The semiconductor laser revolution
PCSELs (Photonic Crystal Surface-Emitting Lasers) are a new type of semiconductor laser that offers a unique combination of advantages: high power, a wide range of wavelengths, and lower manufacturing costs. Because they are made using similar processes as existing lasers, and are easily integrated into electronic devices, they have the potential to improve a wide variety of domestic and industrial applications.
Vector Photonics is at the forefront of the new and developing PCSEL technology. The company’s initial focus is on optical communications, where PCSELs can transmit data through high power, low divergence, surface-emitting lasers. However, the company is also looking at plastic and metal printing applications with LiDAR, mobile consumer and sensing applications close behind.
The performance benefits of PCSELs
The photonic crystal used in PCSELs scatters light linearly, in plane, and orthogonally, out of plane. The out of plane, surface emission offers huge cost and performance advantages for lasers. It makes them easy to manufacture, test, package and incorporate into electronic assemblies. This gives rise to some critical performance features of PCSELs, which are:
- Coherence and coherent arrays
- Solid-state array, beam steering
- Wavelength flexibility
- High speed and data rates
The laser technology evolution.
Fabry Perot lasers are the original, semiconductor laser technology. The laser feedback and emission are both in-plane, so light comes out of the end of the laser and the gain reflection is produced by facet mirrors. DFB lasers also have in-plane feedback and emission, but this time the gain reflection is produced with a grating structure.
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.
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.
EELs compromise
cost and ease of assembly
EEL Lasers (Edge Emitting Lasers) have been in production for more than 40 years, where they have proven their reliability and longevity in telecoms and data systems.
FP (Fabry-Perot) and DFB (Distributed Feedback) lasers are both types of legacy, semiconductor EELs. 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 fibres 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.
VCSELs compromise
wavelength range and power
VCSELs (Vertical Cavity Surface Emitting Lasers) were first produced in the early 1990’s. They have limited operational wavelengths due to the manufacturing challenges caused by the various material systems required for multi-wavelength operation.
A VCSEL is produced by having two Bragg stacks above and below the active region of the laser. The Bragg stack comprises layers of two materials of differing refractive index ‘grown’ on top of one another. The number of layers, and therefore periods and the refractive index contrast, gives rise to the reflectivity. A high number of periods gives high reflectivity, and high index contrast increases reflectivity per layer.
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.
The performance benefits of PCSELs in more detail
Coherence and coherent arrays
The in-plane feedback in PCSELs is two-dimensional and facet free, enabling coherently coupled arrays. The laser elements of the array are joined by a coupler region. This means the in-plane light is linked between laser elements, creating coherence. Using lenses, coherent light can be focused down to a small spot with greater light density, which has advantages in cutting, welding, melting, engraving and drying applications.
Due to the laser elements of the array being coupled in plane, in an ‘n x n’ array, power scaling is also possible. High brightness and the unique geometry enable kilowatts of coherent power, which is simply not achievable with any of the other laser technology.
Solid-state array, beam steering
PCSELs can have an array structure where the coupler region can be controlled in what is called an ‘optical phased array’. By electronically tuning the phase of the coupler region, the laser beam can be steered, in real time, without moving parts. This makes PCSELs applicable to rapidly evolving LiDAR applications, where the beam is steered for imaging. High power, 3D printing in metal and plastic are other applications that could be revolutionised. As a solid-state solution, system size is reduced by 10x, whilst reliability is increased over existing mechanical solutions.
Why 1310nm and 1550nm wavelengths are critical in fibre optics.
Single-mode, fibre optic cables have two wavelength windows which offer high performance for different applications. These occur at 1310nm, the optimal datacoms transmission wavelength and at 1550nm, the optimal telecoms transmission wavelength.
At 1310nm, dispersion in a single-mode, fibre-optic cable is at its minimum. This means a pulse of light transmitted through the fibre will arrive at its destination at mostly the same time and relatively intact. This is important because this intrinsically low dispersion transmission puts less demand on a semiconductor laser for coherence. The downside is that there is greater attenuation, or power loss, in the cable at this wavelength, hence 1310nm is used in datacoms applications where distances are shorter.
At 1550nm, attenuation, or loss, in the fibre optic cable is at its minimum. This makes it the ideal wavelength for long-distance transmission, such as telecoms, where distances of 200km or more are achieved before reamplification. The downside is that there is greater dispersion, meaning a compound semiconductor laser with greater coherence is needed for good system receiver performance.
PCSELs offer a serious breakthrough for all communications applications as they can provide more power and coherence than any other existing laser devices. This means they can be used to increase existing transmission distances, which is especially important in datacoms at 1310nm, and they improve coherence, which is especially important for telecoms at 1550nm.
