In order to realize optical interconnection for high-speed low-latency direct-attached storage in a data center, we developed double-threshold automatic gain control (AGC) that enables reliable optical out-of-band (OOB) transmission in de facto standard storage interfaces such as Serial ATA (SATA) and Serial Attached SCSI (SAS). OOB is quasiternary signaling composed of differential burst signals separated by idle conditions and is poorly compatible with general optical interconnections. It has excessively large gain and substantially affects noise-susceptible idle conditions. 12.5 Gb/s driver and receiver ICs with the double-threshold AGC were fabricated in TSMC 90 nm CMOS process. The double-threshold AGC enabled reliable optical OOB transmission by noise rejection in the idle conditions and by excess gain reduction for suppression of ringing and decay after the burst signals. As a result, optically connected SATA 6 Gb/s transmission was realized for the first time.