Reliability. Yuichi Matsushima. Communications Research Laboratory (CRL) Japan. SubOptic

Transcription

1 SubOptic 2004 Reliability Yuichi Matsushima Communications Research Laboratory (CRL) Japan On April 1,2004 CRL and TAO will be reorganized as the National Institute of Information and Communications Technology

2 High Reliability Technologies for Optical Submarine Cable Systems Highly reliable optical submarine cable systems can be realized by (a) Designing a system with reliability sufficient for requirements, (b) Selecting potentially reliable components, (c) Improving the reliability of the components high enough for submarine applications, (d) Confirming the reliability of the components (e) Quality control of the production line. These issues are closely related to each other, and none of them can be neglected.

3 OUTLINE 1. Design of Reliability for Optical Submarine Cable Systems 2. Reliability of Components < Case Study> # Optical Fiber # Active Device (ex. Laser Diodes) # Passive Device (ex. Optical ADM)

4 System design

5 System design (1) The reliability of the system is guaranteed by careful design and qualification of submersible and terminal equipment components. The system reliability is achieved, established and continues to be improved through : (1) Fault tolerant system design (2) Design capability and qualification program (3) Manufacturing certification programs (4) Experience acquired during design and manufacture of previous submarine systems

6 System design (2) Required reliability for submarine cable systems MTBF (Mean Time Between Failure) more than l0 years Ship repair less than 2 times during the system design life 25 years!!! FIT = {Failure (2) }/ { 30 years( in hour)}x10 9 = Fiber pairs & 100 repeaters => FITs/ fiber pair, repeater

7 System design (3) The submersible plant should be designed to minimize potential ship repairs due to component failures by; (1) Minimizing the number and complexity of repeater components (2) Providing redundancy for devices with insufficient reliability (3) Achieving a high level of component reliability (4) Robust system design tolerant to significant transmission deviation

8 System design (4) Redundancy techniques 1. parallel connection 2. 2 out of 3 system 3. stand-by system Selector Main Sub SW

9 System design (5) Example of Reliability Design PD MIC LD 10 FITs 3 FITs 25 FITs 100 km optical fiber cable 0.05 FITs submarine repeater EDF CP 0.1FIT IOS/ GEQ 1FIT PD 1FIT 0.04FIT 1FIT Pump 15FIT SV/MIC 15FIT

10 Reliability of Components Optical Fiber

11 High Strength Optical Fiber Fracture strength of the optical fiber is about 600 kg/mm2 (7 kg for the optical fiber with a diameter of l25 mm). Although this value is quite higher than piano wires (about 200 kg/mm2), the optical fiber can get flaws on its surface during production process.

12 A distribution of the fracture strength of the optical fiber is shown below. It contains weak portion at some places. These weak points limit the usage of whole the optical fiber to the submarine cable, weak points are desired to be as few as possible. Fracture weight (kg) Length of fiber (m) Longitudinal distribution of fiber

13 The origin of the weak points is internal defects caused by impurity inclusions and flaws on the surface. The following efforts are made in manufacturing process for high strength optical fiber. (1) Elimination of voids, dust particles and impurity inclusion in synthetic fiber material. (2) Surface treatment by acid etching and flame polishing. (3) Sealing up of a drawing room and forcible ventilation for clean drawing operation. (4) Selection of the most suitable sheath material which can not cause damages to the optical fiber.

14 High Strength Optical Fiber Splicing Fracture tensile (%) Fracture possibility (%) (a) normal optical fibers, (b) optical fibers with the sheath chemically removed, (c) spliced optical fibers by a highstrength splicing method and (d) spliced optical fibers by conventional splicing method Fracture weight (kg) Fracture possibility

15 Assurance of Optical Fiber Strength A proof test equipment Dancer roll Optical fiber Pay-out Weight Take-up Belt Drive roll Drive roll The applied tension for the proof-test is determined according to the practical condition and the required reliability. The amount of tensile distortion is about 2 % for a fiber portion and 2.5 % for spliced portion to assure enough strength of long length optical fibers for submarine cables.

16 Reliability of Components Active Device (ex. Laser Diodes)

17 Failure mode of LD is generally categorized into A wear-out failure mode concept of gradual degradation in the course of aging and such degradation is rather unavoidable for the components concerned A random failure mode. sudden failure of the components. The wear-out failure process is rather deterministic, while random failure process is represented by stochastic Poisson process.

18 Case study of LD (1):1470nm pump LD Next figure shows the result of reliability test of 1470nm pump LD chips. The temperature and output power condition of the test were 50 C and 60 mw /facet. 300 pieces of LD chips were tested up to 6,000 hours. Failure criteria is 650 ma (absolute value) of operation current. This failure criteria is severer than twice of initial operation current which is usually used. The life time of each LD was calculated by linearly extrapolating the change of operating current from the beginning to 6,000 hours of each sample. The calculated life times are plotted on a log-norm sheet. The horizontal axis indicates testing time, and vertical axis does cumulative failure rate. Circles of t1, tm and t2 are times when the cumulative failure rates reach 84 %, 50 % and 16 %, respectively.

19 Cumulative failure rate (%) Test condition : 50 C, 60 mw N= 300 pcs t = 6000 hrs Failure Criteria : Iop=650 ma MTTF= 4.3E6 hrs Estimated 10 C (Ea=0.3 ev) MTTF = exp ( µ + s 2 /2 ) s = { ln (t1) - ln (t2)}/2 µ = ln (tm) Estimated 10 C (Ea=0.7 ev) 27 years Time (hours)

20 The following relation is well known between a life time of LD and temperature. Life time? exp ( Ea/kT ) Ea : Activation energy k : Boltzman constant T : Absolute temperature of LD junction Even if 0.3 ev of Ea is adopted, the estimated MTTF at 10 C is 1.8E7 and the cumulative failure rate at 27 years is 0.24 %. FIT value is 10.1, which is calculated as follows. FIT = {Cumulative failure rate / ( 27 years [in hours]) } x 1E9 As for the random failure rate, a failure rate is given by FIT = 2.3 / (n t), according to the Poisson statistics, where n is the number of component and t is the time and the coefficient 2.3 is adopted for 90 % confidence level estimation and for no failure case. When we take n=300, and t=6,000 hours after our life test, we get 1280 FIT at 50 C. The activation energy of 0.7 ev corresponds to an acceleration factor of 35 between 10 C and 50 C. Therefore we obtain 1280/35= 36 FIT

21 Case study of LD (2) Highly Reliable and High Power 980nm Pump Lasers for Undersea Cable Systems cross-sectional view n-algaas current block p-algaas InGaAs/GaAs DQW p -GaAs contact p-algaas 2nd cladding active region n-algaas cladding n-sl-buffer layer GaAs substrate ³ GSMBE / MOCVD two-step growth ³ Planar buried-stripe structure ³ Weakly index guided waveguide ³ Facet coating : I-3 process

22 I-3 Process Back facet Interlayer Ion irradiation facet Front facet Active region Si AlOx AlOx *H. Horie et.al., IEEE J. Sel. QE, vol.5, p.832, 1999

23 L-I characteristics (L=700um) Output Power (mw) L=700µm T=25 C SE=0.89W/A Current (ma)

24 Life test : 120mW APC,50 C Operating Current (ma) group -1 N=108 T=50 C P=120mW Random Fail FIT@10 C Time (hours)

25 300 Operating Current (ma) Operating Current (ma) High Power Life Test N=52 T=50 C P=150mW L=700um TIME (hour) N=103 T=50 C P=170mW L=700um TIME (hour) 120mW module can be realized.

26 L-I characteristics (L=1000um) Output power (mw) L=1000µm T=25 C SE=0.9W/A Current (ma)

27 Higher Power Life test : 350mW APC Operating Current (ma) N=5 T=50 C P=350mW L=1000 mm Time (hours)

28 Co-axial type module Co-axial type PANDA FIBER STEM FBG LD CHIP WINDOW LD COLLIMATOR LENS FIBER COLLIMATOR LENS

29 Spectrum of FBG Locked Module Level [db] Level [db] Level [db] Wavelength [um] 5 C 25 C 45 C

30 Reliability of Components Passive Device (ex. Optical Add and Drop Multiplexer using a Fiber Grating and Optical Circulators)

31 OADM using OC and FG Optical Circulator Fiber Grating Input port Output port Drop port Add port : main signal ( λ 1, λ 2,..., λ n ) Output port Drop port : added and dropped signal ( λ k )

32 Highly Reliable Optical Components for Optical Submarine Cable Systems Selection - potentially reliable components Improvement - material and design Confirmation - reliability test Quality Control - production line Reliability Budgets of Optical ADM is 0.2 FIT

33 Optical Circulators (OC) and Optical Switch (OS) Environmental & Mechanical Tests Vibration Test, Mechanical Shock Test Radiation Proof Test, Hydrogen Proof Test, etc., Temperature Proof Test Long Term Acceleration Test ===================================================== Test Items (Condition) Number of Sample s OC OS Heat cycle (- 40 / 85 C, 4 hours / cycle) High temperature (70 C) Low temperature (10 C) - 5 =====================================================

34 Variation of Insertion Loss of 10 Optical Circulator s after 3,000h heat cycle test (-40/85 C) Variation of insertion loss (db) P1 P2 Variation of insertion loss (db) P3 P Time (Hour) Time (Hour)

35 Weibull Plot of O C heat cycle test (Insertion loss variation P1 -> P2) Failure point : +1.5 db m=1.99 h=2.38 x10 5 FITs for 27 C (acceleration rate : 300) ----> 0.099

36 Long Term Stability of Fiber Gratings Mechanical Strength Insertion Loss Bandwidth Refractive Index Increase induced by Excimer Laser Effect on Center Wavelength &Rejection Ratio

37 Environmental & Mechanical Tests for FG Vibration Test, Mechanical Shock Test Radiation Proof Test, Hydrogen Proof Test, etc., Temperature Proof Test Long Term Acceleration Test for FG ===================================================== Test Items (Condition) Sample Number Heat cycle (- 20 / 70 C, 4 hours / cycle) High temperature-high humidity 10 (70 C, 85 %RH ) 2. High temperature (70 C) Low temperature (10 C) 10 =====================================================

38 Variation of Characteristics of 15 Fiber gratings after 5,000 h heat cycle test (-20/70 C) Variation of insertion loss (db) Time (Hour) Variation of center wavelength (nm) Time (Hour)

39 Environmental & Mechanical Tests of Optical Switch ============================================================== Test Items Condition Vibration Test (Sinusoidal) 20 G, 10 ~ 2000 Hz, 0.5 oct/min, X, Y, Z, axes (Random) MIL-STD-883C, Method 2026, Test Condition I-B, Lower Frequency reduced to 10 Hz 2. Mechanical Shock Test 200 G / 6 ms, 400 G / 3 ms, 5 times /X, Y, Z, axes 3. High Temperature Test 100 C and 120 C, 12 hours / each temperature 4. Low Temperature Test - 60 C & - 80 C, 12 hours / each temperature 5. Tensile Strength Test Parallel : 10 N, Perpendicular : 5 N 6. Radiation Proof Test 60 Co (35 kc), 1 min, Total radiation : 25 rad 7. Hydrogen Proof Test Partial pressure : 10-2 atm, 8 hours 8. Drying Test After kept at 60 C and 20 x10-3 Torr for 70 hours, cooled down to room temperature by 7 hours. 9. Pressure Proof Test 3.5 atm, 5 min ==============================================================

40 Summary Reliability of FG & OC for Optical ADM Environmental Tests & Acceleration Test - OC : ~ 0.1 FITs for 27 years - FG : variation ~ measurement error -> ~ 0.1 FITs for 27 years Applicable to Submersible Branching Units!