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Optical design in Optical Transceivers-the Nature of Coupling

Today we will discuss why it is an “Optical Transceivers“.

Before talking about optical modules, let us popularize one more knowledge point. In modern society, electricity is the closest energy source to human society. It represents the lifeline, and everything can live without electricity.

The first carrier for us humans to generate and transmit information is often electricity, but if electricity is used as a carrier to transmit signals, there are certain disadvantages, such as short transmission distance, low bandwidth, and high energy consumption.

The human industry has been exploring better solutions. After Gao Kun proposed the insertion loss requirements of optical fibers, the use of light as a carrier for signal transmission has become the mainstream, which has allowed the optical communications industry to prosper

1. What are Optical Transceivers?

Let’s go back to this article. We know that electricity is the carrier when information is generated, and light is the carrier when it is transmitted. Then, there must be an exchange process of converting light into electricity and converting electricity into light. We call this an optical module. The optical module, when you hear it literally, is it a module that combines optical components? In fact, the correct translation is optical transceivers. In my opinion, this term is more professional and accurate in representing the industry than the three words optical module.

The optical module is a small but indispensable link in optical fiber communication, used to realize photoelectric and electro-optical conversion. Therefore, one end of it is an electrical port, a switch/network card, etc. connected to it; the other end is an optical port, an optical fiber connected to it.

Due to the difference between photoelectric and electro-optical conversion components, there is a demand for various optical designs in optical modules, the purpose is to couple light into the optical fiber with high efficiency and high quality.

But in the final analysis, the most important thing in optical design is to achieve the following two major functions: coupling or combining and demultiplexing. Here, let’s talk about coupling first.

According to different optical fibers, optical modules can be divided into single-mode and multi-mode. The optical coupling inside the two optical modules is quite different.

Optical coupling in the single-mode optical module

For single-mode optical modules, we can only propagate one mode in the optical fiber. So it is necessary to shape the output light of the electro-optical conversion element. In order to a matched single-mode to achieve high-efficiency coupling.

This sentence is relatively false, and we will disassemble it step by step.

2. What is the mode in optical fiber?

Optical fiber is a constraint to light and is a boundary condition. Only light that meets certain conditions can we stably transmit it inside a single-mode optical fiber. And light that meets all conditions is this mode.

For example, how far an employee can go in the workplace depends on his skills and achievements. Whether they match the requirements of the career path. Assuming that upgrading to a department manager requires ten skills and achievements, if it satisfied an engineer, then he can move forward steadily in this career channel. If only 50% satisfied him, then he can only go to the group leader with a high probability s position.

3. What does single-mode look like?

In the optical communications industry, a large number of single-mode optical fibers are used to meet the G.652 standard. The core diameter is about 9um. The light that can be transmitted stably in this optical fiber fills the entire core. Observed from the cross-section, The intensity of the light is distributed in a Gaussian function: the center is strong and the edges are weak. The indicator for measuring the size of the light field on this section is the mode field diameter, which is about 9um. The mode field diameters of light of different wavelengths are different.

4. What does pattern matching mean?

Mode refers to light that meets certain conditions, and mode matching means that the parameters of the incident light are exactly the same as the modes that the optical fiber can allow to transmit.

Simply put, light can be described by wavelength, amplitude, and phase. To achieve high-efficiency coupling, the amplitude and phase of the incident light must be the same as the light field intensity and phase that the optical fiber can allow to transmit.

For example, to couple the laser light out of the G.652 fiber, we need a lens to transform the laser’s mode field into a light field with a diameter of 9um and a constant phase, which overlaps the fiber end face at this position.

In short, the essence of single-mode optical coupling is mode matching.

5. Optical coupling in multimode optical modules

Unlike the single-mode optical module, the optical coupling mechanism in the multi-mode optical module is more complicated, but the engineering application is simpler.

Multimode fiber can transmit many modes at the same time. For example, we used OM3 multimode fiber on a large scale, which can stably transmit hundreds of modes: in addition to the fundamental mode, there are many higher-order modes.

In a multi-mode optical module, the light source is often a multi-mode VCSEL, and its output light contains multiple modes.

From the perspective of mode matching, it seems that we need to decompose the light source into each mode, and calculate the coupling efficiency of each mode one by one, and obtain the total coupling efficiency after summarizing.

In addition, we should note that the various modes in a multimode fiber will couple with each other, and the incident light of a certain mode may also excite other modes.

Considering these factors comprehensively, the mechanism of multi-mode coupling is very complicated, but in engineering, we do not need to calculate it in this way.

Since there are many modes of multimode fiber, the wave characteristics of light are not significant. At this time, we can use geometric optics to describe and understand optical mode coupling.

6. Summary

Recall the description of optical fiber by geometric optics. The total reflection of incident light inside the optical fiber is the physical basis of the optical fiber light guide. Based on this, when doing multi-mode coupling, it is enough to complete two points:

  1. Reshape the light beam of the light source so that the numerical aperture (incident angle) is not greater than the numerical aperture (receiving angle) of the multimode fiber;
  2. The light spot incident on the multimode fiber after shaping needs to fall completely in the core of the fiber.

In short, we can regard the optical coupling in a multi-mode optical module as numerical aperture coordination.

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