Fabry–Perot Laser Diodes
A Fabry–Pérot laser diode (FP laser diode) is the most common type of laser diode, having a laser resonator which is a Fabry–Pérot interferometer. This means that substantial light reflections occur at both ends, but not within the gain medium. In contrast to that, the resonator of a distributed Bragg reflector laser exhibits a distributed reflection throughout the gain medium, usually created by a grating structure.
In the simplest case, the end reflections in an Fabry–Pérot laser are Fresnel reflections at the interface between the semiconductor device structure and air. Note that the refractive index contrast at these locations is quite high, leading to a substantial reflectivity without any additional measures. If that principle is utilized on both sides, the threshold pump power may already be low enough, and about half of the optical output power is obtained at each side.
In order to obtain the total output power on one side, which is usually preferable, or for optimizing the output power via a lower threshold pump power, one often increases the reflectivity on the side opposite to the output side, e.g. with a semiconductor Bragg mirror structure. In the specific case of a distributed Bragg reflector laser (DBR lasers), increased reflectivities are obtained with Bragg mirrors, or with a Bragg mirror on only one side.
Mode Structure, Beam Quality and Optical Spectrum
A Fabry–Pérot laser diode may emit only on fundamental spatial resonator modes, which leads to a relatively high beam quality, or also on higher-order spatial modes, resulting in a poorer beam quality. The most common example for the latter case is a broad area laser diode. Generally, higher output power (multiple watts) can be achieved when allowing for spatially multimode emission, usually in an active area with increased dimensions.
Even if lasing is restricted to fundamental spatial modes, it may occur on more than one longitudinal mode. Despite the relatively short length of the laser resonator, the free spectral range of the resonator may be small enough compared with the gain bandwidth to allow for that phenomenon. As a result, the laser output spectrum exhibits multiple optical frequencies. (These typically have a spacing of the order of 100 GHz.) It may also happen that emission occurs on a single mode at a time, but temperature changes lead to occasional mode hops to neighbored resonator modes, or also to occasional oscillation on two modes.
For realizing a narrow linewidth laser, one needs to achieve single-mode operation, e.g. by restricting the drive current or the resonator length. (The output can then still contain some weak sub-threshold fluorescence on several other modes.) Another frequently used technique is not to use a Fabry–Pérot design, but instead a distributed Bragg reflector (DFB) design.
Due to the short resonator length, the substantial round-trip losses of the resonator and the moderate intracavity power, the laser linewidth is often substantial (multiple megahertz) even in case of stable single-mode operation. The linewidth may be substantially reduced by coupling to or integration into an external optical resonator. This leads to the concept of an external-cavity diode laser.
Emission Wavelengths of FP Laser Diodes
Fabry–Pérot laser diodes are available with a very wide range of emission wavelengths from the visible region to the mid and far infrared. For the longest output wavelengths, they can be realized as quantum cascade lasers.
There are also semiconductor optical amplifiers which are realized as Fabry–Pérot amplifier. Here, relatively weak and reflections are utilized such that the device stays below the laser threshold. Still, those reflections can substantially increase the amplifier gain.
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