High-power tapered diode lasers utilizing monolithic gratings

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.

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.

Satellites have become essential tools for providing information for weather forecasts, monitoring agriculture and studying climate. In recent years satellite lidar (laser ranging) has come of age, with three missions launched in 2018. These spaceborne lidars are collecting ground-breaking data. They are the only current in-orbit technology capable of directly measuring bare Earth topography under vegetation and of making non-saturating measurements of forest biomass, height and cover. However, the energy requirements of a lidar lead to very sparse coverage. NASA's GEDI mission, the densest sampling lidar yet, is expected to directly sample between 2-4% of the Earth's surface, leading to sampling errors and preventing its use in applications that require continuous coverage without interpolation. The coverage of a lidar satellite is controlled by how much laser power they can emit, collect and make a usable measurement from. This is controlled by the payload power, telescope collecting area, energy per shot needed for an accurate measurement, laser efficiency and detector efficiency. Initial calculations using currently in-orbit technology suggest that global, continuous coverage could be achieved within 5 years at 30 m resolution using ten ICESat-2 class lidar satellites. This would be prohibitively expensive. The Global Altimeter MISSion: GLAMIS project aimed to see whether recent developments in photonics, deployable optics and small-satellites could make continuous coverage lidar more cost-effective. All current lidar satellites use a solid-state laser. These produce short, powerful bursts of light but are only ~5% efficient. Tapered laser diodes are 25% efficient, but cannot emit the same amount of energy in as short a burst as solid state lasers can. Diode laser would need to spread the energy over a longer time, using pulse compressed lidar (PCL) to allow measurements, which would lead to very different noise behaviour to sold state lasers, possibly preventing their use in spaceborne lidar. A satellite lidar simulator was used to determine that PCL is suitable for satellite lidar over a range of biomes. The mirror size and cost for fixed and deployable telescopes were estimated for three different satellite size classes; a 12U satellite, a 150 kg satellite and a 500 kg satellite. The electric power and cost of each of these satellites was also estimated and used, along with the laser efficiencies, to calculate the lidar coverage each could achieve. Orbital constellation simulations were used to determine how many satellites of each configuration would be needed to achieve global coverage within a given timeframe, accounting for loss due to clouds. It was found that deployable optics would allow a more cost effective coverage than fixed optics. It was also found that the larger satellites would be a more cost-effective solution than smaller satellites. In conclusion, using tapered laser diodes in pulse compressed mode and deployable optics are promising technologies to increase the coverage of satellite lidar. For a future constellation of lidar satellites capable of covering the whole Earth, the optimum configuration was found to be a constellation of eleven 150 kg (roughly microwave sized) satellites fitted with foldable mirrors.

Diffraction-limited high-power narrow-spectrum red diode lasers are attractive for many applications, such as photodynamic therapy, laser display, and as a pump source to generate UV light by second harmonic generation (SHG). High-power, diffraction-limited diode lasers can be realized by the technology of lasers with a tapered gain-region (Kintzer et al., 1993; Donnelly et al., 1998; Wenzel et al., 2003; Paschke et al., 2005; Sumpf et al., 2009; Fiebig et al., 2010). The tapered laser devices can be used in applications where narrowspectrum is not needed such as photodynamic therapy, but for other applications such as a pump source for UV light generation, the spectral quality of these devices has to be improved. In order to improve the spectral quality of a tapered laser, different techniques are applied, such as a monolithically integrated master oscillator power amplifier by forming Bragg gratings in the semiconductor material (O’Brien et al., 1993, 1997a), injection locking to an external single-mode laser (Goldberg et al., 1993; Mehuys et al., 1993b; O’Brien et al., 1997b; Wilson et al., 1998; Ferrari et al., 1999; Spieβberger et al., 2011), and different external-cavity feedback techniques (Jones et al., 1995; Cornwell & Thomas, 1997; Morgott et al., 1998; Goyal et al., 1998; Pedersen & Hansen, 2005; Chi et al., 2005; Lucas-Leclin et al., 2008; Tien et al., 2008; Sakai et al., 2009). Up to 1 W output power at 668 nm from a Fabry-Perot tapered diode laser was obtained with a beam quality factor of 1.7, and the spectral width was smaller than 0.2 nm (Sumpf et al., 2007). Around 670 nm, tunable narrow-linewidth diffraction-limited output was also achieved from an injection-locking tapered diode laser system seeded with a single-mode external-cavity diode laser (Haring et al., 2007); the output power was up to 970 mW. A 670 nm micro-external-cavity tapered diode laser system was demonstrated with a reflecting volume Bragg grating as a feedback element; in continuous wave (CW) mode, more than 0.5 W output power was obtained, and in pulse mode, 5 W peak power was obtained with a beam quality factor of 10 and a spectral width below 150 pm (Tien et al., 2008). Up to 1.2 W output power at 675 nm from a tapered laser

We investigate microsecond pulse-mode operation of a micro-integrated high-power diode laser based on volume Bragg grating external-cavity feedback around 808 nm. The laser system contains a tapered amplifier consisting of a ridge-waveguide section and a tapered section with separated electrical contacts. Thus, the diode laser system can be pulsed by modulating the injected current either to the ridge waveguide section (IRW) or to the tapered amplifier section (ITA). With a trigger signal of a 50 µs pulse width and a 10 kHz repetition rate, comparing the modulation depth, peak output power, beam propagation factor, and spectral bandwidth, we conclude that the pulse-mode operation achieved by modulating the ITA gives better results than by modulating the IRW due to the decreased thermal effect. At a constant IRW of 0.2 A and a modulated ITA of 6.0 A, 4.3 W of peak output power is obtained with an emission spectral bandwidth with an upper bound of 0.2 nm, and a beam propagation factor in the slow axis, Mslow2, of 2.6 (1/e2). The modulation depth is almost 100%. The results show that the tapered diode laser system may be a good candidate for microsecond pulse-mode solid-state lasers.

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.

Efficient quasi-three-level Nd:YAG laser at 946 nm pumped by a tunable external cavity tapered diode laser

We present a new and improved method for pumping three-level solid-state lasers. The better beam quality from tapered diode lasers makes them attractive pump sources in configurations that are inherently lossy; in particular, three-level laser systems. We present numerical calculations and experimental comparison of tapered diode laser pump performance against that of traditional broad area diode lasers (BAL).

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.

GaInNAs-based high-power laser diodes with emission in the 1220-1240 nm wavelength range are presented. Broad-area (BA) devices show very low internal losses of only 0.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , allowing high continuous-wave room-temperature output powers of almost 9 W and emission at a wavelength of 1220 nm. Based on the needs for applications like, e.g., pumping of Raman amplifiers, wavelength-stabilized tapered laser diodes are shown with maximum output powers of 1 W together with a high beam quality. Beam propagation factors <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> down to 1.4, high brightness of up to 23 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> middotsr, and nearly spatial single-mode emission have been obtained.

Recently, hybrid integrated compact laser sources with high optical output powers in the visible range around 488 nm were demonstrated using tapered diode lasers. This was done by single-pass second harmonic generation (SHG) using a periodically poled LiNbO3 crystal of 30 mm length. The conversion efficiency depends on the light source but is also a function of the temperature distribution along the length of the crystal. The maximum conversion efficiency of a given beam is theoretically achieved by a homogenous temperature distribution. Experiments have shown that for high power SHG different absorption mechanisms are causing a temperature gradient in the crystal. This gradient leads to an inhomogeneous poling period, which diminishes the effective crystal length and leads to a smaller conversion efficiency. In this paper we present a method for the optimization of the temperature management during the SHG. This is done by a multizone heater package that can be integrated into compact laser sources. This package can be used to create arbitrary temperature distributions and is especially able to compensate an arising temperature gradient.

Tapered diode lasers, composed of an index-guided ridge waveguide and a gain-guided tapered amplifier, are affected by polarization mismatch between the ridge and tapered sections. Beam quality deterioration is caused by TM high-order modes generated in the ridge section. Under high current injection, these TM modes are further amplified in the tapered section due to polarization mismatch, leading to a decrease in the laser output brightness. In this paper, a combination of deep etching in the ridge section and compressive stress in the tapered section is employed to obtain a fundamental mode seed source with a high TE degree of polarization (DOP), simultaneously improving the polarization match between the ridge and tapered sections. The fabricated tapered diode laser achieves a beam quality factor of M2(1/e2) = 1.2 at an output power of 11.57 W. The peak electro-optical efficiency is 59.8%, and the maximum output brightness reaches 936 MW·cm-2·sr-1.

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 &mu;m, 300 &mu;m, 500 &mu;m, and 750 &mu;m were fabricated using selective etching. The contact window for the tapered section was defined applying ion implantation. Devices with a taper angle of 4&deg; 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 &mu;m had the best beam quality (beam waist width 7 &mu;m, far field angle 8.8&deg;, 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 &mu;m laser.

We have demonstrated world record diffraction-limited output from a 1550 nm tapered diode laser. Over 2.7 W continuous wave is achieved at room temperature. The result is enabled by a novel structured contact which permits geometric scaling of the chip dimensions while preserving beam quality.

A wide range of laser medical treatments are based on coagulation of blood by absorption of the laser radiation. It has, therefore, always been a goal of these treatments to maximize the ratio of absorption in the blood to that in the surrounding tissue. For this purpose lasers at 577 nm are ideal since this wavelength is at the peak of the absorption in oxygenated hemoglobin. Furthermore, 577 nm has a lower absorption in melanin when compared to green wavelengths (515 − 532 nm), giving it an advantage when treating at greater penetration depth. Here we present a laser system based on frequency doubling of an 1154 nm Distributed Bragg Reflector (DBR) tapered diode laser, emitting 1.1 W of single frequency and diffraction limited yellow light at 577 nm, corresponding to a conversion efficiency of 30.5p. The frequency doubling is performed in a single pass configuration using a cascade of two bulk non-linear crystals. The system is power stabilized over 10 hours with a standard deviation of 0.13p and the relative intensity noise is measured to be 0.064 p rms.