Fiber Fabry-Perot Tunable Filter: Hirundo FPTF for Narrow Linewidth, Low-Loss WDM Wavelength Scanning
In dense wavelength-division multiplexing (DWDM) and coarse wavelength-division multiplexing (CWDM) networks, the ability to dynamically select a single wavelength channel, without replacing hardware or interrupting live traffic, is a fundamental requirement. The Fiber Fabry-Perot Tunable Filter (FPTF) from Hirundo is engineered to meet exactly that need: a piezoelectric-actuated, fiber-coupled tunable filter that delivers a narrow linewidth of approximately 0.1 nm, low loss, and a wide tuning range of roughly 100 nm, all in a compact, vibration-insensitive package.
How the Fiber Fabry-Perot Tunable Filter Works
The FPTF is based on a fiber tip gap etalon cavity configuration. Two fiber end faces form a low-finesse or high-finesse Fabry-Perot cavity, and the optical path length of that cavity is mechanically adjusted by a piezoelectric actuator. By varying the drive voltage, the resonant transmission wavelength of the etalon shifts, allowing the device to select one target wavelength channel while blocking all others.
Because the tuning mechanism is purely mechanical-optical rather than thermal, the device is inherently vibration-insensitive, which makes it well suited to field-deployed systems and moving platforms. Hirundo offers the FPTF across wavelength ranges spanning 1500 nm to 1600 nm, with typical configurations centered in the 1550 nm to 1600 nm window used by mainstream DWDM and CWDM transmission.
Performance Is a Trade-Off: Linewidth, Tuning Range and Finesse
No tunable filter can maximize every parameter at once, and the FPTF is no exception. The relationship between linewidth, tuning range and finesse is a deliberate engineering trade-off that must be matched to the application:
- Linewidth versus tuning range: if a 0.1 nm linewidth is selected, the tuning range is limited to approximately 10 nm. Applications requiring the full 100 nm tuning range must accept a broader passband.
- Finesse versus insertion loss: higher finesse values deliver sharper filtering and better adjacent-channel rejection, but peak insertion loss rises accordingly.
- Finesse versus power handling: as finesse increases, the maximum optical power that the cavity can safely handle decreases, because more energy is resonantly confined inside the etalon.
Hirundo therefore offers a family of finesse grades, from 400 up to 10000, so that designers can dial in the exact balance of selectivity, loss and power handling their link budget allows.
Key Specifications of the Hirundo FPTF
| Parameter | Unit | Value |
|---|---|---|
| Wavelength Range | nm | 1500 to 1600 |
| Typical FSR | - | 100, 120, 160 or specify |
| Typical Finesse | - | 400, 1000, 2000, 5000, 10000 |
| Max Peak Insertion Loss | dB | 2.5, 3.0, 3.5, 4.0, 4.5 (finesse dependent) |
| Max PDL at Peak Transmission | dB | 0.3 |
| Max Optical Power | mW | 50, 30, 15, 5, 3 (finesse dependent) |
| Max Tuning Frequency | kHz | 1 |
| Fiber Type | - | Singlemode fiber |
| Fiber Jacket | - | 900 um loose tube |
| Max Tuning Voltage per FSR | V | 20 |
| Max Tuning Voltage | V | 70 |
| Operating Temperature | deg C | 0 to +70 |
| Storage Temperature | deg C | -20 to +85 |
| Fiber Length | m | 1 |
Free spectral range (FSR) options of 100, 120 and 160 are available as standard, with custom values on request. The maximum tuning frequency reaches 1 kHz, which is fast enough for swept-wavelength scanning and dynamic channel selection in live traffic-bearing systems.
Flexible Configuration and Ordering
Because the FPTF is an application-specific component, Hirundo builds each unit to order. The ordering code follows the format FFP-TF-XXX-XX, where the first field defines the wavelength and FSR configuration and the second defines the connector and fiber interface.
- Connector options: none (bare fiber), SC/UPC, SC/APC, FC/UPC, FC/APC, LC/UPC, LC/APC, ST/UPC, E2000 and MPO, plus special designs.
- FSR options: 100, 120, 160 or customer-specified.
- Finesse grades: 400, 1000, 2000, 5000 and 10000.
- Fiber jacket options: 250 um bare fiber, 0.9 mm, 2.0 mm and 3.0 mm loose tube, 0.9 mm, 2.0 mm and 3.0 mm tight tube, multimode fiber, polarization-maintaining fiber and special constructions.
Applications: From WDM Channel Selection to Wavelength Scanning
The most common use of the FPTF is dynamic wavelength selection in WDM and DWDM systems. When multiple DWDM or CWDM channels are injected into the input port, the filter passes one selected target channel to the output port and blocks all remaining channels, effectively acting as a remotely tunable channel selector, a reconfigurable add/drop element, or an optical performance monitoring front end.
Beyond telecom, the combination of a wide tuning range and scan speeds up to 1 kHz makes the device a cost-effective engine for:
- Wavelength scanning and swept-source measurement in optical test instruments
- Optical spectrum analysis and channel power monitoring
- Fiber-optic sensing interrogation, including FBG and interferometric sensor arrays
- Laboratory and R&D work requiring rapid, repeatable wavelength selection
Stability, Drift and Feedback Control
Practical Fabry-Perot tunable filters exhibit a degree of wavelength drift over time and with temperature, and the FPTF is designed with this reality in mind. For applications demanding absolute wavelength accuracy, closed-loop feedback control using a wavelength reference or a portion of the tapped output is recommended to lock the passband precisely onto the target ITU grid position. Where relative scanning rather than absolute locking is required, the device can be operated open-loop, which simplifies the drive electronics considerably.
The mechanically robust, vibration-insensitive construction means that, once stabilized, the filter maintains its selected wavelength in environments where thermal or piezo-based alternatives would suffer from alignment shifts.
Why Choose Hirundo
Hirundo designs and manufactures the FPTF as part of a broad portfolio of WDM and passive optical components, including CWDM, DWDM, CCWDM, CDWDM, MWDM, OADM, full-band tunable filters and AWG modules. This integrated product range allows customers to source a complete wavelength management solution from a single supplier, with consistent optical performance and matching interfaces.
The FPTF specifically offers a compelling cost-performance ratio: narrow linewidth, low insertion loss, 1 kHz tuning, and a tuning range up to 100 nm, in a package that is easy to drive and simple to integrate. For engineers building wavelength-scanning systems, reconfigurable optical networks, or high-precision photonic instrumentation, the Hirundo Fiber Fabry-Perot Tunable Filter delivers the selectivity and speed required, at a price point that supports volume deployment.
Conclusion
Selecting the right tunable filter is ultimately a matter of matching finesse, linewidth, tuning range and power handling to the demands of the application. The Hirundo FPTF gives designers that flexibility through a wide choice of FSR values, finesse grades, connectors and fiber jackets, while its piezoelectric fiber-tip etalon architecture keeps the device compact, fast and reliable. Whether the goal is dynamic DWDM channel selection or high-speed wavelength scanning, the FPTF provides a practical, field-proven answer.
