Raman Laser Research Articles

Efficient Raman Lasing and Raman-Kerr Interaction in an Integrated Silicon Carbide Platform.

Implementing stimulated Raman scattering in a low-loss microresonator could lead to Raman lasing. Here, we report the demonstration of an efficient Raman laser with >50% power efficiency in an integrated silicon carbide platform for the first time. By fine-tuning the free spectral range (FSR) of 43 μm-radius silicon carbide microresonators, the Stokes resonance corresponding to the dominant Raman shift of 777 cm-1 (23.3 THz) is aligned to the center of the Raman gain spectrum, resulting in a low power threshold of 2.5 mW. The peak Raman gain coefficient is estimated to be (0.75 ± 0.15) cm/GW in the 1550 nm band, with an approximate full width at half-maximum of (120 ± 30) GHz. In addition, the microresonator is designed to exhibit normal dispersion at the pump wavelength near 1550 nm while possessing anomalous dispersion at the first Stokes near 1760 nm. At high enough input powers, a Kerr microcomb is generated by the Stokes signal acting as the secondary pump, which then mixes with the pump laser through four-wave mixing to attain a wider spectral coverage. Furthermore, cascaded Raman lasing and the occurrence of multiple Raman shifts, including 204 cm-1 (6.1 THz) and 266 cm-1 (8.0 THz) transitions, are also observed. Finally, we show that the Stokes Raman could also help broaden the spectrum in a Kerr microcomb which has anomalous dispersion at the pump wavelength. Our example of a 100 GHz-FSR microcomb has a wavelength span from 1200 to 1900 nm with 300 mW on-chip power.

Sub-Nanosecond Passively Q-Switched Yellow and Orange Raman Lasers

Sub-nanosecond high-peak-power passively Q-switched yellow and orange lasers are compactly developed using a near-concentric resonator with intracavity-stimulated Raman scattering (SRS) as well as second harmonic generation (SHG). The fundamental wave is generated from an a-cut Nd:YVO4 crystal. The Stokes wave for an SHG yellow (579 nm) or orange (589 nm) laser is generated from a Np-cut potassium gadolinium tungstate (KGW) with the Ng or Nm axis parallel to the c-axis of Nd:YVO4 crystal. The optimal cavity length is systematically explored in a near-concentric configuration to achieve sub-nanosecond high-peak-power pulses. The shortest pulse widths for the yellow and orange output pulses are down to 0.67 ns and 0.70 ns, respectively. The highest peak powers for the yellow and orange output pulses are up to 176 and 138 kW, respectively.

Ten-watt-level all-solid-state eye-safe intracavity Raman laser.

We demonstrate the first ten-watt-level eye-safe intracavity crystalline Raman laser, to the best of our knowledge. The efficient high-power eye-safe Raman laser is intracavity-pumped by an acousto-optically Q-switched 1314 nm two-crystal Nd:YLF laser. Benefiting from the unique bi-axial properties of KGW crystal, two sets of eye-safe dual-wavelength Raman lasers operating at 1461, 1645 nm and 1490, 1721nm are achieved by rotating the Raman crystal. Under the launched pump power of 84.9 W and the repetition rate of 4 kHz, the maximum first-Stokes output powers of 7.9 W at 1461 nm and 8.2 W at 1490 nm are acquired with the second-Stokes output powers of 1.4 W at 1645 nm and 1.5 W at 1721nm, respectively, leading to the eye-safe dual-wavelength Raman output powers of up to 9.3 and 9.7 W. Meanwhile, the pulse durations at the wavelengths of 1461, 1490, 1645, 1721nm are determined to be 4.8, 5.5, 4.3, and 3.6 ns, respectively, which give rise to the peak powers approaching about 410, 370, 80, 100 kW. These Stokes emissions are found to be near diffraction limited with M2 < 1.6 across the entire output power range.

High efficiency stimulated rotational Raman scattering of hydrogen pumpedby 1064 nm.

Laser-induced breakdown (LIB) and the competition of other Raman processes are major reasons restricting photon conversion efficiency (PCE) of Raman lasers. In this work, 1064nm was used as the pump source, and stimulated rotational Raman scattering of hydrogen was investigated. The configuration of zooming out and focusing pump beam was applied, and the dimension of the pump beam at the focus spot increased significantly; consequently, LIB was suppressed, and Raman PCE was improved dramatically. With the help of the Raman gas pressure optimization, vibrational Raman could be fully suppressed, and other competition Raman processes could be well controlled. The optimal PCEs of different rotational Raman lasers could be achieved under different conditions. The maximum PCE of the first rotational Stokes (RS1) was improved to 60.7%, and the maximum energy of RS1 reached 204.5mJ. With the increment of hydrogen pressure, the maximum PCE of the second rotational Stokes (RS2) was improved to 28.2%, and the maximum energy of RS2 reached 123.9mJ. Furthermore, a 2.1µm Raman laser was also generated, the maximum PCE of 2.1µm reached 44.8%, and its pulse energy reached 106.1mJ.

Sub-Nanosecond, High Peak Power Yb:YAG/Cr4+:YAG/YVO4 Passively Q-Switched Raman Microchip Laser with the Emission of Multiple Pulses

This paper demonstrates the capability of sub-nanosecond, high peak power Yb:YAG/Cr4+:YAG/YVO4 passively Q-switched Raman microchip lasers at 1134 nm operated in multiple pulses mode under quasi-continuous-wave (QCW) pumping. Total pulse energy for the Stokes laser was 1.8 mJ with a 4 mm YVO4 crystal and TOC = 16%. The corresponding pulse repetition rate reached 225 kHz within a single pumping pulse. By employing a compact plane-concave cavity and 5 mm YVO4 crystal, the single pulse energy for the Raman laser was further scaled up to 44 μJ. The corresponding peak power was 95 kW. A highest output pulse repetition rate of 87.8 kHz and shortest pulse duration of 464 ps were found for the Raman laser. The results indicate that the Raman microchip laser configuration under QCW LD pumping is a promising approach for developing high peak power, commercial and portable Raman lasers with a pulse duration of several hundred-picoseconds at a pulse repetition rate of hundred kilohertz.

Anisotropic Raman spectral analysis and laser output based on large-sized scheelite bismuth molybdate tungstate mixed crystals

This work presented the near decimeter-sized single crystal growth of Bi2Mo2.6W0.4O12 and the systematic analysis of its Raman spectral characteristics.

Two-photon absorption of radiation in potassium-gadolinium tungstate (KGW) crystals and its effect on the operation of high-energy yellow Raman lasers

Two-photon absorption of radiation in potassium-gadolinium tungstate (KGW) crystals and its effect on the operation of high-energy yellow Raman lasers

Exploration of the impact of graphene oxide, acetylenic gemini, and CTAT on the photophysical and aggregation properties of dipolar coumarin 153.

Advanced spectroscopic techniques have been utilized to study the interaction between the laser dye coumarin 153 (C153) and graphene oxide (GO) nanoparticles. GO was synthesized using a modified Hummers' method and characterized by UV-vis spectroscopy, Raman laser spectroscopy, FTIR-ATR spectroscopy, FESEM, HR-TEM, and XRD techniques. The GO@C153 composite was formed by mixing two aqueous solutions of GO and C153 due to their strong interaction through stacking and hydrophobic interactions. In this case, GO acts as an effective fluorescence quencher for C153 molecules, which undergo H-type aggregation in the presence of GO. The Stern-Volmer equation and time-dependent fluorescence studies were utilized to analyse the mechanism of fluorescence quenching. According to the findings, both static and dynamic quenching processes are responsible for the reduction in fluorescence intensity. The effect of surfactants (both cetyltrimethylammonium p-toluenesulfonate (CTAT) and synthesized N,N'-dihexadecyl-N,N,N',N'-tetramethyl-N,N'-but-2-ynediyl-di-ammonium chloride (16-4-16)) on the aggregation and photophysical properties of the dye was investigated using surface tensiometry, conductometry, UV-vis absorption spectroscopy, steady-state fluorescence measurements, DLS, and time-dependent fluorescence spectroscopy. Surfactants change the microenvironment of the C153 dye, leading to spectrum shifting and a higher quantum yield, which causes a rapid rise in fluorescence intensity in the micellar medium. It has been noted that in a micellar medium rather than in an aqueous one, the luminous intramolecular charge transfer (ICT) state of C153 stabilises. Lastly, we investigated the photophysical behavior of the GO-C153-micelle ternary system and discovered that, in the presence of a micellar medium, the quenched and blue-shifted (H-type aggregation) fluorescence peak of C153 (in the presence of GO) began to intensify once more. The main goal of this work is to create an effective and fairly cost powerful fluorescence sensor. Additionally, the ternary system (GO-C153-micelle) analytical idea can be employed to identify the onset of micelle formation. In wastewater treatment analysis, the GO-C153-surfactant ternary system concept can also be used to regenerate the adsorbent (in this case, GO) from dye molecules by allowing the dye molecules to exit the adsorbent and enter the micellar medium.

Eye-safe intra-cavity diamond cascaded Raman laser with high peak-power and narrow linewidth

Eye-safe intra-cavity diamond cascaded Raman laser with high peak-power and narrow linewidth

220 W Raman fiber laser at 1.24 μm based on phosphosilicate fiber

A Raman fiber laser with more than 220 W output at 1.24 μm with phosphorus-doped single-mode fiber as gain medium is reported. The laser consists of a 30 m long piece of fiber and a pair of fiber Bragg gratings with different bandwidth, pumped with a high power 1064 nm Yb fiber laser. The optical conversion efficiency is 64% and the slope efficiency is 71.4%. The full width half-maximum linewidth at the maximum output is 5.6 nm. To the best of our knowledge, the reported power is an order magnitude higher than previous reports in phosphosilicate fiber Raman laser.

The Frequency-stabilized Raman Laser System Based on Isotope Atom Saturated Absorption Spectrum

The Frequency-stabilized Raman Laser System Based on Isotope Atom Saturated Absorption Spectrum

A continuous-wave Nd:YVO 4 -KGW intracavity Raman laser with over 34% diode-to-Stokes optical efficiency

Abstract We demonstrate a continuous-wave (CW) Nd:YVO4-potassium gadolinium tungstate (KGW) intracavity Raman laser with a diode-to-Stokes optical efficiency of 34.2%. By optimizing the cavity arrangement and reducing the cavity losses, 8.47 W Stokes output at 1177 nm was obtained under an incident 878.6 nm diode pump power of 24.8 W. The influence of cavity losses on the power and efficiency of the CW Raman laser, as well as the potential for further optimization, was investigated based on the numerical model. The observation of thermally-induced output rollover was well explained by the calculation of the thermal lensing and cavity stability, indicating that the end-face curvature played an important role when the end-face of the crystal was highly reflective coated to make the cavity. A 10.9 W Stokes output under 40.9 W incident pump was also demonstrated with a cavity arrangement less sensitive to the end-face curvature, which is the highest output power of CW intracavity Raman lasers reported.

SERS behaviors of multi-shape silver nanoparticles on Si substrate – An insight from both experimental and theoretical approaches

In this study, three types of silver nanostructures, including multi-shaped silver nanoparticles (MAgNPs), silver nanospheres (AgNSs), and triangular silver nanoparticles (TAgNPs), are developed, and their functions for surface-enhanced Raman scattering (SERS) application are investigated using both experimental and theoretical approaches. Particularly, the effects of Ag morphologies, excitation wavelength, and inter-particle gap on their SERS performance are focused. Simulation results show that MAgNPs possess a local-electric-field enhancement factor more than AgNSs (ca. 124 folds) and an electric field wider than TAgNPs. Materials characterization results show that the optimal excitation wavelengths of MAgNPs and AgNSs are in the range of 560–590 nm. Thus, employing the Raman laser source (532 nm) is highly suitable for exciting the SERS effects of Ag nanostructures in this study. For demonstrations, the MAgNPs-based SERS substrates are prepared by coating MAgNPs suspension on Si substrates. These substrates demonstrate high sensitivity to Rhodamine B (LOD of 10−13 M) with consistent reproducibility, robust durability, commendable selectivity, and impressive reusability. The results indicate that the experimental outcomes align with the theoretical simulations. These attributes collectively make the MAgNPs-based substrate a promising candidate for practical applications in Raman techniques.

Gigahertz frequency hopping in an optical phase-locked loop for Raman lasers

Raman lasers are essential in atomic physics, and the development of portable devices has posed requirements for time-division multiplexing of Raman lasers. We demonstrate an innovative gigahertz frequency hopping approach of a slave Raman laser within an optical phase-locked loop (OPLL), which finds practical application in an atomic gravimeter, where the OPLL frequently switches between near-resonance lasers and significantly detuned Raman lasers. The method merges the advantages of rapid and extensive frequency hopping with the OPLL’s inherent low phase noise, and exhibits a versatile range of applications in compact laser systems, promising advancements in portable instruments.

Generation of Narrow Modes in Random Raman Fiber Laser Based on Multimode Fiber

We present a new design of a random Raman fiber laser based on a graded-index multimode fiber as the media composing the cavity that allows the generation of narrow spectral lines. We carried out spectral measurements using an optical heterodyning technique by projecting multimode radiation onto the fundamental mode of a standard single-mode fiber. The measurements confirmed the presence of localized ultra-narrow short-lived modes. We measured the powers of the modes and found it to be significantly higher (up to 25 mW) compared with those in a random Raman laser based on a single-mode fiber (&lt;2 mW), while preserving the same typical lifetimes of a few milliseconds.

Order controllable multi-wavelength laser utilizing cascaded diamond Raman conversion

Controllable multi-wavelength gets worse efficiency and quality at high orders and high energy, remaining cascade Raman laser a challenge. Here, we establish an order controllable multi-wavelength cascaded diamond Raman laser to achieve the first-order to fifth-order Raman lasers output with wavelengths of 573.0, 620.3, 676.1, 743.0 and 824.7 nm. At a 532.8 nm pump energy of 1.60 mJ, the total Stokes output energy of 0.51 mJ with a slope efficiency of 36.5% has been achieved. Meanwhile, a Stokes output threshold model of all order is proposed under short pulse pumping for the first time. The threshold energy of the first-order to fifth-order was 256, 298, 320, 494, 1100 μJ separately, which is in consistent with the simulation results perfectly.

Efficient continuous-wave Nd:YVO4/KGW intracavity Raman laser

We demonstrate an efficient Nd:YVO4/KGW intracavity Raman laser in continuous-wave (CW) scheme. With a V-shaped fundamental laser cavity and a short Stokes cavity in it, the oscillating beam sizes are designed to alleviate the thermal effect and to enhance the Raman gain for efficient CW operation. The output power of CW Stokes wave at 1177 nm reached 9.33 W under an incident laser diode pump power of 36.65 W, with corresponding optical efficiency being 25.5%. To the best of our knowledge, these are the highest Stokes output power and conversion efficiency of CW intracavity Raman lasers.

Plasmonic Multi-Layered Built-in Hotspots Nanogaps for Effectively Activating Analytes.

Multi-layered plasmonic nanostructures are able to highly promote the near-field confinement and effectively activate analytes, which are of predominate significance but are extremely challenging. Herein, the semi-open Au core@carved AuAg multi-shell superstructure nanoparticles(multi-Au@Ag-Au NPs, multi = mono, bi, tri, tetra, and penta) are reported with a high designability on electromagnetic field and capability of effectively capturing analytes. By controlling synthetic parameters such as the number of galvanic exchange and Ag growth, multi-Au@Ag-Au NPs are successfully obtained, with tunable layer numbers and asymmetric nanoholes. Due to collective plasmon oscillations of multi-layered built-in nanogaps, the electromagnetic field strength of a single penta-Au@Ag-Au entity reach 48841. More importantly, the penta-Au@Ag-Au NPs show a remarkable light-harvesting capability, which is adaptive to different Raman lasers, supporting high-diversity detection. Additionally, the structural specificity allows analytes to be sufficiently captured into interior hotspots, and further achieve highly sensitive detection with limit of detection down to 3.22 × 10-12 M. This study not only provides an effective pathway for integrating abundant hotspots and activating target molecules in single plasmonic superstructure, but stimulates advancements in SERS substrates for various applications.

Widely-tunable, multi-band Raman laser based on dispersion-managed thin-film lithium niobate microring resonators

Stimulated Raman scattering is an attractive way to extend the operation spectral range of optical sources. However, the spectral extension range of a tunable Raman laser is limited by the Raman frequency shift and pump tuning bandwidth. This makes it challenging to realize chip-scale, widely tunable Raman lasers, as on-chip lasers only provide limited pump power and tuning bandwidth. Here, we tackle this by dispersion engineering of a thin-film lithium niobate microring resonator, where its high-quality factor ( ~ 2.5 million) ensures a sub-milli-watt (0.8 mW) threshold for Raman lasing while its strong normal dispersion with suppressed avoided mode crossing restrains the competing Kerr comb generation process. Combining the multi-wavelength Raman gain response of lithium niobate and cascaded Raman lasing, we demonstrate a widely tunable Raman laser covering 1592–1955 nm, showing a 335-nm spectral extension range from a 94-nm-tuning-bandwidth pump laser. Our demonstration paves the way to realize chip-scale, widely-tunable Raman lasers.

High-power dual-wavelength intracavity diamond Raman laser

Diamond crystals have garnered significant attention for applications in photonics, owing to their characteristics of ultra-high thermal conductivity, extremely broad spectral transmission range, and high Raman gain coefficient. These characteristics are particularly advantageous for the development of high-power, multi-wavelength lasers. In this work, we utilize single crystal diamond as a Raman gain medium, for intracavity wavelength-conversion of emission from a side-pumped, electro-optically Q-switched Nd:YAG module. Cascaded Raman conversion is demonstrated and dual-wavelength, pulsed laser emission at wavelengths of 1.2 μm (first-Stokes wavelength) and 1.5 μm (second-Stokes wavelength) is achieved (with a maximum combined pulse energy of 6.19 mJ). The relative ratio of output energy at the two Stokes orders could be varied by tuning the characteristics of the output coupling mirror and the length of the laser resonator. When operating at maximum pump power, the pulse width of the second-Stokes field was approximately half that of the first-Stokes field (second-Stokes ∼10 ns, first-Stokes ∼18 ns), resulting in maximum peak powers of 174 kW (at 1.2 μm) and 220 kW (at 1.5 μm). To the best of our knowledge, this is the highest reported peak power generated from a dual-wavelength, intracavity diamond Raman laser. This work highlights the diversity and utility of diamond as an effective wavelength-conversion component in high-power lasers, and stands as the current state-of-the-art in high pulse-energy, high peak-power, multi-wavelength intracavity Raman lasers.