Kaoru Minoshima

Terahertz Frequency Metrology Based on Frequency Comb

Two techniques for terahertz (THz) frequency metrology based on frequency comb, namely, a THz-comb-referenced spectrum analyzer and a continuously tunable, single-frequency continuous-wave (CW)-THz generator, are reviewed. Since the frequency comb enables to coherently link the frequency among microwave, optical, and THz regions, it is possible to establish the THz frequency metrology traceable to time of the SI base units. Using a THz-comb-referenced spectrum analyzer based on a stable THz comb generated in a photoconductive antenna for THz detection, the absolute frequency of CW test sources in the sub-THz and THz regions was determined at a precision of 10-11. Furthermore, a continuously tunable, single-frequency CW-THz generator was demonstrated around 120 GHz by photomixing of an accurately tunable CW laser and a tightly fixed CW laser in the optical frequency region, phase locked to two independent optical combs. The combination of the CW-THz generator with the THz-comb-referenced spectrum analyzer will open the door for establishment of frequency metrology in the THz region.

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 2.4 x 10(-11). Furthermore, the potential of the THz spectrum analyzer was effectively demonstrated by real-time monitoring of the spectral behavior of the AFMC source and a photomixing source of two free-running CW lasers at adjacent wavelengths.

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.

Widely and continuously tunable terahertz synthesizer traceable to a microwave frequency standard

We constructed a widely and continuously tunable terahertz frequency synthesizer traceable to a hydrogen maser linked to coordinated universal time. Photomixing of two optical frequency synthesizers, linked to the hydrogen maser via dual optical frequency combs, gave this THz synthesizer frequency uncertainty of 10⁻¹². To demonstrate the potential of wide and continuous tunability in the THz synthesizer, we tuned its output frequency up to 50 GHz discretely and 1.26 GHz continuously in the F-band while maintaining the unprecedented frequency uncertainty by using a uni-traveling-carrier photodiode as a photomixer. This THz synthesizer will be a powerful tool for broadband, high-precision THz spectroscopy and THz frequency metrology.

Continuously tunable, phase-locked, continuous-wave terahertz generator based on photomixing of two continuous-wave lasers locked to two independent optical combs

A continuously tunable, phase-locked, single-frequency, continuous-wave (cw) terahertz generator has been demonstrated around 120 GHz, corresponding to the spectral bandwidth of the F-band unitraveling-carrier photodiode (UTC-PD) used as a photomixer in this study. This cw-terahertz generator is based on photomixing of an accurately tunable cw laser and a tightly fixed cw laser in the optical frequency region, phase locked to two independent optical combs. The continuous tuning range of the presented method was three orders of magnitude around 0.1 THz and two orders around 1 THz broader than that of a previous photomixing method in which two cw lasers are phase locked to a single optical comb, and fully covered the available spectral bandwidth of the F-band UTC-PD. The spectral behavior of the tight locking and continuous tuning of 120 GHz cw-terahertz radiation was confirmed in real time by use of a terahertz-comb-referenced spectrum analyzer. This cw-terahertz generator shows promise as a terahertz clock and synthesizer for terahertz frequency metrology.

Two-color interferometry using frequency combs for high-accuracy self-correction of air refractive index

Long-path interferometry of two-color frequency combs is developed for high-accuracy self-correction of air refractive index fluctuation. Interferometric phase difference was highly consistent with calculation: 3×10−10 for 10-h measurements.

Widely tunable, phase-locked CW-THz radiation by photomixing of two CW lasers locked to two independent fiber combs

Arbitrary single-frequency CW-THz-wave generator based on two-independent femtosecond fiber-combs is presented. Continuous tuning of a 120-GHz CW wave was demonstrated across several MHz, which is limited by electric bandwidth of THz-spectrum analyzer for evaluation.

Accurate, continuously tunable, terahertz synthesizer based on photomixing of two continuous-wave lasers phase-locked to two independent fiber combs

A continuously tunable, phase-locked, THz synthesizer has been demonstrated. This THz synthesizer is based on photomixing of two optical frequency synthesizers composed of CW laser and optical comb, and its output frequency has the same uncertainty as that of a microwave frequency standard used.

Observation of terahertz frequency comb by fiber-based, asynchronous-optical-sampling THz-TDS

We developed a fiber-based, asynchronous-optical-sampling THz-TDS system by combination of mode-locked Er-fiber lasers and photoconductive antennas. Detail structure of the THz comb was clearly observed with a spectral resolution of 5.6 MHz.

Continuous tuning of phase-locked CW-THz radiation by photomixing of two CW lasers locked to two independent optical combs

We present arbitrary single-frequency continuous-wave (CW) terahertz (THz) generator based on photomixing of a scanned CW laser and a fixed CW laser, which are locked to two independent optical combs, respectively. Behavior of tight locking and continuous tuning of its output frequency is monitored at real time by a THz-comb-referenced spectrum analyzer.