By hmimcu Whenever it comes to lasers, people must mention Vcsel, which is a vertical cavity surface emitting laser: Vertical-Cavity Surface-Emitting Laser. In 2017, Apple ‘s iPhone X used vcsel as a 3D sensing technology for Proximity sensor and Face ID modules, which completely made Vcsel popular. Later, it was found that Vcsel has a large application market in LiDAR and gas detection. The market expects the market to expand by more than 10 times in 2023. At the same time, with the construction of optical communication data centers, the use of Vcsel lasers as short-wavelength lasers such as 980nm will also surge. lcd module tft
Application scenarios of Vcsel
market expectation
Major global Vcsel suppliers
2. The structure and principle of Vcsel
As shown in the figure above, we all know that LD is a side-emitting laser. The light source is emitted from the side light-emitting surface, and it is necessary to coat the AR surface with an anti-reflection coating and the HR surface as a high-reflection coating. The light of Vcsel is directly emitted from the P-type or N-type surface, which is a bit like the structure of a red LED.
chip internal structure
The optical cavity of Vcsel is deposited in an epitaxy process using Bragg above and below the active region.
Vcsel shape after TO package
Let’s take the preparation process of Vcsel in the above figure as an example to illustrate:
(1) On the gallium arsenide substrate by MBE or Mocvd, alternately grow GaAs and AlAs, and the alternate growth layers finally form a Bragg mirror. GaAs and AlAs have significantly different refractive indices, but their lattice constants are basically the same, so many layers can be grown alternately without generating dislocations, which is why a mirror effect with high reflectivity can be made.
(2) Then grow several quantum well active regions.
(3) A group of P-doped GaAs/AlAs are grown on it.
(4) A circular area is etched to define a laser with a diameter of several microns in the area.
(5) Usually the metal contact is on the top surrounding device ring, usually in the top mirror stack, by oxidizing the exposed AlAs layer to make the oxide non-conductive, thereby forming a current stop so that the funnel current flows only to the device’s center.
(6) P-type Bragg reflector.
(7) Plating the top P electrode
(8) Thinning to 100um, plating N electrode germanium nickel gold.
3. Advantages and disadvantages of Vcsel
advantage:
(1) There is no need to make a separate mirror, and the process is simple.
(2) The threshold current is only 0.1mA, the device is small in size and small in capacitance, which is suitable for high-speed modulation systems of 10Gbit/s.
(3) The light-emitting surface is circular, the emitted light is also circular, and the height of the vertical cavity is only a few microns, and there is only one longitudinal mode, and when the diameter of the cavity is less than 8um, only one transverse mode exists. It is very convenient to output light coupling light and so on.
(4) The temperature characteristics are good, and no refrigeration is required.
For example: the 850nm Vcsel laser produced by Mergeoptics of Germany, the spectral width is 0.2nm, the average transmit power is -2.17dBm, the extinction ratio is 6.36dB, and the relative intensity noise is -128dB/Hz.
shortcoming:
Low output power and short cavity length. Long-wavelength epitaxy is difficult to do, such as 1310nm and 1550nm for optical communication.
The vcsel of GaAs has an AlAs oxidation process, which can be used to form lateral current confinement. Inp-based materials do not have this process, and the lateral current confinement is not very good.