Classifications

• • 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
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/10038—Amplitude control
• • 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
  • H01S3/0057—Temporal shaping, e.g. pulse compression, frequency chirping
• • 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
  • H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
  • H01S3/2308—Amplifier arrangements, e.g. MOPA
  • H01S3/2316—Cascaded amplifiers

Definitions

• the present invention relates to a frequency drift amplification device for an intense pulsed laser, using the so-called frequency drift amplification technology.
• This technology is used to produce laser pulses of very short duration, for example of the order of a few femtoseconds, and very high peak power.
• Pulse lasers make it possible to reach large instantaneous powers for a very short time, of the order of a few picoseconds (10 "12 s) or a few femtoseconds (10 " 15 s).
• an ultra-short laser pulse is generated in an oscillator before being amplified in an amplifying medium.
• the laser pulse initially produced, even of low energy, generates a great instantaneous power since the energy of the pulse is delivered in an extremely short time.
• Mourou "Compression of amplified chirped optical pulses," (Common Opt 56, 219-221 1985) uses a spectral decomposition of the pulse, making it possible to impose a path of a different length at different wavelengths to shift them temporally.
• FIG. 1 schematically represents the amplification of a laser pulse by this frequency drift amplification method.
• An oscillator 1 emits a laser pulse 91, called an input pulse, of very short duration ⁇ , for example 10 femtoseconds, and relatively low energy E, for example of the order of a few nanoj ounces.
• This input pulse 91 passes through a stretcher 2 which distributes over time the different spectral components as a function of their wavelength.
• FIG. 3 thus represents the arrangement of a stretcher 2 implementing diffraction gratings reflecting the incident light rays with a different orientation according to the wavelength.
• the structure of such a stretcher is described in particular in the article by O.E. Martinez, "3000 times grating compressor with positive velocity dispersion: application to fiber compensation in 1.3-1.6 ⁇ region.” (IEEE Journal of Quantum Electronics, Vol.e-23, pp. 59, 1987.)
• the input pulse 91 is sent to a first network 21 which disperses it spectrally.
• a first network 21 which disperses it spectrally.
• the beam composed by the spectral components, in particular 911, 912 and 913, constituting the laser pulse then passes through an optical system 22 having the effect of converging these different optical components.
• the optical system 22 has a first focal point F1 placed at the rear of the network 21 and a second focal point F '1 placed behind a second network 23, placed at the same distance from the focus F' 1 as the network 21 of home Fl.
• the different spectral components of the laser pulse are returned by this second grating 23 parallel to each other and spread spatially towards a third network 24, symmetrical with the grating 23, which disperses the pulse towards the optical system 25, symmetrical with the optical system 22.
• This optical system 25 having the focal points F2 and F '2 focuses all the spectral components, in particular 911, 912 and 913 on the same point of a fourth network 26, symmetrical of the network 21, which returns all the spectral components in the same direction to form a new laser pulse 92.
• the subsets formed on the one hand by the grating 21, the optical system 22 and the grating 23 and, on the other hand, by the grating 24, the optical system 25 and the grating 26, are symmetrical to each other. 'other. It is therefore possible, according to a conventional embodiment, to use only one of these subassemblies to form the stretcher by placing a dihedral fold between the networks 23 and 24.
• the networks 21 and 23 and the optical system 22 can then respectively play the role of the networks 26 and 24 and the optical system 25.
• the different spectral components, notably 911, 912 and 913, forming the input pulse 91 do not travel the same path in the stretcher 2.
• This difference in path length causes a time shift of the spectral components as a function of their wavelength in the pulse 92, which is called the stretched pulse.
• This stretched pulse 92 consequently has a longer duration than the duration ⁇ of the input pulse 91, which can be for example of the order of 10 5 ⁇ . This greater duration causes a very significant decrease in the instantaneous power of this pulse 92 relative to that of the input pulse 91, which allows its amplification under better conditions.
• Another method used to stretch laser pulses is the propagation of these pulses in optical fibers over long distances.
• the group velocity dispersion of the spectral components of the pulse in the material at the heart of the fiber makes it possible to obtain the desired time elongation.
• This solution is preferably used for relatively long pulses. Indeed, during compression by a compression system implementing diffraction gratings of a very short pulse and stretched, aberrations due to different dispersion laws networks and optical fibers may appear.
• Yet another known method of stretching consists of a Bragg diffraction grating made of a photosensitive material whose pitch is not constant according to the thickness.
• the different spectral components of the laser pulse are then reflected at different depths, which creates a delay in some of the spectral components compared to others and thus stretches the impulse.
• the stretched pulse 92 emerging from the stretcher 2 is then amplified using conventional amplifying media, which increase its power.
• conventional amplifying media which increase its power.
• three amplifying media are shown in FIG.
• the first amplifying medium 3 increases the power of the stretched pulse 92 to give it an energy of the order of 10 6 times the energy E of the incident pulse 91, for example a few millij ou ⁇ es.
• the second amplifying medium 4 and the third amplifying medium 5 each increase the power of the laser pulse so that the amplified stretched pulse 93 has an energy of the order of 10 10 times the energy E of the pulse. input 91, for example 25 joules.
• the pulse has a relatively high energy, its duration is relatively long, so that its peak power is sufficiently low to avoid nonlinear effects in the amplifying media 3, 4 and 5.
• the amplification in the amplifiers 3, 4 and 5 requires a large pumping energy input in these amplifiers. Indeed, the energy gained by the laser pulse during its passage in an amplifying medium is only about 45% of the pumping energy supplied to this amplifying medium.
• the amplifying medium used is most often a stimulated emission amplifying medium, such as, for example, a doped titanium-sapphire crystal.
• the amplification of the laser pulse can be carried out according to a method commonly known in English as "Optical Parametric Chirped Pulse Amplification", which combines the parametric laser pulse amplification with the technique of drift amplification. frequency.
• the amplification of the stretched pulse is made in a material having significant non-linear properties, for example "KDP" (Potassium Dihydrogen Phosphate) "BBO” (Beta Barium Borate) or “LBO” type crystals. (Lithium Triborate).
• the amplification consists of a transfer of energy from the photons of the optical pumping pulse to the photons of the pulse to be amplified.
• the wave vectors of the amplified pulse and the optical pumping pulse must therefore be in phase agreement, and the two pulses must be synchronous.
• Amplifiers employing stimulated emission amplification or parametric amplification are indifferently referred to as "amplifying media" in the rest of the patent application.
• the return of the pulse to a short duration, close to the duration ⁇ of the input pulse, is performed by an optical device called compressor 6 comprising four diffraction gratings 61, 62, 63 and 64 reflecting the light rays. incidents with a different orientation depending on the wavelength.
• a first grating 61 disperses spectrally the stretched pulse 93.
• the three radii 911, 912 and 913, corresponding to two extreme wavelengths of the pulse 910 and at a median wavelength are shown in Figure 1.
• a second grating 62 parallels the spectral components, in particular 911, 912 and 913, constituting the laser pulse, which are thus spatially spread.
• the third network 63 makes it possible to gather these different spectral components on the same point of the fourth network 64, which returns all these spectral components, in particular 911, 912 and 913, in the same direction, to form a new laser pulse 94.
• the compressor 6 is constructed so that the spectral components that have a longer path in the 2, this path length difference causes a time shift of the spectral components as a function of their wavelength opposite to the spectral shift generated by the stretcher 2,
• the spectral components that were temporally delayed in the pulse 92 or 93 catch up, so that all the spectral components are collected temporally in an output pulse 94 having a duration similar to the duration ⁇ of the pulse.
• input 91 for example 20 femtoseconds, and a very large peak power, for example of the order of 10 14 W.
• the different diffraction gratings 61, 62, 63 and 64 composing the compressor 6 each have an energy efficiency in the limited dispersive order, for example of the order of 90%.
• the passage of the pulse 93 by these four networks leads therefore a significant loss of energy. For example, if the energy of the pulse 93 before compression is 25 Joules, the energy of the output pulse 94 may be about 15 Joules.
• the technique of amplification by frequency drift thus allows the production of laser pulses of very high instantaneous power, but generates very high energy loss.
• the present invention aims to overcome these disadvantages of the prior art.
• the object of the invention is to increase the energy efficiency of the frequency-drift laser pulse amplification, so as to make it possible to obtain a high-energy laser pulse with a lower pumping energy.
• the object of the invention is therefore to obtain pulses having the same energy as in the prior art with less important and less expensive installations and less energy consumption.
• Another objective of the invention is to enable the obtaining of laser pulses presenting an energy higher than those obtained in the prior art, without increasing the pumping power, and therefore the size and cost of pumping installations.
• a frequency drift amplification device for a pulse laser comprising successively:
• a stretcher capable of temporally stretching an incident laser pulse
• At least one amplifying medium capable of amplifying the stretched laser pulse capable of amplifying the stretched laser pulse
• a compressor capable of temporally compressing the stretched and amplified laser pulse
• the compressor comprises an amplifying medium, so as to amplify the laser pulse partially temporally compressed.
• the compressor elements placed before the amplifier placed in the compressor must withstand a much lower energy pulse than if the pulse was fully amplified before compression.
• the amplifying medium placed in the compressor is placed at a position where the duration of the laser pulse is substantially half the duration of the pulse entering the compressor.
• the amplifying medium is thus placed between two subsets of the compressor, each performing half of the time compression of the previously strongly stretched pulse.
• the pulse is, at this position, spread spatially.
• the compressor comprises four successive dispersive systems and the amplifying medium which is placed between the second and the third dispersive system.
• the dispersive systems are dispersion networks.
• the first and second dispersive systems of the compressor are placed in the open air, and the third and fourth dispersive systems of the compressor are placed in a vacuum chamber.
• Such a compressor is easier to implement and cheaper than the compressors of the prior art to fully resist the significant energy of a fully amplified pulse before compression, all elements of which should be contained in a chamber. empty.
• the stretcher implements at least one dispersion network.
• the amplifying medium placed in the compressor is constituted by a doped crystal allowing amplification by stimulated emission.
• the amplifying medium placed in the compressor has a doping gradient, so that the different spectral components comprising the laser pulse pass through portions of the amplifying medium having different doping levels.
• At least one of the amplifying media used is constituted by a nonlinear crystal allowing a parametric amplification of the laser pulse.
• the invention also relates to an optical compressor capable of temporally compressing a previously stretched laser pulse, characterized in that it comprises an amplifying medium, so as to amplify the laser pulse partially compressed in time.
• FIG. 1 is a simplified diagram of an amplification device with frequency drift of a laser pulse, according to the prior art
• FIG. 2 is a simplified diagram of an amplification device with frequency drift of a laser pulse according to one embodiment of the invention
• FIG. 3 is a simplified diagram of a stretcher used for frequency drift amplification of a laser pulse.
• FIG. 2 schematically represents a frequency drift amplification device according to one embodiment of the invention.
• the elements of this amplification device which are identical to that of the prior art described in FIG. 1 bear the same references.
• an oscillator 1 emits an input laser pulse 91 which passes through a stretcher 2.
• the pulse 92 stretched temporally, coming out of the stretcher 2, can pass through one or more amplifying media 2 and 3.
• the modification provided by the present invention relates to the compressor 7.
• This compressor comprises, like the compressor 6 of the prior art, four diffraction gratings 71, 72, 73 and 74 having respectively the same roles as the networks 61, 62, 63 and 64 of the prior art.
• an amplifying medium 8 is placed in the compressor 7, between the second diffraction grating 72 and the third diffraction grating 73 constituting this compressor. This amplifier could compensate the energy lost in the diffraction gratings 71 and 72.
• the pulse 96 passing through this amplifying medium 8 has different characteristics of the pulse 95 leaving the amplifier 4 and entering the compressor 7, because of its passage through the first two diffraction gratings 71 and 72. has a duration about half of the duration of the pulse 95, for example of the order of 250 picoseconds if the duration of the pulse 95 is of the order of 500 picoseconds. Moreover, this pulse 96 is spread spatially, the shortest wavelengths being on one side and the longer wavelengths on the other.
• the passage of the pulse 96 in the amplifying medium 8 produces the amplified pulse 97 having the same characteristics of temporal stretching and spatial spreading as the pulse 96.
• This pulse 96 then continues its compression through the networks 73 and 74, spatially recompressing the pulse and completing its compression time, to form the output pulse 98 of short duration and high peak power. Due to the middle position of the amplifier 8 in the compressor, the pulse 97 exiting this amplifying medium undergoes, during its passage through the diffraction gratings 73 and 74, a lower energy loss than if it were passed through the four networks forming the compressor.
• the energy loss of the pulse due to the passage through the two gratings 73 and 74 is 19%.
• obtaining an output pulse 98 of Joules requires a pulse 97 at the output of the amplifying medium 8 of about 18.5 Joules.
• the energy loss of the order of 19% due to the passage of the pulse by the two networks 71 and 72 is very low, of the order of 0.5 joules, because of the low energy of the front beam its passage in the amplifying medium.
• the amplifying media having an energy efficiency of the order of 45%, the total pumping energy to be supplied to these amplifying media is of the order of 40 Joules.
• the compressor 6 may have a structure slightly different from that described. It is possible, for example, in a particular embodiment, for the subsets formed on the one hand by the networks 71 and 72 and, on the other hand, by the networks 73 and 74, to be folded in a conventional manner, putting in a dihedral fold, so that only one subset is traveled twice by the laser pulse.
• the two networks 71 and 72 may have a smaller dimension than the networks 61 and 62 of the compressor of the prior art. It is therefore possible to use less expensive networks, and under more flexible conditions. It is for example conceivable for these two networks 71 and 72 to be in the open air, whereas all the networks composing the compressors of the prior art were to be placed in a vacuum chamber.
• the wavelengths forming this pulse are spatially distributed according to their wavelength.
• the amplifying medium 8 can offer constant amplification in all points, which is obtained when the doping is radially uniform in the crystal.
• the amplifying medium it is possible to implement a variable amplification according to the passage position of each component of the laser pulse in the amplifying medium.
• This different amplification can be done, for example, with an amplifying medium having a doping gradient in a direction perpendicular to the direction of passage of the laser pulse.
• Such a doping gradient exists naturally, for example, in sapphire titanium crystals. It is possible, if necessary, to accentuate this natural radial doping gradient, for example by using large crystals (for example greater than 80 mm in diameter). The doping is then weaker in the center than at the edge of the crystal which generates a less important energy storage in the center than on the edges and thus a lower potential gain in the center.
• This amplification varies according to the spatial position, makes it possible to implement, for spatially spread laser pulses as a function of the wavelength that pass through the amplifying medium 8, a variable amplification as a function of the wavelength.
• the spectral gain of the pulse can indeed be greater for the wavelengths passing in the center of the crystal which is more heavily doped.
• Such a different amplification for the different spectral components of the pulse can be implemented in all cases where the spectral components of the laser pulse are spatially spread. It may be useful, for example, to compensate for the difference in gain of a laser pulse in a titanium sapphire crystal as a function of wavelengths.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Optics & Photonics (AREA)
• Lasers (AREA)
• Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

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• 2010
  • 2010-08-03 FR FR1056414A patent/FR2963707B1/fr not_active Expired - Fee Related
• 2011
  • 2011-08-02 EP EP11755395.8A patent/EP2601713A2/de not_active Withdrawn
  • 2011-08-02 WO PCT/FR2011/051861 patent/WO2012017179A2/fr not_active Ceased
  • 2011-08-02 JP JP2013522284A patent/JP2013535835A/ja active Pending
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