Question

# Basic concepts about fiber coupling

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Hi all,

I would like to ask some basic questions about the fiber coupling.

In the zemax Help file, it is said that the total coupling efficiency is the product of the system efficiency and the receiver efficiency. What is the receiver efficiency? Which factors will affect the receiver efficiency? i.e., how to increase the receiver efficiency?  Thank you in advance.

All the best,

YJ

### 4 replies

Userlevel 4
+3

It describes both the system efficiency S and receiver efficiency T in mathematical terms. Basically, the system efficiency describes the fraction of energy at the entrance pupil which is delivered to the fiber input, and the receiver efficiency describes the fraction of that energy which matches the mode of the fiber. The coupling efficiency is the product.

For a single mode fiber, receiver efficiency is maximized by presenting the fiber input with radiation forming a Gaussian waist that matches the mode field diameter of the fiber.

To add a little more: In designing optics coupling two single mode fibers with the same mode field diameter we need a magnification of one to preserve the mode field diameter, and minimal aberrations to preserve the Gaussian profile. Coupling fibers of different diameters requires an appropriate magnification and minimal aberrations.

Here is my question. I used a pair of collimator and coupler and tried calculating the coupling efficiency. When the distance between the collimator and coupler increased to e.g. 60 meters, the coupling efficiency decreased to 0.32 when polarization is not taken into account ( system efficiency of 0.88 and receiver efficiency of 0.37) and to 0.1 given polarization (system efficiency of 0.28 and receiver efficiency of 0.37).

Then I checked the M2 and it increased to ~10, i.e., it deviated far from the Gaussian beam at a distance of 60 m. In this case, what I can do to increase the  receiver efficiency, to meet the criterion you mentioned “by presenting the fiber input with radiation forming a Gaussian waist that matches the mode field diameter of the fiber”?

Besides, I didn’t get it by “a magnification of one to preserve the mode field diameter, and minimal aberrations to preserve the Gaussian profile”. Which operation needs to be done exactly?

Regards,

YJ

https://cloud.tugraz.at/index.php/s/foD3wFrTJZHackM

Userlevel 4
+3

Hi YJ,

I took a look at your file. I don’t think you’re reading the results correctly.

For fiber coupling without accounting for polarization I see:

System 0.999999, Receiver 0.950921, Coupling 0.950921

Accounting for polarization:

System 0.322407, Receiver 0,955296, Coupling 0.307994

Generally, the most important thing that considering polarization does is cause Fresnel reflections to be modeled. This looks like only 32% of the radiation is making it to the receiver, but that which does couples well to the fiber mode. The most obvious suspect is reflections from the air-glass interfaces.

Running a transmission report confirms this:

Field Pos : 0.0000 (deg)
Transmission at    1.5500:        0.324212919
Transmission at    1.4900:        0.305541476
Total Transmission       :        0.314877198

Chief Ray Transmission Surface By Surface:

Field Pos : 0.0000 (deg)
Wavelength 1: 1.550 µm

Surf        Tot. Tran        Rel. Tran
1         1.000000         1.000000
2         0.815600         0.815600
3         0.664695         0.814977
4         0.614032         0.923779
5         0.566588         0.922735
6         0.523402         0.923779
7         0.482961         0.922735
8         0.393903         0.815600
9         0.321022         0.814977
10         0.321022         1.000000

Field Pos : 0.0000 (deg)
Wavelength 2: 1.490 µm

Surf        Tot. Tran        Rel. Tran
1         1.000000         1.000000
2         0.803808         0.803808
3         0.645716         0.803321
4         0.596397         0.923622
5         0.550351         0.922793
6         0.508317         0.923622
7         0.469071         0.922793
8         0.377043         0.803808
9         0.302887         0.803321
10         0.302887         1.000000

So I think you have a good design, except that the coatings are reflective at your wavelengths. A Coating analysis for surface 2 which has THORA on SFL6 confirms 80% transmission for the surface.

Looking at Thor Labs web page for this part confirms THORA is a visible AR coating. I think you want THORC.

This might have been an error in the library part you loaded.

With THORC coating, the coupling is increased to 69%. So you in fact have a pretty nice design!

I attach the ZAR file in a ZIP.

Userlevel 4
+3

In regard to the two questions I didn’t answer:

Magnification: For coupling a source fiber to a receiver fiber where each have the same mode field diameter, the nominal design of the coupling optics would have a magnification of 1 to image the Gaussian waist at the output of the source to the input of the receiver. If the receiver had twice the MFD, the nominal design would have a magnification of 2. This would also result in reducing the NA to match the receiver.

Aberrations: To get a good quality Gaussian at the receiver we need minimal aberrations. A diffraction limited design is best.

In designing coupling systems, it is best to design a diffraction limited system first, using ordinary sequential design methods. You are trying for a diffraction limited system with an Airy disk comparable to the MFD of the receiver. Then do a final optimization using fiber coupling.