CN112921402A

CN112921402A - Improve CO2Method for frequency doubling efficiency of laser

Info

Publication number: CN112921402A
Application number: CN202110058758.0A
Authority: CN
Inventors: 李强; 边慧征; 王晨旭; 雷訇; 惠勇凌; 朱占达
Original assignee: Beijing University of Technology
Current assignee: Beijing University of Technology
Priority date: 2021-01-16
Filing date: 2021-01-16
Publication date: 2021-06-08
Anticipated expiration: 2041-01-16
Also published as: CN112921402B

Abstract

The invention discloses a method for improving CO₂The method for increasing the frequency doubling efficiency of the laser adopts a mode of increasing the thickness of a single-layer wafer to a magnitude of more than 300 mu m, and the thickness error between wafers can be controlled to be 0.5 lambda to 0.3 mu m, the peak-to-valley value of the flatness is 0.056 lambda, and the root mean square value is 0.009 lambda. The polished crystal is stacked by reversing the polarization direction of the adjacent wafers by 180 degrees, and the wafers are firmly combined together through a high-temperature pressurized thermal bonding process in a vacuum environment. The invention has substantive characteristics and remarkable progress, and is relatively to the CO adopted at present₂The frequency doubling mode of the laser has the following advantages: the wafer processing quality and precision are improved, the thickness error is small, and the roughness and the planeness are reduced(ii) a The bonding layer number can reach more than 50 layers, and the average single-layer interface loss after thermal bonding is low; the clear aperture is large and can reach 20 mm; effectively improve CO₂The output power and the conversion efficiency of the frequency multiplication of the laser.

Description

Improve CO2Method for frequency doubling efficiency of laser

Technical Field

The invention relates to a frequency multiplier prepared by quasi-phase matching gallium arsenide crystal and used for CO₂The laser frequency doubling to produce middle infrared laser belongs to the field of non-linear optics technology.

Background

CO₂The laser frequency doubling light is in a wave band of 3-5 mu m, has the characteristics of low absorption and small scattering in the atmosphere, has an atmospheric transmission rate of more than 70 percent in the wave band, can realize long-distance transmission in the atmosphere, and has important significance in the fields of infrared detection, remote sensing, spectroscopy and the like. Meanwhile, high-power and high-efficiency frequency doubling light is output, and the method has important significance for the application of mid-infrared laser. Compared with most birefringent matching crystals with low thermal conductivity and low laser damage threshold, the quasi-phase matching crystal can achieve higher frequency doubling efficiency. The commonly used quasi-phase matching frequency doubling crystals comprise InSb, GaP, ZnS, ZnSe, InP and the like, and the crystals have the problems of large absorption coefficient, small nonlinear coefficient or high growth difficulty of high-quality single crystals and influence on CO₂The application of the frequency doubling of the laser.

Using first order quasi-phase matched gallium arsenide crystals for CO₂The following problems mostly exist in laser frequency doubling: CO 2₂The output wavelength of the laser is in a band of 9-10 mu m, the gallium arsenide coherence length Lc matched with the output wavelength is in the order of 100 microns, the quality and the precision of wafers of the laser are difficult to guarantee by the existing polishing process, the wafers are easy to deform in the processing process, so that the thickness deviation between the wafers is large, the transmittance of the thermally bonded wafers to fundamental frequency light is not uniform, the loss of a single-layer interface is difficult to reduce, and the frequency doubling efficiency is seriously influenced. In

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1 and 2, the GaAs crystal has a phenomenon of nonuniform transmittance to light with a wavelength range of 4-12 μm, the maximum phase difference can be close to 15%, and CO realizing large aperture₂Difficulty is caused by laser frequency doubling output; in both of

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2 and 3, the GaAs crystal has a problem that the single layer thickness changes greatly after thermal bonding, and the variation in document 2 is 4 to 5 μm, and the variation in document 3 is 2 to 3 μm, and the deviation of the double frequency output from the fundamental frequency light is large. (reference 1, Gordon L, Woods G L.Diffusion-bonded stacked GaAs for quasiphase-matched second-harmonic generation of a carbon dioxide laser[J]Electronics Letters,1993,29(22): 1942-; document 2, Wanghao, Huihongling, Jiangmuihua, et al, quasi-phase matching gallium arsenide crystal preparation process and pulse CO₂Laser frequency doubling study [ J]Laser of china, 2014; document 3, Guo Zhi, Huidong Ling, Borui, et al, GaAs Crystal diffusion bonding Process optimization [ J]Laser and infrared, 2019,049(010) 1206-1211).

It is proposed to increase the thickness of a single layer wafer, making full use of GaAs in CO₂The laser has the advantages of less absorption of the output waveband of the laser, relatively high thermal conductivity, large nonlinear coefficient and the like, the thickness of the wafer is increased to be more than 300 mu m, the deformation of the wafer is small, the thickness error precision and the polishing quality between the wafers can be ensured by the conventional process, the loss of a frequency doubling device is further reduced, and the CO with high conversion efficiency is realized₂And (5) outputting laser frequency multiplication.

Disclosure of Invention

The invention aims to solve the problems that in the current quasi-phase matching wafer processing process, because the wafers are thin and easy to deform, the thickness deviation between the wafers is large, the transmittance of the thermally bonded wafers to fundamental frequency light is not uniform, and the interface loss is difficult to further reduce, and provides a method for improving CO₂A new method for frequency doubling efficiency of laser.

Improve CO₂The method for the frequency doubling efficiency of the laser is integrally divided into three steps of wafer polishing, bonding operation in a vacuum environment and heat treatment. Step 1, gallium arsenide is used as a quasi-phase-matching frequency doubling crystal to be vertical to [0-11]]The crystal orientation cuts and polishes the crystal to a 328-micron wafer, which is advantageous for achieving a high-quality wafer using an existing process, and the thickness deviation between wafers can reach 0.5 λ ═ 0.3 μm, the roughness Ra ═ 0.41nm, the flatness peak-to-valley value of 0.056 λ, and the root mean square value of 0.009 λ (λ ═ 632.8 μm), which are superior to the quality of a first-order quasi-phase-matched crystal (document 3, the roughness of 0.5nm, the flatness peak-to-valley value of 0.06 λ, and the root mean square value of 0.013 λ).

Step 2, removing the crystal from the wafer obtained in the step 1 by using acetone, alcohol and deionized waterThe wafer surface grinding fluid is prepared by pickling in hydrochloric acid mixed solution (hydrochloric acid with purity of 38% and ionized water are mixed according to a volume ratio of 1: 1) for 2 minutes to remove oxide on the wafer surface, performing optical gluing in a vacuum environment by sequentially rotating 180 degrees in a polarization direction, and finally performing vacuum-pumping treatment in a vacuum ball until the vacuum degree is 10^-7Pa, the operation can effectively reduce the loss caused by the absorption of the residue and the oxide on the surface of the wafer to fundamental frequency light and frequency doubling light.

Step 3, placing the crystal obtained in the step 2 in a vacuum hot pressing furnace for heat treatment, controlling the heating speed to be 0.37 ℃/min, heating to 560 ℃, keeping the temperature for 20 hours, and keeping the constant pressure to be 0.27kgf/mm in the heat preservation process²And cooling to room temperature of 25 ℃ to tightly bond the wafers between the layers together, thus obtaining 47 layers of low-loss quasi-phase-matched gallium arsenide crystals, wherein the average single-layer interface loss of the low-loss quasi-phase-matched gallium arsenide crystals is lower than 0.11 percent and is better than the quality of the first-order quasi-phase-matched crystals (document 3 is the best result reported so far, and the average single-layer interface loss of the low-loss quasi-phase-matched gallium arsenide crystals is 0.18 percent.

By increasing the thickness of the wafer, the processing quality and precision of the wafer are improved. According to the quasi-phase matching theory, only odd-order matching modes, namely three-order, five-order and seven-order quasi-phase matching can be adopted, and in consideration of reducing the absorption of fundamental frequency light and frequency doubling light as much as possible, a three-order quasi-phase matching wafer is adopted for frequency doubling experiments. The wafer has a thickness 3 times that of the first order, and has an absorption coefficient of 0.005cm^-1And the absorption of light is less (even if the absorption of light is only 1% in 47 layers of third-order QPM-GaAs crystal), and the same frequency doubling efficiency output value can be realized under the condition of the same first-order layer number.

Compared with other common quasi-phase matching frequency doubling crystals, the gallium arsenide crystal has the advantages of small absorption coefficient, large nonlinear coefficient and high damage threshold. By the diffusion bonding method, the thickness of a single-layer wafer is increased, dozens of layers of large-area bonding can be realized, the aperture of incident fundamental frequency light transmission is improved, the average single-layer thickness change is small after bonding, the single-layer interface loss is further reduced, and a foundation is laid for realizing high-power and high-conversion-efficiency frequency multiplication output.

The invention has substantive characteristics and remarkable progress, and is relatively to the CO adopted at present₂The mode of the invention has the following advantages: (1) the thickness of the wafer is increased, the processing quality and precision of the wafer are improved under the prior art, the thickness deviation between the wafers after polishing is reduced to 0.3 μm, and the roughness and the plane peak-valley value are reduced to 0.41nm and 0.06 λ (λ is 632.8 μm); (2) the wafer breakage rate is reduced in the photoresist process in the vacuum environment, and 50-100 layers of bonding can be realized; (3) compared with the interlayer compression thickness of 3-5 mu m after the first-order quasi-phase matching wafer is subjected to heat treatment, the quality of the wafer is improved by the method, the interlayer compression thickness is as low as 1 mu m, and the average single-layer interface loss is effectively reduced to 0.11%; (4) theoretically, the gallium arsenide crystal with the same layer number as the first-order quasi-phase matching structure can realize the output result with the same efficiency, and the interface loss of the method is lowest, so that the CO can be further effectively improved₂The output power and the conversion efficiency of the frequency multiplication of the laser.

Drawings

Fig. 1 is a schematic stack diagram of a device of the present invention.

Figure 2 is the crystal orientation of a single layer gallium arsenide wafer.

FIG. 3 is CO₂The laser output wavelength corresponds to the coherence length of the GaAs crystal.

Fig. 4 is a measurement result of wafer roughness and flatness.

Fig. 5 is a transmittance curve of a 47-layer quasi-phase-matched crystal.

Fig. 6 is a third order quasi-phase matching frequency multiplication principle.

Fig. 7 is a graph of the first and third order quasi-phase matching frequency multiplication efficiency for the same number of layers.

Fig. 8 is a plot of the frequency multiplication efficiency under third-order quasi-phase matching.

FIG. 9 is a graph of thickness deviation versus frequency doubling efficiency.

Detailed Description

As shown in FIG. 1, the invention adopts third-order quasi-phase matching crystal to prepare multilayer frequency doubling device, and the preparation process can be divided into the following steps. Firstly, determining the crystal orientation of the gallium arsenide wafer, [0-11] is the light passing direction, [0-1-1] is the polarization direction of fundamental frequency light, and [100] is the polarization direction of the crystal. Cutting into slices with the thickness of 2 mm; secondly, thinning and polishing the double surfaces to a specified thickness, and performing pre-bonding under a vacuum condition; and thirdly, carrying out high-temperature thermal bonding.

The absorption coefficient of the gallium arsenide crystal is 0.005cm^-1The second-order nonlinear coefficient is 83pm/V, which is one order of magnitude smaller than that of common quasi-phase matching crystal gallium phosphide and two orders of magnitude smaller than that of indium phosphide. The second order nonlinear coefficient of gallium arsenide is 83pm/V, which is 1.5 times that of zinc selenide.

The cut crystal orientation of gallium arsenide is shown in fig. 2. Three mutually vertical crystal directions in the figure respectively correspond to the incident direction of fundamental frequency light, the polarization direction and the crystal polarization direction, when wafers are stacked, the wafers rotate 180 degrees by taking the polarization direction [0-1-1] as an axis each time, the polarization direction [100] is inverted layer by layer to be stacked, and the wafers are combined into a whole through a thermal bonding method after the stacking is finished.

The shape of the wafer is circular, considering CO after confinement of the fundamental mode₂The output light spot of the laser is circular, and compared with a rectangular shape, the shape can effectively utilize the light transmission area of the crystal and reduce the damage of a sharp edge to the polishing pad in the wafer polishing process. In addition, long and short trimming processing is performed on the circular wafer for distinguishing the polarization direction from the polarization direction. The coherence length of the wafer corresponds to the fundamental wavelength, CO₂The range of the laser outputting the fundamental frequency light is 9.26-10.73 μm, the coherence length is 105.5-109.5 μm, and the corresponding relationship is shown in FIG. 3. Since the wafer thickness is 300 μm or more, the polishing process can improve the wafer quality and precision, and as shown in fig. 4, the thickness process error is 0.5 λ (λ 632.8 μm) to 0.3 μm, the roughness is 0.41nm, the flatness peak-to-valley value is 0.056 λ, and the root mean square value is 0.009 λ (λ 632.8 μm), which are superior to the data of the first-order QPM crystal of the present topic group (Ra 0.5nm, flatness peak-to-valley value is 0.06 λ, and root mean square value is 0.013 λ).

The wafer is thinned and polished to a specified length, and the outermost layer of dozens of layers of frequency doubling devices is not coated with a film, so that the damage threshold of the crystal is reduced due to the coating, and the frequency doubling output is influenced. The polished wafer is firstly cleaned by acetone and alcohol to remove surface grinding fluid and other pollutants so as to reduce interface scattering loss caused by grinding residues, and secondly, mixed solution of hydrochloric acid and deionized water with the volume ratio of 1:1 is used for acid cleaning so as to remove the surface oxide of the wafer and reduce the interface loss caused by absorption of the surface oxide. And finally, carrying out optical cement pre-bonding in a vacuum environment, and carrying out high-temperature pressurizing thermal bonding treatment in a vacuum hot-pressing furnace to finally obtain the low-loss gallium arsenide frequency doubling device. The first-order quasi-phase matching crystal is adopted to prepare a multilayer frequency multiplier, the single-layer thickness of the crystal after thermal bonding treatment is reduced by 2-5 mu m, and the average single-layer interface loss is usually not higher than 0.3%, but after the thickness is increased, the average single-layer thickness is reduced by only 1 mu m, and the average single-layer interface loss is reduced to 0.1% or even lower, as shown in fig. 5.

The wafers are stacked through tens of layers in CO₂In the frequency doubling process of the laser, the conversion of fundamental frequency light energy to frequency doubling light is realized once when the fundamental frequency light passes through one layer of wafer, so that the high-efficiency frequency doubling light conversion is realized. Although the first two coherence lengths do not achieve an increase in the frequency doubled light in each layer of the wafer, gallium arsenide does not cause a particularly large loss of both lights because of its low absorption coefficient for fundamental and frequency doubled light. Fig. 6 is a curve showing the variation of the frequency doubling efficiency with the number of layers of the third-order quasi-phase matching wafer, where the number of layers is larger and the frequency doubling efficiency is higher within a certain range.

As shown in fig. 7, neglecting the crystal absorption, the third-order quasi-phase matching crystal can achieve the same frequency doubling efficiency as the first-order quasi-phase matching crystal in the same number of layers theoretically. Setting the number of layers as 20 (the abscissa is the multiple of the coherent length Lc, the first-order quasi-phase matching wafer thickness is Lc, the third-order wafer thickness is 3Lc), the fundamental frequency light is 9.57 μm, and the optical power density is 5MW/cm²Under the condition, the frequency doubling efficiency obtained in the two cases is 7.57%, and the feasibility and the significance of the third-order quasi-phase matching are demonstrated.

As the number of layers of the wafer increases, the frequency doubling efficiency is also improved. As shown in FIG. 8, the wavelength of fundamental light is set to be 9.57 μm, and the optical power density is set to be 30MW/cm²And obtaining a relation curve of frequency doubling efficiency and the layer number. When the number of layers reaches 100, the frequency doubling efficiency can reach 60%. The damage threshold of the GaAs wafer is 50-70 MW/cm²What is, what isSo that the optical power density can be increased appropriately to improve the frequency doubling efficiency. Meanwhile, as shown in FIG. 9, when the thickness is varied by 1 μm, the fundamental wavelength is 9.57 μm, and the optical power density is 5MW/cm²The first-order QPM wafer drops by 28%, while the third-order drops by only 3%. Therefore, the efficiency of the third-order QPM wafer is reduced less with the same thickness variation between wafers, and the absorption coefficient of GaAs is 0.005cm for fundamental light with a wavelength of 9.57 μm although the absorption of light is increased with an increase in the single-layer thickness of the wafer^-1Gallium arsenide with a length of 2cm also absorbs only 1% of the incident fundamental light. In summary, the third-order QPM-GaAs crystal realizes high-efficiency CO₂And (3) good selection of laser frequency doubling.

Claims

1. Improve CO₂The method for the frequency doubling efficiency of the laser is characterized in that: comprises three steps of wafer polishing, bonding operation in vacuum environment and heat treatment; step 1, gallium arsenide is used as a quasi-phase-matching frequency doubling crystal to be vertical to [0-11]]The crystal orientation cuts and polishes the crystal to a 328 micron wafer, the 328 micron wafer being advantageous for achieving high quality wafers using existing processes, with thickness variation between wafers of 0.5 λ 0.3 μm, roughness Ra 0.41nm, flatness peak-to-valley of 0.056 λ, root mean square of 0.009 λ, superior to the quality of first-order quasi-phase-matched crystals;

step 2, removing the grinding fluid on the surface of the wafer obtained in the step 1 by using acetone, alcohol and deionized water, pickling in a hydrochloric acid mixed solution for 2 minutes to remove oxide on the surface of the wafer, carrying out optical cement in a vacuum environment in a mode of sequentially rotating 180 degrees in a polarization direction, and finally carrying out vacuum-pumping treatment in a vacuum ball until the vacuum degree is 10^-7Pa；

Step 3, placing the crystal obtained in the step 2 in a vacuum hot pressing furnace for heat treatment, controlling the heating speed to be 0.37 ℃/min, heating to 560 ℃, keeping the temperature for 20 hours, and keeping the constant pressure to be 0.27kgf/mm in the heat preservation process²And cooling to room temperature of 25 ℃ to tightly combine the wafers between the layers together to obtain 47 layers of low-loss quasi-phase-matched gallium arsenide crystals, wherein the average single-layer interface loss of the crystals is lower than 0.11 percent and is better than the quality of the first-order quasi-phase-matched crystals.

2. A method of increasing CO according to claim 1₂The method for the frequency doubling efficiency of the laser is characterized in that: by increasing the thickness of the wafer, the processing quality and precision of the wafer are improved; according to the quasi-phase matching theory, only odd-order matching modes, namely three-order, five-order and seven-order quasi-phase matching, can be adopted.

3. A method of increasing CO according to claim 1₂The method for the frequency doubling efficiency of the laser is characterized in that: in consideration of reducing the absorption of fundamental frequency light and frequency doubling light as much as possible, a third-order quasi-phase matching wafer is used for a frequency doubling experiment.

4. A method of increasing CO according to claim 1₂The method for the frequency doubling efficiency of the laser is characterized in that: in step 2, hydrochloric acid with a purity of 38% and ionized water are mixed according to a volume ratio of 1: 1.

5. A method of increasing CO according to claim 1₂The method for the frequency doubling efficiency of the laser is characterized in that: and (3) the polished crystals are stacked by reversing the polarization direction of the adjacent wafers by 180 degrees, and the wafers are bonded together through a high-temperature pressurizing thermal bonding process in a vacuum environment.