\documentclass[12pt]{article} \usepackage{graphicx} \usepackage{times} \usepackage{verbatim} %\usepackage{makeidx} %\usepackage{times,mathptm} %\usepackage[hyperindex,pdfmark,backref,letterpaper]{hyperref} %\def\baselinestretch{1.2} \voffset -1.5cm \oddsidemargin 0.3cm \evensidemargin 0cm \textheight 22cm \textwidth 17cm \footskip 2.2cm \def\Ztitle{\Large\bf Advanced-LIGO Optical Levers \\ \Large\bf Design Requirements} \def\Zauthors{Eric D. Black, Tara Chelermsongsak, Riccardo Desalvo, \\ Zach Korth, Mark Barton, Doug Cook, Cheryl Vorvick, \\ Hiro Yamammoto, Giovanni Salvi, Rob Schofield, Michael Smith} \def\Zdate{\today} \def\Znumber{T0900174} \def\Zversion{02} \def\Zgroup{D} \def\Zfilename{http://www.ligo.caltech.edu/\~\ blacke/{\Znumber}-00.pdf} \begin{document} \input{/Users/blacke/Documents/Templates/tpageTD.tex} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % remove this from the final version % \hfill{Preliminary Version} \hfill{last revised} \hfill{\today} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \tableofcontents \newpage %\section*{ABSTRACT} %xxx %\section*{KEYWORDS} %xxx %\newpage \section{Overview} This document establishes the design requirements for the advanced-LIGO optical lever subsystem. \section{Baseline design} For the purposes of calculation we will assume a baseline design similar to that of initial LIGO (see Figure~\ref{fig:baseline-design}), \emph{i.e.} a low-power visible or near-infrared laser transmitted through a viewport and reflected from the surface of a suspended optic and sensed via a quad-cell photodetector. \begin{figure} \begin{center} \includegraphics{baseline-design.pdf} \caption{\label{fig:baseline-design}Baseline design for an optical lever, common to both advanced- and initial-LIGO.} \end{center} \end{figure} \section{Scope and objectives} \subsection{Objectives} The optical-lever subsystem has the following objectives~\cite{Peter_Fritschel}. \begin{enumerate} \item {\bf Alignment fiducial:} The first goal is to assist in restoring alignment after invasive work. Optical levers may also be used to restore alignment after loss of lock, but only if such a requirement does not drive the system to an overly complex design. \item {\bf Optic drive tuning:} Optical levers will be used to tune the actuator drives on the optics to minimize position-to-angle coupling. They must be sensitive enough to do this to a level of 0.1 rad/m or better, given the drive level available, and in a reasonable time period. \item {\bf Local damping:} While the optlevs are not expected to be used for local damping in low-noise interferometer operation (as they are in iLIGO), we do want the ability to use them for such damping during interferometer commissioning. For this, the optlev noise should be no worse than the iLIGO units, and welcome improvements would be higher frequency and/or better damped structural modes of the optlev supports. % \item Monitor the angular alignment of the large interferometer mirrors. The optical levers should preserve enough alignment information - within the field of view and dynamic range of the optical-lever subsystem - to allow reacquisition of lock, as specified quantitatively in Section~\ref{sec:stability}. % \item Monitor the angular excitation of the large optics for diagnostic purposes to enable the operator or commissioning scientist to tune the actuators to minimize this angular rotation (crosstalk) as desired. \item {\bf HAM Tables:} Monitor the angular alignment of the HAM optical tables. % \item Provide damping of the angular modes of the test masses in science mode. This should, if possible, replace other damping schemes including, OSEM-associated shadow sensors or wavefront sensors. The servos that perform this function should not introduce noise into the gravitational-wave band (noise band, $10$ Hz and above, as described below) at a level any greater than $10 \%$ of the nominal fundamental noise floor of the instrument. \end{enumerate} \subsection{Scope} As described in the ``Baseline design'' section, an optical lever unit consists of a laser transmitter, a receiver for measuring the lateral displacement of the beam, and associated optics and holding structures. \subsubsection{Inclusion} The optical-levers subsystem includes: \begin{enumerate} \item Source lasers, including fiber distribution system, if applicable. \item All optics components required to control the optical-lever beam size and to position the beam on the optic or table being monitored, \emph{except} the sensed optics and vacuum-feedthrough windows. If it is deemed necessary in the future to block stray light from the optical lever lasers, all necessary beam dumps - inside or outside the vacuum - will be included in the subsystem as well. %I hope they won't be, since that would involve us working inside the chamber, which would complicate things a lot. We'll do it if we have to, but I hope we won't. \item Quad photodetectors and the associated readout electronics to measure the lateral displacement of the optical-lever beam. \item Structural pylons for mounting source and receiver hardware \end{enumerate} \subsubsection{Exclusion} The optical-lever subsystem expressly does \emph{not} provide: \begin{enumerate} \item Servo systems, if any, to damp the angular motions of sensed optics \item Initial alignment information. \item Length sensing information. % \item Information about the radius of curvature of the measurement-point mirror (the sensed optic), for example, for thermal compensation. \end{enumerate} \subsubsection{Accessories} \begin{enumerate} \item AR-coated viewports as necessary to allow optical access to the sensed optic \item Cameras as needed for viewing the optical-lever beam to assist setup of the optical lever subsystem \end{enumerate} \section{Input to the design requirements} We had hoped, in the initial stages of setting the design requirements, that the optical levers could allow reacquisition of lock after a loss-of-lock event without the need for dither locking. The question of how much power buildup is required in each part of the interferometer for lock acquisition is, however, not well understood at the time of this writing. In the absence of a well-quantified physical model for lock acquisition as a function of power buildup, we decided to adopt the (seemingly) conservative requirement that the power in any one section of the interferometer be $1/e$ of its maximum value. The assignment of optical levers is based on calculated alignment tolerances for the different suspended optics, the results of which are given in Table~\ref{table:misalignment-tolerances}. These angles are the characteristic angles at which the power in the relevant cavity (arm, MICH, etc.) falls to $1/e$ of its full value, \emph{i.e.\ } \[ \frac{P}{P_0} = e^{-(\theta / \Theta_X)^2} \] where $X$ represents the mirror in question, ITM, ETM, BS, etc. Table~\ref{table:misalignment-tolerances} lists $\Theta_X$ for all of the suspended optics. Derivation of these values is given in an appendix to this document. \begin{table} \begin{center} \caption{\label{table:misalignment-tolerances}Misalignment tolerances, in $\mu \mbox{Rad}$, for the different suspended optics. Optics with a `Y' entry get optical levers. Throw is lever-arm distance in meters.} \begin{tabular}{|c|r|r|c|r|} \hline Optic & Straight & Folded & Lever & Throw (m) \\ \hline \hline ITM & 2.6 & 2.5 & Y & 38.2 \\ ETM & 2.3 & 2.3 & Y & 5.7 \\ ETM-H2 & 2.3 & 2.3 & Y & 3.3 \\ \hline BS & 2.8 & 2.9 & Y & 2.1 \\ \hline PRM & 157.0 & 142.0 & & \\ PR2 & 45.2 & 51.0 & & \\ PR3 & 2.7 & 2.8 & Y & 13.8 \\ \hline SRM & 86.4 & 63.1 & & \\ SR2 & 18.6 & 58.6 & & \\ SR3 & 1.9 & 2.0 & Y & 14.4 \\ \hline SM1 & 305.0 & TBD & & \\ SM2 & 83.1 & TBD & & \\ PMMT1 & 136.9 & TBD & & \\ PMMT2 & 69.4 & TBD & & \\ \hline MC1 & 125.0 & TBD & & \\ MC2 & 78.1 & TBD && \\ MC3 & 125.0 & TBD && \\ \hline FMX & N/A & TBD & Y & 1.9 \\ FMY & N/A & TBD & Y & 1.9 \\ \hline HAM2 & N/A & N/A & Y & 13.5 \\ HAM3 & N/A & N/A & Y & 13.5 \\ HAM4 & N/A & N/A & Y & 13.5 \\ HAM5 & N/A & N/A & Y & 13.6 \\ HAM8 & N/A & N/A & Y & 13.6 \\ HAM9 & N/A & N/A & Y & 13.5 \\ HAM10 & N/A & N/A & Y & 13.5 \\ HAM11 & N/A & N/A & Y & 13.7 \\ \hline \end{tabular} \end{center} \end{table} \section{Specific requirements} \subsection{Interface} The optical lever subsystem shall provide the necessary signals to ISC to reacquire alignment during reacquisition of lock. %All relevant information on the alignment at the measurement point and the signal applied at the actuation point will be recorded as much as is practical within the data acquisition and logging subsystem. Electrical connections shall conform to the standards described in T060123-03, \emph{Standard LIGO Electrical Interfaces,} by Richard Abbott~\cite{Abbott06}. \subsection{Performance} \subsubsection{Stability} \label{sec:stability} All optical lever subsystems shall exhibit no more than \[ 1.0 \mu \mbox{Rad} \] peak-to-peak pointing drift over timescales of minutes to hours, based on single-mirror misalignment calculations described in the appendix. \subsubsection{Noise} The optical levers shall impose no more than \[ %\label{eqn:x-noise-spec} 4 \times 10^{-20} \frac{m}{\sqrt{Hz}} \left( \frac{10 Hz}{f} \right)^2 \] of displacement noise on the sensed optic, in science mode, at frequencies between $10$ Hz and $10$ kHz. One mechanism for such an imposition would be radiation-pressure noise. This requirement is based on the Science Requirement noise curve. %\begin{itemize} %\item \emph{Include averaging time for acquiring and alignment measurement, something like a second or less.} %\item \emph{Does this depend on the lever-arm distance, or will it be entirely contained in the source drift? My guess is source drift, but we should look into it to be sure.} %\end{itemize} %\subsubsection{Reliability} %Optical levers shall not require entering the LVEA/VEA more than once per year per site for maintenance or repair. \subsection{Physical} Optical lever beams shall monitor the angular orientation of the following optics in all interferometers: ITM (X and Y), ETM (X and Y), PR3, SR3, and the beamsplitter (BS). Additionally, optical levers shall monitor the angular orientation of both folding mirrors in the folded interferometer at Hanford. Optical levers shall also be placed on the in-vacuo optical tables in HAMs 2, 3, 4, 5, 8, 9, 10, and 11. This requires a total of thirteen units each for H1 and L1, and fifteen for H2. In addition, at least three and preferably five additional units of each component will be produced for backup and diagnostic purposes. All together, a minimum of forty-four (44) units shall be manufactured, with the preferred total being forty-six (46). %Specifications for each source and receiver shall be identical, but lever arm lengths and pylon heights will depend on the optical layout. A stock of spares of each commonly-replaced component (e.g.\ laser diodes) shall be maintained at each site. %\begin{itemize} %\item \emph{Specify how many ``tall'' and ``short'' pylons we will need, or does that go in Conceptual Design?} %\end{itemize} %\section{Generic requirements} %The optical-lever design shall conform to the generic detector subsystem requirements given in E010613, \emph{Generic Requirements \& Standards for Detector Subsystems}. These requirements are listed below. %\subsection{Mechanical characteristics \& standards} %\subsubsection{Stray-light control workmanship} %All optical-lever components shall be manufactured according to good commercial practice. %\subsubsection{Component naming} %All components shall be identified using the LIGO Naming Convention (LIGO-E950111-A-E). %\subsection{Electrical characteristics \& standards} %Electrical equipment associated with the subsystem shall meet the EMI and EMC requirements of VDE 0871 Class A or equivalent. The subsystem shall also comply with the LIGO EMI Control Plan and Procedures (LIGO-E960036). %\subsection{Vacuum compatibility requirements} %All materials and processes used in the fabrication of in-vacuo beam dumps, if any, shall comply with LIGO Vacuum Compatibility, Cleaning Methods and Procedures (LIGO-E960022-00-D). %\subsubsection{Materials and processes} %The materials and processes used in the fabrication of the in-vacuo beam dumps, if any, shall be compatible with the LIGO approved materials list, LIGO Vacuum Compatible Materials List (LIGO-E960022). %Metal components destined for in-vacuum use shall have appropriate protective finishes on all surfaces, suitable for vacuum installation. %{\bf Processes} %\emph{\ \ \ \ \ Cleaning} %Any materials used inside the vacuum chambers shall be cleaned in accordance with LIGO-E960022-00-D or LIGO-E000007-00, and Specification Guidance for Seismic Component Cleaning, Baking, and Shipping Preparation (LIGO-L970061-00-D). %\subsection{Acoustic requirements} %Optical-lever equipment shall be designed to produce the lowest levels of acoustic noise possible and practical. As a minimum, equipment shall not produce acoustic noise levels greater than specified in Derivation of CDS Rack Acoustic Noise Specifications, LIGO-T960083. %\subsection{Earthquake requirements} %The pylons and all hardware mounted on them shall withstand normal ground seismic disturbance. Earthquake stops and/or other provisions shall be provided so as to minimize catastrophic damage to the optical-lever and adjacent equipment. %\subsection{Operating environment} %The optical-lever assemblies shall be designed to operate in the LVEA and VEA environments during installation and normal operation. In-vacuo beam dumps, if any, shall be designed to operate in the high-vacuum environment of the interferometer. %\subsection{Quality assurance} %\subsubsection{General} %{\bf Responsibility for tests} %AOS shall conduct tests to verify the as-delivered performance specifications of the subsystem. %{\bf Special tests} %\emph{\ \ \ \ \ Engineering tests} %TBD %\emph{\ \ \ \ \ Reliability testing} %TBD %{\bf Configuration management} %Configuration control of specifications and designs shall be in accordance with the LIGO Detector Implementation Plan (LIGO-M050220-02, Guidelines for Advanced LIGO Detector Construction Activities.) %\subsubsection{Quality conformance inspections} %{\bf Inspections} %Manufactured parts with LIGO identification numbers or marks shall be inspected to determine conformity with the procurement specification. %Witness samples will be acceptable proof of the properties of HR and AR coatings applied to the optical surfaces. %{\bf Demonstration} %The required attenuation characteristics of the assembled Faraday isolator shall be demonstrated prior to installation. %The required mechanical characteristics of the suspended assemblies shall be demonstrated before installation. %{\bf Test} %Appropriate tests shall be implemented to verify the specifications of the purchased components. %\subsection{Reliability} %Optical-lever assemblies shall operate reliably and shall not substantially impact the operating duty cycle of the IFO. %\subsection{Maintainability} %Passive optical-lever assemblies shall require no maintenance during normal operation of the IFO. In the event of catastrophic damage, passive optical-lever assemblies shall be repaired by replacement of subassemblies or parts that constitute the subassemblies. %Active optical-lever assemblies, such as light source(s) and photocurrent-to-voltage converters, shall be repaired by replacement at the board level with a minimum down-time of the IFO. %\subsection{Documentation} %The documentation shall consist of working drawings, assembly drawings, and alignment procedures for the optical-lever assemblies. %\subsubsection{Custom specifications} %Manufacturing specifications for custom optical-lever components and assemblies shall be developed. %\subsubsection{Design documents} %The following documents shall be produced: %\begin{itemize} %\item Conceptual design document (including supporting technical design and analysis documentation) %\item Preliminary design document (including supporting technical design and analysis documentation) %\item Final design document (including supporting technical design and analysis documentation) %\item Installation procedures %\end{itemize} %\subsubsection{Engineering drawings and associated lists} %A complete set of drawing suitable for fabrication shall be provided along with Bill of Material (BOM) and drawing tree lists. The drawings shall comply with LIGO standard formats and shall be provided in electronic format. All documents shall use the LIGO drawing numbering system, LIGO-E030350-A, Drawing Requirements. %\subsubsection{Documentation numbering} %All documents shall be numbered and identified in accordance with the LIGO documentation control numbering scheme, LIGO-E030350-A, Drawing Requirements. \section{Documentation} Appropriate documentation shall be maintained at each site \section{Testing} A test plan shall be developed for verifying the specifications of the purchased components, in accordance with LIGO M050220-02, Guidelines for Advanced LIGO Detector Construction Activities. \subsection{Noise and stability performance} The advanced-LIGO optical lever subsystem shall be tested to verify that it's noise and stability are at least as good as, and whenever feasible better than, that of the initial-LIGO optical levers. Baseline performance of the initial-LIGO levers is described in Ref.\ \cite{T1000157} and [ref.\ Sam Waldman's, et al. noise v. frequency study, with the whitening filters Rana called our attention to]. \subsection{OSEM interference} A quantitative measurement of the influence of the optical levers on the OSEMs, or at least an upper bound on it, shall be made and its influence on the interferometer performance evaluated. %\subsection{Transportability} %All optical-lever components shall be transportable by commercial carrier without degradation in performance. %\subsubsection{Preparation} %\begin{itemize} %\item Vacuum preparation procedures as outlined in LIGO-E960022-B LIGO Vacuum Compatibility, Cleaning Methods and Qualification Procedures shall be followed for all components intended for use in vacuum. After wrapping vacuum parts as specified in this document, an additional, protective outer wrapping and provisions for lifting shall be provided. %\item Electronic components shall be wrapped according to standard procedures for such parts. %\end{itemize} %\subsubsection{Packaging} %Procedures for packaging shall ensure cleaning, drying, and preservation methods adequate to prevent deterioration, appropriate protective wrapping, adequate package cushioning, and proper containers. Proper protection shall be provided for shipping loads and environmental stress during transportation, hauling and storage. Passive shock witness gauges shall accompany the crates of damage-susceptibe assemblies during all transits. %\subsubsection{Marking} %Appropriate markings shall be provided for identification, delivery and storage, and to comply with regulations, statutes, and safety. %Identification of the material shall be maintained through all manufacturing processes. Each component shall be uniquely identified in order to maintain a complete history of the component (in association with Documentation ``travelers''). \section{Safety} It is preferred that each source operate at no more than 5 mW of total output power, so as to obviate any need for safety goggles or specialized enclosures for the beam. All components shall meet all applicable NSF and other Federal safety regulations, plus those applicable State, Local, and LIGO safety requirements. A hazard/risk analysis shall be conducted in accordance with guidelines set forth in the LIGO Project Safety Management Plan LIGO-M950046-F, Section 3.3.2. %\section{Acronyms} %\ %AOS - Auxiliary Optics System %AR - Antireflection mirror coating %BRDF - Bi-directional Reflection Distribution Function %BS - Beam Splitter %BSC - Beam Splitter Chamber %CA - Clear Aperture %CDS - Computer Data Systems %COC - Core Optics Component %CP - Compensation Plate %DARM - Differential arm %dB - Decibel %Deg - Degree %ETMx, ETMy - End Test Mass in the interferometer `X' or `Y' arm %GB - Ghost Beam %H - Horizontal %HAM - Horizontal Access Module %HEPI - Hydraulic External Pre-Isolation %HR - High-Reflectance mirror coating %IFO - LIGO InterFerOmeter %IO - Input Optics %ISC - Interferometer Sensing and Control %ITMx, ITMy - Input Test Mass in the interferometer `X' or `Y' arm %LHO - LIGO Hanford Observatory %LIGO - Laser Interferometer Gravitational-wave Observatory %LLO - LIGO Livingston Observatory %m/rtHz, m/ÃHz - meters per root Hertz %mm - millimeter %nm - nanometer %PO - Pick-Off %ppm - parts per million %PRM - Power Recycling Mirror %PSL - Pre-Stabilized Laser %R - Reflectivity %SEI - Seismic %SLC - Stray Light Control %sr - steradian %SRD - Science Requirements Document %SRM - Signal Recycling Mirror %V - Vertical %w - Gaussian beam radius parameter %W - Watt \newpage \begin{thebibliography}{99} \bibitem{Peter_Fritschel} Peter Fritschel, private communication. \bibitem{Tipler} Tipler, \emph{Physics}, 2nd ed., p. 276. \bibitem{Mark} Mark Barton, private communication \bibitem{Abbott06} Richard Abbott, \emph{Standard LIGO Electrical Interfaces,} T060123-03 (last updated November, 2008). \bibitem{T1000157} Tara Chelermsongsak, Riccardo Desalvo, and Eric Black, \emph{Optical Lever: Longterm test of the Laser Diode}, T1000157-v1 (last updated 25 March, 2010). \end{thebibliography} %\newpage %\appendix %\newpage %\section{ \ \ \ \ \ \ Quad suspension modal frequencies} %The following is a list of mechanical resonant frequencies in the quad suspension~\cite{Mark}. %\begin{center} %\begin{tabular}{|r|r|ccc|} \hline %N & $f$ (Hz) & & participation & \\ \hline %1 & 0.32 & pitch3 & pitch2 & \\ %2 & 0.44 & x3 && \\ %3 & 0.46 & y3 && \\ %4 & 0.58 & z3 & z2 & \\ %5 & 0.60 & yaw3 & yaw2 & \\ %6 & 0.82 & roll3 & roll1 & roll2 \\ %7 & 0.99 & x2 & x3 & \\ %8 & 1.04 & y2 & y3 & \\ %9 & 1.20 & pitch0 & pitch3 & pitch2 \\ %10 & 1.34 & yaw3 & yaw1 & \\ %11 & 1.50 & pitch0 & pitch2 & \\ %12 & 1.99 & x0 & x1 & \\ %13 & 2.11 & y0 & y1 & \\ %14 & 2.34 & z0 & z1 & \\ %15 & 2.40 & yaw0 & yaw2 & \\ %16 & 2.69 & roll1 & roll0 & roll3 \\ %17 & 2.94 & pitch1 && \\ %18 & 3.03 & yaw1 & yaw0 & \\ %19 & 3.31 & roll1 & y1 & roll0 \\ %20 & 3.41 & x1 & x0 & \\ %21 & 3.76 & z1 & z0 & \\ %22 & 5.10 & roll0 & y0 & \\ %23 & 8.99 & z2 & z3 & \\ %24 & 12.85 & roll2 & roll3 & \\ \hline %\end{tabular} %\end{center} \end{document}