A simple Fourier transform spectrometer for terahertz applications

Abstract

A simple Fourier transform spectrometer was designed and constructed for the measurement of detectors, sources, passive devices and materials in the terahertz (THz) range. It can be operated at frequencies between 0.3 and 1.5 THz, using a 50-μm-thick Mylar-film beam splitter. The spectral range can be changed by altering the thickness of the beam splitter. The highest frequency resolution is 750 MHz. We studied the properties of heterodyne detectors including superconductor mixers and semiconductor harmonic mixers, direct detectors including an InSb semiconductor bolometer, superconducting tunnel junctions and the Golay cell, and sources including Gunn oscillators and a microwave source with its multipliers, as well as various materials and passive devices including Si wafers and metal mesh filters. Because of its multiplex and etendue advantages, Fourier transform spectroscopy has become the most efficient way to evaluate the properties of signal sources, samples and detectors in the terahertz (THz) range [1][2][3]. The multiplex advantage is that the interferometer receives information from the entire range of a given spectrum during each time element of a scan, whereas a grating spectrometer receives information from only a narrow band of the spectrum. The etendue advantage is that when the required frequency resolution is much lower than that determined by the throughput, the interferometer has the ability to collect a large amount of energy with no strong limitation on the resolution. This is the situation in the THz range. To evaluate the properties of direct detectors, heterodyne detectors (mixers) and filters fabricated in our laboratory, we designed and constructed a simple Fourier transform spectrometer (FTS) system.

Terahertz Direct Detectors Based on Superconducting Hot Electron Bolometers with Microwave Biasing

“…Although the zero-bias HEB direct detector's sensitivity is worse than the constant-voltage bias one, it would be conveniently applied in some applications with lower sensitivity requirements but high response speed without dc bias. After characterizing the performance of the HEB direct detectors, we used the detector combined with the FTS constructed by our lab [13] to measure the transmittance of the VDI mesh filter. The chopping frequency is set at 1.4 kHz, which is limited by the optical chopper and the integral time is set at 30 ms.…”

Theoretical and experimental study on broadband terahertz atmospheric transmission characteristics

“…Plenty of work has been done on measuring the atmospheric absorption characteristics based on terahertz time-domain systems (TDS) and infrared Fourier transform spectrometers (FTIR). [3,[11][12][13][14][15][16] However, most of the previous theoretical and experimental work concentrated on constant normal pressure and temperature (NPT ) only, which is insufficient to depict the accurate width of the atmospheric windows as they should be significantly changed at high-altitude conditions or in the upper atmosphere. Moreover, the lack of research on atmospheric propagation characteristics in the high-frequency range (above 3 THz) limits the applications of some superior THz sources such as quantum cascade lasers (QCL).…”

Photonic Spectrum Analyzer for Wireless Signals in the THz Range

“…An interferometer-based (e.g. Michelson or Martin-Puplett type) SA is reported in [2], [3]. The time trace of the selfhomodyne interferogram generated by adding a delay in one of the two interferometer arms is Fourier transformed in order to obtain the spectral characteristics.…”

“…The maximum travel of the delay stage, its accuracy and speed limit the obtainable frequency range, the resolution and the measurement time of this SA. A frequency range from 100 GHz up to 1.5 THz, a minimum resolution of 750 MHz and a dynamic range of 45 dB have been demonstrated [2], [3]. Despite being a fast and inexpensive architecture of an SA, the low resolution limits the SA to the detection of wideband or low coherence signals only.…”