RU2539113C2 - Method of compensating for thermal bias of interferential-polarisation filter band - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: invention relates to optical instrument-making and a method of compensating for thermal bias of the transmission band of an interferential-polarisation filter. The filter comprises a stack of adjustable elements with half-wave plates, rotation of which adjusts the transmission band of the adjustable filter elements to the measured spectral line of the object. To compensate for thermal bias of the transmission band, a light beam from a reference source is directed through each adjustable element simultaneously with light of the measured spectral line of the object. The beam of the reference source transmitted by the element is split into two beams, an usual and an unusual beam, and the change in the difference of intensities of said beams, caused by change in temperature of the element, is used as a feedback signal for turning the half-wave plate which compensates for thermal bias of the transmission band of the element.

EFFECT: more accurate and simpler method.

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Description

The invention relates to optical instrumentation and measuring equipment and can be used in interference-polarizing filters (IAP) of the Lio type with an adjustable passband to obtain two-dimensional maps and radial velocity measurements by Doppler shift of spectral lines. In the general case, the adjustable IPF elements contain independent blocks installed between the polarizers, consisting of a crystalline plate (successively doubled thickness in adjacent blocks), a stationary quarter-wave plate, and an adjustable half-wave plate. Adjustable half-wave plates are rotated to shift the bandwidth over the spectrum. Typically, radial velocity measurements (simultaneously with magnetic field measurements) are carried out with very narrow-band interference polarization filters. To measure Doppler velocity, a filter passband is scanned relative to the center of the spectral line and images are taken in the blue and red wings of the line. For example, scanning a strip over the spectrum in the IAP [1] is carried out by rotating half-wave plates in adjustable elements. The photometric difference of the two images obtained by scanning in the wings of the line gives a quantitative picture of the Doppler velocities on the measured object.

The main requirement for filters in measurements with scanning is the stability of the initial position of the passband relative to the center of the measured line. However, the traditional drawback of filters is the large dependence of the position of the maximum passband of the filter on temperature. As the temperature changes, the scanning of the strip by the filter will not be symmetrical with respect to the spectral line, and a false line shift and an error in the Doppler velocity measurements will appear.

, что соответствует появлению ложного сигнала в лучевой скорости порядка 180 м/сек (для линии Fel 6173Å). Требуемая точность измерений должна быть в 5-10 раз выше ложного сигнала, тогда при этом точность термостатирования должна быть порядка ±0,001°C. Такую точность не может обеспечить электронный термостат, поэтому в аналоге [2], одновременно с термостатированием, положение полосы пропускания регулируемых элементов корректируют между измерениями. В регулируемые элементы фильтра вместо рабочего исследуемого пучка временно вводят пучок от искусственного источника света. Пучок, после прохождения элементов, выводят на спектрограф, где визуально наблюдают полосу пропускания фильтра и вращением полуволновых пластинок корректируют положение полосы пропускания регулируемых элементов. Однако, между корректировками полосы во время измерения лучевых скоростей, температура фильтра может измениться, и из-за неточного термостатирования происходит смещение полосы пропускания как в нерегулируемых, так и в регулируемых элементах.It is known that to obtain a stable position of the passband and increase the accuracy of measurements, all filter elements, unregulated and adjustable, are placed in a thermostat that maintains the operating temperature. When thermostating with an accuracy of ± 0.01 ° C, the bandwidth of crystalline elements from optical calcite is shifted by $$\mp 0,0037\stackrel{o}{A}$$

, which corresponds to the appearance of a false signal in radial velocity of the order of 180 m / s (for the Fel line 6173Å). The required measurement accuracy should be 5-10 times higher than the false signal, then the temperature control accuracy should be of the order of ± 0.001 ° C. Such an accuracy cannot be ensured by an electronic thermostat, therefore, in analogue [2], simultaneously with thermostatting, the position of the bandwidth of the adjustable elements is adjusted between measurements. Instead of a working test beam, a beam from an artificial light source is temporarily introduced into the adjustable filter elements. The beam, after passing through the elements, is displayed on a spectrograph, where the passband of the filter is visually observed, and the position of the passband of the adjustable elements is adjusted by rotation of the half-wave plates. However, between band adjustments during radial velocity measurements, the filter temperature may change, and due to inaccurate temperature control, the bandwidth is shifted both in unregulated and in adjustable elements.

Another analogue is known - a method for thermooptical compensation of the shift of the passband of an IPF with temperature fluctuations [3]. When the temperature changes, the difference in the path of the rays in the crystal plate changes, and the filter strip shifts. To reduce the change in the path difference, additional crystalline plates with a temperature coefficient of birefringence different from the temperature coefficient of birefringence of the main plates are introduced into the filter elements, and they are set with respect to the main plates to optically subtract the path difference. At a certain ratio of birefringence of the main and additional plates, the change in the stroke difference with temperature changes is compensated, and the requirements for electronic thermal stabilization are significantly reduced. A similar thermooptical compensation of interference-polarization filter steps was used in tandem filters (IAP + Michelson interferometer) in MDI [4] and HMI [5] instruments for measuring magnetic fields and solar formation velocities. These tandem filters are installed at the SOHO and SDO space observatories, respectively. The IPF elements in the tandem filters are made of two crystals: the main plates are made of calcite and the additional plates are made of ADP crystal or KDP crystal. The KDP crystal is used to compensate for the thinnest calcite crystal plates, since the required thickness of the ADP plates is so small that it is not easy to make.

However, the disadvantage of this method is that the thermo-optical compensation of the IPF works in a limited temperature range, and the need to use an electronic thermostat for the most sensitive elements still remains. To stabilize the position of the passband, a Lyo filter with temperature-compensated steps and a Michelson interferometer are placed in a thermostat [4, 5]. The thermostat furnace and on-board controller are designed to maintain temperature stability within 30 ± 0.01 ° C for 1 hour. To do this, use an analog proportional control of the furnace. Moreover, the furnace pre-amplifier and the on-board controller are mounted in their thermostats, thereby making their temperature controlled. Multistage thermal insulation is used in the filter thermostat to ensure that the time scale of any residual temperature changes in the filter is longer than the period of solar p-modes of oscillations, measured using a filter by the shift of the spectral line.

In addition, the temperature stabilization of the passband of Lio filters by compensating for the difference in the path of the rays in the filter steps by selecting crystals with different thermo-optical coefficients is suitable only for a narrow wavelength range due to the dependence of thermo-optical coefficients on the light wavelength. So, the mentioned filters [4, 5] work only in narrow spectral regions: MDI - in the region of the Ni I line 6767.8 Å, and HMI - in the region of the FeI6173 Å line. Thermo-optical stabilization of the filter also does not allow the filter to be tuned to other lines of astrophysical interest by changing the operating temperature. Moreover, the installation for thermooptical compensation of additional crystalline plates increases the optical foot length of the IPF, and this leads to aperture limitations of the filter.

Closest to the proposed one is a method of stabilizing the band of the universal interference-polarization filter SOUP [6] of the SPACELAB space observatory, which eliminates the main problem - the precision thermal regulation of calcite elements - by rejecting traditional electronic heaters and using temperature sensors instead to measure the temperature of each a separate adjustable crystalline element to determine the magnitude of their optical tuning. Of course, instead of maintaining a constant temperature of the elements to stabilize the bandwidth, it is easier to record the temperature and compensate for the offset of the strip by adjusting each element. In the prototype, filter elements are adjusted using a computer algorithm to compensate for any measurable temperature changes. Absolute wavelength calibration is provided using a reference source - He-Ne laser. The temperature value of the adjustable elements and the rotation angles of their half-wave plates are entered into the computer after setting the bandwidth of the elements on the reference line λ 6328 Å of a He-Ne laser, the radiation of which is passed through the entire filter. To tune the filter to any other wavelength other than λ 6328 Å, either change the reference laser or use the current position of the half-wave plates determined by the λ 6328 Å laser to establish a new position of the half-wave plates for a different wavelength. The new positions are tabulated according to an approximate analytical expression obtained on the basis of preliminary measurements of temperature sensitivity and dispersion behavior of the order of interference of calcite crystals from which the filter elements are made. Thus, tuning the position of the passband to the spectral line, scanning it and compensating for the temperature shift of the strip is done by rotating the phase half-wave plates of the adjustable elements according to a computer algorithm taking into account the readings of the temperature sensors installed on the elements.

The disadvantages of the prototype include the fact that to create an algorithm for rotating phase plates with the aim of temperature stabilization of the position of the strip of adjustable elements requires accurate knowledge of the temperature sensitivity of the used crystalline materials. But this is due to the need for laborious, precise laboratory studies and measurements of the optical properties of crystals: birefringence, birefringence dispersion, and thermooptical coefficients. To measure these optical and physical properties of crystals, again, thermostats with an electronic temperature controller are needed. It has been previously shown that the use of a thermostat as a way to stabilize the position of the passband is insufficient due to the instability of thermal control. But this same instability of thermoregulation leads to errors in the measurement of optical and physical characteristics even in a small portion of the spectrum, not to mention measurements in a wide range of the spectrum. It must be borne in mind that measurements should be carried out specifically for those crystal samples from which plates of filter elements will be made. Filter plates made of natural calcite crystals can have a refractive index value that differs in the 5th sign, depending on the crystal growth pyramid. The optical-physical characteristics of calcite crystals in the elements of the prototype were measured both at the Kit Peak Observatory and at the Lockheed Livermol Laboratory [7]. The measurements were different, and average values were taken as an approximation to compile an algorithm for rotating half-wave plates to stabilize the SOUP filter band. Each time you use other crystals, you must perform a measurement procedure. When tuning filter elements and stabilizing their transmission bands on a solar spectral line whose wavelength is different from the reference line, the exact connection between the settings of the passband of the IPF on these lines is lost: it exists through an approximate analytical expression obtained from laboratory measurements of the thermo-optical and dispersion properties of crystalline elements. The adjustment to the measured line may be inaccurate due to errors in the approximation used, as well as due to errors in local temperature measurements of each adjustable element. Because of this, temperature compensation is inaccurate.

The aim of the invention is to compensate for the temperature shift of the filter passband.

The problem is solved due to the fact that the following differences are provided in the method for compensating for temperature mixing of a strip in which the filter passband is adjusted for the measured spectral line and the band offset to compensate for the effect of temperature fluctuations by rotating the half-wave plate of the filter elements relative to the spectral line of the reference light source.

The position of the passband of each adjustable element relative to the measured spectral line is stabilized by turning half-wave plates not according to the approximate algorithm of the temperature sensitivity of the elements, but by photoelectric tracking of the position of the spectral line of the object relative to the reference source line. The light beam of the reference source is not passed through the entire filter, but through each individual adjustable filter element. The light of the reference source transmitted by the element is split into two beams, ordinary and extraordinary, and the change in the intensity difference between the ordinary and extraordinary rays of the reference light source, caused by temperature fluctuations of the element, is used as a feedback signal to control the rotation of the half-wave plate of the adjustable element to compensate for its temperature shift bandwidth from the measured spectral line.

The essence of the proposed method of stabilization of the passband at temperature fluctuations consists in the formation of not only one, common for all adjustable crystalline crystal elements passband (main maximum) on the measured spectral line of the object, but also two passbands per spectral line of the reference light source for each adjustable element in two orthogonally polarized beams, whose intensity difference ensures stabilization of the filter passband position with temperature fluctuations.

The invention is illustrated by figures 1, 2.

Figure 1 shows a diagram of an implementation of a method for compensating the temperature shift of the passband of an IPF using an example of one adjustable element. The scheme includes: a crystalline plate 1, a quarter-wave plate 2, a half-wave plate 3, mounted between two polarizers 4, a source of a reference spectral line 5 with a polarizer 6 and a half-wave plate 7, three reflectors 8, a polarization splitter 9 with photodetectors 10, a control unit 11, and a spectrograph 12.

Figure 2 shows: a - contour of the measured spectral line of the object (the Sun); b - contour of the spectral line of the reference source; c - the bandwidth of the adjustable element, tuned to the measured spectral line of the object; g - bandwidth of the element in the ordinary (solid curve) and extraordinary (dashed curve) rays in the region of the reference spectral line; d - stripes in the ordinary and extraordinary rays, tuned to the reference line.

The proposed method of compensating for temperature displacement is implemented as follows.

For each adjustable element consisting of a crystalline plate 1, a quarter-wave 2, and a half-wave plate 3 with a rotary drive and two polarizers 4, a reference light source 5 with a polarizer 6 (if the source is not polarized) and a half-wave plate 7 with a rotary drive are used. The source is chosen so that it emits a separate emission (reference) spectral line in the spectral region close to the spectral line of the object in which it is supposed to take measurements, while the width of the spectral line of the source should be comparable to the bandwidth of the narrow-band filter element itself.

The light from the source is passed through a half-wave plate 7 and then with two reflectors 8 through a crystalline element near its edge and the third reflector is sent to a polarizing splitter 9, after which ordinary and extraordinary rays fall on the photodetectors 10. The signals of the photodetectors through the controller 11 control the rotation drive of the half-wave plate 3 adjustable element of the optical foot IPF.

In the proposed method, the adjustment of the temperature offset compensation system is carried out simultaneously with the adjustment of the elements on the measured line. To adjust the bandwidth on the measured line, a separate adjustable element of the IFF is installed with the reference light source 5 turned on in front of the entrance slit of the spectrograph 12 and illuminated with sunlight. Thus, the crystalline element is illuminated simultaneously by two light sources - solar and reference. Figure 2 shows the reference spectral line of the laser and the solar line, on which the transmission bands of all crystalline elements must be tuned and the position of the bands must be resistant to temperature changes. The transmission of the adjustable element of sunlight is observed at the focus of the spectrograph in the spectral region with the measured line. The passband of the adjustable filter element is a grooved spectrum, the distance between the maxima of which is inversely proportional to the thickness of the crystal plate of the element. The intensity distribution of the grooved spectrum transmitted by the filter element, excluding absorption and scattering in the optical channels I = Acos ² π (μd / λ + α / 90 °), where d is the crystal thickness, μ is the birefringence index at the wavelength λ, α - the angle of rotation of the half-wave plate of the adjustable element. The position of the maxima of the grooved spectrum relative to the measured line is determined visually or photometrically in the spectrograph and changed by rotation of this half-wave plate. The position of the maxima of the grooved spectra in the o- and e-rays relative to the reference line is determined by the signals of the photodetectors of o- and e-rays. The ray intensities depend on both the angle of rotation a of the half-wave plate of the element and the angle of rotation β of the half-wave plate of the light source. O-ray intensity I _o = cos ² π (µd / λ + α / 90 ° + β / 90 °) - solid line; e-beam intensity I _e = sin ² π (µd / λ + α / 90 ° + β / 90 °) - dashed line. The intensity distributions in o and e rays are additional spectra (I _o + I _e = 1). When the maxima in the spectra of both rays are located symmetrically relative to the reference line (Figure 2), I _o = I _e .

Before starting the adjustment, the maxima of the grooved spectrum of the element can be arbitrarily located relative to the measured and reference lines. By turning the half-wave plate 3 of the crystalline element through an angle α, the groove spectrum is shifted along the spectrum until one of its maxima coincides with the solar spectral line. But for the reference line, it may turn out that I _o ≠ I _e . Immediately, not allowing the temperature of the element to change, turn the half-wave plate 7 of the source 5 until the signals I _o = I _e s are equal and through the controller turn on the feedback circuit: the imbalance of the photometer signals - the angle of rotation of the half-wave plate of the element. The shift of the bandwidth of the element caused by any change in temperature leads to an imbalance in the signals of the photodetectors and, in turn, to the rotation of the half-wave plate 3, which compensates for the shift of the strip of the filter element due to temperature fluctuations. An element set up in this way is replaced on the spectrograph with the following and set them up similarly. All configured elements that are in the active state (with feedback turned on) are assembled together in the stack of the interference-polarization filter. The coincident passbands of the elements form the main passband of the filter on the spectral line, in which the velocities and magnetic fields are measured. Scanning by the spectral band of the line filter for measuring Doppler velocities and magnetic fields is carried out on command from the control unit for the rotation of all half-wave plate elements. Correction due to temperature deviation of the position of the main transmission maximum relative to the reference line is carried out by turning the same plates in almost real time during a brief stop of measurements. The correction frequency is set programmatically depending on the operating conditions of the filter.

Below is described, tested by us, an example of the implementation of the method of temperature compensation.

A plate composed of two crystalline plates was mounted in an adjustable element in such a way that their double refraction is folded: optical calcite with a thickness of 24.912 mm and paratellurite with a thickness of 5.572 mm. The combination of two crystals is used to increase the angular field of the filter. The bandwidths of the grooved spectrum of an element with such crystalline plates have a half-width of 0.12 Å.

An He-Ne LGN-32 laser with λ = 6328Å is installed as a reference source in the adjustable element. The laser light after passing through the element is divided by a birefringent plate of optical calcite into ordinary and extraordinary rays. The rays are separated at a distance of 4 mm, go in parallel, and each falls on its own photodetector type FD265. When the temperature of the element changes, the intensities of the ordinary and extraordinary rays (FD265 signals) change in antiphase, which makes it possible to determine the direction of the strip shift. The strip offset is restored by turning the plate of the adjustable element λ / 2 from the stepper motor ШД200. Due to mechanical reduction from the stepper motor to the rim of the half-wave plate, the engine in one step rotates the half-wave plate of the adjustable element by an angle of 0.34 °.

The temperature of the adjustable stage, placed in the thermostat, gradually changed. The heating rate, which does not cause unacceptable temperature gradients in the crystalline plates, was not more than 0.4 ° C / min. The sensitivity of working out a stepper motor was 1 step. When heated by 1 ° C, the rotation of the half-wave plate was 62.3 °. Compensation of the temperature shift of the strip by rotating the half-wave plate at an angle of 62.3 ° means its shift in the spectrum is 0.166 Å. The return of the strip to the initial rotation of the half-wave plate occurred with an accuracy of 1 step, i.e. about 0.9 mÅ. This accuracy of stabilization of the filter passband is sufficient for measuring the physical parameters of the object. Thus, this proves that in the proposed method, the stabilization of the position of the passband of the IPF can be carried out without precision thermoregulation of the entire filter and individual elements and without placing sensors on individual elements for precision temperature measurements, which are used in the prototype.

Cited literature

[1] P. Sutterlin, R.J. Rutten, V.I. Skomorovsky. Ba II 4554 A speckle imaging as solar Doppler diagnostic ky // Astron. Astrophys. - 2001. - Vol.378, No. 1. - P.251-256.

[2] A. Dollfus, F. Colson, D. Gussaire and F. Lankay. A monochromator for Solar quantitative imagery: the instrument FPSS. Astron Astrophys. 1985, Vol. 151, 235-253.

[3] C. B. Joffe. Interference-Polarizing Filter, A.S. No. 158700, B.I. No. 22, 1963

[4] Sébastien Couvidat Jesper Schou Richard A. Shine, et al. Wavelength Dependence of the Helioseismic and Magnetic Imager (HMI) Instrument onboard the Solar Dynamics Observatory (SDO) Solar Phys (2012) 275: 285-325.

[5] Scherrer P.H., Bogart R.S., Bush R.I., et al. The Solar Oscillations Investigation - Michelson Doppler Imager. Solar Physics 162: 129-188, 1995.

[6] Alan Title THE SOUP AND CIP INSTRUMENTS Adv. Space Res. Vol.4, No.8, pp. 67-74, 1984.

[7] Alan Title, William Rosenberg, Research on spectroscopic imaging, v 1, Technical Discussion April 1979, p.B-22. Lockheed Palo Alto Research Laboratories, 3251 Hanover Street Palo Alto California 94304 http://www.researchqate.net/publication/23897491 Research on spectroscopic imaging. Volume 1 Technical discussion.

Claims (1)

A method of compensating for the temperature shift of the passband of an interference-polarizing filter containing a stack of adjustable elements with half-wave plates, the rotation of which adjusts the passband of the adjustable filter elements to the measured spectral line of the object and corrects the temperature shift of the position of the passband of the adjustable elements relative to the spectral line of the reference source, characterized in that the light beam lines of the reference source are sent through each adjustable the element at the same time as the light is measured by the spectral line of the object, while the beam of the reference source passed by the element is split into two beams, ordinary and extraordinary, and the change in the difference in the intensities of these rays due to the change in the temperature of the element is used as a feedback signal to rotate the half-wave plate compensating for the temperature element bandwidth offset.

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