METHOD AND SYSTEM FOR THE TEMPORAL AND SPECTRAL CHARACTERIZATION OF THE AMPLITUDE AND PHASE OF ULTRASHORT LASER PULSES

Abstract

The method comprises 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.

Claims

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 uses the method of claim 1, wherein it comprises: a replicator means for creating at least two replicas of an input pulse, varying their relative amplitude, and obtaining a range of relative amplitudes, a nonlinear medium, associated to the replicator, which obtains a nonlinear signal for each relative amplitude of the at least two replicas, and an analyzer, associated to the nonlinear medium, for measuring and characterising spectrally each nonlinear signal.

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 analysing means.

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 6, 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 6, 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 comprise 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 comprise 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 comprise an interferometer or an acousto-optic device.

Description

DESCRIPTION OF THE DRAWINGS

[0064] To complement the description being made and in order to aid towards a better understanding of the characteristics of the invention, in accordance with a preferred example of practical embodiment thereof, a ser of drawings is attached as an integral part of said description wherein, with illustrative and non-limiting character, the following has been represented:

[0065] FIG. 1.—Shows, in row 1 simulated, and retrieved in row 2, the nonlinear signal spectra. Different Group Delay Dispersion (GDD) values of the input pulse are represented in the columns.

[0066] FIG. 2.—Shows, in row 1 the simulated spectrum (black), phase (solid dark grey) and retrieved spectral phase (dashed light grey). In row 2 the simulated (solid dark grey) and retrieved (dashed light grey) temporal intensity and phase. Different GDDs are represented in the columns.

[0067] FIG. 3.—Shows, in row 1 the simulated and in row 2, the retrieved nonlinear signal spectra. Different Third Order Dispersion (TOD) values of the input pulse are represented in the columns.

[0068] FIG. 4.—Shows, in row 1, the simulated spectrum (black), phase (solid dark grey) and retrieved spectral phase (dashed light grey). In row 2 the simulated (solid dark grey) and retrieved (dashed light grey) temporal intensity. Different TODs are represented in the columns.

[0069] FIG. 5.—Shows, in column 1, the experimental nonlinear signal spectra; in column 2 the corresponding signal retrieved by the algorithm. The retrieved spectral phases (column 3, solid light grey curve) and time domain pulse intensities (column 4, solid light grey curve) obtained with the present method are compared to the corresponding retrieved spectral phase (column 3, dashed dark grey curve) and time domain pulse intensities (column 4, dashed dark grey curve) obtained from a self-calibrating d-scan. Rows A-D correspond to different pulse compression cases.

[0070] FIG. 6.—Shows a simplified scheme of the system.

PREFERRED EMBODIMENT OF THE INVENTION

[0071] The present disclosure presents a method and system for the temporal and spectral reconstruction and characterization of ultrashort laser pulses, which can be scalar pulses with a constant linear polarization or vector pulses, with spectral and temporal dependent polarization.

[0072] The method comprises steps for creating two replicas (21) of an input pulse (1) to be characterised, varying the relative amplitude of the two replicas (21) along a scan (either scanning or spatially encoded), continuously or step-by-step, creating at each case of said scan a nonlinear signal (31), measuring the spectra of the nonlinear signals (31) and recovering the spectral phase (and possibly the spectral amplitude) of the input pulse (1) and its complex amplitude in the time domain by means of a proper algorithm.

[0073] Specifically, it is disclosed a method for characterizing ultrashort laser pulses, the method comprising: [0074] a pulse manipulation stage, which comprises the steps of: [0075] creating two replicas (21) of an input pulse (1) to be characterized with a temporal delay between them, wherein the two replicas (21) have a relative amplitude; [0076] varying the relative amplitude of the two replicas (21) in order to obtain a range of relative amplitudes between the two replicas (21); [0077] a nonlinear stage, which comprises the steps of: [0078] applying a nonlinear process to the two replicas (21), obtaining a nonlinear signal (31) for each value of relative amplitudes of the at least two replicas (21); [0079] a detection stage, which comprises the steps of: [0080] measuring and acquiring the spectra of the nonlinear signals (31), which depends on the relative amplitude between the two replicas (21), obtaining a two-dimensional trace; [0081] measuring and acquiring the spectral amplitude of the input pulse (1); [0082] a processing and reconstructing stage, which comprises the steps of: [0083] calculating with an algorithm the spectral phase of the input pulse (1), applying the algorithm to the nonlinear signal (31) spectra; and [0084] calculating the temporal amplitude and phase of the input pulse (1) to be characterised applying, preferably, an inverse Fourier transform to the measured linear spectrum and to the retrieved spectral phase.

[0085] The method can additionally comprise the step of overlapping the spectra of the nonlinear signal (31) of the at least two replicas (21) with a remaining intentionally unfiltered part of the input pulse (1) or of the linear signal of the at least two replicas (21), and using it to calculate the absolute phase (carrier envelope phase) of the input pulse (1).

[0086] The present invention also comprises a system, shown in FIG. 6, for the characterization of ultrashort laser pulses. The system comprises the elements that are described below: [0087] means for creating at least two replicas (2) of an input pulse (1) and varying their relative amplitude, obtaining at least two resulting replicas (21), [0088] a nonlinear medium (3), associated to the means for creating at least two replicas (2), which obtains a nonlinear signal (31) for each relative amplitude of the at least two resulting replicas (21), [0089] a first optical element (6), positioned between the means (2) for creating at least two replicas (21) and the nonlinear medium (3), intended to focus the at least two resulting replicas into the nonlinear medium (3), [0090] analysing means (4), associated to the nonlinear medium (3), for measuring and characterising spectrally the nonlinear signal (31) for each resulting pulse, [0091] a filtering element (7) positioned between the nonlinear medium (3) and the analysing means (4), intended to filter the nonlinear signal (31), [0092] a second optical element (8), positioned between the filtering element (7) and the analysing means (4), intended to focus the filtered nonlinear signal (31) into the analysing means (4), [0093] a numerical analysis unit (5), associated to the analysing means (4), for calculating the spectral phase, and [0094] further analysing means (9), associated to the input pulse (1) and to the numerical analysis unit (5), intended to measure the spectral amplitude of the input pulse (1).

[0095] As an example, a first embodiment of the invention consists on: [0096] the means (2) for creating two replicas (21) which comprise a rotating retardation waveplate, and a linear polarizer, [0097] the first optical element (6) which comprises focusing optics, [0098] the nonlinear medium (3) which is a second harmonic generation material, [0099] the filtering element (7) which comprises optics to separate fundamental from nonlinear radiation, and [0100] the analysing means (4) and the further analysing means (9).

[0101] Based on said first embodiment, FIG. 1 shows the simulated (row 1) and retrieved (row 2) nonlinear spectra signal (31) corresponding to the input pulse (1), which has a duration of 100 fs (Fourier-limit condition) full-width half maximum (FWHM) with central wavelength at 800 nm, considering different GDD represented in the columns, GDD=−40000 fs.sup.2, −5000 fs.sup.2, 0 fs.sup.2, +5000 fs.sup.2, +40000 fs.sup.2, respectively. Angle θ represents the orientation of the fast axis of the retardation waveplate. The input pulse (1) is linearly polarized in the x-axis and the linear polarizer in the x-axis is placed after the retardation waveplate and before the second harmonic generation material.

[0102] After applying the algorithm, FIG. 2 presents the comparison between the simulated and retrieved pulses at each of the FIG. 1 GDD cases: (row 1) Simulated spectra (black) and simulated (dark grey) and retrieved (light grey) spectral phase; (row 2) Simulated (dark grey) and retrieved (light grey) temporal intensity and phase. The different GDD values of the input pulse are represented in the columns, GDD=−40000 fs.sup.2, −5000 fs.sup.2, 0 fs.sup.2, +5000 fs.sup.2, +40000 fs.sup.2, respectively. The agreement between simulations and retrievals is good.

[0103] In another example, using the first embodiment of the invention, FIG. 3 shows the simulated (row 1) and retrieved (row 2) nonlinear spectra signal of the input pulse (1) presenting a duration of 100 fs full width half maximum (FWHM) with central wavelength at 800 nm, considering different TOD represented in the columns, TOD=−4000000 fs.sup.3, −1000000 fs.sup.3, 0 fs.sup.3, +1000000 fs.sup.3, +4000000 fs.sup.3, respectively.

[0104] After applying the algorithm, FIG. 4 presents the comparison between both simulations and retrieved pulses at each of FIG. 3 TOD cases: (row 1) Simulated spectra (black) and simulated (dark grey) and retrieved (light grey) spectral phase; (row 2) Simulated (dark grey) and retrieved (light grey) temporal intensity. The different TOD values of the pulse to be characterised are represented in the columns, TOD=−4000000 fs.sup.3, −1000000 fs.sup.3, 0 fs.sup.3, +1000000 fs.sup.3, +4000000 fs.sup.3, respectively. The agreement between simulations and retrievals is good.

[0105] As another example, we show an experimental comparison shown in FIG. 5, between the present method and the d-scan technique in its self-calibrated version [16,19]: (column 1) Experimental nonlinear spectra signal traces; (column 2) corresponding retrieved traces. The retrieved spectral phases (column 3, light grey curve) and time domain pulse intensities (column 4, light grey curve) are compared to the corresponding retrieved spectral phases (column 3, dark grey curve) and time domain pulse intensities (column 4, dark grey curve) from the self-calibrating d-scan. Rows A-D correspond to different pulse compression cases. The agreement between both techniques is good, validating the results.

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