Gain Saturation Research Articles

Observation of energy localization in nonlinear non-Hermitian systems

Energy localization in Hermitian systems has been utilized to generate ultra-sensitivity. Here, we report the interplay between non-Hermitian parity-time (PT) symmetry breaking and the mode localization transition. In our scheme, a PT-symmetric system consists of two coupled LC (inductor–capacitor) resonators: one has a linear loss and the other has a saturated gain described by a nonlinear model. The nonlinear gain is initially set to be slightly higher than the loss, and the system is operated at the exact PT-symmetric phase close to an exceptional point. The capacitance variation applied on the loss resonator, i.e., perturbation, causes PT-symmetry to break, generating complex frequencies. As a result of nonlinear gain, the resonator will grow to reach its steady state and saturate out the gain. This stable oscillation eliminates the complex frequencies, and the mode is ultimately localized at the gain side. We have observed that the voltage amplitude of the gain resonator due to the perturbation has experienced drastic changes. The amplitude ratio before and after the perturbation is sensitive to the perturbation. Our results provide an approach to study perturbation-driven localization phenomena in a PT-symmetric system and pave the way for sensors with ultrahigh sensitivity.

Saturated Gain-Induced Non-Reciprocal Transmission and Broadband On-Chip Optical Isolator

To overcome the limitation of dynamic reciprocity, a new method for designing broadband on-chip optical isolators is proposed and demonstrated based on saturated gain, which is able to support simplex and duplex operation modes. By connecting a saturated gain waveguide to an appropriate linear loss waveguide, broadband isolation is predicted and proved theoretically through saturated gain-induced non-reciprocal transmission. The proposed isolator is numerically demonstrated with an operating band of 59 nm and an isolation ratio of −20 dB at the central wavelength of 1550 nm. It is noteworthy that when the current pump changes, the isolator still works well and keeps the high isolation ratio at a different input power. The footprint of the whole device is 465 μm × 0.35 μm which satisfies the requirement of photonic integrated circuits. The proposed isolator, with the combined advantages of compact footprint, broadband, duplex operation and high isolation, can enable on-chip unidirectional transmission and complex topological routing designation.

Design of Inner Matching Three-Stage High-Power Doherty Power Amplifier Based on GaN HEMT Model.

This paper introduces the structure and characteristics of an internal-matching high-power Doherty power amplifier based on GaN HEMT devices with 0.25 μm process platforms from the Nanjing Electronic Devices Institute. Through parameter extraction and load-pull testing of the model transistor, an EE_HEMT model for the 1.2 mm gate-width GaN HEMT device was established. This model serves as the foundation for designing a high-power three-stage Doherty power amplifier. The amplifier achieved a saturated power gain exceeding 9 dB in continuous wave mode, with a saturated power output of 49.7 dBm. The drain efficiency was greater than 65% at 2.6 GHz. At 9 dB power back-off point, corresponding to an output power of 40.5 dBm, the drain efficiency remained above 55%. The performance of the amplifier remains consistent within the 2.55-2.62 GHz frequency range. The measured power, efficiency, and gain of the designed Doherty power amplifier align closely with the simulation results based on the EE_HEMT model, validating the accuracy of the established model. Furthermore, the in-band matching design reduces the size and weight of the amplifier. The amplifier maintains normal operation even after high and low-temperature testing, demonstrating its reliability. In conjunction with DPD (digital pre-distortion) for the modulated signal test, the amplifier exhibits extremely high linearity (ACLR < -50.93 dBc). This Doherty power amplifier holds potential applications in modern wireless communication scenarios.

10 W super-wideband ultra-low-intensity-noise single-frequency fiber laser at 1 µm.

A 10 W super-wideband ultra-low-intensity-noise single-frequency fiber laser (SFFL) at 1 µm is experimentally demonstrated, based on dual gain saturation effects from semiconductors and optical fibers, together with an analog-digital hybrid optoelectronic feedback loop. Three intensity-noise-inhibited units synergistically work, which actualizes a connection of effective bandwidth and enhancement of noise-suppressing amplitude. With the cascade action of the semiconductor optical amplifier and optical fiber amplifier, the laser power is remarkably boosted. Eventually, an SFFL with an output power of 10.8 W and a relative intensity noise (RIN) below -150 dB/Hz at the frequency range over 1 Hz is realized. More meaningfully, within the total frequency range of 10 Hz to 10 GHz exceeding 29 octaves, the RIN is controlled to below -160 dB/Hz, approaching the shot-noise limit (SNL) level. To the best of our knowledge, this is the lowest RIN result of SFFL within such an extensive frequency range, and this is the highest output power of the near-SNL super-wideband SFFL. Furthermore, a linewidth of less than 0.8 kHz, a long-term stable polarization extinction ratio of 20 dB, and an optical signal-to-noise ratio of over 60 dB are obtained simultaneously. This start-of-the-art SFFL has provided a systematic solution for high-power and low-noise light sources, which is competitive for sophisticated applications, such as free-space laser communication, space-based gravitational wave detection, and super-long-distance space coherent velocity measurement and ranging.

Energetic picosecond 10.2-μm pulses generated in a BGGSe crystal for nonlinear seeding of terawatt-class CO2 amplifiers

We demonstrate what we believe to be a new approach to energetic picosecond 10.2-µm pulse generation based on nonlinear mixing of subnanosecond single-frequency 1338-nm pulses and broadband 1540-nm chirped pulses in a BGGSe crystal followed by a grating compressor for the purpose of seeding high-power CO2 amplifiers. The energy of the 10.2-µm pulses exceeding 60 µJ with 3.4%-rms fluctuation can be routinely obtained. Single-shot pulse duration measurement, performed by Kerr polarization rotation time-resolved by a streak camera, together with the pulse spectrum, indicates the pulse width is between 2.7-3 ps. Numerical calculations show that power broadening and dynamic gain saturation with Rabi-flopping can be induced with such an intense seed in a multi-atmospheric CO2 amplifier. These nonlinear effects greatly suppresses pulse splitting due to the comb-like spectrum of the CO2 molecule. A peak power exceeding 1 TW is expected after multipass of amplification while maintaining an appropriate high intensity by controlling the beam size along the path.

Optimally design of amplifier self-similar pulse in thulium-doped multimode fiber laser

High-energy ultrashort pulses fiber lasers could be widely used in scientific research and industrial applications. Researchers often use two methods to boost the pulse energy, one is to achieve self-similar pulse (SSP) output in a single mode fiber (SMF) laser, and the other is to use multimode fiber (MMF) instead of SMF. Here we combine the above two methods to optimally design the output characteristics of SSP in thulium-doped MMF lasers. The influence of different parameters, including spectral filter bandwidth, fiber length, and gain saturation energy, on the output characteristics of SSP have been analyzed, and an optimal SSP with pulse energy of 63.48 nJ, spectrum width of 52.53 nm, dechirped pulse peak power of 283.34 kW, and dechirped pulse width of 219.74 fs has been obtained.

Optimizing elastic optical networks quality of transmission with an empirical optical amplifier gain saturation model

In this paper, we revisit the problem of adjusting transceiver power, amplifier gain, and nodal gain in optical-bypass-enabled networks, taking into account an empirical model of erbium-doped fiber amplifier (EDFA) gain saturation. We assume that network demands have been routed via the shortest possible paths. A heuristic algorithm, which estimates the degradation of the signal-to-noise ratio (SNR) for each demand, is used to assign the modulation level and spectrum slots to the routed demands. Our optimization process aims to maximize the minimum SNR margins of demands, by adjusting EDFA small-signal gain, variable optical attenuator (VOA) attenuation, and transceiver power. Constraints and impairments imposed by the physical-layer include EDFA and VOA gain limits, transceiver power limits, receiver sensitivity, insertion loss of optical switch input ports, EDFA gain saturation, EDFA noise, and fiber non-linear interference. These are modeled using the Gaussian noise model. The proposed optimization problem is formulated as a geometric programming problem and then converted into an equivalent convex problem.Numerical results suggest that in the optimal solution of our optimization, all demands are served with typically low bit error rates (BER) (<10−4). We calculate the operational gains of the EDFA to set the spans to full compensation whenever possible. Pre-amplifiers and line amplifiers do not experience gain saturation due to relatively small input powers, while approximately 10% of boosters undergo more than a 1 dB gain reduction due to high input powers. We observe that overlooking the effect of EDFA gain saturation and physical-layer-imposed constraints such as nodal gain limits or transceiver power limits during network optimization leads to a considerable number of blocked demands due to SNR degradation or high BER (>10−3).

Neural network control of fractional-order chaotic systems with unknown control direction

In this paper, neural network control of fractional order chaotic systems (FOCSs) with input saturation and unknown sign of the controller gain is addressed by employing the Nussbaum function, where neural networks are utilized to model system uncertainties. To get rid of the limitation that reaching phase should be active before sliding motion in the traditional sliding mode control, a stable sliding surface is constructed. Then, by using the integer order Nussbaum gain control method, a novel controller with neural network sliding mode variable structure is designed. Finally, the practicality of the designed method is confirmed by a simulation experiment.

Observation of Nonlinear Exceptional Points with a Complete Basis in Dynamics.

The exotic physics associated with exceptional points (EPs) is always under the scrutiny of theoretical and experimental science. Recently, considerable effort has been invested in the combination of nonlinearity and non-Hermiticity. The concept of nonlinear EPs (NEPs) has been introduced, which can avoid the loss of completeness of the eigenbasis in dynamics while retaining the key features of linear EPs. Here, we present the first direct experimental demonstration of a NEP based on two non-Hermition coupled circuit resonators combined with a nonlinear saturable gain. At the NEP, the response of the eigenfrequency to perturbations demonstrates a third-order root law and the eigenbasis of the Hamiltonian governing the system dynamics is still complete. Our results bring this counterintuitive aspect of the NEP to light and possibly open new avenues for applications.

Analysis of a Graphene FET-Based Frequency Doubler for Combined Sensing and Modulation through Compact Model Simulation

The ambipolar conduction property of graphene field-effect transistors (GFETs) and the inherent square-like dependence of the drain current on the gate voltage, enable the development of single-device architectures for analog nonlinear radiofrequency (RF) circuits. The use of GFETs in novel RF component topologies allows leveraging graphene’s attractive thermal and mechanical properties to improve the miniaturization and weight reduction of electronic components. These features are specifically appealing for integrated sensing, modulation, and transmission systems. However, given the innovative nature of emerging graphene-based technology, a complete performance analysis of any novel electronic component is essential for customizing the operating conditions accordingly. This paper presents a comprehensive circuital analysis of a GFET-based frequency doubler, exploiting a compact model for GFET circuit simulation to assess the device’s performance parameters, including power conversion gain bandwidth and saturation. The performed analysis proposes to support the design of GFET-based harmonic transponders, offering integrated sensing and signal manipulation capabilities.

Manipulating spectral topology and exceptional points by nonlinearity in non-Hermitian polariton systems

Exceptional points (EPs), with their intriguing spectral topology, have attracted considerable attention in a broad range of physical systems, with potential sensing applications driving much of the present research in this field. Here, we investigate spectral topology and EPs in systems with significant nonlinearity, exemplified by a nonequilibrium exciton-polariton condensate. With the possibility to control loss and gain and nonlinearity by optical means, this system allows for a comprehensive analysis of the interplay of nonlinearities (Kerr type and saturable gain) and non-Hermiticity. Not only do we find that EPs can be intentionally shifted in parameter space by the saturable gain, but we also observe intriguing rotations and intersections of Riemann surfaces and find nonlinearity-enhanced sensing capabilities. With this, our results illustrate the potential of tailoring spectral topology and related phenomena in non-Hermitian systems by nonlinearity. Published by the American Physical Society 2024

Rate equation analysis for deterministic and unidirectional lasing in ring resonators with an S-shaped coupler

Ring resonators are traditionally popular optical devices that apply to various components in photonic ICs. They also play an important role in the on-chip generation of many novel optical states in topological systems and non-Hermitian systems. Unidirectional lasing of ring resonators is used in many such systems to create exotic states of light including optical vortexes and optical skyrmions, but the unidirectional behavior has not been fully understood. Previous research has constructed a simplified model to explain the steady state behaviors of unidirectional ring resonators, but the carrier dynamics and spontaneous emission were omitted. In this work, we give a numerical analysis of unidirectional ring resonators with an S-shaped coupler. We identified the importance of the gain saturation to robustness against backscattering and high unidirectionality by comparing to the model without saturation. We also discuss the effect of asymmetrical coupling on the deterministic realization of unidirectionality.

Grapefruit (Citrus paradisi Mcfad.) and its phytochemical inhibits pancreatic lipase enzyme and modulates the saturated fat-induced obesity model

The prevalence of obesity has reached epidemic proportions and warrants numerous interventions, mostly using functional foods or medicinal plants to combating obesity. This review focuses on medicinal plants that elicit anti-obesity effect targeting pancreatic lipase enzyme (PLE). However, the putative processes of the plants targeting PLE and its consequential effects on triglyceride breakdown remains elusive. The effect of aqueous methanolic extracts, fractions and isolates of selected plants on PLE was tested in-vitro. The potent plant extract was examined for its effect on triglyceride levels in rats administered using saturated fat-rich coconut milk versus non-fat milk model in-vivo. Our observations illustrates that the Citrus paradisi aqueous methanolic extract (CPAME) inhibits PLE and modulate the saturated fat-induced weight gain and triglyceride levels. Whilst, the hesperidin isolated from CPAME has shown potential to inhibit PLE, the observations warrant further investigations on flavonoid and other compounds in grape fruit. Therefore, CPAME could be a promising target in combating obesity by inhibiting PLE.

Nonreciprocity through gain saturation in coupled nanocavities

Nonreciprocity through gain saturation in coupled nanocavities

1 MHz, 273 W average power Ytterbium-doped rod-type fiber chirped pulse amplification system

Ytterbium-doped ultrafast fiber lasers are widely used in scientific research, industrial processing, medical diagnosis, and other fields due to their excellent beam quality and high power output. The larger mode area allows the fiber to transmit higher peak-pulse power. The commercial rod-type Ytterbium-doped fiber with a core diameter of 85 μm, produced by NKT in Denmark, can produce ultra-short pulses on the order of 100 watts and 100 microjoules. Based on this rod-type fiber, we construct a chirped-pulse amplification (CPA) system in which the high-efficiency transmission gratings and temperature-tunable chirped fiber Bragg grating (CFBG) are used to compensate for dispersion. We investigate the effect of power input on the amplified power and pulse compression quality, and find that higher power input slows down the gain saturation and improves amplification efficiency. At power inputs of 20 W and 30 W, we obtain power outputs of 305 W and 323 W respectively, with an amplification efficiency of about 80%. To reduce the accumulation of nonlinear phase shift, we use circular polarization amplification. At low power outputs (less than 160 W), the effect of nonlinear phase accumulation on the compressed pulse is negligible, and the increase in power input increases the amplification efficiency. When the power output exceeds 200 W, the cumulative increase of nonlinear phase shift reduces the pulse compression quality, which implies that the input power is appropriately reduced to the power range between 5 W and 20 W. With a power input of 20 W and pump power of 429 W, the power output can reach 305 W. After pulse is compressed by using a diffraction-grating pair, this rod-type fiber CPA system can deliver 1 MHz, 264 fs pulses with 273 W in average power. These results provide an important experimental basis for optimizing the performance of high-power and high-energy ultrafast fiber lasers.

Large-spatial-scale convective stimulated Raman side scattering under indirect drive conditions

The large-spatial-scale stimulated Raman scattering relevant to the SG-III prototype indirect drive parameters is investigated by using the code PHANTAM, which is based on ray tracing and convective amplification. The simulations show that strong stimulated Raman side scattering processes occur in both empty hohlraum and gasfilled hohlraum. The incident laser spot size is found to be the critical factor affecting stimulated Raman side scattering: under the constant laser intensity conditions, the convective gain of stimulated Raman side scattering increases with the laser spot size increasing in both types of hohlraums. In our simulations, the wavenumber mismatch leads to a saturation of the convection gain of the stimulated Raman side scattering in empty hohlraum, while in gas-filled hohlraum the convection gain of the stimulated Raman side scattering keeps increasing as the spot size increases. Under constant laser power conditions, the convective gain of stimulated Raman side scattering decreases while laser spot size increases, and the convective gain of stimulated Raman side scattering decreases faster in empty hohlraum in our simulations. The convective gain of Raman side scattering can be adjusted by laser spot size.

Effects of white noise on preterm infants in the neonatal intensive care unit: A meta-analysis of randomised controlled trials.

To critically assess the effects of white noise on the pain level, weight gain and vital signs (heart rate, respiratory rate and oxygen saturation) of preterm infants in neonatal intensive care units (NICUs). A systematic review and meta-analysis of randomised controlled trials (RCTs). Ten databases (PubMed, Cochrane Library, Embase, Web of Science, CINAHL, PsycINFO, SinoMed, China National Knowledge Infrastructure, VIP and Wanfang Data) were systematically reviewed from inception to July 2022. Two reviewers evaluated the risk of bias separately using the Cochrane Collaboration criteria and extracted data using a predesigned information form. The meta-analysis included eight eligible RCTs. According to statistical analysis, white noise significantly affected the pain level, weight gain, heart rate, respiratory rate and oxygen saturation in preterm infants. Regardless of the outcome measurement timing, gestational age and birth weight of preterm infants, subgroup analysis demonstrated that white noise reduced the pain level, heart rate and respiratory rate and promoted weight gain in preterm infants in NICUs. White noise is a practical and potentially useful therapy for premature neonates in NICUs. No Patient or Public Contribution.

Research progress of magnetic resonance wireless power transfer based on higher-order non-Hermitian physics

&lt;sec&gt;In recent years, wireless power transfer (WPT) leveraging parity-time (PT) symmetry has made significant progress , in terms of enhancing efficiency, transfer distance, and robustness. This paper overviews magnetic resonance WPT systems utilizing ideal, asymmetric, high-order, and anti-PT symmetry.&lt;/sec&gt;&lt;sec&gt;The first section discusses the second-order PT symmetry, evolving from inductive to resonant WPT. Active tuning and nonlinear saturation gain techniques optimize frequency and spontaneously achieve efficient WPT. These methods improve transmission efficiency, especially with the change of dynamic transfer distance. The second section focuses on the third-order PT and anti-PT symmetry. The third-order PT systems maintain a fixed eigenfrequency, making stable energy transfer possible. Generalized PT symmetry harnesses bandgaps for further efficiency. The BIC in asymmetric systems reveals a pure real mode for stable WPT. The anti-PT symmetry’s ‘level pinning’ maintains stability in dynamic changes. The final section summarizes high-order PT symmetry for long-range WPT. Heterojunction coupling and topologically non-trivial chains enhance efficiency and stability. Examples include long-range WPT via relay coils and directional WPT using asymmetric topological edge states.&lt;/sec&gt;&lt;sec&gt;In summary, this review emphasizes the pivotal role of various forms of PT symmetry in improving the performance and reliability of magnetic resonance WPT systems. By improving transmission efficiency, range, and stability, these symmetries pave the way for wider applications in fields such as smart homes, medical devices, and electric vehicles. The synthesis of current research results provides valuable insights and references for the future development of WPT technology.&lt;/sec&gt;

Loss compensated fiber loop ring-down system driven by random distributed multi-pulse laser

The sensing characteristics of loss compensated fiber loop ring-down (FLRD) system driven by random distributed multi-pulse have been analyzed. We used a random distributed multi-pulse laser to drive a fiber loop with loss compensation of erbium-doped fiber amplifier (EDFA) and analyzed the distortion of exponential attenuation curves, sensitivity and baseline stability of the system. The performance is compared with those of loss compensated FLRD system driven by single pulse and chaotic laser. The loss compensated FLRD theoretical model is discussed by the transmission equation of EDFA and finite element analysis. The performance of loss compensated FLRD system driven by random distributed multi-pulse under different injection optical powers is analyzed under the influence of gain saturation effect of EDF and amplified spontaneous emission (ASE). The results show that the random distributed multi-pulse system has higher R-square of exponential attenuation curves and better baseline stability than the single-pulse system. The increase of injection optical power has the least influence on the sensitivity reduction of random distributed multi-pulse system. The random distributed multi-pulse laser has better performance than single-pulse laser and chaotic laser in loss compensated FLRD system.

Asymmetrical Sequential Load Modulated Balanced Amplifier With Composited Impedance Inverter and Reciprocal Mode for Broadband Applications

In this paper, the design of a broadband asymmetrical sequential load modulation balanced amplifier (ASLMBA) is presented. The composited impedance inverters, which contain the intrinsic elements and the output matching networks (OMNs) of the balanced power amplifier (BPA) transistors, are inserted before the output coupler of the BPA pair. In this way, optimal matching condition of the BPA transistors can be maintained over a wide bandwidth. In addition, to ensure an optimal load modulation trajectory of the control amplifier (CA) without over-saturation, a reciprocal mode is applied to the BPA pair by elaborately changing the turn-on sequence of the sub-amplifiers, which is affected by the composited impedance inverter and the OMN of the control amplifier. It is illustrated that the proposed ASLMBA can effectively maintain high efficiency over an extended dynamic range and a wide bandwidth. As a proof of concept, a broadband ASLMBA working over 1.35-2.55 GHz is designed and measured in this work. Experimental results show that the designed ASLMBA achieves a saturation output power of 43.2-45.1 dBm with a saturation gain of 6.5-10.5 dB and a saturation drain efficiency (DE) of 57.8-77.7%. Meanwhile, the 6 dB and 10 dB back-off DEs of the designed ASLMBA are 51.3-66.6% and 47.7-68.9%, respectively.