How can multimode optical fiber be converted into singlemode optical fiber?

How can multimode optical fiber be converted into singlemode optical fiber?

       Optical fiber is primarily used for data transmission due to its high transmission rate and large capacity. Because of its strong resistance to electromagnetic interference, low dispersion, and low loss, optical fiber has a wide transmission bandwidth and can travel farther than network cables (Ethernet cables). However, whether to use multimode or single-mode fiber depends on the specific network communication requirements.

       Distance is one of the important factors to consider when deploying optical fiber networks. Deploying an optical fiber network typically involves converting the fiber transmission mode, especially the conversion from multimode fiber to single-mode fiber (MMF to SMF). This article will introduce three methods for converting multimode fiber to single-mode fiber.

When is multimode to single-mode conversion necessary?

       Because single-mode and multimode optical fibers support different light transmission modes, their applications differ. Single-mode fiber is primarily used for long-distance, high-bandwidth applications, while multimode fiber is mainly used for short-distance applications.

       However, fiber optic networks don't simply use one type of fiber; fiber transmission mode conversion frequently occurs during signal transmission, such as multimode to single-mode conversion. When the network distance exceeds the maximum transmission distance of multimode fiber, mode conversion between multimode and single-mode fibers is usually required. The specific multimode to single-mode conversion depends on the deployed network equipment and connection type. 

How to convert multimode fiber to single-mode fiber?

① Fiber Optic Transceivers
       As is well known, fiber optic transceivers support multimode to single-mode fiber, dual-fiber to single-fiber, and wavelength conversion. Here, we mainly discuss multimode to single-mode fiber conversion. Fiber optic transceivers can cost-effectively convert multimode fiber to single-mode fiber for transmission distances up to 140 kilometers. For example, an SFP Gigabit Ethernet fiber optic transceiver can convert multimode fiber (maximum transmission distance 550m) to single-mode fiber (maximum transmission distance 20km) for Gigabit Ethernet (1000Mbps transmission rate).

       In practical applications that extend the transmission distance of fiber optic networks, fiber optic transceivers achieve long-distance connections between two Ethernet switches by converting multimode fiber to single-mode fiber, as shown in the following diagram:

② WDM Converter

       WDM converter similar to fiber optic transceivers, have the ability to convert multimode fiber to single-mode fiber. As the name suggests, WDM converter are commonly used in WDM systems, especially DWDM systems, because long-distance DWDM transmissions often require conversion between multimode and single-mode fiber, as shown in the following diagram: 

③Mode-tuned fiber optic patch cords

       Different from the two methods mentioned above in their conversion between multimode and single-mode fiber. Unlike WDM transponders, which change the fiber type to extend network transmission distance, mode-tuned fiber optic patch cords convert multimode to single-mode fiber by altering the light transmission mode.

       The fiber optic connector of a mode-tuned patch cord can emit a single-mode laser beam at the center of the multimode fiber with a specific displacement, allowing the single-mode laser to propagate within the multimode fiber core diameter. This shields against the differential mode delay (DMD) effect that occurs when light propagates in multiple modes within multimode fiber. Furthermore, mode-tuned fiber optic patch cords are only suitable for Gigabit 1000BASE-LX (or 10G Ethernet 10GBASE-LRM and 10GBASE-LX4) devices.

Key Considerations：

Wavelength Matching: Multimode fiber typically uses 850nm/1300nm wavelengths, while single-mode fiber uses 1310nm/1550nm wavelengths. Ensure that the optical transceiver wavelengths are consistent during conversion.

Loss Budget: Calculate the total loss (including conversion loss, fiber loss, and connector loss) to ensure it remains within the receiver’s sensitivity range.

Bidirectional Communication: If bidirectional transmission is required, conversion devices must be deployed at both ends, or the fiber type must be unified across the link.

Summary:

       For hybrid connections in existing networks, opto-to-electrical media converters are a universal and reliable solution; for devices that conform to specific standards (such as 1000BASE-LX), mode conversion patch cords are an economical option; and in the long run and fundamentally, unifying fiber types (all single-mode) and providing matching optical modules is the optimal network architecture choice.