Photonic Crystal Surface Emitting Lasers (PCSELs) are a new class of semiconductor lasers. They utilize a 2D grating structure that scatters light linearly and orthogonally. This makes PCSELs the only laser where feedback is in-plane and light emission is out of plane, emanating from the laser’s top surface. Out of plane, orthogonal, surface emission offers a huge cost advantage for lasers, as it makes them easy to package and incorporate into PCBs and electronic assemblies. The PCSEL structure provides advantages in data rate, wavelength, and power performance when compared to equivalent EELs (Edge Emitting Lasers) or VCSELs (Vertical Cavity Surface Emitting Lasers).
Vector Photonics, a spin-out company from the University of Glasgow has successfully produced 1300nm, all-semiconductor PCSELs (Photonic Crystal Surface Emitting Lasers) targeting 800Gb, datacoms applications. The PCSEL arrays emit four, concurrent, 1300nm wavelengths, with a SMSR (Side-Mode-Suppression Ratio) measurement of around 35dB.
Dr. Richard Taylor, CTO of Vector Photonics, said, “Vector Photonics’ 1300nm, compound semiconductor lasers enable new levels of PCSEL commercialisation for datacoms applications. Unlike incumbent DFB and EEL data centre laser technologies, PCSELs deliver both the high speed and high power required, in the same device. PCSELs are surface emitting, with light coming from the top of the laser not the side, which vastly reduces packaging costs. We now have four, 1300nm wavelengths emitting from a monolithic PCSEL array, rather than four separate chips, simplifying post-production assembly and significantly reducing packaging costs.”
VCSEL technology has out-of-plane gain and emission, where the light emits from the top surface of the laser. This makes both the test and packaging of the lasers much cheaper than EELs. VCSELs compromise power for low cost and high speed. They have limited operational wavelengths due to the manufacturing challenges caused by the various material systems required for multi-wavelength operation. The VCSEL grating structure also has inherent limitations to the single-mode, power levels that can be produced. So, although VCSELs can achieve high speeds and can be produced cost-effectively, their limited single-mode performance makes them unsuitable for high-speed datacoms and long-distance telecoms. These limitations also restrict their use in sensing applications to relatively short distances.
EELs including FP (Fabry-Perot) and DFB (Distributed Feedback) lasers offer speed and power but are expensive to make. EELs offer high levels of single-mode performance, both from optical spectrum range and power perspectives. However, EELs have two significant disadvantages. The first disadvantage is that they must be precisely aligned and handled to be integrated into systems. This is because single-mode light is emitted from the edge, not the front, meaning the lasers must be precisely aligned within subassemblies to re-direct the light into the correct direction for optical fibers or free space. The second disadvantage is that they require complex manufacturing and testing processes. The semiconductor wafers must be split into bars and finished on each side with reflective coatings. Each laser must be tested at bar level before being "singulated" into individual laser devices for system integration. These multiple processing and testing steps increase cost and reduce yield.
PCSELs have the following advantages over VCSELs:
- They do not use a Bragg stack structure, like a VCSEL (or indeed a Facet coating like DFBs), they instead have a 2D grating which is easy to scale.
- They have low contact resistance, ensuring the highest power output of any laser, size for size.
- They can be made in any material, so are suitable for a wide range of wavelengths and applications.
- Their power is single mode, which will scale up proportionally with area, easily enabled by their 2D construction and in-plane feedback.
- They have no Oxide aperture, as used by VCSELs, so do not require specialist oxidation furnaces in manufacture.
The PCSEL is the only laser using in-plane feedback and out-of-plane surface emission. Test and packaging remain cheaper, like VCSELs, but the PCSEL structure provides advantages in data rate, wavelength, and power performance when compared to equivalent-sized EELs or VCSELs. They emit light from the top surface, like VCSELs, making them easy to package and incorporate into PCBs and electronic assemblies. They are also made in a similar way to EELs, so existing, experienced, supply chain capability and capacity can be utilized in their production. Vector Photonics’ PCSELs can be made at any wavelength, so can be used to address a wide breadth of applications.
Click here to watch Richard Taylor’s Photonics West video explaining the commercial journey of PCSELs and their pathway to high power.