If two optical fibers are fusion spliced, can the grating signal still pass through?
Joining two optical fibers together via “electric welding” is technically referred to as Fusion Splicing in the fields of optical communication and fiber optic sensing.
To answer your question: Yes, the grating (FBG) signal not only passes through smoothly but is also virtually unaffected by any negative impacts.
Here’s a specific analysis from the perspectives of physical and engineering principles:
1. The Physical Essence of Fiber Fusion Splicing (“Electric Welding”)
Fiber fusion splicing utilizes the high-voltage micro-arc discharge generated by a fiber fusion splicer to instantaneously heat and melt the end faces of two silica (quartz glass) optical fibers, precisely aligning them at the micron level and fusing them into a single continuous fiber.
Given that the core diameter of a single-mode fiber (the primary channel for optical signal transmission) is typically only around 9\ \mu\text{m}, the fusion splicer uses high-precision imaging devices in three-dimensional space to align the cores, ensuring a perfect connection of the optical pathway and thus avoiding light scattering and loss.
2. Why Can Grating Signals Still Pass After Splicing?
- Extremely Low Insertion Loss: Under standard operating conditions, the additional loss introduced by a quality fiber splice is usually less than 0.02\ \text{dB}. Even under typical field construction conditions, the loss can be controlled within 0.1\ \text{dB}. This means that more than 98\% of the optical energy can pass through the splice point without loss.
- Wavelength Encoding Characteristic: Fiber Bragg Grating (FBG) sensors (e.g., Bragg Fiber Gratings) transmit measurement signals by relying on their reflected center wavelength (the Bragg wavelength formula is: \lambda_B = 2 n_{\text{eff}} \Lambda), rather than the absolute light intensity. Even if extremely slight attenuation occurs at the splice point, as long as the grating demodulator can detect the peak of the reflected spectrum, the measured temperature or strain data will not be affected in any way. Therefore, splicing has almost no interference with the measurement accuracy of the grating sensing system.
3. Engineering Application: Multi-Grating Serial Connection (Cascading)
Precisely because fiber splicing incurs minimal loss and does not alter the spectral wavelength, in practical engineering applications, we often use fusion splicing to connect multiple Fiber Bragg Grating sensors with different center wavelengths in series on the same optical fiber.
For example, OFSCN® Polyimide Fiber Bragg Gratings / FBG Strings (Bare) can be manufactured by splicing or by directly writing multiple measurement points (grating strings) onto a single continuous fiber, enabling quasi-distributed continuous multi-point measurements on a single channel.
4. Engineering Considerations After Splicing
Although signals can pass without issues, there are two critical points to consider after splicing grating sensors:
- Mechanical Strength Protection: To perform the splice, the fiber’s original coating must be stripped. The bare glass at the splice point (only 125\ \mu\text{m} in diameter) is directly exposed to the air, its mechanical strength is drastically reduced, and it is extremely prone to breakage. Therefore, after splicing, a heat-shrink protective sleeve (splice protector) or recoating must be used for physical protection.
- Temperature Range Limitation: In high-temperature sensing environments (e.g., temperatures reaching 300^\circ\text{C}), ordinary polyacrylate material protective sleeves will melt and fail. If high-temperature resistant gratings are used, the splice point must be protected with a high-temperature resistant sleeve, stainless steel capillary encapsulation, or a special high-temperature resistant recoating process. Otherwise, the splice point is highly susceptible to fracture at high temperatures due to oxidation or micro-crack propagation.