We experimentally demonstrate a highly spectral efficient optical flow control technique for intra data center networks. A bi-directional system is implemented for generating flow control signal by reusing label wavelength and the transmission link within the same WDM channel. Dynamic operation shows high-quality flow control signal with 265 ns latency including 220 ns propagation delay and 500 mV amplitude with low input power and low bias current. Error free operation with 0.5 dB penalty for 40 Gb/s payload indicates that no distortion has been caused due the transmission of label and flow control signal.