diode stacks (original) (raw)
Definition: arrangements of multiple diode bars, delivering very high output power
Alternative terms: diode laser stacks, multi-bar module, two-dimensional laser array
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optoelectronics, 
laser devices and laser physics
Related: laser diodesdiode barsfiber-coupled diode laserslaser diode drivershigh-power lasersbrightnesswall-plug efficiencylaser cooling units
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Contents
What are Diode Stacks?
A diode stack (also called diode laser stack, multi-bar module, or two-dimensional laser array) contains a number of diode bars, which are arranged in the form of a stack. The most common arrangement is that of a vertical stack as shown in Figure 1. Effectively this is a two-dimensional array of edge emitters. Such a stack can be fabricated by cleaving linear diode laser arrays (diode bars) from a wafer, attaching them to thin heat sinks, and stacking these assemblies so as to obtain a periodic array of diode bars and heat sinks. There are also horizontal diode stacks (see below).
For the highest beam quality, the diode bars should be as close to each other as possible. On the other hand, efficient cooling requires some minimum thickness of the heat sinks which need to be mounted between the bars. Due to that minimum spacing, the beam quality of the combined output of a diode stack in the vertical direction (and subsequently its brightness) is much lower than that of a single diode bar. There are, however, several techniques for significantly mitigating this problem, e.g. by spatial interleaving of the outputs of different diode stacks, by polarization coupling, or by wavelength multiplexing. Various types of high-power beam shapers and related devices have been developed for such purposes.
Figure 1: Schematic structure of a laser diode stack. The emitters are shown in blue, and the microchannel coolers in dark gray.
Depending on the application, a diode stack may be used with or without attached optics. A common option is the use of fast axis collimation lenses, which are directly attached to the bars (see Figure 2). Further optics can be used for collimation also in the slow axis (horizontal) direction, or even for coupling the output into a multimode fiber.
For more details on beam collimation, see the articles on laser diode collimators and beam collimators.
Performance and Applications
Diode stacks can provide extremely high output powers of hundreds or thousands of watts, as used for pumping of high-power solid-state lasers, or used directly e.g. for laser material processing. There are also fiber-coupled diode stacks, delivering e.g. several kilowatts from a multimode fiber with a core diameter of 600 μm. Some applications such as laser welding of metals or plastics, where a high beam quality is not required, can directly utilize the output of such a laser system (→ direct diode lasers), which can have a very high wall-plug efficiency. This is also attractive for other applications such as laser hardening, alloying, and cladding of metallic surfaces.
Figure 2: Photograph of water-cooled diode stacks, kindly provided by DILAS (now Coherent). The number of diode bars varies from 1 (left-hand side) to 10 (right-hand side). One device has fast axis collimation lenses attached to the bars.
If laser radiation with much higher brightness is required, the laser radiation may be used for pumping a high-power fiber laser based on a double-clad fiber. Such a device can serve as a brightness converter, delivering a somewhat reduced output power but with much higher beam quality.
Figure 3: Photograph of a horizontal diode stack containing 12 bars, kindly provided by Cutting Edge Optronics.
There are also horizontal stacks, where the diode bars are arranged side-by-side, leading to a long linear array of emitters (see Figure 3). Such an arrangement is more easily cooled, and may thus also allow for a higher output power per emitter. The emission pattern of a horizontal stack is suitable for, e.g., pumping of rod lasers, whereas it is probably less convenient when an approximately circular output beam is required. The number of diode bars in a horizontal stack (and thus the total output power) is more limited than in a vertical stack.
The price of laser diode stacks has not dramatically changed over the years, while the output powers have been increased very substantially through the technological progress. Therefore, the price per watt of output power has been reduced substantially. At the same time, the device lifetimes have often been improved, so that the cost per watt-hour has been decreased even more dramatically. It also depends on various factors such as the emission wavelength, included cooling arrangements and the like.
Electric Supply and Cooling
Typically, all diode bars within one stack are electrically connected in series. If one has a stack containing 12 bars, each one emitting 100 W, the laser diode driver will typically have to provide a drive current around 100 A and a drive voltage of the order of 20 V. As a single diode bar would require the same high current, the electrical cables do not have to be stronger for a stack; only the voltage will be higher, but still quite moderate. The higher voltage allows the laser diode driver to reach a higher power conversion efficiency.
The cooling of such diode stacks is somewhat challenging, as a high dissipated power is produced within a small volume, and the means for cooling (e.g., microchannel coolers) should consume as little space as possible. The technical demands for cooling are highest for continuous-wave operation and much more moderate for quasi-continuous-wave operation with pulses of e.g. a few hundred microseconds duration and a pulse repetition rate of some tens of hertz. The latter mode of operation makes it possible to obtain very high peak powers, which can be used e.g. for pumping Q-switched high-power solid-state lasers.
Frequently Asked Questions
What is a diode stack?
A diode stack, or diode laser stack, is a device containing multiple laser diode bars arranged together, typically stacked vertically with intermittent heat sinks. This configuration forms a compact, two-dimensional array of laser emitters to achieve very high optical output power.
Why is the beam quality of a diode stack limited?
The beam quality, particularly in the vertical direction, is limited because the diode bars must be separated by heat sinks for cooling. This spacing makes the overall emitting area much larger than the active area, degrading the beam's focusability and brightness.
What are the main applications of diode stacks?
Diode stacks are used for applications requiring high optical power, such as pumping high-power solid-state lasers and fiber lasers. They are also used directly in material processing for tasks like welding, surface hardening, and cladding.
What is the difference between a vertical and a horizontal diode stack?
In a vertical stack, diode bars are layered on top of each other, creating a compact 2D emitting area. In a horizontal stack, bars are arranged side-by-side in a line, which can be easier to cool but results in a different beam geometry.
How are the laser diodes in a stack typically powered?
The diode bars within a stack are usually connected electrically in series. This allows them to be driven by a power supply providing a single high current at a moderately high voltage, which is more efficient than powering each bar separately.
What is quasi-continuous-wave (QCW) operation of a diode stack?
QCW operation involves running the diode stack in short pulses of high peak power with a low duty cycle. This reduces the average thermal load, enabling very high peak powers useful for applications like pumping Q-switched lasers.
Suppliers
Sponsored content: The RP Photonics Buyer's Guide contains 21 suppliers for diode stacks. Among them:
âš™ hardware
Lumibird manufactures a large range of QCW diode stacks: conduction cooled or water cooled, dual linear stacked array, high temperature/multicolor stacked array, linear bar array, mini stacked array, fast axis collimated stacked array etc.
âš™ hardware
Serving North America, RPMC Lasers offers laser diode stacks with scalable output up to 2400 W in CW & QCW configs, with high reliability, long life, FAC or FAC/SAC lensing, passive/active cooling, and full customization.
These modular, compact, robust laser diode stacks scale power into the kW range with 760 nm — 1070 nm options, vertical stacking of sub-mounts, optimized beam quality via tight 1.0 mm pitch without losing cooling, and flexibility to match your wavelength and power needs.
Proven hard-solder tech and strict production ensure reliability in harsh medical, industrial, or mission-critical uses, with tailored solutions from wafer to module to fit your specs.
Let RPMC help you find the right laser today!
âš™ hardware
Monocrom produces vertical diode laser assemblies. Our vertically structured packages are mounted with a patented solder-free laser bar Clamping™ technique that allows high cooling efficiency and a large operation and storage temperature range. It also eliminates the undesired “smile effect” and prolongs laser lifetime. Versions with an attached prism or full encapsulation are available.
Choose between different architectures with many available wavelengths: 760 nm, 808 nm, 880 nm, 915 nm, 940 nm, 976 nm, 1060 nm, 1470 nm, 1550 nm, 1940 nm.
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