Gain Saturation Research Articles

Deep learning method for predicting the complex nonlinear dynamics of passively mode-locked fiber laser

The dynamic evolution processes are highly complicated nonlinear dynamic processes in passively mode-locked fiber laser systems. Here, an artificial intelligence (AI) model is employed to predict the complex dynamic processes, which uses the long short-term memory network method, serving as an alternative to the numerical calculation of the nonlinear Schrödinger equation (NLSE). We specifically emphasize the complex evolution processes under different gain saturation energies, comparing the results predicted by the AI model with those simulated by the NLSE. The predicted results of the AI model are in good agreement with the simulated results of NLSE. The root mean square errors of test samples in this study are all below 0.15. Furthermore, with GPU acceleration, the AI model achieves a mean simulation time of 0.452 s for 6000 roundtrips, approximately 2391 times faster than the numerical solution of NLSE executed on a CPU.

Inductor-capacitor passive wireless sensors using nonlinear parity-time symmetric configurations

An inductor–capacitor passive wireless sensor is essential to physical, chemical, and biological sensing for scenarios where physical access is difficult. Exceptional points of parity-time symmetric inductor–capacitor systems featuring the linear loss and gain have been utilized for enhancing sensing. However, the exceptional point sensing scheme might bring about fundamental resolution limits and noise enhancement. Here we show, employing a nonlinear saturable gain, the responsivity has a cube-root singularity distinct from a square-root singularity of the linear exceptional point scheme. The saturable gain eliminates the imaginary part of the eigenfrequencies and significantly suppresses the noise. Through an example of inductor–capacitor wireless wearable temperature sensors, we demonstrate the high figure of merit for the nonlinear PT-symmetric configuration. Our results resolve a debate on the effectiveness of the exceptional point sensing scheme for inductor–capacitor sensors and provide a way of enhancing precision for these types of sensors.

Dynamic characteristics and conversion process of solitons in a Mamyshev oscillator

The Mamyshev oscillator (MO) can produce not only a single pulse (SP) but also multi-pulses (MPs). However, most of the research focuses on increasing the output pulse energy and reducing the pulse width, and only some studies reveal the characteristics of MPs output. More research is needed on achieving the conversion from MPs to SP by adjusting the oscillator parameters and determining what intermediate states will be experienced in the conversion process. Here, we study the dynamic characteristics of soliton and the conversion process between different output states of ultrafast MO. By adjusting the gain saturation energy and filter interval, we obtain the relationship between the number of MPs outputs and the oscillator parameters and observe the intermediate process from MPs pulsation to SP. The research results reveal the dynamic characteristics of non-equilibrium optical solitons, assisting in optimizing MO design.

Effects of Gain Saturation on Orbital Instability of Chaotic Laser Diode with External Pseudorandom Signal

In a laser diode (LD) system with optical injection, the effects of gain saturation of the LD on the orbital instability of the system are analyzed numerically. For the optical injection LD system without signal application, it is shown that the effect of optical injection is suppressed in the system with gain saturation and small optical injection, and that a higher amount of optical injection is necessary to obtain similar dynamics. Next, in the optical injection LD system with a pseudo-random signal applied to the LD drive current, it is confirmed that when the dynamics are a periodic window between chaotic and chaotic regions, chaotic dynamics are actualized as the standard deviation of the applied signal becomes larger. Furthermore, it is suggested that this phenomenon can be explained by linear stability analysis, and it is shown by introducing randomly varying tentative gain coefficients that gain fluctuations that lead to an expansion of the chaotic region. Hence, the results of this study provide research on the effects of gain saturation on chaotic oscillation in LDs with pseudo-random signals applied and contribute to the generation of more complex chaotic signals, chaotic secure communication, and random number generation.

Dissipative spatiotemporal soliton in a driven waveguide laser

A distributed Kerr-lens mode locking regime can be realized in a waveguide laser by spatial profiling of the pump beam, thus creating a spatio-temporal soliton. Additional slow temporal modulation of the pump source stabilizes the spatio-temporal solution in a broad range of parameters defined by the dynamic gain saturation. We choose a Cr:ZnS waveguide laser as a practical example, but such a regime is feasible in various waveguide and fiber oscillators. A far-reaching analogy with Bose–Einstein condensates allows this approach to stabilize the weakly dissipative BECs.

Nonadiabatic nonlinear non-Hermitian quantized pumping

We analyze a quantized pumping in a nonlinear non-Hermitian photonic system with nonadiabatic driving. The photonic system is made of a waveguide array, where the distances between adjacent waveguides are modulated. It is described by the Su-Schrieffer-Heeger model together with a saturated nonlinear gain term and a linear loss term. A topological interface state between the topological and the trivial phases is stabilized by the combination of a saturated nonlinear gain term and a linear loss term. We study the pumping of the topological interface state. We define the transfer-speed ratio ω/Ω by the ratio of the pumping speed ω of the center of mass of the wave packet to the driving speed Ω of the topological interface. It is quantized topologically as ω/Ω=1 in the adiabatic limit. It remains to be quantized dynamically unless the driving is not too fast even in the nonadiabatic regime. On the other hand, the wave packet collapses and there is no quantized pumping when the driving is too fast. In addition, the stability against disorder is more enhanced by stronger nonlinearity. Published by the American Physical Society 2024

A Symmetrical Quasi-Synchronous Step-Transition Folded Waveguide Slow Wave Structure for 650 GHz Traveling Wave Tubes.

For the purpose of improving performance and reducing the fabrication difficulty of terahertz traveling wave tubes (TWTs), this paper proposes a novel single-section high-gain slow wave structure (SWS), which is named the symmetrical quasi-synchronous step-transition (SQSST) folded waveguide (FW). The SQSST-FW SWS has an artificially designed quasi-synchronous region (QSR) to suppress self-oscillations for sustaining a high gain in an untruncated circuit. Simultaneously, a symmetrical design can improve the efficiency performance to some extent. A prototype of the SQSST-FW SWS for 650 GHz TWTs is designed based on small-signal analysis and numerical simulation. The simulation results indicate that the maximum saturation gain of the designed 650 GHz SQSST-FW TWT is 39.1 dB in a 34.3 mm slow wave circuit, occurring at the 645 GHz point when a 25.4 kV 15 mA electron beam and a 0.43 mW sinusoidal input signal are applied. In addition, a maximum output power exceeding 4 W is observed at the 648 GHz point using the same beam with an increased input power of around 2.8 mW.

Remote sensing of H2O/HDO in the atmospheric column based on a near-infrared laser heterodyne radiometer suppressing local oscillator relative intensity noise

Laser heterodyne radiometer (LHR) exploits the heterodyne signal generated by solar radiation beated with local oscillator (LO) light to extract information on atmospheric species in the atmospheric column. Most of currently reported LHRs are affected by LO-induced relative intensity noise (RIN), which poses a challenge for high-sensitivity detection. In this work, a near-infrared RIN-suppressed LHR is proposed for simultaneous detection of water vapor and HDO in the atmospheric column. The operability of a home-made sun tracker can be obtained from coarse tracking and fine tracking, which offers an excellent opportunity for unattended observation. A tunable distributed feedback (DFB) diode laser centered at 1552.9 nm is employed as LO, where LO-induced RIN is suppressed by a semiconductor optical amplifier (SOA) operating in the dynamic gain saturation regime. By locking LO power and suppressing LO-induced RIN, the SOA-assisted LHR is dominated by 1.27 times shot-noise and the signal-to-ratio (SNR) of heterodyne signal is increased by 5 times. Based on atmospheric transmission spectra measured with the RIN-suppressed LHR in Hefei, China, the total column abundances of water vapor and HDO are retrieved to be 2050 ppm and 0.6 ppm, respectively, with uncertainty of 2 %.

Synchronization under saturable nonlinearity.

In this theoretical work, we introduce a nonlinear gain saturation law representative of the experimentally observed properties manifested by phenomena ranging from aeroacoustic shear layers in self-sustained cavity oscillations to flame heat release rate in thermoacoustic instabilities. Furthermore, this type of saturable gain may be relevant for a wider class of physical systems, such as active laser media in photonics. The nonlinearity discussed herein governs the fullscale behavior of a self-oscillator exhibiting linear loss under large amplitude perturbations, in contrast to the cubic damping and linear gain of the Van der Pol model. A distinctive characteristic of the proposed equation is the simple, well behaved gain term in the slow timescale dynamics.

Silent white light: Reduction of the second-order intensity correlation coefficient

We investigate the intrawaveguide statistics manipulation of broadband light by combining semiconductor quantum dot physics with quantum optics. By cooling a quantum dot superluminescent diode to the liquid nitrogen temperature of 77 K, Blazek [] have demonstrated a temperature-dependent reduction of the second-order intensity correlation coefficient from 2 for thermal amplified spontaneous emission light to g(2)(0,T=190K)≈1.33. Here, we model the broadband photon statistics assuming amplified spontaneous emission radiation in a pumped, saturable quantum dot gain medium. We demonstrate that, by an intensity increase due to the quantum dot occupation dynamics via the temperature-tuned quasi-Fermi levels, together with the saturation nonlinearity, a statistics manipulation from thermal Bose-Einstein statistics towards Poissonian statistics can be realized, thus producing “silent white light” with a reduced second-order correlation coefficient. Such intensity-noise-reduced broadband radiation is relevant for many applications such as optical coherence tomography, optical communication, or optical tweezers. Published by the American Physical Society 2024

Performance enhancement of optical pulse coding phase-sensitive OTDR system with genetic-optimized code.

Optical pulse coding (OPC) in phase-sensitive optical time-domain reflectometry (φ-OTDR) traditionally relies on standard coding sequences as probe pulse trains, which usually suffer the problem of distorted probe pulse caused by the gain saturation in erbium-doped fiber amplifier (EDFA). To improve the signal-to-noise ratio (SNR) and mitigate distortion caused by the EDFA gain saturation, a genetic-optimized code (Go-code) based OPC φ-OTDR (GOPC φ-OTDR) scheme is proposed in this paper. The unique Go-code sequence generated by the distributed genetic algorithm (DGA) is explored in the field of φ-OTDR for the first time, and the coding sequence was optimized to adapt to the vibration sensing requirements in φ-OTDR. Theoretical analyses focused on EDFA gain saturation effects and coding gain were conducted. The experiment of GOPC φ-OTDR demonstrated significant improvements compared with Golay φ-OTDR, achieving less distortion in decoding response and approximately 7.48 dB phase demodulation SNR enhancement within a 10-km sensing range. This research marks Go-code's pioneering exploration into φ-OTDR, demonstrating substantial advancements in optimization metrics, and opening avenues for exploring advanced OPC technologies.

All-optical wavelength reuse for meter reading application using EDFA saturation and DFB laser

The experiment utilizes the all-optical wavelength reuse for the realization of a cheaper meter reading approach. We demonstrate the all- optical wavelength reuse approach while exploiting a holding beam and the gain saturation of the erbium doped fiber amplifier (EDFA). A 10 Gb/s data is used to modulate a distributed feedback laser (DFB) and is transmitted downstream over 50 km. The saturated EDFA lowered the downstream data’s extinction ratio (ER) from 9.678 dB to 0.701 dB which is significant for both wavelength reuse and data rewrite for upstream transmission of meter reading as it facilitates the direct reuse of the downstream carrier without the addition of another electrical eraser for meter reading application. The variation of ER and the corresponding Peak to Peak power with EDFA input power for a 1550 nm 10 Gb/s DFB laser was demonstrated by Changing the bias current while maintaining the modulation voltage at 0.5 V allowed the input power into the EDFA to be adjusted from −7.278 dBm to 1.079 dBm. The performance analysis for upstream and downstream transmission was done and the receiver sensitivities determined at BER 10−9. The various eye diagrams were also analyzed for back-to-back (B2B) and 50 km upstream link and downstream link transmissions. This work is crucial for offering an all-optical wavelength reuse option for electrical meter reading application. In addition, the technique is all-optical and can therefore be able to support ultrafast systems with higher formats of modulation while ensuring efficiency in power.

Investigating the impact of cavity parameters on an ultrashort pulse in a fiber laser

Fiber lasers have numerous applications in various fields due to their compact design, low noise levels, high efficiency, and robust stability. We present a study focusing on the impact of cavity parameters on ultrashort pulses in fiber lasers, proposing an ultrafast laser that utilizes a MoTe2 saturable absorber (SA). Here numerical simulations allowed us to analyze the diverse pulse dynamics within a laser cavity, incorporating a SA, a segment of passive fiber, and erbium-doped fiber (EDF). Key factors of second-order dispersion, nonlinearity, gain saturation energy, small signal gain coefficient, and modulation depth and saturable energy of the SA were simulated. According to the simulations, we obtained the parameter settings for the minimum output pulse duration using a neural network and genetic algorithm. Then we proposed a normal dispersion laser with a MoTe2 mode-locker, capable of emitting dissipative soliton with a pulse duration of 824 fs and a spectrum bandwidth of 13.2 nm. These simulations provide valuable guidance for the design and optimization of fiber lasers, presenting a versatile and practical approach for applications.

Experimental validation of an analytical transient model for saturated boosting gain in DC–DC converters with variable duty cycle

This paper presents an innovative approach to characterize the boosting gain saturation phenomenon in DC–DC converters with variable duty cycles, a crucial component in energy management applications such as renewable energy systems and IoT devices. While existing literature predominantly relies on steady-state models to explain boosting gain behaviour, this study reveals discrepancies between theoretical predictions and experimental observations, particularly at high duty cycles. The research introduces an analytical model to accurately capture the boosting gain dynamics of a boost chopper converter, addressing the limitations of traditional steady-state analyses. Through experimental validation and numerical simulations using CAD tools, a significant boosting gain saturation of approximately 2.2 is empirically observed, highlighting the necessity for a transient model approach. By providing a comprehensive understanding of the boosting ratio constraints in practical boost converters, this work contributes to advancing the analytical modelling techniques in the field. The mathematical model’s validation through rigorous experimental measurements and simulation analyses underscores its reliability and applicability in real-world scenarios. The structured organization of the paper elucidates the development and verification process, culminating in insightful conclusions that underscore the significance of transient modelling in enhancing the performance and efficiency of DC–DC converters.

Transcatheter Cardiac Interventions in Neonates with Congenital Heart Disease: A Single Centre Experience

Objective: To analyze the outcome among neonates undergoing transcatheter balloon atrial septostomy for various kinds of congenital heart diseases. Study Design: Prospective longitudinal study. Place and Duration of Study: National Institute of Cardiovascular Diseases, Karachi Pakistan, from Feb 2020 to May 2021. Methodology: A total of 28 neonates of both genders with echocardiography proven diagnosis of Tricuspid Atresia with restrictive Patent Foramen Ovale, Dextro-Transposition of Great Arteries with Intact Ventricular Septum, complex cyanotic congenital heart disease with inadequate mixing or hypoplastic left heart syndrome with restrictive PFO and a non-significant Patent Ductus Arteriosus, undergoing transcatheter balloon atrial septostomy were included. Demographic data along with diagnosis, clinical presentation, echocardiographic findings, procedural outcome (successful/unsuccessful), duration of the procedure, complication (if any), post-procedural echocardiographic findings, duration of hospital stay, further treatment plan and 3-months follow up outcomes were recorded among all neonates. Results: Most common diagnosis was Dextro-Transposition of the Great Arteries with IVS observed in 14(50.0%) neonates. Procedural outcome was found to be successful among 23(82.1%) neonates. There were 19(67.9%) neonates who did not show any post-procedure complication. At 3-months follow up, improvement in blood saturation and weight gain were reported among 12(42.9%) neonates each. Overall, mortality was noted in 7(25.0%) neonates, of whom 4(57.1%) neonates were having hypoplastic left heart syndrome. Conclusion: Transcatheter atrial septostomy done in neonatal period was found to have positive outcomes. Mortality remains high among neonates undergoing transcatheter atrial septostomy having hypoplastic left heart syndrome.

Pulsating dynamics of noise-like pulses in a fiber laser with nonlinear optical loop mirror

We experimentally investigate the nature of pulsating noise-like pulses (NLPs) in a fiber laser based on the nonlinear optical loop mirror. By adjusting the intra-cavity polarization, three types of pulsating NLPs can be obtained in the cavity. By utilizing the dispersive Fourier transformation technique, the real-time evolution dynamics of NLP pulsation have been investigated in detail. Different from the conventional pulsating behavior, the NLPs undergo remarkable and periodical variations in their width with slight changes in pulse peak powers during pulsation process. We speculate that the wavelength-dependent gain saturation is involved in the pulsating NLP evolution. Quasi-periodic energy oscillations are associated with cyclic generation and subsequent walkoff of wavelength-shifted components, resulting in the different pulsating dynamics of NLP. Moreover, during pulsation process, the NLP splitting could happen with the increasing of energy. All these findings will help to complement our understanding of NLP pulsation in a fiber laser.

Statistical analysis of bistable feature in a 1.55-[formula omitted] vertical cavity emitting laser

Optical bistability in telecom-band lasers has attracted significant attention because of its immense potential for applications in the fields of optical communications and optical computing. In this paper, we report on an experimental investigation of the statistical properties of a 1.55-μm bistable VCSEL. The VCSEL exhibits mode switching at a current of 5.41 mA and 4.15 mA when the pump continuously increases and decreases, respectively. Both of these switching exhibit a bistable region, indicating a polarization bistability induced by the gain saturation. By calculation of second-order autocorrelation functions with injection current, we identify different regions associated with the dynamics of different dominant modes. In particular, the statistical analysis reveals more properties of the bistable laser, such as the intensity fluctuations induced by different kinds of noise.

Multimode Emission in GaN Microdisk Lasers

AbstractQuantum well nanolasers usually show single‐mode lasing, as gain saturation suppresses emissions in other modes. In contrast, for whispering gallery mode microdisk lasers with GaN quantum wells as active material, above threshold multimode laser emission is observed. This intriguing emission feature is manifested in the fact that several modes simultaneously show the characteristic kink in the input–output curve at the onset of lasing. A quantum theory for nanolasers is used to support the experimental finding and to analyze this behavior in the presence of gain saturation. Coupling effects between neighboring modes are identified as the origin of multimode lasing, which initiate photon exchange between modes via population pulsations similar to classical wave‐mixing effects. A reduction of this type of mode coupling with increasing mode spacing is demonstrated. The results can pave the way for multimode application of nanolasers in integrated photonic circuits.

Asymmetric Doherty power amplifier design considering the effects of peaking power amplifier early conduction

During the design of a Doherty power amplifier (DPA), the phenomenon of peaking PA turning on in advance is commonly observed. This is mainly due to the feedback of the main PA signal to the gate of the peaking PA through the gate-drain feedback capacitor, which results in an increase in its gate voltage. As a result, the overall efficiency of the back-off region is significantly reduced. This paper first describes the phenomenon of peaking PA turning on in advance. The formula for the voltage (referred to as the feedback gate voltage, FGV) imposed on the gate of the peaking PA by the main PA signal is then deduced. On this basis, the FGV is analysed numerically. For verification, an asymmetric DPA with a 9 dB output back-off (OBO) power level is designed and fabricated in this paper using GaN CGH40010F and CGH40025F transistors. According to the measurement results, the saturated drain efficiency of the DPA is 53.5%–63.3% in the 0.75–1.25 GHz band, the efficiency at the 9 dB OBO level is between 45.5%–54%, and the saturated output power is between 45 dBm–45.7 dBm, with a saturation gain of more than 8 dB.

Ultra low-power multistability in [formula omitted]-symmetric periodic structures with saturable coupling, gain and loss

The present work investigates the switching dynamics of conventional fiber Bragg gratings (FBGs) with saturable coupling and parity-time (PT)-symmetric fiber Bragg gratings (PTFBGs) with saturable coupling (SC) and saturable gain-loss (SGL) in the presence of Kerr nonlinearity (KNL) [model - I] and saturable nonlinearity (SNL) [model - II]. Both system models can generate distinct optical multistability (OM) and bistability (OB) curves, including S-shaped, ramp-like, and mixed OM curves in both unbroken and broken PT-symmetric regimes. The impact of each control parameter on controlling switching intensities, hysteresis width, and the number of stable states is thoroughly examined for both of these system models. The direction of right light incidence enables a significant reduction in the switching intensities to below 100 W/cm2, the lowest critical power ever reported in the literature of PTFBGs, thanks to the concept of both saturable coupling, gain and loss.