Method and device for detecting deuterium content of large-size DKDP crystal based on non-critical phase matching

Abstract

The invention discloses a large-size DKDP crystal deuterium content detection method and device based on non-critical phase matching, and the method comprises the steps: carrying out the sum frequency of laser radiation with a fixed wavelength and a tunable laser radiation source based on the non-critical phase matching sum frequency wavelength characteristics of DKDP crystals with different deuterium contents; the deuterium content of the DKDP crystal is calculated by measuring the tunable laser radiation wavelength corresponding to non-critical phase matching and frequency of the DKDP crystal at room temperature, and the spatial distribution of the deuterium content of the DKDP crystal can be obtained by scanning different areas of the crystal. According to the method, high-precision nondestructive measurement of deuterium content distribution of the large-size DKDP crystal can be realized, the operation is simple, and the measurement precision is improved by one order of magnitude compared with that of a traditional spectrum method.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, specifically to a method and device for detecting the deuterium content of large-scale DKDP (deuterated potassium dihydrogen phosphate) crystals based on noncritical phase matching. This method utilizes the sum-frequency wavelength characteristics of DKDP crystals with varying deuterium contents under noncritical phase matching conditions to achieve high-precision, nondestructive measurement of the crystal's deuterium content. The method is particularly suitable for detecting the deuterium content distribution of large-scale DKDP crystals. Background Art

[0002] KDP and DKDP crystals exhibit excellent optical properties, including low half-wave voltage, large electro-optical coefficient, transmission spectrum extending from the deep ultraviolet to the near infrared, large effective nonlinear coefficient, and high damage threshold. They are widely used in the fields of inertial confinement fusion and ultra-intense ultrashort lasers for electro-optical switching and harmonic generation. Furthermore, these chemical and physical properties of DKDP crystals are closely related to their deuterium content, allowing researchers to tailor the deuterium content of the solution during the DKDP crystal growth process to meet the needs of diverse applications.

[0003] Due to the limitations of the laser damage threshold, large-aperture DKDP crystals are indispensable materials for high-energy Nd:YAG laser devices. Compared to KDP crystals, DKDP crystals have a lower transverse stimulated Raman scattering coefficient, which helps avoid damage. For noncritical phase-matched fourth harmonic generation, the refractive index of the DKDP crystal is affected by the deuterium content. To ensure the harmonic generation efficiency and beam quality of high-power laser devices, the deuterium content of large-aperture DKDP crystals must vary by less than 0.4%. Furthermore, broadband third harmonic generation from gradient-deuterium-distributed DKDP crystals, based on a refractive index gradient, is a novel approach to overcoming laser plasma instabilities in inertial confinement fusion. Therefore, accurately measuring the deuterium content and distribution of DKDP crystals is crucial for practical applications.

[0004] Currently, common methods for measuring the deuterium content of DKDP crystals, such as thermogravimetry and ferroelectric methods, can only destructively measure small crystals. Neutron diffraction methods require expensive and complex neutron source devices, which is not conducive to online measurement of large crystals. The measurement accuracy of infrared spectroscopy and confocal Raman spectroscopy is limited by the resolution of the spectrometer. The change in deuterium content per wavenumber is 2.78%, and the measurement accuracy needs to be improved. Summary of the Invention

[0005] To address the challenges faced by the aforementioned methods for detecting the deuterium content of DKDP crystals in high-precision, online, nondestructive measurements, the present invention proposes a method for detecting the deuterium content of DKDP crystals based on noncritical phase matching. This method uses noncritical phase matching to sum-frequency (SF) a fixed-wavelength single-frequency laser, such as a neodymium glass laser, with a wavelength-tunable optical radiation source, such as that obtained through optical parametric oscillation. The tunable wavelength corresponding to the highest noncritical SF conversion efficiency of the DKDP at room temperature is measured. Based on the linear relationship between the noncritical phase-matched deuterium content and the SF wavelength, the deuterium content of the DKDP crystal is inferred. The proposed method also allows the spatial distribution of the deuterium content to be determined by scanning different regions of the crystal. Compared to thermogravimetric methods, this method enables nondestructive testing of the deuterium content distribution of large-diameter crystals. Compared to neutron diffraction, this method significantly reduces testing costs. The measurement accuracy of this method can reach 0.14% / cm. -1 , and the measurement accuracy of the spectroscopy method is 2% / cm -1 Compared with the previous results, the performance has been improved by about one order of magnitude.

[0006] The principles of the present invention are as follows:

[0007] The non-critical type I and type II sum frequency processes of DKDP crystals need to meet the dispersion conditions, which are as follows:

[0008]

[0009] Where n e (D,λ3) represents the refractive index of extraordinary light at wavelength λ3 in a DKDP crystal with a deuterium content of D, n o (D,λ1) represents the refractive index of ordinary light with wavelength λ1 in a DKDP crystal with a deuterium content of D, n o (D,λ2) represents the refractive index of ordinary light at wavelength 2 in a DKDP crystal with a deuterium content of D, n e (D,λ2) represents the extraordinary refractive index at wavelength λ2 in a DKDP crystal with a deuterium content of D.

[0010] Under non-critical phase matching conditions, when the wavelength of λ1 is fixed, the deuterium content D of the DKDP crystal satisfies the linear relationship with λ2:

[0011] D=aλ2+b (3)

[0012] Where a and b are parameters related to wavelength λ1 and ambient temperature. By measuring the wavelength of the tunable laser λ2, which achieves noncritical phase matching with the fixed wavelength λ1 and produces the combined frequency light λ3 in the DKDP crystal under test, the deuterium content of the DKDP crystal under test can be determined according to Equation (3).

[0013] The technical solutions of the present invention are as follows:

[0014] A method for detecting the deuterium content of a large-scale DKDP based on non-critical phase matching is characterized in that it comprises the steps of:

[0015] Combine the fixed wavelength laser λ1 and the tunable laser λ2 and vertically irradiate the DKDP crystal to be tested;

[0016] Adjust the wavelength λ2 of the tunable laser and monitor the energy change of the sum frequency light λ3 in real time;

[0017] Determine the wavelength λ2 of the tunable laser corresponding to when the sum frequency light energy reaches the maximum value;

[0018] The deuterium content of the DKDP crystal to be tested is calculated based on the pre-calibrated linear relationship between the deuterium content D and the wavelength λ2 of the tunable laser: D = aλ2 + b, where a and b are parameters related to the fixed wavelength laser λ1 and the ambient temperature.

[0019] By scanning different regions of the DKDP crystal to be tested and repeating the above steps, the spatial distribution of deuterium content in different regions of the crystal is obtained.

[0020] Furthermore, the DKDP crystal is cut using a non-critical phase matching angle, with the cutting angle being θ = 90° and φ = 45°.

[0021] Furthermore, the fixed wavelength laser λ1 has a line width of less than 0.1 nm, including but not limited to a 355 nm ultraviolet laser generated by a neodymium glass laser through frequency tripling.

[0022] Furthermore, the tunable laser λ2 is generated by, but not limited to, optical parametric oscillation, with a wavelength resolution better than 0.3 nm.

[0023] Furthermore, the parameters a and b in the linear relationship D=aλ2+b are calibrated in the following manner:

[0024] (1) Use KDP crystals with a deuterium content of 0% as the standard sample;

[0025] (2) Measure its characteristic wavelength λ under the same experimental conditions 20 ;

[0026] (3) Establish a calibration curve based on samples with known deuterium content.

[0027] A detection device for implementing the above method is characterized in that it includes:

[0028] Single-frequency laser: provides laser radiation with a fixed wavelength λ1 for the detection device;

[0029] Wavelength tunable laser radiation source: can output laser radiation with continuously tunable wavelength λ2;

[0030] Beam combiner: It can combine two laser beams from a non-coaxial single-frequency laser and a wavelength-tunable laser radiation source, and make the combined beam incident vertically on the DKDP crystal to be tested;

[0031] Beam splitter and energy meter: separate and measure the energy of the sum frequency light λ3 generated in the DKDP crystal to be tested;

[0032] Spectrometer: measures the output wavelength λ2 of the wavelength-tunable laser radiation source when the energy meter reading is maximum;

[0033] To ensure measurement accuracy, the single-frequency laser is a narrow-linewidth laser, including but not limited to radiation sources such as neodymium glass lasers and their harmonics; the wavelength-tunable laser radiation source can achieve narrow-linewidth wavelength continuously adjustable output, and the wavelength tuning method includes but is not limited to optical parametric oscillation and other methods.

[0034] For small crystals that can be mounted in a temperature-controlled furnace, the measurement procedure should include:

[0035] Step 1: Use a temperature-controlled furnace to maintain the DKDP crystal to be tested at a constant temperature. Adjust the angle of the DKDP crystal installed in the temperature-controlled furnace so that the laser output from the single-frequency laser with a wavelength of λ1 and the wavelength-tunable laser radiation source λ2 is incident on the crystal normally.

[0036] Step 2: Adjust the output wavelength λ2 of the wavelength-tunable laser radiation source, measure the energy of the sum frequency light λ3 reflected by the beam splitter and entering the energy meter at different wavelengths of λ2, and record the λ2 spectra corresponding to the different energies of the sum frequency light λ3; fit the relationship curve between the peak value λ2 of different wavelengths and the energy of the sum frequency light to obtain the peak wavelength of the wavelength-tunable laser radiation source λ2 corresponding to the highest energy of the sum frequency light λ3;

[0037] Step 3: Calculate the deuterium content of the DKDP crystal to be tested based on the linear relationship between the deuterium content D and λ2 at the temperature.

[0038] For crystals that are too large to fit in a temperature-controlled furnace, the following steps are included:

[0039] Step 1: Place a KDP crystal with a deuterium content of 0% on a two-dimensional translation stage, and adjust the angle of the KDP crystal so that the lasers output by a single-frequency laser with a wavelength of λ1 and a wavelength-tunable laser radiation source λ2 are incident on the crystal.

[0040] Step 2: Adjust the output wavelength λ2 of the wavelength-tunable laser radiation source, measure the energy of the sum frequency light λ3 reflected by the beam splitter and entering the energy meter at different wavelengths of λ2, and record the λ2 spectra corresponding to the different energies of the sum frequency light λ3; fit the relationship curve between the peak value λ2 of different wavelengths and the energy of the sum frequency light to obtain the peak wavelength of the wavelength-tunable laser radiation source λ2 corresponding to the highest energy of the sum frequency light λ3; calibrate the linear relationship between the deuterium content D and λ2 at the current room temperature based on the λ2 corresponding to the KDP crystal;

[0041] Step 3: Remove the KDP crystal, install the DKDP crystal to be tested, and make the laser output from the single-frequency laser with a wavelength of λ1 and the wavelength-tunable laser radiation source λ2 be incident on the crystal normally;

[0042] Step 4: Adjust the output wavelength λ2 of the wavelength-tunable laser radiation source, measure the energy of the sum frequency light λ3 reflected by the beam splitter and entering the energy meter at different wavelengths of λ2, and record the λ2 spectra corresponding to the different energies of the sum frequency light λ3; fit the relationship curve between the peak value λ2 of different wavelengths and the energy of the sum frequency light to obtain the peak wavelength of the wavelength-tunable laser radiation source λ2 corresponding to the highest energy of the sum frequency light λ3;

[0043] Step 5: Move the crystal in the X and Y directions at regular intervals and repeat step 4 to obtain the peak wavelength of the wavelength-tunable laser radiation source λ2 corresponding to the position and frequency light λ3 when the energy is the highest;

[0044] Step 6: Based on the linear relationship between the calibrated deuterium content D and λ2, the deuterium content distribution of the large-sized DKDP crystal to be tested is calculated.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] 1. Based on the wavelength characteristics of DKDP crystals with different deuterium contents through non-critical phase matching, it is possible to achieve a full range measurement accuracy of 0-100% for different DKDP crystals with a measurement accuracy higher than 0.2% / cm -1 The measurement accuracy is improved by an order of magnitude compared with traditional spectroscopy; the online non-destructive measurement of the uniformity of deuterium content in large-aperture DKDP crystals is realized.

[0047] 2. The wavelength and temperature characteristics of non-critical phase matching of 248nm deep ultraviolet laser generated by sum-frequency generation of DKDP crystals with different deuterium contents are demonstrated, which has important reference value for researchers in the field of deep ultraviolet phase matching performance of DKDP crystals. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 Schematic diagram of the optical path of the third harmonic non-critical sum frequency verification device of the DKDP crystal deuterium content detection method based on non-critical phase matching of the present invention;

[0049] Figure 2 This is a curve showing the corresponding relationship between the tunable wavelength of the third harmonic-based non-critical sum frequency and the deuterium content of the DKDP crystal. Specific implementation instructions

[0050] The present invention will be further described in detail below with reference to the accompanying drawings and examples, but the scope of protection of the present invention shall not be limited thereto.

[0051] The present invention combines the noncritical phase matching wavelength characteristics of DKDP crystals with different deuterium contents and proposes a high-precision nondestructive detection method for the deuterium content of large-aperture DKDP crystals based on the linear relationship between the crystal deuterium content and the noncritical sum frequency wavelength. Example

[0052] Take the third harmonic as an example of a non-critical sum frequency. Figure 1 , Figure 1 This is a schematic diagram of the optical path of a third-harmonic, non-critical sum-frequency verification device for detecting the deuterium content of a DKDP crystal based on non-critical phase matching. As shown, the optical path of the detection device includes a 1064nm laser 1, an energy adjustment module 2, a third-harmonic generation module 3, an OPO wavelength tuning module 4, a spectrometer 5, a beam combiner 6, a DKDP crystal to be tested 7, a beam splitter 8, and an energy meter 9. The crystals contained in the third-harmonic generation module 3, the crystals contained in the OPO wavelength tuning module 4, and the DKDP crystal to be tested 7 are all placed in a temperature-controlled oven with a temperature control accuracy of 0.1°C. The OPO wavelength tuning module 4 can adjust the crystals contained in it to achieve continuously adjustable output wavelengths between 750 and 820nm. The DKDP crystal to be tested 7 is cut using a non-critical phase matching angle (θ = 90°, φ = 45°). The beam combiner 6 is coated with a film that transmits 750-820nm and reflects 355nm, effectively combining beams of different wavelengths. The beam splitter 8 is coated with a film that transmits 355nm and 750-820nm and reflects 240-250nm. The energy meter 9 is used to measure the deep ultraviolet laser energy reflected by the beam splitter 8, and the spectrometer 5 measures the output wavelength of the OPO wavelength tuning module 4.

[0053] During the experiment, the fundamental frequency light generated by the 1064nm laser 1 is split by the energy regulation module 2, and one beam enters the third harmonic generation module 3, where it undergoes frequency doubling and tripling to produce 355nm ultraviolet light. The other beam enters the OPO wavelength tuning module 4, where it undergoes frequency doubling to produce 532nm laser light, which then undergoes an optical parametric oscillation process to output a 750-820nm tunable near-infrared laser. After the 355nm laser and the tunable near-infrared laser are combined by the beam combiner 6, the turntable is adjusted to allow the combined beam to be incident vertically on the DKDP crystal 7 to be tested. By adjusting the output wavelength of the OPO wavelength tuning module 4, the DKDP crystal to be tested can achieve a type of non-critical sum frequency, generating deep ultraviolet light in the 248nm band.

[0054] Experimental data show that when the DKDP crystal 7 to be tested is a KDP crystal with a deuterium content of 0% and the ambient temperature is 30°C, if the output wavelength of the OPO wavelength tuning module is 770.676nm, the reading of the energy meter 7 reaches the maximum value, that is, at this time the sum frequency efficiency is the highest and deep ultraviolet non-critical phase matching is achieved. Similarly, the output wavelength of the OPO wavelength tuning module when the energy meter 7 reaches the maximum value can be measured respectively when the deuterium content of DKDP is 20%, 40%, 60%, 80%, and 98%. Based on the experimental data, the following is drawn: Figure 2 The relationship curve between the output wavelength of the OPO wavelength tuning module 4 and the deuterium content of the DKDP crystal is shown, and the relationship formula is: D=2.54466λ2-1961.21853, where D is the deuterium content of the DKDP crystal, and λ2 is the output wavelength of the OPO wavelength tuning module 4 when the energy meter 8 reads the maximum.

[0055] Further analysis revealed that when the DKDP crystal 7 to be measured is a KDP crystal with a deuterium content of 0%, the maximum energy meter reading corresponds to an optical parametric oscillator crystal angle of 770.676nm; when the DKDP crystal has a deuterium content of 98%, the maximum energy meter reading corresponds to an optical parametric oscillator crystal angle of 809.176nm. Since the output wavelength control accuracy of the OPO wavelength tuning module 4 is higher than 0.3nm, this solution can measure deuterium content with an accuracy of 0.14% / cm -1 .

[0056] Experimental results show that based on the wavelength characteristics of 248nm band generated by non-critical phase matching of DKDP crystals with different deuterium contents, this scheme achieves a full range measurement accuracy of 0-100% for DKDP crystals at different ambient temperatures, which is higher than 0.2% / cm -1By scanning different regions of the crystal, the spatial distribution of its deuterium content can be obtained, which has important application prospects in the selection of DKDP crystals for high-power laser devices. At the same time, this scheme demonstrates the wavelength characteristics of non-critical phase matching of 248nm deep ultraviolet lasers generated by sum-frequency generation of DKDP crystals with different deuterium contents. This will be of great reference value for researchers in the field of deep ultraviolet phase matching performance of DKDP crystals to further leverage the advantages of non-critical phase matching and improve the output performance of high-power deep ultraviolet laser devices.

Claims

1. A method for detecting the deuterium content of a large-scale DKDP based on non-critical phase matching, characterized in that: Including steps: Combine the fixed wavelength laser λ1 and the tunable laser λ2 and vertically irradiate the DKDP crystal to be tested; Adjust the wavelength λ2 of the tunable laser and monitor the energy change of the sum frequency light λ3 in real time; Determine the wavelength λ2 of the tunable laser corresponding to when the sum frequency light energy reaches the maximum value; The deuterium content of the DKDP crystal to be tested is calculated based on the pre-calibrated linear relationship between the deuterium content D and the wavelength λ2 of the tunable laser: D = aλ2 + b, where a and b are parameters related to the fixed wavelength laser λ1 and the ambient temperature. By scanning different regions of the DKDP crystal to be tested and repeating the above steps, the spatial distribution of deuterium content in different regions of the crystal is obtained.

2. The method for detecting deuterium content in large-scale DKDP based on non-critical phase matching according to claim 1, characterized in that: The DKDP crystal is cut at a non-critical phase matching angle of θ = 90°.

3. The method for detecting deuterium content in large-scale DKDP based on non-critical phase matching according to claim 1, characterized in that: The fixed wavelength laser λ1 has a line width of less than 0.1 nm, including but not limited to a 355 nm ultraviolet laser generated by frequency tripling of a neodymium glass laser.

4. The method for detecting deuterium content in large-scale DKDP based on non-critical phase matching according to claim 1, characterized in that: The generation method of the tunable laser λ2 includes but is not limited to optical parametric oscillation, and the wavelength resolution is better than 0.3 nm.

5. The method for detecting deuterium content in large-scale DKDP based on non-critical phase matching according to claim 1, characterized in that: The parameters a and b in the linear relationship D=aλ2+b are calibrated in the following way: (1) Use KDP crystals with a deuterium content of 0% as the standard sample; (2) Measure its characteristic wavelength λ under the same experimental conditions 20 ; (3) Establish a calibration curve based on samples with known deuterium content.

6. A detection device for implementing the method according to any one of claims 1 to 5, characterized in that: include: Single-frequency laser, used to provide fixed wavelength laser λ1; A wavelength tunable laser radiation source, used for outputting laser radiation λ2 with continuously tunable wavelength; Beam combiner, used to combine two laser beams from a non-coaxial single-frequency laser and a wavelength-tunable laser radiation source, and vertically incident on the DKDP crystal to be tested; Beam splitter and energy meter, used to separate and measure the energy of sum frequency light λ3 generated in the DKDP crystal to be tested; The spectrometer is used to measure the output wavelength λ2 of the wavelength-tunable laser radiation source corresponding to the maximum reading of the energy meter.

7. The detection device according to claim 6, characterized in that The single-frequency laser is a narrow-linewidth laser, including but not limited to neodymium glass lasers and their harmonics and other radiation sources; the wavelength-tunable laser radiation source can achieve narrow-linewidth wavelength continuously adjustable output, and the wavelength tuning method includes but is not limited to optical parametric oscillation and other methods.

8. The detection device according to claim 6, characterized in that It also includes a temperature-controlled furnace or a two-dimensional translation stage, which are used for constant temperature measurement of small-sized crystals and spatial scanning of large-sized crystals respectively.

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