Ultrafast laser fabrication of efficient polarization-insensitive demultiplexer circuit in YAG crystal.

Wavelength division multiplexing is a widely used monolithic device with modulating light sources at different wavelengths based on a designed configuration. In this paper, we report an in-chip demultiplexer with a simple design operating at 532/1064 nm in pure YAG crystal. The device is fabricated by femtosecond laser direct writing inside the transparent substrate with just a width of 36 µm. The compact structure is designed based on the principle of self-imaging, and the propagation features have been simulated by utilizing the beam propagation method. The performance of this wavelength demultiplexer has been investigated through an end-face coupling system, which proves the device can separate 532 nm and 1064 nm light into two discrete waveguides polarization-insensitively with an extinction ratio as high as 13 dB. These superior performances manifest this exquisite device can emerge into kinds of photonic applications in the future. Also, this work further proves that femtosecond laser direct writing technology has irreplaceable advantages in processing micro-nano devices in transparent materials for in-chip integration.