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We successfully demonstrate 400 Gbit/s transmission at 1577 nm over an unamplified 40 km standard single-mode fiber (SSMF) link with 11-dB loss target. Commercially available high-bandwidth coherent driver modulator (HB-CDM) and high-bandwidth micro integrated coherent receiver (HB-μICR), designed for C-band are used in combination with two TO-can packaged and thermally tuned low-cost L-band laser sources. With the aid of a memory-less one sample/symbol nonlinear offline digital-signal processing (DSP) correcting for the nonlinear transfer curve of the driver and modulator we can maximize the transmitter optical power, which is critical for unamplified fiber applications. In addition, multistage carrier recovery enables stable performance also with the large phase noise of the used coherent laser source, originally part of an electro-absorption modulated laser module (EML) and designed for …