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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 2 — Jan. 27, 2014
  • pp: 1768–1783
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40 Gb/s optical subassembly module for a multi-channel bidirectional optical link

Jamshid Sangirov, Gwan-Chong Joo, Jae-Shik Choi, Do-Hoon Kim, Byueng-Su Yoo, Ikechi Augustine Ukaegbu, Nguyen T. H. Nga, Jong-Hun Kim, Tae-Woo Lee, Mu Hee Cho, and Hyo-Hoon Park  »View Author Affiliations


Optics Express, Vol. 22, Issue 2, pp. 1768-1783 (2014)
http://dx.doi.org/10.1364/OE.22.001768


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Abstract

A 40 Gb/s bidirectional optical link using four-channel optical subassembly (OSA) modules and two different wavelengths for the up- and down-link is demonstrated. Widely separated wavelengths of 850 nm and 1060 nm are used to reduce the optical crosstalk between the up- and down-link signals. Due to the integration capabilities of silicon, the OSA is implemented, all based on silicon: V-grooved silicon substrates to embed fibers and silicon optical benches (SiOBs) to mount optical components. The SiOBs are separately prepared for array chips of photodiodes (PDs), vertical-cavity surface-emitting lasers (VCSELs), and monitoring PDs, which are serially configured on an optical fiber array for direct coupling to the transmission fibers. The separation of the up- and down-link wavelengths is implemented using a wavelength-filtering 45° mirror which is formed in the fiber under the VCSEL. To guide the light signal to the PD another 45° mirror is formed at the end of the fiber. The fabricated bidirectional OSA module shows good performances with a clear eye-diagram and a BER of less than 10−12 at a data rate of 10 Gb/s for each of the channels with input powers of −8 dBm and −6.5 dBm for the up-link and the down-link, respectively. The measured inter-channel crosstalk of the bidirectional 40 Gb/s optical link is about −22.6 dB, while the full-duplex operation mode demonstrates negligible crosstalk between the up- and down-link.

© 2014 Optical Society of America

1. Introduction

Optical interconnections are utilized to meet the increased level of demand pertaining to internet data traffic and to overcome the limitations of electrical interconnects due to advantages such as a short signal delay, a light weight, lower power consumption, and immunity to electromagnetic interference [1

1. C. L. Schow, F. E. Doany, A. V. Rylyakov, B. G. Lee, C. V. Jahnes, Y. H. Kwark, C. W. Baks, D. M. Kuchta, and J. A. Kash, “A 24-channel, 300 Gb/s, 8.2 pJ/bit, full-duplex fiber-coupled optical transceiver module based on a single “holey” CMOS IC,” J. Lightwave Technol. 29(4), 542–553 (2011). [CrossRef]

3

3. J. Sangirov, I. A. Ukaegbu, T.-W. Lee, M.-H. Cho, and H.-H. Park, “Signal synchronization using a flicker reduction and denoising algorithm for video-signal optical interconnect,” ETRI Journal 34(1), 122–125 (2012). [CrossRef]

]. Optical interconnects improve the speed and quality of data transmissions in short-reach applications such as data processing units, optical storage applications and in data centers. The high-storage data processing units of data centers require high-speed data transmission for rack-to-rack, board-to-board and chip-to-chip interconnections, with a design of high-capacity and compact pluggable optical modules for bidirectional optical links. Thus, careful designs of the TRx modules and optical components are desirable to minimize the optical assembly costs while optimizing their performances.

2. Design of the bidirectional OSA module

2.1 Structure of the bidirectional OSA module

Figure 1
Fig. 1 The structure of the four-channel bidirectional OSA module: (a) top-view and (b) cross-sectional view.
shows the structure of the bidirectional OSA module designed in this work. It consists of a vertical-cavity surface-emitting laser (VCSEL), surface-receiving PD and M-PD components, a TRx IC board, and a fiber array. In the TRx IC board, multi-channel Tx and Rx IC chips are die-bonded onto a small printed circuit board (PCB) on which impedance-matched metal traces are formed for an electrical connection with the main board of the TRx module. VCSEL, PD and M-PD chips are flip-chip bonded onto individual SiOBs, as shown in Fig. 1. These VCSEL, PD and M-PD SiOBs are attached onto an optical fiber array embedded in a V-groove which is formed in a silicon substrate. In the fabrication of these OSA components, we used standard OM3 multi-mode fibers of which the core diameter is 50 μm. For the active components, we used commercially available Tx/Rx IC chips and VCSEL/PD arrays.

Figure 2
Fig. 2 Detailed structure of the SiOB used to package the VCSEL, PD, and M-PD array chips on the fiber array.
shows the detailed structure of the SiOB onto which the VCSEL, PD, and M-PD array chips are mounted. Each 1x4 array chip is mounted on a separate SiOB. In the SiOB, a square groove is formed to contain the chip and metal lines are formed on the bottom and side wall of the groove for an electrical connection with the chip. In the SiOB, optical via holes are also formed; the walls of the via holes utilize metal deposited layers to facilitate light transmission through the silicon substrate. The space between the bottom of the square groove and the PD/VCSEL chip is left as small as possible to reduce any optical crosstalk that could be induced by the divergence and scattering of light beams. When attaching the SiOB to the silicon substrate, alignment marks are formed with copper plating with a thickness of less 8 µm on the SiOB and the silicon substrate. The thickness of the solder ball is less than 5 µm. Thus, the total distance between the SiOB and the silicon substrate is less than 21 µm. When the fiber is buried in the V-groove formed on the silicon substrate, the distance between the fiber and the SiOB is about 21 µm. In the fabrication of the OSA module, we applied a passive alignment method with these alignment marks. Additionally, the via holes are filled with an index matching oil to reduce scattering at the fiber surface and to maintain the reflection effect of the 45°-mirrors. The diameter of the via holes is 70 µm, which is close to the diameter of the fiber core. The height of the via holes is 125 µm and the pitch of the via holes and the fibers is 250 µm. In order to place the via holes of the SiOB at the exact position above the mirrors or at the leakage windows of the fiber array, alignment marks are made on the silicon substrate and on the bottom side of the SiOB substrate.

For an efficient alignment of the fibers and the SiOBs we used the following procedure. Four fibers are placed on the V-grooved Si substrate and mounted using an epoxy. This mounted block with the fibers and the Si substrate is polished to make 45° surfaces on both sides. The 45° surfaces are mechanically polished with a deviation of ± 1° using a minute jig and a commercialized polishing machine [8

8. B. S. Rho, H. S. Cho, J.-Y. Eo, S.-K. Kang, H.-H. Park, Y. W. Kim, Y. S. Joe, and D. J. Yang, “New architecture of optical interconnection using 45°-ended connection rods in waveguide-embedded printed circuit boards,” Proc. SPIE 4997, 71–78 (2003). [CrossRef]

]. Wavelength-filtering layers are deposited on the polished surfaces of the block. The main fibers to couple to mirror-2 of this block are similarly polished mounting on another V-grooved Si substrate and attached to the block using an index-matching epoxy. The SiOBs are placed on the Si substrate using align marks made on the Si substrates and the SiOBs. In a ray trace simulation for the geometries of the 850 nm devices and SiOBs, the alignment tolerance to attain a total coupling loss within 3 dB on both the VCSEL and PD sides is about ± 10 μm in the lateral and longitudinal (along the fiber) directions from the central position of the mirrors. From our alignment procedure, the SiOBs could be placed within ± 3 μm misalignment in the lateral direction and within ± 5 μm in the longitudinal direction, which is expected to be in the range of the 3 dB tolerance for the 850 nm devices. In the rotational angle along the fiber axis, the 45° mirrors can be quite precisely located since the fiber array is already mounted on the Si substrate block and formed the mirrors together.

2.2 Signal transmission in the bidirectional OSA module

3. Implementation of the OSA module and components

The stability, the coupling loss and the crosstalk noise of the four-channel OSA module were measured for error-free and successful data transmission in the bidirectional optical link. Thermal shock tests were also done to evaluate the reliability of the device and to assess the parasitic heating effects with temperature changes. The optical crosstalk between the channels of the OSA module was measured to verify the presence of inter-channel isolation. Further, the coupling losses from the VCSEL to the fiber and from the fiber to the PD were measured to estimate the available power budget and power margin for the transmission of optical signals up to 100 m.

3.1 Wavelength separation of the bidirectional link and crosstalk

3.2 Reliability tests of optical components

The results of the reliability test for the VCSEL and PD are shown in Fig. 8
Fig. 8 Results of the reliability test during the thermal variation assessment. The VCSEL light power and PD responsivities are compared before and after thermal variation: (a) light power at 850 nm, (b) light power at 1060 nm, (c) responsivity at 850 nm, and (d) responsivity at 1060 nm.
. The reliability tests of the VCSEL light power and the responsivity of the PD for the two wavelengths of 850 nm and 1060 nm were assessed during thermal variation of 50 cycles in a range of 0 þC ~70 þC, following the Telcordia reliability assurance requirements (GR-468). After the thermal variation, the change of the VCSEL threshold current, Ith, was less than 0.8 mA. Before the thermal variation, the average output light power of the VCSEL arrays were 2 mW and 0.7 mW at wavelengths of 850 nm and 1060 nm, respectively, and the average responsivities of PD arrays were 0.35 A/W and 0.65 A/W at wavelengths of 850 nm and 1060 nm, respectively. After thermal variation, the change in these VCSEL output light power and PD responsivities are less than 10% changes, as shown in Figs. 8(a)-(d). However, these results indicate that the change of the VCSEL output light power is slightly more severe than the change of the PD responsivity due to the inherent thermal sensitivity of the VCSEL light power [12

12. J. A. Lott, V. A. Shchukin, N. N. Ledentsova, A. Stintz, F. Hopfer, A. Mutig, G. Fiol, D. Bimberg, S. A. Blokhin, L. Y. Karachinsky, I. I. Novikov, M. V. Maximov, N. D. Zakharov, and P. Werner, “20 Gbit/s error free transmission with ~850 nm GaAs-based vertical cavity surface emitting lasers (VCSELs) containing InAs-GaAs submonolayer quantum dot insertions,” Proc. SPIE 7211(14), 1–12 (2009).

, 13

13. A. Mutig, P. Mosera, J. A. Lott, P. Wolf, W. Hofmann, N. N. Ledentsov, and D. Bimberg, “High-speed 850 and 980 nm VCSELs for high-performance computing applications,” Proc. SPIE 7338(19), 1–7 (2011).

].

3.3 Optical link loss measurement

The measured coupling losses with and without M-PD shown in Table 1

Table 1. End-to-end Optical Coupling Loss of the OSA-based Bidirectional Optical Link

table-icon
View This Table
| View All Tables
are taken as the average of all four-channels. The optical coupling loss from the VCSEL to the fiber at a wavelength of 1060 nm is higher than the coupling loss at 850 nm. This result may be due to the higher full-width divergence angle of 33þ in the 1060 nm VCSEL compared to that divergence angle of 19þ in the 850 nm VCSEL. The aperture sizes of the PDs are 60 µm and 50 µm for wavelengths of 850 nm and 1060 nm, respectively, having only a small difference. Thus, it could explain the result of that the coupling losses from the fiber to PD show relatively small differences at 850 nm and 1060 nm.

3.4 Power budget of the optical link

PMM=(PTxPRx)(PCL+PFL).
(2)

4. Experimental results

4.1 Demonstration of the bidirectional optical link

To demonstrate the bidirectional optical link, frequency responses and eye-diagrams were measured using the evaluation boards shown in Fig. 11. An Anritsu MP1763B pulse pattern generator and an Agilent 86100A oscilloscope are used to feed input electrical signals and to measure output electrical signals to/from the evaluation boards. This setup measures the electrical-to-electrical responses received by the PD and receiver IC after light signal transmission from the VCSEL and driver IC. To measure the responses and eye-diagram separately for each channel, the signal is transmitted through one channel while the other channels are stood as turn-on states without the input of modulated signals. The eye-diagrams were measured using a pseudorandom bit stream of 231-1. Differential splitter-baluns (5320B-104) were utilized to convert single-ended signals to differential signals for the measurement of the BER and crosstalk. The BER and eye-diagrams were measured at 10 Gb/s/ch to demonstrate the 40 Gb/s bidirectional optical link.

4.2. Bidirectional optical signal transmission in full-duplex mode

5. Conclusion

Acknowledgments

References and links

1.

C. L. Schow, F. E. Doany, A. V. Rylyakov, B. G. Lee, C. V. Jahnes, Y. H. Kwark, C. W. Baks, D. M. Kuchta, and J. A. Kash, “A 24-channel, 300 Gb/s, 8.2 pJ/bit, full-duplex fiber-coupled optical transceiver module based on a single “holey” CMOS IC,” J. Lightwave Technol. 29(4), 542–553 (2011). [CrossRef]

2.

J.-Y. Park, H.-S. Lee, S.-S. Lee, and Y.-S. Son, “Passively aligned transmit optical subassembly module based on a WDM incorporating VCSELs,” IEEE Photon. Technol. Lett. 22(24), 1790–1792 (2010). [CrossRef]

3.

J. Sangirov, I. A. Ukaegbu, T.-W. Lee, M.-H. Cho, and H.-H. Park, “Signal synchronization using a flicker reduction and denoising algorithm for video-signal optical interconnect,” ETRI Journal 34(1), 122–125 (2012). [CrossRef]

4.

J. D. Ingham, R. V. Penty, and I. H. White, “Bidirectional multimode-fiber communication links using dual-purpose vertical-cavity devices,” J. Lightwave Technol. 24(3), 1283–1294 (2006). [CrossRef]

5.

N. T. H. Nguyen, J. Sangirov, D.-M. Im, M. H. Cho, T.-W. Lee, and H.-H. Park, “Bidirectional optical transceiver integrated with an envelope detector for automatically controlling the direction of transmission,” Proc. ECTC, 2098–2100 (2009).

6.

G.-C. Joo, S.-H. Lee, K.-S. Park, J.-S. Choi, N. Hwang, and M.-K. Song, “A novel bidirectional optical coupling module for subscribers,” IEEE Trans. Adv. Packag. 23(4), 681–685 (2000). [CrossRef]

7.

Y. S. Heo, H.-J. Park, H. S. Kang, and K.-S. Lim, “1/10 Gb/s single transistor-outline-CAN bidirectional optical subassembly for a passive optical network,” Opt. Eng. Lett. 52(1), 010501 (2013). [CrossRef]

8.

B. S. Rho, H. S. Cho, J.-Y. Eo, S.-K. Kang, H.-H. Park, Y. W. Kim, Y. S. Joe, and D. J. Yang, “New architecture of optical interconnection using 45°-ended connection rods in waveguide-embedded printed circuit boards,” Proc. SPIE 4997, 71–78 (2003). [CrossRef]

9.

Y. Nekado and M. Iwase, “1.3-μm range vertical-cavity surface-emitting laser (VCSEL) module,” Furukawa Review 27, 72–78 (2005).

10.

K.-S. Lim, J. J. Lee, S. Lee, S. Yoon, C. H. Yu, I.-B. Sohn, and H. S. Kang, “A novel low-cost fiber in-line-type bidirectional optical subassembly,” IEEE Photon. Technol. Lett. 19(16), 1233–1235 (2007). [CrossRef]

11.

D. Paladino, A. Iadicicco, S. Campopiano, and A. Cusano, “Not-lithographic fabrication of micro-structured fiber Bragg gratings evanescent wave sensors,” Opt. Express 17(2), 1042–1054 (2009). [CrossRef] [PubMed]

12.

J. A. Lott, V. A. Shchukin, N. N. Ledentsova, A. Stintz, F. Hopfer, A. Mutig, G. Fiol, D. Bimberg, S. A. Blokhin, L. Y. Karachinsky, I. I. Novikov, M. V. Maximov, N. D. Zakharov, and P. Werner, “20 Gbit/s error free transmission with ~850 nm GaAs-based vertical cavity surface emitting lasers (VCSELs) containing InAs-GaAs submonolayer quantum dot insertions,” Proc. SPIE 7211(14), 1–12 (2009).

13.

A. Mutig, P. Mosera, J. A. Lott, P. Wolf, W. Hofmann, N. N. Ledentsov, and D. Bimberg, “High-speed 850 and 980 nm VCSELs for high-performance computing applications,” Proc. SPIE 7338(19), 1–7 (2011).

14.

M. Hostut, A. Kilic, S. Sakiroglu, Y. Ergun, and I. Sokmen, “Voltage tunable dual-band quantum-well infrared photodetector for third-generation thermal imaging,” IEEE Photon. Technol. Lett. 23(19), 1370–1372 (2011). [CrossRef]

15.

L. Fu, Q. Li, P. Kuffner, G. Jolley, P. Gareso, H. H. Tan, and C. Jagadish, “Two-color InGaAs/GaAs quantum dot infrared photodetectors by selective area interdiffusion,” Appl. Phys. Lett. 93(1), 013504 (2008).

16.

MaxCap-OM3 - 10 Gb/s multimode optical fiber, high-speed laser-launch multimode fiber (OM3), http://communications.draka.com/sites/usa/Pages/MultiModeFibers_MaxCap.aspx (2013).

17.

A. Aguayo, “Advances in high frequency printed circuit board materials,” Microwave Eng. Europe, December 2009, 11-14 (2009).

OCIS Codes
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems
(200.4650) Optics in computing : Optical interconnects
(230.0230) Optical devices : Optical devices
(080.4035) Geometric optics : Mirror system design
(130.7408) Integrated optics : Wavelength filtering devices

ToC Category:
Optical Communications

History
Original Manuscript: November 8, 2013
Revised Manuscript: December 21, 2013
Manuscript Accepted: January 2, 2014
Published: January 17, 2014

Citation
Jamshid Sangirov, Gwan-Chong Joo, Jae-Shik Choi, Do-Hoon Kim, Byueng-Su Yoo, Ikechi Augustine Ukaegbu, Nguyen T. H. Nga, Jong-Hun Kim, Tae-Woo Lee, Mu Hee Cho, and Hyo-Hoon Park, "40 Gb/s optical subassembly module for a multi-channel bidirectional optical link," Opt. Express 22, 1768-1783 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-2-1768


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References

  1. C. L. Schow, F. E. Doany, A. V. Rylyakov, B. G. Lee, C. V. Jahnes, Y. H. Kwark, C. W. Baks, D. M. Kuchta, J. A. Kash, “A 24-channel, 300 Gb/s, 8.2 pJ/bit, full-duplex fiber-coupled optical transceiver module based on a single “holey” CMOS IC,” J. Lightwave Technol. 29(4), 542–553 (2011). [CrossRef]
  2. J.-Y. Park, H.-S. Lee, S.-S. Lee, Y.-S. Son, “Passively aligned transmit optical subassembly module based on a WDM incorporating VCSELs,” IEEE Photon. Technol. Lett. 22(24), 1790–1792 (2010). [CrossRef]
  3. J. Sangirov, I. A. Ukaegbu, T.-W. Lee, M.-H. Cho, H.-H. Park, “Signal synchronization using a flicker reduction and denoising algorithm for video-signal optical interconnect,” ETRI Journal 34(1), 122–125 (2012). [CrossRef]
  4. J. D. Ingham, R. V. Penty, I. H. White, “Bidirectional multimode-fiber communication links using dual-purpose vertical-cavity devices,” J. Lightwave Technol. 24(3), 1283–1294 (2006). [CrossRef]
  5. N. T. H. Nguyen, J. Sangirov, D.-M. Im, M. H. Cho, T.-W. Lee, H.-H. Park, “Bidirectional optical transceiver integrated with an envelope detector for automatically controlling the direction of transmission,” Proc. ECTC, 2098–2100 (2009).
  6. G.-C. Joo, S.-H. Lee, K.-S. Park, J.-S. Choi, N. Hwang, M.-K. Song, “A novel bidirectional optical coupling module for subscribers,” IEEE Trans. Adv. Packag. 23(4), 681–685 (2000). [CrossRef]
  7. Y. S. Heo, H.-J. Park, H. S. Kang, K.-S. Lim, “1/10 Gb/s single transistor-outline-CAN bidirectional optical subassembly for a passive optical network,” Opt. Eng. Lett. 52(1), 010501 (2013). [CrossRef]
  8. B. S. Rho, H. S. Cho, J.-Y. Eo, S.-K. Kang, H.-H. Park, Y. W. Kim, Y. S. Joe, D. J. Yang, “New architecture of optical interconnection using 45°-ended connection rods in waveguide-embedded printed circuit boards,” Proc. SPIE 4997, 71–78 (2003). [CrossRef]
  9. Y. Nekado, M. Iwase, “1.3-μm range vertical-cavity surface-emitting laser (VCSEL) module,” Furukawa Review 27, 72–78 (2005).
  10. K.-S. Lim, J. J. Lee, S. Lee, S. Yoon, C. H. Yu, I.-B. Sohn, H. S. Kang, “A novel low-cost fiber in-line-type bidirectional optical subassembly,” IEEE Photon. Technol. Lett. 19(16), 1233–1235 (2007). [CrossRef]
  11. D. Paladino, A. Iadicicco, S. Campopiano, A. Cusano, “Not-lithographic fabrication of micro-structured fiber Bragg gratings evanescent wave sensors,” Opt. Express 17(2), 1042–1054 (2009). [CrossRef] [PubMed]
  12. J. A. Lott, V. A. Shchukin, N. N. Ledentsova, A. Stintz, F. Hopfer, A. Mutig, G. Fiol, D. Bimberg, S. A. Blokhin, L. Y. Karachinsky, I. I. Novikov, M. V. Maximov, N. D. Zakharov, P. Werner, “20 Gbit/s error free transmission with ~850 nm GaAs-based vertical cavity surface emitting lasers (VCSELs) containing InAs-GaAs submonolayer quantum dot insertions,” Proc. SPIE 7211(14), 1–12 (2009).
  13. A. Mutig, P. Mosera, J. A. Lott, P. Wolf, W. Hofmann, N. N. Ledentsov, D. Bimberg, “High-speed 850 and 980 nm VCSELs for high-performance computing applications,” Proc. SPIE 7338(19), 1–7 (2011).
  14. M. Hostut, A. Kilic, S. Sakiroglu, Y. Ergun, I. Sokmen, “Voltage tunable dual-band quantum-well infrared photodetector for third-generation thermal imaging,” IEEE Photon. Technol. Lett. 23(19), 1370–1372 (2011). [CrossRef]
  15. L. Fu, Q. Li, P. Kuffner, G. Jolley, P. Gareso, H. H. Tan, C. Jagadish, “Two-color InGaAs/GaAs quantum dot infrared photodetectors by selective area interdiffusion,” Appl. Phys. Lett. 93(1), 013504 (2008).
  16. MaxCap-OM3 - 10 Gb/s multimode optical fiber, high-speed laser-launch multimode fiber (OM3), http://communications.draka.com/sites/usa/Pages/MultiModeFibers_MaxCap.aspx (2013).
  17. A. Aguayo, “Advances in high frequency printed circuit board materials,” Microwave Eng. Europe, December 2009, 11-14 (2009).

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