Automatic alignment of angularly multiplexed beams in excimer laser MOPA system

Accepted Manuscript Title: Automatic alignment of angularly multiplexed beams in excimer laser MOPA system Author: Dahui Wang Hang Qian Xueqing Zhao Yongsheng Zhang Yang Zhu Jun Zhao PII: DOI: Reference:

S0030-4026(16)31523-6 http://dx.doi.org/doi:10.1016/j.ijleo.2016.11.187 IJLEO 58603

To appear in: Received date: Accepted date:

24-4-2016 28-11-2016

Please cite this article as: Dahui Wang, Hang Qian, Xueqing Zhao, Yongsheng Zhang, Yang Zhu, Jun Zhao, Automatic alignment of angularly multiplexed beams in excimer laser MOPA system, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.11.187 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Automatic alignment of angularly multiplexed beams in excimer laser MOPA system Dahui Wang*, Hang Qian, Xueqing Zhao,Yongsheng Zhang, Yang Zhu, Jun Zhao State Key Laboratory of Laser Interaction with Matter, Northwest Institute of Nuclear Technology, P.O.Box 69-26, Xi'an 710024, P.R.China * E-mail: [email protected]

Abstract. We present the study on multiplexed beams automatic alignment of high efficiency excimer laser. He-Cd laser with the wavelength of 325nm is selected as the automatic alignment laser at first. And then, the crosshair arrays close to lens array are proposed and designed as the references of near-field and far-field, whereafter array beams are imaged and processed by specific fluorescence imaging system and region segmentation separately. Experiments are carried out in pre-amplifier II with three beams and double passes in excimer laser MOPA system. Results indicate that accuracy of the automatic alignment beam is 0.54% of the diameter of the windows. Meanwhile, the whole process of automatic alignment just takes 40 seconds, which ensures intelligent and high effective integration of automatic alignment system. Keywords: excimer laser, automatic alignment, crosshair array reference, fluorescence imaging, closed loop feedback control.

1. INTRODUCTION High power laser facility used for inertial confinement fusion (ICF) is the largest scale laser system. By researchers’ investigation and comparison, angular multiplexed excimer laser system with beam smoothing by introduction of spatial incoherence is verified to be a good way to obtain short pulse and perfect target illumination simultaneously. Typically, such excimer laser facility contains a great deal of optical elements, and experiences long distance typically several hundreds of meters or even more. Accordingly, spot size of sub millimeter in diameter is required for most target physics study. In fact, beam pointing is easily influenced by temperature gradient, micro-vibration on ground and optical platforms, mechanical structure worm of the reflectors and lens, etc. In this way, rapid alignment of laser system is needed for operation with high efficiency[1-3]. High power laser facilities such as National Ignition Facility in America[4-6],

Laser Mégajoule in France[10] and ShenGuang III in

China[8-11], outfit modularize and intelligent automatic alignment system mostly. Alignment of angular multiplexed beams should ensure all beams to propagate through amplifiers avoiding optical aberrations even crosstalk occurrence. In this way, the beams from the amplifiers can be transferred along the designated direction and accordingly irradiate on the target with high stabilization and accuracy. Generally, automatic alignment system is made up of irradiation laser, near-field and far-field references, images capturing module and servo control mechanism. In the terms

of the characteristics of excimer laser main oscillator power amplification(MOPA)system, double crosshair arrays close to len arrays used for imaging relay are selected as the references of near-field and far-field, images capturing and processing is accomplished by the means of fluorescence imaging and region segmentation, and closed loop feedback control is accomplished by field programmable gate array and power drive module based on “approaching by one step”. On the basis of above-mentioned technologies, an automatic alignment system applied for multi-passes excimer laser system, which has been verified to be useful and effective in pre-amplifier II with three beams and double passes in excimer laser MOPA system, is designed.

2. KEY TECHNOLOGIES Automatic alignment structure[12-15] is a closed loop control, in which imaging transducers are capturing facilities, dynamoelectric reflectors are executing devices, computers are control hardware, the input errors are the aberration between gravity center of gather beam images and reference images, the output control factors are the steps of step motors, and relation between the input and output is control algorithm. Accordingly, we can get the conclusion that laser automatic alignment system has such primary technologies including as irradiation laser, near-field and far-field references, image capturing module and servo control mechanism. However, the problems of long optical distance, several laser beams and integrated control, in the course of automatic alignment of pre-amplifiers, master amplifies and target optical paths, is urgent to be solved. The analysis of the above mentioned components as follows. 2.1 Alignment laser To ensure the irradiation uniformity of the excimer MOPA system, the laser output by front-end oscillator is focused on the polytetrafluoroethylene (PTFE) plate, from which the scattering light used to irradiate aperture as the original object plane, is collected by the lens. Despite of this, the obtained light intensity is weak and can’t serve to collimate the beams. In this case, another automatic alignment laser is needed to be introduced. In theory, the perfect laser should has the same wavelength and beam transmission characteristics with the feed laser which ensure coaxial transmission and same imaging plane of the double lasers respectively. Therefore, the automatic alignment laser choose He-Ge laser whose output laser wavelength is 325nm approach to of the feed laser whose wavelength is 308nm for the sake of avoiding aberration induced by the difference of the two laser wavelengths. 2.2 Alignment references To make the best use of high irradiation uniformity in high power excimer laser system, technology of Echelon Free Induced Spatial Incoherence (EFISI) is adopted. In this case, spatial beam smoothing is usually achieved by high fidelity image-relaying optical structures in high power laser facilities. Accordingly, the beam exported by oscillators with uniform intensity distribution is transferred serially and re-imaged near the amplification lasers or the frequency conversion facility. The image-relaying structures restore the increment of intensity on the image plane of original input distribution noise to be zero, which achieves the aim of improving the image quality. According to angularly multiplexed design principle, beam filling and energy extraction of amplifiers depends on the parameters of input and output coupled lens arrays which should be selected as the references theoretically. However, lens arrays need to be irradiated on the whole aperture and it is hard to accomplish in engineering. Besides, reposition precision of commercial auto translational stage can reach to micron dimension and can satisfy the demands of beam pointing at present. Therefore, inserted references close to lens arrays can

solve the problems, in which the crosshair on input beam is regarded as the near-field reference while the other on output beam is regarded as the far-field reference. In this system, crosshair arrays close to lens arrays are selected as the alignment references, which not only solve the problem of nonexistence of definite references in excimer laser but also avoid the aberration produced by the adjustment of lens arrays. 2.3

Images capturing

Images capturing is an important component in optical automatic alignment. Generally, the double references arrays irradiated by alignment laser are imaged on double Charge Coupled Devices (CCD) respectively. So CCD is twice the number that multi-beams are. Besides, the laser is in the ultraviolet waveband which needs dear CCD for capturing. As a result, fluorescence imaging means, in which fluorescence plates take the place of double CCD on the near-field and far-field imaging planes and then double cameras take the picture of fluorescence plates, is designed to accomplish the automatic alignment of 18 beams. Automatic alignment of each amplifier, correspondingly, just needs two fluorescence plates , two CCD and one computer. 2.4 Algorithm of feedback control Near-field and far-field beams can get the same barycenter as imaged references by two pairs of motored reflectors according to their error signals usually. Actually for the reason of linkage between the two reflectors and corresponding image planes, rational mathematical model is desiderate to be chosen and appropriate algorithm of feedback control needs to be built. Familiar algorithm of feedback control consists of ‘approached step by step’ and ‘accomplished by one calculation’. For the forward algorithm, Horizontal and vertical directions equilibrium adjustment method is used as the feedback control algorithm to accomplish the convergence of automatic alignment. In this way, the two step-motors on the mirror mounts are driven step by step, which can avoid the phenomenon of beam exceeding the scope of CCD in the course of unsymmetrical adjustment and unexpected pause in the course of adjustment circulation. The alignment error precision is defined according to actual optical path and error demand. For the backward algorithm, the adjustment quantity of the four step motors in servo reflectors system is calculated based on the barycenters of near-field and far-field beams when optical path of automatic alignment is determined. Detailed processes state as follow. The displacement coefficient between reflectors adjustment and the beams at near-field and far-field can be measured in virtue of the direct proportion of above-mentioned parameters. Then, adjustment quantity of step motors can be calculated according to the real aberration. This algorithm has the higher efficiency and less time-consuming, whereas it has the problems of linear approximation, dependence on beam, and unexpected pause. We bring forward one improved ‘accomplished by one calculation’ algorithm which satisfy the needs of high efficiency and unexpected pause. Firstly, adjustment quantity of every step motor is measured. Then, one half of the quantity is executed by every step motor. Repeat the previous two processed until precision of automatic alignment is approached. 2.5 Servo control mechanism As this control mechanism, logical control pulse is generated by field programmable gate array (FPGA) and current drive signals of every step motor are provided by power drive module. Here, the mechanism can not only avoid the problems of signal loss and multi circuit disposal resulted by long distance transmission of the drive signals, but also avoid the uncertain factors induced by relays. As shown in Figure 1, upper computer is used to carry out rotation direction and numbers of every step motor, FPGA provides the drive pulse of every step motor, Single Chip Micyoco (SCM) provides the

interface between upper computer and FPGA, and power drive module generates current to rotate step motors.

3. EXPERIMENTAL RESULTS Automatic alignment experiment of is carried out in pre-amplifier II with three beams and double passes in excimer laser MOPA system, whose optical design is shown in Figure 2. Main three laser beams output from former pre-amplifier are guided to pre-amplifier II by the upper three reflectors of M01and irradiate on lens arrays L01 and L02 including the inserted crosshair reference arrays Cross1 and Cross2, whereafter the beams are transformed onto M01 again in the second pass by motor-driven reflector arrays M02-01 and M02-02. Then, beams transmit into automatic alignment system by the lower three beam splitters of M01, which is combined of the assistant laser, Cross1 and Cross2, the lens arrays L01 and L02, and the measurement segment built by the laser leaked by M01 (including splitting reflector array M03, lens arrays L03 and L04, fluorescence plates FL1 and FL2 and double imaging devices). Cross1 is imaged on FL1 and Camera1 by L03 while Cross2 is imaged on FL2 and Camera2 by L01 and L04. As we know, automatic alignment references are determined by the double inserted crosshair arrays, so the transmission paths of beams in amplifier are similarly determined by lines between the arrays. Spatial layout of crosshair arrays, whose material object is shown as Figure 3. Stainless steel processed by wire cutting is selected as crosshair reference owing to the rigidity and smoothness of the edges demands for materials, which ensure precise crosshair positions and avoid images blur resulted from burrs. The width of crosshair arrays correlative to imaging amplification and resolution of the cameras is designed to be 2 millimeter. 3.1 Positioning and capturing of references The alignment of crosshair arrays in optical path requires a high degree of stability. Crosshair arrays is inserted and exited by step-motor platform. Serving the alignment laser as guide beam, Cross1 and Cross2 is inserted in the input and output pass, hereafter is coincided with the center of feed laser. Adjust the positions of FL1 and FL2, image Cross1 on FL1 by L03, image Cross2 on FL2 by L01 and L04. Imaging cameras take the pictures shown as Figure4 of fluorescence plates respectively. Calculate the centers of crosshair arrays as the references. In addition, repeat inserting and exiting crosshair arrays five times for the validation of repetition positioning precision. Results show that aberration of crosshair arrays centers is no more than 1 pixel. 3.2 Automatic alignment of aberration beams Exit the two crosshair arrays, preset beam deviation, acquire laser beams, and calculate the barycenter coordinates. Then, drive the servo reflector arrays M02-1 and M02-2 by closed loop feedback system in accordance with improved ‘accomplished by one calculation’ algorithm until the coincidence between the barycenters of beam and references is accomplished. Measured results are shown as Table1. Table1 shows that the near-field and far-field coordinates deviate from the crosshair references for several pixels before the alignment, while the maximum deviation is less than 1 pixel after the alignment. At the same time, the whole process of automatic alignment just consumes 40 seconds, which can satisfy the demand of presetting accuracy.

3.3 Accuracy analysis Two crosshairs are used as locating datum for automatic alignment, where r denotes beam radius at the reference, n denotes the pixels of sampling beam on CCD cameras in horizontal or vertical, direction, d denotes the object space resolution of the imaging system and L represents the distance between the two alignment reference planes. In this way, we can show that d 



l pix 2r S  2r / ( )  ,  n l pix

d l pix . Here, S is the length of acquired beam on the CCD cameras in horizontal or vertical  L aL

direction, l pix is the cell pixel value of CCD,  is imaging system magnification of CCD near-field and far-field and  is error angle of optical layout. In view of l pix being a constant and  being determined by imaging system, the distance between the two crosshairs affects the accuracy of automatic alignment. The greater the distance is, the higher the accuracy is. In current optical layout of alignment, Cross1 is imaged on FL1 by L03, in which the distance between Cross1 and L03 is 1500 millimeters, the focus of L03 is 375 millimeters and the distance between L03 and FL1 is 500 millimeters. Therefore, scale factor of the imaging system is calculated to be 1/3. Cross2 is imaged on FL2 by L01 and L04, in which the distance between Cross2 and L01 is 5940 millimeters, the focuses of L01 and L04 are 2970 millimeters and 573 millimeters, the distance between L04 and M03 is 500 millimeters, and the distance between L04 and FL2 is 500 millimeters. Therefore, scale factor of the imaging system is calculated to be 1/7.88. Besides, scale factors of the cameras imaging system are 1/20. The capturing resolution of CCD is 1 pixel so that the object resolutions of Cross1 and Cross2 are 60 pixels and 157.6 pixel respectively got by mutual imaging relations. We can obtain that the total accuracy of the two crosshair arrays is 217.6 pixels by the above analysis. CCD produced by Minton Corporation is chosen whose photosensitive surface size is 4.8 mm×3.6 mm and the pixel value is 6.25 microns. The actual distance between the two crosshairs is 16m, so the alignment

accuracy

is

5940

millimeters,

so

the

alignment

accuracy

is

217.6  6.25μm / 5.94m  228.96μrad . Meanwhile, the distance between forward window of

pre-amplifier II and Cross1 is 2500 millimeters, so is backward window of pre-amplifier II and Cross2. So the maximum aberration of alignment is 107.6 microns. Presume that maximum diameter of transmission beams from pre-amplifier is 20 millimeters, it can be got that the maximum aberration is just 0.54% of beam diameter, which can satisfy the demand of beam pointing precision.

4. CONCLUSION An automatic alignment facility is designed aiming to the demand of multi beams and multi passes amplification in excimer laser MOPA system. Key technologies confronted with alignment including laser, reference, images capturing, feedback control algorithm and servo mechanism, are in contrast and chosen. Based on above analysis, experiment is carried out in pre-amplifier II with three beams and double passes in excimer laser MOPA system. Results show that the error of the automatic alignment beam is 0.54% of the diameter of the windows, which can satisfy the demand of beam alignment. Meanwhile, the structure of this automatic alignment method is compact and the pertinency is better.

The alignment method has been used in the angular multiplexing high power excimer laser targeting test platform.

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Upper computer

Beam Position detection

Serial interface

SCM

Self-defined

FPGA

Switch

Power

Step

drive

line

motor

24V

electric source

module

Fig.1 Theoretic diagram of multiplexed control with FPGA

imaging system II

imaging system I

Alignment module

Output beam of main path

Input beam of main path

Pre-amplifier II

M02-2

-1

Fig.2 Automatic alignment system of the three beams with double paths in pre-amplifier II

Fig.3 Material object of designed crosshair array of pre-amplifier II

Fig.4 Images of crosshair array references (up: Cross1; down: Cross2)

Tab.1 Test results of departure path using automatic alignment Serial number

1st beam

2nd beam

3rd beam

Crosshair references

Aberration beams

Error after alignment

CCD1

CCD2

CCD1

CCD2

CCD1

CCD2

(116, 274)

(650, 303)

(125, 280)

(659, 312)

(0.67, -0.083)

(0.23, 0.36)

(116, 274)

(650, 303)

(106, 266)

(640, 300)

(-0.16, -0.94)

(0.67, -0.53)

(389, 279)

(381, 296)

(370, 276)

(366, 300)

(0.06, -0.16)

(-0.05, 0.19)

(389, 279)

(381, 296)

(395, 276)

(390, 286)

(-0.23, 0.39)

(-0.18, -0.53)

(632, 281)

(121, 307)

(630, 289)

(126, 310)

(-0.50, -0.20)

(-0.12, -0.16)

(632, 281)

(121, 307)

(621, 277)

(126, 301)

(0.15, 0.092)

(-0.077, -0.09)