Hi Steemians, today we will present an optical component which is an optical fiber adapter called "ENDCAP".
In high-peak-power lasers, plasma is generated from the peak power at the point where the light goes out of the fiber, and thus it is possible to see damage from the tip of the fiber (plasma-induced damage , ). It is possible to overcome this problem by expanding the surface area which point the light has left. This can be achieved by splicing the output fiber with a coreless fiber ,  which has a larger surface area, called endcap ,  component for optical fibers.
Endcap component is assembled with the output fiber which transmits the signal. This splicing process is similar to general splicing one. The only difference is that it is necessary to make adjustments with a number of different combinations due to differences about NA and core diameters.
After the fiber splicing  process is completed, the cleaving process is done on the coreless fiber. There are two types of cleaving. These are usually 0-angle cleaving, which is usually done while preparing the inserts before splicer machine, while the other one is angle cleaving and it depends on NA . Angle cleaving has many advantages. The most important of these is to prevent any effect that may occur in the tip of the fiber and also system by preventing the beam from being reflected back.
There are a number of problems encountered in the process. These are ability to withstand high optical average and peak power, protection of the optical properties, and isolation from core to core.
The ability to withstand high optical average and peak power:
The most important feature in this material is the diameter of the output fiber (usually at this point is the gain fiber) coupled with the coreless fiber. How small it is, how to refraction angle inside the coreless fiber is larger thus the air damage limit can be higher.
Preserving the optical properties of the signal:
It is about the no change in polarization and M2 quality after the endcap. These two parameters need to be preserved at the end of the endcap.
If the endcap (coreless fiber) is cleaved at zero angle and there is no optical isolator immediately after optical amplifiers, this will both reduce the efficiency of optical amplifiers, and this reflection will also damage parts of the systems. That's why there are lots of problems with the 0 angle cleaving for the fibers. If it is cleaved by a consistent angle (with respect to NA of the output fiber), it can be isolated up to 60dB. If the output fiber is cleaved by an angle 0, a liquid called AR coating must be used and the isolation with this method can be adjusted up to 50dB.
If your fiber optic amplifier  is backward pumping, the size of the endcap is also important. Because in such a system, both the incoming signal and the forward signal must be transmitted to the endcap, and a large diameter must be emitted from the receiver to prevent damage from the signal.
The length of the endcap is very important for the system. The objective is to have the largest surface area where the light goes out. If the optical fiber amplifier is working as backward pumping, this is the second most important parameter that affects the length of the endcap. Figure-1 shows that the adapter lengths are inadequate and there are damages on the tip of the endcap. The coreless fiber can be used in various lengths to make the endcap (Figure-2).
Figure-1: The damages due to inadequate length for endcap.
Figure-2: Various length of endcap (top-750μm, left-50μm, right-400μm)
In the article, the length of the adapter is seen as 740μm maximum. If the length of the adapter is 740μm the output beam diameter of the light becomes from 20μm to 100μm. In these values you get 25 times the area advantage.
In the all-fiber integrated laser system, with 200ps pulse width and 100μJ pulse energy, 650μm length endcap is applied to the amplifier and at the end of the endcap (figure-3), approximately 25GW / cm2 peak power has been reached. As far as we know, this peak is at a record level in all-fiber integrated laser systems.
Figure-3: 650μm length endcap
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