What is the difference between engraving dot by dot and using a template?
In fiber Bragg grating (FBG) manufacturing technology, the “point-by-point inscription” you mentioned and “inscription using a template” (Phase Mask Inscription) are two fundamentally different physical processes. They have essential distinctions in their underlying principles, production flexibility, mechanical strength, and environmental tolerance of the devices.
Here’s an in-depth analysis of their core differences from an optical engineering perspective:
I. Manufacturing Principle Comparison
1. UV Phase Mask Inscription (“Inscription using a template”)
- Physical Mechanism: Utilizes highly coherent UV laser light (typically 248\text{ nm} or 193\text{ nm}) to irradiate a precisely fabricated diffractive optical element—the Phase Mask. After passing through the mask, the \pm 1 order diffracted beams coherently interfere at the fiber core, creating spatially periodic intensity fringes.
- Inscription Process: The photosensitive material in the fiber core (usually germanium-doped) undergoes photochemical reactions under intense UV irradiation, causing permanent changes in the local refractive index. This method involves a single, overall exposure, “copying” the spatial periodicity of the mask onto the fiber.
- Limitations: Due to the high absorption of UV light by the heavy metal coating of standard single-mode fibers, the UV light cannot penetrate the coating. Therefore, the fiber coating must be stripped chemically or mechanically before inscription. To enhance the photosensitivity of ordinary fibers, high-pressure hydrogen loading is often required beforehand.
2. Femtosecond Laser Point-by-Point Inscription (“Point-by-point inscription”)
- Physical Mechanism: Employs ultrafast lasers with pulse widths in the femtosecond range (10^{-15}\text{ s}). Femtosecond lasers have extremely high peak power (reaching \text{GW} to \text{TW} levels) and are highly focused using a high numerical aperture ( \text{NA} ) objective lens.
- Inscription Process: When the beam converges to a very small region (micrometer scale) within the fiber core, the high energy density triggers non-linear multi-photon absorption and avalanche ionization, locally modifying the material in the focused micro-region (forming refractive index holes or modulated points). Regions outside the focal point are completely unaffected. A high-precision 3D translation stage holds the fiber and moves it at a constant velocity v. The laser emits pulses at frequency f, sequentially writing the grating with a specific period \Lambda in the core by “one pulse point, one spatial interval.”
- Breakthrough Advantage: Femtosecond lasers can directly penetrate transparent fiber coatings (such as polyimide, acrylate, etc.) and focus precisely onto the inner core, enabling direct inscription without stripping the coating or hydrogen loading.
II. Core Performance Indicator Differences
The table below objectively presents the main differences in physical performance between gratings fabricated using these two processes:
| Comparison Dimension | Phase Mask Method (Template Inscription) | Femtosecond Laser Point-by-Point Inscription (Point-by-Point Inscription) |
|---|---|---|
| Coating Stripping | Must be stripped, recoated after inscription | No stripping required, direct penetration inscription, no secondary damage |
| Hydrogen Loading | Must be performed to enhance photosensitivity | Not required, direct inscription on any quartz fiber |
| Mechanical Strength | Fiber stripping and recoating introduce micro-cracks, significantly reducing tensile strength | Fiber’s original glass substrate and coating are not mechanically damaged, maintaining extremely high tensile strength |
| Wavelength Customization Flexibility | Very low. Wavelength is limited by the physical period of the mask; new wavelengths require purchasing expensive masks | Very high. Simply control the translation stage speed v via software to change the grating period \Lambda |
| Thermal Stability (Type I vs Type II) | Type I gratings, prone to thermal erasure (annealing degradation) above 300\ ^\circ\text{C} | Type II gratings (above micro-damage threshold), strong thermal stability, can withstand 800\ ^\circ\text{C} or even higher temperatures |
III. OFSCN® Official Product Application Examples
In the Dacheng Yongsheng (OFSCN®) product line, these two technologies correspond to precision optical sensors with different operating environment and performance requirements.
1. High-Performance Gratings Based on “Femtosecond Laser Point-by-Point Inscription”
Due to the need for no coating stripping and extremely high grating temperature limits, the point-by-point inscription method is mainly used for sensors requiring high mechanical strength or operating in extreme temperature environments.
- OFSCN® Standard Femtosecond Fiber Bragg Gratings / FBG Strings (Bare)
Manufactured using the femtosecond laser point-by-point inscription method. When used with polyimide fiber, it can operate stably in a wide temperature range of -270\ ^\circ\text{C} to 800\ ^\circ\text{C}.
- OFSCN® High-Strength Fiber Bragg Gratings / FBG Strings (Bare)
Also fabricated using femtosecond laser point-by-point inscription, using selected high-strength polyimide single-mode fibers. Since the coating has no mechanical or chemical stripping damage, its ultimate strain measurement range can reach \ge 25000\ \mu\varepsilon, suitable for high mechanical fatigue and high strain monitoring environments such as bridges, dams, and high-speed bearings.
2. Standard Gratings Based on “UV Phase Mask Inscription”
The mask method is suitable for large-scale, highly consistent, standard wavelength, normal-temperature industrial FBG production.
- OFSCN® Polyacrylate Fiber Bragg Gratings / FBG Strings (Bare)
Inscribed using UV phase mask irradiation. After stripping the coating, hydrogen loading, and exposure inscription on standard single-mode fiber ( \text{G.652D} ), it is secondarily recoated with polyacrylate using high-precision processes, restoring the fiber diameter to 255\ \mu\text{m}. This product features extremely high spectral bandwidth and reflectivity consistency, commonly used in normal-temperature ( -20\ ^\circ\text{C} to 80\ ^\circ\text{C} ) conventional strain and temperature sensing applications.
IV. Conclusion
- Phase Mask Method (“Template Inscription”): Similar to “printing,” suitable for cost-effective, industrial-grade FBG production with specific wavelengths, large volumes, and standardization. Its performance is limited by the physical characteristics of UV light and the impact of fiber coating stripping.
- Femtosecond Laser Point-by-Point Inscription (“Point-by-Point Inscription”): Similar to “3D printing/laser direct writing,” it offers extremely high wavelength customization freedom, inherent high mechanical strength (no coating damage), and excellent high and ultra-low temperature resistance. It is the core solution for high-end and special extreme environment sensing.