Self-Written High-Efficiency Single-Mode Optical Link Using a Single Near-Infrared Photopolymerization Step

We present a method for fabricating a self-written waveguide (SWW) between two optical fibers that are single-mode (SM) at 850 nm (780HP/ core diameter: 4.4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m). The basic principle consists in exposing an acrylic photopolymer formulation sensitive in the near-infrared range (NIR) to a laser beam transmitted simultaneously from both fibers placed face to face, to build a continuous, flexible and self-aligned optical link. The specificity of the presented process (NIR-SM-SWW) lies in the use of a writing wavelength identical to that intended for single- mode propagation in the fibers. This enables the creation in a single step of a SWW directly adapted to the fundamental mode to be transmitted. A precise pre-positioning stage is used to optimize the process. For best photochemical conditions, a coupling efficiency as high as 82 % (-0.86 dB loss) is demonstrated for a 300 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m-long link. The effect of fiber-to-fiber axial and lateral distances is also investigated to estimate the propagation loss and misalignment tolerance, respectively. In addition, measurements performed by quantitative phase optical microscopy indicate a homogeneous index profile in the guide. Using these data, optical modeling is performed and compared to experiments, confirming that a high efficiency SM link is actually fabricated, without the need for further fabrication of an external cladding. This method could therefore be easily applied to the SM connection of a SM VCSEL (vertical-cavity surface-emitting laser) to a SM fiber, which is of major interest for the development of compact optical communications and instrumentation systems.