Negligible birefringence in dual-mode ion-exchangedglass waveguide gratings

Sanna Yliniemi, Jacques Albert, Albane Laronche, Jose M. Castro, David Geraghty,and Seppo Honkanen

Polarization dependence of UV-written Bragg gratings in buried ion-exchanged glass waveguides is inves-tigated. A polarization-dependent shift in Bragg wavelength of less than 0.02 nm is measured, both for theeven and the odd modes of a laterally dual-mode waveguide. The measured wavelength shift correspondsto a waveguide birefringence of the order of 10�5, which is negligible for most applications in opticalcommunications. It is observed that the UV-induced birefringence is small, within the limits of the mea-surement accuracy. The thermal stability of the fabricated gratings is also very good. The results are ofparticular importance for devices considered here since they require a polarization-independent mode-converting waveguide Bragg grating. Polarization-independent performance of these gratings enables thefabrication of a new class of integrated optical devices for telecommunication applications. © 2006 OpticalSociety of America

OCIS codes: 130.0130, 230.7380, 260.1440, 050.2770, 260.7190.

1. Introduction

Dense wavelength division multiplexing, in whichthe transmission capacity of a single optical fiber ismultiplied by using several wavelengths, is a well-established key technology in telecommunicationbackbone and metropolitan-area networks. However,there is still a rapidly growing demand to increase thespeed and flexibility of fiber-optic networks. A promis-ing solution to meet the future requirements is to addvarious all-optical functionalities and devices to net-work subsystems.1,2 These all-optical devices eliminatethe need for optical-to-electronic conversion, the so-called electronic bottleneck. Integrated optics, whichuses fabrication technologies similar to the semicon-ductor industry, will be critical for increasing the de-

vice manufacturability to the point where new opticalsolutions are both competitive with state-of-the-art ex-isting technologies and affordable for the end user. Inview of this, we have recently proposed a class of pla-nar devices that is based on ion-exchanged glasswaveguide mode splitters–combiners and mode-converting waveguide Bragg gratings. These devicesinclude an add–drop wavelength filter,3 a (tunable)dispersion compensator,4 and an all-optical packetheader recognition chip.5 All these devices are com-posed of adiabatic asymmetric Y branches, whichhave waveguides of different widths, and UV-writtenwaveguide gratings in a dual-mode waveguide.

A key requirement for the devices mentioned in thepreceding paragraph is a polarization-independentoperation; it is far more difficult to achieve comparedto typical integrated optic devices operating solelywith the fundamental mode. Most importantly, thewaveguide birefringence has to be low in a relativelywide dual-mode waveguide, not only for the funda-mental mode (even mode), but also for the secondlateral mode (odd mode). Higher birefringence is ex-pected for the odd mode, since it is less confined, andtherefore more sensitive to the planar top interfacethat introduces waveguide form birefringence. Usu-ally UV exposure induces some birefringence ondevices.6 In our case, however, the UV-writtengrating must not induce much birefringence. Thepolarization-independent operation would be difficultto achieve with other media for integrated optics,

S. Yliniemi (syliniem@cc.hut.fi) is with the Optoelectronics Labo-ratory, Helsinki University of Technology, Tietotie 3, P.O. Box 3500,Espoo FIN-02150 HUT, Finland. J. Albert and A. Laronche arewith the Department of Electronics, Carleton University, 1125Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada. J. M. Castroand D. Geraghty are with the Department of Electrical and Com-puter Engineering, University of Arizona, 1230 East SpeedwayAvenue, Tucson, Arizona 85721. S. Honkanen is with the Collegeof Optical Sciences, University of Arizona, Meinel Building, 1630East University Boulevard, Tucson, Arizona 85721.

Received 3 January 2006; revised 27 March 2006; accepted 3April 2006; posted 10 April 2006 (Doc. ID 66977).

0003-6935/06/256602-05$15.00/0© 2006 Optical Society of America

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such as silica on silicon, although birefringence issueshave more or less been solved for single-modewaveguides. With the silica-on-silicon platform, lowbirefringence is usually obtained by stress tailoring ofthe cladding layers or by UV trimming resulting inwaveguide birefringence of the order of 10�5.7 Withdirect UV writing, values as low as 5 � 10�6 have beenreported.8 However, silica-on-silicon platforms typi-cally cannot result in nonbirefringent waveguides ofdifferent widths.9 We have recently shown that thefundamental mode of ion-exchanged waveguides canhave very low birefringence for waveguides withwidely varying widths.10 Values of the order of 10�6

were measured when the burial process was followedby thermal annealing. However, we were not able tomeasure the birefringence for the odd mode and nowaveguide grating was included. It is worth mention-ing that our add–drop device showed an unaccept-ably large shift of about 0.25 nm in Bragg wavelengthfor the two polarizations.3

In this paper we show that a polarization-dependentBragg wavelength shift of less than 0.02 nm, for boththe even and the odd modes, is achievable in buriedion-exchanged glass waveguides with UV-writtenBragg gratings. The measured values correspond to awaveguide birefringence of the order of 10�5 or below,which indicates that the UV writing does not signif-icantly increase the waveguide birefringence. It isalso observed that an UV-written Bragg grating doesnot significantly increase the waveguide birefrin-gence. The paper is organized as follows: First, inSection 2 we briefly describe the waveguide and thegrating fabrication method. In Sections 3 and 4 wepresent, respectively, the even and odd mode charac-terization technique and the results. Finally, in Sec-tion 5, we present our results.

2. Fabrication

Waveguides with mask opening widths from 2 to5 �m were constructed in BGG31 glass by Ag�–Na�

ion exchange. Surface-channel waveguides fabricatedin this way were then buried below the glass surfaceto reduce propagation losses and to obtain circularwaveguides matching well with fibers. The burialwas done for 2400 s at T � 250 °C with an appliedfield of 640 V�mm. The burial process also signifi-cantly reduces the birefringence by reshaping theindex profile. After the burial, the sample was an-nealed for 105 min at 230 °C to further reduce bire-fringence. The waveguide fabrication process hasbeen described in more detail elsewhere.10,11

After waveguide fabrication, both weak and strongBragg gratings with a length of 7 mm were exposedinto the waveguides with a pulsed ArF excimer laserat 100 pps with a pulse energy of 80 mJ. In the pre-liminary experiments, it was observed that the grat-ing strength saturated after 8 min of exposure with areflectivity of 60%. This was chosen to be the exposuretime for the strong grating, while the weak gratingwas exposed for only 4 min resulting in a reflectivityof 40%. The phase-mask period of 1065 nm produced

a Bragg grating with a periodicity of 532.5 nm intothe waveguides.

3. Characterization

An UV-exposed grating allows the measurement ofthe polarization-dependent wavelength (PDW) shiftof the reflected light for different modes propagatingin a channel waveguide. Each mode has a character-istic effective refractive index, and therefore they arereflected at slightly different wavelengths accordingto the Bragg condition

� � 2neff�, (1)

where � is the reflected wavelength, � is the period ofthe Bragg grating, and neff is the effective refractiveindex of the mode. The modal birefringence, there-fore, appears as a PDW shift.

The reflected wavelengths for different modes weremeasured using a setup presented in Fig. 1. A broad-band light source is used to simultaneously excite themodes supported by a waveguide. A fiber circulatorcouples the light source to the waveguide throughports 1 and 2, and the reflected spectrum is connectedto the optical spectrum analyzer (OSA) through port3. A fiber polarizer is used to selectively couple eitherthe quasi-TE or TM polarization state to the wave-guide. The power in the unwanted polarization state isminimized by monitoring the transmitted power inthis polarization state, while tuning the polarizationcontroller, as can be seen from Fig. 1. The PDW shift isthe wavelength difference between the quasi-TE-modeand the TM-mode reflection peaks. For each mode, thecorresponding waveguide birefringence, �neff, can bederived from Eq. (1) as

�neff ���

2�, (2)

where �� � �TE � �TM is the measured PDW shift,and � is the period of the Bragg grating. The accuracyof this method is restricted by the OSA resolution andthe wavelength repeatability. According to the man-ufacturer, the OSA (Ando Model AQ6317) used inthese measurements has a resolution of 0.015 nm or

Fig. 1. Measurement setup for determining the polarization-dependent wavelength shift of the channel waveguide Bragg grat-ings for the even and the odd modes. An amplified spontaneousemission (ASE) source is coupled to the waveguide through a fiberpolarizer (FP), a polarization controller (PC), and a circulator. Thereflected light is guided through the circulator to the OSA. Thesetup also includes a free-space rotatable analyzer and an InGaAsdetector to minimize the power in the unwanted polarization stateby monitoring the transmitted light.

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better and a wavelength repeatability of 0.005 nm.This sums up to a minimum detectable PDW shift of0.020 nm or better, which corresponds to a birefrin-gence of 2 � 10�5. This value of birefringence is lowenough for most telecommunications applications.

In a dual-mode waveguide, coupling from an evento an odd mode or vice versa becomes possible if thegrating is slightly tilted.12 This mode conversionoccurs at a wavelength �eo given by

�eo � �z�neff�0� � neff�1��, (3)

where �z is the period of the grating in the propaga-tion direction, and neff�0� and neff�1� refer to the even-and the odd-mode effective refractive indices. In ourproposed devices the grating is purposely positionedat an angle to obtain the even–odd (or odd–even)mode conversion. The grating angle should be opti-mized to maximize the mode conversion in reflec-tion.12

4. Results

The waveguide birefringence for the even and oddmodes as a function of a mask opening width is pre-sented in Figs. 2 and 3, Fig. 2 referring to the mea-sured birefringence with the weak grating and Fig. 3referring to the measured birefringence with thestrong grating. In all the waveguides, the variation inbirefringence for a given waveguide width is less thanthe measurement resolution, which is 2 � 10�5. Thebirefringence of the strong grating appears to beslightly larger than that of the weak grating, whichindicates that some birefringence is induced by theUV writing. This is not conclusive, however, since allthe values are below the OSA accuracy limit. Never-theless, the measured values of birefringence are neg-ligible for most applications, corresponding to a PDWshift of less than 0.02 nm. In most practical applica-tions, reflectivities approaching 100% are required.

This can be achieved by increasing the grating lengthfrom 7 to approximately 12 mm. This would not in-crease the PDW shift, since birefringence is a localeffect independent of the grating length.

Figures 4(a) and 4(b) present typical reflectionspectra obtained from waveguides, respectively, withmask opening widths of 2 and 5 �m for both polar-izations. The reflection spectra for the quasi-TE andTM modes overlap within the measurement accura-cy of 0.02 nm. The waveguide with a mask openingwidth of 2 �m is single mode with a Bragg wave-length of 1549.11 nm (corresponding to an effectiverefractive index of 1.455). For a waveguide with amask opening width of 5 �m, both the even and theodd modes can be observed at Bragg wavelengths of1550.06 nm (1.456) and 1548.49 nm (1.454), respec-tively. In the wider waveguide, a third reflection peakis observed between the even and the odd modes. TheBragg wavelength �1549.25 nm� was calculated tomatch the wavelength obtained from Eq. (3), confirm-

Fig. 2. Measured birefringence �nTE � nTM� from a waveguidewith a weak grating for (a) the even and (b) the odd modes. Circles,squares, and diamonds refer to different measurements fromwaveguides with the same mask opening width.

Fig. 3. Measured birefringence �nTE � nTM� from a waveguidewith a strong grating for (a) the even and (b) the odd modes.Circles, squares, diamonds, and stars refer to different measure-ments from waveguides with the same mask opening width.

Fig. 4. Reflection spectrum for a waveguide with a mask openingwidth of (a) 2 �m and (b) 5 �m. The solid curve refers to thequasi-TE-polarization state and the dashed–dotted curve to thequasi-TM-polarization state.

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ing that this reflection is due to mode conversionbetween the even and the odd modes. This indicatesthat the grating is at a slight angle with respect to thewaveguide.

Mode intensity profiles of the waveguides for thetwo mask opening widths, 2 and 5 �m, are presentedin Fig. 5. The mode intensity profile for a single-modefiber is shown for comparison. The waveguide with amask opening width of 2 �m is a single mode and thewaveguide with a mask opening width of 5 �m sup-ports two modes. The waveguide even-mode profilesare almost circular and slightly smaller than thesingle-mode-fiber mode profile as can be seen fromthe figure. Also, for the waveguide with a 5 �m maskopening width, the profile of the odd mode is shown.

5. Discussion and Conclusions

The results described in Section 4 demonstrate verylow birefringence for the UV-written gratings in bur-ied ion-exchanged glass waveguides. This appears tobe in disagreement with the performance of our add–drop wavelength filter presented in Ref. 3. Thereforewe decided to further investigate the polarizationdependence of the add–drop wavelength filter pre-sented in Ref. 3. Note that the device was fabricatedin a BGG31 glass substrate using similar waveguideand grating fabrication processes as in this study.First, the PDW shift for the even–odd reflection wasconfirmed to be �0.25 nm with the Bragg wavelengthof 1564.60 nm for the quasi-TE polarized light. Toreduce the birefringence we annealed the sample at230 °C in successive steps for a rather long total du-ration of 345 min, and after each annealing step wemeasured the Bragg wavelength for the quasi-TE andTM modes. The measured PDW shift against the an-nealing time is shown in Fig. 6. The annealing de-creases the PDW shift, but it remains above 0.05 nm,which is not low enough for telecommunication ap-plications. Since these gratings are not much stron-ger than the ones fabricated in this study, it isunlikely that the measured PDW shift is due to UV-

induced birefringence. This conclusion is supportedby recent studies on UV-induced birefringence in fi-ber Bragg gratings.13,14 We believe that this nonva-nishing birefringence is due to the relatively shortburial duration of 300 s used in making the add–dropdevice. As a result, the waveguides locate only slight-ly beneath the glass surface. The waveguide modesstill interact with the surface, which increases thewaveguide form birefringence. To confirm this, weused our modeling software for ion-exchanged glasswaveguides15 to determine the form birefringence ina two-mode waveguide fabricated with the parame-ters in Ref. 3. The calculated values of form birefrin-gence are 5 � 10�4 and 2 � 10�4, respectively, for theeven and the odd modes. After a 345 min annealingat 230 °C, these values decrease to 2 � 10�4 and1 � 10�4. These calculated values are in good agree-ment with the measured PDW shift [see Eq. (2)] afterburial. The modeled values after annealing areslightly higher than the measured values. Regardingthe Bragg wavelength, it decreased as the samplewas annealed due to the decrease of the effectiveindex of the modes [see Eq. (3)]. The Bragg wave-length of 1561.54 nm for quasi-TE polarized lightwas measured after annealing for 345 min. Interest-ingly, the reflectivity of the grating dropped only toa value of �90% from �99% after this harsh tem-perature treatment. This implies that these UV-imprinted gratings in BGG31 glass are stable, whichis an important requirement for telecommunicationdevices.

In conclusion, low birefringence, of the order of2 � 10�5 or below for both the even and the odd modes,was observed in UV-written gratings in dual-modeion-exchanged glass waveguides buried deep enoughbelow the glass surface. The UV-exposed Bragg grat-ing had a small effect on the waveguide birefringence.This result has significance in integrated optical com-ponents based on the even–odd mode conversion in-cluding add–drop multiplexers,3 header recognitionchips for optical packet switching,5 and chromaticdispersion compensators.4 These chip-size integrated

Fig. 5. Mode intensity profiles (a) for a single-mode fiber, andfor waveguides with mask opening widths, (b) 2 �m (single mode),(c) 5 �m (even mode), and (d) 5 �m (predominantly odd mode).

Fig. 6. PDW shift of the add–drop wavelength filter as a functionof annealing time. Crosses represent the measured values, and thedashed curve is an exponential fit to the measured values.

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optical devices have the potential to replace the spa-cious fiber-based components utilizing bulky circula-tors in applications requiring numbers of elements.2Another benefit with integrated optical componentsis easy and accurate manufacturing. Standard photo-lithographic techniques combined with an UV-gratingexposure provide control over the grating reflectivity,element spacing, and location. Therefore utilizationof polarization insensitive integrated optical compo-nents could greatly reduce the system’s size and costin optical communication networks.

We thank Brian R. West for his help with thebirefringence modeling. Support from TRIF (Stateof Arizona Photonics Initiative) is appreciated. S.Yliniemi also thanks the Academy of Finland andMagnus Ehrnrooth’s foundation for financial sup-port. This work was performed while S. Yliniemi waswith the College of Optical Sciences at the Universityof Arizona.

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