Depleted Pump Regime Research Articles

Experimental investigation of SRS polarization pulling in depleted pump regime for various pump SOPs

This article presents an experimental investigation of polarization pulling based on stimulated Raman scattering induced by a continuous, fully polarized pump wave in an optical fibre. By means of probe wave states of polarization aggregation, degree of polarization and gain analysis, an assessment of all-optical polarization control, repolarization and amplification in a standard single mode fibre in depleted pump regime is presented. Polarization pulling is proved to be dependent on fibre length, pump power, and pump state of polarization, with respect to pump depletion effect, acting oppositely to the polarization pulling effect. Authors evaluate results aiming to obtain a configuration of stimulated Raman scattering parameters, which allows for the most effective polarization pulling, with respect to statistical stability of the process measured by standard deviation of polarization degree and polarization-dependent gain of the probe wave. The highest polarization attraction efficiency was obtained for 15 km, 0.97 W and the horizontal polarization state of the pump wave.

Accurate derivation of THG conversion efficiency in periodically poled nonlinear medium and optimizing conversion parameters

We have derived a novel equation to accurately predict the third harmonic generation (THG) conversion efficiency in hybrid periodically poled nonlinear medium. Our equation considers the general case that takes both depleted pump regime and phase mismatching cases, resulting in more precise predictions of efficiency. This level of accuracy is crucial for certain applications like high-power THG lasers. Moreover, accurate calculation of THG power density is essential to prevent exceeding the crystal damage threshold. We applied our equation on hybrid MgO:PPLN crystal to determine the optimal SHG region length corresponding to two different power densities, namely, 0.25 and 0.5MW/cm2. The effect of crystal temperature on efficiency was also studies. Furthermore, a comparison between the derived equation and the commonly used nondepleted pump regime equation was performed. We found that the latter equation is significantly less accurate, particularly at high power densities, with the efficiency of the depleted pump regime being 50.6% less than nondepleted one. To demonstrate the effectiveness of the equation, our results were compared with experimental data, and we observed a good agreement between them.

General solution of phase mismatched second harmonic conversion efficiency in periodically poled nonlinear medium

In this work we provide for the first time a novel and more accurate second harmonic generation (SHG) conversion efficiency equation for periodically poled nonlinear medium. We believe the equation derived in this work is more useful than equations reported in previous works which are limited because it is considering the case of either nondepleted pump regime or perfect phase matching. The equation derived in this work tackles the general case: depleted pump regime and phase mismatching which makes it important for theoretical – experimental study. We used MgO:PPLN crystal as an example to illustrate SHG conversion efficiency as function of crystal length, laser power density, crystal temperature and beam divergence. We included fundamental and second harmonic waves reflection at entrance and exit surfaces respectively and performed numerical solution using fifth-order Runge-Kutta method. The numerical results show excellent match with analytical results obtained from the derived formula. We compared our theoretical results with experimental results reported in the literature and the comparison shows a good match. Finally, we performed a comparison between depleted and nondepleted pump regime. The results show a difference of 20% between two efficiencies making nondepleted efficiency equation, which used in many theoretical – experimental comparisons, not accurate at high power densities.

Stable self-pulsing regime in a Brillouin ring fiber laser cavity

This study reports that when solving by full integration a set of coupled ordinary differential equations that describe the spatial evolution of the guided pump and Stokes powers caused by stimulated Brillouin scattering in the regime of pump depletion, a structure of regular pulses emerge from the output of a Brillouin laser cavity characterized as a stable self-pulsing regime. The dependence of this self-pulsing mechanism on the laser cavity length, the coupling factor, and the pump power is investigated revealing the existence of three regimes: no lasing, continuous, and self-pulsing. When compared to records in the scientific literature, this is the first time, to the best of the author’s knowledge, that a stable self-pulsing regime is reported theoretically. Furthermore, we also demonstrated the pumping depletion during the formation of the self-pulsing regime. These results provide not only a better understanding of the dynamics of self-pulsing regimes in a fiber laser cavity but also the possibility of unlocking the mechanisms responsible for the formation of stable pulse trains in a Brillouin fiber-ring laser.

Optical phase conjugation in backward Raman amplification

Compression of an intense laser pulse using backward Raman amplification (BRA) in plasma, followed by vacuum focusing to a small spot size, can produce unprecedented ultrarelativistic laser intensities. The plasma density inhomogeneity during BRA, however, causes laser phase and amplitude distortions, limiting the pulse focusability. To solve the issue of distortion, we investigate the use of optical phase conjugation as the seed pulse for BRA. We show that the phase conjugated laser pulses can retain focusability in the nonlinear pump depletion regime of BRA, but not so easily in the linear amplification regime. This somewhat counterintuitive result is because the nonlinear pump depletion regime features a shorter amplification distance, and hence less phase distortion due to wave-wave interaction, than the linear amplification regime.

Pump depletion in parametric down-conversion with low pump energies.

We report the efficient generation of high-gain parametric down-conversion, including pump depletion, with pump powers as low as 100 µW (energies 0.1 µJ/pulse) and conversion efficiencies up to 33%. In our simple configuration, the pump beam is tightly focused into a bulk periodically poled lithium niobate crystal placed in free space. We also observe a change in the photon number statistics for both the pump and down-converted beams as the pump power increases to reach the depleted pump regime. The experimental results are a clear signature of the interplay between the pump and the down-converted beams in highly efficient parametric down-conversion sources.

High efficiency cascaded third-harmonic generation in a quasi-periodically poled KTiOPO4 crystal

We performed and studied cascaded third-harmonic generation (THG) in a quasi-periodically poled KTP (QPPKTP) crystal allowing simultaneous phase-matching of the two cascading steps ω + ω → 2ω and 2ω + ω → 3ω. The phase-matching was achieved at the fundamental wavelength λω = 1587 nm when the QPPKTP crystal was heated to 95°C. The energy conversion efficiency reached 40% in the picosecond regime for a fundamental energy of 20 µJ that corresponds to an intensity of 1.5 GW/cm2. It is the highest value of THG efficiency ever reported to the best of our knowledge. The modeling in the case of the depleted pump regime accurately described the experiments.

Analytical approach to calculate the gain of Brillouin fiber amplifiers in the regime of pump depletion.

We provide an accurate analytic expression for the Brillouin fiber amplifiers (BFAs) gain in the regime of pump depletion. The solutions are divided into three parts. In the first part, we review the weak pump regime where the highly accurate second-order corrected undepleted-pump-approximation-based solution is adopted; in the high-gain regime where both the fiber loss and nonlinear term are nonnegligible effects and need to be accounted for along with the pump depletion; and in the saturation regime where the Stokes power is at the rear end exceeds critical power. We provide an accurate analytic expression for the BFA gain and show results of experimental validation. The presented analysis can be used to accurately predict the performance of Brillouin fiber amplifiers working in any regime.

Homogenization and Scattering Analysis of Second-Harmonic Generation in Nonlinear Metasurfaces

We propose an extensive discussion on the homogenization and scattering analysis of second-order nonlinear metasurfaces. Our developments are based on the generalized sheet transition conditions (GSTCs) which are used to model the electromagnetic responses of nonlinear metasurfaces. The GSTCs are solved both in the frequency domain, assuming an undepleted pump regime, and in the time-domain, assuming dispersionless material properties but a possible depleted pump regime. Based on these two modeling approaches, we derive the general second-harmonic reflectionless and transmissionless conditions as well as the conditions of asymmetric reflection and transmission. We also discuss and clarify the concept of nonreciprocal scattering pertaining to nonlinear metasurfaces.

Quantum-limited parametric amplification with Josephson circuits in the regime of pump depletion

Linear parametric amplification is a key operation in information processing. Our interest here is quantum-limited parametric amplification, $i.e.$, amplification of quantum signals while adding the minimum amount of noise allowed by quantum mechanics, which is essential for any viable implementation of quantum information processing. We describe parametric amplifiers based on the dispersive nonlinearity of Josephson junctions driven with appropriate tones playing the role of pumps. We discuss two defining characteristics in the architecture of these amplifiers: the number of modes occupied by the signal, idler and pump waves and the number of independent ports through which these waves enter into the circuit. The scattering properties of these amplifiers is also reviewed. The main focus of this work are the computations of the dynamic range and phase-space distributions of the fluctuations of the modes of the amplifiers.

Multifrequency Raman amplifiers.

In its usual implementation, the Raman amplifier features only one pump carrier frequency. However, pulses with well-separated frequencies can also be Raman amplified while compressed in time. Amplification with frequency-separated pumps is shown to hold even in the highly nonlinear, pump-depletion regime, as derived through a fluid model, and demonstrated via particle-in-cell simulations. The resulting efficiency is similar to single-frequency amplifiers, but, due to the beat-wave waveform of both the pump lasers and the amplified seed pulses, these amplifiers feature higher seed intensities with a shorter spike duration. Advantageously, these amplifiers also suffer less noise backscattering, because the total fluence is split between the different spectral components.

Analytical approach of Brillouin amplification over threshold.

We report on an accurate closed-form analytical model for the gain of a Brillouin fiber amplifier that accounts for material loss in the depleted pump regime. We determined the operational model limits with respect to its relevant parameters and pump regimes through both numerical and experimental validation. As such, our results enable accurate performance prediction of Brillouin fiber amplifiers operating in the weak-pump, high-gain, and saturation regimes alike.

Hybrid entanglement caused by depletion of pump on output of spontaneous parametric down converter

We consider novel method for implementation of hybrid entanglement between the states in signal/idler and pumping modes or between the states from different vector spaces which are characterized by different displacement amplitudes on output from spontaneous parametric down converter (SPDC). The entanglement is caused by the depletion of the pump wave. In general, the entangled state is defined by two parameters: squeezing and coupling ones. The generated entanglement is mainly defined by the coupling parameter. We characterize it and show the limits of applicability of the depleted pump regime. The hybrid entanglement is tested by conditional generation of maximally entangled states of two qubits and qutrits. Possibilities to enhance the hybrid entanglement and, as consequence, success probabilities of the conditional generation are considered. The optical scheme for test is realizable in practice and can do without photon number resolving detection (PNRD).

Plasma Wave Seed for Raman Amplifiers.

It is proposed to replace the traditional counterpropagating laser seed in backward Raman amplifiers with a plasma wave seed. In the linear regime, namely, for a constant pump amplitude, a plasma wave seed may be found by construction that strictly produces the same output pulse as does a counterpropagating laser seed. In the nonlinear regime, or pump-depletion regime, the plasma-wave-initiated output pulse can be shown numerically to approach the same self-similar attractor solution for the corresponding laser seed. In addition, chirping the plasma wave wavelength can produce the same beneficial effects as chirping the seed wave frequency. This methodology is attractive because it avoids issues in preparing and synchronizing a frequency-shifted laser seed.

High average power parametric wavelength conversion at 3.31-3.48 m in MgO:PPLN.

We present results of high average power mid-infrared (mid-IR) generation employing synchronized nanosecond pulsed ytterbium and erbium fiber amplifier systems using periodically poled lithium niobate. We generate greater than 6 W of mid-IR radiation tunable in wavelength between 3.31-3.48 μm, at power conversion efficiencies exceeding 75%, with near diffraction limited beam quality (M2 = 1.4). Numerical modeling is used to verify the experimental results in differing pump depletion regimes.

Non-degenerate optical parametric amplification analysis of surface plasmon polariton wave in a silver coated PPLN planar waveguide

Propagation length enhancement of surface plasmon polariton (SPP) wave in telecom c-band through nonlinear χ(2)-based nondegenerate difference frequency generation (ndDFG) process is investigated theoretically. The SPP wave is excited in a silver coated planar periodically poled lithium niobate (PPLN) waveguide. Quasi phase matching is used to compensate the phase mismatch between the interacting excited SPP and guided modes. Domain inversion with proper domain wavelength of Λ = 101.9 μm leads to nonlinear efficient mode interaction of guided↔SPP. With 31 MW/cm coupled pump intensity a SPP amplification of about 16 dB is achievable which results in enhancement of the propagation length about 11 mm. The acceptance SPP amplification bandwidth is pump power dependent. In a low pump depletion regime, a 3-dB bandwidth is about 75 nm whereas in more depletion the bandwidth becomes narrower but not less than 25 nm.

Effect of fabrication errors on multiple second-harmonic generation considering the pump depletion

Second-harmonic generation (SHG) from aperiodic optical superlattices in the regime of pump depletion is investigated, when the influence of typical fabrication errors, which can be introduced by the random fluctuation of the thickness for each domain in the simulation, is considered in accordance with the actual case. It is found that both the SHG conversion efficiencies calculated in the undepleted pump approximation (UPA) and an exact solution decrease when the fluctuation increases; however, the decreasing degree is related to the wavelength of the fundamental wave (FW), and the longer the FW wavelength, the less the decrease in the corresponding conversion efficiency. A relative tolerance with respect to SHG conversion efficiency calculated in the UPA and exact solution is defined as previously [Opt. Express21, 17592 (2013)], in which a typical model based on the relative tolerance curves was proposed to estimate the SHG conversion efficiency. The simulation results show that the relative tolerance curves are basically coincident with the standard curve when the random fluctuation is very small (typically below 1%); however, as the fluctuation increases, the relative tolerance curves exhibit a large deviation from the standard curve, and the deviation is also determined by the wavelength of the FW.

Coherence properties of high-gain twin beams

Twin-beam coherence properties are analyzed in both spatial and spectral domains at high gain, including also the regime of pump depletion. We show an increase in the size of the intensity autocorrelation and cross-correlation areas at increasing pump power, replaced by a decrease in the pump depletion regime. This effect is interpreted as a progressive loss in the mode selection occurring at high-gain amplification. The experimental determination of the number of spatiospectral modes from the measurements of the ${g}^{(2)}$-intensity autocorrelation coefficient confirms this explanation.

Direct fluorescence characterisation of a picosecond seeded optical parametric amplifier

The temporal intensity contrast of high-power lasers based on optical parametric amplification (OPA) can be limited by parametric fluorescence from the non-linear gain stages. Here we present a spectroscopic method for direct measurement of unwanted parametric fluorescence widely applicable from unseeded to fully seeded and saturated OPA operation. Our technique employs simultaneous spectroscopy of fluorescence photons slightly outside the seed bandwidth and strongly attenuated light at the seed central wavelength. To demonstrate its applicability we have characterised the performance of a two-stage picosecond OPA pre-amplifier with 2.8×105 gain, delivering 335μJ pulses at 1054nm. We show that fluorescence from a strongly seeded OPA is reduced by ~500× from the undepleted to full pump depletion regimes. We also determine the vacuum fluctuation driven noise term seeding this OPA fluorescence to be 0.7±0.4photonsps−1nm−1 bandwidth. The resulting shot-to-shot statistics highlights a 1.5% probability of a five-fold and 0.3% probability of a ten-fold increase of fluorescence above the average value. Finally, we show that OPA fluorescence can be limited to a few-ps pedestal with 3×10−9 temporal intensity contrast 1.3ps ahead of an intense laser pulse, a level highly attractive for large scale chirped-pulse OPA laser systems.

A simplified model for quadratic parametric processes considering pump depletion in a layered quasi-phase-matching structure

Quadratic parametric processes in a one-dimensional nonlinear structure were investigated by using a nonlinear transfer matrix and iterative algorithm. Our method can provide an accurate description in the regime of pump depletion. Detailed analysis shows iteration is important for achieving a precise result. In order to simplify the complex iterative derivation, we develop a simplified model for quasi-phase-matched (QPM) second harmonic generation. We extend the simplification strategy to treat other quadratic nonlinear interactions in a QPM structure. The validity of simplification is demonstrated by numerically comparing conversion efficiencies. Our method provides a simple and efficient way to investigate second-order parametric processes in QPM structures.