Ultrafast Laser Writing of In-Line Filters Based on MZI

In-line filters play an important role in optical sensing and mode-locked fiber lasers. Among many different methods of in-line MZI filters, a new type of fiber-core MZI technology has attracted many research interests. However, the current direct-writing method requires the use of a multi-line inscription approach to increase the width of the waveguide, which will increase the difficulty of direct writing and may also reduce the quality of the waveguide. Here, we have proposed a single-line direct-writing method, which enables us to achieve the same MZI filtering effect with just one scan. We also adjusted the distribution of the waveguide positions to expand its application scope. An in-line filter with a length of 516 μm that was based on MZI was successfully written directly. MZI's free spectral range is 141.36 nm with an extinction ratio of 19.665 dB, an insertion loss of 1.122 dB, and the central wavelength of the loss dip is 1089.82 nm. When the same free spectral range was achieved, the direct writing length was reduced by 2.4 times using this strategy. This approach reduces the insertion loss by 2.7 times to achieve the same extinction ratio. Simulation and experimentation are basically the same. We also measured the temperature response curve of the MZI. Lastly, the successful achievement of this result has set the foundation for further development of integrated mode-locked fiber lasers in the future.