Peter G. Zverev

Efficient Raman shifting of picosecond pulses using BaWO4 crystal

Abstract Stimulated Raman scattering of picosecond pulses was investigated in a new BaWO 4 Raman crystal and was compared with SRS in well known KGd(WO 4 ) 2 and KY(WO 4 ) 2 crystals. The first and second harmonic output of a mode-locked Nd:YAG laser system was used as a pump source. Due to transient behaviour, high value of BaWO 4 steady state Raman gain is reduced by a factor of 2.5 for picosecond pump pulses (28 and 40 ps) but remains still higher than that in KGd(WO 4 ) 2 crystal. Maximum first and second Stokes conversion efficiencies for BaWO 4 in a single-pass arrangement were measured to be 30% and 15%, respectively. Measurements of a temporal pulse length showed that the first Stokes pulse was twice as short as the pump pulse. Barium tungstate can thus be considered as a unique Raman crystal for wide variety of pump pulse duration from nanoseconds to picoseconds for the generation of the frequency shifted Raman laser pulses.

Highly efficient picosecond Raman generators based on the BaWO/sub 4/ crystal in the near infrared, visible, and ultraviolet

The stimulated Raman scattering process in a BaWO/sub 4/ crystal was employed to frequency downshift the first, second, and third harmonics of a Nd:YAG actively-passively mode-locked laser system. Single-pass, double-pass, and external cavity configurations were investigated for this purpose. In each experimental arrangement, the Stokes radiation properties were characterized regarding energy, beam profile, spectrum, and temporal development. The peak pump-to-first-Stokes conversion efficiency was measured to be 55% in the near infrared and 85% in the visible spectral region. The BaWO/sub 4/ picosecond Raman gain at a pump wavelength of 355 nm was measured to be 38 cm/GW, and a 15% conversion efficiency was achieved. A study of the Raman-cavity output beam profile development as a function of pumping energy was conducted. This work shows that the BaWO/sub 4/ crystal can be used in picosecond solid-state laser systems as an efficient frequency converter with a wide range of pump radiation wavelengths.

Efficient picosecond Raman lasers on BaWO4 and KGd(WO4)2 tungstate crystals emitting in 1.15 to 1.18um spectral region

Detailed study of Stimulated Raman scattering (SRS) of 1.06- micrometers , 50-ps pulses in BaWO4 and KGW crystals was performed. Single-pass, double-pass and Raman cavity arrangements were compared regarding threshold intensity, conversion efficiency and pulse structure. Single-pass conversion efficiencies reached 25 percent for both BaWO4 and KGW. Double passing of the pump beam slightly improved the efficiency to 35 percent. Placing the BaWO4 crystal into a cavity and optimizing the performance by varying the output coupler spectral characteristics and the resonator length, 55 percent pump-to-first-Stokes efficiency has been obtained. The maximum of the Raman laser output energy was 3 mJ. The output consisted of 2-4 picosecond pulses at the 1180 nm wavelength. Dependence of the BaWO4 Raman laser output beam structure on the pumping energy was studied. The concentric ring profile, typical for transient SRS, was homogenized using a cyclic olefin polymer coated silver hollow glass waveguide. A similar study was performed with KGW crystal, for which the first Stokes wavelengths are 1159 nm or 1177 nm, depending on the sample orientation. Due to lower gain value, Raman laser energy reached 2 mJ and maximum efficiency was 30 percent. Based on our comparative study, a new BaWO4 crystal is highly suitable for utilization in the near IR picosecond Raman lasers.

Stimulated Raman scattering in barium nitrate crystal in tbe external optical cavity

The progress towards an optical soliton telecommunication system is now gathering momentum. This means that design criteria for real fibre deployments must be addressed, urgently. Among these is the question of just how birefringen~el-~ places limits upon the system performance, in terms of its influence on realistic bit patterns and, therefore, upon soliton interactions. A fibre with a constant birefringence can lead to pulse splitting, and it is possible for a fibre to have a distributed birefringence, either intrinsically or perhaps, through the way the fibre is laid. The role of randomly distributed birefringence over randomly distributed sections will, therefore, occupy most of this presentation. Some new work on fibres with a uniform birefringence will also be included, however. Birefringence has been considered, in principle, in the past, but its detailed influence on soliton interactions and a full, generic simulation involving bit patterns has not been attempted before. Mainly, uniform and Gaussian polarisation distributions have been investigated here, but other, more novel, distributions have also been assessed. The fibre has been modelled as randomly distributed sections, with different properties and randomly distributed lengths. We will show that, for a Gaussian distribution, for example, there is a dramatic influence upon solitonic behaviour. In fact, increasing the variance rate significantly reduces interaction between solitons. Two forms of encoding for the multibit patterns have been used, namely pulse code modulation and pulse position modulation, for which it will be demonstrated that realistic birefringence can cause quite a rapid pattern degeneration. Previous work has not introduced soliton interactions in a realistic model of a fibre, which is imperfect due to unexpected birefringence, so in this report we will delineate the restrictions that this type of defect will place on a model of a real communication system. To do this we will use bright solitons and introduce loss and amplification, with both discrete and distributed gain. We permit noise to be added and consider the bounds of the Gordon-Haus effect by calculating jitter and by producing eye diagrams. Techniques for reducing noise are considered, and the progress of periodic trains of bright or dark solitons is monitored. Bit trains with unequal amplitudes and varying degrees of antiphasing are also followed by computer. The propagation of very short pulses, liable to experience two nonlinear time scales (instantaneous and delayed Raman-type), is also simulated. It will be emphasised that all the work will be presented as generic plots, based upon many hours of computation. They will include depolarisation figures and pulse collapse length in interacting systems as a function of average birefringence parameter and initial electric field vector orientation. Mathematical work will be presented that will demonstrate that the generic plots can be underestood in an elegant, and reasonably quantitative, way. Finally, both space and time effects, leading to optical bullets in birefringence fibres, are analysed for their potential use in future systems. *Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8LT, U.K.

Phase conjugation in LiF and NaF color center crystals

A detailed experiment on phase conjugation via degenerate four-wave mixing in LiF:F2- and NaF (F2+)* crystals pumped by pulse-periodic Nd:YAG laser is described. The dependence of the efficiency of phaser conjugation on the energies and polarizations of the interacting beams and on the concentration of F2- CC is described. The mechanism of phase conjugation is connected with the saturation of the absorption in the color center crystals. Phase conjugation was investigated in LiF:F2- crystals that worked simultaneously as a passive Q-switch of a Nd:YAG laser. This allows use of the LiF:F2- crystal as a phase conjugated mirror of a Nd:YAG laser.

Lasing properties of new Nd3+-doped tungstate, molybdate, and fluoride materials under selective optical pumping

The purpose of this work was to determine the relative efficiencies of new Nd3+-doped laser active/Raman - tungstate, molybdate, and fluoride - materials (SrWO4, PbWO4, BaWO4, SrMoO4, PbMoO4, SrF2, and LaF3) under selective longitudinal optical pumping by the alexandrite (~750nm), or diode (~800nm) laser. Crystals with various length, orientations and active ions concentrations were tested. To optimize the output of the tested lasers a set of input dichroic and output dielectric mirrors with different reflectivities were used. For realized lasers operating at pulsed free-running regime, threshold energy, slope efficiency, emission wavelength, and radiation polarization were determined. For each crystal, fluorescence lifetime and absorption coefficient under given pumping were established. The slope efficiency in case of Nd3+:PbMoO4 laser at wavelength 1054nm was measured to be 54.3% with total efficiency of 46% which is the best result obtained for all new tested crystals. For Nd3+ doped SrWO4, PbWO4, and BaWO4 crystals simultaneous laser and self-Raman emission were demonstrated in Q-switched regime. Thus newly proposed laser Raman crystals demonstrate high efficiency for Nd3+ laser oscillations comparable with well known and widely used Nd:KGW crystal. Further improvement in the quality of tungstate and molybdate type crystals should result in further increase in lasing efficiency at 1.06μm wavelength. Self Raman frequency conversion of Nd3+-laser oscillations in these crystals should result in high efficient pulse shortening, high peak power and new wavelengths in 1.2-1.5μm wavelength region.

Stimulated Raman scattering in barium nitrate crystal in the external optical cavity

Barium nitrate Raman laser pumped with nanosecond YAG:Nd laser is investigated. High energy conversion efficiencies to the first, second, and third Stokes components up to 60%, 35%, and 25%, respectively, are obtained. Results on development of Raman laser for the eye safe spectral region are presented.

Nd:SrWO 4 Raman laser

Properties of the laser operation and simultaneously stimulated Raman scattering in the new SRS-active neodymium doped SrWO4 crystal coherently end-pumped by alexandrite 752 nm laser radiation were investigated. The maximum generated energy 90 mJ from the free-running Nd3+:SrWO4 laser at 1057 nm wavelength was obtained with the output coupler reflectivity 52%. The slope efficiency reached s = 0.52, the beam characteristic parameters M2 and divergence q were 2.5 ± 0.1, and 1.5 ± 0.1 mrad, respectively. Maximal output energy of 1.46 mJ for the fundamental wavelength was obtained for Q-switched Nd3+:SrWO4 oscillator with a double Fabry-Perrot as the output coupler (R = 48%), and with the 5% initial transmission of LiF:F2- saturable absorber. Up to 0.74 mJ energy was registered at the first Stokes frequency. The pulse duration was 5 ns and 2.4 ns for the fundamental and Stokes radiation, respectively. The energy of 1.25 mJ at 1170 nm was obtained for closed Raman resonator with special mirrors. For the case of mode-locking, two dye saturable absorbers (ML51 dye in dichlorethan and 3955 dye in ethanol) were used and SRS radiation in the form of pulse train was observed. The influence of the various Raman laser output couplers reflectivity as well as the initial transmissions of passive absorbers were investigated with the goal of the output energy maximization at the Stokes wavelength. In the output, the total measured energy was 1.8 mJ (for ML51 dye) and 2.4 mJ (for 3955 dye). The SRS output at 1170 nm was approximately 20% of total energy.

Picosecond stimulated Raman scattering in new crystals Nd:SrWO 4 and SrMoO 4

New Nd:SrWO4 and SrMoO4 crystals (45 mm and 33 mm of legnth, respectively) were investigated as Raman frequency converters of 50 picosecond Nd:YAG pulses and compared with already previously measured BaWO4 and KGd(WO4)2(KGW) tungstate crystals (33 mm and 40 mm of length, respectively). A forward SRS action was achieved in both new crystals. During the experiment the threshold energy of stimulated Raman process, generated wavelengths, and conversion efficiencies were measured. Single-pass first Stokes conversion efficiencies reached 25% and 21% for Nd:SrWO4 and SrMoO4 crystals, respectively. These values were compared with the maximal first Stokes efficiencies of previously studied crystals measured in the same experimental setup yielding: BaWO4 (η = 25% and KGW (η = 22%). As concerned the threshold - it was comparable for BaWO4, Nd:SrWO4, and SrMoO4. The SRS effect in KGW crystal started for ~25% higher pump energy. Our study shows that new Nd:SrWO4 and SrMoO4 crystals are other promising materials for picosecond Raman generation.

Properties of transient and steady-state stimulated Raman scattering in KGd(WO4)2 and BaWO4 tungstate crystals

BaWO4 and KGd(WO4)2 (KGW) tungstate crystals (33 mm and 40 mm of length, respectively) were investigated as Raman frequency converters of picosecond and nanosecond second-harmonic Nd:YAG pulses. During the experiment the threshold energy of stimulated Raman (SR) process, generated wavelengths, and energy for nanosecond and picosecond pump pulses with both nonlinear crystals were measured. For BaWO4 crystal, the threshold pumping intensity was measured to be 530 MW/cm2 for psec pumping and 200 MW/cm2 for nsec temporal region. The corresponding Raman gain values were 14.3 cm/GW (picosecond pump) and 38 cm/GW (nanosecond gain). For KGW crystal the threshold intensity values 530 MW/cm2 for psec and 340 MW/cm2 for nsec were measured with corresponding Raman gain values of 11.8 cm/GW (psec) and 18.6 cm/GW (nsec). The Stokes components up to the third order in both psec and nsec regions were detected. Temporal length measurements of pump and Stokes pulses in both crystals revealed pulse shortening by a factor of approximately 2 during the SR process. Due to a high value of Raman gain of a new BaWO4 crystal under both nsec and psec pumping, this crystal can be considered as a unique candidate for utilization in solid-state Raman laser systems.