Line-width compression of the distributed feedback laser with an external parallel feedback cavity

Abstract

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. © 2011 SPIE.