e-Photon Summer School 1 Optimization of Wavelength Interleaved Radio-over-Fiber Systems Tiago Silveira, António Teixeira, R. Nogueira, P. André, P. Monteiro, A. Ferreira, S. Stevan, J. Rocha

2e-Photon Summer School Introduction to RoF systems The Wavelength Interleaved Technique Three Scenarios for the RS Simulation and Comparison of Results Simulation and Comparison of Results Hi-Bi Gratings as orthogonal filters Simulation and Comparison of Results Simulation and Comparison of ResultsConclusions Outline

3e-Photon Summer School What is Radio-over-Fiber? Radio Over Fiber –RoF is a technology where Microwave (Electrical) signals are distributed by means of optical components and techniques RoF systems consists normally of a Central Site (CS) and a Remote Site (RS) connected by an optical fiber link or network. Central Station (CS) Remote Sites (RS) WDM RING ADD/DROP Multiplexer λ 1, λ 2, … Remote Node

4e-Photon Summer School Cellular Networks Satellite communications Video Distribution Systems Wireless LANs Vehicle communication and control … Applications of RoF technology

5e-Photon Summer School The Wavelength Interleaved Technique This technique has the possibility of allowing multiple channels to share the spectrum. Can be based on reflective filter add drops which can allow low insertion losses, since no signal splitting is needed. Wavelength interleaving, allows the optical network to achieve a high spectral efficiency, due to its properties.

6e-Photon Summer School Scenarios of RoF systems The CS-RS link is bi-directional, therefore the RS node must be capable of sending information back to the CS. In the following slides we compare 3 possible approaches to be taken at the RS node.

7e-Photon Summer School 1st scenario The RN drops the needed signal optically and sends it to the BS that broadcasts it (electrically) to the users In order to transmit the signals coming from the user, the BS has one emitting laser that generates the dropped carrier and modulates the down link information to the CS COST: HIGH Local laser

8e-Photon Summer School 2nd scenario The RN drops the needed signal and a new clean carrier (generated at the CS), and sends them to the BS, which broadcasts the signal to the users. The information from the users is modulated in the dedicated clean carrier and transmitted to the CS. COST: Average Clean Carrier from CS

9e-Photon Summer School 3rd scenario The RN drops the needed signal optically and sends it to the BS that broadcasts to the users The signal received by the BS comming from the users is modulated by the BS in the reused carrier from the dropped signal and the information is transmitted to the CS COST: LOW Reused Carrier

10e-Photon Summer School Ex: Simulation setup (1st scenario only) The data, RF carrier of 60GHz Use of a Electro-absorption modulator CW DFB lasers Length of the Fiber is set to an average value of 17Km Optical add-drop multiplexer incorporating either fibre Bragg grating (FBG) filters and an optical circulator or Trapezoidal Filters FBG2 responsible for dropping the optical carrier

11e-Photon Summer School Results The Q factor curves for the data recovered at the RS, are presented for trapezoidal filters. Low Bandwidth Interference due to other channels Filter Acceptable bandwidth

12e-Photon Summer School Results The Q factor curves for the data recovered at the RS, are presented for gratings. Low Bandwidth Interference due to other channels Filter Acceptable bandwidth for low Q filter dependence

13e-Photon Summer School Results Introduction of a FBGs instead of ideal optical filters introduces a small penalty for low Bandwidths For higher bandwidths, the system performance starts to be strongly dependent on the filter bandwidth since the lateral lobes of the considered uniform grating start to let a considerable amount of power into the detector causing signal degradation. The peaks noticed in the Q performance occur when the FBG transfer function presents a null over the interfering channels, causing the interference level to be drastically reduced.

14e-Photon Summer School Improved technique The oscillations and degradation in the performance is due to the neighboring channels So, we suggest a technique to improve the quality of the results by polarization multiplexing the carriers and the signals: all the carriers are in one polarization (say X) and all the signals are in the other (say Y). all the carriers are in one polarization (say X) and all the signals are in the other (say Y). If we have polarization sensitive filters, the resulting interference is lowered since the next interferer is twice frequency deviated If we have polarization sensitive filters, the resulting interference is lowered since the next interferer is twice frequency deviated

15e-Photon Summer School Improved technique X X X X X X X X X Y X Y X Y Previous Technique Improved Technique Dropped signal Closest interferers (carriers: High power) Dropped signal Closest interferers (Signals: same power, twice the distance)

16e-Photon Summer School How to implement it in a simple way? The two lobes of the filter are in two different frequencies and in orthogonal polarizations. Hi-Bi FBG’s.

17e-Photon Summer School Filter the signal at the emitting point (CS) Use the same filter for all the dropping in the system With this technique we only need only one filter to drop both the carrier and the signal How to generate and process the signals? X pol Y pol freq X pol Y pol freq Hi-Bi FBG’s

18e-Photon Summer School Results Previous configuration Suggested configuration Input signals Dropped signals

19e-Photon Summer School More results Q(FBG)>Q(HIBI) due to 3dB more power Q(HIBI)>Q(FBG) due to reduced interference

20e-Photon Summer School Conclusions We have presented a Radio over Fiber system where three scenarios for the RS were compared in terms of complexity, cost and performance for two types of filters. The performance dependence on the filters bandwidth was discussed and the main limiting factors identified It can be seen that the system cost and simplicity can be drastically reduced by reusing the carrier frequency at the expense of decreasing the Q of the system By introducing Hi-Bi FBG the system becomes less interference dependent and the simplicity of the add and drop operations can be increased

21e-Photon Summer School Acknowledgments The authors would like to thank FCT project Wo-Net POSI/2001/CPS/ E-Photon One That’s all folks!! Tiago Silveira University of Aveiro