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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.

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.

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).

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.

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.

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 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.

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 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.