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Takushima, Y.,Choi, H.Y.,Chung, Y.C. IEEE 2009 Journal of Lightwave Technology Vol.27 No.6
<P>We develop an adjustment-free differential-phase demodulator based on a delay-interferometer (DI) made of a 3 times 3 optical coupler, which is used as a 120-degree optical hybrid, and demonstrate the possibility of using it as a phasor monitor for the multilevel differential phase-shift keyed (DxPSK) signal. The key features of the proposed demodulator are twofold: (a) in-phase and quadrature (I-Q) components and the phasor diagram can be obtained only by using a single DI, and (b) the phase delay of the DI can be derived from the output power of the DI without the knowledge of the signal under test. These features enable us to demodulate the I-Q components of DxPSK signals without any adjustments. We formulate a theoretical model for deriving the differential phasor from the output signals of the 3 times 3 coupler regardless of unbalances in the phase retardations and the splitting ratios of the 3 times 3 coupler. Thus, the discrepancy from the ideal 120-degree optical hybrid is not a problem, and hence the requirements of the 3 times 3 couplers are greatly relaxed. For a demonstration, we implement the proposed demodulator by using a fiber Michelson interferometer with Faraday rotator mirrors and use it as a differential phasor monitor capable of the plug-and-play (wavelength- and polarization-independent) operation. We show that this phasor monitor can be used for diagnosis of differential phase-shift keyed (DPSK) and differential quadrature phase-shift keyed (DQPSK) transmitters by identifying the sources of impairments from the measured constellation diagram and phasor trajectories. The proposed phasor monitor can also be used for monitoring the optical signal-to-noise ratio (OSNR) of DPSK signal.</P>
Takushima, Akira,Han, Hyung-Suk,Jung, Wei-Bong The Society of Air-Conditioning and Refrigerating 2009 International Journal of Air-Conditioning and Refr Vol.17 No.2
It is well known that a refrigerant-induced noise is caused by two-phase flow in the indoor unit of a heat pump air-conditioner. Especially when the flow pattern in a pipe is intermittent flow, the irregular noise occurs frequently. But it is very difficult to avoid this kind of the noise for the application of air-conditioner. Therefore, in this research, the flow patterns at two-phase flow state in a pipe of the indoor unit for the air-conditioner are researched using cycle simulator at typical cycle conditions. In order to find the relationship between refrigerant-induced noise and flow pattern, the noise patterns are investigated with respect to the estimated flow pattern from the various flow pattern maps. Base on the estimations of the flow patterns by those maps, the refrigerant-induced noise is evaluated as decreasing tube diameter, which can transit the flow pattern from slug to annular flow.
Takushima, Y,Shin, S Y,Chung, Y C Optical Society of America 2007 Optics express Vol.15 No.22
<P>We propose and investigate a ribbon waveguide for difference-frequency generation of terahertz (THz) wave from infrared light sources. The proposed ribbon waveguide is composed of a nonlinear optic crystal and has a thickness less than the wavelength of the THz wave to support the surface-wave mode in the THz region. By utilizing the waveguide dispersion of the surface-wave mode, the phase matching condition between infrared pump, idler and THz waves can be realized in the collinear configuration. Owing to the weak mode confinement of the THz wave, the absorption coefficient can also be reduced. We design the ribbon waveguide which uses LiNbO(3) crystal and discuss the phase-matching condition for DFG of THz wave. Highly efficient THz-wave generation is confirmed by numerical simulations.</P>
Transmission of 108-Gb/s PDM 16ADPSK signal on 25-GHz grid using non-coherent receivers
Takushima, Y.,Choi, H. Y.,Chung, Y. C. The Optical Society 2009 Optics express Vol.17 No.16
<P>We demonstrate the transmission of 108-Gb/s polarization-division-multiplexed (PDM) 16-ary amplitude- and differential phase-shift-keying (16ADPSK) signal by using a non-coherent receiver. We generate the 16ADPSK signal by using a differential 8-ary phase-shift-keying (D8PSK) modulator and a phase-distortion-free amplitude-shift-keying (ASK) modulator. On the other hand, the receiver is implemented by using a delay interferometer based on a 3x3 fiber coupler and the data-aided phase-noise estimation (DAPNE) algorithm. By using these transmitter and receiver, we achieve a nearly quantum-limited receiver sensitivity in the back-to-back condition. In addition, we examine the possibility of transmitting 108-Gb/s signals on a 25-GHz grid without using the coherent detection technology. The results show that we can secure a sufficient optical-signal-to-noise (OSNR) margin after the transmission of 80-km long dispersion managed link. The achieved spectral efficiency is 4.0 bit/s/Hz.</P>
Quality Monitoring of DxPSK Signals by Using Differential Phasor Diagram
Takushima, Y.,Hyeon Yeong Choi,Hoon Kim,Chung, Y.C. IEEE 2009 IEEE photonics technology letters Vol.21 No.18
<P>We describe a new method for quantitative evaluation of the quality of multilevel differential phase-shift-keyed (DxPSK) signals by using a differential phasor monitor. This method measures the phase deviation of the symbols in the differential phasor diagram and estimates the bit-error rate (BER) by using a simple relation based on the <I>Q</I> -factor defined for multilevel DxPSK signals. We demonstrate that the proposed method can accurately estimate the BER of differential quadrature PSK and 8-ary PSK signals.</P>
Takushima, Y.,Hyeon Yeong Choi,Chung, Y.C. IEEE 2009 IEEE photonics technology letters Vol.21 No.19
<P>In this letter, we demonstrate the wavelength- and polarization-independent operation of a receiver for differential quadrature phase-shift-keying and 8-ary phase-shift-keying signals, which utilizes a single delay interferometer made of a 3 times 3 optical coupler. We show that the phase distortion, which occurred in the proposed receiver, can be reduced to less than 2deg without any adjustments. This feature enabled us to achieve nearly quantum-limited sensitivities.</P>
Takushima, Y.,Hyeon Yeong Choi,Chung, Y.C. IEEE 2009 IEEE photonics technology letters Vol.21 No.24
<P>The bit-error-rate performance of a multilevel phase-shift-keying (PSK) signal can be easily degraded by a slight phase distortion induced by the setting errors of the modulator such as the offsets of bias voltages. To solve this problem, we examine the possibility of using the data-aided phase noise estimation (DAPNE) algorithm, which has been originally proposed for the reduction of differential phase noise. The experimental results show that DAPNE can enhance the tolerance to the setting errors of differential quadrature PSK and 8-ary PSK modulators.</P>