An optical fiber coupler directs light from one or more input fibers to one or more output fibers, with power distribution depending on wavelength, polarization, and coupling length.

Basic Concept

A fiber coupler is a device that allows light entering an input fiber to be distributed to one or more output fibers. Common types include 1x2 couplers (one input, two outputs) and 2x2 couplers (two inputs, two outputs) . The coupler operates primarily through evanescent wave coupling, where the cores of two fibers are brought close together over a short interaction length, allowing optical power to transfer between them .

Optical Sequence Diagram Explanation

1. Input Stage: Light enters the coupler through an input port (e.g., Port 1 in a 1x2 coupler). The input can be a single wavelength or multiple wavelengths if using a WDM coupler .
2. Coupling Region: In the central region, the fiber cores are fused or tapered so that their evanescent fields overlap. Light begins to transfer between the fibers. The coupling ratio determines how much power goes to each output and is wavelength-dependent .
3. Output Stage: Light exits through the output ports. In a 1x2 coupler, the power is split between Ports 2 and 3. In a 2x2 coupler, light can either remain in the same fiber or cross over to the other fiber depending on the wavelength and coupling length .
4. Directional Behavior: Most fiber couplers are directional, meaning light entering an input port is mostly transferred to the output ports with minimal back-reflection to the input .
5. Polarization and Wavelength Effects: Polarization-dependent loss (PDL) and wavelength sensitivity can affect the output power distribution. Polarization-maintaining couplers are designed to minimize these effects .

Diagram Representation

A simplified optical sequence diagram for a 1x2 coupler can be represented as:

Input Port (I1) ---> Coupling Region ---> Output Port 1 (O1) ---> Output Port 2 (O2)

For a 2x2 directional coupler, the sequence is:

Input Port I1 ---> Coupling Region ---> Output Port O1 (through) ---> Output Port O2 (cross)Input Port I2 ---> Coupling Region ---> Output Port O2 (through) ---> Output Port O1 (cross)

Numerical simulations of amplitude distribution show that light oscillates between the two cores in the coupling region, with most power eventually exiting through the designated output ports .

Applications

• Power splitting in fiber lasers and amplifiers
• Combining pump and signal light in WDM systems
• Fiber interferometers and sensing applications This sequence diagram helps visualize how light propagates through the coupler, how the coupling ratio affects output power, and how wavelength and polarization influence performance .