The distributed feedback laser (DFB) is a typical laser diode where the active region of the device is periodically
structured as a diffraction grating. The output of a DFB laser has one single longitudinal mode and a relatively narrow
line-width, usually several megahertz or one order less. However, applications such as coherent optical communication
and accurate fiber sensing demand an even narrower line-width. An effective method for compressing a DFB laser is
demonstrated. The line-width of a DFB laser, 200 KHz detected originally, is suppressed to sub kilohertz by adding an
external parallel feedback cavity. The DFB laser is normatively designed but without the output isolator. The parallel
feedback cavity is constructed by inserting several pieces of multimode fiber into a standard linear single-mode fiber
cavity. In multimode fiber, each transverse mode has a different propagation constant. Equivalently, when a light beam
propagates from single mode fiber to multimode fiber, it will split into a few parallel light paths with different
propagation constants. The external cavity and DFB cavity form a compound resonant cavity for light beams. Lasing
light in the compound cavity must fit the restrictions of all light paths, thus line-width of the final output is suppressed.
When a passive external cavity is used, the line-width is suppressed to 1.25 KHz, detected by a delayed self-heterodyne
interferometer with a 100km fiber delay line. By adding an erbium doped fiber amplifier (EDFA) into the external cavity
for loss compensation, the result is updated to 430 Hz. This line-width suppressing method can be applied for other types
of fiber lasers in a similar way. The parallel feedback mechanism is also suggested for general laser cavity designing to
achieve ultra narrow line-width light source.