Nonlinear Optical Loop Mirror Research Articles

Generation of a switchable and tunable rectangular pulse and multi-pulse with different peak powers at the L-band

A novel, to our knowledge, L-band erbium-doped fiber laser, utilizing a nonlinear optical loop mirror (NOLM) as a mode-locker, is presented in this study. Through precise adjustments of the polarization controllers (PCs), the laser achieves the generation of rectangular pulses with distinct single wavelengths, λ1=1593nm and λ2=1571nm, as well as dual-wavelength operation. The laser’s operational mode can extend further to include harmonic mode-locking (HML). Furthermore, the investigation reveals the emergence of trapezoidal pulses and low-peak-power rectangular pulses within proximity of the conventional rectangular pulses. Notably, the evolutions of these low-peak-power pulses with the pump power also adhere to the peak power clamping (PPC) effect. Remarkably, the relative positioning of these pulses remains consistent across varying pump power levels or harmonic orders. Intriguingly, the evolution of the trapezoidal pulse with respect to pump power stands in stark contrast to that of the h-shaped pulse.

Generation and evolution of stable noise-like pulse with varying dispersion in figure-of-eight erbium-doped fiber laser

A stable noise-like pulse (NLP) in a figure-of-eight nonlinear optical loop mirror (NOLM) erbium-doped fiber (EDF) laser is proposed and demonstrated. The work experimentally investigated the effect of dispersion in the generation and evolution of the NLP by varying the length of single mode fiber (SMF) in the NOLM bidirectional loop (bi-loop). The lengths are 5 m, 10 m, 15.2 m, 20.2 m, 25.9 m, 50.1 m, 76.9 m, and 103.7 m with their net cavity dispersion of −0.275 ps2, −0.386 ps2, −0.500 ps2, −0.611 ps2, −0.737 ps2, −1.269 ps2, −1.859 ps2, and −2.447 ps2, respectively. The stability of the NLP was first evaluated by analyzing a temporal trace of the spike and its pedestal from an autocorrelator, followed by a long-term stability test of 350 min. The work demonstrated a stable NLP was obtained at the SMF length of 25.9 m at a threshold pump of 260.6 mW. At the maximum pump power of 344.7 mW, the pulse energy of 7 nJ was obtained with the highest output power of 29.77 mW. The respective spike and pedestal widths were demonstrated to be at 236.87 fs and 65.74 ps, respectively. The net cavity dispersion was −0.737 ps2 with 49.82 m total cavity length, and the corresponding repetition rate was 4.25 MHz. This work presents the importance of dispersion management of the NOLM to generate a stable NLP at low threshold power.

All-Optical format conversion from PAM4 to QPSK based on non-degenerate Phase-Sensitive amplification and pump assisted nonlinear optical loop mirror

An all-optical format conversion (AOFC) scheme from 4-ary pulse amplitude modulation (PAM4) to quadrature phase shift keying (QPSK) based on the non-degenerate phase-sensitive amplification (PSA) and the pump assisted nonlinear optical loop mirror (PA-NOLM) is proposed and numerically simulated for the first time. The input unipolar PAM4 signal with equal amplitude distance (UPAM4-A) is coupled with two input pump lights and injected into the symmetric interferometric fiber optic parameter amplifier (FOPA) to achieve the separation of the UPAM4-A and the new pump light generated at the signal frequency based on the non-degenerate PSA. When the filtered out UPAM4-A is coherent added with the new generated pump, the UPAM4-A signal to the biplolar PAM4 (BPAM4) signal. When the converted BPAM4 is sent into the PA-NOLM after adjusting its power, the BPAM4 signal could be converted into the QPSK signal. The error vector magnitude (EVM) and bit error ratio (BER) performance of the system are measured before and after AOFC with different input and receiver optical signal to noise ratio (OSNR). For the input UPAM4-A signal with an input OSNR of 27 dB, there is about 8.2 dB BER performance improvement in receiver OSNR for the final converted QPSK signal. Additionally, the proposed non-degenerate PSA configuration can be extended to realize the AOFC from one QPSK to two on–off keying (OOK) signals. The scheme can be applied in the optical gateway connecting the long-haul and the short-reach optical networks with suitable modulation formats.

Hybrid mode-locked fiber laser based on nonlinear optical loop mirror and manganese phosphorus triselenide (MnPSe3) coated arc-shaped fiber

This study investigates a hybrid mode-locked fiber laser incorporating manganese phosphorus triselenide (MnPSe3) saturable absorber and nonlinear optical loop mirror (NOLM), yielding profound insights into its potential and performance. The hybrid mode-locked laser showcased an output spectrum centered at 1549 nm with an expanded 3-dB bandwidth of 12.25 nm. It generates remarkably narrow pulses, with a pulse width of 240 fs. The mode-locking operates at a repetition rate of 5.71 MHz, with a high signal-to-noise ratio (SNR) of 58 dB, and achieves an average output power of 299 mW with a pulse energy of 52.36 nJ and peak power of 218 kW. These findings underscore the potential of the hybrid mode-locking configuration utilizing MnPSe3 to enhance mode-locked pulse generation significantly compared to the NOLM configuration.

Raman-scattering-assisted noise-like pulse generation in an all-PM NOLM-based mode-locked fiber laser core-pumped by a 978 nm fiber laser

Stable Raman-scattering-assisted noise-like pulse is experimentally demonstrated in an all polarization-maintaining ytterbium-doped ‘nine-shaped’ mode-locked fiber laser based on nonlinear optical loop mirror. The pump source is a self-made single mode 978 nm fiber laser with a maximum output power of 8.45 W. Different pulse states are investigated through dispersive Fourier transform technology. First and second order Raman pulses are generated effectively with only 7.2 m length large-mode-area fiber. Numerical simulation is conducted to investigate the effect of gain fiber length on output laser wavelength. The output spectrum ranges from 980 nm to 1175 nm, the short-wavelength edge almost extends to the 978 nm pump wavelength, which is benefitted from the core-pumping scheme providing extremely high population inversion for gain fiber, as well as the short length YDF of 0.2 m suppressing reabsorption of radiation laser. The polarization extinction ratio is experimentally measured by a spectral subtraction method to be larger than 28 dB at each wavelength between 1010 nm and 1120 nm.

Self-starting mode-locking in an all-PM Yb-doped fiber laser oscillator enabled by a 3x3 nonlinear optical lossy loop mirror

Ultrafast fiber oscillators based on loop mirror saturable absorbers often suffer from a problematic self-starting mode-locking operation. It is usually necessary to provide an additional phase shift in the loop, guaranteeing repeatable initiation of the pulsed regime. 3x3 even splitting ratio fiber couplers have recently introduced a required phase shift in an all-fiber loop mirror architecture. Until now, mode-locking induced by a 3x3 fiber coupler has been associated only with nonlinear amplifying loop mirrors. Here, we present a self-starting ultrafast dispersion-managed all-polarization-maintaining Yb-doped oscillator that utilizes a nonlinear optical lossy loop mirror instead. We show three ways of inserting asymmetric losses in a loop via a variable optical attenuator, a fiber coupler, and a very simple lossy splice. Complete characterization of all output ports of the oscillator proves significant spectral and temporal breathing of the pulse when circulating through the net normal dispersion cavity, which can deliver nJ-level pulse energy. The system guarantees excellent stability, low noise and performance comparable to nonlinear amplifying loop mirrors while being simpler, cheaper, and providing more usable output ports with different pulse characteristics.

Flexible All-Optical 8QAM Signal Format Conversion Using Pump Assisted Nonlinear Optical Loop Mirror

A flexible all-optical format interconversion scheme based on a pump assisted nonlinear optical loop mirror (NOLM) is proposed and numerically simulated for the first time to our knowledge. In this scheme, input multi-Gbps 8QAM signals are divided into clockwise (CW) and counter-clockwise (CCW) components by a 3-dB optical coupler (OC), which also couples light from the input pump into the NOLM from the CCW direction. The numerical model of the pump assisted NOLM with CW and CCW optical paths is simplified using a nonlinear Mach-Zehnder interferometer (MZI). Optical signals in the upper MZI arm will be mainly affected by the self-phase modulation (SPM) effect when traversing the highly nonlinear fiber (HNLF) and those in the lower MZI arm are impacted by cross-phase modulation (XPM) in addition to SPM when they experience the HNLF with the input pump light. When the upper-arm optical signal with SPM phase shift and the lower arm optical signal with SPM and XPM phase shifts are coherently mixed, a new converted 8QAM signal can be obtained. The power transfer function (PTF) of the pump assisted NOLM and the relative phase shift (RPS) between the input and the output optical signals are theoretically provided and verified. By only changing the input power of the 8QAM signal and the pump light, all-optical format interconversion of square-, standard- and star-shaped 8QAM signals can be achieved. Furthermore, the proposed scheme can achieve format conversion from the 30 Gbps square-shaped 8QAM signal to a 20 Gbps quadrature phase shift keying (QPSK) signal. The scheme performance is analyzed via constellation diagrams, eye diagrams, the error vector magnitude (EVM) and the bit error rate (BER) of the optical signals. The scheme developed can be deployed in optical gateways to connect different optical networks by dynamically selecting their appropriate modulation formats.

Multivariable Analysis of Nonlinear Optical Loop Mirror Operating Parameters Using Jones Matrices and Three-Dimensional Renderings

Nonlinear optical loop mirrors (NOLMs) are used in modern fiber optic devices and optical communications. In this study, we present numerical analyses of the multiple variables involved in the operation of an NOLM in low- and high-power transmissions. The Jones matrix formalism was used to model linear and circular polarization inputs. We used three-dimensional (3D) plots to identify the characteristics required in the experimental operation of the NOLM. These characteristics, including the critical power, low- and high-power transmission, and dynamic range, depend on parameters such as the fiber loop length, input power, angle of retarder plate, and input polarization. A standard single-mode fiber (SMF-28) with high twist loop lengths of 100, 300, and 500 m and input powers of 0–100 W was simulated. Three-dimensional surface graphics provided a comprehensive view of the NOLM transmission and considerably enhanced the optimal transmission by manipulating adjustable device components including the power and polarization control plates. Optimal transmission facilitates its use in integrating ultrafast pulse generation, optical signal processing, optical communication systems, and photonic integrated circuit applications.

Switchable and tunable multi-wavelength random distributed feedback fiber laser based on parallel Lyot filter

A switchable and tunable multi-wavelength random distributed feedback fiber laser based on a parallel Lyot filter is designed, and the transmission characteristics of the filter are deduced in detail by using the transmission matrix theory. The randomly distributed backward Rayleigh scattering generated in single-mode fiber (SMF) serves as a feedback mechanism. The number of output wavelength increases linearly with the increase of pump power. By adjusting the polarization controller (PC) to control the polarization state of the beam into the polarization maintaining fiber (PMF), laser channels with wavelength interval of 0.88 nm or 0.46 nm can be output. Where the wavelength interval depends on the wavelength phase difference caused by the corresponding branch PMF. The intensity-dependent loss (IDL) caused by the nonlinear optical loop mirror (NOLM) in the experimental structure is not only used to suppress the intense mode competition caused by EDF, but also to control the spectral range of the output wavelength to achieve tunable laser output. When the wavelength interval is 0.88 nm, the wavelength tuning ranges of the three and four laser channels are 4.4 nm and 3.7 nm, respectively. When the wavelength interval is 0.46 nm, the wavelength tuning ranges of the three and four laser channels are 3.7 nm and 4 nm, respectively. Subsequently, the influence of using different lengths of SMF on the output wavelength is analyzed. It is found that longer SMF can increase the linewidth of the laser channel, and the side mode suppression ratio (SMSR) will be improved to 39 dB. In addition, the output power fluctuation of the laser is less than 0.32 dB within 30 min, and the power amplitude and SMSR are stable at −4.47dBm and 34.5 dB, respectively, indicating that the laser can achieve flat and stable multi-wavelength output at room temperature.

All-optical wavelength and format conversion of DQPSK-to-PAM4 using DLI and pump assisted NOLM

An all-optical wavelength and format conversion scheme of 20 Gbps differential quadrature phase shift keying (DQPSK)-to-4-ary pulse amplitude modulation (PAM4) signals based on the delay line interferometer (DLI) and the pump assisted nonlinear optical loop mirror (NOLM) is proposed and numerically simulated. The input DQPSK signal is converted into two star-16-ary quadrature amplitude modulation (star-16QAM) signals (namely, star-16QAM-I and star-16QAM-Q) using DLI with the time delay of 1-symbol and different linear phase shifts of π/4 and −π/4, respectively. The star-16QAM signals are converted into the on-off keying (OOK) signals (namely, OOK-I and OOK-Q) using the pump assisted NOLM based on the self-phase modulation (SPM) and the cross-phase modulation (XPM) in the highly nonlinear fiber (HNLF). The two OOK signals can be aggregated into different types of PAM4 signals by adjusting their power ratio (PR) and relative phase shift (RPS) flexibly. Error vector magnitude (EVM) and bit error rate (BER) for the input and output optical signals are calculated to verify the scheme performance. Compared to the DQPSK signal with an input optical signal-to-noise ratio (OSNR) of 30 dB, there are about 2.5 dB penalty of receiver OSNR for the converted OOK-I and OOK-Q signals. For the corresponding aggregated equidistant PAM4 and equiamplitude PAM4 signals, they have about 6.3 dB and 12.1 dB penalty of receiver OSNR. The proposed scheme of DQPSK-to-PAM4 can be deployed at the all-optical gateway to bridge the long-hual and short-reach optical networks.

Dynamics of solitons in NOLM based all-normal dispersion Yb-doped mode-locked fiber laser

The properties of the saturable absorber are critical to the soliton dynamics in the all-normal dispersion Yb-doped mode-locked fiber laser based on the nonlinear optical loop mirror (NOLM). In this paper, it is found that by adjusting the splitting ratio f and cavity length L of the NOLM, self-similar pulse (SSP), soliton molecule (SM), dissipative soliton resonance (DSR), dissipative soliton (DS) and dissipative soliton pair (DSP) can be obtained in such fiber laser system. In addition, the mechanisms and characteristics of these solitons are discussed. These findings will facilitate the in-depth understanding of the characteristics of nonlinear pulses generated by NOLM-based all-normal dispersion Yb-doped mode-locked fiber laser.

Tunable and switchable multi-wavelength pulsed fiber laser based on a nonlinear optical loop mirror and an improved Sagnac filter.

We propose and experimentally demonstrate a tunable and switchable multi-wavelength erbium-doped fiber ring pulsed laser based on a nonlinear optical loop mirror (NOLM) and an improved Sagnac filter. To achieve multi-wavelength pulsed laser output, we adopt a NOLM as a quasi-saturable absorber and an improved Sagnac loop as a wavelength selected filter. The constructed laser has a maximum output wavelength number of five with a pulse repetition frequency of 40.45kHz and pulse duration of 108ns. The laser can output single-wavelength and dual-wavelength pulsed lasers within a certain wavelength tuning range and a five-wavelength pulsed laser with a constant wavelength interval of 3nm by adjusting the polarization controller. Dual-wavelength, three-wavelength, and four-wavelength pulsed lasers with various wavelength intervals are also obtained. The output performance of the constructed laser is tested with a maximum average output power of 127.45µW and minimum pulse duration of 52ns, and the stability of the laser output is also tested with a maximum power fluctuation of 0.62dB and minimum wavelength drift of 0.51nm with pump power of 350mW.

Study of transition dynamics from breathing single pulse to higher-order pulses and hysteresis in a figure-of-9 fiber laser

This study investigates the nonlinear dynamics and hysteresis phenomena in a figure-of-9 fiber laser. The laser cavity consists of an Erbium doped nonlinear amplifying loop mirror (NALM) containing a Dispersion Compensation Fiber (DCF) segment, coupled to a nonlinear optical loop mirror (NOLM). A lumped-element model is developed for the laser cavity accounting for bi-directional light propagation. The model is based on the modified nonlinear Schrödinger equation (NLSE) with coefficients directly related to the parameters of each cavity segment. Gain saturation is taken into consideration for stable laser operation. The numerical results show that self-starting passive mode-locking and self-structuration of soliton lattices occur in the cavity of the laser. The soliton structuring is based on the concept of dissipative solitons, which requires a balance between dispersion-nonlinearity and gain-loss. The gain-loss parameters influence both the temporal pulse interactions and the NALM’s transmission, leading to a large variety of single and multiple breathing dissipative soliton dynamics. In that context, riveting studies of the transition dynamics from breathing single pulses to high-order multiple breathing dissipative solitons and hysteresis with respect to unsaturated amplifier gain in a figure-of-9 all-fiber laser are explored.

Investigation on switchable operation states in a Tm/Ho co-doped fiber laser based on nonlinear optical loop mirror

Investigation on switchable operation states in a Tm/Ho co-doped fiber laser based on nonlinear optical loop mirror

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.