Lasers used in Free Space Optics (FSO) are typically edge-emitting semiconducting lasers, operating at 785, 980 or 1550 nm. These lasers offer high output power, fast modulation rates, and long-term reliability if cooled.

This diagram shows the structure of a typical edge-emitting laser. The active region has dimensions 200 µm long, 2–10 µm wide, 0.1 µm thick).

Current flows from the p-type to n-type semiconductor, injecting electrons and holes into the active region. The double heterostructure means that the large bandgap semiconductor has a lower refractive index than in the active region, giving index guiding in the transverse direction. Gain guiding confines the emission in the plane of the active region, with the refractive index modified by the carrier density. The current flows through a restricted portion of the active region due to the stripe contact separated by semi-insulated regions of proton bombarded semiconductor. This also aids optical confinement in thee plane of the active region.

Population inversion alone is not sufficient to create a laser- a resonant cavity is required for stimulated emission, allowing light to reflect back and forth to build up gain as the light interacts with electrons in the conduction band, until the gain exceeds losses.

Modern heterostructure lasers use complex cavity designs, including Separate Confinement Heterostructure (SCH) or GRaded-INdex Separate Confinement Heterostructures (GRINSCH) with multiple cladding layers to confine carriers. Quantum Well (QW) and Multiple Quantum Well (MQW) active regions have replaced bulk active regions due to their superior performance. A quantum well forms when the active region’s width is comparable to the De-Broglie wavelength (approx. 100 Å), discretising electron states into a few energy levels. The well width controls the number and spacing of these levels, tailoring allowed energy transitions, primarily from the first conduction band level to the first heavy-hole valence band level. Quantum well lasers offer low threshold currents, linear temperature dependence of intrinsic threshold current, and high reliability, with lifetimes exceeding 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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