Large Raman Gain Research Articles

Theoretical Study of Multicascade Raman Microlasers Based on TeO2–WO3–Bi2O3 Glass

The development and investigation of miniature narrow-line coherent light sources based on microresonators with low-power-consumption whispering gallery modes (WGMs) is an actual trend in modern photonics. Raman WGM microlasers can operate at wavelengths inaccessible to traditional laser media and provide a huge pump frequency tuning range. Here, we propose and theoretically study multicascade Raman microlasers based on soft tellurite TeO2–WO3–Bi2O3 glass WGM microresonators (microspheres) which can operate in the near-IR and mid-IR with the pump in the telecommunication range. Thanks to a large Raman gain (120 times exceeding the maximum Raman gain of silica glass) and a huge Raman frequency shift of 27.5 THz for this glass, the Raman waves at 1.83 µm, 2.21 µm, 2.77 µm, and 3.7 µm in the first, second, third, and fourth cascades, respectively, are theoretically demonstrated with a pump at 1.57 µm. We analyze in detail the influence of different factors on the characteristics of the generated Raman waves, such as microsphere diameters, Q-factors, pump powers, and detuning of the pump frequency from exact resonance. We also solve a thermo-optical problem to show that the temperature of a soft glass microresonator heated due to partial thermalization of pump power remains below the glass transition temperature. To the best of our knowledge, mid-IR tellurite glass Raman WGM microlasers have not been studied before.

Manipulating Coupled Field Enhancement in Slot-under-Groove Nanoarrays for Universal Surface-Enhanced Raman Scattering.

Surface-enhanced Raman scattering (SERS) is an ultrasensitive spectroscopic technique that can identify materials and chemicals based on their inelastic light-scattering properties. In general, SERS relies on sub-10 nm nanogaps to amplify the Raman signals and achieve ultralow-concentration identification of analytes. However, large-sized analytes, such as proteins and viruses, usually cannot enter these tiny nanogaps, limiting the practical applications of SERS. Herein, we demonstrate a universal SERS platform for the reliable and sensitive identification of a wide range of analytes. The key to this success is the prepared "slot-under-groove" nanoarchitecture arrays, which could realize a strongly coupled field enhancement with a large spatial mode distribution via the hybridization of gap-surface plasmons in the upper V-groove and localized surface plasmon resonance in the lower slot. Therefore, our slot-under-groove platform can simultaneously deliver high sensitivity for small-sized analytes and the identification of large-sized analytes with a large Raman gain.

Raman Lasing in a Tellurite Microsphere with Thermo-Optical on/off Switching by an Auxiliary Laser Diode.

The generation of coherent light based on inelastic stimulated Raman scattering in photonic microresonators has been attracting great interest in recent years. Tellurite glasses are promising materials for such microdevices since they have large Raman gain and large Raman frequency shift. We experimentally obtained Raman lasing at a wavelength of 1.8 µm with a frequency shift of 27.5 THz from a 1.54 µm narrow-line pump in a 60 µm tellurite glass microsphere with a Q-factor of 2.5 × 107. We demonstrated experimentally a robust, simple, and cheap way of thermo-optically controlled on/off switching of Raman lasing in a tellurite glass microsphere by an auxiliary laser diode. With a permanently operating narrow-line pump laser, on/off switching of the auxiliary 405 nm laser diode led to off/on switching of Raman generation. We also performed theoretical studies supporting the experimental results. The temperature distribution and thermal frequency shifts in eigenmodes in the microspheres heated by the thermalized power of an auxiliary diode and the partially thermalized power of a pump laser were numerically simulated. We analyzed the optical characteristics of Raman generation in microspheres of different diameters. The numerical results were in good agreement with the experimental ones.

Raman amplification at 2.2 μm in silicon core fibers with prospects for extended mid-infrared source generation

Raman scattering provides a convenient mechanism to generate or amplify light at wavelengths where gain is not otherwise available. When combined with recent advancements in high-power fiber lasers that operate at wavelengths ~2 μm, great opportunities exist for Raman systems that extend operation further into the mid-infrared regime for applications such as gas sensing, spectroscopy, and biomedical analyses. Here, a thulium-doped fiber laser is used to demonstrate Raman emission and amplification from a highly nonlinear silicon core fiber (SCF) platform at wavelengths beyond 2 μm. The SCF has been tapered to obtain a micrometer-sized core diameter (~1.6 μm) over a length of 6 cm, with losses as low as 0.2 dB cm−1. A maximum on-off peak gain of 30.4 dB was obtained using 10 W of peak pump power at 1.99 μm, with simulations indicating that the gain could be increased to up to ~50 dB by extending the SCF length. Simulations also show that by exploiting the large Raman gain and extended mid-infrared transparency of the SCF, cascaded Raman processes could yield tunable systems with practical output powers across the 2–5 μm range.

Continuous-Wave Raman Lasing from Metal-Linked Organic Dimer Microcrystals.

Stimulated Raman scattering offers an alternative strategy to explore continuous-wave (c.w.) organic lasers, which, however, still suffers from the limitation of inadequate Raman gain in organic material systems. Here we propose a metal-linking approach to enhance the Raman gain of organic molecules. Self-assembled microcrystals of the metal linked organic dimers exhibit large Raman gain, therefore allowing for c.w. Raman lasing. Furthermore, broadband tunable Raman lasing is achieved in the organic dimer microcrystals by adjusting excitation wavelengths. This work advances the understanding of Raman gain in organic molecules, paving a way for the design of c.w. organic lasers.

Continuous‐Wave Raman Lasing from Metal‐Linked Organic Dimer Microcrystals

AbstractStimulated Raman scattering offers an alternative strategy to explore continuous‐wave (c.w.) organic lasers, which, however, still suffers from the limitation of inadequate Raman gain in organic material systems. Here we propose a metal‐linking approach to enhance the Raman gain of organic molecules. Self‐assembled microcrystals of the metal linked organic dimers exhibit large Raman gain, therefore allowing for c.w. Raman lasing. Furthermore, broadband tunable Raman lasing is achieved in the organic dimer microcrystals by adjusting excitation wavelengths. This work advances the understanding of Raman gain in organic molecules, paving a way for the design of c.w. organic lasers.

Highly Nonlinear Multimode Tellurite Fibers: From Glass Synthesis to Practical Applications in Multiphoton Imaging

Herein, a tellurite multimode optical fiber with a rectangular step‐index core to combine easy coupling of the pump power with the exceptionally cubic nonlinearity of such type of glass is developed. An alkali‐free composition (TeO2–ZnO–La2O3) is prepared to produce a preform with the use of both the stack‐and‐draw and the direct‐bonding methods. The linear and nonlinear optical responses of the fiber are characterized and are then used to generate a homogeneous supercontinuum (SC) in the near‐infrared domain. The spectral broadening, which is obtained in this work in a short fiber sample, is used for imaging mouse kidney cells, labeled with three different fluorochromes, by means of two‐photon absorption (2PA) and three‐photon absorption (3PA). Despite the multimode nature of the output beam, clear images of tubules, actin, and nucleus are collected with a spatial resolution down to 1.2 μm. Although image acquisition at better resolution has already been reported in the literature using relatively long spans of graded‐index silica multimode fibers, here, the combination of high third‐order susceptibility of the glass and the large Raman gain in short segments of tellurite fiber is exploited to obtain nonlinear imaging with adequate spatial resolution.

Improving the Raman amplifier bandwidth and gain using multi-micro ring photonic crystal structures

In this paper, first the improvement in the Raman amplification bandwidth through the self-phase modulation (SPM) effect in the straight photonic crystal structures was compared to the single-ring and 2-ring photonic crystal structures. The resonance effect in micro ring structures, enhances the effective pump power and thus can achieve the same level of Raman gain at a much lower input pump power, but an increase in the amplification bandwidth in a ring structure is limited by the filtering effect of the micro ring. Then, in order to increase the Raman amplification bandwidth and gain, a 10-ring new structure was introduced. The design of this new structure is based on the use of optofluidic materials. To model Raman amplification in these structures, Maxwell equations are solved using finite difference time domain method considering optical nonlinear parameters like two photon absorption, free carrier absorption, Kerr effect and self-phase modulation effect. This multi-ring Raman amplifier yields a large Raman gain (26.79 dB) and a considerable Raman amplification bandwidth (10.33 mm) despite its extremely short amplification length (160 μm) and an extremely low input pump power (0.5 W).

Ultrafast Raman Fiber Laser Based on Cavity Matching Scheme and Heavily Germania-Core Fiber

We experimentally demonstrate the large-energy short-pulsed Raman fiber laser by pumping heavily GeO $_2$ -doped fiber (GDF) with a dissipative soliton resonance (DSR) mode-locked Yb-doped fiber laser. By exploiting the large Raman gain of GDF and the exotic properties of the DSR mode-locked pulses, the cavity matching condition can be facilely fulfilled with high mismatch tolerance and simple cavity design. The Raman fiber laser therefore initiates at 1118 nm and generates pulses with narrow pulse width of 647 ps or pulse energy upto 22 nJ. The novel Raman fiber laser system cannot only achieve synchronization between the Raman laser pulses and the pump waves without delay line, but also features readily tunable pulse duration and peak power. This proof-of-principle work may provide a potential useful route to ultrafast, energy-scalable, and wavelength-versatile (e.g., mid-infrared) pulse generation directly in fiber lasers with low cost, easy-accessibility along with compact structure.

Toward an ideal nanomaterial for on-chip Raman laser

One of the most important applications of stimulated Raman scattering (SRS) is the realization of amplifiers or laser sources in bulk materials, in fiber and in integrated optic format as well. We note that, as a general rule, in all laser gain bulk materials, there is a tradeoff between gain and bandwidth: line width may be increased at the expense of peak gain. This tradeoff is a fundamental limitation toward the realization of micro/nano-sources with large emission spectra. In this paper, in order to clarify the possibility of obtaining new materials with both large Raman gain coefficients and spectral bandwidth, SRS investigations in nanostructures, spanning from nanometrically heterogeneous K2O–Nb2O5SiO2 (KNS) glasses to Si nanocrystals, are reported and discussed.

Nonlinear characterization of GeSbS chalcogenide glass waveguides.

GeSbS ridge waveguides have recently been demonstrated as a promising mid – infrared platform for integrated waveguide – based chemical sensing and photodetection. To date, their nonlinear optical properties remain relatively unexplored. In this paper, we characterize the nonlinear optical properties of GeSbS glasses, and show negligible nonlinear losses at 1.55 μm. Using self – phase modulation experiments, we characterize a waveguide nonlinear parameter of 7 W−1/m and nonlinear refractive index of 3.71 × 10−18 m2/W. GeSbS waveguides are used to generate supercontinuum from 1280 nm to 2120 nm at the −30 dB level. The spectrum expands along the red shifted side of the spectrum faster than on the blue shifted side, facilitated by cascaded stimulated Raman scattering arising from the large Raman gain of chalcogenides. Fourier transform infrared spectroscopic measurements show that these glasses are optically transparent up to 25 μm, making them useful for short – wave to long – wave infrared applications in both linear and nonlinear optics.

Enhanced Raman amplification by conventional and hybrid photonic crystal based ring structure

In this paper, we present new structures based on ring, conventional photonic crystal and hybrid PhC for enhanced Raman amplification. The structures consist of two separate entrances for the pump and signal. In this way, only pump is coupled into the ring and signal passes through the amplified pump without creating coupling noise. Using engineered nano holes filled with optofluidic materials in the signal and pump paths, we reduce pump and signal group velocity to improve the structure and achieve larger Raman gain and less dispersion. The time evolution and propagation of picosecond signal pulses and dispersion inside the device are analyzed and Raman gain, Raman bandwidth and bit rate are studied in one-ring and two-ring structures. To model Raman amplification in these structures, Maxwell equations are solved using finite difference time domain method considering optical nonlinear parameters like two photon absorption, free carrier absorption and Kerr effect.

Phosphate glass-clad tellurium semiconductor core optical fibers

For the first time to the best of our knowledge phosphate glass-clad optical fibers comprising tellurium (Te) semiconductor core have been fabricated using a molten core approach. The cores were found to be highly crystalline and phase-pure as evidenced by X-ray diffraction (XRD) and corroborated by Micro-Raman spectrum. Elemental analysis across the core/clad interface suggests that there is some diffusion of oxygen and phosphorus into the core region and, conversely, diffusion of Te into the cladding region. Unfortunately, the propagation loss of the Te core fibers was too high to measure due to the significant scattering from the grain boundaries, oxygen and phosphor precipitates. However, the larger Raman gain, infrared and terahertz transparency of tellurium over silicon and germanium should make these fibers of significant value for fiber-based mid- to long-wave infrared, terahertz waveguides and Raman-shifted infrared light sources.

Continuous-wave anti-Stokes Raman laser based on phase-matched nondegenerate four-wave mixing.

We demonstrate phase-matched nondegenerate four-wave mixing (FWM) in a high-finesse optical cavity using a gaseous Raman-active medium pumped by two independent continuous-wave lasers. Efficient upconversion is achieved for pump beams at different wavelengths under phase-matched conditions by optimizing the total dispersion of the hydrogen-filled optical cavity. The independent control of the pump-beam polarizations leads to further enhancement of the upconversion efficiency arising from a larger Raman gain than that in degenerate FWM. This approach offers a promising alternative for a narrow-linewidth tunable light source for highly precise laser spectroscopy.

New design of ring-based Raman amplifier using optofluidic materials

We present innovative structures for ring-based Raman amplifier with two separate entrances for the pump and signal. In this way, only the pump is coupled into the ring and the signal passes directly through the path that the amplified pump is propagating inside the ring without a destructive effect on signal pulse. Using engineered nanoholes filled with optofluidic material in the signal and pump paths, we reduce pump and signal group velocity to improve the structure and achieve larger Raman gain. The time evolution and propagation of picosecond signal pulses and dispersion inside the device are analyzed. Maxwell equations are solved using finite difference time-domain method considering optical nonlinear parameters like two photon absorption, free-carrier absorption, and Kerr effect.

Integrated Diamond Networks for Quantum Nanophotonics

We demonstrate an integrated nanophotonic network in diamond, consisting of a ring resonator coupled to an optical waveguide with grating in- and outcouplers. Using a nitrogen-vacancy color center embedded inside the ring resonator as a source of photons, single photon generation and routing at room temperature is observed. Furthermore, we observe a large overall photon extraction efficiency (10%) and high quality factors of ring resonators (3200 for waveguide-coupled system and 12,600 for a bare ring).

Large Raman Gain in a Stable Nanocomposite Based on Niobiosilicate Glass

The optical properties of composite materials can be adjusted by controlling the constituents and morphology of the composite structure. The optical nanocomposite approach offers opportunities to produce high-performance and relatively low-cost optoelectronic media suitable for many applications. In this paper, a nanocomposite material obtained by a high niobium content glass belonging to the K2O–Nb2O5–SiO2 (KNS) glass-forming system is prepared and characterized. We focus on the investigation of spontaneous and stimulated Raman scattering. Strong changes of Raman spectra in nanocomposite material with respect to bulk sample are proved and discussed, while a significant enhancement of Raman gain (up to 25 times higher) and of its bandwidth with respect to SiO2 glass is reported. This family of glasses is a promising candidate for next generation Raman gain applications for use in the format of integrated optics as well.

Supercontinuum generation in highly Ge-doped core Y-shaped microstructured optical fiber

We report experimental results on supercontinuum generation in a highly Ge-doped core Y-shaped microstructured optical fiber using pump pulses of 9 ns duration at 1064 nm. The fiber is easy to fabricate due to its simple structure and shows good compatibility with standard fiber. The generation of nonlinear effects takes advantage of the large nonlinear refractive index and Raman gain of the Ge-doped core, as well as the air hole structure that surrounds the core. Although the fiber was pumped in normal dispersion and relatively far from the zero dispersion wavelength, flat and smooth supercontinuum in the fundamental mode from 550 nm to beyond 1750 nm was generated with a rather moderate value of fiber length and pump peak power product of 13 kW⋅m.

Experimental study on the effect of codirectional Raman gain on system’s performance

We have experimentally investigated the impact of codirectional Raman gains on the performance of distributed fiber Raman amplified systems. The effects of various noise sources, such as optical signal-to-noise ratio (OSNR) degradation, fiber nonlinearities and surviving channel gain variation in different Raman pumping schemes, were evaluated as a function of input power into a fiber span. For measurements, distributed Raman gain was generated by pumping the fiber span with different combinations of Raman pump power between co- and counterdirections. From the results, we found that a large counterdirectional Raman gain assisted by a small codirectional gain could improve the overall static and dynamic performance of distributed Raman amplified systems.

Stimulated Raman X waves in ultrashort optical pulse filamentation

We demonstrate that ultrashort pulse filamentation in liquids with strong Raman gain leads to the spontaneous formation of nonlinear X waves at a Raman-shifted wavelength. We measured as much as 75% energy conversion efficiency into a Raman X wave in ethanol starting from 1 ps pulses due to the group velocity matching between the pump and Raman X pulses. Large Raman gain of a weak seed signal was observed in water, associated with a strong spatiotemporal transformation of the seed into an X wave.