Doped fiber external cavity laser for radio over fiber applications

Radio over fiber is an optical-microwave hybrid technique. With a radio over fiber system, the microwave or radio frequency waves can be generated and transmitted over the fiber optic communication systems to remote locations. The radio over fiber system has the advantages of wireless communications, which are convenience and mobility. Also it has the advantages of fiber optic communications, which are the long distance low loss and high capacity transmission.;Narrow line-width and stable wavelength laser has been studied for radio over fiber communication and fiber optic communication for a long time. Recent studies have shown that Doped Fiber External Cavity Laser (DFECL) has very good performance in line-width and wavelength in its free running state. It is a potential laser source for radio over fiber applications. The detailed analysis of the principle and the experimental studies of the application have not reported before.;This thesis is focused on analysis, design and demonstration of a novel ultra long DFECL, for the purposes of radio over fiber communication applications.;This thesis starts with the study of doped fiber characteristics for an external cavity laser. Although the doped fiber has been extensively studied for applications in fiber amplifiers, the situation has been different for external cavity lasers. This study presents the phenomenon of self-pumped absorption-bleaching and whole-band absorption-bleaching in the in-band pumping regime. The absorption modulation and absorption bleaching can induce a refractive index modulation in the doped fiber. This forms a dynamic grating in the doped fiber inside the external cavity, which has an additional effect for line-width narrowing and wavelength stability.;A radio over fiber system is generally structured with the center stations, the fiber optic communication system and the remote base stations. The microwave or radio waves modulate the light wave and are transmitted on the fiber optic system by optical transmitters. Laser sources are the key optical devices for optical microwave transmission. The stability and noise figure of the laser sources can affect the signal transmission performance for all the radio over fiber systems.;The DFECL is structured with a normal semiconductor laser, a piece of doped fiber and a fiber Bragg grating (FBG). Novel DFECLs have been built in this project. The new DFECLs operated at 1490 nm wavelength, where it is at the edge of erbium doped fiber absorption band. This new laser is studied in depth for its short and long term stability, tunability and modulation capability.;The new laser presented long term stability with fluctuations of only +/-2.5 pm in wavelength and +/-0.025 dBm in power. With self-beating and heterodyne beating methods, the line-width of the laser has been shown to be in the order of kHz, with the side-mode suppression of between 15--20 dB. The DFECL's good stability at 1490 nm indicated that a DFECL could be built anywhere within the absorption band of the doped fiber. The latest DFECL was designed with almost fully doped fiber in the external cavity. The laser light was directly coupled to the fiber lens made on the erbium doped fiber (EDF), which is splicing very close to a FBG. The DFECL operating at ∼1530 nm presented even better performance; the side mode suppression ratio was better than 45 dB, with wavelength stability better than +/-1 pm, and line-width narrower than 350 kHz.;Further studies have pointed out that the DFECL can have either wide or fine tunability. The slow response of the dynamic grating in doped fiber can eliminate fast wavelength fluctuations and stabilize the laser. By slowly and constantly tuning the drive current or temperature of the laser diode, the DFECL wavelength can also be tuned within the bandwidth of the external fiber Bragg grating. The precise tuning rate was 50 MHz/mA, which is very attractive for radio over fiber applications.;Applications of the DFECL have been demonstrated in the thesis for the first time. Due to the saturable absorber inside the external cavity, the DFECL can be directly modulated at high RF carrier frequency; the modulating microwave frequency can be more than the 22nd multiple of its cavity resonant frequency. The experimental results had shown that the direct modulation transmission signal was clean, and can be easily identified. Up to 20 MHz bandwidth FM and OFDM signals have been demonstrated in this thesis. The measured high quality 64-QAM IEEE802.11 a/g OFDM signal transmissions achieved an error vector magnitude (EVM) for the OFDM transmission as low as -40 dB, which is even lower than for most externally modulated transmission systems.