A wavelength-tunable all-fiber Erbium-doped mode-locked fiber laser based on carbon nanotubes and 45° tilted fiber grating (TFG) is demonstrated. We investigated the effect of PDL of 45TFG in the tuning range of a mode locked laser. The central wavelength of the laser can be tuned continuously from 1559.85 nm to 1564.46 nm with a tuning range of 4.6 nm using a weak 45TFG and from 1553.37nm to 1568.63 nm with a tuning range of 15.26 nm using a strong 45TFG. The laser maintains high signal to noise ratio > 50 dB across all the wavelength tuning range.

We systematically study the impact of average cavity dispersion and pulse spectral bandwidth on harmonic mode locking in the long-wavelength band from an Er-doped fiber laser using carbon nanotubes polyvinyl alcohol (CNTs-PVA) film mode locker. By carrying out pulse energy management through optimizing the average cavity dispersion and optical spectral bandwidth, 2.08 GHz repetition rate corresponding to 201 st harmonic with 36.5 dB side mode suppression ratio (SMSR) can be realized under 205 mW pump power at 1597.53 nm. Furthermore, we systematically investigate the characteristics of the harmonically mode locked ultrashort optical pulses with different spectral bandwidths versus pump power under fixed cavity dispersion firstly. Key laser parameters have been studied including pumping efficiency, duration, energy, SMSR and time bandwidth product (TBP). Enlightened by the experimental results, a simple law on preliminary and fast judgment of pulses frequency under fixed pump power is proposed, namely, by monitoring the pulse spectral bandwidth. The laser source featuring high repetition rate and L-band operation is greatly sought-after in some specific applications such as modern optical communication system etc. Index Terms-Fiber lasers, laser mode locking, nanomaterials I. INTRODUCTION OWADAYS, ultra-short fiber lasers with favorable features of high repetition rate at L-band operation are revealed as effective light sources, which contribute to some potential applications such as optical frequency metrology [1], ultra-fast spectroscopy [2], high speed optical sampling [3] and biomedical diagnostics [4] and so on. Especially, the laser sources also make a great extent development of modern Manuscript received XXXXXX.

We demonstrate a passively harmonic mode-locked (PHML) fiber laser operating at L-band using carbon nanotubes polyvinyl alcohol (CNTs-PVA) film. Under suitable pump power and appropriate setting of polarization controller (PC), the 54 th harmonic pulses at L-band are generated with the side mode suppression ratio (SMSR) better than 44dB, and repetition frequency of 503.37 MHz. Further increasing pump power leads to higher frequency of 550 MHz with compromised stability of 38.5 dB SMSR. To the best of our knowledge, this is the first demonstration on the generation of L-band PHML pulses from an Er-doped fiber laser based on CNTs.

A single/dual-wavelength switchable bidirectional Qswitched fiber laser using a bidirectional fiber polarizer is demonstrated. 45° tilted fiber grating (45°TFG) is used as a bidirectional fiber polarizer to induce bidirectional intracavity birefringence filter in both clockwise (CW) and counterclockwise (CCW) directions. Carbon nanotube saturable absorber (CNT-SA) is employed to produce Q-switched pulses. Through adjusting polarization states, switchable single/dual wavelength lasing at 1551 nm, and 1560 nm can be achieved in both CW and CCW directions. To the best of our knowledge, this is the first demonstration of wavelength switchable bidirectional passively Q-switched fiber laser.

Via careful choice of Er-doped fiber length in the cavity, a widely wavelengthtunable L-band dissipative soliton all-fiber Er-doped laser incorporating a L-band optimized polarizing fiber grating device is experimentally demonstrated. The laser delivers 15.38 ps dissipative soliton pulses centered at 1597.34 nm with 3 dB bandwidth of 34.6 nm under 622 mW pump power. The pulse repetition rate is 23 MHz. After using single mode fiber at external cavity, the pulse duration is compressed to 772 fs. With nonlinear polarization rotation-based intracavity comb filter, the central wavelength of the generated dissipative soliton can be tuned from 1567 nm to 1606 nm with a spectral tuning range of 39 nm, which, to the best of our knowledge, is the widest tuning range yet reported for a dissipative soliton fiber laser working in communication band.