Phase-locked pulsed laser for "steady-state micro-beam focusing extreme ultraviolet electron laser device"

Recently, Jingwei Laser (a wholly-owned subsidiary of GZTECH ) and the Tsinghua University Accelerator Laboratory collaborated on a research project, publishing their findings in "Optics Express": They successfully developed a phase-locked high-peak-power sub-nanosecond laser system for verification of quasi-steady-state microbunching (Quasi-SSMB).

The related research results, titled "Phase-locked high-peak-power sub-ns laser for electron modulation in quasi-steady-state microbunching" were published in "Optics Express".

I. Research Background

Steady-state microbunching (SSMB) is an innovative laser source concept that can generate high average power, narrow bandwidth coherent radiation, covering the terahertz to extreme ultraviolet bands. One potential application of SSMB laser sources is as a laser source for future EUV lithography machines. The core of SSMB lies in using lasers to modulate the energy of the electron beam in the storage ring ring, causing the electrons to form a micro-bunch structure, thereby emitting high-power, high-repetition-rate, and narrow-bandwidth coherent radiation.

In 2021, Professor Tang Chuanxiang's research group from the Department of Engineering Physics at Tsinghua University, in collaboration with a team from the Helmholtz-Zentrum Berlin for Materials and Energy (HZB) and the German Federal Institute of Physics and Technology (PTB), published a research paper in "Nature" entitled "Experimental demonstration of the mechanism of steady-state microbunching," reporting the first proof-of-principle experiment of a novel particle accelerator laser source, "Steady-state microbunching" (SSMB).

In 2025, the steady-state microbunching experimental laser system (SSMB PoP II laser), jointly developed by the Tsinghua University team and Jingwei Laser, was successfully delivered and has been transported to Germany for subsequent testing.

Figure 1: Schematic diagram of the SSMB system principle and modulation laser source parameter requirements.

II. Laser System Design

The laser system is based on a main oscillator and power amplifier (MOPA) architecture, using a 1064 nm iodine-stabilized continuous wave single-mode laser as the seed source. Based on a self-developed AWG circuit, it generates a single-frequency pulsed laser with an adjustable pulse width of 0.5–3 ns and a repetition frequency of 3.125–25 MHz through high-speed amplitude modulation technology. By controlling the accumulation of nonlinear effects in the optical fiber, the system achieves a peak power of 35 kW (@6.25 MHz/0.5 ns) laser output. Its core innovation lies in transferring the stability of the iodine-stabilized CW laser to the amplified pulsed laser through a phase-locked loop system, ensuring long-term stability of the modulation phase.

Figure 2. Schematic diagram of the laser system.

Figure 3. Schematic diagram of the laser system.

In addition, the system integrates a pulse selector, composed of three cascaded Pockels cells, which can realize arbitrary time structure control of macroscopic pulse sequences and synchronize with the accelerator. The pulse contrast ratio reaches 53 dB, and the polarization extinction ratio is 26 dB. Remote monitoring and interlock protection functions ensure long-term operational stability, with output power fluctuation better than 0.33% (RMS) and beam center drift less than 2% of the beam diameter within 4 hours.

Figure 4. Output power and back-reflected optical power (left), power stability test results (right)

The output beam parameters of the laser system are as follows:

III. Phase Locking Mechanism

Phase stability is crucial for the success of the quasi-SSMB experiment. Electrons rotate once every 160 ns in the storage ring, and the laser pulse must ensure that the optical phase is fixed during each modulation. The research team achieved phase synchronization between the pulsed laser and the CW reference laser through an optical phase-locked loop (OPLL). The OPLL detects phase errors based on the beat frequency signal and compensates for them through an electro-optic modulator.

In its implementation, after the CW seed laser beam is split, one beam serves as the reference beam, while the other, after modulation and amplification, serves as the signal beam. Both beams generate a beat frequency signal in the fiber combiner, which is converted into an electrical signal by a photodiode. This signal is then processed by a frequency-locking circuit to drive the phase modulator for closed-loop control. The OPLL achieves significant phase noise suppression in the 10 Hz to 10 kHz range. This phase-locked loop system can achieve automatic locking upon power-on and relocking after loss of lock.

Figure 5: Schematic diagram of the phase-locked system principle

Figure 6: Comparison of error signals before and after phase-locked system activation

Experiments show that after phase locking, the beat frequency signal power fluctuation is significantly reduced, and the locked state can be maintained for several hours without loss of lock. The relative intensity noise (RIN) is reduced by approximately 20 dB in the 10 Hz-10 kHz range, and the residual root-mean-square phase jitter is only 0.0031 rad (equivalent to a wavelength of 4.97 × 10⁻⁴), meeting the experimental requirement of 0.05 rad.

IV. Availability and Reliability

GZTECH has extensive industrialization experience in laser manufacturing, enabling it to transform cutting-edge research results into stable and reliable engineering products. In this project, GZTECH was responsible for the integration and debugging of the core components of the laser system, including fiber amplifiers, phase modulators, and control systems. Through modular design, the system not only achieves high performance but also possesses excellent availability and reliability. For example, the laser system employs polarization-maintaining fibers and stress-relief installation technology, effectively avoiding polarization degradation during long-term use and ensuring the continuous stability of the output beam quality.

The industrialization advantages are also reflected in the environmentally adaptable design. To address the needs of international transportation and remote deployment, the team conducted general quality characteristics, electromagnetic compatibility design and testing for the laser system, including electrical aging, high-frequency random vibration, rapid temperature changes, and electromagnetic radiation immunity, ensuring the laser can withstand the challenges of long-distance land and sea transport and the complex electromagnetic environment of accelerator laboratories.

The project team integrated remote control and interlock protection functions into the laser system to ensure system safety under extreme conditions such as abnormal power outages and seed line disconnection. Through a customized visualization software platform, operators can adjust laser parameters (such as macropulse width and repetition frequency) in real time and monitor key indicators such as power, external triggering, and phase locking.

Figure 7: High-frequency random vibration test of the laser in progress

We thank the National Institute of Metrology, China for its technical support in seed laser technology, and the Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences for its technical support in phase-locked circuit technology.