Nonlinear Polarization Rotation Research Articles

Tm–Ho co-doped hybrid mode-locked fiber laser based on Ti3CNTx saturable absorber

We proposed a thulium-holmium (Tm–Ho) co-doped hybrid mode-locking fiber laser utilizing few-layer Ti3CNTx MXene saturable absorber. The Ti3CNTx material exhibits superior optical properties with a saturation intensity of 25 mW and a modulation depth of 18.32%. By integrating nonlinear polarization rotation (NPR) technology with Ti3CNTx-SA, we achieved a self-starting, low threshold, ultra-stable and, efficient hybrid mode-locking pulse. The pulses are generated at a central wavelength of 1959.23 nm, with a 3 dB bandwidth of 5.34 nm and a pulse width of 766 fs. Compared to the single NPR technology, the structure reduces the self-starting threshold for mode-locking, compresses the pulse width by at least 21 fs, and significantly improves the signal-to-noise ratio. The proposed laser underscores the potential application and broad prospects of Ti3CNTx-SA in the 2-μm field of ultrafast fiber lasers.

C-band wavelength tunable mode locked noise-like pulse source using an asymmetrically reflecting Fabry-Perot filter

We report a widely wavelength tunable mode locked noise-like pulse (NLP) source from an erbium doped fiber ring laser. By making use of the reflection of an asymmetric Fabry- Perot interferometer (FPI), continuous tunability of the central wavelength over the entire C-band (1530 nm to 1565 nm), with an independent control over bandwidth (3.5 nm to 17 nm) is demonstrated. Using a single control parameter, i.e. the length of the FPI, it is thus possible to select from a range of output spectral bandwidths and desired central wavelengths. Using the same control on the length of the FPI and intracavity polarization settings, generation of dual wavelength NLPs, temporally shifted noise-like pulse pairs, harmonic mode locked NLPs and loosely bound NLP doublets and NLP triplets are reported. Owing to the simplicity of the tuning mechanism and the adaptable output spectrum, we believe, such a laser will be useful for various applications like sensing, spectroscopy and imaging. The novel combination of a reflecting FPI-based filter, and the complex nonlinear polarization rotation (NPR) based mode locking mechanism, also offers the opportunity to further explore the rich physics of NLPs.

Single-shot characterization of special multi-soliton and noise-like pulse emissions in a mode-locked fiber laser

Single-shot characterization of three categories of multi-pulse emissions in a passively mode-locked fiber laser based on nonlinear polarization rotation is experimentally reported. The mechanism to control the intracavity polarization state relies on a polarizer beam splitter cube together with a half-wave and a quarter-wave retarders. With a precise adjustment of the retarders, the fiber laser can reach different mode locking regimes involving noise-like pulses (NLP) and solitons. These regimes include (i) NLP emission exhibiting broad-bandwidth supercontinuum spectrum, (ii) multiple soliton emission in the form of soliton bunches with rectangular-shaped envelope, and (iii) NLP emission with complex pulse patterns, such as the one obtained from the breakup of a rectangular NLP, or the harmonic mode locking regime exhibiting multiple, uniformly spaced NLP wave packets. The experimental results presented in this work provide useful information for a better understanding of different types of multi-pulse emission from the same fiber laser system, complementing the studies focused on the nature and physical mechanisms underlying NLP emission.

High-power and high-speed all-optical all-fiber wavelength-swept source at 2 μm based on a femtosecond laser and time stretch technique

We present an all-fiber all-optical high-speed wavelength-swept laser source at 2 μm with a 3-dB bandwidth of 33 nm and repetition rate of 18.4 MHz, by using optical time-stretch technology, which to our knowledge is the first report of a 2 μm wavelength-swept source without any bulk, free-space optical elements. The light source is built upon a nonlinear polarization rotation mode-locked femtosecond oscillator. A chirped fiber Bragg grating (CFBG) is employed to realize the all-optical wavelength sweeping. In comparison with the commonly used dispersive fibers, the CFBG possesses outstanding power efficiency and temporal stability, especially for wavelength sweeping in the 2.0 μm wavelength regime. A two-stage fiber amplifier system is utilized to boost the output power up to 1.01 W. This all-fiberized mode-locked swept source with high speed and high stability shows great potential as a valuable source for various spectroscopy applications and swept source optical coherence tomography, particularly for highly scattering medium with low water content.

Study on Spectrum Shifting and Pulse Splitting of Mode-Locked Fiber Lasers Based on NPR Technology.

We conducted a systematic investigation into the spectral and pulse characteristics of C and L-band Nonlinear Polarization Rotation (NPR) mode-locked fiber lasers effectively employing nonlinear polarization rotation technology. In our experimental setup, we achieved a stable mode-locked state at 1560.076 nm, exhibiting a 3 dB spectral bandwidth of 9.1 nm. As the pump power increased, we observed spectral shifts accompanied by shifts in the first Kelly sideband and the generation of new Kelly sidebands. In this paper, the phenomenon of spectral deviation is elucidated through the interplay of self-phase modulation, group velocity drift, and polarization-dependent isolator (PD-ISO) filter effect, with an analysis of the formation and deviation of Kelly sidebands. Notably, spectral shift persisted even when the pump power exceeded 200 mW. However, continuous pump power escalation led to soliton splitting, resulting in the formation of new soliton beams. Based on the simultaneous generation of spectral shift and pulse splitting, our study contributes to an enhanced understanding of soliton dynamics in ultrafast fiber lasers and lays a foundation for the application of high-repetition-frequency harmonic mode-locked lasers with tunable wavelengths.

Yb-doped mode-locked fiber laser with a hybrid structure of NPR and a Lyot filter as the saturable absorber.

Passively mode-locked fiber lasers based on a nonlinear polarization rotation (NPR) have attracted much attention due to their ability to generate short pulses with wide spectra and high peak power. However, environmental perturbations can easily cause the lasers to lose the mode-locked state and make it a challenge for practical application. The aim of this research is to improve the laser stability by inserting a Lyot filter into the mode-locked laser cavity. The experimental results indicate that the mode-locked state can be maintained when the radius of the fiber loop is changed from 7.5 to 1.5 cm, while the signal-to-noise ratio of the fundamental frequency remains almost the same. The tunability of the output power can be achieved by adding a half-wave plate (HWP) in the laser cavity without changing the pump power, while the mode-locked state remains stable. By adjusting the angle of the HWP2, the output power can be adjusted from 3.36 to 66.5 mW at repetition rate of 29.7 MHz.

Switchable dual-wavelength mode-locked cylindrical vector beam fiber laser based on unpumped EDF saturable absorber and nonlinear polarization rotation

A switchable single- and dual-wavelength mode-locked pulse laser with cylindrical vector beam (CVB) based on an unpumped EDF saturable absorber and a nonlinear polarization rotation (NPR) hybrid mode-locked mechanism has been proposed and experimentally demonstrated. An unpumped EDF saturable absorber is utilized to generate a mode-locked operation that generates high-order laser pulses. Utilizing a hybrid mode-locked mechanism, the signal-to-noise ratio (SNR) and operating stability of the laser are improved compared to the mode-locked operation with a single unpumped EDF saturable absorber. Through adjusting the polarization controller, single- and dual-wavelength stable mode-locked pulses can be easily realized. The single-wavelength mode-locked pulse operates with central wavelengths of 1550.16 nm and 1549.80 nm, repetition frequencies of 6.33 MHz and 6.35 MHz, corresponding to pulse intervals of 158 ns and 157 ns. Further, the long-period grating (LPFG) is used as a mode converter to achieve pulse output with a purity exceeding 95 % in the TM01 and TE01 modes at different operating wavelengths. The proposed fiber laser with a simple structure has provided a flexible pulsed CVB laser source for laser processing, time division, and mode division multiplexing applications.

Continuous tuning of pulse parameters in a soliton fiber laser by adjusting the effect of nonlinear polarization rotation: publisher's note.

This publisher's note contains a correction to Opt. Lett.49, 674 (2024)10.1364/OL.509981.

Fiber-interferometric second harmonic generator with dual-color standard quantum-limited noise performance.

We present a novel fiber-interferometric device that achieves dual functionality: simultaneous amplification of the pulsed input signal and generation of its second harmonic while effectively suppressing the intensity noise in both modes, reaching the standard quantum-limit. The underlaying mechanism is based on phase-biased nonlinear polarization rotation coupled with type-I phase-matched second harmonic generation, a concept that is both theoretically investigated and experimentally verified. In the experiment, a fiber-optic system is constructed capable of generating 42 MHz ultra-low noise sub-150 fs output pulse trains simultaneously at 1030 nm and 515 nm, with average powers of 165 mW and 50 mW, respectively. Systematic frequency-resolved intensity noise measurements confirm dual wavelength, quantum-limited noise suppression beyond 100 kHz offset-frequency, with suppression levels up to 14 dB, showing correlation with local maxima in average power in both fundamental and second harmonic mode.

Recent Advances, Applications, and Perspectives in Erbium-Doped Fiber Combs

Optical frequency combs have emerged as a new generation of metrological tools, driving advancements in various fields such as free-space two-way time–frequency transfer, low-noise microwave source generation, and gas molecule detection. Among them, fiber combs based on erbium-doped fiber mode-locked lasers have garnered significant attention due to their numerous advantages, including low noise, high system integration, and cost-effectiveness. In this review, we discuss recent developments in erbium-doped fiber combs and analyze the advantages and disadvantages of constructing fiber combs utilizing different erbium-doped mode-locked fiber lasers. First, we provide a brief introduction to the basic principles of optical frequency combs. Then, we explore erbium-doped fiber combs implemented utilizing various mode-locking techniques, such as nonlinear polarization rotation (NPR), real saturable absorber (SA), and nonlinear amplifying loop mirror (NALM). Finally, we present an outlook on the future perspectives of erbium-doped fiber combs.

Wavelength-tunable spatiotemporal mode-locking in a large-mode-area Er:ZBLAN fiber laser at 2.8 µm.

We report a tunable spatiotemporally mode-locked large-mode-area Er:ZBLAN fiber laser based on the nonlinear polarization rotation technique. A diffraction grating is introduced to select the operating wavelength. Under the spectral and spatial filtering effects provided by the grating and spatial coupling respectively, stable ps-level spatiotemporally mode-locked pulses around 2.8 µm with a repetition rate of 43.4 MHz are generated. Through a careful adjustment of the grating, a broad wavelength tuning range from 2747 to 2797 nm is realized. To the best of our knowledge, this is the first wavelength-tunable spatiotemporally mode-locked fiber laser in the mid-infrared region.

Bright and dark solitons in a photonic nonlinear quantum walk: lessons from the continuum

Abstract We propose a nonlinear quantum walk model inspired in a photonic implementation in which the polarization state of the light field plays the role of the coin-qubit. In particular, we take profit of the nonlinear polarization rotation occurring in optical media with Kerr nonlinearity, which allows to implement a nonlinear coin operator, one that depends on the state of the coin-qubit. We consider the space-time continuum limit of the evolution equation, which takes the form of a nonlinear Dirac equation. The analysis of this continuum limit allows us to gain some insight into the existence of different solitonic structures, such as bright and dark solitons. We illustrate several properties of these solitons with numerical calculations, including the effect on them of an additional phase simulating an external electric field.

Continuous tuning of pulse parameters in a soliton fiber laser by adjusting the effect ofnonlinear polarizationrotation.

We demonstrate that through inserting a short length of highly birefringent small-core photonic crystal fiber (Hi-Bi SC-PCF) into a soliton fiber laser, the nonlinear polarization rotation effect in this laser can be manipulated, leading to continuous tuning of the output pulse parameters. In experiments, we observed that by adjusting the polarization state of light launched into the Hi-Bi SC-PCF and varying the cavity attenuation, the laser spectral width can be continuously tuned from ∼7.1 to ∼1.7 nm, corresponding to a pulse-width-tuning range from ∼350 fs to ∼1.56 ps. During the parameter tuning, the output pulses strictly follow the soliton area theory, giving an almost constant time-bandwidth-product of ∼0.31. This soliton fiber laser, being capable of continuous parameter tuning, could be applied as the seed source in ultrafast laser systems and may find some applications in nonlinear-optics and soliton-dynamics experiments.

Experimental investigation on the h-shaped pulse with leading edge in a Raman fiber laser

We report the experimental investigation on the h-shaped pulse with leading edge in a Raman fiber laser. Based on the nonlinear polarization rotation (NPR) technology, we successfully obtain the h-shaped pulse with leading edge in the all-fiber Raman fiber laser. After that, we experimentally study different affecting factors of pump power and polarization state on the leading edge, even the origin of it. Through adjusting the polarization state or increasing pump power, the leading edge can be extended up to 3907.71 ns or 3554.5 ns respectively without pulse collapse, showing an amazing stability and ultra-wide tunability. Finally, the corresponding spectra show that a left side peak always exists and gets stronger along with the extension of leading edge. After employing a home-made Lyot filter, we confirm that the leading edge is derived from the left side peak. Our work will contribute to the understanding of the h-shaped pulse with leading edge in soliton dynamics and its specific application in the material processing.

Automatic mode-locked fiber laser based on adaptive genetic algorithm

A modified adaptive genetic algorithm (GA) is proposed and implemented in a mode-locked erbium-doped fiber laser (EDFL) based on nonlinear polarization rotation. The algorithm combines GAs with adaptive crossover and mutation rates. As the number of iterations increases during the solving operation, the crossover and mutation rates are dynamically adjusted with the change in the fitness function value. A stretched pulse was automatically generated when the modified adaptive GA was continuously optimized to converge the intracavity polarization state to the target state. The average number of generations required to adjust from other states to stretched pulse locking is two, which is better than that afforded by the general GA. The evolution curves of the mode-locked EDFL with different crossover and mutation rates were investigated, and the results indicate the effectiveness of the proposed modified adaptive GA. This study proposes a new scheme for rapid polarization searching in automatic mode-locked fiber lasers.

Mechanism of noise-like pulse in all-normal dispersion all-fiber laser based on nonlinear polarization rotation.

We investigate the mechanism of changing the polarization state to generate noise-like pulses (NLPs) in the all-normal dispersion (ANDi) all-fiber laser based on nonlinear polarization rotation (NPR). Numerical simulations show that the intracavity positive and negative feedback states change with the polarization state, the peak power of the pulse will be clamped when the negative feedback comes into play, thus facilitating the transition from dissipative soliton (DS) to NLP. Experimentally, the observation of wavelength switching and transition between DS and NLP by simply adjusting the polarization state matches the numerical simulation results. This study contributes to a deeper understanding of the mechanism for generating NLP by changing the intracavity polarization state in ANDi all-fiber lasers based on NPR and offers new possibilities for pulse-switchable light sources.

Repetition Frequency Control of a Mid-Infrared Ultrashort Pulse Laser

In this study, a method for controlling the repetition frequency of a mid-infrared ultrashort pulse laser with a central wavelength of 2.8 µm is developed. A ring cavity that is insensitive to the polarization state of the laser light emitted from the fiber end was constructed to stabilize the oscillation of the mid-infrared ultrashort pulse laser. More oscillation conditions for the ultrashort pulse laser based on nonlinear polarization rotation are found than the conventional method. To confirm that the pulse oscillation is mode-locked, ultrashort pulse oscillation was confirmed by an autocorrelator. The pulse repetition frequency of this robust ultrashort pulse laser was controlled. The control method was based on the phase-locked loop (PLL) control. A wedge window was inserted into the cavity and mounted on a linear stage driven by a piezoelectric transducer. By driving the piezoelectric transducer, the position of the wedge window changed, and the resulting optical path length also changed. The repetition frequency was controlled based on this principle. Optical path length control by the wedge window and temperature control provides an Allan deviation of approximately 1 mHz.

Orbital angular momentum mode femtosecond fiber laser with over 100 MHz repetition rate

Orbital angular momentum (OAM) lasers have potential applications in large capacity communication systems, laser processing, particle manipulation and quantum optics. OAM mode femtosecond fiber laser has become the research focus due to the advantages of simple structure, low cost and high peak power. At present, OAM mode femtosecond fiber lasers have made some breakthroughs in key parameters such as repetition frequency, pulse width, spectrum width, but it is difficult to achieve good overall performance. Besides, the repetition rate is tens of MHz at present. In this paper, a large-bandwidth mode coupler is made based on the mode phase matching principle. In coupler, the first order mode coupler with 3 dB polarization dependent loss is made by the technology of strong fused biconical taper, and the second order mode coupler with 0.3 dB polarization dependent loss is made by the technology of weak fused biconical taper. By combining the nonlinear polarization rotation mode-locking mechanism, OAM mode femtosecond fiber laser with over 100 MHz repetition rate is built. The achievement of the key parameters is attributed to the selection of dispersion shifted fibers that can accurately adjust intracavity dispersion. Comparing with traditional dispersion compensation fibers (DCF), the group velocity dispersion is reduced by an order of magnitude, so it can better adjust intracavity dispersion to achieve the indexes of large spectral bandwidth and narrow pulse width. In addition, the diameter of the fiber is 8 μm, which is the same as that of a single mode fiber. Comparing with DCF, the fusion loss can be ignored, so only a shorter gain Erbium-doped fiber is required, which ensures a shorter overall cavity length and achieves high repetition frequency. The experimental results show that the first order OAM mode fiber laser has 113.6 MHz repetition rate, 98 fs half-height full pulse width, and 101 nm 10 dB bandwidth. Second-order OAM mode fiber laser has 114.9 MHz repetition rate, 60 fs half-height full pulse width, and 100 nm 10 dB bandwidth. Compared with the reported schemes, our scheme has good performance in key parameters such as repetition rate, pulse width and spectral width. We believe that the OAM mode fiber laser with excellent performance is expected to be widely used in OAM communication, particle manipulation and other research fields.

Spatio-temporal study of single and multi-wavelength quasi-continuous-wave generation regimes in an all-normal dispersion ring cavity

In this paper, we report a comprehensive experimental study of quasi-continuous-wave operation in an all-normal-dispersion ring cavity laser with nonlinear polarization rotation based saturable absorber action. Using the temporal mapping technique, we analyze in real time spiking dynamics, characterized by the existence of ephemeral spikes that continually emerge and decay amidst the background radiation. In particular, the effects of the pumping level and its direction of variation on the laser emission dynamics were analyzed. The overall behavior, in particular the well-defined pattern of spike evolution between their emergence until their vanishing, is found to be qualitatively invariant over a broad range of pump power, from 70 mW to 550 mW, albeit the values of spiking parameters vary widely over this range. The spiking phenomenon appears from low pumping powers, just above the laser threshold. A phenomenon of hysteresis was also discovered, as some emission parameters, in particular the optical spectrum, depend on whether pump power is being raised or decreased. 2D autocorrelation analysis also proved insightful, showing that each spike appears at random, having no connection with other spikes. Finally, high pump power settings and birefringence adjustments enabled the discovery of more complex spiking dynamics where emission at various wavelengths occur, some of the lines presenting additional slow dynamics at the millisecond scale. These results indicate that deterministic complexity is, together with stochasticity, an essential ingredient to take into account for a better understanding of the operational modes in fiber lasers far from the pulsed regime.

Generation of pulse bursts with controllable number of pulses and of spectral broadening directly at the laser output from a km-long Yb-doped all-fiber laser

In this work, we study the pulse burst regime of a fiber laser, which allows generating packets of multiple pulses whose number can be precisely controlled by modifying the polarization state by means of two retarder plates (QWR, HWR). The proposed experimental scheme is a highly energetic Yb-doped all-fiber ring pulsed laser operating in the 1064 nm wavelength region and using a nonlinear polarization rotation (NPR)-based passive mode locking technique. Raman scattering effects can be found by increasing pump power to achieve the formation of up to four Stokes, resulting in a spectral broadening over more than 200 nm.