This paper introduces the principle, structure, special type and characteristics of vcsel.

Vertical-Cavity Surface-Emitting Laser (VCSEL for short, also translated as Vertical-Cavity Surface-Emitting Laser) is a kind of semiconductor whose laser is emitted perpendicular to the top surface, which is different from the independent chip process that is generally used for cutting. There is a difference between edge-fired lasers where the laser is emitted from the edge.

A laser resonator consists of two-sided Dispersive Bragg Reflectors (DBR) parallel to the surface of a chip’s active reaction zone, which is composed of one to several quantum wells in which the laser light band exists. A flat DBR consists of several layers of lenses with different high and low refractive indices.

Each layer of the lens is one-quarter the laser wavelength thick and gives more than 99% reflection intensity. To balance the short-axis length of the gain region in a VCSEL, a lens with high reflectivity is necessary.

In a typical VCSEL, the upper and lower two lenses are coated with p-type material and n-type material, respectively, forming a junction diode. In more complex structures, p-type and n-type regions may be buried in the lens, allowing more complex semiconductors to be fabricated on the reaction zone to make circuit connections and to eliminate the loss of electron energy in DBR structures.

The VCSEL lab is doing research using new material systems where the reaction zone can be pumped by short-wavelength external light sources (usually other lasers). This allows VCSELs to be demonstrated without the additional problem of achieving good circuit quality; however such devices are not practical for most applications.

Typical VCSELs with wavelengths from 650nm to 1300nm are based on a gallium arsenide chip composed of a DBR composed of gallium arsenide (GaAs) and [aluminum gallium arsenide (AlxGa(1-x)As). When the composition of the GaAs/AlGaAs system changes due to the lattice constant of the material,

There will be no very strong changes, and it allows multiple lattice paired regeneration layers to grow on the bottom layer of gallium arsenide, so it is very suitable for making VCSELs. However, as the Al molecules increase, the refractive index of AlGaAs becomes stronger, and the minimum number of layers is used to form an effective Bragg mirror compared to other systems.

In addition, in the concentrated part of aluminum, an oxide will form AlGaAs, and this oxide can be used to limit the current in the VCSEL to achieve the purpose of low gate current.

Recently, there are two main methods to limit the current in VCSELs, which are divided into two according to their characteristics: ion-embedded VCSELs and oxidized VCSELs.

In the early 1990s, telecommunications companies tended to use ion-embedded VCSELs. Hydrogen ions H+ are usually implanted into the VCSEL structure, anywhere except the resonant cavity, to destroy the lattice structure around the resonant cavity, so that the current is limited.

In the mid-1990s, these companies proceeded to use oxide VCSEL technology. The oxidized VCSEL uses the oxidation reaction of the material around the VCSEL resonant cavity to limit the current, and the metal layer containing more aluminum inside the VCSEL structure will be oxidized. Oxidation lasers also often use ion embedding techniques. Therefore, in an oxidized VCSEL, the current path is limited by the ion-embedded resonant cavity and the oxidized resonant cavity.

The initial use of oxidized VCSELs encountered many difficulties due to the tension of the oxide layer and other defects that caused the resonant cavity to “popping off.” However, after many tests, it has been proved that the realibilty of VCSEL is very complete. In Hewlett Packard’s research on oxidized VCSELs, it was pointed out that “stress can cause the activation energy and wearout life cycle of oxidized VCSELs to be similar to the output energy of in-cell VCSELs.”

Production difficulties also arise when the industry moves from research and development to the production mode of oxidized VCSELs. The oxidation rate of the oxide layer has a very large relationship with the aluminum content. A slight change in the aluminum content will change its oxidation rate and cause the resonator to be oversized or undersized.

For long-wavelength devices with wavelengths from 1300nm to 2000nm, at least it has been confirmed that the active region is composed of indium phosphide. VCSELs with longer wavelengths are experimental and usually optical pumps. VCSELs at 1310nm are ideal in the minimum wavelength limit of silicon-based fibers.

• Multiple reaction area designs (aka bipolar cascade VCSELs). The difference between the different effective energy values when the feedback is allowed to exceed 100%.
• Channel junction VCSEL: Using channel junction (np), an electron-friendly n-np-p-i-n structure can be created and can affect other molecular structures. (e.g. in the form of a Buried Tunnel Junction (BTJ)).
• Extensive tuning of VCSELs using mechanical (MEMS) tuning mirrors.
• “die-bonding” or “die-fusion” VCSELs: two different semiconductor materials can be used to create substrates with different properties.
• Monolithically optically pumped VCSEL: Two superimposed VCSELs, one of which utilizes optics to pump the other.
• Vertical VCSEL integrated monitor diode: A photodiode is integrated with the back mirror of the VCSEL.
• Lateral VCSEL integration monitoring diode: With proper VCSEL chip lithography, a light-emitting diode can be fabricated to measure the luminous intensity of adjacent VCSELs.
• VCSEL with external cavity, refer to VECSEL or disk laser. VECSELs are optical pumps for conventional laser diodes. This setup allows the device to have a wider area to be pumped and therefore more energy to be absorbed, around 30W. The external resonant cavity also allows intracavity techniques such as frequency multiplication, single frequency operation and femtosecond pulse modelocking.
• Vertical cavity semiconductor optical amplifier VCSOA. Unlike oscillators, this device optimizes amplifiers. Because VOSOA must work under constraints, it will be required to reduce the reflection of the mirror in order to achieve the effect of reducing feedback. In order to maximize the signal, these devices contain a large number of quantum wells (optical pump devices have been shown to have 21-28 quantum wells), resulting in a larger magnitude of signal increase than typical VCSELs (about 5%). . This device operates on narrow linewidth amplifiers (about a dozen GHz) and may have the effect of an enhancement filter.

Because VCSELs emit laser light from the top surface of an integrated circuit, they can be tested directly on the chip before they are split into individual units. This can save the cost and use of equipment in the semiconductor fabrication process. This also allows the fabrication of VCSELs that are not just one-dimensional, but two-dimensional arrangements.

The larger VCSEL output aperture, compared to most edge-emitting lasers, produces a lower divergence angle of the output beam and enables more efficient fiber routing.

Compared with most edge-emitting lasers, a high reflectivity mirror reduces the VCSEL’s gate current, resulting in lower power consumption. To date, however, VESELs emit less energy than edge-emitting lasers. The lower gate current also allows the VCSEL to have inherently high regulation bandwidth.

The wavelength of the VCSEL is in the acquisition band of the reaction zone and can be changed by adjusting the thickness of the reflective layer.

The emission light of early VCSELs is longitudinal multi-mode or filament mode, while today’s VCSELs are mostly single-mode.