Method and assembly for optical analysis of an ultrashort laser pulse

Abstract

An optical analysis method and assembly for analysing an ultrashort laser pulse, the assembly includes a single-shot optical autocorrelator, having a polarity separator for angular separation of an incident laser radiation beam with fundamental frequency (ω) into two laser radiation beams with the fundamental frequency and linear polarities which are orthogonal to one another, the two beams forming angle therebetween so that the beams at least partially overlap at the output of the separator, a type-II nonlinear crystal receives the at least partially overlapping beams and generates, at the output of the crystal, a single laser radiation beam with harmonic frequency (2ω). A spectral filtering device selectively allows the passage of the single laser radiation beam while blocking the laser radiation beams with fundamental frequency. The non-linear crystal, spectral filtering device, and detection system detect an intensimetric single-shot autocorrelation trace of the order of two at the harmonic frequency.

Claims

1. A single-shot optical autocorrelator device for analyzing an ultrashort laser pulse, the single-shot optical autocorrelator device comprising: a polarity separator for angularly separating an incident laser radiation beam with a fundamental optical frequency (ω) into two laser radiation beams with the fundamental frequency (ω) and linear polarities that are orthogonal to one another, said two beams forming an angle between them at the output of said separator, said angle being non-zero so that said beams at least partially overlap at the output of said separator; a type-II non-linear crystal, said non-linear crystal being arranged to receive said at least partially overlapping beams originating from said separator so as to generate, at the output of said crystal, a single laser radiation beam with a harmonic frequency (2ω), which is a second-order autocorrelation trace having an double optical frequency 2ω; at least one spectral filtering device configured to selectively allow the passage of said laser radiation beam with the harmonic frequency (2ω), while blocking said laser radiation beams with the fundamental frequency (ω), said at least one filtering device being placed between said non-linear crystal and a spatially resolved detection system in at least one direction; said non-linear crystal, said at least one spectral filtering device, and said detection system being arranged to detect a second-order intensimetric type single-shot autocorrelation trace at the double optical frequency 2ω.

2. The single-shot optical autocorrelator device as claimed in claim 1, wherein said polarity separator is selected from the group comprising a Wollaston prism, a Babinet prism, and a Rochon prism.

3. The single-shot optical autocorrelator device as claimed in claim 1, wherein with said detection system having a detection plane, said non-linear crystal is at most placed at a distance between ]0, 5] mm from this detection plane.

4. The single-shot optical autocorrelator device as claimed in claim 1, wherein said single-shot optical autocorrelator device is a pre-assembled element, with at least said type-II non-linear crystal and said at least one spectral filtering device being in optical contact.

5. The single-shot optical autocorrelator device as claimed in claim 1, wherein the assembly formed by said polarity separator, said type-II non-linear crystal, and said at least one spectral filtering device is placed in an in-line, contiguous configuration.

6. The single-shot optical autocorrelator device as claimed in claim 1, wherein said at least one spectral filtering device has a recessed output face on at least one portion of its periphery.

7. The single-shot optical autocorrelator device as claimed in claim 1, wherein said detection system comprises a matrix detector or an imaging spectrometer, said imaging spectrometer comprising an inlet slit, a spectrally dispersive optical system, and a spatially resolved two-dimensional detector.

8. The single-shot optical autocorrelator device as claimed in claim 1, wherein the at least one spectral filtering device is a linearly variable spectral filtering device, said spatially resolved detection system being configured to detect a FROG trace, also called spectrally resolved second-order single-shot autocorrelation trace.

9. An optical analysis assembly for analyzing an ultrashort laser pulse, characterized in that it comprises a single-shot optical autocorrelator device as claimed in claim 1.

10. The optical analysis assembly as claimed in claim 9, further comprising one or more of an attenuator device, a polarizer, and an expander for increasing the diameter of the incident laser radiation beam.

11. A method for analyzing an ultrashort pulse implementing an optical analysis assembly as claimed in claim 9, the method comprising the following steps: producing a first laser radiation beam and a second laser radiation beam, said beams having a fundamental optical frequency (ω) and having linear polarities that are orthogonal to one another, said beams propagating while forming a non-zero angle between them; introducing said at least partially overlapping first and second laser radiation beams into a type-II non-linear crystal, said crystal being configured to generate as output the single laser radiation beam with harmonic frequency (2ω), which is the second-order autocorrelation trace having the double optical frequency 2ω; introducing the laser radiation beams exiting the non-linear crystal into the at least one spectral filtering device, said at least one filtering device being configured to selectively allow the passage of the laser radiation beam with harmonic frequency (2ω), while blocking the laser radiation beams with fundamental frequency (ω); introducing the laser radiation beam with harmonic frequency (2ω) into the spatially resolved detection system in at least one direction, said non-linear crystal, said at least one spectral filtering device, and said spatially resolved detection system being arranged to detect the second-order intensimetric type single-shot autocorrelation trace at the double optical frequency (2ω).

12. The method as claimed in claim 11, wherein the at least one spectral filtering device is a linearly variable spectral filtering device and at the output of said non-linear crystal, said laser radiation beam with the harmonic frequency (2ω) is introduced into the linearly variable spectral filtering device, so that, after detection, a FROG trace, also called spectrally resolved second-order single-shot autocorrelation trace, is obtained.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages, aims and particular features of the present disclosure will become apparent from the following description, which is provided by means of a non-limiting explanation, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic representation of a single-shot optical autocorrelator of the prior art;

(3) FIG. 2 schematically shows a single-shot optical autocorrelator device according to a first aspect of the present disclosure;

(4) FIG. 3 is a partial and exploded view of the single-shot optical autocorrelator device of FIG. 2, showing the non-linear crystal, the spectral filtering device and the detector;

(5) FIG. 4 is a screenshot showing an example of a measurement taken with the single-shot optical autocorrelator device of FIG. 2, with the left-hand side of this figure comprising a raw image of a spatially resolved second-order intensimetric single-shot autocorrelation trace and the right-hand side representing this autocorrelation trace after analysis;

(6) FIG. 5 is an exploded view of the raw image shown in FIG. 4;

(7) FIG. 6 is a screenshot showing an example of a measurement taken with a single-shot optical autocorrelator device according to a second aspect of the present disclosure, said autocorrelator device integrating a linearly variable spectral filtering device, the left-hand side of this figure comprising a raw image of a spatially resolved FROG trace, as well as an image of a FROG trace reconstructed by computations using an iterative algorithm, with the right-hand side of this figure representing various curves originating from the analysis of this FROG trace by means of the iterative algorithm;

(8) FIG. 7 is an exploded view of the raw image illustrated in FIG. 6;

(9) FIG. 8 illustrates a partial and exploded view of a single-shot optical autocorrelator device according to a second aspect of the disclosure, the spectral device having been designed to be placed as close as possible to the detection plane of the detector;

(10) FIG. 9 schematically shows a single-shot optical autocorrelator device according to a third aspect of the disclosure, said device being equipped with an imaging spectrometer.

DETAILED DESCRIPTION

(11) Firstly, it is to be noted that the figures are not to scale.

(12) FIGS. 2 and 3 schematically show a single-shot optical autocorrelator device 20 according to a first aspect of the present disclosure.

(13) This single-shot optical autocorrelator device 20 allows the duration of an ultrashort laser pulse to be measured on the basis of the detection of an intensimetric autocorrelation.

(14) Hereafter, an ultrashort laser pulse with fundamental frequency (ω) will be considered, such as a laser pulse generated by a femtosecond laser source.

(15) This autocorrelator device 20 is made up of only the following components: a polarity separator 21; a non-linear crystal 22 allowing type-II phase matching; a spectral filtering device 23; and a detector 24 having a detection plane 31.

(16) This detector 24 is advantageously connected to a processing unit 25 comprising a processor and, preferably, a display means, such as a screen, for displaying the data processed by said processing unit 25.

(17) The elements of this autocorrelator device 20 are mounted in line whilst being accommodated in a casing. The assembly formed by the polarity separator 21, the type-II non-linear crystal 22 and the spectral filter 23 is contiguous. The detector 24 is advantageously placed in the immediate vicinity of the spectral filter 23, which makes the assembly highly compact. Furthermore, by thus placing the detection plane 31 as close as possible to the output face of the type-II non-linear crystal 22, it ensures that the resolution of the autocorrelator device is maintained.

(18) The incident beam 26 advantageously has intensity distribution exhibiting axial symmetry relative to the optical axis 27 of propagation of this beam 26.

(19) The polarity separator 21, which in this case is a Wollaston prism, receives the incident beam 26 and angularly separates this beam into two laser radiation beams 28, 29 with fundamental frequency (ω) and with orthogonal linear polarities between them.

(20) At the output of this separator 21, a first beam 28 propagates in a first direction that is inclined relative to the optical axis 27 of the incident beam 26 and a second beam 29 propagates in a second direction that is inclined relative to the optical axis 27 of the incident beam 26. The first and second directions are symmetrically inclined relative to the optical axis 27.

(21) This polarity separator 21 in this case is configured so that the non-zero angle α formed between the two beams 28, 29 thus generated ensures the at least partial overlapping of these beams 28, 29 in the type-II non-linear optical crystal 22 that is contiguous with the output face of the polarity separator 21.

(22) The beams 28, 29 thus generated are called the replicas of the incident beam 26, or even replicated beams.

(23) This type-II non-linear crystal 22 is configured to ensure frequency doubling of the beam at the fundamental frequency ω. This non-linear optical crystal 22 is, for example, a BBO crystal designed with a phase matching angle θ=42.4°, which allows frequency doubling to be generated for a fundamental frequency ω corresponding to a wavelength λ of 800 nm.

(24) At the output of the type-II non-linear optical crystal 22 an autocorrelation trace of the frequency-doubled incident laser pulse is obtained, which is also called second-order autocorrelation trace with an optical frequency 2ω. This autocorrelation trace 30 propagates along the optical axis 27 of the incident beam 26.

(25) With the two replicated beams 28, 29 at the fundamental optical frequency ω also being present at the output of the type-II non-linear optical crystal, a spectral filter 23 is placed between this type-II non-linear optical crystal 22 and the detector 24 to filter these two replicated beams and to only allow the passage of said laser radiation beam 30 with harmonic frequency (2ω).

(26) This spectral filter 23 in this case is contiguous with the output face of the type-II non-linear optical crystal 22. By way of an example, this spectral filter in this case is formed by a colored filter, such as a BG40 type colored glass.

(27) The image detector 24 thus only receives the second-order autocorrelation trace propagating along the optical axis 27 of the incident laser beam 26, which allows a second-order intensimetric single-shot autocorrelation trace to be measured.

(28) Advantageously, the detector 24 is a spatially resolved camera following two directions (X, Y) transverse to the optical axis 27. This camera preferably is a CCD or CMOS camera operating at a frequency of several ten to several hundred images per second. Of course, it is adapted as a function of the spectral range of the pulse to be measured.

(29) The processing unit 25 connected to the detector 24 processes the second-order intensimetric autocorrelation measurements.

(30) FIGS. 4 and 5 show an example of a measurement taken with the single-shot optical autocorrelator device 20 illustrated in FIG. 2.

(31) The image 40, shown on the left-hand side of FIG. 4 and as an exploded view in FIG. 5, is a raw image of a spatially resolved second-order intensimetric autocorrelation trace.

(32) The abscissa axis represents the axis of time (t) and the ordinate axis represents the diameter (CD) of the measured incident laser beam. The incident laser beam has a Gaussian shape in its spatial dimension as well as in its temporal dimension.

(33) The curve 41 shown on the right-hand side of FIG. 4 is a representation of the integration of the autocorrelation trace thus imaged over its entire diameter, with the abscissa axis representing the axis of time (t) and the ordinate axis representing the measured intensity.

(34) On the basis of the curve 41 that is thus obtained, and following analysis, the duration of the incident laser pulse is determined, which in this case is 310 fs, for a “Gaussian duration”.

(35) FIGS. 6 and 7 illustrate an example of a FROG trace, or spectrally resolved second-order single-shot autocorrelation trace, obtained with an autocorrelator device comprising a linearly variable spectral filter.

(36) FIG. 6 shows a screenshot obtained on the processing unit 25, with the left-hand side of this figure comprising a raw image 50 of a spatially resolved FROG trace, as well as a simulated image 51 of this FROG trace obtained by an iterative algorithm.

(37) This iterative algorithm allows the physical parameters of the incident laser pulse to be determined, whilst ensuring the convergence of the simulated image 51 toward the experimentally acquired raw image 50.

(38) Various curves are shown on the right-hand side of this FIG. 6 that originate from the analysis of this FROG trace by means of the iterative algorithm.

(39) In FIG. 7, which is an exploded view of the raw image 50 of FIG. 6, the abscissa axis represents the axis of time (t) and the ordinate axis represents the spectral axis (wavelength A).

(40) It can be seen that the linearly variable spectral filter has converted the spatial axis into a spectral axis. A specific wavelength is therefore transmitted as a function of the spatial position.

(41) Such a FROG trace thus provides a time/spectrum map, which allows, via the iterative algorithm, the temporal profile of the pulse, the spectral phase and the fundamental spectrum (w) to be found.

(42) Such information relating to the incident laser beam would not be accessible with an intensimetric single-shot autocorrelation trace.

(43) The FROG trace allows the operator to determine the parameters required to reduce the duration of the pulse.

(44) FIG. 8 is a partial and exploded view of a single-shot optical autocorrelator device according to a second aspect of the disclosure. The elements of FIG. 8 using the same reference signs as those of FIGS. 2 and 3 represent the same objects, which will not be described again hereafter.

(45) The autocorrelator device of FIG. 8 differs from that shown in FIGS. 2 and 3 in that the output face of the spectral device 23 has been designed to place said device as close as possible to the detection plane 31 of the detector 24.

(46) However, this output face of the spectral filtering device 23 is not placed directly in contact with this detection plane 31, in this case formed by the external surface of the detection module, to prevent any damage thereto. In this case, the spectral filtering device 23 replaces the protection window of the detection device (CMOS or CCD sensor). With the window forming part of the one-piece manufacture of this type of sensor, a method for removing the window without damaging the sensor has been developed.

(47) FIG. 9 schematically shows a single-shot optical autocorrelator device equipped with an imaging spectrometer. The elements of FIG. 9 using the same reference signs as those of FIGS. 2 and 3 represent the same objects, which will not be described again hereafter.

(48) This imaging spectrometer comprises an inlet slit 60, a first lens 61, a spectrally dispersive optical system 62, a second lens 63 and a spatially resolved two-dimensional detector 64.

(49) By way of an example, the spectrally dispersive optical system 62 comprises a transmission diffraction grating.

(50) The output face of the type-II non-linear crystal 22 is advantageously placed in the immediate vicinity of the inlet slit 60 of the imaging spectrometer.