Laser diode arrays are used in a variety of defense and aerospace applications. Two of the most common uses are illumination and solid-state laser (SSL) pumping, in which the radiation from the diode lasers is used to excite the laser crystal in order to generate light. The SSLs can then be used in a number of configurations and applications, including range finding and target designation.
In many SSL applications it is common to operate the laser diode arrays in pulsed, or quasi-continuous wave (QCW) mode. In this regime the diodes are electrically pumped with a pulse width that is commonly on the order of the upper state lifetime of the laser gain medium. For example, Nd:YAG lasers are typically pumped with pulse widths on the order of 200 us. This pumping scenario leads to efficient laser designs since most of the pump light that is absorbed by the laser crystal can be extracted from the system. The repetition rate of the diode pumps is also defined by the application. Many range finding applications operate in the 10-30 Hz range, and many direct diode illuminations operate at around 60 Hz to match the frame rate on commercial off the shelf (COTS) camera systems. QCW diode pumping holds several advantages over CW diode pumping in SSL systems. First, QCW pumping creates a lower average thermal load in the laser gain medium.
This simplifies the cooling of the system and also enables higher beam quality lasers due to the reduced thermal lensing effects. Second, QCW pumping allows the diodes to be operated at higher peak power than is possible with CW pumping. This leads to SSL systems with higher peak powers.
Northrop Grumman Cutting Edge Optronics (NGCEO) has been manufacturing QCW laser diode arrays for over a decade in a variety of configurations. A schematic of the standard manufacturing process for a QCW diode array is shown in Figure 1. In the first step, a diode bar is soldered to a heatsink. A heatsink with a coefficient of thermal expansion (CTE) near that of the bar (~ 6 ppm/K in the case of GaAs) is used. This allows for the use of hard solders such as eutectic AuSn, which minimizes solder creep and promotes a high degree of reliability. The subassembly created when a bar is soldered to a heatsink is known as a Mounted Bar Assembly (MBA).
In the next step of the standard manufacturing process, a number of MBAs are soldered together and attached to a ceramic backplane and electrical contacts to create a diode array. The electrical contacts also serve as large heatsinks on the end of the array. The bar-to-bar spacing, or pitch, is defined primarily by the thickness of the heatsink and whether there is any space between the MBAs. Pitch values ranging from 350 >m to 2 mm are common in the industry today. The number of bars in the array is determined by the customer specification and is directly related to the desired output power of the device. The array can be subsequently attached to either a water- or conductively-cooled heatsink.
|Figure 1. Assembly process for standard pulsed diode arrays at NGCEO.|
There are several ways to increase the optical power density of a laser diode array. The first is to increase the output power of each of the diode bars that comprises the array. Additionally, optical methods (e.g. interleaving, beam combining) can be used to generate arrays with higher power densities.
NGCEO has created a new array design that eliminates the heatsinks from between the diode bars and drastically reduces the bar-to-bar pitch. This design is called the High Density Stack (HDS). A schematic of this manufacturing process is shown in Figure 2. In this process, a stack of laser diode bars is joined together using AuSn solder. This stack is then attached to a ceramic backplane and electrical contacts in a subsequent soldering step. The resulting array has a bar-to-bar pitch of ~ 150 >m. This pitch is approximately 43% of the smallest industry-standard pitch that can be obtained from the method shown in Figure 1, above. This leads to optical power densities that are approximately 2.3 times higher than can be obtained using standard manufacturing methods.
|Figure 2. Assembly process for High Density Stack arrays at NGCEO.|
|Figure 3. Power density as a function of power/bar for arrays with different pitch values.|
2. EXPERIMENTAL DATA
The bars used for this experiment were designed and fabricated by NGCEO specifically for high power QCW operation. The bars are 1cm in width and have a 1mm cavity length with a fill factor of approximately 83%, and the output facets are passivated to prevent oxidation. These bars are nominally rated as 200W bars and achieve that power level at 175- 180 A. In a standard package, this bar has been shown to have > 50% electrical-to-optical efficiency at temperatures H 70 oC and has also been shown to withstand significant thermal shocks . In addition, this bar/package combination has been shown to have device lifetimes in excess of 13 billion shots . This proven bar is therefore an excellent candidate for use in the HDS arrays.
In order to determine the operating envelope of the HDS arrays, 5-, 10-, and 20-bar arrays were fabricated using the process highlighted in Figure 2, above. All devices were fabricated from the same bar lot in order to allow for direct comparison of the data sets. In the case of the 5- and 10-bar stacks, the stack was built using a single soldering step. In the case of the 20-bar stack, two 10-bar stacks were joined together in a subsequent soldering step.
A picture of a 5-bar HDS array is shown in Figure 4, directly adjacent to a standard array with 400 >m pitch. In this case additional CuW heatsinks were added to the outside of the 5-bar stack so that the entire assembly could maintain the same form factor as the standard assembly for ease of testing. However it is clear that the emission area is significantly smaller for the HDS array. The total emission area of the standard array is 1cm x 1.6mm, while the emission area for the HDS array is 1cm x 0.6mm. For the case of two five-bar arrays operating at the same output power, the HDS array therefore has a 2.6 times greater power density.
|Figure 4. Photograph of two 5-bar stacks placed end-to-end. The High Density Stack is on the op and a standard array with 400 3m pitch is on the bottom.|
|Figure 5. Comparison of example near-field images from a High Density Stack (left) and a standard array with 400 3m pitch (right). Each array contains five bars.|
|Figure 6. Near field images from 5-10-, and 20-bar High Density Stack arrays.|
|Figure 7. Power vs. current for a High Density Stack and a standard array. Each array contains five bars. This|
test was conducted at 20 Hz, 150 3s.
The 20-bar stack produced 94% of the power/bar as the 5- and 10-bar stacks at a pulse width of 150 us. At first glance one would suspect that this was due to the presence of several missing emitters, as seen in Figure 6. However, only ~ 2% of the emitters are emitting significantly less power according to the near field image. Therefore one can conclude that the decrease in output power is likely due to some other factor, most likely excessive heating in the device. At a pulse width of 450 >s, the 20-bar array experienced thermal runaway and no meaningful data could be collected.
|Figure 8. Power/bar as a function of drive pulse width for 5-, 10-, and 20-bar High Density Stacks. All data|
obtained at 20 Hz, 200 A.
The performance of the arrays begins to diverge as the pulse width (and therefore the average heat load) is increased. The 5- and 10-bar arrays have similar output wavelengths over the range of pulse widths in the test. At a pulse width of 450 us, the wavelength of the 10-bar array is only ~ 2 nm higher than the 5-bar array. This corresponds to an average temperature increase of ~ 8K over the entire array. However, the center wavelength of the 20-bar array is a rapidly increasing function of pulse width.
It should be noted that at drive conditions common to Nd:YAG pumping for range finding applications, all three arrays were operating at junction temperatures less than what are typically experienced by CW arrays that are common in the industry today. Therefore at 20 Hz, 150 >s, and 200 A (~ 210 W/bar), all of the arrays are capable of high reliability operation.
|Figure 9. Center wavelength as a function of drive pulse width for 5-, 10-, and 20-bar High Density Stacks. All|
data obtained at 20 Hz, 200 A.
The data presented above was based on NGCEO's 200W laser diode bar. Recent advancements in epitaxial design have led to a bar that is capable of reliable operation at 300W (at approximately 300A). A comparison of the P-I curves for the two epi structures is shown in Figure 10.
|Figure 10. Power vs. current for NGCEO's new epitaxial design, compared to the standard epitaxial design used|
for the High Density Stack arrays in this work.
Northrop Grumman Cutting Edge Optronics has demonstrated a new laser diode array design capable of power densities of ~ 15 kW/cm2 when operating at 200 W/bar. This array design, called the High Density Stack, enables high power densities by packaging the diode bars with a bar-to-bar pitch of ~ 150 >m. NGCEO has collected data for 5-, 10-, and 20-bar arrays in the operating regime common to many Nd:YAG pumping schemes. The data collected from these experiments verifies that the arrays operate at junction temperatures that are suitable for long lifetimes.
Wilmer J. Sánchez