sensors Article

Parallel Imaging of 3D Surface Profile with Space-Division Multiplexing Hyung Seok Lee 1 , Soon-Woo Cho 1 , Gyeong Hun Kim 1 , Myung Yung Jeong 1 , Young Jae Won 2, * and Chang-Seok Kim 1, * Received: 16 November 2015; Accepted: 18 January 2016; Published: 21 January 2016 Academic Editor: Gonzalo Pajares Martinsanz 1

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Department of Cogno-Mechatronics Engineering, Pusan National University, Busan 609-735, Korea; [email protected] (H.S.L.); [email protected] (S.-W.C.); [email protected] (G.H.K.); [email protected] (M.Y.J.) Medical Device Development Center, Osong Medical Innovation Foundation, Cheongju, Chungbuk 361-951, Korea Correspondence: [email protected] (Y.J.W.); [email protected] (C.-S.K.); Tel.: +82-43-200-9714 (Y.J.W.); +82-55-350-5285 (C.-S.K.); Fax: +82-43-200-9609 (Y.J.W.); +82-55-350-5653 (C.-S.K.)

Abstract: We have developed a modified optical frequency domain imaging (OFDI) system that performs parallel imaging of three-dimensional (3D) surface profiles by using the space division multiplexing (SDM) method with dual-area swept sourced beams. We have also demonstrated that 3D surface information for two different areas could be well obtained in a same time with only one camera by our method. In this study, double field of views (FOVs) of 11.16 mm ˆ 5.92 mm were achieved within 0.5 s. Height range for each FOV was 460 µm and axial and transverse resolutions were 3.6 and 5.52 µm, respectively. Keywords: laser and laser optics; swept source; optical frequency domain imaging; three-dimensional imaging; interferometry

1. Introduction Concomitant with the rapid growth of the precision components industry, surface profile measurement has become increasingly important in industrial inspection processes and many nondestructive three-dimensional (3D) surface profiling techniques [1–4] have been developed for product quality control and process efficiency. Further, as demand for cost-effective, noncontact, high-speed, high-accuracy 3D surface profiling technology is on the rise, various optical approaches to 3D surface profile imaging have been widely suggested [5–9]. Axial scanning methods with mechanical displacement change are a common set of approaches used to record the amplitude variance on a sample surface in an optical interferometer [7–9]. However, approaches that employ mechanical movement are susceptible to hysteresis of the mechanical translation, which makes them unsuitable for imaging systems. Alternatives that employ a tunable laser source to build non-mechanical movement optical imaging systems have been extensively studied [10–13]. In particular, optical frequency domain imaging (OFDI) technology, also known as swept source optical coherence tomography (SS-OCT), has been intensively studied as a means of avoiding mechanical movement in axial scanning by using alternative wavelength scanning of the swept source. The principle of OFDI is based on a tunable wavelength and an optical interferometer using it. The output of the light source is split into a reference arm and a sample arm which guides the light reflected from the sample. The interference between the reference and sample arm light is detected with a photo detector while the wavelength of the light source is swept and the path lengths of the reference and sample arm are held constant [11–13]. Sensors 2016, 16, 129; doi:10.3390/s16010129

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Compared to a traditional optical imaging system using a time-domain process with a white-light source, the OFDI image thus obtained also exhibits merit such as enhanced signal-to-noise ratio (SNR) and rapid imaging acquisition [14–16]. As the swept source changes its center wavelength of light to propagate through an optical interferometer, the OFDI system acquires spectrally resolved interference signals to process the displacement information by frequency domain analysis [17]. Because this displacement information along a light path can only be obtained by the wavelength-encoding of the swept source, we are able to successfully image a 3D surface profile in the area dimension using multiple simultaneous light paths when the area beam is illuminated in the OFDI system without any mechanical movement [14,18]. The field of view (FOV) of this area-beam type OFDI system is simply fixed by the dimensions of a single-area beam and a single imaging camera [14]. Recently, as imaging depth range can easily be extended in the conventional point-beam interferometer system, a space division multiplexing (SDM) method was suggested to obtain multiple displacement information from multiple point beams using a single detector in the same time [19,20]. Increase of the 3D surface profiling speed becomes very important in the real-time inspection of industrial fabrication fields such as solid bumps on printed circuit board (PCB), light emitting diode (LED) package, image sensor module (ISM), etc. Since our area-beam type OFDI system is also based on an interferometer, the SDM method can be newly applied to extend from point beam to area beam. It means that simultaneous measurement of multiple-area 3D surfaces can be proved in a new SDM-OFDI system with multiple-area swept source beams. In this study, for the first time, we experimentally demonstrated a SDM-OFDI system with dual-area swept source beams to obtain 3D surface information for two different areas at the same time by using a single camera. We prove our method by measuring 3D surface profiles of two different areas in a coin. 2. Space Division Multiplexing with Dual-Point Swept Source Beams To see the characteristics and capability of our method, we first demonstrated the SDM-OFDI system with dual-point swept source beams as shown in Figure 1. In this experiment, the optical interferometer consists of a swept source, three 3 dB couplers, three collimators, one reference arm, two sample arms, and one photodetector. Two stages are located in both sample arms to change the displacement of the two samples on them. The swept source with a center wavelength of 800 nm was used as a light source. This swept source consisted of a fiber-coupled semiconductor optical amplifier (SOA) module, a wavelength selector, optical isolators, an inline polarization controller (PC), and an output coupler. The sweeping range and maximum output power of the light source were 50 nm and ~20 mW, respectively. The linewidth of the swept source was measured to be less than 0.05 nm; thus, it is a long coherence length greater than can be expected. The swept source laser beam went through a 3 dB coupler and divided into reference and sample arms to realize the interferometer. In the reference and sample arms, each beam was also injected into extra 3 dB couplers to demonstrate SDM method with dual-point swept source beams. In the sample arm, each beam after the extra 3 dB coupler went to each sample after passing through a collimator. In the reference arm, one of two beams after the extra 3 dB coupler was used as a reference. The used optical fiber in the SDM-OFDI system was Corning HI780 to ensure single transverse mode operation. In this system, there were three parts to interfere: (1) reference and sample 1; (2) reference and sample 2; (3) sample 1 and sample 2 such that the distance between them must be carefully considered to distinguish them clearly by our method. By moving the position of each stage, the optical path length of sample 1 and sample 2 can be easily controlled to divide the total coherence length of light source into two depth information regions. The case for the placement of samples and the reference mirror is shown in Figure 2. Here, S11 and S21 represent the distance between the reference mirror and the top of each sample, respectively. S12 and S22 represent the distance between the reference mirror and the bottom of each sample, respectively. In this case, the height information of two samples can be well differentiated with the condition of S21 < S22 < S12 < S11 < S12 + S21.

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SOA

SOA

Wavelength Wavelength Selector Selector

Isolator Isolator

Pol. Cont. Pol. Cont.

Output Output Coupler Coupler

Fiber Fiber 3dB 3dB Coupler Coupler

3dB3dB Coupler Coupler

None None

Swept source Swept source 3dB3dB Coupler Coupler Photo detector Photo detector

Collimator Collimator Reference Reference

Collimator Collimator Sample 1 1 Sample Collimator Collimator

Stage Stage Sample 2 2 Sample Stage Stage

Figure system with dual-point swept source beams. Figure 1.1.SDM-OFDI system with dual-point swept source beams. Figure 1.SDM-OFDI SDM-OFDI system with dual-point swept source beams.

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Figure 2.2.(a) Distance between sample1, sample2 and reference mirror; (b) Parallel measurement ofofof Figure 2. Distance between sample1, sample2 and reference mirror; Parallel measurement Figure (a)(a) Distance between sample1, sample2 and reference mirror; (b)(b) Parallel measurement two 3D surface profiles. two surface profiles. two 3D3D surface profiles.

To To demonstrate thethe parallel acquisition of 3D information forfor twotwo samples, cover glass with a a demonstrate parallel acquisition 3D information samples, cover glass To demonstrate the parallel acquisition ofof3D information for two samples, cover glass with with thickness of 140 µmµm waswas used in both sample arms. TheThe wavelength-encoded optical interference thickness of 140 used in both sample arms. wavelength-encoded optical interference a thickness of 140 µm was used in both sample arms. The wavelength-encoded optical interference signal from thethe twotwo samples andand thethe reference mirror waswas detected by by a photo detector andand converted signal from samples reference mirror detected photo detector converted signal from the two samples and the reference mirror was detected by a aphoto detector and converted intointo thethe electric signal. Then, thethe depth-encoded signal waswas obtained from thethe wavelength-encoded electric signal. Then, depth-encoded signal obtained from wavelength-encoded into the electric signal. Then, the depth-encoded signal was obtained from the wavelength-encoded signal with thethe Discrete Fourier Transform (DFT) method [21]. signal with Discrete Fourier Transform (DFT) method [21]. signal with the Discrete Fourier Transform (DFT) method [21]. Figure 3 shows the depth-encoded signal acquired by thethe SDM-OFDI system with dual-point Figure 3 shows the depth-encoded signal acquired by SDM-OFDI system with dual-point Figure 3 shows the depth-encoded signal acquired by the SDM-OFDI system with dual-point swept source beams. In this experiment, interferometric information was acquired from one cycle sweptsource sourcebeams. beams.InInthis thisexperiment, experiment,interferometric interferometricinformation informationwas wasacquired acquiredfrom fromone one cycle swept cycle scanning of the swept source, as shown in the insets of Figure 3, and it was converted to the depth scanning of the swept source, as shown in the insets of Figure 3, and it was converted to the depth scanning of the swept source, as shown in the insets of Figure 3, and it was converted to the depth range information along about 4000 µm.µm. Sample 1 was placed about 1000 µmµm after thethe reference range information along about 4000 Sample was placed about 1000 after reference range information along about 4000 µm. Sample 1 1was placed about 1000 µm after the reference mirror and sample 2 was placed about 2400 µm before the reference mirror. The depth information mirrorand andsample sample2 2was wasplaced placedabout about2400 2400µm µmbefore beforethe thereference referencemirror. mirror. The depthinformation information mirror The depth duedue to interference between both samples appeared after 3200 µmµm so that thethe depth information forfor to interference between both samples appeared after 3200 so that depth information due to interference between both samples appeared after 3200 µm so that the depth information for twotwo samples cancan be well distinguished by by each oneone with thethe SMD-OFDI system with dual-point samples well distinguished each with SMD-OFDI system with dual-point two samples can bebewell distinguished by each one with the SMD-OFDI system with dual-point swept source beams. Though the single photo detector receives a dual-frequency interferogram, each sweptsource sourcebeams. beams.Though Thoughthe thesingle singlephoto photodetector detectorreceives receivesa adual-frequency dual-frequencyinterferogram, interferogram, each swept each individual frequency contrast is not degraded because the dual frequency fringes are clearly individual frequency contrast is not degraded because the dual frequency fringes are clearly individual frequency contrast is not degraded because the dual frequency fringes are clearly separated separated along thethe depth scale during thethe DFT process, as represented in Figure 3. If3.the coherence separated along depth scale during DFT process, asinrepresented in Figure If length the coherence along the depth scale during the DFT process, as represented Figure 3. If the coherence of the length of the used swept source is increased, the available depth range for two samples is length of the used swept source is increased, the available depth range for two samples used swept source is increased, the available depth range for two samples is proportionally increasedis proportionally increased or the number of samples to measure simultaneously is also increased [18]. the number of samples to simultaneously is also increased [18]. orproportionally the number ofincreased samples toormeasure simultaneously is measure also increased [18].

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(a)

(b)

(c) Figure 3. (a) Depth-encoded signal with only sample1. (b) Depth-encoded signal with only sample 2. Figure 3. (a) Depth-encoded signal with only sample1; (b) Depth-encoded signal with only sample 2; (c) Depth-encoded signal with both sample1 and sample 2. (c) Depth-encoded signal with both sample1 and sample 2.

3. Space Division Multiplexing with Dual-Area Swept Source Beams 3. Space Division Multiplexing with Dual-Area Swept Source Beams The camera-based 3D surface measurement OFDI system with a swept sourced interferometer The camera-based 3D surface measurement system with a swept sourced interferometer has many advantages such as simple design, freeOFDI for depth scan, and fast measurement time due to has many advantages such as simple design, free for depth scan, and fast measurement timemultiple due to parallel data processing with a computer-unified device architecture (CUDA). Moreover, parallel data with a computer-unified device only architecture (CUDA). Moreover, multiple FOVs for 3D processing surface measurement are obtainable despite single camera-based system when the FOVs for 3D surface measurement are obtainable despite only single camera-based system when the SDM method is applied with dual-area swept source beams. SDM The method is applied with source beams. experimental setupdual-area for the swept SDM-OFDI system with dual-area beam illumination is The experimental the SDM-OFDI with dual-area beam illumination is represented represented in Figure setup 4. Thefor used light sourcesystem is the same as the one mentioned in Section 2. It was in Figure 4. The used light source is the same as the one mentioned in Section 2. It was collimated collimated and resized as larger than 10 mm after passing through a collimator and beam-expander. and resizedbeam as larger than 10 mm passing through a collimator and (BS1 beam-expander. Dual-area Dual-area illumination wasafter realized by using two beam splitters and BS2). Most of the beam illumination realized by using two beam splitters (BS1by and Most oflength the interference interference signalswas in the FOV1 area are simultaneously induced theBS2). optical path difference signals inL1 the+ FOV1 induced by the optical pathcomes lengthfrom difference between between L2 andarea L1 +are L3.simultaneously Similarly, the sample information of FOV2 that between L1 L1 + L2 and L1 + L3. Similarly, the sample information of FOV2 comes from that between L1 + L2 and + L2 and L4 + L5 + L6. In order to distinguish the area distance between FOV1 and FOV2 on the same L4 + L5the + L6. In order distinguish the area distance betweensuitably FOV1 and FOV2 the on the same plane, plane, above threetokinds of distances should be adjusted to divide total coherence length of the light source into two height information regions. The 2D interference signals with respect to different wavelengths were passing through two convex lenses (f = 200 mm) and were

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the above three kinds of distances should be adjusted suitably to divide the total coherence length ofSensors the light 2016,source 16, 129 into two height information regions. The 2D interference signals with respect 5to of 7 different wavelengths were passing through two convex lenses (f = 200 mm) and were acquired in order by a Si-type device (CCD) with 8-bit resolution. Theresolution. pixel size ofThe thepixel CCDsize wasof acquired in ordercharge-coupled by a Si-type charge-coupled device (CCD) with 8-bit 5.5the µm ˆ 5.5 µm and the CCD consisted of 2040 ˆ 1088 pixels such that the sensor size of the CCD CCD was 5.5 µm × 5.5 µm and the CCD consisted of 2040 × 1088 pixels such that the sensor size was 11.22 mmwas ˆ 5.98 mm. 170 frames for interference signals withsignals respectwith to of the CCD 11.22 mmIn× this 5.98experiment, mm. In this experiment, 1702D frames for 2D interference different wavelengths were used to extract depth-resolved 3D surface information. Since a frame rate respect to different wavelengths were used to extract depth-resolved 3D surface information. Since a offrame CCD was frames/s, the total acquisition time was 0.5 s. time To protect thes.signal saturation and rate 340 of CCD was 340 frames/s, the total acquisition was 0.5 To protect the signal enhance the interference the output power of light source thelight attenuation of the saturation and enhanceefficiency, the interference efficiency, thethe output power and of the source and the variable attenuator in the reference arm can be optimally controlled. attenuation of the variable attenuator in the reference arm can be optimally controlled.

Figure 4. 4. SDM-OFDI system with dual-area swept source beams. Figure SDM-OFDI system with dual-area swept source beams.

Figure shows two different surface profiles a coin measured the SDM-OFDI system Figure 5a5a shows two different 3D3D surface profiles ofof a coin measured byby the SDM-OFDI system at the same time with dual-area swept source beams, and their cross-sectional profile along the dash at the same time with dual-area swept source beams, and their cross-sectional profile along the dash lineofofeach each3D3D surface profile also represented Figure Comparedwith withthe the conventional line surface profile is is also represented in in Figure 5b.5b.Compared conventional camera-based 3D surface measurement system, the SDM-applied OFDI system with dual-area swept camera-based 3D surface measurement system, the SDM-applied OFDI system with dual-area swept source beams can measure multiple 3D surface profiles of different areas at the same time. can source beams can measure multiple 3D surface profiles of different areas at the same time. It Itcan provide the effect increment FOVs multiple times despite being a single camera-based optical provide the effect forfor increment ofof FOVs multiple times despite being a single camera-based optical system. We obtained two FOVs of 11.16 mm × 5.92 mm with only a single camera-based system and system. We obtained two FOVs of 11.16 mm ˆ 5.92 mm with only a single camera-based system and the acquisition time was 0.5 seconds. To obtain the axial resolution of the depth-encoded signal in the the acquisition time was 0.5 s. To obtain the axial resolution of the depth-encoded signal in the imaging imaging process, we measured cover with a thickness of using 140 µm using the proposed SDMprocess, we measured the cover the glass withglass a thickness of 140 µm the proposed SDM-OFDI OFDI system and found out the digitized resolution of about 3.6 µm in the condition of a 512 data system and found out the digitized resolution of about 3.6 µm in the condition of a 512 data sampling sampling number DFT. The validation of confirmed the resultthat alsothe confirmed that the averageonheight number for DFT. The for validation of the result also average height difference the difference on the coin surface sample (200 points for each top and bottom region) shows a similar coin surface sample (200 points for each top and bottom region) shows a similar value of about 64.8 value of µm about 65.39 µm2,for both FOV 1 and 2, respectively. and 65.39 for64.8 bothand FOV 1 and respectively. Forthe thecomparison, comparison,the the other other profiling scanning interferometry (WSI; NVFor profiling method methodofofwhite whitelight light scanning interferometry (WSI; 2400, Nanosystems, Inc., Daejeon, Korea) was also applied to the same coin sample with a much NV-2400, Nanosystems, Inc., Daejeon, Korea) was also applied to the same coin sample with a much smaller FOV of 466.6 µm × 622.2 µm objective (×10 objective to the obtain theaverage similar height average height smaller FOV of 466.6 µm ˆ 622.2 µm (ˆ10 lens) tolens) obtain similar of 68.6 µm.of 68.6 µm.

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Figure (a) Two different 3D surface profiles a coin measuredbybySDM-OFDI SDM-OFDI system system with with dualFigure 5. (a)5.Two different 3D surface profiles of aofcoin measured dual-area area swept source beams. (b) Cross-sectional profile along the dash line of each 3D surface profile. swept source beams; (b) Cross-sectional profile along the dash line of each 3D surface profile.

In case of the measurement of transverse resolution, we applied this system with the standard

In case the measurement of The transverse resolution, appliedwas this5.52 system with the standard U.S. AirofForce resolution target. measured transversewe resolution µm, which exhibits U.S. Air Force resolution target. measured transverse resolution 5.52 µm, exhibits reasonable agreement with the The theoretical calculation and with the CCDwas dimension andwhich pixel size. Since we can expectwith a single rangecalculation of 920 µm from 256the depth numbers multiplied by a size. reasonable agreement the height theoretical and the with CCD dimension and pixel pixel of 3.6a µm, theheight height range range for eachµm sample sample 2 was about 460 µm by dividing Since depth we can expect single of 920 from1 and the 256 depth numbers multiplied by a depth for two separate height ranges. Since these two FOV images divide the total coherence of the for pixel of 3.6 µm, the height range for each sample 1 and sample 2 was about 460 µmlength by dividing light source into two height information regions, the limit of the height measurement is reduced to two separate height ranges. Since these two FOV images divide the total coherence length of the light half of a single FOV’s. Thus, to keep a similar height measurement region, the longer coherence light source into two height information regions, the limit of the height measurement is reduced to half of source will be preferred. To remove the spiky noise in the data which comes physically from the a single FOV’s.between Thus, to keep a similar height measurement region, theusing longer light source cross-talk CCD pixels, an additional software process is applied thecoherence median filter with will be preferred. To remove the spiky noise in the data which comes physically from the cross-talk a size of 10 × 10 pixels in the 3D surface profile image. As we increase the size of the median filter, between CCD pixels, additional software isbe applied using the median with a size of we can reduce the an spiky noise effectively butprocess there will a trade-off relation betweenfilter the smoothing and measurement accuracy. 10 ˆ effect 10 pixels in the 3D surface profile image. As we increase the size of the median filter, we can reduce the spiky noise effectively but there will be a trade-off relation between the smoothing effect 4. Conclusionsaccuracy. and measurement In this research, we proposed and experimentally demonstrated a modified OFDI system based

4. Conclusions on the SDM method to achieve a parallel imaging of 3D surface profiles. The SDM-OFDI system with dual-area swept source beams makes it possible to measure 3D surface profiles for multiple FOVs

In this research, we proposed and experimentally demonstrated a modified OFDI system based simultaneously, despite being a single camera-based system. The SDM-OFDI method with dual-area on the SDM method to achieve a parallel imaging of 3D surface profiles. The SDM-OFDI system swept source beams can be powerfully applied to industrial fields, especially 3D inspection with equipment, dual-area swept source makes it possible to measure 3D surface profiles for multiple because it hasbeams the many advantages of simultaneous multiple area inspection and FOVsaccurate simultaneously, despite a single camera-based system. SDM-OFDI measurement due tobeing free for depth scanning. By using a singleThe camera, we havemethod acquired with dual-area sweptinformation source beams canareas be powerfully applied to industrial 3D 3D surface for two of a coin at the same time within 0.5fields, s. The especially height range forinspection each equipment, it has the and many of simultaneous samplebecause was about 460 µm theadvantages axial resolution was 3.6 µm. multiple area inspection and accurate measurement due to free for depth scanning. By using a single camera, we have acquired 3D surface Acknowledgments: This work was supported by the National Research Foundation of Korea (NRF) grant information for two areas of a coin at the same time within 0.5 s. The height range for each sample was funded by the Korea government (MSIP: The Ministry of Science, ICT and Future Planning). (NRFabout2015R1A2A1A15056030). 460 µm and the axial resolution was 3.6 µm. Author Contributions: and Young WonNational designed Research and built the system; Hyung Lee,(NRF) Acknowledgments: This Chang-Seok work wasKim supported byJaethe Foundation of Seok Korea Soongovernment Woo Cho performed experiments; YoungICT Jeongand analyzed the Planning). data; grant Gyeong fundedHun by Kim the and Korea (MSIP:the The Ministry Myoung of Science, Future Hyung Seok Lee and Young Jae Won wrote the paper. (NRF-2015R1A2A1A15056030).

Author Contributions: Chang-Seok Kim and Young Jae Won designed and built the system; Hyung Seok Lee, Gyeong Hun Kim and Soon Woo Cho performed the experiments; Myoung Young Jeong analyzed the data; Hyung Seok Lee and Young Jae Won wrote the paper.

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Conflicts of Interest: The authors declare no conflict of interest.

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Parallel Imaging of 3D Surface Profile with Space-Division Multiplexing.

We have developed a modified optical frequency domain imaging (OFDI) system that performs parallel imaging of three-dimensional (3D) surface profiles ...
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