Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/18—Diffraction gratings
  • G02B5/1861—Reflection gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
  • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
  • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
  • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
  • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
  • G01D5/36—Forming the light into pulses
  • G01D5/38—Forming the light into pulses by diffraction gratings
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/08—Mirrors
  • G02B5/0808—Mirrors having a single reflecting layer

Definitions

• the present invention relates to a method for obtaining a reflective diffraction grating. More specifically, the invention relates to a method for obtaining a dielectric diffraction grating optimized for use in particular conditions.
• the invention also relates to the networks obtained by this method of obtaining.
• the invention relates to obtaining such an optimized network for implementing a spectral dispersion of high power laser beam.
• a diffraction grating is an optical device having periodically spaced grooves. It has a number of diffraction orders depending on the incident wavelength, the angle of incidence and its period. In dispersive orders (different from order 0), the reflection angle depends on the wavelength.
• Diffraction gratings are used in many optical systems, and in particular for the amplification of laser pulses by frequency drift.
• 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 by a laser cavity 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 brief time.
• the diffraction gratings used to implement this method must meet several specific requirements. They must have a very good efficiency reflected in a dispersive order, that is to say reflect a very large proportion of the incident light in a dispersive diffraction order, on a spectral interval corresponding to the spectral interval of the impulse laser to be amplified. Frequency drift amplification also requires diffraction gratings having excellent resistance to laser flux, particularly for the recompression of a laser pulse after its amplification.
• Dielectric networks as indicated by the article by MD Perry, RD Boyd, JA Britten, BW Shore, C. Shannon and L. Li, "High efficiency multilayer dielectric diffraction gratings" (Opt Lett 20, 940 -942 - 1995) exhibit better laser flux performance than metallic type networks and better efficiencies. They consist of a stack of thin dielectric layers placed on a substrate and reflecting up to about 99% of the incident light. The upper surface is etched periodically to obtain the diffraction grating.
• each of the layers of this stack are chosen so as to form a Bragg mirror, or "quarter-wave" mirror, in which high refractive index layers n H are alternated with layers of low refractive index. n L.
• the thicknesses t H and t L, respectively, of the layers of high refractive index n H and of the layers of low refractive index n L are determined by the following relationships:
• - ⁇ is the wavelength of the incident light
• - ⁇ and 0L are calculated by the following relationships:
• ⁇ is the angle of incidence of the light on the grating.
• Such a Bragg mirror makes it possible to reflect, thanks to constructive interference phenomena, up to more than 99% of the incident energy for a given wavelength.
• the thicknesses of the different layers are calculated for a single wavelength ⁇ , they do not make it possible to obtain satisfactory results for pulses having a spectral width greater than approximately 20 nm, centered on this wavelength.
• the present invention aims to overcome these disadvantages of the prior art.
• the invention aims to provide a method for obtaining a dispersive reflective diffraction grating optimized for a particular use.
• the invention aims to provide a diffraction grating optimized for use over a wide frequency range of several tens, or even hundreds of nanometers.
• the object of the invention is to make it possible to obtain such an optimized diffraction grating for a frequency drift amplification of an ultra-short pulse laser having a spectral width of several hundred nanometers and a good held with the laser flow.
• a process for obtaining a reflective diffraction grating for the diffraction of a light beam of spectral domain, angle of incidence and defined polarization comprising a stack of at least four planar layers of dielectric materials, an upper layer of dielectric material being etched to form a diffraction grating whose etching period is determined.
• This method implements, according to the invention, the following steps:
• the layers of non-etched dielectric materials are placed on a metal layer, and their number is chosen between 5 and 15.
• the etching parameters whose value varies during the calculation step are the etching depth and the groove width.
• the numerical calculation of the reflection and / or transmission efficiencies of at least one of the diffraction orders is made for a sample of at least 10 frequencies distributed in a spectral range of width greater than 100 nm.
• this spectral range is between 700 and 900 nm.
• the present invention also relates to a reflective diffraction grating comprising:
• At least two of the high refractive index material layers or the low refractive index material layers have distinct thicknesses
• the thicknesses of the high refractive index material layers and the low refractive index material layers, and at least one etching parameter of the upper layer are determined by a sizing method as described above.
• this reflective diffraction grating comprises at least two layers of silica (Si0 2 ) and two layers of hafnium dioxide (Hf0 2 ) alternately, and the etched upper layer consists of silica (SiO 2 ).
• such a reflective diffraction grating for the diffraction of a spectral range light beam between 700 and 900 nm, having an angle of incidence of between 50 ° and 56 °, comprises a substrate on which at least :
• Au gold layer
• hafnium dioxide Hf0 2
• Hf0 2 hafnium dioxide
• hafnium dioxide Hf0 2
• Hf0 2 hafnium dioxide
• hafnium dioxide Hf0 2
• Hf0 2 hafnium dioxide
• a layer of silica Si0 2 ) with a thickness of between 625 nm and 775 nm, etched throughout its thickness so as to form the grating, the etching period d being between 1400 and 1550 lines per mm and the width of etching being such that the ratio c / d is equal to 0.65.
• such a reflective diffraction grating comprises an alumina layer deposited between the last layer of hafnium dioxide (Hf0 2 ) and the etched silica layer (Si0 2 ).
• the invention also relates to such a reflective diffraction grating, comprising a substrate on which are deposited successively: - a layer of gold (Au);
• silica (SiO 2 ) layer with a thickness of 240 nm;
• hafnium dioxide Hf0 2
• Hf0 2 hafnium dioxide
• silica layer Si0 2 ) with a thickness of 380 nm;
• hafnium dioxide Hf0 2
• hafnium dioxide Hf0 2
• Si0 2 silica layer with a thickness of 700 nm, etched throughout its thickness.
• FIG. 1 is a schematic representation in sectional view of a diffraction grating according to the prior art, based on a Bragg mirror;
• FIG. 2 is a diagrammatic representation in sectional view of a diffraction grating according to one embodiment of the invention
• FIG. 3 is a graph showing the reflected efficiency of the diffraction grating shown in FIG. 2 as a function of the wavelength of the incident light;
• FIG. 4 is a graph showing the intensity spectrum of a laser pulse of spectral width of
• FIG. 1 is a diagrammatic sectional view of a diffraction grating according to the prior art, based on a Bragg mirror.
• This network comprises alternating layers 11 of high refractive index and low index of refraction layers 12, deposited on a substrate 13. The thickness of each layer is fixed according to its refractive index n H or n L d on the one hand, and the angle of incidence ⁇ and the wavelength ⁇ of the incident beam on the other hand.
• n H or n L d refractive index
• ⁇ and the wavelength ⁇ of the incident beam on the other hand.
• a gold layer (not shown) can be inserted between the glass substrate 13 and the dielectric stack forming a Bragg mirror in order to reduce the number of thin layers necessary to obtain a reflectivity. high, while guaranteeing a threshold of damage close to those obtained with fully dielectric mirrors.
• this gold layer is much greater than the skin thickness, typically 150 nm, so that the glass substrate has no optical interaction with the laser pulse.
• the number of dielectric layers above the gold deposit can be set by the user but, unlike fully dielectric deposits, it can be reduced to six. This solution is described in the article by N. Bonod and J. Neauport, "Optical performance and laser induced damage threshold In this paper, the following is an example: “(Opt.Commun., Vol 260, Issue 2, 649-655 - 2006).
• the upper layer 15 is etched to form the network.
• the period and the etching geometry are defined in order to collect most of the incident energy in reflection in the dispersive (-1) diffraction order. Only the energy collected in this order (-1) of diffraction will be used in the final laser pulse. The energy emitted in the other orders is lost.
• the period and the etching geometry are generally defined to collect about 95% of the incident energy in reflection in the order (-1) of diffraction.
• Such a network of the prior art can offer good performance only for a given wavelength, and is not suitable, in particular, the dispersion of a laser pulse covering a wide frequency range.
• the present invention is based on the joint optimization of the thickness of the planar layers and the etching profile of the network.
• the thicknesses of the different layers are therefore not those determined for the Bragg mirrors, but are each optimized, in connection with the characteristics of the etching profile, by a numerical optimization method, to present good reflective efficiencies over a wide width. spectral.
• the network to be optimized presents a certain number of parameters which are chosen before implementing the optimization method. These parameters are mainly:
• the number and the nature of the layers of dielectric material being generally limited to less than 20, and preferably less than 15, to avoid the risks of cracking of the network, but to be greater than or equal to 5 for the network to have good reflected efficiency;
• the etching period d which is advantageously determined, knowing the spectral range and the angle of incidence of the laser pulse, so that only the orders 0 (always present) and the order (-1) are propagative diffraction orders, the other orders being evanescent;
• the angle of inclination of the trapezoids forming the engraving profile which is chosen according to constraints related to the manufacture of the network.
• each dielectric layer is the thickness of each dielectric layer
• the engraving depth h which corresponds to the thickness of the etched layer if it is etched over its entire height
• a minimum and a maximum are determined, as is an incrementing step.
• the minimum and the maximum can be chosen in particular according to the constraints of manufacture.
• the step of incrementation is chosen according to the precision of the desired optimization.
• the step of incrementation and the intervals [minimum; maximum] are chosen according to the computing power available to carry out the optimization.
• the number of computations increases in fact when one increases the intervals or when one decreases the steps of incrementation.
• the diffraction grating exhibiting these parameters can be dimensioned, according to the invention, with the method comprising the following steps:
• a plurality of possible diffraction grating configurations corresponding to the parameters mentioned above are determined. For this purpose, all the possible combinations are determined on a computer by varying the thicknesses of each of the layers of dielectric material and the etching parameters of the upper layer in the determined intervals and according to the determined steps.
• the efficiency reflected in the grating diffraction order (-1) is calculated for a sample of frequencies chosen in the spectral range of use of the network to be dimensioned.
• the values of some of the variables can be fixed, to simplify the calculations or if it is not relevant to optimize them.
• the optimization according to the invention can, however, be implemented only by simultaneously optimizing at least one of the etching parameters (engraving height h, angle OC of inclination of the trapezes, width c of the engraved groove) and the thickness of each of the dielectric layers having a strong optical effect, which are four in number minimum.
• this method of numerical optimization therefore takes into account both the thicknesses of each of the layers forming the network, and the etching characteristics of this network.
• the software initializes each variable h, el, e2, e3, e4, e5, e6 and c to their respective minimum value h m i n e lmin e2 m j_ n, e3 m in f e4 m i n e5 m in e6 m in and c m in ⁇
• the reflected efficiency of this first configuration is then calculated by the appropriate method of solving the Maxwell equations.
• the first parameter h is incremented by the value of the step Ah, as long as its value is less than or equal to h max .
• the reflected efficiency of the corresponding configuration is calculated by the appropriate method of solving the Maxwell equations.
• the second parameter el is incremented by the value of step Ael, as long as its value is less than or equal to el max .
• the value of h as described above is varied and the reflected efficiency of all corresponding configurations is calculated by the appropriate method of solving the Maxwell equations.
• each of the following parameters are thus incremented until the reflected efficiencies of all possible configurations of networks whose parameters h, el, e2, e3, e4, e5, e6 and c are between fixed minimum and maximum values, with fixed incremental steps, have been calculated.
• e3 200 nm
• Ae3 10nm, that is 21 possible values for e3
• - e4. 100 nm
• e4 300 nm
• Ae4 10 nm, which is 21 possible values for e4;
• the reflected efficiency of the network can be calculated for several wavelengths previously selected, distributed in a given frequency range.
• the method of calculating the efficiency reflected in the diffraction order (-1) of the configuration of each network configuration, based on a rigorous resolution of the Maxwell equations, is based on the development of the electric and magnetic fields in series of Fourier, which reduces Maxwell's equations to a system of differential equations of the first order.
• the integration of this system from the substrate to the superstrate makes it possible to precisely calculate the reflection and transmission efficiencies of the periodic component.
• a second integration makes it possible to reconstruct the electromagnetic field throughout the space.
• the diffraction grating shown in FIG. 2 is intended for a femtosecond laser pulse frequency amplification amplification amplified by a titanium-sapphire crystal, having a spectral amplitude of 200 nm centered on 800 nm, and a TE polarization ( transverse electric).
• Figure 4 is a measure of the spectral intensity of this laser pulse.
• the angle of incidence of the light on the grating is fixed at 55 °, and the engraving frequency 1 / d of the grating is fixed at 1480 lines per mm.
• the angle of inclination of the trapezoids forming the engraving is chosen at 83 °. This angle is the closest to the angles measured on the networks currently produced by the manufacturers in this type of oxide, and for this type of depth.
• this network has been chosen to manufacture this network with three flat layers 21, 23 and 25 of Si0 2 , alternating with three flat layers 22, 24 and 26 of Hf0 2 , the lower layer 21 of Hf0 2 being laid on a layer of gold 20.
• the incrementation step chosen is 10 nm in a range of [100; 400] nm;
• the incrementation step chosen is 10 nm in a range of [0; 300] nm;
• An additional upper layer 28 of SiO 2 is etched over its entire height.
• a layer 27 of Al 2 O 3 with a thickness of 50 nm is provided between the upper layer 28 of Si0 2 intended to be etched and the upper layer 26 of Hf0 2 to facilitate the etching of the layer 28 of Si0 2 on all its thickness without damaging layer 26 of Hf0 2 .
• This thin layer 27, when it is indispensable for the realization of the network, is taken into account in the calculations of the reflected efficiency of the network as a constant.
• This layer Al 2 0 3 could, of course, not be implemented, or be placed at another position, in other embodiments of the invention.
• the interval chosen for parameter c / d is [0.55; 0.75], with a step of incrementation of 0.1.
• the efficiency reflected in order -1 is calculated for 41 wavelengths between 700nm and 900 nm.
• the number of calculations of the reflected efficiency of the different possible configurations of the diffraction grating is therefore 41 * 3 * 51 * [31] n , where n is the number of plane layers, ie 6.
• This method can of course be used iteratively.
• a first implementation of the method makes it possible to detect optimized network solutions
• one or more new implementations with differently chosen intervals and reduced increment steps make it possible to precisely define the best network solutions.
• the use of the dimensioning method according to the invention thus makes it possible to find different configurations of networks, presenting the parameters described above in relation to FIG. 2, which make it possible to obtain, with an etching depth, the order of 700 nm averages of efficiencies reflected in the order -1 greater than 90% in the spectral range [700; 900] nm.
• One of these configurations corresponds to a network consisting of a glass substrate, on which are deposited successively:
• a gold layer whose thickness is much greater than the skin thickness, typically 150 nm, such that the glass substrate has no optical interaction with the laser pulse.
• hafnium dioxide Hf0 2
• hafnium dioxide Hf0 2
• hafnium dioxide Hf0 2
• the engraving is done so that the value c / d is equal to 0.65.
• FIG. 3 is a graph showing, on the one hand, in full line, the reflected efficiency of this network in the order -1, and on the other hand, in dashed lines, the sum of the efficiencies reflected (order 0 + order -1) of this network, as a function of the wavelength of the incident light.
• the etching parameters have been chosen so that the number of diffraction orders is limited to two (order -1 and order 0) in order to limit the distribution of energy in too many orders.
• the order 0 is not dispersive (the diffraction angle in this order does not depend on the frequency), the order (-1) in which the incident light is dispersed.
• the graph of FIG. 3 shows that minima 30, 31, 32 and 33 appear, but that their spectral width is very fine, so that they do not affect the average of reflected efficiency calculated on the spectral domain.
• FIG. 4 shows, by way of example, the spectral intensity of the laser pulse to be reflected by the network of FIG. 2.
• the criterion used for the selection of the network is the average of the reflected efficiency of the network, weighted by the spectral intensity of the incident wave shown in FIG. 4. This average, calculated on 801 points evenly distributed over the entire spectral range [700 nm; 900 nm], is equal to 94.5% for the network of FIG.
• the fabrication of the network dimensioned according to this method can then be carried out using conventional manufacturing methods known to those skilled in the art for the manufacture of networks based on Bragg mirrors.
• the etching depth of this network is between 625nm and 775 nm, and the number of lines per mm is between 1400 and 1550.
• the intervals in which are included the thicknesses of the layers are:

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