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21
2025
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03
ACS Photonics: Measuring Ultrafast Pulses using Chirplet Transform
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Introduction
Recently,the research group of Dr. Shu Yiqing and Professor Chen Weicheng of Foshan Universityproposed a "Optical Chirplet Transform for Observing Pulse Ultrafast Structures(abbreviated asOCTOPUS)"
This achievement was published inACS Photonics，The title of the paper is "Optical Chirplet Transform for Observing Pulse Ultrafast Structures"Sun Jingxuanis the first author, and the corresponding author is Dr. Shu Yiqing and Professor Chen Weicheng of Foshan UniversityDr. Shu Yiqing、Professor Chen WeichengMacau University of Science and TechnologyProfessor Li Jianqingand Shenzhen UniversityAssociate Researcher Ge Yanqiparticipated in the research work.
Design ideas and working principles
Figure 1 shows the OCTOPUS experimental system and working principle. As shown in Figure 1(a), the measurement system includes a programmable filter, a spectral analyzer, and an autocorrelator. By executing an adaptive phase editing program, the programmable filter can shape the chirped pulse to be measured into a chirp-free Fourier transform-limited pulse, and then obtain the phase of the pulse to be measured through mirror inversion. Figure 1(b) illustrates the relationship between the OCTOPUS trace and the time-frequency domain information of the ultrafast pulse field, and shows the execution process of the OCTOPUS method. The process is roughly as follows: First, the ultrafast pulse is spectrally sliced to generate a series of Chirplet bases in the time domain. Chirplet is a wavelet with chirp characteristics, which can more accurately analyze the time-varying frequency characteristics of the pulse; then, the pulse shaping is realized by adjusting the time domain offset of each Chirplet base; finally, the time domain envelope shape of the pulse can be reconstructed by recombining all Chirplet bases. From Figure 1(b), it can be seen that the projection of the OCTOPUS trace in the wavelength dimension corresponds to the spectrum of the ultrafast pulse, and the summation in the time dimension reflects its time-domain slowly varying envelope.
Figure 1: Working principle of the OCTOPUS measurement system. (a) Experimental setup of the OCTOPUS system. (b) Relationship between OCTOPUS trace and time-frequency domain information of ultrafast pulse field.
Source:ACS Photonics
Figure 2 shows the flowchart of the OCTOPUS pulse field reconstruction algorithm. This algorithm uses the complex Fourier series as the main framework, and decomposes the spectral information of the ultrafast pulse to be measured into a series of Chirplet base components containing several single-frequency components. The phase of each single-frequency component is generated for each Chirplet base using the cubic Hermite interpolation technique, and the time delay and the phase of the single-frequency component of each Chirplet base are adjusted by adaptive phase editing technology. Finally, the OCTOPUS system uses the theoretical chirp-free Fourier transform-limited pulse as a calibration benchmark, and achieves the goal of dechirping the pulse to be measured through AI-assisted dynamic iterative calculation and real-time experimental phase control, and then obtains the phase of the pulse to be measured through the mirror relationship, and realizes pulse field reconstruction.
Figure 2: Flowchart of the OCTOPUS pulse field reconstruction algorithm
Source: ACS Photonics
In the OCTOPUS system, the adaptive phase editing technology is implemented based on a genetic algorithm. As shown in Figure 3, this algorithm optimizes the phase modulation spectrum by simulating the "survival of the fittest" mechanism in biological evolution, thereby achieving chirp elimination and ultimately obtaining mirror information of the phase distribution of the pulse to be measured.
Figure 3: Algorithm flowchart of adaptive phase editing technology in the OCTOPUS system
Source: Sun Jingxuan
Based on the OCTOPUS method, the team measured three groups of ultra-fast pulses with different chirp levels (pulse widths of 2.97 ps, 1.83 ps, and 1.23 ps, respectively), all achieving effective compression and accurate time-domain reconstruction. Experiments show that the OCTOPUS system can adapt to pulses within the range of TBP = 0.342 to 0.800, accurately extracting their slowly varying envelope, instantaneous frequency, and phase characteristics. The results of OCTOPUS reconstructed FROG traces are highly consistent with those of commercial FROG instruments, especially exhibiting higher sensitivity and resolution in weak signal and non-main peak regions, overcoming the problem of low frequency conversion efficiency caused by phase mismatch in traditional measurements. The system can also simultaneously acquire pulse spectral and phase information, revealing the phase distortion trend under changes in resonator parameters. The OCTOPUS trace proposed by the team can further provide intuitive characterization of structural features such as the pulse main peak, Kelly sidebands, and phase distribution, demonstrating excellent full-parameter ultra-fast pulse measurement and diagnostic capabilities.
Figure 4: Information of three groups of ultra-fast pulses measured by an autocorrelator, OCTOPUS system, FROG instrument, and spectral analyzer
Source:ACS Photonics
Summary and Outlook
This research team proposed an OCTOPUS ultra-fast measurement method. Under the constraints of multi-dimensional physical information in the time and frequency domains, the OCTOPUS method can iteratively reshape the ultra-fast pulse to be measured into a chirp-free Fourier transform-limited pulse, and extract the phase information of the original pulse from the phase modulation spectrum applied by the programmable filter. This method can overcome the bottlenecks of traditional measurement methods limited by phase matching conditions and the lack of measured physical information domains, achieving high-precision measurement and complete characterization of pulse time-frequency domain information. Experimental results show that the OCTOPUS method can not only accurately analyze the slowly varying envelope information of the pulse, but also deeply reveal the physical characteristics of its internal fast-varying electric field oscillations, providing a new solution and idea for ultra-fast measurement technology, and having extremely high potential value in the field of ultra-fast science. This method has also been granted an invention patent (Patent name: A device and method for measuring laser pulses using optical chirp wavelet transform, Patent No.: 202410642330.4)
Paper Information
Optical Chirp Transform for Observing Ultrafast Structures of Pulses, Jingxuan Sun, Yiqing Shu, Jianqing Li, Yanqi Ge, and Weicheng Chen,ACS Photonics2024 11 (8), 3441-3446
https://doi.org/10.1021/acsphotonics.4c01011
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