Method for optically recording information in quartz glass using pulses with modulated energy

Abstract

FIELD: optical materials science.SUBSTANCE: invention relates to methods of optical recording of information in quartz glass. A method for optically recording information in quartz glass using pulses with modulated energy involves focusing a packet of femtosecond laser pulses with a wavelength of 513 nm, under the action of which a birefringent voxel with a controlled orientation of the slow birefringence axis is formed, and is characterized in that packets of femtosecond pulses with modulated energy are used, consisting of 10 pulses with a pulse repetition frequency within a packet of 0.2-1 MHz, the first two of which have an energy of 12-16 nJ, and the eight following ones 9-12 nJ, the total energy of the pulse packet lies in the range of 96-128 nJ, which focus to a depth of 100-1300 mcm, and 53 layers of data are recorded on each side of the optical carrier with a distance between layers and voxels of 15-23 mcm and 1.1 mcm, respectively, encoding 3 bits of information in one voxel.EFFECT: reduction in energy, an increase in the number of layers and an increase in precision.1 cl, 4 dwg

Description

[0001] The invention relates to optical materials science, specifically to a method for laser modification of glass volume using a focused femtosecond laser beam and can be used for long-term information storage. The invention enables the recording of voxels—three-dimensional structures with polarization-controlled birefringence—in quartz glass using a femtosecond laser beam. The parameters of these structures are used in multi-bit information encoding. Thus, the result is suitable for creating long-lasting and multi-level optical memory on quartz glass.

[0002] A method for three-dimensional recording of information in quartz glass using a laser beam due to the refractive index contrast is known [Shiozawa, Manabu, et al. Simultaneous multi-bit recording in fused silica for permanent storage / / Japanese Journal of Applied Physics - 52.9S2 (2013). 09LA01]. For recording, 120 fs pulses generated by a titanium:sapphire laser at a wavelength of 800 nm with an energy in the range of 150-1400 nJ were used. The pulse repetition rate was 1 kHz. The pulses were focused into the glass volume using an objective with a numerical aperture of 0.7, and 26 layers were recorded with a distance between layers of 50 μm. The layers consisted of voxels - three-dimensional structures that differ in brightness from unmodified glass when observed through an optical microscope. The minimum voxel height was 25 µm using aberration correction in the focusing objective.Thermal stability of the optical storage medium with recorded information at 1000°C for 2 hours was demonstrated. The main drawbacks of the proposed method are the limited information recording density to one bit per voxel, the recording speed of 1 kbit / s, since the pulse repetition rate is limited to 1 kHz, and the use of light reflected from the voxel for data reading. The latter requires a level of light reflection from the voxel above a certain threshold to ensure reliable voxel reading. When multilayer data recording, the upper layers located in the path of the laser beam to the read layer reflect some of the light, which leads to a gradual attenuation of the read beam and, ultimately, limits either the number of information layers that can be read with an acceptable signal-to-noise ratio, or the recording density of voxels in a layer, which is directly related to the level of attenuation of the read beam, sinceThe higher the density of the voxels adjoining each other, the higher the scattering and reflection of light on the voxels.

[0003] A well-known work [Zhang, Jingyu, et al. "Seemingly unlimited lifetime data storage in nanostructured glass." Physical review letters 112.3 (2014): 033901] is known, which for the first time demonstrates a method for recording voxels, which are anisotropic nanoperiodic structures and suitable for multi-bit information encoding. The essence of the method lies in the formation of birefringent voxels under the action of focused pulses of a femtosecond laser, while the direction of orientation of the slow birefringence axis of the structures can be controlled by changing the orientation of the linear polarization of the recording beam. For recording, laser pulses with a duration of 280 fs and a pulse energy of 63 nJ were used, focused into a spot with a diameter of less than 1 μm using a water-immersion objective with a numerical aperture of 1.2. It was found that by increasing the energy of the recording pulses or their number, it is possible to control the level of phase delay of voxel birefringence.Thus, this method enables encoding information using the slow axis orientation and phase delay level in addition to the three spatial coordinates, increasing the data recording density. To increase the recording speed, a spatial light modulation method was used, which allowed the original laser beam to be split into 100 laser beams, thereby increasing the optical data recording speed. However, the data recording speed was only 6.3 KB / s and was severely limited by the speed of the spatial light modulator, as well as the fact that recording a single voxel requires at least several dozen laser pulses. On the other hand, the use of an immersion lens to focus the laser beam limits the maximum recording depth to no more than 300 µm, which in turn limits the number of possible data layers in the case of multilayer recording.

[0004] The authors of the patent [Patent RU 2710387 C1] have improved the method of multi-bit recording of information in quartz glass by reducing the pulse energy to 20-30 nJ and using dry objectives with a numerical aperture of 0.45-0.9. However, the number of pulses required to form voxel nanogratings imposed limitations on the data recording speed, which amounted to 18.9 KB / s, and the voxel diameter of more than 1 μm limited the data recording density.

[0005] The closest analogues to the claimed invention in technical essence and the achieved result are the scientific work of Lei Y. et al. "High speed ultrafast laser anisotropic nanostructuring by energy deposition control via near-field enhancement" Optica 8.11 (2021): 1365-1371 and the patent [Sakakura M. et al. Method of pulsed laser irradiation with reduced thermal damage: pending patent 20210265797 US. - 2021]. The article was chosen as a prototype. It demonstrated a method for laser recording of information in quartz glass by forming voxels under the action of pulse bursts with modulated energy. Bursts of femtosecond laser pulses with a wavelength of 515 nm, a pulse repetition rate within a burst of 10 MHz, and a pulse duration of 250 fs were focused in the glass volume. The burst included two pulses with an energy of 30 nJ and eight pulses with an energy of 13.5 nJ, so the total energy of the burst was at least 168 nJ.When using a burst of ten equal-energy pulses, significant stress and even localized cracking were observed around the voxels due to laser-induced thermal effects. The absorption of the femtosecond laser pulse burst by the quartz glass results in the formation of voxels, which are elongated submicron cavities with polarization-controlled birefringence. Although the voxel diameter is less than 1 μm, optical data readout methods limit both the minimum inter-voxel distance and the inter-voxel layer distance. Optimizing these parameters during multilayer data recording is critical for recording with maximum density. Thus, a single voxel layer can be reliably read with an inter-voxel distance of 1 μm. However, to record 50 data layers, a voxel pitch of 1.2 μm and a layer pitch of 10 μm were used.The data reading accuracy was 99.5% and 96.3% for the upper and lower layers, respectively.

[0006] The technical result of the invention is a reduction in the total energy of a packet of femtosecond pulses for the formation of birefringent voxels in the volume of glass, an increase in the number of layers, including through the use of the second side of the information carrier and the achievement of high accuracy in reading the recorded information - at least 98% for each recorded layer.

[0007] The specified technical result is achieved by a method of optically recording information in quartz glass using pulses with modulated energy, in which packets of femtosecond laser pulses with a wavelength of 513 nm are focused (the difference in the wavelength of laser radiation in the proposed method and in the prototype is immaterial in the context of the proposed method of optical recording), under the action of which a birefringent voxel with a controlled orientation of the slow birefringence axis is formed, characterized in that packets of femtosecond pulses with modulated energy are used, consisting of 10 pulses with a pulse repetition frequency within a packet of 0.2-1 MHz, the first two of which have an energy of 12-16 nJ, and the eight subsequent ones 9-12 nJ, the total energy of the pulse packet lies in the range of 96-128 nJ, which are focused to a depth of 100-1300 μm, and 53 layers of data are recorded on each side optical carrier with a distance between layers and voxels of 15-23 µm and 1,1 µm respectively, encoding 3 bits of information into one voxel.,

[0008] A Pharos 9W femtosecond laser from Light Conversion was used to record optical information in quartz glass with modulated pulse energy. The laser generated pulses with a wavelength of 513 nm, an energy of up to 1 μJ, a duration of ~240 fs, and a repetition rate of up to 1 MHz. A 0.6 numerical aperture objective was used to focus the laser beam.

[0009] The birefringence of laser-induced voxels was analyzed using an Olympus BX61 optical microscope with an Abrio Microbirefringence attachment [Patent US 7372567 B2. Retardance measurement system and method; https: / / openpolscope.org / ]. Voxel birefringence parameters were recorded, and the readout data, encoded into eight orientations of the slow birefringence axis, were analyzed. A comparison of the histograms of the distribution of the slow axis orientation angles in the case of 3 bits / voxel encoding, where 8 values ​​of the slow axis orientation angles are used (Fig. 1A) and 4 bits / voxel (Fig. 1B), where 16 values ​​of the slow axis orientation angles are used, showed that the maximum value of bits that can be encoded into the slow axis birefringence parameter is 3, since a further increase in the number of bits per voxel leads to a sharp increase in the number of reading errors and the inability to recognize the recorded information.The maximum percentage of errors during reading and decoding information was determined by bitwise comparing the decoded file with the original file for each recorded layer and finding the maximum value of the ratio of the number of errors in a layer to the number of voxels recorded in the layer, multiplied by 100%. The minimum reading accuracy was determined by the formula: 100% minus the maximum percentage of errors.

[0010] The achievement of the claimed technical result is confirmed by the following examples.

[0011] For clarity, the first three examples are illustrated (Fig. 2-4) with graphs of the results of reading previously recorded information using the proposed method for both sides (sides No. 1 and No. 2) of the optical storage medium; the abscissa axis shows the numbers of the recording layers, and the ordinate axis shows the number of errors (reading) in percent.

[0012] Example 1. Information was recorded in a 5 mm thick KU-1 quartz glass plate in the depth range of 100-1300 µm by focused femtosecond laser packets of pulses with a wavelength of 513 nm, a duration of 240 fs, a pulse repetition rate within a packet of 1 MHz and a packet energy of femtosecond pulses of 96 nJ (the first two pulses have an energy of 12 nJ, the next eight have an energy of 9 nJ) with linear polarization of the laser beam at angles of 0; 22.5; 45; 67.5; 90, 112.5; 135 and 157.5°, focused through an objective with a numerical aperture of 0.6 and automatic correction of spherical aberrations. The information was encoded into the azimuth angles of the slow axis. Fifty-three layers were recorded in the glass volume on each side of the optical media, with the distance between information layers being 23 µm and the distance between adjacent voxels being 1.1 µm. The recorded information volume was 66 KB, or 630 B / layer.The maximum number of errors in the layer during reading and decoding of information was 1.75% (Fig. 2), that is, the minimum reading accuracy was 98.25%.

[0013] Example 2. Information was recorded in a 5 mm thick KU-1 quartz glass plate in the depth range of 320-1300 µm by focused femtosecond laser packets of pulses with a wavelength of 513 nm, a duration of 240 fs, a pulse repetition rate within a packet of 1 MHz and a packet energy of femtosecond pulses of 112 nJ (the first two pulses have an energy of 14 nJ, the next eight 10.5 nJ) with linear polarization of the laser beam at angles of 0; 22.5; 45; 67.5; 90, 112.5; 135 and 157.5°, focused through an objective with a numerical aperture of 0.6 and automatic correction of aberrations. The information was encoded into the azimuth angles of the slow axis. Fifty-three layers were recorded in the glass volume on each side of the optical media, with a distance of 15 µm between information layers and 1.1 µm between adjacent voxels. The recorded information volume was 66 KB, or 630 B / layer.The maximum number of errors in the layer during reading and decoding of information was no more than 1.5% (Fig. 3), that is, the minimum reading accuracy was 98.5%.

[0014] Example 3. Information was recorded in a 4 mm thick KU-1 quartz glass plate in the depth range of 100-1300 µm by focused femtosecond laser packets of pulses with a wavelength of 513 nm, a duration of 240 fs, a pulse repetition rate within a packet of 1 MHz and a packet energy of femtosecond pulses of 128 nJ (the first two pulses have an energy of 16 nJ, the next eight 12 nJ) with linear polarization of the laser beam at angles of 0; 22.5; 45; 67.5; 90, 112.5; 135 and 157.5°, focused through an objective with a numerical aperture of 0.6 and automatic correction of aberrations. The information was encoded into the azimuth angles of the slow axis. Fifty-three layers were recorded in the glass volume on each side of the optical storage medium, with a distance of 23 µm between information layers and 1.1 µm between adjacent voxels. The recorded information volume was 66 KB, or 630 B / layer. The maximum error rate per layer during reading and decoding of information was 1.8% (Fig.4), that is, the minimum reading accuracy was 98.2%.

[0015] Example 4. Information was recorded in a 4 mm thick KU-1 quartz glass plate in the depth range of 320-1300 µm by focused femtosecond laser packets of pulses with a wavelength of 513 nm, a duration of 240 fs, a pulse repetition rate within a packet of 1 MHz and a packet energy of femtosecond pulses of 96 nJ (the first two pulses have an energy of 12 nJ, the next eight have an energy of 9 nJ) with linear polarization of the laser beam at angles of 0; 22.5; 45; 67.5; 90, 112.5; 135 and 157.5°, focused through an objective with a numerical aperture of 0.6 and automatic correction of aberrations. The information was encoded into the azimuth angles of the slow axis. Fifty-three layers were recorded in the glass volume on each side of the optical media, with a distance of 20 µm between information layers and 1.1 µm between adjacent voxels. The recorded information volume was 66 KB, or 630 B / layer.The maximum number of errors in the layer during reading and decoding of information was no more than 1.4%, that is, the minimum reading accuracy was 98.6%.

[0016] Example 5. In a 5 mm thick plate of KU-1 quartz glass, in the depth range of 100-1300 μm, information was recorded by focused femtosecond laser packets of pulses with a wavelength of 513 nm, a duration of 240 fs, a pulse repetition rate within a packet of 0.2 MHz and a packet energy of femtosecond pulses of 96 nJ (the first two pulses have an energy of 12 nJ, the next eight 9 nJ) with linear polarization of the laser beam at angles of 0; 22.5; 45; 67.5; 90, 112.5; 135 and 157.5°, focused through an objective with a numerical aperture of 0.6 and automatic correction of aberrations.

[0017] The information was encoded into the azimuth angles of the slow axis. Fifty-three layers were recorded in the glass volume on each side of the optical media, with a distance of 23 µm between information layers and 1.1 µm between adjacent voxels. The recorded information volume was 66 KB, or 630 B / layer. The maximum error rate during reading and decoding was 1.5%, meaning the minimum read accuracy was 98.5%.

[0018] Example 6. Information was recorded in a 5 mm thick KU-1 quartz glass plate in the depth range of 100-1300 µm by focused femtosecond laser packets of pulses with a wavelength of 513 nm, a duration of 240 fs, a pulse repetition rate within a packet of 0.5 MHz and a packet energy of femtosecond pulses of 112 nJ (the first two pulses have an energy of 14 nJ, the next eight 10.5 nJ) with linear polarization of the laser beam at angles of 0; 22.5; 45; 67.5; 90, 112.5; 135 and 157.5°, focused through an objective with a numerical aperture of 0.6 and automatic correction of aberrations. The information was encoded into the azimuth angles of the slow axis. Fifty-three layers were recorded in the glass volume on each side of the optical media, with the distance between information layers being 23 µm and the distance between adjacent voxels being 1.1 µm. The recorded information volume was 66 KB, or 630 B / layer.The maximum number of errors in reading and decoding information was 1.1%, meaning the minimum reading accuracy was 98.9%.

[0019] As can be seen from the examples, the parameters used (distance between voxels, distance between voxel layers, number of bits per voxel) and the recording mode, namely, packets of femtosecond pulses with modulated energy, with a packet energy in the range of 96-128 nJ, make it possible to optically record information in the volume of quartz glass, and then successfully read and decode it.

[0020] The lower energy limit of the burst is determined by the low signal-to-noise ratio during reading and decoding of the recorded information. The upper limit is determined by the appearance of stress around the formed birefringent structures, leading to read errors.

Claims

A method for optically recording information in quartz glass using pulses with modulated energy, in which packets of femtosecond laser pulses with a wavelength of 513 nm are focused, under the action of which a birefringent voxel with a controlled orientation of the slow birefringence axis is formed, characterized in that packets of femtosecond pulses with modulated energy are used, consisting of 10 pulses with a pulse repetition frequency within a packet of 0.2-1 MHz, the first two of which have an energy of 12-16 nJ, and the eight following ones 9-12 nJ, the total energy of the pulse packet lies in the range of 96-128 nJ, which are focused to a depth of 100-1300 μm, and 53 layers of data are recorded on each side of the optical carrier with a distance between layers and voxels of 15-23 μm and 1.1 μm, respectively, encoding 3 bits of information in one voxel.

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