It's all kind of a moot point anyway. Once the missile leaves the bay it's too late. Either you sink the sub before it launches or you watch the fireball. We might detect the sub, but there are other delivery methods. Both us and the Russians have enough nukes to obliterate eachother ten times over. Missile defense systems are basically irrelevant considering the sheer volume of incoming warheads and the technological challenges of hitting a target moving more than 10,000mph.
Wiki on our Trident missiles.
When the third stage motor fires, within two minutes of launch, the missile is traveling faster than 20,000 ft/s (6,000 m/s), or 13,600 mph (21,600 km/h).
They're just too *******ed fast. And one missile can split into multiple incoming warheads. There are quite a few technological breakthroughs to make before a real safety net is remotely feasible. I actually support canceling our current ABM programs - firing a rocket to hit another rocket at such extreme speed, distance, and altitude is an extremely difficult undertaking. Firing hundreds of rockets to hit hundreds of rockets (successfully) is impossible. Closure rates between interceptor and target would be like 6-7 miles
per second. At that speed, you have to account for the clock cycle on the processor that handles guidance or you might miss by hundreds of feet.
Missile defense works for North Korea or Iran's little medium range stuff, but if the United States or Russia decides to end the world right now, the technology does not exist to stop them.
I'm not sure even lasers are going to be able to cross that gap. We can hit small, slower moving stuff with lasers at long range pretty easily, but the speeds we're talking about here mean you have to have extremely accurate tracking equipment. We'll need great leaps in laser output as well as computing/tracking technology. And once you do that, I'm sure someone's going to figure out how to slap a laser-resistant coating on the missile and we're back to the drawing board.
Reading about these things is both fun and scary...