Advances in Test Equipment: Computational imaging streamlines high-power laser system characterization

JIANQIANG ZHU, HUA TAO, XINGCHEN PAN, and CHENG LIU

Although scientists have long known that nuclear fusion is capable of releasing tremendous amounts of energy, achieving controlled fusion on Earth still remains one of the most daunting challenges for human beings in the 21st century. In the process of chasing this magnificent dream, scientists all over the world have expended much time and effort in the study of inertial confinement fusion (ICF)—just one approach to controlled fusion wherein intense laser beams compress and heat a fuel target.

Several pioneering efforts towards achieving controlled fusion with ICF include not only the decades-long research by American scientists at Lawrence Livermore National Laboratory (LLNL; Livermore, CA), but also by Chinese researchers in the ShenGuang (SG) series high-power laser facilities at the National Laboratory of High Power Laser and Physics (NLHPLP) at the Shanghai Institute of Optics and Fine Mechanics (SIOM; Shanghai, China). The NLHPLP facility has specialized in laser fusion research for more than 50 years and recently, the SG-II series laser team is credited with adding several outstanding research achievements.^1,2

In the past decade, SG-II facility research has improved various aspects of ICF, particularly laser beam quality, output stability, and reliability.^3,4 Recently, a computational imaging technique was successfully applied in the SG-II facility to provide complete diagnostics for the entire high-power laser plant using only a single snapshot of diffraction intensity from the system. The technique promises rapid performance evaluation and calibration while replacing most of the legacy near- and far-field measurement methods, and providing even more information on laser beam characteristics than was previously possible.

Diagnostic complexity

The high-power laser plant at SIOM is a large and complex optical system involving several thousands of optical elements-some more than half a meter in diameter (see Fig. 1). The laser system usually has some specially designed optical systems to monitor laser beam quality at different positions along the beam path, quantifying such parameters as the near-field and far-field profile, beam pointing stability, pulse duration, energy, and synchronization of pulses.⁵

The intensity distribution of the light field plays a very important role for the safe operation of the entire facility. Unfortunately, current measurement systems are very complex. For example, near- and far-field measurements reflect the intensity distribution of the laser beam only and not the direct phase across the laser beam.

A more compact and multifunctional monitoring and parameters feedback system is highly desirable. Alternatively, if we obtain the laser beam complex amplitude in terms of its modulus and phase distribution, we can diagnose the laser beam with much more precision. This can also be extended to obtain the cross-sectional intensity distribution of the high-power laser beam at any plane along the light path with known transmission functions of the optical elements. This kind of real-time monitoring based on computational optical imaging is proven to be very accurate on selected planes and can supply the complex amplitude of the laser beam, which is otherwise difficult using traditional devices such as near-field or far-field measurement.

Modulation coherent imaging

The recent emergence of modulation coherent imaging (MCI) techniques make online diagnostics possible through computational optical imaging.^6,7 In MCI, the laser beam to be measured falls on a falls on a random phase plate (RPP) with known structures, and the diffraction pattern intensity is recorded by a charge-coupled device (CCD) camera. Using an MCI algorithm, the complex distribution of the incident laser beam can be calculated through iterative calculations between the CCD plane and the RPP plane.^8-11

To be specific, the light field on the entrance plane is given an initial value at the beginning of the reconstruction. Next, after it is propagated numerically to the recording plane via the RPP plane, the modulus of the calculated light field is replaced by the square root of the recorded intensity and then propagated back to the entrance plane via the RPP plane to get an updated light field. By repeating these forward and backward propagations step by step, the computed light fields will converge to the actual value.

Since the MCI setup is very simple and only a single frame of diffraction pattern intensity is required to compute the complex amplitude of the laser beam, the instrument can be placed at any position—even inside the target chamber—to monitor the quality of the laser beam, even if it is a single pulse.

Because our MCI measurement device is the size of a typical industrial camera, it is small enough for online diagnostic measurements of any high-power laser system via computational imaging. Essentially, a small portion of the main laser beam is sampled out and directed to the RPP through a convex lens. Once the complex amplitude U(x, y) of the light on the RPP plane is computed with the recorded diffraction pattern intensity on the CCD plane, the phase and modulus of light in all other planes present along the optical path can be calculated with the Fresnel formulation. The initial transmission functions of all optical components, such as the spatial filter, amplifier, and final optics, are noted.

In operation

It is worth mentioning that the MCI-based device has already been tested and put into action at the SG-II laser facility, which operates at a wavelength of 1053 nm and has a 350 mm-diameter laser beam. Starting from the initial recorded diffraction pattern intensity, numerical computation can reconstruct the modulus distribution of the beam outside the chamber, the phase distribution, and the focus intensity distribution in the far field (see Fig. 3).

FIGURE 3. Online wavefront measurement results are shown by an MCI device in the SG-II laser facility; (a) is the reconstructed modulus distribution of the laser beam before the convergent lens outside the target chamber, (b) is the corresponding phase distribution, and (c) is the focus intensity distribution in the far field, all obtained through numerical computation starting from the recorded diffraction pattern intensity (d).

To understand and estimate the resolution and the accuracy of the results obtained at the SG-II laser facility, we placed a USAF 1951 target in the laser beam and first computed the modulus of the field (see Fig. 4). The computed image shows a spatial resolution of about 1.5 mm—about 10 times higher than that of a Hartmann sensor. When a standard planar glass plate is also placed in the optical path, the computed phase change of the laser beam has an accuracy of about 0.1 λ—high enough for online monitoring of the SG-II high-power laser facility—compared to the transmission function measured with a Zygo (Middlefield, CT) interferometer.

FIGURE 4. From the computed modulus of the field from a USAF 1951 target placed in the laser beam, the computed image (a) shows spatial accuracy of the MCI technique at about 1.5 mm. From the computed phase change of the laser when a standard glass plate placed in the optical path (c), achieved accuracy is 0.1 λ compared to the transmission function measured with a Zygo interferometer (b).

Potential applications

The MCI-based computational imaging technique has many applications within a high-power laser plant, including measurements of the influence of gravity deformation, the deformation induced by mounting, the complex transmittance of large optical elements, and laser beam temperature changes (see Fig. 5). By analyzing the changes in the computed phase and modulus, the evaluated parameters can be fed back to adaptive devices, including deformable mirrors to make appropriate corrections to the wavefront in real time.

FIGURE 5. Some potential extended applications of the MCI-based technique are shown.

As a very large and complex optical system, high-power laser facilities can seriously benefit from a compact and multifunctional monitoring system to provide accurate information on the laser beam and the status of the whole facility.

The NLHPLP scientists are very hopeful that MCI-based computational optical imaging can resolve most of the current technical problems in the online diagnostics of the SIOM high-power laser facility. The current success of the MCI technique achieved by the pioneering efforts of scientists at the ShenGuang-II facility provides a firm foundation from which to work towards realizing the goal of nuclear-fusion ignition using high-power lasers.

REFERENCES

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3. Y. Li et al., High Power Laser Sci. Eng., 3, e5 (2015).

4. P. Zeng et al., High Power Laser Sci. Eng., 2, e16 (2014).

5. F. Liu et al., High Power Laser Sci. Eng., 1, 1, 29–35 (2013).

6. F. Zhang et al., Phys. Rev. B, 82, 121104 (2010).

7. F. Zhang et al., Opt. Express, 21, 11, 13592–13606 (2013).

8. X. He et al., Laser Phys. Lett., 12, 1, 015005 (2015).

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Jianqiang Zhu is director, Hua Tao and Xingchen Pan are PhD students, and Cheng Liu is professor and group leader of computation optical imaging in the field of high-power laser drivers in the National Laboratory of High Power Laser Physics at the Shanghai Institute of Optics and Fine Mechanics (SIOM) at the Chinese Academy of Sciences, Jiading, Shanghai, China; e-mail: [email protected]; http://sglf.siom.ac.cn.