On-chip integrated optic mirrors in T i : LiNbO3 byion-beam micromachining

R.R.A. SymsR.E.J. Watkins

Indexing terms: Optoelectronics, Integrated optics, Optical waveguides

Abstract: A complete process is describedwhereby on-chip mirrors may be fabricated inTi: LiNbO3 stripe waveguide devices using micro-focal ion-beam machining. Experimental resultsare presented for a device operating at 1.523 /miwavelength, TM mode. An interferometric tech-nique is used to distinguish the reflection from theon-chip mirror from other spurious reflections.Approximately 24% power reflectivity is observed.Results are compared with a simple theoreticalmodel of the chip response under different condi-tions, and good agreement is obtained.

1 Introduction

Mirrors have an important role to play in integratedoptics, as they may act as broadband reflectors or path-folding elements. Already, devices have been fabricatedusing metallised chip-edge reflectors [1-3] or bulk reflec-tors [4], and further devices have been proposed [5, 6].Their true potential will only be realised, however, whenthey may be fabricated at arbitrary positions on chip.Such mirrors have been made in semiconductor materialsfor laser applications (e.g. Reference 7) or for use withstripe waveguide devices [8]; little success has so far beenachieved for diffused waveguides in Ti: LiNbO3.

In principle, a totally reflecting mirror may be con-structed for a diffused waveguide by cutting a trenchorthogonal to the guide into the chip surface. The trenchmay then be metallised to improve reflectivity. It must beof sufficient depth to bisect the guide, and of sufficientwidth for metallisation by, for example, sputter coating.Trench orientation and surface quality are highly impor-tant. Suitable fabrication procedures include ion-beammicromachining [9, 10] and reactive ion etching [11, 12].Both of these require a suitable mask material to bedeposited prior to machining, which will allow sufficientdepth to be reached. An alternative procedure is focusedion-beam micromachining [13, 14], in which a focusedbeam of ions from a liquid metal ion source is used. Nomask is required because the beam may be deflected elec-trostatically to machine features of any desired shape.

Paper 561U (E13/E3), first received 19th January and in revised form21st April 1987R.R.A. Syms holds a Rutherford Atlas Research Fellowship, workingjointly with the EBLF Group, Rutherford Appleton Laboratory,Chilton, Didcot, Oxon, United Kingdom and the Holography Group,Department of Engineering Science, University of Oxford, Parks Road,Oxford OX1 3PJ, United Kingdom. R.E.J. Watkins was formerly withthe Department of Engineering Science and is now with the Depart-ment of Atmospheric Physics, University of Oxford, Parks Road,Oxford OX1 3PJ, United Kingdom

IEE PROCEEDINGS, Vol. 134, Pt. J, No. 5, OCTOBER 1987

We have already presented photographs of typicalstructures that may be made for Ti: LiNbO3 stripewaveguide devices by this method [15]. More recently,similar techniques have been used for laser mirrors [16].The purpose of this paper is to describe our fabricationprocedure and to present experimental measurements ofdevice performance. Approximately 24% power reflec-tivity was observed from a single-waveguide mirror on achip containing several other waveguide components;good agreement is demonstrated between the overall chipresponse under various conditions and a simple theoreti-cal model.

2 Fabrication of Ti : LiNbO3 integrated optic chipscontaining mirrors

Our fabrication procedure follows closely that used pre-viously by many authors for standard Ti: LiNbO3devices, the only difference being the cutting and metal-lisation of the mirrors. First, a waveguide and an elec-trode mask are designed. The latter contains, in additionto the electrodes, small rectangular pads, approximately50 x 20 /mi. These are placed at any potential mirrorsite. This mask allows fabrication of the electrodes andmirror metallisation simultaneously. Both positive andnegative versions are required; the former is used as avisual alignment aid during machining. Secondly, thedevice is part fabricated. Typically, for TM operation at1.523 /im wavelength, 80 nm of Ti in 7 /im wide strips isdiffused for 8.5 hours at 1050°C into Z-cut, Y-propagating LiNbO3 to form the waveguides, and thechip ends are cut and polished. Thirdly, the device iscoated with 100 nm of Al, which is patterned with thepositive electrode mask. This leaves the majority of thesurface metallised, with a high-contrast pattern on it.Mirrors are then fabricated by cutting deep trenches intothe LiNbO3 across the guides, inside the rectangularboxes. Fig. 1 shows a schematic diagram of theequipment used [17]; the sample may be translated intwo directions and also tilted. During machining, thebeam is deflected under computer control. Owing to themetallisation, charging effects are minimised and reflectorsites are easily located using ion or secondary electronimaging. This form of visible alignment would not berequired with more sophisticated positioning and control.Typically, the trenches are 40 x 5 /im machined to adepth of greater than 10 /im. Using In and Ga liquidmetal ion sources [18] with an acceleration voltage of8.5 kV and a beam current of 1 nA, machining takesapproximately 1.5 h. Fig. 2 shows scanning electronmicroscope (SEM) photograph of such a cut, madeacross a polished face to show wall profile and quality.The beam size is approximately 1 /an, which has caused

269

edge rounding and sloping side walls, but overall qualityis high. Owing to the raster nature of the beam scan, oneface (the last visited) is of higher quality. The sample may

deflection plates PS • • • •

source/lens PS

patterned with the negative electrode mask, to remove allAl not required for electrodes or metallisation of mirrors.Any unmachined mirror sites simply remain as metal

view port

liltinghandles

RGA/5IMScontrol unit

.demountable columnand ion source

charged-particle detector

SIMS energyfilter

videoamplifier

-sample stage

Fig. 1diffusion pump

Schematic diagram of equipment used for focused ion-beam micromachining

Fig. 2 SEM photograph of a cut across a polished edge

be tilted to ensure this face is orthogonal to the chipsurface: typical tilt angle was 9°. Fig. 3 shows a mirrorcut across a waveguide inside an alignment box, just aftermachining. Some debris may be seen in a ring-likepattern. Fig. 4 shows a similar cut across the centre of adirectional coupler, fabricated without the aid of analignment box. Both these Figures illustrate the depthand quality that may be achieved. Fourthly, the Al layeris removed, together with any sputtered debris. Thedevice is coated with a buffer layer of 200 nm of SiO2and then sputter coated with 100 nm of Al. In thisprocess, the chip edges are masked off. The device is then

pads on the surface. As the mirror features are deep, athick resist such as Shipley AZ1450J is required for thisstep.

Machining of features is simple and repeatable. Majorproblems are the relatively large beam size and themachining time. The tradeoff between beam size andmachining rate is not clear at present.

3 Interferometric measurement of on-chip mirrorreflectivity

Before presenting experimental results, we consider inthis Section the general problem of the measurement ofon-chip mirror reflectivity with a simple hypotheticalchip geometry. Simple analysis shows that an interfero-metric technique is required to measure the reflectivity,and also that a relatively sophisticated chip is needed todetermine other unknown parameters.

Figs. 5a and b show top and side views of a chip con-taining a waveguide, phase-shifter electrodes and anon-chip mirror M. Light is launched into the waveguideby end-fire coupling using a lens L, and the reflectedpower is measured using a beam splitter BS and a detec-tor D. Knowing the beam-splitter reflectivity, the powermeasured at D when light is launched into the guidemight be an indication of the mirror reflectivity. However(leaving aside the problem of actually obtaining alaunch), we cannot determine the launch efficiency or thewaveguide propagation loss. In addition, the input face ofthe chip will not in general be antireflection coated(though this has, in fact, recently been reported [19]), andso two components will be measured; one correspondingto reflection from the chip input and the other corre-sponding to reflection from M. (At this point, we ignore

270 1EE PROCEEDINGS, Vol. 134, Pt. J, No. 5, OCTOBER 1987

multiple-pass components because of losses). Reflectivityat the chip input will not necessarily correspond to thatof an air | LiNbO3 interface, because the chip input face is

Fig. 3 SEM photograph of a trench cut across a waveguide, inside analignment box

Fig. 4 SEM photograph of a trench cut across the centre of a direc-tional coupler

BS

fromlaser

A Vm

M

-H-

Fig. 5 Hypothetical chip geometry for interferometrxc testing ofon-chip mirror reflectivitya Top viewb Side view

finite in the Z-direction and part of the launch beam willmiss it. Finally, with a laser of sufficient coherence length,the power measured at D will depend on the relativephases of the two components. Using the phase shifter,however, we may determine the maximum and minimumdetected power, and hence at least determine their rela-tive magnitudes. Unfortunately, even this interferenceprocess is complicated, because the clipped beam reflec-ted from the chip input and the emergent reflected guidedbeam do not have the same shape, and so do not fullyoverlap.

First we consider the launch. End-fire coupling is amode-matching problem, which has been treated manytimes (e.g. References 20 and 21). We assume the field atthe focal point of the lens may be adequately representedas a scalar field £,(x, z) travelling in the 7-direction.Similarly, the guided mode is taken as a scalar field £g(x,z). Throughout, we consider fields to be real for simpli-

city, normalised so that

ITJ— oo J — i

E2(x, z) dxdz=l (1)

We take the input field amplitude to be Eo so thatEt{x, z) = Eo £,(x, z). We further assume that the guidedfield Eg decays sharply above the substrate so that £fl — 0for z > 0; this is a reasonable approximation owing tothe large refractive-index change at the substrate/airinterface. In this case, the guided field launched isapproximately

EJx, z) = EotJtEg{x,z) (2)

Here tt is the transmission coefficient at an air/LiNbO3interface, taking ne = 2.2 for the^ LiNbO3, and /, is theoverlap integral between Et and Eg:

£, Eg dx dz (3)

At this point, and subsequently, we ignore the effect ofradiation modes, as their contribution at the detectorswill be small. The guided component will propagate tothe mirror, where it will be reflected. Assuming a propa-gation constant of /? — jcc (complex, to allow for loss),that the chip mirror is distance / from the input, that thetransmission coefficient at a LiNbO3/air interface is t2,and that the mirror reflectivity is | M | , the guided fieldemerging from the chip is then

Erl(x,z) = exp {- -ja)}Eg(x,z) (4)

Calculation of the mirror reflectivity \M\ is again amode-matching problem. We might assume that | M \ ~ 1for a 'perfect' mirror, consisting of a planar surface ofunity reflectivity, excactly orthogonal to the guide. Again,this is because the field Eg decays sharply above the sub-strate so that only a very small portion misses the mirror.If the mirror is not orthogonal, the problem is similar tothat of a sharp directional change in a dielectric wave-guide [22, 23] through an angle twice that of the mirrortilt. For micromachined mirrors, tilt about the Z-axis(Fig. 5) is relatively easy to eliminate by accurate align-ment. Tilt about the X-axis is much more of a problem,and our approach has been to determine tilt angles byexamination of profiles such as Fig. 2. More generally,the mirror will be tilted, nonplanar and of nonunityreflectivity. Ignoring scattering losses due to surface

IEE PROCEEDINGS, Vol. 134, Pt. J, No. 5, OCTOBER 1987 271

roughness, | M | may be found as

°° f Eg(x,z)— oo J— oo

x {/?(x, z) exp (-j<p{x, z))Eg(x, z)} dx dz (5)

Here we have described the mirror locally as the productof two functions: (i) a real function R(x, z) defining localreflectivity, and (ii) a complex phase functionexp (—j(f)(x, z)) defining the local phase mismatch of thereflected wave; for a planar, tilted mirror, (/>(*, z) wouldbe linear in X and Z. Unfortunately, we have no methodat present of determining R and (f> experimentally, and soour investigation is confined to the measurement of | M | :we also ignore any phase change at M for similarreasons. Eqn. 5 implies that for many mirror defects (forexample tilt) our best strategy would be to confine theguided mode as strongly as possible at the mirror so thatR and <j> approach constant values over the importantrange of the integral.

Secondly, we consider the portion of the launch beamreflected from the input face. No power is reflected forz > 0, and we assume uniform reflectivity for z < 0. Wetherefore define a reflected field Er(x, z) as

Er(x, z) = kEt(x, z)= 0

Where

k-i\uy

for z < 0for z > 0

dxdz

(6)

(7)

We take the reflection coefficient for z < 0 as rl9 for anair/LiNbO3 interface. The directly reflected component isthen

Here/r represents the overlap between E{ and Er:Too [*oo

fr=\ ElETdxdz=\/k (9)J - oo J - oo

Thirdly, we consider the power measured by the detectorD, which performs a spatial integration of the localpower. If we correct for the beam-splitter reflectivity andother losses, this figure corresponds to the reflectedpower at the chip input. At this point, the ratio of reflec-ted power to incident power is given by

(10)ir=ik r r < £ M + E ' & E « + E ' F dx dz

* in '-'O J —oo J—oo

Performing the integration, we obtain

^r = {rlfi + tltlff\M\2 exp (-4a/)}+ 2tlt2rlff\M\ exp (-2a/) cos (20/) (11)

Here, we have assumed that, as the guided field decaysquickly above the chip surface, the overlap between theguided and reflected fields may be approximated as

Too C oo

J— oo J — oodx dz ~ kft (12)

Eqn. 11 shows that the reflected power has three ele-ments: (i) A DC term corresponding to the averagepower reflected at the chip input. This is lower than r\because the chip does not extend beyond z = 0. (ii) A DC

term corresponding to the average power reflected fromM. This is lowered by incomplete launch and by sub-sequent propagation loss. If losses are appreciable, thisterm may be neglected as small, (iii) An AC term, corre-sponding to interference between the two components.This last term is lower than might be expected by anadditional factor fv accounting for incomplete overlapbetween the fields Eg and ET at the detector.

More complicated analysis would account formultiple-pass interference effects, but comparison withexperimental results shows this order of analysis to bevalid. Using the phase shifter, we may vary /?/, and thevisibility of the resulting fringes (ignoring the small DCcomponent (ii) above) is found as

Pr(max) - Pr(min) It y t2 ff \ M \ exp ( - 2aQ

2Pr(av) " 7J* (13)

Eqn. 13 shows that fringe visibility is linearly dependenton \M\. By a simple interferometric process we maytherefore in principle determine the reflectivity of on-chipmirrors. In the following Section, we discuss the chipgeometry required to find the other unknown factors inthe equation, and present experimental results.

4 Experimental results

Fig. 6 shows the optical circuit used. The chip containsone stepped-A/? directional coupler [24], two phase

on - chipmirror M2

on-chipmirror Ml

fromlaser

BS•Vc -Vc V m 2

x10 objective •=•

.detector

1detector

I/O device

x10 objective

Ml3

Fig. 6 Geometry of chip actually used for interferometric measurementof on-chip mirror reflectivity

modulators and two on-chip mirrors Ml and M2.Overall chip length is 3.4 cm, and the mirrors are approx-imately 0.5 mm from the chip edge, at either end of theupper arm. The coupler has normalised coupling lengthclose to kd = n/2 [24], and the phase modulators mayapply a phase shift to either the upper or the lower armof the device. Testing was performed using a Spectra-Physics 120S laser at 1.523 fim wavelength. The laser hasa coherence length substantially greater than the chiplength. Light was coupled into and out of the device viamicroscope objective, and detectors measured the trans-mitted and reflected powers. Comparison of the incidentpower with the reflected power measured with a planarbulk optical mirror placed at the launch lens focusshowed that all reflected powers measured should beincreased by a factor of 2.8 to account for the effects ofthe beam splitter, detector differences etc.

The advantages of this more complicated configu-ration are as follows. First, when the switch is in the barstate [24], there is high transmitted output so that launchconditions may be optimised. Secondly, as the chip edgesM3 and M4 reflect light internally, multiple-pass inter-ference fringes may be observed in the transmitted outputwhen the lower phase modulator is driven. These may beused to ascertain the total chip propagation loss and

272 IEE PROCEEDINGS, Vol. 134, Pt. J, No. 5, OCTOBER 1987

hence the launch efficiency/,2. Thirdly, when the lowerphase modulator is driven, there is interference betweenthe back-reflected components from M3 and M4. Thesemay be used to determine the reflection efficiency/2 ofthe launch beam. Fourthly, when the switch is in thecross state [24], there is low transmission, but there isinterference between the back-reflected components fromMl and M4. The reflectivity of the on-chip mirror Mlmay thus be compared with that of M3, and also ascer-tained absolutely.

The second on-chip mirror M2 serves mainly toensure only one reflected output as it acts as a trackbreak. Its contributions to both the transmitted andreflected outputs are neglected, either because theyinvolve higher-order passes, or because they require theswitch transmission and crossover efficiencies to besimultaneously high.

Figs. 7, 8 and 9 show experimental results for a com-pleted device under a variety of conditions. First, launch

reflected

transmitted

time

b

typical phase-modulator voltage (Fig. la) was applied tothe lower and upper phase modulator in turn. Fig. 1bshows results for the former case; here, very little varia-tion is observed in either transmitted or reflected power.

zero coupler volts

transmitted

zero coupler volts

transmitted

Fig. 8 Transmitted and reflected power measured when the coupleronly is driven

a Reflection from on-chip mirror in phase with reflection from chip inputb Reflection from on-chip mirror in antiphase with reflection from chip input

zero coupler volts

transmitted

Pr(av)

timea

reflected

Pr(ov)

timec

Fig. 7 Typical applied signals and measured transmitted and reflectedpowera Typical signals applied to coupler and phase-modulator electrodesb Transmitted and reflected power, measured when the lower phase modulatoronly is drivenc Transmitted and reflected power, measured when the upper phase modulatoronly is driven

conditions were optimised, and an infra-red (IR) videocamera was used to verify that no transmission wasobtained from the upper arm, implying complete bisec-tion of this guide by the micromachining. Secondly,transmitted and reflected powers were measured as the

o.i

zero coupler volts

transmitted

timeb

Fig. 9 Measured transmitted and reflected powera As Fig. 8a, but with the lower phase modulator also drivenb As Fig. 8b, but with the lower phase modulator also driven

Transmission was low, indicating that the coupler wasnearly in the cross state, so the reflected power consistedmainly of components reflected from the chip input M4and the on-chip mirror Ml. Clearly, the relative phases of

IEE PROCEEDINGS, Vol. 134, Pt. J, No. 5, OCTOBER 1987 273

these two components should not be affected by thelower phase modulator. Fig. 1c shows results for thelatter case. Again, little variation was observed in trans-mitted power, but strong oscillations were observed inreflected power. We attribute these oscillations to inter-ference reflections from M4 and Ml.

The phase-modulator voltage was then removed, andthe typical coupler voltage shown in Fig. la was applied,with voltages of alternate sign applied to each section.Fig. 8a shows the results. A strong dip in transmissionoccurs near zero volts, when the coupler is synchronousand thus in the cross state. At this point, a peak isobserved in the reflected output. This is attributed toreflection from the on-chip mirror Ml. The relative phaseof this reflection and that from the chip input are clearlyimportant; in this Figure, they are in phase. The relativephases could be tuned easily by applying a DC bias tothe upper phase modulator, or, as in this example, byvarying the chip temperature slightly using a thermoelec-tric heat pump. Fig. 86 shows similar results, with thereflection from Ml now in antiphase. In regions A andA', there is little variation in reflected power. This mightseem surprising, because the coupler is in the bar statehere, and we would expect interference between reflec-tions from the chip input M4 and output M3. Thisindeed occurs, but the relative phases of the two com-ponents do not change with coupler voltage, and so littlevariation is observed. This is due to a particular propertyof the stepped-A/? coupler, namely that for equal voltagesof alternating sign in the two sections, the transmissionamplitude is real [24],

The phase-modulator voltage was then reapplied,maintaining the coupler voltage. Figs. 9a and b showresults corresponding to those of Figs. 8a and b. In eachcase, there are now additional oscillations in both trans-mitted and reflected outputs. The former are due toFabry-Perot-type interference inside the chip betweenM3 and M4. The latter are due to interference betweenreflected outputs from M3 and M4. Near zero couplervolts, the reflected power is as in Fig. 8 and is again tem-perature tunable.

Results were repeatable, in that mirrors machined atthe same tilt angle gave similar results, though care wasrequired in alignment. A range of tilt angles were tried;mirrors machined at 10° performed considerably worse.We therefore believe there is considerable scope forimprovement in reflectivity simply by more accuratealignment.

5 Comparison with theory

A simple comparison of the size of the oscillations in Fig.1c (due to reflection from the on-chip mirror Ml) and inregions A and A' in Fig. 9 (due to reflections from thechip edge M3) suggests that \M\ is approximately 1.35times r2, the chip edge reflectivity. Assuming r2 = 0.375,we obtain \M\ = 0.487: a power reflectivity of 23.7%.Though a moderate figure, this implies at least partialmetallisation of the mirror, and is a useful demonstrationof future potential.

A simple model of the chip behaviour was made, todetermine the validity of our assumptions, and to checkthe accuracy of the figure found above. For the transmit-ted beam, two components were allowed: one straight-through pass, and one multiple pass via M3 and M4. Forthe reflected beam, three components were included:reflection at the chip input M4, at the chip output M3and at the on-chip mirror Ml. The effects of M2 were

disregarded entirely. Launch and detection of the reflec-ted beams were assumed to be described by the theoryoutlined in Section 3. The coupler operation wasassumed as modelled in Reference 24. Propagation losseswere assumed uniform. Coupler and phase-modulatorvoltages were matched to those used in the experiment.All reflected powers were rescaled by the factor of 2.8mentioned in the preceding Section, to match thoseactually measured.

First, propagation loss was determined from the weakFabry-Perot oscillations in region A of Fig. 9a, asexp ( — 2a/) = 0.193. This implies a propagation loss of2.1 dB/cm, a rather high figure that we ascribe to exces-sive bend loss and to poor quality of the SiO2 bufferlayer. The launch efficiency ff was then found from theaverage transmission with the switch in the bar state, as72%. (There is some variation in this level in Figs. 9a andb, and so an average value was used). The normalisedcoupling length of the switch was found from the size ofthe dip at zero volts, as kd = 1.3. The reflection efficiencyof the launch beam / , was found from the average levelof reflected power in Figs. 1c and 8a, as 84%. The onlyother parameters required were the relative phases of thereflections from Ml and M3; \M\ was taken as 0.487.Fig. 10 shows a simulation of the results of Fig. 8a. Here,

0.1transmitted

time

b

Fig. 10 Theoretical simulation of resultsa Simulation of the results of Fig. 8ob Simulation of the results of Fig. 9a

the reflection from Ml has been taken in phase with thatfrom the chip input, and the reflection from M3 has beentaken as in antiphase. Qualitative agreement is excellent,and though there are some discrepancies in power levels,they are of the same order of magnitude as thoseobserved experimentally, e.g. between Figs. 9a and b. Fig.10b shows a similar theoretical prediction of the results ofFig. 9a, assuming now that the lower phase modulator isdriven as well as the coupler. Agreement here is also verygood. The success of this model encourages belief in the

274 IEE PROCEEDINGS, Vol. 134, Pt. J, No. 5, OCTOBER 1987

validity of our interferometric test and in the figurereached for mirror reflectivity.

6 Discussion

We believe we have demonstrated the application of ion-beam micromachining to fabrication of on-chip mirrorsin LiNbO3 stripe waveguide integrated-optic devices.There is potential for increased reflectivity by more accu-rate angular alignment. Equally, it would be possible toreduce machining times using a heavier or more energeticion species.

As mirror effiency increases, the simple interferometrictest used here will no longer be valid, owing to increasingsignificance of higher-order passes through the device.The best course then would be to antireflection coat thechip end faces and to use a similar design to the chipshown in Fig. 6 in a cavity resonator mode [2]. If theguide propagation loss may be found, the mirror reflec-tivity may be determined from the cavity finesse. Unfor-tunately, we cannot distinguish between the two mirrorsin this test, and so an average value will be found.

If mirror efficiency may indeed be increased, theprocess may have applications beyond the simple fabrica-tion of mirrors. As they may be placed in arbitrary posi-tions, it may be possible to part-fabricate devices withstandard waveguide patterns, which are then 'committed'to a particular optical function by the addition of mirrorsand breaks. This is analogous to the committal of anintegrated electronic uncommitted logic area (ULA) bydirect write. The most obvious application would be inthe configuration of all-optical computer circuits.

7 Acknowledgments

The financial support of the UK Science and EngineeringResearch Council is gratefully acknowledged. Thanks arealso due to Brian French, Tony Pritchard and PhilipJones for assistance with device fabrication.

8 References

1 ALFERNESS, R.C., EISENSTEIN, G., KOROTKY, S.K.,TUCKER, R.S., BUHL, L.L., KAMINOW, IP., BURRUS, C.A.,and VESELKA, J.J.: 'Mode locking a Ti: LiNbO3-InGaAsP/InPcomposite cavity laser with an integrated high speed directionalcoupler switch', Appl. Phys. Lett., 1984, 45, pp. 944-946

2 SYMS, R.R.A.: 'Resonant cavity sensor for integrated optics', IEEEJ. Quantum Electron., 1985, QE-21, pp. 322-328

3 WILSON, M.G.F., and KYPRIOS, G.: 'A single mode stripe wave-guide edge reflector for integrated optics'. Proceedings of 7th Euro-pean Conference on Optical Communications, Copenhagen,Denmark, 8th-llth Sept. 1981

4 VILJANEN, J., and SUHARA, T.: 'Multimode edge reflecting cou-plers', Opt. & Quantum. Electron., 1983,15, pp. 359-362

5 TIETGEN, K.H., and KERSTEN, R.Th.: '180° turns in integratedoptics', Opt. Commun., 1981,36, pp. 281-284

6 SCHLAAK, H.F.: 'Periodic spectral filter with integrated opticaldirectional coupler', Opt. & Quantum. Electron., 1981, 13, pp. 181—186

7 COLDREN, L.A., EBELING, K.J., RENTSCHLER, J.A., andBURRUS, C.A.: 'Continuous operation of monolithic dynamic-single-mode coupled-cavity lasers', Appl. Phys. Lett., 1984, 44,pp. 368-370

8 BUCHMANN, P, KAUFMANN, H., MELCHIOR, H., andGUEBOS, H.: 'Totally reflecting mirrors: fabrication and applica-tion in GaAs rib waveguide devices', in NOLTING, H.-P., andULRICH, R. (Eds.): Proceedings of 3rd European Conference onIntegrated Optics, ECIO'85, Berlin, Germany, 6th-8th May 1985(Springer-Verlag, Berlin, 1985), pp. 135-139

9 GARVIN, H.L., GARMIRE, E, SOMEKH, S., STOLL, H., andYARIV, A.: 'Ion beam micromachining of integrated optics com-ponents' Appl. Opt., 1973,12, pp. 455-459

10 NUTT, A.C.G., BRISTOW, J.P.G., McDONACH, A., and LAY-BOURN, P.J.R.: 'Fiber-to-waveguide coupling using ion-milledgrooves in lithium niobate at 1.3 micron wavelength' Opt. Lett.,1984, 9, pp. 463-465

11 LEE, C.L., and LU, C.L.: 'CF4 plasma etching on LiNbO3\ Appl.Phys. Lett., 1979, 35, pp. 756-757

12 CHUNG, P.S., HORWITZ, CM., and GUO, W.L.: 'Dry etchingcharacteristics of LiNbO3\ Electron. Lett., 1986, 22, (9), pp. 484-485

13 KUBENA, R.L., SELIGER, R.L., and STEVENS, E.H.: 'Highresolution sputtering using a focused ion beam', Thin Solid Films,1982,92, pp. 165-169

14 BROWN, W.L., and WAGNER, A.: 'Finely focussed ion-beams—new tools for technology'. Proceedings of International IonCongress ISIAT '83 and IPAT '83

15 WATKINS, R.E.J., ROCKETT, P., THOMS, S., CLAMPITT, R.C.,and SYMS, R.R.A.: 'Focused ion beam milling'. Low Energy IonBeams 4, 7th-10th April 1986, University of Sussex, UK

16 HARRIOT, L.R., SCOTTI, R.E., CUMMINGS, K.D., andAMBROSE, A.F.: 'Micromachining of integrated optical structures',Appl. Phys. Lett., 1986, 48, pp. 1704-1706

17 WATKINS, R.E.J, THOMS, S., and ROCKETT, P.: 'A low energyion microprobe facility for maskless machining trials'. Oxford Uni-versity Engineering Laboratory Report OUEL 1630/86

18 Oxford Applied Research, Model PR 10019 EISENSTEIN, G., KOROTKY, S.K., STULZ, L.W., VESELKA,

J.J., JOPSON, R.M., and HALL, K.L.: 'Antireflection coatings onlithium niobate waveguide devices using electron beam evaporatedyttrium oxide', Electron. Lett., 1985, 21, (9), pp. 363-364

20 BURNS, W.K., and HOCKER, G.B.: 'End fire coupling betweenoptical fibres and diffused channel waveguides', Appl. Opt., 1977, 16,pp. 2048-2050

21 HUNSPERGER, R.G.: 'Integrated optics' (Springer-Verlag, Berlin,1984) Chap. 6

22 TAYLOR, H.F.: 'Power loss at directional change in dielectricwaveguides', Appl. Opt., 1974; 13, pp. 642-647

23 HUTCHESON, L.D., WHITE, I.A., and BURKE, J.J.: 'Comparisonof bending losses in integrated optical circuits', Opt. Lett., 1980, 5,pp. 276-278

24 KOGELNIK, H., and SCHMIDT, R.V.: 'Switched directional cou-plers with alternating A/3', IEEE J. Quantum., 1976, QE-12, pp. 396-401

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