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Vertical-cavity surface-emitting laser (VCSELs) are a type of semiconducting laser diode used across many optical communications applications. They have been around since 1979, where they were created by Bell Laboratories to help combat the transmission loss in optical fibres. Since then, these lasers have been continuously developing, and in this article, we look at some of the advances of VCSELs within the first five months of 2018.
What Are Vertical-Cavity Surface Emitting Lasers?
A vertical-cavity surface-emitting laser (VCSEL) is a type of semiconducting laser diode that emits a vertical optical beam from its top surface. By contrast, many other edge emitting laser (EEL) diodes emit from surfaces which have been cleaved, and light emitting diodes (LEDs) emit from both the top surface and the sides.
VCSELs use a monolithic laser to emit light perpendicular to the surface of the laser/chip. VCSELs also possess a cavity/resonator that is formed by two semiconductor Bragg mirrors. Located between the two regions are several quantum wells which act as the active region and generate an output power.
VCSELSs can be used in many ways, with the most common being fabricated onto a wafer (where tens of thousands of diodes can be implemented onto a 3” chip), or as a 2-dimensional (2D) array where a single die can have hundreds of individual light sources. VCSELs are useful materials for precision sensing applications and high-speed communications, such as in high-speed optical local-area networks (LANs) and wide-area networks (WANs).
They are often used across multiple applications because they can increase the maximum output power, increase long-term reliability, modulate frequencies above 25 Gbps, whilst providing a controlled and predictable yield alongside low fabrication costs (compared many other laser technologies).
Recent Developments in Vertical-Cavity Surface Emitting Lasers
2018 has seen many publications, and even patents, being published. Below we look at some examples of VCSELs research published so far this year.
Patents
There have been a few VCSELs published by the US Patent and Trademark Office (USPTO) in 2018 so far, although their priority dates date back as far as 2013. A priority date is the date when the patent was first applied for in the patentee’s original application – this is usually in the patentee’s home country. So, the patents are now technically a few years old, but have only just come to light to the public eye.
The first of these patents were going to look at, with patent number US20180054041A1, is a laser system with distributed Bragg reflectors, two mirrors, and an active region between the two. The key defining features of this patent (as set out in the first claim) is that the Bragg reflectors are formed from a multitude of lasers, the laser emission comes from at least one gallium arsenide antimonide nanostructure in the active region, where each of these nanostructures contain more antinomy atoms than arsenic atoms and are comprised of a quantum ring located within a quantum well.
The second patent, with patent number US20180048121A1, is a patent that has only been applied for this year in the US, but has priority in many other countries dating back to 2013. This patent describes a microelectromechanical system (MEMS) tunable vertical cavity surface-emitting laser (VCSEL), which is composed of an air gap, an anti-reflective (AR) coating, and one or more layers of quantum dots situated between two Bragg reflectors.
Publications
There are too many to document already in 2018, however, here is a selection of some of the VCSEL research that has already been published this year.
Haidar et al have developed a single-mode MEMS-tunable VCSEL with an electrothermal tuning range of 64 nm (7.92 THz). The laser possesses a tuning range that is greater than commercially available distributed-feedback laser (DFB) diodes by a factor of 13.
Forman et al have developed an optically polarized m-plane GaN-based VCSEL that uses an ion implanted current aperture, two quantum wells, a tunnel junction intracavity contact and a dual dielectric distributed Bragg reflector. The researchers have achieved continuous-wave operations for over twenty minutes and have found that the VCSEL has a high thermal dependence.
Forman et al have also produced another paper showcasing continuous wave operations in nonpolar GaN-based VCSELs. The basic setup is very similar to their other research, but these VCSELs have an anisotropic gain with a 100% polarization ratio and polarization-locked VCSEL arrays. In a similar theme to their other research, the team was able to showcase a continuous wave operation for longer than twenty minutes.
Ledentsov et al have developed an anti–waveguiding VCSEL which can operate in both longitudinal and transverse optical modes. These VCSELs contain oxygen confined cavities which has enabled the laser to produce error-free transmissions over 2.2 km; with a speed of 54 Gb/s using a digital–multitone (DMT) format and 160 Gb/s using a non–return–to–zero (NRZ) format.
Sources:
- Finisar
- RP Photonics: https://www.rp-photonics.com/vertical_cavity_surface_emitting_lasers.html
- “Vertical-Cavity Surface-Emitting Laser: Its Conception and Evolution”- Iga K., Japanes Journal of Applied Physics, 2008, DOI: 10.1143/JJAP.47.1
- US Patent: Hayne M. and Hodgson P. D., US20180054041A1, Vertical-Cavity Surface Emitting Laser
- US Patent: Makino T., Li T and Eu D., US20180048121A1, Wavelength-tunable vertical cavity surface emitting laser for swept source optical coherence tomography system
- “Systematic characterization of a 1550 nm microelectromechanical (MEMS)-tunable vertical-cavity surface-emitting laser (VCSEL) with 7.92 THz tuning range for terahertz photomixing systems”- Haidar M. T., et al, Journal of Applied Physics, 2018, DOI: 10.1063/1.5003147
- “Continuous-wave operation of m-plane GaN-based vertical-cavity surface-emitting lasers with a tunnel junction intracavity contact”- Forman C. A., Applied Physics Letters, 2018, DOI: 10.1063/1.5007746
- “Continuous-wave operation of nonpolar GaN-based vertical-cavity surface-emitting lasers”- Forman C. A., et al, Proceedings Volume 10532, Gallium Nitride Materials and Devices XIII, 2018, DOI: 10.1117/12.2314885
- “Anti–waveguiding vertical–cavity surface– emitting laser at 850 nm: From concept to advances in high–speed data transmission”- Ledentsov N. N., et al, Optics Express, 2018, DOI: 10.1364/OE.26.000445
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