Raman Oscillator Research Articles

Raman Oscillation Induced by Weak Internal Reflections of Fiber Devices in High-Power Fiber Amplifiers

Raman Oscillation Induced by Weak Internal Reflections of Fiber Devices in High-Power Fiber Amplifiers

All-fiber Raman oscillator with 1.8 kW output power

All-fiber Raman oscillator with 1.8 kW output power

Structure and fluorescence characteristics in highly Dy3+ doped tellurite-fluoride glass for white light fiber lasers

High concentration Dy3+ doped 75TeO2-25YF3-xDyF3 (where x = 8, 10, 12, 14 mo1%) were synthesized by a melt-quenching method. Raman spectra and oscillator strength were utilized to evaluate the structural of glass and radiation properties with differing DyF3 concentrations. The maximum phonon energy of the glass is about 770 cm-1. Ω6 and χ are 3.5 and 0.51 when DyF3 concentration reaches 10 mol%, indicating that the activated ions in the glass material system have a wide fluorescence bandwidth and excellent stimulated emission characteristics. The yellow/blue value enhance with the addition of high concentrations of DyF3, which may be due to the depolymerization of [TeO4], gradually converting into [TeO3+1] and [TeO3], destroying with the local coordination environment and inverting Dy3+ symmetry. Furthermore, under the excitation wavelength of 350 nm, the TYDF2 glass obtained relatively large emission cross-section (35.53 × 10−22 cm2) and gain coefficient (5.88 cm−1) at 4F9/2 → 6H13/2. The CIE coordinates are in the range of white light, and the CCT values are between 4200 and 4400 K. The results show that highly Dy3+ doped tellurite-fluoride glasses prepared in this way have the potential as a gain materials for white light fiber lasers.

High-power free-running single-longitudinal-mode diamond Raman laser enabled by suppressing parasitic stimulated Brillouin scattering

Abstract A continuous-wave (CW) single-longitudinal-mode (SLM) Raman laser at 1240 nm with power of up to 20.6 W was demonstrated in a free-running diamond Raman oscillator without any axial-mode selection elements. The SLM operation was achieved due to the spatial-hole-burning free nature of Raman gain and was maintained at the highest available pump power by suppressing the parasitic stimulated Brillouin scattering (SBS). A folded-cavity design was employed for reducing the perturbing effect of resonances at the pump frequency. At a pump power of 69 W, the maximum Stokes output reached 20.6 W, corresponding to a 30% optical-to-optical conversion efficiency from 1064 to 1240 nm. The result shows that parasitic SBS is the main physical process disturbing the SLM operation of Raman oscillator at higher power. In addition, for the first time, the spectral linewidth of a CW SLM diamond Raman laser was resolved using the long-delayed self-heterodyne interferometric method, which is 105 kHz at 20 W.

Quantum parametric double Raman oscillators with co- and counterpropagating fields: Relative intensity squeezing and spatial photon correlations

We present a comprehensive study of co- and counterpropagating Stokes and anti-Stokes quantum fields in double Raman four-wave mixing parametric oscillators using full quantum Heisenberg-Langevin framework with noise operators. General analytical solutions of the fields operators at any point in the Raman medium are obtained for four cases: two possible copropagating (forward) and two counterpropagating (backward) Stokes and anti-Stokes. We analyze the symmetrical properties of the complex linear and nonlinear susceptibilities spectra of the quantum fields, nonclassicality of two-photon correlation functions, spatial variations of the quantum fields, and the two-mode relative intensity squeezing. We compare the results of forward and backward cases for several limiting double Raman schemes. We find interesting resonant effects of medium length, laser detunings and laser field strengths (Rabi frequencies) for backward-propagating geometries. Analysis of the solutions provide insights on the resonant conditions while computation over multiple variables enables us to identify the values of laser detuning, field strength, and propagation length that give enhanced nonclassical intensity squeezing and persistent correlations. The present work sets the crucial foundations for optimization of the nonclassicality of photons in double Raman systems and would be useful for quantum information storage, quantum nonlinear optics, and quantum spectroscopy.

Broadband yellow-orange light generation based on a step-chirped PPMgLN ridge waveguide.

Yellow-orange lights, valuable in photodynamic therapies, spectroscopy, and optogenetics, are limited by the narrow bandwidth and bulky setup via the conventional Raman or optical parametric oscillation processes. Moreover, flatness in the broad-band spectrum is also important for the aforementioned applications with extended functions. In this paper, by carefully designing grating-periods of a step-chirped PPMgLN ridge waveguide for sum frequency generation (SFG), we report a compact broad-band yellow-orange light with bandwidth of 5.6 nm and an un-reported flatness (<1.5 dB). Correspondingly, the optical conversion efficiency is 232.08%/W, which is the best SFG efficiency for PPMgLN at the yellow-orange region, to the best of our knowledge. The results could also be adopted for other broad-band SFG process toward the vis-infrared region in an integrated structure.

Octave-spanning supercontinuum generation from off-axis Raman oscillation in a monolithic KTP crystal.

We report generation of a visible and near-infrared supercontinuum from a high-gain, ultra-broadband, and mirrorless Raman oscillator in a monolithic KTP crystal. The plane transverse to the pump axis resonates and traps off-axis Stokes waves and their frequency-upconverted components bouncing between two crystal surfaces via total internal reflection. The Raman gain is maximized with the Stokes polarization perpendicular to the plane of reflections. When pumped by a Q-switched Nd:YAG laser, the monolithic oscillator generates quasi-mode-locked Stokes pulses with octave-spanning spectral groups across the visible and near-infrared spectra between 540 and 1800 nm.

Enhanced Raman gain coefficients of semiconductor magneto-plasmas

Considering the origin of stimulated Raman scattering in Raman susceptibility, we obtain expressions for Raman gain coefficients (under steady-state and transient regimes) of semiconductors magneto-plasmas under various geometrical configurations. The threshold value of excitation intensity and most favourable value of pulse duration (above which transient Raman gain vanishes) are estimated. For numerical calculations, we consider n-InSb crystal at 77 K temperature as a Raman active medium exposed to a frequency doubled pulsed CO2 laser. The variation of Raman gain coefficients on doping concentration, magnetic field and its inclination, scattering angle and pump pulse duration have been explored in detail with aim to determine suitable values of these controllable parameters to enhance Raman gain coefficients at lower threshold intensities and to establish the suitability of semiconductor magneto-plasmas as hosts for compression of scattered pulses and fabrication of efficient Raman amplifiers and widely tunable Raman oscillators.

Over 400 W graded-index fiber Raman laser with brightness enhancement.

The power scaling on all-fiberized Raman fiber oscillator with brightness enhancement (BE) based on multimode graded-index (GRIN) fiber is demonstrated. Thanks to beam cleanup of GRIN fiber itself and single-mode selection properties of the fiber Bragg gratings inscribed in the center of GRIN fiber, the efficient BE is realized. For the laser cavity with single OC FBG, continuous-wave power of 334 W with an M2 value of 2.8 and BE value of 5.6 were obtained at a wavelength of 1120 nm with an optical-to-optical efficiency of 49.6%. Furthermore, the cavity reflectivity is increased by employing two OC FBGs to scale the output power up to 443 W, while the corresponding M2 is 3.5 with BE of 4.2. To our best knowledge, it is the highest power in Raman oscillator based on GRIN fiber.

Investigation of high-energy extracavity Raman laser oscillator and single-pass Raman generator based on potassium gadolinium tungstate (KGW) crystal

Herein, a high-energy all-solid-state extracavity Raman oscillator and a high-energy single-pass Raman generator based on potassium gadolinium tungstate (KGW) crystal are theoretically and experimentally investigated. First, a high-energy 1064 nm nanosecond laser system with a maximum output energy of 7.03 J at a repetition rate of 1 Hz is built as the pumping source for stimulated Raman scattering (SRS). Then, the output spectra and energy of multiorder Stokes lasers from 1.16 to 1.49 μm are gradually detected as the 1064 nm pumping laser energy increases for both Raman laser structures. For the extracavity Raman oscillator, under a pumping energy of 1.8 J, three-order Stokes lasers with a maximum energy of 448 mJ are obtained when the electric field direction of the pumping laser is parallel to the Ng optical axis of the KGW crystal (E∥Ng). Moreover, to obtain a higher output of the Raman laser without damaging the cavity mirrors, a single-pass Raman generator is studied for comparison purposes. As the 1064 nm pumping laser energy of 2.8 J is injected, the measured maximum output energy of a two-order Stokes laser is ~ 676 mJ for the E∥Ng condition; this is the highest Stokes energy output of the nanosecond solid-state Raman lasers to the best of our knowledge.

Small nutation of a symmetic gyroscope: two viewpoints

The paper is devoted to the small nutation of an axisymmetric gyroscope in the field of gravity. The expansion of the known solution of the nutation equation as a function of time in powers of the amplitude is obtained. In this case, the frequencies of third order Raman oscillations are both the tripled frequency and the frequency coinciding with the initial one. A formula is found for the nutation amplitude as a function of the integrals of the gyroscope motion. The frequency of zero nutation is also calculated. Another way to obtain the decomposition is to use the results of the general theory of free one-dimensional oscillations. This method is based on the ability to represent the gyro nutation as the movement of a material point of unit mass in a field that cubically-quadratically depends on the coordinate. In this case the only frequency of the third-order Raman oscillation is a triple of the original frequency. Thus, both methods give the same result only for oscillations no higher than second order. In the third approximation, the existing theory of oscillations is insufficient.

Diamond Raman oscillator operating at 1178 nm.

In this contribution, we report high-power Raman frequency downconversion based on an Yb-doped fiber amplifier and a linear external diamond Raman cavity. A maximum output power of 136 W with nearly diffraction-limited beam quality was achieved by pumping in quasi-continuous-wave mode with 10% duty cycle and 10 ms on-time duration. For continuous-wave operation, we achieved record average power of 46 W centered at 1178 nm. The emergence of stimulated Brillouin scattering in diamond is further investigated. This technology shows the potential to extend the spectral range of fiber lasers to reach uncommon wavelengths at high power levels.

Tunable structure and optical properties of single crystal SnS2 flakes

Large-scale hexagonal SnS2 flakes with high quality and tunable structures have been fabricated by an improved chemical vapor deposition method. The flakes follow a spiral growth mechanism and exhibit a two-dimensional structure with a large width-to-height ratio. Raman oscillation induced by laser interference has been observed on the flakes with a pyramid-shaped top. Based on the analysis on oscillatory period and height profile, the index of refraction for the grown SnS2 can be determined. SnS2 flakes with optimized structures have been further designed and prepared successfully, on which enhanced Raman emission can be obtained across the whole flake.

Raman lasing and soliton mode-locking in lithium niobate microresonators

The recent advancement in lithium-niobite-on-insulator (LNOI) technology is opening up new opportunities in optoelectronics, as devices with better performance, lower power consumption and a smaller footprint can be realised due to the high optical confinement in the structures. The LNOI platform offers both large χ(2) and χ(3) nonlinearities along with the power of dispersion engineering, enabling brand new nonlinear photonic devices and applications for the next generation of integrated photonic circuits. However, Raman scattering and its interaction with other nonlinear processes have not been extensively studied in dispersion-engineered LNOI nanodevices. In this work, we characterise the Raman radiation spectra in a monolithic lithium niobate (LN) microresonator via selective excitation of Raman-active phonon modes. The dominant mode for the Raman oscillation is observed in the backward direction for a continuous-wave pump threshold power of 20 mW with a high differential quantum efficiency of 46%. We explore the effects of Raman scattering on Kerr optical frequency comb generation. We achieve mode-locked states in an X-cut LNOI chip through sufficient suppression of the Raman effect via cavity geometry control. Our analysis of the Raman effect provides guidance for the development of future chip-based photonic devices on the LNOI platform.

Coherent bleaching of the medium with Raman scattering under conditions of strong size quantization

We consider stimulated resonant Raman scattering on a system of discretely quantized levels of the superlattice in a quantizing magnetic field, which leads to one-soliton solutions for the Stokes pulses of scattered waves. The consideration is carried out by the method of the equation of motion of the density matrix and perturbation theory. The threshold conditions for the establishment of the Raman oscillations and the formation of stationary one-soliton solutions for the propagation of scattered radiation and possible parametric dependences of the studied processes are revealed.

Single-Frequency BaWO4 Raman MOPA at 1178 nm with 100-ns Pulse Pump

A single-frequency crystalline Raman master oscillator power amplifier (MOPA) at 1178 nm was demonstrated. The pump source was a homemade single-frequency 1062 nm Nd:GGG MOPA system with a pulse width of 104 ns. The BaWO4 Raman oscillator generated seed radiation at 1178 nm with a pulse energy of 7.7 mJ. The highest amplified Raman pulse energy of 41.0 mJ was obtained with a pulse width of 44.1 ns. The linewidth was less than 500 MHz.

Solitons and Josephson-type oscillations in Bose-Einstein condensates with spin-orbit coupling and time-varying Raman frequency

The existence and dynamics of solitons in quasi-one-dimensional Bose-Einstein condensates (BEC) with spin-orbit coupling (SOC) and attractive two-body interactions are described for two coupled atomic pseudo-spin components with slowly and rapidly varying time-dependent Raman frequency. By varying the Raman frequency linearly in time, it was shown that ordinary nonlinear Schr\"odinger-type bright solitons can be converted to striped bright solitons and vice versa. The internal Josephson oscillations between atom-number of the coupled soliton components, and the corresponding center-of-mass motion, are studied for different parameter configurations. In this case, a mechanism to control the soliton parameters is proposed by considering parametric resonances, which can emerge when using time-varying Raman frequencies. Full numerical simulations confirm variational analysis predictions when applied to the region where regular solitons are expected. In the limit of high frequencies, the system is described by a time-averaged Gross-Pitaevskii formalism with renormalized nonlinear and SOC parameters and modified phase-dependent nonlinearities. By comparing full-numerical simulations with averaged results, we have also studied the lower limits for the frequency of Raman oscillations in order to obtain stable soliton solutions.

Intrinsically stable high-power single longitudinal mode laser using spatial hole burning free gain

A fundamental advantage of lasers is their ability to produce a large number of photons in a single optical mode, yet this is achieved in only a minor fraction of devices due to the instability mechanism called spatial hole burning. Here, we exploit the spatial hole burning free nature of a stimulated scattering gain medium to demonstrate single longitudinal mode (SLM) operation in a generic standing wave cavity. A continuous wave diamond Raman oscillator with multi-Watt-level output power and a frequency stability of 80 MHz is demonstrated without use of additional mode-selective elements. Mode stability is addressed by considering the coupling of the Stokes power with thermally induced optical path length changes in the gain medium. The results foreshadow a novel approach for greatly extending the power and wavelength range of SLM laser sources, and with potential advantages for achieving sub-Poissonian intensity noise and sub-Schawlow–Townes linewidths.

Spatial coherence of a Raman wake excited inside a uniform filament

We present results of an investigation of the spatial-temporal structure of the delayed Kerr nonlinearity excited during filamentation of a femtosecond laser pulse inside a crystalline target. It is found that the pulse splitting, which is an inherent property of the filamentation process, constrains the length of the spatial coherence of the Raman wake wave which follows the pump pulse. Each subpulse induces lattice vibrations, which interfere either constructively or destructively depending on the delay between the subpulses. This results in abrupt ``switching'' of the phase of the wake wave along and across the filament volume. For the same reason, the maximal amplitude of the Raman response is observed not at the point of the maximal laser intensity and electron concentration\char22{}it is shifted forward to the end of the filament. This effect should be observed if the period of Raman oscillations is close to the duration of the pump pulse.

Efficient Raman frequency conversion of high‐power fiber lasers in diamond

AbstractWe report high‐power frequency conversion of a Yb‐doped fiber laser using a double‐pass pumped external‐cavity diamond Raman oscillator. Pumping with circular polarization is shown to be efficient while facilitating high‐power optical isolation between the pump and Raman laser. We achieved continuous‐wave average power of 154 W with a conversion efficiency of 50.5% limited by backward‐amplified light in the fiber laser. In order to prove further scalability, we achieved a maximum steady‐state Raman‐shifted output of 381 W with 61% conversion efficiency and excellent beam quality using 10 ms pump pulses, approximately a thousand times longer than the transient thermal time‐constant. No power saturation or degradation in beam quality is observed. The results challenge the present understanding of heat deposition in Raman crystals and foreshadow prospects for reduced thermal effects in diamond than originally anticipated. We also report the first experimental evidence for stimulated Brillouin scattering in diamond. image