JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. IT-2, NO. 6, DECEMBER 1984 975

An Optical Switch for the SL Undersea Lightwave System

STANLEY KAUFMAN, ROBERT L; REYNOLDS, AND GEORGE C . LOEFFLER

Abstract -A 4 X 1 mechanical optical switch has been constructed, which can be used to switch the laser source in regenerators of the SL Undersea Lightwave System.

We describe the design philosophy used to meet the high reliability required of < 2 FIT’S as a passive component, and < probability of not performing the switch-over function when called upon in 25 years.

We describe the grooved silicon chips which were especially designed for this application for the precise alignment of the optical fibers. We also describe a novel fiber end seal. Included are recent loss results for both a 4 X 1 and a 2X 1 mechanical optical switch.

T I. INTRODUCTION

HE SL undersea lightwave system [l] is a single-mode lightwave system operating at 1300 nm. For each

active transmitter, one or more dormant transmitters will be provided. In the event that an active transmitter be- comes marginal in performance, a spare transmitter can be activated. The optical path may be established by either a passive device or a mechanical switch.

A passive polarization independent device suffers an inherent minimum 3-dB bifurcation loss. The primary dis- advantages of a mechanical switch are the moving parts which could cause reliability problems. This paper de- scrjbes a low-loss highly reliable 4x1 electromechanical optical switch which can provide for up to three spares or a total of four transmitters.

Previous work on multimode mechanical optical-fiber switches [2]-[4] were carefully considered for this single- mode application. The switch adopted for the proposed SL Undersea Lightwave System is a moving fiber switch [5] that was first made using preferentially etched silicon chips [6] previously developed foE fiber-optic array splicing [7].

The chips have been subsequently modified for closer tolerances, larger groove spacing, and for fewer and deeper etched grooves to enhance the performance of the switch.

11. RELIABILITY CONSIDERATIONS

The switch must meet a reliability specification of 2 FIT’S as a passive device (2 failures per lo9 device hours) with a device life of 25 years. This equates to 1 failure per 2300 devices over a 25-year period. Also, the probability of not performing the switch-over function in 25 years must be less than 0.001. A switch-over failure is defined as a

Manuscript received April 1,1984; revised September 1, 1984. The authors are with AT&T Bell Laboratories, Allentown, PA 18103.

failure to obey a switching command, even when that command is issued repeatedly.

Obviously for a new device with this low a FIT rate, one cannot rely only on statistics because of the limited num- ber of samples and/or the limited time for testing. The philosophy adopted is to engineer out all defects by design. This approach is enhanced by an ongoing program of subjecting, not only final models, but also where necessary subassemblies, to severe environmental testing.

Test results indicate that there are no adverse interac- tions between the matching fluid (silicone oil) in the switch and other materials in the device that come in contact with the fluid. There is no adhesion of the silicon chips due to long engagement under pressure in the fluid. The fiber seal, which is described in more detail later, has been designed, fabricated, tested and proved to be hermetic relative to the silicone oil.

An accelerated room temperature wear test of early design models gave no indication of significant wear in the mechanically moving parts of the relay. Some 14 relays, which were exercised for some 2.2 million operations total and cycled from 220°C to + 50°C, showed insignificant change in optical performance. Ongoing tests to determine the effect of long-term dormancy on actuation failure have shown no adverse effect to date.

111. PRINCIPLE OF OPERATION OF THE SWITCH

The 4 X 1 optical switch consists of two cascaded grooved silicon-chip fiber switches, a 4 x 2 and a 2 X 1 switch, as shown in Fig. 1. Each switch consists of a movable two-high stack of (positive) grooved silicon chips with one row of fibers and a fixed three-high stack with two rows of fibers. Outside containment of these stacks is provided by silicon-chip assemblies (negative) with inverted grooves (tracks) which mate with those of the stacks and hence provide for alignment of the fibers.

IV. SILICON-CHIP REQUIREMENTS

Of the three types of silicon chips used, the “positive chips” and “positive center chips” have precise parallel grooves etched into the top and bottom surfaces. The negative chips are etched on only one surface to generate two parallel tracks which seat into the two bottom grooves of the positive chips. The top side of the positive chips

0733-8724/84/1200-0975$01.00 01984 IEEE

976 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-2, NO. 6 , DECEMBER 1984

Fig. 1. Elements of a 4 X 1 optical switch.

have five grooves: one matches one of the grooves on the bottom surface for top and bottom mask alignment and four are used for fiber placement. The fifth groove also aids in device assembly by breaking up chip symmetry. The positive center chip has eight grooves (four on. each side) to match the four-fiber greoves of the positive chip.

The tolerances on groove and track widths and on chip thickness may vary by several micrometers for chip-to-chip variation, but are held to 0.5 Fm for groove width varia- tions and 0.25 pm for thickness variations within a given chip. Center-to-center groove distances are held to 0.5-pm tolerances. These tight tolerances provide precise alignment between positive and negative chips and precise fiber placement.

V. FIBER REQUIREMENTS

Variations in fiber-cladding diameter and concentricity of the fiber core must be held to tight tolerances to insure precise alignment of the cores of abutting fibers.

Consider the fiber-groove geometry of Fig. 2. The varia- tion in the distance from center of fiber-to-silicon chip surface due to two abutting fibers of different diameters is given by Ag = Adfi/2. If the core is eccentric to the cladding by an amount “e ” then the misalignment of the core due to diameter variations and eccentricity is 2e + Ad6/2; the factor of 2 is to account for two eccentric abutting fibers. If in addition there is a variation in the groove width “ w”, then the total misalignment q5 which can result is

Assuming a matched fluid, the transmission as a func- tion of fiber offset is given by Marcuse [8] as T = where W is the waist of the beam and is approximately 0.55dc, and where d , = 8.3 pm, the fiber core diameter. The loss in decibels due to offset is

TABLE I

MICRONS

0.22

-I

I (11.0)

Fig. 2. Fiber-groove geometry hreferentially etched silicon chips.

The maximum insertion loss for a 4 X 1 switch is 1.8 dB or approximately 0.9 dB for each of the two switching sections. In addition to offset loss, one might expect smaller contributions from angle variations of abutting fibers, from fiber separation and from pitch vaiiations of mating grooves between positive and negative chips. With this in mind, we shall limit L, to 0.7 dB which results in a maximum allowed offset of

+=8.3,/-=1.8prn. (3)

Hence,

This relationship in terms of the eccentricity is e =

0.9-0.433Ad -0.354AW. If we let Ad, AW take on values of 0.75 pm and 1.0 pm, then the allowed eccentricity is presented in Table I above.

It is not reasonable to expect that the fiber core ec- centricity can be held below 0.2 pm, therefore, variations on both fiber diameter and chip groove width should be held below 0.75 pm. It is extremely important to restrict chip groove and fiber variations to a minimum since inser- tion loss in decibels for offset goes as offset squared. It is our goal to restrict these variations to 0.5 pm.

Variations in fiber diameter are made up of two parts: the contraction and expansion of the fiber diameter along its length; and, the ellipticity (out of roundness) of the fiber. A functional test is used which accounts for the sum

KAUFMAN et a[.: OPTICAL SWITCH FOR THE SL SYSTEM 977

of concentricity (2e) plus ellipticity. This test consists of INPUT FIBERS ~ INPUT CHIP ASSEMBLY

breaking the fiber and butting the two halves at the break in a V-groove, fixing one half, and allowing the other half ASSEMBLY

to rotate. A light source is piaced onto the end of the fixed MAGNET HOLDER

in light power are detected as the fiber is rotated 360". The maximum variation is a measure of the offset due to concentricity and ellipticity. If we restrict these variations INNER HOUSING

to 0.1 dB then the resulting offset is

\ NEGATIVE ,' NER CHIP ASSEMBLY

OUTPUT CHIP ASSEMBLY

half and detected at the end of the rotating half. Variations SHUTTLE ASSEMEL OUTPUT FIBER

8.34- = 0.7 pm. ( 5 )

This substantially meets our goal if we allocate 0.2 pm to ellipticity and 0.5 pm to concentricity (2e). If, in addition, we restrict diameter variations to 0.5 pm over a 25-m length, which is more than sufficient to build a 4 X 1 switch, then our goal of fiber variations is met.

ASSEMBLY

VI. SILICON CHIP-FIBER STACK Fig. 4. A 1 X 4 optical switch.

A schematic of the silicon chip-fiber assemblies (or stacks) for the 4 x 1 optical switch is shown in Fig. 3. The that abut are lapped to produce a Planar surface of fiber, movable two-stack consists of two positive chips with one silicon, and row of fibers. The fixed three-stack consists of a positive chip, a row of fibers, a positive center chip, a second row VII. DEVICE ASSEMBLY of fibers, and a second positive chip. The inner chip assembly consists of a fixed three-stack and a movable Each of the fixed three-stacks is sandwiched between two-stack. For stability of the stacks there are always fibers two negative chip assemblies, right hand and left hand, as epoxied in the outer grooves of the positive chips. Note seen in Fig. 3. The two tracks on each negative chip mate that the two light guide fibers in the inner assembly are with grooves in the positive chips. For structural integrity, horizontal in the fixed three-stack (two rows of fiber) and the negative chip is epoxied to a Kovar backup plate. vertical in the movable two-stack (one row of fibers). Kovar was chosen for its close coefficient of thermal

In order to reduce losses to a minimum, the positive expansion match to silicon which minimizes distortion chips are broken in half along their length. The halves are during epoxy curing. mated at this break in the fixed and movable stack assem- A pin secured by a cantilever spring bears against the blies. The mating ends of the positive chip stack assemblies left-hand negative chip assembly and in turn against the

978 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-2, NO. 6 , DECEMBER 1984

fixed three-stack, the right-hand negative chip assembly and the right-hand stainless-steel wall of the inner housing which is a ground surface. The left-hand negative chip assembly is further supported by a small shim which together with the adjacent shim, is epoxied during final assembly, to the ends of the plate and the housing wall. Longtudinal motion during assembly is prevented by the number 0 torque screw which holds the right-hand negative chip assembly to the inner housing wall. The right-hand negative chip assembly (toward the viewer in Fig. 4) rests on the ground bottom surface of the inner housing. An inner cover (not shown) rests flat on top of the inner housing.

As sketched in Fig. 4, the movable two-stacks are moved by push rods. These push rods and the corresponding reamed holes in the walls of the inner housing are held to close tolerances. Oversize holes are provided in the nega- tive chip assemblies for push rod clearance. The push rods lie between the movable chip stacks and the inner surfaces of the magnet holders which are actuated by coils.

The actuators, consisting of magnet holder assemblies, coils, and magnetic cups, were designed at AT&T Bell Labs, Columbus, Ohio. A 4 X 1 switch has two sets of push rods and magnetic holder assemblies. A magnet holder assembly consists of two magnet holders screwed and epoxied fast to a shaft. The shaft and the push rods, which act as pistons, are fabricated from aluminum bronze. They move in a cylinder of stainless steel (the inner housing) and are somewhat lubricated by the silicone matching fluid.

VIII. EPOXY SEAL FOR GLASS FIBER

The coated glass fibers are inserted through cylindrical holes at the end seals of the outer housing (Fig. 4). Slotted beryllium-copper inserts are placed in the hole with a fiber in each slot. The assembly operation is shown schemati- cally in Fig. 5. Epoxy is inserted into the larger diameter section of the hole and by piston. action the insert forces epoxy through both the smaller section of the hole and back through the slots thus sealing the fibers between the insert and the housing end seal. Alternatively, the inserts may be coated with epoxy by use of a special tool and then placed in the end seal hole with the fibers in place.

Seals of this type were made by one of the authors and thermally cycled at least five times from - 60°C to 100°C. A helium leak test was then performed. The leak rate in each of the five samples was less than cm3/s of helium. Further work recently was completed involving specimens containing matching fluid (silicone oil). All specimens were held nine days at 60°C and tested. The specimens were then thermally shocked at -78OC and again held at 60°C for 19 days. Chromatograms substanti- ated the claim that the proposed fiber seal is sufficiently tight to silicone oil to serve the intended purpose. The switch will be in a dry nitrogen environment under sea. Therefore, it was not necessary to provide a seal to defend against diffusion of water and, hence, no testing for this was performed.

Fig. 5. An SL optical relay fiber to metal epoxy seals four-fiber input.

Ix. OUTER COVER AND MATCHING FLUID

After the fiber seal is made a flexible cover is sealed into the outer housing. Silicone matchng fluid is then vacuum degassed and introduced into the outer housing through a tubulation, and then the tubulation is sealed off. At a wavelength of 1.30 pm the silicone fluid has an index of refraction of 1.39 which sufficiently closely matches the fiber index of 1.46. During temperature fluctuations, the flexible outer cover expands and contracts due to volume changes of the fluid. This cover motion is necessary to track the fluid motion in order to insure that no gas bubbles can form in the fluid which might degrade the performance of the switch. The flexible cover was analyzed by one of the authors using a large deflection finite element program [9]. The stress levels in the flexible cover due to temperature excursions and vacuum filling of the fluid were well below the yield point of the cover material. Even if yielding occurs, the condition is certainly not catastrophic, as the cover would simply conform to the contracted fluid surfaces.

X. RESULTS

4 X 1 switches have been fabricated with insertion losses less than 1.8 dB (specification requirement). In addition to 4 X 1 switches, 2 x 1 switches have been manufactured utilizing only one of the two switching sections (see Fig. 2). The loss specifications for a 2 X 1 switch is 0.9-dB maxi- mum. Fig. 6 depicts insertion-loss hstograms for thirteen 2 X 1 switches and twenty seven 4 X 1 switches at room temperature with a light source wavelength of 1.31 pm. The average insertion loss for the 2 x 1 switches (26 ports) is 0.27 dB. The average insertion loss for the 4 x 1 switches (108 ports) is 0.92 dB. Spectral-loss dependence of a typi- cal 4 X 1 switch has been measured by AT&T Technologies, Clark, NJ, and is shown in Fig. 7. The maximum insertion-loss specification must be met over the wave- length range from 1.29 to 1.33 pm. For this switch three of the four ports show a variation of approximately 0.25 dB from 1.29 to 1.33 pm while the fourth port (upper left hand in Fig. 7) shows only a 0.05-dB variation. Loss variations due to temperature changes from 0°C to 30°C (the specifi-

KAUFMAN et al.: OPTICAL SWITCH FOR THE SL S Y S T ~ 979

l 5 I- 13 - 2 X E L A Y S (26 PORTS)

LOSS = 0.27 dB

LL LL 3

INSERTION LOSS (dB)

u 0 0

15 0 a 10

5

0

2 7 - 4 x 1 RELAYS (108 PORTS) - LOSS = 0.92 dB

INSERTION LOSS (dB)

Fig. 6. Insertion-loss histograms of switches.

” 1.26 1.28 1.30 1.32 1.34 1.36

WAVELENGTH 1.26 1.28 1.30 1.32 1.34 1.36 (MICRONS)

WAVELENGTH (MICRONS)

Fig. 7. Spectral loss-atypical 4 X 1 optical switch (four ports).

cation operation range) show a similar range for insertion loss.

XI. SUMMARY AND CONCLUSIONS

One cannot rely on statistics alone to insure the reliabil- ity requirements of the proposed SL optical switch. The philosophy adopted is to engineer out all defects by design and testing. The design phase has substantially been com- pleted. The ongoing testing program of subjecting subas- semblies and completed switches to severe environmental conditions is well under way.

The reliability of the switch has yet to be proven.’ It is our opinion that the switch will meet all of the require- ments set forth in the specifications, with the present design; and that further reliability testing will substantiate this opinion with perhaps only minor design modifications.

ACKNOWLEDGMENT

We should like to state for the record that the design of the optical switch was truly a team effort, and we would

like to thank all who contributed to its success. The authors are. especially indebted to F. Zwickel, AT&T Technology Systems, whose fabrication and design ingenuity made a lasting imprint on the Optical Switch.

REFERENCES

[l] P. K. Runge and P. R. Trischitta, “SL undersea lightwave system,”

121 P. G. Hale and R. Kompfner, Electron Lett., vol. 12, no. 15, p. 388, pp. 744-753, this issue.

. .

[3] R. B. Kummer, S. C. Mettler, and C: M. Miller, “A mechanically July 22, 1976.

operated four-way optical switch,” in Proc. 6th European Conf. Opt. Commun. (Amsterdam, The Netherlands), Sept. 1979, paper 6.4.

[4] H. Yamamoto and H. Ogiwara, Appl. Opt., vol. 17, no. 22, p. 3675, Nov. 15, 1978.

[5] W. C. Young and L. Curtis, “Cascaded multimode switches for single mode and multimode optical fibers,” Electron. Lett., vol. 17,

161 C . M. Schroeder, “Accurate silicon spacer chips for an optical-fiber no. 16, Aug. 6, 1981.

cable connector,” Bell Syst. Tech. J., vol. 57, no. 1, pp. 91-97, Jan. 1978.

[7] C. M. Miller, “Fiber-optic array splicing with etched silicon chips,” Bell Syst. Tech. J., vol. 57, no. 1, pp. 75-90, Jan. 1978.

[8] D. Marcuse, “Loss analysis of single-mode fiber splices, Bell Syst. Tech. J., vol. 56, no. 5, May-June 1977;

[9] Marc Analysis Rf:earch Corporation, Marc general purpose finite element program, Palo Alto, CA.

Stanley Kaufman received the B.E. in 1948, and the M.S. degree in 1957, from Johns Hopkins University, Baltimore, MD.

He was with Bellcomm from 1968 to 1971; AT&T Bell Laboratories from 1972 to present. At Bellcomm, he developed a model for the stability and performance of the Lunar Roving Vehicle. He is the author of numerous papers on the finite element method and has participated in the development of various piezoelectric and oDtical-fiber devices.

Mr. Kaufman is an Asskiate Fellow of the AIAA.

Robert L. Reynolds attended the University of New Hampshire,

He joined the Piezoelectric Device Department of Bell Laboratories in 1959. He developed cold- welded glass-to-metal sealed quartz crystal en- closures which later became standards for the communication industry, military, and satellite applications. He designed oscillator crystals for submarine cable fault location systems and was a member of the group that conceived and devel- oped monolithic crystal filters. Since 1979, as a

member of an optical component group, he has worked on the develop- ment of optical taps, star couplers, and switches for various communica- tion applications. He has authored several papers and holds patents on crystal packaging and monolithic crystal adjustment techniques.

George C. Loeffler received the B.A. degree in physics from Lehigh University, Bethlehem, PA, in 1953.

He joined Bell Telephone Laboratories as a Member of Technical Staff in 1953. Except for brief assignments with electrolytic capacitors and tantalum thin-film circuits, his principal work has been with extremely highly reliable compo- nents for undersea use, initially oil-impregnated paper capacitors, and now opt0 mechanical switches.

Mr. Loeffler is a member of Phi Beta Kappa. . -. . -. . .