Intracavity-loss controlled wavelength-tunable bidirectional mode-locked erbium-doped fiber laser.

Bidirectional wavelength-tunable mode-locked fiber lasers have demands for many applications. In our experiment, two frequency combs from a single bidirectional carbon nanotube mode-locked erbium-doped fiber laser are obtained. Continuous wavelength tuning is demonstrated in the bidirectional ultrafast erbium-doped fiber laser for the first time. We utilized the microfiber assisted differential loss-control effect on both directions to tune operation wavelength and it presents different wavelength tuning performances in two directions. Correspondingly, the repetition rate difference can be tuned from 98.6 Hz to 32 Hz by applying strain on microfiber within 23 µm stretching length. In addition, a minor repetition rate difference variation of 4.5 Hz is achieved. Such technique may provide possibility to expand wavelength range of dual-comb spectroscopy and broad its application fields.

Observation of RIN reduction via spectral broadening in an NPR-based stretched pulse fiber laser.

We show that an optimum mode-locking state with low relative intensity noise (RIN) can be identified by continuous broadening of an optical spectrum in a stretched-pulse fiber laser based on nonlinear polarization rotation (NPR). Under the premise of keeping the overall spectral shape unchanged, either gradually increasing the pump power or unidirectionally adjusting the polarization controller (PC) can effectively reduce RIN as the optical spectral bandwidth broadens. The optimized intensity noise performance of the laser can be attributed to the increased pulse energy and reduced intra-cavity net dispersion. Moreover, the integrated RIN will further decrease as the maximum 3-dB bandwidth extends. In our experiment, the detected minimum integrated rms RIN is below 0.003% (from 100 Hz to 100 kHz). Our experimental results find that the absolute spectral width is not a necessary key condition for obtaining low RIN mode-locked laser, whereas it may help understand and design versatile low-noise ultrafast laser sources.

Narrow bandwidth spatiotemporal mode-locked Yb-doped fiber laser.

We report a narrow bandwidth spatiotemporal mode-locked (STML) ytterbium-doped fiber laser, based on a homemade carbon nanotube/polyvinyl alcohol composite film and the multimode interference filtering effect. The wavelength-tunable narrow bandwidth STML operations combined with different pulse states are achieved, including single pulse, multiple pulses, and harmonics. The 3-dB bandwidth at the single-pulse state is 103 pm, while at the harmonic state, it is as narrow as 26 pm. To give an insight into the generation of the narrow bandwidth STML pulses, numerical simulations are performed. Such a laser has a wide range of potential applications in fields of optical communication and optical measurement, as well as provides a favorable platform for studying the evolution dynamics of multimode solitons.

Wavelength-tunable noise-like fiber laser based on PbS quantum dot saturable absorber film.

A wavelength-tunable noise-like pulse (NLP) erbium-doped fiber laser incorporating PbS quantum dot (QD) polystyrene (PS) composite film as a saturable absorber (SA) is experimentally demonstrated. The wavelength tuning is implemented via a Lyot filter consisting of a segment of polarization-maintaining fiber (PMF) and a 45° tilted fiber grating. By adjusting the polarization state of the ring cavity, the laser can deliver NLP with a continuous wavelength-tunable range from 1550.21 to 1560.64 nm. During continuous wavelength tuning, the output power varies between a range of 30.88-48.8 mW. Worthwhile noting is that the output power of 48.8 mW is the reported highest output power for wavelength-tunable NLP operation in an erbium-doped fiber laser using composite film as a saturable absorber.

Wavelength-tunable L-band mode-locked fiber laser using a long-period fiber grating.

We demonstrate an L-band wavelength-tunable passively mode-locked fiber laser using a single long-period fiber grating (LPFG) as a narrow-band optical attenuator (NBOA). Through bending the LPFG, the central wavelength can be continuously tuned from 1582.02 to 1597.29 nm, while the output power only varies from 1.465 to 1.057 mW, approximately a rate of 22 µW/nm variation. This is the first time that LPFG is functioned as a NBOA in mode-locked fiber lasers, showing the great advantage of less impact on output power variation reduction. Besides, the total cavity length is 5.08 m, which is the shortest length yet reported in wavelength-tunable mode-locked fiber lasers. The wavelength tuning could also be realized at harmonic mode locking with tuning range of 14.69 nm under 5th harmonic.

Nonlinearity managed passively harmonic mode-locked Er-doped fiber laser based on carbon nanotube film.

We examine the implication of intracavity nonlinearity for harmonic mode locking (HML) by exploiting highly nonlinear fiber in a carbon nanotube film mode-locked Er-doped fiber laser. It is found that the reasonably large nonlinearity is of benefit to increase the extent of harmonic order while the excessive nonlinearity leads to some peculiar multi-pulse patterns such as noise-like pulse and soliton rain. Via appropriate nonlinearity management, nearly 4 GHz repetition rate pulses at the 91st harmonic with 936 fs pulse duration are delivered under the pump power of 280 mW. The pulse stability is evidenced by the super-mode suppression ratio of 35.6 dB. To the best of our knowledge, it is the highest repetition rate yet reported for a passively HML fiber laser based on a film-type physical saturable absorber. Furthermore, the laser exhibits steep pumping efficiency slope of ${\gt}{19}\;{\rm MHz/mW}$, which is also a record among all of the passively HML fiber lasers.

All-fiber passively mode-locked ultrafast laser based on a femtosecond-laser-inscribed in-fiber Brewster device.

We report on an in-fiber Brewster device with a 45° tilted fiber grating (TFG) directly written by a plane-by-plane femtosecond laser inscription method. Up to 10 dB polarization-dependent loss was achieved, proving effective polarizing functionality. Furthermore, we employ it as an in-line polarizer to successfully mode lock a fiber laser through the nonlinear polarization rotation technique. A stable soliton pulse train has been generated at 1563.64 nm with a pulse width of 624 fs and pulse energy of 0.42 nJ. With proper polarization adjustment, the laser also can operate in a noise-like regime. The parameters of this kind of 45°-TFG can be flexibly customized owing to the high flexibility and controllability of the femtosecond laser-inscription approach. In particular, such in-fiber polarizing devices inscribed by femtosecond laser inscription without removing the fiber coating are extremely robust for fiber lasers working at a broad wavelength region including the mid-infrared.