Structural fiber improves the performance of short pulse lasers
more and more high-precision material processing applications need to use short pulse lasers. These applications include micro blind hole drilling on printed circuit boards and flexible circuit boards, semiconductor memory repair, solar cell edge isolation and film graphics, and sapphire substrate scribing in LED manufacturing.  All these applications are typically characterized by increasing miniaturization and/or continuous pressure to reduce manufacturing costs
miniaturization and reduction of feature size are the main reasons for the use of short pulse lasers. In order to reduce the heat affected zone on the workpiece and the subsequent potential damage to nearby components, a pulse width of less than 80ns is usually required. Micron scale features also favor shorter wavelengths, because shorter wavelengths can achieve smaller focal spot sizes. The absorption characteristics of materials are also a key factor to be considered when determining the laser wavelength
in addition, with the reduction of feature size, more features will appear in a single device or in a unit area, so the laser pulse repetition rate must be increased, otherwise the manufacturing cycle of the device will be prolonged. Since the features are made on the substrate, the above requirements become more urgent when the size of the substrate increases at the same time. For example, in the past 10 years, the minimum feature size of semiconductor memory chips has increased from 150 μ M dropped to 60 μ m。 At the same time, the size of silicon wafer has increased from 200mm to 300mm. As a result, the number of features that can be printed on a single wafer has jumped 14 times. In this example, the reduction of feature size also promotes the use of UV wavelength in the processing process to form a smaller spot size. These developments have driven laser developers to set the fundamental frequency wavelength to 1.0 μ The average output power of about M is increased by 10 times, and the application wavelength is moved to 355nm. In other microelectronic applications, the trend related to the performance and quality of deformation, stamping, cutting and other processing with stainless steel as raw material is also very obvious. The increase of surface area and the reduction of processing time promote the further development of solar cell processing technology
at present, most nanosecond pulse applications are realized by diode pumped solid state (DPSS) lasers. The performance of DPSS laser reflects the continuous innovation of more than 20 years, which is difficult to be matched by other laser technologies. However, there are indications that the development of some application requirements may exceed the actual capacity of DPSS lasers. Smaller spot size requirements and material problems are pushing the pulse width into the picosecond region, but even if the pulse repetition rate increases, the required single pulse energy must be maintained. Creative solutions are emerging, such as "double beam" technology, which achieves twice the pulse repetition rate by multiplexing the laser output from two pulse light sources. Another "hybrid" method is to use a low-power, high pulse repetition rate fiber laser to provide light source for DPSS amplifier by separating the two functions of pulse generation and power amplification. Although these solutions are used, they do increase the cost and complexity, and will be limited when they develop to a higher output
in all solutions, fiber laser is an ideal next-generation light source that can meet the requirements of current and developing short pulse applications. The main target specifications are listed in Table 1. Fiber lasers have high one-way gain, which simplifies the design of amplifiers and directly improves the average power, making them attractive for short pulse applications. In these applications, peak power operation is required to achieve the required pulse energy and pulse width, and to achieve high peak power operation, it is necessary to increase the fiber core diameter, which is the difficulty. If the core diameter is not increased, the nonlinear optical effect will cause spectral broadening and output power instability. Currently 20 μ The commercial fiber laser of double clad fiber (DCF) with a core diameter of M can provide a peak power of up to 25kW in 10ns pulses and produce an average power of 25W at a working frequency of 100kHz. This is only a quarter of the target average power in Table 1, and it is also half of the average power that the DPSS laser can achieve at present. One solution that is expected to further improve power includes a unique structure of optical fiber called chiral coupling core diameter or 3C fiber.  Table 1: target index of fiber laser
the core diameter of this 3C fiber is much larger than the traditional double clad, large mode field area fiber, and can achieve single-mode output. The chiral coupling core diameter fiber is composed of a central guide core and at least one spiral satellite core surrounding the central core (see Figure 1). This structural design can selectively couple the high-order optical modes in the central core to the satellite core, and only LP01 mode is transmitted in the central core. Appropriate satellite core parameters and helical period lead to the light mode coupled into the satellite core being scattered into the cladding, so the loss is very high. This concept can be applied to the design of very large core diameter optical fibers (see Figure 2)
Figure 1: chiral coupling core diameter fiber uses a central guide core and at least one spiral satellite core around the central core. The illustration shows the end face of the optical fiber
Figure 2: the calculated core diameter is 35 μ m. Mode loss of a specific 3C fiber with a helical period of 9mm. Among them, the loss of LP01 basic mode is less than 0.2db/m, while the loss of higher-order mode is more than 100dB/m. 
3C optical fibers can be prepared directly, and the preparation process is basically different from the standard DCF. The standard DCF is drawn from a glass preform with a properly doped central core. The size of preform and fiber core shall be matched in proportion in advance, so that when heating and drawing on the optical fiber drawing tower, it will be reduced to the required optical fiber size. The preform of 3C optical fiber includes two doped cores. One core is on the central axis of the preform, and the other satellite core is slightly off the central axis. Next, when the fiber is stretched, it rotates at the same time. This rotation makes the satellite core that deviates from the central axis spiral around the central core, producing the required spiral (see Figure 3)
Figure 3: when 3C optical fiber is drawn, the preform is rotated at the same time, so that the off-axis satellite core spirally surrounds the central core, producing the required spiralAn important attribute of
3c fiber is that its performance does not depend on specific curvature, which is contrary to the standard large mode area fiber. The large mode field area optical fiber is carefully wound to obtain single-mode performance by taking advantage of the difference in loss between the basic mode and the higher-order mode caused by bending. This method has a core diameter of less than 25 μ M fiber is effective. The larger the core diameter, the less effective this method is.  This technology is also problematic for beam transmission and use in optical fiber components. Since mode discrimination does not depend on the curvature of the optical fiber, 3C optical fiber can be used in active or passive optical fiber structures in a straight or curved form
core diameter is 35 μ m. Two kinds of chiral coupling core diameter fibers with ytterbium doped and undoped ytterbium (yb3+) core layer can be used as gain fibers and used in passive fiber component structures. The laboratory test results for the performance of optical fiber in MOPA (master oscillator power amplifier) structure show that the average power generated by it exceeds 100W, the pulse width is 10ns, and the peak power at 100kHz pulse repetition frequency reaches 100kW (see Figure 4). 
Figure 4: the slope efficiency (70%) and beam quality of 3C fiber are measured. The M2 factor of fiber output reaches 1.07
the vast majority of applications of short pulse lasers require visible light and ultraviolet light. It is very important to recognize this, so a suitable fiber laser light source must have a stable polarization output. The polarized light output from optical fibers is usually produced by strong birefringence caused by directional material stress. Polarization output can be realized through the stress bar in the optical fiber, and it is suitable for optical fiber core diameter less than 10 μ M. When the fiber core diameter increases, it becomes more difficult to generate uniform stress in the larger section of the fiber core, which means that it is difficult to obtain high polarization contrast. The resulting polarization performance is very sensitive to thermal and mechanical disturbances, which will cause output instability
in contrast, the design of 3C fiber uses the production process and fiber structure to obtain low birefringence fiber. These low birefringence fibers can maintain the polarization state of the input light very stably (see Figure 5)
Figure 5: incident linearly polarized light into a 4-meter-long ring 3C fiber, heat the fiber from 20 ℃ to 70 ℃, and monitor the polarization state of the output light. The results show that the polarization axis does not rotate, and the polarization extinction ratio remains above 20dB. Under significant mechanical and thermal disturbances, its polarization maintaining performance is still very outstanding
the unremitting pursuit of reducing the size of components and manufacturing costs will continue to promote the demand for short pulse lasers with higher performance. As one of the latest innovations, 3C optical fiber has higher performance, which can meet people's demand for miniaturization and low cost. The performance potential of single-mode fiber with larger core diameter is expected to make it not only applicable to material processing. Three hot scientific research applications using 3C optical fibers have been carried out: directed energy weapons, laser plasma extreme ultraviolet lithography and ultrafast spectroscopy
in the application of directional energy, a larger fiber core diameter is needed to obtain the required continuous wave power while maintaining the narrow spectral linewidth of a single polarization state. Fiber laser has high electro-optical efficiency and small size, which can realize more reliable product assembly. It is an ideal choice for directional energy applications. Extreme ultraviolet lithography is moving forward to mass production of semiconductors that rely on large CO2 and pulsed laser light sources. The research based on large core diameter single-mode fiber shows that by superimposing high-power pulsed fiber laser light source on the spectrum, it is possible to build a more efficient, compact and scalable laser light source.  Finally, large core diameter single-mode fiber is a key factor to provide a small and durable light source for practical ultrafast spectral systems
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