OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 24 — Nov. 22, 2010
  • pp: 24969–24974
« Show journal navigation

All-VCSEL based digital coherent detection link for multi Gbit/s WDM passive optical networks

Roberto Rodes, Jesper Bevensee Jensen, Darko Zibar, Christian Neumeyr, Enno Roenneberg, Juergen Rosskopf, Markus Ortsiefer, and Idelfonso Tafur Monroy  »View Author Affiliations


Optics Express, Vol. 18, Issue 24, pp. 24969-24974 (2010)
http://dx.doi.org/10.1364/OE.18.024969


View Full Text Article

Acrobat PDF (847 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We report on experimental demonstration of a digital coherent detection link fully based on vertical cavity surface emitting lasers (VCSELs) for the transmitter as well as for the local oscillator light source at the receiver side. We demonstrate operation at 5 Gbps at a 1550 nm wavelength with record receiver sensitivity of −36 dBm after transmission over 40 km standard single mode fiber. Digital signal processing compensates for frequency offset between the transmitter and the local oscillator VCSELs, and for chromatic dispersion. This system allows for uncooled VCSEL operation and fully passive fiber transmission with no use of optical amplification or optical dispersion compensation. The proposed system demonstrates the potential of multi-gigabit coherent passive optical networks with extended reach and increased capacity. Moreover, this is, to the best of our knowledge, the first demonstration of coherent optical transmission systems using a low-cost VCSEL as the local oscillator as well as for the transmitter.

© 2010 OSA

1. Introduction

2. All-VCSEL digital coherent receiver

In this paper, we experimentally demonstrate the use of digital coherent detection using a directly modulated VCSEL as light source for the transmitter and another VCSEL for local oscillator. Traditionally, the broad linewidth of VCSELs has made them unsuitable for coherent detection [9

9. M. Seimetz, “Laser linewidth limitations for optical systems with high-order modulation employing feed forward digital carrier phase estimation,” Conference on Optical Fiber Communication (OFC) (1998).

]. By employing digital signal processing algorithms, however, we demonstrate that these limitations can be overcome. Additionally, coherent detection combined with digital signal processing facilitates the use of DSP based chromatic dispersion compensation, thereby eliminating the need for dispersion compensating fiber (DCF) [10

10. K. Prince, M. Ma, T. B. Gibbon, and I. Tafur Monroy, “Demonstration of 10.7-Gb/s transmission in 50-km PON with uncooled free-running 1550-nm VCSEL,” Conference on Lasers and Electro-Optics (CLEO) and Quantum Electronics and Laser Science Conference (QELS) (2010).

]. In combination with the increased sensitivity of coherent detection systems, this makes coherent detection VCSEL based systems a strong candidate for future access networks, while also enabling graceful upgrade due to the ability to operate over the current PON architectures.

Our experimental results show receiver sensitivity up to −36 dBm after 40 km of fiber transmission with no optical amplification or optical dispersion compensation for a 5 Gbps amplitude shift keying (ASK) system. The same system was tested with direct detection, and employing optical dispersion compensation in the form of 6 km of DCF to compensate 40 km SSMF. The measured sensitivity was −23 dBm and −19 dBm, with and without optical dispersion compensation, respectively. This represents an increase in receiver sensitivity of 13 dB for the coherent system when compared with direct detection system with optical dispersion compensation, and 17 dB compared with direct detection without optical dispersion compensation.

3. Experimental setup

Figure 1
Fig. 1 Experimental setup. Digital sampling scope (DSO), digital signal processing (DSP), polarization controller (PC), variable optical attenuator (VOA). The insertion shows the measured combined optical power spectrum of the signal and the LO.
shows a simplified scheme of our experimental setup. A pulse pattern generator directly modulates a 1550 nm VCSEL (VCSEL1) at 5 Gbps. A balanced drive configuration is used for the VCSEL1 with a driving peak-peak voltage of 1 V. The bias current of VCSEL1 is set to 13.6 mA for optimum performance maximizing the extinction ratio while minimizing overshoot.

Figure 2
Fig. 2 Laser linewidth of the VCSEL measured by heterodyne detection with an ECL.
shows the optical spectrum of the transmitter VCSEL measured by heterodyne detection with a 100 kHz linewidth external cavity laser (ECL). The frequency offset between VCSEL1 and the ECL is 5.8 GHz. The measured 3 dB linewidth of VCSEL1 is 350 MHz.

The data pattern used for the full VCSEL coherent detection transmission experiment is a pseudo random binary sequence (PRBS) of length of 215-1. The average output power of the VCSEL1 launched into the fiber is 0.6 dBm. The transmission experiment is over 40 km of SSMF with a total attenuation of 7.7 dB, and a total dispersion of 640 ps/nm. In order to measure the optical power going into the coherent receiver, a variable optical attenuator (VOA), a 20 dB coupler and an optical power meter is placed after the transmission fiber. The coherent receiver consists of a 90° optical hybrid and two pairs of balanced photodiodes. The local oscillator (LO) signal is generated by a free running, continuous wave, 1550 nm VCSEL (VCSEL2). The LO VCSEL2 has been wavelength tuned to match the wavelength of the transmitting VCSEL1 at 1550.34 nm by adjusting the bias level to 16.7 mA for intradyne detection. The insertion in Fig. 1 shows the measured optical power spectrum of the signal and the LO. Due to the intradyne detection scheme, the spectrum of the LO is masked by the signal spectrum.

The wavelength tuning range of VCSEL2 was measured to be 5 nm from 1547 nm to 1552 nm by varying the bias current from 5 mA to 19 mA. This demonstrates the feasibility of bias current wavelength tuning of VCSELs employed as local oscillators in coherent networks. The optical output power of the LO varies from 0.3 mW at 1547 nm to 1.3 mW at 1552 nm.

The in-phase and quadrature components from the coherent receiver are stored with a digital sampling oscilloscope (DSO) with a sampling rate of 40 Gsamples/s. Digital signal processing, consisting of a digital dispersion compensation, synchronization, envelope detection and decision gating, is performed off-line. The detected signals have a random frequency offset. This frequency offset does not affect the demodulation, as the DSP performs envelope detection of the absolute value of in-phase and quadrature components.

4. Results

Figure 3 (a)
Fig. 3 Optical eye diagrams. 5 Gbps, Ibias = 13.6 mA, Vpp = 1 V. (a) Back-to-back. (b) After 40 km SSMF
shows the optical eye diagram at the output of the transmitter. At the bias current of 13.6 mA, beginning overshooting of the VCSEL is observed. The extinction ratio at the transmitter is 6.8 dB measured with an oscilloscope. Figure 3(b) shows the eye diagram after 40 km SSMF transmission. The fiber attenuation has decreased the signal to noise ratio (SNR), and the fiber dispersion has severely distorted the waveform, resulting in significant reduction of eye opening in amplitude as well as time domain.

The bit error ratio (BER) of the 5 Gbps signal back-to-back (B2B) and after 40 km SSMF transmission is plotted in Fig. 4
Fig. 4 BER curves after 40 km SSMF and for B2B configuration. Bit-rate: 5 Gbps, ER: 6.82 dB, LO power: 0 dBm.
. The plot shows the measured BER with direct detection with and without optical dispersion compensation. The plot also shows the BER after coherent detection where the dispersion compensation was performed using DSP.

For direct detection, back to back (B2B) receiver sensitivity at the forward error correction limit of BER<10−3, was measured to be −23. After 40 km of SSMF transmission, the measured sensitivity was −19 dBm, corresponding to 4 dB receiver sensitivity penalty. After DCF for optical dispersion compensation, the measured sensitivity was −23 dBm, and no observable penalty is measured compare to the B2B configuration.

For the coherent detection with DSP based dispersion compensation 2·105 bits were stored. DSP dispersion compensation was used to compensate for the chromatic dispersion of the 40 km of SMF transmission link. Best performance after demodulation was achieved by overcompensation so that the total compensated dispersion was 765 ps/nm. This is attributed to the chirp caused by the direct modulation of the VCSEL at the transmitter. Receiver sensitivity was measured to be −36 dBm B2B as well as after transmission, showing that the DSP effectively compensates dispersion. Comparing to the direct detection cases with and without optical dispersion compensation, sensitivity has been improved by 13 dB and 17 dB, respectively.

5. Conclusions

We present experimental results, for the first time, demonstrating the feasibility of using VCSELs with uncooled and free running operation as transmitter as well as local oscillator laser in a digital coherent receiver.

The transmitter VCSEL was directly modulated at 5 Gbps, and after 40 km SSMF the signal was received using a coherent receiver with a VCSEL as local oscillator. Sensitivity of −36 dBm has been achieved with 28.9 dB power margin and no need for optical dispersion compensation or optical amplification.

A comparison between coherent detection and direct detection approach has been presented. Coherent detection resulted in a sensitivity improvement of 13 dB and 17 dB, comparing with direct detection with and without optical dispersion compensation, respectively. Moreover, in terms of power budget, power margin for direct detection correspond to a passive splitting ratio of 15, while for coherent detection the achievable splitting ratio is 776.

Acknowledgments

The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7) under project 212 352 ALPHA “Architectures for fLexible Photonic Home and Access networks”; and project 224409 GigaWaM “Gigabit access passive optical network using wavelength division multiplexing”.

References and links

1.

P. E. Green, “Fiber to the home: the next big broadband thing,” IEEE Commun. Mag . 42, 100-106 (2004) [CrossRef]

2.

S. Smolorz, H. Rohde, P. Ossieur, C. Antony, P. D. Townsend, T. De Ridder, B. Baekelandt, X. Z. Qiu, S. Appathurai, H.-G. Krimmel, D. Smith, and A. Poustie, “Next generation access networks: PIEMAN and beyond,” International Conference on Photonics in Switching (PS) (2009).

3.

M. Alfani, J. M. Finochietto, and F. Neri, “Efficient multicast bandwidth allocation in TDM-WDM PONs,” Conference on Optical Fiber Communication (OFC) (2009).

4.

A. D. Ellis, and F. C. Garcia Gunning, “Spectral density enhancement using coherent WDM,” IEEE Photonics Technology Letters (2005).

5.

G. Lachs, S. M. Zaidi, and A. K. Singh, “Sensitivity enhancement using coherent heterodyne detection,” J. Lightwave Technol . 12, 1036-1041 (1994). [CrossRef]

6.

X. Zhou, and J. Yu, “Advanced coherent modulation formats and algorithms: higher-order multi-level coding for high-capacity system based on 100 Gbps channel,” Conference on Optical Fiber Communication (OFC) (2010).

7.

S. J. Savory, “Electronic signal processing in optical communications,” Proc. SPIE 7136, 71362C (2008). [CrossRef]

8.

S.-Y. Kim, N. Sakurai, H. Kimura, and H. Hadama, “VCSEL-based coherent detection of 10-Gbit/s QPSK signals using digital phase noise cancellation for future optical access systems,” Conference on Optical Fiber Communication (OFC) (2010).

9.

M. Seimetz, “Laser linewidth limitations for optical systems with high-order modulation employing feed forward digital carrier phase estimation,” Conference on Optical Fiber Communication (OFC) (1998).

10.

K. Prince, M. Ma, T. B. Gibbon, and I. Tafur Monroy, “Demonstration of 10.7-Gb/s transmission in 50-km PON with uncooled free-running 1550-nm VCSEL,” Conference on Lasers and Electro-Optics (CLEO) and Quantum Electronics and Laser Science Conference (QELS) (2010).

OCIS Codes
(000.0000) General : General
(000.2700) General : General science

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: September 9, 2010
Revised Manuscript: October 31, 2010
Manuscript Accepted: November 1, 2010
Published: November 15, 2010

Citation
Roberto Rodes, Jesper Bevensee Jensen, Darko Zibar, Christian Neumeyr, Enno Roenneberg, Juergen Rosskopf, Markus Ortsiefer, and Idelfonso Tafur Monroy, "All-VCSEL based digital coherent detection link for multi Gbit/s WDM passive optical networks," Opt. Express 18, 24969-24974 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-24-24969


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. P. E. Green, “Fiber to the home: the next big broadband thing,” IEEE Commun. Mag . 42, 100-106 (2004) [CrossRef]
  2. S. Smolorz, H. Rohde, P. Ossieur, C. Antony, P. D. Townsend, T. De Ridder, B. Baekelandt, X. Z. Qiu, S. Appathurai, H.-G. Krimmel, D. Smith, and A. Poustie, “Next generation access networks: PIEMAN and beyond,” International Conference on Photonics in Switching (PS) (2009).
  3. M. Alfani, J. M. Finochietto, and F. Neri, “Efficient multicast bandwidth allocation in TDM-WDM PONs,” Conference on Optical Fiber Communication (OFC) (2009).
  4. A. D. Ellis, and F. C. Garcia Gunning, “Spectral density enhancement using coherent WDM,” IEEE Photonics Technology Letters (2005).
  5. G. Lachs, S. M. Zaidi, and A. K. Singh, “Sensitivity enhancement using coherent heterodyne detection,” J. Lightwave Technol . 12, 1036-1041 (1994). [CrossRef]
  6. X. Zhou, and J. Yu, “Advanced coherent modulation formats and algorithms: higher-order multi-level coding for high-capacity system based on 100 Gbps channel,” Conference on Optical Fiber Communication (OFC) (2010).
  7. S. J. Savory, “Electronic signal processing in optical communications,” Proc. SPIE 7136, 71362C (2008). [CrossRef]
  8. S.-Y. Kim, N. Sakurai, H. Kimura, and H. Hadama, “VCSEL-based coherent detection of 10-Gbit/s QPSK signals using digital phase noise cancellation for future optical access systems,” Conference on Optical Fiber Communication (OFC) (2010).
  9. M. Seimetz, “Laser linewidth limitations for optical systems with high-order modulation employing feed forward digital carrier phase estimation,” Conference on Optical Fiber Communication (OFC) (1998).
  10. K. Prince, M. Ma, T. B. Gibbon, and I. Tafur Monroy, “Demonstration of 10.7-Gb/s transmission in 50-km PON with uncooled free-running 1550-nm VCSEL,” Conference on Lasers and Electro-Optics (CLEO) and Quantum Electronics and Laser Science Conference (QELS) (2010).

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Figures

Fig. 1 Fig. 2 Fig. 3
 
Fig. 4
 

« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited