Fiber coupling efficiency and general questions regarding ZEMAX

[erweis]

Hello everyone,

a friend and I are working on our master project at a university chair. The project is to set up a free space optics communication (FSO) to, eventually, transmit data. For this we want to couple our laser beam into a fiber at the receiving end, which is then plugged into our receiver so that we can actually transmit data. It is important to notice that the setup of a lense that focuses the laser beam onto the fiber is necessary and directly 'shooting' into the receiver is no option for multiple reasons.

We started by simulating the FSO in ZEMAX and trying to find suitable lenses. Because of the current situation with the Coronavirus we are not able to enter the lab anymore and build the FSO. Therefore our project was shifted to be purely a simulation of the FSO. Now we want to simulate a given FSO and determine the maximum acceptable variance in position of the different elements in terms of offset (x- & y- displacement) and rotation (angular offset between the optical axis and incoming beam), so the power at the receiver is greater than the sensitivity of the receiver. This means we need to know:

1) what's percentage of total power sent that gets focused on the fiber

2) how much of the power focused onto the fiber is coupled into the fiber

1) the main issue here is that we do not know ZEMAX well enough to know if there is a tool that allows the user to determine how much power is focused on a specific area with respect to Gaussian beam distribution. We found the encircled energy diagram but from our understanding it appears to not show what we expect it should show for a given setup. Maybe we just don't know how to read it properly, so if anyone knows how to read a encircled energy diagram, feel free to educate us!

2) for this part we found the fiber coupling efficiency tool, but we are not sure if we understand it properly either. From our understanding this tools only tells the user what's the percentage of power which gets focused on the fiber that actually couples into the fiber.

Example: Let's say we start with 1mW power and we learn how to read the encircled energy diagram better and see that only 50% of the power actually gets focused on the fiber. Additionally the fiber coupling efficiency tool tells us that we have a system coupling efficiency of 25%. That means that from our 1mW we have 1000 uW * 0,5 = 500 uW (microWatt) on the fiber and 500 uW * 0,25 = 125 uW that actually get coupled in the fiber, right?

Another question that bothers us is that we don't understand why the coupling efficiency drops when the beam diameter gets smaller. Shouldn't it be the other way around? The smaller the beam diameter the less spherical abberation and the better in terms of fitting the N/A of the fiber. Since the ZEMAX manual says that the computation of the coupling efficiency only relies on the spherical abberation and N/A of the fiber we would expect an increase in coupling efficiency if we decrease the beam diameter, which is not what actually happens if we type it into ZEMAX.

In short: We would really appreciate someone who knows ZEMAX.

Thank you in advance for reading and trying to help!

- erweis

 

[Achy47]

Dear erweis,

Thank you for sharing your project with us. It sounds like you are working on a very interesting and important topic in the field of free space optics communication.

In regards to your first question about determining the percentage of total power sent that gets focused on the fiber, there are a few different methods you could use in ZEMAX. One option is to use the "Power Analysis" tool, which allows you to specify a region of interest (ROI) and calculate the total power within that region. This could be useful for determining the percentage of power focused on the fiber.

Another option is to use the "Spot Diagram" tool, which shows the spot size and location of the focused beam. By comparing the spot size to the diameter of the fiber, you could estimate the percentage of power focused on the fiber.

As for reading the encircled energy diagram, it is important to note that this diagram shows the percentage of total power contained within a certain radius from the center of the beam. This means that the power within the first circle (typically 50% of the total energy) is within the specified radius, and the power within the second circle (typically 80% of the total energy) is within twice the specified radius. So, for your example, if you see that 50% of the power is within the first circle, this means that 50% of the total power is within the specified radius, not that only 50% of the power is focused on the fiber.

In regards to your second question about fiber coupling efficiency, it is important to understand that the coupling efficiency is affected by multiple factors, not just the beam diameter. Other factors such as the alignment of the beam with the fiber, the quality of the optics, and the surface roughness of the fiber can also affect the coupling efficiency.

I would recommend consulting the ZEMAX manual or reaching out to their technical support team for further assistance in understanding the results of your simulations. They may also be able to provide suggestions for improving the coupling efficiency in your setup.

Best of luck with your project!