Kiyotaka Murashima

Dispersion effects of FBG filter and optical SSB filtering in DWDM millimeter-wave fiber-radio systems

We study the optical filtering technique for dense wavelength division multiplexing (DWDM) channel allocation of millimeter-wave fiber-radio signals in the optical double-sideband (DSB) format. First, we investigate both theoretically and experimentally the dispersion effect of fiber Bragg grating (FBG) used as the filter on DWDM millimeter-wave optical signal transmissions. This result suggests that the dispersion effect has to be considered in the DWDM channel allocation for millimeter-wave fiber-radio access systems. Next, we propose a DWDM allocation for millimeter-wave fiber-radio systems, which adopts the optical single-sideband (SSB) filtering technique at the receiver side by using a square response of FBG filter. It can realize the minimum WDM channel interval for optical DSB signals, while it enables the optical frequency interleave between the neighboring channels without any serious signal degradation due to the interchannel interference. Then, we experimentally demonstrate the error-free DWDM transmission of two 60-GHz-band, 155.52-Mb/s differential phase-shift keying (DPSK) fiber-radio signals over 25-km-long single-mode fiber (SMF) with the minimum channel interval of 83.6 GHz (=0.68 nm) by using the test-square response FBGs. Finally, we show that based upon the experimental results, in the micro- or pico-cellular DWDM broad-band millimeter-wave fiber-radio access network 1000 antenna base stations (BSs) under the coverage of the single central office (CO) would be feasible by sectorizing the zone.

1.7-μm spectroscopic spectral-domain optical coherence tomography for imaging lipid distribution within blood vessel.

We have developed a spectroscopic optical coherence tomography (OCT) for imaging lipid distribution within blood vessel in order to detect coronary artery plaque. A 1.7-μm spectral-domain OCT with A-scan rate of 47 kHz is fabricated using a broadband light source based on super-luminescent diodes and spectrometers based on extended InGaAs line sensors. We demonstrate imaging of lipid distribution in an in vitro artery model with lipid. The sensitivity and specificity in the differentiation between artery and lipid are 87% and 90% in the training, respectively. The validation test also shows detection of lipid with an accuracy over 90%.

The dispersion-free filters for DWDM systems using 30 mm long symmetric fiber Bragg gratings

The dispersion-free FBG is designed by applying the optimized cosine-progression to the apodization profile. As experimental results show that it offers the suitable filter, with 25 dB crosstalk, 15 ps group delay, for high bit-rate DWDM systems.

Performance improvement by a broadband super-luminescent diode light source in 1.7-μm spectroscopic spectral-domain optical coherence tomography for lipid distribution imaging in a coronary artery

We develop a 1.7-μm optical coherence tomography (OCT) system using a broadband light source based on superluminescent diodes (SLDs) and investigated the possibility of plaque detection by a spectroscopic OCT (S-OCT) method. The SLD-based light source realizes an output power about 20mW and a 3-dB bandwidth over 120nm for optimization of driving current in each SLD. Regarding performance of the 1.7-μm spectral-domain OCT system with the light source, the system sensitivity is 104dB in maximum at the A-scan rate of 47kHz, which is fifty times as high as that in the previous study with a super-continuum light source. Moreover, we perform visualization of lipid distribution at the A-scan frame of 47kHz by an in-vitro artery model which is made of a piece of porcine coronary artery and a lardfilled nylon tube as a plaque phantom. We confirm that the sensitivity and specificity between artery and plaque area in optimal condition for lipid detection at a specific frame image is over 90% and there are high lipid scores at the inside of plaque phantom in other frame images at the same condition. It indicates the possibility of plaque detection in intravascular OCT.

10 Gb/s transmission performance of cascaded group-delay-flattened fiber Bragg gratings

For the purpose of applying 10 Gb/s DWDM systems, the group-delay-flattened FBGs, which have both square spectral response and linear phase response, are developed. The conventional FBGs have the large group delay variation in the reflective bandwidth, and the accumulation of the dispersive effects is serious problem in cascading. On the other hand the group-delay-flattened FBGs has linear phase response, and the power penalty in cascaded 5 FBGs is under 0.1 dB independently of wavelength. These results confirm that the group-delay-flattened FBGs reflect the 10 G/s signals keeping the original waveform well. It therefore results that the group-delay-flattened FBGs are suitable for ADM used in 10 Gb/s DWDM optical transmission system, and these FBGs may be ideal devices for more complicated DWDM network systems.