Results and Discussion

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Fig. 6-2 The block diagram of a control unit

The block diagram of the intensity control unit is shown in Fig. 6-2. Firstly, Controlled the DLP to form uniform distribution of the light intensity and projected the light on the sample. Since there were shiny and dull regions on the surface of this sample, the image captured by the CCD ended up with dark in some areas and possibly with saturated in others. The calibration module received the raw image data from the CCD and an image processing algorithm indicated the boundary of the regions with different values of the surface reflectivity within the field of view and then resulted in the calibration factors for each region. The estimated factors were fed back to the intensity configuration module for the adjustment of the intensity of the fringe images and for the guarantee of the intensity of all regions being within the dynamic region of the CCD camera. For the industrial manufacturing process, the similar inspection condition would be assured that the boundary regions and calibration factors can be loaded from the database. For the phase-shifting algorithm, the modulated fringe images with the revision of the average intensity and the amplitude for different region from calibration factors were sent to DLP projector and projected onto the sample. Therefore several sets of raw images with the surface brightness levels of all pixels could be produced within the dynamic range of CCD camera. The raw images were reconstructed as high dynamic range images according to the calibration factors and were sent to phase-retrieving algorithm. Because of the images with larger signal-to-noise ratio from the high dynamic range, higher quality of 3-D data could be obtained.

To demonstrate the method, measured a slide mounted on a base plane with high reflectivity. Two grooves were formed with low reflectivity on the top of the slide by sand blasting. This study compared the measurement results between the traditional

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fringe-projection moiré and the presented fringe-projection moiré system. The system included a DLP module, a uniform lighting module and a CCD camera. Firstly, aligned the system carefully and measured the relation between the pixel of the DLP and the pixel of CCD camera. Thereafter we could create a mapping table for looking up the intensity of the image which was captured by CCD camera according to the corresponding pixels on the DLP chip. The traditional fringe-projection moiré system and the DLP module provided uniform sine fringe over the whole field of the measurement. For solving the profile of the slide, the DLP module projected three sine-fringe images with a phase shift of 2π/3 to the slide; and then the three-step phase-shifting algorithm was used to solve the profile, as shown in Fig. 6-3. The cross-sectional plot of the marked region of the slide is shown in the left-hand side of Fig. 6-3, and the horizontal axis stands for the pixels of CCD camera and the vertical axis is the height of the slide. The left-bottom image is one of the fringe image captured by CCD camera, indicating that the low reflectivity on the top of the slide made the image lose its contrast, owing to the contrast of the fringe was insufficient to perform the phase-retrieving algorithm. There were many spark noises in the darkest region.

Fig. 6-3 The captured image and retrieved profile of the traditional fringe

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In order to overcome the insufficient contrast, the novel projection moiré system invoked the calibration module calculating the light intensity distribution over the field of view by modifying the edge detection algorithm and by defining the calibration factors for each region. Referring to the mapping table and using mixed-pixels algorithm, the measurement used the DLP module to adjust the average intensity and the average amplitude of the intensity modulation based on the factors and the fringes projected to the slide. The images captured by the CCD camera were then fed in the calibration module for the reconstruction of the high dynamic range images, as shown at the bottom-left of Fig. 6-4. After the process of the phase-retrieving algorithm, it obtained the 3D profile of the slide. Figure 6-4 shows the cross-sectional view of the marked region of the slide. Compared with Fig. 6-3, the contrast is adequate for performing the phase-retrieving algorithm leading to the high dynamic range image obtained from the intensity control unit. Hence the spark noise disappeared. This result demonstrated that the proposed method can successfully measure the objects with large dynamic surface reflectivity.

Fig. 6-4 The captured image and retrieved profile of the regional adjusting fringe projection

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6.4 Conclusion

This chapter present 3D profile systems which are able to acquire a high dynamic range image in one-shot and preserve spatially resolution. An algorithm is proposed for calculating the calibration factors according to the different reflectivity for each region, and then the DLP module is designed for adjusting the illumination.

A CCD camera is used for the capture of the image, and then an algorithm is performed for the reconstruction of the high dynamic range image by a compensation operation according to the calibration factors. This technique can improve the image contrast without reducing the spatial resolution and overcome the wide range of the variation of the surface reflectivity. Because a high dynamic range image is obtained by one-shot, it’s economic without induced errors of multiple exposures. The enhanced fringe contrast by the high dynamic-range image provides the robustness of the phase-retrieving algorithms so that an accurate surface topography of a measured object can be obtained. The proposed method could be applicable to any optical measurement techniques that the reflectivity of the surface of the measured objects varies abruptly.

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References

[1] Frank Chen, Gordon M. Brown, and Mumin Song, “Overview of three-dimensional shape measurement using optical methods,” Opt. Eng., vol. 39, 10-22, (2000).

[2] M. Sjodahl and P. Synnergren, “Measurement of shape by using projected random patterns and temporal digital speckle photography,” Appl. Opt., vol. 38, 1990-1997 (1999).

[3] E. Muller, “Fast three dimensional form measurement system,” Opt. Eng., vol.

34, 2754-2756 (1995).

[4] G. Sansoni, S. Corini, S. Lazzari, R. Rodella, and F. Docchio, “Three dimensional imaging based on gray-code light projection: characterization of the measuring algorithm and development of a measuring system for industrial application,” Appl. Opt., vol. 36, 4463-4472 (1997).

[5] Q. Hu, K. G. Harding, X. Du, and D. Hamilton, “Shiny parts measurement using color separation,” Proc. SPIE, vol. 6000, 6000D1-8 (2005).

[6] R. Kokku and G. Brooksby, “Improving 3D surface measurement accuracy on metallic surfaces,” Proc. SPIE, vol. 5856, 618-624 (2005).

[7] Song Zhang and Shing-Tung Yau, “High dynamic range scanning technique,”

Opt. Eng., vol. 48, 033604 (2009).

[8] Shree K. Nayar and Tomoo Mitsunaga, “High dynamic range imaging: spatially varying pixel exposures,” in IEEE Conf. Computer Vision and Pattern

Recognition, vol. 1, 472-479 (2000).

[9] S. Zhang, X. Li, and S. T. Yau, “Multilevel quality-guided phase unwrapping algorithm for real-time three-dimensional shape reconstruction,” Appl. Opt., vol.

46, 50-57 (2007).

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Chapter 7

Conclusions

Optical measurement techniques have been receiving an increasing attention during recent decades. The most important reason is that they work non-intrusively and therefore do not influence the investigated process. The continuing developments in laser, detector, optical fiber and computer technology will further augment the high applicability and versatility of optical measuring techniques. Therefore, it can be expected that optical techniques will continue to gain in importance in many fields of application. However, the procedures of optical measurement techniques include emitted a light signal, this signal modulated by device under test and analyzed the difference of those signal. The purpose of this dissertation is to discuss what is important in optical measurement technology is to adjust the light source suited with this measurement.

a. Thermo-Optic Tunable Tapered-Fiber Filter:

Broadband light sources with high spectral power density are important for high resolution optical coherence tomography (OCT) in cellular or tissue bio-imaging. The broadband light source with a smooth Gaussian power spectrum is advantageous to achieve low speckle noise, generating from the mutually coherent scattering photons from biological tissues. Echo free OCT imaging can be obtained since a non-Gaussian-spectrum light source will significantly distort the OCT axial point spread function. This dissertation has proposed a new method of achieving widely

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tunable all-fiber broadband Gaussian-shaped spectral filters by concatenating thermo-optic tunable short-pass and long-pass filters. The material and waveguide dispersions are both employed to vary the spectral envelope of short-wavelength-pass filters and long-wavelength-pass filters to respectively fit the right and left wings of the desired Gaussian profile. The achieved spectral contrast can be higher than 40 dB and the filter still keeps Gaussian-shaped during thermo-tuning process. This kind of widely tunable Gaussian filters should be advantageous for optical coherence tomography (OCT) bio-imaging systems using broadband light sources.

This dissertation also presented a new type of thermo-optic tunable short-wavelength-pass fiber filters based on fiber tapering and dispersion engineering has been demonstrated experimentally and analyzed theoretically. Good agreements between the BPM simulation and experimental results are achieved. The effects of material dispersion and waveguide dispersion characteristics have been investigated by examining the spectral response as well as the changing trends of the MFD and the effective mode index. An optimized tapered fiber filter structure that can attain high-cutoff efficiency has been suggested based on the obtained theoretical simulation results. It finds for SMF-28 raw fibers, the uniform tapered waist diameter should be around 35µm, the uniform tapered-waist length should be greater than 30 mm, and the tapered-transition length should be greater than 6 mm. With such an optimized structure, the cutoff slope can be as high as -2.4dB/nm, the rejection efficiency can be as high as 70dB, and the fundamental mode-coupling loss is below 0.3dB. In principle, if different choices of raw fibers can be used, it is possible that the performance can be even more optimized. The analyses presented in the present work should be helpful for developing inline tapered fiber filters based on the dispersion-engineered fundamental-mode cutoff mechanism.

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b. Supercontinuum Generation in a Tapered Fiber:

Supercontinuum generation from 1-µm tapered fiber using the 80 fs Ti:sapphire laser excitation is demonstrated experimentally and studied theoretically.

By properly choosing the exciting wavelength, relatively wide spectra is observed from near UV to near IR only using 1-cm long and 1-µm-diameter optical tapered fiber. Besides, exciting power can be greatly lower down for wide spectra generation extended to near UV by properly connecting two fiber tapers. Split-step FFT method is investigated numerically in order to analyze the spectral response of supercontinuum generation phenomenon corresponding to the wavelength dependent loss occurred at transition region of the tapered fiber. The simulation results agree with the experimental results, and shows that the dispersion and nonlinear effects at transition region of the tapered fiber greatly influences the broaden spectrum shape.

The theoretical result indicates that the zero dispersion cross point located at 2.6 µm so that the pulse width and peak power of the excited pulse is dramatically changed when propagates in transition region, which in term apparently affects the supercontinuum generation spectrum. Hopefully the simulation results in this work provide a helpful viewpoint to analyze the supercontinuum generation in typical tapered fibers.

c. Stable and Tunable Fiber Laser

This dissertation has proposed and investigates experimentally a tunable and stable fiber laser with single-longitudinal-mode output based on double-ring architecture. Double-ring structure provides a fine mode restriction and guarantees a single-longitudinal-mode operation. The output power of larger than −5 dBm and the side-mode suppression ratio of larger than 64.6 dB over the operating range from 1530 to 1560 nm can be obtained. And the maximum output power and side-mode

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suppression ratio of the laser are 4.3 dBm and 70.2 dB at 1536 nm. In addition, the power fluctuation of less than 1 dB and the central wavelength variation of less than 0.04 nm also are observed for lasing wavelength in a short-term observing time.

d. Projection Moiré Profilometry with High-Dynamic Range Image:

This chapter present 3D profile systems which are able to acquire a high dynamic range image in one-shot and preserve spatially resolution. An algorithm is proposed for calculating the calibration factors according to the different reflectivity for each region, and then the DLP module is designed for adjusting the illumination.

A CCD camera is used for the capture of the image, and then an algorithm is performed for the reconstruction of the high dynamic range image by a compensation operation according to the calibration factors. This technique can improve the image contrast without reducing the spatial resolution and overcome the wide range of the variation of the surface reflectivity. Because a high dynamic range image is obtained by one-shot, it’s economic without induced errors of multiple exposures. The enhanced fringe contrast by the high dynamic-range image provides the robustness of the phase-retrieving algorithms so that an accurate surface topography of a measured object can be obtained. The proposed method could be applicable to any optical measurement techniques that the reflectivity of the surface of the measured objects varies abruptly.

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簡 簡 簡 簡 歷 歷 歷 歷

姓 名： 周 森 益 性 別： 男

籍 貫： 台 北 市

學歷：

1990 ~ 1994 國立中山大學 物理系

1994 ~ 1996 國立中山大學 物理所 碩士班

論文題目：具光譜解析能力的共焦顯微鏡

2002 ~ 2012 國立交通大學 光電工程研究所 博士班

論文題目：應用於檢測技術的光源設計

經歷：

1998 ~ 2000 中山科學研究院 材料暨光電技術發展中心

2000 ~ 2004 康石科技股份有限公司

2006 ~ 迄今 工業技術研究院 量測技術發展中心