Strong Gain Saturation Research Articles

Probing the ultrafast gain and refractive index dynamics of a VECSEL

Typically, strong gain saturation and gain dynamics play a crucial role in semiconductor laser mode-locking. While there have been several investigations of the ultrafast gain dynamics in vertical-external-cavity surface-emitting lasers (VECSELs), little is known about the associated refractive index changes. Yet, such refractive index changes do not only have a profound impact on the pulse formation process leading to self-phase modulation, which needs to be compensated by dispersion, but they are also of particular relevance for assessing the feasibility of Kerr-lens mode-locking of VECSELs. Here, we measure both refractive index as well as gain dynamics of a VECSEL chip using the ultrafast beam deflection method. We find that, in contrast to the gain dynamics, the refractive index dynamics is dominated by an instantaneous (∼100 fs) and a very slow component (∼100 ps). The time-resolved measurement of nonlinear refraction allows us to predict a pulse-length dependent, effective nonlinear refractive index n2,eff, which is shown to be negative and on the order of 10−16m2/W for short pulse lengths (∼100 fs). It becomes positive for large excitation fluences and large pulse lengths (few ps). These results agree with some previous reports of self-mode-locked VECSELs for which the cavity design and pulse properties determine sign and strength of the nonlinear refractive index when assuming Kerr-lens mode-locking.

Entanglement and Photon Anti-Bunching in Coupled Non-Degenerate Parametric Oscillators.

We analytically and numerically show that the Hillery-Zubairy’s entanglement criterion is satisfied both below and above the threshold of coupled non-degenerate optical parametric oscillators (NOPOs) with strong nonlinear gain saturation and dissipative linear coupling. We investigated two cases: for large pump mode dissipation, below-threshold entanglement is possible only when the parametric interaction has an enough detuning among the signal, idler, and pump photon modes. On the other hand, for a large dissipative coupling, below-threshold entanglement is possible even when there is no detuning in the parametric interaction. In both cases, a non-Gaussian state entanglement criterion is satisfied even at the threshold. Recent progress in nano-photonic devices might make it possible to experimentally demonstrate this phase transition in a coherent XY machine with quantum correlations.

Mode instabilities in Yb:YAG crystalline fiber amplifiers.

Mode instabilities (MI) threshold in the Yb:YAG crystalline fiber amplifier is simulated by a full numerical model. The propagation of signal fields is simulated by the finite-difference beam-propagation method combined with the rate equations, and the time-dependent heat equation is solved by the alternating-direction-implicit method. Considering the strong temperature-dependent laser performance of Yb:YAG, an iterative method is applied to reach the steady state of Yb:YAG, the crystalline fiber amplifier, before the simulation of MI behavior. The simulated MI thresholds in Yb:YAG crystalline fiber amplifiers are found to be at least 28 times of those in Yb-doped silica-glass fiber amplifiers, up to tens of kilowatts. Simulation results show that, in addition to the expected higher thermal conductivity and lower thermo-optic coefficient, strong gain saturation also plays an important role in the high MI threshold of the Yb:YAG crystalline fiber.

Modeling of ultrashort pulse amplification with gain saturation.

A new method for the realistic modeling of ultrashort pulse amplification under conditions of strong gain saturation has been developed. Both the frequency dependence and the time dependence of the gain are properly taken into account. The method can, for example, be applied to chirped-pulse bulk and fiber amplifiers, but is not restricted to pulses with particular spectral or temporal properties.

Direct amplification of a nanosecond laser diode in a high gain diode-pumped Nd:YVO_4 amplifier

We demonstrate that Nd:YVO₄ can efficiently amplify a nanosecond laser diode in a very simple double-pass configuration. Based on longitudinal pumping with a high brightness fiber-coupled laser diode at 808 nm (60 W, 100 μm, 0.22 NA) and a low Nd-doped (0.2%) temperature controlled Nd:YVO₄ we achieved an optical gain of 62 dB with very low (<2%) parasitic laser emission and an average output power of 10 W. At 15 kHz, we observed a strong gain saturation dynamic resulting in a pulse duration reduction from 100 to 3.5 ns. This effect enhances the peak power by a factor of 18 (130 kW) with an energy of 620 μJ.

Analysis of Gain-Switching Characteristics Including Strong Gain Saturation Effects in Low-Dimensional Semiconductor Lasers

The effects of gain nonlinearities on gain-switched short-pulse-generation characteristics are analyzed via rate equations assuming a nonlinear-gain model including a gain saturation parameter gs to quantitatively describe the strong gain-saturation nonlinearity in low-dimensional semiconductor lasers at high carrier densities. It was found that the minimum pulse width and the delay time are mainly determined by gs rather than a differential gain coefficient g0 and a gain compression factor ε. By tracing the temporal evolution of carrier density, photon density, and material gain during gain switching, distinctly different effects of gs, ε, and cavity lifetime τp on pulse generation were clarified.

Analysis of Gain-Switching Characteristics Including Strong Gain Saturation Effects in Low-Dimensional Semiconductor Lasers

The effects of gain nonlinearities on gain-switched short-pulse-generation characteristics are analyzed via rate equations assuming a nonlinear-gain model including a gain saturation parameter gs to quantitatively describe the strong gain-saturation nonlinearity in low-dimensional semiconductor lasers at high carrier densities. It was found that the minimum pulse width and the delay time are mainly determined by gs rather than a differential gain coefficient g0 and a gain compression factor ε. By tracing the temporal evolution of carrier density, photon density, and material gain during gain switching, distinctly different effects of gs, ε, and cavity lifetime τp on pulse generation were clarified.

Experimental verification of soliton-like pulse-shaping mechanisms in passively mode-locked VECSELs

We present a detailed experimental study on the influence of dispersion on the pulse formation in passively modelocked vertical-external-cavity surface-emitting lasers (VECSELs). We have demonstrated that the shortest pulse duration requires slightly positive dispersion to balance the nonlinear phase shift induced by strong semiconductor gain and absorber saturation. This is in contrast to soliton modelocking in ion-doped solid-state lasers - but similarities remain and will be discussed. Our results are in good qualitative agreement with numerical simulations and confirm the quasi-soliton modelocking mechanism of ultrafast VECSELs.

Time-Domain Modeling of Ultralong Semiconductor Optical Amplifiers

An advanced numerical model for simulating ultralong semiconductor optical amplifiers (UL-SOAs) is presented. The model specifically accounts for the very strong gain saturation and the ultrafast gain dynamics in UL-SOAs. Due to the UL-SOA's length, the weak but fast intraband effects can strongly interact with the input signals. For this reason, UL-SOAs have a tremendous four-wave mixing (FWM) efficiency even for large wavelength detunings. Therefore, it is necessary that the UL-SOA model is a time-domain model so that all FWM products and their interaction with other FWM products are automatically calculated. These FWM products need to be considered in simulations, because they contribute to the UL-SOA's saturation in addition to amplified spontaneous emission. Moreover, due to the time-domain modeling, pseudorandom bit sequence signals can be applied to the model for simulating telecommunication applications. Other aspects that should be considered when modeling UL-SOAs are the gain parametrization and the ultrafast gain dynamics. The wavelength-dependent material gain needs to be properly represented close to the transparency, since most of the device is deeply saturated. The gain dynamics have to be dynamically modeled with rate equations automatically incorporating the limited bandwidths of the different nonlinear gain effects. Although usually not considered for short SOAs, the implementation of weak nonlinear effects like free carrier absorption and two-photon absorption is important due to the long interaction length in UL-SOAs.

Saturated X-ray lasing in Ni-like Sn at 11.9 nm using the GRIP scheme

We report on gain-saturated operation of the 11.9 nm X-ray lasing line in Ni-like Sn using the grazing-incidence pumping scheme (GRIP). The experiments were done with 2-ps duration pump pulses and energies up to 5 J. Strong gain saturation with multi-microjoule output was measured for the Sn laser at a grazing angle of 30° and a pump pulse energy of 3 J. This was achieved with a 4.5%, 2-ps duration prepulse 2.4 ns ahead of the main pulse and also incident at grazing incidence. Increasing laser output was observed at GRIP angles up to 45°. At this angle, the minimum energy required for saturated lasing was determined as ∼2 J.

Amplification of kW peak power femtosecond pulses in single quantum well InGaAs tapered amplifiers

The amplification of ps and fs pulses with peak powers of up to 4.5kW has been investigated in a single quantum well InGaAs tapered amplifier. The pulses with durations of 100fs or 2ps were generated by a modelocked titanium–sapphire laser. The amplified pulses indicate strong gain saturation and carrier generation due to photon absorption in the laser active region which causes a temporal broadening of the amplified pulses as well as modifications of the optical spectrum. The gain recovery time was measured by a pump–probe experiment. The experimental results are analyzed with respect to the sub-ps gain dynamics which is described by a relaxation time approximation.

Mode competition in multi-core fiber amplifier

3D beam propagation numerical code and a newly developed optical mode solver were implemented for studies on the recently experimentally realized Yb fiber amplifier with 7-core hexagonal structure. It was found that the launched seven single-frequency beams matched with the cores converge to a wave field with permanently diminishing space beating patterns, but the result is sensitive to phases of separate beams. Namely, at a spread of phases of individual beams within a few tenths of radian, the convergence of the wave field to the in-phase mode is observed, while at phases random spread on the order of π the out-of-phase mode is dominating. Such behavior is observed at strong gain saturation in the cores. Both effects take place even if the guiding index of the cores fluctuates within a limited range. The mode solver predicts that the in-phase mode has the minimal small signal gain, and the gain of the out-of-phase mode is maximal one. This fact proves that there is no strict correlation between modal gains and amplification in strong saturated multi-core fiber. The smaller core index step significantly improves the convergence of the amplified radiation to the in-phase mode. An approach to the waveguide structure tailoring is described which assures the single in-phase mode stability.

The generation of high efficiency and high quality andhigh stability parametric amplified light

The effects of a series of parameters such as injected signal intensity , pump intensity, crystal length have been analyzed in Optical parametric amplification(OPA). The gain saturation and pump depletion were taken into account. The stability of an OPA system was investigated in both the small signal injection and the large signal injection. Experimental investigations of OPA based on two noncollinear phase-matched LBO OPA under the small signal injection and the large signal injection have been carried out，and the experimental data are in agreement with the theoretical results. The theoretical and experimental results all demonstrated that there exists a strong gain saturation region where parametric light with high efficiency and high quality and high stability can be obtained by properly choosing relevant system parameters.

Experimental study of the optical gain and loss in InAs/GaInSb interband cascade lasers

Optical gain and loss have been measured in type II InAs/GaInSb interband cascade lasers operating in the 3.4–3.6 μm wavelength range. The maximum temperature of cw operation was found to be limited by strong gain saturation due to active region overheating, while the temperature increase of the total optical loss was relatively small. In devices with a longer lasing wavelength and a thinner substrate-side cladding layer, a strong periodic modulation of the optical gain spectra was observed. This effect is consistent with resonant optical leakage into the substrate.

Optical gain and saturation of photoexcited type-II superlattice

The stimulated emission spectra of photoexcited type-II superlattice was measured at room temperature, 77 and 1.8 K as a function of the illuminated stripe length and excitation intensity. Based on these data, the gain spectra and gain saturation behavior were obtained using Agrawal's model for the analysis. Both the spontaneous photoluminescence spectra and the stimulated emission spectra were used in order to analyze the spectral shifts. We used the Kronig-Penney model to approximate the energy level diagram of the samples. We report here for the first time about: (i) a pronounced gain which is a result of X-type radiative recombinations, (ii) a red-shifted emission under CW excitation at all temperatures and (iii) a strong signal gain saturation as a function of temperature and pumping-power.

Chirped pulse amplification in KrF lasers

The performance of a grating scheme for chirped pulse amplification and recompression to 150 fs in a KrF laser is reported. The effects of strong gain saturation and spectral distortion are analysed. The method is shown to have high potential for generating maximum radiance and high contrast ratio in large aperture KrF lasers.

Design of 13-μm GaInAsP surface-emitting lasers for high-bandwidth operation

Important design considerations for high-speed GaInAsP 1.3-microm surface-emitting lasers are described. Modified rate equations for surface-emitting lasers and a small-signal analysis are used to calculate the frequency response versus the mirror reflectivity. For the structure analyzed, the results predict optimum reflectivities of 98% for highfrequency operation and maximum quantum efficiency. Strong gain-saturation effects are predicted owing to the high photon densities that occur in these devices. As the mirror thickness increases, the frequency response of the laser reduces greatly. The effect of heating is found to be important and is included in the analysis.

Generation of picosecond pulses in dye lasers pumped by single pulses

The dynamics of the generation of picosecond radiation pulses by dye lasers operating in the strong gain saturation regime is investigated theoretically and experimentally. Systems are described which are characterized by reliability of operation and simplicity of construction. Generation of bandwidth-limited 3.5 psec duration pulses is reported.