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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 11 — May. 23, 2011
  • pp: 10625–10631
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Image processing method for laser damage probability measurement by single-shot of laser pulse

Jianping Hu, Zhou Xin, Li Shugang, Liu Zhicao, Pan Feng, and Chen Songlin  »View Author Affiliations


Optics Express, Vol. 19, Issue 11, pp. 10625-10631 (2011)
http://dx.doi.org/10.1364/OE.19.010625


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Abstract

An experiment for high efficiency laser-induced damage probability measurement has been implemented using a periodic binary phase grating. With such a grating, a laser beam is transformed into an ensemble array of Gaussian-like spots, which is known as the Fresnel image of the grating. A scientific CCD camera is used to image the laser spot array as well as the damage of the coating sample. The image data is then processed to obtain the peak fluence distribution of the laser spot array based on the calibrated CCD grayscale. By comparing the image of the damaged coating sample with that of the laser spot array, the damage probability of the coating sample can be precisely determined by the use of single-shot of laser pulse.

© 2011 OSA

1. Introduction

The optical components can be damaged catastrophically in the circumstance of high laser fluence level that can be achieved in a high power laser system. The damage threshold of the optical components determines the maximum output level of the laser system, and limits the output power to further improve [1

1. C. A. Haynam, P. J. Wegner, J. M. Auerbach, M. W. Bowers, S. N. Dixit, G. V. Erbert, G. M. Heestand, M. A. Henesian, M. R. Hermann, K. S. Jancaitis, K. R. Manes, C. D. Marshall, N. C. Mehta, J. Menapace, E. Moses, J. R. Murray, M. C. Nostrand, C. D. Orth, R. Patterson, R. A. Sacks, M. J. Shaw, M. Spaeth, S. B. Sutton, W. H. Williams, C. C. Widmayer, R. K. White, S. T. Yang, and B. M. Van Wonterghem, “National Ignition Facility laser performance status,” Appl. Opt. 46(16), 3276–3303 (2007). [CrossRef] [PubMed]

,2

2. C. J. Stolz, “Status of NIF mirror technologies for completion of the NIF facility,” Proc. SPIE 7101, 710115 (2008). [CrossRef]

].

Laser-induced damage test is a method to monitor the damage threshold of optical components for quality assurance [3

3. L. Sheehan, S. Schwartz, C. Battersby, R. Dickson, R. Jennings, J. Kimmons, M. Kozlowski, S. Maricle, R. Mouser, M. Runkel, and C. Weinzapfel, “Automated damage test facilities for materials development and production optics quality assurance at Lawrence Livermore laboratory,” Proc. SPIE 3578, 302–313 (1999). [CrossRef]

,4

4. R. Chow, M. Runkel, and J. R. Taylor, “Laser damage testing of small optics for the national ignition facility,” Appl. Opt. 44(17), 3527–3531 (2005). [CrossRef] [PubMed]

]. At present, the international standard ISO 11254 defines a standard method for 1-on-1 measurement of LIDT (laser-induced damage threshold) of optical components. In such a conventional method, damage threshold measurement is time-consuming and expensive because it is necessary to use not only a large number of shots with different energy densities to determine the LIDT, but also a high quality laser with high output stability less than 5% to ensure the accuracy of LIDT measurement [5

5. Draft International Standard ISO/DIS 11254: Optics and Optical Instruments-Laser and Laser related equipment: Test methods for laser induced damage threshold of optical surface.

,6

6. J. W. Arenberg and W. Riede, “National Round-Robin Test on laser induced damage at 1.064μm: revised data reduction and correlation analysis,” Proc. SPIE 3578, 645–656 (1999). [CrossRef]

]. Instead of the multiple-shots measurement, some researchers have proposed and developed the single-shot measurement. Wiggins et al. reported the use of binary mask (hole gratings) in the small-spot laser damage testing, and discussed the effects of hole size, hole spacing, hole shape, and elliptical hole orientation [7

7. T. A. Wiggins, T. T. Saito, and R. M. Herman, “Hole gratings for laser damage testing,” Appl. Opt. 22(21), 3388–3396 (1983). [CrossRef] [PubMed]

]. Loewenthal et al. demonstrated a single-shot method for the measurement of LIDT on dielectric coatings with a binary mask, but it has the drawback of low transmission (~20%) [8

8. F. Loewenthal, R. Tommasini, and J. E. Balmer, “Single-shot measurement of laser-induced damage thresholds of thin film coatings,” Opt. Commun. 152(1-3), 168–174 (1998). [CrossRef]

]. Christoph et al. improved the transmission up to 99.8% by replacement of the amplitude grating (binary mask) with binary phase grating [9

9. C. Siegel, F. Loewenthal, J. E. Balmer, and H. P. Weber, “Talbot array illuminator for single-shot measurements of laser-induced-damage thresholds of thin-film coatings,” Appl. Opt. 39(10), 1493–1499 (2000). [CrossRef]

].

The critical part of LIDT measurement is the determination of the absolute peak fluence of each spot in the diffraction pattern, and it is the most important factor for practical application of the single-shot LIDT measurement. Christoph et al. adopted the method proposed by Smith et al. [10

10. W. L. Smith, A. J. Degroot, and M. J. Weber, “Silicon vidicon system for measuring laser intensity profiles,” Appl. Opt. 17(24), 3938–3944 (1978). [CrossRef] [PubMed]

], which used the unmodulated (super-Gaussian-like) intensity distribution of the laser beam to determine the calibration constant C of the CCD camera defined as C = Energy/ΣCounts. After calibration of the camera, the phase plate can be inserted and diffraction pattern be recorded. With knowledge of pulse energy and calibration constant C, the peak fluence of each spot in the pattern can be deduced. With this method, however, the flat top pulse (super-Gaussian-like) is required to calibrate the constant C for accuracy, and the energy of the testing pulse must be the same as the calibrated pulse for keeping the constant C the same.

In this paper, we propose a novel method to measure the absolute peak fluence of each spot and their distribution in the diffraction pattern. Assuming that the response of CCD to the laser energy is within the linear dynamic range, the laser peak fluence is directly proportional to the CCD imaging grayscale. If the CCD imaging grayscale has been calibrated versus the laser energy density, we can use the CCD camera to accurately measure the peak fluence of each spot and the peak fluence distribution in the diffraction pattern. We also implemented an experiment of the single-shot measurement for LIDT of optical coatings using a binary phase grating to demonstrate our new method. Using the CCD to image the diffraction pattern and applying the software to process the image data, the statistical determination of the damage threshold can be achieved by inspection of the damage pattern of the testing sample with the energy intensity distribution of laser beam. Compared with the standard ISO11254 LIDT measurement, this single-shot measurement can greatly reduce the time for damage threshold measurement, so it has a great practical value in LIDT test.

2. Experimental setup and results

Figure 1
Fig. 1 Experimental setup of the laser damage test.
shows our experimental setup which mainly consists of a single longitude mode Q switch Nd: YAG laser made by Laser Physics company in Russia, a beam splitter, a binary phase grating, focus lens, and a sample translating stage as well as an energy meter and a CCD of laserCam made by Coherent Corporation. The beam output from the Nd:YAG laser is TEM00 mode with wavelength of 1064nm. The focal length of the lens is 466mm, and L represents the distance between the lens and the test sample or CCD camera.

The binary phase grating was designed for laser wavelength of 1064nm. As shown in Fig. 2
Fig. 2 Structure of the binary phase grating, where ω = 500μm, d = 2ω.
, the grating consists of about 120 holes arranged in a hexagonal grid on the fused silicon substrate with a diameter of 30 mm, each hole has a diameter of 500μm with a phase step of π/2. With such a binary phase grating, a diffraction pattern can be generated in the near field of the lens.

The sample mounted on the translating stage can be moved precisely in X-Y-Z directions, in order to take the experiment for different areas of the sample and different distance L from the focus lens. The laser energy is measured by an EM500 energy meter with precision of ± 1%, and the energy distribution of the laser beam is measured by a CCD camera and analyzed by a computer image processing program written by us. In our experiment, the maximum energy of a single pulse is about 2J with pulse duration of 10ns and a diameter of 10mm.

The laser beam can be transformed into Fresnel diffraction pattern after it passes through a periodic structure of the binary phase grating. With a periodic binary phase grating, a laser beam is transformed into an ensemble array of Gaussian-like spots, which is known as the Fresnel image of the grating [11

11. V. Arrizón, E. López-Olazagasti, and A. Serrano-Heredia, “Talbot array illuminators with optimum compression ratio,” Opt. Lett. 21(4), 233–235 (1996). [CrossRef] [PubMed]

,12

12. V. Arrizón and J. Ojeda-Castañeda, “Multilevel phase gratings for array illuminators,” Appl. Opt. 33(25), 5925–5931 (1994). [CrossRef] [PubMed]

]. The intensity distribution of the diffraction pattern is dependent on the distance from the lens, and the phase and amplitude images of the binary grating are periodically alternated. For example, as shown in Fig. 3
Fig. 3 Image patterns of a laser beam transmitted through the binary phase grating at different distance L from the lens. The patterns at (a) L = 35cm, (b) L = 40cm, (c) L = 65cm are imaged by CCD, and at (d) L = 35cm, (e) L = 40cm, (f) L = 65cm are simulated by use of ASAP.
, when L = 35cm, the diffraction pattern looks similar to that of 65cm, but at L = 40cm the diffraction pattern is seen the same as the structure of the grating. Considering the image size 6.8x8.5mm of CCD and the suitable peak fluence in laser spots, we chose the position of L = 35cm to place the test sample for our damage experiment.

3. Image processing method for LIDT

3.1 Peak fluence measurement

Measurement and calculation of the peak fluence distribution of a laser spot array are important for LIDT measurement of the single-shot laser. Assuming that the laser beam is imaged within the CCD dynamic range of linear response, there is a linear relationship between the peak fluence of the laser beam and the grayscale of the CCD image. Let the peak fluence of the beam and the characteristics grayscales of image have the following relationship:
Dpeak=c×Gpeak+d
(1)
Where Dpeak is the peak fluence with unit J/cm2, the dimensionless Gpeak is the characteristics grayscale of CCD image of the beam, c is the regression coefficient with unit J/cm2, and d is the intercept of vertical axis with unit J/cm2. First, CCD images a standard Gaussian laser beam for calibrating the linear relation between the laser peak fluence and the characteristics grayscale of CCD image. Six groups of standard Gaussian laser beam data are measured by CCD. Using the least square method, the peak fluence of a laser pulse and its corresponding characteristics grayscale of CCD image are fitted linearly. According to Eq. (1), we can easily obtain c=0.0421,d=-0.0907 from the fitting line shown in Fig. 4
Fig. 4 Curve of the peak fluency vs. CCD characteristics grayscale.
. Namely,

Dpeak=0.0421×Gpeak-0.0907
(2)

For the laser spot array formed by the binary phase grating, it is imaged by the calibrated CCD, and the CCD image data is processed to obtain the characteristics grayscale of each spot in the diffraction pattern. According to Eq. (2), we can calculate the peak fluence of each spot, which shows the information of the peak fluence distribution of the laser spot array.

With the in-house CCD image processing software, a spot array image can be opened and segmented, and the parameters of each spot in the diffraction pattern can be calculated. For example, the CCD image of diffraction pattern with about 120 spots produced by 735mj pulse is processed, and the results are shown in Fig. 5
Fig. 5 Peak fluence distribution of a laser spot array.
. We can see that the peak fluence distribution of the laser spot array is between 2 and 16J/cm2.

3.2 Laser damage probability

The experiment samples are HR coating of laser wavelength 1064nm which are deposited on a polished substrate of K9 glass with size 40 × 40mm. Experimental setup has been described in Section 2, and the sample is located at 35cm from the lens. First, a CCD is used to record the image of the intensity distribution of the laser spot array at the position of the sample. The CCD is then taken away and the test sample is placed at the same position as the CCD.

With different laser energy shot at different areas on the test sample, the typical damages induced by the laser spot array are shown in Fig. 6
Fig. 6 The damage induced by laser shot arrays.
. It is clear that the numbers of damage points are dependent on the total energy of single-shot laser. In principle, the damage threshold measurement of single-shot laser is independent of the number of the damage points induced by the laser spot array. As long as the damage occurs and not all laser spots in the array causes the damage, the method may determine the damage threshold of the testing sample by even single shot of the pulse laser.

In our experiment, the test sample is irradiated by different energy of laser pulse on different areas as mentioned above. Using Normaski microscope to take the damage picture, and comparing the damage chart with the laser spot array image recorded by CCD, we can find each corresponding laser spot. The laser damage probability and damage threshold can then be deduced using the image processing program. Figure 7
Fig. 7 Laser damage probabilities of 1-ON-1 and single-shot methods.
shows the results of the damage probabilities that are achieved by different laser energy irradiating on the same test sample. The average damage threshold is about 6.4J/cm 2 within 30% difference for zero damage probability.

For comparison, we also take an experiment to measure the same sample using the conventional 1-on-1 damage threshold measurements. The laser damage probability is also shown in Fig. 7, and the laser damage threshold is about 5.4J/cm2, which is slightly lower than that of single-shot. The fact that each spot size in the array is smaller than the beam size used in the conventional measurement probably resulted in the slightly higher laser damage threshold for single-shot measurement. Otherwise, the image processing would introduce some errors. But overall, single-shot laser damage threshold measurements can give a confident result within the range of certain error.

4. Conclusion

Conventional measurement methods of laser damage threshold is based on a statistical basis, so the measurement process is time consuming and expensive. Adopting a binary phase grating to form a laser spot array, applying the spectrum theory of beam propagation to calculate the energy distribution of the laser spot array, utilizing a calibrated CCD to monitor the peak fluence distribution of the laser spot array and the damage situation of the test sample, and taking image processing on time, we can obtain the laser damage probability and threshold of the test sample. Our experiments show that a certain energy density distribution of the laser spot array generated by a binary phase grating can be used as the detective source in laser damage probability measurement. This approach is also capable of measuring laser damage threshold. The measuring error of single shot would be partly introduced by the CCD image of beam and image processing. However, compared with the laser damage threshold measurement of multi-shots laser, this novel method gives similar results within the error range. In our experiment, the data difference is less than 20% between the conventional and single-shot measurement.

References and links

1.

C. A. Haynam, P. J. Wegner, J. M. Auerbach, M. W. Bowers, S. N. Dixit, G. V. Erbert, G. M. Heestand, M. A. Henesian, M. R. Hermann, K. S. Jancaitis, K. R. Manes, C. D. Marshall, N. C. Mehta, J. Menapace, E. Moses, J. R. Murray, M. C. Nostrand, C. D. Orth, R. Patterson, R. A. Sacks, M. J. Shaw, M. Spaeth, S. B. Sutton, W. H. Williams, C. C. Widmayer, R. K. White, S. T. Yang, and B. M. Van Wonterghem, “National Ignition Facility laser performance status,” Appl. Opt. 46(16), 3276–3303 (2007). [CrossRef] [PubMed]

2.

C. J. Stolz, “Status of NIF mirror technologies for completion of the NIF facility,” Proc. SPIE 7101, 710115 (2008). [CrossRef]

3.

L. Sheehan, S. Schwartz, C. Battersby, R. Dickson, R. Jennings, J. Kimmons, M. Kozlowski, S. Maricle, R. Mouser, M. Runkel, and C. Weinzapfel, “Automated damage test facilities for materials development and production optics quality assurance at Lawrence Livermore laboratory,” Proc. SPIE 3578, 302–313 (1999). [CrossRef]

4.

R. Chow, M. Runkel, and J. R. Taylor, “Laser damage testing of small optics for the national ignition facility,” Appl. Opt. 44(17), 3527–3531 (2005). [CrossRef] [PubMed]

5.

Draft International Standard ISO/DIS 11254: Optics and Optical Instruments-Laser and Laser related equipment: Test methods for laser induced damage threshold of optical surface.

6.

J. W. Arenberg and W. Riede, “National Round-Robin Test on laser induced damage at 1.064μm: revised data reduction and correlation analysis,” Proc. SPIE 3578, 645–656 (1999). [CrossRef]

7.

T. A. Wiggins, T. T. Saito, and R. M. Herman, “Hole gratings for laser damage testing,” Appl. Opt. 22(21), 3388–3396 (1983). [CrossRef] [PubMed]

8.

F. Loewenthal, R. Tommasini, and J. E. Balmer, “Single-shot measurement of laser-induced damage thresholds of thin film coatings,” Opt. Commun. 152(1-3), 168–174 (1998). [CrossRef]

9.

C. Siegel, F. Loewenthal, J. E. Balmer, and H. P. Weber, “Talbot array illuminator for single-shot measurements of laser-induced-damage thresholds of thin-film coatings,” Appl. Opt. 39(10), 1493–1499 (2000). [CrossRef]

10.

W. L. Smith, A. J. Degroot, and M. J. Weber, “Silicon vidicon system for measuring laser intensity profiles,” Appl. Opt. 17(24), 3938–3944 (1978). [CrossRef] [PubMed]

11.

V. Arrizón, E. López-Olazagasti, and A. Serrano-Heredia, “Talbot array illuminators with optimum compression ratio,” Opt. Lett. 21(4), 233–235 (1996). [CrossRef] [PubMed]

12.

V. Arrizón and J. Ojeda-Castañeda, “Multilevel phase gratings for array illuminators,” Appl. Opt. 33(25), 5925–5931 (1994). [CrossRef] [PubMed]

OCIS Codes
(050.1950) Diffraction and gratings : Diffraction gratings
(100.0100) Image processing : Image processing
(140.3330) Lasers and laser optics : Laser damage

ToC Category:
Image Processing

History
Original Manuscript: February 24, 2011
Revised Manuscript: April 23, 2011
Manuscript Accepted: May 4, 2011
Published: May 16, 2011

Citation
Jianping Hu, Zhou Xin, Li Shugang, Liu Zhicao, Pan Feng, and Chen Songlin, "Image processing method for laser damage probability measurement by single-shot of laser pulse," Opt. Express 19, 10625-10631 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-11-10625


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References

  1. C. A. Haynam, P. J. Wegner, J. M. Auerbach, M. W. Bowers, S. N. Dixit, G. V. Erbert, G. M. Heestand, M. A. Henesian, M. R. Hermann, K. S. Jancaitis, K. R. Manes, C. D. Marshall, N. C. Mehta, J. Menapace, E. Moses, J. R. Murray, M. C. Nostrand, C. D. Orth, R. Patterson, R. A. Sacks, M. J. Shaw, M. Spaeth, S. B. Sutton, W. H. Williams, C. C. Widmayer, R. K. White, S. T. Yang, and B. M. Van Wonterghem, “National Ignition Facility laser performance status,” Appl. Opt. 46(16), 3276–3303 (2007). [CrossRef] [PubMed]
  2. C. J. Stolz, “Status of NIF mirror technologies for completion of the NIF facility,” Proc. SPIE 7101, 710115 (2008). [CrossRef]
  3. L. Sheehan, S. Schwartz, C. Battersby, R. Dickson, R. Jennings, J. Kimmons, M. Kozlowski, S. Maricle, R. Mouser, M. Runkel, and C. Weinzapfel, “Automated damage test facilities for materials development and production optics quality assurance at Lawrence Livermore laboratory,” Proc. SPIE 3578, 302–313 (1999). [CrossRef]
  4. R. Chow, M. Runkel, and J. R. Taylor, “Laser damage testing of small optics for the national ignition facility,” Appl. Opt. 44(17), 3527–3531 (2005). [CrossRef] [PubMed]
  5. Draft International Standard ISO/DIS 11254: Optics and Optical Instruments-Laser and Laser related equipment: Test methods for laser induced damage threshold of optical surface.
  6. J. W. Arenberg and W. Riede, “National Round-Robin Test on laser induced damage at 1.064μm: revised data reduction and correlation analysis,” Proc. SPIE 3578, 645–656 (1999). [CrossRef]
  7. T. A. Wiggins, T. T. Saito, and R. M. Herman, “Hole gratings for laser damage testing,” Appl. Opt. 22(21), 3388–3396 (1983). [CrossRef] [PubMed]
  8. F. Loewenthal, R. Tommasini, and J. E. Balmer, “Single-shot measurement of laser-induced damage thresholds of thin film coatings,” Opt. Commun. 152(1-3), 168–174 (1998). [CrossRef]
  9. C. Siegel, F. Loewenthal, J. E. Balmer, and H. P. Weber, “Talbot array illuminator for single-shot measurements of laser-induced-damage thresholds of thin-film coatings,” Appl. Opt. 39(10), 1493–1499 (2000). [CrossRef]
  10. W. L. Smith, A. J. Degroot, and M. J. Weber, “Silicon vidicon system for measuring laser intensity profiles,” Appl. Opt. 17(24), 3938–3944 (1978). [CrossRef] [PubMed]
  11. V. Arrizón, E. López-Olazagasti, and A. Serrano-Heredia, “Talbot array illuminators with optimum compression ratio,” Opt. Lett. 21(4), 233–235 (1996). [CrossRef] [PubMed]
  12. V. Arrizón and J. Ojeda-Castañeda, “Multilevel phase gratings for array illuminators,” Appl. Opt. 33(25), 5925–5931 (1994). [CrossRef] [PubMed]

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