Fig. 1. Schematic of laser near-field complex amplitude measurement based on multistep phase retrieval

Fig. 2. Flow chart of laser far-field focal spot reconstruction algorithm

Fig. 3. Theoretical complex amplitude of near-field. (a) Theoretical amplitude; (b) theoretical phase

Fig. 4. When n＝5, lg E_RMS values of amplitude and phase changed with iterations under different Δz; (a) lg E_RMS of amplitude; (b) lg E_RMS of phase

Fig. 5. When Δz＝2 mm, lg E_RMS values of amplitude and phase changed with iterations under different n. (a) lg E_RMS of amplitude; (b) lg E_RMS of phase

Fig. 6. Retrieved complex amplitude of near-field. (a) Retrieved amplitude; (b) retrieved phase; (c) range of theoretical and retrieved amplitudes; (d) range of theoretical and retrieved phases

Fig. 7. Simulated results of focal spot. (a) Theoretical intensity of focal spot using CZT; (b) retrieved intensity of focal spot using CZT; (c) theoretical intensity of focal spot using FFT; (d) comparison of PIB curves between retrieved and theoretical focal spots under different bucket radii

Fig. 8. Experimental setup. (a) Experimental setup of multistep phase retrieval method; (b) experimental setup of long-focal-length lens imaging method

Fig. 9. Experimental results. (a) Theoretical phase loaded by SLM; (b) retrieved phase by proposed method; (c) comparison of PIB curves between focal spots obtained by proposed method and long-focal-length lens imaging method (lens method) and theoretical focal spot under different bucket radii; (d) theoretical focal spot; (e) retrieved focal spot using proposed method; (f) focal spot obtained by long-focal-length lens imaging method