## Auto bias control technique for optical OFDM transmitter with bias dithering |

Optics Express, Vol. 21, Issue 5, pp. 5833-5841 (2013)

http://dx.doi.org/10.1364/OE.21.005833

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### Abstract

In coherent optical communication systems, the transmitter usually employs an optical in-phase and quadrature (IQ) modulator to perform electrical-to-optical up-conversion. However, some environmental factors, such as temperature and mechanical stress, strongly influence the stability. To stabilize the quality of the transmitted signal, auto bias control (ABC) is essential to keep modulator in optimum bias. In this paper, we present a novel method of ABC for the optical orthogonal frequency division multiplexing (O-OFDM) signal. In the proposed scheme, a small cosine/sine wave dither signal is added on to the I/Q baseband signal, respectively. Based on the power monitoring of the 1st and 2nd harmonics of the dither signal, the biases of the optical IQ modulator for O-OFDM system can be adjusted very precisely. The simulation and experimental results show good performance on locating the optimum bias voltages for the IQ modulator with high precision.

© 2013 OSA

## 1. Introduction

1. G. Li, “Recent advances in coherent optical communication,” Adv. Opt. Photon. **1**(2), 279–307 (2009). [CrossRef]

3. W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett. **42**(10), 587–589 (2006). [CrossRef]

4. W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express **16**(2), 841–859 (2008). [CrossRef] [PubMed]

7. H. Kawakami, E. Yoshida, and Y. Miyamoto, “Asymmetric dithering technique for bias condition monitoring in optical QPSK modulator,” Electron. Lett. **46**(6), 430–431 (2010). [CrossRef]

8. H. Kawakami, T. Kobayashi, E. Yoshida, and Y. Miyamoto, “Auto bias control technique for optical 16-QAM transmitter with asymmetric bias dithering,” Opt. Express **19**(26), B308–B312 (2011). [CrossRef] [PubMed]

10. P. S. Cho and M. Nazarathy, “Bias control for optical OFDM transmitters,” IEEE Photon. Technol. Lett. **22**(14), 1030–1032 (2010). [CrossRef]

10. P. S. Cho and M. Nazarathy, “Bias control for optical OFDM transmitters,” IEEE Photon. Technol. Lett. **22**(14), 1030–1032 (2010). [CrossRef]

10. P. S. Cho and M. Nazarathy, “Bias control for optical OFDM transmitters,” IEEE Photon. Technol. Lett. **22**(14), 1030–1032 (2010). [CrossRef]

*V*) close to the optimum DC condition, the statistical analysis only offers a tiny distinguish ratio of the monitored power (< 0.5 dB). This may be a challenge for the practical implementation. In our proposed scheme, we add a small sine/cosine dither onto the OFDM I/Q signal. Based on the power monitoring of the 1st and 2nd harmonics of the dither signal, the bias of the optical IQ modulator for O-OFDM system can be finely controlled. A numerical simulation is first conducted to investigate the effect of the proposed bias control method. We then carry out a 10 Gb/s single-polarization CO-OFDM experiment to evaluate the system performance under the controlled bias condition. We will demonstrate that the proposed approach successfully enables us to control the system to the optimum bias condition with only negligible dithering penalty in the back-to-back configuration.

_{π}## 2. The principle of auto bias control technique

*E*can be written as:where,

_{o}*φ*is the phase difference between the IQ branches,

_{IQ}*V*is the half-wave switching voltage,

_{π}*V*and

_{bias,I}*V*are the DC bias voltages for the real and imaginary components, respectively. The monitored electrical current of the coupled PD at the output end is proportional to the output optical power (

_{bias,Q}*I*∝

_{m}*P*= |

_{o}*E*|

_{o}^{2}), given by [10

**22**(14), 1030–1032 (2010). [CrossRef]

*φ*= π

_{I(Q)}*V*,

_{I(Q)}/V_{π}*φ*= π

_{bias,I(Q)}*V*, and

_{bias,I(Q)}/V_{π}*P*denotes the input optical power

_{i}*P*= |

_{i}*E*|

_{i}^{2}.

*I*. Thus, we can monitor the power of the monitoring signal

_{m}*I*including both RF and DC components using an electrical low-pass filter, which we call ‘current power’ for brevity. Minimizing the current power by tuning the two biases suppresses the optical carrier, and forces the bias to the null point. Figure 2(a) shows the monitored current power as a function of both

_{m}*φ*and

_{bias,I}*φ*under the optimum

_{bias,Q}*φ*. As shown in Fig. 2(a), for any given

_{IQ}*φ*, we can minimize the current power to find the optimum

_{bias,Q}*φ*. Similarly, we can then find the optimum

_{bias,I}*φ*under the optimum

_{bias,Q}*φ*With a few rounds of tracing, the current power can achieve the minimum value, which indicates the two bias voltages are optimized. To observe the detailed simulation results, we plot the relative current power as a function of

_{bias,I}.*φ*with varying

_{bias,I}*φ*and

_{IQ}*φ*in the Fig. 2(b). Here, we define the distinguish ratio (DR) as the maximum current power variation in a specified range of biases. Within the range of |

_{bias,Q}*△φ*0.1π, the DR is only 0.5 dB, where

_{bias,I}|≤*△φ*=

_{bias,I}*φ*-

_{bias,I}*φ*. As a result,

_{bias,I_optimum}*φ*would be easily affected by the electrical noise or resolution inaccuracy of sampling [10

_{bias,I(Q)}**22**(14), 1030–1032 (2010). [CrossRef]

*φ*using the variance of

_{IQ}*I*as a function of the phase shifter. Similarly, the optimum

_{m}*φ*is hard to be precisely controlled.

_{IQ}*f*should be much smaller than the bandwidth of OFDM subcarriers. The detailed procedure of proposed ABC technique is described as following: I) We first monitor the minimum current power based on [10

_{dith}**22**(14), 1030–1032 (2010). [CrossRef]

*φ*is indicated by monitoring the overall minimum current power at the frequency of 2nd harmonic of dither signal. III) With numbers of iterations, the optimum bias of

_{IQ}*φ*can be estimated by monitoring the minimum current power at the frequency of 1st harmonic of dither signal. Next the fundamental and simulation results of proposed scheme will be introduced. The dither signal is expressed as

_{bias,I/Q}*V*cos(2π

_{dith}*f*t) and

_{dith}*V*sin(2π

_{dith}*f*t) for the real and imaginary branches respectively, where

_{dith}*V*and

_{drift, I}*V*are relatively small compared to

_{drift, Q}*V*. In such a configuration, the biased signal can be expressed as:

_{π}*V*and

_{I}*V*can be written as Eq. (4), note that 2

_{Q}*f*<<

_{dith}*f*:First we set F1, F2 as following:Then we substitute Eq. (3), Eq. (4) into

_{s}*F*and expand using a Taylor series expansion to second order, respectively:

_{1}, F_{2}*f*. Because the frequency of dither signal

_{s}-f_{dither}*f*is much smaller than the subcarrier bandwidth

_{dither}*f*, those cross terms can be digitally or electrically removed by employing a low-pass filter. Therefore, the

_{s}*F*

_{1}can be approximately express as:

*F*

_{2}can be also expressed as:

*φ*≠90°. The 2nd harmonic component only vanishes when

_{IQ}*φ*equals to 90°. Similarly, according to the Eq. (8) the 1st harmonic component vanishes when

_{IQ}*V*and

_{drift, I}*V*equal to zero. Thus, within the 0.1π range based on [10

_{drift, Q}**22**(14), 1030–1032 (2010). [CrossRef]

*φ*to the optimum, which can be achieved by monitoring the power of 2nd harmonic component of dither signal using a narrow band pass filter. Minimizing this power will lead

_{IQ}*φ*to 90°.

_{IQ}*f*versus

_{dith}*φ*of different

_{IQ}*φ*. As shown in the figure, the 2nd harmonic component power reaches the minimum value when

_{bias,I(Q)}*φ*equals to π/2 with any settings of

_{IQ}*φ*. The DR within the range of |

_{bias,I(Q)}*△φ*0.1π is larger than 15dB. Such a high DR signifies that the optimum bias can be accurately set.

_{IQ}|≤*φ*and

_{bias,I}*φ*, where

_{bias,Q}*φ*= π/2. The power of 1st harmonic component reaches the minimum when both of

_{IQ}*φ*and

_{bias,Q}*φ*are optimum. Figure 4(b) shows the power of the 1st harmonic component as a function of

_{bias,I}*φ*with certain values of

_{bias,I}*△φ*. From the Fig. 4(b) we find that the performance of proposed technique is dependent on the resolution of the adjusted bias votages provided by the DACs. For instance, DR is only 5 dB with resolution of 0.02π whereas DR of 17 dB can be achieved with resolution of 0.001π. Currently many commercialized DACs have a resolution up to 16 bits, which can give a tracking resolution of 2π/2^16 = 0.00003π. With such resolution the DR within range of |

_{bias,Q}*△φ*0.1π is more than 20 dB. With a few iterations of tracing the minimum power, we can readily find and control the bias

_{bias,I}|≤*φ*to be close to the optimum.

_{bias,I(Q)}## 3. Experimental setup and results

*V*. A typical coherent receiver is used to detect the optical signal. The received base band signal is digitized by a real-time scope operated at 12.5 GS/s. In the digital signal processing, we use a digital band-pass filter with a bandwidth of 1MHz to remove other frequency components, in order to monitor the RF power which only includes the 1st or 2nd harmonic component.

*△φ*). The

_{bias,I}*φ*is set to

_{IQ}*π/2*. There exists a power floor of the monitored RF power in the region close to the optimum bias, which matches well with the simulation results in Fig. 2. Some local minimum points can be found in the experiment, which is mainly due to the electrical/optical noise and the quantization error of ADC when detecting the small signal. However, by tracking the minimum current power in large variation range of

*φ*and

_{bias,I}*φ*with sufficient DAC resolution we can locate the biases in the range of 0.1π close to optimum.

_{bias,Q}*φ*=

_{IQ}*π/2*, the 2nd harmonic component (~20MHz) vanishes. Hence, minimizing the 2nd harmonic component power will directly lead

*φ*to

_{IQ}*π/2*. The DR is as high as ~13 dB.

*φ*, where

_{bias,I}*φ*and

_{bias,Q}*φ*are set to optimum voltages. Similarly, the 1st harmonic component vanishes when

_{IQ}*φ*reaches to the optimum point. The DR is up to 16 dB when |

_{bias,I}*△φ*0.05π (where the range of 0.29

_{bias,I}|≤*V*). The experimental results are well in agreement with the simulation results discussed in section 2.

^{−3}is ~3 dB. The required OSNRs with and without dither signal are 5.0 dB and 4.8 dB, respectively. The tiny difference is mainly due to additional power of the dithering signal. The inset shows 4-QAM constellation measured at high OSNR (~6 dB/0.1nm) under the dynamically tracked optimum biases.

## 4. Conclusions

## Acknowledgment

## References and links

1. | G. Li, “Recent advances in coherent optical communication,” Adv. Opt. Photon. |

2. | I. B. Djordjevic and B. Vasic, “Orthogonal frequency division multiplexing for high-speed optical transmission,” Opt. Express |

3. | W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett. |

4. | W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express |

5. | T. Sugihara, T. Yoshida, and K. Ishida, “Effect of Modulator Bias Control in the Presence of a Finite Extinction Ratio in DQPSK Pre-equalization Systems,” J. Lightwave Technol. |

6. | K. Sekine, C. Hasegawa, N. Kikuchi, and S. Sasaki, “A Novel Bias Control Technique for MZ Modulator with Monitoring Power of Backward Light for Advanced Modulation Formats,” in |

7. | H. Kawakami, E. Yoshida, and Y. Miyamoto, “Asymmetric dithering technique for bias condition monitoring in optical QPSK modulator,” Electron. Lett. |

8. | H. Kawakami, T. Kobayashi, E. Yoshida, and Y. Miyamoto, “Auto bias control technique for optical 16-QAM transmitter with asymmetric bias dithering,” Opt. Express |

9. | H. Choi, Y. Takushima, H. Y. Choi, J. H. Chang, and Y. C. Chung, “Modulation-Format-Free Bias Control Technique for MZ Modulator Based on Differential Phasor Monitor,” in |

10. | P. S. Cho and M. Nazarathy, “Bias control for optical OFDM transmitters,” IEEE Photon. Technol. Lett. |

11. | Q. Yang, Y. Ma, and W. Shieh, “107 Gb/s Coherent Optical OFDM Reception Using Orthogonal Band Multiplexing,” in |

**OCIS Codes**

(060.1660) Fiber optics and optical communications : Coherent communications

(060.2330) Fiber optics and optical communications : Fiber optics communications

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: December 7, 2012

Revised Manuscript: January 25, 2013

Manuscript Accepted: February 7, 2013

Published: March 1, 2013

**Citation**

Tao Gui, Cai Li, Qi Yang, Xiao Xiao, Linghen Meng, Chao Li, Xingwen Yi, Chao Jin, and Zhaohui Li, "Auto bias control technique for optical OFDM transmitter with bias dithering," Opt. Express **21**, 5833-5841 (2013)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-5-5833

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### References

- G. Li, “Recent advances in coherent optical communication,” Adv. Opt. Photon.1(2), 279–307 (2009). [CrossRef]
- I. B. Djordjevic and B. Vasic, “Orthogonal frequency division multiplexing for high-speed optical transmission,” Opt. Express14(9), 3767–3775 (2006). [CrossRef] [PubMed]
- W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett.42(10), 587–589 (2006). [CrossRef]
- W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express16(2), 841–859 (2008). [CrossRef] [PubMed]
- T. Sugihara, T. Yoshida, and K. Ishida, “Effect of Modulator Bias Control in the Presence of a Finite Extinction Ratio in DQPSK Pre-equalization Systems,” J. Lightwave Technol.29(15), 2235–2248 (2011).
- K. Sekine, C. Hasegawa, N. Kikuchi, and S. Sasaki, “A Novel Bias Control Technique for MZ Modulator with Monitoring Power of Backward Light for Advanced Modulation Formats,” in Proc. of OFC (2007), Paper. OTuH5.
- H. Kawakami, E. Yoshida, and Y. Miyamoto, “Asymmetric dithering technique for bias condition monitoring in optical QPSK modulator,” Electron. Lett.46(6), 430–431 (2010). [CrossRef]
- H. Kawakami, T. Kobayashi, E. Yoshida, and Y. Miyamoto, “Auto bias control technique for optical 16-QAM transmitter with asymmetric bias dithering,” Opt. Express19(26), B308–B312 (2011). [CrossRef] [PubMed]
- H. Choi, Y. Takushima, H. Y. Choi, J. H. Chang, and Y. C. Chung, “Modulation-Format-Free Bias Control Technique for MZ Modulator Based on Differential Phasor Monitor,” in Proc. of OFC (2011), Paper. JWA033.
- P. S. Cho and M. Nazarathy, “Bias control for optical OFDM transmitters,” IEEE Photon. Technol. Lett.22(14), 1030–1032 (2010). [CrossRef]
- Q. Yang, Y. Ma, and W. Shieh, “107 Gb/s Coherent Optical OFDM Reception Using Orthogonal Band Multiplexing,” in Proc. of OFC (2008), Paper. PDP7.

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