In the current chapter we are going to focus on applying the theoretical background studied in the previous chapters to obtain a setup suitable for VCSEL array characterization. The desired setup should provide us with the possibility to measure all the characteristics discussed in the previous section. It should be taken into account that some of the measurements require beam focus on various equipment pieces. Thus, if such a contradicting situation occurs the setup should be easily rearranged to fit the required purpose.
4.1 Chip positioning
To be able to supply a steady current flow to the chip we should establish a proper closed circuit. From the structure of VCSELs we know that p contact is provided by p-side metallization of the top side of the chip and n contact by n-side metallization of the bottom. Several ways of chip positioning in the measurement setup have been reported [25], [26]. Once the chips are separated each array can be mounted and wired to a separate PCB, which itself is connected to the current driver via a basic connector [25].
However, if the wafer has not been yet cleaved or faster and more flexible testing is required the circuit can be established with probing needles [26]. One of the needles is connected to the top side, while the other is placed on the thermal heat sink as close to the testing piece as possible, as illustrated in Figure 10.
Figure 10: Schematics of VCSEL needle probe [26]
While this is a fast way to test chips still in the processing stage, it should be noted that probing needles might damage the chip surface. Thus, mass testing of chips in such a way after the development stage is concluded should be avoided.
4.2 Number of lasing emitters measurements
To analyze the number of emitters in the array, which contribute to the overall power output - we need to obtain the picture of the lasing array. Firstly, VCSEL emission direction suggests camera positioning perpendicular to the array surface. A diode is connected to the camera to provide the possibility to study the array surface when the array itself is not emitting light.
Secondly, once the VCSEL chip starts lasing - the intensity of the light hitting the camera tends to be too strong and no clear picture can be obtained. Neutral density (ND) filter is thus positioned between the beam and the camera providing optical attenuation in the chosen wavelength range. For VCSEL lasing in IR region, 650-1050 nm is an appropriate range. For obtaining higher attenuation levels, several filters might be combined.
The current is provided to the VCSEL array by the current driver. If the driver also has temperature control (TEC), it is connected to the heat sink, so that the temperature of the chip can be controlled throughout testing. The schematics of the setup can be seen in Figure 11. The camera is connected to the computer, where the image is observed and analyzed by the means of ATR.
Figure 11: Setup for measuring the number of working emitters
4.3 LIV and spectrum measurements
For obtaining accurate values for the LIV curve, divergence of the beam should be carefully considered since we have concluded that it is especially crucial for arrays due to the increase in divergence rate caused by their spatial arrangement. Thus, light from the LD is first collimated by passing through a plano-convex lens.
The aim of the setup construction is obtaining measurement versatility along with easiness of reconfiguration. Varying camera position from the setup setting described in the previous subsection would be too cumbersome, thus it would be optimal to insert a high-reflective mirror to change light direction. Then the light is collected by an integrating sphere, a device the interior of which is covered with a diffuse white reflective coating.
Light enters the sphere through a small hole and is afterwards distributed equally all over the inner surface of it [27]. Hence, though the spatial information about the beam is lost, the power is preserved. The photodetector inserted to the side of the integrating sphere and connected to the driver enables the measurements of the LIV curve. Simultaneously, spectrometer is connected to the integrating sphere. The setup shown in Figure 12 enables simultaneous measurements of the LIV curve and spectrum as well as temperature characterization of both.
Figure 12: LIV and spectrum measurement setup
To obtain the most precise measurements the beam position has to be calibrated. This can be done by either varying the LD or integrating sphere position. Since the LD position
is already calibrated with respect to the lens and the camera, adjusting the position of the integrating sphere is a preferable solution.
4.4 Beam profile measurements
For beam profile measurements, the integrating sphere would have to be changed to the camera beam profiler (Figure 13). The profiler is then connected to the computer and by utilizing the appropriate software the beam profile is mapped and analyzed. The adjustments of the temperature settings remain possible.
Figure 13: Beam profile measurement setup