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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.

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.

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.

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.

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.

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.

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.

A 640-GHz heterodyne superconductor-insulator-superconductor (SIS) receiver will be installed in Superconducting Submillimeter-wave Limb Emission Sounder (SMILES) on the Japanese Experiment Module of the International Space Station. SMILES will simultaneously detect twelve emission lines of stratospheric molecules in the limb sounding method. The SIS receiver is operating in single sideband (SSB) mode, and we employ a new type of Martin-Puplett interferometer consisting of two sets of a wire grid and a flat mirror for the SSB filter. The SIS mixers and intermediate-frequency low noise amplifiers will be cooled to 4.5 K and 20 K, respectively, by a Stirling refrigerator combined with a Joule-Thomson circuit used for the first time in space.

THz waves in the 3 THz range have been generated by photonic down-conversion using a Mach-Zehnder-modulator-based flat comb generator. THz waves were detected by a hot electron bolometer mixer with a quantum cascade laser.

In order to generate a reference signal used in phase locking of quantum cascade lasers (QCLs) operating in the THz range, we adopted a photonic down-conversion technique. Optical two-tone signals are created by a combination of a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) and a dispersion-flattened dispersion-decreasing fiber (DF-DDF). THz signals generated from the two-tone signal by using a uni-traveling carrier diode (UTC-PD). We have demonstrated generation of 700 GHz signals, and confirmed that the signal has the C/N ratio of greater than 30 dB and the frequency stability of 10-11 at the averaging time of 1 sec.

We have demonstrated generation of a highly accurate THz continuous wave based on a fs-pulse fiber comb. Applying ultra-narrow optical filtering based on stimulated Brillouin scattering to the fs fiber comb having a repetition rate of 100MHz enables extraction of only two optical modes of the fs fiber laser to generate a highly stable 100GHz wave.

We report on broadband comb generation using a combination of a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) and a dispersion-flattened dispersion-decreasing fiber (DF-DDF), as well as MMW signal generation using the MZ-FCG. For application of quantum cascade lasers operating in the THz range, phase-locking techniques are required to stabilize the frequency and the linewidth. Optical combs are good candidate for a reference source in the phase locking, because of the high stability and accuracy of the comb signals. Generation of a 110 GHz signal by photomixing of two-tone signal extracted from an optical comb signal has been demonstrated as a preliminary experiment.

A Low noise superconducting heterodyne receiver at 3THz is developing for atmospheric or astronomical observations. A NbN HEB(Hot Electron Bolometer) with quasi optical planar (spiral or log-periodic) antenna and a THz-QCL(Quantum Cascade Laser) which is used as a local oscillator are fabricated in our laboratory. The receiver noise temperature in double sideband of ∼2800K which is around 19 times of quantum limit is measured.

A new 17-GHz radio interferometer dedicated for solar observations was constructed in two years at Nobeyama, Nagano. It consists of eighty-four 80-cm-diameter antennas arranged in a tee-shaped array extending 490 m in east-west and 220 m in north-south directions. Since late June of 1992, radio full-disk images of the Sun have been observed for 8 h every day. The spatial resolution is 10" and the temporal resolution is 1 s and also 50 ms for selected events. Every 10 s correlator data are synthesized into images in real time and displayed on a monitor screen. The array configuration is optimized to observe the whole Sun with high spatial and temporal resolution and a high dynamic range of images. Image quality of better than 20 dB is realized by incorporation of technical advances in hardware and software, such as (1) low-loss phase-stable optical-fiber cables for local reference signal and IF signals, (2) newly developed phase-stable local oscillators, (3) custom CMOS gate-array LSTs of 1-b quadraphase correlators for 4/spl times/4 combinations, and (4) new image processing techniques to suppress large sidelobe effects due to the solar disk and extended sources. >

Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) has been operated on orbit since late September 2009. The on-orbit performance of the SMILES submillimeter receiver satisfied the specifications and demonstrated a high capability of superconducting technology for atmospheric science by providing an invaluable low-noise atmospheric limb-emission spectral data in 640-GHz band. The accuracy of the limb spectral data are going to be improved by radiometric calibration based on the instrumental knowledge measured on the ground and measurements on orbit. The observation by SMILES was unfortunately halted because of a failure of the submillimeter-wave local oscillator on 21 April 2010.

Frequency selective surfaces (FSSs) with the Jerusalem-cross array configuration have been developed for simultaneous observations of radio astronomical sources at the 40GHz and 80GHz bands. For supporting material, PET(polyethylene terephthalate: 200×220mm large, 12μm thick, permittivity 3.0) was used, and copper was used for the conducting array. The measured characteristics were in good agreement with the calculated ones. The fabricated FSS has low insertion losses of less than 0.2dB and little distortion of polarization for each band. The characteristics of the developed FSS satisfy radio astronomical requirements.

The bandwidth performance of distributed junction arrays is investigated in this paper. Firstly, the mixing behavior of distributed junction arrays is numerically analyzed with the help of an equivalent circuit model. Secondly, the mixing performances of two respective distributed junction arrays using five and ten junctions are measured. Experimental results are discussed in detail.

The Nobeyama Radioheliograph, originally constructed as a 17 GHz system, was upgraded into a dual-frequency system operating at 17 and 34 GHz on a time sharing basis. For each of the 84 antennas, a frequency-selective sub-reflector, which reflects 17 GHz radio waves into the Cassegrain focus while transmits 34 GHz waves into the primary focus, was installed and a 34 GHz frontend receiver system was mounted in parallel with the existing 17 GHz system. No major modification was introduced to the backend system. Neither were antennas added nor their arrangement changed. With this minimal modification, we have obtained (1) an angular resolution of ∼5″ (at 34 GHz) and (2) a spectral diagnostic capability of cm- to mm-wave emissions from solar flares with temporal resolution up to 100 ms. Daily 8-hour (from ∼22:45 to ∼6:45 UT) operation at dual frequencies started late October, 1995. Final tuning of the new system, such as the calibration and development of image synthesis software tools is still under way. Flare images taken at the dual frequencies are presented and compared with that from the Yohkoh SXT as an example.

An engineering model has been built for a space-borne 640- GHz SIS receiver. It is the key component of Superconducting Submillimeter-Wave Limb-Emission Sounder, which is to be operated aboard the Japanese Experiment Module of the International Space Station in 2005. The receiver includes two Superconductor-Insulator-Superconductor (SIS) mixers cooled at 4.5 K, as well as four High-Electron-Mobility- Transistor (HEMT) amplifiers, two of which cooled at 20 K and the other two at 100 K. These components are integrated in a compact cryostat with two-stage Stirling and Joule- Thomson refrigerators. The receiver components has been successfully cooled and the cryostat has survived random vibration tests. The 640-GHz SIS mixer, which uses a pair of Nb/AlOx/Nb junctions connected in parallel, is built so that a broad RF matching be achieved without mechanical tuners. It is followed by cooled low noise HEMT amplifiers with a noise temperature of less than 17 K. The total receiver noise temperature has been measured around 180 - 220 K over the bandwidth of 5.5 GHz.

We present a characterization of a 1.5 THz waveguide niobium titanium nitride (NbTiN) superconducting hot electron bolometer (HEB) mixer which can be pumped by a commercial solid state tunable local oscillator (LO) source. The NbTiN HEB mixer is made from a 12 nm thick NbTiN thin film deposited on a quartz substrate at room temperature. A gold electrode is formed in situ on the NbTiN thin film without breaking vacuum to ensure good contact. The uncorrected DSB receiver noise temperature is measured to be 1700 K at 1.5 THz, whereas the mixer noise temperature is derived to be 1000 K after corrections for losses of the input optics and the intermediate frequency (IF) amplifier chain. The required LO power absorbed in the HEB mixer is evaluated to be 340 nW by using an isothermal technique. The IF gain bandwidth is supposed to be about 1.3 GHz or higher. The present results show that good performance can be obtained at 1.5 THz even with a relatively thick NbTiN film (12 nm), as in the case of 0.8 THz. In order to investigate the cooling mechanism of our HEB mixers, we have conducted performance measurements for a few HEB mixers with different microbridge sizes both at 1.5 and 0.8 THz. The noise performance of the NbTiN HEB mixers is found to depend on the length of the NbTiN microbridge. The shorter the microbridge is, the lower the receiver noise temperature is. This may imply a contribution of the diffusion cooling in addition to the phonon cooling.