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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 26 — Dec. 12, 2011
  • pp: B203–B208
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30 Gbps 4-PAM transmission over 200 m of MMF using an 850 nm VCSEL

Krzysztof Szczerba, Petter Westbergh, Johnny Karout, Johan Gustavsson, Åsa Haglund, Magnus Karlsson, Peter Andrekson, Erik Agrell, and Anders Larsson  »View Author Affiliations


Optics Express, Vol. 19, Issue 26, pp. B203-B208 (2011)
http://dx.doi.org/10.1364/OE.19.00B203


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Abstract

We present high speed real time, error free 4-PAM transmission for short range optical links based on a VCSEL operating at 850 nm, a multimode fibre and a simple intensity detector. Transmission speeds of 25 Gbps and 30 Gbps are demonstrated, and the maximum fibre reaches were 300 m and 200 m, respectively. The 4-PAM is also compared with OOK transmission at 25 Gbps, and we find that at this bit rate 4-PAM increases the error free transmission distance in the multimode fibre by 100 m, compared to OOK.

© 2011 OSA

1. Introduction

There are constraints for modulation formats complexity in short range optical links, especially those used for data communication, e.g. in supercomputing and data center applications. The limitations come from power consumption, heat generation, and spatial density requirements. 4-PAM offers the lowest implementation complexity of all the modulation formats with spectral efficiency of 2 bits/second/Hz. Real-time hardware for transmitters and receivers for 4-PAM are already developed [7

7. D. Watanabe, A. Ono, and T. Okayasu, “CMOS optical 4-PAM VCSEL driver with modal-dispersion equalizer for 10 Gb/s 500 m MMF transmission,” IEEE International Solid-State Circuits Conference, (2009), 106–107.

, 8

8. T. Toifl, C. Menolfi, M. Ruegg, R. Reutemann, P. Buchmann, M. Kossel, T. Morf, J. Weiss, and M. Schmatz, “A 22 Gb/s PAM-4 receiver in 90-nm CMOS SOI technology,” IEEE J. Solid-State Circuits 41, 954–965 (2006). [CrossRef]

]. In laboratory conditions, 4-PAM modulation can be generated in real time. The bit error rate (BER) measurement can also be done in real time.

In this paper, we present transmission at 25 Gbps and 30 Gbps using 4-PAM over 300 and 200 meters of MMF, respectively. The 4-PAM signal generation and BER measurements were performed in real time. We also compared 4-PAM and OOK modulation at 25 Gbps, and demonstrated that an increase in transmission distance can be achieved by using 4-PAM instead of OOK at the same bitrate. No equalization was used in the receiver, nor was any predistortion used in the transmitter.

2. Experimental setup

2.1. 4-PAM signal generation

The main advantage of the 4-PAM modulation format is its simplicity. The experimental bit error rate measurement of 4-PAM modulation can be done with standard BER test equipment designed for OOK. In our experiments the 4-PAM signal was generated from two binary PRBS data signals of length 27 – 1, at 12.5 Gbps and 15 Gbps for 25 Gbps and 30 Gbps 4-PAM, respectively. With this method, only natural mapping of bits to symbol is possible. The amplitudes of the binary signals were approximately 900 mV and 450 mV, but they were tuned slightly to adjust the resulting 4-PAM signal level spacing. The two binary signals were decorrelated by half of the PRBS pattern length, to reach all possible level transitions in the resulting 4-PAM signal. The OOK signals were phase matched before being coupled together. The 4-PAM generator along with bit to symbol mapping are illustrated in Fig. 1.

Fig. 1 The experimental setup. The outputs of the pattern generator are individually adjustable with regard to amplitude and delay and also phase matched.

2.2. The optical link - VCSEL, fibre and photodetectors used in the experiments

The generated 4-PAM signal was fed to a VCSEL through a bias-T. The VCSEL, having approximately 20 GHz modulation bandwidth, was described in an earlier publication [9

9. P. Westbergh, J. Gustavsson, A. Haglund, A. Larsson, F. Hopfer, G. Fiol, D. Bimberg, and A. Joel, “32 Gbit/s multimode fibre transmission using high-speed, low current density 850 nm VCSEL,” Electron. Lett. 45, 366–368 (2009). [CrossRef]

]. The VCSEL bias and modulating signal amplitude were experimentally optimized for the best performance, the optimal bias current was found to be 7 mA for the driving signal peak to peak voltage of 1.35 V. The modulation current could not be easily and reliably measured. The VC-SEL was operated at room temperature without cooling. Between the VCSEL and the photoreceiver, an OM3+ graded index multimode fiber was used, with a bandwidth-distance product of 4700 MHz·km. A variable optical attenuator was used to vary the optical power. At the receiver, an amplified photoreceiver (PR) with 12 GHz bandwidth was used, which was the most bandwidth-limiting component in the experiment, apart from the transmission fiber.

2.3. Real time bit error rate measurement

The BER measurement was performed using an error analyzer (EA) designed for conventional OOK modulation. For the 4-PAM modulation three decision levels are applicable, each of the levels being between a pair of two adjacent symbol levels. The total symbol error rate was derived from error rate measurements done on each of the 4-PAM threshold levels. Given that we have four symbols denoted as S0, S1, S2 and S3, there are three decision levels between the adjacent symbols: L1, L2 and L3. A symbol error is detected when a symbol Si is transmitted and a symbol Sj is received, where i is not equal to j. The conditional probability of receiving Sj when Si was transmitted is denoted with Pij. In the high signal to noise ratio (SNR) case (which corresponds to low BER), when it can be assumed that only errors between the nearest neighboring symbols happen, and given that natural bit to symbol mapping, the total BER is given by
BER=18(P01+P10+2P12+2P21+P23+P32).
(1)

The error rates at each of the levels were measured with the EA pre-programmed with a pattern corresponding to each of the levels. The aggregate BER can thus be obtained from
BER=12ER1+ER2+12ER3
(3)

The complete experimental setup for 4-PAM measurements is shown in Fig. 1.

2.4. OOK experimental setup

For the experiments with OOK at the bitrate of 25 Gbps the setup was simplified. The same bit pattern generator and error analyser were used, but without generating the 4-PAM signal. The peak to peak amplitude of the signal driving the VCSEL was the same as for the 4-PAM case, which was 1.35 V. Because an amplified photoreceiver for the wavelength of 850 nm with sufficient electrical bandwidth for 25 Gbps OOK operation was not commercially available at the time, a non-amplified photodetector having 25 GHz bandwidth was used. A discrete amplifier was used to amplify the signal from the photodetector. Although this receiver configuration has higher bandwidth that the photoreceiver used for 4-PAM, it has somewhat worse noise performance. The main reason is that a photodiode is a current source and the discrete amplifier is a voltage amplifier, so the power transfer between these two components is not optimized.

3. Results of 4-PAM experiments

The quality of the 25 Gbps 4-PAM signal in the back-to-back (B2B) configuration is illustrated by the eye diagram in Fig. 2a. No predistortion or equalization was done to the signal. The eye diagram of the signal after propagation through 200 m and 300 m of MMF is illustrated in Fig. 2b and Fig. 2c. In Fig. 3a, the eye diagram of the 30 Gbps signal in the B2B case is illustrated, and in Fig. 3b for propagation over 200 m of MMF. Note that because the tran-simpedance amplifier in the photoreceiver was inverting, the modulation level one having the highest power is lowest in the eye diagrams of the figures.

Fig. 2 Eye diagrams at 25 Gbps, after B2B (a) and after propagation over 200 m of MMF (b) and 300 m of MMF (c). The eye diagrams were taken at around 0 dBm received optical power.
Fig. 3 Eye diagrams at 30 Gbps, after B2B (a) and after propagation over 200 m of MMF (b), taken at around 0 dBm received optical power.

Fig. 4 BER results for 4-PAM transmission at 25 Gbps (a) and 30 Gbps (b).

4. Experimental comparison with OOK

The BER results for experiments with OOK at 25 Gbps are illustrated in Fig. 5a. Sensitivities at BER of 10−9 for all measured modulation formats are illustrated in Fig. 5b. The absolute sensitivity, which is around −5 dBm at BER of 10−9, is around 1.5 dB worse that the result achieved with 4-PAM at the same bitrate (25 Gbps). The reason for that is that different receiver configurations were used for each of the modulation formats. For 4-PAM a 12 GHz photoreceiver with an integrated transimpedance amplifier (TIA) was used, but for OOK a 25 GHz photodetector and a discrete voltage amplifier was used. Theoretically, it is expected, that the 4-PAM modulation would require 4.8 dB more optical power at the same symbol rate [12

12. J. Pollard, “Multilevel data communication over optical fibre,” IEE Proc. I 138, 162–168 (1991).

]. At the same bit rate 4-PAM should require only 3.3 dB more optical power [13

13. S. Walklin and J. Conradi, “Multilevel signaling for increasing the reach of 10 Gb/s lightwave systems,” J. Lightwave Technol. 17, 2235–2248 (1999). [CrossRef]

], because of the bandwidth being reduced by half, compared to OOK. This is however offset by the fact, that the configuration of a photodetector and a discrete voltage amplifier has worse noise performance than a photoreceiver with an integrated TIA which was available for 4-PAM because of the reduced symbol rate. The propagation penalties in the multimode fibre are higher for OOK than for 4-PAM. While 4-PAM performance degradation after 100 m of the used MMF is negligible, OOK degrades by 1 dB. After 200 m of the MMF, OOK degrades by 5 dB, while 4-PAM degrades only by around 1 dB. This is also the maximum propagation distance for OOK, i.e. 100 m less than for 4-PAM at the same bit rate.

Fig. 5 BER results for OOK transmission at 25 Gbps (a) and comparison of sensitivities for the three tested cases (b).

5. Conclusions

We show 25 Gbps and 30 Gbps, error free, 4-PAM transmission over 300 m and 200 MMF respectively using directly modulated VCSELs. The BER is measured in real time down to 10−12 and there is a large power margin for error free operation. The VCSEL itself is not a limiting factor but the lack of photoreceivers with sufficient bandwidths is limiting the high-bandwidth performance. In this situation, 4-PAM is a viable alternative. Due to the better spectral efficiency of 4-PAM the transmission distance in MMF is improved over what would be expected from transmission using OOK. The 4-PAM format has also lower complexity than subcarrier formats, which were demonstrated at a higher bitrate [5

5. K. Szczerba, B. Olsson, P. Westbergh, A. Rhodin, J. Gustavsson, A. Haglund, M. Karlsson, A. Larsson, and P. Andrekson, “37 Gbps transmission over 200 m of MMF using single cycle subcarrier modulation and a VCSEL with 20 GHz modulation bandwidth,” European Conference on Optical Communication, (2010), paper We7B2. [CrossRef]

], but with only the transmitter working in real time. We have also showed that 4-PAM has significant benefits compared to OOK at the same bit rate. The transmission distance in MMF is significantly extended, and the hardware requirements are relaxed, so that existing components with lower bandwidth can be used.

Acknowledgments

This work was supported by the Swedish Foundation for Strategic Research (SSF) and the European FP7 project VISIT and we would like to kindly thank for their support.

References and links

1.

P. Westbergh, J. Gustavsson, B. Kögel, A. Haglund, A. Larsson, A. Mutig, A. Nadtochiy, D. Bimberg, and A. Joel, “40 Gbit/s error-free operation of oxide-confined 850 nm VCSEL,” Electron. Lett. 46, 1014–1016 (2010). [CrossRef]

2.

W. Hofmann, P. Moser, P. Wolf, A. Mutig, M. Kroh, and D. Bimberg, “44 Gb/s VCSEL for optical interconnects,” Optical Fiber Communication Conference, (2011), paper PDPC5.

3.

J. Ingham, R. Penty, and I. White, “10 Gb/s & 20 Gb/s extended-reach multimode-fiber datacommunication links using multilevel modulation and transmitter-based equalization,” Optical Fiber communication Conference, OSA Technical Digest (2008), paper OTuO7.

4.

J. D. Ingham, R. V. Penty, I. H. White, P. Westbergh, J. S. Gustavsson, A. Haglund, and A. Larsson, “32 Gb/s multilevel modulation of an 850 nm VCSEL for next-generation datacommunication standards,” in Conference on Lasers and Electro-Optics, (2011), paper CWJ2.

5.

K. Szczerba, B. Olsson, P. Westbergh, A. Rhodin, J. Gustavsson, A. Haglund, M. Karlsson, A. Larsson, and P. Andrekson, “37 Gbps transmission over 200 m of MMF using single cycle subcarrier modulation and a VCSEL with 20 GHz modulation bandwidth,” European Conference on Optical Communication, (2010), paper We7B2. [CrossRef]

6.

S. Lee, F. Breyer, S. Randel, D. Cardenas, H. van den Boom, and A. Koonen, “Discrete multitone modulation for high-speed data transmission over multimode fibers using 850-nm VCSEL,” Optical Fiber Communication Conference, OSA Technical Digest (2009), paper OWM2.

7.

D. Watanabe, A. Ono, and T. Okayasu, “CMOS optical 4-PAM VCSEL driver with modal-dispersion equalizer for 10 Gb/s 500 m MMF transmission,” IEEE International Solid-State Circuits Conference, (2009), 106–107.

8.

T. Toifl, C. Menolfi, M. Ruegg, R. Reutemann, P. Buchmann, M. Kossel, T. Morf, J. Weiss, and M. Schmatz, “A 22 Gb/s PAM-4 receiver in 90-nm CMOS SOI technology,” IEEE J. Solid-State Circuits 41, 954–965 (2006). [CrossRef]

9.

P. Westbergh, J. Gustavsson, A. Haglund, A. Larsson, F. Hopfer, G. Fiol, D. Bimberg, and A. Joel, “32 Gbit/s multimode fibre transmission using high-speed, low current density 850 nm VCSEL,” Electron. Lett. 45, 366–368 (2009). [CrossRef]

10.

W. I. Way, Broadband Hybrid Fiber Coax Access System Technologies, 1st ed., (Academic Press, Inc., Orlando, FL, USA, 1998).

11.

C. Carlsson, H. Martinsson, J. Vukusic, J. Halonen, and A. Larsson, “Nonlinear distortion and dynamic range of red (670 nm) oxide confined VCSELs,” IEEE Photon. Technol. Lett. 13, 358–360 (2001). [CrossRef]

12.

J. Pollard, “Multilevel data communication over optical fibre,” IEE Proc. I 138, 162–168 (1991).

13.

S. Walklin and J. Conradi, “Multilevel signaling for increasing the reach of 10 Gb/s lightwave systems,” J. Lightwave Technol. 17, 2235–2248 (1999). [CrossRef]

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems
(060.4080) Fiber optics and optical communications : Modulation
(140.7260) Lasers and laser optics : Vertical cavity surface emitting lasers

ToC Category:
Access Networks and LAN

History
Original Manuscript: October 3, 2011
Revised Manuscript: October 31, 2011
Manuscript Accepted: October 31, 2011
Published: November 18, 2011

Virtual Issues
European Conference on Optical Communication 2011 (2011) Optics Express

Citation
Krzysztof Szczerba, Petter Westbergh, Johnny Karout, Johan Gustavsson, Åsa Haglund, Magnus Karlsson, Peter Andrekson, Erik Agrell, and Anders Larsson, "30 Gbps 4-PAM transmission over 200 m of MMF using an 850 nm VCSEL," Opt. Express 19, B203-B208 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-26-B203


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References

  1. P. Westbergh, J. Gustavsson, B. Kögel, A. Haglund, A. Larsson, A. Mutig, A. Nadtochiy, D. Bimberg, and A. Joel, “40 Gbit/s error-free operation of oxide-confined 850 nm VCSEL,” Electron. Lett.46, 1014–1016 (2010). [CrossRef]
  2. W. Hofmann, P. Moser, P. Wolf, A. Mutig, M. Kroh, and D. Bimberg, “44 Gb/s VCSEL for optical interconnects,” Optical Fiber Communication Conference, (2011), paper PDPC5.
  3. J. Ingham, R. Penty, and I. White, “10 Gb/s & 20 Gb/s extended-reach multimode-fiber datacommunication links using multilevel modulation and transmitter-based equalization,” Optical Fiber communication Conference, OSA Technical Digest (2008), paper OTuO7.
  4. J. D. Ingham, R. V. Penty, I. H. White, P. Westbergh, J. S. Gustavsson, A. Haglund, and A. Larsson, “32 Gb/s multilevel modulation of an 850 nm VCSEL for next-generation datacommunication standards,” in Conference on Lasers and Electro-Optics, (2011), paper CWJ2.
  5. K. Szczerba, B. Olsson, P. Westbergh, A. Rhodin, J. Gustavsson, A. Haglund, M. Karlsson, A. Larsson, and P. Andrekson, “37 Gbps transmission over 200 m of MMF using single cycle subcarrier modulation and a VCSEL with 20 GHz modulation bandwidth,” European Conference on Optical Communication, (2010), paper We7B2. [CrossRef]
  6. S. Lee, F. Breyer, S. Randel, D. Cardenas, H. van den Boom, and A. Koonen, “Discrete multitone modulation for high-speed data transmission over multimode fibers using 850-nm VCSEL,” Optical Fiber Communication Conference, OSA Technical Digest (2009), paper OWM2.
  7. D. Watanabe, A. Ono, and T. Okayasu, “CMOS optical 4-PAM VCSEL driver with modal-dispersion equalizer for 10 Gb/s 500 m MMF transmission,” IEEE International Solid-State Circuits Conference, (2009), 106–107.
  8. T. Toifl, C. Menolfi, M. Ruegg, R. Reutemann, P. Buchmann, M. Kossel, T. Morf, J. Weiss, and M. Schmatz, “A 22 Gb/s PAM-4 receiver in 90-nm CMOS SOI technology,” IEEE J. Solid-State Circuits41, 954–965 (2006). [CrossRef]
  9. P. Westbergh, J. Gustavsson, A. Haglund, A. Larsson, F. Hopfer, G. Fiol, D. Bimberg, and A. Joel, “32 Gbit/s multimode fibre transmission using high-speed, low current density 850 nm VCSEL,” Electron. Lett.45, 366–368 (2009). [CrossRef]
  10. W. I. Way, Broadband Hybrid Fiber Coax Access System Technologies, 1st ed., (Academic Press, Inc., Orlando, FL, USA, 1998).
  11. C. Carlsson, H. Martinsson, J. Vukusic, J. Halonen, and A. Larsson, “Nonlinear distortion and dynamic range of red (670 nm) oxide confined VCSELs,” IEEE Photon. Technol. Lett.13, 358–360 (2001). [CrossRef]
  12. J. Pollard, “Multilevel data communication over optical fibre,” IEE Proc. I138, 162–168 (1991).
  13. S. Walklin and J. Conradi, “Multilevel signaling for increasing the reach of 10 Gb/s lightwave systems,” J. Lightwave Technol.17, 2235–2248 (1999). [CrossRef]

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