Research and development of a multi-degree colorless, directionless and contentionless reconfigurable optical add-drop multiplexer (CDC-ROADM) has recently been attracting a lot of attention. A large-scale transponder aggregator (TPA) is indispensable for providing high-capacity flexible connections to optical networks. In this paper, we report our study of the requirements for the TPA, which is a key technology for achieving flexible optical networks. To meet the requirements, we have developed an 8×48 TPA prototype based on Si photonics technology. This prototype was made with a few 8×8 Si optical switches and designed to be used with a commercial ROADM system. The 8×8 Si optical switches are made by integrating 152 Mach Zehnder (MZ) Thermo Optoelectronic (TO) 2×2 optical switch elements. A double gate structure is introduced to achieve the high extinction ratio (ER) required for optical communication. To the best of our knowledge, this is the world's first Si-TPA that can be used with a commercial ROADM system. By evaluating the basic optical characteristics utilizing real-time 100Gbps digital coherent detection as one of today's practical technologies and a 4.4THz spectral bandwidth 20Tbps super-channel with digital coherent detection, as a promising future technology, we have confirmed that our prototype Si-TPA has the potential for practical use and future extensibility.