Optical circulator for fiber-optic transceivers

Claudio Zizzo

A highly efficient optical circulator for bidirectional transmission on a single fiber has been developed. A 50%transmitting efficiency and a 70% receiving efficiency have been evaluated for the nonoptimized laboratoryprototype. Its behavior and technology are discussed, together with possible improvements using recentlyannounced optical materials.

1. Introduction

A fundamental component for building a bidirec-tional single optical fiber communication system is atrigate element (Fig. 1). This device allows the pas-sage of light impulses from gate 1, site of the transmit-ter, to gate 2, site of the fiber, while transferring anarriving light signal to the receiver at gate 3. Thistrigate should have low insertion losses. Gates 1 and 3are uncoupled to prevent signals from T, and T2 fromoverlapping at R1.

The use of two distinct wavelengths is the simplestway of transmitting data bidirectionally in a singlefiber.1'3 A trigate in this case is used only to separatethe two wavelengths. There are several physical prop-erties available for achieving this trigate, for example,material dispersion, that is, the dependence of thematerial refractive index on the wavelength used; theutilization of the interference between the wavefrontsin a multilayer dielectric structure; the utilization ofreticle diffraction. There are valid reasons, however,for preferring a single wavelength for the trigate.Both transmitters and receivers on the market todayare optimized for a given wavelength, corresponding tominimum fiber attenuation and chromatic dispersion.Therefore it is advantageous to operate on a singlewavelength both in transmitting and in receiving.

Let us examine some devices that allow a bidirec-tional optical fiber connection using a single wave-length.

A 3-dB directional coupler is a quadrigate in whichgate 1 is coupled with gates 2 and 3, while gate 2 is

The author is with Center for Electronic Research in Sicily, 90046Monreale, Italy.

Received 30 January 1987.0003-6935/87/16347004$02.00/0.© 1987 Optical Society of America.

coupled with gates 1 and 4. If the directional couplerdoes not present insertion losses, 50% of power fromgate 1 goes to gate 2, where the fiber is, and 50% goes togate 3, which is terminated (Fig. 2). Half of the powercoming from the fiber and arriving at gate 2 goes to thetransmitter at gate 1 and half to the receiver at gate 4.

The efficiency of a similar device, excluding unwant-ed reflections and losses, is then 50% in transmissionand 50% in reception.

Such directional couplers have already been madefor optical fibers whose outer cladding has been re-duced to allow evanescent coupling between fibers.4-7

11. High Efficiency Optical Circulator

It is possible to conceive of an optical configurationhaving 100% theoretical efficiency. The characteris-tics are those of an ideal trigate circulator. This circu-lator must contain an anisotropic material with anasymmetric permeability tensor which makes it nonre-ciprocal. There are several magnetooptic materialsthat exhibit these properties if subjected to a magneticfield parallel to the direction in which the light propa-gates.8 ' 9

To utilize the characteristics of the circulator, wemust employ a polarized source such as a laser diode.In this case, due to the relatively poor polarization ofthe laser diode,0' 11 the orthogonal component's powerwill be lost (5% of the total power assuming a 20:1ratio of the total optical power for the two orthogonalpolarizations).

Ideally, at gate 1 (Fig. 1), the light should be alinearly polarized wave. The electromagnetic wavecoming from the optical fiber and entering at gate 2(Fig. 1) will generally, however, be elliptical. To over-come this polarization problem without incurring ap-preciable losses an array of plates at the Brewster angleis proposed (Fig. 3). The plates linearly polarize thelight by reflecting the component at 90° while trans-mitting the other. As can be seen in Fig. 3, the circula-tor contains a rotator which allows the isolation of gate

3470 APPLIED OPTICS / Vol. 26, No. 16 / 15 August 1987

I T+71 2 2 13 3

Fig. 1. Bidirectional single-fiber communication system.

T 1 2 F

4 D 3

X~~~ ~ ~~~~~~~ I IV,

Fig. 2. Directional coupler: D, 3-dB directional coupler; F, fiber;M, matched load.

Fig. 3. Optical signal path to the receiver (solid line).

1 from gate 2. The rotation of the polarization plane ofthe light is given by the relation: 0 = V X B X L, whereB represents the module of the magnetic inductionfield, L is the length of the crystal intersecting field B,and V is the Verdet constant.

Ill. Description of the Optical Circulator

Figure 3 shows the diagram of the optical circulatorwe made. The electromagnetic wave arriving from thefiber passes through a series of lenses that reduce thebeam section. Passing through the first array of platesplaced at the Brewster angle, the parallel wave is trans-mitted, while the normal component is mostly reflect-ed. Given a sufficient number of plates, only a negligi-ble part (of the order of 1%) is transmitted. Thenormal component is thus reflected and sent to thereceiver by means of an inclined mirror and a shortfocus lens.

The component parallel to the plane of incidencepasses through the plates at the Brewster angle andreaches a Faraday rotator which rotates its polariza-tion plane by 45°. This component, exiting the rota-tor (EiI1+45 ' in Fig. 3) reaches a second array of plates; atthis point, however, the incident wave is no longer atthe Brewster angle. Because of the spatial position ofthe plates, the incident wave has a normal electricalfield at the plane of incidence. The light thus reflect-ed by the second array is sent, by a mirror and a lens, tothe receiver. Both components of the electromagneticwave coming from the fiber, therefore, arrive at thereceiver. If the materials used to make the plate ar-rays and the rotator do not absorb and do not diffusethe incident power, all the power from the fiber reachesthe receiver.

The electrical field of the wave leaving the transmit-ter (indicated in Fig. 4 by Eull 4 5 ') is parallel to theincidence plane of the plate array (indicated by array

Fi ber

Hi____ ___________ Xm _~_

Fig. 4. Optical signal path from the laser diode to the fiber (solidline).

2) placed at the Brewster angle. The beam is thustransmitted entirely. Reaching the rotator, the elec-trical field is rotated 450 so that the electrical field ofthe wave coming from the transmitter, when exitingthe rotator, is parallel to the incidence plane of theplates (indicated by array 1). The wave is thus trans-mitted entirely.

In such conditions all the power coming from thetransmitter can reach the optical fiber.

To determine the number of plates necessary foreach array it is sufficient to know the refractive indexof the material used. Indicating this index by N andassuming that the refractive index of air equals 1, theBrewster angle will be given by

O = arcsin[1 N -

In correspondence to this angle, the square of thereflection coefficient of the component whose electricfield is normal at the plane of incidence, assuming amedium of index N of infinite extension, is

r2 I N21 1+N2

If we now assume that we have a plate of losslessmaterial receiving a Gaussian beam with its electricalfield normal at the plane of incidence, we can deter-mine the electrical field reflected as Er = rIEj + r (1- r) 2E + F31 - r)2Ei + .... In terms of powerwe will have Pr = r2Pi+ r2 (1-r2) 2pi+ r 6(1rF)2 Pi +.... The relationship between the incidentpower and that reflected, disregarding the terms ofhigher order, will be given by

R =Pp=r r ( -r2)2

The total reflected power from an array of K plateswill be

= R (1- R)m]

This relation must be close to unity.The higher the refractive index of the material cho-

sen, the lower the number of plates necessary to makean array. The choice of the material, and therefore ofthe number of plates necessary to obtain the desiredreflection, also depends on its absorption index, whichis a function of wavelength. Using five plates of GGG(Gd3Ga5O12, refractive index N = 2) we calculate Prt/Pi to be 0.97. Taking into account material absorp-tion, this value is reduced to 0.9.

15 August 1987 / Vol. 26, No. 16 / APPLIED OPTICS 3471

The rotator was developed using a zinc selenidecrystal (ZnSe). This material has a high Verdet con-stant and low absorption at the wavelength used fortesting the device. The absorption coefficient of aZnSe crystal 13 mm long at a wavelength of 633 nm isequal to 0.18 cm-'.

The magnetic field necessary for the rotation of theplane of polarization is obtained from two 13-mm diamsamarium-cobalt magnets. The magnets have a 2-mmhole drilled at the center to allow the passage of thelaser beam to the interposed crystal (13 X 4 X 4 mm).Because of their limited dimensions, these magnetsgenerate an intense magnetic field only in their prox-imity. To obtain a 450 rotation of the polarizationplane it is necessary to use those parts of the crystalclosest to the magnets. Once the distance between themagnets is fixed, the path length of light through thecrystal is increased using mirrors on both facets of thecrystal itself to obtain multiple reflection (Fig. 5).

To avoid unwanted reflections from the discontinu-ous surfaces, air-crystal and crystal-air, the crystalitself was matched using a cesium fluoride film whoserefractive index (1.63) is close to optimal (1.61), formatching to zinc selenide whose refractive index is 2.6;we thus have 2.6 X 1 = 1.61. The mirrors (Fig. 5),which cover in part the two facets of the crystal, aremade of multidielectric layers by alternately deposit-ing seven high index and seven low index layers of zincsulfide (N = 2.3) and magnesium fluoride (N = 1.38).

The structure of the circulator is schematically de-scribed in Fig. 6. The dimensions of the base whichsupports the two batteries, the rotator, and the twolenses are 12 X 5 cm. The dimensions of the deviceitself are therefore limited and can be easily reduced bya more compact assemblage.

In Fig. 7 the optical circulator is viewed laterially;shown are the second array of plates (indicated byarray 2 in Figs. 3 and 4) and one of the two mirrors,placed in a 450 plane with respect to the base. Thismirror sends the rays reflected by array 2 to the receiv-er also visible on the right-hand side of Fig. 7.

IV. Performance of the Optical Circulator

The theoretical performance of this device is opti-mal. Given lossless material for the rotator, the ab-sence of losses from the mirrors, and a perfect match-ing layer, all the power coming from the transmitterreaches the receiver. The uncoupling of the transmit-ter and the receiver (respectively, T and R in Fig. 4) iscomplete. The materials utilized, however, are notlossless, thus diminishing the ideal performance. Theplates that make up the array reflect, with the polariza-tion described, 90% of the incident light. The mirrorsreflect 95%.

Other losses are introduced by the rotator. Sincethe matching layer lowers the reflection from 20% to<1%, more than 99% of the incident power penetratesthe zinc selenide crystal. The light (A = 633 nm)passes through the length of the crystal three times toachieve 450 rotation, losing through interior scattering32% of the power entering the crystal. We must con-

Fig. 5. Rotator: C, crystal; M, multidielectric mirror; L, matchinglayer.

//

m m~~~~~~~~~~~~~~~~~~I

A, A2

Fig. 6. Optical circulator structure: C, crystal;mirror; A, array of plates.

M, magnet; m,

Fig. 7. Nonoptimized laboratory prototype.

sider, moreover, the losses from the multidielectricmirrors on the crystal. They reflect only 95% of theincident power.

The total efficiency of the prototype was measuredto be 50% in transmission and 70% in reception,assuming the light coming from the fiber to be circular-ly polarized.

V. Possible Improvements

The choice of zinc selenide for the rotator was due tounavailability of new materials that have a lower ab-sorption index and a higher Verdet constant for thewavelength used. These new materials have alreadybeen used in research laboratories in Japan 21 3 and inthe United States.14 The performance of the circula-tor, if such materials were used, can be estimated uti-lizing the available data.

Working with a wavelength of 633 nm, a materialthat offers an interesting performance isCdO.55MnO.45Te. This material has a Verdet constantof 0.15° cm- X G-1 at 633 nm against the 0.006760cm- X G 1 of zinc selenide. The attenuation ofCdMnTe, at 633 nm, is slightly larger than ZnSe. Forthe same magnetic field the high Verdet constant pro-vides 450 rotation for a path length 22 times shorter.With this material a 1.5-mm plate would be sufficientto obtain the 450 rotation. It would, moreover, beunnecessary to use multidielectric mirrors.

The attenuation produced by this rotator, whenmatched to the surfaces of the CdMnTe crystal, would

3472 APPLIED OPTICS / Vol. 26, No. 16 / 15 August 1987

be 1.7%. In fact 98.3% of the incident power would betransmitted. Moreover, using two arrays of platesmade with a material having a high refractive indexbut with fewer losses than GGG, and coated mirrorsproviding 98% reflectivity, the whole device could havean efficiency of 90%.

Up to now we have considered the device as func-tioning at 633 nm, but a very interesting wavelengthfor fiber communications is 1300 nm. At 1300-nmYFeO doped with gadolinium (Gd0.2Y2.8Fe5 012 or moresimply Gd:YIG) can be used to make the rotator of theoptical circulator.

In such an optical isolator,'2 the insertion lossesobtained using this material were 17% for a crystallength of only 2 mm. A highly efficient optical circula-tor, therefore, can be achieved even at 1300 nm.

This work was supported by the Italian C.N.R. Theauthor also holds an appointment with University ofPalermo, Department of Electrical Engineering.

References1. H. F. Mahlein, "Fiber-Optic Communication in the Wave-

length-Division Multiplex Mode," Fiber Integ. Opt. 4, No.4,339(1983).

2. S. Masuda and T. Iwama, "Single-Mode Fiber-Optic DirectionalCoupler," Appl. Opt. 21, 3484 (1982).

3. M. Digonnet and H. J. Shaw, "Wavelength Multiplexing inSingle-Mode Fiber Couplers," Appl. Opt. 22, 484 (1983).

4. J. D. Beasley, D. R. Moore, and D. W. Stowe, "Evanescent WaveFiber Optic Couplers: Three Methods," Proc. Soc. Photo-Opt.Instrum. Eng. 417, 36 (1983).

5. P. Jaccard, B. Scheja, H. Berthou, F. Cochet, and 0. Parriaux,"A New Technique for Low Cost All-Fiber Device Fabrication,"Proc. Soc. Photo-Opt. Instrum. Eng. 479,. 16 (1984).

6. P. A. Bulteel and J. Tillemans, "New Directional Coupler andConnecting Device for Fiber-Optic Local Area Networks," Proc.Soc. Photo-Opt. Instrum. Eng. 479, 76 (1984).

7. R. B. Dyott, V. A. Harderek, and J. Bello, "Polarization HoldingDirectional Couplers Using D Fiber," Proc. Soc. Photo-Opt.Instrum. Eng. 479, 23 (1984).

8. J. A. Wunderlich and L. G. DeShazer, "Visible Optical IsolatorUsing ZnSe," Appl. Opt. 16, 1584 (1977).

9. M. J. Weber, "Faraday Rotator Materials," Lawrence Liver-more Laboratory (Nov. 1981).

10. R. 0. Miles, A. Ceruzzi, and M. J. Marrone, "Attaching Single-Mode Polarization-Preserving Fiber to Single-Mode Semicon-ductor Lasers," Appl. Opt. 23, 1096 (1984).

11. A. Dandridge, R. 0. Miles, and H. F. Taylor, "Polarization-Resolved Low-Frequency Noise in GaAlAs Lasers," IEEE/OSAJ. Lightwave Technol. LT-4, 1311 (1986).

12. T. Aoyama, T. Hibiya, and Y. Ohta, "New Faraday RotatorUsing a Thick Gd:YIG Film Grown by Liquid-Phase Epitaxyand Its Applications to an Optical Isolator and Optical Switch,"IEEE/OSA J. Lightwave Technol. LT-1, 280 (1983).

13. T. Sugie and M. Saruwatari, "Distributed Feedback Laser Diode(DFB-LD) to Single-Mode Fiber Coupling Module with OpticalIsolator for High Bit Rate Modulation," IEEE/OSA J.Lightwave Technol. LT-4, 236 (1986).

14. A. E. Turner, R. L. Gunshor, and S. Datta, "New Class ofMaterials for Optical Isolators," Appl. Opt. 22, 3152 (1983).

OSA 1987DAVID RICHARDSON MEDAL

John W. Evans

OSA fellow John W. Evans of the National Solar Observatoryis the 1987 winner of the 1987 David Richardson Medal of theOptical Society of America. Established in 1966, the Richard-son medal is presented annually for distinguished contributionsto applied optics. The citation to Evans commends him for "alifetime of constant devotion to the advancement of technicaloptics through an understanding of optical principles and thecreative application of these principles to novel and originaldesigns of optical instrumentation."

Evans became director of Sacramento Peak Observatory,now National Solar Observatory, in Sunspot, NM, in 1952 andwas senior scientist there until 1979. Then he became aconsultant to the observatory. Although his career has beenidentified primarily with solar research, he has also madecontributions to technical optics, working on novel and ad-vanced types of optical instrumentation that have significantlyadvanced ground-based solar physics. He was a pioneer inefforts to measure subtle oscillations in the solar atmospherethat are caused by resonant modes of the sun, and he devel-oped a Fourier tachometer. In addition, Evans had led two

eclipse expeditions to observe the height-resolved chromo-spheric spectrum, the first at Khartoum in 1952, and the second

at Puka Puka in the South Pacific in 1958. For these hedesigned two slitless spectrographs and a jumping-film cam-era. The data from the Khartoum eclipse established that thechromosphere is hotter than the photosphere.

Evans received his B.A. degree in mathematics and physicsfrom Swarthmore College in 1932. He received an M.A. and aPh.D. in astronomy from Harvard in 1936 and 1938, respective-ly. In addition, he received honorary D.Sc. degrees from the

University of New Mexico in 1967 and Swarthmore College in1970.

The Richardson award consists of a medal, a scroll, and a

$1000 honorarium. Evans will receive it at Optics '87 (theOSA annual meeting) to be held this year in Rochester, NY, 19-23 October.

15 August 1987 / Vol. 26, No. 16 / APPLIED OPTICS 3473