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Radio-over-fiber (RoF)-based front haul architecture has attracted considerable attention for fifth-generation (5G) cellular systems because of its cost effectiveness, however, it is susceptible to harmonic frequency distortion and intermodulation products (IMs) arising from the nonlinearity of the laser diode. To overcome this limitation, an asymmetric electrical predistortion method, using a single Schottky diode with nonlinear characteristics similar to those of a laser diode, was previously proposed. However, its asymmetric structure causes a phase mismatch, which limits the system bandwidth. In this study, we propose an enhanced electrical predistortion technique using dual-parallel Schottky diode blocks. The proposed technique was validated by experiments involving an IM3 drop of approximately 27 dB, and by using identical dual Schottky diode blocks, the phase mismatch in the previously proposed method is completely eliminated. Thus, the applicable bandwidth is extended to 1.8 GHz. This technique can be used to improve the linearity and increase the bandwidth of RoF systems. Additionally, the simple structure is attractive for low-power applications.

A simple low-current consumption analog predistortion circuit is designed to enhance linearity. The circuit is experimentally verified in a radio-over-fiber (RoF) systems. A Schottky diode is used to compensate for the nonlinearity of RoF systems. An $L$ – $I$ curve for a laser diode is compared with the $I$ – $V$ curve of the Schottky diode and the nonlinear characteristics are analyzed. The predistortion circuit is composed of two paths: 1) the nonlinear circuit with a Schottky diode to compensate for the laser diode nonlinearity and 2) the electrical delay line for a time delay. These two paths are recombined to suppress the IMD3 (third order intermodulation distortion) of the laser diode. Experimental results show that enhancements of $\sim 26$ dBc in the IMD3 and $\sim 7.5$ dB in the spurious-free dynamic range are achieved at 2.4 GHz. The proposed predistortion system has a significant reduction in current consumption compared with previous linearization techniques. In addition, error vector magnitude is satisfied under 4% with 64-quadratic-amplitude modulation.

In this letter, a triangular resonator-based surface plasmon resonance (SPR) sensor that uses total internal reflection (TIR) mirrors to provide SPR excitation for the detection of refractive index (RI) change is proposed and first demonstrated using SoI substrate without any gain medium. The fabricated triangular resonator sensor with a 35-nm-thick Au film on a TIR mirror shows an output power increment of 1.2 dB and a sensitivity of 102 nm/RIU, while that nonmetallic layer on a TIR mirror shows 30 nm/RIU. The sensitivity enhancement is attributed to a phase change in the SPR resulting in a large shift of the resonance peak. This configuration provides significant advantages by increasing the sensitivity of the RI sensor even with the extremely small sensing area of the SPR mirror.

Although the field applications of conventional optical feedforward transmitters were limited due to stationary transmission distance, the proposed optical feedforward transmitter overcame the limitations using vector control theory. A vector control circuit achieved perfectly the third-order intermodulation (IMD3) suppression regardless of distance or dispersion. The experimental results show that the IMD3 for two-tones was suppressed by maximum 38 dB even after 20 km. This work shows that the proposed optical feedforward transmitter is applicable to optical fiber radio transmission system.

The authors report significantly improved performances of a symmetric self-electrooptic-effect device (S-SEED), with high on-off contrast ratio (>30:l) and large optical bistability loop widths ( Delta P=44%) at an applied bias of V/sub a/=0 V, i.e., with no power supply. The S-SEED is made of extremely shallow quantum wells (ESQWs) in an asymmetric Fabry-Perot (AFP) cavity structure. At V/sub a/=5 V Delta P increased by up to 95%, preserving the high contrast ratio. The reflectivity changes at V/sub a/=0 and 5 V were about 15% and 30%, respectively. These are believed to be the largest values ever reported for such structures. >

A novel all-optical bistable double p-i-n structure (p-i-n-i-p) is demonstrated. Two thin intrinsic regions made of extremely shallow quantum wells (ESQWs) allow both large electric field modulation and strong light absorption, enhancing the optical bistability. Due to the noninterferometric nature, the double p-i(ESQWs)-n structure is not sensitive to thickness fluctuation, unlike the previously reported all-optical bistable device of asymmetric Fabry-Perot cavity structure. Uniform and high performance is realized with a contrast ratio of /spl sim/2.1, a reflectivity change of /spl sim/24%, and a bistable loop width of /spl sim/75% at 1 mW optical power without external bias.

We report the effects of a high power light beam on the performance of a self electro-optic effect device, which employs an extremely shallow quantum well in an asymmetric Fabry-Perot etalon structure. For a continuously incident laser beam having 4 mW power and 7 /spl mu/m diameter, for example, we observed the change of reflectivity difference from 25% to 13% and the change of contrast ratio from 16 to 5, both of which we attribute to the rise of local temperature induced by laser heating. Experimental measurements and a simplified thermal analysis show that the device degradation by ohmic heating at reverse bias of -5 V is more pronounced than that by the exciton absorption saturation at forward bias. >