Wikipedia states that Spatial coherence in lasers allows them to be focused to a tight spot and also allows a laser beam to stay narrow over great distances. About temporal coherent laser it states that temporal coherence allows them to emit light with a very narrow spectrum.
I know what temporal and spatial coherence is but i don't understand how the laser formed by them have different properties. What is underlying physics of characteristics shown by above mentioned two laser types?
[Physics] Spatial and temporal coherence in laser
laseropticsquantum-optics
Best Answer
Your question is not very clear at the moment, so let me give some practical ideas.
Spatial coherence means that all portions of the laser beam have all the same phase. When you want to focus this laser (for instance with a lens), the light close to the focus is a combination of all the portions of the initial beam, added constructively. If one portion is out of phase, it may contribute destructively. On the other hand, the light far from the focus is also a combination, but the contributions of different portions add destructively. If one portion is out of phase, it may contribute constructively. In consequence, when a portion is out of phase, it will not contribute in the focus, but it may contribute ouside => it destroys the quality of the focusing.
Temporal coherence means the laser stays in phase compared to the phase it had some time ago. If the phase slowly shifts, it means that the oscillations of the wave have become slightly longer or shorter at some point. In other words, the light wavelength also shifts slowly. This means that the spectrum of the laser is not composed of only one wavelength, but of a bunch of slightly different wavelengths.