Phase-slope and phase measurements of tunable CW-THz radiation with terahertz comb for wide-dynamic-range, high-resolution, distance measurement of optically rough object.

Phase measurement of continuous-wave terahertz (CW-THz) radiation is a potential tool for direct distance and imaging measurement of optically rough objects due to its high robustness to optical rough surfaces. However, the 2π phase ambiguity in the phase measurement of single-frequency CW-THz radiation limits the dynamic range of the measured distance to the order of the wavelength used. In this article, phase-slope measurement of tunable CW-THz radiation with a THz frequency comb was effectively used to extend the dynamic range up to 1.

Absolute distance measurement of optically rough objects using asynchronous-optical-sampling terahertz impulse ranging.

We report on a real-time terahertz (THz) impulse ranging (IPR) system based on a combination of time-of-flight measurement of pulsed THz radiation and the asynchronous-optical-sampling (ASOPS) technique. The insensitivity of THz radiation to optical scattering enables the detection of various objects having optically rough surfaces. The temporal magnification capability unique to ASOPS achieves precise distance measurements of a stationary target at an accuracy of -551 μm and a resolution of 113 μm.

Fiber-based, hybrid terahertz spectrometer using dual fiber combs.

We constructed a fiber-based, hybrid terahertz (THz) spectrometer having two working modes, asynchronous-optical-sampling THz time-domain spectroscopy (AOS-THz-TDS) and multiple-frequency-heterodyning THz comb spectroscopy (MFH-THz-CS), by use of dual fiber-laser-based frequency combs. A spectral range of 2THz and dynamic range of 100 was achieved at the single sweep measurement of 200ms in the AOS-THz-TDS mode, whereas the detailed structure of the THz frequency comb was clearly observed in the MFH-THz-CS mode. The spectrometer features compactness, robustness, flexibility, and cost effectiveness, in addition to high spectral resolution in rapid data acquisition, and has the potential to become a powerful tool for practical applications.

A distance meter using a terahertz intermode beat in an optical frequency comb.

We propose a distance meter that utilizes an intermode beat of terahertz frequency in an optical frequency comb to perform high resolution and high dynamic range absolute distance measurements. The proposed system is based on a novel method, called multiheterodyne cross-correlation detection, in which intermode beat frequencies are scaled down to radio frequencies by optical mixing of two detuned optical frequency combs with a nonlinear optical crystal. Using this method, we obtained a 1.

Real-time monitoring of continuous-wave terahertz radiation using a fiber-based, terahertz-comb-referenced spectrum analyzer.

We propose a fiber-based, terahertz-comb-referenced spectrum analyzer which has the advantages of being a portable, alignment-free, robust, and flexible apparatus suitable for practical use. To this end, we constructed a 1550-nm mode-locked Er-doped fiber laser whose mode-locked frequency was stabilized precisely by referring to a rubidium frequency standard, and used it to generate a highly stable terahertz (THz) frequency comb in a photoconductive antenna or an electro-optic crystal. By standardizing the THz comb, we determined the frequency accuracy of an active-frequency-multiplier-chain (AFMC) source to be 2.

Terahertz spectrum analyzer based on a terahertz frequency comb.

Precision frequency measurements of terahertz (THz) waves are required to establish metrology in the THz spectral region. However, frequency measurement techniques in this region are immature. We propose a THz spectrum analyzer to measure the absolute frequency and spectral shape of continuous-wave THz waves.