Nonlinear Polarization Rotation Research Articles

Comparison of mode-locking regimes in a holmium fibre laser

A comparison has been performed for two mode-locking regimes in an all-fibre holmium laser: locking due to nonlinear polarisation rotation (NPR) and hybrid mode-locking (NPR with application of single-walled carbon nanotubes). The characteristics of pulsed lasing in both regimes are investigated. Pulsed lasing is obtained at wavelengths of 2072 and 2082 nm, with FWHM values of 3.7 and 3.3 nm, respectively. The pulse width in both cases does not exceed 1.5 ps. The pulse energies in the NPR mode-locking and hybrid mode-locking regimes are 0.4 and 0.3 nJ, respectively. It is shown that the signal-to-noise ratio in the case of hybrid mode-locking is 3 dB better than that implemented under conditions of NPR mode-locking. A possibility of self-starting a laser system by applying hybrid mode-locking is indicated.

Passive harmonic mode-locking in an erbium-doped fibre laser

We report an all-fibre erbium laser operating in the regime of passive harmonic mode-locking based on the effect of nonlinear polarisation rotation. The generation of the 67th harmonic with a pulse repetition rate of 5.62 GHz and a duration of 1.8 ps is demonstrated. It is shown that as the harmonic number increases, both the frequency modulation parameters and the pulse duration change.

Artificial neuron based on nonlinear polarization rotation in a semiconductor optical amplifier

Abstract We experimentally demonstrated an optoelectronic artificial neuron model, which can perform sigmoid transfer function. The proposed optoelectronic circuit can mimic the behavior of continuous sigmoidal neuron based on the nonlinear polarization rotation (NPR) in a single semiconductor optical amplifier (SOA). Because the current-induced NPR is anti-parallel to the optical power-induced NPR, excitatory and inhibitory stimuli to the proposed artificial neuron can be implemented by optical injection and electrical modulation, respectively. The response of the proposed neuron to excitatory inputs and inhibitory inputs are related to the initial state of polarization (SOP), bias current and power of probe beam. By adjusting the initial SOP, the neuron can be more likely to be excited or inhibited.

Multiwavelength Er-doped fiber laser using an all-fiber Lyot filter.

We experimentally demonstrated a multiwavelength Er-doped fiber ring laser system by employing an all-fiber Lyot filter (AFLF) and a highly nonlinear fiber (HNLF). The AFLF was employed as a polarizing filter to generate a nonlinear polarization rotation effect and the highly dense and narrow bandwidth comb-like channels. A 1-km-long HNLF was used to enhance the nonlinearity of the laser cavity and suppress the mode competition for multiwavelength operation. In the experiment, 97 laser output channels within a 3dB bandwidth simultaneously were excited under 224mW pump power. The power fluctuation of lasing channels was less than 0.182dB, and the wavelength shift was less than 0.04nm in 100min, after treating the AFLF in a thermostatic ice bath. Meanwhile, the output laser was highly polarized with a degree of polarization up to 99.9%.

Multiplexed ultrafast fiber laser emitting multi-state solitons.

Ultrafast fiber lasers have been serving as an ideal playground for spreading the extensive industrial applications and exploring the optics nonlinear dynamics. Here, we report a bidirectional fiber laser scheme for validating the possibility of a multiplexed laser system, which is passively mode-locked by the nonlinear polarization rotation (NPR) technique. In particular, the proposed fiber laser consists of one main cavity and two counter-propagating branches with different dispersion distributions. Thus, different formation mechanisms are introduced into the lasing oscillator. Consequently, stable conventional solitons (CSs) and dissipative solitons (DSs) are respectively formed in the clockwise (CW) and counterclockwise (CCW) directions of the same lasing oscillator. Moreover, attributing to the strong birefringence filtering effect, the wavelength selection mechanism is induced. Through the proper management of intra-cavity birefringence, wideband wavelength tuning and switchable multi-wavelength operations are experimentally observed. The central wavelength of CS can be continuously tuned from 1560 nm to 1602 nm. Additionally, the evolution process of different multi-wavelength operations is also elucidated. Benefiting from this multiplexed laser scheme, bidirectional lasing oscillation, multi-state soliton emission, wavelength tuning and multi-wavelength operations are synchronously realized in a single laser cavity. To the best of our knowledge, it is the first time for such a multiplexed fiber laser has been reported. The results provide information for multifunctional ultrafast fiber laser system, which is potentially set for telecommunications, fiber sensing and optics signal processing, etc.

Wavelength-tunable Q-switched fiber laser based on a 45° tilted fiber grating

A continuously tunable Q-switched all-fiber Er-doped laser based on a 45° tilted fiber grating and tunable bandpass filter is demonstrated. The 45° tilted fiber grating is used to achieve the nonlinear polarization rotation (NPR) along with two polarization controllers (PCs), Q-switching is realized due to the fact that the NPR effect induced intensity-dependent loss. Under the pump power of 655 mW, the Q-switched optical spectrum can be continuously tuned from 1512 nm to 1552 nm by simply rotating the tunable bandpass filter. During the wavelength tuning process, the average output power increases from 0.282 mW to 4.884 mW while the repetition rate enhances from 27.3 kHz to 119 kHz. To the best of our knowledge, this is the widest continuously tunable range of Q-switched fiber Lasers based on nonlinear polarization rotation effect and spectral bandpass filter.

Efficiently modeling the noise performance of short-pulse lasers with a computational implementation of dynamical methods

Lowering the noise level of short pulse lasers has been a long-standing effort for decades. Modeling the noise performance plays a crucial role in isolating the noise sources and reducing them. Modeling to date has either used analytical or semi-analytical implementation of dynamical methods or Monte Carlo simulations. The former approach is too simplified to accurately assess the noise performance in real laser systems, while the latter approach is too computationally slow to optimize the performance as parameters vary over a wide range. Here, we describe a computational implementation of dynamical methods that allows us to determine the noise performance of a passively modelocked laser within minutes on a desktop computer and is faster than Monte Carlo methods by a factor on the order of 1000. We apply this method to characterize a laser that is locked using a fast saturable absorber---for example, a fiber-based nonlinear polarization rotation---and a laser that is locked using a slow saturable absorber---for example, a semiconductor saturable absorbing mirror.

Wavelength-tunable, dual-wavelength Q-switched Ho3+-doped ZBLAN fiber laser at 1.2 µm

Wavelength-flexible Q-switched Ho3+-doped ZBLAN fiber laser at 1.2 µm is experimentally investigated for the first time. The gain medium is Ho3+-doped ZBLAN fiber, which is pumped by a 1137-nm Raman fiber laser. Nonlinear polarization rotation technique works as the saturable absorber in the ring cavity to provide the intensity modulation. An artificial Lyot filter is inserted into the cavity and its periodic transmission spectrum varied with the polarization allows for the tunability and multi-wavelength operation of the laser. As a result, the laser spectrum tunable from 1190 to 1196 nm and dual-wavelength Q-switched pulses are observed. At a pump power of 2.01 W, stable Q-switched pulse train with the pulse duration of 0.76 µs and pulse energy of 0.62 µJ is achieved.

Timing jitter of high-repetition-rate mode-locked fiber lasers.

We characterize the timing jitter of the pulse trains from 880MHz Yb-doped nonlinear polarization rotation mode-locked fiber lasers based on a balanced optical cross-correlation method. Jitter spectral density at different net-cavity dispersions has been characterized, and the near-zero dispersion shows the lowest rms timing jitter (10fs rms, integrated from 30kHz to 5MHz). The measurements have been compared with analytical models. The comparison shows that the RIN-coupled timing jitter by nonlinearity is the dominated origin of the measured timing jitter below ∼100 kHz, while amplified spontaneous emission noise makes a major contribution in the high frequency range above hundreds of kilohertz. To the best of our knowledge, this is the first high-precision timing jitter characterization for the ∼gigahertz level repetition rate mode-locked fiber lasers. The results will be of great importance for further improving the laser performance for many applications.

Patterned graphene on SiN waveguides for mode locking of fiber lasers

In this paper, we report our results on the use of patterned graphene on SiN waveguides inside an erbium doped fiber ring laser cavity for passive mode locking. We confirm the dominant influence of graphene over non-linear polarization rotation (NPR), arising from polarizing fiber to chip grating couplers, by observing the limits of NPR in high cavity loss fiber lasers. We fabricated waveguides, transferred and patterned graphene to three different lengths, and observed different types of pulses based on the length of graphene, pump power and polarization state of the cavity. We confirmed the success of graphene processing through Raman scattering and saturable absorption measurements, as well graphene absorption simulation. High chip-coupling loss was overcome by using multiple erbium-doped fiber amplifiers. We believe this research will stimulate further interest in combining integrated photonics and fiber laser technologies, and the miniaturization of pulsed laser cavities.

Widely tunable, high optical signal-to-noise ratio erbium-doped photonic crystal fiber laser suitable for acetylene sensing

Narrow line-width fiber lasers are useful in gas-sensing, but systems that are both tunable and stable are rare. Here, we describe a single-wavelength ring cavity erbium-doped photonic crystal fiber laser (ED-PCFL) that can align the acetylene (C2H2) gas absorption peak with high precision and a maximum deviation less than 0.012 nm. Rather than using a combination of erbium-doped fiber and a strong nonlinear fiber, ED-PCF was used as a gain medium to achieve a stable single-wavelength output based on the non-linear polarization rotation effect (NPR). This was selected because ED-PCF has a strong nonlinear effect. We added the dual polarization controllers (PCs) to weaken the homogeneous broadening based on the traditional tunable filter (TF) filtering structure. The results show that these steps improve the tunability of the laser wavelength and enhance the stability of the light source. The final laser had a side-mode suppression ratio as high as 60 dB over 45 nm with a maximum fluctuation in wavelength less than 6 pm. The peak power standard deviation was ∼0.03 dB during the 2 h experimental time. The peak power response of ED-PCFL was studied as a function of C2H2 concentrations, and the calculated R-squared value was 0.993. This means that the ED-PCFL is suitable for C2H2 sensing.

Bidirectional mode-locked thulium-doped fiber laser.

We report a bidirectional mode-locked thulium-doped fiber laser. Mode-locking is enabled by the combination of semiconductor saturable absorption and nonlinear polarization rotation. Two stable mode-locked picosecond pulse trains in opposite directions are generated with a fundamental repetition rate of ∼16.57 MHz. Output wavelengths are tunable over 35nm.

Peak-Power-Clamped Passive Q-Switching of a Thulium/Holmium Co-Doped Fiber Laser

We report on a type of peak-power-clamped passive Q-switching in a thulium/holmium co-doped fiber laser, for the first time to the best of our knowledge. Nonlinear polarization rotation was used as an artificial saturable absorber to initiate the Q-switched operation. The peak-power-clamping (PPC) effect was realized by incorporating a piece of high nonlinear fiber in the fiber resonator and launching high-enough pump power. Normal Q-switching behavior could be observed with the pump power within the range of 0.9–0.95 W, where the pulse peak power monotonically rose with the pump power increasing. However, once the pump power reached a threshold-like level, i.e., ∼1 W, the delivered Q-switched pulse peak power started to be clamped at a nearly constant value of ∼0.4 W, corresponding to an intracavity total peak power of ∼2.22 W. It indicates that, besides typical dissipative soliton resonances in mode-locked fiber lasers, PPC effect can also occur in the passively Q-switched regime with appropriate pump power and cavity nonlinearity managements.

Low repetition rate passively mode-locked thulium-doped fiber laser for LIDAR system

An all-fiber passively mode-locked thulium-doped fiber (TDF) ring laser with a repetition rate of 736 kHz was demonstrated. This is the lowest repetition rate among those of mode-locked TDF lasers to the best of our knowledge. Numerical calculations to find the best solution for successful mode-locking in a highly lossy oscillator were made based on Haus’s master equation with variables such as oscillator loss, small signal gain and modulation depth. Based on the calculation, the laser was configured, where nonlinear polarization rotation (NPR) and two stages of TDF amplifier were properly applied. The overall length of oscillator is about 290 m. The output optical spectrum has a FWHM of 28 nm at 2 μm.

Spatio-spectral dynamics of the pulsating dissipative solitons in a normal-dispersion fiber laser.

For the first time, to the best of our knowledge, we observed the pulsating dissipative solitons in a mode-locked fiber laser at normal dispersion using the dispersive-Fourier transformation technology. The artificial saturable absorbers, as well as the birefringent filter formed by the nonlinear polarization rotation, make the polarization controller an effective component to adjust the laser state from stationary to pulsating. The pulsating dissipative solitons are accompanied with the spectrum breathing and oscillating structures due to the nonlinear pulse propagation. Our results can enhance the understanding of the pulsating solitons in the dissipative systems.

Dissipative soliton with sidebands on spectrum in an all-fibre laser

We experimentally observed a dissipative soliton (DS) in an all-fiber laser with large-normal dispersion. The nonlinear polarization rotation (NPR) technique was used as the mode locker. The DS exhibited obvious sidebands on the spectrum, a feature that is distinctly different to that of the common DS previously reported. The spectral bandwidth was 57.43 nm, and the pulse duration was 2.20 ps. Remarkably, in the whole mode-locked pump regime, the DS remained in the single pulse state without multipulse shaping. Moreover, the pulse was compressed to ~140 fs by a negative dispersion device outside the cavity, and the pulse peak power reached 3.79 KW. We assumed that the spectral sidebands originated from the combined action of the nonlinear phase chirp and NPR-induced transmittance modulation, which may help to keep the mode-locked pulse working in the single pulse state. This work could help us gain a deeper insight into the DS formed in the normal dispersion regime.

All-fiber mode-locked cylindrical vector beam laser using broadband long period grating

We propose and demonstrate an all-fiber passively mode-locked laser based on the mechanism of nonlinear polarization rotation and two-mode fiber long period grating (TMF-LPG) to generate ultrafast cylindrical vector beam (CVB) output. The central wavelength of the laser is 1567.2 nm, and 3 dB bandwidth is measured to be 5.64 nm. The pulse duration of the mode-locked laser is 970 fs and the repetition rate is 20.47 MHz. An eight-period TMF-LPG written in a two-mode fiber by a CO2 laser serves as a broadband transverse mode converter to achieve ultrafast CVBs generation with broader spectrum. The 15.0 dB bandwidth (corresponding to conversion efficiency greater than 96%) of the LPG is 75 nm, which perfectly covers the wide mode-locked spectrum. High purity radially and azimuthally polarized beams are obtained through adjusting the polarization controllers, and the mode purity is calculated to be 94.35%. The slope efficiency of the CVBs’ output is about 5.84%.

Principles of operation of a passively mode-locked fiber ring laser and 3D mapping of ultra-short pulses

In this article, we study the operation and basic elements in a passively mode-locked fiber ring laser (PML-FRL), emphasizing the saturable absorber (SA) effect seen as a nonlinear filter in transmission, which is produced by the non-linear polarization rotation (NPR) and a linear polarizer. Besides, we employ a technique of 3D mapping measurements for characterizing ultra-short pulse dynamics.

Sub-megahertz mode-locked thulium-doped fiber laser

We demonstrated a passively mode-locked thulium-doped fiber (TDF) laser with a repetition rate of 736 kHz, the lowest repetition rate at 2 µm to the best of our knowledge. Numerical simulation was carried out to design the most suitable all-fiber laser configuration. The laser was built in reference to the simulation, where a nonlinear polarization rotation technique and two stages of TDF amplifier were successfully applied. The laser stably delivers fs pulses as short as 173 fs with an average output power of 1 mW. A wide spectrum of 28 nm in FWHM was observed due to large total nonlinearity per roundtrip.

Real-time observation of dissipative soliton formation in nonlinear polarization rotation mode-locked fibre lasers

Formation of coherent structures and patterns from unstable uniform state or noise is a fundamental physical phenomenon that occurs in various areas of science ranging from biology to astrophysics. Understanding of the underlying mechanisms of such processes can both improve our general interdisciplinary knowledge about complex nonlinear systems and lead to new practical engineering techniques. Modern optics with its high precision measurements offers excellent test-beds for studying complex nonlinear dynamics, though capturing transient rapid formation of optical solitons is technically challenging. Here we unveil the build-up of dissipative soliton in mode-locked fibre lasers using dispersive Fourier transform to measure spectral dynamics and employing autocorrelation analysis to investigate temporal evolution. Numerical simulations corroborate experimental observations, and indicate an underlying universality in the pulse formation. Statistical analysis identifies correlations and dependencies during the build-up phase. Our study may open up possibilities for real-time observation of various nonlinear structures in photonic systems.