||Y. Yamanaka, G. Sakano, J. Haruki and K. Kato, THz-wave Phase Shift Measurement by THz-wave Interferometer, Electronics Letters, 53, 13, 868-869, 2017.06, A THz-wave interferometer was devised for measurement of the THz-wave phase shift. In this measurement system, the relative phase shift is deduced from the interference between the THz waves generated by two photomixers. Experimental results with this interferometer revealed that the semiconductor phase modulator could be used as a THz-wave phase shifter by utilizing its chromatic dispersion. .
||Shota Takeuchi, Kazuki Sakuma, Kazutoshi Kato, Yasuyuki Yoshimizu, Yu Yasuda, Shintaro Hisatake, Tadao Nagatsuma, Novel Lightwave-Interferometric Phase Detection for Phase Stabilization of Two-Tone Coherent Millimeter-Wave/Microwave Carrier Generation, IEICE Trans. Electron., Vol. E99-C, 1048-1054, 2016.09, For phase stabilization of two-tone coherent millimeter-wave/microwave carrier generation, two types of phase detection schemes were devised based on lightwave interferometric technique, the Mach-Zehnder interferometric method and the pseudo Mach-Zehnder interferometric method. The former system showed clear eye patterns at both OOK and PSK modulations of 1 Gbit/s on the 12.5-GHz carrier. The latter system demonstrated the error-free transmission at OOK modulation of 11 Gbit/s on the 100-GHz carrier..
||Ryoga Kimura, Yudai Tatsumoto, Kazuki Sakuma, Hirokazu Onji, Makoto Shimokozono, Hiroyuki Ishii, Kazutoshi Kato, Sub-microsecond wavelength stabilization of tunable lasers with the internal wavelength locker, Jpn. J. Appl. Phys., Vol. 55, 2016.07, We proposed a method of accelerating the wavelength stabilization after wavelength switching of the Tunable Distributed Amplification (TDA-) DFB laser using the internal wavelength locker to reduce the size and the cost of wavelength control system. The configuration of the wavelength stabilization system based on this locker was as follows. At the wavelength locker, the light intensity after an optical filter is detected as a current by the photodiodes (PDs). Then, for estimating the wavelength, the current is processed by the current/voltage-converting circuit (IVC), logarithm amplifier (Log Amp) and Field Programmable Gate Array (FPGA). Finally, the laser current is controlled to tune to the desired wavelength with reference to the estimated wavelength. With this control system the wavelength is stabilized within 800 ns after wavelength switching which is even faster than that with the conventional control system..
||Jun Tsuboi, Takeshi Kuboki, Kazutoshi Kato, Wide-capture-range, high-precision wavelength stabilization within ±50 MHz for flexible-grid wavelength division multiplexing by photomixing technique, Jpn. J. Appl. Phys., Vol. 55, 2016.07, The lasers for the flexible-grid wavelength division multiplexing (WDM) system are required to have high precision of wavelength stability. Previously, we proposed the wavelength-controlling system of the Distributed FeedBack Laser Diode (DFB-LD) with the photomixing technique and a microwave filter to precisely measure the optical frequency error from the target value. To enlarge the wavelength-capture range, we improve the system to detect the wavelength error with two different microwave filters in parallel. Experimental results show that the wavelength-capture range is extended up to 4 GHz while the wavelength error is still kept within ±50 MHz..
||Hirokazu Onji, Shota Takeuchi, Yudai Tatsumoto, Nobuhiro Nunoya, Makoto Shimokozono, Hiroyuki Ishii, Kazutoshi Kato, Fast wavelength switching with tunable distributed amplification distributed feedback laser by feedforward control technique, Jpn. J. Appl. Phys., Vol. 53, 2014.07, We proposed the way to accelerate the wavelength switching at the TDA-DFB laser based on the feedforward control. From experimental lasers’ responses, we designed and simulated the feedforward controllers using MATLAB/Simulink so as to reduce the time for wavelength stabilization. We applied this technique to two different situations; one is for the laser drive circuit with a large output current and 50-ns rise time, and the other is for that with a moderate output current and 5-ns rise time. In both situations, we showed the improvement of the wavelength switching times which were from 450 ns to 150 ns and 75 ns to 35 ns, respectively. From these results, it is confirmed that the proposed control technique is efficient for possible practical applications..