magnetooptical 2×2 switch for single-mode fibers

Magnetooptical 2X2 switch for single-mode fibers

Masataka Shirasaki, Fumio Wada, Hisashi Takamatsu, Hirochika Nakajima, and Kunihiko Asama

An optical switch for single-mode fibers is presented in this paper. A novel polarizing prism is introducedto make the 2 input, 2 output optical switch insertion loss low and independent of the polarization state ofthe input fiber. This switch operates by one electric pulse with a voltage of only 2.5 V and a current of 35mA. The insertion loss was 1.2 dB, its fluctuation for different input polarizations was within 0.03 dB, andthe far-end crosstalk was -26 dB at a wavelength of 1.3 /Am. The shock resistance of the switch was con-firmed during an acceleration of 100 g.

I. Introduction

Optical fiber communication systems require a switchthat can change the optical path to select transmissionlines. So far a variety of optical switches have beendeveloped. 1 -5 The majority of the switches previouslyreported for practical use are mechanically operated andcontain mechanical parts, so it is difficult to guaranteehigh switching speed and small switching power.

Previously we developed a nonmechanical opticalswitch for single-mode fibers,6 which uses Faradayrotation of light in yttrium-iron-garnet (YIG) single-crystal thin plate. YIG Faraday rotators are known asefficient polarization rotators.5 7 The switch in Ref. 6exhibited practical characteristics, for example, an in-sertion loss of 1.4 dB. Furthermore, the switch showedhigh reliability which is the stability of the insertion lossafter 108 switching operations. However, the switchhad some disadvantages:

(1) Although it was designed for the insertion loss tobe independent of the incident polarization state, somedependence (0.5 dB) was observed for the fabricatedsamples because of the misalignment of components,that is, the switch was difficult to construct accurately,since it had many components and adjustmentpoints.

(2) A part of the light passing through the switch wasreflected at the end faces of the input and output fibers.This increased the insertion loss, near-end crosstalk,and reflected return light. (The near-end crosstalk,defined as the crosstalk between input ports or between

Masataka Shirasaki, H. Nakajima and K. Asama are with FujitsuLaboratories Ltd., 1677 Ono, Atsugi 243-01, Japan; the other authorsare with Fujitsu Ltd., 1015 Kamikodanaka, Nakahara-ku, Kawasaki211, Japan.

Received 28 January 1984.0003-6935/84/193271-06$02.00/0.© 1984 Optical Society of America.

output ports, is distinguished from the far-end crosstalkdefined as the crosstalk between input and outputports.)

(3) Since two YIG crystals and two magnets wereused in this switch, the structure was complex and twiceas much switching current was needed.

This time we designed an improved switch and fab-ricated it for a wavelength of 1.3 ,um. It uses the Fara-day effect of YIG crystal, and the principle of switchingis similar to that of the prototype we reported in Ref. 6.The new design remedies the minor disadvantages de-scribed above. Furthermore, it has the following ad-vantages:

(1) It is more compact.(2) It has four terminal fibers, two inputs and two

outputs.(3) The magnet is optimized to reduce switching

power.In this paper, the configuration, principle, and charac-teristics of the new switch are presented.

II. Principle of the Optical SwitchThe principle of the new switch design is funda-

mentally similar to that of the prototype switch in Ref.6. Switching is performed by reversing the magneti-zation of a 450 Faraday rotator made of YIG singlecrystal. The YIG Faraday rotator, which is 200 Aumthick and 2.1 mm long, rotates the polarization planeof light passing through it by 450 under a low magneticfield of -100 Oe.

The incident light from the fiber passes through a lensand is separated by a prism into two polarized compo-nents whose polarization planes are perpendicular toeach other. These components pass through the Far-aday rotator and a quartz halfwave plate. The polar-ization plane is then rotated 0 or 90°, corresponding tothe direction of the magnetization. These componentsare combined by another prism and emitted from oneof the two output ports. Hence, an arbitrarily polarized

1 October 1984 / Vol. 23, No. 19 / APPLIED OPTICS 3271

INPUT

MULTI-LAYER FILM

kYIGrX HPOUTPUT 2-Q

HALF-WAVE PLATE

OUTPUT I

Fig. 1. Configuration of the prototype optical switch. To eliminatethe incident polarization dependence of the characteristics, the po-larizations are separated and combined by multilayer interferencefilms. A magnet is placed above the YIG located at each beam

waist.

A Total Reflection Film

/57- Cn)

PolarizationSeparation ilm B

Fig. 2. Configuration of the newly developed polarizing prism. Thisprism has the same functions as those of the prototype and is superior

to the prototype in assembly tolerance accuracy.

Fig. 3. Configuration of the present optical switch. The halfwaveplate is cemented to the prism. Since the light paths between theprisms are near each other (1 mm), only one YIG is needed for the

two light paths.

input light passes efficiently to the output, and the in-sertion loss of this switch is independent of the inputpolarization.

Switching is performed by reversing the magnetiza-tion of the YIG. The YIG magnetization is saturatedby an applied magnetic field generated by an electro-magnet with a semihard magnetic material core. Thiselectromagnet makes the switch bistable.

Ill. Configuration of the Improved Optical Switch

A. Polarizing Prism Features

In a magnetooptical switch, two polarizing prisms areused. Figure 1 shows a switch in which a prototypeconfiguration of such a prism is used. In this switch,fabricated as in Ref. 6, the distance between the twopolarized light beams is 8 mm. This distance that isrelatively large requires precision prisms and accuratestable alignment. Furthermore, one YIG Faraday ro-tator must be used in each of the two light paths, andeach rotator requires one electromagnet. However, theuse of two YIG rotator/magnetic pairs is undesirable forperformance, cost, and reliability considerations.

So we developed a new prism to bring the two polar-ized light beams closer to each other. The configurationof the new prism is shown in Fig. 2. Here, a parallelglass plate is sandwiched between two triangular glassprisms with transparent adhesive. Figure 3 shows theconfiguration of the new switch using these prisms.These prisms reduced the distance between the twolight beams to 1 mm, and the light beams pass througha single YIG rotator.

In the prism shown in Fig. 2, ports A and B are opti-cally connected to the input and output fibers by lenses.(The YIG Faraday rotator is aligned with ports C andD.) The surfaces of the prism where the light pene-trates are antireflection (AR) coated for a wavelengthof 1.3 /-m. Operation of the prism is as follows: Thelight beam entering at port A is separated into twopolarized components at the multilayer thin film forpolarization separation. The s-polarized componentis reflected and the p-polarized component is trans-mitted at the film. Then the s-polarized component(reflected at the total reflection film) exists at port Cwhile the p-polarized component exits at port D. Thelight beam entering at port B is also separated into twopolarized components. Then the s-polarized compo-nent exists at port D and the p-polarized componentexits at port C in a similar way. Here, the polarizationstates of C and D are inverted depending on the inputlight beam from A or B.

This configuration is feasible because the input andoutput fibers are installed in parallel. Furthermore, thehalfwave plate, a 450 invariable polarization rotator, iscemented to the prism surface.

B. Design of the Polarizing PrismIt is important for fabrication of the prism to precisely

control thickness and parallelism of the glass platesandwiched between the two triangular glass prismsshown in Fig. 2. Imperfection in these parametersgenerates spatial shift and angular split of the outputlight beam. These errors increase the insertion loss andits polarization dependence when the output light beamis focused into the output fiber. The optical charac-teristics of single-mode fiber devices are generallysensitive to the accuracy of prisms, but it is compara-tively easy to form the parallel glass plate accurately.

The thickness of the glass plate of the prism is 0.7 mmin this switch. Therefore, the distance between the two

3272 APPLIED OPTICS / Vol. 23, No. 19 / 1 October 1984

vd

light beams from C and D, shown in Fig. 2, is -1 mm.Hence, only one YIG Faraday rotator is needed for thetwo light beams.

The other advantage of this prism is that the spatialshift of the output light beam is much smaller than thatof the prototype prism when the prisms are misalignedby the same angle. The reason is that the spatial shiftis proportional to the products of the tilt angle of theprism and the distance between the two polarized lightbeams. Hence, the distance between the two beams ofthe new prism is about one-eighth of that of the proto-type prism. The increase in insertion loss is negligiblewhen the orientation of the prism is adjusted by aHe-Ne laser beam.

The prism shown in Fig. 2 has two films in it. Onefunctions as a polarization separator and the other asa total reflector of light. The film for polarizationseparation is the same as that of the prototype switchin Ref. 6; it is composed of twenty-three layers of al-ternately stacked TiO2 and SiO2. The total reflectionfilm is composed of thirty-five layers of alternatelystacked TiO2 and Si0 2 . This film has high reflectivityof more than 99% for both s and p polarizations.

These polarization separation and total reflectionfilms are deposited on the glass plate as shown in Fig.2.

C. Lens System

The lens converges the rays from the input fiber andmakes the beam waist in the thin YIG plate. Thenthese rays are focused onto the output fiber end face bythe opposite lens. In this new switch configuration,shown in Fig. 4, graded-index rod lenses were used tomake a beam waist and optically connect the fibers.Rod lens length was determined as 0.278 pitch forpropagated light at 1.3-pum wavelength. This lensmakes the beam waist 15.4 mm from the end face of thelens when the fiber is connected to the lens surface withtransparent adhesive. Since the transparent adhesivehas the same refractive index as fibers and lenses, thelight does not reflect at the fiber-lens interface. Hence,the optical losses are reduced -0.3 dB for the switch.As each lens must be arranged at the optimum position,a margin for adjustment is necessary. So the lens isfixed on the fiber end face, and then the lens-fiber pairis adjusted and fixed. The surface of the lens facing theprism is AR coated. Even if a small amount of light isreflected at this surface, it cannot return to the fiberbecause the light rays passing through this surface arenot parallel. Therefore, this switch does not generatereflected return light. Furthermore, the near-endcross-talk, which is a part of the input light reflected atthe output side, is not generated for the same reason.

Since the light passes through the thin YIG plateshown in Fig. 4, the light beam must be thin near thebeam waist. Figure 5 shows the measurement of thebeam thickness. The horizontal axis indicates thedistance along the light propagation, where scale zeromeans the beam waist. The vertical axis indicates thediameter of the light beam. The two curves show thediameters which encompass more than 90% and 99% of

light energy propagated from fiber to fiber. Thismeasurement shows that the 99% diam at the beamwaist is -87 ,gm. The 99% beam diam is -90 tim atpositions 0.8 mm on either side of the beam waist.These positions correspond to the YIG plate end facesbecause of the YIG high refractive index (2.2) and thedifference of path length between two optical paths.Therefore, it is possible for the light beams to passthrough a 200-Mm thick YIG plate.

D. Faraday Rotator

A 450 Faraday rotator consists of a thin YIG plate andan electromagnet with the core made of semihardmagnetic material as shown in Fig. 4. The YIG plateis 200 ,um thick, 2.1 mm long, and 2.6 mm wide. Lightis propagated in the (111) direction of a YIG singlecrystal. The YIG crystal is grown by the floating zonemethod and has good transparency for the long-wave-length region. 8 The 2.1-mm long YIG plate has -97%transmissivity at 1.3-,um wavelength when AR coatingsare used.

Semi-HardD o_, Magnetic Material,,

YIG

Lens

Fiber

Prism

Fig. 4. Isometric view of the switch. Graded-index lenses are usedto focus the light rays and make the beam waist in the thin YIG plate.The magnet made of semihard magnetic material makes the switch

bistable.

-10 -5 0 +5Distance From BeamWaist X (mm)

Fig. 5. Diameter of the light beam as a function of the distance fromthe beam waist. The upper and lower curves indicate the beam di-ameters including 99% and 90% of the propagating energy,

respectively.


The magnetic field to saturate the YIG plate is ap-plied through the electromagnet. The core of themagnet is made of a semihard magnetic material(Nibcolloy9). Operating voltage and current dependon the parameters of the electromagnet. We deter-mined the coercive force, Hc, to be 9 Oe. The copperwire was wound 1400 turns for the coil. Its electric re-sistance and inductance were then -70 and 7 mH,respectively. The magnetization of this magnet, whenused with a YIG plate, is reversed by one 2.5-V pulse.

Since the core of the magnet is 2 mm wide, the areain which the magnetic field is applied by one magnetcovers the two light beams which are separated by -1mm as shown in Fig. 3.

IV. Characteristics of the Fabricated Optical Switch

We fabricated the new optical switch for 1.3-Mmwavelength. It has two input and two output ports withsingle-mode fibers. The switch as presently developedis shown in Fig. 6; the switch is 50 mm long, 30 mm wide,and 25 mm high excluding the terminal fibers. Thecharacteristics of this switch were measured using thelight from a semiconductor laser at 1.3-Mm wave-length.

First, the switch's electrical characteristics are de-scribed. Since the electromagnet in this switch is madeof semihard magnetic material, the switching is bistable,that is, two states are switched reciprocally by oneelectric pulse to the coil. The minimum switchingvoltage is 2.5 V, and the resulting current is 35 mA.Here, the time constant of current increase is -100 Asecbecause the coil has inductance. This switch is oper-ated by bipolar voltage +2.5 V. It can be switched byunipolar voltage when a three-terminal coil is used;then, twice as much current is needed.

The light transmissivity characteristics are insertionloss, far-end crosstalk, and near-end crosstalk. Here,loss and crosstalk are defined as the ratio of the opticalpower in the output fiber to that in the input fiber.Since this switch has two input and two output ports,there are eight input/output combinations with respectto each state of the switch. Insertion losses and far-endcrosstalk are measured for all combinations, and theresults are shown in Table I. The averages of the in-sertion losses and far-end crosstalk are 1.2 and -26 dB,respectively. The maximum deviations of insertionlosses and far-end crosstalk are -0.1 and 1 dB, respec-tively. Table II indicates the insertion loss details.

Next, the change in the insertion loss induced byvariance of the input polarization state is discussed.This loss change is due to the inequality of lens couplinglosses for two polarization light beams and the differ-ence between the transmissivities for the two lightbeams. Since the new prism is designed so that theformer factor is negligible, the latter affects the losschange. The main factor for the transmissivity dif-ference is the polarization dependence of the reflectivityat the total reflection film. In particular, these differ-ences are doubled when the state of the switch is set sothat the polarization rotations by Faraday rotator and

Fig. 6. Photograph of the 2 X 2 optical switch. The frame is doublestructure. The prisms, YIG plate, magnet, and lenses are constructedon the inner frame. The outer covers of the fibers are fixed to the

outer frame.

Table 1. Insertion Losses and Far-end Crosstalk

Insertion CrosstalkInput port Output port loss (dB) (dB)

1 3 1.18 -25.41 4 1.09 -26.92 3 1.25 -27.12 4 1.18 -25.43 1 1.27 -25.63 2 1.29 -26.34 1 1.20 -26.54 2 1.17 -25.4

Average 1.20 -26.1

Table II. Details of the Insertion Loss

Item Loss (dB)

Lens coupling 0.6Prisms 0.3YIG 0.2Misalignment 0.1Total loss 1.2

halfwave plate cancel out. Namely, the polarizationdependence of the insertion loss should be larger whenlight is transmitted from port 1 to port 3 and from port2 to port 4. Figure 7 shows the polarization dependenceof the insertion loss from port 1 to port 3. The verticalaxis shows the loss change from the mean loss. Typicalvalues are shown with measurement errors. The hori-zontal axis indicates the s-polarization ratio in the inputlight. The ratio was varied by twisting the input fiberand measured by interrupting one of the two polariza-tion light paths in the switch. The s-polarization ratiocould be varied from 0.05 to 0.92 this way. Figure 7shows that the polarization dependence of the insertionloss is about +0.03 dB.

Next, the wavelength dependence of the character-istics is discussed. When the wavelength deviates, in-sertion loss and far-end crosstalk increase. This is


N

_+0.02

IM+0.01C 00 -C-0

-0.02-0.03

0 02 Oh 0.6 _0 1.0

P Polarization Ratio S/(S+P) S

Fig. 7. Input polarization dependence of the insertion loss from port1 to port 3. The horizontal axis indicates s-polarization ratio in the

input light. The vertical axis indicates relative loss change.

Vco0)

-C

nnU

U)

0

0.2

0.1

0

0 10 20 30 40Temperature (C)

Fig. 8. Temperature dependence of the insertion loss. The hori-zontal axis indicates the temperature. The vertical axis indicates theloss change from that at room temperature of 25 0C. Values for four

input/output combinations are represented.

0.

5ms

X-direction Y-direction Z-direction

Fig. 9. Shock resistance of the switch. The lower traces indicatethe accelerations of the switch in the three directions. The uppertraces show that the output power (corresponding to the insertion loss)

is not affected during an acceleration of 100 g.

mainly due to the wavelength dispersion of rotation inthe Faraday rotator, that of phase retardation in thehalfwave plate, that of separation ratio of polarizationseparation film, and that of refractive index of lensmaterial. However, the total effect of these factors forthe insertion loss is theoretically within -0.1 dB in the1.25-1.35-Mtm wavelength region. On the other hand,the far-end crosstalk is comparatively sensitive towavelength deviation, only the effect of the lens isnegligible. The wavelength region, in which thetwenty-three layer polarization separation film theo-retically has crosstalk less than -25 dB for both s andp polarizations, is from 1.25 to 1.35 Am. In the samewavelength region, the total crosstalk due to the YIGFaraday rotator and the quartz halfwave plate is lessthan -25 dB. The value is estimated from measure-ment of the optical circulator,7 because the prisms inthis circulator have low crosstalk in this wavelengthregion.

The values discussed above were measured at roomtemperature (250 C). Figure 8 shows the temperaturedependence of the insertion loss. In this figure, thevertical axis indicates the root mean square of thechanges in insertion loss from that at 250 C for the sevenfabricated switches. The four values at each temper-ature indicate the results for input/output combinations1-3, 1-4, 2-3, and 2-4. The change in insertion loss is-0.1 dB in the temperature range from 5VC to 40'C.

Shock and vibration resistances of this switch weremeasured. Figure 9 shows shock resistance in threedirections, that is, the three lower traces indicate ac-celeration in the X, Y, and Z directions. The uppertraces indicate the intensity of the output light whenthat of the input light is constant. These figures showthat input light is constant. These figures show thatacceleration of 100 g in any direction does not affect theinsertion loss.

Figure 10 shows the vibration resistance. The uppertrace indicates an output intensity similar to that of Fig.9. The lower trace indicates the vibration of the switch,


N1-

, I

E

30Hz Y-directionFig. 10. Vibration resistance of the switch. A 30-Hz vibration of0.75-mm amplitude is applied to the switch. The insertion loss does

not change during vibration.

which is 0.75 mm in amplitude and 30 Hz in frequency.This vibration has a maximum acceleration of -3 g.Figure 9 shows the result in the Y direction; those inother directions are similar. Furthermore, the vibrationresistances for 5-55 Hz were confirmed. From thesemeasurements, it was confirmed that the changes ininsertion loss during a 100-g acceleration and 5-55-Hzand 0.75-mm amplitude vibrations in any direction werenegligible.

Finally, the measurements for the near-end crosstalkare discussed. Since this switch contains a nonreci-procal element (45° Faraday rotator), it functions as afour-port optical circulator. So the reflected light fromthe output side does not return to the input port but isemitted from the other port on the input side prism.That is, this reflected light generates the near-end cross-talk. In Fig. 3, for example, the part of the input lightat port 1 that is emitted from port 2 is near-end cross-talk and was measured to be -17 dB. This near-endcrosstalk is explained as follows: All the input andoutput surfaces of the prisms and the YIG plate are ARcoated and the light rays are not parallel. Hence, thenear-end crosstalk caused by reflection from thesesurfaces is small. A value of -42 dB was measured byinterrupting the light at the output port in front of thelens. Most of the near-end crosstalk is induced by re-flection at the output end face of the output fiber, thatis, the output fiber end face opposite the lens and prism.This was established when near-end crosstalk decreasedto -33 dB after the output end face of the output fiberwas index matched by glycerin.

VI. Conclusion

A superior-characteristics optical switch was pre-sented. We developed a new polarizing prism as onecomponent of this switch; it was used in developing a 2X 2 optical switch for single-mode fibers. This switchhas a YIG Faraday rotator and maintains bistability;it was fabricated for a wavelength of 1.3 gm. The newlydeveloped prism has some advantages over our previousone: this prism makes the insertion loss of the switchvirtually independent of the input polarization state.Moreover, the switch is simple and compact andswitching power consumption is less.

The switching is performed by one electric pulse witha voltage of only 2.5 V and a current of 35 mA. Theinsertion loss from fiber to fiber is 1.2 dB and its changedue to the input polarization state is within 0.03 dB.The change in insertion loss in a temperature rangefrom 5oc to 40C is -0.1 dB. The insertion loss showsno change during an acceleration of 100 g and vibrationswith 5-55 Hz and 0.75-mm amplitude. The far-endcrosstalk is -26 dB at a 1.3-Itm wavelength. The di-mensions of the frame of the switch, excluding the fi-bers, are 50 mm long, 30 mm wide, and 25 mm high.

The authors thank Y. Niiro and H. Wakabayashi ofKDD for useful discussions; S. Kusaka and H. Noda for'the coating process, and Y. Suzuki and W. Yamagishifor supplying the semihard magnetic material. Theyalso are grateful to N. Yoshikubo and M. Motegi forhelpful discussions, and they appreciate the encour-agement of K. Yamagishi, T. Inagaki, and Y. Furukawathroughout this work.

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5. R. E. Wagner and J. Cheng, "Electrically Controlled Optical Switchfor Multimode Fiber Applications," Appl. Opt. 19, 2921 (1980).

6. M. Shirasaki, H. Nakajima, T. Obokata, and K. Asama, "Non-mechanical Optical Switch for Single-Mode Fibers," Appl. Opt.21, 4229 (1982).

7. M. Shirasaki, H. Kuwahara, and T. Obokata, "Compact Polar-ization-Independent Optical Circulator," Appl. Opt. 20, 2683(1981).

8. H. Goto, I. Maeda, T. Nakano, U. Kihara, and M. Torii, "AnEvaluation of The Magneto-Optical Characteristics of YIG SingleCrystals," J. Magn. Magn. Mater. 31-34, 779 (1983).

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magnetooptical 2×2 switch for single-mode fibers

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