Real-time complex amplitude reconstruction method for beam quality M^2 factor measurement

Abstract

We present a real-time complex amplitude reconstruction method for determining the beam propagation ratio M of laser beams based on the transport of intensity equation (TIE). In this work, a synchronous acquisition system consisting of two identical CCDs is established. Once two beam intensity images at different cross-section positions along the optical axis are captured simultaneously by the system, the complex amplitude of the laser beam can be rapidly reconstructed using TIE algorithm. Then the beam intensity distribution at any section position along its propagation direction can be obtained by using angular spectrum (AS) theory. The beam quality M factor is therefore calculated utilizing the second-order moments and hyperbola fitting methods, which conform to the ISO standard. The suitability of this method is verified by the numerical analysis and experiments with the He-Ne and high-power fiber laser sources, respectively. The experimental technique is simple and fast, which allows to investigate laser beams under conditions inaccessible to other methods.

Deep learning enabled superfast and accurate M2evaluation for fiber beams

“…Based on the results above, we can find that our scheme is accurate, robust and very fast for M 2 estimation. Compared with traditional methods [9][10][11][12][13][14][15][16][17][18][19][20][21], our approach only needs a single-shot near-field beam image collected by CCD to perform M 2 estimation, which is very economic and easy to implement under normal condition. Due to the computing efficiency of trained CNN, it only takes about 5ms to perform M 2 estimation for a single input pattern, which is a breakthrough in M 2 measuring efficiency.…”

“…For example, the motion-free variable-focus techniques utilize spatial light modulators [10] or liquid lenses [11], leading to a measuring time below 1 s. In addition, some single-shot schemes have also been reported, with which the beam cross sections can be imaged simultaneously on detectors using a distorted diffraction grating [12] or angled Fabry-Perot filter [13] and these schemes are tested by high power Nd:YAG lasers [12] or high power fiber lasers [13] respectively. Further, complex amplitude reconstruction methods using different kinds of interferometers [14,15] or just Charge-Coupled Device (CCD) [16] are proposed. The latter scheme [16] utilizes two identical CCDs to obtain beam intensity images at different defocused positions for wavefront reconstruction, making the time taken from the image acquisition of He-Ne laser beam to the M 2 value determination only about 0.5 s.…”

“…Further, complex amplitude reconstruction methods using different kinds of interferometers [14,15] or just Charge-Coupled Device (CCD) [16] are proposed. The latter scheme [16] utilizes two identical CCDs to obtain beam intensity images at different defocused positions for wavefront reconstruction, making the time taken from the image acquisition of He-Ne laser beam to the M 2 value determination only about 0.5 s.…”

“…In this paper, we have developed deep CNN for superfast and accurate M 2 prediction. Compared with traditional schemes [9][10][11][12][13][14][15][16][17][18][19][20][21], our approach is very economic, which only needs a CCD camera to obtain near-field beam patterns. The extraordinary time efficiency is another advantage of our scheme.…”

Reconstructing the Spatial Parameters of a Laser Beam Using the Transport-of-Intensity Equation

“…At the same time, a method of real-time reconstruction of the complex amplitude exists for measuring the M 2 parameter by means of transport-of-intensity equation [17][18][19]. The transport-of-intensity equation (TIE) provides a non-interferometric way to obtain quantitative information on the phase of light beams by recording transverse field intensity distributions in two or more cross-sections [20,21].…”