Semiconductor laser brightness milestone: Reach industrial grade thick steel cutting level!

Compared with bulky gas lasers and fiber lasers, semiconductor lasers have the advantages of small size, high energy efficiency, high coherence, and strong controllability. However, this laser that uses semiconductor material as working material to generate stimulated emission also has its own inherent defects: poor temperature characteristics, easy to produce noise, and serious output light divergence. One consequence of these defects is that it is difficult to achieve the brightness levels used for industrial-grade cuts such as thick steel.

But a study published last week in the journal Nature may break that pattern and be a key development:

Led by IEEE fellow Susumu Noda, a team of researchers at Kyoto University in Japan has taken a big step toward overcoming the brightness limitations of semiconductor lasers by altering the structure of photonic crystal surface-emitting lasers (PCSELs).

(Image credit: IEEE Spectrum)

Photonic crystals are made up of regular, nanoscale pores that are perforated on a semiconductor sheet. Photonic crystal lasers are a promising player in the field of high-brightness lasers, but until now, engineers have not been able to elevate them to provide beams bright enough to be used for actual metal cutting and processing. Researchers have been working to optimize the performance of semiconductor lasers, including power conversion efficiency, output power, beam quality, laser level, spectral characteristics, size, robustness to undesirable noise and thermal management, reliability, and more. (Note: Brightness is a measure of the laser output power and beam quality of an indicator, it includes a beam of light focus and divergence degree. The threshold for metalworking is about 1 gigawatt per square centimeter.)

The above research team, led by Academician Susumu Noda, has accumulated more than 20 years of research experience in PCSEL development. In terms of concrete results, they were able to develop a laser with a diameter of 3 mm, which is a 10-fold increase in the area of previous PCSEL devices with a diameter of 1 mm. The innovative laser has an output power of 50W, a significant increase over the 5-10W output of 1mm PCSELs. The new laser has a brightness of about 1GW/cm2 / str, which is sufficient for a range of applications currently dominated by gas and fiber lasers, such as precision smart manufacturing in the electronics and automotive industries. This high brightness level is also sufficient for more specialized applications such as satellite communications and satellite propulsion.

In the process of increasing the size and brightness of photonic crystal lasers, a series of challenges will be encountered. Specifically, semiconductor lasers encounter a bottleneck when their emission region is expanded: a wider laser region means that there is room for continuous oscillations of light in the emission direction and laterally, and these transverse oscillations (known as Higher-order Modes) destroy the quality of the beam. In addition, if the laser is operating continuously, the heat inside the laser will change the refractive index of the device, causing the beam quality to deteriorate further.

The key breakthrough brought by Susumu Noda's research team is that they embedded photonic crystals in the laser and modified the internal reflector to achieve single-mode oscillation over a larger area and compensate for thermal damage. These two changes allow PCsels to maintain high beam quality even when operating continuously.

To embed the photonic crystal, the team designed a pattern of holes in the crystal layer to deflect light in an efficient way, resulting in a beam that diverges very little. They use nanoimprint lithography to make photonic crystals, which speeds up production.

It takes 110 amps to get the laser to its maximum power, which requires many electrodes. (Image credit: IEEE Spectrum)

In a typical photonic crystal laser, these holes, which have a different refractive index to the surrounding semiconductor, deflect the light inside the laser in a precise way. The Susumu Noda team designed the pattern of holes in the crystal so that light is deflected by a set of circular and elliptical holes that are kept a quarter of the laser wavelength apart. Finally, these deflections cause losses in higher-order modes, resulting in a high-quality beam that hardly diverges.

The figure above shows two square modules, one on top of the other. The bottom square has a row of circular modules on top. Photonic crystal surface emitting lasers (PCSELs) emit light from the top, and photonic crystals can increase their brightness.

(Image credit: IEEE Spectrum)

The concept is good enough for a 1mm laser, but scaling it up to a 3mm area will require further innovation. To achieve single-mode oscillations over a larger area, the researchers adjusted the position of the reflector at the bottom of the laser, causing more unwanted mode loss in the vertical direction.

Finally, the Susumu Noda research team also solved the problem that heat changes the refractive index of the device and causes the beam to diverge. They solved this problem by slightly changing the period of the pores in the photonic crystal, so that when the laser is at full power, they work in the right place.

The shape and spacing of pores in photonic crystals cause unwanted laser patterns to refract and interfere with each other (left). For a 3mm laser (right), the bottom mirror of the laser must be used to handle the excess mode source. (Image credit: IEEE Spectrum)

He and his team have established a 1,000-square-meter photonic crystal Surface emitting Laser Center of Excellence at Kyoto University, where more than 85 companies and institutes are involved in the development of PCSEL technology. The team is industrializing their PCSEL design for mass production.

As part of this process, they have completed the conversion from electron beam lithography to nanoimprint lithography to fabricate photonic crystals. Electron beam lithography is accurate, but often too slow for mass production. Nanoimprint lithography, which basically imprints a pattern on a semiconductor, is valuable for quickly creating very regular patterns.

Noda explained that in the future, the team will further expand the diameter of the laser from 3 mm to 10 mm, a size that can produce 1 kilowatt of output power, although this goal can also be achieved by using a 3 mm PCSELs array. He expects that the same technology as the 3mm device can be used to scale up to 10mm (which is expected to produce 1kW of beam), and that using the same design will be sufficient.

Source: OFweek