Tài liệu ADC KRONE - Guide - FO - The challenge in using APC connector ppt

By having the apex of the spherical polish coincide with the center of the ferrule containing the optical fiber, the fibers have a high probability of achieving physical contact when mat

white paper

The Challenges of Apex

Fiber optic connectors provide a low loss means of connecting optical circuits Low loss is achieved by minimizing the lateral offset between optical fibers and by maintaining physical contact between the optical fibers Physical contact can be achieved when the ferrules containing the optical fibers have a convex spherical polish on the endface By having the apex of the spherical polish coincide with the center of the ferrule containing the optical fiber, the fibers have a high probability of achieving physical contact when mated together The parameters that define the endface geometry for a physical contact (PC) ferrule are radius of curvature, height of the fiber with respect to the polished radius, and the distance between the apex of the spherical polish and the center of the fiber (apex offset)

The spherical polish of a ferrule can be applied at an angle to reduce reflectance of the connection The most common angle is 8° Meeting the criteria for fiber height and radius are no more difficult

to do in an angled physical contact (APC) connector than a non-angled PC connector However, achieving low apex offsets proves to be a challenge when the ferrule is polished at an angle

Offset in APC Connectors

Generally speaking, ferrules have a chamfer around the endface that is symmetric with the axis of the ferrule (See Figure 1) When polished at 8°, material is not removed uniformly about the axis of the ferrule (See Figure 2) Because the ferrule is tilted in polish, more material is removed from one side of the ferrule axis than the other As Figure 2 shows, the angle on the left of the chamfer is not

as steep relative to the angle on the right of the chamfer As material is removed in polish, the surface of the endface becomes relatively larger on the left of the ferrule axis as shown in Figure 2 than the right Hence the center of the ferrule endface moves to the left, away from the ferrule axis When polished using a compliant rubber polishing pad, a convex spherical radius is put onto the ferrule endface The peak or apex of this radius will generally be at the center of the surface being polished The distance between the center of the fiber and the center of the spherical surface being polished is the apex offset Because this peak drifts away from the ferrule axis as more material is removed, the apex offset will increase

Figure 1

The relationship between apex offset and the angle the ferrule is being polished at is:

Where:

• y = Ferrule removal during polish, measured along the ferrule axis

• x = Initial ferrule pedestal diameter

• θ= Desired ferrule angle (the angle the endface is viewed at in an interferometer)

• α= Ferrule chamfer angle

X0

Y

R

∝

θ

Ferrule/Fiber

Centerline

Polished

Endface

Apex Offset

Material

Removed in

Polish

2tanα

x 0 + 2 y tan α

x 0

sin (90 - α + θ) 2 cos θ

sin (α)

For a given ferrule geometry (that is, a defined angle of polish, pedestal diameter, and chamfer angle), there will be a minimum apex offset that can be achieved regardless of the quality of the fixtures or methods used In addition, as equation 1 shows, the more the ferrule is polished, the greater the apex offset becomes

Figure 3 shows how the apex offset will vary with the amount of ferrule material removed during polish for several initial pedestal angles As the plots show, the initial pedestal angle has the largest affect on apex offset But minimizing the ferrule chamfer is not easy because international

intermateability standards generally require chamfers of 32.5° to 37.5° Given this, we see that it’s impossible for an APC connector of this design to have an apex offset better than about 25 microns

We have to consider variations in manufacturing processes when determining a maximum permissible apex offset specification If the polishing fixture had no variation between each connector position, one could polish apex offsets with a minimum per Figure 3 and small variations above that point But even the best fixtures and processes have variation that will result in apex offsets greater than 50 microns if the minimum possible apex offset is 25 microns To achieve acceptable yields, the maximum permissible apex offset needs to be greater than 50 microns 65 microns will allow manufacturers to achieve high yields while allowing connectors to maintain physical contact during extreme environmental conditions Considering that PC connectors have an apex offset range of 0-50 microns, having APC connectors with a range of 25 –65 microns still requires a very consistent polishing process Even though APC connectors will have higher average apex offsets than PC connectors, variation in APC connectors

is reduced

Apex Offset vs Ferrule Material Removed for Various Ferrule Chamfers

0

10

20

30

40

50

60

70

80

90

100

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

Ferrule Material Removed in Polishing (mm)

Chamfer = 20∞ Chamfer = 30∞ Chamfer = 40∞

Figure 3 (For X 0 = 585 mm)

A 65 micron apex offset on APC connectors is acceptable for long-term environmental performance as long as the fiber height is ±50 nanometers International standards (Telcordia and IEC) require that apex offset on APC connectors be less than 50 microns But this assumes a fiber height of ±100 nm and a radius of 5 mm to 12 mm These requirements correctly recognize that if both fiber recess and apex offset are too large for a given radius, physical contact could be lost from the pistoning of the fiber within the ferrule But if we reduce the minimum fiber height requirement, we can easily increase the apex offset a small amount and maintain physical contact in temperatures as great as 85°C It has also been shown through experimentation that APC connectors with 65 micron apex offsets and –50nm minimum fiber heights will pass the environmental requirements of Telcordia-GR-326 Given that the minimum possible apex offset on APC connectors of this design is about 25 microns (unless other methods are used), it makes sense to increase the maximum apex offset to 65 microns as it will not inhibit connector performance and greatly increases manufacturing yields

A method to achieve lower apex offsets is to have a ferrule whose chamfer is offset from the axis of the ferrule by an angle equal to the endface angle In this configuration material is removed at an equal rate from both sides of the ferrule axis and no apex drift occurs The result is that an apex offset of zero

is possible However, if one wants to optimize the insertion loss of the connector via tuning, the orientation of this offset is not known until after tuning At this point it’s technically prohibitive to grind

on the chamfer of the ferrule For any APC ferrule that is to be tuned, an offset chamfer cannot be used, forcing a higher minimum and maximum apex offset onto the ferrule So the tradeoff is having lower apex offsets or having a tuned connector Tuning dramatically reduces the loss between two connectors by aligning all fiber eccentricities with the top of the ferrule The average insertion loss for a 6-position tuned connector is 0.08dB versus 0.25dB for untuned (depending on ferrule I.D tolerances) Because the increased apex offset doesn’t inhibit connector performance given tighter fiber heights, it’s preferred to tune without an offset chamfered ferrule as the improvement in loss far exceeds any benefit of a lower apex offset

Figure 4 shows a polished APC ferrule The dashed lines represent the unpolished ferrule and the solid lines are the polished ferrule geometry

Apex offset is the difference between the center of dimension z (the center of the polished surface) and the center of dimension x1(the center of the optical fiber) We can find z by using the law of sines:

c=z

C=2α

A=90- α+θ

b = y +

2tanα

x 0

c = sin(A)

bsin(C)

cosα 1

2 cos α

1

(Using the imaginary lines in Figure 4 to find the length of b)

´

´

Recognizing that apex offset is:

We insert z into apex offset:

Using the relation: 2cosx =

2

z

- , Where x1= x0+ 2ytanα

2cosθ

x 1

2tanα

x 0 + 2 y tan α

x 0

sin x sin 2x

sin (90 - α + θ) 2 cos θ

sin (α)

2tanα

x 0 + 2 y tan α

x 0

sin (90 - α + θ) 2 cos θ

sin (α)

X0 2tan ∝

X0 2

X0

2

∝

+∝

z

b

∝

θ

∝

Ferrule/Fiber Centerline

π 2

π

Figure 4