1mm³ size, the world's first single chip laser module with power breaking KW was born

In recent years, with the continuous development of technology, the volume and cost of Lidar are also decreasing, which has become a key technology concerned by various industries. It is increasingly being used in self-driving cars, LiDAR sensors for atmospheric observations, healthcare (therapeutics and inspection analysis), bio-photonics, and more. The application potential in non-thermal precision cutting, minimally invasive treatment and other laser processing and laser medical scenarios is also increasing. At the same time, the demand for kilowatt high power laser modules is also beginning to increase.

There are many solutions to LiDAR, such as LiDAR, which uses VCSEL (vertical cavity surface emitting laser) light sources, and MEMS, which is based on solid laser technology. However, solid-state lasers and VCsels have their own limitations. The former is powerful but large, while the latter is small but also small. To address the limitations of existing laser technology, SONY has developed a monolithic laser that it claims is the world's first laser solution with peak power up to the kilowatt scale (57.0 kW). Its volume is less than 1 cubic millimeter (equivalent to 1/1000 times that of solid laser), the output pulse duration is 450 picoseconds, and the intensity is 1,000 times that of semiconductor laser.

SONY's modular arrangement is simpler than traditional solid-state laser solutions, and the combination of semiconductor and solid-state laser modules is ideal for LiDAR's need for smaller, high-power laser sources. In addition, small laser units can be arranged side-by-side in a chip, and using existing semiconductor lithography processes, high precision array arrangement (micron scale) can be achieved.

SONY said there is no other laser technology on the market that can meet this demand. Although the usability of high-power laser modules has been proven in the laboratory, there are still cost and size constraints to consider when it comes to market. If such small size, high power ultrashort laser light source can be mass-produced at low cost, it will bring a revolution to the high power laser industry.

In terms of application scenarios, the technology can be used to develop better LiDAR sensors for autonomous vehicles, drones, robotics, 3D sensing, AR/VR and other scenarios.

Reduce the size of the laser cavity

So far, the high power laser source solutions on the market are limited to solid state laser modules, such as disc laser, fiber laser, microchip laser and so on. Solid-state lasers can produce high power pulses because of their long carrier lifetimes (microsecond to millisecond). However, solid-state laser crystals require an external laser to activate them, so the hardware system is large and requires manual assembly. Typically, a solid-state laser system can cost millions of yen, or about $50,000, limiting widespread use of the technology.

Semiconductor laser solutions, on the other hand, are more efficient because they can be activated with an electric current, so they can also be reduced in size to less than 1 square millimeter, and can be mass-produced using existing semiconductor processes. However, semiconductor lasers have a short carrier lifetime of only nanoseconds, making it difficult to produce high power.

Figure 1: y axis is the peak power and x axis is the size of laser cavity. The smaller the size of laser module, the smaller the output power

To address the power and size limitations of laser technology, in 2018 SONY began trying to design a laser crystal that would be compact enough to produce a high enough power output, like a semiconductor laser.

Breakthrough point: cavity overlap

So SONY fused solid-state lasers with semiconductor laser solutions to create a more compact laser technology with higher peak power. At the heart of the technology is a monolithic laser composed of solid-state and semiconductor lasers, as well as a VCSEL (vertical-cavity surface-emitting laser) as the active light source, and the cavity of the VCSEL and solid-state laser (passive Q-switched laser) overlapping.

Figure 2: The blue arrow ranges from the VCSEL cavity (the first cavity) to the passive Q-switched laser cavity (the second cavity). The two cavities are optically coupled

This structure has two advantages. First, solid laser crystals absorb laser heat, which changes the refractive index and makes the laser beam more concentrated. This reduces the need for a focusing lens for solid-state lasers.

Second, because the solid laser crystal is activated in the VCSEL cavity, the pumped laser can be absorbed efficiently even if the one-way absorption rate of the crystal is low. In this way, the thickness of solid-state laser crystals can be reduced to a level that can be achieved by existing cutting processes, and individual lasers can be produced by existing semiconductor manufacturing processes.

Left: traditional high power laser structure; Right: Structure diagram of SONY solution

In other words, when the cavities are overlapped, the internal electric field of the VCSEL cavity can activate the solid laser medium without the need for a concentrating optical system. In addition, this solution is easier to mass produce, without optical alignment, using existing wafer processes to mass-produce high-power lasers at chip size. In contrast, the multi-component components of traditional solid laser modules need to be adjusted in spatial position, which requires micron accuracy and is more difficult to manufacture.

Figure 4: The solid state laser substrate is bonded to the top of the semiconductor laser substrate, then the wafer is cut into individual crystals, which are then wired, installed, and packaged

It is understood that SONY has a deep understanding of the development, installation and manufacturing process of semiconductor devices. Its team has experts in the field of solid laser and semiconductor laser. They have been trying to integrate the new laser technology of VCSEL and solid laser since April 2019, from confirming the laser oscillation to developing the monolithic model. During the development process, the researchers took into account the effects of electricity, light and heat on the laser module to ensure the final mass production.

Figure 5: SONY's laser solution is better than traditional solid-state laser and VCSEL solutions

Source: Qingting net