Photonic generation of high-frequency microwave signals utilizing a multi-transverse- mode vertical-cavity surface-emitting laser subject to two-frequency orthogonal optical injection

We study photonic microwave signal generation obtained when single- and multi-transverse-mode vertical-cavity surface-emitting lasers (VCSELs) are subject to two-frequency orthogonal optical injection. Our calculations show that broadly tunable microwave signals can be obtained in these systems. The response of the multi-transversemode VCSEL is enhanced with respect to that obtained with a similar single-transverse-mode VCSEL subject to the same two-frequency orthogonal optical injection. The extra degree of freedom given by the multi-transverse-mode operation of the VCSEL under two-frequency orthogonal optical injection enhances the performance of the photonic microwave generation system, because the higher-order transverse mode is excited with a much larger amplitude than that of the fundamental transverse mode. Periodic oscillations are obtained for a very wide range of frequency detunings between the optical injections and transverse modes. A relative maximum of the microwave signal amplitude is obtained when the frequency of one of the optical injections is very close to the frequency of the orthogonally polarized fundamental mode of the VCSEL. Periodic oscillations are demonstrated for symmetric and asymmetric values of the injection strengths. Wide tuning ranges, extended into the THz band, are obtained in our system. Our results show that the proposed microwave signal generation mechanism is independent of the polarization of the master lasers. © 2012 Optical Society of America.This work is supported by the Ministerio de Ciencia e Innovación under project TEC2009-14581-C02-02. A. Quirce was supported in part by the Fondo Social Europeo (FSE) under the programme Junta de Ampliación de Estudios (JAE-predoc). H. Lin acknowledges support from the National Science Foundation under Grant No. PHY-1068789.Peer Reviewe

Longer Wavelength GaAs-Based VCSELs for Extended-Reach Optical Interconnects

Data centers of today are increasing in size and are built to accommodate strong traffic demands while providing sustainably by having clients sharing resources under one roof. Their massive scale puts pressure on the server network topology and has incited a need for data transmission links that are energy efficient and capable of operation at high bit rates with reach up to a few kilometers. Optical interconnects (OIs) offer large bandwidth and low attenuation at long distances, and are therefore suitable for this task. The most commonly used OIs, with 850 nm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) and multi-mode fiber (MMF), have a 25 Gb/s reach that is limited to a few hundred meters. However, the fiber chromatic dispersion and attenuation that limit the OI reach can be reduced significantly by increasing the wavelength of this very same technology. The upper limit of the GaAs-based VCSEL technology, with strained InGaAs quantum wells (QWs), is about 1100 nm.With further improved OI performance, new hyperscale data center topologies can be realized and explored. This will lead to a larger number of possible solutions in traffic engineering as well as for power management. 1060 nm VCSELs could soon open up for lane rates of 100+ Gb/s over distances up to 2 km and help reach the Tb/s link speed aim of data center OI standards, in which capacity is built up mainly by employing multiple parallel lanes, increasing symbol rate by going from binary to four-level pulse amplitude modulation (PAM-4), and optimizing with electrical mitigation techniques such as digital signal processing.In this work we show that 1060 nm GaAs VCSELs are suitable light sources for long-reach OIs by first demonstrating their overall stable performance and capability of error-free data transmission up to 50 Gb/s back-to-back and 25 Gb/s over 1 km of MMF. With PAM-4, we show 100 Gb/s error-free capability over 100 m of MMF, suitable for wavelength division multiplexed OIs that can transmit data at several wavelengths from 850 to 1060 nm over the same fiber channel. We also assemble single-mode 1060 nm VCSEL and single-mode fiber links and demonstrate 50 Gb/s error-free transmission over 1 km using pre-emphasis and 40 Gb/s over 2 km without the use of any electrical mitigation techniques. These results stem from careful VCSEL design, including strained InGaAs QWs with GaAsP barriers, doped AlGaAs distributed Bragg reflectors, a short optical cavity and multiple oxide layers. In addition, we show that the fabrication of such a device poses no increase in complexity and can be realized using standard processing techniques

VCSEL Modeling and Parameter Extraction for Optical Link Simulation and Optimization

A single and multimode vertical cavity surface emitting laser (VCSEL) model and the associated parameter extraction using bandwidth and intensity noise measurements was proposed. To meet the growing bandwidth demand, optical VCSEL links are commonly turning to equalization, and multilevel modulation formats. The need for an accurate physics-based rate equation model is critical for the design and analysis of VCSEL links as we move past 25Gb/s. Single and multimode VCSEL models were able to accurately capture the bandwidth and noise characteristics of VCSELs over a large range of operating conditions. Through spatial mode overlaps, which cause inter and intra mode gain saturation, the multimode VCSEL model uncovered a possible source of positive and negative mode noise correlations recently discovered in VCSELs which affect RIN. Through S-parameter VCSEL characterization and intensity noise measurements, our models could extract and simulate extrinsic and intrinsic VCSEL PAM-4 performance for 100Gb/s+ short reach optical links.Ph.D

Multilevel Modulation and Transmission in VCSEL-based Short-range Fiber Optic Links

As the demand for ever higher throughput short-range optical links is growing, research and industry associations have shown increased interest in multilevel modulation formats, such as the four leveled pulse amplitude modulation, referred to as 4-PAM. As on-off keying (OOK) persists to be the choice for low latency applications, for example high performance computing, datacenter operators see 4-PAM as the next format to succeed current OOK-based optical interconnects. Throughput can be increased in many ways: parallel links can be deployed, multicore fibers can be used or more efficient modulation formats with digital signal processing is an alternative. Therefore, to improve link data rates, the introduction of new modulation formats and pre-emphasis are primarily considered in this thesis. In a bandwidth-limited link, turning towards spectrally efficient formats is one of the methods to\ua0 overcome the bandwidth requirements of OOK. Such are the considerations when opting for 3-PAM or 4-PAM schemes. Both require lower bandwidth than OOK and are potential candidates in such scenarios. 4-PAM provides double spectral efficiency and double data rate at the same symbol rate as on-off keying, but, as with any technology transition, new challenges emerge, such as a higher SNR requirement, a lower tolerance to VCSEL nonlinearities and skewing of the signal in the time domain. 3-PAM could potentially be an in-between solution, as it requires 33% less bandwidth than OOK and is less sensitive to VCSEL dynamics which could impair the transmission. A study is presented where 3-PAM has outperformed both OOK and 4-PAM in the same link. Detailed investigation of legacy 25G class VCSELs has shown that devices with moderate damping are suitable for the transition to 4-PAM. The pre-emphasis of signals is a powerful tool to increase link bandwidth at the cost of modulation amplitude. This has been investigated in this thesis for on-offkeying and has shown 9% and 27% increase in bit rate for error-free operation with two pre-emphasis approaches. Similarly, pre-emphasis of a 4-PAM electrical signals has enabled 71.8 Gbps transmission back-to-back with lightweight forward error correction and 94 Gbps net data rate was achieved with the same pre-emphasis and post-processing using an offline least-mean-square equalizer

QUANTUM INSPIRED SYMMETRIES IN LASER ENGINEERING

In this thesis, quantum inspired symmetries including Parity-Time (PT) symmetry and Supersymmetry (SUSY) have been studied in the context of non-Hermitian engineered laser systems. This thesis starts with a short review of semiconductor lasers theory in second chapter, followed by an introduction to quantum inspired symmetries: PT symmetry and SUSY in optics and photonics in chapter three. In chapter four, we have studied the robustness and mode selectivity in PT symmetric lasers. We investigate two important aspects of PT symmetric photonic molecule lasers, namely the robustness of their single longitudinal mode operation against instabilities triggered by spectral hole burning effects, and the possibility of more versatile mode selectivity. Our results, supported by numerically integrating the nonlinear rate equations and performing linear stability analysis, reveals the following: (1) In principle a second threshold exists after which single mode operation becomes unstable, signaling multimode oscillatory dynamics, (2) For a wide range of design parameters, single mode operation of PT lasers having relatively large free spectral range (FSR) can be robust even at higher gain values, (3) PT symmetric photonic molecule lasers are more robust than their counterpart structures made of single microresonators; and (4) Extending the concept of single longitudinal mode operation based on PT symmetry in millimeter long edge emitting lasers having smaller FSR can be challenging due to instabilities induced by nonlinear modal interactions. Finally, we also present a possible strategy based on loss engineering to achieve more control over the mode selectivity by suppressing the mode that has the highest gain (i.e. lies under the peak of the gain spectrum curve) and switch the lasing action to another mode. In chapter five a new scheme for building two dimensional laser arrays that operate in the single supermode regime is proposed. This is done by introducing an optical coupling between the laser array and a lossy pseudo-isospectral chain of photonic resonators. The spectrum of this discrete reservoir is tailored to suppress all the supermodes of the main array except the fundamental one. This spectral engineering is facilitated by employing the Householder transformation in conjunction with discrete supersymmetry. The proposed scheme is general and can in principle be used in different platforms such as VCSEL arrays and photonic crystal laser arrays. Finally, in chapter six we have investigated the laser self-termination (LST) in trimer photonic molecules. It is shown that under the appropriate conditions, LST can exist in complex discrete structures made of three-cavity photonic molecule lasers. We have shown that the phenomenon of LST is a purely linear effect associated with avoided level crossings. Furthermore, our simulations show that the predicted behavior is persistent when gain saturation nonlinearity is taken into account. Conclusion remarks and future works are discussed in the last chapter

1060 nm GaAs VCSELs for Extended Reach Optical Interconnects in Data Centers

The data centers of today are increasing in size and are built to accommodate strong data traffic demands while providing sustainably by having clients sharing resources under one roof. Their massive scale puts pressure on the server network topology and has incited a need for data transmission links that are energy efficient and capable to operate at high bit rates with reach up to a few kilometers. Optical interconnects (OIs) offer large bandwidth and low attenuation at long distances, and are therefore suitable for this task. The most commonly used OIs, with 850 nm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) and multi-mode fiber (MMF), have a 25 Gb/s reach that is limited to a few hundred meters. However, the fiber chromatic dispersion and attenuation that limit the OI reach can be reduced significantly by increasing the wavelength of this very same technology. The upper limit of the GaAs-based VCSEL technology, with strained InGaAs quantum wells (QWs), is about 1100 nm.With further improved OI performance, new hyperscale data center topologies can be realized and explored. This will lead to many more possible solutions in traffic engineering as well as for power management. 1060 nm VCSELs could open up for lane rates of 10, 25 and possibly 50 Gb/s over distances up to 2 km and help reach the Tb/s link speed aim of the Ethernet standard.In this work we show that the 1060 nm GaAs VCSEL is a suitable light source for long-reach OIs by demonstrating its overall stable performance and capability of error-free data transmission up to 50 Gb/s back-to-back and 25 Gb/s over 1 km of MMF. These results stem from careful VCSEL design, including strained InGaAs QWs with GaAsP barriers, doped AlGaAs distributed Bragg reflectors, a short op-tical cavity and multiple oxide layers. We also show that the fabrication of such a device poses no increase in complexity and can be realized using standard processing techniques

Optical techniques for broadband in-building networks

Optical fibres, which can easily handle any bandwidth demand, have been rolled out to more than 32 million consumer’s homes and professional buildings worldwide up to 2010. The basic technological and economical challenges of fibre-to-the-home (FTTH) has been solved. The current FTTH technology can now providing baseband Gbit Ethernet and high definition TV services to the gates of homes. Thus, the bottleneck for delivery of broadband services to the end users is shifting from the access network to the in-building network. In the meantime, the need for high-capacity transmission between devices inside the building, e.g. between desktop PC and data services, are also rapidly increase. How to bring high bandwidth to the mobile terminals such as laptops, PDAs or cell phones as well as to the fixed terminals such as desktop PCs and HDTV equipment in an all-in-one network infrastructure is a challenge we are facing. Building on the flexibility of the wireless access networks and the latent vast bandwidth of a fibre infrastructure, radio-over-fibre (RoF) techniques have been proposed as a cost-effective solution to the future integrated broadband services in in-building networks. This thesis investigates techniques to deliver high data rate wireless services via in-building networks: high capacity RoF links employing optical frequency multiplication (OFM) and sub-carrier multiplexing (SCM) techniques, with single- or multi-carrier signal formats. The orthogonal frequency division multiplexing (OFDM) format is investigated for the RoF transmission system, particularly with regard to the optical system nonlinearity. For low-cost short-range optical backbone networks, RoF transmission over large-core diameter plastic optical fibre (POF) links has been studied, including the transmission of the WiMedia-compliant multiband OFDM UWB signal over bandwidth-limited large-core POF as well as a full-duplex bi-directional UWB transmission over POF. In order to improve the functionalities for delivery of wireless services of in-building networks, techniques to introduce flexibility into the network architecture and to create dynamic capacity allocation have been investigated. By employing optical switching techniques based on optical semiconductor amplifiers (SOA), an optically routed RoF system has been studied. The dynamic capacity allocation is addressed by investigating one-dimensional and two-dimensional routing using electrical SCM and optical wavelengths. In addition, next to RoF networking, this thesis explores techniques for wired delivery of baseband high capacity services over POF links by employing a multi-level signal modulation format, in particular discrete multi-tone (DMT) modulation. Transmission of 10 Gbit/s data over 1 mm core diameter PMMA POF links is demonstrated, as a competitor to more expensive fibre solutions such as silica single and multimode fibre. A record transmission rate of more than 40 Gbit/s is presented for POF whose core diameter is comparable with silica multimode fibre. Finally, from the network perspective, the convergence of wired and wireless multi-standard services into a single fibre-based infrastructure has been studied. Techniques have been designed and demonstrated for in-building networks, which can convey both high capacity baseband services and broadband radio frequency (RF) services over a POF backbone link. The multi-standard RoF signals carry different wireless services at different radio frequencies and with different bandwidths, including WiFi, WiMax, UMTS and UWB. System setups to carry them together over the same multimode optical fibre based network have been designed and experimentally shown. All the concepts, designs and system experiments presented in this thesis underline the strong potential of multimode (silica and plastic) optical fibre techniques for the delivery of broadband services to wired and wireless devices in in-building networks, in order to extend to the end user the benefits of the broadband FTTH networks which are being installed and deployed worldwide