Nonlinear Polarization Rotation Technology Research Articles

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.

Wavelength-Tunable Pulsed Cylindrical Vector Beams in a 1.7-μm Mode-Locking Thulium-Doped All-Fiber Laser

Because of the special absorption peak, pulsed lasers at 1.7 μm have been rapidly developed in medical treatment, biological imaging and so on. Introducing the cylindrical vector beam (CVB) may further promote these special applications due to its unique intensity, phase and polarization characteristics. Herein, we have experimentally demonstrated the generation of wavelength-tunable pulsed CVBs at 1.7 μm based on a thulium-doped all-fiber laser. A bandpass filter with a wide bandwidth combined with nonlinear polarization rotation technology is used to obtain pulsed laser emission at 1.7 μm. By taking advantage of a home-made Lyot filter and mode selective coupler (MSC), pulsed CVBs can be obtained with a wavelength tuning range of 66 nm (1720–1786 nm). The development of wavelength-tunable pulsed CVBs at the 1.7 μm waveband has significant potential applications in deep bioimaging and laser processing.

Generation of controllable bound solitons from a thulium–holmium co-doped mode-locked fiber laser

The characteristics of wavelength tuning and pulse separation changing of soliton pairs at 2 μm are still largely unexploited although they are helpful for the application of bound states in the field of high-capacity soliton fiber communication systems. We report on the experimental observations of stable bound solitons in a thulium–holmium co-doped passively mode-locked fiber laser by using the nonlinear polarization rotation technology. At a fixed pump power, the stable single soliton operation is converted to the soliton pair operation by cautiously adjusting the polarization controllers. Tightly bound soliton pairs with continuously tunable wavelengths from 1960.48 nm to 1989.45 nm, with a soliton separation of ∼4 ps, are observed. Meanwhile, several different pulse separations from 5.2 ps to 11.2 ps of soliton pairs are also realized. With the increasing pump power, a triple-soliton bound state is also observed. The results help in understanding the nonlinear dynamics of fiber lasers.

Visible‐Wavelength Spatiotemporal Mode‐Locked Fiber Laser Delivering 9 ps, 4 nJ Pulses at 635 nm

AbstractSpatiotemporal mode‐locking (STML) in fiber lasers are of interest in applications such as optical communications, nonlinear imaging, and precision machining. To date, STML fiber lasers in the near‐infrared region have been well demonstrated, yet operation at visible wavelengths is still challenging. Here, a STML picosecond fiber laser at 635 nm with the implementation of Pr/Yb co‐doped few‐mode fiber and nonlinear polarization rotation technology is reported. By solving the modified generalized multimode nonlinear Schrödinger equation, the 635 nm STML formation is theoretically predicted and analyzed. The stable 635 nm STML with a 9 ps pulse duration, which is two orders of magnitude narrower than previously reported, is realized experimentally. Moreover, spatiotemporal profiles are illustrated by investigating the locking of transverse and longitudinal modes simultaneously. By further establishing visible ultrafast fiber amplifier, the 635 nm average power is boosted up to 440 mW, corresponding to a maximum pulse energy and peak power of 4 nJ and 280 W, respectively. The experimental results are in good agreement with the numerical simulations. This work helps to understand nonlinear dynamics in STML fiber laser and directly generate large‐energy ultrashort pulses in visible region.

Raman Gain Square‐Wave Noise‐Like Pulse Laser Pumped by a Random Fiber Laser

AbstractMode locking of a long‐cavity Raman fiber laser pumped by a low‐noise random fiber laser (RFL) is demonstrated experimentally, where tunable square‐wave noise‐like pulse (NLP) is generated through nonlinear polarization rotation technology. Pulse duration can be tuned from 26 to 134 ns by controlling the polarization state or pump power at a repetition rate of 93 kHz. A signal‐to‐noise ratio of 64 dB is obtained due to the low‐noise characteristic of RFL. In addition, up to 24th high‐order harmonic mode‐locked square‐wave NLP is observed, which has not been reported in mode‐locked Raman fiber lasers with large normal dispersion. This approach not only provides a good temporal stable pump source for mode locking of Raman fiber lasers, but also enriches the methods of laser pulse generation and regulation.

Generation of soliton rain in a passive mode-locked Tm-doped fiber laser at 2 µm

In this paper, the experimental investigation and theoretical analysis on the soliton rain generated in the Tm-doped fiber laser have been carried out. On the basis of nonlinear polarization rotation technology, the fundamental and harmonic soliton rain pulses were obtained, the influence of net negative dispersion cavity length on the output performances of soliton rain pulse were studied, and the spectral sideband characteristics of soliton rain were numerically analyzed. By comparing the spectra of continuous wave mode-locking and soliton rain states, it can be found that the quasi-periodic modulation exists for the spectral sideband of soliton rain resulted from the complex interaction between the dispersive wave and the soliton wave. The interaction is one of the major factors for the formation of soliton rain. The research results are helpful for the better understanding of the soliton rain dynamics and mechanism in passive mode-locked fiber lasers.

Graphdiyne for Ultrashort Pulse Generation in an Erbium-Doped Hybrid Mode-Locked Fiber Laser

An erbium-doped hybrid passively mode-locked fiber laser based on few-layer graphdiyne (GDY) saturable absorber (SA) has been investigated for the first time. Hybrid mode-locked fiber laser is composed of nonlinear polarization rotation (NPR) technology and GDY-SA. The central wavelength, pulse width and repetition rate of the output pulse are 1530.7 nm, 690.2 fs and 14.7 MHz, respectively. Compared with the passively mode-locked pulse laser with GDY-SA or NPR technology alone, the output pulse width of hybrid passively mode-locked fiber laser is reduced more than 50 fs. It is demonstrated that the performance of GDY can be potentially applied in ultrafast laser.

Improvements of Soliton Pulse Energy and Peak Power in Thulium-Doped Fiber Laser

We report on improvements of mode-locked soliton pulse energy and peak power in a thulium-doped fiber laser. At the pump power of 370 mW, conventional soliton can be first generated using nonlinear polarization rotation technology. By increasing the pump power to 550 mW and appropriately adjusting cavity polarization state, the conventional soliton pulse can be converted into the Gaussian pulse. Due to dispersion management, excess accumulation of nonlinear phase shifts in the cavity can be avoided. In this case, the pulse energy and peak power can be increased from 77 pJ to 1.38 nJ and from 106.9 W to 2.67 kW, respectively. These improvements indicate that the Gaussian pulse operation can effectively enhance the pulse energy and peak power of the thulium-doped fiber laser.

Formation of Various Soliton Molecules in a 2-<inline-formula> <tex-math notation="LaTeX">$\mu$ </tex-math> </inline-formula>m Anomalous-Dispersion Mode-Locked Fiber Laser

We report on the experimental observations of stable single soliton and soliton molecules in an anomalous-dispersion thulium-doped passively mode-locked fiber laser by using the nonlinear polarization rotation technology. The stable single soliton operation can convert to the soliton pair operation by cautiously adjusting the PCs, at the pump power of 350 mW. It is experimentally demonstrated that the formation of stationary soliton pairs is related to the interaction between solitons and dispersive waves. By appropriately adjusting the cavity parameters, more complex soliton molecules composed of three, four, and five bound solitons can be observed. Meanwhile, we experimentally analyze complex nonlinear dynamics of soliton molecules around 2- ${\mu }\text{m}$ .

L-Band Efficient Dissipative Soliton Erbium-Doped Fiber Laser With a Pulse Energy of 6.15 nJ and 3 dB Bandwidth of 47.8 nm

Here, an efficient dissipative soliton mode-locked fiber laser in the L-band using nonlinear polarization rotation technology is demonstrated. With a maximum pump power of 495.3 mW and polarization state optimization, the fiber laser delivers a maximum output power of 68.6 mW with an 11.16 MHz repetition rate and corresponding single pulse energy of 6.15 nJ. The dissipative soliton spectrum is centered at 1599.6 nm with a 3 dB bandwidth of 47.8 nm. The full width at half maximum of the compressed pulse is 156 fs. To the best of our knowledge, this is the highest single pulse energy reported so far for an L-band passively mode-locked all-fiber laser.

Broadband High-Energy All-Fiber Laser at 1.6 $\mu$ m

A broadband high-energy all-fiber laser at 1.6- $\mu \text{m}$ window with erbium-doped fiber mode locked by nonlinear polarization rotation technology is demonstrated. The direct output optical spectrum centered at 1.6 $\mu \text{m}$ has a bandwidth of 52.4 nm. The fiber laser delivers a pulse energy of 3.9 nJ with 9.3-MHz repetition rate at 220-mW 976-nm pump power. Based on it, a wavelength-tunable optical source with a wavelength tuning range from 1.6 to 1.8 $\mu \text{m}$ is demonstrated through soliton self-frequency shift. To satisfy much longer wavelength demands in some special application scenario, e.g., deep brain imaging with multi-photon microscopy, a chirped pulse amplifier was constructed to boost the pulse energy to 14 nJ, and the wavelength can be tuned from 1.6 to 1.94 $\mu \text{m}$ with those amplified pulses. These fiber sources at the $L$ -band and a longer wavelength can explore a wider scope in deep biotissue imaging area for lower water absorption.

Yb-doped mode-locked fiber laser based on pre-chirp managed nonlinear amplification

This paper reports a 100 fs Yb-doped mode-locked fiber laser based on pre-chirp managed nonlinear amplification. The system was first constructed with the mode-locked seed laser by using the nonlinear polarization rotation technology, which could deliver near-linear chirped stable pulse sequence at a repetition rate of 82.3 MHz. After a pre-amplifier and the pre-chirp managed nonlinear amplifier, the average power of the laser output was boosted over 12 W while the pulse width was reduced to less than 100 fs, effectively improved as a comparison to the 180 fs FWHM of the compressed seed pulse.

Generation of stable nanosecond dual-pulse in an erbium-doped fiber laser with normal dispersion

Because of the special needs for nanosecond double pulse applications in micromachining and material processing, we demonstrated a stable nanosecond dual-pulse erbium-doped fiber laser based on nonlinear polarization rotation technology. The fiber laser operates in a normal dispersion regime with a length of 256m OFS-980 fiber. With the pump power of 1120mW, the laser generates a highly stable dual-pulse train with the pulse widths of 6.2ns and 1.2ns respectively. By simply changing the pump power, the pulse width of one of dual-pulse can be tuned from 2.1 to 6.2ns while the other pulse keeping almost the same duration of 1.2ns. The maximum dual-pulse energy is about 72.5nJ with a repetition rate of 783.3kHz. The formation of such nanosecond dual-pulse is attributed to the pulse splitting and depends on the parameters of the laser cavity, such as the cavity-length and the state of polarization controller. The experiment of a single square pulse splitting into different shapes of nanosecond dual-pulse was demonstrated.

Generation of Sub-60 fs Similaritons at 1.6 μm From an All-Fiber Er-Doped Laser

We propose and demonstrate the generation of 1.6 μm similaritons in all-fiber lasers with two segments of different Er-doped fibers (EDFs), which can optimize the gain profile and the dispersion in the cavity. The fiber laser is mode-locked by nonlinear polarization rotation technology. The optimized EDF lengths force the laser operated at 1.6 μm, and the managed cavity dispersion is responsible for the similariton pulse generation. The laser delivers the chirped pulses with the duration of 2.4 ps. The pulses can be dechirped to 57.6 fs by using standard single-mode fibers outside the cavity. The output power is 47 mW and the repetition rate is 36 MHz, corresponding to the pulse energy of 1.3 nJ. The parabolic shape of the spectrum, the feature of the spectral width as a function of the pump power, and the pulse phase provide the evidence of the similariton generation. Furthermore, a simplified numerical model has been used to illustrate the main feature of our laser that can support amplifier similaritons. To the best of our knowledge, it is the first report that an ultrafast Er-doped all-fiber similariton laser operates at 1.6 μm with the pulse duration less than 60 fs.

Experimental research and theoretical study based on gain-guided soliton pulse fiber laser

We use nonlinear polarization rotation technology to realize the self-start passive mode-locking in the erbium-doped fiber ring laser of positive dispersion, which can generate a new kind of gain-guided soliton pulse. The laser generates picosecond pulses with a rectangular-shaped spectrum, the FWHM width of which can be greater than 19 nm. The gain-guided soliton formation mechanism may include laser gain, spectral filtering effect, cavity dispersion and other fiber nonlinear effects. In this paper we also study the chirp property, peak power and other parameters of the pulse. With further increasing the pump power，the laser can operate on dual-pulse output mode.