## 1 Introduction  
Reflective semiconductor amplifiers (RSOAs) are key modules for the realization of next-generation broadband access applications, such as colorless passive optical access networks [1], radio over fiber [2], slow and fast light [3], data erasing and remodulation [4], information packet power equalization [5] and fiber-optical cable television [6], which rely on the manipulation of data flowing in opposite directions. In effect, a single RSOA can simultaneously receive and amplify signals in the downstream while remodulating them with end-user information in the upstream communication link. This multifunctional capability is inherent to RSOAs, which owing to their internal structure use the same device facet both for signal input injection and output extraction. Thus, RSOAs avoid the need of having extra fiber paths, fiber components and optoelectronic elements to support bidirectional transmission, which helps reduce the overall system cost and complexity. However, as a physical byproduct of semiconductor optical amplifiers' finite carrier lifetime [7], the modulation bandwidth, which scales inversely against this parameter [8], of these devices, including RSOAs, is limited to a few GHz. For this reason, an RSOA can be directly modulated by electrical data, which are encoded on a seeding continuous wave (CW) light, only up to speeds which are not sufficient for satisfying the increasing bandwidth needs of the target applications. In the effort to overcome this fundamental limitation towards enabling full-duplex and symmetrical RSOA-based data transfer, several promising options are available, which include electronic equalization [9], advanced modulation formats [10, 11], specially designed RSOA package and driving circuitry [12, 13], and optical notch filtering [14]. In particular, the latter has been widely exploited because of the simple configuration, cost-affordable implementation using off-the-shelfcomponents and passive nature of the underlying signal-equalization mechanism. Furthermore, it is not concerned with challenging issues of the three other alternatives, such as the sensitivity to fiber chromatic dispersion and the requirement of high-speed sophisticated electronics at the transmitter and receiver side, in the first; the inevitable complex signal generation and detection in the second; and the involvement of elaborate fabrication and integration processes in the third. In fact, the two first RSOA bandwidth-extension techniques have even been combined with optical equalization and benefited from its efficiency to further improve their performance [11; 15; 16]. Thus, different schemes that act as optical notch filters on RSOA-encoded outputs have been reported, such as the delay interferometer (DI) [17; 18], the fiber Bragg grating (FBG) [19] and the arrayed waveguide grating (AWG) [20]. In the same technological category also fall schemes that have been applied on conventional directly modulated SOAs, such as the birefringent fiber loop (BFL), either alone [21], in cascade with another BFL [22] or assisted by an optical bandpass filter [23]; and the microring resonator (MRR) [24]. The MRR is a special form of powerful waveguides [25; 26], and exhibits distinctive characteristics over the aforementioned types of employed filters. These include the structural simplicity, the ultra-compact size-which makes it amenable to integration with microelectronic fabrication processes-enhanced wavelength selectivity, fine-tuning capability and availability of different material systems with the potential of copackaging with the SOA in the same hybrid platform. These attributes render the MRR a mostly suitable solution for increasing the operating speed of directly modulated RSOAs as well. In this paper, we investigate and demonstrate, by means of theoretical simulation and analysis, the MRR potential for overcoming the RSOA limited modulation bandwidth and thus for being extended beyond classical filter-oriented applications.