What are the differences between QBH and QCH?

In the application of laser fiber, QBH (Quasi-Boundary-Hole) and QCH (Quasi-Cavity-Hole) are two common fiber design structures. They are mainly used in the gain medium and fiber coupling part of the laser to optimize the laser output performance. Although these two fiber designs are functionally similar, they have some significant differences in structure, application and characteristics. 
Structural Differences
The QBH optical fiber design adopts a different refractive index distribution. It typically has a wider core area of the fiber and uses a lower cladding refractive index. This enables the QBH fiber to effectively transmit laser beams while minimizing the loss of the beam within the fiber. The QCH fiber, on the other hand, has a more compact core structure, with a relatively smaller core area and a more abrupt change in refractive index. This structural design can enhance the coupling efficiency of the fiber, especially in the application of high-power laser devices. 
Application fields
QBH optical fibers are widely used in low to medium power lasers, and are particularly suitable for applications that require high coupling efficiency and stable output. Due to its wide fiber core area, QBH optical fibers can handle larger laser beams well. QCH optical fibers are usually used in high-power laser system, especially in industrial cutting and material processing fields, where they can provide higher beam quality and stronger resistance to high power. 
Performance Characteristics
The QBH optical fiber exhibits excellent stability and low mode mismatch in low-power applications, making it suitable for scenarios requiring high beam quality. Due to its wide core and low loss characteristics, QBH optical fibers typically reduce the expansion and scattering of the light beam within the fiber. The QCH optical fiber, with its compact design, can better focus and transmit high-power lasers, reducing the scattering and loss of the light beam, thereby achieving higher output efficiency and energy density. 
Optical fiber coupling efficiency
The QBH optical fiber, due to its relatively wide core, can achieve a better laser coupling effect, and is particularly suitable for applications of medium and low power lasers. In contrast, the QCH optical fiber, with a smaller core, has a higher energy concentration and usually can achieve a higher coupling efficiency. Especially in high-power laser devices, it can effectively reduce the loss of the light beam. 
In conclusion, although QBH and QCH optical fibers can both be used in laser systems, due to their different fiber structure designs, they differ in terms of laser output power, beam quality, and application fields. Choosing the appropriate fiber type requires considering the specific requirements of the laser and the application environment.