Based on two mutually coupled semiconductor lasers linked by fiber, a chaotic seed signal with a 10 GHz ultra-broadband is obtained experimentally. Adopting an 8-bit analog-to-digital converter, the seed signal is converted into a binary bit stream. By an exclusive-OR operation and the most significant bit rejection, a random bit sequence at a rate up to 17.5 Gbit/s, which has passed both the National Institute of Standard and Technology statistical test and the Diehard test, is obtained.