Towards 640 Gbit/s wavelength conversion based on nonlinear polarization rotation in a semiconductor optical amplifier

Taking into account ultra-fast carrier dynamics, this paper models 640 Gbit/s wavelength conversion scheme based on nonlinear polarization rotation （NPR） in a single semiconductor optical amplifier （SOA） and investigates the performance of this kind of wavelength conversion scheme in detail. In this model, two carrier temperature equations are introduced to substitute two energy density equations, which reduce the complexity of calculation in comparison with the previous model. The temporary gain and phase shift dynamics induced by ultra-short optical pulses are numerically simulated and the simulated results are qualitatively in good agreement with reported experimental results. Simulated results show that non-inverted and inverted 640 Gbit/s wavelength conversions based on NPR are achieved with clear open eye diagrams. To further investigate the performance of the non-inverted wavelength conversion scheme, the dependence of output extinction ratio （ER） on some key parameters used in simulation is illustrated. Furthermore, simulated analyses show that high performance non-inverted wavelength conversion based on NPR can be achieved by using a red-shifted filtering scheme.

Noise-like pulse generation from a thulium-doped fiber laser using nonlinear polarization rotation with different net anomalous dispersion

A mode-locked thulium-doped fiber laser(TDFL) based on nonlinear polarization rotation(NPR) with different net anomalous dispersion is demonstrated. When the cavity dispersion is-1.425 ps^2, the noise-like(NL) pulse with coherence spike width of 406 fs and pulse energy of 12.342 nJ is generated at a center wavelength of 2003.2 nm with 3 dB spectral bandwidth of 23.20 nm. In the experimental period of 400 min, the 3 dB spectral bandwidth variation, the output power fluctuation, and the central wavelength shift are less than 0.06 nm, 0.04 d B, and0.4 nm, respectively, indicating that the NPR-based TDFL operating in the NL regime holds good long-term stability.

Optical sampling system using periodically-poled lithium niobate waveguide and nonlinear polarization rotation mode-locked fiber laser

A novel design of optical sampling system has been developed by using sum-frequency generation （SFG） in a periodically-poled lithium niobate （PPLN） waveguide and using passive mode-locked fiber laser pulses as optical sampling pulses. The system achieved high temporal resolution and high sensitivity using a 30 mm length PPLN with quasi phase match period of 19.3 μm and 151 fs sampling pulses which were generated by passive modelock fiber laser based on nonlinear polarization rotation （NPR）. Clear eye-diagram of 10 Gbit/s non-return-to-zeros （NRZ） pseudorandom binary sequence （PRBS） optical signal were successfully reconstructed by this system.

Investigation of ellipticity and pump power in a passively mode-locked fiber laser using the nonlinear polarization rotation technique

An elliptical initial polarization state is essential for generating mode-locked pulses using the nonlinear polarization rotation technique. In this work, the relationship between the ellipticity ranges capable of maintaining mode-locked operation against different pump power levels is investigated. An increasing pump power, in conjunction with minor adjustments to the polarization controller＇s quarter waveplate, results in a wider ellipticity range that can accommodate mode-locked operation. Other parameters such as noise, pulsewidth, and average output power are also observed to vary as the ellipticity changes.

Temporal contrast enhancement by nonlinear elliptical polarization rotation in a multi-pass cell

We demonstrate the simultaneous temporal contrast improvement and pulse compression of a Yb-doped femtosecond laser via nonlinear elliptical polarization rotation in a solid state multi-pass cell.The temporal contrast is improved to 109,while the pulse is shortened from 181 to 36 fs,corresponding to a compression factor of 5.The output beam features excellent beam quality with M^(2) values of 1.18×1.16.The total efficiency of the contrast enhancement system exceeds 50%.This technique will have wide applications in high temporal contrast ultra-intense femtosecond lasers.

Generation of cavity-birefringence-dependent multi-wavelength bright-dark pulse pair in a figure-eight thulium-doped fiber laser

We experimentally demonstrated a stable multi-wavelength bright-dark pulse pair in a mode-locked thulium-doped fiber laser(TDFL).The nonlinear polarization rotation(NPR)and nonlinear optical loop mirror(NOLM)were employed in a figure-eight cavity to allow for multi-wavelength mode-locking operation.By incorporating different lengths of high birefringence polarization-maintaining fiber(PMF),the fiber laser could operate stably in a multi-wavelength emission state.Compared with the absence of the PMF,the birefringence effect caused by PMF resulted in rich multi-wavelength optical spectra and better intensity symmetry and stability of the bright-dark pulse pair.

A theoretical and experimental study on all-normal-dispersion Yb-doped mode-locked fiber lasers

We report on a theoretical and experimental study of an all-normal-dispersion （ANDi） Yb-doped mode-locked fiber laser, in which nonlinear polarization rotation （NPR） is used to realize mode-locking without any dispersion compensation. Based on the coupled nonlinear Schr6dinger （CNLS） equation, a model simulating the mode-locked process of an all-normal-dispersion ring fiber laser is developed, which shows that the achievement of stable mode-locking depends on the alignment of the polarization controller （PC） along the fast-polarization axis of the fiber, the birefringence intensity, and the net cavity dispersion. According to the theoretical analysis, stable mode-locked pulses with pulse duration 300 ps and average output power 33.9 mW at repetition rate 36 MHz are obtained.

Transition state to mode locking in a passively mode-locked erbium-doped fibre ring laser

The transition state between the continuous wave region and the mode-locked region in a passively mode-locked erbium-doped fibre ring laser has been experimentally observed by utilizing the nonlinear polarization rotation technique. When the pump power reaches the mode-locked threshold, the metastable pulse train with a tunable repetition rate is obtained in the transition from the continuous wave state to the passive mode-locked state via proper adjustment of the polarization controller. A simpie model has been established to explain the experimental observation.

Generation of 12th order harmonic mode-locking in a Nd-doped single-mode all-fiber laser operating at 0.9μm

Based on the Nd-doped single-mode fiber as the gain medium,an all-fiber 12th harmonic mode-locked(HML)laser operating at the 0.9μm waveband was obtained for the first time,to the best of our knowledge.A mandrel with a diameter of 10 mm was employed to introduce bending losses to suppress mode competition at 1.06μm,which resulted in a suppression ratio of up to 54 dB.The 1st–12th order HML pulses with the tunable repetition rate of 494.62 kHz–5.94 MHz were obtained in the mode-locked laser with a center wavelength of∼904 nm.In addition,the laser has an extremely low threshold pump power of 88 mW.To the best of our knowledge,this is the first time that an HML pulse has been achieved in a 0.9μm Nd-doped single-mode all-fiber mode-locked laser with the advantages of low cost,simple structure,and compactness,which could be an ideal light source for two-photon microscopy.

Recent progress in multi-wavelength fiber lasers:principles,status,and challenges

In recent years,multi-wavelength fiber lasers play a significant role in plenty of fields,ranging from optical communications to mechanical processing and laser biomedicine,owing to their high beam quality,low cost,and excellent heat dissipation properties.Benefitting from increasing maturity of optical elements,the multi-wavelength fiber laser has made rapid developments.In this review,we summarize and analyze diverse implementation methods covering continuous wave and pulsed fiber lasers at room temperature conditions:inserting an optical filter device and intensity-dependent loss structure in the resonant cavity,and applying ultrafast nonlinear optical response of materials and a dual-cavity structure.Finally,future challenges and perspectives of the multi-wavelength fiber laser are discussed and addressed.

Study of All-Optical Sampling Using a Semiconductor Optical Amplifier

All-optical sampling is an important research content of all-optical signal processing. In recent years, the application of the semiconductor optical amplifier （SOA） in optical sampling has attracted lots of attention because of its small volume and large nonlinear coefficient. We propose an optical sampling model based on nonlinear polarization rotation effect of the SOA. The proposed scheme has the advantages of high sampling speed and small input pump power, and a transfer curve with good linearity was obtained through simulation. To evaluate the performance of sampling, we analyze the linearity and efficiency of sampling pulse considering the impact of pulse width and analog signal frequency. We achieve the sampling of analog signal to high frequency pulse and exchange the positions of probe light and pump light to study another sampling.