Evaluating the reliability of optical connectors

Evaluating the reliability of optical connectors 

Report 1 


Yuichi Aoki Technical Development Headquarters, Reliability Research Section 

Kishichi Sasaki Reliability Center for Electronic Components of Japan, Environmental Testing Laboratory 

Kiyoyuki Mutaguchi Japan Aviation Electronics Industry, Limited, Connector Division 

W 

e performed the High Temperature Storage Test (Damp Heat) and the 

Temperature Cycling Test on optical connectors in conformance to 

the Telcordia standards. During these reliability tests, automatic 

measurements of the optical characteristics of the specimens were carried 

out and physical changes were compared. During the evaluation following 

the tests, we checked for physical changes in the tips of the optical connectors. 

We found pistoning changes occurring on the tips of the connectors, especially 

during the Temperature Cycling Test. However, these pistoning changes caused 

almost no perceptible changes in optical characteristics. The ferrule tips were 

distorted after 2000 hours at 85°C and 85 percent relative humidity, exhibited 

flattening distortion. 

1 

Introduction 

Next-generation technology infrastructure using optical fiber has been on the march 

toward achieving widespread use during recent years. Acceptance of this technology has 

resulted in demand for lower cost and miniaturized optical components, but such 

improvements must be based on information from reliability testing assuring that quality 

can be maintained. Testing to evaluate reliability of optical components is generally 

performed in conformance to the Telcordia standards. *1 These standards include 

requirements for simultaneously measuring optical characteristics of multiple specimens 

during environmental testing. 1) Measurement methods are not affected by measurement 

margin of error when measuring with the connectors either connected or disconnected. 

Additionally, automatic measuring is capable of efficiently gathering data at short 

measurement intervals. MTF (Mean Time to Failure) calculations are easy to perform using 

this system, for which published examples are available. 2,3) This report will focus on optical 

connectors, which exhibit the basic characteristics of optical components. We shall also 

consider the results of reliability testing that incorporates automatic measuring in 

conformance to the Telcordia standards. 

Fig.1 Overview of optical components 

- 1 - 

Espec Technology Report No20


2 

The Telcordia standards that require simultaneously measuring optical characteristics of 

multiple specimens are mainly concerned with passive optical components. In this report 

the term "passive optical components" refers to such items as fiber, connectors, 

switches, couplers, attenuators, and all types of filters. Table 1 shows reliability tests for 

typical optical components. 

3 

Test standards for passive optical components 

Table 1 Telcordia test standards 

Standard Title 

GR-326-CORE 

Generic Requirements for Single-Mode Optical Connectors and 

Jumper Assemblies 

GR-1209-CORE Generic Requirements for Passive Optical Components 

GR-1221-CORE 

Generic Reliability Assurance Requirements for Passive Optical 

Components 

Structure of optical connectors and factors affecting their reliability 

Optical connectors are 

used to connect optical 

fiber. Photo 1 shows 

some typical 

connectors. Fig.2 

shows the composition 

of the optical 

connectors used in these tests. 

The fiber is inserted into the housing, which is called a zirconia ferrule, and the fiber is 

held in place with a hardening epoxy adhesive. The adhesive used for the connectors is 

Epo-Tek353ND (Epoxy Technology, Inc.). The degradation of this adhesive is a crucial 

element leading to the loss of the reliability of the optical connectors. A major factor 

producing degradation is the absorption of humidity, which causes degradation to occur 

over time. 1~5) Photo 1 Optical connectors 

The High Temperature Storage Test (Damp Heat) is used to evaluate 

humidity-induced degradation. As Fig.2 shows, adhesive degradation resulted in 

pistoning changes to the ferrule tip on the end of the optical connectors. A significant 

amount of pistoning causes such problems as an increase in air layers and an increase in 

leakage of insertion light. These problems result in an increase in optical loss. 

The Temperature Cycling Test was also run to evaluate the progression of cracking 

caused by micro-cracks in the fiber. 

- 2 - 



Fig.2 Structure of optical connectors and factors affecting their reliability 

4 

Environmental test system for optical components 

An environmental test system for optical components was developed to be used in this 

research for testing the reliability of passive optical components. Photo 2 shows the 

system, and Fig.3 shows a block diagram of the system. 

This measurement system selects the light source wavelength using an optical switch 

selector, and is capable of measuring a maximum of 170 channels simultaneously for 

multiple specimens during environmental testing. This capability puts the system in 

conformance to the requirements of the Telcordia standards. The system is capable of 

measuring insertion loss, *4 return loss, *5 and PDL (Polarization Dependent Loss). *6 

Photo 2 Environmental Test System 

- 3 - 



5 

Test method 

Fig.3 System block diagram 

The High Temperature Storage Test (Damp Heat) and the Temperature Cycling Test 

were run using optical connectors as specimens. Table 2 shows the test conditions, and 

Table 3 shows the specimens. The test conditions complied with the 

Telcordia-GR-1221-CORE standards. The specimens consisted of single mode *7 SC and FC 

connectors. As seen in Fig.4, the specimens were connected in groups of three optical 

connectors with fiber and with the measurement points n = 5 (groups). The 

measurement system was used to measure insertion loss and return loss. Table 4 shows 

the measurement conditions. 

The test specimens were measured after each 1000 hours of the High Temperature 

Storage Test (Damp Heat) and after the completion of the Temperature Cycling Test. A 

Ferrule Tip Condition Surveyor (Direct Optical Research Company) was used to measure 

ferrule tip curvature radius, *8 eccentricity, *9 and pistoning. Failure was determined using 

the Telcordia-GR-1221-CORE evaluation standards shown in Table 5. 

The Temperature Cycling Test was not effective for determining degradation, since 

changes occurred in both insertion loss and return loss in the optical fiber temperature 

characteristics in this test. The PVC sheath *10 coating in particular exhibited major 

changes at low temperatures, and so the test was run using Hytrel ® core *11 , which 

exhibited minimal loss at low temperatures as seen in Fig.5. 

- 4 - 



Table 2 Reliability test conditions 

Test items Test Conditions 

High 

Temperature 

Storage Test 

(Damp Heat) 

Temperature 

Cycling Test 

85°C, 85%rh, 

2000 h 

-40°C ←→ 85°C, 

1 hour each, 500 

cycles 

Fig.4 Test method 

Table 4 Measurement conditions 

Items Details 

Measurement items 

Insertion loss and 

return loss 

Light source 

wavelength 

1310 nm, 1550 nm 

Light source power 1 mW 

6 

Measurement 

intervals 

Test results 

Every 10 to 15 min. 

Specimens 

No. of 

specimens 

Evaluation 

method 

- 5 - 

Table 3 Specimens 

FC connectors, SC connectors 

(single mode) 

n = 5 

Insertion loss and return loss 

(optical component 

environmental test system) 

Tip condition measurement 

(pistoning, eccentricity, and 

curvature radius) 

Fig.5 Fiber temperature 

characteristics 

Table 5 Evaluation standards 

(from Telcordia GR-1221-CORE) 

Requirement Objective 

Insertion 

loss 

change 

0.3dB 0.2dB 

Return 

loss 

change 

6-1 Test results from the High Temperature Storage Test (Damp Heat) 

5dB 2dB 

Fig.6 shows the measurement results for the light source wavelength of 1310 nm for the 

first 1000 hours of the 2000-hour High Temperature Storage Test (Damp Heat). After 

1000 hours and again after 2000 hours, we verified that fluctuation did not exceed any 

evaluation standards. In addition, identical trends were exhibited for both light source 

wavelengths, 1310 nm and 1550 nm. No major difference was seen between the FC 

connectors and the SC connectors. 



Test time (h) 

(a) SC connector insertion loss 

Test time (h) 

(c) FC connector insertion loss 

- 6 - 

Test time (h) 

(b) SC connector return loss 

Test time (h) 

(d) FC connector return loss 

Fig.6 Results of the High Temperature Storage Test (Damp Heat) 

(light source wavelength: 1310 nm) 

6-2 Results of the Temperature Cycling Test 

Almost no differences were seen between the FC connectors and the SC connectors in 

the Temperature Cycling Test. Fig.7 shows partial measurement results for the SC 

connectors. Some insertion loss change and return loss change was seen following 

temperature changes in the temperature chamber during the Temperature Cycling Test, 

but these changes did not exceed the evaluation standard limits. The changes included 

insertion loss occurring at low temperatures, but no major fluctuation was seen at high 

temperatures. Return loss, however, tended to increase at high temperatures. Looking at 

the return loss changes over the long term confirms that as soon as return loss increases, 

the fluctuation stabilizes. This is thought to indicate that return loss quality stabilizes due 

to the adhesion of the connectors over time. 



(a) Insertion loss 

(b) Return loss 

Fig.7 Results of the Temperature Cycling Test 

(SC connectors; light source wavelength, 1310 nm) 

6-3 Pistoning measurement results 

Measurement results for the ferrule tip surfaces did not indicate major changes in 

eccentricity and curvature radius, but the measurements confirmed that changes 

occurred in pistoning compared to the initial values. 

Fig.8 shows pistoning measurements after 1000 and 2000 hours of the High 

Temperature Storage Test (Damp Heat), and after 500 cycles of the Temperature Cycling 

Test. This report represents the direction of pistoning as a negative value. In the IEC, the 

standard for tolerance is recommended as -50 to 100 nm, and this level is exceeded, but 

the insertion loss and return loss measurements did not exhibit conspicuous fluctuation. 

On the other hand, optical connectors with powerful insertion light may be subject to 

influence by pistoning. Because of this, we face the challenge of investigating the 

relationship between pistoning and optical characteristics in such areas as manufacturing 

conditions. 

In this research, more pistoning occurred in the Temperature Cycling Test than in the 

High Temperature Storage Test (Damp Heat), regardless of whether the type of fiber 

coating included Hytrel ® core or the PVC sheath. The cause for the increased pistoning is 

thought to include stress in the direction of the pistoning from contraction and peeling of 

the adhesive. This, in turn, is thought to be possibly caused by the effects of humidity and 

the re-hardening of the adhesive at the test temperature (85°C) for reliability testing. 

- 7 - 



Test time (h) 

(85°C, 85%rh, PVC sheath coating) 

Test cycles 

(-40/85°C, 1 hour each, PVC sheath coating) 

Test cycles 

(-40/85°C, 1 hour each, Hytrel ® core) 

Fig.8 Changes in pistoning 

- 8 - 



6-4 Observation of the tip shape 

Fig.9 shows the results of observations made on the shape of the ferrule tip before and 

after the High Temperature Storage Test (Damp Heat). After 2000 hours at 85°C and 85 

percent relative humidity, the surface of the ferrule tip showed signs of pitting and was 

rougher than before testing. This roughness did not appear on the surface of the tip after 

the Temperature Cycling Test, leading to the conclusion that the roughness was caused 

by humidity. Fig.10 shows a three-dimensional computer graphics rendition of the shape 

of the tip. After 2000 hours at 85°C and 85 percent relative humidity, the ferrule tip 

exhibited flattening distortion. However, pistoning measurements with the Ferrule Tip 

Condition Surveyor measured the position of the apex of the tip from the sphericity of the 

ferrule tip, and, using that position as a standard, calculated the amount of pistoning. 

Accordingly, if the position of the tip section differs from these measurements, the 

amount of pistoning could be greater than that calculated. 

(a) Initial period 

(b) After testing 

Fig.9 Ferrule tip observation (85°C, 85%rh, 2000 h) 

(Initial period) (-40 /85°C, 500 cycles) (85°C, 85%rh, 2000 h) 

Fig.10 Three-dimensional shape of the ferrule tip 

- 9 - 



7 

The optical characteristics of the optical connectors reviewed for this study were 

automatically measured during the High Temperature Storage Test (Damp Heat) (at 

85°C and 85 percent relative humidity for 1000 hours) and the Temperature Cycling Test 

(at 85°C / -40°C for 500 cycles of one hour each). We obtained the following results. 

(1)No changes occurred in optical characteristics, but pistoning increased in the 

connectors. 

(2)Increased pistoning of the ferrule tips was observed in both the Temperature Cycling 

Test and the High Temperature Storage Test (Damp Heat), but the major changes 

occurred in the Temperature Cycling Test. 

(3)The post-test condition of the ferrule tips after testing at 85°C and 85 percent relative 

humidity for 2000 hours exhibited flattening distortion. This result could indicate that 

the actual pistoning was greater than the calculated amount. 

8 

Conclusion 

Topics for future discussion 

Many aspects of the relationship between pistoning and optical characteristics have yet 

to be confirmed, and topics such as manufacturing conditions also require research. 

[Terminology] 

*1.Telcordia 

The breakup of the American communications giant AT&T (Bell Telephone) created 

seven regional telephone providers (the so-called "Baby Bells") and established a 

research and development company initially called Bellcore. Later, when this 

subsidiary was sold to a company unrelated to the original Bell, the company became 

known as Telcordia Technologies. In the field of communications in the U.S., the 

Telcordia standards are widely referenced as the baseline standards for this field. 

*2.SC connectors, and *3. FC connectors 

Both of these optical connectors are zirconia ferrule (refer to Fig.2 above) connectors 

developed in Japan that are widely used throughout the world. The stationary part of 

the SC connector is made of plastic, and can be connected by merely being pressed 

on. The stationary part of the FC connector, though, is made of metal and must be 

attached with screws. 

*4.Insertion loss 

The transmission loss occurring when light passes through the part. This value 

compares the insertion light with the outgoing light, and is expressed in decibels. 

*5.Return loss 

This value compares the insertion light power and the reflected light power of the 

reflected light returning inside the part. This value is expressed in decibels. 

- 10 - 



*6. PDL (Polarization Dependent Loss) 

The PDL value measures the change occurring in insertion loss caused by 

polarization. Polarization is a condition of deviation in the vibrational direction of 

light. 

*7. Single mode 

Single mode is a condition which provides light with a single pathway of propagation. 

This mode contrasts with multi-mode, which consists of multiple pathways of light 

propagation. Due to the mode-dispersed loss occurring in multi-mode, single mode 

is used for long-distance transmission. 

*8. Curvature radius 

The length (in mm) of the radius of a circle from a circumference projected from a 

curved surface. 

*9. Eccentricity 

The length (in µm) of the deviation between the center of the ferrule and the center 

of the core of the optical fiber. 

*10.PVC sheath 

This polyvinyl chloride (PVC) sheath, referred to as optical fiber cable, has a 

protective fiber coating. This type of sheath is mainly used indoors. 

*11.Hytrel ® core 

Hytrel ® is a polyester elastomer made of thermoplastic resin that was developed by 

the Dupont Corporation. This substance is an engineering plastic with superb 

features such as strength and heat characteristics. Hytrel ® core is a fiber optic 

stranded conductor coated with this elastomer. 

[Bibliography] 

1) "Generic Reliability Assurance Requirements for Passive Optical Components", 

Telcordia GR-1221-CORE Issue 2, 1999 

2) T. Tomasi, I. De Munari, V. Lista, L. Gherardi, A. Righetti, M. Villa: "Passive optical 

components: from degradation data to reliability assessment-preliminary results", 

Microelectronics Reliability 42, p.1333-1338, 2002 

3) A. Piccirillo, G. Zaffiro, T. Tambosso, G. Gallo: "Reliability of Optical Branching 

Devices", IEEE (Institute of Electrical and Electronics Engineers), Journal of Selected 

Topics in Quantum Electronics, Vol.5 No.5, 1999 

4) F. Caloz, D. Ernst, P. Rossini, L. Gherardi, L. Grassi, J. Arnaud: "Reliability of optical 

connectors - Humidity behavior of the adhesive", Microelectronics Reliability 42, 

p.1323-1328, 2002 

5) Tetso Kumazawa, Makoto Shimaoka, Kazuyuki Fukuda: "Moisture Absorbency of 

Optical Components Resin Adhesive" Journal of Japan Institute of Electronics 

Packaging, Vol.4, No.7, p.621- 624, 2001 

- 11 -

Evaluating the reliability of optical connectors

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?