Enhanced high brightness DBR tapered diode laser based on polarization match.

Numerical simulation and experiment of high brightness tapered lasers

Tapered diode lasers combine high output power and a beam quality near to the diffraction limit resulting in very high brightness. Therefore, they are finding use in a wide range of applications today, such as pumping of rare-earth-doped fibre amplifiers, tunable frequency doubling of diode lasers for blue-green outputs, and non linear spectroscopy. Due to increasing brightness and lifetime tapered lasers even become attractive for material processing and for telecom applications like pumping of Er-doped fiber amplifiers or raman amplifiers. In order to further enhance the brightness of tapered diode lasers the output power has to be increased while simultaneously the beam quality has to be kept near the diffraction limit. For this purpose we have grown low modal gain, single quantum well InGaAs/AlGaAs devices emitting at 976 nm by molecular beam epitaxy. The lateral design of the investigated laser diodes consists of a tapered section and a ridge-waveguide section. Since it has been shown by previous simulations and experiments that longer tapered sections allow higher output power with unchanged beam quality, we use tapered section lengths of 2000 μm, 3000 μm and 4000 μm. The beam quality parameter M2 and output powers as well as the nearfields of the different structures were carefully investigated. For longer devices we reach an optical output power of more than 10 W per single emitter in continuous wave mode (cw) without any distinct thermal rollover.

650 nm tapered laser diodes with nearly diffraction limited beam quality are requested for laser display applications. In this paper, results for 2 mm long 650 nm tapered lasers diodes with different lateral geometries will be presented. The vertical structure is based on a 5 nm thick InGaP single quantum well embedded in AlGaInP waveguide and n- AlInP and p-AlGaAs cladding layers. The ridge waveguides of lengths LRW = 200 μm, 300 μm, 500 μm, and 750 μm were fabricated using selective etching. The contact window for the tapered section was defined applying ion implantation. Devices with a taper angle of 4° were manufactured. The facets were passivated. The rear side was high reflection coated and the taper side anti reflection coated. The devices were mounted p-side down on CVDdiamond heat spreaders and standard C-mounts. All devices reached a maximal output power larger than 1 W. They had a threshold current density of about 700 A/cm² and a slope efficiency of 0.8 W/A. The best conversion efficiency was 20%. The devices with the shortest RW-length LRW = 200 μm had the best beam quality (beam waist width 7 μm, far field angle 8.8°, 85% of the emitted power in central lobe, M² of 1.3 (all values measured at 1/e²-level)) at P = 1 W. The beam quality for devices with longer RW-section deteriorates up to M² = 4.4 for a LRW = 750 μm laser.

A method of beam quality management in multi-stage Master-oscillator power-amplifier (MOPA) laser systems has been presented. Beam quality always gets worse when a laser beam passes though an amplifier due to the thermally induced aberrations in the intensively pumped gain medium. Numerical calculations show that the spherical aberration (SA) coefficient of the aberrated beam wavefront from the amplifier can change the sign in free space propagation. The wavefront can thus be compensated by another amplifier, which has nearly identical pumping parameters with the first amplifier. A single-pass two-stage side-pumped Nd:YAG amplifier is developed in experiments. The seed beam from the oscillator with 42 W output power and beam quality factor of M 2 = 1.21 is scaled up to 74 W by the first stage of the amplifier, while the beam quality is degraded to M 2 = 1.80. The beam quality is improved to M 2 = 1.30 after the second stage by properly wavefront compensation and the output power is 101 W. Experiments are also carried out with a double-pass one-stage amplifier. The output beam with 84 W power and M 2 = 1.29 is achieved. To our best knowledge, it is the first time that beam quality enhancement in a side-pumped MOPA is demonstrated by SA compensation.

We demonstrate monolithic distributed-Bragg-reflector tapered diode lasers having an output power up to 12 W, a small spectral width of below uλ<10 pm and a beam quality close to the diffraction limit. This results in a brightness close to 1 GWcm-2sr-1. Due to these excellent electro-optical characteristics we achieved visible laser light up to P=1.8 W in a single-path second harmonic generation experiment. This allowed us to develop compact Watt-class (P=1.1 W) visible laser modules having an excellent beam quality (M²<3) with a narrow spectrum (uλ<30 pm). The entire device is integrated on a micro-optical bench with a volume below 20 cm³. In another application we demonstrate for the first time a femtosecond gigahertz SESAM-modelocked Yb:KGW laser. Such a laser system benefits from the small spectral emission and the focusability of the developed diode laser. A record peak power of 3.9 kW was achived. At the repetition rate of 1 GHz, 281 fs pulses with an average output power of 1.1 W were generated. This Yb:KGW laser has a high potential for stable frequency comb generation.

A numerical model is established to predict the beam quality factor M2 of fiber laser for the first time. The finite-difference beam propagation method (FD-BPM) is introduced to simulate the beam propagation of thulium-doped fiber lasers (TDFLs) with given design parameters. The output beam profile can be calculated for an active fiber with complex refractive index profiles and various bending radii. The beam quality factor M2 can be obtained by the calculated beam profile. The numerical simulation results agree with the reported experiment data with a deviation of less than 10%, which validates the accuracy of the model. This model provides a convenient and efficient approach for designing the high-beam-quality TDFLs with reduced experiment efforts.

The polarization-dependent transverse mode properties in a few-mode nonpolarization-maintaining (non-PM) fiber amplifier with PM fiber laser seeding are investigated. In addition, the effect of spectral broadening on degeneration of the degree of polarization (DOP) is studied. The experimental results show that a high DOP can be achieved if the output laser of the non-PM fiber amplifier has good beam quality and a narrow linewidth without any broadening. A kilowatt (kW) fiber laser system with PM fiber seed and a non-PM fiber amplifier is demonstrated with the DOP at around 90% by controlling the beam quality and spectrum broadening. With the active polarization control technique, the system can achieve a kW linearly polarized output with 6.5GHz linewidth. The M2 is around 1.1, and the polarization extinction ratio is about 14.5dB. Unfortunately, the random self-pulsing issue arose when the output power exceeded 1093W. We believe that a higher power of linearly polarized laser output based on adaptive polarization control in a non-PM fiber amplifier will be obtained if the self-pulsing issue is solved.

Semiconductor lasers with high beam quality and high optical output power are very attractive for a variety of applications such as molecular spectroscopy, fiber optic communication and frequency conversion. In the used power regime, devices based on tapered gain sections are the most promising candidates to reach these demands. However, two disadvantages of the tapered laser concept are the reduced output power provoked by their additional resonator losses and the astigmatism of these diode lasers. In case of high brightness diode lasers it is important to discuss the methods needed for an advanced output power also from the point of view of beam quality. The knowledge about astigmatism is essential for designing micro-optics. For the experimental results low modal gain, single quantum well InGaAs/AlGaAs devices emitting at 980 nm were grown by molecular beam epitaxy. The influence of the thermal resistance and of the tapered section length on the output power as well as on the beam quality has been investigated. In addition the impact of these parameters on the astigmatism of tapered diode lasers has been analysed. The experimental results have been correlated with simulations of the current-power curves and BPM simulations of the nearfield behaviour.

We report on experimental studies on the beam quality of the 8μm ZGP optical parametric oscillator(OPO). Through conducting a series of comparative experiments, the effects of resonator length, type of phase matching, resonant wavelength and structure of the resonator on the beam quality of ZGP OPO were studied respectively. By using longer resonator length(120mm), type I phase matching, longer resonant wavelength(8μm) and the ring cavity, the output with the beam quality factor of M2∼1.20 and 1.22 in the horizontal and vertical direction were achieved respectively. And this is a rather good beam quality level for 8μm ZGP OPO.

There is ongoing debate about the origin and even the very existence of a high degree of linear polarization of some chromospheric spectral lines observed in solar flares. The standard explanation of these measurements is in terms of the impact polarization caused by non-thermal proton and/or electron beams. In this work, we study the possible role of resonance line polarization due to radiation anisotropy in the inhomogeneous medium of the flare ribbons. We consider a simple two-dimensional model of the flaring chromosphere and we self-consistently solve the non-LTE problem taking into account the role of resonant scattering polarization and of the Hanle effect. Our calculations show that the horizontal plasma inhomogeneities at the boundary of the flare ribbons can lead to a significant radiation anisotropy in the line formation region and, consequently, to a fractional linear polarization of the emergent radiation of the order of several percent. Neglecting the effects of impact polarization, our model can provide a clue for resolving some of the common observational findings, namely: (1) why a high degree of polarization appears mainly at the edges of the flare ribbons; (2) why polarization can also be observed during the gradual phase of a flare; and (3) why polarization is mostly radial or tangential. We conclude that radiation transfer in realistic multi-dimensional models of solar flares needs to be considered as an essential ingredient for understanding the observed spectral line polarization.

Spin polarization of the neutral exciton in a single quantum dot

The development of high-power, high-brightness tapered diode lasers and laser amplifiers is reported in this review. Experimental and theoretical work is described that aims to improve the beam quality of these devices at output power levels well above 1 W. Special emphasis is laid on the dependence of the beam quality on the design of the epitaxial layer structure. For tapered diode lasers as well as for tapered laser amplifiers, the introduction of layer structures with a reduced modal gain leads to an improvement in beam quality by an order of magnitude. At output power levels of 2 W and CW operation beam-quality parameters of M 2 = 2 are achieved. The corresponding far-field widths are near the diffraction limit. Furthermore, a tunable diode-laser system is presented that consists of a tapered amplifier chip and a diffraction grating in an external cavity. This high-power high-brightness diode-laser system provides a narrow-bandwidth emission spectrum together with a wide-range tunability.

High-power and high-beam-quality laser sources are required in numerous applications. Here, we theoretically investigate a tapered laser diode with integrated metalens, which can improve the lateral beam quality. By carefully designing the slot length and width at different lateral positions behind the front facet, the mode mismatch between the ridged cavity and the tapered cavity is overcome while ensuring high transmittance (&gt;96%). The beam quality is greatly improved, and the lateral far-field divergence is reduced. The simulation results show that the lateral far-field divergence (second-moment level) decreases from 7.2° to 5.9°, and the M2 (second-moment level) decreases from 2.3 to 1.2. The proposed structure shows a good thermal stability in the simulation.

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.

A high-power compact continuous-wave (CW) Nd:YAG laser with greatly improved beam quality operating at 946 nm is reported. A simple concave-plane cavity was made with a composite Nd:YAG laser crystal which was pumped by a fiber-coupled laser diode. A maximum output power of 9.63 W was obtained at an incident pump power of 31 W with beam quality factor M2 ~ 5. To the best of our knowledge, this is the highest output power at 946 nm with such a beam quality that was produced from diode-pumped Nd:YAG lasers using bulk crystals.