What is VCSEL?

Light-emitting devices, such as LEDs, are commonly used in traditional photoelectric conversion technology. Because such light-emitting devices are typically edge-emitting and huge in size, they are difficult to integrate with semiconductor technology. After the vertical cavity surface emitting laser (VCSEL) technology matured in the 1990s, it solved the challenge of combining light-emitting devices and semiconductor technology, and it became popular swiftly.

Two-sided emission VCSEL in the middle of the wafer optical lens

Ⅰ. What is VCSEL?

The Vertical-Cavity Surface-Emitting Laser (VCSEL, or Vertical-Cavity Surface-Emitting Laser) is a semiconductor whose laser is emitted perpendicular to the top surface. It differs from an edge-fired laser, which emits the laser from the edge.

VCSEL is a breakthrough light emitting device in optical communications, as well as a new form of optoelectronic technology with enormous development prospects. The edge emitting laser emits in a direction parallel to the substrate's surface and perpendicular to the cleavage surface, whereas the surface emitting laser emits in a direction perpendicular to the substrate's surface, as indicated in the accompanying figure:

Schematic diagram of edge-emitting laser (a) and surface-emitting laser (b)

Its advantages over edge-emitting lasers include: easy two-dimensional planar and optoelectronic integration; circular beams are easy to achieve effective coupling with optical fibers; and circular beams are easy to achieve effective coupling with optical fibers. It is possible to create high-speed modulation, which can be used in long-distance, high-speed optical fiber communication systems. active The area size is extremely small, allowing for high packaging density and low threshold current; no cleavage is required after chip growth, allowing for on-chip tests; it operates in a single longitudinal mode over a large temperature and current range; and it is inexpensive.

The outstanding performance of VCSEL has sparked considerable alarm and has turned it into a global research hotspot. VCSELs have advanced significantly in structure, materials, wavelength, and application domains over the last ten years, and several devices have hit the market.

Ⅱ. VCSEL Basic Structure

The structure of VCSEL is depicted in the graphic below. It is made up of distributed Bragg reflectors (DBR) that are developed alternately with high and low refractive index dielectric materials to form continuous growth of single or multiple quantum well active regions. In order to get the highest stimulated radiation efficiency and enter the oscillating field, 35 quantum wells are typically arranged towards the maximum of the standing wave field. The laser beam is output from the transparent window on the top, and a metal layer is coated on the bottom to improve the optical feedback of the DBR below.

VCSEL Schematic

In fact, a powerful current converging structure must be utilized to complete the low-threshold current operation, similar to a general bar type semiconductor laser, and optical confinement and current intercepting confinement must be done at the same time. The semiconductor multilayer mode mirror DBR of the VCSEL is made of GaAs/AlAs, which is etched into an air-post (mesa) structure, as seen in the above image. The AlAs layer is oxidized in high-temperature water vapor to form an insulating AlxOy layer with a considerably lower refractive index, resulting in a structure that restricts light and carriers vertically. The high reflectivity, low loss DBR, and position of the active region in the cavity are all important factors in VCSEL design.

Ⅲ. Features of VCSEL

Because the VCSEL and the edge-emitting laser have distinct structures, which determines the variations in their characteristics and performance, the basic parameters of the two lasers are shown in the table below.

Comparison of structure and performance between edge-emitting laser and VCSEL

The table shows that the VCSEL active region is small and has a short cavity, making single longitudinal mode, low threshold (sub-milliamp) current operating easier. The active zone, on the other hand, must be larger in order to get a sufficiently high gain. The reflectivity of the hollow mirror must be at least 99 percent. VCSEL is expected to be used in a wide range of high-speed data transfer and optical communications applications due to its high relaxation oscillation frequency. A VCSEL's light emission direction is perpendicular to the surface of the substrate, allowing for good lateral light field restriction. The full wafer test yields a circular beam, and the fabrication of a two-dimensional array is simple. The epitaxial wafer can reduce production costs before the full process is completed.

Ⅳ. The Advantages of VCSEL

l. The outgoing beam is circular with a modest divergence angle, making it simple to couple with optical fibers and other optical components while also being very efficient.

2. It has the ability to perform high-speed modulation and can be used in long-distance, high-speed optical fiber communication systems.

3. Because the active area is tiny, single longitudinal mode and low threshold operation are simple to produce.

4. The electro-optical conversion efficiency could be larger than 50%, implying a longer gadget life. 5. It is simple to implement a two-dimensional array, apply it to a parallel optical logic processing system, achieve high-speed, large-capacity data processing, and use it in high-power devices.

6. The chip can be tested and the product screened before it is packaged, lowering the product's cost significantly.

7. It can be employed in laminated optical integrated circuits with micromechanical technology.

Ⅴ. The History of VCSEL

VCSEL's history is likewise one of constant performance improvement, owing to the efforts of various academic institutions. IGA and the team led by it have played an indelible part in these decades of history, and can be referred to as the IGA professor as one of VCSEL's fathers.

VCSEL's application is becoming more and more widespread as a result of its numerous advantages. And, in order to be suited for these applications, VCSEL is evolving in a number of directions, as illustrated in the diagram, with its key applications being:

VCSEL application fields at various wavelengths

Based on the VCSEL of Soda et al. in 1979, the modern VCSEL is mostly employed in optical transmission. VCSEL development has essentially gone through two stages:

The first stage: from the birth of VCSEL to the end of the 20th century, the stage of wild development.

Numerous organizations have proposed and tested diverse types of VCSELs with various architectures at this point. Due to their numerous advantages, oxide-limited VCSELs ultimately won out.

Huffaker et al. pioneered the use of the mesa structure (Mesa) to intrinsically oxidize AlGaAs and form a buried high-resistance layer of Al oxide to further limit current in 1994. The threshold current can be decreased to 225uA using this arrangement. And this structure is the prototype for the oxide-confined structure that is currently in use.

The first oxide-limited VCSEL

Iga proposed a simple relationship formula for essential VCSEL indicators like threshold current, modulation bandwidth, and active area in 2013:

The threshold current of a VCSEL is the same as that of other semiconductor lasers, and it is proportional to the active area volume:

As can be seen from the formula, lowering the threshold current necessitates reducing the active area volume on a continual basis. When the active area volume of the present VCSEL and the strip laser are compared, it can be seen that while the VCSEL's active area volume is V=0.06um3, the strip laser's active area volume is still V=60um3, which explains why the strip laser's usual threshold current is still tens of mA. The VCSEL threshold current has dropped below one milliamp.

The second stage: Gradually mature stage and optimization stage.

VCSELs with this structure are quickly adopted in optical communications because oxide-limited VCSELs have various advantages, such as low threshold currents.

Because a higher working current can result in better modulation characteristics, it will also increase power consumption, which will result in a rise in temperature, which will have an influence on reliability. In the world of optical transmission, VCSELs have faced significant problems in terms of modulation rate and power consumption.To lower series impedance, YC.Chang et al. used the strategy of raising the number of deep oxide layers to 5 and increasing the p-type doping concentration. The bandwidth/power consumption ratio was just 12.5GHz/mW, which was the most advanced level at the time. The 15GHz modulation bandwidth was reached at 0.9mA current, and the corresponding power consumption is only 1.2mW. The figure depicts the cross-sectional structure of VCSEL:

Deep oxide confined VCSEL

In the same year, Y-C. Chang et al. achieved 35Gbps error-free transmission using the identical VCSEL system.

Petter Westbergh et al. studied the relationship between 850nm oxide-limited VCSEL photon lifetime, resonant frequency, and modulation rate in 2011, finding that a compromise between high resonant frequency and low damping oscillation was achieved to increase the rate: when the photon lifetime was close to 3ps, the modulation bandwidth of the VCSEL could reach 23GHz, and the error rate could reach 40Gb/s.Various interest groups have remained active in high-speed, low-power VCSEL research in recent years. As of 2015, Figure 10 depicts the study findings of several institutions. It can be seen that the current VCSEL back-to-back transmission can achieve 71Gbit/s if the pre-emphasis approach is used.

Recent developments in the field of short-wavelength VCSEL optical interconnection

Ⅵ. The Applications of VCSEL

1. Used for high-speed optical fiber communication

Long-wavelength VCSELs with wavelengths of 1300nm and 1550nm have a large market potential in Gb/s rate optical fiber communications. Because VCSELs with wavelengths of 1300nm and 1550nm have minimal fiber dispersion and attenuation windows, they also have the advantage of high-speed transmission over medium and long distances.

2. Used for digital communication

In short-distance, high-capacity parallel data lines, VCSEL will have a big application market. VCSELs are commonly utilized in data transmission between nodes in a local area network because they are low-cost and high-performance. G-bit Ethernet or a high-speed LAN protocol is necessary to meet the growing demand for LAN bandwidth, and VCSEL can be employed as a low-cost multi-mode optical transmitter.

3. Used for optical interconnection

A crucial component in optical parallel processing, optical identification systems, and optical connector systems is the 1300nm wavelength VCSEL. In the disciplines of optical information processing, optical networking, optical switching, optical computing, neural networks, and so on, VCSEL can fully use the advantages of photonic parallel operation and large-scale integrated area array, and has a wide range of applications.

4.Used for optical storage

The VCSEL can be utilized as a read/write light source for optical storage. To boost the storage density of CD discs, VCSEL can be utilized as the light source, and the disc reading system can be equipped with a separate external photodetector to monitor the reflected light from the disc. A new integrated optical disc read head combining a VCSEL with an internal cavity quantum well absorber has been demonstrated at the University of California in the United States. The VCSEL's CW beam is focused on the optical disc, and the enlarged reflected beam enters the VCSEL optical cavity directly. The cavity absorber acts as a photodetector when reverse biased, and the photogenerated current it generates gives a precise optical feedback variable from the optical disc.

VCSEL is also employed in new types of illuminators, displays, laser printers, and other devices.

The VCSEL has become a crucial device in optoelectronic applications due to its rapid development and inherent advantages. VCSELs have been continually developed in recent years, mostly due to their low threshold current, high output power, high electro-optical conversion efficiency, low operating voltage, high modulation bandwidth, and high yield. With continued research, it is expected that VCSEL will have an increasing number of possible applications.