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● Narrowband Couplers for Use at 405 nm
● Available in Any Split Ratio Starting from 99:1
● Unterminated Fibers, FC/PC Connectors, or FC/APC Connectors
Features
● Wavelength Range: 405 ± 5 nm
● 1x2, 2x2, or 1x4 Configurations Available
● Any Split Ratio Starting from 99:1 Available
● Unterminated Fibers, FC/PC Connectors, or FC/APC Connectors
FOCC offers narrowband couplers for wavelengths of 405 ± 5 nm, with splitting ratios selectable from 99:1. Coupler configurations include 1x2, 2x2, and 1x4. Due to the short wavelength of 405 nm light, several additional factors need to be considered when selecting the appropriate 405 nm coupler type for a specific application.
Key Considerations
Fiber Optics: Two different designs are available, both enabling single-mode transmission at 405 nm wavelength: germanium (Ge)-doped fiber and pure silica-core fiber. Their typical operating wavelength range is 400 nm to ≥635 nm. For a numerical aperture (NA) of 0.12, the core diameter of germanium-doped fiber is 2.1 µm; while for the same NA, the core diameter of pure silica-core fiber is 3.0 µm.
Core Size:
The core diameter required for 405 nm single-mode fiber is more than three times smaller than that required for 1550 nm fiber (e.g., Corning SMF-28). Due to the extremely small core size, these fibers require extremely precise alignment when terminating in FC/PC or FC/APC fiber optic connector ferrules. Any misalignment of the fiber core when connectors are mated together (e.g., in a diaphragm) can cause significant optical loss. Furthermore, fusion splicing of small-core single-mode fibers requires specialized equipment to achieve optimal results.
Attenuation for 405 nm Light Transmission
Fiber loss is inherently greater than that of traditional near-infrared/telecommunications fibers. For example, the attenuation of fiber used to transmit 1550 nm light is ≤0.15 dB/km, while the attenuation of fiber used to transmit 405 nm light is typically ≤30 dB/km.
Due to the small core size, the end faces of single-mode fibers are exposed to intense 405 nm radiation, leading to end-face damage.
In 3 µm pure silica-core fiber, the power density of 5 mW continuous wave light is 70.7 kW/cm²; while in 2.1 µm germanium-doped core fiber, the power density for the same power is as high as 144.4 kW/cm². Any dirt, oil, or grease on the fiber end faces will be exposed to this intense light and undergo chemical decomposition, potentially damaging the fiber or increasing attenuation in the optical path to an unacceptable level. Because of their larger core diameter, pure silica-core fibers are less susceptible to end-face damage compared to germanium-doped fibers. Pure silica-core fibers are generally recommended for power outputs above 5 to 10 mW.
Color Center Formation: Fibers undergo photo-induced absorption (photodarkening) when exposed to light at a wavelength of 405 nm. This means that attenuation caused by absorption or scattering increases over time. The color center formation mechanism involves the interaction of light with dopants or impurities present in the fiber and stress-induced molecular bonds generated during fiber drawing, creating microscopic defects within the fiber. It depends on the fiber type, the wavelength and intensity of the light, and the exposure time. This effect is cumulative but, in some cases, reversible. Photodarkening can significantly impact the performance of any fiber or fiber optic device used at 405 nm wavelength, and may even render the fiber unusable. Pure silica-core fibers exhibit greater resistance to color center formation compared to germanium-doped fibers.
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