Does raster perception of temperature change take one second or microseconds?
The response time of Fiber Bragg Gratings (FBGs) to temperature changes cannot be generalized. It needs to be divided into the intrinsic thermal response time of the bare FBG itself and the thermal response time of the temperature sensor after encapsulation in actual engineering applications.
Overall: The response time of the bare grating itself is at the millisecond (ms) level, and can approach the microsecond (\mu\text{s}) level in specific ideal heat transfer media; while the response time of the encapsulated grating temperature sensor is typically between hundreds of milliseconds to several seconds (s).
Below are the specific physical mechanisms and heat transfer analysis:
I. Intrinsic Thermal Response Time of Bare Gratings (Millisecond Level)
A bare FBG is made of silicon dioxide (quartz glass) with a periodic refractive index modulation inscribed in its core. The outer diameter of standard single-mode fiber bare fiber is only about 125\ \mu\text{m} (typically 155\ \mu\text{m} with coatings like polyimide).
According to the lumped capacitance method of heat conduction, the thermal time constant of the fiber (the time required for the temperature change to reach 63.2% of the external temperature difference) is given by:
\tau = \frac{\rho \cdot V \cdot c}{h \cdot A}
Since the fiber is a cylindrical structure, the ratio of its volume to surface area is:
\frac{V}{A} = \frac{d}{4}
Substituting, we get:
\tau = \frac{\rho \cdot c \cdot d}{4h}
Where:
- \rho is the density of quartz glass (approximately 2.2 \times 10^3\ \text{kg/m}^3);
- c is the specific heat capacity of quartz (approximately 700\ \text{J/(kg}\cdot\text{K)});
- d is the outer diameter of the fiber (e.g., 155\ \mu\text{m}, which is 1.55 \times 10^{-4}\ \text{m});
- h is the convective heat transfer coefficient between the fiber surface and the external medium.
Due to the extremely small diameter d of the fiber, its own thermal capacity (\rho \cdot V \cdot c) is very low. In a medium with good heat transfer (e.g., high-flow liquid, where the convective heat transfer coefficient h is very large):
- The thermal time constant \tau of the bare grating is typically only a few milliseconds to over ten milliseconds.
- In extreme transient environments with high-speed shock waves or instantaneous laser heating, heat can be conducted to the core through the very thin glass medium, and the response time can even approach hundreds of microseconds (\mu\text{s}).
- However, in air, due to the very small convective heat transfer coefficient h of air, even for a bare grating, its thermal response time will be extended to hundreds of milliseconds or even more than one second.
II. Influence of Encapsulation on Thermal Response Time (Hundreds of Milliseconds to Seconds)
In industrial and practical engineering applications, bare gratings are too fragile to be used directly. To protect the fiber from breakage, moisture, or corrosion, it must be encapsulated in a casing.
Once an encapsulation layer is introduced, it brings additional thermal resistance (hindering heat conduction to the interior) and thermal capacity (temperature lag due to heat absorption by metal tubes, filling materials, and adhesives). Therefore, the response time of encapsulated grating temperature sensors usually becomes slower:
- Medium Influence: When measuring in water, the sensor’s response time is typically in the hundreds of milliseconds range; when measuring in still air, it may take several seconds to over ten seconds to reach thermal equilibrium.
- Size Influence: The thicker the outer diameter of the sensor, the more severe the thermal lag.
III. Related OFSCN® Fast Temperature Measurement Products
To provide mechanical protection while maximizing the fast thermal response characteristics of FBGs, Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) has launched the following products by reducing encapsulation size and optimizing heat conduction design:
1. Ultimate Response Speed (Bare FBG Strings)
If your experimental or testing environment allows for direct bonding or sheathless encapsulation, and you pursue millisecond-level extreme response speed, you can choose bare gratings:
- OFSCN® High-Strength Fiber Bragg Gratings / FBG Strings (Bare)
- Technical Features: Uses femtosecond laser point-by-point writing technology without stripping the fiber coating, maintaining extremely high mechanical strength with an outer diameter of only 155\ \mu\text{m}. The operating temperature range is wide, from -200^\circ\text{C} to 300^\circ\text{C}, providing a very short thermal time constant (millisecond level).
2. High Response Speed Encapsulated Sensors (Ultra-thin Outer Diameter)
In temperature measurement scenarios requiring stainless steel armored protection, to control the response time within hundreds of milliseconds, it is recommended to use ultra-thin outer diameter seamless steel tube encapsulated temperature sensors:
- OFSCN® 300°C Fiber Bragg Grating Temperature Sensor
- Technical Features: Default single-layer seamless steel tube encapsulation, with a default outer diameter of 0.9\ \text{mm}, customizable to an extreme minimum outer diameter of 0.5\ \text{mm}. The extremely thin outer diameter minimizes the thermal resistance and thermal capacity of the sensor to the greatest extent, enabling excellent transient temperature tracking capabilities in liquids with a response time of only hundreds of milliseconds.
- Operating Temperature: -200^\circ\text{C} to 300^\circ\text{C}.
- OFSCN® 100°C Fiber Bragg Grating Temperature Sensor
- Technical Features: Standard outer diameter of 0.9\ \text{mm}, customizable to a minimum outer diameter of 0.6\ \text{mm}, also offering excellent response speed.
Summary
The response of gratings to temperature changes is millisecond-level in the bare fiber state; in extreme scientific research experiments such as microsecond and nanosecond laser transient heating, it can exhibit microsecond-level response. In industrial stainless steel encapsulation, the response time is typically hundreds of milliseconds to seconds. To achieve the fastest temperature measurement response, one should choose sensors with thinner outer diameters (e.g., OFSCN®'s 0.5\ \text{mm} custom outer diameter version) as much as possible and improve the heat transfer efficiency between the sensor and the measured medium.