Edge Emitting Laser(FP&DFB)

All laser diodes are based on light-emitting diodes (LEDs). As with the photodiode, the underlying structure is a p-n junction, the only difference is the direction of the applied voltage or bias. The p-n junctions consist of a semiconductor layer (silicon in most photodiodes) doped with atoms carrying extra valence electrons (n-type semiconductors) under a layer doped with atoms carrying one valence electron less than silicon (p-type semiconductor). Charge migration creates a depletion region with an electric field directed toward the p region, allowing current to flow in only one direction. In a photodiode, a reverse bias potential is applied across the diode, preventing current from flowing in the absence of light. With exposure to light, electron-hole pairs are created, generating a current. In a laser diode or a LED, the process is exactly reverse. A positive bias is applied, causing current to flow. As the electrons form the n-type semiconductor flow into the p-type semiconductor, they combine with the holes, releasing energy in the form of light. Diode lasers differ in that light is generated by stimulated rather than spontaneous emission, with the result that the generation efficiency is much higher. A second difference between diode lasers and LEDs is that lasers require a higher current. With higher current, the excited states become more populated than the relaxed states-the condition known as a population inversion. Laser amplification now occurs because each photon on average produces more than one stimulated photon before leaving the laser or being absorbed. Once this laser amplification occurs, the quantum efficiency in converting additional electrical energy into light jumps to values much higher than those for LEDs.

Vertical cavity surface emitting laser (VCSEL)

A vertical cavity surface emitting laser (VCSEL) is a specialized laser diode that promises to revolutionize fiber optic communications by improving efficiency and increasing data speed. The acronym VCSEL is pronounced 'vixel.' Older laser diodes, called edge-emitting diodes, emit coherent light or infrared (IR) energy parallel to the boundaries between the semiconductor layers. The VCSEL emits its coherent energy perpendicular to the boundaries between the layers. The vertical in VCSEL arises from the fact that laser diodes are typically diagrammed showing the boundaries as horizontal planes, so the output of the VCSEL appears to emerge vertically in these drawings. VCSELs have been constructed that emit energy at 850 nanometers (nm) and 1300 nm. These wavelengths correspond to energy in the near infrared (IR) portion of the electromagnetic spectrum. (The longest visible red is at approximately 770 nm.) Optical fibers transmit energy most efficiently at wavelengths around 1550 nm. Materials used to manufacture VCSELs include gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), and indium gallium arsenide nitride (InGaAsN). The VCSEL has several advantages over edge-emitting diodes. The VCSEL is cheaper to manufacture in quantity, is easier to test, and is more efficient. In addition, the VCSEL requires less electrical current to produce a given coherent energy output. The VCSEL emits a narrow, more nearly circular beam than traditional edge emitters; this makes it easier to get the energy from the device into an optical fiber. The main challenge facing engineers today is the development of a high-power VCSEL device with an emission wavelength of 1550 nm. VCSELs have high performance and low cost advantages.  The key features are: The structure can be integrated in two-dimensional array configuration.   Low threshold currents enable high-density arrays.   Surface-normal emission and nearly identical to the photo detector geometry give easy alignment and packaging.   Circular and low divergence output beams eliminate the need for corrective optics.   Passive versus active fiber alignment, combined with high fiber-coupling efficiency.   Low-cost potential because the devices are completed and tested at the wafer level.   Lower temperature-sensitivity compared to edge-emitting laser diodes.   High transmission speed with low power consumption. VCSELs are promising emitter for fiber data communication at speeds higher than 1Gbs. They enable high performance systems in Gigabit Ethernet, Fiber Channel, and ATM markets. Through their integration with original equipment manufacturer's (OEM) systems design, the 850nm VCSELs provide enhanced performance benefits to a variety of applications, such as local area networks(LAN), telecommunication switches, optical storage and other optoelectronic systems.