line coupler, while suppress the second and third harmonics simultaneously. A photograph of the proposed 3-dB branch line coupler is shown in Figure 7 with the compact size of 2.4 Â 1.7 cm 2 .
CONCLUSIONSA simplified design of 3-dB branch line coupler with a compact structure is presented which consists of two pairs of open stubs located at branches. By using Powell's least square method, the optimum values of all imported variables can be obtained. To verify the effectiveness of the proposed design, a 3-dB branch line coupler is implemented, fabricated, and measured. Based on the observation of simulation experiments and actual measurements, this novel type of 3-dB branch line coupler performs well with many added desirable features, such as compact structure, simultaneous second and third harmonic suppression as well as wide stopband with three controllable finite attenuation poles. The compact structure proposed in this article could be adopted in modern communications systems with broad prospect. REFERENCES 1. R. Levy and L.F. Lind, Synthesis of symmetrical branch-guide directional couplers, IEEE Trans Microw Theory Tech, MTT-16 (1968), 80-89. 2. C.Y. Pon, Hybrid-ring directional coupler for arbitrary power division, IRE Trans Microw Theory Tech, MTT-9 (1961), 529-535. 3. E.J. Wilkinson, An n-way hybrid power divider, IRE Trans Microw Theory Tech MTT-8 (1960) ABSTRACT: A scheme for simultaneous extinction ratio enhancement, optical frequency stabilization, and wavelength reuse is proposed for reflective semiconductor optical amplifier-based wavelength-divisionmultiplexed passive optical networks to stabilize the optical channel frequency and enhance bidirectional transmission. This is achieved by simply employing a single Fabry-Perot etalon at the optical line terminal rather than having one at each optical network unit. Compared with the remodulation scheme that uses a 10-Gb/s optical signal with a 3-dB extinction ratio as a downstream optical signal and a seed light, our scheme shows improvements in power penalties greater than 1.8 dB and 1.4 dB for 10-Gb/s downstream and 1.25-Gb/s upstream signals, respectively, after transmission of 25 km at a bit-error-rate ¼ 10
À9. Moreover, a power penalty of only 0.5-dB is observed in comparison to the two 1.25-Gb/s upstream bit-error-rate results based on using a continuous-wave seed light and a data-erased seed light. Key words: extinction ratio; Fabry-Perot etalon; performance enhancement; reflective semiconductor optical amplifier; wavelengthdivision-multiplexed passive optical network (WDM-PON); wavelength reuse
INTRODUCTIONIn the evolution of passive optical network (PON) systems toward higher data rates and longer transmission distances, wavelength-division-multiplexed passive optical networks (WDMPONs) can offer attractive solutions [1, 2]. For upstream signals, the reflective semiconductor optical amplifier (RSOA) is a popular colorless source to be utilized in an optical network unit (ONU) [3, 4]. At the optical line terminal (OLT), directly modulated DFB ...
