Nonlinear Optical Loop Mirror Research Articles

Analysing the temporal amplitude and average output power distribution in the mode-locked regions of an erbium-doped figure-eight fibre laser (theoretical and experimental results)

The noise like pulses (NLPs) are ultrashort light pulses that present very complex dynamics, they are a particular case of the multiple light pulses generated by the partial mode-locked regime. These ultrashort light pulses exhibit very complex behaviour in amplitude and width, in the temporal domain and subsequently in the frequency domain. In this paper we present a theoretical and experimental analysis of temporal amplitude distribution of NLPs within the mode-locked regions of an erbium-doped figure-eight Fibber laser. The mode-locked regions were studied at different settings of the four polarization controllers (HWR1, QWR1) in the ring section and in the nonlinear optical loop mirror (HWR2, QWR2, NOLM) of the fibre laser cavity. We found a wide variety of values of the temporal amplitude of NLPs. The experimental results of the temporal amplitude are compared with simulations generated with the theoretical model based on the theory of polarization of light and the theoretical model of the NOLM. Also, we discovered experimentally that the theoretical model performs well in predicting the complex dynamics of the temporal distribution of amplitude. Finally, we experimentally discovered that the average power output distribution of the cavity has a behaviour which is close to proportional to the temporal amplitude distribution. We analyse NLPs with a carrier wavelength of 1562 nm with typical duration of ns and a repetition rate of 0.9 MHz.

Experimental and theoretical study of a novel input polarization-independent nonlinear optical loop mirror with elliptical birefringence

Experimental and theoretical study of a novel input polarization-independent nonlinear optical loop mirror with elliptical birefringence

Tunable Multiwavelength SOA-Based Fiber Laser

Tunable multiwavelength fiber lasers based on semiconductor optical amplifiers (SOA) have received attracting interest due to their wide prospective applications in dense division multiplexing (DWDM) systems and optical sensing. Using an SOA in a nonlinear optical loop mirror (NOLM), we demonstrate up to 13 lasing peaks by controlling the pump current and the polarization controller. At maximum pump current (450 mA), the emitted multiwavelength is between 1550 nm and 1572 nm with a wavelength spacing of 1.87 nm and 3 dB output linewidth of 0.8 nm with an output power of −7 dBm and 27 dB optical signal-to-noise ratio (OSNR). The multiwavelength output power and multiwavelength peak stability are investigated, and it was found that the power fluctuation of each multiwavelength line is less than 0.2 dB. In addition, by adjusting the polarization controllers (PCs) and SOA temperature, we obtained a tunable multiwavelength emission. The proposed fiber laser offers advantages such as simple structure, low loss, and long-time stable and multiwavelength emission.

Real-time observation of stationary and pulsating noise-like vector pulses in a fiber laser.

Rapid progress in real-time measurement technology has uncovered varieties of transient pulse dynamics. Here, we report the vector nature of noise-like pulse (NLP) in a passive fiber laser based on the nonlinear optical loop mirror (NOLM) as the polarization independent saturable absorber. After achieving the basic operation regime of NLP, various types of vector pulses, namely, the polarization locked noise-like vector pulse (PLNLVP), the group velocity locked noise-like vector pulse (GVLNLVP), and the transitional state of combined characteristics of GVLNLVP and polarization rotation noise-like vector pulse (PRNLVP) are also obtained in the cavity. Besides, by utilizing the Dispersive Fourier transform (DFT) technique, the spectral evolution and the energy vibration of pulsating PLNLVP, GVLNLVP, and the transitional state of combined characteristics of GVLNLVP and PRNLVP are also analyzed in real time. Particularly, the coexisting pulsation vector state of NLP and soliton is also captured. All these findings will help to complement our understanding of noise-like vector pulses (NLVPs) in a fiber laser.

All-optical multi-level amplitude regenerator in a polarization-effect optimized nonlinear-optical loop mirror (PE-NOLM) configuration.

We propose a modified configuration of the nonlinear-optical loop mirror (NOLM) unit by introducing the polarization-effect optimization (PE) into a nonlinear Sagnac interferometer through a polarization-maintaining optical coupler, enabling significant extension of the regeneration region (RR) of the all-optical multi-level amplitude regenerator. We carry out the thoughtful investigations on this PE-NOLM subsystem, and reveal the collaboration mechanism between the Kerr nonlinearity and the PE effect in only one unit. Moreover, the proof-of-concept experiment and its theoretical discussion of multiple-level operation have been performed, observing the 188% enhancement on the RR extending and the consequent 4.5 dB signal-to-noise ratio (SNR) improvement for a 4-level pulse amplitude modulated (PAM4) signal compared to the conventional NOLM scheme.

Robust all-polarization maintaining Yb fiber oscillator with a nonlinear optical loop mirror

We demonstrate an environmentally stable femtosecond Yb-doped all-polarizing maintained (PM) fiber laser with a nonlinear optical loop mirror (NOLM). While applying an unbalance coupling ratio coupler of 70/30 in a NOLM loop, the pulse spectral bandwidth was 20 nm with an output power of 8.95 mW at the repetition rate of 10.7 MHz. The recompressed pulse duration was 220 fs by using a pulse compressor based on pair of gratings. Due to the all-PM architecture, the root means square of output power fluctuation over 3 h was 0.4%. We expect that the developed all-PM Yb fiber oscillator will provide ultrafast broadband time-stretch spectroscopy in the future.

Frequency Comb Generation Based on Brillouin Random Lasing Oscillation and Four-Wave Mixing Assisted with Nonlinear Optical Loop Mirror

A frequency comb generator (FCG) based on dual-cavity Brillouin random fiber lasing oscillation in the 1.5 μm telecon spectral window is established and experimentally demonstrated. In the half-open main cavity of the dual cavity, the stimulated Brillouin scattering in highly nonlinear fiber (HNLF) and Rayleigh scattering in single-mode fiber are employed to provide sufficient Brillouin gain and the randomly distributed feedback, respectively, for random mode resonance. The sub-cavity includes an Er-doped fiber amplifier to couple back and boost lower-order Stokes and anti-Stokes light for the cascade of stimulated Brillouin scattering to generate multiple higher-order Stokes and anti-Stokes light. Meanwhile, efficient four-wave mixing is stimulated in the HNLF-based main cavity, further enhancing the number and intensity of the resonant Stokes and anti-Stokes light. By taking advantages of the unique transmission characteristics of nonlinear optical loop mirrors, the power deviation between Stokes and anti-Stokes lines is further optimized with 17 orders of stable Stokes lines and 15 orders of stable anti-Stokes lines achieved within the 10 dB power deviation, with maximum optical signal-to-noise ratio (OSNR) of ~22 dB and ~17 dB and minimum OSNR of ~10 dB and ~7.5 dB for Stokes and anti-Stokes lines, respectively. In addition, the dynamic characteristics of the proposed FCG have been experimentally investigated. Such an FCG with fixed frequency spacing will find promising applications in fields of optical communication, microwave, optical sensing, etc.

High-speed Multi-channel all-optical de-aggregator based on polarization-assisted phase-sensitive amplification in elastic optical networks

An all-optical de-aggregator for four wavelength-multiplexed channels based on polarization-assisted (PA) phase-sensitive amplification (PSA) is proposed, aiming to improve spectral efficiency in elastic optical networks (EONs). Benefiting from a single bi-directional PSA based on the nonlinear-optical loop mirror (NOLM), signals and their conjugates can be extracted and located at two polarization states respectively. To avoid the interference of undesired harmonics sharing the same frequency with input signals, we introduce a spatial dislocation between signal and interference harmonics by sending signals to different input ports of the NOLM. For each channel, the signal and its conjugate are coherently superimposed by PA-PSA. Each of the four 25/50 Gbaud quadrature phase shift keying (QPSK) signals is de-aggregated into two binary phase shift keying (BPSK) signals. We use the same scheme to de-aggregate 25/50 Gbaud 16-ary quadrature amplitude modulation (16-QAM) into 4-level pulse-amplitude-modulation (PAM-4) signals for four channels to verify the tunability of the scheme. The bit-error-rate (BER) and error vector magnitude (EVM) performance of the scheme are investigated and the corresponding OSNRs for error-free signal recovery are obtained respectively.

Single and Multiple Pulse Pattern Formation in All-Normal All-PM Yb-Doped Fiber Laser Using a Nonlinear Optical Loop Mirror

We report the evolution of single-pulse and multi-pulse in a mode-locked all polarization-maintaining fiber laser based on a nonlinear optical loop mirror (NOLM). The main parameters of the laser were determined by numerically simulating the relationship between the number of pulses and the basic parameters of each element. Experiments demonstrate generation of 1 to 6 pulses per cavity round trip. There is an unstable regime between the stable N and N + 1 pulses states when gradually increasing the pump power, while the intervals of the unstable regime shrink or even disappear when the direction of pump power variation is reversed. Our experimental results are different from those previously reported where the intervals of the unstable pulse regime are almost unchanged regardless of the pump power adjustment direction. We also describe how to obtain fundamental mode-locking from the multi-pulse state. The laser operated in the single-pulse regime shows excellent stability.

Dynamic evolution of multiple solitons and noise-like pulse in a fiber laser based on nonlinear optical loop mirror

We demonstrate the dynamic evolution of multiple solitons (MSs) and noise-like pulse (NLP) in a passively mode-locked fiber laser using the nonlinear optical loop mirror (NOLM). The conversion from MSs to the NLP-soliton hybrid regime, as well as the conversion from the NLP-soliton hybrid regime to NLP, has been investigated with increasing the pump power. When the state converts from the MS regime to the hybrid regime, the NLP first appears as an intermittent feature of the laser emission, whose lifetime progressively increases, until it becomes permanent. While the conversion from the NLP-soliton hybrid regime to NLP is gradual. Moreover, the hybrid mode of dual-wavelength MSs and NLP is also obtained by adjusting the intra-cavity polarization and the pump power. These findings will constitute an impressive illustration of the connections and interactions between solitons and NLP from a passively mode-locked fiber laser.

Dark Rectangular Pulses From a Dumbbell-Shaped Mode-Locked Double-Clad Er:Yb Laser

The formation of dark rectangular pulses from a dumbbell-shaped mode-locked double-clad erbium-ytterbium fiber laser is experimentally demonstrated. The laser operates under a large net-anomalous cavity dispersion regime, with a passive mode locking mechanism that relies on a nonlinear amplifying loop mirror (NALM) along with a nonlinear optical loop mirror (NOLM). This design facilitates the control of nonlinear effects within the cavity and enables the generation of dark pulses as the dominant mode locking regime. The variations of dark-rectangular pulse emission and its spectral evolution are experimentally investigated as a function of the pump power, revealing a dependence analogous to bright dissipative soliton resonance (DSR) pulses, where the dark pulse duration increases with increasing pump power, a behavior that is opposite to previously reported dark rectangular pulses. At the maximum pump power of 9.5 W, the mode-locked fiber laser generates dark rectangular-shaped pulses with duration of 571 ns, average output power of 600 mW, fundamental repetition rate of 320 kHz.

Tunable spatiotemporal mode-locked conventional soliton and soliton molecule in a partial multimode linear-cavity Er-doped fiber laser

In this letter, we have demonstrated the realization of wavelength-tunable spatiotemporal mode locking (STML) in a partial multimode fiber (MMF) laser with a linear cavity. The saturable absorption effect is attributed to both the nonlinear multimode interference (NL-MMI) and the nonlinear optical loop mirror (NOLM). The laser can deliver both the conventional solitons (CSs) and loosely bound solitons (BSs) with a tuning range from ∼1573 nm to ∼1601 nm. The evolutions of these two different STML solitons as the pump power are also given and it is found that the interaction between solitons would have notable affection on the beam distribution of the MM solitons. Furthermore, a spatial instability for the STML CSs is observed within a certain pump power range. These results contribute to our understanding of the nonlinear spatiotemporal dynamics in the MMF laser system.

Soliton macromolecule dynamics in fiber lasers with sinusoidal filtered nonlinear optical loop mirror

We numerically investigated the pulse dynamics of soliton macromolecules (SMs) in erbium-doped fiber lasers mode-locked using a nonlinear optical loop mirror (NOLM) with a sinusoidal spectral filter (SSF). SM regimes with various pulse intervals were proposed by modulating the saturation energy and filter period, distributed discretely owing to the restriction of the cavity length and pulse-to-pulse interaction. Several SM properties were discovered in stable SM states with filter periods of 1.3 and 1.95 nm, including the $\ensuremath{\pi}\text{\ensuremath{-}}\mathrm{phase}$ difference between adjacent pulses, Kelly sidebands in optical spectra, and the alternating change of phases against round trips. The laser worked in transition regimes, such as pulse splitting and pulsating to a stable SM, by increasing the saturation energy with certain filter periods. In pulsating SM regimes, soliton collisions and periodic variations of pulse intervals and peak powers occurred, accompanied by energy transfer between different spectrum components or spectrum shifts. This study reveals that the SSF-based NOLM enables SM generation and provides a method for controlling the SM dynamics in fiber lasers.

All-fiber polarization-maintaining system for noise suppression and signal amplification of picosecond pulses

We present an all–fiber polarization-maintaining system, which consists of an ultralow-intensity-noise picosecond Yb mode-locked fiber oscillator, a Yb-doped fiber amplifier and a noise suppressor based on nonlinear optical loop mirror. After amplification and noise suppression, the optical power is as high as 190 mW, and the relative intensity noise (RIN) measured at an optical power of 30 mW is as low as −161.4 dB Hz−1, which is only 0.3 dB higher than the RIN of the oscillator. We also investigated the temperature dependence of the noise suppression performance, which suggests that guided acoustic-wave Brillouin scattering is one of the factors that limit the performance of the noise suppressor.

Vector Staircase Noise-Like Pulses in an Er/Yb-Codoped Fiber Laser

We report on the experimental observation of vector staircase noise-like pulses (NLPs) produced by an Er/Yb-codoped fiber laser with a nonlinear optical loop mirror (NOLM) as the saturable absorber (SA). The staircase NLPs consist of two rectangular pulses with different energies and durations, such that the two pulse widths both increase linearly with increasing pump power with an almost constant slope while the pulse amplitude remains constant, similar to the case with Dissipative Soliton Resonance pulses. The polarization rotation state of the staircase NLPs has also been investigated along with multiple polarization dynamic patterns, including group velocity locked and polarization locked states. We have also observed harmonic mode-locked (HML) staircase NLPs by carefully adjusting the orientation of the intra-cavity polarization at fixed pump power. It was found that the HML staircase NLPs were identical to the fundamental staircase NLPs as regards the vector character and the changes in pulse duration. The experimental results are expected to enrich understanding of the vector nature of staircase NLPs and may lead to a new laser source for material processing.

Phase-preserving principle of all-optical regenerators with applications to MZI-nested NOLM structure.

The principle of phase-preserving regeneration is revealed by a simple theoretical model, that is, in the regenerated signals the linear phase shift component is dominant over the nonlinear counterpart for phase-preserving amplitude regeneration (PPAR). A Mach-Zehnder- interferometer (MZI)-nested nonlinear optical loop mirror (NOLM) PPAR scheme is proposed and verified by theory and experiment. Our experiment shows that for QPSK regeneration the noise reduction ratio in terms of error vector magnitude (EVM) is linearly dependent on the input signal-to-noise ratio (SNR) with the slope of 0.78 and the average phase disturbation is 4.37 degree, close to the theoretical value of 3.8 degrees. The influence of the optical couplers on the PPAR performance is in detail discussed.

Cavity dynamics of a hybrid mode‐locked figure of eight fiber laser

AbstractMode‐locking is an increased focus area due to the augmented requirement of ultra‐short and high‐intensity optical pulses for varied applications. Tunable mode‐locked lasers capable of operating in the infrared range at high repetition frequency brought innovative opportunities for insensitive and accurate laser surgery. As one important application of optical pulse propagation in an optical fiber, we have discussed a dual‐ring hybrid mode‐locked fiber laser using modulator, and saturable absorber. The active mechanism, saturable absorber, and the nonlinear optical loop mirror result in an output with a peak power of 16 dBm, and 80 fs full‐width at half‐maximum with a high duty cycle. It occupies a small space and is easy to achieve miniaturization and intensification; the laser output can be adjusted by varying multiple parameters such as pump, input, doped fiber length, modulator (including modulation signal), saturable absorber, and other devices.

Experimental demonstration of all-optical aggregation and de-aggregation for a QPSK signal in an elastic optical network.

The aggregation and de-aggregation between one QPSK and two BPSK signals are experimentally demonstrated based on cross-phase modulation (XPM) effects and phase-sensitive amplification (PSA), aiming to improve flexible spectral efficiency in the elastic optical network. Benefiting from the nonlinear-optical loop mirror (NOLM), the two BPSK signals can be extracted without any information redundancy for the de-aggregation scheme. The wavelength of the extracted signals stay the same as the input signal in the NOLM. Moreover, the aggregation from two BPSK signals to one QPSK signal is also successfully achieved. The feasibility of the two schemes can be confirmed by the transfer functions and the input-output constellation. The bit-error-rate (BER) and error vector magnitude(EVM) performance of the two schemes are also investigated and the corresponding OSNRs for error-free signal recovery are obtained respectively. The proposed schemes contribute to the realization of flexible optical networks and can be applied in the future gateway node between optical sub-networks.

Bound-State Soliton and Noise-like Pulse Generation in a Thulium-Doped Fiber Laser Based on a Nonlinear Optical Loop Mirror

We demonstrate a thulium-doped, mode-locked, all-fiber laser capable of operating in two generation regimes: dispersion-managed soliton and noise-like pulse (NLP). Employing a nonlinear optical loop mirror as an artificial saturable absorber, the oscillator generated optical pulses with a fundamental pulse repetition frequency of ~15.795 MHz. The total net dispersion of the laser cavity had a slightly anomalous group delay dispersion value of −0.016 ps2. After appropriate adjustment of a polarization controller, bound states of a dispersion-managed soliton composed of three pulses with fixed soliton separations were also observed. NLP generation, tunable over 35 nm from 1943.5 to 1978 nm, was also presented in the same laser setup. To our knowledge, this is the first report of the generation of tunable NLPs in a mode-locked thulium-doped fiber laser based on a nonlinear loop mirror saturable absorber.

All-Polarization Maintaining Noise-Like Pulse From Mode-Locked Thulium-Doped Fiber Laser Based on Nonlinear Loop Mirror

In this paper, we present a figure-eight all-polarization maintaining thulium-doped mode-locked fiber laser based on a nonlinear optical loop mirror operating at noise-like pulse regimes. The total cavity length is 34.5 m, and the calculated net group delay dispersion is estimated to be ∼−2.616 ps <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . At the maximum pump power, stable noise-like pulse centered at 1943 nm with a spectral full width at half maximum of 32 nm, a pulse energy of 13.51 nJ, and a coherence spike width of 236 fs is generated at a repetition rate of 5.96 MHz with the signal-to-noise ratio ∼64 dB. The polarization extinction ratio is > 15 dB at any mode-locking operation. The slope efficiency and the output power fluctuation of the laser are 5.16% and 0.039%, respectively. The laser does not require polarization state controllers, and it holds very high stability, which is insensitive to temperature changes or mechanical vibrations. Through further amplification, it will be a good laser source for real-time interrogation of fiber Bragg gratings and supercontinuum generation.