Gain Saturation Research Articles

Self-steepening and intrapulse Raman scattering in the presence of nonlinear gain and its saturation

In this paper we present the results of a detailed numerical investigation of the influence of the self-steepening effect on the parameters of the Raman dissipative solitons in the presence of nonlinear gain and its saturation. It has been established that when the effect of self-steepening prevails under the intrapulse Raman scattering it leads to nonlinear changes in the parameters of the Raman dissipative solitons. Comparing the impact of the intrapulse Raman scattering and self-steepening in the presence of nonlinear gain and its saturation on the dissipative solitons, the following differences have been identified: a much slighter change of the amplitude in the presence of self-steepening; different types of nonlinear dependencies of the frequency and velocity in the presence of intrapulse Raman scattering; and a substantial increase of the velocity under the influence of self-steepening. We have found that under the influence of the saturation of the nonlinear gain the observed nonlinear changes of the parameters of Raman dissipative solitons in the presence of the self-steepening effect are weaker. There has been observed a reduction of the maximum values of the amplitude and velocity as well as the maximum absolute values of the frequency of the Raman dissipative solitons.

Amplification of electron-mediated spin currents by stimulated spin pumping

Stimulated emission is a process during which an atomic system gives away energy to create a coherent photon. It is fundamental to the operation of the optical amplifier. Here we propose two mechanisms for amplifying AC spin currents in a solid-state magnetic medium by a stimulated spin pumping process. The first is synchronous and consists of phase-locked pulses that perturb a precessing magnetic moment. The second is asynchronous and is driven by DC spin currents. The amplification relies on a non-adiabatic interaction taking place in a ferromagnetic medium in which the magnetic moment emits spin angular momentum in the form of spin current before equilibrating with the environment. The pumped spin current amplifies or absorbs the injected AC spin current mimicking the operation of the optical gain medium as readily seen from the gain saturation profiles. The mechanisms we propose are a first step towards a realistic spin current amplifier.

Effect of gain saturation on mode capacity of EDFA

Gain saturation in a mode-division-multiplexed erbium-doped fiber amplifier (MDM-EDFA) has a significant impact on system capacity due to complex mode competition. In this paper, the gain saturation of MDM-EDFA at single and multiple wavelengths is investigated. The gain saturation occurs as a certain negative linear correlation between EDF length and pump power. The saturation performance tends to be flat beyond 49 mode groups transmission. In a “WDM + MDM” system with 36 × 36 channels, the normalized root mean square value of the saturation gain is lower than 3. The results provide a theoretical basis for the application of the upcoming “WDM + MDM” system.

Kerr-Lens Mode-Locking: Numerical Simulation of the Spatio-Temporal Dynamics on All Time Scales

We present a complete numerical analysis and simulation of the full spatio-temporal dynamics of Kerr-lens mode-locking in a laser. This dynamic, which is the workhorse mechanism for generating ultrashort pulses, relies on the intricate coupling between the spatial nonlinear propagation and the temporal nonlinear compression. Our numerical tool emulates the dynamical evolution of the optical field in the cavity on all time-scales: the fast time scale of the pulse envelope within a single round trip, and the slow time-scale between round-trips. We employ a nonlinear ABCD formalism that fully handles all relevant effects in the laser, namely—self focusing and diffraction, dispersion and self-phase modulation, and space-dependent loss and gain saturation. We confirm the validity of our model by reproducing the pulse-formation in all aspects: The evolution of the pulse energy, duration, and gain during the entire cavity buildup, demonstrating the nonlinear mode competition in full, as well as the dependence of the final pulse in steady state on the interplay between gain bandwidth, dispersion, and self-phase modulation. The direct observation of the nonlinear evolution of the pulse in space-time is a key enabler to analyze and optimize the Kerr-lens mode-locking operation, as well as to explore new nonlinear phenomena.

A G-Band Broadband Continuous Wave Traveling Wave Tube for Wireless Communications.

Development of a G-band broadband continuous wave (CW) traveling wave tube (TWT) for wireless communications is described in this paper. This device provides the saturation output power over 8 W and the saturation gain over 30.5 dB with a bandwidth of 27 GHz. The maximum output power is 16 W and the bandwidth of 10 W output power is 23 GHz. The 3 dB bandwidth is greater than 12.3% of fc (center frequency). The gain ripple is less than 10 dB in band. A pencil beam of 50 mA and 20 kV is used and a transmission ratio over 93% is realized. The intercept power of the beam is less than 70 W and the TWT is conduction cooled through mounting plate and air fan, which makes the device capable of operating in continuous wave mode. A Pierce’s electron gun and periodic permanent magnets are employed. Chemical vapor deposition diamond disc is used in the input and output radio frequency (RF) windows to minimize the loss and voltage standing wave ratios of the traveling wave tube. Double stages deeply depressed collector is used for improving the total efficiency of the device, which can be over 5.5% in band. The weight of the device is 2.5 kg, and the packaged size is 330 mm × 70 mm × 70 mm.

Charging up studies in thick Gas Electron Multipliers

Gaseous ionization detectors that have insulating media exposed to the active gas volume have issues related to charging up of the insulators during the course of its use[1]. The time-dependent variation of detector response in hole-based Micro-Pattern Gaseous Detectors (MPGDs), especially THick Gas Electron Multipliers (THGEMs), is one of the challenging problems that has been attributed to “charging up” and “charging down”. Experimental studies of stabilization of THGEM gas gain with time in argon-based mixtures under various experimental conditions will be presented here. Effects of different sources with varying irradiation rates on the gain saturation process have been studied. It has been observed that low-rate source shows two-step gain stabilization phenomena, one short-term saturated gain, another long-term saturated gain, whereas high-rate source shows just one-step gain saturation. While this two-step stabilization has been attributed to the charging up of the rim by earlier studies, its effect seems to be subdued for high-rate irradiation according to the present studies. The results provide an insight into the dynamics of gain saturation in THGEMs.

Tunable degree of polarization in a figure-8 fiber laser

We experimentally study a fiber loop laser in a figure-8 configuration and explore the dependency of the degree of polarization on controlled parameters. To account for the experimental observations, a mapping is derived to evaluate the polarization time evolution. The nonlinearity induced by the Kerr effect and gain saturation gives rise to rich dynamics. We find that the degree of polarization can be increased by tuning the system into a region where the mapping has a locally stable fixed point.

Exploration of a Kilowatt-Level Terahertz Amplifier Based on Higher-Order Mode Interaction

The parameters for a kilowatt-level high-power terahertz amplifier were explored based on the higher-order mode and extended-interaction mechanism. It has been demonstrated that the antisymmetric electrical field mode (TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> ) has a unique advantage in achieving high power. Physical factors on power extraction were studied. In particular, the heavily reduced <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}_{{0}}$ </tex-math></inline-formula> has a great effect on the coupling characteristic as well as the output power. It is found that the critical coupling state, i.e., <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}_{e} = {Q}_{{0}}$ </tex-math></inline-formula> , defines an upper limit of the power that can be extracted from the output circuit. The design of a complete interaction circuit was accomplished. The particle-in-cell (PIC) simulations showed that a saturated power of 1.2 kW can be achieved at 220 GHz with a voltage of 45 kV and a total current of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times0.6$ </tex-math></inline-formula> A where two beams were used. The saturated gain is over 30 dB with a 3-dB bandwidth of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 400$ </tex-math></inline-formula> MHz.

Gain Saturation Modified Quantum Noise Effect on Preparing a Continuous-Variable Entanglement

We examine the gain saturation effect in non-Hermitian systems of coupled gain–loss waveguides and whispering-gallery-mode microresonators, through which a continuous-variable (CV) entanglement of light fields is generated. Here, we consider squeezed vacuum inputs for coupled waveguide setup and coherent drive for coupled microresonators, and study the influence from the saturation of the used optical gain. Unlike the ideal situation without gain saturation, it is possible to generate stabilized entanglement measured by logarithmic negativity under gain saturation. Both types of setups realize steady CV entanglement, provided that the gain saturation is sufficiently quick. Particularly, with the coupled microresonators which are pumped by coherent drive, the created CV entanglement is actually out of the gain noise with a squeezing characteristic, under the condition of fast saturation of the initial optical gain.

700 W single-frequency all-fiber amplifier at 1064 nm with kHz-level spectral linewidth

In this work, a single-frequency fiber amplifier with output power of 703 W was demonstrated at 1,064.4 nm in an all-fiber configuration. Cascaded Yb3+-doped fiber structure with different dopant concentration and hybrid 915/976 nm pump scheme were employed in power scaling stage to improve the gain saturation for higher transverse mode instability threshold. An overall optical efficiency of 67.5% was achieved at the maximum output power and the M2 was measured to be ∼1.4. A spectral linewidth of 2 kHz was obtained from the 703-W single-frequency laser. To the best of our knowledge, this is the first time that a single-frequency all-fiber amplifier with kHz-level spectral linewidth is achieved at such high output power.

Over-20-octaves-bandwidth ultralow-intensity-noise 1064-nm single-frequency fiber laser based on a comprehensive all-optical technique.

An over-20-octaves-bandwidth ultralow-intensity-noise 1064-nm single-frequency fiber laser (SFFL) is demonstrated based on a comprehensive all-optical technique. With a joint action of booster optical amplifier (BOA) and reflective Yb-doped fiber amplifier (RYDFA), two-fold optical gain saturation effects, respectively occurring in the media of semiconductor and fiber, have been synthetically leveraged. Benefiting from the gain dynamics in complementary time scales, i.e., nanosecond-order carrier lifetime in BOA and millisecond-order upper-level lifetime in RYDFA, the relative intensity noise (RIN) is reduced to -150 dB/Hz from 0.2 kHz to 350 MHz, which exceeds 20-octaves bandwidth. Remarkably, a maximum suppressing ratio of >54 dB is obtained, and the RIN in the range of 0.09-10 GHz reaches -161 dB/Hz which is only 2.3 dB above the shot-noise limit. This broad-bandwidth ultralow-intensity-noise SFFL can serve as an important building block for squeezed light generation, space laser communication, space gravitational wave detection, etc.

Influence of Gain Saturation Effect on Transverse Mode Instability Considering Four-Wave Mixing

Transverse mode instability (TMI) has been recognized as onse of the primary limiting factors for the average power scaling of high-brightness fiber lasers. In this work, a static model of the TMI effect based on stimulated thermal Rayleigh scattering (STRS) is established while considering the four-wave mixing (FWM) effect. The focus of the model is to theoretically investigate the TMI phenomenon and threshold power dominated by FWM. The gain saturation effect and fiber laser system parameters, such as seed power, pumping direction, and core numerical aperture, which have not been considered in the previous perturbation theory model, are also investigated. This work will enrich the perturbation theory model and extend its application scope in TMI mitigation strategies, providing guidance for understanding and suppressing TMI.

Towards optimal conversion efficiency of Brillouin random fiber lasers in a half-open linear cavity.

We proposed and demonstrated an unprecedented high-efficiency Brillouin random fiber laser (BRFL) by fiber length optimization in a half-open linear cavity. In terms of the trade-off between Brillouin gain saturation and weak distributed Rayleigh feedback strength, optimal laser efficiency associated to proper fiber length in a BRFL was theoretically predicted. As a proof-of-concept, a unidirectional-pumped BRFL with a half-open linear cavity was experimentally conducted, in which a fiber Bragg grating at one end of gain fiber served as a high-reflection mirror while Rayleigh scattering enabled distributed feedback for random lasing resonance. Results show that the optimal fiber length of ∼3.4 km in the BRFL offers sufficient Rayleigh scattered random feedback whilst alleviating the Brillouin gain saturation to a large extent. Consequently, an optimal laser efficiency of 77.0% in the BRFL was experimentally demonstrated, which reaches the state-of-the-art high record. Laser characteristics, including the linewidth, statistics and frequency jitter were also systematically investigated. It is believed that such efficient BRFL could provide a promising platform for inspiring new explorations of laser physics as well as potentials in long-haul coherent communication and fiber-optic sensing.

Beyond the Petermann limit: Prospect of increasing sensor precision near exceptional points

Experiments near the lock-in region in maximally dissipative non-Hermitian systems, e.g., conventional laser gyroscopes near the deadband, have run up against the Petermann limit, where excess noise exactly cancels any scale-factor enhancement resulting in no overall enhancement in precision. As a result, one might be tempted to conclude that exceptional points (EPs) generally cannot be used to increase the precision of laser sensors. Indeed, using a linear eigenmode analysis we show that the Petermann limit applies not just to maximally dissipative systems, but for any type of EP, owing to the fact that EPs are rotationally invariant. It turns out, however, that this restriction comes from the assumption of linearity. We find that nonlinearity breaks the rotation symmetry such that the different types of EPs are no longer equivalent above threshold. In particular, EPs in conservatively coupled systems can lead to an increase in the fundamental precision beyond the Petermann limit as a result of gain saturation. Importantly, we find that only one mode lases under these conditions. We show that the beat note can be recovered by interference with an auxiliary mode, but that this has consequences for the quantum and classical noise that depend on the recovery scheme. Thus, it remains to be seen whether practical experiments can be designed that can take advantage of this enhancement.

Accurate Modeling of Transverse Mode Instability in Fiber Amplifiers

Transverse mode instability is a key limit to power scaling of high-power fiber lasers. Accurate modeling efforts have, however, been hampered by a lack of experimental data to verify a model. Recently, there have been some good experimental studies, making it possible to validate a model. In this work, we developed a model by integrating a 3D fiber amplifier and stimulated thermal Rayleigh scattering. Since we are only interested in the regime where the fundamental mode dominates, our 3D amplifier divides the core into many cylindrical shells. This limits the model to situations where bend-induced mode distortion of the fundamental mode is negligible, but it is still applicable for most practical scenarios. The benefit of this model is high computational efficiency; it can run in minutes on a PC. This 3D amplifier model considers various pumping configurations and amplified spontaneous emission. It can simulate most experimental conditions. Excellent quantitative fit to experimental data was achieved. Additional studies were also conducted to show that gain saturation is a dominating effect in understanding the observed behaviors of transverse mode instability.

Mode-locking induced by coherent driving in fiber lasers.

Mode-locking is a broad concept that encompasses different processes enabling short optical pulse formation in lasers. It typically requires an intracavity mechanism that discriminates between single and collective mode lasing, which can be complex and sometimes adds noise. Moreover, known mode-locking schemes do not guarantee phase stability of the carrier wave. Here, we theoretically propose that injecting a detuned signal seamlessly leads to mode-locking in fiber lasers. We show that phase-locked pulses, akin to cavity solitons, exist in a wide range of parameters. In that regime the laser behaves as a passive resonator due to the non-instantaneous gain saturation.

Gain saturation effects in THz quantum cascade lasers

The effect of gain saturation in quantum-cascade structures with 2–4 quantum wells per period is herein analyzed on the basis of a system of balance equations. It is shown that the nonlinearity parameter decreases with an increase in the relaxation rate of laser levels, but the total current through the structure also increases. The use of the proposed multiphoton designs leads to a decrease in the non-linearity parameter without increasing the operating current. For example, in a two-photon scheme of laser transitions with the same transition probabilities and differential gains, two times slower saturation of the gain with an increase in the photon density is achieved, which leads to a high generation efficiency than in single-photon schemes.

Anomalous Single-Mode Lasing Induced by Nonlinearity and the Non-Hermitian Skin Effect.

Single-mode operation is a desirable but elusive property for lasers operating at high pump powers. Typically, single-mode lasing is attainable close to threshold, but increasing the pump power gives rise to multiple lasing peaks due to inter-modal gain competition. We propose a laser with the opposite behavior: multimode lasing occurs at low output powers, but pumping beyond a certain value produces a single lasing mode, with all other candidate modes experiencing negative effective gain. This phenomenon arises in a lattice of coupled optical resonators with non-fine-tuned asymmetric couplings, and is caused by an interaction between nonlinear gain saturation and the non-Hermitian skin effect. The single-mode lasing is observed in both frequency domain and time domain simulations. It is robust against on-site disorder, and scales up to large lattice sizes. This finding might be useful for implementing high-power laser arrays.

Demonstration of constant-cladding tapered-core Yb-doped fiber for mitigating thermally-induced mode instability in high-power monolithic fiber amplifiers

In this work, a large-mode-area (LMA) step-index constant-cladding tapered-core (CCTC) Yb-doped fiber with a cladding diameter of ∼600 µm is successfully fabricated. The CCTC fiber has a small-core region (diameter of ∼20 µm) at both ends and a large-core region (diameter of ∼36 µm) in the middle. To prove the laser performance of the CCTC fiber, a detailed comparison experiment with conventional uniform fiber with the same effective core diameter is carried out in a multi-kW all-fiber MOPA configuration. The experimental results show that employing the CCTC fiber can effectively mitigate the thermally-induced transverse mode instability (TMI) in both co-pump and counter-pump schemes, and realize high slope efficiency and single-mode beam quality (M2∼1.30). Under the counter-pump scheme, the TMI threshold of the CCTC fiber is observed at ∼2.49 kW with a slope efficiency of 86.2%, while the uniform fiber amplifier exhibits a TMI threshold of ∼2.05 kW. The theoretical analysis based on a semi-analytical model indicates this CCTC fiber can effectively improve the TMI threshold owing to a stronger gain saturation. Our results verify the great potential of such an LMA CCTC fiber to mitigate thermal-induced TMI effect and achieve single-mode operation without sacrifice of laser efficiency in high power monolithic fiber lasers, and the further power scaling is expected by optimizing the fiber design.

Gain-switching in CsPbBr3 microwire lasers

All-inorganic perovskite microwire lasers, which have intrinsic high material gain and short cavity, especially favor the generation of ultrashort optical pulses via gain switching for various potential applications. Particularly, the ultrashort gain-switched pulses may extend perovskite microwires to previously inaccessible areas, such as ultrafast switches, and chipscale microcombs pumping souces in photonic integrated circuits. Here, we show 13.6-ps ultrashort single-mode green pulses from the gain-switched CsPbBr3 microwire lasers under femtosecond optical pumping. The gain-switching dynamics is experimentally investigated by a streak camera system. The excitation fluence dependences of pulse width, delay time and rise time of the output pulses show good agreements with the rate equation simulations with taking gain nonlinearities and carrier recombination ABC model into account. Our results reveal that perovskite microwire lasers have potential for ultrashort pulse generation, while the low transient saturated gain, which may result from the high transient carrier temperature under femtosecond pumping is a significant limitation for further pulse shortening.