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JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U 2 C 0 3

SUB-20 fs SYNCHRONIZATION BETWEEN MODE-LOCKED LASER
AND RADIO-FREQUENCY SIGNAL *J.G. Wang1, B.W. Wu2,3, B. Liu1,†1
Shanghai Advanced Research Institute, Chinese Academy of Science, Shanghai, China
2Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, China
3
University of Chinese Academy of Sciences, Beijing, China
Abstract
The femtosecond synchronization and distribution system of the Shanghai soft X-ray free-electron laser facility(SXFEL) and Shanghai high repetition rate XFEL and extreme light facility (SHINE) are based on the optical pulse
trains generated by passively mode-locked lasers. The passively mode-locked laser has ultralow noise in the high offset frequency (<5 fs, [1 kHz- 1 MHz]). In this paper, we
report precise synchronization of the low-noise passively
mode-locked laser to the radio frequency (RF) master oscillator. RF-based phase-locked loop scheme, the absolutejitter of the phase-locked passively mode-locked laser isless than 20 fs integrated from 10 Hz up to 1 MHz.

INTRODUCTION
Large-scale femtosecond stability has become an urgent
requirement in modern free-electron lasers (FELs) facilities. At SXFEL [1] and for the SHINE, a pulsed opticalsynchronization has been used. The central component ofthe pulsed optical synchronization is the low-noise modelocked laser which operates at telecommunication wavelength(~1550 nm). The use of low noise mode-locked laser
as the optical master oscillator has become an excellent
way to achieve an effectively synchronized network by distributing the optical master oscillator output to the individual end stations by length stabilized and dispersion compensated fiber links. The fiber links are based on a balanced
optical cross-correlation method [2-4].
The optical synchronized network composed of fiber
links distribute high-purity pulsed reference signals to
three different types of end stations along the complete
FEL facility to achieve femtosecond synchronization.
The bunch arrival time monitor based on the electro-optical modulator (EOM) scheme [5] is used to measure thearrival time of electron bunch with femtosecond accuracy
at some special positions along the accelerator and undulator.
The remote lasers synchronization using two-color balanced optical cross-correlation [6, 7] is used to synchronize
remote lasers, such as the photocathode lasers in the injector, the seed lasers near the undulator, and the pump-probe
lasers in the experiment stations, to the output of fiber links
The laser-RF synchronization using a balanced optical.
microwave phase detector enables timing detection between the output fiber link and radio frequency signal zerocrossing for stable RF signal re-generation which is neededin the SRF cavities and beam-diagnostic devices.
___________________________________________* This work is supported by the National Natural Science Foundation ofChina (12105348 and 12275341)† bo.liu@sari.ac.cn

Being the heart of the optical synchronized network, thelow-noise performance of the optical master oscillator is oftremendous importance. Due to the inherent characteristicof the optical master oscillator, the cavity length is easily
affected by environmental fluctuations. The optical masteroscillator is stabilized by the phase-locked loop against theradio frequency master oscillator to ensure long-term stability.

RF-BASED OPTICAL MASTEROSCILLATOR SYNCHRONIZATION
A common microwave phase-locking technique is used
for synchronization between the optical master oscillator
and RF master oscillator.
Figure 1 describes the RF-based experimental setup ofsynchronization between the optical master oscillator andRF master oscillator. For this test setup a commercial
mode-locked laser from Menhir Photonics, which operates
at 238-MHz repetition rate with 1558-nm center wavelength and 170-fs pulse duration, was used as the opticalmaster oscillator. The RF reference signal at 1428-MHz is
supplied by a 1428-MHz RF master oscillator (Rohde &Schwarz).
In order to achieve phase detection for synchronization
purposes, it is necessary to convert pulsed optical signals
into electrical signals. The photodetector generates electrical signals with high spectral purity harmonics of the laser
repetition rate. A power splitter is used to split the electricalsignal from the photodetector into two parts. A sixth harmonic of the photodetected electronic spectrum is filteredout using an RF bandpass filter and a low noise amplifierin combination with an RF phase detector. Meanwhile, another frequency comb with the same repetition rate as theoptical master oscillator is filtered out using another bandpass filter and a low noise amplifier in combination withan RF phase frequency detector. The fundamental frequency is used for coarse tuning for the setting phase point.
The sixth harmonic frequency is used for fine-tuning for
precise synchronization. The lock on the higher harmonic
improves the performance. The phase error signal between
the RF reference signal from the RF master oscillator andoptical master oscillator signal is fed to the digital controller. Once the phase-locked loop (PLL) was established using the RF-based laser-to-RF synchronization setup, a traditional method of measuring the timing jitter of the opticalmaster oscillator was used. The scheme has been widelyadopted by the current state-of-the-art signal source analyzers (SSAs). The phase noise of the obtained RF signal
from the optical master oscillator is converted to ampliTU2C03

05 Longitudinal Diagnostics and Synchronization

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IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U 2 C 0 3

Figure 1: A block diagram of the RF-based synchronization between optical mater oscillator and RF master oscillator.
Abbreviations: OMO: optical master oscillator; RMO: RF master oscillator; BPF: bandpass filter; PFD: phase frequency
detector; ADC: Analogue to Digital Converter; DAC: Digital to Analogue Converter; SSA: signal source analyzer.
tude change by phase detection with a lower noise localoscillator.

MEASUREMENT RESULTAs mentioned before, in order to obtain the test results,
we have built an experimental setup. Simultaneously, wemeasured the phase noise of the RF master oscillator andthe optical master oscillator using a signal source analyzer
APPH20G from Anapico.
The measurement of the RF master oscillator took placein the femtosecond synchronization laboratory in theSXFEL. The measurement of the optical master oscillator
needs a part of the experimental setup. The pulsed opticalsignals from the optical master oscillator are coupled to thefiber-coupled photodetector, and the converted RF signals
are divided into two by the power splitter. One of them isfurther boosted by the high gain low noise amplifier andtransmitted to a custom bandpass filter.
The phase noise of the RMO (in Fig. 2), the free-running
OMO (in Fig. 3), and the synchronized OMO (in Fig. 4)
were measured using a signal source analyzer, respectively.
In the bandwidth from 10 Hz up to 1 MHz the integrated
RMS jitter for the RF master oscillator is 8.0 fs.

Figure 3: Phase noise measurement result of the free-running optical master oscillator and its integrated timing jitter for the frequency range [10 Hz, 1 MHz].
In the bandwidth from 10 Hz up to 1 MHz the integrated
RMS jitter for the free-running optical master oscillator
is 221.5 fs.

Figure 4: Phase noise measurement result of the synchronized optical master oscillator and its integrated timing
jitter for the frequency range [10 Hz, 1 MHz].
Figure 2: Phase noise measurement result of the RF masteroscillator and its integrated timing jitter for the frequency
range [10 Hz, 1 MHz].

In the bandwidth from 10 Hz up to 1 MHz the integrated
RMS jitter for the synchronized optical master oscillator is
9.3 fs.

TU2C03152

05 Longitudinal Diagnostics and Synchronization

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U 2 C 0 3master oscillator follows that of the RF master oscillator,and the phase noise outside the locking bandwidth follows
the absolute phase noise of the extracted RF signal fromthe optical master oscillator. Due to the noise floor of themeasurement setup, the actual phase noise at high offset
frequency is smaller.

ACKNOWLEDGMENTS
This work is supported by the National Natural Science
Foundation of China (12105348 and 12275341).
Figure 5: Timing jitter spectral density of the RF masteroscillator, the free-running optical master oscillator, andthe phase-locked optical master oscillator.
Figure 5 summarizes the phase noise measurement results. Curves RMO and free-running OMO are the absolutephase noise of the RF master oscillator and the optical master oscillator, respectively. By locking the optical masteroscillator to the RF master oscillator with ~2 kHz PLLbandwidth, we can achieve a better absolute phase noise in
optical pulse trains by combining the phase noise of the RF
master oscillator (inside the locking bandwidth) and theoptical master oscillator (outside of the locking bandwidth).
In Fig. 5, it can be seen that the spectral density of freerunning OMO from 1-kHz to 100-kHz offset frequency ishigher than the RMO. This is limited by the fundamental
noise sources such as thermal noise and shot noise. Actually, the optical master oscillator displays ultralow noise
performance at high offset frequency [8, 9].

OUTLOOK
Due to the low sensitivity of the RF-based synchronization, residual phase noise and long-term stability have notyet been carried out. Furthermore, there are several particular problems related to the RF-based microwave phaselocking technique, which involves the use of a photodetector, low-noise amplifier, and bandpass filter. In addition tothe fundamental noise sources mentioned, the RF-based
technique also has several issues. Typical sensitivity of theRF phase detectors on the order of a few μV/fs. Relative
amplitude fluctuations of optical pulse trains lead to phasenoise (AM-to-PM effects) [10-13].
In order to achieve high sensitivity and long-term stability, a balanced optical microwave phase detector (BOMPD)
scheme based on a 3X3 coupler is being studied and tested.
In the future, the RF-based synchronization will introduce an out-of-loop measurement setup based on theBOMPD to achieve a more comprehensive stability evaluation.

CONCLUSION
In this paper, we have presented the use of an RF-based
phase-locking technique to achieve sub-20 fs synchronization accuracy between mode-locked laser and RF master
oscillator. The RMS timing jitter of the RF-locked opticalpulse train is 9.3 fs when integrating from 10 Hz to 1 MHz
offset frequency. As expected, inside the locking bandwidth (~1 kHz), the absolute phase noise of the optical

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