We experimentally demonstrate linewidth-tolerant real-time $40\text{-}\mathrm{Gbit}∕\mathrm{s}$ $(10\text{-}\mathrm{Gsymbol}∕\mathrm{s})$ 16-quadrature amplitude modulation. We achieved bit-error rates of $<{10}^{-9}$ using an external-cavity laser diode with a linewidth of $200\mathrm{kHz}$ and $<{10}^{-7}$ using a distributed-feedback laser diode with a linewidth of $30\mathrm{MHz}$ , thanks to the phase-noise canceling capability provided by self-homodyne detection using a pilot carrier. Pre-equalization based on digital signal processing was employed to suppress intersymbol interference caused by the limited-frequency bandwidth of electrical components.