Nonlinear Optical Loop Mirror Research Articles

Polarization dynamics of dissipative-soliton-resonance pulses in passively mode-locked fiber lasers.

We present a comprehensive numerical investigation of the polarization dynamics of dissipative-soliton-resonance (DSR) pulses in a passively mode-locked Yb-doped fiber laser with a nonlinear optical loop mirror (NOLM). In our simulations, the NOLM's saturable absorption effect is modeled by its transmission function. Depending on the strength of cavity birefringence, various types of vector DSR pulses, including the polarization-locked DSR (PL-DSR), the polarization rotation-locked DSR (PRL-DSR), and the group velocity-locked DSR (GVL-DSR) states are obtained in the laser cavity. The characteristics of these vector DSR pulses are shown. We also analyze the polarization evolution and internal polarization dynamics of PRL-DSR pulses within the cavity. Our simulation results offer insight into the vector nature of DSR pulses in polarization-insensitive mode-locked fiber lasers.

Tunable and switchable harmonic h-shaped pulse generation in a 303 km ultralong mode-locked thulium-doped fiber laser

We experimentally demonstrated a type of tunable and switchable harmonic h-shaped pulse generation in a thulium-doped fiber (TDF) laser passively mode locked by using an ultralong nonlinear optical loop mirror. The total cavity length was ∼3.03 km, the longest ever built for a TDF laser to our best knowledge, which resulted in an ultralarge anomalous dispersion over −200 ps2 around the emission wavelength. The produced h-shaped pulse can operate either in a fundamental or in a high-order harmonic mode-locking (HML) state depending on pump power and intra-cavity polarization state (PS). The pulse duration, no matter of the operation state, was tunable with pump power. However, pulse breaking and self-organizing occurred, resulting in high-order HML, when the pump power increased above a threshold. At a fixed pump power, the order of HML was switchable from one to another by manipulating the PS. Switching from the 8th up to the 48th order of HML was achieved with a fixed pump power of ∼4.15 W. Our results revealed the detailed evolution and switching characteristics of the HML and individual pulse envelope with respect to both the pump power and PS. We have also discussed in detail the mechanisms of both the h-shaped pulse generation and the switching of its HML. This contribution would be helpful for further in-depth study on the underlying dynamics of long-duration particular-envelope pulses with ultralarge anomalous dispersion and ultralong roundtrip time.

Unusual Evolutions of Dissipative-Soliton-Resonance Pulses in an All-Normal Dispersion Fiber Laser

We experimentally report the novel evolutions of dissipative-soliton-resonance (DSR) pulses with respect to the pump power in an all-normal-dispersion fiber laser based on a nonlinear optical loop mirror. By solely increasing the pump power, pulse breaking of the DSR pulses was observed apart from the typical pulse broadening at a fixed pulse peak power. Instead of arbitrarily broadening with the increase in the pump power or directly losing the mode-locking state, the process of pulse breaking was accompanied by the multipulse state and harmonic mode-locking pulses evolving from the original DSR pulse. In addition, to the best of our knowledge, a narrowing of the DSR pulse with the increase in the pump power was observed for the very first time. Further results show that these unusual evolutions of DSR pulses could be attributed to the changes in several laser parameters resulting from the increasing pump power under specific operating conditions.

Mode-locked thulium-fluoride fibre laser with an adjustable pulse width using a nonlinear optical loop mirror

A mode-locked thulium-fluoride fibre laser with a tunable pulse width using a nonlinear optical loop mirror (NOLM) for operation in the S-band region is proposed and demonstrated. Stable dissipative soliton resonance (DSR) mode-locked pulses are obtained at a threshold pump power of 186.8 mW and maintain stability until a maximum pump power of 249.7 mW. Pulse width modulation of between 70.8 ns and 120.3 ns is obtained over the operating pump power range, with a fundamental frequency at 97.4 kHz. A maximum output power of 0.99 mW and a pulse energy of 10.21 nJ are obtained. The output pulse has a signal-to-noise ratio of 69 dB at the maximum pump power, indicating a highly stable output. The variable pulse width will find significant applications as modulated light sources for various optical and optoelectronic systems.

Rectangular Pulse Generation From a Mode Locked Raman Fiber Laser

Rectangular and step-like pulses are generated in a mode-locked Raman fiber laser with a nonlinear optical loop mirror. The Raman laser has a typical figure-8 configuration. Stable rectangular pulses with a fundamental repetition rate of 733 kHz are obtained. As the pump power increases, the Raman pulse evolves to a step-like rectangular pulse. The maximum pulse energy is 64.1 nJ with a pulsewidth of 25.1 ns. After spectrally resolved measurements, the step-like pulse is proved to be induced by the dual wavelength oscillation. It is the first report of rectangular pulsed output in the mode-locked Raman fiber laser to the best of our knowledge.

Influence of spectral filtering on mode-locking operation of figure-eight Er-doped fiber laser

Over the last decades, passive mode-locked fiber laser has received considerable attention because of ultrashort pulse, compactness, and low cost. As a saturable absorber, nonlinear optical loop mirror (NOLM) has shown the advantages of high damage threshold, possibility of all-PM fiber implementation, short response time and therefore potentially low intrinsic noise. Spectral filtering plays an important role in NOLM mode locked fiber laser, but the influence of filtering parameters on mode locking operation is rarely reported. In this paper, the influence of filtering bandwidth on mode locking operation and on output pulse characteristics are experimentally investigated. A 2 × 2 optical coupler with a splitting ratio of 10 : 90 is introduced at one end of fiber loop to form a loss-imbalanced NOLM, and extracts 90% of intracavity pulse energy as outputs. With this architecture, an all polarization-maintaining figure-8 Er-doped fiber ultrafast laser is achieved. A home-made bandwidth and wavelength tunable bandpass filter is utilized in the cavity, and the filtering bandwidth is defined by 10 dB bandwidth. The clockwise and counter-clockwise mode locked output power are 8.4 mW and 8.6 mW, respectively, with a repetition rate of 2.734 MHz. With a spectral bandwidth of 2.1 nm, the intracavity pulse is shaped by spectral filtering and soliton effect. The 3 dB bandwidth of the clockwise and counter-clockwise mode locked output pulse are 10.1 nm and 1.8 nm, and the values of corresponding full width at half maximum (FWHM) of the direct outputs are 583.7 fs and 2.94 ps, respectively. As the filtering bandwidth increases, the role of filter in spectral shaping weakens, and the parameters of two output pulses become close. When spectral bandwidth is larger than 7.3 nm, the intracavity pulse is shaped by gain spectrum and soliton effect. Both of the clockwise and counter-clockwise output pulses become the transform-limited pulses with almost the same FWHMs of 440 fs. Besides, the wavelength of the figure-8 fiber laser can be adjusted in a range larger than 30 nm by modulating the wavelength of the filter. The tunable mode-locked fiber laser has great potential applications in various application fields.

Multiwavelength Fiber Laser Using Erbium-Doped Twin-Core Fiber and Nonlinear Optical Loop Mirror

A multiwavelength fiber laser based on erbium-doped twin-core fiber (ED-TCF) and nonlinear optical loop mirror (NOLM) is proposed and demonstrated in this paper. The ED-TCF is employed as comb filter and gain medium simultaneously. The NOLM serves as an intensity equalizer. A fiber laser with 8 wavelength outputs is achieved at room temperature. The optical signal to noise ratio (OSNR) is larger than 40 dB. The fluctuation of peak power and wavelength is less than 1 dB and within 0.015 nm, respectively. This fiber laser has the advantages of simple structure and stable operation.

All-Optical Multilevel Amplitude Regeneration Based on Polarization-Orthogonal Continuous-Wave-Light-Assisted Nonlinear-Optical Loop Mirror (PC-NOLM) Subsystem

We demonstrate a multilevel amplitude regeneration experiment for non-return-to-zero 4-level pulse amplitude modulated (NRZ-PAM4) signals in the proposed polarization-orthogonal continuous-wave-light-assisted nonlinear-optical loop mirror (PC-NOLM) subsystem. A uniform regenerative behavior including the same noise-suppression efficiency and the noise-handling capability is observed across multiple amplitude levels. The pre-phase modulation is carried out for the input signal enabling the multilevel amplitude regeneration directly on highly-spectral-efficient modulated formats. An overall improvement of 2.33 dB is achieved by optimizing the launched optical power and the distortion strength in the all-optical NRZ-PAM4 regeneration experiment. The uniform regenerative performance for the proposed PC-NOLM regenerator is also analyzed numerically to reveal the regenerative mechanism and the influence from the operational parameters.

High-power dissipative soliton resonance fiber laser with compact linear-cavity configuration

An ultra-compact mode-locked Yb-doped fiber laser under the operation regime of dissipative soliton resonance (DSR) has been demonstrated, which exports sub-nanoscond pulses with a maximum output average power of 2.4 W. The linear-cavity configuration of the proposed fiber laser is formed by a highly reflective fiber Bragg grating at 1066 nm and a nonlinear optical loop mirror (NOLM) consisting of a 5/95 optical coupler. As a result, DSR pulses with maximum average power of 2.4 W and repetition rate of 10.119 MHz have been obtained, when pump power is 10 W. The compact DSR Yb-doped fiber laser with high output power have great potential applications in material processing, optical communications, and so on.

Dissipative soliton resonance in a passively mode-locked praseodymium fiber laser

Dissipative soliton resonance (DSR) operation in a passively mode-locked praseodymium-doped fluoride fiber laser with a nonlinear optical loop mirror is proposed and demonstrated. A praseodymium-doped fluoride fiber is employed as the gain medium to allow lasing in the 1.3 μm region, and a stable square pulse emission at 1303 nm is observed. The width of the square pulse broadens with the increment of pump power, from 83 ns to 239 ns, whereas the peak power remains at a comparable level. The highest single pulse energy is recorded at around 2.3 nJ without the appearance of pulse breaking, validating the predicted DSR operation. To the best of author’s knowledge, this work is the first study of DSR operation in the 1.3 μm region.

Wavelength-spacing-controllable multi-wavelength fiber laser based on a Lyot-Sagnac filter.

A wavelength-spacing-controllable multi-wavelength polarization-maintaining erbium-doped fiber laser (MW-PM-EDFL) in a linear cavity configuration is presented based on a Lyot-Sagnac filter and a nonlinear optical loop mirror (NOLM). Using a Lyot-Sagnac filter as the wavelength selective filter makes the wavelength spacing controllable by adjusting the effective length of polarization-maintained fiber (PMF) segments. An NOLM inducing wavelength-dependent cavity loss can be used as an amplitude equalizer, which is employed to suppress the mode competition resulting from the homogeneous line broadening effect of the erbium-doped fiber in the cavity. Thus, a stable wavelength-spacing-controllable MW-PM-EDFL is realized. In this experiment, 20 stable lasing wavelengths are obtained with a pump power of 170mW at 980nm when the length of the PMF is 13m. When the length of the PMF is 8m, 10 stable lasing wavelengths are obtained at 185mW pump power. The power fluctuations and wavelength shifts are less than 2dB and 0.02nm in 1h at room temperature. The demonstrated multi-wavelength fiber laser has great potential for applications in optical communications and optical sensing systems.

Single-channel 10.2 Tbit/s (2.56 Tbaud) optical Nyquist pulse transmission over 300 km.

We describe a single-channel 10.2 Tbit/s online transmission using non-coherent ultrashort optical Nyquist pulses. A 10.2 Tbit/s signal was generated at a symbol rate of as fast as 2.56 Tbaud with a polarization-multiplexed DQPSK format. We developed a new ultrafast optical sampler for Nyquist OTDM demultiplexing with a nonlinear optical loop mirror, an RZ-CW conversion technique to improve the SNR, and an active stabilization technique providing stable long-term demultiplexing operation. With precise higher-order dispersion compensation up to fourth order, a 10.2 Tbit/s signal was transmitted over 300 km for the first time as a real-time demonstration with a spectral efficiency of 2.5 bit/s/Hz. We also report a 10.2 Tbit/s transmission over 225 km with a spectral efficiency of 3.7 bit/s/Hz, which we realized by reducing the roll-off factor to zero.

Dissipative Soliton Resonances in a Mode-Locked Holmium-Doped Fiber Laser

We demonstrate what is believed to be the first dissipative-soliton-resonance mode-locking of a holmium-doped fiber (HDF) laser. A telecommunication band fiber source was used to pump a piece of thulium-doped fiber, resulting in 1.9-μm band lasing, which was subsequently used to intra-cavity-pump another piece of HDF, resulting in lasing beyond 2 μm. A nonlinear optical-loop mirror incorporating two pieces of high nonlinear fiber (HNLF) was employed as an artificial saturable absorber, and, meanwhile, to enhance the cavity nonlinearity. By varying the pump power from ~2.1 to ~4 W, the produced pulse could be tuned from ~4.85 to ~20.8 ns in duration while maintaining a nearly constant peak power, i.e., peak power clamping (PPC) effect. By further incorporating different lengths of HNLF, it was found that the longer the HNLF, the larger the pulse duration and the lower the pulse peak power, due to the expected stronger PPC effect. Meanwhile, spectral blue-shifting effect has occurred due to the increased fiber attenuation loss. Based on our preliminary experiments using the HDF laser as a test bed, a higher pulse energy at a longer wavelength could be expected with further enhanced pumping and gain-tailoring designs.

Multiwavelength Thulium-Doped Fiber Laser Using a Micro Fiber-Optic Fabry–Perot Interferometer

A compact, all-fiberized multiwavelength thulium-doped fiber laser based on a micro fiber-optic Fabry–Perot interferometer (FPI) and a nonlinear optical loop mirror (NOLM) has been demonstrated. The FPI with a free spectral range of ∼26 nm was used as an effective wavelength selective filter, which was constructed by splicing a ∼71.4- μ m long air-filled hollow-core fiber with single-mode fibers at both ends. An NOLM, served as an amplitude equalizer, was used for stabilizing multiwavelength laser operation. Besides switchable single-wavelength operations (1948.00 and 1972.24 nm), stable dual-wavelength operations (1948.02/1974.25 and 1974.16/1975.30 nm) and triwavelength operation (1947.94/1973.03/1974.14 nm) have been obtained. The optical signal-to-noise ratio of each emission laser line is larger than ∼47 dB.

Numerical study of multiple noise-like pulsing in a dispersion-managed figure-eight fibre laser

We study numerically noise-like pulsing in a long passively mode-locked figure-eight fibre laser with a polarization-imbalanced nonlinear optical loop mirror (NOLM) and a strong dispersion map. Contrary to the common scenario where a single noise-like pulse develops, it is found that multiple small bunches tend to form in the cavity, sharing roughly the same characteristics, which are determined by the dispersion parameters. The wave packets are subject to ample periodic variations of their duration and peak intensity as they pass through sections of opposite dispersion signs, displaying a breathing dynamics analogous to the one undergone by solitons in stretched-pulse fibre lasers. Multiple noise-like pulsing operation can be understood by considering the strong compression and corresponding intensity increase of small bunches as they travel through the NOLM, which yields higher transmission through the NOLM and thus smaller cavity losses for these small units than for larger wave packets. The present results could serve as a basis for a deeper study of the dynamics taking place in partially mode-locked fibre sources, which is particularly complex when multiple bunches of radiation coexist and interact in the cavity.

Multi-wavelength mode-locked thulium-doped fiber laser based on a fiber-optic Fabry–Perot interferometer and a nonlinear optical loop mirror

An all-fiberized multi-wavelength mode-locked thulium-doped fiber laser (TDFL) based on fiber-optic Fabry–Perot interferometer (FPI) and nonlinear optical loop mirror (NOLM) has been presented. The fiber-optic FPI played the roles of wavelength selective filter and output mirror, which constructs by a single mode-hollow core-single mode fiber structure (SMF-HCF-SMF). A NOLM with wavelength-dependent cavity loss was employed to alleviate the mode competition in thulium-doped fiber and enabled the multi-wavelength mode-locking. With proper manipulation of the polarization controller and the pump power, besides the single-wavelength (~1948 nm) fundamental and high orders harmonic mode-locked operations, switchable dual-wavelength (1924.67/1949.16 nm and 1949.64/1972.68 nm) and stable three-wavelength (1924.80/1951.42/1972.14 nm) mode-locked operations are obtained. The multi-wavelength mode-locked TDFL exhibits a good stability with maximum peak power fluctuations of less than 0.5 dB in 2 h.

High-Energy Noiselike Pulses in an all-PM Double-Clad Er/Yb-Codoped Fiber Laser

We demonstrate an all-polarization maintaining double-clad Er/Yb-codoped fiber laser with a nonlinear optical loop mirror, which delivers noiselike pulses (NLPs). The proposed configuration is featured for two different lengths of cavities (~210 and ~510 m), corresponding to the repetition rate of 985 and 402 KHz, respectively. The maximum pulse energy has reached to $1.54~\mu \text{J}$ , which is, to the best of our knowledge, the highest value increased by an order of magnitude than the previous work for NLPs directly generated from the laser output. The central wavelengths for both configurations have been obtained at 1616 and 1612.3 nm, which could fully cover the absorption lines of the ethane (C2H6) gas and could be applied in ethane gas detection. Simulation results are in good agreement with the experiment.

All-optical multilevel amplitude regeneration in a single nonlinear optical loop mirror.

We experimentally demonstrate all-optical amplitude regeneration of 4-level pulse amplitude modulated signals (PAM4) based on a single nonlinear optical loop mirror (NOLM). Four power-plateau regions are achieved using return-to-zero (RZ) pulses of narrow pulse-width, enabling large nonlinear phase shifts within the highly nonlinear fiber (HNLF). We quantify noise suppression characteristics at each amplitude level and obtain an overall EVM improvement of 0.92dB by optimizing input power and distortion strength. A theoretical analysis has been also carried out matching the experimental results and revealing the design characteristics of the regenerator's nonlinear transfer function.

Supercontinuum generation in an Er-doped figure-eight passively mode-locked fiber laser.

A supercontinuum source based on a figure-eight Er-doped fiber ring cavity has been experimentally demonstrated with low repetition rate. The proposed configuration of the experiment is a figure-eight fiber laser grounded in Nonlinear Optical Loop Mirror (NOLM) technique. A broad spectrum of approximately 410 nm spanning the range 1315-1725 nm at the level of 30 dB can be obtained at a given average power of 2.6 mW and without any amplifier. Such wide spectrum can be directly achieved in the fiber resonator, which makes the structure compact and robust. By changing the pump power, the temporal pulse width can be adjusted accordingly. The pulse width is about 4 ns at the pump power of around 240 mW. The broadband spectrum was generated directly from the fiber resonator, which can be easily applied in numerous areas, such as the optical frequency metrology, optical spectroscopy, optical coherence tomography, optical communications, and medical sciences.

2 µm high-power dissipative soliton resonance in a compact σ-shaped Tm-doped double-clad fiber laser

We report direct generation of a high-power, large-energy dissipative soliton resonance (DSR) in a 2 µm Tm-doped double-clad fiber laser. A compact σ-shaped cavity is formed by a fiber Bragg grating and a 10/90 fiber loop mirror (FLM). The 10/90 FLM is not only used as an output mirror, but also acts as a nonlinear optical loop mirror for initiating mode locking. The mode-locked laser can deliver high-power, nanosecond DSR pulses at 2005.9 nm. We further perform a comparison study of the effect of the FLM’s loop length on the mode-locking threshold, peak power, pulse energy, and optical spectrum of the DSR pulses. We achieve a maximum average output power as high as 1.4 W, a maximum pulse energy of 353 nJ, and a maximum peak power of 84 W. This is, to the best of our knowledge, the highest power for 2 µm DSR pulses obtained in a mode-locked fiber laser.