DEVELOPMENT STATUS OF RF REFERENCE PHASE STABILIZATIONSYSTEM FOR SUPERKEKB INJECTOR LINAC
N. Liu∗ ,1 , T. Miura1,2 , T. Matsumoto1,2 , T. Kobayashi1,2 , F. Qiu1,2 , D. Arakawa2 , H. Katagiri2 ,Y. Yano2 , S. Michizono1,2 , B. T. Du1 , 1 SOKENDAI, Kanagawa, Japan, 2 KEK, Tsukuba, Japan
Abstract
SuperKEKB injector linear accelerator (LINAC) has
600 m beam lines which consist of 8 sectors. The 2856 MHzRF reference signals are distributed to each sector with longphase stabilized optical fiber (PSOF). The RF reference
phase stability requirement is estimated to be 0.2° (RMS)
corresponding to 200 fs. The prototype of RF reference
phase stabilization system with single mode optical circulator was implemented and demonstrated in the laboratory.
The returned phase drift is compensated by a piezo-driven
fiber stretcher. The transmitted phase through 120 m PSOF
is stabilized to 41 fs (pk-pk), which fulfilled the requirement.
This paper introduces the RF reference phase stabilization
system and reports the preliminary feedback result.

INTRODUCTION
SuperKEKB injector linear accelerator (LINAC) has
600 m beam lines which consist of 8 sectors (sector A-C, 15) [1]. The accelerating structures at each sector are operated
at 2856 MHz (S-band). The S-band radio frequency (RF)reference signals are distributed to each sector by phase stabilized optical fiber (PSOF). Different lengths of PSOF (max.
380 m) are located in LINAC gallery where the temperaturefluctuation is controlled within ±1 ∘C and the humidity varies
from 10%RH to 50%RH without control. During longdistance transmission, PSOF suffers the slow phase driftdue to the temperature and humidity changes. The thermal
propagation delay of PSOF is -1.92 ps/km/∘C at 1550 nm [2].
The transmitted phase through different lengths of PSOF
drifts varing from 3° to 5° at sector 2 to 5 [3]. SuperKEKB
injector LIANC has been upgraded from KEKB. The bunchcharge is increased from 1 nC to 4 nC and more tight energy
spread is required, e.g. 0.125% to 0.07% for electron beamat the end of LINAC. Stabilization of RF reference phaseover long distance transmission is very important for stable
RF operation. The RF reference phase stability requirement
is estimated to be 0.2° (RMS) at 2856 MHz, corresponding
to 200 fs, including short-term timing jitter and long-term
phase drift. A RF reference phase stabilization system is
proposed to compensate the long-term phase drift.
The reference phase stabilization system is implemented
by using two single mode optic circulators (SMOC) and apiezo-driven fiber stretcher (PDFS) is adopted to compensate
the phase drift. The same optical wavelength (1550 nm) isused for the forward and backward signals. In this paper, anoverview of the reference phase stabilization system is presented firstly. Then the main optical devices are introduced.
Finally, the long-term performance is evaluated.
∗ liuna@post.kek.jp

SETUP OF RF REFERENCE PHASESTABILIZATION SYSTEM
A prototype of reference phase stabilization system with
SMOC is proposed. The schematic diagram is shown inFig. 1. The 2856 MHz reference signal (REF) is generated
by a signal generator. The electric signal is converted to theoptical signal with 1550 nm wavelength by a optical transmitter (E/O). The optical signal is transmitted with 120 mPSOF, which is situated inside the temperature and humidity controlled chamber (THC) so that we can simulate theenvironment changes near PSOF as that of LINAC gallery.
Finally, the optical signal is converted to the electric signal
by a optical receiver (O/E). This transmitted signal (TRS) is
further used as local oscillator (LO) for low-level RF (LLRF)
system and the reference signal for beam diagnostic system
in SuperKEKB injector LINAC.
Usually, the reference signal is distributed with hundredsof meters PSOF from master oscillator (MO) to the receiver.
Two SMOCs are used to return the signal. The phase drift ofthe returned signal (RTN) is expected to be twice that of TRS.
The RTN phase drift is compensated by the active opticallength control with PDFS so that the TRS phase is stabilized.
The same optical wavelength (1550 nm) is applied for thetransmitted and returned line.
The S-band frequency is down-converted to the intermediate frequency (IF, 14.28 MHz) by mixing with LO(2870.28 MHz). The IF signal is sampled by 16-bit analogto-digital converter (ADC) with the sampling rate (SR)114.24 MHz using oversampling technique (SR = 8*IF). Finally, the RF phase is obtained [3]. The phase error between
RTN and REF is calculated and fed to a PDFS so that theoptical length is adjusted to compensate the phase drift. An
integral (I) controller is applied for precise feedback control.
The TRS phase is monitored to evaluate the system performance. Beside this, the phase drift of E/O, O/E modules isalso measured to study its phase drift contribution. The E/O
and O/E modules are provided by TAMAGAWA Inc. and thespecification is listed in [3, 4]. To reduce the system phasedrift due to the temperature and humidity changes, the monitor system and all the RF/optic devices are situated inside
the temperature stabilized chamber (TC) with ±0.1 ∘C.

Single Mode Optic Circulator
The transmitted signal is sent back by single mode opticcirculator. The SMOC is non-reciprocating, one directional,
three-port devices. It allows light to travel only from port 1
to port 2, port 2 to port 3 with minimal loss. It is providedby Thorlabs Inc. [5] and the specification is listed in Table 1.

Figure 1: Schematic diagram of RF reference phase stabilization system with two optic circulators.
Table 1: Specification of Single Mode Optic Circulator
Specification
Wavelength Range
Insertion LossIsolation
Directivity (port 1 to port 3)
Return Loss
Max. Optical PowerOperating Temperature
Operating Relative Humidity

Value
1525-1610 nm
0.8 dB Typ./1.0 dB Max.
>40 dB>50 dB⩾50 dB
500 mW
0 to 70 ∘C
5% to 60%

Piezo-Driven Fiber Stretcher
To compensate the phase drift of long optical link causedby the temperature and humidity fluctuation, the mechanicalvariable optical delay line (VODL) and PDFS are widelyused for optical length control. The VODL is driven by astepping motor with low resolution and large optical delay
range up to hundreds of milimeters. The PDFS is driven
by a electrical piezo with high resolution and small opticaldelay range limited to several milimeters. In our system, theRF phase at 2856 MHz is very sensitive to the mechanicalstructure. 0.25° phase jump was observed when the stepper motor changed the direction of rotation [2]. For precise
optical length control, PDFS is adopted for the phase driftcompensation. The PDFS is provided by General Photonics
Inc. [6]. The optical delay range is up to 3 mm with maximum (max.) input voltage 4.7 V. The device is controlled
by four independent piezoelectric ceramics (PZTs) so that
we have the option to drive each PZT individually for highresolution or collectively for large stroke. In addition, each
PZT can be controlled either with an analog signal or a 12bit transistor-transistor logic (TTL) signal. In our case, 4Figure 2: Phase change as a function of the input voltage ofPDFS.
cascaded PZTs are driven by a 16-bit digital-to-analog converter (DAC). Figure 2 shows the relationship between theRF phase change and the input voltage of PDFS. The phaserange of PDFS is up to 10.5° at 2856 MHz, which covered
the phase drift of optical link in the gallery (max. 5°). The
sensitivity is 2.23°/V.

PERFORMANCE EVALUATIONTo evaluate the performance of the phase stabilization system, a test feedback system is implemented in the laboratory
as shown in Fig. 1. The temperature inside THC is changed
from 25 ∘C to 40 ∘C and down to 25 ∘C at stable humidity
40%RH. The temperature step is 5 ∘C per 2 hours. Figure 3
shows the phase drift without feedback (DAC=0V). The TRSand RTN phase are changed 5.16° and 10.3°, respectively.
The RTN phase is twice the TRS phase as expected. The
feedback result is shown in Fig. 4. It is clear to see that thephase drift of the long optical link is compensated by PDFS.
The RTN and TRS phase are stabilized within 0.73 fs (RMS)Figure 3: Long-term phase drift without compensation.

Figure 5: Long-term phase drift with compensation for 72hours.
within 0.73 fs (RMS) and the transmitted phase was stabilized to 41 fs (pk-pk) which fulfilled the requirement 200 fs(RMS). We found that the transmitted phase is influenced
by outside humidity because all the optical components are
only temperature stabilized, not humidity controlled. It isconsidered to seal the optical components with more stable
humidity to improve the transmitted phase stability. The
other two different phase stabilization systems will be implemented and tested. Finally, the long-term performanceof these three phase stabilization systems will be compared
to be a candidate scheme for SuperKEKB injector LINAC.

REFERENCES
Figure 4: Long-term phase drift with compensation for 18hours.
and 111 fs (pk-pk). Figure 5 shows the long-term feedback
result for 72 hour. The TRS phase is stabilized to 41 fs (pkpk). In the above measurements, all the system phase drift isrejected from TRS and RTN phase. The RTN phase can becontrolled very well but small phase drift still reminds forTRS. We found that TRS phase has weak correlation withthe temperature and humidity inside TC, moderate correlation with outside humidity, very weak or no correlation withoutside temperature. Thus, the humidity effect on the opticdevices is the main reason for the residual TRS phase drift.
The moisture can enter through the holes or the small gaps
from the outside into the thermostats. It is considered to seal
the optical devices with stable humidity.

CONCLUSION
A phase stabilization system with optic circulator is developed and demonstrated. The returned phase was controlled[1] M. Akemoto et al., “The KEKB injector linac”, Prog. Theor.
Exp. Phys., 03A002, 2013.
[2] N. Liu et al., “Performance Evaluation of the RF Reference
Phase Stabilization System on Fiber-optical Link for KEKe-/e+ Injector LINAC”, in Proc. 29th Linear Accelerator
Conf. (LINAC’18), Beijing, China, Sep. 2018, pp. 230–232.
doi:10.18429/JACoW-LINAC2018-MOPO107[3] N. Liu et al., “Measurement of the RF Reference Phase
Stability in the SuperKEKB Injector LINAC”, in Proc.
9th Int. Particle Accelerator Conf. (IPAC’18), Vancouver,
Canada, Apr.-May 2018, pp. 4815–4818. doi:10.18429/
JACoW-IPAC2018-THPML073[4] N. Liu et al., “The consideration of RF phase reference distribution system for the injector LINAC of SuperKEKB”, inProc. 14th Annual Meeting of Particle Accelerator Society ofJapan (PASJ’2017), Hokkaido, Japan, Aug. 2017, WEP065,
pp. 1015-1018.
[5] Thorlabs Inc., https://www.thorlabs.com/[6] General Photonics Inc., http://www.generalphotonics.
com/