Raman Conversion Research Articles

Efficient high-power 1.9 µm picosecond Raman laser in H2-filled hollow-core fiber without generation of rotational lines

In this paper, an efficient high-power 1.9 µm picosecond Raman laser through pure vibrational stimulated Raman scattering (SRS) in H2-filled hollow-core fiber (HCF) is demonstrated. The maximum Raman conversion efficiency of 34 % and the Raman power of 7.3 W (36.5 µJ) is achieved within a 130 cm length of our in-house fabricated fiber. By controlling the H2 pressure, the high-order Stokes and rotational Stokes/anti-Stokes will not be generated, thereby enhancing the efficacy of the 1st Stokes conversion. In addition, the vibrational anti-Stokes are produced inevitably, which can be explained by the phase-matching of higher-order modes. Our results show SRS in H2-filled HCF to be a promising method for generating high-power 1.9 µm picosecond pulses, which are valuable in spectroscopy, defense, and efficient nonlinear conversion.

Multiwavelength highly transient stimulated Raman scattering on dual Raman modes in Sr(MoO4)0.8(WO4)0.2 and Sr(MoO4)0.4(WO4)0.6.

For the first time to our knowledge, multiwavelength, highly transient, single-pass stimulated Raman scattering with a low wavelength spacing on dual (stretching and bending) Raman modes in Sr(MoO4)0.8(WO4)0.2 and Sr(MoO4)0.4(WO4)0.6 solid solutions in a range of 1000-1300 nm (transparence window of biological tissue) under ultrafast chirped pulse laser pumping is comparatively investigated in the interests of multicolor two-photon imaging of a living tissue. For both the solid solutions, the optimum range (1-5 ps) of chirped pump pulse durations for multiwavelength Raman conversion on dual Raman modes was wider than for SrMoO4 (2-3 ps) due to the higher integral cross section of the bending Raman mode. Higher efficient SRS conversion took place at negative chirping of the pump pulse with its stretching from 0.25 ps up to 5 ps due to the compensation of a positive chirp caused by nonlinear phase modulation with total Raman conversion efficiency of up to 36% for Sr(MoO4)0.8(WO4)0.2 and 49% for Sr(MoO4)0.4(WO4)0.6. The highest number (five) of Stokes components in the desired range (1000-1300 nm) was observed in the optimum Sr(MoO4)0.4(WO4)0.6 solid solution, which has the Raman modes with comparable intensities.

External-cavity tunable yellow-red laser based on Ng-cut KGW Raman conversion and frequency mixing

External-cavity tunable yellow-red laser based on Ng-cut KGW Raman conversion and frequency mixing

15.5 W, pulsed 630 nm generation based on Raman fiber laser and second-harmonic generation.

We present a high-power, nanosecond 630 nm beam generation based on Raman conversion and second-harmonic generation (SHG). 116.2 W, single-mode 1080 nm fiber laser based on 10/125 µm optical fiber is used as a pump source for Raman conversion and the 1260 nm seed laser diode helps the amplification of third-order Raman conversion, which results in 63.7W at 1260 nm with 54.8% Raman conversion efficiency. SHG to 630 nm is based on type-I noncritical phase-matching conditions with bismuth triborate (BIBO) nonlinear crystal. The average power of 630 nm is 15.5 W at a repetition rate of 9.26 MHz, a pulse width of 16.0 ns, and a SHG efficiency of 24.4%. This result can facilitate the generation of a high-power visible light source with good beam quality at a specific wavelength.

Ultra-narrow linewidth in continuous-wave external cavity diamond Raman laser

A continuous-wave narrow linewidth diamond Raman laser based on an intracavity frequency doubling structure was demonstrated. Linewidth narrowing and power enhancement were realized by tuning the phase matching in the second-harmonic generation element through temperature control. A maximum output power of 5 W at 592 nm with a linewidth of 72 MHz was obtained using a Yb doped fiber laser with a power of 40 W and a linewidth of >4 GHz, an enhancement of the linewidth by a factor of about 65. The results indicate that high-power and narrow linewidth lasers can be produced utilizing diamond Raman conversion combined with the phase matching in the second-harmonic generation element.

Cooperative properties of multiple quantum scattering: II. Coherentlasing

The description model of the multiple scattering lasers using the superposition states between the generated photons in the ensemble of bi-modes of the resonator field, we introduced a concept of indistinguishable energy portions generated in the resonator following multiple scattering. Each of these quasi-quanta has energy equal to the difference between the pumping and scattering quantum energies at each step of multiple scattering. The conversion of the photons in the external electromagnetic mode of the resonator destroys this cooperative correlation between the bi-modes and established conservation lows during the cooperative emission. The master equation containing two parameters connected with the gain of quasi-energy portions, and their annihilation due to the losses from the resonator, is proposed. The competition between these two processes is numerically studied. An attractive problem is connected with established quantum correlations between the photons belonging to non-adjacent modes of the cavity. The time behavior of the evolution of correlations between such modes is observed. This conception may be used for the teleportation of information in other modes during the multiple Raman conversion.

Cooperative properties of multiple quantum scattering: I quantum nutation

The cooperative models of the bimodal field in the multiple quantum scattering nutations are discussed and proposed for possible detections in open cavities. We proposed two types of cooperation between the converted photon processes in these multiple steps of scattering nutation in the cavity. One of them takes into consideration the cooperative process between the photons of each step of the multiple steps of Raman conversion. The second cooperative process takes place between the photons belonging to different steps of multiple scattering conversions. The proposed novel bimodal entangled sources take into consideration both the coherence and collective phenomena between the photons belonging to the system of the bimodal field obtained in multiple scattering emissions. The application of higher-order multiple Raman bimodal coherent field in quantum information is proposed.

High power tunable Raman fiber laser at 1.2 μm waveband

Development of a high power fiber laser at special waveband, which is difficult to achieve by conventional rare-earth-doped fibers, is a significant challenge. One of the most common methods for achieving lasing at special wavelength is Raman conversion. Phosphorus-doped fiber (PDF), due to the phosphorus-related large frequency shift Raman peak at 40 THz, is a great choice for large frequency shift Raman conversion. Here, by adopting 150 m large mode area triple-clad PDF as Raman gain medium, and a novel wavelength-selective feedback mechanism to suppress the silica-related Raman emission, we build a high power cladding-pumped Raman fiber laser at 1.2 μm waveband. A Raman signal with power up to 735.8 W at 1252.7 nm is obtained. To the best of our knowledge, this is the highest output power ever reported for fiber lasers at 1.2 μm waveband. Moreover, by tuning the wavelength of the pump source, a tunable Raman output of more than 450 W over a wavelength range of 1240.6–1252.7 nm is demonstrated. This work proves PDF’s advantage in high power large frequency shift Raman conversion with a cladding pump scheme, thus providing a good solution for a high power laser source at special waveband.Graphical

Multiple visible wavelength switchable cascaded self-Raman laser based on selective wave-mixing mechanism

A diode-pumped solid-state laser with watts-level five switchable wavelengths spanning green to red is first experimentally demonstrated. The selective wave-mixing mechanism was introduced for multiple visible wavelength switchable output. The five visible wavelengths are generated by wave-mixing of the fundamental wave, the first-Stokes wave, and the second-Stokes wave in a Nd:YVO4 self-Raman laser. A β-barium borate crystal with a critical phase-matching angle cut is used for the wavelength conversion, and the angle-tuning method was used to achieve fast wavelength switching. A diffusion bonded YVO4/Nd:YVO4/YVO4 crystal was used to reduce the thermal lens effect and increase the effective length of the Raman gain medium for high output power and efficient Raman conversion. Under a pump power of 19.5 W, five visible laser output powers achieved for the green at 532 nm, lime at 559 nm, yellow at 588 nm, orange-red at 620 nm, and deep red at 657 nm are 4.01, 1.76, 2.54, 1.41, and 2.82 W, respectively. This laser system provides a convenient way for visible wavelength-switchable coherent light generation and may find potential applications where multiple visible wavelengths are required.

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.

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.

Broadband tunable Raman fiber laser with monochromatic pump.

Raman fiber laser (RFL) has been widely adopted in astronomy, optical sensing, imaging, and communication due to its unique advantages of flexible wavelength and broadband gain spectrum. Conventional RFLs are generally based on silica fiber. Here, we demonstrate that the phosphosilicate fiber has a broader Raman gain spectrum as compared to the common silica fiber, making it a better choice for broadband Raman conversion. By using the phosphosilicate fiber as gain medium, we propose and build a tunable RFL, and compare its operation bandwidth with a silica fiber-based RFL. The silica fiber-based RFL can operate within the Raman shift range of 4.9 THz (9.8-14.7 THz), whereas in the phosphosilicate fiber-based RFL, efficient lasing is achieved over the Raman shift range of 13.7 THz (3.5-17.2 THz). The operation bandwidths of the two RFLs are also calculated theoretically. The simulation results agree well with experimental data, where the operation bandwidth of the phosphosilicate fiber-based RFL is more than twice of that of the silica fiber-based RFL. This work reveals the phosphosilicate fiber's unique advantage in broadband Raman conversion, which has great potential in increasing the reach and capacity of optical communication systems.

Highly transient stimulated Raman scattering in SrMoO4 under ultrafast laser pumping with a controllable chirp.

For the first time, to the best of our knowledge, we demonstrate highly transient, multiwavelength, single-pass Raman generation with combined frequency shifts on two Raman modes of an SrMoO4 crystal with high total Raman conversion efficiency of up to 48% in conditions of competition with self-phase modulation (SPM). A 58-mm-long SrMoO4 crystal was used as the active medium under pumping by the 1030-nm, 40-µJ laser pulses with controllable dispersive stretching in a range of 0.25-6 ps at negative and positive chirping. The pump pulse chirping was optimized for both high- and low-frequency Raman shifts on the primary (888 cm-1) and secondary (327 cm-1) Raman modes of the crystal. At the optimal conditions, four Stokes components of stimulated Raman scattering (SRS) radiation with high- and low-frequency Raman shifts at the wavelengths of 1066, 1134, 1177, and 1261 nm were efficiently generated.

Grating-incoupled waveguide-enhanced Raman sensor.

We report a waveguide-enhanced Raman spectroscopy (WERS) platform with alignment-tolerant under-chip grating input coupling. The demonstration is based on a 100-nm thick planar (slab) tantalum pentoxide (Ta2O5) waveguide and the use of benzyl alcohol (BnOH) and its deuterated form (d7- BnOH) as reference analytes. The use of grating couplers simplifies the WERS system by providing improved translational alignment tolerance, important for disposable chips, as well as contributing to improved Raman conversion efficiency. The use of non-volatile, non-toxic BnOH and d7-BnOH as chemical analytes results in easily observable shifts in the Raman vibration lines between the two forms, making them good candidates for calibrating Raman systems. The design and fabrication of the waveguide and grating couplers are described, and a discussion of further potential improvements in performance is presented.

Multimode Graded Index Fiber with Random Array of Bragg Gratings and Its Raman Lasing Properties

Light propagation in multimode fibers is known to experience various nonlinear effects, which are being actively studied. One of the interesting effects is the brightness enhancement at the Raman conversion of the multimode beam in graded index (GRIN) fiber due to beam cleanup at Raman amplification and mode selective feedback in the Raman laser cavity based on fiber Bragg gratings (FBGs) with special transverse structure. It is also possible to explore random distributed feedback based on Rayleigh backscattering on natural refractive index fluctuations in GRIN fibers, but it is rather weak, requiring very high power multimode pumping for random lasing. Here, we report on the first realization of femtosecond pulse-inscribed arrays of weak randomly spaced FBGs in GRIN fibers and study Raman lasing at its direct pumping by highly multimode (M2~34) 940-nm laser diodes. The fabricated 1D–3D FBG arrays are used as a complex output mirror, together with the highly reflective input FBG in 1-km fiber. Above threshold pump power (~100 W), random lasing of the Stokes beam at 976 nm is obtained with output power exceeding 28 W at 174 W pumping. The beam quality parameter varies for different arrays, reaching M2~2 at the linewidth narrowing to 0.1–0.2 nm due to the interference effects, with the best characteristics for the 2D array.

Cladding-pumped Raman laser with M2 of 1.3 and 400 µJ first Stokes energy

Cladding-pumped Raman lasers have often been cited for their potential for brightness enhancement, but so far have never achieved high power and beam quality simultaneously. By utilizing a fiber geometry with a larger cladding to core ratio than current high power Raman fiber lasers for brightness enhancement (BE-RFLs), a noise-seeded Raman fiber laser pumped by a 100 ns pulsed laser with record beam quality is achieved with M2=1.3 and 0.4 mJ first Stokes output, with an instantaneous brightness enhancement of 60. The results here support the hypothesis that the limiting factor to high beam quality in BE-RFLs is Raman conversion in the pump cladding, rather than higher order core modes as in most conventional fiber lasers.

Order controllable enhanced stimulated Brillouin scattering utilizing cascaded diamond Raman conversion

We report on the design and operation of a laser, which outputs wavelengths in the 1.2 and 1.5 μm ranges by leveraging two non-linear processes of stimulated Raman scattering and stimulated Brillouin scattering in diamond. By precisely controlling characteristics of the laser resonator formed around the diamond crystal, we are able to selectively control the onset of each non-linear process so as to tailor laser output characteristics both in way of wavelength and output power. This work demonstrates the high degree of flexibility and power-handling capacity of diamond for wavelength conversion of common laser wavelengths (such as 1064 nm as used in this work) and the generation of a span of discrete wavelengths (with up to eight cascaded orders being demonstrated in this work).

High beam quality yellow laser at 588 nm by an intracavity frequency-doubled composite Nd:YVO4 Raman laser.

High beam quality 588 nm radiation was realized based on a frequency-doubled crystalline Raman laser. The bonding crystal of YVO4/Nd:YVO4/YVO4 was used as the laser gain medium, which can accelerate the thermal diffusion. The intracavity Raman conversion and the second harmonic generation were realized by a YVO4 crystal and an LBO crystal, respectively. Under an incident pump power of 49.2 W and a pulse repetition frequency of 50 kHz, the 588 nm power of 2.85 W was obtained with a pulse duration of 3 ns, corresponding to a diode-to-yellow laser conversion efficiency of 5.75% and a slope efficiency of 7.6%. Meanwhile, a single pulse's pulse energy and peak power were 57 µJ and 19 kW, respectively. The severe thermal effects of the self-Raman structure were overcome in the V-shaped cavity, which has excellent mode matching, and combined with the self-cleaning effect of `Raman scattering, the beam quality factor M2 was effectively improved, which was measured optimally to be Mx 2 = 1.207, and My 2 = 1.200, with the incident pump power being 49.2 W.

Stimulated Raman scattering of H2 in hollow-core photonics crystal fibers pumped by high-power narrow-linewidth fiber oscillators.

The stimulated Raman scattering (SRS) process in gas-filled hollow-core fiber is mostly used to realize the wavelength conversion, which has the potential to produce narrow-linewidth and high-power fiber laser output. However, limited by the coupling technology, the current research is still at a few watts power level. Here, through the fusion splicing between the end-cap and the hollow-core photonics crystal fiber, several hundred watts pump power can be coupled into the hollow core. Homemade narrow-linewidth continuous wave (CW) fiber oscillators with different 3 dB linewidths are used as the pump sources, then the influences of the pump linewidth and the hollow-core fiber length are studied experimentally and theoretically. As the hollow-core fiber length is 5 m the H2 pressure is 30 bar, 109 W 1st Raman power is obtained with a Raman conversion efficiency 48.5%. This study is significant for the development of high-power gas SRS in hollow-core fibers.

Efficient Raman pulse fiber laser pumped by a dissipative soliton resonance pulse near 2 µm.

A high-efficiency Raman conversion from 1.987 µm to 2.177 µm is demonstrated experimentally in 45 m GeO2-doped silica fiber, adopting a dissipative soliton resonance (DSR) rectangular pulse as the pump. Over the entire spectral distribution, the spectral purity of the first-order Raman pulse is up to 96.8%, suggesting a nearly complete pump depletion before the onset of cascaded Raman shifts. The corresponding pump-to-Raman conversion efficiency of 67.4% is the highest up to date in this spectral region. Meanwhile, a large Raman pulse energy of 1.03 µJ was obtained at the repetition rate near MHz level, corresponding to 0.893 W average power. In the total output, the Raman-dominated spike has a Full Width Half Maximum (FWHM) of 1.18 ns far narrower than DSR's pulse duration of 10.25 ns. The results indicate that DSR is a promising candidate for developing efficient Raman nanosecond pulse fiber laser in mid-infrared (MIR) region.