Closed-loop Cavity Research Articles

Integrated closed-loop cavity of a tunable laser

In this paper, a closed-loop cavity of a tunable laser integrated onto a silicon chip is demonstrated. The closed-loop cavity consists of a semiconductor optical amplifier chip, two separated micro-ring resonators, and a U-shaped waveguide sub-loop, enabling dominating lasing in the counterclockwise direction. The lasing wavelength is tuned by varying the effective refractive index of the thermal ring-resonators. It has achieved wide tuning range (55.4 nm), high spectral purity (50-dB side mode suppression ratio), ∼1-mW output power, and 36-dB counter-propagation power suppression ratio. The integrated tunable laser has high potential in applications such as optical network, optical sensing, and integrated optoelectronic systems.

A self-optimising injection moulding process with model-based control system parameterisation

The consideration of the pvT-behaviour (pressure, specific volume and temperature) of the plastic material in combination with closed-loop cavity pressure control allows for compensation of variable boundary conditions in the injection moulding process.By suitably implementing cavity pressure control, repeatability and product quality in injection moulding processes can be improved. However, there are still obstacles for industrial application. As the process behaviour is greatly dependent on the mould – which is interchangeable and typically designed and manufactured independently of the machine’s control system – challenges arise in designing a cavity pressure controller that yields high performance and at the same time is robust enough to be suitable for universal use.The use of a model predictive controller (MPC) for cavity pressure control is being researched and found to be helpful to overcome these issues. Unlike controllers such as proportional-integral-derivative controllers, the control output is not determined using a well-tuned, but mathematically relatively simple algorithm. Instead, it performs an online optimisation based on a process model in order to obtain the control outputs. In order to operate as intended, the model used by the MPC has to be adjusted with every significant change of the system, in particular the machine, material and mould.Therefore, a process model as well as a suitable strategy for in-process identification of the necessary parameters is developed and presented.For automated parameterisation, a strategy based on two experiments is suggested and first experimental results are presented.

Optically controlled light-pulse propagation in closed-loop configuration with saturable erbium-doped fiber

We report experimental results on the 1550 nm modulated light propagation in a closed-loop all-fiber resonance configuration, which includes a segment of the erbium-doped fiber with saturable optical absorption. It is shown that by a simple increase of the average incident light power, one can switch from the undercoupling to the overcoupling with the closed-loop cavity, which leads to a dramatic change from advancement to delay in the transmitted optical signal with the resonance carrier frequency. In the reported experiments with the ≈4.4 m long closed-loop cavity (with a 2.4 m long EDF segment) we observed a switch from ≈0.26 fractional advancement to ≈0.65 fractional delay in 166 ns periodic light pulses with a resonance frequency in the 1550 nm wavelength range when the average incident power was changed from 0.09 to 0.6 mW.

250 mJ, self-adaptive, diode-side-pumped Nd:YAG slab laser

A scheme of self-adaptive, closed-loop, diode-side-pumped Nd:YAG slab laser was presented. As a result of four-wave mixing of standing waves intersecting at a small angle in a closed-loop cavity, the self-adaptive process of beam cleaning leading to fundamental mode operation despite severe asymmetry of the inversion profile and thermal-optical distortion was achieved. The output beam was extracted from the cavity as the first- order diffraction beam on the dynamic gain gratings created in an active medium. The near-diffraction-limited (parameter M(2)<1.4, divergence of 1 mrad) output beam of 250 mJ energy and the optical slope efficiency of 30% at a repetition rate of up to 25 Hz in a free-running regime were demonstrated.

Optical Fiber Ring Cavity Sensor for Label-Free DNA Detection

An outcladding sensitized label-free DNA biosensor is developed based on tilted fiber Bragg gratings. The biosensor, functionalized with peptide nucleic acid (PNA) probes, is based on a double tilted fiber Bragg grating that forms a modified Fabry-Perot core-cladding closed-loop cavity. Interference is set up between an injected guided mode and the reflected core mode and cladding modes created by the light scattered by the tilted gratings, leading to the generation of interference fringes within both these spectral notches. When the DNA binds to the PNA probes attached onto the fiber cladding, a refractive index change occurs at the cladding-PNA interface and the fringe visibility changes accordingly. Real-time spectral measurement results are reported, showing that a 10-nM DNA solution induces a 10% modulation of the corresponding fringes visibility. Cycling tests are performed for measuring and checking the repeatability of the sensor.

High-temperature lasing characteristics of randomly assembled ZnO nanowires with a ridge waveguide

High-temperature lasing characteristics of randomly assembled ZnO nanowires at ultraviolet wavelength are investigated. Lasing emission was observed from the randomly assembled ZnO nanowires for temperature up to 500 K. The corresponding lasing mechanism is attributed to the coherent random lasing action via the formation of closed-loop cavity modes. Furthermore, ridge waveguide lasers using the randomly assembled ZnO nanowires as the active medium were fabricated. It is found that the width of the ridge waveguides restricted the size of the closed-loop cavity modes such that the corresponding characteristic temperature can be increased by 10 K.

Dynamic Compensation of a Synthetic Jetlike Actuator for Closed-Loop Cavity Flow Control

Actuation devices are crucial components of closed-loop flow control schemes. Synthetic jetlike actuators, which are commonly employed in cavity flow control, exhibit a dynamic response that, if ignored, may significantly affect the overall characteristics of the closed-loop system. This paper presents the development and implementation of a dynamic compensator for a synthetic jetlike compression driver actuator which has been successfully implemented for feedback control of subsonic cavity flows. A time-delay model of the actuator dynamics is obtained from experimental data using subspace-based identification methods. The model is designed to match the frequency response of the physical system in a frequency range of interest that covers the resonance frequencies of the cavity. The model is then used for the synthesis of a dynamic controller which employs a Smith predictor in conjunction with an H ∞ mixed-sensitivity design. Order reduction is applied to obtain a low-order digital controller amenable to real-time applications. The compensator is retrofitted to an existing cavity flow control architecture, and used to force the actuator output to closely follow the input commands, thereby compensating undesirable actuator dynamics. Experiments show that the integration of the actuator compensator within the cavity control system significantly improves closed-loop performance.