We demonstrate high-speed terahertz (THz) time-domain spectroscopy based on electronically controlled optical sampling (ECOPS). The ECOPS system utilizes two synchronized Ti:sapphire femtosecond lasers with a $100\mathrm{MHz}$ repetition frequency. The time delay between the two laser pulses is demonstrated to be rapidly swept at a scan rate of $1\mathrm{kHz}$ on a time delay window of $77\mathrm{ps}$ by using an external offset voltage applied to a locking electronics. It is shown that a THz pulse can be exactly measured by ECOPS, as is done by asynchronous optical sampling (ASOPS), and the measurement time is shortened by a factor of 50 by using ECOPS compared with ASOPS in the case of employing $100\mathrm{MHz}$ repetition-rate lasers.

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We demonstrate a terahertz (THz) spectrum analyzer based on frequency and power measurement. A power spectrum of a continuous THz wave is measured through optical heterodyne detection using an electromagnetic THz frequency comb and a bolometer and power measurement using a bolometer with a calibrated responsivity. The THz spectrum analyzer has a frequency precision of $1\times {10}^{-11}$, a frequency resolution of $1\mathrm{Hz}$, a frequency band up to $1.7\mathrm{THz}$, and an optical noise equivalent power of ~1 $\mathrm{pW}/{\mathrm{Hz}}^{1/2}$.

High-speed frequency-domain terahertz (THz) coherence tomography is demonstrated using frequency sweeping of continuous-wave THz radiation and beam steering. For axial scanning, THz frequency sweeping with a kHz sweep rate and a THz sweep range is executed using THz photomixing with an optical beat source consisting of a wavelength-swept laser and a distributed feedback laser diode. During the frequency sweep, frequency-domain THz interferograms are measured using coherent homodyne detection employing signal averaging for noise reduction and used as axial-scan data via fast Fourier transform. Axial-scan data are acquired while scanning a transverse range of 100 × 100 mm² by use of a THz beam scanner with moving neither sample nor THz transmitter/receiver unit. It takes 100 s to acquire axial-scan data for 100 × 100 points with 5 averaged traces at a sweep rate of 1 kHz. THz tomographic images of a glass fiber reinforced polymer sample with artificial internal defects are presented, acquired using the tomography system.

We present a new technical implementation of a high-speed broadband frequency sweep of continuous-wave terahertz (THz) radiation. THz frequency sweeping with a kHz sweep rate and a THz sweep range is implemented using THz photomixing in which an optical beat source consists of a wavelength-swept laser and a distributed feedback laser diode. During the frequency sweep, frequency-domain THz interferograms are measured using the coherent homodyne detection employing signal averaging for noise reduction, which can give time-of-flight information via fast Fourier transform. Multiple reflections in a Si wafer and the thickness of the wafer are measured to demonstrate the potential of this method for fast THz tomography and thickness measurement.

Monolithic self-pulsating semiconductor lasers called amplified feedback lasers (AFLs) can generate high-frequency self-pulsations according to the concept of a single-mode laser with shortly delayed optical feedback, which consist of a distributed-feedback (DFB) laser, a phase control, and an amplifier section. Since mode degeneracy of the DFB section, which should operate as a single-mode laser, affects the self-pulsation, single-mode characteristics of the DFB section are critical for the self-pulsation. The effect of a complex coupling in the DFB section on the self-pulsation is numerically analyzed to reveal that the complex coupling provides a wide operation range for the self-pulsation. Also, self-pulsating AFLs based on a loss-coupled DFB laser are experimentally demonstrated to verify the self-pulsation characteristics and the capability for all-optical clock recovery.