Raman Oscillator Research Articles

Effect of nanosizing on some properties of one-dimensional polyacetylene chains

Density functional theory at the B3LYP/6-31G* level is used to calculate equilibrium geometry of the one-dimensional trans polyacetylene chains (C2H2)n from n=2 to 20 which are of nanometer length. The Raman frequencies, intensities, optical absorption , oscillator strength, ionization potential, electron affinity and binding energy per monomer are calculated as a function of the chain length in the nanometer range. The results show with the exception of the oscillator strength of the absorption that there is a small critical length less than 4nm at which changes of properties become significant and this length is property dependent.

Numerical simulations of pulsed and quasi-cw regimes of synchronously pumped Raman oscillators

Synchronously pumped Raman oscillators allow for an efficient frequency conversion of pump pulses whose power is well below the threshold for extracavity stimulated Raman scattering (SRS). Numerical simulations show that the cavity mismatch of pump laser and Raman oscillator is critical for maximum conversion efficiency and minimum pulse duration for transient and stationary SRS. At a certain mismatch the simulations predict a quasi-cw Raman oscillator output even at moderate cavity quality factors. Optimum operation conditions are discussed for pump pulse trains of finite lengths. The results of the numerical calculations are in qualitative agreement with a simple phenomenological model.

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.

Picosecond parametric Raman laser based on KGd(WO4)2 crystal

Simultaneous frequency up and down conversion using a biharmonically pumped parametric Raman laser (PRL) based on a KGd(WO4)2 (KGW) crystal has been demonstrated experimentally. Collimated beams have been generated for both the 1st anti-Stokes (1AS) at 511 nm and 2nd Stokes (2S) at 579 nm by four-wave parametric Raman interaction. To excite PRL biharmonically, we pumped the KGW crystal with 20 ps pulses at the fundamental 530 nm and its 1st Stokes component at 555 nm. The 1st Stokes pump beam was generated using stimulated Raman scattering in a separate KGW Raman oscillator. The energy conversion efficiencies reached experimentally were 10% for the 2S and 4% for the 1AS.

Enhanced stimulated Raman scattering in optical parametric oscillators from periodically poled KTiOPO4

Enhanced Raman scattering and concurrent Raman oscillation have been observed in nanosecond optical parametric oscillators in the near-infrared spectral region for certain periodicities of periodically poled KTiOPO4. The increased Raman activity is associated with direct excitation of the phonon overtone band by the idler wave. The decrease in efficiency of stimulated Raman scattering at crystal temperature above 80 °C is explained by higher dephasing rate of coherently driven lattice polarization due to interaction with thermally generated phonons.

Theoretical characterization of Raman oscillation with intracavity pumping of fiber lasers

Theoretical results relating to the generation of continuous-wave (CW) output from fiber lasers that are internally pumped with light generated from the stimulated Raman effect are presented. This investigation establishes the important fiber and resonator parameters, such as the fiber length and glass composition, dopant concentration, and pump power required to realize this new form of fiber laser arrangement. Three examples are studied: the Ho/sup 3+/-doped silica fiber laser that is pumped at a wavelength of 1.15 /spl mu/m, the Er/sup 3+/-doped silica fiber laser which is pumped at 1.48 /spl mu/m and, the Tm/sup 3+/-doped silica fiber laser which Is pumped at 1.625 /spl mu/m. These three examples cover first Stokes pumping, second Stokes pumping, and first Stokes pumping with direct dopant absorption of the pump light, respectively. The simulations involve the use of simple numerical models comprising the spatially dependent field propagation equations (under the slowly varying field approximation) and the rate equations for the population densities. It is established that intracavity Raman pumping of fiber lasers with first Stokes radiation is efficient when the losses at the pump, Stokes and laser wavelengths are kept low (<10 dB/km). It is also established that second Stokes pumping is, even with direct absorption of the pump light, theoretically quite efficient and, as a result, the Er/sup 3+/-doped silica fiber laser which is pumped with second Stokes radiation at 1.48 /spl mu/m may provide the best demonstration of intracavity Raman pumping.

Raman oscillation with intracavity pumping of a fibre laser

We introduce the concept of cascaded resonant Raman pumping of fibre lasers. The pump scheme utilises the relatively large intracavity Stokes field that is generated within a Raman fibre laser to excite a lanthanide ion that is doped within the core of the fibre providing the Raman gain. In order to illustrate the general characteristics of the pump method and, to establish the design parameters necessary for the realisation of the pump scheme, calculations from a theoretical model that is used to simulate the generation of laser output from a fibre laser that is resonantly pumped with first Stokes radiation is presented. Specifically, the 2.1 μm output from a Ho 3+-doped silica fibre laser that is pumped with 1.15 μm first Stokes radiation is calculated with the use of a relatively simple numerical model. For a launched pump power of 20 W at 1.07 μm, a fibre laser output of ∼3 W is predicted for a nominal intrinsic loss of 1.5 dB/km at 2.1 μm, however, this low value of the intrinsic loss at 2.1 μm can be significantly relaxed when the length of the Ho 3+-doped silica fibre laser resonator is made considerably shorter than the Stokes resonator.

High efficiency nanosecond Raman lasers based on tetragonal PbWO 4 crystals

We report on laser action in linear and ring Raman oscillators based on novel χ (3)-active PbWO 4 crystals with scheelite structure ( ω R=901 cm −1). These resonators are pumped by the fundamental and the second harmonic wavelengths, respectively, of a Q-switched Nd 3+:Y 3Al 5O 12 laser. In these preliminary measurements, conversion efficiencies as high as 20% are observed. To our knowledge, this is the first such demonstration of Raman lasing in lead tungstate in the nanosecond regime. Raman lasers based on PbWO 4 are very promising for several applications, including the generation of wavelengths in the uv (in conjunction with other nonlinear processes) for ozone differential absorption lidar. This material has optical transparency from ≈0.33 to ≈5.5 μm, making it also potentially useful for trace gas detection in the infrared. Furthermore, it has a high damage threshold and is not hydroscopic. Some new data on picosecond high-order Stokes and anti-Stokes generation are also presented. In particular, our SRS experiments with a 1-μm Nd 3+:Y 3Al 5O 12 pump laser demonstrate multiple anti-Stokes emission up to the 10th component with λ ASt10=0.5433 μm.

Phase Relations, Quasicontinuous Spectra and Subfemtosecond Pulses in High-Order Stimulated Raman Scattering with Short-Pulse Excitation

High-order stimulated Raman scattering for pumping by ps and sub-ps pulses is studied in the frame of a time-domain approach without the slowly varying envelope approximation. Formation of pulse trains with sub-fs durations in the ps-pump regime is demonstrated using external phase compensation. For sub-ps excitation a novel broadening mechanism of Raman lines is predicted that leads to a quasicontinuous spectrum and permits one to generate single sub-fs pulses. We predict also essential shortening of a delayed fs-probe pulse by the Raman oscillations without phase control.

Intracavity Raman conversion for 16-μm Stokes pulse generation in para-hydrogen

16-μm Stokes pulses were directly generated for the first time to our knowledge by an intracavity configuration for the para-hydrogen Raman laser. We have analyzed Stokes field growth using a focused gain model and designed a pump/Stokes cavity to satisfy CO2 pump power and pulse duration requirements for Raman oscillation. The CO2 laser oscillation with circular polarization was realized by seeding externally circularly polarized CO2 radiation. An output energy of 2.4 mJ was obtained with the output coupler of 0.5% transmittance, which indicated that 420 mJ of Stokes pulse energy was stored inside the cavity. This suggests that a much higher energy can be extracted by the optimization of cavity parameters.

Raman oscillation with intracavity sum-frequency generation

We use a plane-wave analysis to examine a Raman oscillator containing an intracavity sum-frequency interaction that frequency sums the circulating first-order Stokes radiation with the pump radiation. We find that there is an optimum ratio between the nonlinear coupling in the Raman medium and the nonlinear coupling in the sum-frequency generator. We also find that higher order Stokes radiation should be suppressed with the optimum choice of nonlinear coupling in the sum-frequency interaction. Numerical integration of the equations containing transverse effects predicts a time-averaged power-conversion efficiency of 61.4% for conversion of 532- to 273.5-nm radiation using a CW mode-locked frequency-doubled Nd:YAG laser with Ba(NO/sub 2/)/sub 3/ for the Raman material and CsLiB/sub 6/O/sub 10/ (CLBO) for the sum-frequency material.

High resolution spectral measurements of Raman shifts in barium nitrate

High resolution spectral measurements of the first, second, and third Stokes shifts in barium nitrate are reported. The laser source for the experiment is a frequency-doubled, injection-seeded Nd:YAG laser. Spectra for both single-pass conversion and for a Raman oscillator are compared. Significant gain narrowing of the spectrum is observed.

Optical-phonon behavior inGa1−xInxAs:The role of microscopic strains and ionic plasmon coupling

We present an experimental and theoretical investigation of long-wavelength optical-phonon behavior in ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{As}$ alloys. We propose a model which accounts quantitatively for both phonon frequencies and Raman intensities. The transverse-optical-phonon behavior is shown to be determined by mass disorder and microscopic strains. It is shown that Raman scattering provides a means of measuring local bond distortions in mixed crystals. Coupling by the macroscopic ionic polarization is shown to determine the longitudinal optical-phonon behavior. The model accounts for the so-called anomalies of the Raman intensities and oscillator strengths observed in ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{As}.$ Our analysis is supported by a thorough Raman-scattering study. The frequencies, Raman intensities and symmetries of both the transverse- and longitudinal-optical modes have been determined accurately over the whole compositional range, by using different crystal orientations ([001], [110], and [111]).

Generation of the second and third harmonics of the radiation of a subpicosecond neodymium laser with a 1012 contrast on a metal target

A neodymium laser with a master Raman oscillator, generating pulses of 0.8 ps duration and of 1012 contrast, was used to study harmonic generation by the interaction of radiation of 1015—1016 W cm-2 intensity with a metal target. When the laser pulse had the p polarisation (the angle of incidence of the radiation on the target was 45°C), the second and third harmonics were generated by anharmonic motion of electrons on the plane plasma—vacuum interface. The energy efficiencies of conversion to the second and third harmonics depended weakly on the laser radiation intensity I (they were proportional to I0.2) and amounted to ~1.5×10-5 and ~0.8×10-5, respectively. When the pulse had the s polarisation, the generation efficiency fell by a factor of 10 for the second harmonic and by a factor more than 103 for the third harmonic.

Observation of stimulated Raman scattering in leucosapphire α-Al2O3

Picosecond pumping was used to excite for the first time multifrequency parametric Raman oscillation in a crystal of leucosapphire α-Al2O3 based on the SRS-active vibrational mode Ag1 with νR ≈ 419 cm-1. The recorded Stokes (up to seven) and anti-Stokes (up to six) SRS components were identified.

Raman oscillation with intracavity second harmonic generation

We use a plane-wave analysis to examine a Raman oscillator containing an intracavity second harmonic interaction that frequency doubles the circulating first-order Stokes radiation. We find that there is an optimum ratio between the nonlinear coupling in the Raman medium and the nonlinear coupling in the frequency doubler. We also find that higher order Stokes radiation should be suppressed with the optimum choice of nonlinear coupling in the frequency doubler. We present numerical integration results that model the stimulated Raman scattering and second harmonic generation in three spatial dimensions. Quantum efficiencies as large as 48% are predicted from planewave theory and 43% are obtained from numerical integration of the equations containing transverse effects.

Experimental investigations of an equidistant mode distribution and of intermode beat stabilisation via the discharge current in an He—Ne/Ne laser with full self-mode-locking

An experimental investigation was made of the extent to which the mode distribution was equidistant in an He—Ne laser with a neon absorption cell and full self-mode-locking. A method was developed for stabilisation of the intermode frequency when this laser was self-mode-locked. This was done by phase locking to the frequency of an external oscillator via the discharge current. The stability of the pulse repetition (intermode) frequency was governed by stability of the frequency of an external rf standard. An analysis was made of the feasibility of using lasers with self-mode-locking stabilised at the intermode frequency as secondary standards of length utilising a Raman oscillator.

Comparative analysis of optically pumped intersubband lasers and intersubband Raman oscillators

A new tunable source of the far-infrared radiation based on intersubband electronic Raman scattering in semiconductor quantum wells is proposed. The gain and threshold of the proposed Raman oscillator are estimated and compared with the intersubband laser.

Raman-amplification and polarization-dependence measurement in a methane Raman shifter seeded by a liquid Raman oscillator

We perform Raman amplification in a high-pressure methane cell seeded by a low-threshold liquid Raman oscillator. The Raman frequency of the oscillator medium, dimethyl sulfoxide (DMSO), is tuned when it is mixed with water to fit the Raman frequency of methane. The Raman gain in this configuration is not so sensitive to the methane pressure, and the conversion efficiency is high. Using this configuration, we measure the polarization dependence of the Raman amplifier. An experiment is performed for three typical polarization states. Complete gain suppression is expected in some polarization states, but there is a little gain because of the incompleteness of the polarization state. Theoretically the results are analyzed when the x and the y components of the scattered waves are coupled.

Some comments on ‘quenching of stimulated Raman scattering by two‐photon absorption in fuels’

AbstractA recent paper by Golombok et al. in this Journal provides an explanation for the appearance in stimulated Raman scattering from certain aromatic liquids of oscillation on many Raman modes in place of the single Raman mode that generally dominates such spectra; this explanation is based on two‐photon absorption of the resulting Raman radiation. It is suggested in this paper that this process, although undoubtedly significant, is not the dominant process in suppressing completely oscillation on certain strong, sharp Raman transitions which would be expected to dominate the spectra. Of greater significance in suppressing such oscillations will be sum‐photon absorption with one photon being absorbed from each of the incident laser and the Stokes‐shifted Raman radiation. Calculated growth curves of the Raman radiation generated show that this process has the correct functional dependence to prevent Raman oscillation totally whereas two‐photon absorption of the Raman radiation created can only limit, but not totally suppress, the growth of the first Stokes wave.