偏振旋轉器

ARCoptix偏振旋轉器是一種扭曲向列液晶類的偏振旋轉器，它能有效調整光波的線性極化方向，當光線通過扭曲向列液晶的時候，它的極化方向會因液晶中的分子的性質而旋轉，與傳統的石英延遲片相比，扭曲向列液晶偏振控制器有更大的波長適用范圍和更大的接收角，且價格更為合理。而且，通過控制對液晶施加電壓的與否，能控制偏轉器的"開關"，因此90°旋轉器也可以被用作光學快門。

本產品有多種規格的液晶偏振旋轉器，供不同需要的客戶進行選擇，我們既提供用于科研和工業的45°或90°的標準產品，也可以根據客戶的需要定制其他特殊角度的產品.

電源（可選）

偏振控制器需要一個交變的方波電信號來控制極化方向的變化，用戶可根據需要選擇ARCoptix LC Driver，它是一款雙獨立輸出的電源，usb端口連接電腦精確控制輸出。

Liquid Crystal twisted nematic polarization rotator (TN cell) is very useful when one wants to rotate the orientation of a linear polarization by a fix amount of typically 45° or 90°. When light is traversing LC twisted nematic cell its polarization follows the rotation of the molecules (see figure below). The screens of any laptop computer is based on the same effect (polarization rotator tutorial). In optical systems the polarization is often rotated by quartz retardation plates (l/2 or l/4 plates). Quartz plates shows high quality and good transmission performances especially in the UV region. However such plates present also some disadvantages: They are expensive, functions only for a narrow spectral bandwidth and have a small incidence angle acceptance (field of view less then 2°). The liquid crystal nematic cells have therefore a large acceptance angle, function over a very large spectral range from VIS to NIR (if they are thick enough) and are less expensive. Optionally, by applying a voltage on the TN cell, the polarization rotation can be “switched off”. Also when placing a 90° twisted cell between crossed polarizers it can be used as a shutter.

• l/2 plate for a very broadband  range of wavelengths   

• Optionally rotation effect can be electrically switched off    

• In combination with two crossed polarizers it can be used as an optical shutter

 

A twisted nematic liquid crystal cell consists essentially of a liquid crystal layer placed between two treated glass substrates.
The inner-surfaces of the cell is composed of two layers: The first layer is a transparent electrode (mostly ITO). It permits to apply an electrical field across the cell and switch the cell between the OFF and the ON state. The second layer is responsible for the homogenous alignment of the LC. It is generally a rubbed polyimide layer of about 100nm.
The liquid crystal alignment at both sides of the cell is hence defined during cell manufacturing. By careful control, any twist-angle can therefore be induced in the helical structure across the liquid crystal layer. With a twist-angle of exactly 90°, the standard 90° twisted nematic (TN) cell is formed. Twist-angles of less than 90° form the low-twist (LT) cell whereas by definition, super-twist cells are cells that possess twist-angles exceeding 180°.The two glass substrates are separated by spacers with a well defined size (usually between 3mm and 20mm) and sealed with glue.

 

When the polarization rotator is in the off state, the helical structure formed by the LC molecules rotates the entrance polarization as shown in figure 1. In the ON state the polarization rotary power is suspended and the polarization state of the light entering normally to the entrance surface is not altered by the TN cell.100% efficient rotation of a linear entrance polarization can only be obtained in the limit of large cell thickness and in general the exiting light becomes elliptically polarized with components oscillating in directions lying both parallel and perpendicular to the exit liquid crystal molecules. Furthermore, it is the optical-path-difference in the liquid crystal cell that affects the overall magnitude of the polarization efficiency for the TN cell. The optical-path-difference is given by the Dnd parameter, where Dn is the anisotropic index of refraction for the liquid crystal material and d is the cell-gap. The following equation shows the transmission of a TN 90° cell as function of a normalized retardation parameter u. It assumes that the TN cell is placed between two parallel orientated ideal polarizers.

   

The best extinction (which means also the best rotation efficiency) is obtained with the highest optical path difference. So for optimal rotation of the entrance polarization over a broad spectral range it is better to use a TN cell with a high optical path difference (which means a large cell gap and a high anisotropy).
However one must be aware that higher cell gaps decrease drastically the switching time of the TN cell. So rapid switching times and high efficiency over a brad spectral range cannot be obtained. Notice that the curves shows some minimum and a custom made TN cell can be optimized to have a good rotatory efficiency (low transmission) and a rapid switching time (minimal cell gap) for a narrow range of wavelength .
In application where switching time does not matter it is better to choose a TN cell with a high optical path difference.

 

SpecificationsThe table below summarizes the principal characteristics of the device:

Rotation angle	90° or 45° (custom any angle possible for scientific and industrial grade)
wavelength range	350-1700 nm
Active area	scientific grade:10 mm or 20 mm (diam.) Industrial grade:22 mm (square)
Transmission	About  85% (VIS)
Retarder material	Nematic Liquid-Crystal Dn=0.28
Substrates	Glass
wavefront distortion	scientific grade: < lamda/4 (over 10 mm) Industrial grade: < 2lamda (over 23 mm)
temperature range	15°-35°
Rotation accuracy	+/-1° (wavelength dependent)
Maximum modulation frequency of the phase shift	< 10Hz
Save operating limit	500 W/cm2 CW 300 mJ/cm2 10 ns, visible 200 mJ/cm2 10 ns, 1064 nm
Anti-reflection coating (scientific grade only)	Broadband for VIS.
Total size (with housing)	Scientific grade: 25 mm diameter, 15mm long Industrial grade: 31mmx25mmx2.2mm (without housing).

 

 

Driver (optional)

The Polarization rotator can be driven with a standard labor function generators but it can also be driven by the USB ARCoptix LC Driver.The Arcoptix LC (Liquid Crystal) driver is a USB computer controlled electrical power supply optimized for driving one or two Polarization rotators.

The LC driver has two independent outputs (Lemo connectors). They are controlled via a simple windows compatible software. The output has a variable square amplitude with polarity inversion and a frequency of 1.6 KHz. This guarantees  a homogenous variation of the LC layer inside the cell. An external trigger input can be provided on demand.

                                              

 

Prices

Devices	Prices
Industrial grade 45° or 90° (23mm ap. without housing)	390 Euro
Scientific grade any angle (10mm ap.with housing)	790 Euro
Scientific grade (20mm ap.with housing)	1190 Euro
USB LC Driver (2 outputs)	590 Euro

 

 

 

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