Prof. Werner H. Hofmann Profile

Prof. Werner H. Hofmann

at Deutsches Patent- und Markenamt

SPIE Involvement:

Conference Chair | Editor | Author

Area of Expertise:

Vertical-Cavity Surface-Emitting Lasers , High-Contrast-Grating VCSEL , VCSEL Modelling , High-Speed VCSEL , Optical Interconnects

Websites:

Company Website

Publications (20)

Proceedings Article | 1 April 2020 Presentation + Paper

Mode resolved modulation response and the impact of mode intensity nonuniformity on VCSELs dynamics

Wissam Hamad, Marwan Bou Sanayeh, Mohammad Radi, Elie Zoghbi, Joseph Maroun, Maya Al Ajami, Pascal Nasr, Werner H. E. Hofmann

Proceedings Volume 11356, 1135618 (2020) https://doi.org/10.1117/12.2555934

KEYWORDS: Modulation, Vertical cavity surface emitting lasers, Data modeling, Performance modeling, Nanophotonics

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To investigate the origin of the deep-notch in the modulation response of multi-mode vertical cavity surface emitting lasers (VCSELs) that occurs at low frequencies, the carrier transport and the mode intensity nonuniformity effects are considered in this work. The model that includes the carrier transport effect is a possible candidate to explain the deep-notch behavior, as it introduces a first-order low-pass filter to the pure intrinsic transfer function. Nevertheless, the expected dynamic characteristics of this filter for high-performance VCSELs, disfavor the transport effect as a possible candidate. Therefore, to fully rule out its impact, we optimized the transport theory by first considering both the transverse and longitudinal carrier transport effects, and second by including the frequency dependence of the transport factor into the derivation of the VCSELs’ dynamic transfer function. Taking all these factors into consideration, however, did not result in a significant improvement in the resulting transfer function form and order. Thus, a second effect, which is the mode intensity nonuniformity effect, is favored as a possible candidate to explain the deep-notch phenomenon. In this effect, the spatial nonuniformity of the transverse optical mode in the radial direction results in concentric disk-like regions with different dynamic properties under small signal modulation. At low driving currents, the model predicts a notch ahead of the relaxation oscillation frequency. This notch is caused by a low frequency roll-off resulting from the spatial nonuniformity of the optical transverse mode. The proposed model introduces a ratio containing a pole and a zero to the pure intrinsic transfer function and can very accurately fit the deep-notch in the measured modulation response. Furthermore, this fitting results in the extraction of very reliable performance indicators. A second topic that is also presented in this work is about the dynamics of the mode resolved modulation response. To achieve a higher bandwidths, it is necessary to study the mode ensemble resolved modulation characteristics of the intrinsic dynamics. Based on the novel carrier-reservoir splitting approach, analytical expressions for the dynamics in the two spatially separated central and peripheral regions are derived.

Proceedings Article | 1 April 2020 Presentation + Paper

Device physical parameters extraction of high-speed VCSELs

Marwan Bou Sanayeh, Wissam Hamad, Mariane Mansour, Reem Al Fata, Elsie Nakhle, Anthony Youssef, Mustapha Hamad, Semaan Georges, Werner Hofmann

Proceedings Volume 11356, 1135619 (2020) https://doi.org/10.1117/12.2555955

KEYWORDS: Vertical cavity surface emitting lasers, Modulation, Data modeling, Mirrors, Optical resonators, Nanophotonics, Efficient operations, Performance modeling

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Realizing that vertical cavity surface emitting lasers (VCSELs) are continuously breaking higher bandwidth limits, it is essential to understand their basic material constituting properties and extract their physical characteristics, in order to fine tune their high-speed performance. Throughout the past decade, the device performance was continuously optimized towards higher bandwidths, faster data rates, and efficient operation. Therefore, for a successful further optimization of their dynamic characteristics, the extraction of a reliable set of their physical parameters becomes indispensable. Consequently, the main objective of this work is to provide accurate physical parameter values of cutting-edge high-speed VCSELs. The extraction process of these set of parameters is based on the novel intrinsic and extrinsic dynamic models that were recently developed by our research group. For the intrinsic dynamics, the advanced split carrier reservoir multimode model is employed. Furthermore, the pure intrinsic modulation response is de-embedded from the total measured device response using our recently proposed novel parasitic-network model. Moreover, the extraction of these device physical parameters is based on the device’s performance indicators, such as the relaxation oscillation frequencies and damping coefficients obtained by fitting the intrinsic model to the measured modulation data. Furthermore, since these performance indicators can be expressed in terms of a combination of these physical parameters, a precise estimation of their values and their possible ranges, along with a carefully designed fitting process, is crucial. Consequently, for extracting a reliable set of data, the accurate prior estimation of these parameters was conducted based on the device physical structure and reported material properties, leading to the establishment of an accurate set of parameters along with their possible ranges. These final calculations are based on simple device geometrical considerations, reported physical and material properties and device static performance measurements. Finally, the estimated values and their ranges are further validated by comparing them to ones in standard literature.

Proceedings Article | 6 November 2018 Paper

High-power 910 nm vertical-cavity surface-emitting laser arrays for light detection and ranging

Jianwei Zhang, Yongqiang Ning, Kefu Liu, Jian Qiu, Xing Zhang, Yingying Yan, Zhiwen Chen, Hao Wu, Hongbo Zhu, Werner Hofmann, Li Qin, Lijun Wang

Proceedings Volume 10812, 1081219 (2018) https://doi.org/10.1117/12.2502184

KEYWORDS: Vertical cavity surface emitting lasers, LIDAR, Integrated circuits, High power lasers, Resistance, Inductance, Capacitance, Integrated circuit design, Absorption, Sensors

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Proceedings Article | 6 November 2018 Paper

Modulation response, impedance, and equivalent circuit of ultra-high-speed multi-mode VCSELs

Wissam Hamad, Marwan Bou Sanayeh, Bassel Aboul Hosn, Vana Ajemian, Rami Yehia, Eliane Zouein, Mustapha Hamad, Werner Hofmann

Proceedings Volume 10812, 1081203 (2018) https://doi.org/10.1117/12.2500166

KEYWORDS: Vertical cavity surface emitting lasers, Circuit switching, Modulation, Data modeling, Capacitance, Oxides, Resistance, Microwave radiation, Capacitors, Device simulation

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Proceedings Article | 6 November 2018 Paper

Advanced modulation response analysis of high-speed VCSELs

Wissam Hamad, Marwan Bou Sanayeh, Mustapha Hamad, Werner Hofmann

Proceedings Volume 10812, 108120N (2018) https://doi.org/10.1117/12.2501216

KEYWORDS: Vertical cavity surface emitting lasers, Modulation, Analytical research, Diffusion, Performance modeling, Nanophotonics, Data modeling, Solid state physics, Modulation transfer functions, Phase modulation

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Proceedings Article | 9 May 2018 Paper

Equivalent circuit modeling of the dynamic operation of ultra-high speed multi-mode VCSELs

Wissam Hamad, Elio Nakhle, Oliver Daou, Marwan Bou Sanayeh, Mustapha Hamad, Werner Hofmann

Proceedings Volume 10682, 106821R (2018) https://doi.org/10.1117/12.2306823

KEYWORDS: Vertical cavity surface emitting lasers, Modulation, Data modeling, Device simulation, Performance modeling, Modeling, Solid state physics

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Proceedings Article | 9 May 2018 Paper

De-embedding device parasitics of ultra-high speed VCSELs

Wissam Hamad, Nataly Dalal, Serena Bou Nassar, Marwan Bou Sanayeh, Mustapha Hamad, Werner Hofmann

Proceedings Volume 10682, 106821T (2018) https://doi.org/10.1117/12.2306920

KEYWORDS: Vertical cavity surface emitting lasers, Data modeling, Modeling

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To establish highly performing vertical cavity surface-emitting lasers (VCSELs), it is essential to have an adequate understanding of the intrinsic laser dynamics of these devices. However, this is done while bearing in mind that extrinsic parasitic elements in VCSELs play an important role in limiting the intrinsic modulation bandwidth. In this work, we analyse different electrical parasitic equivalent circuit models in the aim of comparing them and selecting the one that can best describe and represent the physical properties of our high-performance VCSELs. Through measuring the microwave reflection coefficient S₁₁(f), then fitting it with the calculated one from the equivalent circuit impedance model, the parasitic components of the equivalent circuit model can be extracted. The S₁₁(f) data was collected over different ranges of operating bias currents and using a 7 μm oxide aperture diameter VCSEL. This allows us to observe the variations of these circuit elements with respect to the current and compute the transfer function and the resulting parasitic cut-off frequencies (bandwidth limitation) for each model. After plotting and comparing the transfer functions of the different models together, under the same driving current, it was found that the discrepancy between the two curves, in a specific frequency range, is rather small over the VCSEL bandwidth of interest, hence allowing us to use the first-order low pass filter to de-embed the parasitic contributions and separate them from the device intrinsic response. However, over higher frequency ranges, the deviation is found to be substantial and the extract parasitic transfer function should be taken into consideration.

Another issue to be addressed is the reliability of the simple circuit models to extract accurate circuit component values, especially when the deviation between the measured microwave reflection coefficient S₁₁(f) and the fitted model is substantially large. This discrepancy is due to the oversimplification imposed on the equivalent circuit model, leading to a high level of uncertainty in the extracted circuit component values. Thus, sufficient modelling and accurate fitting strategies are needed for a reliable parasitic de-embedding approach.

Proceedings Article | 17 May 2017 Paper

Small-signal analysis of ultra-high-speed 30 GHz VCSELs using an advanced multi-mode approach

Wissam Hamad, Marwan Bou Sanayeh, Hassan Hamad, Mustapha Hamad, Semaan Georges, Werner Hofmann

Proceedings Volume 10242, 102420Y (2017) https://doi.org/10.1117/12.2265834

KEYWORDS: Mathematical modeling, Solid state physics, Nanophotonics, Vertical cavity surface emitting lasers, Modulation, Data modeling, Optical interconnects, Mirrors, Data communications, Physics, Analytical research, Binary data

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Proceedings Article | 17 March 2017 Paper

Multi-mode rate-equation analysis of VCSELs with >30GHz bandwidth

Wissam Hamad, Stefan Wanckel, Werner Hofmann

Proceedings Volume 9757, 975709 (2017) https://doi.org/10.1117/12.2212065

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Proceedings Article | 15 March 2013 Paper

Rigorous, highly-efficient optical tools for HCG-VCSEL design

Pierluigi Debernardi, Renato Orta, Werner Hofmann

Proceedings Volume 8633, 86330A (2013) https://doi.org/10.1117/12.2007465

KEYWORDS: Reflectivity, Vertical cavity surface emitting lasers, Oxides, Polarization, Refractive index, Quantum wells, Mirrors, 3D modeling, Finite-difference time-domain method, Electromagnetism

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Proceedings Article | 29 November 2012 Paper

VCSELs for exascale computing, computer farms, and green photonics

Werner Hofmann, Philip Moser, Philip Wolf, Gunter Larisch, Hui Li, Wei Li, James Lott, Dieter Bimberg

Proceedings Volume 8552, 855205 (2012) https://doi.org/10.1117/12.2006273

KEYWORDS: Vertical cavity surface emitting lasers, Optical interconnects, Data centers, Energy efficiency, Semiconductor lasers, Computing systems, Computer security, Photonics, Metals, Data communications

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The bandwidth-induced communication bottleneck due to the intrinsic limitations of metal interconnects is inhibiting the performance and environmental friendliness of today´s supercomputers, data centers, and in fact all other modern electrically interconnected and interoperable networks such as data farms and "cloud" fabrics. The same is true for systems of optical interconnects (OIs), where even when the metal interconnects are replaced with OIs the systems remain limited by bandwidth, physical size, and most critically the power consumption and lifecycle operating costs. Vertical-cavity surface-emitting lasers (VCSELs) are ideally suited to solve this dilemma. Global communication providers like Google Inc., Intel Inc., HP Inc., and IBM Inc. are now producing optical interconnects based on VCSELs. The optimal bandwidth per link may be analyzed by by using Amdahl´s Law and depends on the architecture of the data center and the performance of the servers within the data center. According to Google Inc., a bandwidth of 40 Gb/s has to be accommodated in the future. IBM Inc. demands 80 Tbps interconnects between solitary server chips in 2020. We recently realized ultrahigh bit rate VCSELs up to 49 Gb/s suited for such optical interconnects emitting at 980 nm. These devices show error-free transmission at temperatures up to 155°C and operate beyond 200°C. Single channel data-rates of 40 Gb/s were achieved up to 75°C. Record high energy efficiencies close to 50 fJ/bit were demonstrated for VCSELs emitting at 850 nm. Our devices are fabricated using a full three-inch wafer process, and the apertures were formed by in-situ controlled selective wet oxidation using stainless steel-based vacuum equipment of our own design. assembly, and operation. All device data are measured, recorded, and evaluated by our proprietary fully automated wafer mapping probe station. The bandwidth density of our present devices is expected to be scalable from about 100 Gbps/mm² to a physical limit of roughly 15 Tbps/mm² based on the current 12.5 Gb/s VCSEL technology. Still more energy-efficient and smaller volume laser diode devices dissipating less heat are mandatory for further up scaling of the bandwidth. Novel metal-clad VCSELs enable a reduction of the device's footprint for potentially ultrashort range interconnects by 1 to 2 orders of magnitude compared to conventional VCSELs thus enabling a similar increase of device density and bandwidth.

Proceedings Article | 9 May 2012 Paper

High-speed VCSELs for energy efficient computer interconnects

Philip Moser, Philip Wolf, James Lott, Gunter Larisch, Alexey Payusov, Alex Mutig, Waldemar Unrau, Nikolay Ledentsov, Werner Hofmann, Dieter Bimberg

Proceedings Volume 8432, 843202 (2012) https://doi.org/10.1117/12.923054

KEYWORDS: Vertical cavity surface emitting lasers, Optical interconnects, Modulation, Multimode fibers, Optical fibers, Temperature metrology, Data transmission, Data centers, Binary data, Standards development

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Proceedings Article | 7 February 2012 Paper

Energy-efficient vertical-cavity surface-emitting lasers (VCSELs) for "green" data and computer communication

Philip Moser, James Lott, Philip Wolf, Gunter Larisch, Alexey Payusov, Gerrit Fiol, Nikolay Ledentsov, Werner Hofmann, Dieter Bimberg

Proceedings Volume 8276, 82760J (2012) https://doi.org/10.1117/12.906856

KEYWORDS: Vertical cavity surface emitting lasers, Modulation, Data transmission, Multimode fibers, Optical fibers, Data communications, Optical interconnects, Temperature metrology, Energy efficiency, Semiconducting wafers

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Proceedings Article | 7 February 2012 Paper

980-nm VCSELs for optical interconnects at bandwidths beyond 40 Gb/s

W. Hofmann, P. Moser, P. Wolf, G. Larisch, W. Unrau, D. Bimberg

Proceedings Volume 8276, 827605 (2012) https://doi.org/10.1117/12.908466

KEYWORDS: Vertical cavity surface emitting lasers, Optical interconnects, Modulation, Receivers, Mirrors, Binary data, Semiconducting wafers, Photonics, Gallium arsenide, Temperature metrology

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Proceedings Article | 28 November 2011 Paper

High-speed 850 and 980 nm VCSELs for high-performance computing applications

Alex Mutig, Philip Moser, James Lott, Philip Wolf, Werner Hofmann, Nikolay Ledentsov, Dieter Bimberg

Proceedings Volume 8308, 830818 (2011) https://doi.org/10.1117/12.903698

KEYWORDS: Vertical cavity surface emitting lasers, Optical interconnects, Data transmission, Oxides, Quantum wells, Multimode fibers, Light sources, Gallium arsenide, Binary data, Temperature metrology

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Proceedings Article | 8 February 2011 Paper

High-speed highly temperature stable 980 nm VCSELs operating at 25 Gb/s at up to 85 °C for short reach optical interconnects

Alex Mutig, James Lott, Sergey Blokhin, Philip Moser, Philip Wolf, Werner Hofmann, Alexey Nadtochiy, Dieter Bimberg

Proceedings Volume 7952, 79520H (2011) https://doi.org/10.1117/12.876156

KEYWORDS: Vertical cavity surface emitting lasers, Oxides, Modulation, Temperature metrology, Optical interconnects, Quantum wells, Laser stabilization, Signal processing, Eye, Data transmission

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Proceedings Article | 21 August 2009 Paper

Optical glucose monitoring using vertical cavity surface emitting lasers (VCSELs)

Sahba Talebi Fard, Werner Hofmann, Pouria Talebi Fard, Ezra Kwok, Markus-Christian Amann, Lukas Chrostowski

Proceedings Volume 7397, 739704 (2009) https://doi.org/10.1117/12.828467

KEYWORDS: Glucose, Vertical cavity surface emitting lasers, Calibration, Absorption, Sensors, Spectroscopy, Blood, Error analysis, Data modeling, Statistical analysis

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Proceedings Article | 29 January 2008 Paper

Monolithic 2D high-power arrays of long-wavelength VCSELs

W. Hofmann, M. Görblich, M. Ortsiefer, G. Böhm, M.-C. Amann

Proceedings Volume 6908, 690807 (2008) https://doi.org/10.1117/12.763009

KEYWORDS: Vertical cavity surface emitting lasers, High power lasers, Optics manufacturing, Continuous wave operation, Semiconductor lasers, Data transmission, Mirrors, Gold, Headlamps

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Proceedings Article | 20 December 2007 Paper

Applications of 1.55 μm optically injection-locked VCSELs in wavelength division multiplexed passive optical networks

Elaine Wong, Xiaoxue Zhao, Connie Chang-Hasnain, Werner Hofmann, Marcus Amann

Proceedings Volume 6783, 67832M (2007) https://doi.org/10.1117/12.750154

KEYWORDS: Vertical cavity surface emitting lasers, Modulation, Signal detection, Transmitters, WDM-PON, Backscatter, Channel projecting optics, Receivers, Signal attenuation, Sensors

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Proceedings Article | 25 September 2007 Paper

Long-wavelength VCSELs for optical networks and trace-gas monitoring

W. Hofmann, G. Böhm, M. Ortsiefer, M. Görblich, C. Lauer, N. H. Zhu, M.-C. Amann

Proceedings Volume 6766, 67660F (2007) https://doi.org/10.1117/12.736660

KEYWORDS: Vertical cavity surface emitting lasers, Modulation, Absorption, Picosecond phenomena, Quantum wells, Coarse wavelength division multiplexing, Optical interconnects, Telecommunications, Optical networks, Molecules

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