Fiber Fusion Splicing Determines The Upper Limit of Performance for High-power Lasers.

In recent years, kilowatt-level lasers have gradually become more widespread. The industry's focus has always been on the output power, beam quality, and light-to-light conversion efficiency of the lasers, but it often overlooks the hidden core process that determines the life and death limit of the laser - optical fiber fusion splicing.

Poor splicing quality can lead to a decrease in the laser's light conversion efficiency and cause transverse mode instability (TMI). In severe cases, it can directly result in the burning and destruction of the optical fiber, and even cause permanent damage to the entire laser platform.

This article discusses the influence of high-power laser splicing performance and explores how to select reliable fiber optic fusion splicer.

1. Why is fiber fusion splicing the "life or death line" for high-power lasers?

Checking fiber fusion involves simply using a high-voltage electric arc to fuse the ends of two optical fibers, then using a highly precise motion mechanism to smoothly push them together, allowing the two fibers to merge into one, achieving the connection of optical fibers and the complete closed transmission of the optical path. For all-fiber structure lasers, the entire optical path is entirely composed of optical fibers and fiber components, and all components need to be connected through fusion. The quality of the fusion directly determines the performance lower limit and life upper limit of the entire laser.

In the traditional low-power communication fiber scenarios, a fusion splicing loss of less than 0.2 dB is sufficient to meet the usage requirements. However, in high-power resonant cavity fiber lasers with power exceeding 3 kW, even a fusion splicing loss of 0.1 dB can lead to fatal thermal accumulation, becoming the trigger for the failure of the laser.

1. Directly lowering the core performance of the laser: abnormal power and efficiency loss

High-power resonator structure fiber lasers have much higher requirements for the quality of fiber fusion than in low-power scenarios and amplifier structures. The fusion of ytterbium-doped dual-clad active and passive fibers inevitably involves certain inherent losses, and poor fusion quality will cause these losses to be exponentially magnified.

According to the actual measurement data, when the splicing is not good, in the high-power condition, the optical-to-optical conversion efficiency of the laser will be 1% to 3% lower, and the surface temperature of the cladding power stripper (CPS) will be 5 to 10 degrees Celsius higher.

In the experiment, for the sample with the poorest fusion quality, the optical-to-optical conversion efficiency dropped by 2.28%, and the CPS temperature soared by 11.2℃. In more severe cases, when the pump power reached 3800W, the signal light power would stagnate, the CPS temperature would rapidly rise, directly triggering the TMI effect, causing the laser to completely lose the ability to output high-power stably. Here, a core industry concept is added: lateral mode instability.

(TMI) is one of the core bottlenecks for high-power fiber lasers to achieve output of over ten thousand watts. In simple terms, as the pump power increases, the fundamental mode of the laser will undergo nonlinear coupling with higher-order modes, resulting in a sharp deterioration of beam quality and significant fluctuations in output power. And the thermal effect caused by the fusion splicing defect is one of the core triggers for TMI.

2. Fatal Risk: The burning of the optical fiber break point and the damage to the platform are more serious than power abnormalities. They are caused by poor fusion quality, resulting in the optical fiber break point and burn. This is also one of the most common failure modes in the production and testing of high-power lasers.

The fusion point is located within the laser resonator, at both ends of the active optical fiber. It is precisely the position within the entire optical path where the energy density is the highest, and the uniformity of refractive index and stress is the poorest. When the fusion quality deteriorates further, excessive local energy will cause the core to heat up sharply, resulting in intense thermal effects, ultimately triggering the core fuse effect, leading to fragmentation of the core layer in the fusion section, the appearance of bullet-shaped cavities, or even direct melting.

From this, we can conclude that a good optical fiber fusion machine should have the following features: 1. Clear imaging, capable of displaying the details of the optical fibers, preparing for a good alignment. 2. The system and the clamping system require high precision. During the process of precise alignment of the optical fibers, they need to remain stable and minimize errors. 3. The structural precision is extremely demanding. The optical fiber fusion machine has numerous components, and if the precision is low, it will cause fatal errors in the overall process of core placement and adjustment. 4. The structure design is reasonable, and it has strong resistance to high temperatures and vibrations. Ensure robustness for long-term use.