Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J11/00—Measuring the characteristics of individual optical pulses or of optical pulse trains
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/0014—Monitoring arrangements not otherwise provided for
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
  • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation

Definitions

• the field of the invention relates to optical self-correlating devices for characterizing laser pulses. More specifically, the field of the invention relates to devices for characterizing lasers in the infrared spectral band and neighboring infrared bands. Finally, the field of the invention relates to methods and devices for measuring the duration of a femtosecond laser pulse. STATE OF THE ART
• the first category incorporate delay lines, also called delay line, so as to create from a laser pulse two sub pulses.
• This solution is complex and requires a configuration difficult to implement, for example, in terms of optical alignment and mechanical design.
• these devices do not make it possible to perform so-called “single-shot” measurements to characterize a laser pulse, that is to say based on the analysis of a single pulse.
• so-called single-shot devices allow such a measurement to be dispensed with by the use of a delay line. In these devices, the analysis of the temporal characteristics of the pulse are reported in the spatial domain and then deduced from a spatial measurement.
• the measurement of a pulse duration generally requires the creation of a nonlinear effect producing traces on a detector to deduce the time characteristics of the pulse.
• a widespread method is the use of a non-linear crystal that generates a quadratic optical signal in intensity and thus produces a non-linear optical effect which is then converted into an electrical signal by a detector.
• This method is generally based on the generation of a second order harmonic. This phenomenon is also known as a two-photon light emission phenomenon.
• the principle of such a method for generating non-linearity is to modify the color of an incident beam.
• non-linear crystal nevertheless remains expensive.
• the non-linear crystal is, moreover, easily subject to damage.
• the use of nonlinear crystals reduces the spectral range of use and complicates the devices to be implemented.
• the self-correlating devices give a partial characterization of a laser pulse. Generally, they do not allow to estimate the spectrum of the pulse and the temporal and spectral phases.
• the current devices are often complex and they do not make it possible to extract all the data making it possible to characterize a laser pulse.
• An object of the invention comprises producing a nonlinear effect by absorbing two photons directly into a detector from beam interferences separated by a Fresnel bi-prism, the detector generating and then detecting the effect. non-linear. Mathematical functions then make it possible to process the resulting image of the nonlinear effect in the frequency domain.
• An object of the invention relates to a device for the characterization of a laser pulse.
• the device comprises:
• a sensor comprising at least one semiconductor line sensor disposed in an overlap region in which the split beams interfere and generate a trace by absorption of two photons at least two beams separated by the Fresnel biprism.
• the generated trace includes information characterizing the laser pulse (Pu). Different methods allow the analysis of the generated trace to deduce parameters from the laser pulse.
• a computer processes a signal from the detector produced by the generation of the trace to deduce a laser pulse duration.
• One advantage is to offer a device with great compactness and simplicity of implementation.
• the device avoids the use of an expensive and easily damaged nonlinear crystal.
• the Fresnel bi-prism separates the incident pulse into two identical sub-pulses recombining spatially to form a trace on each pixel of the detector, the signal detected by each pixel being dependent on the delay between the two pulses. interfering on each of the pixels.
• an optimized position of the detector with respect to the position of the bi-prism is calculated according to the Apex and refractive index of the Fresnel bi-prism, the optimization of the position making it possible to generate a trace on the detector whose accessible time range is optimized.
• the device of the invention comprises an optical system for producing:
• ⁇ a beam homogenizing function in a direction perpendicular to the edge of the Fresnel bi-prism and / or;
• the first function makes it possible to generate an extended laser beam at least in the direction transverse to the edge of the bi-prism and spatially homogeneous at the input of the Fresnel bi-prism from a laser source.
• the optical system comprises a telescope for performing this homogenization function.
• One advantage is to obtain a uniform illumination of the incident beam on the upstream side of the bi-prism and homogeneous spatially at the entrance of the Fresnel bi-prism.
• the optical system TSP such as a telescope, includes:
• a divergent cylindrical mirror and a convergent cylindrical mirror for example Ag or Au;
• One advantage is that the optical system TSP to homogenize the beam allows in particular an optimal use of the useful surface of the bi-prism.
• the device of the invention also comprises an optical system LC for focusing the beam in a direction parallel to the edge of the Fresnel bi-prism in the plane of the detector so as to to increase the sensitivity of the device.
• An optical system is used to focus the beam in a direction parallel to the edge of the Fresnel bi-prism in the plane of the detector to increase the sensitivity of the device.
• the optical system for focusing LC comprises: either a cylindrical lens or a cylindrical mirror.
• the optical system for harmonization such as a telescope, may be used in conjunction with an optical system to focus the beam in a direction parallel to the edge of the bi-prism.
• the two optical systems TSP and LC can be combined in a single optical system making it possible both to perform the function of homogenizing the beam in a direction perpendicular to the edge of the Fresnel bi-prism and perform a function of focusing the beam in a direction parallel to the edge of the Fresnel bi-prism.
• An example is an optical system comprising a diverging cylindrical mirror and a convergent spherical mirror.
• the spherical mirror allowing the function of the cylindrical mirror in one dimension to achieve homogenization in the direction perpendicular to the edge of the bi-prism and focusing in a direction parallel to the axis of the edge of the Fresnel bi-prism .
• This embodiment makes it possible to obtain a more compact device.
• the detector comprises at least: A line of pixels;
• One advantage is to have a detector that performs the following two functions: the generation of the non-linear effect and the detection of the non-linear effect thus generated.
• the detector is a CCD or CMOS camera and the semiconductor material is silicon or InGaAs.
• a filter is associated with the detector so as to filter the frequencies producing a linear effect on the detector.
• the calculator performs:
• ⁇ filter in the frequency domain of the dimensional Fourier transform so as to identify contributions from at least one experimental oscillating function at different frequencies and the resultant effect obtained two photons on the detector;
• the spectral composition of the pulse is determined from the Fourier transform of at least one experimental oscillating function.
• Another subject of the invention relates to a method for the characterization of a femtosecond laser pulse, characterized in that it comprises: ⁇ An acquisition of an image of a sensor produced by absorption of two photons from beams of interference emitted during a femtosecond laser pulse, said beams being separated by a Fresnel biprism;
• ⁇ filter in the frequency domain at least a resulting line of the Fourier Transform so as to identify at least a contribution of at least one experimental oscillating function of the resulting image formed of the effect obtained on two-photon the detector;
• ⁇ at least one inverse transform of at least a filtered Fourier transform of at least one filtered oscillating function defining at least an experimental feature
• ⁇ a computing at least one theoretical oscillating function from a theoretical model of the laser pulse
• An advantage of the method of the invention is that it makes it possible to perform mathematical operations from the image produced to characterize the laser pulse. Another advantage is the fact of requiring a simple parameterization. In particular, the calculation of theoretical functions can be performed with predefined pulse profiles. Another advantage is that the process steps can be automatically generated so as to provide a result as soon as the image is produced on the detector.
• the filtering step is optimized from optical parameters comprising at least the Apex angle of the biprism, the detector sensitivity band, the two-photon absorption band of the detector, the pixel size of the detector.
• these parameters can be preconfigured in an interface so as to make the process totally automatic and independent of an intervention.
• Another advantage is to allow a great flexibility of change or modification of the device. For example, when another bi-prism is chosen, only a few parameters can be redefined.
• the theoretical model comprises the definition of a hypothesis of the shape of the laser pulse field profile.
• the adjustment step comprises determining a pair of parameters of at least two theoretical oscillating functions, said pair of parameters determining an optimum of likelihood between the theoretical functions and the experimental functions.
• One advantage is to calculate an optimum according to two parameters, which makes the adjustment step particularly efficient.
• FIG. 1 an optical assembly according to the invention to make interfering signals separated by a Fresnel bi-prism to obtain a mark on a detector by two-photon absorption;
• ⁇ Figure 2 an optical system for obtaining a collimated laser beam, lying at least in the direction normal to the edge of the Fresnel biprism;
• Figures 3A, 3B a linear optical detector comprising a set of pixels before and after the application of the laser beam;
• the invention relates to a device for the characterization of a femtosecond laser pulse also called "optical autocorrelator".
• the device of the invention makes it possible to measure ultra-short laser pulses ranging from a few femtoseconds to several hundred femtoseconds.
• the device of the invention makes it possible, on the one hand, to create two identical sub-pulses variably offset in time from a Fresnei bi-prism and, on the other hand, to observe a non-linear effect induced by a two-photon absorption effect by combining these two sub-pulses on a detector comprising a semiconductor.
• FIG. 1 represents a Fresnei bi-prism, denoted BPF, which is arranged in the optical axis of a laser beam to separate an incident beam Fsi into emerging beams F se .
• BPF Fresnei bi-prism
• the emerging beams then interfere in an interference zone, denoted Zi, also called emergent beam overlap zone F se .
• the Fresnei BPF bi-prism comprises an upstream face F am occurring in a plane perpendicular to the optical axis, that is to say perpendicular to the incident beam Fsi emitted by the laser.
• the Fresnei BPF biprism has a downstream face F av comprising different inclined portions. Each inclined portion of the downstream face is oriented at an angle defined by the Apex A Fresnei BPF bi-prism which makes it possible to modify the exit angle of the emerging beam F se .
• the beams In the upper part of the downstream face F av of the Fresnei BPF bi-prism, the beams converge at an angle of - ⁇ with respect to the optical axis and in the lower part of the downstream face F av of the Fresnei bi-prism BPF, the beams converge at an angle of ⁇ with respect to the optical axis.
• An example of a Fresnel BPF bi-prism that can be used is a bi-prism in fused silica or NaCl, BaF2, CaF2.
• the Fresnel BPF bi-prism in the configuration of the invention is an optical element making it possible to generate two sub-pulses, that is to say two replicas of the pulse whose duration is to be measured, intersecting with an angle 2 ⁇ with respect to the initial propagation axis of the incident beam Fsi.
• n is the refractive index of Fresnel BPF bi-prism at the wavelength considered.
• the incident beam F si is then uniformly distributed on the upstream face F am of the Fresnel BPF bi-prism.
• the device of the invention comprises an optimal configuration for which the generation and detection of a linear effect are maximum on a detector disposed downstream of the Fresnel BPF bi-prism.
• the optimal distance "d" at which an AC detector can be placed to generate a non-linear effect with the greatest amplitude by combining two emerging beams can be expressed in the following analytical form: with H the half-height of the Fresnel BPF bi-prism.
• This time range is determined by integrating, for all the heights x, the value of the delay r between the two sub-pulses. The size of an optimal linear detector is thus obtained for a length of L detector.
• a CAM detector is arranged in the interference zone Zi to detect the interferometry traces of the recombinant beams.
• the Fresnel BPF bi-prism allows, by the creation of two sub-pulses, to transpose the temporal properties of the emerging beams of the Fresnel BPF bi-prism into spatial information by generating interferometry traces on the pixels.
• a CAM detector a CAM detector.
• Fresnel BPF bi-prism An interest in using a Fresnel BPF bi-prism is to obtain a compact and space-saving device.
• the invention however relates to other elements whose function is to separate a beam into two crossed beams traveling on average over the same distance.
• the use of two mirrors oriented with an angle equivalent to that of the angle ⁇ of the emerging beams of the Fresnel BPF bi-prism and forming a reflective system can be used to perform the same functions as the Fresnel BPF bi-prism.
• the Fresnel BPF bi-prism makes it possible to associate, at each pixel of a line of the detector CAM, a different delay between the two beams, the temporal increment between two pixels being constant.
• the two beams incident on the pixel then produce a nonlinear effect by two-photon absorption.
• An advantage of the device and method of the invention is to create a relationship between the spatial intensity generated on each pixel relative to the delay of two beams interfering at the CAM detector. Optical editing of upstream formatting
• optical system arranged upstream of the bi-prism along the optical axis may be associated with the latter.
• optical system DO which comprises the two upstream formatting systems TSP and LC, more generally the optical system which includes the functions of beam harmonization and focusing of the beam.
• FIG. 2 represents an exemplary case illustrating such an assembly intended to standardize the illumination of the upstream face F am of the Fresnel BPF bi-prism.
• Such an assembly can be achieved by a TSP optical telescope.
• the optical telescope TSP comprises at least two mirrors or spherical or cylindrical lenses Md and Me, the first of which is divergent Md and the second is convergent Me.
• the focal length of the diverging mirror M d can be chosen substantially equal to 5cm and the focal length of the convergent mirror M c can be chosen substantially equal to 20cm.
• the optional LC lens allows to focus the beam in the direction parallel to the edge of the bi-prism in order to locally increase the light intensity in a few lines.
• One advantage is to obtain a non-linear signal recorded on the detector covering the entire dynamics of the latter and improves the sensitivity of the device.
• FIG. 2 shows a beam F sg coming from a laser whose impulse is to be characterized, the beam F ss coming out of the TSP telescope passes through an LC lens and intercepts the upstream face F am of the Fresnel BPF bi-prism by a incident beam F SI .
• Another more compact optical system for performing the beam harmonization function and the focusing function can be proposed by using a spherical mirror or a spherical lens of the telescope avoiding the use of the cylindrical lens LC.
• the device of the invention is an autocorrelator to the extent that the autocorrelation produced is called "interferometric".
• the traces produced on the CAM detector of the invention form an interferogram recorded by interferometric autocorrelation. This interferometric autocorrelation is based on two-photon absorption.
• the two-photon absorption phenomenon is a phenomenon different from that of two-photon light emission which is, for example, produced with a nonlinear crystal.
• FIG. 4 represents a schematic diagram of the two-photon absorption mechanism in which the energy bands of the semiconductor material are represented as a function of the electron wave vector k.
• the conduction band Bc and the valence band Bv are separated by a Gap noted G.
• the silicon Gap is about 1120 nm.
• the detector is not subjected to radiations whose wavelength belongs to its range of sensitivity, for example by carrying out the process in the dark, that is to say ie without visible light, either by the use of a filter.
• the detector in the connected band and greater than the sensitivity band of the detector, that is to say for silicon, the band of [1200-2200 nm], the detector then produces a photo-current only due to the nonlinear two-photon absorption phenomenon.
• the sensitivity band of a detector corresponding to the detection of linear effects
• the two-photon band of a detector corresponding to the detection of non-linear effects.
• the photocurrent recorded is then only due in this zone to absorption at 2 photons, ie the simultaneous absorption of 2 photons.
• the photo-current l ph is then proportional to:
• an advantage is to be able to generate the non-linear phenomenon directly on the detector since it is by the two-photon absorption phenomenon that the detector itself creates the non-linearity which is exploited. then by the steps of the method of the invention.
• the device of the invention advantageously comprises a detector at least linear, that is to say at least formed of a row of pixels.
• a linear detector comprises a line of pixels, denoted p xi , on which the interferometry traces are formed from the combination of the two pulses created by the Fresnel BPF bi-prism.
• the detector is a matrix of pixels.
• the signal delivered by the detector is designated by the term "image" in the following, unidimensional image in the first case and two-dimensional in the second.
• the size of the pixels is defined so as to allow sufficient resolution to sample the different signal intensities as a function of the delay ⁇ .
• An interesting property of the device of the invention is the generation of a non-linear effect created by the two-photon absorption phenomenon. This phenomenon makes it possible to generate a non-linearity resulting in a luminous intensity created on each pixel p xi of the detector CAM when the two sub-pulses are combined.
• CAM may be Silicon or InGaAs.
• the semiconductor material is chosen and adapted according to the spectral range of the laser that it is desired to characterize. Each semiconductor material comprises a two-photon absorption range.
• the device and the method of the invention thus, it is possible to determine a semiconductor material of the detector that is in the two-photon range according to the frequency range of the laser to be characterized.
• the device of the invention comprises a detector performing two functions. The first function is the generation of a nonlinear effect and the second function is the detection of this effect.
• the optical detector may be configured not to detect the waves in its linear sensitivity range.
• the method of the invention can be implemented without parasitic light source, for example in a dark room in which the only light source is the laser.
• the detector may be coupled to a light filter for filtering and thus to discourage the capture of light waves in its linear sensitivity range.
• the filtering may be selective and let pass only the waves corresponding to the wavelengths of the waves emitted by the laser.
• An interest of the filtering of the light waves is to be freed from a perturbation of the measurements by the capture of linear effects resulting from the detection of the waves in the visible domain.
• the filter may advantageously be configured to filter the waves having a wavelength lower than 1.2 ⁇ m so as to protect the sensor from any visible radiation which would disturb the measurements.
• the objective of such a filtering is to suppress the contribution of the linear signal on the CAM detector which would disturb the measurements of the non-linear effect making it possible to characterize the laser pulse.
• the device of the invention is configured, in one example, to measure the infrared pulse duration of the laser in the two-photon absorption wavelength range of the detector in the band [1.3 ⁇ m; 2.4 pmj. This range corresponds to the absorption zone at two Silicon photons.
• the sensitivity range of the detector is about [400 nm; 1200 nm]
• the method of the invention makes it possible, starting from the two-photon absorption phenomenon, to measure the effect produced by the detector during the capture of photons from the Femtosecond laser.
• the sensitivity range of the detector is from 0.9 ⁇ m to 1.7 ⁇ m.
• the two-photon absorption range of the detector is 1.8 ⁇ m to 3.4 ⁇ m.
• the invention relates to any type of semiconductor material of a detector.
• the properties of the CAM detector are chosen so as to characterize a laser pulse emitted in a given range of wavelengths.
• the operating ranges including the spectral ranges of the laser, the acquisition time windows of the detector, the temporal resolution of the detector, the detector sensitivity and the time range of the laser to be characterized, can be adapted by modifying the Apex angle of the Fresnel BPF bi-prism and the type of CAM detector.
• the traces formed on the CAM detector of the invention produced by interferometric autocorrelation comprise at least three different information items, hereafter designated G 2 , Fi and F 2 , resulting from the nonlinearities generated on the detector. This information can be described as oscillating functions.
• an ad hoc pulse shape i.e. a generated laser pulse profile
• An assumed pulse profile may be, for example, a Gaussian profile. This assumed profile is then used to calculate a theoretical oscillating function.
• the oscillating functions also make it possible to deduce the spectrum of the pulse.
• the method of the invention also makes it possible to deduce the minimum duration of the pulse obtainable from this spectrum.
• the method of the invention therefore makes it possible to determine a frequency slip parameter at the sign close to the spectrum of the laser pulse to be characterized.
• the oscillating function G 2 can also be used independently of the functions Fi and / or F 2 to determine the pulse duration of the laser.
• the oscillating function G 2 can therefore be used to obtain a second value of the pulse. This can be used to check the value obtained with the functions Fi and F 2 . It can also be used to establish an average value of the pulse duration.
• the function G 2 is more sensitive than the functions Fi and F 2 to measurement artifacts related to the spatial inhomogeneities of the laser to be characterized. As a result, the value calculated by the oscillating function G 2 can be obtained in a greater uncertainty range than the values obtained by the functions Fi or F 2 . If we consider a complex E field compressed at the limit of
• the shape of the field is considered Gaussian.
• the acquired signal is a function of the delay ⁇ .
• the delay ⁇ is set for each pixel of the CAM detector.
• the signal strength of each pixel is a combination of the three functions for a given delay ⁇ .
• the functions Fi, F 2 and G 2 are connected to temporal and therefore spatial modulations which are linked to different frequencies.
• the method of the invention makes it possible to apply a one-dimensional Fourier transform of at least one line of the image. Simulation of the complete device
• the method of the invention makes it possible to define the electric field after passing through the Fresnel BPF bi-prism.
• An electric field of the form is considered:
• E (x, t) exp exp [- iat 2 ] ⁇ [x]
• F [x] is the spatial distribution of E.
• Each step of the method of the invention can be carried out by means of a computer to perform the operations necessary to perform each function whether it relates to signal processing operations, image processing or algorithms to deduce one or more values.
• the direct and inverse Fourier transforms can be performed by a computer.
• one or more memories can be used (s) to save data during calculations, or to record values obtained by the method.
• of the Display means can also be used to observe intermediate results of the process, the lines obtained or the values calculated by the method.
• the method comprises a step of generating an image, denoted T1JMG, which aims to quantify the luminous intensity acquired on each pixel.
• the image obtained represents a distribution of intensities of a two-photon signal on each of the pixels.
• Fig. 3A shows a linear detector comprising a pixel line p xi before the laser pulse is emitted.
• FIG. 3B represents the same linear detector after the acquisition of the two-photon signals by each pixel p xi .
• Each pixel Pxi receives a signal intensity corresponding to a delay ⁇ varying on the detector line according to recombinations of emerging beams F se Fresnel BPF bi-prism. It will be understood, in the light of FIG. 3B, that each pixel p xi receives a signal whose intensity varies as a function of the shades of gray represented. The case of Figure 3B is shown without taking into account a particular configuration of the device. b) Fourier Transform
• the method or device of the invention then allows the realization of a one-dimensional Fourier transform (FFT) for all the lines of pixels of the CAM detector.
• FFT Fourier transform
• the Fourier transform makes it possible to pass into the frequency space and to obtain frequency lines. This step is represented by the step FFT 1 D of FIG.
• the Fourier transform of the traces acquired on each pixel can be visualized by means of a display in the form of spectral lines.
• At least three main lines are obtained and correspond to the contributions of oscillating functions Fi, F 2 and G 2 .
• the three lines are obtained for each of the following pulsations:
• the method of the invention comprises a step of filtering each of the lines obtained by the Fourier transforms.
• An advantage of the filtering step makes it possible to isolate the contribution of each of the oscillating functions in the signal acquired by each pixel.
• the filtering can be for example a selective filtering around each line so as to obtain the main contribution of each function
• step F This step is represented in FIG. 5 by step F.
• the filtering step is F is configured according to the optical parameters of the device of the invention.
• these parameters we find: the type of detector, its ranges of sensitivity and absorption at two photons, the apex angle and the size of the pixels.
• These parameters are data that make it possible to configure an optimized filtering of the obtained lines. d) inverse transform
• the method of the invention comprises a step for calculating the inverse Fourier transform of the three filtered frequency responses in the filtering step.
• This step therefore comprises at least one operation which comprises at least one inverse transform (FFT 1 ) of a previously filtered line.
• FFT 1 inverse transform
• the inverse transform function makes it possible to switch back to the time domain.
• the invention makes it possible to determine the inverse Fourier transform of one, two or three lines corresponding to the oscillating functions F 1 , F 2 and G 2 .
• This step therefore comprises the sub-steps noted in FIG. 5: FFT "1 (Fi), FFT " 1 (F 2 ), FFT "1 (G 2 ).
• the method of the invention comprises an adjustment step, denoted COMP ( ⁇ , ⁇ ⁇ ).
• This step aims at adjusting the experimental curves obtained by the inverse Fourier transform with the theoretical functions f 1 , f 2 and g 2 .
• the adjustment function of the method corresponds to a comparison, for example, of an experimental function F1 and of a theoretical function f1 for which certain parameters vary to determine an optimum of the likelihood of the two functions.
• the theoretical functions f1, f2, g2 are determined mathematically by assuming a theoretical time form of the laser pulse.
• a theoretical time form of the laser pulse By way of example, several forms of pulses can be used, for example a Gaussian or hyperbolic form. This form is determined either because it is known or by what it is supposed to be.
• the shape of the pulse can be defined also by another mathematical function.
• the method of the invention therefore comprises the definition of a hypothesis of the shape of the pulse.
• the adjustment step makes it possible to determine the theoretical function most likely to the experimental function obtained after the inverse transform steps. This step makes it possible to deduce the pulse duration and the spectral width.
• the adjustment step comprises a parameter variation defining the theoretical laser pulse that can be performed by an adjustment algorithm. Such an algorithm is known and can be chosen according to a given configuration of the device.
• the adjustment step makes it possible to determine the parameters K and ⁇ ⁇ which are parameters of the analytical form of the theoretical functions f 1 , f 2 and g 2 .
• the adjustment step allows to deduce the best pair ⁇ K, ⁇ ⁇ ⁇ for which the theoretical functions and the experimental functions are closest to each other.
• the adjustment step of the method of the invention makes it possible to adjust the experimental functions Fi and F 2 at the same time.
• the best compromise is sought so that the theoretical functions f1 and f2 are as close as possible to the experimental functions F1 and F2. This mode is particularly advantageous for accurately obtaining a good estimate of the duration of the pulse.
• the Fourier transform of at least one oscillating function makes it possible to deduce the spectrum of the laser.
• the method of the invention also optionally comprises means for storing and displaying the results of the processing of the oscillating functions.
• the results obtained, including the current laser duration, the frequency slip parameters and the minimum duration of the pulse if the frequency slip is compensated, can also be memorized and displayed by means of a memory and a display. .
• An advantage of the invention is that it is applicable to low speed and high energy lasers and high speed and low energy lasers.

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• 2015
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• 2016
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