A compact optical system is provided. The system includes a first optical module, a first substrate, a second optical module, and a second substrate. The first optical module is utilized to modulate a laser beam. The first substrate supports the first optical module, and the first substrate defines a first optical via such that the laser beam can pass through the first substrate through the first optical via. The second optical module receives the laser beam from the first optical via for modulating the laser beam. The second substrate is disposed parallel to the first substrate and away from the first substrate with a first predetermined distance and utilized to support the second optical module. An ultrafast laser thereof is further provided.