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Narrow linewidth fiber lasers are intensively studied during these years, for their wide use in coherent
optical communication, optical fiber sensing, high-precision spectroscopy and many other industrial and military
applications. A usual technique to suppress linewidth is narrow band filtering in the resonating structure of the laser.
However, it is hard in fabrication for a tradition optical filter to achieve an ultra narrow pass band such as
kilo-Hertz. In this paper, the parallel feedback structure is proposed and experimentally studied. A fiber laser with
ultra narrow linewidth (15Hz detected) is achieved in laboratory, basing on the parallel feedback mechanism in a
fiber ring cavity. In multimode fiber, each transverse mode has a different propagation constant. Equivalently, when
a light beam propagates form single mode fiber to multimode fiber, it will split into a few parallel light paths with
different propagation constants. The parallel feedback structure of a fiber ring laser is carried out by introducing
one or more pieces of multi-mode fiber into its resonant cavity. Lasing light in the cavity must fit the restrictions of
all light paths, thus linewidth of the laser output is suppressed. Ultra narrow linewidth can be achieved by carefully
adjusting the amount and length of multi-mode fiber pieces. The narrowest linewidth achieved in laboratory is
15Hz detected by a delayed self-heterodyne interferometer with a 100km fiber delay line. In this work, we focus on
pressuring linewidth, and mechanism of selecting and stabilizing mode isn't employed. Therefore, the narrow
linewidth fiber laser isn't single-longitudinal-mode (SLM). A similar ring laser structure with bidirectional output
is also experimental implemented, achieving a bandwidth of the same order. The bidirectional ring laser is the basic
component of a laser gyroscope, and its linewidth is one key limiting factor of the gyroscope accuracy. This narrow
linewidth bidirectional ring laser is a good candidate for accurate fiber laser gyroscope. Theory analysis and
experiment shows the fiber ring laser based on parallel feedback structure is robust against environmental
disturbance, achieving high stability and ultra narrow linewidth. The parallel feedback mechanism in optical
circuits can also used in other applications demanding narrow linewidth.
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