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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 25 — Dec. 11, 2006
  • pp: 12028–12038
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PAM-4 Signaling over VCSELs with 0.13µm CMOS Chip Technology

J.E. Cunningham, D. Beckman, Xuezhe Zheng, Dawei Huang, T. Sze, and A. V. Krishnamoorthy  »View Author Affiliations


Optics Express, Vol. 14, Issue 25, pp. 12028-12038 (2006)
http://dx.doi.org/10.1364/OE.14.012028


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Abstract

We present results for VCSEL based links operating PAM-4 signaling using a commercial 0.13µm CMOS technology. We perform a complete link analysis of the Bit Error Rate, Q factor, random and deterministic jitter by measuring waterfall curves versus margins in time and amplitude. We demonstrate that VCSEL based PAM–4 can match or even improve performance over binary signaling under conditions of a bandwidth limited, 100meter multi-mode optical link at 5Gbps. We present the first sensitivity measurements for optical PAM-4 and compare it with binary signaling. Measured benefits are reconciled with information theory predictions.

© 2006 Optical Society of America

1. Introduction

2. Experimental setup

Fig. 1. (a) and (b). Left, Fig. 1(a) experimental setup. Right, Fig. 1(b). L-I-V characteristic measured for the Emcore 10Gbps VCSEL used in experiments described in the text. The inset shows the approximate mapping of the PAM-4 signal levels onto this VCSEL. The horizontal dotted line is the approximate bias voltage applied to the VCSEL. The vertical dotted lines shows the approximate voltage where the PAM–4 swing maps onto the I-V VCSEL characteristic. The dotted sloping line represents a linear output power versus current relationship for this VCSEL.
Fig. 2. Small signal response from a 2.5 and 10Gbps VCSEL. The bias current for the 2.5Gbps VCSEL was 14 mA and the peak at 8GHz is from the resonant frequency of the VCSEL. The bias current for the 10Gbps VCSEL was 6 mA.

An example L-I-V characteristic for a 10Gbps VCSEL is shown in Fig. 1(b) and illustrates how the PAM-4 signals map onto the VCSEL. The characteristic response of the Emcore 2.5Gbps VCSEL, although different, qualitatively aligns to the PAM-4 levels much as Fig. 1(b) depicts. The threshold current in Fig.1 (b) is about 1.1 mA and deviation from a perfectly linear L-I response sets in above 7mA. This nonlinear VCSEL response could arise from thermal rollover or even mode coupling into the fiber. Effort was taken to minimize the penetration into this nonlinear region of the VCSEL’s response. The voltage characteristics in Fig. 1(b) are typical for this model (aperture size) of VCSEL which further can be easily biased for use with the CML output driver from the CMOS chip. The upper level PAM-4 transition as shown in Fig. 1(b), the 11 transition, most likely incurs mild degradation from the non-linearity of the VCSEL. We expect the other PAM transitions to be in the strictly linear regimen of the VCSEL.

Fig. 3. (a) and (b). Top (a), BER at 4.9Gbps versus amplitude margins comparing PAM-4 and bottom (b), binary signaling when detected by the eye tool on the CMOS chip. Both eye patterns is shown for each case, again taken with a scope tool included on chip.

3. Results

Fig. 4. (a) and (b). (a) BER at 4.9Gbps versus timing margins comparing PAM-4 and (b) binary signaling when detected by the eye tool on the CMOS chip.

Fig. 5. BER versus attenuated power for sensitivity comparing PAM-4 and binary using the 10Gbps VCSEL.

penalty=(M1)(ln(M))12
(1)

The numerator accounts for reducing the eye amplitude by three in PAM-4 where as the denominator factors in the longer integration time the receiver detects the signal in multilevel signaling [11

11. G. P. Agrawal, “Fiber Optics Communication Systems,” 2nd ed., John Wiley and Sons, p. 180–182, (1997).

]. One reason the optical sensitivity penalty in our experiment may exceed theory associates with non-linearities in VCSEL output that acts to compress the PAM levels. Further, our chip BER algorithm deploys the worst of the three eyes in accomplishing the error detection analysis. The level of compression is directly observed in eye picture of Fig. 3 for the PAM-4 case as is signified by the reduction in level spacing for the lower of the three eyes. This eye picture is inverted in sense as the lower level coincides with the 11 PAM-4 level shown in Fig. 1(b). Compression penalties for optical PAM-M have been estimated in reference [6

6. David Cunningham, “Multilevel Modulation for 10GbE:Link and Component Specification,” see http://grouper.ieee.org/groups/802/3/10G_study/public/july99/cunningham_1_0799.pdf.

].

Fig. 6. An 8 Gbps PAM-4 optical eye using the 10Gbps VCSEL.

In the case of the 2.5Gbps transmitter at 5 Gbps experiments our PAM-4 measurements resulted in a net 3-4 dB advantage over binary signaling. Here, the PAM-4 approach not only overcame the SNR penalty of 9.5dB expected from theory (see eqn. 2) but also a significant bandwidth roll off of the VCSEL that overly degraded binary signaling. Because true PAM-4 signaling is performed using half the bandwidth deployed under binary signaling then a significant advantage should be realized in our experiment. Judging from the small signal results in Fig. 2 the advantage that PAM-4 has over binary is distinctly obvious for the 2.5Gbps transmitter case. In fact, based on an analysis of Fig. 3, one may expect the net gain for PAM-4 with the 2.5 Gbps transmitter to be far greater than that extracted from our margin measurement of Fig. 3 (i.e., 3-4dB). Again, our PAM-4 chip measures BER with the worst of the compressed eye and hence the PAM-4 advantage over binary may fall short of the full theoretical expectation. Furthermore our chip’s PAM-4 algorithm eliminates transitions spanning more than two levels and hence has a reduced multilevel functionality.

Commensurate with the 3-4 dB PAM-4 gain based on the amplitude margin analysis of Fig. 3 we also observe a significant jitter advantage in Fig. 4. One might anticipate in advance that the jitter benefit is entirely a deterministic jitter improvement for multilevel signaling since pattern dependant jitter always accompanies bandwidth limited transport (impairing the 5Gbps binary in our case). It would further be tempting to equate the degradation in jitter metric with an amplitude penalty akin to jitter penalty treatments found in literature [12

12. P. Voois, N. Swenson, and T. Lindsay, “Extending the Ethernet Link Model for EDC and Modulation Format,” http://grouper.ieee.org/groups/802/3/10GMMFSG/public/mar04/voois_1_0304.pdf

]. One notable observation of Fig. 4 is that PAM-4 random jitter did not significantly change relative to the binary signaling case. In addition, we are not aware of any treatments quantifying the overhead for PAM-4 versus binary signaling in terms of a jitter penalty in dB. Nevertheless, the PAM-4 amplitude gain is only 3 dB out of 26 dB and hence represents a relatively small change in signal to noise ratio. But this is precisely the issue when quantifying the PAM-4 advantage from a fundamental analysis. From a theoretical perspective, Q and random jitter should be impacted on similar footing since improvements in signal to noise translates into a commensurate reduction in random jitter. From this perspective the improvements in Q and random jitter are modestly similar within our experimental uncertainty [13

13. We have not factored in the effects from the CMOS circuitry which could be the dominate source of rms jitter and thereby mask out other components that contribute to this metric.

]. However, improvements in deterministic jitter should arise from the bandwidth roll off characteristics within the link. This supports the reason DJ is found to be far better for the PAM-4 signaling case whereas RJ is relatively unchanged. Therefore our preliminary analysis of the data appears well founded and reconciled with information theory predictions.

20log(221)=9.54dB
(2)

To deliver the same data throughput, the baud rate of binary signaling needs to be doubled resulting in a bandwidth spectrum of the signal twice as wide as PAM–4. This condition could lead to a transmitter bandwidth limit and hence a SNR degradation that we estimate can be written as

Fig. 7. (a) Simulated small signal response for a 10Gbps VCSEL (b) Simulated NRZ eye diagram at 10Gbps (c) Simulated PAM4 eye diagram at 20Gbps. Resonance frequency of device is 10GHz. Damping ratio varies (0.25, 0.5, 0.75 and 1).
SNRdegradation=log(2)SNRslope
(3)

where SNRslope is the bandwidth roll off of a transmitter’s response. From Eq. (2) it is clear that PAM–4 becomes advantageous over binary when the following conditions develop.

SNRslope>32dBdec
(4)

Acknowledgments

This material is based upon work supported by DARPA under Contract No. NBCH3039002.

References and links

1.

R. Farjad-Rad, C.-K. K. Yang, and M. A. Horowitz, “A 0.3- mm CMOS 8-GS/s 4-PAM Serial Link Transceiver,” IEEE J. Solid-State Circuits 35, 757–764, (2000). [CrossRef]

2.

S. Walkin and J. Conradi, “Multilevel Signaling for increasing the reach of 10Gbps Systems,” J. Lightwave Technol. 17, 2235–2248 (1999). [CrossRef]

3.

K. Yonenage and S. Kuwano, “Dispersion tolerant optical transmission using doubinary transmitter and binary receiver,” J. Lightwave Technol. 15, 1530–1537, (1997). [CrossRef]

4.

A. Adamiecki, M. Duelk, J.H. Sinsky, and M. Mandich: “Scalability of Duobinary Signaling to 25Gb/s for 100 GbE Applications,” IEEE 802.3ap Task Force Meeting, (November 2004). http://www.ieee802.org/3/ap/public/nov04/adamiecki_01_1104.pdf

5.

H. Wu, J. Tiemo, P. Pepeljugoski, J. Schaub, S. Gowda, J. Kask, and A. Hajimir, “Differential 4-tap and 7-tap Transverse Filters in SiGe for 10Gh/s Multimode Fiber Optic” Link equalization” 2003 IEEE Intemational Solid State Circuit Conference. pp 180–181, February 2003. [CrossRef]

6.

David Cunningham, “Multilevel Modulation for 10GbE:Link and Component Specification,” see http://grouper.ieee.org/groups/802/3/10G_study/public/july99/cunningham_1_0799.pdf.

7.

C. Pelard, E. Gebara, A. J. Kim, M. Vrazel, E. J. Peddi, V. M. S. Hietala, Bajekal, S. E. Ralph, and J. Laska, “Multilevel signaling and equalization over multimode fiber at 10 Gbit/s,” Gallium Arsenide Integrated Circuit (GaAs IC) Symposium, 2003. 25th Annual Technical Digest 2003. IEEE, p.197–199, (2003) [CrossRef]

8.

John. E. Cunningham, David K. McElfresh, Leon D. Lopez, Dan Vacar, and Ashok V. Krishnamoorthy, “Scaling vertical-cavity surface-emitting laser reliability for petascale systems,” Appl. Opt. 45, 6342–6348 (2006). [CrossRef] [PubMed]

9.

Richard J. S. Bates, Daniel M. Kuchta, and Kenneth P. Jackson, “Improved Multimode Fiber Link BER Calculations due to Modal Noise and Non Self-Pulsating Laser Diodes,” Opt. Quantum Electron. 27, 203–224 (1995). [CrossRef]

10.

See Sang-Hoon Lee, Yong-Sang Cho, Bon-Jo Koo, and Sang-Kook Han, “Optimization of a small to-can package electrical design for SFF/SFP optical transceiver module,” Microwave Opt.l Technol. Lett. 40, 239–242, (2003). [CrossRef]

11.

G. P. Agrawal, “Fiber Optics Communication Systems,” 2nd ed., John Wiley and Sons, p. 180–182, (1997).

12.

P. Voois, N. Swenson, and T. Lindsay, “Extending the Ethernet Link Model for EDC and Modulation Format,” http://grouper.ieee.org/groups/802/3/10GMMFSG/public/mar04/voois_1_0304.pdf

13.

We have not factored in the effects from the CMOS circuitry which could be the dominate source of rms jitter and thereby mask out other components that contribute to this metric.

14.

Dawei Huang, “Free Space Optical Interconnect For Computing System- Design, Optimization and Implementation,” PhD Thesis, University of California, San Diego, to be published Dec (2006)

15.

Vladimir Stojanovic, “Channel-Limited High-Speed Links: Modeling, Analysis And Design, PhD Thesis, Stanford University,” 2004

16.

Larry A. Coldren and Scott W. Corzine, “Diode Lasers and Photonic Integrated Circuits,” pg 200, John Wiley and Sons (1995)

17.

Only small improvement in VCSEL frequency response have developed to date compared with earlier reports of a 21 GHz result dated in 1997. see K. L. Lear, M. Ochiai, V. M. Hietala, H. Q. Hou, B. E. Hammons, J. J. Banas, and J. A. Nevers, “Small and large signal modulation of 850nm oxide-confined vertical-cavity surface-emitting lasers,” Advances in Vertical Cavity Surface Emitting Lasers in Trends in Optics and Photonics Series , 15, 69–74, (1997).

OCIS Codes
(060.4510) Fiber optics and optical communications : Optical communications
(200.4650) Optics in computing : Optical interconnects

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: August 28, 2006
Revised Manuscript: November 21, 2006
Manuscript Accepted: November 21, 2006
Published: December 11, 2006

Citation
J. E. Cunningham, D. Beckman, Xuezhe Zheng, Dawei Huang, T. Sze, and A. V. Krishnamoorthy, "PAM-4 Signaling over VCSELs with 0.13µm CMOS Chip Technology," Opt. Express 14, 12028-12038 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-25-12028


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References

  1. R. Farjad-Rad, C.-K. K. Yang and M. A. Horowitz, "A 0.3- mm CMOS 8-GS/s 4-PAM Serial Link Transceiver," IEEE J. Solid-State Circuits 35, 757-764, (2000). [CrossRef]
  2. S. Walkin and J. Conradi, "Multilevel Signaling for increasing the reach of 10Gbps Systems," J. Lightwave Technol. 17, 2235-2248 (1999). [CrossRef]
  3. K. Yonenage and S. Kuwano, "Dispersion tolerant optical transmission using doubinary transmitter and binary receiver," J. Lightwave Technol. 15, 1530-1537, (1997). [CrossRef]
  4. A. Adamiecki, M. Duelk, J.H. Sinsky, M. Mandich: "Scalability of Duobinary Signaling to 25Gb/s for 100 GbE Applications," IEEE 802.3ap Task Force Meeting, (November 2004). http://www.ieee802.org/3/ap/public/nov04/adamiecki_01_1104.pdf
  5. H. Wu, J. Tiemo, P. Pepeljugoski, J. Schaub,S. Gowda, J. Kask, and A. Hajimir, "Differential 4-tap and 7-tap Transverse Filters in SiGe for 10Gh/s Multimode Fiber Optic" Link equalization" 2003 IEEE Intemational Solid State Circuit Conference. pp 180-181, February 2003. [CrossRef]
  6. David Cunningham, "Multilevel Modulation for 10GbE:Link and Component Specification," see http://grouper.ieee.org/groups/802/3/10G_study/public/july99/cunningham_1_0799.pdf.
  7. C. Pelard, E. Gebara, A. J. Kim, M. Vrazel, E. J. Peddi, V. M. S. Hietala, Bajekal, S. E.  Ralph, and J. Laska, "Multilevel signaling and equalization over multimode fiber at 10 Gbit/s," Gallium Arsenide Integrated Circuit (GaAs IC) Symposium, 2003. 25th Annual Technical Digest 2003. IEEE, p.197-199, (2003) [CrossRef]
  8. John. E. Cunningham, David K. McElfresh, Leon D. Lopez, Dan Vacar, and Ashok V. Krishnamoorthy, "Scaling vertical-cavity surface-emitting laser reliability for petascale systems," Appl. Opt. 45, 6342-6348 (2006). [CrossRef] [PubMed]
  9. RichardJ. S. Bates, Daniel M. Kuchta, Kenneth P. Jackson, "Improved Multimode Fiber Link BER Calculations due to Modal Noise and Non Self-Pulsating Laser Diodes," Opt. Quantum Electron. 27,203-224 (1995). [CrossRef]
  10. See Sang-Hoon Lee, Yong-Sang Cho, Bon-Jo Koo, Sang-Kook Han, "Optimization of a small to-can package electrical design for SFF/SFP optical transceiver module," Microwave Opt.l Technol. Lett. 40, 239-242, (2003). [CrossRef]
  11. G. P. Agrawal, "Fiber Optics Communication Systems," 2nd ed., John Wiley and Sons, p. 180-182, (1997).
  12. P. Voois, N. Swenson, and T. Lindsay, "Extending the Ethernet Link Model for EDC and Modulation Format," http://grouper.ieee.org/groups/802/3/10GMMFSG/public/mar04/voois_1_0304.pdf
  13. <other>. We have not factored in the effects from the CMOS circuitry which could be the dominate source of rms jitter and thereby mask out other components that contribute to this metric.</other>
  14. Dawei Huang, "Free Space Optical Interconnect For Computing System- Design, Optimization and Implementation," PhD Thesis, University of California, San Diego, to be published Dec (2006)
  15. Vladimir Stojanovic, "Channel-Limited High-Speed Links: Modeling, Analysis And Design, PhD Thesis, Stanford University," 2004
  16. LarryA.  Coldren and Scott W. Corzine, "Diode Lasers and Photonic Integrated Circuits," pg 200, John Wiley and Sons (1995)
  17. Only small improvement in VCSEL frequency response have developed to date compared with earlier reports of a 21 GHz result dated in 1997. see K. L. Lear, M. Ochiai, V. M. Hietala, H. Q. Hou, B. E. Hammons, J. J. Banas, and J. A. Nevers, "Small and large signal modulation of 850nm oxide-confined vertical-cavity surface-emitting lasers," Advances in Vertical Cavity Surface Emitting Lasers in Trends in Optics and Photonics Series,  15, 69-74, (1997).

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