Your requirements are similar to those of computer processors, except in that case the voltage can be even lower and the current higher. The total power is comparable. Switch-mode power supplies (buck converters) are mandatory for reasonable efficiency--with 12V input, it can be above 90%.
Obviously, in these applications, it is difficult to provide sufficient filtering without either requiring huge output capacitors/inductors, or operating your buck converter at a ridiculously high frequency. In fact, the former is often not possible because of the requirement to be able to rapidly change the output voltage or current. The other major problem is that one normally wishes to minimize the size of the output inductor and switching transistors for cost and other reasons.
As a result, computers usually employ multiphase buck converters. This not only allows one to replace expensive, high-power single devices with many cheaper ones in parallel, but also leads to better controllability because less energy is stored in the output inductors and filtering components than would be for a single-phase buck regulator. At the same time, ripple is reduced by offsetting the conduction of each phase.
There is not much difference in principle between a constant-voltage or constant-current buck converter apart from the nature of the feedback signal. Since a laser diode need not be referenced to ground, you can use either high-side or low-side current sensing. Some (current-mode control) buck controllers even include an average current output and cycle-by-cycle current limiting, which could simplify the design.
Some devices that could be useful for you are LM5119 (dual-channel, but lacking the average current monitoring) or and LM5117 (with current monitor--see section 8.4 of the datasheet--but single-channel, although synchronizable to an external clock for multiphase implementations). Both also provide soft-start functionality.
I suggest you start by reading the datasheets in detail, although you will soon appreciate that this design is not going to be a trivial undertaking. Using devices like those mentioned above, with integrated high- and low-side gate drives and much of the necessary logic already integrated, will make your life easier. Still, you should pay very close attention to loop stability over the whole current range, especially with multiphase implementations.
