Multimode Fiber is a type of optical fiber communication transmission medium that allows light signals to be transmitted inside the fiber in multiple modes. Below is an introduction to the classification, numerical aperture, modes, and methods for measuring transmittance of multimode fibers.
1. Classification of Multimode Fiber
Multimode fiber can be divided into two categories based on the refractive index distribution:
Step-Index Fiber: The refractive index of the core and cladding changes abruptly, rather than gradually.
Graded-Index Fiber: The refractive index of the core gradually decreases from the center outwards, forming a gradient, which helps reduce modal dispersion.
2. Numerical Aperture of Multimode Fiber
The Numerical Aperture (NA) is a dimensionless quantity used by fiber manufacturers to determine the acceptance angle of the fiber, expressed as:
The refractive index of the cladding nclad is the refractive index of the optical fiber's cladding, n is the refractive index of the external medium, and θacc is the acceptance half-angle of the fiber.
For multimode fibers, this equation can be used to directly calculate the NA. The NA can also be experimentally determined by tracking the far-field beam distribution and measuring the angle between the beam center and the point where the light intensity is 5% of the maximum intensity.
3. Modes of Multimode Fiber
The number of modes (single-mode or multimode) supported by the fiber. The normalized frequency, commonly referred to as the V-number or V-value, is a dimensionless number proportional to the free space frequency but scaled by the guiding properties of the fiber. The V-value is expressed as:
Where V is the normalized frequency (V-value), a is the core radius, and λ is the free space wavelength.
For multimode fibers with a large V-value, the number of modes supported can be approximated using the following equation:
4. Transmission Performance of Multimode Fiber
Transmittance testing typically involves measuring the efficiency of the fiber to transmit light at specific wavelengths. Here are some common methods to test the transmittance of multimode fibers:
Light Source and Power Meter Method:
Use a stable light source (such as an LED or laser) connected to one end of the fiber. At the other end of the fiber, use a power meter to measure the light power passing through the fiber. Record the output power of the light source Pin and the reading of the power meter Pout.
Calculate the transmittance (T) using the following formula:
Insertion Loss Test:
This test is usually conducted simultaneously with the transmittance test. It indirectly evaluates transmittance by measuring the insertion loss in the fiber link. Use a reference fiber (with known transmittance) and the test fiber for comparison. Determine the insertion loss of the test fiber by comparing the power differences between the two.
OTDR (Optical Time-Domain Reflectometer) Method:
OTDR sends a pulse of light into the fiber and measures the reflected and transmitted light to analyze the characteristics of the fiber. While OTDR is primarily used for measuring the length and loss of the fiber, it can also provide transmittance information.
Optical Fiber Attenuation Tester:
Use a specialized optical fiber attenuation tester, which can automatically measure the transmittance of the fiber. The tester will emit light with known power and measure the light power after it passes through the fiber.
Post time: Aug-08-2024