Based on the reported specs of a Blu-Ray disc (BD) - physical dimensions, data capacity - I ran through some math, and calculated the physical width of a single bit of information stored on a BD to be about 550 nanometers (this assumes a BD bit has approximately equal length and width). This agrees pretty closely with the reported laser spot size of 580 nanometers, so I figure I’m not too far off.
CD’s, DVD’s, and BD’s are molded plastic discs that are designed to be removed/reinstalled on a player’s spindle by the user. Whereas the platter on a PC’s hard drive is mounted on a precision spindle bearing exactly once and held to that center of rotation for the life of the drive, there’s no way that a removable optical disc can be recorded at the factory (or in a PC’s burner-drive), and then placed in a player on exactly the same center of rotation; it’s pretty much guaranteed that during every playback, the recorded track on the disc will be eccentric, conceivably by several thousanths of an inch. In the case of a BD, an eccentricity of a thousandth of an inch would be approximately 25 times the track width.
With a BD rotational speed of several thousand RPM, how is the read-laser able to follow that eccentric track? Is the read-head able to mechanically wobble back and forth at 100+ cycles per second precisely enough to accomplish this, or are they doing some kind of electro-optical beam steering?