Working Principle and Application of Fiber Directional Coupler

Fiber directional coupler is an optical device that can realize the distribution and combination between different optical fibers. It is made of optical fiber and has a simple structure.
The main features of fiber directional coupler:
① The main body of the device is optical fiber.
② The coupling effect of the mode in the optical fiber is used to realize the distribution of light power and direction.
③ The transmission direction of the optical signal is fixed.

1.Typical fiber directional couplers can be divided into the following three categories:
① Transmission type

② Reflection type

③ Transmissive type

2. Working principle of fiber directional coupler
The core diameter of a general single-mode optical fiber is 8-10um, and the cladding is 125um. Normally, two optical fibers are arranged together, and the distance between their fiber axes is much larger than their core diameters. At this time, no energy coupling occurs between the two optical fibers. When the distance between the two fiber axes is less than 5um, the energy in one fiber will be coupled to the other fiber. This phenomenon is due to the Gaussian distribution of the optical power in the fiber core. Under this distribution, the light is not completely confined to the core for transmission. Its tail part will penetrate into the cladding, and its energy accounts for about 20% of the total optical power. When the distance between the two fiber axes is less than 5um, the waveguide field in one fiber will cause the polarization of the other fiber medium, resulting in the excitation of the conduction mode in the fiber. The energy of the two fibers penetrates and overlaps each other, and the energy is coupled and transmitted according to certain rules.

1×2 fiber directional couplers provide a variety of splitting ratios, including 50:50, 75:25, 90:10, 99:1 or 99.9:0.1, among which 99:1 or 99.9:0.1 is often called tap.

1×4 fiber directional couplers use three 50:50 couplers to evenly distribute the incident light to four output ports.

3. Application of fiber directional couplers

As the core functional unit of optoelectronic systems, fiber directional couplers are used in three major fields: communication, sensing, and precision measurement. In optical communications, they achieve dynamic distribution and combining of optical power through flexible splitting ratio design (such as 1×2, 1×4, or asymmetric splitting), and are widely used in splitting nodes of passive optical networks (PON), wavelength selective coupling of wavelength division multiplexing (WDM) systems, and resonant cavity feedback control of fiber lasers. At the same time, its low insertion loss (<0.3 dB), wide bandwidth (1260~1650 nm), and high stability (temperature drift <0.005 dB/℃) make it a key device for optical signal routing in 5G fronthaul networks, data center interconnection, and smart grid monitoring. In the field of sensing, fiber optic directional couplers can accurately demodulate changes in physical quantities such as temperature, strain, and vibration by constructing interference structures such as Mach-Zehnder, Michelson, and Sagnac, becoming the core component of fiber optic gyroscopes, hydrophones, and distributed fiber optic sensors. Their high-precision phase response capability (up to 10^-6 rad) provides reliable support for aerospace navigation and marine environment monitoring.