High efficiency mode field adapters fabricated with CO2 laser splicer

Fiber lasers have attracted great attention in recent decades due to their high efficiency, low cost, compactness, and excellent beam quality. To achieve high power single-mode beam in fiber lasers, mode field adapters (MFAs) are usually required as they can significantly reduce the detrimental nonlinear effects. Traditionally a combination of thermally expanded core (TEC) technique and fiber tapering is used to fabricate MFAs. Due to narrow heating zones or short lifetime of electrodes and filaments, it is not very practical to directly expand the mode field diameter (MFD) of the single-mode fiber (SMF) to match that of the large mode area (LMA) fiber. Thus a combination of TEC and fiber tapering is needed to obtain matched MFDs between the two fibers. In this paper we present a very simple method for fabricating MFAs with CO₂ laser splicer. The wide heating zone provided by CO₂ laser splicer is ideal for TEC process. The MFD of the SMF can be easily expanded to around 20 μm, which is sufficient to match the MFDs of many commonly used LMA fibers. The SMF with TEC is directly spliced to the LMA fiber. Low insertion loss less than 0.2 dB and high beam quality with M² < 1.1 are achieved. The highly reliable and repeatable fabrication process may only take a few minutes, which is vital for volume production.