【作者】 郑之伟；

【导师】 余建军；

【作者基本信息】 湖南大学 ， 计算机应用技术， 2012， 博士

【摘要】 太赫兹(THz)波是指0.1～10THz频段之间的电磁波,它在电磁波谱中位于微波和红外光之间。低频太赫兹波是指频率范围在0.1-0.3THz之间。近年来,由于太赫兹波在材料、通信、成像和国防等众多领域有着良好的应用前景,受到了各国研究机构的重视。如何获得低噪声、低成本、高功率的太赫兹波以满足实际应用中的需求是一个亟待解决的问题。与传统的电子学技术相比,微波光子技术更为适合被用来解决这一难题。微波光子技术能够有效融合微波技术与光子技术,利用光子学的技术手段实现微波和太赫兹波的产生、传输、控制和处理。外调制技术是一种典型的微波光子技术,非常适合产生低相位噪声的太赫兹波。它是利用光调制器的非线性响应,产生相干的高阶光边带,再将高阶边带在光电探测器(PD)拍频,转换为太赫兹信号。通过使用成熟和高性价比的光通信器件,能有效地降低太赫兹系统的成本,提升输出的功率本文理论分析了基于外调制技术产生0.1THz低频太赫兹波的功率影响因素,利用商用的光电器件将输出功率提升到毫瓦量级,将该太赫兹源分别用于研究石墨烯的可饱和吸收特性和光载太赫兹高速通信系统(TOF)。本论文的主要成果和创新点如下：第一,提出了一种基于外调制技术产生高功率的0.1THz连续太赫兹波的方法,理论和实验研究了影响太赫兹功率提升的因素。通过实验验证了当调制器的调制深度小于2和商用PIN型的光电探测器(PD)的光输入功率小于6dBm时,太赫兹波输出功率随着调制深度和PD的光输入功率的增加而增强。商用PD的光电饱和效应是提升功率的一个瓶颈问题,通过在PD后端增加一个商用的电放大器和一个增益系数为25dBi的W波段的高频天线,可以进一步提升输出功率通过太赫兹功率计测量,在离天线距离2cm处的0.1THz太赫兹波输出功率值超过1mW。第二,首次实验发现石墨烯在0.1THz频段附近具有可饱和吸收的特性。通过外调制技术产生可调谐的低频太赫兹波源,调谐频率范围为96～100GHz。随着太赫兹波的入射功率增加,石墨烯对太赫兹波的可饱和吸收逐渐降低,当功率超过80μW后,饱和吸收降低到一个恒值,这证明了石墨烯具有可饱和吸收特性。经实验测量和拟合计算得出,石墨烯样品的饱和功率范围为28.8～79.6μW,饱和强度为0.016～0.045mW/cm2,调制深度范围为4.58%～12.77%。同时,比较了光频段的石墨烯可饱和吸收特性,通过开孔Z扫描技术在光通信波段和1053nm波段实验验证了相同石墨烯样品的可饱和吸收特性,在1550nm和1053nm的波段的饱和吸收强度分别为7.89MW/cm2,10.32MW/cm2。将相同的石墨烯样品作为可饱和吸收体放置在光纤激光器腔内,通过调节偏振控制器实现了锁模激光脉冲的输出,脉冲重复为1.21MHz。通过这些工作,实验验证了石墨烯是一个能工作在低频太赫兹和光波段的宽频段范围的可饱和吸收体。第三,提出了两级光调制产生0.1THz光载太赫兹信号的新方案,实现了无线速率为2-5Gb/s的高速通信,经无线信道传输0.1m距离后,接收单元通过相干解调,得到清晰的无线信号眼图。分别将波长重利用和光本振远程传送技术用于简化全双工的TOF系统的基站,提出了基于光波长重利用技术的全双工0.1THz的TOF系统的新方案,实验实现了下行传输速率为5Gb/s和上行传输速率为2.5Gb/s在10km色散位移光纤中的传输;提出基于半导体光放大器(SOA)四波混频效应的0.1THz的光本振远程传送的新方案,并在40GHz频段对该方案进行了实验验证,实验结果表明,经过20km标准单模光纤传输后,仍能得到清晰的40GHz的光本振信号的波形。更多还原

【Abstract】 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.更多还原