What is Cisco QSFP to QSFP Active Optical Cables ?

Cisco QSFP to QSFP copper direct-attach 40GBASE-CR4 cables are suitable for very short distances and offer a flexible way to connect within racks and across adjacent racks. Active optical cables are much thinner and lighter than copper cables, which makes cabling easier. Active optical cables enable efficient system airflow and have no EMI issues, which is critical in high-density racks. Cisco currently offers active optical cables in lengths of 1, 2, 3, 5, 7, 10, 15, 20, 25 and 30 meters.