DE4434921C2 - Achromatic phase delay element - Google Patents

Description

The invention relates to an achromatic phase ver delay element in the case of multiple liquid crystal layers are arranged one after the other and with the especially the state of polarization of light with different wavelengths can be monitored.

In addition to the wavelength, the spread and the intensity is the polarization an essential parameter elek tromagnetic radiation. To influence the bottom state of larization in the range of ultraviolet, Visible and infrared light are common λ / 2 and λ / 4 plates that are birefringent Crystals such as calcite, potassium dihydrogen phosphate (KDP) or quartz are used.

A plate that has been ground once is only for dimensioned a certain wavelength, and the  Influencing the state of polarization, for example of several wavelengths is not possible.

Such plates are out of phase fixed. An active influence on the phase ver delay to compensate for temperature effects or to regulate the phase delay to differ Che values are not possible.

Only electro-optical modulators, for example from potassium dihydrogen phosphate (KDP) allow one Change in phase delay. Are already there with small apertures, voltages in the kV range are required those that are not safe and only difficult to handle are cash.

A uniform influencing of the polarization state in a wide spectral range requires achromatic phase plates that have a fixed phase delay between the extraordinary and ordinary chem beam over a certain wavelength interval [λ _beginning , λ _end ).

One use case for this are astronomical polari station measurements, where due to the lack of light wide spectral ranges for many observation objects are integrated in order to obtain evaluable signals.

Another option is the con trolls of the state of polarization with an ultra-cure zen intense laser pulses generated white light con tinuums.

Achromatic plates can consist of several layers of different crystal materials with the same  or the 90 ° position of the fast axes (D. Clarke, Optica Acta 14, 343 (1967)), several of different thicknesses and with certain angles of fast axes cemented layers of the same Materials (S. Pancharatnam, Proc. Ind. Acad. Sci. A 41, 130 and 137 (1955)) or from several of the same thickness Layers of the same material with different orientations of the fast axes (C.M. Mc Intyre, S.E. Harris, J. Opt. Soc. At the. 58, 1575 (1968)). However, this only applies to the first method, the location of the fast Ach the whole element regardless of the waves length.

With a combination of using multiple layers different materials with the same or around 90 ° rotated position of the fast axes and the ver application of layers of different thicknesses are cemented together at certain angles, are so-called "super achromatic" plates Lich over the entire visible spectrum Rich almost constant phase lag and location of the have fast axes.

However, the phase plates designed in this way are costly intensive and from a technical point of view only with one small diameter of a few centimeters bar.

This small diameter requires when used in astronomy on large telescopes (surface polar trie) the arrangement of the element in a high col limited beam path. This means that there is no axis parallel beams of incidence to Surface normals of the element unacceptably large and  there will be a deviation from the desired phases delay caused by this in addition is increased that the individual elements manufacture are phase plates of higher order for technical reasons (i.e. delay of e.g. (m + 1/2) λ with m »1).

Of each. Stockley, G.D. Sharp, D. Doroski, K.N. John son "High-speed analog achromatic intensity modula Tor ", Optics Letters, Vol. 19, No. 10 (1994), pp. 758-760, becomes an achromatic intensity modulator, based on two identical, between crossed Polarizers arranged layers smectically Liquid crystals are described for a medium Wavelength of the achromatic range as λ / 2 plate act. By applying control voltages can change the angles between fast axes of the rivers sigcrystals and polarization direction of the input polarizers can be changed. This results in for the combination of liquid crystal layers and Polarization a behavior similar to that of a rotatable ren, achromatic λ / 2 plate between crossed Polarizers. The transmission of the combination for Light emitted in the forward polarisa torsion swings, can be between almost 0% and 100% be managed.

Without detection of the incoming and outgoing Polarisa direction through the polarizers are the egg properties of an achromatic λ / 2 plate, however not realized. For example, it is not possible, clockwise circularly polarized light in to transfer left-handed polarized light. Also the production of an achromatic λ / 4 plate, at for example to transfer linearly polarized Light in circular polarized light is based  on the Ge described in the above publication advises not possible.

Furthermore, DE 43 14 349 A1 contains a film on com described an optical phase delay ben. For variable compensation of an optical Phase delay is a liquid crystal in one Polymer layer ver in a spherical state divides and electrodes are made on the polymer layer transparent, conductive material, a translucent casual faceplate and a rear pane brought. The orientation angle of the river sigkristalls in the polymer layer through to the elec tread voltage can be changed.

It is therefore an object of the invention to create an achroma table phase delay element to create that a uniform influence on the polarization enabled in a wide spectral range and simple and inexpensive in sufficient size can be produced.

According to the invention, this task is characterized by the drawing part of claim 1 mentioned features solved.

Liquid crystals, preferably different, nemati are liquid crystals, are in several Layers arranged so that the fast axes are oriented identically or orthogonally (90 °).

Building on known technologies, such Phase delay elements inexpensive and with large Aperture to be produced.  

A use of a trained according to the invention Phase delay element is as a phase modulator, Polarization analyzer (in connection with a downstream polarizer) and after addition with two upstream and downstream polarization filters possible as an intensity modulator.

The order m of the individual layers can be determined by Aus selection of suitable liquid crystals should be kept to a minimum and therefore the usable opening angle accordingly turn out big. The additionally arranged elec  tric conductive elements between which the rivers sig crystal layers are made possible by the Applying a relatively low voltage a rain phase delay for each layer separately different. This can be used to tune the phase ver Delay of the entire element from a maximum value to almost zero. Besides, one is Compensation of environmental influences (e.g. change birefringence due to temperature fluctuations) possible Lich.

The phase delay designed according to the invention elements offer to the conventional phase plates made of crystalline materials an almost universal usable alternative, the layer thicknesses that Number of layers and the selected liquid crystals can be optimized for the special application can.

The achromasia of the phase delay elements is through a combination of several liquid crystals layers (k = number) determined.

The thicknesses and the number of individual liquid crystal layers can be determined mathematically. For the desired phase shift δ (in rad), e.g. B. δ = π for a λ / 2 plate, and Δn _i (λ) the wavelength-dependent birefringence of the i-th liquid crystal layer (i = 1, 2,..., K) becomes the desired wavelength interval [λ _start , λ _End ] preferably equidistantly divided into k-1 parts [λ₁, λ _{i + 1} ). In this case, the following system of equations (1.1-1.k) always has a solution (d₁,... D _k ) for liquid crystals with different dispersions of birefringence:

Δn₁ (λ₁) _* d₁ + Δn₂ (λ₁) _* d₂ +. . . + Δn _k (λ1) _* d _k = δ _* λ₁ / 2π (1.1)

Δn₁ (λ₂) _* d₁ + Δn₂ (λ₂) _* d₂ +. . . + Δn _k (λ2) _* d _k = δ _* λ₂ / 2 (1.2)

Δn₁ (λk) _* d₁ + Δn₂ (λ _k ) _* d₂ +. . . + Δn _k (λk) _* d _k = δ _* λ _k / 2π (1.k)

The values d _{i can be} positive or negative. The thickness of the i-th liquid crystal layer is the absolute amount of d _i . In the event that d _{i is} positive, the i-th layer is aligned parallel to the x-axis (vertical axis) and, on the contrary, parallel to the y-axis (horizontal axis). In this case, the desired phase shift δ for the selected wavelengths λ₁, λ₂,. . . λ _k exactly reached.

The finer the division of the wavelength interval [λ _start , λ _end ] is selected, the better the achromasia of the element. However, this is connected with an increasing number of liquid crystal layers.

A phase delay element with the electrically lei tendency electrodes has the possibility to change tion of the phase delay with suitable changes tensions. Above the response voltage applies in a suitable interval:

Δn _i (λ, V _i ) = Δn _i (λ, V _i = 0) _* (const _{i, 1} -const _{i, 2 *} V _i ),

where V _{i is} the amplitude of the alternating voltages of the i th layer.

With the choice of suitable voltages, solutions of the system of equations (1.1-1.k) for various ne phase delays δ can be found without the layer thicknesses d _i or liquid crystal layers having to be changed. Due to the small layer thicknesses, only low voltages in the volt range are required to effect sufficiently large changes.

With a suitable control loop for the created In contrast, tension at the respective layer can sentence to the known crystalline phase plates Ab deviations from the setpoints for the phase delays tion due to manufacturing tolerances or tempe influence, who compensates the. In addition, a targeted tuning of the Phase delay possible. For example, it can be a Element with the phase delay π to a neutra len element can be detuned because of high changes stress amplitudes the birefringence of the liquid crystal layers disappears. With targeted lowering of the respective AC voltage amplitude Phase delay of the entire phase delay element mentes switched on, whereby at small to ver decreasing amplitude assumes its maximum value.

Manufacturing process for multilayer liquid crystals Stall cells are sufficient for display applications known (US 42 60 224, US 44 31 270). The invention according phase delay element differs however, in that the individual liquid crystals are aligned in parallel in the individual layers and there is no "twist" arrangement. Depending on the optical properties of the for the individual layer The liquid crystals used are also the thicknesses to determine the layers. Here it can System of equations (1.1-1.k) can be used.  

Arranged between the liquid crystal layers Like the substrates, th separating layers are made of optical high quality material (e.g. quartz glass) to make an over the entire aperture uniform layer thickness, free voltage birefringence and wavefront ver to ensure strain with low losses.

The usable diameter of the phase delay element can be made using the existing manufacturing tech technologies for liquid crystal displays, with defined and uniform thickness of each liquid crystal layers easily in the range of 10 cm lie, compared to the complex and high quality manufacturing of crystal requiring legal requirements Linen delay elements cost a lot cheaper and more flexible production is possible.

Contrary to the limits that apply when using dop fur refractive crystalline materials (availability of sufficiently large and pure crystals) and their machinability (polishability with exact Layer thickness) exist, liquid crystals are in known in various forms and usable. So are for example in Landolt-Börnstein, "Numerical Data and Functional Relationships in Science and Techno logy ", New Series, Group IV: Macroscopic Properties of Matter, Vol. 7a-d: Liquid Cristals, ed. J. Thiem, Springer, Berlin / Heidelberg (1992/1993) approx. 50,000 known liquid crystals listed. With that on organic components based structure of the rivers Sig crystals arise in addition to those already known also other possibilities for the synthesis of further ge suitable liquid crystals.  

In the following, the invention will be described with reference to Aus management examples are explained in more detail.

It shows

Fig. 1 is a partial perspective Dar position of an achromatic phase delay element;

Fig. 2 is a diagram of the determined phase delay for a plate made of quartz and magnesium fluoride

Fig. 3 is a diagram of the calculated phase delay of a phase delay element according to the invention.

The partial perspective view shown in FIG. 1 of an example of an achromatic phase delay element according to the invention has, on the outer sides, as seen in relation to the beam path of the light, transparent substrates S with surfaces of optical quality and parallel orientation. In between, liquid crystal layers LC₁ to LC _k are arranged, at the outer interfaces of which there are pairs of opposite orientation layers R₁ to R _k and, in an improving manner, additional transparent, electrically conductive electrode pairs E₁ to E _k are present. Between these layers there are separating layers T made of a material which is transparent in the spectral region of interest and has surfaces of optical quality.

The thickness d _{i of} the individual liquid crystal layer LC _i is given by spacers A _i , which consist of glass fibers, My larfolien or evaporated quartz glass, before. The spacers A _i preferably have the desired thickness dimension d _i for the i th liquid crystal layer. The same distance is also specified for the mutually facing surfaces of the orientation layer R _i with the respective spacer A _i .

These orientation layers R₁ to R _k , which determine the direction of the liquid crystals in the respective liquid crystal layers LC₁ to LC _k , are preferably formed from brushed polyimide. The thickness of these orientation layers R 1 to R _k is to ensure a high transmittance in the region of about 100 nm. The rubbing direction is parallel for the orientation layers R _i, each including a liquid crystal layer LC _i . The respective direction of friction of a pair follows either the x-axis or y-axis shown in FIG. 1, depending on the result obtained with the system of equations (1.1-1.k) for the liquid crystal layer LC _{i in} question.

Just like the rubbing direction and thus the orientation of the liquid crystal layers LC _i , the thickness of the respective liquid crystal layers LC _{i is} dependent on the optical properties of the liquid crystals used for the layer and the selected spectral range with the equation system mentioned (1.1-1.k) certainly. The thickness of the liquid crystal layers LC _{i is} in the µm range.

The liquid crystals used can each selected according to requirements.

To compensate for temperature fluctuations, manufacturing tolerances or tuning the phase delay δ, control voltages U _{i can be applied} to the respective pair of transparent electrodes E _i , which can optionally be influenced by a control or regulating circuit, not shown. To suppress electrochemical reactions, AC voltages of approx. 1 kHz are used without a DC offset.

Evaporated indium tin oxide is used as the material for the electrodes E _i in layer thicknesses in the range from 5-150 nm, preferably from 10-100 nm, in order to ensure high transmission from the ultraviolet to the infrared spectral range .

FIG. 3 shows an example of a calculated phase delay δ of a two-layer phase delay element with the liquid crystals NP-5, manufacturer Merck Chemicals, Germany, and RO-TN-403, manufacturer BDH chemicals, England, and in FIG. 2, an example shown for a plate made of quartz and magnesium fluoride.

The necessary birefringence data for the liquid crystals became Shin-Tson Wu, Phys. Rev. A 33, 1270 (1986) and S.T. Wu, A.M. Lackner, U. Efron, Appl. Opt. 26, 3441 (1987).

The figure, thickness and birefringence for quartz and Magnesium fluoride are K. Serkowski, "Methods of Expe rimental physics ", ed. N. Carleton, Academie-Press York / London, Vol. 12A, 361 (1974).

The layer thickness for NP-5 is 4.6 µm, and for RO- TN-403 is 6.5 µm. This corresponds to a  Wavelength of 750 nm of an order of m = 1 for at de Liquid crystal types.

The corresponding crystal plate points at 500 nm an order of m = 6 for magnesium fluoride with a Thickness of 0.262 mm and an order of m = 5 for quartz with a thickness of 0.304 mm.

It follows that the usable opening win against the use of suitable liquid crystals Significantly improved over the crystal plates becomes. The red shift of the achromatic area is in the case of the liquid crystal pair under consideration by crossing the double dispersion curves refraction at about 750 nm for the liquid crystals and at about 500 nm for quartz and magnesium fluoride founds.

A change / shift in the achromatic world length range can also be used other liquid crystals.

As shown in this example, are different Pairs or a plurality of liquid crystals with very different dispersions of the doubles refraction available in order to achromatize with the described method the order m of each Layers and so the angle dependence of the phases delay compared to the conventional in the kri stall optics used pairs of quartz and magnesium fluo rid to decrease very.

A further improvement of the achromasia of an invented phase delay element according to the invention can with newly synthesized liquid crystals  chen, in addition to the prominent absorption in the Ultra violet another prominent absorption in the near Have infrared. The dispersion of the double bread chung with the wavelength of these liquid crystals in Visible deviates from normal behavior (at increasing wavelength strictly falling monotonously without Turning point) in that either the double refraction in red or a turning point is there. In both cases, Kombina tion with a conventional liquid crystal layer a better adaptation to the optimal course (Dop refraction proportional to λ, cf. System of equations (1.1-1.k)).

A further improvement of achromasia by Hin tereineinerschaltung several of the described Ele elements with different orientations of the fast axes, based on those of Pancharatnam in proc. Ind. Acad. Sci. A 41, 130 u. 137 (1955) and Mc Intyre / Harris in J. Opt. Soc. At the. 58, 1575 (1968) proposed procedures is also possible Lich. However, it should be noted that with a Change of the target delay of z. B. λ / 2 to λ / 4 in addition to the phase delays of the individual liquids crystal layers that yes about the created bills are adjustable, also the orientations of the fast axes must be changed to each other. This will either be the possibility of regulation the phase lag is lost, or it must be any Element can be mechanically adjusted individually nen.

Claims (16)

1. Achromatic phase delay element with several liquid crystal layers, characterized in that the liquid crystal layers (LC 1 -LC _k ) arranged one behind the other in the beam path of light are formed from different liquid crystals, the fast axes of the individual liquid crystal layers (LC 1 -LC _k ) being parallel and / or are oriented orthogonally to one another. 2. Achromatic phase delay element according to claim 1, characterized in that the liquid crystal layers (LC₁-LC _k ) in the direction of the radiation beam of light have different thicknesses (d₁-d _k ). 3. achromatic phase delay element according to claim 1 or 2, characterized in that the liquid crystal layers (LC₁-LC _k ) in the direction of the rays of light on the front and back depending on orientation layers (R₁-R _k ) are summarized. 4. Achromatic phase delay element according to one of claims 1 to 3, characterized in that a substrate layer (S) is applied to the front in the beam path of light, the first liquid crystal layers (LC₁) and on the rear side of the last liquid crystal layer (LC _k ) . 5. Achromatic phase delay element according to claim 3, characterized in that on the light path in the front orientation layer (R₁) of the first liquid crystal layer (LC₁) and on the rear orientation layer (R _k ) of the last liquid crystal layer (LC _k ) a substrate layer (S) is brought up. 6. Achromatic phase delay element according to one of claims 1 to 5, characterized in that the liquid crystal layers (LC₁-LC _k ) in the direction of the rays of light on the front and back of transparent, conductive electrodes (E₁-E _k ) are bordered . 7. achromatic phase delay element according to one of claims 1 to 6, characterized in that in the direction of the beam path for each liquid crystal layer (LC₁-LC _k ) on the front of the front orientation layer and on the back of the rear orientation layer transparent, conductive electrodes (E₁-E _k ) are applied. 8. Achromatic phase delay element according to at least one of claims 1 to 7, characterized in that between one of a liquid crystal layer (LC₁-LC _k ), a pair of orientation layers (R₁-R _k ) and a pair of transparent, conductive electrodes ( E₁-E _k ) formed layer combinations transparent elements (T) are arranged. 9. achromatic phase delay element according to claim 3, characterized in that the orientation of the liquid crystal layers (LC₁-LC _k ) is predetermined by rubbing the orientation layers (R₁-R _k ) in a certain direction be. 10. Achromatic phase delay element according to one of claims 3 to 9, characterized in that the orientation layers (R₁-R _k ) are made of brushed polyimide ge. 11. Achromatic phase delay element according to at least one of claims 1 to 10, characterized in that the conductive electrodes (E₁-E _k ) are applied to the substrates (S) and the transparent elements (T). 12. Achromatic phase delay element according to one of claims 1 to 11, characterized in that the conductive electrodes (E₁-E _k ) are vapor-deposited indium-tin oxide to which control voltages can be applied. 13. Achromatic phase delay element after at least one of claims 1 to 12, characterized, that AC voltages without DC offset can be created. 14. Achromatic phase delay element according to claim 1, characterized in that the thicknesses (d₁-d _k ) of the liquid crystal layers (LC₁-LC _k ) with inter mediate the orientation layers (R₁-R _k ) arranged spacers (A₁-A _k ) are predetermined . 15. Achromatic phase delay element according to claim 14, characterized in that the spacers (A₁-A _k ) are formed from glass fibers, Mylar foils or vapor-coated quartz glass. 16. Achromatic phase delay element according to at least one of claims 1 to 15, characterized in that the liquid crystal layers (LC₁-LC _k ) are formed from nematic liquid crystals.