High beam quality, high power vertical cavity surface emitting laser

1 Introduction
Vertical-cavity surface-emitting lasers (VCSELs) have surface light transport
Easy two-dimensional integration, circular symmetry of the beam, high fiber coupling efficiency, and work
Advantages such as low consumption and long service life[1-3]
At present, it has been widely used in optical interaction
Connection, optical communication, laser fuze, laser display, optical signal processing and core
Chip-level atomic clocks and other fields[4-7]
。 Not only that, but based on VCSEL laser
Radar (LIDAR) systems for driverless cars and 3D imaging-based
DOI： 10.3788/CJL230977
Title; Surface etching gratings and photonic crystals increase the loss of higher-order modes
It is necessary to optimize the light field, but it is often complex to prepare
process and precise control, otherwise it will increase optical loss and reduce optical output
power, which leads to higher costs, limits its push in some application areas
Wide; By adjusting the size and distribution of injected protons, it is possible to optimize the light
field, to obtain a higher beam quality, but there are also some limitations, injection
The number and distribution range of protons are difficult to accurately control, and it is easy to cause the device
The performance of the piece deteriorates and the service life is reduced.
Smartphones have attracted much attention in recent years [8-11]
。 With VCSEL application
The expansion of the scene, the output power and beam quality of the VCSEL
There is also a higher requirement to obtain light while increasing the power of the device
The light field with better beam quality is a technology that needs to be solved urgently by scientific researchers
Puzzle.

With the development of VCSELs with large aperture and two-dimensional arrays,
The output power of the VCSEL has been significantly improved. But for large apertures
In the case of VCSEL, the carrier aggregation effect and the spatial burning effect can cause excitation
The optical power density is unevenly distributed, which has a great impact on the light output quality of the device
Sound. Therefore, researchers at home and abroad have conducted in-depth research and exploration on this issue
Cable, which is currently achieved by adding optics to the outer cavity, can be achieved for high power
Manipulation of VCSEL light fields, such as microlenses [12] and diffractive optical elements
(DOE), filters, etc., but with the help of external optics will make the device
Flexibility is poor, which is not conducive to miniaturization. In addition to this, you can also pass the excellent
The structure of the device increases the power and improves the light field distribution: the surface is prepared with silver nano
Rice wire thin film [13], surface etching grating [14-16], photonic crystal structure [17-20], quality
Combination of sub-injection and oxide pore size [21], etc. However, on the surface of the device
Although the preparation of silver nanowire films improves the uniformity of the carriers, the preparation of silver nanowire films is not effective in When preparing a large area, the uniformity of nanowires is difficult to control to a certain extent
It increases the complexity of fabrication and integration, and is prone to introduce thermal coupling problems. Surface etching gratings and photonic crystals increase the loss of higher-order modes
It is necessary to optimize the light field, but it is often complex to prepare
process and precise control, otherwise it will increase optical loss and reduce optical output
power, which leads to higher costs, limits its push in some application areas
Wide; By adjusting the size and distribution of injected protons, it is possible to optimize the light
field, to obtain a higher beam quality, but there are also some limitations, injection
The number and distribution range of protons are difficult to accurately control, and it is easy to cause the device
The performance of the piece deteriorates and the service life is reduced.

Through theoretical analysis, on the basis of not introducing external structures,
A VCSEL with a multi-annular cavity structure was designed to inject an electric current
The region is optimized into multiple concentric ring electrode structures to accommodate the uniformity of the carriers
Homogeneous distribution, thus optimizing the light field distribution of large-aperture VCSELs. while
The maximum continuous output of the device at room temperature at an injection current of 0.8 A
The power is 140 mW, which is nearly 56% higher than the traditional structure, and the far-field is present
Gaussian distribution.

2. Theoretical analysis and structural design
For small pore size VCSELs (typically Φ<25 μm diameter), analysis
When distributing currents in a VCSEL, it is common to assume that the area below the limit zone is assumed
The current density distribution is uniform. But for large aperture VCSELs
This does not apply. The cylindrical coordinate system is selected, and the light window is used as the origin, z
The direction is perpendicular to the window and pointing towards the substrate, and r is the distance to the z-axis. according to
The Poisson equation for carrier transport gives the radial direction of the current density
Distribution[22]:     

where J(r,z) is the current density at (r,z); ρa for the active area of electricity resistivity; V is the junction voltage.

The electrode voltage is assumed to be V0
, then the boundary condition of Poisson's equation is

where: w is the window radius of the VCSEL; σi and σj are the conductances of the ith and j-layers of the Bragg Mirror (DBR) distributed at the junction.

The carrier rate equation is 

where D is the carrier diffusion coefficient; N(r) is the carrier density; J(r) is the current density at r; τ is the carrier lifetime; B is the spontaneous radiation recombination coefficient; νg
is the group velocity; S(r) is the photon density; e is the amount of charge; d is the quantum well thickness. From Eq. (1), it can be seen that the current density injected into the center of the active region (r=0) is small, and increases along the radial off-axis, and the closer to the edge of the active region the current density increases. As can be seen from Eq. (3), when a current is injected, the load
The carriers are non-uniformly distributed, and the carrier density increases the closer to the edge of the active region. Therefore, when the operating current is injected, the closer to the edge of the electrode, the higher the current density, resulting in the carriers in the active region mainly gathering near the electricity
The edge area of the pole makes the distribution of laser power density uneven, and with the
This non-uniformity is exacerbated by the increase in the aperture of the light. Coupled with the effect of the spatial perforation effect [23], the maximum intensity of the VCSEL light field moves from the center to the edge, and the lasing spot appears in a ring shape. The simulation results in Figure 1 also verify this phenomenon.

To solve this problem, we designed a multi-annular cavity structure (Fig. 2(b)) to separate the injected current region into multiple regions to suppress the carrier aggregation effect, where P-DBR is P-type DBR and N-DBR is N
DBR. The finite difference time domain (FDTD) method was used to simulate the light field by adjusting the size of the annular cavity and the proportion of light output area (PLEA), as shown in Figure 3. As can be seen from Figure 3, the intensity of the light field of structure A is evenly distributed, but due to the transverse size of the annular cavity and the light output area
The proportion (50%) is small, and the utilization rate of effective light-emitting area is low. Due to the large proportion of light output area (75%) and transverse size, the uniformity of the light field deteriorates and the intensity of the light field is also weakened. The light output area of the C structure accounts for 67%, the light field is evenly distributed, and the space utilization rate of the light emitting area is relatively high.Good, it is good for obtaining high power output.

3 Device preparation
Using the same chip and under the same process conditions, high-power VCSEL devices with the same traditional structure and new structure with the same outer diameter of the optical aperture were fabricated. For the epitaxial structure, the metal-organic compound chemical vapor precipitation (MOCVD) technique was used to grow on n-type GaAs substrates, and the structure included 41 pairs of Si-doped N-type DBR and 24 pairs of Be-doped P-type
DBR, active region contains 3 GaAs/AlGaAs quantum well structures, made
It is the high Al layer of the oxidation restriction layer, Al0.98
Ga0.02As is located in P-type DBR with between active regions. After epitaxial growth, UV lithography, inductively coupled plasma (ICP) etching, wet oxidation, growth of SiO2 masks, sputtering electrodes, metal stripping, and thinning packaging are carried out, as shown in Figure 4.

Compared with the traditional structure, some key technologies in the preparation process of VCSEL with multi-ring cavity structure are worthy of attention, for example, for multi-ring structures with complex graphics, pattern alignment and interlocking are required, so accurate overlay alignment lays the foundation for the preparation of good devices; For the ICP etching process, it is necessary to study the influence of etching process parameters on the steepness of the sidewall of the countertop, which is helpful for the preparation of a uniform oxidation limiting layer. For the wet oxidation process, in progress
During multi-epoxide, it is necessary to optimize the oxidation temperature and study the effect of temperature on the oxidation rate and the shape of the oxidation pore, so as to accurately control the size and shape of the oxidation pore.

4 Results and Discussion
Figure 5 shows a near-field image of a conventional structure and a new structure VCSEL tested under the same injection conditions. The test results show that the light field distribution uniformity of the traditional VCSEL [Fig. 5(d)] is very poor, and only the annular region near the electrode ring emits light. However, each light-emitting region of the three annular cavity structures of A, B and C is completely luminous, and the light field distribution is relatively uniform, which greatly improves the phenomenon of extremely poor light field distribution caused by the carrier aggregation effect and the spatial hole burning effect of the traditional structure. Through the comparison, it can be seen that the light field uniformity of the C structure is good, the utilization rate of the luminous area is high, and more importantly, its light field is the strongest, which is consistent with the theoretical simulation results. Figure 6 shows the far-field distribution and spectral pattern of C-structure VCSELs. It can be seen that the central strength of the far field is strong and shows a Gaussian distribution. The lasing spectroscopy also verifies its excellent single-mode performance, with a peak wavelength of 805.03 nm and a full width at half maximum
At 0.82 nm, the device exhibits very good lasing characteristics. The beam parameters such as the waist of the C-structure VCSEL were measured by using the charge-coupled device (CCD) imaging technique, and the results are shown in Fig. 7, and the values of the transverse beam quality factor (M2x) and longitudinal beam quality factor (M2y) were obtained by fitting the Gaussian equation of laser beam propagation to 1.226 and 1.126, respectively.

It can be seen that when the injection current reaches the threshold current, the output power increases with the increase of the current, and when the current increases to a certain level, the output power reaches the maximum, and with the further increase of the current, the output power decreases. This is due to the fact that with the injection of current, a large amount of heat is generated inside the device, and the temperature of the device increases, especially in the active region, so the various non-radiative recombinations in the device are enhanced, the internal and external quantum efficiency is reduced, and more injected electrical power is converted into thermal power, and then the output power is reduced. In addition, conventional VCSELs (Fig. 5(d)) have a maximum continuous output power of 90 mW and a threshold current of 80 mA at an injection current of 0.7 A. Compared with the traditional structure devices, the output power and slope efficiency of the new structure are improved, and the threshold current is reduced. The distribution of carriers will directly affect the optical gain of the optical cavity, and the spatial burning effect caused by the uneven distribution of carriers in the traditional structure will reduce the gain in the center of the device and increase the gain at the edge, and more current needs to be injected to achieve the threshold gain, so as to achieve lasing. The new multi-annular cavity structure makes the carriers evenly distributed, effectively suppresses the spatial burning effect, and increases the radiation recombination rate generated by the electron-hole pairs injected into the active region, reduces the carrier loss, and thus reduces the threshold. The threshold current of the new structure C is 49 mA, and the maximum continuous output power is 140 mW, which is nearly 56% higher than the traditional structure. It shows that the new structure is of great significance to the improvement of device performance.

5 Conclusions
By analyzing the reasons for the uneven distribution of carriers in large aperture VCSEL, a multi-annular cavity structure VCSEL was designed and the light field of the new structure was simulated, and the results showed that the light field distribution could be optimized by optimizing the size of the annular cavity and the proportion of the light output area. On this basis
On the same epitaxial wafer, the traditional structure and the new structure VCSELs with the same outer diameter of the exit aperture were prepared at the same time, and their light field uniformity and output characteristics were compared and analyzed. The results show that the new structure improves the uneven distribution of the light field caused by the carrier aggregation effect of the traditional structure, and the new multi-annular cavity structure with a duty cycle of 67% has the best near-field distribution and the threshold current is reduced. When the injection current is 0.8 A, the continuous output power at room temperature reaches up to 140 mW, which is nearly 56% higher than that of the traditional structure, and the far-field presents a Gaussian distribution and the beam quality is better. The research results meet the demand of VCSEL for high-power and high-beam quality semiconductor laser sources in the field of optical communication, and further expand the application range of VCSEL in the field of smart devices.