## Performances improvement in radio over fiber link through carrier suppression using Stimulated Brillouin scattering

Optics Express, Vol. 18, Issue 11, pp. 11827-11837 (2010)

http://dx.doi.org/10.1364/OE.18.011827

Acrobat PDF (1666 KB)

### Abstract

The performances of radio-over-fiber (RoF) link with fixed incident optical power on photodetector (PD) are improved through carrier suppression method. Firstly, a precise analytical model is proposed to quantify the relationship between the improvement of link gain, noise figure (NF), spur-free dynamic range (SFDR) and the carrier suppression ratio *x*, in which, the modulation index *m* is fully considered for the first time to our knowledge. Then the optimum optical carrier-to-sideband ratio (CSR) for RoF link performances in both double-sideband and single-sideband modulation is obtained from the optimum *x* for the link performances. Finally the experiments with the carrier subtraction method realized by Stimulated Brillouin scattering (SBS) are carried out and the experimental results show good agreement with the simulation ones.

© 2010 OSA

## 1. Introduction

1. E. Ackerman, S. Wanuga, D. Kasemset, A. S. Daryoush, and N. R. Samant, “Maximum dynamic range operation of a microwave external modulation fiber-optic link,” IEEE Trans. Microw. Theory Tech. **41**(8), 1299–1306 (1993). [CrossRef]

2. R. C. Williamson and R. D. Esman, “RF photonics,” J. Lightwave Technol. **26**(9), 1145–1153 (2008). [CrossRef]

2. R. C. Williamson and R. D. Esman, “RF photonics,” J. Lightwave Technol. **26**(9), 1145–1153 (2008). [CrossRef]

4. C. H. Cox III, E. I. Ackerman, G. E. Betts, and J. L. Prince, “Limits on the performance of RF-over-fiber links and their impact on device design,” IEEE Trans. Microw. Theory Tech. **54**(2), 906–920 (2006). [CrossRef]

5. A. Karim and J. Devenport, “Noise Figure Reduction in Externally Modulated Analog Fiber-Optic Links,” IEEE Photon. Technol. Lett. **19**(5), 312–314 (2007). [CrossRef]

5. A. Karim and J. Devenport, “Noise Figure Reduction in Externally Modulated Analog Fiber-Optic Links,” IEEE Photon. Technol. Lett. **19**(5), 312–314 (2007). [CrossRef]

8. W. K. Burns, G. K. Gopalakrishnan, and R. P. Moeller, “Multi-octave operation of low-biased modulators by balanced detection,” IEEE Photon. Technol. Lett. **8**(1), 130–132 (1996). [CrossRef]

9. M. J. LaGasse, W. Charezenko, M. C. Hamilton, and S. Thaniyavarn, “Optical carrier filtering for high dynamic range fibre optic links,” Electron. Lett. **30**(25), 2157–2158 (1994). [CrossRef]

10. K. J. Williams and R. D. Esman, “Stimulated Brillouin scattering for improvement of microwave fiber-optic link efficiency,” Electron. Lett. **30**(23), 1965–1966 (1994). [CrossRef]

5. A. Karim and J. Devenport, “Noise Figure Reduction in Externally Modulated Analog Fiber-Optic Links,” IEEE Photon. Technol. Lett. **19**(5), 312–314 (2007). [CrossRef]

10. K. J. Williams and R. D. Esman, “Stimulated Brillouin scattering for improvement of microwave fiber-optic link efficiency,” Electron. Lett. **30**(23), 1965–1966 (1994). [CrossRef]

11. Y. C. Shen, X. M. Zhang, and K. S. Chen, “Stimulated Brillouin scattering for efficient improvement of radio-over-fiber systems,” Opt. Eng. **44**(10), 105003 (2005). [CrossRef]

12. C. Lim, M. Attygalle, A. Nirmalathas, D. Novak, and R. Waterhouse, “Analysis of optical carrier-to-sideband ratio for improving transmission performance in fiber-radio links,” IEEE Trans. Microw. Theory Tech. **54**(5), 2181–2187 (2006). [CrossRef]

13. R. D. Esman and K. J. Williams, “Wideband efficiency improvement of fiber optic systems by carrier subtraction,” IEEE Photon. Technol. Lett. **7**(2), 218–220 (1995). [CrossRef]

12. C. Lim, M. Attygalle, A. Nirmalathas, D. Novak, and R. Waterhouse, “Analysis of optical carrier-to-sideband ratio for improving transmission performance in fiber-radio links,” IEEE Trans. Microw. Theory Tech. **54**(5), 2181–2187 (2006). [CrossRef]

13. R. D. Esman and K. J. Williams, “Wideband efficiency improvement of fiber optic systems by carrier subtraction,” IEEE Photon. Technol. Lett. **7**(2), 218–220 (1995). [CrossRef]

*m*. Both double-sideband and single-sideband modulation schemes are included. The optimum carrier suppression ratio and best CSR for the link gain, NF and SFDR are theoretically calculated and experimentally verified through carrier subtraction by SBS. As carrier suppression method will increase even-order distortion; it is only useful in sub-octave bandwidth applications. Therefore the result of the second-order harmonic relative to fundamental component with regards to carrier suppression ratio

*x*is also considered in this paper.

## 2. Theory

*m*in the RoF link can be expressed asHere,

*L*is the RoF link total loss,

*x*is the carrier suppression ratio and

13. R. D. Esman and K. J. Williams, “Wideband efficiency improvement of fiber optic systems by carrier subtraction,” IEEE Photon. Technol. Lett. **7**(2), 218–220 (1995). [CrossRef]

*m*is small enough that

*x*)

^{−2}(supposing

*x*)

^{2}. However, when the modulation index is not so small, the above approximation for the Bessel function is not exact, and the result is not precisely true.

*x*under different modulation index

*m*supposing

*x*)

^{−2}is also shown as curve a. It can be seen that the smaller

*m*is, the more the curve of link gain overlaps with curve a. Besides, the link gain increases with the increasing of carrier suppression ratio

*x*only when

*m*is small enough and

*x*is smaller than a certain value, beyond which the RF gain drops quickly.

*m*is small. While

*m*>0.5, it has little impact, which can be seen from curve e and f. In order to obtain a simple expression of the optimum

*x*and CSR for this method when

*m*is small, the above unexpanded expressions are simplified by ignoring the third-order and all the higher order sidebands, and leaving only the first item of Bessel function Taylor series. Thus, we can get the approximated expressions as below when

*m*is small.

12. C. Lim, M. Attygalle, A. Nirmalathas, D. Novak, and R. Waterhouse, “Analysis of optical carrier-to-sideband ratio for improving transmission performance in fiber-radio links,” IEEE Trans. Microw. Theory Tech. **54**(5), 2181–2187 (2006). [CrossRef]

*m*, the optimum carrier suppression ratio

**54**(5), 2181–2187 (2006). [CrossRef]

*k*is Boltzmann’s constant,

*T*

_{0}is room temperature (290 K), and

*Noise_out*is the output noise power from PD, which includes three main noise sources, thermal noise, RIN and shot noise [14

14. C. H. Cox III, G. E. Betts, and L. M. Johnson, “An analytic and experimental comparison of direct and external modulation in analog fiber-optic links,” IEEE Trans. Microw. Theory Tech. **38**(5), 501–509 (1990). [CrossRef]

15. C. Cox III, E. Ackerman, R. Helkey, and G. E. Betts, “Techniques and Performance of Intensity-Modulation Direct-Detection Analog Optical Links,” IEEE Trans. Microw. Theory Tech. **45**(8), 1375–1383 (1997). [CrossRef]

*x*under different

*m*, in which the laser-RIN is considered as −165 dB/Hz. The solid lines of curve a and b are the simulation results with

*m*= 0.3 and

*m*= 0.1, respectively, both of which have incident optical power on PD of 2 mW. While curve c and d are the dashed lines with the incident optical power on PD of 5 mW, and

*m*= 0.3 and m = 0.1, respectively. We can see that the parameter

*m*determines the shape of the NF curve, while the incident optical power on PD just determines the NF value at

*x*= 0 with no carrier suppression. Thus, if the modulation index

*m*is determined, the impact of carrier suppression on the improvement of NF and link gain is decided. And the smaller modulation index

*m*is, the greater improvement of NF and link gain can be obtained through the carrier suppression method, just as what is shown in Fig. 1 and Fig. 2

*IP*

_{3}is the input RF signal power when the third-order intermodulation output equals to the fundamental output, which only depends on the half-wave voltage of the modulator [7], having nothing to do with the carrier suppression ratio

*x*. The output noise also keeps unchanged when the incident optical power on PD is constant. Therefore, it can be concluded that the increase of link gain leads to an improvement of SFDR. And the best carrier suppression ratio

*x*for link gain is also the best point for SFDR with constant PD incident optical power. In such case, SFDR can be improved through increasing the link gain without lowering the noise level.

*x*increasing, the second-order harmonic component increases. Therefore this method is only limited in sub-octave bandwidth applications.

## 3. Experiments and results

*x*are demonstrated in Fig. 4 , which are shown as solid lines and dots, respectively. The RF input signal is 18 GHz,

*m*= 0.1 and incident optical power on PD is 2 mW. The simulation results of NF in Fig. 4 are much larger than Curve b in Fig. 2 with the same

*m*and the same PD incident optical power. The main reason is that we lack of high-power and low-noise laser source, and the laser RIN in the experiment is much larger than the simulation parameter of −165 dB/Hz in Fig. 2. However, we can still get great improvement of link performances and good agreement between simulation and experiments. From Fig. 4, we can see the optimum carrier suppression ratio for link gain is also the optimum one for NF. When the carrier suppression ratio

*x*is less than 0.93, the value corresponding to CSR of 3 dB for

*m*= 0.1, an amount of carrier suppression results in an equal amount of RF gain, also an equal amount of NF reduction. When

*x*>0.93, link gain drops while NF increases. There is a small deviation between the experimental results and the simulation results after the optimum carrier suppression ratio

*x*= 0.93. The reason lies in that, in order to obtain deep carrier suppression, the Stokes wave feeding back into the tail of DSF will be large enough to induce the second-order stokes, which reduces the useful part in the PD incident optical power and causes the performances worse. It also can be seen from Fig. 4 that there is almost 18 dB improvement of link gain and NF when CSR is 3 dB, which verifies the validity of the carrier subtraction method for the performances improvement of RoF link.

*m*= 0.3 and the incident optical power on PD of 2 mW. Just as the theory predicts, the larger

*m*is, the less improvement of link gain and NF we can get. In Fig. 5, there is less than 10 dB improvement of link gain and NF. However, good agreement between theory and experimental results is also obtained. The optimum carrier suppression ratio for

*m*= 0.3 is

*x*= 0.788 that corresponds to the optimum CSR of 3 dB. The experimental results with 9 GHz RF input and

*m*= 0.5 is shown in Fig. 6 , which only has less than 5 dB improvement. All the results in Fig. 4 to Fig. 6 show that the optimum carrier suppression ratio

*x*for link gain is the same one for NF and the modulation index

*m*is a key parameter for the performances improvement of RoF link with carrier subtraction method.

*x*is shown in Fig. 7 , in which RF signal frequency is 9 GHz and

*m*= 0.3. The solid line is the simulation results based on Eq. (21) and the diamond dots are the experimental ones. Good agreement is obtained. Obviously, the carrier subtraction method will increase the second-order harmonic, which should be limited in sub-octave applications.

^{2/3}when EDFA output is 23.3 dBm.

## 4. Conclusion

*m*is an important factor to determine whether this method is effective or not to improve the link performances. When

*m*is small enough, all figures of merit including link gain, NF and SFDR can be optimized by choosing a proper carrier suppression ratio. The optimum CSR for double-sideband and single-sideband modulation is 3 dB and 0 dB, respectively. However the carrier subtraction method will increase the second-order harmonic distortion, which may limit its applications. Besides, SBS is an effective way to function as a carrier filter, not only because it can realize carrier subtraction conveniently, but also it is a common nonlinear effect which may need to be avoided in other cases, as it will easily occur when the RoF link is longer and the input power is a bit higher, for example in the low-biasing scheme.

## Acknowledgements

## References and links

1. | E. Ackerman, S. Wanuga, D. Kasemset, A. S. Daryoush, and N. R. Samant, “Maximum dynamic range operation of a microwave external modulation fiber-optic link,” IEEE Trans. Microw. Theory Tech. |

2. | R. C. Williamson and R. D. Esman, “RF photonics,” J. Lightwave Technol. |

3. | C. H. Cox III, E. I. Ackerman, and J. L. Prince, “What do we need to get great link performance?” |

4. | C. H. Cox III, E. I. Ackerman, G. E. Betts, and J. L. Prince, “Limits on the performance of RF-over-fiber links and their impact on device design,” IEEE Trans. Microw. Theory Tech. |

5. | A. Karim and J. Devenport, “Noise Figure Reduction in Externally Modulated Analog Fiber-Optic Links,” IEEE Photon. Technol. Lett. |

6. | M. L. Farewell, W. S. C. Chang, and D. R. Huber, “Increased linear dynamic range by low biasing the Mach-Zehnder modulator,” IEEE Photon. Technol. Lett. |

7. | J. Devenport and A. Karim, “Optimization of an externally modulated RF photonic link,” Fiber Integr. Opt. |

8. | W. K. Burns, G. K. Gopalakrishnan, and R. P. Moeller, “Multi-octave operation of low-biased modulators by balanced detection,” IEEE Photon. Technol. Lett. |

9. | M. J. LaGasse, W. Charezenko, M. C. Hamilton, and S. Thaniyavarn, “Optical carrier filtering for high dynamic range fibre optic links,” Electron. Lett. |

10. | K. J. Williams and R. D. Esman, “Stimulated Brillouin scattering for improvement of microwave fiber-optic link efficiency,” Electron. Lett. |

11. | Y. C. Shen, X. M. Zhang, and K. S. Chen, “Stimulated Brillouin scattering for efficient improvement of radio-over-fiber systems,” Opt. Eng. |

12. | C. Lim, M. Attygalle, A. Nirmalathas, D. Novak, and R. Waterhouse, “Analysis of optical carrier-to-sideband ratio for improving transmission performance in fiber-radio links,” IEEE Trans. Microw. Theory Tech. |

13. | R. D. Esman and K. J. Williams, “Wideband efficiency improvement of fiber optic systems by carrier subtraction,” IEEE Photon. Technol. Lett. |

14. | C. H. Cox III, G. E. Betts, and L. M. Johnson, “An analytic and experimental comparison of direct and external modulation in analog fiber-optic links,” IEEE Trans. Microw. Theory Tech. |

15. | C. Cox III, E. Ackerman, R. Helkey, and G. E. Betts, “Techniques and Performance of Intensity-Modulation Direct-Detection Analog Optical Links,” IEEE Trans. Microw. Theory Tech. |

**OCIS Codes**

(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems

(060.4510) Fiber optics and optical communications : Optical communications

(060.4256) Fiber optics and optical communications : Networks, network optimization

(060.5625) Fiber optics and optical communications : Radio frequency photonics

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: November 30, 2009

Revised Manuscript: March 26, 2010

Manuscript Accepted: May 13, 2010

Published: May 20, 2010

**Citation**

Lan Liu, Shilie Zheng, Xianmin Zhang, Xiaofeng Jin, and Hao Chi, "Performances improvement in radio over fiber link through carrier suppression using Stimulated Brillouin scattering," Opt. Express **18**, 11827-11837 (2010)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-11-11827

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### References

- E. Ackerman, S. Wanuga, D. Kasemset, A. S. Daryoush, and N. R. Samant, “Maximum dynamic range operation of a microwave external modulation fiber-optic link,” IEEE Trans. Microw. Theory Tech. 41(8), 1299–1306 (1993). [CrossRef]
- R. C. Williamson and R. D. Esman, “RF photonics,” J. Lightwave Technol. 26(9), 1145–1153 (2008). [CrossRef]
- C. H. Cox III, E. I. Ackerman, and J. L. Prince, “What do we need to get great link performance?” Microwave Photonics, International topical meeting (Germany, 1997), pp. 215–218.
- C. H. Cox, E. I. Ackerman, G. E. Betts, and J. L. Prince, “Limits on the performance of RF-over-fiber links and their impact on device design,” IEEE Trans. Microw. Theory Tech. 54(2), 906–920 (2006). [CrossRef]
- A. Karim and J. Devenport, “Noise Figure Reduction in Externally Modulated Analog Fiber-Optic Links,” IEEE Photon. Technol. Lett. 19(5), 312–314 (2007). [CrossRef]
- M. L. Farewell, W. S. C. Chang, and D. R. Huber, “Increased linear dynamic range by low biasing the Mach-Zehnder modulator,” IEEE Photon. Technol. Lett. 5(7), 779–782 (1993). [CrossRef]
- J. Devenport and A. Karim, “Optimization of an externally modulated RF photonic link,” Fiber Integr. Opt. 27(1), 7–14 (2008).
- W. K. Burns, G. K. Gopalakrishnan, and R. P. Moeller, “Multi-octave operation of low-biased modulators by balanced detection,” IEEE Photon. Technol. Lett. 8(1), 130–132 (1996). [CrossRef]
- M. J. LaGasse, W. Charezenko, M. C. Hamilton, and S. Thaniyavarn, “Optical carrier filtering for high dynamic range fibre optic links,” Electron. Lett. 30(25), 2157–2158 (1994). [CrossRef]
- K. J. Williams and R. D. Esman, “Stimulated Brillouin scattering for improvement of microwave fiber-optic link efficiency,” Electron. Lett. 30(23), 1965–1966 (1994). [CrossRef]
- Y. C. Shen, X. M. Zhang, and K. S. Chen, “Stimulated Brillouin scattering for efficient improvement of radio-over-fiber systems,” Opt. Eng. 44(10), 105003 (2005). [CrossRef]
- C. Lim, M. Attygalle, A. Nirmalathas, D. Novak, and R. Waterhouse, “Analysis of optical carrier-to-sideband ratio for improving transmission performance in fiber-radio links,” IEEE Trans. Microw. Theory Tech. 54(5), 2181–2187 (2006). [CrossRef]
- R. D. Esman and K. J. Williams, “Wideband efficiency improvement of fiber optic systems by carrier subtraction,” IEEE Photon. Technol. Lett. 7(2), 218–220 (1995). [CrossRef]
- C. H. Cox, G. E. Betts, and L. M. Johnson, “An analytic and experimental comparison of direct and external modulation in analog fiber-optic links,” IEEE Trans. Microw. Theory Tech. 38(5), 501–509 (1990). [CrossRef]
- C. Cox, E. Ackerman, R. Helkey, and G. E. Betts, “Techniques and Performance of Intensity-Modulation Direct-Detection Analog Optical Links,” IEEE Trans. Microw. Theory Tech. 45(8), 1375–1383 (1997). [CrossRef]

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