Advanced applications in optical frequency metrology demand improved tunable lasers with high coherence. Herein, a tunable single-frequency fiber laser based on the longitude-purification induced by the distributed feedback has been demonstrated. The key device in the proposed laser structure is a distributed self-injection feedback structure (DSIFS) which function as a mode selector to have a robust frequency-domain mode suppression and wavelength adaptability. Our work is distinctly focused on the theoretical analysis of the formation process of single-longitudinal-mode (SLM) laser output and the wavelength adaptive characteristics in the DSIFS. In this experiment, the side mode suppression of the resonant longitudinal modes can be achieved in a fiber ring laser without any accurate control of the main cavity. The fiber laser obtained a compressed laser linewidth of 855 Hz, enhanced side mode suppression ratio (SMSR) of ∼67 dB, and frequency noise suppression of 40 dB. Furthermore, the fiber laser can be tuned over the entire flat gain region. The measured output power, wavelength and SMSR variations of the proposed laser over a long-term observation are less than 0.034 mW, 0.028 nm and 2.53 dB, respectively. In addition, the SLM operation of the laser can also be obtained at different pump power for every wavelength channel. Moreover, this method is applicable to any other gain-type lasers, indicating that the longitude-purification mechanism has a broad application prospect in other highly coherent tunable lasers.

An acousto-optic tunable bandpass filter was proposed and fabricated based on acoustic-flexural-wave-induced single-mode fiber birefringence via coupling the core mode to a single-cladding vector mode. In the experiment, the resonant wavelength and insertion loss could be electrically tuned with a span of nearly 100 nm and a lowest insertion loss of $-1.7\mathrm{dB}$. The structure gains advantages of a large tuning range, fast speed, easy fabrication, low insertion loss, and zero frequency shift.

Acousto-optic interactions, employed in the ultrafast laser regulation, possess remarkable advantages for fast tuning performance in a wide spectral range. Here, we propose an ultrafast fiber laser whose wideband tunability is provided by an acousto-optic structure fabricated with an etched single-mode fiber. Because of the laser polarization conversion induced by the coupling between the core and cladding vector modes in the etched fiber, a band-pass characteristic of the acousto-optic interaction is achieved to effectively regulate the inner-cavity gain range. Cooperating with a saturable absorber based on single-wall carbon nanotubes (SWCNTs) with polarization robustness, a soliton operating state is achieved in the tunable erbium-doped fiber laser. By controlling the acoustical wave frequency from 1.039 to 1.069 MHz, this soliton laser can be conveniently tuned in a wide spectral range from 1571.52 to 1539.26 nm. Meanwhile, the laser pulses have near-transform-limited durations stably maintaining less than 2 ps at different wavelength channels, owing to the broadband nonlinear absorption of SWCNTs.

We demonstrate an all-fiber single-longitudinal-mode (SLM) narrow-linewidth ring laser stabilized by a microsphere resonator and fiber Bragg gratings (FBGs) with a large continuous wavelength tuning range from 1540 nm to 1570 nm. In the experiment, stable lasing with a linewidth smaller than 5 kHz was obtained. The laser wavelength was linearly and continuously tuned in the range of 0.15 nm by increasing the pump power and discretely tuning with a step of 0.1 nm by stress-controlled FBGs. The tuning range of the proposed laser configuration was determined by the FBG, and the SLM state was ensured by the coupling between the few-mode tapered fiber and the microsphere, which was simpler in alignment than other ring lasers utilizing microresonators.

An ultra-widely tunable acousto-optic add-drop filter is proposed and demonstrated based on the acoustically-induced polarization conversion of cladding-etched single-mode fiber, which covers wavelengths from 1081 nm to 1582 nm with a tuning band larger than 500 nm. The constructed filter has reconfigurable band-pass and band-rejection filtering characteristics, and obtains the advantages of ultra-wide wavelength tuning range, fast tuning speed, simple fabrication and zero frequency shift. It has potential applications in the fields of WDM communication, ultra-wide tunable laser, signal processing, swept-source optical coherence tomography (SS-OCT) and wideband spectroscopy.