Fiber Type Selection
Fiber Bragg grating (FBG) sensors commonly used in bridge and tunnel monitoring projects are generally built on G.657.A1 or G.657.A2 fiber, and the two grades are mutually compatible. When extending fiber during construction, G.657.A1 single-mode single-core armored fiber should be the first choice. For long-distance transmission, multiple channels can be aggregated into the acquisition station via a trunk optical cable (e.g., GYTA-B1.3).
The table below compares common single-mode fiber categories for reference:
| No. | IEC Class | Description | ITU-T Class |
|---|---|---|---|
| 1 | B1.1 | Non-dispersion-shifted fiber | G.652.A/B |
| 2 | B1.2 | Cutoff wavelength-shifted fiber | G.654 |
| 3 | B1.3 | Low water peak (wavelength-extended) non-dispersion-shifted fiber | G.652.C/D |
| 4 | B2 | Dispersion-shifted fiber | G.653 |
| 5 | B4a | Non-zero dispersion-shifted fiber | G.655.A |
| 6 | B4b~e | Non-zero dispersion-shifted fiber | G.655.B~E |
| 7 | B5 | Wide-band non-zero dispersion-shifted fiber | G.655 |
| 8 | B6a | Bend-insensitive fiber for access networks | G.657.A1~A3 |
| 9 | B6a | Bend-insensitive fiber for access networks | G.657.B3 |
In-Channel Loss Control
Most manufacturers allow an in-channel attenuation of no more than 20 dB; during construction it is recommended to keep it within 12 dB. Attenuation comes mainly from three sources, calculated as follows:
Total fiber channel attenuation = intrinsic fiber loss + splice loss + connector insertion loss
Notes on each component:
- Intrinsic fiber loss: related to fiber length and wavelength. Taking the 1550 nm wavelength as an example, the attenuation coefficient is about 0.19 dB/km, meaning a 10 km fiber run contributes roughly 1.9 dB of loss
- Splice loss: approximately 0.05~0.1 dB per fusion splice, depending on workmanship quality
- Connector insertion loss: generally estimated at 0.5 dB per connector. Pay attention to connector type matching: FBG interrogator ports are typically FC/APC, while stable light sources and optical power meters are usually FC/UPC — confirm this in advance and prepare the appropriate adapters
Field measurement method: use a stable light source and an optical power meter to measure the total attenuation of the fiber channel in either direction. Measuring in both directions and averaging the results is recommended for better accuracy.
Practical tip: field measurements show that excessive splice loss is common, mainly caused by aging fusion splicer electrodes that are not recalibrated in time. It is recommended to perform an OTDR comparison calibration on the splicer monthly, which can effectively keep splice loss below 0.05 dB.
Bend Radius Recommendations
Although G.657.A1 fiber supports micro-bending with a minimum bend radius down to 3 mm, sufficient margin should be left in actual engineering installation:
Keep the bend radius no less than 10 times the fiber diameter. For example, a 5 mm diameter single-mode single-core armored fiber (field-deployable fiber) should be installed with a bend radius of at least 50 mm.
The main reason: continuous micro-bending introduces cumulative additional loss into the fiber channel, which is difficult to detect during construction. Problems discovered only after bridge completion often require slotting the deck to fix. Leaving adequate margin in advance effectively reduces rework risk.
Related Standards
- YD/T 2001-2009 Fiber Bragg gratings for fiber deformation sensors
- GB/T 34065-2017 Technical code for monitoring of building and bridge structures