Integrated-optical Mach–Zehnder add–drop filterfabricated by a single UV-induced grating exposure

Turan Erdogan, Thomas A. Strasser, Michele A. Milbrodt, Edward J. Laskowski,Charles H. Henry, and Glenn E. Kohnke

A four-port bandpass filter is demonstrated with good performance by use of UV-induced Bragg gratings inP-doped channel waveguides without UV trimming. The simple, single-step fabrication procedure isexperimentally and theoretically analyzed, and tolerances are calculated for achieving practicalwavelength-division-multiplexed communication system performance. © 1997 Optical Society of America

1. Introduction

Dense wavelength-division-multiplexed optical com-munications systems require the ability to passivelymultiplex and demultiplex channels at the link endsand, in some architectures, to add and drop channelsat selected points on the link. Various schemes foraccomplishing passive channel multiplexing have beendemonstrated, including bulk-optical, integrated-optical, and all-fiber methods. One recently proposedmethod uses UV-induced fiber gratings1 in the arms ofan all-fiber Mach–Zehnder ~MZ! interferometer toform a single-channel multiplexer.2,3 Multichannelfunctions are accomplished by concatenation of singledevices. This method has the advantages of provid-ing sharp, accurate filter characteristics and very lowexcess loss. Greater than 20 dB of adjacent-channelisolation and return loss and less than 0.5 dB of excessloss have been demonstrated in these devices for 100-GHz channel spacing.3

Similar grating–MZ filters were demonstrated inan integrated-optical format with Ge-doped channelwaveguides.4 The advantages of basing such filterson integrated optics arise from performance, fabrica-tion, and packaging issues. From a performancestandpoint, it is preferable to form interferometricdevices on a single, thermally uniform, and mechan-ically and thermally stabilized substrate. In terms

T. Erdogan is with the Institute of Optics, University of Roch-ester, Rochester, New York 14627. T. A. Strasser, M. A. Milbrodt,E. J. Laskowski, C. H. Henry, and G. E. Kohnke are with LucentTechnologies, Bell Laboratories, 600 Mountain Avenue, MurrayHill, New Jersey 07974.

Received 17 January 1997.0003-6935y97y307838-08$10.00y0© 1997 Optical Society of America

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of fabrication, because add–drop filters for multiplechannels can be combined on a single chip and anexcimer laser and mask can be used to write thegratings, as we demonstrate here, it is conceivablethat once the MZ interferometers for a multichanneldevice are fabricated, the gratings for all channelscould be defined with a single exposure step. Fi-nally, regarding packaging, the definition of mul-tichannel filters on a single chip permits single-step,fiber-ribbon connection and the use of a single ther-moelectric cooler on a simple, mechanically stablepackage. We note that device characteristics forintegrated-optical filters have not yet approached thepractical values demonstrated in the all-fiber device.

For both the all-fiber and the integrated-opticalfilters demonstrated previously, UV trimming of theinterferometer balance after grating writing was nec-essary to compensate for nonidentical UV-inducedgratings in the two arms. However, filters that re-quire UV trimming are undesirable for a number ofreasons. First, UV trimming substantially compli-cates the fabrication process. It requires a localized,controlled UV exposure with parameters that dependon real-time monitoring of the device characteristics.It is conceivable that, with a large-area excimer-laserbeam and phase mask, UV-induced gratings could befabricated with a single step on multiple devices on achip without real-time monitoring—a very manufac-turable process. In contrast, UV trimming must bedone on each individual device, thus spoiling the par-allel nature of the manufacturing process. Further-more, many efficient multichannel layouts are notpossible when monitoring the balance of individualinterferometers is necessary, as is required by UVtrimming. In addition, valuable chip real estatemight be used by the extra interferometer arm lengthrequired for the trimming location. Finally, UV

trimming must be done while photosensitizing H2 orD2 is present for some waveguide materials, such asthe P2O5-doped SiO2 waveguides described here.But the presence of these molecules can alter thewaveguide refractive index and hence the couplerproperties, thus making accurate characterizationand therefore trimming of final ~after molecular out-diffusion! device characteristics difficult.

For these reasons it is highly desirable to investi-gate the possibility of fabricating grating–MZ filterswith a single grating exposure step. In this paperwe investigate integrated-optical grating–MZ filtersthat use a single, simultaneous grating exposure ofboth interferometer arms to maintain balance auto-matically and thus avoid the need for UV trimming.Both theoretical considerations and experimentaldemonstration of devices in P2O5-doped SiO2waveguides are described.

2. Model

A schematic of the add–drop filter investigated here isshown in Fig. 1. An illustration of the drop operation

Fig. 1. Schematic of a channel-waveguide grating Mach–Zehnder-interferometer add–drop filter.

of an ideal filter3 is shown in Fig. 2. Light at a num-ber of channel wavelengths ~represented by the solidvertical lines! is input through port 1. Wavelengthsnot resonant with the grating are transmitted out port4 ~T3!, and those reflected by the grating are trans-mitted out port 2 ~R3!, where transmitted channels arerepresented by the dashed vertical lines. In the idealcase no light escapes out ports 1 ~R5! and 3 ~T5!. Forthis illustration a 4-mm-long, uniform grating with acoupling–length product of kL 5 4 was chosen, wherek is the grating coupling coefficient and L is the lengthof the grating. Practically, an apodized grating withreduced sidelobes is desirable. For the add operationthe existing channels are again input through port 1,and the channels to be added, which must be resonantwith the grating, are input through port 3; all channelsare then transmitted out port 4.

In practice the ideal add–drop filter characteristicscan be achieved only approximately. There arethree main sources of imperfect operation. The firsttwo of these are imperfect 3-dB couplers and noniden-tical gratings in the two arms. While it is also pos-sible to have unequal path lengths for the arms, weignore this effect here, since equal path lengths canbe achieved to good tolerances in integrated-opticalMZ interferometers, and otherwise the effects can bemodeled by imperfect 3-dB couplers. These twomain sources of imperfect operation affect both thetransmission and reflection characteristics of theaddydrop filter. The third source is a different rel-ative position of the gratings along the waveguideaxes in each arm of the interferometer. For a singlegrating exposure that covers both arms simulta-

Fig. 2. Transmission and reflection spectra, demonstrating ideal drop operation of a filter with 4-mm long, kL 5 4 gratings. Solidvertical lines represent input channels, and dashed lines represent output.

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neously, this position difference could arise if eitherthe grating lines ~i.e., the phase mask! or the noncir-cular exposing beam were tilted with respect to thewaveguide axes. This source of imperfect operationaffects only the reflection characteristics of the filter.

To understand these three problem sources, it isinstructive to model the spectral transmission andreflection of the add–drop filter. Consider first thetransmission spectrum. By our keeping track of theamplitudes and phases of the electric fields in each ofthe two waveguides as light propagates through thefilter, the transmission spectrum can be written as

T5~l! 5 ~cos4 u!TG1~l! 1 ~sin4 u!TG2~l! 2 2~cos2 u!

3 ~sin2 u!ÎTG1~l!TG2~l! cos DFT~l!,

T3~l! 5 ~cos2 u!~sin2 u!$TG1~l! 1 TG2~l!

1 2@TG1~l!TG2~l!#1y2 cos DFT~l!%, (1)

where u [ sin21~=TC3! is an angle that determinesthe coupling efficiency of the 3-dB couplers ~TC3 isthe cross transmission of the waveguide coupler!,such that u 5 py4 implies perfect 3-dB coupling, andwhere TG1~l! and TG2~l! are the power transmissionspectra for the gratings in each of the two arms of theinterferometer. If the amplitude transmission spec-tra for the gratings are written t1,2~l! 5 @TG1,2~l!#1y2

3 exp$i phase@t1,2~l!#%, then DFT~l! is the relativephase difference in transmission for the two gratings,defined by DFT~l! [ phase@t2~l!# 2 phase@t1~l!#.Similarly, the reflection spectra can be written as

R5~l! 5 ~cos4 u!RG1~l! 1 ~sin4 u!RG2~l! 2 2~cos2 u!

3 ~sin2 u!@RG1~l!RG2~l!#1y2 cos@2w 1 DFR~l!#,

R3~l! 5 ~cos2 u!~sin2 u!$RG1~l! 1 RG2~l!

1 2@RG1~l!RG2~l!#1y2 cos@2w 1 DFR~l!#%, (2)

where DFR~l! is the analogous relative phase differ-ence in reflection and the term w 5 2pndyl is thephase accumulated by light traveling the distance dby which one grating is shifted relative to the otheralong the direction of propagation. Note that for d 50 the cosinusoidal dependence on the phase terms inEqs. ~1! and ~2! implies that it does not matter whicharm contains the stronger or weaker grating. Fornonzero relative grating position d, the reflectionspectra are affected by the location of the differentstrength gratings.

When imperfect 3-dB couplers and nonidenticalgratings at different locations occur simultaneously,then the full Eqs. ~1! and ~2! must be used to estimatethe resulting spectral effects. However, we can gainan appreciation for the magnitude of these effects byconsidering the three sources independently. First,consider the case of an add–drop filter with identicalgratings at identical locations but with imperfect3-dB couplers. Equations ~1! and ~2! then reduce to

R5~l! 5 RG~l!cos2 2u, T5~l! 5 TG~l!cos2 2u,

R3~l! 5 RG~l!sin2 2u, T3~l! 5 TG~l!sin2 2u, (3)

7840 APPLIED OPTICS y Vol. 36, No. 30 y 20 October 1997

where RG~l! and TG~l! are the power reflection andtransmission spectra, respectively, for both gratings.It is immediately apparent from Eqs. ~3! that if, forexample, R5 and T5 must be less than 220 dB for alll, we must have uu 2 py4u # 0.05, or 0.45 ~23.5 dB! #TC3 # 0.55 ~22.6 dB!.

Second, consider the case of a filter with perfect 3-dBcouplers, but nonidentical gratings in the two arms.For the UV-induced gratings considered here, the factthat UV exposure raises the average effective refrac-tive index of the waveguide mode leads to two differentclasses of nonidentical gratings. The first class con-sists of gratings of equal length L but of different cou-pling coefficients k1,2 5 pdn1,2yl and accordinglydifferent average refractive indexes n 1 dn1,2 in thegrating regions, where n is the effective index of thewaveguide mode and dn is the UV-induced effectiveindex change averaged over a grating period, andwhere we assume 100% index modulation. The sec-ond class comprises gratings of equal coupling coeffi-cient k but of different lengths L1,2. The effects ofnonidentical gratings are not as simple to quantify asthose of imperfect coupling. Mathematically, onemust calculate the spectra given by Eqs. ~1! and ~2! foru 5 py4. As an example, Fig. 3 shows the bar andcross reflection and transmission spectra of an add–drop filter with uniform, nonidentical gratings of thefirst class. The average coupling–length product ofthe gratings is ~kL!avg 5 1⁄2 ~k1L 1 k2L! 5 2, wherek1,2L are the products for the individual gratings.The spectra are plotted versus the normalized differ-ence in the coupling–length product between the twoarms defined by D~kL! 5 ~k2L 2 k1L!y~kL!avg, wherethe length L is fixed at 2 mm. Note that the spectraare asymmetric with respect to the center wavelength.This asymmetry occurs because the stronger gratinghas a longer Bragg wavelength, lB 5 2~n 1 dn!L, thanthe weaker grating.

The plots in Fig. 3 are meant to show qualitativelythe effects of nonidentical gratings on the spectra.To quantify these effects, Fig. 4 shows the calculatedreflection and transmission at the Bragg ~center!wavelength of the average grating, characterized by~kL!avg, as a function of the normalized grating dif-ference D~kL!. Figures 4~a! and 4~b! are plots for~kL!avg 5 2 ~L 5 2 mm! and ~kL!avg 5 4 ~L 5 4 mm!,respectively. The results show that, to maintain baroutputs below 220 dB, the relative difference be-tween coupling–length products of the two gratingsshould not exceed ;10%, as limited mainly by R5.We do not plot results for the second class of noniden-tical gratings, for which k is fixed and L is different.The magnitude of the modifications to the add–dropfilter spectra are similar to those shown in Figs. 3 and4. However, for gratings with different lengths Lthe spectral changes are symmetric about theaverage-grating Bragg wavelength, unlike for grat-ings with different k’s.

A simple method for estimating the effects of non-identical gratings involves looking at the transmis-sion at wavelengths not resonant with the grating.Well outside the grating bandwidth we find TG1~l! '

Fig. 3. Theoretical reflection and transmission spectra at the four ports of an add–drop filter with uniform, 2-mm-long nonidenticalgratings with average kL 5 2. D~kL! is the normalized difference in kL between the two gratings.

TG2~l! ' 1 and RG1~l! ' RG2~l! ' 0. If we thenassume perfect 3-dB couplers, the transmission spec-tra are given by

T5~l! 5 sin2~DFT~l!y2!,

T3~l! 5 cos2~DFT~l!y2!, (4)

where for the first class of nonidentical gratingsDFT~l! ' 2p~dn2 2 dn1!Lyl and for the second classDFT~l! ' 2pdn~L2 2 L1!yl. Thus, to maintain T5

below 220 dB, we must ensure that D~dn! [ dn2 2dn1 # 0.03lyL or that DL [ L2 2 L1 # 0.03lydn.For 4-mm-long gratings with kL 5 4 at 1550 nm, wemust maintain D~dn! # 1 3 1025 ~for dn 5 5 3 1024!or DL # 100 mm.

The third source of nonideal operation, a relative

position shift of the two gratings along the directionof propagation, affects only the reflection spectra.Consider the case of perfect 3-dB couplers and iden-tical gratings that are positioned along the arms ofthe interferometer at different points separated by adistance d along the direction of propagation. Thereflection spectra are then given by

R5~l! 5 RG~l!sin2 w,

R3~l! 5 RG~l!cos2 w, (5)

where the phase w 5 2pndyl. Thus, to keep R5

below 220 dB, we must have d # 0.016lyn. For l 51550 nm and n 5 1.45, we require d # 20 nm. Thepossibility of attaining this tolerance in practice isaddressed in Section 4.

Fig. 4. Theoretical reflection and transmission at the four ports of an add–drop filter with uniform, nonidentical gratings in the two arms.The gratings are ~a! 2-mm long with average kL 5 2 and ~b! 4-mm long with average kL 5 4. D~kL! is the normalized difference in kLbetween the two gratings.

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3. Experiments

Our main goal in experiments on integrated-opticaladd–drop filters was to determine how well thesources of imperfect operation just described could becontrolled. The devices consisted of waveguideMach–Zehnder interferometers fabricated in P2O5-doped SiO2 channel waveguides on silicon substrates.The channel waveguide fabrication and characteris-tics are similar to those described elsewhere.5 The5 3 5 mm2 waveguide core is buried 15 mm below thesample surface, and the relative core–cladding indexdifference is D 5 0.6%. The arms of the MZ inter-ferometers are 0.5-cm long and are separated later-ally by 30 mm. This narrow separation was chosento permit a single grating exposure to write nearlyidentical gratings in both arms simultaneously but topreclude coupling caused by lateral overlap of theevanescent mode tails.

Loading the waveguides with several mole percentof molecular deuterium ~D2! enhanced the photosen-sitivity of the P2O5-doped core.6,7 The gratings wereexposed with a 193-nm ArF excimer laser operatingat 30 Hz. A 20-cm focal-length cylindrical lens andslit were placed ;18 cm from the waveguides in thebeam path. The lens focused the beam to a 0.2-cmwidth measured perpendicular to the waveguideaxes. Diffraction of the poor-spatial-coherencebeam from the slit resulted in a smooth, Gaussian-like intensity profile with a width of 0.2 cm measuredparallel to the waveguide axes. The resulting flu-ence per pulse was approximately 100 mJycm2. Thegratings were written by exposure of the sample forseveral minutes with a fused-quartz surface-reliefphase mask8,9 in physical contact with the sample’ssurface. A typical grating reflection spectrum isshown in Fig. 5 ~solid curve!, along with a theoreticalfit to the spectrum ~dashed curve! that assumes aGaussian profile of the period-averaged UV-inducedindex change. The fit parameters include a Gauss-ian FWHM of 0.18 cm, a peak period-averaged indexchange of 1.3 3 1023, and a modulation of 66%. The

Fig. 5. Typical experimentally measured grating reflection spec-trum ~solid curves! and theoretical model for the spectrum ~dashedcurves!, assuming a Gaussian UV-induced index profile.

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disagreement in Fig. 5 occurs because the experimen-tal grating profile is not actually Gaussian and themeasurement resolution was 0.1 nm. Note that thesidelobes on the short-wavelength side of the spec-trum result from Fabry–Perot effects associated withwavelengths that are resonant with the tails but notthe center of the grating10; these effects are a funda-mental consequence of a nonuniform grating forwhich the average refractive index increases in pro-portion to the grating strength. Because the spec-tral width was growing unreasonably wide, thegrating growth for this example was halted before thereflectivity exceeded 99%. This problem is a conse-quence of the short grating length chosen to avoidspilling of the grating tails into the 3-dB couplers. Asharper spectrum could be obtained with longer grat-ings, which could be achieved by chopping off thegrating tails or by lengthening the arms of the MZinterferometers. Neither solution is ideal: Theformer produces spectral sidelobes whereas the latterrequires excessive, valuable chip real estate. Amore careful investigation of add–drop filter spec-trum optimization is in progress.

Filter characterization was performed both duringgrating exposure and after the fabrication was com-pleted. The in situ monitoring permitted the effectsof nonidentical gratings to be bounded by measure-ment of the change in T3. After the gratings wereexposed, the fabrication was completed by annealingof the waveguides for 24–48 h at 120 °C. This stepensured the stability of the gratings and out-diffusionof the residual D2. Full filter characterization wasperformed after the annealing step because the pres-ence of free H2 or D2 for photosensitization alters thewaveguide effective index and thus the coupler char-acteristics, including u or TC3. Bar and cross reflec-tion and transmission spectra measured for a typicaladd–drop filter are shown in Fig. 6. Measurementswere made by butt-coupling fiber ribbon connectors tothe waveguides and using a broadband edge-emittinglight-emitting diode ~ELED! and an optical spectrumanalyzer ~OSA!. Bar reflection was measured with a3-dB fiber coupler used between the source and thedevice. By measuring the power at all ports of theadd–drop filter and the fiber coupler, the effects ofbutt-coupling efficiencies and reflections could be re-moved from the measured spectra, thus isolating thespectral characteristics of the add–drop filter itself~as shown in Fig. 6!.

Since these devices are envisaged as components ina system employing optical fibers, it is important tocharacterize their polarization properties. Crosstransmission ~T3! spectra were measured for both TEand TM incident polarizations with a free-space po-larizer and a microscope objective used for couplinginto the waveguide. Output light was collected by asingle butt-coupled fiber. The results are presentedin Fig. 7. The TE spectrum is shifted to shorterwavelengths by 0.3 nm relative to the TM spectrum.This shift corresponds to a birefringence of approxi-mately 3 3 1024 and agrees with the hypotheses thatthe birefringence of the waveguide core results from

Fig. 6. Typical experimentally measured reflection and transmission spectra at the four ports of an add–drop filter after the device hasbeen annealed: Effects of fiber-channel waveguide coupling efficiencies and spurious reflections have been removed from the data. Thespectra are for ~a! R5, ~b! R3, ~c! T5, ~d! T3.

material strain. The spectral structure on the short-wavelength side of the Bragg resonance is associatedwith coupling to radiation modes.10 In addition to awavelength shift, both the position and the amplitudeof these spectral features exhibit a polarization de-pendence, which is a consequence of interference ef-fects resulting from reflection of the radiation modes

Fig. 7. Measured polarization dependence of the cross transmis-sion of an add–drop filter. Radiation-mode coupling loss appearsat wavelengths below 1565 nm.

off of noncore interfaces in the waveguide. Alterna-tively, one can think of the features as resulting fromcoupling of the core mode to cladding modes. Forsilica waveguides on silicon substrates, the cladding-mode guiding geometry is highly asymmetric, whichmakes the cladding modes polarization dependent.Since radiation mode coupling affects the transmis-sion but not the reflection spectra, the polarizationdependence of the reflected ports consists entirely ofthe birefringent Bragg wavelength shift.

4. Analysis

With data such as those presented in Fig. 6, it ispossible to calculate the individual contributions ofthe three problem sources discussed above to the non-ideal operation of the experimental add–drop filters.The procedure for this calculation is described here,along with data for the example device shown in Fig.6. First, the in situ monitoring spectra are used todetermine the changes in T3 and T5 caused by writ-ing the gratings. Before grating exposure, Eqs. ~1!are used with TG1 5 TG2 5 1 and DFT 5 0 to deter-mine a value for u ~which can change after the resid-ual D2 is outgassed!. After exposure, the newmeasured values of T3 and T5 at a wavelength far offgrating resonance ~1569 nm! are used to determineDFT~l!. The expressions for DFT~l! given after Eq.~4! are then used to estimate D~dn! 5 dn2 2 dn1 orDL 5 L2 2 L1. In practice, it proved difficult to

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Fig. 8. Theoretical model of the reflection and transmission spectra of Fig. 6, assuming nonidentical gratings ~solid curves! and identicalgratings ~dashed curves!. The spectra are for ~a! R5, ~b! R3, ~c! T5, ~d! T3.

monitor accurately the low power transmitted out theT5 port, and thus we relied mainly on the measuredchange in T3. T3 was typically observed to decreaseby ;0.1 dB; unfortunately, such a decrease couldeasily be attributed to UV-induced loss. Thereforeat best we can put bounds on D~dn! or DL by assumingthat the 0.1 dB change in T3 was caused either en-tirely by nonidentical gratings @D~dn! 5 D~dn!max orDL 5 DLmax and no UV-induced loss# or entirely byUV-induced loss @identical gratings with D~dn! 5DL 5 0#. For the example, we find 0 # D~dn! #D~dn!max 5 4.5 3 1025 or 0 # DL # DLmax 5 140 mm,depending upon which class of nonidentical gratingswe have. We note that these maxima are largerthan the values D~dn! # 0.03lyL 5 2.6 3 1025 orDL # 0.03lydn 5 36 mm that would be required tomaintain T5 # 220 dB according to Eqs. ~4!.

The next step is to use the known value of DFT~l!at a nonresonant wavelength ~1569 nm! in conjunc-tion with a postannealing measurement of T3 and T5

and Eqs. ~1! to determine u and hence the couplercross coupling efficiency TC3. For the example wefind TC3 5 0.39 ~24.1 dB! when we assume identicalgratings with D~dn! 5 DL 5 0, and TC3 5 0.42 ~23.7dB! for nonidentical gratings with D~dn! 5 D~dn!maxor DL 5 Dlmax. Clearly a significant limitation onthe performance of the devices studied here is theimperfect nature of the 3-dB couplers.

With the information gathered so far, we can nowmodel the transmission properties of the experimen-

7844 APPLIED OPTICS y Vol. 36, No. 30 y 20 October 1997

tal add–drop filters by using Eqs. ~1! and the theo-retically calculated grating spectrum that includesboth amplitude and phase10 ~see Fig. 5!. Figures8~c! and 8~d! show the calculated transmission spec-tra of the add–drop filter example considered here.The dashed lines were obtained for identical gratings,and the solid lines are for nonidentical gratings withD~dn! 5 D~dn!max and DL 5 0. When these spectraare compared with the measured spectra in Figs. 6~c!and 6~d!, it is clear that the calculation for noniden-tical gratings is in better agreement with the exper-imental data, particularly at wavelengths just to thelong-wavelength side of the grating center wave-length.

The final step in the calculation of the contribu-tions of the individual problem sources is to deter-mine the relative position shift d from thepostannealing reflection spectra. This is accom-plished by comparing the peak measured reflectivityvalues to calculations of Eqs. ~2! for various w, usingthe now known value u and the function DFR~l! de-termined from the now known D~dn! and the theoret-ical model for the grating spectrum. For theexample we find values of d 5 46 nm for the case ofidentical gratings and of d 5 38 nm for nonidenticalgratings with D~dn! 5 D~dn!max. Calculated reflec-tion spectra for these two cases are plotted in Figs.8~a! and 8~b!, where again the solid ~dashed! curvecorresponds to the case of nonidentical ~identical!gratings. As for the transmission spectra, the cal-

culation for nonidentical gratings agrees more favor-ably with the experimental reflection spectra.Based on the 30-mm separation of the two MZ inter-ferometer arms, these values of d correspond to tiltangles of the grating lines ~phase-mask lines! withrespect to the waveguide axis normals of 0.072° and0.088°. It was argued earlier that a value of d # 20nm ~tilt angle #0.038°! would be necessary to main-tain R5 # 220 dB. We note that the alignment ofthe phase mask to the waveguide axes was donesomewhat crudely by simple reliance on visual refer-encing of flats to surfaces. We predict that with amore careful alignment scheme a tilt angle of ;0.01°should be attainable, thus maintaining d ,, 20 nm.

5. Conclusion

To summarize the experimental results, we empha-size that the specific devices investigated here arenot candidates for high-performance dense wave-length division multiplexer functions for two mainreasons: First, the 3-dB couplers are significantlynonideal, and, second, the gratings are too short toyield both sharp, narrow spectral characteristicsand high ~.99%! reflectivity. Nevertheless, theexperiments on these devices are valuable for as-sessing the feasibility of using a single-grating ex-posure per MZ interferometer to imprint gratings inboth arms without upsetting the balance of the in-terferometer. As mentioned in Section 1, there arenumerous advantages of making devices in this waythat relate to fabrication complexity and cost. Theresults of the experiments are not clearly positivebut look promising. In particular, the problem ofpositioning the gratings at the same position ineach arm with a single exposure appears to be sur-mountable. However, we experimentally observenonidentical gratings in the two arms with a differ-ence in UV-induced index change somewhat largerthan the allowed tolerance for maintaining a220-dB specification for the bar port outputs. It ispossible that with a more uniform phase mask anda more homogenized laser beam ~without nonuni-formities resulting from diffraction and focusing!the required minimum deviation from the ideal,identical-grating case could be achieved. It shouldbe pointed out that, as in the case of the analogousall-fiber device, UV trimming of the interferometerbalance could be performed after grating fabrica-tion,3,4 but this necessity drastically complicatesthe fabrication of multichannel devices, as de-scribed in Section 1, and is thus undesirable.

In conclusion, we have experimentally demon-strated integrated-optical grating-MZ add–drop fil-ters with a single grating fabrication exposure,covering both arms of the interferometer, and with nofurther modifications for balancing the device. Weinvestigated the practical capability of producinghigh-performance multiplexing devices subject tothree identified sources of nonideal performance.

The actual devices investigated do not meet thesehigh-performance requirements, but experimentswith these devices enabled us to analyze whethersuch high-performance devices are realizable. Inparticular, we show that, if integrated-optical MZinterferometers with nearly ideal 3-dB couplers arefabricated ~as has been demonstrated,11!, then it isfeasible to fabricate gratings in the two arms at es-sentially identical locations by use of a single phase-mask grating and an excimer laser. We did notexperimentally demonstrate gratings that are suffi-ciently identical in the two arms such that the inter-ferometer balance is maintained well enough to meeta practically reasonable specification of 220-dB ex-tinction. However, it is conceivable that such toler-ances could be met. Multichannel multiplexingdevices based on the simple add–drop device de-scribed here could possess numerous advantages overother passive multiplexing approaches relating tofabrication simplicity, low loss, and sharp, accuratespectral characteristics.

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