Speckle reduction in multimode fiber with a ...

Speckle reduction in multimode fiber with a piezoelectric transducer in radial vibration for fiber laser marking and display applications W. Haa, S. Leea, Y. Jungb, J. Kimb, and K. Oh*a Institute of Physics and Applied Physics, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, South Korea; b Department of Information and Communications, GIST, 1 Oryong-dong, Buk-gu, Gwangju 500-712, South Korea a

ABSTRACT We propose and experimentally demonstrate an effective method to reduce far-field speckle noise in multimode fiber with a short cylindrical piezoelectric transducer (PZT) vibrating in radial direction. In this study, the fiber was coiled as tightly as possible around the mandrel of the PZT and periodic stretching effect was caused by the radial oscillations of the actuator. This technique can be adapted at a high modulation frequency, so the speckle patterns can be timeaveraged. The output of the optical fiber was intensively observed by a CCD camera. By counting all the pixels corresponding to relative intensity graded 256 levels in selected area and by calculating the mean value and standard deviation of the intensity, we can measure the speckle contrast and vibration effect in quantitative measurands. It was clearly observed that the characteristics of the speckle pattern in vibration-ON-state were significantly reduced than that of vibration-OFF-state by comparing the proposed measurands as well as direct CCD images. We expect that the proposed speckle reduction technology would find viable applications in realization of fiber laser, laser marking, optical trapping and projection display systems. Keywords: speckle reduction, multimode fiber, piezoelectric transducer, radial vibration, time-averaged smoothing

1. INTRODUCTION Present display industry is concentrate on the extension of large screen led by flat panel display technologies, LCD, PDP, and FED. And future trend is emphasizing the importance of mobility with rapid progress in mobile consumer electronics. Therefore research in display devices is also converging to small size, high resolution, and low power consumption like OLED. For the hand-held device there are practical limits to size and resolution of the display. The small size of the display cannot produce the discernible information due to the resolution limit1 and yet large size display with the enough resolution requires a suitable distance between the viewers. In order to cope with these demands, light beam scanning display technology is being intensively developed2. The Scanning Photonic System (SPS) converts electrons to photons before scanning the image, so a phosphor screen is eliminated to result in high potential for miniaturization. Thus far SPS technologies have been based on bulk optics for color generation and delivery. Recently a compact optical design using a fiber scanning method has been reported3. In the report fiber scanner using dual bimorph piezoelectric actuators was implemented for small form factor yet the system only rendered monochromatic display. In recent years authors have reported about fiber optic full color synthesizer which is the color synthesis using several ports waveguide optical combiner4 because fiber delivery offers significant benefits in terms of reduced optical access, ease of alignment, separation of the source from the vibrating test rig, and safety (as the beam is enclosed)5. The problem of forming speckle patterns in multimode optical fibers was still, however, not solved. The speckles in optical fiber may have significant impacts in laser marking applications especially when the laser output is being delivered using multimode fibers. Spatial and temporal randomness in laser intensity speckle are major obstacles to all-fiber solutions in projection displays and fiber laser marking applications. In this study, we propose and experimentally demonstrate an effective method to reduce speckle noise in multimode fiber (MMF) especially for the far-field region with a piezoelectric transducer (PZT) vibrating in radial direction. * [email protected]; phone 82 2 2123-5608; fax 82 2 365-7657; http://allwise.yonsei.ac.kr

Fiber Lasers V: Technology, Systems, and Applications, edited by Jes Broeng, Clifford Headley, Proc. of SPIE Vol. 6873, 68731V, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.774889

Proc. of SPIE Vol. 6873 68731V-1 2008 SPIE Digital Library -- Subscriber Archive Copy

2. THEORY AND PREVIOUS WORKS When a coherent light is guided along a MMF, a speckle pattern is formed at the waveguide output6 and typical pattern is shown in Fig. 1. These speckles are caused by interference between guided modes traveling within the fiber that result in degrading the laser beam quality in applications of laser marking and projection display as well as medical surgeries. In fact, the fiber speckle patterns are extremely sensitive to external perturbations6, externally applied fiber stresses and laser diode wavelength changes7. Therefore, previously various attempts have been made to overcome the speckle problem in guided optics. The simplest way to reduce the speckle contrast was to increase the number of high-order modes excited within a fiber by increasing the fiber length, the launched NA, or the number of bends. While this is effective for smoothing speckle in conventional step index fibers, it would not be constructive for some specialty fiber as higher-order modes exhibit large losses within these fibers. Other published techniques rely on creating some variation in the speckle pattern and averaging it over time. This may be done by vibrating the fiber by using a rotating diffractive optical element (DOE)8, 9, or even a varying magnetic field10. While effective for continuous wave illumination, such techniques are not applicable to delivery of pulsed lasers.

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SINGLEMODE FIBER

MULTIMODE FIBER

Fig. 1. Output beam patterns of 635 nm He-Ne laser at the end of (a) conventional SMF and (b) 62.5 µm standard graded index MMF

Other speckle reduction techniques have been published that are not applied to optical fibers but are still of interest as they apply to pulsed laser sources. An echelon11 or dispersing element12 may be used to create a variation in optical path length across the wavefront. Providing the path-length variation is longer than the coherence length of the laser, and then this has the effect of changing a short temporal coherence to a short spatial coherence. This leads to reduced interference across the wavefront and a smoothing of any speckle patterns normally present. A time-domain technique was reported to divide a laser pulse and launch separate components into a series of delay lines13-16. By carefully selecting the length of each delay line, a series of pulse may be produced that are each delayed by more than the coherence length of the laser and therefore do not interact coherently. By using diffusers the beams may be averaged over a short time to produce a smooth intensity pattern. An acousto-optic modulator (AOM) has been reported to reduce the speckle contrast from a MMF17. The AOM was used to split in two and then recombine a beam while imparting a variation in frequency, polarization state, and angle of divergence. When the two beams were recombined and launched into a single fiber, each beam coupled into a different set of modes giving two different overlapping speckle patterns, which were mutually incoherent and so add linearly5. In this paper, we introduce an efficient method to reduce speckle noise in MMF with a short cylindrical PZT. The fiber is coiled around the cylindrical mandrel of the actuator and periodic stretching effect is caused by the radial oscillations of the actuator. As a primary light source, the He-Ne laser was used and launched into a MMF. Approximately 2 m-long graded index MMF with core diameter of 50 µm and graded refractive index profile were tested to investigate the effectiveness and dependence of speckle reduction ratio. We examined the speckle reduction ratio by adjusting different driving frequency of the modulation actuator. The proposed technique can be utilized in high frequency modulation up to several MHz of the actuator so that the speckle patterns can be averaged over time18.

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3. EXPERIMENTS AND RESULTS When a multimode waveguide is bent, coupling will occur between guided modes according to Marcuse19, and Kingsley and Davis showed that the speckle pattern is reduced by the phase modulation if the waveguide is vibrated20. Based on these two models, Kajenski et al. showed speckle patterns can be affected by mode coupling and phase modulation simultaneously6. They also found that modal noise is dominated by the mode coupling when low coherent source, like laser diode (LD), is used. Modulation of speckle was found to be dominated by the phase modulation when highly coherent source, such as He-Ne laser, is used. Vibrating the fiber has been found to be, of course, a fruitful method, but Kajenski’s model had a weak point that their system was not compatible to integrated optical circuits. As a solution of this, Povilus et al. proposed a novel method using cylindrical PZT in radial vibration18. By winding the fiber around the mandrel of the actuator, bending effect and phase modulation are occurred simultaneously and speckle patterns can be time-averaged. According to their paper, to obtain effective time-averaged smoothing of the multiple spatial modes in the fiber, the phase of each mode should be varied by more than 2π on time scales faster than the response time. Povilus applied this to magneto-optic trap yet detailed analysis on speckle reduction has not been fully described yet. In this paper, we propose quantitative measurands to characterize the speckle pattern out of MMF and report the effect of speckle reduction by using radial vibrating cylindrical PZT especially for the far-field region. Actual images of vibration-ON/OFF-state captured by color CCD camera are provided and the patterns are analyzed with National Instrument IMAQ Vision Builder by counting all the pixels of 256-level intensity in selected area.

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1:" Fig. 2. Piezo-fiber assembly vibrating in radial direction. 2 m-long graded index MMF of 50 µm core size was wound 20 rounds around the mandrel of actuator as tightly as possible. Fiber-stretching effect was caused by the radial oscillation of the actuator of which radial displacement is less than 0.4 µm/V.

Properties Mechanical

Values 7.5×103 kg / m3 Mass density, ρ E Elastic compliance, S33 18.3×10-12 m2 / N Young's modulus, E (= 1 / S33E) 54.6×109 Pa Dielectric 6.13×10-9 C2 / N m2 Permittivity, ε 692 Relative permittivity, ε / ε0 Electromechanical charge constant, d33 380×10-12 m / V piezoelectric stress constant e (=d33 / S33E) 20.77 C / m2

Table 1. Material Properties of EC-65 produced by EDO Corporation. The above values are nominal; actual production values may vary +/- 10 %

The mechanism of the piezo-fiber assembly is shown in Fig. 2. Cylindrical PZT used in this experiment was EC-65 produced by EDO Corporation. This product has an outer diameter of 38 mm, height of 38 mm, and 2 mm thickness and is made of lead zirconate titanate. The detail properties of this PZT are shown in Table 1. Note that some values are given and the other is converted accordingly21, 22. From these specifications, we can estimate that the PZT will have

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radial displacement less than 0.4 µm/V. With this actuator, core size of 50 µm standard graded index MMF is wound 20 rounds around the axis of the transducer as tightly as possible, and bonded by instant glue of Henkel Loctite. To find more detail properties of this MMF, see Table 2. Attenuation (dB/km)

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Table 2. Detail performance of MMF used in this experiment manufactured by LS cable, Korea.

Throughout the experimental process, the power of the laser source was maintained as 2.55 mW. Because beam pattern is very sensitive to perturbation along the optical path, external vibrations were minimized on a floated optical table. We used line- and field-scanning methods to measure the speckle contrast. The integration time of the camera system could be varied by the frame-grabbing system and adapted to the time constant of the observer’s eye8. Schematic experimental setup is shown in Fig. 3. The focused beam goes into color CCD camera (Toshiba IK-642F), and one can record the movie and capture the image in high resolution by running National Instrument IMAQ Vision Builder. In addition to restoring as various type of multimedia file, it can serve as various analytical and statistical tools like histogram distribution, line profile, to name a few. To drive cylindrical PZT, Aglient 33120A arbitrary waveform generator is connected it. This RF signal generator could have amplitude of 10 volt peak to peak, and with one more generator we could obtain amplitude of 20 volt peak to peak by using the principle of instructive interference between two sinusoidal waves. The lowest resonant frequency in radial vibration could by evaluated according to the paper of Kim21, and the PZT, thus, will have the frequency near 24~25 kHz. Therefore the measurement was considered in the frequency range from 22 to 26 kHz to find the lowest resonant frequency.

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Fig. 3. Schematics of the experimental setup in this study. 635 nm He-Ne highly coherent laser was used as a primary source and RF sinusoidal wave of which frequency is 20 ~ 30 kHz radio was driven with 10 ~ 20 volt peak to peak. To focus the beam into CCD camera, x60 objective lens (NA 0.85) was used.

National Instrument IMAQ Vision Builder can count all the pixels of 256-level intensity graded from 0 to 255 in selected region. By taking all the images at the modulation frequency f from 24 kHz to 26 kHz with ∆f=0.01 kHz, we obtained total of 401 images with all the other experimental conditions fixed. Analysis for each picture was performed over the exactly same spatial region. For the circular area of 26,976 pixels, Vision Builder counted the number of pixels, evaluated the mean value, and calculated the standard deviation. Note that the distance of objective lens between nearand far-field was 100 µm. We can think the standard deviation of intensity as the uniformity of the beam. In other words, if the deviation has large value, the beam has high contrast and thus apparent speckles will be increase. On the other hand, small deviation means the uniform distribution of beam intensity in selected area, so that beam will be smoothed and the speckles will be reduced. The resultant plot of the standard deviation and mean value of intensity are shown in Fig. 4. Only one signal generator was used at the driving voltage of 10 volt peak to peak. Fig. 4-(a) shows that the lowest resonant frequency of the PZT is about 24.16 kHz. On both sides of the resonant peak, medium peaks were shown and sinusoidal noise pattern was observed around the peak frequency. Far-field beam pattern

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is more sensitive than near-field image because the interference between the guided modes is correlated by the external conditions until the light reached at screen. The overall frequency response was rather noisy with many side robes, and it is expected that higher driving voltage with a suitable RF amplifier would improve signal to noise ratio.

Fig. 4. (a) Standard deviation of intensity as a function of frequency between 22 and 26 kHz with ∆f=0.01 kHz. At the frequency of 24.16 kHz the lowest deviation was observed. Data points show unclear sinusoidal noise pattern guessed by the low driven voltage. (b) Mean value of intensity as a function of frequency. It is interesting that the shape was roughly turned upside down though the mean value does not have the highest value at the resonant frequency.

Captured images of the vibration-OFF-state and the vibration-ON-state at resonant frequency of 24.16 kHz are shown in Fig. 5. We see speckle noise with high contrast in Fig. 5-(a). On the other hand, the speckles are reduced by the vibration and the contrast between bright and dark region is modulated by time-averaged smoothing in Fig. 5-(b). In this case, two function generator were used instead of using one.

(b)

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Fig. 5. Captured images through color CCD camera for (a) the vibration-OFF-state and (b) the vibration-ON-state at frequency of 24.16 kHz. The contrast of the image was reduced for (b). From now, we investigate the reduction effect quantitatively by surveying all the pixels in selected area. 26976 pixels in the bright circular line of Fig. 5 were analyzed. Note that exactly same location was also observed to obtain the data of Fig. 4. By grading that identically black color has an intensity of zero and that white color has an intensity of 255, the intensity of output beam is can be identified among 256 levels. After acquiring all data, the intensity was rescaled from zero to unity. The result of this is plotted in Fig. 6.

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Quantitative comparison between vibration-ON- and vibration-OFF-state is given in Fig. 6-(a) by counting the number of pixels for all 256-graded intensity. The difference between them can not be clearly seen in linear scale, so the logarithmic scale was adopted. Apparent difference is shown in the figures; the too much dark pixels were removed and so much bright pixels were reduced by the vibration of actuator, thus it will be expected that the standard deviation of the intensity will be decreased and the mean value of the intensity will be increased. In other words, the cylindrical PZT can play a role as a beam homogenizer. To understanding more deeply and clearly, we arranged this data by calculating the reduction ratio. This can be achieved by dividing the difference of the number of pixels between two states into the number of vibration-OFF-state. High positive reduction ratio means many pixels corresponding to the intensity were vanished and negative reduction ration means the pixels of corresponding intensity were increased. As shown in the Fig. 6-(b), distinguished feature is revealed; the dark region was perfectly removed, the ratio begin to decrease from the intensity of 0.2, the number of pixels increases in the range from 0.5 to 9.5, and hot pixels (intensity near 1.0) are decreased to drop the deviation. The summary of this result is arranged at Table 3.

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Minimum Maximum Mean value value value vibration-OFF 0 1 0.66286 vibration-ON 0.18824 1 0.68213 Type

Note

Standard deviation 0.20855 0.17358

Area [pixels] 26976 26976

2.9071 % ↑ 16.76816 % ↓

Table 3. Effect of the vibration at resonant frequency (24.16 kHz) with 20 volt peak to peak and power of 2.55 mW.

We easily guess that the line profile of the vibration-OFF-state will have dirty jagged form. It is expected that piezo-fiber assembly should mitigate this unstable profile into pulse shape or Gaussian type. The measurement was taken at the line remarked in Fig. 5, and the result is shown in Fig. 7. You can see that the line profile somewhat changed as a form with reduced noise in this figure. At the position near 50, 80, 100, 150, and 175, the homogeneity of the beam was improved by the phase modulation and become more flattened beam type. In this paper, the analysis was restricted to far-field area because to discuss about the display application. Of course, we could not achieve the perfectly flat beam type. Because time-averaged smoothing depends on the performance of the piezoelectric actuator, the reduction ratio and the homogeneity of the beam surly will be improved by adopting wellvibrating material and RF signal amplifier to raise the driven voltage. However the authors find the potential as the beam homogenizer by using the PZT vibrating in radial direction and this can be applied to develop as integrated circuit because the mechanical size of operation was reduced considerably.

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4. CONCLUSION In this paper, we have experimentally studied the effective speckle noise reduction method in MMF using phase modulation and mode coupling with piezoelectric actuator vibrating in radial direction. Quantitative analysis was investigated by digital images using a CCD camera and National Instrument IMAQ Vision Builder in terms of mean, deviation of pixel counts. It was shown that the proposed method can reduce speckles significantly especially in low intensity range. It is expected that larger vibration amplitudes with a higher performance of the piezoelectric material and amplified RF voltage would further improve the speckle reduction efficiency. Further investigations on near-field speckle are being pursued by the authors as well as for other types of optical fibers such as 62.5 µm core size MMF, hollow optical fiber (HOF), and hard polymer cladding optical fiber (HPCF). We experimentally confirmed that the proposed speckle reduction technology would find various viable applications in fiber laser, laser marking, optical trapping and projection display systems.

ACKNOWLEDGMENTS This work was supported in part by the KOSEF (Program Nos. R01-2006-000-11277-0 and R15-2004-024-00000-0), the KICOS (Program No. 2007-8-0536), and the Brain Korea 21 Project of the Ministry of Education.

REFERENCES 1

M. B. Spitzer, P. M. Zavracky, J. Crawford, P. Aquilino, and G. Hunter, “Eyewear platforms for miniature displays,” SID Symposium Digest 32, 258-261 (2001). 2 T. M. Lippert, V. Andron, and T. E. Sanko, “Overview of light beam scanning technology for automotive projection displays,” the 8th Annual Symposium on Vehicle Display Session 1.1, 15-16 (2001). 3 E. J. Seibel, S. S. Frank, M. Fauver, J. Crossman-Bosworth, J. R. Senour, and R. Burstein, “Optical fiber scanning as a microdisplay source for a wearable low vision aid,” SID Symposium Digest 33, 338-341 (2002). 4 Y. S. Jeong, S. C. Bae, Y. Jung, J. W. Lee, and K. Oh , “Fiber optic full color synthesizer,” SID Symposium Digest 36, 907-909 (2005).

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J. P. Parry, J. D. Shephard, J. D. C. Jones, and D. P. Hand, “Speckle contrast reduction in a large-core fiber delivering Q-switched pulses for fluid flow measurements,” Appl. Optics 45, 4209-4218 (2006). 6 P. J. Kajenski, P. L. Fuhr, and D. R. Huston, “Mode coupling and phase modulation in vibrating waveguides,” J. Lightwave Technol. 10, 1297-1301 (1992). 7 Ken-Ichi Sato and Koichi Asatani, “Speckle noise reduction in fiber optic analog video transmission using semiconductor laser diodes,” IEEE T. Commun. COM-29, 1017-1023 (1981). 8 L. Wang, T. Tschudi, T. Halldórsson, and P. R. Pétursson, “Speckle reduction in laser projection systems by diffractive optical elements,” Appl. Optics 37, 1770-1775 (1998). 9 J. I. Trisnadi, “Hadamard speckle contrast reduction,” Opt. Lett. 29, 11-13 (2004). 10 L. I. Ardasheva, N. D. Kundikova, M. O. Sadykova, N. R. Sadykov, and V. E. Chernyakov, “Speckle mode rotation in a few mode optical fiber in a longitudinal magnetic field,” Opt. Spectrosc. 95, 645-651 (2003). 11 R. H. Lehmberg, A. J. Schmitt, and S. E. Bodner “Theory of induced spatial incoherence,” J. Appl. Phys. 62, 26802701 (1987). 12 D. Veron, H. Ayral, C. Gouedard, D. Husson, J. Lauriou, O. Martin, B. Meyer, M. Rostaing, and C. Sauteret, “Optical spatial smoothing of Nd-glass laser beam,” Opt. Commun. 65, 42-46 (1988). 13 P. F. Michaloski and B. D. Stone, “Laser illumination with speckle reduction,” U.S. patent 6,191,887 (2001). 14 Y.-H. Chuang and J. J. Armstrong, “Peak power and speckle contrast reduction for a single laser pulse,” U.S. patent 6,693,930 (2004). 15 W. N. Partlo and W. G. Oldham, “Method and means for reducing speckle in coherent laser pulses,” U.S. patent 5,233,460 (1993). 16 E. V. Ivakin, A. I. Kitsak, M. U. Karelin, A. M. Lazaruk, and A. S. Rubanov, “Transformation of the spatial coherence of pulsed laser radiation in a delay line,” Quantum Electron. 33, 255-258 (2003). 17 V. M. Kotov, G. N. Shkerdin, D. G. Shkerdin, A. N. Bulyuk, and S. A. Tikhomirov, “Decrease in the contrast of the speckle of the optical field using Bragg diffraction of light by sound,” Quantum Electron. 31, 839-842 (2001). 18 A. P. Povilus, S. E. Olson, R. R. Mhaskar, B. -K. Teo, J. R. Guest, and G. Raithel, “Time averaging of multimode optical fiber output for a magneto-optical trap,” J. Opt. Soc. Am. B 22, 311-314 (2005). 19 D. Marcuse, Theory of dielectric waveguides, New York, Academic, 1974. 20 S. A. Kingsley and D. E. N. Davies, “Multimode optical fibre phase modulators and discriminators: I-Theory,” Electron. Lett. 14, 322-325 (1978). 21 Jin O. Kim, Kyo Kwang Hwang, and Hyung Gon Jeong, “Radial vibration characteristics of piezoelectric cylindrical transducers," J. Sound Vib. 276, 1135-1144 (2004). 22 A. J. Masys, W. Ren, G. Yang, and B. K. Mukherjeea, “Piezoelectric strain in lead zirconate titante ceramics as a function of electric field, frequency, and dc bias,” J. Appl. Phys. 94, 1155-1162 (2003).

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