Jun Amagai

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.

Spin motion of the AJISAI satellite derived from spectral analysis of laser ranging data

The spin rate of the geodetic satellite AJISAI was derived through a spectral analysis of laser range data obtained by precise high-return-rate tracking stations. This new analysis method utilizes the directional dependence of the target optical depth resulting from the sparse distribution of corner cube reflectors on the satellite surface. The third and sixth harmonics of the spin rate were found to be dominant in the pass-by-pass spectral analysis of the full-rate range residuals from multiphoton laser ranging systems. Accumulating hundreds of passes, the analysis results gave the slowdown of AJISAIs spinning from 1997 to 1998. Its trend was almost identical to the conventional flash observation. Although the spin rate derived from this method was not as precise as the conventional one, it can provide global spatial coverage and continuous time coverage to monitor the spin motion. This is a new approach to research satellite orientation and orbit and is applicable to other satellites.

Development of a GPU-Based Two-Way Time Transfer Modem

We have developed a new two-way time transfer modem to improve the time transfer precision of remote clock comparison. As a timing signal, we apply a binary offset carrier, which is similar to those signals used for the next-generation Global Navigation Satellite Systems. We took advantage of versatile A/D and D/A converters, and most of the digital signal processing stages were realized by software, running on an off-the-shelf PC. This enabled us to realize the complete system with cheaper equipment, leading to an affordable low-cost modem. For the real-time digital signal processing stages implemented in software, we relied on a graphics processing unit (GPU) developed for computer game enthusiast. The developed modem can receive four channels at the same time with a single GPU card. We performed two-way satellite time transfer experiments using these modems between Japan and Taiwan. The obtained results are consistent within 200 ps with respect to the results of GPS carrier phase time transfer. As a consequence, we improved the time transfer precision by nearly one order of magnitude as compared to a conventional two-way modem without increasing the connection fees caused by commercial communication satellites.

The center-of-mass correction of the geodetic satellite AJISAI for single-photon laser ranging

Detection timing of laser pulse returned from the satellite AJISAI is modeled for a single-photon laser ranging system. The average distribution of ten thousand return pulses simulated under their model agrees well with the distribution of residuals of full-rate range data taken from Herstmonceux station that uses the single-photon detection concept. Their model indicates that the center-of-mass correction for AJISAI will differ by 28 mm between multiphoton detection and single-photon detection. The ranging data of AJISAI and LAGEOS satellites from 1995-1996 was analyzed to estimate the range bias for each laser station, and Herstmonceux station was found to have a 26 mm difference between AJISAI and LAGEOS. The other multiphoton stations have smaller differences. The results show that the difference between the multiphoton system and the single-photon system is detected and close to that predicted by our AJISAI model.

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.

Delay Difference Calibration of TWSTFT Earth Station Using Multichannel Modem

Delays in signal transmission and reception paths of an Earth station must be calibrated accurately, and their variation should be reduced in the two-way satellite time and frequency transfer (TWSTFT). We have developed a portable TWSTFT station using a modem built by the National Institute of Information and Communications Technology (NICT) and performed calibration between NICT and Telecommunication Laboratories. An uncertainty of about 1 ns in the differential calibration was achieved. In addition, we have monitored the delay variation in the Earth station using a measurement system installed at NICT to improve accuracy of the TWSTFT. The contributed instability was 50 ps/day

Detection of Philippine Sea Plate Motion by very long baseline interferometry

The Geographical Survey Institute (GSI) deployed a mobile Very Long Baseline Interferometry (VLBI) system at an established site in Chichijima (the biggest island in the Bonin islands), twice in an interval of two years. Two sessions were conducted in the experiments each year by connecting the reference base station at Kashima Space Research Center of Communications Research Laboratory (CRL). The results from the data of four experiments indicate that the Chichijima site moved 7.4±0.8cm in the direction of N67°W in the two years. This is the first successful direct measurement of the motion of Philippine Sea plate (PHS) by any method. Our result is consistent with the predicted rigid motion of PHS from the newest plate velocity model (NUVEL-1).

Carrier-phase-based two-way satellite time and frequency transfer

We performed measurements of carrier-phase-based two-way satellite time and frequency transfer (TWST-FT) with an A/D sampler and conventional TWSTFT system. We found that an instability resulting from a local signal at the satellite transponder was negligible. The short-term stability of 4 × 10-13 at 1 s was achieved in a short-baseline measurement. The results showed good agreement with the GPS carrier phase.

Comparison Study of GPS Carrier Phase and Two-Way Satellite Time and Frequency Transfer

A dual frequency GPS and a geostationary communication satellite based time and frequency transfer network have been established for the Asia-Pacific region. Some stations link to Europe with both methods. Consequently, we can compare the performance of GPS carrier phase time or frequency transfer to a two-way satellite time and frequency transfer, and vice versa. The difference between the two methods shows good consistency in the regional network. On the other hand, for intercontinental links there is an obvious variation with the diurnal and secular changes.

Development of a GPU based two-way time transfer modem

We have developed a new two-way time transfer modem, which adopts a dual pseudo-random noise (DPN) code. The usage of DPN signal dramatically improves time transfer precision without increasing connection fees of commercial communication satellites. In addition, the new modem is based on software-defined radio technology. As the only drawback of such a software implementation would be the processing speed, we overcome this problem by using a graphics processing unit (GPU) as a parallel platform. The usage of GPU enables real-time signal processing with the new modem.