Optical Switch Array Using Banyan Network - CiteSeerX

IEICE TRANS. COMMUN., VOL.E82–B, NO.2 FEBRUARY 1999

365

PAPER

Joint Special Issue on Photonics in Switching: Systems and Devices

Optical Switch Array Using Banyan Network∗ Hideaki OKAYAMA† , Member, Yutaka OKABE† , Takeshi KAMIJOH† , Nonmembers, and Nobuyoshi SAKAMOTO† , Member

SUMMARY A large scale optical switch array based on guided-wave technology using banyan network architecture is demonstrated. Banyan network architecture is the simplest N×N network connecting a input port to all the output ports. A banyan network optical switch array serves as a base for constructing many classes of switch networks, as we propose in this report. We fabricated a 32×32 switch and measured its characteristics. Drive voltage was about ±12 V and extinction ratio was 18 dB, and the average insertion loss was 18 dB. Preliminary experiments were conducted on a 64×64 device. The use of proton exchanged waveguides makes a 10 mm radius of curvature feasible. key words: optical switch, switch matrix, waveguide, LiNbO3

1.

Introduction

The optical switch array [1]–[3] is a basic component of any system, such as cross-connects or local area networks, routing optical signals in an optical form. These systems, require large-scale optical switch arrays that handle numerous input and output ports. The largest scale optical switch array has been 16×16 switch [1], [4]. For devices based on silica waveguides the crossbar architecture (Fig. 1(b)) is most suitable. The crossbar architecture provides low power consumption. Large number of switch element stage (long waveguide) is not problematic for a low loss silica waveguides. On the other hand, the propagation loss tends to be high in a waveguide using electro-optic materials such as LiNbO3 , semiconductor and polymer. For these materials, architectures with less switch element stages like tree structure, Benes or banyan (Fig. 1) are desirable. In this report, we demonstrate devices using banyan architecture to implement switches exceeding 32×32. Banyan network architecture is the simplest network, having a route from every input port to an output port. This structure enables a device with a relatively large network to be fabricated on a single chip. A banyan network optical switch array serves as a base for constructing many classes of switch networks, as proposed in this report. Using devices with a banyan architecture Manuscript received June 14, 1998. Manuscript revised September 21, 1998. † The authors are with R&D Group, Oki Electric Industry, Co., Ltd., Hachioji-shi, 193-8550 Japan. ∗ This paper is also published in IEICE Trans. Electron., Vol.E82-C, No.2, pp.313–320, February 1999.

enables a nonblocking network to be constructed using combined interconnects to reduce the essential number of interconnects handled. In Sect. 2, we compare the banyan network to other architectures and describe its properties. In Sect. 3, we describe some architectures using banyan networks. Experimental results from devices fabricated in LiNbO3 are shown in Sect. 4. The paper is concluded in Sect. 5. 2.

Optical Switch Array Architectures

Three main schemes implement guided-wave devices that enable reconfigurable connections between input and output ports [1]. The first uses deflectors that perform the same as phased array antennas [5]; the output port is selected by changing the deflection angle. The second uses optical gates [1]; all interconnects between input and output ports are made with optical gates on every route and input and output ports are connected by opening gates. The third (optical switch array), discussed in this report, uses an array of multistage 2×2 switch elements interconnected with waveguides. Of the three schemes, the third has been studied the most because it has the lowest insertion loss and crosstalk. Optical switch arrays are classified by their connection capability—strictly nonblocking, wide-sense nonblocking, rearrangeably nonblocking, and blocking [3]. Examples of basic optical switch array architectures are shown in Fig. 1. Some advanced architectures with more complex structure such as rearrangeable dilated Benes, wide-sense nonblocking double crossbar and strictly nonblocking extended generalized shuffle (Fig. 2) have been proposed [3] to reduce crosstalk. Tree, crossbar and simplified-tree structures [6] are used as strictly nonblocking—strictly speaking the crossbar is wide-sense nonblocking. The Benes network is rearrangeable nonblocking and the banyan network is blocking. Network complexity increases with its connection capability. Thus, device total length is longer for a device with more connection capability. Figure 3 shows the calculated total lengths for the tree, simplified-tree , cross-bar, Benes and banyan architectures. The banyan architecture provides the shortest device. In calculating the total length, the centers of adjacent switch elements were assumed to be separated by 100 µm. The curvature radius of curved waveguides

IEICE TRANS. COMMUN., VOL.E82–B, NO.2 FEBRUARY 1999

366

Fig. 1

Fig. 2

Optical switch array architectures.

Optical switch array with reduced crosstalk.

was assumed to be 40 mm. The switch element was 6 mm long. The total length of the 32×32 banyan switch was 62.5 mm. If the device length is the only concern, a switch of up to 64×64 can be implemented on a 4-inch substrate, and an 128×128 switch of about 10 cm long is feasible. If a 2-mm-long switch element and a waveguide curvature radius of 10 mm are used, the total length of the 128×128 switch would be about 30–40 mm. In this design, input and output waveguide spacing was not adjusted to the conventional fiber array. Commercially available fanout waveguide arrays are used to connect the device to fiber arrays. Calculated crosstalk characteristics are shown in Fig. 4. Crosstalk characteristics of banyan architecture fall between those of the simplified-tree and Benes. The crosstalk of banyan network is relatively low due to the small number of optical switch element ranks.

Fig. 3 Total length as a function of port number (network size) for some optical switch array architectures.

OKAYAMA et al: OPTICAL SWITCH ARRAY USING BANYAN NETWORK

367

3.

Network Using Banyan Architecture

Although the banyan architecture is a blocking network, banyan networks can be connected to produce a network with greater connection capability. Networks using banyan architecture can be classified by the number of device stages or interconnection stages. A device is composed of several stages of switch elements integrated on a substrate to implement multiport switching function in a single chip. The chip is connected to optical fiber arrays and packaged as a module. In this section we describe architectures using at most four module stages. The insertion loss increases along with the number of module stages. When the same function can be implemented, an architecture with less number of module stages is more preferable. The total module number should be smaller to reduce cost. 3.1 Two-Module Stage Architecture Two banyan networks can, for example, be connected to form a network used as a Benes network. We propose an architecture using a banyan network (Fig. 5). A total of 2 m thinned-out-banyan networks (N/m×N ) are con-

nected to construct a N×N network. The thinned-outbanyan is a network using only N/m input or output ports of the N ×N banyan network (Fig. 5). Connection capability for m is shown in Table 1. For m = 2, this is equivalent to the dilated Benes network (Fig. 2) [3]. The switch size of a single chip is half of the conventional structure, although interconnection is needed. For m = N , it is equivalent to a tree network. For m = N/4 or N/2, two thinned-out banyan networks are interconnected to implement a switch network at the middle four or two stages (Table 1), known to be wide-sense nonblocking [3], [7] or strictly nonblocking making whole the network wide-sense nonblocking or strictly nonblocking. For other m number, the structure belongs to extended generalized shuffle network. Fiber to waveguide insertion loss is reduced in a two module stage structure, compared to the conventional three module structure. As examples show, many kinds of networks, some with low crosstalk and high connection capability, are constructed using banyan network fabricated in a chip. For m = N/4 a wide-sense nonblocking network is constructed (Fig. 6 (a) 8×8, (b) 16×16) as we propose in this report. In this structure, interconnects between thinned-out-banyan switches are groups of four same-pattern interconnects. By using a fiber bundle

Table 1

Two-module-stage architecture features.

Fig. 4 Signal to crosstalk ratio (SXR) as a function of port number (network size) for optical switch array architectures.

Fig. 5

Thinned-out banyan switch and two-module-stage network.

IEICE TRANS. COMMUN., VOL.E82–B, NO.2 FEBRUARY 1999

368

Fig. 7 Equivalent circuit of Figs. 6 and 8. An extending method of the network size.

(a)

(a)

(b) Fig. 6 Two-module-stage network using thinned-out banyan switch m = N/4.

having the same pattern as interconnects groups, we reduce the essential complexity and number of connectors. A 4×4 switch network shown in Table 1 is widesense nonblocking, when all of the four inner switch is never allowed to be in the same state [3], [7]. There are redundunt switches to exchange routes, on a route from input to output port. A switch element which meets above condition is selected to exchange routes. The whole network becomes equivalent to that of Fig. 7 with 4×4 wide-sense nonblocking switch network used as N/m0×N/m0 switch. Output ports are grouped into m0 groups. A N/m0 ×N/m0 switch is connected to output ports of each group. An 1×m0 switch is connected to m0 N/m0 ×N/m0 switches. An 1×m0 switch selects a N/m0 × N/m0 switch connected to desired group of output ports. N/m0×N/m0 switch is drived to set up a route to a desired output in the group. The total switch element for m = N/4 is N 2 − 2N which is 2N less than crossbar architecture. For N = 8, the total number of switch element is 48 which is, to our knowledge, the smallest number reported for a 8×8 wide-sense nonblocking network. Crosstalk can, for example, be reduced to the same

(b) Fig. 8 Two-module-stage network using thinned-out banyan switch with reduced crosstalk.

level as tree architecture for network using N×N banyan network as N/2 × N/2 switch and nonblocking is attained using only four of the input and output ports of the N/2 × N/2 switch. The structure is shown in Fig. 8 (a) 8×8, (b) 16×16. Total of N/2 thinned-out banyan device (4 × N ) is used to implement a N × N switch network. Each thinned-out banyan device is connected by bundle of 4 interconnections. The number of the interconnections in a bundle is selected to ensure that all the signal from the four input ports in a thinned-out banyan device can be routed simultaneously to four output ports of another thinned-out banyan device. The number of interconnection bundles is N 2 /16 which is 1/16 of that in tree structure. One output of the last rank 2×2 switch in a input thinned-out banyan device is connected to dif-

OKAYAMA et al: OPTICAL SWITCH ARRAY USING BANYAN NETWORK

369

Fig. 9

Fig. 10 Four-module-stage rearrangeable network architecture.

Dilated wide-sense nonblocking 4×4 switch.

ferent output thinned-out banyan device. One input of the first rank 2×2 switch in a output thinned-out banyan device is connected to different input thinnedout banyan device. An equivalent circuit obtained from architecture of Fig. 8 is that of Fig. 7 with dilated 4×4 wide-sense nonblocking switch network (Fig. 9) used as N/m0 × N/m0 switch. A 2×2 switch element can be decomposed into 2×1 and 1×2 switches connected by a single guide, when using an algorithm in which only one signal is allowed to pass the 2×2 switch element to avoid crosstalk. These 1×2 and 2×1 switches (second and third stages, sixth and seventh stages) are merged together to form 2×2 switch. Total switch element used in the architecture is 2(N 2 − N ) less than double crossbar [3] architecture (crossbar using redundant switch elements to reduce crosstalk: Fig. 2). The crosstalk is calculated to be SXR (dB) = −10 log10 [4 + log2 (N/4)] + 2XS

Fig. 11 Routing of four-module-stage rearrangeable architecture.

(1)

with XS as the crosstalk of a switch element. 3.2 Three-Module Stage Architecture Three module stage architecture is a extended generalized shuffle (EGS) network of Ref. [8] (Fig. 2). With N being port number in the network, 1×m optical switches are placed at each input port and m×1 optical switches at each output port. A total of m banyan switches with N input and N output are connected to 1×m and m×1 optical switches respectively. 3.3 Four-Module Stage Architecture Four-module stage architecture is a method for extending the port number of the two-module architecture described in Sect. 3.1. Rearrangeable nonblocking architecture: The architecture for 8×8 network is shown in Fig. 10. The network consists of four-stage multiport switch or arrays (Fig. 10). Input and output 1 × m switch arrays are allocated to each port. N/m m×N/m switch arrays form a middle-rank N ×N switch.There are m N ×N switches. The total number of switch arrays is 4N . A

banyan network switch can permutate freely for one input signal at a time. Only a single input light or output light is allowed in a single m×N/m switch array to keep crosstalk low. Crosstalk to first-order approximation is SXR (dB) = −10 log10 m + 2XSA

(2)

with XSA as the crosstalk of a switch array. In this architecture, N 2 /m or mN (larger one) parallel interconnections are used in the network, compared to N ×N in a conventional tree structure. A permutation is done as shown in Fig. 11. When we interchange two output ports A and B, we need to consider only two of the N × N switches containing corresponding optical signal paths. Overlapping all paths in two N ×N switches yields a new interconnection pattern. Since only two paths are connected to a single m×N switch, all interconnections shown can be decomposed into new two sets of interconnections in which a single m×N switch array handles only one optical signal path. 1×m switch array should be switched to connect input and output ports to set up new interconnections corresponding to new sets of interconnection patterns in the two N × N switches. We obtain any desired permutation by repeating this process, making

IEICE TRANS. COMMUN., VOL.E82–B, NO.2 FEBRUARY 1999

370

this architecture rearrangeably nonblocking. For m = 2 and N = 4, the architecture becomes 4×4 dilated Benes [3]. The routing function of the banyan network in this architecture can be also realized with a multiport optical switch such as the multileg Mach-Zehnder interferometer [9] or a switch array composed of a star coupler and optical gate switches, but at the cost of high crosstalk or loss. Strictly nonblocking architecture: The strictly nonblocking four-module stage optical switch network architecture can be constructed using architecture in Sect. 3.1 together with Fig. 7 to extend the network size. 4.

Experiment

4.1 32×32 Switch [10] Switch matrix architecture is a banyan network using a perfect shuffle interconnect (Fig. 12) [8] that enables interconnecting waveguides to have large crossing angles. Design specifications in Sect. 2 were used. Waveguide interconnections between switch elements were constructed using waveguides with two different widths to reduce coupling between waveguides in the section near switch elements where two waveguides run nearly parallel with a relatively small separation. No dummy

Fig. 12

32×32 banyan switch.

waveguide intersection was used in this prototype to measure excess loss at waveguide intersection. A two-mode interference switch composed of two mode splitters [10] was used as the switch element (Fig. 13). We chose this type of switch because it is fabrication-tolerant and has low drive voltages. The switch element was 6 mm long, including the 0.5-mmlong fanout sections at both ends. The two mode splitters were connected to construct a 5-mm-long interferometer. The calculated ∆βL/π was 1.6; ∆β is the change in asynchronism required to switch the device, and L is the interferometer length. The calculated switching curve was obtained using beam propagation method (BPM) (Fig. 13). Fabrication and measurement results: A singlemode waveguide was fabricated by indiffusing a 70-nmthick Ti film into a z-cut LiNbO3 substrate at 1050◦ C for 8 hours. A 350-nm-thick SiO2 film was used as a buffer layer. The Au/Ti electrode was placed above waveguides. Device characteristics were measured at a wavelength of 1.3 µm in the TM mode. Drive voltages of the switch element for the cross state averaged 14 V and for the bar state averaged −10 V with a standard deviation of 4 V. The extinction ratio at output ports averaged −18 dB with a standard deviation of 2.4 dB. Light was fed into the outmost input waveguide and output ports were scanned by driving switch elements to measure path-dependent insertion loss (Fig. 14). The

Fig. 14

Measured loss characteristics.

Fig. 13 Two-mode interference switch composed of mode splitters used as optical switch elements in Fig. 12.

OKAYAMA et al: OPTICAL SWITCH ARRAY USING BANYAN NETWORK

371

Fig. 15

64×64 banyan switch.

maximum loss deviation was 10 dB. Insertion loss increases with the number of waveguide intersections and curved waveguides. The maximum number of waveguide intersections the light traversed was 26, indicating an average excess loss at an intersection of 0.4 dB. The insertion loss of the switch matrix averaged 11 dB higher than that of the straight reference waveguide. 4.2 Preliminary Experiment on 64×64 Switch [11] As discussed in Sect. 2, switches exceeding 64×46 can be constructed by using more steeply curved waveguides. We implemented 10-mm-radius curved waveguides using proton exchange (PE). The structure of 64×64 switch is shown in Fig. 15. The total length and optical switch element length were the same as for the 32×32 switch. Part of structure (thick line in Fig. 15) was fabricated. The curved waveguides of the last three stages were fabricated using proton exchange done at 200◦ C for 2 hours; subsequent annealing was at 450◦ C for 0 to 2 hours. The total loss difference between the route with the most curved waveguides and the route with no curved waveguides was estimated to be less than 5 dB at 2 hours of annealing (Fig. 15). The total loss includes curved waveguide, waveguide crossing, and PE waveguide to Ti waveguide junction loss. The loss of the curved waveguide was 0.5 dB and 0.02 dB for the waveguide crossing. 5.

Conclusion

We demonstrated large-scale optical switch arrays based on guided-wave technology. Simple banyan architecture is used to implement a device with a large number of input and output ports on a single chip. We proposed a two-stage architecture to implement nonblocking network employing devices with a banyan ar-

chitecture. With this scheme, the group of interconnects between devices is bundled to decrease the number of connectors. We also proposed a four-stage architecture for a blocking multiport switching elements. We fabricated a 32×32 switch and measured characteristics. Drive voltage was about ±12 V, the extinction ratio was 18 dB, and the insertion loss was 18 dB. Preliminary experiments on a 64×64 device were conducted. The use of proton exchanged waveguides makes a 10 mm radius of curvature feasible. References [1] I.P. Kaminow and T.L. Koch, eds., “Optical Fiber Telecommunications IIIB,” Chapter 10, Academic Press, London, 1997. [2] R.A. Spanke, “Architectures for guided-wave optical switching systems,” IEEE Commun. Mag., vol.25, pp.42– 48, May 1987. [3] H.S. Hinton, “An Introduction to Photonic Switching Fabrics,” Chapter 3, Plenum Press, New York, 1993. [4] T. Goh, M. Yasu, K. Hattori, A. Himeno, M. Okuno, and Y. Ohmori, “Low-loss and high-extinction-ratio silica-based strictly nonblocking 16×16 thermooptic matrix switch,” IEEE Photon. Tech. Lett., vol.10, pp.810–812, June 1998. [5] H. Okayama and M. Kawahara, “Experiment on deflectorselector optical switch matrix,” Electron. Lett., vol.28, no.7, pp.638–639, March 1992. [6] H. Okayama, A. Matoba, R. Shibuya, and T. Ishida, “Optical switch with simplified N ×N tree structure,” J. Lightwave Tech., vol.7, no.7, pp.1023–1028, July 1989. [7] V.E. Benes and R.P. Kurshan, “Wide-sense nonblocking network made of square switches,” Electron. Lett., vol.17, p.697, Sept. 1981. [8] E.J. Murphy, T.O. Murphy, A.F. Ambrose, R.W. Irvin, B.H. Lee, P.P. Gaylord, W. Richards, and A. Yorkins, “16×16 strictly nonblocking guided-wave optical switching system,” J. Lightwave Tech., vol.14, no.3, pp.352–358, March 1996. [9] M. Bachmann, Ch. Nadler, P.A. Besse, and H. Melchior, “Compact polarization insensitive Multi-leg 1 × 4 MachZehnder switch in InGaAsP/InP,” Proc. 20th European

IEICE TRANS. COMMUN., VOL.E82–B, NO.2 FEBRUARY 1999

372

Conf. on Opt. Commun., pp.519–522, Italy, Firenze, Sept. 1994. [10] H. Okayama and M. Kawahara, “Prototype 32×32 optical switch matrix,” Electron. Lett., vol.30, no.14, pp.1128– 1129, July 1994. [11] H. Okayama, Y. Okabe, and T. Kamijoh, “Large-scale optical switch array,” Tech. Digest Photonics in Switching, no.PThB2, pp.174–175, Sendai, Japan, April 1996.

Hideaki Okayama received B.S. and M.S. degrees in applied physics and physics from Waseda University, Tokyo, Japan, in 1981 and 1983, respectively. He received the Ph.D. degree in 1994. In 1983 he joined the Oki Electric Industry Co., Ltd., Tokyo, Japan, where he has been engaged in research on guided-wave optoelectronics and optical communication subsystems. Dr. Okayama is a member of IEEE, the Japan Society of Applied Physics, and the Optical Society of America.

Yutaka Okabe received B.S. and M.S. degrees in applied chemical from Waseda University, Tokyo, Japan, in 1983 and 1985, respectively. In 1985 he joined the Oki Electric Industry Co., Ltd., Tokyo, Japan, where he has been engaged in research on semiconductor device processing, organic material and guided-wave optoelectronics. Mr. Okabe is a member of the Society of Polymer Science, Japan and the Japan Society of Applied Physics.

Takeshi Kamijoh received the B.S. degree in 1976 and the D.Eng. degree in 1982 both from the Hosei University. In 1982 he joined the Oki Electric Industry Co., Ltd., Tokyo, Japan, where he is currently project organizer of the advanced photonic devices project. His research interests include optical semiconductor materials and optical device development. Dr. Kamijoh is a member of the IEEE, Japan Society of Applied Physics, the Japan Physics Society and the American Institute of Physics.

Nobuyoshi Sakamoto received the M.S. degree of electronic engineering from Kyushu Institute of Technology in 1975. In 1975 he joined the Research Laboratory of Oki Electric Industry Co., Ltd., Tokyo, Japan and has been engaged in research on SAW devices and waveguide devices. He is currently project organizer of optical waveguide devices project.