Optical fiber parameters and influence on measurement method

Fiber Attenuation

To achieve the best system performance, it is important to choose optical fibers that transmit well over your full wavelength range of interest. This will minimize the amount of light lost through fiber coupling and reduce attenuation of some wavelengths over others. When working in the ultraviolet portion of the spectrum, particularly below 300 nm, it is important to use solarization-resistant fibers, as other fibers will become less transmissive over time at those wavelengths (an effect known as solarization).

Take a look below to find the attenuation spectrum that best suits your application or contact one of our application engineers for guidance. Keep in mind that 1 dB is equivalent to ~21% of light being lost in transmission.

What parameters can I specify?

• Patch cords provide 1:1 routing of light using a single fiber, although more than one fiber can be bundled into a fiber assembly jacket.
• Bifurcated fibers provide 1:2 routing in a Y-configuration. Two fibers are bundled at one end, branching out into separate “legs.” This allows fibers of the same or different core diameter or wavelength range to be used for each leg.
• Fiber splitters provide 1:2 routing as well but using three separate fiber segments. In this case, the fiber from each leg overlaps partially with the third fiber at a mechanical junction. Transmission is lower for this type of Y-configuration, but light from each fiber leg is efficiently mixed while traveling the common end.
• Probes end in a ferrule that can interface directly with your sample, either for reflection/backscattering or fluorescence, or for immersion in solution for transmission measurements.

Wavelength Range

Determining the wavelength range in the application to be measured is very important when choosing an optical fiber. Suitable optical fibers ensure light transmission efficiency, minimizing loss in the wavelength region to be measured.

• VIS/NIR is multimode step index fiber with a low OH fused silica core and glass cladding (400 – 2100 nm)
• UV/VIS is multimode step index fiber with a high OH fused silica core and glass cladding (300 – 1100 nm)
• SR is multimode step index fiber with a high OH fused silica core and glass cladding (200 – 1100 nm)
• XSR is multimode step index fiber with a high OH fused silica core and fluorine-doped silica cladding (180 – 900 nm)
• Single mode fiber is Corning SMF-28e+ fiber optimized for telecom use (1260 - 1700 nm)
• Single-mode performance ceases below the cutoff wavelength of λc = 1260 nm

The assembly’s boot color lets you know the fiber type and the most efficient wavelength range for using your fiber.

Diameter

Fiber core diameter is a basic parameter that affects the intensity of light transmitted through the measurement sample and led to the spectrometer.

The assembly’s color band tells you the fiber core diameter.

Jacketing

The fiber assembly jacketing is designed to protect the fiber and provide strain relief. The jacket layer increases the durability of the optical fiber, and there are many options to suit the operating environment and measurement application of the system.

Bend Radius & Mechanical Specifications

Optical fiber works by guiding light down the fiber core due to variations in index of refraction between the core and cladding. A flexible buffer material in one or more layers is then applied to improve flexibility and protect the glass core/cladding. Even with this additional coating, there are still limits on how tightly the fiber can be bent without being prone to microscopic fractures that can lead to breaks.

LTBR (long term bend radius): Observe as a minimum radius allowed for storage conditions.

STBR (short term bend radius): Observe as a minimum radius allowed during use and handling.

Mechanical Specifications: VIS/NIR, UV/VIS, SR fibers

Mechanical Specifications: XSR Fibers

Mechanical Specifications: Single mode fibers

Numerical Aperture

Optical fibers are designed to transmit light from one end of the fiber to the other with minimal loss of energy. The principle of operation in an optical fiber is total internal reflection. When light passes from one material to another, its direction is changed. According to Snell’s Law, the new angle of the light ray can be predicted from the refractive indices of the two materials. When the angle is perpendicular (90º) to the interface, transmission into the second material is maximum and reflection is minimum. Reflection increases as the angle gets closer to parallel to the interface. At the critical angle and below the critical angle, transmission is 0% and reflection is 100% (see figure below).

Snell’s Law can be formulated to predict critical angle and also the launch or exit angle θmax from the index of refraction of the core (n1) and cladding (n2) materials. The angle also depends on the refractive index of the media (n).

The left side of equation is called the numerical aperture (NA) and determines the range of angles at which the fiber can accept or emit light.

There are many types of fibers available, with a variety of numerical apertures.  While a fiber with a larger numerical aperture will collect more light than a fiber with a smaller numerical aperture, it is important to look at both ends of the system to ensure that light exiting at a higher angle can be used. In optical sensing, one end is gathering light from an experiment and the other is directing light to a detector. Any light that does not reach the detector will be wasted.

Solarization Effects

Ultraviolet radiation below 300 nm degrades transmission in silica fibers, resulting in solarization (increased light absorption in the fiber that occurs over time and impacts data). For applications below 300 nm, we recommend solarization-resistant assemblies.

XSR Fibers for High Transparency and Durability

Xtreme solarization-resistant (XSR) optical fiber and probe assemblies for spectroscopy are manufactured using a proprietary process for enhanced UV transmission (signal will transmit to 180 nm) and remarkable resistance to UV degradation, making it ideal for deep-UV applications (<300 nm). Ocean Optics is the only spectroscopy manufacturer to offer XSR Fiber.