Terahertz metrology is becoming more and more important along with the fast development of terahertz technology. This paper reviews the research works of the groups from the physikalisch-technische bundesanstalt (PTB), National institute of standards and technology (NIST), National physical laboratory (NPL), National institute of metrology (NIM) and some other research institutes. The contents mainly focus on the metrology of parameters of power, frequency, spectrum and pulse. At the end of the paper, the prospect of terahertz metrology is predicted.
Apodization function is very important to the spectral accuracy in the phase error correction procession of Fourier transform spectrometer. So a novel apodization function and a high efficient phase error correction method are developed in this paper. The short double sided interferogram data and zero-crossing single sided interferogram data are gained by sampling and this data need zero-padding in order to have the same resolution. Then we reconstruct a complex number sequence, which the real part is the data of zero-crossing single sided interferogram and the imaginary part is the data of short double sided interferogram. The complex number sequence multiplied by the new apodization function and then the discrete Fourier transform is used to attain time domain data. We can obtain high precision spectrum based on the conjugate symmetry properties of discrete Fourier transform in the end. The results of experiments and theory analyzing demonstrate that proposed apodization function could suppress sidelobe and decrease the error due to the short double sided interferogram data have been used twice effectively, and the spectral inversion algorithm based on the new apodization function has not need interpolation computing, only has once fourier transform and once apodization, so the efficiency is improved greatly. Theory analysis and experiments shows the method is reasonable and efficient.
The center frequency of Brillouin scattering spectrum is easily influenced by the noise and the measurement accuracy
of optical fiber strain is reduced. So a novel denoising method which can be applied in the Brillouin scattering spectrum
is developed in this article. The Brillouin scattering spectrum is decomposed into multi-scale detail coefficients and
approximation coefficients by using the wavelet transform. The wavelet decomposition detail coefficients are threshold
quantified by utilizing the threshold algorithm. At the same time, the wavelet decomposition approximation coefficients
are trained and simulated by using the BP neural network in order to remove noise hided in the approximation
coefficients. So the novel method can reduce the wavelet decomposition scales. The Brillouin scattering spectrum which
has a better denoising effect can be gained by using the inverse wavelet transform, and the measurement accuracy of
optical fiber strain is enhanced also. The results of simulation and experiment demonstrate that the proposed method can
suppress noise better; accordingly, the new method can gain more precision optical fiber strain and reduce the wavelet
decomposition scales effectively than the conventional wavelet denoising method. Theory analysis and experiment show
that the method is reasonable and efficient.
In order to evaluate the performance of themal image system, an infrared simulation method of 4-bar
target is put forward based on blackbody radiation characteristics. These simulated targets are efficient
inputs for performance testing and evaluating the minimum resolvable temperature difference and
spatial resolution of the infrared computer simulating system. Finally, the infrared simulation images of
standard 4 bars target, which have tunable spectral band, adjustable temperature difference and
different spatial frequency, are realized by utilizing Matlab.