PICs With Lower Bend Loss

Scheme diagram of writing compact PICs with low bend loss using a femtosecond laser. (Source: Science China Press)

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 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.

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 at the waveguide bend was sub­stantially increased. By opti­mizing the geo­metrical para­meters of the bend-loss-suppression walls, 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,”, comments Yongfeng Lu, the Past President of Laser Insti­tute of America. (Source: Science China Press)

Reference: Z. Liu et al.: Suppression of bend loss in writing of three-dimensional optical waveguides with femtosecond laser pulses, Sci. China Phys. Mech. Astron. 61, 070322 (2018); DOI: 10.1007/s11433-018-9202-0

Link: State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, China

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