MANUFACTURING METHOD OF SPATIALLY MODULATED WAVEPLATES

Abstract

The invention relates to volume modification of transparent materials by means of ultrashort laser pulses. A method for manufacturing of highly transparent spatially variant waveplates includes focusing Gaussian laser beam with pulse duration 500 fs to 2000 fs inside of material transparent to laser wavelength building self-organizing structures of nanoplates. The workpiece is moved in three coordinates relatively to beam focus along desired line. A combination of focus area, pulse repetition rate, energy and velocity of movement is selected to locate said structures inside of the workpiece for acting as birefringent optical elements with specific retardance. Energy of pulses exceeds the threshold of building nanoplates in part of the focal area limited by −σ/2 and σ/2 where σ is standard deviation from maximum of Gaussian function. Energy of pulses creating nanoplates is accumulated in said area from the sequence of 1000 to 2000 pulses in total not exceeding 0.2-0.3 μJ.

Claims

1. Method for manufacturing of spatially variant waveplates, including: focusing of linearly polarised ultrashort pulse laser radiation (USPLR) beam (2) with Gaussian intensity distribution in the material of a workpiece (5) that is transparent to USPLR beam (2), performing controlled displacement of the transparent material workpiece (5) with respect to a focused focal point of the USPLR beam (2) in accordance with the predetermined rule, while simultaneously changing a direction of USPLR polarization in the workpiece material, depending on the USPLR beam (2) focal point coordinates in the workpiece (5), wherein formation of nano-plates in spots of the workpiece (5) material affected by the focused USPLR beam (2) and their self-organization into periodic structures with a period shorter than USPLR wavelength take place, wherein the formed periodic structures are oriented perpendicularly to the USPLR polarization and covers a region in the workpiece material along the direction of the USPLR propagation, that is longer than the said wavelength of the USPLR more than 100 times, selecting of the focused USPLR beam focal area, a frequency of pulse repetition, energy thereof, and the workpiece (5) moving velocity so that the formed nano-plate structures (6) would position in the workpiece material space and function as birefringent optical elements with their characteristic phase delay, characterized in that a pulse duration of the USPLR pulses focused in the workpiece (5) material is from 500 fs to 2000 fs, their repetition period is from 1 μs to 50 μs, wherein a density of the focused USPLR pulse energy exceeds the threshold (10) determined by properties of the affected material only in the part of the focal area, the linearly polarized pulses of USPLR beam are delivered into the workpiece in sequences, wherein number of pulses in a sequence (16) is chosen to ensure the formation of the nano-plate structure (6) in the workpiece material.

2. Method according to claim 1, characterized in that the part of the focal area, in which the USPLR beam pulse energy density exceeds the threshold (10) determined by the properties of the affected material, is defined by the deviation of the intensity distribution from the peak position, and the said deviation is within the range from −σ/2 to σ/2.

3. Method according to claim 1 or claim 2, characterized in that energy of the sequence comprising USPLR beam pulses, accumulated in the part of the focal area, in which the periodic nano-plate structure (6) is formed, is from 0.2 μJ to 0.3 μJ.

4. Method according to any one of claims 1-3, characterized in that the number of linearly polarized USPLR pulses in the sequence (16) for the formation of a nano-plate structure (6) is selected in the range from 1000 to 2000.

Description

[0016] The invention is explained with more details through the drawings, where

[0017] FIG. 1 shows a principal block chart of the device used to implement the proposed method of the spatially modulated waveplate manufacturing;

[0018] FIG. 2 shows the distribution of the focused USPLR beam intensity, depending on the deviation from the beam axis; if the coordinate deviates from the axis by 0.5σ, where σ is the average deviation, the intensity is 0.88 from the maximum in the axis.

[0019] FIG. 3 shows the portion of the focused USPLR beam intensity distribution required for the formation of periodic structures from nanoplates.

[0020] FIG. 4 shows the effect of USPLR impulse energy accumulation in defects.

[0021] FIG. 5 shows the spectral bandwidth of an optical element described in a way proposed in this application, by exceeding the threshold for the formation of periodic structures by 10%, and accumulating energy of 1000 pulses, as well as the bandwidth of ultraviolet glass UVFS of which the workpiece of measured element is made.

[0022] FIG. 6 shows an optical element manufactured in the manner proposed in the application, its spectral bandwidth is shown in FIG. 5.

[0023] FIG. 7 shows an example of spatial distribution of exit light beam obtained from Gaussian entry beam.

AN IMPLEMENTATION EXAMPLE OF THE PROPOSED INVENTION

[0024] The proposed method for the manufacturing of spatially modulated wafeplates includes the following sequence of operations: focuses the radiation beam of the ultrashort pulse laser radiation mode TEM.sub.00 (USPLR) with the intensity distribution according to the Gauss law and linear polarization, in a workpiece of a material transparent for the said beam. The additional elements set directions of the polarization vector. The duration of the USPLR focused in the workpiece material is selected within the range from 500 fs to 2000 fs, and their repetition period is selected within the range from 1 μs to 50 μs. The energy of single pulses and the area of the focal waist are chosen so that only a small part of the focal area will exceed the threshold for the formation of structures from nano-planes. The energy density of these pulses is no more than 15% above the threshold determined by the properties of the affected material in the said part of the focal area, defined by the deviation of the intensity distribution from the maximum position in the range from −σ/2 to σ/2. The workpiece is moved in relation to the focal point according to the trajectory set, at each point of that trajectory setting the required direction of the focused USPLR polarization and the orienting the nano-plane structures. The area of the focused USPLR beam focal point, the frequency of pulse repetition, the velocity of their energy and workpiece movement is selected in such a way that the resulting nano-plane structures would be arranged in the space of the workpiece material, and would act as birefringent optic elements with the phase delay that is characteristic to them. This way, one or more layers of nano-planes are recorded. Impulse energy accumulated in the said part of the focal area, where the periodic nano-plane structure is formed, is in range from 0.2 to 0.3 μJ. The formation of a nano-plane structure requires a linearly polarized USPLR pulse sequence in which the number of pulses is in the range from 1000 to 2000.

[0025] FIG. 1 shows a principal block chart of the device used to implement the proposed method of the spatially modulated waveplate manufacturing. The device includes a laser source 1, generating the beam of ultrashort pulse laser radiation of the Gaussian intensity distribution 2, in the optical path of which a half-wave (λ/2) phase plate 3 for setting the direction of the polarization vector in the USPLR beam, is placed. A focusing optic 4 is arranged behind the plate 3 to direct the laser radiation beam 2 into a workpiece 5 of a material transparent for the USPLR beam, in it a self-organizing periodic structures of nano-planes 6 are created, they are arranged in the set trajectory 7. The positioning device to move the workpiece in three spatial directions 8 is also provided.

[0026] In the manufacturing method of the spatially modulated waveplates proposed according to the invention, the defects created in the material are accumulated by creating them with pulses the intensity of which in the focused beam focal point is distributed according to the Gaussian (normal) law 9, and the energy only marginally (no more than 15%) exceeds the nano-plane formation and self-organizing threshold 10. The pulses of such intensity are directed at a workpiece of a material transparent to the affecting light wave and are periodically repeated until the nano-plane structure of the required optical activity is formed. The repetition period is chosen such that, during the time between pulses, all processes related to the formation of defects would end: the release of the electrons−the formation of excitons, the self-trapping of the excitons (formation of STEs), the energy transfer to the grating (thermal processes), and the relaxation of the silicon-oxygen bonds. At least 1 μs, i.e., the laser pulse repetition frequency must not exceed 1 MHz, in order for all of these processes to end. The operation of the optical element is based on the layout of the nano-plane structures in space, where, at each point of the element, the nano-planes are oriented according to the law, set by the requirements of the distribution of laser radiation energy and phase in the laser beam. The energy part 11 located below the nano-plane structure formation threshold influences the accumulation of the described effects such as the formation of centers, but the birefringence of light occurs only due to the pulse peak 12, the area of which does not exceed the Gaussian distribution part, limited to the half of the average deviation σ/2. In order to be able to orient the nano-plane structure, which affects that beam in the most effective way, first of all we must accumulate material defects in the spot where the structure 13 is being created, and then, by aiming the energy 11, which exceeds the threshold 10, at that spot, we achieve that nano-plane structure would form and self-organize in the target, its orientation is perpendicular to polarization of the pulse exceeding the said threshold. This is achieved by moving the workpiece in relation to the beam focal point. Then, at the beginning of the successively following impulses with the convex energy corresponding to the Gaussian distribution 14, in the increasing order, the required defects are accumulated in the material until a pulse exceeding the structure formation and self-organization threshold 10 moves into the target region 15, and the sequence 16 of such pulses creates the nano-plane structure of the desired direction and efficiency. Subsequent laser pulses continue to accumulate defects in descending order, these increase the optical efficiency of the structure. It is important that these residual effects do not accumulate too much, as this results in undesirable light absorption and diffusion centers. Proper structure performance without increasing losses in them is achieved when the number of structure forming pulses is between 1000 and 2000. By selecting proper combination of light focusing area, pulse repetition frequency, energy thereof and workpiece movement speed, it is possible to achieve that the created nano-plane structures would function with maximum efficiency as birefringent and the light diffusion and absorption would be minimal. The effectiveness of such recording is shown by an spectral transparency 17 of the optical element of a curve, described in a way proposed in this application, by exceeding the threshold for the formation of periodic structures by 10%, and accumulating energy of 1000 pulses, as well as the transparency 18 of ultraviolet glass UVFS of which the workpiece of measured element is made, and the image 19 of the optical element manufactured as proposed in the application.