Is a DVD sensor a photodiode or an array of them?
The signal is picked up with a high-speed photodiode array of several sensors to allow the tracking circuitry and focusing to work. Using closed-loop control allows the mechanical parts to have relatively loose tolerances compared to even an LP record turntable. As one Japanese Engineer said you could make the player chassis out of chopsticks and it would still work. 



Here's the sensor array used in a Sony pickup: