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Abstract
The photocathode drive laser system in the Institute of High Energy Physics (IHEP) has been upgraded. An all-fiber drive laser system has been developed using photonic crystal fibers and photonic crystal rods as the main gain medium. This system has been operated stably. The output infrared (IR) power reaches 116.2 W. The pulse width and maximum output power of the green laser generated by the second harmonic generation (SHG) are less than 2 ps and about 39.4 W, respectively. The SHG efficiency exceeds 60%. This paper introduces the development of the drive laser system and reports the measurement results of the performance test.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photocathode has been widely used in free-electron lasers (FELs) and energy recovery accelerators (ERLs). Controlled by the drive laser, it could produce electron beams with ultra-low emittance and varying bunch lengths. Due to these facts, most FELs, operating and under construction worldwide use photocathode electron guns as the electron source [1]. High average current and high average power electron beams are the advanced research forefronts of photocathode electron guns. Cornell University firstly proposed a photocathode electron gun with 100 mA of current as its goal and has achieved 75 mA of electron beam by far based on its ERL. This is currently the highest indicator in this field [2]. The drive laser system, a green laser, of this photocathode gun has been upgraded to improve the quality of the electron beam. The maximum average power of the green laser has reached 167 W [3]. High Energy Accelerator Research Organization (KEK) also proposed a high average current electron gun for its ERL, which has achieved a 10 mA of electron beam [4]. The development of a high average power drive laser system plays a key role in achieving a 100 mA of average current. Replacing microsecond-pulse trains with high repetition rate continuous pulses could significantly increase the average current of the electron gun. Such a solution requires a high repetition rate and high average power laser.
The pattern and quality of the laser pulses determine the distribution of the electron beam produced by the photocathode electron source [5]. Theory and experiments have proved that laser shaping, such as flat-top, 3D ellipsoidal laser beams [1], plays a significant role in reducing the electron beam emittance. Laser shaping techniques have been extensively investigated by major accelerator laboratories. Coherent shaping developed at the Peking University could provide various shaped laser pulses, such as almost flat top, triangle and others [6]. Researches of 3D ellipsoidal shaping have been carried out by Deutsches Elektronen-Synchrotron (DESY) on Photo Injector Test Facility (PITZ) [7]. However, due to the reflection at the end face of the shaping crystal and the absorption in the crystals, usually only 10% of the laser energy is left after the shaping process [8]. This power loss is caused by the connatural attribute of optical elements and is hard to overcome for a drive laser system. Considering the power loss during the shaping, one effective and direct way to gain high laser power on the cathode is to increase the total laser power.
The photocathode electron gun developed at the institute of high energy physics (IHEP) of China has realized an electron beam with an average current of 5 mA [9]. The drive laser system of the gun has two operating modes, with repetition rates of 100 MHz and 1.3 GHz [10], respectively. In recent years, all-fiber laser systems are trended to be used in photocathode drive laser systems to gain high average power in the world [11]. This paper reports a photocathode drive laser system with high average power developed at the IHEP. Ytterbium-doped photonic crystal rod fiber is applied as the main amplifier. The average infrared power has exceeded 100 W. Higher output power brings sufficient flexibility to subsequent pulse compression and shaping.
2. Overview of the IHEP drive laser system
The layout of the drive laser system is shown in Fig. 1. The oscillator is a femtosecond mode-locked laser from MenloSystems. Its output repetition rate can reach 81.25 MHz, which can be synchronized with an external RF signal. The center wavelength of the laser is 1030 nm with a spectral width of 28 nm. The oscillator has two out ports, a free space out port and a fiber out port. The laser pulse from the free space port has a 300 fs of pulse width and 60 mW of average power, while 6 ps of pulse width and 30 mW of average power from the other port. The fiber port is selected as the seed laser source in our case to make an all-fiber drive laser system. After an optical isolator and a 2-nm optical spectrum filter centered at 1030 nm, the seed laser pulses are amplified in the first stage of fiber pre-amplifier (FPA). The Pump (model S30-7602-480, LUMENTUM) in FPA-1 operates at a center wavelength of 976 nm with a maximum output power of 480 mW. The pump power is coupled into the gain-fiber (model PM-YSF-HI, NUFERN) via wavelength division multiplexer (WDM). Mode field diameter (MFD) of the gain fiber is 7.5 µm, and core absorption is 250 dB/m at 975 nm. The output of the amplifier is 105 mW when the pump current is 500 mA. Laser pulses then pass through a fiber which is used as a pulse stretcher to increase the pulse width for subsequent high-power amplification. The length of stretch fiber (model PM980-XP, NUFERN) is 250 m. After the pulse is stretched, it enters FPA-2. The parameter of the pump and gain-fiber of FPA-2 are same as FPA-1. At the pump current 760 mA, FPA-2 output is 179.7 mW.
The main amplification modules adopt a 1.5-m Yb-doped photonic crystal fiber and a Yb-doped photonic crystal rod fiber as the main gain medium. The MFD of the photonic crystal fiber (model DC-200-40-PZ-Yb, NKT) is 29 µm. At 976 nm, the pump (model dst11-t192, OSTECH) absorption is 10 dB/m, and the maximum output power is 75 W. The core diameter of the pump is 200 µm, and its numerical aperture (NA) is 0.22. The Rod fiber (model areoGAIN-ROD- module2.1, NKT) has a 3300 µm2 mode field area. The core diameter and effective gain fiber length of the rod are 85 µm and 804 mm, respectively. The pump of rod (model D4F2S22-976, DILAS) has high pump absorption about 30 dB/m at 976 nm with a maximum power of 250 W.
The amplified laser pulses are then sliced and passes through an electro-optic modulator to form a small duty macro-pulse. The repetition rate of the pulses varies from 100 Hz to 10 kHz. The widths of the amplified laser pulses are expanded by the fiber to reduce the peak power. Then, after the amplifiers, a pair of transmission gratings are used to compress the pulses in order to increase the laser beam intensity. As the SHG crystal, a lithium triborate (LBO) crystal is used to generate green laser pulses with a wavelength of 515 nm. Finally, the laser beam is projected on the photocathode.
3. Main amplifier of infrared laser
As described in the previous section, photonic crystal fiber and rod are used in the main amplifier. Their small loss and high damage threshold make them very suitable for high-power laser amplification. The output laser beam quality factor, M2, could be lower than 1.3. Besides, their large NA and extremely high pump absorption rate could lead to a high efficiency for the amplifier. Furthermore, the joint usage of the photonic crystal fiber and photonic crystal rod fiber in the drive laser system could greatly improve the stability of the system and the quality of the output laser beam.
Precision adjustment of coupling on the seed laser and pump of the photonic crystal fiber is required. The main amplification unit (MMU) with high precision adjustment was developed. In the MMU, a two-stage pre-amplificated laser, the seed laser, passes through a collimator and isolator, and then is coupled into the photonic crystal fiber by a focal lens. The focal length of the lens is 25 mm. The 976-nm pump laser is reversely coupled into the photonic crystal fiber by two lenses. The residual unabsorbed pump laser is absorbed by a beam dump after passing through a dichroic mirror. 5-dimensional adjustment fixtures are adopted for the coupling lens and both end surfaces of the photonic crystal fiber to achieve high coupling matching.
The measured output power of the photonic crystal fiber is shown in Fig. 2. The output power starts to increase only when the pump current is above 8 A due to the power absorption of the fiber. Both the output power and pump power increase almost linearly with the pump current. The output laser power reaches 10.42 W at 28 A of pump current. To ensure high stability of the whole laser system, the pump current was set at 20 A. The system was continuously monitored for about 3 hours, and the results are shown in Fig. 3. The output power is 5.35 W with 0.03W of root mean square (RMS) error.
After being expanded by two lenses and passing through the isolator, the output amplified laser is coupled into the rod photonic crystal fiber. In the meantime, the pump laser is reversely coupled into the rod through two lenses with focal lengths of 26 and 20 mm, respectively. The rod is cooled by 25°C circulating water. The measured output power of the rod is shown in Fig. 4. As indicated by the figure, the output power increases linearly with the pump current. The output power is 116.2 W when the pump current is 35 A. The laser output spot pattern is nearly gaussian. The system has been running stably for more than 2 months.
4. Laser pulse compressing and SHG
Non-linear effects and spectrum change could be induced when high-power laser pulses while transmitting through fibers. Therefore, the widths of laser pulses need to be expanded to reduce the peak power during the amplification process. However, low peak power is not efficient for frequency doubling and longitudinal shaping for the laser beam. The expanded positive dispersion laser pulse is then compressed before SHG using a pair of gratings. The pulse widths after the expansion and compression are shown in Fig. 5, which indicates that the full width at half maxima (FWHM) of the expanded and compressed laser pulses are 22 ps and 2.4 ps, respectively. The peak power of the laser increases about an order of magnitude. The frequency of the compressed laser is then doubled by an LBO. The compression and SHG parameters are shown in Fig. 6. The compression efficiency increases slowly from 50% to 60% as the pump current of the rod changes. The theoretical diffraction efficiency of the transmission pulse compression grating (mode FSTG-PCG-1250-1064, Ibsen Photonics) is about 90%. The laser beam passes though the grating 4 times during the whole compression process. Thus, the total theoretical efficiency is about 65.61%. Considering the combined effects of polarization, reflection and absorption of the crystal, the power loss during compression is reasonable.
The SHG efficiency of the laser, with a wavelength of 515 nm, is below 50% at a low rod pump current. As the IR laser power increases, the efficiency increases to about 60%. Even with optimized temperature matching condition, the SHG efficiency at different IR powers still fluctuates slightly. The maximum output power of the green laser is about 39.4 W. At this power, the frequency doubling efficiency exceeds 60%. The FWHM pulse width of the green laser is measured to be less than 2 ps and the beam quality of transverse mode is detected by beam monitor as shown in Fig. 7. The distribution of transverse mode is uniform as a whole, but part of energy dispersion in the upper left corner. The green laser power was monitored for more than 1 hour with about 20 W of output power. The result is shown in Fig. 8, showing a 0.417 W of RMS error of the whole laser system.
5. Conclusions
The drive laser system, with photonic crystal fiber and rod photonic crystal fiber as the main gain medium of the high-power amplifier, has been operated stably. The output power of the IR laser can reach more than 100 W. The pulse width and maximum output power of the green laser generated by the SHG are less than 2 ps and about 39.4 W, respectively. The SHG efficiency exceeds 60%. The high-power green laser beam can be used to increase the average current of the photocathode electron source. It also provides an excellent laser source for shaping experiments. The system will be further optimized to increase the power and stability.
Funding
National Natural Science Foundation of China (11675174, 11475199); State Key Laboratory of Nuclear Physics and Technology, Peking University (NPT2020KFY15).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. F. Stephan and M. Krasilnikov, “High Brightness Photo Injectors for Brilliant Light Sources,” in Synchrotron Light Sources and Free-Electron Lasers: Accelerator Physics, Instrumentation and Science Applications (Springer International Publishing, Cham, 2020), pp. 603–646.
2. A. Bartnik, C. Gulliford, I. Bazarov, L. Cultera, and B. Dunham, “Operational experience with nanocoulomb bunch charges in the Cornell photoinjector,” Phys. Rev. Spec. Top.--Accel. Beams 18(8), 083401 (2015). [CrossRef]
3. Z. Zhao, B. M. Dunham, and F. W. Wise, “Generation of 167 W infrared and 124 W green power from a 1.3-GHz, 1-ps rod fiber amplifier,” Opt. Express 22(21), 25065–25070 (2014). [CrossRef]
4. M. Akemoto, D. Arakawa, S. Asaoka, E. Cenni, M. Egi, K. Enami, et. al., “Construction and commissioning of the compact energy-recovery linac at KEK,” Nucl. Instrum. Methods Phys. Res.- A 877, 197–219 (2018). [CrossRef]
5. F. Zhou, A. Brachmann, P. Emma, S. Gilevich, and Z. Huang, “Impact of the spatial laser distribution on photocathode gun operation,” Phys. Rev. Spec. Top.--Accel. Beams 15(9), 090701 (2012). [CrossRef]
6. F. Liu, S. Huang, S. Si, G. Zhao, K. Liu, and S. Zhang, “Generation of picosecond pulses with variable temporal profiles and linear polarization by coherent pulse stacking in a birefringent crystal shaper,” Opt. Express 27(2), 1467–1478 (2019). [CrossRef]
7. S. Y. Mironov, A. V. Andrianov, E. I. Gacheva, V. V. Zelenogorskii, A. K. Potemkin, E. A. Khazanov, P. Boonpornprasert, M. Gross, J. Good, I. Isaev, D. Kalantaryan, T. Kozak, M. Krasilnikov, H. Qian, X. Li, O. Lishilin, D. Melkumyan, A. Oppelt, Y. Renier, T. Rublack, M. Felber, H. Huck, Y. Chen, and F. Stephan, “Spatio-temporal shaping of photocathode laser pulses for linear electron accelerators,” Phys.-Usp. 60(10), 1039–1050 (2017). [CrossRef]
8. I. Will and G. Klemz, “Generation of flat-top picosecond pulses by coherent pulse stacking in a multicrystal birefringent filter,” Opt. Express 16(19), 14922–14937 (2008). [CrossRef]
9. X. P. Li, J. Q. Wang, J. Q. Xu, S.-L. Pei, O.-Z. Xiao, D.-Y. He, K. Lv, X.-C. Kong, and X.-H. Peng, “Constructions and Preliminary HV Conditioning of a Photocathode Direct-Current Electron Gun at IHEP,” Chin. Phys. Lett. 34(7), 072901 (2017). [CrossRef]
10. L. Xiaoshen, X. Jinqiang, and S. Darui, “Drive laser system for a photocathode at IHEP,” High Power Laser and Particle Beams 30(2), 18–21 (2018).
11. Z. Zhao, A. Bartnik, F. W. Wise, I. V. Bazarov, and B. M. Dunham, “High-power fiber lasers for photocathode electron injectors,” Phys. Rev. Spec. Top.--Accel. Beams 17(5), 053501 (2014). [CrossRef]
