Edge-emitting semiconductor laser arrays, also called laser bars, are probably the most widely known and widespread architecture when it comes to high-power diode lasers (HPDLs).
At present, when utilized as part of an electron-pumping scheme, such structures are capable of generating up to 500 W of CW optical power while maintaining an overall active material volume in the area of fewer than 0.01 mm3.
The electro-optical efficiency of these devices can easily exceed 50%, especially when working with GaAlAs- and InGaAs-based diode lasers. Unfortunately, the portion of energy that is not converted into light can be almost as high, percentage-wise.
This means that a large amount of energy has to be dissipated from the laser device in the form of heat, as the active medium would otherwise likely melt in a matter of microseconds.
Heat dissipation is therefore of vital importance when it comes to HPDL mounting technology. Heat generated is transmitted to the surrounding substrate volume by means of conduction.
The volume of this substrate is usually between 1 and 4 mm3. However, the heat rate is too high for such a limited volume. This means that other heat dissipating steps are required, via a larger volume of material, before this is eventually removed by the environment via forced convection towards air or water.
The process needs to be rapid enough to rule out extreme temperature rises in the active medium.
Copper is a widely used option for this type of application, not just due to its excellent electron conductivity but also because it demonstrates the second largest thermal conductivity among all metals (k~385 W·m-1·K-1). It is only slightly surpassed by silver (k~405 W·m-1·K-1). However, the latter is not generally a viable option cost-wise.
Copper has been the sub-mount material of choice since semiconductor laser technology was initially developed. A technical challenge has since appeared, however: the challenge of establishing a proper interface between the laser bar and copper heat sink.
At first, this was achieved by using soldering techniques and while this remains the most commonly adopted technology, there are a number of inherent issues with this approach.
The melting temperature of the interface material selected for soldering purposes needs to be comfortably under the melting points of both the heat sink and the laser bar. In addition, the melting temperature needs to be high enough to ensure thermal and mechanical stability over the laser diode’s operating temperature, which is usually between 15 ºC and 80 ºC. Indium, with its Tmelt of ~157 ºC, was initially utilized for this purpose.
Soldering is a process whereby a solid joint is created between the laser bar and the copper heat sink, achieving this at approximately the melting temperature of indium.
As the three materials present each exhibit different coefficients of thermal expansion (CTE), any device will invariably experience a degree of residual stress as it cools down to room temperature.1
Smile in laser diode arrays
An important consequence of this is known as the smile phenomena, where the bar suffers a curvature. This can results in the emitters within the array not lasing completely in parallel to the horizontal axis. A separation in height ranging from 2 μm to 5 μm often appears from the bottom to the top emitter (Figure 1).
This distance is often the optimal measure of smile magnitude. The potential presence of smile in laser bars is a vital consideration in instances where external resonator configurations are necessary or where a maximum degree of brightness is expected from the fast axis.
Figure 1. Theoretical emission intensity pattern after fast axis collimation and slow axis imaging of a 19-emitter laser bar. Top image corresponds to a smile-free laser bar. The two bottom images correspond to two different kind of smile effect. Image Credit: Monocrom
External resonator configurations
When constructing an external resonator with a laser bar, a widely employed technique is to increase the spectral brightness or power brightness of the laser bar itself.
A high spatial brightness (W·cm-2·sr) is an important aspect for high power diode lasers utilized in a range of industrial applications, such as soldering, metal cutting, and drilling. Low wavelength thermal shift (nm·K-1) and high spectral brightness (W·cm-2·sr·nm-1) are also associated with solid-state laser pumping applications.
Figure 2. Basic scheme illustrating the spectral beam combining principle. Spatially separated emitters emitting at slightly different wavelengths impinge the diffraction grating at different incident angles. However, the diffracted angle is common to all of them (different colours are used here just to illustrate the difference in wavelength but they are not representative of the wavelength itself). Image Credit: Monocrom
The power brightness’s external feedback is generated by employing a reflective diffraction Bragg grating (Figure 2). This technique results in the effective spatial superimposition of all the laser beams from the laser bar, in much the same manner as if intensity was generated by just a single emitter.
This means spatial brightness is increased by an order of magnitude.
This effect can also be obtained by broadening the emission bandwidth, with a resultant lower spectral brightness, while accepting a certain percentage of optical losses and power losses; for example, an overall 20% to 40% reduction in power w.r.t. free beam operation.
These optical losses are linked to the diffraction grating’s efficiency and the lenses’ transmission and are regarded as unavoidable and inherent.
It is important to note that optical losses are also impacted by how the laser beam returns to the emitter after some of its intensity is reflected back to the outcoupling mirror.
Figure 3. The emitters of a laser bar with no smile are contained within the plane defined by the fast axis and the optical axis of the system. As a consequence, the emitted laser beams and its partially-reflected counterpart (feedback) are spatially coincident in an external resonator configuration. Image Credit: Monocrom
Figure 4. Most of the emitters in a laser bar with smile are partially or totally out of the plane defined by the fast axis and the optical axis of the system. This results in a partial lack of optical feedback in an external resonator configuration (most of the emitted and feedback beams are partly or totally non-coincident). Image Credit: Monocrom
In essence, this is feedback emission from the external resonator, and the greater the smile effects, the higher the resulting losses (Figures 3 and 4).
Placing Volume Bragg gratings (VBGs) in front of a fast axis collimated laser bar can lead to improved spectral brightness. The Bragg grating needs external feedback, in this instance serving to narrow and ‘lock’ the emission wavelength of the entire laser bar.
This provides a large reduction in the wavelength shift (w.r.t.) temperature, from 0.3 nm·K-1 to less than 0.08 nm·K-1, a measurement similar to that of distributed feedback lasers that have been applied on the diode structure itself or are connected using an optical fiber with integrated grating.
When it comes to working with laser diode arrays, the avoidance of smile is vital to ensure uniform feedback on each emitter, especially considering that the optical elements are common to every emitter (Figures 3 and 4).
Fast axis brightness
Laser emission in the fast axis is diffraction limited (M2~1) in almost every instance, offering a clear advantage in applications where maximum brightness is needed along a line-shaped laser spot, such as offset laser printing (computer-to-print machines).
The smile effect in a laser bar has the potential to impair overall fast axis brightness by between 50% and 80% in cases where it is present, as the apparent height of a laser source in the fast axis increases in proportion to the smile.
By utilizing a fast axis collimator (FAC) lens in front of the laser bar, there will be either a larger focused spot or a higher residual divergence (Figure 6).
Figure 5. Near-field representation of a 10-emitter laser bar with no smile (top) and with 3 μm smile effect (bottom). The apparent size (represented by the dashed frames below) is enlarged in the fast axis due to the smile. Image Credit: Monocrom
Figure 6. Different smile patterns (left) yield different fast axis intensity profiles under fast axis collimation (middle). The far field intensity profile along the fast axis is the result of superimposing as many line-shaped spots as emitters within the laser bar (right). Image Credit: Monocrom
Alternatives to smile suppression
Despite the obstacles highlighted above, the HPDL industry has succeeded in overcoming the smile effect. One widely known option is called ‘hard-soldering,’ where an AuSn alloy is employed as the interface material and CuW is used as the heat sink (Figure 7, left).2
The hard soldering approach results in the CTE of the semiconductor, heat sink metal and soldering interface becoming more aligned than this would be using indium (Figure 7, center).
It also leads to the smile effect being minimized, but at a cost, as CuW shows a marked decrease in thermal conductivity in comparison to copper alone (approximately 50% lower), adding cost while offering a much lower degree of machinability.
As a result, CuW is only utilized as an intermediate volume between the heat sink (which is generally manufactured from copper) and the laser bar. While CuW hard soldering provides additional thermal resistance jumps to the laser diode package, its benefits are limited to a small number of applications.
When it comes to approaches involving indium and hard soldering, Monocrom has demonstrated the advantages of Clamping™ for over twenty years. This distinctive method relies on the straightforward principle of mechanical pressure.
This does not mean that the engineering behind the principle is straightforward. However, and Monocrom is still the only company able to apply this type of approach to laser diode packaging.
Mononcrom’s Clamping™ technology primarily depends on the presence of a superior surface finish on the copper heat sink, plus establishing a direct thermal and electrical contact with the laser bar. This approach is enhanced by the application of mechanical force (Figure 7, right).
The soldered bars are connected to the application's p-side by the heat sink (anode). Wire bonding is used to connect to the n-side (cathode). Clamped bars are ‘sandwiched’ from both sides, using the bulky heat sinks that function as the anode and cathode.
Figure 7. Comparison between the most common soldering approaches for laser bar packaging and ClampingTM technology. Image Credit: Monocrom
The advantages of Clamping™ are numerous:
- Cold process: No residual stress is caused by dissimilar CTE so that the smile is limited to the surface flatness achieved in the heat sink itself, usually μm over 1 cm2 (Figure 8).
- Minimum thermal resistance jumps: As heat is directly evacuated to the heat sink from the laser bar, this means it is necessary to account only for the contact resistance between copper and the semiconductor material.
- As heat is dissipated from both the p- and n-sides, this provides an additional path for heat dissipation. In CW and high duty cycle QCW operations, this facilitates the employment of water/glycol cooling channels machined on both electrodes. It reduces the need for micro-channel cooling, as millimeter-sized channels can be utilized, leading to reduced corrosion sensitivity while enabling minimal maintenance requirements.
- Simplicity and cost reduction: No interface materials or soldering equipment are required. The required mechanical force is applied by means of a stainless steel screw, and the perfect surface finish of the copper heat sink is also a key factor.
- Superior performance in pulsed mode: The lack of horizontal residual stress removes the risk of fatigue in the joint and laser bar during successive on/off cycles. This can prolong the device’s service life.
Figure 8. Typical intensity profile of the individual emitters (fast axis collimation plus slow axis imaging) within a soldered laser bar (left) and a clamped laser bar (right). Image Credit: Monocrom
Clamping™ is an intelligent solution to a formidable packaging issue. Monocrom has effectively implemented this solution as part of the standard manufacturing process.
- CTEIn = 33 μm·m-1·K-1; CTECu = 17 μm·m-1·K-1; CTEGaAs = 5 μm·m-1·K-1
- CTEAu(80)Sn(20) = 16 μm·m-1·K-1; CTECuW = 5-9 μm·m-1·K-1
Monocrom is established in 1993 in Vilanova i la Geltrú, Barcelona area. Since then, their vision has been to grow as a global market leader in the semiconductor laser industry serving high-quality laser diode systems and custom laser solutions. A continued improvement and commitment to excellence are the basis of our business practices.
Monocrom is a UNE EN ISO 9001:2015 registered laser company located in one of Europe’s pioneering regions in photonics. Their portfolio of diode lasers comprises standard and custom-made solutions for the medical, industrial, and defense industries among others. Their laser systems are designed to meet the most demanding requirements in material processing, ophthalmology, dermatology and aesthetics, imaging, security, life science, and research applications.
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