CN102566197B - Optical parametric frequency multiplication conversion device with phase matching condition insensitive to temperature - Google Patents

Abstract

The invention belongs to the field of laser technology, and specifically relates to an optical parameter frequency multiplication conversion device capable of realizing phase matching conditions that are not sensitive to temperature. The device is mainly composed of two different nonlinear crystals in a cross-cascaded manner. For the optical parametric frequency doubling process of a specific wavelength, the sign of the first-order partial derivative of the phase mismatch due to the change of the operating temperature of the two crystals with respect to temperature is opposite. When the operating temperature of the crystal deviates from its phase-matching temperature, the phase mismatch accumulated in the first crystal will be compensated in the second crystal cascaded behind it. This device can effectively reduce the temperature sensitivity of the phase matching condition in the process of optical parametric frequency doubling conversion. Moreover, this device also has the advantage that the working wavelength can be designed, and can be used to construct a high average power frequency-doubling conversion laser system with different wavelengths.

Description

Optical parametric frequency doubling conversion device with phase matching condition insensitive to temperature

technical field

The invention belongs to the field of laser technology, and in particular relates to an optical parameter frequency multiplication conversion device capable of realizing that the phase matching condition is not sensitive to temperature.

Background technique

Optical parametric frequency doubling conversion technology can expand the working wavelength of various laser sources, and it can obtain a series of coherent radiation with different wavelengths from a fixed-frequency laser source to meet the needs of various practical applications. Now, optical parametric frequency doubling conversion technology has been widely used in laser sources with single pulse output or low average power. However, the low conversion efficiency and unsatisfactory beam quality due to thermal effects seriously restrict the application of frequency doubling conversion technology in high average power laser sources. There are many reasons why frequency-doubled light cannot achieve high average power output. One of the main reasons is that nonlinear crystals will absorb the energy of fundamental-frequency light and frequency-doubled light to generate thermal gradients, which will cause temperature differences inside them. Since the refractive index of the crystal will change with the temperature, the uneven distribution of the refractive index on the crystal cross section will make the whole crystal unable to meet the phase matching conditions at the same time, resulting in the decrease of the frequency conversion efficiency and the deterioration of the beam quality. Excessive thermal gradients can even cause damage to the crystal.

In order to control the thermal load and compensate the thermal effects in nonlinear crystals used for high average power frequency conversion, many research institutions represented by Livermore Laboratory (LLNL) in the United States have proposed various methods, including Beam shaping and multi-chip thin crystal design with light-transmitting surface air cooling and other methods. However, because these technologies need to use a lot of extra devices, resulting in complex structures, and the overall effect is not very obvious, making little progress in frequency doubling conversion with high average power. In recent years, the appearance of YCOB crystals has substantially promoted the development of high average power frequency doubling conversion technology. However, in general, a nonlinear crystal can only achieve frequency doubling conversion for one wavelength, and the phase matching condition is not sensitive to temperature. In other bands, the frequency doubling of high average power is still limited by the thermal effect of the crystal. Therefore, this method, which relies on the optical properties of the nonlinear crystal material itself, is not universally applicable.

Contents of the invention

Aiming at the phase mismatch problem caused by uneven temperature distribution faced by current high-average-power frequency-doubling conversion, the purpose of the present invention is to provide an optical parametric frequency-doubling conversion device applicable to high-average power laser sources.

The frequency doubling conversion device proposed by the present invention, which can realize that the phase matching condition is not sensitive to temperature, is specifically composed of two different nonlinear crystals in a cross-cascaded manner. Among them, for the optical parametric frequency doubling process of a specific wavelength, the signs of the first-order partial derivatives of the phase mismatch caused by the change of the operating temperature of the two crystals with respect to the temperature are opposite.

The nonlinear crystal will absorb the energy of the fundamental frequency light and the frequency doubled light to generate a thermal gradient, and then cause the crystal to have an inhomogeneous distribution of temperature on its cross section. Only those parts of crystals working at the phase-matching temperature can satisfy the phase-matching condition. For most of the remaining crystals, the deviation from the phase matching temperature means that the phase mismatch ?k between the input fundamental frequency light and the output frequency doubled light will be introduced. Figure 1 shows several typical nonlinear crystals when the phase matching temperature is 20 ^o C: KH ₂ PO ₄ (KDP) (short dashed line), LiB ₃ O ₅ (LBO) (dotted line), BaB ₂ O ₄ (BBO) (dotted line) and YCOB (xy plane: dotted line; xz plane: solid line), the first-order partial derivative of the phase mismatch due to the change of operating temperature varies with the wavelength of the incident fundamental frequency light and changing graphs. Of particular importance to the present invention is the ability to find (i.e. exist) two opposite-signed first-order partial derivatives of the thermally induced phase mismatch with respect to temperature over a wide spectral range (from ~700 nm to greater than 2 μm). Crystals, such as 1064 nm KDP crystals and LBO crystals, and 1550 nm BBO crystals and YCOB crystals.

The structure used in the present invention is shown in Figure 2. According to different needs, two or more of these two different nonlinear crystals can be used as a working medium for nonlinear frequency conversion. Due to the existence of thermal effects, when the operating temperature of the crystal deviates from its phase matching temperature, the two crystals no longer satisfy the phase matching condition (?k ≠ 0), and, as the degree of deviation increases, the resulting phase loss Matching will be more serious. However, if the signs of the thermally induced phase mismatch of the two crystals are opposite, that is, one ?k is greater than 0 and the other ?k is less than 0, then the accumulated phase mismatch in the first crystal The amount is compensated in the second crystal cascaded after it. The higher the number of crystals used, the less sensitive the phase-matching condition is to temperature. In order to ensure that the frequency doubling conversion process can be carried out stably, generally two to four crystals can be used in a cross-cascaded design. Shown in the figure is a structure consisting of four crystals cross-connected.

During use, corresponding to different actual situations, there will be an optimal value for the length ratio of two adjacent different crystals, so that the phase matching condition of this optical parametric frequency doubling conversion device is the least sensitive to temperature. This optimal value is determined by the thermally induced phase mismatch of the crystal and the effective nonlinear coefficient, which can be roughly obtained by computer numerical simulation to solve nonlinear coupled wave equations. According to different use cases, this optimal value will be different, and there is no connection between these optimal values. The present invention has no requirement on the ratio of the widths of two adjacent crystals, and only needs the aperture of the crystal to allow the light beam to pass through completely. This device can effectively reduce the temperature sensitivity of the phase matching condition in the process of optical parametric frequency doubling conversion. Moreover, it also has the advantage that the working wavelength can be designed, and can be used to construct a high average power frequency-doubling conversion laser system with different wavelengths.

The frequency multiplication conversion device of the present invention not only has a simple structure, does not need additional devices, but also has obvious effects. More importantly, this device is wavelength-designable. As long as a suitable crystal is selected, it can realize phase-matching condition-insensitive frequency conversion at the wavelength of interest.

Description of drawings

Figure 1 shows the curves of the first-order partial derivative of the phase mismatch due to the change of the operating temperature of several typical nonlinear crystals as a function of the wavelength of the incident fundamental frequency light.

Fig. 2 is a schematic diagram of an optical parametric frequency multiplication conversion device designed according to the present invention with phase matching conditions insensitive to temperature.

FIG. 3 is a schematic diagram of an experimental setup for a proof-of-principle experiment conducted in order to demonstrate the present invention.

Fig. 4 is a graph showing the variation of frequency conversion efficiency of different frequency conversion systems with temperature obtained from experiments.

In the figure: 1 is the nonlinear crystal A, 2 is the nonlinear crystal B adjacent to the nonlinear crystal A, 3 is the narrow-band picosecond laser source, 4 is the picosecond fundamental frequency light, 5 is the first nonlinear crystal, 6 is the frequency-doubled laser and the remaining fundamental-frequency laser after passing through the first nonlinear crystal, 7 is the second nonlinear crystal, 8 is the frequency-doubled laser and the remaining fundamental-frequency laser after passing through the second nonlinear crystal, and 9 is Optical filter, 10 is frequency-doubled laser, and 11 is a laser power meter.

Detailed ways

Further describe the present invention below in conjunction with accompanying drawing.

Fig. 2 shows an optical parametric frequency multiplication conversion device designed according to the present invention, whose phase matching condition is not sensitive to temperature. The device includes two different nonlinear crystals (A and B), adopts the design scheme of cross-connection of two or more of these two different nonlinear crystals, and is used as a working medium for nonlinear frequency doubling conversion. Among them, for the optical parametric frequency doubling process of a specific wavelength, the signs of the first-order partial derivatives of the phase mismatch caused by the change of the operating temperature of the two crystals with respect to the temperature are opposite. The following mainly describes the situation when there are only two crystals.

Assume that at room temperature (20 ^o C), the two nonlinear crystals used for frequency doubling meet the phase matching conditions, and as the high average power frequency doubling conversion process proceeds, due to the gradual accumulation of thermal effects, the nonlinear crystal The temperature inside will deviate from the original temperature, destroying the phase matching condition that has been satisfied.

Due to the phase mismatch between the incident fundamental frequency light and the double frequency light, when the cumulative phase difference between them reaches π, that is to say, when the action length in the crystal is equal to its coherence length ( _LC = π/?k), the new The generated frequency-doubled light will coherently cancel out the previously generated frequency-doubled light, and the reverse process of frequency doubled—backflow will begin. As the crystal action length continues to increase, the doubled frequency light will be converted back to the fundamental frequency light, and the conversion efficiency will gradually decrease.

However, the signs of the thermally induced phase mismatch of the two frequency-doubling crystals used in the present invention are opposite, that is to say, one of them ?k is greater than 0 and the other ?k is less than 0, then in the first crystal A The phase difference (Δφ ₁ = ?k ₁ L ₁ ) between the accumulated fundamental frequency light and the doubled frequency light will be compensated in the second crystal B cascaded behind it (Δφ ₂ = ?k ₂ L ₂ ). Therefore, in this case, as long as the phase difference in the two different nonlinear crystals is less than π during the entire frequency conversion process, the generated frequency doubled light can always maintain coherent growth, corresponding to the frequency conversion positive process. Therefore, the sensitivity of the frequency doubling conversion efficiency to temperature changes will be greatly reduced, and then the optical parametric frequency doubling conversion whose phase matching condition is not sensitive to temperature can be realized. Also, the higher the number of crystals used, the less the conversion efficiency will be affected by temperature changes.

FIG. 3 is a diagram of an experimental setup for a proof-of-principle experiment conducted in order to demonstrate the present invention.

The device includes an optical parameter frequency doubling conversion system designed according to the invention, whose phase matching condition is insensitive to temperature, and a laser power meter for measurement. in:

The picosecond laser 4 emitted by the narrow-band picosecond laser source 3 passes through the first nonlinear crystal 5 satisfying a positioning matching angle, and undergoes optical parametric frequency doubling conversion (SHG). Then, pass the obtained frequency-doubled laser light and the remaining fundamental-frequency laser light 6 through the second nonlinear crystal 7 to perform SHG again. After the remaining fundamental-frequency laser light that does not participate in the function is filtered out by the filter 9 , the finally obtained frequency-doubled light 10 is measured by a laser power meter 11 . In the specific experiment, this device uses two reasonably designed KDP crystals and LBO crystals as the first nonlinear crystal 5 and the second nonlinear crystal 7 respectively, which are used as the working medium of the optical parametric frequency doubling conversion system. It can be seen from Figure 1 that for the frequency doubling process at 1064 nm, the signs of the thermally induced phase mismatch of the two nonlinear crystals are opposite. At the same time, the two crystals are respectively placed in two digital temperature-controlled furnaces. By adjusting the operating temperature of the nonlinear crystal in the temperature-controlled furnace, the curve of frequency-doubling conversion efficiency with temperature can be finally obtained.

Figure 4 shows the curve of the frequency doubling conversion efficiency obtained in the experiment as a function of temperature. At the same time, for the traditional frequency doubling conversion system using a single crystal as a nonlinear medium, the output frequency doubling conversion efficiency is also shown in the figure. Curves as a function of temperature. In order to compare the sensitivity of the phase matching conditions of two different frequency multiplication conversion systems to temperature fairly and intuitively, each frequency multiplication conversion system in the figure is designed to obtain comparable frequency multiplication conversion efficiency when the phase matching conditions are met. . It can be seen from the figure that as the operating temperature of the nonlinear crystal gradually deviates from its phase matching temperature, the occurrence of phase mismatch will lead to a decrease in frequency doubling conversion efficiency, and the more sensitive to temperature, the faster the decrease. However, if two kinds of KDP crystals (triangle symbols) and LBO crystals (rectangle symbols) that are sensitive to temperature changes at 1064 nm frequency doubling are cross-connected (circle symbols), it is possible to Improve the frequency conversion efficiency within the range. Likewise, this method can also be used at other doubled wavelengths, such as 1550 nm. However, there is still no frequency-doubling crystal that can realize phase matching conditions in this band and is insensitive to temperature.

It can be seen that the cross-cascade connection of two different nonlinear crystals proposed by the present invention as a frequency multiplication conversion device used as a nonlinear frequency multiplication conversion working medium can better solve the problem caused by uneven temperature distribution. Phase mismatch, resulting in the reduction of frequency doubling conversion efficiency and the deterioration of beam quality. It is not only simple in structure, does not need to use additional devices, but also has obvious effects. More importantly, this device can be designed for the wavelength, as long as the appropriate crystal is selected, it can realize the phase-matching condition in the band of interest and the frequency conversion that is not sensitive to temperature. It can be used to construct high average power frequency-doubling conversion laser systems with different wavelengths.

Claims (2)

1. the position temperature-resistant optical parameter doubling conversion device of condition that matches, is characterized in that intersecting cascade by two kinds of different nonlinear crystals forms, as the actuating medium of non-linear frequency-doubled conversion, wherein:

For the optical parameter frequency multiplication process of specific some wavelength, the symbol of the single order local derviation of the position phase amount of mismatch that these two kinds of crystal cause because working temperature changes to temperature is contrary.

2. according to claim 1 the temperature-resistant optical parameter doubling conversion device of condition that matches, is characterized in that the length ratio of adjacent two blocks of different nonlinear crystals exists an optimal value.

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