Edited by Jun Umezu

A1. Time and Frequency Standards and Time Transfer Technique

NRLM (National Research Laboratory of Metrology) developed the optically pumped cesium beam frequency standard named NRLM-4. The frequency stability is 5x10-13/t1/2 and the total uncertainty is 2.9x10-14. NRLM-4 is operated regularly and used for the calibration of the international atomic time (TAI)[Nakadan et al.1996, Hagimoto et al.1998]. The sapphire loaded cavity oscillator is included as the local oscillator in the Cs frequency standard system [Koga et al.1998]. Aiming an accuracy of 10-16, an atomic fountain frequency standard is being developed. A magnetic field, which had a uniformity of 0.4% in the 100nT level and light source system were constructed [Costanzo et al.1997]. A scheme using a pyramidal mirror is also investigated.

CRL (Communications Research Laboratory) and NIST have jointly developed an optically pumped Cesium frequency standard CRL-O1. The frequencies of CRL-O1 and NIST-7 agreed within 1x10-15 [Hasegawa et al. 1998a,b], and the preliminary result shows that the uncertainty is 2x10-14. In order to develop an atomic fountain standard, CRL has started the research on laser cooling and succeeded to cool Cs atoms down to a few mK using the polarization gradient cooling, of which characteristics has been also investigated [Fukuda et al. 1997]. Launching of Cs atoms by moving molasses method is also succeeded. CRL has started the research on a spaceborne hydrogen maser and is now developing a sapphire loaded cavity.

CRL keeps UTC (CRL) and disseminates the time and frequency standard in Japan on routine base, as well as the GPS time transfer [Imae, 1996]. NAO(National Astronomical Observatory Mizusawa) and NRLM continue routine base GPS time transfers. They send weekly data to BIPM via e-mail to contribute to the Coordinated Universal Time(UTC). CRL is also developing a two-way satellite time transfer network, collaborating with other time and frequency standard institutes in the Asian and Oceanian region. Some other experiments on time and frequency transfer were also made by CRL, using VLBI or a broadcasting satellite [Kiuchi et al. 1997, Morikawa et al. 1997].

Using an originally developed millisecond pulsar observation system, CRL has continued weekly observations of PSR1937+21 since 1997, and also succeeded to detect two other weak but stable millisecond pulsars for the first time in Japan.

(Yasuhiro Nakadan and Takao Morikawa)

A2. Laser Stabilization and Frequency Measurement

In these several years, many studies have been done on compact laser sources such as solid state lasers and extended cavity laser diodes (ECLD). They are powerful light sources for wavelength / optical frequency standards. On the other hand, many tools for frequency measurements in optical region such as a frequency comb generator, an OPO and a mode lock laser have been also ready for use for metrology. The increase of the number in the list of the "recommended radiations" (revision of the mise en pratique of the definition of the meter in 1997, BIPM) reflects prosperity in this field. Several papers concerning the frequency stabilization and the optical frequency measurement have been published from universities and institutes in Japan. An accurate optical frequency atlas of 1.5mm band of acetylene has been reported with an accuracy of about 10-9. Using an optical frequency comb generator and a frequency standard of the Rb two-photon transition at 778 nm, they could precisely measure the optical frequencies of an ECLD stabilized on the saturation-dips of ro-vibrational lines 12C2H2 and 13C2H2 molecule [K. Nakagawa et al., 1996].

Based on above results, an attempt to establish an more accurate and working frequency standard has been also made. Using an sealed-off acetylene cell and an electro optical modulator (EOM), they have attained long term stability up to 2 x 10-12 with an integration time of 100 s [A. Onae et al., 1998].

A first international comparison of I2-stabilized Nd:YAG lasers has been made between National Research Laboratory of Metrology (NRLM, Japan) and Joint Institute for Laboratory Astrophysics (JILA, USA). From the comparison, they have confirmed that the Allan variance of the portable NRLM laser has reached <3 x 10-13 when the integration time is larger than 100 s [Hong et al., 1998].

A compact iodine stabilized He-Ne laser at 633 nm has been developed. Through specially designed mechanical construction and short laser tube, they have realized a compact transportable laser with good mechanical and thermal stabilities [Ishikawa et al., 1996]. A hybrid laser system using a visible diode laser has been also developed for thepurpose of improving the performance of iodne stabilized He-Ne lasers [Kurosu et al., 1998].

A doubly resonant, cw monolithic optical parametric oscillator (OPO) for use as a light source for optical measurement has developed. KTiOPO4 is used as a nonlinear crystal and obtained output power is 6 mW for a pump power of 40 mW, with a pump threshold of 7mW. They also have demonstrated the feasibility of frequency control by phase locking the beat frequency between the signal and the idler [Ikegami, et al., 1998].

An optical frequency comb generator (OFCG) can be used to measure the difference frequency of two lasers. The OFCG is based on a Fabry-Perot electro-optic modulator. The accuracy of the frequency intervals between the sidebands is given by the accuracy of the microwave oscillator that drives the OFCG. They have increased the width of an optical frequency comb up to 30 THz by using self-phase modulation in an optical fiber. They also compare the resulting spectrum to numerical simulations [Imai, et al., 1998].

(Atsushi Onae )

A3. Realization of Electrical Units

A 10-V JJAVS (Josephson Junction Array Voltage Standard) system has been successively developed since we were successful to fabricateour own 10-V JJA chips in the end of 1993. Using the 10-V JJAVS system, we demonstrated the estimated accuracy of a typical measurement of the 10-V output of the ZDRS (Zener diode-based reference standard) is 6x10-9 [Murayama et al., 1997]. A comparison between JJAVS systems (which are similar systems in a mean that each system is composed of a 1-V JJAVS and a 10:1 voltage divider) of KRISS (Korea) and ETL(Japan) was performed by transporting a ZDRS. As a result, the values of the 10-V output of the ZDRS measured by the both JJAVS systems agreed within 1x10-7 or less [Kim et al., 1997].

Behavior of the JJA has been investigated in detail in the aspect of the microwave-strip-line design [Yoshida et al., 1997] and biasing the operating current on a desired voltage step [Ogita et al., 1997], [Ojha et al., 1998].

A second generation system of the QHE (Quantum Hall Effect) resistance standard has been developed. In the system, a CCC (Cryogenic Current Comparator) is used for the any resistance ratio measurement. The system is designed so that the size of the total system is reasonably small. The basic operation of the system was evaluated [Nakanishi et al., 1996]. The noise performance of the DC SQUID used in the CCC has been investigated [Nakanishi, 1997].

In order to reconstruct the primary capacitance standard at ETL, a series of systems composed of a resistance-capacitance chain from the QHR (Quantum Hall Resistance) to 0.1 pF has been developed since 1995. In the course of the development of the total system, new measurement techniques for the ratio calibration of a 10:1 ratio transformer and the 1 kHz operation of a quadrature bridge have been developed [Nakamura et al., 1997], [Nakamura et al., 1998].

In the course of studying the SET (Single Electron Tunneling), a filtering technique was developed [Fukushima et al., 1997].

In order to be responsible for a new requirement of calibration of the voltage ratio which is a measurement quantity in the strain-gauge transducer instrument used as the pressure gauge, a calibration method using inductive voltage dividers has been developed [ Bohacek J et al., 1998].

(Tadashi Endo )

A4. EM Field, Power Density and Antenna Measurement

Current topics of microwave EMI antennas and measurements including Japanese papers were reviewed in the special issue on EMC/EMI Problems in Microwave Frequency Range, IEICE Trans. on Communications. This review presents recent developments in small-sized broadband antennas for EMI measurements, antennas and measurement techniques of fast transient fields, and calibration techniques for EMI antennas used in VHF/UHF bands [Sugiura et al., 1997].

As a new measure of antenna characteristics for field measurements, the complex antenna factor (CAF) was proposed. In this parameter, the phase is added to the conventional antenna factor in the EMI measurements. For measuring the CAF of a monopole antenna, the near-field corrected 3-antenna method was introduced in order to reduce the effects of the measurement site and background noises [Ishigami et al., 1996]. To evaluate the CAF of a dipole antenna, a field transfer factor was newly proposed that theoretically estimates the transmission coefficient at a distance satisfying the far-field condition from the measured transmission coefficients in the near-field.Its effectiveness was experimentally confirmed. The evaluated results were also compared with those given by the near-field corrected 3-antenna method [Fujii et al., 1997]. The CAF of a dipole antenna with a balun was evaluated by calibrating the S parameters of the balun and calculating the effective length of the antenna element. The result was compared with that of the 3-antenna method with the field transfer factor. The CAF values obtained by the two methods fairly agree [Hosoyama et al., 1998].

A linearly polarized conical log-periodic spiral (CLS) antenna for microwave EMC/EMI measurements was developed. The proposed antenna called the double-wire CLS has two additional elements to the conventional circular polarized CLS antenna. The calculated gain and antenna factor were presented [Wakabayashi et al., 1997].

The two-dimensional electromagnetic field distributions inside waveguide-type planar circuits were measured with a semi-rigid cable probe. The measurement results are in good agreement with the results of the numerical analysis based on the normal mode expansion method [Anada et al., 1997].

(Takashi Iwasaki)

A5. Power, Attenuation and Impedance Measurement

Calibration techniques of power meters are studied in the radio frequency (RF) of 10 MHz-18 GHz for the 7 mm coaxial waveguide with APC-7 type connector [Inoue et al, 1998].

The measurement principle of RF power is based on an isothermal control type micro-calorimeter. It measures the effective efficiency of the standard thermistor mount used as the RF load. Calibration of a power meter is performed by comparing it with the standard thermistor mount of which effective efficiency is known. The signal source is composed of a leveling loop with a broadband synthesizer, a power splitter and a monitor power meter. An automated measurement system was fabricated for both calorimetric measurement and comparison measurement. The combined standard uncertainty was found to be 0.0008 -0.0071 within the frequency region.

A new broadband microwave radiometer of the differential type has been designed for microwave noise measurement by adopting a correlation scheme with a quadrature hybrid coupler (QHC) and without devices that limit the radiometer bandwidth. And its usefulness experimentally evaluated over 6 to 12.4 GHz. [Nakano et al., 1998]

A Homodyne system based on single sideband modulation is studied for precision measurement of attenuation. The radio frequency attenuation is transformed to 1 kHz attenuation and is compared with 1 kHz signal from a standard inductive voltage divider. Radio frequency phase adjustment in the carrier and subcarrier channel is an important parameter in homodyne systems using balanced modulation and ordinary amplitude modulation. The homodyne system using single sideband modulation does not require the phase adjustment but measurement error depend on suppression of unwanted sideband [Kawakami, 1997].

A broadband attenuation calibration system based on parallel intermediate substitution technique has been studied in radio frequency region up to 40 GHz. The dynamic range of the system is 60 dB in single step for the fundamental mixer mode and 40 dB in single step for the third harmonic mixer mode.

(Hiroyoshi Yajima)

A6. Optical Communication System Measurement

Optical time domain reflectometer (OTDR) is a commonly used measurement technique for fault location and characterization in optical fiber transmission systems. One research target in this period is to extend the dynamic range of OTDR. Coherent detection OTDR is a promising technique but suffers from amplitude fluctuation. A stochastic description of the amplitude fluctuation and a theoretical estimation are described [Izumita et al., 1997].

Wavelength division multiplexing (WDM) is a promising technique for optical communication systems that will offer huge transmission capacity and wide bandwidth. In WDM systems, devices to add/drop target signals to/from multiplexed optical signals are very important. Arrayed waveguide gratings (AWG's) are indispensable in WDM systems. Ways to improve their characteristics should be presented in this period. Two reports discuss environmental wavelength stability [Ishii et al., 1998] and digitally tunable filter configuration [Ishida et al., 1997], both of which involve AWG usage. Crosstalk performance is reduced by using either cascaded AWG's [Kawai et al., 1997] or phase-error compensation [Yamada et al., 1998]. The combination of an AWG multiplexer and an array of optical space-division switches has been described. This configuration can provide a simple and compact multiplexer using only one AWG for any number of channels and wavelengths, and makes it possible to integrate the circuits on one chip [Suzuki et al., 1998]. The AWG can be used to simultaneously monitor multiple wavelengths as well [Teshima et al., 1998].

One of the key functions in WDM networks is wavelength conversion for signal routing. The most practical method is the configuration of an optical receiver and tunable DBR laser pairs [Teshima et al., 1998]. The multiplexed optical signals are first converted into electrical signals in the optical receivers. The electrical signals then modulate the CW light emitted from DBR lasers to recreate the original optical signal at a different wavelength. The other method of wavelength conversion is accomplished through the lasing-mode gain-suppression of lasers by injected signal light [Yasaka et al., 1997]. This method can convert the wavelength of 10 Gb/s signals over wavelength range from 1530 to 1560 nm. Another method is based on dual-wavelength-pumped Raman-resonant four-wave mixing [Uchida et al., 1998]. A cascaded fiber Raman converter can achieve conversion from 1.31 to 1.55 mm with 7% conversion efficiency at 5 Gb/s.

Fiber amplification in three wavelength bands has been presented in this period. The reports of 1520 nm band [Kani et al., 1998] and 1650 nm band [Masuda et al., 1988] amplification describe the use of fiber Raman amplifiers. Fiber amplifiers using Er3+-doped silica fibers have been presented. A 20 dB signal gain over the signal bandwidth from 1528 to 1611 and a low flat noise figure of better than 6.5 dB from 1539 to 1624 can be obtained [Mor et al., 1998]. A wide amplification bandwidth (70 nm) and a low noise figure (better than 5 dB) have been realized with a hybrid configuration of 980 nm forward-pumped and 1480 nm bidirectionally-pumped amplifiers and by adjusting the length of the Er3+-doped silica fibers [Yamada et al., 1998].

(Masami Kihara)


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Bohacek J., Y. Nakamura, K. Yoshihiro and T. Endo [1998], "Calibration of precision measuring amplifiers for strain gauge transducers," CPEM98 Digest, pp. 120-121

Costanzo, G.A., S.Ohshima, K.Hagimoto, Y.Nakadan and Y.Koga [1997], "A New Laser System for the Cs Fountain Frequency Standard at NRLM", Proc.11th European Frequency and Time Forum, Neuchatel, March 4-6, pp.151-155

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Uchida, A., M. Takeoka, T. Nakata and F. Kannari [1998], "Wide-range all-optical wavelength conversion using dual-wavelength-pumped fiber raman converter," IEEE J. Lightwave Technol., vol. 16, pp.92-99

Wakabayashi, R., K. Shimada, H. Kawakami, and G. Sato, "Linearly polarized conical log-periodic spiral antenna for microwave EMC/EMI measurement," IEICE Trans. on Communications, vol.E80-B, 5, pp.692-698

Yamada, H., K. Takada and S. Mitachi [1998], "Crosstalk reductiion in a 10-GHz spacing arrayed-waveguide grating by phase-error compensation," IEEE J. Lightwave Technol., vol. 16, pp.364-371

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Yasaka, H., H. Sanjoh, H. Ishii, Y. Yoshikuni and K. Oe [1997], "Finely tunable wavelength conversion of high bit-rate signals by using a superstructure-grating distributed Bragg reflector laser," IEEE J. Lightwave Technol., vol. 15, pp.334-341

Yoshida H., Y. Murayama and T. Endo [1997], "A new 10-V array of Josephson junctions based on low-frequency designs scaled up to 94 GHz," IEEE Trans. Instrum. & Meas., vol. 46, 1, pp. 72-78