Detailed explanation of the Core Dip indicator for MT and MPO connectors in optical module products
1. Core Dip indicator overview
1) Core Dip Description: Because the core of the fiber is softer than the cladding material, it is easier to be cut during the grinding process, forming a depression of the core (relative to the cladding), called "Core Dip". As shown below, the fiber core "Core Dip" of multimode MT/MPO products
2) Core Dip effect: The inner recess of the fiber core will cause the “Air Gap gap” between the fibers when the MT/MPO product is terminated, which directly (mainly) affects the “Return Loss Loss” indicator of the system.
3) Measurement of Core Dip indicator: Based on the definition of IEC 61300-3-30, it is recommended to use red light, at least green interferometer, which is more suitable for measuring microscopic continuous surfaces, which can improve measurement accuracy, repeatability and reproducibility. .
4) Correspondence between Core Dip indicator and Return Loss return loss:
Remarks: Return Loss is defined as the MT/MPO product docking test of the same specification, rather than the direct reflection of air. Remarks: Core Dip indicator positive means “sag” and negative number means “bulge”
2. Multimode high-speed optical module requirements for termination return loss indicators
1) 40G/100G SR4 optical module
Signal system: 10G/25GNRZ signal
RL return loss requirements: 20 ~ 30dB
Core Dip Specifications: <150nm
2) 400G SR8 optical module
Signal system: 50GPAM4 signal
RL return loss requirement: >40dB
Core Dip Specifications: <50nm
3) Introduction to the current situation of the industry
As the signal format of the high-speed optical module transitions from the NRZ signal to the PAM4 signal, it can be intuitively seen from the eye diagram that the system is more sensitive to "noise" and reduces the back-reflection of the system termination. Improving Return Loss) has become an important factor to consider
The actual "terminated" return loss of a multimode high speed optical module at the customer's end is determined by two factors:
A, Core Dip indicator of optical module optical interface (MT)
B, Core Dip indicator of MPO/MTP Patchcord connector purchased by the end customer
Optical module manufacturers can require their MT cable connector suppliers to manage the Core Dip metrics, but the MPO/MTP Patchcord quality assessment for their end users (such as Data Center customers) is unknown.
Therefore, we see a compromise solution in the 400G SR8 optical module based on PAM4 signal, in order to solve the hidden trouble of system return loss, is the multi-mode MT/PC grinding form, adjusted to multi-mode MT/APC grinding form, description as follows:
A, multimode MT/MPOAPC grinding type termination return loss (Return Loss) > 40dB
B, PC type MPO/MTP
PC vs APC end face reflection reflection
3. MT/MPO grinding process and Core Dip capacity reasons
1) Ceramic ferrule for connectors such as conventional LC/SC/FC, one fiber in one ferrule, in order to ensure complete contact of the fiber during docking, the ceramic ferrule adopts "spherical grinding technology" as shown below
2) Because the MT ferrule in the MPO/MTP connector is a fiber array structure, if the spherical surface is purchased, the intermediate fiber will be able to be docked, and the fibers on both sides will not be touched. Therefore, MPO/MTP products can only be used for "planar grinding".
3) MPO uses plane grinding, which brings a problem: although we say that the PC surface is 0 degrees, it is actually tolerance, that is, +/- 0.2 degrees, and it is the long axis and the short axis. There are angular tolerances in all directions. Then, when two MPO products (planar) are docked, because there is a grinding angle tolerance, the fibers will not be in contact with each other, forming a "butt gap".
4) Grinding angle tolerance is inevitable, then how to solve the fiber mating gap problem. Therefore, it is necessary to make the fiber protrude from the end face of the MT ferrule. The figure below shows the definition of fiber height for IEC 61755-3-3 and the 3D/2D pattern of fiber height measured by the MT interferometer:
5) In order to realize the fiber protruding MT ferrule end face during the grinding process, the flannel is generally used for grinding. Because the fiber material is hard, and the MT ferrule is made of PPS plastic material, it is softer. Therefore, during the lint polishing process, the fluff + abrasive particles can achieve a larger cutting amount for the plastic MT ferrule than the optical fiber, thus forming a "bump" effect.
6) However, in view of the difference in the “softness and hardness” difference between the fluff and the abrasive particles, the fiber core Fiber Core is softer than the fiber cladding Cladding, so in the process of grinding the convex fiber, Incidentally, the Core Dip depression is a "nm level unit", which is the capacity mechanism of Core Dip.
7) Commonly repaired MT/MPO Core Dip process
Back-cut grinding process: by adding a SiO/CeO polishing, the Fiber spherical area is smoothed as much as possible, and the Core Dip core recess is lowered. As shown in the following figure, the core may be slightly convex.
Flock Film Grinding Process: Reduces the formation of Core Dip by reducing the effect of fluffing of MT/MPO embossed fibers
4. to sum up
Market demand is the driving force for all technological advancement. Optical module products are welcoming the demand of new markets such as 400G and 5G. The transformation of product technology is an inevitable trend, and the coordination and integration of active and passive technologies will be closer.