Beam Quality Parameter M2 Research Articles

High-average-power thin-tapered-rod ultrafast laser amplifier with gradient-doped Yb:YAG crystal

Gradient doping in thin-rod and thin-tapered-rod active elements was proposed and implemented. Thermal and laser processes in such elements were studied theoretically and experimentally. It was shown that the use of tapered active elements with gradient doping enables simultaneous gain enhancement and reduction of the temperature of the active medium, thereby increasing amplification efficiency and improving beam quality. An amplifier with unique cumulative parameters: an average power of 150 W and an amplification efficiency of 35% with beam quality parameter M2=1.62×1.53 was developed.

2 kW high-efficiency Raman fiber amplifier based on passive fiber with dynamic analysis on beam cleanup and fluctuation.

In this paper, we study the power scaling in high power continuous-wave Raman fiber amplifier employing graded-index passive fiber. The maximum output power reaches 2.087 kW at 1130 nm with an optical conversion efficiency of 90.1% (the output signal power versus the depleted pump power). To the best of our knowledge, this is the highest power in the fields of Raman fiber lasers based merely on Stokes radiation. The beam quality parameter M2 improves from 15 to 8.9 during the power boosting process, then beam spot distortion appears at high power level. This is the first observation and analysis on erratic dynamic properties of the transverse modes in high power Raman fiber amplifier.

Greater than 2 kW all-passive fiber Raman amplifier with good beam quality

Abstract We report a 2 kW all-fiberized Raman fiber amplifier with efficient brightness enhancement based on the graded-index fiber. The maximum power output reaches up to 2.034 kW centered at 1130 nm, with a conversion efficiency of 79.35% with respect to the injected pump power. To the best of our knowledge, this is the highest conversion efficiency obtained for any Raman laser system using graded-index fiber. An optimized fiber combiner adopting graded-index fiber as the pigtail fiber was fabricated, enabling the preservation of the seeding brightness in the core-pumped Raman fiber amplifier, and further enhancing the ultimate brightness of the output laser after amplification. At the maximum power output, the beam quality parameter M2 is 2.8, corresponding to a signal-to-pump brightness enhancement factor of 11.2. As far as we know, we obtain the highest brightness enhancement among Raman fiber lasers of over 100 W, and the best beam quality for graded-index Raman fiber lasers of over 150 W.

The concept and design of navigation systems based on solid-state and semiconductor lasers

The concept and design of the navigation system based on solid-state and semiconductor lasers is presented. The possibility of using a laser with a self-reversal of a radiation wavefront with a beam quality parameter M2 ⩽ 1.2 and a single-frequency pulse power of more than 20 MW for constructing a master oscillator of a solid-state laser emitter is considered. The possibility of using a 100 W multi-cell continuous semiconductor laser for constructing a master oscillator of a semiconductor laser emitter with a fiber output is demonstrated.

High-power, frequency-doubled Nd:GdVO4 laser for use in lithium cold atom experiments.

We report on an 888 nm-pumped Nd:GdVO4 ring laser operational over a wavelength range from 1340.3 nm to 1342.1 nm with a maximum output power of 7.4 W at 1341.2 nm and a beam quality parameter M2<1.1. To our knowledge this is the highest single-longitudinal-mode power obtained with a Nd:GdVO4 crystal laser. We use a commercial frequency-doubling cavity to achieve 1.2 W at 671.0 nm and 4.0 W at 670.6 nm for use in lithium cold atom experiments. Respectively, these wavelengths are approximately resonant with and 250 GHz blue-detuned from the lithium D-lines. Thus, this source provides ample power for laser cooling of lithium atoms while also offering substantial power for experiments requiring light 10's to 100's of GHz blue-detuned from the primary lithium transitions.

Experimental demonstration of anamorphic M2 laser transformation in polar coordinates

Transforming the beam of a highly multimode laser into a narrow annular shape with almost diffraction-limited ring width is a potentially useful technique for certain material processing applications, such as drilling and cutting. In a previous article, we have described and simulated a theoretical method of realizing such transformation. Here, we provide experimental verification of this method, based on transforming the laser beam quality parameter M2 in polar coordinates. The transformation is done using a dedicated diffractive optical element, which we call a “spinner.” A comparison of the results for the spinner optical system versus a standard axicon ring shaping has been done, both in the focal plane and in defocus positions. Measured results have been compared with the theoretical model and simulations.

High-efficiency LD-pumped all-fiber Raman laser based on a 100 µm core graded-index fiber

We demonstrate a high-power, high-efficiency Raman laser based on a 100 µm core graded-index (GRIN) fiber directly pumped by 915 nm multimode laser diode modules in all-fiber configuration. Optimization of GRIN fiber length and pumping scheme was performed. As a result, 62 W of CW power has been obtained at a wavelength of 954 nm with a slope efficiency of 85%. The joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of the GRIN fiber core results in significant beam quality enhancement for the generated Stokes beam ( < 3) in comparison with that of the pump radiation ( > 30). In addition, the exact value of the operating wavelength near the Raman gain maximum was varied. To the best of our knowledge, this is the first study of the impact of a Raman gain spectral profile on the power, spectrum and beam quality parameter M2 of multimode Raman laser. It appears that they very much dependent on the spectral region of Raman gain at which we work, when the wavelength is selected by a fiber Bragg grating inscribed in a multimode GRIN fiber.

Quality of spatial entanglement propagation

We explore, both experimentally and theoretically, the propagation dynamics of spatially entangled photon pairs (biphotons). Characterization of entanglement is done via the Schmidt number, which is a universal measurement of the degree of entanglement directly related to the non-separability of the state into its subsystems. We develop expressions for the terms of the Schmidt number that depend on the amplitude and phase of the commonly used double-Gaussian approximation for the biphoton wave function, and demonstrate migration of entanglement between amplitude and phase upon propagation. We then extend this analysis to incorporate both phase curvature in the pump beam and higher spatial frequency content of more realistic non-Gaussian wave functions. Specifically, we generalize the classical beam quality parameter M2 to the biphotons, allowing the description of more information-rich beams and more complex dynamics. Agreement is found with experimental measurements using direct imaging and Fourier optics.

A Single -Longitudinal-Mode Nd:Ce:YAG Q-Switched Laser Based on a Three-Plan Resonant Reflector

We present an efficient and compact passively Q-switched flash-lamp-pumped Nd:Ce:YAG singlelongitudinal-mode (SLM) laser system. With Cr4+:YAG as a saturable absorber, we design a three-plan resonant reflector for generating smooth SLM Q-switched pulses. We provide a theoretical calculation and optimization of the resonant reflector for improving the longitudinal-mode selection ability. We obtain a stable SLM output with a single-pulse energy of 10 mJ and a pulse width of 10.7 ns at 10 Hz. The near-diffraction-limited beam-quality parameter M2 is less than 1.5. The system can operate with a repetition rate from 1 to 10 Hz. We achieve the stable laser operation with less than 3% fluctuation of the pulse energy within 10,000 shots.

750 nm 1.5 W frequency-doubled semiconductor disk laser with a 44 nm tuning range.

We demonstrate 1.5W of output power at the wavelength of 750nm by intracavity frequency doubling a wafer-fused semiconductor disk laser diode-pumped at 980nm. An optical-to-optical efficiency of 8.3% was achieved using a bismuth borate crystal. The wavelength of the doubled emission could be tuned from 720 to 764nm with an intracavity birefringent plate. The beam quality parameter M2 of the laser output was measured to be below 1.5 at all pump powers. The laser is a promising tool for biomedical applications that can take advantage of the large penetration depth of light in tissue in the 700-800nm spectral range.

Passively Q-switched Nd:YAG laser with diffractive output resonator

Passive Q-switching is presented in a novel, closed-loop, diode side-pumped Nd:YAG slab laser. Q-switched generation provided five pulse series with total energy of 120 mJ, 1 MW peak power and 24 ns single pulse width for maximal pump energy of 840 mJ. Beam quality parameter M2 was less than 2. Generation parameters in both four-wave mixing and classic linear resonator with similar length were compared.

Optically pumped long external cavity InGaN/GaN surface-emitting laser with injection seeding from a planar microcavity

Optically pumped InGaN/GaN quantum well vertical-external-cavity surface-emitting laser emitting at 420 nm has been realized. Lasing at external cavity lengths of up to 50 mm is demonstrated, making integration of practical sized intracavity elements possible. Spectral and beam profile measurements indicate best operation conditions in a semiconfocal cavity configuration. Lasing threshold of 20.9 W is achieved for a 49 mm long cavity with output beam quality parameter M2 not exceeding 1.1.

355-nm UV Generation by Intracavity Frequency Tripled Passively $Q$-Switched Nd:YAG/Cr$^{4+}$:YAG Laser

In this letter, intracavity frequency-tripling of a diode-end-pumped passively <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -switched Nd:YAG/Cr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> :YAG composite laser has been demonstrated for the first time. Under the absorbed pump power of 7.2 W, the maximum UV output power of 110 mW is obtained with the beam quality parameter <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of 2.95. The minimum pulsewidth and the highest repetition rate are 23.9 ns and 19.6 kHz, respectively. The corresponding single pulse energy and pulse peak power are 5.6 μJ and 0.23 kW. The fluctuation of the average output power at 110 mW within an hour operation is tested and determined to be 4.7%.

Optical quality of high-power fiber laser beams propagating through collimating systems

Within the framework of the ray optics theory, the effects of heating effect and geometric aberration on optical quality of fiber laser beam are calculated and analyzed. An analytical expression for the beam quality parameter M2 of fiber laser beam propagating through a multilens optical system is derived as a function of the system parameter. The optical quality of high-power laser beam propagating through a collimating system is investigated. It is shown that a best optical quality can be found as the laser power increases and the corresponding laser power depends on the collimating system parameter. The investigation presents a reference for the design of high-energy fiber laser collimating systems.

Management of the high-order mode content in large (40 μm) core photonic bandgap Bragg fiber laser

Very large-mode-area Yb(3+)-doped single-mode photonic bandgap (PBG) Bragg fiber oscillators are considered. The transverse hole-burning effect is numerically modeled, which helps properly design the PBG cladding and the Yb(3+)-doped region for the high-order mode content to be carefully controlled. A ratio of the Yb(3+)-doped region diameter to the overall core diameter of 40% allows for single-mode emission, even for small spool diameters of 15 cm. Such a fiber was manufactured and subsequently used as the core element of a cw oscillator. Very good beam quality parameter M(2)=1.12 and slope efficiency of 80% were measured. Insensitivity to bending, exemplified by the absence of temporal drift of the beam, was demonstrated for curvature diameter as small as 15 cm.

Optical quality of high-power laser beams in lenses

We analyze the propagation of a high-power laser beam through a lens and calculate the optical beam quality resulting from geometrical aberrations and thermal nonlinearities. We present a general ray-optics formulation, including diffraction effects, for propagation through a nonlinear medium. An analytical expression for the beam quality parameter M2 is derived that includes the lowest-order effects of geometrical and thermal aberrations in a thin lens. A ray-optics simulation, including thermal effects, is used to model propagation through a multilens optical beam expander of the type used in recent long-range, high-power fiber laser experiments.

Clarinet laser: Semiconductor laser design for high-brightness applications

High-power and high-brightness continuous-wave (cw) operation has been achieved with an optimized design of fully index-guided tapered laser emitting at 975 nm. The device achieves simultaneously negligible astigmatism and stable low divergence in the lateral axis at high-power operation. By using a quasi-three-dimensional simulation model, the different mechanisms modifying the slow axis beam divergence at high power have been carefully balanced in the clarinet design, easing the use of collective optics in laser bars. The devices consist of a relatively long ridge-waveguide filtering section coupled to a relatively short tapered section with an aperture angle of 2°. InGaAs∕InGaAsP lasers were fabricated with this design, demonstrating an output power of 1 W cw, a maximum wall-plug efficiency of 50%, negligible astigmatism, a slow-axis far-field divergence (measured at 1∕e2) of 5° at 1 W and beam quality parameter M2<3.

Off-axis parabolic mirrors: A method of adjusting them and of measuring and correcting their aberrations

This paper proposes a method of adjusting off-axis parabolic mirrors, based on the minimization of beam-quality parameter M². This method requires no additional adjustment elements and possesses adequate accuracy: 1% for the angular orientation of the mirror and 2.3% for the rms deviation of the M² value. An experimental technique is given for measuring and correcting the aberrations of strongly focusing off-axis optical elements. The technique is based on measuring the wave front by the Shack-Hartmann method. The aberrations were corrected using a controllable flexible mirror in the manual and automatic regimes by the aperture-probing and phase-conjugation methods, respectively. © 2005 Optical Society of America

Pulsed single-mode dye laser for coherent control experiments

We describe a pulsed single-mode dye laser whose output characteristics satisfy the stringent requirements of coherent control experiments. This simple, low cost laser exhibits 1.6× transform-limited frequency performance with a single-shot linewidth of 58 MHz, and a time averaged linewidth of 240 MHz. The spatial mode is nearly Gaussian and has a beam quality parameter M2=1.2. We modified the pulse shape using saturable absorption in the dye amplifier. The resultant pulses are temporally smooth and have a duration of approximately 20 ns.

Time varyingM 2 inQ-switched lasers

Numerical simulations of the pulse build up in aQ-switched laser reveal a considerable change in the beam spatial profile during the pulse evolution, on a nanosecond time scale. Such changes were measured experimentally, and the measured temporal evolution of the beam profile is shown to be in good agreement with the theoretical results. The time evolution of the spatial profile translates into an evolution of the beam quality parameterM2; for typical experimental conditions, in a Nd:YAG laser utilizing a variable-reflectivity mirror resonator,M2 is very close to a value of 1 at the beginning of the pulse, and increases smoothly throughout the pulse to reach a value of approximately 2 at the pulse trailing edge.