Sony Semiconductor Solutions Corporation(SSS) is attempting to utilize the characteristics of VCSELs, which can be easily converted into two-dimensional arrays, to develop a wide range of array solutions for both industrial and consumer use. We have already succeeded in commercializing the world’s first*1 32ch-VCSEL arrays in red wavelength bands for laser printers, and are implementing them in high-speed, high definition on-demand printers that are far superior to others.
SSS has also recently developed various applications for smartphones and are engaged in VCSEL-related technical development to suit a wide range of applications. In addition to the development of light emitting elements, we are making contributions to sensing solutions that are unique to SSS through coordination with engineers related to the development of laser drivers, optical elements, and modules.
In preparation for the expansion of sensing applications in the near future, we are carrying out development in close coordination with the Sony Group’s R&D Center while also considering horizontal deployment to automotive LiDAR and equipment for driver monitors.
*1) Sony research in April 2014.
32ch polarized light control single mode array and laser driver solutions
Our “polarized light control red single mode VCSEL array” has 32 emitters which are all controlled to have a uniform polarization direction, and also operates with a single transverse mode. By overcoming issues with temperature properties through the use of solutions that use laser drivers unique to SSS, we became the first in the world to mass-produce red wavelength band surface emitting lasers with an oscillation wavelength of 680 nm for laser printers.
680 nm band, 32ch polarized light control single mode VCSEL arrays
Red wavelength band VCSELs have the characteristic of a large carrier overflow caused by the properties of materials used in their elements. Therefore, even when their current is driven by rectangular pulses, the optical response does not follow them, so they are known to display blunted optical waveforms. At low-temperature low-output conditions, optical waveforms with “reverse droop,” where the optical output increases gradually, can be observed. Comparatively, at high-temperature high-output conditions, waveforms show “droop,” where the optical output decreases in association with a rise in self-heating. Both of these waveforms can lead to a degradation of image quality, so there is a need to adjust the optical waveforms to square waves. SSS has successfully overcome this issue by introducing an “active layer temperature predicting VCSEL driver.” True to its name, it is a laser driver that predicts the temperature of the active layer of a VCSEL, while applying an optimal correction current to the VCSEL. In order to predict the temperature of the active layer, it is necessary to identify the temperature of the environment of the VCSEL chip, and SSS was the first*1 to implement a temperature detection element built-in to the VCSEL. Also, there is a need to consider self-heating for fine detection of the active layer temperature, so incorporating a VCSEL heat circuit model into the driver has made it possible to predict the active layer temperature accurately. As shown in the diagram, applying an optimal correction current according to the active layer temperature can adjust a blunted optical waveform to a square wave.
VCSEL optical waveform correction by a laser driver
*1) Adjusts the FFP (Far Field Pattern) to a Gaussian shape.
*2) The “VCSEL based Thermo-Sensor” in Figure 8 is equivalent to a temperature detection element. It uses the current-voltage temperature properties of a diode element that does not emit light as a thermometer.
Distance image sensors used in applications such as smartphones and gaming devices will require further expansion of measured distances and improvements in distance measurement precision in the future. One form of technology that can achieve such advancements is VCSEL multi-junction conversion. This technology can connect multiple laser diodes in series via tunnel junctions, but requires highly advanced epitaxial growth technology and device design technology.
Multi-junction VCSELs that include three active layers, which are currently under development by SSS, have been confirmed to have a slope efficiency that is exactly three times that of single junctions. It has additionally been confirmed that their current-voltage characteristics are approximately the same as if a built-in diode were added, with an extremely small increase in series resistance. From here on, SSS intends to implement high optical output drives (long-distance measurement) and high-speed pulse response (outstanding distance measurement precision) in mobile sensing, through combinations with VCSEL drivers that have higher withstand voltages.
In order to connect three active layers in series, two tunnel junctions are inserted into the cavity.
It can be seen that conversion to a triple junction has led to a three-fold increase in SE. The WPE and 8W also exceed 50%.
Related Products & Solutions
Find out more about the laser diode products using this technology.
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