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A novel on-line beam diagnostic method for continuous-wave high energy laser (HEL) is presented. The system based on this method is mainly consisted of a scanning circular reflector and a photodetector array disposed spatially. Laser beam passes through the system except a little part of whole beam is sampled and reflected into the detector array by the circular reflector. Through the arithmetic of spatial mapping and image restoration with the output signal of detector array, the spatial-temporal distribution parameters of the laser beam are obtained. The HEL beam of several hundred millimeters in diameter can be on-line measured with spatial resolution of 2 mm and temporal resolution of 30~50ms.
©2007 Optical Society of America
1. Introduction
Spatial-temporal intensity distribution is the basic characteristics of laser beams. Accurate measurement of this parameter is very important for diagnosing laser beam quality, evaluating the state of laser system and studying the atmospheric transmission effects. But for high energy laser (HEL), it is difficult to fulfill this measurement due to the high energy output and large beam aperture.
In recent years, several beam diagnostic systems [1–8] have been developed specially to meet the need above. Burn-ins on plexiglass, by vaporizing the plexiglass material, can provide qualitative estimates of the beam profile pattern only and the Plexiglas vapors released in the process are toxic and distort the image of the laser beam. CCD camera [1], always used with a scattering screen together [2,3], has both high spatial and temporal resolution, but it can only gives a relative intensity distribution. Matrix detector arrays, including thermal-electrical array and photoelectric array, are frequently used for measure intensity distribution of HEL beams, and the resolution depends on the characteristic and amount of the detectors. These methods above are used in different situations but they shared the same disadvantage of blocking the whole laser beam and can not be used for beam monitoring, thus severely limited their applications.
Beam splitter [4–6] is a good choice for on-line measurement of laser beams in normal optical experiments. The sampled low-power beam can be measured using CCD camera [4], matrix [5] or linear [6] detector arrays, but when consider large aperture and field environments, the method loses its convenience and not to mention the phase distortion owing to the properties of the beam splitter.
Hollow needle technique [7, 8], which scan the laser beam via a pinhole in a rotating hollow needle, is widely used in beam monitoring of laser material processing. But this method suffers the same limit of only several tens millimeters in beam diameter and can not be easily scaled up to hundreds of millimeters.
In this paper, we present a novel on-line HEL beam diagnostic method whose measure aperture can reach to several hundred millimeters. Based on our method, the absolute intensity distribution of the HEL beam can be on-line measured with spatial resolution of 2 mm and temporal resolution of 30–50ms.
2. Configuration and principle of the system
Figure 1 shows the schematic diagram of the sampler. A circular 45° slant reflector and a rotational cylinder are fixed together and revolve at high speed. When HEL beam passes through the cylinder along the cylinder axis, only a little part of whole beam is blocked and reflected at definite directions toward respective detectors by the reflector. Totally 256 detectors and attenuators are fixed along the circumference of an aluminous pan evenly. The circular reflector is intentionally placed to make more than half of the detectors in the array could obtain the reflected light at the same time, as shown in Fig. 1(b) and Fig. 1(c).
By processing with the signal from the detector array, we can get the irradiance distribution parameters of laser along the circular reflector. After the beam reflector finish a round trip and get back to the original position, we can restore the spatial power distribution of laser beam through a special arithmetic. With the process repeated at high speed of 1200~1800 round per minute, the spatial-temporal power distribution of the HEL beam can be obtained. Other parameters such as total energy, beam cross-section, beam drifts in X-Y plane, variation of beam diameter with time, and etc. can be calculated as well.
The largest beam aperture which can be measured by the system is defined by the diameter of the rotational cylinder. The maximal measurable power density mainly depends on the coefficient of the attenuators and saturation threshold of the detectors. The diameter of the aluminous pan and the number of the detectors commonly define the spatial resolution of the system. The temporal resolution can be adjusted by using different motor speed.
3. Arithmetic of spatial mapping and image restoration
As mentioned above, the sampled light is reflected at definite directions toward respective detectors by the reflector. It is necessary to build a spatial mapping arithmetic for restoring the laser beam profiles.
3.1 Spatial mapping
First we calculate the mapping spatial position of an arbitrary detector when circular reflectoris at the -x axis position, as showed in Fig. 2. Totally η detectors are fixed on the aluminous pan evenly, and detector A can be defined as numbered i anti-clockwise from +x axis.
The angle of OA with +x axis is
According to the principle of the light reflection, the light reflected toward detector A is from C, that means C is the mapping spatial point of detector A for the given condition above. We suppose |BC|=|BO=r| is the radius of the circular reflector, and |OA|=R is the radius of the aluminous pan, ∠ABO=α. Then we can get
Because A is the arbitrary detector of the array, we get
Therefore the polar coordinate of the C(ρ,β) is
R and r are constants, so the coordinate of C is only correlative with θ. Thus we can get the mapping polar coordinates of all the detectors from Eqs. (1), (4) and (5) by changing θ. When the reflector is rotated to θ ′ from the position of -x axis, that also can be regard as detector array is rotated θ ′ relatively, θ should be changed to θ+θ ′. Then we get a matrix of power density of mapping points of all detectors after the circular reflector finished a round trip.
Repeating process, the spatial-temporal power distribution can be obtained as
It should be noted that the spatial mapping arithmetic above is used under the assumption of laser power density value is equal in the spatial field which can be resolved by the system. According to the principle of the system, when the reflector rotates continuously, the mapping point of the detector will blur to a small region due to the time response characteristic of the photodetector. Because the detector’s integration time is less than 1µs, and the dimension of blurring region is calculated to be several 10-2 mm which can be neglected compared to the system’s spatial resolution of 2 mm, the arithmetic above can be used in not only static condition but also when the reflect rotate continuously.
The spatial mapping arithmetic is simulated under the condition of 128 detectors and 300mm rotational cylinder diameter. As the result shown in Fig. 3, the distributions of mapping points produce a series of concentric circles and sampled the whole aperture well.
3.2 Image restoration
In order to get quantitative visual images of the laser beam, an arithmetic of square grid matrix segment is adopted, by changing the polar coordinates matrix of mapping points into orthogonal coordinates matrix according to the spatial resolution of the measure system.
Assuming that k is the dimension of the pixel, so there are (d/k)·(d/k) pixels distributed evenly in the measurement field of d·d. The process of image restoration and display is shown as Fig. 4.
More than one point will map to a single pixel of F ″ (x ′,y ′) in the course of the conversion, which means more than one power density value is “given” to a pixel. So a counter is built to every pixel of F ″ (x ′,y ′), and the power density values are averaged in the same pixel. Thus we get the power density value of every pixel, and the beam profiles of the measured laser beam as well.
4. Experimental applications
An experimental setup to validate the method of image restoration is shown in Fig. 5(a). An aluminum board with a hole of 60mm in diameter is placed in front of the measure system to shape the incoming HEL beam and leak part of the beam on the upper and left. As shown in Fig. 5(b), the measure result gives a good agreement with experimental setup. A series of high energy laser beam spatial distributions measured in different time are also shown in Fig. 6.
5. Conclusion
We present a novel on-line beam diagnostic method for continuous-wave HEL based on scanning circular reflector. The arithmetic of spatial mapping and image restoration is introduced and validated by the experiment. This method has been succeeded in measurement of the HEL beam, and is valuable to analyze and evaluate the state of the laser system.
References and links
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