Optical and Quantum Electronics 34: 1137–1144, 2002. Ó 2002 Kluwer Academic Publishers. Printed in the Netherlands.
Single mode lasers based on monolithic integration of ridge waveguides with 2D photonic crystal waveguides T.D. HAPP*, M. KAMP, F. KLOPF AND A. FORCHEL Technische Physik, University of Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany (*author for correspondence: E-mail: [email protected]) Received 5 April 2001; accepted 7 May 2002 Abstract. The monolithic combination of active light sources with photonic crystal (PC) waveguide components is a key building block for future highly integrated photonic circuits. We demonstrate the coupling of light from an InGaAs/AlGaAs ridge waveguide laser to a monolithically integrated 2D PC waveguide. The PC guide is formed by removing three or ﬁve rows in a triangular lattice of air rods etched into the semiconductor. A tapered ridge waveguide geometry is demonstrated to improve coupling eﬃciency, so that maximum output powers of up to 10 mW from the PC waveguide are achieved. The resulting coupled cavity laser shows single mode emission with side mode suppression ratios >35 dB over a broad range of injection currents. Key words: integrated optics, monomode lasers, photonic crystal waveguides
1. Introduction After the pioneering work addressing the suppression of spontaneous emission (Yablonovitch 1987) and localization of light in solids (John 1987), the control of light propagation with photonic crystals (PCs) has drawn ever increasing attention. In particular PC based waveguides show interesting properties for a tight light control: It has been theoretically predicted and experimentally shown that electromagnetic waves can be eﬃciently guided even around sharp bends (Joannopoulos et al. 1995; Mekis et al. 1996; Lin et al. 1998; Chutinan and Noda 2000). This makes them an attractive component for highly integrated photonic circuits. From the fabrication viewpoint, especially the 2D variant combined with an index waveguide providing light conﬁnement in the third dimension (Labilloy et al. 1997) is a promising approach: Their compatibility with standard planar semiconductor processing technology and the possibility for an integration with active optoelectronic devices such as semiconductor lasers make possible a future use in complex integrated photonic devices. One of the crucial building blocks for
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such PC based integrated photonic circuits is the coupling of electrically pumped light sources with a PC network. As a ﬁrst step, we demonstrate in this work the monolithic integration of a ridge waveguide with a straight 2D PC waveguide in a combined laser cavity. The ridge waveguide section provides the gain necessary for lasing, while the PC waveguide is a passive section. In order to improve the coupling eﬃciency between the diﬀerent waveguide types, a tapered section of the ridge waveguide is introduced (Xu et al. 2000). The performance of these devices is analyzed and compared to devices without taper. In addition, a closer analysis of the spectral properties of these two-section coupled cavity lasers is presented. The concept is demonstrated for an InGaAs/AlGaAs-based GRINSCH quantum dot laser emitting at wavelengths around k ¼ 935 nm, but could be easily ported to diﬀerent material systems and wavelengths.
2. Design and fabrication The laser cavity consists of two coupled sections, a 710–740 lm long classical ridge waveguide which is electrically pumped and an unpumped, 10–40 lm long straight PC waveguide section, as can be seen from Fig. 1. The PC
Fig. 1. Schematics of the two-segment lasers with an SEM micrograph showing the end of an untapered ridge waveguide butt-coupled to the W3 PC waveguide.
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waveguide is formed by removing three (W3) or ﬁve (W5) rows along the CKdirection of a triangular lattice of air cylinders. The air ﬁll factor was chosen to be f 30%, yielding a photonic TE band gap for normalized frequencies of a=kvacuum ¼ 0:21–0:28 (Joannopoulos et al. 1995). The PC lattice constant was chosen to a ¼ 250 nm, so that the laser wavelength of k ¼ 935 nm lies well within the band gap. As there exists a signiﬁcant mode mismatch between the 2 lm wide ridge waveguide and the 0.650–1.05 lm wide PC waveguide, tapering of the ridge waveguide was investigated. The shallow etched ridge waveguide only provides weak lateral conﬁnement, therefore it is not possible to completely narrow it down to the PC waveguide width: After a certain minimum width, further width reduction does not reduce the lateral mode extension but rather squeezes the mode from the horizontal index waveguide into the substrate. Using a 2D ﬁnite element simulation (Coldren and Corzine 1995), the mode distribution in the ridge waveguide was modeled and the optimum taper end width determined to 1.4 lm, resulting in a minimum lateral mode width (FWHM of intensity) of 1.8 lm. The length of the taper section is 200 lm. The devices are fabricated on an InGaAs/AlGaAs GRINSCH laser structure emitting at 935 nm in TE-polarization (~ Ek GRINSCH). The 920 nm thick lower Al0.63Ga0.37As cladding is followed by the 420 nm Al0.15–0.3Ga0.85–0.7As GRINSCH waveguide containing the InGaAs quantum dots, followed by the 820 nm thick upper Al0.63Ga0.37As cladding terminated by a 100 nm thick, highly p-doped GaAs contact layer. A more detailed description of the concept of such laser structures can be found in (Klopf et al. 2000). The fabrication is done in two steps: First, the ridge waveguides pattern is lithographically deﬁned and etched down to a distance of 60 nm from the GRINSCH waveguide, using a Cl2/Ar electron cyclotron resonance reactive ion etch step (ECR-RIE) in conjunction with an electron beam evaporated BaF2/Ni etch mask. In a second step, the PC is fabricated: First a 150 nm thick SiO2 layer is sputtered on the sample, providing a sturdy high resolution etch mask required for the PC etch. The triangular PC pattern with a ¼ 250 nm lattice constant is deﬁned using 100 kV high resolution electron beam lithography in a 500 nm thick spin-coated polymethylmethacrylate (PMMA) resist. The required high alignment accuracy which can be seen in the top-view SEM micrograph in Fig. 1 is achieved using an interferometrically controlled positioning system. After development in 1:3 methylisobutylketone/propanol, the PC pattern is transferred into the SiO2 layer using a CHF3/Ar RIE step. The PC is ﬁnally etched into the semiconductor using a low-pressure Cl2/Ar ECR–RIE process, yielding 700 nm deep holes with vertical sidewalls. Here the penetration of the lower cladding layer is crucial to minimize scattering losses of the guided wave in the PC (D’Urso et al. 1998). After mask removal, the sample is planarized with benzocyc-
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lobutene (BCB) followed by contact metallization and cleaving to the bar level for characterization.
3. Results and discussion For an assessment of the performance of the diﬀerent devices, cw power characteristics have been obtained from the PC waveguide facet of the lasers. The results are plotted in Fig. 2 for devices with 10 lm long PC waveguides: For the devices with untapered ridge waveguides (dotted lines), slope eﬃciencies of 0.05 W/A in the case of the W3 waveguide and 0.09 W/A for the W5 are observed. The maximum power levels obtained for these devices in the investigated current regime up to 100 mA exceed 3 mW for the W3 and 6 mW for the W5 waveguide. The threshold currents of the devices are all around 17 mA. This indicates rather small overall losses, so that the lasers operate in the region of steep increase of the logarithmic gain – current dependence (Coldren et al. 1995). For the devices with ridge waveguide tapers, an increase in coupling eﬃciency can be observed (solid lines): The slope eﬃciencies are increased to 0.08 and 0.12 W/A for the W3 and W5 PC waveguide, respectively, resulting in output powers of up to 5 and 10 mW. The deviation of the W3 power characteristics from a straight line for high injection currents indicates changes in the longitudinal mode selection and therefore a redistribution in the ratio of the output power from the two diﬀerent facets. From the cleaved facet on the ridge waveguide side slope
Fig. 2. Power characteristics obtained from the PC waveguide side under cw operation for lasers with tapered (solid)/untapered (dotted) ridge waveguide and 10 lm long W3/W5 PC waveguides, respectively.
MONOLITHIC INTEGRATION OF RIDGE WAVEGUIDES
eﬃciencies about 0.24 W/A are obtained, with no clear dependency on the PC waveguide width. The maximum output powers here are 18 mW for pump currents up to 100 mA. For lasers with 40 lm long PC waveguide sections it is still possible to achieve output powers of more than 2 mW for the W3 and more than 5 mW for the W5 PC waveguide, despite of the expected higher absorption in the unpumped PC area. Assuming a rather high value of 100 cm)1 for this reabsorption, one would expect a decrease by 10% for 10 lm and by 33% for 40 lm, respectively. Of course it should be easily possible to increase the output power from the PC waveguide by applying a high reﬂectivity coating to the ridge waveguide cleaved factet. Due to their better performance, we will focus on the devices with tapered ridge waveguide for the further discussion. A typical spectrum of a laser with 40 lm long W5 PC waveguide can be seen in Fig. 3. The emission is single mode with a side mode suppression ratio (SMSR) of more than 40 dB. This monomode emission is stable over rea-
Fig. 3. Spectrum of a laser with a 40 lm long W5 PC waveguide biased at 75 mA (cw), the SMSR is 41 dB.
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sonable current ranges, starting with 30 dB SMSR closely above threshold which increases to values over 40 dB at injection currents >60 mA. At higher currents exceeding 80 mA, the gain peak is shifted to larger wavelengths due to thermal eﬀects and multimode emission at diﬀerent wavelengths occurs. The temperature dependence is somewhat similar: From room temperature up to 50 °C the single mode emission shifts due to the changing refractive index by 0.62 A˚/K to larger wavelengths, until the faster shifting gain peak causes a mode jump. Particularly interesting in the spectrum is the obvious strong modulation of the sidemodes, from which it is possible to extract information about the feedback mechanism in the laser. As the spectral width of the quantum dot gain region only probes a tiny fraction of the broad photonic band gap, the transmission of the PC waveguides can be assumed to be constant. In order to clarify the longitudinal mode selection mechanism, a Fourier transform reﬂectometry analysis of a subthreshold spectrum as described in Ackermann et al. (1998) was performed, which can be seen in Fig. 4. The Fourier transformed spectrum, which is dispersion corrected and calibrated in optical cavity length, shows principal cavity lengths of 39, 710 and 749 lm. Given the geometrical distance between the two cleaved facets of 750 lm and the designed length of the PC waveguide of 40 lm, the interpretation of these
Fig. 4. Fourier transform analysis of the position of reﬂective planes in the laser cavity of a subthreshold spectrum (inset) of a laser with a 40 lm long W5 PC waveguide: the strong modulation in the spectrum is due to the two coupled laser segments.
MONOLITHIC INTEGRATION OF RIDGE WAVEGUIDES
Table 1. Averaged modal reﬂectivity at the entry of the PC waveguide as obtained from an eﬀectivemirror model Avg. reﬂectivity at PCW entry W3 Taper + W3 W5 Taper + W5
29% 24% 12% 6%
results is straightforward: The main cavity in this laser is the ridge waveguide section between the cleaved facet and the entry of the PC waveguide with a length of 710 lm. The 39 lm long cavity is the PC waveguide itself, with the same rather strong reﬂection at the interface between ridge waveguide and PC waveguide. The second cleaved facet is also visible, but the magnitude of this feedback seems to be much smaller compared to the waveguide interface reﬂectivity. However, as the absorption in the PC waveguide is not taken into account in this analysis, the absolute values of the reﬂectivity cannot be reliably assessed with this technique. In order to achieve a more quantitative description of the reﬂectivity at the interface between the ridge waveguide and the PC waveguide, a diﬀerent analysis is necessary: Altogether, the diﬀerent sections in these lasers can be described as coupled cavites, which provide the longitudinal mode selection from the overlap of their diﬀerent Fabry–Perot mode combs. An established analysis of such coupled cavity lasers uses the eﬀective mirror model (Coldren et al. 1995), where the interface reﬂectivity can be extracted from the measured diﬀerential external quantum eﬃciencies from the two diﬀerent output facets. A summary of the results is presented in Table 1: As there is some scattering in the data for individual lasers, averaged values are presented. For the W3 PC waveguides, the interface reﬂectivity is of the order of 30%, which is slightly reduced with the use of the tapered ridge waveguide. For the W5 case, the reﬂectivity is signiﬁcantly smaller and also the improvement from the tapered ridge is clearly visible: the remaining interface reﬂectivity is only 6% for this case.
4. Conclusions We have successfully integrated a ridge waveguide with 2D triangular W3 and W5 PC based waveguides in an electrically pumped laser demonstrator. The coupled cavity lasers show single mode emission for cw output powers as high as 10 mW with SMSRs up to 41 dB. It is demonstrated that the coupling between ridge waveguide and PC waveguide can be signiﬁcantly improved by tapering the end of the ridge waveguide. A closer analysis using a Fourier transform reﬂectometry analysis and output power calculations in
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aneﬀective mirror model shows that the interface reﬂectivity is still of the order of 25% for the W3 waveguide, however, power levels coupled through the PC waveguide reach 5 mW for the W3 and 10 mW for the W5 in cw operation. As the concept of PC optics allows a very compact realization of various functions such as waveguide bends, combiners or ﬁlters, this indicates the possibility of a monolithic integration on a single epitaxy wafer together with the active devices with tolerable absorption losses.
Acknowledgements The authors would like to thank C. Kilian and M. Fischer for valuable technical assistance during sample fabrication. The ﬁnancial support from the EC research project IST-PCIC is gratefully acknowledged.
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