The process of a Red Light Pen (Visual Fault Locator, abbreviated VFL) locating the physical position of a broken optical fiber is based on the physical effect of leakage and scattering of high-brightness visible laser light at points of geometrical discontinuity in the fiber.
Its specific working mechanism and physical processes are as follows:
1. Light Source Injection and Total Internal Reflection Transmission
A Red Light Pen typically integrates a high-power semiconductor laser diode (LD), which emits a stable red visible laser (typical wavelength \lambda \approx 650\text{ nm} \pm 10\text{ nm} ).
In a normal, intact optical fiber, due to the core refractive index n_1 being slightly greater than the cladding refractive index n_2 , the injected red light satisfies the condition of Total Internal Reflection (TIR) at the interface between the core and the cladding. The light energy is strictly confined within the core and propagates axially forward, with almost no light observable escaping from the exterior of the fiber.
2. Refraction and Scattering Mechanism at Fault Points
When the optical fiber experiences breaks, macrobends (excessive bending radius), or poor splices, the physical conditions for total internal reflection are disrupted:
- Breaks and Cracks: The fiber core becomes discontinuous at the break point, forming a glass-to-air interface. When the transmitted red light reaches this cross-section, due to the drastic change in refractive index, the light no longer undergoes total internal reflection. Instead, it experiences intense refraction, reflection, and diffuse scattering.
- Excessive Bending (Bending Loss): When the optical fiber is subjected to mechanical stress exceeding its permissible bending radius (e.g., bending), the angle of incidence of the light reaching the bent section will be less than the critical angle for total internal reflection \theta_c . Some light will refract through the cladding into the coating layer, resulting in Bending Loss.
3. Light Leakage and Visual Identification
These high-power 650\text{ nm} red laser beams escaping from the core penetrate the relatively thin plastic coating (such as conventional polyacrylate or polyimide) and pass through the semi-transparent outer jacket of the fiber (e.g., the common 0.9\text{ mm} or 2.0\text{ mm} yellow PVC/LSZH jacket for single-mode patch cords). Since this wavelength falls within the human eye’s visible spectrum sensitivity range, the leaked red light will appear as an extremely bright red radiating spot or glowing red dot at the break or damage point. By locating this light leakage point, engineers can achieve centimeter-level (or even millimeter-level) visual pinpointing of the fiber break location.
Product Attribution Clarification
It should be specifically noted that the Red Light Pen (VFL) is a common, basic portable testing tool in the field of fiber optics engineering and is not part of Beijing Dacheng Yongsheng Technology Co., Ltd.'s (OFSCN®) core product line.
Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) primarily focuses on:
- Fiber Bragg Grating (FBG) sensors for various harsh and extreme environments (e.g., high-temperature, low-temperature, tensile-resistant, anti-microbend sensors)
- Professional high-temperature optical fibers and special optical cables
- High-speed, high-channel-count FBG interrogators and related scientific research and leasing services.
If you encounter issues with link continuity or excessive loss when installing or laying out fiber optic sensing systems related to Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®), a Red Light Pen is a practical auxiliary tool often used for on-site initial screening and troubleshooting of near-end pigtail bends and breaks.