## All optical discrete Fourier transform processor for 100 Gbps OFDM transmission

Optics Express, Vol. 16, Issue 6, pp. 4023-4028 (2008)

http://dx.doi.org/10.1364/OE.16.004023

Acrobat PDF (436 KB)

### Abstract

Optical orthogonal frequency division multiplex (OFDM) symbol generation by all-optical discrete Fourier transform (DFT) is proposed and investigated for 100-Gbps transmission performance. We discuss a design example for a 4 × 25Gbps OFDM transmission system and its performance comparison with that for a 100-Gbps single-channel return-to-zero data transmission in an optically amplified system.

© 2008 Optical Society of America

## 1. Introduction

1. H.C. Bao and W. Shieh, “Transmission of wavelength-division-multiplexed channels with coherent optical OFDM,” IEEE Photon. Tech. Lett. **19**, 922–924 (2007). [CrossRef]

2. B. J. Schmidt, A. J. Lowery, and J. Armstrong, “Experimental demonstrations of 20 Gbit/s direct-detection optical OFDM and 12 Gbit/s with a colorless transmitter,” Natl. Fiber Optic Eng. Conference (NFOEC) 2007 (OSA, 2007), paper PDP18, http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2007-PDP18.

3. A. J. Lowery, L. B. Du, and J. Armstrong, “Performance of optical OFDM in ultralong-haul WDM lightwave systems,” J. Lightwave Technol. **25**, 131–138 (2007). [CrossRef]

7. W. Shieh, R. S. Tucker, W. Chen, X. Yi, and G. Pendock, “Optical performance monitoring in coherent optical OFDM systems,” Opt. Exp. **15**, 350–356 (2007). [CrossRef]

9. W. Shieh, W. Chen, and R. S. Tucker, “Polarization mode dispersion mitigation in coherent optical orthogonal frequency division multiplexed systems,” Electron. Lett. **42**, 996–997 (2006). [CrossRef]

## 2. All-optical discrete Fourier transform circuit

*ε*and

_{m}*E*are the time and frequency domain samples at the

_{k}*m*-th and

*k*-th positions, respectively. Integer number

*N*is the total number of samples, and 0≤

*k*,

*m*<

*N*. The corresponding frequency and time positions are given by

*t*=

_{m}*mτ*and

*ω*=

_{k}*kδ*, where

*τ*and

*δ*are the sampling spaces of the time and frequency, respectively, such that

*δτ*= 2

*π*/

*N*. Here

*ω*’s correspond to the optical frequency of subcarriers. Investigating DFT expressions, one can infer that an optical circuit implementation of DFT is merely phase delays by precise optical path length adjustments combined by a power combination by a power coupler as shown in Fig. 1(a). The overall design of the inverse DFT looks like a wavelength division multiplexer, except that there is a careful arrangement of time delays and relative phase tuning in each path, so all wavelength components that correspond to the subcarriers of an OFDM are orthogonally multiplexed into one output port, as illustrated in Fig. 1(b). Note that the phase delays are defined with respect to the optical carrier frequency. In this arrangement, a short laser pulse can be used as data input at every subcarrier input. Only one part of the spectral component that corresponds to an optical OFDM subcarrier has constructive interference at the output port, while all other spectral components fed to the same input port have completely destructive interference at the output port. The narrow input pulse can be modulated to transmit information. If we consider the system transfer function of the IDFT processor, it manifests as a simple wavelength division multiplexer (WDM). However, the proposed IDFT maintains deterministic phase relations among subcarriers, so that one can manipulate the transmission system performance using OFDM techniques. A forward DFT can be constructed in the same way, as the circuit model is retro-propagation invariant except for the phase conjugation, requiring phase shifts negative to, and delays complementary to those in the inverse DFT.

_{k}## 3. 100-Gbps transmission application

*f*

_{C}, which corresponds to the wavelength of a continuous-wave source laser diode.

*f*

_{1}and

*f*

_{3}, or even-number channels at

*f*

_{2}and

*f*

_{4}, are selectively multiplexed. Here

*f*

_{1},

*f*

_{2},

*f*

_{3}, and

*f*

_{4}are referred to as subcarrier detuning frequencies, which are located at -37.5, -12.5, +12.5, and +37.5 GHz, respectively. The device technology how one can accurately control subcarrier channel frequencies in inverse DFT and DFT sets the limit how to closely pack the subcarriers. The current arrayed-waveguide grating on silicon substrate can maintain channel accuracies within approximately 10 GHz, and the subcarrier spacing need to be greater than 10 GHz. This limits the scalability of the number of subcarriers.

*f*

_{1},

*f*

_{2},

*f*

_{3}, and

*f*

_{4}, which equalizes receiver performances against optical amplifier noise and inter-subcarrier crosstalk penalties. Different choices of signs of the pre-emphasis change the waveform of OFDM symbols: Pre-emphasis with the same sign produces a single strong peak the center of an OFDM symbol waveform. In our case, there is a null at the centers located at 20 and 60 ps in Fig. 3(b), spreading the peak power to the neighboring 10-ps features. An optimized choice of pre-emphasis can offer for mitigation of fiber nonlinearity impairments.

## 4. Conclusions and Discussions

^{−12}. Comparisons with electronic OFDM may reveal other interesting merits and demerits, but we leave it as further studies.

## Acknowledgments

## References and links

1. | H.C. Bao and W. Shieh, “Transmission of wavelength-division-multiplexed channels with coherent optical OFDM,” IEEE Photon. Tech. Lett. |

2. | B. J. Schmidt, A. J. Lowery, and J. Armstrong, “Experimental demonstrations of 20 Gbit/s direct-detection optical OFDM and 12 Gbit/s with a colorless transmitter,” Natl. Fiber Optic Eng. Conference (NFOEC) 2007 (OSA, 2007), paper PDP18, http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2007-PDP18. |

3. | A. J. Lowery, L. B. Du, and J. Armstrong, “Performance of optical OFDM in ultralong-haul WDM lightwave systems,” J. Lightwave Technol. |

4. | A. J. Lowery, “Fiber nonlinearity pre- and post-compensation for long-haul optical links using OFDM,” Opt. Exp. |

5. | A. J. Lowery, “Fiber Nonlinearity mitigation in optical links that use OFDM for dispersion compensation,” IEEE Photon. Technol. Lett. |

6. | W. Shieh, X. Yi, Y. Ma, and Y. Tang, “Theoretical and experimental study on PMD-supported transmission using polarization diversity in coherent optical OFDM systems,” Opt. Express |

7. | W. Shieh, R. S. Tucker, W. Chen, X. Yi, and G. Pendock, “Optical performance monitoring in coherent optical OFDM systems,” Opt. Exp. |

8. | A. J. Lowery and J. Armstrong, “Orthogonal frequency division multiplexing for dispersion compensation of long-haul optical systems”, Opt. Express |

9. | W. Shieh, W. Chen, and R. S. Tucker, “Polarization mode dispersion mitigation in coherent optical orthogonal frequency division multiplexed systems,” Electron. Lett. |

10. | I. B. Djordjevic, “PMD compensation in fiber-optic communication systems with direct detection using LDPC-coded OFDM,” Opt. Exp. |

11. | N. Cvijetic, L. Xu, and T. Wang, “Adaptive PMD compensation using OFDM in long-haul 10Gb/s DWDM systems,” Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest, (OSA, 2007), Paper OTuA5. [PubMed] |

12. | A. Sano, H. Masuda, E. Yoshida, T. Kobayashi, E. Yamada, Y. Miyamoto, F. Inuzuka, Y. Hibino, Y. Takatori, K. Hagimoto, T. Yamada, and Y. Sakamaki, “30 × 100-Gb/s all-optical OFDM transmission over 1300 km SMF with 10 ROADM nodes,” Technical Digest of ECOC 2007, Paper PDS1.7 (2007) |

**OCIS Codes**

(060.0060) Fiber optics and optical communications : Fiber optics and optical communications

(060.4230) Fiber optics and optical communications : Multiplexing

(070.7145) Fourier optics and signal processing : Ultrafast processing

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: January 3, 2008

Revised Manuscript: February 27, 2008

Manuscript Accepted: February 29, 2008

Published: March 11, 2008

**Citation**

Kyusang Lee, Chan T. D. Thai, and June-Koo Kevin Rhee, "All optical discrete Fourier transform processor
for 100 Gbps OFDM transmission," Opt. Express **16**, 4023-4028 (2008)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-6-4023

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### References

- H. C. Bao and W. Shieh, "Transmission of wavelength-division-multiplexed channels with coherent optical OFDM," IEEE Photon. Tech. Lett. 19, 922-924 (2007). [CrossRef]
- B. J. Schmidt, A. J. Lowery, and J. Armstrong, " Experimental demonstrations of 20 Gbit/s direct-detection optical OFDM and 12 Gbit/s with a colorless transmitter," Natl. Fiber Optic Eng. Conference (NFOEC) 2007 (OSA, 2007), paper PDP18, http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2007-PDP18.
- A. J. Lowery, L. B. Du, and J. Armstrong, "Performance of optical OFDM in ultralong-haul WDM lightwave systems," J. Lightwave Technol. 25, 131-138 (2007). [CrossRef]
- A. J. Lowery, "Fiber nonlinearity pre- and post-compensation for long-haul optical links using OFDM," Opt. Express 15, 12966-12970 (2006).
- A. J. Lowery, "Fiber Nonlinearity mitigation in optical links that use OFDM for dispersion compensation," IEEE Photon. Technol. Lett. 19, 1556-1558 (2007). [CrossRef]
- W. Shieh, X. Yi, Y. Ma, and Y. Tang, "Theoretical and experimental study on PMD-supported transmission using polarization diversity in coherent optical OFDM systems," Opt. Express 15, 9936-9947 (2007). [CrossRef] [PubMed]
- W. Shieh, R. S. Tucker, W. Chen, X. Yi, and G. Pendock, "Optical performance monitoring in coherent optical OFDM systems," Opt. Express 15, 350-356 (2007). [CrossRef]
- A. J. Lowery and J. Armstrong, "Orthogonal frequency division multiplexing for dispersion compensation of long-haul optical systems," Opt. Express 14, 2079-2084 (2006). [CrossRef] [PubMed]
- W. Shieh, W. Chen, and R. S. Tucker, "Polarization mode dispersion mitigation in coherent optical orthogonal frequency division multiplexed systems," Electron. Lett. 42, 996 - 997 (2006). [CrossRef]
- I. B. Djordjevic, "PMD compensation in fiber-optic communication systems with direct detection using LDPC-coded OFDM," Opt. Express 15, 3692-3701 (2007). [CrossRef]
- N. Cvijetic, L. Xu, and T. Wang, "Adaptive PMD compensation using OFDM in long-haul 10Gb/s DWDM systems," Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest, (OSA, 2007), Paper OTuA5. [PubMed]
- <other>. A. Sano, H. Masuda, E. Yoshida, T. Kobayashi, E. Yamada, Y. Miyamoto, F. Inuzuka, Y. Hibino, Y. Takatori, K. Hagimoto, T. Yamada, and Y. Sakamaki, "30x100-Gb/s all-optical OFDM transmission over 1300 km SMF with 10 ROADM nodes," Technical Digest of ECOC 2007, Paper PDS1.7 (2007).</other>

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