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We have successfully demonstrated the simultaneous phase-locking of two terahertz quantum cascade lasers (THz-QCLs). These phase-locked THz-QCLs are used as local oscillators of a hot electron bolometer mixer (HEBM). The HEBM was simultaneously pumped by two phase-locked THz-QCLs for the multifrequency heterodyne detection of emission line spectra of methanol [CH3OH] at 2 THz using a gas cell. The phase-locking of THz-QCLs with arbitrary frequencies using an optical comb and a photomixer to generate THz references was also demonstrated. An advantage of this technique is that it expands the RF bandwidth of an HEBM.

The impact of the response speed of a microelectromechanical system (MEMS) bolometer on the quality of terahertz (THz) imaging with rapid scan has been investigated and compared with a pyroelectric detector (PED). The imaging system was an active imaging composed of the MEMS bolometer, THz quantum cascade laser, and two-dimensional translation stage. The scanning process of the imaging system consisted of continuous scans in the X direction and step scans in the Z direction. The MEMS bolometer had a beam resonator with a resonant peak of 422 kHz, and it was operated in the frequency modulation detection mode. The frequency shifts in the resonant peak due to the irradiation of the THz waves were tracked using the phase-locked loop system built in the lock-in amplifier. In the measurement of the response characteristics using a 100-μm pinhole, the peak intensity was almost constant against the stage speed in the case of the MEMS bolometer. For the PED, there was a decrease in the peak intensity and delay in the peak positions with increasing stage speed due to the insufficient response speed. When the images were acquired using the MEMS bolometer with a stage speed of 25 mm/s, fine structures of a metal bookmark were clearly observed and no degradation in the resolution was observed with changing stage speed. In the case of the PED, the resolution gradually degraded with increasing stage speed.

We are developing a waveguide-type hot electron bolometer mixer (HEBM) at 2 THz. A membrane-type Ni-NbN HEBM device on a Si3N4 substrate was fabricated in a clean room facility of our laboratory. The membrane bridge with a size of W55 μm x L820 μm x t1 μm was supported by a Si frame with a size of 3 mm by 3 mm and a thickness of 200 μm. The device was installed in a waveguide-type mixer mount with a WR-0.51 waveguide and a diagonal horn. The measured receiver noise temperature and IF gain bandwidth are ~1,690 K in DSB and ~4 GHz which must be improved by getting the HEBM device closer to the design. We are also trying to fabricate a mixer block with a corrugated horn at 2 THz.

Using a frequency-tunable optical comb generated from a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) and a nonlinear optical fiber, we demonstrated a frequency measurement of continuous terahertz wave sources with the frequency of 0.1 and 0.6 THz by an electro-optic sampling method. We clearly observed beat signals between the terahertz source and the optical two-tone extracted from the optical comb, allowing us to determine the absolute frequency. Owing to the wide comb spacing of the MZ-FCG, this method has a high potential for the high-speed measurement of the frequency of terahertz wave sources.

We have successfully demonstrated a continuous phase-locking of a terahertz quantum cascade laser (THz-QCL) in ∼5 days by suppressing a slow drift of the lasing frequency in free running from ~150 to 5~10 MHz/day. The THz-QCL was covered with an enclosure to prevent the gas adsorption. The holding time of the phase-locking is expected to be longer.

To expand the intermediate frequency (IF) band and improve the sensitivity of a hot-electron bolometer mixer (HEBM), we proposed and examined a new HEBM structure using a magnetic thin film. We found that it was possible to suppress the superconductivity of the 5-nm-thick niobium nitride (NbN) thin film by the addition of a 1.8-nm-thick nickel (Ni) thin film. It was also confirmed that the superconductivity disappeared in the Au (70 nm)/Ni (1.8 nm)/NbN (5 nm) trilayer for forming the electrodes of the HEBM. By using the magnetic thin film for the electrodes, we suggested that the superconductivity of the HEBM strip would be affected and that hot spots would be formed near the electrodes. This approach is effective for shortening the hot-electron drift length and will lead to the expansion of the IF bandwidth. We think that the new structure lowers the required local oscillator power and improves the HEBM sensitivity by suppressing the proximity effect under the electrode. The IF bandwidth of the fabricated Ni-HEBMs was evaluated at 1.9 THz. We confirmed that the IF bandwidth expands, and the evaluated bandwidths were in the range of 5.1–5.7 GHz at 4 K. Ni-HEBMs with 0.1-μm strip length were also fabricated and evaluated. The IF bandwidth was about 6.9 GHz at 4 K.

We are developing a hot electron bolometer mixer (HEBM) at 2 THz for the atmospheric observations. We demonstrated an integrated HEB mixer in a cryostat with a cryogenic local oscillator (LO). An alignment of the LO into an HEB mixer was made using a cryogenic rotator. We have detected a beat note using this receiver system at 2 THz.

The Superconducting Submillimeter-Wave Limb-Emission Sounder-2 (SMILES-2) is a satellite mission for observation of the stratosphere, mesosphere, and lower thermosphere with superconducting submillimeter-wave receivers, being proposed to the announcement of opportunity for M-class missions run by the Japan Aerospace Exploration Agency (JAXA). The SMILES-2 notable features are extremely high sensitivity and observation capability of atmospheric diurnal cycles. Those are requested from a scientific requirement for the investigation of the 4-D space-time structure of diurnal variations in the whole atmosphere. Such features require SMILES-2 to have 4–K cooled superconducting devices for high sensitivity and to be aboard a satellite platform on a non-sunsynchronous orbit. The mechanical 4–K cooling system is a power consuming instrument. Satellite on a non- sunsynchronous orbit gets a large sun-shade ratio in most of season, and has limitations of power supply to the mission instrument. The issue of the SMILES-2 mission design is mainly to fit the 4–K cooling system to the limited power resource of M-class satellite. This paper discusses our development status on designing the SMILES-2 mission payload. SMILES-2 will be proposed to JAXA in the beginning of the next year.

We proposed a terahertz (THz) communication system operating in the 3 THz range using an optical-comb-based THz source and a heterodyne receiver system using a hot electron bolometer mixer (HEBM) along with a quantum cascade laser (QCL) operating at 3 THz for a local oscillator. The THz source is composed of a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG), and a uni-traveling-carrier photodiode (UTC-PD). By using a phase-locked loop (PLL) system, the oscillation frequency of the QCL was highly stabilized. By using this system, generation and detection of coherent THz carriers in the 3 THz range have been demonstrated, which had a phase noise of -70 dBc/Hz at an offset frequency of 100 kHz.

In this paper, we report on THz imaging using a microelectromechanical system (MEMS) resonator-based bolometer and a quantum cascade laser (QCL) operating at 3 THz. The MEMS bolometer has high sensitivity and high-speed response, and detects THz waves as the change in the resonant frequency, so that THz imaging systems with short acquisition time can be constructed. As a preliminary experiment, THz imaging of an etched metal bookmark fabricated with very small holes has been demonstrated. An estimated spatial resolution was less than 200 µm.

We have successfully demonstrated to extend stabilizing duration of a phase-locking of a THz-QCL to 48 hours. In addition to an ordinary phase-locking, a frequency drift of a THz-QCL was compensated by controlling bias voltage of a THz-QCL. A PLL out voltage was used as a trigger control.

SMILES-2 is a proposed spaceborne mission using THz superconducting receiver for measurement of the Earth's atmospheric thermal limb emission. The mission concept is prepared for the call-for-proposals for JAXA/ISAS M-class scientific satellite mission. SMILES-2 will measure wind, temperature, atmospheric compositions in the stratosphere, mesosphere and lower thermosphere, and their diurnal cycles. Three-band receiver at 638 GHz, 763 GHz, and 2 THz can cover the unprecedented wide altitude range of wind and temperature observation. A technical concern of the mission is a large power consumption requirement to the resource of an M-class satellite orbiting a non-sun-synchronous orbit. Large power is necessary to operate cryocooler for cooling superconducting devices that work at temperature of around 4 K. We are studying measures to reduce the power consumption and enhance the feasibility of the mission.

The Superconducting Submillimeter- Wave Limb-Emission Sounder 2 (SMILES-2) is a satellite mission to observe the stratosphere, mesosphere, and lower thermosphere with superconducting submillimeter receivers. SMILES-2 aims at studying the dynamical, chemical, and electromagnetic coupling of the middle and upper atmosphere with an unprecedented high-precision observations of multiple atmospheric parameters from a platform on a non-sunsynchronous orbit. SMILES-2 consists of superconducting receivers at frequencies between 619 GHz and 2.06 THz. The GHz receivers observe mainly the temperature, wind, and chemical compositions in the stratosphere and mesosphere. The frequency band of 640 GHz receiver includes the band investigated by JEM/SMILES mission, which was operated during the period from 2009 to 2010. The THz receiver, a new channel of SMILES-2, is devoted to measurement of atomic oxygen for observation of the lower thermospheric temperature and wind. The SIS mixer of GHz receiver and HEB mixer of THz receiver are cooled down to around 4 K by a mechanical cryocooler, which is designed to have 3 year lifetime and 5 year goal. The SMILES-2 payload is estimated to be less than 200 kg and consume power of 300 to 400 W. SMILES-2 is proposed for a mission installed on 500 kg size satellite of the Japan Aerospace Exploration Agency (JAXA).

We have demonstrated the phase-locking of a THz quantum cascade laser (THz-QCL) toward the realization of an accurate and stable local oscillator for a high-resolution receiver. The beat note between the THz-QCL and a THz reference was obtained by heterodyne mixing in a superconducting hot electron bolometer mixer (HEBM) and used for stabilizing the phase of 3.1 THz radiation from the THz-QCL. The phase-locked 3.1 THz radiation was fully evaluated with a superlattice harmonic mixer operating in the THz band, and this revealed that the THz-QCL synchronized with the microwave reference with a fractional frequency instability of 3×10 -15 at an averaging time of 100 s, corresponding to a center frequency deviation within 1 mHz, and the imposed phase noises during the heterodyne mixing were negligibly small.

Simultaneous stabilization of two quantum cascade lasers operated at 2 and 3 THz has been demonstrated by using a multi-frequency THz source based on Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) combined with a programmable optical filter.

We have developed a phase-locking system of a terahertz quantum cascade laser (THz-QCL) using a hot electron bolometer mixer (HEBM) and a terahertz reference such as an amplifier/multiplier chain (AMC) or an optical-comb generator followed by a UTC-PD. We have demonstrated simultaneous phase-locking of two THz-QCLs. The phase-locked THz-QCLs can be used as local oscillators (LOs) of an HEBM. This technique is useful for atmospheric and astronomical observations.

We observed ozone (O3) in the vertical region between 250 and 0.0005 hPa (~ 12–96 km) using the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on the Japanese Experiment Module (JEM) of the International Space Station (ISS) between 12 October 2009 and 21 April 2010. The new 4 K superconducting heterodyne receiver technology of SMILES allowed us to obtain a one order of magnitude better signal-to-noise ratio for the O3 line observation compared to past spaceborne microwave instruments. The non-sun-synchronous orbit of the ISS allowed us to observe O3 at various local times. We assessed the quality of the vertical profiles of O3 in the 100–0.001 hPa (~ 16–90 km) region for the SMILES NICT Level 2 product version 2.1.5. The evaluation is based on four components: error analysis; internal comparisons of observations targeting three different instrumental setups for the same O3 625.371 GHz transition; internal comparisons of two different retrieval algorithms; and external comparisons for various local times with ozonesonde, satellite and balloon observations (ENVISAT/MIPAS, SCISAT/ACE-FTS, Odin/OSIRIS, Odin/SMR, Aura/MLS, TELIS). SMILES O3 data have an estimated absolute accuracy of better than 0.3 ppmv (3%) with a vertical resolution of 3–4 km over the 60 to 8 hPa range. The random error for a single measurement is better than the estimated systematic error, being less than 1, 2, and 7%, in the 40–1, 80–0.1, and 100–0.004 hPa pressure regions, respectively. SMILES O3 abundance was 10–20% lower than all other satellite measurements at 8–0.1 hPa due to an error arising from uncertainties of the tangent point information and the gain calibration for the intensity of the spectrum. SMILES O3 from observation frequency Band-B had better accuracy than that from Band-A. A two month period is required to accumulate measurements covering 24 h in local time of O3 profile. However such a dataset can also contain variation due to dynamical, seasonal, and latitudinal effects.

Superconducting devices were used to make atmospheric limb observations from space for the first time. The Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) is a superconductor-insulator-superconductor (SIS) receiver in frequency bands of 625 and 650 GHz. SMILES was deployed on the International Space Station. SMILES observed atmospheric limb spectra for six months from October 2009 to April 2010. The sensitivity of the receiver is the most important performance parameter for microwave atmospheric limb observation, in which the receiver measures, sometimes very weak, thermal line emissions. The SMILES SIS receivers demonstrated limb observations with a sensitivity more than one order of magnitude better than that of conventional limb sensors. The sensitivity of the SMILES receivers in space was 315 K-322 K in a definition of single-sideband system noise temperature at the antenna; this met the instrument requirement with a large margin. The SMILES-receiver stability also met the requirement; the stability time of the receiver was 8 s (and 500 s for spectroscopic data) in a frequency resolution of about 1.1 MHz. Although the stability time is shorter than the calibration period (53 s) in operational observation, the variance increment by the drift noise is found to be insignificant. The temperature resolution for the continuum signal is estimated to be better than 0.27 K. There was no evidence that the stability of the SIS receiver was influenced by the temperature fluctuation of the 4-K cooling system, which consists of a two-stage Stirling cooler and a Joule-Thomson cycle cooler. The suppression of baseline ripples is another important performance parameter of the receiver for spectral measurement like the SMILES receivers. As a result of our design of low-standing-wave optics, we found no baseline ripple in the observed spectra of SMILES in practical level.

By using frequency-stabilized multi-frequency terahertz (THz) source system composed of a Mach-Zehnder-modulator-based flat comb generator, simultaneous phase-locking of two quantum cascade lasers was successfully demonstrated.

Broadband THz detection technique is required for high precision molecular gas spectroscopy, wireless communications, and so on. In this work, Sensitivities of electrooptic (EO) sampling method for the frequency measurement of continuous-wave terahertz (THz) sources were compared from 0.1 to 3.1 THz. By using a femtosecond fiber laser to gate the EO sampling, we observed beat signals between the harmonics of laser repetition frequency and the THz sources. The signal to noise ratio linearly decreases as the power of the THz sources decreases, indicating that the sensitivity is almost constant throughout this frequency range.

For frequency calibration of a Terahertz (THz) signal at a distant site we developed a THz frequency reference transfer system using an optical fiber link. Transmission of a 2.8THz frequency reference was demonstrated using 51km optical fiber including a 1km actually-laid fiber link, and used for evaluating a phase-locked THz quantum cascade laser. Depending on the condition of the fiber link, the transmitted THz frequency reference degrades. In the case of our 1km fiber link, imposed noise was negligibly small (10−15 level), having an adequate accuracy to calibrate the distant THz frequency standard.

For simultaneous stabilization of a set of cw-THz source devices oscillating at different frequencies, a number of frequency reference sources are required. In this paper, we propose a multi-frequency reference source based on Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) combined with a programmable optical filter (POF). An optical comb generated by the MZ-FCG was spectrally-synthesized by the POF. By photomixing of the synthesized comb modes using a high-speed photodiode, multi-frequency THz waves are generated. As a preliminary experiment, generation of multi-frequency THz wave in the W-band has been demonstrated.

We have developed a superconducting low noise heterodyne receiver at 3-THz band based on a hot electron bolometer mixer (HEBM) with a phase-locked THz Quantum Cascade Laser (THz-QCL) as a local oscillator for atmospheric or astronomical observations. We have demonstrated gas cell measurement of emission line spectra of methanol [CH 3 OH] at 3.7-THz band. We have also measured methanol line at 2.07-THz band.

NICT has commenced development of an accurate and stable frequency standard for THz frequency band. We succeeded in phase-locking a single-mode 3THz quantum cascade laser (THz-QCL) to a THz reference. Using the phase-locked THz-QCL and a hot electron bolometer mixer, a THz wave generated by photomixing was evaluated. The result showed that the phase-locked THz-QCL and the optically generated THz wave had the same sability as the microwave references at the 10−15 level and also that imposed noises during heterodyne mixing were negligibly small.

We have developed the 3.1-THz heterodyne receiver using an NbTiN Hot-Electron Bolometer (HEB) mixer and a THz Quantum Cascade Laser (THz-QCL) as a local oscillator. A quasi optical twin-slot antenna is adopted for the coupling of the RF signal with the mixer. The receiver noise temperature is measured to be 5600 K in DSB. When the optical loss is corrected, it is as low as 2100 K. This result demonstrates that the NbTiN HEB mixer works with the equivalent level of performance at 3.1 THz in comparison with the NbN HEB mixers usually employed in this frequency region.

We are developing quasi-optical superconducting hot-electron bolometer (HEB) mixer receivers for astronomical and atmospheric remote sensing applications. The microbridge of the HEB mixer was fabricated at room temperature from a 6.8-nm-thick niobium titanium nitride (NbTiN) film deposited on a 20-nm-thick aluminum nitride (AlN) buffer layer, using a helicon sputtering technique at a slow deposition rate. The mixer was cooled to 4.2 K using a closed-cycled mechanical 4 K pulse-tube cryocooler with a temperature fluctuation of ±1.6 mK. The stability of a large-volume NbTiN HEB mixer was studied at 1.47 THz by changing local oscillator (LO) power with the mixer bias voltage fixed. The intermediate frequency (IF) signal measured at 1.5 GHz had a maximum peak at a certain mixer bias current. The receiver noise temperature was lowest at around the IF maximum peak. It was also found that the IF signal was most stable at around the IF maximum peak under conditions in which the instability of LO pumping level, induced by small mechanical vibrations of the cryostat, remained in the optical system.

In this paper, we present the development of the waveguide niobium titanium nitride (NbTiN) superconducting hot electron bolometer (HEB) mixers, cryogenically cooled by a 4 K close-cycled refrigerator. The NbTiN thin film is formed on a crystalline quartz substrate by sputtering an NbTi target with the Ar and N2 gas at room temperature. The HEB mixer element is fabricated by using the 12 nm NbTiN film, and is mounted on a waveguide block. Measurement of a Fourier transform spectrometer shows that the response of the mixer is centered near 810 GHz with a bandwidth of about 500 GHz. The uncorrected DSB receiver noise temperature is measured to be 500 K, and the noise bandwidth is to be 1.4 GHz at 810 GHz. The present result shows that a good noise performance can be obtained for the NbTiN HEB mixer even with a relatively thick film (12 nm) fabricated at the room temperature.

We propose a frequency-stabilized multi-frequency terahertz (THz) source system, which is composed of THz sources frequency-locked to a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG). We report on phase-locking of a THz-QCL by using the MZ-FCG.

Satellite missions for measuring winds in the troposphere and thermosphere will be launched in a near future. There is no plan to observe winds in the altitude range between 30-90 km, though middle atmospheric winds are recognized as an essential parameter in various atmospheric research areas. Sub-millimetre limb sounders have the capability to fill this altitude gap. In this paper, we summarize the wind retrievals obtained from the Japanese Superconducting Submillimeter Wave Limb Emission Sounder (SMILES) which operated from the International Space Station between September 2009 and April 2010. The results illustrate the potential of such instruments to measure winds. They also show the need of improving the wind representation in the models in the Tropics, and globally in the mesosphere. A wind measurement sensitivity study has been conducted for its successor, SMILES-2, which is being studied in Japan. If it is realized, sub-millimeter and terahertz molecular lines suitable to determine line-of-sight winds will be measured. It is shown that with the current instrument definition, line-of-sight winds can be observed from 20 km up to more than 160 km. Winds can be retrieved with a precision better than 5 m s(-1) and a vertical resolution of 2-3 km between 35-90 km. Above 90 km, the precision is better than 10 m s(-1) with a vertical resolution of 3-5 km. Measurements can be performed day and night with a similar sensitivity. Requirements on observation parameters such as the antenna size, the satellite altitude are discussed. An alternative setting for the spectral bands is examined. The new setting is compatible with the general scientific objectives of the mission and the instrument design. It allows to improve the wind measurement sensitivity between 35 to 90 km by a factor 2. It is also shown that retrievals can be performed with a vertical resolution of 1 km and a precision of 5-10 m s(-1) between 50 and 90 km. RAGAM A, 1953, PHYSICAL REVIEW, V92, P1448

Generation of cw-THz waves with high frequency accuracy was demonstrated by using a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG). A combination of the MZ-FCG and a highly nonlinear dispersion shifted fiber generated broadband optical combs, and cw-THz waves in the range of 3 THz were generated by photonic down-conversion of two-tone signals extracted from the broadband combs.

We are developing a low noise superconducting heterodyne receiver based on a hot electron bolometer mixer (HEBM) and a THz Quantum Cascade Laser (THz-QCL) as a local oscillator at 3THz for atmospheric and astronomical observations. The best value of the uncorrected receiver noise temperature of 1,200 K (DSB) which corresponds to 8 times quantum limit was achieved. We demonstrated phase-locking of a 3 THz-QCL using the HEBM and evaluated its performance.

We characterized a 3THz continuous wave generated by photomixing two optical modes of a broadband optical frequency comb. This optically generated THz wave and the output beam of a 3THz quantum cascade laser phase-locked to a THz reference were coupled onto a hot electron bolometer mixer operated at 4K and the beat signal was precisely evaluated. This direct comparison at 3THz reveals that the optically generated 3THz wave has fractional frequency instability of 4×10−15 at 100sec averaging time and the power spectral density of −10 dBc/Hz at 1Hz Fourier frequency.

A balloon-borne superconducting submillimeter-wave limb-emission sounder (BSMILES) was developed to observe thermal emission lines from stratospheric minor constituents. BSMILES carries a 300-mm-diameter offset parabolic antenna, a 624-639-GHz superconductor-insulator-superconductor (SIS) receiver, a three-axis fiber-optical gyroscope, and an acousto-optical spectrometer. BSMILES was launched from the Pacific Coast of Japan. All systems operated properly and emission line spectra of stratospheric gases, such as O/sub 3/, HCl, HO/sub 2/, and O/sub 3/ isotopes were measured. The system noise temperature in double sideband (DSB) during the flight was less than 460 K over the observing bandwidth with a best value of 330 K that is 11 times as large as the quantum limit (11h/spl nu//k/sub B/). After the observation, the gondola splashed down in the Pacific Ocean and was retrieved. Almost all instruments were waterproofed, and it has been proved that they are reusable.

Stratospheric O/sub 3/ and ClO were simultaneously observed off the northeastern coast of Japan by the Balloon-Borne Superconducting Submillimeter-Wave Limb-Emission Sounder (BSMILES) developed at National Institute of Information and Communications Technology. BSMILES is a highly sensitive submillimeter radiometer that exploits the superconductor-insulator-superconductor (SIS) technology for atmospheric research. This paper presents the first BSMILES spectra, and describes the details of the calibration process. The vertical profiles of O/sub 3/ and ClO have been also retrieved. In spite of some calibration uncertainties the obtained profiles are in relatively good agreement with previous and other available measurements.

A fitting method is presented here for the accurate characterization of the IF noise contribution of a sub-millimeter SIS receiver. By fitting the mixer's IF output power response and junction's IV curve of an SIS mixer without LO pumping, we can obtain the IF noise contribution, the physical temperature of the isolator connected just behind the SIS mixer, the output mismatching of the mixer, and the total gain of the EF chain. Differing from a conventional method, which only uses the normal-state (linear) branch of the junction's IV curve, the method proposed here also includes the nonlinear portion around the gap voltage, The dynamic resistance in this portion is varied dramatically, providing us a good probe to characterize the output mismatching of the mixer, as well as other parameters.

We demonstrated rapid measurement of spectra of terahertz (THz) radiation from a quantum cascade lasers. THz waves generated by photonic down-conversion using Mach-Zehnder-modulator-based flat comb generator were used for local oscillator signals.

We have developed a phase-locking system of a 3.1THz QCL (Quantum Cascade Laser) using a low noise hot electron bolometer mixer (HEBM) and a THz reference. The THz reference was generated by photomixing two optical modes of a frequency comb. The THz-QCL and HEBM devices are fabricated in our laboratory. A line width of the phase-locked QCL of narrower than 1Hz was achieved.