Fused couplers are one of the most important optical passive components used in fiber optic communication systems. The reason why they are used is that they allow you to do light branching and splitting in passive networks.
These passive components are made by joining two separate optical fibers that work on the principle of coupling between parallel optical waveguides. Their claddings are fused over a small area. In addition to light branching and splitting, fused couplers are also used in various other applications, such as:
- Wavelength multiplexing or de-multiplexing
- Filtering
- Polarization selective splitting
- Wavelength-independent splitting and more
These components work on the principle of energy transfer between optical fiber cores after fusion.
What are Single Mode Fused Couplers?
The most basic form of a fused coupler is a 2×2 waveguide directional coupler made by placing parallel single-mode optical waveguides. Hence, this type of coupler is commonly called a single-mode fused coupler. Sometimes, this component also represents which fibers it is made of. For instance, a 2.0μm single-mode fused coupler clearly indicates it is made using 2.0μm single-mode fibers.
How do Single Mode Fused Couplers work?
The primary operation of this device involves a complete or partial transfer of optical power between two wavelengths. The transfer of optical power occurs because of the optical coupling between the evanescent tail of one waveguide’s guided mode in which light is launched and that of the natural mode of the second waveguide. You can think of this optical interaction as the beating between symmetric and asymmetric super mode.
An important role in the coupling process is also played by the parallel interaction region, which has a longitudinally constant structure.
As soon as the light is launched into one of the waveguides and it is coupled into one of the waveguides, it excites a linear combination of both types of modes. Since each mode has a different propagation constant, the fields propagating in the system also develop a relative phase difference between the distance of propagation.
When the accumulated phase difference between the two modes over a certain length becomes pi (π), the superposition of both modal fields cancels the field amplitudes in the input waveguide and an addition in the second waveguide. This is referred to as coupled state and the associated interaction length is called coupling length.
When the interaction length extends beyond the coupling length, then reverse coupling occurs and for propagation over double coupling length, there develops a phase difference of 2π. As a result, optical power is restored in the input waveguide. This happens periodically across the entire time of wavelength of propagation.
If both waveguides are identical, there can occur a transfer of complete optical power. However, if both waveguides are non-identical, only a certain amount of maximum power transfer can take place.
At DK Photonics, we manufacture a wide range of fused couplers, including 2.0μm single-mode fused couplers, 1.0μm single-mode fused couplers, 1XN single-mode coupler modules, and more. We also offer customization of single-mode fused couplers. If have any queries related to single-mode fused couplers, please get in touch with us.