Development of the RF Phase Shifter with Femtosecond Time
Delay Resolution for the PAL-XFEL Laser System
D.C. Shin†, G.J. Kim, H.-S. Kang, G. Mun, C.-K. Min‡
Pohang Accelerator Laboratory, Pohang, South Korea
Abstract

RPS Block Diagram

We will introduce the RF Phase Shifter (RPS) used in the Pohang Accelerator
Laboratory X-ray Free-Electron Laser (PAL-XFEL) optical laser system. This equipment isdesigned to finely adjust the laser signal in femtosecond time by manipulating the phase ofthe RF reference signal using a couple of Direct Digital Synthesizer (DDS) devices.
Furthermore, it is designed with low phase noise and low phase drift feature in order tominimize the impact on the system.
Currently these units are installed at the Injection site, Hard X-ray and Soft X-ray
Beamline. They are installed for feedback control of the electron beam at the Injector andfor use in pump-probe experiments at the Beamline. This paper will introduce the design,fabrication, and experimental results of the RPS, as well as its usage status at PAL-XFEL.

The figure below depicts the block diagram of the RPS.
The output phase of the RPS is controlled by adjusting the Phase Offset of the DDS #2.
The phase change of the IF signal is maintained in the RF signal .

Motivation of Development
The time delay feature is an important functionality in the PAL-XFEL laser system.
Typically, controlling the time delay of lasers involves adjusting the physical length of thelaser path using a delay stage, which consists of a motion stage and mirrors. However, thisapproach, which relies on the use of a delay stage, can lead to degradation in laser quality
due to vibrations and shifts in laser focus. Furthermore, adjusting the time delay pulse bypulse is challenging and can result in significant wastage of XFEL or probe laser energy inpump-probe experiments. Therefore, we explored a digitally controlled method with a fast
response speed, leading us to develop the RF Phase Shifter (RPS)."
Figure 1: Block Diagram of the PALXFEL injector laser system.

Figure 2: RPS Block Diagram

Phase vs Time Phase Resolution = 360° / (DDS Phase Offset (2^16)) ≈ 0.00549°
 Time Resolution = T(Period of the 2856MHz REF) / (DDS Phase Offset (2^16)) ≈ 5.34 fs
During the PLL (Phase-Locked Loop) process, changes in the phase of theReference(REF) signal are monitored and tracked in real-time. These changes are reflectedin the output laser signal with a time delay, ensuring synchronization between the output
signal and the REF signal.

In the left image, we planned toreplace the ‘Delay Line’ system withan ‘RF Phase Shifter’.

Fabrication
Figure 3: RPS PhotosSpecification
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The temperature-sensitive RFcircuits are placed within acontrolled temperature environment,
while the digital circuit is exposed toroom temperature. Water cooling isused for temperature control.

Phase Control Resolution : 0.0055°
Control Range : (-2^30 ~ 2^30) * resolution
Linearity(0~360 degrees) : ≤ ±0.05°
Update Rate : ≥ 10 kHz
Stability degrees/day : ≤ 0.1°
Phase Noise : ≤ -150 dBc@100kHz ~ 1MHz

Test Result and Usage of PAL-XFEL

Figure 7: Resolution Test
Figure 4: Long-term stability by Phase Detector

Figure 9: Installation
Figure 8: Linearity Test

Figure 5: Long-term stability by LLRF

Figure 6: Phase Noise-------------------------------† dcshin@postech.ac.kr‡ minck@postech.ac.kr

Figures 4 and 5 depict the results of long-term stability experiments in which phasevariations were measured over one week, remaining within a range of ±0.05 degrees.
Figure 4 displays measurements obtained using I-tech's Phase Detector, while Figure 5
presents measurements acquired using the LLRF device installed at PAL-XFEL.
Figure 6 shows the phase noise measurement results. It confirms that the additive phasenoise specification of the RPS unit remains below -150 dBc from 100 kHz to 1 MHz.

Figure 10: Feedback in Injector

Figure 7 represents the results of the resolution test, displaying measurements obtained using LLRF while incrementing the phasevalue of the RPS by 0.0055 degrees. The 0.0055-degree variations are clearly distinguishable
Figure 8 shows the linearity measurement results, obtained using LLRF while incrementing the phase of the RPS in intervals of 0 to
10 degrees up to 350 degrees. The relative error remains within ±0.05 degrees, and it is noticeable that the slope of the correlationgraph is very close to 1.
Figure 9 depicts the installation of RPS in the beamline and Injector laser room. Figure 10 illustrates how the RPS is currently being
used in the feedback control of the electron beam in the Injector section. In the case of the beamline, users still tend to prefer delaylines. We plan to promote more active utilization of this device in the future.

Pohang Accelerator Laboratory

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