Hirundo FBT Coupler Selection Guide: 1x2/2x2 Mini, PM, and FBT-WDM Solutions for FTTH and CATV

Fused biconic taper (FBT) couplers remain one of the most reliable and cost-effective ways to split or combine optical signals in passive optical networks. Whether you are designing a compact optical module, upgrading a CATV distribution plant, or rolling out an FTTH access layer, choosing the right FBT coupler directly affects insertion loss budgets, polarization stability, and long-term reliability. Hirundo has built a comprehensive FBT portfolio that covers the mainstream needs of modern optical networks — from mini 1x2/2x2 dual-window couplers, to polarization-maintaining (PM) couplers, to fused WDM couplers. This guide walks through the key specifications, splitting ratios, packaging options, and typical applications so engineers can make fast, confident decisions.

What Is an FBT Coupler?

An FBT coupler is fabricated by fusing two or more optical fibers together and then tapering the fusion region to a precise diameter. Light from the input fiber is gradually coupled into the adjacent fiber along the tapered section, so the energy is distributed according to the target splitting ratio. Because the coupling mechanism is purely passive and happens entirely in the glass, FBT couplers offer very low excess loss, high return loss, and excellent environmental stability. This makes them a natural fit for passive optical components used in communication systems, CATV, and FTTH deployments.

The 1x2 / 2x2 Mini Size FBT Coupler

The 1x2/2x2 mini size FBT coupler from Hirundo is designed for compact optical modules and space-constrained communication systems. Its most distinguishing feature is a dual-window design: it maintains the same coupling ratio at both the 1310 nm and 1550 nm communication windows, so a single component can serve multiple transmission bands without additional engineering changes. The device also delivers low excess loss and low polarization-dependent loss (PDL), which are essential for maintaining a clean link budget in high-speed and analog systems.

Key Optical Specifications

Split Ratio and Insertion Loss

Hirundo offers a broad selection of splitting ratios so designers can match the coupler to the exact power budget of the network. Typical insertion loss values for the two grades are summarized below.

Insertion loss is specified without connectors. Because the mini package uses a Ø2.4 × L25 mm housing and supports 250 µm bare fiber or 900 µm loose tube pigtails by default, it can be integrated directly into dense optical engine layouts.

PM Coupler: Preserving Polarization in Coherent Systems

For coherent transmission, sensing, and fiber laser applications, polarization state matters as much as optical power. Hirundo PM couplers split or combine signals while maintaining the polarization state of the transmitted light, which helps control polarization-dependent gain, reduce noise, and stabilize system performance. The PM series covers 1310 nm and 1550 nm operating windows as well as the 980/1030/1040/1060 nm and 780/850 nm bands, with a ±15 nm operating bandwidth. Typical excess loss is ≤0.3 dB for Grade P and ≤0.4 dB for Grade A in the 1310/1550 band, and the polarization extinction ratio (PER) reaches ≥20 dB for Grade P and ≥18 dB for Grade A. Return loss and directivity are both ≥50 dB, and maximum optical power is rated at ≤500 mW. Available configurations span 1x2, 1x3, 1x4, 2x2, 3x3, and 4x4, with splitting ratios ranging from 50/50 all the way to 0.1/99.9. Packaging options include miniature steel tubes (3×40, 3×45, 3×54, 3×60, 4×64, 4×75), 90×20×10 ABS boxes, and 100×80×10 ABS boxes for rack-level deployment.

FBT-WDM: Fused Wavelength Division Multiplexing Couplers

Fused biconic WDM couplers merge and separate single-mode signals from two different wavelength intervals. Hirundo's FBT-WDM portfolio supports popular wavelength pairs such as 980/1550, 1310/1550, 1310/1490, 1480/1550, 1060/1310, 1550/1620, 980/1060, and 980/1310 nm, with 1x1 and 1x2 configurations. Representative performance for the 1310/1550 nm pair includes insertion loss ≤0.2 dB (Grade P) and ≤0.3 dB (Grade A), isolation ≥17 dB and ≥16 dB respectively, and PDL ≤0.05 dB. The 1480/1550 nm version reaches insertion loss ≤0.3 dB (Grade P) and ≤0.35 dB (Grade A), while the 980/1550 nm version achieves insertion loss down to ≤0.15 dB (Grade P). All versions share ≥50 dB return loss and directivity and operate over -40 to +85 °C. Pigtail options cover 250 µm bare fiber, 900 µm loose tube, and 900 µm/2 mm/3 mm loose tube, with package sizes including Ø3.0 × L54 mm and a 90 × 20 × 10 mm module.

Packaging and Ordering Options

Beyond the optical parameters, Hirundo supports a full range of mechanical formats so the coupler fits cleanly into the target enclosure. Standard options include 3×40, 3×45, 3×54, 3×60, and 4×64/4×75 steel tubes, ABS boxes, and 1U rack-mount or LGX-style chassis. Fiber type choices span 250 µm bare fiber, 0.9 mm loose tube, 2.0 mm loose tube, 3.0 mm loose tube, and their tight-tube equivalents, plus multi-mode and PM fibers. Connector types include SC/UPC, SC/APC, FC/UPC, FC/APC, LC/UPC, LC/APC, ST/UPC, E2000, and MPO. For the 1x2/2x2 mini FBT coupler, the standard part number follows the format FBT-XXXXXXXX-XX, with the first eight characters defining the optical and mechanical variant and the final two characters representing the serial number.

Typical Applications

• FTTH access networks: Dual-window 1x2 and 2x2 couplers distribute downstream and upstream signals across the access layer with a low insertion loss budget.
• CATV and RF-over-glass: Stable coupling ratio and low PDL keep analog carriers clean and reduce distortion.
• Optical modules and transceivers: The mini Ø2.4 × L25 mm package fits inside compact pluggable and embedded optical engines.
• Fiber lasers and amplifiers: PM couplers and fused WDM couplers deliver pump-signal combining and wavelength separation with preserved polarization.
• Coherent and sensing systems: PM configurations support interferometric and polarization-sensitive architectures.

How to Choose the Right FBT Coupler

Start with the wavelength plan. If the network operates in both the 1310 nm and 1550 nm windows, the dual-window mini FBT coupler removes the need for a second component. Next, define the splitting ratio based on the remaining power budget — a 90/10 or 95/5 tap is common for monitoring ports, while 50/50 is used for balanced distribution. Then confirm whether polarization must be preserved; if so, select a PM coupler and verify the PER grade. Finally, choose the packaging, fiber jacket, and connector style that match the enclosure and installation environment. Because all three families share the same FBT manufacturing platform — tight process control during taper drawing, low excess loss, high return loss, and stable operation from -40 to +85 °C — they integrate seamlessly into a single passive optical design.

With 1x2/2x2 mini dual-window couplers, PM couplers, and FBT-WDM couplers under one portfolio, Hirundo gives network designers a flexible toolkit for FTTH, CATV, and communication systems. Matching the right configuration to the right link is the fastest path to a lower loss budget, a more stable polarization state, and a simpler bill of materials.