Cut-off filters

Details Cut-off filters Cut-off filters

1999-10-29

2001-08-07

Chang, Audrey (Department: 2872)

Optical: systems and elements

Light interference

Produced by coating or lamina

C359S580000, C359S490020, C359S490020, C359S490020, C359S506000

Reexamination Certificate

active

06271968

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of optics, and more particularly to cut-off filters, particularly suitable for use at long wavelengths.
BACKGROUND OF THE INVENTION
There is a need in science and technology for short- and long-wavelength cut-off filters. An ideal cut-off filter would have a very sharp transition between the transmission and rejection regions. It would have very small losses in the transmission region and a high attenuation or reflectance in the rejection region over an extended spectral range. At oblique angles of incidence its performance would be independent of the polarization of the incident light and it would have a reasonable angular field.
For the visible range of wavelengths there is a very complete set of short-wavelength cut-off filters based on absorption in colored glasses or organic dyes. They are relatively cheap and convenient to use. Many companies provide such filters.
Unfortunately, a corresponding set of long-wavelength cut-off filters does not exist for the visible and adjacent near infrared spectral regions. In the past, solutions to this problem have been sought that are based on the interference of light in thin films. The thin film layers are usually deposited onto a plate glass substrate and they may or may not be protected with an additional cover plate. When the light is incident at an oblique angle, a second type of solution is possible in which the coating is deposited at the interface of two cemented prisms. Interference cut-off filters can be constructed with the transition occurring at any desired wavelength, and almost any desired rejection can be achieved provided that a sufficiently large number of layers is used. The disadvantage of these devices is that, in order to achieve a rejection over a wide range of wavelengths, a number of contiguous layer stacks have to be used and consequently the total number of layers in the resulting system can be quite large. Another disadvantage is that the range of wavelengths over which the transmittance is high can be relatively narrow, unless special designs are used, in which case the number of layers required is even larger.
SUMMARY OF THE INVENTION
According to the present invention there is provided an optical filter with a sharp cut-off at a predetermined wavelength &lgr;, comprising a pair of optical media respectively defining an inlet port and an outlet port and having an inclined interface such that a light beam passing between said inlet and outlet ports strikes said interface at a non-normal angle of incidence &thgr;; a functional core comprising at least one material sandwiched between said optical media at said interface, said functional core having a least one optical constant which, in a transition region, varies with wavelength, giving rise to a transition between transmission and rejection at said predetermined wavelength, and wherein said angle of incidence &agr; is selected such that a sharp cut-off in the transmittance of the filter occurs at said predetermined wavelength &lgr;.
In this specification, the term optical includes light in both the visible and infrared, consistent with common practice in the art.
In accordance with the principles of the invention, use is made of light at non-normal angles of incidence at least one coating material with special properties. Such a material should be transparent in the intended transmission region, but its refractive index n or extinction coefficient k should undergo a significant decrease or increase in this and in the adjoining longer or shorter wavelength spectral region. The dispersion in n or k gives rise to important properties, namely the transition from transmission to rejection, which becomes sharper at non-zero angles of incidence and the ability to change the cut-off wavelength by changing the angle of incidence.
Materials with such properties include indium tin oxide (ITO), doped tin oxide (for example, SnO
2
:Sb), and cadmium tin oxide (Cd
2
SnO
4
). These materials are often used as transparent conducting films in electro-optical devices. They have also been proposed for use in various energy conversion and conservation applications. It is well known that substrates coated with these materials are transparent in the visible part of the spectrum, but that the coatings reflect strongly in the near infrared.
The invention also provides a method of filtering light comprising the step of passing a light beam at a non-normal angle of incidence through an active core sandwiched between a pair of optical media at an interface, said active core having a refractive index which has high dispersion in the vicinity of a predetermined cut-off wavelength &lgr; and an extinction coefficient which is high on the cut-off side and low on the transmission side of said predetermined wavelength &lgr;; said non-normal angle of incidence &agr; being selected such that total internal reflection occurs at said predetermined wavelength &lgr; to bring about a sharp cut-off in the transmittance of the filter at said predetermined wavelength.


REFERENCES:
patent: 5124841 (1992-06-01), Oishi
patent: 5808798 (1998-09-01), Weber et al.
patent: 5912762 (1999-06-01), Li et al.

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