Free Space Optics: Laser Technology
Typical lasers used in Free Space Optics (FSO, Laser, Infrared, Optical Wireless) are edge-emitting semiconductor devices, running at 785, 980 or 1550nm. Various output powers and wavelengths are available. Lasers have the advantages of high output powers, high modulation rates, and if cooled, long-term reliability.
This diagram shows the structure of a typical edge-emitting laser. The dimensions of the active region are 200 µm in length, 2-10 µm lateral width and 0.1 µm in transverse dimension. In reality there are many different designs of edge-emitting lasers.
Current flows from the p to the type semiconductor with electrons and holes being injected in the active region. A further advantage of the double heterostructure is that the large bandgap semiconductor has a lower refractive index than that in the active region giving index guiding in the transverse direction. In the plain of the active region the emission is confined by gain guiding, where the refractive index is modified by the carrier density. The formation of a stripe contact separated by a semi-insulating regions of proton bombarded semiconductor allows the current to flow through a restricted portion of the active region. This also aid the optical confinement in the plane of the active region.
Population inversion is not enough to create a laser. In order for stimulated emission to become significant, the light must interact with the electrons in the conduction band. This is achieved by creating a resonant cavity in which the light is reflected back and forth many times before leaving the cavity. If the gain equals loss, lasing will occur.
The design of the cavity structures for modern heterostructure lasers can be much more complicated incorporating more than one set of cladding layers to confine the carriers (Separate Confinement Heterostructure SCH) or GRaded-INdex Separate Confinement Heterostructures (GRINSCH). Quantum well and Multiple Quantum Well (MQW) active regions have superseded bulk active-regions because of the advantages that they offer. A quantum well is formed when the width of the active region of the laser becomes comparable with the De-Broglie wavelength, (approx. 100 Å). In this situation, the electron states are no longer quasi-continuous but become separated until only a few states lie within the well. The width of the well determines the number and separation of the energy levels within the well, thus the allowed energy transitions. Radiative recombination in the quantum well is predominantly from the first energy level in the conduction band to the first energy level in the heavy-hole valence band. Therefore, the separation of the energy levels can be tailored by careful design of the well width. Another advantage of quantum well lasers is that the temperature dependence of the intrinsic threshold current (i.e. only including properties that are intrinsic to the gain medium.) is linear with temperature. Well designed quantum well lasers have low threshold currents and are very reliable with estimated lifetimes of greater than 106 hours.
Wave propagation through a semiconductor cavity
Above: Wave propagation through the semiconductor cavity. An incident wave of amplitude is partially transmitted with ratio t1 and the right hand facet of the cavity the amplitude has attenuated exponentially and this amplitude is transmitted with ratio t2. Subsequent reflections from the ends of the cavity are summed at the right hand facet from which the threshold conditions can be calculated.
The diagram below shows the emission spectra of an edge emitting laser just below threshold. The closely space modes are superimposed on the spontaneous emission profile. As the current is increased to just above threshold one lasing mode becomes dominant.
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