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Tunable Principles

retarder as the waveplate enables the birefringence to be a function of the applied voltage. Thus by applying a different voltage, the location of the peaks can be moved and a filter can be tuned.

A single Lyot Stage is generally not particularly useful, but if several of them are stacked together, the free spectral range can be increased, while the pass band remains about the same. The following figure shows how many Lyot Stages can work in con- cert to diminish every peak except for the one of interest (top line). Modulators Spatial Spatial

Meadowlark Tunable Optical Filters facilitate spectral analysis of not only a collimated beam, but also more impor- tantly an entire object. The Tunable Filter passes only a narrow bandwidth of light while blocking all others within the desig- nated spectral range. The pass band can be shifted to a new in the blink of an eye. This combination of pass band and speed is equivalent to thousands of dichroic or interference filters on hundreds of filter wheels. This enables the user to acquire images at thousands of different in a short amount of time. Applications of Tunable Optical Filters include Tunable Optical Filters Optical Tunable , absorption microscopy, Raman microscopy and solar .

Tunable Optical Filter Basics Fig. 7-1 Example of multi-stage transmission behavior A Tunable Optical Filter works on the principle of dispersion. Simply stated, when light passes through a wave- plate (e.g. a liquid crystal variable retarder), it will be retarded Solc Stages, like Lyot Stages, are also useful in building Tunable by a certain number of waves. When light of a different wave- Filters. A Solc Stage consists of multiple waveplates at pre- Polarimeters length passes through the same waveplate, it will be retarded scribed angles between parallel (or crossed) . One by a different number of waves. advantage of using a Solc Stage over a Lyot Stage is that Solcs have a wider null and a narrower peak than a comparable Lyot. To illustrate this effect, consider a Lyot Stage. This Lyot Stage Therefore, when using stacks of Solc Stages to build a Tunable consists of two parallel (or crossed) polarizers placed around a Filter, it is possible to reduce the total number of stages needed. waveplate, which is oriented at 45 degrees to the polarizers as shown. Transmission through the Lyot Stage obeys the follow- Meadowlark Optics uses Lyot Stages, Solc Stages and other pro- ing equation, prietary types of stages to make Tunable Filters. This permits a Tunable Filter to have optimum peak transmission with a mini- T(λ) = T cos2(bt/λ) 0 mum of leakage.

where T(λ) is the transmission at λ , T 0 is the maxi- mum peak , b is the waveplate birefringence and t is its thickness. The spectral response is shown in Fig 7-1 for a 1300 nm retarder as stage one. Using a liquid crystal variable

65 www.meadowlark.com • (303) 833-4333 • [email protected] Tunable Optical Filters A. B.

Document Verification:

Modulators A. Ordinary Illumination Spatial Light Spatial B. Through Filter

Solar Astronomy

Fluorescence Microscopy Tunable Optical Filters Optical Tunable Custom Tunable Optical Filters Other specifications include field of view, which defines the applicable half-cone angle from the optical axis for which the Meadowlark Optics excels at providing customized Tunable filter performance is guaranteed. Thus the rays entering the Filters for all kinds of applications. Whether you are doing Tunable Filter should be kept less than this angle. If this is not astronomical imaging, microscopy, or remote imaging, we can find possible, the main result will be a decrease in the out-of-band a solution that meets your needs. Since all components are made rejection. Out-of-band rejection is the measure of how much light, in-house, a wide range of customization is possible. Whether you which is not within the pass band, gets through the filter; it is would like a larger clear aperture, a narrower pass band, a wider usually desirable to have the out-of-band rejection as high as Polarimeters field of view, a variable pass band, or more, we can do it. When possible. Other specifications including the tuning accuracy of the deciding on specifications for a custom filter, it is important to peak transmission can be optimized in Custom Filters. remember that not all specs can be chosen completely Meadowlark Optics has several proprietary and patent-pending independently. The most important spec to consider is the finesse, designs for Tunable Filters. If you are interested in a Custom f, which is defined as is the ratio between the free spectral range Tunable Filter with a very wide field-of-view, a variable FWHM (i.e. (FSR) of the filter and the full width at half maximum (FWHM) of several different FWHMs possible at a particular wavelength) or the pass band. Higher finesses are achieved by adding more stages any important specifications, please call us to discuss custom filter and thus more optics, which leads to a more expensive filter and requirements. also to lower peak transmission. Peak transmission is another important specification. This is defined as the maximum transmission of the pass band curve. Applications: Additionally peak transmission is generally reported for light that • Remote Sensing is polarized along the transmission axis of the first in the • Solar Astronomy Tunable Filter. If unpolarized light is incident on the filter then the • Document Verification peak transmission will be half of the polarized peak transmission. • Hyperspectral Imaging The peak transmission is a function of the center wavelength of • Fluorescence Microscopy the pass band as well as the number of optics in the Tunable Filter. • Biological/Chemical Imaging This means that the peak transmission is highly dependent on the • Pharmacological Development optical finesse, since higher finesses have more optics and hence • Chromographic Analysis more absorptive elements. • Filter wheel replacement

66 www.meadowlark.com • (303) 833-4333 • [email protected] Selectable Bandwidth Tunable Optical Filters

Key Benefits • Tuning Range • Selectable Bandwidth FWHM • ¼” -20, C-mount threads • Turnkey system included • Multiple control interface options Spatial Light Modulators

By utilizing multiple liquid crystal variable retarders and polarizers, this tunable filter allows the user to switch between any wavelength from

420 to 730 nm. In addition to the tunability of wavelength, this filter also Tunable Optical Filters allows the user a choice of three different bandwidth sizes at each wavelength. Selecting a wider bandwidth, one can increase the overall transmission of broadband light whereas the narrow FWHM can pass only a few wavelengths from a broadband light source. All tunable optical filters from Meadowlark Optics come with three components: the optical head, the controller, and the software necessary to run the tunable optical filter. This software can be installed and run on any Windows based PC through either the USB or serial port while using the provided FilterDRIVE software or commands in C++, MatLab or LabVIEW. Meadowlark optics excels at providing customized Tunable Optical

Filters for a variety of applications. This filter can also be customized to Polarimeters achieve different bandwidths, wavelength ranges or any of the other specifications. Please contact your Meadowlark Optics Sales Engineer for assistance or a custom quote.

The three different bandwidth sizes: wide (red), medium (green) and narrow (blue).

[email protected] • (303) 833-4333 • www.meadowlark.com 67 SPECIFICATIONS Wavelength Range 420 - 730 nm

Clear Aperture Diameter 20 mm Typical FWHM at 550nm * Narrow 12 nm Medium 24 nm Wide 39 nm Spatial Light Modulators Polarized Peak Transmission** 5 - 50% Tuning Resolution 0.1 nm Tuning Accuracy FWHM/10

Field of View (Half Cone Angle) ±6°

Switching Speed Long to Short Wavelengths < 100 ms

Short to Long Wavelengths < 500 ms Tunable Optical Filters Temperature Range 10°C - 35°C

Out-of-Band Blocking (Average) > OD2 Optics Head Dimensions (L x W x H) 2.50 x 3.50 x 2.08 in. Controller Dimensions (L x W x H) 9.50 x 6.25 x 1.50 in. Total System Weight 4 lbs. Minimum Systems Requirements • PC with Windows™ • USB or RS232 COM Port • Use of LabVIEW Instrument Library requires LabVIEW version 6.1 or

higher Polarimeters

* FWHM is dependent on wavelength. Please see previous page for more information The TOF-SV filter can be mounted in three different ways. ** Transmission is dependent on wavelength. Please see previous page There are 8-32 and ¼” -20 threads on two faces and the front for more information. face has M3 threads for optical cage systems. An optional adapter has C-mount threads centered around the clear Custom versions are available. Please contact our Sales Department to aperture. obtain a price quote or your Meadowlark Optics Sales Engineer for assistance.

[email protected] • (303) 833-4333 • www.meadowlark.com 68