Μm Raman Laser Research Articles

High beam quality and high peak power 1.6 µm deuterium Raman laser.

In this work, a 1.67 µm laser was generated in the pressurized deuterium pumped by a pulsed 1064 nm laser via the stimulated vibration + rotation Raman scattering (SV-RRS). By the optimization of the deuterium pressure, focal length, and polarization of the pump laser, a 66.2 mJ Raman laser was achieved with the configuration of 1.7 MPa deuterium pressure, 2245 mm focal length, and 220 mJ circularly polarized pump laser. Correspondingly, the photon conversion efficiency was 47.8%, the energy conversion efficiency was 30.5%, the pulse width was 5.4 ns, and the maximum peak power was 12.3 MW. In addition, SV-RRS was found to improve beam quality significantly: from Mx 2 = 2.67 and My 2 = 3.07 for the pump laser to Mx 2 = 1.42 and My 2 = 1.53 for the 1.67 µm Raman laser.

Miniaturization of high beam quality 1.543 μm Raman laser with backward stimulated Raman scattering

It is very challenging to make high peak power (big pulse energy) Raman lasers compact, due to laser induce breakdown (LIB) effect; and it is even more difficult to achieve a decent beam quality meanwhile. In this work, a pulsed 1064 nm laser was used as pump source; pressurized methane was used as Raman active medium, which had the largest ratio of backward/forward Raman gain coefficient among all gaseous media; and backward first Stokes (BS1) Raman laser of 1543 nm with good beam quality was realized. When an f=0.5 m lens was used to focus pump beam into a 0.9 m long Raman cell filled with 3.5 MPa methane, 276.1 mJ BS1 was achieved, the corresponding photon conversion efficiency was 70.8% and the peak power was 83.7 MW. BS1 beam quality was measured to be Mx2=2.54,My2=2.28, which was significantly better than that of pump laser. In a setup of f=0.3 m focal lens and 0.5 m long Raman cell, 197.8 mJ BS1 was achieved, with the company of serious LIB. In order to meliorate LIB and improve BS1 conversion efficiency, the focal lens was tilted by 20°, pump laser beam waist and depth of focus increased significantly, BS1 was improved to 225.0 mJ, the corresponding photon conversion efficiency was 58.5%. An even short focal lens and Raman cell with reasonable BS1 energy was possibly achieved by tilting a shorter focal lens by a larger angle. This work also demonstrated that the increment of BS1 conversion may help to reduce the effect of LIB on SRS process.

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.

1.7 µm - 1.73 µm tunable ultrafast Raman fiber laser pumped by 1.6 µm dissipative soliton pulses.

Here, we report an all-fiber tunable ultrafast Raman laser synchronously pumped by a home-made 1.6 µm dissipative soliton (DS) picosecond (ps) laser, which produces Stokes light beyond 1.7 µm. The Raman gain medium is a segment of highly germanium-doped (Ge-doped) fiber offering a high Raman gain coefficient at the target wavelength. Once the Raman conversion cavity is synchronized with the pump light, a stable 1.7 µm Raman laser (the first Stokes light) can be obtained at a low pump threshold. The maximum output power of the 1.7 µm Raman laser can reach ∼ 22.62 mW. The wavelength tuning operation is independent of tunable pump source and intra-cavity filter. By adjusting the intra-cavity delay line simply, the different spectral component within the broad Raman gain bandwidth can be selectively synchronized with the pump light so that the Raman laser wavelength can be tuned continuously from 1702.6 nm ∼ 1728.84 nm. This tunable 1.7 µm waveband ultrafast laser will have potential applications in multiphoton microscopy for e.g. deep bio-imaging.

Comprehensive modeling and design of Raman lasers on SOI for mid-infrared application

In this article, we present a comprehensive model for a CW Raman laser in a silicon-on-insulator (SOI) platform. We validate this model by comparing its output with experimental results from the literature. We then design a 2.232 μm Raman laser with an on-chip ring resonator cavity. Numerical analysis of laser characteristics, such as threshold power and output power, are conducted. The optimized values for the cavity length and power coupling ratios are determined taking into account fabrication variations. Finally, by designing a tunable directional coupler (DC) for the laser cavity, a robust design of Raman laser is presented. We show that the reduction of propagation loss down to 0.6 dB/cm for the silicon waveguides and free carriers removal, by the use of a p-i-n junction, will enable a Raman laser with a threshold power of 16 mW and a slope efficiency of 62%.

Modeling and simulation of a mid-IR 43 µm Raman laser in chalcogenide glass fibers

Raman lasers based on mid-infrared (IR) fibers that operate at the 3–5 µm atmospheric transparency windows are attractive sources for many important applications. In this work, a Raman laser operating in the mid-IR range at 4.3 µm based on an As2Se3 fiber pumped by a 3.92 µm fiber laser has been designed and systematically optimized. The influences of pump power, fiber length, and output coupling efficiency on laser performance are investigated. Simulation results show that a fiber length of 0.75–1.3 m and an output coupler reflectivity of 89%–98% could obtain the optimized output power. The maximum output power of 0.269 W was obtained at a pump power of 1.5 W. The simulated result can be used for theoretical guidance and design optimization of practical chalcogenide fiber Raman lasers.

1.56 μm and 2.86 μm Raman lasers based on gas-filled anti-resonance hollow-core fiber

We report here a single-pass 1.56 μm fiber gas Raman laser in a deuterium-filled hollow-core fiber and a 2.86 μm cascade fiber gas Raman laser with methane in the second stage. The maximum output powers at 1.56 and 2.86 μm are 27 and 8.5 mW with Raman conversion efficiency of 30% and 42%, respectively. The results offer a new method to produce a 1.5 μm fiber source and prove the potential of the cascade fiber gas Raman laser in extending the available wavelength.

2.9, 3.3, and 3.5 μm Raman Lasers Based on Revolver Hollow-Core Silica Fiber Filled by 1H2/D2 Gas Mixture

Mid-infrared Raman laser based on silica fiber is demonstrated. Hollow-core silica fiber filled by the mixture of 1H2 and D2 gases was used as an active medium. Pumped by nanosecond pulses of a high-power Er-doped 1.56 μ m fiber laser, the Raman laser generates radiation at 2.9, 3.3, and 3.5 μ m wavelength, which depends on gas composition and pump power. Quantum conversion efficiency as high as ∼10% is achieved. The possibilities to optimize the performance of the Raman laser are discussed.

The Design Optimization and Experimental Investigation of the 4.4 μm Raman Laser Basedon Hydrogen-filled Revolver Silica Fiber

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High peak power 2.8 μm Raman laser in a methane-filled negative-curvature fiber.

We demonstrate a 2.8 μm gas Raman laser in a methane-filled hollow-core negative-curvature fiber with average power of 113 mW, pulse energy of 113 μJ and estimated peak power of 9.5 MW. Raman quantum efficiency of 40% has been reached from the pump source at 1.064 μm to the 2nd order vibrational Stokes at 2.812 μm using 1.8 MPa methane gas. To our knowledge, this is the first high peak power fiber-based gas Raman laser in mid-infrared region and a range of applications in supercontinuum generation, laser surgery, molecular tracing and gas detection are in prospect.

1.88 μm, Ba(NO3)2-based Raman laser pumped by a potassium-titanyl-phosphate-based optical parametric oscillator laser

A 1.88 μm Ba(NO3)2-based stimulated Raman scattering (SRS) laser pumped by a potassium-titanyl-phosphate-based optical parametric oscillator (OPO) laser is presented. Under the pumping energy of a 130 mJ 1064 nm Q-switched Nd:YAG laser, a 40 mJ 1.57 μm laser is achieved; the maximum output energy of the 1.88 μm Raman laser reaches 7.5 mJ with a pulse duration of 5.3 ns at repetition rates of 10 Hz, and the corresponding total optical conversion efficiency is about 5.8%. The combination of the SRS and OPO techniques significantly extends the wavelength from 1.064 to 1.57 μm (OPO) and then demonstrates Stokes-shifting to 1.88 μm (SRS).

Towards ten-watt-level 3-5 µm Raman lasers using tellurite fiber.

Raman lasers based on mid-infrared fibers operating at 3-5 µm atmospheric transparency window are attractive sources for several applications. Compared to fluoride and chalcogenide fibers, tellurite fibers are more advantageous for high power Raman fiber laser sources at 3-5 µm because of their broader Raman gain bandwidth, much larger Raman shift and better physical and chemical properties. Here we report on our simulations for the development of 10-watt-level 3-5 µm Raman lasers using tellurite fibers as the nonlinear gain medium and readily available continuous-wave (cw) and Q-switched erbium-doped fluoride fiber lasers at 2.8 µm as the pump sources. Our results show that a watt-level or even ten-watt-level fiber laser source in the 3-5 µm atmospheric transparency window can be achieved by utilizing the 1st- and 2nd-order Raman scattering in the tellurite fiber. The presented numerical study provides valuable guidance for future 3-5 um Raman fiber laser development.

CW-pumped short pulsed 1.12 μm Raman laser using carbon nanotubes

We demonstrate passive mode-locking of a Raman fiber laser at 1.12 μm, using a nanotube-based saturable absorber. A regular train of pulses, with a duration of 236 ps at the fundamental repetition frequency of the cavity, are generated by the all-normal dispersion oscillator. Importantly, this simple system is pumped with a continuous wave Yb fiber laser, removing the need for complex synchronous pumping schemes, where pulse-shaping depends on the action of the saturable absorber and a balance of dissipative processes. These results illustrate the flexibility of combining Raman amplification with a nanotube-based absorber for wavelength versatile pulsed sources.

Stimulated rotational Raman scattering at multiwavelength under tea CO2 laser pumping with a multiple-pass cell

Stimulated rotational Raman scattering (SRRS) at multiwavelength pumped by TEA CO2 laser was demonstrated in this paper. Raman mediums were cooled by liquid-N2 and a multiple-pass cell (MPC) with 25 passes was designed and used. When the para-H2 was pumped by single-longitudinal-mode (SLM) circular polarized TEA CO2 laser on 10P(20), 9P(20), and 10R(20), 50 mJ 16.95 μm, 350 mJ 14.44 μm, and 536 mJ 16.9 μm radiations were obtained, corresponding to energy conversion efficiency of 1.2, 11.7, and 13.4%, respectively. When the ortho-D2 was pumped by CO2 laser on 10R(18), 108 mJ 12.57 μm Raman laser was obtained with energy conversion efficiency of 2.9%.

Thermal and optical properties of Nd3+:SrWO4: a potential candidate for eye-safe 1.517 μm Raman lasers

Nd3+ doped SrWO4 single crystal with a dimension of ϕ 30 mm×80 mm and a good optical quality was grown by Czochralski method. The refractive indices in a function of wavelength and the principal coefficient of thermal expansion were precisely measured. The three principal coefficients of thermal expansion along (001), (010), (100) are 2.39×10-5 °C-1, 1.09×10-5 °C-1, and 0.97×10-5 °C-1, respectively. In the framework of the Judd–Ofelt theory, the intensity parameters, radiative probabilities, radiative branching ratios and radiative lifetime were calculated. We also present end-pumped self-stimulated Raman scattering in the two laser channels 4F3/2→4I11/2 and 4F3/2→4I13/2 to generate 1.18 and 1.52 μm Self-stimulated Raman scattering radiations, the latter being close to the eye-safe domain. This material is a potential material for eye-safe Raman lasers.

Cross section of supercooled238UF6 in multiphoton absorption induced by 16 micrometer Raman-laser radiation

Measurements of multiphoton absorption of 16 µm Raman-laser radiation in supercooled238UF6 at 90 K were performed by using a pulsed Laval nozzle with an optical path length of 50 cm. The laser fluence was varied between 50 and 500 mJ/cm2 for four frequencies in the range from 625 to 629 cm−1. The energy absorbed by238UF6 molecules was investigated as a function of laser frequency or fluence, and highly accurate results were obtained with the use of the nozzle whose optical path length is much greater than that of nozzles used before. The results indicated that the absorption cross section at the peak absorption frequency (627.8cm−1) was proportional to the −1/3 power of the fluence.

Multiphoton Absorption of 16 μm Raman Laser Radiation in238UF6

Abstract Measurements of multiphoton absorption of 16 μm Raman laser radiation in static UF6(with 99.3% 238UF6) have been performed at room temperature and —35°C. The laser fluence has been varied between 10 and 300 mj/cm2 for six laser frequencies in the range from 615 to 629 cm−1. Laser frequency and fluence dependence of energy absorbed by UF6 molecules have been investigated. The results indicate that the number of photons absorbed per molecule η(φ) at a mean fluence φ at several frequencies is in proportion to φ where α≈2/3.

The CW optically pumped 12.08µm Raman laser theory and experiments for high power-conversion efficiency

A model is developed to predict the performance of the continuous-wave CO 2 -pnmped 12.08 μm NH 3 laser which is operated by a two-photon or Raman-like process. The local gain and pump absorption are determined from a two-wave three-level treatment based on the density matrix formalism. A ring cavity configuration is considered and interaction of the two intensities inside the cavity are described using coupled wave equations. The subsequent 12.08μm output intensity is calculated for a wide range of operational parameters (injected pump intensity, NH 3 gas pressure, pump frequency offset, gain length, output coupling,... ). For a well optimized system, power-conversion efficiencies of 10-30 percent should be realistically obtained by pumping with a conventional CW CO 2 laser. Experiments illustrating the major conclusions are described.

12 µm Raman lasers in NH3pumped by low-power CO2laser pulses

12 µm Raman lasers in NH₃pumped by low-power CO₂laser pulses