US-12627110-B2 - Method and system for the temporal and spectral characterization of the amplitude and phase of ultrashort laser pulses

US12627110B2US 12627110 B2US12627110 B2US 12627110B2US-12627110-B2

Abstract

A method includes steps for creating at least two replicas of an input pulse to be characterised, varying the relative amplitude of the two replicas within a range, creating a nonlinear signal at each case of said amplitude variation, measuring the spectra of the nonlinear signals and recovering the spectral amplitude and phase of the input pulse with a proper algorithm. The system includes a replicator for creating at least two replicas of the input pulse and varying their relative amplitude within a range of relative amplitudes, a nonlinear medium, which obtains a nonlinear signal for each relative amplitude, and an analyzer, associated to the nonlinear signal, for measuring and characterising spectrally each nonlinear signal.

Inventors

Iñigo Juan SOLA LARRAÑAGA
Benjamín ALONSO FERNÁNDEZ

Assignees

UNIVERSIDAD DE SALAMANCA
Sphere Ultrafast Photonics, S.L.

Dates

Publication Date: 20260512
Application Date: 20201216
Priority Date: 20191219

Claims (15)

1 . A method for the temporal and spectral characterization of the amplitude and phase of ultrashort laser pulses, wherein the method comprises the steps of: a pulse manipulation stage, which comprises the steps of: creating at least two replicas of an input pulse to be characterized, with a temporal delay between them, having the at least two replicas a relative amplitude; varying the relative amplitude of the at least two replicas, between a lower limit and an upper limit, in order to obtain a range of relative amplitudes of the at least two replicas; a nonlinear stage, which comprises the steps of: applying a nonlinear process to the at least two replicas, obtaining a nonlinear signal for each value of relative amplitudes of the at least two replicas; a detection stage, which comprises the steps of: measuring and acquiring a spectrum of each nonlinear signal, obtaining a two-dimensional trace; a processing and reconstructing stage, which comprises the steps of: recovering the temporal and spectral amplitude and phase of the input pulse, applying an algorithm to the two-dimensional nonlinear signal spectra.
2 . The method of claim 1 , further comprising the step of measuring and acquiring the spectrum of the input pulse to be used in the processing and reconstruction stage.
3 . The method of claim 1 , wherein varying the relative amplitude of the at least two replicas can be in the module of the amplitude or in the module and phase of the complex amplitude.
4 . The method of claim 1 , further comprising the step of overlapping the spectra of the nonlinear signal of the at least two replicas with a remaining intentionally unfiltered part of the input pulse or of the linear signal of the at least two replicas, and using it to calculate the absolute phase of the input pulse.
5 . The method of claim 1 , wherein measuring the linear spectrum of the at least two replicas as a function of their varying amplitudes, to be used in the processing and reconstruction stage.
6 . A system for the characterization of ultrashort pulses, which comprises: a replicator for creating at least two replicas of an input pulse, varying their relative amplitude, and obtaining a range of relative amplitudes, wherein the at least two replicas have a temporal delay between them and a relative amplitude, and wherein varying comprises varying the relative amplitude between a lower limit and an upper limit; a nonlinear medium, associated to the replicator, configured to apply a nonlinear process to the at least two replicas which produces a nonlinear signal for each relative amplitude of the at least two replicas; and an analyzer, associated to the nonlinear medium, for measuring and characterizing spectrally each nonlinear signal, the analyzer configured to measure and acquire a spectrum of each nonlinear signal, obtaining a two-dimensional trace, and to recover the temporal and spectral amplitude and phase of the input pulse by applying an algorithm to the two-dimensional non-linear signal spectra.
7 . The system of claim 6 , further comprising a first optical element, positioned between the means for creating at least two replicas and the nonlinear medium.
8 . The system of claim 6 , further comprising a filtering element positioned between the nonlinear medium and the analyzer.
9 . The system of claim 6 , further comprising a second optical element, positioned between the nonlinear medium and the analyzer.
10 . The system of claim 6 , further comprising a numerical analysis unit with an electronic data processor, associated to the analyzer, intended for calculating the spectral and temporal amplitude and phase of the input pulse.
11 . The system of claim 10 , comprising further the analyzer, associated to the input pulse and to the numerical analysis unit, intended to measure the spectral amplitude of the input pulse.
12 . The system of claim 10 , comprising further the analyzer, associated to the replicator of the input pulse and to the numerical analysis unit, intended to measure the spectrum of the two or more replicas as a function of the varying relative amplitudes.
13 . The system of claim 6 , wherein the replicator comprises a moving birefringent element, a set of anisotropic elements and a polarizing element or set of polarizing elements.
14 . The system of claim 6 , wherein the replicator comprises a static set of optical components, as birefringent wedges, anisotropic elements and polarizing elements, intended to introduce the variation of the relative amplitude between the at least two replicas with respect to a spatial coordinate, being compatible with the analysing means operating in a single acquisition.
15 . The system of claim 6 , wherein the replicator comprises an interferometer or an acousto-optic device.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY This patent application claims priority from PCT Application No. PCT/ES2020/070798 filed Dec. 16, 2020, which claims priority from Spanish Patent Application No. P2019 filed Dec. 19, 2019. Each of these patent applications are herein incorporated by reference in their entirety. DESCRIPTION Object of the Invention The present disclosure relates to laser systems and laser pulse characterization methods and presents a method and system for the characterization of ultrashort laser pulses. BACKGROUND OF THE INVENTION The arising of ultrafast optics and the increasing of its applications came in parallel to the need of characterizing ultrashort laser pulses [1]. As those are turning into increasingly complex and short (to the extreme of single-cycle regime [2,3] or even shorter [4]), the pulse measurements are becoming more and more demanding. The first characterization techniques were based on the pulse autocorrelation [5], obtained from scanning the time delay between two pulse replicas and the measurement of the power of a nonlinear signal depending on the time overlap of both replicas. These methods give an approximate idea of the pulse characteristics but are not able to reconstruct the actual pulse or to provide its spectral phase. Later, the FROG technique [6] used a similar scheme but acquiring the nonlinear signal spectra, instead of the overall nonlinear signal power. The so-called FROG spectrograms, consisting in the nonlinear spectra depending on the replicas delay, encode information of the spectral phase and, by means of reconstruction algorithms [7,8], succeed in reconstructing the pulses. Some years later, the SPIDER technique [9] was invented using spectral interferometry, where both replicas remained at a fixed time delay and a spectral shearing was introduced within a nonlinear process. Thus, the derivative of the pulse spectral phase is encoded in the spectral interference of both replicas and extracted by means of Fourier analysis. Another evolution of the spectral interferometry strategy is the self-referenced spectral interferometry (Wizzler) [10], where the reference pulse is cleaned in time, obtaining a flat spectral phase. A different strategy was presented in 2004 with pulse characterization using phase scanning, the so-called multiphoton intrapulse interference phase scan (MIIPS) [11,12]. The general idea consists in introducing a known set of spectral phases in the test pulse and to observe the second order harmonic generation (SHG) signal of the resulting pulse. The unknown pulse group delay dispersion (GDD) can be therefore extracted at a given wavelength by calculating the amount of GDD within the scan range needed to optimize the SHG signal at that wavelength. Later, the d-scan technique [13] used the spectral phase scan concept with some practical modifications and introduced retrieval algorithms [14-17] to reconstruct the spectral phase of the test pulse. A related technique was proposed in [18], using an acousto-optic programmable dispersive filter (AOPDF) for the known spectral phase scan and an algorithm to reconstruct both the spectral amplitude and phase of the pulse. In general, the main part of the time pulse characterization operates under laboratory stability conditions. A major challenge nowadays is to implement characterization set-ups robust and simple enough to work under less controlled conditions. Thus, one of the main goals of the present work is to study the idea and implementation of reconstruction systems capable of facing those demands. On the other hand, for designing a characterization set-up, it is needed to take into account the time duration ranges and central wavelength of the pulse to reconstruct, since they are major conditionings for the system implementation. Here, we aimed to develop a simple and robust system for pulse characterization, presenting an in-line configuration, being able to be adapted easily to a broad range of pulse durations. Here we present a different approach to the pulse reconstruction by using some delayed replicas and varying the relative amplitude between them. The time delay can be chosen in such a way that the two replicas can overlap on time. By varying the relative amplitude between them, the resulting time evolution changes because of the pulse interference in the time domain. If subsequently the resulting pulse generates a nonlinear signal (e.g., but not limited to, by second harmonic generation, SHG), the nonlinear spectrum will depend strongly on its time evolution. Therefore, by scanning the relative amplitude between the two replicas the spectrum of the nonlinear signal may change, encoding information about the original input pulse. DESCRIPTION OF THE INVENTION The present invention discloses a method and system for the temporal and spectral characterization of the amplitude and phase of ultrashort laser pulses. Some reconstruction techniques (e.g., autocorrelati