Recently, optical differential phase shift keying (DPSK) has attracted much interest in high-speed optical transmission systems. It features constant intensity and thus is less vulnerable to the fiber nonlinearities. In addition to high-speed data transmission, it is desirable to carry relatively low-speed supervisory information simultaneously to facilitate the system monitoring and network management. In this paper, we propose and investigate an orthogonal modulation scheme for carrying relatively low bit-rate supervisory amplitude shift keying (ASK) data on a high-speed (10-Gb/s) optical DPSK data stream. The supervisory data is amplitude-modulated onto the constant intensity optical DPSK signal, thus no dedicated time slots are needed and this eliminates the bandwidth overhead. We investigate both experimentally and numerically the optimized operation conditions for such orthogonal modulation. The results suggest the design guidelines of the proposed ASK/DPSK orthogonal modulation for supervisory information dissemination.