Full Duplex Radio Over Fiber System Based On Polarization Multiplexing And Wavelength Reuse

Radio over fiber(RoF)system has received great popularity as it combines the advantages of mobile communication and optical fiber communication.It combines the advantage of the anti-electromagnetic interference?low cost?good mobility?large bandwidth?low signal crosstalk?small optical fiber size and long stable transmission distance.A general RoF consisting of the center station and base station.Generally speaking,RoF technology is using the center station' s low frequency RF signal modulated onto the optial carrier through Mach-Zehnder modulator,and then transmit to the base station via single mode fiber.Then a photodetector is used to perform optical-electrical conversion,converting the optical signal into the corresponding electrical signal,then it is transmitted into the air via antenna.In order to make use of the free license 5GHz in the 60GHz millimeter-wave signal,researchers have published various reports.Firstly,as 60GHz electrical devices are too expensive to commercialize and it is rarely used.The most common method is modulating the low frequency RF signal into the optical carrier and process the modulated signal in the optical domain,which is easy and flexible.As the Spatial light modulators' implement and market are more sophisticated in recent years,this method will be much more popular.Furthermore,lithium niobate modulator has played a key role in the RoF system as its non-fluctuate response of frequency and steady performance of transmission.It makes the electrical signal modulated onto the optical domain,reducing the cost of the overall system.Lithium niobate modulator consisting of phase modulator and intensity modulator.The phase modulator is stable and can generate all sidebands easily by changing the phase of the signal.The intensity modulator biased at the minimum transmission point,maximum transmission point,quadrature transmission point to realize double sideband modulation,single sideband modulation,double sideband with optical carrier suppression modulation,or suppress all even-order sidebands modulation.Then optical filter,fiber bragg grating or interleaver can be used to suppress undesired sidebands to generate double,quadruple or even higher multiplication factor millimeter-wave signal generation.However,as the optical filter is wavelength-dependent,limiting the bandwidth of the system and reduce the quality of generated signal.Based on the above discussion,we have proposed a new method of using the polarization multiplexed and correlated signals to suppress the optical carrier,generating double,quadruple frequency of the RF signal without any optical filtering technique,while the recovered pure optical carrier is wavelength reused to provide light source for the uplink,further reduce the cost of base station.The main innovations in this article are as follows:1.A simple and versatile method to realize optical generation of microwave signal using carrier polarization-suppressed double sideband modulation and no optical filtering is proposed and experimentally demonstrated.By simply adjusting the polarization direction of the polarizer,the optical carrier generated by the single-drive intensity modulator along one polarization direction is polarization-suppressed by the pure optical carrier along another orthogonal polarization direction.A high spectrum purity 20 GHz electrical signal with more than 36 dB optical harmonic distortion suppression ratio is generated when the driving signal is 10 GHz.The system is simple,cost-effective and fast frequency tuned as no optical filtering technique is required,which may be attractive to the radio over fiber system.2.A full duplex radio over fiber system based on polarization multiplexing and carrier polarization-suppressed double sideband modulation using a phase modulator is theoretical analyzed and experimentally demonstrated.In the center station(CS),the optical carrier along one polarization direction is fed into a phase modulator driven by downlink signal,while the optical carrier along the other polarization direction is keep unmodulated.Then the two signals are polarization multiplexed by the polarization beam combiner(PBC).In the base station(BS),the optical carrier is polarization suppressed by simply adjusting the polarization angle between the principle axis of the polarizer and one principle axis of the PBC to be 135° and the millimeter-wave signal is generated when it is sent into a photodiode for optical/electrical conversion.On the other hand,when the principle axis of polarizer is aligned at an angle of 90 degrees to one principle axis of PBC,the original unmodulated pure optical carrier is recovered,which can be wavelength reused to provide light source for the uplink and reduce the cost of BS unit while not increase the complexity of CS as conventional methods.A proof of concept experiment is carried out to verify the feasibility of the proposed system.Experimental results show that the power penalties of bidirectional 1Gb/s signal are less than 0.5 dB after transmitted over 10 km single mode fiber.The system is simple cost-effective and may be attractive to the radar and telecommunication network.3.A full duplex radio over fiber system with frequency quadrupled millimeter-wave signal generation based on polarization multiplexing using a single-drive intensity modulator is proposed.In the central station,downstream signal modulated on the single-drive intensity modulator is polarization multiplexed with pure optical carrier by a PBC.The single-drive intensity modulator is biased at the maximum transmission point to generate optical carrier and two second-order sidebands.In the base station,by simply adjusting the difference angle between the principle axis of polarizer and one principle axis of PBC to be 135 degrees,a frequency quadrupled millimeter-wave signal is generated.In addition,when the difference angle between the principle axis of polarizer and one principle axis of PBC is adjusted to be 90°,the original pure optical carrier is recovered,which is wavelength reused to provide light source for the uplink to deliver upstream signal.A proof-of-concept experiment is performed.Pure optical carrier and 40 GHz millimeter-wave signal with 20 dB optical harmonic suppression ratio are obtained.The power penalties of the bidirectional links are less than 0.3 dB after transmitted over 10.5 km single mode fiber.The measured power fluctuation of the generated millimeter-wave is less than 1 dB in one hour,showing the proposed scheme is relatively stable for long-distance transmission system.4.We experimentally demonstrated a frequency quadrupling technique for RoF system to realize baseband signal,20 GHz MW signal and 40 GHz MMW signal generation simultaneously.In the proposed scheme,the two second-order sidebands are generated when the optical carrier along one polarization axis is polarization suppressed by the optical carrier along the other orthogonal polarization axis,and the frequency quadrupled 40 GHz MMW signal is generated when it is sent to a high speed photodiode for detection.In addition,pure optical carrier is wavelength reused to provide light source and modulated with the uplink signal.As a result,the 1.5 Gb/s Datal,1 Gb/s Data2 and 1 Gb/s Data3 are delivered successfully with less than 0.5 dB power penalty.