Mode field diameter conversion FA provides a method of low loss coupling to a waveguide with a small mode field. A small section of ultra-high numerical aperture single-mode fiber (UHNA) is spliced to the tail fiber of standard SM or PM fiber to realize mode field conversion. The product is an ideal solution for connection on high-speed silicon photonic transceiver module, and can provide customized mode field diameter, such as 3.2μm/3.3μm/4μm/5.5μm to 9μm. It adopts special assembly and polishing process and high-precision V-groove substrate to realize low insertion loss. It is made of all quartz and resistant to wide temperature.