Ultracompact optical filter based on a stubresonator in GaInAsP/InP optical wire technology
Sophie Maricot,1,* Marie Lesecq,1 Jean-Pierre Vilcot,1 Christiane Legrand,1
Marc Francois,1 and Maxime Beaugeois2
1Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN), UMR 8520, Universitédes Sciences et Technologies de Lille, Avenue Poincaré, BP60069, 59652, Villeneuve d’Ascq cedex, France
2Laboratoire de Physique des Lasers, Atomes et Molécules, UMR 8523, Université des Scienceset Technologies de Lille, 59655 Villeneuve d’Ascq cedex, France
Microresonators are versatile elements for integratedoptics; they can be used to create filters, add/dropmultiplexers/demultiplexers, and all other kinds ofwavelength selective devices. Optical wire technology[1] based on III-V technology allows the fabrication ofvery compact optical structures. For several years,InP- and GaAs-based microresonators have beendemonstrated by numerous groups [2–4]. Thanks tothe high light confinement, filtering with high qualityfactor was obtained for ring or disk diameter smallerthan 15 �m. Recently, we reported on the optical andelectrical tunability on such InP-based microdiskresonators [5,6].
In this Letter, we report on the fabrication andcharacterization of a kind of resonator in which the-oretical concept has been previously reported [7,8].This concept is based on the reproduction in the op-tical domain of the stub resonator, which is used inthe electrical (microwave) domain.
Our basic waveguiding structure is a narrow deep-etched waveguide or optical wire. Its fabrication pro-cess as well as characterization has been fully devel-oped in [1]. The epilayer structure, which is grown onan InP substrate, is composed of a 0.3-�m-thick In-GaAsP quaternary material guiding layer (cutoffwavelength: 1.3 �m) surrounded by two 1.2-�m-thickcladding layers. The fabrication process [1] can beroughly summarized by an e-beam lithography stepthat is used to define the etching mask (using a one-step process based on hydrogen sielses quioxane re-sist), which is followed by an inductively coupledplasma dry-etching step. In the vertical direction(perpendicular to the epilayer planes), confinement ismainly due to the difference of material refractive in-dex between GaInAsP and InP (around 0.24), whichleads to an almost low confinement value. In the hori-zontal direction (in the epilayer planes), a very highconfinement is obtained owing to the difference(around 2.3) between the effective index of waveguid-ing and air. The width of waveguides is 1 �m; theirdepth is close to 2.7 �m.
The stub resonator takes the form of a rectangular
cavity disposed on each side of a straight waveguide;
its width is the same as the one of the straight wave-guide (i.e., 1 �m), and its length is calculated de-pending on the desired resonance wavelength [7,8]. Itwas designed in order to get the resonance at1550 nm, and its length is so close to 1.6 �m.
The fabrication process is the same as the one thatwas previously reported for optical wires [1]. A scan-ning electron microscope picture is presented inFig. 1.
The theoretical approach [7,8] showed that the useof an as-etched stub, like the one presented in Fig. 1,leads to very poor resonance efficiency. Effectively, ifwe stick on the comparison with the microwave do-main, the latter uses either pure short or open circuitto realize the resonator. The as-etched optical stub isthen a very pale copy of this, since the reflectivity co-efficient at its end is around 0.3, which is far awayfrom the optimum value, i.e. the closer to 1 it is, thehigher the finesse of the resonator is. This theoreticalapproach also demonstrated that a metal sidewallcovering on the stub allows reaching a much higherfinesse value. Following the statement just men-
Fig. 1. Scanning electron microscope view of a stub reso-nator. The waveguide width and depth are 1 �m and2.7 �m, respectively. The stub length is 1.6 �m. Figure
tioned above, this is easily understandable. We havealso developed a technological process to define local-ized metallic patterns on the stubs. We shall depositmetallization over the sidewalls of the stubs in orderto improve their behavior. Practically, this is almostdifficult to obtain on such tiny devices, and metal isdeposited all over the stubs, i.e. also on the top ofthese. This shall have no or little influence on stubbehavior, since the optical mode profile does notreach the top surface of the waveguide [1]. Else-where, sidewall metal produces much higher absorp-tion on the light propagation within the waveguide.
First, a 300-nm-thick gold film is deposited all overthe wafer using a sputtering technique. The picturein Fig. 2 shows a cross section, made using focus ionbeam etching, of the metalized waveguide. It con-firms the isotropic metal deposition, even if the thick-ness is different depending on the sidewalls of thewaveguide. Resist patterns are then defined to coveronly the stubs. Because of the size of these patternsand the important aspect ratio of the structures, wedeveloped a specific e-beam lithography process us-ing AZ nLof 2070 resist. A thick �5.6 �m� film of thisnegative tone resist is deposited all over the wafer,and an e-beam lithography is used to define the zonesthat shall not be covered by metal, i.e. almost all thesurface area of the wafer except the stubs. They arethen cleared using a wet chemical etching based on aKI solution. The remaining resist film is finally re-moved in NANO REMOVER PG solvent stripper.Figure 3 shows a scanning electron microscope viewof a finished structure. We can note that metallic ar-eas are almost well localized on the stubs only. How-ever, we observe metallic deposition on the substratearound the stubs, but this should not have significanteffect, since no optical propagation is present in thispart.
The measurement chain is almost simple; it con-sists in an optical source, a micropositioning testbench allowing fiber couplings to the device, and an
Fig. 2. Cross section of a metalized waveguide (made us-
ing a focused ion beam). Figure courtesy of IEMN.
optical power meter. A supercontinuum optical source(from 1470 nm up to 1600 nm) has been used, andlight injection is made using a lensed fiber. The out-put optical beam is collected by an as-cleaved fiberconnected to an optical power meter. Input and out-put facets of the waveguides are tapered in order tolaunch and to recover the light beams with a greaterefficiency [1]. Since TM polarization has shown lowerpropagation loss than TE polarization [1], all subse-quent measurements have been made in TM polar-ization. Except for the optical source, the character-ization bench and the measurement procedure arethe same as the ones reported in [1]. Transmissionmeasurements were made on a straight waveguide,which acts as the reference device and on a wave-guide incorporating a stub resonator. Both wave-guides have the same length �1.5 mm�, since they arelocated on the same cleaved bar of samples. Thetransmission measurements of the stub graftedwaveguide were de-embedded from the ones of thestraight waveguide, and the result is given in Fig. 4.The resonance of the stub is clearly observed at1516 nm with a peak of around 12 dB. The supple-mentary insertion losses, due to the stub, are closeto −7 dB.
In comparison with the theoretical calculations, a32 nm offset is obtained in the resonance wave-length. This discrepancy can be linked to two mainorigins: stub length and metal thickness. Concerningstub length, such an offset shall need a 530 nm erroron stub length definition. This is really unlikely tak-ing into account the accuracy of the e-beam lithogra-phy process that has been used and the post-processing check procedures. On the other hand,modeling (2D finite difference time domain) showed arather large dependence of the resonance wavelengthon the metal thickness (Fig. 5). Design was made tak-ing into account a 200 nm metal film thickness (goldrefractive index used for modeling: 0.18–10.2i at1.55 �m wavelength), and this thickness was depos-ited. But, as it can be seen in Fig. 2, sidewall metalthickness is smaller than the one deposited on thetop; it can reach a factor of 2 and even more. The
Fig. 3. Scanning electron microscope view of the finisheddevice. Figure courtesy of IEMN.
modeling results of Fig. 5 show that by using a gold
film thickness of 100 nm instead of 200 nm, the reso-nance wavelength is shifted by −25 nm. This is ingood agreement with the observed experimental phe-nomenon. Moreover, modeling predicts a contrastvalue close to 10 dB; we experimentally obtained a12 dB contrast. This can be explained by a slightlyoverestimated loss value of the metal in themodeling.
In conclusion, we reported the first fabrication (toour knowledge) of a very compact optical filter in in-tegrated optics based on the stub resonator concept.It was designed to get a resonance wavelength at1550 nm, and its length is only 1.6 �m. The experi-mental characterization showed a 1520 nm reso-nance wavelength. Feedback on design showed thatthis can be mainly explained by the thickness of themetallic film that wraps the stubs in order to en-hance their resonant behavior. Taking this into ac-
Fig. 4. Experimental transmission spectrum of the stub-based optical filter (0 dB corresponds to the transmission ofa straight waveguide of same length).
count, both resonance frequency and FWHM of the
resonance peak are in agreement with the theoreticalresults. A 12 dB contrast has been experimentallyobtained.
References
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Fig. 5. Normalized theoretical transmission for differentmetal thickness deposited on the stub (dotted curve,100 nm; dashed curve, 200 nm; continuous curve, 500 nm).
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