YAG Crystal Rod Research Articles

Improving performance prediction of diode end-pumped solid-state Nd:YAG rod amplifiers by incorporating pump mode evolution.

Master Oscillator Power Amplifier (MOPA) systems find extensive use in laser development to increase the optical power of laser emissions from a Master Oscillator (MO). Commonly used are the cylindrical rod MOPAs that are optically excited using a multimode fiber-coupled (FC) diode laser emission in an end-pumped configuration. Current analytical 3D models that incorporate thermal effects, gain saturation, and iterative Fourier beam propagation methods, collectively, rely on static approximations of the evolution of the FC pump beam profile over the longitudinal volume of the amplifier crystal. Furthermore, in general, the spectral behavior of the FC diode emission is assumed to be static, and the thermal wavelength shift is not accounted for in the simulation. In this work, we demonstrate a novel approach for accurate modeling of the multimode FC pump beam emission as a complex field using a phase-only Gaussian to flat-top (FT) diffractive optical element, thus allowing for the inclusion of the pump beam into the iterative propagation method. Additionally, we present a method for precise calibration of the model using simple experimental measurements of the diode emission spectrum. The theoretical model is experimentally validated using an end-pumped Nd:YAG crystal rod to perform single-pass amplification of a Gaussian beam, showing excellent agreement with predicted output powers over the calibrated range of pump powers. Furthermore, we provide experimental data that exhibits a strong correlation between the Gaussian to FT phase-only transformation and the multimode FC diode evolution in free-space propagation.

Investigation of dependence of solar-pumped laser power on laser medium length

The efficiency of solar-pumped lasers (SPLs) is limited when the length of the laser medium is unsuitable. This is because superfluous regions in the laser medium introduce losses and contribute slightly to the stimulation of radiation in the laser resonator. Before designing an SPL, an appropriate length of laser medium is critical. We present a method to calculate the optimal length of the gain medium in SPLs by exploring the relationship between the absorbed solar power and material loss for different laser medium lengths. Thus, the lengths of Nd:YAG crystals with diameters of 3 to 6 mm were optimized, and the output characteristics were calculated numerically. The maximum collection efficiency (CE) (40.1 W / m2) was obtained for the 5.5-mm diameter Nd:YAG crystal rod of length 21.1 mm, which was 1.7 W / m2 higher than the previous numerical record. The optimal length of the 6-mm diameter Nd:YAG crystal rod was found to be 21.9 mm. For a laser rod of this length, a CE of 36.3 W / m2 is expected. This value is 1.13 times greater than the existing experimental record for the Nd:YAG crystal of the same diameter, which highlights the importance of optimizing the length of the laser rod.

Highly Efficient Solar Laser Pumping Using a Solar Concentrator Combining a Fresnel Lens and Modified Parabolic Mirror

Solar-pumped lasers (SPLs) allow direct solar-to-laser power conversion, and hence, provide an opportunity to harness a renewable energy source. Herein, we report significant improvements in end-side-pumped solar laser collection efficiency and beam brightness using a novel 1.5-m-diameter compound solar concentrator combining a Fresnel lens and modified parabolic mirror. A key component of this scheme is the off-axis-focused parabolic mirror. An original dual-parabolic pump cavity is another feature. To determine the dependence of the SPL performance on the distance between the focus and central axis of the modified parabolic mirror, several systems with different distances were optimized using TracePro and ASLD software. It was numerically calculated that end-side pumping a 5-mm-diameter, 22-mm-long Nd:YAG crystal rod would generate 74.6 W of continuous-wave solar laser power at a collection efficiency of 42.2 W/m2, i.e., 1.1 times greater than the previous record value. Considering the laser beam quality, a brightness figure of 0.063 W was obtained, which is higher than that of other multimode SPL designs with end-side pumping. Thus, our SPL concentrator offers the possibility of achieving a beam quality as high as that obtainable via side pumping, alongside highly efficient energy conversion, which is characteristic to end-side pumping.

Fabrication of yttrium aluminosilicate fibers with high Yb3+ doping from Yb:YAG ceramic nanopowders and its application in single-frequency fiber lasers

A yttrium aluminosilicate fiber with a Yb doping concentration of 4.53 wt.% was fabricated by powder-in-tube method, in which the preform core was Yb:YAG ceramic nanopowders synthesized by co-precipitation method. Compared with the method of using YAG crystal rod as fiber core, the fiber composition design can be more flexible and a series of crystal rod processing technology can be circumvented. The absorption coefficient of the Yb-doped YAS fibers was measured 28.4 dB/cm at 976 nm. A single-frequency fiber laser was constructed with the YAS fiber length of 1cm. To our knowledge, this is the first time that rare earth (RE) doped YAS fiber prepared by powder-in-tube method was used in fiber laser.

Superior performance of a 2 kHz pulse Nd:YAG laser based on a gradient-doped crystal

Herein, we report a homemade new Nd:YAG crystal rod that contains a gradient dopant of 0.39–0.80 at.% Nd 3 + from end to end, achieving superior performance of a 2 kHz Nd:YAG pulse laser at 1064 nm. The optical-to-optical conversion efficiency reached 53.8%, and the maximum output power of the laser was 24.2 W, enhanced by 35.9% compared with a uniform crystal rod with the same total concentration of Nd 3 + . Significantly, our experiments revealed that the gradient concentration crystal produced a relatively even pumping distribution along the rod axis, greatly reducing the temperature gradient as well as having a smaller thermal effect. The pump and thermal distribution smoothing obviously improved the features of laser oscillation and output.

Multiwavelength diode array end-pumped, thermally stabilized, Nd:YAG pulsed laser

An electro-optic Q-switched multiwavelength laser diode array end-pumped Nd:YAG laser without temperature control is proposed and demonstrated in this paper. Using a 3.2 kW multiwavelength laser diode array as the pump source, a lens duct as the coupling system, RTP crystals as the Q-switch, and an end-pumped Nd:YAG crystal rod (Ø 6 × 60 mm2) as the gain medium, the laser system obtained a maximum output pulse energy of 72 mJ at room temperature with a pulse width of 15.8 ns, a repetition rate of 20 Hz, and an optic-optic conversion efficiency of 11.25%. Meanwhile, the output energy increased from 69 mJ at −40 °C to 78 mJ at 60 °C, with an energy instability of less than 10% and a pulse width instability of less than 15% over the full temperature range of 100 °C. The divergence angles of the output laser in the horizontal and vertical directions were measured as 2.5 and 2.6 mrad, respectively. The work the authors have done will provide important references for other researchers to carry out similar research studies.

Exceeding 50% slope efficiency DBR fiber laser based on a Yb-doped crystal-derived silica fiber with high gain per unit length.

We fabricated a Yb-doped yttrium aluminium garnet (Yb:YAG) crystal-derived silica fiber (YCDSF) using an assembly consisting of a YAG crystal rod and silica tube on a CO2 laser-heated drawing tower. The fiber has a Yb concentration of 5.66 wt%, and absorption coefficient of 32 dB/cm at 980 nm. The figure of merit of the unsaturated absorption and gain per unit length of the YCDSF are 93% and 4.4 dB/cm, respectively. Based on the results of the numerical simulation, an all-fiber distributed Bragg reflector (DBR) laser using only a 1.5-cm-long YCDSF is experimentally demonstrated to have a maximum output power of 360 mW with a pump threshold power of 21 mW. The fiber laser also achieved an optical signal-to-noise ratio of 80 dB, a beam quality factor of 1.022 in two orthogonal directions and a slope efficiency of up to 50.5%. These results indicate that the all-fiber DBR laser has potential applications in high-quality seed sources and coherent optical communications.

The characteristics of high-quality Yb:YAG single crystal fibers grown by a LHPG method and the effects of their discoloration.

Single crystal fibers (SCFs), especially ytterbium (Yb) doped crystal fibers, have great potential in the field of high-power lasers. Colorless Yb:YAG single crystal fibers were fabricated using a laser heated pedestal growth (LHPG) method with a diameter fluctuation of less than 2% and a length to diameter ratio greater than 320 : 1. An abnormal color issue exists with respect to Yb:YAG crystals. The origin of coloration was studied via density functional theory, single-crystal X-ray diffraction, XPS and Raman spectroscopy and it was confirmed that the cyan coloration of Yb:YAG crystals is due to oxygen vacancies. Yb:YAG SCFs prepared via the LHPG method could avoid this type of defect due to the large specific surface area and melt convection caused by surface tension. The fundamental properties of the cyan Yb:YAG crystal source rod and colorless Yb:YAG SCFs were systematically investigated. The colorless Yb:YAG SCFs have higher infrared transmittance and thermal conductivity. The distributions of Yb3+ along the radial and axial directions were also measured. Meanwhile we demonstrated the propagation loss and a fiber laser using the colorless Yb:YAG SCFs, obtaining a minimum loss coefficient of 0.008 dB cm−1 and a maximum continuous-wave (CW) output power of 3.62 W. The colorless Yb:YAG SCFs with good thermal conductivity, low propagation loss, wide transparency and uniform ion distribution show promise for acting as the host material in single-mode lasers.

Compact 20 W femtosecond laser system based on fiber laser seeder, Yb:YAG rod amplifier and chirped volume Bragg grating compressor.

In this work we presented a compact femtosecond laser system based on Yb doped fiber seed laser and efficient Yb:YAG crystal rod power amplifier. Matched pair of chirped fiber Bragg grating stretcher and chirped volume Bragg grating compressor were used to obtain high fidelity - Strehl ratio 76%, pulses of 764 fs duration, 104 µJ energy at 200 kHz repetition rate at the output of the laser system.

5.04% system slope efficiency solar-pumped Nd:YAG laser by a heliostat–parabolic mirror system

We demonstrated a solar-pumped laser with a heliostat–parabolic mirror system. A designed conical pump cavity with a water-filled quartz tube lens was used to couple efficiently the concentrated solar radiation from the focal zone of the primary concentrator into a 5.5-mm-diameter, 95-mm-length grooved Nd:YAG crystal rod within a gold-plated conical pump cavity. For 1.0-m2 effective solar radiation collection area, 20.1-W continuous-wave (CW) output laser power was obtained, corresponding to a total system slope efficiency of 5.04%. This value was 1.31 times higher than the previous with the same solar facility. For 1.5-m2 effective collection area, 34.6-W CW output laser power was achieved at the total system slope efficiency of 4.51%. A strong dependency of laser power on laser rod mounting position was also found.

Development of longer Nd:LGGG crystal for high power laser application

In order to further improve the Nd3+:(LuxGd1-x)3Ga5O12 (Nd:LGGG) crystal performance in high power laser field, a long Nd:LGGG crystal with dimensions of Φ 23×112mm3 has been grown successfully by the Czochralski (Cz) method for laser rod fabrication. Compared with the normal size LGGG crystals (like 30–50mm in length), we overcame several difficulties in the growth of longer ones, including crystal cracking by a large longitudinal temperature gradient, spiral growth by a small radial temperature gradient, and growth instability and even constitutional super cooling by Ga2O3 volatilizing continuously. The doping concentrations of Nd3+ and Lu3+ in the as-grown crystal and the crystal optical quality have been measured. The performance of diode-side-pumped Nd:LGGG rod laser has been preliminarily tested for the first time, simply by replacing the Nd:YAG crystal rod inside a commercial laser module. Under an incident pump power of 160W, the maximum continuous wave output power of 38Whas been obtained, corresponding to an optical-optical conversion efficiency of 23.8% and a slope efficiency of 40.8%, respectively.

Multiplexing and Amplification of 2-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math> </inline-formula> Vortex Beams With a Ho:YAG Rod Amplifier

The multiplexing and the amplification of 2- $\mu \text{m}$ vortex beams are experimentally verified in a Ho:YAG crystal rod amplifier. Spatially multiplexed vortex beams are studied and the amplification results are presented and discussed. The integrity of the launched vortex beams is well maintained through the amplification process. Further discussions are provided to increase the gain as well as for power scaling. Due to the nature of Ho:YAG material and the geometry of the rod amplifier, our system shows the potential of working as a power amplifier for vortex beams.

Concentric vortex beam amplification: experiment and simulation.

The amplification of a 2 μm concentric vortex beam in a Ho:YAG crystal rod amplifier is simulated and experimentally verified. Different concentric vortex beams are studied and the amplification results are compared with simulation. The integrity of the launched vortex beams is well maintained through the amplification process. Further discussions are provided to increase the gain as well as for power scaling. Due to the nature of Ho:YAG material and the geometry of the rod amplifier, our system shows potential of working as a power amplifier for vortex beams.

Few-Mode Cr-Doped Crystalline Core Fibers for Fiber Amplifier

A few-mode Cr-doped crystalline core fiber (FMCDCCF) is demonstrated. The fiber fabrication is a modified laser-heated pedestal growth (LHPG) technique that consists of over cladding a Cr:YAG crystal rod with silica and then fine tuning its core size with multiple core reduction through the LHPG system. The FMCDCCF exhibits a core diameter of 2 μm and a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> -value around 3.77 to provide the few-mode characteristics. The optical properties of the FMCDCCF are similar to that of a pristine Cr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> :YAG crystalline. A gross gain of the FMCDCCF is measured around 2.3 dB at a wavelength of 1.4 μm.

Q-switched neodymium-doped Y_3Al_5O_12-based silica fiber laser

We present pulsed laser operation in a Nd-doped, Y3Al5O12-based silica fiber. The fiber was fabricated using the rod-in-tube technique with a Nd:YAG crystal rod as the core material and a silica tube for the cladding material. A spectroscopy study revealed that the core region had become amorphous in the process of fiber drawing. Q-switched pulsed laser operation was realized at a wavelength of 1058 nm when the fiber was cladding pumped at a wavelength of 808 nm. The laser delivered 38 μJ of energy in 65 ns pulses. The extracted energy was limited due to the multimodal operation of the fiber. Laser slope efficiency in continuous wave operation reached 52%. The spectroscopic properties of the fabricated fiber are discussed and compared to a Nd:YAG crystal and a Nd:Al-doped silica fiber.

Multi-wavelength operation of LD side-pumped Nd:YAG laser

A dual-wavelength laser at 1064nm and 1319nm is obtained by a single Nd:YAG crystal rod. On the basis of 1064nm and 1319nm dual-wavelength laser installation, the second harmonic waves at 532nm and 660nm can be achieved by using non-linear frequency conversion technology. When 1064nm and 1319nm lasers oscillate simultaneously, the maximum output power is 30.5W and 8.78W, respectively. When the 1319nm laser is restrained, we obtain a 35.6W maximum output power at 1064nm and by contrary 11.2W at 1319nm. The maximum output powers of 532nm and 660nm lasers are 5.34W and 1.353W when oscillating simultaneously. With one of them restrained, the maximum output power is 6.72W at 532nm and 1.90W at 660nm. The optimum repetition rate of the acousto-optic Q-switch is 10.5KHz and 20.5KHz for 532nm and 660nm lasers, respectively. The optical-to-optical conversion efficiency from the fundamental waves to the harmonic waves is 17.5% and 15.4%. The instability is less than 2%.

Relationship between thermally induced lensing and output power for a high‐power laser‐diode side‐pumped solid‐state laser

AbstractThe variable focal length of thermal lensing in grains of a thermally loaded Nd:YAG crystal rod have been calculated and compared with the measurements of our system. By use of the space‐dependent rate equations, the analytical expression of the output power on the pump power, which consists of the factors of threshold volume and the focal length of thermal lensing, is presented. We investigate the threshold pump power, and the relationships between the output power and the pumping power under different lengths of thermal lensing. Experimental results concerning the output power of fundamental mode and the pump power, which are related to the thermal lensing and agreeable to the theoretical analysis are given. Finally, the optimized experimental output power of our laser system is obtained. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 361–365, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20305

High efficiency diode laser side-pumped Nd:YAG rod laser

High efficiency laser performance of 2 mm diameter and 20 mm long Nd:YAG crystal rod transversely pumped by laser diode array is presented. Using the directly side-coupled geometry enclosed by cylindrical pump cavity plated with gold, we obtained long pulse energy of 4.6 mJ corresponding to optical-to-optical slope efficiency of 37%. The complete solid-state Nd:YAG laser is described to be simple, compact and highly efficient. Q-switching of the laser is also reported.