The low-repetition-rate and high-energy laser systems under development for studying inertial fusion energy and high-energy-intensity physics use a multipass amplifier architecture to reduce the costs and physical size of facilities. This technology combines a low-repetition-rate Pockels cell (a device that exploits the so-called Pockels electro-optic effect) and a thin-film polarizer. The result is an optical switch that is key to suppressing self-oscillations and controlling the number of passes that the laser-pulse train (a sequence of laser pulses) makes through the amplifier cavity. However, for low-repetition-rate and high-energy applications, optical absorption—though weak in the case of a Pockels cell—can lead to thermal effects that degrade performance. These include wavefront distortion, thermal stress, and stress-induced depolarization. These undesirable effects must be carefully controlled in a high-performance laser system. In addition, since the area of the laser beam is commonly as large as several centimeters, the switch must also be able to scale to a large aperture.
The plasma-electrode Pockels cell (PEPC), devised at the Lawrence Livermore National Laboratory in 1984, uses thin crystals and can be scaled as suggested. However, the thermal load in the electro-optic crystal cannot be dissipated efficiently because of the low-pressure environment. We propose and demonstrate a reflecting Pockels cell (RPC) that has thermal properties that make it advantageous for low-repetition-rate and high-energy laser systems.
Figure 1 .A photograph of the reflecting Pockels cell (RPC) installed on an adjustable mechanical support (a). A schematic drawing shows the main components of the RPC (b). KD*P: Potassium deuterated phosphate. C0: Voltage dividing capacity. R: Resistance.
Figure 2. Circuit diagram of a single-pulse-driven RPC. CPC: Capacity of the Pockels cell. Z: Impedance of the transmission cable.
The device,1 shown in Figure 1, is a reflective, longitudinally driven, 40×40×5mm3 thin-crystal KD*P (potassium deuterated phosphate) Pockels cell, constructed with a plasma chamber that provides an incident-side electrode and a copper-substrate mirror serving three purposes: mirror, back-side electrode, and heat sink. The heat-sink role is the most important relative to the scientific advancement we claim, as it allows the heat generated by laser absorption in the KD*P crystal to be efficiently removed. This mitigates thermal birefringence effects that would otherwise degrade the device at high average power.
Figure 3. An oscillogram of the chopped wave and the voltage pulse. T: Oscillogram edge trigger.
Figure 4. Depolarization distribution at thermal equilibrium.
To rotate the polarization of a linearly polarized laser beam by 90° using the Pockels effect in the RPC, a 3kV voltage would need to be applied across the KD*P through the copper electrode and the highly conductive transparent plasma formed in the discharge chamber. However, based on our previous experience, the breakdown voltage of helium is equal to or greater than 11kV. We designed a capacity-dividing method to realize a single-pulse-driven RPC.2 Figure 2 shows the circuit diagram for the device described in Figure 1(b). By adjusting C0 (the voltage dividing capacity) and the output voltage of the switch pulse generator, the higher breakdown voltage of the RPC and the lower quarter-wavelength voltage required in potassium dideuterium phosphate (DKDP, a popular optical crystal), can both be satisfied.
With the air pressure 3500Pa at the far end of the inlet and 12.5Pa at the far end of the outlet, neon gas breaks down stably when the output voltage of the power supply is greater than 11kV. The discharge chamber is filled with plasma over the entire clear aperture (i.e., the area traversed by the laser). Figure 3 shows the chopped wave (a pulse from a CW laser) as well as the switch-pulse voltage wave during the gas discharge. The blue curve exhibits the voltage shape typical of a single-pulse-driven RPC. The green curve shows the chopped wave from a 1064nm CW laser.
The ‘cold’ (i.e., in the absence of high energy or high RF laser irradiation) extinction ratio and low-power switching performance are important first steps in verifying the basic functionality of a new device. Measuring both the static extinction ratio and switch efficiency makes it possible to estimate the performance of the RPC. Our results indicate that the static extinction ratio is greater than or equal to 971:1 locally in the clear aperture and that the dynamic switching efficiency is greater than 99.6%.
Figure 5. Wavefront distribution (λ=1064nm). λ: Wavelength.
‘Hot’ (i.e., subjected to high energy or high RF laser irradiation) performance is studied by numerical analysis. Suppose that the incident laser flux is 5J/cm2 with repetition rate 10Hz, wavelength 1064nm, and beam dimension 30×30mm. Figures 4 and 5 show the depolarization and wavefront distribution, respectively, at thermal equilibrium. The maximum depolarization is 0.82%, and the average value in the whole beam area is 0.13%. For wavefront distortion, the PV (peak-to-valley) value is 0.24λ (λ=1064nm, where λ is the wavelength). Modeling predicts acceptable performance of the device at high average power.
In summary, experimental measurements on cold performance and modeling results of hot performance indicate that the idea of a reflective Pockels cell with thermal qualities is well suited to high-average-power applications. However, neither the thermal performance of the device nor the damage threshold has been determined experimentally, and this will be our next step in pursuing the development of this technology.
Jun Zhang, Xiongjun Zhang, Dengsheng Wu, Xiaolin Tian, Jiangang Zheng, Wentao Duan, Feng Jing
Research Center of Laser Fusion
Chinese Academy of Engineering Physics
1. Jun Zhang, Xiongjun Zhang, Dengsheng Wu, Xiaolin Tian, Mingzhong Li, Feng Jing, A reflecting Pockels cell with aperture scalable for high average power multipass amplifier system, Opt. Express
18, pp. A185-A191, 2010. doi:10.1364/OE.18.00A185
2. Xiongjun Zhang, Dengsheng Wu, Jun Zhang, One-pulse driven plasma electrodes Pockels cell with DKDP crystal for repetition-rate application, Opt. Express
17, pp. 17164-17169, 2009. doi:10.1364/OE.17.017164