer with uncertainties below 10􀀀19 over several hundred km [19, 20]. It allows comparison of optical signals with accuracy and stability down to 10􀀀20 level, comparison of remote clocks with unprecedented uncertainty [21, 22] opening doors to new stringent tests of fundamental physics and relativity [23] and chronometric geodesy [24]. In the RF and time domain, the link from AOS to GUM in Poland is running almost continuously from 2013 with state of the art time uncertainty [25]. Applications to VLBI were explored in the RF and optical domain [26, 27, 28, 29, 30].
Time transfer was by comparison less intensively studied. A seminal experiment  was carried out by imprinting a modulation on the optical carrier as in [31, 32] and an absolute time accuracy of 250 ps and long-term timing stability of 20 ps was achieved for a 540 km public telecommunication optical fiber network [31]. Another significant time transfer experiment was carried out by transferring a femto-second laser over a 159 km installed fiber network [33]. They achieved a time deviation of 300 fs at 5 s and an accuracy at the 100 ps level.
One of the most mature technique is the active stabilization of the propagation delay for joint time and frequency transfer, also referred as the ELSTAB technique [34, 35]. They extend and test this system up to 600 km long fibers installed in an urban Polish telecommunication network [36]. They demonstrate a time stability below 1 ps for averaging time up to 103 seconds which increases to about 3 ps for longer averaging times and a time calibration with accuracy well below 50 ps [36].

Network Time Protocol (NTP)

NTP was developed by David Mills [37] and is the most widely utilized Internet protocol. It uses only the highest OSI layer [38] of telecommunication, which is the software layer. NTP is a client-server service. There is no modification at all of the telecommunication backbone and it only requires software installation at the end user. Any computer with commonly used operating system can run NTP daemon and synchronize its clock to the time reference. It works on a wide area network (WAN). The use of NTP is for free. For these reasons, it is extremely popular. But this service may not be traceable and accurate, as the propagation delay is not constant and is not precisely known, and depends on the data traffic. NTP is a two way technique based on packet exchange process between the Client (local time) and the Server (reference time). Fig. 1.8 displays the NTP packet exchange process. The Client initiates the message exchange process by sending a packet including the value of time t1, which is the time at which the message was sent. This message is received by the Server at a time t2 in its timescale. Then this process is reversed, the Server sends a packet timestamped with t3 along with the t1, t2 values, which is received at time t4 by the Client. The Client thus has the knowledge of the timestamps needed to evaluate the time offset between the Client and Server timescales.

SONET and SDH

Seminal work was done by Jefferts et al. in the early 90’s on time and frequency over Synchronous Ethernet, using SONET/SDH frames [40]. Their SONET two way time transfer system demonstrated stabilities less than 10 ps over short distances (km). Relatively few experiments were carried out over the following decade as Global Navigation Satellite System (GNSS) solutions fulfilled most needs and covered wide area (the free space advantage). In 2010, significant work was done on two way optical passive time transfer based on packet over SONET/SDH utilizing the Swedish telecommunication network. This work performed long distance time transfer over a 560 km fiber link with precision (relative to the GPS link) less than 1 ns for over several months of measurement[41].

White Rabbit PTP (WR-PTP)

White Rabbit (WR) is an extension of PTP with Synchronous Ethernet and digital phase measurements to achieve much higher performance [42]. White Rabbit was developed by CERN and other scientific laboratories as a successor of their dedicated timing system for CERN’s accelerators. It integrates synchronization with the scalability and flexibility of regular Ethernet networks. The initial White Rabbit experiment by the CERN team was a short range synchronization between a WR Master and three WR Slaves in a daisy chain configuration [42]. Each Slave was synchronized by a 5 km fiber spool to the previous stage. The accuracy and precision of the synchronization between the Master and each of the Slave is displayed in Fig. 1.9. The histogram depicts the master to slave offset below 1 ns for each of the Slaves. The accuracy is within 200 ps and standard deviation for each of the Slaves is in the order of 6 ps.

Table of contents :

1 Introduction 
1.1 Time and frequency metrology
1.1.1 Accuracy
1.1.2 Stability
1.2 The Oscillator signal
1.3 Noise analysis
1.3.1 Frequency domain
1.3.2 Time domain
1.3.2.1 Allan Variance
1.3.2.2 Overlapping Allan Variance
1.3.2.3 Modified Allan Variance
1.3.2.4 Time Variance
1.4 Clocks and timescales
1.5 Introduction to time transfer methods
1.5.1 One way time transfer method
1.5.1.1 GNSS time dissemination
1.5.1.2 GNSS time transfer methods
1.5.2 Two way time transfer method
1.5.3 Two way Satellite Time and Frequency transfer (TWSTFT)
1.5.4 Time and frequency transfer over Optical fiber links
1.5.5 Time over Internet
1.5.5.1 Network Time Protocol (NTP)
1.5.5.2 Precision Time Protocol
1.5.5.3 SONET and SDH
1.5.5.4 White Rabbit PTP (WR-PTP)
1.5.6 Performance comparison of some time transfer methods
1.6 Implemented and Potential applications of White Rabbit
1.6.1 For Scientific experiments
1.6.2 Calibration in the RF domain
1.6.3 Smart power grids
1.6.4 5G mobile networks
1.6.5 Financial transactions and timestamping
1.7 Outline of the thesis
2 Introduction to White Rabbit Precision Time Protocol (WR-PTP) 
2.1 Introduction to WR project
2.2 Introduction to WR-PTP
2.2.1 Precision time Protocol (PTP)
2.2.2 Synchronous Ethernet
2.2.3 Digital Dual Mixer time difference (DDMTD) phase detector
2.3 A typical White Rabbit Network
2.4 Synchronization in White Rabbit
2.4.1 Syntonization
2.4.2 Link Delay measurement
2.4.3 Link asymmetry evaluation
2.4.4 Clock offset evaluation
2.5 Unification of White Rabbit into PTP
2.6 Components of a White Rabbit Network
2.7 White Rabbit equipment
2.7.1 The White Rabbit Switch
2.7.2 The White Rabbit Nodes
2.8 Optical emitters
2.9 The transmission medium – Optical fibers
2.10 White Rabbit clocking Scheme
2.11 The potential performance limitations
2.12 Outlook
3 Improving the White Rabbit Switch performance 
3.1 Introduction
3.2 The White Rabbit Switch in Grandmaster mode
3.2.1 Experimental setup
3.2.2 Phase noise power spectral density
3.3 Improving the Grandmaster WRS performance
3.3.1 Phase noise Power Spectral Density and Allan Deviation
3.3.2 Time Stability performance
3.4 Performance of a Slave White Rabbit Switch
3.4.1 Testing Optical link configurations
3.4.2 Experimental setup
3.4.3 Effect of Chromatic dispersion
3.4.4 Results
3.5 Improving the performance of a Slave WRS
3.5.1 Phase Locked Loops (PLL)
3.5.2 Experimental Setup
3.5.3 Phase locked loop Bandwidth variation
3.5.4 A mid range White Rabbit link
3.5.5 Phase locked loop bandwidth optimization
3.5.6 Frequency and Time stability performance
3.6 The Local Oscillator performance
3.7 Increasing the PTP message exchange rate
3.8 Summary
4 Towards long range time and frequency dissemination using White Rabbit 
4.1 Introduction
4.2 A realistic Telecommunication network span
4.2.1 Experimental Setup
4.2.2 Phase locked loop Bandwidth optimization
4.2.3 Experimental results
4.2.4 Fiber thermal noise
4.2.5 Limitations for the time stability performance
4.3 Effect of Chromatic Dispersion
4.4 Tackling Chromatic Dispersion
4.5 Cascaded White Rabbit links using DWDM technique
4.5.1 Experimental Setup
4.5.2 Phase locked loop Bandwidth optimization for cascaded stages
4.5.3 Frequency and time stability performance
4.5.4 Extension to a Cascaded 400 km White Rabbit link
4.5.4.1 Phase locked loop bandwidth optimization
4.5.4.2 Frequency stability performance
4.5.4.3 Time stability performance
4.6 A long haul telecommunication span White Rabbit link
4.6.1 Experimental setup
4.6.2 Frequency stability performance
4.6.3 Time stability performance
4.6.4 Effect of reduced PTP rate
4.6.5 Effect of reduced Bandwidth of locking
4.7 A multi user 4×125 km White Rabbit link
4.7.1 Experimental Setup
4.7.2 Frequency and time stability performance
4.8 Conclusion
5 Deployments 
5.1 Introduction
5.2 A short range in-campus dissemination network
5.3 A mid range suburban WR link using dark fiber network
6 Time Accuracy 
6.1 Introduction
6.2 Calibration of fiber spools using White Rabbit
6.2.1 Calibration: A sensitive task
6.2.2 Calibration by forcing asymmetry in a uni-directional link
6.3 Wavelength swapping technique
6.3.1 Implementation and Results
6.4 Fiber swapping technique
6.4.1 Implementation and Results
6.5 Dual wavelength technique
6.5.1 Implementation and Results
6.6 Summary
7 Conclusion 
7.1 Summary
7.2 Perspectives
Bibliography

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