Stimulated Raman Scattering Threshold Research Articles

Effect of laser linewidth and fiber length on self-pulsing dynamics and output stabilization of single-mode Yb-doped double-clad fiber laser

We report an experimental study of the effect of fiber length and laser linewidth on self-pulsing dynamics and output stabilization of a single-mode Yb-doped double-clad CW fiber laser. It is found that initiation of self-pulsing under low-level pumping conditions is due to relaxation oscillations and saturable absorption in the weakly pumped region of the doped fiber, irrespective of the fiber length and the laser linewidth. However, with an increase in pump power, depending on fiber length and laser linewidth, the pulses initiated due to relaxation oscillation get amplified, and result in short-duration giant pulses due to either stimulated Brillouin scattering (SBS) or stimulated Raman scattering (SRS). In the case of fiber lasers that employ a broadband mirror and wherein the fiber length is sufficient to reach the SRS threshold, the giant self-pulses are generated by SRS, whereas in the case of fiber lasers using a fiber Bragg grating, characterized by narrowband reflection and with sufficient fiber length to reach the SBS threshold, the giant self-pulses are generated by SBS. Output stabilization and, hence, elimination of self-pulsations can be achieved either by suppressing the relaxation oscillations with the addition of an appropriate length of a passive fiber to sufficiently increase the cavity photon lifetime, or by increasing the pump power to achieve gain uniformity along the doped fiber such that relaxation oscillations and reabsorption effects are suppressed.

High-Power Laser Beam Transfer through Optical Relay Fibers for a Laser Guide Adaptive Optics System

Abstract We are developing a laser guide star adaptive optics (LGSAO) system for the Subaru Telescope on Mauna Kea, Hawaii. The AO188 system will dramatically increase the observable sky area. This system differs from systems used at other large telescopes in that it utilizes the combination of an all-solid-state mode-locked sum-frequency generation (SFG) laser as a light source and single-mode optical fiber for beam transference. Optical fibers transfer laser light from the source, located at the Nasmyth platform, to the laser launching telescope more flexibly and more easily than do mirror relay optics. However, optical fibers induce nonlinear scattering effects, such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), beyond certain threshold levels in high-power lasers. We measured the laser transmission characteristics of a photonic crystal fiber (PCF) whose mode field diameter (MFD) was 11$\mu$m, and a step index fiber (SIF) cable whose MFD was 4.2$\mu$m to evaluate the threshold levels for non-linear effects. We observed SRS in the 200-m-long SIF when we input 1.3 W. However, SRS and SBS were not induced in the 200-m-long PCF, even for an input power of 5.3 W. As a result, we estimated the threshold of SRS to be 33 W for the 35-m-long PCF designed for the Subaru LGSAO system. Given that the fiber will carry a laser beam of about 30 W with a pulse width of less than 1 ns, we conclude that nonlinear scattering will pose no problems for this application.

Dependence of the stimulated Raman scattering threshold in droplets on the temporal stretching of a nanosecond laser pulse

AbstractFor linear Raman spectroscopy in micro‐droplets the excitation laser energy and therefore the signal strength is limited either by the threshold for the excitation of nonlinear processes in the droplets or by the generation of a plasma (gas breakdown) in the vicinity of the droplets. These processes are due to the enhancement of the excitation field by morphology‐dependent resonances (MDRs) which are built up in the cavity synthesised by the droplets. As these stimulated signals disturb the detection of the linear signal it is of high interest for different applications to suppress the stimulated Raman scattering (SRS) or to use an excitation intensity below the SRS threshold with a sufficiently large integrated laser energy to obtain an appropriate signal‐to‐noise ratio for the linear Raman process. In this paper the effect of temporal stretching of the incoming laser pulse with the help of an optical pulse stretcher on the SRS threshold is described and discussed. Copyright © 2008 John Wiley & Sons, Ltd.

Picosecond Stimulated Raman Scattering of SrWO4 Crystal

Stimulated Raman scattering (SRS) of picosecond pulses is investigatedin a new crystal SrWO4. The second harmonic generation of amode-locked Nd:YAG laser system is used as the pump source. In anexternal single-pass configuration, the SRS thresholds for the first tothe fourth Stokes lines are measured. For the first Stokes line, thesteady-state gain coefficient of the SrWO4 crystal is calculatedto be 15.96 cm/GW. In our experiment, as many as five Stokes lines(559.23 nm, 589.61 nm, 623.49 nm, 661.50 nm, 704.44 nm) and threeanti-Stokes lines (506.97 nm, 484.34 nm, 463.65 nm) are observed, andthe total conversion efficiency is as high as 62%.

Stimulated Raman scattering in high-power double-clad fiber lasers and power amplifiers

Theoretical analyses of stimulated Raman scattering (SRS) in high-power ytterbium-doped double-clad fiber lasers and power amplifiers with bidirectional end pump and distributed pump are comprehensively presented. The Raman effects are quantitatively compared in these schemes with different fiber lengths, core diameters, signal wavelengths, pump absorption, facet reflections, etc., and the related restrictions are also illustrated. Fiber power amplifiers, characteristically with low facet reflections, have the advantage of significantly higher SRS thresholds than fiber laser resonators, which makes master oscillator power amplifiers better candidates for kilowatt single-transverse-mode fiber laser sources.

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.

New Crystals for Raman Lasers

Steady-state and transient stimulated Raman scattering (SRS) in crystals is analyzed. The basic laws of an increase in the SRS gain in crystals are revealed, and methods for searching and creating new SRS laser materials are developed. New crystals for picosecond and nanosecond SRS lasers are proposed, fabricated, and characterized. These materials have the highest SRS cross section, a low SRS threshold, and a wide spectral range of operation.

Theoretical and Experimental Study of Stimulated and Cascaded Raman Scattering in Ultrahigh-<tex>$Q$</tex>Optical Microcavities

Stimulated Raman scattering (SRS) in ultrahigh-Q (UHQ) surface-tension-induced spherical and chip-based toroid microcavities is considered both theoretically and experimentally. These microcavities are fabricated from silica, exhibit small mode volume (typically 1000 /spl mu/m/sup 3/) and possess whispering-gallery type modes with long photon storage times (in the range of 100 ns), significantly reducing the threshold for stimulated nonlinear optical phenomena. Oscillation threshold levels of less than 100 /spl mu/W of launched fiber pump power, in microcavities with quality factors of 100 million are observed. Using a steady-state analysis of the coupled-mode equations for the pump and Raman whispering-gallery modes, the threshold, efficiencies and cascading properties of SRS in UHQ devices are derived. The results are experimentally confirmed in the telecommunication band (1550 nm) using tapered optical fibers as highly efficient waveguide coupling elements for both pumping and signal extraction. The device performance dependence on coupling, quality factor and modal volume are measured and found to be in good agreement with theory. This includes analysis of the threshold and efficiency for cascaded Raman scattering. The side-by-side study of nonlinear oscillation in both spherical microcavities and toroid microcavities on-a-chip also allows for comparison of their properties. In addition to the benefits of a wafer-scale geometry, including integration with optical, electrical, or mechanical functionality, microtoroids on-a-chip exhibit single mode Raman oscillation over a wide range of pump powers.

Stimulated raman scattering in fiber-optic communication lines

Stimulated Raman scattering (SRS) in a long-distance fiber-optic communication line with wavelength-division multiplexing (WDM) is studied theoretically at a high power of the signal transmitted. A new criterion for determining the SRS threshold is used to calculate the critical input power versus the number of optical channels and frequency separation between them. The theoretical model is verified experimentally. SRS interaction between two channels in a communication line with an SRS amplifier for which the optical waveguide of the line serves as a nonlinear medium is measured.

Stimulated Raman scattering in a spherical microparticle

An electrodynamic approach is developed to solve the problem of stimulated Raman scattering (SRS) in a transparent dielectric spherical particle. In this approach it is proposed that optical fields for time-dependent amplitudes of coupled waves at Stokes and pump frequencies in a spherical particle be represented as an expansion in terms of eigenfunctions of the stationary scattering problem in which the expansion coefficients determine the temporal behavior of the field and comply with inhomogeneous differential equations. Solutions of these equations for initial phase SRS and under steady-state conditions are analyzed. The SRS threshold is determined, and the threshold for steady-state SRS at a given intensity is found for when there is double resonance between the fields. It is shown that, to excite SRS, one should compensate for the loss of the Stokes wave that is due to absorption and emission through the particle surface. To provide steady-state SRS generation it is necessary to compensate additionally for the energy loss that is due to depletion of pump intensity.

On cavity modification of stimulated Raman scattering

We study theoretically stimulated Raman scattering (SRS) in a nonlinear dielectric microcavity and compare SRS thresholds for the cavity and the bulk material it is made of. We show that cavity SRS enhancement results solely from the intensity build up in the cavity and from the differences of the SRS dynamics in free and confined space. There is no significant modification of the Raman gain due to cavity QED effects. We show that the SRS threshold depends significantly on the nature of the dominating cavity decay as well as on the coupling technique with the cavity used for SRS measurements.

Stimulated Raman scattering in alkaline-earth tungstate crystals

Results on spontaneous Raman spectroscopy of alkaline-earth tungstate crystals are presented. The frequency of the modes active in stimulated Raman scattering (SRS) modes was found to increase and their width to decrease with increasing cation radius and mass in the series of alkaline-earth (Ca, Sr, Ba) tungstates. High peak values of the Raman scattering cross section for barium and strontium tungstate crystals — new media offering promise for SRS — were predicted and observed in the experiments. Laser experiments on the SRS threshold showed that the SRS gain for the barium tungstate crystal was close to the record value observed for the barium nitrate crystal, which is extensively used in SRS.

Linear Raman spectroscopy on aqueous aerosols: influence of nonlinear effects on detection limits

The influence of stimulated Raman scattering on the characterization of aqueous aerosols by linear Raman spectroscopy was studied experimentally. Raman scattering from a chain of microdroplets was excited by an argon ion laser. Holographic Raman filters, a spectrograph and a cryogenically cooled CCD detector were used to record the spectra. A procedure to estimate the detection limits of Raman instrumentation is presented. For the set-up used here, the detection limit for nitrogen in a gas mixture was 3 ppm. The detection limit for aqueous aerosol particles was limited to 100 mg m −3 by the occurrence of stimulated Raman scattering (SRS). The first measurement of SRS with continuous-wave excitation in water droplets is reported. The SRS thresholds for water droplets in the size range of 15–75 μm were 0.3 to 1 MW cm −2. For droplets, which consist of a 5 M NaNO 3 solution, the threshold was 0.2 MW cm −2. Continuous-wave excited four-wave mixing in microdroplets of NaNO 3 solution is reported. 1998 Elsevier Science Ltd. All rights reserved

STIMULATED RAMAN SCATTERING IN PLASMAS PRODUCED BY LOW-ENERGY LASER

The result of experimental studies of stimulated Raman scattering (SRS) was report in this paper- When the disk target was irradiated by low-energy laser, the plasma density scale length was short, and the SRS threshold intensity was very high, SRS only occurred in those local ranges where electron density was homogeneous- Multi-material targets and filamentation will produce more those local range, these will increase SRS light-

Threshold and conversion experiments on seeded stimulated Raman scattering

High energy-conversion efficiencies in Stimulated Raman Scattering (SRS) are demonstrated both in experiments and by simulations for pump powers below SRS threshold. The scattering is induced by a short seed pulse at the Stokes frequency, the pulse width of which is much shorter than the pump pulse width and which is comparable with the medium's dephasing time.

The stimulated Raman scattering threshold for a nondiffraction-limited pump beam

The threshold power of stimulated Raman scattering (SRS) has been calculated for various nondiffraction-limited pump beams using a numerical model. These calculations were performed for the following pump beam profiles: Gaussian-Hermite, Gaussian-Laguerre, Gaussian-Schell-model (GSM), and several superpositions of Gaussian-Hermite modes. It is shown that the threshold power, for a tightly focused beam, depends on the M/sup 2/ factor, which is a dimensionless factor that compares the divergence of a given beam with the divergence of a (diffraction-limited) Gaussian beam, related to the pump beam quality. The influence of the exact pump beam profile on the threshold is small. It is shown that the measured SRS thresholds in methane for two pump beams, one with an M/sup 2/ of 1.12 and the other with M/sup 2/=8.7, correspond with the calculated thresholds for two GSM beams with the same M/sup 2/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Stimulated Raman-scattering threshold behavior of binary mixture micrometer-sized droplets

Stimulated Raman scattering (SRS) from binary liquid mixture micrometer-sized droplets irradiated by nanosecond laser pulses exhibits previously unreported features. SRS emission at wavelength shifts corresponding to combination frequencies of the individual component Stokes shifts are observed in droplets but not in bulk mixtures. Furthermore, droplet SRS thresholds are determined by component refractive indices and concentrations, in contrast to bulk thresholds, for which self-focusing likely plays a dominant role.

Stimulated Raman scattering from individual water and ethanol droplets at morphology-dependent resonances

Stimulated Raman scattering (SRS) from an individual liquid droplet (~30-microm radius) has been observed for H(2)O, D(2)O, and ethanol. The occurrence of a series of spectrally narrow peaks that are regularly spaced in wavelength is consistent with morphology-dependent resonances of the droplet, which acts as an optical resonator. The input intensity required to achieve the SRS threshold for the droplet is considerably less than that for the liquid in an optical cell.

Multiple-pass Raman gain cell

The application of a multiple-pass Herriott cell to stimulated Raman scattering (SRS) is evaluated and demonstrated. The cell combines a long optical path length with periodic refocusing to enhance Raman gain. This technique is especially useful for reducing SRS threshold pump power in the IR where the Raman gain is low. This paper presents an analysis and design procedure for a multiple-pass Raman gain cell and gives experimental results for SRS in H(2) gas.

Stimulated Raman Scattering Thresholds for Ultra-short Excitation

S.R.S. thresholds in different liquids have been measured with a 25 ps pulse duration. An important rise, compared with the measured thresholds in the nanosecond range, has been noted, and it cannot be completely explained by the transient self-focusing or Raman effects as they are usually calculated. Several assumptions are reviewed in order to explain the observations.