EP1166064A1 - Systeme de diagnostic integre pour modulateur photoelastique - Google Patents

Systeme de diagnostic integre pour modulateur photoelastique

Info

Publication number
EP1166064A1
EP1166064A1 EP00919988A EP00919988A EP1166064A1 EP 1166064 A1 EP1166064 A1 EP 1166064A1 EP 00919988 A EP00919988 A EP 00919988A EP 00919988 A EP00919988 A EP 00919988A EP 1166064 A1 EP1166064 A1 EP 1166064A1
Authority
EP
European Patent Office
Prior art keywords
diagnostic
optical element
light
primary
pem
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP00919988A
Other languages
German (de)
English (en)
Inventor
Paul Kadlec
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Hinds Instruments Inc
Original Assignee
Hinds Instruments Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Hinds Instruments Inc filed Critical Hinds Instruments Inc
Publication of EP1166064A1 publication Critical patent/EP1166064A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/0128—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on electro-mechanical, magneto-mechanical, elasto-optic effects
    • G02F1/0131—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on electro-mechanical, magneto-mechanical, elasto-optic effects based on photo-elastic effects, e.g. mechanically induced birefringence
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/0121—Operation of devices; Circuit arrangements, not otherwise provided for in this subclass

Definitions

  • This application relates to an integrated optical diagnostic system for a photoelastic modulator.
  • a resonant photoelastic modulator is an instrument that is used for modulating the polarization of a beam of light.
  • a PEM employs the photoelastic effect as a principle of operation.
  • the term "photoelastic effect” means that an optical element that is mechanically strained (deformed) exhibits birefringence that is proportional to the amount of strain induced into the element. Birefringence means that the refractive index of the element is different for different components of polarized light.
  • a PEM includes an optical element, such as fused silica, that has attached to it a piezoelectric transducer for vibrating the optical element at a fixed frequency, within, for example, the low-frequency, ultrasound range of about 20 kHz to 100 kHz.
  • the mass of the element is compressed and extended as a result of the vibration.
  • the compression and extension of the optical element imparts oscillating birefringence characteristics to the optical element.
  • the frequency of this oscillating birefringence is the resonant frequency of the optical element and is dependent on the size of the optical element, and on the velocity of the transducer-generated longitudinal vibration or acoustic wave through the optical element.
  • Retardation or retardance represents the integrated effect of birefringence acting along the path of electromagnetic radiation (a light beam) traversing the vibrating optical element. If the incident light beam is linearly polarized, two orthogonal components of the polarized light will exit the optical element with a phase difference, called the retardance. For a PEM, the retardation is a sinusoidal function of time. The amplitude of this phase difference is usually characterized as the retardance amplitude or retardation amplitude of the PEM.
  • Both the size and acoustic wave velocity of a PEM depend on the optical element's temperature. Consequently, the resonant frequency of a PEM will also depend on the device's temperature. In general, this temperature depends on two factors: (1 ) the ambient temperature, and (2) the amplitude of the stress oscillations in the optical element. At high stress amplitudes, the amount of acoustic (mechanical) energy absorbed in the optical element can become significant. As the absorbed acoustic energy is converted to heat within the mass of the element, significant temperature increases and corresponding shifts in the PEM's resonant frequency can occur.
  • the system's operating frequency is determined by the PEM's resonant frequency and, as explained above, thus depends on both ambient temperature and the amplitude at which the PEM is driven. This results in an operating frequency that drifts with ambient temperature, as well as during warm- up and after changes in the set retardation amplitude. Such a situation may be undesirable in certain applications where the PEM's operating frequency, as well as its amplitude, must be kept constant.
  • the present invention is generally directed to a diagnostic system for a PEM.
  • the system provides optically determined information about the retardance characteristics induced by the PEM.
  • the diagnostic system is integrated with the PEM so that the PEM performance may be diagnosed or monitored during operation of the PEM.
  • the diagnostic system is used alongside an optical setup that employs a primary light beam for conventional purposes such as polarimetry, optical metrology, etc.
  • the diagnostic system includes its own diagnostic light source that is directed through the optical element of the PEM at a location remote from the primary aperture of the PEM.
  • the diagnostic system and the primary PEM operation can be undertaken simultaneously, with one not interfering with the other.
  • Fig. 1 is a diagram of a preferred embodiment of the integrated diagnostic system for a photoelastic modulator configured in accordance with the present invention.
  • Fig. 2 is a diagram showing one side of a part of a photoelastic modulator component of the present system.
  • FIG. 1 depicts a photoelastic modulator (PEM) 20 that is used in a primary optical setup 22 (described more below) and that has integrated with it a diagnostic system 24 configured in accordance with present invention.
  • PEM photoelastic modulator
  • the block 22 shown in dashed lines in Fig. 1 is intended to represent any of a variety of optical setups with which a PEM may be employed.
  • One such setup is described in international application number PCT/US99/03481 , hereby incorporated by reference (the '481 application).
  • the setup employed in the '481 application is used for precise measure of the birefringence characteristics of an optical sample. That setup is summarized next for the purposes of explaining an exemplary, primary setup 22 with which the later-described diagnostic system may be integrated.
  • the primary setup 22 includes a primary light source 26 such as a HeNe laser that has a wavelength of 632.8 nanometers (nm).
  • the light beam 28 emanating from the source has a cross sectional area or "spot size" of approximately 1 millimeter (mm).
  • the primary light beam 28 is directed to be incident on a polarizer 30 that is oriented with its polarization direction at +45° relative to a baseline axis.
  • a high-extinction polarizer such as a Glan-Thompson calcite polarizer, is preferred. It is also preferred that the polarizer 30 be secured in a precision, graduated rotator.
  • the polarized light from the polarizer 30 is incident on the optical element 32 of the PEM 20.
  • the PEM is one manufactured by Hinds Instruments, Inc., of Hillsboro, Oregon, as a low-birefringence version of Model PEM-90 I/FS50.
  • the primary beam 28 is directed through the primary aperture 38, which, in a bar-type optical element, is located at the center of the optical element 32 and provides the maximum oscillating birefringence effect (hence, producing the greatest retardance amplitude in the primary beam).
  • the birefringence effect diminishes in the direction away from the aperture, toward the ends of the optical element.
  • the PEM 20 has its birefringent axis oriented at 0° and is controlled by a controller 40 that imparts an oscillating birefringence to the optical element 32, preferably at a nominal frequency of 50 kHz.
  • the controller 40 drives two quartz transducers 34 between which the optical element 32 is bonded with an adhesive.
  • the oscillating birefringence of the PEM 20 introduces retardance characteristics into the primary beam 28.
  • retardance can be considered as the time-varying phase difference between the orthogonal components of the polarized light that propagates through the PEM.
  • the peak amplitude of this phase shift is referred to as the PEM's retardance, and is measurable in units of length, such as nanometers.
  • the PEM controller 40 provides controls for adjusting the amplitude of the retardance introduced by the PEM.
  • the retardation amplitude may be selected to be 0.383 waves (242.4 nm).
  • the beam of primary light 28 propagating from the optical element 32 of the PEM is directed through the transparent sample 36.
  • the sample is supported in the path of the beam by a sample stage that is controllable for moving the sample in a translational sense along orthogonal (X and Y) axes.
  • the stage may be any one of a number of conventional designs.
  • the motion controllers of the sample stage are driven to enable scanning the sample 36 with the beam, thereby to arrive at a plurality of retardance and orientation measurements across the area of the sample.
  • the sample 36 will induce its own retardance into the primary beam that passes through it. In the exemplary primary optical setup 22, it is this retardance value that is determined.
  • the beam emanating from the sample 36 is directed through a beam splitter 42, a two-channel detector 44, and processing hardware 46 that includes a lock-in amplifier that relies on a reference signal 48 from the PEM controller 40 for signal capture.
  • the diagnostic system 24 includes its own discrete light source 50, which is a monochromatic laser, which makes the diagnostic system inherently self- calibrating.
  • the beam 52 emanating from that source (hereafter referred to as the diagnostic light beam 52) is directed through a collimating lens 54.
  • a polarizer 56 is located next in the setup for the purpose of removing unwanted birefringence effects that may have been introduced by the optical components of the source 50 or lens 54.
  • the diagnostic light 52 then passes through the optical element 32.
  • the diagnostic beam 52 is not directed through the primary aperture 38. Rather, the beam 52 is directed through a secondary, "diagnostic" aperture 58 that is remote from the primary aperture 38. This enables the simultaneous operation of the both the primary and diagnostic optical system without one interfering the other. As will be explained more fully below, this real-time availability of diagnostic information can be (after suitable processing) displayed and/or used for feedback control of the PEM.
  • a secondary aperture for the diagnostic beam 52 is not critical. That is, the present invention contemplates that the primary aperture 38 may be used for both the primary and diagnostic beam. This could be accomplished in a situation where, for example, the primary beam 28 could be intermittently halted while the diagnostic beam is directed through the primary aperture 38. Movable mirrors may be employed to direct the particular beam (diagnostic and primary) to the corresponding detector and processing. As noted, however, separate apertures are preferred.
  • the diagnostic beam 52 emanating from the PEM optical element 32 passes through an optional waveplate 60 that reduces the operating output frequency of the PEM to equal to the eigenfrequency (resonant frequency) of the PEM.
  • An analyzer 62 then eliminates the unwanted complex component from the diagnostic beam.
  • the waveplate 60 and analyzer 62 are combined and provided as a commercially-available circular polarizer.
  • the diagnostic beam 52 next impinges on a detector 64, such as a photodiode.
  • the detector 64 produces as output a current signal representative of the time varying intensity of the received diagnostic light.
  • the wavelength of the diagnostic beam 52, as well as the location where that beam strikes the optical element 32, is preferably selected to achieve optimum sensitivity to the modulation level of the detector 64, while avoiding interference with the operating frequency of the primary beam 28.
  • the detector output is delivered to a preamplifier 66.
  • the preamplifier 66 provides output to a phase sensitive device (preferably a lock-in amplifier 70) in the form of a low-impedance intensity signal V A c, and a DC intensity signal V D c, which represents the time average of the detector signal.
  • the lock-in amplifier is provided with the reference signal 48 of the PEM controller 40 for the purpose of capturing the output from the detector. This captured signal reflects the retardance induced into the diagnostic beam 52 by the optical element (that is, the portion of the optical element at the diagnostic aperture 58). In the preferred embodiment, this diagnostic-retardance signal is converted to DC signals and provided to a microprocessor 72.
  • the microprocessor 72 initially addresses the task of converting the diagnostic-retardance signal into a signal representative of the retardance induced in the primary beam 28 (which passes through the primary aperture -the region of greatest induced retardance). Put another way, the retardance induced in the diagnostic beam 52 is relatively smaller than the retardance induced in the primary beam. The signal indicative of the diagnostic-beam retardance, therefore, is processed to arrive at a corresponding value of the primary-beam retardance.
  • ⁇ b is the retardance of the fixed waveplate 60
  • ⁇ m is the peak retardance of the PEM 20 at the location of the diagnostic beam's central ray
  • IQ is the peak intensity at the detector 64
  • m is the resonant frequency of the
  • ⁇ !> is the extrapolated retardance of the PEM at the center of the PEM primary aperture 38
  • D is the distance from the center of the primary aperture 38 to the center of the diagnostic aperture 58
  • L is the length of the PEM optical element 32.
  • the dimensions D and L are illustrated in Fig. 2.
  • the distance D is 20 millimeters (mm) and the length L of the optical element is 57. 3 mm.
  • the constants used in the foregoing equations are stored in the firmware of the diagnostic system, such firmware being illustrated as memory 74 in Fig. 1. That memory is preferably a read-only (ROM) type, the contents of which are programmed during factory calibration of the diagnostic system.
  • ROM read-only
  • the apertures 38, 58 have been illustrated as circles in the Fig. 2, it is pointed out that, as respects the optical element 32, there is no indicia on that component representing the aperture.
  • the optical element 32 is mounted to and enclosed in a housing.
  • the circles appearing in Fig. 2 can be considered as representing the openings in the otherwise opaque housing, which openings expose the apertures 38, 58 to the respective diagnostic and primary light beams.
  • the opening for the primary aperture 38 is a circle with a diameter of 14 mm.
  • the opening for the diagnostic aperture 58 is a circle having a 5 mm diameter.
  • most of the components of the diagnostic system are mounted directly to the PEM housing, with the electrical components being carried on an associated printed circuit board that is also mounted in the housing.
  • the primary-beam retardance characteristic (such as retardance amplitude) that is generated by the processor 72 is provided to the PEM controller 40. That signal is then displayed 76, real-time, as the primary system 22 is operated.
  • the diagnostic signal indicating a retardance characteristic of the primary beam which is provided by the processor 72 to the PEM controller 40, is converted into a PEM control signal. That is, the central processing unit of the PEM controller compares the retardance characteristic (such as retardance amplitude) selected by the user with the "actual" retardance characteristic received from the diagnostic processor 72. In instances where these two values are different, an error signal corresponding to that difference is generated and used to change the drive signal applied to the transducers 34 until the actual value reaches the selected.
  • the retardance characteristic such as retardance amplitude

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)
  • Testing Of Optical Devices Or Fibers (AREA)

Abstract

Selon cette invention, un système de diagnostic (24) destiné à un modulateur photoélastique (20) fournit des informations obtenues par procédé optique sur les caractéristiques de retard induites par le modulateur photoélastique (20). Le système de diagnostic (24) est fait intégralement avec le modulateur photoélastique (20), ce qui permet de diagnostiquer ou de surveiller les performances du modulateur photoélastique (20) lors du fonctionnement du modulateur photoélastique (20). Plus concrètement, le système de diagnostic (24) est utilisé parallèlement à un ensemble optique (22) qui utilise un rayon de lumière primaire (28) pour des tâches traditionnelles telles que la polarimétrie, la métrologie optique, etc. Le système de diagnostic (24) comprend sa propre source de lumière de diagnostic (50) qui est dirigée à travers l'élément optique (32) du modulateur photoélastique (20) dans un endroit éloigné de l'ouverture primaire (38) du modulateur photoélastique (20). Ainsi, on peut faire fonctionner en simultané le système de diagnostic (24) et le modulateur photoélastique primaire (20) sans créer d'interférences entre les deux. La sortie du système de diagnostic reflète la caractéristique de retard réel fournie par le modulateur photoélastique (20) et peut être utilisée comme signal de rétroaction pour régler selon les besoins la commande du modulateur photoélastique.
EP00919988A 1999-03-31 2000-03-31 Systeme de diagnostic integre pour modulateur photoelastique Withdrawn EP1166064A1 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
US12726399P 1999-03-31 1999-03-31
US127263P 1999-03-31
PCT/US2000/008625 WO2000058699A1 (fr) 1999-03-31 2000-03-31 Systeme de diagnostic integre pour modulateur photoelastique

Publications (1)

Publication Number Publication Date
EP1166064A1 true EP1166064A1 (fr) 2002-01-02

Family

ID=22429189

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EP00919988A Withdrawn EP1166064A1 (fr) 1999-03-31 2000-03-31 Systeme de diagnostic integre pour modulateur photoelastique

Country Status (4)

Country Link
EP (1) EP1166064A1 (fr)
JP (1) JP2002540417A (fr)
AU (1) AU4059200A (fr)
WO (1) WO2000058699A1 (fr)

Families Citing this family (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
AU4470200A (en) * 1999-04-20 2000-11-02 Ceptyr, Inc. Dsp-8 dual-specificity map kinase phosphatase
DE102008009601A1 (de) * 2008-02-15 2009-08-20 Carl Zeiss Smt Ag Optisches System für eine mikrolithographische Projektionsbelichtungsanlage sowie mikrolithographisches Belichtungsverfahren
CN103280972B (zh) * 2013-05-22 2016-01-20 中北大学 反相串接式弹光调制干涉具驱动电路
CN104967444A (zh) * 2015-07-20 2015-10-07 中北大学 一种用于弹光调制的数字锁相硬件电路
CN106354052B (zh) * 2016-08-04 2018-08-17 北京航空航天大学 一种用于原子陀螺的光弹调制控制器
JP6351893B1 (ja) * 2018-02-26 2018-07-04 日本分光株式会社 位相差制御装置

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Publication number Priority date Publication date Assignee Title
US4362387A (en) * 1980-08-22 1982-12-07 Rockwell International Corporation Method and apparatus for measuring visibility from the polarization properties of the daylight sky
US4480916A (en) * 1982-07-06 1984-11-06 The United States Of America As Represented By The Secretary Of The Navy Phase-modulated polarizing interferometer
JPH0518858A (ja) * 1991-07-12 1993-01-26 Casio Comput Co Ltd 薄膜の光学特性測定方法
FR2714970B1 (fr) * 1994-01-12 1996-03-29 Centre Nat Rech Scient Ellipsomètre spectroscopique modulé.
US5652673A (en) * 1994-06-24 1997-07-29 Hinds Instruments, Inc. Elimination of modulated interference effects in photoelastic modulators
US5956147A (en) * 1997-06-13 1999-09-21 Lockheed Martin Energy Research Corporation Two modulator generalized ellipsometer for complete mueller matrix measurement

Non-Patent Citations (1)

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Title
See references of WO0058699A1 *

Also Published As

Publication number Publication date
AU4059200A (en) 2000-10-16
WO2000058699A1 (fr) 2000-10-05
JP2002540417A (ja) 2002-11-26

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