Gain Recovery Research Articles

Dynamic gain driven mode-locking in GHz fiber laser

Ultrafast lasers have become powerful tools in various fields, and increasing their fundamental repetition rates to the gigahertz (GHz) level holds great potential for frontier scientific and industrial applications. Among various schemes, passive mode-locking in ultrashort-cavity fiber laser is promising for generating GHz ultrashort pulses (typically solitons), for its simplicity and robustness. However, its pulse energy is far lower than the critical value of the existing theory, leading to open questions on the mode-locking mechanism of GHz fiber lasers. Here, we study the passive mode-locking in GHz fiber lasers by exploring dynamic gain depletion and recovery (GDR) effect, and establish a theoretical model for comprehensively understanding its low-threshold mode-locking mechanism with multi-GHz fundamental repetition rates. Specifically, the GDR effect yields an effective interaction force and thereby binds multi-GHz solitons to form a counterpart of soliton crystals. It is found that the resulting collective behavior of the solitons effectively reduces the saturation energy of the gain fiber and permits orders of magnitude lower pulse energy for continuous-wave mode-locking (CWML). A new concept of quasi-single soliton defined in a strongly correlated length is also proposed to gain insight into the dynamics of soliton assembling, which enables the crossover from the present mode-locking theory to the existing one. Specifically, two distinguishing dynamics of Q-switched mode-locking that respectively exhibit rectangular- and Gaussian-shape envelopes are theoretically indicated and experimentally verified in the mode-locked GHz fiber laser through the measurements using both the standard real-time oscilloscope and emerging time-lens magnification. Based on the proposed criterion of CWML, we finally implement a GDR-mediated mode-locked fiber laser with an unprecedentedly high fundamental repetition rate of up to 21 GHz and a signal-to-noise ratio of 85.9 dB.

The impact of changes in physical activity on functional recovery for older inpatients in post-acute rehabilitation units.

The effect of increased physical activity duration on functional recovery in older inpatients in subacute settings is not well established. This study aimed to investigate the relationship between physical activity and functional recovery in older patients receiving post-acute and subacute care. We analyzed cohort data of hospitalized older patients (age ≥ 65years) in the post-acute rehabilitation units. The main outcome was functional independence measure (FIM) gain. Physical activity was measured using a triaxial accelerometer. Changes in sedentary behavior and total physical activity time from admission to discharge were measured as changes in each physical activity time. Logistic regression analysis was performed to examine the relationship between changes in physical activity and FIM gain. A total of 210 patients were eligible for analysis. The mean age of the study patients was 83.6 ± 7.2years, and 63.8% (n = 134) were female. According to the multivariate regression analysis, changes in sedentary behavior time were significantly associated with high recovery of FIM gain (odds ratio [OR] 0.996, 95% confidence interval [CI]: 0.993-1.000; p = 0.026), and changes in total physical activity time also showed a similar association (OR 1.006, 95% CI 1.000-1.011; p = 0.041). Decreased sedentary behavior time and increased total physical activity time were significantly associated with high functional recovery in post-acute rehabilitation units. These results suggest that interventions for physical activity duration may be effective in improving activities of daily living in older post-acute and subacute patients.

Gain Depletion and Recovery as a Key Mechanism of Long-Range Pulse Interactions in Soliton Fiber Laser

Gain Depletion and Recovery as a Key Mechanism of Long-Range Pulse Interactions in Soliton Fiber Laser

Flat-topped optical spectrum as a specific marker of multi-pulse grouping in a soliton fiber laser.

We report the experimental observation of a stable generation regime in a soliton fiber laser, characterized by a distinct flat-topped optical spectrum. Notably, in multi-pulse generation, this specific spectrum shape prevents the harmonic mode-locking state, instead connecting the solitons into bound complexes or tight chaotic bunches. Physically, this suggests that in the observed regime, long-range attractive forces dominate over the inter-pulse repulsion across the entire laser cavity. Our experimental findings align with numerical simulations, which demonstrate that the predominance of a long-range inter-pulse attraction is due to a complex interaction mechanism. This mechanism combines the generation of dispersive waves with dissipative forces arising from gain depletion and recovery.

On the selection of humidification dehumidification desalination system cycle for high productivity, low energy consumption and low capital cost

Humidification Dehumidification (HDH) desalination is a convenient process to produce drinking water from low temperature saline water in waterfront areas. High energy consumption of the system remains as a limiting factor, besides many enhancements done in improving the system performance. Though HDH system cycles are classified on the basis of air and water cycles, detailed performance analysis of different HDH cycles is absent till date. The present study aims at carrying out parametric analysis of possible HDH layouts and finally proposing a layout with high Gain Output Ratio (GOR) and Recovery Ratio (RR) at the low equipment volumes. Simulations are carried out using the Scilab® software. Simulation results are validated using the experimental data obtained from a pilot plant of open-air open-water single stage system. Variation of system parameters like humidifier volume, dehumidifier volume, GOR and RR were analysed in detail for each individual layouts. Out of the eight different layouts investigated, Layout 8 is found to be the best based on GOR, RR, equipment volume and specific energy consumption. Hence it is recommended to use Layout 8 with an inlet temperature of 74 °C and MFR of 2.4 for productivity of 90 LPH.

Double harmonic mode-locking in soliton fiber ring laser acquired through the resonant optoacoustic coupling

Passive harmonic mode-locking of a soliton fiber laser locked to optoacoustic resonance (OAR) in the cavity fiber ensures high-frequency laser operation, high pulse stability, and low timing jitter. However, the pulse repetition rate (PRR) of such lasers is limited to ∼1 GHz for standard fibers due to the available acoustic modes. Here, we address these limitations by demonstrating a soliton fiber laser built from standard fiber components and subjected to double harmonic mode-locking (DHML). As an example, the laser adjusted to operate at the 15th harmonic of its cavity matching the OAR at ∼199 MHz could be driven to operate at a high harmonic of this particular OAR frequency, thus reaching ∼12 GHz. This breakthrough is made possible through controllable optoacoustic interactions in a short, 50 cm segment of unjacketed cavity fiber. We propose that the precise alignment of the laser cavity harmonic and fiber acoustic modes leads to a long-lived narrow-band acoustic vibration. This vibration sets the pace for the pulses circulating in the cavity by suppressing modes that do not conform to the Vernier principle. The surviving modes, equally spaced by the OAR frequency, in cooperation with the gain depletion and recovery mechanism, facilitate the formation of stable high-frequency pulse sequences, enabling DHML. In this process, the OAR rather than the laser cavity defines the elementary step for laser PRR tuning. Throughout the entire PRR tuning range, the soliton fiber laser exhibits enhanced stability, demonstrating supermode suppression levels better than ∼40 dB and picosecond pulse timing jitter.

Mechanisms and predictors of menses resumption once normal weight is reached in anorexia nervosa

BackgroundIn cases of Anorexia Nervosa (AN), achieving weight gain recovery beyond the lower limits set by the World Health Organization and normalizing classical nutritional markers appears to be essential for most patients. However, this is not always adequate to restore menstrual cycles. This discrepancy can cause concern for both patients and healthcare providers, and can impact the medical management of these individuals. Thus, the purpose of this study was to assess the ability of anthropometric and hormonal factors to predict the resumption of menstrual cycles in individuals with anorexia nervosa upon reaching a normal body weight.MethodPatients with AN who had achieved a normal Body Mass Index but had not yet resumed their menstrual cycles (referred to as ANRec) were evaluated on two occasions: first at visit 1 and then again 6 months later, provided their body weight remained stable over this period (visit 2). Among the 46 ANRec patients who reached visit 2, they were categorized into two groups: 20 with persistent amenorrhea (PA-ANRec) and 26 who had regained their menstrual cycles (RM-ANRec). Anthropometric measurements, several hormone levels, Luteinizing Hormone (LH) pulsatility over a 4-h period, and LH response to gonadotropin-releasing hormone injection (LH/GnRH) were then compared between the two groups at visit 1.ResultsPatients in the RM-ANRec group exhibited higher levels of follicular stimulating hormone, estradiol, inhibin B, LH/GnRH, and lower levels of ghrelin compared to those in the PA-ANRec group. Analysis of Receiver Operating Characteristic curves indicated that having ≥ 2 LH pulses over a 4-h period, LH/GnRH levels ≥ 33 IU/l, and inhibin B levels > 63 pg/ml predicted the resumption of menstrual cycles with a high degree of specificity (87%, 100%, and 100%, respectively) and sensitivity (82%, 80%, and 79%, respectively).ConclusionsThese three hormonal tests, of which two are straightforward to perform, demonstrated a high predictive accuracy for the resumption of menstrual cycles. They could offer valuable support for the management of individuals with AN upon achieving normalized weight. Negative results from these tests could assist clinicians and patients in maintaining their efforts to attain individualized metabolic targets.Trial registrationIORG0004981.

Temporal Cavity Soliton Interaction in Passively Mode-Locked Semiconductor Lasers

Weak interactions of temporal cavity solitons resulting from gain saturation and recovery in a delay differential model of a long cavity semiconductor laser were studied numerically and analytically using an asymptotic approach. This paper shows that in addition to the usual soliton repulsion leading to a harmonic mode-locking regime, soliton attraction is also possible in a laser with a nonzero linewidth enhancement factor. It is shown numerically that this attraction can lead either to pulse merging or to pulse bound-state formation.

Gain recovery in heavily Irradiated Low Gain Avalanche Detectors by high temperature annealing

Studies of annealing at temperatures up to 450 °C with Low Gain Avalanche Detectors (LGAD) irradiated with neutrons are described. It was found that the performance of LGADs irradiated with 1.5e15 n/cm2 was already improved at 5 min of annealing at 250 °C. Isochronal annealing for 30 min in 50 °C steps between 300 °C and 450 °C showed that the largest beneficial effect of annealing is at around 350 °C. Another set of devices was annealed for 60 min at 350 °C and this annealing significantly increased depletion voltage of the gain layer (Vgl). The effect is equivalent to reducing the effective acceptor removal constant by a factor of ∼ 4. Increase of Vgl is the consequence of increased effective space charge in the gain layer caused by formation of electrically active defects or re-activation of interstitial boron atoms.

Distortion-free amplification of 100 GHz mode-locked optical frequency comb using quantum dot technology.

Semiconductor mode-locked optical frequency comb (ML-OFC) sources with extremely high repetition rates are central to many high-frequency applications, such as dense wavelength-division multiplexing. Dealing with distortion-free amplification of ultra-fast pulse trains from such ML-OFC sources in high-speed data transmission networks requires the deployment of semiconductor optical amplifiers (SOAs) with ultrafast gain recovery dynamics. Quantum dot (QD) technology now lies at the heart of many photonic devices/systems owing to their unique properties at the O-band, including low alpha factor, broad gain spectrum, ultrafast gain dynamics, and pattern-effect free amplification. In this swork, we report on ultrafast and pattern-free amplification of ∼100 GHz pulsed trains from a passively ML-OFC and up to 80 Gbaud/s non-return-to-zero (NRZ) data transmission using an SOA. Most significantly, both key photonic devices presented in this work are fabricated from identical InAs/GaAs QD materials operating at O-band, which paves the way for future advanced photonic chips, where ML-OFCs could be monolithically integrated with SOAs and other photonic components, all originated from the same QD-based epi-wafer.

Non-linear dynamics of multimode semiconductor lasers: dispersion-based phase instability and route to single-frequency operation.

Emission dynamics of a multimode broadband interband semiconductor laser have been investigated experimentally and theoretically. Non-linear dynamics of a III-V semiconductor quantum well surface-emitting laser reveal the existence of a modulational instability, observed in the anomalous dispersion regime. An additional unstable region arises in the normal dispersion regime, owing to carrier dynamics, and has no analogy in systems with fast gain recovery. The interplay between cavity dispersion and phase sensitive non-linearities is shown to affect the character of laser emission with phase turbulence, leading to regular self-excited oscillations of mode intensity, self-mode locking, and single-frequency emission stabilized by spectral symmetry breaking. Such physical behavior is a general phenomenon for any laser with a slow gain medium relative to the round trip time, in the absence of spatial inhomogeneities.

Dietary Fat Exacerbates Liver Disease in Bile Duct-Ligated Rats

Previous studies have suggested that the mortality of bile duct-ligated (BDL) rats is related to the amount of dietary fat consumed. We investigated the influence of dietary fat concentration on liver disease in BDL rats. Groups of rats were fed for 4 wk either a low fat diet (LF, 0.92 kJ/g; 3% of total energy from fat), a high fat diet (HF, 1.07 kJ/g; 30% fat), a high fat diet with energy density equivalent to that of the LF diet (HFIB, 0.92 kJ/g; 30% fat) or a diet based on the composition of commercial nonpurified diets (COMP, 0.90 kJ/g; 10% fat). Energy intake, body weight gain, plasma biochemical indices and hepatic histology were compared in BDL and sham-operated control rats. Bile duct-ligated animals consuming the LF diet showed a faster recovery of energy intake and greater body weight gain following surgery than did BDL animals fed the other three diets. Plasma alkaline phosphatase activity was significantly greater in BDL animals fed either of the high fat diets than in those fed the LF or COMP diet starting 1 and 2 wk, respectively, after surgery. Hepatic fibrosis and bile duct proliferation at d 28 post-ligation were greater in rats fed a 30% fat (HF) diet than in rats fed the 3% fat (LF) diet. These results suggest that dietary fat concentration can influence the severity of liver dysfunction in extrahepatic biliary obstruction.

Gain recovery dynamics in active type-II semiconductor heterostructures

Type-II heterostructures as active layers for semiconductor laser devices combine the advantages of a spectrally broad, temperature stable, and efficient gain with the potential for electrical injection pumping. Their intrinsic charge carrier relaxation dynamics limit the maximum achievable repetition rates beyond any constraints of cavity design or heat dissipation. Of particular interest are the initial build up of gain after high-energy injection and the gain recovery dynamics following depletion through a stimulated emission process. The latter simulates the operation condition of a pulsed laser or semiconductor optical amplifier. An optical pump pulse injects hot charge carriers that eventually build up broad spectral gain in a model (Ga,In)As/GaAs/Ga(As,Sb) heterostructure. The surplus energies of the optical pump mimic the electron energies typical for electrical injection. Subsequently, a second laser pulse tuned to the broad spectral gain region depletes the population inversion through stimulated emission. The spectrally resolved nonlinear transmission dynamics reveal gain recovery times as fast as 5 ps. These data define the intrinsic limit for the highest laser repetition rate possible with this material system in the range of 100 GHz. The experimental results are analyzed using a microscopic many-body theory identifying the origins of the broad gain spectrum.

2. Weight Gain and Psychological Recovery in Patients with Atypical Anorexia Nervosa: Findings from the Study of Refeeding to Optimize iNpatient Gains (StRONG)

2. Weight Gain and Psychological Recovery in Patients with Atypical Anorexia Nervosa: Findings from the Study of Refeeding to Optimize iNpatient Gains (StRONG)

Pseudo-static tests of reinforced concrete pier columns confined with pre-tensioned superelastic shape memory alloy wires

To enhance the seismic performance and self-centring capacity of reinforced concrete (RC) pier columns, a novel self-centring shape memory alloy (SMA) wire-confined–RC-composite column was proposed and experimentally tested in this study. One common RC-pier specimen and eight SMA-wire-constrained–RC-pier specimens were constructed for experimentation. The effects of the axial-compression ratio, SMA pre-strain level, and SMA configuration rate on the seismic performance of the RC piers were investigated. The failure mode, hysteresis curve, skeleton curve, bearing capacity, ductility, stiffness, energy-dissipation capacity, and self-centring capacity of the novel column are illustrated and discussed. The test results show that, compared with the shear failure of normal RC-pier columns, the failure mode of the SMA-wire-confined–RC-pier columns were transformed into bending failure or bending-shear failure with certain ductility. With an increase in the SMA-wire configuration rate, the shear bearing capacity, ductility, energy-dissipation capacity, and self-centring capacity of the RC-pier columns were improved. Pre-tensioned SMA wires were attributed to the earlier activation of force restraint and provided a greater binding force, providing more shear restraint and improving the deformation capacity of the core concrete. The ductility and bearing capacity of the strengthened specimens decreased with an excessively large axial-compression ratio; however, they were higher than those of unreinforced specimens. The research results provide new ideas and an experimental basis for the external strength gain and rapid recovery of existing RC piers after an earthquake.

Direct measurement of an ultrafast sub-bandwidth-limited signal in mid-infrared quantum cascade lasers

Frequency comb lasers with fast gain recovery times naturally favor the emission of frequency mod-ulated periodic signals, which are useful for multiple applications in spectroscopy and communications. Models and phase measurements predict an ultrashort strong intensity spike at the instantaneous frequency discontinuity of the cavity cycle. Here we experimentally study the ultrashort spike of fast-gain frequency modulated combs through direct upconversion sampling, and measure a width below that of a transform-limited pulse. Specifi-cally, we demonstrate a mid-infrared quantum cascade laser which inherently lases in a frequency modulated comb and produces a spike with full-width at half-maximum below 600 fs, which is below the Fourier-limit derived from the corresponding spectrum. We believe these ultrashort spikes can be highly beneficial for sub-bandwidth time domain measurements. Using mean-field theory based simulations, we confirm the occurrence of such features as well as further optimize our system for going even further below the Fourier-limit.

Enhanced Catalytic Palladium Embedded Inside Porous Silicon for Improved Hydrogen Gas Sensing

In this work, we reported on room temperature porous silicon (PS) and embedding PS using simple and economical techniques of electrochemical etching and thermal evaporation. The PS substrate was prepared using the technique of electrochemically etching the n-type Si (100) wafer at a constant current density of 10 mA/cm2 for 10 min under the illumination of incandescent white light. After PS formation, Ge pieces were thermally evaporated onto the two PS substrates in a vacuum condition. This was then followed by the deposition of the ZnO layer onto the Ge/PS substrate by the same method using commercial 99.9% pure ZnO powders. The three samples were identified as PS, Ge/PS and ZnO/Ge/PS samples, respectively. Pd finger contacts were deposited on the PS and embedding PS (Ge/PS and ZnO/Ge/PS) to form Pd on PS hydrogen sensors using RF magnetron sputtering. SEM and EDX suggested the presence of substantial Ge and ZnO inside the uniform circular pores for Ge/PS and ZnO/Ge/PS samples, respectively. Raman spectra showed that good crystalline Ge and ZnO nanostructures embedded inside the pores were obtained. For hydrogen sensing, Pd on ZnO/Ge/PS Schottky diode exhibited a dramatic change of current after exposure to H2 as compared to PS and Ge/PS devices. It is observed that the sensitivity increased exponentially with the hydrogen flow rate for all the sensors. The ZnO/Ge/PS showed more sensitivity towards H2 than that of PS and Ge/PS especially at high flow rate of H2 with higher current gain (69.11) and shorter response (180 s) and recovery times (30 s).

Ultrafast Buildup Dynamics of Terahertz Pulse Generation in Mode-Locked Quantum Cascade Lasers

Ultrashort terahertz pulse generation is essential for a range of proven terahertz applications, from time-resolved spectroscopy of fundamental excitations to nondestructive testing and imaging. Recently, it has been shown that semiconductor-based terahertz quantum cascade lasers (QCLs) can be used to generate pulses as short as a few picoseconds through active mode locking. However, further progress for subpicosecond and high peak power pulse generation is hampered by poor knowledge on how the electric field actually forms in these lasers. Here, we theoretically and experimentally show the amplitude- and phase-resolved buildup of pulse generation through active mode locking, from initiation of pulse generation to the nanosecond steady state. The experimental results, using an ultrafast coherent seeding technique to probe the laser from femtosecond to nanosecond time scales, are in full agreement with the theoretical calculations based on a theoretical model using multimode reduced rate equations. In particular, we show that the electric field buildup to achieve short pulse operation is extremely fast, requiring only a few photon round trips, owing to the ultrafast gain dynamics of the lasers. Further, this shows a gain recovery time of the order of a few picoseconds, an order of magnitude smaller than the photon round-trip time, highlighting that terahertz QCLs are categorically class-A lasers. This demonstration marks an important formulism for future progress towards exploring the ultrafast pulse generation buildup dynamics of these complex semiconductor lasers.

Dynamic power and chirp measurements of a quantum dash semiconductor optical amplifier amplified picosecond pulses using a linear pulse characterization technique

Quantum Dash (QDash) SOAs are of interest as an alternative to quantum dot SOAs, since they have some dot-like properties and are easier to fabricate such as to operate in the 1550 nm region, albeit with longer gain recovery times in the range of hundreds of picoseconds. QDash-SOAs are promising core component is optical subsystems for applications in next generation coherent communications and optical signal processing, which often involve pulse amplification. It is of interest to measure both the dynamic power, phase and associated chirp and thereby the spectrum of typical QDash-SOA amplified pulses. Non-linear measurement techniques are appropriate for pulsewidths less than 5 ps but have low sensitivity for wider pulsewidths often encountered in practice. This paper describes the use of a modified linear pulse characterization technique to measure the dynamic power and phase of the identical pulses of a QDash-SOA amplified 20 GHz repetition rate 19 ps pulsewidth pulse train. The pulse chirp and power from which the pulse chirp and pulse train power spectrum is calculated. The amplified pulse structure is strongly dependent on the amplifier gain and the input pulse shape and energy.

Antioxidative potential and ameliorative effects of green lentil (Lens culinaris M.) sprouts against CCl4-induced oxidative stress in rats.

The present study is aimed to investigate the antioxidative potential and ameliorative effects of Lens culinaris Medikus sprouts hydroalcoholic extract (LSHE) on CCl4-induced oxidative stress in rats. The research has been carried out in two successive stages. Firstly, the highest phenolic content and antioxidant activity of L. culinaris sprouts were assessed at 20 ± 1°C and 90-93% RH during sprouting. Total phenolic content (TPC), total carotenoids (TC), total flavonoids (TF), total flavonols (TFL), DPPH-RSA, and vitamin C contents of L. culinaris seeds and 6-days sprouts were determined. Subsequently, phenolics by HPLC analysis of L. culinaris seeds, 3rd and 6th-day sprouts were identified and quantified. Results indicated that 6th-day sprouts contained considerable phenolics with superior antioxidant capacity, thus selected to be examined for biological activity in a rat's module consisting of five groups. G1, normal rats orally received distilled water. G2 received 1.0 mL kg-1 of CCl4 and olive oil (1:1) intraperitoneally (i.p.) twice a week. G3 received CCl4 (i.p.) and 50 mg GAE kg-1 of LSHE daily/orally. G4 received CCl4 (i.p.) 100 mg kg-1 of LSHE orally/daily. G5 (reference group) treated by intramuscular injection (i.m.) of vit. E+Selenium (Vit. E+Se, 50 mg kg-1 twice a week). The weight gain, relative weight of organs, hypoglycemic and hypolipidemic efficiencies, liver's and kidneys' functions, and antioxidant biomarkers were examined. LSHE enhanced the weight gain recovery % and significantly reduced fasting blood glucose. The hypolipidemic effect of LSHE was dramatically reduced triglycerides (TG), total cholesterol (CHO), high- and low-density lipoproteins (HDL-c and LDL-c), and very-low-density lipoproteins (VLDL-c). Administration of 50 and 100 LSHE mg kg-1 ameliorated liver and kidney function in dose-dependent manure. Intriguingly, LSHE considerably reduced malondialdehyde (MDA) while significantly raising reduced glutathione (GSH), superoxide dismutase (SOD), and catalase (CAT) in a dose-dependent manner. In conclusion, biochemical examinations confirmed the therapeutic efficacy of LSHE as a functional product. It encouraged us to recommend L. culinaris sprout production for attenuating hepatotoxicity and nephrotoxicity, as well as being beneficial and profitable for controlling oxidative stress complications.