Front Mirror Research Articles

Local readout enhancement for detuned signal-recycling interferometers

High power detuned signal-recycling interferometers currently planned for second-generation interferometric gravitational-wave detectors (for example Advanced LIGO) are characterized by two resonances in the detection band, an optical resonance and an optomechanical resonance which is upshifted from the suspension pendulum frequency due to the so-called optical-spring effect. The detector's sensitivity is enhanced around these two resonances. However, at frequencies below the optomechanical resonance frequency, the sensitivity of such interferometers is significantly lower than non-optical-spring configurations with comparable circulating power; such a drawback can also compromise high-frequency sensitivity, when an optimization is performed on the overall sensitivity of the interferometer to a class of sources. In this paper, we clarify the reason for such a low sensitivity, and propose a way to fix this problem. Motivated by the optical-bar scheme of Braginsky, Gorodetsky, and Khalili, we propose to add a local readout scheme which measures the motion of the arm-cavity front mirror, which at low frequencies moves together with the arm-cavity end mirror, under the influence of gravitational waves. This scheme improves the low-frequency quantum-noise-limited sensitivity of optical-spring interferometers significantly and can be considered as an incorporation of the optical-bar scheme into currently planned second-generation interferometers. On the other hand it can be regarded as an extension of the optical-bar scheme. Taking compact binary inspiral signals as an example, we illustrate how this scheme can be used to improve the sensitivity of the planned Advanced LIGO interferometer, in various scenarios, using a realistic classical-noise budget. We also discuss how this scheme can be implemented in Advanced LIGO with relative ease.

ITER ECRH-ECCD System Capabilities for Extended Physics Applications

The ability of ITER electron cyclotron (EC) wave launchers to drive localized current at various plasma locations is analyzed by means of beam-tracing codes, looking at extended physics application of EC current drive in ITER and at possible synergy between the two launchers. Calculations for an improved design of the upper launcher, based on four upper ports and front steering mirrors allowing both optimum focusing of the beams and an extended plasma deposition region, show that narrow, high peak current density profiles may be maintained over the radial range 0.4 ≤ ρp ≤ 0.9. Calculations for the equatorial launcher, where the control of the deposition location is achieved by varying the toroidal injection angle β, point out that because of poor localization and incomplete power absorption at large toroidal angles (β > 40 deg), the power deposition and current drive location by this launcher is limited to ρp ≤ 0.55. Moreover, it is shown that performance close to the center can be improved with a poloidal tilt of the low and top front mirrors. The main aim of this study is to provide guidance to the design of both launchers in order to optimize their performance, depending on the physics application.

Lithographic nano-patterning of colloidal nanocrystal emitters for the fabrication of waveguide photonic devices

In this work, we show the fabrication of colloidal nanocrystals (NCs) based waveguide photonic devices by exploiting a new lithographic approach for the nano-positioning of NCs. Our approach relies on the dispersion of CdSe/ZnS core/shell NCs into a layer of electro-sensitive resist (poly-methyl methacrylate, PMMA), which is subsequently patterned by means of electron beam lithography (EBL). This localization technique has been demonstrated by detecting high-resolution photoluminescence (PL) maps on an e-beam nano-patterned PMMA/NCs film through a confocal microscope. This technique has been exploited for the fabrication of distributed feedback (DFB) structures and distributed Bragg reflectors (DBR) suitable for the realization of in-plane waveguide lasers. A DFB structure has been obtained by patterning a periodical grating on a PMMA/NCs ridge waveguide. Two DBR have been similarly fabricated by creating quarter-wavelength thick alternated stripes of air and active blend; these structures have been exploited as front and back mirrors of an in-plane PMMA/NCs ridge cavity. In both cases, no following etching processes have been required, thus simplifying the steps needed for the fabrication of NCs based active photonic devices.

Investigation of dark line defects induced by catastrophic optical damage in broad-area AlGaInP laser diodes

The authors present a detailed investigation of defects generated during catastrophic optical damage (COD) in high-power 650nm AlGaInP lasers using microphotoluminescence (μ-PL) mapping, focused ion beam (FIB) microscopy, and deep-etching techniques. High-resolution μ-PL images demonstrated that during COD, nonradiative dark line defects (DLDs) originate from the front mirror of the laser and propagate in several branches into the laser perpendicular to the output facet. Furthermore, FIB microscopy identified the epitaxial layers affected by COD, revealing that DLDs are confined to the active region. In addition, deep etching confirmed that these defects have a noncrystalline nature.

Beam Steering Type of 1 : 4 Optical Switch Using Thermo-Optic Effect

We propose a beam steering type optical switch employing a phase shifter. It consists of collimating waveguide mirrors, an arrayed-waveguide which has deep trenches incorporating polymer materials, and input/output waveguides. The incident light is guided to the front mirror, where it is then collimated, and input to the arrayed waveguide. The number of trenches filled with polymer linearly increases in order. The refractive index of the polymer can be controlled with temperature, and the propagation direction of the output light from the arrayed waveguide can also be controlled. A switch with 15 waveguides in the array has been fabricated. The chip size is about 2:5 × 9:0 mm2, the insertion loss is 10 dB, and the extinction ratio is about 10–13 dB.

Large enhancement of GaN‐UV light emission using silver mirror resonator

AbstractIn this paper we used new microcavity resonator design of extracting light with high efficiency from a high index material (GaN), n > 2 for use in light emitting diode (LED) and laser diode (LD). A GaN/sapphire structure as an active layer was sandwiched between two silver mirrors. For the study two types of microcavity were fabricated: (air/GaN/sapphire/silver) and (silver/GaN/sapphire/silver). Via photo‐luminescence measurements we observed 2‐fold intensity enhancement in the UV region (364 nm) at room temperature by using 400 nm silver back mirror compared with uncoated sample. The amplitude of the photoluminescence is enhanced 10‐times when we used 400 nm back coated together with 50 nm silver as front mirror. Further, a tremendous enhancement was obtained when a small hole was made in the front mirror with 50 nm thickness. The large increment of GaN/sapphire light emission in this microcavity could be caused by few important factors: increasing absorption in the cavity, optical field enhancement due to the resonator and coupling of plasmon in the metallic layer with the UV light. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Neodymium random laser with external mirrors

We have studied the influence of the front and the rear mirrors on the emission properties of neodymium random lasers at different thicknesses of the powder layer. Introduction of external mirrors drastically reduces the lasing threshold of the random laser, with the effect of the front mirror being more dominant. External mirrors also influence the location of the stimulated emission region within the powder volume. The experimental results are in a good agreement with the results of the Monte-Carlo simulation.

Less than 5-ns wavelength switching with an SG-DBR laser

We achieve fast wavelength switching between all channel combinations of a 64-channel sampled-grating distributed Bragg reflector laser in less than 5 ns. The laser was designed to have low tuning currents and efficient heat extraction in order to reduce parasitic thermal effects. We also report on the behavior of the front and back mirror pre-emphasis amplitudes required for the fast switching.

Tunable GaInNAs lasers with photonic crystal mirrors

We present results of tunable GaInNAs lasers with photonic crystal (PhQ mirrors, fabricated from GaAs-AlGaAs layers with a double GaInNAs quantum well emitting at IR wavelength. The devices are realized as ridge waveguide lasers with two coupled segments and a total length between 240 and 580 pm. PhC blocks with different thicknesses are used for the back and front mirror as well as for the intermediate reflector between the two segments. The lasers have threshold currents around 20 mA and output powers up to 6 mW. Tuning of the laser emission over 30 nm is achieved by a variation of the currents injected into the two segments.

Design of the mm-wave system of the ITER ECRH upper launcher

The design work for the ITER ECRH remote-steering upper-port launcher is described. The goal of the system is the stabilization of neoclassical tearing modes (NTMs). The required scanning range in the plasma (more than 20°) is achieved remotely, far away from the hostile plasma environment. This is done behind a diamond window in a separate vacuum environment. Scanning of the beam is done by a mirror that both rotates and translates to steer the beam in the proper way through the window and an isolation valve into a square corrugated waveguide. At the output of this 4.4 m long waveguide, the beam comes out at the same angle and is reflected by a fixed front mirror into the plasma [A.G.A. Verhoeven, et al., The ITER remote-steering ECW upper-port launcher, in: N. Hiromoto (Ed.), The 28th International Conference on Infrared and MM Waves, Otsu, Japan, September 29–October 3, 2003 (JSAP Catalog No. 031231)].

Resonator effects of Ar2+ ionic excimer pumped by electron beam

The third continuum of argon from 200 to 270 nm is obtained by electron beam pumping argon. With the front mirror and the rear mirror, the resonator effects are observed at 240 and 220 nm. Compared with the intensity of fluorescence, the intensity near 240 nm increases more than 10 times when the resonator is formed. In order to explain the origin of the continuum, the kinetic model on Ar2+ and Ar22+ ionic excimers pumped by electron beam is built and calculated. Based on the theoretical results, the Ar2+ ionic excimer should be responsible for the continuum around 240 nm.

Reflectivity Modelling of All-Porous-Silicon Distributed Bragg Reflectors and Fabry-Perot Microcavities

Herein, the problem of nanocrystaline silicon laser and its importance in microelectronics are discussed upon. The features of vertical Fabry-Perot microcavities made on the base of porous silicon are described. The responses of the reflectivity of the distributed reflection Bragg mirrors and Fabry-Perot microcavities were found using transfer matrixes method for this purpose. Inherent optical parameters of porous silicon, deposited by electrochemical etch, were used in the calculations. The calculation of the reflectivity of the distributed reflection Bragg mirrors with front active layer of nanostructural porous silicon had been examined. In the second part, the features of Fabry-Perot microcavities on variation of the number of layers of the front or rear mirrors are described. The impact of the thickness of the active nanocrystaline silicon spacer between two distributed reflection Bragg mirrors upon the spectra of optical reflectivity of Fabry-Perot microcavities in the wavelength range of 0.4–0.9 µm had been examined as well. The made conclusions are important for improvement of the thickness of the active porous silicon spacer in front of Bragg mirror and the features of Fabry-Perot microcavities.

Integration of a dog-leg beam routing for the remote steering upper port launcher for ITER

In the course of the development of a remote steering ECRH upper port launcher for ITER, it became clear that a modification could be introduced in the conceptual design in order to solve a number of structural weaknesses. Up to that point, all conceptual layouts were based on each remote steering beamline having a single front mirror placed in front of the square waveguide to aim the beam towards its resonance surface in the plasma. By placing an additional mirror per line inside the front shield of the upper port plug - effectively creating a dogleg routing - a number of structural issues were solved. This modification allows for a decrease of the heat load on the front mirrors and a shift downwards of the launching point. Additionally, through correct placement and focusing of the mirrors, the front shield penetration could be reduced by a factor of 4 and the cut in the blanket module below the upper port could be reduced significantly, while the level of overall performance could be increased as well. In order to visualise this new concept accurately, a more detailed design of the beam propagation was required. Through concerted effort within our institute, two different approaches were made to come to this new design; further advancements of the 3D-model and an Excel-based 2D simulation. This dual approach, together with beam tracing calculations done by affiliated institutes have indicated that the dogleg can prove to be a reliable design for a RS upper port launcher.

The Tore Supra Front Steering Long Pulse ECRH Antenna. Past Experience - Present Evolution - Future Experiments

The long pulse capability of Tore Supra and its ECRH system makes it an ideal machine to prove steady state feedback control as required in ITER. Although Neoclassical Tearing Modes (NTMs) have not yet been observed on Tore Supra, the control of other MHD modes represents a very similar task from a control point of view and the stabilisation of such modes for long periods using ECRH will provide essential experience for the implementation of such control schemes on ITER. For this work to progress on Tore Supra, it must be possible to vary the injection angles in real time under feedback control from measured plasma parameters. At Tore Supra the front mirror position - and hence the injection angles - is adjusted using stepper motors controlled through a serial link. The use of a serial link limiting the sampling time for the control system to 50-100 ms and the dynamic response of the stepper motors results in a system frequency response <5 Hz. For following the evolution of the current profile this seems fully acceptable though it could prove somewhat slow for reacting to fast beta changes. In any case the time constants typically associated with growth rate of NTMs on ITER would not require a significantly faster system. Initial commissioning of such real time control was performed in 2004. Unfortunately, following a limited number of cycles and prior to using the system for plasma experiments, one water bellows in the antenna ruptured indicating a major weakness in the design of the water-cooling system for the antenna. As a consequence the water-cooling of the mobile mirror has been redesigned. Careful calculations and subsequent tests have been used to optimise the trajectories of the flexible water connections and a more robust layout is being implemented, ready for operation in the summer 2005.

Measurement and calculation of the `Electron efficiency' on the `CLIO' Free-Electron Laser

This paper describes the recent measurements of ‘Electron efficiency’ which has been done with the ‘CLIO’ Free-electron laser (FEL). The ‘Electron efficiency’ is the relative energy loss of the electron beam in the undulator during the FEL interaction. It gives an absolute measurement of the optical power produced by the FEL, which is then compared to the direct measurement using a power meter at the exit of the FEL. An analytical expression of the ‘Electron efficiency’ is given here, and it is compared to the measurements. THE CLIO FREE ELECTRON LASER The ‘CLIO’ Free Electron Laser (FEL) is an infrared tuneable laser source [1]. It uses a linear accelerator, which produces an electron beam of adjustable energy from 13MeV to 50MeV. The laser spectral range is tuneable from 5µm to about 120µm. Such a wide broadband is achieved by using, for λ >100µm, a special set of toroidal mirrors. Also, in order to reduce the cavity losses at large wavelength, a waveguide is installed in place of the vacuum chamber in the undulator section. The laser extraction of the cavity is achieved by hole coupling in the front mirror. The best configuration of radius of curvature for the cavity mirrors has been obtained with a numerical code [2] [3]. This code calculates the propagation of the laser wave front A(x,y) in the optical cavity. It uses an iterative process of wave propagation in the cavity, which converges to a steady state laser mode As(x,y) corresponding to the FEL saturation regime. It takes into account the design of the cavity (mirrors, vacuum chamber,…) including the waveguide effect in the undulator section, and the hole coupling in the front mirror. This code gives the amplitude distribution As(x,y) of the laser mode at saturation, in any point of the cavity, and it gives all related parameters : the cavity losses L, the ‘extraction rate’ Tx of hole coupling, the optical mode cross-section Σo,…

290 fs switching time of Fe-doped quantum well saturable absorbers in a microcavity in 1.55μm range

Ultrafast all-optical switch has been demonstrated by inserting Fe-doped multiple quantum wells (MQWs) in asymmetric Fabry-Perot microcavities. Heavy Fe doping during the InGaAs∕InP MQW epitaxial growth is a well-controlled technique to reach subpicosecond optical time constants. An asymmetric Fabry-Perot microcavity using gold metal as a back mirror and air/InP interface as a front mirror is realized. Pump-probe experiments using a conventional scheme on such switching devices are investigated. The device reveals an ultrafast response time, as low as 290fs, for an iron concentration of 2×1019cm−3, a contrast ratio of 8dB, a threshold switching fluence of 3.5μJ∕cm−2, and a 37-nm 3-dB bandwidth in the 1.55-μm telecommunication spectral range.

Noninverted interference fringes upon light reflection from a Fabry-Perot interferometer with an asymmetric diffraction mirror

This paper describes the calculation, fabrication, and experimental investigation of a reflection two-mirror interferometer whose front mirror is asymmetric in reflection and made as a thin-layer diffraction metal structure (Al, Ag) in combination with a dielectric coating (MgF2, ZnS). In reflection, this interferometer creates in the zero diffraction order a transmission-like pattern similar to that produced by an ordinary Fabry-Perot interferometer operating in transmitted light.

GaAs -based 1.3μm microlasers with photonic crystal mirrors

We present results of GaAs-based microlasers with photonic crystal mirrors. The lasers are fabricated from GaAs∕AlGaAs layer structures with a doub1e GaInNAs quantum well emitting at 1.3μm wavelength. The devices are realized as ridge waveguide lasers with two coupled cavities and a total length between 175 and 600μm. Photonic crystals are used to define the front and back mirrors of the lasers. Threshold currents around 40mA and output power levels of 80mW were achieved. Single-mode emission with 30dB side-mode suppression ratio is obtained due to mode interference between the two cavities.

80&amp;lt;tex&amp;gt;$^circ$&amp;lt;/tex&amp;gt;C Continuous-Wave Operation of 2.01-&amp;lt;tex&amp;gt;$mu$&amp;lt;/tex&amp;gt;m Wavelength InGaAlAs–InP Vertical-Cavity Surface-Emitting Lasers

Electrically pumped buried tunnel junction InGaAlAs-InP vertical-cavity surface-emitting lasers (VCSELs) with self-adjusted lateral current and optical confinement and record emission wavelengths beyond 2 /spl mu/m are presented. Front and back side mirrors are realized using 31.5 epitaxial layer pairs of alternating InGaAs-InAlAs and a dielectric 2.5 pair CaF/sub 2/-a-Si layer stack. The devices show single-mode continuous-wave operation up to heat sink temperatures over 80/spl deg/C. The maximum output power at 20/spl deg/C reaches 0.43 mW, threshold current and voltage are as low as 0.66 mA and 0.73 V, respectively. To reach the long emission wavelength, we use an optimized active region comprising heavily strained quantum wells. High-resolution X-ray diffraction and photoluminescence measurements reveal excellent material quality without relaxation in the quantum wells.

Multiplexing thermography for International Thermonuclear Experimental Reactor divertor targets

The concept of multiplexing thermography is applied to the design of the divertor thermography system for International Thermonuclear Experimental Reactor (ITER). The combination of the front mirror with multiellipticity and a Czerney–Turner spectrometer with a 0.2 mm pitched multichannel detector enables a spatial resolution of 3 mm and a time resolution of 20 μs above a target temperature of 300 °C to be achieved. This should be sufficient to measure ELM heat fluxes to the targets in ITER. To satisfy the measurement requirement, it is very important to keep an accurate alignment around the optical axis against movement of the vessel during the plasma discharges. Several key engineering problems, such as the survivability of components against mirror coating by redeposited divertor material, remain to be solved. Potential solutions have been identified.