Femtosecond laser direct writing has been recently considered as a promising technology for fabrication of photonic integrated chips mainly due to its intrinsic capability of three-dimensional (3D) prototyping in transparent substrates. Currently, the difficulty in inducing large refractive index changes smoothly distributed in the laser irradiated regions is the major obstacle for producing compact photonic integrated circuits (PICs). Recently, researchers in China proposed a solution to suppress the bend loss of the waveguide at small radii of curvatures by more than one order of magnitude, opening a new avenue to downsizing of 3D photonic integrated circuits (See Figure 1). Their work, entitled "Suppression of bend loss in writing of three-dimensional optical waveguides with femtosecond laser pulses", was published in Sci China-Phys Mech Astron 2018, Vol. 61, No. 7.
PICs manufactured by mature photolithographic technologies are being used extensively in sensing, optical communications, optical signal processing and biophotonics. As an intrinsically planar fabrication technology, increasing the integration density in the photolithography mainly depends on reducing the sizes of individual components. Alternatively, PICs of geometrically complex 3D configurations can now be fabricated using femtosecond laser direct writing, which potentially provides high integration density and extreme flexibility in terms of integrated multifunctional systems such as optofluidics and optomechanics.
Currently, waveguides inscribed in fused silica glass have been demonstrated to support single-mode transmission with propagation losses as low as 0.1 dB/cm at 1550 nm wavelength. However, the typical refractive index increase induced in fused silica by femtosecond laser irradiation is on the order of ~10-4-~10-3, giving rise to large bending losses at small radii of curvatures. This has become a major obstacle for producing compact photonic devices with the 3D waveguides written by femtosecond laser pulses.
To solve this challenging problem, the researchers inscribe multiple modification tracks in fused silica by femtosecond laser direct writing, arranged into two arrays to form a pair of vertical modification walls on the two sides of the curved waveguide. The modification structures produce a strong localized densification of the material as well as significantly enhanced structural stress in the guiding region. As a result, the refractive index contrast of at the waveguide bend was substantially increased. By optimizing the geometrical parameters of the bend-loss-suppression walls (BLSWs), they successfully reduced the bend loss of curved waveguides with a bending radius of 15 mm from ~3 dB to ~0.3 dB.
"Since small radii of curvatures benefit large-scale integration of PICs on a single chip, it is expected that the technique can be highly valuable for construction of chip-based photonic devices." Commented by Dr. Yongfeng Lu, the Past President of Laser Institute of America and Lott Distinguished Professor at the University of Nebraska-Lincoln.
This research was funded by the Major Program of National Natural Science Foundation of China (Grant No. 61590934), the Project of Shanghai Committee of Science and Technology (Grant No. 17JC1400400) and the National Basic Research Program of China (Grant No. 2014CB921303).
See the article:
Z. M. Liu, Y. Liao, Z. W. Fang, W. Chu, and Y. Cheng, Suppression of bend loss in writing of three-dimensional optical waveguides with femtosecond laser pulses, Sci. China-Phys. Mech. Astron. 61, 070322 (2018), https:/