CRL (Communications Research Laboratory), NAO (National Astronomical Observatory) and NRLM (National Research Laboratory of Metrology) continue routine base GPS time transfers each other. They send weekly data to BIPM via e-mail to contribute to the Coordinated Universal Time (UTC).
CRL cesium beam primary frequency standard Cs1, whose purpose is to contribute to UTC (CRL) and TAI. The design, experimental production and measurement of special characteristics are discussed, as well as the sources of uncertainties in accuracy evaluation [Nakagiri et al., 1992].
Aiming at realization of an operational primary standard, an optically pumped Cs frequency standard is performed at NRLM. [Ohshima et al., 1994]. Optically pumped cesium frequency standards are under development at the CRL and NTT (Nippon Telegraph and Telephone Corporation) [Umezu et al., 1992; Hisadome et al., 1994].
In order to develop a high resolution Ramsey resonance spectrometer has been developed by employing the technique of launching of laser cooled Cs atoms (atomic fountain) is being performed [Ohshima et al., 1995a].
A new vertical type cesium beam frequency standard is under construction at Kinki University. Its main features are vertical low velocity beam selection by recycling beam collimator and 4-pole electromagnets, solenoid
coil C-field and a H-bend Ramsey cavity [Nakagiri, to be published].
The effects of relative intensity noise and residual FM-noise of a diode-laser and the light shift effect on the frequency stability of a laser-pumped Rb gas cell standard were investigated. The measurement showed the best stability is characterized as 1x10-12t-1/2 for t < 200 s, 7x10-14 for 200 s < t < 1500 s and 2x10-15t1/2 for 1500 s < t [Saburi et al., 1994].
At the NRLM, a single Yb+ ion, trapped in a rf trap, was laser-cooled to a temperature of 150 mK. Quantum jumps were observed with laser-cooled trapped Yb+ ions. The Yb+ ions excited in the metastable states reacted with H2 or O2 molecules into YbH+ or YbO+. This effect is one of the causes of the disappearance of fluorescence from the trapped Yb+ ions [Sugiyama et al., 1995].
Ca+ ions were trapped and laser-cooled to below 1 K. To observe the electric-quadrupole-allowed transitions (clock transitions), the laser diode operated at low temperature was developed. The clock transitions were observed using the shelved electron double resonance method [Urabe et al., 1994]. CRL's new ring ion-trapping system for frequency standard which can store a large number of ions with low susceptibility to the second-order Doppler effect was proposed [Kajita et al., 1993].
Accuracy and stability of the 657 nm Ca optical frequency standard were investigated, in which a dye laser was frequency stabilized to the optical Ramsey fringe center observed in a thermal Ca atomic beam. High resolution diode laser spectrometer has been also developed and applied to the high sensitive detection of high resolution Ca Ramsey fringes [Ito et al., 1994].
The two-way time transfer experiments using an INTELSAT Ku-band satellite in eastern Asia in cooperation between the (CRL, Japan), Korea Research Institute of Standard and Science (KRISS, Korea), and the Telecommunication Laboratory (TL, Taiwan) were carried out during 1992. The CRL-KRISS experiment attained a short term precision of better than 1 ns [Engelkemier et al., 1995].
CRL has started a project which applies millisecond pulsars to time and frequency standards, and developed a basic observation system. Using this system, the authors can observe a millisecond pulsar, PSR 1937+21, and obtain an observation precision of 10 ms/t which is primarily influenced by system noise [Hanado et al., 1994].
1) Stabilization of Laser Diodes
A 1.56 mm DFB laser diode was, at first time, stabilized to the saturated absorption spectrum of the HCN. The low pressure absorption gas cell was set in a 20-cm-long Fabry-Perot cavity and 8 mW of LD power used for the experiment [de Labachelerie et al., 1994]. An GaAlAs LD was stabilized to the saturation absorption line of Rb D2 by using the Faraday effect. The reference spectrum were Doppler-free and so the direct laser frequency modulation was not necessary. The improvement of the stability was obtained by changing the biased magnetic field strength [Ueno et al., 1993]. An GaAlAs laser diode(780 nm) was stabilized by using the zerocrossing signal of the Zeeman line from the 85Rb cell which was placed in the solenoid coil. As the zerocrossing signal was obtained by modulating the magnetic field of the coil, so the method did not need the modulation of the laser frequency. The obtained stability was between 2x10-12 and 2x10-14 at the averaging time of 100 ms < t < 100 s [Tanaka et al., 1994].
2) LD Pumped Nd:YAG Stabilization
LD pumped Nd:YAG laser was stabilized to the high-finesse(25600) Fabry-Perot cavity as a reference. By using the high-power acceptance photodetector, the shot-noise limit of 4.2x10-5 Hz/Hz1/2 below the Fourier frequency of 500 Hz. By beating two such stabilized lasers, 16 Hz linewidth of the beat signal was obtained [Uehara et al., 1994]. The LD pumped Nd:YAG was also stabilized by using an extra-cavity electro-optic modulator as a fast phase transducer and shot-noise limit of 7x10-5 Hz/Hz1/2 with cavity input power of 30 mW was obtained. By improving the short-term frequency stability by vibrational isolation of the reference cavities and by reducing the noise lower than 1 Hz/Hz1/2 at Fourier frequency higher than 5 Hz, minimum noise obtained was 7x10-3 Hz/Hz1/2. And Allan variance about 10-14 for t = 0.01 s was obtained [Nakagawa et al., 1994a].
3) Stabilization of the Other Lasers
Optically-pumped far infrared(CH3OH, HCOOH, CH3I) lasers were stabilized by adopting concave dichroic output couplers and adding the buffer gas. By comparing with a harmonics of PLL'ed Gunn oscillator, they measured the FWHM of 10 kHz after stabilization. They also tried stabilization of the laser by using the personal computer [Hori et al., 1993].
Picometer interferometry of moving objects is demonstrated experimentally over the frequency range 0 to 160 Hz [Fujimoto et al., 1995]. Active feedback is adopted to reduce the lowest frequency noise component due to coupling between geometrical fluctuations of the laser beam incident on the interferometer optics and imperfections in its optical elements. Passive measures to reduce the noise due to thermally induced change of optical elements both in the geometrical arrangement and in the optical properties are introduced by decreasing the sensitivity to temperature change. With these methods the resultant noise is reduced to 3 pm.
A simpler method is presented for measuring the refractive index of air, which uses two frequency-doubling crystals as optical components of an interferometer [Matsumoto et al., 1994]. It is based on a two-color method utilizing a fundamental wave of a laser-diode-pumped YAG laser and its second-harmonic wave. Their waves are input into another frequency-doubling crystal for generating interference fringes. The refractive index of air is measured with a resolution better than 1x10-7 at a traveling distance near 50 cm.
Novel method for simultaneous 3-D imaging using chirped ultrashort light pulses is reported[Minoshima et al., 1994]. Simultaneous imaging without scanning is possible by means of the conversion between the time and the color axes mediated by the chirped pulse. The efficacy of this method is proved by measuring an area composing three flat steps made of three gauge blocks of different lengths with an accuracy of better than 0.3 mm. This technique is applicable for imaging of the shape of moving objects, or surface testing of inspection.
A comparison among three sets of similar 1-V automated JJAVS systems was carried out to confirm that each system can be operated with assigned uncertainty of about 1x10-8 for daily calibration work of the Zener diode-based reference standards (ZDRSs) [Iwasa et al., 1994].
A 10-V calibrating system in which an automated voltage divider and a 1-V JJAVS system are combined has been used for daily calibration of 10-V ZDRSs at ETL since January 1991 [Sakamoto et al., 1993].
A 10-V Josephson junction array has been developed at ETL. It has been confirmed that the 10-V array generates voltage steps up to 14 V and the 10-V array should be used to set up a voltage standard system of 10-V [Murayama et al., 1995] [Endo et al., 1995].
A dc-SQUID (Superconducting QUantum Interference Device) magnetometer has been developed as a sensitive current detector [Nakanishi et al., 1994]. A resistance ratio bridge has been developed using a CCC (Cryogenic Current Comparator) in addition to the dc-SQUID magnetometer [Nakanishi et al., 1995].
Stable operation of a SET (Single Electron Tunneling) device with three gates has been theoretically analyzed under a condition that the device is operated by sinusoidal gate voltages [Fukushima et al., 1995].
Co-tunneling rate has been theoretically calculated for a SET device operated by sinusoidal gate voltage [Iwasa et al., 1995].
In order to re-evaluate the present ac-dc transfer standard of ETL, a new system has been developed combining a comparator system of thermal converters and a fast-reversed dc source [Sasaki et al., 1993].
The optical transmission technology was applied to a spherical dipole antenna for electromagnetic field measurements. An optical-electric (for a transmitting antenna) or an electric-optical (for receiving one) converter was built in the spherical antenna and an optical fiber was used as a signal line. Using the transmitting and receiving antennas, the site attenuation of a Electromagnetic Interference (EMI) test site can be evaluated without cable disturbances [Murakawa et al., 1994].
Characteristics of commonly used dipole antennas in EMI measurements were experimentally investigated. It was found that the antenna balun may not seriously affect the height patterns, but the antenna type (tuned/half-wave or shortened) causes great effects on measurement results, especially for the case of vertical polarization [Morikawa et al., 1994].
The near-field 3-antenna method for measuring antenna factors was proposed. The attenuation levels of three transmission systems were measured in the near-field region. The antenna factors were calculated from the attenuation values and theoretically obtained near-field correction factors [Iida et al., 1995].
The complex antenna factor (CAF) with phase information was introduced to measure fast transient fields. CAF of a monopole antenna was measured in the frequency range up to 6 GHz by the modified near-field 3-antenna method [Ishigami et al., 1995].
To improve the standard site method (SSM) for EMI antenna calibration, it was proposed that transmitting and receiving antennas were fixed at a same height during the site attenuation measurements. It was concluded that the proposed method was more accurate than the conventional SSM [Sugiura et al., 1995].
A homodyne system for measuring attenuation at 10 GHz and 30 MHz is constructed and evaluated. The radio frequency attenuation is converted to 1 kHz attenuation with homodyne detection and balanced modulation. The converted attenuation is measured by the 1 kHz receiver. A 1 kHz voltage ratio standard is also proposed to check the accuracy of the receiver for maintain the long term accuracy [Kawakami et al., 1993]. A complex attenuator consisting of a variable attenuator and two fixed attenuator (or two isolators) was presented for use as a transfer standard of attenuation in the radio frequency. The mismatch error in calibrations were analyzed [Kawakami, 1992]. An attenuator with polarization free was developed for an intercomparison of attenuation value in 1.3 mm single-mode system. The maximum attenuation change due to the center wavelength change 0.01 mm was within 0.05 dB/10 dB.
A novel method is presented for calibrating a six-port reflectometer using only one sliding load of unknown reflection coefficient and one short as a calibration standard, The validity and utility of the method were confirmed in the frequency range of 8.5-12 GHz [Yakabe et al., 1994].
Optical frequency-domain reflectometer (OFDR) has a potential for high spatial resolution. In coherent OFDR, lightwave frequency is swept linearly with respect to time so that an optical path difference between a backscattered lightwave and and a reference lightwave is identified by their frequency difference. A coherent OFDR using a 1.55-mm linewidth-narrowed laser diode was demonstrated [Tsuji et al., 1995]. The Rayleigh backscattering profile for a 16.4-km-long fiber was measured. An in-service fiber line identification scheme based on OFDR was proposed and demonstrated for single star, survivable, and passive double star architectures [Katsuyama, 1995]. The system can measure a wide range of length differences from 4 cm to 3 km.
A new reflectometry, optical coherence domain reflectometry by synthesis of optical coherence function (OCDR), for high resolution measurements of unwanted back-reflections using a laser diode as a light source [Hotate, 1993]. This method has no mechanical moving parts, and the oscillation frequency of the laser diode is directly controlled. A millimeter-resolution optical low coherence reflectometer (OLCR) that exhibits reduced jaggedness in the Rayleigh backscatter signal was reported [Takada et al., 1994]. The spatial resolution was 1.2 mm, and the hidden spaces defined by the full-widths at -20 dB and -60 dB maximum of a Fresnel response of the OLCR were 6 mm and 1.2 cm, respectively.
Gain monitoring by detecting the spontaneous emission emitted by 0.98-mm laser diode pumped erbium-doped fiber amplifiers was investigated [Masuda, 1993]. The minimum gain accuracy was less than +-0.21 dB for an EDFA with launched pump power from 10 to 30 mW. The noise figure of a cascaded in-line EDFA is monitored with an accuracy of 0.05 dB and a reproducibility of +-0.03 dB [Masuda, 1993].
Optical fiber communication systems have advantages in its high-speed and wide-bandwidth characteristics. The timing jitter due to both the carrier phase noise of laser diode pulse sources and the Gordon-Haus effect in soliton transmission was analyzed [Iwatsuki et al., 1995]. A technique to measure the difference in the carrier linewidth of two pulse sources was proposed. Frequency control techniques for cold start and channel recognition functions in coherent multichannel local area networks were reported [Ohshima et al., 1995b]. The frequency deviation of the optical transmitter was estimated to be less than +-1 GHz.
In fixed radio communications, the development on high capacity digital radio-relay systems has already matured. The general trend of research is the semi-fixed wireless access systems which support multimedia services provided by B-ISDN [Kohiyama, 1995]. The frequency band will be above 20 GHz and the transmission characteristic was also studied [Manabe et al., 1993]. In relation to the wireless access systems, the study on WLL (Wireless Local Loop) has been started [Minami, 1995].
N-STAR, the communication satellite for domestic use, was launched in 1995. N-STAR adopts multibeam system and covers S-band, C-band, Ku-band and Ka-band [Nakagawa et al., 1994b]. The domestic mobile satellite communications system operating in S-band was developed and was accommodated in N-STAR satellite. This system is used for maritime telephone service and complements the terrestrial cellular mobile systems [Hagiwara et al., 1994].
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