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Edge-emitting semiconductor laser arrays are also called laser rods. These devices are undoubtedly the most widely known and widely used architectures in high power diode lasers (HPDL).
When used under electronic pumping schemes, these structures are currently capable of generating up to 500 W of CW optical power while maintaining a total active material volume of less than 0.01 mm3.
Although the photoelectric efficiency of these devices can easily exceed 50%-especially with diode lasers based on GaAlAs and InGaAs-the part of the energy that is not converted to light is almost as high.
This is converted into a large amount of energy that must be dissipated from the laser device in the form of heat; otherwise, the active medium may melt within a few microseconds.
Heat dissipation is the most pressing issue in HPDL installation technology. Any heat generated is initially transferred to the surrounding substrate volume by conduction. The volume of the substrate is usually between 1 and 4 mm3.
However, for such a limited volume, the heat rate is too high, which means that a further heat dissipation step is required to dissipate heat through a larger volume of material before it is finally removed by the environment-usually by forced convection to water or air.
This process must be fast enough to avoid extreme temperature increases in the active medium. Due to its excellent electronic conductivity, copper is a popular choice for such applications.
Copper also exhibits the second largest thermal conductivity of all metals (k~385 W·m-1·K-1), which is only slightly higher than that of silver (k~405 W·m-1·K-1). Generally not due to cost impact, a viable option.
Since the first development of semiconductor laser technology, copper has been used as the preferred base material. However, a new technical challenge has emerged: establishing an appropriate interface between the laser rod and the copper heat sink.
This was originally achieved through welding, which is still the most commonly used technique. However, welding brings several inherent problems.
The interface material selected for welding purposes requires a melting temperature lower than the melting point of the laser rod and the heat sink.
It should also be high enough to ensure thermal and mechanical stability within the operating temperature range of the laser diode-usually between 15ºC and 80ºC. The first material used for this purpose is Indium-Tmelt ~157ºC.
The goal of the welding process is to create a strong joint between the laser rod and the copper heat sink, achieving this near the melting temperature of indium.
Since each of the three materials that exist exhibits a different coefficient of thermal expansion (CTE), the device will always experience a certain degree of residual stress when it is cooled to room temperature. 1
One of the key implications of this is the so-called smile phenomenon, that is, the curvature of the strip causes the emitters in the array to emit lasers not completely parallel to the horizontal axis.
Conversely, there is usually a height gap of 2 μm to 5 μm from the bottom to the top emitter (Figure 1).
This distance is usually the best measure of the magnitude of a smile. When an external resonator configuration is required or a fast axis with maximum brightness is required, the smile in the laser bar is an important consideration.
Figure 1. Theoretical emission intensity pattern of 19 emitter laser bars after fast-axis collimation and slow-axis imaging. The top image corresponds to the smile-free laser bar. The two images at the bottom correspond to two different smile effects. Image source: Monocrom
Building an external resonator with a laser rod is a common method to increase the spectral brightness or power brightness of the laser rod itself. High spatial brightness (W·cm-2·sr) is an important factor for high-power diode lasers used in industrial applications such as metal cutting, welding or drilling.
High spectral brightness (W·cm-2·sr·nm-1) and low-wavelength thermal shift (nm·K-1) are usually also related to solid-state laser pumping applications.
The external feedback of the power supply brightness is generated using a reflective diffraction Bragg grating (Figure 2). This method results in the effective spatial superposition of all laser beams from the laser rod, as if the intensity were generated from a single emitter within the laser rod.
Figure 2. Basic scheme illustrating the principle of spectral beam combination. Spatially separated emitters that emit at slightly different wavelengths strike the diffraction grating at different angles of incidence. However, the diffraction angle is common to them (different colors are used here just to illustrate the difference in wavelength, but they do not represent the wavelength itself). Image source: Monocrom
Therefore, the spatial brightness has increased by an order of magnitude.
This can also be achieved by expanding the emission bandwidth (resulting in lower spectral brightness) while accepting a certain percentage of power and optical loss; for example, the overall power for beamless operation is reduced by 20% to 40%.
These unavoidable inherent optical losses are related to the efficiency of the diffraction grating and the transmittance of the lens.
Figure 3. The emitter of the laser bar without a smile is contained in a plane defined by the fast and optical axes of the system. Therefore, the emitted laser beam and its partially reflected counterpart (feedback) coincide spatially in the external resonator configuration. Image source: Monocrom
Figure 4. Most of the emitters in the smile laser bar are partially or completely outside the plane defined by the system's fast axis and optical axis. This results in a partial lack of optical feedback in the external resonator configuration (most of the emitted beam and the feedback beam partially or completely do not coincide). Image source: Monocrom
Crucially, the optical loss is also affected by the way the laser beam returns to the emitter after some of its intensity is reflected by the out-coupling mirror. This is essentially a feedback emission from an external resonator, and a greater smile effect will lead to higher losses (Figures 3 and 4).
The volume Bragg grating (VBG) is usually placed in front of the fast-axis collimated laser rod to increase the spectral brightness. The Bragg grating requires external feedback, in this case it is used to shrink and "lock" the emission wavelength of the entire laser bar.
The result is a significant reduction in wavelength drift and temperature, from 0.3 nm·K-1 to less than 0.08 nm·K-1, similar to those found in distributed feedback lasers, which are either applied to the diode structure itself or connected to an optical fiber .
When using laser diode arrays, avoiding smiles is an important factor in ensuring consistent feedback for each emitter, especially considering that the optical components are common to each emitter (Figures 3 and 4).
Laser emission on the fast axis is almost always limited by diffraction (M2~1). This has significant advantages in applications where maximum brightness is required along the line-shaped laser spot, such as offset laser printing (computer to printer).
The presence of the smile effect in the laser bar may reduce the overall fast axis brightness by 50% to 80%. This is because the apparent height of the laser source on the fast axis increases in proportion to the smile.
Placing a fast axis collimator (FAC) lens in front of the laser rod will result in a higher residual divergence or a larger focus point (Figure 6).
Figure 5. Near-field representation of a 10-emitter laser bar without smile (top) and 3 μm smile effect (bottom). Due to the smile, the apparent size (represented by the dashed box below) is magnified on the fast axis. Image source: Monocrom
Figure 6. Different smile modes (left) produce different fast axis intensity distributions under fast axis collimation (middle). The far-field intensity distribution along the fast axis is the result of superimposing as many linear spots as the transmitter in the laser bar (right). Image source: Monocrom
Despite the aforementioned challenges, the HPDL industry has found a way to overcome the smile effect. A prominent solution is called "brazing", which involves the use of AuSn alloy as the interface material and CuW as the heat sink (Figure 7, left). 2
Hard soldering sees the CTE of the semiconductor, heat sink metal, and solder interface become closer, resulting in a more reliable connection than using indium (Figure 7, middle). This method leads to the minimization of the smile effect, but at a price.
Compared with copper, the thermal conductivity of CuW is significantly reduced (about 50% lower), as well as much lower processability and much higher cost.
Therefore, CuW is only used as an intermediate volume between the laser rod and the heat sink, which is usually made of copper. This method adds an additional thermal resistance jump to the laser diode package, but the benefits of this method are limited to a few applications.
One method stands out from those involving indium and brazing. Monocrom has proven the advantages of Clamping™ for more than two decades. This method relies on the extremely simple principle of mechanical pressure (Figure 7, right).
A simple concept does not always mean simple engineering design. Monocrom is still the only company that can introduce this revolutionary method to laser diode packaging.
Clamping™ technology mainly relies on the excellent surface finish on the copper heat sink and the direct thermal and electrical contact with the laser rod. These factors are then enhanced by applying mechanical force.
The electrode is in contact with the p-side through the heat sink (anode) and is connected to the n-side (cathode) using wire bonding. Use bulky heat sinks used as anode and cathode to "clamp" the clamping rod from both sides.
Figure 7. Comparison between the most common soldering method for laser bar packaging and ClampingTM technology. Image source: Monocrom
The advantages of Clamping™ are many:
Figure 8. Typical intensity distribution (fast-axis collimation plus slow-axis imaging) of a single emitter in a welding laser bar (left) and a clamped laser bar (right). Image source: Monocrom
Clamping™ can be understood as an intelligent solution for solving challenging packaging problems, and Monocrom has effectively implemented it as part of the standard manufacturing process.
This information is derived from materials provided by Monocrom and has been reviewed and adapted.
For more information on this source, please visit Monocrom.
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