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s data transmission with TO-packaged multimode GaAs VCSELs over 1 m long polymer waveguides for optical backplane applications
1 June 2002
Optics Communications 206 (2002) 309–312 www.elsevier.com/locate/optcom
10 Gb/s data transmission with TO-packaged multimode GaAs VCSELs over 1 m long polymer waveguides for optical backplane applications F. Mederer a
a,*
, R. Michalzik a, J. Guttmann b, H.-P. Huber b, B. Lunitz b, J. Moisel b, D. Wiedenmann c
Optoelectronics Department, University of Ulm, Albert-Einstein-Allee 45, D-89069 Ulm, Germany b DaimlerChrysler Research Center, D-89069 Ulm, Germany c U-L-M photonics GmbH, D-89069 Ulm, Germany Received 17 December 2001; received in revised form 21 March 2002; accepted 3 April 2002
Abstract TO-packaged GaAs vertical-cavity surface-emitting lasers (VCSELs) are being investigated for high-bit-rate data transmission over 103 cm of a 250 200 lm2 core size polymer-based optical waveguide to be employed in optical backplanes. The bit-error rates for 5 and 10 Gb/s transmission over the waveguides incorporating 45° deflection mirrors are better than 10 12 . For the first time, the measured bandwidth-length-product for a polymer-based optical backplane exceeds 10 GHz m. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.55.S; 42.55.P; 42.79.S; 42.82.E Keywords: Vertical-cavity surface-emitting laser; Optical backplane; Optical communication; GaAs
1. Introduction The demand for high data throughput capacity caused by increasing operation speed of computers can be satisfied using optical interconnects on printed-circuit boards and backplanes [1,2]. Optical interconnects overcome the problems of electrical strip-lines like electromagnetic interference * Corresponding author. Tel.: +49-731-50-26037; fax: +49731-50-26049. E-mail address: [email protected] (F. Mederer). URL: http://www-opto.e-technik.uni-ulm.de.
sensitivity, short link length and high crosstalk at high data rates [3]. We can classify intrasystem optical interconnects [4] by the transmission distance from the longest to the shortest as rackto-rack, board-to-board, inter-multi-chip-module (MCM), chip-to-chip, and intra-chip, where optical backplanes fall into the board-to-board category. Optical backplane concepts can be distinguished between discrete fiber cabled [5], freespace straight path [6], free-space zig-zag path [7], direct interboard [8] , and integrated waveguides backplanes [2,9]. The optical backplane concept discussed in [2] fulfills the needed requirements and adds a plug-in-capability of stacked printed-circuit
0030-4018/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 2 ) 0 1 4 2 0 - 7
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F. Mederer et al. / Optics Communications 206 (2002) 309–312
boards to be used in, e.g., airplane racks. Using large core multimode waveguides with integrated mirrors and lenses, a high alignment tolerance in excess of 500 lm for 1 dB loss has been achieved which helps to reduce cost and increase system lifetime. For low-cost systems obviously transmitters must also be cheap. Vertical-cavity surfaceemitting lasers (VCSELs) are ideal transmitters for such short-distance optical interconnects. GaAsbased VCSELs emitting at a wavelength around 850 nm show excellent modulation behaviour, low threshold currents, high wallplug efficiency [10], operation over a wide temperature range [11] and the possibility of heterogeneous integration with electronics and micro-optics [12].
2. Backplane Fig. 1 shows a photograph of a polymer-based optical waveguide sample used in the DaimlerChrysler’s Blackplanee concept. To save space the waveguide has a spiral shape with a total length of 103 cm. The multimode waveguide features a core size of 250 200 lm2 with a numerical aperture of 0.3. In the backplane, 45° gold mirrors are integrated for light deflection. The total measured attenuation from the inner to the
Fig. 1. Sample of polymer waveguides with 250 200 lm2 cross-section and 103 cm length. Light in- and out-coupling is performed through integrated 45° mirrors.
outer end of the spiral is as low as 5 dB at a wavelength of 850 nm. The transmission bandwidth of the 103 cm long waveguide exceeded the small-signal measurement setup limit of 10 GHz and therefore we suppose a bitrate-length-product of more than 10 Gb=s m.
3. VCSELs Top-emitting selectively oxidised GaAs based 852 nm emission wavelength VCSELs fabricated by solid source molecular beam epitaxy are used for transmission experiments. An AlAs layer included in the top mirror is selectively oxidised to define a current aperture of 12 lm. The laser is mounted on a TO-46 socket capable for data rates up to 10 Gb/s and is wire bonded to a bondpad on a polyimide passivation. Output characteristics of the VCSEL are given in Fig. 2. Threshold current and threshold voltage are 1.8 mA and 1.7 V, respectively. The maximum output power at a driving current of 17 mA is 5.8 mW. As seen in the inset of Fig. 2, the output spectra for a CW driving current of 10 mA and under modulation with a 10 Gb/s 27 1 pseudo random bit sequence (PRBS) signal are highly multimode and centered around 852 nm. In both cases the spectral width is around 2 nm. The measured 3 dB bandwidth is 7.5 GHz,
Fig. 2. Driving characteristics of a selectively oxidised GaAs VCSEL with 12 lm active diameter.
F. Mederer et al. / Optics Communications 206 (2002) 309–312
making the device capable for 10 Gb/s data transmission.
311
ceived optical power of )13.0 dBm and a power penalty for waveguide transmission of 0.5 dB was observed.
4. Data transmission experiments 5. Conclusion For transmission experiments, TO-packaged VCSELs are mounted on a SMA socket and biased with 10 mA current. A 27 1 PRBS nonreturn-to-zero signal with a modulation voltage of 0.9 V is added. This leads to a measured on–offratio of 6.5 dB at 10 Gb/s data rate. The light was then collimated and focussed on the inner mirror. After transmission over the waveguide the light exiting the outer mirror was again collimated and focussed onto a 62:5 lm multimode fiber-pigtailed InGaAs-PIN photodetector with a modulation bandwidth of 8 GHz. The eye diagram at a data rate of 10 Gb/s after transmission over the waveguide at a bit-error rate (BER) of 10 12 is shown in the inset of Fig. 3. The eye is wide open and shows no relaxation oscillations. The results of the BER measurements at 5 and 10 Gb/s are summarised also in Fig. 3. For back-to-back (BTB) transmission of 5 Gb/s, a minimum received optical power of )17.3 dBm is necessary. A power penalty of only 0.35 dB for transmission over the waveguide was measured. For 10 Gb/s and BTB, we need a minimum re-
We have successfully demonstrated 5 and 10 Gb/s PRBS data transmission over 1 m long multimode polymer optical waveguides with integrated beam deflection. The total waveguide attenuation including two 45° mirrors is as low as 5 dB. We have shown that TO-packaged multimode VCSELs with 3 dB bandwidths of 7.5 GHz are capable for data transmission up to 10 Gb/s. Due to a bitrate-length-product of more than 10 Gb=s m the power penalties for transmissions of 5 and 10 Gb/s are as low as 0.35 and 0.5 dB, respectively. Measured behaviours of VCSELs and waveguides used in the optical backplane concept prove the advantages of multi-Gb/s data transmission over even more than 1 m multimode optical waveguides.
Acknowledgements This work was supported by the German Ministry of Education and Research (BMBF) under contract No. 01 BP 804/4.
References
Fig. 3. Eye diagram at 10 Gb/s PRBS at a BER of 10 12 recorded after transmission over the 103 cm long polymer waveguide (WG) and bit-error rate measurements at 5 and 10 Gb/s with word lengths of 27 1.
[1] E. Griese, A. Himmler, K. Klimke, A. Koske, J.-R. Kropp, S. Lehmacher, A. Neyer, W. S€ ullau, in: M.R. Taghizadeh, H. Thienpont, G.E. Jabbour (Eds.), Micro- and Nanooptics for Optical Interconnection and Information Processing, Proc. SPIE, 4455, 2001, p. 243. [2] J. Moisel, J. Guttmann, H.-P. Huber, O. Krumpholz, M. Rode, R. Bogenberger, K.-P. Kuhn, Opt. Eng. 39 (2000) 673. [3] D.A.B. Miller, Int. J. Optoelectron. 11 (1997) 155. [4] R. Michalzik, in: C. DeCusatis (Ed.), Fiber Optic Data Communication: Technological Trends and Advances, Academic Press, San Diego, 2002, p. 216. [5] G.Y. Zhou, Y. Zhang, W. Liu, Proc. IEEE 88 (2000) 856. [6] T.H. Szymanski, H.S. Hinton, Appl. Opt. 35 (1996) 1253. [7] G. Kim, X. Ham, R.T. Chen, IEEE J. Lightwave Technol. 18 (2000) 1477.
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F. Mederer et al. / Optics Communications 206 (2002) 309–312
[8] K. Hirabayashi, T. Yamamoto, S. Hino, Opt. Eng. 37 (1998) 1332. [9] Y.S. Liu, IEEE Circuits Dev. 14 (1998) 23. [10] K. Iga, IEEE J. Select. Topics Quantum Electron. 6 (2000) 1201.
[11] F.H. Peters, M.H. MacDougal, IEEE Photon. Technol. Lett. 13 (2001) 645. [12] Y. Liu, in: 51st Electron. Comp. and Technol. Conf., ECTC, Orlando, FL, USA, 2001, p. 864.
Optics Communications 206 (2002) 309–312 www.elsevier.com/locate/optcom
10 Gb/s data transmission with TO-packaged multimode GaAs VCSELs over 1 m long polymer waveguides for optical backplane applications F. Mederer a
a,*
, R. Michalzik a, J. Guttmann b, H.-P. Huber b, B. Lunitz b, J. Moisel b, D. Wiedenmann c
Optoelectronics Department, University of Ulm, Albert-Einstein-Allee 45, D-89069 Ulm, Germany b DaimlerChrysler Research Center, D-89069 Ulm, Germany c U-L-M photonics GmbH, D-89069 Ulm, Germany Received 17 December 2001; received in revised form 21 March 2002; accepted 3 April 2002
Abstract TO-packaged GaAs vertical-cavity surface-emitting lasers (VCSELs) are being investigated for high-bit-rate data transmission over 103 cm of a 250 200 lm2 core size polymer-based optical waveguide to be employed in optical backplanes. The bit-error rates for 5 and 10 Gb/s transmission over the waveguides incorporating 45° deflection mirrors are better than 10 12 . For the first time, the measured bandwidth-length-product for a polymer-based optical backplane exceeds 10 GHz m. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.55.S; 42.55.P; 42.79.S; 42.82.E Keywords: Vertical-cavity surface-emitting laser; Optical backplane; Optical communication; GaAs
1. Introduction The demand for high data throughput capacity caused by increasing operation speed of computers can be satisfied using optical interconnects on printed-circuit boards and backplanes [1,2]. Optical interconnects overcome the problems of electrical strip-lines like electromagnetic interference * Corresponding author. Tel.: +49-731-50-26037; fax: +49731-50-26049. E-mail address: [email protected] (F. Mederer). URL: http://www-opto.e-technik.uni-ulm.de.
sensitivity, short link length and high crosstalk at high data rates [3]. We can classify intrasystem optical interconnects [4] by the transmission distance from the longest to the shortest as rackto-rack, board-to-board, inter-multi-chip-module (MCM), chip-to-chip, and intra-chip, where optical backplanes fall into the board-to-board category. Optical backplane concepts can be distinguished between discrete fiber cabled [5], freespace straight path [6], free-space zig-zag path [7], direct interboard [8] , and integrated waveguides backplanes [2,9]. The optical backplane concept discussed in [2] fulfills the needed requirements and adds a plug-in-capability of stacked printed-circuit
0030-4018/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 2 ) 0 1 4 2 0 - 7
310
F. Mederer et al. / Optics Communications 206 (2002) 309–312
boards to be used in, e.g., airplane racks. Using large core multimode waveguides with integrated mirrors and lenses, a high alignment tolerance in excess of 500 lm for 1 dB loss has been achieved which helps to reduce cost and increase system lifetime. For low-cost systems obviously transmitters must also be cheap. Vertical-cavity surfaceemitting lasers (VCSELs) are ideal transmitters for such short-distance optical interconnects. GaAsbased VCSELs emitting at a wavelength around 850 nm show excellent modulation behaviour, low threshold currents, high wallplug efficiency [10], operation over a wide temperature range [11] and the possibility of heterogeneous integration with electronics and micro-optics [12].
2. Backplane Fig. 1 shows a photograph of a polymer-based optical waveguide sample used in the DaimlerChrysler’s Blackplanee concept. To save space the waveguide has a spiral shape with a total length of 103 cm. The multimode waveguide features a core size of 250 200 lm2 with a numerical aperture of 0.3. In the backplane, 45° gold mirrors are integrated for light deflection. The total measured attenuation from the inner to the
Fig. 1. Sample of polymer waveguides with 250 200 lm2 cross-section and 103 cm length. Light in- and out-coupling is performed through integrated 45° mirrors.
outer end of the spiral is as low as 5 dB at a wavelength of 850 nm. The transmission bandwidth of the 103 cm long waveguide exceeded the small-signal measurement setup limit of 10 GHz and therefore we suppose a bitrate-length-product of more than 10 Gb=s m.
3. VCSELs Top-emitting selectively oxidised GaAs based 852 nm emission wavelength VCSELs fabricated by solid source molecular beam epitaxy are used for transmission experiments. An AlAs layer included in the top mirror is selectively oxidised to define a current aperture of 12 lm. The laser is mounted on a TO-46 socket capable for data rates up to 10 Gb/s and is wire bonded to a bondpad on a polyimide passivation. Output characteristics of the VCSEL are given in Fig. 2. Threshold current and threshold voltage are 1.8 mA and 1.7 V, respectively. The maximum output power at a driving current of 17 mA is 5.8 mW. As seen in the inset of Fig. 2, the output spectra for a CW driving current of 10 mA and under modulation with a 10 Gb/s 27 1 pseudo random bit sequence (PRBS) signal are highly multimode and centered around 852 nm. In both cases the spectral width is around 2 nm. The measured 3 dB bandwidth is 7.5 GHz,
Fig. 2. Driving characteristics of a selectively oxidised GaAs VCSEL with 12 lm active diameter.
F. Mederer et al. / Optics Communications 206 (2002) 309–312
making the device capable for 10 Gb/s data transmission.
311
ceived optical power of )13.0 dBm and a power penalty for waveguide transmission of 0.5 dB was observed.
4. Data transmission experiments 5. Conclusion For transmission experiments, TO-packaged VCSELs are mounted on a SMA socket and biased with 10 mA current. A 27 1 PRBS nonreturn-to-zero signal with a modulation voltage of 0.9 V is added. This leads to a measured on–offratio of 6.5 dB at 10 Gb/s data rate. The light was then collimated and focussed on the inner mirror. After transmission over the waveguide the light exiting the outer mirror was again collimated and focussed onto a 62:5 lm multimode fiber-pigtailed InGaAs-PIN photodetector with a modulation bandwidth of 8 GHz. The eye diagram at a data rate of 10 Gb/s after transmission over the waveguide at a bit-error rate (BER) of 10 12 is shown in the inset of Fig. 3. The eye is wide open and shows no relaxation oscillations. The results of the BER measurements at 5 and 10 Gb/s are summarised also in Fig. 3. For back-to-back (BTB) transmission of 5 Gb/s, a minimum received optical power of )17.3 dBm is necessary. A power penalty of only 0.35 dB for transmission over the waveguide was measured. For 10 Gb/s and BTB, we need a minimum re-
We have successfully demonstrated 5 and 10 Gb/s PRBS data transmission over 1 m long multimode polymer optical waveguides with integrated beam deflection. The total waveguide attenuation including two 45° mirrors is as low as 5 dB. We have shown that TO-packaged multimode VCSELs with 3 dB bandwidths of 7.5 GHz are capable for data transmission up to 10 Gb/s. Due to a bitrate-length-product of more than 10 Gb=s m the power penalties for transmissions of 5 and 10 Gb/s are as low as 0.35 and 0.5 dB, respectively. Measured behaviours of VCSELs and waveguides used in the optical backplane concept prove the advantages of multi-Gb/s data transmission over even more than 1 m multimode optical waveguides.
Acknowledgements This work was supported by the German Ministry of Education and Research (BMBF) under contract No. 01 BP 804/4.
References
Fig. 3. Eye diagram at 10 Gb/s PRBS at a BER of 10 12 recorded after transmission over the 103 cm long polymer waveguide (WG) and bit-error rate measurements at 5 and 10 Gb/s with word lengths of 27 1.
[1] E. Griese, A. Himmler, K. Klimke, A. Koske, J.-R. Kropp, S. Lehmacher, A. Neyer, W. S€ ullau, in: M.R. Taghizadeh, H. Thienpont, G.E. Jabbour (Eds.), Micro- and Nanooptics for Optical Interconnection and Information Processing, Proc. SPIE, 4455, 2001, p. 243. [2] J. Moisel, J. Guttmann, H.-P. Huber, O. Krumpholz, M. Rode, R. Bogenberger, K.-P. Kuhn, Opt. Eng. 39 (2000) 673. [3] D.A.B. Miller, Int. J. Optoelectron. 11 (1997) 155. [4] R. Michalzik, in: C. DeCusatis (Ed.), Fiber Optic Data Communication: Technological Trends and Advances, Academic Press, San Diego, 2002, p. 216. [5] G.Y. Zhou, Y. Zhang, W. Liu, Proc. IEEE 88 (2000) 856. [6] T.H. Szymanski, H.S. Hinton, Appl. Opt. 35 (1996) 1253. [7] G. Kim, X. Ham, R.T. Chen, IEEE J. Lightwave Technol. 18 (2000) 1477.
312
F. Mederer et al. / Optics Communications 206 (2002) 309–312
[8] K. Hirabayashi, T. Yamamoto, S. Hino, Opt. Eng. 37 (1998) 1332. [9] Y.S. Liu, IEEE Circuits Dev. 14 (1998) 23. [10] K. Iga, IEEE J. Select. Topics Quantum Electron. 6 (2000) 1201.
[11] F.H. Peters, M.H. MacDougal, IEEE Photon. Technol. Lett. 13 (2001) 645. [12] Y. Liu, in: 51st Electron. Comp. and Technol. Conf., ECTC, Orlando, FL, USA, 2001, p. 864.