## 2 Microscope Imaging Apparatus
### Ultra-Fast Illumination Solutions
#### 2.1.2 Multi-Beam Vertical-Cavity Surface-Emitting Diode Laser Array  
To answer the needs for more efficient light transmission, we have developed VCSEL-based light sources as a means to achieve high quality illumination that is uniform and free of laser speckle [21]. The devices were originally developed for data communication and local-area networks and were designed to replace edge-emitting lasers. The lasers consist of layers of semiconductor material, which is grown on top of each other on a substrate. In the lasing medium of a VCSEL, the light oscillates perpendicular to the layers and exits through the top of the device. This laser technology offers a number of advantages over edge-emitting laser diodes, such as higher quality beam, practically easier implementation, and their ability to operate at relatively high temperatures, thus not requiring dedicated cooling. For aircraft icing applications, the laser may be in an environment close to the probe heaters so the temperature insensitivity is important. These lasers can deliver very high power per unit area reaching approximately 1200 W/cm\({}^{2}\). They emit a circular beam which can be shaped to have a Gaussian intensity distribution. This simplifies the optics needed for transforming the beam to a near top hat intensity distribution. High-power VCSEL devices combine hundreds or thousands of individual VCSEL sources. As mentioned above, beam quality is one of the many advantages of VCSEL technology compared to laser diodes. Beam divergence of bare VCSEL devices is approximately 20\({}^{\circ}\), which is substantially narrower than LEDs, providing a significant advantage for light transmission. For this research, we selected a VCSEL device (FLIR CWV-HP) equipped with integrated optics to control beam divergence and provide a near-Gaussian distribution with a dispersion angle on the order of 7\({}^{\circ}\). The relatively small emitting area and low divergence enable the beam output to be collimated for transmission over larger distances, a key advantage compared to LEDs, as stated above. We used 100 mm focal length plano-convex lenses to collect and nearly collimate the radiation coming from each VCSEL in the multi-beam assembly. The six beams are converged at a 2\({}^{\circ}\) angle, and merged by a plano-concave lens with a -200 mm focal length. This produces a circular distribution on the order of 10 mm in diameter at the object plane located 130 mm away from the plano-concave lens (represented in red), and 12 mm at the receiving optics' effective front element (dark gray rectangle). The partial results of the ray tracing simulations carried out to optimize this optical arrangement is provided in Figure 3.  
As suggested by the ray tracing simulations, the output of the VCSEL illumination system is a slightly diverging beam, which is fully collected by the receiving optics, as demonstrated by the dark gray rectangle on the right side of the diagram. Compared to the LED simulation results, the ray tracing diagram of Figure 3 indicates that over 20% of the laser radiation is illuminating the probe volume, compared to less than 1% for the LED system, despite the condenser lens. Note that the distances are much greater than for the LED system to achieve optimal illumination distribution with the VCSEL system. The length of the arrangement shown in Figure 3 is more than twice that of the LED-based arrangement, and it does not include another 150 mm necessary for proper VCSEL collimation. In total, the VCSEL-based illumination system is approximately 550 mm long to the object  
Figure 3: Ray tracing simulation showing the output radiation of the multi-beam laser illumination system. The collimated and oriented laser beams enter from the left and are merged together by a -200 mm plano-concave lens. The probe volume is shown in red and the microscope objective’s effective optical collection is represented by a dark gray rectangle. plane.