June 1981 / Vol. 6, No. 6 / OPTICS LETTERS 275

Low-cross-talk 2 X 2 optical switch

Richard A. Soref

Sperry Research Center, Sudbury, Massachusetts 01776

Received February 17, 1981

An electro-optical 2 X 2 switch for fiber-optic applications has been developed. Optical cross talk is reduced signif-icantly by inserting two 2 x 1 switches in each optical path. A compact structure is used to contain the four 2 X 1'sthat are required. We observed -27-dB cross talk and 2.5-dB insertion loss in both switching states. The deviceis controlled by a 5-V twisted-nematic liquid-crystal cell. Multimode, unpolarized light is switched.

The attainment of low cross talk and low insertion lossare two of the most challenging problems in multimodeoptical switching. Results in electromechanicalswitching have been impressivel- 5 but progress inelectro-optical switching has lagged behind in thecross-talk and loss areas. This Letter reports a tech-nique for obtaining significant reductions in cross talk(twice the isolation in decibels of previous state-of-the-art devices) together with low loss and low-voltagecontrol-a result that puts electro-optic switches morenearly on an equal footing with their electromechanicalcompetitors.

The idea is to build a 2 X 2 reversing switch from four1 X 2 "subswitches" in the manner shown in Fig. 1, afunctional diagram of the compound switch. A pair of1 X 2's (open circles) is connected by four optical pathsto a pair of 2 X 1's (closed circles). Note that two opticalpaths in the midregion cross over each other while theother two paths are parallel. The elemental switchesopen and close in unison. Assuming that an elementalswitch in Fig. 1 has a fractional optical cross talk of b,note that cross-talk light from an input switch travelsthrough a second switch in reaching an output port (truein either the ® or the a state). Thus, in theory, thecross-talk intensity arriving at either output is 62.

The Fig. 1 switch has been implemented in a compact,low-voltage structure. We have extended and modifiedthe electro-optic design of Wagner and Cheng, 6 whoused polarizing beam splitters (PB's), right-angleprisms, and the polarization rotation from a liquid-crystal (LC) cell to provide 2 X 2 switching. The volt-age-off and voltage-on states of our new 2 X 2 structureare shown in Fig. 2. In the left half of the structure, two1 X 2's are contained side by side in two PB's and twoprisms. Similarly, the right half includes two 2 X l'slaterally displaced from each other within two PB's andtwo prisms. These elemental switches are actuallytruncated 2 X 2's. The unused input and output portsterminate in optically absorptive material (the twoshaded regions seen in Fig. 2 at the lower portion of theswitch).

Three transfer prisms at the upper portion of theswitch provide the required optical crossover, which isseen in Fig. 2 by following the dotted and solid rays(voltage-on state).

A useful feature of Fig. 2 is that the LC cell is uniform.

Electrodes in the cell are not segmented, and the entireLC area turns on and off as a unit. Operation of theswitch is understood by ray tracing. The orthogonals- and p-polarization components are first separatedon entering the device and are later recombined. Bothpolarizations are switched, giving low loss. With volt-age off, the LC cell rotates each incoming polarizationby 900. This leads to the cross state. With voltage on,the LC loses its rotary power and all polarization statesare unchanged in traversing the LC, leading to the barstate. The 2 X 2 switch is fail safe because opticalcontinuity is maintained in the power-off condition.

Figure 3 shows a laboratory prototype of the Fig. 2

-.

INPUTS a x , OUTPUTS

Fig. 1. Low-cross-talk 2 X 2 optical reversing switch madefrom four 1 X 2's. Lines represent optical paths.

VOLTAGE OFF

Fig. 2.switchcell.

V - 3V

VOLTAGE ON

Two operating states of compound 2 X 2 opticalbased on Fig. 1. Switch includes an electro-optic LC

0146-9592/81/060275-03$0.50/0 © 1981, Optical Society of America

276 OPTICS LETTERS / Vol. 6, No. 6 / June 1981

switch. All components (1.3-cm cubes) were designedfor the 6328-A He-Ne laser wavelength. The 45°-90°450 prisms were antireflection coated, but the PB'swere not. One transfer prism is half the size of theothers. Although components are closely packed, noeffort was made to eliminate air gaps between them.The LC cell, 1.6 cm X 6.4 cm X 0.3 cm, consisted of a10-,Mm layer of 90"-twisted nematic LC, specifically, themixture ZLI-1132 from EM Industries.

Optical cross talk and insertion loss in the Fig. 3switch were measured using 6328-A laser beams7 pola-rized at 450 to the PB edges (50% s and 50% p) to sim-ulate the effect of unpolarized fiber light. Figure 4shows the optical intensity at both outputs of the switchas a function of the control voltage amplitude (rms valueof a 4-kHz square wave) when the first optical input isexcited. The dashed curves show the results when lightimpinges upon the second input. The ordinate in Fig.4 is a decibel scale normalized to the optical inputlevel.

The switch is off from zero to 1.3 V, is fully on at 5 V,and exhibits the desired 0 and (® behavior. Insertionloss is 2.5 dB in both on and off states. About 1.7 dB ofthis loss is due to reflection losses among the 12 con-stituents. Approximately 1.3 dB of the Fresnel loss canbe eliminated by interposing a transparent film ofindex-matching adhesive between each pair of opticalparts. The optical cross talk is 26-27 dB below theinput, the lowest cross talk obtained thus far to ourknowledge in any LC switch. The turn-on time of theswitch is 50 msec, and the natural decay time is 150msec.

Fig. 3. Laboratory version of 2 X 2 optical switch of Fig. 2.

OPTICALOUTPUTPOWEROFSWITCH{dB)

-10

-20

-10a 1 2 3 4 5

CONTROL VOLTAGE (VOLTS RMSI

Fig. 4. Experimentally observed switching characteristicsof Fig. 3 device.

OPTICAL 'INPUTS _

MULTIMODEFIBER

- OPTICALc OUTPUTS

ABSORBER

Fig. 5. Proposed structure of low-cross-talk 2 X 2 fiber-optical switch.

For fiber-optic applications, the switch would becoupled to four multimode guides by quarter-pitchGRIN rod lenses, as shown in Fig. 5. Here, the opti-cal-beam crossover is provided by a multifaceted roofprism shown at the top of the structure. This 2 X 2switch is completely reciprocal (bidirectional) and ismade rugged with optical cement at the interfaces. TheSelfoc lenses introduce about 0.8-dB coupling loss be-tween each fiber pair.8 The expected insertion loss is0.8 dB + 2.5 dB - 1.3 dB, or 2.0 dB.

The extinction ratio of each PB is defined as es + Ep,where es is the transmitted s fraction and Ep is the re-flected p fraction.6 Our PB's had es = 0.005 and ep =0.030. Thus, in accordance with the above discussion,we predict that the compound 2 X 2 will have a crosstalk of (0.035)2, or -29.1 dB. The discrepancy betweenthe theoretical and experimental result is attributed torotation errors in the LC cell.6 Wagner and Cheng6

possessed PB's with es + ep = 0.010. With such com-ponents, we would anticipate a cross talk of -35 dB inan experimental version of our switch [-40 dB from (es+ Ep)2 plus 5 dB from LO rotation errors and LC scat-tering].

In summary, we have demonstrated 2 X 2 opticalswitching with -27-dB cross talk (both states), 2.5-dBinsertion loss (both states), 5-V control, uniform elec-trodes, and fail-safe operation, which is the best com-bination of performance specifications ever reported toour knowledge for a multimode electro-optic switch.The present switch is, however, more complex thanprevious 2 X 2's. As we have discussed, an optimizedfiber-optic switch would have -35-dB cross talk, 3-Vcontrol, and 2-dB insertion loss in both states. Furtherdetails on this switch are given in an unpublished con-tract report.9

This research was supported by the Rome Air De-velopment Center, Deputy for Electronic Technology,Solid State Sciences Division, Hanscom Air Force Base,Massachusetts 01731, under contract F19628-80-C-0147.

References1. C. Dahne and A. L. Harmer, Electron. Lett. 16, 647

(1980).2. C. M. Miller, R. B. Kummer, S. C. Mettler, and D. N.

Ridgway, Electron. Lett. 16, 783 (1980).3. E. G. Rawson and M. D. Bailey, Opt. Eng. 19, 628

(1980),4. M. Nunoshita and Y. Nomura, Appl. Opt. 19, 2547

(1980).

June 1981 / Vol. 6, No.6 / OPTICS LETTERS 277

5. J. Minowa et al., Electron. Lett. 16, 422 (1980).6. R. E. Wagner and J. Cheng, Appl. Opt. 19, 2921 (1980).7. The switch is expected to function properly at the 0.84- and

1.3-Mm fiber-optic communication wavelengths becausethe liquid crystal is highly transparent at those wave-lengths.

8. This performance is based on +0.7' beam collimation in

the switch, which is easily obtained with 3-mm focal-lengthlenses (Selfoc SLS, 2-mm diameter) for 50-Mgm core-di-ameter fibers. GRIN rods with 6-mm focal length arerecommended for 100-Mm core fibers.

9. R. A. Soref, Quarterly Status Report No. 2 on Contract No.F19628-80-C-0147 (January 1981), available from SperryResearch Center, Sudbury, Mass., as Rep. No. CR-81-3.