Murasawa Kengo, Sato Koki, Hidaka Takehiko
Department of Applied Computer Sciences, Shonan Institute of Technology, Fujisawa, Kanagawa 251-8511, Japan.
Rev Sci Instrum. 2011 May;82(5):053104. doi: 10.1063/1.3589859.
A new method for measuring optical-beat frequencies in the terahertz (THz) region using microwave higher harmonics is presented. A microwave signal was applied to the antenna gap of a photoconductive (PC) device emitting a continuous electromagnetic wave at about 1 THz by the photomixing technique. The microwave higher harmonics with THz frequencies are generated in the PC device owing to the nonlinearity of the biased photoconductance, which is briefly described in this article. Thirteen nearly periodic peaks in the photocurrent were observed when the microwave was swept from 16 to 20 GHz at a power of -48 dBm. The nearly periodic peaks are generated by the homodyne detection of the optical beat with the microwave higher harmonics when the frequency of the harmonics coincides with the optical-beat frequency. Each peak frequency and its peak width were determined by fitting a Gaussian function, and the order of microwave harmonics was determined using a coarse (i.e., lower resolution) measurement of the optical-beat frequency. By applying the Kalman algorithm to the peak frequencies of the higher harmonics and their standard deviations, the optical-beat frequency near 1 THz was estimated to be 1029.81 GHz with the standard deviation of 0.82 GHz. The proposed method is applicable to a conventional THz-wave generator with a photomixer.
提出了一种利用微波高次谐波测量太赫兹（THz）波段光拍频的新方法。将微波信号施加到通过光混频技术在约1太赫兹频率下发射连续电磁波的光电导（PC）器件的天线间隙上。由于偏置光电导的非线性，在PC器件中会产生具有太赫兹频率的微波高次谐波，本文对此进行了简要描述。当微波在-48 dBm的功率下从16 GHz扫频到20 GHz时，在光电流中观察到了13个近似周期性的峰值。当谐波频率与光拍频一致时，这些近似周期性的峰值是由光拍与微波高次谐波的零差检测产生的。通过拟合高斯函数确定每个峰值频率及其峰值宽度，并使用光拍频的粗略（即较低分辨率）测量来确定微波谐波的阶数。通过将卡尔曼算法应用于高次谐波的峰值频率及其标准偏差，估计1太赫兹附近的光拍频为1029.81 GHz，标准偏差为0.82 GHz。所提出的方法适用于带有光混频器的传统太赫兹波发生器。
