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There are many different types of lasers in use today across a wide range of applications. While there are many types of well-known lasers, vertical-cavity surface emitting lasers often fall under the radar. In this article, we look at what vertical-cavity surface emitting lasers are.
Vertical-cavity surface-emitting lasers (VCSELs) have been around since 1979 and are a type of semiconducting laser diode. Since their creation by Bell Laboratories, they have been used to prevent the loss of transmission in optical fibers and have since found use in various optical communication applications, among others.
VCSELs emit a vertical optical beam from their top surface. This way of operating is very different to other laser diodes that emit at their edges/surfaces, because many other laser diodes emit from both the top and side surfaces.
The emittance from only a single surface enables VCSELs to have the maximum output power possible, long-term reliability and stability, a uniform wavelength, an ability to produce modulated frequencies, and a high-quality and symmetrical laser beam, as well as a low fabrication cost. VCSELs are also known to produce a circular beam so are easy to couple with other optical components as no complicated beam-shaping optics are required between components.
VCSELs employ a laser diode and a monolithic resonator so that the light is emitted in the direction perpendicular to the surface laser and/or chip. The resonator, which is also referred to as the cavity (hence the name), is often composed of two semiconducting Bragg mirrors. In between these two Bragg mirrors is another region which is only a few micrometers thick and is known as the active region.
The active region possesses multiple quantum wells and gets pumped with several tens of milliwatts to generate an output power of up to 5 mW (per each diode). The power that is applied to this region is often done so using a ring electrode, in which the output beam is then extracted, and the current is confined to the resonator region.
VCSELs can be fabricated in many ways, but the most common is through being fabricated on to a wafer where these wafers can incorporate thousands of diodes. Another common way is through a 2D dimensional array, in which each array can still have hundreds of diodes. The implementation of multiple arrays is a common way of increasing the output power of VCSELs, but if too many are utilized in a small area, the beam quality can become reduced.
Applications of VCSELs
As previously stated, VCSELs are used throughout a wide range of optical applications. As mentioned, one of the biggest applications of VCSELs is their use in optical communications, and this often takes the form of a transmitter in single optical fibers, multimode fibers, and free-space communications across both short-range and long-range communication lines.
Another big application area of VCSELs is in gas sensing applications. These applications use wavelength-tunable VCSELs—which are often constructed using microelectromechanical systems (MEMS)—and possess a separate output coupling mirror.
This mirror can be tuned using various stimuli such as thermal expansion or electrostatic forces, and relies on the gaseous molecules absorbing the laser at specific wavelengths. Because the determination of a specific gaseous molecules relies on the wavelength intensity, VCSELs can only be used to detect one type of gaseous molecule at one time. Oxygen is the easiest to detect with VCSELs, but water vapor, methane, and carbon dioxide are also possible.
Other applications include being used in computer mice as the light source to facilitate a highly precise tracking position, in optical clocks where the laser beam probes atomic transitions in caesium ions, and as pumps for solid-state lasers.
References and Further Reading
RP Photonics: https://www.rp-photonics.com/vertical_cavity_surface_emitting_lasers.html
Princeton Optics: http://www.newmetals.co.jp/pdf/234.pdf