Kalosha V.P.,TU Berlin |
Bimberg D.,TU Berlin |
Bimberg D.,King Abdulaziz University |
Ledentsov N.N.,VI Systems
IEEE Journal of Quantum Electronics | Year: 2013
Cold-cavity 3-D transverse-vertical modes of 850 nm GaAs/AlGaAs vertical-cavity surface-emitting lasers with thick oxide aperture layers were simulated. Single fundamental mode operation at large aperture diameters is achieved by proper positioning of the layer with low Al content and thickness beyond a quarter-wavelength in the top distributed Bragg reflector above the aperture layers. This modification promotes the lateral leakage of the transverse higher-order modes along the oxidized layers and their discrimination due to increased gain thresholds at certain aperture diameters. At high leakage, even the fundamental mode exhibits confined spatially-modulated states due to coupling with the leakage field component. © 2013 IEEE.
Ledentsov N.N.,VI Systems
Semiconductor Science and Technology | Year: 2011
Discovery of self-organized epitaxial quantum dots (QDs) resulted in multiple breakthroughs in the field of physics of zero-dimensional heterostructures and allowed the advancement of optoelectronic devices, most remarkably, lasers. The most advanced and well-understood results are obtained for lasers based on Stranski-Krastanow InGaAs-GaAs three-dimensional QDs; even significant progress in the understanding of basic lasing properties is also achieved for QDs made of II-VI materials and 'native' QDs formed by nanoscale alloy phase separation in the InGaN-AlGaN material system. © 2011 IOP Publishing Ltd.
Kalosha V.P.,TU Berlin |
Ledentsov N.N.,VI Systems |
Bimberg D.,TU Berlin |
Bimberg D.,King Abdulaziz University
Applied Physics Letters | Year: 2012
The output modal content of the oxide-confined vertical-cavity surface-emitting lasers (VCSELs) crucially depends upon the thickness of the low-index oxide aperture, its position with respect to the standing waves of the transverse-longitudinal modes and the separation from the cavity. Three-dimensional cold-cavity optical modes of typical AlGaAs/GaAs VCSELs at 850 nm were simulated to study these dependencies quantitatively taking into account the field diffraction and the material dispersion. Modification of one or two periods of the distributed Bragg reflector by positioning the thin oxidized aperture layers in the mode nodes allows single-mode regime to extend to the aperture diameters as large as 10 μm. © 2012 American Institute of Physics.
Lateral photonic integration of oxide-confined leaky vertical-cavity surface-emitting lasers enables their application in data communications and sensing. Vertical-cavity surface-emitting lasers (VCSELs) that operate at 850nm and are based on oxide-confined apertures are widely used in optical interconnects in data centers, supercomputers, wireless backbone networks, and consumer applications.1 As the processor productivity in these applications increases, it is necessary to continuously improve performance and scale transmission speeds accordingly. In recent years, developers have produced a generation of devices capable of transmitting 40Gb/s at moderate current densities,2, 3 and they have recently demonstrated 54Gb/s non-return-to-zero transmission through 2.2km of multimode fiber.4 Now, 108Gb/s per wavelength transmission can be realized over 100–300m of multimode fiber through the use of advanced modulation formats: discrete multi-tone,5 multiCAP,6 and PAM4.7 All of these achievements are made possible through the use of VCSELs operating in a single transverse and longitudinal mode (SM VCSELs). When manufacturing SM VCSELs, developers typically make the oxide aperture in a VCSEL very small (around 2–3μm in diameter). This approach, however, may result in very low optical power, high resistance, and low manufacturing yield. To extend single-mode behavior toward more conventional aperture sizes (5–7μm), several alternative approaches have been proposed, including surface patterning, etching, overgrowth, and ion implantation in combination with photonic crystals.8, 9 These approaches require additional processing steps that must be precisely aligned (oxide aperture and surface pattern). The resulting complexity can reduce the yield and increase the cost of manufacturing. Our approach uses oxide-confined leaky VCSELs, which—through the application of proper epitaxial design—enable the generation of high optical leakage losses for high-order transverse modes. Using these devices, we extend the single-mode behavior of the laser toward large oxide aperture diameters. With our approach, we aim to create an additional cavity at a wavelength longer than the VCSEL cavity mode. Upon oxidation, the relative intensity distribution of the optical field between the coupled cavities can be strongly affected in the oxidized section. This induces a break in the orthogonality of the VCSEL mode and the second cavity mode (when at a certain tilt angle), which enables in-plane leakage to occur. High-order modes with the field intensity maxima close to the oxide periphery have thus much higher leakage losses.10 We have designed and manufactured oxide-confined leaky VCSELs and observed their leakage process through tilted narrow lobes in the far-field spectrum. The emission comes from the area outside the aperture, and thus does not suffer from diffraction-induced broadening. To model the VCSELs in 3D, we applied finite element analysis based on Maxwell's vector equations in a rotational symmetric system.11 Figure 1 shows a cross section of the simulated electric field of the fundamental and first excited optical modes of an oxide-confined aluminum gallium arsenide-based leaky VCSEL. A simulated far-field profile of the excited mode can be seen in Figure 2. The simulations show that the leakage effect results in a specific tilted emission over the VCSEL surface at ∼35–37°. Most of the intensity of the leaking light is channeled in the direction parallel to the surface. Figure 1. Radial distribution of the simulated electric field of oxide-confined leaky vertical-cavity surface-emitting laser (VCSEL) optical modes. (a) Fundamental optical mode. (b) First excited mode. An active region (magenta line) placed within the cavity is confined by aluminum gallium arsenide distributed Bragg reflectors. The structure contains oxide apertures (white lines). A semiconductor-air interface is shown as a dotted line in the figure. arb. u.: Arbitrary units. Figure 2. Far-field profile simulation of the excited VCSEL mode presented in Figure 1. We manufactured and tested VCSELs according to our design. The far-field measurements of the devices at two current densities can be seen in Figure 3, which shows that at high current densities during multimode operation, narrow lobes arise at ∼35° angles. These lobes are related to the leakage process (see Figure 2). Figure 3. Far-field profiles of a leaky VCSEL operating in fundamental mode (blue, 10kA/cm2) and multi-mode (red, >25kA/cm2). Electroluminescence spectra of the leaky VCSEL at different current densities are shown in Figure 4. We concluded that the VCSEL was predominantly single mode at all the current densities examined, despite the relatively large aperture diameter (5μm). In contrast, the non-leaky VCSEL with thick oxide apertures was heavily multimode, with the excited modes dominant even at small current densities.12 Figure 4. Electroluminescence spectra of an oxide leaky VCSEL with a 5μm aperture. The graph shows dominance of the fundamental mode up to high currents (5.5mA, red). Insert: An optical eye diagram (PRBS7) at 32Gb/s. To summarize, we have shown that it is possible to significantly improve VCSEL spectral quality without involving any additional processing steps. Furthermore, we confirmed the occurrence of in-plane leakage through leakage lobes in the far-field profile of the device. Our findings create opportunities for engineering photonic integrated circuits, for example, by coherent coupling of two or more devices. Therefore, it may be possible to use the technique for beam steering.13 By operating one VCSEL in a couple under reverse bias, it is possible to realize an on-chip integrated monitor photodiode, thus drastically reducing the cost of packaging (since fewer of the elements require alignment and assembly). Our future work will focus on optimization of the leakage effect in order to manufacture high- power and high-speed single-mode VCSELs. This project received funding from the European Union's Horizon 2020 research and innovation program under grant 666866. VI Systems GmbH Nikolay Ledentsov Jr. received his MSc in physics at the Technical University of Berlin while developing indium gallium arsenide-based LEDs. At VI Systems he is responsible for the design and numerical simulation of optoelectronic devices, and operates an automated testbed for spectral and high-speed characterization. Vitaly Shchukin received a diploma in physics and engineering in the field of semiconductor physics from St. Petersburg State Polytechnical University, St. Petersburg, Russia, and a PhD (1987) and doctor of science (1999) in physics and mathematics from the Ioffe Physical-Technical Institute, Russian Academy of Sciences, St. Petersburg. He is co-author of more than 190 published papers, and holds 22 patents and a monograph. Joerg Kropp holds a doctor of science in the field of atomic physics with optical spectroscopy and laser applications. He has more than 25 years' experience in industry in the field of optical communications through management positions with Siemens and Infineon. Mikel Agustin received a diploma in telecommunications engineering from the Public University of Navarra, Spain, and completed his education at the Institute of Telecommunications, Warsaw University of Technology, Poland. At VI Systems he is responsible for developing energy-efficient ultrafast vertical-cavity surface-emitting lasers and photodetectors. Nikolay N. Ledentsov received a diploma in electrical engineering from the Electrical Engineering Institute in Leningrad (LETI, now Electrotechnical University, St. Petersburg, Russia) in 1982. He obtained his PhD (1987) and doctor of science (1994) in physics and mathematics from the Ioffe Physical-Technical Institute. He has been professor of electrical engineering at LETI since 1994 and professor of physics and mathematics at the Ioffe Physical-Technical Institute since 2005. 1. T. R. Fanning, J. Wang, Z.-W. Feng, M. Keever, C. Chu, A. Sridhara, C. Rigo, et al., 28-Gbps 850-nm oxide VCSEL development and manufacturing progress at Avago, Proc. SPIE 9001, p. 900102, 2014. doi:10.1117/12.2039499 3. S. A. Blokhin, J. A. Lott, A. Mutig, G. Fiol, N. N. Ledentsov, M. V. Maximov, A. M. Nadtochiy, V. A. Shchukin, D. Bimberg, 850nm VCSELs operating at bit rates up to 40Gbit/s, Electron. Lett. 45, p. 501-503, 2009. 4. G. Stepniak, A. Lewandowski, J. R. Kropp, N. N. Ledentsov, V. A. Shchukin, N. Ledentsov, G. Schaefer, M. Agustin, J. P. Turkiewicz, 54 Gbit/s OOK transmission using single-mode VCSEL up to 2.2km MMF, Electron. Lett. 52, p. 633-635, 2016. 5. B. Wu, X. Zhou, Y. Ma, J. Luo, K. Zhong, S. Qiu, Z. Feng, et al., Close to 100 Gbps discrete multitone transmission over 100m of multimode fiber using a single transverse mode 850nm VCSEL, Proc. SPIE 9766, p. 97660K, 2016. doi:10.1117/12.2208901 6. R. Puerta, M. Agustin, L. Chorchos, J. Tonski, J.-R. Kropp, N. Ledentsov, V. A. Shchukin, et al., 107.5Gb/s 850nm multi- and single-mode VCSEL transmission over 10 and 100m of multi-mode fiber, OSA Opt. Fiber Commun. Conf. Th5B, p. Th5B.5, 2016. 7. G. Stepniak, L. Chorchos, M. Agustin, J.-R. Kropp, N. N. Ledentsov, V. A. Shchukin, N. N. Ledentsov, J. P. Turkiewicz, Up to 108Gb/s PAM 850nm multi and single mode VCSEL transmission over 100m of multi mode fiber, 2016. Paper accepted at the 42nd Euro. Conf. Opt. Commun. in Düsseldorf, 18-22 September 2016. 8. E. Haglund, A. Haglund, J. Gustavsson, B. Kögel, P. Westbergh, A. Larsson, Reducing the spectral width of high speed oxide confined VCSELs using an integrated mode filter, Proc. SPIE 8276, p. 82760L, 2012. doi:10.1117/12.908424 10. V. Shchukin, N. N. Ledentsov, J. Kropp, G. Steinle, N. Ledentsov, S. Burger, F. Schmidt, Single-mode vertical cavity surface emitting laser via oxide-aperture-engineering of leakage of high-order transverse modes, IEEE J. Quantum Electron. 50, p. 990-995, 2014. 11. N. Ledentsov, V. A. Shchukin, N. N. Ledentsov, J.-R. Kropp, S. Burger, F. Schmidt, Direct evidence of the leaky emission in oxide-confined vertical cavity lasers, IEEE J. Quantum Electron. 52, p. 1-7, 2016. 12. N. N. Ledentsov, J. Xu, J. A. Lott, Future Trends in Microelectronics: Frontiers and Innovations, ch. Ultrafast nanophotonic devices for optical interconnects, Wiley, 2013. doi:10.1002/9781118678107.ch11
Shchukin V.,VI Systems |
Shchukin V.,RAS Ioffe Physical - Technical Institute |
Ledentsov N.,VI Systems |
Ledentsov N.,RAS Ioffe Physical - Technical Institute |
Rouvimov S.,University of Notre Dame
Physical Review Letters | Year: 2013
A new method for the formation of three-dimensional (3D) strained islands in lattice-mismatched (B on A) heteroepitaxy is proposed. Once B forms a wetting layer of a subcritical thickness, material C is deposited, which is lattice matched to A and does not wet B. Then B and C phase separate forming local B-rich and C-rich domains on the surface. The thickness of B-rich domains thus exceeds locally that of the initial film of B, and 3D islands may form as it is demonstrated by modeled phase diagrams of the C/B/A system. We show that the growth of the subcritical InAs/GaAs(100) film followed by the deposition of AlAs results (i) in the formation of Al-rich and In-rich domains in the wetting layer, confirmed by chemically sensitive scanning transmission electron microscopy, and (ii) in the stimulated onset of 3D islands, as evidenced both by high resolution transmission electron microscopy and by a significant redshift of the photoluminescence spectrum, which is in agreement with the proposed model. © 2013 American Physical Society.