High Raman Gain Research Articles

Nanocavity-Enhanced Giant Stimulated Raman Scattering in Si Nanowires in the Visible Light Region.

Silicon photonics has been a very active area of research especially in the past two decades in order to meet the ever-increasing demand for more computational power and faster device speeds and their natural compatibility with complementary metal-oxide semiconductor. In order to develop Si as a useful photonics material, essential photonic components such as light sources, waveguides, wavelength convertors, modulators, and detectors need to be developed and integrated. However, due to the indirect electronic bandgap of Si, conventional light emission devices such as light-emitting diodes and lasers cannot be built. Therefore, there has been considerable interest in developing Si-based Raman lasers, which are nonlinear devices and require large stimulated Raman scattering (SRS) in an optical cavity. However, due to the low quantum yield of SRS in Si, Raman lasers have very large device footprints and high lasing threshold, making them unsuitable for faster, smaller, and energy-efficient devices. Here, we report strong SRS and extremely high Raman gain in Si nanowire optical cavities in the visible region with measured SRS threshold as low as 30 kW/cm2. At cavity mode resonance, light is confined into a low mode volume and high intensity electromagnetic mode inside the Si nanowire due to its high refractive index, which leads to strong SRS at low pump intensities. Electromagnetic calculations reveal greater than 6 orders of magnitude increase in Raman gain coefficient at 532 nm pump wavelength, compared to the gain value at 1.55 μm wavelength reported in literature, despite the 108 higher losses at 532 nm. Because of the high gain in such small structures, we believe that this is a significant first step in realizing a monolithically integrable nanoscale low-powered Si Raman laser.

Continuous-wave intracavity YVO4/BaWO4 Raman laser pumped by a wavelength-locked 878.9 nm laser diode

In this paper, the composite Nd:YVO4 laser crystal is in-band pumped by a wavelength-locked laser diode at 878.9 nm, with the purpose of reducing thermal effects and improving pump absorption simultaneously. By using the YVO4 and BaWO4 crystals as Raman media, the influences of crystal properties, resonator structure and stability on the performance of continuous-wave intracavity Raman laser are investigated experimentally and theoretically. The results show that the resonator stability greatly affects laser performance due to the long cavity length of intracavity Raman laser. By choosing the Raman medium with high Raman gain, we can not only obtain higher Raman conversion efficiency, but also reduce the thermal effect to a certain extent. Furthermore, the smaller the curvature radius of the output mirror in the plano-concave cavity structure, the greater the power density of the fundamental laser in the Raman crystal is and the wider the dynamic stability region of the resonator, and hence the higher output power of the Raman laser can be achieved. Finally, by using 30-mm BaWO4 crystal as Raman medium, a highest Raman output of 3.02 W is obtained at a pump power of 25.1 W, corresponding to a diode-to-Stokes optical conversion efficiency of 12%.

Stimulated lasing and self-excited stimulated Raman scattering of Nd3+ doped silica microsphere pumped by 808 nm laser

Self-stimulated Raman lasers have attracted more and more interest, because they have no need of additional Raman device, and they are compact in structure and also economical in cost. Self-stimulated Raman lasers are always emitted from crystalline mediums such as Nd3+:KGd(WO4)2, Nd3+:PbWO4 that are commonly used as laser host materials and proved to be available Raman-active mediums. The Nd3+ doped crystals possess high stimulated emission cross-section for laser emission and high Raman gain coefficients for Raman transitions, but the required pump powers (typically hundreds of milliwatts) are large in those experiments.The whispering-gallery mode (WGM) of silica microsphere cavity has achieved the highest Q factor (8×109) to date. The high Q factor and small mode volume make it possible to realize a resonant buildup of high circulating optical intensities, thereby drastically reducing the threshold powers for laser oscillation and stimulated nonlinear process. The coupler with optical fiber taper allows the excitation of WGMs with ultralow coupling loss, which significantly improves the overall efficiency to produce stimulated Raman laser. In this paper, we report the observation of ultralow threshold self-stimulated Raman laser operating in an Nd3+ doped silica microsphere, and the wavelength range can be extended to O-waveband 1143 nm.A high Q microsphere is fabricated with a thin Nd3+ doped silica layer covered by sol-gel method, in which smooth surface is formed by electrical arc-heating. An optical taper fiber is employed to couple the 808 nm laser into Nd3+ doped microsphere (NDSM) to form whispering gallery mode, which acts as the pump light. Based on 4f electron of neodymium ion transmission and optical oscillation in microsphere, the stimulated laser with a wavelength band of 1080 nm-1097 nm is excited. Due to high power density of the excited laser near the surface of orbit in microsphere, the first order self-stimulated Raman laser with a wavelength range of 1120-1143 nm is stimulated in the high Q microsphere. In a theoretical model, the formulas for calculating the output power and the threshold power of the oscillation laser and the self-stimulated Raman scattering are derived. In experiment, we succeed in getting single-mode and multi-mode laser oscillation due to the 4f layer electron transitions of Nd3+ ions, pumped by 808 nm laser. The results show that the NDSM emits a typical single-mode output laser at 1116.8 nm with a pump power of 8.33 dBm, also the relationship between the 1116.8 nm output power and the pump power with a threshold pump power of 3.5 mW. The multi-mode laser spectrum dependent on the microsphere morphology characteristics is observed, which varies by changing the couple position of the optical fiber taper with microsphere. The characteristics of the laser are discussed including the output power, threshold power, spectral line width, side-mode suppression ratio, etc. The NDSM will have many potential applications in new compact lasers. It is beneficial to wavelength converter and optical amplifier in O band.

Raman Spaser in a Plasmonic Nanoantenna Embedded with Raman-Active Nanoparticle

We report on a novel type of spaser which is based on the nonlinear optical Raman gain rather than stimulated emission. It consists of a plasmonic nanoantenna as the cavity and a highly Raman-active diamond nanoparticle as the gain medium. Ultrasmall mode volume accompanied by strong field enhancement and high Raman gain enable Raman spaser-based nanolasing. By using the model describing the oscillation of Raman mode in micro/nanocavity and the calculations for plasmonic nanocavity characteristics, we theoretically study the dependency of spasing threshold on the structural parameters. We believe that our work will trigger the extensive studies of nonlinear optical spasers.

Proposal for ultrasmall deep ultraviolet diamond Raman nanolaser

We propose diamond nanoparticle Raman laser operating in the spectral range of deep ultraviolet. High Raman gain and low cavity loss of diamond nanoparticles enable low-threshold Raman lasing. Based on the coupled-mode theory, we numerically study its lasing dynamics. For the diamond nanoparticle with a radius of about 130 nm, the lasing threshold energy is below 10 pJ for a pump spot size of 1 μm.

Generation of a vacuum ultraviolet to visible Raman frequency comb in H2-filled kagomé photonic crystal fiber.

We report on the generation of a purely vibrational Raman comb, extending from the vacuum ultraviolet (184 nm) to the visible (478 nm), in hydrogen-filled kagomé-style photonic crystal fiber pumped at 266 nm. Stimulated Raman scattering and molecular modulation processes are enhanced by higher Raman gain in the ultraviolet. Owing to the pressure-tunable normal dispersion landscape of the "fiber + gas" system in the ultraviolet, higher-order anti-Stokes bands are generated preferentially in higher-order fiber modes. The results pave the way toward tunable fiber-based sources of deep and vacuum ultraviolet light for applications in, e.g., spectroscopy and biomedicine.

Raman gain of Ge-Sb-Se chalcogenide glass

Previously reported chalcogenide glass Raman fiber lasers are made of glass compositions such as As2S3 or As2Se3. However, due to the high toxicity of the element arsenic, there is a potential risk in the glass preparation, fiber drawing, and testing processes. Therefore, we need to explore new environmentally friendly chalcogenide glasses that do not contain As for Raman fiber lasers. Studies have shown that the chalcogenide glasses of Ge-Sb-Se system have excellent infrared transmissions and good environmental friendliness, and thus they are excellent candidates for chalcogenide glass Raman fiber lasers. However, their Raman gains have not been reported. Then Raman gain coefficients can be obtained by experimental measurements and theoretical analyses. The experimental method requires expensive laboratory equipments, a complex optical path, and precision adjustments. Therefore, the design and preparation of new chalcogenide glass fiber with high Raman gain require the theoretical analysis of the Raman gain characteristics in a particular glass component glass. In this work, four chalcogenide glasses, respectively, with compositions of As2S3, As2Se3, Ge20Sb15Se65 and Ge28Sb12Se60 (mol%) are prepared. Refractive indices, infrared transmission and Raman spectra of these glass samples are measured. By using spontaneous Raman scattering theory combined with the measured Raman spectral data, the values of Raman gain coefficient gR of the chalcogenide glasses are calculated and calibrated by a quartz glass sample. Results show that the gR of As2S3 glass is 6010-13 m/W at 230 cm-1 Raman shift and the gR of As2Se3 glass is 22310-13 m/W at 340 cm-1 Raman shift, which are consistent with the experimental results reported in the literature. Compared with the traditional method, the present method used for calculating the fiber Raman gain coefficient provides great convenience for exploring new chalcogenide glasses with high Raman gain. By using this method, we obtain the gR values of Ge20Sb15Se65 and Ge28Sb12Se60glasses at 200 cm-1 Raman shift, which are 21510-13 m/W and 11110-13 m/W respectively. Meanwhile, we analyze the effects of composition and network structure of chalcogenide glass samples on the Raman gain coefficient and gain spectrum. There are two Raman peaks at 165 cm-1 and 200 cm-1 Raman shift, which are attributed to Ge-Ge bond vibration and Ge-Se bond vibration of common apex GeSe4/2 tetrahedral structure respectively. It could be found that the Raman gain coefficient of Ge20Sb15Se65 glass is bigger than that of Ge28Sb12Se60glass at 200 cm-1 Raman shift because of more Ge-Se bonds. By further optimizing the ratio of components of Ge-Sb-Se chalcogenide glass, we could obtain higher Raman gain coefficient at a particular frequency shift. These results show that the Raman gain coefficient of Ge-Sb-Se chalcogenide glass without poisonous element is up to over 200 times that of the ordinary quartz glass, which provides a new possibility for environment-friendly Raman fiber laser material.

Monoclinic m-LaVO4—a novel high Raman gain SRS-active crystal

Monoclinic m-LaVO4, having a monazite-type structure and being known as a host crystal for Nd3+-lasants, is an attractive χ(3)-nonlinear optical material for Raman laser converters. This paper presents the stimulated Raman scattering (SRS) spectroscopy of m-LaVO4 in the visible and near-IR spectral range using picosecond laser excitation. High-order SRS and Raman-induced four-wave parametric mixing processes result in an almost two-octave-wide Stokes and anti-Stokes frequency comb. All the measured spectral lines were assigned to the participating molecular vibration modes of the studied vanadate. Finally, an estimation of the steady-state Raman gain coefficient for first Stokes generation in near-IR was performed.

Compositional and thermal treatment effects on Raman gain and bandwidth in nanostructured silica based glasses

Investigations of new materials possessing both higher Raman gain and larger spectral bandwidth than fused silica are becoming mandatory in order to satisfy increasing telecommunications demands. Herein, silica–niobia based glasses have been prepared and characterized by ellipsometry and Raman spectroscopy. A fine glass structural characterization of bulk system has been performed and the Raman gain has been quantified. Our silica-based glass system (K2O–Nb2O5–SiO2 (KNS)) provides via thermal treatment a nanocomposite material of easy and cheap fabrication. In our glass composition a strong Raman gain (up to 30 times higher than fused silica) and a large bandwidth have been measured and demonstrated to be related to nanostructured material domains.

Supercontinuum generated in all-normal dispersion photonic crystal fibers with picosecond pump pulses

The supercontinuum (SC) generation in all-normal dispersion (ANDi) photonic-crystal fiber (PCF) pumped by high power picosecond pulses are investigated in this paper. Our results show that an octave SC may be achieved by pumping the ANDi PCF with picosecond pump pulses. However, the PCF length required may have to be lengthened to several tens of centimeters, which is much longer than that with femtosecond pump pulses. The relatively long PCF gives rise to much higher Raman gain and stronger Raman frequency shift compared to those with femtosecond pump pulses, which in turn not only cause a distorted temporal waveform and an un-flattened spectrum, but also severely degrade the coherence of the generated SC.

Designs of photonic crystal nanocavities for stimulated Raman scattering in diamond

Diamond is a good candidate for producing Raman laser due to its high first-order Raman gain coefficient. Since its Raman shift (~1,332.5 cm−1) is large compared to other solid-state materials, it is possible to produce a Raman frequency converter using diamond crystals. Photonic crystals can be employed for confining photons within periodic structures, the scale of which is on the order of the incident wavelength, making it convenient for integrating all-optical circuits. Combining the merits of both diamond and photonic crystals, we present two designs of photonic crystal nanocavities (in hexagonal and square lattice structures) which can produce stimulated Raman lasing with low-threshold power. After optimizing the photonic bandgaps, triple resonant modes with high Q and small modal volume are realized in each design by tuning the radii of dot defects in the nanocavities. Numerical simulations show that for such designs, the threshold power for generating Raman lasers in the range of a few hundred nano-Watts can be achieved.

Compositional effect of WO3, MoO3, and P2O5 on Raman spectroscopy of tellurite glass for broadband and high gain Raman amplifier

In order to obtain broadband and high Raman gain coefficient in the tellurite glass, a detailed study of the effects of WO3, MoO3, and P2O5 in TeO2-ZnO-Na2O-Nb2O5 (TZNN) glass system on the thermal stabilities and Raman spectroscopic was performed. It was found that both WO3 and MoO3 improved the glass thermal stability and enhanced the bandwidth significantly. Higher Raman gain coefficients and broader bandwidth were realized in the MoO3 modified glasses than those of WO3 added glass. The tellurite glass containing 15 mol. % MoO3 exhibits the bandwidth 1.7 times larger than the silica glass and the Raman gain coefficient is as high as 38 times that of the silica glass. Moreover, with the addition of P2O5 in TeO2-ZnO-Na2O-Nb2O5-WO3 (TZNNW) and TeO2-ZnO-Na2O-Nb2O5-MoO3 (TZNNM) glass systems, the thermal stabilities can further be improved. On the other hand, the Raman bandwidth can further be broadened, especially in the TZNNMP glass system. The tellurite glass containing 15 mol. % MoO3 and 15 mol. % P2O5 shows the bandwidth 1.9 times larger than the silica glass and maintains high Raman gain coefficient which is as high as 37 times that of the silica glass, indicating this glass is a promising candidate as new gain media for broadband Raman fiber amplifier.

1.06 $\mu$m Picosecond Pulsed, Normal Dispersion Pumping for Generating Efficient Broadband Infrared Supercontinuum in Meter-Length Single-Mode Tellurite Holey Fiber With High Raman Gain Coefficient

We investigate efficient broadband infrared supercontinuum generation in meter-length single-mode small-core tellurite holey fiber. The fiber is pumped by 1.06 μm picosecond pulses in the normal dispersion region. The high Raman gain coefficient and the broad Raman gain bands of the tellurite glass are exploited to generate a cascade of Raman Stokes orders, which initiate in the highly normal dispersion region and quickly extend to longer wavelengths across the zero dispersion wavelength with increasing pump power. A broadband supercontinuum from 1.06 μm to beyond 1.70 μm is generated. The effects of the pump power and of the fiber length on the spectrum and on the power conversion efficiency from the pump to the supercontinuum are discussed. Power scaling indicates that using this viable normal dispersion pumping scheme, 9.5 W average output power of infrared supercontinuum and more than 60% conversion efficiency can be obtained from a 1 m long tellurite fiber with a large mode area of 500 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

A Highly Nonlinear Spiral Photonic Crystal Fiber for Tailoring Two Zero Dispersion Wavelengths in the Visible Region

A dispersion flattened, spiral silica photonic crystal fiber (SSPCF) is presented here for supercontinuum generation in the visible region. Two zero dispersion wavelengths (ZDWs) (570nm & 630nm) are obtained in the visible region for a core diameter of 600nm and the range of the anomalous dispersion increasing with the increment of the core diameter. The fiber shows high nonlinearity parameter (1433W -1 km -1 ), high Raman gain (698.478W -1 km -1 ), ultraflattened dispersion (-0.05064 ps/nm 2. km) and very low confinement loss (0.00161 dB/km) at 600 nm. The proposed SSPCF shows improvement over the dispersion control of a hexagonal PCF, and low damage threshold of a highly nonlinear (HN) soft glass PCF. So it can be an excellent candidate for generating supercontinuum in the visible region with HeNe laser of pump wavelenght at 612nm. Full Text: PDF

High raman gain of dispersion compensation fiber using RZ‐DPSK format for long‐haul DWDM transmission system

AbstractWe investigated the transport of a 16 channels 40 Gb/s dense wavelength division multiplexing (DWDM) system which uses high Raman gain of dispersion compensation fiber (DCF) for long‐haul DWDM transmission. Using the return to zero differential phase shift keying (RZ‐DPSK) modulation format with an optimized dispersion compensation format, we demonstrate DWDM transmission with a capacity of 640 Gb/s with 0.4 nm channel spacing over 4500 km of transmission fiber. The transmission system structure uses 120 km single mode fiber and 30 km DCF for thirty spans in the C band wavelength range and a Raman Amplifier with high Raman gain to achieve long‐haul transmission. We also used enhanced forward error correction for high capacity transmission over several thousand kilometers with the RZ‐DPSK modulation format. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:2548–2551, 2010; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25552

Self-tunable chalcogenide Raman laser

We have developed a pervious Raman extracavity laser intensity equation by taking into account the nonlinear Kerr effect and group velocity dispersion and by considering the chalcogenide glass as an active material for its high Raman gain. The propagation of fundamental dark solitons is simulated by the combination of the Crank-Nicolson scheme, FDTD, and the Gause-Seidel iteration scheme. The results show that for a given Kerr nonlinearity there exists a critical minimum input peak power that makes a laser self-tunable such that the different frequency component is achieved by control of the input intensity above the critical intensity.

Widely tunable ring-cavity tellurite fiber Raman laser

We report a widely tunable ring-cavity tellurite fiber Raman laser covering the S+C+L+U band. A tunable range (1495-1600 nm, limited by the tunable optical bandpass filter) over 100 nm is obtained by using a single-mode tellurite fiber with high Raman gain coefficients (55 W(-1)km(-1)) and large Raman shift (approximately 22.3 THz) as the gain medium. Furthermore, the free-running 1665 nm Raman fiber laser is achieved from the ring cavity by removing the tunable optical bandpass filter, which shows that such a tellurite fiber has potential for constructing a widely tunable fiber Raman laser covering the S+C+L+U band. A high optical signal-to-noise ratio of over 60 dB for almost all of the tunable range is also demonstrated.

Observation of a high Raman gain induced by a pulsed pump laser in a Λ system of three fine-structure levels

We have observed a high Raman gain in a quasi-stationary state realized by a single strong-pulsed-pump laser. The Raman gain, originating from the population inversion in the dressed state picture, was as large as 1700% in a Lambda system of three fine-structure levels of atomic samarium, corresponding to a gain cross section comparable with the linear absorption cross section for the same transition.

Analysis of a realistic and idealized dispersion compensating photonic crystal fiber Raman amplifier

In the present paper, the highly efficient Raman amplification properties of a realistic and idealized dispersion compensating photonic crystal fiber (DCPCF) are described numerically. We have used an accurate full-vectorial finite element modal solver with hybrid curvilinear edge/nodal elements and anisotropic perfectly matched layers for an accurate modal characterization of the realistic as well as idealized DCPCF. A good agreement is observed between numerically evaluated and experimentally [P.J. Roberts, B.J. Mangan, H. Sabert, F. Couny, et al., J. Opt. Fiber Commun. Rep. 2 (2005) 435–461] measured dispersion values. A high peak Raman gain efficiency of 10.5 W −1 km −1 is obtained at 13.1 THz frequency shift for a 1455 nm depolarized pump. A DCPCF module of 1-km length can compensate for the dispersion accumulated over 70-km of conventional single mode fiber link with a residual dispersion of ±50 ps/nm and can provide an average gain of 7.6 dB with ±0.9 dB gain ripples over C-band.

Soliton transients and supercontinuum generation in high-Raman-gain materials

Modification of the Raman response of a nonlinear-optical material can dramatically change the spectral and temporal evolution of ultrashort pulses, leading to new interesting and practically significant regimes of short-pulse dynamics. We show that, in fibers with a high Raman gain, solitonic spikes can arise only as transient features in the fast dynamics of ultrashort field waveforms. Enhanced Raman scattering induces a fast frequency downshift of these transients, preventing the formation of temporally and spectrally isolated solitons and giving rise to new highly efficient regimes of supercontinuum generation.