Objective Current developments in pulsed lasers have accelerated developments in fields, such as inertial confinement fusion, plasma emission, and high-energy-density physics. Fully characterizing these laser pulses is critical for its wide applications. Currently, many useful techniques exist, such as frequency-resolved optical gating, spectral phase interferometry for direct electric-field reconstruction (SPIDER), and double-blind holography for fully characterizing ultrashort laser pulses. However, these techniques are either incapable or ineffective for nanosecond laser pulse diagnostics because its narrow spectral width is always beyond most spectrometers’ resolution capability. For the nanosecond laser pulse diagnostic technique, the temporal intensity distribution can be obtained using a photodetector and oscilloscope, and the temporal phase distribution can be obtained using a heterodyne measurement approach. However, no applicable technique can provide the temporal intensity and phase distributions using a single detector.
Methods In the proposed technique, the measured pulse is divided into two, with a delay of hundreds of picoseconds. One replica received a proper amount of frequency shift using an acousto-optic frequency shifter. The temporal shearing interferogram is recorded using a normal photodiode and oscilloscope after combining the delayed and frequency-shifted pulses. The measured pulse’s temporal phase distribution is reconstructed using the Fourier transform method, like that used in the SPIDER technique. In this paper, an amplitude reconstruction algorithm based on Fourier transform is proposed to reconstruct the measured pulse’s temporal amplitude distribution using the recorded temporal interferogram. Fig. 2 shows a flowchart of the detailed amplitude reconstruction algorithm. Thus, the proposed amplitude reconstruction algorithm can fully characterize the nanosecond laser pulse with a single photodiode.
Results and Discussions From the systematic analysis of the amplitude reconstruction algorithm principle, four nanosecond laser pulses with different intensity distributions are simulated to evaluate the feasibility algorithm (Fig. 3). The numerical simulation results showe that the temporal intensity distribution of nanosecond pulses could be reconstructed from the temporal interferogram using the proposed amplitude reconstruction algorithm. Figs. 4 and 5 show the influence of signal-noise ratio (SNR) of the recorded interferogram, relative time delay, the relative intensity between delayed and frequency-shifted pulses, and relative frequency response ratio of photodiodes on the reconstructed results. When SNR of the recorded interferogram, the relative time delay between two pulses, is between 0.002 and 0.6, the algorithm can well reconstruct the temporal intensity distribution. Furthermore, the experiment verifies the algorithm’s feasibility. In the experiment, a nanosecond-pulsed laser diode with a central wavelength of 640 nm generates laser pulses with time durations varying from 5 ns to 39 ns at a repetition rate from 1 MHz to 10 MHz, with a peak power of 50 mW. An acousto-optic frequency shifter is used to obtain a frequency shift of 1.16 GHz. A fast photodiode (PDA2.5GA3KSFA) with a bandwidth of 2.5 GHz and a 20 GSa/s oscilloscope (LeCroy WaveRunner 620Zi) are used to record the temporal heterodyne signal intensity S(t) (Fig. 6). The results showe that the reconstructed temporal intensity of the measured pulses correlated well with those recorded directly using a photodiode.
Conclusions In this paper, online measurement technology based on self-referencing temporal shearing interferometry is proposed to reconstruct the complex amplitude distribution of nanosecond pulses from the interferogram. In this proposed scheme, a temporal amplitude reconstruction algorithm is proposed to reconstruct the measured pulse’s temporal intensity using the recorded temporal interferogram. Simulation is conducted to determine the influence of SNR, relative time delay, the relative intensity between two pulses, and the relative frequency response ratio of photodiode on the reconstructed results. Furthermore, an experiment is conducted to evaluate the reliability of the proposed algorithm. An additional measurement is conducted to record the temporal intensity distribution of measured pulses using a photodiode as a comparison to the reconstructed intensity distribution. The results showe that the measured and reconstructed temporal intensity distributions of the measured pulses matched well. The proposed amplitude reconstruction algorithm simplifies the experimental setup and fully characterizes nanosecond laser pulses within a single shot. The proposed algorithm is simple and has wide applications. It provides a new measurement method for fully characterizing nanosecond laser pulses. Furthermore, the proposed amplitude reconstruction algorithm could reconstruct the amplitude distribution in the SPIDER technique.
