Abstract
Precise alignment of femtosecond laser tracks in standard single mode optical fiber is shown to enable controllable optical tapping of the fiber core waveguide light with fiber cladding photonic circuits. Asymmetric directional couplers are presented with tunable coupling ratios up to 62% and bandwidths up to 300 nm at telecommunication wavelengths. Real-time fiber monitoring during laser writing permitted a means of controlling the coupler length to compensate for micron-scale alignment errors and to facilitate tailored design of coupling ratio, spectral bandwidth and polarization properties. Laser induced waveguide birefringence was harnessed for polarization dependent coupling that led to the formation of in-fiber polarization-selective taps with 32 dB extinction ratio. This technology enables the interconnection of light propagating in pre-existing waveguides with laser-formed devices, thereby opening a new practical direction for the three-dimensional integration of optical devices in the cladding of optical fibers and planar lightwave circuits.
Highlights
With the preponderance of fiber optics in telecommunication, biomedical and sensing applications, there has been significant attention towards adding functionality directly within the fiber to avoid the packaging and assembly to discrete components
This paper explores the design and fabrication of femtosecond laser-written asymmetric directional couplers (DCs) in single mode fiber (SMF) to form optical fiber taps that connect the core waveguide light with cladding waveguides without damage or modification to the core waveguide
A cross-sectional view of an optical fiber embedded with a DC is shown in the inset image in Fig. 1 for the example case where the writing laser, incident from the top surface, formed a waveguide at a lateral offset distance of S = 9 μm from the SMF core waveguide
Summary
With the preponderance of fiber optics in telecommunication, biomedical and sensing applications, there has been significant attention towards adding functionality directly within the fiber to avoid the packaging and assembly to discrete components. The coupling coefficient and birefringence dependence as a function of the waveguide offset are examined and applied to tailor DC devices with variable coupling ratios that can serve as broadband couplers with minimal polarization dependence at shorter lengths to strongly isolating polarization selective taps of narrow bandwidth at longer lengths In this way, femtosecond laser writing of DCs offers a precise method to create new types of in-fiber polarizers and spectral filters, as well as open opportunities to design and build fiber cladding photonics that connect with the fiber core waveguide
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