A novel scheme to achieve linewidth depth narrowing and control of a single frequency Distributed Bragg Reflector (DBR) fiber laser based on distributed external feedback. Herein, the feedback signal can match the laser phase in real time to avoid the phase jump, thus realizing the laser mode is greatly suppressed. Moreover, it can also effectively suppress the unavoidable spontaneous radiation of the laser active medium, thus narrowing the linewidth in depth during the oscillation process. Based on this, the master laser can be regulated in an external all-optical approach by using a fiber device with controllable physical parameters. Eventually, an ultra-narrow linewidth laser with a spectral signal-to-noise ratio of 64 dB, a side mode suppression ratio of 83 dB, an output linewidth of 115 Hz. In particular, the Lorentz linewidth of the laser can be continuously adjusted from 115 Hz to 8.2 kHz by controlling the intensity of the distributed feedback signal. The proposed controllable mechanism of laser linewidth also provides a new perspective for extreme regulation of laser parameters of other types of lasers.
A new method to analyze the wavelength-swept laser spectrum is proposed with the laser energy decomposition to a series of chirp-frequency signals, which is realized by Fractional Fourier Transform (FRFT). It can be promising to obtain a spectrum-concentrated energy distribution. In the proof-of-concept experiment, we use the Delayed Self-Heterodyne (DSH) coherent detection system to get the wavelength-swept laser signal, and the narrow linewidth of a commercial wavelength-swept laser is tested with different sweep speeds from 0.93 nm/s to 1000.03 nm/s. We explore the influence of the time window width of FRFT with different sweep speeds. Specially, we further compare the spectra by FRFT and FT in different time window widths. The narrowest linewidths calculated by FRFT, and FT are 0.14 MHz and 0.84 MHz, respectively. Finally, we compared the dynamic analysis of FRFT and traditional Fourier transform. The proposed analysis method of laser dynamical parameters will promote the understanding of laser dynamics, and benefit for the optical precision measurement applications.
In this work, we demonstrate an ultra-narrow linewidth SOA fiber laser with single-longitudinal-mode (SLM) operation, wherein the side mode is suppressed by the Rayleigh backscattering (RBS) in a distributed self-injection feedback structure (DSIFS). Herein, the high scattering fiber (HSF) with a large scattering coefficient is employed as the DSIFS to soar the amount of RBS signal. In this experiment, a hybrid laser is constructed by the HSF serving as the feedback medium of ring main cavity. The SLM operation and linewidth narrowing of the proposed laser at different pump currents under the premise of stable laser output are discussed. Our experimental results show that the side mode suppression ratio of the SOA fiber laser is 70 dB with an output linewidth is less than 540 Hz. The 50 m HSF is employed to achieve laser side mode suppression and deep compression of linewidth, which greatly simplifies the system and improves performance of the laser
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