An oscillator-type free electron laser three-hole coupled output method and device

Abstract

The present invention relates to a method and device for three-hole coupled output of an oscillator-type free electron laser. Three circular holes are opened on the downstream mirror of the optical resonator, and their position distribution and size can be determined according to the transverse optical field distribution in the resonator. When the free electron laser operates at the spectral gap wavelength, the saturated optical field in the resonator will form three or more optical field power peaks in the vertical direction under the influence of the optical waveguide. Under this optical field distribution, three small holes are opened on the downstream mirror of the optical resonator, arranged vertically. The middle hole is located at the center of the mirror, and the positions of the two side holes correspond to the positions of the two optical field power peaks adjacent to the center of the mirror. The diameter of the small holes is selected according to the optical field distribution on the mirror at the spectral gap wavelength, generally not larger than the diameter of the central hole. Without affecting the coupled output power of wavelengths outside the spectral gap, the present invention greatly improves the problem of low coupled output power at the spectral gap, thereby enhancing the overall performance of the oscillator-type free electron laser device.

Description

Technical Field

[0001] The present invention relates to a three-hole coupling output method and device for an oscillator-type free electron laser, belonging to the field of free electron lasers, and can be applied to oscillator free electron lasers in the infrared and terahertz bands. Background Art

[0002] The oscillator-type free electron laser has been widely used as a scientific research light source around the world for many years, mainly for generating lasers in the infrared and terahertz bands. Most of the currently operating infrared and terahertz band free electron laser devices in the world also adopt the oscillator mode. The oscillator-type free electron laser light source can provide laser pulses with continuously tunable wavelength, quasi-monochromatic, and high peak power, and is applied to scientific research such as condensed matter physics, physical chemistry, and biophysics (B.N. Murdin, Contemporary Physics, 50, 2 (2009), O'Shea, P.G., and H.P. Freund, Science 292, 5523 (2001)). In order to obtain a laser more suitable for scientific research, the requirements for the coupling output of the oscillator-type free electron laser are wide tuning and high efficiency and high power.

[0003] The coupling output of the oscillator-type free electron laser is realized through an optical resonator. In principle, as Figure 1 shown, the optical resonator is mainly composed of two high-reflectivity mirrors that are centered on each other in a high vacuum. It stores the spontaneous radiation light field generated when the electron beam passes through the undulator, and makes the light field reflect back and forth inside it, continuously interacting with the subsequent electron beam, so that the light field accumulates and amplifies until saturation.

[0004] Common coupling output methods of oscillator-type free electron lasers include: central hole coupling output, partial transmission mirror coupling output, near-Brewster angle flat plate coupling output, and side coupling output. For partial transmission mirror coupling output, it hardly disturbs the light field distribution in the optical resonator, but the tuning range is not wide enough. For example, the composite metal mesh grating coupling output mirror (Kong Wei-Peng, et al., J. Infrared Millim. Waves 34, 5 (2015)). For near-Brewster angle flat plate coupling output, it has a total reflection cavity mirror with a high mirror damage threshold, wide tuning, and adjustable coupling efficiency. However, the disadvantage is multi-beam output (D. Paboeuf, et al., Appl Phys B 108, 289–293 (2012)). Side coupling output is flexible, can adjust the Q value of the optical resonator, has wide tuning, and relatively small diffraction losses (Dou Yu-Huan, et al., High Power Laser and Particle Beams 15, 12 (2003)). Currently, oscillator-type free electron lasers mostly adopt central single-hole coupling output. The advantages are high coupling efficiency, wide tuning, good output light quality, the reflector of the optical resonator can use a metal substrate to improve the mirror damage threshold, and the small hole can also serve as the electron beam channel. The disadvantages are that the hole cannot be too large, high-order modes are easily generated, and the coupling output rate is related to the wavelength. For long wavelengths, the diffraction losses are relatively large (B. Faatz, et al., IEEE Journal of Quantum Electronics 29, 7 (1993), Jiang Yun-Qing, Wang Yuan-Zhang, High Power Laser and Particle Beams 8, 3 (1996)).

[0005] In oscillator-type free electron lasers in the infrared and terahertz bands, rectangular optical waveguides are mostly used to overcome diffraction losses. However, due to the addition of rectangular optical waveguides, a spectral gap phenomenon will occur during use (R. Prazeres, et al., Physical Review Special Topics-Accelerators and Beams 12, 010701 (2009)). As Figure 5 shown, the spectral gap phenomenon is that near some resonant wavelengths, the optical waveguide causes a change in the light field mode in the optical resonator, resulting in a very weak light field intensity near the central hole on the coupling output mirror, and then leading to a rapid decrease or even zero of the central hole coupling output power, affecting the wavelength tuning range of the output laser.

[0006] The existence of this spectral gap has a highly adverse impact on a free electron laser device because spectral scanning is a commonly used scientific research method by users. Especially when the spectral gap is at certain critical wavelengths, the impact is even more severe, which may lead to the inability to conduct many scientific experiments. Therefore, developing a coupling output method that can increase the output power at the spectral gap is of great significance for the infrared oscillator free electron laser light source. Summary of the Invention

[0007] The technical problem solved by the present invention: Overcoming the deficiencies of the prior art, providing a three-hole coupling output method and device for an oscillator-type free electron laser, which has the advantages of being easy to implement, high coupling efficiency, and wide tuning for infrared free electron laser coupling output. By adopting three-hole coupling output on the downstream mirror of the optical resonator, the output power of the free electron laser at wavelengths near the spectral gap is significantly increased, and the adverse impact of the spectral gap phenomenon on the infrared oscillator free electron laser device is improved.

[0008] The technical solution of the present invention: A three-hole coupling output method for an oscillator-type free electron laser, characterized in that: three holes are opened on the downstream mirror of the oscillator optical resonator.

[0009] The three holes are circular holes, arranged vertically. The middle hole of the three holes is located at the center of the mirror, and its diameter is designed according to the oscillator-type free electron laser, and the total loss of the free electron laser in the entire working band needs to be considered; the positions of the two side holes of the three holes are symmetric about the center hole in the vertical direction, and the centers of the two side holes correspond to the positions of the two optical field power peaks adjacent to the center on the light field distribution on the mirror at the wavelength of the spectral gap caused by the waveguide mode. The diameters of the two side holes are determined according to the light field distribution on the mirror at the spectral gap and are smaller than the diameter of the center hole.

[0010] A three-hole coupling output device for an oscillator-type free electron laser of the present invention includes an upstream mirror, an undulator, an optical waveguide, a downstream mirror, and a three-hole array; both the upstream mirror and the downstream mirror are placed in a high-vacuum chamber and are connected to the optical waveguide through a vacuum pipeline. The line connecting the centers of the upstream mirror and the downstream mirror coincides with the line connecting the foci to form an optical resonator; the optical waveguide is symmetrically placed in the middle position of the line connecting the centers of the upstream mirror and the downstream mirror; the upper and lower magnetic arrays of the undulator are placed on the upper and lower sides of the optical waveguide, and three circular holes for coupling output are opened on the downstream mirror. The central circular hole of the three circular holes is located at the center of the downstream mirror, and the other two holes are located on both sides of the center hole in the vertical direction.

[0011] The present invention has the following advantages and positive effects compared with the prior art:

[0012] (1) The present invention can effectively improve the output power of free electron lasers at wavelengths near the spectral gap and improve the overall output performance of free electron laser devices over the entire wavelength range. The prior art uses single-hole coupling output, and it is also possible to increase the output power of free electron lasers at wavelengths near the spectral gap by enlarging the diameter of the single hole. However, a relatively large single-hole diameter is usually required, which may have a very serious impact on the laser coupling output at wavelengths outside the spectral gap. Especially when the free electron laser operates at relatively short wavelengths, the light spot on the mirror will be relatively small. An overly large coupling output hole will result in an excessively high coupling output ratio at this wavelength, making the light field stored in the optical resonator too small, thereby leading to a low saturation power of the free electron laser or even failure to start oscillation. Compared with the prior art, the solution of the present invention can increase the output power of free electron lasers at the spectral gap wavelength by about four times. At the same time, for wavelengths outside the spectral gap, not only will it not affect their normal start-up and saturation power levels, but it can also effectively increase the power of the coupled output free electron lasers.

[0013] (2) The present invention is easy to fabricate: It is relatively easy to implement in engineering to open three small holes on the downstream mirror, and the manufacturing cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic diagram of the three-hole coupling output of an oscillator-type free electron laser, that is, the schematic diagram of the solution of the present invention;

[0015] Figure 2 It is a schematic diagram of the downstream mirror with three coupling small holes in the present invention;

[0016] Figure 3 It is the distribution diagram of the saturated light field in front of the mirror at the resonance wavelength of 25.8 μm (outside the spectral gap) obtained by simulation;

[0017] Figure 4 It is the distribution diagram of the saturated light field in front of the mirror at the resonance wavelength of 21.7 μm (i.e., the spectral gap) obtained by simulation;

[0018] Figure 5 It is a comparison diagram of the free electron laser output power between the three-hole coupling output method and the central single-hole coupling output method obtained by simulation.

[0019] Among them: 1 is the upstream mirror, 2 is the undulator, 3 is the optical waveguide, 4 is the downstream mirror, 5 is the three-hole array, and 6 is the electron beam. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be described in detail below with reference to the accompanying drawings and embodiments. The content not described in detail in the embodiments of the present invention belongs to the prior art well-known to those skilled in the art.

[0021] AsFigure 1 As shown in Figure 1 , an oscillator-type free electron laser three-hole coupling output device of the present invention includes an upstream mirror 1, an undulator 2, an optical waveguide 3, a downstream mirror 4, a three-hole array 5, and an electron beam 6. The upstream mirror 1 and the downstream mirror 4 are both placed in a high-vacuum chamber and connected to the optical waveguide 3 through a vacuum pipe. The line connecting their centers coincides with the line connecting the foci, forming an optical resonator. The optical waveguide 3 is symmetrically placed at the center of the optical resonator. The upper and lower magnetic arrays of the undulator 2 are placed on the upper and lower sides of the optical waveguide 3. The electron beam deflects into the optical waveguide 3 from the upstream-side vacuum chamber of the optical waveguide, interacts with the stored optical field in the optical resonator in the magnetic field environment of the undulator to amplify the optical field, and deflects out of the optical resonator from the downstream-side vacuum chamber of the optical waveguide. Three circular holes for coupling output are opened on the downstream mirror. The central circular hole is located at the center of the downstream mirror, and the other two holes are located on both sides of the central hole along the vertical direction.

[0022] A method for the three-hole coupling output of an oscillator-type free electron laser of the present invention is to open three circular holes on the downstream mirror of the optical resonator, and their position distribution and size can be determined according to the transverse optical field distribution in the resonator. In an oscillator-type free electron laser, when the wavelength is short, the transverse dimensions of the optical field stored in the optical resonator are not large everywhere in the cavity, and the saturated optical field distribution on the mirror shows a Gaussian distribution. However, when the wavelength is long, the spot sizes everywhere in the cavity will increase accordingly, while the size of the flat vacuum chamber in the vacuum pipe, especially in the undulator, is relatively small, and the diffraction loss increases. Therefore, it is necessary to replace the flat vacuum chamber in the undulator with an optical waveguide with a smooth inner wall and extremely high conductivity. The optical waveguide will have an impact on the optical field in the cavity and produce a phenomenon of spectral gaps at certain specific wavelengths. When the free electron laser operates at the spectral gap wavelength, the saturated optical field in the resonator will be divided into three or more optical field power peaks in the y-axis (vertical) direction, as Figure 4 shown. Under this optical field distribution, three holes, all circular holes, are opened on the downstream mirror of the optical resonator, arranged along the vertical direction. The middle hole is located at the center of the mirror, and its diameter is designed according to a general oscillator-type free electron laser, that is, it is necessary to reasonably consider the total loss of the free electron laser in the entire working wavelength range. The positions of the two side holes are symmetric about the central small hole along the vertical direction, and their centers correspond to the positions of the two optical field power peaks adjacent to the center of the optical field distribution on the mirror at the spectral gap. The diameters of the two side holes need to be determined according to the optical field distribution on the mirror at the spectral gap, and generally do not exceed the diameter of the central hole. Without affecting the coupling output power in the resonance wavelength range other than the spectral gap, the three-hole coupling output method of the optical resonator can improve the problem of low coupling output power at the spectral gap, thereby expanding the tuning range of the coupling output.

[0023] The present invention can improve the coupling output rate of an oscillator-type free electron laser. In particular, for the optical field caused by an optical waveguide, the waveguide mode distribution in the y (vertical) direction appears on the mirror. The waveguide mode will cause the optical field stored in the optical resonator to have a very small optical field distribution at the central hole position when reaching the mirror at some free electron laser wavelengths, while most of the optical field energy is distributed on both sides of the central hole along the y direction. Therefore, if a single central hole coupling output is used, the output power of the free electron laser coupled out is extremely low, and this wavelength will be a spectral gap.

[0024] The solution of the present invention is to add two holes on both sides of the central hole along the y direction, which can effectively improve the output power of the free electron laser at the spectral gap wavelength. The diameter of the central hole is designed according to the general oscillator-type free electron laser, that is, it is necessary to reasonably consider the total loss of the free electron laser in the entire working wavelength range. The positions of the two side holes are symmetric about the central small hole along the vertical direction, and their centers correspond to the positions of the two optical field power peaks adjacent to the central one in the optical field distribution on the mirror at the spectral gap. The diameters of the two side holes need to be determined according to the optical field distribution on the mirror at the spectral gap, and generally should not be larger than the diameter of the central small hole. One is to ensure an appropriate coupling output rate so that the total loss of the oscillator in one pass is not too large, and the other is to keep an appropriate distance between the two side holes and the central hole as much as possible so that the operation of the free electron laser at wavelengths outside the spectral gap is not greatly affected.

[0025] Embodiment

[0026] In this example, the structure shown in Figure 1 is adopted. Taking an infrared oscillator-type free electron laser with a resonance wavelength range of 15 - 50 μm as an example, the vacuum chamber in the undulator adopts an optical waveguide structure, and a three-hole coupling output method is used on the downstream mirror. The main simulation parameters are as follows: the electron energy of the electron beam is 25 MeV, and the root-mean-square energy spread is 200 keV; the period length of the undulator is 46 mm, and the number of periods is 50; the cavity length of the optical resonator (the distance between the centers of the upstream and downstream mirror surfaces) is 5.04 m, the mirror reflectivity of the cavity is 98.5%, and the radius of curvature of the mirror is 2.756 m; the inner diameter length and width of the cross-section of the optical waveguide are 30 mm and 10 mm respectively, and the length is the same as that of the undulator.

[0027] In this example, as shown in Figure 2 a coupling output method of opening three circular holes along the y-axis direction is adopted. The central hole is located at the center of the downstream mirror, that is, its center is located on the central optical axis of the optical resonator. The diameter of the central hole is designed according to the general oscillator-type free electron laser and needs to be determined according to the gain and total loss in the entire working wavelength range of the free electron laser. The coupling output rate of the central hole is the main component of the total loss. According to the design of the general oscillator-type free electron laser, the diameter of the central small hole is 1.5 mm.

[0028] The selection of the center positions and diameters of the two side small holes is based on the distribution of the saturated optical fields at wavelengths outside the spectral gap and at the spectral gap wavelength as shown in Figure 3 and Figure 4 on the downstream mirror. From Figure 4 , the two optical field power peaks adjacent to the center of the optical field distribution on the mirror at the spectral gap are respectively located 3.8 mm on both sides of the mirror center along the y-axis. Therefore, the center positions of the two side small holes are respectively set at 3.8 mm in the positive y-axis direction of the center hole and -3.8 mm in the y-axis direction. From Figure 4 , it can also be found that the sub-light spots corresponding to the two optical field power peaks adjacent to the mirror center are both ellipses with a major axis of 16 mm and a minor axis of 4 mm. After optimization, we select the diameters of the two side small holes to be the same as that of the center small hole, which is 1.5 mm.

[0029] Figure 5 Fig. shows the relationship between the free electron laser power of the above three-hole coupled output obtained by simulation and the wavelength, and a comparison is made with the case of the center single-hole coupled output method. In the simulation, the center small hole diameter of the center single-hole coupled output method is the same as that of the three-hole coupled output method, and the difference is only in the presence or absence of the upper and lower two holes. In this way, it can be compared that the three-hole coupled output method improves the free electron laser output power in the wavelength range of the spectral gap. At the center wavelength of the spectral gap of 22.5 μm, the free electron laser output power is increased from 0.1 MW to 0.4 MW. At wavelengths outside the spectral gap, the free electron laser output power also has a significant increase.

[0030] Therefore, it can be seen that the three-hole coupled output method of the present invention can improve the influence of the spectral gap of the free electron laser of the infrared oscillator with a waveguide. On the basis of having the advantages of the center hole coupled output method, it improves the tuning ability of the coupled output, and is a coupled output method for an optical resonator with wide tuning, high efficiency and high power.

[0031] The above embodiments are provided only for the purpose of describing the present invention, and are not intended to limit the scope of the present invention. The scope of the present invention is defined by the appended claims. All equivalent substitutions and modifications made without departing from the spirit and principles of the present invention shall be covered within the scope of the present invention.

Claims

1. A method for three-hole coupled output of an oscillator-type free electron laser, characterized in that: three holes are opened on the downstream mirror of the oscillator optical resonator; the output power of the free electron laser at the spectral gap wavelength is increased by 4 times; the three holes are circular holes arranged vertically. The middle hole of the three holes is located at the center of the mirror, and its diameter is designed according to the oscillator-type free electron laser, and the total loss of the free electron laser in the full working band needs to be considered; the positions of the two side holes of the three holes are symmetric about the center hole in the vertical direction, and the centers of the two side holes correspond to the positions of the two optical field power peaks adjacent to the center in the optical field distribution on the mirror at the spectral gap wavelength caused by the waveguide mode. The diameters of the two side holes are determined according to the optical field distribution on the mirror at the spectral gap and are smaller than the diameter of the center hole.

Images