Laser diodes are ubiquitous today. They are the simplest elements that convert electrical energy into laser power. Laser diodes are based on several semiconductor assembly materials (gallium arsenide, INP, or other more complex structures such as gallium nitride). Single-mode laser diodes are low-power laser diodes (typically <1W);Multimode laser diodes are much higher power devices (typically 10 W to several kilowatts).
Fiber type: It is important to know that there are two types of active fibers, which are commonly used to couple light coming in from laser diodes
a.Single-mode fibers typically have a core of a few meters (e.g., a core of 1 m is about 6 m, while a wavelength of 15 m of about 9 m, about 6 times).
b.Multimode fibers are fibers with a larger diameter that can handle higher levels of optical power. The core diameter of the standard version is usually up to >1000 m. The smaller the diameter, the easier it is to focus the light from the fiber onto a small spot with a lens or microscope objective.
Figure 1: Principles of single-mode or multimode fiber. The version of the multimode fiber has a larger core. Polarization-maintaining fibers: Single-mode laser diodes can be standard (SMF) or polarization-maintaining (PM) versions. In the polarization-maintaining version, the fiber has a special cladding structure that maintains the polarization of the light over the entire length of the fiber. Therefore, polarization-maintaining fibers are generally used in applications that are sensitive to polarization states, such as interferometers, lasers, or connections between light sources and external modulators. As the wavelength decreases, the core diameter becomes very small. When taking into account its cut-off wavelength and this cut-off wavelength 1At wavelengths between 5 times, single-mode fibers work well. Below this range, the fiber becomes multimode fiber, and above this range, light can easily leave the fiber when the fiber is bent.
1.Single-mode fiber-coupled laser diodes
1.1 Package Type:This type of laser diode is commonly used in two packages, a "butterfly" package that integrates a TEC temperature-controlled cooler and a thermistor. Single-mode fiber-coupled laser diodes are typically capable of reaching several hundred MW to 15 W of output power.
Figure 2: Example of a single-mode fiber-coupled butterfly-packaged laser diode with an emission wavelength of 976nm (10-pin mini-butterfly (left) and 14-pin standard butterfly (right)). These laser diode modules include a TEC temperature-controlled cooler, a thermistor, and a backside monitor (BFM) to measure optical power levels. One is the "coaxial" package, which is commonly used in laser tubes that do not require TEC temperature control, and the coaxial package is also available with TEC. 
Figure 3: Coaxial laser tube package1.2 Laser Tube Type:Class 3 laser diodes commonly found on the market. VCSEL laser diodes are generally not fiber-coupled. They are common types of laser diodes in large-scale diffusion sensing applications, such as computer mouse devices or smartphone 3D sensing facial recognition. DFB and FP are edge emitters, usually fiber-coupled. 
Figure 4: Class 3 laser diode emitters commonly used in the industry.
a.FP (Fabry-Perot) Fabry Pero laser diodeFP lasers are the most common and common semiconductor lasers, which are semiconductor light-emitting devices that use FP cavity as a resonant cavity to emit multi-longitudinal mode coherent light. The process is very mature and widely used. However, the spectral characteristics of FP are not good, and there are problems of multiple edge modes and dispersion. Therefore, it can only be used for low-to-medium rates (below 1-2G) and short-range applications (less than 20 km).
In order to reduce the emission bandwidth and improve the overall stability of the laser diode, laser diode manufacturers often add fiber Bragg gratings within the output fiber. The Bragg grating adds a few percent of the reflectance to the laser diode at a very precise wavelength. This results in an overall reduction in the emission bandwidth of the laser diode. The emission bandwidth is typically 3-5 nm in the absence of a Bragg grating, and much narrower (<0.) in the presence of a Bragg grating1nm）。In the absence of a Bragg grating, the wavelength spectral temperature tuning coefficient is typically 035 nm °C, which is much smaller in the presence of a Bragg grating.
Figure 5: Principle of adding a Bragg grating to an optical fiberb. dfb（distributed feedbackDistributed feedback lasersLaser tubes.
DBR (Distributed Bragg Reflector).Laser.
The DFB DBR laser diode device integrates the wavelength stabilization part of the Bragg grating directly into the gain medium inside the laser diode, and forms a mode selection structure in the resonator, which can achieve complete single-mode operation. This provides DFB with a narrower emission wavelength, typically 1MHz (i.e., 10-5nm) instead of about 01nm。Therefore, the spectral characteristics are very good, which can avoid the influence of dispersion in length transmission, and is widely used in long-distance and high-speed applications. The wavelength spectral temperature tuning coefficient is typically 006 nm/°c。
Figure 6: DFB and dbr laser diode principles.
Comparison of the performance of FP laser tube and DFB laser tube2.Multimode fiber-coupled laser diodes
2.1 4 multimode fiber-coupled laser diode seriesThe multimode fiber-coupled laser diode is based on the wide-area side, and the laser diode chip it emits was originally designed and fabricated by semiconductor wafers. There are 4 types of multimode fiber-coupled laser diodes: Single emitter: When the laser diode chip is isolated, it is assembled on a sub-base and individually packaged in a laser diode module. What we are talking about here is 15W of power coupled into a 105 (core) 125 m (cladding) laser diode. 
Figure 7: Single-element laser diode.
Multi-emitter: When multiple emitters are separated and optically coupled with other isolated emitters in a multimode fiber. As a result, output power levels can be scaled to hundreds of watts, and the size of the fiber can be kept small, such as 100 or 200 m cores.
Figure 8: Multi (3) element laser diode single bar: When multiple emitters act as a single bar and are assembled in a single laser diode module. What we are talking about here is about 50 W of power coupled into a 200 m (core) and 240 m (cladding) laser diodes.
Figure 9: Individual elements assembled on a base (left) or a single bar made of 19 emitters.
Dobar: When multiple bars are assembled in a large water-cooled package and coupled in a large-diameter multimode fiber. We're talking here about 100 W or even kW coupled to, for example, 600 or 800 m core multimode fibers.
2.2 It is worth noting how typical voltage and current levels vary when various series are taken into account:A typical single transmitter has a voltage level of 15V at 15A. For multi-emitter laser diodes, the emitters are assembled in series. This means that the current level does not change (usually up to 15 A), but the voltage increases as the number of transmitters increases. (For example, 4.)5 W laser diode at 15V) A laser diode bar assembles all emitters in parallel. As a result, the voltage level remains the same, but the current level can easily reach 45 or 50 A. Similarly, when multiple bars are assembled together, they are assembled in series, so the current level (e.g. 45 A) does not change, but the voltage increases regularly as the number of bars increases.