An optical fused coupler is a fiber optics device designed to connect one or more fiber ends to transmit light signals in multiple paths. It can be used to either combine two or more inputs into a single output or divide a single input into two or more outputs.
In this blog, you will learn the working principle of optical fused couplers and how they work.
Before you understand the working principle, it is critical to understand the formation of optical fused couplers. So, let’s first understand how optical fused couplers are formed.
The Formation of Optical Fused Couplers
A fused fiber optical coupler is typically formed by using two fibers that are placed adjacent to each other and then fused together. In each fiber, there is a long tapered section, and then comes a uniform section of a certain length where both fibers are fused together. After that uniform section, the fibers taper back to the original fiber size.
The main reason behind fusing the fiber is to create a fused coupling region. Optimal care is taken and continuous monitoring is done during the drawing process to achieve the desired coupling ratio. This coupling takes place when the two cladding modes interact with each other. During the drawing process, the power output values from the output ports are closely monitored and the process is stopped as soon as the desired coupling ratio is achieved.
Working Principle of Optical Fused Couplers
Optical fused couplers are based on the working principle that involves the leaking and coupling of the evanescent wave.
An evanescent wave is a near-field wave with an intensity that decays exponentially (instead of sinusoidally) with distance from the boundary at which it is formed. Here, the term ‘evanescent’ means tending to vanish.
In an optical fused biconical coupler, the two cores are so close that the evanescent waves can leak from one fiber into the other fiber. The energy transfer that takes place depends upon the separation distance and the interaction length.
When the coupling region (or interaction length) is long enough, complete energy transfer can occur from one fiber to the other. When the length still continues, the process will continue to occur, shifting the complete energy back to the original fiber core.
Thus, the fibers are tapered gradually so that only a negligible fraction of optical power from either input port reflects back and almost complete power is coupled into the two output ports. That’s why these couplers are sometimes called directional couplers.
By using this technique, different types of optical fused couplers can be designed that use the fact that the optical power from one fiber that is coupled to the power of the other fiber can be controlled by changing three things, namely, the length of the coupling region, the core separation distance, and the difference in the core radii in the coupling region.
The coupling region length usually varies with the light wavelength, and this fact makes it possible to make a wavelength division multiplexer or demultiplexer.
We hope this article has helped you understand the working principle behind optical fused couplers.
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