CN107024760A - A kind of transmission-type light splitting focusing system for VUV LASER - Google Patents

Abstract

A transmissive spectroscopic focusing system for vacuum ultraviolet laser proposed by the present invention belongs to the technical field of beam splitting and focusing. The system is composed of three or four convex lenses and a diaphragm. The optical axis is set; wherein, the first convex lens and the second convex lens are arranged on the same side of the diaphragm, and the remaining convex lenses are located on the other side of the diaphragm. This system splits and focuses the vacuum ultraviolet laser (VUV) light source generated by four-wave mixing. The splitting ratio of the fundamental frequency light reaches more than 99.99%, and the total transmittance of VUV light (125nm‑150nm) can reach more than 10%. . When focusing, a high-precision aspheric convex lens can also be used to offset spherical aberration, so that the system reaches the diffraction limit, thereby achieving submicron spot focusing.

Description

A transmission type spectroscopic focusing system for vacuum ultraviolet laser

technical field

The invention belongs to the technical field of beam splitting and focusing, and in particular relates to a transmissive splitting focusing system for vacuum ultraviolet lasers.

Background technique

High-intensity vacuum ultraviolet laser (VUV, wavelength range 120-150nm) can be generated by using four-wave mixing technology. The laser light source has the characteristics of high single-photon energy, narrow laser linewidth, high energy density, and small divergence angle. It is widely used in the fields of atomic and molecular excited state energy level structure measurement, molecular reaction dynamics, and mass spectrometry imaging. Due to the short wavelength of vacuum ultraviolet laser, it can provide sub-micron spatial resolution-in theory, VUV light can be focused to a spot with a diameter of 100nm. Therefore, VUV light has far-reaching significance for submicron biological cell imaging and microstructure analysis of nanomaterials.

The four-wave mixing technology is based on the principle of nonlinear optics. After a part of the input light (that is, the fundamental frequency light, including ultraviolet light and visible light) is converted into VUV light in the mixing medium, the remaining fundamental frequency light will be combined with the VUV light. common output. The VUV wavelength is shorter than the fundamental frequency light, and the single photon energy is higher than the fundamental frequency light, but the resulting VUV light intensity is only one percent or less of the fundamental frequency light. Since the output light is also mixed with high-intensity fundamental-frequency light, if the VUV light is not separated from the high-intensity fundamental-frequency light, the sample to be tested will be damaged, or the effect of the VUV light will be masked. Therefore, it is necessary to separate the VUV light from the fundamental frequency light.

The separation of VUV light and fundamental frequency light generally uses two independent optical systems for splitting and focusing respectively. The conventional technology of light splitting is to use a prism or a grating to separate the VUV light and the fundamental frequency light in the output light. This method takes advantage of the fact that prisms or gratings have different refraction or diffraction angles for different wavelengths of light, but the outgoing VUV light path changes direction, which greatly increases the difficulty and stable use of the experimental device.

One of the prism splitting systems reported in the literature (D. Riedel, Appl. Phys. A 69, 375-380, 1999) splits light and then achieves focusing through a single lens. The direction of propagation of the output light changes after passing through the prism. Different wavelengths of light are shifted at different angles to achieve light splitting. The visible light path is not on one axis, which is very inconvenient to use.

The current focusing system for VUV light can adopt reflective focusing. Reflective focusing mirrors have the advantage of no chromatic aberration; however, they are expensive, bulky, and have a low damage threshold for reflective coatings. In addition, the surface photolysis chemical reaction will generate a pollution layer on the optical surface, and the reflectivity will decrease with the working time, resulting in the loss of VUV light intensity; and it is difficult to clean the reflective coating. Especially when used for high-intensity VUV light, the above-mentioned problems existing in the reflective focusing system will be more prominent.

To sum up, there is no transmissive spectroscopic focusing system for high-intensity vacuum ultraviolet laser.

Contents of the invention

The object of the present invention is to provide a transmission type spectroscopic focusing system for vacuum ultraviolet laser in order to overcome the shortcomings of the prior art. The system can use as few as three lenses to achieve simultaneous light splitting and submicron near-diffraction-limit focusing.

A transmissive spectroscopic focusing system for vacuum ultraviolet laser proposed by the present invention includes three convex lenses and a diaphragm, each convex lens and diaphragm are arranged perpendicular to the optical path and have a common optical axis; wherein, the first convex lens and the second convex lens The front and back are arranged on the same side of the diaphragm, and the third convex lens is located on the other side of the diaphragm;

Let the distance between the first and second convex lenses, the distance between the second convex lens and the diaphragm, and the distance between the diaphragm and the third convex lens be L ₁ , L ₂ , and L ₃ respectively, and let the first, second, and The focal lengths of the third convex lens in the vacuum ultraviolet laser band are f ₁ , f ₂ , and f ₃ respectively, and the parameters of each element satisfy the following formula:

1.5×(f ₁ +f ₂ )＞L ₁ ＞f ₁ +f ₂ (1)

L ₁ >2f ₁ (2)

f ₂ ≥ f ₁ (3)

L ₃ >f ₃ (5-1)

The working distance WD of the third convex lens is calculated according to formula (6):

Another transmissive spectroscopic focusing system for vacuum ultraviolet laser proposed by the present invention includes four convex lenses and an aperture, each convex lens and aperture are arranged perpendicular to the optical path and have a common optical axis; wherein, the first convex lens, the second The convex lenses are arranged on the same side of the diaphragm, the third convex lens and the fourth convex lens are arranged on the other side of the diaphragm;

Let the distance between the first and second convex lenses, the distance between the second convex lens and the diaphragm, and the distance between the diaphragm and the third convex lens be L ₁ , L ₂ , and L ₃ respectively, and let the first, second, and The focal lengths of the third and fourth convex lenses in the vacuum ultraviolet laser band are f ₁ , f ₂ , f ₃ , and f ₄ respectively, and the parameters of each element satisfy the following formula:

1.5×(f ₁ +f ₂ )＞L ₁ ＞f ₁ +f ₂ (1)

L ₁ >2f ₁ (2)

f ₂ ≥ f ₁ (3)

L ₃ =f ₃ (5-2);

The fourth convex lens is arranged anywhere behind the third lens, and the working distance of the fourth convex lens is equal to the focal length f ₄ of the lens.

The diameter of the central hole of the aperture is 1-50 μm.

The characteristics and beneficial effects of the present invention are: simple system structure, low cost, small volume, coaxial optical path, high damage threshold of optical surface, and convenient pollution cleaning. The invention uses magnesium fluoride and lithium fluoride to make optical elements for VUV light band, and uses at least three spherical or aspheric convex lenses to split and focus the VUV light source generated by four-wave mixing. The light splitting rate of fundamental frequency light can reach more than 99.99%, and the total transmittance of VUV light (125nm-150nm) can reach more than 10%. When focusing, cooperate with the aspheric lens to offset the spherical aberration, so that the system reaches the diffraction limit, so as to realize the submicron diffraction limit spot focusing.

Description of drawings

FIG. 1 is a schematic diagram of the optical path structure of a transmissive spectroscopic focusing system proposed by the present invention.

Fig. 2 is a schematic diagram of the optical path structure of another transmissive spectroscopic focusing system proposed by the present invention.

detailed description

Below in conjunction with accompanying drawing and specific embodiment the present invention is described in further detail:

The present invention proposes a transmissive spectroscopic focusing system for vacuum ultraviolet laser, which consists of three convex lenses and a diaphragm. The overall structure is shown in Figure 1. Each convex lens is made of magnesium fluoride or lithium fluoride. Each convex lens The first convex lens 1 and the second convex lens 2 are arranged on the same side of the diaphragm 5, and the third convex lens 3 is located on the other side of the diaphragm 5. The output light obtained by four-wave mixing is used as the incident light of this spectroscopic focusing system. After the incident light passes through the first and second convex lenses, the vacuum ultraviolet laser in the incident light converges and passes through the central hole of the diaphragm 5. The fundamental frequency The light is blocked by the diaphragm 5 due to the dispersion of the convex lens; the vacuum ultraviolet laser is focused through the third convex lens.

The setting distance of each component is as follows:

Let the distance between the first and second convex lenses, the distance between the second convex lens and the diaphragm, and the distance between the diaphragm and the third convex lens be L ₁ , L ₂ , and L ₃ respectively, and let the first, second, and The focal lengths of the third convex lens in the vacuum ultraviolet laser band are f ₁ , f ₂ , and f ₃ , then L ₁ , L ₂ satisfy the following formulas, which can achieve the designed light splitting effect while ensuring that the diaphragm and the first and second convex lenses will not damaged by:

1.5×(f ₁ +f ₂ )＞L ₁ ＞f ₁ +f ₂ (1)

L ₁ >2f ₁ (2)

f ₂ ≥ f ₁ (3)

The distance L between the _second convex lens and the diaphragm can be determined by formula (4):

At this time, the spot size of the fundamental frequency light at the position of the second convex lens is larger than 1/3W, and the optical density of the fundamental frequency light is less than 9 times of the incident fundamental frequency light, wherein W is the beam diameter of the incident light. When the second lens 2 with a longer focal length f ₂ is used, the optical density value of the fundamental frequency light at the second lens can be made smaller.

The diameter D of the central hole of the aperture can be 1-50 microns, and the size can be selected according to the thermal damage threshold of the sample to be tested. The aperture can simultaneously split the light and improve the quality of the VUV beam. The attenuation rate Q of the diaphragm to the fundamental frequency light (the ratio of the light energy through the central hole of the diaphragm to the total energy of the incident light, the greater the attenuation rate, the better the spectral effect), can be calculated by the following formula: Q=1-(D /w) ² . When the diameter of the incident light beam is 5 mm, the attenuation rate is between 99.99% and 99.99999%. Although the smaller the diameter of the central hole of the diaphragm, the better the light splitting effect, but the adjustment accuracy requirements for the position of the diaphragm will increase accordingly.

In order to ensure that the light beam converges behind the third convex lens ₃ , the distance L3 from the third convex lens to the diaphragm and the focal length f3 of the _third convex lens should satisfy:

L ₃ >f ₃ (5-1)

The working distance WD of the third convex lens should be calculated according to formula (6):

Another transmission-type spectroscopic focusing system for vacuum ultraviolet laser proposed by the present invention is composed of four convex lenses and a diaphragm. The overall structure is shown in Figure 2. Basically, while keeping the parameters and positions of the first convex lens 1, the second convex lens 2 and the diaphragm 5 unchanged, the distance L ₃ from the third convex lens to the diaphragm is adjusted to the focal length f ₃ of the third convex lens, so that the light beam passes through Collimated light is formed behind the third convex lens, and a fourth convex lens 4 is coaxially arranged at any position behind the third convex lens to achieve final focusing. The working distance of the fourth convex lens is equal to the focal length of the fourth convex lens. The attenuation rate of the fundamental frequency light of the spectral focusing system constituted by the four convex lenses is consistent with that of the spectral focusing system of the above three convex lenses. The advantage of the four-convex lens splitting and focusing system compared to the three-convex lens splitting and focusing system is that the final VUV focus position is adjustable, that is, the length of L4 in Figure ₂ can take any positive value, and the total length of the optical path is more flexible.

The technical solution of the present invention will be further described below through two specific examples. Both of these two embodiments can achieve the effect of integrating light splitting and focusing and coaxial optical paths. Moreover, the structure is simple, the maintenance is convenient, and the cost is low.

Embodiment 1 is the spectroscopic focusing system of three convex lenses, and the specific composition is as follows:

In the present embodiment, the spot diameter of the incident light is 4 mm, the central hole diameter D of the diaphragm is 10 μm, the wavelength of the VUV light is 125 nm, and the wavelength of the fundamental frequency light is 255-640 nm; the first convex lens, the second convex lens, and the third convex lens The focal lengths are respectively f ₁ =50mm, f ₂ =80mm, f ₃ =40mm, the distance between the first convex lens and the second convex lens L ₁ =140mm, the distance between the second convex lens and the diaphragm L ₂ =183mm, The distance between the third convex lens and the diaphragm is L ₃ =450mm, and the parameters of the working distance of 52mm and the diffraction-limited spot size of 500nm can be obtained.

The convex lenses in this embodiment are all made of magnesium fluoride, the transmittance to 125nm VUV light is 13%, and the attenuation rate to 255-640nm fundamental frequency light is 99.93%. At this time, the spot size of UV light (ultraviolet light) with a wavelength of 255 nm at the diaphragm is 12 mm. The first convex lens, the second convex lens and the third convex lens can adopt spherical mirrors. Since spherical mirrors will produce spherical aberration, aspheric lenses can also be used instead of spherical mirrors to reduce spherical aberration and achieve near-diffraction-limit performance.

Embodiment 2 is the spectroscopic focusing system of four convex lenses, and concrete composition is as follows:

On the basis of Embodiment 1, the third convex lens is replaced by a convex lens with a focal length of 450mm, and the parameters and positions of the first and second convex lenses and the diaphragm remain unchanged. The function of the third convex lens is to collimate the divergent VUV light into a collimated beam with a diameter of 20 mm. Then place the fourth convex lens 4 with a focal length of 52 mm at a position 50 mm behind the third convex lens 3 to obtain the parameters of a working distance of 52 mm and a diffraction-limited spot size of 500 nm. The convex lens in this embodiment is all made of magnesium fluoride, the transmittance to 125nm VUV light is 10%, and the attenuation rate to 255-640nm fundamental frequency light is 99.93%. The first, second, third and fourth convex lenses can be spherical mirrors. Since spherical mirrors will produce spherical aberration, aspheric lenses can also be used instead of spherical mirrors to reduce spherical aberration and achieve near-diffraction-limit performance.

Claims (3)

1. a kind of transmission-type light splitting focusing system for VUV LASER, it is characterised in that including three pieces of convex lens and one Individual diaphragm, each convex lens and diaphragm are each perpendicular to light path and common optical axis is set；Wherein, before and after the first convex lens, the second convex lens Diaphragm the same side is arranged in, the 3rd convex lens are located at diaphragm opposite side；

Make the distance between the distance between first and second convex lens, the second convex lens and diaphragm, diaphragm and the 3rd convex lens The distance between be respectively L₁、L₂、L₃, make first, second, third convex lens distinguish in the focal length of VUV LASER wave band For f₁、f₂、f₃, then the parameter of each element meet below equation：

1.5×(f₁+f₂) ＞ L₁＞ f₁+f₂ (1)

L₁＞ 2f₁ (2)

f₂≥f₁ (3)

L₃＞ f₃ (5-1)

The operating distance WD of 3rd convex lens is calculated according to formula (6)：

2. a kind of transmission-type light splitting focusing system for VUV LASER, it is characterised in that including four pieces of convex lens and one Individual diaphragm, each convex lens and diaphragm are each perpendicular to light path and common optical axis is set；Wherein, before and after the first convex lens, the second convex lens Diaphragm the same side is arranged in, the 3rd convex lens, the 4th convex lens are in tandem in diaphragm opposite side；

Make the distance between the distance between first and second convex lens, the second convex lens and diaphragm, diaphragm and the 3rd convex lens The distance between be respectively L₁、L₂、L₃, make first, second, third and the 4th convex lens VUV LASER wave band Jiao Away from respectively f₁、f₂、f₃、f₄, then the parameter of each element meet below equation：

1.5×(f₁+f₂) ＞ L₁＞ f₁+f₂ (1)

L₁＞ 2f₁ (2)

f₂≥f₁ (3)

L₃=f₃(5-2)；

4th convex lens are arranged on the 3rd lens rear any place, and the operating distance of the 4th convex lens is equal to the convex lens Focal length f₄。

3. transmission-type light splitting focusing system according to claim 1 or 2, it is characterised in that the centre bore of the diaphragm is straight Footpath is 1-50 μm.