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Study Of Power Promotion And Applications Of Low-frequency Terahertz Wave Generated By Microwave Photonic Technology

Posted on:2013-07-23Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z W ZhengFull Text:PDF
GTID:1268330425483966Subject:Computer application technology
Abstract/Summary:PDF Full Text Request
The terahertz (THz) wave consists of the electromagnetic waves at frequencies from0.1THz to10THz, at the frequency gap between infrared light and microwave. The THz wave at low frequency band, also known as low-frequency THz wave, ranges from0.1THz to0.3THz. A large strong wave of research interest on Terahertz wave has been initiated internationally, as encouraged by bright prospects for the application in material, communication, imaging, and national defense. To achieve low-phase noise, high power, and cost-effective THz waves for practical applications is now located at the central part of current research wave. Microwave photonic techniques are considered to possess more advantages than conventional techniques, which are normally based on electronic devices.Microwave photonic technology, a marriage of the microwave and optical technology, covers diversive aspects, including but not limted by the following: photonic generation, processing, control and distribution of micorwave and THz wave signals. External modulation is a typical microwave photonic technology, with the unique advantage of producing low phase noise THz wave. This technology originates from the inherent nonlinearity of the response of the optical modulator for generating high-order optical sidebands. The generation of THz wave can directly achieved through beating two high-order optical sidebands inside a photodiode (PD). Furthermore, by taking the advantage of mature and cost-effective optical telecommunication components, one can potentially reduce the cost and increase the output power in THz systems.In this thesis, the0.1THz wave generation mechamism is theoretically studied. The THz wave output power is enhanced at milliwatt magnitude by commercial optoelectronic devices. The THz source can be used for probing the saturable absorption of graphene and terahertz over fiber (TOF), respectively. My main contribution are summarized as follows:Firsty, we proposed a scheme to generate high power low-frequency terahertz wave based on external modulation technique. The parameters in determining the power of THz wave are theoretically and experimentally verified. Here, we found that1) the THz wave power scales with the depth of the IM with less than2;2) the commercial PIN-PD input power with less than6dBm increase. To solve the power limitation problem caused by commercial PD saturation, a low-noise electrical amplifier and a W-band antenna with a gain of25dBi are employed to amplify the output power of a commercial PD. The detected power exceeds1mW at a2-cm distance from the antenna once an absolute THz power meter is used.Secondly, the first experiments on saturable absorption in graphene at0.1THz band are performed. A tunable terahertz source is generated by using external modulation technique with a tunable frequency range is from96to100GHz. Almost independent of the incident frequency, microwave absorbance of graphene always decreases with increasing the power and reaches at a constant level for power larger than80μW, evidencing the microwave saturable absorption property of graphene. By using experimental measurement and fitting calculation, the results show that the saturable power and intensity is varied from28.8μW to79.6μW and0.016~0.045mW/cm2, respectively. The modulation depth is from4.58%~12.77%. Broadband optical saturable absorption of the same graphene sample was also characterized. Optical saturable absorption of the same graphene sample was also experimentally confirmed by an open-aperture Z-scan technique by one laser at telecommunication band and another pico-second laser at1053nm, respectively. Saturable intensity of about7.89MW/cm2at the wavelength of telecommunication band and10.32MW/cm2at wavelength at1053nm are respectively obtained. When the same grapheme sample is used as a saturable absorber in fiber laser cavity, a mode locking laser pulse is generated by adjusting polarization controller. The repetition rate of pulse is1.21MHz. Here, we are able to conclude that graphene is indeed a broadband saturable absorber that can operate at both low-terahertz and optical band.Finally, a novel optical cascaded modulation scheme for optical0.1THz wave signal generation is proposed and the wireless rate of2~5Gb/s high-speed communication is realized. After a wireless transmission distance of0.1m, the eye diagram of wireless data remains still clear by coherent demodulation at the receiving unit. Optical wavelength reuse and local oscillator distribution technologies are applied to simplify base station of a full-duplex TOF. A novel scheme for the full-duplex0.1THz TOF based optical wavelength resue is proposed. The5Gb/s downlink and2.5Gb/s uplink data are successfully transmitted over10-km dispersion shift fiber (DSF), respectively. A novel scheme for0.1THz optical oscillating (LO) distribution based on four wave mixing (FWM) in a semiconductor optical amplifier (SOA) is proposed. We experimentally demonstrated40GHz optical local oscillating (LO) distribution by this proposed scheme. The experimental results show that the clear waveform of40GHz optical LO is obtained after20km single-mode fiber (SMF) transmission.
Keywords/Search Tags:Microwave phtonics, terahertz wave, external modulation, graphene, terahertz over fiber
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