Optical Filters Based on Ring Resonators With Integrated Semiconductor Optical Amplifiers in GaInAsP InP

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1 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 6, NOVEMBER/DECEMBER Optical Filters Based on Ring Resonators With Integrated Semiconductor Optical Amplifiers in GaInAsP InP Dominik G. Rabus, Michael Hamacher, Ute Troppenz, and Helmut Heidrich Abstract A key device in all-optical networks is the optical filter. A ring resonator filter with an integrated semiconductor optical amplifier (SOA) on the basis of GaInAsP InP has been investigated and fabricated. A required passband shape of loss-compensated ring resonator filters can be custom-designed by the use of multiple coupled resonators. Results of single-, double-, and triple-ring resonators with integrated SOAs with free spectral ranges of 12.5, 25, and 50 GHz, respectively, are presented. A box-like filter response is obtained by the double- and triple-ring resonators using specific coupling coefficients. Index Terms Filter, GaInAsP, resonator, ring, semiconductor optical amplifier (SOA). I. INTRODUCTION WAVELENGTH division multiplexing (WDM), especially dense wavelength division multiplexing (DWDM), communication systems require optical components which can de-/multiplex closely spaced channels. A filter referred to as an add/drop filter is required to separate the channel to be dropped from those that pass through unaffected. Channel dropping filters on the basis of ring resonators are of great interest due to their compactness and high wavelength selectivity [1] [4]. Closer channel spacing (e.g., 25 GHz and 50 GHz) requires sharper filter responses and ON OFF ratios of more than 20 db to separate the channels without introducing crosstalk from the other channels. Recently, ring resonators have been demonstrated for many other applications, such as single-mode lasers [5], tunable lasers [6], modulators [7], [8], switching devices [9], biosensors [10], and dispersion compensators [11]. The most obvious application for ring resonators is for demultiplexing very closely spaced channels. The realization of high-performance optical filters based on planar ring resonators requires low-loss waveguides with a strong optical confinement to allow low bending radii. In the case of periodic filters, in our case ring resonators, it is important to fit the transmission curve to the defined channel spacing. The performance of passive ring resonators for filter applications is limited by internal losses. The incorporation of a semiconductor optical amplifier (SOA) enables additional Fig. 1. Model for a single-ring resonator. functionality (e.g., switchability), including the compensation of internal losses. Ideally, a filter should have a box-like (rectangular) amplitude response and zero dispersion which corresponds to a linear spectral phase over the filter s passband. The passband width of the filter must accommodate fabrication tolerances on the filter and laser center wavelengths as well as their polarization, temperature, and aging characteristics. The realization of a box-like filter response requires the use of multiple coupled resonators [12]. The passband shape depends sensitively on the interactions between all resonators and the couplers. In order to achieve a box-like filter response, a general design rule is given in this paper for engineering filter shapes using double- and triple-coupled ring resonators. II. RING RESONATORS THE FILTER DESIGN A. Single-Ring Resonator The model of a single-ring resonator (SRR) with one bus waveguide is shown in Fig. 1. The circumference of the ring is ( ; the radius is ), and the power coupling factor is. The intensity attenuation coefficient of the ring is. The intensity insertion loss coefficient of the coupler is denoted with. The wave propagation constant is. The intensity relation for the output port is given by [13] Manuscript received August 27, 2002; revised October 1, This work was supported in part by the Deutsche Forschungsgemeinschaft under Grant HE 3366/2-1. The authors are with the Fraunhofer Institut Nachrichtentechnik Heinrich- Hertz-Institute, Berlin, Germany ( rabus@hhi.de). Digital Object Identifier /JSTQE (1) X/02$ IEEE

2 1406 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 6, NOVEMBER/DECEMBER 2002 where is the transmitted and is the inserted electric field and is the transmitted and is the inserted intensity. The minimum transmission will be achieved if the following equation is satisfied: Minimum transmission on resonance [ ] can be realized for fixed values of and by adjusting the coupling factor to the intensity attenuation coefficient or vice versa. The value of can only be changed significantly by the implementation of an SOA within the ring resonator or by using an all-active ring resonator. The implementation of SOAs in ring resonators is even more important if ring resonators with input and output waveguides are used. In this case, lossless resonators are required for an ideal filter response. (2) (3) B. Double- and Triple-Ring Resonators Ring resonators coupled in series enable the realization of all kinds of transmission characteristics including a box-like filter shape. The superposition of all possible resonating paths in multiple coupled ring resonators has to be considered. There are various combinations of resonating paths, for example, the first and the second ring resonator can be combined to form another resonating path. In an ideally designed multiple coupled ring resonator configuration, all possible optical resonator lengths are identical or a multiple of each other. The synthesis of multiple-ring resonator filters is described in [14] for the serially coupled configuration and in [15] for the parallel configuration. The model for multiple serially coupled ring resonators is shown in Fig. 2. There are basically three important figures when designing a filter response: 1) the 3 db bandwidth or the full-width at half-maximum (FWHM); 2) the ON OFF ratio; 3) the shape factor. For a lossless ring resonator configuration, the 3-dB bandwidth depends mainly on the coupling coefficients and the optical round-trip length. The ON OFF ratio for the drop port, which is the ratio of the on-resonance intensity to the OFF-resonance intensity, is calculated for a lossless double-ring resonator (DRR) with symmetrical outer couplers. Then the ON OFF ratio is given by db (4) Fig. 2. Model for multiple serially coupled ring resonators. There are basically two types of filter responses which can be realized with a DRR by a proper choice of the inner coupler: a Lorentzian-like filter response and a box-like filter response. It is helpful to describe the filter response by a shape factor defined as Shape factor Bandwidth Bandwidth db Bandwidth db Bandwidth In the case of a Lorentzian-like response this factor is limited by the predetermined value of In contrast, the ideal response shape of a box-like filter with a flat passband and a step-like roll-off from passband to stopband will have a shape factor of 1. The improvement of the shape factor can be realized by multistage filter architectures. For practical use we restrict our considerations on DRR and triple-ring resonator (TRR) devices. In this case, there is an essential tradeoff between bandwidth and residual amplitude ripple. Furthermore, the technological issues for reproducible fabrication have to be taken into account. A lossless DRR configuration with a shape factor of 0.5 is exemplarily designed using the transfer function as described in [16]. This device would depict ring resonators in series with for the outer couplers. Such 3-dB couplers can be realized with high fabrication tolerances using MMI devices. For this setup, can be realized within the range by using codirectional couplers. The achievable ON OFF ratio for this configuration is larger than 20 db. As a second example, a serially TRR is used to realize a shape factor up to 0.6. For simplification, we set and. (5)

3 RABUS et al.: OPTICAL FILTERS BASED ON RING RESONATORS WITH INTEGRATED SOAs IN GaInAsP InP 1407 TABLE I LAYER SEQUENCE OF THE SOA Fig. 3. SEM picture of a deeply etched passive waveguide in the curvature. Fig. 4. SEM photograph of the active passive interface. For an ON OFF ratio of more than 30 db, we determined coupling coefficients of about 0.7 for the outer couplers and within for the couplers in the center. Another possibility for realizing a box-like filter is using coupling coefficients of and. III. DESIGN AND FABRICATION The performance of passive ring resonators for filter applications is limited by internal losses. Therefore, an SOA which is butt coupled to the passive waveguide has been implemented in the ring resonators. The SOA length has been designed to compensate for the butt-coupling losses and the ring losses. A standard ridge waveguide laser structure was used for the SOA section, which required an additional epitaxial growth step. The layer sequence of the SOA structure is as follows (Table I). The gain structure consists of six quantum wells (QWs) with a bandgap wavelength of m. The barrier layers are made of n-gainasp with a bandgap wavelength of m. The width of the SOA is 2.2 m. A scanning electron microscope (SEM) picture of the passive waveguide in the curve is shown in Fig. 3. The bandgap wavelength of the quaternary material is m. The waveguide ridge was deeply etched on the outer side of the waveguide in the curve for index enhancement [17]. The waveguide width is 1.8 m. The waveguide design assures both a monomodal propagation of the light in the waveguide and low bending losses. The facets of the input and output waveguides have been antireflection-coated to avoid Fabry Perot resonances in the straight waveguides. The SEM photograph of an active passive interface is shown in Fig. 4. The butt-coupling losses at the active passive waveguide interface have been calculated by the finite difference method (Fig. 5). The calculated vertical and lateral offset between the Fig. 5. Calculated butt-joint losses. active and passive waveguide results in a minimum theoretical coupling loss of 1dB. The starting point for the calculation of the horizontal shift has been chosen so that the passive structure is located symmetrically in the center of the active section. The calculation of the vertical shift starts with the passive active section, butt coupled at the position where the rib starts for the passive structure and in the middle of the active layers for the SOA section. The butt-joint losses are determined using passive straight waveguides with integrated SOAs of different lengths (Fig. 6) and are measured similarly to the cut-back method. The SOA sections of different length are driven at a constant current density. The insertion losses of the SOA integrated waveguides measured with tapered fibers are shown in Fig. 7. With an experimentally determined fiber-chip coupling loss of 5 db, an SOA butt-joint loss is calculated to be less than 3 db each. Recent results show that this value can be decreased below 2 db by improved height adjustment. The back reflections from the butt-joints have been determined using optical lowcoherence reflectometry [18] and are below 50 db. This shows that the butt-joints do not form a parasitic cavity and the back reflection can be neglected in the simulation.

4 1408 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 6, NOVEMBER/DECEMBER 2002 Fig. 6. Fabricated straight passive waveguides with integrated SOAs. Fig. 8. Photograph of an SRR with an integrated SOA. The waveguide-integrated Pt-resistors enable a fine tuning of the filter response [19], [20]. This functionality was not used in the experiments. Fig. 7. Determination of butt-joint losses. IV. EXPERIMENTAL RESULTS The measurement of the filter devices is performed connecting an external cavity laser (ECL) and a fiber integrated polarization controller in front of the input interface. The transmitted signal is detected in an external photo diode. The ECL signal is coupled to the input waveguide by using a tapered fiber, which can be adjusted by a three-axis piezo drive. The near-field pattern of the output waveguide is focused on the photodiode by using a microscope lens. The specimen is placed on a Peltier cooler so that all measurements are performed at a well-defined temperature. The devices with integrated SOAs are measured at 15 C. The SOAs favor TE polarization due to the specific QW structure, therefore all measurements are performed using TE-polarized light. A. SRR With Two Bus Waveguides and an Integrated SOA The photograph of a fabricated SRR with an integrated SOA and Pt-resistors is shown in Fig. 8. The measurement of the filter response of an SRR with two input/output waveguides, a radius of 780 m, an SOA length of 800 m, and coupler lengths of 225 m(gap m) is shown in Fig. 9. The SOA was operated at a current of 100 ma. The achieved ON OFF ratio is more than 20 db. The coupling factors have been determined from the simulation to be. The free spectral range (FSR) is 12.5 GHz as designed. The FWHM is determined to be nm, leading to a factor for this device of. The shape factor of the drop port is Fig. 9. SRR with two straight input/output waveguides and codirectional couplers (length = 225 m, gap = 0:9 m), R = 780 m, SOA length = 800 m, and FSR = 12:5 GHz. The sidelobes are caused by parasitic multiresonances. approximately The finesse for a lossless SRR with input and output waveguides and symmetrical couplers is given by where and. The finesse of approximately 15 of the ring resonator was derived by parameter extraction from the experimental filter response. The ring resonator is lossless, which can be seen from the response of the throughput port. Here the insertion loss at resonance is as high as the insertion loss measured from the throughput port off resonance. SRRs with integrated SOAs with an FSR of 25 and 50 GHz are demonstrated in [19]. B. DRR With Two Bus Waveguides and Integrated SOAs The photograph of a DRR with integrated SOAs is shown in Fig. 10. The DRR is made up of two straight input/output waveguides, two 3-dB MMI couplers for the outer couplers (length m, width m), and a codirectional coupler in the center (length m, gap m), m, SOA length m which results in an FSR GHz. The measured response from the drop port is shown in Fig. 11. The driving current for each SOA is 50 ma. The shape factor (6)

5 RABUS et al.: OPTICAL FILTERS BASED ON RING RESONATORS WITH INTEGRATED SOAs IN GaInAsP InP 1409 Fig. 10. Photograph of a DRR with integrated SOAs. Fig. 13. Measured response of the drop port of a serially coupled TRR with integrated SOAs, R = 325 m, an SOA length of 400 m, and an FSR of 25 GHz. Fig. 11. Drop-port response from a DRR with two straight input/output waveguides, two MMI couplers, and a codirectional coupler (length = 150 m, gap = 1 m),r = 347 m, SOA length = 500 m, and FSR = 25 GHz. Fig. 14. Measured response of the drop port of a serially coupled TRR with integrated SOAs, R = 133 m, an SOA length of 300 m, and an FSR of 50 GHz. Fig. 12. Photograph of a serially coupled TRR with integrated SOAs. achieved for this configuration is 0.5, which is the designed value. The FWHM is measured to be 0.04 nm, leading to a finesse of 5. C. TRR With Two Bus Waveguides and Integrated SOAs The serially coupled TRR (cf. Fig. 12) has a radius of 323 m. The length of each SOA is 400 m, the coupler length is 325 m, and the coupling gaps for the couplers are 0.8, 1, 1, and 0.8 m. The shape factor of the simulated drop-port filter response of the serially coupled TRR is calculated to be 0.6. The FSR achieved is 25 GHz. The driving current for each of the three SOAs is 50 ma. The filter characteristic of the drop port is shown in Fig. 13. The shape factor for the drop port is measured to be The insertion loss is as for the DRR, which is approximately 10 db. This is due to the higher number of couplers in the ring resonator configuration compared to an SRR. The FWHM is determined to be 0.06 nm, leading to a finesse of 3. The FSR is increased to 50 GHz in the following TRR. The measured drop-port response of a TRR with 3-dB MMI couplers for the outer couplers and a codirectional coupler in the center with a gap of 1 m is shown in Fig. 14. The driving current for each SOA is 30 ma. The transmission is normalized to the insertion loss. The shape factor for the drop port is determined to be 0.52, using the 5 db and 15 db bandwidths. The FWHM, which is nearly doubled compared to the previously described TRR, is measured to be 0.1 nm, leading to a finesse of 4. D. Outlook Concerning the device feature in the time domain, the filter s system performance depends on the cavity response time of the resonator and the switching speed of the SOA. The application of our devices in high-bit-rate systems is mainly limited by the time required to charge/deplete the ring cavity. The respective response time scales with the round-trip time and is increased

6 1410 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 6, NOVEMBER/DECEMBER 2002 by lowering the power coupling factor. So far, the response is characterized in the wavelength domain. System experiments including dispersion measurements will be performed in a following step. The implementation of SOAs in ring resonators opens the door to a wide range of active and passive devices. The FSR of the fabricated devices can be increased in multiple coupled ring resonators owing to the Vernier effect [21], [22]. V. CONCLUSION We have demonstrated optical multistage filters using loss-compensated ring resonator configurations with integrated SOAs, MMI couplers, and codirectional couplers for performing well-specified coupling conditions between the resonators. FSRs of 12.5, 25, and 50 GHz and ON OFF ratios for the drop port of 20 db for the serially coupled ring resonators have been achieved. Accurate variation of the interresonator coupling strength modified the filter passband shape and demonstrated the expected flattened response. Exemplary designs were given to achieve a specific shape factor for the drop port for the doubleand triple-ring resonator configurations using appropriate coupling factors. The GaInAsP material system facilitates the integration of active elements like SOAs which are essential for realizing this type of optical filter with the demonstrated filter response. The use of multiple coupled ring resonators provides additional design flexibility to achieve various filter responses. ACKNOWLEDGMENT The authors would like to thank R. Steingrüber and C. Weimann (mask fabrication), H. Schroeter-Janssen (SA-MOVPE), F. Reier and H. Barsch (epitaxy), I. Tiedke, Ch. Schultz, and B. Reinsperger (photolithography), and R. Schmidt and R. Tuerck (antireflection coating). D. G. Rabus would like to thank K. Petermann, H. G. Weber, and H. 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Lett., vol. 14, pp , June Dominik G. Rabus received the Dipl.-Ing. in electrical engineering from the University of Stuttgart, Stuttgart, Germany, in 1999 and Dr.-Ing. degree in computer sciences from the Technical University of Berlin, Berlin, Germany, in He joined Heinrich-Hertz-Institut für Nachrichtentechnik Berlin GmbH, Berlin, Germany, in 1999, where he started his thesis on ring resonators. He is currently engaged in research on photonic ICs based on InP. Michael Hamacher received the Dipl.Ing. degree from the Nat.-Techn. Akademie of Isny in He then joined Heinrich-Hertz-Institut für Nachrichtentechnik Berlin GmbH, Berlin, Germany, in Since then, he has been engaged in the development, fabrication, and characterization of optoelectronic/photonic integrated circuits on InP with different functionalities for future optical communication systems.

7 RABUS et al.: OPTICAL FILTERS BASED ON RING RESONATORS WITH INTEGRATED SOAs IN GaInAsP InP 1411 Ute Troppenz received the Diploma Degree in physics and the Ph.D. degree from Humboldt University, Berlin, Germany, in 1981 and 1985, respectively. After working at the Institutes of the Academy of Science, GDR, she has been with Heinrich-Hertz-Institut für Nachrichtentechnik Berlin GmbH, Berlin, Germany, since Formerly working on electroluminescent phosphors for display applications, her work is now focused on laser modeling and on the development of passive and active InP based devices for photonic ICs. Helmut Heidrich received the Dipl. Phys and Dr.-Ing. degree in physics from Technische Universität Berlin, Berlin, Germany, in 1973 and 1979, respectively. After three years work at Standard Electric Lorenz AG, Berlin, he joined Heinrich-Hertz-Institut für Nachrichtentechnik Berlin GmbH (HHI), Berlin, Germany, in 1982, where for the first five years he headed a group working in integrated optics LiNbO devices. For over one decade, he has been engaged in research on photonic ICs (PICs) based on InP where he was and is heading several national and European projects within HHI on the realization of complex PICs, e.g., heterodyne receivers, optical sweepers, bi-directional WDM-transceivers, and ps-source optoelectronic ICs. He is the author and coauthor of more than 100 publications.