The past and future of high power semiconductor lasers

With the continuous improvement of efficiency and power, laser diodes will continue to replace traditional technology, change the processing of existing things, and promote the birth of new things.
Traditionally, economists believe that technological progress is a gradual process. More recently, the industry has focused more on disruptive innovation areas that can cause discontinuities. These innovations, known as general technology (GPTs), are "profound new ideas or technologies that may have important impact on many aspects of the economic field". General technology usually takes decades of development, or even longer, to bring productivity up. At the very beginning, they were not well understood. Even after the commercialization of technology, production adoption also had a long lag. Integrated circuits are a good case. The transistor was first demonstrated in the early twentieth Century, but it was widely commercially available until late in the late stage.
Gordon Moore, one of the founders of Moore's law, predicted in 1965 that semiconductors would develop at a faster rate, "bringing the popularity of electronics and pushing this science into many new fields." Despite his bold and unexpected prediction, it has been through decades of continuous improvement before productivity growth and economic growth are achieved.
Similarly, there is a limited understanding of the dramatic development of high-power semiconductor lasers. In 1962, the industry was the first to demonstrate the conversion of electrons into lasers, and a great deal of progress has followed. These advances have led to a significant improvement in the conversion of electrons into high-yield laser processes. These improvements can support a series of important applications, including optical storage, optical network and a wide range of industrial applications.
Looking back on these developments and the many improvements they have brought, we have highlighted the possibility that it will bring greater and more general influence on many aspects of the economic field. In fact, with the continuous improvement of high-power semiconductor lasers, the scope of important applications will increase and will bring profound impacts on economic growth.
History of high power semiconductor lasers
In September 16, 1962, the research team led by Robert Hall (Robert Hall) of the Ge Corp demonstrated the infrared emission of gallium arsenide (GaAs) semiconductor, which has a "strange" interference pattern, meaning the birth of the coherent laser - the first semiconductor laser. Holzer initially thought that semiconductor lasers were "long-range" because the efficiency of light-emitting diodes at that time was very low. At the same time, he was also skeptical about that. Because the laser that was confirmed and existed two years ago needs "exquisite mirror".
In the summer of 1962, Holzer said he was quite shocked by the highly efficient GaAs light-emitting diode developed by the Lincoln Laboratory of Massachusetts Institute of Technology. He then said he was lucky to be able to test with some of the high quality GaAs materials and, using his experience as an amateur astronomer, developed a method to polish the edge of the GaAs chip and form a cavity.
Holzer's successful demonstration is based on the design of radiation bounces back and forth on the interface instead of vertical rebound. He said modestly that no one had happened to put forward this idea before. In fact, Holzer's design is a lucky coincidence, that is, the semiconductor material that forms waveguide has the property of limiting the bipolar carrier at the same time. Otherwise, it is impossible to achieve a semiconductor laser. By using dissimilar semiconductor materials, a planar waveguide can be formed to make the photon overlap with the carrier.
These preliminary demonstrations at Ge Corp are a major breakthrough. However, these lasers are far from practical devices. In order to promote the birth of high-power semiconductor lasers, we must integrate different technologies. The key technology innovation starts with the understanding of direct band gap semiconductor materials and crystal growth technology.
Later developments include the invention of double heterojunction lasers and the subsequent development of quantum well lasers. The key to further enhance these core technologies lies in the improvement of efficiency, as well as the development of cavity surface passivation, heat dissipation and packaging technology.
brightness
Innovation in the past decades has brought exciting improvements. In particular, the improvement in brightness is excellent. In 1985, the most advanced high-power semiconductor laser at that time could couple 105 MW power to 105 micron core diameter optical fiber. The most advanced high-power semiconductor laser now can generate more than 250 watts, and has a single wavelength of 105 micron optical fiber, which is 10 times every eight years.
Moore conceived "fixing more components on the integrated circuit". Then, the number of transistors per chip increased by 10 times every 7 years. Coincidentally, high power semiconductor lasers integrate more photons into optical fibers at a similar exponential rate.
The improvement of the brightness of high-power semiconductor lasers has promoted the development of various unforeseen technologies. Although this trend continues to require more innovation, there is reason to believe that the innovation of semiconductor laser technology is far from complete. Physics, which is well known, can further improve the performance of semiconductor lasers through continuous technological development.
For example, compared with the current quantum well devices, quantum dot gain medium can significantly improve efficiency. Slow axis brightness provides another magnitude of improvement potential. The new packaging material with improved heat dissipation and expansion matching will provide continuous power consumption adjustment and simplify the enhancement function required for thermal management. These key developments will provide a roadmap for the development of high power semiconductor lasers in the coming decades.Diode pumped solid-state and fiber laser
The improvement of high power semiconductor lasers makes the development of the downstream laser technology possible. In the field of downstream laser technology, semiconductor lasers are used to stimulate (pumped) doped crystals (diode pumped solid-state lasers) or doped fibers (fiber lasers).
Although semiconductor lasers provide efficient and low-cost laser energy, there are two key limitations: they do not store energy and brightness is limited. Basically these two lasers need to be used in many applications: one is used to convert electricity into laser emission, and the other is used to enhance the luminance of the laser.
A diode pumped solid state laser. In the late 1980s, the application of diode laser pumped solid state lasers began to popularize in commercial applications. Diode pumped solid-state laser (DPSSL) greatly reduces the size and complexity of the thermal management system (mainly circulating cooler), and obtains a module that has always been used to pump solid-state laser crystals with a arc lamp.
The wavelength selection of a semiconductor laser is based on their overlapping with the spectral absorption characteristics of the solid-state laser gain medium; compared with the wide-band emission spectra of the arc lamp, the thermal load is greatly reduced. Due to the popularity of 1064nm neodymium laser, 808nm pumped wavelength has become the largest number of wavelength in semiconductor lasers for more than 20 years.
With the enhancement of the luminance of the multimode semiconductor laser and the ability to use the body Prague grating (VBGs) to stabilize the narrow emission line in the mid 2000, the second generation improved diode pump efficiency has been realized. The weaker absorption and narrower absorption characteristics of 880nm become the research focus of high brightness pump diodes. These diodes can achieve spectral stability. These higher performance lasers can directly stimulate the upper level 4F3 / 2 of the laser in neodymium, reducing the quantum defects, thus improving the higher average power of the base mode extraction, otherwise the thermal lens will be limited.
By the beginning of 2010, we witnessed the high power scaling of the single transverse mode 1064nm laser and the related series of frequency conversion lasers working in the visible and ultraviolet bands. Due to the long high energy states of Nd:YAG and Nd:YVO4, the Q switching operation of these DPSSL provides high pulse energy and peak power, which is very suitable for the processing of ablative materials and the application of high precision micro machining.
Fiber laser. Fiber laser provides a more effective way to transform the brightness of high power semiconductor lasers. Although wavelength multiplexed optical devices can convert relatively low luminance semiconductor lasers into brighter semiconductor lasers, this is at the expense of increasing spectral width and optical mechanical complexity. Fiber lasers have been proved to be especially effective in photometric conversion.
The double cladding fiber introduced in 1990s uses a single mode fiber surrounded by a multimode cladding, which can put a higher power, lower cost multimode semiconductor pumped laser into the fiber efficiently, creating a more economical way to convert high power semiconductor lasers into brighter lasers. For ytterbium doped (Yb) doped fiber, the pump excite the narrow-band characteristic with a width of 915 nm or around 976 nm. With the pump wavelength approaching to the lasing wavelength of the fiber laser, the so-called quantum defect will be reduced, so the efficiency will be maximized and the surplus heat dissipation will be minimized.
Fiber lasers and diode pumped solid state lasers all depend on the improvement of diode laser brightness. Generally speaking, with the continuous improvement of diode laser brightness, the proportion of laser power pumped by them is also increasing. The brightness enhancement of semiconductor lasers is conducive to more efficient brightness conversion.
As we expect, space and spectral luminance will be necessary for future systems, which will make it possible for the dense wavelength multiplexing scheme of low quantum defect pumping and direct semiconductor laser applications with narrow absorption characteristics in solid lasers.
Market and Application
The development of high-power semiconductor lasers makes many important applications possible. These lasers have taken the place of many traditional technologies and implemented a new product category.
With the improvement of cost and performance more than 10 times every ten years, high power semiconductor lasers destroy the normal operation of the market in an unpredictable way. Although it is difficult to accurately predict future applications, it is very meaningful to review the development process of the past thirty years and to provide the framework for the next ten years of development.
1980s: optical storage and initial niche applications. Optical storage is the first large-scale application of semiconductor laser industry. Shortly after Holzer's initial display of infrared semiconductor lasers, the Ge Corp's Nick Holonyak also displayed the first visible red light semiconductor laser. Twenty years later, the optical disk (CD) was launched into the market, and then the optical storage market came into being.
The continuous innovation of semiconductor laser technology brings the development of optical storage technology, such as the injection of digital versatile disk (DVD) and Blu ray disk (BD). This is the first big market for semiconductor lasers, but normally moderate power levels limit other applications to relatively small niche markets, such as thermosensitive printing, medical applications, and selected aviation and defense applications.

1990s: optical networks prevail. In 1990s, semiconductor laser became the key of communication network. Semiconductor lasers are used to transmit signals through the optical fiber network, but the high power single mode pumped lasers used in optical amplifiers are critical to the realization of the scale of optical networks and the real support for the growth of Internet data.
It brings far-reaching influence to the telecom industry. Taking Spectra Diode Labs (SDL) as an example, one of the pioneers of the high-power semiconductor laser industry is an example. Founded in 1983, SDL is a joint venture of Spectra-Physics (spectral Physics) and Xerox (Xerox), a laser brand under the Newport group of the United States, and is listed in 1995, with a market value of about $100 million. Five years later, SDL sold to JDSU at the price of more than 40 billion dollars in the peak period of telecom industry, which is also one of the biggest technology acquisitions in history. Soon after, the telecom bubble burst and destroyed trillions of dollars of capital. Now it is regarded as the biggest bubble in history.
2000s: laser is a tool. Although the burst of telecom market bubble is devastating, the huge investment in high-power semiconductor lasers has laid the foundation for wider adoption. With the improvement of performance and cost, these lasers begin to replace traditional gas lasers or other energy conversion sources in various processes.
Semiconductor lasers have become a widely used tool. Industrial applications range from traditional manufacturing processes, such as cutting and welding, to new advanced manufacturing technologies, such as 3D printing metal parts, etc. Micro manufacturing applications are more diverse, because key products such as smart phones have been commercialized through these lasers. Aerospace and defense applications involve a wide range of key mission applications, and may also include the next generation of directional energy systems in the future.
summary
More than 50 years ago, Moore didn't put forward a new basic law of physics, but made a great improvement on the integrated circuit that was first studied ten years ago. His prediction lasted for decades, and brought a series of disruptive innovations, which were unthinkable in 1965.
When Hall demonstrated the semiconductor laser more than 50 years ago, it triggered a technological revolution. No matter how Moore's law is, no one can predict the rapid development of the high-power semiconductor laser's brightness after a lot of innovation.
There is no fundamental rule in physics to control these technological improvements, but continuous technological progress may promote the improvement of laser brightness. This trend will continue to replace traditional technology and further change the way of development. More important for economic growth is that high power semiconductor lasers will also promote the birth of new things.