can people with back problem do yoga?

because i have lower back pain, i stopped working out in the gym as the doctor said to rest and relax.
But now i have put on too much weight and the doctor is saying that if i lose a little weight, my back ache may reduce.
he had suggested a healthy diet and i am already following that. but its not making me lose any weight as i have absolutely no exercise. so i want to start some low intensity work out like yoga. I have never done yoga before but i have heard that yoga is very safe and relaxing..more of stretches and stuff. so is it okay for people with back pain to do yoga? then i will go and join a yoga class.......