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How do fiber optic connectors achieve precise alignment between fibers

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How do fiber optic connectors achieve precise alignment
between fibers?
When the two optical fibers are connected, due to the difference in position, shape and structure
of the two optical fibers, the energy cannot be 100% transferred from one fiber to the other, that
is, the connection loss will occur. The two connected fibers must be precisely aligned to ensure
low loss.The main function of the optical fiber connector is to connect two optical fibers quickly,
so that the optical signal can be continuous to form an optical path. How do fiber optic
connectors achieve precise alignment between optical fibers?
The types of fiber optical connectors are variable. However, the basic warranty for precise
alignment between optical fibers depends on two factors. The first is ceramic ferrules with
precise inner diameter, outer diameter and concentricity, which is warranted by mechanical
fabrication of ceramics. The second is ceramic sleeves with a cleft, which is a clever design. Fig.1
shows how two optical fibers are aligned through a ceramic sleeve. The inner diameter of sleeve
is a little smaller than the outer diameter of the ferrules. The ferrules can be inserted into the
sleeve because of the cleft. The expanded sleeve constricts the ferrules to ensure precise
alignment.
Fig.1
Alignment between two optical fibers
The core size of the single mode fiber (SMF) is about 8~10μm. The two connected fibers must be
precisely aligned to ensure low loss. Fig.2 shows how much the lateral offset between two optical
fibers influence the IL. The curve is exponential. A small lateral offset such as 2.4 μ m will
introduce IL of 1dB. Thus the lateral offset between the two fibers must be kept <0.5μm for SMF
fiber optic connectors.
Fig.2
IL vs the lateral offset between two optical fibers
Physical Contact between Fibers
However, only precisely alignment is not enough for optical fiber connection. As we know, back
reflection happens at the interface between two different medias. The refractive index (RI) of
quartz fiber @1.55μm is about 1.455. Thus the BR at the fiber endface is 3.4%. The BR light will
affect the performance of the communication system. Meanwhile, an IL of 0.15dB is introduced
at each quartz-air surface. Thus a fiber connection will add to 0.3dB signal loss.
Anti-reflection (AR) coating is usually employed to reduce BR at interface. However, it is not
considered for fiber optical connectors. Firstly, AR coating will add to the cost. Secondly, the fiber
connection is not fixed. Repeated plugging is required for fiber optical connectors, which will
damage the AR coating. Can we have the fiber endfaces AR coated and separated. Fig.3 shows
how the longitudinal gap between two optical fibers influences the IL. A gap of 50 μ m will
introduce an excess IL of nearly 1dB, which is intolerable in optical fiber communication system.
Thus we get the common view that the fibers must be in contact and the endfaces can’t be AR
coated. BR happens at the interface between two different medias. The air between fiber
endfaces must be squeezed out and thus the fiber endfaces are in physical contact (PC) as a
uniform media. Because the optical fibers are fixed at the center of ceramic ferrules, any
roughness on the ceramic surfaces will affect the physical contact between optical fibers. In order
to keep the physical contact between fibers, the ferrule endface is spherically polished with the
fiber endface at the top of the sphere, which is the second clever design. As shown in Fig.1, the
ferrules are inserted into the sleeve and the endfaces are deformed under pressure. The
deformation of endfaces ensures the physical contact. The physical contact depends on
deformation of the endface. Ceramic material is abrasion resistant and elastic, which are why it is
selected instead of glass.
Fig.3 IL vs the longitudinal gap between two optical fibers
Further Reduction of Back Reflection
The physical contact between fibers can ensure low loss in fiber connection, while the RL is
limited to about 55dB. For some applications requiring higher RL, fiber optic connectors with
angled polished endface are introduced, which is called angled physical contact (APC). Fig.4
shows the relationship between the added return loss and angle of the endface. The endfaces of
fiber optic connectors are usually polished by an angle of 8°, which adds to an excess RL of 36dB.
Thus the total RL of APC connectors are >65dB.
Fig.4 Relationship between the added return loss and angle of the endface
Fiber optic connectors are the most basic optical passive components for optical fiber
communication systems. The basic specification requirements for them are low insertion loss (IL)
and high return loss (RL), i.e. low back reflection (BR). However, as the most widely employed
components, low cost and easy connection are equal important as the specifications.
About HYC
HYC Co., Ltd was founded in 2000. It is a leading passive optical device OEM / ODM and solution
provider in the global industry, focusing on R&D, manufacturing, sales and service of passive
devices for optical communication. The company's main products are: optical fiber connectivity
products, WDM wavelength division multiplexers, PLC optical splitters, and MEMS optical
switches, which is widely used in 4G / 5G networks, Telecom, Data centers, etc.
http://www.hyc-system.com
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