FBG (Fiber Bragg Grating) Filters

FBG filters, one kind of applications of fiber Bragg grating, have become very important for commercial applications because of its compactness and low cost.

As the picture shows, it is made by using ultraviolet light with phase mask to write gratings in the photosensitive fiber. When the gratings are formed by the external UV light source, it is a small perturbation of fiber refractive index to occur. This fiber grating will reflect all the light beams have a certain wavelength range if the Bragg condition is met.

At here, the Bragg condition is: 2Λ = λ/n (assign the theta is half of pi) and Λ is the spatial period of grating, λ/n is the wavelength in the propagation medium. For example, we need to reflect light at 1550nm in the opposite direction then we can let Λ be nearly 530nm and consider n to be about 1.46 in a typical single mode fiber.

Optical Isolator

The main use of the isolating device is to protect the laser and an unstable system that the reflection of amplifier leads to, and its function lies in high return loss and causes the reflecting energy that is generated where it was a discontinuous medium to reduce its power.

One of common manufacturing methods is to utilize the magnetic field effect of Faraday crystal. As picture show that, at the beginning, the light source is inputted by the left end then the light is formed a very linearly polarization that we assume it is vertical when it passes through the polarizer.

After the Faraday rotator, the polarization is rotated 45 degrees. What follows is being lined by passing the second polarizer of 45 degree. Finally, the light that it has the polarization of 45 degree is outputted.

If we are viewed from reflecting view, as the light source being from the output end enters the polarizer and is lined to 45 degree. Later, it is rotated 45 degree again that causes the backward light is rotated total 90 degree and makes a transition to parallel light. At this time, the polarizations are perpendicular to one another, the front polarizer is vertical and the light is parallel, that it causes no light to pass through the polarizer. In short, the light can not pass through backward because it will be reduced to near zero.

For this reason, we can understand that isolator is an irreversible component and has different insertion loss between forward and backward directions. Generally speaking, it is 0.2 to 2dB forwardly and the backward is 20 to 40dB, and its return loss is greater than 60dB.

WDC Wavelength Dependent Coupler (or WDM Coupler)

The point of difference between WDC and WIC is that WDC regards wavelength as the basis of assigning input light to output channels but WIC stands on the ratio.

The picture shows the common component of three ports, among them regard the common end as the end of input from the light source, then the two output ports can have alternatives basing on different wavelengths. The main applications were traditional WDM, Wavelength Dependent Multiplexer component (1310/1550nm), and pumping power coupler of EDFA, Erbium Doping Fiber Amplifier (1480/1550nm or 980/1550nm), and so on.

So far as its applications is concerned, we maybe want to know how good it is to select the light of different wavelengths from light source at output ports. The parameter, Isolation, was useful to be defined:
Isolation = -10log(P2/P1) and should be greater than 20 to about 50dB.

Because the light is reversible, the WDC can be used for dividing waves for the light source (DeMux, the common end is regarded as the input end) or mixing waves (Mux, the common end is regarded as the output end) in a optical communication system.

Generally, its internal main structure is the thin film filter where regarded as the wavelength selector.

WIC Wavelength Independent Coupler

The function of wavelength independent coupler was to couple the light from each channel to the other one. It was applied to distribute suitable optical power in our optical circuits and the ratio of distribution was independent wavelength.

General specifications of couplers could be classed two categories, one was 50% to 50% (3dB) four ports another was 5%/95% and 1%/99% three ports.

The former mainly applied on the distribution of communication system channels, latter one may used to monitor the energy stability of transmission system and the optical characteristic parameters.

It had several kinds to manufacture coupler, what was more common method (fusion) to pulling four ends of two fibers while heating up till suitable coupling ratio.

Theoretically, a good coupler, the total input power should be same with the total output energy, but we known that it was imposable.
So, the Excess Loss was defined as: EL = -10log[summation (Pout)/Pin]. The extra loss value (EL Excess Loss) should reduce as far as possible, generally was 0.2 to 0.5dB about.

Return loss of Optical Passive Component

The reflection of light occur in passive component when the advancing light meet any cut sections or the discontinuous place of refracting rate, air gap, misalignment, …etc.

The reflecting question can cause two-piece, the first is the noise of reflection can cause S/N ratio to be worse. The second is the reflecting energy will cause laser source (maybe optical transmitor) to be unstable.

So, we defined a parameter, Return loss:
Return loss = -10log(Pr/Pin).As we know, the greater its value is, the more difficult to produce it is.

Insertion Loss of Optical Passive Component

At the first we should know that the optical power unit was used to be defined:
10log(P/1mW) and named dBm. P is the power measured by power meter or OSA.
Example:
If P is 30mW then we say it is 14.77dBm (10log(30mW/1mW)).

The second, we cared about the power was decreased when it passed through component so we defined the insertion loss as -10log(Pout/Pin) and called it dB. In here the negative symbol used was because the number, 10log(Pout/Pin), was always negative.
Example:
If Pin is 30mW (or 14.77dBm) and Pout is 25mW (13.98dBm) then we can calculate the insertion loss is:
– (13.98-14.77) = 0.79 (dB) or – (10log(25/30)) = 0.79 (dB).
It is natural that we should calculate it by the front, it is easier.

Fiber bending test

A macro bend experiment
Theory Geometry optics analysis for Macro-bending：
In here, we can define two parameters for this system as θc is the critical angle and θ is incident.
Then, we can understand that will be two states happened at the bending area.One is the light pass through bending area fully similar to ray A when θ＞θc.Another is light out of bending area because the refraction was happened if θ＜θc.

Thinking and a simple experiment
ThinkingBased on the theory to think about our product, maybe we have a worse detection direction of photodiode to lead bigger insertion loss.Like the figure 1 showing, the maximum power direction perhaps is direction A not B, so I think that we can adjust the detection direction to reduce insertion loss.

A simple experiment
I do a simple experiment this morning, put a power meter at direction A and direction B then measure optical power individually：Direction A is more or less -25dBm.And direction B is the neighborhood of -27dBm.