A thermo-optically tuneable periodic wavelength filter having a rectangular wavelength response with a 50 GHz free spectral range, based on a Mach-Zehnder interferometer with ring resonator has been fabricated in SiON waveguide technology

de Ridder, R.M.

Roeloffzen, C.G.H.

Sengo, G.

Worhoff, Kerstin

English

University of Twente Research Information

Fibers and wavegulde components P1.20Passband flattening and rejection band broadening of a periodic Mach-Zehnder wavelength filter by adding a tuned ring resonatorC.G.H. Rbeloffzen, R.M. de Ridder, G. Sengo, K. Wdrhoff and A. DriessenLightwave Devices Group, MESA Research Institute, University of Twente,P.O. Box 217, 7500 AE Enschede, The Netherlands, c.g.h.roeloftzen§utwente.nlAbstract A thermo-optically tuneable periodic wavelength filter having a rectangular wavelength response with a50 GHz free spectral range, based on a Mach-Zehnder interferometer with ring resonator has been fabricated inSION waveguide technology.IntroductionIn order to efficiently use the available bandwidth inmulftwavelength optical networks, filters with arectangular-shaped wavelength transfer function areneeded. Filters which are periodic in frequency orwavelength (slicers or interleavers) can be used asbuilding blocks in a binary tree arrangement in orderto realize more complicated filter functions like Add-Drop multiplexers [1]. Such composite filters can befully tuned if each of the component slicers istuneable over one free spectral range (FSR).In this paper, we demonstrate a thermo-opticallytuneable slicer having a nearly rectangular-shapedwavelength transfer function, with 50 GHz FSR,consisting of a Mach-Zehnder Interferometer (MZI)filer with a ring resonator coupled to one of itsbranches [2, 3].Device structure and designFigure 1 shows the lay-out of the MZI + ring filter. Thelength difference of the two branches of the MZI is3,996 mm resulting in a FSR of 50 GHz. A ring isconnected to the short channel of the MZI using adirectional coupler (DC) having a power couplingcoefficient of 0.82. The roundtrip length of the ring(7.992 mm) is twice the length difference of the MZI.The MZI input and output couplers each consist oftwo 3dB DC's interconnected by equal lengthchannels. Heaters are located on the long MZI branch(for tuning the filter central frequency), on the ring (forsynchronising it with the MZI), and on one of thechannels in each of the input and output couplers (foradjusting the coupling ratio). Using 1.5 mm longheaters, a 2ir phase change could be easily obtained.(a)20mm(b)Figure 1:Scemaffc (a) and real (b) Layout of the MZI +ring filter.Figure 2 shows two simulated filter responses. Thesolid curve is the desired response (power coupling tothe ring is 0.82). The isolation is 29 dB and the widthof the stop band 31 % of the FSR @ -25 dB. Thedashed curve is for a smaller power coupling of 0.59.The filter is still passband flattened but the isolationhas degraded to 15 dB. Channel loss of 0.5 dB/cmwas included in the calculations.o 4 , _ IW.5-15-20-300.0 0.5 10 15 20normaflzed frquencyFigure 2: Calculated filter curves for power couplingcoefficients of 0.82 (solid line) and 0.59 (dashed line)respectively. The roundtnip loss ofthe ring is 0.4 dB.The device has been fabricated using PECVD SiONtechnology [4] where the layers were annealed tolower the N-H absorption peak at 1510 nm (5]. Thewaveguide is a buried waveguide channel as shownin figure 3. These channels have propagation loss0.4-0.5 dB/cm @1550nm and 1.3 dBlcm at thehydrogen absorption peak (1510 nm), and allow bendradii down to 0.6 mm. The waveguide is designed tohave low birefringence.Fibers and waveguide components P1.20n-1.45/M1"M $ 0.8jpm3.2 umFigure 3: Cross-section of the buried waveguidechannel.Device characterizationNot yet fully optimised devices have been realizedand evaluated. The index contrast came out to be toolarge for the desired power coupling ratio. In the caseof the 3 dB couplers, it could be compensated. Butnot for coupling to the ring. The coupler between thering and the MZI branch could not be of theadjustable type (due to space considerations).Therefore, the desired 0.89 power coupling to the ringcannot be obtained and the best filter had a powercoupling of 0.59 for TM-polarized light, resulting in atheoretical filter curve as shown in figure 2 (dashedline). For TE, the coupling was found to be 0.52.Using a tuneable laser, the cross and bar transmis-sion spectra of the slicer have been measured for TMlight, as shown in figure 4. First, the MZI couplerswere tuned to 3 dB. Then the phase of the ring wastuned to obtain the optimum filter curve. The bar andcross spectra are measured without retuning. Themeasured isolation is 15 dB. Passband flattening andstop band broadening is clearly observed. Repeatingthe rneasurement for TE polarized light withoutretuning the heaters resulted in bad filter characteris-tics. Better curves, as shown in figure 5, were onlyobtained after retuning the ring. The measuredisolation is 12 dB, which can be explained as a resultof the even smaller coupling coefficient between ringand MZI branch. Comparing the TE and TM spectralposition of the curves, one is tempted to conclude thatthe birefringence is low. However, there is a possibil-ity that the curves have been translated over a fullFSR..25 |-15490 1549.5 1550.0 IS50.5 1551.Wavelengt (nm]Figure 4: Measured bar (grey) and cross (black) filterresponse for TM polarzed light.V.~10p~-¶5i-25 I1549.0 15495 1550.0 1550.5 1551.Wavelength rnm)Figure 5: Measured bar (grey) and cross (black) filterresponse for TE-polanized light The dashed curvewas found without retuning the nng.It is difficult to make filters with 50 GHz FSR that arefree of birefringence, since ANffTmM1E=2 10"4 results ina frequency shift of 25 GHz. This is independent ofthe FSR of the filter. The power needed to tune overone FSR is 0.9 W.ConclusionsA passband flattened wavelength slicer, having aFSR of 50 GHz, based on an MZI with a ringresonator coupled to one of its branches, has beendesigned, fabricated and measured. The isolation.was designed to be 29 dB, but the measured valuewas 15 dB for TM polarized light The main cause forthis deviation is a not yet optimised value of thecoupling coefficient between the ring and MZI branch,caused by too strong index contrast. Although thestraight waveguide was designed to have lowbirefringence, polarization dependent behaviour of thedevice was observed. Work on progress in realizingdevices having correct index contrast.AcknowledgementThis work is funded by the Dutch TechnologyFoundation STW. We thank L. Hilderink for growingthe PECVD SiON layers and M.G. Hussein for thethermal treatment of the layers.References[1] C.G.H. Roeloffzen, et al., IEEE Photon. Technol.Lett., 12 (2000), 1201.[2] K. Oda, et al., J. Ughtwave Technol., 16 (1988),1016.[31 M. Kohtoku et al., IEEE Photon. Technol. Lett.,12 (2000), 1174.[4) R.M. de Ridder et al., IEEE J. Select. TopicsQuantum Electron., vol. 4, pp. 930, 1998.[51 M.G. Hussein, et al., in Proc. Symp. IEEEILEOSBenelux Chapter, 265, 2001.

Passband flattening and rejection band broadening of a periodic Mach-Zehnder wavelength filter by adding a tuned ring resonator

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