In recent years, the need for kW-class lasers with high-peak-power output is increasing in various fields. Its use is expanding to, for example, self-driving vehicles and atmospheric observation in which LiDAR is used for long-range sensing, and to the healthcare and biophotonics industries for use in treatments and inspection analysis. Additionally, its potential in non-thermal precision cutting, minimally invasive treatments, and other laser processing and laser medical care processes is growing.
Up to now, high-peak-power output laser light sources were limited to solid-state lasers, like disk lasers, fiber lasers, and microchip lasers. Solid-state lasers can output high peak power pulse due to their long carrier lifetimes, ranging from microseconds to milliseconds. However, the solid-state laser crystal’s need for a separate laser to excite it has been preventing its widespread use because it makes the requirement for a large system unavoidable, requires manual assembly, and costs millions of yen for just one laser.
On the other hand, semiconductor lasers are highly efficient because they can be excited with an electrical current, they can be reduced to a size of less than one millimeter squared, and they are able to be mass-produced using the same manufacturing process as for semiconductors. However, they have difficulty outputting high peak power due to their short carrier lifetime on the order of nanoseconds.
Image 1 shows the characteristics of solid-state and semiconductor lasers. The y-axis shows peak power: The higher the line goes up, the stronger the output becomes. The x-axis shows the size of the laser cavity: The further the line goes to the right, the smaller the cavity gets.
In winter 2018, a revolutionary idea was born: Could we solve the peak power and cavity size trade-off issue, a fundamental issue surrounding laser light sources, by creating a laser chip that was compact like the semiconductor laser, but was capable of high peak power output?
Image 1: The positioning of solid-state and semiconductor lasers
By fusing a solid-state and a semiconductor laser, we could create a more compact laser with higher peak power than ever. This is the chip-scale high-peak-power laser concept we are aiming for.
The core idea was to create a monolithic laser by fusing a solid-state and a semiconductor laser and to overlap the VECSEL (vertical external cavity surface emitting laser) cavity as an excitation light source and a solid-state laser (passively Q-switched laser) cavity.
Image 2 is a schematic drawing of a chip-scale high-peak-power laser. The blue arrow indicates the VECSEL cavity (first cavity), and the red arrow indicates the passively Q-switched laser cavity (second cavity). The two cavities are optically coupled.
There are two advantages to this structure. Firstly, the heat generated in a solid-state laser crystal by excitation light absorption changes the solid-state laser crystal’s refractive index and causes the laser beam to gather. This allows for the focusing of the pump laser’s light and eliminates the solid-state laser’s usual need for a focusing lens.
The other is that because the solid-state laser crystal is excited inside the VECSEL cavity, even if the crystal’s single-pass absorption rate is low, the pump laser can be absorbed efficiently. Therefore, the solid-state laser crystal’s thickness can be reduced as thin as the dicing process will allow, and individual lasers can be produced by the same process used to manufacture semiconductors.
Image 2: Schematic drawing of a chip-scale high-peak-power laser
By overlapping the two cavities, the VECSEL cavity’s strong internal electrical field can be used to excite the solid-state laser medium, eliminating the need for a condensing optical system. In addition, compared to the solid-state laser that needs to have the spatial position of its multiple components adjusted with micron precision, it is possible to conduct batch production of chip-scale high-peak-power lasers on the wafer-level without performing optical alignment.
Image 3: Structures of a traditional and a chip-scale high-peak-power laser
Image 4 shows the mass production manufacturing process. The solid-state laser substrate is bonded on top of the semiconductor laser substrate. Then, the wafer is diced into individual chips. Finally, it is wired, mounted, and packaged.
The prototype chip-scale high-peak-power lasers made so far are the world’s first single-chip laser capable of extreme peak power in the kWs. In the journal article, it is recorded as having a volume of 1 mm3, a peak output power of 57.0 kW, and an output with a pulse duration of 450 ps.
Image 4: The manufacturing process
Image 5: A new laser that exceeds previous common knowledge
What was it that made it possible for us to make the unique fusion of solid-state and semiconductor lasers a reality and to create a new technology?
One of the big reasons is Sony’s various technological assets they have developed over the years. When it comes to semiconductor lasers, Sony has many years of experience in their utilization, including their use in making the world’s first laser for CD players. They also have solid-state laser technology that ranges from basic to applied use and human resources that support such technology. Furthermore, Sony has a deep knowledge of the semiconductor device, its mounting, and its process technologies.
In addition, the solid-state and semiconductor laser specialists respect each other and have a relationship in which they can freely exchange their opinions. There is also a company culture that supports research by encouraging the testing of off-the-wall ideas that, if realized, could lead to breakthroughs.
This technology’s research began in April 2019, but we did not start making prototypes until 2020. The first year was spent at the whiteboard constantly repeating thought experiments like “What kind of spatiotemporal energy conversion process would be necessary to change the injected electrical energy into light and heat?” and “What do we need to do to obtain laser oscillation?” The structural design was our first obstacle. This is because we were working to create a never-before-seen laser cavity structure, and there was no precedent to be used as a reference. (The thought experiments were proven to be correct after the completion of the theoretical model.) In the example of heat, when a device is operated and the temperature rises, the difference in the coefficient of thermal expansion can change the amount the material expands, causing a risk in breakage. In order to create a device based on the three perspectives of electricity, light, and heat, we designed the device from scratch through trial-and-error with the final mass production process in mind.
Conversely, after finalizing the design, we were able to leverage the expertise we developed during the development and production of the VCSEL (vertical cavity surface emitting laser) to finish development, from the confirmation of laser oscillation to the development of the single chip model, in about one year.
Chip-scale high-peak-power laser during laser oscillation
Oscillating beam of chip-scale high-peak-power laser
Laser oscillation experiment
Our success in developing a laser with a volume 1/1000 the size of existing solid-state lasers and a laser 1000 times stronger than semiconductor lasers came as a huge shock to those in the laser industry.
The article summarizing our research’s success was published in the famous British science journal Nature Communications and was selected as an Editor’s Highlight as a notable article. Following the article’s publication, we have received inquiries from colleges and businesses across the world, demonstrating a high level of interest.
We are currently investigating element technology for use in future mass production. We believe that if we succeed in mass-producing an ultrashort laser light source of high-peak-power at a mini-scale size and a low cost, it will bring about a revolution of the laser market. This technology will significantly raise the popularity of laser using applications previously not popularized due to the problems of size and cost, so we are going to continue challenging the realization of this game changing technology.
If we are able to implement this research, I am expecting it to have as big an impact on society as the change from light bulbs to LEDs did. We first came up with the idea for this laser four years ago, and now, professionals of not only technology but of various fields within Sony Group are gathering and working together as we strive for its development. I believe this a rewarding place to work for young people with innovative ideas, talent, and a desire to make the world a better place through technology because of the opportunity to research and develop the world’s most advanced optical devices.
I studied mechanical engineering in university and had no experience with lasers, but now I full immersed in the laser industry. Sony has a work environment in which someone with no experience can learn after joining the company. I also believe the process of developing an interest in a phenomenon, forming a hypothesis from a general principle, and then proving that hypothesis is the same no matter the field. I have opportunities to use the skills I have developed over the years to do various tasks, such as designing and developing necessary experimental systems, on my own. Our strength lies in our ability to gather people with various expertise to work together for a specific goal. We look forward to working with us who want to make use of their curiosity and expertise.
