Beam alignment is crucial to high-power laser facilities and is used to adjust the laser beams quickly and accurately to meet stringent requirements of pointing and centering. In this paper, a novel alignment method is presented, which employs data processing of the two-dimensional power spectral density (2D-PSD) for a near-field image and resolves the beam pointing error relative to the spatial filter pinhole directly. Combining this with a near-field fiducial mark, the operation of beam alignment is achieved. It is experimentally demonstrated that this scheme realizes a far-field alignment precision of approximately 3% of the pinhole size. This scheme adopts only one near-field camera to construct the alignment system, which provides a simple, efficient, and low-cost way to align lasers.
© 2017 Optical Society of America
Laser-driven inertial confinement fusion (ICF) has attracted persistent attention since the 1960s [1–4]. It is a complex scientific and technological investigation aiming to produce extreme conditions of fusion, under which the fuel will ignite and burn, liberating more energy than is required to initiate the fusion reactions [5–7]. The primary requirement of laser fusion research is developing high-power laser drivers with a MJ class output energy, and a number of facilities have been proposed or constructed in the world, for example, the operating National Ignition Facility (NIF) [8–10] in the USA, the Laser Megajoule (LMJ) [11–13] in France, the Shenguang-III Facility (SG-III)  in China, the UFL-2M facility (UFL-2M)  in Russia, the High Power Laser Energy Research Facility (HiPER) [16, 17] in the United Kingdom and the Fast Ignition Realization Experiment (FIREX-I)  in Japan. In such facilities, the alignment of each amplifier channel is one of the most important problems for reaching the required energy and uniformity of radiation at the target. For the requirements of shot accuracy and system operational safety, all beams need to be precisely and quickly adjusted before being run by an automatic alignment system.
Currently, the conventional beam alignment systems are based on what can be called the reference approach. The approach is based on video monitoring of the mutual positions of the center of the alignment beam, the centers of optical elements in the near-field, and the centers of pinholes of spatial filters in the far-field. During closed loop alignment, the centering and pointing reference features are identified and recorded. The control system computes commands for the reflector mount motors to eliminate any centering and pointing errors, and the process is iterated until errors are less than a predetermined amount. The diode source and the spatial filter pinhole are used as the near- and far-field references in NIF. The position of the pointing and centering references are successively obtained in the near- and far-field camera, and then the position errors are calculated to align the centering and pointing of the laser beam [19–22]. In LMJ, the position errors between the alignment beam and the center of the cavity mirror is obtained using two images captured by a near-field camera. The positions of the far-field beam spot and pairs of reference laser sources are simultaneously collected by a far-field camera. Then the beam spot is moved to the midpoint of the line joining the barycenter of the two references pairs, which eliminates the pointing error [23–26]. In the SG-III facility, both the beam focal spot compared to a spatial filter pinhole reference and the beam position compared to a fiducial mark crosshair are acquired, and the errors are eliminated in every pass of light [27–29]. The UFL-2M facility is similar to that of the LMJ and adopts a cavity mirror and pairs of reference laser sources as the centering and pointing reference . The reference approaches mentioned above require independent detection units for both the near- and far-field, and acquisition of alignment errors is split up into reference recording and beam identifying, which is complicated and expensive.
In this paper, we develop a novel beam alignment method with single detection units for a near-field in the high-power laser system. By employing data processing of the two-dimensional power spectral density (2D-PSD) of the near-field image, the beam pointing error relative to a spatial pinhole could be resolved immediately, and then combined this with a near-field fiducial mark to achieve closed-loop alignment operation. Different from the traditional reference approach, this scheme acquires the pointing error directly, thus giving rise to the name of error approach. It adopts only one near-field camera to form an alignment system, which provides a simple, efficient, and low-cost method for beam alignment.
The paper is organized as follows: Section 2 states out the principle of the novel alignment method and gives out the analytical derivation and numerical simulations. In Section 3, an experimental setup is established and experimental results are discussed. And the final section is devoted to the conclusions.
2. Proposed scheme and simulations
High-power laser drives for ICF research are typically composed of amplifiers, spatial filters, transport mirrors, frequency conversion crystals and an alignment system. In such facilities, far-field alignment is implemented to ensure that laser pulses propagate unhindered through the spatial filter pinhole as the far-field reference. Figure 1 depicts the optical schematic of an alignment system based on the novel method, which contains two motorized mirrors M0 and M1, a crosshair as the centering reference, and a near-field camera placed in the relay plane. The far-field alignment is achieved based on frequency characteristics, which show on the 2D-PSD image of output in the near-field.
A confocal lens pair L1 and L2 is used in the spatial filter to meet these imaging properties in the image relaying optical system in Fig. 1. The object plane distance from the first lens L1 is approximately , and the image plane distance from the last lens L2 is approximately . The beam complex amplitude in the object plane is . Frequency spectrum imaging on the focal plane occurs after the light passes through lens L1. We can derive the result of according to Fourier optics as:Equation (1) reveals the relative distribution of the amplitude of except the phase factor is the Fourier transform of the beam in the object plane. To reduce computational complexity, the spatial filter system can assume a 4f image system, namely, , and then we can simplify the frequency spectrum as:
Because the back focal plane of lens L1 coincides with the front focal plane of lens L2, the image plane in the back focal plane of lens L2 is the Fourier transforms of the frequency spectrum according to the above derivation, and the complex amplitude of the image plane is given by:
The constant factor can be ignored because a relative distribution is used in the analysis of the PSD. Based on the convolution theorem, complex conjugation and the symmetry theorem of the Fourier transforms, Eq. (6) can be rewritten as:
The amplitude of the frequency spectrum of the output laser pulse is always a larger direct component (DC) than any other frequency, one-dimensional (1D) distribution of which is logarithmically shown in Fig. 2. Since the 2D-PSD is symmetrical about the DC, the following analysis is done only for the right part. Based on Eq. (7), the 2D-PSD of the near-field image is calculated and presented, which appears with two steps in the right part. The space between the outermost step and the DC is , the width of the filter pinhole, and the internal step is . When beam pointing is at the right side of the pinhole with distance , the logarithmic amplitude of the frequency spectrum, the window function, the frequency spectrum after the filter pinhole and the 2D-PSD of the near-field image are presented in Fig. 3. The 2D-PSD contains three steps because of the shift between the DC and the center of the filter pinhole. The space between the outermost step and the DC is also . The distance between the two internal steps and the DC is dependent on the pointing shift, which is and for each individual step.
To intuitively grasp the relationship between the 2D-PSD image and the pointing error, simulations are conducted. The input beam is assumed as a circular beam with a 16th-order super-Gaussian profile including spatial noise, and the filter pinhole is assumed to be the size of 30 diffraction limits. Figure 4 shows the results of the beam pointing at the center of the pinhole, which contains the beam in the object plane, the frequency spectrum after the pinhole and the 2D-PSD of the near-field image. The frequency spectrum and the PSD are logarithmic to enhance the details of the display. When the beam points at the center of the pinhole, the DC of the frequency spectrum coincides with the center of the pinhole. Based on the Eq. (7), the 2D-PSD of the near-field image is also a circle. The results for the beam pointing at the left, upper and upper left sides of the pinhole are shown in Fig. 5. There are two overlapping circles in the direction of the pointing offset in the 2D-PSD, the central distance of which is twice that of the pointing shift.
According to the one-dimensional and two-dimensional simulation results, the beam pointing is judged by the direction in which the circles overlap and the central distance in the 2D-PSD of the near-field image. If two circles coincide, the pointing error is non-existent, meaning that beam is passing through the center of the pinhole. If there is an overlap of the two circles, the pointing error is shown by the direction in which the circles overlap and the central distance.
3. Experimental results and discussion
3.1 Experimental setup and results
Figure 6 depicts an experimental setup to validate the performance of the novel alignment method. The experiments will affect by external and internal factors. The high-frequency drifting driven by external disturbance that frustrates the alignment process is isolated by structure design. There are three major internal factors of this system. One is the sampling rate and pixel size of CCD, that low sampling rate may lose frequency information and big pixel size lowers alignment precision. The second is the step accuracy of step motor which also affects the alignment precision. The third is the image processing method.
A cw 1053 nm beam from a fiber launcher is used for alignment. The circular beam with 72 mm diameter is obtained in the experiment. The mirrors M0 and M1 are fixed at the two-dimensional motorized mirror mount with a ± 1.5° range and 1μrad stepping precision. Beam injection into the main laser is accomplished by matching the 4130 mm-focal-length spatial filter lenses with a spatial filter pinhole of optical size of 30 diffraction limits, the diameter of which is 4.42 mm. A lens group is employed to reduce the beam diameter to fit the near-field CCD with a 12.3 mm width. The insertable mark is a crosshair placed in the relay plane as the near-field alignment reference. The near-field images are captured and processed by a computer to provide the feedback to the motorized mirror mounts to center and to point the beam. In contrast, the far-field CCD is also adopted to align the beam pointing based on the conventional alignment method. The pixel size of the CCDs used in the experiment is 12 μm, and the sampling accuracy is 12 bits.
The first alignment step is to manually adjust the mirror M0 and M1 to make the beam pass through the appropriate pinhole positions, which ensures that beam can be detected in the near-field camera and is not necessary for far-field CCD. Once the beam can be detected, the alignment can work.
The near-field images that acquired by near-field CCD, their 2D-PSD images and the binary processed images are presented in Fig. 7(a)-7(l) as the beam points at the center, lower, left, and upper sides of the filter pinhole. There are two steps for PSD image process: firstly, the background noises are removed in the image; secondly, the values in the image exceeding a properly preset threshold are set to 1 and others are set to 0. Then we can get the images such as shown in Fig. 7(i)-7(d). Binary images can highlight the steps in the 2D-PSD. Additionally, the pointing offset, within acceptable limits, can be considered to point at the center of the pinhole due to the limits of accuracy of the stepping motors. It can be seen that the 2D-PSD of the near-field image demonstrates the two overlapping circles introduced by the filter pinhole and that the two circles coincide when the beam points to the center of the filter pinhole; otherwise, they overlap in the offset direction.
The conventional reference method is applied in the near-field alignment process. The crosshair appeared in the near-field image is used to align the centering of the beam. It does not affect the pointing analysis of the 2D-PSD because this alignment approach is based on frequency characteristics. The center position of the crosshair and beam is identified and recorded. Then the control system computes the commands for mirror M0 and M1 mount motors to eliminate any centering errors, and the near-field alignment is finished until errors are less than the predetermined amount.
In the experiments, when the deflection of the tilt and roll direction of mirror M1 is and , respectively, the beam pointing moves in the horizontal direction and in the vertical direction. Figure 8 shows the linear relationship of the central distance of the two overlapping circles with the pointing offset in both the horizontal and the vertical directions, which coincides well with the simulation results. Therefore, the pointing beam is controlled by changing the tilt and roll direction of the mirror.
The alignment algorithm is designed and implemented to align the beam based on the linear relationship of the central distance with the pointing offset. Table 1 shows the experimental results of 8 experimental sequences of the central positions of two overlapping circles in the 2D-PSD and the pointing error before and after alignment. The beam alignment can be accomplished with an accuracy of approximately 3% of the pinhole size in the far-field within one minute for the single beam-line.
The experimental results verify the correctness of the principle of the novel alignment method. When the beam pointing deviates from the center of the pinhole, there are two overlapping circles in the deviation direction in the 2D-PSD, the central of which is linear with respect to the pointing deviation.
The far-field alignment is also achieved by conventional reference method for comparison. A concave lens is used to light up the spatial filter pinhole and the alignment reference is the central position of pinhole image. The constraint condition of the alignment close loop is 5 pixels below which the alignment may not convergence. The alignment precision of traditional method in our experimental setup is approximately 2%. The alignment of single beam-line is accomplished within one minute.
Compared to the conventional reference method, the precision and efficiency of this novel alignment method is equivalent. But the novel alignment method has wider range of correcting the pointing error. Conventional reference method goes invalid under the situation of Fig. 7d because of nothing acquired by the far-field CCD. By contrast, even if a tiny beam in the near-field is acquired, the alignment still works on the basis of the holographic characteristics of the near-field. Besides, the novel alignment method saves expense by only requiring one near-field camera rather than two sets of imaging detection systems to construct an alignment system.
Since the 2D-PSD approach is based on the spatial frequency spectrum of the laser near-field, its validity is not affected by laser spatial profile. Two overlapping circles appear in the 2D-PSD due to the removal of the high frequency component by the pinhole. So no matter what the beam profile is, the alignment approach of PSD is valid under the effect of the pinhole.
This paper introduces a novel beam alignment method based on beam frequency characteristics, which is different from the traditional reference approach of successively providing the position of the near- and far-field reference and the alignment beam and then calculating the alignment error. This method employs data processing of the 2D-PSD of the near-field image and resolving the beam pointing error relative to the spatial pinhole directly. The beam pointing error is demonstrated by the two overlapping circles in the 2D-PSD, which are induced by the spatial pinhole edge. The two circles coincide when the pointing error is eliminated; otherwise they overlap in the direction of the pointing offset. The experimental results show that this scheme realizes a far-field alignment precision of less than 3% of the pinhole size in one minute, which is comparable to the traditional far-field reference approach. This scheme adopts only one near-field camera to form an alignment system, which provides a simple, efficient, and low-cost method of alignment for laser facilities.
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