Lidar origin VCSEL

According to Yole Developpement's recent VCSEL -- 2021 Technology and Market Trends report, driven by data communications and mobile applications, the global VCSEL market will reach $2.4 billion in 2026, with a compound annual growth rate of 13.6%; The current largest market is mobile phones and consumer electronics, which is expected to reach $1.7 billion in the same period, representing a compound annual growth rate of 16.4%. Data communications is VCSEL's second-largest market and is expected to generate $430 million in revenue in 2021 and $566 million in 2026, representing a compound annual growth rate of 5.6%.

According to the report, multi-junction technology represents the next leap in the VCSEL industry and will accelerate the application of VCSEL in the automotive sector. LiDAR will be the typical application scenario, and 5G networks will increase the demand for high-speed optical modules, which will bring more market opportunities for VCSEL. However, some technical challenges remain.

Where did the VCSEL come from?

From the perspective of VCSEL structure, the biggest difference between VCSEL and ordinary EEL laser is that in the cavity, the optical field oscillation is in the vertical direction, and the direction of current injection is also vertical, so the direction of current injection and optical field oscillation is parallel, while the oscillation and current of the edge-emitting structure are perpendicular.

Basic structure of VCSEL

Thanks to Dr. Mo Qingwei, chief scientist of Eagle Semiconductor, provided an old photo, which shows that it is really not easy for VCSEL to come to today. In 1965, Ivars Melngailis from Lincoln Laboratory in the United States first proposed the concept similar to VCSEL, which is to add a highly reflective electrode to the ordinary IGBT laser and make a cut off, so that the forced current is injected from right to left, and the light field is forced to oscillate back and forth between left and right, which has the basic form of modern VCSEL.

The birth of the VCSEL

The resulting laser looks like a side emitter, but the direction of light actually comes from the bottom, which is essentially close to the prototype of a modern VCSEL laser. Of course, it was based on indium antimonide (InSb) material system, and the parameters were not good, 50ns pulse, and the current threshold was 20A (600A/mm2) at 10K ultra-low temperature, but it was not easy.

The modern true VCSEL is credited to Kenichi lga, a professor at Tokyo Institute of Technology. In 1977, he drew the VCSEL pattern in a lab notebook. Later, he made many very important contributions to the development of VCSEL.

From the development history, the earliest VCSEL started from the infrared wavelength semiconductor laser in 1962. In 1964, the first longitudinal current injection surface emission laser close to VCSEL appeared. Kenichi lga realized its 1977 concept in the laboratory in 1979; Room temperature pulses were realized in 1984, and are getting closer to being practical. Before that, pulse plus low temperature. Since then, many people at Bell LABS have gone on to do DBR (Distributed Bragg Reflection), ion implantation, and all kinds of VCsels. Wet oxidation technology was implemented in Austin in 1994, and the VCSEL was finally brought to the practical stage by Honeywell, whose VCSEL business was later acquired by Finisar.

VCSEL Development History

Different usage scenarios have different requirements

In addition to data communication, the vast majority of VCSEL usage scenarios are related to 3D sensing. The important working modes of 3D sensing include: passive binocular, active binocular, structured light and flight time, which basically covers the application scenarios of 3D sensing of VCSEL.

Passive binocular is to use two cameras to simulate two human eyes, look at the same object from two different points, and then use the brain to perform some simple geometric calculations, which is a function of human years of evolution, and is now implemented with VCSEL or computer.

Basic kinds of 3D sensing

The active binocular is also two cameras, with projectors to actively project light spots or light, with two cameras to receive these spots.

The structured light retains a camera, and the other is a projector that projects light spots to the camera to receive them. This is the rationale behind Apple's Face ID.

There are several types of flight time, including direct flight time (dToF) and indirect flight time (iToF). Time of flight is essentially the rationale for automotive lidar.

iToF&dToF

In different application scenarios, the above three structures have different advantages. For example, the precision of the depth information of structured light is proportional to the square of the distance. It is very accurate in short distance, but the accuracy will decline after a certain distance (about 1 meter), which is also the reason why Apple Face ID chooses structured light. Because the precision of structured light is very high within 1 meter and about half a meter, the accuracy of the distance is not as good as other schemes.

The second scheme is iToF, which counts the different phase differences, and dToF, which counts the time it takes to go out and come back. iToF, which monitors the phase difference between emitted and received light, is less accurate at first than structured light and better than structured light after 1 meter. So a lot of smart access control or mobile phone equidistant applications are often used.

dToF is almost not sensitive to distance, especially for long distances, such as more than 10 meters and its accuracy is very good, so dToF is used in car lidar more often, which is why Apple lidar scanner later selected it to build the external 3D VR environment.

The above VCSEL schemes do not compete with each other, but have different choices in different scenarios, and VCSEL will play an important role.

What is a polyjunction structure?

As mentioned earlier, multi-junction technology represents the next big leap for the VCSEL industry, as multi-junction launch performance will be much better, currently the industry can do up to five. The multi-junction technique involves stacking several PN junctions vertically, unlike a common multi-quantum well, which is a single PN junction with several quantums distributed roughly equally. The band band of multi-junction VCSEL uses the tunneling principle to change the electrons in valence band of the previous PN junction into conduction band electrons in the next PN junction. In this way, the cycle starts again, but it will not go on forever. Generally, other problems will occur when the number reaches a certain extent.

Laser radar with multiple junction VCSEL technology

The advantage of multi-junction VCSEL is that higher power density can be obtained, which is very important for Lidar. Slope efficiency can also be obtained, because multiple VCsels share only one DBR, avoiding multiple losses. In addition, it is easier or cheaper for a power supply or drive to always have high voltage and low current than high current and low voltage at the same power. Multi-junction VCSEL through the current unchanged, increased voltage, drive and power are friendly changes, it is the major breakthrough of the two years VCSEL, let its power density from tens of watts/square millimeter or hundreds of watts/square millimeter into a few kilowatts/square millimeter, thus becoming a car radar "series player".

The significance of multi-junction VCSEL is that if several quantum Wells of edge-emitting laser are stacked, or several edge-emitting lasers are connected in series, and then VCSEL is made into multi-junction, from the optical point of view, it is the so-called "no change". If three quantum Wells of edge-emitting laser are stacked together, The Area Solid Angle product becomes triple; If you put three devices in series it's three times as much; The product of area solid Angle does not change, but the optical density and the far field become three times. In this way, multiple junctions can gain many optical benefits at relatively low cost. The benefits far outweigh the negative costs, which is a very important logic behind the long VCSEL.

Comparison of multi-junction VCSEL with multi-junction EEL and multi-chip EEL

The VCSEL light has come down to earth

The adoption of VCSEL on iphones and ipads has taken VCSEL to the next level, resulting in an order of magnitude increase in 3D sensing applications. iPad Pro's VCSEL solution based on dToF technology can help users build 3D virtual scenes and scan the surrounding environment, which is the starting point of Apple's future VR ecology.

As mentioned before, VCSEL is a semiconductor device, its laser is emitted perpendicular to the top surface, unlike the conventional cut-open stand-alone chip technology, the laser is emitted from the edge of the edge laser. It integrates many advantages of red outer emission laser, using a better laser source, not only like infrared LED is very suitable for large-scale wafer level production, low process and packaging costs, but also has the edge emission laser very good spectrum and high optical density characteristics; It also has the characteristic of very low temperature drift, the typical drift of each group of VCsels from low to high temperature is only 0.07 nm/K. This is hard to do with other light sources, which is why Apple chose Face ID as its light source. It is the architecture used for VCSEL that makes it a winner over many light sources.

Advantages of VCSEL technology

In short, VCSEL has the advantages of high photoelectric conversion efficiency, small divergence Angle, good beam quality, good wavelength stability, high reliability, low threshold current, low power consumption, and easy to coupling with fiber, easy to single longitudinal mode emission and high modulation frequency, plus easy to prepare two-dimensional luminous array, mass production cost controllable, is a key component of 3D imaging and recognition sensing module. It is widely used in optical communication and interconnection, data acquisition and transmission, consumer electronics 3D imaging, data centers and cloud computing, Internet of Things, autonomous vehicles, biomedicine, industry and other fields.

Currently, compact all-solid-state lidar based on VCSEL and SPAD (single photon array) has been mass-produced. For example, Ouster has introduced nine lidar models in three series, OS0, OS1 and OS2, using this pure chip-based architecture without mobile mechanical structure. It has recently extended its digital lidar technology to industrial and robotics verticals in Japan and South Korea; And join NVIDIA to accelerate the deployment of autonomous vehicles, based on NVIDIA DRIVE to provide dedicated NVIDIA DriveWorks plug-in to help customers integrate their digital lidar into autonomous vehicles.

Another company, Ibeo, also uses a VCSEL and SPAD array-based solution to achieve two-dimensional scanning. Its ibeoNEXT solid-state lidar solution is very small, and it is already in the Great Wall Mocha SUV.

There's Valeo, which launched its second generation of SCALA in 2021, and a near-field Lidar that replaces existing millimeter-wave or ultrasonic radar at four corners of the vehicle for collision avoidance, obstacle avoidance, and reversing. This is of great significance for the application of VCSEL in automobiles, which can make the usage scenario more abundant. This VCSEL plus SPAD compact solution can be integrated into the rearview mirror, which is both beautiful and practical.

Where does VCSEL want to go?

VCSEL application Trend

Yole's vision of the VCSEL's history and future shows that when it was first industrialized in the 1990s, it was mainly used for data communication, such as 850nm high-speed lasers, and later it was used in optical mice. Until Apple drove the second wave, increasing 3D sensing by an order of magnitude. The expected next wave is lidar for cars, and there will be plenty of them. What's after the lidar? Vcsels are used in IoT, artificial intelligence, or smart connected applications. LED first started from the backlight of mobile phones, then TV backlight, and then lighting. Car lighting is another wave. Now the market size and application of Mini LED and Micro LED photoelectric devices are gradually enlarged in wave after wave of batch applications.

Insight's market analysis shows that, based on the development of VCSEL technology, more and more power, more and more distance, will gradually be used in a variety of situations. From the earliest data communications, mobile sensors, to vehicle monitoring or security, or autonomous vehicles. Therefore, its application scenarios will become richer and broader with the development of VCSEL technology or the improvement of performance.

Application scenario: Power and distance

Challenges remain

It's a long way from the edge-shot laser to the current VCSEL, which Apple put on the industry's radar. But the challenges are many.

Dr. Liao Mingzhi, general manager of VIGO Asia Pacific Region, pointed out that there are still many technical routes of lidar, such as 905nm, 1500nm and MEMS, the use of semiconductor materials are different, the performance of each has advantages and disadvantages.

In the past few years, the VCSEL market has been further extended and penetrated, especially the on-board applications also put forward more and higher requirements for the performance, reliability and cost of VCSEL. In addition, there are several important parameters to consider, including field of view, launch Angle, detection range, and object. Compared with consumer products, Lidar has significantly different requirements for its emission range. The former can meet the needs with a range of 10 meters and a small power, while VCSEL used for autonomous vehicles must greatly improve its detection range and power, as well as its performance and reliability requirements.

In-car applications are more demanding

At present, the application and promotion is affected by the lack of relevant standards for VCSEL and Lidar. Manufacturers have to meet many conditions, and can only customize according to customer requirements, which affects the output and cost efficiency. Therefore, he hopes to form consensus as far as possible through more cooperation with application manufacturers and upstream and downstream manufacturers, improve the consumption of VCSEL, and realize the marketization and scale of application.