Optocoupler Basics: Definition, Types, and Features

An optocoupler is a coupling device used to couple optical signals. It’s primarily employed to combine and split signals in optical networks, and it’s also referred to as a directional coupler.

Image alt: Optocoupler-Optical coupler

The figure above depicts a 2x2 coupler with two input ports and two output ports. The simplest and most common coupler is made by fusing two optical fibers at their middle sections. These are known as fused fiber couplers.

As shown, a fractional part (let’s say $\alpha$) of the power from the IN1 port passes through the OUT1 port, and the remaining part, $1 - \alpha$, exits from the OUT2 port. Here, $\alpha$ represents the coupling ratio. The fraction $1 - \alpha$ of the power from IN2 is distributed to the OUT1 port, and the leftover power goes to the OUT2 port.

Optical couplers are designed to be either wavelength-selective or wavelength-independent. They typically operate over a broad range of wavelengths, referred to as the bandwidth.

In wavelength-selective types, $\alpha$ depends on the wavelength, while in wavelength-independent types, $\alpha$ remains constant regardless of the wavelength. Wavelength-selective optical couplers are commonly used to combine signals at wavelengths of 1310 nm and 1550 nm into an optical fiber without signal loss. Wavelength-dependent couplers are also used to combine 980 nm or 1480 nm pump signals along with a 1550 nm signal into an EDFA (Erbium-Doped Fiber Amplifier).

The 4N46 optocoupler from Avago Technologies is a good example of an optical coupler that offers high gain with a Darlington output.

Optical Coupler Features

• High isolation
• No signal reflectivity
• Low coupling power loss
• No added polarization effects
• No added dispersion effects
• No signal absorption
• No added noise
• No through-phase shift