Terahertz wave transmission in flexible polystyrene-lined hollow metallic waveguides for the 2.5-5 THz band.

A low-loss and low-dispersive optical-fiber-like hybrid HE11 mode is developed within a wide band in metallic hollow waveguides if their inner walls are coated with a thin dielectric layer. We investigate terahertz (THz) transmission losses from 0.5 to 5.5 THz and bending losses at 2.85 THz in a polystyrene-lined silver waveguides with core diameters small enough (1 mm) to minimize the number of undesired modes and to make the waveguide flexible, while keeping the transmission loss of the HE11 mode low. The experimentally measured loss is below 10 dB/m for 2 < ? < 2.85 THz (∼4-4.5 dB/m at 2.85 THz) and it is estimated to be below 3 dB/m for 3 < ? < 5 THz according to the numerical calculations. At ∼1.25 THz, the waveguide shows an absorption peak of ∼75 dB/m related to the transition between the TM11-like mode and the HE11 mode. Numerical modeling reproduces the measured absorption spectrum but underestimates the losses at the absorption peak, suggesting imperfections in the waveguide walls and that the losses can be reduced further. © 2013 Optical Society of America

Investigation of hollow cylindrical metal terahertz waveguides suitable for cryogenic environments

The field of terahertz (THz) waveguides continues to grow rapidly, with many being tailored to suit the specific demands of a particular final application. Here, we explore waveguides capable of enabling efficient and accurate power delivery within cryogenic environments (< 4 K). The performance of extruded hollow cylindrical metal waveguides made of un-annealed and annealed copper, as well as stainless steel, have been investigated for bore diameters between 1.75 - 4.6 mm, and at frequencies of 2.0, 2.85 and 3.4 THz, provided by a suitable selection of THz quantum cascade lasers. The annealed copper resulted in the lowest transmission losses, < 3 dB/m for a 4.6 mm diameter waveguide, along with 90° bending losses as low as ~2 dB for a bend radius of 15.9 mm. The observed trends in losses were subsequently analyzed and related to measured inner surface roughness parameters. These results provide a foundation for the development of a wide array of demanding low-temperature THz applications, and enabling the study of fundamental physics.Engineering and Physical Sciences Research Council (Grant No. EP/J017671/1, Coherent Terahertz Systems)

Modes in silver-iodide-lined hollow metallic waveguides mapped by terahertz near-field time-domain microscopy

Thin dielectric layers inside hollow metallic waveguides are used to improve the waveguide transmission characteristics as the dominant waveguide mode changes into the hybrid HE11 mode. We investigate the effect of 1 μm thick silver iodide (AgI) coatings on the fundamental modes in cylindrical waveguides at terahertz (THz) frequencies, in the regime of the dielectric layer being thinner than the optimal thickness hopt(2 THz) ̃ 20 μm. In the region of 1-3.2 THz, the lowest-order modes are similar in profile to the TE11 and TM11 modes, as determined by the timeresolved near-field measurements and verified numerically. Higher-order modes are detected experimentally as mode mixtures due to the multimode propagation. Numerical electromagnetic modeling is applied to resolve the mode structure ambiguity, allowing us to correlate experimentally detected patterns with a superposition of the TM11 and the higher-order mode, TE12. Mode profiles determined here indicate that in the regime of ultrathin dielectric (h c 0.1γeff ), the dielectric layer does not transform the dominant mode into the low-loss HE11 mode. Experimental mode patterns similar to the HE11 and the TE01 modes nevertheless can be formed due to mode beating. The results indicate that the Ag/AgI waveguides can be used for guiding THz waves in the TE01 mode or the TE12 mode with high discrimination against other modes. © 2012 Optical Society of America

Multimode VIS–NIR transmission through silver coated hollow optical waveguides

Multimode transmission of continuous wave 633 nm radiation and 1064 nm Q-switched Nd:YAG pulses using silver coated hollow core optical waveguides (HCWs) with bore diameters of 700 μm and 1000 μm is reported. The effect of launch conditions, input beam polarization and waveguide bore diameter on the pulse energy transmission and potential for focussing the beam effectively at the HCW exit is detailed. An optimal launch f-number range of 155–165 is identified for minimizing the exit angle

Mechanically robust cylindrical metal terahertz waveguides for cryogenic applications

As the ambition behind THz quantum cascade laser based applications continues to grow, abandoning free-space optics in favor of waveguided systems promises major improvements in targeted, easy to align, and robust radiation delivery. This is especially true in cryogenic environments, where illumination is traditionally challenging. Although the field of THz waveguides is rapidly developing, most designs have limitations in terms of mechanical stability at low temperatures, and are costly and complicated to fabricate to lengths > 1 m. In this work, we investigate readily available cylindrical metal waveguides which are suitable for effective power delivery in cryogenic environments, and explore the optimal dimensions and materials available. The materials chosen were extruded un-annealed and annealed copper, as well as stainless steel, with bore diameters of 1.75, 2.5, and 4.6 mm. Measurements were performed at three different frequencies, 2.0, 2.85 and 3.2 THz, with optimal transmission losses 1, and forms a comprehensive investigation of cryogenically compatible THz waveguides and optical couplers, paving the way for a new generation of systems to utilize THz QCLs for a host of low-temperature investigations

Policy regularized model predictive control framework for robust legged locomotion

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 67-71).A novel Policy Regularized Model Predictive Control (PR-MPC) framework is developed to allow general robust legged locomotion with the MIT Cheetah quadruped robot. The full system is approximated by a simple control model that retains the key nonlinearities characteristic to legged contact dynamics while reducing the complexity of the continuous dynamics. Nominal footstep locations and feedforward forces for controlling the robot's center of mass are designed from simple physics-based heuristics for steady state legged movement. By regularizing the predictive optimization with these policies, we can exploit the known dynamics of the system to bias the controller towards the steady state gait while remaining free to explore the cost space during transient behaviors and disturbances. The nonlinear optimization makes use of direct collocation on the simplified dynamics to pose the problem with a highly sparse structure for fast computation. A generalized approach to the controller design is independent from specific gait pattern and reference policy and allows stabilization of aperiodic locomotion. Simulation results show dynamic capabilities in a variety of gaits including trotting, bounding, and galloping, all without changing the set of algorithm parameters between experiments. Robustness to sensor and input noise, large push disturbances, and unstructured terrain demonstrate the ability of the predictive controller to adapt to uncertainty.by Gerardo Bledt.S.M

Implementing Regularized Predictive Control for Simultaneous Real-Time Footstep and Ground Reaction Force Optimization

© 2019 IEEE. This work presents a successful implementation of a nonlinear optimization-based Regularized Predictive Control (RPC) for legged locomotion on the MIT Cheetah 3 robot platform. Footstep placements and ground reaction forces at the contact feet are simultaneously solved for over a prediction horizon in real-time. Often in academic literature not enough attention is given to the implementation details that make the theory work in practice and many times it is precisely these details that end up being critical to the success or failure of the theory in real world applications. Nonlinear optimization for real-time legged locomotion control in particular is one of the techniques that has shown promise, but falls short when implemented on hardware systems subjected to computation limits and undesirable local minima. We discuss various algorithms and techniques developed to overcome some of the challenges faced when implementing nonlinear optimization-based controllers for dynamic legged locomotion