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Novel wavefront measurement method for high power laser devices
Release time:
2023-11-17
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Novel wavefront measurement method for high power laser devices
With the development of high-power laser devices, people have higher and higher requirements for the output capability and beam quality of the device, and also put forward new requirements for optical field measurement.
At present, the direct imaging method can only obtain the intensity distribution of the near-field and far-field of the beam on a limited fixed plane, and the Hartmann sensor can only measure the low-frequency wave-front information, and the precise medium and high-frequency wave-front information cannot be obtained. In the process of laser transmission, the triple frequency damage and multi-wavelength effect also make the traditional measurement method stretched. Therefore, there has been no suitable technology to measure the intensity and phase of high-power laser beam on-line with high enough accuracy, and the comprehensive on-line detection of laser beam light field has not been realized.
Based on years of technology accumulation, a new wavefront measurement method-coherent modulation imaging (CMI) method is proposed in the High Power Laser Physics Joint Laboratory.
coherent modulation imaging (CMI) method
In principle, different from the existing measurement technology, the coherent modulation imaging (CMI) method uses a phase plate with a known structure and highly random distribution to modulate the phase of the wave front, records a single diffraction spot by CCD, and then reconstructs the amplitude and phase of the light by iterative algorithm. The idea of reconstruction is to transmit the guessed incident light field back and forth between the incident window surface and the CCD surface, and impose constraints on the amplitude on the two surfaces, so that the calculated light field gradually converges to the true value. How do you ensure that the convergence is the true complex amplitude?
From the spatial point of view, the strong scattering effect of the random phase plate makes the points of the wave front to be measured related to each other, so some differences on the wave front to be measured will produce great differences in the diffraction spots collected by CCD (as shown in the figure below). Therefore, the final calculation of the light field can only converge to the real light field through the limitation of the diffraction spot intensity and the airspace of the incident window surface.
Explained from the frequency domain, the spectrum of the randomly distributed phase plate is wide, because its effect with the incident light spectrum is a convolution process. The wider the spectral range of the phase plate, the more equations can be constructed, and when the number of equations is greater than the number of unknowns (incident light spectrum), a unique solution can be obtained.
In short, the emphasis on the production of the random phase plate strengthens the limitations of the existing airspace and frequency domain, and enhances the ability to filter the wavefront, so that the true amplitude and phase can be obtained. After obtaining the true distribution of the wavefront to be measured, the required information such as near-and far-field intensity distribution, beam direction, and energy concentration can be further extracted to achieve a comprehensive diagnosis of the laser beam.
Application of the CMI Method
The instrument developed by using the CMI method has many advantages such as compact structure, fast measurement speed and high precision, and can be placed in any position in the driver for real-time precision detection of the beam wavefront. It overcomes the shortcomings of the existing direct imaging method and Hartmann sensor, and solves the problem that the high spatial frequency can only be measured offline by interferometer. At present, the method has been successfully verified by experiments on the XXX national laser device, and the near-and far-field optical field distribution of a single pulse is measured, and the near-field resolution of 500 μm is achieved.
As a new type of wavefront measurement technology, the CMI method is also suitable for the detection of optical components. After adding light source and collimation module, the function of interferometer can be realized, and the measurement accuracy can reach 0.05 λ. Considering its high dynamic range, low environmental stability requirements, and no reference beam, this technology has a good development prospect in the field of component measurement. In addition, based on the breakthrough of principle, CMI method is also expected to be used in interdisciplinary fields such as microscopic imaging and plasma state measurement.
CMI wavefront measuring instrument has a wide range of applications in national defense engineering and civil fields, and can break the technical monopoly of developed countries in the field of large-scale optical precision measuring instruments. A few days ago, the project won the gold medal of the first "China dual-use Technology Innovation and Application Competition" and was highly praised by the jury. High Power Laser Physics Joint Chamber Confident in Future of CMI Wavefront Gauge
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