Mengmeng Han

Highly Efficient Rectangular Pulse Emission in a Mode-Locked Fiber Laser

Dissipative soliton resonance (DSR) has been experimentally investigated in a nonlinear polarization rotation (NPR) mode-locked Yb-doped fiber laser with a long cavity. The adjustable nonlinear transmission function of the NPR and the relatively large nonlinearity of the long cavity play key roles in the formation of high energy rectangular pulses in the DSR region. The intracavity optical-to-optical efficiency can be as high as 46%, and the maximum pulse energy can reach 54.6 nJ with the relatively low pump power of 299 mW. This is so far the highest efficiency in a DSR laser operated at 1-μm wavelength yet achieved to our knowledge. Furthermore, while maintaining the pulse amplitude almost constant and without the appearance of wave-breaking phenomena, the pulse duration could be tuned from 54 to 91 ns by increasing only the pump power.

Different polarization dynamic states in a vector Yb-doped fiber laser.

Different polarization dynamic states in an unidirectional, vector, Yb-doped fiber ring laser have been observed. A rich variety of dynamic states, including group velocity locked polarization domains and their splitting into regularly distributed multiple domains, polarization locked square pulses and their harmonic mode locking counterparts, and dissipative soliton resonances have all been observed with different operating parameters. We have also shown experimentally details of the conditions under which polarization-domain-wall dark pulses and bright square pulses form.

Polarization domain wall pulses in a microfiber-based topological insulator fiber laser

Topological insulators (TIs), are novel two-dimension materials, which can act as effective saturable absorbers (SAs) in a fiber laser. Moreover, based on the evanescent wave interaction, deposition of the TI on microfiber would create an effective SA, which has combined advantages from the strong nonlinear optical response in TI material together with the sufficiently-long-range interaction length in fiber taper. By using this type of TI SA, various scalar solitons have been obtained in fiber lasers. However, a single mode fiber always exhibits birefringence, and hence can support two orthogonal degenerate modes. Here we investigate experimentally the vector characters of a TI SA fiber laser. Using the saturated absorption and the high nonlinearity of the TI SA, a rich variety of dynamic states, including polarization-locked dark pulses and their harmonic mode locked counterparts, polarization-locked noise-like pulses and their harmonic mode locked counterparts, incoherently coupled polarization domain wall pulses, including bright square pulses, bright-dark pulse pairs, dark pulses and bright square pulse-dark pulse pairs are all observed with different pump powers and polarization states.

L-band wavelength-tunable dissipative soliton fiber laser.

A tunable L-band dissipative soliton (DS) fiber laser with nonlinear polarization rotation (NPR) playing the roles of both a saturable absorber (SA) and a tunable filter has been demonstrated experimentally and numerically. By appropriate adjustment of the states of the polarization controllers (PCs) and the pump power, DSs with continuously tunable wavelengths have been observed over the wavelength range from 1583.0 to 1602.4 nm with a 3-dB spectral bandwidth of around 20 nm and from 1581.9 nm to 1602.6 nm with a 3-dB spectral bandwidth of around 4 nm. In addition, we have observed that by increasing the pump power, the 3-dB spectral bandwidth of the DS could be increased without pulse breaking. Numerical results for the characteristics of the DSs are in accord with the experimental data.

Polarization dynamic patterns of vector solitons in a graphene mode-locked fiber laser

Multiple polarization dynamic patterns of vector solitons, including fundamental solitons, bunched solitons, loosely or tightly bound states and harmonic mode locking have been observed experimentally in an erbium-doped fiber ring laser with graphene as a saturable absorber. By carefully adjusting the pump power and the orientation of the intra-cavity polarization controller, either polarization rotation or polarization locked operation have all been achieved for the above vector solitons. This is the first time that high order harmonic mode locking of polarization rotation vector solitons has been achieved. The signal to noise ratio of our system was ~51 dB, which indicates that the laser operated with high stability.

Tunable and channel spacing precisely controlled comb filters based on the fused taper technology

We propose and demonstrate an adjustable all-fiber comb filter with precisely controlled channel spacing by employing a tapered fiber in one arm of a Mach-Zehnder interferometer (MZI) for the first time. Using fused taper technology to draw the fiber, we can precisely control the optical path difference between the two arms of the MZI, thus realizing a precisely controllable channel spacing. By rotating the polarization controller state in the other arm of the MZI, the transmission spectrum wavelength can be continuously tuned. Comb filters with controllable channel spacings from 0.2 to 3.0 nm have been numerically studied and achieved in experiment. Applications of a filter based on a multi-wavelength tunable all-fiber laser source are also demonstrated.

Flexible operability and amplification of gray pulses.

We have investigated experimentally the flexible production and amplification of gray pulses for the first time to our knowledge. Switchable wavelengths, tunable pulse-widths, and adjustable contrasts have all been obtained in a fiber laser. Amplification of gray pulses was also experimentally investigated in detail. The contrast of the pulses could also be increased in an amplifier. The robust stability that results from the interactions between adjacent harmonic mode locking counterparts of gray pulses was found to last for up to ten hours. To the best of our knowledge, the gray pulses trains we have generated are the most stable achieved to date in an all-fiber laser system. This finding can be used as a guide for the establishment of robust gray pulses as laser sources.

Fine-structure oscillations of noise-like pulses induced by amplitude modulation of nonlinear polarization rotation

The generation and evolution processes of noise-like pulses (NLPs) in an all-normal-dispersion nonlinear polarization rotation (NPR) mode-locked Yb-doped fiber laser are numerically and experimentally investigated. We demonstrate that the amplitude modulation induced by reverse saturable absorption and the peak-power-limiting effect of the NPR plays a key role in the formation of fine-structure oscillations of NLPs. The width variation of the NLPs is also discussed in detail.

Polarization Domains and Polarization Locked Vector Solitons in a Fiber Laser

We have experimentally observed, for the first time, the coexistence of polarization domains (PDs) and polarization locked vector solitons (PLVSs) in an Er-doped fiber laser with microfiber-based graphene as a saturable absorber (SA). Taking advantage of the high nonlinearity and the saturable absorption of the microfiber-based graphene SA, we observed both PDs and their splitting into regularly or irregularly distributed multiple PDs under relatively high pump power; at lower pump power with careful adjustment of the intra-cavity polarization controllers, we observed PLVSs either as disordered soliton bunch or with harmonic mode locking. The conditions under which these patterns formed have also been experimentally investigated in detail.

Wavelength Tunable Q-Switched Fiber Laser Using Variable Attenuator Based on Tapered Fiber

We demonstrate a wavelength tunable, Q-switched Er-doped fiber laser using a mixture of gold nanorods and nanospheres as a saturable absorber. The laser has a very low Q-switched threshold pump power of 22 mW, and at a pump power of 47.86 mW, a pulse with a maximum pulse energy of 36.01 nJ, repetition rate of 37.50 kHz, and pulse width of