Moto Kinoshita

Calorimetric measurement of absolute terahertz power at the sub-microwatt level

We developed a calorimeter for determining absolute terahertz (THz) power. The calorimeter is based on a DC substitution method using an isothermal temperature-control technique. A neutral-density optical filter glass was used as a volume absorber, and its THz absorption was evaluated by a time-domain spectrometer. Highly sensitive measurement of the absolute THz power was experimentally achieved in the range from the submicrowatt to microwatt level at room temperature. Power measurement of a continuous-wave THz source using a photomixer was demonstrated. This calorimeter is expected to demonstrate the traceability of THz power at a submicrowatt level.

Determination of the Microwave Field Strength Using the Rabi Oscillation for a New Microwave Power Standard

This paper describes the development of a new type of microwave power standard based on atomic resonances. An atomic Rabi frequency is proportional to the magnetic field strength of the resonant microwave; the proportionality constant is determined only from fundamental constants and atomic quantum theory. The microwave field strength that corresponds to microwave power is thus uniquely determined by the Rabi frequency. The proportionality constant can thus serve as the basis for a new microwave power standard. This paper confirmed that the Rabi frequency of cesium-133 (133Cs) atoms in a rectangular cell is proportional to the incident microwave field strength. A rectangular Cs glass cell was developed and inserted into a WR90 waveguide-an industrially practical waveguide that permits the impedance analysis necessary to measure microwave power. The microwave field strength in the Cs cell can be adjusted to arbitrary values proportional to the Rabi frequency. The relative expanded uncertainties of the effective microwave magnetic field strength H eff for the coverage factor k = 2, which is typically less than 7%, averaged 4%. The dynamic range of the available Rabi frequency was 31.9 dB.

Validation of Power Linearity in Terahertz Time-Domain Spectroscopy Using a Programmable Step Attenuator

We have constructed a programmable terahertz step attenuator by stacking metallized-film attenuators (MFAs) of different transmittances. Sputter deposition of Inconel alloy on thin polyester films is used to fabricate the MFAs. Good repeatability and flatness of the step attenuator are confirmed in transmittance measurements by a terahertz time-domain spectroscopy (THz-TDS) system. A method to evaluate the linearity of the THz-TDS system is proposed by using the step attenuator over a wide dynamic range. Calibration curves are analyzed by measuring the transmittance of maltose in different concentrations, and the validity of the proposed method is discussed.

Power Linearity Measurement in Terahertz Time-Domain Spectroscopy Using Metalized Film Attenuators

We describe a method for evaluating the power linearity over a wide dynamic range of a terahertz time-domain spectroscopy (THz-TDS) system. The dynamic range is achieved by means of metalized film attenuators (MFAs). The evaluation was based on repeated measurements of transmittance in a particular sample at different incident power levels. We apply the method to both focused- and collimated-beam systems and show that the method can be applied to reflectance measurements as well.

Atomic Microwave Power Standard Based on the Rabi Frequency

The Rabi frequency for an atomic hyperfine structure transition is proportional to the magnetic field strength of the incident electromagnetic wave. Using the known proportionality constant, an atomic microwave power standard based on the Rabi frequency can be realized. In this paper, the Rabi frequency of cesium atoms in a glass cell inserted in a microwave waveguide was measured using the atomic-candle method. The Rabi frequency was converted into the incident microwave power via the magnetic field strength by correcting the effects of the glass cell and impedance mismatch. The relative expanded uncertainty was approximately 4% at a coverage factor of 2. The microwave power measured from the Rabi frequency was compared with that measured by a calorimetric method. These results coincided within their uncertainties.

A Broadband Waveguide Calorimeter in the Frequency Range From 50 to 110 GHz

A new broadband rectangular waveguide calorimeter for radio-frequency (RF) power meter calibration in the millimeter-wave region is reported. The calorimeter has a quasi-twin structure exploiting the WR-10 and WR-15 rectangular waveguides, and its operational frequency ranges from 50 to 110 GHz. This calorimeter is characterized by its high measuring speed and sensitivity. A thermal feedback control system reduces the typical reading time per frequency to less than 10 min, and the minimum measurable incident RF power is 0.2 mW. A new technique for reducing the equivalence error by utilizing a flat millimeter-wave absorber is also discussed.

Frequency Calibration of Terahertz Time-Domain Spectrometer Using Air-Gap Etalon

Although various terahertz (THz) technologies have been rapidly developed in recent years, their metrology standards are yet to be established. We have developed a simple calibration method for the frequency of a THz time-domain spectrometer (TDS) using an optically transparent THz air-gap etalon (OT-TAGE). The OT-TAGE can be used in both the near-infrared (NIR) and THz regions. Therefore, a frequency measurement using THz waves can be directly compared with one using NIR light via the OT-TAGE. In this study, the frequency of a commercial THz-TDS was calibrated by referring to the NIR wavelength was demonstrated. The obtained frequency errors were within ± 3 GHz, with relative uncertainties within 1%.

Optimization of the Atomic Candle Signal for the Precise Measurement of Microwave Power

To use the atomic candle signals of Cs atoms for the measurement of microwave power, the main parameters such as a standing or running microwave in the glass cell containing Cs atoms, the probing position in the waveguide, the intensity of the probing laser, the pressure of the

Rabi frequency measurement for microwave power standard using double resonance spectrum

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Electromagnetic Field Sensor Based on Atomic Candle

buffer gas, and the beam diameter were optimized. As a result, the average uncertainty in obtaining the Rabi frequency, whose accuracy is important for the measurement of microwave power, was reduced several-fold. We demonstrated the measurement of microwave power using the optimized atomic candle signal in our previous work. In this paper, we discuss the optimization process in detail and the utility of optimizing the atomic candle signal for the precise measurement of microwave power.