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News Article | August 22, 2016
Site: www.spie.org

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.

Fiol G.,TU Berlin | Lott J.A.,VI Systems | Ledentsov N.N.,VI Systems | Bimberg D.,TU Berlin
Electronics Letters | Year: 2011

Error-free 25Gbit/s optical fibre communication links at 850nm with vertical-cavity surface-emitting lasers (VCSELs) via bit error ratio tests are demonstrated. At 25°C through 300m of fibre a VCSEL dissipated-energy-to- bit-rate ratio of only 122.4mW/Tbits is achieved. The energy efficient VCSELs reduce the energy required for data and computer communication. © 2011 The Institution of Engineering and Technology.

Ledentsov N.N.,VI Systems | Lott J.A.,VI Systems
Physics-Uspekhi | Year: 2011

Researchers have considered vertical-cavity surface-emitting lasers (VCSEL) as the main element of modern short-range optical communication lines. Efforts are being made to increase bit rate of these vertical-cavity surface-emitting lasers according tom the requirements of computational systems in the future. VCSELs play a dominant role in the existing optical data communication systems where their radiation is emitted perpendicular to the substrate surface. Individual devices of such a design and their arrays can be manufactured by using planar technology and have a very small size in the plane of a wafer. Layers containing quantum wells, quantum wires, and quantum dots are being used most extensively, as they improve the properties of lasers significantly. InAs quantum dots allow the fabrication of VCSELs and stripe lasers emitting at 1.3 μm by using the standard and economical arsenide-gallium technology.

Moser P.,TU Berlin | Hofmann W.,TU Berlin | Wolf P.,TU Berlin | Lott J.A.,VI Systems | And 5 more authors.
Applied Physics Letters | Year: 2011

Extremely energy-efficient oxide-confined high-speed 850 nm vertical-cavity surface-emitting lasers for optical interconnects are presented. Error-free performance at 17 and 25 Gb/s via a 100 m multimode fiber link is demonstrated at record high dissipation-power-efficiencies of up to 69 fJ/bit (<0.1 mW/Gbps) and 99 fJ/bit, respectively. These are the most power efficient high-speed directly modulated light sources reported to date. The total energy-to-data ratio is 83 fJ/bit at 25 °C and reduces to 81 fJ/bit at 55 °C. These results were obtained without adjustment of driving conditions. A high D -factor of 12.0 GHz/ (mA) 0.5 and a K -factor of 0.41 ns are measured. © 2011 American Institute of Physics.

Moser P.,TU Berlin | Lott J.A.,VI Systems | Wolf P.,TU Berlin | Larisch G.,TU Berlin | And 4 more authors.
IEEE Photonics Technology Letters | Year: 2012

We present extremely energy-efficient oxide-confined 850-nm single-mode vertical-cavity surface-emitting lasers (VCSELs) for optical interconnects. Error-free transmission at 17 Gb/s across 1 km of multimode optical fiber is achieved with an ultra-low energy-to-data ratio of 99 fJ/bit, corresponding to a record-low energy-to-data-distance ratio of 99 fJ/(bit$\cdot$km). This performance is achieved without changing any of the driving parameters up to 55 \$ \circC. To date our VCSELs are the most energy-efficient directly modulated light-sources for data transmission across all distances up to 1 km of multimode optical fiber. © 2006 IEEE.

Tan M.P.,Urbana University | Fryslie S.T.M.,Urbana University | Lott J.A.,VI Systems | Lott J.A.,TU Berlin | And 3 more authors.
IEEE Photonics Technology Letters | Year: 2013

With the separation of optical and current apertures, photonic crystal vertical-cavity surface-emitting lasers can reach a 3-dB small-signal modulation bandwidth of >18 GHz while lasing in the fundamental mode. Because of reduced chromatic dispersion, such devices enable error-free transmission over 1-km OM4 multimode fiber at a data rate of 25 Gb/s and operating at a current density of 5.4 kA/cm2. This can potentially lead to a laser source that is useful for rack-to-rack transmissions in large data centers and potentially long device lifetime. © 1989-2012 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.