Investigation Of Transmission Methods And Photonic Microwave Signal Processing Technologies In Radio-over-fiber System

In view of the burgeoning growth of high-speed and large-bandwidth streaming media technology based services such as Internet Protocol Television(IPTV)?High-definition Television(HDTV)?video conferences driven by smart phones or Internet services,the bandwidth and data rate requirement in wireless access networks is rapidly increasing.Benefiting from the huge bandwidth and low-loss of optical fiber transmission,radio over-fiber(RoF)technologies have been considered as an efficient delivery of high-speed multimedia services to remote base stations from central office(CO).This paper mainly focuses on the research of signal transmission methods and access technologies in RoF system as well as microwave photonics technologies.Three kind of novel transmission methods in RoF system as well as the photonic microwave signal mixing technologies and frequency and phase tunable OEO using photonic filter are studied in this paper.The highlights of this work are summarized as follow:1.A RoF system with colorless upstream transmission based on phase-coherent orthogonal lightwave generation(POLG)is proposed.POLG is realized by using a polarization rotator(PR)and the polarization-sensitive properties of Mach-Zehnder modulator(MZM),achieves polarization-orthogonality between the optical carrier and subcarriers generated by radio frequency signals.Then the scheme is mathematically formulated and experimentally demonstrated.In the scheme,by adjusting a polarization controller(PC)in the remote access units(RAU),different modulation schemes can be flexibly implemented,e.g.optical double-sideband(ODSB)modulation for low radio frequency service,optical carrier suppression(OCS)modulation for millimeter-wave(MMW)service.In the meantime,the optical carrier(OC)can be reused for the upstream transmission without any filtering and additional PC.Thus,this scheme realizes low frequency and MMW services and colorless upstream transmission successfully.2.A full-duplex MMW RoF access architecture employing polarization-insensitive RAU is proposed.In this scheme,the upstream signals are generated by using polarization-orthogonal lights,and the performance of the upstream signals is independent of the polarization state of the MZM input lights.Hence,it is not necessary to align the polarization state of the optical signal with the principal axis of the MZM crystal in the RAU.The RAU is polarization insensitive.The proposed system is mathematically formulated and experimentally demonstrated.The 1.2-Gb/s quadrature phase-shift keying(QPSK)orthogonal frequency-division multiplexing(OFDM)downstream signal at the 60-GHz band and 5-Gb/s on-off keying(OOK)polarization-insensitive upstream signal over a 15-km standard single-mode fiber(SSMF)are demonstrated.The upstream signal performance with BER about 10-9 can be roughly obtained,hence forward error correction(FEC)is not necessary.Compared to the conventional scheme,our proposed scheme has better BER performance with a more flat time trace.3.Two schemes to compensate the dispersion-induced power fading effects are proposed.The first scheme is based on an optical single sideband(OSSB)modulation.The OSSB modulation with tunable optically carrier-to-sideband ratio(OCSR)is achieved by using the Sagnac loop and modulator.This scheme is experimentally demonstrated.In the experiment,an OSSB-modulated signal with tunable OCSR from-10 dB to 27 dB is achieved by adjusting the PC.The proposed scheme is wavelength independent,which can support low microwave bandwidth and achieve flexible wavelength tunability.The second method is based on the POLG technique.At the CO,a PR and a MZM is used to achieve POLG technique.By adjusting a PC in the RAU,the phase difference between the orthogonal polarization carrier and two sidebands can be controlled,and the frequency response can be flexibly shifted for individual radio frequency service.In the experiment,the destructive interference can be shifted to the constructive interference at various frequencies for 25 km and 30 km fiber links.4.According the Sagnac loop and polarization dependent modulation of MZM,a new photonic-assisted microwave mixer is proposed and experimentally demonstrated.In this scheme,a dual-parallel Mach-Zehnder modulator(DPMZM)is incorporated in a Sagnac loop,where light is optical single-sideband(OSSB)modulated along one direction with no modulation along the opposite direction due to the traveling-wave velocity mismatch in DPMZM.The OSSB-modulated light and unmodulated OC with opposite propagation directions and orthogonal polarizations are combined via a polarization beam combiner(PBC),and thus,a partial orthogonal OSSB-modulated signal is obtained,which is injected into a Mach-Zehnder modulator(MZM),followed by a polarizer and a photodetector for optical-to-electrical conversion,a phase-stable microwave signal is obtained.By adjusting the bias point of MZM,clear eye diagrams,and waveforms of 1 Gb/s OOK,amplitude-shift keying(ASK),and binary-phase-shift keying(BPSK)signals at 8 GHz are experimentally achieved.5.A scheme to realize optoelectronic oscillator(OEO)with simultaneously frequency and phase tunability is proposed and experimentally demonstrated.In the proposed OEO,a carrier phase-shifted double-sideband modulated optical signal is generated using a DPMZM.The stimulated Brillouin scattering effect is used to select the oscillation frequency which equals to the Brillouin frequency shift.The tunable phase of the generated microwave signal is achieved by controlling the bias voltage of the DPMZM to change the phase difference between the optical carrier and the sideband.Thanks to the wavelength-dependent effect of the Brillouin frequency shift,the tunable frequency is realized by tuning the frequency of the laser source.Microwave signals with a tunable frequency from 8.950 to 9.351 GHz and a tunable phase from 0° to 360°are experimentally generated.