WO1999047966A1 - Controlling resonant photoelastic modulators - Google Patents
Controlling resonant photoelastic modulators Download PDFInfo
- Publication number
- WO1999047966A1 WO1999047966A1 PCT/US1999/005586 US9905586W WO9947966A1 WO 1999047966 A1 WO1999047966 A1 WO 1999047966A1 US 9905586 W US9905586 W US 9905586W WO 9947966 A1 WO9947966 A1 WO 9947966A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- drive
- frequency
- signal
- amplitude
- photoelastic
- Prior art date
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Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/30—Polarising elements
- G02B5/3083—Birefringent or phase retarding elements
-
- 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
- G02F1/11—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 acousto-optical elements, e.g. using variable diffraction by sound or like mechanical waves
- G02F1/113—Circuit or control arrangements
-
- 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
- G02F1/11—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 acousto-optical elements, e.g. using variable diffraction by sound or like mechanical waves
Definitions
- This application relates to a system and method for precise control of resonant photoelastic modulators.
- 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.
- the value of the retardation amplitude is selectable by the user. Because resonant PEMs are typically driven at their resonant frequency, stress oscillations, which are induced by the transducer, can exhibit relatively large amplitudes. However, driving PEMs at their resonant frequency prevents the user from controlling the oscillation frequency.
- 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 3
- PEMs having high retardance amplitudes are required, for example, in Fourier Transform spectral analysis (see, for instance, US Patents 4,905,169 and 5,208,651).
- spectral resolution is proportional to the PEM's retardation amplitude, and useful spectral resolutions are achieved at high amplitudes, which cannot be reached by a conventional (single) PEM.
- the total retardation amplitude of the stack may be less than the sum of the maximum retardation amplitudes of each of the PEMs in the stack. This is because even a relatively small spread in the resonant frequencies of the individual PEMs, which is fully consistent with manufacturing specifications, results in most of the PEMs not being driven exactly at resonance. As a result, the phases of the oscillations of the individual PEMs are not the same, even though the PEMs are driven at the same frequency, and, therefore, the total retardation amplitude is less than the sum of the individual amplitudes.
- the phases of the individual PEM oscillations may further diverge as the driving amplitude is increased or as the system warms up. This may lead to a further decrease in the efficiency of the PEM stack.
- the problem of drifting operating frequencies is not limited to stacked PEM arrangements.
- 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 directed to a practical system and method for exploiting the dependence of a PEM's resonance frequency on temperature (attributable to driving amplitude and to ambient temperature) to generally improve the performance of PEMs.
- each of the PEMs in the stack are 5
- the common-resonance-frequency PEMs operate like a single PEM, but at a much higher retardation amplitude than that available from conventional single-element PEMs. This result makes the multiple PEM arrangement amenable to applications, such as the above-mentioned Fourier Transform spectral analysis, that require large retardation amplitudes.
- a single-element PEM is controlled in a manner to account for the subtle changes in the PEM's resonant frequency.
- a control method and system is provided to keep constant both the retardation amplitude of the PEM and the actual operating frequency of the PEM, which frequency need not necessarily be the resonant frequency.
- Fig. 1 is a diagram of an exemplary system for controlling multiple PEMs in accordance with one aspect of the present invention.
- Fig. 2 is a block diagram of a global controller and one of the local controllers employed with a preferred embodiment of the present invention. 6
- Fig. 3 is a diagram of a preferred embodiment of a low-pass filter employed as an operator in the global controller.
- Fig. 4 is a diagram of a preferred embodiment of a low-pass filter employed as an operator in the local controllers.
- Fig. 5 is a diagram of one preferred embodiment of a circuit for generating a drive voltage waveform for a PEM.
- Fig. 6 is a diagram of a system for controlling a single PEM in accordance with another aspect of the present invention.
- Fig. 7 is a diagram of a system for controlling a single PEM in accordance with another aspect of the present invention.
- FIG. 1 depicts an exemplary system for controlling multiple photoelastic modulators (PEMs) in accordance with one aspect of the present invention.
- PEMs photoelastic modulators
- a series of three PEMs 20 are depicted.
- the PEMs are aligned so that light from a source 22 travels in a beam 24 through each of the PEMs 20 to reach a detector 26.
- the optical aspects of the setup shown in Fig. 1 have been greatly simplified (polarizers, etc. being omitted) for the purposes of this description.
- Such a setup may be used, for example, in the Fourier Transform spectral analysis mentioned above.
- the polarization of the light 24 is modulated by the PEMs to impart a retardation amplitude in the beam that reaches the detector.
- optical element 28 of each PEM is vibrated by an attached piezoelectric transducer 30, thereby to introduce into the optical element (such as fused silica) the oscillating birefringence discussed above (as well as the attendant contribution to the total retardation in the light that reaches the detector).
- the optical element such as fused silica
- a system for controlling the multiple PEM arrangement just described so that all of the PEMs operate at a resonant frequency and provide a total retardation amplitude (i.e., the sum of the individual values of the retardation provided by each PEM) that precisely matches a desired reference amplitude.
- the preferred system is primarily embodied as local controllers 32, one of which is associated with each PEM 20, and a global (system) controller 34.
- a computer 36 is, optionally, provided to facilitate operation of the global and local controllers, which computer 36 may also receive the total retardation information collected by the detector 26.
- the phase error relates the driving frequency to the resonant frequency of the PEM.
- the phase error is zero when the two frequencies are equal; that is, when the PEM is driven at resonance.
- the current-voltage phase difference corresponding to resonance driving will have a non-zero, but fixed, value. In such a case, the phase error will be the difference between the current-voltage phase difference and that fixed value.
- the retardation amplitude error is the difference between actual and selected retardation amplitudes, and vanishes when the two amplitudes are equal.
- the retardation amplitude error can be optically determined, or approximated by the driving current amplitude error. In the following portion of this description, the latter approximation is employed, although the former is certainly usable.
- a stable feedback loop can be implemented so that the two control parameters (driving voltage and driving frequency) always change in such a way as to cancel the two error signals.
- the PEM operates at resonance and provides the selected retardation amplitude.
- Fig. 1 depicts the use of multiple (stacked) PEMs. Three PEMs are shown, although the following description will apply to groups of two or more PEMs. Thus, one can consider the series of PEMs as comprising any of a number "n" PEMs, which number is selected to suit the retardation amplitude needs of a particular application. 9
- n+1 control parameters the driving voltage amplitude for each of the "n” PEMs and the common driving frequency. It would appear that 2n independent error signals must be cancelled (the amplitude and phase errors for each of the "n” PEMs). This would require 2n control parameters, such as “n” individual driving voltage amplitudes and "n” individual driving frequencies.
- the control system requires only n+1 control parameters in the system. As a result, the PEM stack is controlled (i.e., all PEMs reaching a common resonant frequency while maintaining a selected total retardation amplitude) with only n+1 control parameters.
- each local controller 32 monitors its PEM's current-voltage phase difference, as well as reference signals generated by the global controller 34, which reference signals are described more fully below. Using these signals, the local controller adjusts the voltage amplitude and frequency of the PEM's driving signal.
- each local controller 32 generates a current-voltage phase error signal that is provided to the global controller 34.
- the local controller also sends a local current amplitude signal to the global controller 34.
- each local controller 32 locks the phase of its PEM current to a common phase reference, preferably the phase of the common frequency signal generated by the global controller 34, as will be described. 10
- the exemplary one of "n" local controllers 32 drives its PEM by applying the following feedback equation 1 :
- V -D7 ⁇ - ⁇ '
- ⁇ ' a value corresponding to the difference between a selected retardation amplitude (here approximated by a current amplitude) and the "average" retardation amplitude actually applied by the PEMs and to the sum of the phase error signals of all PEMs; this signal is the time derivative of the common driving frequency, and is supplied by the global controller.
- the local controller 32 employs a voltage-controlled oscillator (VCO) 40 receiving on line 42 a frequency input signal that, as described more below, sets the frequency of the constant amplitude signal that emanates from the VCO. That signal is applied to a multiplier 110 (shown as "X" in the figure and controlled as described below). The multiplier multiplies the amplitude of the VCO output signal. Thus, on line 46 there appears the drive voltage signal that is applied to drive the transducer of the PEM 20.
- VCO voltage- controlled oscillator
- the VCO and multiplier elements can be considered as the PEM's drive waveform generator.
- each local controller 32 is provided with a phase comparator 50 that receives as input the drive voltage signal (via line 52) and a signal (line 54) that is proportional to the current applied to the PEM.
- This current signal is developed by a current pickup "I" shown at 56.
- the output of the comparator 50 (line 60) comprises the phase error signal
- inverter 64 directed to a summing circuit 66 as one of the input signals of
- global controller 34 (as are the comparable signals from the other PEMs) for processing as described below.
- the summing circuit 66 receives as its other input the signal ⁇ '. This
- ⁇ a coefficient determined by the physical characteristics of the PEMs in the system
- D ⁇ an integral operator, which functions as a low-pass filter
- ⁇ l the signal representing the current error
- v the coefficient determined by the physical characteristics of the PEMs in the system. 13
- each local controller 32 provides to the global
- controller 34 a signal indicative of the current amplitude that is applied to that particular PEM. This is done by directing the output of the above described current pickup 56 to a root-mean-square (rms) converter 70. This converter generates a voltage signal proportional to the square root of the mean signal value of the PEM current. This voltage signal, therefore, is a good measure of
- from all of the PEMs are provided to a
- the signal representing ⁇ l passes to an operator D, which can be
- a low-pass filter 80 (Fig. 3) for which the following relationship exists between output and input voltages:
- the output of the low-pass filter 80 is multiplied by the coefficient ⁇ and
- summing circuit 82 receives the sum (via summing
- the signal ⁇ ' is output from the summing circuit 82 and applied on line 90
- this signal is comprised (see equation 2, above) of two components, one corresponding to the difference between a selected retardation amplitude and the "average" retardation amplitude actually applied by the PEMs and another to the sum of the phase error signals of all PEMs. As will become clear, this signal is used as a control parameter of the driving voltages of the individual PEMs.
- the output of the summing circuit 82 is also applied to an integrator 92 that provides its output to a voltage controlled oscillator 96 (VCO) carried by the global controller 34.
- VCO voltage controlled oscillator
- the VCO 96 places on line 100, which is common to all
- This voltage output is an amplitude control signal applied, via multiplier 110, to modulate the constant amplitude output of the local controller VCO 40 so that the voltage amplitude applied to the PEM (via line 46) is that for matching the overall or "average" resonant frequency of all of the system PEMs.
- the PEMs operate in unison (at the same resonant frequency), even though the individual retardation amplitudes are neither monitored nor directly controlled.
- retardation amplitude is certain to equal the sum of the individual PEM amplitudes.
- the local controllers 32 employ another phase comparator 120. That comparator compares the phases of the PEM current (as provided by the signal from the current pickup 56) with that of the common drive
- controller's VCO 40 thereby locking the local PEM to the common, system drive frequency.
- Fig. 5 depicts one preferred embodiment of a waveform generator (primarily represented by components 40 and 110 of the local controller 32) for each PEM.
- the control voltage V from the integrator 106 is used to control the output of two high-voltage power supplies 130 and associated programmable regulators 132 of opposite polarity.
- the desired waveform is then obtained via a switch controller 134 that is driven by the output of the VCO to switch the output between the two power supplies.
- the resulting waveform has the same phase and frequency as the VCO output.
- the switch is implemented with high-speed, high-voltage, MOSFET transistors, which results in a simple circuit that does not introduce significant additional phase errors.
- This preferred implementation uses rectangular driving waveforms. Moreover, as long as the frequency of the rectangular waveform is relatively close to the resonance frequency of the PEMs, the very narrow band pass characteristics of the latter (narrow resonance response) filter out the higher harmonics in the driving waveform. Consequently, there is no significant difference between driving the PEMs (near resonance) with rectangular and sinusoidal signals. Of course, using rectangular driving waveforms allows for simpler control of the amplitude and phase of the driving signal.
- equations 1 and 2 force the individual PEM resonant frequencies to converge to the common driving frequency.
- the feedback system generates a driving frequency that tracks an "average" of the individual PEM resonant frequencies.
- the net result is that the common driving frequency is always within the spread of resonant frequencies and that this spread tends to collapse toward that common driving frequency.
- all resonant frequencies and the driving frequency soon coalesce at a common value. From that point on, all PEMs in the stack operate in unison and are driven at resonance.
- both the driving voltage amplitudes and the common driving frequency also depend on the total amplitude error. This dependence is such that the PEMs, while operating in unison, also track the selected total retardation amplitude. The net result is a PEM stack that operates like a single PEM, but at a much higher retardation amplitude.
- the common driving frequency generated by the control system can be modified so as to drive most efficiently the PEMs farthest in resonant frequency from the average. This can result in an initial decrease in the spread of resonant frequencies as the system warms up, to the point where the method described above can be successfully applied.
- Fig. 6 illustrates a controller that may be used with a single PEM 220 so that the PEM is controlled in a manner to account for subtle changes in the
- the single-PEM controller 232 keeps constant both the retardation amplitude of the PEM and the actual operating frequency of the PEM, which frequency need not necessarily be the resonant frequency.
- controller 232 may be a modified version of a local controller 32 as described above, but switchable, as by control of the computer 19
- the computer 36 may also be employed to supply certain reference signals to the controller 232.
- controller components can be the same as those used in the local controller embodiment 32 and, as such, carry the same reference numbers in Fig. 6. A detailed description of these components, therefore, is not repeated here.
- the key to simultaneous control of the PEM amplitude and operating frequency is the separate control of the phase and the amplitude of the voltage waveform used to drive the PEM.
- the individual PEM 220 is driven by the square waveform generated as described above in connection with Fig. 5.
- the drive voltage signal on line 52 and the current pickup on line 54 are used as inputs to amplitude and phase feedback loops.
- phase-lock loop circuit includes the phase comparator 50, which receives as input the voltage signal on line 52, and the
- the error signal ⁇ is summed with an external phase
- this drift is attributable, for example, to the power dissipation into the PEM (with attendant temperature change) when the PEM amplitude is increased.
- the functional relationship between the drive amplitude and resonant frequency changes may be employed, for example as a look-up table processed by the computer 36 or in a control loop (described below) to
- this is achieved by directing the PEM current signal from the rms 70, after inverting at 270, to an input of a differential amplifier 272.
- the other input to the amplifier is from a user-
- Fig. 7 diagrams an analog system that phase-locks the PEM driving voltage or current to a frequency reference signal 21
- phase control input can be used to adjust the phase between the PEM current
- a digital outer feedback loop that has a frequency counter for receiving an A/DC- converted PEM driving voltage or current.
- the output of the frequency counter is compared to a set frequency by a digital processor, which correspondingly
- the stability, accuracy, and resolution of the frequency counter only depend on how accurately and finely one wants to adjust the PEM's operating frequency.
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Nonlinear Science (AREA)
- Mechanical Light Control Or Optical Switches (AREA)
Abstract
Description
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Priority Applications (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
AU30881/99A AU3088199A (en) | 1998-03-16 | 1999-03-16 | Controlling resonant photoelastic modulators |
US09/623,954 US6970278B1 (en) | 1998-03-16 | 1999-03-16 | Controlling resonant photoelastic modulators |
EP99912519A EP1064582A1 (en) | 1998-03-16 | 1999-03-16 | Controlling resonant photoelastic modulators |
CA002323871A CA2323871A1 (en) | 1998-03-16 | 1999-03-16 | Controlling resonant photoelastic modulators |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US7806998P | 1998-03-16 | 1998-03-16 | |
US60/078,069 | 1998-03-16 |
Publications (1)
Publication Number | Publication Date |
---|---|
WO1999047966A1 true WO1999047966A1 (en) | 1999-09-23 |
Family
ID=22141740
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/US1999/005586 WO1999047966A1 (en) | 1998-03-16 | 1999-03-16 | Controlling resonant photoelastic modulators |
Country Status (4)
Country | Link |
---|---|
EP (1) | EP1064582A1 (en) |
AU (1) | AU3088199A (en) |
CA (1) | CA2323871A1 (en) |
WO (1) | WO1999047966A1 (en) |
Cited By (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP1197764A2 (en) * | 2000-10-10 | 2002-04-17 | JDS Uniphase Corporation | Strain-stabilized birefringent crystal |
US6867863B1 (en) * | 1999-03-31 | 2005-03-15 | Hinds Instruments | Integrated diagnostic for photoelastic modulator |
WO2007087670A1 (en) * | 2006-01-31 | 2007-08-09 | Endeavour Instruments Pty. Ltd. | A photoelastic modulator |
US7920318B2 (en) | 2005-01-28 | 2011-04-05 | Endeavour Instruments Pty. Ltd. | Photoelastic modulator system |
Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4905169A (en) * | 1988-06-02 | 1990-02-27 | The United States Of America As Represented By The United States Department Of Energy | Method and apparatus for simultaneously measuring a plurality of spectral wavelengths present in electromagnetic radiation |
US5652673A (en) * | 1994-06-24 | 1997-07-29 | Hinds Instruments, Inc. | Elimination of modulated interference effects in photoelastic modulators |
US5744721A (en) * | 1995-10-25 | 1998-04-28 | Hinds Instruments, Inc. | Electronic control system for an optical assembly |
-
1999
- 1999-03-16 EP EP99912519A patent/EP1064582A1/en not_active Withdrawn
- 1999-03-16 CA CA002323871A patent/CA2323871A1/en not_active Abandoned
- 1999-03-16 WO PCT/US1999/005586 patent/WO1999047966A1/en not_active Application Discontinuation
- 1999-03-16 AU AU30881/99A patent/AU3088199A/en not_active Abandoned
Patent Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4905169A (en) * | 1988-06-02 | 1990-02-27 | The United States Of America As Represented By The United States Department Of Energy | Method and apparatus for simultaneously measuring a plurality of spectral wavelengths present in electromagnetic radiation |
US5652673A (en) * | 1994-06-24 | 1997-07-29 | Hinds Instruments, Inc. | Elimination of modulated interference effects in photoelastic modulators |
US5744721A (en) * | 1995-10-25 | 1998-04-28 | Hinds Instruments, Inc. | Electronic control system for an optical assembly |
Cited By (8)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US6867863B1 (en) * | 1999-03-31 | 2005-03-15 | Hinds Instruments | Integrated diagnostic for photoelastic modulator |
EP1197764A2 (en) * | 2000-10-10 | 2002-04-17 | JDS Uniphase Corporation | Strain-stabilized birefringent crystal |
EP1197764A3 (en) * | 2000-10-10 | 2003-11-26 | JDS Uniphase Corporation | Strain-stabilized birefringent crystal |
US7920318B2 (en) | 2005-01-28 | 2011-04-05 | Endeavour Instruments Pty. Ltd. | Photoelastic modulator system |
WO2007087670A1 (en) * | 2006-01-31 | 2007-08-09 | Endeavour Instruments Pty. Ltd. | A photoelastic modulator |
GB2447595A (en) * | 2006-01-31 | 2008-09-17 | Endeavour Instr Pty Ltd | A photoelastic modulator |
US7768687B2 (en) | 2006-01-31 | 2010-08-03 | Endeavour Instruments Pty. Ltd | Photoelastic modulator |
GB2447595B (en) * | 2006-01-31 | 2011-02-23 | Endeavour Instr Pty Ltd | A photoelastic modulator |
Also Published As
Publication number | Publication date |
---|---|
AU3088199A (en) | 1999-10-11 |
EP1064582A1 (en) | 2001-01-03 |
CA2323871A1 (en) | 1999-09-23 |
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