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Switchable dual-polarization external cavity tunable laser Tim Mueller, Alejandro Maese-Novo, Ziyang Zhang,* Andrzej Polatynski, David Felipe, Moritz Kleinert, Walter Brinker, Crispin Zawadzki, and Norbert Keil Fraunhofer Heinrich-Hertz Institute, Einsteinufer 37, Berlin 10587, Germany *Corresponding author: [email protected] Received November 17, 2014; accepted December 16, 2014; posted December 23, 2014 (Doc. ID 226919); published February 2, 2015 An external cavity laser is demonstrated based on the hybrid integration of an InP-based gain element, a half-wave plate, and thermally drivable polymer waveguide circuits. The laser has one oscillation region but two outputs for TE and TM emissions. The central wavelength can be tuned 20 nm at 20 mW heater electrical power. The TM path undergoes a 1.4 dB power penalty due to the presence of the half-wave plate. However, the on-chip thermo-optic switch (TOS) can compensate for this imbalance and steer the laser into an equal TE and TM output power. The TOS can also be adjusted to prefer one polarization path over the other with ∼10 dB extinction ratio. © 2015 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (310.5448) Polarization, other optical properties; (140.3600) Lasers, tunable; (130.5460) Polymer waveguides. http://dx.doi.org/10.1364/OL.40.000447
Highly functional, low-cost, and power-efficient tunable laser sources are highly desired in optical communication networks. They can be used to construct a colorless optical network unit in the wavelength-divisionmultiplexing passive optical networks [1,2], or function as local oscillators in coherent detection schemes [3]. For a higher spectral efficiency, polarization multiplexed transmission methods are adopted, such as dualpolarization quadrature phase-shift keying and dualpolarization 16-quadrature amplitude modulation. A tunable laser source with polarization diversity is then sought after to mix signals of each polarization. Furthermore, to be able to adaptively handle the potential swing of the data capacity in the network, the optical transceivers need to provide certain flexibility and allow functionality switching at multilevel complexities. The last decade has witnessed the fast development of polymer-based photonic devices and the associated hybrid approach for photonic integration [4,5]. Components with excellent performance have been demonstrated, including external cavity tunable lasers [6,7], thermo-optic switches (TOSs) [4,8,9], and variable optical attenuators (VOAs) [10,11], among others. Though it is possible to fabricate lasers with desired polarization (TE or TM) on the InP monolithic platform [12], it may prove more straightforward and cost-effective to manipulate the polarization of light on the polymer platform by using vertically integrated thin-film elements [5,6,13]. In this Letter, an external cavity laser with switchable dual-polarization emission is designed, assembled, and characterized on the polymer platform. The layout of the device is sketched in Fig. 1. A TE-emitting, InP-based multiquantum well gain chip (GC) is butt-coupled to the polymer board. The left facet of the GC is high-reflection coated against air and the right facet is antireflection coated against polymer. The thermally tunable polymer Bragg grating functions as the front reflector. The GC and the polymer Bragg grating comprise the laser oscillator region. A 1 mm long waveguide buffer is reserved to prevent any optical feedback from the TOS back into the oscillator for a stable single-mode operation. The laser 0146-9592/15/040447-04$15.00/0
emission is then split into two outputs: the lower path remains at the TE polarization, while the upper path goes through a half-wave (λ∕2) plate and gets the polarization state rotated to TM emission. The TOS can steer the paths for either a balanced output or a preferred output at one polarization. Though there has been much research activity in polymer Y -branch-based TOSs [4,8,9], two issues need to be addressed carefully before they can be integrated into this application. (1) The refractive index contrast in most published works is below 1%. With this weak index contrast, it is difficult to realize compact devices due to the large bending radius and long Bragg grating sections. (2) The Y -branch angle is kept very small (≪1°) to ensure a slow adiabatic transformation of the eigenmode in the asymmetric (switched) Y -junction, which results in a TOS length of a few millimeters at least. A compact TOS design is needed to bring down the footprint of the fully integrated device. The polymer waveguide in this Letter features a square buried core of 3.2 μm × 3.2 μm. The index contrast is raised to 2%, with polymer cladding refractive index of 1.45 and core index of 1.48. The heater electrode is buried ∼2.5 μm below the waveguide, and deep air trenches are added to confine the thermal power [14]. This layout, though not the most efficient in generating a temperature gradient to drive the TOS [15,16], proves
Fig. 1. Schematic layout of the polarization switchable external cavity tunable laser. © 2015 Optical Society of America
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to be beneficial for the long-term operation of the tunable Bragg grating against heater electrical breakdowns [6,14]. The Y -branch angle is increased to 1.2° and the TOS length is limited to ∼400 μm. The compromise to the TOS performance is made mainly in terms of the optical loss and extinction ratio (ER) of the switched output powers, to ensure a relatively compact device. The compromised TOS is investigated in the following way. The steady-state temperature distribution of the proposed structure is first simulated in COMSOL. Figure 2(a) shows an example of such distribution at an electrode power of 15 mW. The induced temperature change with respect to the ambient temperature (20°C) is then translated into refractive index variations considering the polymer thermo-optic coefficient of −1.14 × 10−4 K−1 . Finally, the index distribution is implemented into the transmission simulation in R-Soft (BeamPROP) to evaluate the performance of the TOS. Figure 2(b) shows the optical power variation at the Y branch outputs with respect to different heater electrical powers. At 30 mW heater power, the TOS appears to have 0.5 dB optical loss and the ER reaches ∼10 dB. The light intensity profile in the Y branch is displayed in the inset of Fig. 2(b). The TOS can be further driven beyond 30 mW for a lower loss and higher ER, but the local temperature at the waveguide will increase beyond 300°C, the certified degradation temperature of the chosen material (ZPU series from ChemOptics). The polymer cladding is spin-coated and cured on a standard 4-inch silicon wafer to form a thick layer of ∼46 μm. The heater electrodes, consisting of 20 nm Ti
and 100 nm Au, are structured by a standard liftoff process. A second cladding layer of 2.5 μm is cured before the core layer is added. The double-side corrugated polymer waveguide Bragg grating is raised to the third order of around 1.62 μm, allowing a standard mask aligner (at 320 nm) to be used for the photolithography process. A thin Ti metal of 100 nm is evaporated on the top cladding and structured as the hard mask. The air trenches and the slot to hold the λ∕2 plate are then etched by oxygen plasma for a total depth of 45 μm, i.e., 38 μm below the waveguide. The Ti mask is then removed and finally a galvanic process is performed to grow ∼3 μm Au on the opened contact pads to ease the wire-bonding process as well as to limit the unwanted heating at these regions. The λ∕2 plate is ∼13.5 μm thick. The etched slot has a width of ∼14 μm, allowing the λ∕2 plate to be inserted easily and passively without generating much excess losses. The λ∕2 plate is secured by UV-curable indexmatching epoxy. The GC is first placed on a submount and then actively coupled to the polymer waveguide under an automatic alignment setup. Both the optical power and the laser spectrum at the upper (TM) output are monitored during the coupling. Once a stable lasing point is reached, the facet is fixed by the index-matching and mechanically supporting epoxies. The assembled device is shown in Fig. 3. The P–I and U–I curves of the laser are plotted in Fig. 4. The two outputs are measured subsequently in
Fig. 3. Photo of the assembled laser. The GC and the λ∕2 plate are secured by epoxies on the polymer board.
Fig. 2. (a) Temperature distribution at the TOS cross-section, simulated in COMSOL. (b) TOS behavior at different heater powers, simulated in R-Soft. Inset: light intensity distribution at 30 mW heater power.
Fig. 4. P–I and U–I curves of the dual-polarization emitting laser. The common GC and the laser oscillation region account for the identical U–I characteristics and threshold current for both outputs. The power imbalance between the TM and TE paths is caused by the λ∕2 plate.
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a standard single-mode fiber. The threshold current is ∼5 mA. At ∼32 and 62 mA, the longitudinal mode hopping disturbs the P–I curve, but this can be avoided by adjusting the additional phase electrode in front of the polymer Bragg grating section. The λ∕2 plate on the TM path introduces ∼1.4 dB extra loss, as can be seen in the output power imbalance between the TM and TE P–I curves. The polymer Bragg grating electrode is biased and the heater power is scanned to obtain the tuning characteristics of the laser. The phase electrode is left unbiased. The GC current is fixed at 25 mA. The overlaid spectra are displayed in Fig. 5(a) for the TE polarization and Fig. 5(b) for the TM polarization. Apart from the power imbalance, the two outputs appear to have the same tuning behavior. When the heater electrode power increases to 20 mW, the wavelength shifts continuously from 1561 to 1541 nm, i.e., 20 nm wavelength tuning range is achieved, amounting to a tuning efficiency of 1 nm/mW. The side mode suppression ratio is larger than 35 dB in all the wavelengths obtained and can be further enhanced to above 40 dB by adjusting the phase electrode to avoid the mode-hopping positions. The TOS is tested to verify the switching performance from the design. The lower electrode is biased to direct the light to the TM path. The results are displayed in Fig. 6(a). At ∼5 mW electrical heater power, the TM and TE paths are adjusted to have the same output power. Further on at 31 mW, the TM output power increases from −3.2 to −0.7 dBm while the TE output power
decreases from −1.8 to −8.8 dBm. An optical loss of ∼0.5 dB and an extinction ratio of 9.5 dB of the TOS are obtained. When the upper electrode is biased, the TE power increases from −1.8 to 0.7 dBm and the TM output is suppressed from −3.2 to −12.2 dBm, as shown in Fig. 6(b). The experimental results are in good agreement with the design. The switching speed is limited to a few tens of milliseconds, similar to the polymerbased thermo-optic components published previously [6–11,13–16]. To reduce the λ∕2 plate-induced scattering loss, waveguide tapers can be added at the interface to increase the mode field size and therefore limit the beam broadening in the slot region. Compact VOAs based on 1 × 1 MMIs can be inserted in both paths to further suppress the nonselected output power and add another 30 dB dynamic extinction ratio at the cost of ∼0.5 dB extra loss [16]. To conclude, a switchable dual-polarization external cavity laser has been demonstrated on the polymer-based hybrid photonic integration platform. The laser can be tuned 20 nm and the on-chip TOS can steer the laser into balanced TE/TM output power or select one polarization path with ∼10 dB suppression over the other. This Letter may trigger novel polarization-diversified laser designs on other photonic integration platforms as well as
Fig. 5. Overlaid laser spectra during tuning: (a) TE path, (b) TM path.
Fig. 6. TOS performance: (a) lower electrode biased to prefer TM output and (b) upper electrode biased to prefer TE output.
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advanced designs of low-cost local oscillators for the dual-polarization coherent receivers. This work has been mainly funded by the European Commission under EU FP-7 PANTHER (619411). The authors would like to thank Sven Mueller, Larissa Schmidt, and the HHI-TZL team for their assistance in the wafer fabrication. References 1. R. Urata, C. Lam, H. Liu, and C. Johnson, National Fiber Optic Engineers Conference, OSA Technical Digest (Optical Society of America, 2012), paper NTh3E.4. 2. Z. Zhang, D. Felipe, W. Brinker, M. Kleinert, A. Maese Novo, C. Zawadzki, M. Moehrle, and N. Keil, Proceedings of the European Conference on Optical Communication (IEEE, 2014), paper Mo.4.4.4. 3. P. Dong, X. Liu, S. Chandrasekhar, L. Buhl, R. Aroca, and Y. Chen, IEEE J. Sel. Top. Quantum Electron. 20, C1 (2014). 4. Z. Zhang, N. Mettbach, C. Zawadzki, J. Wang, D. Schmidt, W. Brinker, N. Grote, M. Schell, and N. Keil, IET Optoelectron. 5, 226 (2011). 5. D. Felipe, Z. Zhang, W. Brinker, M. Kleinert, A. Maese-Novo, C. Zawadzki, M. Moehrle, and N. Keil, IEEE Photon. Technol. Lett. 26, 1391 (2014).
6. K. H. Yoon, B. S. Choi, O. K. Kwon, S. H. Oh, K. S. Kim, D. M. Kang, Y. T. Han, and H. K. Lee, IEEE Photon. Technol. Lett. 26, 47 (2014). 7. N. Keil, H. Yao, and C. Zawadzki, Electron. Lett. 32, 1470 (1996). 8. M. H. Lee, Y. H. Min, S. Park, J. J. Ju, J. Y. Do, and S. K. Park, IEEE Photon. Technol. Lett. 14, 615 (2002). 9. H. Ma, A. K.-Y. Jen, and L. R. Dalton, Adv. Mater. 14, 1339 (2002). 10. Y. O. Noh, M. S. Yang, Y. H. Won, and W. Y. Hwang, Electron. Lett. 36, 2032 (2000). 11. X. Jiang, X. Li, H. Zhou, J. Yang, M. Wang, Y. Wu, and S. Ishikawa, IEEE Photon. Technol. Lett. 17, 2361 (2005). 12. S. Nicholes, J. Raring, M. Dummer, A. Tauke-Pedretti, and L. Coldren, IEEE Photon. Technol. Lett. 19, 771 (2007). 13. J. W. Kim, S. H. Park, W. S. Chu, and M. C. Oh, Opt. Express 20, 12443 (2012). 14. A. Liu, Z. Zhang, D. Felipe, N. Keil, and N. Grote, IEEE Photon. Technol. Lett. 26, 313 (2014). 15. Z. Zhang, A. Maese Novo, E. Schwartz, C. Zawadzki, and N. Keil, Opt. Lett. 39, 5170 (2014). 16. A. Maese-Novo, Z. Zhang, G. Irmscher, A. Polatynski, T. Mueller, D. de Felipe, M. Kleinert, W. Brinker, C. Zawadzki, and N. Keil, Appl. Opt. 54, 569 (2014).
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Switchable dual-polarization external cavity tunable laser Tim Mueller, Alejandro Maese-Novo, Ziyang Zhang,* Andrzej Polatynski, David Felipe, Moritz Kleinert, Walter Brinker, Crispin Zawadzki, and Norbert Keil Fraunhofer Heinrich-Hertz Institute, Einsteinufer 37, Berlin 10587, Germany *Corresponding author: [email protected] Received November 17, 2014; accepted December 16, 2014; posted December 23, 2014 (Doc. ID 226919); published February 2, 2015 An external cavity laser is demonstrated based on the hybrid integration of an InP-based gain element, a half-wave plate, and thermally drivable polymer waveguide circuits. The laser has one oscillation region but two outputs for TE and TM emissions. The central wavelength can be tuned 20 nm at 20 mW heater electrical power. The TM path undergoes a 1.4 dB power penalty due to the presence of the half-wave plate. However, the on-chip thermo-optic switch (TOS) can compensate for this imbalance and steer the laser into an equal TE and TM output power. The TOS can also be adjusted to prefer one polarization path over the other with ∼10 dB extinction ratio. © 2015 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (310.5448) Polarization, other optical properties; (140.3600) Lasers, tunable; (130.5460) Polymer waveguides. http://dx.doi.org/10.1364/OL.40.000447
Highly functional, low-cost, and power-efficient tunable laser sources are highly desired in optical communication networks. They can be used to construct a colorless optical network unit in the wavelength-divisionmultiplexing passive optical networks [1,2], or function as local oscillators in coherent detection schemes [3]. For a higher spectral efficiency, polarization multiplexed transmission methods are adopted, such as dualpolarization quadrature phase-shift keying and dualpolarization 16-quadrature amplitude modulation. A tunable laser source with polarization diversity is then sought after to mix signals of each polarization. Furthermore, to be able to adaptively handle the potential swing of the data capacity in the network, the optical transceivers need to provide certain flexibility and allow functionality switching at multilevel complexities. The last decade has witnessed the fast development of polymer-based photonic devices and the associated hybrid approach for photonic integration [4,5]. Components with excellent performance have been demonstrated, including external cavity tunable lasers [6,7], thermo-optic switches (TOSs) [4,8,9], and variable optical attenuators (VOAs) [10,11], among others. Though it is possible to fabricate lasers with desired polarization (TE or TM) on the InP monolithic platform [12], it may prove more straightforward and cost-effective to manipulate the polarization of light on the polymer platform by using vertically integrated thin-film elements [5,6,13]. In this Letter, an external cavity laser with switchable dual-polarization emission is designed, assembled, and characterized on the polymer platform. The layout of the device is sketched in Fig. 1. A TE-emitting, InP-based multiquantum well gain chip (GC) is butt-coupled to the polymer board. The left facet of the GC is high-reflection coated against air and the right facet is antireflection coated against polymer. The thermally tunable polymer Bragg grating functions as the front reflector. The GC and the polymer Bragg grating comprise the laser oscillator region. A 1 mm long waveguide buffer is reserved to prevent any optical feedback from the TOS back into the oscillator for a stable single-mode operation. The laser 0146-9592/15/040447-04$15.00/0
emission is then split into two outputs: the lower path remains at the TE polarization, while the upper path goes through a half-wave (λ∕2) plate and gets the polarization state rotated to TM emission. The TOS can steer the paths for either a balanced output or a preferred output at one polarization. Though there has been much research activity in polymer Y -branch-based TOSs [4,8,9], two issues need to be addressed carefully before they can be integrated into this application. (1) The refractive index contrast in most published works is below 1%. With this weak index contrast, it is difficult to realize compact devices due to the large bending radius and long Bragg grating sections. (2) The Y -branch angle is kept very small (≪1°) to ensure a slow adiabatic transformation of the eigenmode in the asymmetric (switched) Y -junction, which results in a TOS length of a few millimeters at least. A compact TOS design is needed to bring down the footprint of the fully integrated device. The polymer waveguide in this Letter features a square buried core of 3.2 μm × 3.2 μm. The index contrast is raised to 2%, with polymer cladding refractive index of 1.45 and core index of 1.48. The heater electrode is buried ∼2.5 μm below the waveguide, and deep air trenches are added to confine the thermal power [14]. This layout, though not the most efficient in generating a temperature gradient to drive the TOS [15,16], proves
Fig. 1. Schematic layout of the polarization switchable external cavity tunable laser. © 2015 Optical Society of America
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to be beneficial for the long-term operation of the tunable Bragg grating against heater electrical breakdowns [6,14]. The Y -branch angle is increased to 1.2° and the TOS length is limited to ∼400 μm. The compromise to the TOS performance is made mainly in terms of the optical loss and extinction ratio (ER) of the switched output powers, to ensure a relatively compact device. The compromised TOS is investigated in the following way. The steady-state temperature distribution of the proposed structure is first simulated in COMSOL. Figure 2(a) shows an example of such distribution at an electrode power of 15 mW. The induced temperature change with respect to the ambient temperature (20°C) is then translated into refractive index variations considering the polymer thermo-optic coefficient of −1.14 × 10−4 K−1 . Finally, the index distribution is implemented into the transmission simulation in R-Soft (BeamPROP) to evaluate the performance of the TOS. Figure 2(b) shows the optical power variation at the Y branch outputs with respect to different heater electrical powers. At 30 mW heater power, the TOS appears to have 0.5 dB optical loss and the ER reaches ∼10 dB. The light intensity profile in the Y branch is displayed in the inset of Fig. 2(b). The TOS can be further driven beyond 30 mW for a lower loss and higher ER, but the local temperature at the waveguide will increase beyond 300°C, the certified degradation temperature of the chosen material (ZPU series from ChemOptics). The polymer cladding is spin-coated and cured on a standard 4-inch silicon wafer to form a thick layer of ∼46 μm. The heater electrodes, consisting of 20 nm Ti
and 100 nm Au, are structured by a standard liftoff process. A second cladding layer of 2.5 μm is cured before the core layer is added. The double-side corrugated polymer waveguide Bragg grating is raised to the third order of around 1.62 μm, allowing a standard mask aligner (at 320 nm) to be used for the photolithography process. A thin Ti metal of 100 nm is evaporated on the top cladding and structured as the hard mask. The air trenches and the slot to hold the λ∕2 plate are then etched by oxygen plasma for a total depth of 45 μm, i.e., 38 μm below the waveguide. The Ti mask is then removed and finally a galvanic process is performed to grow ∼3 μm Au on the opened contact pads to ease the wire-bonding process as well as to limit the unwanted heating at these regions. The λ∕2 plate is ∼13.5 μm thick. The etched slot has a width of ∼14 μm, allowing the λ∕2 plate to be inserted easily and passively without generating much excess losses. The λ∕2 plate is secured by UV-curable indexmatching epoxy. The GC is first placed on a submount and then actively coupled to the polymer waveguide under an automatic alignment setup. Both the optical power and the laser spectrum at the upper (TM) output are monitored during the coupling. Once a stable lasing point is reached, the facet is fixed by the index-matching and mechanically supporting epoxies. The assembled device is shown in Fig. 3. The P–I and U–I curves of the laser are plotted in Fig. 4. The two outputs are measured subsequently in
Fig. 3. Photo of the assembled laser. The GC and the λ∕2 plate are secured by epoxies on the polymer board.
Fig. 2. (a) Temperature distribution at the TOS cross-section, simulated in COMSOL. (b) TOS behavior at different heater powers, simulated in R-Soft. Inset: light intensity distribution at 30 mW heater power.
Fig. 4. P–I and U–I curves of the dual-polarization emitting laser. The common GC and the laser oscillation region account for the identical U–I characteristics and threshold current for both outputs. The power imbalance between the TM and TE paths is caused by the λ∕2 plate.
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a standard single-mode fiber. The threshold current is ∼5 mA. At ∼32 and 62 mA, the longitudinal mode hopping disturbs the P–I curve, but this can be avoided by adjusting the additional phase electrode in front of the polymer Bragg grating section. The λ∕2 plate on the TM path introduces ∼1.4 dB extra loss, as can be seen in the output power imbalance between the TM and TE P–I curves. The polymer Bragg grating electrode is biased and the heater power is scanned to obtain the tuning characteristics of the laser. The phase electrode is left unbiased. The GC current is fixed at 25 mA. The overlaid spectra are displayed in Fig. 5(a) for the TE polarization and Fig. 5(b) for the TM polarization. Apart from the power imbalance, the two outputs appear to have the same tuning behavior. When the heater electrode power increases to 20 mW, the wavelength shifts continuously from 1561 to 1541 nm, i.e., 20 nm wavelength tuning range is achieved, amounting to a tuning efficiency of 1 nm/mW. The side mode suppression ratio is larger than 35 dB in all the wavelengths obtained and can be further enhanced to above 40 dB by adjusting the phase electrode to avoid the mode-hopping positions. The TOS is tested to verify the switching performance from the design. The lower electrode is biased to direct the light to the TM path. The results are displayed in Fig. 6(a). At ∼5 mW electrical heater power, the TM and TE paths are adjusted to have the same output power. Further on at 31 mW, the TM output power increases from −3.2 to −0.7 dBm while the TE output power
decreases from −1.8 to −8.8 dBm. An optical loss of ∼0.5 dB and an extinction ratio of 9.5 dB of the TOS are obtained. When the upper electrode is biased, the TE power increases from −1.8 to 0.7 dBm and the TM output is suppressed from −3.2 to −12.2 dBm, as shown in Fig. 6(b). The experimental results are in good agreement with the design. The switching speed is limited to a few tens of milliseconds, similar to the polymerbased thermo-optic components published previously [6–11,13–16]. To reduce the λ∕2 plate-induced scattering loss, waveguide tapers can be added at the interface to increase the mode field size and therefore limit the beam broadening in the slot region. Compact VOAs based on 1 × 1 MMIs can be inserted in both paths to further suppress the nonselected output power and add another 30 dB dynamic extinction ratio at the cost of ∼0.5 dB extra loss [16]. To conclude, a switchable dual-polarization external cavity laser has been demonstrated on the polymer-based hybrid photonic integration platform. The laser can be tuned 20 nm and the on-chip TOS can steer the laser into balanced TE/TM output power or select one polarization path with ∼10 dB suppression over the other. This Letter may trigger novel polarization-diversified laser designs on other photonic integration platforms as well as
Fig. 5. Overlaid laser spectra during tuning: (a) TE path, (b) TM path.
Fig. 6. TOS performance: (a) lower electrode biased to prefer TM output and (b) upper electrode biased to prefer TE output.
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advanced designs of low-cost local oscillators for the dual-polarization coherent receivers. This work has been mainly funded by the European Commission under EU FP-7 PANTHER (619411). The authors would like to thank Sven Mueller, Larissa Schmidt, and the HHI-TZL team for their assistance in the wafer fabrication. References 1. R. Urata, C. Lam, H. Liu, and C. Johnson, National Fiber Optic Engineers Conference, OSA Technical Digest (Optical Society of America, 2012), paper NTh3E.4. 2. Z. Zhang, D. Felipe, W. Brinker, M. Kleinert, A. Maese Novo, C. Zawadzki, M. Moehrle, and N. Keil, Proceedings of the European Conference on Optical Communication (IEEE, 2014), paper Mo.4.4.4. 3. P. Dong, X. Liu, S. Chandrasekhar, L. Buhl, R. Aroca, and Y. Chen, IEEE J. Sel. Top. Quantum Electron. 20, C1 (2014). 4. Z. Zhang, N. Mettbach, C. Zawadzki, J. Wang, D. Schmidt, W. Brinker, N. Grote, M. Schell, and N. Keil, IET Optoelectron. 5, 226 (2011). 5. D. Felipe, Z. Zhang, W. Brinker, M. Kleinert, A. Maese-Novo, C. Zawadzki, M. Moehrle, and N. Keil, IEEE Photon. Technol. Lett. 26, 1391 (2014).
6. K. H. Yoon, B. S. Choi, O. K. Kwon, S. H. Oh, K. S. Kim, D. M. Kang, Y. T. Han, and H. K. Lee, IEEE Photon. Technol. Lett. 26, 47 (2014). 7. N. Keil, H. Yao, and C. Zawadzki, Electron. Lett. 32, 1470 (1996). 8. M. H. Lee, Y. H. Min, S. Park, J. J. Ju, J. Y. Do, and S. K. Park, IEEE Photon. Technol. Lett. 14, 615 (2002). 9. H. Ma, A. K.-Y. Jen, and L. R. Dalton, Adv. Mater. 14, 1339 (2002). 10. Y. O. Noh, M. S. Yang, Y. H. Won, and W. Y. Hwang, Electron. Lett. 36, 2032 (2000). 11. X. Jiang, X. Li, H. Zhou, J. Yang, M. Wang, Y. Wu, and S. Ishikawa, IEEE Photon. Technol. Lett. 17, 2361 (2005). 12. S. Nicholes, J. Raring, M. Dummer, A. Tauke-Pedretti, and L. Coldren, IEEE Photon. Technol. Lett. 19, 771 (2007). 13. J. W. Kim, S. H. Park, W. S. Chu, and M. C. Oh, Opt. Express 20, 12443 (2012). 14. A. Liu, Z. Zhang, D. Felipe, N. Keil, and N. Grote, IEEE Photon. Technol. Lett. 26, 313 (2014). 15. Z. Zhang, A. Maese Novo, E. Schwartz, C. Zawadzki, and N. Keil, Opt. Lett. 39, 5170 (2014). 16. A. Maese-Novo, Z. Zhang, G. Irmscher, A. Polatynski, T. Mueller, D. de Felipe, M. Kleinert, W. Brinker, C. Zawadzki, and N. Keil, Appl. Opt. 54, 569 (2014).
