A simple and effective approach to semiconductor laser linewidth narrowing via mutual injection locking is proposed and demonstrated in both short and long delay regimes. A theoretical analysis is presented to investigate the linewidth behavior of semiconductor lasers under mutual injection locking. Experimental demonstrations in short and long delay regimes are implemented by integrated devices and a fiber link system, respectively. Locking condition and dependence of laser linewidth on coupling parameters in both regimes are studied, confirming mutual injection locking as a practical method for linewidth narrowing. For the short-delayed integrated lasers, a linewidth narrowing factor of 13 is demonstrated and sub-MHz linewidth is achieved, while for the long-delayed lasers coupled by fiber link, the intrinsic linewidth is reduced to sub-100 Hz.

The frequency noise characteristic of lasers under mutual injection locking is investigated. Pronounced frequency noise suppression is demonstrated for two distributed feedback lasers mutually coupled through a fiber link with 60ns delay, confirmed by both local oscillator heterodyne and delayed self-heterodyne methods. Locking can be secured for weak coupling regime and a locking map is derived. The frequency noise of the laser under mutual injection locking is found to improve with increasing coupling strength or injection current. The single-sided white frequency noise of the laser is estimated to be below 3Hz2/Hz, corresponding to an intrinsic linewidth narrower than 9.4Hz.

Pronounced linewidth narrowing in mutually injection locked distributed feedback lasers is confirmed by local oscillator heterodyne and delayed self-heterodyne measurements. White frequency noise of 3 Hz²/Hz is recorded, corresponding to 10 Hz intrinsic linewidth.