DE202012005809U1 - Optical filter unit with adjustable spectral transmission - Google Patents

Abstract

Optical filter unit, characterized in that it consists of two rotatable optical filters whose total transmission (spectral width, intensity and central wavelength) can be adjusted by rotating the filter.

Description

For different applications, it is necessary to spectrally filter light: for illumination with light of low spectral bandwidth or for detection of light of low spectral bandwidth.

Currently, in light sources, spectral filtering by grating monochromators is commonly used. Since, for the filtering of light of high spectral bandwidth, both the entrance and exit slits of the monochromator must be reduced in order to reduce the spectral bandwidth, the intensity of the transmitted light decreases quadratically with the spectral bandwidth of the transmitted light. Therefore, only a very low light output is transmitted when filtering with low spectral bandwidth. Likewise, it is not possible to spectrally filter images or fiber bundles in a simple manner. Other methods do not provide adjustable spectral bandwidth (AOMs, optical filters with gradients in transmission characteristic, rotatable optical filters). With rotatable optical filters, light with fixed spectral bandwidth can be filtered, the adjustment of the central filtered wavelength being determined by the angle of incidence of the light on the filter.

The filter unit described here allows the adjustment of the central filtered wavelength and - at the same time - the spectral bandwidth, the intensity of the transmitted light decreasing only linearly with the spectral bandwidth. The exact transmission efficiency depends on the optical filters used. A beam offset, as it occurs when passing through optical filters in non-normal incidence, can be compensated. The spectral filtering of images or fiber bundles is possible.

The inventive spectral filtering is based on the and described. Two optical filters ( . and ) are arranged rotatably one behind the other. Parallel or nearly parallel light of large spectral width ( . ) is passed through the filter 1 ( . ), so that only part of the light is transmitted (exemplary transmission curve in FIG . ). After passing through the filter 2 ( . ) another part of the light is spectrally filtered (exemplary transmission curve in . ), so that now only in the overlap area of the transmission curves (overlap area in . ) of the two filters light is transmitted through the entire system. Due to the free rotation of the two filters different angles of incidence to the light can be selected (resulting transmission in . and ). This ensures that the spectral width of the respective overlap region of the transmission (overlap regions of the transmission in . and , spectral width in . and ) and the central wavelength ( . and ) can be freely adjusted within the range tunable by oblique incidence of light. Thus, light with adjustable spectral bandwidth and central wavelength can be filtered. With sufficiently steep flanks of the filter transmission curves and high filter transmission, the transmitted intensity decreases only linearly with the spectral bandwidth. Other rotatable or fixed filters can be added for improved unwanted-light rejection.

If the transmission of the filters and thus of the overall system is a priori unknown, the entire transmitted light, or only a part thereof, can be transmitted via a beam splitter ( . ) in a detector, namely a spectrometer and / or power meter ( . ), and the filter system can be adjusted by turning the filters to the desired value (central wavelength, spectral bandwidth or intensity). Through the beam splitter, this can be done during the use of the filter system, ie during a measurement.

When light passes through optical elements (here filters 1 and 2) with interfaces which are not perpendicular to the incident light and whose refractive index differs from that of air, refraction will cause light to deviate from the original direction (beam offset). This problem is reinforced in the described filter system when both filters are rotated in the same direction: the beam offset ( . ) becomes larger as it passes through each of the two filters. If both filters are twisted in opposite directions, the beam offset is partially compensated ( . ). However, complete compensation would only be possible with the same type of filters if the same angle was used, but this would not allow an adjustable spectral bandwidth. If the partial compensation of the beam offset is not sufficient, then a third rotatable optical element (a plane parallel plate or another filter in . ) are inserted into the beam path, by which the beam offset is completely compensated ( . ).

LIST OF REFERENCE NUMBERS

1

Optical filter 1

2

Optical filter 2

3

light beam

4

Beam splitter (optional)

5

Detector (spectrometer and / or power meter) (optional)

6

Transmission of filter 1

7

Transmission of filter 2

8th

Out ( 6 ) and ( 7 ) resulting total transmission curve (hatched)

9

Spectral width of the transmission curve ( 8th )

10

Central wavelength of the transmission curve ( 8th )

11

Transmission of filter 1 at a rotation angle different from ( 6 )

12

Transmission of filter 2 at a rotation angle different from ( 7 )

13

Out ( 11 ) and ( 12 ) resulting total transmission curve (hatched)

14

Spectral width of the transmission curve ( 13 )

15

Central wavelength of the transmission curve ( 13 )

16

Beam offset after transmission through optical filter 1 ( 1 ) and 2 ( 2 ) and additional optical element ( 12 )

17

Additional optical element

Claims (10)

Optical filter unit, characterized in that it consists of two rotatable optical filters whose total transmission (spectral width, intensity and central wavelength) can be adjusted by rotation of the filters. Optical filter unit according to claim 1, characterized in that it consists of a combination of filters longpass / shortpass, longpass / bandpass, bandpass / shortpass or bandpass / bandpass or one of these combinations plus additional filters. Optical filter unit according to one of the preceding claims, characterized in that it largely or completely compensates for the beam offset, which occurs during passage of the light through the oblique optical filter by opposite rotation of the optical filter or by a further rotatable optical element. Lighting system, characterized in that light of large spectral width is optically filtered with an optical filter unit according to one of the preceding claims. Lighting system according to claim 4, characterized in that a part of the transmitted light is detected (spectrometer and / or power meter) and the rotation of the filters of the optical filter unit can be adjusted accordingly. Detection system, characterized in that it spectrally filters the light to be detected with an optical filter unit according to claim 1 to 3. Detection system according to claim 6, characterized in that the light originates from a single fiber (optical fiber) and has been directed in parallel in front of the optical filter unit. Detection system according to claim 6, characterized in that it filters spectrally parallel light from a fiber bundle, wherein the spatial resolution of the individual fibers is maintained after passing through the optical filter system. Detection system according to Claim 6, characterized in that light directed in parallel, starting from an object or from an image, is spectrally filtered, and in which an image of the object or image can be generated by means of a further optical system. Detection system according to claim 9, characterized in that it is installed in a microscope and is used for spectral filtering of the microscope image.

Images