A birefringence compensation device and method for improving extraction efficiency of a laser amplifier

Abstract

The application discloses a birefringence compensation device and method for improving extraction efficiency of a laser amplifier, wherein two end-pumped Yb:YAG crystal rods are connected in series through a 4f system and a quartz optical rotation crystal, signal laser is amplified twice after passing through the 4f system, and thermal-induced birefringence is precisely compensated; meanwhile, a beam expanding 4f imaging system is adopted to reduce the size of signal laser and pump laser in the first rod, so as to improve the extraction efficiency, and meanwhile, the signal spot in the second rod is ensured to be large enough to prevent damage of high-energy laser to optical elements. Compared with the traditional birefringence compensation method, the application designs parameters according to the disadvantages of Yb:YAG crystal quasi-three-level, so that the extraction efficiency is improved while the same compensation effect is achieved, and higher-energy laser pulses are outputted, which is beneficial to industrial processing.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of solid laser technology, and in particular to a birefringence compensation device and method for improving the extraction efficiency of a laser amplifier. BACKGROUND

[0002] High-power and high-energy laser systems have important applications in industrial processing and other fields. Solid-state lasers have the advantages of simple structure, stable operation, low maintenance cost and long service life. However, the thermal effect of the gain medium in a solid-state laser cannot be ignored. Yb:YAG crystal greatly reduces the thermal effect in the laser amplification process due to its excellent quantum efficiency, and has become the most promising candidate for gain medium. However, due to its quasi-three-level characteristics, there will be reabsorption of signal laser during laser amplification, which reduces the extraction efficiency. Moreover, there is also a birefringence effect, which causes distortion of the wavefront of the amplified light beam and causes depolarization loss.

[0003] The traditional birefringence compensation method connects two crystal rods with similar birefringence effects in series and places a 90° quartz rotator in the middle. For example, a low-energy large-power double-rod serial flat-cavity green laser is disclosed in Chinese patent document CN1972034A. However, due to the focusing effect of the crystal thermal lens, the propagation of light is not parallel to the optical axis, so the propagation area of light in the two rods is different, and the compensation effect is not good.

[0004] A method derived later combines a 90° quartz rotator with a 1:1 ratio 4f system, so that the light beam of the first crystal rod is imaged on the second crystal rod, so that the propagation area of light in the two rods is the same, and the compensation effect of birefringence is good. For example, a uniform time waveform narrow linewidth 1319nm pulse laser is disclosed in Chinese patent document CN108054627A. However, this method is difficult to apply in Yb:YAG laser amplifiers that produce high-energy lasers. This is because in order to protect the laser device, a larger spot area is usually required for high-energy lasers to meet the damage threshold of the laser device. However, due to the special quasi-three-level characteristics of Yb:YAG crystal, a larger spot area will significantly reduce the extraction efficiency in the laser amplification process. The commonly used 1:1 ratio 4f system method will make the spot area in the first crystal rod too large, thereby reducing the extraction efficiency. Low extraction efficiency means that higher pump power is needed to obtain the same energy of signal laser, which will significantly increase the thermal effect. SUMMARY

[0005] In view of the shortcomings of the above-mentioned birefringence compensation methods, the present application provides a birefringence compensation device and method for improving the extraction efficiency of a laser amplifier, which can accurately compensate for the thermal birefringence effect of Yb:YAG crystal.

[0006] A birefringence compensation device for improving the extraction efficiency of a laser amplifier, comprising a seed source, a first lens, a first total reflection mirror, a first pump source, a first optical coupling system first lens, a first optical coupling system second lens, a first dichroic mirror, a first laser gain medium, a second dichroic mirror, a 4f imaging system first lens, a quartz optical rotation crystal, a 4f imaging system second lens, a second pump source, a second optical coupling system first lens, a second optical coupling system second lens, a third dichroic mirror, a second laser gain medium, a fourth dichroic mirror, a polarization beam splitter;

[0007] The seed source, the first lens and the first total reflection mirror are sequentially arranged along the optical path; the first pump source, the first optical coupling system first lens, the first optical coupling system second lens, the first dichroic mirror, the first laser gain medium, the second dichroic mirror are sequentially arranged along the optical path; the second pump source, the second optical coupling system first lens, the second optical coupling system second lens, the third dichroic mirror, the second laser gain medium, the fourth dichroic mirror are sequentially arranged along the optical path.

[0008] The first dichroic mirror is placed in the reflection path of the first total reflection mirror; the 4f imaging system first lens, the quartz optical rotation crystal and the 4f imaging system second lens are sequentially placed between the second dichroic mirror and the fourth dichroic mirror along the optical path; and the polarization beam splitter is placed in the reflection path of the third dichroic mirror.

[0009] Further, the laser wavelength output by the seed source is 1030 nm, the beam quality is less than 1.3, and the spot roundness is greater than 90%.

[0010] Further, the first laser gain medium and the second laser gain medium are doped laser materials, and the doped laser material is a Yb:YAG crystal.

[0011] Further, the focal length f2 of the 4f imaging system second lens is greater than the focal length f1 of the 4f imaging system first lens.

[0012] Further, the polarization rotation angle of the quartz optical rotation crystal is 90°.

[0013] Further, the first dichroic mirror, the second dichroic mirror, the third dichroic mirror and the fourth dichroic mirror are placed at an angle of 45° with the optical path.

[0014] Further, the reflectivity of the first dichroic mirror, the second dichroic mirror, the third dichroic mirror and the fourth dichroic mirror to the amplified laser is greater than 99.5%, and the reflectivity to the pump laser is less than 5%.

[0015] A birefringence compensation method for improving the extraction efficiency of a laser amplifier, using the above-mentioned birefringence compensation device, comprising the following steps:

[0016] S1, the signal laser generated by the seed source enters the first laser gain medium, is amplified, passes through the 4f imaging system composed of the first lens and the second lens of the 4f imaging system and the quartz optical rotatory crystal placed in the 4f imaging system, then enters the second laser gain medium for secondary amplification, and is output after the depolarized laser is filtered out by the polarization beam splitter;

[0017] Wherein, the focal length of the first lens of the 4f imaging system is f1, the focal length of the second lens of the 4f imaging system is f2, the distance between the right end face of the first laser gain medium and the first lens of the 4f imaging system is L1, the distance between the first lens of the 4f imaging system and the second lens of the 4f imaging system is L2, and the distance between the second lens of the 4f imaging system and the right end face of the second laser gain medium is L3, wherein L1 = f1, L2 = f1+f2, L3 = f2, and the 4f imaging system images the light beam A at the right end face of the first laser gain medium at the light beam B at the right end face of the second laser gain medium;

[0018] S2, the spot size ratio of the light beam A at the right end face of the first laser gain medium and the light beam B at the right end face of the second laser gain medium should be equal to f1 / f2, the spot size ratio of the pump laser generated by the first pump source and the second pump source at the left end face of the first laser gain medium and the left end face of the second laser gain medium should be equal to f1 / f2, and the waist size ratio inside the first laser gain medium and the second laser gain medium should also be equal to f1 / f2; since f2>f1, the signal laser spot size in the second laser gain medium should be larger than the signal laser spot size in the first laser gain medium, so that the second laser gain medium with high signal laser power is not damaged and the extraction efficiency of the first laser gain medium for signal light amplification is improved;

[0019] S3, the quartz optical rotatory crystal is placed between the first lens and the second lens of the 4f imaging system, which makes the radial polarization light in the first laser gain medium rotate by 90° to become a tangential component, and the tangential component rotates to become a radial component, and the radial and tangential polarization components of all points on the cross section of the first laser gain medium are exchanged point by point in the second laser gain medium, so that the sum of the phase differences generated by the radial and tangential components is zero after passing through the two laser gain media; the 4f imaging system makes the light beams propagate in the same area in the two laser gain media, that is, the phase delay characteristics are the same, thereby effectively compensating the birefringence effect.

[0020] Compared with the prior art, the present application has the following beneficial effects:

[0021] 1, the present application adopts a compact device and fewer optical elements to realize accurate compensation of the thermal-induced birefringence effect of the Yb:YAG crystal, and the compensated light beam has high roundness and no distortion.

[0022] 2、The application compensates birefringence and improves the extraction efficiency of Yb:YAG amplifier generating high-energy laser, solves the problem of low extraction efficiency after low-energy laser amplification, reduces the pump power and lowers the heat accumulation in the crystal.

[0023] 3、The application improves the extraction efficiency of the first-stage amplifier and enlarges the signal laser spot in the second-stage amplifier, effectively protecting the optical elements from being damaged by high-energy laser.

[0024] 4、The application compensates birefringence and effectively amplifies the laser power, outputs higher-energy laser pulses and is beneficial to industrial processing. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the application, the drawings needed to be used in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor.

[0026] Figure 1 It is a light path schematic diagram of the birefringence compensation device for improving the extraction efficiency of the laser amplifier.

[0027] Figure 2 It is a relationship diagram of the extraction efficiency of the signal laser after the first amplification of the 4f system with a ratio of 1:1.5 and the extraction efficiency of the signal laser after the first amplification of the 4f system with a ratio of 1:1 and the pump power.

[0028] Figure 3 It is a measurement diagram of the beam waist size before and after the birefringence compensation and the corresponding position spot image after the first amplification of the signal laser of the 4f system with a ratio of 1:1.5.

[0029] Figure 4 It is a relationship diagram of the extraction efficiency of the signal laser after the second amplification of the 4f system with a ratio of 1:1.5 and the extraction efficiency of the signal laser after the second amplification of the 4f system with a ratio of 1:1 and the pump power.

[0030] Figure 5 It is a measurement diagram of the beam waist size before and after the birefringence compensation and the corresponding position spot image after the second amplification of the signal laser of the 4f system with a ratio of 1:1.5. DETAILED DESCRIPTION

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] It should be noted that, unless otherwise specified, the features in the following embodiments and implementation methods can be combined with each other.

[0033] like Figure 1 As shown, a birefringence compensation device for improving the extraction efficiency of a laser amplifier includes a seed source 1, a first lens 2, a first total reflection mirror 3, a first pump source 4, a first optical coupling system first lens 5, a first optical coupling system second lens 6, a first dichroic mirror 7, a first laser gain medium 8, a second dichroic mirror 9, a 4f imaging system first lens 10, a quartz optical rotator crystal 11, a 4f imaging system second lens 12, a second pump source 13, a second optical coupling system first lens 14, a second optical coupling system second lens 15, a third dichroic mirror 16, a second laser gain medium 17, a fourth dichroic mirror 18, and a polarization beam splitter 19.

[0034] Among them, seed source 1, first lens 2, and first total reflection mirror 3 are arranged sequentially along the optical path; first pump source 4, first optical coupling system first lens 5, first optical coupling system second lens 6, first dichroic mirror 7, first laser gain medium 8, and second dichroic mirror 9 are arranged sequentially along the optical path; second pump source 13, second optical coupling system first lens 14, second optical coupling system second lens 15, third dichroic mirror 16, second laser gain medium 17, and fourth dichroic mirror 18 are arranged sequentially along the optical path.

[0035] The first dichroic mirror 7 is placed on the reflected light path of the first total reflection mirror 3; between the second dichroic mirror 9 and the fourth dichroic mirror 18, the first lens 10 of the 4f imaging system, the quartz optical rotator crystal 11, and the second lens 12 of the 4f imaging system are placed sequentially along the light path; a polarizing beam splitter 19 is placed on the reflected light path of the third dichroic mirror 16.

[0036] The laser output from seed source 1 has a wavelength of 1030 nm, a beam quality of less than 1.3, and a spot circularity of more than 90%.

[0037] The first laser gain medium 8 and the second laser gain medium 17 are doped laser materials, and the doped laser materials are Yb:YAG crystals.

[0038] The focal length of the second lens 12 in the 4f imaging system needs to be greater than the focal length of the first lens 10 in the 4f imaging system.

[0039] The polarization rotation angle of the quartz optical rotation crystal 11 is 90°.

[0040] The first dichroic mirror 7, the second dichroic mirror 9, the third dichroic mirror 16 and the fourth dichroic mirror 18 are placed at 45° to the optical path. The reflectivity of the first dichroic mirror 7, the second dichroic mirror 9, the third dichroic mirror 16 and the fourth dichroic mirror 18 to the amplification laser is greater than 99.5%, and the reflectivity to the pump laser is less than 5%.

[0041] A birefringence compensation method for improving the extraction efficiency of a laser amplifier, using the above-mentioned birefringence compensation device, comprising the following steps:

[0042] S1, the signal laser generated by the seed source 1 enters the first laser gain medium 8 after amplification, passes through the 4f imaging system composed of the first lens 10 and the second lens 12 of the 4f imaging system and the quartz optical rotation crystal 11 placed therein, and then enters the second laser gain medium 17 for secondary amplification, and then is output after the depolarized laser is filtered out by the polarization beam splitter 19;

[0043] The focal length of the first lens 10 of the 4f imaging system is f1, the focal length of the second lens 12 of the 4f imaging system is f2, the distance between the right end face of the first laser gain medium 8 and the first lens 10 of the 4f imaging system is L1, the distance between the first lens 10 of the 4f imaging system and the second lens 12 of the 4f imaging system is L2, and the distance between the second lens 12 of the 4f imaging system and the right end face of the second laser gain medium 17 is L3, wherein L1 = f1, L2 = f1+f2, L3 = f2, and the 4f imaging system images the light beam A at the right end face of the first laser gain medium 8 at the light beam B at the right end face of the second laser gain medium 17;

[0044] S2, the spot size ratio of the light beam A at the right end face of the first laser gain medium 8 and the light beam B at the right end face of the second laser gain medium 17 should be equal to f1 / f2, the spot size ratio of the pump laser generated by the first pump source 4 and the second pump source 13 at the left end face of the first laser gain medium 8 and the left end face of the second laser gain medium 17 should be equal to f1 / f2, and the waist size ratio inside the first laser gain medium 8 and the second laser gain medium 17 should also be equal to f1 / f2; since f2>f1, the signal laser spot size in the second laser gain medium 17 should be larger than the signal laser spot size in the first laser gain medium 8, so as to protect the second laser gain medium 17 with high signal laser power from being damaged and improve the extraction efficiency of the first laser gain medium 8 for signal light amplification;

[0045] S3, the quartz optical rotatory crystal 11 is placed between the first lens 10 of the 4f imaging system and the second lens 12 of the 4f imaging system, which makes the radial polarization light in the first laser gain medium 8 turn 90° into a tangential component, and the tangential component is rotated into a radial component, and the radial and tangential polarization components of all points on the cross section of the first laser gain medium 8 are exchanged point by point in the second laser gain medium 17, so that the sum of the phase difference generated by the radial and tangential directions after passing through the two laser gain media is zero; the 4f imaging system makes the light beams propagate in the same area in the two laser gain media as much as possible, that is, the phase delay characteristics are the same, thereby effectively compensating the birefringence effect.

[0046] In the embodiment of the application, the seed source 1 adopts a hybrid architecture of optical fiber + solid, the output wavelength is 1030 nm, the average power is 2.4 W, the repetition frequency is 20 kHz, the single pulse energy is 0.12 mJ, the pulse width is 200 ps, and the beam quality is less than 1.1. The first pump source 4 and the second pump source 13 are InGaAs semiconductor laser diodes with an output wavelength of 969 nm, and the pump power is 140 W. The first optical coupling system is composed of two lenses with focal lengths of 12 mm and 75 mm. The second optical coupling system is composed of two lenses with focal lengths of 12 mm and 100 mm. The focal length of the first lens 2 is 200 mm. The first dichroic mirror 7, the second dichroic mirror 9, the third dichroic mirror 16 and the fourth dichroic mirror 18 are placed at an angle of 45° with the optical path, the reflectivity of the signal laser is 99.9%, and the transmittance of the pump laser is 98%. The first laser gain medium 8 and the second laser gain medium 17 are both Yb:YAG crystals, the ytterbium doping concentration of the crystal is 1 at.%, and the Yb:YAG crystal is a cuboid with a size of 4*4*26.

[0047] The focal length of the first lens 10 of the 4f imaging system is 100 mm, the focal length of the second lens 12 of the 4f imaging system is 150 mm, the distance between the right end face of the first laser gain medium 8 and the first lens 10 of the 4f imaging system is L1=100 mm, the distance between the first lens 10 of the 4f imaging system and the second lens 12 of the 4f imaging system is L2=250 mm, and the distance between the second lens 12 of the 4f imaging system and the right end face of the second laser gain medium 17 is L3=150 mm. The polarization rotation angle of the quartz optical rotatory crystal 11 to the signal laser is 90°.

[0048] The signal laser is converged after passing through the first lens 2 before entering the first laser gain medium 8, and is diverged into the first laser gain medium 8, with a spot diameter of 0.65 mm at the left end face. After the first amplification, the spot diameter at the right end face of the first laser gain medium 8 is 0.53 mm. The beam waist diameter of the pump laser generated by the first pump source 4 in the first laser gain medium 8 is 0.46 mm after passing through the first optical coupling system, which is 9.1 mm away from the left end face of the first laser gain medium 8, and the spot diameter of the pump laser at the left end face is 0.62 mm. After the beam expansion of the signal laser by the 4f imaging system, the spot diameter at the right end face of the second laser gain medium 17 is 0.81 mm, and after the second amplification, the spot diameter at the left end face of the second laser gain medium 17 is 0.68 mm. The beam waist diameter of the pump laser generated by the second pump source 13 in the second laser gain medium 17 is 0.69 mm after passing through the second optical coupling system, which is 18.2 mm away from the left end face of the second laser gain medium 17, and the spot diameter of the pump laser at the left end face is 0.93 mm. The pump laser and the signal laser are in the same direction in the first laser gain medium, and the pump laser and the signal laser are in opposite directions in the second laser gain medium 17.

[0049] After the amplification of the first laser gain medium 8, the extraction efficiency of the amplified signal laser is measured as shown in Figure 2 . Compared with the traditional 1:1 4f compensation method, the results show that the extraction efficiency is obviously improved under different pump powers. The spot measurement of the signal laser before and after the beam waist after single amplification is shown in Figure 3 . The beam quality and measured in the horizontal and vertical directions of the amplified laser are 1.09 and 1.07 respectively, which does not deteriorate compared with the beam quality of the laser before amplification, but the birefringence effect makes the spot shape elliptical. The extraction efficiency of the signal laser after the amplification of the second laser gain medium 17 is measured as shown in Figure 4 . Under the premise that the peak power density at the laser outlet is consistent, the extraction efficiency of the method of the present application is still obviously improved compared with the traditional method. In addition, the compensation effect of birefringence is obvious, as shown in Figure 5 , the roundness of the spot before and after the beam waist is obviously improved, and there is almost no distortion. The beam quality of the signal laser after the second amplification is slightly deteriorated, and the beam quality and measured in the horizontal and vertical directions are 1.15. In addition, the depolarization power after compensation is measured as 0.07 W.

[0050] The above-described embodiments have described the technical solutions and beneficial effects of the present application in detail, and it should be understood that the above-described is only a specific embodiment of the present application and is not used to limit the present application, and any modification, supplement and equivalent replacement made within the principle range of the present application should be included in the protection range of the present application.

Claims

1. A birefringence compensating device for improving the extraction efficiency of a laser amplifier, characterized by, The seed source (1), the first lens (2), the first all-reflection mirror (3), the first pump source (4), the first optical coupling system first lens (5), the first optical coupling system second lens (6), the first dichroic mirror (7), the first laser gain medium (8), the second dichroic mirror (9), the 4f imaging system first lens (10), the quartz optical rotatory crystal (11), the 4f imaging system second lens (12), the second pump source (13), the second optical coupling system first lens (14), the second optical coupling system second lens (15), the third dichroic mirror (16), the second laser gain medium (17), the fourth dichroic mirror (18), and the polarization beam splitter (19) are sequentially arranged along the optical path. The first pump source (4), the first optical coupling system first lens (5), the first optical coupling system second lens (6), the first dichroic mirror (7), the first laser gain medium (8), and the second dichroic mirror (9) are sequentially arranged along the optical path. The first dichroic mirror (7) is arranged on the reflection path of the first all-reflection mirror (3); the 4f imaging system first lens (10), the quartz optical rotatory crystal (11), and the 4f imaging system second lens (12) are sequentially arranged along the optical path between the second dichroic mirror (9) and the fourth dichroic mirror (18); the polarization beam splitter (19) is arranged on the reflection path of the third dichroic mirror (16); and the focal length f2 of the 4f imaging system second lens (12) is greater than the focal length f1 of the 4f imaging system first lens (10).

2. The birefringence compensation device for improving the extraction efficiency of a laser amplifier according to claim 1, characterized in that, The seed source (1) outputs laser with a wavelength of 1030 nm, a beam quality less than 1.3, and a spot roundness greater than 90%.

3. The birefringence compensating apparatus for improving the extraction efficiency of a laser amplifier according to claim 1, wherein The first laser gain medium (8) and the second laser gain medium (17) are doped laser materials, and the doped laser material is Yb:YAG crystal.

4. The birefringence compensating apparatus for improving the extraction efficiency of a laser amplifier according to claim 1, wherein The polarization rotation angle of the quartz optical rotatory crystal (11) is 90°.

5. The birefringence compensating apparatus for improving the extraction efficiency of a laser amplifier according to claim 1, wherein The first dichroic mirror (7), the second dichroic mirror (9), the third dichroic mirror (16), and the fourth dichroic mirror (18) are arranged at an angle of 45° with the optical path.

6. The birefringence compensating apparatus for improving the extraction efficiency of a laser amplifier according to claim 1, wherein The reflectivity of the first dichroic mirror (7), the second dichroic mirror (9), the third dichroic mirror (16), and the fourth dichroic mirror (18) to the amplified laser is greater than 99.5%, and the reflectivity to the pump laser is less than 5%.

7. A birefringence compensation method for improving the extraction efficiency of a laser amplifier, characterized by, The birefringence compensation device of any one of claims 1-6 is used, and the following steps are included: S1, the signal laser generated by the seed source (1) enters the first laser gain medium (8) after amplification, then passes through the 4f imaging system composed of the 4f imaging system first lens (10) and the 4f imaging system second lens (12) and the quartz optical rotatory crystal (11) arranged therein, and then enters the second laser gain medium (17) for secondary amplification, and then is output after the depolarized laser is filtered out by the polarization beam splitter (19). Wherein, the focal length of the first lens (10) of the 4f imaging system is f1, the focal length of the second lens (12) of the 4f imaging system is f2, the distance between the right end face of the first laser gain medium (8) and the first lens (10) of the 4f imaging system is L1, the distance between the first lens (10) of the 4f imaging system and the second lens (12) of the 4f imaging system is L2, and the distance between the second lens (12) of the 4f imaging system and the right end face of the second laser gain medium (17) is L3, wherein L1 = f1, L2 = f1 + f2, L3 = f2, and the 4f imaging system images the light beam A at the right end face of the first laser gain medium (8) to the light beam B at the right end face of the second laser gain medium (17); S2, the spot size ratio of the light beam A at the right end face of the first laser gain medium (8) and the light beam B at the right end face of the second laser gain medium (17) is equal to f1 / f2, the spot size ratio of the pump laser generated by the first pump source (4) and the second pump source (13) at the left end face of the first laser gain medium (8) and the left end face of the second laser gain medium (17) is equal to f1 / f2, and the waist size ratio inside the first laser gain medium (8) and the second laser gain medium (17) is also equal to f1 / f2; since f2 > f1, the signal laser spot size in the second laser gain medium (17) is larger than the signal laser spot size in the first laser gain medium (8), thereby protecting the second laser gain medium (17) with high signal laser power from being damaged and improving the extraction efficiency of the first laser gain medium (8) for signal light amplification; S3, the quartz optical rotation crystal (11) is placed between the first lens (10) of the 4f imaging system and the second lens (12) of the 4f imaging system, which makes the radial polarized light in the first laser gain medium (8) turn 90° to become a tangential component, and the tangential component rotates to become a radial component. The radial and tangential polarization components of all points on the cross section of the first laser gain medium (8) are exchanged point by point in the second laser gain medium (17), so that the sum of the phase differences generated by the radial and tangential components after passing through the two laser gain media is zero. The 4f imaging system makes the propagation areas of the light beams in the two laser gain media the same, that is, the phase delay characteristics are the same, thereby effectively compensating the birefringence effect.

Images