Preparation method of PIT-OPO middle and far infrared laser frequency conversion device

Abstract

The invention discloses a preparation method of a PIT-OPO middle and far infrared laser frequency conversion device, relates to a preparation method of a middle and far infrared laser frequency conversion device, and aims to solve the technical problem that a proper frequency conversion device for middle and far infrared laser output is lacked at present. The method comprises the following steps: cutting a PIT crystal into a cuboid according to the azimuth angle of 30 degrees and the phase matching angle of 36-62 degrees, or cutting the PIT crystal into a cuboid according to the azimuth angle of 0 degree and the phase matching angle of 45-90 degrees, and then polishing, cleaning and drying to obtain the middle and far infrared laser frequency conversion device. A Q-switched or mode-locked 2.9 [mu] m Er laser, a gain switch or mode-locked 2.5 [mu] m Cr laser and a Q-switched 2.1 [mu] m Ho or 1.9 [mu] m Tm laser are used as incident light sources to irradiate the crystal device, middle and far infrared laser covering 3-5 [mu] m or 8-14 [mu] m is output, the absorption coefficient is lower than 0.1 cm <-1 >, and the crystal device can be used in a middle and far infrared solid laser.

Description

Technical Field

[0001] The invention relates to a method for preparing a mid- and far-infrared laser frequency conversion device. Background Art

[0002] Coherent tunable laser sources in the mid-infrared and far-infrared (2.5-30μm) bands have important applications in key equipment such as optoelectronic countermeasures, chemical identification, biomedical diagnosis, and terahertz spectroscopy. At present, most high-power, tunable, and relatively universal solid-state laser sources operate in the near-infrared (0.7-3μm) band, and the wavelength can be converted to the mid-infrared and far-infrared and terahertz regions through a frequency down-conversion process. At present, there is a lack of high-quality nonlinear optical crystals for mid-infrared and far-infrared laser output. Summary of the invention

[0003] The present invention aims to solve the technical problem that there is a lack of suitable and efficient frequency conversion devices for mid- and far-infrared laser output at present, and provides a method for preparing a PIT-OPO mid- and far-infrared laser frequency conversion device. The present invention utilizes OPO technology to achieve mid- and far-infrared laser output of PIT crystals covering two important atmospheric windows of 3-5μm and 8-14μm.

[0004] The preparation method of the PIT-OPO mid-to-far infrared laser frequency conversion device of the present invention comprises the following steps:

[0005] 1. PbIn 6 Te 10 (PIT) crystal, by azimuth The phase matching angle of type I is 30° and the phase matching angle of type I is 36° to 62°. The device is cut into a rectangular block with a phase matching angle of 0° and a type II phase matching angle θ of 45° to 90° to obtain a semi-finished device;

[0006] 2. Physically polishing the crystal surface of the semi-finished device to a roughness of less than 1.0 nm and a flatness of less than λ / 6, where λ = 632.5 nm;

[0007] 3. Mix hydrogen peroxide with a mass percentage concentration of 5%, ammonia water with a mass percentage concentration of 25% and deionized ultrapure water in a volume ratio of 1:(1-2):(10-20) to obtain a cleaning solution; immerse the semi-finished device treated in step 2 in the cleaning solution and perform ultrasonic treatment for 5-30 seconds to remove the oxide layer and adherent impurities on the surface of the crystal after polishing, so that the smoothness is better than 40 / 20; then clean it with deionized ultrapure water and ethanol in turn, and dry it to obtain a PIT-OPO mid-to-far infrared laser frequency conversion device.

[0008] Furthermore, in step 1, PbIn 6 Te 10(PIT) crystal, according to the azimuth angle is 30°, the class I phase-matching angle (θ) is 46° - 62°, cut into a cuboid, or according to the azimuth angle is 0°, the class II phase-matching angle (θ) is 55° - 90°, cut into a cuboid, to obtain a semi-finished device; this device is a semi-finished device suitable for the incident pumping of a Q-switched Tm laser (ns pulse, ~1.9 μm).

[0009] Furthermore, in step one, PbIn 6 Te 10 (PIT) crystal, according to the azimuth angle is 30°, the class I phase-matching angle (θ) is 45° - 51°, cut into a cuboid, or according to the azimuth angle is 0°, the class II phase-matching angle (θ) is 45° - 90°, cut into a cuboid, to obtain a semi-finished device; this device is a semi-finished device suitable for the incident pumping of a Q-switched Ho laser (ns pulse, ~2.1 μm).

[0010] Furthermore, in step one, PbIn 6 Te 10 (PIT) crystal, according to the azimuth angle is 30°, the class I phase-matching angle (θ) is 37° - 42°, cut into a cuboid, or according to the azimuth angle is 0°, the class II phase-matching angle (θ) is 45° - 70°, cut into a cuboid, to obtain a semi-finished device, this device is a semi-finished device suitable for the incident pumping of a gain-switched or mode-locked Cr laser (ns, ps or fs pulse, ~2.5 μm).

[0011] Furthermore, in step one, PbIn 6 Te 10 (PIT) crystal, according to the azimuth angle is 30°, the class I phase-matching angle (θ) is 36° - 40°, cut into a cuboid, to obtain a semi-finished device, this device is a semi-finished device suitable for the incident pumping of a Q-switched or mode-locked Er laser (ns or ps pulse, ~2.9 μm).

[0012] Furthermore, the PIT-OPO far-infrared laser frequency conversion device described in step three also needs to coat infrared antireflection films on its incident surface and exit surface;

[0013] Furthermore, the materials of the infrared antireflection films on the incident surface and exit surface of the PIT-OPO far-infrared laser frequency conversion device described in step three are SiO 2 , ThO 2 or Ta 2 O 5 .

[0014] Further, the thickness of the infrared antireflection film on the incident surface and the exit surface of the PIT-OPO mid-infrared laser frequency conversion device described in step three is 0.3 to 2.5 μm.

[0015] Further, the antireflection range of the infrared antireflection film on the incident surface and the exit surface of the PIT-OPO mid-infrared laser frequency conversion device described in step three includes the incident laser wavelength and the operating frequency conversion output laser wavelength range. The absorption coefficient of the device in the 1.8 - 3 μm, 3 - 5 μm, and 8 - 14 μm wavelength bands is lower than 0.1 cm -1 .

[0016] Further, the reflectivity of the infrared antireflection film layer on the incident surface and the exit surface of the PIT-OPO mid-infrared laser frequency conversion device described in step three is lower than 2%.

[0017] Further, the laser-induced damage threshold of the infrared antireflection film on the incident surface and the exit surface of the PIT-OPO mid-infrared laser frequency conversion device described in step three is higher than 0.5 J / cm 2 .

[0018] Place the PIT-OPO mid-infrared laser frequency conversion device in the laser resonator cavity, and the output mirror in the laser resonator cavity is a 50% reflection mirror. Use the pump laser with a wavelength of 2.097 μm as the incident light to irradiate the PIT-OPO mid-infrared laser frequency conversion device, and output mid-infrared laser covering 3 - 5 μm or 8 - 14 μm. Among them, the pump laser with a wavelength of 2.097 μm is generated by a Ho:YAG crystal laser; the output mirror in the laser resonator cavity can also be set as a total reflection mirror to increase the laser power density in the cavity, thereby increasing the PIT-OPO conversion efficiency. At the same time, there is no need for beam transformation of the laser to the PIT crystal, reducing the debugging difficulty.

[0019] The present invention provides a preparation method for an angle phase matching PIT-OPO mid-infrared laser frequency conversion device. Angle phase matching is to control the propagation of the laser beam in a specific direction in the crystal .

[0020] In the optical parametric oscillation process, the incident high-frequency laser is called the pump light, and two low-frequency lights, namely the signal light and the idler light, are generated by gain frequency conversion. The wave vector mismatch factor Δk of the three light waves is 0, that is, equation (1):

[0021] n p ω p -n s ω s -n i ω i = 0 (1)

[0022] n p 、n s, n i are the refractive indices of the pump light, signal light, and idler light respectively, ω p , ω s , ω i are the refractive indices of the pump light, signal light, and idler light respectively.

[0023] The plane where the optical wave vector transmission direction and the crystal optical axis are located is called the crystal principal plane. When the polarization direction of the optical wave vector is perpendicular to the principal plane, it is called the ordinary light (o - light); when the polarization direction of the optical wave vector is in the principal plane, it is called the extraordinary light (e - light). PIT is a positive uniaxial crystal, the refractive index of e - light is greater than that of o - light, the refractive index of o - light does not change with the angle θ between the transmission direction and the optical axis; the refractive index of e - light is determined by the angle θ between the transmission direction and the optical axis and can be expressed as:

[0024]

[0025] The refractive indices of different wavelengths in the PIT crystal can be obtained from the Sellmeier equation.

[0026]

[0027] For the positive uniaxial PIT crystal, when the incident high - frequency pump light is o - light, the polarization directions of the two optical - frequency electric - field components generated by frequency conversion are parallel to each other and belong to the same type of polarization state, denoted as o→e + e (oee type), and this matching method is called type - I phase matching.

[0028] When the incident high - frequency pump light is o - light, the polarization directions of the two optical - frequency electric - field components generated by frequency conversion are perpendicular to each other and belong to two different types of polarization states, denoted as o→e + o (oeo type), and this matching method is called type - II phase matching.

[0029] For the positive uniaxial crystal, the phase - matching angle θ m refers to the angle between the optical wave vector k and the optical axis inside the crystal, that is, the angle between the normal of the crystal incident surface and the crystal optical axis. Generally speaking, only crystals with relatively large birefringence and relatively small dispersion can achieve phase matching, especially type - II phase matching.

[0030] The PbIn 6 Te 10 (PIT) crystal of the present invention, when cut at an azimuth angle of 30° and a phase - matching angle (θ) of 45° - 51°, is for the first - type phase matching of the oee type; when cut at an azimuth angle It is cut at 0° and the phase matching angle (θ) is 45° to 90°, which is the second type of phase matching of the oeo type. It realizes that the PIT output covers the 3-5μm and 8-14μm infrared windows. The average transmittance in the 3-5μm band is 59.7% to 62.2%, and the average transmittance in the 8-14μm band is 60% to 64%. Using MATLAB software to simulate the light-light conversion efficiency of the PIT crystal, when the input wavelength is 2.097um pump laser, the light-light conversion efficiency of the output infrared light can reach more than 60%.

[0031] The present invention utilizes the optical parametric oscillation (OPO) technology. The prepared PIT-OPO mid-far infrared laser frequency conversion device has many advantages such as a wide tunable wavelength range, high conversion efficiency, simple structure, and narrow linewidth output, and can be used in mid-far infrared lasers. Brief Description of the Drawings

[0032] Figure 1 For PbIn in Step 1 of Example 1 6 Te 10 (PIT) crystal near-infrared absorption coefficient curve graph;

[0033] Figure 2 It is a photo of the PIT-OPO mid-far infrared laser frequency conversion device in Step 3 of Example 1;

[0034] Figure 3 It is the infrared transmission spectrum diagram of the PIT-OPO mid-far infrared laser frequency conversion device of Example 1;

[0035] Figure 4 It is the type I phase matching curve graph of the PIT-OPO device prepared in Example 1;

[0036] Figure 5 It is the infrared transmission spectrum diagram of the PIT-OPO mid-far infrared laser frequency conversion device of Example 2

[0037] Figure 6 It is the type II phase matching curve graph of the PIT-OPO device prepared in Example 2. Detailed Implementation Modes

[0038] The beneficial effects of the present invention are verified by the following examples.

[0039] Example 1: The preparation method of the PIT-OPO mid-far infrared laser frequency conversion device in this example is carried out according to the following steps:

[0040] 1. Use the "cross orientation method" for PbIn 6 Te 10(PIT) crystals are directionally cut to obtain a (060) crystal plane. Subsequently, azimuthal cutting is performed by rotating 30° along the (060) crystal plane of the PIT crystal, and after cutting, a 3-mm wafer is cut along the plane normal direction; cutting is performed after rotating 40° along the (006) crystal plane of the wafer to obtain a 3×3×11.5-mm 3 semi-finished product of type-I phase-matched PIT-OPO device;

[0041] Second, the crystal planes of the semi-finished device are physically polished with emery to a roughness of 0.4 nm and a flatness of λ / 6, where λ = 632.5 nm;

[0042] Third, hydrogen peroxide with a mass percentage concentration of 5%, ammonia water with a mass percentage concentration of 25%, and deionized ultrapure water are mixed evenly according to a volume ratio of 1:1.5:10 to obtain a cleaning solution; the semi-finished device treated in the second step is immersed in the cleaning solution and ultrasonically maintained for 25 s to remove the oxide layer and adhered impurities on the crystal surface after polishing, and then cleaned successively with deionized ultrapure water and ethanol and dried to obtain a PIT-OPO mid-infrared laser frequency conversion device.

[0043] The surface finish of the PIT-OPO mid-infrared laser frequency conversion device obtained in this embodiment is 40 / 20. It is a PIT-OPO mid-infrared laser frequency conversion device suitable for the incident pumping of a 2.1-μm Q-switched Ho laser (ns pulse).

[0044] Using a Ho:YAG laser for pumping to output mid-infrared laser, near-infrared region spectral testing is performed on the PIT crystal described in the first step, and the obtained near-infrared absorption coefficient curve of the crystal is as Figure 1 shown. As can be seen from Figure 1 , the absorption coefficient of the PIT crystal in the near-infrared region is less than 0.1 cm -1 , especially at 2.097 μm, the absorption coefficient is less than 0.05 cm -1 .

[0045] The photo of the 3×3×11.5-mm 3 type-I phase-matched PIT-OPO device obtained in the third step of this embodiment is as Figure 2 shown.

[0046] The azimuth is 30°, and the PIT-OPO mid-infrared laser frequency conversion device with a phase-matching angle (θ) of 50° is placed in the laser resonator. The output mirror in the laser resonator is a 50% reflector; a laser with a wavelength of 2.097 μm, a repetition rate of 100 Hz, and a pulse width of 20 ns is used as the incident light to irradiate the PIT-OPO mid-infrared laser frequency conversion device, and the infrared transmission spectrum of the device is asFigure 3 As shown in Figure 3 it can be seen that the average transmittance in the 3 - 5μm band is 59.7%, and the average transmittance in the 8 - 14μm band is 60.4%. Figure 4 is the type - I phase - matching curve of the PIT - OPO device prepared in Example 1; as shown in Figure 4 it can be seen that under the condition of a pump spot radius of 0.5mm, the average power threshold of the pump laser exceeds 4.9W, and the output covers the 3 - 5μm mid - infrared window band laser.

[0047] Example 2: The preparation method of the PIT - OPO mid - far - infrared laser frequency - conversion device in this example is carried out according to the following steps:

[0048] I. Use the "cross - orientation method" to orient and cut the PbIn 6 Te 10 (PIT) crystal to obtain the (060) crystal plane. Subsequently, cut along the (060) crystal plane of the PIT crystal at an azimuth angle of 0°, and then cut a 3mm wafer along the plane normal direction after cutting; along the (006) crystal plane of this wafer, cut at a phase - matching angle (θ) of 45° to obtain a type - II phase - matching semi - finished PIT - OPO device with dimensions of 3×3×11.5mm 3 ;

[0049] II. Use emery to physically polish the crystal plane of the semi - finished device to a roughness of 0.4nm and a flatness of λ / 6, where λ = 632.5nm;

[0050] III. Mix hydrogen peroxide with a mass percentage concentration of 5%, ammonia water with a mass percentage concentration of 25%, and deionized ultrapure water in a volume ratio of 1:1.5:10 to mix them evenly to obtain a cleaning solution; immerse the semi - finished device processed in step II into the cleaning solution and ultrasonically maintain it for 25s to remove the oxide layer and adhered impurities on the crystal surface after polishing, and then wash it clean with deionized ultrapure water and ethanol in sequence, and dry it;

[0051] IV. Coat the incident surface and the exit surface of the semi - finished device processed in step III with an infrared antireflection film made of Ta 2 O 5 with a thickness of 0.5μm to obtain a PIT - OPO mid - far - infrared laser frequency - conversion device.

[0052] The azimuth angle A PIT-OPO mid- and far-infrared laser frequency conversion device with an angle of 0° and a phase matching angle (θ) of 45° is placed in a laser resonator. The output mirror in the laser resonator is a 50% reflector. A Ho laser with a wavelength of 2.097 μm, a repetition rate of 100 Hz, and a pulse width of 20 ns is used as the incident light to irradiate the PIT-OPO mid- and far-infrared laser frequency conversion device. The infrared transmission spectrum of the device is as shown in Figure 5 shown. It can be seen from Figure 5 that the average transmittance in the 3 - 5 μm band is 61%, and the average transmittance in the 8 - 14 μm band is 63%. Figure 6 is the type-II phase matching curve graph of the PIT-OPO device prepared in Example 2. It can be seen from Figure 6 that under the condition of a pump spot radius of 0.5 mm, the average power threshold of the pump laser exceeds 6.3 W, and the output covers the far-infrared window band laser in the 8 - 12 μm range.

[0053] The crystal PbIn 6 Te 10 (PIT) is a positive uniaxial crystal belonging to the trigonal crystal system. It has good light transmittance in the mid- and far-infrared spectral range of 1.5 - 31 μm, and there is no absorption loss (not exceeding 0.05 cm -1 ). The refractive index varies in the range of 3 - 3.2, the birefringence is ~0.05, and the nonlinear coefficient d 11 = 51 pm / V. Combining its broad light transmission band and sufficient birefringence, it becomes a unique material for nonlinear frequency conversion in the mid- and far-infrared spectral ranges, and can be used for both up-conversion and down-conversion. Most attractively, parametric oscillators pumped by nanosecond pulses and parametric generators pumped by picosecond or femtosecond pulses can achieve tunable output of long-wave lasers. The pump source can be selected from Q-switched or mode-locked 2.9 μm Er lasers (ns or ps pulses), gain-switched or mode-locked 2.5 μm Cr lasers (ns, ps or fs pulses), and Q-switched high-power Ho or Tm lasers (ns pulses, ~2 μm).

Claims

1. A method for preparing a PIT-OPO mid-to-far-infrared laser frequency conversion device, characterized in that: The method comprises the following steps:

1. PbIn6Te 10 Crystals, by azimuth The phase matching angle of type I is 30° and the phase matching angle of type I is 36° to 62°. The device is cut into a rectangular block with a phase matching angle of 0° and a type II phase matching angle θ of 45° to 90° to obtain a semi-finished device; 2. Physically polishing the crystal surface of the semi-finished device to a roughness of less than 1.0 nm and a flatness of less than λ / 6, where λ = 632.5 nm; 3. Mix hydrogen peroxide with a mass percentage concentration of 5%, ammonia water with a mass percentage concentration of 25% and deionized ultrapure water in a volume ratio of 1:(1-2):(10-20) to obtain a cleaning solution; immerse the semi-finished device treated in step 2 in the cleaning solution and perform ultrasonic treatment for 5-30 seconds to remove the oxide layer and adherent impurities on the surface of the crystal after polishing, so that the smoothness is better than 40 / 20; then clean it with deionized ultrapure water and ethanol in turn, and dry it to obtain a PIT-OPO mid-to-far infrared laser frequency conversion device.

2. The method for preparing a PIT-OPO mid-to-far infrared laser frequency conversion device according to claim 1, characterized in that: In step 1, PbIn6Te 10 Crystals, by azimuth The phase matching angle of the type I phase is 30° and the phase matching angle of the type I phase is 46° to 62°. The device is cut into a rectangular block with a phase matching angle θ of 0° and a type II phase matching angle θ of 55° to 90° to obtain a semi-finished device suitable for Tm laser pumping.

3. The method for preparing a PIT-OPO mid-to-far infrared laser frequency conversion device according to claim 1, characterized in that: In step 1, PbIn6Te 10 Crystals, by azimuth The phase matching angle of the first type is 30° and the phase matching angle of the second type is 45° to 51°, which can be cut into a rectangular block or cut into a rectangular block according to the azimuth angle. The device is cut into a rectangular block with a phase matching angle of 0° and a type II phase matching angle θ of 45° to 90° to obtain a semi-finished device suitable for Ho laser pumping.

4. The method for preparing a PIT-OPO mid-to-far infrared laser frequency conversion device according to claim 1, characterized in that: In step 1, PbIn6Te 10 Crystals, by azimuth The phase matching angle of type I is 30° and the phase matching angle of type I is 37° to 42°. The device is cut into a rectangular block with a phase matching angle of 0° and a type II phase matching angle θ of 45° to 70° to obtain a semi-finished device suitable for Cr laser pumping.

5. The method for preparing a PIT-OPO mid-to-far infrared laser frequency conversion device according to claim 1, characterized in that: In step 1, PbIn6Te 10 Crystals, by azimuth The device is cut into a rectangular block with a phase matching angle of 30° and a type I phase matching angle θ of 36° to 40° to obtain a semi-finished device suitable for Er laser pumping.

6. The method for preparing a PIT-OPO mid-to-far infrared laser frequency conversion device according to claim 1, characterized in that: The PIT-OPO mid-to-far infrared laser frequency conversion device described in step 3 also needs to be coated with an infrared anti-reflection film on its incident surface and output surface.

7. The method for preparing a PIT-OPO mid-to-far infrared laser frequency conversion device according to claim 2, characterized in that: The anti-reflection range of the infrared anti-reflection film on the incident surface and the output surface of the PIT-OPO far-infrared laser frequency conversion device described in step three includes the wavelength range of the incident laser and the wavelength range of the working frequency conversion output laser.

8. The method for preparing a PIT-OPO mid-to-far infrared laser frequency conversion device according to claim 2, characterized in that: The laser damage threshold of the infrared anti-reflection film is higher than 0.5 J / cm 2 .

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