Hitoshi Kiuchi

High Extinction Ratio Mach–Zehnder Modulator Applied to a Highly Stable Optical Signal Generator

Research into optical modulators has made remarkable progress in recent years. This paper discusses the possibility of applying the high extinction ratio optical modulator to a high-stability and high-frequency (over 100 GHz) optical reference signal generator. High-frequency reference signals are generated by a highly stable optical two-tone generator, which is used for high-rate communication and astronomical application. One method to generate two optical signals is producing them from a pair of laser sources using an optical phase-locked loop for feed back control; however, the optical phase-locked loop has a stability problem in its operation. A good alternative method to the optical phase-locked scheme is the LiNbO3 Mach-Zehnder (MZ) optical intensity modulator, which is capable of generating two highly stable optical signals (upper sideband and lower sideband components) by applying a sinusoidal microwave signal to an input laser signal. The two optical signals require phase stability better than 10-13 in the Allan standard deviation, vibration robustness, and polarization maintaining capability. The signal coherence loss estimated from the phase stability of the two optical signals generated by the MZ modulator shows that the optical MZ modulator has the ability to generate highly stable optical signals.

Quadruple Frequency Double Sideband Carrier Suppressed Modulation Using High Extinction Ratio Optical Modulators for Photonic Local Oscillators

We proposed a novel scheme for the fourth order harmonic generation with two Mach-Zehnder (MZ) optical modulators. High extinction ratio intensity modulation technique with active trimmers is used for high spurious suppression ratio. Each modulator has a pair of trimmers in the arms of the MZ structure. The trimmers can compensate amplitude imbalance in the MZ structure, and suppress residual carrier in double sideband suppressed carrier (DSB-SC) modulation. A photonic local signal of 42GHz was generated from a 10.5GHz modulating signal, where spurious component intensity was 41.8dBc without using any optical or electric filters for spurious suppression. The upper limit of modulating frequency in the state-of-the-art is about 50GHz. Thus, we can generate high-purity stable 200GHz photonic local signals by using this technique, which would be useful for large scale millimeter wave antenna arrays.

Highly Stable Millimeter-Wave Signal Distribution With an Optical Round-Trip Phase Stabilizer

Highly stable millimeter-wave signal distribution technique is required for a recent radio interferometry. In the photonic local system, two coherent optical signals are transmitted and converted into a millimeter-wave signal by a photomixer in the antenna. During the signal transmission through the fiber cable, the cable length delay fluctuation is caused together with polarization mode dispersion, which will impact the performance of coherent signal distribution. In this paper, we present the test results of a prototype roundtrip phase stabilizer.

Automated geodetic Very Long Baseline Interferometry observation and data analysis system

A precise geodetic measurement network using three modern space geodetic techniques, i.e. Very Long Baseline Interferometry, Satellite Laser Ranging, and Global Positioning System, is being established around Tokyo, Japan by the Communications Research Laboratory. The Key Stone Project, which is the name of the project, was started to obtain precise relative positions of four stations using these three space geodetic techniques on a daily basis. The system was designed to make frequent observations possible with minimum human operations and to provide analyzed results as fast as possible. This paper describes various aspects of new features and the performance of the automatic geodetic Very Long Baseline Interferometry observation and data analysis system designed for the Key Stone Project. This automated design has allowed daily Very Long Baseline Interferometry experiments to be conducted since January 1995 and the results to be immediately made available for public use after each experiment.

Evaluation of repeatability of baseline lengths in the VLBI network around the Tokyo metropolitan area

Since 1995, VLBI measurements using fixed VLBI stations around the Tokyo metropolitan area have been continually producing data of station positions and baseline lengths. The accuracy of baseline length measurements is evaluated in terms of repeatability, conventionally defined as a standard deviation of those obtained by five continuous sessions. Continuous improvement both in system hardware and in the observation method have resulted in a remarkable improvement in measurement accuracy. Repeatability reaches about a 2-mm level in baseline length in our VLBI network.

Japan-US time comparison experiment for realizing better than 1-ns accuracy by using a radio interferometric technique

A zero baseline interferometry (ZBI) experiment was conducted to determine the differential instrumental delay between two stations (Kashima, Japan, and Richmond, USA). It enabled the authors to find an absolute time comparison with an accuracy of better than 1 ns. ZBI experiments in Japan were conducted in winter and summer to find the delay dependency on temperature. >

Development of GPS positioning system 'PRESTAR'

A precise relative positioning system using GPS, called PRESTAR, has been developed and is designed to give accurate relative position measurements, precise time synchronization, and precise orbit determinations. PRESTAR uses a high-gain beam antenna to obtain highly accurate range data through the high signal-to-noise (S/N) ratio of the received signal. It makes use of a beam antenna, necessitating a single channel sequential system. Its performance gives accurate relative position measurements, precise time synchronization, and precise orbit determinations have been demonstrated with a 55-km baseline experiment. Longer baseline positioning experiments and orbit determination experiments are being planned. The authors outline the PRESTAR system and some results of experiments. >

Real-time VLBI system using ATM network

The Communications Research Laboratory (CRL), Tokyo, Japan, and the Telecommunication Network Laboratory Group, Nippon Telegraph and Telephone Corp., Tokyo, Japan, have developed a highly precise, very long baseline interferometry (VLBI) system using a high speed asynchronous transfer mode (ATM) network. The observed data is transmitted through a 2.488-Gbps ATM network [STM-16/OC-48] instead of being recorded onto magnetic tape. The system was specially designed for the Key Stone Project (KSP), a project begun in 1994 to measure crustal deformation in the Tokyo metropolitan area. Cross-correlation processing and data observations are carried out simultaneously by one operator. It takes about one hour to analyze the data after the observations and correlations are completed. In regular geodetic VLBI experiments run every other day for 24 h, a horizontal position uncertainty of about 2 mm and a vertical position uncertainty of about 10 mm were achieved. The system was designed to enable automated operation throughout the entire process. The results obtained are available to the public via the Internet at http:ksp.crl.go.jp. This system is a significant advance in VLBI and should provide more precise information about crustal deformation in the Tokyo metropolitan area.

Wide Frequency Range Optical Synthesizer With High-Frequency Resolution

The generation of high-frequency signals including those in the terahertz region has become indispensable in state-of-the-art communication systems. A synthesizer as a measurement device generates signals of desired frequencies with high-frequency resolution. Electric signals (microwaves or millimeter waves) can be generated by a photomixer performing optical-electrical energy conversion based on the phase difference between two coherent optical signals. Recently, the frequency bandwidth for photomixers has reached the terahertz region and photomixers are now more readily available. The main purpose of this letter is to provide an optical synthesizer with a high-frequency resolution (better than 1 Hz) and with seamless frequency coverage from the microwave to the terahertz-wave regions using one apparatus. The system is designed in such a way that the mean of the resolution frequency includes the frequency instability of the generated signal.

A highly stable crystal oscillator applied to the VLBI reference clock

Research into crystal oscillators has made remarkable progress in recent years, and the stability of selected crystal oscillators can reach 3/spl times/10/sup -13/, a value comparable to the stability of the atmosphere. Instead of the hydrogen maser, a carefully selected crystal oscillator which is phase locked to a cesium frequency standard for 100-second time scales was adopted to the time and frequency standard of a geodetic VLBI experiment. The frequency stability of the integrated frequency standard system is poorer than that of the normally used hydrogen maser, but the overall results of geodetic VLBI measurements are comparable to those obtained from the normal experiments, provided a properly designed observation schedule is used. VLBI experiments were carried out for four years from 1988 using this system at one of the 1000 km baselines, and we successfully measured the position of Marcus Island within 5 mm and its site velocity within 5 mm/year. The first VLBI experiment using this system on the 11000 km baseline between Japan and Antarctica was carried out successfully in January 1990.