CN111129935A

CN111129935A - Sum frequency method

Info

Publication number: CN111129935A
Application number: CN201911348814.3A
Authority: CN
Inventors: 王家赞; 朱光
Original assignee: Beijing Keyi Hongyuan Photoelectric Technology Co ltd
Current assignee: Beijing Keyi Hongyuan Photoelectric Technology Co ltd
Priority date: 2019-12-24
Filing date: 2019-12-24
Publication date: 2020-05-08

Abstract

The invention provides a sum frequency method, which comprises the following steps: frequency doubling the fundamental frequency light to form a frequency doubled light and a frequency tripled light respectively; firstly, rotating the polarization direction of the double-frequency light, and then enabling the double-frequency light to pass through a polaroid; combining the double-frequency light with triple-frequency light with the same optical path as the light to form a first combined light; the first combined beam of light is passed through a quintuple frequency crystal to sum the frequency of the two and three times frequency lights to form a quintuple frequency light. Compared with the prior art, the invention has the beneficial effects that: by using the method, the working efficiency of sum frequency can be improved, and the quintuple frequency laser can be efficiently obtained.

Description

Sum frequency method

Technical Field

The invention relates to the technical field of optics, in particular to a sum frequency method.

Background

In the application field of lasers, a 213nm deep ultraviolet solid laser is mainly used for on-line monitoring equipment of wafer defects, wherein the laser with the wavelength of 213nm is generated by multiple frequency doubling and sum frequency of the laser with the wavelength of 1064 nm.

In the prior art, the sum frequency of the fundamental frequency light and the quadruple frequency light is generally carried out to obtain 213nm laser. However, this sum frequency scheme has drawbacks, and the main drawbacks are that the selection range of crystals and coating products in the laser is narrow and the working efficiency is not high because 266nm quadruple light laser is involved. Thus, the application of such sum frequency schemes is limited.

Therefore, it is necessary to design another method for generating frequency quintuplex laser by sum frequency, so that the working efficiency of the 213nm deep ultraviolet solid state laser can be improved.

Disclosure of Invention

In view of this, in order to solve the problem that the application of the sum frequency scheme in the prior art is limited, the invention provides a sum frequency method, which includes the following steps:

step S1, frequency doubling the fundamental frequency light to form a frequency-doubled light and a frequency-tripled light respectively;

step S2, firstly, rotating the polarization direction of the double frequency light, and then making the double frequency light pass through the polaroid;

step S3, combining the double frequency light with triple frequency light with the same optical path to form the first combined light;

step S4, the first combined beam is passed through a quintuple frequency crystal to sum the frequency of the two-frequency light and the three-frequency light to form a quintuple frequency light.

Preferably, before the step S1 is executed, the fundamental light is subjected to optical path shrinking by a beam expander.

Preferably, the step S1 is implemented by:

step S10, the fundamental frequency light passes through the frequency doubling crystal and the frequency tripling crystal respectively, and the fundamental frequency light is separated to form a second beam combining light with the frequency doubling light and the frequency tripling light;

in step S11, the second combined beam is split into the second and third doubled beams.

Preferably, in step S10, the fundamental light is split by the second beam splitting device.

Preferably, in step S11, the beam splitting is performed by the first beam splitting device.

Preferably, before step S2 is executed, the optical path length compensation device performs optical path length compensation on the frequency-doubled light.

Preferably, in step S2, the polarization direction of the frequency-doubled light is rotated by a half-wave plate.

Preferably, before step S3 is executed, the optical path length and the propagation direction of the triple-frequency light are adjusted by adjusting the mirror.

Preferably, the light-emitting end face of the quintupling frequency crystal is provided with a Brett angle.

Preferably, the step S4 is followed by: and the quintupling frequency light passes through a triangular prism to be subjected to beam collimation.

Compared with the prior art, the invention has the beneficial effects that: by using the method, the working efficiency of sum frequency can be improved, and high-quality quintuple frequency laser can be efficiently obtained.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the embodiments or the prior art descriptions will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without inventive exercise.

Fig. 1 is a diagram illustrating a location distribution of each device in a sum frequency method according to an embodiment of the present invention;

fig. 2 is a schematic diagram of light paths of light emitted in parallel through a triangular prism according to an embodiment of the present invention;

fig. 3 is a flow chart of a sum frequency method according to an embodiment of the present invention;

fig. 4 is a trend graph of BBO frequency doubling efficiency normalization varying with crystal length according to an embodiment of the present invention.

Detailed Description

The above and further features and advantages of the present invention are described in more detail below with reference to the accompanying drawings.

In the description of the present invention, it is to be understood that the terms "first", "second" and the like are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implying any number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, "a plurality" means at least two unless specifically defined otherwise.

In the present invention, unless otherwise expressly stated or limited, the terms "mounted," "connected," "secured," and the like are to be construed broadly and can, for example, be fixedly connected, detachably connected, or integrally formed; may be mechanically coupled, may be electrically coupled or may be in communication with each other; they may be directly connected or indirectly connected through intervening media, or they may be connected internally or in any other suitable relationship, unless expressly stated otherwise. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.

In the description herein, references to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention.

In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular system structures, techniques, etc. in order to provide a thorough understanding of the embodiments of the invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known systems, devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, various embodiments or examples and features of different embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction.

In order to explain the technical means of the present invention, the following description will be given by way of specific examples.

Example one

Fig. 3 shows a schematic implementation flow diagram of the sum frequency method provided in the present application, which includes the following steps:

Before the step S1 is executed, the fundamental light is subjected to optical path reduction by a beam expander.

The specific implementation steps of step S1 are as follows:

step S10, the fundamental frequency light passes through the frequency doubling crystal and the frequency tripling crystal respectively, and the fundamental frequency light is separated by the second beam splitting device, so as to form a second beam combining light with the frequency doubling light and the frequency tripling light;

in step S11, the second combined beam of light is split into the second and third doubled light by the first beam splitting device.

Before step S2 is executed, the optical path length compensation device is used to perform optical path length compensation on the frequency-doubled light.

In step S2, the polarization direction of the frequency-doubled light is rotated by using a half-wave plate.

Before step S3 is performed, the optical path length and the propagation direction of the frequency-tripled light are adjusted using the adjustment mirror.

And the light-emitting end face of the quintuple frequency doubling crystal is provided with a cloth-cultured angle.

The step S4 is followed by:

the quintupling frequency light is collimated by a triangular prism.

The method provided by the present application is described below with reference to specific examples:

fig. 1 is a diagram of the location distribution of the devices in the sum frequency method according to the present invention, which includes a main optical path 10, a frequency-doubled optical path 20, a frequency-tripled optical path 30, and a beam combining optical path. The main light path 10 is sequentially provided with a beam expander 1, a frequency doubling crystal 2, a frequency tripling crystal 3, a second beam splitter 7 and a first beam splitter 6. The light is split by the first beam splitting device 6 to form a frequency doubling light branch 20 and a frequency tripling light branch 30. The triple-frequency light branch 30 is provided with an adjusting reflector 19. The double-frequency light branch 20 is sequentially provided with an optical path compensation device, a half-wave plate 13 and a polaroid 15. The optical path lengths of the triple-frequency optical branch 30 and the double-frequency optical branch 20 are the same. The frequency-doubled optical branch 20 and the frequency-tripled optical branch 30 are combined by the beam combining device 16 to form a combined optical path. That is, the frequency-doubled light in the frequency-doubled light branch 20 and the frequency-tripled light in the frequency-tripled light branch 30 are combined by the beam combining device 16 to form a combined light. The two-frequency light and the three-frequency light in the combined beam light are subjected to sum frequency by a quintuple frequency crystal 5 provided at the end of the combined beam light path, thereby forming quintuple frequency light.

Preferably, an adjusting mirror 19 for adjusting the propagation direction and the optical path of the light beam is provided between the optical path compensating device and the first beam splitting device 6. One or more adjusting mirrors 19 for adjusting the propagation direction and optical path of the light beam are arranged between the polarizer 15 and the beam combining device 16. The second beam splitting means 7 and the first beam splitting means 6 are dichroic beam splitters. The beam combining means 16 is a dichroic beam splitter.

Preferably, the light of the second frequency passing through the optical path length compensation device has the same propagation direction but the optical path length is increased. Further, optical path compensation arrangement includes two and decides end reflector 11, two and move end reflector 12 and displacement platform, and two are decided end reflector 11 and two and are moved end reflector 12 and arrange according to the rectangle structure, and two are moved end reflector 12 and place on displacement platform, and two are decided end reflector 11 and two and are moved end reflector 12 combined action and form fluted type light path. By moving the displacement platform, the optical path of the frequency-doubled light inside the optical path compensation device can be changed, increased or decreased, so that the frequency-tripled light in the frequency-doubled light branch 30 and the frequency-doubled light in the frequency-doubled light branch 20 can be ensured to have the same optical path.

Preferably, frequency doubling crystal 2 and frequency tripling crystal 3 are lithium triborate crystals, i.e. LBO. The quintupling crystal 5 is preferably a barium metaborate crystal, BBO. The ultraviolet frequency doubling crystals comprise BBO, LBO, PPLN, CLBO, KBBF and the like, and each crystal has respective advantages and disadvantages in the frequency doubling process. Because quintuple frequency laser is generated by frequency doubling light and frequency tripling light sum frequency, BBO and KBBF are preferably used as crystals, but the KBBF crystal has more deep ultraviolet absorption and has the defects of difficult processing, few purchasing ways and the like, so the BBO is selected for quintuple frequency optical design. BBO was originally developed successfully by Fujian material structure of Chinese academy of sciences, and is one of the most excellent nonlinear crystals for frequency doubling in the deep ultraviolet band so far. When the BBO crystal is used for frequency quintuple, the scheme is flexible, and the sum frequency can be carried out by using the fundamental frequency light and the frequency quadruplicate, and the sum frequency can be carried out by using the frequency doubling light and the frequency triplex light. Since the frequency doubling schemes of frequency doubling and light tripling are more excellent, the corresponding frequency doubling and frequency tripling schemes are realized when BBO crystal class I phase matching is used.

Preferably, a brewster angle of the quintuple frequency laser is cut on the light-emitting end face of the quintuple frequency crystal 5, three beams of light are opened at the light-emitting position, the two-frequency light and the three-frequency light respectively enter the garbage can to be collected, and the quintuple frequency light passes through a triangular prism 9 to be used for collimation of the beams.

The sum frequency method provided by the embodiment of the application comprises the following specific steps:

the front end of the main optical path 10 is provided with a laser seed source, and the laser seed source can emit seed laser to the beam expander 1 on the main optical path 10. The seed laser needs a high-power laser, and the high-power laser can be a high-power nanosecond laser, a high-power nanosecond picosecond laser or a high-power nanosecond femtosecond laser.

Firstly, the seed laser is subjected to optical path beam shrinkage through the beam expander 1, and the optical path beam shrinkage has the beneficial effects that: which can increase the peak power density of the back end. The seed laser belongs to fundamental frequency light.

Then, the fundamental frequency light continues to propagate in the main optical path 10, the second frequency light is generated after the fundamental frequency light passes through the second frequency doubling crystal 2, and the third frequency light is generated after the fundamental frequency light passes through the third frequency doubling crystal 3, so that the second frequency light, the second frequency doubling light and the fundamental frequency light are respectively propagated in the main optical path 10 at the rear end of the third frequency doubling crystal 3 and at the front end of the second beam splitting device 7, but the fundamental frequency light at this time cannot be utilized by the subsequent sum frequency, and can be separated, after the light beam containing the second frequency doubling light, the second frequency doubling light and the fundamental frequency light passes through the second beam splitting device 7, the second frequency doubling light and the third frequency doubling light continue to propagate backwards along the main optical path 10, and the fundamental frequency light is reflected by the second beam splitting device 7 to the light blocking plate 8 or the garbage can. When the frequency-doubled light and the frequency-tripled light which continuously propagate in the main light path 10 through the second beam splitting device 7 encounter the first beam splitting device 6, the frequency-doubled light and the frequency-tripled light are separated, the frequency-doubled light enters the frequency-doubled light branch 20, and the frequency-tripled light enters the frequency-tripled light branch 30. Since the optical paths of the triple-frequency light branch 30 and the double-frequency light branch 20 are the same, the optical paths of the double-frequency light and the triple-frequency light are the same before beam splitting and beam combining. The frequency doubled light and the frequency tripled light are split at the first beam splitting means 6 and combined at the beam combining means 16. The optical path of the frequency-doubled light and the optical path of the frequency-tripled light are the same, and the beneficial effects are as follows: thus, the phase matching condition of frequency multiplication can be met.

The triple frequency light branch 30 is provided with an adjusting reflector 19, and the triple frequency light can smoothly reach the beam combining device 16 under the reflection action of the adjusting reflector 19. The adjustment mirror 19 in the triple-frequency optical branch 30 can adjust the optical path length and the propagation direction of the triple-frequency light.

The optical path length of the double frequency light propagating in the double frequency light branch 20 is adjusted by the optical path length compensation device in the double frequency light branch 20. The optical path compensation device is used for compensating the optical path difference problem in the frequency doubling optical branch 20 and the frequency tripling optical branch 30, if the optical path difference is too large, the frequency doubling phenomenon cannot occur, generally, the optical path difference has little influence on the nanosecond level, but when the sum frequency of the picosecond or femtosecond level light is performed, the influence of the optical path difference is great, so that the optical path compensation device is required to compensate the pulse dislocation in the time.

The double-frequency light passes through the optical path compensation device and then passes through a system consisting of a half-wave plate 13 and a polaroid 15, the polaroid 15 is used for controlling the polarization direction of the final double-frequency light, and because the polarization directions of the front and the back of the polaroid 15 are different, the polarization direction needs to be rotated by using a half-wave 13, and meanwhile, under the normal condition, the light-emitting power of the double-frequency light is higher than the light-emitting power of the triple frequency, so that the optimal photon number ratio can not be achieved in the process of generating the quintuple light by the double-frequency light and the triple frequency light, the light-emitting power of the double-frequency light can be freely controlled by using the combination of the half-wave plate and the polaroid, and the frequency doubling efficiency and the final light-emitting power can be.

The double-frequency light and the triple-frequency light are finally superposed at the position of the beam combining device 16, the adjusting reflecting mirrors 19 in the double-frequency light branch 20 and the triple-frequency light branch 30 are simultaneously controlled, two beams of reflected light passing through the beam combining device 16 are completely superposed in space, the superposed combined light finally passes through the quintuple frequency crystal 5, thus quintuple frequency light is obtained by frequency mixing, a Brewster angle of the quintuple frequency laser is cut at the light-emitting end face of the quintuple frequency crystal 5, the three beams of light are opened at the light-emitting position, the double-frequency light and the triple-frequency light respectively enter a garbage can to be collected, and the quintuple frequency light is used for collimating the light beams through a triangular prism 9.

BBO's exit surface can all design into the cloth angle cutting, and because the refracting index is different at BBO crystal's exit surface like this, the light-emitting direction just splits naturally, then places a prism at the light-emitting mouth, can let emergent light level outgoing behind the prism. And placing a prism for the exit light, wherein the incident plane is at the Brewster angle, and then, the laser is horizontally emitted. The initial roundness of the 212.8nm laser is about 75%, the laser is horizontal in the walking direction in a BBO, the horizontal ellipse is changed, after the laser is emitted from the inclined plane of the BBO, a light spot is compressed and horizontally contracted, then the laser is collimated by a triangular prism arranged at a certain angle, the light spot is amplified, the finally emitted light spot is close to the light spot in the crystal, the final roundness is about 75%, and the corresponding power loss is about 5%.

Example two

The difference between the present embodiment and the above embodiments is that LBO is a nonlinear optical crystal with excellent performance, and the LBO crystal has the characteristics of high optical damage threshold, moderate nonlinear coefficient, stable chemical performance, high mechanical hardness, wide product application range, and the like, so that LBO is used for frequency doubling related to double frequency light and triple frequency light.

When LBO is used for frequency doubling, class I phase matching of the LBO crystal is used here, and the temperature is set to 149 ℃, mainly aiming at keeping the fundamental light and the frequency doubled light from going away after LBO.

When the temperature is 149 ℃, the angle tolerance and the acceptance angle range of the corresponding crystal are very large, so that the crystal has very high containment to laser and very high frequency doubling efficiency.

Because the repetition frequency of the laser is very high and the corresponding peak power density is not very high, the laser needs to be converged first, the peak power density of the laser is improved, and then the corresponding frequency doubling calculation is performed.

In order to prevent the appearance of singular points in light spots, the damage resistance power of a corresponding beam-shrinking system is higher, and the peak power of a coating film is usually required to be more than 3 times larger than the peak power, so that the peak power of a light-emitting lens of the beam-shrinking system is required to be more than 1.5GW/cm ^ 2.

When a crystal with a proper length is used, the overall 532nm laser power is 30W, and the rear-stage matching power is basically met. When the 532nm laser power in the later stage is too high, the corresponding frequency doubling efficiency can be reduced by adjusting the crystal temperature.

Specifically, in this embodiment, a frequency tripler laser of 354.7nm can be obtained by performing sum frequency using fundamental light of 1064nm wavelength and frequency doubled light of 532nm wavelength.

Due to the highly reliable type of lithium triborate crystal, the sum frequency 354.7nm laser still uses LBO. Because the front stage is the fundamental frequency light and the frequency doubling light without walk-off, the sum frequency of the two beams can be realized by directly inputting the two beams into LBO.

When using LBO for frequency tripled sum frequency, class II phase matching of the crystal is used, with the temperature set at high temperature, primarily to prevent deliquescence of the crystal.

The walk-off of 532nm laser in the crystal is 9.49mrad, the corresponding walk-off is not very large, the influence on the sum frequency efficiency is limited, and the 354.7nm laser and the 1064nm laser are collinear and do not walk off, so that the beam quality of the 354.7nm laser can be ensured.

Specifically, performing sum frequency with 532nm laser light and 354.7nm laser light can obtain five times frequency laser light of 212.8 nm. Since the sum frequency of 532nm laser and 354.7nm laser is used, the BBO crystal performs the sum frequency using class I phase matching. When using BBO crystals for quintuple frequency summation, the temperature is set to a high temperature, mainly aimed at preventing the BBO crystals from deliquescing.

The corresponding sum frequency efficiency can be calculated by adjusting the crystal temperature to reduce the light output power of 354.7nm laser and carrying out quintupling frequency sum frequency calculation. When the total incident power is not changed, the temperature of the frequency doubling crystal 2 and the frequency tripling crystal 3 is properly adjusted, and the light emitting power of 532nm laser can be controlled.

The BBO crystal has a comparatively large absorption effect on deep ultraviolet laser light, in which the absorption coefficient is very large at the 213nm position. Therefore, when the sum frequency of the 354.7nm laser and the 532nm laser is used for generating the 212.8nm laser, the crystal also absorbs corresponding heat along with the increase of the conversion efficiency. When the absorption condition is considered, a correction term of the absorption coefficient needs to be increased, and the corresponding light-emitting power is reduced. When the sum frequency is carried out, the temperature control of the crystal is required to be reduced, the design temperature of the crystal is high, the frequency doubling efficiency with high middle peak value can be guaranteed to be the highest when the edge temperature is low, but the BBO crystal is not very sensitive to the temperature when the BBO crystal is matched in the class I mode, corresponding matching can be achieved within a large temperature range, and if the temperature change is large, the corresponding temperature difference can be compensated through angle adjustment.

The final sum frequency optical power is about 1.26W, and if the incident power is higher than the above condition, the sum frequency optical power is increased simultaneously.

The above-mentioned embodiments are only used for illustrating the technical solutions of the present invention, and not for limiting the same; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; such modifications and substitutions do not substantially depart from the spirit and scope of the embodiments of the present invention, and are intended to be included within the scope of the present invention.

Claims

1. A sum frequency method, characterized in that it comprises the steps of:

2. The sum frequency method according to claim 1, wherein the fundamental light is optically down-converted by a beam expander before performing step S1.

3. The sum frequency method according to claim 1, wherein the step S1 is implemented by the following steps:

4. Sum frequency method according to claim 3, characterized in that in step S10 the fundamental light is split by a second beam splitting means.

5. Sum frequency method according to claim 3, characterized in that in step S11 the splitting is performed by a first splitting means.

6. Sum frequency method according to claim 1, characterized in that the double frequency light is path length compensated by path length compensation means before step S2 is performed.

7. The sum frequency method according to claim 1, wherein the polarization direction of the double frequency light is rotated by a half wave plate in step S2.

8. The sum frequency method according to claim 1, wherein the optical path length and the propagation direction of the triple-frequency light are adjusted by adjusting the mirrors before performing step S3.

9. The sum frequency method according to claim 1, wherein the light exit end face of the quintuplex crystal is provided with brewster's angle.

10. Sum frequency method according to claim 1, characterized in that said step S4 is followed by further comprising:

and the quintupling frequency light passes through a triangular prism to be subjected to beam collimation.