What’s the difference between a VCSEL and PCSEL?

When it comes to laser technology, the differences between a vertical cavity surface-emitting laser (VCSEL) and a photonic crystal surface-emitting laser (PCSEL) are crucial. While VCSELs rely on a three-dimensional structure for light emission, PCSELs incorporate a 2D photonic crystal layer to manipulate light emission. These structural variances result in distinct performance and application possibilities.

One noticeable distinction among semiconductor lasers is their beam patterns. Both VCSELs and PCSELs emit light perpendicular to the chip surface, but PCSELs produce a smaller beam pattern compared to VCSELs.

On the other hand, light-emitting diodes (LEDs) also emit light perpendicular to the chip surface, but their emissions spread out laterally, creating a more diffuse beam. Edge-emitting lasers (EELs) come in two common designs: Fabry-Perot (FP) and distributed feedback (DFB), both producing oblong beams from the edge of the die.

Figure 1. Comparison of major solid-state laser technologies. The beam patterns (right-hand column) are a key differentiator. (Image: Vector Photonics)

Table of Contents

Performance comparison

Key performance differences between VCSELs and PCSELs include:

VCSELs have smaller active areas to maintain single-mode operation, limiting their maximum output power. In contrast, the PhC layer in PCSELs can support a larger active area, resulting in higher output power and better single-mode emissions.

VCSELs have less flexibility in emission wavelengths compared to PCSELs.

While the costs of VCSELs have decreased over time due to production longevity, PCSELs, as an emerging technology, currently incur higher costs.

In lower-power applications like optical communications, facial recognition, and time-of-flight (ToF) sensors, VCSELs are more suitable for portable devices. Conversely, PCSELs are anticipated to excel in higher-power and higher-speed optical communications, industrial and automotive LIDAR, and biomedical sensors.

Getting under the hood

The vertical structure of VCSELs results in emissions perpendicular to the device surface, simplifying system integration and optical fiber coupling, thereby reducing system costs. This simplicity was a driving factor in the development of PCSELs.

Both VCSELs and PCSELs typically feature multiple quantum well (MQW) gain sections that utilize the quantum confinement effect within multiple thin semiconductor material layers to amplify light.

The primary differentiator lies in how optical confinement is achieved by the two technologies. While VCSELs use distributed Bragg reflectors (DBRs), PCSELs replace them with a PhC, as shown in Figure 2.

Figure 2. The 3D DBR structure in a VCSEL (a) is replaced with a 2D PhC in a PCSEL (b) (images not to scale). (Image: Optical Materials Express)

PCSEL promises

PCSELs are poised to revolutionize solid-state laser technology in several ways. They can deliver high, coherent power, offer phase control for beam steering and high-speed modulation, and their straightforward fabrication suggests low costs once mass production is achieved.

Arrays of PCSELs can generate high power levels by linking in-plane light between individual lasers, creating coherence that results in a small spot of high-intensity laser light.

PCSEL arrays have the potential to produce several kilowatts of coherent power, a feat not achievable with VCSELs or other technologies, making them valuable for industrial processes like metal cutting, welding, and melting.

Moreover, the individual elements in a PCSEL array can be steered in real time as a phased array, opening up possibilities in applications such as LIDAR and high-power 3D printing of metals and plastics.

PCSELs outpace VCSELs by 2.5 times in terms of speed, making them ideal for high-speed data transmissions in the multi-gigabit range.

This speed advantage in high-speed communications is partly due to the wavelength flexibility of PCSELs. Their structure allows for fabrication from various semiconductor materials capable of producing different wavelengths.

While gallium arsenide (GaAs) is commonly used for high-volume VCSEL production emitting at 850 nm, producing VCSELs with indium phosphide (InP) is challenging. In contrast, PCSELs can be easily fabricated using InP and can emit at 1310 and 1550 nm for faster data communication.

Summary

PCSELs are set to build on the success of VCSELs with expanded functionality, replacing the 3D DBR structure in VCSELs with a 2D PhC element. PCSELs offer tighter beam patterns, higher power, faster modulation, and support for a wider range of wavelengths compared to VCSELs.