Introduction to Non-Contact MPO (MNC™)

1. Non-Contact MPO (MNC™) Fiber Connectors

Fiber communication networks are the cornerstone of the modern information society. Endless fiber optic links require a tremendous number of fiber connectors.

Today, single fiber connectors are inadequate to meet the fiber connection density requirements. High density multi-fiber connectors are required. The number of fibers in a high-density fiber optic connector ranges from 8, 12, 24, 48. In the future, it can increase to 144, 576, or even 1024.

However, the current working principle of high-density fiber connectors has big problems because it relies on physical contact between the fiber end faces to transmit light signals. For example, if there are 48 fibers, the end faces of each pair of fibers must be in contact with each other, which is clearly virtually impossible to achieve.

The non-contact MPO fiber connector which we invented makes it ideal for transmitting optical signals without contact between the fiber end faces, therefore perfectly solving this major technical problem. Figure 1 shows a 48 channel non-contact MPO fiber connector. The non-contact MPO fiber connector has a trademark of MNC™.

The MNC™ fiber optic connector is based on the MPO fiber connector. The form factor of the MNC™ is the same as that of the regular MPO, the only difference is in the fiber surface. The MNC fiber optic connector has two key elements: A) an anti-reflection coating on the fiber end face (as shown in Figure 2), and B) all fiber end faces are slightly recessed from the connector plastic ferrule surface (see Figure 3). The anti-reflection film can prevent multiple reflection of light, and the recessed end face of the fiber can ensure that the end face of the fiber is not damaged when mated.

Figure 2. Non-contact MPO (MNC™) fiber connector (left) with anti-reflection coating on the fiber end face, regular MPO fiber connector (right)	Figure 3. All fiber endfaces of a MNC™ fiber connector are recessed from the plastic ferrule surface. Typical recess depth is 1 micron to several microns.

2. Comparison of working principles between MPO and MNC™ fiber connectors

Regular MPO fiber connectors require simultaneous physical contact between all fiber ends. In general, since all fiber ends are not always on a flat plane after being polished, when the MPO fiber connectors are mated, a slight air gap is naturally formed between fiber endfaces. Dust and other contaminant between the end faces of the fibers can also create air gaps. A regular MPO with an air gap is schematically shown in Figure 4(a).

Light has a 4% reflection at the fiber/air interface. When there is an air gap between the fibers, light is reflected multiple times between two fiber ends to form a Fabry-Perot cavity, as shown in Figure 4(b). Figure 4(c) shows the theoretical calculation of connector insertion loss as a function of air gap. It is unacceptable that the insertion loss value will rapidly oscillate with the change of the air gap, which seriously affects the stability of the signal. Therefore, the MPO fiber connector must ensure that the fiber end face is in close physical contact at all times, otherwise there is a serious problem.

Figure 4 (a) MPO connector mating diagram	Figure 5 (a) MNC™ connector mating diagram
Figure 4 (b) MPO connector with reflection	Figure 5 (b) MNC™ connector without reflection
Figure 4 (c) MPO insertion loss shows rapid oscillation with respect to gap.	Figure 5 (c) MNC™ insertion loss shows smooth variation with respect to gap.

MPO fiber connectors must eliminate the tiny air gaps that exist naturally to achieve physical contact. To ensure that there is no air gap, firstly, all the fiber end faces are prominently protruded from the surface of the plastic ferrule by grinding, and then a large working pressure is applied by the spring on the MPO fiber connector, so that the fiber surface is sufficiently deformed to achieve the effect of physical contact of many fiber ends at the same time.

The presence of large amounts of dust and other contaminants in the actual working environment can stop the physical contact between the fibers, making the connection stability of this fiber connector poor. The greater the number of fibers in a contact MPO fiber connector, the worse the stability of the optical connection.

The working pressure applied on the contact MPO fiber connector is proportional to the number of fibers. The more the fiber, the greater the working pressure. The 48 ch MPO requires too high working pressure.

Therefore, the working principle of the contact MPO fiber connector is a big problem.

Compared with the physical contact MPO fiber optic connector, the fiber endface of our MNC™ fiber connector is lower than the surface of the plastic ferrule, and the depth of the fiber recess is controlled at about 0.6 μm. The recessed fiber endface ensures that it is not damaged when mated. An anti-reflection film is deposited on all the fiber end faces, and after the connectors are mated, an air gap of 1.2 μm is generated between the end faces of the fibers, as shown in Fig. 5(a).

The anti-reflection film prevents multiple reflections of light so that a Fabry-Perot cavity is not formed, as shown in Figure 5(b).

Figure 5(c) shows the theoretical calculation of the insertion loss of the MNC™ connector as a function of the air gap. When the air gap increases, the insertion loss value is stable and substantially zero. For example, assuming that the additional air gap due to uneven grinding is 0.5 microns, and the original air gap is 1.2 microns, the final air gap is 1.7 microns. According to Fig. 5(c), the insertion loss caused by the air gap changing from 1.2 micrometers to 1.7 micrometers is negligible.

Therefore, the signal transmission of the MNC™ connector is not affected by the inevitable small air gap between the fiber end faces, and is the most ideal high-density fiber optic connector. The MNC™ connector works on single-mode and multimode fiber.

Because there is no physical contact between the optical fibers, only a small spring pressure is needed. Contact can be formed between the plastic ferrule surfaces. The springs used in our 24ch and 48ch MNC™ fiber connectors are 10N springs which we used in 12ch MNC™. MNC™ connectors do not require protruding fiber ends, which greatly simplifies the grinding process.

3. Test results of MNC™ connector

Fig. 6(a) Insertion loss distribution of 12ch SM MNC™	Fig. 6(b) Return loss distribution of 12ch SM MNC™

Fig. 6(a) and Fig. 6(b) show the test results of the insertion loss and the return loss. The connector is a 12ch SM MNC™ fiber connector. The insertion loss is slightly lower (better) than that of the regular MPO, and the return loss of MNC™ is about 10 dB higher than that of the regular MPO. This further verifies the working principle of the MNC™, and the insertion loss does not increase substantially when the light beam propagates in the air gap between the end faces of the fibers.

Figure 7 is a direct comparison of the insertion loss repeatability of the MPO and MNC™ connectors. Four kinds of MPO connectors (SM MPO, MM MPO, SM MNC™, MM MNC™) were mated 100 times with no cleaning in between, and measurement was taken after every five matings.

(a) MM MPO	(b) MM MNC™
(c) SM MPO	(d) SM MNC™
Figure 7. MPO fiber optic connector continuous docking 100 insertion loss test results

Comparing the test results of MM MPO and MNC™ (Figure 7(a) and Fig. 7(b)), it can be found that the insertion loss test results of the physical contact MM MPO are highly unstable. The insertion loss test result of the MNC™ is very stable. Comparing the test results of single-mode MPO and MNC™, the same results can be obtained by the two figures in Fig. 7(c) and Fig. 7(d).

It can be found that MNC™ has an absolute advantage over traditional MPO in terms of repeatability. The stable reliability of optical connections is the most fundamental reason why our MNC™ fiber connectors are actively adopted by customers.

4. Performance advantages of MNC™ fiber optic connectors

The MNC™ fiber connector uses the same components and is innovative in its working principle, resulting in a much more reliable high-density fiber connector than the regular MPO. Its many advantages come from its more reasonable and simple working principle.

MNC™ fiber connectors have the following significant performance advantages:

• Low insertion loss: Average 0.06dB;
• High return loss: 75dB (single mode);
• Exceptional insertion loss repeatability: <0.01dB;
• Long connector mating life: >2,000 matings (10 times that of MPO);
• Interchangeability: guaranteed connection between any pair of connectors;
• Not sensitive to dust;
• Small spring force: 10N for 48ch MNC™;
• MNC™ can mate MPO;
• MNC does not damage device under test due to no physical contact;
• Cost of production slightly higher than regular MPO connectors.