Double-clad fiber and single-mode fiber

Double-clad single-mode fiber features a single-mode core for signal propagation surrounded by a multimode inner cladding for pump light, enabling high-power amplification and efficient light delivery.

Structure and Function

A double-clad fiber (DCF) consists of a single-mode core—typically around 9 µm in diameter—surrounded by a larger multimode inner cladding (e.g., 105 µm) and an outer cladding with a lower refractive index to confine light . The core supports single-mode light, ensuring high beam quality, while the inner cladding allows multimode light to propagate, which is particularly useful for delivering pump light in fiber lasers and amplifiers . This design enables cladding pumping, allowing high-power multimode pump sources to efficiently excite the active ions in the core without degrading the single-mode output .

Applications

Double-clad single-mode fibers are widely used in:

• High-power fiber lasers and amplifiers: The single-mode core provides diffraction-limited output, while the multimode cladding allows high-power pumping from diode bars or multimode sources .
• Telecommunications and CATV: Passive double-clad fibers at 1550 nm are used for pump/signal combiners and laser delivery, offering low splice loss and excellent signal confinement .
• Fluorescence imaging and light delivery: The dual-cladding structure allows efficient collection and delivery of light in scientific and medical applications .

Splicing and Termination

Double-clad fibers can be spliced to either single-mode fibers (matching the core) or multimode fibers (matching the inner cladding) with low splice loss, typically below 0.1 dB . Standard connectors such as FC/PC, FC/APC, or SMA905 can be used for termination, and specialized kits are available for proper fiber handling and connectorization .

Advantages

• High-power handling: The large inner cladding allows efficient coupling of high-power pump light.
• Maintained beam quality: Single-mode core ensures diffraction-limited output.
• Versatility: Can be used in both passive and active fiber systems, including Er/Yb-doped fibers for 1550 nm applications .
• Low splice loss: Compatible with standard single-mode and multimode fibers for easy integration into optical systems . Double-clad single-mode fibers are essential in modern fiber laser technology, enabling high-power, high-quality output while maintaining flexibility for various optical applications.