Dual Window Coupler 1310 1550 | Hirundo FBT Coupler for FTTH, CATV & Data Center Networks
In fiber optic networks that carry both 1310 nm traffic and 1550 nm services over the same span, engineers need a passive component that behaves consistently in both windows. The dual window FBT (fused biconical taper) coupler from Hirundo is engineered for exactly this requirement: it provides the same coupling ratio at 1310 nm and 1550 nm, with low excess loss, low polarization dependent loss (PDL), and a miniaturized steel tube package that fits compact optical modules and communication systems.
Why a Dual Window Design Matters
A conventional single window splitter is optimized around one center wavelength. In real deployments, however, the same fiber link often has to serve 1310 nm upstream signals, 1550 nm downstream signals, and sometimes both simultaneously. If the coupler behaves differently in each window, the splitting ratio drifts, link budgets become unpredictable, and network operators are forced to stock two separate part numbers. A dual window coupler solves this by maintaining the same coupling ratio across both communication windows, supported by an operating bandwidth of ±40 nm around each center wavelength. Because the device remains essentially wavelength insensitive inside that range, it is equally suited to access networks, hybrid fiber coaxial plants, and optical monitoring paths where multiple wavelengths share a single fiber.
Key Optical Performance of Hirundo Dual Window Couplers
Hirundo offers the 1x2 and 2x2 mini size dual window FBT coupler in two performance grades so that designers can match the component to their link budget. Grade P targets demanding links where every fraction of a decibel counts, while Grade A provides a cost optimized alternative for less critical paths. Both grades share the same mechanical footprint and the same environmental ratings, which simplifies qualification and inventory management.
| Parameter | Grade P | Grade A |
|---|---|---|
| Operating Wavelength (nm) | 1310 & 1550 | 1310 & 1550 |
| Operating Bandwidth (nm) | ±40 | ±40 |
| Typical Excess Loss (dB) | ≤0.07 | ≤0.1 |
| PDL (dB) | ≤0.15 | ≤0.2 |
| Return Loss (dB) | ≥50 | ≥50 |
| Directivity (dB) | ≥50 | ≥50 |
| Max Optical Power (mW) | ≤500 | ≤500 |
| Operating Temperature (°C) | -40 ~ +85 | -40 ~ +85 |
| Storage Temperature (°C) | -40 ~ +85 | -40 ~ +85 |
The combination of high return loss and high directivity keeps back reflections and unwanted crosstalk under control, an important consideration in analog CATV transport and in high sensitivity receiver front ends. Low PDL also means the device adds negligible polarization dependent variation to the link, which helps when the coupler sits in front of a coherent or polarization sensitive receiver.
Splitting Ratios from 1/99 to 50/50
The dual window FBT coupler is available with any splitting ratio from 1/99 up to 50/50. This flexibility allows the same product family to serve as a symmetric power divider in a PON head end or as a low ratio tap for live fiber monitoring, where only a small fraction of the light should be diverted to a detector. The table below lists the guaranteed insertion loss for each splitting ratio in both grades. Values are tested without connectors.
| Splitting Ratio | Insertion Loss Grade P (dB) | Insertion Loss Grade A (dB) |
|---|---|---|
| 50/50 | ≤3.6 | ≤3.8 |
| 45/55 | ≤4.2 / 3.2 | ≤4.4 / 3.4 |
| 40/60 | ≤4.7 / 2.7 | ≤4.9 / 2.9 |
| 35/65 | ≤5.4 / 2.4 | ≤5.7 / 2.6 |
| 33/67 | ≤5.7 / 2.2 | ≤6.0 / 2.4 |
| 30/70 | ≤6.0 / 1.9 | ≤6.3 / 2.1 |
| 25/75 | ≤7.0 / 1.7 | ≤7.3 / 1.9 |
| 20/80 | ≤7.9 / 1.3 | ≤8.4 / 1.4 |
| 15/85 | ≤9.5 / 1.0 | ≤10.0 / 1.2 |
| 10/90 | 9.2~11.2 / ≤0.75 | 8.80~11.40 / ≤0.8 |
| 5/95 | 12.05~14.15 / ≤0.4 | 11.60~14.60 / ≤0.5 |
| 3/97 | 14.1~16.5 / ≤0.35 | 13.60~17.05 / ≤0.45 |
| 2/98 | 15.75~18.45 / ≤0.3 | 15.15~19.00 / ≤0.4 |
| 1/99 | 18.6~21.6 / ≤0.25 | 17.95~22.25 / ≤0.35 |
For asymmetric ratios, the first figure refers to the low power output port and the second figure to the high power output port. Because the same values apply at both 1310 nm and 1550 nm, a single part number can be qualified once and reused across multiple wavelength plans.
Miniature Packaging, Fiber and Connector Choices
The mini size construction is the defining mechanical feature of this coupler. With dimensions of only Ø2.4 × L25 mm, it can be embedded inside compact optical modules, transceiver housings, and dense distribution panels where standard coupler bodies would not fit. Hirundo supports a broad range of build options so that the coupler can be delivered as a drop in part for an existing design:
- Configuration: 1x2 or 2x2 for the dual window mini type, with 1x1, 1x3, 1x4, 3x3 and 4x4 structures available elsewhere in the FBT coupler family.
- Fiber type: single mode, multimode, and polarization maintaining (PM) fiber options, including 250 µm bare fiber and 0.9 mm loose tube constructions.
- Fiber length: 1 m standard, or other lengths on request to match panel routing and splice trays.
- Wavelength variants: the wider FBT coupler range covers 780 nm, 850 nm, 980 nm, 1310 nm, 1490 nm, 1550 nm, and multi window combinations such as 1310 & 1490, 1310 & 1550, 1490 & 1550, and 1310 & 1490 & 1550.
- Packaging formats: mini steel tube packages such as 3x30 mm, 3x25 mm, 2.4x25 mm and 2.0x15 mm, plus LGX boxes, ABS boxes and 1U rack mount shelves for central office deployment.
Each coupler is identified with an ordering code in the form FBT-XXXXXXXX-XX, where the number encodes the wavelength, ratio, fiber and package combination required by the project.
Applications Across Access, CATV and Data Center Networks
The wavelength independence of a dual window coupler makes it a natural fit in several network layers:
- FTTH and PON networks: passive splitting of a single feeder fiber toward multiple subscribers, with the same device handling upstream and downstream windows without additional inventory.
- CATV and video overlay: distributing analog and digital video alongside data services on one fiber, where low excess loss and stable coupling ratio protect carrier to noise performance.
- Data center optical interconnect: aggregating and distributing signals between servers, switches and patch panels, supporting high density cabling where the small form factor is a decisive advantage.
- Fiber optic sensing systems: coupling sensing signals with reference light in perimeter security, pipeline monitoring and infrastructure health diagnostics.
- High power laser and amplifier systems: beam combining and splitting in industrial processing and medical laser platforms, supported by a maximum optical power rating of 500 mW.
Selection and Customization Guidance
Choosing the right dual window coupler comes down to four decisions. First, define the splitting ratio from the power budget: symmetric 50/50 for balanced distribution, or a low ratio such as 1/99 or 5/95 for monitoring taps. Second, select the performance grade; Grade P is the better choice when excess loss and PDL directly affect reach, while Grade A controls cost in short reach or non critical links. Third, confirm the fiber and pigtail construction, including whether bare fiber is acceptable or a loose tube is needed for handling and strain relief. Fourth, confirm the packaging and fiber length against the available space in the enclosure. Hirundo also produces custom wavelengths, custom splitting ratios and custom package sizes on request, so a specification that falls outside the standard table can usually still be manufactured.
Summary
Hirundo's dual window FBT coupler brings together the three qualities that optical network designers ask for most often: identical coupling behavior at 1310 nm and 1550 nm, low excess loss with low PDL in a mature fused biconical taper process, and a mini steel tube footprint that fits into compact modules. With splitting ratios spanning 1/99 to 50/50, single mode, multimode and PM fiber options, and packaging choices from mini steel tubes to rack mount shelves, it is a practical building block for FTTH, CATV and data center networks alike.
