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Description
Versatile metrology of ultrashort pulses using amplitude swing technique
Íñigo Sola, a,b Cristian Barbero, a,b Miguel López-Ripa, a,b and Benjamín Alonso a,b
a Grupo de Investigación en Aplicaciones del Láser y Fotónica (ALF), Universidad de Salamanca, 37008 Salamanca, Spain.
b Unidad de Excelencia en Luz y Materia Estructuradas (LUMES), Universidad de Salamanca, 37008 Salamanca, Spain.
E-mail Presenting Author: ijsola@usal.es
In ultrafast optics, accurately measuring the temporal profile of ultrashort pulses is crucial, as the control of light-matter interactions relies on knowing the pulse phase, spectrum, temporal structure, and duration. Despite the availability of various characterization methods, there remains a demand for techniques that merge high precision with operational simplicity to handle complex setups. Our group has developed the amplitude swing (a-swing) technique [1] to meet this demand, offering a simpler, highly robust alternative. This versatile diagnostics tool is tailored for diverse ultrafast optics applications, e.g., nonlinear optics, and fiber laser applications, featuring a design that minimizes alignment errors for highly reliable performance.
The a-swing method works by interfering two temporally delayed replicas of a pulse, using the modulation of their relative amplitude [1] as the phase-encoding control parameter to record a 2D trace (Fig. 1). Experimentally, this is achieved via a compact in-line setup utilizing birefringent crystals (multi-order waveplates) and a linear polarizer. This configuration acts as a common-path interferometer, providing exceptional inherent stability [2]. The framework has also been generalized to alternative amplitude modulation patterns, allowing flexible adaptation to different experimental configurations [3].
Several algorithmic methods have been developed to retrieve pulse profiles from the second-harmonic traces, evolving from global optimization routines to a ptychographic iterative algorithm [4–6]. This ptychographic approach enables the rapid, simultaneous reconstruction of both the spectral phase and temporal intensity of the electric field, significantly improving the retrieval process.
The versatility of a-swing is demonstrated by its broad operating capabilities. It operates across a multi-octave spectral range from the visible to the near-infrared [6], and can characterize pulses down to the few-cycle regime [7]. Beyond scalar fields, a-swing is specially sensitive to vector pulses (i.e., pulses with time-evolving polarization), allowing researchers to determine the temporal evolution of ellipticity and orientation from just a single trace [5]. Furthermore, its robustness enables the simultaneous retrieval of the pulse and the spectral response of the nonlinear stage through the marginale [2].
Applications of a-swing range from fundamental studies to the diagnostics of commercial laser platforms. It has been successfully used to optimize Chirped Pulse Amplifier (CPA) compressors, analyze post-compression systems, study time-evolving polarization sources, and characterize tunable Optical Parametric Amplifiers (OPAs), as well as diagnostics of unstable sources. Additionally, it has been applied to industrial-grade high-power lasers and commercial Erbium-doped fiber lasers, establishing a-swing as a comprehensive metrology tool for process control in diverse ultrafast technology applications.
Figure 1. Measured and retrieved a-swing traces (left panel) and spectral and temporal amplitude (black) and phase (red) retrievals (right panel).
Notes and References
1 Alonso, B.; Holgado, W.; Sola, Í. J. Opt. Express 2020, 28, 15625
2 Sola, Í. J.; Alonso, B. Sci. Rep. 2020, 10, 18364
3 López-Ripa, M.; Sola, Í. J.; Alonso, B. Opt. Express 2023, 31, 34428
4 Barbero, C.; Sola, Í. J.; Alonso, B. Opt. Laser Technol. 2025, 188, 112939
5 Barbero, C.; Alonso, B.; Sola, Í. J. Opt. Express 2024, 32, 10862
6 López-Ripa, M.; Sola, Í. J.; Alonso, B. Opt. Laser Technol. 2023, 164, 109492
7 López-Ripa, M.; Pérez-Benito, Ó.; Alonso, B.; Weigand, R.; Sola, Í. Opt. Express 2024, 32, 21149