What is a broad contact semiconductor laser

What is a broad contact semiconductor laser
Lasers are used in almost all distributed fiber optic sensing systems (DOFS) because they can efficiently inject high-power lasers into fibers. However, there are various types of lasers, and here the main types are introduced in order of increasing coherence length, i.e. decreasing spectral width. Generally speaking, a laser is composed of a gain medium that amplifies the light propagating through it and provides feedback to send the amplified light back into the gain medium for gain cycling. Therefore, starting from spontaneous emission within the gain medium, stronger and more coherent light waves will be generated, whose characteristics depend on both the gain medium and the feedback properties. Usually, feedback provides a resonant structure that forms one or more modes, each emitting within a narrow spectral range.
The broad contact semiconductor laser, the simplest and earliest semiconductor laser, consists of a semiconductor gain region formed by a p-n junction of a direct bandgap material (such as GaAs), with feedback provided by reflection from the laser chip’s cleaned facets; The length of laser chips is usually several hundred micrometers. This connection structure is designed to simultaneously constrain charge carriers and light propagating within the gain region. As diodes, these devices have electrical contacts on both sides. In a broad contact device, the top electrode width is several tens of micrometers (typically around 75 μ m); Therefore, the light emission forms a line along this width, which is single-mode in the plane perpendicular to the junction, and presents a complex multi-mode light field distribution in the plane parallel to the junction. Due to this emission mode, wide contact lasers can only be efficiently coupled into multimode fibers. However, the power of the broad contact laser is relatively high, and under pulsed operating conditions, the 75 μ m electrode device can typically output peak power exceeding 10 W.
Broad contact laser diodes are suitable for certain types of Raman multimode distributed temperature sensing systems, as these systems do not require high spectral purity. They are also applicable to loss based sensors, such as those systems that utilize controlled micro bending as a sensing mechanism.