なぜレーザーの光は平行光か？
つまり？
集光生がよいか？
（えー、これは同義語なの？）
それはね
The high degree of collimation arises from the fact that the cavity of the laser has very nearly parallel front and back mirrors which constrain the final laser beam to a path which is perpendicular to those mirrors. The back mirror is made almost perfectly reflecting while the front mirror is about 99% reflecting, letting out about 1% of the beam. This 1% is the output beam which you see. But the light has passed back and forth between the mirrors many times in order to gain intensity by the stimulated emission of more photons at the same wavelength. If the light is the slightest bit off axis, it will be lost from the beam.
http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/qualig.html
なんだ、それだったら
光空洞からでないレーザー光は
コヒーレントじゃないの？
たとえば
レーザーダイオードではそうならないのでは？