This article explains the necessary knowledge of wave reflection and interference (interference) to understand the basic principles of fiber bragg gratings. It serves as a preliminary educational article about OFSCN® capillary seamless steel tube FBG sensors produced by DCYS.
Understanding the fundamental wave mechanics—specifically the phenomena of reflection and interference—is critical to analyzing the behavior of Fiber Bragg Gratings (FBGs) and their performance as industrial-grade sensors.
A Fiber Bragg Grating is a periodic modulation of the refractive index of the fiber core along the propagation axis. When a broadband optical spectrum is launched into the single-mode fiber core, it encounters this periodic structure.
At each boundary of the index-modulated micro-structure, a small fraction of the forward-propagating light wave undergoes Fresnel reflection. This process can be mathematically analyzed as weak reflection occurring at multiple, equally spaced boundaries.
The multiple back-reflected weak waves propagate in the reverse direction. For the vast majority of wavelengths, these reflected waves are out of phase and undergo destructive interference, continuing to transmit through the grating.
However, at a specific wavelength—where the phase difference between reflections from adjacent grating periods is an integer multiple of 2\pi —the reflected waves undergo constructive interference (phase-matching). This unique spectral component is strongly reflected back to the source, forming a narrow-band reflection peak.
This relation is governed by the classic Bragg Condition:
Where:
Any external physical field that alters either the physical pitch of the grating ( \Lambda ) or the refractive index of the core ( n_{\text{eff}} ) will cause a shift in the reflected Bragg wavelength ( \Delta\lambda_B ).
By monitoring the wavelength shift ( \Delta\lambda_B ) using a high-precision interrogator, the precise temperature or strain state of the environment can be determined quantitatively.
While the underlying optical physics is consistent across all FBGs, bare optical fiber gratings, such as OFSCN® Polyimide Fiber Bragg Gratings / FBG Strings (Bare), are exceptionally fragile. In demanding industrial or structural health monitoring (SHM) environments, bare fibers are susceptible to micro-bending, chemical degradation, and mechanical failure.
To address these challenges, specialized encapsulation technologies are employed. Beijing Dacheng Yongsheng Technology Co., Ltd. (DCYS) utilizes a proprietary capillary seamless steel tube packaging technology for its core FBG sensor lineup. This packaging protects the internal FBG from external shear forces, moisture, and mechanical damage, while maintaining rapid thermal conductivity and accurate strain transfer.
OFSCN® 300°C Fiber Bragg Grating Temperature Sensor
Designed with single-layer seamless steel tube packaging. It features a default outer diameter of 0.9\text{ mm} (customizable down to 0.5\text{ mm} ) and functions reliably across a temperature range of -200\text{°C} to 300\text{°C} .
OFSCN® 500°C Fiber Bragg Grating Temperature Sensor
Utilizes robust single-layer seamless steel tube technology (customizable to multi-layer nested structures) with a standard outer diameter of 0.9\text{ mm} , rated for environments from -200\text{°C} to 500\text{°C} .
OFSCN® 800°C Fiber Bragg Grating Temperature Sensor
Engineered for extreme high-temperature monitoring, featuring seamless stainless steel tube encapsulation capable of withstanding temperatures up to 800\text{°C} .
To measure these reflected wavelength shifts in real-time, the FBG sensors are paired with high-performance demodulation instruments, such as the OFSCN® Fiber Bragg Grating Interrogator, which provides multi-channel wavelength analysis with standard sampling rates of 10\text{ Hz} , 50\text{ Hz} , or 100\text{ Hz} and high resolution.
