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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 24 — Nov. 26, 2007
  • pp: 15767–15775
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High-efficiency and stable optical transmitter using VCSEL-direct-bonded connector for optical interconnection

Do-Won Kim, Tae-Woo Lee, Mu Hee Cho, and Hyo-Hoon Park  »View Author Affiliations


Optics Express, Vol. 15, Issue 24, pp. 15767-15775 (2007)
http://dx.doi.org/10.1364/OE.15.015767


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Abstract

A high-efficiency optical transmitter module for PCB (printed circuit board)-based interconnections was fabricated using a bottom-emitting VCSEL. The bottom-emitting VCSEL was directly bonded by an epoxy on a 90°-bent fiber connector which is inserted into the PCB to couple to the fiber layer embedded in the board. A ray trace simulation indicates that close contact between the VCSEL and the connector removes most of the losses due to Fresnel reflection and beam divergence. This tendency was experimentally identified. Thermal dissipation through the epoxy layer and the connector also improves significantly the power characteristics of the VCSEL. The VCSEL after bonding on the connector shows about 40% higher power compared to that of the bare VCSEL at the current showing a peak power before bonding. The results indicate that direct bonding improves both optical and electrical efficiencies. A successful eye diagram at the speed of 5 Gb/s/ch with 850 nm was accomplished from the VCSEL-direct-bonded transmitter module.

© 2007 Optical Society of America

1. Introduction

High speed and high capacity data transmissions are in great demand for future computer memory systems and mobile communications. Optical interconnections have preferred possibility over electrical interconnections to meet this requirement because of certain advantages like wider bandwidth, lower power consumption, no existence of electromagnetic interference (EMI) effects, and higher packaging density [1

1. J. Wu, J. Wu, J. Bao, and X. Wu, “Soft-lithography-based optical interconnection with high misalignment tolerance,” Opt. Express 13, 6259–6267 (2005) [CrossRef] [PubMed]

, 2

2. R. Lytel, H. L. Davidson, N. Nettliton, and T. Sze, “Optical interconnections within modern high-performance computing systems,” in Proceeding of IEEE. 88, 758–763 (2000) [CrossRef]

]. They also have THz carrier modulation and are easier to control over reflections [3

3. E. D. Kyriakis-Bitzaros, N. Haralabidis, M. Lagadas, A. Georgakilas, Y. Moisiadis, and G. Halkias, “Realistic end-to-end simulation of the optoelectronic links and comparison with electrical interconnections for system-on-chip applications,” IEEE J. Lightwave Technology 19, 1531–1542 (2001) [CrossRef]

, 4

4. M. R. Feldman, S. C. Esener, C. C. Guest, and S. H. Lee, “Comparison between optical and electrical interconnects based on power and speed considerations,” Appl. Opt. 27, 1742–1751 (1988) [CrossRef] [PubMed]

].

In this paper, we employ the direct bonding technique to improve the coupling efficiency in the optical transmitter module for the optical interconnection system. Bottom emitting VCSEL array is directly bonded on the top surface of the 90°-bent fiber. This module has no free space gap at the interface between VCSEL and 90°-bent fiber connector, which can reduce the optical loss at the interface. A thermal conduction through the connector can also help room temperature operation of the VCSEL device. Furthermore, the module tightly bonded on the connector can work more stably over a long period of time because it can be protected from mechanical vibrations and external environments.

2. Package structure

Figure 1(a) shows the optical interconnection system we developed previously [7

7. S. H. Hwang, M. H. Cho, S.-K. Kang, H.-H. Park, H. S. Cho, S.-H. Kim, K.-U. Shin, and S.-W. Ha, “Passively assembled optical interconnection system based on an optical printed circuit board,” IEEE Photo. Technol. Lett. 18, 652–654 (2006) [CrossRef]

]. We used silica fiber layers for a wave-guiding medium in the optical PCB. The 90°-bent fiber connectors and the transmitter/receiver modules were assembled in the optical PCB using ferrule-type guide pins. In this system, a top-emitting VCSEL array was used, for which gold wiring was applied on the top side of the chip for electrical connection with driver ICs. Since gold wires can be easily damaged by physical contact, it required a gap of at least 100 µm to protect from contact with the connector. This free space gap induces some losses due to scattering and beam divergence. Figure 1(b) shows an improved system where the VCSEL is directly bonded on the 90°-bent fiber connector to reduce the optical losses. To setup this system we chose a bottom emitting VCSEL array of which all bump patterns for p and n contacts are formed to the top side. Since there is no electric pad on the bottom side, it can be closely bonded on the connector. In the experiment, the VCSEL chip was tightly bonded using an epoxy by pressing the chip on the connector.

Fig. 1. (a) Previous optical transmission system in which a transmitter module is assembled on a 90°-bent fiber connector with a free space gap. (b) Improved optical transmission system with a transmitter module in which VCSEL is directly bonded on a 90°-bent fiber connector.

3. Effects of alignment

A ray trace simulation [8

8. B. S. Rho, S.-K. Kang, H. S. Cho, H.-H. Park, S.-W. Ha, and B.-H Rhee, “PCB-compatible optical interconnection using 45° -ended connection rods and via-holed waveguides,” J. Lightwave Technol. 22, 2128–2134 (2004) [CrossRef]

] was performed using the LightTools simulator to investigate the loss changes by longitudinal and lateral misalignments at the interface of the VCSEL and 90°-bent fiber connector. In the simulation, the parameters were applied similarly with those of the devices used in the experiment. For the connector, a multi-mode silica fiber was applied with dimensions of 100/110 µm in core/clad diameters and refractive indices of 1.47/1.422 at 850 nm. The bending radius of the fiber was 3 mm. The VCSEL has an aperture diameter of 10 µm, a divergence angle of 15°, and an emitting wavelength of 850 nm. For the lateral x (or z) direction, as defined in Fig. 1, simulation was carried out within a misalignment range from -50 µm to +50 µm by a 5 µm step. For the longitudinal y direction, the range was 0 to +500 µm. The effect of filling the gap with index-matching material at the interface was also simulated.

The effect of misalignment on the loss was also experimentally measured. To control the alignment accurately we used a fiber-tailed VCSEL module as a light source, in which driver IC was installed. The aperture of the polished fiber tip was aligned to the aperture of the 90°-bent fiber connector. The tailed fiber has a core diameter of 9 µm, similar with the window size of the VCSEL that we used in the main transmitter module. The divergence angle emitted from the fiber tip was also near that of the VCSEL, 15°. As a filling material, index-matching oil having an index of 1.514 was used.

Figure 2 shows comparisons between the results of simulation and experiment. Note that in this figure all data commonly include the bending losses in the connector. However, in the simulation the bending loss of the fiber with a 3 mm radius of curvature was negligibly low near 0.02 dB. In the measurement also it was within the experimental error. In the longitudinal direction, the simulated curve for the free-space gap shows little loss until 150 µm distance and beyond this distance the loss abruptly increases, as seen in Fig. 2(a). This trend originates from the Gaussian intensity profile of the laser beam. In the case that the index-matching material is filled, the loss increase appears at a longer distance near 220 µm. This tendency appears similarly in the experiment result, but the measured losses show overall lower values than the calculated values. The critical distance showing the abrupt loss increase is shifted from about 60 µm to 220 µm. We note that the loss beyond the critical distance mainly arises from the effect of beam divergence [11

11. B. Schwarz, M. Grüttner, and W. Röhle,“Beam attenuation measurement of hydrosols by means of a new measuring technique,” Meas. Sci. Technol. 1, 1102–1105 (1990) [CrossRef]

]. When the index-matching material (n=1.514) is filled, the divergence angle is calculated to be reduced to about 10° from 15°. Thus, for a large gap, the loss due to the beam divergence can be significantly reduced by filling the index-matching material. By this effect, the misalignment tolerance for 3dB degradation increases from 360~380 µm to 580~650 µm by the filling. If the distance is very small, the effect of beam divergence becomes negligible and the effect of Fresnel reflection [11

11. B. Schwarz, M. Grüttner, and W. Röhle,“Beam attenuation measurement of hydrosols by means of a new measuring technique,” Meas. Sci. Technol. 1, 1102–1105 (1990) [CrossRef]

] appears dominantly. The loss due to the Fresnel reflection is only 4% for a free and plat surface of a glass fiber. Thus, the loss curves in Fig. 2(a) show very small and almost constant values within the critical distance.

Figure 2(b) shows loss curves for the lateral z direction when the distance is 10 µm for example as a shallow gap. The curves show almost symmetric degradation tendency for the negative and positive misalignments. The 3 dB degradation tolerances in the simulation are near +50 µm for both cases without and with the index-matching material. As discussed in Fig. 2(a), the filling effect does not clearly appear at the shallow gap. In the experiment, the 3 dB degradation tolerances are +34~+42 µm. This difference in the tolerances might not be so significant, considering the experimental error range.

Colligating the results of Figs. 2(a) and (b), close contact in the longitudinal alignment between the VCSEL and the connector is essential to obtain a high optical coupling efficiency. Once the close contact is achieved within a critical distance, the filling effect is negligible in the loss and lateral alignment.

Fig. 2. Comparison of optical losses obtained from ray trace simulation and experimental measurement for (a) longitudinal (y direction) and (b) lateral (z direction) misalignments.

4. Fabrication and characterization of transmitter module

Figure 3 shows the fabrication procedure of the transmitter module with the VCSEL-direct-bonded connector. It was prepared through the following procedures. First, to fabricate the 90°-bent fiber connector, 12 strands of glass fibers having 100 µm core diameter and 110 µm clad diameter are inserted into two 12-channel MT-ferrules as shown in Fig. 3(a). Then, bending the fibers, two MT-ferrules are assembled in an aluminum block whose dimension is 8 mm×6 mm×6 mm as seen in Fig. 3(c). After pasting and thermal-curing the polymeric adhesive to bond the assembled MT-ferrules and the aluminum block, projected parts of the MT-ferrules were cut and two cut surfaces were polished. The fabricated 90°-bent fiber connector is shown in Fig. 3(c).

Secondly, for the fabrication of the optical transmitter module, we pasted the transparent UV-curable epoxy resin, whose model name is ZEOM105 made by CHEMOPTICS in Korea, on the top surface of the connector. This UV-curable epoxy adhesive is produced as a core material for polymeric waveguide fabrication and has a refractive index of 0.512 at 1550 nm. Then we set a 1×4 bottom-emitting VCSEL array chip by adjusting each aperture of the VCSEL array to each center of the fibers on the connector. The bottom-emitting VCSEL chip for 850 nm wavelength was manufactured by ULM PHOTONICS in Germany for 5 Gb/s/ch operation. All bump patterns were formed on the top [12

12. M. Grabherr, R. Jäger, R. King, B. Schneider, and D. Wiedenmann, “Fabricating VCSELs in a high tech start-up,” in Proceedings of SPIE, VCSELs and Optical Interconnects 4942, 13–24 (2003)

]. In this VCSEL chip the substrate material was exchanged from GaAs, absorbing 850 nm wavelength, to a transparent glass. The substrate thickness is about 100 µm and antireflection (AR) layers were coated on the bottom side. Figure 3(d) shows the connector bonded with the VCSEL chip. The upper part of Fig. 3(d) shows the top side of the connector where the bonded VCSEL chip is magnified. The alignment of the VCSEL chip was done by using microscope and manual positioning 3-directional stages. Optical microscope BX51TRF made by OLYMPUS Corporation with image analyzing software TOMORO ScopeEye 3.5 was used for bonding the VCSEL manually. Ultra Violate Light Sources DYMAX 2000-EC was used for curing of UV-curable epoxy resin pasted for bonding of the bottom-emitting VSCEL. Then, we packaged the VCSEL driver IC on a transmitter PCB board as shown in Fig. 3(e). The transmitter board was then bonded on the edge of the top side of the connector, exposing the metal pads on the top surface of the VCSEL. After bonding the board, electrical connection was performed using gold wires between the VCSEL on the connector and the driver IC on the board. The VCSEL chip was originally designed for flip-chip bonding. However, there was no available driver IC chip designed for flip-chip bonding, so that the VCSEL chip was connected by wire-bonding with the drive IC chip, as illustrated in Fig. 1(b). We made molding over gold wires with UV-curable epoxy resin for protection and then assembled SMA connectors to the board. The finished transmitter module is shown in Fig. 3(f).

Fig. 3. Fabrication procedure of the transmitter module with the VCSEL-direct-bonded connector; (a) aluminum block and MT-ferrules with inserted fibers, (b) Assembled MT-ferrules by bending fibers into aluminum block, (c) fabricated 90°-bent fiber connector after cutting out the projected parts and polishing the surfaces, (d) VCSEL bonding on the connector with 0.73 µm misalignment tolerance, (e) transmitter PCB on which driver IC is bonded, (f) completed transmitter module where the connector is bonded to the PCB.

Figure 4 shows a photograph of light emitting from the VCSEL-direct-bonded connector and whole transmitter module. The bottom picture in Fig. 4 shows a magnified image of light spots observed from the fiber connector by an infra red (IR) camera. The output power of this transmitter module was -0.6 dBm.

Fig. 4. Photographs of the transmitter module bonded with the 90°-bent fiber connector. Bottom picture shows a magnified image of light spots observed from the fiber connector.

An eye diagram was successfully measured at the bit rate of 5 Gb/s/ch. Anritsu Pulse pattern generator MP1763B generated pseudo random bit sequences (PRBS) non return to zero (NRZ) pulses with the PRBS/zero substation of 231-1 and PRBS mark ratio of 1/2. An Agilent 86100A wide-bandwidth oscilloscope was used. Figure 5 shows an eye diagram measured by the electrical detector of the oscilloscope. The total jitter was 78.89 ps. The average power was 537.57 µW and the total jitter was 66.7 ps when measured by the optical detector of the oscilloscope. The bit error rate (BER) of this optical transmission system, which was detected by using the Anritsu error detector MP1764A, was 10-9 at the bit rate of 5 Gb/s/ch. This relatively low BER performance might be attributed to the distant wire-bonding between the VCSEL and the driver IC chips. If flip-chip bonding is applied for both chips, the dynamic characteristics could be improved.

Fig. 5. Eye diagram measured at 5Gb/s/ch.

5. Thermal effect

When the bottom-emitting VCSEL is directly bonded on a thermally conductive material, the AR coating and thermal impedance of the VCSEL should be considered as important factors influencing the optical output power. After filling the epoxy at the interface, the output power itself of the VCSEL could be changed. Since the VCSEL chip we used has an AR-coating layer which is designed to be exposed to air, covering the AR layer with the epoxy having a higher index should degrade the output power. In addition, one can expect that the thermally conductive UV-cured epoxy layer and aluminum connector assembled with MT-ferrules can work as heat sinks. This thermal effect may help increasing the power of the VCSEL if the VCSEL structure is optimized for room temperature operation. To confirm the power change due to these effects, we measured L-I-V curves for the VCSEL bonded by the epoxy to a conventional MT-ferrule. In the ferrule, linear fiber segments were inserted and both sides of the ferrule were polished. For comparison, the curves for the bare state VCSEL before coupling with the MT-ferrule were measured. Figure 6 shows L-I-V curves measured by dc probes before bonding (bare VCSEL) and after bonding for the same VCSEL device. Other VCSEL devices in the 1×4 array showed similar curves. The measurement was done by using a precision laser diode current source and an optical power meter. In Fig. 6, the output power through the ferrule is about 40% higher than the power of the bare VCSEL at the current showing peak power. The power of the VCSEL bonded on the ferrule increases continuously up to 10mA. This result indicates that high thermal conductivity of the epoxy layer and the ferrules improves significantly the power characteristics of the VCSEL we used. Such effects were reported in other works [13

13. C.-K. Lin, S.-W. Ryu, and P. D. Dapkus, “High-performance wafer-bonded bottom-emitting 850–nm VCSEL’s on the undoped gap and sapphire substrates,” IEEE Photo. Technol. Lett. 11, 1542–1544 (1999) [CrossRef]

]. Thus, we can conclude that the effect of thermal dissipation through the connector overcomes the interference effect from the mismatched AR layer in power changes.

Fig. 6. Comparison of L-I-V curves of the bare VCSEL and the VCSEL bonded on the connector.

If the VCSEL is packaged with the drive IC which controls the input voltage, it is hard to know the real current injected to the VCSEL and the exact output power of the VCSEL. The current is sensitively changed at the same voltage by the resistivity change which is dependant on the thermal dissipation, as expected from the I–V curves in Fig. 6. So that, in the transmitter module fabricated in Fig. 4, it is difficult to measure accurately the change of optical loss after filling the gap. This is one of the reasons that in Fig. 2 the optical power is supplied through the fiber tip, instead of the drive-IC-packaged module. Since the VCSEL source is far from the connector, the thermal effect is completely excluded and only the effects of alignment and filling on the loss can be measured. The bonding effect on the electrical efficiency can be roughly estimated from the result of Fig 6. Comparing the slopes of two L-I curves in Fig. 6, the optical power increases from minimum +0.3 dB to maximum +3.3 dB at the same current below 7 mA after bonding. Considering the current increase at a constant voltage by the effect of thermal dissipation, further increase is expected. This effect can override the optical loss at the interface and present positive gains in the close contact region as shown in Fig. 2.

6. Conclusions

A high-efficiency transmitter module for optical PCB-based optical interconnections has been demonstrated using a bottom-emitting VCSEL which is closely bonded with an epoxy on the fiber connector. This module can minimize the optical losses which occur at the interface of the VCSEL and the connector. The effective heat dissipation through the connector also increases significantly the output power of the VCSEL, thus increasing electrical efficiency. From this module, 5 Gb/s/ch operation was demonstrated. Tight bonding of the VCSEL and the connector can protect from mechanical vibrations and external environments, near to hermitic sealing. The VCSEL-direct-bonded transmitter module can be a good candidate for the construction of stable and high-efficiency optical interconnection systems. To obtain the best performance from this module, flip-chip bonding structures are required for the driver IC chip as well as the VCSEL chip.

Acknowledgments

This work was supported by the national program Tera-level nanodevices as a 21st Century Frontier R & D Project funded by the Korean Ministry of Science and Technology (MOST).

References and links

1.

J. Wu, J. Wu, J. Bao, and X. Wu, “Soft-lithography-based optical interconnection with high misalignment tolerance,” Opt. Express 13, 6259–6267 (2005) [CrossRef] [PubMed]

2.

R. Lytel, H. L. Davidson, N. Nettliton, and T. Sze, “Optical interconnections within modern high-performance computing systems,” in Proceeding of IEEE. 88, 758–763 (2000) [CrossRef]

3.

E. D. Kyriakis-Bitzaros, N. Haralabidis, M. Lagadas, A. Georgakilas, Y. Moisiadis, and G. Halkias, “Realistic end-to-end simulation of the optoelectronic links and comparison with electrical interconnections for system-on-chip applications,” IEEE J. Lightwave Technology 19, 1531–1542 (2001) [CrossRef]

4.

M. R. Feldman, S. C. Esener, C. C. Guest, and S. H. Lee, “Comparison between optical and electrical interconnects based on power and speed considerations,” Appl. Opt. 27, 1742–1751 (1988) [CrossRef] [PubMed]

5.

T. Happel, M. Franke, H. Nanai, and J. Schrage, “Demonstration of optical interconnection-and assembly technique for fully-embedded optical PCB at data rates of 10 G bps/ch,” in Proceeding of IEEE Electronics Systemintegration Technology Conference 1, 247–252 (2006) [CrossRef]

6.

M. H. Cho, S. H. Hwang, H. S. Cho, and H.-H. Park, “ High-coupling-efficiency optical interconnection using a 90°-bent fiber array connector in optical printed circuit boards,” IEEE Photo. Technol. Lett. 17, 690–692 (2005) [CrossRef]

7.

S. H. Hwang, M. H. Cho, S.-K. Kang, H.-H. Park, H. S. Cho, S.-H. Kim, K.-U. Shin, and S.-W. Ha, “Passively assembled optical interconnection system based on an optical printed circuit board,” IEEE Photo. Technol. Lett. 18, 652–654 (2006) [CrossRef]

8.

B. S. Rho, S.-K. Kang, H. S. Cho, H.-H. Park, S.-W. Ha, and B.-H Rhee, “PCB-compatible optical interconnection using 45° -ended connection rods and via-holed waveguides,” J. Lightwave Technol. 22, 2128–2134 (2004) [CrossRef]

9.

I.-K. Cho, K. B. Yoon, S. H. Ahn, M. Y. Jeong, H.-K. Sung, B. H. Lee, Y. U. Heo, and H.-H. Park, “Board-to-board optical interconnection system using optical slots,” IEEE Photo. Tech. Lett. 16, 1754–1757 (2004) [CrossRef]

10.

A. L. Glebov, J. Roman, M. G. Lee, and K. Yokouchi, “Optical interconnect modules with fully integrated reflector mirrors,” IEEE Photo. Technol. Lett. 17, 1540–1542 (2005) [CrossRef]

11.

B. Schwarz, M. Grüttner, and W. Röhle,“Beam attenuation measurement of hydrosols by means of a new measuring technique,” Meas. Sci. Technol. 1, 1102–1105 (1990) [CrossRef]

12.

M. Grabherr, R. Jäger, R. King, B. Schneider, and D. Wiedenmann, “Fabricating VCSELs in a high tech start-up,” in Proceedings of SPIE, VCSELs and Optical Interconnects 4942, 13–24 (2003)

13.

C.-K. Lin, S.-W. Ryu, and P. D. Dapkus, “High-performance wafer-bonded bottom-emitting 850–nm VCSEL’s on the undoped gap and sapphire substrates,” IEEE Photo. Technol. Lett. 11, 1542–1544 (1999) [CrossRef]

OCIS Codes
(130.3120) Integrated optics : Integrated optics devices
(220.1140) Optical design and fabrication : Alignment
(220.4610) Optical design and fabrication : Optical fabrication

ToC Category:
Integrated Optics

History
Original Manuscript: September 11, 2007
Revised Manuscript: October 31, 2007
Manuscript Accepted: October 31, 2007
Published: November 13, 2007

Citation
Do-Won Kim, Tae-Woo Lee, Mu Hee Cho, and Hyo-Hoon Park, "High-efficiency and stable optical transmitter using VCSEL-direct-bonded connector for optical interconnection," Opt. Express 15, 15767-15775 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-24-15767


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References

  1. J. Wu, J. Wu, J. Bao, and X. Wu, "Soft-lithography-based optical interconnection with high misalignment tolerance," Opt. Express 13, 6259-6267 (2005). [CrossRef] [PubMed]
  2. R. Lytel, H. L. Davidson, N. Nettliton, and T. Sze, "Optical interconnections within modern high-performance computing systems," in Proceeding of IEEE. 88, 758-763 (2000). [CrossRef]
  3. E. D. Kyriakis-Bitzaros, N. Haralabidis, M. Lagadas, A. Georgakilas, Y. Moisiadis, and G. Halkias, "Realistic end-to-end simulation of the optoelectronic links and comparison with electrical interconnections for system-on-chip applications," J. Lightwave Technol. 19, 1531-1542 (2001). [CrossRef]
  4. M. R. Feldman, S. C. Esener, C. C. Guest, and S. H. Lee, "Comparison between optical and electrical interconnects based on power and speed considerations," Appl. Opt. 27, 1742-1751 (1988). [CrossRef] [PubMed]
  5. T. Happel, M. Franke, H. Nanai, and J. Schrage, "Demonstration of optical interconnection-and assembly technique for fully-embedded optical PCB at data rates of 10 G bps/ch," in Proceeding of IEEE Electronics System integration Technology Conference 1, 247-252 (2006). [CrossRef]
  6. M. H. Cho, S. H. Hwang, H. S. Cho, and H.-H. Park, "High-coupling-efficiency optical interconnection using a 90o-bent fiber array connector in optical printed circuit boards," IEEE Photon. Technol. Lett. 17, 690-692 (2005). [CrossRef]
  7. S. H. Hwang, M. H. Cho, S.-K. Kang, H.-H. Park, H. S. Cho, S.-H. Kim, K.-U. Shin, and S.-W. Ha, "Passively assembled optical interconnection system based on an optical printed circuit board," IEEE Photon. Technol. Lett. 18, 652-654 (2006). [CrossRef]
  8. B. S. Rho, S.-K. Kang, H. S. Cho, H.-H. Park, S.-W. Ha, and B.-H Rhee, "PCB-compatible optical interconnection using 45° -ended connection rods and via-holed waveguides," J. Lightwave Technol. 22, 2128-2134 (2004). [CrossRef]
  9. I.-K. Cho, K. B. Yoon, S. H. Ahn, M. Y. Jeong, H.-K. Sung, B. H. Lee, Y. U. Heo, and H.-H. Park, "Board-to-board optical interconnection system using optical slots," IEEE Photon. Technol. Lett. 16, 1754-1757 (2004). [CrossRef]
  10. A. L. Glebov, J. Roman, M. G. Lee, and K. Yokouchi, "Optical interconnect modules with fully integrated reflector mirrors," IEEE Photon. Technol. Lett. 17, 1540-1542 (2005). [CrossRef]
  11. B. Schwarz, M. Grüttner, and W. Röhle,"Beam attenuation measurement of hydrosols by means of a new measuring technique," Meas. Sci. Technol. 1, 1102-1105 (1990). [CrossRef]
  12. M. Grabherr, R. Jäger, R. King, B. Schneider, and D. Wiedenmann, "Fabricating VCSELs in a high tech start-up," Proc. SPIE 4942, 13-24 (2003).
  13. C.-K. Lin, S.-W. Ryu, and P. D. Dapkus, "High-performance wafer-bonded bottom-emitting 850-nm VCSEL’s on the undoped gap and sapphire substrates," IEEE Photon. Technol. Lett. 11, 1542-1544 (1999). [CrossRef]

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