Fiber Lasers And High Performance Photodetectors For THz Generation

THz-wave is receiving intensive research interest due to its broad applications inimaging technology, material analysis, biological science and ultra-high speedcommunications. Photomixing with dual-frequency lasers, which is able to generatetunable CW THz-wave, becomes an important method for THz generation. The keytechnologies of THz generation by photomixing are stable dual-frequency lasers and highspeed photodetectors, which are theoretically and experimentally investigated. In addition,silicon-based high responsivity photodetectors are also studied as an extension to highspeed photodetectors. The detailed content is as follows:(1) Fiber Bragg gratings (FBG) are studied as wavelength filters for fiber lasers.The transmission spectra of phase-shifted uniform FBGs are investigated, and thefeasibility of its applications in the dual-wavelength single-longitudinal-mode (SLM)lasers is discussed. A novel dual-Ï€-phase-shifted chirped FBG, which has a dual-channelnarrow-linewidth transmission spectrum with 8nm wavelength spacing, is designed andfabricated.(2) THz optical beating and dual-wavelength SLM erbium doped fiber lasers(EDFL) are theoretically studied. The principles and conditions of THz beat-notegeneration are discussed, and the autocorrelation trace of THz beat-notes in anautocorrelator is theoretically deduced. The mechanism for dual-wavelength SLM EDFLis studied. The influence of the spectrum hole-burning of erbium-doped fiber lasers ondual-wavelength lasing is analyzed, and the influence of the narrow-band filtering of thesaturable absorber on SLM oscillation is also analyzed.(3) Dual-wavelength SLM EDFL and THz optical beating are experimentallystudied. SLM, multiwavelength, and dual-wavelength SLM performance of the EDFL isexperimented. Stable dual-wavelength SLM EDFL is achieved with the frequency spacingof 1 THz. 1 THz beat-note is also successively observed with the help of an autocorrelator.(4) High speed photodetectors including photoconductors with ultrashort carrierlifetime and uni-traveling-carrier photodiodes (UTC-PD) are theoretically studied. Thetheoretical basis and numerical models of semiconductor photodetectors are analyzed. Theanalytical expressions for 1D photoconductor are deduced, and both 1D and 2Dphotoconductors are numerically simulated by the commercial simulation tool COMSOL.A special UTC-PD based on materials of ultrashort carrier lifetime is designed and analyzed, which takes advantage of the merits of both UTC-PD and photoconductors withultrashort carrier lifetime, but solves the problems of large dark current ofphotoconductors and slow diffusion process of UTC-PDs.(5) High responsivity photodetectors with resonant cavity enhancement (RCE) andnano-pillar are theoretically and experimentally studied. A special double-cavity RCEdetector is designed and analyzed, which carries high quantum efficiency and narrow-banddetection spectrum. A novel nano-pillar detector is invented for weak light detection,which not only has very high internal gain, but also overcomes the shortcomings ofavalanche photodiodes (APD), such as high bias voltage, hot carrier crosstalking, etc. Thenano-pillar detector is theoretically analyzed and numerically simulated by the commercialsemiconductor simulation software DESSIS. The process flow is designed, the keyprocess technologies are studied, and parts of the process are finished.