Kenichi Okumura

Compact Two-Dimensional Array of Stressed GE:GA Detectors

We have developed a 4 × 8 array of stressed Ge:Ga detectors. This array detector has a high density format ofentrance pupils so that we can minimize the size of the cameraoptics. The cutoff wavelength of the detector is about 170 μm, and the detectors NEP is better than 1016 WHz-1/2. We are going apply this array detector toballoon-borne astronomical observations. Furthermore, we aredeveloping this detector into a 5 × 15 array detector that will be placed onboard the IRIS satellite to be launched in 2003.

Flight performance of the Far-Infrared Line Mapper (FILM)

The far-infrared line mapper (FILM) is a far-infrared spectrometer and in one of four focal plane instruments of the infrared telescope in space (IRTS), FILM was designed for wide area intensity mapping of far-infrared emission from interstellar gas and dust in the galaxy. The targets are the [CII] 158 micrometer line of the ionized carbon, the [OI] 63 micrometer line of the oxygen atom, and the continuum emission at 155 and 160 micrometer from the interstellar dust grain. A cylindrically concave varied line-space grating and a linear array of stressed Ge:Ga were successfully developed and allowed us to make a compact spectrometer compatible to severe limitations of the small cryogenic telescope. The IRTS, onboard the space flyer unit (SFU), was launched by a HII rocket on March 18, 1995 and was recovered by a STS on January 13, 1996. The FILM worked very well during four weeks allocated for the IRTS observation and produced a lot of valuable data. The sensitivity and the spatial resolution for the [CII] line are an order of magnitude better than the previous work.

Development of stressed Ge:Ga 2D array

We are developing a stressed Ge:Ga 2D array detector that will be used for balloon-borne and satellite-born astronomical observations at wavelength between 100 and 200 micrometers . We have succeeded in making a 4 X 8 element stressed array detector with a stress of 600 N/mm2 and responsivity peak wavelength moved to about 165 micrometers . This has the largest number of pixels at the present time. The responsivities of the detector are high enough as well as those currently in use. This detector has a compact structure and a small total pixel size, and thus, it can be used for satellite-born instruments that have severe space limitation.

Submillimeter-wave telescope onboard a sounding rocket

We have made a submillimeter-wave telescope for a Japanese sounding rocket, S- 520-17, which is dedicated for an observation of cold dust in Orion molecular cloud region. The system is now under test for launch in Jan. 1995. The telescope include an off-axis Gregorian telescope with aperture of 30 cm, focal plane bolometer array, cryogenic cooling system down to 0.3 K, and a star sensor using charge modulation device. A very low emissivity optical arrangement of less than 1% is achieved using pure aluminum mirror, off axis reflector and cold optics. Single moded conical feed horn is effectively coupled with bolometers with efficiency of more than 90%. The focal plane array consists of 12-element bolometers, six for 250 micrometers observation and six for 500 micrometers observation. NEPs of the bolometers are 5 X 10-17W/√Hz which is read out by AC bridge read-out circuit. Total system gives sensitivities of about 10-12W/cm2 X sr for diffuse objects or 2 Jy for compact objects at 500 micrometers over 100 deg2 region with a beam sizes of 10 arcmin. This observation gives unique data on cold dust distribution, which is believed to dominate the dust mass distribution, over Orion Molecular Cloud region.

Evaluation of cryogenic readout circuits with GaAs JFETs for far-infrared detectors

The characteristics of gallium arsenide junction field- effect transistors (GaAs JFETs) and the performance of cryogenic readout circuits using GaAs JFETs with various gate sizes ranging form W/L equals 5micrometers /0.5micrometers to 200micrometers /200micrometers to systematically measure their static characteristics and low-frequency noise spectra. We found that the low-frequency noise voltage depends on the device size in the saturation region of GaAs JFETs at 4.2 K, and the power density of the noise voltage is inversely proportional to the gate are. These findings allowed us to determine the Hooge parameter of the GaAs JFET at 4.2 K to be 4 by 10-5, assuming that the carrier mobility is 1.5 by 103 cm2/Vs. On the other hand, we did not find the obvious correlation between the low-frequency noise and gate size in the ohmic region of GaAs JFETs. Based on these measurements for GaAs JFETs, we fabricated and tested a dual GaAs JFET, a source-follower-per-detector (SFD) circuit, and a 20 by 3 channel SFD circuit array. The Common-Mode-Rejection-Ratio (CMRR) of the dual GaAs JFET with W/L equals 50micrometers /20micrometers at 4.2 K was determined to be 40-60 dB under small power dissipation. The performance of SFD circuits and 20 by 3 channel SFD arrays for 2D far-IR Ge:Ga detector readouts are currently being evaluated.

Development of GaAs JFETs for cryogenic electronic circuits

GaAs JFET with various gate sizes, which ranges from 0.5 to 200 micrometers in gate length and from 2 to 200 micrometers in gate width, are fabricated and their DC characteristics and low- frequency noise spectra are measured at low temperatures in order to develop cryogenic electronic circuits for a far-IR detector array on board a satellite such as the Japanese IR satellite. We obtained the following results from the noise measurements: (1) Noise spectra of GaAs JFETs are dominated by a 1/f noise and include some generation-recombination noises in low-frequencies. (2) The 1/f noise voltage is found to remarkably depend on both the gate length and the drain-source voltage, but the gate width and the gate-source voltage have not almost concern with the 1/f noise voltage. Therefore, we suppose that the electric field in the channel of the GaAs JFET mainly contributes to the 1/f noise. By using these characteristics, the GaAs JFET having very low power dissipation and very low noise will be designed for cryogenic readout circuits at low temperatures.

Heterojunction field effect transistors (HJFETs) for a readout circuit of a cryogenically cooled far-infrared detector

Deep cryogenic field effect transistors (FETs) which are able to operate under liquid helium temperatures have significant advantages over conventional cryogenic Silicon- Junction-FETs or Si-metal-oxide-semiconductor-FETs as readout circuits of a far-IR focal plane array detector: simple operation, simple system structures, and large transconductance. We report the testing of an InGaAs-channel heterojunction field effect transistor (HJFET) operating at 4.2 K designed for a readout circuit of a cryogenically cooled far-IR detector. In this report, we present current- voltage characteristics, transconductance, low-frequency noise (LFN) characteristics, and the influence of the gate leakage current on the LFN characteristics of the HJFET. Input-referred noise voltage as low as a few hundred nanovolts at 1 Hz was measured for the HJFET with a 100 X 100 micrometers 2 gate area. We discuss further possibilities for the fabrication of HJFETs with an extremely small input current of less than 10-15 A.

Rocketborne submillimeter-wave telescope and related technologies

We have made a submillimeter-wave telescope for a Japanese sounding rocket, -520-17, which is dedicated for an observation of cold dust in Orion molecular cloud region. The submillimeter-wave telescope was launched on January 23, 1995. The telescope include an off-axis Gregorian telescope with aperture of 30cm, focal plane bolometer array, cryogenic cooling system down to 0.3K, and a star sensor using charge modulation device. A very low emissivity optical arrangement of less than 1% is achieved using pure aluminum mirror, off axis reflector and cold optics. Single moded conical feed horn is effectively coupled with bolometers with efficiency of more than 90%. The focal plane array consists of 12-element bolometers, six for 250 micrometers observation and six for 500 micrometers observation. NEPs of the bolometers are 5 X 10-17 W/(root)Hz which is read out by AC bridge read-out circuit. Total system gives sensitivities of about 10-12 W/cm2 (DOT) sr for diffuse objects or 2Jy for compact objects at 500 micrometers over 100deg2 region with a beam size of 10arcmin. This observation should give unique data on cold dust distribution, which is believed to dominate the dust mass distributional, over Orion Molecular Cloud region. Also discussed in the last section is a development of array detectors for future space mission in far-infrared and submillimeter-wave region.

Flight performance of submillimeter-wave telescope on board S-520-17

Submillimeter-wave telescope on board S-520-17 was launched on January 23, 1995. The telescope is dedicated for observation of cold dust within Orion molecular cloud. In this paper, a brief description of the telescope system and review through the environmental test, preflight calibration, and flight performance is described. Also discussed in the last section is a development of array detectors for future space mission in far-infrared and submillimeter- wave.

Eight-element-stressed Ge:Ga linear array: development and performance

We have developed an eight-element stressed Ge:Ga linear array. It has a compact and stable structure. The pixel size is 0.9 X 0.9 X 0.9 mm3, the pitch of the array is 1.0 mm, and the total sensitive area is 8.0 mm X 1.0 mm. The longer wavelength cut-off is 200 microns, and the peak responsivity is 100 A/W in a high-background condition including cavity efficiencies. It has been demonstrated that this array has a useful performance in the high-background condition, such as for airborne and balloon-borne instruments. The structure of the stress assembly is provably extendable up to sixteen and more.