M2 Factor Research Articles

Single-mode oscillations of diode-pumped mid-infrared Er:Y2O3 ceramic microchip lasers at 2.7 μm.

Two compact mid-infrared microchip lasers at 2717 and 2740 nm have been demonstrated using a Er:Y2O3 ceramic as laser gain medium with thickness of 800 μm, for the first time to our knowledge. Under a 976-nm diode laser pumping, the 2717 nm microchip laser with linewidth of about 0.16 nm is achieved with a maximum output power of 234.8 mW and slope efficiency of about 10.9%. The laser beam quality expressed by M2 factor is measured to be about 1.23 and 1.45 in x and y directions. A single wavelength at 2740 nm with linewidth of about 0.15 nm is also achieved with maximum output power of 102 mW and slope efficiency of about 4.9%. Beam quality of the 2740 nm laser is found to be about 1.15 and 1.26 in x and y directions. Using a mechanical chopper to modulate the pump laser for thermal mitigation, the maximum output powers can be further improved to 312 mW for 2717 nm laser and 145 mW for 2740 m laser at higher pump powers. Such a mid-infrared microchip laser source with very compact size could be have great potential in various eye-safety-related applications.

Role of the Features of Focused Laser Beam at Pulsed Laser Cutting

Abstract In this article investigation of the roles of two important factors of focused laser beam, the focal spot diameter and the Rayleigh length as determining variables of the beam quality were made. The equations of these two factors are based on those most commonly used in the literature. The exchange between three different beam quality numbers were shown. It is proven on the basis of the scientific literature, that the beam quality degrades compared to the original data given by the factory of laser. The causes of the beam quality degradation are lens aberrations in the optical path of the given laser, and the shifting of the beam propagation ratio (M2) to higher values. A new equation for estimation of the new, lowest value for M2 factor is presented, based on the comparison of the laser cut material thickness to the depth of focus, which is two times the Rayleigh length.

High-order cylindrical vector beams with tunable topological charge up to 14 directly generated from a microchip laser with high beam quality and high efficiency

Large topological charge optical vortex beams carrying orbital angular momentum have potential applications on optical trapping, optical communication with high capacity, quantum information processing. However, the beam quality is degraded in vortex beams generated with spiral phase plates or resonator mirrors with defect spots and optical conversion efficiency in solid-state lasers is sacrificed by controlling the loss of resonator. It is a big challenge for generating high beam quality, high-order cylindrical vector beams with large topological charge in compact solid-state lasers. Here, high-order cylindrical vector beams [Laguerre-Gaussian (LG) modes with zero degree and order of l, LG0,l] with tunable topological charges up to 14 have been generated in an annular beam pumped Yb:YAG microchip laser by manipulating the pump power-dependent population inversion distribution. Efficient performance with optical efficiency of 17.5% has been achieved. The output power is 1.36 W for a vector-vortex laser with 14 topological charges. The pump power dependent wavelength tunable and dual-wavelength laser oscillation in vector-vortex beams has been observed by controlling the reabsorption loss at 1030 nm. Wavelength tunable, dual-wavelength (1030 and 1050 nm) laser oscillation has been achieved for vector-vortex beams with topological charges of 8, 9, and 10. The laser beam quality factor M2 close to the theoretical value (l + 1) has been achieved for LG0,l vector-vortex beams with tunable topological charges up to 14. This work provides a new effective method for generating large topological charge high-order cylindrical vector beams in solid-state microchip lasers with high efficiency and high beam quality.

LD end-pumped single-ended bonding Tm:LuAG laser

A diode end-pumped Tm:LuAG laser at room temperature was reported. The thermal lens effect of Tm:LuAG crystal was analyzed, the distribution of the light field and thermal field in the crystal was simulated moreover. In this paper, by using plane-concave resonator, the continuous 2 μm laser output with maximum output power of 10.8 W is obtained. The slope efficiency is 19.56% and the optical-to-optical conversion efficiency is 25.71%. The beam quality factors M2 are 1.24 and 1.28 in the x and y directions respectively.

High-pulse-quality Yb-fiber amplifier generation of 10 μJ, 250 fs pulses at 500 kHz repetition rate

A high-pulse-quality femtosecond chirped pulse amplifier system based on tunable chirped fiber Bragg grating (CFBG) stretcher is reported. Dispersion of CFBG is fine-tuned designed to match the compressor. By controlling the dispersion of CFBG, net dispersion of the system approaches zero. Laser pulses with 10 μJ pulse energy and duration time of 250 fs after compression at a repetition rate of 500 kHz are obtained. The measured output energy fluctuation of root mean square is 0.3% in 1 h and the M2 factor is 1.07. These results indicate that the laser amplifier exhibits a great potential in industrial processing, military defense and scientific research.

Pulse duration adjusted by changing the cavity length with a multi-pass cavity in a Q-switched Nd:YAG laser.

We report a compact, long nanosecond (ns) pulse duration stretched laser source by a multi-pass cavity (MPC). Based on the combination of the MPC and pump power, a high-power high beam quality 1064nm Q-switched Nd:YAG laser with a pulse duration adjustable over the range of 160-1000ns was obtained at a pulse repetition frequency of 10kHz for the first time, to the best of our knowledge. At a typical pulse width of 560ns, an average output power of 10.6W was successfully achieved. The beam quality factor M2 was measured to be 1.45 with a good Gaussian mode.

A high pulse energy single-pass picosecond master oscillator power amplifier system with output power 35.7 W

We have demonstrated a high pulse energy single-pass picosecond (ps) master oscillator power amplifier (MOPA) system of 1064 nm at a pulse repetition rate of 500 kHz. A 500 kHz, 32.38 μJ picosecond laser was used as the signal laser. Through a single-pass traveling-wave amplifier, a maximum output power of 35.7 W at a pump power of 100.6 W was generated with the optic-optic efficiency of 19.39%. The output pulse duration was 16.9 ps, corresponding to the pulse energy of 71.4 μJ, while simultaneously the beam quality factor M2 was measured to be 1.36 and 1.32 along the x, y axis direction, respectively.

Extremely high-brightness tapered photonic crystal diode laser with narrow-emitting aperture

High-brightness tapered photonic crystal diode lasers in the 980 nm range are investigated. The tapered lasers with a 20 μm wide light-emitting aperture achieved 2.55 W at 3.2 A. The beam quality factor M2 in the vertical and lateral directions are 1.16 and 1.48 at 2.5 A. Maximum brightness of 126.8 MW cm−2 · sr at the second moment level is achieved. The devices also shows a narrow far-field of 17.1° × 6.5° in the vertical and lateral directions and a high efficiency of 53%. The proportion of the first-order mode in the lasing emission is also investigated based on the measured beam quality factor.

996 nm high-power single-longitudinal-mode tapered gain-coupled distributed feedback laser diodes.

A high-power single-longitudinal-mode regrowth-free tapered gain-coupled distributed feedback laser diode based on periodic current injection is achieved at 996 nm. It enhances the output power without beam quality degrading. A continuous-wave output power of over 1.12 W is achieved at 3 A. The maximum output power in single-longitudinal-mode operation is up to 0.56 W at 1.4 A. The power conversion efficiency is over 24%, and the slope efficiency is 0.58 W/A. The side mode suppression ratio is over 38 dB; the 3 dB spectral linewidth is less than 2.4 pm. The lateral far-field divergence angle is only 14.98°, and the beam quality factor M2 is 1.64, achieving a near-diffraction-limit emission. Our device has great potential in commercial applications and the experimental study of high-power near-diffraction-limit laser diodes for its low-cost fabrication technique and narrowband single-longitudinal-mode emission at high power.

10.3 W diode-pumped passively mode-locked Nd:YAG laser at 1319 nm with a semiconductor saturable absorber mirror

A high-power diode-end-pumped continuous-wave (CW) passively mode-locked picosecond (ps) 1319 nm Nd:YAG laser was demonstrated with a semiconductor saturable absorber mirror (SESAM). The oscillator cavity was accurately designed to optimize the laser beam radii in the centre of crystal and on the SESAM. At an absorbed pump power of 102 W, the maximum mode-locked output power of 10.3 W was achieved with pulse width of 27.1 ps and a beam quality factor M2 = 1.38 at the repetition rate of 76 MHz, corresponding to an optical-optical efficiency of 10.1%. To the best of our knowledge, this is the first demonstration of diode-pumped passively CW mode-locked ps Nd:YAG laser at 1319 nm with a SESAM.

Experimental Study of 5-kW High-Stability Monolithic Fiber Laser Oscillator With or Without External Feedback

In this paper, we construct a high power monolithic fiber laser oscillator based on a commercial ytterbium-doped fiber with core/inner cladding diameter of 25/400 μ m. The output performances of the fiber oscillator are experimentally investigated in the conditions of either with or without an external feedback. In the presence of external feedback, a very strong stimulated Raman scattering (SRS) (∼9 dB below the signal light) emerges at the output power of ∼3.56 kW, where the onset of the transverse mode instability (TMI) limits further power scaling. By large angle cleaving the signal arm of the forward combiner to avoid external feedback, the maximum output power of 5.07 kW is achieved with no sign of TMI. The Raman stokes light is ∼35 dB below the signal light and the M2 factor is measured to be ∼1.6. To directly characterize its power stability and engineering capability, the fiber oscillator is tested for continuous 5-h operation at ∼5.07 kW with power fluctuation less than 0.42%. During the test process, the output power, spectrum, beam quality, and temporal signal perform excellent stability. The experimental results reveal that thresholds of SRS and TMI strongly depend on the external feedback in high power fiber oscillators.

High-Brightness Low-Divergence Tapered Lasers with a Narrow Taper Angle**Supported by the National Key Research and Development Program of China under Grant Nos 2016YFB0402203 and 2016YFA0301102, and the National Natural Science Foundation of China under Grant Nos 61535013 and 91850206.

High-brightness tapered lasers with photonic crystal structures are designed and fabricated. A narrow taper angle is designed for the tapered section. The device delivers an output power of 3.3 W and a maximum wall-plug efficiency of 43%. The vertical beam divergence is around 11° at different currents. Nearly diffraction-limited beam qualities for the vertical and lateral directions are obtained. The lateral beam quality factor M2 is below 2.5 and the vertical M2 value is around 1.5 across the whole operating current range. The maximum brightness is 85 MW·cm−2sr−1. When the current is above 3.3 A, the brightness is still above 80 MW·cm−2sr−1.

Intra-cavity cascaded pumped 912nm/1030 nm dual wavelength laser output

A dual-wavelength laser operating at 912 nm/1030 nm for the first time using a compact intracavity cascade pumping structure is set up The laser system model was designed and built for testing. The proposed system employs a coaxial linear cavity structure and uses a fiber-coupled 808 nm laser diode (LD) end-pumped Nd:GdVO4 crystal to obtain a 912 nm laser, which then pumped Yb:YAG crystal directly in the cavity to produce a 1030 nm laser. When the LD pump power at 808 nm is 25 W, a 0.97 W laser at 912 nm and a 1.33 W laser at 1030 nm are obtained, respectively. The corresponding optical to optical conversion efficiency is 9.2%. The beam quality factors M2 of 912 nm and 1030 nm lasers at maximum output power is measured to be 1.19 and 1.23.

Comparative study on transverse mode instability of fiber amplifiers based on long tapered fiber and conventional uniform fiber

Compared with conventional uniform fibers, long tapered fibers provide an effective way to maintain beam quality and suppress the generation of nonlinear effects in fiber lasers. In this work, the transverse mode instability (TMI) of two amplifiers based on ytterbium-doped fiber (YDF) with uniform core diameter and tapered core diameter has been investigated experimentally. For a reasonable comparison of TMI in these two fiber amplifiers, the same effective core diameters and design parameters are applied to both. In the uniform YDF amplifier, the TMI threshold is around 1046 W, and the beam quality M2 factor increases with the power and reaches 3.2 when the power is around 1500 W. In a tapered YDF (T-YDF) amplifier, the maximum output power is up to 2170 W with a beam quality of M2 ~ 2.2 and no sign of TMI is observed. To the best of our knowledge, this is the first reported all-fiber tapered fiber laser with a 2 kW-level average output power. The experimental results and theoretical analysis indicate that the TMI threshold of the T-YDF amplifier is higher than the uniform YDF amplifier in spite of sharing the same effective core diameter.

M2 factor of a vector Schell-model beam

Extending existing scalar Schell-model source work, we derive the M2 factor for a general electromagnetic or vector Schell-model source to assess beam quality. In particular, we compute the M2 factors for two vector Schell-model sources found in the literature. We then describe how to synthesize vector Schell-model beams in terms of specified, desired M2 and present Monte Carlo simulation results to validate our analysis.

High-Power Dual-End-Pumped Monolithic Tm:YAP Microlaser

We report a high-power monolithic Tm:YAP microlaser dual-end-pumped by two fiber-coupled LDs emitting at 792 nm. The microlaser operated in the continuous wave (CW) mode, with a wavelength of 1,936.82 nm. We obtain a maximum output power of 37.8 W with a beam quality factor M2 of 9.6 at a total incident power of 174.9 W, corresponding to a slope efficiency of 29.2% and an optical–optical conversion efficiency of 21.6%.

About the validity of the parabolic approximation in Kerr lensing effect

The usual parabolic approximation used for modelling the focusing properties of a Kerr lens induced by a Gaussian laser beam, through optical Kerr effect, consists to roughly approximate a Gaussian profile by a parabola. The Kerr focal length deduced from the ABCD formalism is found to not correctly provide the focal plane position. The latter has been numerically determined from a numerical diffraction analysis, and we have empirically found that the right focal length is obtained by multiplying, by a factor equal to 3, the focal length given by the usual parabolic approximation. The expression of the focal length associated with the Kerr lens that we have obtained is in a good agreement with the expression of the focal length determined on the basis of a Zernike decomposition. In addition, it is demonstrated that the emerging beam from the Kerr lens is no longer Gaussian since its propagation factor M2 is larger than unity. It is found that the M2 factor increases linearly with the on axis phase shift.

Short pulse close to round-trip time generated by cavity-less high-gain Nd:GdVO4 bounce geometry.

In this paper, pulsed output with pulse-widths approaching the round-trip time were generated by utilizing a cavity-less high-gain Nd:GdVO4 bounce geometry. By adopting an EOQ (electro-optics Q-switch) device, pulse-widths of 1.36 ns, 1.82 ns, and 2.39 ns were achieved at three effective cavity lengths, respectively. All these pulse-widths were close to the round-trip time of corresponding effective cavity lengths. Moreover, watt-level output power at kHz-level repetition rate was achieved, as well as the good beam quality with M2 factor less than 1.3. The output had a time-averaged continuous spectrum with 10 dB linewidth of 0.2 nm.

Design and demonstration of a passive-cooled, Innoslab-based Nd:glass regenerative amplifier with high beam quality

We report on a high-energy and passive-cooled Nd:glass regenerative amplifier based on Innoslab laser technology. At a total pump power of 4.5 kW at 802 nm and pulse energies up to 27.2 mJ are generated with a pump pulse duration of 450 µs. The energy stability at 16.3 mJ is 1.5% (rms) over 2.5 hours at a 1 Hz repetition rate with beam propagation factors M2 in the horizontal direction and the vertical direction of 1.12 and 1.14, respectively. To the best of our knowledge, this is the first Innoslab-based Nd:glass regenerative amplifier without active cooling.

Thin-disk laser resonator design: The dioptric power variation of thin-disk and the beam quality factor

This paper investigates the general limitations of thin-disk laser (TDL) resonators due to the dioptric power variation of thin-disk (TD) crystal to attain the good beam quality. For this reason, the generalized formulation of TDL resonator in geometric optics theory is analytically derived. The analytical model is developed by considering two general configurations of stable resonators based on the position of the TD crystal inside the resonator; end-disk and fold-disk resonators. The calculation results reveal best M2 factor at operation pump power (OPP) from a specified disk is proportional to the variation interval of the dioptric power of the TD and it depends on the general configuration of resonator. For each configuration, the initial radius of curvature of the TD determines the resonator shape to attain the minimum acceptable M2 factor.