The description of optical fiber connector is mainly composed of two parts, such as SC/UPC connector, SC represents the interface type of the connector, and UPC represents the ferrule end face shape.
Connector interfaces are divided into SC, FC, LC, ST, MPO and other types. The commonly used in engineering are LC, FC and SC. The main shapes of ferrule end face are PC, UPC and APC.
Difference of APC PC UPC
PC Physical Contact
The polished front end surface of the ferrule is spherical shape, it ensure that full contact of the the physical end surface when the two connectors are connected, and eliminate the impact of Fresnel reflection on the fiber end surface on the system and reduce the return loss (hereinafter referred to as RL) value reaches more than 40dB.
UPC Ultra Physical Contact
Compared with PC end face, UPC optical fiber end face have geometric size requirements in industry standards, making the return loss reach more than 50dB. UPC is based on PC with more optimized end-face polishing and surface finishing, and the end-face looks more dome-shaped.
APC Angled Physical Contact
The fiber end face is usually ground into an 8° bevel, and the reflected light is reflected to the cladding through the bevel angle instead of returning directly to the light source, thereby minimizing back reflection and making the return loss reach more than 60dB.
The 3 difference end face diagrams are as follows. Since the UPC connector has a higher return loss than the PC connector, it has completely replaced the PC connector, and the PC connectors commonly referred to as UPC connectors are now used.
Therefore, there are two main types of fiber optic connector end face currently in use: APC and UPC. The difference between APC and UPC is that the APC end face has an 8° bevel and the return loss is larger. In appearance, the main color of these two connector are obviously different, UPC is blue and APC is green.
RL of optical connectors
What is return loss?
The difference in transmission characteristics between APC and UPC is the difference of return loss. Return Loss (abbreviation: RL) refers to the number of decibels of the retro reflected optical power relative to the incident optical power of the optical signal at the connection point.
This formula can also be written as: RL=10lg(P1/P0). The larger the RL value, the smaller the reflected light power entering the optical fiber; the optical power reflected by APC into the optical fiber is smaller than that reflected by UPC, so the value of RL is larger. Obviously, the larger the RL, the better.
The impact of return loss on optical communications
In the analog optical communication system, the intensity of the optical signal changes continuously. If the intensity of the reflected signal in the optical fiber is large, it will inevitably affect the normal signal. Therefore, the larger the RL value of the active connector in the analog optical communication system, the better.
In a digital optical communication system using NRZ codes, there are only two intensities of optical signals (representing “1” and “0” respectively). As long as the intensity of the reflected signal in the optical fiber is not enough to affect the judgment of the signal “1”, then It will not affect optical signal transmission. Therefore, in digital optical communication systems using NRZ codes, there are no special requirements for the RL value of the movable connector.
Currently, for 50G, 200G, and 400G optical communication systems with single optical modules in metropolitan area networks, the line code mainly uses PAM4 code. There are 4 signal strengths of PAM4 code, representing 11, 10, 01 and 00 respectively. When the intensity of the reflected signal in the optical fiber is large, it may affect the judgment of the “01” signal. Therefore, communication systems that use PAM4 codes as line codes have certain requirements for the RL of the connector.
In an actual optical transmission system, there are usually multiple active connections in an optical fiber transmission link. The optical power of the reflected signal generated by each active connection will accumulate in the optical fiber link. Therefore, the greater the number of active connections in the optical fiber link, the return loss requirements for each connector are also higher. For example, for a 40km optical fiber link using PAM4 code, when the number of active connectors in the link is 2/4/6/8, the return loss of the active connectors should be no less than 27dB/32dB/35dB/37dB respectively.
The impact of ferrule end face contamination on return loss
When the end face of the connector is contaminated, it will cause an increase in return loss. Through testing the insertion loss and return loss of UPC active connectors in the existing network, it was found that the return loss of the connector is more sensitive to end-face contamination. About 20% of connector return loss in the current network are lower than the standard value due to end-face contamination. Only when the end-face of the connector is seriously contaminated will the insertion loss be affected.
The insertion loss and return loss of UPC connectors with contaminated end face are shown in the table below.
| S/N | Insertion loss (dB) | Return loss (dB) |
| 1 | 0.06 | 48.5 |
| 2 | 0.08 | 47.2 |
| 3 | 0.34 | 35.9 |
| 4 | 0.32 | 20.1 |
| 5 | 0.06 | 45.4 |
| 6 | 0.12 | 43.3 |
| 7 | 0.69 | 40.3 |
| 8 | 0.01 | 45.6 |
Application of APC connectors
Since the APC connector has a larger return loss, it is suitable for use in optical communication systems that require higher return loss. like:
(1) Analog optical communication systems such as CATV;
(2) Metro 50G, 200G, 400G and above optical communication systems with line code type PAM4;
The previous analysis shows that PAM4 signals do not have high requirements for connector return loss (when there are 8 active connections in a 40km link, the connector return loss is not less than 37dB). But the end face contamination is a common problem, so recommend to use APC connectors. For example, there is a 50G SPN system opened by an operator using the access optical cable network (a total of 8 active connections in the entire link), due to the use of PAM4 line codes and UPC connectors, some systems experienced an increase in pre-correction error counts, Local-Fault and 50G port flashing were reported. After the connector was replaced with APC, the fault was eliminated.
- WDM system using Raman fiber amplifier
The APC connector end face is an 8°bevel, and its cross-sectional area is larger than that of the UPC connector. The larger optical fiber cross-sectional area and larger return loss make APC connectors more suitable for carrying higher optical power. At present, Raman fiber amplifiers are commonly used in single-carrier WDM systems exceeding 100G; the reverse output optical power of Raman fiber amplifiers is as high as 30dBm. If a UPC connector is used, it is easy to burn out the end face of the connector; therefore, equipment manuals for Raman fiber amplifiers usually clearly require the use of APC connectors.
Conclusion
APC and UPC connectors with the same interface have basically the same size. Although they can be connected physically, due to the difference in pin end faces, the insertion loss after connection is larger (more than 4.0dB). Therefore, APC and UPC connectors are Cannot be mixed.
Currently, UPC active connections are commonly used in operators’ optical wiring equipment. With the increase in the use of communication systems with line code type PAM4 and WDM systems using Raman fiber amplifiers, the shortcomings of UPC active connections are increasingly exposed, and optical fiber active connectors should transition from UPC to APC as soon as possible.
FAQ
What end face for multimode fiber?
Mulitmode fiber use UPC ferrule, APC is only for single mode fiber.
UPC and APC ferrule are compatible?
Connector connections need to have the same end face structure. For example, the end face of APC is ground to an 8-degree angle. APC cannot be combined with UPC, which will cause the performance of the connector to decrease. However, the optical fiber end faces of PC and UPC are both flat, and the difference lies in the quality of grinding. Therefore, the mixed connection of PC and UPC will not cause permanent physical damage to the connector.
Is ferrule equal to fiber?
No. Ferrule is ceramic material, white color, and in the middle there is hole to insert the bare fiber, it is only to hold the fiber and make it in place. They are not the same. The ferrule concentricity will affect the fiber insertion loss.
What is the insertion loss of connector?
Normally, the insertion loss will be decided by the ferrule and the polishing artworks. The international standard insertion loss will be less than 0.3dB for both single mode fiber and multi mode fiber, the lower, the better. But there are some project only request 0.5dB the most. It is ok. And if you need lower at 0.1dB, that is ok to do, the cost will be higher.
How do you know test the return loss?
We mainly use the insertion loss and return loss tester to check, it can directly show the data of the fiber optic connector on screen, if fail, will return back to polish and grinding again. please refer to our quality management page for more details.
What is PC UPC APC?
PC UPC APC is 3 different end face shape of the ceramic ferrule. PC is physical contact, UPC is ultra physical contact, APC is angel physical contact. Please read article for more information: Optical Fiber End Face Grinding Polishing Method PC UPC APC
What is the difference between UPC and PC?
UPC is Ultra Physical Contact. Compared with PC end face, UPC optical fiber end face have geometric size requirements in industry standards, making the return loss reach more than 50dB. UPC is based on PC with more optimized end-face polishing and surface finishing, and the end-face looks more dome-shaped.
Is APC better than UPC?
APC return loss is better than UPC, but we can’t say who is better than the other, it mainly depends on the actual application.
PC is the most common polishing method for fiber optic connectors and is widely used in Telecom operator equipment. UPC is usually used in fiber optic Ethernet equipment (such as ODF fiber optic distribution frames, media converters and fiber optic switches, etc.), digital, quantum and telephone systems, etc. APC is generally used in high wavelength range optical RF applications such as CATV, and also in optical passive applications such as PON network structures or passive optical relays.
How to clean the dust or oil on the ferrule end face?
There are many type of connector cleaning kits, like cleaner pen, cleaner cassette, and please refer to link to check how to use them.
