Stimulated Raman Scattering Suppression Research Articles

Experimental investigations on multi-kilowatt all-fiber distributed side-pumped oscillators

3 kW and 4 kW all-fiber distributed side-pumped oscillators, based on a distributed side-coupled cladding-pumped fiber, are demonstrated for the first time, to the best of our knowledge. Using three cascaded gain sections, the 2.95 kW output power with 77.4% slope efficiency and more than 52 dB suppression of stimulated Raman scattering (SRS) is obtained. However, the power scaling is mainly limited by the pump power. By adding one more gain section, the output power is scaled up to 3.96 kW, the slope efficiency is about 73.0% and the SRS suppression is about 48 dB. The transverse mode instability is observed in the 4 kW fiber oscillator, and its dependence on the output power is also measured in experiment.

Suppression of stimulated Raman scattering in a high-power fiber amplifier by inserting long transmission fibers in a seed laser

We demonstrate the suppression of stimulated Raman scattering (SRS) in a kilowatt fiber amplifier by reducing the temporal fluctuations of the seed oscillator, which is achieved by simply inserting a long piece of transmission fiber between the seed source and amplifier. We experimentally investigate the dependence of the amount of Raman light in the amplifier on the seed oscillator’s transmission fiber length. We also numerically analyze the temporal features of the corresponding seed sources and amplifiers. Experiments and simulations indicate that long transmission fibers will flatten temporal fluctuations of the seed laser, and thereby the SRS effect in the amplifier can be mitigated. The difference of the SRS suppression mechanisms between the method proposed here and the conventional method employing spectrally wider fiber Bragg grating is also presented.

Filtering of stimulated Raman scattering in a monolithic fiber laser oscillator using chirped and tilted fiber Bragg gratings

We have demonstrated here, for the first time to the best of our knowledge, the filtering of stimulated Raman scattering (SRS) in a monolithic fiber laser oscillator, using chirped and tilted fiber Bragg gratings (CTFBGs). We designed and inscribed CTFBGs in large-mode-area fibers according to the operating wavelength of the fiber laser oscillator. With a CTFBG inserted before the output coupling FBG in the oscillator, a maximum SRS suppression ratio more than 10 dB was achieved in spectrum with 976 nm and 915 nm laser diode pumps respectively at a given power level. Obvious decreasing of the Stokes power and increasing of the equivalent Raman threshold was observed. By reducing the insertion loss and improving the transmission spectrum of the CTFBG, an improvement in laser efficiency could be achieved. This work provides a novel idea for SRS filtering in a high-power all-fiber oscillator system, which is very useful for the power enhancing of a fiber oscillator.

Analytical study on the stimulated Raman scattering threshold in distributed-pumped fiber amplifiers.

In this paper, the stimulated Raman scattering (SRS) threshold of the distributed-pumped fiber amplifier consisting of multiple sub-amplifiers is analytically studied based on the rate-equation model. The SRS threshold formula is presented where the predictions agree well with the numerical results. With the help of the formula, the effect of the pumping scheme on the SRS suppression of the distributed-pumped fiber amplifier is systematically investigated. It is found that with a given number of pump injection ports, the SRS suppression can be optimized by making only some of sub-amplifiers counter-pumped with the others bi-directional-pumped. It is also suggested that the first sub-amplifier should be adopted to be counter-pumped for suppressing SRS if only one sub-amplifier can be single-directional-pumped. The variation of SRS threshold with the number of sub-amplifiers is also discussed in detail. We believe that these results can provide significant guidance on understanding and designing distributed-pumped fiber lasers and amplifiers.

Effective suppression of stimulated Raman scattering in half 10 kW tandem pumping fiber lasers using chirped and tilted fiber Bragg gratings

The average power of fiber lasers has been scaled deeply into the kW regime in the past years. However, stimulated Raman scattering (SRS) is still a major factor limiting further power scaling. Here, we have demonstrated for the first time, to the best of our knowledge, the suppression of SRS in a half 10 kW tandem pumping fiber amplifier using chirped and tilted fiber Bragg gratings (CTFBGs). With specially self-designed and manufactured CTFBGs inserted between the seed laser and the amplifier stage, a maximum SRS suppression ratio of >15 dB in spectrum is observed with no reduction in laser efficiency. With one CTFBG, the effective output power is improved to 3.9 kW with a beam quality M2 factor of ∼1.7 from 2; with two CTFBGs, the effective laser power reaches 4.2 kW with an increasing ratio of 20% and an M2 factor of ∼1.8, and further power improvement is limited by the power and performance of the 1018 nm pump sources. This work provides an effective SRS suppression method for high-power all-fiber lasers, which is useful for further power scaling of these systems.

Mitigation of stimulated Raman scattering in kilowatt-level diode-pumped fiber amplifiers with chirped and tilted fiber Bragg gratings

The average power of diode-pumped fiber lasers has been developed deeply into the kW regime in the past years. However, stimulated Raman scattering (SRS) is still a major factor limiting the further power scaling. Here, we have demonstrated the mitigation of SRS in kilowatt-level diode-pumped fiber amplifiers using a chirped and tilted fiber Bragg grating (CTFBG) for the first time. The CTFBG is designed and inscribed in large-mode-area (LMA) fibers, matching with the operating wavelength of the fiber amplifier. With the CTFBG inserted between the seed laser and the amplifier stage, an SRS suppression ratio of ${\sim}10~\text{dB}$ is achieved in spectrum at the maximum output laser power of 2.35 kW, and there is no reduction in laser slope efficiency and degradation in beam quality. This work proves the feasibility and practicability of CTFBGs for SRS suppression in high-power fiber lasers, which is very useful for the further power scaling.

Fabrication of kW-level chirped and tilted fiber Bragg gratings and filtering of stimulated Raman scattering in high-power CW oscillators

Suppression of stimulated Raman scattering (SRS) by means of chirped and tilted fiber Bragg gratings (CTFBGs) has become a key topic. However, research on high-power systems is still lacking due to two problems. Firstly, after the inscription, there are a large number of hydroxyl compounds and hydrogen molecules in CTFBGs that cause significant heating due to their strong infrared absorption. Secondly, CTFBGs can couple Stokes light from the core to the cladding and the coating, which causes serious heating in the coating of the CTFBG. Aimed at overcoming these bottlenecks, a process that combines constant-low-temperature and variable-high-temperature annealing is used to reduce the thermal slope of the CTFBG. Also, a segmented-corrosion cladding power stripping technology is used on the CTFBG to remove the Stokes light which is coupled to the cladding, which solves the problem of overheating in the coating of the CTFBG. Thereby, a CTFBG with both a kilowatt-level power-carrying load and the ability to suppress SRS in a fiber laser has been developed. Further, we establish a kW-level CW oscillator to test the CTFBG. Experimental results demonstrate that the power-carrying load of the CTFBG is close to 1 kW, the thermal slope is lower than $0.015\,^{\circ }\text{C}/\text{W}$, and the SRS suppression ratio is nearly 23 dB.

Impact of Stimulated Raman Scattering on the Transverse Mode Instability Threshold

Quantum defect originating from the stimulated Raman scattering (SRS) effect is a heat source that induces refractive index grating and, thus, significantly decreases the threshold of transverse mode instability (TMI), which is the main limiting factor for the power scaling of high-power fiber lasers. A semi-analytical model that takes the TMI and SRS effects into account was developed, and the impact of the Raman content variation due to changes in the injected seed and fiber length on the TMI threshold was investigated. Experimental demonstrations of the impact of the SRS effect on the TMI threshold are presented. We proved that TMI affects the beam quality of the signal light rather than the Raman light. Taking these facts into account, the suppression of SRS could be an effective way to counter the TMI of fiber lasers.

Selective Suppression of Stimulated Raman Scattering with Another Competing Stimulated Raman Scattering.

A three-beam femtosecond stimulated Raman scattering (SRS) scheme is formulated and demonstrated to simultaneously induce two different SRS processes associated with Raman-active modes in the same molecule. Two SR gains involving a common pump pulse are coupled and compete: As one of the Stokes beam intensities increases, the other SRS is selectively suppressed. We provide theoretical description and experimental evidence that the selective suppression behavior is due to the limited number of pump photons used for both of the two SRS processes when an intense depletion beam induces one SRS process. The maximum suppression efficiency was ∼60% with our experimental setup, where the SR gain of the ring breathing mode of benzene is the target SRS signal, which is allowed to compete with another SRS process, induced by an intense depletion beam, of the CH stretching mode. We anticipate a potential of this new switching-off concept in super-resolution label-free microscopy.

Raman-noise enhanced stimulated Raman scattering in high-power continuous-wave fiber amplifier

The stimulated Raman scattering (SRS) in high-power continuous-wave (CW) fiber amplifier is investigated with the seeded Raman noise taken into account. It is revealed that the seeded Raman noise, as a portion of seed light, should enhance SRS in high-power CW fiber amplifier even when it is much weaker than the seeded signal light. The analytic formula for predicting the Raman threshold of bi-directional pumping fiber amplifier is given with the seeded Raman noise taken into account, and the predictions agree well with the numerical results. It is also revealed that the co-pumping amplifier suffer from the lowest Raman threshold, and the counter-pumping one is more beneficial to the SRS suppression than the bi-directional pumping one when the pump absorption of active fiber is larger than 8.7dB. These results will provide guidance for the design of high-power CW fiber amplifier.

Two-photon interband absorption coefficients in tungstate and molybdate crystals

Two-photon absorption (TPA) coefficients were measured in tungstate and molybdate crystals – BaWO4, KGW, CaMoO4, BaMoO4, CaWO4, PbWO4 and ZnWO4 upon different orientations of excitation polarization with respect to the crystallographic axes. Trains of 25ps pulses with variable radiation intensities of third (349nm) harmonics of passively mode-locked 1047nm Nd:YLF laser were used for interband two-photon excitation of the crystals.It was suggested that in the case, when 349nm radiation pumping energy exceeds the bandgap width (hν>Eg), the nonlinear excitation process can be considered as two-step absorption.The interband two-photon absorption in all the studied crystals induces the following one-photon absorption from the exited states, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the excitation intensity. This fact was taken into account under analysis of the experimental dependences of the reciprocal transmission on the excitation intensity. Laser excitation in the transparency region of the crystals caused stimulated Raman scattering (SRS) not for all the crystals studied. The measured nonlinear coefficients allowed us to explain the suppression of SRS in crystals as a result of competition between the SRS and TPA.

Transient stimulated raman scattering in crystals during motion of populations of vibrational states

Transient stimulated Raman scattering (SRS) in crystals is analyzed taking into account the motion of populations of vibrational states under the action of subpicosecond (shorter than the dephasing time) pump pulses. Analytic expressions describing the dynamics of excitation of vibrations in SRS are derived. It is found that for a small wavelength of SRS interaction and high intensities of pump radiation observed for femtosecond SRS in crystals, avalanche excitation of vibrations can be responsible for SRS suppression. It is shown that when phase matching of Stokes-anti-Stokes parametric coupling in transient SRS is ensured, it is possible to elevate the efficiency of frequency conversion under conditions of motion of populations of vibrational states; this explains recent successful results in the experimental implementation of femtosecond SRS in crystals pumped by a Bessel beam.

Growth of ring ripple in a collisionless plasma in relativistic-ponderomotive regime and its effect on stimulated Raman backscattering process

A theoretical and numerical study has been made of the propagation of a ring rippled laser beam in collisionless plasma with dominant relativistic ponderomotive nonlinearity and its effect on the excitation of electron plasma wave and stimulated Raman backscattering process. The growth of ring ripple, riding on an intense Gaussian laser beam in plasma has also been studied. A paraxial-ray and WKB approximation has been invoked to understand the nature of propagation of the ring rippled Gaussian laser beam in plasma, electron plasma wave and back reflectivity under the influence of both nonlinearities. The growth rate and focusing of a ring rippled beam is found to be considerably affected by the power of the main beam and the phase angle between the electric vectors of the main beam and the ring ripple. It has also been observed that the focusing is released by the coupling of relativistic and ponderomotive nonlinearities, which significantly affected the dynamics of the excitation of electron plasma wave and back reflectivity of stimulated Raman scattering (SRS). Due to the strong coupling between ring rippled laser beam and the excited electron plasma wave, back reflectivity of SRS is enhanced. It has been observed from the computational results that the effect of the increased intensity leads to suppression of SRS back reflectivity. The results have been presented for established laser and plasma parameters.

Kilowatt high average power narrow-linewidth nanosecond all-fiber laser

AbstractA high power narrow-linewidth nanosecond all-fiber laser based on the master oscillator power amplifier (MOPA) configuration is demonstrated. A pulsed seed with high repetition rate of 10 MHz was generated by modulating a continuous-wave (CW) single-frequency fiber laser at $\sim $1064 nm by using an electro-optic intensity modulator (EOIM). After multi-stage cascaded power amplification, the average power was boosted to be kilowatt level. The pulses from the main amplifier had a pulse width of $\sim $3 ns and an average/peak power of 913 W/28.6 kW. Further power scaling of the pulses was limited by stimulated Raman scattering (SRS) for the moment, method for SRS suppression and further power scaling was briefly discussed.

Two-photon absorption of high-power picosecond pulses in PbWO4, ZnWO4, PbMoO4, and CaMoO4 crystals

The nonlinear process of two-photon interband absorption is studied in tungstate and molybdate oxide crystals excited by a sequence of high-power picosecond pulses with a wavelength of 523.5 nm. The transmission of the crystals is measured for the excitation pulse intensity up to 100 GW/cm2. The pulse intensity in the crystals initially transparent at a wavelength of 523.5 nm is strongly limited due to two-photon absorption (TPA), and the reciprocal transmission in PbWO4 and ZnWO4 crystals reaches 50–60. In all crystals, TPA induces long-lived one-photon absorption, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the laser excitation intensity. Absorption dichroism manifests itself in a significant difference in the transmission intensities when the principal orthogonal optical axes of the crystals are excited. The TPA coefficients are determined during the excitation of two optical axes of the crystals. TPA coefficients β for the crystals vary over a wide range, namely, from β = 2.4 cm/GW for PbMoO4 to β = 0.14 cm/GW for CaMoO4, and the values of β can differ almost threefold when different optical axes of a crystal are excited. Good agreement is achieved between the measured intensities limited by TPA and the estimates calculated from the measured nonlinear coefficients. Stimulated Raman scattering (SRS) upon excitation at a wavelength of 523.5 nm is only detected in two of the four crystals under study. The experimental results make it possible to explain the suppression of SRS by its competition with TPA, and the measured nonlinear coefficients are used to estimate this suppression.

Suppression of stimulated Raman scattering employing long period gratings in double-clad fiber amplifiers

We report on the suppression of stimulated Raman scattering (SRS) in a double-clad fiber amplifier using long-period gratings (LPGs). The LPGs, fabricated with a CO(2) laser, achieve SRS suppression by coupling the Stokes wavelength from the active core into the cladding. With only three LPGs inserted into a fiber pulse amplifier, the extractable Raman-free output power was nearly doubled. A numerical simulation of the setup shows good agreement with the experimental results.

Suppression of Stimulated Raman Scattering of a Laser in a Rippled Density Plasma

The effect of a stationary electron density ripple on stimulated Raman scattering (SRS) of a laser in a plasma is studied. The density ripple couples with the primary Langmuir wave, generated in the Raman process, to produce a secondary Langmuir wave of larger wave number that is strongly Landau damped on the electrons. As a consequence, the growth rate of the Raman process is suppressed. In a special case of density ripple produced by two counterpropagating lasers of intensity ∼1015W∕cm2 at 1μm wavelength in a 6% critical density plasma of 1keV temperature, the reduction in growth rate is substantial. The effect of density ripple wavelength on SRS is studied at different plasma temperatures. Up to ∼70% suppression in SRS is observed for different plasma temperatures at different ripple wavelengths. However, for higher temperatures we also get ∼10% of suppression for the other ripple wavelength values.

Suppression of first order stimulated Raman scattering in erbium-doped fiber laser based LIDAR transmitters through induced bending loss

A model is presented for suppression of first-order stimulated Raman scattering (SRS) in pulsed Er-doped fiber amplifiers with wavelength-selective induced bending loss. The model estimates the effectiveness of SRS suppression through fiber coiling in a pulsed amplifier, considering both the single- and dual-clad fiber cases. The basic approach is to fit analytical expressions to the evolution of a quasi-CW signal in the amplifier. Then, assuming that SRS does not deplete the signal, exact analytical expressions for the propagation of the Stokes wave are determined, from which the SRS threshold is estimated. Finally, example calculations are presented, and the model is used to determine a fiber geometry for the suppression of SRS through fiber coiling.

Suppression of stimulated Raman scattering from microdroplets by seeding with nanometer-sized latex particles

Stimulated Raman scattering (SRS) in laser-irradiated microdroplets is suppressed by the addition of nanometersized latex particles. The microdroplets consist of either pure ethanol or a solution of Rhodamine 6G dye in ethanol, seeded with latex particles having diameters of 50 < d < 500 nm. SRS emission occurs at droplet morphology-dependent resonances (MDR's) after either direct pumping by the incident 532-nm laser or indirectly whereby the pump laser first initiates dye lasing, which in turn pumps SRS. For large latex, we observe SRS suppression at a near-coincident threshold concentration independent of the presence of dye, whereas, for small latex, adding dye reduces the threshold concentration by more than an order of magnitude. These findings are consistent with the interpretation that for large latex ~1 particle must occupy the MDR mode volume at threshold, whereas for small latex the addition of particles facilitates Förster-assisted annihilation of both 532-nm and dye-lasing MDR pump photons.