1. INTRODUCTIONIntegrated optical devices based on waveguide structureshave a wide range of application in optical communicationsystems1 and in chemical and biochemical sensingsystems.2,3 As the development of integrated optics ad-vances, it is of major importance to have the appropriatecharacterization tools so that the stages of design, fabri-cation, and testing will be completed and feedback be-tween them will be possible. In the case of waveguide de-vices, it is important to characterize the influence of thetopographic features of the waveguide (such as surfaceroughness and rib height) on the propagation of the light.This information will improve the design so as to avoidscattering losses, to obtain monomode behavior, and toachieve maximum sensitivity and coupling efficiency.
One of the most powerful tools for this kind of charac-terization is the scanning near-field optical microscope(SNOM), as has been recently demonstrated.4–6 SNOMallows the performance of nondestructive local measure-ments of the topography of integrated waveguide simul-taneously with the acquisition of the evanescent field.This field is obtained by scanning a sharp optical fiber afew nanometers above the waveguide. The tip–surfacedistance is regulated by a feedback system based on de-tecting the shear force. SNOM images of the evanescentfield show the main characteristics of the modal propaga-tion of the light inside the waveguide as well as the mea-surement of the effective refractive index.4 More specifi-cally, by observing the evanescent tail of the light outside
the core, it is possible to measure the decay length andthe intensity of the light at the surface of the waveguide,which are important parameters for evaluating the sensi-tivity of sensor devices.
In this paper we present the results of applying theSNOM technique to various antiresonant reflecting opti-cal waveguide (ARROW) devices, measuring different to-pographic and optical parameters in situ ARROW’s7,8 areamong the most interesting choices. ARROW structuresare fabricated with standard silicon technology (CMOScompatible), the cladding of the waveguide is relativelysmall (typically half of the core), and the diameter of thecore is similar to that of single-mode optical fibers. TheARROW structure presents a quasi-single-mode behaviorwith large core size, simplifying the fiber–waveguide lightcoupling.
2. FABRICATIONARROW’s were first described in 1986 by Duguay et al.8
In Fig. 1 the structure of our ARROW’s is presented. Thecore and the second cladding are silicon oxide (SiO2) lay-ers with a refractive index of 1.46, where the core is 4 mmthick and the second cladding is half of the core (2 mm).The first cladding is a 120-nm layer of silicon nitride(Si3N4) with a refractive index of 2.00. The optical con-finement of light in these waveguides is based on the totalinternal reflection at the air–core interface and a veryhigh reflectivity (99.9% at l 5 633 nm) at the two inter-
2000 Optical Society of America
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ference claddings beneath the core. This structure doesnot support guided modes; rather, it is a kind of leakywaveguide with an effective single-mode behavior be-cause of the high losses in the high-order modes and thevery low losses in the fundamental mode. Figure 2 de-picts a calculation of the mode attenuation as a functionof the Si3N4 thickness for l 5 633 nm. To perform thecalculations a one-dimensional finite-difference method(FDM) was implemented.9 The fundamental mode TE0has a very low attenuation when the proper dimension ofthe layer is chosen. The TM modes are not shown in thisfigure, but they present higher losses than TE modes, andtherefore ARROW’s are sensitive to polarization.
The fabrication process involves the following steps:First, a SiO2 layer, which corresponds to the second clad-ding, is thermally grown over the silicon substrate. Thefirst cladding, the Si3N4 layer, is deposited by low-pressure chemical vapor deposition. Finally, the core isdeposited by plasma-enhanced chemical vapor deposition,which allows growing of SiOx with a refractive index vary-ing from 1.46 to 1.72. A more detailed description of thetechnology of growing process can be obtained in Ref. 10.To confine the light in the lateral direction, a reactive ionetching is applied to the SiO2 core, defining the finalstructure of the designed device. According to Ref. 10,the rib is set higher than 0.8 mm and lower than 3.5 mm toavoid high losses due to insertion or to scattering, respec-tively; thus we have chosen 2.5 mm for the rib waveguidesand the Y junction, and 1 mm for the directional coupler.
Fig. 1. Geometry and refractive index structure of the ARROW.The width, W, and the height, H, of the waveguide are varied de-pending on the device.
Fig. 2. Calculation of the attenuation versus thickness of thefirst cladding (Si3N4 layer) of the ARROW for a wavelength l5 633 nm. Only the fundamental mode TE0 has a very low at-tenuation that achieves a virtual monomode propagation.
Along with the fabrication facilities due to the use ofstandard integrated-circuit technology process, the AR-ROW waveguides present a monomode behavior with acore thickness the same as that of the core of a single-mode optical fiber; thus an efficient coupling end fire isperformed, solving the problem of the high insertionlosses in optical-fiber–guide interconnections.
3. EXPERIMENTAL RESULTS ANDDISCUSSIONThe structures described above were characterized withuse of the home-made SNOM described in Ref. 4. Thedesigned structures were optimized for a light wavelengthof 633 nm. We have divided the study into three sectionsaccording to the device investigated: rib waveguides, a Yjunction of a Mach–Zehnder interferometer, and a direc-tional optical integrated coupler.
A. Rib WaveguideA precise characterization of simple straight waveguidesis the first step toward understanding more-complexstructures. Figure 3(A) shows the geometry of a ribwaveguide and a profile of the structure. The width ofthe waveguide is 5 mm and the height is 2.5 mm. In Fig.3(B) a zoom at the top of the waveguide shows the surfaceroughness, with a rms value of the surface of 15 nm (lessthan 0.4% of the size of the core); thus one of the mainsources of attenuation is greatly reduced. Anothersource of scattering losses is the sidewall roughness andangle. With the SNOM we evaluated the sidewall angle,giving in the ARROW a measured value of 60° [Fig. 3(A)],similar to scanning electron microscopy measurements.Because fiber tips with a conus angle better than 30° canbe easily fabricated, the common convolution effect be-tween the tip and the sample can be neglected.
For the operation of interferometers and couplers, it isimportant to have both vertical and lateral single-modepropagation. Figure 4(A) shows the light propagation ofthe structure presented in Fig. 3. As can be clearly seen,for this 5-mm-wide waveguide there are two peaks of thelight inside the waveguide [see the profile in Fig. 4(B)],which means that the propagation corresponds to at leasta first-order mode. One can also observe a high localscattering on the right side of the waveguide at the top ofthe image, which is probably due to damage of the lateralwall of the waveguide.
In Fig. 5 the optical propagation corresponds to a 4-mm-wide waveguide. In this case the propagation is mono-mode, although a second peak of the light is seen on theleft side of the rib of the waveguide. This peak is also ob-served in Fig. 4 (see profile), and it corresponds to a con-volution between the topography and the evanescent fieldof the lateral mode, which is the responsible for the cou-pling in optical couplers. Poweleit et al.11 viewed this ef-fect for the first time in 1996, showing single lateral scanssuch as the one represented in Fig. 5(B).
To understand the formation of this peak properly, weshould consider Fig. 6. In Fig. 6(A) is shown a two-dimensional FDM simulation of the optical intensity pro-file of the ARROW described in Fig. 1, with a rib of 2.5 mmand a width of 4 mm, like the experimental one of Fig. 5.
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In Fig. 6(B) a zoom on the rib region is shown. The heavydotted curve corresponds to one topographic scan of thetip following the sample at a constant distance. Then,when the tip approaches the waveguide laterally, the op-tical intensity detected by the tip increases. When thetip climbs the rib of the waveguide, the intensity profilelines decreases, which results in an apparent vanishing ofthe light. Finally, at the top of the waveguide the tip fol-lows the mode profile, increasing the intensity again.Hence the resulting optical image is a convolution of theoptical intensity profile and the topography of the wave-guide.
However, this effect is observed only on one side of thewaveguide, owing to a feedback effect: The tip climbs therib of the waveguide faster than it descends from the top,because of the proportional-integrator-derivative control-ler; thus the descending path is a convolution betweenthis effect and the topography. Then, because the imagetakes into account only one scanning direction (from leftto right), we see only one side of the lateral evanescent
Fig. 3. (A) 28-mm 3 88-mm topographic image of an ARROWand a profile showing waveguide dimensions. (B) 3.7-mm3 3.7-mm zoom over the top of the waveguide.
field: If we look at the acquired image in the reverse di-rection, the same effect takes place on the other side ofthe waveguide. One way to avoid this effect is to scan atlower speed, which means a larger acquisition time forthe whole image; thus drift and vibration instabilitiesprevent us from accurately performing several imagesthat follow the device structure, and some drift effects caneven affect one single image.
Finally we note that the lateral evanescent field is de-tected 2 mm from the waveguide, which means that thelateral evanescent field goes several micrometers into theupper cladding (the air). This agrees with the theoreticalcalculations, which give a decay length of 450 nm for thelateral evanescent field, and thus it extends up to severalmicrometers.
B. Y Junction of a Mach–Zehnder InterferometerA Y junction is a splitting point of a waveguide where itseparates into two different branches. Very careful de-sign and fabrication must be performed to preserve thepropagation conditions. This Y junction can be used inmany configurations. When it is used in a cascade con-
Fig. 4. (A) 18-mm 3 45-mm optical image corresponding to thewaveguide of Fig. 3, showing an optical beating along the wave-guide. (B) Topographic and optical profiles, clearly showing theintensity corresponding to the mode beating.
Fig. 5. (A) 17-mm 3 40-mm optical image of a 4-mm-wideARROW. (B) Profile showing single-mode behavior. The lat-eral peak remains out of the waveguide and corresponds to thelateral evanescent field.
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Fig. 6. Two-dimensional FDM simulation of the optical intensity of the fundamental mode of the ARROW superimposed on the topog-raphy of the waveguide. The structure corresponds to Fig. 1 with a rib of 2.5 mm and a width of 4 mm. The resulting effective refractiveindex is neff 5 1.456. (B) Magnification of the rib zone of the waveguide showing the real profile made by the fiber tip (heavy dottedcurve).
figuration, an optical waveguide beam splitter isobtained.12 The Y junction can also be used in a Mach–Zehnder interferometer. The Mach–Zehnder interferom-eter offers a highly sensitive interferometric scheme todetect the variation of the light path between its twobranches. In communications, this variation can be pro-duced by a phase shifter to obtain wavelength division orfrequency division.12 On the other hand, changes due tochemical reactions at the waveguide surface in onebranch of the interferometer can be measured when it isused as a sensor.13
Figure 7(A) shows a 300-mm-long topographic image ofthe splitting point of the Mach–Zehnder interferometer.Figure 8 shows an optical image of the propagation along400 mm of the structure of Fig. 7. Several optical profilesshowing the intensity distribution across the structureare also depicted in Fig. 8. One can observe how thelight injected from the top is divided at each branch of theY junction. From the topographic profiles [Fig. 7(B)], themeasured waveguides are 2.5 mm high. Each branch ofthe interferometer is 7 mm wide. Far enough before thesplitting point (not shown in this image), the waveguide is7 mm wide, but owing to the technological process, thewaveguide becomes broader while approaching the split-ting point: In the upper profile of Fig. 7(B) the wave-guide is already 12.5 mm wide. This effect produces op-tical interferences, as can be observed in Fig. 8. Thesplitting measured angle is 1.5°. Observing the highscattering at this point, it can be stated that the angleshould be reduced to avoid scattering losses. In fact, tosustain the light level during imaging, the photomulti-plier gain for amplifying the light coming from the opticalfiber tip was increased after the splitting point.
In determining the sensibility of the interferometer, itis important to characterize the decay length of the eva-
Fig. 7. (A) 300-mm-long topographic image of a Y junction. Themeasured splitting angle is 1.5°. (B) Profiles showing the di-mensions of the structure before (upper profile) and after (lowerprofile) the splitting point.
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nescent field. In Figure 9 the intensity of the evanescentfield at the top of three different points of the waveguideare depicted: one before the splitting point and the oth-ers at each branch of the junction. The technique used toperform the curves is as follows: The tip is retractedfrom the surface of the waveguide controlling the tip–sample distance; meanwhile, the optical intensity isacquired.4 The three curves are parallel in a log plot,which indicates an exponential decay that is due to theevanescent field with the same decay length: 42.5 nm.
C. Optical Integrated CouplersOptical directional couplers are widely used in communi-cation systems. Two waveguides are placed near eachother in such a way that the lateral evanescent field pen-etrates from one waveguide into the other. Initially thelight is coupled to one waveguide, and at some predeter-mined distance the light is totally confined to the otherwaveguide. This distance is called the coupling length(LC) and depends on the structure, on the geometry of thecoupler, and on the wavelength.
Fig. 8. Optical image of the Y junction measured in Fig. 7.Lateral dimensions are the same as in Fig. 7, and the vertical di-mension is 400 mm. The lateral profiles show the optical inter-ference pattern produced along the waveguide.
Fig. 9. Plot of the evanescent field. The log plot reveals the ex-ponential nature that is due to the evanescent field. The threecurves correspond to a point before the splitting and at eachbranch of the Y junction after the splitting. The decay lengthmeasured is 42.5 nm.
Fig. 10. Topographic and optical images of a directional coupler.(A) Before the coupling zone: Dimensions are 48.4 mm3 88.4 mm. (B) In the coupling region without apparent cou-pling: Dimensions are 30.4 mm 3 297.6 mm for the topographicimage and 22.7 mm 3 146.5 mm for the optical image. (C) 400mm farther from the previous image in the coupling region. Inthis case a partial coupling can be seen. Dimensions are38.3 mm 3 297.6 mm for the topographic image and 30.4 mm3 297.6 mm for the optical image. (D) Topographic and opticalprofile of Fig. 10(C).
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Figure 10 displays topographic and optical images atdifferent places along an optical coupler. Figure 10(A)corresponds to a zone before the coupling region. Eachbranch of the coupler is 5 mm wide. The distance be-tween the waveguides is 18.5 mm (much larger than theevanescent field calculated in Subsection 3.A), and thusthe light is confined in one waveguide, as can be seen inthe optical image.
The measured rib height is 1 mm. The theoretical cou-pling length (LC) for this structure is 2.1 mm. It can alsobe noted that in this case the lateral evanescent field isvery intense. This is because when the rib is madesmaller, the lateral refractive-index contrast diminishes,and therefore there is less lateral confinement of thelight. In this case the theoretical value is Dneff5 0.0015, which is 4.4 times smaller than for thewaveguides with a 2.5-mm rib. The calculated decaylength of the lateral mode is 770 nm, almost two timeslarger than for the 2.5-mm rib guide, which explains thelarger evanescent field in the images, and thus a more ef-ficient coupling is obtained.
When the waveguides are closer together, the light con-finement oscillates from one waveguide to the other. Theimages in Figs. 10(B) and 10(C) correspond to the cou-pling region of the directional coupler, with a distance of400 mm between the two series of images. The distancebetween the waveguides is 2.5 mm. In the first case [Fig.10(B)], although there is a large lateral evanescent field,there is no apparent coupling, while in the second case[Fig. 10(C)] there is a partial coupling of the light into thesecond waveguide, as can be seen in the profile in Fig.10(D). To perform an experimental calculation of thecoupling length, a larger measurement is necessary to en-sure a higher coupling between the two branches of thecoupler.
4. CONCLUSIONSIn this paper we have presented the results of the char-acterization of different ARROW devices by means of aSNOM. With this technique we have performed a high-resolution on-line characterization of optical integrateddevices. First we studied rib waveguides, distinguishingmonomode from multimode behavior as well as the lateralevanescent field of the waveguide. We also made topo-graphic measurements for a better understanding of theoptical images. A Y junction of a Mach–Zehnder inter-ferometer was also studied, showing high losses at thesplitting point, with a measured angle of 1.5°. The eva-nescent decay length measured in this structure is 42.5nm, which is useful information when the device is to beused as a sensor. Finally, a partial optical coupling in a
directional coupler device was imaged, and a topographiccharacterization of the structure was given.
ACKNOWLEDGMENTThis work has been supported by Comision Interministe-rial de Ciencia y Tecnologıa project TIC98-0499 of theSpanish government.
*Corresponding author Xavier Borrise can bereached at the address on the title page or byphone, 34-93-581-3218; fax, 34-93-581-1350; ore-mail, [email protected].
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