Electrostatically driven micromechanical 2 X 2 optical switch

Kazuo Hogari and Takao Matsumoto

This paper describes a new electrostatically driven micromechanical 2 X 2 optical switch which has advan-

tages in terms of its small size and low driving power. Insertion losses, which are important to the fabrication

of a low loss switch, are theoretically and experimentally evaluated. Based on the results, the relationshipbetween cross angle and gap length is obtained in order to minimize insertion loss. The switch is demonstrat-

ed and its switching performance is examined. Insertion losses of less than 3.1 dB and crosstalk of less than

-40 dB were obtained.

1. Introduction

Optical switches are important to construct flexibleand reliable optical transmission systems and opticalsignal processing systems. Small size and low drivingpower switches especially are required for optical sub-scriber transmission systems and optical parallel sig-nal processing systems.

Electrostatically driven micromechanical switcheshave advantages in terms of their small size and lowdriving power characteristics.1-5 The switches report-ed so far have been fabricated by using selective etch-ing techniques of silicon substrates. 5 These switchesoperate by means of membrane movements that de-flect the optical beams. However, such switches arenot suitable for fiber optics, because they are extreme-ly difficult to align with optical fibers and because thealignment optics are much larger than the switch ele-ment itself. Thus, in order to obtain a small sizeswitch for fiber optics, an electrostatically driven fiberoptic micromechanical switch is proposed.6 Thisswitch has an on/off function to transmit light betweenthe fibers and it cannot change the optical path of thetransmitted light.

In this paper, a new type of electrostatically drivenmicromechanical 2 X 2 optical switch is described.The insertion loss of this switch is theoretically andexperimentally investigated. Based on the results, therelationship between cross angle and gap length isobtained to minimize insertion loss. This switch isdemonstrated and its switching performance is exam-ined.

The authors are with NTT Transmission Systems Laboratories, 1-2356 Take, Yokosuka, Kanagawa 238-03, Japan.

Received 20 February 1990.0003-6935/91/101253-05$05.00/0.C) 1991 Optical Society of America.

II. Switch Structure and Operation Principle

The proposed structure for the electrostaticallydriven micromechanical 2 X 2 optical switch is shownin Fig. 1. This switch consists of a metal membrane, ametal substrate, an insulator, and two optical wave-guides. The two optical waveguides intersect and asmall gap is made in their cross region. The mem-brane tip is bent at a right angle and inserted into thegap. When no voltage is applied, since the membranetip is in the gap, the transmitted light is reflected bythe bending part of the membrane tip and injected intothe other waveguide.

When voltage is applied between the membrane andthe substrate, the membrane is bent upward by theelectrostatic force. The light can then be transmittedthrough the gap without interruption in the samewaveguide. Thus, it is possible to obtain a 2 X 2optical switch that can transmit light between twowaveguides. As with other electrostatically driven op-tical switches, the proposed switch has similar advan-tages in terms of small size and low driving power.

Ill. Switch Insertion Loss

A. Insertion Loss from Transmission and Reflection

It is important in switch designs to analyze the inser-tion losses that occur because of transmission and re-flection. The approximate analysis used here is basedon the plane wave expansion method, which is oftenused to analyze the transmission and reflection prob-lem of a Gaussian beam at the dielectric interface.The calculation model for insertion losses from trans-mission and reflection is shown in Fig. 2. Incident,transmitted and reflected waves are represented by7-9

Ei(Xi,Yi) = (1/27r)2 f f F(k.,ky) exp[-j(k.Xi + kyYi)ldkxdky,

(1)

1 April 1991 / Vol. 30, No. 10 / APPLIED OPTICS 1253

METALSUBSTRATE\

t .. t. . . ~~i...<. -

ELECTRICALSWITCH I

\ _POWERSOURCE.- ':'a ':', , , ,r T

A

A

TRANSMISSION

_ REFLECTIONFig. 1. Electrostatically driven micromechanical 2 X 2 optical

switch structure.

Fig. 2. Calculation model for insertionand reflection.

losses from transmission

Et(XY,) = (/2r) 2f 5 T1(kxky)T2(kxky)F(k.,ky)

X exp[j(kxX, + kY, + b1)]dkdky, (2)

E,(X,,Y,) = (1/27r)2 f 5 T1 (k.,ky)T 2 (k.,ky)R(k,,ky)

X F(k,,ky) exp[-j(kxXr + kyYr + 4'l)]dkdky, (3)

where F(kxky) is the spectral amplitude, Tl(kxky) isthe Fresnel transmission coefficient from the wave-guide to the air, T2(kx,ky) is the Fresnel transmissioncoefficient from the air to the waveguide, R(kxky) is

the Fresnel reflection coefficient, k and ky are thewavenumbers along the Xi- and Yi-axes, respectively,and Al is the field expansion caused by diffraction. is given by

b = 2w tan~k, + 2G tanD±(k, cosO + k, sin.)

+ 2G[(nok) 2- (kX cosO + k, sinO)2 - ky]"2 , (4)

k = [(nk -)2 -k-k2y]/2, (5)

where ko is the free space wavenumber (=27r/X), X is thewavelength, no is the refractive index of the air, n isthe refractive index of the waveguide, 2G is the gaplength, 20 is the cross angle of the two waveguides, Ot isthe transmitted angle when light is transmitted in theair, and 2w is the le field width of the incident beam.

The insertion losses of the switch from transmissionand reflection can be calculated by the field overlapintegral. The insertion losses are given by

L,= -10 loglo[I f f E,(XtYt)Eto(XtYt)dXtdYtl

o S E2(XYt)dXtdy12], (6)

Lr = -10 og10[I f f Er(XrYr)EO(XrYr)dXrdYr/

f S E%2(Xr,Y)dXrdYr2 (7)

where Eto(Xt,Yt) and Ero(Xr,Yr) are transverse fieldsof the fundamental mode of the output waveguidesandEto(Xt,Yt) =Ero(XYr) = Ei(Xi,Y). The incidentwave is assumed to be a Gaussian beam when theinsertion losses are calculated.

Calculated results of the insertion losses are shownin Fig. 3. Insertion losses from transmission and re-flection for the TE and TM waves are different, be-cause the Fresnel transmission and reflection coeffi-cients are dependent on polarization. Moreover, theinsertion losses rapidly increase when the cross angleapproaches the critical angle.

Insertion losses from transmission and reflectionwere examined using single-mode fibers as the wave-guides in Fig. 1. The mode field diameter of the fiberwas 11.0 ,um at 1.5 5-Mm wavelength. A fiber is fixed ona glass substrate and the substrate facet is polished atan angle. The two glass substrates are then arrangedand fixed at the desired angle and a facet of the com-bined substrates is polished. When the insertion lossfrom transmission is measured, the two fixed glasssubstrates are arranged with a gap. Also, when theinsertion loss from reflection is measured, an alumin-ium coated mirror and a fixed glass substrate are ar-ranged with a gap. A fiber optic endlessly rotatablefractional wave device is used to control polarization.10

Measurement results are also shown in Fig. 3. It isfound that the measured values are in agreement withthe calculated values. Thus, it is clear that insertionlosses from transmission and reflection can be predict-ed by Eqs. (1)-(7).

B. Insertion Loss From Transversal Displacement

In case of reflection in the 2 X 2 optical switch,reflected waves, which are transversely displaced asthe result of the membrane thickness as shown in Fig.4, are transmitted. The reflected wave of the trans-

1254 APPLIED OPTICS / Vol. 30, No. 10 / 1 April 1991

TRANSVERSALDISPLACEMENT

TRANSVERSALDISPLACEMENT

Fig. 4. Calculation model of reflected insertion loss with transver-sal displacement.

3.0

m

Sa

-oI

0zU)U)0-J

2.5

2.0

1.5

1.0

0.5

0

-0.510 20 30 40 50 60 70 80 90

I I 4 6 8 1 GAP LENGTH 2G (Jm)

0 20 40 60 80 100 Fig. 5. Relationship between gap length and loss increased.

GAP LENGTH 2G (pm)

Fig. 3. Calculated results of insertion losses from transmission andreflection: (a) relationship between cross angle and insertion loss;

(b) relationship between gap length and insertion loss.

versal displacement based on the previously describedmethod is given by

Erd(XrYr) = (1/2r)2 5 5 T 1(k,,ky)T2(k.,ky)R(k.,ky)

X F(k.,ky) exp[-j(kxXr + kyr + '2)JdkdkY, (8)

'P2 = - t[(noko) 2 - (k. cosO + k, sinO)2 -k2y]1/2 (9)

where t is the membrane thickness. Insertion lossfrom transversal displacement is also calculated by thefield overlap integral, which is given by

Lrd = -10 log10[I 5 5 Erd(XrYr)EO(XrYr)dXrdYr/

5 5 E%0(XtYr)dXrdYrj2] (10)

The calculated results are shown in Fig. 5. As themembrane thickens, the loss from transversal dis-

1 April 1991 / Vol. 30, No. 10 / APPLIED OPTICS 1255

1 6

14

12

10

8

6

4

2

00 20 40 60 80 100

CROSS ANGLE 20 (DEGREE)

6

5

4

3

2

1-

U)U)0Iz0

U)z1

0

150

140

130

120

- 11015 100E !,

N 90

l 80

Z 70

- 60a.

50

40

30

20

100 20 40 60 80

CROSS ANGLE 20 (DEGREE)Fig. 6. Design diagram of cross angle and gap length.

placement increases proportionally. On the otherhand, it is found that insertion loss decreases to gaplength, because the transmitted beam in the gap ex-pands in proportion to gap length.

C. Design of Cross Angle and Gap Length

It is important to determine cross angle and gaplength when designing a low loss 2 X 2 optical switch.When the maximum insertion loss Lmax of the switch isdetermined, the cross angle and gap length can bedesigned using Eqs. (1)-(10). The calculated valuesare shown in Fig. 6. If maximum insertion loss Lma is<1.0, 2.0, or 3.0 dB, when the membrane used is 5 Mmthick,6 the region for cross angle and gap length isindicated by the region on the hatched side of thecurves.

Insertion loss depends on polarization. The differ-ence between the insertion losses for the TE and TMwaves, LTETM, is also calculated by Eqs. (1)-(10).These calculated results are shown in Fig. 6. If themaximum difference between the insertion losses forthe TE and TM waves is <0.2,0.3, or 0.5 dB, the regionfor cross angle and gap length is the hatched side of thecurves.

From the previous discussion, both cross angle andgap length should be selected from the hatched regionsin Fig. 6 to design the 2 X 2 optical switch.

IV. Switching Performance

To evaluate the performances of the proposedswitch, the 2 X 2 optical switch was demonstratedaccording to the results described in the previous sec-tion. Figure 7 shows a photo of the switching elementand the aligned optical fibers. The membrane, sub-strate, and insulator of the switching element are madeof titanium, brass, and epoxy resin, respectively. Toobtain a switching time <2 ms and a displacement ofthe membrane tip >15 ,um,6the membrane should be 5Am thick, 1 mm wide, and 3.5 mm long, respectively.Both surfaces of the bent part of the membrane tip arecoated with aluminum. Optical fibers are used as thewaveguides in Fig. 1. A fiber is fixed on a glass sub-strate and the substrate facet is polished slantingly.The two substrates are then arranged and fixed at a30° cross angle and a facet of the fixed substrates ispolished for a 50 -Am gap. The fixed substrate pair isarranged with a 50-Mm gap. The fibers are single-mode fibers with a mode field diameter of 11.0 ,m at1.55 Am wavelength.

Waveforms for the applied voltage and optical out-puts of the 2 X 2 optical switch are shown in Fig. 8.Just after voltage is applied or turned off between themembrane and the substrate, optical power is unstablefor a few milliseconds because of chattering. To sup-press chattering, a multistep voltage is used,61 asshown in Fig. 8. By this method, chattering can besuppressed and switching time can be <2 ms.

The insertion loss and crosstalk of the switch weremeasured with a switching voltage of 100 V. A fiberoptic endlessly rotatable fractional wave device is usedto control polarization. 0 Measured insertion lossesare shown in Table I. Insertion losses of <3.1 dB wereobtained. The measured insertion losses were slightlylarger than the theoretical values. This discrepancybetween the measured values and the theoretical val-ues is caused by perpendicular misalignment of thetwo aligned fibers in the same fixed substrate andoptical beam scattering is the result of surface rough-ness of the membrane. Moreover, crosstalk of theswitch was confirmed to be less than -40 dB at 1.55-,m wavelength.

FIBER _________ FIBER

t - hi 5 5A -

GLASSSUBSTRATE

Fig. 7. Switching element andaligned optical fibers for the 2 X 2

optical switch. 10mm ALIGNED FIBERS

SWITHIGELEMBRANE

SWITCHING ELEMENT

1256 APPLIED OPTICS / Vol. 30, No. 10 / 1 April 1991

APPLIED VOLTAGE

TRANSMISSIONPort I

OPTICAL - Port 3OUTPUT REFLECTION

Port 1- Port 4

F-AlmS

Table I. Insertion Losses of the 2 X 2 Optical Switch

Insertion Loss(dB)

I T E M

TM TE

Portl - Port3(transmission) 2.3 2.5

Portl - Port4(reflection) 2.8 3.0

Port2 -Port4(transmission) 2.4 2.5

Port2 - Port3(reflection) 2.9 3.1

V. Conclusion

An electrostatically driven micromechanical 2 X 2optical switch, which has advantages in terms of itssmall size and low driving power, has been built andtested. The insertion loss of this switch was theoreti-cally and experimentally evaluated. A design proce-dure based on insertion losses from transmission andreflection in the switch was clarified, and the relation-ship between cross angle and gap length was obtainedin order to minimize insertion loss. Based on theresults, the switch was demonstrated and its switchingperformance was evaluated. Insertion losses of lessthan 3.1 dB and crosstalk of less than -40 dB wereobtained. The 2 X 2 optical switch can be used toconstruct flexible and reliable optical subscriber trans-

1-100V0V

-100PWOpW

-120uW1 0oW

Fig. 8. Applied voltage and optical output wave-

forms for the 2 X 2 optical switch.

mission systems and optical parallel signal processingsystems because of its small size and low driving power.

The authors wish to thank H. Kimura for his supportand encouragement and K. Noguchi for his helpfulcomments and discussions.

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