High-power Diode Laser Bars Research Articles

Beam concentration and homogenization for high power laser diode bar

A novel optical element is presented and applied for beam concentration and homogenization of laser diode (LD) bar. It consists of a tapered SiO2-rod with twisted surfaces which is designed and optimized by the optical system design software and fabricated by a curved surface grinder and an optical polishing lathe. Results show that a rectangular output beam spot with uniform intensity distributions both at slow and fast axis is obtained at the output facet of the rod and the beam size is only 1.3mm∗0.8mm.

WITHDRAWN: Non-imaging concentration and homogenization for high power laser diode bar

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Efficient Operation of a Cesium-Vapor Laser Longitudinally Pumped by a Fine-Tunable Bandwidth-Narrowed Laser-Diode Bar

We report on a high-power cesium-vapor (Cs-vapor) laser longitudinally pumped by a tunable external-cavity broad-area laser-diode bar. The external cavity consisting of a beam-transformation system and an off-axis volume Bragg grating forced the high-power laser-diode bar to emit a fine-tunable narrow-bandwidth beam. Using the tunable narrowed-bandwidth beam, we effectively pumped a Cs-vapor laser with a flat-concave cavity and obtained strong emission (output peak power, 7 W; slope efficiency, 14%).

Gradual degradation of red-emitting high-power diode laser bars

The authors analyze early stages of gradual degradation in highly reliable 650nm emitting high-power diode laser arrays with continuous wave emission powers of 2.5W (facet load of 4mW∕μm). In all cases the edges of the metallized emitter stripes are identified as the starting points of gradual degradation. The magnitude of the observed degradation signatures, however, is highly correlated with the bar-specific packaging-induced strain at each emitter. We find a bar-specific effect, namely, the presence of packaging-induced strain, to be the driving force of gradual degradation. Our findings point to the significance of proper strain management in advanced device structures.

Diode laser bar beam shaping by optical path equalization

A beam shaping technique for a high power diode laser bar is presented. This optical design reshapes the highly asymmetric and astigmatic radiation of a high power diode laser bar, equalizing the beam quality in the orthogonal axes without loss of brightness. The optical paths of each sub-beam are equal, realizing the conservation of the beam quality in a system just a third of the size of previous solutions with similar performances. We present the results of a simulation and the production of a prototype for focusing the beam of a QCW bar to a spot size of about 300 µm with an NA of 0.28. The method for the realization of the optical elements is explained in detail.

Miniaturization and Power Scaling of Fundamental Mode Optically Pumped Semiconductor Lasers

We demonstrate that TEM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> mode optically pumped semiconductor lasers (OPSLs) may be scaled to tens of watts in the visible wavelength range using laser cavities an order of magnitude smaller than those of conventional solid-state lasers. In particular, we show that the output power may be scaled linearly by increasing the number of optically pumped semiconductor (OPS) devices and derive a unique solution for a dynamically stable resonator that is independent of the physical cavity length and internal design. This enables miniaturization of high-power OPS lasers to ~1 cm footprints without compromising many resonator performance metrics. The results are applied to demonstrate a 15-mm footprint cavity producing 7.3-W output at 486 nm, and a cavity with two OPS chips with 24-W output at 561 nm. In addition, we show that efficient TEM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> mode performance may be realized using free-space-coupled, high-power laser diode bars. Single-frequency operation is also demonstrated, and an rms noise level less than 0.01% is achieved.

Transient thermal properties of high-power diode laser bars

The transient thermal properties of high-power diode laser bars with active and passive cooling are analyzed. Both thermal imaging and the analysis of the thermal wavelength tuning behavior are employed to extract the device temperature as a function of time. A steady-state thermal situation is established with rise times of about 10 and 60ms for active and passive cooling, respectively. The latter number, however, is substantially increased by the particular properties of the external heat sink. Such results are confirmed by model calculations based on the finite element method.

Spatially resolved and temperature dependent thermal tuning rates of high-power diode laser arrays

Thermal tuning properties of passively cooled 808nm emitting high-power diode laser bars are analyzed. Data from standard devices packaged on Cu heat sinks and identical devices mounted on expansion-matched Cu–W heat sinks are compared. For a standard device, we find up to one-fifth of the thermal tuning rate of −(0.56±0.04)meVK−1 to be caused by pressure tuning driven by the relaxation of compressive packaging-induced stress for increasing temperatures. For devices packaged on expansion-matched heat sinks the observed tuning rate of −(0.46±0.01)meVK−1 represents almost the genuine thermal tuning rate of the semiconductor device structure. Thus this technology potentially leads to improved device properties.

Correction of beam errors in high power laser diode bars and stacks

The beam errors of an 11 bar laser diode stack fitted with fast-axis collimator lenses have been corrected by a single refractive plate, produced by laser cutting and polishing. The so-called smile effect is virtually eliminated and collimator aberration greatly reduced, improving the fast-axis beam quality of each bar by a factor of up to 5. The single corrector plate for the whole stack ensures that the radiation from all the laser emitters is parallel to a common axis. Beam-pointing errors of the bars have been reduced to below 0.7 mrad.

Screening of high-power diode laser bars by optical scanning

High-power diode laser bar arrays (808 nm) with very uniform emission properties are inspected by the laser-beam-induced current technique (LBIC). Setting the excitation energy to 50 meV below the lasing energy, we observe distinctive signatures within the LBIC scans at certain locations on the devices. After 1000 h of high-power operation, we observe degradation at exactly those positions that previously showed a characteristic LBIC signature. A cathodoluminescence analysis reveals that the quantum wells of these anomalous device sections suffer from the existence of spots with reduced luminescence efficiency. Additionally, a concomitant 2.3 meV redshift of the quantum-well cathodoluminescence spectrum is also observed. Our measurements demonstrate the efficiency of the LBIC approach as a screening tool as well as its capacity for predicting device failure well before any degradation of the emission properties is observed.

Measurement systems for the investigation of soldering quality in high-power diode laser bars

Thermal and spatial-spectral imaging methods are used to investigate the soldering quality in high-power laser-bar devices. The temperature distribution and the wavelength profile across a driven laser bar correlate with the local thermal resistance between laser and cooler and, thus, with the uniformity of the solder layers. Measurements with a properly soldered and a deficiently soldered laser bar are made for the illustration and comparison of the methods.

Deep level emission from high-power diode laser bars detected by multispectral infrared imaging

Broadband imaging thermography is applied to investigate the infrared (IR) emission from high-power diode laser bars. We demonstrate the capabilities of a multispectral imaging system that operates in two optical channels, the near IR at 1.5–2μm and the mid IR at 2.4–5.5μm. In the near-IR region, deep level luminescence of the device contributes significantly to the thermo-images. A complementary measurement of the IR emission spectrum shows a broad defect band with maxima at E1=0.94eV and E2=1.05eV. Analysis of IR transients in both spectral channels makes a distinction of thermal radiation and deep level emission possible. In the mid IR, thermal radiation dominates, allowing for an analysis of thermal properties of the device with measurement times of fractions of a second only. This makes imaging thermography highly attractive for device screening.

High-Power Operation of 1 cm Laser Diode Bars on Funryu Heat Sink Cooled by Fluorocarbons

We have investigated the properties of fluorocarbons as coolants for a high-power laser diode bar mounted on a funryu-jet heat sink. The thermal conductivities of fluorocarbons are about ten times lower than that of water, but they are less corrosive to the funryu copper heat sink and their boiling points may be higher than that of water. Since the cooling efficiency of such a high power laser diode (LD) bar may be a result of the dynamic viscosity, the heat capacity, the thermal resistance, and the boiling temperature of the coolant, we have compared two types of “Fluorinert”, 3M's FC77 and FC43, and water as coolants. Wavelength distributions along a 1 cm bar, operated under a 52 A driving current, are measured to be ±1.1 nm for FC77, ±2.1 nm for FC43 and ±0.6 nm for water. From their thermal and material characteristics given by 3M, the lower viscosity of FC77 is important for our funryu heat sink, which has a high flow resistance. A thermal resistance of 0.7°C/W is achieved at a flow rate of 1.0 L/min for FC77. FC77 is adopted as the coolant for CW 30 W operation and has operated continuously for over 800 h.

Highly efficient high-power quasi-continuous diode laser bars for pumping solid-state lasers based on Yb-containing active media

Highly efficient high-power quasi-continuous diode laser bars (DLBs) emitting in the region of λ=0.95 μm have been developed for pumping solid-state lasers based on Yb-containing active media. The parameters of DLBs and the results of long-term tests are presented. The proposed DLBs are implemented in a solid-state laser based on an Yb-Er glass operating in the 1.5 mm range.

Simultaneous quantitative determination of strain and defect profiles within the active region along high-power diode laser bars by micro-photocurrent mapping

We present microscopically-resolved photocurrent spectroscopy as a new powerful analytical tool for the simultaneous detection of packaging-induced strain and defects in GaAs-based high-power laser diode arrays (cm-bars). Using the Fourier-transform (FT) technique we measure photocurrent (PC) spectra with a high spatial resolution of better than 50 μm at the active layer of the device. By analyzing this data, spatially resolved distributions of strain as well as of defects are obtained. So far, PC measurements at cm-bars have only provided an overview of the distribution of strain and defects in the device on an emitter-by-emitter scale. The high spatial resolution now allows examination of the distribution of strain and defects even within one single emitter. This is essential for obtaining insights into device failure mechanisms and also makes a substantial contribution for improving device performance and reliability.

Effective suppression of beam divergence for a high-power laser diode bar by an external-cavity technique.

We describe effective suppression of beam divergence for a high-power laser diode bar by use of an external-cavity technique. Nineteen off-axis external-cavity laser diodes of the high-power laser diode are formed by feedback with a stripe mirror. At three times the threshold current, the diverging angle (1/e2) of the external-cavity laser diode bar is reduced to 1.5 degrees from 6.6 degrees (free running) with 14.1-W peak output power and 70.4% of the radiated power of the free-running state without the external cavity. This technique effectively improves the beam quality of the high-power laser diode bar.

Spatially resolved spectroscopic strain measurements on high-power laser diode bars

We compare a number of strain-sensitive spectroscopic techniques, namely, micro-Raman, micro-photoluminescence, photocurrent, and electroluminescence by applying them to two AlGaAs/GaAs high-power diode laser arrays, so-called “cm-bars.” Both devices are fabricated from the same bar batch and considered “identical,” but experienced two different packaging procedures that resulted in different intentionally adjusted “packaging-induced” compressive strains along the array, namely, about −(3.0±0.5)×10−4 and −(5.0±0.7)×10−4 strain difference between edges and centers of the bars. The methods are primarily evaluated with respect to their ability to monitor and quantify the different spatial strain distributions along the devices. In addition their potential for defect detection is demonstrated. Pros and cons of the techniques are summarized and discussed.

Investigation of reliability issues in high power laser diode bar packages

As applications continue to demand increasingly higher optical output power and longer lifetime, thermo-mechanical stresses on dissimilar materials interfaced for packaging pose an ever-growing challenge for the realization of a durable system. Particularly important for an epitaxy-down configuration is the die-attachment interface, which is desired to be defect free and stress managed for reliable optical alignment. A knowledge of the changes in the physical defect density and magnitude of the thermo-mechanical stress present in the active region as a function of the fabrication process and aging is crucial to an understanding of the influence of the process parameters and operating conditions on device performance and reliability. In this study, we investigated high power laser diode array packages aged under various conditions. Microscopic defect analyses of the die attachment interface and device stress were carried out using primarily metallography, scanning electron microscopy, scanning acoustic microscopy, microhardness, and micro-Raman spectroscopy. It was noted that the intermetallic compounds and microscopic physical defects at the die attach interface are detrimental to transient heat transfer, and thus, overall package reliability. Using micro-Raman spectroscopy, we found that tensile stress near the bar-package interface increases with aging for the first few hundred hours and then decreases with further aging.

Micro thermal management of high-power diode laser bars

Lifetime and reliability of high-power diode laser bars are sensitively related to operating temperature, mounting stress, and solder electromigration. These three factors have been taken into account for the development of a new packaging technology for 1 cm laser bars of gallium arsenide. We examine the use of chemical-vapor-deposited (CVD) diamond as heatspreaders in order to reduce thermal resistance of a microchannel cooler for liquid cooling. We show that it is possible to perform hard soldering on a CVD-diamond with a new technique. Additionally, we present a controlled water cooling system fit to the flow characteristics of the cooler. It permits one to adjust the emission wavelength of the diode lasers by changing the water flux.

100-Watt laser bars based on phase-locked arrays

High-power diode laser bars (DLBs) based on InGaAlAs quantum-confinement heterostructures with an output optical power of no less than 100 W in a quasi-continuous wave regime (pulse duration, 200–400 μs; repetition rate, 50–100 Hz) were fabricated and investigated. A bar of this type consists of a set of strip emitters, each representing an optimized phase-locked array of single-mode emitters. This design provides for a considerably improved temporal and spatial stability of the DLB emission, with a simultaneous decrease in the optical noise level.