I want to ask what is the meaning of these parameters :
1. What is Numerical Aperture? Why is it a dimensionless number?
2. What is Normalized Frequency? What is the reference to divide a frequency to get a normalized frequency?
3. What is fundamental mode spot size?
Hello Stevan
1) Numerical Aperture: This value is dimensioness because: NA=sinQ=sqrt(n1^2-n2^2), where n1 is a refractive index of core, n is a- refractive index of cladding.
2) Normalized frequency is a parameter that plays an important role in determining
the cut off conditionis defined as: V=(pi*a/lambda)*NA, where where a is the core radius, lambda is the wavelength in vacuum. The cutoff wavelength is the shortest wavelength at which the fibre will be single-moded. Wavelengths shorter than the cutoff will travel in multiple modes whereas wavelengths longer than the cutoff will travel in a single mode. For single-mode operation is required that V < 2.4048, which is the first root of the Bessel function J0.
3) Optical power (in single-mode fibre) travels in both the core and the cladding. In many situations, not the least being when we join fibre, we need a number that will give us a measure of the extent of the region that carries the optical signal. In single-mode fibre the core diameter is not sufficient.
The mode field (MFD) is defined as the distance between the points where the strength of the electric field is decayed to 0.37 (1/e=0.37) of the peak.
We introduce the concept of the “mode field”. The mode field can be considered the effective core of the fibre although the real core size is typically somewhat smaller. One important point is that there is no abrupt boundary that defines the extent of the mode field. Even though the signal decays exponentially there is no exact cutoff point. Therefore we must choose an arbitrary point as the boundary.
The MFD of standard SMF at 1550 nm is between 10.5 microns and 11 microns depending on the fibre. In the 1310 nm band the MFD of standard fibre is 9.3 microns.
The spot size is also sometimes used to characterise single-mode fibre. The diameter of the spot is just the radius of the mode field.
volodymyr's answer is very complete and fine. but i can tell you about NA in another way. it might have useful for you too. Total angle over which light in the air at the end of the fiber can propagate into the fiber, named acceptance angle. Sin ( theta-half acceptance angle ) = NA
good luck
@Volo : May I know why the normalized frequency is a dimensionless number? What parameter does it divide with?
As I read, the normalized frequency and the fundamental mode spot size is used for calculating quantities such as the single-mode cut-off, mode radius and splice losses. I used a single mode fiber, and from my calculation, I get the V value > 2.4048, is it possible? I used that to calculate the loss caused by splice.
From what you explain about MFD, what I get is that it's the effective area of the fiber optic cable? Am I right?
So, what is fundamental mode spot size? I still don't get it, since you explain about MFD and spot size separately.
@Sara : Oh, I get it, so I must count the NA from the refractive index formula, to get the half acceptance angle right?
yes, but this formula comes from ray optic and it's not fully appropriate for describing the operation details of optical fibers. However, it gives at least some estimate concerning how large an incidence angle may be for efficiently launching light.
I think more helpful will be read some books in the area of the fiber optics. For example like these:
1. Agrawal G. Fiber-Optic Communication Systems // A John Wiley & Sons, Inc., Publication, (Third edition), 2002.
2. H. Dutton. Understanding Optical Communications // Prentice Hall PTR, Upper Saddle River, New Jersey, 1998.
3. Agrawal G. NONLINEAR FIBER OPTICS
@Sara : ok then, thank you very much.
@Volo : ok, I'm going to search the ebook. Thanks. =)
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