Light Field Homogenization Technology Based on Beam Scanning
Objective As the core instrument for developing ultra-strong and ultrashort laser devices,the load capacity of the holographic grating entirely depends on the effective aperture of the pulse-width compression grating and the anti-laser damage threshold.Large-aperture holographic gratings are currently only available to the United States and Europe,with China facing an embargo.Domestic and international methods such as beam scanning exposure,static interference field transmission exposure,exposure splicing,and mechanical scribing have not been able to manufacture bidirectional meter-level gratings.Recently,researchers have applied large-aperture off-axis reflective collimators to the double-beam exposure system,overcoming the aperture limitations of large-aperture collimating lenses and offering convenient periodic adjustment advantages.Our study paves a new way for fabricating large-aperture diffraction gratings and lays a technical foundation for developing bidirectional meter-level pulse compression gratings required for hundred-petawatt-level high-power laser devices.However,the off-axis reflective exposure system presents an uneven exposure dose issue.Traditional methods address this by employing Gaussian spot center interception to achieve a highly uniform light field,which results in low energy utilization and increased exposure time.The increase in exposure time not only increases the stability requirements of the exposure system but also reduces the efficiency of the production grating.The rise of beam shapers has seen widespread use of schemes that convert Gaussian beams into flat-top beams in Lloyd's mirror exposure systems.However,after shaping by the beam shaper,the Gaussian beam can no longer filter out stray light and high-order mode interference in the optical path using a spatial filter,affecting grating manufacturing quality.In this study,we propose a novel beam scanning homogenization method based on the existing large-aperture off-axis reflection exposure system.Post-scanning interference lithography allows for an evenly distributed light field intensity,equivalent to uniform exposure dose distribution.Methods Combining the principle of light homogenization of new beam scanning,we numerically simulate the influence of different scanning parameters on the uniformity distribution of light field.A moving mirror mounted on an air-flotation translation stage achieves the two-dimensional scanning movement of the beam at the substrate position.Using a complementary metal oxide semiconductor(CMOS)sensor,we capture the light field distribution at the substrate position after beam scanning.Based on the new beam scanning interferometric exposure system,we conduct scanning homogenization experiments under various beam radii.We introduce reference light to lock fringe drift and analyze the regularity variation of fringe drift at different positions from time and frequency domain perspectives.Results and Discussions Our simulations indicate that when the ratio of the scanning beam radius to the step distance is less than or equal to 1.2,the uniformity of the light field after scanning stitching exceeds 99.50％(Fig.3).With beam radius of 5 mm,8 mm,and 15 mm corresponding to ratios of 1.1,1.2,and 1.3,respectively,pattern data processing yields light intensity distribution curves(Figs.6-7).The superimposed beam scanning quantitatively describes the uniformity of the light field's intensity distribution,showing a uniformity better than 99.3％when the ratio of scanning beam radius to the step distance is less than or equal to 1.2(Table 1).Excluding environmental factors,experimental and simulation results are generally consistent.Under identical proportional-integral-derivative(PID)parameters,fringe drift within a 150 mm× 75 mm area is controlled at±0.02λ(3σ),with a phase change less than±0.02 interference fringe period namely(Fig.8).Spectral analysis reveals that the introduction of closed-loop control effectively suppresses the 50 Hz low-frequency error at different positions(Fig.8).Time-domain analysis of fringe drift shows that with an optimized exposure system,the phase change across a φ200 mm range is less than±0.015 interference fringe periods(Fig.9).Closed-loop control effectively suppresses the 20 Hz low-frequency error causing fringe drift(Fig.9).Introducing reference light for fringe drift-locking significantly reduces environmental error impact during beam scanning.Conclusions We introduce a novel beam scanning homogenization method that controls the beam's two-dimensional scanning motion via two one-dimensional mirrors mounted on an air-floating translation stage.The beam passes through the optical path system,which carries out the scanning superposition movement of the large spot on the substrate.This approach offers advantages such as reduced load and noise compared to moving large substrates,especially meter-sized ones.To verify the method's feasibility,we conduct the homogenization experiment of beam scanning and the locking experiment of fringe drift.Results show that when the beam radius to the step distance is less than or equal to 1.2,the intensity uniformity of the light field surpasses 99％,indicating that the method can achieve a high uniform distribution of light intensity while making full use of the laser energy.Under identical PID control,the amount of fringe drift at different positions within a φ200 mm range is confined to±0.015λ(3σ),effectively mitigating low-frequency errors caused by fringe drift.Enhancing the uniform distribution of the light field significantly improves the large-aperture off-axis reflection exposure system's applicability,addressing uneven exposure dose and elevating grating manufacturing quality.
gratingsholographic gratingexposure systembeam scanninglight field uniformityfringe drift-locking
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