A vertical cavity surface-emitting laser (VCSEL) relies on a three-dimensional (3D) structure for light emission. A photonic crystal surface-emitting laser (PCSEL) incorporates a 2D photonic crystal (PhC) layer to manipulate light emission. Those structural differences lead to different performance and application possibilities.
An obvious difference between the various types of semiconductor lasers is the beam patterns. VCSELs and PCSELs emit light perpendicular to the chip surface, but PCSELs produce a smaller beam pattern than VCSELs.
Light-emitting diodes (LEDs) also emit light perpendicular to the chip surface, but emissions spread out laterally for a more diffuse beam. Edge-emitting lasers (EELs) are available in two common designs: Fabry-Perot (FP) and distributed feedback (DFB). Both produce oblong beams from the edge of the die (Figure 1).
Figure 1. Comparison of major solid-state laser technologies. The beam patterns (right-hand column) are a key differentiator. (Image: Vector Photonics)
Performance comparison
Some key performance differences between VCSELs and PCSELs include:
- To maintain single-mode operation, VCSELs have smaller active areas that limit their maximum output power. The PhC layer in PCSELs can support a larger active area, resulting in higher output power and better single-mode emissions.
- VCSELs are generally less flexible in emission wavelengths compared with PCSELs.
- VCSELs have been in production long enough that their costs have declined. PCSELs, on the other hand, are still an emerging technology, resulting in higher costs.
- In portable devices, VCSELs are better suited for lower-power applications like optical communications, facial recognition, and time-of-flight (ToF) sensors. Anticipated applications for PCSELs include higher-power and higher-speed optical communications, industrial and automotive LIDAR, and biomedical sensors.
Getting under the hood
The vertical structure of VCSELs produces emissions that are normal to the device surface. That simplifies system integration and coupling VCSELs to optical fibers, reducing system costs, and was one of the inspirations for developing PCSELs.
Typical designs of VCSELs and PCSELs include multiple quantum well (MQW) gain sections that use the quantum confinement effect within multiple thin layers of semiconductor material to amplify the light.
The key differentiator is how the two technologies achieve optical confinement. In a VCSES, the distributed Bragg reflectors (DBRs) are replaced with a PhC in PCSELs (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 promise to advance solid-state laser technology in several ways. They can deliver high, coherent power, phase control that enables beam steering, and high-speed modulation, and their straightforward fabrication promises low cost once high production volumes are achieved.
Arrays of PCSELs can support high power levels. The in-plane light is linked between the individual lasers, creating coherence. That coherent light can produce a small spot of high-intensity laser light.
PCSEL arrays can produce several kW of coherent power, which is not possible with VCSELs or other technologies. This makes PCSEL arrays useful for metal cutting, welding, melting, and other industrial processes.
In addition, the individual elements in a PCSEL array can be steered in real time as a phased array. This has potential applications in systems like LIDAR and high-power 3D printing of metals and plastics.
PCSELs are 2.5x faster than VCSELs. That makes them useful for high-speed data transmissions in the multi-Gb range.
The advantage in high-speed communications derives partially from PCSELs’ wavelength flexibility. Their structure enables them to be fabricated from a range of semiconductor materials that can produce different wavelengths.
High-volume VCSEL production is currently possible using gallium arsenide (GaAs) emitting 850 nm light. VCSELS are difficult to produce using indium phosphide (InP). PCSELs are readily fabricated using InP and can produce 1310 and 1550 nm emissions for higher-speed data communication.
Summary
PCSELs are being developed to build on the success of VCSELs with expanded functionality. They replace the 3D DBR structure in a VCSEL with a 2D PhC element. PCSELs can deliver tighter beam patterns, higher power, faster modulation, and support a wider variety of wavelengths compared to VCSELs.
References
A Simple Method to Build High Power PCSEL Array with Isolation Pattern Design, MDPI crystals
Demonstration of high-power photonic-crystal surface-emitting lasers with 1-kHz-class intrinsic linewidths, Optica
Performance Analyses of Photonic-Crystal Surface-Emitting Laser: Toward High-Speed Optical Communication, Nanoscale Research Letters
Photonic crystal lasers: from photonic crystal surface emitting lasers (PCSELs) to hybrid external cavity lasers (HECLs) and topological PhC lasers, Optical Materials Express
The semiconductor laser revolution, Vector Photonics
The Tiny Ultrabright Laser that Can Melt Steel, IEEE Spectrum
What is the difference between laser diodes and VCSELs?, RPMC Lasers
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