Light sources used for fiber optic transmission need to meet several criteria: they must be the right wavelength, be able to be modulated fast enough to transmit data and be efficiently coupled into fiber. Lasers have proven to be ideal light sources because of their high bandwidth capability and narrow spectral output.
There are 3 main types of lasers in the market today for fiber optic transmitters – VCSELs, FP lasers and DFB lasers.
Types of Lasers
VCSELs: The vertical-cavity surface-emitting laser, or VCSEL is a type of semiconductor laser diode which emits laser beams perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also in-plane lasers) which emit from laser cavities in the middle of a chip.
Transceivers typically use 850nm wavelength VCSELs. VCSELs are cheap to make and are quite adequate for distances of around 500 meters or less.
Because VCSELs emit from the top surface of the chip, they can be tested on-wafer, before they are cleaved into individual devices unlike edge emitters which can be tested only after production. This reduces the fabrication cost of the devices. Additionally, because VCSELs emit the beam perpendicular to the active region of the laser as opposed to parallel as with an edge emitter, tens of thousands of VCSELs can be processed simultaneously on a three-inch gallium arsenide wafer.
VCSELs have the advantage of low power consumption and high coupling efficiency with optical fibers.
Vitex has been providing 850nm VCSELs for our customers. Please contact firstname.lastname@example.org for more information.
FP Lasers: Single spatial mode lasers which can support multiple longitudinal modes are called Fabry-Perot (FP) lasers. In this device, two parallel ends of the semiconductor are cleaved along the crystal axis, creating reflective mirrors forming a Fabry-Perot laser cavity with the semiconductor as the gain medium. The characteristics of such devices are large output power, smaller divergence angle, narrow spectrum and high modulation rate. They are suitable for longer distances than VCSELs.Because FP lasers produce discrete wavelengths, they cannot support transmission over very long distances or transmission over WDM systems.
DFB lasers: DFB lasers incorporate a grating into one of the cladding layers surrounding the active layer of a laser diode. The grating reflects only a specific wavelength back into the cavity and allow others to pass through. It is essentially feeding back the desired wavelength into the cavity. Therefore, the feedback is “distributed,” and thus the name distributed feedback laser. DFBs are more expensive than FP lasers but ideal for long distance transmission.